W.C. Mahaney (editor)
elsevier
Developments in Palaeontology and Stratigraphy, 7
quatentary dating m d s Edited by
WILLIAM C. MAHANEY Department of Geography York University Downsview, Ont. M3J 2 R 7 (Canada)
E LSEV IE R Amsterdam - Oxford - New York - Tokyo 1984
ELSEVIER SCIENCE PUBLISHERS B.V. 1, Molenwerf, P.O. Box 21 1,1000 AE Amsterdam, The Netherlands Distribution for the United States and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY INC. 52, Vanderbilt Avenue New York, N.Y. 10017, U.S.A.
ISBN 0-444-42392-3 (V01.7) ISBN 0-444-41142-9 (Series) 0 Elsevier Science Publishers B.V., 1984
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V./Science & Technology Division, P.O. Box 330, 1000 AH Amsterdam, The Netherlands. Special regulations for readers in the USA - This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the publisher. Printed in The Netherlands
VII
PREFACE Dating of geological materials is of universal concern to earth scientists working on various problems within the Quaternary. If we are to adequately determine the ages of diverse materials in numerous Quaternary sequences all over the world, we must bring together specialists from several disciplines who are working, in some cases, with the newest dating techniques, and in others, with the older "classical" methods. In many research situations the use of a combination of dating methods provides the greatest return f o r time and labor s-oent in the field and laboratory. Over one hundred research scientists, from many parts of North America, convened at York University in May, 1981, to discuss dating methods of interest to Quaternary scientists. The publication of this proceedings volume should be viewed as a statement of where we are at present and where we intend to go in the future. Papers are organized into groups dealing with absolute, relative, and multiple dating methods. The first group o L p a p e r s on absolute dating (AD) methods opens with discussions of radiocarbon by J. Terasmae and J . Rucklidge. They report on some of the limitations and problems using gas counting methods in radiocarbon dating, and a new particle accelerator under construction at the University of Toronto. Both authors stress the importance of detecting very small concentrations of 1 4 C to permit dating of materials older than 40,000 years. Uranium series dating is discussed by H.P. Schwarcz, M. Gascoyne and C.E. Stearns. They summarize the problems of dating different materials, such as corals, mollusca, and speleothems, as well as the importance of using U-series dates in reconstructing sea level and climatic fluctuations. The application of 4oAr/39Arto dating young volcanic rocks provides a topic of discussion for C. Hall and D. York. They summarize numerous problems, including mineral separations and enrichment of radiogenic 40Ar. They stress that 4 0 A r can be used for very young samples down to zero age, and further, that results are often superior to K/Ar determinations. In a much younger time frame of approximately the last hundred years, "'Pb dating of lake sediments is examined by R.W. Durham and S.R. Joshi. N.D. Naeser and C.W. Naeser describe fission track dating where ages are determined from tracks counted in separate grains, and tephra characterization as it applies to dating continental sequences. R.W. Barendregt discusses paleomagnetism as a technique for relative and absolute age dating of a variety of deposits. Short-lived excursions in the continental paleomagnetic record will likely provide a useful correlative tool as more work is carried out on terrestrial sequences. A second contribution outlining the problems of matching polarity transitions in undated sequences of deposits with the radiometrically dated magnetic polarity time scale is given by M. Stupavsky and C.P. Gravenor. They investigate the problems of using small amplitude time variable characteristics (excur,sions) in successful dating and correlation of sediments. Improved sampling techniques, magnetic cleaning, screening and smoothing of sediment magnetic remanence help to refine the magnetic calendar. Dating obsidian by measuring hydration thicknesses on clasts and determination of rates of hydration is discussed by I. Friedman and F. Trembour. They stress the immense time frame over as well as the prewhich the method operates - several million years cision of measuring hydration thickness, and factors controlling the rate of hydration. Thermoluminescence dating is reported on by M. Lamothe, A. Dreimanis, M. Morency and A. Raukas. They explore the use of quartz particles in the 100-140 pm and 40-70 pm sizes and polymineralic fine silts 4-11 pm to determine TL ages over most of latePleistocene time.
-
VIII
Amino acid dating of mollusks which involves the racemization reaction where the L-form (left-handed) develops into an equilibrium mixture (50-50) of D (right-handed) and L amino acids provides the subject of discussion for J . Wehmiller. He evaluates the method utilizing samples within radiometrically dated sections and stresses that several genera give increasing D/L values with older ages, and that each genera achieves racemic equilibrium in samples ranging from early Pleistocene to Miocene age. The utility of using amino acid D/L ratios in dating wood samples is reviewed by N.W. Rutter and R.J. Crawford. They assess ratios based on aspartic acid, but caution that correlation of ratios in sedimentary sequences must be based on assumptions of similar climatic and environmental histories. The use of amino acid ratios in wood does not appear to be applicable to units older than Sangamonian age. The principle, limitations and applications of tree-ring dating are discussedby M.L. Parker, L.A. Jozsa, S.G. Johnson and P.A. Bramhall. They review the method and stress the importance of X-ray densitometry in measuring ring width and density as well as computer cross-dating employed to match samples that are difficult to date using other matching techniques. P.E. Calkin and J.N. Ellis examine the successful use of lichen measurements to date Holocene geologic events in the Brooks Range, Alaska. The problems of determining a time-size curve for slow growing R h i z o c a r p o n g e o g r a p h i c u r n form the central part of their discussion.
In the second group of papers contributors discuss various individual relative dating (RD) methods including landform characteristics, till sheet characteristics, surface and buried paleosols, pollen, and vertebrate fossils as they are used in age assignment. The use of landform characteristics in determining the relative ages of landscapes is reviewed by D.R. Coates. He synthesizes the use of various criteria such as drainage density changes, hypsometric integrals, bifurcation ratios, stream junction angles, spur morphology and hillslope geometry in arriving at relative age relationships. The use of paleosols as a relative age dating tool is examined by W.J. Vreeken. He reviews several principles of age relationships between soils and surfaces, and summarizes the use of soil properties as time indicators. A. MacS. Stalker discusses till sheet characteristics from several sections in the southwestern Canadian Prairies. Deposits are differentiated on postglacial chemical alteration, compaction, structure, jointing, style of breakage and oxidation features. C.W. Fink1 investigates the chronological ordering of pedological episodes vital to the reconstruction of paleoenvironments. He stresses the importance of micromorphological data as a tool in establishing chronological sequences of soils, especially in deeply weathered tropical terrain. Using pollen assemblages to check the accuracy of radiocarbon and for relative dating beyond the range of 5 O , O O O yrs., A.M. Davis explores numerous problems resulting from variations in pollen spectra, redeposition of Dollen from older sediments, size of the pollen sum, and use o f numerical clustering methods. Changes in vertebrate faunal groups from Blancan, through Irvingtonian to Rancholabrean Land Mammal ages is discussed by C.R. Harington. He reviews the evidence for the first appearance of Bison in North America at the end o f the Irvingtonian Age as well as conflicting absolute age determinations on the same materials. The third group of papers centers on the use of multiple criteria used in age assignment. Here contributors stress the importance o f using several relative dating methods to establish geological sequences. An evaluation of multiple dating methods used to differentiate Quaternary deposits in the Wind River and Teton Ranges, Wyoming, by W.C. Mahaney, D. Halvorson, J. Piegat and K. Sanmugadas centers on the importance of lichenometry, weathering characteristics and soils as age indicators. They pay particular attention to the use of F e ratios (oxalate extractable/dithionite extractable iron) and quartz/feldspar ratios, clay mineral composition, and organic/chemical properties in soils for age differentiation. Relative and absolute dating methods applied to dating late glacial sediments in the Lake Agassiz Basin provide the basis f o r discussion by R.W. Klassen. He assesses the discrepancy between absolute and relative chronologies, and suggests the oldest 1 4 C dates may be contaminated with dead carbon. W . J . Wayne
Ix d e s c r i b e s t h e u s e o f s e v e r a l r e l a t i v e d a t i n g methods u s e d i n t h e Rfo B l a n c o B a s i n , Cordon d e l P l a t a , Mendoza P r o v i n c e , A r g e n t i n a , t o d a t e g l a c i a l and p e r i g l a c i a l d e p o s i t s . These methods i n c l u d e z i r c o n f i s s i o n t r a c k , d e p o s i t morphology, l o e s s t h i c k n e s s e s , s o i l p r o f i l e development, v e g e t a t i o n c h a r a c t e r i s t i c s and l i c h e n s . The p a p e r s w e r e f o l l o w e d by a p a n e l d i s c u s s i o n on S a t u r d a y e v e n i n g . The p a n e l was c h a i r e d b y C.S. C h u r c h e r ( R o y a l O n t a r i o Museum, T o r o n t o ) , D . R . C o a t e s , H.B.S. Cooke ( V a n c o u v e r , B . C . ) and J . T e r a s m a e . They r e viewed and a s s e s s e d t h e i n f o r m a t i o n p r e s e n t e d by v a r i o u s s p e a k e r s . T h i s ended w i t h a s h o r t b u t l i v e l y d i s c u s s i o n l e d by C . Kolb ( L o u i s i a n a S t a t e U n i v . , Baton Rouge, L a . ) on t h e s u i t a b i l i t y of s a l t domes i n Louisiana f o r nuclear waste d i s p o s a l . The i m p o r t a n c e of u s i n g v a r i o u s d a t i n g methods t o d e t e r m i n e t h e age o f s t r e a m t e r r a c e s c u t i n l a t e T e r t i a r y s e d i m e n t s o v e r l y i n g t h e s a l t domes formed t h e m a j o r t h r u s t o f discussion. On t h e l a s t day of t h e Symposium, I . P . M a r t i n i , M.E. B r o o k f i e l d ar,d Q.H.J. Gwyn l e d a f i e l d t r i p to t h e B o w n a n v i l l e B l u f f s , a l o n g t h e n o r t h s h o r e o f Lake O n t a r i o . I n the f i e l d t r i p guide they discuss t h e s e q u e n c e o f W i s c o n s i n a n t i l l s a n d i n t e r l a y e r e d l a c u s t r i n e and f l u v i a l sediments. The l a t e H a r r y S . Crowe, Dean o f A t k i n s o n C o l l e g e a n d t h e York U n i v e r s i t y Ad-Hoe Fund a s s i s t e d w i t h f i n a n c i a l s u p p o r t f o r t h e Symposium and p u b l i c a t i o n o f t h i s volume. THe t e c h n i c a l and o r g a n i z a t i o n d e t a i l s f o r t h e Symposium w e r e worked o u t by A . Cote and h i s f a c i l i t i e s s t a f f . E . Y a t e s t y p e d some o f t h e p a p e r s and h a n d l e d numerous p r e r e g i s t r a t i o n p r o b l e m s . G . Wahab s u p e r v i s e d r e g i s t r a t i o n . Only m i n i m a l e d i t o r i a l c h a n g e s were made to t h e m a n u s c r i p t s t o c r e a t e a u n i f o r m d e s i g n through-o u t t h e vol.ume. E v e l y n Cassalman d i l i g e n t l y t y p e d a n d p r o o f r e a d t h e a b s t r a c t s - w i t h - p r o g r a m a n d f i e l d g u i d e as w e l l a s t h i s p r o c e e d i n g s volume. L i n d a Mahaney p r o o f r e a d t h e e n t i r e volume i n d r a f t and camerar e a d y copy s t a g e s . Milo Dowden p r o v i d e d e n c o u r a g e m e n t i n t h e p r e p a r a t i o n o f t h e p r o c e e d i n g s . I a m most g r a t e f u l to a l l o f t h e abovenamed i n d i v i d u a l s f o r t h e i r v a l u a b l e c o n t r i b u t i o n and s u p p o r t t o w a r d t h e p u b l i c a t i o n o f t h i s volume. F i n a l l y , a number o f s t u d e n t s a s s i s t e d w i t h t h e p l a n n i n g , o r g a n i z a t i o n and l o g i s t i c s f o r t h e Symposium. I n p a r t i c u l a r I would l i k e t o t h a n k G . Berssenbrugge', V. E l c h u k , R . F o x a l l , L . J . Gowland, P . J u l i g , J . K o l o d i z , D . MeWilliams, R . P r u k n e r and G . Yamada. S e s s i o n s were York U n i v e r s i t y . Some i l l u s t r a t i o n s were t a p e d by J . B r i g g s , D . I . A . R . , p r e p a r e d by G . B e r s s e n b r u g g e . A l l i l l u s t r a t i o n s w e r e p h o t o g r a p h i c a l l y r e p r o d u c e d by J . Dawson and J . N o l t y , D . I . A . R . , York U n i v e r s i t y . 'The f o l l o w i n g p a p e r s were p r e p a r e d by members o f t h e U . S . l o g i c a l Suyvey and a r e n o t s u b j e c t t o c o p y r i g h t :
Geo-
F i s s i o n Track Datinp The P r e s e n t S t a t u s of O b s i d i a n H y d r a t i o n D a t i n g WILLIAM C.
MAHANEY
Downsview O n t .
XI
CONTENTS Preface Contributing Authors Sessions Chairmen Panel Discussion Radiocarbon Dating: J . Te r asmae
VII XIII
XVI XVI Some Problems and Potential Developments
1
Radioisotope Detection and Dating with Particle Accelerators J . C . Ru c klidge
17
Uranium-Series Dating of Quaternary Deposits H. Schwa r cz and M. Gasco y ne
33
Uranium-Series Dating and the Histor y of Sea Level C . E . Stearn s
53
The Applicability of 40 Ar/ C . M. Hall and D. York
67
39
Ar Dating to Young Volcanics
Lead-210 Dating of Sediments from Some Northern Ontario Lakes R . W. Durham and S . R . Joshi
75
Fission Tr a ck Da ting
N.D . Naese r and C. W. Naese r
87
Using Paleomagnetic Remanence and Magneti c Susceptibility Data for the Differentiation, Relati v e Corr e lation and Absolute Dating o f Quat e rnar y Se diment s R . W. Barendreg t
101
Paleomagnetic Dating o f Quaternary Sediments: M. Stupavsky an d C . P . Gravenor
123
A Re v iew
The Present Status o f Obsidian Hy drati o n Datin g F . Trembou r and I . Friedman
141
Thermoluminescence Datin g of Quaternary Sediments M. Lamot h e , A . Dreimanis , M. Morency and A . Rau k a s
153
Relative and Absolute Dating of Quaternary Mollusks with Amino Acid Ra cemization: Evaluation, Applications, and Questions J . F . Weh mille r
171
Utilizing Wood in Amino Acid Dating N. W. Rutter and R . J . Crawford
195
Tree-Ring Dating in Canada and the Northwestern U.S. M. L . Parker , L. A . J o zsa , S . C . J o hnson and P .A. Br am ha l l
211
Development and Application of a Lichenometric Dating Curve, Broo k s Range, Alaska P . E . Calkin and J . M. Ellis
227
Landforms and Landscapes as Measures o f Relative Time D. R . Coates
247
Relative Dating of Soils and Paleosols W. J . Vreeken
2 69
XII
Dating With Pollen:
Methodology, Applications, Limitations
A. M. Da vi s
283
Mammoths, Bison and Time in No rth America
C. R. Haring t on
299
Fi eld Use of Macrofeatures for Correlating Tills and Estimating Their Ages: A Rev iew
A . MaeS . Stalker
31 1
Evaluation of Relative Pedostratigraphic Dating Methods, With Special Reference to Quaternary Successions Ov erl y ing Weathered P latform Materials
C. W. Finkl , Jr .
323
Ev aluation of Dating Methods Used to Assign Age s Ln the Wind Ri ve r and Teton Ranges, Western Wyomi ng
W. C. Mahaney , D. Halvorson , J . Piegat and K. Sanmugadas
355
Dating Me t hods Applicable to Late Glacial Deposits of the Lake Agassiz Ba sin, Manitoba
R. W. Klassen
375
The Quaternar y Succession in the Rio Blanco Basin, Cor don De l Plata, Me ndoza Pro v ince, Argent i na: An Application of Mult iple Relative-Dating Techniques ~l . J .
Wayne
389
Panel Di scussion
C. S . Chu re her , D. R. Coates , H.B . S . Cooke and J . Te r asmae
407
Qu ate r nary Stratigraphy of the Coastal Bluffs of Lake Ontario East of Os hawa
I . P. Martin i, M. E. Brookfield and Q.H.J. Gwyn
4 17
In dex
429
XI11
CONTRIBUTING AUTHORS R. W. Barendregt
Lethbridge University Department of Geography Lethbridge, Alberta
P.A.
Bramha 2 Z
F o r i n t e k Canada L t d . Vancouver, B r i t i s h Columbia
M. E.
Brookfield
U n i v e r s i t y of Guelph D e p a r t m e n t o f Land R e s o u r c e S c i e n c e Guelph, Ontario
P. Ca Z k i n
S t a t e U n i v e r s i t y of N e w York Department of G e o l o g i c a l S c i e n c e s 14226 B u f f a l o , New York
D.R.
Coates
S t a t e U n i v e r s i t y of N e w York Department of Geological Sciences Binghamton, New York 13901
R. J .
Crawford
U n i v e r s i t y of A l b e r t a Department of Chemistry Edmonton, A l b e r t a
A.M.
Davis
University of Toronto Department of Geography T o r o n t o , O n t a r i o M5S 1 A l
A.
Dreimanis
University of Western Ontario D e p a r t m e n t of G e o l o g y London, O n t a r i o N6A 5 R 7
R. W.
Durham
N a t i o n a l Water R e s e a r c h I n s t i t u t e C a n a d a C e n t r e f o r I n l a n d Waters E n v i r o n m e n t Canada Burlington, Ontario L7R 4A6
J.M.
EZZis
S t a t e U n i v e r s i t y of New York Department of Geological Sciences B u f f a l o , N e w York 14226
C.W. F i n k Z , J r .
Nova U n i v e r s i t y Ocean S c i e n c e s C e n t e r I n s t i t u t e of C o a s t a l S t u d i e s 8000 North Ocean D r i v e Dania, Florida 33004
I . Friedman
U.S.
M.
McMaster U n i v e r s i t y Department of Geology Earnilton, Ontario
Gascoyne
C. P.
Gravenor
Q.H.J. G w y n
Geological Survey Denver F e d e r a l C e n t e r Denver, Colorado
U n i v e r s i t y of Windsor Department of Geology W i n d s o r , O n t a r i o M9B 3 P 4 Dgpartement de g6ographie Universitg de Sherbrooke S h e r b r o o k e , Qudbec
XIV
HaZZ
C.M.
D.
HaZvorson
TJniver s F t y o f T o r 0 n t o D e p a r t m e n t of P h y s i c s Toronto, Ontario U n i v e r s i t y of F o r t h Dakota Department of Geology Grand F o r k s , N o r t h Dakota
C . R. Harington
N a t i o n a l lluseums o f Canada Paleobiology Division Ottawa, Ontario
5’. G . J o h n s o n
F o r i n t e k Canada L t d . Vancouver, B r i t i s h Columbia
58201
S.R.
Joshi
Na t i o n a 1 IJa t e r R e s e a r c h I n s t i t u t e C a n a d a C e n t r e f o r I n l a n d Waters Environment Canada Burlington, Ontario L7B 4A6
L.A.
Jozsa
F o r i n t e k Canada L t d . Vancouver, B r i t i s h Columbia
R. W . K Z a s s e n
M.
Lamothe
G e o l o g i c a l Survey of Canada 3 3 0 3 c 3 3 r d S t r e e t V o r t h West Calgary, Alberta U n i v e r s i t y of Irestern O n t a r i o D e p a r t m e n t of Geology L o n d o n , O n t a r i o P?6A 5 B 7
W.C.
Mahaney
York U n i v e r s i t y Atkinson College D e p a r t m e n t of Geography 4700 K e e l e S t r e e t Downsview, O n t a r i o P13J 2 R 7
I.P.
Martini
U n i v e r s i t y of Guelph D e p a r t m e n t of Land R e s o u r c e S c i e n c e Guelph, Ontario
M.
Morency
U n i v e r s i t ; du Qu6bec a l l o n t r g a l Dgpartement d e s S c i e n c e s d e l a Terre M o n t r e a l , Q u e b e c U3C 3 P 8
Naeser
U.S. G e o l o g i c a l Survey Denver F e d e r a l C e n t e r Denver, Colorado
N . D . Naeser
U.S. Geological Survey Denver F e d e r a l C e n t e r Denver, Colorado
C.W.
M.L.
J.
Parker
Piegat
M.L. P a r k e r C o . , B o x 6 3 8
P o i n t R o b e r t s , Washington 98281
Purdue University Department of Geosciences Vest Lafayette, Indiana 47907
A . Raukas
A c a d e m y o f S c i e n c e s o f T s t o n i a n SSR I n s t i t u t e of Geology 200101 T a l l i n , E s t o n i a , U . S . R . E .
J . C . RuckZidge
Vniver s it y of Toronto Department of Geology T o r o n t o , O n t a r i o 115s 1 A l
xv Rutter
N.W.
U n i v e r s i t y of A l b e r t a Department of Geology Xdmonton, A l b e r t a
K. S a n m u g a d a s
York U n i v e r s i t y Department of Geography 4.70c) K e e l e S t r e e t D o w n s v i e w , O n t a r i o 1135 1 P 3
H. Schwarcz
If c l l a s t e r Un i v e r s i t y Department of Geology Familton, Ontario
A . MacS. S t a l k e r
G e o l o g i c a l Survey of Canada 6 0 1 Booth S t r e e t Ottawa, O n t a r i o K 1 A 038
C. E.
Stearns
M. S t u p a v s k y
Tufts Yniversity Department of Geology Xedford, liassachusetts
U n i v e r s i t y of Idindsor Department of Geology FJindsor, O n t a r i o P?93 3 ? 4
.
T e r a sma e
Brock U n i v e r s i t y Deoartrnent of Geology St. Catharines, Ontario
F.
Trembour
U.5. Seolopical Survey 3enver Federal Center Denver, Colorado
J
W. J .
Vreeken
Queens U n i v e r s i t y 3 e p a r t n e n t of Geography Kingston, Ontarlo
W. J.
Wayne
U n i v e r s i t y of Nebraska Department of Geology T,i n c o 1n , E? e b r a s k a
J.F.
Wehmiller
U n i v e r s i t y of Delaware Department o f Geology Rewark, P e l a w a r e 19711
D.
York
02155
C n i v e r s i t y of Toronto Department of Physics Toronto, Ontario
XVI
SESSIONS CHAIRMEN 2133, 154 Street White Rock, B r i t i s h Columbia
H.B.S. Cooke
Guelph U n i v e r s i t y Department of Geography Guelph, Ontario
Fahey
B.D.
McMaster U n i v e r s i t y Department o f Geography Hamilton, Ontario
Ford
D.
D.R.
Grant
V . K . Prest,
A.
Retired
MacS. S t a l k e r
Geological Survey of 6 0 1 Booth S t r e e t Ottawa, Ontario
Canada
Geological Survey of 601 Booth S t r e e t Ottawa, Ontario
Canada
Geological Survey of 601 Booth S t r e e t Ottawa, Ontario
Canada
J . Terasmae
Brock U n i v e r s i t y Department of G e o l o g i c a l Sciences S t . Catharines, Ontario
J . We Z s t e d
Brandon U n i v e r s i t y Department of Geography Brandon, Manitoba
PANEL DISCUSSION C. S . C h u r c h e r
D.R.
Coates
H . B . S . Cooke
J
.
Te r a s m a e
U n i v e r s i t y of Toronto Department of Zoology Toronto, Ontario S t a t e U n i v e r s i t y of N e w York Department of Geological Sciences Binghamton, N e w York 13901
2133 1 5 4 Street White Rock, B r i t i s h Columbia V4A 4S5 Brock University Department of G e o l o g i c a l S c i e n c e s L 2S 3 A 1 St. Catharines, Ontario
1
RADIOCARBON DATING: SOME PROBLEMS AND POTENTIAL DEVELOPMENTS
J. TERASMAE
ABSTRACT R e s e a r c h on t h e r a d i o c a r b o n d a t i n g method d u r i n g t h e l a s t 2 0 y e a r s h a s i n c r e a s e d a l m o s t e x p o n e n t i a l l y i n t e r m s o f b o t h volume a n d d i v e r s i t y , and t h e r e h a s b e e n a l s o a n i n c r e a s e i n t h e number of p r o b l e m s r e The m u l t i d i s c i p l i n a r y l a t i n g t o v a r i o u s a s p e c t s of radiocarbon d a t i n g . s c o p e of many r a d i o c a r b o n d a t i n g p r o b l e m s has e e q u i r e d i n v o l v e m e n t of e x p e r t i s e from s e v e r a l f i e l d s o f r e s e a r c h and t h e p r o b l e m s can b e s e c u l a r v a r i a t i o n s of radiocarbon, g r o u p e d i n t o a few b r o a d c a t e g o r i e s : l a b o r a t o r y t e c h n i q u e s , r e f e r e n c e s t a n d a r d s , sample c o n t a m i n a t i o n , c a l i b r a t i o n a n d d a t a r e p o r t i n g , r a d i o c a r b o n i n o c e a n s , f r e s h w a t e r , and s o i l s , c o r r e c t i o n and e v a l u a t i o n o f r a d i o c a r b o n d a t e s , d a t i n g of v a r i o u s materials, e t c . T h i s a b u n d a n c e o f p r o b l e m s h a s l e d t o some n e g a t i v e c r i t i c i s m of e i t h e r t h e whole or c e r t a i n a s p e c t s o f t h e m e t h o d . On t h e p o s i t i v e s i d e , however, t h e r e c o g n i t i o n o f p r o b l e m s has l e d t o a b e t t e r u n d e r s t a n d i n g o f t h e method and s i g n i f i c a n t improvements h a v e b e e n made w i t h r e g a r d t o b o t h p r e c i s i o n and a c c u r a c y o f r a d i o c a r b o n d a t i n g . C u r r e n t a c t i v e r e s e a r c h on many p r o b l e m s by d i f f e r e n t l a b o r a t o r i e s , and e s p e c i a l l y t h e concerted e f f o r t s d i r e c t e d towards s o l v i n g s p e c i f i c p r o b l e m s by g r o u p s o f l a b o r a t o r i e s c l e a r l y i n d i c a t e t h e h e a l t h y c o n d i t i o n of t h e r a d i o c a r b o n d a t i n g f i e l d . Some e x p e c t e d d e v e l o p m e n t s a r e l i k e l y t o i n c l u d e a d d i t i o n a l s t a n d a r d i z a t i o n o f l a b o r a t o r y methods a n d p r o c e d u r e s , improved s t a n d a r d s of c a l i b r a t i o n a n d c o r r e c t i o n o f r a d i o c a r b o n d a t e s , b e t t e r u n d e r s t a n d i n g o f s e c u l a r v a r i a t i o n s o f r a d i o c a r b o n , a n d some e x t e n s i o n o f t h e r a n g e o f t h e method a l t h o u g h t h e common p r a c t i c a l l i m i t w i l l probably remain a t 30,000 t o 50,000 years BP. INTRODUCTION
R a d i o c a r b o n d a t i n g i s t h e method o f a g e d e t e r m i n a t i o n t h a t i s p r o b a b l y c o n s i d e r e d f i r s t i n most s t u d i e s o f l a t e Q u a t e r n a r y e v e n t s where a t i m e s c a l e or a c h r o n o l o g i c a l s e q u e n c e n e e d s t o b e e s t a b l i s h e d . T h i s i s n o t b e c a u s e i t i s t h e o n l y method a v a i l a b l e f o r s u c h s t u d i e s , o r e v e n t h e most a p p r o p r i a t e one j n many c a s e s , b u t r a t h e r b e c a u s e t h e p o t e n t i a l u s e r s a r e more f a m i l i a r w i t h r a d i o c a r b o n d a t i n g t h a n w i t h many o f t h e o t h e r new d a t i n g m e t h o d s . R a d i o c a r b o n d a t i n g h a s b e e n i n u s e f o r a b o u t 3 0 y e a r s s i n c e Libby
( 1 9 5 2 ) e s t a b l i s h e d t h e b a s i c p r i n c i p l e s and t e c h n i q u e s of t h e method.
I n more r e c e n t y e a r s t h e n u m b e r s o f b o t h r a d i o c a r b o n d a t i n g l a b o r a t o r i e s a i d r a d i o c a r b o n d a t e s have i n c r e a s e d almost e x p o n e n t i a l l y , and according t o a n estimate i n 1 9 7 6 a b o u t 1 5 , 0 0 0 r a d i o c a r b o n d a t e s were produced a n n u a l l y ( L i b b y , 1979). A s c o u l d b e e x p e c t e d , t h e number of p r o b l e m s r e l a t i n g t o r a d i o c a r b o n d a t i n g a l s o i n c r e a s e d as e x p e r i e n c e a c c u m u l a t e d f r o m d a t i n g
2
d i f f e r e n t k i n d s o f s a m p l e s from d i f f e r e n t e n v i r o n m e n t s a n d r a d i o c a r b o n d a t e s w e r e c h e c k e d a g a i n s t o t h e r d a t i n g methods and h i s t o r i c a l r e c o r d s . These p r o b l e m s h a v e b e e n t h e o b j e c t of a c t i v e r e s e a r c h from t h e t i m e of t h e 1 s t I n t e r n a t i o n a l R a d i o c a r b o n C o n f e r e n c e i n 1 9 5 4 . The 1 0 t h Radioc a r b o n C o n f e r e n c e was h e l d i n 1 9 7 9 a n d i f t h e number of p a g e s o f conf e r e n c e p r o c e e d i n g s i s any i n d i c a t i o n o f t h e amount o f r e s e a r c h ( a b o u t 1 0 0 0 pages i n t h e 1 0 t h Conference Proceedings) t h e n t h i s r e s e a r c h w i l l c e r t a i n l y c o n t i n u e and e v e n i n c r e a s e i n t h e f u t u r e y e a r s . Of c o u r s e , a l a r g e volume of a d d i t i o n a l d a t a has b e e n p u b l i s h e d i n v a r i o u s j o u r n a l s and r e p o r t s , i n c l u d i n g 3 a d i o c a r b o n , t h a t c o n t a i n s d a t e l i s t s from a number of l a b o r a t o r i e s a n d s p e c i f i c t e c h n i c a l n o t e s . I t would b e q u i t e i m p o s s i b l e t o c o v e r f u l l y a l l p r o b l e m s o f r a d i o c a r b o n d a t i n g i n t h i s b r i e f p a p e r and t h e r e f o r e I h a v e i n t e n t i o n a l l y c h o s e n o n l y a few p r o b l e m s f o r d i s c u s s i o n w h i l e r e a l i z i n g t h a t o t h e r p e o p l e most l i k e l y would h a v e made a d i f f e r e n t s e l e c t i o n of p r o b l e m s . R a d i o c a r b o n d a t i n g i s by no means u n i q u e w i t h r e g a r d t o p r o b l e m s of methodology b u t t h e r a n g e of t h i s d a t i n g method c o v e r s t h e t i m e of human a c t i v i t i e s and h i s t o r i c a l r e c o r d s a n d , t h e r e f o r e , some e r r o r s i n a c c u r a c y a r e e a s i e r t o d e t e c t t h a n i n t h e c a s e of o t h e r m e t h o d s . F u r t h e r m o r e , t h e o v e r e n t h u s i a s t i c u s e r s have e x p e c t e d g r e a t e r a c c u r a c y t h a n t h e method c a n n o r m a l l y o f f e r and t h i s h a s l e d t o some d i s a p p o i n t ment and sometimes r a t h e r u n w a r r a n t e d c r i t i c i s m .
T h e r e i s s t i l l some c o n f u s i o n a b o u t t h e m a t t e r of what a r a d i o carbon d a t e r e a l l y r e p r e s e n t s . The u s e r s h o u l d remember t h a t a d a t e i s b a s e d s i m p l y on a b e s t e s t i m a t e of r a d i o c a r b o n c o n t e n t of t h e s a m p l e s u b m i t t e d t o t h e l a b o r a t o r y . I t d o e s n o t i n c l u d e any o f t h e p o s s i b l e s a m p l i n g e r r o r s and n o r m a l l y none of t h e p h y s i c a l and b i o l o g i c a l e r r o r The r e p o r t e d d a t e i s a mean v a l u e s o u r c e s t h a t can a f f e c t t h e d a t e . w i t h a s t a t e d e r r o r f i g u r e , f o r example 1 1 , 1 8 0 t 1 8 0 y e a r s BP (C-SC-649) where t h e d a t a i n b r a c k e t s i d e n t i f y t h e l a b o r a t o r y a n d s a m p l e number. A s p o i n t e d o u t by Ogden ( 1 9 7 7 ) t h e s t a t i s t i c s of n u c l e a r d i s i n t e g r a t i o n a p p o x i m a t e a P o i s s o n d i s t r i b u t i o n which means t h a t t h e sample e v e n t s , r e c o r d e d as c o u n t s i n a r a d i a t i o n d e t e c t o r , a r e a s y m m e t r i c a l l y g r o u p e d a b o u t t h e mean. F o r a g e s of l e s s t h a n a b o u t 2 5 , 0 0 0 y e a r s t h e d i f f e r e n c e i s s m a l l enough s o t h a t t h e t o t a l c a l c u l a t e d e r r o r r a n g e ( a t t h e one sigma l e v e l ) i s s i m p l y d i v i d e d by 2 and t h i s v a l u e i s r e p o r t e d a s t h e one sigma e r r o r for t h e r a d i o c a r b o n d a t e . However, for g r e a t e r a g e s t h e a s y m m e t r y e f f e c t of a P o i s s o n d i s t r i b u t i o n i s s i g n i f i c a n t and t h e s e d a t e s a r e r e p o r t e d a s , f o r e x a m p l e , 4 8 , 3 0 0 t 5 0 0 , - 4 0 0 y e a r s BP ( G r N - 6 6 9 5 ) . The r e p o r t e d e r r o r i s b a s e d on t h e s t a t i s t i c s of c o u n t i n g a l o n e and n o r m a l l y i n c l u d e s u n c e r t a i n t i e s r e l a t i n g t o measurement of modern, b a c k g r o u n d , and sample a c t i v i t i e s .
Owing t o t h e s t a t i s t i c s of r a d i o c a r b o n d e t e r m i n a t i o n , t h e p r o b -
a b i l i t y t h a t t h e t r u e a g e of t h e sample i s e x a c t l y t h e r e p o r t e d mean i s z e r o , and a l l t h a t i s i m p l i e d i s t h a t t h e r e p o r t e d r a d i o c a r b o n a g e o f
t h e s u b m i t t e d s a m p l e h a s two c h a n c e s o u t of t h r e e of b e i n g w i t h i n t h e quoted l i m i t s .
I t i s i n t e r e s t i n g t o n o t e t h a t , a c c o r d i n g t o Ogden ( 1 9 7 7 ) , f e w e r t h a n 50% of t h e r a d i o c a r b o n d a t e s from g e o l o g i c a l and a r c h a e o l o g i c a l s a m p l e s i n n o r t h e a s t e r n N o r t h America have b e e n a d o p t e d as " a c c e p t a b l e " by t h e u s e r s . To m e t h i s means t h a t t h e r e i s a n e e d f o r d i s c u s s i o n o f t h e p r o b l e m s c o n c e r n i n g t h e r a d i o c a r b o n d a t i n g method and t h a t t h e u s e r s s h o u l d b e e n c o u r a g e d t o e x p l a i n why some r a d i o c a r b o n d a t e s a r e r e j e c t e d w h e r e a s o t h e r s a r e c o n s i d e r e d a c c e p t a b l e . One m i g h t w e l l wonder w h e t h e r t h e s a y i n g "my mind i s made u p , p l e a s e do n o t c o n f u s e me w i t h f a c t s " may h a v e some r e l e v a n c e i n t h i s c o n t e x t . THE SIMPLE MODEL AND SOME BASIC ASSUMPTIONS
I n p r i n c i p l e t h e r a d i o c a r b o n d a t i n g method i s r a t h e r s t r a i g h t i s produced f o r w a r d . A s d e s c r i b e d by Libby (1952), r a d i o c a r b o n ("C) i n t h e u p p e r a t m o s p h e r e by r e a c t i o n between cosmic r a d i a t i o n a n d n i t r o . g e n . R a d i o c a r b o n combines w i t h oxygen t o f o r m r a d i o a c t i v e c a r b o n d i o x i d e t h a t i s u n i f o r m l y mixed t h r o u g h o u t t h e a t m o s p h e r e and i n c o r p o r a t e d
3
i n t o t h e b i o s p h e r e ( p r i m a r i l y through p h o t o s y n t h e s i s ) and exchangedwith the hydrosphere r e s u l t i n g i n a g l o b a l equilibrium s t a t e of r a d i o c a r b o n the i n i t i a l radiocarbon a c t i v i t y . When a s u b s y s t e m ( a t r e e , a s e a s h e l l , e t c . ) i s i s o l a t e d from t h e g l o b a l s y s t e m (i.e. a t r e e i s c u t down, o r d i e s a n d i s b u r i e d i n s e d i m e n t ) t h e n n o more r a d i o c a r b o n i s a d d e d t o i t a n d t h e i n i t i a l a c t i v i t y ( a m o u n t o f 14C i n t h e s u b s y s t e m o r s a m p l e ) b e g i n s t o d e c r e a s e a c c o r d i n g t o laws o f r a d i o a c t i v e d e c a y . Prcv i d e d t h a t t h e r a t e o f d e c a y i s known ( r e f l e c t e d by t h e h a l f - l i f e o f t h e i s o t o p e ) t h e a c t i v i t y of r a d i o c a r b o n i n t h e s a m p l e i s d e t e r m i n e d and t h e l e n g t h o f t i m e ( a g e ) t h a t t h e sample has been i s o l a t e d from t h e g l o b a l e q u i l i b r i u m s t a t e c a n b e c a l c u l a t e d . The a b o v e d e s c r i p t i o n c a n b e c o n s i d e r e d as " t h e s i m p l e model" o f r a d i o c a r b o n d a t i n g . The v a l i d i t y o f t h i s m o d e l d e p e n d s on s e v e r a l b a s i c a s s u m p t i o n s which i n c l u d e : 1.
The p r o d u c t i o n o f r a d i o c a r b o n i n t h e a t m o s p h e r e h a s b e e n
2.
The m i x i n g , u p t a k e , a n d e x c h a n g e o f r a d i o c a r b o n i n t h e a t mosphere - b i o s p h e r e - h y d r o s p h e r e s y s t e m have been uniform ar?d r a p i d o n a g l o b a l s c a l e t o p r o v i d e t h e same i n i t i a l a c t i v i t y for a l l s a m p l e s u s e d f o r r a d i o c a r b o n d a t i n g .
3.
The d e c a y r a t e o f r a d i o c a r b o n h a s b e e n c o n s t a n t .
4.
No " y o u n g " o r " o l d " c a r b o n h a s b e e n a d d e d t o t h e s a m p l e s i n c e i t was i s o l a t e d f r o m t h e g l o b a l e q u i l i b r i u m s t a t e .
5.
No i s o t o p i c f r a c t i o n a t i o n h a s o c c u r r e d t o a l t e r t h e s t a n d a r d 1 4 C : 1 3 C : "C r a t i o s i n t h e sample.
c o n s t a n t f o r t h e l a s t 50,000
--
100,000 y e a r s .
Radiocarbon a n a l y s t s , i n c l u d i n g Libby, r e a l i z e d almost from t h e b e g i n n i n g t h a t a l t h o u g h t h e a b o v e a s s u m p t i o n s were g e n e r a l l y v a l i d t h e y were n o t e x a c t l y c o r r e c t a n d s u b s e q u e n t r e s e a r c h h a s b e e n d i r e c t e d towards i n v e s t i g a t i o n of problems t h a t can a f f e c t v a r i o u s a s p e c t s of t h e r a d i o c a r b o n d a t i n g m e t h o d . T h e s e p r o b l e m s c a n b e g r o u p e d i n a few general categories: ( a ) s e c u l a r f l u c t u a t i o n s of radiocarbon, (b) l a b o r a t o r y t e c h n i q u e s , ( c ) r e f e r e n c e s t a n d a r d s , ( d ) sample contamina t i o n , ( e ) c a l i b r a t i o n and d a t a r e p o r t i n g , ( f ) r a d i o c a r b o n i n o c e a n s , f r e s h w a t e r , a n d s o i l s , (g) c o r r e c t i o n a n d e v a l u a t i o n o f r a d i o c a r b o n d a t e s , and ( h ) d a t i n g of v a r i o u s m a t e r i a l s . R A D I O C A R B O N FLUCTUATIONS
Although p r o b a b l e t e m p o r a l r a d i o c a r b o n f l u c t u a t i o n s i n t h e a t mosphere - b i o s p h e r e h y d r o s p h e r e s y s t e m were s p e c u l a t i v e l y a n t i c i p a t e d d u r i n g t h e e a r l y y e a r s of r a d i o c a r b o n d a t i n g , i t r e q u i r e d t e c h n o l o g i c a l , t h e o r e t i c a l , and e m p i r i c a l advancements i n s e v e r a l d i s c i p l i n e s ( i n c l u d i n g a s t r o p h y s i c s , oceanography, and geochemistry) before some o f t h e h y p o t h e s e s c o u l d b e t e s t e d a n d a t l e a s t p a r t l y q u a n t i f i e d .
-
R e c e n t l y Damon e t 0 2 . radiocarbon fluctuations. 1.
( 1 9 7 8 ) summarized t h e p o s s i b l e c a u s e s o f
V a r i a t i o n s i n t h e r a t e of r a d i o c a r b o n p r o d u c t i o n i n t h e a t m o s p h e r e .
a.
V a r i a t i o n s i n t h e cosmic-ray f l u x throughout t h e s o l a r system. a . Cosmic-ray b u r s t s from s u p e r n o v a e a n d o t h e r s t e l l a r phenomena. b. I n t e r s t e l l a r modulation of the cosmic-ray f l u x .
b.
M o d u l a t i o n o f t h e c o s m i c - r a y f l u x by s o l a r a c t i v i t y .
c.
M o d u l a t i o n of t h e c o s m i c - r a y f l u x b y c h a n g e s i n t h e g e o magnetic f i e l d .
d.
P r o d u c t i o n by a n t i m a t t e r m e t e o r i t e c o l l i s i o n s w i t h t h e e a r t h .
4
e.
Production by nuclear weapons testing and nuclear technology.
2. Variations in the rate of exchange of radiocirbon between various geochemical reservoirs and changes in the relative carbon dioxide content of the reservoirs.
a.
Control of CO, solubility and dissolution as well as residence times by temperature variations.
b.
Effects of sea-level variations on ocean circulation and capacity.
c.
Assimilation of CC, by the terrestrial biosphere in proportion to biomass and CO, concentration, and dependence of CO, on temperature, humidity and human activity.
d.
Dependence of CC, assimilation by the marine biosphere upon ocean temperature and salinity, availability of nutrients, upwelling of C0,-rich water, and turbidity of the mixed layer of the ocean.
3. Variations in the total amount of carbon dioxide in the atmosphere, biosphere and hydrosphere. a.
Changes in the rate of introduction of CO, into the atmosphere by volcanism and other processes that result in CO, degassing of the lithosphere.
b.
The various sedimentary reservoirs serving as a sink of CC, and I4C. Tendency f o r changes in the rate of sedimentation to cause changes in the total CO, content of the atmosphere.
c.
Combustion of fossil fuels by human industrial and domestic activity.
There is no longer any doubt that natural radiocarbon fluctuations have, indeed, occurred through time, and in addition to those caused by human activities (burning of fossil fuels and nuclear weapons testing). However, the causes, magnitude, and possible cycles of the natural 1 4 C fluctuations are still the object of active research, as are, for example, the probable linkages of these fluctuations with solar activity and climate change. Dendrochronology and radiocarbon dating have had a long association because tree-ring series have provided a time scale against which radiocarbon dates could be checked, as well as providing samples of wood from the same series that could be used for radiocarbon activity measurements. For example, Suess (1955) used wood samples to demonstrate the decreasing atmospheric radiocarbon activity due to combustion of fossil fuels, and de Vries (1958) showed that atmospheric radiocarbon concentration had fluctuated due to natural causes during the last few hundred years, using samples from dendrochronologically dated tree-ring series. Subsequent research has confirmed the results of these early studies and perhaps the best known example is the study, about 7,500 years long, of the bristlecone pine tree ring series established in the southwestern United States by Ferguson (1970). Some 550 to 600 samples from this series have been analysed for radiocarbon by five different laboratories (Damon e t a Z . , 1978, Suess, 1980). It appears that both short-term (decades and centuries) and long-term (millennia) fluctuations of natural radiocarbon concentration have occurred and range from about one or two percent for the short events and about 10% for the last 10,000 years. A 10% change in radiocarbon concentration would cause an approximately 1000 years change in a corresponding radiocarbon date. Although these fluctuations comprise a problem in radiocarbon dating they do not invalidate the method because when the nature of the fluctuations is sufficiently l*iell known they can be corrected for in the radiocarbon age calculation.
5
The n u c l e a r w e a p o n s t e s t i n g c a u s e d a s u d d e n i n c r e a s e i n t h e a t mospheric r a d i o c a r b o n c o n c e n t r a t i o n ( a l m o s t 1 0 0 % ) i n t h e e a r l y 1960-s and t h e r e h a s b e e n a s t e a d y d e c l i n e i n m o r e r e c e n t y e a r s ( N y d a l e t aZ., 1979, Stenhouse and B a x t e r , 1 9 7 9 ) . This u n i n t e n t i o n a l ' e x p e r i m e n t ' has been used t o s t u d y t h e d i s p e r s a l , r e s i d e n c e t i m e , u p t a k e , and exchange of r a d i o c a r b o n i n t h e a t n o s p h e r e - b i o s p h e r e - h y d r o s p h e r e s y s t e m . On t h e o t h e r h a n d , human a c t i v i t i e s have a l t e r e d t h e n a t u r a l r a d i o c a r b o n c o n c e n t r a t i o n t o t h e e x t e n t t h a t it i s no l o n g e r p o s s i b l e t o u s e t h e p r e s e n t 1 4 C c o n c e n t r a :ioi? a 3 " n o r m a l " f o r c o m p a r i s o n w i t h p a s t c o n c e n t r a t i o n s a n d t h i s z e r t a i n l y has c r e a t e d a problem f o r r a d i o c a r b o n d a t i n g . REFERENCE STANDARDS F i e f e r e n c e s t a n d a r d s i n r a d i o c a r b o n d a t i n g a r e n e c e s s a r y for c a l i o r a t i o n and p e r i o d i c c h e c k i n g o f t h e g r o p e r o p p r a t i o n o f l a b o r a t o r y equipment, as w e l l as f o r t h e c a l c u l a t i o n of r a d i o c a r b o n d a t e s t h a t a r e r e p o r t e d t o t h e u s e r . The u s e o f r e f e r e n c e s t a n d a r d s a s s u r e s t h e u n i f o r m i t y of r a d i o c a r b o n a n a l y s i s i n i n d i v i d u a l l a b o r a t o r i e s a n d p r o v i d e s ?. common b a s e f o r c o m p a r i n g r a d i o c a r b o n d a t e s p r o d u c e d by d i f f e r e n t laboratories.
For many y e a r s t h e o x a l i c a c i d s t a n d a r d , p r o v i d e d by t h e U.S. N a t i o n a l B u r e a u o f S t a n d a r d s ( f i r s t b a t c h o f 1 0 0 0 p o u n d s was p r o d u c e d i n 1957) h a s b e e n u s e d by r a d i o c a r b o n d a t i n g l a b o r a t o r i e s . The c a r b o n 14 a c t i v i t y s t a n d a r d i s d e f i n e d a s 9 5 % o f t h e c a r b o n - l b a c t i v i t y i n t h e UBS O x a l i c A c i d S t a n d a r d . The r a p i d i n c r e a s e i n t h e number o f r a d i o carbon d a t i n g l a b o r a t o r i e s and t h e c o r r e s p o n d i n g i n c r e a s e i n u s e r demand f o r t h e O x a l i c A c i d S t a n d a r d h a d d e p l e t e d t h e s u p p l y by a b o u t 1978. 4 new b a t c h ( 1 0 0 0 p o u n d s ) was p r o d u c e d f o r t h e NBS by f e r m e n t a t i o n o f Trench b e e t m o l a s s e s f r o m t h e 1 9 7 7 s p r i n g - s u m m e r - f a l l h a r v e s t . Howeve?, zhe 1 3 C a b u n d a n c e i n t h e new s t a n d a r d was f o u n d t o b e h i g h e r by 1 . 5 1 ? O . l 7 % , a n d h e n c e , t h e new s t a n d a r d i s n o t i d e n t i c a l w i t h t h e d e p l e t e d supply of the o l d s t a n d a r d . A number o f s t u d i e s o f t h e s t a b l e i s o t o p e s o f c a r b o n ( I 3 C a n d " C ) , - a r r i e d o u t s i n c e t h e 1 9 5 0 - s , h a v e shown t h a t t h e r a t i o o f 1 3 C / 1 2 C i s l o t t h e same i n a l l s a m p l e m a t e r i a l s u s e d f o r r a d i o c a r b o n d a t i n g . ' I ' h e r e f o r e , a n o t h e r s t a n d a r d was r e q u i r e d f o r u s e a s r e f e r e n c e i n r e s p e c t L O t h e c a r b o n i s o t o p e y a t i o s , a n d for c o r r e c t i n g r a d i o c a r b o n d a t e s d u e 50 t h e variance of t h e carbon i s o t o p e r a t i o s . It h a d b e e n e s t a b l i s h e d , furthermore, t h a t t h e f r a c t i o n a t i o n of I 4 C i s v e r y c l o s e l y t w i c e t h a t ?or 1 3 C , and because o f t h i s r e l a t i o n s h i p t h e measured 1 3 C / I L C r a t i o -an be used f o r c o r r e c t i n g t h e i n i t i a l 1 4 C a c t i v i t y o f t h e s a m p l e . F o r . > h i s p u r p o s e t h e PDB C a r b o n a t e S t a n d a r d (or " C h i c a g o L i m e s t o n e S t a n d a r d " ) was e s t a b l i s h e d ( C r a i g , 1 9 5 7 ) . T h i s s t a n d a r d i s b a s e d on b e l e m n i t e s 'BeZemniteZZa americanal f r o m t h e P e e d e e F o r m a t i o n ( C r e t a c e o u s ) i n ,:outh C a r o l i n a . The 1 3 C / ' 2 C r a t i o o f a s a m p l e i s c o m p a r e d w i t h t h e .,ame i s o t o p i c r a t i o i n t h e PDB C a r b o n a t e S t a n d a r d a n d e x p r e s s e d a s 3' 3 ~ p D Bi n p e r m i l l e (O/oo).
I n a d d i t i o n t o t h e o x a l i c a c i d and b e l e m n i t e s t a n d a r d s , o t h e r . - e f e r e n c e s t a n d a r d s h a v e b e e n i n t r o d u c e d by d i f f e r e n t l a b o r a t o r i e s f o r d i f f e r e n t r e a s o n s ( P o l a c h , 1979) , f o r example: 1.
A r i z o n a 1 8 5 0 Wood, p r i m a r y r a d i o c a r b o n d a t i n g s t a n d a r d . 1.2 k i l o g r a m s o f t e n r i n g s ( A D 1846-1855) from d o u g l a s f i r growing n e a r Tucson. 6 l 3 C p ~ *= - 2 3 . 0 f 1 O/oo ( r a n g e - 2 2 t o - 2 5 O/oo).
2.
ANU ( A u s t r a l i a n N a t i o n a l U n i v e r s i t y ) S u c r o s e , s e c o n d a r y r a d i o carbon d a t i n g standard. 1000 kilograms of a n a l y t i c a l l y pure s u c r o s e produced from s u g a r cane ( S e p t . 1965 - J u n e 1 9 7 1 ) . A 1 0 t o n b a t c h o f raw s u g a r was p r o c e s s e d . 6 1 3 C p= ~ ~-11 ? 1 . 5 o/oo ( r a n g e - 0 to -13 o/oo).
3.
S c i e n t i s t s i n New Zealand have i n t r o d u c e d contemporary r e f e r ence s t a n d a r d s f o r m a r i n e s h e l l s and f r e s h - w a t e r s h e l l s , A n t a r c t i c s p e c i m e n s , t e r r e s t r i a l bone specimens and l a n d s n a i l s
6
w h i l e s t m d a r d s f o r s o i 1 s a r e b e i 5 g i n v e s k i g a t e d (3iafter e t a l . , 1 9 7 2 ) . The 6 I 3 C p ~ 3v a l u e s h a v e b e e n d e t e r m i n e d f o r t h e s e s t a n d a r d s t h a t a r e b a s e d or, c o l l e c t i o n s made in t h e 1 9 5 0 - 3 w h i c h a r e no', s u b j e c ' , t o t h e P u c l e a r h7eapor.s t e s t i r g e f f e c t s .
.
Otker a d d i t i o n a l refererice s t a r d a r d s have been i n t r o d u c e d by some l a b o r a ' , o r i e s p r i m a r i l y f o r t h e i r own u s e .
The p r o l i f e r a t i o n oi" r e f e r e r - c e s t a n d a r d s , e s p e c i a l l y i f i t C O P t i n u e s a t a n i r , c r e a s i n g r a t e , can c e r t a i n l y l e a d t o problems f o r r a d i o = a r b o n d a t i n g . Some s t a n d a r d s a r e s r o d u c e d i n s v a l l q d a n t i t y t h a t w i l l 3e q u i c k l y d e p l e t e d a n d new b a t c h e s may or may p o t b e i d e n t i c a l t o t h e ?revlous ones. Tne m a i n p r o b l e m , h o w e v e r , w i i l b e t h e u s e o f v a r i o u s s t a n d a r d s for c o r r e c t i n g r a d i o c a r b o n d a t e s . A l t h o u g h a d a t i n g l a b o r ? , t o r y n o r m a l l y p r o v i d e s a d e q u a t e docurcentation on t h e r e p o r t e d r a d i o z a r b o n d a t e s , e s p e c i a l l y i n c a s e s when s a m p l e p r e p a r a t i o n o r a g e c a l = E l a t i o n d i f f e r s from t h e g e n e r a l i y f o l l o w e d p r a c t i c e , s u c h document? t i o n i s commonly o m i t t e d by many u s e r s who p u b l i s h d a t e s w i t h o u t t h e l u a l i f y i n g d a t a p r o v i d e d by t h e l a b o r a t o r y . ' d i t h o u t q u a l i f y i n g d a t a -;ke r e a d e r h a s n o way o f k n o w i n g w h i c h d a t e s h a v e b e e n c o r r e c t e d ( i f :hey were c o r r e c t e d ? ) u s i n g w h i c h r e f e r e n c e s t a n d a r d s (or p r o c e d u r e s ) . 2 u b s e q u e n t u s e o f s u c h u n q u a l i f i e d p u b l i s h e d d a t e s w i l l o n l y compound the confusion. C A R B O N ISOTO?E FRACTIOXATION
Recent s t u d i e s have demonstrated t h a t i s o t o p i c f r a c t i o n a t i o n of -,arbon i s o t o p e s o c c u r s d u r i n g t h e growth p r o c e s s e s o f a l l o r g a n i c m a t e r i a l s . Hundreds of samples o f d i f f e r e n t s p e c i e s ( e s p e c i a l l y p l a n t s ) nave been a n a l y s e d f o r carbon i s o t o p e f r a c t i o n a t i o n and i t h a s been Tound t h a t i s o t o p e r a t i o s v a r y b e t w e e n s p e c i e s a n d a l s o b e t w e e n d i f ' e r e n t p a r t s o f t h e same o r g a n i s m . P l a n t s c a n b e d i v i d e d i n t o two g r o u p s a c c o r d i n g t o t h e e x t e n t o f -,arbon i s o t o p e f r a c t i o n a t i o n t h a t i s d i r e c t l y c o r r e l a t e d w i t h t h e p a t h way o f c a r b o n m e t a b o l i s m ( T r o u g h t o n , 1 9 7 2 ) . Most t e m p e r a t e - c l i m a t e h a r d w o o d s a n d c o n i f e r s o p e r a t e on t h e C a l v i n o r C 3 c a r b o n - f i x a t i o n c y c l e , w h e r e a s many a r i d - l a n d p l a n t s o p e r a t e on t h e S l a c k - H a t c h o r C 4 cycle. However, a t h i r d g r o u p o f p l a n t s ( t h e CAM p l a n t s , r e f e r r i n g t o C r a s s u l a c e a n Acid M e t a b o l i s m ) can o p e r a t e e i t h e r as C 3 o r C 4 p l a n t s dep e n d i n g on e n v i r o n m e n t a l c o n d i t i o n s . There i s c o n s i d e r a b l e v a r i a t i o n i n r e s p e c t t o carbon i s o t o p e composition w i t h i n each of t h e s e groups of p l a n t s (Lerman, 1 9 7 2 ) and t h e i s o t o p e f r a c t i o n a t i o n c l e a r l y i s a n o t h e r problem i n r a d i o c a r b o n d a t i n g . I t i s recommended t h a t r a d i o c a r b o n a c t i v i t i e s o f a l l s t a n d a r d s a n d s a m p l e s b e n o r m a l i z e d t o a s t a n d a r d 1 3 C / 1 2 C r a t i o , o r 6 I 3 C v a l u e on t h e PDB s c a l e : - 2 5 o/oo f o r o r g a n i c m a t t e r a n d - 1 9 o/oo f o r t h e o x a l i c a c i d s t a n d a r d (Damon e t al., 1 9 7 8 ) . T h i s n o r m a l i z a t i o n w i l l c o m p e n s a t e for e n v i r o n m e n t a l , b i o l o g i c a l , a n d l a b o r a t o r y i s o t o p e f r a c t i o n a t i o n . iowever, only about 2 0 % of l a b o r a t o r i e s p u b l i s h l i s t s o f c o r r e c t e d d a t e s ( L e r m a n , 1 9 7 2 ) . T h e r e f o r e t h e u s e r s h o u l d b e aware o f t h i s problem and be c e r t a i n whether a d a t e has o r has n o t been c o r r e c t e d f o r isotope fractionation.
From a p r a c t i c a l v i e w p o i n t , t h e c o r r e c t i o n s f o r f r a c t i o n a t i o n a r e l n a t h e r i n s i g n i f i c a n t on a 3 0 , 0 0 0 y e a r o l d sample b e c a u s e a c o r r e c t i o n o f e v e n 400 y e a r s i s u n l i k e l y t o b e s e e n w i t h i n t h e q u o t e d p l u s o r m i n u s e r r o r f o r t h a t s a m p l e . However, t h e s e c o r r e c t i o n s become i n c r e a s i n g l y s i g n i f i c a n t f o r yourger samples. F i g u r e 1 i l l u s t r a t e s t h e magnitude o f c o r r e c t i o n s f o r v a r i o u s k i n d s o f s a m p l e s t h a t r e s u l t f r o m i s o t o p e f r a c t i o n a t i o n . The c o r r e c t i o n s f o r a p p a r e n t r a d i o c a r b o n a g e s a r e shown r e l a t i v e t o " a v e r a g e wood" t h a t b e l o n g s i n t h e C 3 g r o u p o f p l a n t s a n d h a s a 6 I 3 C p ~ v a~l u e o f - 2 5 Voo. One s h o u l d r e m e m b e r t h a t i t i s i m p o r t a n t to i d e n t i f y t h e d a t e d sample t o t h e s p e c i e s o f p l a n t o r a n i m a l i f a c o r r e c t i o n f o r i s o t o p e
7
SOME E X A M P L E S OF
-s? .a
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PLANTS ( C rossulacean A c i d Metabolism cycle)
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200 I00 T O O YOUNG
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Figure 1
APPARENT
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V a r i a t i o n i n t h e a p p a r e n t r a d i o c a r b o n a g e of d i f f e r e n t kinds of samples due t o carbon isotope f r a c t i o n a t i o n , p l o t t e d r e l a t i v e t o " a v e r a g e wood" ( C J p l a n t s ) t h a t h a s a 6I3CpDB v a l u e o f -25 o / o o (modified a f t e r Stuckenrath, 1977).
fractionation is to be made at a later date on the basis of available data on fractionation. Detrital samples (composed of fragments of many species of plants, for example) present an unresolved problem in respect to correction for isotope fractionation. Lake sediments, peat, soils, and alluvial plant-bearing deposits are all detrital and collectively comprise a large proportion of all radiocarbon dated samples. CORRECTION AND REPORTING OF RADIOCARBON DATES Technological advancements in the radiocarbon dating method since the 1950-s improved significantly the precision of the method and also confirmed the discrepancy between radiocarbon dates and other time scales such as the Egyptian chronology (based on historically dated samples) and dendrochronologically dated wood. Research by Pearson e t al. (1977) has demonstrated that it is possible to make accurate measurements of radiocarbon in dendrochronological samples with an overall precision of less than 25 years standard deviation. The general conclusion is that "radiocarbon years" do not correspond exactly to calendar years or the absolute time scale and, therefore, attempts have been made to establish methods for correcting the radiocarbon chronology. The problem is that correction for natural atmospheric radiocarbon variation is only one of several corrections that have been necessary to relate the radiocarbon assay of organic sample materials to a calendar time scale. Carbon isotope fractionation has been mentioned above, and there are many reports that deal with the problems of sample contamination, including the growth processes of organisms and postdepositional addition of either "old" or "young" carbon, and the effects of such contamination on radiocarbon dates (for example, Blake, 1981;
8
Karrow a n d A n d e r s o n , 1975; Mangerud a n d G u l l i k s e n , 1 9 7 5 ; O g d e n , 1 9 7 7 ; C l s o n a n d Q r o e c k e r , 1958; O l s s o n , 1970, 1974; O l s s o n a n d B l a k e , 19616 2 ; O l s s o n a n d F l o r i n , 1983; F a r d i a n d T J a r c u s , 1 9 7 7 ; S i s s o n s , 1981; S t u c k e n r a t h , 1 9 7 7 ; S t u c k e n r a t h e t al., 1 9 7 9 ; S t u i v e r a n d B o r n s , 1 9 7 5 ; Thurber, 1972). The a t t e m p t s o f c o r r e c t i n g r a d i o c a r b o n d a t e s i n r e s p e c t t o t h e v a r i o u s s o u r c e s o f e r r o r h a v e r e s u l t e d i n many p r o p o s e d m e t h o d s o f c a l i b r a t i o n so that radiocarbon d a t e s could be expressed i n calendar y e a r s or a s a n a b s o l u t e t i m e s c a l e or c h r o n o l o g y . One oJ" t h e b e s t known m e t h o d s o f c a l i b r a t i o n i s b a s e d on t h e d e n d r o c h r o n o l o g y o f b r i s t l e c o n e p i n e ( S u e s s , 1980). X o w e v e r , Damon e t al. (1978) l i s t r e f e r e n c e s f o r n o l e s s t h a n 13 r a d i o c a r b o n c a l i b r a t i o n s c h e m e s (or " c u r v e s " ) t h a t f o l l o w t h e same o v e r a l l t r e n d b u t d i f f e r i n d e t a i l . A t t h e moment t h e r e i s n o " o f f i c i a l c a l i b r a t i o n c u r v e " for c o r r e c t i n g r a d i o c a r b o n dates and S t u c k e n r a t h (1977) c a u t i o n e d t h a t p u b l i c a t i o n of d a t e s " c o r r e c t e d " by o n e s c h e m e or a n o t h e r w o u l d m e r e l y a d d t o t h e e x i s t i n g confusion because "eventually, those l c o r r e c t e d l daies w i l l h a v e t o b e u n c o r r e c t e d i n o r d e r t o b e r e c o r r e c t e d i n o r d e r t o b e correct". The r e p o r t i n g o f r a d i o c a r b o n d a t e s a l s o p r e s e n t s sorne p r o b l e m s . By i n t e r n a t i o n a l a g r e e m e n t , a l l l a b o r a t o r i e s u s e a common s e t o f s t a n d a r d s f o r c a l c u l a t i o n and r e p o r t i n g o f r a d i o c a r b o n dates s o t h a t t h e p u b l i s h e d r e s u l t s w i l l be comparable. The NBS O x a l i c A c i d S t a n d a r d I n ada n d t h e PDB C a r b o n a t e S t a n d a r d w e r e a l r e a d y m e n t i o n e d a b o v e . d i t i o n , t h e " L i b b y " h a l f - l i f e o f 5 5 6 8 f 30 y e a r s i s u s e d i n s t e a d o f t h e more r e c e n t l y d e t e r m i n e d v a l u e o f 5730 t 4 0 y e a r s , a r d 1950 AD i s u s e d a s t h e " z e r o y e a r " ( o r " p r e s e n t " ) when r a d i o c a r b o n d a t e s a r e e x p r e s s e d U n f o r t u n a t e l y , however, a s t h e number o f y e a r s b e f o r e p r e s e n t , or B ? . complete u n i f o r m i t y w l t h r e s p e c t t o t h e u s e of s t a n d a r d s h a s n o t been a c h i e v e d a n d d i f f e r e n c e s e x i s t ir. s a m p l e p r e p a r a t i o n p r o c e d u r e s , t h e age c a l c u l a t i o n s t a t i s t i c s , ' a n d the r e p o r t i n g format of d a t e s . For e x a m p l e , F i g u r e 2 i l l u s t r a t e s t h e s i g n i f i c a n c e of' d e s i g n a t i o n s A C , '22 a n d B P , e s p e c i a l l y w i t h r e f e r e n c e t c r e p o r t i n g r e l a t i v e l y yoKng r a d i o carbon ages.
R E P O R T I N G OF R A D I O C A R B O N D A T E S AS
N
AD, BC,BP.
YEARS
E
E X A M P L E : T h e a g e of event
i s " 6 0 0 years''
( r e l a t i v e t o 1980)
6 0 0 years A D = 1980- 600 = 1380 years o l d
600 years B C = 1980 t 6 0 0 = 2 5 8 0 years o l d 6 00 years
-
B P = ( I 9 8 0 1950)
Radiocarbon
dates are commonly reported as ages
in years before present means
(BPI
where " P R E S E N T "
1950 A D . II 180
Figure 2
+ 600 = 6 3 0 years old
f
180 yr B P ( G S C - 6 4 9 )
T h e e f f e c t o f d e s i g n a t i o n s A D , B C , a n d BP i n r e p o r t i n g r e l a t i v e l y young r a d i o c a r b o n d a t e s .
9
The ___-__9 t h I n t e r r a t i o n a l 2adiocarbdn Conference (Berger and S u e s s ,
1979) passed t h e following r e s o l u t i o n s . 1.
No c h a n g e i s recommended i n t h e u s e o f c o n v e n t i o n a l 1 4 C y e a r s . P, c o n v e n t i o n a l 1 4 C y e a r i m p l i e s t h e u s e o f t h e L i b b y h a l f - l i f e of 5562 y e a r s .
2.
The r e f e r e n c e s t a n d a r d r e m a i n s 9 5 % of t h e NE3S o x a l i c a c i d a c t i v i t y , c o r r e c t e d f o r i s o t o p i c f r a c t i o n a t i o n t o a 6°C value o f -19.0 o/oo w i t h r e g a r d t o PDS. The y e a r A D 1950 c o n t i n u e s t o be t h e r e f e r e n c e y e a r f o r conventional 14c ages i n y e a r s BP. The u s e of l o w e r c a s e b p was r e j e c t e d .
3.
I t i s r e c o m n e n d e d t h a t 195C b e n o l o n g e r s u b t r a c t e d f r o m c o n v e n t i o n a l 1 4 C a g e s i n o r d e r t o a r r i v e a t a s o - c a l l e d AD/BC a g e . AD*/BC* n o m e n c l a t u r e i s t o b e u s e d a f t e r a p p l i c a t i o n of o n e of t h e a v a i l a b l e a g e c o r r e c t i o n c u r v e s or t a b l e s . The a s t e r i s k i n d i c a t e s a t r e e - r i n g c a l i b r a t e d age ( e . g . , 1 2 5 0 BC*), whereas t h e t e x t should s p e c i f y t h e curve o r t a b l e used.
4.
I t i s recommended t h a t a format for r e p o r t i n g r a d i o c a r b o n d a t e s compat i b l e w i t h c ornp u t e r -b a s e d r e t r i e v a 1 s y s t ems b e e s t a b 1i s h e d.
S u b s e q u e E t w o r k s h o p s h a v e d e a l t w i t h s e v e r a l a r e a s of c o n c e r n , for examp:e t h e c a l i b r a t i o n cf t h e r a d i o c a r b o n t i m e s c a l e (3amon e t aZ., L980), t h e u s e o f t h e new NBS O x a l i c A c i d S t a r d a r d ( S t u i v e r , 198C), a n d t h e r e p o r t i n g of marine sample a g e s . It i s e x p e c t e d t h a t contir,ued reEearch and interna:icr!al c o o p e r a : i o n w i l l r e s o l v e sor?e o f t h e p r o b l e m s t h a t can a l l t o o e a s i l y l e a d t o a c h a o t i c s i t u a t i o n i n r a d i o c a r b o n eating. The u s e r o f r a d i o c a r b o n d a t e s a l s o h a s a r e s p o n s i b i l i t y i n h e l p i n g t o avoid p o s s i b l e c o r f u s i o n i n r e s p e c t t o p u b l i s h e d d a t e s . A s mentioned e a r l i e r , l a b o r a t o r i e s normally p r o v i d e adequate background i n f o r m a t i o n (sample p r e p a r a t i o n , age c a l c u l a t i o n , c o r r e c t i o n s ) f o r r a d i o c a r b o n d a t e s r e p o r t e d t o t h e u s e ? , p a r t i c s l a r l y when s p e c i a l p r o c e d u r e s h a v e b e e n L n v o l v e d . l i o w e v e r , many u s e r s commonly o m i t t h e r a t h e r e s s e n t i a l s u p p o r t i n g d a t a , p r o v i d e d by t h e l a b o r a t o r y , when t h e y p u b l i s h r a d i o c a r b o n d a t e s and a " t h i r d g e n e r a t i o n " u s e r of a l r e a d y p u b l i s h e d d a t e s h a s no way o f k n o w i n g w h a t q u a l i f y l n g s t a t e m e r . t s were made by t h e L a b o r a t o r y - i n i t i a l l y . This can c e r t a i n l y l e a d t o problerrs. It i s important, therefore, t h a t users publish radiocarbon dates n r o p e r i y , f o r e x a m p l e , 11,189 2 1 8 0 y e a r s BF ( s S C - 6 h 9 ) , i n c l u d i n g t h e l a b o r a t o r y sample i d e n t i y i i c a t i o n d a t a and any q u a l i f y i n g i n f o r m a t i o n :hat t h e d a t i n g l a b o r a t o r y p r o v i d e s . C o r r e c t i d e n t i f i c a t i o n of t h e .;ample m a t e r i a l i s also a n i m p o r t a n t m a t t e r . A l l t h i s informatics is e s s e n t i a l i f a radiocarbon date i s t o be c o r r e c t e d at a l a t e r t i m e when t h e a p p r o p r i a t e p r o c e d u r e s h a v e beer, e s t a b l i s h e d a n d a c c e p t e d , o r i f i n t e r m e d i a t e c o r r e c t i c n s a r e d e s i r a b l e for some p a r t i c u l a r p u r p o s e . .:lithout a d e q u a t e b a c k g r o u n d d z t a n o r r . e a n i r . g f u l c o r r e c t i o n of r a d i o -,arbon d a t e s i s p o s s i b l e . COIVINENTS ON SOKE CTHER PRO3LEI:IS
Sample c o n t a m i n a t i o n h a s b e e n a n e v e r p r e s e n t p r o b l e m i n r a d i o carbon d a t i n g . I t r a n g e s f r o m t h e o b v i o u s - moderr. r o o t s , wood ? r a g x e n t s w i t h p a i n t on t h e m , a n d a p i e c e of wood w i t h a r u s t y n a i l i n i t t o t h e v i s u a l l y u n d e t e c t a b l e , such as an a p c h a e o l o g i c a l sample o f charc o a l p i e c e s d e r i v e d f r o m a m i x t u r e o f m u . l t i p l e a p e wood ( f o r e x a n p l e , p r e v i o u s s t r u c t u r e s , d r i f t w o o d , e t c . ) . U s u a l l y t h e mixed sample p r o b l e m c a n b e r e s o l v e d by d a t i n g a s e t of s a m p l e s from t h e same s t r a t i g r a p h i c l e v e l b u t f r e q u e n t l y a s i n g l e sample i s c o l l e c t e d t h a t , f u r t h e r m o r e , m i g h t b e s o s m a l l t h a t o n l y o n e a g e d e t e r m i n a t i o n car. b e made. The c o s t of d a t i n g ( a b o u t $180 p e r s a n p l e c h a r g e d by c o n n m e r c i a l l a b o r a t o r i e s ) may a l s o b e a p r o b l e m d u e t c l i m i t e d Funds f o r a p r o j e c t o r t h e l o n g t i m e t h a t c a r e f u l d a t i n g of a s e t o f s a m p l e s c a n r e q u i r e . A g e n e r a l s u r v e y o f t h e i n r l u e n c e of some p o s s i b l e a l i o c h t h c n o u s
10
m a t e r i a l o f d i f f e r e n t o r i g i n on t h e a p p a r e n t a g e o f r a d i o c a r b o n d a t e d s a m p l e s c a n b e summarized a s f o l l o w s . 1.
2.
3.
A p p a r e n t a g e t o o low.
a.
D i s s o l v e d humus p r o d u c t s f r o m s t r a t i g r a p h i c a l l y h i g h e r levels.
b.
Modern r o o t s .
c.
Sample c o n t a i n e r s ( p a p e r b a g s , c l o t h b a g s boxes).
d.
C a r e l e s s s a m p l i n g a n d sample s t o r a g e can c o n t r i b u t e younger or modern c a r b o n t o t h e s a m p l e .
,
cardboard
Apparent age t o o h i g h . a . D i s s o l v e d humus p r o d u c t s from o l d e r d e p o s i t s . b.
Dissolved groundwater carbonate.
c.
Older organic p a r t i c l e s .
d.
Eroded or r e w o r k e d d e p o s i t s ( e s p e c i a l l y c a r b o n a t e r o c k particles).
e.
Coal, l i g n i t e , g r a p h i t e p a r t i c l e s .
"Mixed" o r " a v e r a g e " a p p a r e n t a g e .
a.
S o i l organic m a t t e r (from s o i l p r o f i l e s ) .
b.
A l l u v i a l d e p o s i t s ( e r o s i o n and r e d e p o s i t i o n ) .
c.
Mixing o f d e p o s i t s b y b i o t u r b a t i o n , f r o s t a c t i o n , or human activities.
The sample c o n t a m i n a t i o n p r o b l e m s can b e a t l e a s t p a r t l y a l l e v i a t e d by u s e r a w a r e n e s s of a l l a s p e c t s o f t h i s m a t t e r , i n c l u d i n g t h e r e c o g n i t i o n of p o t e n t i a l c o n t a m i n a t i o n s o u r c e s i n t h e f i e l d . The v a l u e of c a r e f u l o b s e r v a t i o n and f i e l d n o t e s c a n n o t b e o v e r e m p h a s i z e d b e c a u s e $ h e a g e d e t e r m i n a t i o n i n t h e l a b o r a t o r y can b e o n l y a s good a s t h e q u a l i t y o f t h e s u b m i t t e d s a m p l e , and s u p p o r t i n g f i e l d d a t a can b e v e r y h e l p f u l i n d e a l i n g with contamination problems. Sample s i z e i s a n o t h e r common p r o b l e m . Normally a b o u t 5 t o 1 0 T r a m s of c a r b o n i s r e q u i r e d for a r a d i o c a r b o n a g e d e t e r m i n a t i o n . To a t t a i n h i g h a c c u r a c y i n r a d i o c a r b o n measurements P e a r s o n e t a l . (1977) u s e d 1 8 0 - 2 0 0 gram s a m p l e s of wood. U n f o r t u n a t e l y , many u s e r s s u b m i t s a m p l e s f o r d a t i n g t h a t are w e l l below t h e minimum r e q u i r e d w e i g h t . Ins u f f i c i e n t sample s i z e can a f f e c t a r a d i o c a r b o n age d e t e r m i n a t i o n i n a t l e a s t two d i f f e r e n t w a y s . It w i l l n o t b e p o s s i b l e t o p r o p e r l y p r e t r e a t (or c h e m i c a l l y c l e a n ) t h e s a m p l e , and t h e e r r o r l i m i t s of t h e d a t e w i l l b e g r e a t e r for a s m a l l s a m p l e ( F i g u r e 3 ) e v e n a t l o n g c o u n t i n g t i m e s . T h i s p r o b l e m i s a g g r a v a t e d by t h e commonly v e r y low o r g a n i c c a r b o n cont e n t of many P l e i s t o c e n e d e p o s i t s ( e s p e c i a l l y g l a c i a l d e p o s i t s ) a n d i t i s s i m p l y n o t f e a s i b l e t o o b t a i n s u f f i c i e n t l y l a r g e samples f o r r a d i o c a r b o n d a t i n g w i t h i n t h e p r a c t i c a l l i m i t a t i o n s o f t i m e and c o s t i n t h e field. I n a r c h a e o l o g i c a l r e s e a r c h i t can b e a d i f f i c u l t d e c i s i o n w h e t h e r t o s u b m i t one or more v a l u a b l e o b j e c t s t h a t c o m p r i s e a s u f f i c i e n t l y l a r g e s a m p l e f o r r a d i o c a r b o n d a t i n g ( t h a t w i l l d e s t r o y t h e s a m p l e ) or s u b m i t a v e r y s m a l l s a m p l e t h a t c a n n o t y i e l d t h e d e s i r e d a c c u r a c y of the date. SOME POTENTIAL DEVELOPMENTS
One o f t h e most e x c i t i n g r e c e n t d e v e l o p m e n t s i n r a d i o c a r b o n d a t i n g
11 400-
\
\ \
\
"0.7L counter \
"2.0 L counter
\ \
ot 2 atmospheres of CH4
3004
+
9 W
n \
a
a n z looa v,
.
*SMALL SAMPLE
\
'x..
D
I-
... .
/ - - -- - -- ---- _ _ -. ''.. -- --- - - -- ----__ -- - - - _ _- - _ _ 7 - ----
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>
5 x 1 0 yr
IOxId;r----
-3-
t .
LARGE
- - - - _- - - - - - - - _ _ _
5x10 y r
SAMPLE
I000 COUNTING
Figure 3
3
2000 TIME
3000
4000
(MINUTES)
The d i f f e r e n c e b e t w e e n s m a l l a n d l a r g e s a m p l e s w i t h r e s p e c t t o age, e r r o r l i m i t s , and counting t i m e (modified a f t e r Stuckenrath, 1977).
has b e e n t h e u s e o f t e c h n i q u e s f r o m n u c l e a r p h y s i c s f o r c o u n t i n g r a d i o carbon atoms d i r e c t l y i n t h e sample, r a t h e r t h a n t h e d i s i n t e g r a t i o n e v e n t s ( M u l l e r , 1977; L i t h e r l a n d , 1 9 7 9 ) . S i n c e t h e d e t a i l s of t h i s method w i l l b e d e s c r i b e d i n a n o t h e r p a p e r p r e s e n t e d a t t h i s c o n f e r e n c e , o n l y a few p o t e n t i a l a p p l i c a t i o n s w i l l b e m e n t i o n e d h e r e .
P e r h a p s t h e g r e a t e s t a d v a n t a g e o f t h i s new m e t h o d i s i t s c a p a b i l i t y of u s i n g v e r y s m a l l s a m p l e s ( o n l y a f e w m i l l i g r a m s ) f o r d a t i n g a n d t h u s y e s o l v e one of t h e d i f f i c u l t problems of c o n v e n t i o n a l r a d i o c a r b o n dating. Other a d v a n t a g e s i n c l u d e t h e s h o r t c o u n t i n g t i m e t h a t , c o u p l e d w i t h t h e s m a l l s a m p l e s i z e c a p a b i l i t y , c a n make i t f e a s i b l e t o o b t a i n s e t s o f d a t e s ( a s s u m i n g t h a t t h e c o s t i s n o t p r o h i b i t i v e ) f r o m , for e x a m p l e , d e t r i t a l s a m p l e s a n d s o r t o u t some o f t h e c o n t a m i n a t i o n p r o b l e m s , a n d t h e p o t e n t i a l p o s s i b i l i t y of e x t e n d i n g t h e r a n g e of r a d i o c a r b o n d a t i n g . S t u i v e r ( 1 9 7 8 ) has co m p are d t h e new a to m c o u n t i n g me th o d a n d t h e c o n v e n t i o n a l b e t a c o u n t i n g method, and one s h o u l d te mp e r o v e r e n t h u s i a s m w i t h r e a l i s m i n r e s p e c t t o t h e new m e t h o d b e c a u s e i t d o e s n o t i n s t a n t l y ' ; ol ve a l l p r o b l e m s o f r a d i o c a r b o n d a t i n g a n d make t h e c o n v e n t i o n a l m e t hod o b s o l e t e . I m p r o v e m e n t s h a v e b e e n made a l s o i n t h e t e c h n i q u e s u s e d i n t h e c o n v e n t i o n a l (or b e t a r a d i a t i o n c o u n t i n g ) me th o d o f r a d i o c a r b o n d a t i n g . - > o r e x a m p l e , H a r b o t t l e e t al. ( 1 9 7 9 ) d e s c r i b e a me th o d t h a t makes i t n o s s i b l e t o e x t e n d t h e c o n v e n t i o n a l r a d i o c a r b o n d a t i n g b y means of a s p e c i a l l y d e s i g n e d small g a s p r o p o r t i o n a l c o u n t e r t o samples c o n t a i n i n g as l i t t l e a s 1 0 m i l l i g r a m s o f c a r b o n . The a d v a n t a g e o f t h i s me th o d i s > h a t i t i s much l e s s e x p e n s i v e t h a n a n a to m c o u n t i n g f a c i l i t y w h i c h ( c o s t s w e l l i n e x c e s s o f $5OO,OOO f o r t h e b a s i c i n s t a l l a t i o n a l o n e . Another improvement i s t h e i s o t o p i c enrichment of r a d i o c a r b o n b y :herma1 d i f f u s i o n t h a t e x t e n d s t h e d a t i n g r a n g e t o a b o u t 7 5 , 0 0 0 y e a r s 3 P ( G r o o t e s , 1 9 7 8 ; S t u i v e r e t aZ., 1 9 7 8 ) . However, t h i s t e c h n i q u e i s
12
r a t h e r c o m p l i c a t e d , t i m e consuming, c o s t l y , and c e r t a i n l y n o t w i t h o u t p r o b l e m s as i l l u s t r a t e d by t h e f o l l o w i n g e x a m p l e . The Salmon S p r i n g s n o n g l a c i a l i n t e r v a l ( p e a t ) n e a r Sumner, W a s h i n g t o n , was d a t e d a t 71,500 + 1 7 0 0 , - 1 4 0 0 y e a r s BP ( Q L - 1 1 0 ) by S t u i v e r e t a l . ( 1 9 7 8 ) . On t h e o t h e r h a n d , E a s t e r b r o o k e t aZ. ( 1 9 8 1 ) o b t a i n e d f i s s i o n t r a c k a g e s ( c o u p l e d w i t h p a l e o m a g n e t i c s t u d i e s ) o f 0 . 6 6 f 0 . 0 4 m i l l i o n y e a r s (m.y.), 0 . 8 4 f 0 . 2 1 m . y . a n d 0 . 8 7 f 0 . 2 7 m . y . from t h e same s t r a t i g r a p h i c s e q u e n c e , i n d i c a t i n g t h a t t h e Salmon S p r i n g s G l a c i a t i o n i s o f m i d d l e Quaternary age. I n o t h e r c a s e s , t h e i s o t o p i c e n r i c h m e n t method seems $0 y i e l d a g e s t h a t a r e i n a g r e e m e n t w i t h i n d e p e n d e n t s t r a t i g r a p h i c and c h r o n o l o g i c a l e v i d e n c e ( S t u i v e r e t aZ., 1 9 7 8 ) . The r a d i o c a r b o n f l u c t u a t i o n s c a u s e d by c h a n g e s i n t h e g l o b a l c a r b o n cycle probably w i l l be the o b j e c t of f u r t h e r r e s e a r c h . A box-diffusion model f o r t h e c a r b o n c y c l e , p r o p o s e d b y S i e g e n t h a l e r e t aZ. ( 1 9 8 0 ) , i n d i c a t e s t h a t c a r b o n - c y c l e - i n d u c e d 1 4 C v a r i a t i o n s may h a v e b e e n s i g n i f i c a n t d u r i n g t h e t r a n s i t i o n e p i s o d e from g l a c i a l t o p o s t g l a c i a l c o n d i t i o n s when r a t h e r d r a s t i c e n v i r o n m e n t a l c h a n g e s o c c u r r e d w i t h i n a s h o r t t i m e . S i g n i f i c a n t v a r i a t i o n s o f r e s e r v o i r p a r a m e t e r s c a n b e exp e c t e d d u r i n g p e r i o d s o f m a j o r c l i m a t i c c h a n g e s , and S i e g e n t h a l e r e t aZ. ( 1 9 8 0 ) b e l i e v e t h a t c h a n g e s i n t h e s u p p o s e d o c e a n i c c i r c u l a t i o n may b e t h e most i m p o r t a n t f a c t o r (in a d d i t i o n t o t h e a t m o s p h e r i c p r o d u c t i o n r a t e o f 1 4 C ) a f f e c t i n t h e g l o b a l d i s t r i b u t i o n o f r a d i o c a r b o n . They also p o s t u l a t e t h a t g 4 C v a r i a t i o n s due t o c h a n g e s of t h e a t m o s p h e r i c C02 l e v e l o r t h e a i r - s e a exchange p r o b a b l y d i d n o t e x c e e d one t o a few p e r c e n t , and t h a t f l u c t u a t i o n s o f t h e f o r e s t b i o m a s s w h i c h may h a v e o c c u r r e d between g l a c i a l and p o s t g l a c i a l c o n d i t i o n s h a r d l y a f f e c t e d t h e 14C c o n c e n t r a t i o n o v e r a l o n g t e r m . These m a t t e r s a r e c l e a r l y o f i m p o r t a n c e t o improvement of t h e a c c u r a c y o f r a d i o c a r b o n d a t i n g . Geomagnetic s t u d i e s w i l l p r o b a b l y p l a y a g r e a t e r r o l e i n f u t u r e r a d i o c a r b o n r e s e a r c h . F o r example, B a r b e t t i ( 1 9 8 0 ) p o s t u l a t e d t h a t l o n g - t e r m v a r i a t i o n s i n t h e 1 4 C t i m e - s c a l e a r e r e a d i l y e x p l a i n e d by known g e o m a g n e t i c c h a n g e s , and S t u i v e r and Quay ( 1 9 8 0 ) p o s t u l a t e d t h a t s o l a r wind m a g n e t i c a s w e l l a s g e o m a g n e t i c f o r c e s m o d u l a t e t h e i n coming cosmic r a y f l u x a n d e x p l a i n t h e main f e a t u r e s of t h e a t m o s p h e r i c 14C r e c o r d . They a r g u e t h a t c l i m a t i c f l u c t u a t i o n s a r e n o t a dominant cause o f atmospheric I 4 C changes. A r e c e n t s t u d y by E i c h e r e t al. ( 1 9 8 1 ) , c o n c e r n i n g p o l l e n and oxygen i s o t o p e a n a l y s e s o f l a t e Q u a t e r n a r y l a c u s t r i n e d e p o s i t s i n F r a n c e , i n d i c a t e s some i n t e r e s t i n g t r e n d s i n t h e s e i n d e p e n d e n t p a l e o If s u c h s t u d i e s can b e c o r r e l a t e d w i t h g e o m a g n e t i c climatic records. r e s e a r c h and c a r b o n i s o t o p e r a t i o measurements i t i s p o s s i b l e t h a t some u s e f u l i n f o r m a t i o n c a n be o b t a i n e d on t h e c l i m a t e - r a d i o c a r b o n geomagnetic ( s o l a r a c t i v i t y ) r e l a t i o n s h i p s .
The m o n i t o r i n g o f t h e r a d i o c a r b o n c o n c e n t r a t i o n ( a c t i v i t y ) i n t h e biosphere - hydrosphere m u l t i p l e components o f t h e g l o b a l a t m o s p h e r e s y s t e m w i l l p r o b a b l y b e e x p a n d e d s i g n i f i c a n t l y i n t h e f u t u r e and hopef u l l y w i l l improve o u r u n d e r s t a n d i n g o f t h e g l o b a l c a r b o n c y c l e . Such m o n i t o r i n g i s n o t w i t h o u t some " f r i n g e b e n e f i t s " as p o i n t e d o u t by Ogden ( 1 9 7 7 ) b e c a u s e b o t h w h i s k e y and wine h a v e b e e n u s e d a s s a m p l e s f o r t r a c i n g t h e a t m o s p h e r i c bomb c a r b o n f l u c t u a t i o n s i n S c o t l a n d , Belgium and F r a n c e .
-
I wish t o conclude with t h e o p t i m i s t i c p r e d i c t i o n t h a t radioc a r b o n r e s e a r c h w i l l p r o c e e d v i g o r o u s l y on many f r o n t s i n f u t u r e y e a r s and w i l l c o n t i n u e t o i n v o l v e s c i e n t i s t s from many d i s c i p l i n e s f o r t h e b e n e f i t o f a l l c o n c e r n e d . When we u n d e r s t a n d t h e c a u s e s o f r a d i o c a r b o n f l u c t u a t i o n s i t may w e l l h a p p e n , f o r e x a m p l e , t h a t t h e r a d i o c a r b o n r e c o r d from t r e e - r i n g s c a n h e l p t h e a s t r o p h y s i c i s t l e a r n more a b o u t I t seems c e r t a i n t h a t t h e more we know a b o u t t h e past solar activity. p r o b l e m s o f r a d i o c a r b o n d a t i n g t h e b e t t e r c h a n c e we h a v e t o improve t h e method, make i t more a c c u r a t e , and more a p p l i c a b l e t o a g r e a t e r v a r i e t y o f u s e r s and d i s c i p l i n e s .
13
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Blake,W., J r . , 1 9 8 1 , G l a c i a l h i s t o r y of S v a l b a r d and t h e problem of t h e Barents Shelf i c e sheet: comments: Boreas, v. 1 0 , p . 125-131. Craig, H . , 1957, I s o t o p i c s t a n d a r d s f o r carbon and oxygen and corr e c t i o n f a c t o r s f o r mass-spectrometric a n a l y s i s of carbon d i o x i d e : v . 1 2 , p . 133-149. G e o c h i m i c a e t C o s m o c h i m i c a &a., Damon, P . E . , L e r m a n , J . C . a n d L o n g , A . , 1 9 7 8 , T e m p o r a l f l u c t u a t i o n s o f atmospheric I 4 C : c a u s a l f a c t o r s and i m p l i c a t i o n s : Annual Review of E a r t h a n d P l a n e t a r y S c i e n c e s , v . 6 , p . 4 5 7 - 4 9 4 . Damon, P . E . , L e r m a n , J . C . , L o n g , A . , B a n n i s t e r , B . , K l e i n , J . a n d L i n i c k , T.W., 1980, R e p o r t on t h e workshop on t h e c a l i b r a t i o n of the radiocarbon t i m e scale: R a d i o c a r b o n , v . 2 2 , n o . 3 , p . 947949 * De V r i e s , H 1 . , 1 9 5 8 , V a r i a t i o n i n c o n c e n t r a t i o n o f r a d i o c a r b o n w i t h t i m e and l o c a t i o n on e a r t h : K . Ned. Akad. Wet., P r o c . S e r . B , v . 6 1 , p . 94-102. Easterbrook, D.J., Briggs, N.D., Westgate, J . A . and Gorton, M.P., 1981, Age o f t h e S a l m o n S p r i n g s G l a c i a t i o n i n W a s h i n g t o n : G e o l o g y , v . 9, p . 87-93. E i c h e r , U., S i e g e n t h a l e r , U. a n d W e g m c l l e r , S . , 1 9 8 1 , P o l l e n a n d o x y g e n i s o t o p e a n a l y s e s on l a t e - and p o s t - g l a c i a l s e d i m e n t s of t h e T o u r b i e r e de- C h i r e n s (Dauphine; F r a n c e ) : Quaternary Research, v . 1 5 , p. 160-170. 1 9 7 0 , Dendrochronology of b r i s t l e c o n e p i n e , Pinus E s t a b l i s h m e n t of a 7484-year chronology i n t h e White Mountains of e a s t e r n - c e n t r a l C a l i f o r n i a : p . 237-259 i n O l s s o n , I . U . , Radiocarbon V a r i a t i o n s and A b s o l u t e Chronology: WileyI n t e r s c i e n c e , 657 p .
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diary:
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Stuckenrath, R . , M i l l e r , G.H. and Andrews, J . T . , 1979, Problems of r a d i o c a r b o n d a t i n g Holocene o r g a n i c - b e a r i n g s e d i m e n t s , Cumberland Canada: A r c t i c and Alpine RePeninsula, Baffin Island, N.W.T., s e a r c h , v . 11, n o . 1 , p . 1 0 9 - 1 2 0 . S t u i v e r , M . , 1978, Carbon-14 d a t i n g : a comparison of b e t a and i o n counting: Science, v . 202, p . 881-883. no.
, 1 9 8 0 , Workshop on I 4 C d a t a r e p o r t i n g : 3 , p . 964-966.
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22,
S t u i v e r , M. and Borns, H . W . , J r . , 1 9 7 5 , L a t e Q u a t e r n a r y m a r i n e i n v a s i o n i n Maine: i t s chronology and a s s o c i a t e d c r u s t a l movements: Geol. SOC. A m e r . B u l l . , v . 8 6 , p . 99-104. S t u i v e r , M . , H e u s s e r , C . J . and Yang, I . C . , h i s t o r y extended t o 75,000 y e a r s ago:
1978, North American g l a c i a l S c i e n c e , v . 200, p. 16-21.
S t u i v e r , M. and Quay, P.D., 1980, P a t t e r n s of Radiocarbon, v . 22, no. 2 , p . 166-176.
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I 4 C
1 9 5 5 , R a d i o c a r b o n c o n c e n t r a t i o n i n modern wood: Suess, H.E., V. 1 2 2 , p . 415-417.
, 1 9 8 0 , The r a d i o c a r b o n r e c o r d i n t r e e r i n g s o f 8000 y e a r s : R a d i o c a r b o n , v. 2 2 , n o . 2 , p . 200-209.
changes: Science,
the last
T h u r b e r , D . L . , 1 9 7 2 , P r o b l e m s o f d a t i n g non-woody m a t e r i a l f r o m c o n t i n e n t a l environments: i n Bishop, W.W. and Miller, J . A . , e d ~ . , C a l i b r a t i o n of Hominid E v o l u t i o n , Univ. T o r o n t o Press, T o r o n t o , p . 1-17. Troughton, J . H . , 1 9 7 2 , Carbon I s o t o p e f r a c t i o n a t i o n by p l a n t s : i n Rafter, T.A. and Grant T a y l o r , T . , e d s . , 8 t h I n t e r n . Radiocarbon Conf. P r o c . , Royal S o c i e t y of New Z e a l a n d , W e l l i n g t o n , N . Z . , p. 420-438.
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RADIOISOTOPE DETECTION AND DATING WITH PARTICLE ACCELERATORS J. C.R UCK L I DG E
ABSTRACT Radiocarbon d a t i n g i s c o n v e n t i o n a l l y p e r f o r m e d b y t h e accuratemeasuren e n t o f t h e b e t a r a y s e m i t t e d d u r i n g t h e d e c a y o f 1 4 C . The d i r e c t d e t e r m i n a t i o n o f "C a t o m s may a l s o b e a c c o m p l i s h e d by u l t r a s e n s i t i v e m a s s spectrometry using a p a r t i c l e a c c e l e r a t o r . S i g n a l t o n o i s e r a t i o s i n excess of i O 1 5 : l are r e q u i r e d f o r t h i s t a s k , and can o n l y be a t t a i n e d i f t h e b a c k g r o u n d f r o m t h e i s o b a r 1 4 N a n d from i n t e r f e r i n g m o l e c u l a r i o n s i s e l i m i n a t e d . By u s i n g a t a n d e m Van d e Graaff a c c e l e r a t o r as a m o l e c u l e d i s i n t e g r a t o r a n d a c c e p t i n g o n l y n e g a t i v e i o n s , t h i s c o n d i t i o n may be a t t a i n e d a n d i n d i v i d u a l 1 4 C a t o m s may t h u s b e c o u n t e d d i r e c t l y . The h i g h e r c o u n t i n g r a t e s i n h e r e n t i n t h i s method, and t h e a b i l i t y t o u s e m c h s m a l l e r sample s i z e s o f f e r s i g n i f i c a n t a d v a n t a g e s o v e r t h e convent i o n a l method. I n addition, other naturally occurring radioactive isot o p e s w i t h h a l f l i v e s r e l e v a n t t o Q u a t e r n a r y d a t i n g may b e m e a s u r e d I'C, s i n g t h e same a p p r o a c h . A t t h e p r e s e n t t i m e t h e i s o t o p e s " B e , 2 6 A 1 , 3 2 S i , 3 6 C l a n d '''1 h a v e b e e n d e t e c t e d a t n a t u r a l l e v e l s w i t h o u t p r i o r i s o t o p e e n r i c h m e n t by u s i n g n e g a t i v e i o n s , m o l e c u l a r d i s s o c i a t i o n - d i t h tandem a c c e l e r a t o r s and atom c o u n t i n g . R a t i o s of 1 4 C / 1 2 C and 3 6 C l / c11 n e a r have been reached d u r i n g experiments b e i n g c a r r i e d o u t t o develop r a d i o c a r b o n and 3 6 C l d a t i n g o f milligramme s a m p l e s . With t h e s b i l i t y t o u s e such small s a m p l e s , v a l u a b l e g e o l o g i c a l and a r c h a e o l o g i c 11 s p e c i m e n s w i l l b e d a t a b l e w i t h a u t s i g n i f i c a n t d a m a g e , a n d t h e c o s t s N e w f a c i l i t i e s d e d i c a t e d t o t h i s t y p e of i n v o l v e d may b e r e d u c e d . neasurement are b e i n g c o n s t r u c t e d a t v a r i o u s c e n t r e s , and are e x p e c t e d t o begin producing r o u t i n e radiocarbon dates i n 1982. INTRODUCTION
Several r a d i o i s o t o p e s of l i g h t elements are produced through spal'Lation p r o c e s s e s c a u s e d by c o s m i c r a y s i n t e r a c t i n g w i t h t h e common como o n e n t s o f t h e a t m o s p h e r e . O f t h e s e t h e b e t t e r known a r e 1 4 C ( h a l f l i f e If equi5 7 3 0 a ) f r o m I4N, a n d 3 6 C l ( h a l f l i f e 3 . 0 1 x 1 0 5 a ) f r o m 3 6 A r . librium i s assumed, t h e r a t e a t which t h e r a d i o a c t i v e i s o t o p e i s prol u c e d i s b a l a n c e d by t h e r a t e o f i t s l o s s d u e t o b e t a d e c a y . The e q u i librium concentration of 1 4 C i n l i v i n g organic matter i s about 1 . 2 ~ 1 0 - l ~ s t o m s p e r a t o m o f "C. After t h e organism's d e a t h t h e 1 4 C i s no l o n g e r ConT e p l e n i s h e d , a n d i t d e c a y s away w i t h a h a l f l i f e o f 5 7 3 0 y e a r s . -Jentional r a d i o c a r b o n d a t i n g , as p i o n e e r e d by Libby (Anderson e t aZ., 1 9 4 7 ) i s c a r r i e d o u t by m e a s u r i n g t h e r a d i o a c t i v i t y o f t h e s a m p l e d u e 50 t h e b e t a d e c a y of t h e 1 4 C a t o m s i n i t . For c o n t e m p o r a r y c a r b o n t h e s p e c i f i c a c t i v i t y i s a b o u t 1 5 d i s i n t e g r a t i o n s p e r minute p e r gram, though e a c h gram o f m a t e r i a l c o n t a i n s 6 x 1 0 ' ~ a t o m s o f 1 4 C . C l e a r l y , a n y :echnique w h i c h c a n i d e n t i f y t h e s e I 4 C a t o m s d i r e c t l y , e v e n w i t h v e r y -nodest c o l l e c t i o n e f f i c i e n c i e s o f , s a y , lom3 w o u l d b e s o much more e f f i c i e n t than r a d i o a c t i v e measurements ( t h e f r a c t i o n a l d i s i n t e g r a t i o n o f 14C i s 1 . 4 x l O - * / h o u r ) t h a t t h e s i z e o f t h e s a m p l e n e e d e d f o r d i r e c t a t o m - 0 u n t i n g would b e d r a m a t i c a l l y l e s s t h a n t h a t r e q u i r e d b y t h e c o n v e n t i o n a l m e t h o d . A t t h e same t i m e , h i g h e r a c c u r a c y i s i n p r i n c i p l e p o s s i b l e , b e c a u s e many more c o u n t s s h o u l d b e r e c o r d e d i n much s h o r t e r
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c o u n t i n g times. Even s a m p l e s as o l d as 100,300 y e a r s c o n t a i n 3 . 3 5 x l o 5 a t o m s 1 4 C / g r a m w h i c h s h o u l d p r o v i d e e n o u g h c o u n t s to o b t a i n s a t i s f a c t o r y s t a t i s t i c s on s p e c i m e n s more t h a n t w i c e a s o l d a s t h o s e w h i c h c a n b e d a t e d by t h e b e t a d e c a y m e t h o d . C o n t a r r i n a t i o n o f s a m p l e s would p r o b a b l y p r o v e to b e t h e f a c t o r l i m i t i n g t h e a g e w h i c h c o u l d b e m e a s u r e d reliably. To e m p h a s i z e t h e p r o b l e m s o f d e t e c t i n g o n e 1 4 C a t o m i n a n o c e a n o f 10'' a t o m s o f 1 2 C a n d 1 3 C , i t i s i n s t r u c t i v e t o c o n s i d e r t h e a n a l a g o u s p r o b l e m o f i d e n t i f y i n g a s i n g l e s p e r i f i c g r a i n o f s a n d , 0.25mm d i a m e t e r , ir. a p i l e w i t h a v o l u m e o f 88 c u b i c m e t r e s , e g u Z v a l e n t t o t h e c o n t e n t s of f i v e loaded duap t r u c k s ! One o f t h e m a j o r p r o b l e m s i n p e r f o r r n i n g t h i s s o r t i n g by c o n v e r . t i a n a 1 mass s p e c t r o m e t r y i s t h e e x i s t e n c e o f t h e i s o b a r o f 1 4 C , I4N w h i c h i s u b i a u i t o u s i n c o m p a r i s o n , a n d w h i c h d i f r e r s f r o m 1 4 C i n mass b y o n l y o n e p a r t i n l o 5 , h e n c e a n e x t r e m e l y h i g h r e s o l u t i o n mass s p e c t r o m e t e r w o u l d b e n e e d e d t o s e p a r a t e t h e few 1 4 C a t o m s f r o m t h e many 1 4 N a t o m s . An a d d i t i o n a l p r o b l e m i s t h e e x i s t e n c e o f m o l e c u l e s s u c h a s 1 2 C H 2a n d l 3 C 9 w h i c h h a v e masses e v e n c l o s e r t o 14C a n d h e n c e c o n t r i b u t e to a n o v e r w h e l m i n g b a c k g r o u n d w h i c h p r e v e n t s t h i s s i m p l e mass s p e c t r o m e t r i c a p proach from b e i n g s u c c e s s f u l . One e a r l y a t t e m p t w h i c h came c l o s e t o a t t a i n i n g t h e n e c e s s a r y s e n s i t i v i t y of 1 i n 1012 to d e t e c t 1 4 C i n c o n t e m p o r a r y c a r b o n was t h a t o f Anbar e t a % . i n 1 9 7 4 ( A n b a r , 1 9 7 8 ) . The s t a b l e m o l e c u l e 14C15N- was u s e d t o d i s c r i m i n a t e a g a i n s t I4NI5N- i o n s w h i c h were e x p e c t e d t o b e u n s t a b l e . The l i m i t a t i o n s w e r e b a c k g r o u n d s from "C,HS-, 12C160H-, "Sia n d "SiH-. By c a r e f u l e l i m i n a t i o n o f h y d r o g e n f r o m t h e s y s t e m a s e n s i t i v i t y o f 1 i n 10" was o b t a i n e d , b u t t h e u l t i m a t e s t u m b l i n g b l o c k proved t o be "Si-. The b r e a k t h r o u g h i n t h i s p r o b l e m h a d to await t h e d e v e l o p m e n t o f C- i o n s o u r c e s a n d t h e r e a l i z a t i o n t h a t p a r t i c l e a c c e l e r a t o r s , as used i n n u c l e a r p h y s i c s exp e r i m e n t s , c o u l d b e made t o f u n c t i o n a s u l t r a s e n s i t i v e mass s p e c t r o m e t e r s , a n d i n t h i s mode o f o p e r a t i o n t h e t w o m a j o r d i f f i c u l t i e s o u t l i n e d a b o v e c o u l d b e o v e r c o m e . R e v i e w s o f t h e u s e o f a c c e l e r a t o r s for u l t r a s e n s i t i v e a n a l y s i s may b e f o u n d i n a r t i c l e s b y L i t h e r l a n d 1 9 8 0 , a n d L i t h e r l a n d e t aZ. ( 1 9 8 1 ) . ACCELERATOR MASS SPECTROMETRY
The p r o b l e m o f m o l e c u l a r i n t e r f e r e n c e s i n t h e s t u d y o f r a r e i s o t o p e s by mass s p e c t r o m e t r y c a n b e s o l v e d by t h e u s e o f a t a n d e m a c c e l e r a t o r a s a molecule d i s i n t e g r a t o r ( P u r s e r , 1977). Negative molecular ions diss o c i a t e r a p i d l y a n d c o m p l e t e l y a f t e r l o s i n g t h r e e or more e l e c t r o n s d u r i n g charge changing c o l l i s i o n s i n t h e c e n t r a l e l e c t r o d e of a tandem accelerator. The a n a l y s i n g a n d s w i t c h i n g m a g n e t s t h a t u s u a l l y f o l l o w a t a n d e m a c c e l e r a t o r c a n t h e n b e u s e d t o remove m o s t o f t h e m o l e c u l a r fragments leaving t h e atomic species alone f o r study. The s u c c e s s of t h i s p r o c e d u r e i n t h e d e t e c t i o n o f 1 4 C a t n a t u r a l l e v e l s was f i r s t d e m o n s t r a t e d i n 1 9 7 7 a t t h e U n i v e r s i t y o f R o c h e s t e r ( P u r s e r e t aZ., 1977). The i n s t a b i l i t y o f 14N- a n d t h e s t a b i l i t y o f 14C was a l s o u s e d t o e l i m i n a t e t h e i n t e r f e r i n g mass 1 4 i s o b a r . S h o r t l y a f t e r w a r d s t h e 1 4 C i s o t o p e was d e t e c t e d i n g r a p h i t e s a m p l e s w i t h 1 4 C / 1 2 C l e s s t h a n l o - ' $ ( B e n n e t t e t a % . , 1 9 7 7 ) . The a b i l i t y t o make s u c h measurements on c a r b o n i s o t o p e s a l s o r e q u i r e d t h e development o f h i g h c u r r e n t n e g a t i v e i o n s p u t t e r s o u r c e s for n u c l e a r p h y s i c s . These i o n s o u r c e s i n v o l v e t h e s p u t t e r i n g of s o l i d m a t e r i a l s a n d c o n s e q u e n t l y h a v e a v e r y low memory e f f e c t w h i c h i s e s s e n t i a l for t h e m e a s u r e m e n t o f s u c h l a r g e and v a r i a b l e i s o t o p e r a t i o s . Almost s i m u l t a n e o u s l y a n d i n d e p e n d e n t l y , m e a s u r e m e n t s on t h e t a n d e m a c c e l e r a t o r a t McMaster U n i v e r s i t y ( N e l s o n e t al., 1 9 7 7 ) showed t h a t 1 4 C a t n e a r n a t u r a l l e v e l s could be d e t e c t e d and a t Berkeley a c y c l o t r o n was u s e d , f o l l o w i n g a s u g g e s t i o n by M u l l e r ( 1 9 7 7 ) , t o d e m o n s t r a t e t h a t 1 4 C a t o r n e a r n a t u r a l l e v e l s c o u l d be d e t e c t e d u s i n g p o s i t i v e i o n s and r a n g e s e p a r a t i o n of t h e 1 4 C f r o m t h e i n t e n s e i n t e r f e r i n g I4N beams ( S t e p h e n s o n e t a%., 1 9 7 9 ) . The t h r e e p i o n e e r i n g e x p e r i m e n t s on 1 4 C h a v e now b e e n e x t e n d e d t o many s t a b l e a n d r a d i o i s o t o p e s , a n d t h e s i g n i f i c a n c e o f t h e new p r i n c i ~ l e sfor r a r e i s o t o p e a n a l y s i s by mass s p e c t r o m e t r y h a s b e e n p e r c e i v e d
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rapidly. New s p e c i a l i z e d tandem a d c e l e r a t o r s and t h e i r p e r i p h e r a l equipment a r e b e i n g d e v e l o p e d t o e x p l o i t t h e d i s c o v e r i e s ( P u r s e r e t al., 1980). Tandem A c c e l e r a t o r The l a y o u t o f t h e i o n beam t r a n s p o r t s y s t e m o f a tandem a c c e l e r a t o r i s i l l u s t r a t e d i n F i g u r e 1. I n t h e n e g a t i v e i o n s p u t t e r s o u r c e t h e sample i s bombarded b y a beam o f C s t i o n s which e r o d e s t h e sample s u r f a c e , r e l e a s i n g a t o m s a s b o t h n e u t r a l and c h a r g e d ( b o t h p o s i t i v e and
A p o s i t i v e e x t r a c t i o n e l e c t r o d e draws n e g a t i v e i o n s negative) ions. away from t h e s a m p l e and i n t o t h e i n f l e c t i o n magnet which s e l e c t s i o n s w i t h a p a r t i c u l a r M/q for i n j e c t i o n i n t o t h e a c c e l e r a t o r . The h i g h v o l t a g e e l e c t r o d e a t t h e c e n t r e o f t h e tandem i s m a i n t a i n e d a t a h i g h p o s i t i v e p o t e n t i a l o f t h e o r d e r o f m e g a v o l t s , RMV b e i n g t h e v o l t a g e used i n t h e i n i t i a l 1 4 C e x p e r i m e n t s . N e g a t i v e i o n s a c c e l e r a t e d t o t h i s p o i n t p a s s t h r o u g h a g a s c a n a l c o n t a i n i n g Ar, where c h a r g e c h a n g i n g collisions take place. E l e c t r o n s a r e s t r i p p e d from t h e i o n s i n t h e beam, t h e p r e c i s e number o f e l e c t r o n s s t r i p p e d d e p e n d i n g on t h e a c c e l e r a t i n g v o l t a g e and g a s p r e s s u r e . The n e g a t i v e i o n s a r e t h u s c o n v e r t e d t o p o s i t i v e , b u t b e c a u s e o f t h e i r h i g h e n e r g i e s and h i g h r i g i d i t y r e m a i n e s s e n t i a l l l y u n d e v i a t e d from t h e i r p a t h s . I n t h e i r new m u l t i p l e p o s i t i v e change s t a t e t h e y a r e a c c e l e r a t e d f u r t h e r t h r o u g h t h e r e m a i n d e r o f t h e a c c e l e r a t o r , a s t h e e l e c t r i c f i e l d g r a d i e n t i s now r e v e r s e d . Eme r g i n g a t t h e h i g h e n e r g y e n d t h e p o s i t i v e i o n s p a s s t h r o u g h a 90" magnet which s e l e c t s a s p e c i f i c mass e n e r g y p r o d u c t ( M E / q 2 ) . A 45" s w i t c h i n g magnet d i r e c t s t h e p a r t i c l e s down t h e a p p r o p r i a t e d e t e c t i o n l i n e , and a 1 0 " e l e c t r o s t a t i c a n a l y z e r d e f i n e s E / q s o t h a t i o n s e n t e r i n g t h e f i n a l d e t e c t o r h a v e u n i q u e v a l u e s o f M / q , or what i s e q u i v a l e n t , u n i q u e l y d e f i n e d v e l o c i t i e s . The f i n a l d e t e c t o r i s a g a s f i l l e d heavy i o n c o u n t e r d e v e l o p e d b y S h a p i r a e t al. ( 1 9 7 5 ) which p r o v i d e s s e v e r a l dE/dx s i g n a l s from s e p a r a t e e l e c t o d e s a l o n g t h e p a t h o f t h e i o n , a s w e l l as t h e t o t a l e n e r g y d e p o s i t e d . When t h e v e l o c i t y of t h e i o n s i n c i d e n t on t h e c o u n t e r i s d e f i n e d , a measurement of dE/dx d e f i n e s t h e i o n ' s nuclear charge Z . T h i s p r o v i d e s v a l u a b l e i n f o r m a t i o n a n d sometimes c o n c l u s i v e i d e n t i f i c a t i o n of t h e i s o b a r of i n t e r e s t .
Molecular Destruction M o l e c u l e s a r e bound b y t h e i r o u t e r e l e c t r o n s a n d i f enough o f t h e s e e l e c t r o n s a r e removed t h e p o s i t i v e l y c h a r g e d components w i l l r e p e l e a c h o t h e r and t h e m o l e c u l e w i l l f r a g m e n t . I t i s e a s y t o show t h a t m o l e c u l e s w i t h two o r more e l e c t r o n s removed s h o u l d b e d e s t r o y e d by t h e coulomb r e p u l s i o n o f t h e components. However, i t was f o u n d ( L i t h e r l a n d 1978) t h a t t h e m o l e c u l e 1 2 C H 2 f 2 was s t a b l e enough t o i n t e r f e r e w i t h " C t 2 detection. I t i s t h e r e f o r e n e c e s s a r y t o s e l e c t t h e c h a r g e s t a t e t3 a s t r i p l y c h a r g e d m o l e c u l e s a r e unknown e x c e p t for some t r a n s i t i o n m e t a l elements. For t h i s reason i n t h e carbon experiments t h e high energy a n a l y s i s s t a g e s w e r e t u n e d t o a c c e p t 1 4 C t 3 for which t h e r e a r e no i n t e r f e r i n g molecular species. The m o l e c u l a r f r a g m e n t s c a n be s e l e c t i v e l y e l i m i n a t e d b y a s u i t a b l e c o m b i n a t i o n of e l e c t r i c and m a g n e t i c f i e l d s ( P u r s e r e t al., 1 9 7 9 ) l e a v i n g o n l y t h e a t o m s t o b e a n a l y z e d . -However, if low r e s o l u t i o n (M/AM<400) mass s p e c t r o m e t e r s a r e u s e d , c e r t a i n a m b i g u i t i e s a r e p o s s i b l e . The e n e r g y , E , of a n i o n a f t e r a c c e l e r a t i o n i n a tandem a c c e l e r a t o r d i v i d e d b y t h e c h a r g e , g , i s g i v e n by
where m i s t h e mass of t h e n e g a t i v e i o n i n j e c t e d , and M i s t h e mass o f t h e i o n a c c e l e r a t e d t o e n e r g y E . V and v a r e t h e v o l t a g e o f t h e c e n t r a l e l e c t r o d e o f t h e tandem and t h e i n j e c t i o n v o l t a g e . e i s the electronic charge. C l e a r l y i f M and g have common f a c t o r s t h e n i o n s w i t h t h e same v a l u e s of M / q and E/q w i l l be t r a n s m i t t e d t h r o u g h a l l mass s p e c t r o m e t r i c e l e m e n t s o f low r e s o l u t i o n . An example m i g h t b e t h e i o n s1 9 5 p t t s 1 5 6 ~ ~ + 4 1, 1 7 s n t 3 J , 7 8 S e t 2 and 3 9 K t 1 a l l of which would be
20
p r e s e n t f o l l o w i n g t h e i n j e c t i o n i n t o a tandem a c c e l e r a t o r o f n e g a t i v e i o n s o f mass 1 9 5 . I n p r i n c i p l e s u c h a m b i g u i t i e s c a n b e e l i m i n a t e d f o r s t a b l e i s o t o p e s by a s i m p l e e n e r g y measurement or 3 y a n o t h e r c h a r g e c h a n g i n g c o l l i s i o n f o l l o w e d b y m a g n e t i c or e l e c t r l c s p e c t r o m e t r y . !lowe v e r , f o r t h e much r a r e r r a d i o i s o t o p e s a c h a r g e s t a t e g r e l a t i v e l y p r i m e to M s h o u l d b e c h o s e n , for example t 7 when m e a s u r i n g 3 6 C l (Elmore e t a l . , 1 9 7 9 ) . The optimum e n e r g y for m o l e c u l a r d e s t r u c t i o n h a s b e e n d i s c u s s e d by L i t h e r l a n d ( 1 9 8 0 ) and c o r r e s p o n d s s i m p l y t o t h e tandem a c c e l e r a t o r v o l t a g e n e e d e d to o p t i m i z e t h e a p p r o p r i a t e c h a r g e s t a t e of the positive ions. I s o b a r S e l e c t i o n Methods A l l r a d i o i s o t o p e s e v e n t u a l l y d e c a y to s t a b l e i s o t o p e s and i n t h e c a s e o f b e t a d e c a y t h e m a s s e s o f t h e r a d i o i s o t o p e and s t a b l e i s o t o p e I 4 C and I 4 N a r e members o f a r e so close t h a t they are called isobars. a w e l l known p a i r . P.t p r e s e n t t h e r e i s no u n i v e r s a l method for s e p a r a t i n g i s o b a r s o t h e r t h a n e x t r e m e l y h i g h r e s o l u t i o n mass s p e c t r o m e t r y and a s t h i s i m p l i e s low atom e f f i c i e n c y i t i s u n s u i t a b l e for r a r e i s o t o p e s . However, a number o f p r o c e d u r e s have b e e n u s e d w i t h v a r y i n g d e g r e e s of s u c c e s s .
1. The u s e o f n e g a t i v e i o n s p r o v i d e s a s p e c t a c u l a r s o l u t i o n to some i s o b a r p r o b l e m s . I n t h e c a s e of I 4 C and 1 4 N t h e a p p a r e n t c o m p l e t e i n s t a b i l i t y of N- makes i s o b a r s e p a r a t i o n p o s s i b l e a t t h e i o n s o u r c e for 1 4 C / ' 2 C h a v e b e e n ( L i t h e r l a n d , 1 9 8 0 ) . I s o t o p e r a t i o s below m e a s u r e d ( B e n n e t t e t al., 1 9 7 7 ) , and o t h e r e x a m p l e s a r e d i s c u s s e d b e l o w . F o r t u n a t e l y many s t u d i e s o f n e g a t i v e i o n s h a v e b e e n made i n r e c e n t y e a r s ( M i d d l e t o n , 1 9 7 7 ) p r i m a r i l y to d i s c o v e r how b e s t to p r o d u c e them f o r tandem a c c e l e r a t i o n . Cesium n e g a t i v e i o n s p u t t e r s o u r c e s h a v e b e e n dev e l o p e d w h i c h o p t i m i z e t h e p r o d u c t i o n of n e g a t i v e i o n s , and which a r e s u i t a b l e f o r mass s p e c t r o m e t r y w i t h tandem a c c e l e r a t o r s . 2. I s o b a r s c a n a l s o b e s e p a r a t e d c h e m i c a l l y , b u t t h i s method i s of l i m i t e d u s e b e c a u s e of t h e v e r y h i g h d e g r e e o f p u r i t y needed i n t h e r e a g e n t s . The p a r t i a l c h e m i c a l s e p a r a t i o n of s u l p h u r and c h l o r i n e h a s s o f a r p r o v e d e s s e n t i a l f o r t h e d i s t i n g u i s h i n g 3 6 S and which b o t h form negative ions readily.
3 . The r a n g e o f t h e i s o b a r w i t h l o w e r n u c l e a r c h a r g e , 2 , i s l o n g e r i n m a t t e r t h a n t h a t of t h e i s o b a r w i t h l a r g e r 2 , and t h i s makes t h e i r s e p a r a t i o n p o s s i b l e ( F u l l e r , 1 9 7 7 ) . The method i s v e r y u s e f u l f o r t h e s e p a r a t i o n of "Be a n d l o B and i s t h e o n l y r e l i a b l e method a t p r e s e n t . However, f o r 1 4 C a n d 14N s e p a r a t i o n i t i s more d i f f i c u l t ( S t e p h e n s o n e t a Z . , 1 9 7 9 ) and t h i s method i s p r o b a b l y l i m i t e d t o l i g h t i s o t o p e s b e c a u s e of r a n g e s t r a g g l i c g . 4 . I n d i v i d u a l i o n s can be i d e n t i f i e d by dE/dx measurements i n t h e heavy i o n c o u n t e r a t multi-MeV e n e r g i e s , and t h i s method i s of g r e a t v a l u e p r o v i d e d t h e r e l a t i v e i n t e n s i t y of t h e i n t e r f e r i n g i s o b a r i s n o t too g r e a t . A r e l a t i v e i n t e n s i t y o f up to l C 5 seems to b e n e a r t h e l i m i t f o r d i s c r i m i n a t i o n by r a t e o f e n e r g y loss (Elmore e t al., 1 9 7 9 ) .
5. I f the isobar with the larger Z i s the rare radioactive isobar t h e n c o m p l e t e s t r i p p i n g o f t h e e l . e o t r o n s from t h e n u c l e u s makes i s o b a r selection possible. T h i s method can b e a p p l i e d a t i n t e r m e d i a t e e n e r g i e s ( K i l i u s and L i t h e r l a n d , 1 9 7 8 ) or a t Very h i g h e n e r g i e s ( R a i s b e c k e t a l . , 1 9 7 9 ) b u t i s p r o b a b l y o f l i m i t e d u s e b e m d s e o f t h e c o m p l e x i t y and c o s t o f t h e e q u i p m e n t . However, t h e s e p a r a t i o n o f 7 B e + 4 a n d 7 L i + 3 c a n b e a c c o m p l i s h e d a t low ( a few MeV) e n e r g i e s and h a s r e c e n t l y b e e n done ( K u t s c h e r a , p e r s . commun.). The s e p a r a t i o n o f 2 6 A l t 1 3 a n d 2 E M g t 1 2 r e q u i r e s a t l e a s t lO0MeV i o n s ( R a i s b e c k e t al., 1 9 7 9 ) . I t i s i n t e r e s t i n g to n o t e t h a t t h e r a r e i s o t o p e 3 H e was f i r s t i d e n t i f i e d by t h i s method i n 1 9 3 9 by A l v a r e z a n d C o r n o g . O t h e r methods of s e p a r a t i n g i s o b a r s have b e e n d i s c u s s e d b y L i t h e r l a n d
( 1 9 8 0 ) , b u t a t p r e s e n t i t a p p e a r s t h a t e a c h i s o b a r i c p a i r h a s t o b e cons i d e r e d s e p a r a t e l y to f i n d t h e b e s t m e t h o d .
21
RADIOCARBON DATING The best known experiments in direct radioisotope measurements have been on 14C (Bennett e t aZ., 1977, 1978; Gove e t aZ., 1980) and have been summarized in a more general article by Bennett (1979). Themethod of sample preparation was originally to prepare amorphous carbon mixed with an equal amount of KBr which served as a binder. This waspressed into an aluminium cylinder which was mounted on the negative ion source wheel. In more recent measurements, a mixture of amorphous carbon and finely powdered silver or copper was pressed into a 2mm diameter shallow re-entrant hole in the aluminium cylinder. The weight of carbon included in the sample ranged from less than lmg up to lOmg, although in most cases the major part of the material still remained after the measurement was completed. Attempts are presently being made in several lakcratcries to crack carbon I'rom methane or acetylene cnto metal substrates which can be mounted on the source wheel. This will produce a graphite-like material which, in the cesium sputter source, should give substantially higher currents of C - .
NEGPTIVE C IW AMLYSIS
MOLEWLM DISINTEGRATi
MP TANDEM ACCELERATOR
I
90' MAGNETIC ANAL QER
POSIrIVE /ON MawmruM FC2 ANALYSIS
10' ELECTROSTAT
1
msInvE
_J
Figure 1
ION ENERGY ANALYSIS
The i o n beam t r a n s p o r t s y s t e m o f t h e U n i v e r s i t y o f R o c h e s t e r tandem a c c e l e r a t o r u s e d as a n u l t r a s e n s i t i v e mass s p e c t r o m e t e r i s s h o w n s c h e m a t i c a l l y . I o n - b e a m d e f i n i n g a p e r t u r e s a r e d e s i g n a t e d AP a n d Faraday cups a r e d e s i g n a t e d FC.
Figure 2 shows a typical mass spectrum of negative ions extracted from the sputter source, prior to injection into the accelerator. It can be seen how the peak at mass 14, which is made LID almost entirely of the molecules "CH2- and 13CH- is four orders of maqcitude lower than 1 2 C at mass 12. The subsequent molecular elimination removes all but 0n.e part in 10' of this mass 1 h peak in order to reveal the miniscule component of 14C. Measurement times of one to six hours have been found to be adequate to obtain the counts necessary to achieve accuracy conparable with the standard method. If higher currents can b e produced by improved sample preparations, then these counting times may be sLSstan5ially reduced.
22
FIELD OF INFLECTION M A G N E T Figure 2
O u t p u t o f t h e n e g a t i v e i o n beams f r o m t h e s p u t t e r source using a carbon sample, p r i o r t o i n j e c t i o n into the accelerator. The i o n s h a v e a n e n e r g y o f 2 0 keV a n d a r e m a g n e t i c a l l y a n a l y z e d .
F i g u r e 3 shows a t h r e e d i m e n s i o n a l p l o t o f t h e o u t p u t o f ',he f i n a l heavy i o n d e t e c t o r when a s a m p l e o f c a r b o n from M t . S h a s t a , d a t e d by t h e U . S . G e o l o g i c a l S u r v e y a t 4 5 9 0 2 250 y e a r s o l d , was r u n i n t h e i o n : > o u r c e . The two p a r a m e t e r s ETOTAL and E F I N A L r e f e r t o t h e s i g n a l s from t h e d e t e c t o r corresponding t o t h e t o t a l energy deposited i n t h e detector from t h e i o n s a n d t h e e n e r g y f r o m some f r a c t i o n o f t h e t r a c k n e a r t h e c.nd o f i t s r a n g e . The p e a k s a r e p l o t t e d on a l o g a r i t h m i c s c a l e . The c a r b o n i o n e n e r g y was 40MeV. I n F i g u r e 4 t h e g r e a t s e n s i t i v i t y o f t h e e y s t e m i s a g a i n i l l u s t r a t e d i n two p u l s e h e i g h t s p e c t r a f o r c a r b o n s a m p l e s . The one from M t . Hood, 2 2 0 y e a r s o l d , i s a l m o s t c o n t e m p o r a r y , w h i l e t h e H i l l s d a l e s a m p l e i s 4 0 , 0 0 0 y e a r s o l d . The 1 4 C / 1 2 C r a t i o f o r t h e M t . Hood Sam l e i s a b o u t lom1' w h i l e t h e H i l l s d a l e s a m p l e i s a b o u t E x The ' 2 C and 1 3 C p e a k s come from "CH2and I'CH- m o l e c u l e s from t h e s o u r c e , which a r e d i s s o c i a t e d i n t h e t e r m i n a l and t h e n u n d e r g o improbable c h a r g e changes d u r i n g t h e i r a c c e l e r a t i o n i n t h e second h a l f c f t h e tandem, s o t h e y have t h e same ME q 2 as t h e 14C i o n s . These c e a k s c o u l d b e e l i m i n a t e d w i t h an e l e c t r o s t a t i c d e f l e c t o r , b u t a s t h e y a r e c l e a r l y r e s o l v e d from t h e 14C p e a k t h e r e i s no n e e d t o be conc e r n e d w i t h t h e m . The I 4 N a r i s e s from n e g a t i v e i o n m o l e c u l a r h y d r i d e s which a l s o u n d e r g o low p r o b a b i l i t y c h a r g e c h a n g e s and s l i p t h r o u g h t o t h e d e t e c t o r , b u t a g a i n t h e p e a k i s w e l l r e s o l v e d , and o f f e r s no interference. The a b u n d a n t i s o t o p e o f c a r b o n , "C, i s m e a s u r e d by i t s c u r r e n t i n a F a r a d a y cup a f t e r t h e 90" m a g n e t , a s t h e beam i s f a r t o o i n t e n s e for atom c o u n t i n g . The e x p e r i m e n t a l u r o c e d u r e i s t o c y c l e r e p e a t e d l y t h e t u n i n g of t h e i n f l e c t i o n magnet and a c c e l e r a t o r v o l t a g e t h r o u g h t h e
23
W - 3 7 0 3 MT. SHASTA 4590
* 250 y r s . ( U S G S )
Figure 3
I3c
A t h r e e dimensional r e p r e s e n t a t i o n of d a t a from t h e heavy i o n counter used a t Rochester. The v e r t i c a l a x i s i s t h e l o g a r i t h m o f t h e number o f c o u n t s . i s t h e t o t a l i o n energy measured i n t h e E~~~~~ c o u n t e r and E is t h e energy measured i n t h e FINAL is related to f i n a l s e c t i o n o f t;ie c o u n t e r . E~~~~~ t h e i o n r a n g e and t h e r a t e o f e n e r g y l o s s dE/dX.
mass range 12 to 14. While tuned to masses 12 and 13 current measurements are made for short periods and then at mass l h counts f r o m 1 4 C are accumulated so that the rstio I 4 C I 2 C can be derived. Each micro-. ampere o f 12C yields about 450 ions o f 1 4 C per minute for a sample of contemporary carbon. Up to 10 microarnps of 12C- may be extracted from the ion source under favourable conditions, so with a transmission efficiency of 10% 14C count rates in the order o f hundreds per minute may be expected. This must be compared with the rate of 15 beta rays emitted per minute per gram for the conventional method. Results from some of the early measurements made at Rochester are shown in Figure 5 and details listed in Table 1. The deviation of the first M t . Shasta determination was due to inhomoFeneities in the sample. When a larger fraction of the charcoal sample .&as taken and homogenized by pulverization, the second set of data was recorded. The repeat measurements were in exceilent agreement with the expected age, and there is an important lesson to be learned here, While the new method is able to date milligram quantities o f material, it must be established clearly that this small sample is t r u l y reDresentative o f the sample as a whole. Obviously, for priceless archaeological specimens a few milligrams may be all that can be spared, but where more material is available it should be carefully homogenized, or the results interpreted accordingly.
In June 1977 an intact, completely frozen baby mammoth (named Dima) was uncovered by a bulldozer operator in the Magadan region of northeastern Siberia. A sample of 1.3g o f muscle was made available to M.
24
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GRAPHITE 2C I?
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8 a
8 :
*
cn
-I
Z
3
0
E -FI N AL Figure 4
The pulse height spectra from the final section of t h e gas i o n i z a t i o n c h a m b e r , w i t h d E / d X s e l e c t i o n for c a r b o n , a r e s h o w n for two- c a r b o n s a m p l e s . The d i f f e r e n c e b e t w e e n t h e I 4 C c o u n t s is s i g n i f i c a n t .
Goodman o f t h e Anatomy Department a t Wayne S t a t e U n i v e r s i t y . A t h i s r e q u e s t a s a m p l e of a b o u t 0.9mg ( o b t a i n e d from a b o u t 4mg of m u s c l e ) was d a t e d and t h e a g e shown i n T a b l e 1 was m e a s u r e d (Gove e t a Z . , , 1980). The a c c u r a c y o f t h e most r e c e n t m e a s u r e m e n t s , b a s e d on r e p r o d u c i b i l i t y , a p p r o a c h e s 1 % . I n o r d e r t o compete w i t h t h e h i g h l y d e v e l o p e d b e t a r a y c o u n t i n g method a n a c c u r a c y o f a t l e a s t +1% ( 2 8 0 y e a r s on r e c e n t m a t e r i a l ) must b e a t t a i n e d c o n s i s t e n t l y . T h i s h a s n o t b e e n e a s y to a c h i e v e w i t h e x i s t i n g a p p a r a t u s , and t h e p r o b l e m s seem t o b e as-
s o c i a t e d with v a r i a t i o n s i n transmission e f f i c i e n c y through t h e e l e c t r o n
25
I
MT HOOD ,MT. SHAS7-A
I
n i
u.I
~~~
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10
1
20
I
30
I
40
KNOWN AGE OF SAMPLE THOUSANDS OF YEARS Figure 5
The r e s u l t s o f d a t i n g m i l l i g r a m s a m p l e s by a t o m c o u n t i n g compared t o t h e a g e s m e a s u r e d by b e t a - d e c a y c u r r e n t. counting. The 14C i s compared w i t h t h e ' * C -
s t r i p p i n g c a n a l due t o v a r i a t i o n s i n i o n s o u r c e ernittsn,7p d u r i n g a m e a s u r e m e n t . T h i s p r o b l e m c a n b e s o l v e d i n p r i n c i p l e by i n c r e a s i n g t h e d i a m e t e r o f t h e s t r i p p i n g c a n a l or b y c a r e f u l l y c o n t r o l l i n g t h e e m i t t a n c e o f t h e i o n beams i n j e c t e d i n t o t h e a c c e l e r a t o r . Yowever, t h e i s o t o p e r a t i o s a l s o a7spear t o v a r y w i t h t i m e ( T o m b r e l l o a n d K e l l o g g , 1 9 7 9 ) due t o t h e n a t u r e of t h e c e s iu m s p u t t e r i n g p r o c e s s s o t h a t g r e a t c a r e w i l l have t o be t a k e n t o e n s u r e t h a t a l l t h e v a r i c u s f r a c t i o n a t i o n p r o c e s s e s a t a l l s t a g e s a r e under c o n t r o l . F o r t u n a t e l y measurements of i s o t o p e r a t i o s a r e made w i t h r e s D e c t t o some s t a n d a r d s o t h a t i t i s necessary only t o ensure t h a t t h e various f r a c t i o n a t i o n processes during measurement a r e t h e same f o r t h e unknown s a m p l e as f o r t h e c a l i b r a t i o n sample. C a r e f u l c o n t r o l of c o n t a m i n a t i o n i s a l s o n e c e s s a r y . Up t o t h e p r e s e n t t h e u s e o f a t a n d e m f o r r a d i o c a r b o n d a t i n g o f a r c h a e o l o g i c a l samples has n o t been p o s s i b l e as t h e accuracy r e q u i r e d (021%) has n o t b e e n r e a c h e d c o n s i s t e n t l y . However t h e m o t i v a t i o n f o r t h e t a n d e m w o r k , a t l e a s t a t R o c h e s t e r , was t o c a r r y o u t r e s e a r c h a n d d e v e l o p m e n t t o w a r d s t h e d e s i g n o f m a c h i n e s s p e c i f i c a l l y for r a d i o c a r b o n d a t i n g . I n t h i s r e s p e c t , t h e work has b e e n q u i t e s u c c e s s f u l a s a d e d i c a t e d 3MV m a c h i n e , t h e F a c d e t r o n , i s ncw c o n s t r ~ c t e db y G e n e r a l Ionex C o r p o r a t i o n . Such i n s t r u m e n t s w i l l be i n s t a l l e d a t t h e U n i v e r s i t i e s o f O x f o r d , A r i z o n a , T o r o n t o a n d Nagoya, a n d may b e e x p e c t e d t o begin producing r o u t i n e d a t e s i n 1 9 8 2 . I n t h e meantime, w h i l e s e v e r a l l a b o r a t o r i e s are p r e p a r i n g t o u s e e x i s t i n g tandems f o r d a t i n g , r e l a t i v e l y few r e s u l t s h a v e b e e n p u b l i s h e d .
26 T a b l e 1.
"C
r e s u l t s from the'Toronto-Rochester-General
Sample
M t . Hood (Oregon)
Mt. Shasta (California) L. Agassiz (N. Dakota)
Hillsdale (Michigan) Graphite
I o n e x Group
Run T i m e (min)
Rochester A e ( y e a r s BP)
90
220 2 300
220 f 150
180
5700 ? 400
4590 f 250
90
8800 f 600
9150 ? 250
400
41,000 f 1200
39,500 f 1000
65
48,000 f 1400
i
E x p e c t e d Age
M t . Shasta (repeat)
4580 f 90
4590 ? 250
Bull.Mummy c l o t h (Dahshur, Egypt)
2200 f 150
2050 f 200
27,000 ? 1000
B a b y w o o l l y mammoth (USSR) a
~
~
BP = y e a r s b e f o r e 1 9 5 0 .
b
Available sample t o o s m a l l f o r conventional dating
~~
14C h a l f - l i f e
b A s m e a s u r e d by Meyer Rubin o f
t h e U.S.
assumed
t o b e 5568 years
Geological Survey
T u c k e r e t al. ( 1 9 8 1 ) r e p o r t e d r e c e n t l y o n e a r t h q u a k e d a t i n g by 1 4 C a t o m c o u n t i n g o n a t a n d e m a c c e l e r a t o r i n Z u r i c h . T r e n c h e s were c u t a c r o s s f r a c t u r e l i n e s a s s o c i a t e d w i t h t h e blasatch f a u l t i n Utah, and ~ . i l l i g r a ms i z e d s p e c i n ; e r . s of d e t r i k a l c h a r c o a l w e r e c o l l e c t e c ! ?ram e a c h s i d e i n t h e s u r f a c e s o i l l a y e r s . The c h a r c o a l was a s s u m e d t o b e a s s o c i a t e d w i t h p r e h i s t o r i c e a r t h q u a k e s a l o n g t h e f a u l t , a n d 1 4 C / 1 2 C were m e a s u r e d w h i c h c o r r e s n o n d t o a g e s of 7 8 0 0 , 8 8 0 0 a n d 9000 y e a r s B P w i t h uncertainties of f600 years. These r e s u l t s w e r e c o n s i s t e n t w i t h g e o l o g i c a l e v i d e n c e and one c o n v e n t i o n a l b e t a count from a l a r g e r sample. I n t h i s s t u d y t h e p o i n t was s t r e s s e d t h a t o n e of t h e m a j o r a d v a n t a g e s of t h e a t o m c o u n t i n g m e t h o d was t h a t r e s u l t s c o u l d b e o b t a i r . e d w h i l e c o l l e c t i n g p a r t i e s were s t i l l i n t h e f i e l d , a n d s o t h e y coul? b e d i r e c t e d t o c o l l e c t more m a t e r i a l g u i d e d b y t h e i n i t i a l r e s u l t s , r a t h e r t h a n h a v i n g t o w a i t for t h e n e x t Y i e l d s e a s o n b e f o r e f o l l o w i n g i t u p . O t h e r g r o u p s w h i c h h a v e r e p o r t e d on t h e e x p e r i m e n t a l m e a s u r e m e n t o f u s i n g a t a n d e m a c c e l e r a t o r i n c l u d e N e l s o n e t 01. ( 1 9 7 7 ) a t McMaster U n i v e r s i t y , who s u c c e s s f u l l y o b s e r v e d I 4 C a t c o n t e m p o r a r y l e v e l s ; Andrews e t aZ. ( 1 9 7 8 ) a t C h a l k R i v e r d e t e c t e d a b a c k g r o u n d Of 1 4 C c o r r e s p o n d i n g t o a n a g e of 6 0 , 0 0 0 y e a r s ; B a r r a t t e t a2. (1978) a t O x f o r d U n i v e r s i t y d e t e c t e d I 4 C i n o l d a n d new c a r b o n . 14C
A n o t h e r a p p r o a c h t o a t o m c o u n t i r . g w i t h a c c e l e r a t o r s h a s b e e n nade u s i n g a c y c l o t r o n b y a g r o u p a t t h e U n i v e r s i t y of C a l i f o r n i a a t B e r k e l e y ( M u l l e r , 1 9 7 7 ) . The c y c l o t r o n a c t s as a m u l t i s t a g e mass s p e c t r o m e t e r , a n d b e c a u s e of t h e r e s o n a n t n a t u r e of t h e a c c e l e r a t i o n p r o c e s s t h i s h a s t h e e f f e c t of improving t h e r e s o l u t i o n f u n c t i o n by c o m p l e t e l y removing loris w h i c h a r e off r e s o n a n c e . ?he c y c l o t r o n i t s e l f i s t h e r e f o r e s u p e r i o r t o a s i n g l e or d o u b l e mass s p e c t r o m e t e r w i t h t h e same mass r e s o l u t i o n . The f a c t t h a t t h e i o n s m u s t t r a v e l a b o u t lkm d u r i n g a c c e l e r a t i o n un: o r t x a t e l y makes t h e t r a n s r c i s s i o n r z k h e r s e n s i t i v e t o vacuum p r e s s . d r e . I n a d d i t i o n , t h e p r o b l e m s of g e t t i n g t h e i o n s i n t o a n d o u t of t h e c y c l c t r o n a r e more d i f f i c u l t t h a n t h o s e of t h e t a n d e m a c c e l e r a t o r a n d t h i s
27
c o m p l i c a t e s a c c u r a t e i s o t o p e r a t i o , m e a s u r e m e n t s . Up t o t h e p r e s e n t , t h e hope t h a t t h e c y c l o t r o n w o u l d b e u s e f u l for r a d i o i s o t o p e d a t i n g , e s p e c i a l l y w i t h 1 4 C , has n o t b e e n r e a l i z e d . However, t h e B e r k e l e y g r o u p s u c c e s s f u l l y d e t e r m i n e d a ' b l i n d ' measurement of a sample a l r e a d y d a t e d by t h e c o n v e n t i o n a l m e t h o d a t 6 0 0 0 y e a r s ( M u l l e r e t aZ., 1 9 7 8 ) , t h o u g h a second ' b l i n d ' measurement o f a n 8000 y e a r o l d sample y i e l d e d a n e r raneoLis a g e of 1 8 , 0 0 0 y e a r s ( b y u l l e r , 1 9 7 8 ) . OTHER i3A3IGISOTOPES
'"C i s t h e r a d i o i s o t o p e w h i c h h a s f o u n d w i d e a p p l i c a t i o n i n d a t i n g a r c h a e o l o g i c a l o b j e c t s ar,d Q u a t e r n a r y s t r a t a . The u n i q u e f e a t u r e o f -arbon i s i t s a s s o c i a t i o n w i t h o r g a n i c r e m a i n s s o t h a t i t has d i r e c t yelevarce i n t h e d a t i n g of f o r m e r l y l i v i n g organisms, and secondly i t s k , ; a l f - l i f e I s s u i t e d to d e t e r m i n i n g a g e s i n t h e a p p r o p r i a t e t i m e f r a m e , r.amely b a c k t o a b o u t 5 0 , 0 0 0 y e a r s BP. 1 4 C i s p r o d u c e d b y t h e a c t i o n o f c o s m i c r a y s on ' " N i n t h e a t m o s p h e r e , b u t t h e r e a r e s e v e r a l o t h e r s p a l l a t i o n s from c o s m i c r a y s w h i c h g i v e r i s e to r a d i o i s o t o p e s w i t h d i f f e r e n t hali?-lives and d i f f e r e n t chemical a f f i n i t i e s . Ber y 11i u m , -1 0 Ana l y s i s "Be i s g e n e r a t e d by t h e a c t i o n o f c o s m i c r a y s o n n i t r o g e n a n d oxygei-! ir? t h e a t m o s p h e r e , ar,d h a s a h a l f - l i f e o f 1 . 6 x 1 0 6 a . B e c a u s e the advantage o f c o u n t i n g atoms i n c r e a s e s i n d i r e c t p r o p o r t i o n t o t h e k e l f - l i f e o f t h e r a d i o n u c l i d e , t h e o n l y e f f e c t i v e way c f m e a s u r i n g "Be i s t h r o u g h u l t r a s e n s i t i v e mass s p e c t r o m e t r y ( R a i s b e c k e t ai?., 1 9 7 8 a ) . A f t e r b e i n g p r e c i p i t a t e d o u t o f t h e a t m o s p h e r e , c o s m o g e n i c "Be i s - de-. p o s i t e d i? g e o l o g i c a l r e s e r v o i r s , s u c h a s s e d i m e n t a r y r o c k s a n d p o l a r i c e s h e e t s . Here i s aE i s o t o p e t h r o u g h w h i c h s u c h s t r a t a a ? e w m i l l i o n y e a r s o l d may b e d a t e d , i f t h e g e o c h e m i c a l b e h a v i o u r o f b e r y l l i u m c a n Se a s c e r t a i n e d w i t h s u f f i c i e n t p r e c i s i o n .
Man37 o f t h e m e a s u r e m e n t s o f "Be h a v e b e e n made u s i n g a c y c l o t r o n a s p a r t of a mass s p e c t r o m e t e r , a n d t h e i n t e r f e r i n g i s o b a r , " B y h a s b e e n r e j e c t e d by t h e r a n g e t e c h n i q u e . The f e a s i b i l i t y o f t h e m e t h o d was e s t a b l i s h e d w i t h s a m p l e s a l r e a d y d e t e r m i n e d b y b e t a d e c a y m e a s u r e r a n g i n g from m e n t s ( E i a i s b e c k e t aZ., 1 9 7 8 b ) , a n d r a t i o s o f "Be/'Be lo-' t o 1 0 - l " were c o n f i r m e d . S a m p l e s c o n t a i n i n g a s few as l P 7 "Be a t o m s , w h i c h w o u l d o n l y e m i t 5 b e t a r a y s p e r y e a r , c o u l d b e m e a s u r e d by t h i s m e t h o d . T h e s e e a r l y t e s t s were f o l l o w e d by a s e r i e s o f e x p l o r a t o r y measurements which i n c l u d e d t h e d e t e c t i o n of Be i n A n t a r c t i c i c e ( R a i s b e c k , Yioux, F r u n e a u , i i e u v i n a n d L o u i s e a u x , 1 9 7 9 ) , "Be v a r i a t i o n s i n marine s e d i m e n t s ( R a i s b e c k , Yioux, Fruneau, L o u i s e a u x . L i e u v i n , R a v e l a n d H a y s , 1 9 7 9 ) a n d a s t u d y of t h e r e s i d e n c e t i m e o f "Be a n d i t s c o n c e n t r a t i o n i n t h e o c e a n s u r f a c e l a y e r ( R a i s b e c k , Yioux, F r u n e a u , Louiseaux and L i e u v i n , 1 9 7 9 ) . "Be h a s s u b s e q u e n t l y b e e n m e a s u r e d i n t a n d e m a c c e l e r a t o r s , b_ut i s v e r y low t h e m o l e c u l a r i o n 1 0 B e 1 6 0 was b e c a u s e t h e y i e l d o f "Bes e l e c t e d for a c c e l e r a t i o n . A f t e r t h e b r e a k u p o f t h i s m o l e c u l e ' ' B e s 3 was a c c e l e r a t e d f u r t h e r a n d c o u n t e d - , b e i n g s e p a r a t e d f r o m t h e i n t e r f e r i n g 1 ° B by t h e r a n g e t e c h n i q u e ( K i l i u s e t aZ., 1 9 8 0 ) . I t a p p e a r s r a t i o s a s low as 7 x c a n b e made t h a t o b s e r v a t i o n of " B e / ' B e without e x c e s s i v e d i f f i c u l t y , and i t i s t o b e expected t h a t a p p l i c a t i o n s of t h e ' " B e m e t h o d w i l l become more common i n t h e n e a r f u t u r e . Chlorine-36 Analysis j 6 C 1 ( h a l f - l i f e 3 . 1 x 1 0 5 a ) i s c r e a t e d i n t h e a t m o s p h e r e by c o s m i c r a y s , from whence i t j o i n s t h e s t a b l e i s o t o p e s o f c h l o r i n e , 3 5 C 1 a n d 3 7 C l , a n d for t h e m o s t p a r t e n t e r s t h e h y d r o s D h e r e . The m a i n u s e o f j 6 C 1 s e e m s t o be i n h y d r o g e o l o g i c a l s t u d i e s , and t h r o u g h i t r e l i a b l e m e t h o d s o f d a t i n g g r o u n d w a t e r s may b e d e v e l o p e d . An i m p o r t a n t a p p l i c a t i o n may b e i n t h e e v a l u a t i o n o f s i t e s p r o p o s e d f o r n u c l e a r waste s t o r a g e . Experiments w i t h a tandem a c c e l e r a t o r have r e s u l t e d i n 36Cl b e i n g d e t e c t e d i n g r o u n d w a t e r , a n d some o f t h e s p e c t r a o b t a i n e d i n t h i s work a r e p r e s e n t e d i n F i g u r e 6 ( E l m o r e e t aZ., 1 9 7 9 ) .
28
Figure 6
A) A t h r e e dimensional r e p r e s e n t a t i o n , s i m i l a r t o t h a t shown i n F i g u r e 3 , o f d a t a f r o m t h e h e a v y i o n counter at Rochester. The i o n s a r e o b s e r v e d f r o m a n AgCl s a m p l e f r o m L a k e O n t a r i o when t h e a p p a r a t u s w a s a d j u s t e d t o t r a n s m i t o n l y mass-37 c h a r g e - 7 i o n s The 3 5 C l and 3 7 C l p e a k s a r e p a r t of t h e background and t h e 36S peak i s due t o t h e r e s i d u a l s u l p h u r i n t h e p u r i f i e d sample. B ) The 3 6 C l + 7 i o n s f r o m a z o n e r e f i n e d r e a g e n t - g r a d e AgCl s a m p l e a r e shown t o g e t h e r w i t h t h e b a c k g r o u n d ions. The few c o u n t s shown-for 3 6 C l r e p r e s e n t a 3 6 C l / C l r a t i o o f ( 3 + 1 ) x 10 1 5 .
3 6 C l has to be distinguished from the isobars 3 6 A r and 3 6 S as well as from molecules s6ch as 1 2 C 3 and "0,. Ar- is unstable and does not enter the accelerator, and molecules are of course broken up. 3 6 S , although a rare isotope (0.014% of sulphur) forms negative ions readily and is the major difficulty in the measurement. Samples have to be carefully purified to minimize sulphur contamination, and are prepared in the form of A C1. In the final analysis the residual 3 6 S is distinguished from "Cl by the range technique. In the measurement of 3 6 C l in Antarctic meteorites by Nishiizumi e t a7Y. (1979) the lower limit reached was 7 x for 36Cl/Cl, and this is the smallest radioisotope ratio yet measured by this technique.
Aluminium-26 Analysis 2 6 A 1 (half-life 7.2 x 105a) is a l s o created by cosmic rays in the
29
atmosphere by t h e s p a l l a t i o n o f a r g o n , b u t b e c a u s e i t s p r o d u c t i o r y i e l d i s a b o u t 1 0 0 t i m e s l e s s t h a n t h a t o f ” B e i t i s much more d i f f i c u l t t o s t u d y . The s e p a r a t i o n o f 2 6 A l a n d i t s i s o b a r 26Yfg c a n b e p e r f o r m e d a n d K i l i u s e t aZ. s a t i s f a c t o r i l y by n e g a t i v e i o n mass s p e c t r o m e ’ r y , ( 1 9 7 9 ) h a v e m e a s u r e d 2 b A 1 / A 1 r a t i o s l e s s t h a n 1 0 l 4 i r l rnilligrans a m p l e s . T h i s i s c o m p a r a b l e t o m e a s u r e m e n t s T a d e on i r r g e s a m p l e s b v c o u n t i n g t h e gamma r a y s f o l l o w i n g t h e b e t a d e c a y to 26Mg. The a r e a s where a p p l i c a t i o n s h a v e b e e n a t t e m p t e d i n c l u d e s t u d i e s o f t h e g r o w t h r a t e s of m a n g a n e s e n o d u l e s on t h e o c e a n f l o o r ( G u i c h a r d e t aZ. (1978), and d e t e r m i n a t i o n o f t h e c o n s t a n c y o f c o s m i c r a y i n t e n s i t y i n t h e Dast (Hampel a n d S c h a e f f e r , 1 9 7 9 ) . I t seems o o s s i b l e t h a t m e a s u y i n g 2 6 P L l / ”Be on s m a l l s a m p l e s i n t h e same a p p a r a t u s may D r o v i d e a s e v e r a l m i l l i o n y e a r c h r o n o l o g y f o r o c e a n s e d i m e n t s which would be i n d e p e n d e n t o f f l u c t u a t i o n s i n c o s m i c r a y f l u x ( L a l , 1 9 6 2 ; R e y s s e t aZ., 1976). Iodine-129
Analysis
I2’I ( h a l f - l i f e 1 . 6 x 10’a) occurs i n m e t e o r i t e s through the a c t i o n o f cosmic r a y s . It can a l s o be produced i n t h e sDontaneous f i s s i o r of 2 3 8 U and i n t h e n e u t r o n i n d u c e d f i s s i o n o f 2 3 5 U . Neutron a c t i v a t i o n a n a l y s i s p r o v i d e s a s a t i s f a c t o r y method f o r s t u d y i n g 1 2 ’ 1 , b u t r e c e n t e x p e r i m e n t s by E l m o r e e t aZ. ( 1 9 8 0 ) h a v e shown t h a t n e g a t i v e ior. t a n d e m mass s p e c t r o m e t r y w i t h a t i m e - o f - f l i g h t d e t e c t i o n s y s t e m c a n d e t e r m i r e 1 2 y I / 1 2r7a 1 t i o s less than ir? m e a s u r e m e n t t i m e s l e s s ‘char. h a l f a n h o u r . The d e t e c t i o n of I 2 ’ I i n t w o m e t e o r i t e s a m p l e s has b e e r , a c complished t h i s way.
CONCLLSIONS The e x p e r i m e n t s d e s c r i b e d i n t h i s a r t i c l e h a v e b e e n m a i n l y o f a n e x p l o r a t o r y n a t u r e . The f e a s i b i l i t y o f m e a s u r i n g t h e v a r i o u s r a d i o i s o t o p e s by a c c e l e r a t o r mass s p e c t r o m e t r y h a s b e e n e s t a b l i s h e d , b u t r o u t i n e p r o d u c t i o n o f r e s u l t s f o r a p D l i c a t i o n to a r c h a e o l o g y a n d q u a t e r n a r y g e o l o g y m u s t await t h e i m m i n e n t e s t a b l i s h m e n t o f l a b o r a t o r i e s I n t h i s s e c o n d d e v e l o p m e n t s t a g e much a t d e d i c a t e d to t h e s e t a s k s . t e n t i o n w i l l b e d e v o t e d t o improving t h e p r e c i s i o n w i t h which t h e m e a s u r e m e n t s c a n b e made, s o t h a t p r e s e n t u n c e r t a i n t i e s w i l l b e e l i m i n a t e d . There i s every i n d i c a t i o n t h a t w i t h i p p r o v e d d e s i g n of i o n s o u r c e s , i o n beam o p t i c s a n d d e t e c t o r s , p r e c i s i o n s b e t t e r t h a n +1% may be o b t a i n e d . I n a d d i t i o n i t may b e p o s s i b l e t o e x t e n d t h e r a n g e o f r a d i o i s o t o p e s s t u d i e d t o some n o t m e n t i o n e d h e r e . One w h i c h h o l d s This isp r o m i s e for a r c h a e o l o g i s t s a n d Q u a t e r n a r y g e o l o g i s t s i s “ G a . t o p e , w h i c h has a h a l f - l i f e o f 1 . 3 x 1 0 5 a , p r e s e n t s p r o b l e m s i n i t s i d e n t i f i c a t i o n because of t h e d i f f i c u l t y i n d i s c r i m i n a t i n g a g a i n s t the isobar ’lK. H o w e v e r , t h e r e may b e t h e p o s s i b i l i t y o f u s i n g t h e m o l e c u l e ‘lCaFf t o d i s c r i m i n a t e a g a i n s t t h e u n s t a b l e 41KFf ( R a v n , 1 9 7 9 ) , i n which c a s e t h e i m p l i c a t i o n s f o r t h e d a t i n g o f c a l c i f e r o u s organisms a r e obvious. A f u r t h e r a s p e c t o f t h e a d v e n t o f t h i s method o f u l t r a s e n s i t i v e a n a l y s i s i s i n t h e a s s a y o f s t a b l e i s o t o p e s a t u l t r a - l o w l e v e l s . The r e c e n t l y p u b l i c i s e d i r i d i u m anomaly a t t h e C r e t a c e o u s - T e r t i a r y boundary ( A l v a r e z e t aZ., 1 9 8 0 ) may r e p r e s e n t a g l o b a l e v e n t w h i c h h a s l e f t i t s mark i n c o n t e m p o r a n e o u s s t r a t a a r o u n d t h e w o r l d . C o r r e l a t i o n of s u c h s t r a t a by m a t c h i n g t r a c e e l e m e n t p a t t e r n s i n d i f f e r e n t l o c a t i o n s may o f f e r a n o t h e r a p p r o a c h to g e o c h r o n o l o g y . However t h e m e t h o d a n d its a p p l i c a t i o n s e v o l v e , t h e r e can b e l i t t l e doubt t h a t a n i m p o r t a n t b r i d g e h a s b e e n c r o s s e d , a n d t h e p a t h on t h e o t h e r s i d e may b e e x p e c t e d to l e a d to e x c i t i n g new f i e l d s .
ACKNOWLEDGEMENTS I would l i k e to t h a n k my c o l l e a g u e s a t t h e U n i v e r s i t i e s of T o r o n t o and R o c h e s t e r f o r t h e o p p o r t u n i t y t o t a k e p a r t i n t h e s e e x c i t i n g d e velopments. I n p a r t i c u l a r I t h a n k A . E . L i t h e r l a n d f o r many d i s c u s s i o n s I a l s o acknowledge t h e and a d v i c e i n t h e p r e p a r a t i o n o f t h i s a r t i c l e . r e c e i p t of g r a n t s f r o m t h e N a t u r a l S c i e n c e s a n d E n g i n e e r i n g R e s e a r c h C o u n c i l , Ottawa.
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R a i s b e c k , G.M., Y i o u , F . , F r u n e a u , L o i s e a u x , J.M., L i e u v i n , N., R a v e l , J . C . and Hays, J . D . , 1979, A search i n a marine sediment core f o r %10&Be c o n c e n t r a t i o n v a r i a t i o n s d u r i n g a g e o m a g n e t i c f i e l d r e v e r s a l : Geophys. R e s . L e t t . , v . 6 , p . 717-719. R a i s b e c k , G.M., Y i o u , F . , F r u n e a u , L o i s e a u x , J.M. a n d L i e u v i n , M . , 1 9 7 9 , "Be c o n c e n t r a t i o n and residence t i m e i n t h e ocean s u r f a c e l a y e r : E a r t h a n d P l a n e t a r y S c i e n c e L e t t e r s , v . 43, p . 237-240. Ravn, H . L . , 1 9 7 9 , E x p e r i m e n t s w i t h i n t e n s e s e c o n d a r y beams o f r a d i o active ions: P h y s i c s R e p o r t s , v . 54, p . 201-259. R e y s s , J . L . , Yokayama, Y . a n d T a n a k a , S . , 1 9 7 6 , Aluminum-26 sediments: S c i e n c e , v . 193, p. 1119-1121.
i n deep-sea
S h a p i r a , D . , D e V r i e s , R . M . , F u l b r i g h t , H.W., T o k e , J. a n d C l o v e r , M . R . , 1 9 7 5 , The R o c h e s t e r h e a v y - i o n d e t e c t o r : Nuclear I n s t r u m e n t s and Methods, v . 1 2 9 , p . 123-130. S t e p h e n s o n , E . J . , Mast, T . S . and M u l l e r , R . A . , 1979, Radiocarbon dating N u c l e a r I n s t r u m e n t s and M e t h o d s , v . 1 5 8 , p . 571with a cyclotron: 577. 1979, Simulation experiments and Tombrello, T.A. and Kellogg, W . K . , p l a n e t a r y s p u t t e r i n g phenomena: Tenth Lunar and P l a n e t a r y S c i e n c e C o n f e r e n c e , A b s t r a c t s , p . 1233-1235. T u c k e r , A . B . , B o n a n i , G . , S u t e r , N. a n d W o e f l i , W . , 1 9 8 1 , E a r t h q u a k e d a t i n g b y I 4 C a t o m c o u n t i n g , P r o c . Symp. o n A c c e l e r a t o r Mass Spectrometry: A r g o n n e N a t i o n a l L a b o r a t o r y P u b l i c a t i o n ANL/PHY-81-1, p . 285-292.
33
URANIUM-SERIES DATING OF QUATERNARY DEPOSITS
H. SCHWARCZ and M. GASCOYNE
ABSTRACT A wide v a r i e t y o f Q u a t e r n a r y d e p o s i t s c a n b e d a t e d by U - s e r i e s methods, o v e r t h e t i m e r a n g e from 400 ky to 1 k y . I n o r d e r t o b e d a t a b l e , t h e d e p o s i t s must c o n t a i n m a t e r i a l w h i c h :
a)
c r y s t a l l i z e d a t the time of deposition;
b)
i n i t i a l l y c o n t a i n e d some U ( 0 . 1 ppm o r m o r e ) b u t no T h o r P a ;
c)
has r e m a i n e d a c h e m i c a l l y - c l o s e d s y s t e m s i n c e t h e t i m e o f deposit,ion.
I n c o n t i n e n t a l r e g i m e s t h e b e s t s u c h m a t e r i a l s a r e t r a v e r t i n e s formed e i t h e r i n c a v e s ( s p e l e o t h e m s ) o r as s p r i n g d e p o s i t s . The l a t t e r a r e often intercalated with d e t r i t a l e l a s t i c deposits containing vertebrate f o s s i l s , or r e l a t a b l e to g l a c i a l s t r a t i g r a p h y . The t r a v e r t i n e s may have been p o r o u s a t t h e t i m e o f d e p o s i t i o n , and s u b s e q u e n t f i l l i n g of p o r e s b y l a t e r c a r b o n a t e c a n r e s u l t i n s p u r i o u s l y young a g e s . C a l c r e t e ( c a l i c h e ) formed i n s o i l p r o f i l e s can b e d a t e d ; however c a l c r e t e d h o r i z o n s grow c o n t i n u o u s l y o v e r l o n g p e r i o d s , and m a t e r i a l must be s e l e c t e d t o be r e p r e s e n t a t i v e o f d i s c r e t e d e p o s i t i o n a l e v e n t s . S p e l e o them i s o f t e n i n t e r c a l a t e d w i t h f l u v i a l s e d i m e n t s i n t h e deep i n t e r i o r sf c a v e s , m a r k i n g p e r i o d s o f f l o o d i n g . I n cave-mouth d e p o s i t s , windolown d e t r i t u s i s a l s o p r e s e n t , as w e l l a s c u l t u r a l m a t e r i a l s t o which a g e s can b e a s s i g n e d by d a t i n g of s p e l e o t h e m . L a c u s t r i n e marls a r e 3 a t a b l e i f t h e c a r b o n a t e component i s d o m i n a n t l y a u t h i g e n i c . M o l l u s c a n - , a r b o n a t e , p r e s e n t a s s n a i l or clam s h e l l s i n l a c u s t r i n e s e d i m e n t s , a p p e a r s to g i v e l e s s r e l i a b l e a g e s . Bone, a l t h o u g h c o n t a i n i n g t h e q i g h e s t c o n c e n t r a t i o n s of U (up to 1 0 0 0 ppm) h a s g i v e n q u e s t i o n a b l e r e s u l t s i n some c a s e s , e v i d e n t l y due to "open s y s t e m " , c o n t i n u e d u p t a k e of U from g r o u n d w a t e r . U - - s e r i e s d a t e s of m a r i n e , b i o g e n i c c a r b o n a t e s have been u s e d t o e s t a b l i s h h i g h s e a - s t a n d s r e l a t e d t o e u s t a t i c f l u c t u - a t i o n s . Low s e a - s t a n d s c a n be e s t a b l i s h e d by d a t i n g s p e l e o t h e m s f o u n d i n c a v e s t h a t h a v e b e e n drowned by r i s e of s e a l e v e l . These d a t i n g methods c a n b e r e l a t e d to c l i m a t i c f l u c t u a t i o n s i n various ways. S p e l e o t h e m , d a t e d by U - s e r i e s methods c o n t a i n s a n i s o t o p i c r e c o r d o f p a l e o t e m p e r a t u r e and v a r i a t i o n i n i s o t o p i c composi t i o n of r a i n f a l l . D a t e d t r a v e r t i n e s i n a r i d r e g i o n s mark a n c i e n t p e r i o d s of h i g h e r r a i n f a l l and l a k e l e v e l s . Calcretes deposited i n t e r , i i t t e n t l y i n s o i l s e q u e n c e s o r a t v a r y i n g d e p t h s may r e f l e c t c h a n g e s i n p a s t r a i n f a l l i n t e n s i t y . G l a c i a l outwash d e p o s i t s i n some l o c a l i t i e s i n t h e U . S . R o c k i e s h a v e b e e n b u r i e d b y or i n t e r s t r a t i f i e d w i t h t r a v e r t i n e s formed by warm-water s p r i n g s . S p e l e o t h e m formed n e a r c a v e *iouths can e n t r a p p o l l e n which can b e u s e d t o d e f i n e t h e c l i m a t i c e o n d i t i o n s a t t h e t i m e of d e p o s i t i o n . T h i s would p e r m i t t h e con; t r u e t i o n of d a t e d p o l l e n p r o f i l e s beyond t h e r a n g e o f I 4 C .
34
INTR 0DU C TI0N B e f o r e t h e d i s c o v e r y of a b s o l u t e r a d i o m e t r i c d a t i n g m e t h o d s , t i m e s c a l e s for t h e e v o l u t i o n o f g e o m o r p h i c f e a t u r e s h a d t o b e e s t i m a t e d on t h e b a s i s o f o b s e r v a t i o n of m o d e r n e r o s i c n r a t e s a n d e x t r a p o l a t i D n t o e a r l i e r times. T h i s method s u f f e r e d , a t l e a s t i n t e m p e r a t e and a r c t i c regimes, from t h e problem t h a t erosiori r a t e s have a l t e r e d d r a s t i c a l l y d u r i n g t h e p a s t few h u n d r e d s o f t h o h s a n d s of y e a r s d u e t o i n t e n s e changes i n c l i m a t e t h a t have accompanied g l o b a l , c o n t i n e n t a l g l a c i a t i o n . If ma.1 h a d e v o l v e d d u r i n g a p e r i o d o f z e r s i s t e n t l y s t a b l e c l i m a t e , t h e n h e w o u l d h a v e b e e n a b l e t o u s e t h i s m e t h o d more e f f e c t i v e l y . In fact, i t i s now a p p a r e n t t h a t o n e o f t h e c h i e f t a s k s o f t h e g e o m o r p h o l o g i s t i s n o t o c l y t o e s t i m a t e t h e r a t e s of e v o l u t i o n of l a n d f o r m s , b u t a l s o t o n o t e t h e f l u c t u a t i o n s of t h e s e r a t e s t h r o u g h t i m e , and from them, t o e v o l v e a p i c t u r e of c l i m a t i c change. T h i s r e c o r d can be compared i n t u r n w i t h t h e r e c o r d of f a u n a l a n d f l o r a l c h a n g e a n d w i t h t h e now w e l l c a l i b r a t e d r e c o r d o f g l o b a l i c e volume o b t a i n e d from t h e i s o t o p i c s t u d y o f d e e p s e a c o r e s ( S h a c k l e t o n a n d Ondyke, 1976). I n v i e w o f t h e i n t e r - e s t i n t h i s problem of t i m e s c a l e s i n georrorphology i t i s f o r t u n a t e t h a t we now h a v e a t our d i s p o s a l a number o f g e o c h r o n o m e t e r s a p p l i c a b l e t o t h e m e a s u r e m e n t of t h e a g e o f s u c h y o u n g d e p o s i t s . T h e s e a r e summ a r i z e d i n F i g u r e 1, w h i c h c o m p a r e s t h e t i m e r a n g e s o v e r w h i c h e a c h i s a p p l i c a b l e . T h i s p a p e r i s s p e c i f i c a l l y d e v o t e d t o t h o s e which a r i s e f r o m t h e d i s e q u i l i b r i u m b e t w e e n t h e d a u g h t e r i s o t o p e s o f U-238 a n d U-235, a n d t h e i r r e s p e c t i v e D a r e n t s .
1
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-
I - 1
1
K-Ar Fission Track
t
=
lo3 Figure 1
-
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lo4
iU-series I
lo5 YEARS
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lo6
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lo7
D a t i n g m e t h o d s appl-icable to t h e q u a t e r n a r y . N R M : n a t u r a l r e m a n e n t m a g n e t i s m ; n o r m a l ( b l a n k ) and reversed (shaded) chrons are shown; Mat = Matuyama; amino acid raceGa = G a u s s ; G i = G i l b e r t . B A R : m i z a t i o n ; TL: t h e r m o l u m i n e s c e n c e ; ESR: electron spin resonance; U-series range is shown f o r 2 3 o T h / 2 3 4 Ud a t i n g .
U r a n i u m s e r i e s d a t i n g i s b a s e d on t h e d e c a y o f t h e d a u g h t e r s o f U i s o t o p e s by e m i s s i o n o f a l p h a p a r t i c l e s . I n e a c h method ( 2 3 0 T h / 2 3 4 U , 2 3 4 U / 2 3 8 U , 2 3 1 P a / 2 3 5 U e t c . ) t h e f o l l o w i n q a s s u m D t i o n s a r e made: 1) a t t h e t i m e o f f o r m a t i o n o f t h e d a t e d sample t h e p a r e n t i s o t o p e ( s ) was p r e s e n t b u t t h e d a u g h t e r i n q u e s t i o n was e i t h e r a b s e n t o r was p r e s e n t a t a known a c t i v i t y l e v e l : 2 ) a f t e r a t i m e w h i c h i s lone; w i t h r e s p e c t to t h e h a l f - l i f e o f t h e d a u g h t e r , t h e a c t i v i t y of t h e d a u g h t e r b e c o m e s e q u a l t o t h a t of t h e p a r e n t ( t h a t i s , t h e s a m p l e a c h i e v e s s e c u l a r
35
r a d i o a c t i v e e q u i l i b r i u m ) ; 3 ) t h e sample h a s n o t been c h e m i c a l l y d i s t u r b e d s i n c e t h e t i m e of i t s f o r m a t i o n . The h a l f l i f e of t h e d a u g h t e r s o f t h e uranium i s o t o p e s r a n g e from a few m i l l i s e c o n d s t o 2 . 5 x 1 0 5 y , and a l l o w us t o e s t i m a t e t h e a g e o f s a m p l e s o v e r t h e mid- and u p p e r P l e i s t o c e n e though w i t h v a r y i n g d e g r e e s o f p r e c i s i o n . The a p p l i c a b i l i t y of t h e method r e s t s on t h e o b s e r v a t i o n t h a t t h e d a u g h t e r i s o t o p e s behave c h e m i c a l l y q u i t e d i f f e r e n t l y from t h e i r respective p a r e n t s . Therefore, i n low-temperature d e p o s i t i o n a l proc e s s e s such a s g r o w t h o f c a l c i t e from s o l u t i o n , t h e p a r e n t u r a n i u m may be t r a p p e d a s a t r a c e e l e m e n t i n a c r y s t a l w h i l e t h e d a u g h t e r s may b e excluded o r a t l e a s t g r e a t l y d e p l e t e d r e l a t i v e to t h e i r e q u i l i b r i u m v a l u e . The v a r i o u s d e c a y s e r i e s i n q u e s t i o n a r e shown i n F i g u r e 2 . Thorium-230, t h e d a u g h t e r of 2 3 4 U ( a n d i n t u r n , 2 3 8 U i s v e r y i n s o l u b l e i n ground and s u r f a c e - w a t e r ) . T h e r e f o r e c a l c i t e p r e c i p i t a t e d i n c o n t i n e n t a l or m a r i n e e n v i r o n m e n t s i s f o u n d to c o n t a i n up t o s e v e r a l ppm of U b u t n e g l i g i b l e amounts o f T h . Therefore i t s age of c r y s t a l l i z a t i o n can be d e t e r m i n e d by measurement o f t h e amount o f 2 3 0 T ht h a t Similar relations are has grown i n t o e q u i l i b r i u m w i t h t h e U p a r e n t . observed between 2 3 1 P a ( a l s o i n s o l u b l e i n s u r f a c e w a t e r s ) a n d i t s parent 'U The a g e s o f s a m p l e s a r e g i v e n by t h e f o l l o w i n g e q u a t i o n s :
.
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.qhere 2 3 4 U , e t c . , a r e t h e r e s p e c t i v e a c t i v i t i e s i n d i s i n t e g r a t i o n s p e r minute p e r gram o f s a m p l e , X 2 3 0 , e t c . a r e t h e d e c a y c o n s t a n t s o f t h e i s o t o p e s , and t i s t h e t i m e s i n c e c r y s t a l l i z a t i o n . Note t h a t e q u a t i o n (1) i s t r a n s c e n d e n t a l and must b e s o l v e d by i t e r a t i v e a p p r o x i m a t i o n s . Figure 3 i s a g r a p h of s o l u t i o n s o f t h i s e q u a t i o n which shows t h a t t h e l i m i t o f t h e a g e d e t e r p i n a b l e b y t h i s method i s a b o u t 4 0 0 ky d e p e n d i n g 3n t h e 234U/238Ur a t i o . The D r e c i s i o n o f measurement o f t h e a c t i v i t y r a t i o s depends on t h e c o n c e n t r a t i o n o f U i n t h e s a m p l e , and t h e chemical y i e l d of t h e e x t r a c t i o n method. Generally 2 3 1 P a / 2 3 5 U ages can o n l y be o b t a i n e d on s a m p l e s c o n t a i n i n g more t h a n 1 ppm U: due t o t h e much l o w e r a b u n d a n c e o f t h e p a r e n t 2 3 5 U ( 2 3 5 U / 2 3 8 U = 1/137.8 on atom basis). The r a t i o 2 3 4 U / 2 3 8 Us h o u l d b e e q u a l t o u n i t y i n r o c k s and w a t e r s , s i n c e t h e s e a r e two i s o t o p e s of t h e same heavy e l e m e n t and t h u s n o t s u b j e c t to i s o t o p e f r a c t i o n a t i o n l i k e l i g h t e l e m e n t s . However, i t h a s neen found t h a t 2 3 4 U i f p r e f e r e n t i a l l y l e a c h e d from r o c k s d u r i n g t h e w e a t h e r i n g c y c l e , s o t h a t t h e 2 3 4 U / 2 3 8 Ur a t i o i n g r o u n d w a t e r s i s n o r m a l l y somewhat g r e a t e r t h a n u n i t y ( a n d r a n g e s up to v a l u e s o f a b o u t 2 0 i n r a r e i n s t a n c e s ) w h i l e t h e r a t i o i n s e d i m e n t s i s g e n e r a l l y somewhat l e s s t h a n u n i t y . The v a l u e of t h i s r a t i o i n a Q u a t e r n a r y d e p o s i t which h a s behaved a s a c l o s e d s y s t e m s i n c e i t s f o r m a t i o n , c o u l d b e u s e d t o e s t i m a t e t h e a g e o f t h e s e d i m e n t . The r a t i o c h a n g e s w i t h t i m e a c c o r d i n g to t h e r e l a t i o n :
liowever, t h e i n i t i a l r a t i o , ( 2 3 4 U / 2 3 8 U ) o , c a n n o t b e i n d e p e n d e n t l y Itnown, i n most c a s e s . For m a r i n e s a m p l e s ( c o r a l s , e t c . ) i t c a n b e assumed t o have b e e n n e a r t h e p r e s e n t - d a y v a l u e o f 1 . 1 4 , b u t t h e r a t i o o f
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Decay chains € o r 2 3 8 U and 2 3 5 U . Heavy borders are around longer-lived isotopes, useful for dating Quaternary sediments.
m a r i n e s a m p l e s a p p r o a c h e s u n i t y w i t h i n a b o u t 3 0 0 k y , which i s w i t h i n t h e r a n g e of t h e 2 3 0 T h / 2 3 4 U method. For o l d e r c o n t i n e n t a l d e p o s i t s one c a n sometimes assume t h a t t h e i n i t i a l r a t i o h a s r e m a i n e d c o n s t a n t i f y o u n g e r d e p o s i t s , d a t a b l e by 2 3 0 T h , d e m o n s t r a t e t h i s f a c t . However, many examples o f s i t e s a t which t h e i n i t i a l r a t i o has v a r i e d d r a s t i c a l l y w i t h t i m e a r e now known, e . g . , f o r s t a l a g m i t e d e p o s i t i o n i n c a v e s ( s e e below). The i d e a l s a m p l e f o r U - s e r i e s d a t i n g i s one which was c h e m i c a l l y p r e c i p i t a t e d from a l o w - t e r m p e r a t u r e s o l u t i o n . A l a r g e v a r i e t y o f poetentially-datable materials s a t i s f y t h i s c r i t e r i o n . Principally, t h e s e a r e v a r i o u s forms of c a l c i u m c a r b o n a t e , e i t h e r c a l c i t e , a r a g o n i t e o r some o t h e r polymorph o r h y d r a t e d s p e c i e s . We s h a l l r e f e r t o a l l of them h e r e a s c a l c i t e , t h o u g h t h e same methods a r e a p p l i c a b l e t o o t h e r The c a l c i t e may o c c u r a s t r a v e r t i n e , polymorphs or s t a t e s o f h y d r a t i o n . t h a t i s , m a s s i v e , c h e m i c a l l y - p r e c i p i t a t e d l i m e s t o n e formed e i t h e r i n a c a v e ( s p e l e o t h e m ) or a t a l i m e - r i c h s p r i n g . I f t h e t r a v e r t i n e i s i n i t i a l l y d e n s e , n o n - p o r o u s , and f r e e of d e t r i t a l C o n t a m i n a n t s , t h e n i t g e n e r a l l y s a t i s f i e s t h e a s s u m p t i o n s o f t h e m e t h o d . However, many s u r f i c i a l d e p o s i t s o f t h i s s o r t ( t u f a ) a r e p o r o u s . and c o n t a i n d e t r i t u s . They may b e r e c r y s t a l l i z e d or l e a c h e d b y w a t e r p e r m e a t i n g t h r o u g h them l o n g a f t e r t h e y were d e p o s i t e d ; t h e y may also c o n t a i n n o n - r a d i o g e n i c
37
Figure 3
Graph of activity ratios 2 3 4 U / 2 3 8 U u s f o r c l o s e d s y s t e m s of v a r y i n g i n i t i a l
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Isochrons are nearyvertical curves; sub-horizontal c u r v e s a r e l o c i o f s a m p l e s of c o n s t a n t i n i t i a l 2 3 4 U / 2 3 8 U ratio. A p p r o x i m a t e e r r o r c r o s s is s h o w n Effective limit f o r a s a m p l e c o n t a i n i n g 0.5 p p m U . o f d a t i n g m e t h o d i s 350,000 y e a r s .
2 3 0 T h (common Th) t r a p p e d on t h e d e t r i t a l p a r t i c l e s . a s w e l l a s p a r t i c l e s of l i m e s t o n e which b e h a v e c h e m i c a l l y l i k e t h e t r a v e r t i n e , b u t which c o n t a i n e d u r a n i u m i n s e c u l a r e q u i l i b r i u m . R e c r y s t a l l i z a t i o n w i l l make t h e sample a p p e a r t o be a n o m a l o u s l y young, w h i l e d e t r i t a l conr tamination has t h e opposite e f f e c t .
O t h e r t y p e s o f p o t e n t i a l l y d a t a b l e m a t e r i a l s a r e : c a l c i t e dep o s i t e d i n t h e s o i l as c a l c r e t e ( c a l i c h e ) ; s h e l l s of m o l l u s c s or o t h e r i n v e r t e b r a t e organisms; bones of v e r t e b r a t e s ; e g g - s h e l l s o f l a r g e birds. A l l o f t h e s e d e p o s i t s c a n i n i t i a l l y c o n t a i n s i g n i f i c a n t amounts o f uranium or a c q u i r e them s o o n a f t e r d e D o s i t i o n from g r o u n d w a t e r . Peat d e p o s i t s , w h i l e i n i t i a l l y l a c k i n g i n u r a n i u m , c a n a l s o a d s o r b t h i s element from g r o u n d w a t e r some t i m e a f t e r d e p o s i t i o n , and i f t h e y a r e s u b s e q u e n t l y d r i e d o u t , and compacted t h e y may b e h a v e a s c l o s e d systems. Other t y p e s o f c h e m i c a l d e p o s i t s s u c h a s . p h o s p h a t i c c o n c r e t i o n s , gypsum, e v a p o r i t e s i n g e n e r a l , and even d e p o s i t s o f f e r r i c or manganese h y d r o x i d e s ( e . g . c o n c r e t i o n s ) c a n c o n t a i n s i g n i f i c a n t amount of co-. p r e c i p i t a t e d u r a n i u m a l t h o u g h i n many i n s t a n c e s t h e r e w i l l a l s o b e s i z a b l e amounts o f common t h o r i u m e i t h e r i n t h e same c r y s t a l l i n e materi a l , or a d s o r b e d on d e t r i t u s . The a n a l y s i s o f i s o t o p e r a t i o s f o r a g e d e t e r m i n a t i o n i s u s u a l l y c a r r i e d o u t by d i s s o l v i n g a s a m p l e o f t h e m a t e r i a l i n q u e s t i o n , i n n i t r i c a c i d or some o t h e r o x i d i z i n g medium. T h i s e n s u r e s t h a t uranium remains (or i s c o n v e r t e d t o ) t h e 6 t o x i d a t i o n s t a t e , s o i t can b e exI n t h e c a s e of c a r b o n a t e s t r a c t e d b y i o n e x c h a n g e from t h e s o l u t i o n . ( t r a v e r t i n e , e t c . ) t h e i n s o l u b l e r e s i d u e may b e s i m p l y d i s c a r d e d , OF i t may be s e p a r a t e l y d i s s o l v e d i n h y d r o f l u o r i c a c i d , and added t o t h e remaining s o l u t i o n . The HN0; i n s o l u b l e p o r t i o n commonly c o n t a i n s t h e d e t r i t a l component r e s p o n s i b l e f o r t h e - p r e s e n c e o f common Th or P a ,
38
a n d i n some c a s e s i t c a n b e s e p a r a t e l y a n a l y s e d t o p e r m i t a c o r r e c t i o n for t h e component (Ku e t aZ., 1 9 7 9 ) . The s o l u t i o n i s t h e n p r o c e s s e d t o s e p a r a t e U a n d Th by i o n e x c h a n g e ; t h e s e e l e m e n t s a r e p l a t e d o n t o metal d i s c s and t h e a c t i v i t y of t h e v a r i o u s a l p h a - e m i t t i n g i s o t o p e s i s d e t e r m i n e d by a l p h a s p e c t r o m e t r y . U s u a l l y a t r a c e r c o n t a i n i n g a r t i f i c i a l n u c l i d e s i s a d d e d ( e . 9 . "*Th i n e q u i l i b r i u m w i t h 2 3 2 U ) t o d e t e r m i n e t h e c h e m i c a l y i e l d s o f U and T h y and t h u s a l l o w s u c h r a t i o s a s 2 3 0 T h / 2 3 4 Ut o b e d e t e r m i r e d . The a g e i s o b t a i n e d b y s o l u t i o n o f e q . ( 1 ) , e t c . , u s i n g a c o m p u t e r . E r r o r e s t i m a t e s a r e b a s e d on t h e number o f a l p h a d i s i n t e g r a t i o n s c o u n t e d , a s s u m i n g P o i s s o n s t a t i s t i c s .
If t h e s a m p l e c o n t a i n s much i n s o l u b l e d e t r i t u s . or i f I t d i s p l a y s t h e p r e s e n c e of 2 3 2 T hi n t h e a l p h a spectrum, t h e n t h e spectrum i s l i k e l y t o c o n t a i n some common 2 3 0 T h a n d p o s s i b l y some common U l e a c h e d from t h e d e t r i t u s . If t h e a b u n d a n c e o f common Th i s h i g h , as shown by a low 2 3 0 T h / 2 3 2 T ha c t i v i t y r a t i o ( t y p i c a l l y l e s s t h a n 201, t h e n i t i s n e c e s s a r y t o c o r r e c t f o r t h i s c o m p o n e n t . V a r i o u s s c h e m e s for m a k i n g t h i s c o r r e c t i o n have been d e v i s e d , and a r e summarized e l s e w h e r e (Schwarcz, 1 9 8 0 ) .
.
I n t h e f o l l o w i n g s e c t i o n s we s h a l l d e s c r i b e some o f t h e a p p l i c a t i o n s of t h e s e d a t i n g methods t o Q u a t e r n a r y c o n t i n e n t a l d e p o s i t s , w i t h s p e c i a l emphasis t o t h o s e a p p l i c a t i o n s which r e v e a l i n f o r m a t i o n about c l i m a t i c c h a n g e . We s h a l l o m i t t h e d a t i n g o f c o r a l r e e f s , w h i c h w i l l be d i s c u s s e d by S t e a r n s ( t h i s v o l u m e ) . TRAVERTINE
Speleothems C a v e s a r e f o r m e d i n l i m e s t o n e d u e t o d i s s o l u t i o n o f t h e r o c k by waters c o n t a i n i n g c a r b o n i c a c i d from t h e s o i l . S u b s e q u e n t l y , t h e caves may b e p a r t l y f i l l e d by v a r i o u s f o r m s o f c h e m i c a l l y - p r e c i p i t a t e d c a r bonate: s t a l a g m i t e s , s t a l a c t i t e s , f l o w s t o n e , e t c . , which a r e c o l l e c t i v e l y c a l l e d speleothems. When d e p o s i t e d i n t h e d e e p i n t e r i o r o f c a v e s , a n d a b o v e t h e l e v e l w h e r e streams c a n t r a n s p o r t d e t r i t u s , t h e s e d e p o s i t s c a n b e e x c e p t i o n a l l y p u r e and f r e e of d e t r i t a l c o n t a m i n a n t s . S u c h s p e l e o t h e m s a r e t y p i c a l l y q u i t e i m p e r m e a b l e , a n d a r e composed o f a g g r e g a t e s o f a f e w v e r y l a r g e c r y s t a l s , up t o s e v e r a l cm i n s m a l l e s t d i m e n s i o n . They a r e t h e r e f o r e i d e a l l y s u i t e d t o U - s e r i e s d a t i n g , a s w e l l as t o o t h e r t y p e s o f geochemical a n a l y s e s (Gascoyne e t a Z . , 1 9 7 8 ) . N o r m a l l y t h e c o n t e n t o f U i n s p e l e o t h e m i s b e l o w 1 ppm. Therefore ages However, c a n g e n e r a l l y o n l y b e o b t a i n e d u s i n g t h e 2 3 0 T h / 2 3 4 Um e t h o d . i f t h e r e a r e l a y e r s o f s h a l e o r o t h e r d e t r i t a l s e d i m e n t a r y rocks h y d r o l o g i c a l l y s i t u a t e d s o as t o c o n t r i b u t e U t o i n f i l t r a t i n g g r o u n d w a t e r s , t h e n s p e l e o t h e m s p r e c i p i t a t e d f r o m t h e s e w a t e r s may c o n t a i n up t o 1 0 0 ppm U . The 2 3 4 U / 2 3 8 U r a t i o o f w a t e r d r i p p i n g f r o m a s i n g l e s o u r c e i n a c a v e i s r e l a t i v e l y c o n s t a n t a l t h o u g h some s h o r t t e r m v a r i a t i o n h a s b e e n o b s e r v e d ( P . Thompson e t a Z . , 1 9 7 5 ) . The p o s s i b l e u s e o f t h i s r a t i o i n s p e l e o t h e m a s a m e t h o d of a g e d e t e r m i n a t i o n has b e e n i n v e s t i g a t e d by P . Thompson e t aZ. ( 1 9 7 5 ) , G . Thompson e t aZ. ( 1 9 7 5 1 , a n d Harmon a n d S c h w a r c z ( 1 9 8 2 ) . The l a t t e r a u t h o r s f o u n d t h a t a t t w o d i s c r e t e s i t e s w i t h i n a s i n g l e cave a f e w hundred m e t r e s a p a r t t h e b e h a v i o u r of t h e 2 3 4 U / 2 3 8 U r a t i o v a r i e d markedly. A t one s i t e t h e i n i t i a l r a t i o r e mained e s s e n t i a l l y c o n s t a n t o v e r a p e r i o d o f 2 5 0 , 0 0 0 y e a r s , w h i l e a t t h e o t h e r s i t e t h e r a t i o v a r i e d o v e r t h e same t i m e r a n g e , a n d was s i g n i f i c a n t l y d i f f e r e n t f r o m t h a t a t t h e f i r s t s i t e . G . Thompson e t a Z . (1975) found t h a t t h e measured 2 3 4 U / 2 3 8 U r a t i o d e c r e a s e d r e g u l a r l y from T h i s a l l o w e d them t o t h e t o p t o t h e b a s e of a s i n g l e speleothem. d e t e r m i n e i t s g r o w t h h i s t o r y b u t t h e a g e s w h i c h t h e y o b t a i n e d were much o l d e r t h a n t h e a p p a r e n t 2 3 0 T h / 2 3 4 Ua g e s f o r t h e same s a m p l e ( s e e Harmon e t aZ., 1 9 7 8 a f o r a d i s c u s s i o n o f t h e s e r e s u l t s ) . A b s o l u t e a g e s o f s p e l e o t h e m s c a n b e u s e d i n v a r i o u s ways t o h e l p d e f i n e t h e Q u a t e r n a r y h i s t o r y of a r e g i o n . F i r s t , t h e y p r o v i d e a b a s i c If they are i n t e r chronology f o r t h e caves i n which t h e y a r e formed. c a l a t e d b e t w e e n e l a s t i c d e t r i t u s c a r r i e d i n t o t h e c a v e s by m e l t w a t e r streams from g l a c i e r s , t h e n t h e y g i v e p o t e n t i a l i n f o r m a t i o n a b o u t t h e t i m i n g of g l a c i a t i o n (Ford, 1 9 7 3 ) . I f t h e y a r e f r o m c a v e s t h a t h a v e
39
been a e v e i o p e d a t p r o g r e s s i v e l y d e e p e r l e v e l s i n a l i m e s t o n e f o r m a t i o n due t o l o w e r i n g o f l o c a l b a s e l e v e l , t h e n t h e a g e o f a g i v e n d e p o s i t i s a minimum e s t i m a t e o f t h e t i m e t h e b a s e l e v e l s t o o d a t t h e e l e v a t i o n of the s?eleothem. T h i s h a s b e e n u s e d by F o r d e t aZ. (1981) t o d e t e r m i n e t h e a v e r a g e r a t e o f d o w n c u t t i n g of t h e c a r b o n a t e - b a s e d p a r t of t h e Canadian 3 o c k i e s . Xere a number of p h r e a t i c c a v e s h a v e b e e n l e f t h i g h and d r y by l o w e r i n g o f v a l l e y f l o o r s . Many o f t h e c a v e s a r e r e p r e sented t o d a y o n l y by remanent t u n n e l s t r u n c a t e d a t b o t h e n d s by s t e e p m o u n t a i n c l i f f s ( F i g u r e 4 ) . W i t h i n some o f t h e s e c a v e s s p e l e o t h e m s a r e f o g 2 6 r a - g i n g ir:age f r o n 8 few “,Gasand y e a r s t o b e y o n d t h e r a n g e of 2 3 0 T h / 2 3 4 Ud a t i n g . One s a m p l e h a s i n f a c t b e e n shown by p a l e o m a g n e t i c m e a s u r e m e n t s t o b e o l d e r t h a n 700 k y , s i n c e i t p r e s e r v e s a r e v e r s e d c a t u r a l r e m a n e n t m a g n e t i s a t i o n ( L a t h a m e t aZ., 1979). Sy r e l a t i n g t h e e l e v a t i o n s a n d a g e s of t h e s e s p e l e o t h e m s , i t h a s b e e n p o s s i b l e t o c o n s t r u c t a m o d e l f o r t h e t i m i n g of t h e u p l i f t a n d d o w n c u t t i n g o f t h e n o u p - t a i n r a n g e , a n d t h E s t o e s t i m a t e t h e a g e of l o c a l r e l i e f i n t h e e a s t e r n p a r t o f t h e C a n a d i a n Rocky N o u n t a i n s . A summary o f t h e s e d a t a i s g i v e n i n T a b l e 1. U s i n g t h e h e i g h t s of t h e c a v e m o u t h s a b o v e t h e a d j a c e n t v a l l e y floors, a n d t a k i n g i n t o a c c o u n t t h e h y d r a u l i c g r a d i e n t s t h a t must h a v e e x i s t e d b e t w e e n t h e m o u t h s of t h e c a v e s a n d t h e i n n e r p o r t i o n s r e p r e s e n t e d by t h e p r e s e r v e d r e n a n e n t s , t h e maximum r a t e o f d o w n c u t t i n g was b e t w e e n 0 . 1 3 a n d 2 . 1 m p e r 1 0 0 0 y w h i l e t h e minimum r a t e r a n g e d b e t w e e n 3.04 a n d 0 . 0 7 m/1000 y. The mean r e l i e f o f 1 . 3 km t h a t e x i s t s t o d a y i n t h i s r e g i o n would have d e v e l o p e d i n a t i m e o f between 6 and 1 2 m i l l i o n y e a r s .
Figure 4
V i e w from i n s i d e of e r o s i o n a l cave remanent, c r e s t o f Rocky M o u n t a i n s , A l b e r t a .
near
The e x i s t e n c e of s p e l e o t h e m d e p o s i t s i n a c a v e i m p l i e s t h a t t h e r e was l i q u i d w a t e r i n t h e s o i l a b o v e t h e c a v e a n d t h a t o r g a n i c a c t i v i t y t o p r o d u c e C O z was o c c u r r i n g i n t h e s o i l . T h e r e f o r e , t h e r a t e o f d e p o s i t i o n o f s p e l e o t h e m i s m o d u l a t e d by c h a n g e s i n c l i m a t e , a n d t h e f r e q u e n c y o f o c c u r r e n c e of s p e l e o t h e m a g e s i n a r e g i o n g i v e s a n i n d i - a % i o n o f t h e i n t e n s i t y of t h e c l i m a t e t h r o u g h t i m e . B o t h a r i d i t y a n d i n t e n s e p r o l o n g e d c o l d c o u l d h a v e t h e same e f f e c t on g r o w t h r a t e s b u t
40 T a b l e 1.
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i n a g i v e n r e g i o n i t w i l l g e n e r a l l y be a p p a r e n t which of t h e s e f a c t o r s was r e s p o n s i b l e , I n a l p i n e c a v e s o f w e s t e r n N o r t h America we have obt a i n e d a f r e q u e n c y d i s t r i b u t i o n o f s p e l e o t h e m a g e s o v e r t h e l a s t 300 ky (Harmon e t al., 1977) t h a t p a r a l l e l s c l i m a t e v a r i a t i o n s a s i n f e r r e d from t h e d e e p - s e a i s o t o p e r e c o r d . I n England a s i m i l a r p a t t e r n s l i g h t l y d i s p l a c e d i n t i m e , h a s b e e n o b t a i n e d (Gascoyne e t al., 1 9 8 2 ) ( F i g u r e
5).
A s n o t e d , U - s e r i e s d a t e s on s p e l e o t h e m s can a l s o b e u s e d a s a t i m e b a s e f o r v a r i o u s s t u d i e s which t r e a t s Q e l e o t h e m s as r e p o s i t o r y r e c o r d s of t h e p a s t . For e x a m p l e , t h e i r s t a b l e i s o t o p i c c o m p o s l t i o n c a n b e u s e d t o i n f e r p a s t c l i m a t e (Harmon e t al. , 1978b; Gascoyne e t al., 1981), t h e i r p a l e o m a g n e t i c r e c o r d can b e d e t e r m i n e d and compared w i t h r e c o r d s from o t h e r t y p e s of s a m p l e s ( L a t h a m e t al., 1979), and t h e y have b e e n shown t o c o n t a i n p o l l e n which may a l s o p r e s e r v e a p a l e o c l i m a t i c r e c o r d ( B a s t i n , 1 9 7 8 ) . S p e l e o t h e m from t h e mouths o f c a v e s can b e i n t e r s t r a t i f i e d w i t h d e t r i t a l s e d i m e n t s c o n t a i n i n g a r c h a e o l o g i c a l r e m a i n s , a s w e l l a s f a u n a and f l o r a ( i n c l u d i n g p o l l e n ) , a l l o f which c a n b e s e t i n a c h r o n o l o g i c a l framework b y u s e o f t h e d a t e d s p e l e o t h e m l a y e r s ( S c h w a r c z , 1980; B l a c k w e l l e t al., 1 9 8 2 ) . I
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Figure 5
H i s t o g r a m of d a t e s o f s p e l e o t h e m s from c a v e s i n Lancashire, England. Note absence of speleothem d e p o s i t s d u r i n g g l a c i a l maxima a n d p e a k s i n f r e quency d u r i n g i n t e r g l a c i a l s and i n t e r s t a d i a l s . From G a s c o y n e e t a l . . 1 9 8 2 .
41
Spring-Deposited T r a v e r t i n e s L i m e - r i c h w a t e r s e m e r g i n g f r o m s p r i n g s i n a r e a s of c a r b o n a t e b e d r o c k , or i n v o l c a n i c t e r r a i n , c a n d e p o s i t t e r r a c e - l i k e a c c u m u l a t i o n s o f c a l c i u m c a r b o n a t e , w h i c h a r e commonly f o u n d t o b e i n t e r c a l a t e d w i t h o t h e r t y p e s o f s e d i m e n t . The c a r b o n a t e s , commonly d e s c r i b e d as t u f a , a r e t y p i c a l l y q u i t e p o r o u s , and i n c l u d e l a r g e amounts of v e g e t a b l e m a t t e r d e r i v e d from t h e growth o f r e e d s , mosses and o t h e r p l a n t s , a s w e l l a s a l g a l o r b a c t e r i a l mats t h a t may b e p a r t l y r e s D o n s i b l e f o r c a r b o n a t e p r e c i p i t a t i o n . The o r g a n i c m a t e r i a l i s l a r g e l y l o s t by oxi d a t i o n and decay w i t h i n a few t h o u s a n d y e a r s a f t e r d e p o s i t i o n , a d d i n g t o t h e p o r o s i t y of t h e d e p o s i t . F i n a l l y , f u r t h e r p r e c i p i t a t i o n of c a l c i t e may o c c u r d u e t o p a s s a g e o f w a t e r t h r o u g h t h e p o r o u s mass, f i l l i n g p r i m a r y p o r e s w i t h s e c o n d a r y c a l c i t e whose a b s o l u t e a g e i s much y o u n g e r t h a n t h e t i m e o f d e p o s i t i o n o f t h e t e r r a c e . N e v e r t h e i e s s , i n i d e a l circumstances, it i s p o s s i b l e t o obtain meaningful U-series d a t e s on s u c h t r a v e r t i n e s . Optimally t h e sample material s h o u l d be c o l l e c t e d f r o m l a y e r s w h i c h a p p e a r i n i t i a l l y t o h a v e b e e n as d e n s e (non-porous) as p o s s i b l e , and l a c k i n g i n d e t r i t a l c o n t a m i n a n t . Study of some modern c a r b o n a t e - d e p o s i t i n g s D r i n g s s u g g e s t s t h a t t h e d e t r i t u s p r o b l e m i s a l l e v i a t e d by t h e a c t i o n o f C,he f l o w i n g s p r i n g w a t e r c a r r y i n g away a l l o c h t h o n o u s s e d i m e n t t h a t i s b l o w n i n or c a r r i e d i n by animals.
Dates o f s u c h t r a v e r t i n e s c a n b e s i g n i f i c a n t i n s t u d i e s o f g e o Torphic e v o l u t i o n . Where t h e t r a v e r t i n e forms a b r o a d , s h e e t - l i k e d e p o s i t , i t s e r v e s as a c h r o n o s t r a t i g r a p h i c m a r k e r w h i c h may b e d e p o s i t e d . o n , or i n t e r c a l a t e d w i t h , o t h e r Q u a t e r n a r y s e d i m e n t s w h o s e a g e s a r e s o u g h t . ? o r e x a m p l e , t e r r a c e s o f t u f a may b e s u p e r i m p o s e d on f l u v i a l t e m a c e s , arid may t h u s p r o v i d e a n a g e for t h e c b t t i n g o f t h e l a t t e r . T h i s s i t u a t i o n i s f o u n d i n n o r t h w e s t e r n Hungary where t e r r a c e s have b e e n c u t a t v a r i o u s s t a g e s o f d e v e l o p m e n t o f t h e 3 a n u b e R i v e r a n d i t s t r i b u t a r i e s . W a r m - s p r i n g - d e p o s i t e d t u f a mounds a r e b u i l t up o n t h e s e p l a t f o r m s a t many p o i n t s w e s t o f S u d a p e s t , a n d some h a v e b e e n d a t e d by t h e 2 3 0 T h / 2 3 4 Um e t h o d . The f i r s t s t u d i e s w e r e by C h e r d y n t s e v e t al. (1965) who w e r e c o n c e r n e d w i t h t h e a r c h a e o l o g i c a l s i t e s a t T a t a and V g r t e s s E l l 6 s . L a t e r , P e ' c s i (1973) p o i n t e d o u t t h e r e l a t i o n b e t w e e n t h e s e t u f a d e p o s i t s a n d t h e f l u v i a l t e r r a c e s , ar,d c o r r e l a t e d t h e m w i t h d i s c r e t e g l a c i a l s t a g e s ( T a b l e 2 ) ; d a t e s on t h e t u f a s o b t a i n e d by J . K . Osmond ( q u o t e d i n P e ' c s i , 1973), c o n f i r m t h e g r e a t e r a n t i q u i t y o f :he h i g h e r t u f a m o u n d s , s h o w i n g t h a t i n g e n e r a l t h e y a p p e a r t o h a v e been d e p o s i t e d n o t l o n g a f t e r t h e t e r r a c e c u t t i n g . ?lore r e c e n t d a t a on two of t h e s e t e r r a c e s by o u r s e l v e s a r e a l s o s h o w n , i n d i c a t i n e : t h a t :he t u f a s u r f a c e a t V 6 r t e s s z 6 l l G s c a n n o t b e as o l d as was s u g g e s t e d by 2 6 c s i . N o t e h o w e v e r t h a t t h e a g e o f s u c h a t u f a g i v e s a mir,imum a g e f o r c u t t i n g o f t h e f l u v i a l t e r r a c e al?d t h e t r u e a g e c a n b e much o l d e r , s i n c e t h e r e i s no s p e c i a l t e n d e n c y f o r t h e t e r r a c e c u t t i n g t o s t i m u l a t e formation of w a r m s p r i n g s . Another t y p e o f s e t t i n g i n which t r a v e r t i n e a g e s a r e a p p l i c a b l e , l e s s commonly h o w e v e r , i s w h e r e g l a c i a l , g l a c i o f l u v i a l or g a l c i o l a c u s t F i n e d e p o s i t s a r e c a p p e d by s p r i n g d e p o s i t s . I n t h e w e s t e r n U n i t e d S t a t e s t h e g l a c i a l o u t w a s h d e p o s i t s o f some o f t h e a l D i n e g l a c i e r s o f t h e Rocky YIoun5ains a r e l o c a l l y c a p p e d by w a r m - s p r i n g d e p o s i t e d t r a v e r t i n e s w h i c h a r e w i d e s p r e a d i n t h i s r e g i o n ( F e t h a n d B a r n e s , 1979). 3ne e x a m p l e of t h i s phenomenon w h i c h we h a v e s t u d i e d i s on t h e n o r t h s l o p e o f t h e Wind R i v e r Range o f Wyoming w h e r e Richmond (1976) h a s mapped a s e r i e s o f l o b e s o f d r i f t w h i c h h a v e b e e n D u s h e d n o r t h w a r d down t h e c a n y o n s o f t h i s r a n g e by s u c c e s s i v e t o n g u e s of i c e e m e r g i n g from a l p i n e i c e c a p s o n t h e rar,e:e, d u r i n g e a c h o f t h e p a s t g l a c i a l s t a g e s . The d r i f t l o b e s c a n b e t r a c e d i n t u r n t o s h e e t s o f f l u v i a l outwash d e p o s i t s , w h i c h p a r t l y f i l l t h e v a l l e y o f t h e Wind F i i v e r , n o r t h 3f t h e r a n g e . These outwash d e p o s i t s a r e themselves c u t i n t o a s e r i e s o f f l u v i a l t e r r a c e s , and are l o c a l l y capped w i t h s h e e t s o f warm-springf e d t r a v e r t i n e . We h a v e d a t e d some o f t h e s e t r a v e r t i n e s i n o r d e r t o e s t a b l i s h a c h r o n o l o g y f o r t h e g l a c i a l o u t w a s h e v e n t s . The C i r c l e Terrace i s b e l i e v e d t o b e e q u i v a l e n t i n a g e t o t h e emplacement o f t h e B u l l Lake m o r a i n e . T r a v e r t i n e r e s t i n g on t h e C i r c l e T e r r a c e , d a t e d by 230Th/234Uy , i e l d s a g e s r a n g i n g f r o m k2 ky a t t h e b a s e t o 30 ky a t t h e
42 Table 2 .
T e r r a c e s of
t h e Tata R i v e r , Hungary.
The T a t a i s a s m a l l t r i b u t a r y o f t h e Danube: i t s f l o o d p l a i n i s b e l o w t h e f l o o d l e v e l o f t h e Danube. The f o l l o w i n g t e r r a c e s a r e a t t r i b u t e d t o e r o s i o n a l s t a g e s of t h e P l e i s t o c e n e ( h e i g h t s i n metres, measured i n v i c i n i t y of V 6 r t e s s z S l l g s ) .
Terrace No
Height above flood plain
I
0
.
Proposed c o r r e l a t i v e glacial-stage
Absolute age of t r a v e r t i n e (KY)
modern
---
IIa
4-5
Wiirm
IIb
8-10
R i s s I I -R i s s / bJu r m
120-78(a)
I11
20-25
Riss I
190 2 45(b)
IV
c.
Mind e l
V
55-65
40
-210 t l O ( a )
?
>270 Sources of
data:
Schwarcz and A.
a)
H.P.
b)
Pgcsi,
(b)
Latham,
unpublished data,
1981
1973.
top. These a g e s l i e w i t h i n t h e m i d d l e - W i s c o n s i n g l a c i a l s t a g e and t h e r e f o r e c a n n o t b e v e r y c l o s e t o t h e a g e s of t h e outwash e v e n t s , a l t h o u g h t h e y p u t a minirxun a g e on t h e e v e n t . Cutwash c o r y e l a t e d w i t h t h e y o u n g e r P i n e d a l e m o r a i n e i s capped by a p o r o u s t r a v e r t i n e t h a t y i e l d s a n age of 1 2 ? 3 k y , c o n s i s t e n t w i t h i t s f o r m a t i o n d u r i n g t h e m e l t i n g o f t h e i c e cap p r e s e n t on t h e c r e s t of t h e r a n g e d u r i n g t h e Wisconsin. While t h e s e methods h a v e s o f a r o n l y b e e n a p p l i e d t o s p r i n g d e p o s i t e d t r a v e r t i n e s , t h e r e i s a n o t h e r c l a s s of c a r b o n a t e d e p o s i t s which s h o u l d b e i n v e s t i g a t e d . Pany r i v e r s f e d by C a - r i c h s p r i n g w a t e r s w i l l d e p o s i t CaC03 where t h e w a t e r becomes s u f f i c i e n t l y a e r a t e d t o l o s e C C 2 and becorre s u p e r s a t u r a t e d i n c a l c i t e (or a r a g o n i t e ) . T u f a mounds a c c u m u l a t e a l o n g t h e t h a l w e g and t h e i r p r e s e n c e s e r v e s as a turbulence-producing s t r u c t u r e that a c c e l e r a t e s the a e r a t i o n process. They can t h u s grow i n t o s i z e a b l e s t r u c t u r e s which m a r k a s t a g e i n t h e e v o l u t i o n of t h e s t r e a m . G e n e r a l l y t h e s e mounds of s t r e a m - d e p o s i t e d t r a v e r t i n e a r e themselves highly s u s c e p t i b l e t o erosion during f u r t h e r l o w e r i n g o f b a s e l e v e l . ? o r e x a m p l e , i n t h e Wadi P a r a n , i n t h e Negev D e s e r t o f I s r a e l , l a r g e mounds of t r a v e r t i n e a r e b u i i t up on t h e s i d e s and c r e s t s of i n t e r m i t t e n t l y a c t i v e w a t e r f a l l s . These a c c u m u l a t i o n s p r o b a b l y d e v e l o p e d d u r i n g t h e W i s c o n s i n g l a c i a l s t a g e which was a p e r i o d o f h i g h r a i n f a l l i n t h i s r e g i o n ( B o r o w i t z 1 9 7 9 ) . No U - s e r i e s d a t e s have y e t b e e n o b t a i n e d on t h e s e d e p o s i t s . S i m i l a r t e s t s of p l u v i a l / i n t e r p l u v i a l c l i m a t e c o u l d be o b t a i n e d from s p e l e o t h e m s obt a i n e d from c a v e s i n s u c h r e g i o n s . LACUSTRINE LIYESTONES
C a l c a r e o u s s e d l m e n t s which w e r e d e p o s i t e d i n p l u v i a l l a k e s a r e known from s e v e r a l c o n t i n e n t s . Both c a l c a r e o u s b o t t o m s e d i m e n t s , formed p a r t l y by c h e m i c a l p r e c i p i t a t i o n from t h e l a k e w a t e r s , and t u f a a c c u m u l a t i o n s a l o n g t h e l a k e s h o r e s , a r e known f r o m s u c h e n v i r o n m e n t s , and h a v e i n some i n s t a n c e s b e e n d a t e d b y K - s e r i e s m e t h o d s , a s w e l l a s by t h e I 4 C m e t h o d .
43
The P l e i s t o c e n e l a k e s L a h o n t a n and B o n n e v i l l e i n Utah and Nevada accumulated a c o n s i d e r a b l e t h i c k n e s s o f m a r l , i n which i s c o n t a i n e d a n abundant f a u n a o f o s t r a c o d s and g a s t r o p o d s : t u f a mounds d a t i n g from former h i g h l a k e - s t a n d s o c c u r a l o n g t h e m a r g i n . These d e p o s i t s have been a n a l y s e d by I 4 C and U - s e r i e s methods by Kaufman a n d B r o e c k e r (1965). T h e r e was a g r e e m e n t b e t w e e n t h e two methods f o r t h o s e s a m p l e s l a c k i n g i n e x c e s s 2 2 6 R ar e l a t i v e t o u r a n i u m ; t h e d a t e s l a y i n t h e i n t e r v a l 25 t o 1 0 k y , a s would b e e x p e c t e d f o r t h e l a s t p l u v i a l p e r i o d in t h i s region. Some s a m p l e s p o s s i b l y d a t i n g t o o l d e r p l u v i a l s were also r e c o g n i z e d . E v a p o r i t i c l a k e d e p o s i t s i n t h e S e a r l e s Lake b a s i n of C a l i f o r n i a w e r e d a t e d b y Feng e t aZ. ( 1 9 7 8 ) , a l s o u s i n g b o t h U1s e r i e s and I4C, a n d y i e l d i n g a g e s i n t h e l a s t p a r t o f t h e W i s c o n s i n g l a c i a l s t a g e ( 3 3 t o 9 k y ) . I n t e r e s t i n g l y , t h e r e was s a t i s f a c t o r y con cordance between I 4 C and 2 3 0 T h a g e s a l t h o u g h some c o r r e c t i o n f o r d e t r i t a l m a t t e r h a d t o be made t o t h e l a t t e r . I n I s r a e l , Kaufman ( 1 9 7 1 ) was a b l e t o u s e 2 3 0 T h / 2 3 4 Ur a t i o s t o determine t h e t i m e of a c c u m u l a t i o n o f m a r l y s e d i m e n t s o f t h e L i s a n Formation which w e r e d e p o s i t e d i n t h e J o r d a n g r a b e n , where t h e s m a l l e r Dead Sea now r e m a i n s . The L i s a n s e d i m e n t s ( F i g u r e 6 ) y i e l d e d a g e s i n t h e i n t e r v a l from 6 2 t o 2 0 k y , a g a i n showing t h a t l a k e l e v e l s were h i g h during t h e m i d s t of t h e l a s t , Wisconsin, G l a c i a l s t a g e i n t h i s r e g i o n . S i g n i f i c a n t c o r r e c t i o n s h a d t o b e made f o r common 2 3 0 T h i n t h e a n a l y s i s of t h e s e s e d i m e n t s , which c o n s i s t of f i n e l y i n t e r l a m i n a t e d , c h e m i c a l l y p r e c i p i t a t e d c a l c i t e , and d e t r i t a l s i l t b r o u g h t i n t o t h e l a k e by s p r i n g f l o o d s . The U - s e r i e s a g e s for t h e y o u n g e r p a r t s o f t h e s e q u e n c e were i n e x c e l l e n t a g r e e m e n t w i t h I4C a g e s . Agreement i n t h e a b s o l u t e a g e s o f s t r a t i g r a p h i c a l l y c o r r e l a t i v e p a r t s o f t h e L i s a n F o r m a t i o n was a l s o o b s e r v e d , o v e r d i s t a n c e s of t e n s o f k i l o m e t e r s . T a y l o r D r y V a l l e y i n A n t a r c t i c a , a t p r e s e n t a p o l a r d e s e r t , cont a i n s a s e r i e s of p r o g l a c i a l l a k e s e d i m e n t s i n c l u d i n g a u t h i g e n i c c a r b o n a t e s and s u l p h a t e s . A b s o l u t e d a t e s o f t h e s e by 2 3 0 T h / 2 3 4 Ushow t h a t t h e y were d e p o s i t e d d u r i n g a t l e a s t t h e l a s t t h r e e i n t e r g l a c i a l s t a g e s , p o s s i b l y a t t h e v e r y end of t h e i n t e r g l a c i a l s d u r i n g a t i m e o f t h i c k . . e n i n g and e x p a n s i o n o f t h e a d j a c e n t i c e s h e e t s (Hendy e t aZ., 1 9 7 9 ) . Hollin ( 1 9 8 0 ) has used t h e s e dates i n support of h i s proposal t h a t a s u r g e of t h e E a s t A n t a r c t i c i c e s h e e t b r o u g h t a b o u t r a p i d c o o l i n g o f t h e g l o b a l o c e a n and t h e o n s e t o f t h e f o l l o w i n g g l a c i a l s t a g e . D a t i n g o f p l u v i a l l a k e s e q u e n c e s i n a r i d r e g i o n s r e m a i n s one o f t h e most p o w e r f u l t e c h n i q u e s f o r d e f i n i n g c l i m a t i c change i n t h e s e r e g i o n s . I n t h e N o r t h A f r i c a n d e s e r t many s u c h b a s i n s e x i s t , some c o n t a i n i n g e x t e n s i v e f a u n a l and a r c h a e o l o g i c a l r e m a i n s a s w e l l as c a r b o n a t e s e d i m e n t s . One b a s i n which h a s begun t o y i e l d s u c h d a t a i s t h e S h a t i Cardium L a k e i n L i b y a , from w h i c h a g e s b e t w e e n 2 0 0 and 8 0 ky have been o b t a i n e d (Gaven e t aZ., 1 9 8 1 ) . I n t h i s a n d o t h e r p l u v i a l l a k e s e q u e n c e s s p e c i a l c a r e must b e t a k e n t o s o r t o u t t h e i s o t o p i c e f f e c t s a t t r i b u t a b l e t o a d m i x t u r e o f d e t r i t a l s e d i m e n t and t o p o s s i b l e r e c r y s t a l l i z a t i o n o f p r i m a r y a r a g o n i t e t o c a l c i t e , or f i n e - g r a i n e d magnesian c a l c i t e t o c o a r s e r , s p a r r y low-Mg c a l c i t e . Such r e c r y s t a l l i z a t i o n e f f e c t s w e r e i n f e r r e d by Szabo and B u t z e r ( 1 9 7 9 ) i n o r d e r t o account f o r a n a p p a r e n t i n v e r s i o n i n t h e a g e s o f s u c c e s s i v e c a r b o n a t e deposits i n a South African calc-pan. CALCRETE, AND OTHER PEDOGENIC CARBONATES
A u t h i g e n i c c a l c i t e c a n form i n s o i l s o f a r i d or s e m i - a r i d e n v i r o n ments. I f t h e c a r b o n a t e i s s u f f i c i e n t l y a b u n d a n t , i t may o c c u r as a d i s c r e t e l a y e r i n t h e s o i l ( c a l c r e t e , c a l i c h e ) or as d i s c r e t e conThe t i m e o f f o r m a t i o n o f a n y p a r t i c u l a r p a r t o f c r e t i o n a r y masses. such a l a y e r or c o n c r e t i o n can b e d e t e r m i n e d r o u g h l y by U - s e r i e s d a t i n g . M i c r o s c o p i c a n a l y s i s o f s u c h d e p o s i t s g e n e r a l l y shows t h e m t o be e x t e n s i v e l y i c t e r s p e r s e d w i t h d e t r i t a l p a r t i c l e s w h i c h o r i g i n a l l y comprised t h e s o i l . These w i l l i n v a r i a b l y c o n t r i b u t e b o t h U and Th , t o any a n a l y s i s o f t h e c a r b o n a t e . and must be e x t e n s i v e l y c o r r e c t e d f o r . Ku e t aZ. ( 1 9 7 9 ) h a v e u s e d m u l t i p l e a n a l y s e s o f c o e v a l s a m p l e s t o determine t h e t i m e o f f o r m a t i o n of c a l c r e t e s from a l l u v i a l s o i l s i n t h e Mojave D e s e r t , C a l i f o r n i a . M u l t i v a r i a t e r e g r e s s i o n techniques allowed
44
L E A N STRATIGRAPHIC SECT IONS unit A
Age (ky)
unit B
Age(ky)
unit C Age (ky)
I7 +2
Depth 0
24t3 (2521)
13
47t2
3723
.... . . .: . .. . . . . . . .. .
4525
6126
rTA Upper Lisan sediments 1-1.. Lower Lisan sediments I
Iunexposed
c 6 0 t l ky230Th/%
age
4 2 0 f l ) ky 14C age Figure 6
C h r o n o s t r a t i g r a p h y of l a c u s t r i n e s e d i m e n t s o f t h e L i s a n f o r m a t i o n , showing c o m p a r i s o n b e t w e e n 1 4 C and 2 3 0 T h / 2 3 4 Ua g e s . D a t a f r o m K a u f m a n (1971).
them t o i n f e r t h e 2 3 0 T h / 2 3 4 U and 234U/238Ur a t i o s o f t h e a u t h i g e n i c component o f t h e s o i l . The main p r o b l e m i n u s i n g s u c h d a t a map u l t i m a t e l y b e t h e q u e s t i o n o f geomorphological s i g n i f i c a n c e of t h e d e p o s i t . S o i l s , and c a l c r e t e
l a y e r s i n p a r t i c u l a r , a r e t h e p r o d u c t of s l o w dvelopment o v e r l o n g t i m e p e r i o d s . T h e r e f o r e i t i s somewhat d u b i o u s t o s p e a k o f t h e " a g e " o f a c a l c r e t e , u n l e s s t h e p r o c e s s o f p e d o g e n e s i s was i n i t i a t e d and i n t e r rupted at discrete points i n t i m e . T h i s s i t u a t i o n can be o b s e r v e d , f o r e x a m p l e , where soils h a v e formed on tox, o f v o l c a n o g e n i c s e d i m e n t ( s u c h a s a s h - f a l l t u f f s ) . Here p e d o g e n e s i s can b e assumed t o h a v e begun a t t h e t i m e of e r u p t i o n , and i n many c a s e s w i l l h a v e b e e n t e r m i n a t e d a t t h e time o f t h e next e r u p t i o n . This s i t u a t i o n i s described, f o r e x a m p l e , by Williams e t a Z . ( 1 9 7 9 ) i n t h e Gadeb r e g i o n o f E t h i o p i a , where s u c c e s s i v e a s h b e d s a r e e a c h capped by s o i l s w i t h w e l l - d e v e l o p e d Ca-horizons. Another s e r i o u s p r o b l e m i n t h e d a t i n g of c a l c r e t e s i s t h e r e q u i r e ment t h a t t h e s e d i m e n t from which t h e s o i l d e v e l o p e d must h a v e b e e n lacking i n calcareous d e t r i t a l p a r t i c l e s , since these w i l l contribute a n a p p a r e n t l y i n f i n i t e - a g e component t o t h e a n a l y s i s o f t h e c a l c r e t e , one t h a t c a n n o t b e compensated f o r b y a n a l y t i c a l methods s u c h a s t h o s e u s e d b y Ku e t a Z . T h i s i s n o t a problem where t h e c a l c r e t e has dev e l o p e d from a v o l c a n i c a s h .
45
O R G A N I C DEPOSITS
U n q u e s t i o n a b l y one o f t h e most w i d e s p r e a d and a b u n d a n t s o u r c e s o f primary carbonate p r e c i p i t a t e s a s s o c i a t e d w i t h Quaternary sediments, This both f l u v i a l and l a c u s t r i n e , i s m o l l u s c a n s h e l l m a t e r i a l . m a t e r i a l i s e v e n more a b u n d a n t i n m a r i n e s e t t i n g s where i t o c c u r s , f o r example, on r a i s e d b e a c h e s m a r k i n g o l d h i g h - s e a - s t a n d s . Yany examples of U - s e r i e s d a t i n g o f m o l l u s c s o c c u r i n t h e l i t e r a t u r e . However, t h e v a l i d i t y o f s u c h d a t e s h a s b e e n s e v e r e l y c r i t i c i z e d by Kaufman e t aZ. ( 1 9 7 1 ) who g i v e c l e a r e v i d e n c e f o r post-mortem m i g r a t i o n o f r a d i o n u c l i d e s i n t o and o u t o f m o l l u s c t e s t s . T h i s i s shown f o r example by t h e o b s e r v a t i o n t h a t t h e s h e l l s o f l i v i n g clams c o n t a i n l e s s t h a n 0 . 2 ppm U w h i l e f o s s i l clam s h e l l s a few t e n s o f t h o u s a n d s of y e a r s o l d While Szabo and R o s h o l t ( 1 9 6 9 ) h a v e a t t e m p t e d c o n t a i n 2 ppm or more U . t o c o r r e c t f o r t h i s m i g r a t i o n by u s e of an open s y s t e m m o d e l , Kaufman e t aZ. ( 1 9 7 1 ) c o n c l u d e t h a t t h i s model was n o t v a l i d f o r most c a s e s of n u c l i d e m i g r a t i o n , and t h a t e v e n c o n c o r d a n c e b e t w e e n 2 3 0 T h / 2 3 4 Uand 2 3 1 P a / 2 3 5 Ua g e s d i d n o t e n s u r e t h a t t h e a g e s were c o r r e c t . N e v e r t h e l e s s , some o f t h e most i m p o r t a n t e s t i m a t e s o f c l i m a t i c e v e n t s o f t h e Q u a t e r n a r y h a v e b e e n made t h r o u g h d a t e s on m o l l u s c s . S t e a r n s and T h u r b e r ( 1 9 6 7 ) u s e d 2 3 0 T h / 2 3 4 Ua g e s of m o l l u s c s f r o m a n c i e n t s t r a n d l i n e s a r o u n d t h e M e d i t e r r a n e a n to o b t a i n d a t e s of h i g h s e a - s t a n d s a t g r e a t e r t h a n 2 5 0 , 2 0 0 , 1 2 0 a n d a b o u t 8 0 k y . Andrews e t aZ. ( 1 9 7 5 ) have u s e d d a t e s o f m o l l u s c s from a r c t i c s h o r e l i n e s to d e f i n e t h e a g e However, e x t r e m e c a u t i o n s h o u l d of l a s t i n t e r g l a c i a l h i g h s e a - s t a n d s . be t a k e n i n u s i n g s u c h d a t a as d e f i n i t i v e e s t i m a t e s o f Q u a t e r n a r y c h r o n o l o g y . F u r t h e r work o u g h t to be u n d e r t a k e n t o r e v i e w t h e methods of d a t i n g s h e l l s and t h e s e l e c t i o n of s a m p l e s f o r d a t i n g . S i m i l a r p r o b l e m s seem to p l a g u e t h e e q u a l l y p r o m i s i n g f i e l d o f d a t i n g a n c i e n t b o n e s . L i k e m o l l u s c s h e l l s , t h e s e o r g a n i c d e p o s i t s of c a l c i u m p h o s p h a t e a r e i n i t i a l l y f r e e of a l l b u t m i n u t e t r a c e s o f uranium. A f t e r a f e w t h o u s a n d y e a r s o f b u r i a l i n t h e s o i l t h e y may a c q u i r e from a few ppm t o as much as s e v e r a l t h o u s a n d ppm U . The upt a k e of U i s p o o r l y u n d e r s t o o d a n d may i n c l u d e b o t h t h e r e p l a c e m e n t of Ca by U i n t h e a p a t i t e c r y s t a l s t r u c t u r e , and t h e r e d u c t i o n o f s o l u b l e U6+ t o i n s o l u b l e U 4 + by o r g a n i c m a t t e r ( c o l l a g e n ) of t h e b o n e . Szabo (1980) has u s e d c o m p a r i s o n b e t w e e n I 4 C and U - s e r i e s a g e s to d e t e r m i n e t h a t U-uptake c e a s e s a f t e r a b o u t 2700 y e a r s i n b o n e s i n some c o n t i n e n t a l s e d i m e n t s . However, i t i s q u e s t i o n a b l e t h a t t h i s i n t e r v a l i s u n i v e r s a l l y a p p l i c a b l e t o a l l s e t t i n g s i n which f o s s i l b o n e s o c c u r . Szabo h a s u s e d t h e d a t e s on b o n e s from c o n t i n e n t a l , f l u v i a l d e p o s i t s t o p l a c e l i m i t s on t h e i r t i m e of d e p o s i t i o n ( T a b l e 3 ) . Szabo e t aZ. ( 1 9 7 3 ) give references t o s e v e r a l o t h e r U-series s t u d i e s of f o s s i l bones. F i n a l l y we s h o u l d n o t e t h a t some p o s s i b i l i t y e x i s t s f o r d a t i n g t h e o r g a n i c m a t t e r f o u n d i n some Q u a t e r n a r y s e d i m e n t a r y d e p o s i t s . Cherdyntsev ( 1 9 7 1 ) f o u n d t h a t , f o r p e a t d e p o s i t s w i t h s u f f i c i e n t l y h i g h 230Th/232Th r a t i o s ( i n d i c a t i n g low c o n t a m i n a t i o n w i t h d e t r i t u s ) t h e r e was a good c o r r e l a t i o n between I 4 C and 2 3 0 T h / 2 3 4 Ua g e s . It i s presumed t h a t U i s a d s o r b e d by t h e o r g a n i c m a t t e r s o o n a f t e r d e p o s i t i o n , from groundwater or t h e swamp w a t e r from which t h e p e a t was d e p o s i t e d a f t e r compaction, t h e m i g r a t i o n of U i n t o t h e p e a t c q a s e s . Vogel and Kronf e l d ( 1 9 8 0 ) have r e c e n t l y d e s c r i b e d a s u c c e s s f u l a t t e m p t a t d a t i n g p e a t b y t h e 2 3 0 T h / 2 3 4 Um e t h o d . Samples from Emmen and P e e l o , t h e N e t h e r l a n d s ; T e n a g i P h i l i p p o n , Macedonia; and Z e l l a m I n n and G r o s s w e i l i n S . B a v a r i a , were d a t e d by f i r s t a s h i n g and t h e n l e a c h i n g w i t h d i l u t e All s a m p l e s , e x c e p t t h o s e from Emmen, c o n t a i n e d hydrochloric a c i d . high U c o n c e n t r a t i o n s ( 7 t o 2 1 . 5 ppm) and 2 3 0 T h / 2 3 2 T hr a t i o s r a n g i n g C l o s e a g r e e m e n t was f o u n d between 1 4 C and 2 3 0 T h / 2 3 4 U from 5 t o 36. ages f o r t h e s e s a m p l e s , a n d a c l e a r e r a g e d e t e r m i n a t i o n was o b t a i n e d f o r t h e S . B a v a r i a n p e a t s (85 t o 9 0 k y ) t h a n o b t a i n e d by 1 4 C . I n a s e p a r a t e a t t e m p t a t d a t i n g Sangamon p e a t d e p o s i t s from B a f f i n I s l a n d and T o r o n t o , Canada, Gascoyne ( u n p u b . r e s u l t s ) f o u n d h i g h 2 3 2 T h cont e n t s i n a l l samples, accompanied b y low 2 3 0 T h / 2 3 2 T hr a t i o s , r a n g i n g No a t t e m p t was made t o c o r r e c t f o r t h i s l a r g e between 0 . 7 and 1 . 9 . d e t r i t a l component and t h e s a m p l e s w e r e r e g a r d e d as u n d a t a b l e by t h e method u s e d .
46 Table 3.
Comparison o f U - s e r i e s and I 4 C a g e s on bones from different archaeological sites U-series
Location
1 4
ape
(KY1
C age
I 4 c ape-U series age (KY)
(KY)
Lindenmeier S i t e , Colorado
4250f
Lindenmeier S i t e , Colorado
55002 500
107802 375
Dent S i t e ,
7700f
1 1 2 0 0 2 500'
3500
Colorado
107802 375b
500a
500
b
6530 5280
Lehner S i t e , Arizona
7700+1500d
111152 500
3415
Domebo Mammoth S i t e , Oklahoma
11500f2000d
1 1 0 4 5 2 647'
-455
Murray S p r i n g s S i t e , Arizona
112302 340e
Caulapan, Mexico
3060021000R
Caulapan, Mexico
21850t
Medicine H a t ,
Canada
980 12100
850'
850
112002 2ooi
1700
4U a n d 'Pa/' 'U a g e bHaynes and Agogino aAverage concordant 'Th/' 1 9 6 0 , F o l s o m l e v e l ; 'Haynes 1 9 6 7 , C l o v i s l e e l s : ' u n p u b l i s h e d r e s u l t s of B.J. S z a b o ; eH a y n e s 1 9 6 8 , C l o v i s . l e v e 1 ; 'Szabo e t al., 1 9 6 9 ; ' K e l l e g e t al., 1 9 7 8 ; h S z a b o e t az., 1 9 7 3 ; t a i n e d on d e n t i n e ,
1
Lowdon a n d B l a k e 1 9 6 8 ; J d a t e obnow c o n s i d e r e d u n r e l i a b l e f o r d a t i n g . From S z a b o
(1980) w i t h p e r m i s s i o n
DETFiITAL SEDIMENTS
I n a l l o f t h e a b o v e examples o f d a t i n g o f c h e m i c a l l y - D r e c i p i t a t e d 3 a t u r a l s u b s t a n c e s ( c a l c i t e , a p a t i t e , e t c . ) we h a v e c o n s i d e r e d t h a t t h e main s o u r c e of c o n t a m i n a t i o n i s t h e l a r g e l y s i l i c e o u s d e t r i t a l , e l a s t i c s e d i m e n t t h a t i s p r e s e n t a s a c o n t a m i n a n t i n s a m p l e s c o l l e c t e d for Us e r i e s a n a l y s i s . It i s p o s s i b l e , i n p r i n c i p l e a t l e a s t , t o t u r n t h e argument a b o u t and u s e s u r f i c i a l , d e t r i t a l s e d i m e n t s a s t h e s a m p l e . A l t h o u g h s u c h m a t e r i a l s a r e n o t l i k e l y t o form a c l o s e d s y s t e m , i t i s p o s s i b l e t h a t some s o r t o f r e g u l a r b e h a v i o u r c o u l d b e d e t e c t e d t h r o u g h c o m p a r i s o n of s e v e r a l c o e v a l a i i a u o t s w h i c h h a v e some common c h e m i c a l c h a r a c t e r i s t i c s r e v e a l i n g t h e t i m e t h e y were l a i d down. R o s h o l t ( 1 9 8 0 ) had d e r i v e d s u c h a method, w h i c h h e c a l l s "uranium t r e n d a n a l y s i s " where p l o t s o f 'J and T h i s o t o p e r a t i o s w i t h r e s p e c t t o one a n o t h e r g e n e r a t e l i n e s whose s l o p e s can t h e n b e i n t e r p r e t e d i n t e r m s o f t h e t i m e s i n c e t h e s y s t e m came i n t o e x i s t e n c e ( t h e s e d i m e n t was d e p o s i t e d ) . R o s h o l t assumes t h a t t h e s y s t e m h a s b e e n c o n t i n u o u s l y modified a f t e r d e p o s i t i o n , by a combination o f p r o c e s s e s of l e a c h i n g A n a r a m e t e r "F" i s and u p t a k e ( b y a d s o r p t i o n ) of r a d i o - i s o t o D e s . i n t r o d u c e d which r e p r e s e n t s t h e f l u x o f U t h r o u p h t h e system-but which i s i t s e l f d e s c r i b e d as a decaying f u n c t i o n of t i m e , implving t h a t t h e i n t e n s i t y of U t r a n s p o r t t h r o u g h t h e s y s t e m d e c r e a s e s from a n i n i t i a l maximum a t or n e a r t h e t i m e o f d e p o s i t i o n . He has o b t a i n e d a g e s a s g r e a t a s 9 0 0 ky by t h i s m e t h o d , by which n o i n t i t i s o n l y c h a n g e s i n t h e 2 3 s U / 2 3 8 U r a t i o which d e t e r m i n e t h e a p e o f t h e s y s t e m . It i s n e c e s s a r y t o c a l i b r a t e t h e method f o r e a c h s e d i m e n t a r y s e q u e n c e , i n o r d e r t o e v a l u a t e t h e F v a l u e for t h a t s e a u e n c e . Also, s e v e r a l a n a l y s e s o f v a r y i n g c o m p o s i t i o n a r e n e c e s s a r y from a s i t e i n o r d e r t o o b t a i n a
47
single age. U l t i m a t e l y we may s u p p o s e t h a t w h e r e a n y p r i m a r y , a u t h i g e n i c p h a s e i n a s o i l o r sediment h a s rernained a c l o s e d system s i n c e f o r m a t i o n , it might b e d a t a b l e , a s l o n g a s some c o n t r o l on i t s i n i t i a l i s o t o p i c comp o s i t i o n i s p o s s i b l e . T h l s c o u l d , f o r e x a m p l e . b e u s e d on c l a y m i n e r a l s or f e r r i c hydroxide minerals i n a s o i l , i f they could be separated c l e a n l y f r o m t h e s o i l , a n d i f t h e y c o u l d b e shown to h a v e h a d a w e l l d e f i n e d i n i t i a l i s o t o p i c c o m p o s i t i o n . The m e t h o d i s , i n p r i n c i p l e , a n a l o g o u s t o t h e d a t i n g o f d e e p - s e a s e d i m e n t s or m a n g a n e s e n o d u l e s by s t u d y o f t h e d e c a y o f e x c e s s 2 3 0 T h or 2 3 1 P a (Ku, 1976), b u t h a s y e t t o be t r i e d o u t on s o i l s . V O L C A N I C ROCKS
V o l c a n i c r o c k s l e s s t h a n l o 5 y o l d c a n i n some c a s e s b e d a t e d by U - s e r i e s m e t h o d s more p r e c i s e l y t h a n by t h e more c o n v e n t i o n a l K / A r method. Condomines ( 1 9 7 8 ) h a s u s e d t h e 2 3 0 T h / 2 3 8 Um e t h o d to d a t e t h e l a v a s i n w h i c h were r e c o r d e d t h e Laschamp m a g n e t i c r e v e r s a l a t 39 k y . We h a v e a p p l i e d t h e m e t h o d t o s e v e r a l o t h e r v o l c a n i c s e q u e l ? c e s a n d g e n e r a l l y o b t a i n c o n s i d e r a b l y b e t t e r p r e c i s i o n t h a n o b t a i n a b l e by K / A r o v e r t h e same t i m e i n t e r v a l . The a s s u m p t i o n i n t h e m e t h o d i s t h a t a t the time of e x t r u s i o n and c r y s t a l l i z a t i o n o f t h e v o l c a n i c r o c k , a l l mine'rals i n t h e r o c k h a v e t h e same 2 3 0 T h / 2 3 2 T hr a t i o b u t d i f f e r e n t T h e n , w i t h p a s s a g e o f t i m e , 2 3 d T hgrows i n t o e a c h 2 3 4 U / 2 3 2 T hc o n t e n t s . nnlneral p h a s e o f t h e r o c k i n p r o p o r t i o n to t h e amount o f U i n t h a t p h a s e . M i n e r a l s a n d w h o l e r o c k s c r y s t a l l i z e d f r o m t h e same magma r e s e r v o i r a t t h e same t i m e w i l l t h e n l i e a l o n g a n i s o c h r o n ( F i g u r e 7). T h i s method i s a p p l i c a b l e to r o c k s a s o l d a s 350 k y , a n d c a n b e u s e d t o make a c r o s s - c h e c k on K/Ar a n a l y s i s o f r o c k s s u s p e c t e d of c o n t a m i n a t i o n w i t h small amounts of o l d r o c k s . T h i s c o n t a r n i n a t i o n would h a v e a l a r g e e f f e c t on K / A r d a t e s , b u t w o u l d h a v e a s m a l l e r , or n e g l i g i b l e e f f e c t o n the U - s e r i e s methods , depellding on t h e U-content of t h e contaminant. CA2 Minerals
-
1
Auaite
0 Figure 7
0.5
/
Maanetite
1
I
1
1.5
2
(238U/232Th
L
2.
P l o t o f 2 3 0 T h / 2 3 2 T hU S 2 3 8 U / 2 3 2 T hf o r m i n e r a l s e p a r a t e s and t o t a l r o c k f o r an a n d e s i t e v o l c a n i c flow from Tierra Blanca, Costa Rica. Souiline would b e l o c u s of p o i n t s i f s a m p l e were a t s e c u l a r equilibrium. Slope of b e s t - f i t l i n e t o d a t a points g i v e s 2 3 0 T h / 2 3 8 Ur a t i o f r o m w h i c h a g e h a s b e e n c a l culated. Data from A l l e g r e and Condomines, 1 9 7 6 .
48
CONGLUSIONS We h a v e s e e n t h a t t h e U - s e r i e s methods a r e w e l l - s u i t e d to d a t i n g a wide v a r i e t y o f c o n t i n e n t a l d e p o s i t s formed d u r i n g t h e l a s t 3 5 0 , 0 0 0 y e a r s o f t h e Q u a t e r n a r y P e r i o d . The methods d e s c r i b e d h e r e c a n b e a p p l i e d t o p r o b l e m s o f geomorphic e v o l u t i o n , p a l e o c l i m a t e , d a t i n g o f a r c h a e o l o g i c a l , f a u n a l and f l o r a l a s s e m b l a g e s , and to q u e s t i o n s o f ancient sea level. I n r e g a r d to t h e l a t t e r , i t s h o u l d b e n o t e d t h a t t h e d a t i n g o f s p e l e o t h e m s f r o m c a v e s on o c e a n i c i s l a n d s s u c h a s Bermuda or t h e Bahamas a l l o w s t h e p o s s i b i l i t y o f d e t e r m i n i n g n o t o n l y t h e l e v e l of h i g h s e a - s t a n d s as a r e c o n v e n t i o n a l l y o b t a i n e d from U . s e r i e s d a t e s on c o r a l r e e f s . I n a d d i t i o n , i t i s p o s s i b l e , by d a t i n g o f "drowned" s t a l a g m i t e s and s t a l a c t i t e s , t o p l a c e l i m i t s on t h e t i m e o f low seas t a n d s and t o t h e r e b y f o l l o w t h e c u r v e o f c h a n g i n g s e a l e v e l ( a n d cont i n e n t a l i c e v o l u m e ) t h r o u g h t h e p a s t few g l a c i a l s t a g e s (Gascoyne e t aZ., 1 9 7 9 ; Harmon e t aZ., 1 9 8 1 ) . The f u t u r e o f U - s e r i e s methods would a p p e a r to l i e i n t h e more r e l i a b l e a p p l i c a t i o n o f t h e s e methods t o s a m p l e m a t e r i a l s w h i c h h a v e h e r e t o f o r e c a u s e d t h e g r e a t e s t p r o b l e m s : a n i m a l b o n e s and m o l l u s c a n s h e l l s . The f o r m e r h a v e b e e n shown to g i v e r e s u l t s which a r e conc o r d a n t w i t h I 4 C a g e s on one end o f t h e s c a l e , a n d w i t h U - s e r i e s d a t e s on c o r a l s , on t h e o t h e r . Eowever, a s i g n i f i c a n t number o f c a s e s o f g r o s s d i s e q u i l i b r i u m have a l s o b e e n i d e n t i f i e d , a s i n d i c a t e d by d i s c o r d a n c e b e t w e e n 2 3 0 T hand 2 3 1 P a d a t e s for t h e same s a m p l e , and f a i l u r e of t h e d a t e s t o a g r e e with s t r a t i g r a p h i c sequence (Cherdyntsev, 1971). The o t h e r f i e l d f o r improvement i n t h e m e t h o d s , i s i n t h e p o s s i b l e app l i c a t i o n o f 2 3 4 U / 2 3 8 Ur a t i o s to a g e d e t e r m i n a t i o n . T h i s r a t i o c a n i n p r i n c i p l e be u s e d t o m e a s u r e a g e s UD to a b o u t 1 0 0 0 k y , i f some c o n t r o l on t h e i n i t i a l r a t i o i s a v a i l a b l e . While t h e method o f R o s h o l t ( 1 9 8 0 ) shows some p r o m i s e a l o n g t h e s e l i n e s a n o t h e r a p p r o a c h which m i g h t p r o v e t o b e more v a l u a b l e i s to t a k e a d v a n t a g e of t h e o b s e r v a t i o n t h a t 2 3 4 U atoms i n a c r y s t a l c a n be d i v i d e d i n t o two c a t e g o r i e s : " h o t " atoms which grew in situ from p a r e n t 2 3 8 U a t o m s . a n d " c o l d " atoms t h a t w e r e p r e s e n t when t h e c r y s t a l f o r m e d . With t h e p a s s a g e o f t i m e , " c o l d " atoms d i s a p p e a r , and " h o t " atoms grow, w h i l e t h e t o t a l r a t i o o f h o t t c o l d t o t o t a l 2 3 8 U atoms ( m e a s u r e d , a s u s u a l , i n a c t i v i t y r a t i o s ) a p p r o a c h e s u n i t y . By s e l e c t i v e l e a c h i n g o f t h e s a m p l e , i t may b e p o s s i b l e to i d e n t i f y t h e h o t and c o l d atoms s e p a r a t e l y and t h u s to d e t e r m i n e t h e e x t e n t of a p p r o a c h t o e q u i l i b r i u m (Schwarcz and W i n t l e , 1980) ACKNOWLEDGEMENTS We acknowledge t h e f i n a n c i a l a s s i s t a n c e o f t h e N.S.E.R.C. and t h e S.S.H.R.C. Much o f t h e r e s e a r c h d e s c r i b e d i n t h i s p a p e r was c a r r i e d 3 u t by u s i n c o l l a b o r a t i o n w i t h Dr. D . C . F o r d , whose a s s h t a n c e we T r a t e f u l l y acknowledge. REFERENCES C I T E D X l l e g r e , C . 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197813, L a t e Pleistocene paleoclimates o f North America as inferred from stable isotope studies of speleothems: Quaternary Research, v. 9 , p. 54-70. Harmon, R.S., Land, L.S., Mitterer, R.M., Garrett, P., Schwarcz, H.P. and Larson, G.J., 1 9 8 1 , Bermuda sea level during the last interglacial: Nature, v. 289, p. 481-483. Harmon, R.S. and Schwarcz, H.P., 1982, Isotopic studies of speleothems from a cave i n southern Missouri: Submitted to Lull. Geol. S O C . America. Hendy, C.H. , Healy, T.R. , PLayner, E.11. , Shaw, J . and Vilson, A.T. , 1979, Late Pleistocene glacial chronology of the Taylor Valley, Antarctica, and the global climate: Quaternary Research, _- v . 1 1 , p . 172-
50
184. Hollin, J., 1980, Climate and sea level in isotope stage 5: an East Antarctic ice surge at 95,000 B D ? : Natur.e, v. 283, p . 629-633.
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Horowitz, A., 1979, The Quaternary of Israel:
N.Y.,
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Kaufman, A., 1971, U-series dating of Dead Sea Basin carbonates: Geochim. Cosmochim. Acta, v. 35, p. 1269-1281. Kaufman, A. and Broecker, W . S . , 1965, Comparison of T h Z 3 " and CI4 ages for carbonate materials from Lakes Lahontan and Bonneville: Jour. Geophys. Res., v. 7 0 , p . 4039-4054. Kaufman, A., Broecker, W.S., Ku, T.-L. and Thurber, D.L., 1971, The status of U-series methods of mollusk dating: Seochim. Cosmochim. V. 35, p . 1155-1183.
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K u , T.-L., 1976, The uranium series methods o f age determination: Ann. Rev. Earth Plan. Sci.., v. 4 , p . 347-380. Ku, T.-L., Bull, W.G., Freeman, S.T. and Knauss, K.G., 1979, T h 2 3 0 / U 2 3 4 dating of pedogenic carbonates in gravelly desert soils of Vidal Valley, Southeastern California: Geol. S O C . Amer. Bull., v. 980, p . 1063-1073. Latham, A.G., Schwarcz, H.P., Ford, D.C. and Pearce, G.W., 1979, Palaeomagnetism of stalagmite deposits: Nature, v. 280, p . 383385. Pgcsi, M., 1973, Geomorphological position a n d absolute age of the lower Paleolithic site at V&rtessz8llos, Hungary: Masyar Tud. Akad., FEldrajztud. Kutato Intgzet, FEldrajzi Kb;z., v , 2, p . 109119. Peng, T.-H., Goddard, J.G. and Broecker, U . S . , 1978, A direct comparis o n of I4C and 2 3 0 T h ages at Searles Lake, California: -Quaternary Research, v. 9 , p. 319-329. Richmond, G.M., 1976, Pleistocene stratigraphy and chronology in the mountains o f western Wyoming, in Mahaney, W.C., ed., Quaternary Stratigraphy of North America: Stroudsburg, Dowden, Hutchinson and R O S S , p. 353-379. Rosholt, J.N., 1980, Uranium-Trend Dating of Quaternary sediments, U.S. Geol. Surv., Open-File P,ep. 80-1087. Schwarcz, H.P., 1980, Absolute age determination o f archaeological sites by uranium series dating of travertines: Archaeometry, v. 22, no. 1, p . 3-24. Schwarcz, H.P. and Wintle, A.G.? 1980, 2 3 s L J / 2 3 8 Udating o f speleothem by hot-atom method. Tech. Memo 80-1, Dept. of Geology, McMaster Univ., Samilton, Ont., Canada. Shackleton, F.J. and Opdyke, P.D., 1976, Oxygen isotope and paleomagnetic stratigraphy of Pacific core V28-239, Late Pliocene to latest Pleistocene, in Cline, R . M . and Hays, J.D., eds., Investigation of Late Quaternary Paleoceanography and Paleoclimatology, Geol. SOC. Amer. Vem. no. 1 4 5 , p. 449-464. Stearns, C.E. and Thurber, D.L., 1967, T h 2 3 0 / U 2 3 4dates o f late Pleistocene marine fossils from the Yediterranean and Moroccan littorals: Progress in Oceanography, v. 4 , p . 293-305. Szabo, B.J., 1980, Results and assessment of uranium-series dating of vertebrate fossils from Quaternary alluviums in Colorado: Arctic Alpine Iies., v. 12, p . 95-100..
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Szabo, B . J . , S t a l k e r , A . MacS. a n d C h u r c h e r , C . S . , 1 9 7 3 , Uraniums e r i e s a g e s of some Q u a t e r n a r y d e p o s i t s n e a r M e d i c i n e H a t , A l b e r t a , Canada: Can. J o u r . E a r t h S c i . , v . 1 0 , p . 1464-1469. Szabo, B . J . and R o s h o l t , J . N . , 1 9 6 9 , Uranium s e r i e s d a t i n g o f P l e i s t o cene molluscan s h e l l s from S o u t h e r n C a l i f o r n i a An o p e n s y s t e m model: J o u r . o f Geophys. Res., v. 74, n o . 1 2 , p . 3253-3260.
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1979, Uranium-series d a t i n g of l a c u s t r i n e Szabo, B . J . and B u t z e r , K . W . , limestones from pan d e p o s i t s with f i n a l Acheulian assemblage a t Rooidam, K i m b e r l e y D i s t r i c t , S o u t h A f r i c a : Quaternary Research, V. 11, p . 2 5 7 - 2 6 0 . Thompson, G . M . , Lumsden, D . M . , Walker, R.L. and Carter, J . A . , 1 9 7 5 , Uranium series d a t i n g o f s t a l a g m i t e s f r o m B l a n c h a r d S p r i n g s Caverns, U.S.A.: Geochim. Cosmochim. A c t a , v . 3 9 , p . 1211-1218. T h o m p s o n , P . , F o r d , D . C . a n d S c h w a r c z , H.P., 1 9 7 5 , 2 3 4 U / 2 3 8 U r a t i o s i n l i m e s t o n e c a v e s e e p a g e w a t e r s a n d s p e l e o t h e m s f r o m West V i r g i n i a : Geochim. Cosmochim. A C E , v . 3 9 , p . 661-669. V o g e l , J . C . a n d K r o n f e l d , J . , 1 9 8 0 , A new m e t h o d f o r d a t i n g p e a t : South A f r i c a n J o u r . of S c i . , v . 76, p . 557-558. W i l l i a m s , M.A.J., W i l l i a m s , F.M., Gasse, F . , C u r t i s , G . H . a n d Adamson, D . A . , 1979, P l i o - P l e i s t o c e n e e n v i r o n m e n t s a t Gadeb p r e h i s t o r i c site, Ethiopia: N a t u r e , v . 282, p . 29-33.
53
URANIUM-SERIES DATING AND THE HISTORY OF SEA LEVEL CHARLES E. STEARNS
ABSTRACT U-series d a t i n g of c o r a l a s s o c i a t e d w i t h former s h o r e l i n e s has provided t h e a b s c i s s a o f t i m e f o r diagrams of f l u c t u a t i o n i n sea l e v e l O r d i n a t e s of amplitude d u r i n g , e s p e c i a l l y , t h e l a s t 250,000 y e a r s . have b e e n p r o v i d e d by m o r D h o s t r a t i g r a p h i c d e t a i l , b y a s s u m p t i o n s o f r a t e s o f u p l i f t , or by "0 v a l u e s i n e i t h e r l i t t o r a l d e - o o s i t s or d e e p s e a s e d i m e n t s . The l a t t e r c a s e p r o v i d e s m u t u a l r e i n f o r c e m e n t o f "0 and s e a l e v e l c h r o n o l o g i e s . The most c o m p l e t e r e f e r e n c e s e c t i o n s come f r o m t h e Huon p e n i n s u l a of New G u i n e a . T w e l v e s e p a r a t e r e e f c r e s t s h a v e b e e n d i s t i n g u i s h e d a n d d a t e d : f o u r i n t h e i n t e r v a l 250 k a to 1 8 0 k a ( c o r e s t a g e 7), s e v e n i n t h e i n t e r v a l 1 4 0 k a t o 2 8 ka ( c o r e s t a g e s 5-4-3), a n d o n e i n t h e i n t e r v a l 9 k a t o 5 k a . Were 2 3 0 T h / 2 3 ' U d a t i n g s u f f i c i e n t l y a c c u r a t e , s e a l e v e l h i s t o r i e s f r o m o t h e r p a r t s o f t h e w o r l d m i g h t be c o m p a r e d t o t h e New G u i n e a r e c o r d , e v e n w i t h t h e i d e n t i f i c a t i o n o f i n d i v i d u a l s e a l e v e l events. Such, a l a s , i s n o t t h e c a s e . A t l e a s t t h e f o l l o w i n g s o u r c e s of e r r o r p l a g u e u s . S t a t i s t i c a l e r r o r s i n 2 3 0 T h / 2 3 4 U v a l u e s a r e commonly q u o t e d a s t 0 . 0 2 , b u t a n i n t e r l a b o r a t o r y c o m p a r i s o n s u g g e s t s t h a t f 0 . 0 4 may b e a f a i r e r index of p r e c i s i o n . This i s f 15 ka a t 1 2 5 k a , n e a r l y e q u a l t o Thus, i n d i t h e 17 k a i n t e r v a l b e t w e e n s e a l e v e l maxima on N e w G u i n e a . v i d u a l s a m p l e s f r o m o t h e r a r e a s c a n n o t b e a s s i g n e d to a s p e c i f i c e p i s o d e by " c l o s e d - s y s t e m " a p e s a l o n e . C l o s e c o r r e l a t i o n must depend upon p o s i t i o n i n a m o r p h o s t r a t i w a p h i c s e q u e n c e a s w e l l . Corals a r e n o t i d e a l c l o s e d systems. 2 3 0 T h a g e s may b e e i t h e r too o l d o r too y o u n g . C o n c o r d a n c y of 2 3 0 T h a n d 2 3 1 P a a g e s i s t h e only u s e f u l check, b u t 2 3 1 P a / 2 3 5 U i s seldom measured. Molluscan d a t a are n o t o r i o u s l y u n r e l i a b l e and would n o t be s t u d i e d , were we n o t c u r i o u s a b o u t t h e e x t r a - t r o p i c a l s e a s . C o e x i s t i n g c o r a l and m o l l u s c a c a n y i e l d i d e n t i c a l 2 3 0 T ha g e s . More commonly, m o l l u s c a n ages a r e s i g n i f i c a n t l y y o u n g e r . Concordant 2 3 0 T hand 2 3 1 P a a g e s can be a s l i t t l e as 5 0 % o f t h o s e f r o m c o e x i s t i n g c o r a l . The m o l l u s c a n s y s t e m s have b e e n " c l o s e d " to l o s s o f d a u g h t e r p r o d u c t s , b u t " o p e n " t o a d d i t i o n of p a r e n t U . We a r e m e a s u r i n g t h e a v e r a g e a g e o f t h e p a r e n t , w h i c h enters molluscan s h e l l a f t e r d e a t h .
Both c o r a l a n d m o l l u s c a may a l s o b e " o p e n s y s t e m s " t o w h i c h 2 3 4 U ,
2 3 0 T h , and/or 2 3 1 P a have been added i n e x c e s s of t h a t Droduced w i t h i n t h e s h e l l . I n some c a s e s , a d d i t i o n may h a v e b e e n s y s t e m a t i c . The S-R
model, w h i c h a l l o w s c o r r e c t i o n t o " c o n c o r d a n t " a q e s , i s n o t d e m o n s t r a b l y u s e f u l , b u t a l t e r n a t i v e models might be developed from s u f f i c i e n t d a t a . T h e r e c a n b e n o d o u b t t h a t " d a t i n g " t h e maximum s e a l e v e l o f t h e l a s t i n t e r g l a c i a l a t 1 2 5 , 0 0 0 f 1 0 , 0 0 0 y e a r s , and c o n f i r m a t i o n of i t s wide-spread p r e s e r v a t i o n a r e a c h i e v e m e n t s o f U-series d a t i n g . D e t a i l e d
54
s e a l e v e l r e c o r d s o f t h e " l a s t i n t e r g l a c i a l " ( c o r e s t a g e 5), b a s e d on l o c a l r n o r p h o s t r a t i g r a p h i c e v i d e n c e , s t i l l show c o n f l i c t s ( d i f f e r e n c e s ? ) which c a n n o t b e r e s o l v e d by U - s e r i e s d a t i n g . I N T R O D UCT I 0 N
U - s e r i e s d a t i n g of c o r a l a s s o c i a t e d w i t h f o r m e r s h o r e l i n e s h a s p r o v i d e d t h e a b s c i s s a of t i m e f o r diagrams of f l u c t u a t i o n i n s e a l e v e l O r d i n a t e s of a m p l i t u d e d u r i n g , e s p e c i a l l y , t h e l a s t 250,000 y e a r s . h a v e b e e n p r o v i d e d by m o r p h o s t r a t i g r a p h i c d e t a i l , by a s s u m p t i o n s o f r a t e s o f u p l i f t , or b y l80 v a l u e s i n e i t h e r l i t t o r a l d e p o s i t s or deeps e a s e d i m e n t s . The l a t t e r c a s e p r o v i d e s m u t u a l r e i n f o r c e m e n t of '*O and s e a l e v e l c h r o n o l o g i e s . Were 2 3 a T h / 2 3 4 U d a t i n g s u f f i c i e n t l y a c c u r a t e , s e a l e v e l h i s t o r i e s from v a r i o u s p a r t s of t h e w o r l d m i g h t be compared, even t o t h e i d e n t i f i c a t i o n of i n d i v i d u a l sea l e v e l e v e n t s . Such, a l a s , i s n o t t h e c a s e . 2 3 0 T h a g e s a r e s u b j e c t t o e r r o r s a r i s i n g from (1) f a i l u r e o f c a l c a r e o u s f o s s i l s t o b e i d e a l c l o s e d s y s t e m s , and ( 2 ) l a b o r a t o r y p r e c i s i o n l e s s t h a n t h a t i n p l l e d by cmnmocly q u o t e d s t a t i s t i c a l e r r o r s o f m e a s u r e m e n t . I n t h e u s e o f 2 3 0 T h a g e s , we must h o l d r e a l i s t i c l i m i t s o f e r r o r cons t a n t l y i n mind. THE CLOSED SYSTEY MODEL A l i v i n g o r g a n i s m may i n c o r p o r a t e U b u t n e i t h e r 2 3 0 T h nor 2 3 1 F a i n i t s c a l c a r e o u s s k e l e t o n . U , commonly w i t h 2 3 4 U i n e x c e s s o f e q u i l i b r i u m w i t h 2 3 8 U ( a c t i v i t y r a t i o 1 . 1 4 2 0.03), i s p r e s e n t i n s e a w a t e r . 2 3 0 T h and 2 3 1 P a a r e n o t . A f t e r d e a t h , t h e f o s s i l s k e l e t o n may r e m a i n c l o s e d t o f u r t h e r a d d i t i o n of U , b u t r e t a i n d a u g h t e r p r o d u c t s o f U a l r e a d y i n c o r p o r a t e d . T h i s i s t h e model c o n v e n t i o n a l l y u s e d i n conn e c t i o n w i t h u n r e c r y s t a l l i z e d c o r a l : t h e c l o s e d - s y s t e m m o d e l . Ages ( t i m e e l a p s e d s i n c e d e a t h ) may b e e s t i m a t e d f r o m t h e g r o w t h o f d a u g h t e r p r o d u c t s toward e q u i l i b r i u m w i t h t h e i r v a r e n t s . A c t i v i t y r a t i o s r e c o r d t h i s g r o w t h and a r e s i m p l e r t o d e t e r m i n e t h a n i n d i v i d u a l q u a n t i t i e s .
U s e f u l n e s s o f t h e model w i t h u n r e c r y s t a l l i z e d c o r a l h a s b e e n conf i r m e d by ( a ) c o n c o r d a n c y o f 2 3 0 T h / 2 3 4 U ,2 3 1 P a / 2 3 5 U , a n d 2 3 4 U / 2 3 8 U ages, ( b ) c o n s i s t e n c y w i t h r n o r p h o s t r a t i g r a p h i c s e q u e n c e , and ( c ) c o n s i s t e n c y w i t h i n d e p e n d e n t e s t i m a t e s of a g e (Ku, 1 9 7 6 ) . R e a s o n a b l e 2 3 0 T h / 2 3 4 U r a t i o s a l o n e , h o w e v e r , do n o t p r o v e t h e c l o s e d - s y s t e m a s s u m n t i o n . Common c h e c k s on r e l i a b i l i t y a r e l a r g e l y n e g a t i v e : a b s e c c e o f r e c r y s t a l l i z a t i o n p l a u s i b l e U and Th c o n c e n t r a t i o n s , 234U/238U0= 1.14 f 0 . 0 3 , h i g h 2 ' o T h / 2 3 7 T hr a t i o s . I f a n y o f t h e s e b e s u s p e c t , t h e c l o s e d I f t h e y b e r e a s o n a b l e , however, t h e y s t i l l s y s t e m model i s s u s p e c t . do n o t g u a r a n t e e r e l i a b i l i t y . A t p r e s e n t , t h e b e s t c h e c k o f r e l i a b i l i t y i s c o n c o r d a n c y between 2 3 a T h / 2 3 4 Uand 2 3 1 P a / 2 3 5 U a g e s (Ku, 1 9 6 8 , 1 9 7 6 ) . These r a t i o s r e l a t e t o d i f f e r e n t d e c a y s e r i e s . I f t h e y y i e l d t h e same a g e , t h e y s t r o n g l y s u p p o r t t h e i n f e r e n c e t h a t t h e two d a u g h t e r p r o d u c t s have b e e n p r o d u c e d b y a common s o u r c e ( c o n t a i n i n g b o t h p a r e n t s ) a n d t h a t t h e y have b e e n q u a n t i t a t i v e l y r e t a i n e d i n t h e sample. It i s s t i l l important t o remember t h a t w e a r e m e a s u r i n g t h e a g e of t h e u r a n i u m s o u r c e , b u t i t -nay a l s o b e t h e a g e o f t h e f o s s i l . The c h e c k i s seldom made, b e c a u s e s n a l y s i s for 2 3 1 P a i s t e d i o u s .
Oahu d a t a (Ku e t a l . , 1974) s u g g e s t t h a t 2 3 1 P a may l e a k somewhat. age, I n e l e v e n s a m p l e s , mean 2 3 1 P a / 2 3 5 U a g e i s l e s s t h a n mean 2 9 0 T h ; / 2 3 4 U m d t h e spread i n 2 3 1 P a / 2 3 5 Uages i s g r e a t e r . Thus, a l t h o u g h 2 3 1 P a / 2 3 5 U - . a t i o s p r o v i d e t h e b e s t c o n f i r m a t i o n o f " c l o s e d s y s t e m " a p e s , t h e y may :hemselves t e n d t o b e t o o low. 2 3 0 T h / 2 3 4 Ui s t h e more p r e c i s e m e a s u r e of a g e . C o r a l from B a r b a d o s h a s y i e l d e d c o n c o r d a n t a g e s f o r f i v e d i s c r e t e r e e f - t e r r a c e s (Ku, 1 9 6 8 , James e t al.,, 1 9 7 1 ) . I n a d i a g r a m o f 2 3 0 T h / 2 3 4 U v s 2 3 1 P a / 2 3 ' U ( F i g u r e lA), t h e y p r o v i d e u s w i t h f i v e r e f e r e n c e p o i n t s o f "known" a g e s . They a l s o d e f i n e a c o n c o r d i a ( f o r 2 3 r U / 2 3 8 U = 1.11 k O . 0 2 ) , or l o c u s of p o i n t s r e p r e s e n t i n g ' e q u a l 2 3 0 T h and 2 3 1 F a a g e s .
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The p l o t has b e e n h e l p f u l t o m e a s a b a s e - l i n e a g a i n s t w h i c h t o compare and e v a l u a t e o t h e r d a t a . Oahu d a t a , for e x a m p l e , p l o t s m o s t l y t o t h e l e f t of c o n c o r d i a . Szabo ( 1 9 7 9 a ) has m e a s u r e d b o t h 2 3 'Th/' 'U a n d 3 1 P a / 2 3 5 U r a t i o s i n f o u r c o r a l and one m o l l u s c from R o t t n e s t I s l a n d , A u s t r a l i a ( F i g u r e 1B). W i t h i n r e a s o n a b l e l i m i t s of e r r o r a l l , i n c l u d i n g t h e m o l l u s c , y i e l d concordant and i n d i s t i n g u i s h a b l e closed-system a g e s . One c o r a l and two e c h i n o d e r m s f r o m M a g d a l e n a , Baja C a l i f o r n i a (Omura e t a z . , 1979) a r e more c l o s e l y c o n c o r d a n t . I n a t l e a s t t h e s e c a s e s , 2 3 0 T h / 2 3 4 U i s a " r e l i a b l e " b a s i s for e s t i m a t i o n of a g e . N o t e , h o w e v e r , t h a t Ma g d a le n a f a l l s midway b e t w e e n t w o B a r b a d o s r e f e r e n c e p o i n t s a n d c o u l d n o t b e s p e c i f i c a l l y c o r r e l a t e d w i t h o n e or t h e o t h e r by i s o t o p i c r a t i o s a l o n e . Bender e t aZ. ( 1 9 7 9 ) h a v e shown t h a t B a r b a d o s c o r a l o l d e r t h a n 1 5 0 k a h a v e d e m o n s t r a b l y e x c e s s 2 3 0 T h a n d t h a t t h o s e o l d e r t h a n 2 5 0 ka
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h a v e d e m o n s t r a b l y e x c e s s 2 3 4 U . 2 3 ! P a was n o t s t u d i e d . T o t a l U does n o t i n c r e a s e p e r c e p t i b l y w i t h a g e . These c o r a l a r e n o t i d e a l c l o s e d systems b u t , r a t h e r , a c c e p t a t l e a s t d a u g h t e r p r o d u c t s from t h e e x t e r n a l environment a f t e r d e a t h . S z a b o ( 1 9 7 9 b ) has m e a s u r e d b o t h 2 3 0 T h / 2 3 4 U a n d 2 3 1 P a / 2 3 5 U ir! f i v e c o r a l a n d t h r e e m o l l u s c a f r o m a J a m a i c a n d e p o s i t ( F i g u r e 1C). O n l y one c o r a l y i e l d s a n e a r l y c o n c o r d a n t "c lo s e d -s y s te m" a g e . In the o t h e r s , 2 3 0 T h / 2 3 4 Ua n d 2 3 1 P a / 2 3 5 Ua r e b o t h t o o h i g h : i n d i v i d u a l s a m p l e s d r i f t t o t h e r i g h t a n d u p w a r d , away f r o m c o n c o r d i a . These y o u n g c o r a l have a c c e p t e d d a u g h t e r p r o d u c t s from t h e e x t e r n a l environment, a d d i t i o n a l t o t h o s e p r o d u c e d w i t h t h e s h e l l . It i s i m p o r t a n t to n o t e t h a t t h e s e s a m p l e s meet a l l o r d i n a r y c r i t e r i a o f " r e l i a b i l i t y " : calcite < 3 % , 2 3 0 T h / 2 3 2 T h1 0 0 , U >2.25 < 3 . 9 0 ppm, 2 3 4 ~ / 2 3 8 >1.05 ~ <1.18. 2 3 1 ~ a / 2 3 5 U s h o u l d b e r e g u l a r l y c h e c k e d f o r c o n c o r d a n c y i n c o r a l as w e l l a s mollusca
.
S z a b o a n d R o s h o l t (1969) p r o p o s e d t h a t d i s c o r d a n t a g e s , i n w h i c h 2 3 1 P a a g e > 2 3 0 T h a g e (or 2 3 1 P a / 2 3 5 U >l), m i g h t b e " c o r r e c t e d " b y a model i n which: ( a ) U , once i n c o r D o r a t e d , i s r e t a i n e d i n t h e s h e l l , ( b ) d a u g h t e r p r o d u c t s produced w i t h i n t h e s h e l l a r e r e t a i n e d , and ( c ) d a u g h t e r p r o d u c t s of U i n a n " e x t e r n a l " environment ( i n t e r s t i t i a l f l u i d or s o l f d m a t r i x ) a r e a l s o f i x e d by i m p l o s i o n a n d / o r a b s o r p t i o n . The s y s t e m i s " o p e n " t o a d d i t i o n of d a u g h t e r D r o d u c t s , b u t " c l o s e d " t o their loss. I n my r e f e r e n c e d i a g r a m , d a t a p o i n t s l i e t o t h e r i g h t o f c o n c o r d i a ( F i g u r e lC, J a m a i c a c o r a l ) . With a p p r o p r i a t e a s s u m p t i o n s , i t m i g h t b e p o s s i b l e t o s e g r e g a t e t h e two c o m p o n e n t s o f d a u g h t e r p r o d u c t s added t h r o u g h t i m e , s u b s t r a c t t h o s e components added from t h e e x t e r n a l e n v i r o n m e n t , and r e c o v e r t h e c o n c o r d a n t 2 3 0 T h and 2 3 1 P a a g e s ofUfixed i n the shell. Szabo and R o s h o l t (1969) proposed a s p e c i f i c model, b u t i n t h e a b s e n c e o f i n d e p e n d e n t e v i d e n c e o f a g e of t h e C a l i f o r n i a m o l l u s c a f r o m w h i c h d a t a were t h e n a v a i l a b l e , w e r e u n a b l e t o e v a l u a t e i t s u s e f u l n e s s . A s p o i n t e d o u t by Kaufman e t aZ. ( 1 9 7 1 ) , i t i s s i m p l y n o t p o s s i b l e t o t e l l w h e t h e r a p p a r e n t a g e s d e r i v e d from t h e model a r e p r e f e r a b l e t o t h o s e b a s e d on o t h e r m o d e l s o f m i g r a t i o n . R e t u r n i n g t h e J a m a i c a n d a t a ( F i g u r e 1 C ) t o c o n c o r d i a by a b e s t - f i t t i n g s t r a i g h t l i n e d o e s a s w e l l . F o s s i l m a r i n e m o l l u s c a t y p i c a l l y c o n t a i n more U t h a n do l i v i n g m o l l u s c a (Kaufman e t aZ., 1 9 7 1 ) , w h i c h i n t u r n c o n t a i n l e s s t h a n l i v i n g c o r a l . Thus, most o f t h e U i n f o s s i l m o l l u s c a i s i n c o r p o r a t e d a f t e r d e a t h . Kaufman e t aZ. ( 1 9 7 1 ) c o n c l u d e d t h a t U c o n c e n t r a t i o n s m i g h t i n c r e a s e b y a f a c t o r o f 25 w i t h i n 1 0 k a a n d by a n o t h e r 5 0 p e r c e n t i n t h e I t i s o f p a r a m o u n t i m p o r t a n c e t o remember t h a t , i f a n y next 100 ka. t h i n g , o n e may b e a b l e t o m e a s u r e t h e a g e o f t h e u r a n i u m i n a f o s s i l m o l l u s c . Even f o r t h i s l i m i t e d o b j e c t i v e , c o n c o r d a n c y o f 2 3 0 T h a n d 2 3 1 P a a g e s i s t h e o n l y c h e c k on " r e l i a b i l i t y " . L i m i t e d d a t a f r o m c o e x i s t i n g c o r a l a n d m o l l u s c a (James e t aZ., S z a b o , 1 9 7 9 a ; F i g u r e 1B) show t h a t t h e l a t t e r c a n y i e l d c o n c o r d 2 3 0 T h ages of a n t 130Th and 2 3 1 P a a g e s i n d i s t i n g u i s h a b l e from c o r a l . m o l l u s c a a r e more commonly l e s s t h a n t h o s e o f c o e x i s t i n g c o r a l (Veeh a n d C h a p p e l l , 1 9 7 0 , Hoang e t aZ., 1 9 7 4 , 1 9 7 8 ) . 2 3 0 T h a g e s o f m o l l u s c a i n a V i r g i n i a d e p o s i t , b e l i e v e d t o b e a t l e a s t 5 0 0 ka o l d ( 2 3 0 T h / 2 3 4 U s h o u l d have r e a c h e d t r a n s i e n t e q u i l i b r i u m ) , r a n g e from 5 t o 160 k a (Wehmiller e t aZ., 1 9 8 0 ) . None h a v e r e a c h e d e q u i l i b r i u m .
1971
S z a b o (197913; F i g u r e 1C) has shown t h a t m o l l u s c a c a n y i e l d conc o r d a n t 2 3 0 T h a n d 2 3 1 F a a g e s w h i c h d i f f e r among t h e m s e l v e s a n d may b e 2s l i t t l e as f i f t y p e r c e n t o f c o e x i s t i n g c o r a l a g e s . T h i s s i n g l e s t u d y makes t h e i m p o r t a n t s u g g e s t i o n t h a t m o l l u s c a n s h e l l may h a v e b e e n "open" t o a d d i t i o n of U , b u t " c l o s e d " i n t h e r e t e n t i o n of d a u g h t e r p r o d u c t s produced w i t h i n t h e s h e l l and r e f u s a l of d a u g h t e r p r o d u c t s HoanF ( p e r s o n a l c o m m u n i c a t i o n ) h a s now from t h e e x t e r n a l e n v i r o n m e n t . 'Pa/' j 5 U i n t h r e e molluscan samples neasured both 0 T h / 2 34U a n d from O u l j i a n d e p o s i t s n e a r A g a d i r , Morocco ( F i g u r e 1D). E s t i m a t e d s g e s f o r e a c h a r e c o n c o r d a n t w i t h i n r e a s o n a b l e l i m i t s of e r r o r , a n d n o t s i g n i f i c a n t l y d i f f e r e n t from t h o s e o f , S z a b o ' s "young" m o l l u s c a from
57
-
Jamaica. F u r t h e r m o r e , two s a m p l e s from a n o l d e r t e r r a c e a t Agadir .ield a c t i v i t y r a t i o s n e a r e q u i l i b r i u m - n o t i n e x c e s s suggesting that daughters have n o t been added from t h e e x t e r n a l e n v i r o n m e n t , ,chile d a u g h t e r s p r o d u c e d i n t h e s h e l l h a v e b e e n r e t a i n e d . I n both s e t s of s a m p l e s , we a r e m e a s u r i n g t h e a v e r a g e a g e o f u r a n i u m a d d i t i o n and n o t t h e f u l l a g e s i n c e d e a t h o f t h e o r g a n i s m . I n summary, f o r 2 3 0 T h " a p e s " to be m e a n i n c f u l , some r e a s s u r a n c e must be p r o v i d e d t h a t i s o t o p i c a c c u m u l a t i o n i n f o s s i l s h e l l has b e e n s y s t e m a t i c . The n e g a t i v e r e a s s u r a n c e t h a t r e c r y s t a l l i z a t i o n h a s n o t o c c u r r e d i s i n s u f f i c i e n t . A t p r e s e n t , c o n c o r d a n c y o f 2 3 0 T ha n d 2 3 1 P a a g e s i s t h e b e s t c h e c k on t h e a s s u m p t i o n t h a t a s y s t e m h a s r e m a i n e d " c l o s e d " ; i t i m p l i e s t h a t d a u g h t e r p r o d u c t s h a v e b e e n p r o d u c e d by U fixed i n t h e s h e l l . I n c o r a l , i f U b e > 2 <4 ppm a n d 2 3 4 U / 2 3 8 U 0b e 1 . 1 4 2 0 . 0 3 , " c l o s e & system" a g e s may b e t r u e a g e s . I n m o l l u s c a , concordant ages a r e always minimum a g e s . They may b e i n d i s t i n g u i s h a b l e f r o m t r u e a g e s ( R o t t n e s t , T i g u r e lB), b u t t h e y may a l s o b e a s l i t t l e a s f i f t y p e r c e n t o f t r u e age ( J a m a i c a , F i g u r e 1C). F u r t h e r m o r e , c o e x i s t i n g m o l l u s c a n e e d n o t y i e l d t h e same a g e . Presumably, t h e h i g h e s t concordant v a l u e i n a s e r i e s o f s a m p l e s f r o m o n e l o c a l i t y i s c l o s e s t t o t r u e a g e , b u t Us e r i e s d a t a p r o v i d e no b a s i s f o r e s t i m a t i n g t h e d i s c r e p a n c y . M i l d d i s c o r d a n c y , w i t h 2 3 1 P a a g e < 2 3 0 T h age may r e f l e c t i n complete r e t e n t i o n o f 2 3 1 ? a . I n s u c h s a m p l e s , 1 3 a T h a g e i s p r o b a b l y t h e b e t t e r e s t i m a t e o f t r u e ( c o r a l ) o r a p p a r e n t ( m o l l u s c a ) a g e . The S z a b o - R o s h o l t ( 1 9 6 9 ) m o d e l c o r r e c t i o n d o e s n o t a p p l y to s u c h s a m p l e s .
D i s c o r d a n c y w i t h 2 3 1 P a a g e > 2 3 0 T h a g e (or 2 3 1 P a / 2 3 5 U>1) i m p l i e s t h a t d a u g h t e r p r o d u c t s f r o m s o u r c e s e x t e r n a l to t h e s h e l l h a v e b e e n added t o t h o s e p r o d u c e d by U f i x e d w i t h i n t h e s h e l l . 2 3 0 T h a g e w i l l b e h i g h . I f t h e g a i n c o u l d b e shown to b e s y s t e m a t i c , i t m i g h t b e c o s s i b l e t o r e d u c e t h e d a t a t o d i s c o r d a n c y . U s e f u l n e s s o f t h e SzaboF o s h o l t ( 1 9 6 9 ) m o d e l h a s n o t b e e n d e m o n s t r a t e d (Kaufman e t al., 1971) and a t b e s t n e e d s s e r i o u s r e e v a l u a t i o n w i t h a n e n l a r g e d d a t a b a s e o f s a m p l e s o f known a g e . SEA LEVEL D U R I N G THE "LAST INTERGLACIAL"
( C O R E STAGE 5 )
There c a n b e n o d o u b t t h a t " d a t i n g " t h e maximum s e a l e v e l o f t h e L a s t i n t e r g l a c i a l a t 1 2 5 , 0 0 0 If: 1 0 , 0 0 0 y e a r s a n d c o n f i r m a t i o n o f i t s r i d e - s p r e a d r e c o r d a r e a c h i e v e m e n t s o f U - s e r i e s d a t i n g . "is episode i s commonly v i e w e d a s a b r i e f i n t e r v a l , i . e . , a few t h o u s a n d y e a r s , It f o l l o w e d c u r i n g w h i c h s e a l e v e l s t o o d a few m e t e r s h i g h e r t h a n now. e r a p i d r i s e f r o m s y n g l a c i a l low l e v e l s , T e r m i n a t i o n 11, e s t i m a t e d by b a r i o u s m e t h o d s to c e n t e r a t 1 2 8 k a , 138 k a , or 1'15 k a i n c o r e V 2 8 - 2 3 8 [Kominz e t al., l979), a n d was i n t u r n f o l l o w e d p2 a d r o p t o l o w e r r e v e l s . It has b e e n s p e c i f i c a l l y c o r r e l a t e d t o 0 c o r e s u b s t a g e 5e, :he b r i e f p e a k o f w h i c h i s a b o u t 0 . 5 p e r m i l l i g h t e r t h a n a n y o t h e r p a r t of c o r e s t a g e 5 .
S t a t i s t i c a l e r r o r s i n 2 3 0 T h / 2 3 4 Uv a l u e s a r e commonly q u o t e d a s 2 C.02, b u t a n i n t e r l a b o r a t o r y c a l i b r a t i o n (Harmon e t az., 1 9 7 9 ) s u g g e s t s that t 0 . 0 4 may b e a f a i r e r m e a s u r e o f p r e c i s i o n . T h i s i s f 15 k a a t 125 ka. 2 3 0 T h / 2 3 4 U v a l u e s ( a n d a g e s ) of s a m p l e s u s e d i n t h e s t u d y and t i t t r i b u t e d t o t h e 1 2 5 k a e v e n t a r e 0 . 7 5 ( 1 3 9 k a ) f r o m W i n d l e y Key, Florida 0 . 6 9 ( 1 2 4 k a ) from Curacao, and 0 . 6 6 ( 1 1 8 k a ) from Barbados. 2 3 1 P a / 2 ' 5 Uwas n o t s t u d i e d . " I t i s l i k e l y t h a t t h e o b s e r v e d v a r i a t i o n i s due i n p a r t t o b o t h u n c e r t a i n t y d e r i v e d f r o m c o u n t i n g s t a t i s t i c s and d i f f e r e n t d i a g e n e t i c m o d i f i c a t i o n o f t h e s a m p l e s . " (Harmon e t al., 1 9 7 9 ) . An e q u i v a l e n t r a n g e o f v a l u e s a n d a g e s h a s b e e n r e p o r t e d f r o m 1). l o c a l i t i e s s c a t t e r e d a r o u n d t h e w o r l d (!able
It s h o u l d b e p o i n t e d o u t t h a t t h e v a l u e s 0 . 6 6 ( 1 1 8 k a ) a n d 0 . 6 3 1105 k a ) , f r o m t w o d i s c r e t e t e r r a c e s on B a r b a d o s (Harmon e t al., 1 9 7 9 ) , a r e n o t s t a t i s t i c a l l y d i f f e r e n t ; t h e y c o u l d n o t b e d i s t i n g u i s h e d by i s o t o p i c d a t a a l o n e . We h a v e a l r e a d y n o t e d ( F i g u r e iB) t h e p r o b l e m o f c o r r e l a t i o n of t h e s i n g l e d e p o s i t a t Magdalena, Mexico, w i t h one o r t h e
58 T a b l e 1.
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0.72
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H a r m o n e t a Z . , 1 9 7 9 , b D a i P r a a n d S t e a r n s , 1 9 $ 9 , 'Veeh a n d Chappell, 1970, dSzabo, 1979a, eSzabo, 1979b, Ku e t al., 1 9 7 4 , g M e s o l e l l a e t aZ., 1 9 6 9 , h O m u r a e t az., 1 9 7 9 .
N o t e t h a t v a l u e s f o r B a r b a d o s I11 ( 0 . 6 6 ) are not s t a t i s t i c a l l y distinguishable.
a n d B a r b a d o s 11 ( 0 . 6 3 )
o t h e r Barbados t e r r a c e . 2 3 u T h / 2 3 ' Uv a l u e s a t b o t h extremes of t h e r a n g e f o r t h e "125 ka e v e n t " h a v e a l s o b e e n r e p o r t e d f r o m a t l e a s t f i v e l o c a l i t i e s : Oahu (Ku e t aZ., 1 9 7 4 1 , J a m a i c a ( r o o r e a n d S o m a y a j u l u , 1 9 7 4 ) , t h e i n n e r b a r r i e r o f G i p p s l a n d ( M a r s h a l l a n d Thorn, 1 9 7 6 ) , N e w G u i n e a a n d A t a u r o ( C h a p p e l l and Veeh, 1 9 7 8 ) where t h e y r e p r e s e n t two d i s c r e t e ''high s t a n d s " , s e p a r a t e d by d i s c o n f o r m i t i e s r e c o r d i n g an i n t e r v a i of lower s e a l e v e l . I n t h e s e l o c a l i t i e s , t h e 125 k a e v e n t i s d o u b l e a n d , i n c l u d i n g b o t h " h i g h s t a n d s " , l o n g e r t h a n we h a d i n m i n d . Individual samples from o t h e r l o c a l i t i e s , where only one "high s t a n d " i s r e c o r d e d , cannot be a s s i g n e d t o a s p e c i f i c " h i g h s t a n d " by 2 3 0 T ha g e s a l o n e . In f a c t , I suppose, i n p r o p o s i n g c o r r e l a t i o n of a n i s o l a t e d "high s t a n d " w i t h t h e 125 k a e v e n t , w e may r e a l l y b e c o r r e l a t i n g i t w i t h a "low s t a n d " s e p a r a t i n g two h i g h s t a n d s . Gaven a n d V e r n i e r ( 1 9 7 9 ) h a v e s p e c i f i c a l l y invoked t h i s p o s s i b i l i t y t o account f o r t h e p r e s e r v a t i o n o f 125 k a a n d 105 k a r e e f s a t t h e same e l e v a t i o n i n t h e I l e s G l o r i e u s e s . The p o i n t t o remember i s t h a t i t i s n e i t h e r a c l o s e c o r r e l a t i o n w i t h i n t h e f i n e s t r u c t u r e of t h e "Last I n t e r g l a c i a l " n o r a p a r t i c u l a r l y acc u r a t e s t a t e m e n t o f age. The m o s t c o m p l e t e r e f e r e n c e s e c t i o n f o r t h e 1 2 5 k a e v e n t comes f r o m N e w G u i n e a a n d A t a u r o ( C h a p p e l l a n d V e e h , 1 9 7 8 ) . On t h e s e h i g h u p l i f t c o a s t s , t h e o l d e r "high starid" a p p e a r s t o have been lower t h a n t h e younger. On t h e low u p l i f t c o a s t s of Oahu, J a m a i c a , a n d G i p p s l a n d , t h e t w o " h i g h s t a n d s " a r e now n o more t h a n a m e t e r a p a r t . B o t h " h i g h s t a n d s " on N e w G u i n e a , h o w e v e r , w e r e s i g n i f i c a n t l y h i g h e r t h a n y o u n g e r " h i g h s t a n d s " a t 1 0 5 k a a n d 8 2 k a (Bloom e t aZ., 1 9 7 4 ) , n o t r e c o g n i z e d on Oahu, J a m a i c a , a n d Q i p p s l a n d . T h u s , t h e New G u i n e a r e f e r e n c e c u r v e f o r 1 4 0 k a t o 75 k a ( e n v e l o p i n F i g u r e 2 ) i n c l u d e s two " h i g h s t a n d s " somewhat a b o v e p r e s e n t s e a l e v e l , f o l l o w e d by t w o y o u n g e r " h i g h s t a n d s " somewhat b e l o w p r e s e n t s e a l e v e l . A h a r o n e t aZ. ( 1 9 8 0 ) h a v e p a r t i a l l y d e c o u p l e d t h e N e w G u i n e a c u r v e of sea l e v e l from "0 f l u c t u a t i o n s w i t h i n c o r e s t a g e 5 . I have superi m p o s e d t h e i r "0 v a l u e s ( c i r c l e s i n F i g u r e 2 ) on t h e i r s e a l e v e l c u r v e by a r b i t r a r y s c a l e . The o l d e r " h i g h s t a n d " ( 1 3 4 k a ) i s i s o t o p i c a l l y
59
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Sea l e v e l h i s t o r i e s f r o m v a r i o u s l o c a l i t i e s . Redrawn from s o u r c e s i d e n t i f i e d i n t e x t .
l i g h t e s t , t h e y o u n g e r ( 1 2 0 k a ) 0 . 5 p e r m i l h e a v i e r . On1.y a f t e r sea l e v e l has f a l l e n f r o m i t s s e c o n d “ h i g h s t a n d ” d i d ’*O v a l u e s r i s e t o v a l u e s o n l y 0 . 2 5 p e r m i l h e a v i e r t h a n t h o s e a t 1 3 4 k a . C o mp a ra b le v a l u e s were r e g a i n e d a t lo5 k a ; v a l u e s a t 82 ka a r e n o t r e p o r t e d . Aharon e t a Z . ( 1 9 8 0 ) s u g g e s t t h a t t h e d e c o u p l i n g a t 1 2 0 k a may r e f l e c t a massive A n t a r c t i c s u r g e . One may i n f e r t h a t 1 3 4 k a o n N e w G u i n e a , i s o t o p i c a l l y l i g h t e s t , i n c l u d e s t h e p e a k o f c o r e s t a g e 5 e , a l s o i s o t o p i c a l l y l i g h t e s t . The 1 2 5 k a r e e f on B a r b a d o s , a l s o i s o t o p i c a l l y l i g h t e s t ( S h a c k l e t o n a n d Matthews, 1 9 7 7 , F a i r b a n k s a n d Matthews, 1 9 7 8 ) , may t h u s b e e q u i v a l e n t t o 134 k a on N e w G u i n e a . What t h e n o f t h e 1 0 5 k a r e e f o n B a r b a d o s ? I s i t e q u i v a l e n t t o 1 2 0 k a or 1 0 5 k a on N e w G u i n e a ? P r o b a b l y t h e l a t t e r , b u t I r a i s e t h e q u e s t i o n t o emphasize t h a t t h e q u e s t i o n cannot be a ns w e r ed b y 2 3 0 T h d a t i n g a l o n e . 230Th/234Uactivity ratios for 120 ka and 1 0 5 k a a r e n o t s t a t i s t i c a l l y d i s t i n g u i s h a b l e . Rather, choice must be made i n t h e c o n t e x t o f m o r p h o s t r a t i g r a p h i c s e q u e n c e s a n d a n y other p o s s i b l e argument. C o n s t r u c t i o n o f e o l i a n i t e d u n e s on Bermuda i m p l i e s t h a t t h e Bermuda p l a t f o r m ( - 2 0 m ) was s u b m e r g e d for much o f t h e i n t e r v a l 1 4 0 t o
60
75 k a ( V a c h e r , 1 9 7 3 , Harmon e t a Z . , . 1 9 7 8 , 1 9 8 1 ) . A s i n g l e p e a k a t a b o u t t 5 m a b o u t 1 2 5 k a i s r e c o r a t - i bT: t h e m a r i n e D e v o n s h i r e Y o r m a t i o n . One or more y o u n g e r maxima a r e r e c o r d e d by s t o r m d e p o s i t s ( " c l i f f p l a s t e r s " ) somewhat a b o v e s e a l e v e l . C o r a l g r o w t h i m p l i e s t h a t t h e Bahamas p l a t f o r m ( - 2 0 m ) was s u b m e r g e d ~ G much T of t h e i n t e r v a l 140 t o 1 0 0 k a , w i t h a s i n g l e d e f i n a b l e maximum a t t 5 . 6 m a b o u t 1 2 5 k a (Neumann a n d Moore, 1 9 7 5 ) . 2 3 u T ha g e s f r o m c o r a l i n c l a s s i c a l E u t y r r h e n i a n d e p o s i t s n e a r T a r a n t o , I t a l y , i m p l y t h a t t h e Mare P i c c o l o was c o n t i n u o u s l y submerged above t 5 m between 1 5 6 ka and 9 0 k a , r e a c h i n g a s i n g l e i d e n t i f i a b l e maximum a b o u t 1 4 0 k a , now t 2 8 t o t 3 5 m.. Harmon e t a Z . ( 1 9 7 8 , 1 9 8 1 ) h a v e d e v e l o p e d a d e t a i l e d r e c o r d o f s e a l e v e l f l u c t u a t i o n on Bermuda ( 3 i g u r e 2 ) w i t h t h e a d d i t i o n a l c o n t r o l o f 2 3 u T h a g e s f r o m s u b a e r i a l s p e l e o t h e m s now or f o r m e r l y s u b m e r e d . C o r a l in s i t u f r o m t h e e m e r g e n t D e v o n s h i r e F o r m a t i o n y i e l d s ' 3 0 T h a g e s 1314 k a t o 1 1 8 k a . A s t a l a c t i t e was s u b m e r g e d t o t h a t t h e same t i m e ; b r a c k e t i n g d a t e s a r e 130 k a a n d 1 1 0 k a . O t h e r s n e l e o t h e m s r e c o r d c o n t i n u o u s s u b a e r i a l d e p o s i t i o n a t -7 m y 195 k a t o 150 k a a n d 1 2 0 k a t o 1 0 0 ka. One s t a l a g m i t e m a y h a v e grown c o n t i n u o u s l y a t -15 m b e t w e e n 1 1 0 ka and 1 0 ka. Two p r i n c i p a l e p i s o d e s of d u n e - b u i l d i n g a r e i n f e r r e d f r o m a m i n o a c i d r a c e m i z a t i o n a g e s of 1 0 5 k a a n d 85 k a f r o m t w o d i f f e r e n t o u t c r o p s . There a r e a l s o r e d e p o s i t e d c o r a l i n t h e d i s c o n t i n u o u s storm d e p o s i t s ( " c l i f f p l a s t e r s " ) , t h r o w n up t o p r e s e n t e l e v a t i o n s a b o v e s e a l e v e l unknown d i s t a n c e s a b o v e c o n t e m p o r a n e o u s s e a l e v e l ( a s much as 2 0 m i f t h e s t a l a g m i t e a t - 1 5 m was n e v e r s u b m e r g e d . These c o r a l y i e l d one c l u s t e r of 2 3 0 T h a g e s comparable t o t h o s e from t h e Devonshire Formation a n d two c l u s t e r s c o m p a t i b l e w i t h l o 5 k a a n d 85 k a d u n e - b u i l d i n g e v e n t s . No d i s c o n t i n u i t y i s r e c o r d e d i n t h e 1 2 5 k a e v e n t . The s h o u l d e r a t - 8 m a n d 1 2 0 k a i n F i g u r e 2 , h o w e v e r , i s b a s e d on o n e s p e l e o t h e m a g e of l l g k 1 3 , n o t s t a t i s t i c a l l y d i s t i n g u i s h a b l e f r o m o t h e r " c o n t r o l l i n g " a g e s of 1 1 0 k 14, 111 k 0 , a n d 1 1 3 k 1 2 a t t h e same d e p t h . The H a r r i n g t o n F o r m a t i o n , 1 2 1 t 9 by 2 3 0 T h / 2 3 4 U , was i n t e r p r e t e d by V a c h e r (1973) a s a n e m e r g e n t " b a c k - b e a c h " d e p o s i t . 2 3 0 T h a g e s a l l o w a much b r o a d e r 1 2 5 k a p e a k t h a n t h a t drawn by Harmon e t a Z . ( 1 9 8 1 ; F i g u r e 2 ) .
T h u s , e v i d e n c e f r o m t h e Bahamas, Bermuda, a n d T a r a n t o i m p l i e s t h a t s e a l e v e l was a b o v e - 2 0 m for m o s t o f t h e " L a s t I n t e r g l a c i a l ' ' ( c o r e It r o s e f r o m s y n g l a c i a l low s t a g e 5 ) , b e t w e e n 1 4 0 k a a n d 75 k a a g o . l e v e l s r a p i d l y ( T e r m i n a t i o n 11) t o l e v e l s h i g h e r t h a n p r e s e n t a b o u t 125 k a ; t h e m o s t c o m p l e t e l o c a l r e c o r d s ( N e w G u i n e a - A t a u r o ) show a d o u b l e p e a k , 135 k a a n d 1 1 8 k a , s e p a r a t e d b y a d i s c o n f o r m i t y r e c o r d i n g a n i n t e r v e n i n g low. T h e r e a f t e r it f e l l below i t s p r e s e n t l e v e l t o a t l e a s t -20 m by 110 k a . The y o u n g e r p e a k s a t 1 0 5 k a a n d 82 k a a r e c l o s e l y b r a c k e t e d b y t h e n e c e s s i t y o f s u b m e r g i n g t h e Bermuda p l a t f o r m (-20 m ) w i t h o u t submerging a s t a l a g m i t e (-15 m ) . S i m i l a r l e v e l s of sea l e v e l a t 1 0 5 k a a n d 8 2 k a were e s t i m a t e d b y a s s u m p t i o n s of u n i f o r m u p l i f t on Barbados ( M e s o l e l l a e t aZ., 1 9 6 9 ) and New Guinea (BloometaZ., 1 9 7 4 ) . I f i n d it d i f f i c u l t , however, t o v i s u a l i z e throwing t h e " c l i f f p l a s t e r s " t o e l e v a t i o n s 2 0 m a b o v e c o n t e m p o r a n e o u s s e a l e v e l . Were t h e y a s s o c i a t e d w i t h t r u l y " b r i e f " h i g h e r s e a l e v e l e v e n t s not r e c o r d e d i n t h e s t a l a g m i t e (or i n t h e B a r b a d o s a n d N e w G u i n e a r e e f t r a c t s ) ? The A n t a r c t i c s u r g e s e n v i s i o n e d by H o l l i n ( 1 9 7 7 , 1 9 8 0 ) a r e j u s t s u c h b r i e f e v e n t s ( n x 1 0 0 y r ) , w h i c h m i g h t b e s u p e r i m p o s e d on b r o a d e r p e a k s ( n x 1 0 0 0 y r ) r e c o r d e d b y t h e c o n s t r u c t i o n o f d u n e s on Bermuda a n d reefs elsewhere. I h a v e t h e b e g i n n i n g s o f a r e f e r e n c e c u r v e ( F i g u r e 2), n o t d i s s i m i l a r f r o m o t h e r s (Bloom e t aZ., 1974). Where c o m p l e t e l y r e c o r d e d , high i t s c h a r a c t e r i s t i c s i n a t u r e i s a s e r i e s of p e a k s : h i g h low - l o w . B e c a u s e 2 F 0 T h a g e s h a v e a p r e c i s i o n l e s s t h a n t h e i n t e r v a l between p e a k s , i s o l a t e d r e c o r d s of i n d i v i d u a l sea l e v e l e v e n t s cannot be p l a c e d c l o s e l y i n t h e g e n e r a l c u r v e . P l a u s i b l e c h o i c e s w i l l be made i n t h e c o n t e x t o f m o r p h o s t r a t i g r a p h i c s e q u e n c e , i f a t a l l .
-
Cronin e t aZ. ( 1 9 8 1 ) s u g g e s t t h a t t h r e e "age c l u s t e r s " of c o r a l f r o m t h e s o u t h e a s t e r n U . S . c o a s t a l p l a i n may r e p r e s e n t 1 2 5 k a , 1 0 5 k a ,
61
and 8 2 k a e v e n t s ( F i g u r e 2 ) . Two ( 1 2 0 k a , o n e s a m p l e , 9 4 k a , t h r e e samples) a r e from l i t t o r a l d e p o s i t s i n South C a r o l i n a which have been a s s i g n e d t o a s i n g l e f o r m a t i o n (Wando F o r m a t i o n , M c C a r t a n e t aZ., 1 9 8 0 ) , p r o b a b l y e q u i v a l e n t t o t h e " P r i n c e s s Anne u n i t " of C o l q u h o u n ( 1 9 7 4 ) and t h e S o c a s t e e F o r m a t i o n of D u b a r e t 0 2 . ( 1 9 7 4 ) . Low 2 3 0 T h 2 3 2 T h r a t i o s ( 2 . 0 t o 13.0) c a s t d o u b t o n t h e r e l i a b i l i t y o f t h e s p e c i f i c a g e s . The t h i r d " c l u s t e r " o f f o u r s a m p l e s f r o m t h e N o r f o l k F o r m a t i o n o f V i r g i n i a r a n g e s from 6 2 k a t o 7 9 k a : s i m i l a r a g e s were r e p o r t e d by Oaks and Coch ( u n p u b l i s h e d , b u t q u o t e d b y F l i n t , 1 9 7 1 ) . The 6 2 k a s a m p l e , which d i f f e r s f r o m t h e o t h e r s b y more t h a n o n e s t a n d a r d d e v i a t i o n , a l s o y i e l d s a c o n c o r d a n t 2 3 1 P a a g e , s u g g e s t i n g t h a t i t may b e a f a i r m e a s u r e o f t h e a v e r a g e a g e of t h e c o n t a i n e d u r a n i u m ( n o t n e c e s s a r i l y t h e a g e o f t h e f o s s i l ) . Amino a c i d r a t i o s s u g g e s t t h a t t h e N o r f o l k F o r m a t i o n i s i n d e e d y o u n g e r t h a n t h e Wando a n d S o c a s t e e F o r m a t i o n s ( W e h m i l l e r , t h i s p a p e r , and p e r s o n a l communication). The r e g i o n of s o u t h e r n Chesapeake B a y p r o b a b l y has a r e c e n t h i s t o r y o f e m e r g e n c e ( u p l i f t ? ) d i f f e r e n t f r o m t h a t i n S o u t h C a r o l i n a (Mixon e t a l . , 1 9 7 4 ; Demarest and B e l k n a p , 1 9 8 0 ) . T h u s , 2 3 0 T h a g e s a l o n e , f r o m t w o s e p a r a t e a r e a s , provide n e i t h e r f i r m b a s i s f o r c o r r e l a t i o n w i t h a s t a n d a r d sea l e v e l h i s t o r y n o r s u p p o r t f o r t h e s u g g e s t i o n t h a t sea l e v e l was a t t h e same elevation a t t h r e e d i f f e r e n t times. The c l a s s i c a l E u t y r r h e n i a n ( c o u c h e 3 S t r o m b e s ) o f t h e M e d i t e r r a n e a n (= e a r l y O l u j i a n ofMorocco) i s a s s u r e d l y i h e 125 ka e v e n t (Brancaccio
e t al., 1 9 7 8 , Dai P r a a n d S t e a r n s , 1979). Our p o s t u l a t e o f a y o u n g e r e p i s o d e 7 5 k a t o 90 k a ( S t e a r n s a n d T h u r b e r , 1 9 6 5 , 1 9 6 7 ) f r o m 2 3 0 T h a g e s o f m o l l u s c a a l o n e was p r e m a t u r e . Most o f t h e " y o u n g " 2 3 0 T h a g e s c i t e d ( i d e m . , Kaufman e t a l . , 1 9 7 1 , B e r n a t e t al., 1 9 7 8 , Hoang e t al., 1 9 7 8 ) probably b e l o n g t o t h e 125 ka e v e n t ( S t e a r n s , i n p r e s s ) .
N e v e r t h e l e s s , a younger e p i s o d e ( N e o t y r r h e n i a n = l a t e O u l j i a n ) can be s e p a r a t e d f r o m t h e E u t y r r h e n i a n l o c a l l y by f i r m m o r p h o s t r a t i g r a p h i c e v i d e n c e , commonly a d i s c o n f o r m i t y a s s o c i a t e d w i t h r e d d i s h s o i l s . We have no s e c u r e a g e f o r t h e N e o t y r r h e n i a n . One of t h e b e s t e x a m p l e s from t h e T u n i s l a n S a h e l ( P a s k o f f a n d S a n l a v i l l e , 1 9 7 6 , 1 9 7 9 , 1 9 8 0 ) i s illustrated i n Figure 2 .
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The s e a l e v e l s i g n a t u r e ' ' h i g h " ( D o u i r a ) "high" ( R e j i c h e ) "low" (Chebba) r e s e m b l e s t h e 1 2 5 k a e v e n t ( d o u b l e ) p l u s one o f t h e younger e v e n t s (105 k a or 8 2 k a ) , y e t " h i g h " a n d ''lev" s t a n d s d i f f e r Because of contemporaneous u p l i f t , i n e l e v a t i o n by o n l y 8 or 1 0 m . t h i s i s a maximum v a l u e f o r t h e d i f f e r e n c e i n c o n t e m p o r a e n o u s s e a levels. Similar r e l a t i o n s h i p s a t o t h e r Neotyrrhenian l o c a l i t i e s led me t o q u e s t i o n t h e p o s t u l a t e d 2 5 m d i f f e r e n c e i n " h i g h " a n d ''low" s e a l e v e l peaks d u r i n g t h e l a s t i n t e r g l a c i a l ( S t e a r n s , 1 9 7 6 ) .
Do t h e d i f f e r e n c e s r e f l e c t d i f f e r e n c e s i n u p l i f t h i s t o r y o n t h e s e d i f f e r e n t c o a s t s ? Does t h e N e o t y r r h e n i a n r e c o r d o n e or more s u r g e s imposed on b r o a d e r sea l e v e l p e a k s a t 1 0 5 k a a n d / o r 82 k a ? N e o t y r rhenian d e p o s i t s are i n f a c t c h a r a c t e r i s t i c a l l y p o o r l y s o r t e d c o a r s e l i t t o r a l c o n g l o m e r a t e s , c o n t r a s t i n g w i t h t h e more e x t e n s i v e and b e t t e r differentiated Eutyrrhenian. These a r e q u e s t i o n s which can be r a i s e d b y 2 3 0 T hd a t i n g , b u t n o t a n s w e r e d . The d e t a i l e d h i s t o r y o f sea l e v e l d u r i n g t h e " L a s t I n t e r g l a c i a l " c a n b e e x t e n d e d o n l y by 2 3 0 T h a g e s placed s e c u r e l y i n t h o r o u g h l y documented m o r p h o s t r a t i g r a p h i c sequences SEA LEVEL FLUCTUATIONS D U R I N G THE BRUNHES R e a s o n a b l e e x t r a p o l a t i o n s from y o u n g e r d a t e d t e r r a c e s ( > 2 5 0 k a ) on Barbados a n d N e w G u i n e a t h r o u g h f l i g h t s of h i g h e r t e r r a c e s , u s i n g t h e a s s u m p t i o n of u n i f o r m u p l i f t , show t h a t f l u c t u a t i o n s o f s e a l e v e l during t h e l a s t T O O k a h a v e b e e n as numerous as h a v e f l u c t u a t i o n s o f l R O values i n deep-sea c o r e s . A t l e a s t t h e m a j o r odd-numbered " 0 s t a g e s a r e r e p r e s e n t e d , g e n e r a l l y by m u l t i p l e t e r r a c e s r e f l e c t i n g " 0 substages as w e l l . On B a r b a d o s , a f i r s t t e x t o f d a t i n g by t h e a c cumulation o f h e l i u m has b e e n t r i e d . If He4 p r o d u c e d b y a l p h a e m i s s i o n b e q u a n t i t a t i v e l y r e t a i n e d i n f o s s i l s h e l l , i t s a c c u m u l a t i o n ( i n c r e a s e i n He-U r a t i o ) i s a b a s i s f o r
62
e s t i m a t e of a g e ( F a n a l e and S c h a e f f e r , 1365). Allowance must f o r b o t h i n i t i a l ( " i n h e r i t e d " ) and modern ( " a t m o s p h e r i c " ) He, t o t h a t p r o d u c e d i n t h e s a m p l e . Both of t h e s e s h o u l d b e l e s s s o u r c e s of e r r o r w i t h i n c r e a s i n g a g e , Allowance must f u r t h e r f o r He4 p r o d u c e d by U and d a u g h t e r p r o d u c t s added p o s t m o r t e m e x t e r n a l environment.
be made unrelated important b e made _from t h e
The a s s u m p t i o n t h a t He4 i s q u a n t i t a t i v e l y r e t a i n e d i s s u p p o r t e d by a c o m p a r i s o n o f He-U a g e s w i t h s t r a t i g r a p h i c a g e s of c o r a l from t h e s o u t h e a s t e r n U n i t e d S t a t e s , r a n g i n g i n a g e from Eocene t o l a t e P l i o c e n e (Bender, 1 9 7 0 , 1 9 7 3 , B l a c k w e l d e r , 1 9 8 1 ) . Less s a t i s f a c t o r y agreement h a s b e e n o b t a i n e d f r o m Y i o c e n e c o r a l i n t h e r e g i o n o f Chesapeake B a y ; He-U loss i s p r o b a b l e ( B e n d e r e t aZ., 1 9 7 9 , p . 5 8 9 ) . E l e v e n r e e f t r a c t s o l d e r t h a n t h e 125 k a r e e f on Barbados have b e e n " d a t e d " on t h e a s s u m p t i o n s t h a t He4 h a s b e e n q u a n t i t a t i v e l y r e t a i n e d and t h a t a c o r r e c t i o n can b e made for H e 4 p r o d u c e d b y 2 3 4 U and 2 3 0 T h added p o s t m o r t e m . Errors a r e e s t i m a t e d t o b e ? lo%, e q u i v a l e n t t o one c o r e s t a g e for t h e o l d e s t t e r r a c e s . R e s u l t s g e n e r a l l y c o n f i r m t h e i n f e r e n c e made b y e x t r a p o l a t i o n w i t h t h e a s s u m p t i o n o f u n i f o r m upl i f t . The d a t e d t e r r a c e s a r e B r u n h e s , and t h e y r e c o r d f l u c t u a t i o n s i n s e a l e v e l a s f r e q u e n t a s "0 s u b s t a g e s i n t h e d e e p - s e a r e c o y d . They a r e p r o b a b l y n o t u s e f u l f o r c o r r e l a t i o n s of s p e c i f i c i n d i v i d u a l t e r r a c e r e m n a n t s , e v e n on Barbados i t s e l f . CONCLUSION U - s e r i e s d a t i n g i s u s e f u l , a l t h o u g h n o t a s a c c u r a t e a s we might w i s h (or a s some u s e r s i m p l y ) . One must a l w a y s remember t h a t i t " d a t e s " u r a n i u m . n o t f o s s i l s . P?o:luscan "aBes" a r e n o t o r i o u s l v unr e l i a b l e , b u t c o r a l s a r e a l s o n o t i d e a l c l o s e d s y s t e m s . Concordancy of 2 3 1 P ai s 2 3 0 T h a n d 2 3 1 P a a g e s i s t h e o n l y t r u e check on " r e l i a b i l i t y " . commonly n o t m e a s u r e d , b e c a u s e t h e a n a l y s i s i s t e d i o u s . It s h o u l d be m e a s u r e d more o f t e n . Those o f u s who a r e n o t c h e m i s t s n u s t a l s o l e a r n t o s c r e e n t h e a v a i l a b l e c h e m i c a l d a t a f o r r e l i a b i l i t y a s f a r as possible.
-
The s e a l e v e l maximum o f t h e " L a s t I n t e r g l a c i a l ' ' i s s e c u r e l y d a t e d a t l25,OOO 2 1 0 , 0 0 0 y e a r s a g o . I t was a d o u b l e p e a k , a l t h o u g h many l o c a l i t i e s do n o t r e c o r d t h i s . We h a v e t h e b e g i n n i n g s o f a d e t a i l e d h i s t o r y of s e a l e v e l b e t w e e n 1 4 0 k a and 75 k a , b a s e d u p o n good e v i d e n c e from a v e r y few l o c a l i t i e s . T h e r e were f o u r s e a l e v e l maxima, n o t a l l of them s e p a r a t e l y r e c o r d e d . " h e i r c h a r a c t e r i s t i c s i g n a t u r e a p p e a r s t o l l h i g h l l - " h i g h " - r l l ~ ~" 1l0 wl" . The " h i g h s " e q u a l l e d or e x c e e d e d p r e s e n t sea l e v e l . The lows d i d n o t , and a r e n o t a p t t o be r e c o r d e d Individual 'Th a g e s on " s t a b l e " o r l o w - u p l i f t (>O. 1 m/ka) c o a s t s . h a v e a p r e c i s i o n no g r e a t e r t h a n t h e i n t e r v a l b e t w e e n s e a l e v e l p e a k s . R e a s o n a b l e e s t i m a t e s a r e 135 k a , 118 k a , 1 0 5 k a , and 85 k a , b u t i s o l a t e d s e a l e v e l events cannot be assigned t o s p e c i f i c p o s i t i o n s i n t h e s e q u e n c e by 2 3 0 T h d a t i n g a l o n e . E s t i m a t e s of t h e l e v e l of s e a l e v e l i n i n d i v i d u a l p e a k s a r e n o t s e c u r e enough t o s a y w h e t h e r o r n o t there are significant local differences. Sea l e v e l f l u c t u a t i o n s d u r i n g the. Brunhes h a v e b e e n as numerous as T h i s h a s b e e n c o n f i r m e d on h i g h u p l i f t c o a s t s by r e a s o n a b l e e x t r a p o l a t i o n s from t h e 125 k a t e r r a c e and t h e assumption of uniform u p l i f t . 2 3 0 T ha g e s may a l l o w a s s i g n m e n t t o c o r e s t a g e 7 (ca. 225 k a ) , b u t n o t t o one o f i t s s u b s t a g e s . Beyond 250 k a , He-U a g e s h a v e a p r e c i s i o n n o t much g r e a t e r t h a n one c o r e stage.
'*O f l u c t u a t i o n s i n d e e p - s e a c o r e s .
ACKNOWLEDGEMENTS I a m p a r t i c u l a r l y g r a t e f u l t o Barney Szabo and Hoang Chi T r a c h f o r t h e i r p a t i e n c e w i t h m e w h i l e t r y i n g t o l e a r n how b e t t e r t o move t h r o u g h c h e m i c a l a m b i g u i t i e s . Both h a v e h e l p e d me s t u m b l e l e s s , b u t n e i t h e r i s r e s p o n s i b l e f o r c o n t i n u i n g i m p e r f e c t i o n s i n my g a i t .
63
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Veeh, H . H . a n d C h a p p e l l , 1970, A s t r o n o m i c a l t h e o r y o f c l i m a t i c c h a n g e : S c i e n c e , v . 167, p . 862-865. support from New Guinea: Wehmiller, J . P . , Goddard, J . G . , B e l k n a p , D.F. a n d K e e n a n , E . , 1980, Comparison of U-series and amino a c i d a g e e s t i m a t e s f o r q u a t e r n a r y
66 m o l l u s k s , T ' s C o r n e r , V i r g i n i a (Delmarva p e n i n s u l a ) : A m e r . A b s t r . w i t h Programs, v . 1 2 , p . 88-89.
Geol.
SOC.
67
THE APPLICABILITY OF 4oAr/3gAr DATING TO YOUNG VOLCANICS
CHRIS M. HALL and DEREK YORK
ABSTRACT The 4 o A r / 3 9 A r s t e p - h e a t i n g m e t h o d c a n b e a p p l i e d s u c c e s s f u l l y t o a n y sample w h i c h i s s u i t a b l e for K - A r d a t i n g . The s t e p - h e a t i n g a p p r o a c h i s p a r t i c u l a r l y v a l u a b l e f o r Q u a t e r n a r y r o c k s s i n c e i t p r o v i d e s d a t a concerning t h e fundamental assumptions used i n t h e K-Ar technique ( ? : . e . a t m o s p h e r i c i n i t 5 . a l Ar, n o e x c e s s 40Ar,e t c . ) w h i c h a r e v i r t u a l l y una t t a i n a b l e by a n y o t h e r m e a n s . A b r i e f o u t l i n e o f t h e m e t h o d i s g i v e n , along with i t s advantages and d i s a d v a n t a g e s f o r d a t i n g Q u a t e r n a r y rocks. The r e s u l t s o f a s i n g l e s t e p - h e a t i n g a n a l y s i s f o r a s a m p l e f r o m M t . Pupuke, N . Z . a r e p r e s e n t e d . T h i s c a s e h i s t o r y d e m o n s t r a t e s a few o f t h e method's p o t e n t i a l f o r a l l e v i a t i n g p r o b le ms e n c o u n t e r e d by t h e conventional K-Ar technique. INTXODUCTION 4 0 A r / 3 9 A r d a t i n g i s an e x t r e m e l y p o w e r f u l v a r i a n t of t h e s t a n d a r d K-Ar d a t i n g t e c h n i q u e . S i n c e i t s development i n t h e mid-1960's ( M e r r i h u e , 1 9 6 5 a n d M e r r i h u e a n d T u r n e r , l g 6 6 ) , t h i s m e t h o d has p r o v e n t o be a most v e r s a t i l e g e o c h r o n o l o g i c a l t o o l . Not o n l y c a n a w i d e v a r i e t y of materials be d a t e d u s i n g t h i s t e c h n i q u e , b u t a l s o , a v a s t range of ages can be measured, s p a n n i n g from l e s s t h a n 50,000 y e a r s ( H a l l a n d York, 1 9 7 8 a n d H a l l , 1 9 8 2 ) t o more t h a n four b i l l i o n y e a r s (York e t a Z . , 1 9 7 2 ) . T h i s p a p e r w i l l c o n c e n t r a t e o n t h e s p e c i a l problems a n d b p p o r t u n i t i e s a s s o c i a t e d w i t h a p p l y i n g t h e 40Ar/39Ar method t o s a m p l e s o f Q u a t e r n a r y a g e .
Since impossible e v e r , some provide us Quaternary
t h i s a p p l i c a t i o n of 4 0 A r / 3 9 A r d a t i n g i s s o v e r y new, i t i s t o g i v e many c o n c r e t e e x a m p l e s a n d f i r m p r e d i c t i o n s . Hows u c c e s s h a s a l r e a d y been a c h i e v e d and on-going s t u d i e s can w i t h some b a s i s f o r o u t l i n i n g t h e m e t h o d ' s p o t e n t i a l f o r dating,
Basic P r i n c i p l e s The K - A r d a t i n g m e t h o d r e l i e s o n t h e f a c t t h a t 4 0 K , w h o ' s a b u n d a n c e i s 0 . 0 1 1 6 7 a t o m % o f n a t u r a l K , s p o n t a n e o u s l y d e c a y s by e l e c t r o n c a p t u r e d e c a y s i n t o "Arj t h e r e s t d e i n t o 4 0 A r . A c t u a l l y , o n l y 1 0 . 5 % o f "K c a y s i n t o 4 0 C a , a common i s o t o p e o f c a l c i u m . The K - A r m e t h o d r e q u i r e s t h e measurement o f t h e 4 0 K c o n c e n t r a t i o n f r o m o n e a l i q u o t o f a s a m p l e , u s u a l l y by f l a m e p h o t o m e t r y o r some o t h e r w e t - c h e m i s t r y t e c h n i q u e . From a n o t h e r a l i q u o t , t h e r a d i o g e n i c 4 0 A r ( o r 4 0 A r * ) c o n c e n t r a t i o n i s measured by f u s i n g t h e s a m p l e i n a n u l t r a - h i g h vacuum s y s t e m a n d York a n d m e a s u r i n g t h e e v o l v e d A r i s o t o D e s i n a mass s p e c t r o m e t e r . Farquhar ( 1 9 7 2 ) and Dalrymple a n d Lanphere ( 1 9 6 9 ) d i s c u s s e d d e t a i l s o f t h e e x p e r i m e n t a l t e c h n i q u e s t h a t a r e commonly u s e d . Assuming t h a t a l l o f t h e 4 0 A r * was r e m o v e d f r o m t h e r o c k d u r i n g i t s f o r m a t i o n , t h e a g e of t h e s a m p l e i s g i v e n b y :
68
where A , t h e t o t a l decay c o n s t a n t f o r 4 0 K , i s 5 . 5 4 3 x l o - ’ ’ year-’ and X e , t h e c o n s t a n t f o r 4 0 K d e c a y v i a e l e c t r o n c a p t u r e i n t o 40Ar, i s 0 . 5 8 1 x lo-” y e a r - ’ ( S t e i g e r a n d J g g e r , 1 9 7 7 ) . For a g e s l e s s t h a n a b o u t 20 m i l l i o n y e a r s , E q . 1 c a n b e a p p r o x i m a t e d b y :
The a m o u n t s o f 4 0 A r p r o d u c e d i n t y p i c a l Q u a t e r n a r y v o l c a n i c r o c k s , t h o u g h s m a l l , a r e e a s i l y m e a s u r a b l e , a n d good K - A r d a t e s as low as 10,000 years have been found (Dalrymple, 1 9 6 7 ) . U n f o r t u n a t e l y , r a d i o g e n i c 40Ar i s n o t t h e o n l y “Ar i n a v o l c a n i c r o c k . The a r g o n i n t h e a t m o s p h e p e , w h i c h c o m p r i s e s a b o u t 1%o f t h e t o t a l a t m o s p h e r i c p r e s s u r e , i s 9 9 . 6 % 4 0 A r . Whole-rock b a s a i t s and K - f e l d s p a r m i n e r a l s e p a r a t e s t y p i c a l l y h a v e a b o u t 1 x lo-’ ccSTP/g o f t r a p p e d o r d i s s o l v e d a t m o s p h e r i c 40Ar ( D a l r y m p l e a n d L a n p h e r e , 1 9 7 1 ) . The amount o f a t m o s p h e r i c “Ar i n a s a m p l e c a n b e e s t i m a t e d by m e a s u r i n t h e c o n c e n t r a t i o n o f 36Ar a n d n o t i n g t h a t t h e a t m o s p h e r i c 40Ar t o ’6Ar r a t i o i s 2 9 5 . 5 . Thus t h e r a d i o g e n i c “Ar i n t h e s a m p l e i s g i v e n b y :
40Ar* =
measured -295.5
OAr
(3)
36Ar
Now Eq. 2 c a n b e r e w r i t t e n a s :
F o r Q u a t e r n a r y s a m p l e s , t h e d i f f e r e n c e b e t w e e n “Ar/36Ar i n t h e s a m p l e and 295.5 can be e x t r e m e l y small and t h i s i s t h e fundamental f a c t o r l i m i t i n g t h e p r e c i s i o n o f t h e m e t h o d . B e c a u s e t h e 36Ar a b u n d a n c e i s always v e r y small, t h e e r r o r i n i t s measurement i s t h e dominant s o u r c e o f e r r o r i n t h e c a l c u l a t e d a g e . The e r r o r i n t h e a g e d u e to a n e r r o r i n t h e 36Ar p e a k i s a p p r o x i m a t e l y : at
t
=
‘’ (
295.5 Ar/’
measured -295.5)
.
A36Ar 6Ar
(5)
T h u s a s a m p l e t h a t has 9 5 % a t m o s p h e r i c a r g o n ( t y p i c a l for a b a s a l t l e s s t h a n 1 0 0 , 0 0 0 y e a r s o l d ) w i t h a 36Ar p e a k m e a s u r e d w i t h a n a c c u r a c y o f 0 . 5 % , w i l l have a n a p p r o x i m a t e a g e e r r o r of 1 0 % . The a b o v e d i s c u s s i o n d e a l s w i t h t h e c o n v e n t i o n a l K-Ar s y s t e m a n d n o t t h e 40Ar/39Arm e t h o d , b u t f o r Q u a t e r n a r y r o c k s , many o f t h e p r o b l e m s a r e t h e same. S p e c i f i c a l l y , t h e s e n s i t i v i t y o f b o t h m e t h o d s t o e r r o r s from t h e 36Ar m e a s u r e m e n t a r e v e r y s i m i l a r , b u t i t i s much e a s i e r t o show t h i s w i t h t h e c o n v e n t i o n a l K-Ar m e t h o d t h a n i t i s w i t h t h e 40Ar/39Art e c h n i q u e . THE
’A R /
AF. METHOD
I n t h e 40Ar/39Arm e t h o d , b o t h t h e Ar a n d t h e K m e a s u r e m e n t s a r e p e r f o r m e d on t h e same a l i q u o t o f t h e s a m p l e . T h i s i s d o n e by i r r a d i a t i n g t h e sample w i t h f a s t n e u t r o n s i n a n u c l e a r r e a c t o r t o produce 39Ar f r o m t h e r e a c t i o n 3 9 K ( n , p ) 3 9 A r . 39Ar i s a r a d i o a c t i v e i s o t o p e of A r which does n o t o c c u r n a t u r a l l y i n s i g n i f i c a n t q u a n t i t i e s . S i n c e t h e 39Ar c o n c e n t r a t i o n i s p r o p o r t i o n a l t o t h e 3 9 K i n t h e s a m p l e , a n d s i n c e 3 9 K i s i n t u r n p r o p o r t i o n a l t o t h e 4 0 K , t h e 3 9 A r peak from t h e mass s p e c t r o m e t e r r u n i s a m e a s u r e o f t h e 4 0 K i n t h e s a m p l e . Thus b o t h t h e 40Ar* a n d t h e ‘OK c o n c e n t r a t i o n s c a n b e d e t e r m i n e d s i m u l t a n e o u s l y
69
from a s i n g l e mass s p e c t r o m e t e r a n a l y s i s . I n p r a c t i c e , t h e p r o p o r t i o n a l i t y constant r e l a t i n g 3 9 A r with 4 0 K i s determined by i r r a d i a t i n g a s t a n d a r d sample of known K-Ar a g e a l o n g w i t h t h e unknown. U n f o r t u n a t e l y , t h e r e a r e unwanted i n t e r f e r e n c e r e a c t i o n s which complicate t h i s s i m p l e p i c t u r e . S p e c i f i c a l l y 3 9 A r a n d 36Ar a r e b o t h produced from n u c l e a r r e a c t i o n s w i t h Ca, and “ A r i s g e n e r a t e d from a slow-neutron r e a c t i o n w i t h 40K. The p r o d u c t i o n of e x t r a n e o u s 3 6 A r from Ca i s e s p e c i a l l y w o r r y i n g for Q u a t e r n a r y r o c k s , a s t h i s i s o t o p e i s c r i t i c a l f o r the atmospheric argon c o r r e c t i o n . Fortunatel-y, neutron r e a c t i o n s w i t h Ca p r o d u c e much more 3 7 A r t h a n e i t h e r 3 6 A r o r 3 9 A r . 3 7 A r , l i k e 3 9 A 1 1 , i s a s h o r t - l i v e d i s o t o p e o f A r which d o e s n o t o c c u r n a t u r a l l y and h e n c e i t can b e u s e d t o c o r r e c t for t h e p r e s e n c e o f Cad e r i v e d 3’Ar and 3 9 A r . A s p o i n t e d o u t b y T u r n e r ( 1 9 7 1 ) , by o p t i m i z i n g t h e n e u t r o n f l u x t h a t t h e sample r e c e i v e s , i t i s p o s s i b l e t o r e d u c e t h e Ca i n t e r f e r e n c e e f f e c t s t o n e g l i g i b l e l e v e l s . Even when s a m p l e s r e ceive a l a r g e neutr on f l u x , t h e 3 7 A r c o r r e c t i o n s a r e very s u c c e s s f u l i n a c c o u n t i n g for C a - d e r i v e d 3 6 A r a n d 3 9 A r i n young b a s a l t s , a s shown b y Dalrymple and Lanphere ( 1 9 7 1 ) . The 4 o A r / 3 9 A r v e r s i o n o f E q u a t i o n 1 i s :
tU-
A
in
11 t J ( 4 0 A r * / 3 9 A r K ) u )
(6a)
where
OAra/3 9 A r K = (1-f,)(40Ar/39Ar)m-295.5 (l-f,)(36Ar/39Ar)m-(40Ar/39Ar)K
f, =
1/ 11-(37Ar/39Ar)ca(39Ar/37Ar)m/
f, = f ,
11-(36Ar/39Ar)Ca(39Ar/36Ar)ml
(6d) (6e)
Here t h e s u b s c r i p t s have t h e f o l l o w i n g m e a n i n g s : m
i s a q u a n t i t y as measured;
u
means t h e unknown, o r t h e sample u n d e r s t u d y ;
s
i s the standard, o r f l u x monitor;
Ca,K
a r e t h e c a l c i u m and p o t a s s i u m d e r i v e d i s o t o p e s r e s p e c t i v e l y .
Note t h a t t s i s t h e K - A r a g e o f t h e s t a n d a r d which i s i r r a d i a t e d w i t h t h e unknown. The v a l u e s o f t h e i n t e r f e r e n c e c o r r e c t i o n f a c t o r s a r e determined by i r r a d i a t i n g and t h e n f u s i n g p u r e K and Ca s a l t s . The values t h a t we c u r r e n t l y u s e ( B o t t o m l e y , 1982), f o r s a m p l e s i r r a d i a t e d i n t h e McMaster U n i v e r s i t y n u c l e a r r e a c t o r a r e : (37Ar/39Ar)Ca
=
1536.1
(36Ar/39Ar)Ca
=
0.390
(40Ar/39Ar)K
=
0.0156
(7c)
Although E q u a t i o n s 6a t o 6e a r e c o n s i d e r a b l y more i n v o l v e d t h a n Equation
70
4 , t h e c r u c i a l c o r r e c t i o n f o r young r o c k s i s s t i l l t h a t due t o atmosp h e r i c “ A r i n E q u a t i o n 6 c . From e x t e n s i v e a n a l y s i s of t h e e r r o r p r o p a g a t i o n e f f e c t s from measurements o f 4 u A r , 3 9 A r , 3 7 A r and 3 6 A r i n H a l l ( 1 9 8 2 ) , i t c a n b e shown t h a t e r r o r s i n t h e a g e o f Q u a t e r n a r y s a m p l e s a r e d o m i n a t e d b y e r r o r s i n t h e 3 6 A r p e a k . F o r v e r y young s a m p l e s ( a few h u n d r e d t h o u s a n d y e a r s or l e s s ) , E q u a t i o n 5 i s a good a p p r o x i m a t i o n o f t h e p r e c i s i o n o b t a i n a b l e from 40Ar/39Ard a t i n g . ADVANTAGES OF THE 4 0 A R / 3 9 A R METHOD The 40Ar/39Armethod t o t a i l y e l i m i n a t e s t h e wet c h e m i s t r y s i d e of I n t h e c o n v e n t i o n a l K-Ar t e c h n i q u e , t h e 4 0 K a n d 4 0 A r * c o n c e n t r a t i o n s a r e m e a s u r e d from d i f f e r e n t a l i q u o t s of t h e same s a m p l e . I f t h e sample i s n o t p r e c i s e l y homogeneous, t h e m e a s u r e d 4 0 A r * t o 4 0 K r a t i o w i l l be i n e r r o r , l e a d i n g t o an i n c o r r e c t age. T h i s i s p a r t i c u l a r l y t r o u b l e s o m e when w o r k i n g w i t h whole-rock b a s a l t s w h i c h c a n h a v e s i g n i f i c a n t K h e t e r o g e n e i t y . T h i s problem does n o t e x i s t w i t h 4 0 A r / 3 9 A r d a t i n g , h o w e v e r , s i n c e b o t h t h e K and “ A r * measurements a r e done on t h e same a l i q u o t of t h e s a m p l e . t h e K-Ar Method.
A l t h o u g h s i n g l e f u s i o n 4 0 A r / 3 9 A r a g e s , which a r e a n a l o g o u s w i t h a g e s , c a n b e m e a s u r e d , t h e r e a l power of t h e 4 0 A r / 3 9 A r method i s r e a l i z e d o n l y w i t h t h e s t e p - h e a t i n g v a r i a n t of t h e t e c h n i q u e . I n s t e a d of ? u s i n g t h e s a m p l e i n one s t e p , g a s f r a c t i o m a r e c o l l e c t e d , p u r i f i e d , and a n a l y s e d from a s e q u e n c e o f i n c r e a s i n g t e m p e r a t u r e s t e p s . S i n c e “Ar* and 3 9 A r a r e s i m u l t a n e o u s l y r e l e a s e d a t e a c h t e m p e r a t u r e s t e p , i t i s p o s s i b l e t o c a l c u l a t e a n a p p a r e n t a g e for e a c h g a s f r a c t i o n . An i d e a l , u n d i s t u r b e d m i n e r a l s h o u l d g i v e e s s e n t i a l l y t h e same a p p a r e n t age over t h e e n t i r e gas r e l e a s e , t h u s y i e l d i n g a “ p l a t e a u “ age. A m a j o r d i s t u r b a n c e i n t h e a g e s p e c t r u m i s a good d i a g n o s t i c i n d i c a t o r of n o n - i d e a l b e h a v i o u r of t h e s a m p l e ( i . e . a r g o n loss, a r g o n g a i n , K m o b i l i t y , m u l t i p l e p h a s e s w i t h d i f f e r e n t a p p a r e n t a g e s , etc.). Any s i n g l e c o n v e n t i o n a l K - A r d a t e d o e s n o t h a v e s u c h a n i n t e r n a l c h e c k , and i f t h e sample’s behaviour i s reproducible, multiple conventional K-Ar a n a l y s e s m i g h t n o t d e t e c t a n y p r o b l e m s w i t h a d i s t u r b e d m i n e r a l . For f i n e - g r a i n e d w h o l e - r o c k b a s a l t s , t h e s t e p - h e a t i n g method can b e p a r t i c u l a r l y a t t r a c t i v e . A s shown i n H a l l and York ( 1 9 7 8 ) , t h e s t e p - h e a t i n g t e c h n i q u e can d e g a s d i f f e r e n t m i n e r a l s a t d i f f e r e n t t e m p e r a t u r e s , t h e r e b y e f f e c t i v e l y p e r f o r m i n g a m i n ? r a l s e p a r a t i o n on a r o c k for which m e c h a n i c a l m i n e r a l s e p a r a t i o n i s n o t p r a c t i c a l . A l s o , i t was f o u n d t h a t t h e b u l k of t h e a t m o s p h e r i c “ A r i n a b a s a l t i c sample i s o f t e n r e l e a s e d a t t h e v e r y h i g h e s t t e m p e r a t u r e s . From t h e r a t i o o f 3 7 A r t o 3 9 A r i n t h e s e g a s f r a c t i o n s , i t can b e shown t h a t t h e m i n e r a l s t h a t degas a t t h e h i g h e s t t e m p e r a t u r e s a r e d e f i c i e n t i n K and have a h i g h Ca t o K r a t i o . The b u l k o f t h e 3 9 A r and 40Ar* i s a c t u a l l y r e l e a s e d a t low t o i n t e r m e d i a t e t e m p e r a t u r e s (i.e. 650-900 d e g r e e s C e l s i u s ) . T h e r e f o r e , s t e p h e a t i n g can s e p a r a t e t h e a t m o s p h e r i c argon and r a d i o g e n i c a r g o n c o m p o n e n t s , and a s can b e s e e n from t h e d i s c u s s i o n on e r r o r p r o p a g a t i o n a b o v e , t h i s can improve t h e p r e c i s i o n o f t h e a g e e s t i m a t e beyond t h a t o b t a i n a b l e b y c o n v e n t i o n a l K - A r d a t i n g . K-Ar
When d a t i n g v o l c a n i c t u f f s , f e l d s p a r m i n e r a l s e p a r a t e s a r e t h e most w i d e l y d a t e d m a t e r i a l . K - f e l d s p a r s a r e p a r t i c u l a r l y u s e f u l , and t h e y u s u a l l y g i v e r e l i a b l e K - A r a g e s for Q u a t e r n a r y and P l i o c e n e v o l c a n i c s ( E v e r n d e n a n d C u r t i s , 1 9 6 5 ) . However, 4 a A r / 3 9 A r d a t i n g c a n make a v e r y r e a l c o n t r i b u t i o n w i t h t h e s e s a m p l e s by d i r e c t l y d e m o n s t r a t i n g w h e t h e r or n o t t h e f e l d s p a r s a r e as w e l l - b e h a v e d as o n e assumes i n conv e n t i o n a l K - A r d a t i n g . A r e c e n t example i s t h e work of McDougall (1981) on t h e KBS t u f f p r o b l e m . The n e a r l y p e r f e c t p l a t e a u x from t h e KBS a n o r t h o c l a s e added c o n s i d e r a b l e w e i g h t t o t h e p r e v i o u s l y d e t e r m i n e d age of 1 . 8 8 Ma d e t e r m i n e d from c o n v e n t i o n a l K - A r measurements (McDougall e t aZ., 1980 and Drake e t a Z . , 1 9 8 0 ) . I n a s t u d y on d i f f e r e n t m a t e r i a l s i m i l a r r e s u l t s w e r e o b t a i n e d b y R a d i c a t i d i Brozolo e t aZ. ( 1 9 8 1 ) . I n t h i s s t u d y , 40Ar/39Arp l a t e a u a g e s for Q u a t e r n a r y s a m p l e s w e r e f o u n d t o b e c o n c o r d a n t w i t h Rb-Sr i s o c h r o n a g e s from t h e same r o c k s . The f u n d a m e n t a l p o i n t i s t h a t 4 0 A r / 3 9 A r s t e p - h e a t i n g a g e s can b e o b t a i n e d on a l m o s t any sample t h a t c a n b e d a t e d b y t h e c o n v e n t i o n a l K - A r t e c h n i q u e , and a few 4 0 A r / 3 9 A r a n a l y s e s c a n p r o v i d e more i n f o r m a t i o n t h a n many c o n v e n t i o n a l K-Ar r u n s on t h e same m a t e r i a l .
71
R e c e n t l y , t h e r e h a s been a u n i q u e a p p l i c a t i o n o f t h e “ A T ? / ~ ’ A ~ s t e p - h e a t i n g method t o t h e d a t i n g o f Q u a t e r n a r y r o c k s which h a s n o d i r e c t analogy i n conventional K r A r d a t i n g . G i l l e s p i e e t a t . ( 1 9 8 2 ) a n a l y s e d m i c r o c l i n e x e n o c r y s t s which had b e e n i n c o r p o r a t e d w i t h i n young volcanics. The h e a t o f t h e e r u p t i o n had c a u s e d p a r t i a l , b u t n o t t o t a l , r a d i o g e n i c Ar l o s s from t h e t r a p p e d c r y s t a l s . Conventional K - A r a n a l y s e s on t h e s e x e n o c r y s t s would h a v e y i e l d e d a g e s c o n s i d e r a b l y i n e x c e s s o f t h e e r u p t i o n a g e . However, a s shown by G i l l e s p i e e t aZ. ( 1 9 8 2 ) , t h e l o w e s t t e m p e r a t u r e f r a c t i o n s from a 40Ar/39Ars t e p - h e a t i n g r u n s h o u l d r e c o r d t h e a g e of t h e v o l c a n i c e r u p t i o n and t h i s was conf i r m e d by e x p e r i m e n t . DISADVANTAGES OF THE 40AR/39AF, SYSTEN I m p l i c i t i n t h e 40Ar/39Ara g e e q u a t i o n s i s t h e a s s u m p t i o n t h a t t h e s t a n d a r d m i n e r a l o f known K-Ar a g e r e c e i v e s t h e same n e u t r o n d o s e as t h e unknown s a m p l e . F o r Q u a t e r n a r y r o c k s , s a m p l e masses a r e n o r m a l l y l a r g e - t y p i c a l l y 1 t o 1 0 g , and i t i s o f t e n not p o s s i b l e t o h a v e a p e r f e c t l y uniform neutron f l u x over t h e space r e q u i r e d t o hold t h e s a m p l e and t h e s t a n d a r d . T h i s p r o b l e m c a n be r e d u c e d b y ( 1 ) c h o o s i n g i r r a d i a t i o n l o c a t i o n s w i t h i n t h e r e a c t o r which h a v e low n e u t r o n f l u x g r a d i e n t s and ( 2 ) a v e r a g i n g t h e J v a l u e s from s e v e r a l s t a n d a r d s i n t h e same i r r a d i a t i o n c a n . I n any c a s e , u n c e r t a i n t i e s i n t h e r e l a t i v e n e u t r o n d o s e s r e c e i v e d b y t h e s a m p l e s and s t a n d a r d s can u s u a l l y be k e p t below l % ,a n d t h i s i s a l m o s t a l w a y s l e s s t h a n t h e e x p e c t e d e r r o r i n Q u a t e r n a r y a g e s f o u n d b y t h e 40Ar/39Armethod.
A much more s e r i o u s p r o b l e m i s c a u s e d by Ar r e c o i l . A s shown b y T u r n e r and Cadogan ( 1 9 7 4 ) , a 4 0 K n u c l e u s which d e c a y s i n t o “ A r m i g h t move l e s s t h a n a n a n o m e t e r , b u t a 3 9 K n u c l e u s which i s c o n v e r t e d i n t o 3 9 A r by a n n , p r e a c t i o n c a n move up t o a b o u t 1 0 0 nm. 3 7 A r , 3 9 A r and 3 6 A r which a r e p r o d u c e d from Ca can a l s o be e x p e c t e d t o r e c o i l , b u t t h e d i s t a n c e s t h e y would t r a v e l have n o t y e t b e e n a c c u r a t e l y e s t i m a t e d . I f t h e i r r a d i a t e d m i n e r a l s a r e homogeneous on a s c a l e o f a b o u t 5 0 m i c r o n s , r e c o i l p r e s e n t s no p r o b l e m s , as any t r a n s p o r t o f Ar n u c l e i w i l l a v e r a g e o u t . Only a v e r y t h i n ( a b o u t 0 . 1 u m ) l a y e r s u r r o u n d i n g t h e m i n e r a l g r a i n w i l l h a v e any n e t e f f e c t from r e c o i l . U n f o r t u n a t e l y , t h e K - r i c h p h a s e s i n v o l c a n i c r o c k s sometimes a r e (1) v e r y s m a l l , and ( 2 ) s u r r o u n d e d by K-poor r e g i o n s . I n s u c h c a s e s , t h e r e can be a s i g n i f i c a n t n e t t r a n s p o r t o f 3 9 A r from t h e K-rich p h a s e i n t o t h e K-poor phase. As n o t e d a b o v e , t h e d i f f e r e n t m i n e r a l s d e g a s a t d i f f e r e n t tempe r a t u r e s . Thus some g a s f r a c t i o n s w i l l have t h e i r a p p a r e n t a g e s changed by t h e i r r a d i a t i o n . I f t h e small K-rich p h a s e s a r e n o t i n i n t i m a t e c o n t a c t w i t h o t h e r m i n e r a l s , i t i s e v e n p o s s i b l e to have a n e t l o s s of ’Ar.
One c a n a t l e a s t check t o s e e i f t h e r e i s a n e t l o s s of 3 9 A r by comparing t h e i n t e g r a t e d or t o t a l f u s i o n 40Ar/39Ara g e , w i t h t h e coriv e n t i o n a l K - A r a g e from t h e same s a m p l e . If t h e r e i s o n l y r e d i s t r i b u t i o n o f 39Ar,t h e two a g e s w i l l a g r e e w i t h i n e x p e r i m e n t a l e r r o r . I t i s o u r e x p e r i e n c e ( H a l l and York, 1 9 7 8 and H a l l , 1 9 8 2 ) t h a t whole-rock b a s a l t s do r e t a i n a l l o f t h e i r 3 9 A r . It i s l i k e l y t h a t a s l o n g a s r o c k s a r e n o t c r u s h e d t o a f i n e powder, r e c o i l i s n o t l i k e l y t o a f f e c t t h e i n t e g r a t e d 40Ar/39Ara g e o f a s a m p l e . I n c o m p a r i s o n w i t h f l u x g r a d i e n t s , r e c o i l e f f e c t s a r e more l i k e l y
to c o m p l i c a t e i n t e r p r e t a t i o n by a l t e r i n g t h e s t r u c t u r e o f t h e a g e s p e c t r u m d e r i v e d from a 4oAr/3yArs t e p - h e a t i n g e x p e r i m e n t . However, any e x p l a n a t i o n of odd a g e s p e c t r a f e a t u r e s b a s e d upon r e c o i l i s s e v e r e l y c o n s t r a i n e d by mass b a l a n c e c o n s i d e r a t i o n s and by t h e l i k e l y
r e c o i l ranges. T h i s r e s t r i c t s t h e use of r e c o i l as an e x p l a n a t i o n f o r any n o n - i d e a l b e h a v i o u r by a s a m p l e .
A Case H i s t o r y
F i g u r e 1 shows t h e r e s u l t s from a s i n g l e 40Ar/39Are x p e r i m e n t . The s a m p l e i s a n o l i v i n e b a s a l t from M t . Pupuke, N . Z . It i s t h e sample G A 2853 t h a t was s t u d i e d by t h e c o n v e n t i o n a l K-Ar method by McDougall
72 F i g u r e 1 The r e s u l t s o f a 4 0 A r / 3 9 A r experiment o n s a m p l e GA 2 8 5 3 f r o m M t . Pupuke, N.Z. The fraction temperatures a r e i n increasing order: 500, 700, 950, 1060, 1170, 1250 and 1600 degrees Celsius.
37Co /39K PUPUKE I
t ha
5
a ) 3 7 A r C a / 3 9 A r K Us 3 9 A r K released. The h i g h e s t temperature fraction is off-scale. It has a value of 39.5. b)Apparent age Us 3 9 A r K released. The h i g h e s t temperature fraction is off-scale with an apparent a g e o f 900 i 420 k a .
2
I / FRACTION OF 3 9 RELEASED ~
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PUPUK€
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c ) ~ ’ A ~ /U S~ 3~ 6 A A r ~r e leased. 1
1
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A l l isotope concentrations have been corrected f o r K a n d Ca i n t e r f e r e n c e A l l error effects. estimates are la.
50 2 0 0 W
0
a
150100-
50 -
6oor
FRACTION OF 3 9 ~ RELEASED
IcJ 140/36J PUPUKE I
300
204
;
; ; 4
;
.6
+
8
9
io
FRACTION OF 36s RELEASED
e t aZ. (1969).
Figure la is a graph of the 37Ar to 39Ar ratio as a function of 39Ar released. The 37Ar to 39Ar ratio is proportional to the Ca to K ratio. Figure la shows that the K.-rich phases preferentially degassed in the low and intermediate temperature steps. Figure lb is a standard 40Ar/39Arage spectrum while figure lc shows “ A P / ~ ~ A ~ as a function of 36Ar released. The bottom figure graphically illustrates which fractions contain the bulk of the radiogenic “AT. The volume of radiogenic 40Ar in a fraction is directly proportional to the area between the fraction’s 40Ar/36Arbox and a horizontal line running through 295.5 ( i . e . the atmospheric value of 40Ar/36Ar). As can be seen from figure lc, most of the radiogenic “ A r resides in the same K-rich phases which degas at low and intermediate temperatures. A l s o
13
n o t e t h a t a l a r g e p r o p o r t i o n of t h e was r e l e a s e d a t h i g h t e m p e r a t u r e s .
36
A r (and hence atmospheric argon)
I t i s i n t e r e s t i n g t o n o t e t h a t t h e c o n v e n t i o n a l K - A r o f GA 2 8 5 3 was f o u n d t o b e 2 5 5 Ka ( Y e D o u g a l l e t aZ., 1 9 6 9 ) , w h i l e t h e i n t e g r a t e d 40Ar/39Ara g e f r o m t h e r u n shown i n F i g u r e 1 i s 260 2 29 Ka. However, M e D o u g a l l e t aZ. ( 1 9 6 9 ) i r t e r p r e t e d t h i s a g e , a n d o t h e r a g e s f r o m t h e fiuckland v o l c a n i c f i e l d , t o be anomalously o l d due t o t h e p r e s e n c e of e x c e s s 40Ar. From F i g u r e 1, i t i s c l e a r t h a t t h e r e i s e x c e s s “A? in the higher temperature fractions. The s e c o n d f r a c t i o n ( 7 0 0 d e g r e e s C) h a s a n a p p a r e n t a g e o f 207 t 29 k a , w h i c h may b e a b e t t e r a g e e s t i m a t e t h a n t h a t f r o m t h e c o n v e n t i o n a l K-Ar t e c h n i q u e . It a p p e a r s t h a t t h e s t e p - h e a t i n g method h a s s e p a r a t e d t h e r a d i o g e n i c “ A r from t h e b u l k o f t h e i n i k i a l argor. t r a p p e d w i t h i n t h e sample.
CONCLUSIONS Any s a m p l e w h i c h c a n b e a r i a l y s e d by t h e c o n v e n t i o n a l K-Ar m e t h o d c a n a l s o b e d a t e d by t h e 40Ar/39Ars t e p - h e a t i n g t e c h r i i q u e . In a large m a j o r i t y o f c a s e s , t h e 40Ar/39Art e c h n i q u e y i e l d s much more u s e f u l i n f o r m a t i o n t h a n d o e s t h e c o n v e n t i o n a l K - A r s y s t e m . The s t e p - h e a t i n g p r o c e s s can e f f e c t i v e l y s e p a r a t e d i f f e r e n t m i n e r a l p h a s e s and i t can s e p a r a t e r a d i o g e n i c 4 0 A r f r o m much o f t h e s a m p l e ’ s c o n t a m i n a t i n g i n i t i a . 1 fir. I t may n o t b e p o s s i b l e i n a l l c a s e s t o d e t e r m i n e a g e o l o i c a l l y m e a n i n g f u l a g e f r o m e a c h s a m p l e , b u t i n many i n s t a n c e s , t h e 4FAr/39fir t e c h n i q u e can s e e t h r o u g h problems which would d e f e a t t h e c o n v e n t i o n a l K-Ar system. J u s t a s i m p o r t a n t l y , t h e 4oAr/39Arm e t h o d h a s i n t e r n a l c o n s i s t e n c y checks which can o f t e n d e t e r m i n e whether a sample i s s u i t able f o r d a t i n g a t a l l . REFERENCES CITED Bottomley, R.J., 1 9 8 2 , 4 0 A r * 3 9 A r dating of melt r o c k from impact craters: Ph.D. Thesis, University of Toronto. Dalrymple, G.B., 1 9 6 7 , Potassium-argon ages of recent rhyolites of the Mono and I n y o craters, California: Earth and Planetary Science L e t t e r s , v. 3 , p . 289-298. D a l r y m p l e , G.B. and Lanphere. M.A., 1 9 6 9 , Potassium-Argon D a t i n g , San Francisco, W.H. Freeman, 2 5 8 p .
, 1 9 7 1 , 4 0 A r / 3 9 A r technique of K / A r dating: a comparison with the conventional techniques: Earth and Planetary Science L e t t e r s , v. 1 2 , p . 300-308. D r a k e , R.E., C u r t i s , G.H., C e r l i n g , T.E., Cerling, B.W. and Hampel, J . , 1 9 8 0 , KBS Tuff dating and geochronology of tuffaceous sediments i n the K o o b i F o r a and Shungura F o r m a t i o n s , East Africa: Nature, v. 283, p‘?\268-372. Evernden, J.F. and Curtis, G.H., 1 9 6 5 , The Potassium-Argon Dating of Late Cenozoic Rocks i n East Africa and Italy: Current Anthrop o l o g y , V . 6 , p. 343-385. Gillespie, A.R., H u n e k e , J.C. and Wasserburg, G.J., 1 9 8 2 , Dating Pleistocene Basalts by 4 0 A r - 3 9 A r Analysis of Granitic Xenoliths: EOS Transactions, American Geophysical Union, v. 6 3 , p . 4 5 4 . H a l l , C.M., 1 9 8 2 , The Application of K-Ar and 4 0 A r / 3 9 A r methods to the dating of recent volcanics and the Laschamp event: (Ph.D. Dissert.): University of Toronto. H a l l , C.M. and Y o r k , D., 1 9 7 8 , K-Ar and 4 0 A r / 3 9 A r Age of the Laschamp geomagnetic polarity reversal: N a t u r e , v. 274, p . 462-464.
74 McDougall, I., 1 9 8 1 , 4 0 A r / 3 9 A r age. spectra from the KBS Tuff, Koobi Fora Formation: Nature, v. 2 9 4 , p. 1 2 0 - 1 2 4 . McDougall, I., Maier, R., Sutherland-Hawkes, P. and Gleadow, A.J.W., 1 9 8 0 , K-Ar age estimate for the KBS Tuff, East Turkana, Kenya: Nature, v. 2 8 4 , p. 2 3 0 ~ 2 3 4 . McDougall, I., Polach, H . A . and Stipp, J.J., 1 9 6 9 , Excess radiogenic argon in young subaerial basalts from the Auckland volcanic field, New Zealand: Geochimica et Cosmochimica Acta, v. 3 3 , p. 1 4 8 5 1520.
Merrihue, C.M., 1 9 6 5 , Trace-element determinations and potassium-argon dating by mass spectroscopy of neutron-irradiated samples: EOS Transactions, American Geophysical Union., v. 4 6 , p . 1 2 5 . Merrihue, C.M. and Turner, G., 1 9 6 6 , Potassium-argon dating by activation with fast neutrons: _Journal of Geophysical Research, v. 7 1 , p. 2 8 5 2 - 2 8 5 7 .
Radicati di Brozolo, F., Huneke, J.C., Papanastassion, D.A. and Wasserburg, G.J., 1 9 8 1 , 4 0 A r - 3 9 A r and Rb-Sr age determinations on Quaternary volcanic rocks: Earth and Planetary Science Letters, V.
53,
p. 4 4 5 - 4 5 6 .
Steiger, R.H. and Jager, E., 1 9 7 7 , Subcommission on geochronology: Convention on the use of decay constants in geo- and cosmochronology, Earth and Planetary Science Letters-, v. 3 6 , p. 3 5 9 ~ 3 6 2 . Turner, G., 1 9 7 1 , Argon4O-Argon3’ Dating: The optimization of irradiation parameters: Earth and Planetary Science Letters, v. 10, p. 2 2 7 - 2 3 4 . Turner, G. and Cadogan, P.H., 1 9 7 4 , Possible effects o f 3 9 A r recoil in 4 0 A r - 3 9 A r dating: Proceedings of the Fifth Lunar Conference, Geochimica et Cosmochimica Acta, Supplement 5 , v. 2, p. 1 6 0 1 - 1 6 1 5 . York, D. and Farquhar, R.M., 1 9 7 2 , The Earth’s Age and Geochronology: Oxford, Pergamon, 1 7 8 p. York, D., Kenyon, W.J. and Doyle, R.J., 1 9 7 2 , 4 0 A r - 3 9 A r ages of Apollo 1 4 and 1 5 samples: Proceedings of the Third Lunar Conference: Geochimica et Cosmochimica Acta, Supplement 3 , v. 2 , p. 1 6 1 3 - 1 6 2 2 .
75
LEAD210 DATING OF SEDIMENTS FROM SOME NORTHERN ONTARIO LAKES
R.W. DURHAM and S.R. JOSHI
AB STR A CT
F o u r t e e n s e d i m e n t c o r e s e x t r a c t e d from t w e l v e s m a l l l a k e s i n n o r t h e r n and e a s t e r n O n t a r i o were s e c t i o n e d a t 0 . 5 cm i n t e r v a l s , f r e e z e d r i e d , and t h e p a n a l y z e d f o r " ' P b , 2 2 6 R a ,a n d 1 3 7 C s . The e x c e s s "'Pb (i.e., t o t a l *"Pb l e s s t h a t p r o d u c e d from t h e d e c a y o f i n s i t u 2 2 6 R a ) c o n c e n t r a t i o n p r o f i l e s o b t a i n e d were u s e d t o c a l c u l a t e a n n u a l s e d i mentation r a t e s and t h u s t h e a g e p r o f i l e s of t h e c o r e s . The d a t i n g model assumes a c o n s t a n t f l u x o f "'Pb a t t h e l a k e s u r f a c e from t h e d e cay o f a t m o s p h e r i c 2 2 2 R n , f o l l o w e d b y a d s o r p t i o n on s u s p e n d e d p a r t i c u l a t e s which c a r r y t h e " ' P b t O tl-e l a k e s e d i m e n t s a t a c o n s t a n t r a t e . The "'Pb h a l f - l i f e o f 2 2 . 2 6 y e a r s a l l o w s s e d i m e n t s t o be d a t e d back i n time about 1 0 0 y e a r s . 1 3 7 C s concentration p r o f i l e s are used t o corr o b o r a t e t h e a n n u a l s e d i m e n t a t i o n r a t e s s i n c e t h e h o r i z o n f o r 30 y e a r h a l f - l i f e 1 3 ' C s i n l a k e sediments corresponds t o about 1958, t h e o n s e t of l a r g e - s c a l e t e s t i n g of n u c l e a r weapons. A l l t h e c o r e s had v e r y h i g h w a t e r c o n t e n t w i t h p o r o s i t i e s n e a r t h e sediment/water i n t e r f a c e v a r y i n g from 0 . 9 5 t o 0 . 9 8 and o n l y one l a k e showed any c o m p a c t i o n of t h e s e d i m e n t i n t h e t o p 1 0 em. The a n n u a l s e d i m e n t a t i o n r a t e s were g e n e r a l l y v e r y low, r a n g i n g from 0 . 4 t o 1 . 6 mm y - ' . One c o r e showed e v i d e n c e of s e v e r e p e r t u r b a t i o n o f t h e t o p two em of s e d i m e n t .
INTRODUCTION
P r e c i p i t a t i o n s c a v e n g i n g o f t h e d e c a y p r o d u c t s from a t m o s p h e r i c '*'Rn r e s u l t s i n a n a t u r a l s u p p l y o f "'Pb t o surface water resources where, f o l l o w i n g r a p i d a d s o r p t i o n o n t o s e t t l i n g p a r t i c u l a t e s , i t i s i n c o r p o r a t e d i n t h e a c c u m u l a t i n g s e d i m e n t s . The r a d i o a c t i v e d e c a y o f t h i s "'Pb w i t h a 22.26 y r h a l f - l i f e c o n s t i t u t e s a g e o l o g i c a l c l o c k f o r e s t a b l i s h i n g t h e c h r o n o l o g y of r e c e n t s e d i m e n t s . S i n c e t h e f i r s t app l i c a t i o n of "'Pb measurements i n c a l c u l a t i n g s e d i m e n t a t i o n r a t e s for r e c e n t m a r i n e a n d l a k e s e d i m e n t s (Krishnaswamy e t aZ., 1 9 7 1 ) , a number of i n v e s t i g a t o r s h a v e s u c c e s s f u l l y u t i l i z e d t h e t e c h n i q u e for d a t i n g purposes (Koide e t a l . , 1973; Robbins and E d g i n g t o n , 1 9 7 5 ; F a r m e r , 1978; O l d f i e l d e t aZ., 1 9 7 8 ; Durham and J o s h i , 1 9 8 0 a ) . The u s e of t h e t e c h nique i s i n c r e a s i n g r a p i d l y and v a r i o u s a p p l i c a t i o n s i n c l u d e s t u d i e s 3f e u t r o p h i c a t i o n i n l a k e s ( A l l a n e t a l . , 1980), the recent history of heavy m e t a l p o l l u t i o n ( C h r i s t e n s e n and C h i e n , 1 9 8 1 ) , t h e p e r s i s t e n c e o f 3 r g a n o t o x i c s (Brownlee e t a l . , 1 9 7 7 ) , a n d s o i l e r o s i o n ( G o l d b e r g e t a l . ,
1978).
I n t h i s p a p e r w e d e s c r i b e t h e d a t i n g of s e d i m e n t s from t w e l v e l a k e s i n n o r t h e r n and e a s t e r n O n t a r i o u s i n g t h e 2 1 0 P b t e c h n i q u e . T h i s work i s p a r t of a j o i n t s t u d y w i t h F i s h e r i e s and Oceans Canada of heavy i e t a l and a c i d r a i n i n p u t s t o t h e s e l a k e s i n t h e r e c e n t p a s t a n d t h e noncomitant i m p a c t on d i a t o m and c h i r o n o m i d p o p u l a t i o n s . The s e d i m e n t Zores were s e c t i o n e d and a n a l y z e d for " " P b a n d 2 2 6 R a t o o b t a i n " e x c e s s " "'Pb p r o f i l e s . The " e x c e s s " ' l O P b i s t h a t o b t a i n e d by s u b t r a c t i n g
76
t h e ‘ I O P b produced from 2 2 6 R a decay i n d e t r i t a l m a t e r i a l i n t h e s e d i m e n t i n g p a r t i c u l a t e s from t h e t o t a l ‘ l O P b . A p l i c a t i o n o f a n exponp r o f i l e s gave s e d i e n t i a l r a d i o a c t i v e d e c a y model t o t h e e x c e s s “ ‘ P b m e n t a t i o n r a t e s , ‘ l O P b a t m o s p h e r i c f l u x e s and age p r o f i l e s o f t h e s e d i m e n t s . C o r r o b o r a t i o n o f t h e a g e p r o f i l e s was o b t a i n e d by d e t e r m i n i n g 1 3 7 C s p r o f i l e s f o r t h e c o r e s . The 1 3 7 C s i n p u t t o t h e l a k e s were from n u c l e a r weapons t e s t i n g i n t h e a t m o s p h e r e which s t a r t e d on a l a r g e s c a l e i n 1 9 5 8 . The f a l l o u t from t h e s e t e s t s p e a k e d i n 1 9 6 3 - 6 4 a n d t h e n d e c r e a s e d s h a r p l y f o l l o w i n g t h e a g r e e m e n t by most o f t h e n u c l e a r powers t o end a t m o s p h e r i c weapons’ t e s t s . The 1 3 7 C s h o r i z o n i n s e d i m e n t c o r e s i s thus about 1958. EXPERIMENTAL TECHNIQUES
Sediment c o r e sampling S u r f ‘ i c i a l s e d i m e n t c o r e s were o b t a i n e d w i t h a B e n t h o s c o r e r a t t h e s t a t i o n s shown i n F i g u r e 1. C o r e s w e r e s t o r e d a t 4 ° C u n t i l s e c t i o n e d a t 0 . 5 cm i n t e r v a l s . While s e c t i o n i n g , t h e o u t s i d e 0 . 5 cm o f e a c h c o r e was r e j e c t e d t o a v o i d p o s s i b l e c o n t a m i n a t i o n o f d e e p e r l y i n g m a t e r i a l d u r i n g c o r i n g . P r e c i s e sample t h i c k n e s s e s were o b t a i n e d b y d i v i d i n g t h e wet mass o f t h e s e d i m e n t a l i q u o t by i t s b u l k d e n s i t y a n d a r e a . Figure 1
Sampling l o c a t i o n s i n northern Ontario.
Measurement o f s e d i m e n t p o r o s i t y The wet s e d i m e n t s e c t i o n s w e r e f r e e z e - d r i e d a n d t h e w a t e r c o n t e n t d e t e r m i n e d from t h e w e i g h t loss. P o r o s i t y ( 9 ) v a l u e s w e r e o b t a i n e d using the relationship
where Mw and M a r e t h e masses and p (1 g and p s ( 2 . 4 5 g c ~ n - ~ ) a r e t h e d e n s i t y e s of water and s o l i d % i n t h e s e d i m e n t , r e s p e c t i v e l y . A l l t h e c o r e s had v e r y h i g h w a t e r c o n t e n t w i t h p o r o s i t i e s n e a r t h e s e d i m e n t / w a t e r i n t e r f a c e v a r y i n g from 0 . 9 5 t o 0 . 9 8 and o n l y one l a k e
77
(Lake Opeongo) showed a n y c o m p a c t i o n ' o f t h e s e d i m e n t i n t h e t o p 1 0 em. Measurement o f 2 1 0 P b , 2 2 6 R a , and 1 3 7 C s a c t i v i t i e s The d r i e d s e c t i o n s were a n a l y z e d f o r ' l O P b , 2 2 6 R a ya n d 'j7Cs u s i n g p r o c e d u r e s d e s c r i b e d e a r l i e r ( J o s h i a n d Durham, 1 9 7 6 ) . B r i e f l y , t h e ,nethod i n v o l v e d s e p a r a t i o n o f 13'Cs f r o m * l O P b a n d 2 2 6 R a w i t h a c a r b o n a t e f u s i o n f o l l o w i n g a d d i t i o n of r e s p e c t i v e c a r r i e r s and removal of s i l i c a by c y c l i c HN03-HF t r e a t m e n t s . The Pb a n d B a c a r r i e r s , c o n t a i n i n g 2 1 0 P b a n d 2 2 6 R a , were s e p a r a t e d f r o m Ca u s i n g 70-72% H N O j p r e c i p i t a t i o n and a l c o h o l - e t h e r s e p a r a t i o n t e c h n i q u e s b e f o r e m u t u a l s e p a r a t i o n o f Pb and Ba u s i n g a n i o n e x c h a n g e . 2 1 0 P b a n d 2 2 6 R awere d e t e r m i n e d by C - a n d a - c o u n t i n g o f Pb a n d B a c h r o m a t e p r e c i p i t a t e s , r e s p e c t i v e l y , a f t e r allowing s u i t a b l e ingrowth p e r i o d s f o r t h e i r d a u g h t e r s . 1 3 7 C s was r e moved from t h e c a r b o n a t e f u s i o n f i l t r a t e by a b s o r p t i o n on ammonium n o l y b d o p h o s p h a t e . F o l l o w i n g r a d i o c h e m i c a l p u r i f i c a t i o n i t was p r e c i 7 i t a t e d as C s 3 B i 2 1 9 a n d @ - c o u n t e d . DETERMINATION OF SEDIMENTATION RATES
Krishnaswamy e t aZ., ( 1 9 7 1 ) h a v e shown t h a t i n h o m o g e n e o u s l y d e p o s i t i n g sediments t h e e x c e s s ' l O P b c o n c e n t r a t i o n A ( x ) i n pCi g - ' , a t d e p t h x b e l o w t h e s e d i m e n t / w a t e r i n t e r f a c e i s g i v e n by
vhere P i s t h e f l u x o f e x c e s s '"Pb a t t h e sediment/water i n t e r f a c e ( p C i em-' y - ' ) , A i s t h e d e c a y c o n s t a n t o f '"Pb ( 0 . 0 3 1 1 y-'), D i s t h e { n s i t u d e n s i t y of t h e sediment ( g e m q 3 ) , and S t h e s e d i m e n t a t i o n r a t e (em y - ' ) . R o b b i n s a n d E d g i n g t o n (1975) r e f i n e d t h i s m o d e l t o t a k e i n t o sccount compression o f t h e s e d i m e n t a r y m a t e r i a l u s i n g a r e l a t i o n s h i p 3etween c o m p a c t i o n a n d p o r o s i t y ( A t h y , 1 9 3 0 ) . They m o d i f i e d e q u a t i o n (1) t o g i v e
w i t h I$ b e i n g ';he p c r c s i t y a t t h e s e d h e n k / w a k e r i r , t e r f a c e ar,d So t h e surfacg s e d i m e n t a t i o n r a t e . As d i s c u s s e d i n a n o t h e r p u b l i c a t i o n (Durham a n d J o s h i , 1 9 8 0 a ) w e have p r e f e r r e d t o u s e t h e f o l l o w i n g e x p r e s s i o n , a l s o g i v e n b y Athy (1930), t o e v a l u a t e e q u a t i o n ( 3 ) a n a l y t i c a l l y :
where
6 is a constant.
S u b s t i t u t i o n of ( 4 ) i n (3) g i v e s
Substitution of ( 5 ) i n ( 2 ) then gives
L i n e a r r e g r e s s i o n a n a l y s i s o f t h e l o g t r a n s f o r m o f e q u a t i o n (6) g i v e s v a l u e s o f S o y t h e l i n e a r s u r f a c e s e d i m e n t a t i o n r a t e , P , t h e 2 1 'Pb f l u x a t t h e s e d i r n e n t / w a t e r i n t e r f a c e , a n d w, t h e mass s e d i m e n t a t i o n r a t e
(=so(l-@o)Ps).
78
Assuming a constant supply of, atmospheric 'lOPb and a constant sedimentat ion rate, the age profile of the sediment core is represented by
Since the sediment surface 'l'Pb concentration, A(o), corresponds to P/So(l-Q, ) p s , it follows from (6) and (7) that the corFespondence between the age of a core section and its depth is given by
As these cores showed little change in porosity with depth in the surface layers there was no compaction and t approximates x/So. RESULTS 'I
'Pb Profiles
The excess "'Pb profiles are given in Figure 2. The errors in the sedimentation rates were evaluated by linear regression analysis of the core data. The regression lines for each core A
Yi
=
a
+ bXi
(9)
were obtained by log transformation of the excess 'lOPb concentrations to give values of Yi and calculation of values of f(x) from equation (5) to give Xi values. It can be seen from equation (7) that the slope of the regression line, b y corresponds to ?,/Soy while the intercept, a , when X.=O, is P/w. The regression lines are a l s o plotted in Figure 2, while the computed values of P, S , and w are given in Table 1. As discussed earlier (Durham and JosRi, 198l), the standard deviations given in Table 1 were obtained using the following equations for the variances of So and w :
Age profiles of the sediment cores have been calculated using equation (8) and are given in Figure 3. 1 3 7 ~ measurements s
The 1 3 7 C s profiles obtained for each core are depicted in Figure
4. The profiles of all fourteen cores.show the usual shape for lake
sediments consisting of a relatively high concentration for the top portion followed by a drop in concentration below a few cm. Since large scale atmospheric testing of nuclear weapons started in 1958, it would be expected that this drop in 1 3 7 C s concentration corresponds to a r ' 1 3 7 Chorizon" ~ and would coincide with this date. The depths corresponding to 1958, obtained from the age-versus-depth curves in Figure 3 , are in reasonably good agreement with the observed 1 3 7 C s horizon in each case except for Barker Lake where diffusion of 1 3 7 C sis observed in the top few cm.
DISCUSSION The results presented in Table 1 show that the surface sedimentation rates in these small lakes vary from 0.37 to 1.65 mm y-l (3.9 to 39.3 mg cm-'y-'). These values are similar to those obtained earlier
Table 1 . Lake
Parameters from excess '"Pb
Location Lat.N. L0ng.W.
Porosity o f surface sediment (Q0)
and 1 3 7 C s profiles of sediment cores Sedimentation rate, (mm y-'1
F l u x of excess Hass sedimentation 137cs 'lOPb, P rate, ~ = s ~ ( l - @ ~ )Horizon p ~
(pCicm-'y-')
(mp. cm-'y-l f
(em)
Antoinne
45'00'
76'44'
0.982
1.28
? 0.11
0.143
5.6
0.5
3.2
Ardoch
44'56'
76'52'
0.979
1.04
A 0.17
0.048
5.3 ? 0.9
1.8
Barker
45'07'
77'23'
0.941
1.17
A 0.14
0.526
16.9
Opeongo E . Arm
45'42'
78'23'
0.957
0.37
A 0.04
0.072
3.9
Opeongo N. Arm
45"42'
78'23'
0.968
1.55
f
0.25
Opeongo S . Arm
45'42'
78"23'
0.969
1.55
South Bay
45'39'
81'50'
0.885
Sucker
45'43'
81"52'
Windfall
45'42'
Upper Headwater 4 7 ' 0 4 '
? 2.0 f
Diffuse ( 2 - 4 )
0.4
2.3
0.132
1 2 . 3 f 2.0
3.3
? 0.28
0.092
11.8
f: 2 . 1
2.5
1.39
? 0.24
0.655
39.3
f
6.8
3.0
0.984
1.65
f
0.54
0.021
6.5
+_
2.1
2.4
82'04'
0.985
1.26
f
0.35
0.051
4.6
f
1.3
2.6
84"24'
0.971
0.96
? 0.06
0.933
6.9
? 0.4
3.0
Turkey
47'03'
84'25'
0.956
0.67
? 0.06
0.620
7 . 0 f: 0 . 6
3.5
Little Turkey
47'03'
84'25'
0.972
0.79
f
0.04
0.400
5.5
? 0.3
3.0
Savanne
48'50'
90'06'
0.948
0.49
f
0.05
0.068
6.2
? 0.6
1.5
Henderson
48"49'
90"18'
0.953
1.48
? 0.35
0.077
17.1 ? 4.0
4.8 ___-
80
1001 h
I%\+ 1
Antoinne
'\\\
-I-
4
\.
10
Barker
Ardoch
1i
F i g u r e 2a-2e
Excess
*l0pb profiles
i n sediment cores
1001
Opeongo North Arm
Opeongo South Arm + + I -
\\\ '\
+\\\,
+f\\\\
$\, -I++ +*\
0
It
It
I
I!
yt
I
2
4
6
8
i
10
I2 0
7
2
4
6
8
10
12
0 14
Measured depth in core (cm)
F i g u r e 2b
2
4
6
8
lb
1'2
81
101
L
'
Windfa II
Sucker
South Bay
t
.01 li
0
I
2
I
4
I
6
8
I
10
I
12 0
I
I
I
2
4
6
1
0 8
I
2
I
4
1
6
I
8
Measured depth in core (cm) F i g u r e 2c W
l
Upper Headwater
t
F i g u r e 2d
82
c \
Savanne
Henderson
\y
4-
f+i
\
I
2
\
I
4
t
I
I
I
I
I
6 0
2
4
6
a
Measured depth in core (cm) F i g u r e 2e
-'
(Durham a n d J o s h i , 1 9 8 0 b ) f o r s m a l l l a k e s Mata ami ( 0 . 8 1 mm y-'; 59 mg y-l) i n northwest em 2 y I ) a n d Q u e v i l l o n ( 0 . 5 2 mm y-'; 35 mg cm Quebec, b u t are c o n s i d e r a b l y lower t h a n t h o s e d e r i v e d f o r f o u r small p r a i r i e l a k e s ( 4 . 7 t o 6 . 6 mm y-') i n S a s k a t c h e w a n (Durham e t aZ., 1 9 8 0 ) w h e r e s p r i n g snow-melt p r o v i d e s h i g h s e d i m e n t l o a d i n g . These s e d i m e n t a t i o n r a t e s a r e a l s o s i m i l a r t o t h o s e r e p o r t e d f o r much l a r g e r a n d d e e p e r l a k e s - i n t h e Great L a k e s s y s t e m w h i c h h a v e r a t e s r a n g i n g f r o m 0 . 1 - 2 . 1 mm y ( 4 . 8 - 9 3 mg em-" y f o r L a k e s Huron (Durham a n d J o s h i , 1980a), O n t a r i o (Farmer, 1 9 7 8 ) , Michigan (Robbins and Edgington, 1 9 7 5 ) a n d S u p e r i o r @mp e t aZ., 1 9 7 8 ) . A much w i d e r r a n g e o f 0 . 2 - 8 . 5 mm y-l ( 1 3 - 2 0 4 mg ern y-l) was m e a s u r e d f o r Lake E r i e ( N r i a g u e t aZ., 1 9 7 9 ) w h i c h i s much s h a l l o w e r t h a n t h e o t h e r Great L a k e s . One c o r e (Opeongo n o r t h arm) c o n t a i n e d v e r y l i t t l e e x c e s s 2 1 0 P b i n t h e t o p 2 em c o m p a r e d t o t h e n e x t f e w em. T h i s may b e d u e t o s e d i ment r e w o r k i n g i n t h e r e c e n t p a s t whereby o l d e r s e d i m e n t a r y m a t e r i a l h a s o v e r l a i n t h a t d e p o s i t e d e a r l i e r . T h i s a p p a r e n t l o s s o f 2 1 0 P b can not be a t t r i b u t e d t o p o s s i b l e b i o l o g i c a l o r chemical r e a c t i o n s occurr i n g , s u c h as m e t h y l a t i o n t o v o l a t i l e t e t r a m e t h y l - l e a d , i n t h e t o p f e w em s i n c e 13'Cs - a n i s o t o p e h a v i n g d i f f e r e n t c h e m i c a l b e h a v i o u r a n d b i o l o g i c a l i n t e r a c t i o n s t h a n 2 1 0 P b - a l s o shows a s i m i l a r d r o p i n concentration i n t h i s region (Figure h ) .
83
AGE versus DEPTH PROFILES
8,
&Henderson
Vndfa"
p Opeongo North Arm
1860
Opeongo East Arm
1830
\
1800-
0
1
2
3
4
5
6
7
8
9 1 0
11
Little Turkey
Turkey Lake 1 2 1 3 1 4
Depth in core (cm)
Figure 3
'lOPb a g e v e r s u s d e p t h in c o r e .
The "'Pb fluxes at the sediment/water interface range between 3.021 and 0.933 pCi em-' y-' for these lakes. Moore and Poet (1976) have calculated direct atmospheric 'lOPb fluxes from measurements of 'lOPb concentrations in rainfall. They have concluded that the atmospheric 'l"Pb flux ranges from 0.24 to 0.92 pCi em-' y-' for the north temperature zone. Previous measurements of 'lOPb fluxes in Lake Huron (Durham and Joshi, 198@a), Lake Superior (Durham and Joshi, 1981), Lake Matagami in Quebec (Durham and Joshi 1980b), and those reported by Robbins and Edgington (1975) for Lake Michigan cover a range similar to that reported by Moore and Poet (1976). However, several of the
84
f
ol 0
. ....................... 2
d
B
B
0
0
2
1
B
8
DO
2
1
B
8
0
$20
2
I
6
B
a 1 2 0
2
4
B
8
0 1 2
i
Figure 4
I3'Cs
...
0
2
1
b
8
0 1 2
0
2
4
6
8
W
L
p r o f i l e s i n sediment cores.
n o r t h e r n O n t a r i o l a k e s ( A r d o c h , O-oeongo, S u c k e r , ! d i n d f a l l , S a v a n n e , a n d f l u x e s lower t h a n e x n e c t e d from t h e d a t a of H e n d e r s o n ) d i s p l a y "'Pb Moore a n d P o e t . It i s u n l i k e l y t h a t t h e atmospheric f l u x of 'l'Pb w o u l d b e d e p r e s s e d s o much i n t h e v i c i n i t y o f t h e s e l a k e s . A m o r e p r o b a b l e r e a s o n for t h e l o w e r f l u x v a l u e s a t t h e s e d i m e n t / w a t e r i n t e r by a m i n o a c i d s i n t h e l a k e w a t e r t o g i v e f a c e i s c o m p l e x a t i o n of "'Pb s o l u b l e l e a d Compounds ( R i c k a r d a n d N r i a g u , 1 9 7 8 ) . The l a k e s i n v o l v e d i n t h i s s t u d y are a l l l o c a t e d i n f o r e s t e d areas w i t h t h e d r a i n a g e b a s i n s p r o v i d i n g s o u r c e s of o r g a n i c a c i d s from t h e decay of v e g e t a t i o n .
A C KN 0WL EDGEM ENT S We would l i k e t o t h a n k Dr. M.G. J o h n s o n a n d Dr. R . L . Thomas f o r p r o v i d i n g t h e s e d i m e n t c o r e s and r a i s o n d l @ t r ef o r t h i s s t u d y and R . Dawson, L . R o l s t o n , J . A . F i t z G e r a l d a n d S . L i v e r m o r e f o r c a r r y i n g o u t t h e a n a l y t i c a l work. REF E R E N C E S C I T E D A l l a n , R . J . , W i l l i a m s , J.D.H., J o s h i , S . R . a n d W a r w i c k , W . F . , 1 9 8 0 , H i s t o r i c a l changes and r e l a t i o n s h i p t o i n t e r n a l l o a d i n g of s e d i ment p h o s p h o r u s forms i n h y p e r t r o p h i c P r a i r i e l a k e s : Jour. Environ. Qual., v . 9, p . 199-206. Athy, L.F., rocks:
1930, D e n s i t y , p o r o s i t y , and compaction of sedimentary Bull. Amer. Assoc. P e t . Geol., v. 14, p. 1-24.
S t r a c h a n , W . M . J . and J o s h i , S . R . , 1977, D i s Brownlee, B . , Fox, M.E., t r i b u t i o n of dehydroabietic a c i d i n sediments adjacent t o a k r a f t J o u r . F i s h . Res. Board v . 34, p. 838p u l p and p a p e r m i l l : 843.
Can:,
C h r i s t e n s e n , E.R. and Chien, N . K . , 1981, F l u x e s of a r s e n i c , a n d c a d m i u m t o G r e e n Bay a n d L a k e M i c h i g a n s e d i m e n t s : S c i . T e c h n o l . , v . 1 5 , p . 553-558.
lead, zinc, Environ.
Durham, R.W. a n d J o s h i , S . R . , 1 9 8 0 a , R e c e n t s e d i m e n t a t i o n r a t e s , "'Pb f l u x e s , and p a r t i c l e s e t t l i n g v e l o c i t i e s i n Lake Huron, L a u r e n t i a n Great L a k e s : Chem. G e o l . , v . 3 1 , p . 5 3 - 6 6 .
, 1 9 8 0 b , The ' l O P b a n d 1 3 7 C s p r o f i l e s i n s e d i m e n t c o r e s f r o m Lakes Matagami and q u e v i l l o n , n o r t h w e s t Quebec, Canada: Can. Jour. E a r t h S c i . , v. 1 7 , p . 1746-1750. , 1981, S e d i m e n t a t i o n r a t e s i n w e s t e r n Nipigon Bay, Lake S u p e r i o r u s i n g t h e "'Pb method: Sci. Total Environ. (Submitted).
85
Durham, R.W., Joshi, S.R. and A l l a n , , R . J . , 1980, Radioactive dating of sediment cores from four contiguous lakes in Saskatchewan, Canada: Sci. Total Environ., v. 1 5 , p. 65-71. Farmer, J.G., 1978, The determination of sedimentation rates i n Lake Ontario using the "'Pb dating method: Can. Jour. Earth Sci., v. 15, p . 431-437. Goldberg, E.D., Hodge, V., Koide, M., Griffin, J . , Gamble, E., Bricker, O.P., Matisoff, G . 9 Holden, G.R. and Braun, R., 1 9 7 8 , A pollution history of Chesapeake Bay: Geochim. Cosmochim. A c t a , v. 4 2 , p. 1413-1425. Joshi, S.R. and Durham, R.W., 1 9 7 6 , Determination of "'Pb, I3'Cs in sediments: Chem. Geol., v. 1 8 , p. 155-160.
2 2 6 R a , and
Kemp, A.L.W., Williams, J.D.H., Thomas, R.L. and Gregory, M.L., 1978, Impact o f man's activities o n the chemical composition of the sediments of Lakes Superior and Huron: Water, Air and Soil P o l Jution, v. 10, p. 381-402. Koide, M., Bruland, K.W. and Goldberg, E.D., 1 9 7 3 , Th--228/Th-232 and Pb-210 geochronologies in marine and lake sediments: Geochim. Cosmochim. Acta, v. 37, p. 1171-1187. Krishnaswamy, S., L a l , D., Martin, J.M. and Maybeck, M . , 1971, Geochronology of lake sediments: Earth Planet Sci. Lett., v. 11, p. 407-414. Moore, H.E. and Poet, S.E., 1 9 7 6 , "'Pb fluxes determined from "'Pb and 2 2 6 R a soil profiles! Jour. Geophys. Res., v . 8 1 , p. 10561058. Nriagu, J.O., Kemp, A.L.W., Wong, H.K.T. and Harper, N., 1 9 7 9 , Sedimentary record of heavy metal pollution in Lake Erie: Geochim. Cosmochim. Acta, v. 4 3 , p . 247-258. Oldfield, F., Appleby, P.G. and Battarbee, R.W., 1978, Alternative 'lOPb dating: results from the New Guinea Highlands and Lough Erne: Nature - (London), v. 271, p. 339-342. Eickard, D.T. and Nriagu, J.O., 1 9 7 8 , Aqueous environmental chemistry of lead, in Nriagu, J.O., ed., The Biogeochemistry o f Lead in the Environment, Part A: Elsevier/Holland Biomedical Press. riobbins, J . A . and Edgington, D.N., 1 9 7 5 , Determination of recent sedimentation rates in Lake Michigan using Pb-210 and Cs-137; Geochim. Cosmochim. Acta, v. 39, p. 285-304.
FISSION-TRACK DATING
NANCY D. NAESER and CHARLES W NAESER
ABSTRACT F i s s i o n t r a c k s a r e z o n e s o f i n t e n s e damage t h a t r e s u l t when f i s s i o n fragments p a s s through a s o l i d . S e v e r a l n a t u r a l l y o c c u r r i n g i s o t o p e s u n d e r g o s p o n t a n e o u s f i s s i o n , b u t o n l y 2 3 8 Up r o d u c e s a s i g n i Spontaneous f i s s i o n o c c u r s f i c a n t number o f t r a c k s o v e r g e o l o g i c t i m e . a t a known r a t e , a n d s o by d e t e r m i n i n g t h e number o f t r a c k s a n d t h e amount o f u r a n i u m i n a m i n e r a l or g l a s s , i t i s p o s s i b l e t o c a l c u l a t e i t s age. Many m a t e r i a l s c o n t a i n u r a n i u m , b u t b e c a u s e o f f a c t o r s s u c h as wanium abundance, t r a c k r e t e n t i o n , and r e l a t i v e a b u n d a n c e , o n l y g l a s s and z i r c o n a r e r o u t i n e l y d a t e d i n Q u a t e r n a r y r o c k s . Advantages o f f i s s i o n t r a c k d a t i n g f o r Q u a t e r n a r y m a t e r i a l s i n Disadvantclude low c o n t a m i n a t i o n and t y p i c a l l y small s a mp le s n e e d e d . a g e s i n c l u d e few t r a c k s i n s a m p l e s l e s s t h a n 1 0 0 , 0 0 0 y e a r s o l d , a n d a n n e a l i n g of t r a c k s i n g l a s s a t a m b i e n t s u r f a c e t e m p e r a t u r e s . Q u a t e r n a r y m a t e r i a l s d a t e d by f i s s i o n t r a c k s i n c l u d e v o l c a n i c a s h , archaeological m a t e r i a l s , t e k t i t e s and impact g l a s s , and n a t u r a l clinker. TEEORY AND METHODS A f i s s i o n t r a c k i s t h e z o n e o f i n t e n s e damage f o r m e d when a f i s sion fragment p a s s e s t h r o u g h a s o l i d . S e v e r a l n a t u r a l l y o c c u r r i n g i s o t o p e s u n d e r g o s p o n t a n e o u s f i s s i o n , b u t o n l y 2 3 8 Uh a s a f i s s i o n h a l f life (9.9 x y e a r s ) t h a t i s s u f f i c i e n t l y s h o r t t o produce a s i g n i f i c a n t number o f t r a c k s o v e r g e o l o g i c t i m e . Yet e v e n t h e s p o n t a n e o u s f i s s i o n o f 2 3 8 U i s a r a r e e v e n t . More t h a n a m i l l i o n 2 3 8 U a t o m s d e c a y by alpha e m i s s i o n f o r e a c h f i s s i o n d e c a y . When a n a t o m s u c h as 2 3 8 U f i s s i o n s , t h e n u c l e u s b r e a k s up i n t o t w o l i g h t e r n u c l e i , o n e a b o u t 9 0 atomic mass u n i t s a n d t h e o t h e r a b o u t 135 a . m . u . , w i t h t h e l i b e r a t i o n 3f a b o u t 2 0 0 MeV o f e n e r g y . The two h i g h l y c h a r g e d n u c l e i r e c o i l i n opposite d i r e c t i o n s and d i s r u p t t h e e l e c t r o n b a l a n c e of t h e atoms i n t h e m i n e r a l l a t t i c e or g l a s s a l o n g t h e i r D a t h . T h i s d i s r u p t i o n c a u s e s the p o s i t i v e l y charged i o n s i n t h e l a t t i c e t o r e p u l s e each o t h e r and f o r c e t h e m s e l v e s i n t o t h e c r y s t a l s t r u c t u r e , f o r m i n g t h e t r a c k or damage zone ( F l e i s c h e r e t a l . , 1975). The new t r a c k i s o n l y a few angstroms w i d e a n d i s a b o u t 1 0 - 2 0 u m i n l e n g t h . The t r a c k i s l o n g e r i n low d e n s i t y m i n e r a l s a n d g l a s s e s t h a n i n d e n s e m i n e r a l s s u c h as z i r c o n . A t r a c k i n i t s n a t u r a l s t a t e can only b e observed w i t h a n e l e c t r o n m i c r o s c o p e , b u t a c h e m i c a l e t c h a n t c a n e n l a r g e t h e damage z o n e s o t h a t I t car, b e o b s e r v e d i n a n o p t i c a l m i c r o s c o - , e a t i n t e r m e d i a t e m a g n i f i c a t i o n s (x2OO-500) ( F i g u r e 1 ) . Common e t c h a n t s u s e d i n c l u d e : n i t r i c a c i d ( f o r a p a t i t e ) , h y d r o f l u o r i c a c i d ( f o r g l a s s a n d m i c a s ) , conoen?rated b a s i c s o l u t i o n s ( f o r sphene, and b a s i c f l u x e s ( f o r z i r c o n ) ( F l e i s c h e r e t a l . , 1 9 7 5 , T a b l e 2 - 2 ; Gleadow e t a l . , 1 9 7 6 ) .
88
Figure 1
F i s s i o n t r a c k s i n a p a t i t e from t h e r a d i a l d i k e a t S h i p r o c k , Few M e x i c o .
T r a c e a m o u n t s of 2 3 8 U o c c u r i n a number o f common m i n e r a l s a n d g l a s s e s . B e c a u s e 2 3 8 Uf i s s i o n s s p o n t a n e o u s l y a t a c o n s t a n t r a t e , f i s s i o n t r a c k s c a n b e u s e d t o d a t e t h e s e n a t e r i a l s . The t e c h n i q u e s u s e d for d a t i n g h a v e b e e n d e v e l o p e d by p h y s i c i s t s a n d g e o l o g i s t s o v e r t h e l a s t 20 y e a r s . E a r l y d e v e l o p m e n t o f t h e m e t h o d h a s b e e n r e v i e w e d b y F l e i s c h e r e t a l . ( 1 9 7 5 ) a n d N a e s e r ( 1 9 7 9 ) . The a g e o f a m i n e r a l o r g l a s s c a n b e c a l c u l a t e d f r o m t h e amount o f u r a n i u m a n d number o f s p o n Spontaneous t r a c k d e n s i t y i s u s u a l l y taneous t r a c k s that it contains. determined by: (1) p o l i s h i n g t h e s u r f a c e o f t h e s p e c i m e n , ( 2 ) e n l a r g i n g t h e f i s s i o n t r a c k s i n t e r s e c t i n g t h i s s u r f a c e by e t c h i n g , a n d ( 3 ) c o u n t i n g t h e number o f t r a c k s p e r u n i t a r e a u n d e r a n o p t i c a l m i c r o s c o p e , g e n e r a l l y a t ma n i f i c a t i o n s o f x500 t o x2500. The r e l a t i v e abundance of 2 3 8 U and ‘35U i s c o n s t a n t i n r o c k s , and t h u s t h e e a s i e s t a n d m o s t a c c u r a t e way t o d e t e r m i n e t h e amount o f u r a n i u m i s t o c r e a t e a new s e t o f f i s s i o n t r a c k s by i r r a d i a t i n g t h e s a m p l e i n a n u c l e a r r e a c t o r w i t h t h e r m a l n e u t r o n s , w h i c h i n d u c e f i s s i o n i n 2 3 5 i J , The r e s u l t i n g i n d u c e d t r a c k d e n s i t y i s a f u n c t i o n o f t h e amount o f u r a n i u m i n The d o s e t h e sample and t h e n e u t r o n dose it r e c e i v e d i n t h e r e a c t o r . may b e d e t e r m i n e d by i n c l u d i n g w i t h t h e s a m p l e s a s t a n d a r d o f known u r a n i u m c o n t e n t ( C a r p e n t e r a n d Reimer, 1 9 7 4 ; F l e i s c h e r e t al., 1 9 7 5 , T a b l e 11-11. A f i s s i o n - t r a c k age i s c a l c u l a t e d using the spontaneous t r a c k ‘U ( p s ) , t h e n e u t r o n - i n d u c e d t r a c k d e n s i t y 2 f r o n 5U d e n s i t y from ( p i ) , a n d t h e t h e r m a l n e i l t r o n f l i l e n c e (0, I n n e u t r o n s cm ), ( ? r i c e and W a l k e r , 1963; Naeser, 1 9 6 7 ) :
A
=
-1 I n [l A,
t
d
p A
014
---I6 - 3 PAX i2’
X A = t o t a l d e c a y c o n s t a n t for 2 3 8 U ( 1 . 5 5 1 x 1 0 - ” y r - l ) d
A,
f
= decay c o n s t a n t
for s p o n t a n e o u s f i s s i o n o f 2 3 8 U
( 6 . 8 5 x 1 0 - 1 7 y r - 1F; l e i s c h e r a n d P r i c e , 1Q64a)
( 7 . 0 3 x i0-17yr-’; F i o b e r t s e t aZ., 1 9 6 8 ) ’ ( 8 . 4 2 x l ~ ) - - ’ ~ y r - - ‘S; p a d a v e c c h i a a n d F a h n , 1 9 6 7 ) ’Value p r e f e r r e d by a u t h o r s .
89
cr
= cross-section
(580 x
f o r t h e r m a l n e u t r o n - i n d u c e d f i s s i o n of
2 3 5 TU7
1 0 - ~ 4 C m ~ )
I
= isotopic ratio
A
= age i n y e a r s
2 3 5 U / 2 3 8 U (7.252 x
and
S e v e r a l f a c t o r s d e t e r m i n e i f a sample can b e d a t e d b y t h e f i s s i o n t r a c k method. F i r s t l y , t h e sample m u s t c o n t a i n a m i n e r a l or g l a s s o f a p p r o p r i a t e uranium c o n t e n t . I n Q u a t e r n a r y s a m p l e s t h e r e must be enough u r a n i u m t h a t a s t a t i s t i c a l l y s i g n i f i c a n t number of t r a c k s can be counted i n a r e a s o n a b l e t i m e . S e c o n d l y , t r a c k s must b e r e t a i n e d once t h e y a r e f o r m e d , or t h e a p p a r e n t a g e w i l l b e a n o m a l o u s l y young. S e v e r a l e n v i r o n m e n t a l f a c t o r s c a n c a u s e t h e l o s s or " a n n e a l i n g " of s p o n t a n e o u s t r a c k s once t h e y a r e formed ( F l e i s c h e r e t aZ., 1975; H a r r i s o n e t aZ., 1979), b u t by f a r t h e most common c a u s e i s h e a t i n g , which c a n c a u s e p a r t i a l to c o m p l e t e f a d i n g of s p o n t a n e o u s f i s s i o n t r a c k s .
Data on t h e t e m p e r a t u r e s r e q u i r e d f o r a n n e a l i n g h a v e b e e n d e t e r mined by (1) e x t r a p o l a t i n g l a b o r a t o r y h e a t i n g e x p e r i m e n t s t o g e o l o g i c time ( N a e s e r and F a u l , 1969) and (2) m e a s u r i n g a g e - d e c r e a s e w i t h i n c r e a s i n g d e p t h and t e m p e r a t u r e i n deep d r i l l h o l e s from a r e a s where t h e r c c k s have u n d e r g o n e h e a t i n g of known d u r a t i o n ( N a e s e r , 1981). Such s t d d i e s h a v e shown t h a t t h e a n n e a l i n g t e m p e r a t u r e depends on t h e mine r a l involved--different minerals anneal at d i f f e r e n t temperatures-and t h e d u r a t i o n of h e a t i n g ; t h e l o n g e r a m i n e r a l i s h e a t e d , t h e l o w e r t h e t e m p e r a t u r e t h a t i s r e q u i r e d t o a n n e a l i t s t r a c k s . The t r a c k i s s t a b l e i n most non-opaque m i n e r a l s a t t e m p e r a t u r e s o f 8 0 C or l e s s b u t f i s s i o n t r a c k s i n n a t u r a l g l a s s e s a r e a f f e c t e d a t much l o w e r t e m p e r a t u r e s ( S e w a r d , 1979; N a e s e r e t a l . , 1980b). By d a t i n g a m i n e r a l i t i s o f t e n p o s s i b l e to d e t e r m i n e i f i t has e v e r been h e a t e d above i t s c r i t i c a l t e m p e r a t u r e and when i t l a s t c o o l e d below t h i s t e m p e r a t u r e . Although a n n e a l i n g c a n c a u s e p r o b l e m s i n det e r m i n i n g t h e p r i m a r y a g e o f s a m p l e s , i t i s a o o w e r f u l method f o r s t u d i e s of t h e i r t h e r m a l h i s t o r y . These s t u d i e s h a v e b e e n d i r e c t e d m a i n l y a t o l d e r r o c k s , for d e t e r m i n i n g u D l i f t r a t e s and t h e r m a l h i s t o r y o f s e d i m e n t a r y b a s i n s and m i n e r a l i z a t i o n (e.g., Wagner e t aZ., 1977; N a e s e r , 1979; B r y a n t and N a e s e r , 1980; N a e s e r e t a l . , 1 9 8 0 a ; B r i g g s e t aZ., 1981), b u t s e v e r a l Q u a t e r n a r y s t u d i e s have a l s o made u s e o f annealing.
E t c h i n g s t u d i e s h a v e shown t h a t t r a c k s c a n b e r e v e a l e d i n more t h a n 1 5 0 m i n e r a l s and g l a s s e s ( F l e i s c h e r e t aZ., 1975), b u t t h e comb i r - a t i o n of s u c h f a c t o r s as typical uranium content, annealing chara c t e r i s t i c s , and r e l a t i v e abundance r e s u l t s i n v e r y few m i n e r a l s being used f o r d a t i n g . Z i r c o n and g l a s s a r e t h e o n l y m a t e r i a l s t h a t a r e dated routinely i n Quaternary rocks. Z i r c o n a n d g l a s s r e q u i r e d i f f e r e n t methods b e c a u s e g l a s s from a s i q g l e s o u r c e t e n d s t o have uniform uranium c o n t e n t whereas z i r c o n c r y s t a l s from t h e same s o u r c e t e n d t o h a v e v e r y inhomogeneous u r a n i u m distributions. Z i r c o n r e q u i r e s t h e u s e o f t h e e x t e r n a l d e t e c t o r method ( N a e s e r , Because u r a n i u m c a n be d i s t r i b u t e d inhomogeneously b o t h w i t h i n a n d between z i r c o n c r y s t a l s , i t i s n e c e s s a r y t o c o u n t t h e i n d u c e d t r a c k s p r o d u c e d from t h e same a r e a s of a c r y s t a l i n which f o s s i l t r a c k s a r e counted. I n t h e e x t e r n a l d e t e c t o r method t h e f o s s i l t r a c k s a r e c o u n t e d of t h e c r y s t a l and t h e i n d u c e d t r a c k s a r e c o u n t e d i n a d e t e c t o r t h a t c o v e r e d t h e c r y s t a l mount d u r i n g n e u t r o n i r r a d i a t i o n . (
1979) ( F i g u r e 2).
Glass c a n b e d a t e d by t h e p o p u l a t i o n method ( N a e s e r , 1979) ( F i g u r e 3 ) . Because a l l o f t h e g l a s s f r o m a s i n g l e s o u r c e h a s a s i m i - . l a r u r a n i u m c o n c e n t r a t i o n , i t i s p o s s i b l e t o d e t e r m i n e t h e f o s s i l and i n d u c e d t r a c k d e n s i t i e s from d i f f e r e n t s p l i t s of t h e s a m p l e . One s p l i t i s mounted i n e p o x y , p o l i s h e d , and e t c h e d f o r t h e f o s s i l t r a c k d e n s i t y
90 SAMPLE
I
1
SEPARATE MINERAL
m 1
MOUNT IN EPOXY OR TEFLON GRIND & POLISH
I
4
1-1
ETCH
Figure 2 Steps involved in obtaining a fission-track age using the external detector method.
ADD EXTERNAL DETECTOR I
+ STAN'DARDS
I
ETCH DETECTOR
MOUNT ON SLIDE ~~
4
PI
\ /
COUNT
X
NEUTRON 'DOSE (+)
1
AGE
d e t e r m i n a t i o n . The s e c o n d s p l i t i s i r r a d i a t e d , and t h e n mounted, p o l i s h e d , and etched ( i t i s standard p r a c t i c e t o e t c h both groups a t t h e same t i m e ) . The i r r a d i a t e d s p l i t c o n t a i n s b o t h f o s s i l and i n d u c e d t r a c k s ; p r e i r r a d i a t i o n a n n e a l i n g t o remove f o s s i l t r a c k s i s n o t recommended b e c a u s e i t c a n a l t e r t h e e t c h i n g c h a r a c t e r i s t i c s and c h e m i s t r y o f g l a s s . The f o s s i l t r a c k d e n s i t y (p,) i s s u b t r a c t e d from t h e t o t a l t r a c k d e n s i t y i n t h e i r r a d i a t e d sample ( p s -!- p i ) t o a r r i v e a t t h e i n d u c e d t r a c k d e n s i t y ( p i ) . The amount o f g l a s s t h a t must b e c o u n t e d d e p e n d s on s e v e r a l f a c t o r s , i n c l u d i n g i t s u r a n i u m c o n t e n t , a g e , and vesicularity. Naeser ( 1 9 7 6 ) d e t a i l s t h e l a b o r a t o r y methods. ADVANT AGES AND L I M I T ATIONS
One a d v a n t a g e o f f i s s i o n - t r a c k d a t i n g i s t h a t c o n t a m i n a t i o n i s m i n i m i z e d . I n c o n v e n t i o n a l r a d i o c a r b o n and K - A r d a t i n g , b u l k s a m p l e s must b e a n a l y z e d . C o n t a m i n a t i o n o f a I 4 C s a m p l e w i t h r e c e n t c a r b o n r e s u l t s i n a y o u n g e r a g e , and a few o l d e r d e t r i t a l g r a i n s i n a K - A r s a m p l e c a n h a v e a s i g n i f i c a n t e f f e c t on a K - A r a g e (Naeser e t aZ.,1981). F i s s i o n - t r a c k d a t i n g i s a g r a i n - d i s c r e t e method i n which i n d i v i d u a l g r a i n s a r e s c a n n e d and c o u n t e d . I n t h e c o u r s e o f d a t i n g a sample u s i n g z i r c o n s , a n a g e i s o b t a i n e d on e a c h g r a i n t h a t i s c o u n t e d . T h e r e f o r e o l d e r g r a i n s show u p a s c o n t a m i n a t i o n ; a g r a i n w i t h a Miocene a g e i n a P l e i s t o c e n e sample i s obvious b e c a u s e of i t s o l d e r a g e . I n a d d i t i o n , primary z i r c o n g r a i n s i n P l e i s t o c e n e t e p h r a s can g e n e r a l l y be i d e n t i f i e d
91 SAMPLE
I
I
SEPARATE
+
GLASS
/
I
REACTOR
I
ETCH
1-
COUNT
X NEUTRON DOSE (+)
I I
AGE Figure 3
Steps involved i n obtaining a fission-track age u s i n g t h e p o p u l a t i o n Inethod.
4 ) . Contamination i s u s u a l l y not problem w i t h g l a s s s h a r d s , a l t h o u g h N . D . N a e s e r has r e c o g n i z e d o l d e r g l a s s i n a t e p h r a s a m p l e from S a s k a t c h e w a n . It c o m p r i s e d l e s s t h a n O w l $ o f t h e g l a s s b u t was o b v i o u s b e c a u s e o f i t s h i g h e r t r a c k d e n s i t y ; t h i s % l a s s i s shown i n Westgate a n d Gorton ( 1 9 8 1 , F i g u r e 5 ) .
b y t h e g l a s s a d h e r i n g t o them ( F i g u r e 2
A major problem of f i s s i o n - t r a c k d a t i n g i n t h e Q u a t e r n a r y i s t h a t very young s a m p l e s (<100,000 y e a r s ) c o n t a i n v e r y few t r a c k s . T h i s l e a d s t o l o n g c o u n t i n g t i m e s and t o a g e s w i t h l a r g e a n a l y t i c a l u n c e r t a i n t i e s . Lierd and N a e s e r ( 1 9 7 4 ) d e t e r m i n e d a z i r c o n t o b e a b o u t 1 0 0 , 0 0 0 y e a r s o l d , w i t h a 4 0 % s t a n d a r d d e v i a t i o n ; i n 45 z i r c o n s a t o t a l of 1 5 t r a c k s ‘dere o b s e r v e d . B r i g g s and W e s t g a t e (1978) r e p o r t e d one g l a s s s a m p l e i n which t h e y d i d n o t s e e any s D o n t a n e o u s t r a c k s i n t h o u s a n d s o f s h a r d s . Thus for young s a m p l e s t h e a n a l y t i c a l u n c e r t a i n t y i s l a r g e , b u t even Lhen t h e r e s u l t m i g h t a n s w e r a c e o l o g i c a l q u e s t i o n .
Z i r c o n s , a l t h o u g h p r e f e r a b l e t o qlass, a r e n o t p r e s e n t i n a l l samples. I n t e p h r a s , t h e i r p r e s e n c e d e p e n d s upon t h e c h e m i s t r y o f t h e p a r e n t magma and t h e d i s t a n c e downwind from t h e e r u p t i o n v e n t . Experi e n c e has s h o w n t h a t a c i d i c t e p h r a s h a v e more u s a b l e z i r c o r , s t h a n b a s i c x e p h r a s . Z i r c o n s t h a t a r e e x t r e m e l y f i n e g r a i n e d ( < 7 5 p m ) a r e t o o small t o be d a t e d by f i s s i o n t r a c k c o u n t i n g . T h i s i s o f t e n t h e c a s e i n k e p h r a sampled a l o n g d i s t a n c e from i t s v e n t .
92
Figure 4
Zircons separated from a contaminated tephra deposit. The primary zircons (arrowed) are glass mantled; the grains without glass a r e detrital.
N a t u r a l g l a s s e s h a v e beer, d a t e d e x t e n s i v e l y b e c a u s e of t h e i r abundance i n Q u a t e r n a r y t e p h r a and t h e i r a r c h a e o l o g i c a l and o t h e r a p p l i c a t i o n s . However, g l a s s e s p r e s e n t s p e c i a l n r o b l e r n s , t h e g r e a t e s t of which i s t h e e a s e w i t h which t h e y l o s e s p o n t a n e o u s t r a c k s b y a n n e a l i n g ( F l e i s c h e r e t at., 1 9 6 5 a ; S t o r z e r and Wagner, i969; ?CacCougaii, i 9 7 6 ; Seward, 1 9 7 9 ) . H y d r a t e d g l a s s , which i s f o u n d i n t h e t y p i c a l t e p h r a d e p o s i t , i s p a r t i c u l a r l y s u s c e p t i b l e t o a n n e a l i n g ( L a k a t o s and M i l l e r , 19721, b u t r e c e n t work b y Naeser e t aZ. (1980b) h a s shown t h a t b o t h h y d r a t e d and n o n h y d r a t e d g l a s s can l o s e s p o n t a n e o u s t r a c k s a t a m b i e n t surface temperatures over geologic time. I n a s t u d y o f 1 4 t e p h r a s from upper Cenozoic (<30 rn.y.) d e p o s i t s of t h e w e s t e r n U n i t e d S t a t e s , o ~ l y one g l a s s h a d a f i s s i o n - t r a c k a g e c o n c o r d a n t w i t h t h e f i s s i o n - t r a c k age o f c o e x i s t i r , g z i r c o n ( F i g u r e 5 ) . All o t h e r sarnples had a g e s t h a t were s i g n i f i c a n t l y younger thar, t h e z i r c o n a g e s . Sewrrd ( 1 9 7 9 ) showed t h a t a b o u t 6 0 % of t h e g l a s s f i s s i o n - t r a c k a g e s of Q u a t e r n a r y t e p h r a s i n h e r s t u d y i n New Z e a l a n d were s i g n i f i c a n t l y y o u n g e r t h a n t h e f i s s i o n - t r a c k age s of t h e c o e x i s t i n g z i r c o n s . Two p r o c e d u r e s a r e a v a i l a b l e f o r t r e a t i n g g l a s s t o check f o r p a r t i a l a n n e a l i n g and t o c o r r e c t t h e r e s u l t i n g l o w e r e d f i s s i o n - t r a c k a g e s . These a r e t h e t r a c k d i a m e t e r measurement method ( S t o r z e r and Wagner, 1 9 6 9 ) and t h e p l a t e a u a n n e a l i n g method ( S t o r z e r and Poupeau, 1 9 7 3 ) . The p l a t e a u a n n e a l i n g method s e r v e s b e t t e r f o r Q u a t e r n a r y s a m p l e s b e c a u s e i t i s p o t e n t i a l l y more n r e c i s e f o r young g l a s s e s of low t r a c k d e n s i t y (Naeser e t aZ., 1 9 8 0 b ) . I n t h i s method, d i f f e r e n t s p l i t s of t h e i r r a d i a t e d a n d n o n i r r a d i a t e d g l a s s a r e h e a t e d t o g e t h e r i n a f u r n a c e f o r one h o u r i n t e r v a l s a t p r o g r e s s i v e l y h i g h e r t e m p e r a t u r e s , and an age i s determined a f t e r each h e a t i n g s t e p . I f t h e g l a s s has prev i o u s l y b e e n p a r t i a l l y a n n e a l e d , t h e a g e w i l l i n c r e a s e t h r o u g h t h e lower h e a t i n g s t e p s u n t i l i t r e a c h e s a p l a t e a u , which i s u s u a l l y t h e p r i m a r y age. I f no a n n e a l i n g h a s o c c u r r e d , p r o g r e s s i v e h e a t i n g d o e s n o t a f f e c t t h e a g e ; i t r e m a i n s t h e same a s t h e a g e of t h e u n t r e a t e d g l a s s , a l t h o u g h t h e i n d u c e d and s p o n t a n e o u s t r a c k d e n s i t i e s d e c r e a s e . F i g u r e 6 shows t h e r e s u l t s of p l a t e a u a n n e a l i n g f o r t w e l v e upper T e r t i a r y g l a s s e s , i n c l u d i n g b o t h o b s i d i a n s and t e p h r a s h a r d s .
93
15
25
20
35
30
FISSION-TRACK ZIRCON AGE ( X I 0 6 YEARS)
Figure 5
P l o t showing t h e f i s s i o n - t r a c k a g e s of c o e x i s t i n g g l a s s s h a r d s and z i r c o n s from upper Cenozoic t e p h r a d e p o s i t s of t h e w e s t e r n U n i t e d S t a t e s (from Kaeser e t a l . , 1980b).
30
1
EXPLANATION K-Ar glass age vs natural state fissiontrack glass age o K-Ar glass age vs plateau-annealing fission-track glass age A Fission-track zircon age vs plateau annealing fission-track glass age A Fission-track zircon age vs natural state fission-track glass age I
I I
I I I I I
I I
I
o0v Figure 6
I
5
I
10
I
15
1
28
I
25
K-Ar GLASS AGE OR ZIRCON FISSION-TRACK AGE (X106 YEARS)
I
30
-1
35
Diagram showing a p p a r e n t and p l a t e a u c o r r e c t e d a g e s o f g l a s s e s ( o b s i d i a n s and t e p h r a s h a r d s ) compared t o z i r c o n f i s s i o n - t r a c k o r K - A r a g e s ( f r o m Naeser e t al., 1 9 8 0 b ) .
Both the diameter measurement and plateau annealing methods are difficult to apply to Quaternary glass because the small number of tracks present generally prevents statistically significant measureTents. A l s o , even a corrected age will still be too young if any of
94
t h e t r a c k s i n t h e g l a s s were t o t a l l y a n n e a l e d (YacDougall, 1 9 7 6 ; Naeser e t aZ., 1 9 8 0 b ) . Because o f t h e ease w i t h which g l a s s e s a n n e a l and t h e u n c e r t a i n t y o f c o r r e c t i n g t h e i r a g e s , g r e a t c a r e must b e u s e d i n t h e i n t e r p r e t a t i o n o f a l l g l a s s f i s s i o n - t r a c k a g e s . They s h o u l d a l w a y s b e c o n s i d e r e d minimum a g e s . A n o t h e r l i m i t a t i o n for d a t i n g g l a s s s h a r d s r e l a t e s t o t h e i r g r a i n s i z e and v e s i c u l a r i t y . L a r g e b u b b l e - j u n c t i o n , p l a t y s h a r d s a r e by f a r t h e e a s i e s t t o d a t e . I n v e r y f i n e - g r a i n e d o r pumiceous s h a r d s (Westg a t e a n d B r i g g s , 1 9 8 0 , F i g u r e 4 ) , i t i s d i f f i c u l t t o c o u n t t r a c k s , and t o d e t e r m i n e g l a s s a r e a by t h e c o n v e n t i o n a l method o f a g r i d i n t h e m i c r o s c o p e e y e p i e c e . The p r o b l e m of d e t e r m i n i n g a r e a can b e overcome for many s a m p l e s b y u s e o f a p o i n t - c o u n t i n g t e c h n i q u e a n a l a g o u s t o t h a t u s e d i n p e t r o g r a p h i c modal a n a l y s i s ( S e w a r d , 1 9 7 4 ; B r i g g s and W e s t g a t e , 1 9 7 8 ; N a e s e r e t aZ., 1 9 8 2 ) . Even s o , d a t i n g s u c h g l a s s i s v e r y t i m e consuming.
Glass c o n t a i n i n g a b u n d a n t m i c r o l i t e s may be d i f f i c u l t or i m p o s s i b l e t o d a t e b e c a u s e o f t h e c l o s e r e s e m b l a n c e of t h e e t c h e d m i c r o l i t e s t o f i s s i o n t r a c k s . T h i s problem i s r a r e l y e n c o u n t e r e d i n g l a s s s h a r d s , b u t i s common i n d a t i n g o b s i d i a n s . Despite t h e s e problems, g l a s s has been used t o d a t e Quaternary s a m p l e s b e c a u s e i n some c a s e s , i t i s t h e o n l y d a t a b l e p h a s e p r e s e n t a n d , i n some s i t u a t i o n s , e v e n minimum d a t e s p r o v i d e u s e f u l i n f o r m a t i o n . APPLICATIONS Tephrochronology The m a j o r c o n t r i b u t i o n o f f i s s i o n - t r a c k d a t i n g t o Q u a t e r n a r y c h r o n o l o g y has b e e n i n t h e f i e l d o f t e p h r o c h r o n o l o g y . F i s s i o n t r a c k s h a v e p r o v i d e d t h e most s u i t a b l e method f o r d a t i n g t e p h r a s , p a r t i c u l a r l y t h o s e o l d e r t h a n t h e 4 0 , 0 0 0 t o 5 0 , 0 0 0 - y e a r BP l i m i t of r a d i o c a r b o n d a t i n g . An a d v a n t a g e of t h e f i s s i o n - t r a c k m e t h o d , o v e r K-Ar and r a d i o c a r b o n d a t i n g , i s t h a t t h e p r o b l e m of c o n t a m i n a t i o n i s g r e a t l y m i n i mized ( s e e d i s c u s s i o n a b o v e ) . T h i s i s p a r t i c u l a r l y i m p o r t a n t f o r d a t i n g t e p h r a s , which commonly c o n t a i n d e t r i t a l c o n t a m i n a n t s . Also, g l a s s s h a r d s c a n n o t b e d a t e d r e l i a b l y by t h e I ( - A r method b e c a u s e e x c e s s r a d i o g e n i c a r g o n i s o f t e n p r e s e n t , - p r o d u c i n g ages t h a t a r e t o o o l d , and p o t a s s i u m g a i n a n d / o r a r g o n loss d u r i n g or a f t e r h y d r a t i o n can p r o duce a g e s t h a t a r e t o o young. T h e r e f o r e , i t i s n o t p o s s i b l e t o know i f K - A r a g e s o f g l a s s s h a r d s a r e t o o o l d , t o o young, or, ~ o s s i b l yf o r t u i t o u s l y , c o r r e c t ( N a e s e r e t aZ., 1 9 8 1 ) . D u r i n g t h e l a s t d e c a d e t h e r e have b e e n a number o f e x a m p l e s i n t h e l i t e r a t u r e of f i s s i o n - t r a c k d a t i n g o f z i r c o n a n d / o r g l a s s i n Q u a t e r n a r y t e p h r a s ( N a e s e r e t aZ., 1 9 7 3 ; Seward, 1 9 7 4 , 1 9 7 5 , 1 9 7 9 ; I z e t t and N a e s e r , 1976; Aronson e t aZ., 1 9 7 7 ; B r i g g s a n d b i e s t g a t e , 1978, W e s t g a t e e t aZ., 1 9 7 8 ; Gleadow, 1 9 8 0 ) . S t u d i e s on t h e P e a r l e t t e f a m i l y a s h b e d s and t h e B i s h o p a s h bed i l l u s t r a t e s t h e u s e o f f i s s i o n - t r a c k d a t i n g i n tephrochronology. V o l c a n i c a s h b e d s of t h e P e a r l e t t e f a m i l y o c c u r i n P l e i s t o c e n e d e p o s i t s o f w e s t e r n N o r t h America ( I z e t t e t al., 1 9 7 0 , 1 9 7 2 ) . B e f o r e 1 9 7 0 , t h e P e a r l e t t e a s h was c o n s i d e r e d t o b e a s i n g l e a s h b e d , and i t was u s e d a s a t i m e m a r k e r for many m i d c o n t i n e n t Q u a t e r n a r y d e p o s i t s . Then I z e t t e t aZ. ( 1 9 7 0 , 1 9 7 2 ) r e c o g n i z e d minor c h e m i c a l d i f f e r e n c e s among some o f t h e P e a r l e t t e a s h l o c a l i t i e s , and f o u n d t h a t t h e a s h e s could be c o r r e l a t e d geochemically w i t h d e p o s i t s of t h e t h r e e major ashf l o w e r u p t i o n s t h a t had o c c u r r e d i n t h e r e g i o n o f Y e l l o w s t o n e N a t i o n a l P a r k , Wyoming. These e r u p t i o n s have b e e n d a t e d a t 2 . 0 2 f 0 . 0 8 m . y . ( H u c k l e b e r r y R i d g e T u f f ) , 1 . 2 7 ? 0 . 1 m . y . (Mesa F a l l s T u f f ) , and 0 . 6 1 6 ? 0 . 0 0 8 m.y. (Lava Creek T u f f ) u s i n g t h e K - A r method (J.D. O b r a d o v i c h , w r i t t e n c o m m u n i c a t i o n , 1 9 7 3 ) . N a e s e r e t aZ. ( 1 9 7 3 ) d a t e d z i r c o n s from two o f t h e t h r e e t y p e s o f t e p h r a and o b t a l ? e d a g e s o f 1.9 ? 0 . 1 n . y .
95
+
f o r ash' c o r r e l a t e d w i t h t h e Huckleberry Ridge Tuff and 0 . 6 0 . 1 m.y. for a s h 2 c o r r e l a t e d w i t h t h e Lava Creek T u f f . T h e s e a g e s m a t c h e d t h e ages i n t h e s o u r c e r e g i o n a n d c o n f i r m e d t h e g e o c h e m i c a l e v i d e n c e o f I z e t t e t aZ. ( 1 9 7 0 , 1 9 7 2 ) t h a t t h e r e were t h r e e P e a r l e t t e a s h e s r a t h e r than j u s t o n e . Dates o b t a i n e d for t h e H u c k l e b e r r y R i d g e a s h b e d g i v e an e x c e l l e n t example o f t h e p r o b l e m s c a u s e d by t r a c k a n n e a l i n g i n glass (Table 1 ) . T a b l e 1.
F i s s i o n - t r a c k a g e s of z i r c o n and g l a s s from t h e Euckleberry Ridge a s h bed
Investigator Naeser e t a l .
(1973, 1980b)
Seward ( 1 9 7 9 ) N.D.
Naeser a
(unpub.
Zircon"
Glassa
data)
1 . 3 2 0.17 m.y.
1 . 9 i 0 . 1 m.y.
1 . 3 9 f 0.08 m.y.
1 . 9 3 f 0.16 m.y.
1.21f 0 . 0 7 m . y .
1.9
2 0 . 2 5 m.y.
E r r o r shown i s 2 1 s i g m a .
The Bishop a s h b e d , l i k e the P e a r l e t t e f a m i l y o f a s h e s , i s a widespread a i r f a l l t e p h r a i n t h e w e s t e r n U n i t e d S t a t e s ( I z e t t e t a l . , 1 9 7 0 ; Izeti; arc! Y ~ e s e r , 1 9 7 5 ) . I z e i ; t e t aZ. (197c) :-.ave i d e r y i f i e d i t a s t h e a i r f a l l e q u i v a l e n t of t h e B i s h o p T u f f , which o r i g i n a t e d i n t h e Long Valley C a l d e r a i n C a l i f o r n i a a h o u t 700,000 y e a r s a g o . D a l r y m p l e e t a l . ( 1 9 6 5 ) r e p o r t e d a n a v e r a g e K - A r a g e of 0 . 7 0 8 f 0 . 0 1 5 m.y. ( + a t t h e 9 5 % confidence i n t e r v a l ) on m i n e r a l s f r o m t h e Bishop T u f f i n i t s s o u r c e a r e a . Both g l a s s a n d z i r c o n from t h e Bishor, a s h have b e e n d a t e d by t h e f i s s i o n - t r a c k method ( T a b l e 2 ) . These a g e s a l s o show t h e p r o b l e m caused b y t r a c k a n n e a l i n g i n g l a s s . Table 2.
Fission-track ages of Bishop a s h bed
z i r c o n and g l a s s from t h e
Investigator I z e t t a n d Naeser ( 1 9 7 6 ) Naeser (unpub. d a t a )
N.D.
a
0.56
Glassa
zircona
--
0 . 7 4 ? 0 . 0 3 m.y.
2 0 . 0 5 m.y.
--
E r r o r shown i s ? 1 s i g m a .
I n s e v e r a l c a s e s f i s s i o n - t r a c k d a t i n g has d e m o n s t r a t e d t h a t t e p h r a s a r e c o n s i d e r a b l y o l d e r t h a n i n f e r r e d by r a d i o c a r b o n d a t i n g o f a s s o c i a t e d m a t e r i a l . For e x a m p l e , t h e Salmon S p r i n g s D r i f t a t i t s t y p e l o c a l i t y , a t Salmon S p r i n g s , W a s h i n g t o n , c o n s i s t s o f two d r i f t s h e e t s s e p a r a t e d by about 1 . 5 m o f p e a t , s i l t , and v o l c a n i c ash (Lake Tapps t e p h r a ) ( C r a n d e l l e t al., 1 9 5 8 ; E a s t e r b r o o k e t aZ., 1 9 8 1 ) . The p e a t g r a d e s downward w i t h d e c r e a s i n g o r g a n i c c o n t e n t i n t o a b o u t one m e t e r of s i l t , which i n t u r n g r a d e s i n t o t h e v o l c a n i c a s h ( D . J . E a s t e r b r o o k , w r i t t e n commun., 1 9 8 0 ) . The p e a t h a s b e e n r a d i o c a r b o n d a t e d a t 7 1 , 5 0 0 f 11 74 00 00 y e a r s BP b y t h e e n r i c h m e n t method ( S t u i v e r e t a l . , 1 9 7 8 ) , and t h e d r i f t s h e e t s were t h u s c o n s i d e r e d e a r l y W i s c o n s i n i n a g e . However, f i s s i o n t r a c k d a t a on t h e a s h , and on c o r r e l a t i v e a s h a t Auburn, W a s h i n g t o n , along w i t h p a l e o m a g n e t i c and t e D h r o c h r o n o l o g i c a 1 e v i d e n c e , show t h a t t h e Salmon S p r i n g s D r i f t i s much o l d e r ( E a s t e r b r o o k e t aZ., 1 9 8 1 ) (Table 3 ) . 'Huckleberry Ridge a s h bed of I z e t t and Wilcox (1982); = P e a r l e t t e t y p e B a s h o f L z e t t e t al. ( 1 9 7 3 ) . 2Lava C r e e k B a s h b e d o f I z e t t a n d W i l c o x ( 1 9 8 2 ) ; = P e a r l e t t e t y p e 0 a s h o f I z e t t e t aZ. ( 1 9 7 0 ) a n d N a e s e r e t aZ. ( 1 9 7 3 ) .
96 Table 3.
Comparison of f i s s i o n - t r a c k a g e of t h e Lake Tapps t e p h r a , Washington, and r a d i o c a r b o n a g e of a s s o c i a t e d peat
Locality
Dating Method
Salmon Springs
C-14 F-T
peat zircon
71,500 t1 4 0 0 840,000 2 210,000
Auburn
F-T F-T
zircon glass
870,000 f 2 7 0 , 0 0 0 660,000 f 40,000
a
Vat er i a l Dated
Radiocarbon a g e from S t u i v e r e t aZ. from E a s t e r b r o o k e t a l . ( 1 9 8 1 ) .
Agea ( y e a r s B P 510) 1 7 0 0
(1978);
fission-track
ages
Archaeology F i s s i o n t r a c k s a r e F o t e n t i a l l y u s e f u l for d a t i n g a v a r i e t y o f a r c h a e o l o g i c a l m a t e r i a l s ( s e e r e v i e w by Wagner, 1 9 7 8 ) . They h a v e b e e n u s e d (1) t o d a t e s t r a t i g r a p h i c l a y e r s , s u c h a s t e p h r a b e d s a s s o c i a t e d w i t h a r c h a e o l o g i c a l r e n a i n s ( F l e i s c h e r e t aZ., 1 9 6 5 ~ ;A r o n s o n e t a Z . , 1 9 7 7 ; G l e a d o w , 1 9 8 0 ; S t e e n - M c I n t y r e e t aZ., 1981); ( 2 ) t o d a t e t h e m a t e r i a l u s e d t o manufacture a r t i f a c t s ; and ( 3 ) t o d a t e h e a t i n g e v e n t s i n w h i c h g l a s s e s 3r m i n e r a l s were h e a t e d t o a h i g h e n o u g h t e m p e r a t u r e t o t o t a l l y anneal t h e i r spontaneous t r a c k s . B i g a z z i a n d Bonadonna ( 1 9 7 3 ) , S u z u k i ( 1 9 7 3 , 1 9 7 4 ) , D u r r a n i e t a Z . (1971), a n d o t h e r s h a v e u s e d f i s s i o n t r a c k s t o d e t e r m i n e t h e a p e , a n d ir! some c a s e s t h e u r a n i u m c o n t e n t , o f t h e o b s i d i a n u s e d t o make a r t i -
f a c t s i n o r d e r t o t r a c e t h e o b s i d i a n back t o i t s g e o l o g i c s o u r c e and t h u s map a n c i e n t t r a d e r o u t e s . F l e i s c h e r e t aZ. ( 1 9 6 5 b ) d a t e d a mesol i t h i c k n i f e whose w i l t e d s h a p e i n d i c a t e d t h a t i t h a d b e e n h e a t e d t o a h i g h t e m p e r a t u r e a t some t i m e a f t e r i t s m a n u f a c k u r e ; f i s s i o n t r a c k s d a t e d t h e h e a t i n g e v e n t a t 3700 f 9 0 0 y e a r s B P . W a t a n a b e a n d S u z u k i ( 1 9 6 9 ) determined t h e time o f h e a t i n g of v a r i o u s a r t i f a c t s from J a p a n , i n c l u d i n g a g l a s s g l a z e on a b o w l f r a g m e n t w h i c h t h e y d a t e d a t 5 2 0 t 110 years BP. M i l l e r a n d Wagner (1981) d e t e r m i n e d b o t h t h e g e o l o g i c a l a g e of o b s i d i a n a n d t h e t i m e o f i t s m a n u f a c t u r e i n t o a r t i f a c t s f o r s e v e r a l s a m p l e s f r o m S o u t h A m e r i c a . The y o u n g e s t a r c h a e o l o g i c a l m a t e r i a l t o b e d a t e d t h u s f a r i s manmade " u r a n i u m g l a s s " m a n u f a c t u r e d w i t h i n t h e l a s t 1 4 0 y e a r s ( W a g n e r , 1976, 1 9 7 8 ) . Most a r c h a e o l o g i c a l s a m p l e s a r e v e r y y o u n g a n d composed o f m a t e r i a l t h a t i s r e l a t i v e l y low i n u r a n i u m , s o t h a t t r a c k d e n s i t i e s a r e g e n e r a l l y e x t r e m e l y low. T h i s means t h a t c o u n t i n g a s t a t i s t i c a l l y s i g n i f i c a n t number of s p o n t a n e o u s t r a c k s i s a t e d i o u s , t i m e - c o n s u m i n g p r o c e s s t h a t o f t e n i n v o l v e s r e p e a t e d s t e p s o f p o l i s h i n g , e t c h i n g , and c o u n t i n g . Some e x a m p l e s o f t h i s a r e d i s c u s s e d by F l e i s c h e r e t aZ. ( 1 9 7 5 ) . Other C o a t e s a n d Naeser ( 1 9 8 1 ) h a v e d ' e m o n s t r a t e d t h e u s e o f f i s s i o n t r a c k d a t i n g of z i r c o n s from s a n d s t o n e s ( c l i n k e r ) h e a t e d by n a t u r a l b u r n i n g of c o a l t o s t u d y t h e r a t e of Q u a t e r n a r y landform development i n t h e e a s t e r n Powder R i v e r B a s i n o f Wyoming. F i s s i o n - t r a c k d a t i n g a n d K-Ar d a t i n g h a v e h e l p e d e s t a b l i s h t h e a g e a n d e x t e n t of m a j o r t e k t i t e s t r e w n f i e l d s , i n c l u d i n g t h e 1 - m . y . - o l d I v o r y Coast and 0.7-m.y.-old A u s t r a l a s i a n f i e l d s ( s e e f o r example F l e i s c h e r a n d P r i c e , 1 9 6 4 b ; S t o r z e r a n d Wagner, 1969; G e n t n e r e t aZ., 1967, 1 9 6 9 ) . F i s s i o n t r a c k s have been p a r t i c u l a r l y u s e f u l i n d a t i n g m i c r o t e k t i t e s f r o m d e e p sea c o r e s w h e r e t h e r e i s t o o l i t t l e m a t e r i a l for K-Ar d a t i n g ( G e n t n e r e t aZ., 1 9 7 0 ) . One o f t h e y o u n g e s t n a t u r a l m a t e r i a l s t o b e d a t e d by t h e f i s s i o n - t r a c k m e t h o d i s a n i m p a c t g l a s s from t h e Kofels s t r u c t u r e i n A u s t r i a , which y i e l d e d a n age of 8 . 0 2 6 . 0 x lo3 y e a r s ( S t o r z e r e t aZ., 1 9 7 1 ) .
91
REFERENCES C I T E D Aronson, J . L . , S c h m i t t , T . J . , Walter, R . C . , Johanson, D.C., Naeser, C.W. a n d I J a i r n , chronologic and paleomagnetic d a t a f o r Formation of E t h i o p i a : Nature, v . 267,
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101
USING PALEOMAGNETIC REMANENCE AND MAGNETIC SUSCEPTIBILITY DATA FOR THE DIFFERENTIATION, RELATIVE CORRELATION AND ABSOLUTE DATING OF QUATERNARY SEDIMENTS RENE W BARENDREGT
ABSTRACT Paleomagnetism is used in Quaternary stratigraphic studies as a tool for correlation and relative age dating of equivalent strata or f o r the absolute dating of deposits. The method is based on the dekection of changes in the earth's magnetic field and especially changes of nolarity that are recorded by ferromagnetic sediments at the time of deposition. Dating by paleomagnetic characterization and geomagnetic polarity ,n.lstory is a relatively new technique. The large-scale features of the earth's magnetic field character have been well worked out for the past 5 nlllion years or so. The detailed small-scale features for this neriod are still being discovered and defined through analysis of terrestrial sediments. Because of the much greater sedimentation rate on i a ~ d ,these are more likely to show short-lived events and record the excursions which ultimately will become useful correlative tools. Fine-grained sediments, lava flows, and baked pottery are the media most frequently used. Because reversals have occurred repeatedly in the past their identification within incomplete sedimentary records is only possible through comparison with other stratigraphic o r radiorletric data collected for similar or related sedimentary sequences. Continuously deposited marine or terrestrial sediments which show a nigh sedimentation rate provide isochrons which can be used for worldwide correlation. The recent flourishing of research activity into the secular variation of the earth's non-dipole field promises to greatly refine and embellish the geomagnetic timetable for the Quaternary. The only practical way of demonstrating the validity of interpreted Tagneto-stratigraphy is to show that results are reproducible in widely separated sections with different lithology and sedimentation rates. Ir Canada where Pleistocene deposits are largely glacial in origin, and were thus episodic, one must be aware that these deposits may only have recorded the earth's magnetic field in short time intervals. Possible subsequent alteration of these sediments by the processes outlined in this paper, must be borne in mind. Great Lake sediments and other glacial lake sediments provide excellent opportunity for magnetostratigraphic correlation and dating. INTRODUCTION Dating of Pleistocene sediments beyond the range of radiocarbon dating as well as the dating of sediments which contain no dateable carbon material has always been a problem. The new and the perfected absolute methods (potassium-argon, fission track, amino acid racemization and accelerated carbon-14) require media which are often lacking in Pleistocene sediments.
A new method of dating based upor, the paleomagnetic characterization
102
of s e d i m e n t s a n d r o c k s o f f e r s g r e a t p r o m i s e for t h e p a r t i a l a l l e v i a t i o n of t h i s p r o b l e m . I n t h e p a s t t e n y e a r s c o n s i d e r a b l e u s e h a s b e e n made of t h i s t e c h n i q u e i n Canada, U.S.A., J a p a n , The N e t h e r l a n d s , B r i t a i n , The U . S . S . R . , and e l s e w h e r e . I t s m a j o r v a l u e i n P l e i s t o c e n e s t r a t i g r a p h i c work w i l l come i n t h e n e a r f u t u r e , when a more d e t a i l e d r e c o r d o f t h e s e c u l a r v a r i a t i o n of t h e n o n - d i p o l e f i e l d becomes a v a i l a b l e .
A wide r a n g e of m a t e r i a l s has b e e n employed w i t h t h e m e t h o d . Silt t o f i n e s a n d - s i z e d s e d i m e n t s work b e s t , b u t i n g e n e r a l any f i n e - g r a i n e d s e d i m e n t c a n p o t e n t i a l l y b e a n a l y z e d . Marine a s w e l l a s t e r r e s t r i a l s e d i m e n t s have been u s e d . I n a d d i t i o n , l a v a flows s e r v e as e x c e l l e n t r e c o r d e r s o f t h e e a r t h ‘ s p a s t m a g n e t i c f i e l d and can b e sampled w i t h r e l a t i v e e a s e . Baked p o t t e r y a n d o t h e r o b j e c t s can a l s o b e u s e d i f t h e c l a y o f which t h e y a r e made t o o k on a t h e r m a l r e m a n e n t m a g n e t i z a t i o n d u r i n g t h e t i m e of l a s t b a k i n g . Archaeomagnetic s t u d i e s p r o v i d e i n f o r m a t i o n a b o u t s e c u l a r c h a n g e s o f t e n more d e t a i l e d t h a n t h a t which h a s b e e n o b t a i n e d from l a k e s e d i m e n t s , b u t a r e l i m i t e d by t h e i n t e r m i t t e n t r i s e and f a l l of p a s t c i v i l i z a t i o n s . G E O M A G N E T I C DYNAMO THEORY
I t i s w i d e l y a c c e p t e d t h a t t h e m a g n e t i c f i e l d o f t h e e a r t h and s u n a r e p r o d u c e d by dynamos. I n t h e c a s e of t h e e a r t h t h e dynamo a c t i o n i s t h o u g h t t o be p r o d u c e d b y m o t i o n s i n t h e e l e c t r i c a l l y c o n d u c t i n g f l u i d c o r e ( F i g u r e 1 ) . E d w a r d B u l l a r d ( 1 9 7 2 ) s t a t e s t h a t t h e r e i s no d i r e c t d e m o n s t r a t i o n of t h e e x i s t e n c e of t h e s e dynamos; t h e r e a s o n s f o r bel i e v i n g i n them a r e t h e a b s e n c e of any o t h e r s a t i s f a c t o r y t h e o r y and r a t h e r vague q u a l i t a t i v e a r g u m e n t s a b o u t t h e i r p o s s i b l e p r o p e r t i e s . The m a g n e t i c f i e l d for which t h e t h e o r y a t t e m p t s t o a c c o u n t i s a somewhat i d e a l i z e d v e r s i o n of t h e a c t u a l f i e l d and i s d e s c r i b e d by B u l l a r d as a f i e l d t h a t i s a p p r o x i m a t e l y (i20%) a d i p o l e f i e l d , t h a t c h a n g e s by a l a r g e f r a c t i o n o f i t s e l f i n a few h u n d r e d y e a r s , t h a t has r e p e a t e d l y a n d r a t h e r a c c u r a t e l y r e v e r s e d i t s d i p o l e component, and t h a t t h e nond i p o l e p a r t ( F i g u r e s 2 and 3 ) and i t s r a t e of change have l e n g t h s c a l e s of a f e w t h o u s a n d k i l o m e t e r s , a r e q u i t e complex, have a t e n d e n c y t o d r i f t w e s t w a r d , and a r e n o t c o r r e l a t e d w i t h g e o l o g y or geography ( e x c e p t f o r a s y s t e m a t i c t e n d e n c y t o s m a l l r a t e s of change o v e r t h e P a c i f i c ) . I n t h e dynamo t h e o r y t h e t o p o g r a p h y of t h e n o n - d i p o l e f i e l d i s p r o b a b l y associated with eddies i n the convection p a t t e r n s i n the f l u i d core. B u l l a r d c o n c l u d e s t h a t t h e r e a r e n o t many ways i n which a c o m p l i c a t e d changing, r e v e r s i n g f i e l d can b e produced i n a s p h e r e of molten i r o n The o n l y p l a u s i b l e one i s t o assume t h a t shut i n a r i g i d container. t h e r e a r e m o t i o n s i n t h e m a t e r i a l and t h a t t h e s e a c t a s a s e l f - e x c i t i n g dynamo. He s t a t e s t h a t i n a bounded, s t a t i o n a r y e l e c t r i c a l l y c o n d u c t i n g b o d y , any s y s t e m of e l e c t r i c c u r r e n t s w i l l d e c a y e x p o n e n t i a l l y w i t h i t s own t i m e c o n s t a n t . The t i m e c o n s t a n t i s p r o p o r t i o n a l t o t h e e l e c t r i c a l c o n d u c t i v i t y and t h e c h a r a c t e r i s t i c l e n g t h r e p r e s e n t i n g t h e d i s t a n c e i n which t h e f i e l d c h a n g e s by a n a p p r e c i a b l e p a r t o f i t s e l f . B u l l a r d h a s c a l c u l a t e d t h i s t i m e c o n s t a n t t o b e 1 5 , 0 0 0 y e a r s for t h e e a r t h ( a s s u m i n g a r a d i u s o f 3500 km and a c o n d u c t i v i t y of a b o u t 3x1050hm-’m’’). As this p e r i o d i s v e r y s h o r t from a g e o l o g i c a l p o i n t o f v i e w , i t i s e s s e n t i a l t o m a i n t a i n t h e f i e l d , which i n a s e l f - e x c i t i n g dynamo i s done by t h e c u r r e n t s p r o d u c e d by e l e c t r o m a g n e t i c i n d u c t i o n r e s u l t i n g from t h e movement of t h e c o n d u c t i n g m a t e r i a l t h r o u g h t h e f i e l d . B u l l a r d g o e s on t o s t a t e t h a t i t i s f a r from o b v i o u s w h e t h e r a g i v e n m o t i o n can m a i n t a i n a f i e l d , or w h e t h e r a n y m o t i o n c a n . He l i s t s t h r e e non-dynamo t h e o r e m s t h a t p r o h i b i t p a r t i c u l a r t y p e s o f m o t i o n or f i e l d i n a s p h e r e from a c t i n g a s dynamos a n d d i s c u s s e s t h e work o f H e r z e n b e r g ( 1 9 5 8 ) , B a c k u s (1958), C h i l d r e s s (1969) and R o b e r t s ( 1 9 7 0 ) . He s t a t e s t h a t R o b e r t s h a s shown t h a t a l m o s t a l l m o t i o n s s p a t i a l l y p e r i o d i c i n t h r e e d i m e n s i o n s a c t as dynamos. Of t h r e e - d i m e n s i o n a l m o t i o n s s p a t i a l l y p e r i o d i c i n two d i m e n s i o n s , h e has shown t h a t a b o u t h a l f a c t as dynamos. It i s a l m o s t c e r t a i n t h a t t h e r e i s m o t i o n i n t h e c o r e . W i t h o u t i t t h e r e c a n b e no dynamo, and i t seems i m p o s s i b l e t o a c c o u n t f o r t h e s h o r t t i m e s c a l e of t h e m a g n e t i c v a r i a t i o n s . If t h e r e i s m o t i o n , t h e r e must b e f o r c e s t o m a i n t a i n i t . The c o r e i s w e l l p r o t e c t e d from e x t e r n a l i n f l u e n c e s , s o no g r e a t v a r i e t y of f o r c e s can p l a u s i b l y b e s u p p o s e d t o produce t h e motion. E a r l i e r workers suggested thermal convection
103
Figure 1
F a r a d a y D i s k Dynamo g e n e r a t e s e l e c t r i c c u r r e n t s ( s h o r t a r r o w s ) when a c o p p e r d i s k i s t u r n e d t h r o u g h t h e m a g n e t i c l i n e s of f o r c e of a b a r magnet o r through a c o i l of w i r e ( a s shown). In a selfs u s t a i n i n g dynamo t h e g e n e r a t e d e l e c t r i c c u r r e n t s s e r v e t o r e i n f o r c e t h e magnetic f i e l d of t h e c o i l s o t h a t no e x t e r n a l s u p p l y o f m a g n e t i s m i s n e e d e d beyond t h a t which o r i g i n a l l y s e r v e d t o t r i g g e r t h e dynamo. The m e t a l l i c l i q u i d i n t h e c o r e of t h e e a r t h i s b e l i e v e d t o f l o w i n s u c h a way a s t o a c t a s a m e c h a n i c a l dynamo i n g e n e r a t i n g m a g n e t i c f i e l d s of t h e e a r t h . (From C a r r i g a n a n d G u b b i n s , 1 9 7 9 ) .
r e s u l t i n g from r a d i o a c t i v e h e a t i n g . W h e t h e r , i n f a c t , t h e r a d i o a c t i v i t y o f t h e c o r e i s h i g h enough and w h e t h e r t n e p r o c e s s e s f o r removing h e a t from t h e o u t s i d e o f t h e c o r e a r e s u f f i c i e n t l y e f f e c t i v e t o p r o d u c e t h e t e m p e r a t u r e g r a d i e n t n e e d e d t o i n i t i a t e c o n v e c t i o r i s unknown. The main requirement i s t h a t t h e t e m p e r a t u r e g r a d i e n t e x c e e d t h e a d i a b a t i c ; t h e heat flow r e q u i r e d f o r t h i s g r e a t l y e x c e e d s t h e e n e r g y a b s o r b e d by t h e dynamo ( B k l l a r d , 1972). The q u e s t i o n o f r e v e r s a l s o f t h e e a r t h ' s m a g n e t i c f i e l d i s a c h a l lenge t o any t h e o r y o f t h e o r i g i n o f t h e e a r t h ' s f i e l d . A t w o - d i s k dynamo c o n n e c t e d t o g e t h e r s o t h a t e a c h d i s k f e e d s c u r r e n t t o t h e c o i l o f $he o t h e r has S e e n d e v i s e d t a d e m o n s t r a t e t h a t r e v e r s a l s c r n b e p r c duced. Although t h e d i s k dynamo i s f a r from a n y t h i n g t h a t c a n b e i m agined t o e x i s t i n t h e e a r t h ' s c o r e , t h e r e i s a c e r t a i n s i m i l a r i t y i n t h e s t r u c t u r e o f t h e e q u a t i o n s t h a t c o n t r o l i t and t h e e q u a t i o n s cons t r u c t e d f o r magneto-hydrodynamic dynamo t h e o r i e s . I f t h e e q u a t i o n s give u r j t a b l e s o l u t i o n s t h a t f l i p from one d i r e c t i o n t o t h e o p p o s i t e o n e , t h e n a r e v e r s a l h a s no s p e c i f i c " c a u s e " , i t i s s i m p l y a c o n s e q u e n c e o f t h e u n f o l d i n g i n t i m e of t h e s o l u t i o n o f t h e e q u a t i o n . It i s not c e r t a i n t h a t t h i s i s t h e r e a l s t a t e o f a f f a i r s ; i t might be t h a t t h e r e v e r s a l s were a r e s u l t o f d i s t 1 x b a n c e oI" t h e R o t i o n b y s o n e c a k a s t r o c h i c e v e n t , s u c h a s t h e f a l l of a v e r y l a r g e m e t e o r i t e . It i s a l s o p o s s i b l e t h a t t h e c a u s e i s s t a t i s t i c a l and i s a s s o c i a t e d w i t h random f l u c t u a t i o n s i n t h e e l e c t r i c c u r r e n t s and t h e m o t i o n s or t h e f o r c e s . I t i s known t h a t random e m f ' s c a n c a u s e r e v e r s a l s i n a disk-dynamo ( B u l l a r d , 1955). Cox ( 1 9 6 8 ) has s u g g e s t e d t h a t random f l u c t u a t i o n s i n the n o n - d i p o l e p a r t o f t h e f i e l d may p r e c i p i t a t e a r e v e r s a l . He h a s developed a p a r t i c u l a r model i n some d e t a i l and h a s shown t h a t i t l e a d s t o a Poisson d i s t r i b u t i o n o f t i m e i n t e r v a l s between r e v e r s a l s .
104
I
Figure 2
Map o f n o n - d i p o l e f i e l d , v e r t i c a l c o m p o n e n t , f o r epoch 1945, Contours labelled i n milligauss. P o s i t i o n s a r e shown of t h e r a d i a l d i p o l e s w i t h which, together with a central dipole, Alldredge & Hurwitz (1964) r e p r e s e n t e d the 1945 f i e l d : symbols 0 r e p r e s e n t p o s i t i o n s of d i p o l e s p o i n t i n g down w h i l e symbols W r e p r e s e n t p o s i t i o n s of d i p o l e s A l l the r a d i a l dipoles are located a t pointing up. .025 Earth r a d i i . The a r r o w s i n d i c a t e w h e t h e r the d i p o l e s d r i f t e d eastwards o r westwards between epochs 1945 and 1955. (From Creer, 1 9 7 7 ) .
C o m p a r i s o n w i t h t h e o b s e r v e d d i s t r i b 1 J t l o n :her?. l e a d s t c a n e s t b a t i c r . o f t h e p a r m e t e r s o f ‘,he m o d e l . L n e v i t a b l y t h e m o d e l i s b a s e d o n i n t u i t i o n s a b o u t t h e dynamo p r o c e s s r a t h e r t h a n o n a n y f u n d a m e n t a l t h e o r y ; i t i s x o n e t h e l e s s s u g g e s t i v e , and i t seems p o s s i b l e , t h a t t h e r e v e r s a l s o f t h e f i e l d a r e a s s o c i a t e d w i t h c h a n g e s i n t h e complex n o n - d i p o l e f i e l d ( B u l l a r d , 1 9 7 2 ) . An e x c e l l e r i t d i s c u s s i o n o f t h e s o u r c e o f t h e e a r t h ’ s m a g n e t i c f i e l d i s f o u n d i n C a r r i p a n 3rd J u b b i n s ( 1 9 7 9 ) . PALEOMASNETIC REMANENCE A N 3 SUSCEPTIBILITY ANALYSIS Yhe e a r t h ’ s m a g n e t i c f i e l d h a s two s t a b l e s t a t e s : i n i t s n o r m a l s t a t e t h e f i e l d i s b e l i e v e d t o wobble s e v e r a l t e n s of d e g r e e s a b o u t a d i p o l e d i r e c t i o n which, over t h e e a r t h ‘ s s u r f a c e , i s d i r e c t e d toward t h e north. I n i t s r e v e r s e d s t a t e , i t wobbles about a s o u t h o o l a r d i r e c t i o n . T r a n s i t i o n from one s t a t e t o a n o t h e r t a k e s a n a v e r a g e o f 1 0 , 0 0 0 y e a r s . A r e c o r d o f t h e s e c h a n g e s i s l e f t b e h i n d ir! t h e f o r m o f w h a t i s t e r m e d a remanent m a g n e t i z a t i o n of s e d i m e n t s and r o c k s . I n t h e c a s e o f v o l c a n i c r o c k s , t h i s remanence, c a l l e d t h e r m o r e m a r e n t .nag-etism (‘TRV) i s very s t r o n g . Because of t h e s h a r p n e s s o f t h e t r a n s i t i o n zones, t h e y p r o v i d e v e r y u s e f u l s t r a t i g r a p h i c markers which are e a s i l y + , r a c e a b l e and mappable i n t h e f i e l d .
Two d i f f e r e n t t y p e s o f g e o m a g n e t i c p o l a r i t y t i n e w i t s c a n b e d i s t i n g u i s h e d on t h e b a s i s of t h e i r d u r a t i o n . The 1 o r . g e r o f t h e two i s t e r m e d a g e o m a g n e t i c p o l a r i t y e p o c h a n d is d e f i n e d a s a t i m e i n t e r v a l d u r i n g w h i c h t h e e a r t h ‘ s f i e l d was e n t i r e l y o r p r e d o m i n a n t l y o f o n e
105
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Figure 3
Secular v a r i a t i o n of t h e geomagnetic f i e l d d i r e c t i o n n e a r London a n d P a r i s . A l l values have beenreduced t o t h e p r e s e n t o b s e r v a t o r i e s a t H a r t l a n d (50°59.7'N, 350'31.0'E) and Chambon-la-ForGt ( 4 8 " 0 1 . 4 ' N , 2'15.6' E). The s o l i d c u r v e s a r e t a k e n p r i m a r i l y f r o m c o n tinuous observatory records, whereas t h e dashed c u r v e s a r e b a s e d upon h i s t o r i c a l v a l u e s g i v e n by Gaibar-Puertas (1953). For t h e d a t e s i n parenthes i s , t h e v a l u e s o f t h e d e c l i n a t i o n were o b t a i n e d by extrapolation. (From S k i l e s , 1 9 7 0 ) .
polarity to that of the epoch. The time epochs and events, constructed by Cox e t cther workers (see Figure 4) is based on paleomagnetic measurements from about 60 Hawaii, Alaska, Europe and Africa.
scale for geomagnetic polarity a2. (1964) and modified by the potassium-argon dates and lava flows from California,
The lines of force in the earth's magnetic field are directed toward the magnetic poles and the angle o f any point between true North end the direction o f the field is called the declination. The lines of force are also directed (except at the equator) toward or away from the center of the earth, and the angle above o r below the horizontal is called the inclination (see Figure 5). It is along these lines of force that 'memory elements' have been oriented. The memory elements are rragnetic domains (local small zones within the ferromagnetic material that have a large uniform spontaneous magnetization within the various iron and titanium grains).
A 'good' sample from the sedimentary record should be fine-grained, strongly magnetized, free from secondary mineralization or weathering and unlikely to have a history of lightndng strikes. Fine-grained samples are essential because minute grains of ferromagnetic material in the sediment must be oriented by the earth's ambient magnetic field rather than by the geologic agent that transported and deposited the minerals at the sampling site. Magnetization resulting from these oriented mineral grains is called detrital remanent magnetization (DRM) and conveys a record of the earth's magnetic field at the time of deposition. This DRM may be destroyed or weakened through weathering, secondary mineralization, or lightning strikes. These secondary magnetizations may be an unstable part of the original remanent magnetization and are referred to as a viscous remanent magnetization (VRM). The unwanked secondary magnetizations must be removed to isolate any existent primary magnetization which recorded the ancient earth's field.
106 Figure 4 Revised Late Cenozoic p o l a r i t y t i m e scale reconciled t o the new K - A r c o n s t a n t s . Each short horizontal l i n e ind i c a t e s a potassium-argon a g e and m a g n e t i c p o l a r i t y determined f o r one volcanic cooling unit. Stippled pattern indicates periods of normal p o l a r i t y with coarse stippled pattern indicating events whose l i m i t s a r e p o o r l y defined. Arrows i n d i c a t e possible brief polarity events. The t i m e s c a l e is based on v o l c a n i c c o o l i n g u n i t s and deepsea c o r e s . (From Mankinnen and Dalrymple,
1979).
I f paleomagnetism i s t o provide a valuable d a t i n g t o o l , t h e magnetic r e c o r d must b e s t a b l e a n d o f a s i n g l e component.
It has b e e n shown b y A s and Z i j d e r v e l d ( 1 9 5 8 ) and one c a n n o t s i m p l y d i v i d e s a m p l e s i n t o m a g n e t i c a l l y s t a b l e and u n s t a b l e o n e s , b u t t h a t e a c h rock usually contains several n a t u r a l magnetizations of d i f f e r e n t s t a b i l i t y a n d o f t e n d i f f e r e n t d i r e c t i o n s . The t o t a l n a t u r a l r e m a n e n t m a g n e t i z a t i o n (NRM) o b t a i n e d by s i m p l y m e a s u r i n g a s a m p l e i s t h e
107 NP
Figure 5
C a l c u l a t i o n of p a l e o m a g n e t i c p o l e f r o m a v e r a g e The sampling d i r e c t i o n s o f m a g n e t i z a t i o n ( D m , I*). s i t e S h a s l a t i t u d e and l o n g i t u d e ( A , @ ) and t h e (From McElhinny, pole P has coordinates ( A ' , @ ' ) .
1973).
resultant of these different magnetizations. If a stable magnetization remains after 'quality' cleaning it is inferred to be primary. However, a primary magnetization need not necessarily be stable if, for example, it was recorded by soft multi-domain particles. Various field and laboratory tests have been devised to determine stability of magnetization and are outlined in all textbooks of paleomagnetism. The most commonly used method involves alternating field (a.f.) demagnetization and is best described by Zijderveld (1975). If a sample is placed in a magnetic field of a certain intensity, a part of its remanent magentization will become unblocked from its fixed position and aligned in this ambient field. Upon intensifying this ambient field, more and more of the remanence will be unblocked and redirected. When the applied field is alternating and is slowly decreased from its peak value the unblocked grains are randomized (eliminated) leaving behind the more stable remanence fractions. The sample can be demagnetized stepwise with increasingly stronger alternating magnetic fields by which fractions with higher and higher unblocking fields are randomized. Complete measurement after each treatment gives the magnetization remaining af'ter each demagnetization step. By vector subtraction the direction and intensity components are determined. The strength of the alternating field entirely eliminating the secondary magnetization lies at the point where the resultant vector stops changing direction. This is an important value since it is just with an alternating magnetic field of that strength, that the sample is said to be magnetically 'cleaned' at which time the remaining vector is assumed to be the DRM. It must be pointed out however that vector subtraction will only yield the direction and intensity of the subtracted vector when a single component is removed. Components of remanencemay have overlapping coercivity spectra so that two or more components may be removed simultaneously in the same proportion. Furthermore, 'cleaning' may in some cases leave a stable secondary remanence as for example in sediments where the smallest grains ( i . e . of magnetite) were finally fixed in position sometime after the main DRM. These smallest
108
g r a i n s may b e t h e most s t a b l e component t h r o u g h b e i n g s i n g l e dornained. Chemical o v e r p r i n t s u c h a s d e u t e r i c o x i d a t i o n may s i m i l a r l y l e a v e a s t a b l e s e c o n d a r y remanence. To b e t t e r u n d e r s t a n d t h e means b y w h i c h s e d i m e n t s b e c o m e m a g n e t i z e d
i t i s necessary t o i d e n t i f y t h e magnetic minerals responsible f o r t h e NRM ( s e e L b v l i e e t a;., 1 9 7 1 ) . If f o r e x a m p l e , t h e p r i n c i p l e m a g n e t i c m i n e r a l i s m a g n e t i t e and t h e d e p o s i t s a r e n o t s u b s t a n t i a l l y w e a t h e r e d , t h e i r remanence i s most p r o b a b l y a d e t r i t a l r e m a n e n t m a g n e t i z a t i o n ( D R M ) a n d d a t e s f r o m t h e t i m e of d e p o s i t i o n of t h e s e d i m e n t . I n a d d i t i o n t o m a g n e t i c remanence c h a r a c t e r i s t i c s , s e d i m e n t s ex-
h i b i t m a g n e t i c s u s c e p t i b i l i t y c h a r a c t e r i s t i c s which can b e used i n t h e i r
d i f f e r e n t i a t i o n . M a g n e t i c s u s c e p t i b i l i t y i s a m e a s u r e of t h e d e g r e e t o w h i c h a s u b s t a n c e i s a t t r a c t e d t o a m a g n e t , t h a t i s , t h e r a t i o of t h e i n t e n s i t y of m a g n e t i z a t i o n t o t h e m a g n e t i c f i e l d s t r e n g t h i n a m a g n e t i c c i r c u i t . I n a m i n e r a l g r a i n t h i s s u s c e p t i b i l i t y may b e i s o t r o p i c or a n i s o t r o p i c d e p e n d i n g on t h e s h a p e ‘ a n d c r y s t a l l o g r a p h y of t h e g r a i n . I n g e n e r a l t h e maximum s u s c e p t i b i l i t y o f a n i y r e g u l a r - s h a p e d g r a i n l i e s a l o n g i t s g r e a t e s t d i m e n s i o n and minimum s u s c e p t i b i l i t y a c r o s s i t . C e r t a i n e x c e p t i o n s may e x i s t however. The o r i e n t a t i o n o f any p a r t i c l e w h i c h h a s a m a g n e t i c a n i s o t r o p y may b e a f f e c t e d by t h e m a g n e t i c f i e l d of t h e e a r t h d u r i n g d e p o s i t i o n . Also, t h e magnetic p a r t i c l e s of a s e d i m e n t may h a v e a d i f f e r e n t s i z e or s h a p e d i s t r i b u t i o n from t h a t o f t h e non-magnetic f r a c t i o n and s o may b e a f f e c t e d d i f f e r e n t l y b y hydrodynamic f o r c e z . M a g n e t i t e , which i s i s o t o p i c , has no s t r o n g c r y s t a l l o g r a p h i c a n i s o t r o p y of s u s c e p t i b i l i t y . Any a n i s o t r o p y i n m a g n e t i t e i s due t o t h e d e v i a t i o n i n s h a p e o f t h e g r a i n from a s p h e r e . T h e r e f o r e , u n l e s s most m a g n e t i t e g r a i n s i n a s e d i m e n t a r e e q u a n t , t h e s u s c e p t i b i l i t y anisotropy w i l l r e f l e c t t h e i r alignment. I n a r o c k or s e d i m e n t however, t h e e f f e c t s a r e o b s e r v e d o n l y i f e i t h e r t h e c r y s t a l l o g r a p h i c or s h a p e a x e s o f many g r a i n s a r e a l i g n e d . A p r i m a r y f a b r i c i s a s s o c i a t e d w i t h c o n d i t i o n s a t t h e t i m e o f t h e d e p o s i t i o n and i s g e n e r a l l y c h a r a c t e r i z e d by minimum s u s c e p t i b i l i t y a l i g n e d a b o u t t h e normal t o t h e bedding plane. Thus a s t r o n g a n i s o t r o p y i n m a g n e t i c s u s c e p t i b i l i t y t e n d s t o r o t a t e t h e d i r e c t i o n of m a g n e t i z a t i o n i n t o t h e p l a n e of m a x i mum s u s c e p t i b i l i t y ( U y e d a e t u Z . , 1 9 6 3 ) . G r a v e n o r e t aZ. ( 1 9 7 3 ) and B a r e n d r e g t e t a Z . ( 1 9 7 6 ) h a v e shown t h a t b u l k m a g n e t i c s u s c e p t i b i l i t y v a l u e s of g l a c i a l s e d i m e n t s can be u s e d a s a r a p i d a n d a c c u r a t e d i a g n o s t i c t e c h n i q u e for t h e g e o l o g i s t t o : (1) i d e n t i f y t h e s o u r c e a r e a of s e d i m e n t s and ( 2 ) d i f f e r e n t i a t e dep o s i t s . G r a v e n o r and S t u p a v s k y ( 1 9 7 4 ) a l s o showed t h a t t h e r e i s a s i g n i f i c a n t c o r r e l a t i o n b e t w e e n t h e amount o f m a g n e t i t e a n d t h e amount o f heavy m i n e r a l s i n a t i l l , t h u s a l l o w i n g m a g n e t i c s u s c e p t i b i l i t y m e a s u r e m e n t s t o r e p l a c e t h e t e d i o u s t a s k of heavy m i n e r a l s e p a r a t i o n , i d e n t i f i c a t i o n a n d c o u n t i n g which i s o f t e n r e q u i r e d for t i l l d i f f e r entiation. ABSOLUTE A G E D A T I N G I n t h e p a s t , g e o c n r o n o l o g i c a l d e t e r m i n a t i o n s u s i n g paleomagnetism h a v e b e e n c a r r i e d o u t a l m o s t e n t i r e l y on d e e p - s e a s e d i m e n t s and l a v a f l o w s . A l t h o u g h some e v i d e n c e for r e v e r s a l s o f t h e e a r t h ’ s f i e l d was r e c o g n i z e d l o n g a g o , i t was n o t u n t i l 1963 t h a t a t t e m p t s were made t o d e f i n e t h e g e o m a g n e t i c p o l a r i t y h i s t o r y . R e v e r s a l s a r e g l o b a l phenomenon and t h u s p r o v i d e u s e f u l marker h o r i z o n s for t h e s t r a t i g r a p h e r . S i n c e s e v e r a l r e v e r s a l s h a v e occu”rred i n t h e l a s t 2 m . y . s o m e o t h e r d a t i n g t o o l or s t r a t i g r a p h i c c o n t r o l must i d e n t i f y t h e r e v e r s a l b e f o r e a n a b s o l u t e c o r r e l a t i o n can b e made. The p o l a r i t y t i m e s c a l e and ass o c i a t e d n o m e n c l a t u r e was f i r s t o u t l i n e d i n d e t a i l b y Cox ( 1 9 6 9 ) a n d h a s b e e n r e f i n e d b y a sub-commission of t h e I n t e r n a t i o n a l Commission on S t r a t i g r a p h y ( I n t e r n a t i o n a l Union o f G e o l o g i c a l S c i e n c e s ) . A new v e r s i o n of t h e t i m e s c a l e ( F i g u r e 4) has b e e n b a s e d on more c o m p l e t e K - A r d a t a (Mankinen and D a l r y m p l e , 1 9 7 9 ) . The b e s t r a d i o m e t r i c a g e i n t h e s c a l e o b t a i n e d by m u l t i p l e K.-Ar d a t e s of ashes and l a v a s , f i x e s t h e b a s e o f t h e O l d u v a i e v e n t a t 1 . 8 t .1 m.y. BP ( C u r t i s a n d H a y , 1 9 7 2 ) . O t h e r d a t e s marking b o u n d a r i e s between normal and r e v e r s e epochs a r e
109
much less accurate. Opdyke e t aZ. (1977) have shown from work in Anza Borrego State Park in California that faunal changes from Blancan to Irvingtonianland mammal ages can be accurately dated using the magnetic record of sediments in which the bones occur. Johnson e t a%. (1975) have shown from work in California, Texas and Kansas, that the oldest Irvingtonian based on the occurrence of Lepus(hare) and other small mammals occurs within the Matuyama reversed polarity epoch in the region o f theOlduvai event, while Blancan faunas occur in the Gauss normal p,olarity epochand range into the lower Matuyama epoch. This correlation has also been found in Saskatchewan, Canada by Foster and Stalker (1976). Nany more fossil ages will undoubtedly become fixed with continued correlation between the paleomagnetic and fossil records in terrestrial sediments (Barendregt and Stalker, 1978). CORRELATION AND RELATIVE AGE DATING In addition to absolute dating, paleomagnetic studies can be used to correlate equivalent horizons and $0 relative age-date horizons provided they show one or more o f the following: (1) a secular variation (oscillations of the local magnetic vector originating principally from the non-dipole field' which can be recognized over a relatively large area); (2) comparable total intensity oscillations of the NFM; and (3) similar magnetic susceptibility characteristics (Gravenor and Stupavsky, 1974; Barendregt e t a t . , 1976). In these cases magnetostratigraphy affords a convenient and simple means of correlating Pleistocene deposits o f either terrestrial or marine origin (see Figure 6). One of the more exciting prospects emerging f r o m such chronologic correlations is calibration of evolution rates and directions of dispersal in restricted mammalian species (Opdyke e t a%., 1977; Lindsay e t a l . , 1976; and Johnson e t a%., 1975). The secular variation record whose basic period generally varies from 4 t o l o 4 years (McElhinny and Merrill, 1975) has been shown to be o f some use for regional correlations by Stober and Thompson, 1977 (see Figure 6) as well as by Turner and Thompson (1979) and Vitorello and van der Voo (1977). 'Turner and Thompson (1979) have shown that secular variation is not caused by wobbling of the main geomagnetic dipole as suggested by Kawai and Hirooka (1967) but rather results dominantly from more localized non-dipole changes involving both westward and eastward drift (see also Denham, 1974). They arrive at this conclusion because their 0-7000 year BP secular variation record f r o m Loch Lomond sediments in Scotland does not compare with Japanese archaeomagnetic records or with North American sediment data f r o m this same period. The cause of long or short period oscillations of the geomagnetic secular variation record remains unknown. The oscillations may be due to the main dipole field wobble, westward or eastward drift of non-dipole sources, fluctuations in intensity o f stationary non-dipole sources, or combinations of each of these. Xowever, any one o f these mechanisms would produce different secular changes in widely separated localities. It is likely that secular variation generally must be due to both non-dipole and dipole changes. The matter is still under intensive discussion in the literature. Verosub (1979) has shown that the geomagnetic pole can make large shifts of 10" or more in less that 200 years. This implies that a sudden change in the paleomagnetic directions as recorded by sediments 'The n o n - d i p o l e f i e l d i s t h e r e s u l t of l o c a l c o m p l e x i t i e s i n t h e m a g n e t i c f i e l d , h a v i n g an a v e r a g e v a l u e o f a b o u t 5 % o f t h e m a i n f i e l d a t t h e e a r t h ' s s u r f a c e , a n d s h o w i n g some e i g h t r e g i o n s , o f c o n t i n e n t a l dimensions d i s p l a y i n g p o s i t i v e o r n e g a t i v e v a l u e s w i t h a n a m p l i t u d e of around .15 O e . The n o n - d i p o l e f i e l d h a s a w e s t e r l y d r i f t o f some . 2 t o .3 d e g r e e s o f l o n g i t u d e p e r y e a r a n d i s t h o u g h t t o o r i g i n a t e f r o m r e g i o n s n e a r t h e core-mantle boundary, where l o c a l c e n t e r s of f l u i d m o t i o n may d i s t o r t t h e m a i n t o r o i d a l f i e l d l o c a l l y . ( F i g u r e s 2 and 3)
110
-
VUOKONUARVI 2 OECLlN AT ION ( 2 0 0 0 c 1 DECLINATION
W INOERMERE OECL INAT ION
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.
*. * .'.:.-',.". . .;..*. *
.-O
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Figure 6
-4500
229 5 0
Paleomagnetic r e l a t i v e d e c l i n a t i o n logs f o r a c o r e from Lake Windermere ( E n g l a n d ) , V u o k o n j a r v i ( F i n l a n d ) c o r e 2 (NRM a n d p a r t i a l l y d e m a g n e t i z e d a t 200 Oe) a n d V u o k o n j a r v i c o r e 1 ( 2 0 0 Oe d e m a g . ) Windermere d a t a i s from Mackereth (1971). ( F i g u r e i s f r o m S t o b e r and Thompson, 1 9 7 7 ) .
may n o t b e i n t e r p r e t e d a s e v i d e n c e f o r t h e o c c u r r e n c e o f a d i a s t e m ( m i n o r d e p o s i t i o n a l b r e a k ) . Many s t u d i e s h a v e shown n o m a g n e t i c anoma l i e s i n l o c a t i o n s a n d s e d i m e n t s where t h e y w o u l d b e e x p e c t e d o n t h e b a s i s o f o t h e r c o r r e l a t i o n s . Though g a p s i n t h e r e c o r d may b e p r e s e n t a n d d a t i n g e r r o r s a r e p o s s i b l e , i t i s n o t u n l i k e l y t h a t some a p p a r e n t p o l a r i t y e x c u r s i o n s ' a r e only very l o c a l i z e d geomagnetic o c c u r r e n c e s o r a r e t h e r e s u l t o f n e c h a n i c a l d i s t u r b a n c e o f s t r a t a . Much more d a t a are needed b e f o r e c o n c l u s i o n s about u s i n g s e c u l a r f i e l d o s c i l l a t i o n s f o r s t r a t i g r a p h i c c o r r e l a t i o n can b e made.
'The I n t e r n a l commission on s t r a t i g r a p h y of t h e I n t e r n a t i o n a l Union of G e o l o g i c a l S c i e n c e s d e f i n e s p o l a r i t y e x c u r s i o n as "a s e q u e n c e of v i r t u a l g e o m a g n e t i c p o l e s w h i c h may r e a c h i n t e r m e d i a t e l a t i t u d e s a n d w h i c h may e x t e n d b e y o n d 1 3 5 " o f l a t i t u d e f r o m t h e p o l e s , f o r a s h o r t i n t e r v a l of t i m e , before returning t o t h e o r i g i n a l p o l a r i t y " (Watkins, 1976)
111
Correlation on the basis of intensity fluctuations presents yet another relative dating tool. Cox (1968) constructed a curve showing the variation in the dipole moment (inte-nsity) of the geomagnetic field. The composite curve obtained by Opdyke e t aZ. (1972) from deep-seacores is similar. One must of course be confident that intensity changes are Hot related to lithologic or mineralogical changes. Correlation on the basis of intensity requires correction to a common datum such as the equator, since intensity varies according to latitude. It was found ‘;hat a gradual drop in sample intensity often corresponds with shallowing of inclination and declination swings. Kean e t aZ. (1979) have <-howrithat bog and lake cores from Cedarburg bog and Lake Michigan have lntensity records which afford good correlation having similar long and s h o r t wavelength features, and both records are correlatable with r’ecords from Lake St. Croix described by Lund and Banerjee (1979). The ratio between natural remanent magnetization (NRM) intensity :?rd initi.ai susceptibility (x) sometimes referred to as the modified Koenigsberger ratio or Q ratio which in deep-sea sediments often re”lects changes in intensity of the geomagnetic field (Harrison, 1966; Lpdyke, 1972; Creer, 1974) has been used by others (Thompson, 1975; ievi and Banerjee, 1976) to correlate cores from lake sediments. Thompson concludes that the Q ra.tio of sediments does not correlate between lakes or in certain cases even within lakes and states that in c,pparently uniform lake sediments and possibly also in deep sea cores lit is important to have shown that NRM intensity and (x) (initial suszeptibility) are due to the same magnetic minerals before using normalized intensities as indicators of ancient field intensities. Levi and aarerjee state that before u s i ~ ga particular sediment core for relative Faleointensity determinations, the sediment’s remanence properties must be established, because only those sections with similar remanence pronerties can be compared for relative paleointensities. They found that :homogeneity can be established from similarities in the NRM properties cf the sediment and from similarities of properties of laboratory inG-uced remanences such as ARM (anhysteretic remanent magnetization) and I R M (isothermal remanent magnetization). On the other hand they found that magnetic susceptibility and saturation magnetization are not favored as normalizing parameters, because they are likely to activate ‘1 disproportionately large fraction of the superparamagnetic and multidomain particles which are relatively less important as stable NRM carriers. The non-dipole portion of the geomagnetic field is presently o b :le~vedt o be drifting westward about . 2 ” per year (Bullard e t aZ., 2959). Drift in a westward direction is in accord with magnetohydrodynamic models in which convective and Coriolis forces control the dominant processes in the fluid core, with electromagnetic coupling occurring between the core and mantle (Denham 1974). However, some features of the field show other than westward motion, as for example at Sitka, Alaska, where the three principle orthogonal field components are all drifting eastward at the present time (Skiles, 1970). Eastward drift has also occurred in the past as can be seen from the paleomagnetic record. These non-dipole magnetic fluctuations may cause the local field vector to move about in a looping fashion, with periods on “ne order of hundreds to thousands o f years. These vector loops may also provide a means of sediment correlation and thus can be considered as a relative dating tool. Working with a suggestion of Runcorn (1959), Skiles (1970) discussed the method for inferring the drift direction using paleomagnetic data from a single site and showed that the study zf vector loops could be valuable for gaining insight into the drift of the ancient geomagnetic field. The procedure is to view the unitvectors of magnetic direction along their axis from negative toward positive, and note whether they tend to trace a clockwise or a counterclockwise loop as time advances (Figure 7). In a normally polarized -r.ain field, such loops were shown to correspond to westward or eastward drift respectively, independent of the source polarity or the latitude of the source and observation site (Denham, 1974). Skiles also notes ‘,hat if westward drift persists through a field reversal as the theories of Bullard e t al. (1950), Hide (1966) and Skiles (1969) indicate, then during a reversal we should look for a counterclockwise
112
a)
Ash
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I
Northwest S i t e
x 90"E
!
Southeast S i t e
Figure 7 Equal-area stereographic p r o j e c t i o n of paleom a g n e t i c d i r e c t i o n s a t two s i t e s a r o u n d Mono L a k e f r o m Denham ( 1 9 7 4 ) . Each r e c o r d exhibits counterclockwise r o t a t i o n of t h e magnetic v e c t o r a l t h o u g h d e t a i l s of t h e two r e c o r d s a r e d i f f e r ent. The t r i a n g l e r e p r e s e n t s the present field direction. t h e arrow d e n o t e s t h e posi t i o n of a d i s t i n c t a s h l a y e r used i n c o r r e l a t i n g between Arrows i n t h e two s i t e s . dicate counterclockwise l o o p i n g motion as t i m e advances. (From V e r o s u b , 1977).
60"
I
,90"E
r o t a t i o n o f t h e f i e l d v e c t o r a t a l l o b s e r v a t o r - 3 s ( S k i l e s , 1 9 7 0 ) . Acc o r d i n g to H i d e (1966) s l o w e a s t w a r d m o t i o n s on t h e c o r e a r e t h e o r e t i c a l l y p o s s i b l e , s o l o n g as t h e o v e r a l l a v e r a g e d r i f t i s m a i n t a i n e d t o wards t h e west. I n t u i t i o n s u g g e s t s t h a t t h e l i f e - s p a n o f a n eastward d r i f t i n g s o u r c e i s i n v e r s e l y r e l a t e d t o i t s r a t e o f d r i f t . Motions c o n t r a r y t o t h e o v e r a l l w e s t w a r d d r i f t may h a v e a low p r o b a b i l i t y o f t r a v e r s i n g g r e a t d i s t a n c e s b e f o r e dying o u t . Thus, it i s q u i t e p o s s i b l e t h a t t h e l i f e - s p a n s o f t h e l o o p a n d t h e p e r t u r b i n g s o u r c e were s i m i l a r (Denham, 1 9 7 4 ) . A s more d e t a i l e d s e d i m e n t a r y d a t a become a v a i l a b l e , d i s t i n c t e v e n t s o f p a l e o - s e c u l a r v a r i a t i o n , s u c h as t h e Mono L a k e c o u n t e r c l o c k w i s e l o o p i n g d e s c r i b e d by Denham ( 1 9 7 4 ) may p r o v e u s e f u l f o r Quaternary s t r a t i g r a p h i c c o r r e l a t i o n over r e l a t i v e l y l a r g e areas. C r e e r e t al. (1972) f o u n d a l o n g - p e r i o d c o u n t e r c l o c k w i s e l o o p i n g o f V . G . P . ' s a b o u t t h e g e o g r a p h i c p o l e f o r t h e i r Lake W i n d e r m e r e d a t a . T h i s data f u r t h e r m o r e s u g g e s t s a h a l f - p e r i o d of a b o u t 1 1 , 0 0 0 y e a r s and t h e y s t a t e t h a t i t i s tempting t o a s s o c i a t e t h i s w i t h t h e p e r i o d of p r e c e s s i o n o f t h e e q u i n o x e s , or some 2 5 , 8 0 0 y e a r s . (Due t o t h e g r a v i t a t i o n a l a t t r a c t i o n of t h e moon a n d t h e s u n on t h e e a r t h ' s e q u a t o r i a l b u l g e , t h e e a r t h ' s s p i n a x i s d e s c r i b e s a cone around t h e c e l e s t i a l p o l e w i t h a p e r i o d of a b o u t 25,800 y e a r s . A s one f a c e s n o r t h t h e p r e c e s s i o n a l m o t i o n i s c o u n t e r c l o c k w i s e , i.e. i n t h e o p p o s i t e s e n s e to t h e r o t a t i o n C r e e r e t aZ., 1 9 7 2 ) . Abrahamsen a n d Readman ( 1 9 8 0 ) a l s o d e s c r i b e a V . G . P . w h i c h moves f i r s t i n a c l o c k w i s e and t h e n i n a c o u n t e r c l o c k w i s e d i r e c t i o n as time a d v a n c e s which t h e y i n t e r p r e t as i n d i c a t i n g e i t h e r a r a t h e r sudden c h a n g e i n t h e s e n s e o f d r i f t o f t h e n o n - d i p o l e f i e l d f r o m westward t o e a s t w a r d , or a f a i r l y r a p i d g r o w t h or d e c a y o f l o c a l d i p o l e c o n f i g u r ations i n the outer core. They u s e t h i s v e c t o r i a l r o t a t i o n p a t t e r n as a p o s s i b l e d a t i n g t o o l and s u g g e s t t h a t t h e Older Yoldia c l a y s n e a r N o r r e L y n g b y , Denmark, may h a v e a n a g e o f 2 4 , 0 0 0 y r s . B P , s i m i l a r t o t h e Mono Lake s e d i m e n t s c o n t a i n i n g s i m i l a r V . G . P . l o o p i n g p a t t e r n s .
113
PROBLEKS AEID L I N I T A T I O N S I N PALEOMAGNETIC ANALYSIS There are a number of possible reasons why the natural remanent magnetization of sediments should not always faithfully reproduce the directions of the ambient magnetic field in which they were deposited. These are outlined by van Montfrans (1971), Verosub (1975), Verosub and Banei,jee (1977) and are summarized here. The reasons include the following: (1) Inclinations may be systematically low due to the preferential alignment o f elongated, tabular or flat shaped magnetic particles. Inclination error may also be due to compaction after deposition. (2) The presence of currents during the deposicion of the sediment, may result in systematically deviating directions of magnetization. ( 3 ) The presence of magnetic material that is too coarse to be aligned by the ambient field during or shortly after deposition and thus fails to record the directions of the magnetic field. (4) Bioturbation after deposition of the sediment results in randomly distributed directions of magnetization or a re-alignment of particles in a field direction different from the one present during original deposition. (5) Collapsing of sediment on a small scale during decalcification. (6) The presence of cryoturbatic structures resulting in randomly distributed directions in these structures. (7) Deformation of sediments as a result of glacial pushing (over-riding), turbidity currents, slumping, liquifaction or some other process which moves sedime'nts away from their original depositional orientation spoils the record. (8) Possible self-reversal, as a result of Dost-secondary alterations ir, the ionic ordering of the crystal framework of the magnetic constituents of the sediment (Irving, 1964). (9) Formation of new Tagnetic minerals during a tlme when the polarity of the geomagnetic field was different or opposite to the period of original deposition. (10) Instability of the natural remanent magnetization, allowing for viscous components of magnetization to set in. (li) Small inaccuracies in field sampling and nreoaration of specimens in the laboratory ( i . e . drying). (12) Errors and misorientations, made during field sampling r,r labelling of samples. Corinq operations may result in breaking and twisting of' the sediments which may then be interpreted as an excursion. These errors can usually be traced. For the above reasons, some sample data must be discarded because +he analytical results Show: (1) a remanent magnetization which makes too large an angle with the direction which would result from the field of an axial geocentric dipole, taking into account secular variation;
(2)
the sediments were not stably magnetized; and
(3)
the present of a stable secondary magnetization.
The glacial sediments of the Quaternary represent special problems in magnetostratigraphic correlation. Just as glaciers picked up, transported and deposited bedrock blocks (Stalker, 1976) they may well have picked up, transported and deposited blocks of frozen glacial sediments and placed them in anomalous stratigraphic positions. A l s o , over-riding of glacial sediments by renewed glaciation may have induced stress deformation (Stupavsky e t al., 1979) and given rise to structures which will yield 'apparent' magnetic excursions, such as those described by Verosub (1975). Other mechanisms of deformation and disturbance of glacial (lake) sediments include seismic activity, turbidity flows, density currents, and periglacial activity such as cryoturbation and congelifluction. Glacial sediments often lack continuous fine-grained sediments or contain large hiatuses or were subject to erosion, all of which represent interruptions in their paleomagnetic record. Glacialtills also represent special problems (Barendregt e t a Z . , Time stratigraphic lines within tills are often n o t horizontal due to different rates of erosion and/or deposition. Stupavsky e t aZ. (1979) in a discuesion of the Meadowcliffe till show a lack of correlation of the remanence characteristics over short interprofile separations leading them to conclude that the till deposition was non-uniform h-ith rapid and variable rates of deposition from point to point within the till. In a study of the Seminary till Gravenor e t a Z . (1979) and
1977).
114 Symons e t aZ. ( 1 9 8 3 ) h a v e p r o p o s e d t h a t t h e t h i x o t r o p i c e f f e c t r e p o r t e d b y Garzes ( 1 9 7 7 ) f o r a d o b e b r i c k s , i s a p r o b a b l e c a ~ l s efor s c n e r e v e r s e d d i r e c t i o r - s . According t o them, t h e l a c k o f c o r r e l a t i o n i n remanenee d i r e c t i o n s b e t w e e n t w o p r o f i l e s .7 m a p a r t a n d t h e l a r g e w i t h i n - c o r e v a r i a t i o n of 1 3 " between specimen p a i r s : s u g g e s t t h e p o s s i b i l i t y o f r e m a n e n c e r e s e t t i n g c a u s e d by t h e hammer b l o w s o r t h e p l a s t i c t u b e s a . n p l e r which t h e y u s e d .
I t h a s b e e n s u g g e s t e d . t h a t t i l l s e x n i b i i ; a m a g n e t i c r e m a n e n c e ',hat i s l a r g e l y t h e r e s u l t o f t h e d i r e c t i o n o f i c e movement,' a n d n o t d u e t o t h e p r e s e n c e o f a l a y e r o f water a t t h e i c e / s e d i m e n t i n t e r f a c e which w o u l d h a v e a l l o w e d t h e m a g n e t i c g r a i n s t o become a l i g n e d i n t h e e a r t h l s a m b i e n t f i e l d . The f o r m e r h a s b e e n r u l e d o u t by a n i s o t r o p y o f m a g r e t i c s u s c e p t i b i l i t y measurements c a r r i e d o u t by Gravenor and Stupavsky ( 1 9 7 4 ) who h a v e shown t h e r e m a n e n c e a n d t h e a n i s o t r o p y o f m a g n e t i c s u s c e p t i b i l i t y of t i l l s t o have d i s t i n c t l y differer,t d i r e c t i o n s . I n c l i n a t i o n e r r o r s a r e a l s o commonly r e p o r t e d f o r t i l l s . L a b o r a t o r y e x p e r i m e n t s c o n d u c t e d by V e r o s u b e t aZ. ( 1 3 7 9 ) showed t h e p a r t i a l r e a l i g n m e n t o f a r t i f i c i a l s e d i m e n t s l , x m i e s on s t i r r i n g by ! s h e a r induced l i q u i f a c t i o n ' t o give a post-depositional DRY. I n t h e same s t u d y , i n c l i n a t i o n e r r o y s o f up t o 30" were p r o d g e e d i n t h i n - t i l l s l u r r i e s w h i l e i n t h i c k - t i l l s l u r r i e s n o s i g n i f i c a n t e r r o r s w e r e n o t e d . The a b o v e m e n t i o n e d v a r i a b i l i t y as w e l l a s t h e s h o c k - i n d u c e d t h i x o t r o p i c r e s e t t i n g a s a r e s u l t o f s a m p l i n g p r o c e d u r e s may e x p l a i r . t h e f a r - s i d e d V . G . P . ( V i r t u a l G e o m a g n e t i c P o l e ) p o s i t i o n s for t i l l s s a m p l e ? , i n O n t a r i o (Symons e t aZ., 1 9 8 0 ) a n d i n A l b e r t a ( B a r e n d r e g t e t aZ., 1 9 7 7 ) . I n c l i n a t i o n e r r o r due t o t h e p r e f e r e n t i a l a l i g n m e n t of i r r e g u l a r - s h a p e d m a g n e t i c p a r t i c l e s n a y a;so e x p l a i n some o f t h e f a r - s i d e d V.G.?. positions. A r e c u r r i n g problem f a c i n g t h e s t u d e n t of Q u a t e r n a r y paleomagnetism t o d a y , i s t h e l a r g e number o f r e p o r t e d e x c u r s i o n s w h i c h c a n n o t b e prop e r l y c o r r e l a t e d or a r e b a s e d o n m e a s u r e m e n t s o f d o u b t f u l a c c u r a c y . C o n c l u s i o n s a r e s o m e t i m e s d r a w n f r o m Lncorr-Dlete d a t a (Opdyke , 1 9 7 6 ) or a r e b a s e d o n i n s u f f i c i e n t s a m p l i r , g s i t e s ( K u k l a a n d Nakagawa, 1 9 7 7 ) . For t h e n o v i c e t h e f o l l o w i n g g u i d e l i n e m i g h t w e l l b e u s e f u l : "a n e g a t i v e i n c l i n a t i o n d o t h n o t a r e v e r s a l m-ake".
I n s e v e r a l p a r t s of t h e w o r l d , t e r r e s t r i a l paleorr.agr.etlc r e c o r d s show e x c u r s i o n s d u r i n g t h e S r u n h e s r o r m a l p o l a r i t y e p o c h . T h e s e e x c u r s i o n s a r e - r e p o r t e d i n g r e a t numbers f o r s e d i m e n t s which a r e between 8,000 and 2 0 , 0 0 0 y e a r s i n a p e . However, i n deep-sea r e c o r d s and t h e Aegean S e a , t h e r e i s n o e v i d e n c e f o r a r e v e r s a l or e x c u r s i o n o f t h e f i e l d d u r i n g t h i s t i m e (Opdyke e t aZ., 1 9 7 2 ) . T r u e r e v e r s e d m a g n e t i z a t i o n s i n s e d i m e n t s a r e c a u s e d by a r e v e r s a l of t h e main d i p o l e f i e l d and s h o u l d be s e e n world-wide. Excursions of t h e f i e l d ( d e v i a t i o n s of t h e d i r e c t i o n of t h e f i e l d g r e a t e r t h a n t h e n o r m a l s e c u l a r v a r i a t i o n o f a f e w t e n s o f d e g r e e s ) codld also b e c a u s e d b y t h e m a i r d i p o l e f i e l d t i l t i n g a t a l a r g e a n g l e t o t h e r o t a t i o n a x i s and t h e n r e t u r n i n g t o t h e same o r i e n t a t i o n a s b e f o r e . S u c h e x c u r s i o n s s h o u l d a l s o b e s e e n w o r l d w i d e . Kowever, s o u r c e s of t h e n o n - d i p o l e f i e l d c o u l d a l s o p r o d u c e a n e x c u r s i o n t h a t c o u l d c h a n g e t h e d i r e c t i o n o f t h e f i e l d a t o n e p o i n t by 1 8 0 0 , r e s u l t i n g i n a n a p p a r e n t r e v e r s a l a t t h a t p o i n t . Such c o n d i t i o n s would be i n d i s t i n g u i s h a b l e from a r e v e r s a l o f t h e d i p o l e f i e l d , u n l e s s o b s e r v a t i o n s a t e x a c t l y t h e same t i m e i n t e r v a l f r o m a n o t h e r p a r t o f t h e e a r t h do n o t show a r e v e r s a l . I n d e t e r m i n i n g t h e z o n e o f d i s t u r b a n c e o f a d i p o l e f i e l d c a u s e d by ( i . e . one n o t i n v o l v i n g t h e main d i p o l e f i e l d ) a t one p o i n t i n t h e e a r t h l s s u r f a c e , H a r r i s o n and Ramirez ( 1 9 7 5 ) developed a model which p l a c e d a v e r t i c a l d i p o l e a t t h e s u r f a c e o f t h e e a r t h ' s c o r e whose m a g n e t i c f i e l d i s o p p o s i t e t o t h a t o f t h e main d i p o l a r f i e l d . They show t h a t t h e a r e a l c o v e r a g e o f t h e d i s t u r b e d m a g n e t i c f i e l d a r o u n d s u c h a p s e u d o - r e v e r s a l c a n b e q u i t e s m a l l , s u c h t h a t o b s e r v a t i o n s made o n l y a f e w t h o u s a n d km away w o u l d show n o a n o m a l o u s d i r e c t i o n . If t h i s m o d e l i s a p p l i c a b l e t h e r e a r e n o c o n t r a d i c t o r y o b s e r v a t i o n s o f t h e exi s t e n c e o f a r e v e r s a l a t Laschamp a n d o t h e r l o c a t i o n s h a v i n g s i m i l a r a g e , p r o v i d e d %hey a r e p s e a d o - r e v e r s a l s c a u s e d by n o n - d i p o l e f i e l d
a pseudo-reversal
115
s o u r c e s . H a r r i s o n and Ramirez, f u r t h e r show t h a t i f t h e v e r t i c a l d i p o l e model f o r t h e n o n - d i p o l e f i e l d i s c o r r e c t , t h e n p s e u d o - r e v e r s a l s s h o u l d be much l e s s common a t low l a L i t u d e s t h a r , a t h i g h l a t i t u d e s . Geomagnetic e x c u r s i o n s d u r i n g t h e Brunhes e p o c h a r e more and more b e i n g c o n s i d e r e d as m a g n e t o s t r a t i g r a p h i c m a r k e r s . These e x c u r s i o n s a r e a p p a r e n t l y s h a r p movements o f t h e V i r t u a l Geomagnetic P o l e ( V G P ) t o w a r d s or beyond t h e e q u a t o r f o l l o w e d by a r e t u r n t o t h e s t a b l e p o s i t i o n , a l l w i t h i n t h e t i m e s p a n o f a few t h o u s a n d y e a r s . Table 1 provides a l i s t of some w e l l documented e x c u r s i o n s . A u s e f u l d i s c u s s i o n o f some o f t h e s e g e o m a g n e t i c e x c u r s i o n s i s p r o v i d e d by Verosub and B a n e r j e e (1977) who view w i t h c a u t i o n t h e p a l e o m a p e t i c e v i d e n c e p r o v i d e d by t h e v a r i o u s authors. T a b l e 1.
Name
Paleomagnetic P o l a r i t y Excursions and Proposed Ages D u r i n g B r u n h e s Epoch
a
Proposed age ( y r s . BP)
Researchers
Laschamp e v e n t
Bonhommet a n d B a b k i n s ( 1 9 6 7 ) Bonhommet a n d Z a h r i n g e r ( 1 9 6 9 ) H a l l , Y o r k a n d Bonhommet ( 1 9 7 9 ) Heller (1980)
8,000-20,000
Mono L a k e Excursion
Denham a n d Cox Denham ( 1 9 7 4 )
2 4 ,0 0 0
Gothenburg Event
M o r n e r e t at. ( 1 9 7 1 ) Morner and L a n s e r (1974) Morner (1977)
12,350-12,400
Blake Event
Smith and F o s t e r (1969) Denham e t at. ( 1 9 7 6 ) Denham e t al. ( 1 9 7 7 )
1 0 5 , 000
L a k e Mungo E v e n t
B a r b e t t i and McElhinny
L a k e Biwa Excursions
N a k a j i m a e t at. ( 1 9 7 3 ) Y a s k a w a e t al. ( 1 9 7 3 )
18,000 and 104,000-117,000
Erieau Excursion
C r e e r e t at. ( 1 9 7 6 )
7,600-14,000
Lake Michigan Excursions
V i t o r e l l o a n d v a n d e r V oo
Gulf of Mexico Excursions
Freed and Healy (1974) C l a r k and Kennett (1973)
15,000-18,000 30,000-33,000
Mott and F o s t e r
1 2 ,5 0 0
Maple H u r s t Lake and Basswood Road Lake E x c u r s i o n
(1971)
(1976)
(1977)
(1973)
2 9 ,5 0 0
7,500 and 13,000
Meadowcliffe Excursion
s t u p a v s k y e t at. ( 1 9 7 9 )
3 0 , 500
P o r t Dover Excursion
Morner
13,300
Maelifell Event
P e i r c e and C l a r k (1978)
Rubjerg Excursion
Abrahamsen and Knudsen
(1979)
23 ,000-40
N o r r e Lyngby Excursion
A b r a h a m s e n and Readman
(1980)
23,000-40,000
Kipp E x c u r s i o n
Barendregt and S t a l k e r ( i n p r e s s ) 24,000(?)
(1976)
and
40,000 (?)
a L o c a t i o n names a r e u s e d o n l y t o i d e n t i f y t h e r e p a r t e d a r e n o t n e c e s s a r i l y e s t a b l i s h e d names.
,0 0 0
e x c u r s i o n s and
116
The G o t h e n b u r g e v e n t ( l a b e l J e d a s a m a g n e t i c ' f l i p ' i n t h e l i t e r a t u r e ) i s e s p e c i a l l y problematic since t h e authors suggest t h a t the e v e n t l a s t e d o n l y some t e n s o f y e a r s w h i l e t h e i n c l i n a t i o n s w i t c h o n l y t o o k a few y e a r s ( M o r n e r , 1 9 7 7 ) . T h i s would make i t t h e most r a p i d The mechanism f o r s u c h a n m a g n e t i c p o l a r i t y change known a t p r e s e n t . abr?.ormally r a p i d ' f l i p ' i s d i f f i c u l t t o i m a g i n e . R e c e n t l y Thompson and 3 e r g l u n d ( 1 9 7 6 ) r e p o r t e d f i n d i n g no e v i d e n c e f o r t h e Gothenburg e v e n t i r . a d e t a i l e d s t u d y of c o r e s from Sweden. They a t t r i b u t e p r e v i o u s r e p o r t s o f anomalous d i r e c t i o n s as a n example o f t h e ' r e i n f o r c e m e n t syndrome' ( W a t k i n s , 1 9 7 2 and W a t k i n s , 1 9 7 6 ) . From c o n t i n u o u s r e c o r d s of t h e g e o m a g n e t i c f i e l d for t h e p e r i o d 0 - 1 6 , 0 0 0 y e a r s BP o b t a i n e d from t h e s e d i m e n t s o f two p o s t g l a c i a l l a k e s i n M i n n e s o t a and a r e - e x a m i n a t i o n o f t h e p r i m a r y d a t a r e l a t e d to t h e Gothenburg e v e n t a n d E r i e a u e x c u r s i o n , B a n e r j e e and Lund ( 1 9 7 9 ) c o n c l u d e t h a t b o t h e x c u r s i o n s m i g h t i n f a c t be a r t i f a c t s o f t h e l i t h o l o g y ( s e e a l s o B a n e r j e e e t al., 1 9 7 9 ) . T h u s i t becomes o b v i o u s t h a t p a l e o m a g n e t i c r e c o r d s a s r e p o r t e d i n t h e l i t e r a t u r e must b e c r i t i c a l l y w e i g h e d . Even from d e e p - s e a c o r e s t a k e r , o v e r t h e p a s t 2 0 y e a r s , t h e r e i s e v i d e n c e f o r ' s t a b l e ' and ' u n s t a b l e ' c o r e s . A c c o r d i n g t o Verosub and a a n e r j e e ( 1 9 7 7 ) : "A s t a b l e c o r e i s one w h i c h g i v e s a c l e a n p a l e o m a g n e t i c r e c o r d w i t h s i m p l e f e a t u r e s t h a t are generally consistent with the accepted r e v e r s a l seq u e n c e . An u n s t a b l e c o r e on t h e o t h e r hand g i v e s a complex p a l e o m a g n e t i c r e c o r d w i t h many a p p a r e n t c h a n g e s i n p a l e o m a g n e t i c d i r e c t i o n . These cannot b e c o r r e l a t e d w i t h t h o s e i n o t h e r c o r e s . " H a r r i s o n (1974) c o n c l u d e s t h a t many of t h e d e e p - s e a s e d i m e n t c o r e s show s h o r t - p e r i o d e v e n t s which c a n n o t b e c o r r e l a t e d w i t h t h e p r e s e n t l y known t i m e s c a l e of r e v e r s a l s a n d t h e r e f o r e s h o r t - p e r i o d e v e n t s a r e n o t r e l i a b l e s t r a t i graphic markers. O t h e r c o r e s i n which t h e r e i s a c o r r e l a t i o n between d i r e c t i o n a n d / o r i n t e n s i t y o f m a g n e t i z a t i o n , and c l i m a t i c i n d i c a t o r s , do n o t seem t o h a v e a c c u r a t e l y r e c o r d e d D a r a m e t e r s of t h e E a r t h ' s magnetic f i e l d . I t i s hoped t h a t t h r o u g h d e t a i l e d work on t e r r e s t r i a l s e d i m e n t s , t h e mechanism whereby s e d i m e n t s become m a g n e t i z e d w i l l b e b e t t e r u n d e r s t o o d and some of t h e d i s c r e p a n c i e s s o l v e d .
On l a n d , N o r t h American s t u d i e s o f l a k e s e d i m e n t s h a v e o f t e n b e e n made on s i n g l e s a m p l e s p e r h o r i z o n , I 4 C d a t e s a r e few and f r e q u e n t l y have l a r g e e r r o r l i m i t s . Verosub and B a n e r j e e ( 1 9 7 7 ) o b s e r v e t h a t : "Most d i s t u r b i n g o f a l l i s t h e f a c t t h a t t h e o b s e r v e d e x c u r s i o n s a r e f r e q u e n t l y o b s e r v e d n e a r l i t h o l o g i c b o u n d a r i e s r e f l e c t i n g a change from g l a c i a l t o p o s t g l a c i a l times w i t h concomitant r a p i d f l u c t u a t i o n s expected i n t h e depositional conditions i n the lakes." H a r r i s o n and Ramirez ( 1 9 7 5 ) h a v e shown t h a t t h e s m a l l e s t l a t e r a l d i s t a n c e o v e r which a g e o m a g n e t i c f l u c t u a t i o n would b e o b s e r v e d i s a r o u n d 1 0 0 0 km. For t h i s r e a s o n , a n e x c u r s i o n s h o u l d b e r e c o r d e d i n n e a r b y a r e a s i n s e d i m e n t s o f s i m i l a r a g e and as Verosub and B a n e r j e e (1977) c l e a r l y p o i n t out: "Failure t o d e t e c t a paleomagnetic excursion i n a n a d j a c e n t l a k e or o c e a n b a s i n i s a n i m p o r t a n t r e s u l t which s h o u l d n o t go u n r e p o r t e d b e c a u s e i t i s u n i n t e r e s t i n g . " U s i n g a s i m i l a r model t o t h a t of H a r r i s o n and R a m i r e z , Denhamet aZ. ( 1 9 7 6 ) showed t h a t t h e B l a k e e v e n t may n o t h a v e b e e n f e l t o v e r m o r e t h a n 9 % o f t h e e a r t h ' s s u r f a c e . On t h e o t h e r h a n d , S m i t h ( 1 9 6 7 ) and Verosub and Cox (1971) h a v e shown t h a t p a l e o m a g n e t i c e x c u r s i o n s may r e f l e c t g l o b a l g e o m a g n e t i c phenomena r e s u l t i n g from t h e change i n t h e r e l a t i v e s i z e s o f t h e d i p o l e and n o n - d i p o l e f i e l d s .
A l l t h i s i s not t o say t h a t localized excursions a r e not useful. T h e Q u a t e r n a r y s t r a t i g r a p h e r must f i r s t come t o r e a l i z e t h a t t h e y can b e l o c a l i n n a t u r e o r may h a v e a w o r l d w i d e o c c u r r e n c e , and t h e n b e a b l e t o d e t e r m i n e t h e i r a g e s o t h a t t h e y can b e u s e d a s s t r a t i g r a p h i c m a r k e r s . I f a n e x c u r s i o n c a n b e shown t o b e o f g l o b a l e x t e n t and d i p o l a r i n
o r i g i n , a v e r y u s e f u l marker h o r i z o n h a s i n d e e d been found.
F i n a l l y , i f e a c h of t h e r e p o r t e d e x c u r s i o n s l i s t e d i n Table 1 r e p r e s e n t s a d i s t i n c t e x c u r s i o n , t h e n t h e g e o m a g n e t i c f i e l d i s much l e s s s t a b l e t h a n had p r e v i o u s l y b e e n t h o u g h t . Such a h i g h d e g r e e of i n s t a b i l i t y when e x t e n d e d b a c k o v e r g e o l o g i c a l time would p r o d u c e a
117
m a g n e t i c f i e l d f a r more c o m p l e x t h a n h a s b e e n o b s e r v e d i n t h e r e c o r d ( V e r o s u b , 1975). C 0 NC iED I NG RE MARKS
D a t i n g by p a l e o m a g n e t i c c h a r a c t e r i z a t i o n a n d g e o m a g n e t i c p o l a r i t y h i s t o r y i s a r e l a t i v e l y riew t e c h n i q u e . The l a r g e - s c a l e f e a t u r e s o f t h e e a r t h ‘ s m a g n e t i c f i e l d c h a r a c t e r have been well worked o u t f o r t h e p a s t 5 m i l l i o n y e a r s o r s o . The d e t a i l e d s m a l l - s c a l e f e a t u r e s f o r t h i s p e r i o d a r e s t i l l b e i n g d i s c o v e r e d and d e f i n e d t h r o u g h a n a l y s i s of t e r r e s t r i a l s e d i m e n t s . B e c a u s e o f t h e much g r e a t e r s e d i m e n t a t i o n r a t e on l a n d , t h e s e a r e n o r e l i k e l y t o show s h o r t - l i v e d e v e n t s a n d r e c o r d t h e e x c u r s i o n s w h i c h u l t i m a t e l y w i l l become o s e f o i c o r r e l a t i v e t o o l s . The o n l y p r a c t i c a l way of d e m o n s t r a t i n g t h e v a l i d i t y o f i n t e r p r e t e d m a g n e t o - s t r a t i g r a p h y i s t o show t h a t r e s u l t s a r e r e p r o d u c i b l e i r ? w i d e l y s e p a r a t e d s e c t i o n s w i t h d i f f e r e n t l i t h o l o g y and s e d i m e n t a t i o n r a t e s . I n Canada where P l e i s t o c e n e d e p o s i t s a r e l a r g e l y g l a c i a l i n o r i g i n , a n d w e r e t h u s e p i s o d i c , o n e m u s t b e aware t h a t t h e s e d e p o s i t s may o n l y h a v e recorded t h e earth’s magnetic f i e l d i n s h o r t time i n t e r v a l s . P o s s i b l e s u b s e q u e n t a l t e r a t i o n of t h e s e s e d i m e n t s by t h e p r o c e s s e s o u t l i n e d e a r l i e r , must be b o r n e i n mind. Great Lake s e d i m e n t s , g l a c i a l l a k e s e d i m e n t s a n d d e p o s i t s s u c h as t h o s e d e s c r i b e d b y S t a l k e r i n w e s t e r n Canada ( F o s t e r a n d S t a l k e r , 1 9 7 6 ) p r o v i d e e x c e l l e n t o p p o r t u n i t y f o r m a g n e t o - s t r a t i g r a p h i c c o r r e l a t i o n and d a t i n g .
ACKNOWLEDGEMENTS The a u t h o r ’ s w o r k i n Q u a t e r n a r y p a l e o r n a g n e t i s m h a s b e e n s u p p o r t e d by g r a n t s f r o m t h e N a t i o n a l R e s e a r c h C o u n c i l o f C a n a d a a n d t h e D e p a r t ment o f E n e r g y , M i n e s a n d R e s o u r c e s . I a m g r a t e f u l t o Drs. J.H. F o s t e r , K . L . V e r o s u b , D . P a c k e r , a n d A . M . S t a l k e r for t h e i r c o n s i d e r a b l e h e l p i n b r o a d e n i n g my u n d e r s t a n d i n g o f p a l e o r n a g n e t i s m a s a u s e f u l t o o l i n I a m i n d e b t e d t o D r s . A . L a t h a m a n d H . C . Palmer Quaternary research. for c r i t i c a l l y r e v i e w i n g t h e m a n u s c r i p t a n d m a k i n g many h e l p f u l c o m r n e n t s . REFERENCES C I T E D A b r a h a m s e n , N . a n d K n u d s e n , K . L . , 1 9 7 9 , I n d i c a t i o n o f a g e o m a g n e t i c lowi n c l i n a t i o n excursion i n supposed middle Weichselian I n t e r s t a d i a l m a r i n e c l a y a t R u b j e r g , Denmark: P h y s i c s of t h e E a r t h and P l a n e t a r y I n t e r i o r s , v . 1 8 , p . 238-246. Abrahamsen, N . a n d Readman, P . W . , 1 9 8 0 , G e o m a g n e t i c v a r i a t i o n s r e c o r d e d i n O l d e r ( 2 2 3 , 0 0 0 BP) a n d Y o u n g e r Y o l d i a c l a y ( - 1 4 , 0 0 0 BP) a t N o r r e Lyngby, Denmark: G e o p h y s . J. R . A s t r . S O C . , v . 6 2 , p . 345366. A l l d r e d g e , L . R . and H u r w i t z , L . H . , 1964, R a d i a l d i p o l e s as t h e s o u r c e s of t h e E a r t h ’ s main m a g n e t i c f i e l d : J . G e o p h y s . Res.., v . 6 9 , n o . 1 2 , p . 2631-2640. As,
and Z i i d e r v e l d , J . D . A . , i n paleomagnetic research: 78, p. 1-56.
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1958, A c l a s s of s e l f - s u s t a i n i n g d i s s i p a t i v e s p h e r i c a l Ann. P h y s . , v . 4 , p . 3 7 2 - 4 4 7 .
B a n e r j e e , S.K. and Lund, S . P . , 1979, Gothenburg and E r i e a u e x c u r s i o n s q u e s t i o n s r e g a r d i n g t h e i r v a l u e as m a g n e t o s t r a t i g r a p h i c markers: EOS T r a n s . Am. G e o p h y s . U . , v . 6 0 , n o . 1 8 , p . 2 3 8 . B a n e r j e e , S . K . e t az., 1 9 7 9 , G e o m a g n e t i c r e c o r d i n M i n n e s o t a l a k e sediments-absence of t h e Gothenburg and E r i e a u e x c u r s i o n s : Geology V. 7 , p. 588-591. 1 9 7 6 , T h e L a k e Nungo g e o m a g n e t i c B a r b e t t i , N.F. a n d M c E l h i n n y , M . C . , London, v . 281, p . 515-542. excursion: P h i l . T r a n s . R . Soc,,
118 B a r e n d r e g t , R.W. e t a l . , 1 9 7 6 , D l f f e r e n t i a t i o n o f t i l l s i n t h e PakowkiP i n h o r n area of s o u t h e r n A l b e r t a on t h e b a s i s o f t h e i r magnetic susceptibility: Geol. S u r v . Can. P a p e r 76-1C, p . 189-190.
, 1977, Paleomagnetic remanence c h a r a c t e r i s t i c s of t i l l s found i n Pakowki-Pinhorn area of s o u t h e r n A l b e r t a : Can. P a p e r 77-1B, p . 271-272.
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Creer, K.M., 1974, Geomagnetic v a r i a t i o n s f o r t h e i n t e r v a l 7,000-25,000 y r . BP a s r e c o r d e d i n a c o r e o f s e d i m e n t f r o m S t a t i o n 1 4 7 4 o f t h e B l a c k S e a c r u i s e o f A t l a n t i s 11: E a r t h P l a n e t . S c i . L e t t . , v . 2 3 , p . 34-42.
, 1977, Geomagnetic s e c u l a r - v a r i a t i o n s during t h e l a s t 25,000 y e a r s : a n i n t e r p r e t a t i o n of d a t a o b t a i n e d from r a p i d l y deposited sediments: Geophys. J . R . A s t r o n . SOC., v . 4 8 , p . 91-109. C r e e r . K . M . e t al.. 1 9 7 2 , G e o m a-g n e t i c s e c u l a r v a r i a t i o n r e c o r d e d i n t h e s t a b l e m a g n e t i c remanence of Recent s e d i m e n t s : Earth Planet. Sci. L e t t . , v . 1 4 , p. 105-127.
, 1976, Late Quaternary geomagnetic s t r a t i g r a p h y recorded i n E a r t h P l a n e t . S c i . L e t t . , v . 3 1 , p . 37-47. Lake Erie sediments: C u r t i s , G.H. and Hay, R.L., 1972, F u r t h e r g e o l o g i c s t u d i e s and K-Ar Bishop, W.W. d a t i n g of O l d u v a i Gorge and Ngorogoro Crater, i n and Miller, J . A . , e d s . , C a l i b r a t i o n of Hominoid E v o l u t i o n : Edinburgh, S c o t t i s h Academic P r e s s , p . 289-302.
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Denham, C . R . , 1 9 7 4 , C o u n t e r c l o c k w i s e m o t i o n o f 2 4 , 0 0 0 y e a r s a g o a t Mono L a k e , C a l i f o r n i a : V. 26, p . 487-498.
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Denham, C . R . a n d C o x , A . , 1 9 7 1 , E v i d e n c e t h a t t h e L a s c h a m p p o l a r i t y event did not occur 13,300-30,400 years ago: Earth Planet. Sci. L e t t . , v. 13, p . 181-190. D e n h a m , C . R . e t al., 1 9 7 6 , B l a k e p o l a r i t y e p i s o d e i n t w o c o r e s f r o m t h e Greater A n t i l l e s o u t e r r i d g e : Earth Planet. Sci. L e t t . , v. 29, p . 422-434.
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Garnes, K.P., 1 9 7 7 , The m a g n i t u d e of t h e p a l e o m a g n e t i c f i e l d : a new non-thermal, n o n - d e t r i t a l method u s i n g sun-dried b r i c k s : Geophys. J . o f t h e Roy. A s t r o n . S O C . , v. 48, p . 315-329. G r a v e n o r , C.P. a n d S t u p a v s k y , M . , 1 9 7 4 , M a g n e t i c s u s c e p t i b i l i t y o f t h e s u r f a c e t i l l s of southern Ontario: C a n . J . E a r t h S c i . , v . 11, N o . 5 , p . 658-663. Gravenor, C.P. e t a z . , 1 9 7 3 , Paleomagnetism a n d i t s r e l a t i o n s h i p t o till deposition: Can. J . E a r t h S c i . , v . 1 0 , p . 1068-1078.
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Utrecht
123
PALEOMAGNETIC DATING OF QUATERNARY SEDIMENTS: A REVIEW M. STUPAVSKY and C.P. GRAVENOR
ABSTRACT Taleomagnetic dating of Quaternary sediments is a secondary dating method that is based on matching polarity transitions, excursions and secular variations of the paleomagnetic field recorded in the sediments with their radiometrically dated equivalents in the magnetic polarity time scale. Approximately 10 polarity reversals (epochs and events) are known tc have occurred during the Quaternary and, in many cases, these have been used successfully to date deep-sea and terrestrial sediments. In ccntrast, only a few sediments have been dated successfully by their peleosecular variation and oolarity excursion records. The limited success rate of the use of these small amplitude time variable charazteristics of the paleomagnetic field recorded in sediments results from the introduction of "magnetic noise" of comparable amplitude into t k e sediment magnetic record during sedimentation or sampling. At present, the probability of successfully dating a sedimentary sequence by paleomagnetic methods is low (approximately 20% for deepsea cores). The factors involved in this l o w success rate include, sedimentation processes which create distortions in the record, unrecognized sediment deformation after remanence acquisition, remanence resetting caused by shock induced thixotropy ( e . g . , during sampling or b ; ~ earthquakes), diagenetic production of secondary minerals and magnetic remanence instability. Nevertheless, the success rate is being izproved significantly by the use of better sampling methods, sampling strategy, magnetic cleaning, and screening and smoothing of sediment renanence data to reject "magnetic noise". These improved procedures are designed to identify and reject spurious observations and refine the geomagnetic calendar. This refinement of the geomagnetic time scale is an ongoing process and relative to other sediment dating techniques that span the Quaternary. Successful paleomagnetic dating has the highest relative and absolute time resolution. INTRODUCTION Paleomagnetic dating of sediments is a secondary dating method in that it is dependent upon a radiometrically dated magnetic polarity time scale and master secular variation curves to determine the magnetic remanence age of a sediment (Creer e t a Z . 1976a; Cox, 1969; Dodson e t a Z . , 1977; Lund e t aZ., 1976; Mankinen and Dalrymple, 1979; Thompson,
1975, 1978).
Thas, the first requirement in the da5ing prccess Is to neasure paleomagnetic field variations in rocks that have been dated radiometrically and, from this information, build up global paleomagnetic polarity time scale and regional master secular variation curves. The
124 p a l e o m a g n e t i c v a r i a t i o n s i n r o c F s o f urlknown a g e a r e t h e n m e a s u r e d a n d t h e r e s u l t s m a t c h e d a g a i n s t p o l a r i t y t i m e s c a l e or m a s t e r s e c u l a r v a r i a t i o n curves i n o r d e r t o d e t e r m i c e t h e p a l e o m a g n e t i c a n d a b s o l u t e a g e o f t h e r o c k s ir. q u e s t i o n . I t i s t h e p u r p c s e o f t h i s r e p o r t to p ~ c v i d ea " s t a t e - o f - t k e - a r " ' r e v i e w o f t h e p a l e o n a g r > e t i c d a t i n g o f Qua'ernary s e d i m e n t s 3 r d t o show t h e F o t e n t i a l arA3l i m i % a 5 F c n s sf t h e t , e c h n i ~ l ~ss e zfi a?:! Ir, 5 ~ 5 T n ga-.d c o r r e l a t i r g s e 6 , l a e z t s ~ : ? i c hh a v e beer, s e g c s i t e d c v e r t h e z a s t l.? nilliov. years.
F L,Ec::'IA\ ::;JzI'Z I;IFL3 y 1L*:E P a l e o r n a g n e t i c s t d d l e s c n rocks r z i i g i - g Ir, age f r c n t h e e a r l l e s t P r e c a m b r i a n t c t h e &1J.aternary shc1.r t h a t 5ke Earth's r r a g r . e t l c f i e l d exh i b i t s p o L a r 5 t y r e v e r s a l s at f r e = _ i i e n tI-tervals t h r c x g h o u t g e o l c g i c t i n e . Tine i n t e r v a l s l a s t i r i g a p p r c x i n a ' e l y 106 y e a y s d ' d r i n g w h i c h t h e m a g n e t i c p o l a r i t y I s p r i m a r i l y o f o r e p o l a r i t y are c a l l e d e p o c h s and named a f t e r e a r l y w o r k e r s i n g e o m a g n e t i s r . ( 3 r u r . h e s , i ~ I a ' , u y ~ r r,a Gauss, Gllbert, ......). T a b l e 1. Process
Geomagnetic F i e l d V a r i a t i o n s
Origin
Geo g r a p h i c extent
Amplitude
continental extent global
< < 1%o f main d i p o l e field
Time Scale
micropulsations solar storms di u rna1 annual 11 y r
external meteorological
secular variation
non d i p o l e field
continental extent
AD2.30" A1 % 15%
polarity excursions
main d i p o l e field
global
AD >> 30" A1 >> 15%
lo2 - 1 0 3
AD A1
103
-
105
- 107
101 0O
lo1
polarity events
I1
polarity epochs
II
Note:
(y)
I1
I1
2. %
180" 200%
11
A D , (AI) i s t h e v a r i a t i o n i n t h e d e c l i . n a t i o n of t h e geomagnetic f i e l d d i r e c t i o n . -
lo2 -
103
105
(inclination)
! v i t h i n a p o l a r i t y e p o c h , t h e r , a l e c f i e l d ma:: h a v e or,e o r n c r e g l o b a l s o l a r l t y r e v e r s a l s , l a s t i n g t h e c r d e r c f l o 4 t o 1 0 5 years, terned e v e n t s , a n d / o r o n e c r more g l c b a l c r r e g i c n a l l a r g e p a l e o f i e l d m p l i t u d e d e v i a t i o n s f r o m t h e mear! t i n e a v e r a g e v a l u e , l a s t i n g f o r less t h a n lo4 y e a r s , which a r e termed e x c u r s i o n s . At a l l t i m e s , t h e E a r t h ' s m a g n e t i c f i e l d d i s p l a y s v a r i a t i o r s o f srnsll a m p l i t u . d e , t h e o r d e r o f 30" i n dec l i r a t i o n a n d 10" i n i n c l i n a t i o n , w h i c h l a s t ?ran a b c u t 500 to 3 0 0 0 y e a r s p e r c y c ? e , w h i c h i s t e r m e d s e c u l a r v a r i a t i o r . a n d is t h e s m a l l e s t v a r i a t i o n i n t h e E a r t h ' s r a g n e t i c f i e l d w h i c h c a n b e ~ s e dfor d a t i n g purpcses. 7
7
.
These p a l e o m a g n e t i c f i e l d v a r i a t i o n s , t h a t r a n g e i n t i m e s p a n from t o a f e w h u n d r e d years, d e t e r m i n e t h e p c s s i b l e p l a e o m a g n e t i c d a t i n g Thus, t h e a g e of a resolution f o r a particular sedinentary record. sediment and i t s r a t e of s e d i m e n t a t i o n , a l o n g w i t h o t h e r f a c t o r s which w i l l b e d i s c u s s e d , combine t o deterrr.ine which o f t h e f e a t u r e s of t h e p a l e o m a g n e t i c f i e l d c a n b e used f o r d a t i r i g purposes.
lo5
125
DATINS OF RECENT SEDIMENTS Thick sequences of Recent s e d i m e n t s which c o n t a i n abundant amounts o f d a t a b l e o r g a n i c m a t t e r a r e commonly f o u n d o n m o s t p a r t s o f t h e E a r t h . As a r e s u l t , t h e s e s e d i m e n t s h a v e b e e n e x a n i n e d e x t e n s i v e l y i n t h e p a s t d e c a d e t o d e t e r m i n e t h e r e g i o n a l p a l e o s e c u l a r v a r i a t i o n s i n t h e magnet i c f i e l d ( e . g . , Lund a n d B a n e r j e e , 1 9 7 9 ) . T h e s e s t u d i e s h a v e b e e n Qsed t o d e r i v e s e v e r a l d a t e d p a l e o s e c u l a r v a r i a t i o n master c u r v e s which c a n b e u s e d for r e g i o n a l c o r r e l a t i o n a n d d a t i n g p u r p o s e s ( T h o m p s o n , ' 9 7 5 , 1 9 7 8 ; Dodson e t aZ., 1 9 7 7 ; C r e e r e t aZ., 1 9 7 6 a ; B a n e r j e e e t a Z . ,
7979).
T h u s , t h e a g e o f a R e c e n t s e d i m e n t c a n b e d e t e r m i n e d by m e a s u r i n g i t s s e c u l a r v a r i a t i o n r e c o r d and matching t h e r e c o r d w i t h t h e r e g i o n a l m a s t e r c u r v e s . However, s u c c e s s f u l m a t c h i n g a n d , h e n c e , d a t i n g , i s dep e n d e n t u p o n a number o f f a c t o r s , s u c h a s : (1) t h e p a l e o f i e l d i s a c c u r a t e l y r e c o r d e d i n t h e s e d i m e n t
(2)
t h e magnetic record i s s t a b l e w i t h t i m e
(3) n o d e f o r m a t i o n o f t h e s e d i m e n t h a s t a k e n p l a c e a f t e r t h e p a l e o f i e l d r e c o r d was s e t
(4) t h e m a g n e t i c r e c o r d shows s e v e r a l s e c u l a r v a r i a t i o n c y c l e s t o a c h i e v e a r e l i a b l e match
(5)
t h e m a g n e t i c r e c o r d h a s n o t b e e n a l t e r e d by e a r t h q u a k e s , by
d e f o r m a t i o n o f t h e s p e c i m e n d u r i r ' g s a m p l i n a or by new m a g n e t i c r e m a n e n c e c a r r i e r s c r e a t e d by d i a g e p e t i c c h e m i c a l r e a c t i o n s .
I n many c a s e s , o n e or m o r e o f t h e s e c o n d i t i o n s i s n o t n e t , w i t h t h e r e s u l t t h a t t h e "unknown" m a g n e t i c r e c o r d s c a n n o t b e s u c c e s s f u l l y T s t c h e d w i t h t h e master s e c u l a r v a r i a t i o n c u r v e s a n d , h e n c e , c a n n o t be d a t e d by p a l e o m a g n e t i c means ( L u n d a n d B a n e r j e e , 1 9 7 9 ) . F u r t h e r i n f o r m a k i o n or s a m p l i n g t e c h n i q u e s , r e m a n e n c e m e a s u r e p e n t , d a t a s m o o t h i n g and p r e s e n t a t i o n , a n a l y s i s to t e s t for r e m a n e n c e s t a b i l i t y a n d t h e a n a l y t i c a l o b j e c t i v e method f o r c o r r e l a t i n g a m a g n e t i c r e c o r d w i t h m a s t e r c i x v e s i s c o r t a i n e d i n two r e c e z t a r t i c l e s by L w d a n d S a n e r j e e ( 1 9 7 9 ) z n d Thompson ( 1 9 7 9 ) . D A T I N G OF PLEISTOCENE SEDIMENTS
P a l e o m a g n e t i c d a t i p g o f P l e i s t o c e n e s e d i m e n t s , s u c h as t h o s e f o u n d i n lake b o t t o m s , d r y - l a k e s e d i m e n t s , ocean-bottom s e d i m e n t s and g l a c i a l C e p o s i t s , i s p o s s i b l e by t h e u s e o f p o l a r i t y c h a n g e s a n d e x c u r s i o n s kcown t o e x i s t i n t h e g e o m a g n e t i c t i m e s c a l e . S u c c e s s f u l d a t i n g by t h e L1se o f t h e s e f e a t u r e s r e q u i r e s t h a t : (1) t h e s e d i m e n t a c c u r a t e l y r e c o r d s o n e or more o f t h e s e f e a t u r e s (2)
t h e r e c o r d h a s n o t b e e n a l t e r e d by s e d i m e n t d e f o r m a t i o c a f t e r t h e p a l e o f i e l d r e c o r d was s e t or by r e m a n e n c e r e s e t t i n g by e a r t h q u a k e s or d u r i n g s a m p l i n g
(3)
t h e r e c o r d e d f e a t u r e s can be unambiguously c o r r e l a t e d w i t h
t h e p o l a r i t y time s c a l e .
If t h e P l i o c e n e - P l e i s t o c e n e b o u n d a r y i s p l a c e d a t a b o u t 1 . 8 Y y a , t h e n a s shown i n F i g u r e 1, t h e e a r l y P l e i s t o c e n e u p to 0 . 7 3 Mya i s i n t h e Matuyama E p o c h w i t h i n w h i c h t h e r e a r e e i g h t d a t e d p o i n t s c o r r e s p o n d i n g t o m a g n e t i c p o l a r i t y c h a n g e s . I n t h e p o s t 0 . 7 3 Mya B r u n h e s E p o c h , t h e r e a r e t w o d a t e d p o i n t s , t h e B l a k e E v e n t a t 110,000 y a a n d t h e L a k e I n a d d i t i o n , t h e r e are s e v e r a l o t h e r poten?v.ungo E v e n t a t 3O,O@O y a . t i a l p o i n t s w i t h i n t h i s time frame which might be used f o r d a t i n g p u r r ,Lacs ?^ b u t t h e s e s t i l l r e q G i r e c o n f i r m a t i o n on a g l o b a l s c a l e .
The s t a t u s o f v a r i o u s e x c u r s i o n s a n d e v e n t s t h a t h a v e b e e n d e t e c t e d i n s e d i m e n t s , v o l c a n i c r o c k s and baked s e d i m e n t s d e p o s i t e d o v e r t h e p a s t 3@O,OOO y e a r s h a s b e e n c r i t i c a l l y r e v i e w e d b y V e r o s u b a n d S a n e r j e e ( 1 9 7 7 ) . They p o i n t o u t t h a t i n o r d e r t o d a t e a s e d i m e n t ,
126
w
AGE OF POLARITY E V E N T (EXCURSION) BOUNDARY (Ma) 03 :I0 -18
:so
.48
(LAKE MUNGO) BLAKE (BIWA I )
\!I% 5\
(SNAKE RIVER)
zz
a0 0 J
on n w
v, W
I
5 a m
.73
OLDUVAI
Figure 1 P o l a r i t y reversal t i m e scale for the quaternary. (After, Mankinen and Dalrymple, 1979)
events and excursions in the sedimentary record must have internal consistency. In order to meet this requirement, the excursion or event with the same characteristics must be found repeatedly in the same sediment. In addition, it is desirable to show that the event or excursion has spatial consistency in sediments of the same age and it has been suggested that regional geomagnetic field fluctuations should be observed over lateral distances exceeding 1000 km (Harrison and Ramirez, 1975; Verosub and Banerjee, 1977). Many of the excursions listed in Table 2 lack the requirement of spatial consistency. For example, the Starno event (Noel and Tarling, 1975), the Gothenburg event (M6rner and Lanser, 1974), the Erieau excursion (Creer e t a t . , 1976b), an excursion in Lake Michigan sediments (Vitorello and Van der Voo, 1977), have not been observed in time equivalent sediments from Recent lake sediments in Minnesota (Banerjee e t a t . , 1979). This does not mean that all of these events or excursions are spurious but rather that spatial consistency has yet to be demonstrated so that they can be accepted for dating purposes.
In a given sedimentary record, only one event o r excursion may be present and, obviously, this can present difficulties in matching the
127
e v e n t or e x c u r s i o n w i t h a c c e p t e d p o l a r i k y changes found in other sedin e n t a r y r e c o r d s . I n some i n s t a n c e s , h o w e v e r , t h e c h a r a c t e r i s t i c s i g n a t u r e o f a g l o b a l e v e n t or e x c u r s i o n may b e u s e f u l i n i d e n t i f y i n g w h i c h e v e n t or e x c d r s i o n h a s S e e n f o u n d . Thus, a ~ o l a r i t ye x c u r s i o n r e c o r d e d i n sediments from w i d e l y s e p a r a t e d g e o g r a p h i c l o c a t i o n s s h o u l d g i v e s i v i l a r v i r t d a l geomagnetic p o l e s (VGP). Y0-r example, the e x c u r s i o n r e c o r d e d i n t h e M e a d o w c l i f f e Till a t Y o o r o n t o , O n t a r i o , d a t e d a t a p p r o x i m a t e l y 3 0 , 0 0 0 y a ( S t u p a v s k y e t al., 1979). y i e l d s a s i m i l a r VGP t o t h a t r e c o r d e d i n some N e w E n g l a n d t i l l s ( S o l o y a n i s a n d S r o w n , 1979) a n d b o t h g i v e 'JG? w h i c h c o r r e l a t e w e l l w i t h t h o s e g i v e n by t h e Lake 'vlungc e x c u r s i o n r e c o r d e d i n baked s e d i m e n t s i n A u s t r a l i a ( B a r b e t t i and I / I c E l h i n n y , 1976). Table 2.
Reported P o l a r i t y Excursions i n t h e P a s t 500,000 Y r .
Locality
Y e a r s BP
S o u t h e r n Sweden
2800
S o u t h e r n Sweden
12,077-12,103
Gothenburg,
12,350-12,400
Sweden
1 2 , 000
Czechoslovakia
Noel and T a r l i n g , I1
I1
1975 11
I1
Mijrner a n d L a n s e r , Bucha,
1974
1973
Lake Michigan c o r e s
8000-10,000
V i t o r e l l o a n d Van d e r V o o , 1977; C r e e r e t aZ., 1 9 7 6 a
Lake E r i e c o r e s
8000-14,000
C r e e r e t aZ., 1 9 7 6 b
Lake Biwa,
Japan
Imuruk Lake, Alaska Gulf
of Mexico
R u b j e r g , Denmark Mono L a k e ,
Gulf
o f Mexico
Y a s k a w a e t al., 1 9 7 3 ; Yaskawa, 1974
1 8 , 000
Noltimier and Colinvaux, 1976
15,000-18,000
Clark and Kennett,
23,000-40,000
Abrahamsen and Knudsen, 1978
2 5 ,0 0 0
California
L a k e Mungo, A u s t r a l i a
18,000
1973
D e n h a m a n d C o x , 1 9 7 1 ; Denham, 1 9 7 4 ; L i d d i c o a t a n d Coe,1979
28,000-31,000
B a r b e t t i and McElhinny, 1976
30,000-33,000
Freed and Healy,
Toronto, Ontario
1974
30,000
S t u p a v s k y e t aZ., 1 9 7 9
New England, U . S . A .
?J
30,000
S o l o y a n i s a n d Brown,
Laschamp,
%
33,000
Bonhommet a n d Z a h r i n g e r , 1969; V a l l a d a s e t al., 1 9 7 7
France
Washington, Lake B i w a ,
U.S.A. Japan
deep-sea c o r e s (Blake event) Lake Biwa, Lake B i w a , Lake B i w a , Snake River
Japan Japan Japan
> 45,000 4 9 , 000 104,000-117,000
% 110,000 ) 176,000-186,000) 292,000-298,000)
4 8 0 ,0 0 0
1979
E a s t e r b r o o k , 1975; Easterbrook and Othberg, 1975 Y a s k a w a e t al., 1 9 7 3 ; Yawkawa , 1 9 7 4 Smith and F o s t e r , 1969; D e n h a m , 1 9 7 6 ; C r e e r e t al., 1980
K a w a i e t ai?., 1 9 7 2 ; C r e e r e t al., 1 9 8 0 C h a m p i o n e t al., 1 9 7 9
To sum u p , a n e x c u r s i o n o r e v e n t t h a t l a c k s i n t e r n a l a n d s p a t i a l c o n s i s t e n c y a n d h a s a s i g n a t u r e w h i c h c a n n o t b e c o r r e l a t e d w i t h known e x c u r s i o n s i s p r o b a b l y u n r e l a t e d to a g e o m a g n e t i c p o l a r i t y e x c u r s i o n . I n many i n s t a n c e s , t h e s e s p u r i o u s e x c u r s i o n s o r e v e n t s l i k e l y r e s u l t Trom p r o b l e m s r e l a t e d t o d i s t u r b a n c e o f t h e s e d i m e n t w h i c h h a v e b e e n
128
m e n t i o n e d above and w i l l b e d i s c u s s e d i n more d e t a i l .
SAMFLING AND MEASUEENEPJT P3OCEDTJ3ES T e r r e s t r i a l s e d i m e n t s may b e s a m p l e d b y a v a r i e t y o f t e c h n i q u e s b u t , i n o r d e r to p r o v i d e e v i d e n c e o f i n t e r n a l c o n s i s t e n c y , i t i s n o r m a l p r a c t i c e t o c o n t i n u o u s l y s a m p l e t w o o r more t i m e - e q u i v a l e n t p r o f i l e s . S a m p l e s a r e u s u a l l y o b t a i n e d by c u t t i n g o u t o r i e n t e d s m a l l c u b i c s p e c i (1 2 . 2 em on a s i d e ) or o r i e n t e d b l o c k s ( 1 0 cm o r more c n a s i d e ) mens f r o m w h i c h s m a l l e r c u b e s or c y l i n d e r s a r e l a t e r c u t c u t o r d r i l l e d o u t i n t h e l a b o r a t o r y . I f t h e s e d i m e n t i s s u f f i c i e n t l y s o f t and n o i s t , t h e s m a l l e r c u b e s c a n b e o b t a i n e d i n t h e f i e l d by p r e s s i n q hollow p l a s t i c c u b e s i n t o t h e s e d i m e n t . The v e r t i c a l s p a c i n g between. c o n s e c u t i v e s a m p l e s w i l l d e p e n d , i n p a r t , on t h e s e d i m e n t a t i o n r a t e b u t , i n g e n e r a l , 1 0 ern s p a c i n g i n t e r v a l s may b e u s e d i f t h i s a m o u n t o f s e d i m e n t was d e p o s i t e d o v e r a p p r o x i m a t e l y 50 t o 1 0 0 y e a r s . When s a m p l i n g l a k e or o c e a n b o t t o m s e d i m e p t s , l o n g c o n t i n u o u s c o r e s a r e o b t a i n e d u s i n g o n e o f t h e many c o r i n g d e v i c e s a v d l a b l e . The r e l a t i v e m e r i t s o f t h e s e c o r i n g d e v i c e s h a s b e e n d i s c u s s e d i n d e t a i l by Lund a n d B a n e r j e e (1979) a n d Thompson (1979). The r e m a n e n c e o f t h e c o r e may b e m e a s u r e d c o n t i n u o u s l y 32 a l o n g core m a g n e t o m e t e r ( F o l y n e a u x e t al., 1 9 7 2 ; Dodson e t aZ. , 1.974), or by r e m o v i n g s p e c i m e n s by p r e s s i n g h o l l o w p l a s t i c c u b e s i n t o t h e c o r e a t i n t e r v a l s c f a b o u ? 2 t o 5 em. I f two s a m p l e s a r e r e m o v e d f r o m e a c h t i m e - e q u i v a l e c t p o s i t i c q w i t h i n t h e c o r e and two n a r a l l e l t i m e - e a u i v a l e n t c o r e s a r e used t h e n i t s h o u l d b e p o s s i b l e , f r o v r e m a n e n c e v a r i a n c e a n a l y s i s , to s e g r e q a t e t h a t f r a c t i o n o f t h e o b s e r v e d s e d i m e n t remanence which i s due t c v a r i a t i o n s of t h e p a l e o m a g n e t i c f i e l d from t h a t f r a c t i o n which r e s u l t s from d i s A t u r b a n c e o f t h e s e d i m e n t or c t h e r “ n o i s e ” i n t h e m a g n e t i c r e c o r d . simple e s t i m a t e of t h e palecmagnetic f i e l d v a r i a t i o n ( s i g n a l ) content t h a t i s r e c o v e r e d f r o m t h e s e d i m e n t r e c o r d i s t h e d i f f e r e n c e o f mean b e t w e e n c o n s e c u t i v e h o r i z o n s p e c i m e n s a n d mean w i t h i n h o r i z o n s a m p l e angular deviation. Any d i f f e r e n t i a l v a r i a t i o n b e t w e e n t h e p r o f i l e s i n t h e f i d e l i t y o f
t h e s e d i m e n t as a r e c o r d e r of t h e p a l e o f i e l d and s e d i m e n t d e f o r m a t i o n
a f t e r r e m a n e n c e a c q u i s i t i o n i s r e v e a l e d by a n o n - z e r o a n g u l a r d e v i a t i o n b e t w e e n t i m e - e q u i v a l e n t s a m p l e s f r o m two p r o f i l e s (Symons e t aZ., 1980). An a n a l y t i c a l c r o s s - v a l i d a t i o n s m o o t h i n g o f t h e r e m a n e n c e d a t a f r o m a s i n g l e p r o f i l e h a s b e e n d e v i s e d by C l a r k a n d Thompson (1978) t o r e c o v e r t h e p a l e o m a g n e t i c s i g n a l a n d i t s c o n f i d e n c e l i m i t s from t h e o b s e r v e d It s h o u l d b e p o i n t e d o u t , h o w e v e r , t h a t t h e c o r sediment record. r e l a t a b l e b e t w e e n c o r e v a r i a t i o n may b e s p u r i o u s i f i t o r i g i n a t e s f r o m r e g i o n a l c l i m a t e - c o n t r o l l e d s e d i m e n t o l o g i c f a c t o r s which can a l t e r the a c c u r a c y w i t h which the sediment r e c o r d e d the paleomagnetic f i e l d ( H a r r i s o n , 1 9 7 4 ; V e r o s u b , 1977). O v e r t h e p a s t few y e a r s , t h e r e s u l t s o f p a . l e o m a g n e t i c work on s e d i m e n t s has become m o r e r e l i a b l e as v a r i o u s t e s t s a r e u s e d t o d e t e r m i n e t h e n a t u r e o f s e d i m e n t r e m a n e n c e a n d i t s s t a b i l i t y . These i n c l u d e : (1) a l t e r n a t i n g f i e l d (AF) d e m a g n e t i z a t i o n t o r e m o v e v i s c o u s r e m a n e n t m a g n e t i z a t i o n (VFIF‘!) a c q u i r e d d u r i n g s t o r a g e i n t h e
ambient magnetic f i e l d
(2)
s t o r a g e t e s t s i n t h e ambient l a b o r a t o r y magnetic f i e l d t o d e t e r m i n e t h e r a t e o f VRM a c q u i s i t i o n
(3)
measurement o f m a g n e t i c s u s c e p t i b i l i t y t o d e t e r m i n e t h e amount of m a g n e t i c m i n e r a l s p r e s e n t i n a s p e c i m e n and t o determine Koenigsberger r a t i o s
(4) m e a s u r e m e n t
of t h e a n i s o t r o p y of m a g n e t i c s u s c e p t i b i l i t y ( w h i c h i s a m e a s u r e of m a g n e t i c f a b r i c ) t o i d e n t i f y d e f o r m a t i o n i n s e d i m e n t s (Verosub, 1977; L d v l i e and H o l t e d a h l , 1 9 8 0 )
(5)
t h e measurement o f s a t u r a t e d i s o t h e r m a l remanent m a g n e t i z a t i o n
129
(SIRM) to identify magnetic minerals
(6) shock tests to determine the significance of remanence resetting from shock induced thixotropy during sediment sampling (Symons e t aZ., 1980).
After all due care has been taken in the sampling and measurement procedures, the remanence data may be screened to select only homogeneously (reliably) magnetized specimens for interpretive purposes. The screened data is smoothed by either using a simple moving mean direction of a number of adjacent sample directions or using an analytical cubic spline smoothing technique developed by Clark and Thompson (19783. Finally, the smoothed declination (if available) and inclination of the remanence is plotted against stratigraphic position. Alteratior of the Faleomagnetic Record by Sedimentation Processes The processes by which a sediment acquires a remanent magnetization are complex and have been reviewed by Verosub (1977). The processes involved are given in Table 3. The initial detrital remanent magnetization (DRM) is developed when the magnetic moments of the remanence carriers are oriented in the magnetic field as they fall through the water column. The remanence thus acquired may vary significantly from the magnetic field due to a number of sedimentologic factors, such as ',he action of bottom currents, sediment grain size distribution and slope of bedding planes. After initial sedimentation, diagenetic effects, such as bioturbation and dewatering, may result in distortion o r l o s s of part or all of the original DRM and the development of p o s t depositional remanent magnetization that is parallel to the ambient field present when the disturbance took Dlace. Table 3. Magnetization
detrital remanent ma gn e t i za t ion (DRM)
Origin o f NRM of Sediments Process
Accuracy of record of the geomagnetic field
settling through a water column
poor, A 1 ~ 2 0 " virtually AD Q 4 O o instantaneous
post deposition shear induced remanent magnetization liquefaction during (PDRM) bio turbat ion, sediment deformation, earthquakes, sediment coring
chemical remanent magnetization (CRM) Note:
A1
chemical Droduction of magnetic minerals in the sediment
'inclination error'
=
AD = 'declination error'
=
=
Time interval of NRM acquisition
103
excellent
excellent
- 104
after deposition depending on the rate of dewatering in deep-sea sediment 10 - 100 y after deposition in lake sediments over the age of the sediment
deviation o f the inclination of the WRM from the ambient magnetic field deviation o f the declination of the
NRM from the ambient magnetic field
As a result of these and other factors (see Table 4), the remanent ragnetization of a sediment may consist Dartly of the initial DRM and Dostdepositional remanent magnetization. Thus, while varved clays may
130
r e t a i n up t o 100% o f t h e i n i t i a l DRM, up t o l O O $ o f t h e r e m a n e n t magneti z a t i o n i n b i o t u r b a t e d s e d i m e n t s may h a v e b e e n a c q u i r e d a f t e r d e p o s i t i o n . Table 4 .
Sediment NRM = f (DRM, PDRM, CRM)
Sediment type glacial varved deposits:
NRM magnetization type NRM
100% DRM
Record of paleomagnetic field poor record of the paleomagnetic field. Sedimentologic factors produce spurious variations of comparable amplitude to paleosecular variations. (Figure 2)
good deep-sea sediment cores: good lake sediment cores:
NRM
100% PDRM
excellent record of paleomagnetic field retaining the sequence of polarity excursions, events and epochs in accord with the polarity reversal time scale. (Figures 3 - 4 )
NRH
30-100% CRM
poor record of paleomagnetic field. The sequence o f polarity excursions, events and epochs has been overprinted by intense and stable CRM acquired in the Recent geomagnetic field.
NRM
30-100% PDRM
poor record of paleomagnetic field. Paleomagnetic field record has been completely or partially overprinted by recently acquired PDRM as a result of shockinduced thixotropy during recent earthquakes or during sediment sampling. (Figure 5 , Table 5)
poor deep-sea sediment cores: poor lake sediment
I n addition t o those syn-depositional d i f f i c u l t i e s , postd e p o s i t i o n a l d e f o r m a t i o n s p r e s e n t a d i f f e r e n t s e t of p r o b l e m s . For e x a m p l e , v a r v e d c l a y s u s u a l l y r e t a i n t h e i r o r i g i n a l DRM d u r i n g p o s t d e p o s i t i o n a l d e f o r m a t i o n which can b e p r o v e n b y a f o l d t e s t (Graham, 1 9 4 9 ; J o h n s o n e t aZ., 1 9 4 8 ; V e r o s u b , 1 9 7 5 ) . On t h e o t h e r h a n d , i n o t h e r s e d i m e n t s t h e remanence may be a c q u i r e d --a f t e r d e f o r m a t i o n (Keen, 1 9 6 3 ) . Between t h e s e two e x t r e m e s , some s e d i m e n t s may a c q u i r e p a r t of t h e i r RM b e f o r e d e f o r m a t i o n a n d a f t e r d e f o r m a t i o n ( N i i t s u m a , 1 9 7 7 ) . Sediment d e f o r m a t i o n may b e d e t e c t e d i n some c a s e s b y v i s u a l e x a m i n a t i o n o f t h e s e d i m e n t , X-ray a n a l y s i s or by m e a s u r i n g t h e a n i s o t r o p y o f m a g n e t i c s u s c e p t i b i l i t y ( L b v l i e and H o l t e d a h l , 1980). Q u i t e o f t e n , s e d i m e n t d e f o r m a t i o n i s l o c a l i n c h a r a c t e r a n d , i f two p a r a l l e l c o r e s h a v e b e e n t a k e n and a l a r g e a m p l i t u d e p a l e o f ' i e l d v a r i a t i o n i s
131
Declination p
lnclinat ion
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# # declination inclination from Geomagnetic field records
I Figure 2
Remanence i n c l i n a t i o n a n d d e c l i n a t i o n o f v a r v e d s e d i m e n t s d e p o s i t e d between 1700 t o 1800 AD from Hagavatn, I c e l a n d . The two s i t e s were a b o u t 1 0 0 m a p a r t . + , s i t e 1, o c h r e , A , s i t e 1, p r e y ; , site 2, ochre, 0 , site 2, grey. These varves provide a poor record of the paleomagnetic f i e l d . The o b s e r v e d v a r i a t i o n s a r e c l e a r l y s p u r i o u s ( A f t e r G r i f f i t h e t aZ., 1 9 6 0 ) .
I NCLINATION
Figure 3
II
I05
INCLINATION
171
Remanence i n c l i n a t i o n s f r o m two ' g o o d ' d e e p - s e a s e d i m e n t c o r e s (V-20 c o r e 105; RC-11 c o r e 1 7 1 ) . The s e q u e n c e o f p o l a r i t y r e v e r s a l s a r e i n a c c o r d w i t h t h e p o l a r i t y reversal time scale. ( A f t e r Opdyke, 1 9 7 2 )
132
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R e m a n e n c e d e c l i n a t i o n v a r i a t i o n s (A-L) v s r a d i o carbon ages f o r 6 B r i t i s h lakes. Dashed l i n e shows t h e p r e f e r r e d a g e s of t h e geomagnetic f l u c t u a t i o n s (A-L). The good w i t h i n l a k e and b e t w e e n l a k e paleomagnetic correlation suggests t h a t these sedim e n t s p r o v i d e a good r e c o r d o f t h e p a l e o m a g n e t i c field. ( A f t e r Thompson, 1 9 7 9 )
f o u n d i n one c o r e b u t n o t a t t h e same t i m e - e q u i v a l e n t p o s i t i o n i n t h e s e c o n d c o r e , t h e n t h e s p u r i o u s r e s u l t s can b e r e j e c t e d . REMANENCE RESETTING BY EARTHQUAKES AND SAMPLING PROCEDURES
L a b o r a t o r y s t u d i e s i n d i c a t e t h a t s e d i m e n t s a c q u i r e a s t a b l e RM a b o u t 1 0 d a y s a f t e r d e p o s i t i o n when t h e w a t e r c o n t e n t i s r e d u c e d below a c e r t a i n c r i t i c a l l e v e l ( L b v l i e , 1 9 7 4 ) . On d e D o s i t i o n , s e d i m e n t s have a h i g h w a t e r c o n t e n t a n d , h e n c e , t h e m a g n e t i c t o r q u e on most o f t h e remanence c a r r i e r s e x c e e d s t h e v i s c o u s d r a g f o r c e a n d , c o n s e q u e n t l y , t h e m a g n e t i c moments a r e a l i g n e d p a r a l l e l t o t h e m a g n e t i c f i e l d . As a s e d i m e n t l o s e s w a t e r and t h e v i s c o s i t y i n c r e a s e s , t h e m a g n e t i c t o r q u e on l a r g e m u l t i - d o m a i n c a r r i e r s which h a v e t h e l o w e s t m a g n e t i c moment p e r u n i t volume, f a l l s below t h e v i s c o u s d r a g f o r c e , and t h e remanence of t h e s e c a r r i e r s i s f i x e d . A s dewatering proceeds, t h e magnetic t o r q u e on s i n g l e domain c a r r i e r s which have t h e h i g h e s t m a g n e t i c moment p e r u n i t volume f a l l s below v i s c o u s d r a g f o r c e s and t h e i r remanence i s f i x e d . Thus, most o f t h e p o s t - d e p o s i t i o n a l RM i s a c q u i r e d when t h e sediment i s i n t h e f i n a l s t a g e s of dewatering. When a s e d i m e n t i s s u b j e c t e d t o a s h o c k , s u c h as a n e a r t h q u a k e or d u r i n g s a m p l i n g , i t s v i s c o s i t y may m o m e n t a r i l y d e c r e a s e from lo4 t o 1 0 p o i s e s ( F r a n c i s , 1 9 7 1 ) . During t h i s shock i n d u c e d t h i x o t r o p y , t h e m a g n e t i c moment of some c a r r i e r s , p a r t i c u l a r l y t h o s e which h a v e h i g h m a g n e t i c moment p e r u n i t volume, a r e r e o r i e n t e d i n t h e a m b i e n t f i e l d . Thus, t h e s e d i m e n t RM may be p a r t i a l l y or w h o l l y r e s e t d e p e n d e n t on t h e f r a c t i o n s of c a r r i e r s which a r e r e a l i g n e d .
133
Remanence resetting by earthquake induced thixotropy was first proposed to account for the observation that many deep-sea cores which span several million years are magnetized normally (Francis, 1971). He suggested that deep-sea sediments found in earthquake-prone areas may have their remanence reset at about 400 y intervals. Games (1977) showed that the FM found in mud bricks is acquired when the bricks undergo thixotrophy during injection into a mould. Similarly, laboratory studies show that some sediments are very susceptible to remanence resetting when subjected to mechanical shock (Symons e t a Z . , 1980; Stupavsky e t aZ., 1979). Consequently, any sampling process which produces mechanical shock or an effect similar to injecting clay into moulds may cause remanence resetting. Sediments which are susceptible to remanence resetting by shock can be readily identified by a simple laboratory mechanical shock test (Symons e t aZ., 1980). This laboratory test should be used on a routine basis so that, if required, sampling procedures can be modified to prevent remanence resetting during sampling (Stupavsky e t al., 1979). CHEMICAL PRCDUCTlON OF REMANENCE CARRIERS Diagenetic chemical reactions in sediments are well known and, in some instances, these reactions may produce remanence carriers which give the sediment a chemical remanent magnetization (CRM). In addition to the production of new carriers, old carriers which carried the original DRM may be destroyed. It is evident that if a significant amount of CRM is developed in a sediment that the sediment will be unsuitable for dating purposes. For example, it has been found that the reason that some deep-sea cores are unsuitable for dating is because of the presence of excess amounts of CRM (Harrison, 1974). INHERENT LIMITATIONS ON THE ACCURACY OF PALEOMAGNETIC DATING ARISING FROM THE NATURE OF THE NRM OF SEDIMENTS The inherent limitations on sediment dating are: (1) The NRM is acquired over a variable period of time after deposition. In deep-sea sediments, PDRM is acquired 1 0 3 - 1 0 4 y after deposition depending on the rate of dewatering. In lake sediment, it is acquired 10-100 y after deposition. (2)
The accuracy of the paleomagnetic field recorded in the sedinent NRM is variable. Thus, the accuracy of secular variation sediment dating that is based on small amplitude variations of the paleomagnetic field is p o o r .
(3)
The paleomagnetic field recorded by the sediment NRM is subject to magnetic overprinting by the Recent geomagnetic field. Magnetic overprinting is a serious problem. In deepsea sediments, % 80% of cores have been overprinted by CRM or PDRM acquired in the Recent geomagnetic field and are thus unsuitable for dating. Ir, lake sediments, magnetic overprinting problems are likely even more serious. Even partial magnetic overprinting during sampling will introduce spurious variation in the sediment record of comparable magnitude to the paleosecular variation by which the sediment is to be dated. SUCCESS RECORD OF PALEOMAGNETIC DATING OF QUATERNARY SEDIMENTS
The limitations and problems discussed in.this paper have an effect on the success record of absolute ape paleomagnetic dating of sediments. F o r example, the absolute paleomagnetic age of a sediment refers to the time when the sediment acquired its stable RM. Thus, if the sedimentation rate is low and the RM is fixed after the effects of bioturbation and dewatering, then the age obtained is likely much younger than the true age of the sediment. In deep-sea sediments, sediment rates are the order of 1 ~ m / l @y~ and the depth of bioturbation may be the order
134
o f 1 0 cm ( G a r t n e r , 1 9 7 2 ) . I n t h i s c a s e , t h e p a l e o m a g n e t i c a g e would be a b o u t l o 4 y y o u n g e r t h a n t h e a g e of d e p o s i t i o n .
The s u c c e s s of p o l a r i t y - r e v e r s a l p a l e o m a g n e t i c d a t i n g o f s l o w l y d e p o s i t e d o c e a n i c s e d i m e n t s which s p a n p e r i o d s o f a b o u t l o 6 y e a r s i s w e l l documented and i s w e l l below u n i t y . For e x a m p l e , from a n a n a l y s i s o f 2 1 6 d e e p - s e a c o r e s t h a t p e n e t r a t e t h e Brunhes-Yatuyarna b o u n d a r y , H a r r i s o n ( 1 9 7 4 ) showed 43 or 2 0 % a r e s u i t a b l e for d a t i n g . The r e m a i n i n g 8 0 % o f t h e c o r e s e i t h e r showed no p o l a r i t y r e v e r s a l s o r t h e p o l a r i t y r e v e r s a l p a t t e r n d i d not c o r r e l a t e w i t h t h e polarity,-reversal time scale. The f r e q u e n c y o f s u c c e s s f u l p o l a r i t y - r e v e r s a l d a t i n g o f t e r r e s t r i a l s e d i m e n t s i s n o t known b u t i s D r o b a b l y n o t s i g n i f i c a n t l y h i g h e r t h a n t h e 2 0 % of d e e p - s e a s e d i m e n t s . N e v e r t h e l e s s , t h e low s u c c e s s r a t e emp h a s i z e s t h e need t o e x e r c i s e c o n s i d e r a b l e c a r e i n o b s e r v i n g t h e l i m i t a t i o n s imposed by d i f f i c u l t i e s a r i s i n g from t h e n a t u r e o f s e d i m e n t a t i o n and s a m p l i n g p r o c e d u r e s . A s i d e from t h e w e l l documented Brunhes-Matuyama boundary and t h e B l a k e , J a r a m i l l o a n d O l d u v a i e v e n t s , a t p r e s e n t t h e r e a r e o n l y two p o l a r i t y e x c u r s i o n s w i t h i n t h e Q u a t e r n a r y w h i c h can b e u s e d f o r d a t i n g p u r p o s e s . For e x a m p l e , p o l a r i t y e x c u r s i o n d a t e s o f a b o u t 2 9 , 0 0 0 y have b e e n o b t a i n e d from t h e M e a d o w c l i f f e T i l l a t T o r o n t o and from some New England t i l l s ( S t u p a v s k y e t aZ., 1 9 7 9 ; S o l o y a n i s and Brown, 1 9 7 9 ) . The e x c u r s i v e VGP r e c o r d e d by t h e s e t i l l s c o r r e l z t e v e r y w e l l w i t h t h e Lake Mungo VGP which h a s b e e n d a t e d a t a b o u t 2 9 , 0 0 0 y ( B a r b e t t i and McElhinny, 1 9 7 6 ) . F i n a l l y , a l t h o u g h many a t t e m p t s h a v e b e e n made t o d a t e R e c e n t s e d i m e n t s by p a l e o s e c u l a r v a r i a t i o n , t h e o n l y e x p l i c i t d a t i n g of Recent s e d i m e n t s has b e e n made by Thompson ( 1 9 7 9 ) i n h i s t e s t of p a l e o s e c u l a r variation dating
.
TIME RESOLUTION OF PALEOMASNETIC D A T I N G
A l t h o u g h one o f t h e p u r p o s e s o f t h i s p a p e r i s t o p o i n t o u t t h e l i m i t a t i o n s of paleomagnetic d a t i n g , i t i s appropriate t o a l s o consider c e r t a i n of t h e p o s i t i v e a s p e c t s of t h i s d a t i n g t e c h n i q u e . Of a l l t h e sediment d a t i n g techniques t h a t span t h e Quaternary, paleomagnetic d a t i n g p r o b a b l y has t h e b e s t r e l a t i v e and a b s o l u t e t i m e r e s o l u t i o n . For e x a m p l e , i f p r o p e r p r o c e d u r e s a r e u s e d and r e g i o n a l m a s t e r s e c u l a r v a r i a t i o n c u r v e s d e v e l o p e d , i t h a s b e e n shown t h a t i t i s p o s s i b l e t o p r o v i d e d a t e s on s e d i m e n t s of unknown a g e which a r e i n good a g r e e m e n t w i t h a c c e p t e d r a d i o c a r b o n d a t e s (Thompson, 1 9 7 9 ) . P o l a r i t y c h a n g e s and g l o b a l p o l a r i t y e x c u r s i o n s p r o v i d e a n unp a r a l l e l e d o p p o r t u n i t y t o c o r r e l a t e and d a t e Q u a t e r n a r y t e r r e s t r i a l and d e e p - s e a s e d i m e n t s on a g l o b a l s c a l e . The t i m e r e q u i r e d f o r a p o l a r i t y t r a n s i t i o n i s a b o u t lo3 y ( F u l l e r e t aZ., 1 9 7 9 ) and f o r a p o l a r i t y exT h u s , t h e a g e o f any s e d i m e n t which r e c o r d s c u r s i o n a b o u t 10’ - lo3 y . a g l o b a l e x c u r s i o n ( e . g . , t h e Lake Mungo e x c u r s i o n ) or a p o l a r i t y change ( e . g . , t h e B l a k e e v e n t or t h e Brunhes-Matuyama b o u n d a r y ) c a n b e d e t e r m i n e d w i t h a r e l a t i v e a c c u r a c y o f between 10’ and l o 3 y . Obviously, t h e a b s o l u t e p a l e o m a g n e t i c age o f a sediment i s l i m i t e d by t h e a c c u r a c y o f t h e p o l a r i t y t i m e s c a l e . A t p r e s e n t , t h e t i m e s c a l e f o r t h e p a s t 5 my i s b a s e d on 354 K-Ar d a t e s on v o l c a n i c r o c k s (Mank i n e n and D a l r y m p l e , 1 9 7 9 ) . The a g e s of t h e p o l a r i t y r e v e r s a l s h a v e e i t h e r b e e n d i r e c t l y m e a s u r e d from v o l c a n i c r o c k s t h a t r e c o r d p o l a r i t y A s new e x c u r s i o n s a r e r e v e r s a l s or a r e known w i t h i n t i g h t l i m i t s . v e r i f i e d and d a t e d , i t i s e v i d e n t t h a t t h e p o t e n t i a l o f p a l e o m a g n e t i c d a t i n g w i l l improve. RECOMMENDATIONS FOR R O U T I N E SCREENING OF SEDIMENT DATA I n examining t h e v a r i o u s published p a l e o s e c u l a r v a r i a t i o n s t u d i e s on s e d i m e n t s f o r t h i s r e v i e w a r t i c l e , a s e r i o u s d e f i c i e n c y i s o b s e r v e d i n the data treatment procedure. I n most p u b l i c a t i o n s , a l l t h e m e a s u r e d d a t a i s a c c e p t e d f o r i n t e r p r e t a t i o n . No r o u t i n e s c r e e n i n g o f t h e d a t a i s c a r r i e d out t o r e j e c t u n r e l i a b l y magnetized d a t a b e f o r e
135
interpretation despite the strong evidence that a significant fraction may be unreliably magnetized so that its interpretation has no paleomagnetic significance. The evidence includes large remanence variations within and between time equivalent samples, erratic remanence direction changes of large amplitude between approximately time equivalent samples, and the remanence variations of pilot specimens during step demagnetization. The rejection of the unreliable data through a suitable screening procedure will increase the validity of any interpretation. In rock paleomagnetic studies, the remanence data is routinely screened at the core and site level to select only the reliably magnetized data for interpretation. This two-tiered screening procedure that requires remanence homogeneity at the core and site levels may reject more than 50% of the data as unreliable in some studies. The screening criteria devised 'for rock paleomagnetic studies are directly applicable to sediment studies. SPECIMEN LEVEL SCREENING The remanence measurement procedure on the various magnetometers is such that each Cartesian remanence component is measured several times. From these redundant measurements, the within specimen remanence variation can be computed. Only homogeneously magnetized specimens having remanence angular standard deviations smaller than some chosen value ( e . g . , 5") are accepted for interpretation (Harrison, 1980; Briden and Arthur, 1981; Lowrie e t aZ., 1980). This within specimen remanence variation provides a measure, a N , of the sDurious remanence variations found in rapidly deposited sediments. CORE LEVEL SCREENING The remanence angular deviation between two specimens from a single c0r.e; i . e . , two time equivalent samples if the coring is horizontal may also be used as a screening criterion. Thus, only homogeneously magnetized cores or samples having an angular deviation between the directions of the two specimens or samples which is smaller than some chosen value ( e . g . , 5") are accepted for interpretation. This within core remanence variation also provides a measure for o N of the spurious remanence variations in rapidly deposited sediments. FURTHER REMANENCE VARIANCE ANALYSIS OF SEDIMENT DATA The measured sediment remanence consists in part of the paleomagnetic field variations (signal) and spurious variations (noise) arising from variable sedimentation conditions, variable degree of partial remanence overprinting during sediment thixotropy, variable degree of CRM acquisition, e t c . An estimate of the signal to noise content of the record may be determined by computing the between consecutive core (specimen) angular remanence deviation. When the consecutive samples are separated by more than about 100 years in time, then this provides a measure of the signal variations, as, together with noise variations, ON. The mean signal variation content, o s , for the whole profile is the difference of the mean between core (specimen) and the mean within core (specimen) angular remanence variation. The mean signal to noise ratio ZS/ON provides a measure of the reliability of the sediment's remanence. High values ( > 2) indicate good remanence reliability. Conversely, low values ( < 1) indicate the sediment record is unsuitable for paleomagnetic interpretation since the measured variations are almost completely spurious. Applying this type of remanence 1 0.4 variance analysis to the data in Figure 5, (Table 5), a low is obtained that indicates the observed variations are largely spurious and have no paleomagnetic significance.
:,/aN
OTHER EFFECTIVE SCREENING CR ITER I0N When the sediment is sampled at two time equivalent profiles, the angular remanence variation between time equivalent samples from the two
136
O 0' *>
.O'
I NCLlNATlON
D ECLINAT1ON Figure 5
Remanence v a r i a t i o n s f r o m t h e S e m i n a r y T i l l , Toronto. T h i s sediment p r o v i d e s a poor r e c o r d of t h e paleomagnetic f i e l d . Variance a n a l y s i s of t h e d a t a ( T a b l e 5) shows t h a t t h e o b s e r v e d v a r i a t i o n s are spurious. ( A f t e r S y m o n s e t al., 1 9 8 0 ) .
****** Table 5 . Remanence variation
Remanence A n g u l a r V a r i a n c e A n a l y s i s
Number of c o r e s
Mean a n g u l a r Standard d e v i a t i o n ( d e g r e e s ) d e v i a t i o n (degrees)
within core
43
13
11
between c o n s e c u t i v e cores
48
18
10
mean p a l e o s e c u l a r v a r i a t i o n e s t i m a t e = mean b e t w e e n =
mean
= 5",
-
((signal s.d.
=
within core deviation noise) noise)
+
-
21'
Thus t h e o b s e r v e d v a r i a t i o n h a s no d a t i n g v a l u e b e c a u s e i t i s m o s t l y spurious.
137
p r o f i l e s p r o v i d e s a m e a s u r e of O N w h i c h n a y b e u s e d a ? s o a n a n e f f e c t i v e s c r e e n i r , g c r i t e r i o n . S u c h s c r e e n i n g e r , s u r e s good b e t w e e n p r o f i l e remanence c o r r e l a t i o n . The most s u i t a b l e a n d e f f e c t i v e sedirr.ent s c r e e n i n g p r o c e d u r e s t i l l needs t o b e d e t e r m i n e d . kiowever, i t i s a p p a r e n t t o t h e a u t h o r s t h a t r0utir.e s c r e e n i n g and d e t a i l e d renanence a r , a l y s i s of sediment d a t a can s i g n i f i c a n t l y irnprove i t s v a l i d i t y . SUW4A.F Y
T h r o u g h o u t t h i s p a p e r , we h a v e a t t e m n t e d t o d e m o n s t r a t e C e r t a i n of t h e l i a i t a t i o n s i n h e r e n t i n t h e p a l e o m a g n e t i c d a t i n g of Q u a t e r n a r y sedirnents. These l i m i t a t i o n s a r e r e l a t e d , i n p a r t , t o t h e complex synand p o s t - d e p o s i t i o n a l p r o c e s s e s b y w h i c h t h e r e m a n e n t m a g n e t i z a t i o n i s recoraed i n a sediment. O t h e r l i m i t a t i o n s r e l a t e to t h e r e l a t i v e l y s m a l l number of w e l l - d o c u m e n t e d p o l a r i t y e x c u r s i o n s a n d e v e n t s w h i c h have t a k e n p l a c e o v e r t h e p a s t 1.8 ny. A l t h o u g h t h e s u c c e s s r a t e for t h e p a l c o m a g n e t i c d a t i n g i s low, t h i s w i l l u n d o u b t e d l y i m p r o v e a s more a t t e n t i o n i s p a i d t o t h o s e f a c t o r s which e f f e c t t h e a c q u i s i t i o n of a n a c c u r a t e 2 M i n s e d i m e n t s . I n add i t i o n , b e t t e r i n s t r u m e n t a t i o n i s now a v a i l a b l e w h i c h w i l l h e l p t o i s o l a t e t h e o r i g i n a i remanent m a g n e t i z a t i o n i n s e d i m e n t s .
In s h o r t , t h e paleomagnetic d a t i n g of sediments i s going through "growing p a i n s " w h i c h h a v e b e e n e n c o u n t e r e d i n o t h e r m e t h o d s of d a t i n g s e d i n e n t s . I t h a s t h e i n h e r e r i t a d v a n t a g e of b e i n g i n d e p e n d e n t of f i n d i n g d a t a b l e m a t e r i a l , such as o r g a n i c m a t t e r and t e p h r a a n d , where s u c c e s s f u l , h a s t h e h i g h e s t r e l a t i v e a n d a b s o l u t e t i m e r e s o l u t i o n of any o t h e r d a t i n g m e t h o d . H e n c e , d e s p i t e t h e l i m i t a t i o n s , p a l e o m a g n e t i c d a t i n g of Q u a t e r n a r y s e d i m e n t s has a b r i g h t f u t u r e and w i l l p l a y a strong r o l e i n t h e c o r r e l a t i o n and d a t i n g of Q u a t e r n a r y s e d i m e n t s .
AC KNOWLEDGEVENT S We would l i k e t o t h a n k t h o s e p e o p l e who w e r e k i n d e n o u g h to s e n d us r e p r i n t s , p r e p r i n t s a n d comments o n t h e i r m o s t r e c e n t s t u d i e s . REFERENCES C I T E D Abrahamsen, N . a n d K n u d s e n , K . L . , 1 9 7 8 , I n d i c a t i o n o f a g e o m a g n e t i c low-inclination excursion i n supposed middle Weichselian i n t e r s t a d i a 1 m a r i n e c l a y a t R u b j e r g , Denmark: P h y s i c s of t h e E a r t h and P l a n e t . I n t e r i o r s , v . 1 8 , p . 238-246. Banerjee, S . K . , Lund, S . P . and L e v i , S . , 1979, Geomagnetic r e c o r d i n Minnesota l a k e s e d i m e n t s - Absence of t h e Gothenburg and E r i e a u Excursions: Geology, v. 7 , p . 588-591. 1 9 7 6 , T h e L a k e Munpo g e o m a g n e t i c B a r b e t t i , N.F. a n d M c E l h i n n e y , M . W . , P h i l . T r a n s . R . SOC. London, v . 281, p . 5157542. excursion: Bonhommet, N . a n d Z a h r i n g e r , J . , 1 9 6 9 , P a l e o m a g n e t i s m a n d p o t a s s i u m a r g o n a g e d e t e r m i n a t i o n s of t h e Laschamp geomagnetic p o l a r i t y event: E a r t h P l a n e t . S c i . L e t t . , v . 6 , p . 43-46. Briden, J . C . and A r t h u r , ent magnetization:
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141
THE PRESENT STATUS OF OBSIDIAN HYDRATION DATING
FRED TREMBOUR and IRVING FRIEDMAN
ABSTRACT Obsidian hydration dating is being used by both archaeologists and geologists to date events ranging in age from a few hundred to several million years. The method requires thar, a measurement of hydration thickness or the depth of penetration of water into obsidian be measured, and a rate of hydration be known. The measurement can be made optically, using a thin section of the sample and an ordinary microscope or it can be made by more sophisticated methods using particle accelerators. The rate of hydration is a function of the chemistry of the obsidian and of the temperature (s) that the sample was exposed to. This paper will be mainly concerned with methods to measure or estimate the rates of hydration as a function of temperature, and devices to measure or record integrated temperature. These latter devices include the Pallmann sucrose inversion cell, and the modified Ambrose cells developed by a group at the USGS in Denver. Data will be given comparing the performance of these various types of temperature integrating devices. We will also discuss the problem of direct measurement of hydration rates on obsidian powder. Formation and Hydration of Obsidian The natural glass called obsidian owes its origin either to the rapid cooling of volcanically erupted acidic magmas of rhyolitic composition (approx. 70 to 78% SiO,) or to the welding together of explosively erupted glass shards. When thus exposed to the conditions at the Earth's surface the solidified glass begins to react with its environment and eventually forms perlite. From its inception this change is characterized by the absorption of ambient moisture which builds up as a distinct and progressively thickening hydration layer or rind. While the initial H,O content of most obsidians is from 0.1 to 0 . 3 % , the saturation level after hydration reaches about 3 to 4% by weight. Any fresh surface formed later, as by fracture or cracking, on the pristine obsidian will proceed to acquire a hydration layer of its own in the same way. The hydration layers, Figure 1, are firmly adherent to the parent glass and resistant to chemical dissolution under neutral conditions. The hydrated layer is under strain and exhibits strain birefrlngence underpolarized light (Figure 2). The buildup of strain usually causes the hydration rim to spa11 o f f after about 50 micrometers of thickness have formed; however the authors have measured rinds as deep as c a . 100 micrometers on the geological cortex of some obsidian samples. Because its isotropic nature favors controllable shaping of sharp edged tools and weapons, by skilled chipping. obsidian was a desired material among stone age people. It was widely used in the industries of all early cultures that inhabited most of the earth's volcanic regions, as far back as several hundred thousand years ago in the Paleolithic of East Africa.
142
The r i n d i s 4
Figure 1
Hydration rind i n plain l i g h t . micrometers thick.
Figure 2
Hydration rind i n polarized l i g h t .
Hydration Yeasurement and D a t i n g I n a 1960 p u b l i c a t i o n , F r i e d m a n a n d S m i t h r e p o r t e d a m e t h o d o f a s s e s s i n g t h e a g e o f m e a s u r a b l e h y d r a t i o n r i n d s on o b s i d i a n , a n d i n a companion a r t i c l e Evans and Meggers (1960) d i s c u s s e d a f i r s t a p p l i c a t i o n of t h e method t o a r c h a e o l o g i c a l l i t h i c s from Ecuador. The k e y r e l a t i o n s h i p b e t w e e n h y d r a t i o n d e p t h (D) a n d t i m e ( t ) a t a n y g i v e n t e m p e r a t u r e was f o u n d by F r i e d m a n a n d S m i t h t o b e :
w h e r e k i s a c o n s t a n t t h a t i n c o r p o r a t e s t h e e f f e c t o f c h e m i c a l comp o s i t i o n o f t h e m a t e r i a l a n d t e m p e r a t u r e on t h e d i f f u s i o n r a t e . For a
143
given o b s i d i a n and a g i v e n t e m p e r a t u r e t h e h y d r a t i o n r a t e i s u s u a l l y e x p r e s s e d i n t e r m s of m i c r o p e t e r s 2 / 1 @ O 0 y e a r s . The b a s i c m e a s u r e m e n t o f r5r.d d e p t k as v i c r o r e k e r s ( p m ) i s p e r formed on a s a m p l e i n t h e l a b o r a t o r y . I n b r i e f , a s l i c e a b o u t 1 m i l l i meter t h i c k i s c u t and broken from t h e s e l e c t e d o b s i d i a n s u r f a c e w i t h twc n c J r r r i a i ; ; l n d p a r a l l e l c u t s o f a t h i n k d i a m o n d saw. The removed s l i c e , whlch car, b e a s s m a l l a s 2 x 2 q i l l i n e t e r s , i s mouRted on a g l a s s s l i d e 2nd made i n t o a t h i c k e r t h a n s t a n d a r d p e t r o g r a p h i c t h i n - s e c t i o n ( c a . 3 . 1 mm t h i c k ) by conrnon l a b p r o c e d u r e s for e x a m i n a t i o n by t r a n s m i t t e d l l g n t i n a petrographic microscope. The h y d r a t i o n l a y e r c a n b e i d e n t i f i e l d b y its b r i g h t b i r e f r i n g e n c e i n p o l a r i z e d l i g h t ( c r o s s e d n i c o l s ) ( F i g u r e 2). Accurate r i n d measurement from edge t o i E t e r n a l boundary can t h e n be made i n p l a i n l i g h t i l l u m i n a t i o n w i t h s u i t a b l e e y e p i e c e a c c e s s o r i e s s u c h as a V i c k e r s s p l i t - i m a g e m i c r o m e t e r or a f i l a r m i c r o m e t e r . With - . a r e , u s e f u l t h i c k n e s s m e a s u r e m e n t s down t o a l i m i t of 0 . 2 um c a n b e achieved b y t h e o p t i c a l method. A good l a b o r a t o r y s e t - u p a l l o w s t h e rrhole p r o c e d u r e i n c l u d i n g p r e p a r i n g t h i s s e c t i o n a n d m e a s u r i n g t h e r i m t o b e c a r r i e d o u t i n a m a t t e r o f 1 0 or 15 m i n u t e s . Some v e r y d a r k z o l o r e d n a t u r a l g l a s s e s c a l l for e x t r a t h i n g r i n d i n g ( t o - 0 . 0 5 m m ) t o secure s u f f i c i e n t l i g h t t r a n s m i s s i o n f o r measurement. R e s e a r c h e s w i t h a v a r i e t y o f m a t e r i a l s h a v e shown t h a t t h e i n t r i n s i c h y d r a t i o n r a t e s o f o b s i d i a n s v a r y o v e r a r a n g e a s w i d e as 2 0 t o 1, ( 8 r i e d m a n a n d L o n g , 1 9 7 6 ) . A s a c o n s e q u e n c e , for e x a m D l e , two p i e c e s 9 f o b s i d i a n f r o m t h e same f i n d s p o t w i t h i d e n t i c a l h y d r a t i o n a m o u n t s w i l l n e c e s s a r i l y b e o f t h e same a g e o n l y i f t h e y a r e o f t h e same c h e m i c a l zomposition. The m e t h o d i s u s e f u l i n t h e t i m e i n t e r v a l of a few h u n d r e d y e a r s 50 s e v e r a l m i l l i o n y e a r s . S i n c e t h e i n c e p t i o n o f h y d r a t i o n d a t i n g , many i n v e s t i g a t o r s , e s p e c i a l l y a r c h a e o l o g i s t s have e n t e r e d t h e f i e l d and ',heir work h a s l e d t o s c o r e s o f p u b l i c a t i o n s . R e l a t i v e l y f e w of t h e s e have d e a l t w i t h g e o l o g i c a l p r o b l e m s ( e . g . F r i e d m a n e t aZ., 1 9 7 3 ) . a n d these papers w i l l be d i s c u s s e d l a t e r . I n g e n e r a l most s t u d i e s have i n volved d i r e c t a t t e m p t s a t d a t i n g a r c h a e o l o g i c a l c o n t e x t s t h a t c o n t a i n e d s u i t a b l e o b s i d i a n a r t i f a c t s ( C l a r k , 1961; Johnson, 1969; Layton, 1 9 7 2 ) , but o t h e r s h a v e b e e n d e v o t e d t o improvement and e x t e n s i o n o f t h e method i t s e l f (Lee e t aZ., 1 9 7 4 ; T s o n g e t a Z . , 1 9 7 8 ) . The p r e s e n t a u t h o r s published a review a r t i c l e i n 1 9 7 8 (Friedman and Trembour) and a n o t h e r rras w r i t t e n by M i c h e l s a n d T s o n g i n 1 9 8 0 . I n t h i s t r e a t m e n t w e s h a l l c o n s i d e r some c u r r e n t r e s e a r c h t r e n d s . ? u r c h i e f i n t e r e s t h a s b e e n t h e p r o b e r e v a l u a t i o n of t h e t w o f a c t o r s t h a t d e t e r m i n e t h e h y d r a t i o n r a t e of any o b s i d i a n p i e c e i n q u e s t i o n : (1) t h e e f f e c t i v e h y d r a t i o n t e m p e r a t u r e a t i t s f i e l d l o c a t i o n a n d ( 2 ) 5he c o m p o s i t i o n o f t h e g l a s s . The T e m p e r a t u r e F a c t o r According t o t h e Arrhenius e q u a t i o n
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h y d r a t i o n p r o c e s s ( c a l o r i e s p e r m o l e ) , and R i s t h e g a s c o n s t a n t ( c a l o r i e s p e r d e g r e e p e r m o l e ) . Under c o n d i t i o n s p r e v a i l i n g i n n a t u r e t h e s b s i d i a n h y d r a t i o n r a t e i n c r e a s e s r o u g h l y 10% f o r e a c h l 0 C i n c r e a s e i n t e m p e r a t u r e . Hence, under f l u c t u a t i n g t e m p e r a t u r e c o n d i t i o n s ( d i u r n a l a n d a n n u a l ) t h e e f f e c t i v e h y d r a t i o n t e m p e r a t u r e ( E H T ) of t h e r a n g e i s tiot t h e a r i t h m e t i c t e m p e r a t u r e mean b u t a n i n t e g r a t e d v a l u e a t some iigher temperature level (Figure 4 ) .
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F r o m ( N o r t o n a n d F r i e d m a n , 1 9 8 1 , F i g u r e 1). Plots of t h e t e m p e r a t u r e r a n g e e x p e r i e n c e d by a s a m p l e versus the effective temperatures calculated for the Pallmann r e a c t i o n , t h e o b s i d i a n hydration r e a c t i o n a n d t h e Ambrose d i f f u s i o n p r o c e s s . The c u r v e s were c a l c u l a t e d from t h e A r r h e n i u s e q u a t i o n and assume a sinusoidal temperature variation within the range, and r e s p e c t i v e r e a c t i o n a c t i v a t i o n e n e r g i e s of 27kcal/mole, 19.7kcal/mole and 11.7kcal/mole.
145
I t i s t h i s EYT l e v e l t h a t was e x p e r i e n c e d by t h e s a m p l e where i t was f o u n d w h i c h m u s t b e e v a l u a t e d c l o s e l y i f a r e l i a b l e a g e c o n v e r s i o n f r o m h y d r a t i o n r i n d d e p t h i s to be o b t a i n e d .
When t h e o b s i d i a n h y d r a t i o n t e c h n i q u e was f i r s t d e v e l o p e d , t e m p e r a t u r e s b a s e d on t h e g r o s s t h e r m a l s u b d i v i s i o n s p r o v i d e d by t h e e a r t h ' s c l i m a t i c z o n e s were u s e d . R e s e a r c h e r s q u i c k l y saw t h a t e v e r n a r r o w e r d e f i n i t i o n s o f t e m p e r a t u r e w e r e demanded to r e s o l v e r e c u r r i n g i n c o n s i s t e n c i e s t h a t a r o s e , e . g . , i n the comparison o f h y d r a t i o n d a t e s w i t h I 4 C d a t e s or w e l l - e s t a b l i s h e d c u l t u r a l a g e i n d i c e s . P u b l i s h e d l o n g t e r m d a t a f r o m n e a r e s t w e a t h e r s t a t i o n s were s o o n r e s o r t e d to a s a b a s i s f o r i m p r o v e d e s t i m a t i o n s . A d d i t i o n a l r e f i n e m e n t s h a v e b e e n a p p l i e d to t h e s e a r e a e s t i m a t e s o f mean a r m u a l a i r t e m p e r a t u r e t o c o m p e n s a t e f o r s i t e a l t i t u d e , f i n d s p o t d e p t h ( a t or) b e l o w s u r f a c e , s e a s o n a l snow c o v e r , g e o t h e r m a l e f f e c t s and o t h e r s (Friedman and Long, 1 9 7 6 ) . While t h i s a p p r o a c h r e m a i n s t h e b e s t f e a s i b l e way to c o p e w i t h t h e t e m p e r a t u r e f a c t o r i n many i n s t a n c e s , t h e o b v i o u s a l t e r n a t i v e o f m a k i n g p r e s e n t d a y o n - s i t e measurements a t p o i n t s o f i n t e r e s t h a s been g a i n i n g a t t e n t i o n . T h i s way o f i n v e s t i g a t i n g t h e m i c r o e n v i r o n m e n t , s o t o s p e a k , o n b e h a l f o f h y d r a t i o n d a t i n g seerns e s s e n t i a l E o r d e a l i n g w i t h s u c h s t i l l u n a p prehended s o u r c e s of e r r o r a s s o i l c o n d u c t i v i t y , a l b e d o , p l a n t and t r e e c o v e r , s l o p e o f t e r r a i r , w i n d c h i l l a n d more. Eow a r e m e a s u r e m e n t s o f this k i n d a t p r e c i s e f i e l d s p c t s c a r r i e d c u t ? The n e e d h e r e , o f c o w s e , i s f o r r e a l l y l o w - c o s t , s e l f - c o n t a i n e d , no-maintenance f i e l d i 2 s t a l l a t i o n s t h a t p r o v i d e a n i n t e g r a t e d p a r a n e t e r of' c o n t i n u o u s l y v a r y i n g t e m p e r a t u r e s o v e r a l o n g p e r i o d o f t i m e . We h a v e w o r k e d w i t h t w o d i f f e r e n t t y p e s o f comlsact s e n s o r s t h a t f i t t h i s descriptlon (Figure 5 ) .
Figure 5
P h o t o o f Ambrose c e l l s . From l e f t t o r i g h t . (1) o r i g i n a l s p h e r i c a l c e l l d e s c r i b e d by Ambrose. (2) all p l a s t i c m o d i f i e d Ambrose c e l l . (3) metal c e l l w i t h p l a s t i c end windows on metal c e l l . (5) m e t a l c e l l s e a l e d i n p l a s t i c t u b e f i l l e d w i t h water.
One i s t h e P a l l m a n n g l a s s c e l l i n w h i c h a s u c r o s e s o l u t i o n w i t h c o n t r o l l e d ph' i n v e r t s a t a t e m p e r a t u r e d e p e n d e f i t r a t e i n t o o t h e r s u g a r forms, a n d t h e d e g r e e o f i n v e r s i o n i s m e a s u r e d a s o p t i c a l r o t a t i o n by
146
polarimetry at the end of the fie'ld run (O'Brien, 1971). With careful laboratory calibration, temperatures can be measured to f .Og"C. The other type is the plastic (acrylic resin) diffusion cell originated by Ambrose (1976) which absorbs and traps water inside its plastic walls at a temperature dependent rate and permits the uptake rate of water to be determined easily as weight gain over time. In both sensing methods, calibration of rate of sugar inversion or of weight change v s . temperature is done in the laboratory. The energy source for field operation in both cases is simply the thermal ambience at the site. Both types of cell are plain tubular objects a few centimeters in length and about 15 millimeters in diameter, and both require only a protective cover of sturdy plastic pipe for emplacement at the site. The one field visit necessary after deployment is for retrieval following the allotted time (commonly a year); then a single laboratory measurement for each cell provides the data for integrated temperature conversion. Sub-freezing conditions do not interrupt the operation. Figure 6 shows typical calibration curves for an Ambrose diffusion cell. Typical time-weight change curves for acrylic d i f f u s i o n (Ambrose) cells
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The use of the Pallmann technique-, including details of a method of emplacement of the two meter long probes using explosives is given in a recent publication by Norton and Friedman (1981). Beginning in May 1979 Trembour and F. Smith (unpublished) developed a variation of the Ambrose method in which the plastic cell is filled with pure water. The cell is surrounded by a desiccant. The water diffuses g& of the cell at a rate which is temperature dependent. Filling the cell with water rather than with desiccant allows the use of a smaller cell and longer service life. To achieve standardized field performance the diffusion cells must be precisely machined and assembled from raw material stock such as extruded plastic rod. Otherwise, variability in a manufactured lot will cause the need for extensive individual cell calibration after completion. However, experiments with certain commercially available
147
p l a s t i c c o n t a i n e r s h a v e shown some p r o m i s e f o r t h e a p p l i c a t i o n o f molded p r o d u c t s a s c e l l b o d i e s a l s o . An e x a m p l e i s t h e ( W h e a t o n ' ) 43 mm-longx 1 2 mm-diameter p o l y p r o p y l e n e v i a l w i t h s c r e w c a p . W a t e r - f i l l e d and s e a l e d w i t h a p r o p r i e t a r y p l a s t i c g l u e ( s u c h as S c o t c h G r i p b y 3 M ) t h i s c e l l v a r i e t y h a s shown a low d i f f u s i o n loss r a t e t h a t c o u l d make i t u s e f u l f o r v e r y long f i e l d r u n s and/or h i g h ambient t e m p e r a t u r e s . I n attempting t o date obsidian hydration t h a t i s o l d e r than about 1 0 , 0 0 0 y e a r s BP we n e e d t o c o r r e c t p r e s e n t d a y t e m p e r a t u r e s t o t a k e I n t h e i r p a p e r comi n t o account c l i m a t i c f l u c t u a t i o n s i n the p a s t .
p a r i n g o b s i d i a n h y d r a t i o n d a t e s o f r h y o l i t e f l o w s w i t h 1 4 C a n d K-Ar d a t e s o n t h e same f l o w s , F r i e d m a n a n d O b r a d o v i c h ( 1 9 8 1 ) e s t i m a t e d t h e d u r a t i o n and i n t e n s i t y o f p a s t t e m p e r a t u r e c h a n g e s b a s e d upon t h e " 6 0 c o m p o s i t i o n of f o r a m i n i f e r a from d a t e d deep sea c o r e s .
A n o t h e r a p p r o a c h was u s e d by P i e r c e e t a Z . , 1 9 7 6 , i n d a t i n g c r a c k s i n o b s i d i a n c a u s e d by g l a c i a l t r a n s p o r t . I n t h i s r e s e a r c h . K - A r d a t e s o n o b s i d i a n f l o w s f o u n d n e a r t h e g l a c i a l d e p o s i t s were u s e d t o c a l i brate the hydration r a t e . This c a l i b r a t i o n of hydration rate allowed t h e s e a u t h o r s t o d a t e t h e l a s t two g l a c i a t i o n s i n t h e Y e l l o w s t o n e P a r k a r e a . Figure 7 i l l u s t r a t e s the percussion cracks i n obsidian caused by g l a c i a l t r a n s p o r t . F i g u r e 8 shows t h e c u r v e r e l a t i n g h y d r a t i o n t h i c k n e s s t o a g e f r o m w h i c h t h e d a t i n g o f t h e g l a c i a l e v e n t s was obtained.
Figure 7
Photomicrograph of t h i n s e c t i o n o f o b s i d i a n p e b b l e showin'g h y d r a t i o n a l o n g c r a c k s . Cracks ,uch as t h e s e p r o b a b l y f o r m e d d u r i n g g l a c i a l a b r a s i o n when t h e o b s i d i a n p e b b l e impinged a g a i n s t a n o t h e r r o c k fragment. ( F r o m P i e r c e e t al., 1 9 7 6 ) .
The C h e m i c a l C o m p o s i t i o n F a c t o r O b s i d i a n i s composed m a i n l y o f 1 0 e l e m e n t s . The s e q u e n c e i n d e c r e a s i n g c o n c e n t r a t i o n , l i s t e d as o x i d e s , i s s i l i c a ( S i O z ) , alumina (A1203), p o t a s s i u m o x i d e (K20) s o d i u m o x i d e ( N a z O ) , i r o n o x i d e ( F e z 0 3 a n d F e O ) , c a l c i u m o x i d e ( C a O ) , m a g n e s i a (PlgO), t i t a n i u m d i o x i d e ( T i O z ) a n d w a t e r (H20t). I
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thickness (pm)
T h e r a t e o f o b s i d i a n h y d r a t i o n f o r t h e West Y e l l o w s t o n e B a s i n i s d e t e r m i n e d by h y d r a t i o n r i n d s on c o o l i n g c r a c k s o f t h e West Y e l l o w s t o n e f l o w , K - A r dated as 179,000 f 3,000 y e a r s o l d . The b l a c k l i n e g i v e s t h e a v e r a g e r a t e b a s e d on t h e two d a t e d f l o w s . The w h i t e l i n e i s d r a w n t o a c c o u n t f o r v a r i a t i o n i n h y d r a t i o n r a t e due t o c l i m a t i c change d u r i n g t h e l a t e Quaternary. (From d a t a by P i e r c e e t a t . , 1 9 7 6 ) .
The m i n o r o r t r a c e e l e m e n t a n a l y s e s o f o b s i d i a n h a v e a l s o b e e n s t u d i e d e x t e n s i v e l y by m o d e r n i n v e s t i g a t o r s - o f t e n a t low l e v e l s of p a r t s p e r m i l l i o n - for " f i n g e r p r i n t i n g " p u r p o s e s t o i d e n t i f y g e o l o g i c s o u r c e of a r t i f a c t m a t e r i a l . The i n f o r m a t i o n s e r v e s t o t r a c e a n c i e n t t r a n s p o r t a n d c u l t u r a l e x c h a n g e p a t t e r n s . The a n a l y t i c a l m e t h o d s u s e d a r e commonly of t h e r a d i a t i o n e m i s s i o n t y p e s s u c h as o p t i c a l s p e c t r o s c o p y , n e u t r o n a c t i v a t i o n a n d X-ray f l u o r e s c e n c e ( C a n n e t a l . , 1 9 7 0 ) . N a t u r a l g l a s s e s c o v e r a wide r a n g e i n chemical c o m p o s i t i o n and a t p r e s e n t a comprehensive u n d e r s t a n d i n g o f t h e water d i f f u s i o n p r o c e s s f o r a l l n a t u r a l g l a s s e s , i n c l u d i n g o b s i d i a n , i s s t i l l l a c k i n g ( e . 9 . Findlow, 1 9 7 7 ) . Recent r e s e a r c h ( e . g . , H a l l e r , 1 9 6 3 ; Doremus, 1975; M i c h e l s and B e b r i c h , 1 9 7 1 ; Hench a n d C l a r k , 1 9 7 8 ) h a s b r o u g h t o u t new f e a t u r e s o f h y d r a t i o n d i f f u s i o n s u c h a s i o n exchange r e a c t i o n s and c o n c e n t r a t i o n d e p e n d e n t as w e l l as i n d e p e n d e n t d i f f u s i o n p a t t e r n s f o r s p e c i f i c g l a s s e s . For t h e t i m e b e i n g it might be prudent t o s u s p e c t t h a t g l a s s e s of q u i t e d i f f e r e n t chemical composition ( e . g . , b a s a l t - r h y o l i t e ) have d i f f e r e n t h y d r a t i o n mechanisms. O b s e r v i n g d i f f e r i n g s e t s o f h y d r a t i o n a m o u n t s among s t r a t i f i e d f l a k e c o l l e c t i o n s f r o m V a l l e y of M e x i c o a r c h a e o l o g i c a l s i t e s , C l a r k ( 1 9 6 1 ) deduced t h a t t h e g r e e n - c o l o r e d v a r i e t y h y d r a t e d f a s t e r t h a n t h e g r a y one u n d e r l i k e c o n d i t i o n s b e c a u s e o f c h e m i c a l d i f f e r e n c e s between t h e two o b s i d i a n s o u r c e d e p o s i t s . S u z u k i ( 1 9 7 3 ) a t t e m p t e d a c o r r e l a t i o n of h y d r a t i o n r a t e a n d c h e m i s t r y of o b s i d i a n s from f i v e d i f f e r e n t s o u r c e s of t h e Kanto d i s t r i c t of J a p a n . A l t h o u g h h e c o n s i d e r e d h i s f i n d i n g s h i g h l y p r o v i s i o n a l h e f a v o r e d t h e r a t i o of t h e K20/A1203 p e r c e n t a g e s a s t h e b e s t among t h r e e chemical parameters t h a t he i n v e s t i g a t e d .
149
In 1976 Friedman and Long published their "chemical index" based on studies of rhyolitic glass source samples from 9 widely separated North American proveniences and one from Iceland. Their best correlation with experimental hydration data was expressed as a function 45(Ca0 t of the following sum of chemical conbent percentages: SiOn MgO) - 20H20t. In reviewing efforts of the kind just described one may suppose that the empirical results achieved correlations useful within a limited subgroup of all obsidians while not illuminating the underlying causes of differences among them. However, a recent study by Michels e t aZ. (1981), involving experimental hydration and chemical analyses on a selection of 12 Sardinian artifact flakes and source samples from the Monte Arci flow system on the island, confirmed the chemical index of Friedman and Long (1976). The hydration rate pre-. dieted by the chemical index differed from their laboratory result, 4.0 (pm)2/1000 years at 19"C, by a mere 1%.
-
CONCLUSIONS In closing this section of the review we wish to mention the application of substitute parameters f o r precise chemical analysis. An example is refractive index of obsidian which has been resorted to for hydration rate approximations when warranted to save time and cost. Using a few grains of powdered obsidian, a series of index-calibrated immersion oils and glass slides and covers, the specimen is examined in a microscope at low magnification with illumination by the focal masking technique. Using index oils of high dispersion (Wilcox, 1964) and observing interface Becke-line phenomena between plass particle and oil, a match of refractive indices accurate to .001 can be obtained in a matter o f 5 to 15 minutes per specimen. This measurement can be converted to estimated hydration rate at say 10°C from the experimental correlation shown in Figure 9. Here again we think that this empirical short-cut might best be used to obtain a first apDroximation of the glass's hydration rate only if its refractive index falls within the range (1.483-1.494) o f a group investigated by Friedman and Long.
0
z
I 1.490-
X
W
a
5 w
->
k-
0
a a a
1.485-
I . 4801
0
Figure 9
I
I
I
I
I
I
3 4 5 HYDRATION RATE AT l O * C - ( p r ~ /103YRS )~ 2
I
6
I
7
H y d r a t i o n r a t e a t 10°C a s a f u n c t i o n o f r e f r a c t i v e i n d e x (Nd) f o r o b s i d i a n s a m p l e s f r o m 1 2 s o u r c e s . (from Friedman and Long, 1976, F i g u r e 6 ) .
150
C u r r e n t l a b o r a t o r y work by t h e a u t h o r s i n c l u d e s t h e a p p l i c a t i o n o f o b s i d i a n powder s a mp l e s t o a c c e l e r a t e d h y d r a t i o n t e s t s a t c o n t r o l l e d The p r o c e d u r e s a r e a d a p t e d t e m p e r a t u r e s i n t h e normal ambient r a n g e . f r o m t h o s e p r o p o s e d b y Ambrose ( 1 9 7 5 ) . The p o w d e r s a m p l e s a r e 1 t o 3 gms, 2 5 0 t o 4 0 0 s i e v e mesh s i z e a n d a b o u t 1 s q . gram i n s u r f a c e a r e a . The p o w d e r s a r e h y d r a t e d i n a c l o s e d t u b e for a p e r i o d o f s e v e r a l d a y s or weeks b e t w e e n s a m p l e w e i g h i n g s o n a n e l e c t r o b a l a n c e t h a t i s s e n s i t i v e t o l o m 6 gm. The e x p e r i m e n t s r e q u i r e 2 or 3 s p a c e d w e i g h t r e a d i n g s t o y i e l d t h e l i r i e a r c a l i b r a t i o n p l o t s o f t h e k i n d shown f r o m Ambrose ( 1 9 7 6 , Figures 5.4). The d i r e c t i o n s o f t h e r e s e a r c h d i s c u s s e d i n t h e p r e c e d i n g p a r a g r a p h h a v e b e e n p a r a l l e l e d t o some e x t e n t by t h e d e v e l o p m e n t s i n h y d r o g e n p r o f i l e m e a s u r e m e n t s by n u c l e a r r e a c t i o n t e c h n i q u e s ( a s by L a u r s e n a n d L a n f o r d , 1 9 7 8 ) . T h i s m e t h o d i s e s p e c i a l l y s u i t e d for t h e d e t e c t i o n of v e r y t h i n h y d r a t i o n d e p o s i t s on a r t i f a c t s of low a g e a n d s l o w hydr a t i o n r a t e , s u c h as abound f o r example i n New Z e a l a n d ( L e a c h , 1 9 7 7 ) . I t may o p e n new f r o n t i e r s f o r d a t i n g a s w e l l a s f o r r e s e a r c h i n t h e mechanism o f h y d r a t i o n . REFERENCES C I T E D Ambrose, W.R., 1976, I n t r i n s i c hydration rate determinations,in Taylor, R . E . , e d . , Advances i n O b s i d i a n Glass S t u d i e s : N o y e s P r e s s , p . 81105. Cann,
J . R . , Dixon, J . E . and Renfrew, C . , 1 9 7 0 , O b s i d i a n a n a l y s i s and t h e o b s i d i a n t r a d e , i n B r o t h w e l l , D . and H i g g s , E . , e d s . , S c i e n c e i n Archaeology: Praeger P u b l i s h e r s , p. 578.
C l a r k , D . , 1 9 6 1 , The a p p l i c a t i o n o f t h e o b s i d i a n d a t i n g method t o t h e archaeology of c e n t r a l C a l i f o r n i a : (Ph.D. D i s s e r t . ) , S t a n f o r d Univ., 160 p. Doremus, R . H . , 1 9 7 5 , I n t e r d i f f u s i o n of hydrogen and a l k a l i i o n s i n a glass surface: J o u r n a l of N o n - c r y s t a l l i n e S o l i d s , v . 1 9 , p . 1 3 7 . E v a n s , C . a n d M e g g e r s , B . J . , 1 9 6 0 , A new d a t i n g m e t h o d u s i n g o b s i d i a n : P a r t 11: A m e r i c a n A n t i q u i t y , v . 25 p . 523-537. F i n d l o w , F . J . , 1 9 7 7 , A r e v i s i o n i n t h e Government M o u n t a i n - S i t g r e a v e s Peak, Arizona, o b s i d i a n hydration r a t e : T h e K i v a , v . 4 3 , n o . 1, p . 27-28. Friedman, I. and Long, W . , V. 1 9 1 , p . 347-352.
1976, Hydration rate of obsidian:
Science,
F r i e d m a n , I . a n d O b r a d o v i c h , J., 1 9 8 1 , O b s i d i a n h y d r a t i o n d a t i n g o f volcanic events: Q u a t e r n a r y R e s e a r c h , v . 1 6 , p . 37-47. F r i e d m a n , I . a n d S m i t h , R . L . , 1 9 6 0 , A new d a t i n g m e t h o d u s i n g o b s i d i a n , American A n t i q u i t y , v . 25, p . 476-493. P a r t I: F r i e d m a n , I . a n d Trembour, F.W., 1 9 7 8 , O b s i d i a n : A m e r i c a n S c i e n t i s t , v . 6 6 , n o . 1, p . 44-51.
the dating stone:
Obradovich, J . D . and Long, W . , Friedman, I . , P i e r c e , K.L., Obsidian hydration dates g l a c i a l loading, Science, v. 734.
1973, 180, p.
733-
Haller, W . , 1963, Concentration-dependent d i f f u s i o n c o e f f i c i e n t of water i n g l a s s : P h y s i c s and Chemistry of Glasses, v . 4 , p . 217220. Hench, L.L. and C l a r k , D.E., 1978, P h y s i c a l c h e m i s t r y of J o u r n a l of N o n - c r y s t a l l i n e S o l i d s , v . 28, p . 83.
glass s u r f a c e s :
J o h n s o n , LeRoy, J r . , 1 9 6 9 , O b s i d i a n h y d r a t i o n r a t e f o r t h e Klamath B a s i n of C a l i f o r n i a and Oregon: S c i e n c e , v . 165. p . 1354-56.
151 Laursen,
v.
T. and Lanford, 276, p . 153-156.
W.A.,
1978, . H y d r a t i o n of
Obsidian:
Nature,
L a y t o n , T . N . , 1 9 7 2 , L i t h i c c h r o n o l o g y i n t h e F o r t Rock V a l l e y , Tebiwa, v . 1 5 ( 2 ) , p . 1-20.
Oregon:
L e a c h , B . F . , 1 9 7 7 , New p e r s p e c t i v e s on d a t i n g o b s i d i a n a r t e f a c t s i n New Zealand: New Zealand J o u r n a l of S c i e n c e , v . 20, p . 123-138. L e e , R . , L e i c h , D . , T o m b r e l l o , T . . E r i c s o n , J. a n d F r i e d m a n , I . , 1 9 7 4 , Obsidian hydration p r o f i l e measurements using a nuclear r e a c t i o n t e c h n i q u e : N a t u r e , v . 250, p . 44-47. Flichels, J . W . and Bebrich, C.A., 1971, Obsidian hydration d a t i n g , i n Michael, H . N . and Ralph, E . K . , e d s . , Dating Techniques f o r t h e A r c h a e o l o g i s t , MIT P r e s s , p . 164-221. 1980, Obsidian hydration dating: a FLichels, J . W . and Tsong, I . S . T . , coming of a g e , i n S c h i f f e r , M . B . , e d . , Advances i n A r c h a e o l o g i c a l Method and T h e o r y , Academic P r e s s , v o l . 3 , p . 405-444. A t z e n i , E . , Tsong, I . S . T . and S m i t h , G . A . , 1 9 8 1 , S a r Michels, J . W . , d i n i a n Archaeology and Obsidian Dating. R e p o r t , M a t e r i a l s Research Laboratory, Pennsylvania State University. h'orton, D . R . a n d Friedman, I . , 1 9 8 1 , Ground t e m p e r a t u r e measurements, 111. G r o u n d t e m p e r a t u r e s i n a n d n e a r Y e l l o w s t o n e N a t i o n a l P a r k . U . S . G e o l o g i c a l S u r v e y P r o f . P a p e r s 1 2 0 3 , C h a p . A . , p . 1-11. O ' B r i e n , P.J., 1 9 7 1 , P a l l m a n n m e t h o d f o r m a s s s a m p l i n g o f s o i l , w a t e r or air temperatures: G e o l . SOC. A m e r . B u l l . , v . 8 2 , p . 2927. P i e r c e , K . , O b r a d o v i c h , J. a n d F r i e d m a n , I . , 1 9 7 6 , O b s i d i a n h y d r a t i o n d a t i n g and c o r r e l a t i o n of B u l l Lake and P i n e d a l e g l a c i a t i o n s n e a r West Y e l l o w s t o n e , M o n t a n a : Geol. S O C . A m e r . B u l l , v . 8 7 , p . 701710. S u z u k i , M . , 1 9 7 3 , C h r o n o l o g y o f p r e h i s t o r i c human a c t i v i t y i n K a n t o , Japan. J o u r n a l o f t h e F a c u l t y o f S c i e n c e , U n i v . o f Tokyo ( 5 ) , v . 4 , p . 241-318. Tsong, I . S . T . , H o u s e r , C.A.,Yusef, N . A . Messier, R . F . , W h i t e , W.B. and Michels, J.W., 1 9 7 8 , O b s i d i a n h y d r a t i o n p r o f i l e s m e a s u r e d by sputter-induced optical emission: S c i e n c e , v . 201, p . 339-341. 1964, Immersion l i q u i a s of r e l a t i v e l y s t r o n g d i s p e r s i o n Wilcox, R.E., i n t h e low r e f r a c t i v e i n d e x r a n g e (1.46-1.52): American Mineral: o g i s t , V. 49, p . 683-688.
153
THERMOLUMINESCENCE DATING OF QUATERNARY SEDIMENTS M. LAMOTHE, A. DREIMANIS, M. MORENCY and A. RAUKAS
AB STF.A C T In any geological environment, natural radiation induces free electrons in minerals that can be trapped into lattice defects. They may escape upon heating and recombine with holes at luminescent centers. Energy will then be released in the form of light. By recording the thermoluminescence (TL) o f a mineral, the last drainage of the traps can be dated, assuming a constant radiation level, by the following equation: EQUIVALENT DOSE (rads) AGE (years = DOSE-RATE (rads/yearrThe equivalent dose is the dose that can produce the natural TL level and is found by irradiation from known beta or gamma sources. The dose-rate is computed from the weight o f the radioactive elements in the sample to which may be added a small cosmic-ray contribution.
A specific geological problem is the determination o f the initial TL level at the time of sedimentation. It appears that, during the sedimentary cycle, exposure to sunlight bleaches all but a fraction o f the primary TL. This residual signal may give apparent ages of many thousands o f years. Simulation o f the sunlight process may be achieved by means o f a sunlamp, permitting an evaluation of this primary level. The "sedimentary" equivalent dose is thus the difference between this "residual" and the total natural dose. Stability of the equivalent dose is insured by the plateau test. Two main methods are now used in geology: a)
The "quartz-inclusion" method in which large inclusions of quartz (40-70 um or 88-125 um) are extracted and etched in HF to eliminate the short range alpha-induced TL. The age equation is then dependent only on the beta and gamma contribution.
b)
The "fine-grain" method in which determinations are made on poiymineralic fine silts (4-11 urn) that have received the full alpha dosage. Because of low efficiency in inducing TL compared to the beta and gamma radiation, a correction factor is applied to the alpha contribution, the total dose-rate being expressed in beta equivalent, in the age equation. INTRODUCTION
The light emitted by a crystal when heated is termed Thermoluminescence (TL). This thermally stimulated process has been used for dating archaeological objects f o r over two decades. Its successful application in geochronology is more recent.
154
The TL d a t i n g t e c h n i q u e i s p a r t of a g r o u p of methods b a s e d on s o l i d s t a t e p h y s i c s , t o g e t h e r w i t h E l e c t r o n S p i n Resonance (ESR) and T h e r m a l l y S t i m u l a t e d C u r r e n t (TSC). They m e a s u r e t r a p p e d e l e c t r o n s i n m i n e r a l s , t h i s b e i n g m a i n l y a consequence o f t h e l i t h o s p h e r i c d e c a y . A s e a r l y as 19115, R a n d a l l a n d W i l k i n s s u g g e s t e d t h a t e l e c t r o n s i n t r a p s h a v e a "Maxwellian" d i s t r i b u t i o n o f t h e r m a l e n e r g i e s t h u s e s t a b l i s h i n g t h e p h y s i c a l b a s i s o f t h e phenomenon. A few y e a r s l a t e r , D a n i e l s e t al. ( 1 9 5 3 ) p r o p o s e d many p o t e n t i a l u s e s of t h e TL p r o c e s s i n c l u d i n g c o r r e l a t i o n a n d d a t i n g . R e s e a r c h was t h e n d i r e c t e d t o w a r d s Indeed, i n 1966, c a l c a r e o u s m a t e r i a l s a n d met w i t h many d i f f i c u l t i e s . a t t h e Thermoluminescence o f G e o l o g i c a l M a t e r i a l s C o n f e r e n c e (McDougall, 1 9 6 8 ) , few p a p e r s r e p o r t e d a g e d e t e r m i n a t i o n s o f g e o l o g i c a l s a m p l e s . One o f t h e m a j o r p r o b l e m s was t h e o c c u r r e n c e o f n o n - r a d i a t i o n i n d u c e d TL ( " S p u r i o u s " ) . A i t k e n e t al. ( 1 9 6 3 ) s u g g e s t e d t h a t h e a t i n g i n a n o x y g e n - f r e e a t m o s p h e r e o f a n i n e r t g a s ( n i t r o g e n or a r g o n ) would e r a d i c a t e t h i s p r o b l e m . The t e c h n i q u e t h e n e v o l v e d q u i t e r a p i d l y . Today t h e TL d a t i n g method i s u s e d on a r o u t i n e b a s i s i n a r c h a e o l o g y a s r e s e a r c h i s now d i r e c t e d t o w a r d s r e f i n e m e n t o f v a r i o u s a s p e c t s o f t h e t e c h n i q u e (PACT, 1 9 7 8 ) . I n g e o l o g y , we must n o t e t h e p i o n e e r work o f S h e l k o p l y a s ( 1 9 7 1 ) w h i c h k e p t a l i v e t h e i n t e r e s t t o w a r d s t h e t e c h n i q u e i n s p i t e o r methodo l o g i c a l m i s c o n c e p t i o n s . R e c e n t l y , new h o p e s h a v e b e e n g e n e r a t e d by a s e r i e s of i n v e s t i g a t i o n s ( H E t t a n d R a u k a s , 1 9 7 7 ; W i n t l e and H u n t l e y , 1 9 8 0 ; W i n t l e , 1 9 8 1 ) p r o p o s i n g s o l u t i o n s t o t h e s p e c i f i c problems of geological dating. We s h a l l o u t l i n e h e r e t h e g e n e r a l p r i n c i p l e s of t h e t h e r m o l u m i n e s c e n c e p r o c e s s , r e v i e w t h e a g e e q u a t i o n s o f t h e two t e c h n i q u e s commonly u s e d i n T i d a t i n g , f o l l o w e d by a d i s c u s s i o n o f t h e a p p l i c a t i o n s t o Q u a t e r n a r y g e o l o g y . F i n a l l y , some c o n c e p t s r e l a t e d t o t h e u p p e r and lower t e r m s o f t h e g e n e r a l age e q u a t i o n ( e . g . t h e e q u i v a l e n t dose and t h e d o s e - r a t e ) a r e i l l u s t r a t e d by t h e s e n i o r a u t h o r ' s own work on t h e Upper T h o r n c l i f f e s e d i m e n t s ( M i d d l e W i s c o n s i n ) . D e t a i l e d r e s u l t s o f t h e TL d a t i n g p r o j e c t o f t h e T o r o n t o P l e i s t o c e n e w i l l be p u b l i s h e d a t a later date. This paper i s r e s t r i c t e d t o sediments. References t o o t h e r rock t y p e s w i l l be found i n r e v i e w p a p e r s o f V a l l a d a s ( 1 9 7 9 ) , W i n t l e ( 1 9 8 0 ) and Whippey ( 1 9 8 0 ) . THE TL PROCESS
An e x h a u s t i v e t r e a t m e n t o f t h i s s u b j e c t can b e found i n Levy (1974), A i t k e n ( 1 9 7 4 ) and F l e m i n g ( 1 9 7 9 ) . Most m i n e r a l s p r e v i o u s l y a f f e c t e d by i o n i z i n g r a d i a t i o n w i l l e m i t l i g h t when h e a t e d . S i n c e t h i s i s a n i r r e v e r s i b l e phenomenon t h e m i n e r a l w i l l n o t e m i t l i g h t when h e a t e d a g a i n . Moreover, a t m o d e r a t e d o s e s , t h e i n t e n s i t y o f t h e l i g h t i s d i r e c t l y r e l a t e d t o t h e i r r a d i a t i o n d o s e ( F i g u r e 1 ) . T h e r e f o r e , TL can r e f l e c t t h e t o t a l r a d i a t i o n a m a t e r i a l has received. Radiation I n t h e n a t u r a l e n v i r o n m e n t , t h e r a d i a t i o n comes m a i n l y from t h e l i t h o s p h e r i c d e c a y w i t h a m i n u t e c o n t r i b u t i o n from t h e cosmic r a y s . Alpha (a) and b e t a ( 6 ) r a y s a r e p a r t i c l e - l i k e r a d i a t i o n s w h e r e a s t h e gamma ( y ) r a y s a r e p a r t o f t h e e l e c t r o m a g n e t i c s p e c t r u m . They d i f f e r l a r g e l y i n t e r m s o f e l e c t r o n - h o l e p a i r s p r o d u c t i o n e f f i c i e n c y , t h e main consequence b e i n g t h a t t h e a l p h a c o n t r i b u t i o n , because of i t s small r a n g e ( = 2 2 p m ) , i s r e s t r i c t e d t o t h e s u r f a c e o f q u a r t z g r a i n s , if f r e e of i n t e r n a l r a d i o a c t i v i t y . The b e t a and gamma c o n t r i b u t i o n s may b e here considered equivalent. I n a s i m p l i f i e d scheme, non l u m i n e s c e n t and some l u m i n e s c e n t m i n e r a l s ( f e l d s p a r s , c a l c i t e ) , s u r r o u n d i n g t h e q u a r t z g r a i n s , emit t h e r a d i a t i o n .
155
TL
QUARTZ
INTENSITY (mv) 60-
Pre-heated
1200
v
p i 1.1 ' C I S
N2 50-
10-
4980 rads
y
30-
20-
BLACK BODY RADIATION
9 i
200
Figure 1
TEMPERATURE I'CI
400
A r t i f i c i a l g l o w c u r v e s f o r q u a r t z p r e - h e a t e d at 7 0 0 ' C for 3 h o u r s s h o w i n g t h a t t h e r m o l u m i n e s c e n c e and i r r a d i a t i o n a r e d i r e c t l y r e l a t e d . N o t e that q u a r t z e x h i b i t s t w o p e a k s and o n e s h o u l d e r . T h e low-temperature peak has faded as measurements were carried 8 months after irradiation.
S t o r a g e of Energy The l u m i n e s c e n t m i n e r a l s ( q u a r t z , f e l d s p a r s , c a l c i t e ) i n s e d i m e n t s a b s o r b i n c o m i n g r a d i a t i o n . They b e h a v e a s n a t u r a l d o s i m e t e r s i f i o n i z i n g r a d i a t i o n c a n e x p e l a n e l e c t r o n from i t s n o r m a l o r b i t , a n d i f t h i s e l e c t r o n becomes t r a p p e d a t a l a t t i c e c h a r g e d i s e q u i l i b r i u m s i t e , thereby c a l l e d an electron-trap. On t h e o t h e r h a n d , b y m u t u a l exchanges o f p e r i p h e r a l e l e c t r o n s between atoms, a n "absence o f e l e c t r o n " ( a " h o l e " ) w i l l t r a v e l i n t h e l a t t i c e and e v e n t u a l l y b e t r a p p e d . Imm e d i a t e r e c o m b i n a t i o n o f t h e e l - e c t r o n w i t h t h e p a r e n t atom or n e i g h b o u r i n g h o l e i s n e v e r t h e l e s s , b y f a r , a more f r e q u e n t p r o c e s s . The t r a p s a r e vacancies, i m p u r i t i e s , i n t e r s t i t i a l atoms, e t c . (Marfunin, 1 9 7 9 ) ; f o r i n s t a n c e , i t has b e e n s u g g e s t e d t h a t aluminium i m p u r i t i e s a c t a s h o l e t r a p s i n q u a r t z , w i t h l i t h i u m , sodium and sometimes germanium a s c h a r g e c o m p e n s a t o r s t o g e t h e r w i t h oxygen v a c a n c i e s a c t i n g as e l e c t r o n t r a p s (MeMorris, 1971).
R e l e a s e o f t h e Energy When t h e n a t u r a l d o s i m e t e r i s h e a t e d , t h e e l e c t r o n s g a i n enough e n e r g y t o e s c a p e from t h e t r a p s . The e l e c t r o n r e l e a s e r a t e i s t e m p e r a t u r e d e p e n d e n t and i s e x p r e s s e d b y t h e R a n d a l l - W i l k i n s e q u a t i o n :
-dn - dt
- n s e -E/kT
Where
n = t r a p c h a r g e c o n c e n t r a t i o n a t time t s = "attempt t o escape" frequency k = Boltzman c o n s t a n t E = A c t i v a t i o n e n e r g y f o r t h e r m a l charge release T = temperature
The b a n d model i s a n e n e r g y r e p r e s e n t a t i o n o f t h i s p r o c e s s (Figure 2). Having g a i n e d f r e e d o m , t h e e l e c t r o n w a n d e r s i n t h e c o n d u c t i o n band. If t h e e l e c t r o n recombines a t a luminescent c e n t e r , u s u a l l y a h o l e trap, e n e r g y w i l l t h e n b e r e l e a s e d i n t h e form o f l i g h t . T h i s c o n d u c t i o n band c h a r g e t r a n s f e r i s n o t t h e o n l y p o s s i b l e r o u t e f o r t h e e l e c t r o n .
For i n s t a n c e , r a d i a t i o n l e s s r e c o m b i n a t i o n ( w i t h o u t l i g h t e m i s s i o n ) may
156
Figure 2
The b a n d m o d e l , m o d i f i e d f r o m A i t k e n ( 1 9 7 4 ) . In t h e n a t u r a l environment (A), i o n i z a t i o n b r i n g s e l e c t r o n s i n t h e c o n d u c t i o n b a n d ; t h e y may b e trapped i n c r y s t a l ' s defects a t d i f f e r e n t "depths" Upon corresponding t o d i f f e r e n t energy levels. thermal excitation' (B), t h e electron i s ejected from t h e t r a p and wanders i n t h e conduction band. I t may r e c o m b i n e a t a l u m i n e s c e n t c e n t e r a n d e n e r g y will t h e n b e e n c o u n t e r e d a s l i g h t . Many o t h e r routes are possible.
occur a t " k i l l e r " c e n t r e s . I n a s i , n p l i . f i e d way , m i n e r a l s r e c e i v e i o n i z i n g r a d i a t i o n . They b e h a v e a s n a t u r a l d o s i m e t e r s i f t h e y c a n s t o r e t h i s e n e r g y o v e r geol o g i c a l t i m e and r e l e a s e i t when h e a t e d . THE A G E E Q U A T I O N
From t h e p r e c e e d i n g s e c t i o n , i t f o l l o w s t h a t i f t h e r a d i a t i o n doser a t e and t h e number o f t r a p s a r e c o n s t a n t , t h e l i g h t e m i t t e d upon h e a t i n g may b e a m e a s u r e o f t i m e . T h e r e f o r e , i f one can f i n d t h e d o s e t h a t c a n r e p r o d u c e t h e n a t u r a l
TL l e v e l ( E D ) a n d compute t h e a n n u a l d o s e from t h e r a d i o a c t i v e e l e m e n t s
f o u n d i n t h e s a m p l e , a n a g e r e l a t i o n s h i p can b e e s t a b l i s h e d : AGE ( y e a r s )
EQUIVALENT DOSE ( r a d s ) ( r a d s / m
= DOSE-RATE
Sue t o d i f f e r e n t m i c r o d o s i m e t r i e s , two t e c h n i q u e s have b e e n developed: The Q u a r t z I n c l u s i o n T e c h n i q u e ( F l e m i n g , 1 9 7 0 ) T h i s method makes u s e o f q u a r t z g r a i n s (40-70 p m ) i s o l a t e d from t h e m a t r i x by heavy l i q u i d s a n d / o r m a g n e t i c s e D a r a t i o n . By e t c h i n g t h e s a m p l e i n HF, t h e o t h e r l i g h t m i n e r a l s ( m a i n l y c a l c i t e and f e l d s p a r s ) a r e removed and m o r e o v e r , t h e o r i g i n a l s u r f a c e o f t h e q u a r t z g r a i n s i s dissolved, thereby eliminating t h e alpha dose c o n t r i b u t i o n .
ED1 AGE = D t D t D c B Y
Where EDi
D
P'
e q u i v a l e n t d o s e for i n c l u s i o n dating
:
D
Y'
Dc:
dose.-rate from t h e 6, y and cosmic r a y s .
157
The Fine-Grain Technique (Zimmerman, 1971) This method involves very fine grains (b-11 urn) sedimented on small aluminium disks. In this fashion, as compared to loose sand grains, samples are found to be more homogeneous and easier to handle (particularly for irradiation purposes). For this particle size, the alpha dosage attenuation is negligible and the gamma and beta contribution importance is reducecl. An efficiency factor is then introduced in the age equation and defined as: TL per rad of a radiation a = TL per rad of @ radiation so that the dose-rate is expressed in beta equivalent. For sediments, the alpha radiation is generally 10 times less efficient in inducing TL compared to beta radiation. The age equation in fine-grain dating is then: EDn-
l&
AGE
=
aDat D t r t x B Y APPLICATION TO QUATERNARY GEOLOGY
Specific problems related to the application of the technique to Quaternary geology are discussed in Dreimanis e t a t . (1978). They concern the inherited signal at time of sedimentation, the type ofmaterial used for dating and the age limits. Inherited thermoluminescence Archaeological dating assumes complete drainage of the previously acquired TL due to firing of the ceramic. For secondary detrital deposits, this thermal event has not likely happened. However, Morozov (1969), Shelkoplyas (1971) and Vlasov e t aZ. (1978b) suggested that sunlight bleaches part of the initial TL signal. This may be attributed to photon interaction with trapped electrons (Huntley, pers. commun.). Indeed, Vlasov e t al. (1978a) encountered a residual TL for sediments naturally exposed to sunlight from river bars and terraces which yield apparent TL ages of up to 9,400 years (Table 1). Thus an initial lowlevel of non-bleached thermoluminescence (residual) is inherited at the time of sedimentation. From that time on, in a continuous manner, the radioactive elements supply electrons to trapping centers until the sediment is remobilized in the sedimentary cycle (Figure 3). Residual contributions must be subtracted from the natural glow curves in order to date the last sedimentation event. By means of a sunlamp, Wintle and Huntley (1980) proposed several methods to simulate the sunlight exposure. TL dates on oceanic sediments and loesses (Wintle 1981) obtained by this technique were found to be in good agreement with stratigraphical evidence. However, whether or not the sunlamp bleaching reproduces exactly the last sunlight exposure to be dated remains open. The zero point problem also impedes assessment of error limits in TL dating of sediments. It is probably in the order of -f 20%. Material In Quaternary sedimentary deposits, due to its resistance to the weathering processes, quartz is a stable ubiquitous mineral. Being almost free of internal radioactivity and easy to isolate from the other minerals in inclusion dating by routine laboratory techniques, it is a target dosimeter. Quartz typically exhibits a glow curve with three peaks (Figure 1) and the high temperature part of the glow curve is stable over geological time (Fleming, 1969). Feldspar shows a very high TL sensitivity.
However, it exhibits
158
Table 1.
Residual Thermoluminescence o f Sediments Naturally Exposed to Sunlight (From Vlasov e t a l . ,
Sample 2011
20/2
Area IssikViliskaya Basin II
21
I1
22
I1
Type of deposits
ED (rads)
1978a)
-~
TL AGE (years)
Modern bar o f Tong River
4200
4700
Recent beach deposits Tongsky Gulf
2300
1900
800
900
1500
1400
I1
10m from strandline 11
25m from strandline (Upper Holocene) 24
II
Lower Pleistocene terrace
9400
5400
25
11
Middle Pleistocene lacustrine sediments
4100
3400
Alluvial sediments surface
5400
2800
K1
Klyazi Valley
K2
11
same, 1 cm lower
6400
2800
K3
11
same, 10 cm lower
6900
3900
Alluvial sediments surface
15000
8400
"1 owe r
16900
9400
B1 B2
Moscow Valley 11
Figure 3
Hypothetical TL level versus time. Any event permitting the sediment to be exposed to sunlight ( e . g . remobilization in the sedimentary cycle) reduces the thermoluninescence to a residual level. Today's heating drains the sample.
"anomalous fading" (next section) and moreover, extraction may be difficult .
In calcite, which is also used in TL dating, sample inhomogeneity in the distribution of internal radioactive elements in respect to luminescent centers in calcite gave rise to discrepancies from the
159
u r a n i u m d i s e q u i l i b r i u m s e r i e s d a t e s o f up t o 5 2 0 0 % i n a s p e l e o t h e m s t u d y by W i n t l e ( 1 9 7 8 ) . W a l t o n a n d Debenham ( 1 9 8 0 ) d e m o n s t r a t e d t h i s inhomogeneity d u r i n g l i g h t e m i s s i o n i n n a t u r a l c a l c i t e s u s i n g a highg a i n image i n t e n s i f i e r . I s o l a t i o n of s p e c i f i c m i n e r a l s i s rarely achieved i n f i n e - g r a i n d e t i n g , s o t h a t m e a s u r e m e n t s a r e made on p o l y r n i n e r a l i c f r a c t i o n s . The s h a p e o f t h e glow c u r v e i s t h e n d e p r i v e d o f a c u t e p e a k s ( F i g u r e 4). I s o l a t i o n o f q u a r t z i s a d i f f i c u l t a n d t i m e c o n s u m i n g t a s k . However, B e r g e r e t aZ. ( 1 9 8 0 ) a c h i e v e d a d e g r e e o f p u r i t y o f 9 5 % q u a r t z w i t h a h y d r o f ~ u o r o s i ~ i c iacc i d t r e a t m e n t . TL d a t i n g h a s b e e n p e r f o r m e d on many t y p e s o f s e d i m e n t s : (a) o c e a n i c (Huntley and Johnson, 1976; W i n t l e and Huntley, 1 9 8 0 ) ; ( b ) g l a c i o - l a c u s t r i n e ( T r o i t s k y e t aZ. 1 9 7 9 , t h i s p a p e r ) ; ( c ) l o e s s e s ( L i e t QZ., 1977, S h e l k o p l y a s 1971; W i n t l e 1 9 8 1 ) ; ( d ) s o i l s ( S h e l k o p l y a s , 1571; a n d ( e ) t i l l s ( T r o i t s k y e t aZ., 1 9 7 9 ) , w i t h v a r y i n g d e g r e e s o f s ~ i c c e s s . Because o f obvious exposure t o s u n l i g h t d u r i n g i t s t r a n s p o r t , l c e s s s h o u l d p r o v e t o be t h e b e s t t y p e of sediment t o be d a t e d .
Figure 4
N a t u r a l glow c u r v e o f Upper T h o r n c l i f f e s e d i m e n t s a n d g l o w c u r v e s o f t h e same s e d i m e n t s t o w h i c h h a s b e e n a d d e d known i r r a d i a t i o n d o s e s .
Age L i m i t s T h e o r e t i c a l l y , t h e u p p e r l i m i t o f t h e TL d a t i n g m e t h o d d e p e n d s on t h e s t a b i l i t y of t h e electrons i n t h e t r a p s . The l i f e t i m e o f t h e s e c h a r g e s i s g i v e n b y :
t
=
s -l e E/kT
160
Fleming (1969) found lifetime values for quartz, a2 room temperature, of 3.5 hours for the 110°C peak, of 3,000 years f o r the 325°C peak and of 4x107 years for the 375°C peak (heating rate: 20°C/sec). On the other hand, Hctt e t aZ. (1977) suggested, on the basis of saturation doses achieved in quartz from various sedimentary deposits, that the TL range should cover l o 6 years. Experience shows that a practical upper limit may be set at250,000 years, mainly because of early saturation of the traps. Hctt and Smirnov (manuscript) hope to extend the range to 500,000 years by using feldspars, on the basis of TL growth curve studies on this mineral.
A lower limit may actually be set at 5000 years because of increased importance of the residual signal for young samples. THE EQUIVALENT DOSE The upper term in the TL age equation, the equivalent dose (ED) is defined as the artificial dose that can simulate the natural TL level. It is expressed in rads. Determination of the ED The calculation of the ED is normally achieved by the additive method in which the artificial dose is gradually added to the natural sample; the equivalent dose is found at-the interception of the TL -~ growth curve with the abcissa (Figure 4 and 5).
I n t e n s i t y
( m v )
E Q U I V A L E N T
b e 0 -
D 0S E
A
I R R A D I A T I O N
D O S E
#
0 -
i
1 2 4 0 0
Figure 5
6 2 0 0 i r a d s )
6 2 0 0
1 2 4 0 0
The a d d i t i v e m e t h o d u s e d t o f i n d t h e e q u i v a l e n t d o s e (ED).
For sediments, the following data have to be found (Figure 6): 1) the natural TL intensity
2)
the hypothetical TL intensity at time of sedimentation by exposing sets of natural samples under a Sylvania sunlamp ( e . g . one hour; 275 watts; distance: 40 cm)
3)
the TL intensity acquired by such light-exposed samples when artificially irradiated (6200 rads y ; 12,400 rads y).
For the Upper Thorncliffe sediments, the ED1 found at the interception of the natural level with the TL growth curve was 8,800 rads at 289°C. In the preliminary state of the dating project, we considered as a maximum value for the ED, the projection of the TL growth curve to the TLintensity given by sets of samples exposed to the sunlamp for a very long time (18 hours: E D l 8 ; Figure 7).
161
UPPER
TL INTENSITY
THORNCLIFFE
( my1
NATURAL
2oo
100
Figure 6
12400
Figure 7
SEDIMENTS
Fine -grain
technique
+ 1hr
TEMPERATURE ( ' C )
3 00
Glow curves o f Upper Thorncliffe sediments showing t h e v a r i o u s s t e p s t o f i n d t h e s e d i m e n t a r y dose.
6200
6200 IRRADIATION DOSE (rads 11 < EDI--------,
T h e TL g r o w t h c u r v e , at 2 8 9 O C ; E D 1 8 determined as shown.
12400
v a l u e s of ED1 and
162
A t t h e I n s t i t u t e of Geology,' i n T a l l i n ( E s t o n i a ) t h e e q u i v a l e n t d o s e i s f o u n d by means of a n i t e r a t i v e e q u a t i o n ( H G t t and Smirnov, manuscript ) :
I:
Ip{l
-
e - a (ED
t B ( ED t A D ) }
AD'
Where ED = e q u i v a l e n t d o s e AD = a r t i f i c i a l d o s e
I .
= natural level
I
= a r t i f i c i a l level
a and 8 = c o n s t a n t s c o n n e c t e d w i t h T L k i n e t i c s S t a b i l i t y o f t h e ED A s a f u n c t i o n of t e m p e r a t u r e , t h e ED n o r m a l l y i n c r e a s e s t o a p l a t e a u v a l u e b e g i n n i n g i n t h i s c a s e around 2 6 0 ° C ( h e a t i n g r a t e : 1 . 1 " C I s e c ) . T h i s P l a t e a u Test (Figure 8 ) (Aitken, 1974) i d e n t i f i e s t h e s t a b l e p a r t o f t h e glow c u r v e , where l e a k a g e of e l e c t r o n s s h o u l d n o t have happened d u r i n g g e o l o g i c a l t i m e . A sample which f a i l s t o e x h i b i t a p l a t e a u a n d , y e t , h a v e s u f f e r e d l e a k a g e from t r a p s o f any d e p t h by wave-mechanical " t u n n e l l i n g " . T h i s phenomenon c a n b e i m p o r t a n t . I n d e e d , W i n t l e (1973) r e p o r t e d 4 0 % f a d i n g i n 1-5 h r s . f o r f e l d s p a r s . By k e e p i n g a sample a t room t e m p e r a t u r e for a p e r i o d of a few m o n t h s , one c a n c h e c k i f t h e s a m p l e shows t h i s k i n d o f f a d i n g . As a n e x a m p l e , sample 340-343 of t h e RC8-39 c o r e from t h e W i n t l e and H u n t l e y ( 1 9 8 0 ) s t u d y r e v e a l e d 1 4 % f a d i n g o v e r 2 0 d a y s s o t h a t t h e TL i n t e n s i t y was c o n s i d e r e d t o b e minimum. The a g e of t h e sample must b e g r e a t e r t h a n what i s e f f e c t i v e l y measured ( > 7 6 , 0 0 0 B P ) .
Upper T h o r n c l i f f e s e d i m e n t s showed a good p l a t e a u f o r b o t h e q u i v a l e n t d o s e s ( E D 1 = 8835 r a d s y ; E D 1 8 = 1 2 , 4 0 0 r a d s y ; F i g u r e 8 ) . A s a f i r s t a p p r o x i m a t i o n , we p r o p o s e a n a v e r a g e s e d i m e n t a r y dose o f 10,600 r a d s y , t o be used i n t h e age e q u a t i o n . L i n e a r i t y o f t h e TL growth c u r v e A t m o d e r a t e d o s e ( 4 0 , 0 0 0 r a d s y ) , t h e TL g r o w t h c u r v e i s f a i r l y l i n e a r . For low d o s e s , t h e g r o w t h c u r v e i s , i n some c a s e s , slowed down b e c a u s e of c o m p e t i t i o n between TL p r o d u c i n g and n o n - p r o d u c i n g t r a p s ( T i t e , 1 9 6 6 ) . T h i s l e a d s t o u n d e r e s t i m a t i o n of t h e e q u i v a l e n t d o s e , t o w h i c h must t h e n b e added a s u p r a l i n e a r i t y c o r r e c t i o n (I). W i n t l e However, a s t h e s e ( 1 9 8 1 ) computed s u c h a c o r r e c t i o n f o r l o e s s e s . s a m p l e s were n o t h e a t e d a t t i m e o f s e d i m e n t a t i o n , we s h o u l d check i f s u c h a s e n s i b i l i t y change c o u l d b e g e n e r a t e d by s o l a r b l e a c h i n g .
On t h e o t h e r h a n d , t h e TL g r o w t h c u r v e f l a t t e n s a t h i g h d o s e s and may e v e n s a t u r a t e . T r o i t s k y e t aZ. ( 1 9 7 9 ) h a d many s a t u r a t i o n p r o b l e m s of t h i s k i n d s o t h a t many of t h e i r d a t e s i n S p i t s b e r g e n were c o n s i d e r e d as " g r e a t e r t h a n " . DOSE-RATE The l o w e r t e r m i n t h e TL a g e e q u a t i o n may b e d e f i n e d a s t h e a n n u a l r a d i a t i o n a sample h a s r e c e i v e d from n e i g h b o u r i n g r a d i o a c t i v e e l e m e n t s It i s e x p r e s s e d i n r a d s - y e a r - ' . and t h e a t m o s p h e r i c cosmic r a y s . Potassium - 4 0 ,
and t h e r a d i o a c t i v e c h a i n s o f Thorium-232, Uranium-
2 3 8 and 2 3 5 , a n d , i n some c a s e s , Thorium-230and P r o t a c t i n i u m - 2 3 1 , a r e t h e c o n t r i b u t i n g i s o t o p e s . The f i r s t i s e a s i l y d e t e r m i n e d by a t o m i c
a b s o r p t i m . The o t h e r s may b e f o u n d by gamma-ray s p e c t r o m e t r y , n e u t r o n a c t i v a t i o n a n a l y s i s or, more commonly, by a l p h a - c o u n t i n g ( H u n t l e y , 1 9 7 7 ) . B e l l ( 1 9 7 9 ) computed t h e most r e c e n t v a l u e s o f d o s e r r a t e cont r i b u t i o n s for u n i t q u a n t i t y of' e a c h e l e m e n t . A s d o s e - r a t e s a r e exp r e s s e d i n b e t a - e q u i v a l e n t , t h e c o r r e c t i o n f a c t o r ( a D a t D +D +D , a s p r e v i o u s l y d e f i n e d ) must b e d e t e r m i n e d i n f i n e g r a i n d a t f n g ? i i y p i c a l
163
PLATEAU TEST
UPPER
Figure 8
THORNCLIFFE
SEDIMENTS
The p l a t e a u t e s t f o r Upper T h o r n c l i f f e s e d i m e n t s ; a n a v e r a g e v a l u e of 1 0 , 6 0 0 r a d s y was u s e d i n t h e age equation.
d o s e - r a t e v a l u e s i n s e d i m e n t s a r e of t h e o r d e r o f few h u n d r e d m i l l i r a d s per year. Thus, t h e d o s e - r a t e v a l u e i s computed e a s i l y . The p o s s i b i l i t y remains,. however, t h a t t h i s d o s e - r a t e c o u l d h a v e f l u c t u a t e d d u r i n g g e o l o g i c a l t i m e b e c a u s e o f v a r i o u s f a c t o r s t h e most c r i t i c a l b e i n g t h e w a t e r c o n t e n t o f t h e s e d i m e n t . It a c t s a s a b a r r i e r f o r a l l t h r e e t y p e s It d e c r e a s e s t h e e f f e c t i v e d o s e - r a t e of r a d i a t i o n a t v a r i o u s d e g r e e s . by a f a c t o r o f :
where
H =
r a t i o r e l a t e d t o t h e s t o p p i n g power o f w a t e r
A =
water t o s o l i d weight r a t i o i n t h e sample.
For s e d i m e n t s f o u n d i n t h e u n s a t u r a t e d z o n e , t h e e v a l u a t i o n o f t h e a v e r a g e w a t e r c o n t e n t can b e f a i r l y s p e c u l a t i v e . For t h e Upper Thornc l i f f e s e d i m e n t s , t h e i n s i t u A v a l u e was 9 % , t h e s a t u r a t i o n A v a l u e was 1 8 % . A s t h i s s e q u e n c e was u n d e r g l a c i a l l a k e w a t e r s and t h e n undercn e a t h t h e L a u r e n t i d e I c e S h e e t , we may s u g g e s t t h a t t h e s e s i l t y s a n d s were a t s a t u r a t i o n for most o f t h e t i m e , g i v i n g a n a v e r a g e w a t e r cont e n t o f 1 5 % . We b e l i e v e t h e r e a d e r c a n i m a g i n e t h e d a n g e r s o f s u c h s p e c u l a t i o n s f o r s a m p l e s o f unknown a g e ! On t h e o t h e r h a n d , Radon-222 i s a g a s e o u s p r o d u c t h i g h l y m o b i l e i n d r y s o i l . I t s d e p l e t i o n i s i m p o r t a n t b e c a u s e 98% of t h e y c o n t r i b u t i o n i n t h e u r a n i u m s e r i e s i s p r o d u c e d beyond r a d o n . However, w a t e r The r a d o n e m a n a t i o n i s r o u g h l y c o u n t e r greatly prevents i t s mobility. b a l a n c e d by t h e g r o u n d w a t e r d i l u t i o n e f f e c t and by t h e i n c o m i n g r a d o n from t h e l o w e r l e v e l s . D i s e q u i l i b r i u m of t h e r a d i o a c t i v e c h a i n s , r e m o v a l of r a d i o a c t i v e e l e m e n t s b y t h e g r o u n d w a t e r and e v e n i n h o m o g e n e i t y o f t h e r a d i a t i o n
164
field (Sutton, 1979) are also possible. To limit those uncertainties to a lesser extant, phosphor dosimetry
( e . 9 . CaS04-Dy) at the sampling site can be done over periods of many months to account f o r seasonal variations. This is now done as a routine by the Tallin TL laboratory and HGtt e t a l e (1978) reported significant differences between i n s i t u measurements and laboratory determinations from the weight o f the radioactive elements, of up to
50% in non-homogeneous profiles.
The dose-rate measured for Upper Thorncliffe sediments is dominated by potassium which contributes two-thirds o f the total 3 2 4 mrads/year value. The cosmic-ray estimation of 14 mrads/year was taken from Wintle (1981). An CY efficiency factor of 0.1 has been postulated. SOME RECENT TL DATINGS OF GLACIGENIC DEPOSITS The applicability of the TL dating method may be tested by TL age determinations of samples from sections that have been dated by some other methods. Two such test cases have been discussed recently from areas in USSR where the relative stratigraphy is known from palynologic studies: (1) Pasva section in the district of Archangelsk (Deviatova e t aZ., 1981) and (2) the Kraslava area of S.E. Latvia (Meirons e t aZ., 1981). The TL dates were obtained at the TL laboratory of the Institute o f Geology in Tallin, by using the quartz inclusion technique. Pasva section This deposit is on River Vaga in northern USSR, in an area that was not covered by the ice sheet of the last or Valdai ( = Wisconsin) Glaciation. Two adjoining sections were investigated palynologically, and they contained peat and clay of Mikulino ( = Sangamon) Interglacial age overlain by alluvial, lacustrine and glacio-lacustrine sediments of the Valdai Glaciation, and underlain by glacio-lacustrine sediments and till of probable Riss ( = Illinoian) age. The TL and 14C dates of both sections are listed in Table 2. Table 2 . Section No.
TL a n d
I 4 C
d a t e s , Pasva s e c t i o n s .
e t aZ., 1 9 8 1 )
Depth (m>
TL o r L a b . No.
Material
(After Deviatona date d a t e , BP
14C
1.7
S i l t y sand
Tln-TL-1
26,500
2.7
Clay
Tln-TL-5
40,000
8.2
S i l t y sand
Tln-TL-2
45,000
9.3
Sand
Tln-TL-6
3 2 ,0 0 0
Sand wedge Tln-TL-3 injected into peat below
62,000
11.25
AGE
VALDAI
2
11.90-11.95
Woody p e a t
2
12.20-12.25
Wood
Tln-216
MIKULINO INTER3 6 , 5 0 0 2 750 GLACIAL
12.45-12.50
Woody p e a t
Tln-217
3 4 , 0 0 0 + 750
12.45-12.50
Woody p e a t
Tln-226
>49,700
T i l l
Tln-TL-4
151,000
19.8 Note:
.
Tln-215
34,500*1100
PREM I K U L I NO
14C d a t e s h a v e l a b . N o s . w i t h o u t T L .
The 14C dates Tln-215, in-216, and Tln-217 are definintely too young, because of contamination, and the infinite date of >49,700 yrs BP does n o t specify the absolute ape, since the Mikulino Interglacial
165
i s beyond t h e I4C d a t i n g r a n g e . The T L d a t e s o f t h e p o s t - M i k u l i n o s e d i m e n t s a p p e a r r e a s o n a b l e f o r t h e Middle t o E a r l y V a l d a i , e x c e p t f o r Tln-T1-6 t h a t i s o u t - o f - l i n e i n t h e s e q u e n c e o f t h e downward i n c r e a s i n g a g e s . The TL d a t e Tln-TL-4 of t h e u n d e r l y i n g t i l l ( 1 5 1 , 0 0 0 ) a l s o a p p e a r s r e a s o n a b l e , i f t h e t i l l i s of R i s s a g e a s s u g g e s t e d by t h e s e d i m e n t o l o g i c a l a n d p a l y n o l o g i c a l s e q u e n c e s from t i l l t o t h e o v e r lying sediments.
K r a s l a v a Area T h i s a r e a i s w e l l known f o r i t s i n t e r g l a c i a l d e p o s i t s s i n c e t h e mid-19th c e n t u r y . The s t r a t i g r a p h i c c r o s s - s e c t i o n t h r o u g h t h e a r e a (Meirons e t aZ., 9981, p . 2 9 ) i s b a s e d upon a c o n s i d e r a b l e number o f t e s t d r i l l i n g s a n d n a t u r a l e x p o s u r e s i n v e s t i g a t e d by s e v e r a l a u t h o r s . The l a s t or B a l t i c G l a c i a t i o n ( = V a l d a i = Wiirm = W i s c o n s i n ) i s r e p r e s e n t e d by one t o two r e d d i s h - b r o w n t i l l s h e e t s and some s t r a t i f i e d d r i f t d e p o s i t s . A s i l t y sediment underlying t h i s t i l l a t f i d i n i (15 km W . o f K r a s l a v a ) i s p r o b a b l y o f E a r l y B a l t i c a g e ; i t h a s b e e n d a t e d by TL a s 97,150 y e a r s o l d (Tln-TL-45). The g r e y t i l l o f p r o b a b l e Kurzeme G l a c i a t i o n ( = R i s s = I l l i n o i a n ) h a s b e e n d a t e d from two t e s t d r i l l i n g s :
- a t i i d i n i (Tln-TL-42):
> 6 8 , 8 0 0 y r s B?
- a t R o b e i n i e k i (Tln-TL-h9):
106,250 y r s BP.
The a b s e n c e o f warm-climate i n t e r g l a c i a l d e D o s i t s o f t h e F e l i c i a n o v a ( = M i k u l i n o = Eemian = Sangamon) a g e e i t h e r above or below t h i s g r e y t i l l makes i t s s t r a t i g r a p h i c p o s i t i o n u n c e r t a i n . However, i f i t b e l o n g s t o t h e Kurzeme G l a c i a t i o n , t h e n t h e f i n i t e TL d a t e of 1 0 6 , 2 5 0 y r s B P i s a few t e n s of t h o u s a n d s of y e a r s t o o y o u n g . The s t o n e l e s s c l a y r i g h t u n d e r n e a t h t h e t i l l a t f i d i n i p r o d u c e d a s t r a n g e TL d a t e 5150 y r s BP (Tln-TL-43) - p r o b a b l y due t o some m i s t r e a t m e n t o f t h e c l a y sample. T h i s d a t e does n o t h e l p t o f i n a l i z e t h e o v e r l y i n g t i l l d a t e > 6 8 , 8 0 0 y r s B P ) , b u t i t w a r n s of t h e p o s s i b i l i t y of some (Tln-TL-42: e r r a t i c and c o m p l e t e l y u n r e a s o n a b l e d a t e s , a s i n a n y d a t i n g p r o c e d u r e . The n e x t l o w e r l a y e r d a t e d was t h e famous K r a s l a v a or, more s p e c i f i c a l l y , Adomova I n t e r g l a c i a l d e p o s i t p a l y n o l o g i c a l l y i n v e s t i g a t e d and c o n s i d e r e d t o b e o f P u l v e r n i e k i I n t e r g l a c i a l ( = L i k h v i n = Mindel9 i s s = Yarmouth) a g e by t h e m a j o r i t y o f i t s i n t e r n r e t e r s . I t s TL d a t e i s > 1 6 1 , 5 5 0 y r s BP (Tln-TL-48). This i n f i n i t e date i s i n t h e r i g h t r e l a t i v e o r d e r , i n comparison w i t h t h e d a t e s o f t h e o v e r l y i n g t i l l s . I f we d i s m i s s t h e o b v i o u s l y wrong (Tln-TL-43) d a t e o f 5 1 5 0 y e a r s B ? , a l l t h e o t h e r s appear i n t h e r i g h t r e l a t i v e order: t h e deeper t h e l a y e r , t h e o l d e r t h e d a t e , b u t h a l f of them a r e n o t f i n i t e and t h e i r a b s o l u t e values remain u n c e r t a i n . S o u t h e r n England A t h i r d s e t of r e c e n t l y p u b l i s h e d T L d a t e s on t e r r e s t r i a l m a t e r i a l s a r e t h e s i x age determinations by Wintle (1981) of l o e s s e s i n S . England t h a t a r e most p r o b a b l y of L a t e D e v e n s i a n a g e . They r a n g e from 14.5 t o 1 8 . 8 t h o u s a n d y e a r s BP 2 2 0 % . W i n t l e ( 1 9 8 1 , p . 4 8 0 ) m e n t i o n s t h a t t h e y a r e “ e a s i l y d i s t i n g u i s h e d from o l d e r l o e s s e s ” , and t h e r e f o r e t h e s e d a t e s a p p e a r t o b e i n t h e r i g h t o r d e r of m a g n i t u d e .
R e c e n t l y p u b l i s h e d f i n i t e TL d a t e s from t h r e e a r e a s e i t h e r a g r e e w i t h t h e i r e x p e c t e d a g e s , or o c c a s i o n a l l y , some a r e e r r o n e o u s . The a b s o l u t e v a l u e s q u o t e d i n S o v i e t l i t e r a t u r e may b e t o o o l d a s some r e s i d u a l TL was p r o b a b l y s t i l l p r e s e n t d u r i n g d e p o s i t i o n of t h e n a t e r i a l d a t e d . Whenever s e v e r a l s t r a t i g r a p h i c u n i t s o f some a g e d i f f e r e n c e s a r e d a t e d , e . g . a t Pasva and K r a s l a v a , most d a t e s a r e i n t h e r i g h t r e l a t i v e a g e s e q u e n c e . T h i s i s i n a g r e e m e n t w i t h t h e conc l u s i o n s on t h e v a r i o u s d a t e s p u b l i s h e d and d i s c u s s e d by D r e i m a n i s e t aZ.
(1978).
166
C OmNC LU SI0N then:
The thermoluminescence age equation of Quaternary sediments is AGE =
c(
'K20
SEDIMENTA3Y DOSE -
~
+
(. ~ 0205/1+H ~
Y
A)} ~
~
)
tWUC(.2783a/ltHuA) t (.0146/l+HgA) t (.0127/1+H A ) )
Y
tWTh{(.0740a/ltHuA)
t (.0029/1tHgA) t (.0050/1tH
Y
A)}
tDc/ltHyA where W: weight in per cent for K20 and in ppm for U and Th; thevalues introduced are the u , 6 and y contributions for unit quantity of each element in mrads/year (Bell, 1979; Wintle and Huntley, 1980). Therefore, with a sedimentary dose of 10,600 rads, for a dose-rate of 324 mrads/year, a TL date of 32,700 ? 5500 BP for the Upper Thorncliffe sediments is suggested. This value correlates reasonably well with radiocarbon dating (Table 3 and Figure 9). T a b l e 3. "K20
wU 'Th DC
H
A
Y
T h e r m o l u m i n e s c e n c e d a t i n g o f U p p e r T h o r n c l i f f e sediments.
=
2.67
f
.05%
=
1.6
f
0.3
ppm
=
2.9
f 0.8
pprn
=
14 m r a d s l y e a r
=
1.50
=
1.25
SEDIMENTARY DOSE 10,600 rads
DOSE-RATE 3 2 4 mradslyear
= 1.00
15% TL A G E = 3 2 , 7 0 0 y e a r s
Still uncontrolled variables in TL dating are the determination of the residual signal at time of sedimentation and the fluctuations of the water content. Studies of modern environment or calibration of sunlamp exposure from chronologically controlled sites may tide over the former. Careful sampling should limit the latter. As TL dating is performed on detrital materials, it provides an overwhelming advantage over many other techniques, and should lslay a major role in deciphering the time-stratigraphy of the late Pleistocene. ACKNOWLEDGEMENTS The authors wish t o thank D . J . Huntley who made determinations of the radioactive elements by alpha-counting and who collaborated with M. Lamothe over a one week visit to his laboratory at Simon Fraser University. C. Hillaire-Marcel and P. Pag6 kindly reviewed the manuscript. ?'his research was supported by a "Fonds F.C.A.C. pour l'aide et le soutien 2 la recherche-Qu6bec" scholarship to M. Lamothe and N.S.E.R.C. grant A4251 to A. Dreimanis. Micheline Lacroix typed the manuscript.
167
la h r s .
3 aooo,,
36000,-
34000,-
Aver age
32000,-
30000,-
28000
__
I
1 hr##
I
I
I4c Figure 9
Thermoluminescence date (32,700 i 5500 BP) versus radiocarbon dates (28,300 i 600 GSC-1082; 32,000 ? 690 GSC-1221) for Upper Thorncliffe sediments; error bars for the TL date are arbitrarily chosen as the ages given by ED1 and ED18, respectively; variations i n the TL date for different water to solid weight ratios (9, 15 and 18%) are also shown.
4ich5le L a i t h i e r ' s d r a f t i n g g r e a t l y enhanced t h e p a p e r . APPENDIX
The s a m p l e d a t e d was t a k e n a t t h e H i s e c t i o n o f Karrow (1967), i n a one m e t r e l a y e r o f s i l t s , f o u n d b e t w e e n t h e M e a d o w c l i f f e a n d L e a s i d e t i l l s . T h i s u n i t r e p r e s e n t s p r o b a b l y t h e "number 9 l a y e r " o f MGrner (1971). T h e s e s i l t s s h o u l d l i e b e t w e e n t h e 2 8 , 3 0 0 a n d 3 2 , 0 0 0 BP r a d i o carbon d a t e d p e a t y s a n d s of B e r t i ( 1 9 7 5 ) . The p e a t y l a y e r s were n o t o b s e r v e d a t t i m e o f s a m p l i n g . The s e d i m e n t s were p r o t e c t e d from s u n l i g h t i n an opaque b a g . REFERENCES CITED Aitken, M . J . ,
1974, Physics and Archaeology:
Clarendon Press, Oxford,
168
291 p. Aitken, M.J., T i t e , M . S . and Reid, J., 1 9 6 3 , Thermoluminescent dating progress report: Azchaeom,, v. 6 , p. 65-75. Bell, W.T., 1 9 7 9 , Thermoluminescence dating: A s h a e o m e t r v , v. 21, p. 243-245.
radiation dose-rate data:
Berti, A.A., 1 9 7 5 , Paleobotany of Wisconsinian Interstadials, Eastern Great Lakes Region, North America: Quaternary Research, v. 5 , p . 591-619. Berger, G.W., Mulhern, P.J. and Huntley, D.J., 1 9 8 0 , Isolation o f siltsized quartz from sediments: kncient T L , n. 11, p. 8-9. Daniels, F . , Boyd, C.A. and Saunders, D . F . , 1953, Thermoluminescence as a research tool: Science, v. 117, p. 343-349. Deviatova, E.I., Raukas, A.V., Raiamaie, R.A. HGtt, G . I . , 1981, Verkhnepleistotsenovii razrez Pasva (r. Vaga, Arkhangelskaia oblast) i ego stratigraficheskoe znachenie: Bulleten Komissii P O izucheniiu chetvertichnogo periods, N o . 5 1 , p . 38-50 (in Russian). Dreimanis, A., H u t t , G., Raukas, A. and Whippey, P.W., 1 9 7 8 , Dating methods of Pleistocene deposits and their problems: I Thermoluminescence dating: Geoscience Canada, v. 5 , p. 55-60. Fleming, S . J . , 1969, The acquisition of radioluminescence by ancient ceramics, Unpublish. D. Phil. Thesis, Oxford Univ.
, 1 9 7 0 , Thermoluminescence dating: refinement of the quartz inclusion method: Archaeom., v. 1 2 , p . 135-146. , 1 9 7 9 , Thermoluminescence techniques in archaeology: Clarendon Press, Oxford, 227 p . Huntley, D.J., 1977, Experiences with an alpha counter: ._Ancient TL_, no. 1 , p . 3-6. Huntley, D.J. and Johnson, H.P., 1976, Thermoluminescence as a potential means of dating siliceous ocean sediments: Canadian Journal o f Earth Sciences, v. 1 3 , p. 593-596. Hutt, G. and Raukas, A., 1977, Potential use of the thermoluminescence method for dating Quaternary deposits: B u l l . Comm. Stud. Quat,. Per., v. 4 7 , p . 77-86 (in Russian). H u t t , G. and Smirnov, A.V., manuscript, Thermoluminescent dating of sediments in the Soviet Union. 1 2 p . Hutt, G., Smirnov, A.V. and Punning, Y - V . K., 1978, In s i t u dosimetry by means of CaS04-Dy for determination of the annual dose-rate: Tallin’ 7 8 , p. 124-125 (in Russian). Hutt, G., Vares, K. and Smirnov, A.V., 1977, Thermoluminescent and dosimetric properties of quartz from Quaternary deposits, Izv. USSR: Chem. Geol. ser., v. 26, p. 275-283 (in Russian). Karrow, P.F., 1967, Pleistocene Geology of the Scarborough Area: Ontario Department Mines, Geol. Rep. 46. L e v y , P.W., 1974, Physical Principles ofthermoluminescence and recent developments in its measurement: N . Y . Upton, Brookhaven Lab., 18 p. L i , J.L., Pei, J . X . , Wang, 2.2. and L u , Y.C., 1977, A preliminary study of both thermoluminescence of the quartz, powder in loess and the age determination of the loesses layers: K w e Tanebau, v. 2 2 , p. 498-502.
169
McDougall, D.J., 1968, Thermoluminescence of Geological Materials: London, Acad. Press. McMorris, D.W., 1 9 7 1 , Impurity color centres in quartz and trapped electron dating: ESR, TL studies: Jour. o f Geophys. Res,, v. 76, p. 7875-7887. Marfunin, A.S., 1 9 7 9 , Saectroscopv, Luminescence and radiation center.? in minerals: N.Y., Springer-Verlag. Meirons, Z., Punning, J.M., HGtt, G., 1981, Results obtained through the TL dating of South-East Latvian Pleistocene deposits, Eesti NSV Teaduste Akadeemia Toimetised, Geologia, 3011, p. 28-33 (in Russian, with Estonian and English summaries). Morozov, G.V., 1 9 6 9 , The dating o f Quaternary Ukranian sediments by thermoluminescence: ZIIIth International Quaternary Association Conxress. Morner, N.-A., 1 9 7 1 , The Plum Point Interstadial: a p e , climate and subdivision: Canadian Journal of Earth Sciences, v . 8 , p . 14231431. Pact, 1978, A specialist seminar o n thermoluminescence dating, Journal of the European Study Group o n Physical, Chemical and Mathematical techniques applied to Archaeology, 2 volumes, Oxford. Randall, J.T. and Wilkins, M.H.F., 1945, Phosphorescence and electron traps: Proc. Roy. S O C . of London, v. A184, p. 366-407. Shelkoplyas, V.N., 1 9 7 1 , Thermoluminescence method in Quaternary deposits dating, in Zubakov, V.A. and Kotchegura, V.V., e d s . , Chronology of the glacial age: Leningrad. p. 115-159 (in Russian). Sutton, S., 1 9 7 9 , Thermoluminescence dating of ancient heated rocks: A progress report and sample request: S.A.S. Newsletter, v. 3 , no. 2, p . 1-2. Tite, M.S., 1 9 6 6 , Thermoluminescent dating of ancient ceramics: reassessment: Archaeoml, v. 9 , p. 155-169.
a
Troitsky, L., Punning, J.M., H i t t , G. and Rajamae, R . , 1 9 7 9 , Pleistocene glaciation chronology o f Spitsberpen: B o r e a s , v. 8 , p . 401-407. Valladas, G., 1979, La datation des roches par la thermoluminescence, Applications; Bull. de 1'Association Francaise pour l'gtude du Quaternaire, p. 43-52. Vlasov, V.K., Kulikov, O.A. and Karlov, N . A . , 1 9 7 8 a , Determination of the residual TL in quartz from surficial deposits: Tallin '78, p. 23-25 (in Russian).
, 1978b, The zero-point problem
Tallin
78,p.
in thermoluminescence dating:
26-28 (in Russian).
Walton, A.J. and Debenham, N.C., 1980, Spatial distribution studies of thermoluminescence using a high-pain image intensifier: Mature, V . 284, p. 42-44. Whippey, P.W., 1 9 8 0 , Applications of thermoluminescence to problems in -Geoloery, manuscript, 27 p . Wintle, A.G., 1 9 7 3 , Anomalous fading of thermoluminescence i n mineral samples: Nature., v. 244, p. 143-144.
, 1 9 7 8 , A thermoluminescence dating study of some Quaternary calcite: potential and problems: Canadian Journal of Earth Sciences, v. 1 5 , p. 1977-1986.
170
, 1980, Thermoluminescence dating: a review of recent a p p l i c a t i o n s to n o n - p o t t e r y m a t e r i a l s : A ~ c h a e o m e t r y , v. 22-2, p. 113-122. , 1 9 8 1 , T h e r m o l u m i n e s c e n c e d a t i n g of l a t e D e v e n s i a n l o e s s e s i n s o u t h e r n E n g l a n d : N a t u r e , v. 2 8 9 , p . 4 7 9 - 4 8 0 . W i n t l e , A.G. a n d H u n t l e y , D.J., 1 9 8 0 , T h e r m o l u m i n e s c e n c e d a t i n g o f o c e a n s e d i m e n t s : C a n a d i a n J o u r n a l of E a r t h S c i e n c e s , v . 1 7 , p . 348-360. Z i m m e r m a n , D.W., 1 9 7 1 , T h e r m o l u m i n e s c e n t d a t i n g u s i n g f i n e g r a i n s f r o m pottery: A r c h e o m . , v. 1 3 , p . 29-52.
171
RELATIVE AND ABSOLUTE DATING OF QUATERNARY MOLLUSKS WITH AMINO ACID RACEMIZATION: EVAL UAT I 0N, A PPLICA T I 0NS AND QUESTIONS
JOHN F. WEHMILLER
ABSTRACT Amino acids are entrapped in living skeletal carbonates as components of the structural protein upon which the carbonate phase forms. After death of the organism, the protein undergoes a complex array of reactions, involving hydrolysis into short-chain polypeptides and free amino acids, decomposition and/or leaching, and racemization of the amino acids in both free and bound forms. The racemization reaction involves conversion of each amino acid from its original 100% L form ("left-handed") into an equilibrium mixture (usually 5C-50) of D ("righthanded"? and i amino acids. The chronological utility of amino acid racemization has been studied in several genera of mollusks from more than 200 Quaternary marine localities along both the Atlantic and Pacific coasts of the United States. In these studies, both relative and absolute age estimates have been proposed for a large number of previously undated localities. In, addition, a number o f criteria for the evaluation of the method have been developed and tested, using samples with some form of absolute or relative age control. These criteria include: i)
acceptable precision ( 5 to 10% depending upon the amino acid) for multiple analyses of shells o f the same genus from the same out crop ;
2)
mineralogical and structural preservation;
3)
ability of each genus of interest to consistently demonstrate increasing D/L values in samples o f increasing relative age (Such as vertical terrace sequences or in superposed strata);
4) ability of each genus of interest to achieve racemic equilibrium in "old" samples (early Pleistocene to Miocene, depending on temperature);
5)
consistent relative rates of racemization of different amino acids in the same samDle;
6) increasing D/L values with decreasing latitude (increasing temperature) in samples o f known age equivalents.
Genera that do not meet one or more of these criteria must only be used with caution in chronological applications. Multiple genera should be employed in chronological studies because systematic generic effects on apparent racemization kinetics do exist and because only rarely would one genus be found in abundance at all localities of interest. The use of multiple genera provides cross-checks on age estimates, and occasionally reveals ambiguities that would be unrecognized if only singlegenera were used. The simplest application of amino acid racemization is asarelative
172
stratigraphic tool for closely spaced (but discontinuous) localities that can be assumed to have had similar or identical temperature histories. Results from southern California and the mid-Atlantic coastal plain provide good examples of this approach.
If amino acid data are available for at least one absolutely dated locality from within local "aminostratigraphic" sequences, then absolute age estimates can be derived from the D/L values using one of several kinetic models of diagenetic racemization that are under consideration. Correlation of separate aminostratigraphic sequences between twowidelyspaced regions (hence with different temperature histories) requires estimates of the differences in temperature histories as well as a quantitative kinetic model of the temperature dependence of racemization in samples of equal age. These kinetic models, and their inherent assumptions, have been calibrated and successfully tested in a few cases, mostly with samples of known age from a broad latitude range ( 3 5 " N to 25" N) along the Pacific coast of North America. On the Atlantic coast, some conflicts between U-series coral ages and both relativeand kinetic model age estimates have been encountered. INTRODUCTION Amino acids are found in fossil Quaternary mollusks as the remains of the original protein upon which the calcareous matrix was formed. Diagenesis of this protein-aceous material involves hydrolysis of high molecular weight components, l o s s of amino acid material by chemical destruction and/or leaching and diffusion, and racemization of the amino acids. This latter process is the conversion of the original Lamino acids ( Z e v o , or "left-handed") into a mixture of D- ( d e z t r o , or "right-handed") and L-amino acids. In principle, the enantiomeric ratio (D/L value) for each amino acid retained by the fossil will increase with time from an initial value of 0.0 to an equilibrium value that is usually 1.0. The general pattern of these diagenetic reactions is shown in Figure 1. The extent of the racemization reaction is usually determined on the total amino acid mixture, though several laboratories also routinely analyze the free amino acid component for enantiomeric ratios. As the measured D/L value is the combined result of a number of contributing reactions (with variable rates of hydrolysis, racemization, leaching or decomposition for different amino acids in different molecular weight components), it is difficult to predict theoretically the rates of racemization for different amino acids in different genera or sample types. From a variety of field and laboratory experiments, it is known that D/L values in fossils are a measure of the age of the fossil, but that these D/L values are also dependent on temperature, amino acid, genus (or sample type), and contamination or other physical-chemical diagenetic effects. In this respect there are many similarities between racemization and obsidian hydration dating methods. If well-preserved samples of a single genus are available, thenthe racemization dating method can be used to determine relative ages for closely spaced but discontinuous deposits that can be reasonably assumed to have had similar temperature histories. The racemization method can be used as an absolute dating tool if suitable calibration is available along with an appropriate model for the overall kinetics of racemization in the sample type being used. Temperature assumptions are an inherent component of either relative or absolute applications of the method. Enantiomeric ratios in Quaternary mollusks have been used as dating tools for coastal deposits of the Atlantic (Hare and Mitterer, 1967; Mitterer, 1974; 1975; Belknap, 1979; Belknap and Wehmiller, 1980), Pacific (Wehmiller e t aZ., 1977; 1978a; Kennedy, 1978; Lajoie e t a l . , 1979; 1980; Wehmiller and Emerson, 1980), and Arctic (Miller e t aZ., 1977; Miller and Hare, 1980a) coasts of North America, as well as the coast of Great Britain (Miller e t al., 1979). Many of these studies have developed basic information that aids in the evaluation of the reliability of the method, such as the demonstration of typical precision
173
Figure 1
Amino A c i d D i a g e n e d i s
in Fossils
1
Hi CJ h Mo l e c u l a r We i g h t P o 1y p e p t i de s
Low M o l e c u l a r W e i g h t P o l y p e p t i d e s 1;
Y
H-N-C-c
Y B
-N-C-c
y
1;
1;
I'
I I -N -C -C -N-& -c-OH
-011
0
a-
E c"
o u ..c-I
GJC
2:
GI
F r e e Amino A c i d s
L
H
H
HOOC
a-
D COOH
RACEMIZATION
(Interior, terminal, free positions)
D/L I n c r e a s e s from 0.0
t o 1.0 F r e e / B o u n d I n c r e a s e s f r o m S O t o >10 Common Amino A c i d s : R =
-H -CH3 -CH(CH3) -CH,COOH
Glycine Alanine Valine A s p a r t i c Acid
-CH CH(CH3), - C H ? C H ~ ) (C2H5) -CH2CH2COOH -CH, (CeH5)
H I
N
H C/-COOH
I
Leu c i n e Isoleucine Glutamic acid P h e n y 1alanine
Proline
of multiple analyses (Kvenvolden e t al., 1979), intrageneric and intergereric relative apparent rates of racemization (Lajole e t a l . , 1980), aric the "ability" of several genera to show increasing D/L values in sarrples of known increasing stratigraphic age (Hare and Mitterer, 1967; Weklmiller e t aZ., 1977; 1978a; Lajoie e t aZ., 1980. Belknap, 1979). Studies of samples of equal age (as established by radiometric calibraticn) along north-south coastlines ha-Je been particularly useful in de7,eloping a framework for regional correlation, and for quantification of the temperature effect on racemization (Wehmiller e t aZ., 1977; Wetmiller and Belknap, 1978: Wehmiller and Emerson, 1980). These latter studies have also been important in discussions of optional kinetic mociels for racemization in Quaternary mollusks (Wehmiller, 1981a). The present paper will review some of the conclusions developed in these prpvious studies and will discuss some unresolved questions about the reIiability of the method. A comprehensive discussion of the amino
174
acid datir-g method is f o u r d ir! 3are, D.E., e t al. (19eC).
LOCALITIES AND S A W L Z S Atlantic and Pacific coast Quaternary localities from which molluscan enantiomeric ratio data are available are shown in Figure 2.
Figure 2
Quaternary f o s s i l l o c a l i t i e s f o r which amino a c i d enantiomeric r a t i o d a t a are a v a i l a b l e . Labelled l o c a l i t i e s have radiometric o r s t r a t i g r a p h i c control.
P a c i f i c Coast PL C
sc V
LA
SNI SD TB
M
Puget Lowland: Holocene radiometric loc. Cayucos: 1 2 5 ka U-series l o c . ; F i g s . 4 , 6. Sea C l i f f : H o l o c e n e r a d i o metric loc.; strat. loc.: Figs. 4 , 6. Ventura: S t r a t . l o c . , Figs. 4 , 6. Los Angeles Basin: strat: loc., Figs. 4 , 6. San N i c o l a s I s l a n d : 120 ka U-series l o c . ; s t r a t . loc.; Figs. 4 , 6. S a n D i e g o : 1 2 0 k a U-series loc.; strat. loc., Figs. 5 , 6. T u r t l e Bay: s t r a t . l o c . , F i g . 6. M a g d a l e n a B a y : 1 2 0 k a Useries l o c . , Fig. 6.
A t l a n t i c Coast RR R a p p a h a n o c k R i v . : 1 9 0 ka U-series l o c . , F i g . 9. N N o r f o l k : 7 4 k a U-series l o c . , Fig. 9. NR N e u s e R i v e r , F l a n n e r Beach Fm., F i g . 9 . MB Y y r t l e B e a c h ; s t r a t l o c s . Fig. 9. Ch C h a r l e s t o n : 9 0 ks U series l o c s . , Fig. 9. S F S o u t h F l o r i d a : 1 2 0 Useries l o c s . , F i g . 9.
Specific locality information is found in previously-cited references for these areas. A few of the localities shown in Figure 2 have absolute age control (U-series dates on corals, or I4C) for rigorous
175
c a l i b r a t i o n o f a m i n o a c i d k i n e t i c s i n a g i v e n r e g i o n , a n d a few o f t h e l o c a l i t i e s shown i n F i g u r e 2 a c t u a l l y r e p r e s e n t a s e r i e s o f u n i t s w i t h u n a m b i g u o u s s t r a t i g r a p h i c r e l a t i o n s h i p s . S a m p l e s from t h e s e t y p e s o f l o c a l i t i e s a r e p a r t i c u l a r l y i m p o r t a n t i n t h e e v a l u a t i o n o f t h e amino a c i d d a t i n g method. Molluscan g e n e r a t h a t have b e e n employed most e x t e n s i v e l y i n t h e s e studies are: Family Veneridae:
Protothaca ~azidomus T i v e Za Chione Mereenaria
F a m i l y T e l l i n i d a e : Macoma
(Pacific coast) (Pacific coast) (Facific coast) ( P a c i f i c and A t l a n t i c c o a s t s ) (Atlantic coast) ( P a c i f i c and A t l a n t i c c o a s t s )
Other genera t h a t have been a n a l y z e d less f r e q u e n t l y i n c l u d e : Z'pilucina, Cumingia, TeguZa, Diodora, P o Z i n i c e s , H i a t e l l a , Mya, S p i s u l a , 3usycon, C r a s s o s t r e a , and O s t r e a . A N A L Y T I C A L PETHODS
M e t h o d s e m p l o y e d for e n a n t i o m e t r i c r a t i o d e t e r m i n a t i o n r e l y upon e i t h e r g a s c h r o m a t o g r a p h i c r e s o l u t i o n o f t h e d e r i v a t i v e s o f t h e D- a n d ;-amino a c i d s ( K v e n v o l d e n e t a l . , 1 9 7 2 ; W e h m i l l e r e t a l . , 1 9 7 7 ; Hoopes e t aZ., 1 9 7 8 ; S m i t h a n d W o n n a c o t t , 1 9 8 0 ; W e h m i l l e r a n d E m e r s o n , 1980), o r l i q u i d c h r o m a t o g r a p h i c r e s o l u t i o n o f amino a c i d s , e i t h e r i n m i x t u r e s (Hare a n d G i l - A v , 1 9 7 9 ) or i n d i v i d u a l l y , e i t h e r d e r i v a t i z e d ( B a d a a n d IIan, 1 9 8 0 ) o r u n d e r i v a t i z e d (Hare a n d Y i t t e r e r , 1 9 6 7 ; H a r e , 1 9 6 9 ; I 3 i l l e r a n d Hare, 1 9 8 0 a ) . T h i s l a t t e r a p p r o a c h h a s b e e n a p p l i e d t o t h e y e s o l u t i o n o f L - i s o l e u c i n e f r o m D - a l l o i s o l e u c i n e ( p r o d u c e d by t h e r a c e n i z a t i o n o f L - i s o l e u c i n e a t t h e a - c a r b o n ) , a n d i t was t h e r e c o g n i t i o n -\ft n e p r e s e n c e o f 3 - a l l o i s o i e ~ ~ c i n ien f o s s i l s t h a t l e d t o t h e p r o p o s a l t h a t r a c e m i z a t i o n r e a c t i o n s b e u s e d i n g e o c h r o n o l o g y (Hare a n d M i t t e r e r , 1 9 6 7 ) . A s D-alloisoleucine/L-isoleucine d e t e r m i n a t i o n s c a n b e p e r f o r m e d w i t h s t a n d a r d a m i n o a c i d a r a l y z e r s , t h i s a n a l y t i c a l m e t h o d has b e e n m o s t ' r e q u e n t l y u s e d by most w o r k e r s . Recent developments i n i n s t r u m e n t a t i o n ( s e e Hare a n d G i l - A v , 1 9 7 9 , a n d r e f e r e n c e s t h e r e i n ) h a v e made t h i s m e t h o d h i g h l y s e n s i t i v e , r a p i d , a n d q u i t e i n e x p e n s i v e . The i n s t r u m e n t a t i o n h a s e v e n b e e n made p o r t a b l e s o t h a t i t may b e u s e d i n t h e f i e l d ( M i l l e r a n d Hare, 1 9 8 0 b ) . I n a d d i t i o n t o t h e advantages l i s t e d above f o r t h e D-alloisoleucine/ L - i s o l e u c i n e method, t h i s a n a l y t i c a l a p p r o a c h a l s o p e r m i t s q u a n t i f i c a t i o r i o f a l l t h e a i n i n c a c i c ? s i r i t h e a n a l y z e d f o s s i l . Its p r i c c i p l e d i s a d v a n t a g e i s t h a t i t y i e l d s D/; i n f o r m a t i o n f o r o n l y o n e a m i n o a c i d i n t h e sample, though i t h a s u s u a l l y b e e n assumed t h a t t h e exter!t of r a c e m i z a t i o n o f i s o l e u c i n e was r e p r e s e n t a t i v e o f t h e e x t e n t o f r a c e T i z a t i o n o f a l l amino a c i d s i n a s a m p l e . C-alloisolezcine/L-Lsoleucir,e v a l c e s have prover, t o b e q u i k e v a r i a b l e ( 1 5 % u n c e r t a i n t y ) i n i n t e r l a b o r a t o r y comparisons (Kvenvolden, 1 9 8 0 ) , and a s i g n i f i c a n t range (between about 1 . 0 5 and 1 . 4 0 ) i n t h e " e q u i l i b r i u m " D-alloisoleucine/L-isoleucine v a l u e h a s b e e n r e p o r t e d (Hare a n d M i t t e r e r , 1 9 6 7 ; K i t t e r e r , 1 9 7 4 ; ?/lasters a n d B a d a , 1977), though most o f t h e r e p o r t e d e q u i l i b r i u m v a l u e s a r e w i t h i n 1 0 % o f 1 . 3 0 .
Gas c h r o m a t r o g r a p h i c m e t h o d s o f f e r t h e g e n e r a l a d v a n t a g e o f : ; i e l d i n g e n a n t i o m e r i c r a t i o d a t a on s e v e r a l a m i n o a c i d s ( s i x or m o r e ) I n a s a m p l e . A s e a c h a m i n o a c i d r a c e m i z e s a t i t s own r a t e , m u l t i p l e D/L v a l u e s i n a s i n g l e s a m p l e c a n b e u s e d t o e v a l u a t e i n t e r n a l c o n s i s t e n c y . T h e s e m e t h o d s a r e , h o w e v e r , somewhat m o r e t i m e - c o n s u m i n g a n d e x p e n s i v e t h a n i s t h e i s o l e u c i n e m e t h o d , a n d i t i s much m o r e d i f f i c u l t t o o b t a i n q u a n t i t a t i v e a n a l y s e s by g a s c h r o m a t o g r a p h y . Two g a s c h r o m a t o g r a p h i c m e t h o d s h a v e b e e n e m p l o y e d i n t h i s laboratory:
176
1)
NT?A-(t)-2-butyl method, in which the mixture of amino acids to be analyzed is esterified with an optically active alcohol (thereby introducing a second center of asymmetry into the amino acid molecule). Resolution of this mixture of derivatized amino acids is accomplished with chromatographic columns coated with the liquid phases OV225, Carbowax 20M, or UCON 75H 9 0 , O O C . Examples of the chromatographic results of these procedures are found in Kvenvolden e t aZ., 1972; Hare and Hoering, 1973; Kvenvolden e t aZ., 1979; Belknap and Wehmiller, 1980.
2)
NTFA-isopropyl method, in which the mixture of amino acids is esterified with an optically inactive alcohol. Resolution of this mixture of derivatized amino acids is accomplished with a chromatographic column coated with an optically active phase. Examples of this approach are found in Smith and Wonnacott (1980) and Wehmiller and Emerson (1980).
In both analytical approaches, the elution time for a particular D- or L-amin o acid is a function of its solubility in the liquid phasecoating the interior of the column and the volatility of the derivative. Detection of the amino acid derivatives eluted from the chromatographic columns in the above methods nas usually been by flame ionization detectors (FID), which ionize and detect all eluted carboncontaining molecules. Nitrogen-specific detectors (NPD) permit the specific detection of amino acids, often with several orders of magnitude more sensitivity than with flame ionization detectors. Using a variety of derivatives, columns, and detectors, it is possible to determine the D/L value f o r each of seven amino acids (leucine, glutamic acid, alanine, valine, proline, phenylalanine, and aspartic acid) by at least two gas chromatographic schemes, therIeby reducing the possibility that measured D/L values have been affected by interfering chromatographic peaks. Reported D/L values are usually the ratios of D- and L- peak heights or areas. Figure 3 shows a high-resolution chromatogram, obtained in our laboratory, of NTFA-(t)-2-butyl esters of a 100 meter glass capillary column, coated with OV225, using simultaneous flame ionization (FID) and nitrogen-specific (NPD) detectors. The extreme high resolution of this method permits baseline separation of D-alloisoleucine and Lisoleucine. This separation has always been difficult by gas chromatography, thereby preventing confident comparisons of gas chromatographic and liquid chromatographic analyses (Kvenvolden, 1980). On-going work in this laboratory indicates that D-allisoleucine/L-isoleucine values determined by the method shown in Figure 3 are within 5% of the Dalloisoleucine/L-isoleucine values determined by the conventional liquid chromatographic methods. The use of dual detection systems, as shown in Figure 3, occasionally has revealed the presence of nonnitrogenous compounds, some of which actually co-elute with individual amino acid peaks, thereby invalidating the measured FID D/L value. Interlaboratory comparison of gas ?hromatographic methods has revealed a range of uncertainty between about 5% and 15% for different amino acids (Kvenvolden, 1980). This variability (partially related t o different analytical methods) is somewhat more than would be expected for multiple analyses in a single laboratory: regular repeated analysis of a homogeneous powdered fossil mollusk in this laboratory has revealed a range of 3 - 5% for leucine, valine, alanine, and glutamic acid D/L values. Uncertainties for proline, phenylalanine, and aspartic acid are between 8 and 15%. Comparable precision is reported by most workers for multiple analyses of single molluscan genera at a single outcrop (Kvenvolden e t aZ., 1979; Miller and Hare, 1980a). The sensitivities of the methods outlined above are such that as little as 5 mg of carbonate material can be analyzed, given typical abundances of amino acids in most Pleistocene fossils. It is important, however, that the analysis be representative of the sample being studied,
177
0 CD
0 t
\
3 SNOd S 3 k l
U 0 1 3 3 13a
as inhomogeneities in D/L values have been observed when small ((0.2gm) fragments have been analyzed from large ( > 3 0 gm) bivalves. At least part of this problem can be eliminated if individual layers of mollusks are analyzed (Hare and Mitterer, 1969). As a working rule, we have found that a fragment representing at least 5% of the weight of the shell should be analyzed, even though analytical sensitivity might permit samples several orders of magnitude smaller. In all cases, samples should be extensively cleaned (mechanically and with dilute acid and base washes) to remove contamination and altered surficial layers. Sample preservation should be evaluated to the fullest extent
178
possible. Macroscopic examination of most bivalves can identify gross leaching or physical alteration, but nicrcscopic and nineralsgic examination are also important. It has generally been assumed (often with supportive evidence) that the primary effect of poor sample creservation would be to lower the enantioneric ratios from their true values, because contamination would introduce amiqo acids with low D/L values and leaching would remove privzrily free amir.0 acids which are the most extensively racemized amino acids in the sample. Though this assumption is probably valid in most cases, a few examples have beer encountered where "poorly preserved" samples (leached, chalky, or recrystallized) have yielded higher D/L values than expected (h'ehmiller e t aZ., 1976). In addition to the above chemical and physical criteria for sample selection, it is important that samples be collected from great enough depths so that significant exposure to high temperatures in the shallow soil zone has not occurred. The nature of this exposure is highly dependent on local climates, moisture, and vegetative cover. It has been shown (Wehmiller, 1977) that exposure to high temperatures at depths of less than 1 meter (or on the soil surface) can significantly affect the measured extent of racernization, if the duration of exposure is long in proportion to the absolute age of the sample. This problen is especially important for Holocene samples, archaeological samples, and all samples found in arid climates (Wehmiller e t aZ., 1979). INTER-
AND INTEAGENERIC EYFECTS ON FACEYIIZATION
Individuai amino acids have their own characteristic rates of racemization in aqueous solutions (Schroeder and Bada, 1976; Williams and Smith, 1977). Similarly, a regular pattern of characteristic relative rates of racemization of the different amino acids is observed in diff'erent genera of mollusks, corals, and foraminifera (Lajoie e t aZ., 1980; Wehmiller, 1980; Wenmiller e t al., T976; Kvenvolden e t aZ., 1973). These intrageneric relative apparent rates of racemizatior are important in the evaluation of the internal consistency of any given sample analysis. The uniformity of these relative intrageneric rates among different fossil types suggests a common diaaenetic Dathway for the calcified prateins ir, these sanples (Wehrxiller, 1980). Though intrageneric relative racemization kinetics might be similar or identical in different molluscan Eenera, intergeneric effects on racemization among these genera are potentially significant (Wehmiller e t aZ., 1977; Belknap, 1979; Lajoie e t aZ., 1980). Comparison of individual amino acid apparent rates o r racemization between coexisting samples of different genera has revealed that at least two kinetic groups of mollusks exist: the "slow-racemizing" group, comprised of the venerids P r o t o t h a c a , C h i o n e , Saxidomus, T i v e l a , and M e r e e n a r i a , and the "fast-racemizing" grour, comprised of other bivalves or gastropods ( E p i z u c i n a , C u r n i n g i a , Macoma, B u s y c o n , T e g u l a ) (Wehmiller e t aZ., 1977; Belknap, 1979; Lajoie e t aZ., 1980). Additional studies of oyster samples coexisting with samples from the above groups (Kvenvolden e t a Z . , 1979; Belknap, 1979) suggest that oysters (either C r a s s o s t r e a or O s t r e a ) form a still slower racemizing group. These relative intergeneric rates of racemization have been duplicated in laboratory pyrolysis experiments (Keenan and Wehmiller, in prep.) and have also been observed in foraminifera (King and Hare, 1972: King and Neville, 1977). Wehmiller (1980) has suggested that these intergeneric kinetic effects can be explained by the relative abundance of stable peptide bonds in the various genera, with the slow-racemizing groups containing a greater proportion of stable bonds which hydrolyze more slowly, thereby producing free amino acids (which are more extensively racemized) more slowly. The three categories of mollusks that can be recognized by their relative racemization kinetics can also be grouped identically according to their relative abundances of aspartic acid (Wehmiller, 1980), which apparently form stable peptide bonds in calcareous matrices (Hare e t aZ., 1975). Thus there appears to be a "chemical taxonomy" that roughly parallels the classical taxonomic organization of the mollusks that have been studied.
179
Generic effects on racemization kinetics complicate the chronologic application of the method because results f r o m different genera may n o t be directly comparable, unless the genera involved are known to belong to the same kinetic group. Nevertheless, siuch generic effects provide useful cross-checks on the relative age estimates that might be developed from one genus. As it is rare for a single genus to be abundant at all the localities of potential interest, multiple genera would need to be employed for any regional chronological study. In order to establish the framework for such a study, relative intergeneric kinetics for all the genera of interest should be established using coexisting samples from selected outcrops (see Lajoie e t a Z . , 1980, for example). Occasional examples of inversions of intergeneric relationships have been encountered (Wehmiller e t a Z . , 1977; l978a; Lajoie e t aZ., 1980; Yerkes e t aZ., 1980; Wehmiller, unpubl.). Usually these are minor inversions among genera of the same kinetic group, but at least one example of a major inversion betweer, groups has been encountered (Wehmiller, unpubl.; discussion in Yerkes e t a Z . , 1980). In several cases the most likely explanation f o r these inversions is reworking (which can also be recognized by multiple analyses of samples of a single genus), but in a few cases the reason for the inversion is more enigmatic, and the derived age estimate is less certain (Yerkes e t aZ., 1980). STRATIGRAPHIC EVALUATION AND APPLICATION The simplest approach to evaluation or application of amino acid enantiomeric ratios is as a stratigraphic tool ("aminostratigraphy" see Miller and Hare, 1980a) in a local region where it can be reasonably assumed that all samples (which might be at present-day temperatures within 1" C of each other) have had similar or identical temperature histories. The "resolving power'' (ability to distinguish aminostratigraphic units) of this approach depepds upon tke accuracy of this temperature assumption. Examples of this approach, from the Pacific coast of the United States, are summarized in Figure 4. Other similar examples can be found in Hare and Mitterer (1967); Mitterer (1974; 1975); Miller e t aZ. (1979); Karrow and Bada (1980); and Miller and Hare (1980a). In those cases where a clear, unambiguous stratigraphic relationship exists among the analyzed fossil localities, the reliability of amino acid enantiomeric ratios as relative dating tools canbe evaluated. The best examples of this type of evaluation are the results for vertical terrace sequences at San Nicolas Islagd, Palos Verdes Hills, San Joaquin Hills, Point Loma (San Diego), and Sea Cliff (all southern California) (Wehmiller e t a t . , 1977; Lajoie e t al., 1979; 1980), and superposed stratigraphic sequepces in the L o s Angeles Basin (Wehmiller e t a Z , , 1977; Lajoie e t aZ., 1980), near Ventura (Wehmiller e t al., 1978a, b ; Lajoie e t a Z . , 19791, and in San Francisco Bay (Atwater e t a z . , 1981). In these cases it has been possible to show that several genera ( P r o t o t h a c a , S a x i d o m u s , C h i o n e , T i v e l a , Macoma, T e g u Z a , and O s t r e a ) exhibit steadily increasing D/L values in samples of known increasing age. Some of the deposits are old enough so that certain samples have reached racemic equilibrium ( C / L values at "effective" unity, 2 0.95). In all but a very few cases, all of these genera have exhibited proper enantiomeric ratio trends when tested witliin a rigorous stratigraphic framework. Some genera (particularly the gastropods D i o d o r a and P o Z i n i c e s ) have repeatedly shown decreasing D/L values with increasing stratigraphic age, thereby failing the most basic test of the method. Occasionally other genera ( P r o t o t h a c a , E p i Z u c i n a , and T i v e Z a ) have shown decreasing D/L values with increasing age, but these anomalous result: can usually be ascribed to poor sample preservation. Even wellpreserved D i o d o r a , though, have failed to meet these simple tests of reliability. In many cases where stratigraphic relationships among discontinuoutcrops are unclear, amino acid enantiomeric ratios can be used to propose an aminostratigraphic framework (Mitterer; 1974; 1975; GUS
180
SOUTHERN CALIFORNIA S T R A T I G R A P H I C E V A L U A T I O N S , D/L LEUCiNE
VENTURA ( P R O TO THACA)
m
700 ka ash
t
- 2 , 0 0 0 rn
i
30r rn
>
L . A. BASIN
0-
6
( S A X I D 0 MU S )
.0 . - 0
1.0
1 3i
.71/
2100 oo m
SAN NICOLAS ISLAND (SAXIDOMUS)
120 k a
0.
Figure 4
Stratigraphic tests; southern California. D/L l e u c i n e t r e n d s r e p r e s e n t t h o s e f o r o t h e r amino acids. V e n t u r a s e c t i o n i n c l u d e s Macoma d a t a c o n v e r t e d t o e q u i v a l e n t Protothaca.
b J e h m i l l e r e t a Z . , 1977; M i l l e r e t a l . , 1979; L a j o i e e t a Z . , 1979; B e l k n a p a n d W e h m i l l e r , 1 9 8 0 ; Karrow a n d B a d a , 1980). F i g u r e 5 shows hnw a number o f d i s c r e k e ( a l l s t a t i s t i c a l l y s i g n i f i c a n t ) a m i n o s t r a t i y a p h i c " e v e n t s " a r e r e c o r d e d i n t h e a r e a o-f S a n D i e g o , C a l i f o r n i a ( W e h m i l l e r e t aZ., 1977; L a j o i e e t a l . , 1979; W e h m i l l e r , 1981b), a n d i n i 3 r t h e a s t e r n S o u t h C a r o l i n a ( B e l k n a p , 1979; B e l k n a p , 1 9 8 0 ) . I n b o t h o f t h e s e c a s e s , m u l t i p l e d e p o s i t i o n a l e v e n t s would b e e x p e c t e d on t h e b a s i s of c o n v e n t i o n a l g e o m o r p h i c a n d s t r a t i g r a p h i c i n f o r m a t i o n , b u t t h e number o f e v e n t s w o u l d b e d i f f i c u l t t o s p e c i f y . Amino a c i d d a t a a p p e a r t o cont - i b u t e t o answering t h i s q u e s t i o n . Eowever, i n t h e c a s e o f t h e South
181
STRATIGRAPHIC APPLICATIONS LEUCINE
D/L
S A N DIEGO AREA (PROTOTHACA) W E P t . Lorna
.3a -
404
62 72
WJ
0
%?AD9%2*
120 k a Nestor
-
'1.0
3"-4
k m
S O U T H C A R O L I N A (MERCZNARIA)
W
E
30 7
.'
0 - 2 - 4
Figure 5
k m
Apparent aminostratigraphic groups, D/L l e u c i n e , San Diego, C a l i f o r n i a and South C a r o l i n a ( M y r t l e Beach a r e a ) . O t h e r a m i n o a c i d s follow s i m i l a r trends. San Diego area i n c l u d e s t h e 120 k a N e s t o r T e r r a c e , o n P t . Loma ( s e e F i g s . 6 a n d 7 ) .
Carolina work, the number of apparent aminostratigraphic events is not in agreement with at least one lithostratigraphic interpretation. REGIONAL CORRELATIONS
-
EVALUATION AND APPLICATION
A logical extension of the local aminostratigraphic approach to evaluation and application is the regional approach, involving comparisons of aminostratigraphic sequences at different latitudes (with different present-day mean annual temperatures). Quaternary marine terrace localities along the Pacific coast have proven to be an ideal setting in which to study this aspect of racemization, as several widely spaced localities with good chronologic control (U-series dates on corals) are available f o r documentation of the latitudinal (temperature) gradient of enantiomeric ratios in samples of equal age. Figure 6 (modified from Wehmiller and Emerson, 1980, and Emerson
e t aZ., 1981) shows this relationship for one amino acid, leucine, between latitudes 2 5 ' N and 35" N along the Pacific coast. Other amino
182
-
1.0
q
-
*.
.9-
g2 - 0 -
4'7-
2 .6-
-
Q -5-
g.4-
-
0
3
b
' -+-+ 0
0
b
/
-3
Q
w -1.2-I
5
/+ *:STAGE
-
\*I n
f,"
.. .
EARLY ST
sc-2
sc-1
-
STAGE I
-t
0-LOCS: C
I
LA SNI SC,VSD PC I
1
I
1
PSR I
I
16 18 20 PRESENT MEAN ANN. TEMP., O C 14
Figure 6
TB
1
M
1
22
.
P l o t o f D / L l e u c i n e i n Protothaca ( o r o t h e r g e n e r a c o n v e r t e d t o e q u i v a l e n t Protothaca) us. p r e s e n t m e a n annual temperature, Pacific coast localities. Pres e n t t e m p e r a t u r e i s a smooth f u n c t i o n of l a t i t u d e , l o c a l i t i e s r a n g e from a p p r o x . 35" N t o 24" N . O t h e r a m i n o a c i d s show s i m i l a r t r e n d s . Early Stage 5 i s o c h r o n c o n n e c t e d t o 1 2 0 k a l o c a l i t i e s w i t h Useries c o n t r o l ; l a t e Stage 5 isochron connected t o two d a t a p o i n t s f o r t e r r a c e s w i t h s t r a t i g r a p h i c r e l a t i o n s h i p s t o 1 2 0 k a l o c s . ( E m e r s o n et az., 1981).
+
L o c a l i t y w i t h 1 2 0 k a U-series c o n t r o l o r Holocene 14c control a L o c a l i t y w i t h no r a d i o m e t r i c c o n t r o l
L o c a l i t y a b b r e v i a t i o n s as i n Figure 2: PC = P u n t a Carnal; (Valentine, 1980) PSR = P u n t a S a n t a R o s a l i l l i t a ( W o o d s , 1 9 8 0 ) . I
acids would show a similar trend. D/L leucine values are plotted u s . present mean annual temperature (because temperature is the control of kinetics), though it is known that present temperatures are not representative of the long-term temperature history of the samples. Because present-day temperatures are a nearly linear function of latitude in this region, it is possible to assume that differences in long-term temperature histories would be similar in magnitude to present-day ';emperatwe differences. The solid line in Figure 6 connects D/L leucine data f o r four localities which represent early Stage 5, roughly 120,000 to 130,000 years BP (120 - 130 ka), according to U-series coral dates at these localities. The trend of this line is consistent with the hypothesis that D/L values should increase in samples of equal age at lower latitudes (higher temperatures): consequently these observations become an additional test of the validity of enantiomeric ratios as age or temperature indices. The slope o f the line can only be predicted by
183
kinetic modelling and temperature assumptions (Wehmiller and Emerson, 1980), but in those cases where several calibration localities are available, such kinetic models are not necessary for estimation of ages if the D/L data for intermediate localities fall on or near the interpolated 120 ka isochron. It is only necessary to assume that localities with present-day temperatures intermediate to those of calibration localities have always had intermediate temperatures, i . e . , that latitudinal gradients of temperature have not changed significantly (though temperatures certainly would have dropped) during Pleistocene climatic cycles. Shown in Figure 6 are the results for four calibration localities (converted by the intergeneric regressions of Lajoie e t a Z . , 1980 to equivalent genera) and the data for Pacific coast stratigraphic sequences shown in Figures 4 and 5. Results from Baja. California localities have been discussed by,Woods (1980), Valentine '(1980), Wehmiller and Emerson (1980), and Emerson e t a Z . (1981). The late Stage 5 isochron, roughly parallel to the early Stage 5 isochron and connecting data points for terraces at San Diego and Turtle Bay, is supported by both stratigraphic and faunal relationships with nearby terrace deposits that show slightly higher degrees of racemization, and which are either calibrated with or correlated to early Stage 5 U-series coral dates (Kern, 1977; Lajoie e t a Z . , 1979; Emerson e t a Z . , 1981). Data for one locality (Sea Cliff, Second Terrace .- SC-2; see Figure 4) plotted in Figure 6 fall well below the late Stage 5 isochron but above the data for Holocene marine terrace samples (SC-1). Sea Cliff 2 is estimated to be a Stage 3 (40 - 50 ka) terrace by both qualitative and kinetic model age estimates (Wehmiller e t a Z . , 1978a, b ; Lajoie e t a Z . , 1979). Collectively the ages and elevations of these two terraces (see Figure 4) imply unusually rapid uplift rates, up to approximately 6 m/1000 years (Lajoie e t a l . , 1979). KINETIC MODELLING Estimation of absolute ages f o r data points in Figure 6 that lie well above or below the calibrated isochrons requires a kinetic model that can quantify age and temperature relationships for calibrated and uncalibrated racemization data. The development and testing of kinetic models requires the availability of both Holocene and Pleistocene calibration samples, and paleoclimatic information with which to estimate the temperature history (usually referred to as the Effective Quaternary Temperature, or EQT) for Pleistocene samples. Age estimates proposed for undated localities also rely upon the assumption, discussed previously, that present temperature differences between localities are a measure of the differences in EQT for these localities i . e . , that gradients of EQT have been similar to present temperature gradients. The validity of this assumption depends upon the intensity and uniformity of climatic change in a region of study.
-
Recent discussion (Wehmiller, 1981a; Kvenvolden e t a Z . , 1981) of kinetic modelling has centered on the kinetics of leucine racemization in the venerid genera P r o t o t h a c a and S a x i d o m u s , two closely related mollusks that have very similar relative kinetics (Lajoie e t a Z . , 1980). Further evidence (Lajoie e t a Z . , 1980; Wehmiller, 1980; unpublished) indicates that whatever kinetic model is appropriate for P r o t o t k a c a and S a x i d o m u s should also be appropriate for other venerids such a s C h i o n e and M e r e e n a r i a . Figure 7 summarizes the basic issue of leucine kinetic modelling. Figure 7 shows the variable (XE - X)/XE for leucine, in a logarithmic format, where X = D/(DtL) at any time T and XE = D/(DtL) at equilibrium; for leucine, XE = 0.50. Traditional discussions of racemization kinetics have used this (or similar) graphical format because simple first order reversible racemization (L D) would be represented by a straight line in this format. Deviations f r o m linearity would then indicate more complex kinetics. Two models are shown in Figure 7:
184
MODEL LEUCINE KtNETICS
0
-,,o w
Y
.2u 0 Figure 7
I
I
40
I
I
80
I
1
I20
I
T I M E , 1000 yrs.
I
160
I
Comparison o f kinetic model options for leucine X = D/(D+L) at time racemization i n P r o t o t h a c a . T , XE = X at equilibrium. F o r l e u c i n e , XE = 0.50. (XE - x)/xE would be a linear function o f time, i n this format, if racemization kinetics w e r e simple first-order reversible. Two data p o i n t s , SC-1 (Figs. 4 and 6) and Nestor (Figs. 5 and 6) are used to compare models because these points represent Holocene and late Pleistocene calibrated localities with similar present temperatures. Solid lines represent Model B , the extended linear model of Dashed lines represent Kvenvolden e t a z . (1979). model A , the non-linear model o f Wehmiller e t al. (1977). Model B r a t e constants for SC-1 and N (ksc and kn), i n combination w i t h present temperature of Sea Cliff (15.3"C) would require that the Effective Quaternary Temperature (EQT) o f Nestor be about 5" C. Model B would require that the EQT of Nestor be about 13.5" C . See text, Wehmiller (1981b) and Kvenvolden e t a z . (1981), for further discussion.
Model A , t h e " n o n - l i n e a r " m o d e l oI" I,;ehmiller e t a l . ( 1 9 7 7 ) ; W e h m i l l e r a n d B e l k n a p (19781, w h i c h h a s two l i n e a r c o m p o n e n t s w i t h a t r a n s i t i o n z o n e b e t w e e n . The e a r l y p o r t i o n o f t h e r a c e m l z a t i o n c u r v e i s l i n e a r t o a 2/L l e u c l n e v a l u e o f a b o u t 0.2, w i t h a s t e a d y d e c r e a s e i n s l o p e s o t h a t b e y o n d 2/L l e u c i n e of a b o u t 0 . 3 5 , t h e s l o p e i s a b o u t 1 0 % o f t h e i n i t i a l slope. T h i s m o d e l was d e r i v e d f r o m a c t u a l k i n e t i c s o b s e r v e d i n f o r a m i n i f e r a (1t;ehmiller a n d E a r e , 1 9 7 1 ; K v e n v o l d e n e t aZ., 1973) a n d r e q u i r e s e x t r a p o l a t i o n o v e r a r a t h e r s m a l l t e m p e r a t u r e r a n g e (10 - 15") t o b e a D p l i c a b l e t o t h e ambient t e m p e r a t u r e s of molluscan r a c e m i z a t i o n . K o d e l E, t h e " e x t e n d e d l i n e a r " m o d e l o f R v e n v o l d e n e t al. (1979; 1 9 8 1 ) w h i c h i s l i n e a r up t o a D/i l e u c i n e v a l u e o f a b o u t 0 . 5 5 , w i t h l e s s e r s l o p e s ( p o o r l y d e f i n e d ) beyond t h i s D/L v a l u e . This m o d e l h a s b e e n d e r i v e d from k i n e t i c s o b s e r v e d i n h i g h t e m p e r a t u r e ( 1 4 0 - 1600 C) p y r o l y s i s e x p e r i m e n t s a n d r e q d i r e s e x t r a p o l a t i o n over l a r g e temperature ranges ( c a . 130") i n order t o be applicable t o t h e ambient t e m p e r a t u r e s o f molluscan r a c e m i z a t i o n . The t e m p e r a t u r e d e p e n d e n c e ( c a . 18%/" C) i s a s s u m e d to b e t h e same for b o t h m o d e l s , a s t h e r e d o e s n o t a p p e a r to b e s i g n i f i c a n t d i s a g r e e m e n t
over t h e a c t i v a t i o n energy f o r leucifle racemization (see d i s c u s s i o n i n k'ehmiller, 1981a).
185 D/L l e u c i n e r e s u l t s for Protothaca i n t w o s o u t h e r n C a l i f o r n i a c a l i b r a t i o n l o c a l i t i e s ( w i t h s i m i l a r p r e s e n t - d a y t e m p e r a t u r e s ) a r e shown i n Figure 7. SC-1 i s t h e H o l o c e n e ( I 4 C d a t e s ) t e r r a c e l o c a l i t y shown N e s t o r i s t h e 1 2 0 k a u p p e r t e r r a c e on P o i n t Lorna, i n F i g u r e s 4 and 6 . San D i e g o ( F i g u r e s 5 a n d 6 ) . 'The N e s t o r d a t a , when c o m p a r e d w i t h t h e SC-1 d a t a i n a l i n e a r m o d e l f o r m a t (Model B y s o l i d l i n e s , F i g . 7 ) , i n d i c a t e t h a t t h e N e s t o r r a t e c o n s t a n t ( c a l c u l a t e d w i t h e q u a t i o n 1 of K v e n v o l d e n et aZ., 1 9 7 9 ) i s a p p r o x i m a t e l y 2 0 % o f t h e r a t e c o n s t a n t obs e r v e d for t h e H o l o c e n e SC-1 s a m p l e s . U s i n g t h e a c c e p t e d t e m p e r a t u r e d e p e n d e n c e of l e u c i n e r a c e m i z a t i o n , t h i s a p p a r e n t l i n e a r r a t e c o n s t a n t for t h e N e s t o r s a m p l e s w o u l d i m p l y a n EQT a b o u t 1 0 " C l e s s t h a n t h e A s t h e EQT i s t h e i n t e g r a t e d p r e s e n t t e m p e r a t w e (15.3" C ) a t SC-1. k i n e t i c e f f e c t 0-C a l l t e m p e r a t u r e s t o w h i c h t h e N e s t o r s a m p l e s h a v e b e e n e x p o s e d , i t c a n b e shown ( W e h m i l l e r e t aZ., 1 9 7 7 ; W e h m i l l e r , 1 9 8 1 2 ) t h a t a 10' C c o o l e r EQT w o u l d r e q u i r e t h a t f u l l - g l a c i a l t e m p e r a t u r e s a t t h e N e s t o r l o c a l i t y were a s low a s 0 " C , l o w e r b y a t l e a s t 8 " C t h a n most g l a c i a l age t e m p e r a t u r e e s t i m a t e s f o r c o a s t a l s o u t h e r n C a l i f o r n i a ( J o h n s o n , 1 9 7 7 ; F e t e r s o n et aZ., 1 9 7 9 ) . S i m i l a r t e m p e r a t u r e a m b i g u i t i e s a r i s e when o t h e r E o l o c e n e - P l e i s t o c e n e c a l i b r a t i o n s a m p l e s a r e c o m p a r e d ( W e h m i l l e r et aZ., 1 9 7 7 ; W e h m i l l e r , 1 9 8 1 a ) .
The n o n - l i n e a r m o d e l ( M o d e l A ) i s shown a s d a s h e d l i n e s i n F i g u r e K i n e t i c p a t h w a y s f o r t e m p e r a t u r e s o f 1 3 , 14 a n d 1 5 " C a r e s h o w n . The N e s t o r d a t a i n d i c a t e t h a t a t e m p e r a t u r e o f a b o u t 13.5" C w o u l d b e t h e EQT v a l u e i n f e r r e d f r o m t h e n o r . - l i n e a r m o d e l f o r t h i s l o c a l i t y . T h i s EQT v a l u e , a n d t h e a s s o c i a t e d e x t r a p o l a t i o n o f t h e n o n - l i n e a r model, w o u l d form t h e b a s i s f o r a g e e s t i m a t i o r . o f more e x t e n s i v e l y r a c e m i z e d s a m p l e s f r o m t h e same t e m p e r a t u r e r e g i o n . A r e d u c t i o n o f a b o u t 2 . 5 " C i n 3QT for t h e X e s t o r l o c a l i t y ( r e l a t i v e t o its p r e s e n t t e m p e r a t u r e of 16") i m p l i e s a f u l l - g l a c i a l t e m p e r a t u r e r e d u c t i o n o f 4 - 5" C , much s m a l l e r t h a n t h e 15' C f u l l - g l a c i a l r e d u c t i o n r e q u i r e d b y Model B a n d m o r e c o n s i s t e n t w i t h p a l e o c l i m a t i c i n f o r m a t i o n for s o u t h e r n C a l i f o r n i a ( J o h n s o n , 1977; P e t e r s o n e t al., 1979).
7.
The c o n f l i c t b e t w e e r t h e t w o m o d e l s d i s c u s s e d a b o v e c o n c e r n s t h e p o s i t i o n o f t h e b r e a k i n s l o p e of' t h e k i n e t i c s . There i s gene-a1 agreemezt t h a t t h e b r e a k i n s l o p e , w h e r e v e r 5: i s l o c a t e d , i s r e l a t e d t o t h e o h a n g i n g r b u n d a n c e s o f d i f f e r e K t FDleciAlar w e i g h t ccrnponents i n t h e f o s s i l s , w i t h e a c h c o m p o n e n t k a v l n p i t s owc r a t e of r a c e m i z a t i o n ( s e e , f o r e x a m p l e , ~ d e h m i l l e r a n d H a r e , 1971; 3 a d a a n d S c h r o e d e r , 1 9 7 2 ; K r i a u s a k u l a n d K i t t e r e r , 1 9 7 8 ; 1980; W e h m i l l e r , 1 9 8 0 ) . I n a d d i t i o n , i t i s known t h a t t h e i n i t i a l l y r a p i d r a t e o f r a c e m i z a t i o n ( r o u g h l y 1 O X t h a t o f f r e e a m i n o a c i d s i n s o l u t i o n ) i s s i g n i f i c a n t l y a f f e c t e d by h y d r o l y s i s and t h e p r o d u c t i o n of e x t e n s i v e l y r a c e m i z e d f r e e amino a c i d s ( W e h m i l l e r a n d Hare, 1 9 7 1 ) . Model A h a s o b s e r v e d k i n e t i c s i n d e e p - s e a s e d i m e n t s as i t s b a s i s , and though i t r e q u i r e s e x t r a p o l a t i o n o v e r a s m a l l t e m p e r a t u r e r a n g e , i t r e l i e s upon t h e a s s u m p t i o n t h a t f o r a m i n i f e r a and m o l l u s k s have s i m i l ' a ~ k i p e t i c p a t h w a y s . PIodel 3 h a s h i g h t e m p e r a t u r e m o l l u s c a n k i n e t i c s a s i t s b a s i s , hence d o e s n o t depend upon a s s u m p t i o n s r e g a r d i n g t h e s i m i l a r i t y o f f o r a m i n i f e r a and mollusk r a c e m i z a t i o n k i n e t i c s . Nevertheless Model B r e l i e s u p o n t h e a s s u m p t i o n t h a t t h e k i n e t i c s o b s e r v e d a t t e m p e r a t u r e s of r o u g h l y 150" C a r e an a c c u r a t e model of n a t u r a l d i a g e n e t i c k i n e t i c s a t ambient temperatures. U n t i l t h i s i s s u e i s r e s o l v e d , s i g n i f i c a n t d i f f e r e n c e s i n a g e e s t i m a t e s or p a l e o c l i m a t i c c o n c l u s i o n s w i l l b e d e r i v e d f r o m t h e two k i n e t i c m o d e l s o u t l i n e d a b o v e . S e v e r a l a d d i t i o n a l a p p r o a c h e s c a n b e t a k e n t o e v a l u a t e t h e s e two k i n e t i c m o d e l s . One i s t o c o m p a r e a g e e s t i m a t e s d e r i v e d f r o m e a c h m o d e l for p r e - 1 2 0 k a s a r r p l e s ( t h o s e t h a t p l o t w e l l a b o v e t h e 1 2 0 k a i s o c h r o n i n ? i g u r e 6 ) . These a g e e s t i m a t e s r e l y upon t h e b a s i c a s s u m p t i o n t h a t EQT v a l u e s h a v e b e e n s i m i l a r t h r o u g h t i m e i n a r e g i o n of s i m i l a r p r e s e n t t e m p e r a t u r e s . Though rigorous i n d e p e n d e n t a g e c o n t r o l f o r s a m p l e s i n t h i s a g e r a n g e i s n o t f r e q u e n t l y a v a i l a b l e , n o n - l i r e a r model a g e estimates a p p e a r more c o n s i s t e n t w i t h g e o l o g i c a l c o n s t r a i n t s f o r t h e V e n t u r a , S a n N i c o l a s I s l a n d , a n d Los A n g e l e s B a s i n s t r a t i g r a p h i c s e q u e n c e s shown i n ? i g u r e 4 ( W e h m i l l e r et aZ., 1 9 7 8 a ; L a j o i e e t aZ., 1 9 8 0 ) .
186
The format of Figure 6 can also be used to test the two models shown in 3igure 7. This approach is shown in Figure 8, in which 120 ka isochrons are shown for the two models for effective temperature equal to present temperatures and for 5 " C lower effective temperatures. The lower position of the lower-temperature isochron reflects the slower kinetics that would be expected at lower temperatures. The positions of the linear model isochrons are approximate because the kinetics beyond D/L leucine of 0.55 are poorly defined by this model. Superimposed upon these two model isochron pairs is the 120 k a calibrated isochron f r o m Yigure 6. It is clear that this calibrated isochron plots in the kinetic-temperature range that is more consistent with the non-linear node1 (model A) of Figure 7. Figure 8 reinforces the point made previously that the extended linear model (model B) can o n l y be reconciled with Zolocene and Pleistocene calibration data if extremely cold effective temperatures are invoked for the late Pleistocene paleoclimate of coastal western North America.
r 1.0-
1 120 k a , Const. T ..... ......... .......... 120 ka, T 5 O low
,9-
9.8-
1
T
3.7k
2.6 -
.......
0 k.5Q \
4-
w' z 3 -33
5 2-
tj .I A
0 -L O C S : C SNI L I
14 Figure 8
M
SO I
1
I
I
I
1
1
I
16 18 20 22 PRESENT MEAN ANN. TEMF?,OC.
I
C o m p a r i s o n o f k i n e t i c m o d e l s shown i n F i g u r e 7 i n t h e f o r m a t o f F i g u r e 6 , P a c i f i c c o a s t Protothaca. Only 1 2 0 ka c a l i b r a t e d d a t a p o i n t s from F i g u r e 6 a r e shown. The two m o d e l s (Model A , e x t e n d e d l i n e a r ; Model B , n o n - l i n e a r ) h a v e b e e n u s e d t o p r e d i c t t h e p o s i t i o n s of 1 2 0 k a i s o c h r o n s f o r two selected temperatures: e q u a l t o p r e s e n t temperThe a t u r e s (const. T), o r 5" c o o l e r (5" low). Model A i s o c h r o n s b o t h p l o t h i g h e r t h a n t h e Model B i s o c h r o n s because of t h e g r e a t e r long-term r a t e of r a c e m i z a t i o n p r e d i c t e d by Model A . Isochrons for cooler temperature f a l l below those f o r t h e warmer t e m p e r a t u r e because of reduced rates of racemiza t i o n a t lower temperatures. The 1 2 0 ka c a l i b r a t e d i s o c h r o n from F i g u r e 6 f a l l s w i t h i n t h e envelope of n o n - l i n e a r m o d e l i s o c h r o n s , f o r t e m p e r a t u r e s between 3" l o w e r t h a n p r e s e n t t e m p e r a t u r e s , approximately 1 w i t h g r e a t e r r e d u c t i o n s i n t e m p e r a t u r e s a t more northerly latitudes.
-
187
r,he calibrated 12C ka isockroc m d the non-linear model isochrons shown in Figure 8 car, be used to infer gradients of ZffectiveQuaternary Temperature. The relation of the ,-series calibrated isochron to the 120 ka model isochrons for 0 and 5" temperature reductions indicates that greater temperature reductions have occurred at mo?e northerly latitudns - i . e . , that latitudinal temperature gradients have been steeger than present gradients durirg the late 7ieistocene.
SISNIFICANT QUESTIONS Secause of the consistency of enantiomeric ratio trends with stratigraphic and latitudinal controls for several venerid genera on the Pacific coast, it has been assumed that the venerid M e r e e n a r i a , common to Atlantic coastal plain localities, would serve as an equally reliable fossil for aminostratigraphic applications. In fact, this massive, well-preserved bivalve has been shown to meet all of the criteria discussed previously for establishing the credibility of a given genus (Hare and Mitterer, 1967; Mitterer, 1974; 1975; Selknap, 1979; Belknap and Wehmiller, 1980). Nevertheless, recent work on the Atlantic coastal plain has revealed some significant conflicts between apparent arninostratigraphy and independent stratigraphic or radiometric data (Bellmap, 1979; Belknap and Wehmiller, 1980). These conflicts have far-reaching implications f o r the general reliability of the amino acid racemization dating method. These conflicts relate to the comparison of enantiomeric ratios (in M e r e e n a r i a ) and V-series dates on solitary corals (as reported by Cronin e t a Z . , 1981) from the same localities. Figure 9 demonstrates this comparison in the same format as Figure 6, for localities between Virginia and Florida (see Figure 2 for locations). It can be seen in Figure 9 that a smooth latitudinal trend of increasing enantiomeric ratios with decreasing latitude for samples of equal (Stage 5) age, as seen on the Pacific coast, is not seen on the Atlantic coastal plain (points 6, 2 and 1). At least part of this difference in enantiomeric ratio trends along the two coasts might be explained by greater steepening of the Atlantic coastal plain latitudinal gradients during the late Pleistocene because of the proximity of continental ice sheets. Nevertheless, a dramatic reversal of the expected latitudinal trend of enantiomeric ratios is seen for points 7 and 5, in Figure 9. The trend for these two points represents a serious conflict between U-series and amino acid data, and even the younger age ( c a . 200 ka) proposed by Cronin (1980) for the Flanner Beach locality (point 5) could not reconcile the inversion in enantiomeric ratios for points 5 and 7. In addition, the different D/L leucine values (0.28 v s . 0.65) for the 74 ka and 190 ka localities (points 6 and 7, respectively, Figure 9 ) is far greater than would be expected from Pacific coast samples (with similar temperatures) with this age difference. Though local temperature variations (up to 10%) might be expected to introduce slight variations around smooth latitudinal trends in enantiomeric ratios (Wehmiller e t a Z . , 1979), major inversions and deviations like those shown in Figure 9 raise serious questions about the temperature assumptions inherent to any relative or absolute chronologic application of amino acid enantiomeric ratios. Present temperature trends along both coasts are nearly linear functions of latitude (between 25" N and 50" N) and the Pacific coast data certainly indicates that this trend has not been grossly altered during the late Pleistocene. If the U-series dates for all the localities shown in Figure 9 are correct, and if there are no unknown chemical effects on racemization in M e r e e n a r i a , (all of the points shown in Figure 9 represent multiple analyses with good precision), then extreme temperature variations (both temporally and latitudinally) must be invoked to reconcile the amino acid data with the U-series data. Both latitudinal inversions of temperature gradients and significant local variations in Effective Quaternary Temperatures (more than c a . 1" C) contradict the basic temperature assumptions of aminostratigraphy.
188
24 Figure 9
1
2 3
4 5
6 7
18 16 14 P R E S E N T MEAN ANN.TEMP.,OC.
22
20
leucine values i n Mercenaria, plain localities. Temperature i s o f l a t i t u d e b e t w e e n 25" N a n d 39' shown h e r e . Localities as given i p l a n a t i o n of d a t a p o i n t s :
D/L
Atlantic coastal a smooth f u n c t i o n N, the region n Fig. 2. Ex-
South F l o r i d a 120 ka c a l i b r a t e d d a t a , converted from r e s u l t s of Mitterer (1975). C h a r l e s t o n , S . C . $95 k a c a l i b r a t e d d a t a , l o c s . 91 2 o f C r o n i n e t aZ. (L981). M y r t l e B e a c h , S . C . Waccamaw F m . , r e v e r s e d m a g n e t i c p o l a r i t y (Cronin, 1980). Myrtle Beach, S. C. Canepatch Fm., 1 440 ka, l o c . 8 o f C r o n i n e t aZ. ( 1 9 8 1 ) . Neuse R i v e r , N . C . F l a n n e r Beach Fm., > 5 0 0 k a , l o c . 7 o f C r o n i n e t aZ. ( 1 9 8 1 ) o r $ 2 0 0 k a ( C r o n i n , 1 9 8 0 ) . N o r f o l k , V a . 74 ? 4 k a c a l i b r a t e d d a t a , l o c . 2 o f C r o n i n e t az. ( 1 9 8 1 ) . Rappahanock R i v e r , V a . $190 k a U - s e r i e s c o r a l d a t a , l o c . 1 of Cronin e t aZ. (1981).
Data p o i n t s 2 , 3 and 4 a r e c o n s i s t e n t w i t h s t r a t i g r a p h i c relationships (Fig, 5). Dashed l i n e s a r e c o r r e l a t i o n s d i c t a t e d by t h e radiometric d a t a given above.
CONCLUSIONS Amino acid racemization in Quaternary mollusks has been used in a number of situations for both relative and absolute age estimation. A rumber of criteria developed in these studies are universally applicable to the evaluation of any new genus or sample type that might be employed in chronostratigraphic applications of amino acid racemization: 1) mechanical, mineralogical, and chemical preservation; absence of contamination;
189
2)
good analytical precision, single and/or multiple genera;
3)
stratigraphic and thermal consistency; ability of samples to demonstrate increasing racemization with increasing age, at constant temperature; or, for isochronous samples, with increasing temperature.
4) ability to reach equilibrium D/L values, and to maintain them. Occasional inversions of intergeneric relations have been encountered, and a few genera of gastropods have been rejected because they consistently violate several of the criteria listed above. Nevertheless, the vast majority of samples studied from the Pacific coast (mostly of the bivalve family Veneridae) appear totally consistent with the few unambiguous stratigraphic or latitudinal tests that are available. Stratigraphic and latitudinal f'rameworks have Dermitted the testing and evaluation of kinetic models of racemization. These models are necessary to quantify age-temperature relationships between calibrated and uncalibrated localities. Yodels of racerization kinetics in molluscan genera, and the resolution of the question of the similarity of molluscan and foraminifera kinetics, should aid in understanding the overall mechanism of diagenetic racemization in calcified matrices. Presently available kinetic models for molluscan racemization result in significantly different age and temperature conclusions, hence they cannot be applied without thorough consideration of their implications. In the application of any kinetic model, one must make two related assumptions that are inherent to arnir,o acid racemization dating in all forms : 1)
present temperature differences are a measure of past temperature differences, and localities with sinilar temperatures today, though with different apparent ages, would be assumed to have had similar Effective Quaternary Temperatures;
2)
Effective Quaternary Temperatures can be estimated using paleclimatic information in combination with racemization data for Holocene and late Pleistocene calibration samples.
Intheabsence of any independent calibration data for a region, amino acid enantiomeric ratios can be used for both relative and absolute age estimation if Effective Quaternary Temperatures can be independently estimated, and if one of the kinetic models discussed herein can be assumed to be valid for the genera being consldered. M e r e e n a r i a , a robust venerid mollusk that is comrcon in Atlantic coastal plain Quaternary localities, has been utilized for its reliability in amino acid racemization applications. Though fewer stratigraphic tests have been available from the Atlantic coast, most of the available data has suggested that Mercenaria conformed to stratigraphic tests as well as Pacific coast venerids. U-series (solitary coral) dating of Atlantic coastal plain localities, compared with amino acid enantiomeric ratios, has raised some major conflicts (independent of kinetic model issues) that challenge some of the basic temperature assumptions of simple stratigraphic applications of amino acid racemization. The resolution of these conflicts, and a better understanding of their implications, will be important steps in the further development of the racemization dating method.
ACKNOWLEDG EPENTS This research has been supported by grants from the U . S . Geological Survey, numbers 14-08-0001-G-248 and 4-08-0001-G-592. Many of the observations and publications discussed herein have resulted from collaboration and discussion with the following individuals, all of whom are gratefully acknowledged for their efforts and contributions: D.F. Belknap, T.M. Cronin, J . M . Demarest, W.K. Emerson, E . M . Keenan, G . L . Kennedy, K.R. Lajoie,L. McCarten, R. Morris, R.B. Mixon, and J.P.Owens.
190
Special appreciation g o e s to D . Y . ' B . f o r the use of some unpublished field and laboratory data in the construction and discussion of Figure
9.
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M i l l e r , G . H . a n d H a r e , P . E . , 1 9 8 0 a , Amino a c i d c h r o n o l o g y : integrity of t h e c a r b o n a t e m a t r i x and p o t e n t i a l of molluscan f o s s i l s : in, Hare P . E . , H o e r i n g , T . C . a n d K i n g , K . , J r . , e d s . ,f o Amino A c i d s : N . Y . , W i l e y , p . 415-443.
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, 1 9 7 5 , Ages a n d d i a g e n e t i c t e m p e r a t u r e s o f P l e i s t o c e n e d e p o s i t s o f F l o r i d a b a s e d upon i s o l e u c i n e e p i m e r i z a t i o n i n Mereenaria: E a r t h a n d P l a n e t a r y S c i e n c e L e t t e r s , v . 2 8 , p . 275282. P e t e r s o n , G.M., W e b b , T . 111, K u t z b a c h , J . E . , v a n d e r Hammen, T . , W i j m s t r a , T . A . a n d S t r e e t , F . A . , 1 9 7 9 , The c o n t i n e n t a l r e c o r d o f e n v i r o n m e n t a l c o n d i t i o n s a t 1 8 , 0 0 0 y r BP: an i n i t i a l evaluation: Q u a t e r n a r y R e s e a r c h , v . 1 2 , p . 47-82. S c h r o e d e r , R.A. and Bada, J . L . , 1976, A r e v i e w of t h e geochemical app l i c a t i o n s of t h e amino a c i d r a c e m i z a t i o n r e a c t i o n : Earth Science Reviews, v. 1 2 , p . 347-391. 1 9 8 0 , The r e s o l u t i o n o f e n a n t i o m e r i c Smith, G.G. and Wonnacott, D . M . , i n Hare, P . E . , H o e r i n g , T . C . amino a c i d s by g a s chromatography: a n d K i n g , K . , Jr., e d s . , B i o g e o c h e m i s t r y o f Amino A c i d s - : N.Y., Wiley, p . 203-214. Valentine, J.W., California:
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W e h m i l l e r , J . F . , 1 9 7 7 , Amino a c i d s t u d i e s o f t h e D e l M a r , C a l i f o r n i a , midden s i t e : a p p a r e n t r a t e c o n s t a n t s , ground t e m p e r a t u r e models, Earth and P l a n e t a r y S c i e n c e and c h r o n o l o g i c a l i m p l i c a t i o n s : L e t t e r s , v . 37, p . 184-196.
, 1980, I n t e r g e n e r i c d i f f e r e n c e s i n apparent racemiza t i o n k i n e t i c s i n mollusks and f o r a m i n i f e r a : implications for m o d e l s o f d i a g e n e t i c r a c e m i z a t i o n , i n Hare, P . E . , H o e r i n g , T . C . a n d K i n g , K . , J r . , e d s . , B i o g e o c h e m i s t r y o f Amino A c i d s : N.Y., Wiley, p . 341-355. , 1 9 8 1 a , K i n e t i c m o d e l o p t i o n s f o r i n t e r p r e t a t i o n of comments amino a c i d e n a n t i o m e r i c r a t i o s i n Q u a t e r n a r y m o l l u s k s : G e o c h i m i c a e t Cosmochimica o n a p a p e r b y K v e n v o l d e n e t al. ( 1 9 7 9 ) :
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, 1 9 8 1 b , Amino a c i d a g e e s t i m a t i o n o f Q u a t e r n a r y m o l l u s k s : downtown San D i e g o a n d C o r o n a d o I s l a n d : i n T r e n c h i n g t h e R o s e Canyon F a u l t Zone, by E . A r t i m and D . S t r e i f f , F i n a l T e c h . R e p o r t , U.S. G e o l o g i c a l S u r v e y C o n t r a c t No. 14-08-0001-19118, p . D-2 D-27. W e h m i l l e r , J . F . and Hare, P . E . , 1 9 7 1 , R a c e m i z a t i o n o f amino a c i d s i n marine sediments: S c i e n c e , v . 1 7 3 , p . 907-911. Wehmiller, J . F . and Belknap, D.F., 1978, A l t e r n a t i v e k i n e t i c models f o r t h e i n t e r p r e t a t i o n of amino a c i d e n a n t i o m e r i c r a t i o s i n P l e i s t o c e n e mollusks: examples from C a l i f o r n i a , Washington and F l o r i d a : -ternary R e s e a r c h , v . 9 , p . 330-348. W e h m i l l e r , J . F . and Emerson, W . K . , 1 9 8 0 , C a l i b r a t i o n of amino a c i d r e s u l t s from Magdalena racemization i n late Pleistocene mollusks: Bay, Baja C a l i f o r n i a S u r , Mexico, w i t h d a t i n g a p p l i c a t i o n s and paleoclimatic implications: The N a u t i l u s , v . 9 4 ( 1 ) , p . 31-36. Wehmiller, J . F . , Hare, P.E. and K u j a l a , G . A . , 1 9 7 6 , Amino a c i d s i n fossil corals: r a c e m i z a t i o n ( e p i m e r i z a t i o n ) r e a c t i o n s and t h e i r i m p l i c a t i o n s f o r d i a g e n e t i c r e a c t i o n s and c h r o n o l o g i c a l s t u d i e s : Geochimica e t Cosmochimica A c t a , v . 40, 763-776. Wehmiller, J . F . , L a j o i e , K . R . , Kvenvolden, K . A . , P e t e r s o n , E . , Belknap, D . F . , K e n n e d y , G . L . , A d d i c o t t , W . O . , V e d d e r , J . G . and W r i g h t , R.W., 1977, C o r r e l a t i o n and chronology of P a c i f i c c o a s t marine t e r r a c e s o f c o n t i n e n t a l U n i t e d S t a t e s by amino a c i d s t e r e o c h e m i s t r y t e c h n i q u e e v a l u a t i o n , r e l a t i v e a g e s , k i n e t i c model a g e s , and geologic implications: U.S. G e o l o g i c a l S u r v e y O p e n F i l e R e p o r t 7 7 6 8 0 , p . 1 1 9 6 . -Wehmiller, J . F . , L a j o i e , K . R . , Sarna-Wojcicki, A.M., Yerkes, R.F., K e n n e d y , G . L . , S t e p h e n s , T . A . a n d K o h l , R . F . , 1 9 7 8 a , Amino a c i d r a c e m i z a t i o n d a t i n g of Quaternary m o l l u s k s , P a c i f i c c o a s t United States: i n Zartman. R . E . . e d . . S h o r t PaDers of t h e F o u r t h I n t e r n a t i o n a l C o n f e r e n c e , Geochronology, Cosmochronology and I s o t o p e G e o l o g y 1 9 7 8 , U.S. G e o l o g i c a l S u r v e y O p e n F i l e R e p o r t 7 8 - 7 0 1 , p . 445-448. Wehmiller, J . F . , Lajoie, K.R., Sarna-Wojcicki, A.M. and Yerkes, R.F., 1978b, Unusually h i g h rates of c r u s t a l u p l i f t i n Ventura County, C a l i f o r n i a , i n f e r r e d from Quaternary marine t e r r a c e chronology: Geol. SOC. Amer. A b s t r a c t s with Programs, v. 1 0 ( 7 ) , p. 513. Wehmiller, J . F . , L a j o i e , K . R . and Kennedy, G . L . , 1979, R o l e of t h e r m a l h i s t o r y u n c e r t a i n t i e s i n amino-acid racemization age e s t i m a t i o n of g e o l o g i c a l and a r c h a e o l o g i c a l samples: G e o l . S O C . A m e r . A b s t r a c -. ts with Programs, v. 1 1 ( 7 ) , p. 536. W i l l i a m s , K.M. a n d S m i t h , G . G . , 1977, A c r i t i c a l e v a l u a t i o n of t h e app l i c a t i o n o f amino a c i d r a c e m i z a t i o n t o g e o c h r o n o l o g y and geothermometry: O r i g i n s of L i f e , v . 8 , p . 91-144. Woods, A . J . , 1980, Geomorphology, d e f o r m a t i o n and chronology o f m a r i n e t e r r a c e s a l o n g t h e P a c i f i c c o a s t o f c e n t r a l Baja C a l i f o r n i a , Mexico: Q u a t e r n a r y R e s e a r c h , v 1 3 , p . 346-364. T i n s l e y , J . C . and L a j o i e , K.R., 1980, Yerkes, R.F., Greene, H.G., S e i s m o t e c t o n i c s e t t i n g of t h e Santa Barbara c h a n n e l area, s o u t h e r n U . S . G e o l o g i c a l S u r v e y Open F i l e S e p o r t 8 0 - 2 9 9 , p . California: 1 - 3 9 ; s u p e r s e d e d by U . S . G e o l o g i c a l S u r v e y R e p o r t MF-1169, p . 1 - 2 5 .
195
UTILIZING WOOD IN AMINO ACID DATING
N.W. RUTTER and R.J. CRAWFORD
ABSTF.ACT
D/L r a t i o s of wood h a v e b e e n d e t e r m i n e d on s a m p l e s f r o m v a r i o u s Q u a t e r n a r y u n i t s i n t h e u n g l a c i a t e d O l d Crow a r e a , Yukon a n d A l a s k a , m o s t of w h i c h , h a v e b e e n s u b j e c t e d t o l o n g p e r i o d s of p e r m a f r o s t c o n d i t i o c s . Tke p u r p o s e was t o e v z l E a t e t h e E s e f Q l n e s s of D / L r a t i o s oi' a m i n o a c i d s i n wood a s a n a i d i n c o r r e l a t i n g e q u i v a l e n t s t r a t i g r a p h i c u n i t s , d e t e r m i n i n g r e l a t i v e a g e s , and i n a g e n e r a l way, t h e a b s o l u t e a g e s of u n i t s . The t o t a l a m o u n t s of a m i n o a c i d s were d e t e r m i n e d u s i n g a g a s c h r o m a t o g r a p h e q u i p p e d w i t h a FID D e t e c t o r a n d C h i r a s i l - v a l c a p i l l a r y column ( 2 5 m ) a n d c o n t r o l l e d by a d i g i t a l m i c r e p r o c e s s o r t e r n i n a l w h i c h r e p o r t s p e a k a r e a s by a u t o m a t i c i n t e r g r a t i o n . D/L r a t i o s of a s p a r t i c a c i d h a v e p r o v e d to b e u s e f u l a n d r e l i a b l e .
R e s u l t s show t h a t D / L r a t i o s of a s p a r t i c a c i d of wood a r e u s e f u l i n c o r r e l a t i n g e q u i v a l e n t u n i t s i n w i d e l y s p a c e d s e c t i o n s , some o v e r 300 km f r o m e a c h o t h e r . T h i s i s p r o v i d e d t h e s e d i m e n t s h a v e h a d similar c l i m a t i c and e n v i r o n m e n t a l h i s t o r i e s and t h a t u n i t s above and below t h o s e b e i n g c o r r e l a t e d a r e of w i d e l y v a r y i n g a g e . It a p p e a r s t h a t s p e c i e s d i f f e r e n t i a t i o n i s n o t n e c e s s a r y for gross c o r r e l a t i o n s . D / L r a t i o s o f a s p a r t i c a c i d for H o l o c e n e s e d i m e n t s v a r y m o s t l y from a p L a t e and Pid-Wisconsin 0 . 1 4 t o 0 . 2 4 and proximately 0 . 0 1 t o 0.08. 3 . e s u l t s from samples o l d e r e a r l y W i s c o n s i n a n d Sangamon 0 . 2 4 to 0 . 3 6 . t h a n Sangamon a r e d i f f i c u l t t o o b t a i n w i t h a c c u r a c y . A l t h o u g h t h e e x p l a n a t i o n i s n o t c l e a r l y u n d e r s t o o d , more a c c e p t a b l e r a t i o s a r ' e obt a i n e d by u s i n g a g r e a t e r amount of s a m p l e t h a n i s u s u a l i n r o u t i n e a n a l y s i s . D/L r a t i o s of a s p a r t i c a c i d of 0 . 5 0 h a v e s o f a r b e e n obtained.
INTF.02UCT I0N Many v a r i e t i e s of f o s s i l s a r e now r o u t i n e l y a n a l y z e d t o d e t e r m i n e t h e D / L r a t i o s of a m i n o a c i d s for r e l a t i v e a n d a b s o l u t e d a t i n g o f s e d i m e n t s . Most i n v e s t i g a t i o n s h a v e c e n t e r e d o n a v a r i e t y o f m a r i n e m o l l u s c s a n d m a m m a l b o n e s b r o u g h t a b o u t m a i n l y by t h e a v a i l a b i l i t y of m a t e r i a l a n d s u c c e s s i n a n a l y s e s . Wood, h o w e v e r , has n o t b e e n w i d e l y a n a l y z e d (L ee e t aZ., 1 9 7 6 ; E n g e l e t al., 1 9 7 7 ; R u t t e r e t aZ., 1980), as i t i s d i f f i c u l t t o p r e p a r e s a m p l e s f o r a c c u r a t e a n a l y t i c a l r e s u l t s . The p r o c e d u r e s o u t l i n e d b e l o w h a v e l a r g e l y e l i m i n a t e d s p u r i o u s r e s u l t s . The o b j e c t i v e of t h i s i n v e s t i g a t i o n i s to e v a l u a t e t h e r e l i a b i l i t y of D / L r a t i o s of a s p a r t i c a c i d i n wood a s a tool i n s t r a t i g r a p h i c c o r r e l a t i o n a n d r e l a t i v e a g e d a t i n g by c o m p a r i n g r e s u l t s f r o m t h e same s t r a t i g r a p h i c u n i t s of n e a r b y s e c t i o n s a n d w i d e l y s p a c e d s e c t i o n s . T h i s w i l l a i d i n e v a l u a t i n g o v e r how b r o a d a n a r e a r e l i a b l e c o r r e l a t i o n s c a n b e made i n a r e g i o n t h a t h a s b e e n s u b j e c t e d t o s i m i l a r p a l e o c l i m a t e s a n d e n v i r o n m e n t s . A t t h e same t i m e i t w i l l c o n f i r m , r e f i n e o r v o i d Another o b j e c t i v e i s to estimate p a r t s of t h e p r o p o s e d s t r a t i g r a p h y . t h e a b s o l u t e a g e o f s e d i m e n t s by c o m p a r i n g D/L r a t i o s o f a s p a r t i c a c i d i n wood f r o m s e d i m e n t s o f known or e s t i m a t e d a g e .
196 The p r e s e n t w o r k i s b a s e d on o v e r 7 5 a n a l y s e s o f m o s t l y Q u a t e r n a r y a g e wood f r o m t h e O l d Crow a n d a d j a c e n t b a s i n s i n t h e N o r t h e r n Yukon ( F i g u r e 1 ) . T h i s a r e a was s e l e c t e d b e c a u s e , 1) t ’ e r e i s a n a b u n d a n c e o f wood i n u n i t s o f w i d e l y v a r y i n g a g e a n d b r o a d J i s t r i b u t i o n , 2 ) a g e o l o g i c a l f r a m e w o r k has b e e n e s t a b l i s h e d for t h e r e g i o n , a l t h o u g h problems s t i l l remain and, 3) t h e area i s i n t h e Continuous Permafrost Zone a n d l i t t l e i s known a b o u t t h e e f f e c t s on t h e r a t e o f r a c e m i z a t i o n o f amino a c i d s o f f o s s i l s s u b j e c t e d t o l o n g p e r i o d s of p e r m a f r o s t conditions.
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?4ETH0D S Preparation Volatile enantiomers a r e prepared i n order t o record the various a m i n o a c i d s b y g a s c h r o m a t o g r a p h y . Each wood s a m p l e i s t h o r o u g h l y c l e a n e d w i t h d i s t i l l e d w a t e r , a n d t h e n a i r d r i e d on a p l a s t i c w e i g h i n g dish. Next t h e s a m p l e i s b r o k e n i n t o s m a l l f r a g m e n t s a n d c r u s h e d u s i n g a m o r t a r and p e s t l e . P e r i o d i c a l l y t h e m a t e r i a l i s s i e v e d through a 20 mesh s c r e e n a n d c o l l e c t e d i n a p l a s t i c w e i g h i n e b o a t . The s i e v e d wood p a r t i c l e s a r e washed t w i c e i n a p l a s t i c d i s p o s a b l e c e n t r i f u g e t u b e
u s i n g 2 N HCR a n d t w i c e w i t h d o u b l e d i s t i l l e d w a t e r . B e t w e e n w a s h i n g s , t h e s a m p l e i s s o n i f i e d , c e n t r i f u g e d a n d d e c a n t e d . The c l e a n e d p a r t i c l e s a r e t r a n s f e r r e d t o a Buchner f u n n e l c o n n e c t e d t o a water-vacuum t a p and 4 . 2 5 cm) a n d w a s h e d f i t t e d w i t h Whatman g l a s s f i b r e p a p e r (GF/A s e v e r a l t i m e s w i t h d o u b l e d i s t i l l e d w a t e r . The f i l t r a t e i s d i s c a r d e d The and t h e washed p a r t i c l e s a r e c o l l e c t e d i n small p l a s t i c v i a l s . v i a l s a r e c o v e r e d a n d p l a c e d i n a d e s i c c a t o r f o r vacuum d r y i n g . T h i s usually takes several hours. D e r i v a t i z a ti o n About 1 0 0 m g o f w a s h e d , d r i e d s a m p l e i s p l a c e d i n a g l a s s s c r e w Added t o t h i s i s a b o u t 6 t o 8 m l 5.5N t o p c u l t u r e t u b e ( 1 3 x 100 m m ) . H C R ( c o n s t a n t b o i l i n g ) . The m i x t u r e i s a l l o w e d t o r e f l u x a t 1 0 8 ° C f o r 24 h r s . i n a heating block. A f t e r h e a t i n g , t h e t u b e i s removed and a l l o w e d t o c o o l t o room t e m p e r a t u r e . It i s t h e n c e n t r i f u g e d a f t e r c o o l i n g t o remove p a r t i c u l a t e m a t t e r . The s u p e r n a t a n t l i q u i d i s c o l l e c t e d u s i n g a P a s t e u r p i p e t t e and t r a n s f e r r e d t o a c l e a n c u l t u r e t u b e . The s a r n p l e i s t h e n e v a p o r a t e d t o d r y n e s s i n a S p e e d Vac C o n c e n t r a t o r . The r e s i d u e i s d i s s o l v e d i n 1 t o 2 m l d o u b l e d i s t i l l e d w a t e r a n d a d d e d t o f r e s h l y r e g e n e r a t e d c a t i o n e x c h a n g e r e s i n (Dowex A G 50W-X8, 50-100 m e s h ) . Next 4 b e d v o l u m e s o f d o u b l e d i s t i l l e d w a t e r a r e a d d e d a n d t h e effluent discarded. Two b e d v o l u m e s o f 3 N NHQOH a r e a d d e d t o e l u t e t h e amino a c i d s . About 1 0 m l o f a m i n o a c i d e l u a t e a r e c o l l e c t e d i n a c l e a n l 3 x 1 0 0 mm s c r e w - t o p c u l t u r e t u b e when t h e s o l v e n t f r o n t i s a b o u t 1 . 5 t o 2 . 0 cm f r o m t h e b o t t o m o f t h e c o l u m n . A s u b s t a n t i a l amount o f h e a t i s e m i t t e d f r o m t h e amino a c i d barid. The e x c e s s NH,OH i s e v a p o r a t e d It u s u a l l y t a k e s more t h a n 8 h o u r s t o u s i n g a S p e e d Vac C o n c e n t r a t o r . evaporate t o dryness. E s t e r i f i c a t i o n i s c a r r i e d o u t by a d d i n g 0 . 1 m l i s o p r o p a n o l / 3 . 5 N
HCR t o t h e d r i e d e l u a t e . T h i s i s s o n i f i e d u n t i l h o m o g e n e o u s , t h e n A f t e r e v a p o r a t i o n t o dryh e a t e d a t 100°C f o r 1 5 m i n u t e s i n a o i l b a t h .
n e s s , a b o u t 2 h o u r s i n t h e c o n c e n t r a t o r , t h e s a m p l e i s a c y l a t e d by a d d i n g 0 . 1 m l PFPA ( p e n t a f l u o r o p r o p i o n i c a n h y d r i d e ) a n d 0 . 3 m l d i s t i l l e d CH2CG2 (methylene c h l o r i d e ) . The s a m p l e i s s o n i f i e d u n t i l d i s s o l v e d a n d t h e n h e a t e d i n a n o i l b a t h a t 1 0 0 ° C f o r 5 m i n u t e s . The e x c e s s FFPA a n d C H 2 C R 2 a r e c o l d e v a D o r a t e d on a Biichi r o t a r y e v a p o r a t o r u s i n g l i q u i d N2. N e x t t h e s a m p l e i s washed w i t h 0 . 5 t o 0 . 1 m l C H z C R 2 a n d a f t e r a l lowing t h e r e s i d u e t o d i s s o l v e completely, i t i s t h e n c o l d e v a p o r a t e d t o d r y n e s s u s i n g a r o t a r y e v a p o r a t o r . The s a m p l e i s t h e n d i l u t e d i n 0.5 m l C H Z C R Z a n d f i l t e r e d t h r o u g h a Gelman a l p h a - 2 0 0 , 0 . 2 0 um m e t r i c e l f i l t e r . The d e r i v a t i v e i s now r e a d y t o b e i n j e c t e d i n t o t h e g a s chroma t o g r a p h . The s a m p l e may b e d i l u t e d w i t h a d d i t i o n a l C H ~ C R Zd e p e n d i n g upon i t s c o n c e n t r a t i o n upon i n j e c t i o n . About 0 . 2 t o 1 . 0 p 1 i s i n j e c t e d . The g a s c h r o m a t o g r a p h u s e d i s a H e w l e t t - F a c k a r d Model 5840A e q u i p p e d w i t h FID D e t e c t o r a n d C h i r a s i l - v a l c a p i l l a r y c o l u m n ( 2 5 m) a n d cont r o l l e d by a d i g i t a l m i c r o - p r o c e s s o r t e r m i n a l w h i c h r e p o r t s p e a k a r e a s by a u t o m a t i c i n t e g r a t i o n . Results D/L r a t i o s o f a l a n i n e , v a l i n e , l e u c i n e , and a s p a r t i c a c i d are r o u t i n e l y determined. be t h e most u s e f u l b e c a u s e o f t h e r e l a t i v e l y a n d r e l i a b i l i t y ( K v e n v o l d e n , 1 9 8 0 ) . O n l y D/L are reported here.
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PHYSICAL ENVIRONMENT Introduction The r e s u l t s r e p o r t e d h e r e a r e f r o m s e c t i o n s i n t h e O l d Crow, B e l l , B l u e f i s h a n d B o n n e t Plume B a s i n s a n d f r o m s e c t i o n s on t h e P o r c u p i n e R i v e r i n A l a s k a ( F i g u r e 1 ) . The F r o c u p i n e R i v e r d r a i n s t h e B e l l , B l u e - . f i s h , a n d O l d Crow b a s i n s . It flows northward t h r o u g h t h e B e l l Basin, t h e n w e s t w a r d s e p a r a t i n g t h e O l d Crow B a s i n t o t h e n o r t h a n d t h e B l u e f i s h Basin t o the south before continuing southwestward t o Alaska. Having e s c a p e d g l a c i a t i o n , s e d i m e n t s have b e e n a b l e t o a c c u m u l a t e w i t h
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a mir,imum a n o u n t o f e r o s i o n a n d t h e r e f o r e , c o n t a i n s e v e r a l h u n d r e d m e t r e s cf Quaternary or o l d e r m a t e r i a l . The B o n n e t Plume S a s i n h a s b e e ? p a r t i a l l y g l a c i a t e d a n d i s d r a i p e d by t h e e a s t w a r d f l o w i n g P e e l River. g o w n c u t t i n g by m a j o r r i v e r s a n d t r i b u t a r i e s w i t h i n t h e b a s i n s h a v e e x p o s e d s e c t i c n s on t h e order cf 5 0 m y c o n s i s t i n g mostl17 o r l a c u s t r i n e a n d f l u v i a l sed.imer,t c o ~ t a i n i n ga b u n d a n t f o s s i l s a n d o r g a n i c m a t e r i a l . T h e s e f a c t o r s h a v e g e r e r a t e d d e t a i l e d i n v e s t i g a t i o n s by a nurnber of" Q u a t e r n a r y s c i e n t i s t s a n t i c i p a t i n g a n e a r l y c o m p l e t e r e c o r d of Q u a t e r n a r y e z v i r o n m e n t s and e v e n t s i n c l u d i n g e l u c i d a t i o n of t h e m i g r a t l o n o f Ran i n t o N o r t h A m e r i c a ( S a g h e s , 1972; I r v i n g , 1978: "!orlan, 197e; W,rlar! a n d I C a t t h e w s , 1 9 7 8 ) . :vIajor p r o b l e m s i n s t r a t i g r a p h i c i n v e s t i g a t i o n s have been i n c o r r e l a t i p g and d a t i g g e q x i v a l e n t u n i t s b e t w e e n s e c t i o n s . T h i s i s h a m p e r e d by t h e s m a l l number o f w i d e s p r e a d s t r a t i g r a p h i c m a r k e r s s u c h a s v o l c a n i c a s h , t h e s i r r i l a r i t y o f u n i t s of v a r i o u s a g e s , l a t e r a l f a c i e s changes o f e q u i T r a l e n t u n i t s , and t h a t m o s t u n i t s a r e t o o o l d t o b e d a t e d by 1 4 C r r e t h o d s .
OLD C;ilC5J BASIN - NEA.3SY SECTIONS Stratigraphy The d e t a i l s o f t h e g e o l o g y o f t h e O l d Crow a n d a d j a c e n t b a s i n s a r e s u b j e c t s o f a s e r i e s o f p a p e r s i n p r e p a r a t i o n . Only t h e e l e m e n t s of t h e s t r a t i g r a p h y t h a t a r e n e c e s s a r y t o i l l u s t r a t e t h e u t i l i z a t i o n o f D/L ratios o f a m i n o a c i d s i n wood w i l l b e d i s c u s s e d . I n t h e O l d Crow B a s i n s e v e r a l m a j o r s e c t i o n s a r e l o c a t e d a l o n g a 10 km s t r e t c h o f t h e O l d Crow R i v e r a b o u t 30 km n o r t h o f t h e v i l l a g e o f O l d Crow ( F i g u r e 1). T h r e e o f t h e s e h a v e b e e n i n v e s t i g a t e d a n d s a m p l e d i n d e t a i l ( l o c a t i o n s 11, 1 2 , a n d 15). Although problems p e r s i s t i n c o r r e l a t i n g equivalent u n i t s , a b a s i c s t r a t i g r a p h y has been e s t a b l i s h e d A s i m p l i f i e d composite s e c t i o n i s i l l u s t r a t e d i n Figure 2 . The l o w e r m o s t u n i t , 1, c o n s i s t s o f m a s s i v e l a c u s t r i n e c l a y w i t h s i l t . The u n i t e x t e n d s below water l e v e l i n a l l s e c t i o n s s o t h i c k n e s s e s a r e d i f f i c u l t t o e s t i m a t e b u t 1 to 3 m a r e commonly e x p o s e d a t low w a t e r l e v e l . The u p p e r s u r f a c e o f t h i s u n i t i s i r r e g u l a r , u n c o n f o r m a b l y o v e r l a i n by r e worked l a c u s t r i n e c l a y , U n i t 2 , and c o n t a i n i n g abundant m o l l u s c s and wood. T h i s u n i t r e a c h e s a maximum t h i c k n e s s o f o n l y a few m e t r e s a n d i s m i s s i n g i n some s e c t i o n s . The o v e r l y i n g u n i t , U n i t 3 , w i t h t h i c k n e s s e s o n t h e o r d e r o f 2 0 m c o n s i s t s m o s t l y of w e l l - b e d d e d , l a m i n a t e d s i l t a n d s a n d a l l u v i u m w i t h m i n o r g r a v e l , c o n t a i n i n g wood, p e a t , mol-l u s c s and bone. L a t e r a l and v e r t i c a l f a c i e s and s t r u c t u r a l changes have h i n d e r e d c o r r e l a t i o n of beds between s e c t i o n s . T h e r e f o r e , subd i v i d i n g t h i s u n i t h a s n o t been p o s s i b l e . I n several s e c t i o n s , t h e u p p e r c o n t a c t i s m a r k e d by a w e l l d e f i n e d u n c o n f o r m i t y , A , t h a t h a s b e e n t h e s o u r c e o f a b u n d a n t f o s s i l s i n c l u d i n g b o n e s some o f w h i c h may b e a r t i f a c t s . U n i t 4 , c o n s i s t i n g m o s t l y o f w e l l - b e d d e d s i l t a few m e t r e s t h i c k o v e r l i e s U n i t 3 . I t i s c h a r a c t e r i z e d by s e v e r a l h o r i z o n s m a r k e d by c r y o t u b a t e d z o n e s , i c e r w e d p e c a s t s , p e a t l a y e r s , m i n o r u n c o n f o r m i t i e s a n d l a y e r s c o n t a i n i n g wood. m o l l u s c s a n d b o n e . Unit 4 i s o v e r l a i n by U n i t 5 , a z o n e o f s i l t a n d f i n e s a n d . U n i t 6 c o n s i s t s of w e l l - b e d d e d g l a c i a l l a c u s t r i n e c l a y a n d s i l t , commonly v a r v e d a n d l e s s than 7 m thick. No wood or o t h e r f o s s i l s h a v e b e e n f o u n d . T h i s u n i t a l o n g w i t h o v e r l y i n g p e a t f o r m s t h e u w p e r s u r f a c e o v e r much o f t h e O l d Crow B a s i n . I n p l a c e s , f l u v i a l s i l t and sand have been d e p o s i t e d i n high l e v e l channels. A s e r i e s o f 14C d a t e s a n d f i s s i o n t r a c k d a t a f r o m t e p h r a h a v e The t e p h r a , l y i n g j u s t b e l o w Unconformity A i n Unit 3 h a s been estimated a t between 50 ka and 1 0 0 ka ( J . A . W e s t g a t e , U . o f T o r o n t o , p e r s . commun., 1 9 7 7 ) . 1 4 C d a t e s from o v e r l y i n g U n i t s 4 a n d 5 h a v e y i e l d e d d a t e s f r o m a b o u t 3 2 k a t o >53 k a ( G S C 2507, 2 5 7 4 , 2 6 7 6 , 2 7 3 9 ) and a 1 2 , 4 6 0 y e a r o l d d a t e ( G S C 3 5 7 4 ) h a s b e e n o b t a i n e d f r o m t h e u p p e r m o s t s i l t a n d s a n d u n i t a b o v e U n i t 6 . From t h e s e dates and o t h e r g e o l o g i c a l e v i d e n c e i t seems r e a s o n a b l e t o p l a c e a l a t e W i s c o n s i n a g e ( 1 0 k a t o 2 5 k a ) on U n i t 6 , w h i c h was d e p o s i t e d when C o n t i n e n t a l g l a c i e r s c o v e r e d a r e a s t o t h e e a s t . U n i t s 4 a n d 5 , w h e r e m o s t o f t h e d a t e d m a t e r i a l h a s come f r o m i s m a i n l y m i d - W i s c o n s i n ( 2 5 k a t o 65 k a ) w i t h t h e s t r a t i g r a p h i c a l l y l o w e r b e d s p e r h a p s o l d e r aided i n d a t i n g t h e younger s e d i m e n t s .
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than mid-Wisconsin. Below the tephra in Unit 3, no finite dates are available, so the entire unit may be all or in part mid-Wisconsin o r older. Unit 2 appears to have resulted from the thawing and redeposition of ice-rich sediments derived from Unit 1 below. This, and the presence of abundant wood suggests a relatively warm time perhaps the Sangamon Interglaciation (100 ka). At the present time, little can be said about the age of Unit 1, or its association with glacier activity to the east. In summary, the sediments of the composite section are waterlain,
200
r e p r e s e n t i n g a r e l a t i v e l y long p e r i o d of time, n o s t l i k e l y over 100 k a . I t seems l i k e l y t h a t t h e s e d i m e n t s h a v e b e e n s u b j e c t e d t o l o n g p e r i o d s of c l i m a t e s i m i l a r t o , or c o l d e r t h a n t o d a y a n d t h e r e f o r e , u n d e r p e r m a frost conditions. 2 e s u l t s of Amino A c i d A n a l y s e s F i g u r e 2 i n d i c a t e s t h a t mcst samples f o r amino a c i d a n a l y s e s a r e from U n i t s 3 a n d 4 w i t h a few from U n i t s 2 , 5 and 6 . Two o u t of t h r e e of t h e r e s u l t s i n U n i t 2 have lower r a t i o s ( 0 . 2 7 , 0 . 2 0 ) t h a n would be e x p e c t e d when c o m p a r e d t o r e s u l t s o b t a i n e d i r , t h e o v e r l y i n g u n i t . We have no s a t i s f a c t o r y e x p l a n a t i o n f o r t h i s anomaly. It may b e d u e t o d i a g e n e t i c c h a n g e s t h a t t a k e p l a c e i n wood w i t h t i m e r e s u l t i n g i r , a p a u c i t y o f a m i n o a c i d s t h a t make a c c u r a t e d e t e r m i n a t i o r L s o f D/L r a t i o s d i f f i c u l t . The r a t i o of 0 . 3 6 , i s p r o b a b l y a n a l y t i c a l l y c o r r e c t ar,d a p p e a r s r e a s o n a b l e for U p i t 2 , i f t h e u g p e r p a r t o f U n i t 2 i s s i m i l a r i n a g e t o t h e l o w e r p a r t o f U n i t 3 . S t r a t i s r a p h i e d a t a from o t h e r s e c t i o n s o f f e r some v a l i d i t y t o t h i s i n t e r p r e t a t i o n . S a m p l e s f r o m U n i t 3 were t a k e n a t s e v e r a l s t a t i o n s f r o m t h r e e s e c t i o n s a t widely spaced i n t e r v a l s w i t h i n each u n i t . In general, 3/L r a t i o s of a s p a r t i c a c i d i n c r e a s e w i t h i n c r e r s i n g age of t h e s e d i m e n t , v a r y i n g from 0 . 2 4 t o 0 . 3 6 w i t h an a v e r a g e of 0 . 2 9 . C c r r e l a t i o n of i n d i v i d u a l b e d s b e t w e e n s t a t i 0 P . s o r s e c t i o n s car.r,ot b e made w i t h a n y a c c u r a c y s o D / L r a t i o s o b t a i n e d frcm wood a t a c e r t a i n e l e v a t i o n a t o n e s t a t i o n may b e a t a d i f f e r e r i t s t r a t i g r a p h i c p o s i t i o n tha.;: D/L r a t i o s o f wood o b t a i n e d a t a b o u t t h e same e l e v a t i o n et a n o t h e r s t a t i o n . A s ment i o n e d a b o v e , a l l t h a t c a n b e s a i d a b o u t t h e a g e o f thls u n i t , i s t h a t t h e u p p e r p a r t is > 5 3 k a . I f U n i t 2 b e l o w c o n t a i n s Sangamon a g e s e d i mer.ts, t h e n U n i t 3 c o u l d i n c l u d e p a r t m i d - W i s c o r s i n a n d e a r l y W i s c o n s i n a g e s e d i m e n t s , i . e . b e t w e e n a p p r c x l m a t e l y 50 t o 1 0 0 - 1 2 0 k a . The s p r e a d of r a t i o s c o n t a i n e d i n U n i t 3 g i v e s an i r d i c a t i o n of what a s p a r t i c a c i d D/L r a t i o s t o e x p e c t for t h i s t i m e r a n g e i n t h i s r e g i o n . Furthermore, e v e n t h o u g h C n i t 3 i s c o r r e l a t e d b e t w e e n s e c t i o n s from l i t h o g r a p h i c e v i d e n c e , D / L r a t i o s o f a s p a r t i c a c i d when corr?pared t o D / L r a t i o s o f t h e o v e r l y i n g u n i t , i n d i c a t e t h a t c o r r e l a t i o n o f ' J n i t 3 between s e c t i o n s i s p o s s i b l e w i t h D / L r a t i o s a l o n e , a l t h o u g h p r o b l e m s may a r i s e when s e p a r a t i n g t h e o l d e r beds of Unit 3 w i t h t h o s e of a n d e r l y i n g Gnit 2 . Wood s a m p l e s were a n a l y s e d f r o y Y r c o n f o r m i t y A a t s e v e r a l s t a t i o n s a t L o c a t i o n 15 a n d t w o f r o m a b o v e t h e u n c o c f o r m i t y i n L o c a t i o n 11 a n d 1 2 i n U n i t s 4 a n d 5 . C/S r a t i o s o f a s p a r t i c a c i d v a r y b e t w e e n 9.15 arid 0 . 2 4 w i t h a n a v e r a g e o f 0 . 2 1 . Not er.ou&h r e s u l t s a r e a v a l l a b l e from U n i t s 4 and 5 t o r e a c h d e f i n i t e c o n c l u s i o r s , b u t t h e r e s u l t s s u g g e s t t h a t c o r r e l a t i o n s o f U n i t s 4 a n d 5 c a n b e made b e t w e e n s t a t i o n s a n d n e a r b y s e c t i o n s . The D/L r a t i o v a r i a t i o p s o b t a i n e d a l o n g U n c o n f o r m i t y 4 a n d from U n i t s 4 and 5 a r e p r o b a b l y r e p r e s e n t a t i v e o f wkat t o e x p e c t i n s e d i m e n t s o f a g e s b e t w e e n a o p r o x i m a t e l y 2 5 k a t o 65 k a (mid-Wisconsin) i n t h i s region. O n l y t w o D/L r a t i o s of a s p a r t i c a c i d a r e a v a i l a b l e f r o m U n i t 6 . R a t i o s of 0 . 1 5 a n d 0 . 1 8 a p p e a r t o b e t y p i c a l for d e o o s i t s b e t w e e n a b o u t 1 0 ka t o 25 ka ( l a t e Wisconsin).
From t h e a b o v e , ? t c a n b e c o n c l u d e d t h a t i n t h e O l d Crow B a s i n : 1) - ; h e r e a r e p r o b l e m s i n o b t a i n i n g a c c u r a t e 5/L r a t i o s o f a s D a r t i c a c i d i n ',he o l d e r u n i t s o f s e c t i o n s ; 2 ) 3/L r a t i o s of a s p a r t i c a c i d c a n b e :ised t o r o u g h l y i n d i c a t e m i d - ( a n d e a r l y ? ) and l a t e ' d i s c o n s i n s e d i m e n t s and 3 ) m a j o r u n i t s can be c o r r e l a t e d between s t a t i o n s w i t h i n a s e c t i o n , a n d b e t w e e n n e a r b y s e c t i o n s u s i n g D / L r a t i o s of a s p a r t i c a c i d i f t h e - 7 a r i a t i o n s o f r a t i o s w i t h i n a u n i t a r e known. ClLD CROW EASIN
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WIDELY SDACED SECTIOXS
Stratigraphy L o c a t i o n s 3 2 , 4 4 and X r e p r e s e n t s e c t i o n s t h a t l i e a t a d i s t a n c e 20 km, 3 0 km a n d 1 0 km f r o m L o c a t i o n 15 r e s p e c t i v e l y ( F i g u r e s 1 a n d 2 ) . The o b j e c t i v e o f s a m p l i n g t h e s e s e c t i o n s was t o t e s t t h e f e a s i b i l i t y
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o f c o r r e l a t i n g u n i t s by D/L r a t i o s o f a s D a r t i c a c i d o v e r g r e a t e r d i s t a n c e s t h a n n e a r b y s e c t i o n s b u t w i t h i n t h e same s e d i m e n t a r y b a s i n u n d e r similar paleoenvironments. Although: t h e s e s e c t i o n s have n o t been i n v e s t i g a t e d i n d e t a i l , r o u g h l i t h o l o g i c c o r r e l a t i o n h a s b e e n made w i t h u n i t s of t h e composite s e c t i o n g i v e n above ( F i g u r e 2 ) . T h e r e i s no problem i n c o r r e l a t i n g U n i t s 1 and 6 b u t t h e d e p o s i t s between U n i t s 1 and 6 c o n t a i n d i f f e r e n t c h a r a c t e r i s t i c s t h a n t h o s e o f t h e c o m p o s i t e s e c t i o n , b u t a r e t e n t a t i v e l y c o r r e l a t e d w i t h t h o s e b a s e d on s i m i l a r stratigraphic position. R e s u l t s o f Amino A c i d A n a l y s e s U n i t 2 a t L o c a t i o n 4 4 y i e l d e d D/L r a t i o s o f a s p a r t i c a c i d o f 0 . 3 6 and 0 . 2 1 ( F i g u r e 2 ) . The f o r m e r r a t i o i s i n k e e p i n g w i t h w h a t i s e x pected f o r Unit 2 whereas 0 . 2 1 i s probably erroneous f o r t h e reasons c i t e d i n t h e l a s t s e c t i o n . At l o c a t i o n X , a r a t i o o f 0 . 3 6 was o b t a i n e d f r o m w h a t was t h o u g h t to b e t h e l o w e r D a r t o f U n i t 3 . The r a t i o o f 0 . 3 6 , h o w e v e r , i n d i c a t e s t h a t p e r h a p s t h e s a m p l e came f r o m b e d s t h a t a r e a c t u a l l y e q u i v a l e n t to 5 n i t 2 .
Most s a m p l e s a n a l y s e d were f r o n U n i t 3 . 3 a t i o s frorn L o c a t i o n s 32 a n d 44 v a r y b e t w e e n 0 . 2 4 a n d 0.34, a v e r a g i n g 0.27, The d a t a a v a i l a b l e s u g g e s t t h e r e i s no a p p a r e n t o v e r a l l i n c r e a s e i n D/L r a t i o s w i t h deDth, a s t h e r e w a s a t L o c a t i o n 15. The r a t i o s a r e s i m i l a r , h o w e v e r , s u g g e s t i n g t h a t c o r r e l a t i o n of U n i t 3 between L o c a t i o n 1 5 and L o c a t i o n s 3 2 and 4 4 i s p o s s i b l e . D / L r a t i o s o f 0.15 w e r e o b t a i n e d from s a r n p l e s l o c a t e d i n t h e u p p e r p a r t o f U n i t 6 a n d j u s t o v e r l y i n g U n i t 6 a t L o c a t i o n s 32 a n d X r e s p e c t i v e l y . The r a t i o s a r e c o m p a t i b l e w i t h r a t i o s for l a t e W i s c o n s i n s e d i m e n t s f o u n d a t L o c a t i o n s 11 a n d 15.
I n conclusion, with the data available it appears t h a t correlation of u n i t s , u s i n g D/L r a t i o s o f a s p a r t i c a c i d i s p o s s i b l e between w i d e l y s p a c e d s e c t i o n s w i t h i n t h e O l d Crow E j a s i n .
Introduction U s i n g L o c a t i o n 1 5 i n t h e O l d Crow a s a b a s e , D / L r a t i o s o f s a m p l e s f r o m s e c t i o n s i n n e a r b y b a s i n s a n d r i v e r s were c o m p a r e d . The o b j e c t i v e was t o t e s t f o r l o n g d i s t a n c e c o r r e l a t i o n s o f u n i t s i n r e g i o n s w h e r e p a l e o c l i m a t e s may h a v e v a r i e d b u t p r o b a b l y n o t e n o u g h t o c a u s e m a j o r d i f f e r e n c e s i n D/L r a t i o s o f s a m p l e s o f a b o u t t h e same a g e . A s s e e n i n F i g u r e 1, two s e c t i o n s a r e l o c a t e d i n t h e B l u e f i s h B a s i n , 1 2 Mile B l u f f a n d B l u e f i s h ; or?e s e c t i o n i n t h e B o n n e t Plume B a s i n a t Hungry Creek; a n d two s e c t i o n s a l o n g t h e P o r c u p i n e R i v e r , a t T u s k 13luff a n d Canyon B l u f f . The f u r t h e s t s e c t i o n i s Hungry C r e e k a b o u t 3 2 0 km s o u t h e a s t o f L o c a t i o n 15. W i t h few e x c e p t i o n s , e q u i v a l e n t u n i t s c a n n o t b e c o r r e l a t e d by l i t h o l o g i c c h a r a c t e r i s t i c s i n t h e s e w i d e l y s p a c e d s e c t i o n s , a l t h o u g h I4C d a t a , t e p h r a al?d s t r a t i g r a p h i c p o s i t i o n s h a v e p r o v e d somewhat h e l p f u l . D/L r a t i o s were c o m p a r e d a n d e v a l u a t e d w h e r e s t r a t i g r a p h i c p o s i t i o n s o f u n i t s were known a n d i n s0T.e c a s e s , were u s e d to d e t e r m i n e t h e s t r a t i g r a p h i c p o s i t i o n o f u n i t s b a s e d o n t h e s u c c e s s w i t h i n t h e O l d Crow S a s i n . Stratigraphy
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1 2 Mile E l u _ ” f S e c t i o n ( B l u e f i s h B a s i n )
About 45 km s o u t h of L o c a t i o n l 5 i n t h e O l d Crow B a s i n , l i e s 1 2 Mile B l u f f S e c t i o n i n t h e B l u e f i s h B a s i n ( F i g u r e 1 ) . T h i s s e c t i o n w i t h t h i c k n e s s e s up to 2 0 0 m, i s e x p o s e d on t h e s o u t h b a n k o f t h e P o r c u p i n e R i v e r , e x t e n d i n g f o r more t h a n 3 km. The c h a r a c t e r a n d t h i c k n e s s e s o f u n i t s v a r y f r o m l o c a t i o n t o l o c a t i o n . A. s i n p l i f i e d c r o s s - s e c t i o n i s i l l u s t r a t e d i n F i g u r e 3 . E n i t s t h a t h a v e b e e n c o r r e l a t e d by l i t h o l o g y , 14C d a t e s , t e p h r a a n d s t r a t i g r a p h i c p o s i t i o n , w i t h u n i t s w i t h i n t h e O l d Crow S a s i n a r e d e s i g n a t e d by t h e same u r i t n u m b e r . The l o w e r u n i t s c o n s i s t o f s a n d , silt a n d g r i t b e d s w i t h a b u n d a n t
202
; r e e t r u n k s , roots, b r a n c h e s , t w i g s a n d c o n e s , i c e wedge c a s t s a n d c r y -.turbat,ed z o n e s . Although t h e age is u n c e r t a i n , t h e p l a n t remains sug; e s t t h a t t h e u n i t s a r e p r e - Q u a t e r n a r y . The o v e r l y i n g c l a y , s i l t u n i t i s e q u a t e d t o U n i t 1 i n t h e O l d Crow B a s i n . O v e r l y i n g u n i t s may b e e q u i v a l e n t t o S ' n i t 3 i n t h e O l d Crow B a s i n , b u t d i r e c t c o r r e l a t i o n i s questionable. Near t h e u p p e r D a r t o f t h e u n i t , h o w e v e r . t h e same ' e D h r a t h a t c r o n s o u t i n t h e O l d Crow E a s i n i s p r e s e n t s o t h a t a r o u g h , o r r e l a t i o n c a n b e made w i t h t h e u p p e r p a r t o f U n i t 3 . A l t h o u g h t h e :.rominent u n c o n f o r m i t y ( U n c o n f o r m i t y A ) a b o v e t h e t e p h r a i s n o t i d e n t i I'ied a t 1 2 T i l e 3 l u f f , t h e c h a r a c t e r i s t i c s o f t h e o v e r l y i n g d e p o s i t s h r e s i m i l a r t o t h o s e o f t h e Old Crow s e c t i o n s t h a t t h e s e may w e l l b e P q u i v a l e n t a t l e a s t i n D a r t t o U n i t s 4 a n d 5 . The o v e r l v i n f f d e D o s i t c o n s i s t s of w e l l bedded. n a r t l v v a r v e d , R l a c i a l l a c u s t r i n e c l a v and L i l t . m o s t l i k e l v e a u i v a l e n t t o U n i t 6 i n t h e O l d Crow B a s i n . R e s u l t s o f Amino A c i d A n a l v s e s
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1 2 Mile B l u f f S e c t i o n
F i g u r e 3 shows D/L r a t i o s o f a s n a r t i c a c i d d e t e r m i n e d f r o m u n i t s c t t w o l o c a t i o n s i n t h e B i g B l u f f s e c t i o n . O n l y o n e r e l i a b l e D/L r a t i o f r o m t h e l o w e r u n i t s was o b t a i n e d . T h i s r e a c h e d 0 . 5 0 m a k i n g i t the f i g h e s t r a t i o o b t a i n e d i n t h e e n t i r e r e g i o n , s u g g e s t i n g t h a t t h e lower i l n i t s a r e o l d e r t h a n a n y u n i t s f o u n d i n t h e O l d Crow B a s i n . T h i s a g e C i f f e r e n c e was s u s p e c t e d from o t h e r s t r a t i g r a p h i c c r i t e r i a a n d was conf i r m e d b y D/L r a t i o s . A s u i t e o f r a t i o s was o b t a i n e d from s e d i m e n t s a b o v e a n d b e l o w t h e t e p h r a i n i r n i t 3 ( ? ) . The r e s u l t s s h o u l d b e r o u g h l y t h e same a s t h o s e f o u n d i n a b o u t t h e same s t r a t i p a c h i c p o s i t i o n a t L o c a t i o n 1 5 . T e p h r a i s f o u n d j u s t b e l o w G n c o n f o r m i t y A a t L o c a t i o n 15 w h e r e a s i n t h e 1 2 T i l e B l u f f s e c t i o n , t h e u n c o n f o r m i t y h a s n o t b e e n i d e n t i f i e d . A t Locat i o n 15 t h e r a t i o s a b o v e t h e u n c o n f o r m i t y a r e l o w e r t h a n t h o s e f o u n d t e l o w . About t h e same r a t i o s ( b e t w e e n 0 . 2 7 a n d 0 . 2 9 ) a r e f o u n d i n t h e s e d i m e n t s i n r o u g h l y e q u i v a l e n t b e d s a t S t a t i o n 2 i n t h e 1 2 Mile B l u f f s e c t i o n , whereas i n S t a t i o n 1 t h e r a t i o s a r e l o w e r ( b e t w e e n 0 . 1 7 a n d C . 2 0 ) j u s t a b o v e , a t , a n d b e l o w t h e a s h a n d a b o u t t h e same ( 0 . 2 8 ) 3 m below t h e ash. T h e r e f o r e , t h e only r e a l d i s c r e p a n c i e s i n t h e r a t i o s a r e f o r t h o s e a t , or j u s t b e l o w t h e a s h a t 1 2 M i l e B l u f f ; t h e y s h o u l d b e h i g h e r to b e c o m p a t i b l e w i t h t h o s e a t L o c a t i o n 1 5 . Analytically the The d i s c r e p a n c y c a n o n l y b e e x r a t i o s a r e b e l i e v e d to b e c o r r e c t . p l a i n e d by d i f f e r e n t r a c e m i z a t i o n r a t e s u n d e r d i f f e r e n t c o n d i t i o n s .
Stratigraphy
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Bluefish Section (Bluefish Basin)
The o t h e r s e c t i o n s a m p l e d i n t h e B l u e f i s h B a s i n i s named t h e E l u e f i s h S e c t i o n l o c a t e d a b o u t 6 0 km s o u t h w e s t o f L o c a t i o n 15 i n t h e O l d Crow B a s i n on t h e w e s t b a n k o f t h e B l u e f i s h R i v e r ( F i g u r e 1 ) . The Above t h e l o w e r m o s t u n i t , s 2 c t i o n i s a b o u t 25 m t h i c k ( F i p u r e 4 ) . T e r t i a r y sediment w i t h c o a l , l i e s a u n i t of a l l u v i a l s i l t and s a n d , ~ i t h g r a v e l n e a r t h e b a s e , c o n t a i n i n g wood p i e c e s t h r o u g h o u t . Overl r i n g t h i s u n i t unconformably, i s a l l u v i a l g r a v e l and sand t h a t underl i e s l a c u s t r i n e s i l t , t h e uppermost u n i t . The l a c u s t r i n e s i l t i s c o r r e l a t e d w i t h U n i t 6 o f t h e O l d Crow c o m p o s i t e s e c t i o n b a s e d on s i m i l a r l i t h o l o g y and s t r a t i g r a p h i c p o s i t i o n . The u n i t s b e l o w , h o w e v e r , c a n n o t b e c o r r e l a t e d with a n y a s s u r a n c e . The age o f t h e s i l t , s a n d u n i t i s o l d e r t h a n 5 3 k a b a s e d u p o n a 1 4 C d a t e (GSC 2 3 7 3 - 3 ) o n wood f r o m t h e o*/erlying g r a v e l , sand u n i t . SamDles f o r a m i n o a c i d a n a l y s e s were t i k e n s y s t e m a t i c a l l y throughout the s i l t , sand u n i t .
( R e s u l t s o f Amino A c i d A n a l y s e s
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Bluefish Section (Bluefish Basin)
The a m i n o a c i d a n a l y s e s f r o m t h e silt, s a n d u n i t were r e p o r t e d e , i r l i e r b u t a r e d i s c u s s e d h e r e i n r e l a t i o n to l o n g d i s t a n c e c o r r e l a t i o n s ( ' ? u t t e r e t al., 1 9 8 0 ) . All s a m p l e s a n a l y s e d were Picea.
A s can be s e e n i n F i g u r e 4 , r e s u l t s v a r y between 0 . 1 7 and 0 . 3 1 , a'reraging 0 . 2 4 . Although t h e s e d i m e n t s cannot be c o r r e l a t e d l i t h o l o g i c Y - l y w i t h s e d i m e n t s a t 1 2 Mile B l u f f or t h o s e i n t h e O l d Crow B a s i n , t h e D/L r a t i o s , w i t h f e w e x c e p t i o n s a r e s i m i l a r t o t h o s e o b t a i n e d f o r s l m p l e s f r o m t h e u p p e r p a r t o f U n i t 3 ( ? ) a t 1 2 Mile B l u f f a n d t h e u p p e r
203
--
Bluefish Basin - 12 Mile Bluff Section 601
Stn. 1
7
Stn 2
Unit 6 Glacial Lacustrint
> 3 7 000 yrs > 53 000 yrs B P
32 400,
451
Tephra (50-100 000 yrs B P 1
?-7-7-7-7-7-7-
Sand Silt
.28 28.29 ~.
.17.20 . 2 0 .18 .28
.27
Sand, Fine Gravel Unit 3? Silt. Fine Sand
Sand Fine Gravel Silt
-
30r
Unit 1 Lacustrine Clay. Silt
0 0 0 Concretions
15r
/ Figure 3
Cryoturbated Units
1 2 Mile B l u f f composite s e c t i o n showing sample l o c a t i o n s a n d D/L r a t i o s o f a s p a r t i c a c i d .
p a r t o f U n i t 3 a t L o c a t i o n 15 t h a t a r e b e t w e e n a b o u t 50 and 1 0 0 k a .
Stratigraphy
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Hungry Creek S e c t i o n ( B o n n e t Plume B a s i n )
The Hungry Creek s e c t i o n i n t h e Bonnet Plume B a s i n i s l o c a t e d a b o u t 3 2 0 km s o u t h e a s t of L o c a t i o n l 5 i n t h e Old Crow B a s i n , a much g r e a t e r distance i n r e l a t i o n t o other sections discussed (Figure 1). No a t t e m p t h a s b e e n made t o l i t h o l o g i c a l l y c o r r e l a t e u n i t s f o r t h e Old Crow B a s i n b u t 1 4 C d a t e s have a i d e d i n i d e n t i f y i n g e q u i v a l e n t d e p o s i t s and v e r i f y i n g t h e amino a c i d r e s u l t s o b t a i n e d . The l o w e s t u n i t c o n s i s t s o f a g r a v e l u n i t more t h a n 4 m t h i c k ( F i g u r e 5 ) . T h i s i s o v e r l a i n b y a b o u t 11 m o f s i l t , c l a y a n d s a n d w i t h a b u n d a n t o r g a n i c l a y e r s i n c l u d i n g wood which u n d e r l i e s a b o u t 3 m of t i l l . The t i l l i s d e r i v e d from g l a c i e r s t h a t f l o w e d from t h e e a s t termi n a t i n g n o t f a r w e s t o f t h i s s e c t i o n . Near t h e t o p o f t h e s i l t , c l a y and s a n d u n i t , a s m a l l r o u n d e d f r a g m e n t .of wood h a s b e e n d a t e d a t 36,900 3 0 0 BP (GSC 2 4 2 2 ) . T h i s d a t e i s e s p e c i a l l y i m p o r t a n t b e c a u s e
*
204
Bluefish Basin - Bluefish Section
5; 6m
6m
Gravel, Sand
.29 .25 Silt, Sand
llm
.19 .28 .25
.31 .17
Gravel 2m
Dolomite
Figure 4
B l u e f i s h S e c t i o n s h o w i n g s a m p l e l o c a t i o n s a n d D/L r a t i o s of a s p a r t i c a c i d .
i.t g i v e s a n i n d i c a t i o n o f t h e maximum a g e o f t h e l a s t g l a c i a t i o n t o The v a l i d i t y o f t h i s d a t e was q u e s t i o n e d on s t r a t i e f f e c t t h i s area. g r a p h i c g r o u n d s a n d t h e p o s s i b i l i t y t h a t t h e wood h a d b e e n r e d e p o s i t e d ( H u g h e s e t a Z . , 1 9 8 1 ) . Above t h e t i l l i s a b o u t 2 . 5 rn o f s a n d y s i l t o v e r l a i n by p e a t .
R e s u l t s o f Amino A c i d A n a l y s e s - Hungry C r e e k S e c t i o n ( B o n n e t Plume B a s i n ) To a i d i n e v a l u a t i n g t h e v a l i d i t v o f t h e 1 4 C d a t e , s e v e r a l wood s a m p l e s were t a k e n f r o m b e l o w t h e d a t e d s a m p l e b u t w i t h i n t h e same u n i t f o r a m i n o a c i d a n a l y s i s . The D/L r a t i o s o f a s n a r t i c a c i d d e r i v e d f r o m t h i s u n i t v a r y b e t w e e n 0 . 1 2 a n d 0 . 2 1 w i t h a n a v e r a g e o f 0 . 1 6 ( F i g u r e 5). Yhese r a t i o s a r e c o m p a t i b l e w i t h t h o s e i n t h e O l d Crow B a s i n f o r d e p o s i t s w i t h i n t h e r a n g e o f t r a d i t i o n a l I4C d a t i n g o l d e r t h a n Holocene. It i s s u g g e s t e d t h e n t h a t t h e 14C d a t e i s c o r r e c t . Therefore, t h e unit i s r o u g h l y e q u i v a l e n t t o t h e uDper p a r t o f U n i t 4, a n d 5 i n t h e O l d Crow B a s i n , a n d t h e t i l l i s l a t e W i s c o n s i n a n d e q u i v a l e n t to U n i t 6 i n t h e O l d Crow B a s i n . POFiCUPINE R I V E R
-
ALASKA
Introduction I n e a s t e r n A l a s k a , on t h e b a n k s o f t h e P o r c u p i n e R i v e r a number of t h i c k Q u a t e r n a r y a n d L a t e T e r t i a r y s e c t i o n s c r o p o u t ( F i g u r e 1 ) . Two o f t h e s e s e c t i o n s h a v e b e e n d e s c r i b e d by R . T h o r s e n ( p e p s . comm., 1980) a n d l a t e r s a m p l e d f o r amino a c i d a n a l y s i s . The p u r p o s e was to a i d Thorsen i n h i s s t r a t i g r a p h i c s t u d i e s and a g a i n t o t e s t l o n g d i s t a n c e c o r r e l a t i o n w h i l e t r y i n g t o c o r r e l a t e e q u i v a l e n t u n i t s w i t h t h o s e of t h e O l d Crow R e g i o n .
20 5
Bonnet Plume Basin - Hungry Creek Section
26 n
15 17 1 9
!O 12 1 7 1 21 16 13
18
14
Hungry Creek
Figure 5
Hungry Creek s e c t i o n showing sample l o c a t i o n s and D/L r a t i o s of a s p a r t i c a c i d .
S t r a t i g r a p h y - Canyon B l u f f S e c t i o r , ( P o r c u p i n e F . i v e r ? The Canyon B l u f f S e c t i o n i s l o c l t e d a b o u t 1 3 0 k m s o u t h w e s t o f L o c a t i o n 15 i n t h e O l d Crow B a s i n on t h e s o u t h b a n k o f t h e P o r c u p i n e R i v e r ( F i g u r e 1 ) . The s e c t i o n c o n s i s t s o f a b o u t 25 m o f T e r t i a r y ( ? ) s a n d s u n d e r l y i n g a b o u t 1 2 m o f Q u a t e r n a r y d e p o s i t s . The l o w e r Q u a t e r n - . a r y d e p o s i t c o n s i s t s o f a b o u t 7 m o f g r a v e l , w i t h some s a n d a n d s i l t , commonly c r y o t u r b a t e d . The o v e r l y i n g u n i t c o n s i s t s o f a b o u t 1 m o f s i l t . Above t h e s i l t u n i t i s a b o u t 2 m o f c r y o t u r b a t e d g r a v e l w i t h i n c r e a s i n g s i l t c o n t e n t t o w a r d t h e top. T h i s i s o v e r l a i n by a b o u t 1 t o 2 m o f f l u v i a l g r a v e l w h i c h i n t u r n i s o v e r l a i n by a t h i n b l a n k e t o f s i l t containing a cryoturbated s o i l horizon.
R e s u l t s o f Amino A c i d A n a l y s e s - Canyon B l u f f S e c t i o n (Porcupine River )
Wood s a m p l e s for a m i n o a c i d a n a l y s i s were t a k e n f r o m t h r e e w i d e l y s p a c e d h o r i z o n s w i t h i n t h e g r a v e l u n i t ( F i g u r e 6). R e l a t i v e l y h i g h D/L r a t i o s o f a s p a r t i c a c i d o f 0 . 3 7 , 0 . 3 8 a n d 0 . 4 2 were o b t a i n e d . T h i s u n i t a p p e a r s t o b e r e l a t i v e l y o l d a n d may b e e q u i v a l e n t t o t h e l o w e r p a r t o f U n i t 3 a n d U n i t 2 i n t h e O l d Crow B a s i n . A r a t i o o f 0.30 was o b t a i n e d f r o m t h e o v e r l y i n g s i l t u n i t i n d i c a t i n g a r e l a t i v e l y o l d a g e b u t younger t h a n t h e u n d e r l y i n g u n i t .
206
Porcupine River - Canyon Bluff Section
35 m
Cryoturbated
25m Ice Wedge Casts
15m
5m
Porcupine River
Figure 6
Canyon B l u f f
s e c t i o n showing sample l o c a t i o n s and
D/L r a t i o s o f a s p a r t i c a c i d .
Stratigraphy
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Tusk B l u f f S e c t i o n ( P o r c u p i n e R i v e r )
The s e c o n d s e c t i o n s a m p l e d on t h e P o r c u p i r , e F . i v e r i n Alaska is named T u s k B l u f f . I t i s l o c a t e d a b o u t 1 3 0 km f r o m L o c a t i o n . 1 5 i n t h e O l d Crow B a s i n a n d a b o u t 5 km d o w n s t r e a m . f r o m t h e Canyon B l u f f S e c t i o n ( F i g u r e 1 ) . The Q u a t e r n a r y g e o l o g y i s c o n s i d e r a b l y d i f f e r e n t f r o m Canyon B l u f f a n d i s c o n s i d e r e d by T h o r s e n (rsers. commun., 1980) t o b e r e l a t i v e l y younger. A s w i t h Canyon B l u f f t h e Q u a t e r n a r y d e p o s i t s a r e u n d e r l a i n by T e r t i a r y ( ? ) s e d i m e f i t s ( F i g u r e 7 ) . About 4 m o f T e r t i a r y ( ? ) s e d i m e n t s a r e e x p o s e d above water l e v e l o v e r l a i n by a b o u t 2 1 m of i n t e r b e d d e d s a n d , s i l t , p e b b l y s a n d a n d g r a v e l . Near t h e t o p o f t h e u n i t a I 4 C b o n e c o l l a g e n d a t e o f a b o u t 2 9 k a was o b t a i n e d . G v e r l y i - g t h i s u n i t i s about 7 m of c r y o t u r b a t e d s i l t .
R e s u l t s o f Amino 4 c i d A n a l y s e s - Tusk S l u f f S e c t i o n (Porcupine F i v e r ) D/L r a t i o s of a s c a r t i c a c i d w e r e d e t e r r r i n e d f r o m s a m l e s c o l l e c t e d from t h e T e r t i a r y ( ? ) s e d i m e n t s . F a t i a s o f 0 . 3 0 a n d 0 . 3 8 d e m o n s t r a t e -,hat t h i s d e p o s i t i s o l d , b u t n a t T e r t i a r y . These sediments are n r o b a b l y e q u i v a l e n t t o t h e g r a v e l u n i t a t Canyon B l u f f a n d may b e e q u i v a l e n t t o t h e l o w e r D a r t o f U n i t 3 a n d ,nit 2 i n t h e Cld Crow B a s i n . lO/L r a t i o s o f a s p a r t i c a c i d o f 0 . 1 9 a n d 0 . 1 8 h e r e d e t e r m i n e d f r o m wood i b o u t 1 m b e l o w t h e 29 k a d a t e i n t h e c e b b l y s a p d s u b u n i t . T h e s e r a t i o s 'it i n w e l l w i t h r a t i o s d e t e r m i n e d f o r f i 3 i t e d a t e d s e d i m e n t s , s l i g h t l y i l d e r t h a n l a t e Wisconsin a t t h e Eungry Creek s e c t i o n a n d , even though ' , h e r e i s n o t s i m i l a r d a t a t o d r a h o n , for s i m i l a r a g e d s e d i m e n t s i n t h e O l d Crow B a s i n .
Stratigraphy
-
Upper P o r c u p i n e S e c t i o n ( B e l l S a s i n )
The U p p e r P o r c u p i n e S e c t i o n i s l o c a t e d i m t h e B e l l B a s i n a b o u t 150
::m s o u t h e a s t o f L o c a t i o n 15 i n t h e Old Crow B a s i n ( F i g J r e 1).
Samples
207
Porcupine River - Tusk Bluff Section
Figure 7
Tusk B l u f f s e c t i o n showing sample l o c a t i o n s D/L r a t i o s of a s p a r t i c a c i d .
and
were c o l l e c t e d from a Holocene s e c t i o n t h a t forms a t e r r a c e l o c a t e d b e s i d e t h e Upper> P o r c u p i n e R i v e r . R e s u l t s o f t h i s i n v e s t i g a t i o n were r e p o r t e d e a r l i e r ( R u t t e r e t a Z . , l 9 8 0 ) , b u t a r e r e p e a t e d h e r e t o comp a r e Z o l o c e n e D/L r a t i o s of a s p a r t i c a c i d w i t h t h o s e o b t a i n e d from wood i n older deposits. The s e c t i o n c o n s i s t s of 9 m o f f l u v i a l s a n d and s i l t w i t h a b u n d a n t wood f r a g m e n t s t h r o u g h o u t , o v e r l a i n b y a b o u t 3 m of p e a t P i g u r e 8). The only 14C d a t e a v a i l a b l e , 9 1 9 0 y e a r s BP (GSC 2 4 6 1 ) , i s d e r i v e d from wood t a k e n from some o f t h e o l d e s t s e d i m e n t s w i t h i n t h e unit. It i s a maximum age for t h e i n i t i a t i o n o f d e p o s i t i o n o f t h e m a t e r i a l t h a t forms t h e t e r r a c e . The p a u c i t y of t r e e s i n t h e r e g i o n t o d a y compared w i t h t h e abundance of wood f o u n d t h r o u g h c u t t h e s e c t i o n ( P o p u Z u s and S a l i x ) , s u g g e s t s t h e s e s e d i m e n t s were d e p o s i t e d d u r i n g a p e r i o d warmer t h a n t o d a y . R e s u l t s o f Amino Acid A n a l y s e s - Upper P o r c u p i n e S e c t i o n (Bell Basin) Samples were t a k e n a t v a r i o u s i n t e r v a l s t h r o u g h o u t t h e e n t i r e s e c t i o n a l l s t r a t i g r a p h i c a l l y above t h e o l d e s t d a t e d b e d s . R a t i o s v a r y between 0 . 0 1 and 0 . 0 8 w i t h a n a v e r a g e o f 0 . 0 6 , b u t a r e p r o b a b l y a l l a b o u t t h e same a g e , b e c a u s e t h e t e r r a c e s e d i m e n t s r e p r e s e n t l a r g e s c a l e p o i n t b a r d e p o s i t s , and t h e r e f o r e , d e p o s i t e d r e l a t i v e l y f a s t , s a y i n a few h u n d r e d y e a r s . The r a t i o s seem r e a s o n a b l e f o r Holocene wood, when compared w i t h wood from o l d e r s e d i m e n t s . DISCUSSION AND CONCLUSIONS
All i n a l l , d e t e r m i n i n g D/L r a t i o s o f t h e a s p a r t i c a c i d i n wood h a s been u s e f u l as a n a i d i n c o r r e l a t i n g e q u i v a l e n t d e p o s i t i o n a l u n i t s , d e t e r m i n i n g r e l a t i v e a g e s , and i n a g e n e r a l w a y , t h e a b s o l u t e a g e o f u n i t s . Long r a n g e c o r r e l a t i o n h a s b e e n s u c c e s s f u l p r o v i d i n g t h e s e d i ments have h a d s i m i l a r c l i m a t i c and e n v i r o n m e n t a l h i s t o r i e s and t h a t u n i t s above a n d below t h o s e b e i n g c o r r e l a t e d a r e of w i d e l y v a r y i n g a g e , D/L r a t i o s o f a s p a r t i c a c i d from Holocene s e d i m e n t s v a r y m o s t l y from a p p r o x i m a t e l y 0 . 0 1 t o 0 . 0 8 , l a t e and mid-Wisconsin 0 . 1 4 t o 0 . 2 4 , and e a r l y W i s c o n s i n and Sangamon 0 . 2 4 t o 0 . 3 6 . R e s u l t s from s a m p l e s o l d e r t h a n Sangamon a r e d i f f i c u l t t o o b t a i n w i t h a c c u r a c y . Although t h e exp l a n a t i o n i s n o t c l e a r l y u n d e r s t o o d , more a c c e p t a b l e r a t i o s a r e o b t a i n e d
208
Bell Basin
- Upper Porcupine Section
3m
.08 .01
.07 Sand, Silt, Wood 9m
.08 .06
.08
9190 yrs B.P. Figure 8
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Upper P o r c u p i n e s e c t i o n showing sample l o c a t i o n s and D/L r a t i o s of a s p a r t i c a c i d .
( t h a t i s , on t h e o r d e r of 0.50) by u s i n g a g r e a t e r amount o f o r i g i n a l sample t h a n u s u a l i n r o u t i n e a n a l y s i s .
D i f f e r e n c e s i n D / L r a t i o s i n wood of t h e same a g e c a n be c a u s e d b y d i a g e n e t i c c h a n g e s b e f o r e and a f t e r b u r i a l , c o n t a m i n a t i o n , v a r i a t i o n s i n a n a l y t i c a l procedures, v a r i a t i o n i n paleoenvironments (including t e m p e r a t u r e and c l i m a t e ) , and v a r i a t i o n i n t h e r a t e a t which v a r i o u s I t i s d i f f i c u l t to comment on what d i a g e n e t i c changes species racemize. may h a v e t a k e n p l a c e b e f o r e o r a f t e r b u r i a l , or what s a m p l e s may have b e e n c o n t a m i n a t e d . However, comments can b e made on o t h e r a s p e c t s . We b e l i e v e t h e r e s u l t s of our a n a l y t i c a l p r o c e d u r e s a r e a c c u r a t e t o w i t h i n 5 t o 7 % . T h i s i s b a s e d on o v e r 1 0 0 0 s a m p l e s a n a l y z e d i n a variety of materials. Accuracy and p r e c i s i o n a r e checked by f r e q u e n t l y d e t e r m i n i n g amino a c i d s o f s t a n d a r d s , a n a l y z i n g modern wood and by I n a d d i t i o n , we h a v e made a n a l y z i n g t h e same s a m p l e more t h a n o n c e . i n t e r l a b o r a t o r y c o m p a r i s o n s by a n a l y z i n g t h e same s a m p l e . Paleoenvironmental f a c t o r s , p a r t i c u l a r l y temnerature v a r i a t i o n s h a v e p r o b a b l y had t h e g r e a t e s t e f f e c t on D / L r a t i o s o f s p e c i m e n s o f t h e same a g e . The o l d e r t h e s a m p l e s , t h e more l i k e l y t h e y a r e t o show v a r i a t i o n . Long t e r m t e m p e r a t u r e f l u c t u a t i o n s i n t h e Q u a t e r n a r y a r e w e l l known. I n a d d i t i o n , l o c a l f a c t o r s r e l a t e d t o t h e p r e s e n c e o r abs e n c e o f p e r m a f r o s t may c a u s e v a r i a t i o n . F o r i n s t a n c e , how l o n g h a v e t h e s p e c i m e n s b e e n u n d e r p e r m a f r o s t c o n d i t i o n s , and a t what tempera t u r e s ? What s p e c i m e n s have b e e n s u b j e c t e d to a c t i v e l a y e r m e l t i n g and w h a t s p e c i m e n s have n o t ? A l t h o u g h d i f f i c u l t to a n s w e r , i t i s s a f e t o s a y t h a t most s p e c i m e n s have b e e n s u b j e c t e d t o l o n g p e r i o d s of permaf r o s t c o n d i t i o n s . T h e r e f o r e , we a s s u m e r a c e m i z a t i o n h a s b e e n r e t a r d e d .
209
Although r a c e m i z a t i o n r a t e s are s p e c i e s s p e c i f i c f o r s h e l l , bone, and some o t h e r s u b s t a n c e s , t h i s d o e s n o t a p p e a r t o b e t h e c a s e i n our s t u d i e s o f wood. The r a c e m i z a t i o n r a t e d e p e n d s upon t h e p o s i t i o n o f t h e a s p a r t i c a c i d r e s i d u e i n t h e p o l y p e p t i d e c h a i n ( S m i t h and S i l v a de Sol, 1 9 8 0 ) . It i s a l s o d e p e n d e n t on t h e n a t u r e o f t h e a d j a c e n t p e p t i d e u n i t s ( R . M o i r , p e r s . commun., 1982), t h u s i t i s p o s s i b l e t h a t t h e p r o t e i n m a t e r i a l i n wood i s s i m i l a r from one s p e c i e s to a n o t h e r w h e r e a s t h e amino a c i d s e q u e n c e v a r i e s from one s p e c i e s to a n o t h e r s p e c i e s i n s a g , s h e l l s . I t i s a l s o known t h a t t h e m a t r i x i n which p r o t e i n i s i n t e r c a l a t e d c a n c a u s e a v a r i a t i o n i n t h e amount of r a c e m i z a t i o n . I n t h e wood s a m p l e s , t h e a l i q u o t s a r e f r o m s t r u c t u r a l m a t e r i a l and do n o t have a p o s s i b l e f u n c t i o n a l v a r i a t i o n as i n non-plant m a t e r i a l . ACKNOWLEDGEMENTS We would l i k e t o t h a n k t h e team o f t h e Yukon Refugium P r o j e c t for h o u r s o f d i s c u s s i o n on t h e s t r a t i g r a p h y o f t h e Old Crow R e g i o n . These i n c l u d e Drs. O.L. Hughes and J . V . Matthews Jr., ( G e o l o g i c a l Survey of C a n a d a ) ; R . E . Morlan and C.R. H a r i n g t o n ( N a t i o n a l Museums of C a n a d a ) ; and C.E. Schweger ( U n i v e r s i t y of A l b e r t a ) . The p r e s e n t work h a s b e e n f i n a n c e d by g r a n t s from t h e N a t u r a l S c i e n c e and E n g i n e e r i n g R e s e a r c h C o u n c i l and t h e G e o l o g i c a l Survey o f Canada. REFERENCES C I T E D Engel, M.H., Zumberge, J . E . a n d Nagy, B . , 1 9 7 7 , K i n e t i c s o f amino a c i d A n a l . Biochem., r a c e m i z a t i o n i n S e q u i o a d e n d r o n giganteurn h e a r t w o o d : V. 8 2 , p . 415-422. H u g h e s , O . L . , 1 9 7 2 , S u r f i c i a l g e o l o g y of t h e n o r t h e r n Yukon T e r r i t o r y and n o r t h w e s t e r n D i s t r i c t of Mackenzie, Northwest T e r r i t o r i e s : G e o l o g i c a l Survey of Canada P a p e r 69-36. Hughes, O . L . , H a r i n g t o n , C . R . , J a n s s e n s , J . A . , Matthews, J . V . , J r . , Morlan, R . E . , R u t t e r , N.M. and Schweger, C . E . , 1 9 8 1 , Upper P l e i s t o cene s t r a t i g r a p h y , paleoecology and archaeology of t h e n o r t h e r n Arctic, Y u k o n i n t e r i o r , e a s t e r n B e r i n g i a 1. B o n n e t P l u m e B a s i n : V. 34, p . 329-365. I r v i n g , W.H., 1978, Pleistocene archaeology i n e a s t e r n Beringia, i n B r y a n , A . L . , e d . , E a r l y Man i n A m e r i c a - f r o m a C i r c u m - P a c i f i c Perspective: A r c h a e o l o g i c a l R e s e a r c h e s I n t e r n a t i o n a l , Edmonton, p. 96-101. Kvenvolden, K.A., 1 9 8 0 , I n t e r l a b o r a t o r y c o m p a r i s o n of amino a c i d racem i z a t i o n i n a P l e i s t o c e n e m o l l u s k ; S a x i d o m u s giganteus, i n H a r e , P . E . , e d . , B i o g e o c h e m i s t r y o f Amino A c i d s , N . Y . , W i l e y , p . 2 2 3 232. Lee,
C . , Bada, J . L . f o s s i l woods:
and P e r s o n , E . , Nature., v . 259,
1 9 7 6 , Amino a c i d s i n m o d e r n a n d p . 183-186.
1 9 7 8 , E a r l y man i n n o r t h e r n Y u k o n T e r r i t o r y : p e r s p e c t i v e s Morlan, R.E., a s o f 1 9 7 7 , i n B r y a n , A . L . , e d . , E a r l y Man i n A m e r i c a - f r o m a Circum-Pacific Perspective: Archaeological Researches Internationa l , Edmonton, p . 78-95. Morlan, R.E. and Matthews, J . V . , GEOS, 2-5, W i n t e r 1978.
J r . , 1 9 7 8 , N e w d a t e s f o r e a r l y man:
C r a w f o r d , R . J . a n d H a m i l t o n , F.., 1 9 8 0 , C o r r e l a t i o n a n d R u t t e r , N.W., r e l a t i v e a g e d a t i n g o f q u a t e r n a r y s t r a t a i n t h e c o n t i n u o u s permaf r o s t z o n e o f n o r t h e r n Y u k o n w i t h D/L r a t i o s o f a s p a r t i c a c i d o f wood, f r e s h w a t e r m o l l u s c s , a n d b o n e , i n Hare, P . E . , e d . , g e o c h e m i s t r y o f A m i n o A c i d s , N.Y., W i l e y , p . 4 6 3 - 4 7 5 .
G-
S m i t h , G . G . and S i l v a d e S o l , B . , 1 9 8 0 , R a c e m i z a t i o n o f amino a c i d s i n d i p e p t i d e s shows COOH-NH2 f o r n o n - s t e r i c a l l y h i n d e r e d r e s i d u e s : S c i e n c e , v . 207, p . 765-767.
211
TREE-RING DATING IN CANADA AND THE NORTHWESTERN U S .
M.L. PARKER, L.A. JOZSA, SANDRA G. JOHNSON and PAUL A. BRAMHALL
ABSTRACT
Dendrochronology h a s b e e n u s e d r e c e n t l y i n Canada and t h e n o r t h western United S t a t e s t o d a t e a r c h a e o l o g i c a l s i t e s , driftwood acc u m u l a t i o n s o f a l a k e c r e a t e d by a s u r g i n g g l a c i e r , d r i f t w o o d on r a i s e d b e a c h e s , g l a c i a l m o r a i n e s , r a t e s o f a l l u v i a t i o n , f l o o d i n g , i c e jamming and f o r e s t f i r e s . A r c h a e o l o g i c a l t r e e - r i n g s a m p l e s h a v e b e e n d a t e d from a c a r i b o u - t r a p s i t e i n t h e n o r t h e r n Yukon, O z e t t e V i l l a g e on t h e Olympic P e n i n s u l a , Kitwanga N a t i o n a l H i s t o r i c S i t e , B r i t i s h Columbia and t h e B e l l S i t e n e a r L i l l o o e t , B r i t i s h Columbia. D a t e s a l s o were o b t a i n e d from l o g c a b i n s a t S i l v e r C i t y , a g o l d - r u s h g h o s t - t o w n i n t h e Yukon. C o a s t a l , h i g h - e l e v a t i o n of h i g h - l a t i t u d e s i t e s h a v e g e n e r a l l y been c o n s i d e r e d to b e t h e o n e s t h a t p r o d u c e t r e e - r i n g m a t e r i a l of p o o r e r d e n d r o c h r o n o l o g i c a l q u a l i t y t h a n do t h e s e m i - a r i d s i t e s . However, i t i s now p o s s i b l e to o b t a i n d a t e s from t h e D o o r e r - q u a l i t y s a m p l e s from t h e s e s i t e s b e c a u s e o f t h e development o f s e v e r a l new t e c h n i q u e s . Xr a y d e n s i t o m e t r y i s u s e d t o m e a s u r e r i n g d e n s i t y as w e l l as r i n g w i d t h . Maximumring d e n s i t y h a s b e e n u s e d t o p r o v i d e c r o s s d a t i n g on t h e c o a s t a l , h i g h - l a t i t u d e and h i g h - e l e v a t i o n wood s a m p l e s t h a t c o u l d n o t be d a t e d u s i n g r i n g w i d t h a l o n e . Computer c r o s s d a t i n g h a s b e e n u s e d t o match t r e e - r i n g s a m p l e s t h a t h a v e b e e n d i f f i c u l t t o d a t e by o t h e r m a t c h i n g t e c h n i q u e s . U s i n g t h e s e computer c r o s s d a t i n g t e c h n i q u e s , t r e e - r i n g p a t t e r n s have b e e n matched o v e r d i s t a n c e s g r e a t e r t h a n 500 kilometers. Some l i v i n g t r e e s i n Washington and B r i t i s h Columbia exc e e d 1 3 0 0 y e a r s i n a g e . T h e r e i s a p o t e n t i a l for b u i l d i n g t r e e - r i n g c h r o n o l o g i e s , u s i n g b o t h l i v i n g and d e a d t r e e m a t e r i a l , b a c k i n t i m e f o r t h o u s a n d s of y e a r s . I N T R O D U C T I O N AND BACKGR-OUND
The r e c o r d e d o b s e r v a t i o n o f t h e r e l a t i o n s h i p b e t w e e n t i m e and t h e d i a m e t e r g r o w t h of t r e e s d a t e s b a c k a t l e a s t two m i l l e n i a t o Theop h r a s t u s , t h e s t u d e n t and s u c c e s s o r of A r i s t o t l e . A number o f botan-. i s t s , f o r e s t e r s and a s t r o n o m e r s i n Europe and America i n t h e 1 8 t h a n d 1 9 t h C e n t u r i e s u n d e r s t o o d t h a t r i n g s i n t r e e s a r e a n n u a l i n n a t u r e and some e v e n u n d e r s t o o d t h e p r i n c i p l e of c r o s s d a t i n g between t r e e - r i n g s e r i e s , S t u d h a l t e r , 1 9 5 9 . However, i t was n o t u n t i l t h e work o f A . E . Douglass ( 1 9 1 9 , 1 9 2 9 , 1 9 3 5 , 1937) i n t h e 20th Century t h a t dendrochronology developed as a r e c o g n i z e d and c o n t i n u i n g f i e l d o f s c i e n t i f i c i n v e s t i g a t i o n . I n 1 9 0 1 , t h e i d e a o c c u r r e d t o Douglass t h a t t h e growth o f t h e p i n e s and j u n i p e r s i n n o r t h e r n A r i z o n a s h o u l d depend on t h e y e a r ' s , a l w a y s l i m i t e d , m o i s t u p e s u p p l y and t h a t t h i s might b e r e f l e c t e d i n t h e w i d t h of t h e a n n u a l r i n g s ( G i d d i n g s , 1 9 6 2 ) . I n 1 9 0 4 D o u g l a s s t e s t e d t h i s i d e a by m a t c h i n g t h e p a t t e r n of wide a n d n a r r o w a n n u a l r i n g s formed i n t h e c r o s s s e c t i o n s o f stems o f d i f f e r e n t t r e e s g r o w i n g i n t h e same a r e a . I n t h e y e a r s t h a t f o l l o w e d ,
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D o u g l a s s d e v e l o p e d t e c h n i q u e s t o f a c i l i t a t e c r o s s d a t i n g and c h r o n o l o g y b u i l d i n g ( t h e s u m m a r i z i n g and t e m p o r a l e x t e n s i o n o f a n n u a l r i n g - w i d t h r e c o r d s from d i f f e r e n t t r e e s i n t o a s i n g l e " t r e e - r i n g c h r o n o l o g y " ) . The p o t e n t i a l f o r t r e e - r i n g d a t i n g was r e c o g n i z e d e a r l y by t h e southwestern archaeologists. I n 1 9 1 6 , Douglass f i r s t r e c e i v e d samples 3f p r e h i s t o r i c wood. C r o s s d a t i n g o f wood and c h a r c o a l s a m p l e s from a number o f d i f f e r e n t s o u t h w e s t e r n I n d i a n r u i n s p r o c e e d e d u n t i l by t h e 1 9 2 0 ' s a number o f " f l o a t i n g c h r o n o l o g i e s ' ' e x i s t e d . T h e s e f l o a t i n g c h r o n o l o g i e s p r o v i d e d c r o s s d a t i n g between s a m p l e s o f t h e same s i t e and between s i t e s b u t were n o t t i e d down t o c a l e n d a r y e a r s . T h i s 585-year-long f l o a t i n g c h r o n o l o g y , t h a t Douglass c a l l e d t h e " R e l a t i v e D a t i n g S e r i e s " , was c o m p i l e d f r o m beams from A z t e c , P u e b l o 3 o n i t o , C l i f f P a l a c e , B e t a t a k i n and o t h e r I n d i a n r u i n s from t h e s o u t h d e s t . Haury ( 1 9 6 2 ) h a s r e c o r d e d t h e d r a m a t i c moment, when i n 1 9 2 9 , a z h a r c o a l beam from a r u i n i n Showlow, A r i z o n a , p r o v i d e d t h e r i n g s e 2uence r e q u i r e d t o c r o s s d a t e t h e R e l a t i v e D a t i n g S e r i e s w i t h t h e t r e e r i n g c h r o n o l o g y t h a t D o u g l a s s h a d c o m p i l e d from l i v i n g t r e e s from t h e s r e a . T h i s e v e n t marked t h e c l i m a x o f y e a r s o f t e c h n i q u e d e v e l o p m e n t , 2 r o s s d a t i n g and c h r o n o l o g y b u i l d i n g by D o u g l a s s and h i s a s s o c i a t e s . 4t t h i s one moment i n t i m e , t h e c a l e n d a r - y e a r d a t i n g o f many m a j o r r u i n s i n t h e s o u t h w e s t was a c c o m p l i s h e d . T h i s s t o r y has b e e n r e p e a t e d many t i m e s , i . e . , u s i n g t h e c r o s s d a t i n g t e c h n i q u e , b u i l d i n g l i v i n g - t r e e c h r o n o l o g i e s and c h r o n o l o g i e s o f u n d a t e d s a m p l e s , and f i n a l l y , m a t c h i n g t h e two s e r i e s t o p r o v i d e c a l e n - d a r d a t e s o f p r e v i o u s l y u n d a t e d s a m p l e s ( F i g u r e 1 ) . A good example o f T h i s i s when G i d d i n g s ( 1 9 5 2 ) b u i l t a f l o a t i n g c h r o n o l o g y from t r e e - r i n g s a m p l e s from p r e h i s t o r i c s i t e s on t h e Kobuk R i v e r i n A l a s k a and t h e n d a t e d t h e s e s i t e s by m a t c h i n g t h i s f l o a t i n g c h r o n o l o g y w i t h a l i v i n g ;ree chronology. The work begun by D o u g l a s s h a s c o n t i n u e d and has b e e n g r e a t l y expanded upon b y E . Schulman, B . B a n n i s t e r , C . W . F e r g u s o n , H . C . Fritts, C . W . S t o c k t o n , J . Dean, W . R o b i n s o n , T . P . H a r l a n , M . A . S t o k e s , V.C. Lamarche and many o t h e r s a t t h e L a b o r a t o r y of T r e e - r i n g Fiesearch i n m L u c s o n , A r i z o n a ( S t o k e s and Smiley 1 9 6 8 ; F r i t t s 1976; s e e a l s o i s s u e s of t h e T r e e - R i n g B u l l e t i n ) . T r e e - r i n g l a b o r a t o r i e s have b e e n e s 5 a b l i s h e d i n many p a r t s o f t h e w o r l d u s i n g t h e t e c h n i q u e s and p r i n c i p l e s developed i n Arizona. The s u c c e s s f u l t r e e - r i n g d a t i n g r e p o r t e d i n t h i s p a p e r h a s r e s u l t e d , t o a l a r g e e x t e n t , from a p p l y i n g t h e t e c h n i q u e s d e v e l o p e d by D o u g l a s s and h i s s t u d e n t s . I n a l l c a s e s , however, t h i s d a t i n g was a c h i e v e d by a p p l y i n g two a d d i t i o n a l and r e l a t i v e l y new t e c h n i q u e s : (1) X-ray d e n s i t o m e t r y , and ( 2 ) computer c r o s s d a t i n g . A l l t r e e s a r e not of equal q u a l i t y f o r crossdating purposes. !here i s a wide r a n g e i n d e n d r o c h r n n o l o g i c a l q u a l i t y of r i n g s e r i e s from t h o s e t h a t d i s p l a y good r i n g - t o - r i n g v a r i a t i o n and p r o v i d e good d a t i n g o f c h r o n o l o g i e s , t o t h o s e t h a t a r e of s u c h p o o r q u a l i t y t h a t ?hey c a n n o t b e d a t e d . T r e e s growing i r , t h e s e m i a r i d r e g i o n s o f w e s t e r n N o r t h America, from t h e Y r a s e r F i v e r V a l l e y i n B r i t i s h Columbia t o Mexico, h a v e a l i m i t e d m o i s t u r e s u p p l y and u s u a l l y p r o d u c e goodq u a l i t y t r e e - r i n g r e c o r d s . These r i n g s e r i e s t h a t h a v e marked r i n g - t o C o n v e r s e l y , t r e e s growing i n r i n g v a r i a t i o n a r e known a s " s e n s i t i v e " . c o a s t a l , h i g h - e l e v a t i o n or h i g h - l a t i t u d e e n v i r o n m e n t s u s u a l l y p r o d u c e r i n g s e r i e s w i t h l i t t l e ring-to-ring width v a r i a t i o n . o r "complacent" 5ree-ring chronologies (Figure 2 ) . T h i s p a p e r d e a l s w i t h t h e u s e o f t h e methods o f X-,ray d e n s i t o m e t r y and computer c r o s s d a t i n g ( a u g m e n t i n g t h e t e c h n i q u e s d e v e l o p e d by D o u g l a s s ) t o d a t e t h e p o o r e r q u a l i t y t r e e - r i n g s a m p l e s from Canada and t h e northwestern United S t a t e s .
TWO METHODS THAT IMPROVE CROSSDATING POTENTIAL F o r y e a r s , d e n d r o c h r o n o l o g i c a l r e s e a r c h h a s been c o n c e n t r a t e d
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Figure 1
H i s t o r i c a n d p r e h i s t o r i c wood s t r u c t u r e s c a n b e d a t e d by m a t c h i n g t h e i r r i n g p a t t e r n s a g a i n s t t h o s e f o u n d i n l i v i n g t r e e s o f known a g e .
m a i n l y on t h o s e t r e e s from t h e s e m i - a r i d r e g i o n s t h a t p r o d u c e goodq u a l i t y t r e e - r i n g w i d t h s e r i e s . However, e x t e n s i v e and s p e c t a c u l a r t r e e - r i n g m a t e r i a l i s a v a i l a b l e from more m e s i c s i t e s s u c h a s O z e t t e V i l l a g e on t h e Olympic P e n i n s u l a . We now know t h a t by a p p l y i n g s e v e r a l new t e c h n i q u e s , i t i s p o s s i b l e t o o b t a i n t r e e - r i n g d a t e s from t h e s e s i t e s . We c a n look f o r w a r d t o r e p e a t i n g i n many new a r e a s , t h e s c e n a r i o f o l l o w e d b y D o u g l a s s y e a r s ago i n t h e s o u t h w e s t , o f c r o s s d a t i n g , c h r o n o l o g y b u i l d i n g and t h e n t h e c a l e n d a r p y e a r d a t i n g o f t r e e r i n g samples. X-Ray D e n s i t o m e t r y V a r i o u s r a d i a t i o n , l i g h t and m e c h a n i c a l t e c h n i q u e s h a v e b e e n dev e l o p e d t o s t u d y t h e w i t h i n - r i n g d e n s i t y v a r i a t i o n s i n wood s a m p l e s (Cameron e t al., 1 9 5 9 ; Marian and Stumbo, 1 9 6 0 : Green 1 9 6 4 , 1 9 6 5 ; Green and W o r r a l l , 1964; H a r r i s , 1 9 6 9 ; Kawaguchi, 1 9 6 9 ) ; b u t p e r h a p s t h e most s u c c e s s f u l method, X - r a y d e n s i t o r n e t r y . was p i o n e e r e d by P o l g e ( 1 9 6 3 , . 1 9 6 5 and 1 9 6 6 ) d u r i n g t h e 1 9 6 0 ' s . The t e c h n i q u e s i n X-ray d e n s i t o m e t r y u s e d i n t h e r e s e a r c h d e s c r i b e d i n t h i s p a p e r were dev e l o p e d i n O t t a w a , O n t a r i o , w i t h t h e G e o l o g i c a l Survey o f Canada
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Figure 2
Complacent r i n g s e r i e s from t r e e s on t h e m o i s t B r t i s h Columbia c o a s t have r i n g s t h a t a r e more uniform i n width than a r e t h e r i n g s of the sensit i v e chronologies of trees from t h e p r o v i n c e ' s dry i n t e r i o r or t h e Northwest T e r r i t o r i e s .
between 1967 and 1970 (Parker 1969a, 196913, 1970a, 1970b; Parker and Henoch 1969 and 1971; Jones and Parker 1970; Parker and Meleskie. 1970) and in Vancouver, British Columbia with the University of British Columbia; the Western Forest Products Laboratory, Canadian Forestry Service; and Forintek Canada Corp. between 1970 and the present (Parker 1972 and 1976; Parker and Jozsa 1973a, 1973b, 1977a and 197713; Parker and Kennedy 1973; Parker, Jozsa and Brue. 1973; Parker, Schoorlemmer and Carver, 1973; Parker, Barton and Jozsa 1974; Parker, Bunce and Smith 1974; Parker e t aZ., 1976, 1977, 1979, 1980 and 1981; Heger e t a Z . , 197'1; Johnson, 1982). The X-ray densitometry system currently in use in Vancouver uses an in-motion X-ray technique to produce radiographs of wood and charcoal samples and an on-line computerized scanning densitometer to collect and process data from the radiographs (Parker e t aZ., 1980). Detailed data of earlywood and latewood components of both width and
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d e n s i t y a r e r a p i d l y and a c c u r a t e l y c o l l e c t e d and s t o r e d on m a g n e t i c t a p e . The o n - l i n e computer and p e r i p h e r a l d e v i c e s g r e a t l y f a c i l i t a t e p r o c e s s i n g , p r i n t i n g , p l o t t i n g and d a t i n g o f t r e e - r i n g s e r i e s . I f we compare t h i s method of r e c o r d i n g t r e e - r i n g d a t a w i t h t h o s e methods u s e d by D o u g l a s s i n t h e e a r l y y e a r s , w e c a n n o t e t h a t p r e p a r i n g a g i v e n sample b y t h e X - r a y d e n s i t o m e t r y method i s much more t i m e consuming. I n many c a s e s , a d e n d r o c h r o n o l o g i s t w o r k i n g i n t h e s o u t h w e s t e r n U.S. can b r e a k a p i e c e of a r c h a e o l o g i c a l c h a r c o a l by h a n d , examine i t w i t h a hand l e n s or low-power m i c r o s c o p e and d a t e i t by t h e memory method ( D o u g l a s s , 1 9 4 6 ) i n a few m i n u t e s . If t h e memory method does n o t p r o d u c e r e s u l t s , t h e s a m p l e may be d a t e d by t h e s k e l e t o n - p l o t t e c h n i q u e ( S t o k e s and S m i l e y , 1 9 6 8 ) . T h i s method a l s o i s a f a i r l y r a p i d o n e . Even i n t h e s o u t h w e s t , however, where g o o d - q u a l i t y t r e e r i n g s a m p l e s e x i s t , many s a m p l e s c a n n o t be d a t e d b e c a u s e o f t h e i r p o o r quality. I n most of Canada and t h e n o r t h w e s t e r n U . S . , t h e v e r y good q u a l i t y t r e e - r i n g w i d t h s a m p l e s a r e r a r e . Most r i n g - w i d t h s e r i e s a r e complacent. I t h a s b e e n d e m o n s t r a t e d , however, t h a t t h e c o a s t a l h i g h e l e v a t i o n and h i g h - l a t i t u d e s p e c i m e n s may b e s e n s i t i v e w i t h r e s p e c t t o r i n g - t o - r i n g d e n s i t y v a r i a t i o n a n d a r e , t h e r e f o r e , o f good q u a l i t y for d a t i n g or for c l i m a t i c s t u d i e s i f d e n s i t y i s u s e d ( P a r k e r and Henoch, 1 9 7 1 ; P a r k e r 1976; P a r k e r e t a Z . , 1 9 8 1 , 1 9 8 2 ) . Year-to-year v a r i a t i o n s i n t e m p e r a t u r e a r e r e f l e c t e d e s p e c i a l l y i n a n n u a l r i n g maximum d e n s i t y ( F i g u r e 3 ) . T r a d i t i o n a l methods of r i n g - w i d t h a n a l y s i s a r e n o t adeq u a t e f o r d a t i n g most o f t h e s e wood s a m p l e s . F o r t h i s r e a s o n X-ray d e n s i t o m e t r y i s t h e most a p p r o p r i a t e e x i s t i n g method t o b e u s e d t o d a t e t r e e - r i n g s a m p l e s f r o m Canada and t h e n o r t h w e s t e r n U.S. Computer C r o s s d a t i n g The computer c r o s s d a t i n g program i n u s e a t F o r i n t e k , t h e S h i f t i n g Unit D a t i n g Program (SUDP), was d e v e l o p e d by P a r k e r a t t h e L a b o r a t o r y o f Tree-Ring R e s e a r c h ( P a r k e r , 1 9 6 7 ) , m o d i f i e d a t t h e G e o l o g i c a l Survey of Canada ( P a r k e r , 1 9 7 0 a and 1 9 7 0 b ) , and m o d i f i e d a g a i n a t t h e Western Forest Products Laboratory w i t h t h e a s s i s t a n c e of Paul Bramhall. T h i s program u s e d computer t e c h n i q u e s t o c r o s s d a t e t r e e - r i n g s e r i e s . A p o r t i o n ( u n i t ) of a n u n d a t e d t r e e - r i n g s e r i e s i s c o r r e l a t e d with a dated master tree-ring s e r i e s i n a l l possible positions. The p o s i t i o n s and c o r r e l a t i o n s o f t h e t h r e e b e s t m a t c h e s between t h e u n i t and t h e m a s t e r s e r i e s a r e r e c o r d e d . Successive u n i t s (of designated l e n g t h and i n c r e m e n t ) a r e c o r r e l a t e d w i t h t h e master o f a l l p o s s i b l e l o c a t i o n s u n t i l t h e e n t i r e u n d a t e d s e r i e s h a s b e e n matched w i t h t h e e n t i r e d a t e d s e r i e s . The v a l i d i t y o f t h e c r o s s d a t i n g i s e v a l u a t e d by t h e s e q u e n t i a l placement of t h e u n i t s and t h e v a l u e s of t h e c o r r e l a t i o n c o e f f i c i e n t s . T h i s t e c h n i q u e has b e e n u s e d s u c c e s s f u l l y t o c r o s s d a t e t r e e - r i n g s e r i e s o v e r d i s t a n c e s g r e a t e r t h a n 5 0 0 km ( F i g u r e 4) f o r (1) C a l i f o r n i a and t h e Four C o r n e r s a r e a ( P a r k e r , 1 9 6 7 ) . s a m p l e s from: ( 2 ) O n t a r i o and Nova S c o t i a ( P a r k e r . 1 9 7 0 b ) . a n d ( 3 ) s e v e r a l a r e a s a l o n g t h e B r i t i s h Columbia c o a s t ( F o r i n t e k Canada C o r p . , no d a t e ) . Computer c r o s s d a t i n g was u s e d t o d a t e t h e t r e e - r i n g m a t e r i a l , d e s c r i b e d i n a f o l l o w i n g s e c t i o n , from g e o l o g i c a l , a r c h a e o l o g i c a l and h i s t o r i c s i t e s i n t h e Yukon T e r r i t o r y , N o r t h w e s t T e r r i t o r i e s , B r i t i s h Columbia, Manitoba and W a s h i n g t o n . METHOD USED FOR STANDARDIZING AND SUMMARIZING TREE-RING DATA A m a j o r p r o b l e m t h a t d e n d r o c h r o n o l o g i s t s f a c e i n d e a l i n g w i t h most t r e e - r i n g d a t a i s t h e r e m o v a l o f n o n - c l i m a t i c t r e n d s , s u c h as t h e g r o w t h t r e n d , and a v e r a g i n g t h e s e d a t a i n some s t a n d a r d form w i t h o u t i n a d v e r t e n t l y removing f e a t u r e s t h a t s h o u l d b e r e t a i n e d . O b j e c t i v e s a r e t o p r o d u c e t r e e - r i n g c h r o n o l o g i e s t h a t a r e o f good q u a l i t y f o r d a t i n g p u r p o s e s and t h a t a c c u r a t e l y r e f l e c t t h e i n f l u e n c e of c l i m a t e .
The "raw d a t a " p r o d u c e d by t h e X - r a y d e n s i t o m e t r y s y s t e m d e s c r i b e d here consist o f : (1) r i n g - w i d t h v a l u e s i n 0 . 0 1 mm u n i t s , and ( 2 )
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Figure 3
T r e e - r i n g d e n s i t y of t h e h i g h - e l e v a t i o n s p r u c e i s shown t o b e c l o s e l y r e l a t e d t o A u g u s t t e m p e r a t u r e and r i v e r r u n o f f . The two l o w e r g r a p h s show t h e c l o s e r e l a t i o n s h i p o f May t h r o u g h A u g u s t t e m p e r a t u r e s and p r e c i p i t a t i o n t o t r e e - r i n g d e n s i t y i n t h e B r i t i s h Columbia c o a s t a l D o u g l a s - f i r .
maximum r i n g - d e n s i t y v a l u e s i n g/cm3 measured a t 0 . 0 1 mm i n c r e m e n t s a l o n g t h e s c a n n e d p o r t i o n o f a t r e e - r i n g s a m p l e . These r a w d a t a a r e " s t a n d a r d i z e d " by c o n v e r t i n g them i n t o i n d i c e s ( r a t i o of o b s e r v e d -Jalue t o f i t t e d t r e n d , y i e l d i n g v a l u e s w i t h a mean o f 1 . 0 0 ) by removing t h e g r o w t h t r e n d ( a n d sometimes o t h e r t r e n d s o r f l u c t u a t i o n s ) . The s t a n d a r d i z e d d a t a ( i n d i c e s ) f o r a l l s a m p l e s a r e t h e n "summarizedf1, o r a v e r a g e d , t o p r o d u c e a summary o r " m a s t e r " c h r o n o l o g y for t h e s i t e . This procedure i s i l l u s t r a t e d i n Figure 5 . I n a previous public a t i o n ( P a r k e r e t aZ., 1 9 8 1 ) t h e "A" , "B" and "C" components h a v e been described: "A" d e f i n e d as t h e g r o w t h t r e n d ; " B r r , t h e s h o r t - t e r m f l u c t u a t i o n s g r e a t e r t h a n 1 0 y e a r s i n l e n g t h ; and " C " , t h e y e a r - t o b e a r v a r i a t i o n s . V a r i o u s data p r o c e s s i n g programs a r e used t o produce a n "A , a " B r f , a "C" and a ''B & C " c h r o n o l o g y from t h e raw d a t a o f e a c h t r e e - r i n g s a m p l e . These d a t a a r e t h e n a v e r a g e d w i t h d a t a from o t h e r t r e e - r i n g sample s e r i e s t o p r o d u c e t h e summary c h r o n o l o g i e s . If t r e e - r i n g d a t a a r e a v e r a g e d w i t h o u t removing t h e g r o w t h t r e n d , n o n - c l i m a t i c f l u c t u a t i o n s r e l a t e d t o t h e a g e of t h e t r e e w i l l b e i n c l u d e d i n t h e c h r o n o l o g y . T h e r e f o r e , it i s e s s e n t i a l t h a t t h e growth t r e n d b e removed. However, v a r i a t i o n due t o c l i m a t e may be removed
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Figure 4
L i v i n g t r e e s up to 4 m e t r e s i n d i a m e t e r h a v e b e e n s a m p l e d w i t h a p o w e r - d r i v e n i n c r e m e n t borer. Using computer crossdating techniques, tree-ring patterns h a v e b e e n m a t c h e d o v e r d i s t a n c e s g r e a t e r t h a n 500 kilometers.
i n a d v e r t e n t l y , i f proper techniques a r e not used i n t h i s procedure. One method for removing t h e g r o w t h t r e n d ( t h e " A " component) from t r e e - r i n g s e r i e s , u t i l i z e d a d i g i t a l - f i l t e r w i t h a l e n g t h of 60 y e a r s ( P a r k e r , 1 9 7 0 a ) . T h i s method i s d e s i g n e d t o remove t h e g r o w t h t r e n d and t h e n o n - c l i m a t i c s u r g e s i n g r o w t h , s u c h a s t r e e g r o w t h r e l e a s e a f t e r a f o r e s t f i r e . T r e e - r i n g i n d i c e s ( i n t h e f o r m of o u r "B & C " c h r o n o l o g y ) a r e p r o d u c e d b y c a l c u l a t i n g d e v i a t i o n s from a g r o w t h - t r e n d l i n e t h r o u g h t h e raw d a t a v a l u e s . The "B and C " c h r o n o l o g i e s a r e o b t a i n e d from t h e t r e e - r i n g i n d i c e s ( t h e "B & C " c h r o n o l o g y ) . T h i s i s done b y u s i n g a d i g i t a l - f i l t e r t e c h n i q u e w i t h a l e n g t h o f 1 3 y e a r s ( P a r k e r , 1 9 7 0 a ) . The " B r r c h r o n o l o g y i s t h e e s t i m a t e d c u r v e which i s a w e i g h t e d r u n n i n g mean. Devia t i o n s from t h i s l i n e a r e t h e y e a r - t o - y e a r f l u c t u a t i o n s , or t h e " C " c h r o n o l o g y . The ''C" c h r o n o l o g y tyDe has p r o v e n t o b e t h e form most useful f o r dating purposes. SUCCESSFUL CROSSDATING I N NORTHERN AND WESTERN NORTH A M E R I C A
X-ray d e n s i t o m e t r y and computer c r o s s d a t i n g were u s e d i n a l l c a s e s of s u c c e s s f u l d a t i n g o f a r c h a e o l o g i c a l o r d r i f t w o o d t r e e - r i n g m a t e r i a l I n n e a r l y a l l c a s e s maximum r i n g from Canada or t h e n o r t h w e s t e r n U . S .
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d e n s i t y was t h e p a r a m e t e r u s e d . U o r t h e r n Yukon Adze-cut Stump The f i r s t s u c c e s s f u l d a t i n g o f a n a r c h a e o l o p i c a l t r e e - - r i n g sample i n Canada was t h e d a t i n g of a n a d z e - c u t stump from a c a r i b o u t r a p l o c a t e d i n t h e Old Crow a r e a i n t h e n o r t h e r n Yukon T e r r i t o r y . T h i s s a m p l e was s u b m i t t e d t o t h e G e o l o g i c a l Survey o f Canada b y W i l l i a m I r v i n g . The SUDP was u s e d t o match t h e r i n g s e r i e s o f t h i s s a m p l e s g a i n s t r i n g s e r i e s from a s i n g l e w h i t e s p r u c e ( P i c e a gZauca (Moench) 7 7 0 s s ) t r e e from n e a r I n u v i k , NWT, a d i s t a n c e o f a b o u t 250 k m . A d a t e 2f 1 6 4 7 ( v e r y v a r i a b l e o u t s i d e ) was o b t a i n e d from b o t h r i n g w i d t h and naximum r i n g d e n s i t y . The s i g n i f i c a n c e o f t h i s i s t h a t i t d e m o n s t r a t e s t h a t t r e e - r i n g d a t e s can b e o b t a i n e d i n t h i s n o r t h e r n r e g i o n b y c r o s s d a t i n g samples d e r i v e d from areas l o n g d i s t a n c e s a p a r t and e v e n by u s i n g s i n g l e - t r e e chronologies. 3 e l l S i t e n e a r L i l l o o e t , B r i t i s h Columbia Woodand c h a r c o a l s a m p l e s o f D o u g l a s - f i r ( P s e u d o t s u g a r n e n z i e s i i ( M i r b . ) F r a n c o ) , l o d g e p o l e p i n e ( P i n u s c o n t o r t a D o u g l . ) and p o n d e r o s a c i n e ( P i n u s p o n d e r o s a L a w s . ) from t h e B e l l S i t e were s u b m i t t e d t o t h e I.:estern F o r e s t P r o d u c t s L a b o r a t o r y b y A . H . S t r y d f o r t r e e - r i n g a n a l y s i s
219
( S t r y d , 1 9 7 9 ) . The B e l l S i t e i s a p r e h i s t o r i c p i t h o u s e v i l l a g e s i t e i n t h e B r i t i s h Columbia i n t e r i o r n e a r L i l l o o e t . A number o f t r e e - r i n g s a m p l e s c r o s s d a t e d w i t h one a n o t h e r and a composite c h r o n o l o g y d e r i v e d from 9 s a m p l e s was c o n s t r u c t e d . A d a t e o f 1854 was o b t a i n e d f o r t h e most r e c e n t r i n g - y e a r on t h i s c o m p o s i t e b y u s i n g SUDP t o match t h i s c h r o n o l o g y a g a i n s t a m a s t e r c h r o n o l o g y cons t r u c t e d from D o u g l a s - f i r t r e e s c o l l e c t e d n e a r P a v i l i o n Lake b y H . C . F r i t t s i n 1 9 6 6 . The s u c c e s s f u l m a t c h was o b t a i n e d b y u s i n g r i n g w i d t h r a t h e r t h a n maximum r i n g d e n s i t y . T h i s s u g g e s t s t h a t a l t h o u g h maximum r i n g d e n s i t y i s t h e b e t t e r p a r a m e t e r t o u s e w i t h s a m p l e s from t h e w e t t e r and c o l d e r s i t e s , r i n g w i d t h i s b e t t e r f o r c r o s s d a t i n g s a m p l e s from t h e d r y i n t e r i o r s i t e s .
A d d i t i o n a l c r o s s d a t i n g was o b t a i n e d b e t w e e n o t h e r s a m p l e s from d i f f e r e n t h o u s e s a t t h e s i t e a n d b e t w e e n t h e B e l l S i t e s a m p l e s and a n o t h e r s i t e i n t h i s r e g i o n , b u t no c a l e n d a r - y e a r d a t e s were d e t e r m i n e d p r o b a b l y b e c a u s e t h e m a t e r i a l i s t o o o l d t o b e matched w i t h t h e e x i s t i n g master c h r o n o l o g y . Kitwanga N a t i o n a l H i s t o r i c S i t e , B r i t i s h Columbia Wood and c h a r c o a l s a m p l e s from t h e Kitwanga N a t i o n a l H i s t o r i c S i t e , B a t t l e H i l l , w e r e s u b m i t t e d t o F o r i n t e k Canada Corp. f o r t r e e - r i n g a n a l y s i s ( J o z s a , P a r k e r , B r a m h a l l , K e l l o g g a n d Rowe. 1 9 8 0 ) . T r e e - r i n g m a s t e r c h r o n o l o g i e s w e r e b u i l t from r e c e n t l y - f e l l e d w e s t e r n hemlock ( T s u g a h e t e r o p h y Z Z a ( R a f . ) S a r g . ) and w e s t e r n r e d c e d a r ( T h u j a p l i c a t a Donn) t r e e s from t h e Kitwanga a r e a . Although a l a r g e p r o p o r t i o n of t h e f o r e s t i n t h a t a r e a c o n s i s t s o f w e s t e r n hemlock, none o f t h e e i g h t s p e c i e s r e p r e s e n t e d i n t h e a r c h a e o l o g i c a l t r e e - r i n g c o l l e c t i o n was hemlock. The t r e e r i n g s a m p l e s from t h e B a t t l e H i l l s i t e were g e n e r a l l y of very poor d e n d r o c h r o n o l o g i c a l q u a l i t y - c o m p l a c e n t , s h o r t and f r a g m e n t a r y . However, one d a t e ( 1 7 4 9 ) was o b t a i n e d , u s i n g SUDP, f o r a w e s t e r n r e d c e d a r s a m p l e . O z e t t e V i l l a g e S i t e , Washington A v e r y l a r g e number o f t r e e - r i n g s a m p l e s h a v e b e e n removed from t h e p r e h i s t o r i c v i l l a g e of O z e t t e l o c a t e d on t h e west c o a s t o f t h e Olympic P e n i n s u l a i n W a s h i n g t o n . A n i l o t s t u d y was u n d e r t a k e n b y F o r i n t e k Canada Corp. f o r t h e Washington A r c h a e o l o g i c a l R e s e a r c h C e n t e r (WARC) t o d e t e r m i n e i f t r e e - r i n g dates c o u l d b e d e r i v e d from t h i s m a t e r i a l ( J o z s a , P a r k e r and Bramhall, 1 9 8 0 ) .
S i x t e e n a r c h a e o l o g i c a l t r e e - r i n g s a m p l e s were a n a l y z e d and one d a t e ( 1 6 1 3 ) was o b t a i n e d by u s i n g SUDP t o c r o s s d a t e a w e s t e r n r e d c e d a r s a m p l e w i t h a c o m p o s i t e c h r o n o l o g y b u i l t from two r e c e n t l y . f e l l e d western red cedar t r e e s . Under a more r e c e n t c o n t r a c t between F o r i n t e k and W A R C , b e t t e r q u a l i t y m a s t e r c h r o n o l o g i e s h a v e b e e n b u i l t f o r D o u g l a s - f i r and w e s t e r n r e d c e d a r t r e e s from t h e Olympic P e n i n s u l a . An a d d i t i o n a l t r e e - r i n g d a t e was o b t a i n e d ( 1 7 1 9 v v ) by m a t c h i n g a n a r c h a e o l o g i c a l sample w i t h t h e western r e d cedar master chronology. There are l i v i n g w e s t e r n r e d c e d a r t r e e s i n t h e a r e a t h a t exceed 1 3 0 0 y e a r s i n a g e , however, some o f t h e s e a r e v e r y c o m p l a c e n t and l a c k c i r c u i t u n i f o r m i t y . These t r e e s a r e o f t o o p o o r q u a l i t y t o b e u s e d f o r d e n d r o c h r o n o l o g i c a l s t u d i e s even i f X-ray d e n s i t o m e t r y i s employed.
However, t h e good q u a l i t y o f some t r e e - r i n g s e r i e s , t h e a g e o f t h e t r e e s a n d t h e e x t e n s i v e n e s s o f t h e a r c h a e o l o g i c a l m a t e r i a l , make t h i s a p o t e n t i a l l y f r u i t f u l area f o r t r e e - r i n g r e s e a r c h . S i l v e r C i t y , Yukon T e r r i t o r y I n 1 9 7 9 , e l e v e n t r e e - r i n g s a m p l e s were c o l l e c t e d from s p r u c e l o g s u s e d i n t h e c o n s t r u c t i o n o f c a b i n s i n t h e g o l d - r u s h town o f S i l v e r C i t y
2 20
i n t h e Yukon ( J o z s a e t aZ., 1 9 8 1 ; C l a g u e e t aZ., 1 9 8 2 ) . X--ra,y d e n s i t o m e t r y was u s e d t o a n a l y z e t h e s e s a m p l e s arld t h e t r e e - r i n g d a t a were m a t c h e d a g a i n s t t r e e - r i n g c h r o n o l o g i e s d e r i v e d from l i v i n g w h i t e s p r u c e t r e e s growing i n t h e v i c i n i t y . E a t e s w e r e p r o d u c e d for a l l e l e v e n s a m p l e s i n d i c a t i n g t h a t t h e l o g - c a b i n t r e e s w e r e c u t i n 1905 or s h o r t l y t h e r e a f t e r . The s i g n i f i c a n c e of t h i s i s t h a t t h e t r e e . ; - r i n g s e r i e s o f t r e e s i n t h a t r e g i o n a r e o f e x c e l l e n t q u a l i t y for d a t i n g p u r p o s e s and i t s h o u l d b e p o s s i b l e t o o b t a i n d a t e s f r o m t h e many h i s t o r i c s t r u c t u r e s i n t h i s n o r t h e r n a r e a . N e o g l a c i a l Lake A l s e k , Yukon T e r r i t o r y The i n t e r m i t t e n t damming o f t h e A l s e k R i v e r b y L o w e l l G l a c i e r a t v a r i o u s t i m e s i n t h e p a s t h a s r e s u l t e d i n t h e f o r m a t i o n o f numerous beaches. Some o f t h e s e b e a c h e s c o n t a i n d r i f t w o o d s t r a n d l i n e s . D r i f t wood s a m p l e s were c o l l e c t e d i n 1979 from b e a c h e s n e a r H a i n e s J u n c t i o n , Yukon T e r r i t o r y ( J o z s a e t aZ., 1 9 8 1 , C l a g u e e t aZ., 1 9 8 2 ) . S i x t e e n t r e e - r i n g d a t e s w e r e o b t a i n e d from d r i f t w o o d s a m p l e s from s e v e r a l of t h e more r e c e n t b e a c h e s . These d a t e s r a n g e d from 1675 t o 1 8 4 8 . All d a t e s w e r e o b t a i n e d t h r o u g h t h e u s e o f X - r a y d e n s i t o m e t r y and computer c r o s s d a t i n g . R a i s e d Beaches o n The Hudson Bay C o a s t I n 1 9 8 1 , d r i f t w o o d s a m p l e s w e r e c o l l e c t e d from r a i s e d b e a c h e s n e a r t h e mouth of t h e Owl R i v e r which f l o w s i n t o Hudson B a y midway between C h u r c h i l l a n d York F a c t o r y , Manitoba ( P a r k e r e t aZ., 1982). X-ray d e n s i t o m e t r y a n d c o m p u t e r c r o s s d a t i n g w e r e u s e d t o d a t e t w e l v e of t h e s e t r e e - r i n g s a m p l e s . The d a c e s r a n g e from 1849 t o 1 9 6 1 .
Figure 6
Tree r i n g s can be used t o d a t e events such as lands l i d e s by c h a n g e s i n g r o w t h p a t t e r n s a n d c o m p r e s s i o n wood.
221
Almost a l l o f t h e s a m p l e s p r o c e s s e d from t h e y o u n g e s t b e a c h e s were dated. This demonstrates t h a t i t i s p o s s i b l e t o d a t e driftwood i n t h e Hudson Bay a r e a . Some d r i f t w o o d i n t h e r e p i o n i s t h o u s a n d s o f y e a r s o l d , making i t a good a r e a for f u t u r e d e n d r o c h r o n o l o g i c a l r e s e a r c h . O t h e r Tree-Ring D a t i n g i n Canada I n a d d i t i o n t o t h e above e x a m p l e s , d e n d r o c h r o n o l o g y h a s b e e n u s e d i n a number o f m i n o r p r o j e c t s s u c h as t h e d a t i n g o f a b o n f i r e u s e d t o consume t h e body o f a m u r d e r v i c t i m , t h e d a t i n g o f a d r i f t w o o d s a m p l e from t h e H o r t o n R i v e r i n t h e n o r t h e r n N o r t h w e s t T e r r i t o r i e s , t h e d a t i n g of f o r e s t f i r e s and a number o f l a n d s l i d e s ( F i g u r e 6). D a t i n g methods o t h e r t h a n X-ray d e n s i t o m e t r y h a v e sometimes b e e n u s e d . These methods i n c l u d e r i n g c o u n t s t h e u s e o f c o m p r e s s i o n wood, s c a r s , t r a u m a t i c r e s i n c a n a l s or o t h e r t e c h n i q u e s . These methods a r e s i m p l e b u t u s e f u l for d a t i n g s u c h e v e n t s as f l o o d i n g or i c e jamming ( F i g u r e 7 ) ( P a r k e r , J o z s a a n d B r u c e , 1973). CONCLUSIONS There a r e l i v i n g t r e e s i n t h e n o r t h w e s t e r n U n i t e d S t a t e s and Canada t h a t e x c e e d 1 3 0 0 y e a r s i n a g e . T h e r e a r e t r e e - r i n g s a m p l e s i n g e o l o g i c a l and a r c h a e o l o g i c a l c o n t e x t s t h a t a r e t h o u s a n d s o f y e a r s o l d . The r e c e n t d e v e l o p m e n t o f new t e c h n i q - u e s , p a r t i c u l a r l y X-ray d e n s i t o - . m e t r y , and computer c r o s s d a t i n g , h a v e b e e n u s e d t o augment o l d e r t e c h n i q u e s t o o b t a i n many t r e e - r i n g d a t e s d u r i n g t h e l a s t s e v e r a l y e a r s . There a r e e c o n o m i c a l l y s i g n i f i c a n t a p p l i c a t i o n s o f d e n d r o c h r o n o l o g i c a l
Figure 7
The s c a r s on t h e s e t h r e e t r e e s e c t i o n s i n d i c a t e t h e ‘ y e a r i n which f l o o d i n g o r i c e jamming o c c u r r e d a l o n g t h e river where t h e trees grew.
222
r e s e a r c h t o c l i m a t i c , wood q u a l i t y and e n v i r o n m e n t a l s t u d i e s . The corn-b i n a t i o n of t h e a v a i l a b i l i t y o f good-quality t r e e - r i n g m a t e r i a l t o s t u d y , t h e d e v e l o p m e n t o f new t e c h n i q u e s and t h e e x i s t e n c e of many u s e f u l a p p l i c a t i o n s , makes Canada a n d t h e n o r t h w e s t e r n U n i t e d S t a t e s l o g i c a l r e g i o n s i n which t o conduct t r e e - r i n g r e s e a r c h .
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Bull.,
a n d S m i l e y , T . L . , 1 9 6 8 , An I n t r o d u c t i o n t o Tree-Ring C h i c a g o , U n i v e r s i t y of C h i c a g o P r e s s , 73 p .
Stryd, A.H., 1 9 7 9 , The L i l l o o e t a r c h a e o l o g i c a l p r o j e c t l a b o r a t o r y A statement t o the Social Sciences a n a l y s i s program (Part 2): and Humanities Research Council f o r Canada f o r r e s e a r c h g r a n t #S75-1241. C a r i b o o C o l l e g e , Kamloops, B . C . , p . 21-24.
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DEVELOPMENTAND APPLICATION OF A LICHENOMETRIC DATING CURVE, BROOKS RANGE, ALASKA
PARKER E. CAlKlN and JAMES M. ELLIS
ABSTRACT Lichenometry is a dating techniaue that uses lichens to obtain estimates of relative or absolute ages of rock-bearing substrates. It is based on the general assumptions that: a) stabilization and succeeding colonization of lichen-free rocks occur shortly after deposition and b) subsequent lichen growth occurs with a predictable pattern as a function of time. Species often reach ages of several thousand years, perhaps even to -9000 yr BP. Searches of bouldery Holocene moraines frontinp small cirque glaciers in the arctic and alpine terrain of the central Brooks Range reveal patterns of selected lichen species consistent on the basis of maximum thallus diameters. These Datterns show close Darallelism to ridges marking successive ice-marginal Dositions. They have beenmaDped along with lichen trimline and density data as isophyses or graphed as frequency distributions to yield relative ages and modes of movement for over 50 glaciers. Absolute ages have been derived through develoDment of a lichen growth curve based largely on the commonly used species R h i z o c a r p o n geographicum s . Z . , and the fast-growing A l e c t o r i a m i n u s c u l a / p u b e s c e n s . The R . g e o g r a p h i c u m curve has a "great (rapid) Deriod" of growth -200 years. This part of the curve is controlled indirectly by historic and dendrochronologic data as well as by direct measurements of the A Z e c t o r i a species which grows seven times faster. The subsequent linear (slower) growth phase of R . g e o g r a p h i c u r n approximates 3 mm per century and is calibrated to 1300 yr BP by radiocarbon dates. This l i c h e n factor of " 3 " is comparable to those from southern Alaska, Baffin Island, Swedish Lapland, and the Colorado Front Range. A colonization period of -30 years following substrate stabilization is built into the curve. We have computed surface ice velocity by measuring lichens along 1200 m of a supraglacial boulder train. Lichenometric maps of moraines have been used to reconstruct former glaciers. Lichenometry and radiocarbon analysis of vegetation emerging f r o m beneath a retreating ice margin has provided a chronology of glacial advance and retreatsDanning the last 2500 years. A regional synthesis of data suggests a complex glacial chronology involving seven periods of expansion during the past 4500 y r BP. In addition to glacial studies, we have made considerable use of lichens to indicate substrate stability and to estimate age of rock glaciers as well as other periglacial landforms. INTRODUCTION Purpose The intent of this paper is to review the basic conckpts and proc,edures of the technique of lichenometry in the context of their practical application to dating Holocene glacial features of the central
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Brooks Range. While we cannot cover all of the aspects of lichenometry that may prove useful in individual cases, we hope to provide an introduction for those unfamiliar with lichenometry and/or f o r those whohave yet to apply the technique. For a more comprehensive, single text coverage of lichenometry, we refer readers to Locke e t al. (1979) or to a series of articles in Arctic and Alpine Research, 5(4), 1973. Need in the Brooks Range Lichens have world-wide distribution (Brown e t al., 1976) but they are particularly useful as a Holocene dating tool in polar and alpine regions. Here materials for other methods such as radiocarbon or tree ring dating are sparse, historical records are short or fragmentary, ar.d long-living species particularly useful for dating are widespread on rock surfaces. Such is the case along the Continental Divide of the east-west trending Brooks Range (Figure 1).
Figure 1
L o c a t i o n map s h o w i n g t h e B r o o k s R a n g e and Atigun P a s s f i e l d area.
Our initial studies of Holocene glaciation undertaken in 1977 in the Atigun Pass area of the east-central Brooks Range (Figure 1; Ellis and Calkin, 1979) revealed that high bouldery moraines of the small cirque glaciers bore a lichen cover with selected species surprisingly consistent in maximum thallus diameters along individual ridges. In addition, these lichens were markedly smaller and more consistently sized than those on boulders of the older, Pleistocene, drift sheets extending beyond downvalley. Therefore, we used lichenometry to date these deposits and a variety of associated forms (Calkin and Ellis,
2 29
1 9 8 0 ) . We have e x t e n d e d o u r a p p l i c a t i o n s from t h e A t i g u n pass a r e a i n t o t h e w e s t - c e n t r a l Brooks Range ( E l l i s e t a Z . , 1 9 8 1 ) a n d i n 1 9 8 1 e a s t ward 3 0 0 km t o t h e h i g h g l a c i e r i z e d a r e a s o f t h e e a s t e r n Brooks Range. THE STUDY AREA
Geologic S e t t i n g The s t u d i e s r e f e r r e d t o i n t h i s p a p e r were u n d e r t a k e n p r i n c i p a l l y between e l e v a t i o n s o f 1 0 0 0 m and 2 0 0 0 m f l a n k i n g t h e C o n t i n e n t a l D i v i d e a s i t r i s e s g e n t l y n o r t h e a s t w a r d from t h e s p e c t a c u l a r , g r a n i t e A r r i g e t c h Peaks i n t h e w e s t , t h r o u g h t h e s e d i m e n t a r y t e r r a i n s o f Anaktuvuk Pass and A t i g u n Pass. R e p e a t e d g l a c i a t i o n s i n c e a t l e a s t e a r l y P l e i s t o c e n e time (Hamilton and P o r t e r , 1 9 7 5 ) h a s l e f t a rugged a l p i n e topography w i t h s h a r p p e a k s a n d c o l d i v i d e s t h a t r e a c h 1 0 0 0 t o 1500 m above t h e f l o o r s o f i n t e r v e n i n g U-shaped v a l l e y s . S c a t t e r e d n o r t h - o r i e n t e d c i r q u e g l a c i e r s l e s s t h a n two k i l o m e t e r s l o n g , r e m a i n below p e a k s n o r t h o f t h e Continental Divide. Climate L i c h e n o m e t r y was most s u c c e s s f u l above a n d n o r t h o f t h e s p r u c e t r e e l i n e w i t h i n t h e zone o f c o n t i n u o u s p e r m a f r o s t ( F e r r i a n s , 1 9 6 5 ) . Mean t e m p e r a t u r e s r i s e above f r e e z i n g i n May t h r o u g h S e p t e m b e r (Haugen, 1 9 7 9 ) . I n A t i g u n P a s s , t h e main a r e a o f l i c h e n s t u d y , t h e mean a n n u a l t e m p e r a Annual p r e c i p i t a t i o n a t A t i g u n t u r e a t 1450 m a l t i t u d e i s about - 1 4 O C . Pass r a n g e s b e t w e e n 4 0 0 and 7 0 0 mm o f which a b o u t 5 0 p e r c e n t i s snow ( C a l k i n a n d E l l i s , 1 9 8 0 , F i g u r e 2 ) . A r e a s n e a r or below t h e c i r q u e g l a c i e r s a r e g e n e r a l l y s n o w - f r e e from l a t e J u n e t h r o u g h A u g u s t . Prec i p i t a t i o n may b e o f s i m i l a r m a g n i t u d e t o t h e s o u t h w e s t i n t h e A r r i g e t c h P e a k s a l t h o u g h w i n t e r s n o w f a l l may b e h e a v i e r t h e r e ( E l l i s e t a Z . , 1 9 8 1 ; H a m i l t o n , 1981a).
Figure 2
P h o t o g r a p h s o f m a j o r l i c h e n s u s e d in t h e c e n t r a l Brooks Range s t u d y , i n c l u d i n g a ) AZectoria minuscula; b ) Rhizocarpon geographicurn s . 1 . ; a n d c ) R h i z o c a r p o n eupetraeoides/inarense. See Calkin and Ellis ( 1 9 8 0 , F i g u r e 3 ) f o r v o u c h e r specimens.
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COLLECTION AND O R G A N I Z A T I O N OF LICHEN DATA
Development and B a s i c Assumptions o f L i c h e n o m e t r y The b a s i c d e v e l o p m e n t o f l i c h e n o m e t r y t h r o u g h i t s a c c e p t a n c e as a v a l i d , r e l a t i v e and a b s o l u t e d a t i n g t o o l may b e a s c r i b e d t o Roland B e s c h e l (1950, 1961), a l t h o u g h t h e t e c h n i q u e was r e p o r t e d as e a r l y as 1 9 3 3 ( s e e Locke e t a Z . , 1979). I t s b a s i c c o n c e p t s , l i m i t a t i o n s , and i t s a p p l i c a t i o n t o g l a c i a l and c l i m a t i c s t u d i e s a r e p a r t i c u l a r l y w e l l p r e s e n t e d i n s p e c i a l i z e d s t u d i e s by Andrews a n d Webber (1964), B e n e d i c t (1967, 1968) , Orwin (1970), Worsley (1973), J o c h i m s e n ( 1 9 7 3 ) , and K a r l 6 n (1973). I n A l a s k a , our i n v e s t i g a t i o n s i n t h e Brooks Range ( C a l k i n and E l l i s , 1980) were p r e c e d e d by l i c h e n o m e t r i c work i n t h e A l a s k a Range by Reger and P6w6 (1969) and t h a t i n t h e S t . E l i a s a n d ‘ d r a n g e l l Mountains f a r t h e r s o u t h by Denton and K a r l 6 n (1973). L i c h e n o m e t r y may b e d e f i n e d a s a d a t i n g method t h a t u s e d l i c h e n s t o i n d i c a t e r e l a t i v e and a b s o l u t e a g e s of r o c k - b e a r i n g s u b s t r a t e s . I t i s b a s e d on a s s u m p t i o n s t h a t : a ) r o c k d e b r i s i s f r e e o f l i c h e n s when deposited; b ) colonization occurs s h o r t l y a f t e r surface s t a b i l i z a t i o n ; c ) s u b s e q u e n t l i c h e n g r o w t h ( a s e x p r e s s e d by i n c r e a s e o f w e i g h t , or most commonly d i a m e t e r or a x e s ) o c c u r s w i t h a p r e d i c t a b l e p a t t e r n ; and d) w i t h i n a n a r e a o f s i m i l a r c l i m a t e , g r o w t h i s a f u n c t i o n of t i m e p a s s e d s i n c e c o l o n i z a t i o n ( B e s c h e l , 1 9 6 1 ; Karle‘n 1973). T h e r e f o r e , t h e diameter of t h e l a r g e s t l i c h e n s h o u l d b e p r o p o r t i o n a l t o t h e age o f t h e substrate i n question. The l a t t e r p r e s u p p o s e s t h a t t h e l i f e s p a n o f t h e s p e c i e s measured i s g r e a t e r t h a n t h e age o f t h e s u b s t r a t e s u r f a c e . B e s c h e l (1957) n o t e d t h a t t h e a c t u a l g r o w t h o f l i c h e n s p r o c e e d s i n s m a l l s t e p s . Annual i n c r e a s e s i n t h a l l u s s i z e o f slow-growing s p e c i e s a r e s u b j e c t t o f l u c t u a t i o n s w h i c h a r e smoothed o u t o v e r p e r i o d s o f s e v e r a l y e a r s . Furthermore, t h e frequency d i s t r i b u t i o n of l i c h e n s i z e c l a s s e s on any one s u r f a c e shows a r e g u l a r r e l a t i o n s h i p ; r e c e n t work s u g g e s t s t h i s may b e l o g n o r m a l (Anderson and S o l l i d , 1971). L i c h e n S p e c i e s and Problems o f I d e n t i f i c a t i o n L i c h e n s c o n s i s t o f a l g a e and f u n g i g r o w i n g i n c l o s e a s s o c i a t i o n . ? h o s e s p e c i e s u s e d i n l i c h e n o m e t r y h a v e a p l a n t body ( t h a l l u s ) t h a t a t t a c h e s i t s e l f t o r o c k s u b s t r a t e s and t e n d s toward a n e a r - c i r c u l a r g r o w t h p a t t e r n t h a t c a n b e r e a d i l y m e a s u r e d . A t l e a s t 35 s p e c i e s or s u b s p e c i e s h a v e b e e n u s e d t o d a t e (Locke e t a Z . , 1979). D e t a i l s o f morphology a n d h e l p i n f i e l d i d e n t i f i c a t i o n o f t h e s e may b e o b t a i n e d from v a r i o u s m a n u a l s ( H a l e , 1 9 6 9 ) . However, f o r t h e a v e r a g e f i e l d w o r k e r i t i s d e s i r a b l e t o follow p r e l i m i n a r y f i e l d i d e n t i f i c a t i o n by c o l l e c t i n g v o u c h e r s p e c i m e n s . These can b e v e r i f i e d u n d e r l a b o r a t o r y c o n d i t i o n s by a l i c h e n o l o g i s t . We h a v e m e a s u r e d s i x l i c h e n s p e c i e s i n our s t u d i e s i n t h e Brooks Range ( s e e C a l k i n and E l l i s , 1980 f o r a m p l i f i c a t i o n ) . A l l a r e wides p r e a d i n t h i s a r e a on s i l i c e o u s e l a s t i c , c r y s t a l l i n e , and t o u g h metas e d i m e n t s u b s t r a t e s . The f i r s t f o u r , a s i d e n t i f i e d b e l o w , were among s p e c i e s s t u d i e d on B a f f i n I s l a n d s i n c e 1 9 5 0 . ( A n d r e w s and Webber, 1964; 1 9 6 9 ; M i l l e r and Andrews, 1972; G . H . M i l l e r , 1973; Andrews and B a r n e t t ,
1979).
A l e c t o r i a rninuscula ( F i g u r e 2 a ) i s a b l a c k f i b r o u s l i c h e n u s e d i n Specimens conjunction w i t h t h e similar f r u t i o o s e AZectoria pubescens. r e a c h d i a m e t e r s g r e a t e r t h a n 1 4 0 mrn; t h e s e o f t e n a r e d e t e r i o r a t e d i n t h e c e n t e r , perhaps i n d i c a t i n g t h e o n s e t of senescence (end of l i f e ) . Urnbilicaria proboscidea i s a dark gray t o b l a c k f o l i o s e l i c h e n with d e l i c a t e l e a v e s r a d i a t i n g from a c e n t r a l s t r u c t u r e which i s a t t a c h e d t o t h e s u b s t r a t e . These t h r e e s p e c i e s a r e f a s t g r o w i n g , h a v e s i m i l a r g r o w t h r a t e s , and a r e common on f r e s h d e b r i s s u r f a c e s . We h a v e u s e d t h r e e y e l l o w - g r e e n s p e c i e s o f t h e c r u s t o s e g e n e r a R h i z o c a r p o n , one o f t h e most a b u n d a n t l y r e p r e s e n t e d l i c h e n g e n e r a i n t h e A r c t i c (Thomson, 1967, 1979). These a r e a l l slow-growing s p e c i e s and i n c l u d e R . geographicurn ( F i g u r e 2 b ) , t h e s p e c i e s most commonly r e p o r t e d
231
in lichenometric studies. This is a rather variable species which is described by many different names and even subdivided as a group (Thomson, 1967, 1979). Difficulty in field identification of these variants has led us and other field workers to use the species name in the general sense as R h i z o c a r p o n geographicurn s e n s u Z a t o ( s . 2 . ) . R. s u p e r f i c i a l e is very similar in appearance and may have been included in some o f our measurements (Calkin and Ellis, 1980, Figure 3c). R h i z o c a r p o n e u p e t r a e o i d e s (Nyl.) Blomberg and Forssell and R . i n a r e n s e (Vainio) v a i n i o are particularly noticeable on late Pleistocene and early Holocene substrates in the central Brooks Range. They may only be clearly distinguished from each other by d. chemical iodine test on the medulla (interwoven fungal thread between the algal layer) (Thomson, 1967, p. 441). Both belong to the R h i z o c a r p o n group Alpicola (Runem.) which include R. a l p i c o l a found in Europe and Asia and used extensively in lichenometric studies in Scandinavia by Denton and Karlen (1973, Figure 7). R . e u p e t r a e o i d e s / i n a r e n s e (Figure 2c) displays more vivid yellow-green aureoles (tiny button-like reproductive parts at the thallus surface) than R . geographicurn (Figure 2b).
There are difficulties and frustrations in the field identification of the various species of R h i z o c a r p o n . For example, we found them difficult to distinguish when less than 5 or 6 mm in diameter. In addition, R . geographicurn lichens were difficult to distinguish, if present at all, on surfaces where thalli resembling R . e u p e t r a e o i d e s / i n a r e n s e in the 2 0 to 40 mm size range were common. Our references to R . geographicurn may generally be applied to R . e u p e t r a e o i d e s / i n a r e n s e for thallus diameters to 150 mm. Above 150 mm, R . geographicurn thalli were often found to coalesce or disintegrate, while the R . e u p e t r a e o i d e s / i n a r e n s e thalli were more circular and distinct. We were able tomeasure distinct R . geographicurn thalli to 250 mm and R . e u p e t r a e o i d e s / i n a r e n s e to 450 mm. These maxima are very similar respectively to thosemeasured for R . geographicurn on Baffin Island (Andrews and Barnett, 1979) and for R. a l p i c o l a in Swedish Lapland (Denton and Karlgn, 1973) where they are believed to reach ages on the order of 8,000 to 9,000 years. Measurement and Sampling General For the purposes of this study, we followed the experience of Webber (Andrews and Webber, 1964; personal communication, 1977), Benedict (1968) and Karlgn (1973) who adapted the fundamental assumption of Beschel (1950). That is, only the maximum diameter (longest axis) of the largest lichen thallus is an indicator of substrate age, because it is assumed to be the oldest and to represent the optimum growth rate for the site studies. Some variants of this procedure are common in the literature, F o r example, Locke e t a Z . (1979) advocate measurement of the shortest axis (largest inscribed circle) of the largest lichens. Some workers have used dry weight, particularly when measurement was more difficult in the case of some foliose or fruticose species. The measurement of lichen area is tedious, but this additional technique is particularly useful when recording increases in thallus size during direct measurements of lichen growth rate. Many studies now average a group o f the largest lichens, for example, the largest five (Karlgn, 1979). Size-frequencv measurements (Benedict, 1967; Lindsay, 1973) take considerable time but are desirable when surface age is a goal and helz, to reveal "the effects of climatic change, substrate disturbance, lichen senility, and comDetition" (Locke e t a Z . , 1979, D. 14) as well as to exclude anomalous-sized thalli. We excluded anomalous thalli by discounting those where maximum diameters were about 20 percent or more above the next largest diameter orcluster of diameters. Only lichens with distinct thallus margins were used, and of these, only those slightly elliptical to circular-shaped. We recorded the minimumaswell as maximum diameters to the nearest 1 mm along with the lichaen species, orientation, and its substrate inclination and lithology.
232
D u r i n g o u r l i c h e n o m e t r i c mapping we a p p o r t i o n e d o u r t i m e t o c o v e r t h e a r e a as t h o r o u g h l y a s p o s s i b l e , ' l o o k i n g a t b o u l d e r s and c o b b l e s f o r t h e l a r g e s t m e a s u r a b l e l i c h e n . Measurements o f t h e 1 0 t o 15 l a r g e s t l i c h e n s were r e c o r d e d i n s m a l l a r e a s o f 5 t o 1 0 in2 or c o n t i n u o u s l y a l o n g l i n e a r landforms. V a l i d r e s u l t s f o r d a t i n g of t h e N e o g l a c i a l moraines u s u a l l y r e q u i r e d mapping a r e a s i n e x c e s s o f 500 m 2 K a r l 6 n ( 1 9 7 3 ) . Fixed area s e a r c h e s , s h o r t s e l e c t i v e t r a v e r s e s a c r o s s t h e t e r r a i n , o r t i m e l i m i t s on s a m p l i n g ( B i r k e l a n d , 1973; Rampton. 1 9 7 0 ) h a v e b e e n u s e d b y o t h e r w o r k e r s t o p r o v i d e d a t a for s t a t i s t i c a l a n a l y s i s . Some E n v i r o n m e n t a l C o n s i d e r a t i o n s E n v i r o n m e n t a l f a c t o r s p l a y a p a r t i n s a m p l i n g t e c h n i q u e s and t h e i r r e l a t i v e i m p o r t a n c e i s v a r i a b l e from r e g i o n t o r e g i o n . The a v a i l a b i l i t y o f s u n l i g h t , m o i s t u r e , and s t a b i l i t y o f t h e s u b s t r a t u m w e r e c r i t i c a l f a c t o r s i n t h e c e n t r a l Alaska Range, 5 0 0 k m t o t h e s o u t h o f t h e A t i g u n P a s s ( R e g e r a n d P6w6, 1 9 6 9 ) . Rock s u r f a c e s b e a r i n g t h e l a r g e s t l i c h e n t h a l l i i n t h i s a r e a d i s p l a y a dominant s o u t h w e s t o r i e n t a t i o n ; however, i n t h e F r o n t Range of C o l o r a d o ( B i r k e l a n d , 1 9 7 3 ) or t h e S w i s s Alps ( 3 a e b e r l i e t aZ., 1 9 7 9 ) Rhizocarpon g e o g ra p h ic u rn d o e s n o t o c c u r on s o u t h - f a c i n g s u r f a c e s . We f o u n d no s t r o n g r e l a t i o n s h i p s b e t w e e n l i c h e n g r o w t h and o r i e n t a t i o n or h e i g h t above t h e m o i s t g r o u n d s u r f a c e i n o u r Brooks Range s t u d i e s . On m o i s t e r s l o p e s a n d o v e r l o c a l a r e a s o f abunda n t m o i s t u r e , such as t h e A r r i g e t c h Peaks, r e d - c o l o r e d T r e n t e p h o Z i a i o Z i t h u s a l g a e w e r e u b i q u i t o u s on new r o c k s u r f a c e s . T h i s a p p e a r e d t o i n h i b i t l i c h e n c o v e r as i t p r e c e d e s l i c h e n c o l o n i z a t i o n ( D . Cooper, p e r s o n a l communication, 1 9 8 0 ) . S u b s t r a t e s t a b i l i t y s e v e r e l y c o n t r o l s l i c h e n g r o w t h and c r e e p or t u m b l i n g of b o u l d e r y r u b b l e i s a p e r s i s t e n t p r o b l e m i n t h e Brooks Range a s i n o t h e r a l p i n e a r e a s a n d i s commonly r e l a t e d t o m e l t i n g o f subs u r f a c e i c e m a s s e s ; t h i s i n t u r n , i s more p r e v a l e n t i n t h e t o p o g r a p h i c a l l y - s h a d e d a r e a s s u c h as t h e s t e e p g r a n i t e - w a l l e d c i r q u e s o f t h e A r r i g e t c h P e a k s ( E l l i s e t a Z . , 1 9 8 1 ) t h a n i n more open a r e a s . While i t was i m p o s s i b l e t o c o m p l e t e l y a v o i d u n s t a b l e d e p o s i t s i n s u c h a r e a s , w e were c a r e f u l t o make l i c h e n o m e t r i c c o m p a r i s o n s b e t w e e n a r e a s or forms o f maximum s t a b i l i t y . Moraine r i d g e c r e s t s b o r e t h e most c o n s i s t e n t l i c h e n d i a m e t e r s on s t a b l e s u r f a c e s ( E l l i s a n d C a l k i n , 1 9 7 9 ) . On u n s t a b l e s u b s t r a t e s , t h e l o w e r a r e a s y i e l d e d t h e more r e l i a b l e l i c h e n o m e t r i c e v i d e n c e . However, a t t h e same t i m e , s p e c i a l e f f o r t s w e r e t a k e n t o s a m p l e w e l l - d r a i n e d t o p o g r a p h i c a r e a s where snow d i d n o t l i n g e r . S e v e r a l r e c e n t w o r k e r s ( 9 i l l i n g s , 1974; F l o c k , 1 9 7 8 ; H a e b e r l i e t aZ., 1 9 7 9 ; K o e r n e r , 1 9 8 0 ) h a v e e m p h a s i z e d t h a t l i c h e n g r o w t h i s s t r o n g l y i n h i b i t e d by l o n g e r l a s t i n g , b u t s t i l l a n n u a l snow f i e l d s . T h e s e o f t e n p r o d u c e s h a r p t r i m l i n e s . V a r i a t i o n s i n s u b s t r a t e l i t h o l o g y a f f e c t l i c h e n growth b u t d e t a i l s
o f t h i s c o n t r o l a r e n o t c l e a r l y u n d e r s t o o d . T h e r e f o r e we and o t h e r s h a v e a t t e m p t e d t o m i n i m i z e t h e s e unknowns b y m e a s u r i n g l i c h e n s on r o c k s of s i m i l a r composition. L i t h o l o g y i s a f i r s t - o r d e r c o n t r o l of topography a n d b e c a u s e o u r s t u d i e s were p r i n c i p a l l y c o n c e r n e d w i t h t h e h i g h r e s i s t a n t u p l a n d s , we w e r e a b l e t o work w i t h q u i t e u n i f o r m a n d r e s i s t a n t
r o c k s u b s t r a t e s . T h e s e c o n s i s t e d of r e s i s t a n t s i l i c e o u s c o n g l o m e r a t e s , and s a n d s t o n e s o r q u a r t z i t e s t h r o u g h o u t most o f t h e c e n t r a l Brooks Range and u n i f o r m g r a n i t i c r o c k s i n t h e A r r i g e t c h P e a k s . Our measurements show no c l e a r r e l a t i o n s h i p o f g r o w t h w i t h s i l i c a c o n t e n t . The m a j o r l i m i t a t i o n of l i c h e n growth w i t h t h e s e r o c k s i s t h e i r r e s i s t a n c e t o w e a t h e r i n g . F o r e x a m p l e , R . g e o g ra p h ic u rn o n l y r a r e l y r e a c h e d d i a m e t e r s o f g r e a t e r t h a n 2 5 mm on e a s i l y - w e a t h e r e d s h a l e s and p h y l l i t e s . The s p e c i e s u s e d i n t h i s s t u d y do n o t grow on l i m e s t o n e s and t h i s must b e t a k e n i n t o a c c o u n t when p l a n n i n g l i c h e n o m e t r i c s t u d i e s (Osborne and T a y l o r , 1 9 7 5 ) . Below o u t c r o p s o f l i m e s t o n e w e s e a r c h e d f o r t h e s i l i c e o u s e x o t i c s t o n e s or m e a s u r e d l i c h e n t h a l l i on c h e r t n o d u l e s w h i c h occurred within the limestones.
233
I n t e r s p e c i f i c Ratios We m e a s u r e d t h e r a t i o s o f t h a l l u s d i a m e t e r s for a r a n g e of s i z e s among four o f t h e l i c h e n s p e c i e s u t i l i z e d i n t h i s s t u d y . Such r a t i o s a r e i n v a l u a b l e f o r t r a n s f e r of p o t e n t i a l d i a m e t e r s and a g e r e l a t i o n s h i p s from one s p e c i e s t o a n o t h e r . I n our s t u d y i t has a l l o w e d c o m p a r i s o n t o work on B a f f i n I s l a n d (Andrews a n d Webber, 1 9 6 4 ; 1 9 6 9 ; M i l l e r and Andrews, 1 9 7 2 ; G . H . M i l l e r , 1 9 7 3 ) . F u r t h e r m o r e , a f i e l d c o n c e p t of t h e s i z e r a t i o s b e t w e e n t h e s p e c i e s p r o v i d e d a handy t o o l t o check f o r anomalous t h a l l u s d i a m e t e r m e a s u r e m e n t s . Only t h e maximum d i a m e t e r l i c h e n s c h a r a c t e r i s t i c of t h e a g e o f a l a n d f o r m were u s e d i n our r a t i o determinations. The s i z e r a t i o o f A . rninuscula t o A . p u b e s c e n s was 1 . 0 4 and A . r n i n u s c u l a was 1 . 0 6 ; t h e r e f o r e , we u s e d t h e s e s p e c i e s i n t e r c h a n g e a b l y i n l i c h e n o m e t r i c d a t i n g . The r a t i o o f A . r n i n u s c u l a / p u b e s c e n s t o R . g e o graphicurn was i n t h e r a n g e of 6 . 5 t o 7 . 0 ( F i g u r e 3 ; C a l k i n and Ellis, 1 9 8 0 , T a b l e 1). The r a t i o s a r e s i m i l a r t o t h o s e a t t a i n e d on B a f f i n
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Sample of growth r a t e r e l a t i o n s h i p between
A l e c t o r i a rninuscula a n d R h i z o c a r p o n g e o graphicurn s . 2 . T a k e n from m e a s u r e m e n t s a t f i v e g l a c i e r s n e a r A t i g u n Pass.
234
I s l a n d (Locke e t al., 1 9 7 9 , T a b l e 2). However, d a t a of Andrews and Xebber ( 1 9 6 9 ) , M i l l e r and Andrews ( 1 9 7 2 ) and G . H . M i l l e r ( 1 9 7 3 , F i g u r e 2) who u s e d d i r e c t m e a s u r i n g methods h a v e shown t h a t t h e i n t e r s p e c i f i c r a t i o o f A . m i n u s c u l a t o R . geographicurn c h a n g e s c o n t i n u o u s l y w i t h t i m e . A frequency curve of such r a t i o s d i s p l a y s a broad, although symmetrical p a t t e r n ( s e e Locke e t al., 1 9 7 9 , F i g u r e 3 ) . T a b l e 1.
Sample of d i r e c t growth measurement d a t a f o r
A l e c t o r i a minuscula a n d A l e c t o r i a pubescens e x t r a p o l a t e d t o R h i z o c a r p o n geographicurn.
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minuscuZa a n d A . p u b e s cen s
Maximum d i a m e t e r s (mm) 1977
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Planimetricallydetermined diameter change (mm/yr)a 1977 - 1979
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( a r e a / v ) % f r o m t r a c i n g s made i n e a r l y A u g u s t of
1977
bAn i n t e r s p e c i f i c r a t i o o f 7 : 1 f o r A . r n i n u s c u l a / p u b e s c e n s : R . geographicurn i s assumed ( s e e C a l k i n and E l l i s , 1980, T a b l e s 1 and 2 ) .
One p r o b l e m i n making o u r m e a s u r e m e n t s was t h e p o t e n t i a l d i f f e r e n c e i n c o l o n i z a t i o n p e r i o d b e t w e e n s p e c i e s , Our s t u d i e s s u g g e s t t h a t R . geographicurn a n d A . rninuscula c a n c o l o n i z e or become v i s i b l e t o t h e una i d e d e y e on s u r f a c e s c o n t e m p o r a n e o u s l y a l o n g w i t h A . p u b e s c e n s and U r n b i Z i c a r i a p r o b o s c i d e a ; t h i s i s i n c o n t r a s t t o r e s u l t s of work by Andrews and Webber ( 1 9 6 4 ) , H a l e ( 1 9 6 7 ) , and G . H . M i l l e r ( 1 9 7 3 ) . RELATIVE D A T I N G FROM L I C H E N MEASUREMENTS
L i c h e n s may b e u s e d t o p r o v i d e r e l a t i v e a g e s e i t h e r by d i r e c t como a r i s o n o f t h a l l u s d i a m e t e r s or by p e r c e n t a g e of l i c h e n c o v e r . Our measurements o f t h e p e r c e n t a g e o f l i c h e n c o v e r i n t h e c e n t r a l Brooks Range h a v e s o f a r y i e l d e d o n l y v e r y c r u d e r e l a t i v e a g e d a t a . Some exs n p l e s o f t h i s u s e a r e g i v e n i n F i g u r e s 4 and 5 . The v a r i a t i o n o f lichen-cover percentage i s not only t i m e dependent b u t i s a f f e c t e d i n a l p i n e g l a c i a t e d a r e a s s u c h a s t h e Brooks Range by a v a r y i n g d e g r e e o f r e s t r i c t i o n o f e a c h o f t h e v a r i o u s f a s t - or slow-growing s p e c i e s t o c e r t a i n f a v o r a b l e s i t e s . These i n t u r n , v a r y i r r e g u l a r l y i n s p a c e and t i m e b e c a u s e o f t o p o g r a p h y , snow c o v e r , b o u l d e r s i z e , and i n s t a b i l i t y d e r i v e d from i c e - c o r e s . F r e q u e n t l y t h e d e g r e e o f c o v e r can b e 2nd or 3rd degree recycling. T h e r e f o r e , a g e a l o n e , was d i f f i c u l t t o d e r i v e i n t h e s h o r t mapping p e r i o d a v a i l a b l e for e a c h d e p o s i t . Given t h e f i e l d t i m e , t h e s e p r o b l e m s m i g h t b e a p p r o a c h e d i n t h e manner o f H a e b e r l i e t a l . ( 1 9 7 9 ) . They a p p l i e d l i c h e n c o v e r s t u d i e s t o a n a c t i v e r o c k g l a c i e r a f t e r f i r s t mapping t h e e c o l o g i c a l v a r i a b l e s c o n t r o l l i n g l i c h e n d i s t r i b u t i o n . B e n e d i c t ( 1 9 6 7 , 1 9 6 8 ) as w e l l a s B i r k e l a n d ( 1 9 7 3 ) h a v e b e e n more s u c c e s s f u l i n u s i n g t h e p e r c e n t a g e of t o t a l l i c h e n c o v e r i n ' J o l o r a d o where i n s t a b i l i t y and p o s s i b l y e c o l o g i c a l c h a n g e s a r e l e s s o f zi p r o b l e m . B i r k e l a n d ( 1 9 7 3 ; Burke and B i r k e l a n d , 1 9 7 9 ) u s e d a n e s t i m a t e of t h e p e r c e n t a g e of l i c h e n c o v e r on 50 b o u l d e r s i n t h e M t . S o p r i s a r e a
235
as a mapping t e c h n i q u e , b u t f o u n d t h a t t h e mean c o v e r for t h e s e h a d a l i n e a r r e l a t i o n s h i p w i t h t h e maximum c o v e r on one b o u l d e r , a much more r a p i d o b s e r v a t i o n t o make.
Figure 4
Photographs showing t y p i c a l s u b s t r a t e l i c h e n c o v e r a t H a r l e q u i n Duck G l a c i e r w i t h a ) E a r l y Holocene-aged rock g l a c i e r s e c t i o n near terminus; and b ) upper p o r t i p n of d e b r i s l o b e i n t r a n s i t i o n zone where moraines are -400 l i c h e n y e a r s o l d . Note t h a t w h i l e t h e o v e r a l l l i c h e n cover i s n e a r l y s i m i l a r i n t h e two a r e a s , t h e p e r c e n t of c r u s t o s e ( l i g h t e r tone) lichen i s g r e a t e r on the older surf a c e a n d w e a t h e r i n g a n d s o i l d e v e l o p m e n t much f u r t h e r advanced. See F i g u r e 5 f o r l o c a t i o n s and further d e t a i l of lichen cover.
G E N E R A T I O N OF A L I C H E N GROWTH CURVE
Introduction I n o r d e r t o a s s i g n a b s o l u t e a g e s t o s u r f a c e s on which l i c h e n t h a l l i s i z e s a r e r e c o r d e d , i t i s n e c e s s a r y t o c o n s t r u c t growth c u r v e s f o r t h e major l i c h e n s p e c i e s used i n t h e r e g i o n . T h i s i s n o t always a simple t a s k b e c a u s e t h e e n v i r o n m e n t a l f a c t o r s which i n f l u e n c e growth r a t e o f a s p e c i e s may h a v e c h a n g e d somewhat t h r o u g h t h e l i c h e n a g e . Even u n d e r c o n s t a n t e n v i r o n m e n t a l c o n d i t i o n s , t h e growth r a t e v a r i e s o v e r t h e l i c h e n l i f e s p a n ( B e n e d i c t , 1 9 6 7 ) . I n i t i a l g r o w t h of t h e s p e c i e s u s e d i n t h i s a n d most s i m i l a r s t u d i e s i s r a p i d . For a c o n t r a s t i n g t r e n d s e e Armstrong ( 1 9 7 6 ) . T h i s " g r e a t p e r i o d " o f g r o w t h ( B e s c h e l , 1 9 5 0 ) l a s t s from 40 t o 5 0 0 y r d u r i n g which r a t e s d e c e l e r a t e u n t i l a g e n e r a l l y l i n e a r , long-term p e r i o d of growth i s a t t a i n e d ( F i g u r e 6 ) . I n t h e c a s e o f R h i z o c a r p o n g eo g r a p h i cu r n or r e l a t e d s p e c i e s , t h i s may e x t e n d o v e r a l m o s t 9 , 0 0 0 y r s . ( D e n t o n a n d K a r l g n , 1 9 7 3 ; Andrews and B a r n e t t , 1 9 7 9 ) . L i c h e n f a c t o r s ( g r o w t h r a t e s e x p r e s s e d i n mm/100 y r ) c o m p i l e d from
29 R . g e o g r a p h i c u r n c u r v e s b y Webber and Andrews ( 1 9 7 3 ; Locke e t a Z . ,
1 9 7 9 ) show t h a t e n v i r o n m e n t a l f a c t o r s o f p r e c i p i t a t i o n , g r o w i n g s e a s o n , and h i g h t e m p e r a t u r e v a r y d i r e c t l y w i t h l i c h e n g r o w t h r a t e d u r i n g t h e g r e a t g r o w t h p e r i o d ( P o r t e r , 1 9 8 1 ) . B e s c h e l ( 1 9 6 1 ) a n d Ten B r i n k ( 1 9 7 3 ) made t h i s p a r t i c u l a r l y c l e a r by d i r e c t m e a s u r e m e n t s o v e r a 1 2 - y e a r p e r i o d i n west G r e e n l a n d when g r o w t h r a t e s w e r e " h i g h l y s e n s i t i v e t o
236
60-70% lichen cover 45% crustose cover
75% R.geogrophicurn cover
W-70% lichen cover
20-30% lichen cover
50% crustose cover Lower L i m i t of
Transition Zone
10% lichen cover
15% crustose cover
10% black lichen 2% crustose cover
maximum diameter
Figure 5
L i c h e n o m e t r i c map o f g l a c i e r - c o r e d d e p o s i t s o f H a r l e q u i n Duck G l a c i e r ( F i g u r e 1 ) . I n t h e n e a r g l a c i e r area of s t a b l e , Neoglacial moraines ( c a l l e d t h e " t r a n s i t i o n z o n e " a n d shown by d i a g o n a l p a t t e r n ) i s o p h y s e s c o n n e c t p o i n t s of e q u a l lichen diameter. The p r e d o m i n a n t l y , g r a v i t y mobilized rock d e p o s i t s downslope of t h e t r a n s i t i o n z o n e , show i r r e g u l a r l i c h e n a g e s . Crustose lichen w h i c h u s u a l l y i n c l u d e s R . geographicurn, i s d i s t i n g u i s h e d f r o m g e n e r a l l i c h e n c o v e r w h i c h may i n c l u d e many f a s t - g r o w i n g f r u t i c o s e o r f o l i o s e t a x a . L o c a t i o n s o f F i g u r e 4a a n d 4b a r e shown.
l o c a l c l i m a t e a n d i n v e r s e l y p r o p o r t i o n a l - t o c o n t i n e n t a l i t y " (Ten B r i n k , 1973, p . 3 2 3 ) . S i m i l a r l y , G.H. M i l l e r ( 1 9 7 3 ) f o u n d e l e v a t i o n a l l y . ' > e l a t e d c l i m a t i c c o n t r o l i m p o r t a n t t o g r o w t h on e a s t e r n B a f f i n I s l a n d . C l i m a t i c d i f f e r e n c e s may h a v e l e s s e f f e c t on t h e l o n g - t e r m ( l i n e a r ? ) g r o w t h r a t e of R . g e o g r a p h i c u r n a l t h o u g h t i g h t e n i n g o f t h e a b s o l u t e C o n t r o l for t h i s p a r t o f t h e c u r v e may e v e n t u a l l y p r o v e o t h e r w i s e . C u r v e s d e r i v e d f r o m B a f f i n I s l a n d ( M i l l e r and Andrews, 1972), t h e C o l o r a d o F r o n t Range ( B e n e d i c t , 1 9 6 7 ) , s o u t h e a s t e r n A l a s k a (Denton and K a r l g n , 1 9 7 3 ) a n d t h e Brooks Range ( C a l k i n and E l l i s , 1 9 8 0 a n d t h i s p a p e r ) a l l y i e l d g r o w t h r a t e s ( l i c h e n f a c t o r s ) of a p p r o x i m a t e l y 3 m m / 1 0 0 y r f o r i n t e r v a l s of o v e r 1 0 0 0 y r ( F i g u r e 6 ) .
231
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Long-term growth
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1
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Radiocarbon Years Since Substrate Stabilization Figure 6
Growth c u r v e s f o r Rhizocarpon geograpkicum/aZpicoZa (A. D e n t o n a n d K a r l g n , 1 9 7 3 ) a n d R h i z o c a r p o n geograpkicum (B. Luckman, 1 9 7 7 ; C . D e n t o n a n d K a r l g n , 1973; D . B e n e d i c t , 1967; and G. Miller and Andrews, 1972). In general decreasing precipitation, temperature, and l e n g t h of growing season r e s u l t A f t e r C a l k i n and i n a slower e a r l y growth rate. E l l i s , 1 9 8 0 r.
D i r e c t Measurements o f Growth R a t e The g r o w t h r a t e s o f A Z e c t o r i a m i n u s c u l a a n d A . p u b e s c e n s b e t w e e n t h e d i a m e t e r s o f 30 mm and 72 mm h a v e b e e n m e a s u r e d d i r e c t l y i n t h e A t i g u n Pass a r e a b y b o t h d i r e c t t r a c i n g ( H a l e , 1 9 5 9 ) a n d a m o r e a c c u r a t e p h o t o g r a p h i c method o f Hooker and Brown, ( 1 9 7 7 ) ( T a b l e 1). I n t h e l a t t e r method, we p h o t o g r a p h e d t h e t h a l l i from a p e r p e n d i c u l a r p o s i t i o n u s i n g a 35 mm camera w i t h a s t a n d a r d 50 mm l e n s . A c o i n s e r v e d a s s c a l e . G . H . M i l l e r ( 1 9 7 3 ) and Locke e t aZ. ( 1 9 7 9 ) h a v e made d e t a i l e d r e c o m m e n d a t i o n s f o r measurement t h a t w i l l a l l o w f o r more u n i f o r m f u t u r e d u p l i c a t i o n b y o t h e r w o r k e r s . A r e a s i n b o t h e n l a r g e d t r a c i n g s and p h o t o g r a p h i c p r i n t s were d e t e r m i n e d b y p l a n i m e t e r . These i n t u r n were c o n v e r t e d t o t h e o r e t i c a l d i a m e t e r s ( 2 (area/* )%) assuming c i r c u l a r t h a l l i t o d e t e r m i n e growth between times o f f i e l d r e c o r d i n g s . Measurements of l i c h e n t h a l l i were made on b o u l d e r y N e o g l a c i a l d r i f t a s s o c i a t e d w i t h G r i z z l y G l a c i e r (1680 m ) and B u f f a l o G l a c i e r (1615 m ) ( F i g u r e 1) o v e r a c o m p o s i t e p e r i o d o f t h r e e g r o w i n g s e a s o n s (19777 9 ) . R e c o r d s a n d m e a s u r e m e n t s were made on w e l l - d r a i n e d , r e p r e s e n t a t i v e s i t e s , and d u r i n g t i m e s when t h e l i c h e n s w e r e d r y t o m i n i m i z e s w e l l i n g o r s h r i n k a g e ( G . H . M i l l e r , 1 9 7 3 ) . These d a t a ( C a l k i n and E l l r s , 1 9 8 0 , T a b l e 1) w e r e u s e d t o h e l p c o n s t r u c t a n A Z e c t o r i a m i n u s c u l a / p u b e s c e n s growth c u r v e and t h e n e x t r a p o l a t e d t o t h e i n d i r e c t l y - c o n t r o l l e d growth c u r v e o f t h e slow-growing R h i z o c a r p o n g e o g r a p h i c u m by u s i n g t h e i n t e r s p e c i f i c r a t i o o f 7 : l ( F i g u r e 7 ) . A s d i a m e t e r s o f R . geographicurn i n c r e a s e d f r o m 5 . 0 t o 1 0 . 1 mm. t h e g r o w t h r a t e d e c r e a s e d from a b o u t 25 m m / l O O y r t o 1 4 mm/100 y r . The r a n g e i n g r o w t h r a t e s o f t h e two A Z e c t o r i a s p e c i e s r a n g e d between 1 and 1 . 9 mm/yr i n t h e A t i g u n Pass a r e a ; t h i s i s on t h e same o r d e r of m a g n i t u d e as r a t e s d e t e r m i n e d b y G . H . M i l l e r ( 1 9 7 3 , T a b l e 2 ) on B a f f i n I s l a n d . The v a r i a b i l i t y i n r a d i a l g r o w t h r a t e s b e t w e e n some f a s t - g r o w i n g , f o l i o s e l i c h e n s o f e q u a l s i z e may b e h i g h , e v e n u n d e r s t a n d a r d g r o w t h c o n d i t i o n s ( A r m s t r o n g , 1 9 7 6 ) . However, e x p e r i e n c e and t h e l i t e r a t u r e s u g g e s t t h a t e n v i r o n m e n t a l f a c t o r s on v a r i o u s s c a l e s form t h e m a j o r l i m i t a t i o n s o f l i c h e n c u r v e s g e n e r a t e d b y d i r e c t measurement. The c l i m a t i c c o n d i t i o n s i n c l u d i n g t h o s e o f a n n u a l snowcover u n d e r which l i c h e n g r o w t h a r e m e a s u r e d , may b e a t y p i c a l of t h o s e c o n t r o l l i n g l o n g term g r o w t h . I n a d d i t i o n , d i r e c t measurements a r e u s u a l l y made on a
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Lindo Creek (0) Arrigetch Peaks (HI Lindo Creek (HI 4 Linda Creek ( H I 5 Anigonigurok (HI 6 Dietrich River (D) 7 MS-112-2 ( R ) 8 Buffolo Glacier ( R ) 9 Dietrich River (D) 10 M S - I I O 2 ( R ) A l s c t o r l p direct O measurement points I 2 3
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Figure 7
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5000
R h i z o c a r p o n geographicurn a n d A Z e c t o r i a rninuscuZa/ pubscens growth c u r v e s f o r t h e c e n t r a l Brooks Range. Data o n A . rninuscuZa/pubscens c u r v e show d i r e c t measurements.
C o n t r o l p o i n t s 1-10 f o r R .
geographicurn a r e i n d i c a t e d a s d e n d r o c h r o n o l o g i c ( D ) ,
h i s t o r i c ( H ) , o r r a d i o c a r b o n (R) a n d a r e l o c a t e d on F i g u r e 1 w i t h e l e v a t i o n s . The c u r v e h a s n o t b e e n a d j u s t e d s i n c e p l a c e m e n t o f A t i g u n Camp r a d i o c a r b o n p o i n t 11 o f 1 3 0 0 y r / 5 0 m m . Shaded zone i n d i c a t e s q u a l i t a t i v e *20 p e r c e n t a g e r e l i a b i l i t y . Curve i s n o t f i x e d t o any g i v e n y e a r ; t o c o n v e r t 1 4 C g i v e n o n a b s c i s s a ts I 4 C y r BP ( b e f o r e A D 1 9 5 0 ) , s u b t r a c t AD 1 9 5 0 f r o m c u r r e n t AD d a t e a n d d e d u c t t h i s v a l u e from g r a p h ' s 14C age. Frequency h i s t o gram a t r i g h t shows number o f m o r a i n e s c h a r a c t e r i z e d b y R . geographicurn o f i n d i c a t e d maximum t h a l l u s diameter. The g r a p h s u g g e s t 7 t o 8 c l u s t e r s o f m o r a i n e s t h a t may mark w i t h d r a w a l f r o m a d v a n c e d g l a c i a l positions (glacial advances?).
v a . r i e t y o f l i c h e n t h a l l i t h a t have lower a v e r a g e growth r a t e s t h a n t h a t l s r g e s t l i c h e n u s e d f o r d a t i n g a n d p e r h a p s a p r o d u c t of a more f a v o r a b l e g r o w t h e n v i r o n m e n t . T h e r e f o r e , t h e c u r v e y i e l d s e s t i m a t e s of t h e maximum s d r f a c e a g e s (Locke e t a Z . , 1 9 7 9 ) . I N D I R E C T C O N T R O L OF GROWTH RATE
Curves from O t h e r A r e a s S i n c e c l i m a t e i s a p r i m a r y c o n t r o l o f l i c h e n growth r a t e , p a r t i c u l a r l y d u r i n g t h e " g r e a t p e r i o d " w e assumed t h a t a f i r s t a p p r o x i m a t i o n of a R . geographicurn c u r v e f o r t h e c e n t r a l Brooks Range a r e a s h o u l d
239
resemble t h e r e l a t i v e l y w e l l - c o n t r o l l e d curve of c e n t r a l B a f f i n I s l a n d ( F i g u r e 6). T h e r e , mean a n n u a l t e m p e r a t u r e i s - 1 0 ° C and a n n u a l p r e c i p i t a t i o n i s a b o u t 350 t o 4 0 0 mm (Andrews and Webber, 1 9 6 4 ) . Radiocarbon Dates T h r e e r a d i o c a r b o n d a t e s h a v e b e e n u s e d a s c o n t r o l p o i n t s i n cons t r u c t i o n o f b o t h g r o w t h c u r v e s ( F i g u r e 7 ) and a f o u r t h p o i n t (11) h a s b e e n p l o t t e d . T h e s e c o n t r o l p o i n t s ( l o c a t e d on F i g u r e 1) a r e d e r i v e d from o r g a n i c m a t e r i a l s u n d e r l y i n g u n i f o r m , s t a b l e s u r f a c e s b e a r i n g b o u l d e r s w i t h a w e l l d e v e l o p e d c o v e r of R . g e o g r a p h i c u r n . R e p r e s e n t a t i v e maximum d i a m e t e r s w e r e d e t e r m i n e d from t h i s c o v e r . The most u s e f u l and r e l i a b l e d a t e was 320 2 1 0 0 y r BP (BGS-522) for control point 8 o b t a i n e d on woody v e g e t a t i o n b u r i e d by d e p o s i t i o n of a n o u t e r m o s t Holocene l a t e r a l m o r a i n e a t B u f f a l o G l a c i e r ( C a l k i n and E l l i s , 1980, F i g u r e 6 ) . T h i s d a t e was t h e f i r s t d i r e c t c o n t r o l o f
N e o g l a c i a l m o r a i n e d e p o s i t i o n o b t a i n e d i n t h e Brooks Range. Control p o i n t s 7 ( 2 1 0 t 9 0 y r BP (BGS-547) of A l y e s k a , s i t e MS-112.2) and 1 0 ( 8 0 0 t 1 0 0 y r BP (BGS-547) a t MS-110.2) as w e l l as p o i n t 11 ( 1 3 0 0 100 y r BP (BGS-670) n e a r A t i g u n Camp) were o b t a i n e d from p e a t l a y e r s which o c c u r r e d w i t h i n s a n d and g r a v e l a l l u v i u m 0 . 6 5 , . 9 5 and . 8 5 m r e s p e c t i v e l y below p r e s e n t v e g e t a t e d s u r f a c e s . The maximum d i a m e t e r s o f l i c h e n c h a r a c t e r i z i n g t h e s e s u r f a c e s ( F i g u r e 6 ) must r e p r e s e n t minimum f i g u r e s . T h i s i s b e c a u s e o f t h e t i m e f a c t o r f o r d e p o s i t i o n , e r o s i o n , o r nond e p o s i t i o n w h i c h may h a v e i n t e r v e n e d b e t w e e n b u r i a l o f p e a t and l i c h e n c o l o n i z a t i o n on t h e o v e r l y i n g s u b s t r a t e s . +_
D e n d r o c h r o n o l o g i c Data T r e e r i n g c o u n t s t a k e n a t t h e w h i t e s p r u c e t r e e l i n e l ' j km s o u t h and 750 m below A t i g u n Pass ( F i g u r e l), a f f o r d a p p r o x i m a t e a g e s o f t h e l a r g e s t R . g eo g r a p h i cu r n l i c h e n s on a l l u v i a l f a n s and m e l t w a t e r c h a n n e l s u r f a c e s ( c o n t r o l p o i n t s 1, 6 and 9 ) . A t t h e g o l d m i n i n g a r e a o f L i n d a C r e e k ( F i g u r e 1) a 39 y r o l d w h i t e s p r u c e s a p l i n g growing o u t o f a t a i l i n g p i l e c h a r a c t e r i z e d b y maximum 7 mm R . g e o g ra p h ic u rn p r o v i d e d c o n t r o l p o i n t 1. Historic Controls The o p e r a t i o n s o f hand-worked g o l d p l a c e r s a t t h e L i n d a Creek g o l d m i n i n g c e n t e r s o u t h and 1 0 0 0 m below A t i g u n Pass a r e v e r y w e l l documented (Maddren, 1 9 1 3 ; Cobb and K a c h a d o o r i a n , 1961; and Cobb, 1 9 7 6 ) . T h e r e f o r e , t h e abandonment o f t a i l i n g p i l e s o r g r a v e l c l e a r i n g s may be d a t e d t o t h e y e a r a s i n c o n t r o l p o i n t s 3 and 4 ( C a l k i n and E l l i s , 1 9 8 0 , F i g u r e 5 ) . L i c h e n d a t i n g a t t h i s s i t e i s s u b j e c t o n l y t o t h e unknown period of colonization. S i n c e t h e summer's g r o w i n g s e a s o n i s warmer h e r e t h a n i n t h e main a r e a o f a p p l i c a t i o n (Haugen, 1 9 7 9 ) , t h e d i a m e t e r s o f R . g e og r a p h i c u r n m e a s u r e d i n t h i s m i n i n g a r e a a r e c o n s i d e r e d maximum v a l u e s f o r t h e l i c h e n growth c u r v e .
R i f l e c a r t r i d g e s l y i n g a b o u t t h e abandoned Nunamiut Eskimo v i l l a g e o f A n i g a n i g u r a k ( c o n t r o l p o i n t 5 ; F i g u r e 1) i n d i c a t e o c c u p a t i o n no e a r l i e r t h a n 1 8 7 3 a n d abandonment b e t w e e n 1 8 8 0 and 1890 ( C o r b i n , 1 9 7 1 ) . A v e r y l i m i t e d p o p u l a t i o n o f m e a s u r a b l e R . g e o g ra p h ic u rn on s c a t t e r e d s a n d s t o n e e r r a t i c s showed e v i d e n c e o f b e i n g o v e r t u r n e d d u r i n g t h i s occupation. S e q u e n t i a l p h o t o g r a p h s w e r e t a k e n on t h e r e t r e a t i n g m a r g i n s o f s e l e c t e d c i r q u e g l a c i e r s i n t h e A r r i g e t c h Peaks i n 1 9 1 1 , 1 9 6 2 ( H a m i l t o n , 1 9 6 5 ) , and i n 1 9 7 9 ( E l l i s e t u Z . , 1 9 8 1 ) . These h a v e p r o v i d e d minimum No v i s i b l e l i c h e n c o l o n i z p e r i o d s o f c o l o n i z a t i o n and growth r a t e s . a t i o n o f R . g e o g r a p h i cu r n was f o u n d i n a n i c e - m a r g i n a l zone e x p o s e d d u r i n g t h e 17 y r s i n c e 1962. However, t h e r e was a g e n e r a l down-valley i n c r e a s e o f R . g e o g r a p h i cu r n t h a l l i d i a m e t e r s t o a b o u t 9 mm w i t h i n t h e zone e x p o s e d by r e t r e a t i n t h e t o t a l 5 1 y r b e t w e e n 1 9 1 1 and 1 9 6 2 . B o u l d e r s d e p o s i t e d d i r e c t l y on b e d r o c k a n d a p p a r e n t l y a t a b o u t t h e t i m e o f t h e 1911 p h o t o g r a p h i c s u r v e y , d i s p l a y e d R . g e o g ra p h ic u rn up t o 11 mm, R . e u p e t r a e o i d e s / i n a r e n s e t o 1 0 mm and A l e c t o r i a p u b e s c e n s t o 6 2 mm.
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C o n s t r u c t i o n o f t h e L i c h e n Curve We h a v e u s e d d i r e c t measurement e x t r a p o l a t e d f r o m A . r n i n u s c u l a / p u b s e e e n s , t h e l i c h e n c u r v e s from a r e a s o f s i m i l a r c l i m a t e , r a d i o c a r b o n d a t e s , t r e e r i n g c o u n t s , a g e s d e r i v e d from a r c h e o l o g i c a l s i t e s , h i s t o r i c r e c o r d s o f m i n i n g a r e a s , and s e q u e n t i a l p h o t o g r a p h s t o c o n s t r u c t a R h i z o c a r p o n geographicurn g r o w t h c u r v e f o r t h e c e n t r a l Brooks Range. A l l d a t e s from t h e s e c o n t r o l p o i n t s a r e a d j u s t e d t o AD 1979 and a r e p r e s e n t e d i n r a d i o c a r b o n y e a r s BP on F i g u r e 7 . A d j u s t m e n t t o r a d i o c a r b o n y e a r s i s a common p r o c e d u r e b e c a u s e t h e c r i t i c a l o l d e r s e c t i o n s o f g r o w t h c u r v e s a r e b a s e d on 1 4 C d a t e s which do n o t c o n v e r t a c c u r a t e l y t o c a l e n d a r y e a r s (Locke e t a Z . , 1 9 7 9 ) . The c u r v e i s drawn t h r o u g h t h e v a r i o u s l i c h e n c o n t r o l p o i n t s s o as t o i n c o r p o r a t e e s t i m a t e s o f t h e t i m e l a g s e p a r a t i n g d e a t h or b u r i a l o f t h e c a r b o n - d a t e d v e g e t a t i o n , t h e s t a b i l i z a t i o n of o v e r l y i n g s u r f a c e and l i c h e n c o l o n i z a t i o n . P e r i o d of C o l o n i z a t i o n E v i d e n c e from d i r e c t measurement c i t e d i n t h e l i t e r a t u r e s u g g e s t t h a t t h e t i m e l a g b e t w e e n s u b s t r a t e s t a b i l i z a t i o n and i n i t i a l l i c h e n c o l o n i z a t i o n f o r t h e s p e c i e s u s e d i n t h i s s t u d y r a n g e b e t w e e n 1 0 and 5 0 y r u n d e r s i m i l a r c l i m a t i c r e g i m e s (Andrews and Webber, 1 9 6 4 ; B e n e d i c t , 1968; Rampton, 1 9 7 0 ; M i l l e r a n d Andrews, 1 9 7 2 ; B e s c h e l and W e i d i c k , 1973; K a r l g n , 1 9 7 3 ) . I n t h e A r r i g e t c h P e a k s a r e a , A l e c t o r i a m i n u s c u Z a a n d A . p u b e s c e n s as w e l l as R . geographicurn a p p e a r e d b e t w e e n 1 7 and 6 8 y r f o l l o w i n g d e g l a c i a t i o n ( s e e H i s t o r i c C o n t r o l ) . One hundred k i l o m e t e r s n o r t h w e s t of A r r i g e t c h P e a k s , W.P. Brosge ( p e r s o n a l communication, 1 9 8 0 ) p h o t o g r a p h e d R . geographicurn l i c h e n s to 2 mm d i a m e t e r on a g r a n i t i c r o c k s u r f a c e s c r a p e d c l e a n b y h i m 2 9 y e a r s b e f o r e . A t t h e r e c e d i n g C h a m b e r l i n G l a c i e r i n t h e n o r t h e a s t e r n Brooks Range c o m p a r i s o n o f a 1958/59 p h o t o g r a p h (Holmes and L e w i s , 1 9 6 1 ) and t h e 1 9 8 1 i c e - m a r g i n a l p o s i t i o n d e m o n s t r a t e d no l i c h e n c o l o n i z a t i o n a f t e r - 2 3 y e a r s o f s u b s t r a t e e x p o s u r e . Based on t h e s e d a t a , we h a v e assumed a c o l o n i z a t i o n p e r i o d o f 30 y r w h i c h i s i n c o r p o r a t e d i n t h e growth c u r v e s of F i g u r e 7 . G r e a t Growth and L i n e a r Growth P e r i o d s The e x a c t p a t t e r n s and p e r i o d o f d e c e l e r a t i o n d u r i n g t h e " g r e a t p e r i o d " o f growth v a r i e s b u t o u r c u r v e i s s i m i l a r i n s h a p e t o o t h e r s ( F i g u r e 6 ) t o t h e e x t e n t t h a t i t shows a n i n t e r v a l of v e r y r a p i d g r o w t h f o l l o w e d b y a t a p e r i n g o f f where a n e x p o n e n t i a l l y d e c r e a s i n g g r o w t h r a t e i s approximated (see P o r t e r , 1981). This extends through t h e f i r s t 2 0 0 y r ; a t t h i s p o i n t , t h e Brooks Range c u r v e , ( a n d o t h e r s ) a p p e a r s t o become l i n e a r w i t h g r o w t h a t a p p r o x i m a t e l y 3 m m / 1 0 0 y r . The R h i z o c a r p o n geographicurn c u r v e f o r t h e C e n t r a l Brooks Range i s l i n e a r l y e x t r a p o l a t e d beyond a 1 , 3 0 0 ? 1 0 0 y r BP d a t e t o 5 0 0 0 y r BP b a s e d on measurement of R . geographicurn i n o t h e r a l p i n e a r e a s o f t h e w o r l d ( B e n e d i c t , 1 9 6 7 ; M i l l e r and Andrews, 1 9 7 2 ; Denton a n d K a r l g n , 1973; Andrews and B a r n e t t , 1 9 7 9 ) . T h e r e i s some e v i d e n c e from t h e Brooks Range s u p p o r t i n g one p o i n t on t h e c u r v e e x t e n s i o n . T h i s i s b a s e d on a c o r r e l a t i o n o f Holocene c o n s t r u c t i o n of m o r a i n e s and s t r e a m t e r r a c e s t h a t a l l o w s maximum R . geographicurn t h a l l u s d i a m e t e r s o f 1 4 5 mm t o b e t i e d t o d a t e s o f - 4 0 0 0 y r B P , r e s p e c t i v e l y ( C a l k i n and E l l i s , 1 9 8 0 ; H a m i l t o n , 1 9 8 1 b ) . A q u a l i t a t i v e 5 2 0 p e r c e n t a g e r e l i a b i l i t y i s shown b y t h e s h a d e d zone s u r r o u n d i n g t h e g r o w t h c u r v e o f F i g u r e 7 . T h e r e i s no s t a t i s t i c a l b a s i s f o r t h i s ; however, i t i s i n s e r t e d f o l l o w i n g t h e p r o c e d u r e o f M i l l e r a n d Andrews ( 1 9 7 2 , p . 1 1 3 7 ) as ''a s u b j e c t i v e i n t e r p r e t a t i o n o f t h e r e l i a b i l i t y of t h e c u r v e a t p r e s e n t . "
The R h i z o c a r p o n geographicurn g r o w t h c u r v e can b e a p p l i e d w i t h g r e a t reservation t o t h a l l u s diameters of R. eupetraeoides/inarense which a r e l e s s t h a n 1 5 0 mm i n d i a m e t e r . The A . m i n u s c u Z a g r o w t h c u r v e ( F i g u r e 7 ) g e n e r a t e d e n t i r e l y by d i r e c t or i n d i r e c t m e a s u r e m e n t s on t h i s s p e c i e s a l s o r e p r e s e n t s A . p u b e s c e n s a n d can b e u s e d for d a t i n g d e p o s i t s t o 4 0 0 y r BP. UrnbiZicaria p r o b o s c i d e a i s i n c o r p o r a t e d i n t h i s
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l a t t e r curve but t h e s p e c i e s i s only u s e f u l f o r d e p o s i t s less than 1 0 0 y r old. APPLICATIONS AND PROBLEMS OF D A T I N G WITH LICHEN Most o f our a p p l i c a t i o n s o f l i c h e n o m e t r y s o f a r i n t h e Brooks Range have b e e n d i r e c t e d t o w a r d r e c o n s t r u c t i n g and d a t i n g o f g l a c i e r s or r o c k g l a c i e r p o s i t i o n s d u r i n g t h e H o l o c e n e . C i r q u e g l a c i e r s i n t h e c e n t r a l Brooks Range a r e p r e s e n t l y w a s t i n g away w i t h e q u i l i b r i u m l i n e s w i t h i n 1 0 0 m o f t h e g l a c i e r h e a d s ( E l l i s and C a l k i n , 1 9 7 9 ) . L i c h e n o m e t r i c maps o f Holocene m o r a i n e s h a v e b e e n u s e d t o r e c o n s t r u c t f o r m e r g l a c i e r s a n d t h e i r e q u i l i b r i u m - l i n e a l t i t u d e s . The u s e o f l i c h e n t r i m l i n e s f o r t h e s e r e c o n s t r u c t i o n s i s d i f f i c u l t because c a r e has t o be t a k e n t o a v o i d c o n f u s i o n of t r u e i c e - t r i m l i n e s w i t h zones c o n t a i n i n g d e p l e t e d l i c h e n p o p u l a t i o n s . These l a t t e r may o n l y r e f l e c t contemporary b a l a n c e w i t h l i n g e r i n g , a n n u a l snow c o v e r ( s e e H a e b e r l i e t a Z . , 1 9 7 9 ; Koerner, 1980). L i c h e n s f r e q u e n t l y s u r v i v e s u p r a g l a c i a l t r a n s p o r t , p a r t i c u l a r l y on l a r g e b o u l d e r s . A t s e v e r a l o f more t h a n 50 g l a c i e r s mapped, we obs e r v e d s u p r a g l a c i a l t o i c e - c o r e d s i t u a t i o n s where m e d i a l a n d l a t e r a l m o r a i n e s w e r e p o p u l a t e d by t a x a r e p r e s e n t i n g 1 0 t o 2 0 l i c h e n o m e t r i c y e a r s o f g r o w t h . On Twin G l a c i e r , n o r t h o f A t i g u n Pass ( F i g u r e l), we r e c o r d e d t h e p r o g r e s s i v e i n c r e a s e o f R . g e o g ra p h ic u rn t h a l l i d i a m e t e r s on b o u l d e r s from t h e i r c o l o n i z a t i o n below t h e c i r q u e h e a d w a l l t o a p o i n t 1 2 0 0 m d o w n g l a c i e r a l o n g a n a c t i v e f l o w l i n e . From t h i s we der i v e d a mean v e l o c i t y of g l a c i e r ,flow o f 6 m / y r , a f i g u r e c o m p a t i b l e w i t h m e a s u r e d f l o w r a t e s e l s e w h e r e i n t h e Brooks Range (Wendler e t a l . , 1975). Some o f t h e l i c h e n s o f s u p r a g l a c i a l d e b r i s s u r v i v e d e p o s i t i o n , p a r t i c u l a r l y on s h r i n k i n g i c e m a s s e s (Matthews, 1973). T h i s r a r e ' l y o c c u r s d u r i n g g l a c i a l a d v a n c e s i n t h e Brooks Range. P r e s e r v a t i o n o f l i c h e n u n d e r a c t i v e g l a c i e r s i s much l e s s common t h a n t h i s s u r f a c e t r a n s p o r t ; however, i t h a s b e e n r e p o r t e d from t h e C a n a d i a n A r c t i c I s l a n d s a s w e l l a s G r e e n l a n d ( s e e K o e r n e r , 1 9 8 0 1 , and now i n t h e c e n t r a l Brooks Range ( C a l k i n and E l l i s , 1 9 8 1 a ) . C o n d i t i o n s f a v o r a b l e f o r p r e s e r v a t i o n of l i c h e n s b e n e a t h g l a c i e r s a r e a p p a r e n t l y r e l a t e d t o t h e frozen basal conditions of t h e s e p o l a r o r subpolar g l a c i e r s a n d , o f c o u r s e , t h e l a c k o f b u r i a l by g l a c i a l d r i f t . Recess i o n o f Golden E a g l e G l a c i e r ( F i g u r e 1) from a N e o g l a c i a l maximum marked by R h i z o c a r p o n g eo g r a p h ic u rn t h a l l i o f up t o 1 9 m m , i s c u r r e n t l y e x p o s i n g a n 8 0 0 m2 s u r f a c e w i t h u n d i s t u r b e d l i c h e n c o v e r e d b o u l d e r s s u r r o u n d e d b y d e a d moss a n d u n s o r t e d p a t t e r n e d g r o u n d . R . g e o g r a p h i c u r n up t o 72 mm d i a m e t e r , b r i g h t l y c o l o r e d and v i s u a l l y i n d i s t i n g u i s h a b l e from l i v i n g f o r m s , o c c u r n e a r t h e t e r m i n u s o f Golden E a g l e G l a c i e r b u t become p r o g r e s s i v e l y b l e a c h e d and d e t e r i o r a t e d 40 m beyord t h e g l a c i e r t o e . A s p r i n k l i n g o f b o u l d e r s which h a v e m e l t e d o u t from s u p r a g l a c i a l p o s i t i o n s a r e d i s t i n g u i s h e d by t h e i r more a n g u l a r and r e l a t i v e l y l i c h e n - f r e e c h a r a c t e r . Radiocarbon a n a l y s i s o f t h e dead e m e r g e n t moss s u r r o u n d i n g t h e b o u l d e r s d a t e s a N e o g l a c i a l a d v a n c e a c r o s s t h e s i t e a t 1120 k 180 y r BP. Maximum d i a m e t e r s o f t h e p r e s e r v e d l i c h e n s i n d i c a t e t h a t a minimum i c e - f r e e i n t e r v a l o f 1 5 0 0 t o 2 5 0 0 y r preceded t h i s g l a c i a l expansion.
C o n s i s t e n t p a t t e r n s of l i c h e n d i a m e t e r s a r e u s e d t o d i s t i n g u i s h s t a b l e , i c e - c e m e n t e d or i c e - c o r e d m o r a i n e s o f open t e r r a i n from uns t a b l e , g l a c i e r - c o r e d d e p o s i t s ( E l l i s and C a l k i n , 1 9 7 9 ) . F u r t h e r m o r e , t h e y h e l p s u b s t a n t i a t e t h e r e l i a b i l i t y o f t h e l i c h e n o m e t r i c method i n t h i s a r e a . The s t a b l e , l o o p e d m o r a i n e s o f Holocene a g e which f r o n t t h e s m a l l c i r q u e g l a c i e r s o f t h e c e n t r a l Brooks Range, a r e c h a r a c t e r i z e d b y : a ) a g e n e r a l l a c k o f l i c h e n c o v e r on r e c e n t l y d e p o s i t e d i c e marg i n a l r i d g e s ; b ) l i c h e n t h a l l i t h a t become s u c c e s s i v e l y l a r g e r i n d i a m e t e r .outward from t h e m a r g i n c o r r e s p o n d i n g t o s u c c e s s i v e l y g r e a t e r s u b s t r a t e a g e a s b a s e d on geometry and i n d e p e n d e n t g e o l o g i c c r i t e r i a ; and c ) a l i c h e n c o v e r b e a r i n g a s u b s t a n t i a l number o f t h a l l i c l o s e t o t h e maximum measured d i a m e t e r on i n d i v i d u a l r i d g e s . V a r i a t i o n o f t h e
242 maximum d i a m e t e r o f t e n f a l l s w i t h t n a 2 t o 3 mm r a n g e a l o n g r i d g e c r e s t s up t o 1 km l o n g . ? h e r e s i i l t s o r t h l s s a m p l i n g h a v e S e e n r e c o r d e d o n maps w h i c h d i s p l a y a r e a s s e g m e n t e d on t h e b a s i s of l i c h e n a g e s ( s e e also D e n t o n a n d K a r l g n , 1973) a n d i s o p h y s e s w h i c h c o n n e c t p o i n t s o f e q u a l l i c h e n d i a m e t e r s ( F i g u r e 5 ; C a l k i n a n d E l l i s , 1 9 8 0 , F i g u r e 8 ) . Data f r o m t h e s e maps may b e p r e s e n t e d as h i s t o g r a m s p l o t t i n g t h e f r e q u e n c y o f i n d i v i d u a l m o r a i n e r i d g e s b e a r i n g d i a m e t e r s of maDped l i c h e n s a t i n t e r v a l s . T h e s e i n t u r n may b e c o r r e l a t e d w i t h t h e l i c h e n c u r v e a s i n F i g u r e 7 .
We h a v e c o n s t r u c t e d a H o l o c e n e g l a c i a l c h r o n o l o g y b a s e d l a r g e l y on such lichenometric d a t a t o g e t h e r w i t h f i v e I 4 C d a t e s d i r e c t l y a s s o c i a t e d w i t h m o r a i n e s and r e l a t i v e w e a t h e r i n g and s o i l c r i t e r i a ( E l l i s and C a l k i n , 1 9 8 1 ) . T h i s c h r o n o l o g y i s f a r more c o m p l i c a t e d t h a n f o r m e r l y e n v i s i o n e d ( D e t t e r m a n e t a Z . , 1958: P o r t e r and De n to n , 1 9 6 7 ) and i n c l u d e s s e v e n m a j o r e x p a n s i o n s of s i m i l a r m a g n i t u d e e x t e n d i n g f r o m 4500 jrr B P t o 350 y r B P ( C a l k i n a n d E l l i s , 1 9 8 1 b ) .
A l e s s d i s t i n c t b u t c o m p l i m e n t a r y c h r o n o l o g y i s d e f i n e d by l i c h e n m e t r i c a n a l y s i s of r o c k g l a c i e r d e p o s i t s i n t h i s a r e a ( E l l i s a n d C a l k i n , 1 9 7 9 ) . G r a v i t y - c o n t r o l l e d movement o f t h e s e d e p o s i t s d i s r u p t s o r p r e ~ l u d e st h e c o n t i n u i t y of l i c h e n t h a l l i d i a m e t e r s a t s u c c e s s i v e d i s t a n c e s I'rom g l a c i e r h e a d s ( F i g u r e 5 ) . However, maximum l i c h e n d i a m e t e r s n e a r t h e s n o u t s of ice-cored rock g l a c i e r tongues ( F i g u r e 5 ) are i n d i c a t i v e of g l a c i a l d e v e l o p m e n t arid s u r f a c e movement i n e a r l y E o l c c e c e t h e . " T r a n s i t i o n z o n e s " ( F o s t e r and Holmes, 1 9 6 5 ) o f d i s t i n c t m o r a i n a l r i d g e s o c c u r between downslope r o c k g l a c i e r tongues and u p s l o p e s m a l l c i r q u e g l a c i e r remnants ( F i g u r e 5 ) . These c a r r y a l i c h e n c o v e r l i m i t e d t o t h e l a t e H o l o c e n e ( s e e F i g u r e 4b) b u t r e f l e c t a c h r o n o l o g y , s i m i l a r t o t h a t of s t a b l e moraine r i d g e s u n a s s o c i a t e d w i t h rock g l a c i e r s . D e t a i l s of l i c h e n o m e t r i c s t u d i e s of r o c k g l a c i e r d e p o s i t s a r e d i s c u s s e d b y C . D . ibliller ( 1 9 7 3 ) , Luckman a n d O s b o r n e ( 1 9 7 9 ) , H a e b e r l i e t a2. ( 1 9 7 9 ) , a n d ,ohnson ( 1 9 8 0 ) . ACKNOWLEDGENENTS T h i s s t u d y was s u p p o r t e d b y NS9 g r a n t s f r o m t h e D e p a r t m e n t o f p o l a r P r o g r a m s t h r o u g h t h e R e s e a r c h F o u n d a t i o n of t h e S t a t e U n i v e r s i t y of New York. P h o t o g r a p h i c work was p a r t l y f u n d e d by Sigma X i . Botanical i d e n t i f i c a t i o n s were p r o v i d e d b y W.A. Webber, P.J. Webber, J.W. Thomson, C. C o p p e r , a n d R . Z a n d e r . We a r e g r a t e f u l t o T . D . H a m i l t o n a n d t h e USGS, P l a c i d O i l C o . , a n d ALASCOM f o r l o g i s t i c a l s u p p o r t . F i e l d ass i s t a n t s were M . B r u e n , S . W a l t i , a n d T . L o w e l l . REFERENCES CITED A n d e r s o n , J.T. a n d S o l l i d , J.L., 1 9 7 1 , G l a c i a l c h r o n o l o g y a n d glacial. geomorphology in the m a r g i n zones of the glaciers, Midtdalsbreen and N i g a r d s b r e e n , s o u t h Norway: N o r s k G e o g r . Tidsskr., v . 25, p. 1-38. A n d r e w s , J.T. a n d B a r n e t t , D . M . , 1 9 7 9 , H o l o c e n e ( N e o g l a c i a l ) m o r a i n e a n d p r o g l a c i a l l a k e c h r o n o l o g y , B a r n e s - I c e C a p , C a n a d a : B o r e a s , v. 8 , p. 341-358. A n d r e w s , J.T. a n d W e b b e r , P.J., 1 9 6 4 , A l i c h e n o m e t r i c a l s t u d y o f t h e northwestern m a r g i n of the Barnes I c e Cap: A geomorphological technique: G e o g r a . B u l l . , N o . 2 2 , p. 8 0 - 1 0 4 .
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246 P o r t e r , S.C. and Denton, G . H . , 1967, Chronology of N e o g l a c i a t i o n i n t h e A m e r . J o u r . S c i . , v. 265, p. 177-210. North American C o r d i l l e r a : Rampton, V . , 1 9 7 0 , N e o g l a c i a l f l u c t u a t i o n s o f t h e N a t a x h a t and K l u t l a n g l a c i e r s , Yukon T e r r i t o r y , C a n a d a : Canadian Jour. Earth S c i . , v. 5 , p. 1236-1263. R e g e r , R . D . a n d P6w6, T . L . , 1 9 6 9 , L i c h e n o m e t r i c d a t i n g i n t h e c e n t r a l A l a s k a R a n g e , i n P6w6, T . L . , e d . , The P e r i g l a c i a l E n v i r o n m e n t : M o n t r e a l , McGill-Queens Univ. P r e s s , p . 223-247. 1 9 7 3 , L i c h e n g r o w t h r a t e s i n West G r e e n l a n d : Ten B r i n k , N . W . , and A l p i n e R e s e a r c h , v . 5 , p . 323-331. Thomson, V.
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Webber, P . J . and Andrews, J . T . , 1973, Lichenometry: A r c t i c and A l p i n e R e s e a r c h , v . 5 , p . 295-302.
Arctic
NovaHedwigia, Buffalo,
A commentary:
Wendler, G . , Benson, C . , F a h l , C . , Ishikawa, N . , T r a b a n t , D . and W e l l e r , G . , 1 9 7 5 , G l a c i o - M e t e o r o l o g i c a l s t u d i e s of McCall G l a c i e r , Weller, G . and Bowling, S . A . , eds., C l i m a t e of t h e A r c t i c in (24th Alaska Science Conference, 1973). Geophysical I n s t i t u t e , U n i v e r s i t y o f A l a s k a , p . 334 338. W o r s l e y , P . , 1 9 7 3 , An e v a l u a t i o n o f t h e a t t e m p t t o d a t e t h e r e c e s s i o n o f T u n d s b e r g d a l s b r e e n , s o u t h e r n Norway, by l i c h e n o m e t r y : G e o g r a f i s k a A n n a l e r , v . 54A, p . 1 3 7 - 1 4 1 .
247
LANDFORMS AND LANDSCAPES AS MEASURES OF RELATIVE TIME
DONALD R. COATES
ABSTRACT
W.M. D a v i s d e v e l o p e d t h e f i r s t r e l a t i v e a g e t e r r a n e models w i t h
h i s g e o g r a p h i c a l c y c l e s o f y o u t h , m a t u r i t y , a n d o l d a g e for f l u v i a l
l a n d s c a p e s i n humid and a r i d r e g i o n s . L a t e r workers followed h i s l e a d d e l i n e a t i n g r e l a t i v e a g e p h a s e s for c o a s t a l , k a r s t , and p e r i g l a c i a l e n v i r o n m e n t s . T h e s e were d e s c r i p t i v e m o d e l s , b u t t h e newer t o o l s of q u a n t i t a t i v e geomorphology c a n P r o v i d e i n some c a s e s a n u m e r i c a l b a s e s u c h as d r a i n a g e d e n s i t y f o r d e t e r m i n i n g r e l a t i v e age of l a n d s c a p e s and h y p s o m e t r i c i n t e g r a l s .
....
R e l a t i v e a g e c o m p a r i s o n s a r e o n l y v a l i d when l a n d f o r m and l a n d s c a p e s e t s have t h e same g e o l o g y , h y d r o l o g y , c l i m a t e , and t o p o g r a p h i c framework. P h y s i o g r a p h i c b o u n d a r i e s c a n n o t b e c r o s s e d for p u r p o s e s o f contrasting age. C a u t i o n i s a l s o needed b e c a u s e o f such c o n s t r a i n t s as t h e p r i n c i p l e of e q u i f i n a l i t y . S i n c e s i m i l a r - a p p e a r i n g l a n d f o r m s c a n be c r e a t e d b y d i f f e r e n t p r o c e s s e s , a knowledge of l a n d f o r m g e n e s i s i s c r u c i a l . F u r t h e r m o r e l a n d f o r m s t h a t may h a v e b e e n i n i t i a t e d a t t h e same t i m e , may p r o g r e s s d i f f e r e n t l y i f m a g n i t u d e and f r e q u e n c y o f a c t i n g processes are not similar. R e l a t i v e d a t i n g r u l e s of c r o s s - c u t t i n g f e a t u r e s , s u p e r p o s i t i o n , and d e g r e e of c o m p l e x i t y c a n p r o v i d e g u i d e l i n e s f o r d e v e l o p i n g a c h r o n o l o g y o f e v e n t s . A l s o when a p p r o p r i a t e l y a n a l y z e d t h e geomorphic i n d i c a t o r s of l a n d f o r m s i z e , h i l l s l o p e s t e e p n e s s and s h a p e , and l a n d s c a p e f a b r i c can p r o v i d e u s e f u l age d e t e r m i n a t i o n . Each g e o l o g i c p r o c e s s t h a t i n f l u e n c e s t h e e a r t h ' s s u r f a c e , w h e t h e r e n d o g e n i c o r e x o g e n i c , h a s i t s own c h a r a c t e r i s t i c f i n g e r p r i n t t h a t a i d s i n r e v e a l i n g t h e s e q u e n c e o f p r o c e s s e s . F l u v i a l , g l a c i a l , and f a u l t e d t e r r a n e s a r e used t o i l l u s t r a t e these b a s i c p r i n c i p l e s . I n f l u v i a l s y s t e m s , b a r b e d t r i b u t a r i e s , wind g a p s , p o i n t b a r s , t e r r a c e s , a l l u v i a l f a n s , and d e l t a s can p r o v i d e i m p o r t a n t i n f o r m a t i o n i n d a t i n g r e l a t i v e G l a c i a l landforms s u c h as presequences of e r o s i o n o r d e p o s i t i o n . g l a c i a l s t r a n d l i n e s , u m l a u f b e r g s , e r o s i o n a l s l u i c e w a y s , and m o r a i n e s may c o n t a i n s p e c i a l f e a t u r e s t h a t a s s i s t i n d i a g n o s i n g t h e i r r e l a t i v e a g e . F a u l t e d a r e a s may c o n t a i n many l a n d f o r m s t h a t i n d i c a t e r e l a t i v e d e v e l o p m e n t , s u c h as d e g r e e of s p u r a n d f a c e t m o d i f i c a t i o n , w a t e r t a b l e r e a d j u s t m e n t , and s t r e a m c h a n n e l p r o f i l e c h a n g e s . Landform a n a l y s i s i n t h e B a s i n and Range p r o v i n c e h a s b e e n s u c c e s s f u l l y u s e d t o p l a c e r e l a t i v e c h r o n o l o g i e s on e v e n t s t h a t r a n g e from a b o u t 1 0 0 , 0 0 0 t o 1 m i l l i o n years ago. INTRODUCTION
Geomorphology i s c o n c e r n e d w i t h r e s e a r c h on t h e a g e of m a t e r i a l s and s u r f a c e s . Since t h e l a t e 1 9 6 0 ' ~ however, ~ t h e r e h a s been an even g r e a t e r e m p h a s i s on t i m e b e c a u s e o f renewed i n t e r e s t i n e n v i r o n m e n t a l a f f a i r s . H i s t o r i c a l l y a wide v a r i e t y of d a t i n g methods h a v e b e e n u s e d t o develop c h r o n o l o g i e s u s e f u l i n d e t e r m i n i n g r a t e s and t y p e s o f changes
248
to l a n d f o r m s a n d t h e p r o c e s s e s t h a t c r e a t e t h e m . The new u r g e n c y i n c a l c u l a t i n g a c c u r a t e t i m e f r a m e s sterns f r o m g e o l o g i c h a z a r d s on human a c t i v i t i e s a s w e l l as human i m p a c t s to r . a t u r a 1 s y s t e m s . !he u s e o f " h i r 1 d c a s t i n g " t e c h r , i q u e s to p r e d i c t a n d f o r e c a s t f u t u r e e v e n t s h a s become a n e c e s s i t y i n l a n d u s e m a n a g e m e n t , a n d i s a s t a n d a r d f u n c t i o n i n t h e U n i t n d S t a t e s for p r e p a r i n g e n v i r o n m e n t a l i m p a c t s t a t e m e n t s ur.der t h e N a t i o n a l E n v i r o n m e n t a l P o l i c y A c t of 1 9 7 0 . Thus i t h a s b e c o m e i m p o r t a n t t o a s s e s s t h e t i m e f a c t o r for d e t e r m i n i n g r e c u r r e n c e i n t e r v a l s of s u c h phenomena a s e a r t h q u a k e s , f l o o d i n g , l a n d s l i d i n g , a n d v o l c a n i c a c t i v i t y . A l t h o u g h t h i s p a p e r i s c e n t e r e d a r o u n d r e l a t i v e t i m e , many o f t h e a p p r o a c h e s t h a t a r e d i s c u s s e d h e r e h a v e r e l e v a n c e to t h e b r o a d e r i s s u e s of g e o l o g i c t i m e . A b a s i c t e n e t o f geomorphology h a s b e e n t h a t i f b o t h t h e e x o g e n i c and endogenic f o r c e s and c o n d i t i o n s remain c o n s t a n t a c h a r a c t e r i s t i c s y s t e m of l a n d f o r m s w i l l d e v e l o p . S u c h a n i d e a was f i r s t e n u n c i a t e d by G i l b e r t (1877) a n d l a t e r r e s t a t e d by Hack ( 1 9 6 0 , 1 9 7 5 ) as t h e c o n c e p t of d y n a m i c e q u i l i b r i u m . T i m e was v i e w e d a s o n l y i n c i d e n t a l a n d emp h a s i s was p l a c e d on t h e s t a b i l i t y of l a n d f o r m s s o l o n g as t h e v a r i a b l e s r e m a i n e d c o n s t a n t . An a l t e r n a t e m o d e l of l a n d s c a p e d e v e l o p m e n t was i n t r o d u c e d by D a v i s i n 1 8 9 9 w h e r e b y t i m e was r a n k e d , a l o n g w i t h s t r u c t u r e a n d p r o c e s s v a r i a b l e s , a s b e i n g i n t h e t r i n i t y of f a c t o r s t h a t influence terrain character.
W i l l i a m M o r r i s Davis d e s e r v e s c r e d i t f o r i n t r o d u c i n g t h e most p o p u l a r s y s t e m f o r t h e r e l a t i v e d a t i n g of l a n d s c a p e s . He undoubtedly r e a s o n e d t h a t i f D a r w i n ' s m o d e l for e v o l u t i o n i n t h e o r g a n i c w o r l d was v a l i d , t h e r e must b e a s i m i l a r d e v e l o p m e n t a l model o f i n o r g a n i c f e a t u r e s f o r t h e e v o l u t i o n of topography w i t h t h e passage of t i m e . Davis (1899) f i r s t used t h e model i n t h e d e s c r i p t i o n of changes t h a t p r o g r e s s i n a humid c l i m a t e , a n d a s s i g n e d t h e c l a s s e s of c h a n g e i n t h e a n t h r o p o m o r p h i c i m a g e s o f y o u t h , m a t u r i t y , a n d o l d a g e ( F i g u r e 1 ) . The p r e m i s e t h a t r e s u l t e d i n t h i s v a l u e j u d g m e n t was t h a t a l a n d m a s s was r a p i d l y upl i f t e d d u r i n g a n i n i t i a l p h a s e , a n d t h a t t h e r e a f t e r b a s e l e v e l was s t a t i c . With s u c h p o s t u l a t e s f l u v i a l t o p o g r a p h y e v o l v e d t h r o u g h time f r o m c o n d i t i o n s o f l a r g e e x p a n s e s of u n d i s s e c t e d u p l a n d s i n y o u t h , t o a t i m e of m a t u r i t y when a l l t e r r a i n was i n maximum s l o p e . The "geog r a p h i c a l c y c l e " was c o m p l e t e d d u r i n g o l d a g e when r e l i e f was g r e a t l y subdued t o a " p e n e p l a i n " c o n d i t i o n . Davis, and a l s o h i s f o l l o w e r s ( C o t t o n , 1 9 4 7 ) e x p a n d e d o n t h e t h e m e to i n c l u d e b o t h c l i m a t i c a n d v o l c a n i c " a c c i d e n t s " t h a t c a u s e i n t e r r u p t i o n s to t h e " n o r m a l " c y c l e . For example, b a s e l e v e l changes would l e a d t o renewed e r o s i o n , s o t h a t a p o l y c y c l i c l a n d s c a p e forms c o n t a i n i n g e l e m e n t s of b o t h d e s t r o y e d o r p a r t i a l l y d e s t r o y e d e a r l i e r s u r f a c e s . The r e l i c t f e a t u r e s were t h e n u s e d t o d i a g n o s e a r e l a t i v e s e q u e n c e of e v e n t s . The o r d e r l y p r o g r e s s i o n o f l a n d f o r m s a s p r o p o u n d e d by D a v i s s e e m e d to t h e m a j o r i t y of o b s e r v e r s t o b e s o p r o b a b l e a n d l o g i c a l t h a t i t was
w i d e l y a d o p t e d by m o s t g e o m o r p h o l o g i s t s . D a v i s ( 1 9 0 5 ) e x t e n d e d h i s i d e a s t o i n c l u d e a n e v o l u t i o n of f e a t u r e s d u r i n g a n a r i d c y c l e , a n d o t h e r w o r k e r s e x t r a p o l a t e d t h e t h e s i s to c o v e r n e a r l y a l l g e o m o r p h i c p r o c e s s e s a n d c l i m a t e s . Beede ( 1 9 1 1 ) and C v i j i c ( 1 9 1 8 ) d e v e l o p e d t h e e r o s i o n cycle f o r k a r s t t e r r a n e . Johnson (1919) i n s t i t u t e d c o a s t a l e r o s i o n c y c l e s for s h o r e l i n e s t h a t were e i t h e r u n d e r g o i n g e m e r g e n c e o r s u b m e r g e n c e . P e l t i e r ( 1 9 5 0 ) a d o p t e d t h e theme f o r p e r i g l a c i a l r e g i o n s . T h i s p r o c e s s a p p r o a c h a s s i g n e d to e a c h e n v i r o n m e n t a l s y s t e m i t s own i n d i v i d u a l a s s e m b l a g e o f l a n d f o r m s t h a t g r a d u a l l y c h a n g e d . The wide spectrum o f landforms from d i v e r s e t e r r a n e s and c l i m a t e s o c c u p i e s n i c h e s i n a continuum of f e a t u r e s t h a t e v o l v e through t i m e . Each t e r r a n e i s v i e w e d as h a v i n g u n i q u e s i g n a l s t h a t mark i t as h a v i n g d e v e l o p e d d u r i n g a p a r t i c u l a r t i m e framework i n t h e t o t a l h i e r a r c h y o f l a n d s c a p e s . The r e l a t i v e t i m e c y c l e f o r l a n d s c a p e d e v e l o p m e n t h a s r e c e i v e d s e v e r a l i m p o r t a n t c h a l l e n g e s , a n d t h e s e a r e s u m m a r i z e d by F l e m a l ( 1 9 7 1 ) . F o r e x a m p l e , P e n c k ( 1 9 5 3 , a l t h o u g h f i r s t w r i t t e n i n German i n 1 9 2 4 ) a n d King ( 1 9 5 3 ) c h a l l e n g e d t h e D a v i s i a n i d e a of s l o p e s becoming g e n t l e r w i t h time and t h e p r o p o s i t i o n t h a t denudation i s p r o p o r t i o n a l t o relief. The D a v i s i a n c y c l e s were a l s o a t t a c k e d by s u c h a r g u m e n t s a s (1) t h e e a r t h i s too m o b i l e w i t h t o o many c l i m a t e a n d b a s e l e v e l f l u c t u a t i o n s
2 49
LANDFORM
YOUNG
Figure 1
S I Z E
AND
RELIEF
OLD
If e n v i r o n m e n t a l a n d g e o l o g i c c o n d i t i o n s a r e cons t a n t , a l a r g e r l a n d f o r m w i l l be a r e f l e c t i o n o f a younger age.
t o form s t a n d a r d l a n d s c a p e s , ( 2 ) l a n d f o r m s c a n n o t b e a s c r i b e d t o c r e a t i o n by a s i n g l e mechanism, ( 3 ) n a t u r a l s y s t e m s a r e open and n o t c l o s e d , (4) t h r e s h o l d e v e n t s a r e n o t s u f f i c i e n t l y a c c o u n t e d f o r w i t h u n i f o r m e v o l u t i o n o f f e a t u r e s , and ( 5 ) dynamic e q u i l i b r i u m can o c c u r w i t h i n s h o r t e r i n t e r v a l s t h a n t h e m a t u r e p h a s e of D a v i s ' c y c l e s . I n a n a t t e m p t t o r e s o l v e t h e q u e s t i o n of c y c l e s and t i m e , Schumm and L i c h t y ( 1 9 6 5 ) d e s i g n e d a s y s t e m t h a t c o n t a i n e d t h r e e d i f f e r e n t s c a l e s of t i m e . C y c l i c t i m e was o p e r a t i v e f o r p e r i o d s > l o 6 t o 10' r a n g e , and s t e a d y t i m e o c c u r r e d from 10' t o lo-'. With s u c h a s y s t e m , t h e i n d e p e n d e n t and d e p e n d e n t v a r i a b l e s t h a t i n t e r a c t i n l a n d s c a p e dev e l o p m e n t c a n change w i t h t h e p a s s a g e of t i m e . For e x a m p l e , i n s t e a d y t i m e , t h e r e l i e f or t h e volume w i t h i n a landmass above b a s e l e v e l w i l l be an independent v a r i a b l e , but with c y c l i c time such f a c t o r s w i l l be dependent. The s t a g e i s now s e t f o r t h e e v a l u a t i o n o f t h o s e f e a t u r e s t h a t may form by p r o c e s s e s o p e r a t i n g t h r o u g h t i m e , a n d t o p r o v i d e some g u i d e l i n e s I t must b e made c l e a r t h a t o u r t h a t b e a r on t h i s t y p e o f a n i n v e n t o r y . p r e s e n t q u e s t i s f o r t h e e l u c i d a t i o n o f phenomena t h a t a r e p e r t i n e n t t o t h e development o f r e l a t i v e t i m e i n d i c a t o r s a s r e v e a l e d by t h e t o p o g r a p h y . I t would b e e r r o n e o u s t o assume t h i s i s t h e a c t u a l and o n l y way geomorphologists analyze f e a t u r e s . Instead t h e " e a r t h d e t e c t i v e " w i l l use a l l a v a i l a b l e dating techniques at h i s disposal i n order t o unravel t h e c o m p l e t e , a n d when p o s s i b l e i n a b s o l u t e t e r m s . c h r o n o l o g y of e v e n t s t h a t h a v e a f f e c t e d t h e e a r t h ' s s u r f a c e form a n d m a t e r i a l s . TIME AND GEOMORPHOLOGY A p r i n c i p a l o b j e c t i v e f o r t h e g e o m o r p h o l o g i s t i s t h e a n a l y s i s of t h e s c u l p t u r i n g o f t h e e a r t h ' s s u r f a c e , which p r o d u c e s l a n d f o r m s and landscapes. B a t e s and J a c k s o n ( 1 9 8 0 ) d e f i n e a l a n d f o r m a s "Any p h y s i c a l , r e c o g n i z a b l e form or f e a t u r e o f t h e E a r t h ' s s u r f a c e , h a v i n g a c h a r a c t e r i s t i c s h a p e , and p r o d u c e d by n a t u r a l c a u s e s . . . . ' I ( p . 3 4 9 ) . I n s i m i l a r f a s h i o n , l a n d s c a p e i s d e f i n e d a s "The d i s t i n c t a s s o c i a t i o n o f l a n d f o r m s , e s p e c i a l l y a s m o d i f i e d by g e o l o g i c f o r c e s t h a t can b e s e e n i n a s i n g l e v i e w , e . g . g l a c i a l l a n d s c a p e " ( p . 3 4 9 ) . Thus a h i l l i s a l a n d f o r m , and a v a l l e y i s a l a n d f o r m , and t h e manner i n which t h e y a r e r e l a t e d c a n c o n s t i t u t e a l a n d s c a p e . It i s t h e s l o p e o f t h e t e r r a i n t h a t g i v e s i t i n d i v i d u a l i t y . Thus a b a s i c component o f geomorphology i s t h e a n a l y s i s of sloDe l e n g t h , s t e e p n e s s , s h a p e , d i r e c t i o n , and t h a t c o n s t i t u t e s t h e b u i l d i n g b l o c k s of t h e t o p integrated fabric o g r a p h y . F u r t h e r m o r e t h e g e n e s i s and r a t e of change o f s l o p e c o m p r i s e t h e t i m e f r a m e t h a t i s n e c e s s a r y f o r a c o m p l e t e u n d e r s t a n d i n g of t h e
....
....
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geornorphological personality of the area being investigated. For this comprehension it is important to establish both absolute and relative time indices, but this paper includes only the latter which can still reveal an evolutionary sequence of features. Systems and Theory The evaluation of time necessarily involves a systems approach because for any given input of energy at a specific time there is a resulting output whereby the energy transformation is manifest in work accomplished. In natural systems the processes of feedback and linkage become intermixed so that when the time dimension is added the output from one system may become the input to another system. The complexity of these interrelationships have been shown in the classic paper by Schumm and Lichty (1965). The adoption of a strategy to evaluate the relation of time with space and form becomes both a philosophical and theoretical exercise. Landforms might be interpreted as tending toward a variety of semiequilibrium states, such as static, metastable, or steady-state (dynamic equilibrium). Another approach would be to select a model that most closely fits one's perception of the landscape system (Thornes and The deterministic model might be used by the pragBrunsden, 1977). matist because temporal changes then become closely related to process and response, input to output, and cause to effect. However, one might argue this is not realistic because there is so much variability and uncertainty in natural systems that only a stochastic model can establish "truth". Such models incorporate degrees of randomness among the variables, and provide results in terms of statistical solutions. or probabilities. The stochastic model can be designed to be sirnple or complex, and the accuracy becomes a function of the precision of the data input. Boundary Conditions It is now important to establish some guidelines that will aid us to steer a course for the determination of time and terrain change. Kirkby (1980) set some rules for landform change and developed the idea of dominance domains. The character of these determine whether a landform, such as the stream head, will underqo uniform Drogressive change or be subjected to a threshold condition, whereby an extraordinary transformation is produced. One measure of aeomorphic effectiveness that can be useful in calculating dominance domains for different processes is sediment transport. This acts in an analogous manner to landform evolution, much as the flow of money being an indicator of economic conditions. The next set of parameters that need inclusion for terrain assessment involve the magnitude, frequency, and sequence of environmental events that are required to produce landscape evolution. These domains of geomorphic activity possess a set of forces that change over space and time. Their interaction with materials of contrasting resistance produce measurable sediment yield responses. Differences in terrain character may contain memory features that reveal past events. Identification of these memory features (special elements in the landscape) comprise the tools that aid to unravel the time dimension. Topographies that contain only short memory systems are more likely to obey rules of the stochastic model, whereas those topographies with long memories are deterministic. The search for conditions that favor change, and thereby create the fingerprints used for differentiating relative time sequences, ultimately rests with forces and forms, and the production of inequalities. The keys to this analysis involve the concepts of thresholds and relaxation time (Coates and Vitek, 1980). There is a wide range of different threshold types, but my principal concern in this paDer is the combination of events that have triggered a memorable response within the geomorphic system. The responses within this system to drastically change impulses are subject to feedback mechanisms that either dampen (negative)
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or e n h a n c e ( p o s i t i v e ) t h e c h a n g e . The amount and d i r e c t i o n o f l a n d s c a p e m o d i f i c a t i o n i s a r e f l e c t i o n of t h e t o t a l environment s i t u a t i o n . . . . r o c k t y p e and s t r u c t u r e , l a n d m a s s s i z e and s h a p e , r e l i e f and b a s e l e v e l , c l i m a t e and v e g e t a t i o n e t c . Brunsden ( 1 9 8 0 ) h a s c h a r a c t e r i z e d t h e r e l a t i o n s i n t h e form o f two e q u a t i o n s . The l a n d s c a p e change f a c t o r ( F L C )
FLC
- m a g n i t u d e o f b a r r i e r s t o change magnitude of d i s t u r b i n g f o r c e s
( a ) mobile s t a t e s w i t h high T h e r e a r e two end members i n t h i s s y s t e m : s e n s i t i v i t y t o c h a n g e s u c h as b e a c h e s , and ( b ) s l o w l y r e s p o n d i n g s t a t e s which a r e l a r g e l y i n s e n s i t i v e to s h o r t - r a n g e change s u c h a s p l a t e a u s . The t r a n s i e n t form r a t i o ( T F r ) r e l a x a t i o n t i m e of t h e s57stem T F r = mean mean r e c u r r e n c e t i m e o f e v e n t s c a u s i n g change Within t h i s c o n t e x t r e l a x a t i o n t i m e i s t h e t i m e n e c e s s a r y f o r a system t o a c h i e v e a new e q u i l i b r i u m f o l l o w i n g a n i m p u l s e t h a t h a s changed t h e o r i g i n a l c h a r a c t e r of t h e s y s t e m . When t h e r a t i o i s <1.0 t h e i m p u l s e s of change a r e i n s u f f i c i e n t t o p r o h i b i t r a p i d r e c o v e r y t o a c h a r a c t e r i s t i c s t a t e of t h e s y s t e m . WheA t h e r a t i o i s >1.0 t h e i m p u l s e s a r r i v e s o r a p i d l y t h e system cannot a d j u s t . Thus a n a l y s i s o f l a n d s c a p e e v o l u t i o n and c h r o n o l o g y must r e c o g n i z e t h e s e q u e n c e i n which a d j u s t m e n t s have o c c u r r e d . The d e g r e e o f s u c c e s s o f t h i s u n d e r t a k i n g d e p e n d s upon t h e s e n s i t i v i t y of t h e t e r r a i n t o Some l a n d f o r m s a r e s o i n s e n s i t i v e a s t o a l m o s t p o s s e s s a memory. t r a n s c e n d t i m e , s u c h a s m a j o r e s c a r p m e n t s and i n s e l b e r g s , and t h u s a r e u s e l e s s a s Q u a t e r n a r y t i m e i n d i c a t o r s . O t h e r l a n d f o r m s m a y be s o f r a g i l e a s t o be e r a s e d w i t h n e a r l y e v e r y p a s s i n g e v e n t , s u c h a s b e a c h c u s p s . I t i s t h e m i d d l e g r o u n d t h a t i s most h e l D f u l t o t h e Q u a r t e r n a r i s t . . . . t h o s e s i t u a t i o n s where c h a n g e s a r e c a u s e d a n d / o r p r e s e r v e d w i t h i n t h e lo-' t o 3 x lo6 r a n g e . Timescales I n t h e most r e c e n t book on t h i s t o p i c ( C u l l i n g f o r d , Davidson and Lewin, 1 9 8 0 ) geomorphic t i m e i s d i v i d e d i n t o t h r e e o r d e r s : s h o r t 1 0 ' t o l o 2 , medium lo3 t o l o 4 , and l o n g lo4 t o l o 5 . Cambers ( 1 9 7 6 ) a p p l i e d t h e Schumm and L i c h t y model f o r c o a s t a l l a n d f o r m s and d e t e r m i n e d t h a t c l i f f s i n s o f t rock c o n s t i t u t e d steady time because they a t t a i n e d e q u i l i b r i u m i n 2 t o 1 0 0 y r . For more r e s i s t a n t c o a s t a l f e a t u r e s K i r k ( 1 9 7 7 ) c a l c u l a t e d i t t o o k s h o r e p l a t f o r m s lo2 t o lo3 yr t o e v o l v e t o a s t a b l e , or s t e a d y t i m e form. The r e c e n t s t u d y by Brunsden and J o n e s ( 1 9 8 0 ) o f c o a s t a l c l i f f s d e v e l o p e d on s o f t r o c k s a t West D o r s e t , England, i n d i c a t e d a time p e r i o d of 1 0 ' y r f o r i n t e g r a t i o n o f p r o c e s s and form t o e s t a b l i s h a dynamic e q u i l i b r i u m s y s t e m . The mass movement phenomena t h e y i n v e s t i g a t e d showed t h e r e were d i a g n o s t i c l a n d f o r m ass e m b l a g e s t h a t c o u l d i n d i c a t e s t a g e s of d e v e l o p m e n t i n a n o v e r a l l s e q u e n c e of e v e n t s . Wolman and Gerson ( 1 9 7 8 ) h a v e a t t e m p t e d t o q u a n t i f y l a n d f o r m t i m e c h a n g e s b y t h e d e t e r m i n a t i o n of e f f e c t i v e n e s s o f an e v e n t f o r c r e a t i n g In change a s r e l a t e d t o t h e r e l a x a t i o n t i m e , or r e t u r n p e r i o d . m o u n t a i n a r e a s t h e y c a l c u l a t e d l a r g e r a n g e s f o r t h e t r a n s i e n t form r a t i o (TFr), w i t h a r a t i o o f 5 . 0 f o r Tanasawa M o u n t a i n s , J a p a n , and o f 0.25 f o r t h e A p p a l a c h i a n s . The i n f e r e n c e of t h e s e numbers i s t h a t low r a t i o s a r e a s s o c i a t e d w i t h r e l a t i v e l y l o n g g e o l o g i c t i m e s p a n s . For e x a m p l e , t h e D o r s e t s e a c l i f f s h a v e a 2 . 5 r a t i o when composed o f c l a y , and a 1 0 r a t i o i n s a n d . T i m e s c a l e s f o r r e l a t i v e d a t i n g o f t h e Q u a t e r n a r y c a n be d e v e l o p e d from: (1) d i r e c t f i e l d m e a s u r e m e n t s o f p r o c e s s and f o r m , b u t g r e a t c a r e ' m u s t be e x e r c i s e d t h a t t h e forms b e i n g s t u d i e d w e r e made by t h e p r o c e s s b e i n g m e a s u r e d , ( 2 ) u s e of h i s t o r i c a l d a t a , ( 3 ) c o m p a r a t i v e
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qualitative analysis of earth features, and (4) comparative quantitative analysis using field and map numbered data. RELATIVE AGE DETERMINANTS There are several fundamental principles that can be used as indicators o f the relative age of the terrain. The validity for some of these guidelines is increased when the origin of features being compared is known, and environmental conditions have been similar throughout the1.r development. Stratigraphic Indicators Some of the concepts from stratigraphic geology can be borrowed and can be equally useful for deciphering relative form sequences in the topography of an area. 1. Law of superposition . . . . .that younger materials rest on older materials. Thus, any landform that can be seen to rest on a landform that is now partly hidden by the topmost landform is younger than the one that is partly obscured. Alluvial fans, deltas, and moraines provide good examples where this technique can be used.
2. Law of cross-cutting relations.....that materials had to be in place and predate any events that intrude or cut them. Wind gaps, barbed tributaries, and intrusive troughs (as in the Finger Lakes region, N.Y.) provide examples of this concept.
3. Law of complexity . . . . . that those rock sequences which contain the greatest number of diverse features and structures are older than less complicated rock masses. This rule can be especially applied to those landscapes that have undergone multiple cycles of development. Remnant features in polygenetic terrane are particularly helpful when using this law. Again the Finger Lakes provide a good example because there are vestiges of glacial and fluvial events, and of glacial episodes, interglacial events, postglacial, and later Holocene features. Geomorphic Indicators When used with care, there are several suggestive terrain signatures that can provide strong evidence for the development of sequential landscape chronologies. Each of the following statements should be prefaced with the disclaimer that "other things beine equal such features may be an important indication of relative time." 1. Landform size. As in all other measures, if two different landforms were initially the same height and mass, then a diminution in these variables should be a reflection of age difference (Figure 1). 2. Hillslope steepness. Although the amount of slope is dependent on many factors, many studies have shown that hillslopes trend toward an equilibrium steepness through time. Davis contended that hillslopes were steepest during the "mature stage" of the geographical cycle. My studies, and that of many colleagues, have shown there is a maximum slope of equilibrium for each lithologic type. Deviations from this steepness provide a signal of relative age f o r the feature.
3. Hillslope shape. The subject of hillslope shape is much too involved to be discussed adequately, but a few generalizations are relevant. Convex hillslopes may be considered as "gravity controlled", whereas concave slopes are "wash, or water controlled". Prior to inception of fluvial activity, the principal denudational process would be gravity movement, thus producing convex slopes. With continued development, slopes undergo a transition to concave shapes as stream networks work headward. In this scenario only the transportation and erosional parts of slopes have been considered f o r it is assumed that waste products are being completely evacuated from hillslope locations.
4.
Landscape fabric.
The total array of features that constitute
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t h e l a n d s c a p e can p r o v e h e l p f u l i n r e v e a l i r , g r e l a t i v e a g e s o f d i v e r s e elements. T h i s i s e s p e c i a l l y t r u e when m u l t i . . g e n e r a t i o n e v e n t s h a v e It i s t h e o c c u r r e d and b e e n i n d e l i b l y i n s c r i b e d i n t o t h e t e r r a i n . a n a l y s i s o f t h e s e l a n d s c a p e memory s y s t e m s t h a t p r o v i d e s t h e r e l a t i v e d a t i r 2 g c l u e s . Barbed t r i b u t m i e s a r e one s u c h f e a t u r e . With convent i o n a l ne5WGPkS s t r e a m j u n c t i o n a n g l e s a r e a c u t e and p o i n t downstream. However, when a c o m p e t i n g s y s t e m d r a i n i n g t h e o p p o s i t e d i r e c t i o n capt u r e s t h e h e a d w a t e r s o f a d i s a d v a n t a g e d s y s t e m , t h e uppermost t r i b u t a r i e s a r e i n c o r p o r a t e d i n t o t h e newer s y s t e m . ?he r e s u l t i n g p a t t e r n d i s p l a y s p i r a c y b e c a u s e t h e j u n c t i o r , a n g l e s a r e now " b a r b e d " and t h e o r i g i n a l a c u t e a n g l e PO l o n g e r p o i n t s i n t h e d i r e c t i o n o f v a l l e y lowering. Such a g e o n e t r y t h u s r e v e a l s r e v e r s a l and shows t h e t r i b u t a r i e s antedated t h e master stream (Figure 2 ) .
BARBED
TRIBUTARIES
A
PHASE I Figure 2
The u s e of b a r b e d t r i b u t a r i e s a s a n a g e c r i t e r i o n . Two c o m p e t i n g a n d o p p o s i t e l y f l o w i n g d r a i n a g e s y s t e m s a r e s h o w n i n P h a s e 1. I n P h a s e 2 t h e s o u t h f l o w i n g master stream h a s c a p t u r e d p a r t of t h e Stream segments previously north-flowing drainage. A a n d B a r e now ' ' b a r b e d " b e c a u s e t h e y p o i n t i n o p p o s i t e d i r e c t i o n t o f l o w of master stream.
Water g a p s and wind g a p s a r e o t h e r examples t h a t may i n d i c a t e a p o l y - p h a s e d l a n d s c a p e . T h u s , wind g a p s may r e v e a l a p r i o r h i s t o r y t h a t i s no l o n g e r p a r t o f t h e c u r r e n t f l u v i a l e n v i r o n m e n t . Such f e a t u r e s t h a t are not linked t o present conditions provide evidence f o r t h e i r g r e a t e r a n t i q u i t y . M i s f i t s t r e a m v a l l e y s and o t h e r o u t - o f - a d j u s t m e n t f o r m s g i v e c l u e s t o t h e i r e a r l i e r a n c e s t r y . I n some c a s e s , as des c r i b e d by Dury ( 1 9 6 4 ) s e v e r a l r i v e r s i n England now f l o w i n v a l l e y s whose form d e v e l o p e d by t h e g r e a t l y e n l a r g e d f l o w s d u r i n g P l e i s t o c e n e g l a c i a l e p i s o d e s . The Binghamton, N.Y. a r e a c o n t a i n s many s u c h v a l leys that are similarly underfit. I n a d d i t i o n . some s t r e a m v a l l e y s a r e Although c e r t a i n o v e r f i t , such as t h e Tioughnioga R i v e r V a l l e y . s e c t i o n s a r e wider and r e p r e s e n t t h e u s u a l t o p o g r a p h i c f l a v o r of t h e r e g i o n , o t h e r p a r t s c o n t a i n n a r r o w , i n c i s e d v a l l e y s , and a r e " o v e r f i t " i n r e l a t i o n t o t h e t o t a l s y s t e m . The o v e r a l l a p p e a r a n c e of s u c h " b e a d e d v a l l e y s " i n d i c a t e s d i f f e r e n t a g e s of d e v e l o p m e n t . C o a t e s and K i r k l a n d . ( 1 9 7 4 ) h a v e t h e r e f o r e d e s c r i b e d s u c h t o p o g r a p h y as a m u l t i cycle sluiceway.
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Q u a n t i t a t i v e Morphology A c o n s i s t e n t theme i n t h e g e o g r a p h i c a l c y c l e o f D a v i s and o t h e r s who f o l l o w e d h i s l e a d e r s h i p i s t h e a b s e n c e o f q u a n t i t a t i v e i n d i c e s or numbers t h a t s i g n a l d i f f e r e n t d e v e l o p m e n t a l s t a g e s ( a n d t h e r e b y r e l a t i v e a g e s ) . It i s i m p o r t a n t t o n o t e t h a t Davis d e l i b e r a t e l y chose t h i s a p p r o a c h , i n s p i t e of h i s a b i l i t y t o h a n d l e n u m b e r s . . . . a s e v i d e n c e d b y h i s e a r l y works i n a s t r o n o m y and m e t e o r o l o g y . He s i m p l y p r e f e r r e d t o u s e h i s e x c e p t i o n a l command o f p r o s e t o p r e s e n t i d e a s i n a n e l e g a n t d e s c r i p t i v e manner. Although t h e r e were e x c u r s i o n s i n t o numbers b y v a r i o u s geornorp h o l o g i s t s p r i o r t o 1 9 4 5 , i t was l a r g e l y b e c a u s e o f t h e p i o n e e r work o f H o r t o n ( 1 9 4 5 ) t h a t t h e " q u a n t i t a t i v e r e v o l u t i o n " was i n i t i a t e d i n H i s e a r l i e s t d i s c i p l e s were A . N . S t r a h l e r and t h e geomorphology. Columbia " s c h o o l o f geomorphology". S e v e r a l landform a n a l y s i s param e t e r s t h a t were i n i t i a t e d d u r i n g t h e 1 9 4 5 - 5 2 p e r i o d a r e a p p l i c a b l e f o r u s e i n a s s i g n i n g numbers t o t h e a p i n g p r o c e s s i n f l u v i a l t e r r a n e s .
D r a i n a g e d e n s i t y ( t h e r a t i o o f t o t a l s t r e a m l e n g t h s t o a r e a ) was one o f t h e e a r l i e s t methods t o b e s u c c e s s f u l l y a o p l i e d t o a n a g e r e l a t e d problem. Ruhe ( 1 9 5 2 ) showed t h a t t h e l o c a t i o n of d i f f e r e n t a g e t i l l s h e e t s corresponded w i t h appropriate d i s s i m i l a r drainage densities The Kansan, or o l d e s t d r i f t , h a s a 7 . 9 t o 1 0 . 2 d r a i n a g e d e n s i t y , whereas t h r e e s u c c e s s i v e l y younger Wisconsinan d r i f t s have d r a i n a g e d e n s i t i e : of 6 . 1 t o 7 . 7 , 4 . 7 t o 5 . 4 , a n d 1 . 9 t o 2 . 1 ( F i g u r e 3 ) . H y p s o m e t r i c a n a l y s i s ( t h e r e l a t i o n of r e m a i n i n g l a n d m a s s above b a s e l e v e l t o a t o t a l volume w i t h t h e same a r e a and h e i g h t ) h a s b e e n u s e d b y S t r a h l e r ( 1 9 5 2 ) and o t h e r s a s a t e c h n i q u e for d e t e r m i n i n g r e l a t i v e a g e . T h e h y p s o m e t r i c i n t e g r a l c a n b e viewed as a p e r c e n t o f l a n d mass r e m a i n i n g . S t r a h l e r e q u a t e d h i g h numbers t o a n " i n e q u i l i b r i u m s t a g e ( y o u t h ) , medium numbers a r o u n d 5 0 p e r c e n t a s t h e " e q u i l i b r i u m s t a g e " , and low numbers a s t h e "monadnock p h a s e " . S e v e r a l o t h e r q u a n t i t a t i v e methods w i l l a l s o y i e l d numbers t h a t c a n be p l a c e d i n t o a s y s t e m f o r o r d e r i n g t h e r e l a t i v e a g e of a l a n d scape. Stream b i f u r c a t i o n r a t i o s i n c r e a s e through time t o an e q u i l i b r i u m s t a t e . For e x a m p l e , t y p i c a l b i f u r c a t i o n r a t i o s (number o f t r i b u t a r y b r a n c h i n g s p e r m a s t e r stream o f n e x t h i g h e s t o r d e r ) i n d e n d r i t i c s y s t e m s u s u a l l y f a l l w i t h i n t h e 3 . 5 t o 4 . 5 r a n g e . Thus if t h e b i f u r c a t i o n r a t i o i s c o n s i d e r a b l y l e s s i t most p r o b a b l y i n d i c a t e s a b a s i n t h a t i s n o t f u l l y e x t e n d e d , and t h u s immature i n d e v e l o p m e n t . However e x c e p t i o n s may o c c u r i n e x t r e m e l y a n c i e n t and p r e - P l e i s t o c e n e terranes. The d e g r e e o f s t r e a m s t r a i g h t n e s s , or i t s s i n u o s i t y r a t i o ( c h a n n e l l e n g t h u s v a l l e y s t r a i g h t l i n e l e n g t h ) , i s a n o t h e r i n d e x t h a t can p r o v i d e i n s i g h t i n t o t h e r e l a t i v e a g e of a s t r e a m . I f s t r u c t u r a l c o n t r o l i s n o t a f a c t o r , most s t r e a m s i n c r e a s e c u r v a t u r e w i t h t i m e . Anyone who h a s c h a n n e l i z e d a s t r e a m knows t h i s r u l e b e c a u s e a f t e r c h a n n e l i z a t i o n t h e s t r e a m i n v a r i a b l y a t t e m p t s t o r e - e s t a b l i s h i t s o r i g i n a l meandering c o u r s e . S t r a i g h t s t r e a m s e n d u r e o n l y when e r o s i o n a l f o r c e s expand a l l e n e r g y i n d o w n c u t t i n g and t h e r e i s c o m p l e t e e v a c u a t i o n of v a l l e y d e b r i s . U l t i m a t e l y , however, v a l l e y w i d e n i n g e n s u e s whereby t h e c h a n n e l i s widened by l a t e r a l m i g r a t i o n o f t h e t h a l w e g w i t h accompanying d e v e l o p ment o f a f l o o d p l a i n . S i n u o s i t y c a n t h e n be a m e a s u r e of r e l a t i v e a g i n g i n t h i s p r o c e s s . Low numbers r e f l e c t e a r l i e r d e v e l o p m e n t a l s t a g e s t h a n h i g h e r numbers.
CONSTRAINTS TO A SYSTEM FOR RELATIVE D A T I N G T h e r e a r e two r e a l m s o f c o n s t r a i n t s t h a t r e s t r i c t t h e o p e r a t i o n o f a u n i v e r s a l s y s t e m whereby r e l a t i v e d a t e s can be e s t a b l i s h e d f o r a l l t e r r a n e s . . . . p r i m a r y c o n s t r a i n t s and s e c o n d a r y c o n s t r a i n t s . The p r i n c i p a l r u l e i s t o compare t o p o g r a p h i e s t h a t h a v e t h e most e n v i r o n m e n t a l f a c t o r s i n common. For example i t would b e e r r o n e o u s t o a t t e m p t c o r r e l a t i o n o f r e l a t i v e a g e s o f t h e S c o t l a n d and West V i r g i n i a h i g h l a n d s . T h e r e a r e t o o many d i s s i m i l a r i t i e s s o t h e c o m p a r i s o n would b e i n v a l i d .
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> d h -
o
-
w-0 4 C C d
3rimary C o n s t r a i n t s 1. L i t h o l o g y and s o i l s . The c o r n p o s i t i 3 n cf m a t e r i l l s d e t e r m i r - e s t h e e f f e c t i v e n e s s s? d e n l d a t i z n a l p r o c e s s e s . They a l s c : i n f l u e r c e t y p e &?A amount o f v e g e t a ” o r , i n f i l t r a t i o n a n d runof!’ P r o p e r t L e s c_” p r e ~ L p i t a t i c n , a n d cherrcter o f weathering e t c .
2. S t r u c t u r e . Eere we include both t h e g r c s c r e l a t i o n s such E S t h e n a t u r e o f f o l d i n g a n d f a u l k i n g , a n d t?e srrallzr s c s l e Teatllres cf b e a d i n g , foliation, j o i z t i n g , c l e a v a g e , e t c . A h i g h l y fyactured
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t e r r a n e w i l l progress di"erert1y discontinuities.
t h r o u g h t i m e t h a n o t h e r s w i t h Fewer
3 . G e o l o g i c p r o c e s s e s . Not o n l y d o t h e e n d o g e n i c f o r c e s of v o l canism and d i a s t r o p h i s m produce d i f f e r e n t t e r r a n e s , b u t t h e exogenic f o r c e s a l s o d i f f e r , among t h e m s e l v e s . . . . . r u n n i n g w a t e r , g r o u n d w a t e r , g r a v i t y movements, s h o r e l i n e a c t i v i t y , g l a c i e r s , and w i n d .
4 . L a n d f o r m m a g n i t u d e . For b e s t c o m p a r i s o n s o f r e l a t i v e t i m e whenever p o s s i b l e o n l y t h o s e f e a t u r e s t h a t s t a r t e d t h e i r development w i t h similar landmass volume and r e l i e f s h o u l d b e u s e d . The p r o g r e s s i o n o f f e a t u r e s i n a l a r g e l a n d m a s s may d i f f e r f r o m t h o s e o f a s m a l l e r one. 5 . C l i m a t e . The i d e a o f h o m o l o g o u s l a n d s c a p e d e v e l o p m e n t i n d i f f e r e n t c l i m a t e s a s p r o p o s e d by K i n g ( 1 9 5 3 , 1 9 6 7 ) i s u n a c c e p t a b l e . The c o n c e p t o f m o r p h o g e n e t i c r e g i o n s a s e x p r e s s e d by P e l t i e r ( 1 9 5 0 ) i s m o r e a p p l i c a b l e . Even D a v i s ( 1 9 3 5 ) r e c o g n i z e d t h a t d i f f e r e n t c r i t e r i a m u s t b e e m p l o y e d t o e v a l u a t e t h e a g i n g p r o c e s s i n h u m i d a r e a s a s cont r a s t e d w i t h a r i d a r e a s . C l i m a t e i n f l u e n c e s n o t o n l y water a v a i l a b i l i t y a n d v e g e t a t i o n , b u t some r o c k t y p e s , s u c h a s l i m e s t o n e h a v e i n v e r s e t o p o g r a p h i c r e l a t i o n s ( v a l l e y - f o r m i n g i n humid t e r r a n e , a n d cliff-forming under dry c o n d i t i o n s ) . 6 . P h y s i o g r a p h i c p r o v i n c e . When a l l f a c t o r s a r e summed, t h e i r blend i s t h e production of d i s t i n c t i v e physiographic provinces. Such d i s t i n c t i v e r e g i o n s s h o u l d n o t b e c r o s s e d for c o m p a r a t i v e p u r p o s e s . T h u s l a n d f o r m s as d e v e l o p e d i n t h e F o l d e d A p p a l a c h i a n s rxay b e a r a d i f f e r e n t developmental h i s t o r y t h a n t h o s e i n t h e Appalachian P l a t e a u . Secondary C o n s t r a i n t s The s e c o n d a r y l i m i t s t h a t s h o u l d b e c o n s i d e r e d when e v a l u a t i n g t o p o g r a p h i c r e l a t i v e t i m e sequences are n o t s o p e r v a s i v e and u n i v e r s a l as p r i m a r y c o n s t r a i n t s , b u t y e t can c r e a t e p r o b l e m s i f n e g l e c t e d . 1. P r i n c i p l e o f e q u i f i n a l i t y . A s applied t o landforms, t h e m e a n i n g of t h i s c o n c e p t i s t h a t s i m i l a r a p p e a r i n g f e a t u r e s may h a v e f o r m e d by d i f f e r e n t p r o c e s s e s . T h u s , t h e r e may b e n o u n i q u e s o l u t i o n f o r some c a s e s . A 5 0 m h i g h h i l l w i t h 2 0 " s l o p e s may b e c r e a t e d by many d i f f e r e n t p r o c e s s e s , a n d i t s e v o l u t i o n may h a v e t a k e n e n t i r e l y d i f f e r e n t t i m e p e r i o d s . T h e r e f o r e when d e t e r m i n i n g r e l a t i v e a g e , a p r i m a r y r e q u i r e m e n t i s knowledge o f t h e g e n e s i s o f t h e l a n d f o r m . 2. P o l y g e n e t i c t e r r a n e s . Many l a n d s c a p e s r e p r e s e n t h y b r i d s y stems t h a t d e v e l o p e d f r o m t h e i n t e r a c t i o n o f s e v e r a l d i f - f e r e n t p r o c e s s e s . A l t h o u g h i n some r e g i o n s t h i s may c r e a t e s p e c i a l p r o b l e m s ( e s p e c i a l l y when t h e d i f f e r e n c e s a r e n o t r e c o g n i z e d ) , i n o t h e r a r e a s s u c h c o m p l e x i t y may p r o v e a b l e s s i n g . I n s o u t h - c e n t r a l N.Y. t h e s e q u e n c e of g l a c i a l e p i s o d e s , a n d i n t e r g l a c i a l p e r i o d s h a v e e a c h l e f t s u f f i c i e n t l y x n i q x e l a n d f o r m a r r a y s t h a t t h e t o t a l l a n d s c a p e c a n be placed i n t o a r e l a t i v e - d a t i n g chronology.
3 . R e l i c t t e r r a n e s . One s h o u l d n o t h a v e a b l i n d f a i t h i n t h e law of u n i f o r m i t a r i a n i s n . I t i s e a s y f o r t h e f i e l d o b s e r v e r to l o o k a t a n d m e a s u r e t e r r a i n f e a t u r e s , a n d to a s s u m e p r e s e n t c o n d i t i o n s h a v e
c r e a t e d t h e l a n d s c a p e p e r s o n a l i t y h e now w i t n e s s e s . However t h e magnit u d e a n d f r e q u e n c y o f a c t i v i t y may h a v e c h a n q e d a n d t h e D r o c e s s e s may h a v e a l t e r e d . T h u s , i f t h e s l o p e s were a n i n h e r i t a n c e f r o m a v a s t l y d i f f e r e n t environment s u c h as " h i n d c a s t i n g " p r o c e d u r e w i l l p r o v i d e a f a l s e f o u n d a t i o n f o r f o r e c a s t i n g f u t u r e d e v e l o p m e n t of t h e s l o p e s . The m o s t d i f f i c u l t f e a t u r e s t o a s s e s s The a b s e n c e of i n t e r v e n i n g m a r k e r s r e d u c e s t h e number o f y a r d s t i c k s t h a t c a n b e u s e d f o r c o m p a r ison purposes.
4.
Single cycle landforms.
are those with uninterrupted h i s t o r i e s .
5. T o p o g r a p k L c f e a t u r e s t h a 5 y e v e r s e w i t h tirr.e. A I % k o u g h r e s t r i c t e d t o Quaternary t i m e i n t h i s d i s c u s s i o n , under c e r t a i n conditions
257
l a n d f o r m r e v e r s a l s c a n o c c u r w i t h i n t h i s t i m e s p a n . H i l l s l o p e s may b e g i n w i t h convex p r o f i l e s ( F i g u r e 4), t h e n change t o concave. and r e v e r s e t o convex a g a i n a t a d v a n c e d s t a g e . . . . e s p e c i a l l y when d e p o s i t i o n a l m a t e r i a l s a r e i n v o l v e d a s i n t h e p e d i m e n t s l o p e s n e a r Phoenix, A r i z o n a (b!oss, 1977). H i l l s l o p e s may b e i n i t i a l l y g e n t l e , s t e e p e n w i t h a g e , and a g a i n d e c l i n e i n a n g l e o f p r o f i l e a t some s u b s e q u e n t a g e . O f c o u r s e t h i s was a b a s i c component o f D a v i s ' humid g e o g r a D h i c c y c l e . ;$any q u a n t i t a t i v e s t u d i e s o f h i l l s l o D e s i n d i f f e r e n t s t r e a m o r d e r s y s t e m s , show t h a t f i r s t and s e c o n d o r d e r s l o p e s a r e more p e n t l e t h a n t h i r d and fourth o r d e r s l o p e s .
S L O P E R E V E R S A L WITH T I M E Phase
2.
Figure 4
Slope Type
Process
CONVEX
GRAVl T Y
CONVEXCONCAVE
GRAVITYWASH
CONCAVE
WASH
CONVEX
G R A V I T Y WITH DEEP WEATHERING
Slope r e v e r s a l with t i m e . During i n i t i a l Phase 1 t h e h i l l s i d e has a convex slope p r i o r t o e s t a b l i s h ment o f r u n n i n g water d e v e l o p m e n t . With i n c r e a s i n g l y g r e a t e r i n f l u e n c e of w a t e r t h e s l o p e becomes A deep i n c r e a s i n g concave i n Phases 2 and 3 . weathering Phase 4 s t a g e caused slope t o again r e v e r t t o a convex form.
6 . T h r e s h o l d s e v e n t s . C o n d i t i o n s b e f o r e and a f t e r t h e s h o l d s have o c c u r r e d may r e v e a l e x c e p t i o n a l l y d i v e r s e t e r r a i n c h a r a c t e r i s t i c s . F a i l u r e t o r e c o g n i z e t h e h i e r a r c h y can l e a d t o i n a c c u r a t e l a n d s c a p e analysis. 7 . R a t e of change o f v a r i a b l e s . P h a s e s i n l a n d f o r m develol3ment r e f l e c t t h e m a g n i t u d e and r a p i d i t y o f f o r c e s t h a t Droduce c h a n g e . T h e r e f o r e , t h e end p r o d u c t s may a p p e a r s i m i l a r , b u t g r e a t e r f o r c e a c t i n g more o f t e n w i l l c r e a t e t h e same form i n a f r a c t i o n o f t h e t i m e i t w i l l t a k e a weaker p r o c e s s . 8 . The a b b r e v i a t e d Q u a t e r n a r y l e n g t h o f t i m e . Some l a n d s c a p e s may b e s o i n s e n s i t i v e t h a t t h e y f a i l t o r e v e a l s i g n i f i c a n t c h a n g e s w i t h i n t h e 3 my o f Q u a t e r n a r y t i m e . 9 . Human l a n d s c a p e i m p a c t s . The f u l l s c a l e and c h a r a c t e r of topOgraphTC c h a n g e s t h a t can b e a t t r i b u t e d t o human a c t i v i t y r a i s e s important questions. I n some c a s e s , human i m p a c t s may h a v e b e e n s o s e v e r e a s t o b l u r n a t u r a l changes i n t h e l a n d - w a t e r e c o s y s t e m .
258
RELATIVE DATING OF FLUVIAS LANDFORMS
Although e v e r y g e o l o g i c p r o c e s s t h a t i n t e r a c t s w i t h t h e e a r t h ' s s d r f a c e h a s i t s own s e t of r e l a t i v e a g e l a n d f o r m i n d i c a t o r s , e x a m p l e s cf t h r e e of t h e s e p r o c e s s e s w i l l b e e m o h a s i z e d . . . . f l u v i a l , g l a c i a l , and f a u l t e d t e r r a n e . S t r e a m Channel Development The r e l a t i v e a g e of a c h a n n e l c a n b e d e t e r m i n e d f r o m b o t h t h e d e p o s i t s and t h e c h a n n e l changes i n meandering r i v e r s . When a b e p d c r o g r e s s i v e l y m i g r a t e s a c r o s s a f l o o d p l a i ? a s e r i e s of p o i n t b a r s may h e l e f t i n i t s wake t h a t i n d i c a t e s b o t h r e l a t i v e t i m e a n d p o s i t i o n of formation. I n a d d i t i o n , d u r i r p flooding t h e channel c o n s t r i c t i o n of a m e a n d e r b e n d c a n b e s e v e r e d w i t h i n t r u s i o n of a new c h a n n e l t h r o u g h t h e o l d e r d e p o s i t s ( F i g u r e 5 ) . H i c k i n ( 1 9 7 4 ) a n d H i c k i n a n d Nansor, (1975) h a v e u s e d s u c h c r i t e r i a t o d e t e r m i n e c h a n n e l s e q u e n c e s for rreandering r i v e r s .
Figure 5 P r i n c i p l e of c r o s s c u t t i n g r e l a t i o n s as r e v e a l e d by f o u r s u r v e y s of t h e Mississippi River. Younger s u r v e y s show the r i v e r has cut through channels f o r m e r l y o c c u p i e d by earlier river positions. After U . S . Army C o r p s o f E n g i n e e r s and A.N. St r a h l e r
.
259
Stream Terraces Terraces carved in valley fill sediments provide a systematic technique to determine relative age. Davis (1902) in his classic study o f New England terraces, and Fairbridge (1968) outline the use of such landforms, If there is no upset in normal terrace evolution, the higher the terrace the older its formation (Figure 6). Fluvial history chronologies have thus been analyzed by numerous authors such as Jenkins (19641, Everitt (1968?, and Womack and Schumm (1977). Glaciofluvial terraces have also been studied by Peltier (1949) and Brunnacker (1975).
Figure 6
R e l a t i v e a g e of a l l u v i a l terraces can b e i n f e r r e d by t h e i r p o s i t i o n i n t h e v a l l e y . Older terraces are higher than younger t e r r a c e s . (After Strahler,
1951).
Alluvial Fans and Deltas These stream depositional forms have many common elements. They both form at a drainage terminus and are composed of sediments discharged from an initiating channel. The sediment regime changes reflect the channel shifting over the landform, leaving a legacy that reveals the sequence of formation (Gole and Chitale, 1966). The youngest depositing event is the one that has covered the greatest number of different prior depositional sites (Figure 7). In similar manner the youngest channel is the one that has incised through the most former channel sites. These criteria have been used in the alluvial fan studies of Bull (1964), Hunt and Mabey (1966), and Ruhe (1967). ‘Kolb and Van Lopik (1966) provide classic w o r k for dating of the Mississippi River delta, and Coleman (1968) synthesizes the geomorphic evolutionary development in deltaic environments (Figure 8).
260
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Map v i e w a n d e x p l a n a t i o n o f s e d i m e n t s c o m p r i s i n g J o h n s o n Canyon a l l u v i a l € a n i n D e a t h V a l l e y , C a l i f o r n i a ( a f t e r Hunt a n d Mabey, 1 9 6 6 ) . The younger €an g r a v e l s o c c u r superposed on o l d e r gravels.
Figure 8
M i s s i s s i p p i River d e l t a t i m e sequences ( a f t e r Kolb a n d Van L o p i k , 1 9 6 6 ) . Respective ages for deltas a r e S a l e - C y p r e m o r t > 4 6 0 0 yBP; C o c o d r i e 4600-3500 yBP; T e c h e 3 5 0 0 - 2 8 0 0 yBP; S t . B e r n a r d 2 8 0 0 - 1 0 0 0 yBP; L a F o u r c h e 1 0 0 0 - 3 0 0 yBP; P l a q u e m i n e 550-500 yBP; B a l i z e < 5 5 0 y r s .
261
RELATIVE D A T I N G OF G L A C I A L LANDFORVS S i z e and form o f t e r r a i n f e a t u r e s c r e a t e d by g l a c i a l a c t i o n a r e p a r t i c u l a r l y vexing i n t h e i r i n a b i l i t y t o contain r e l a t i v e age i n d i cators. Such l a n d f o r m s a s e s k e r s , kames, d r u m l i n s , c i r q u e s , Uvshaped v a l l e y s , e t c . l a r g e l y r e f l e c t t h e i n t e n s i t y and d u r a t i o n o f t h e p r o c e s s t h a t formed them. However, some g l a c i a l l a n d f o r m s a r e u s e f u l f o r r e l a t i v e time chronologies. Strandlines These s u r f a c e s r e p r e s e n t t h e l a n d - w a t e r i n t e r f a c e of p r o g l a c i a l l a k e s and can be used ( E l s o n , 1 9 6 7 ) as t i m e - l i n e i n d i c a t o r s . P o s t g l a c i a l rebound p r o d u c e s a t i l t i n g of t h e former h o r i z o n t a l s t r a n d l i n e s , and t h e g r e a t e r t h e d i s t o r t i o n o f t h e s t r a n d l i n e s t h e o l d e r t h e forma t i o n of t h a t s u r f a c e ( F i g u r e 9 ) . +
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G l a c i a l rebound produced t h e s e upwarped s t r a n d l i n e s of g l a c i a l Lake A g a s s i z . Four of t h e former shorel i n e s were r a d i o c a r b o n d a t e d a s b e i n g 1 1 , 7 0 0 yBP, 9200 yBP, 8000 yBP, a n d 7000 yBP. Note t h a t t h e older strandlines contain the steepest gradients. (After Elson, 1967).
Um l a u fb e r g s
C o a t e s (1974) p r o v i d e d a b r i e f d e s c r i p t i o n o f t h e s e forms i n t h e g l a c i a t e d A p p a l a c h i a n P l a t e a u , and i n d i c a t e d t h a t d i f f e r e n c e s i n t h e i r development m i g h t r e v e a l t h e number o f c y c l e s t h e y e x p e r i e n c e d ( F i g u r e 10). I n t h i s r e g i o n , u m l a u f b e r p s d e v e l o p when i c e b l o c k s m e l t w a t e r d r a i n a g e p a t h w a y s , f o r c i n g w a t e r t o i n c i s e a new c h a n n e l which s e v e r s t h e h i l l from i t s o r i g i n a l u p l a n d s e t t i n g . F e a t u r e s t h a t can be u s e d (1) The p r e s e n c e o f t o d e c i p h e r t h e c y c l e s of e v o l u t i o n i n c l u d e : t r i b u t a r i e s . I f t h e r e a r e no t r i b u t a r i e s , t h e u m l a u f b e r g would b e f i r s t c y c l e b e c a u s e i n s u f f i c i e n t p o s t g l a c i a l t i m e has n o t a l l o w e d stream d e v e l o p m e n t . (2) H e i g h t of u m l a u f b e r g comDared w i t h uDland height. The g r e a t e r t h e h e i g h t d i s p a r i t y , t h e o l d e r t h e u m l a u f b e r g , and ( 3 ) S i z e of v a l l e y s . Younger u m l a u f b e r g s g e n e r a l l y h a v e l a r g e d i f f e r e n c e s i n w i d t h o f t h e two v a l l e y s . F o r example w i t h r e D e a t e d g l a c i a t i o n s or w i t h l o n g e r t i m e s p a n s , i s o l a t i o n becomes i n c r e a s e d for t h e bedrock o u t l i e r .
262
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4 . S t a r t o f i n t e r g l a c i a l time w i t h remnant o f f i r s t c y c l e umlaufberg. 5 . With prolonged i n t e r g l a c i a l e r o s i o n new t r i b u t a r i e s form on t h e umlaufberg and i n t h e a d j a c e n t highlands.
Umlaufberg development.
Sluiceways Sluiceways, as used i n t h i s p a p e r , a r e major c o u r s e s f o r g l a c i a l meltwater r i v e r s . I n a d d i t i o n , I have r e s t r i c t e d d i s c u s s i o n t o t h o s e w i t h p r o n o u n c e d e r o s i o n . They d i f f e r from o v e r f l o w c h a n n e l s o r s p i l l ways by v i r t u e o f d e n u d a t i o n t o t h e g e n e r a l b a s e l e v e l p o s i t i o n o f f l u v i a l t o p o g r a p h y , When e a c h g l a c i a l and i n t e r g l a c i a l e p i s o d e l e a v e s i t s i m p r i n t , t h e n a chronology o f s l u i c e w a y c y c l e s can b e c a l c u l a t e d . F i r s t c y c l e s l u i c e w a y s d e v e l o p when m e l t w a t e r c a r v e s a new v a l l e y or combines e l e m e n t s from p r e - e x i s t i n g v a l l e y s i n a new f l o w p a t t e r n .
T h i s s t a g e c a n b e r e c o g n i z e d by t h e a b s e n c e o f w e l l i n t e g r a t e d t r i b u - . t a r i e s and o f t i l l i n t h e v a l l e y .
Second c y c l e s l u i c e w a y s c o n t a i n b e t t e r d e v e l o p e d t r i b u t a r i e s b e c a u s e d u r i n g i n t e r g l a c i a l e p i s o d e s s u f f i c i e n t t i n e has e l a p s e d f o r t h e i r creation. I f i c e c o v e r e d t h e a r e a , t i l l may h a v e b e e n l e f t i n p a r t s of t h e v a l l e y . Multicycle sluiceways i n v a r i a b l y c o n t a i n t o t a l v a l l e y systems t h a t a r e beaded as a l r e a d y d e s c r i b e d f o r t h e Tioughnioga R i v e r . P a r t s of t h e s y s t e m w i l l c o n t a i n w i d e v a l l e y s , o t h e r s w i l l be n a r r o w . P a r t s w i l l have f u l l y i n t e g r a t e d t r i b u t a r y development, o t h e r p a r t s w i l l have a p a u c i t y of stream j u n c t i o n s .
263
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S e v e r a l o t h e r g l a c i a l l a n d f o r m s may o f f e r p o s s i b i l i t i e s f o r r e l a t i v e d a t i n g p u r p o s e s , For e x a m p l e , w i t h i n a s e r i e s o f r e c e s s i o n a l m o r a i n e s , a s i n m i d w e s t e r n U . S . , t h e o v e r l a p of r i d g e s can d e t e r m i n e t h e i r sequence. I n c o r r e l a t i n g moraines, t h e s t r a i g h t n e s s of t h e r i d g e may o f f e r d a t i n g c l u e s . Such s t r a i g h t n e s s may b e a f u n c t i o n o f i c e t h i c k n e s s , or i n d i c a t e t h e e f f e c t i v e n e s s o f p o s t g l a c i a l e r o s i o n . T h u s , s t r a i g h t m o r a i n e s would have b e e n formed b y icc. t h a t was t h i c k e r , t h e r e f o r e o l d e r , w h e r e a s s e r r a t e r i d g e s would r e p r e s e n t e i t h e r t h i n n e r i c e , o r landforms t h a t have undergone c o n s i d e r a b l e e r o s i o n . RELATIVE D A T I N G OF E N D O G E N I C PROCESSES The e a r t h ' s i n t e r i o r f o r c e s of v o l c a n i s m and d i a s t r o p h i s m can p r o d u c e some l a n d f o r m s t h a t r e v e a l t h e r e l a t i v e a g e o f t h e p r o c e s s o f f o r m a t i o n . Lava f l o w s c a n b e p l a c e d i n t o a c h r o n o l o g i c s e q u e n c e b e c a u s e t h e y obey t h e r u l e s o f s u p e r p o s i t i o n . The b r e a c h i n g of c r a t e r s , a s i n c i n d e r c o n e s c a n p r o v i d e i n s i g h t i n t o t h e e f f e c t i v e n e s s , and perhaps length of time, of the denudational process. Cross-cutting of j o i n t s e t s can d e t e r m i n e t h e i r s e q u e n c e , and many w o r k e r s h a v e shown t h e i n f l u e n c e o f j o i n t s i n t e r r a i n development u n d e r c e r t a i n e n v i r o n m e n t a l c o n d i t i o n s . I n t r u s i o n b y s a l t domes c a n d e v e l o p d i a g n o s t i c f e a t u r e s r e g a r d i n g t h e i r r e l a t i v e a g e , and w h e t h e r t h e y a r e s t i l l a c t i v e o r n o t . T h i s becomes a p r a c t i c a l p r o b l e m . b e c a u s e s a l t domes h a v e b e e n viewed a s a p o s s i b l e r e c e p t i c l e f o r n u c l e a r w a s t e s . The s a l t domes i n Texas h a v e b e e n shown t o c o n t a i n y o u n g e r f e a t u r e s n e a r t h e c o a s t and t o b e r e l a t i v e l y o l d e r f a r t h e r i n l a n d .
Faults Knowledge o f p o s s i b l e f a u l t i n g , a n d t h e r e c u r r e n c e i n t e r v a l o f s e i s m i c a c t i v i t y , i s c r u c i a l i n l a n d p l a n n i n g of u n s t a b l e r e g i o n s . The s i t i n g o f n u c l e a r power p l a n t s i s e s p e c i a l l y c r i t i c a l , and r e g u l a t i o n s t h a t g o v e r n t h e i r l o c a t i o n r e q u i r e knowledge o f t h e f a u l t a g e . The f o l l o w i n g l i s t i n d i c a t e s some of t h e t y p e s of g e o m o r p h i c e v i d e n c e t h a t can b e u s e d t o a s s e s s t h e r e l a t i v e d a t e s o f f a u l t s : (1) C r o s s c u t t i n g r e l a t i o n s ; ( 2 ) D e g r e e o f change o f s p u r s and f a c e t s ( F i g u r e 11); ( 3 ) R e t r e a t o f e s c a r p m e n t from o r i g i n a l f a u l t p o s i t i o n ; ( 4 ) Degree o f r e - e s t a b l i s h m e n t o f w a t e r t a b l e t h a t may h a v e b e e n c u t b y v e r t i c a l f a u l t movement; ( 5 ) D e g r e e of a d j u s t m e n t i n r e - e s t a b l i s h m e n t o f s t r e a m equilibrium p r o f i l e s .
F i g u r e 11
Evolutionary erosional development showing relative age differences in topographic sequences of a t i l t e d f a u l t b l o c k . ( A f t e r W.M. D a v i s a n d A.N. S t r a h l e r ) .
B u l l ( 1 9 7 7 , 1 9 7 8 ) and Hamblin, Damon, a n d B u l l ( 1 9 8 1 ) h a v e p r o v i d e d scme g u i d e l i n e s i n t h e r a p i d l y g r o w i n g d i s c i p l i n e o f t e c t o n i c geom'orphology. They show how a s t r a i g h t m o u n t a i n f r o n t m i g h t b e i n d i c a t i v e of more r e c e n t and a c t i v e f a u l t movement. R e s i d u a l knobs o f b e d r o c k p r o t r u d i n g above t h e e r o s i o n a l p l a i n o f a p e d i m e n t i s g e n e r a l l y
264
a s s o c i a t e d w i t h a more s i n u o u s m o u n t a i n f r o n t , and b o t h e l e m e n t s a r e s u g g e s t i v e of r e l a t i v e t e c t o n i c q u i e s c e n c e . I n t h e San G a b r i e l M o u n t a i n s , C a l i f o r n i a , B u l l (1978) u s e d s t r e a m t e r r a c e s t o d e t e r m i n e t h e f r e q u e n c y and m a g n i t u d e o f l o c a l f a u l t i n g e p i s o d e s . Where t e r r a c e s c r o s s f a u l t z o n e s , t h e y o u n g e s t t e r r a c e s were g e n e r a l l y n o t f a u l t e d , w h e r e a s o l d e r t e r r a c e s r e v e a l e d p r o g r e s s i v e l y more g r o u n d r u p t u r e . A t a s p e c i f i c p o i n t on a s t r e a m t h a t p o s i t i o n a c t s as a l o c a l b a s e l e v e l , a f f e c t i n g s l o p e s and s t r e a m s upstream and downstream. When a n e v e n t o c c u r s t o change e l e v a t i o n o f t h a t p o i n t t h e b a s e l e v e l c o n d i t i o n s become c h a n g e d . A combination of t e c h n i q u e s were u s e d i n c l u d i n g c h a r a c t e r i s t i c s o f t e r r a c e s , s o i l p r o f i l e s , and a l l u v i a l f a n d e p o s i t s t h a t e n a b l e d B u l l t o d e t e r m i n e r e l a t i v e a g e s o f f a u l t i n g and m a g n i t u d e s d u r i n g t h e l o 5 t o l o 6 y r I s e r i o d . I n t h e Mojave D e s e r t , B u l l ( 1 9 7 7 ) was p a r t i c u l a r l y i n t e r e s t e d i n d e t e r m i n i n g which f a u l t s had b e e n a c t i v e d u r i n g t h e p e r i o d from 5 O 0 , O O O t o 1 m i l l i o n y r a g o . H i s l a n d s c a p e s t u d i e s showed t h a t t h e e a s t e r n and w e s t e r n p a r t s had been t e c t o n i c a l l y i n a c t i v e whereas t h e c e n t r a l a r e a had r e c e i v e d r e p e a t e d f a u l t movements. Along t h e e a s t e r n m a r g i n s o f t h e B a s i n and Range p r o v i n c e , Harnblin, Damon, and B u l l ( 1 9 8 1 ) combined K-Ar d a t i n g o f Cenozoic b a s a l t s and l a n d f o r m a n a l y s i s t o d e t e r m i n e t h e d e f o r m a t i o n r a t e , and i t s v a r i a n c e i n t i m e and s p a c e . R a d i o m e t r i c d a t i n g made p o s s i b l e t h e e s t i m a t i o n f o r r a t e o f r e g i o n a l and l o c a l u p l i f t b y comparing p r o f i l e s o f a n c i e n t s t r e a m s p r e s e r v e d b e n e a t h t h e i s o t o p i c a l l y d a t e d l a v a f l o w s w i t h prof i l e s of p r e s e n t s t r e a m s . A g g r a d a t i o n and d e g r a d a t i o n p r o c e s s e s d i r e c t l y r e f l e c t t h e t e c t o n i c s o f t h e l o c a l e where a l a v a f l o w i s extruded. I f t h e s t r e a m c h a n n e l i s l o w e r i n g t h e l a v a becomes b u r i e d i n alluvium, but i f t h e stream i s s t a b l e t h e d i s p l a c e d channel approaches i t s p r e v i o u s p r o f i l e . T o p o g r a p h i c i n v e r s i o n u p s t r e a m from a f a u l t w i l l o c c u r when t h e d i s p l a c e m e n t c a u s e s a c c e l e r a t e d e r o s i o n . These s t u d i e s showed t h a t t h e Grand Wash a r e a i s r i s i n g 2 6 m / m y , t h e a r e a between t h e Grand Wash and H u r r i c a n e f a u l t s i s r i s i n g 9 0 m / m y , a n d t h e b l o c k e a s t o f t h e H u r r i c a n e f a u l t i s r i s i n g 390 m / m y . CONCLUSIONS A l t h o u g h r e l a t i v e d a t i n g d o e s n o t h a v e t h e p r e c i s i o n of a b s o l u t e d a t i n g , i t can s t i l l p r o v i d e i m p o r t a n t and n e c e s s a r y i n s i g h t s i n t o t h e c h r o n o l o g y o f e v e n t s t h a t s h a p e t h e e a r t h ' s s u r f a c e . The t e c h n i q u e s used t o e s t a b l i s h t h e r e l a t i v e d a t e s of e v e n t s a r e b e i n g i n c r e a s i n g l y u s e d b e c a u s e o f t h e n e c e s s i t y t o e s t a b l i s h an e n v i r o n m e n t a l data b a s e for l a n d u s e p l a n n i n g . REFERENCES CITED B a t e s , R.L. a n d J a c k s o n , J.A., eds., 1 9 8 0 , G l o s s a r y of G e o l o g y , 2nd ed.: F a l l s C h u r c h , V a . , A m e r . G e o l . I n s t . , 749 p . 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, 1 9 7 7 , T e c t o n i c geomorphology o f t h e Mojave Desert, C a l i f o r n i a , C o n t r a c t r e p o r t f o r t h e O f f i c e of Earthquake S t u d i e s , U . S . Geol. S u r v e y , Menlo P a r k , C a l i f o r n i a , 188 p . , 1978, South f r o n t of t h e San G a b r i e l Mountains, s o u t h e r n California, Contract report f o r the Office of Earthquake Studies, U.S. G e o l . S u r v e y , M e n l o P a r k , C a l i f o r n i a , 1 0 0 p . Cambers, G . , 1976, Temporal s c a l e s i n c o a s t a l e r o s i o n systems: I n s t . B r i t . Geog.: N e w S e r i e s , v . 1, p . 2 4 6 ~ 2 5 6 .
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C . R . a n d Van L o p i k , R . R . , 1966, D e p o s i t i o n a l environments of t h e M i s s i s s i p p i River and d e l t a i c p l a i n s o u t h e a s t e r n L o u i s i a n a , i n S h i r l e y , M.L. a n d R a g s d a l e , J . A . , e d s . , D e l t a s a n d t h e i r G e o l o g i c Framework, Houston Geol. S O C . , p . 17-61.
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269
RELATIVE DATING OF SOILS AND PALEOSOLS W.J. VREEKEN
ABSTRACT Relative dating of a soil includes establishing the succession and duration of pedogenic processes involved in the principal stages of its development, as much as the duration of various environmental and geomorphic conditions that may have successively prevailed in the soillandscape, apart from determining the total duration of the formation of that soil. Practices of relative dating include dating of geomorphic surfaces by stratigraphical means, and the use of pedogenic indices, whether as direct measures of soil age, or indirectly, wheninterpreting soils as environmental indicators. These practices are reviewed and numerous principles, bearing on them, are formulated. Application of the stratigraphical approach towards pedochronological analysis of functional soil-landscape units is illustrated with examples from enclosed and open hill-slope systems. INTRODUCTION Relative dating of an object amounts to reconstructing conditions, processes and events thought to bear on its nature. Relative dating of a soil places it in an historical framework of geologic, geomorphic, climatic and biotic conditions, processes and events, some of which may have been anthropogenically influenced. Relative soil dating involves simultaneous consideration of soil age and soil genesis. A soil (Ruhe, 1972) is a naturally occurring three-dimensional body with morphology and properties resulting from effects of climate, flora and fauna, parent rock materials and topography imparted through time (Figure 1A). A soil occupies a portion of the land surface, is mappable, and commonly is composed of horizons that parallel the ground surface. A vertical section downward through all the horizons of the soil is called a soil profile.
A paleosol is a soil that began forming on a landscape in the past (Ruhe,1965). Three basic kinds of paleosols are relict, buried and exhumed soils. Relict soils (Figure 1B) began forming on landscapes in the past that escaped destruction or burial and persist as relict geomorphic surfaces within the present-day topography (Thornbury, 1969). Because relict soils remained subaerially exposed during and after changes in geomorphic regimen, pedogenesis continued and younger pedogenic features were superprinted on their older, relict features. A soil that developed under var.ious climatic and biotic, but essentially unchanged geomorphic, regimens (Figure lC), also may be regarded as a relict soil, because it may have retained imprints from past pedogenic regimens. Normally however, such a soil is called a polygenetic soil. Actually, most soils and paleosols are polygenetic because climatic and biotic factors have changed during their development. A buried soil (Figure 1D) formed on a preexisting landscape and was subsequently buried by younger sediment or rock. An exhumed or
270
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-----------YEARS BEFORE PRESENT (B P )
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FORMATION OF A GROUN DSOlL
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b
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F i g u r e 1A-lB S c h e m a t i c i l l u s t r a t i o n s o f t h e F o r m a t i o n o f G r o u n d s o i l s (A) and R e l i c t S o i l s ( B ) .
r e s u r r e c t e d s o i l ( F i g u r e 1E) i s a s o i l t h a t , once b u r i e d , h a s b e e n ree x p o s e d on t h e p r e s e n t - d a y t o p o g r a p h y by e r o s i o n o f t h e c o v e r i n g m a n t l e . A s o i l , once b u r i e d under t h i n o v e r b u r d e n , can l a t e r be s u b j e c t e d t o p o s t b u r i a l s o i l formation, extending through t h a t overburden, whether t h e l a t t e r h a s b e e n p a r t l y e r o d e d or n o t ( F i g u r e 1F). The r e s u l t i s c a l l e d a w e l d e d p r o f i l e (Ruhe and O l s o n , 1 9 8 0 ) . S o i l f o r m a t i o n cont i n u i n g d u r i n g g r a d u a l or i n t e r m i t t e n t b u r i a l p r o c e s s e s g i v e s r i s e t o a curnulic p r o f i l e . A b u r i e d s o i l t h a t h a s n o t b e e n s u b j e c t e d t o s o i l f o r m a t i o n a f t e r f i n a l b u r i a l may b e c a l l e d a f o s s i l s o i l . The a g e o f a s o i l c o r r e s p o n d s t o t h e d u r a t i o n o f i t s e x p o s u r e t o f a c t o r s , i.e. t h e l e n g t h of i t s pedogenic i n t e r v a l . For a n o n b u r i e d s o i l , t h i s i s d e f i n e d by d i s t a n c e on t h e t i m e a x i s between t h e p o i n t m a r k i n g i n c i p i e n c e ( t i m e z e r o ) o f s o i l f o r m a t i o n and t h e p o i n t t h a t marks t h e p r e s e n t ( F i g u r e lA,B,C). The a g e o f a b u r i e d s o i l i s d e f i n e d by t h e d i s t a n c e b e t w e e n t h e p o i n t s t h a t mark i n c i p i e n c e and c e s s a t i o n ( b y b u r i a l ) o f s o i l f o r m a t i o n ( F i g u r e 1D). Changes i n c u r r e d by a s o i l w h i l e n o t exposed t o t h e s o i l - f o r m i n g f a c t o r s are called diagenic (Figure lD,E.F.). The a g e of a n exhumed s o i l i s t h e sum o f t h e a g e a t t a i n e d b e f o r e b u r i a l and t h e d u r a t i o n o f e x p o s u r e s i n c e e x h u m a t i o n ( F i g u r e 1E). S i m i l a r l y , t h e a g e o f t h e b u r i e d p o r t i o n af a w e l d e d p r o f i l e i s a c o m p o s i t e a g e ( F i g u r e 1F). the soil-forming
271
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More time benchmarks are needed than those delimiting pedogenic intervals, to answer the basic pedochronological questions (Ruellan, 1971): 1.
What were the principal stages in the formation o f a given soil?
2.
Did the soil evolve in environments or physical landscapes that differed from those prevailing now?
3.
What was the succession o f processes responsible f o r profile differentiation observable now?
Pedochronological analysis serves to resolve soil characteristics, timetransgressively acquired and .superprinted on one another as they are, in terms of the historical environmental record that is contained in coeval sedimentary sequences. Firm chronological control on such correlation is provided only by physically tracing the soil towards those sediments. Such linking requires stratigraphical analysis of the broader soil-landscape. Soil-sediment continua in the landscape are hydrophysical and hydrochemical in nature. Destruction of soil profiles by erosion and mass-wasting provides detritus for accumulation of clastics in colluvia
272
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Tim'e of Resurrection of Buried Soil
Sequum Superprinted on Exhumed Profile
HEMATIC PROFILE OF N EXUMED SOIL I
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HEMATIC PROFILE OF WELDED SOIL
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b
F i g u r e 1E-lF S c h e m a t i c i l l u s t r a t i o n s o f t h e F o r m a t i o n of E x h u m e d S o i l s (E), a n d I f e l d e d S o i l s (F).
and alluvia. The hydrophysical soil-sediment continuum ranges from i n - s i t u soil, through transported soil material in which the soil fabric is retained, to raw sediment (Gerasimov, 1971). Similarly, chemical leaching and precipitation of constituents from and within members of the hydrophysical continuum provide for a hydrochemical continuum ranging from i n - s i t u profiles including the soil solution, to organochemical sediments that may accumulate in suitable depositories. Continuity of these processes results in superprinting of their impacts in the soil-landscape. This complexity-is accounted for in Milne's catena: Soil catena refers to the sequence of soils encountered between the crest of a low hill and the floor of the adjacent swamp or thalweg, the soil profile changing from point to point of this traverse in accordance with conditions of drainage and past history of the land surface. Two variants of the catena can be distinguished in the field. In one, the topography was modeled, by denudation or other process, from a formation originally similar in lithologic character. Soil differences were then brought about by drainage conditions, differential transport of eroded material, and leaching, translocation, and redeposition of mobile chemical constituents. In the other variant, the topography was carved out of two or more superposed formations which
273
differ lithologically (!!ilne,
1336).
Pedogenic hysteresis al?d pedogenic inertia are historical factors that compound the progressively cumulative and therefore timetransgressive way in which soil features are acquired. Pedogenic hysteresis means that "a particular factor in soil development may be operating at a changed value, or may have beer. eliminated altogether, but its effects are erased only slowly, or may persist indefinitely if they were of an irreversible or destructive kind'' (Twlilne, 1936). Pedogenic inertia (Bryan and Teakle, 1946) states "that specialized soilforming processes, o ~ c eestablished, tend to continue (or persist) in spite of changes in original conditiocs of formation. Cnce .cast certain physical and chemical thresholds, the pedogenic regimen continues by virtue of its inertia'' (Finkl, 198C). These considerations are related to the fundamental pedochronological questions. They must be kept in mind as underscoring complexities that may be involved both ir. soil dating and in using soils as age criteria in the Quaternary. D A Y I I N S SOILS BY DAFIXS SUYFACES
A soil is maximally as old as the geomorphic surface on which it began forming, and minimally as old as the base of its overburden. Soil changes incurred after burial remain under diagenesis as long as pedogenesis imposed on the overburden does not extend beneath its base.
A geomorphic surface, unit, or complex of surfaces 1.
is a part of the land that is specifically defined in space and time and may include many landforms;
2.
is a mappable feature whose geographic distribution is portrayed on maps or aerial photographs and ,whose geometric dimensions are specified and analyzed;
3.
has an association with other geomorphic units (surfaces) which is defined in order to place it in a space and time sequence ;
4.
has an association with rock or sediment below it, or on it, which must be specified;
6.
is labeled with a geographic name (FLuhe and Vreeken, 1976).
Time zero of soil formation on a given surface reflects the nature of that surface. Siihen a surface, whether constructional or erosional, stabilizes at some point in time, time zero of soil formation is marked by the onset of surface stabilization. If the surface remains instable, erosion or deposition may dominate weathering and soil-forming processes, and time zero of soil formation cannot be assigned to a specific moment. A l s o , if pedogenic processes dominate, but erosional or depositional processes continue, time zero cannot be pinpointed because it transgresses time. Cessation of soil formation again reflects the nature ofgeomorphic processes responsible for burial, and may be abrupt or gradual. Abrupt cessation, suggested by a sharp upper boundary of the buried soil, often was preceded by partial truncation. In such a case, the burial event provides a minimum date for the end of soil development and there may have been a considerable pedogenic hiatus. Gradual cessation, suggested by pedogenic features within the basal increments of the overburden, still may have been preceded by partial truncation of' the pre-burial soil profile. Here, the beginning of burial provides again a minimum date for the end of development of the older soil, and it provides a maximum date for the time-transgressive time zero of the subsequent cumulic soil.
214
Relative d a t i n g o f geomorphic s u r f a c e s i n v o l v e s combined u s e o f t h e P r i n c i p l e o f S u p e r p o s i t i o n , i . e . y o u n g e r b e d s a r e on t o p o f o l d e r b e d s p r o v i d i n g t h e y h a v e not b e e r , o v e r t u r n e d , a n d o f t h e P r i n c i p l e o f C r o s s C u t t i n g R e l a t i o n s h i p s , i.e. i n a c r o s s - c u t t i n g r e l a t i o n s h i p , t h e feature t h a t i s cut i s older than the feature t h a t cuts across i t . A p p l i e d to t h e l a n d s c a p e , t h i s r e s u l t s i n t h e f o l l o w i n g P r i n c i p l e s for D a t i n g o f Geomorphic S u r f a c e s : 1.
A s u r f a c e i s younger t h a n t h e youngest material t h a t i t c u t s ;
2.
A s u r f a c e i s younger t h a n any s t r u c t u r e t h a t i t b e v e l s ;
3.
A s u r f a c e i s younger t h a n any material of which t h e r e a r e
4.
A s u r f a c e i s contemporaneous w i t h a l l u v i a l d e p o s i t s t h a t l i e
d i s t i n g u i s h a b l e f r a g m e n t s or f o s s i l s i n a l l u v i a l d e p o s i t s t h a t a r e now o n t o p o f t h a t s u r f a c e ;
on i t , a n d i t i s t h e same a g e a s , or o l d e r t h a n o t h e r t e r r e s t r i a l d e p o s i t s l y i n g on i t ;
5.
A s u r f a c e i s o l d e r t h a n v a l l e y s c u t below i t ;
6.
A s u r f a c e i s younger t h a n e r o s i o n remnants above i t ;
7.
A s u r f a c e i s o l d e r t h a ? d e p o s i t s i n v a l l e y s c u t below i t ;
8. A s u r f a c e i s y o u n g e r t h a n a n y e r o s i o n s u r f a c e s t a n d i n g a t a higher level;
9.
A s u r f a c e i s o l d e r t h a n a n y e r o s i o n s u r f a c e a t a Lower l e v e l
(Ruhe, 1 9 6 9 a ) .
T h e s e p r i n c i p l e s a r e f o u n d i l l u s t r a t e d i n n u m e r o b s p u b l i c a t i o n s ar,d h a v e b e e n s u m m a r i z e d by Ruhe (1969a) m d b y C a n i e l s e t al. (1971). D A T I N G SOILS BY S:A.Y
03 SOIL PROPEFITIES
I n d i c e s o f soil d e v e l o p m e n t h a v e beer, al?d c o n t i n u e b e i n g u s e d a s n e a s u r e s o f r e l a t i v e s o i l a g e . The a p p r o a c h i s a s f o l l o w s . One or more p r o p e r t i e s f r o m a s e r i e s o f s o i l s d i f f e r i n g i n a g e , a r e k e y e d t o r e l a t i v e a g e v a l u e s t o produce a r e f e r e n c e chronosequence. P.lterna t i v e l y , complete s o i l p r o f i l e s a r e a r r a n g e d i n t o development sequences, i n w h i c h s o i l s a r e d e s i g n a t e d a s f u l l y m a t u r e , h a l f - m a t u r e , e t c . (;enr.y, > 9 4 1 ) , or a s i n f a n t i l e , j u v e n i l e , v i r i l e a n d s e n i l e (biohr a n d 7,'an B a r e n , 1 9 5 4 ) . S u b s e q u e n t l y , n o n - d a t e d soils a r e a s s i g n e d a c e r t a i n a g e upon comparison w i t h t h e referefice sequence, w h i l e a p p l y i n g t h e P r i n c i p l e of U n i f o r m i t a r i a n i s m , i . e . "The same p r o c e s s e s ar,d l a w s t h a : o p e r a t e t o d a y , operated throughout geologic time, although not necessarily with t h e same i n t e n s i t y a s now" ( T h o r n b u r y , 1 9 6 9 ) . P r i n c i p a l c h r o n o s e q u e n c e s w e r e d i s c u s s e d b y V r e e k e n ( 1 9 7 5 ) . Bockheim ( 1 9 8 0 ) g a v e t h e m o s t r e c e - t review of s o i l chronofunctions. T h e r e a r e two v a r i a n t s i n t h e a p p l i c a t i o n o f t h i s a p p r o a c h . The f i r s t may b e c a l l e d D a t i r , g by P e d o g e n i c I r d e x . H e r e , t h e r e f e r e f i c e s e q u e n c e i s u s e d u n d e r t h e a s s u m p t i o n t h a t s o i l s d i r f e r i n g i n terms o f t h i s i n d e x " r e p r e s e n t t h e c o n s e c u t i v e s t a g e s o f some s o i l f o r m i n g p r o c e s s " and t h a t " t h e s e s o i l s , o c c u r r i n g as t h e y do a t one end o f t h e e v o l u t i o n s e r i e s , h a v e i n t h e p a s t g o n e t h r o u g h s t a g e s c h a r a c t e r i z e d by some p r e c e d i n g y o u n g e r n e m b e r o f t h e same s e r i e s , i . e . t h e y h a v e b e e r , endowed e a r l i e r w i t h f e a t u r e s w h i c h a r e now t h e g r o p e r t y o f s o i l s t h a t s t a n d a t t h e b e g i n n i n g o f t h e e v c l u t i o n a r y s e r i e s " ( R o d e , 1 9 6 1 ) . Thxs, environmental constancy i s i m p l i c i t . ?wc s u b v a r i a n t s may b e i d e n t i f i e d , i . e . D a t i n g b y A v e r a g e O e d c g e n i c I n d e x a n d D a t i n g by V a r i a b l e F e d o g e r i c Index. The s e c o n d v a r i a n t may b e c a l l e d C a t i r i g by P a l e o p e d o g e n l c I p d e x . I i e r e d a t i n g i s b a s e d a g a i n on a r e f e r e n c e s e q u e n c e , b u t on t h e ass u m p t i o n t h a t c e r t a i n p r o p e r t i e s o f t h e older s o i l n e m o e r s c o u l d n o t have developed under t h e preser,t enviro?menk, s u c h anomalous p r o p e r t i e s
275
are then ascribed to paleopedogenesis (Butler, 1967). The anomalous properties, when encountered in undated soils, become a criterion for their age. Thus, environmental change is emphasized. Dating by Average Pedogenic Index In this method, the soil is viewed as a result of the relative effective age (duration) of weathering of its parent material (Ruhe, Y969b). Average rate of soil change is estimated by dividing the difference between the value of a property for a soil and that of its presumed parent material, through an appropriate absolute or relative time Teasure. Relative effective age of another soil is then determined by dividing the value of its index property through that rate value. Index zroperties used include depth of carbonate leaching (Smith, 1942), solum thickness (Hutton, 1951), and molar ratios of CaO/ZrOn (Beavers e t al., 1963). Such relative effective age values, which actually are developTent ratios, have been subsequently used to explain between-profile differences in other soil properties. Implicit assumptions are those 3f monogenetic soil development, environmental constancy, and that all types of soil development have been replicated throughout geologic time. Soil monogenesis implies that the soil-forming factors remained 2oristant during the course of soil formation. Using this premise, functional analysis of modern soils, introduced by Jenny (1941) and UDdated by Birkeiand (1974), has abundantly illustrated the limiting effectiveness on soil development of factors other than time. Soils of similar age tend to differ in accordance with climatic, vegetational . . . e t c . setting. When adding to this the more realistic notion that soil"orming factors tend to interact and change through time, one arrives at the view of soil as the product of a functional synthesis of the soil-forming factors (Stephens, 1947). This view implies a p r i o r i assumption of potential soil polygenesis and can be combined with conclusions reached l r o m functional analysis into the Principle of Limiting ?edogenic Factors The degree of development of a soil profile may reflect the limiting effectiveness of one, or of more than one soil-forming factor, but soil development must be evaluated with a view on the integrated action of the soil-forming factors, changeable as these may be through time. This Principle implies that differences in expression of soilf'orming factors can partly substitute for one another and yet effect similar pedogenic imprints. That is the element of equifinality that limits the diagnostic value of pedogenic indices as measures for soil ige. In soil formation, the significance of the factor time as asimple 'neasure of process duration is probably outstripped by its r o l e as a zime-space or frame for successive environmental conditions and prozesses. Thus, the very effort of constructing average rate measures of long term soil change is questionable, let alone their use for the purpose of soil dating. Finally, when transplanting rates of processes backwards into time f o r pdrposes of dating soils from the more distant past by this method, the assumption of environmental and pedogenic replication throughout time clashes with the Principle of Environmental Uniqueness
No environment in geological history is ever exactly duplicated. Especially progressive organic evolution, continental drift, and plant succession during climatic change are not replicated (After Nairn, 1965). In conclusion, caution expressed by Ruhe (1969b) against Dating by Average Pedogenic Index is formulated here as the Principle of Fallibility of Pedogenic Indices as Estimators of Soil Age: Using a property of soil to construct a time measure that, in turn, is used to show that soils differ because of age, involves a circular argument, unless some independent criterion of environmental
276
history relevant to soils is brought in. 3atir,g by Variable Tedogenic Ir iex This me5hod acknowledges that a newly exposed. surficial material may adju-t, by internal change, at variable rate, to attair, a state that is more in equilibrium with the prevailing ervironment. AP. initially rapid rate of adjustment would progressively decrease as a hypothetical equilibrium state would be approached, if soil development were comparable to a thermodynamic system adjusting to change (Lavkulich, 1965). This then could apply to pedogenesis ur.der environmental conditions permitting monogenesis. If environmental coriditions were to change during pedogenesis, rates of pedogenic adjustment could change again (Yaalon, 1971), to follow a new trend, a shifted pedogenic course. According to this concept, reference chronosequences could be envisaged, to typify rate variations of monogenetic processes under constant and specified environmental conditions. Age benchmarks to peg the sequences would necessarily have to be obtained using nonpedogenic criteria. Then, the age of an undated soil that developed under the same conditions, could be picked off the reference curve, on the strength of its pedogenic index property. F’rovided, of course, that the latter value would not plot on the fiat, i.e. time-independent portion of the curve depicting the chronofunction, and provided that the curve was ascending only. Sags in the reference curve would allow for multiple age options, if a single index were to be used. Sags in such a reference curve could reflect, for example, the depletion of nutrients, e.g. P, necessary to maintain levels of biotic productivity and of related pedogenic indices (Walker, 1965). Fiate variations of processes under changing environmental conditions would be again more variable. The corresponding compounded reference chronosequences would be less useful for purposes of dating unknown soils. Pedologists have constructed chronosequences ever since Dokuchaev’s first attempt in 1883 (Dickson and Crocker, 1954). Their primary objective was pedochronologically oriented. The traditional approach has been to employ soils of different age, present in the groundsoil continu.um of regions with multiple geomorphic surfaces, and to interpret them as a time series. These so-called post-incisive chronosequences (Vreeken, 1975) constitute the vast majority of the sequences, reviewed by Bockheim (1980), and represent a basis for contemporary pedological models. Regrettably, the pedochronological significance of these postincisive sequences is highly overrated. Basic limitations are imposed by the kind of soil evidence selected in the field (Vreeken, 1975). Also, their traditional interpretation is confused by monogenetic bias and circular logic, oblivious of cautions expressed by Rode (1961) and in Nairn’s (1965) Principle of Environmental Uniqueness. Unless apostincisive soil chronosequence can be proven to reflect monogenesis, and until someone can prove that soil history repeats itself, this sequence does not provide unambiguous information on rate variations of pedogenetic processes through time, because it does not provide time-lapse information. These traditionally used chronosequences are unfit for the testing of thermodynamics-inspired models. Neither do they provide the pedochronological information ascribed to them. The secondary objective of chronosequence studies is to use them for purposes of dating formations and surfaces of unknown age. To this end, the same approach is followed as outlined before, but the reference sequences are used in a purely empirical manner. Examples can be found in Birkeland (1974, 1978) and Birkeland et a l . (1979, 1980). The main problem that may arise is the problem of equifinality, i.e.whereby soil-forming factors can partly substitute for one another or operate differently through time and yet effect similar pedogenetic imprints. This includes the potential overprinting of impacts from pedogenic hysteresis and inertia. Clearly, such empirical reference sequences are not likely to be
277
e x p o r t a b l e f a r beyond t h e r e l a t i v e l y s m a l l a r e a i n which t h e y were e s t a b l i s h e d , b e c a u s e e n v i r o n m e n t a l f a c t o r s and t h e i r h i s t o r y may s p a t i a l l y d i f f e r . On g r o u n d s o f s i m i l a r p e d o g e n i c i n d i c e s , two s o i l s might b e equated t h a t a r e a c t u a l l y q u i t e a p a r t i n age. In addition, s i t e s e l e c t i o n i s c r u c i a l l y i m p o r t a n t , b o t h when e s t a b l i s h i n g t h e r e f e r e n c e s e q u e n c e and when s a m p l i n g t h e u n d a t e d s o i l . S i t e s t a b i l i t y , with r e f e r e n c e t o geomorphic p r o c e s s e s , i s one p r e r e q u i s i t e and would d i c t a t e t h e u s e o f l e v e l u p l a n d or i n t e r f l u v e s i t e s , t h u s r e s t r i c t i n g t h e t e c h n i q u e t o o n l y one h i l l s l o p e e l e m e n t from t h e c a t e n a r y s o i l - l a n d s c a p e (Ruhe, 1 9 6 0 ) . A l s o , t h e d a t e d as w e l l a s u n d a t e d soils s h o u l d a l w a y s have o c c u p i e d t h i s c o m p a r a b l e h i l l s l o p e p o s i t i o n . If n o t , one r i s k s comparing p r o f i l e s t h a t d i f f e r b o t h i n a g e and c a t e n a r y p o s i t i o n , hence p r o c e s s s e t t i n g . T h e s e two b a s i c a p p r o a c h e s t o w a r d s e x p l a i n i n g s o i l t o p o g r a p h y r e l a t i o n s h i p s w e r e a l r e a d y ( i n 1 9 3 0 ) r e v i e w e d b y Norton and S m i t h . A c t u a l l y t h e y complement one a n o t h e r (Ruhe and W a l k e r , 1 9 6 8 ; Vreeken, 1 9 7 3 ) a c c o r d i n g t o t h e o r i g i n a l c a t e n a concept (Milne, 1 9 3 6 ) . The r e s t r i c t i o n t o g e o m o r p h i c a l l y s t a b l e s i t e s i m p l i e s t h a t t h e s o i l s t o b e used a r e a l m o s t a u t o m a t i c a l l y p o l y g e n e t i c , i f n o t even r e l i c t paleosols. P o l y m o r p h i c (Simonson, 1 9 7 8 ) and b i s e q u a l p r o f i l e s may c h a r a c t e r i z e t h e o l d e r s o i l s , a d d i n g c o m p l e x i t y t o c o m p a r i s o n s between p r o f i l e s of d i f f e r e n t a g e . I n d i c e s m e a s u r i n g d e g r e e o f s o i l development a r e b e i n g u s e d a s i n d i c e s f o r r e l a t i v e age i n t h e absence of r e f e r e n c e chronosequences ( B i r k e l a n d , 1 9 7 4 ) . T h i s i s done i n a r e a s where a l t e r n a t i v e a g e c r i t e r i a a r e a b s e n t o r have n o t b e e n e x p l o r e d a s y e t . Q u a l i t a t i v e as t h e r e s u l t s of t h i s a p p r o a c h may b e a t t h e b e s t , s o i l s s h o u l d n o t b e u s e d as a g e c r i t e r i a , t o t h e e x c l u s i o n o f o t h e r r e l a t i v e d a t i n g methods. I b i d e m Pedogenic i n d i c e s remain f a l l i b l e as e s t i m a t o r s of s o i l a g e . D a t i n g by Paleopedogenic Index Most s o i l - l a n d s c a p e s a r e p o l y c y c l i c , h a v i n g d e v e l o p e d u n d e r v a r i o u s g e o m o r p h i c , c l i m a t i c a n d / o r b i o t i c r e g i m e n s . Thus p a l e o p e d o g e n i c i m p r i n t s a r e n o r m a l phenomena, w h e t h e r a s r e l i c t or p o l y g e n e t i c s o i l f e a t u r e s , or as b u r i e d s o i l s i n r e s t r i c t e d l a n d s c a p e p o s i t i o n s . T h i s a p p l i e s both t o soil-landscapes t h a t a r e l a r g e l y s u b a e r i a l l y exposed, and t o t h o s e t h a t h a v e b e e n b u r i e d i n toto. Such i m p r i n t s h a v e b e e n u s e d a s r e l a t i v e a g e i n d i c e s f o r t h e geomorphic s u r f a c e s on which t h e y a r e f o u n d . However, p r e r e q u i s i t e s must be m e t . Diagnosis of s o i l f e a t u r e s as paleopedogenic i s t h e f i r s t prerequisite. T h i s r e q u i r e s c r i t e r i a t o d i s t i n g u i s h between f e a t u r e s o r i g i n a t i n g f r o m s u c c e s s i v e p e d o g e n e t i c r e g i m e n s , and i s n o r m a l l y comp l i c a t e d by t h e d i f f e r e n t i a l p e r s i s t e n c e o f o l d e r f e a t u r e s when s u b j e c t e d t o f u r t h e r p e d o g e n e s i s or d i a g e n e s i s . It a l s o r e q u i r e s d i s t i n c t i o n b e t w e e n s o i l , w e a t h e r e d zone and s e d i m e n t . A n a l y t i c a l and m i c r o m o r p h o l o g i c a l c r i t e r i a c a n be h e l p f u l b u t , t a k e n by t h e m s e l v e s , a r e o f l i m i t e d d i a g n o s t i c v a l u e when r e f e r r i n g t o s i n g l e p r o f i l e s or v e r t i c a l s e c t i o n s o n l y . The g e n e t i c s o i l c o n c e p t r e q u i r e s s o i l t o e x h i b i t l a t e r a l v a r i a b i l i t y within t h e three-dimensional landscape, i n accordance w i t h d i f f e r e n c e s i n t h e soil-forming f a c t o r s . Catenary t r e n d s o f v a r i a b i l i t y p r o v i d e t h e p r i m a r y d i s t i n c t i o n . b e t w e e n s o i l and not-soil. C r i t e r i a for t h e i d e n t i f i c a t i o n o f b u r i e d s o i l s were r e viewed by V a l e n t i n e and D a l r y m p l e ( 1 9 7 6 ) , b u t no r e v i e w i s a v a i l a b l e for relict soils. I n b o t h c a t e g o r i e s , t h e r e i s a s h o r t a g e of informa t i o n on h y d r o p h y s i c a l and h y d r o c h e m i c a l s o i l - s e d i m e n t c o n t i n u a . T h i s i s n o t p r o n o u n c e d f o r a q u a t i c e n v i r o n m e n t s n o t t r a d i t i o n a l l y s t u d i e d by p e d o l o g i s t s (Buurman, 1 9 7 5 ) . These i n c l u d e p a l u d a l , l a c u s t r i n e , d e l t a i c , e s t u a r i n e , t i d a l T l a t and o t h e r m a r i n e e n v i r o n m e n t s . These g a p s may c a u s e much p a l e o p e d o g e n i c e v i d e n c e t o go u n d e t e c t e d . Buried Soil-Landscapes Buried soil-landscapes a r e s i g n i f i c a n t markers i n t e r r e s t r i a l s t r a t i g r a p h i c a l s t u d i e s and s e r v e as m a c r o c r i t e r i a f o r t h e r e l a t i v e a g e o f t h e i r o v e r b u r d e n s and s u b s t r a t e s . Reviews a r e i n B i r k e l a n d ( 1 9 7 4 ) , Mahaney and Fahey ( 1 9 7 6 ) and F i n k 1 ( 1 9 8 0 ) . Prerequisite i s t h a t these s o i l - l a n d s c a p e s h a v e b e e n a s s e s s e d i n t e r m s o f t h e geomorphic s u r f a c e s .
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c o n t a i n e d , a n d i n terns o f c a t e n a r y t r e n d s o f v a r i a b l l i t y w i t h i . ? t h e s e s u r f a c e s , as w e l l a s t h e d i f f e r e n c e s i n t h e s e t r e n d s b e t w e e n a d j a c e n t s u r f a c e s , b o t h on t h e l o c a l a n d r e g i o n a l s c a l e . B e c a u s e s u c h p r e r e q u i s i t e s o f t e n a r e n o t m e t , t e l e c o r r e l a t i o n o f b u r i e d s o i l s on t h e basis of r e l a t i v e d e g r e e of p r o f i l e development a l o n e , i s a d v i s e d against (Birkeland, l97b). I s o l a t e d b u r i e d s o i l p r o f i l e s or, c o n v e r s e l y , t h e i r l o c a l a b s e n c e s h o u l d n o t f o r m a b a s i s f o r r e g i o n a l l a c d s c a p e i n f e r e n c e s . For e x a m p l e t h e t y p e l o c a l i t y for t h e Y a r m o u t h I n t e r g l a c i a l r s a l e o s o l , e2 y e a r s l a t e r , was shown t o c o n t a i r , a p r e - I l l i n o i a n a p e s o i l a t t h e t y p e l e v e l . ?he Y a r m o u t h s o i l was s t r a t i g r a p h i c a l l y l o w e r ( 3 a l l b e r g e t al., 1980). On t h e o t h e r h a n d , a b s e n c e o f p a l e o s o l s f r o m c e r t a i n b u r i e d g e o m o r p h i c s u r f a c e s i n n o r t h e a s t I o w a , w h i l e a t t r i b u t e d t o t h e Iowan G l a c i a t i o n d u r i n g more t h a n 70 y e a r s , was f i n a l l y shown $0 r e f l e c t a s u b a e r i a l e r o s i o n c y c l e ( R u h e e t al., 1968). The u n i f o r m i t a r i a n a p p l i c a t i o n o f w h a t i s known a b o u t t h e g r o u n d s o i l c o n t i n u u m e n t a i l s t h e P r i n c i p l e o f Regional Paleogeomorphic S i g n i f i c a n c e of Paleopedogenic Evidence p e r s e . A s much a s s o i l f o r m a t i o n , e r o s i o n a n d d e p o s i t i o n a r e s i m u l t a n e o u s l y a c t i v e w i t h i n p r e s e n t day l a n d s c a p e s , p a i e o p e d o g e n i c e v i d e n c e f r o m a l i m i t e d number o f s i t e s i s i n s u f f i c i e n t t o i n f e r r e g i o n a l / l a n d s c a p e s t a b i l i t y . Absence o f p a l e o p e d o g e n i c e v i d e n c e from l i m i t e d s i t e s i s i n s u f f i c i e n t t o i n f e r r e g i o n a l i n s t a b i l i t y . Detailed soil-landscape analyses are a p r e r e q u i s i t e . S u b a e r i a l l y Exposed S o i l - L a n d s c a p e s P o l y c y c l i c s o i l - l a n d s c a p e s c o n t a i n v a r i o u s geomorphic s u r f a c e s and, correspondingly, an age mosaic of s o i l s , i n c l u d i n g r e l i c t p a l e o s o l s . The p a l e o p e d o g e n i c i n d e x m e t h o d o f r e l a t i v e d a t i n g p r e s u m e s t h a t e n v i ronment of s o i l f o r m a t i o n c a n b e read from t h e c o n s t i t u t i o n o f s c i l s t h e m s e l v e s : A s s u m i n g t h a t m o r p h o l o g y a n d p r o p e r t i e s of r e l a t i v e l y y o u n g s o i l s s h o u l d r e f l e c t t h e c u r r e n t l y o p e r a t i v e e n v i r o n m e r t a l framework, t h e s e s o i l s s e r v e as an e n v i r o n m e n t a l rorm t o gauge t h e f e a t u r e s o f o t h e r s o i l s i n t h e same a r e a . I f l a t t e r f e a t u r e s a r e d i s s i m i l a r t o t h e norm, t h e y a r e r e g a r d e d a s a n o m a l o u s a n d i n t e r p r e t e d t o o r i g i n a t e trorn a d i f f e r e n t e n v i r o n m e n t a l Tramework. T h u s , t h e y a r e i n t e r p r e t e d as p a l e o p e d o g e n i c , and i n d i c a t i v e of g r e a t e r a g e . The f i r s t q u e s t i o n r e g a r d i n g t h i s m e t h o d i s : What s c i l s u n a m b i g u o u s l y r e f l e c t t h e c u r r e n t l y o p e r a t i v e e n v i r o n n e n t a l framework, and c o u l d s e r v e a s e n v i r o n m e n t a l norms? I n t e r p r e t a t i o n o f s o i l s as environm e n t a l i n d i c a t o r s i s r o o t e d i n t h e C l i m a t i c S o i l Z o n a l i t y C o n c e p t . But, most s o i l s showing a z o n a l d i s t r i b u t i o n r e s e m b l i n g t h a t o f b i o c l i n a t i c d i v i s i o n s a r e p o l y g e n e t i c , a s much a s d i s t r i b u t i o n a n d e x p r e s s i o n o f l a t t e r d i v i s i o n s a r e r e s u l t a n t s of change and s u c c e s s i o n . T h e r e f o r e , a s l o n g a s t h o s e s o i l s , u s e d as e n v i r o n m e n t a l n o r m s , a r e n o t p e d o c h r o n o l o g i c a l l y and g e n e t i c a l l y f u l l y u n d e r s t o o d , t h e i r normative v a l u e for p a l e o - e n v i r o n m e n t a l r e c o n s t r u c t i o n , w i t h a l l a t t e n d a n t i n f e r e n c e s on a g e , i s d o u b t f u l . T h i s r e s e r v a t i o n may b e t e r m e d t h e Non-Axiom o f Climatic S o i l Zonality: The p r i n c i p a l d i f f e r e n c e s b e t w e e n - s o i l s , a n d c e r t a i n l y b e t w e e n paleosols, are not n e c e s s a r i l y c l i m a t i c a l l y zonal, b u t present c l i m a t e a l o n e does n o t c o n t r o l and i n f l u e n c e t h e b i o l o g i c a l , c h e m i c a l and most of t h e p h y s i c a l p r o c e s s e s i n s o i l f o r m a t i o n . D i f f e r e n c e s due t o o t h e r s o i l - f o r m i n g f a c t o r s , i n c l u d i n g t h e v a r i a b l e i m p a c t o f a l l s o i l - f o r m i n g f a c t o r s t h r o u g h t i m e may b e more i m p o r t a n t . A s e c o n d q u e s t i o n i s , whether one r e a l l y c o u l d i n t e r p r e t t h e c h a r a c t e r o f a n o l d s o i l i n terms o f p r e s e n t - d a y e n v i r o n m e n t , u s i n g s o i l The P r i n c i p l e o f E n v i r o n m e n t a l U n i q u e n e s s s u g g e s t s t h a t evidence alone. s u c h i n t e r p r e t a t i o n s can be t e n t a t i v e o n l y . Thus, s u p p o r t from indep e n d e n t , n o n - p e d o l o g i c a l p a l e o - e n v i r o n m e n t a l c r i t e r i a w i l l be r e q u i r e d ( B u t l e r , 1967). A l t h o u g h S r y a n a n d A l b r i t t o n ( 1 9 4 3 ) r e c o n s t r u c t e d e n v i r o n m e n t f r o m b u r i e d p a l e o s o l s i n N o r t h A m e r i c a , Ruhe ( 1 9 6 5 ) d r e w a t t e n t i o n t o e q u i f i n a l D e d o g e n i c f e a t u r e s when s u b s t i t u t i n g t i m e a n d
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various other soil-forming factors,, thus underscoring the Principle of Limiting Pedogenetic Factors. Butler (1967) warns against recon-. structing environmental histories from soil-derived evidence, where applied to non-buried soils in Australia. CONCLUSION
In conclusion, as much as pedogenic criteria are fallible as measures of soil age, they are fallible as paleoenvironmental and corresponding age indicators. Stratigraphically supported criteria for the environmental significance of soil features must be established first. This involves soil-landscape analysis and the delineation of geomorphic surfaces. REFERENCES
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DATING WITH POLLEN: METHODOLOGY, APPLICATIONS, LIMITATIONS
A. M. DA VIS
ABSTRACT A l t h o u g h t h e d a t i n g f u n c t i o n o f p o l l e n has b e e n l a r g e l y s u p e r c e d e d by r a d i o c a r b o n , t h e f o r m e r i s s t i l l v a l u a b l e D a r t i c u l a r l y f o r d a t i n g b e y o n d t h e r a n g e of I 4 C , i n d e t e r m i n i n g t h e c h r o n o l o g y o f r e c e n t a n d Pollen s h o r t - t e r m e v e n t s , a n d i n s e d i m e n t s where 1 4 C c a n n o t b e u s e d . s t r a t i g r a p h i e s may b e u s e f u l a s c h e c k s on t h e a c c u r a c y o f r a d i o c a r b o n dating. The b a s i s f o r d a t i n g w i t h p o l l e n i s t h e r e c o g n i t i o n o f a s s e m b l a g e s o r z o n e s , u s u a l l y from t h e p e r c e n t a g e s o f t h e t a x a , b u t sometimes from p o l l e n i n f l u x . The d e t e r m i n a t i o n o f z o n e s i s i n f l u e n c e d by a number o f s i t e - r e l a t e d v a r i a b l e s a n d by p r e p a r a t i o n a n d a n a l y t i c a l c o n s t r a i n t s . P o l l e n s p e c t r a vary w i t h t h e environment of d e D o s i t i o n . P o l l e n r e c o r d s from l a k e s e d i m e n t s t e n d t o r e p r e s e n t r e g i o n a l i n p u t s , w h i l e t h o s e from T h e r e may b e c o n s i d e r p e a t s a r e u s u a l l y d o m i n a t e d by l o c a l p r o d u c e r s . a b l e i n t e r n a l v a r i a b i l i t y t h a t i n l a k e s r e f l e c t s s i z e and s h a p e , and t h e p o s i t i o n of t h e s e q u e n c e w i t h i n t h e b a s i n . P o l l e n s D e c t r a from s o i l s , c a v e s , i n s h o r e , a n d d e e p w a t e r m a r i n e d e p o s i t s may a l l b e d i s t i n c t i v e a n d d i f f e r e n t . A l l c a n b e z o n e s , b u t t h e t a x a on w h i c h z o n a t i o n i s b a s e d may v a r y a n d t h e z o n e s may n o t b e s y n c h r o n o u s .
The p o l l e n s p e c t r a may a l s o b e i n f l u e n c e d by d i f f e r e n t i a l p r e s e r v a t i o n a n d by r e d e p o s i t i o n f r o m o l d e r s e d i m e n t s . The r e c o r d may b e d i s t o r t e d by m i s i d e n t i f i c a t i o n , p a r t i c u l a r l y a problem i n t h e e a r l y d a y s o f p o l l e n a n a l y s i s . The v a l i d i t y o f z o n a t i o n i s i n p a r t c o n d i t i o n e d by t h e s i z e a n d c o m p o s i t i o n o f t h e p o l l e n sum. Z o n a t i o n o f p o l l e n s e q u e n c e s c a n b e d e t e r m i n e d s u b j e c t i v e l y or o b j e c t i v e l y by s e v e r a l n u m e r i c a l m e t h o d s : c l u s t e r i n g and d i v i s i v e t e c h n i q u e s , p r i n c i p a l components a n a l y s i s , s e q u e n t i a l c o r r e l a t i o n , e t c . The l a t t e r m e t h o d s a l l o w d i v i s i o n i n t o z o n e a n d s u b - z o n e s by i d e n t i f y i n g t h e l o c a t i o n s and ‘powers’ of groupings and/or b o u n d a r i e s . Subjective and o b j e c t i v e approaches seldom y i e l d d i s s i m i l a r r e s u l t s . The b r o a d s y n c h r o n e i t y o f p o l l e n z o n e s i n H o l o c e n e s e q u e n c e s f r o m e a s t e r n N o r t h America and n o r t h w e s t Europe i l l u s t r a t e t h e v a l u e o f z o n a t i o n as a d a t i n g d e v i c e . However, e f f e c t i v e n e s s a n d s y n c h r o n e i t y may b e much l e s s o b v i o u s i n s e q u e n c e s from s e d i m e n t a r y e n v i r o n m e n t s other than lakes. Z o n a t i o n may b e e x t e n d e d as a r e l a t i v e d a t i n g d e v i c e b e y o n d t h e r a n g e o f 14C. F o u r - o r f i v e - p h a s e m o d e l s o f g l a c i a l / i n t e r g l a c i a l p o l l e n s p e c t r a have been a p p l i e d i n Europe and North America. T h e r e c o g n i t i o n o f t h e s e d e p e n d s l a r g e l y on t h e a s s u m p t i o n o f t h e e c o l o g i c a l s t a b i l i t y o f t h e t a x a on w h i c h z o n a t i o n i s b a s e d . P o l l e n i n f l u x has been used t o estimate t h e l e n g t h of i n t e r g l a c i a l s . Pollen a n a l y s i s i s valuable i n determining t h e chronology of r e c e n t or s h o r t - t e r m e v e n t s o u t s i d e o f t h e r e s o l u t i o n o f 14C.
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I N T 3 O D i l CTION
W i t h i n t h e r a n e of r a d i o c a r b o n , p o l l e r . s t r a t i p a p h i e s a r e g e n e r a l l y s u p p o r t e d by "C d a t e s . E o w e v e r , f o r m o s t s e q u e n c e s , t h e number of d a t e s i s s m a l l , a n d d u e t o e n v i r o n m e n t a l a n d p r o c e d u r a l c o n s t r a i n t s many o f t h e c h r o n o l o g i e s a r e u n r e l i a b l e . Ogden (1977) e s t i m a t e d t h a t t h e r e were t h e n a p p r o x i m a t e l y 300 s e d i m e n t c o r e s f r o m e a s t e r n N o r t h A m e r i c a . Less t h a n 30 of t h e s e h a d more t h a n t h r e e r a d i o c a r b o n d a t e s . Terasmae ( t h i s s y m p o s i u m ) i n d i c a t e s t h a t less t h a n 5 0 5 of 1 4 C d a t e s a r e a c c e p t e d by t h o s e s u b m i t t i n g t h e m a t e r i a l . Some o f t h e r e j e c t i o n s a r e p r e s u m a b l y d u e t o c o l l e c t i o n or p r o c e s s i n g p r o b l e m s ; o t h e r s t o t h e uns u i t a b i l i t y of m a t e r i a l s ( O h l s s o n , 1 9 7 9 ) . L a k e s e d i r r e n t s o f t e n y i e l d unduly o l d d a t e s b e c a u s e of r e s e r v o i r e f f e c t s (Ohlsson and F l o r i n , 1 9 8 0 ) . S h o t t o n ( 1 9 7 2 ) has d e m o n s t r a t e d t h e e f f e c t o f ' h a r d w a t e r ' e r r o r on d a t i n g . I n a s e d i m e n t composed of a n a l g a l mud m a t r i x w i t h i n c l u s i o n s of wood a n d s m a l l t w i g s , t h e l a t t e r d a t e d 1 7 0 0 y e a r s y o u n g e r t h a n t h e m a t r i x . The d a t e s from wood a n d t w i g s w e r e i n a g r e e m e r . t w i t h the pollen stratigraphy r e l i a b l y dated elsewhere. The ' o l d ' d a t e on t h e a l g a l mud was t h e r e s u l t o f s y n t h e s i s of c a r b o n a t e by a l g a e . Dates f r o m c a v e s e d i m e n t s a n d a r c h a e o l o g i c a l s i t e s a r e o f t e n e q u i v o c a l . The a c c u r a c y o f d a t e s f r o m p e a t d e p e n d s much on t h e c a r e u s e d t o e l i m i n a t e c o n t a r n i n e t i o n by t h e d e e p r o o t s of p l a n t s y o u n g e r than the matrix. R a d i o c a r b o n i s r o u t i n e l y u t i l i z e d t o ca. k 0 , O O O B F . Beyond t h a t , p o l l e n s t r a t i g r a p h i e s may p r o v i d e c r u d e r e l a t i v e d a t e s , i . e . g l a c i a l vs. i n t e r g l a c i a l , o r p o s i t i o n w i t h i r ! t h e p l a c i a l / i n t e r g l a c i a l c y c l e . Podern or s h o r t - d u r a t i o n e v e n t s c a n n o t b e a d e q u a t e l y d a t e d by r a d i o c a r b o n . P o l l e n a n a l y s i s may a l l o w d e t e r m i n a t i o n of a b s o l u t e a g e w i t h i n a s t a n d a r d d e v i a t i o n o f I4C. P o l l e n i n f l u x may p r o v i d e a ' f l o a t i n g ' a b s o l u t e chronology w i t h i n t h e r a n g e of 14C o r beyond i t ( g a b r o w s k i , 1971; Mehringer e t a l . , 1 9 7 7 ) . Z O N A T I O N AND D A T I N S The m o s t common a p p r o a c h t o d a t i n g w i t h p o l l e n i s t h r o u g h t h e r e c o g n i t i o n of b i o s t r a t i g r a p h i c s n i t s : p o l l e n zones. P r i o r t o t h e d e v e l o p m e n t o f r a d i o c a r b o n d a t i n g : , t h i s was t h e o n l y m e t h o d . I t remaifis i m p o r t a n t d e s p i t e t h e a v a f l a b i l i t y of I4C. Z o n a t i o n of p o l l e n d i a g r a m s s h o u l d b e d e t e r m i n e d e n t i r e l y on t h e o b s e r v e d p o l l e n a s s e m b l a g e s . However, o t h e r c r i t e r i a h a v e b e e n u s e d . 3 i r k s a n d S i r k s ( 1 3 7 9 ) s u g g e s t t h a t p o l l e n z o n e s h a v e b e e n d e f i n e d as u n i t s o f i n f e r r e d p a s t v e g e t a t i o n , u n i t s of i n f e r r e d p a s t c l i n a t e , as u n i t s of s e d i m e n t l i t h o l o g y , a n d a s u n i t s of i n f e r r e d t i m e .
The a s s o c i a t i o n of z o n e s w i t h t i m e - b o u n d c l i m a t e a n d v e g e t a t i o n c h a n g e s p r e d a t e s t h e u s e of 1 4 C . The B l y t t - S e r n a n d e r s c h e m e of' l a t e g l a c i a l a n d F i o l o c e n e c l i m a t i c c h a n g e was b a s e d on p e a t s t r a t i g r a p h y Inevitably, pollen analysis a n d e m p h a s i z e d m o i s t u r e as a c o n t - 0 1 . d e v e l o p e d b y v o n P o s t a f t e r 1 9 1 6 was a l s o climatically-constrained. Von P o s t , r e c o g n i z i n g t e m p e r a t u r e a s a m a j o r c o n t r o l , p r o p o s e d a t r i p a r t i t e H o l o c e n e w i t h a n e p i s o d e o f i n c r e a s i n g w a r m t h , f o l l o w e d by a c l i m a t i c o p t i m u m , w h i c h was r e p l a c e d b y a n e p i s o d e of d e c r e a s i n g warmth. S e a r s ( 1 9 3 5 ) a t t e m p t e d t o a p p l y t h e European p o l l e n / c l i m a t e He r e c o g n i z e d f o u r z o n e s ( B o r e a l , A t l a n t i c , sequence t o North America. S u b b o r e a l , S u b a t l a n t i c ) . These zones were c o n s i d e r e d t o be b i o s t r a t i graphic u n i t s with time-parallel boundaries. I n c r e a s i n g u s e of 1 4 C t o d a t e p o l l e n s e q u e n c e s h a s shown t h a t t h e a s s u m p t i o n o f t i m e - p a r a l l e l z o n e s i s d o u b t f u l . The p o l l e n r e c o r d s f r o m N e w E n g l a n d show a l a c k o f s y n c h r o n e i t y i n t h e a p p e a r a n c e of m a j o r t a x a . The c l i m a t i c o p t i m u m , or h y p s i t h e r m a l a p p e a r s t o b e t i m e - t r a n s g r e s s i v e ( W r i g h t , 1976). R i c h a r d (1977) h a s shown t h a t t h e t u n d r a z o n e a t t h e b a s e o f p o l l e n s e q u e n c e s i n s o u t h e r n Q u e b e c d a t e s a t 1 1 , 4 0 0 BP a t Mount Smith and P i l c h e r S h e f f o r d a n d a t 7 2 0 0 BP i n t h e s o u t h e r n L a u r e n t i d e s . ( 1 9 7 3 ) have d e m o n s t r a t e d t h a t t h e Alnus r i s e i n B r i t a i n , assumed t o mark t h e s t a r t of t h e A t l a n t i c a t ca.'7500 BP t r a n s g r e s s e s some 2 0 0 0 y e a r s . The d i f f i c u l t i e s o f i d e n t i f y i n g z o n e s w i t h r e g i o n a l a p p l i c a t i o n h a v e become o b v i o u s a s t h e number o f p o l l e n s e q u e n c e s h a s i n c r e a s e d . I n B r i t a i n , t h e z o n a t i o n s c h e m e p r o p o s e d by Godwin ( 1 9 4 0 ) i s s t i l l
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used, but has frequently been replaced or complemented by local schemes. In North America, zonation schemes often have a regional application, but interregional comparison may be difficult (Figure 1).
Figure 1
Comparison of p o l l e n z o n a t i o n schemes f r o m eastern North America. Some similarities are evident, d e s p i t e t h e l a c k of a c o m m o n z o n a t i o n nomenclature. K.M. ( W r i g h t e t al., 1 9 6 3 ) ; S . L . ( O g d e n , 1 9 6 6 ) ; M.L. ( F o t t a n d Y a r l e y G i l l , 1 9 7 8 ) ; L . P . (Davis, 1 9 6 7 ; B.P. (Ilott, 1 9 7 7 ) .
Pollen Behaviour and Site Variables Clearly, the derivation o f any zonation scheme is dependent on the variations in the pollen spectra. The pollen record from any site is determined by many pollen behaviour and site variables. These include pollen productivity, dispersal, deposition/redeposition, pollen preservation, site size and character. Analytical constraints, particularly the number o f grains counted, and the composition of the pollen sum, influence representation of the pollen assemblage, hence zonation. A brief review o f these variables is presented below. Birks and Birks (1979) presented a more detailed synthesis. The pollen rain at any location is largely determined by productivity and dispersal. Janssen (1973) recognized four components: local, extralocal, regional, and extraregional pollen. Most pollen has a short dispersal distance, consequently local comDonents. i . e . those growing at the site or within a few meters o f it, should dominate the spectra. Such domination is usually obvious in records from peatlands and small lakes. Local input is small in the centre o f large lakes and also in environments where local productivity is small. In tundra areas, the pollen rain contains a large extraregional component due to the low inputs from other components of the pollen rain. The incorporation of pollen grains into sediments may involve a number of physical and chemical processes each o f which will contribute
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t o t h e c h a r a c t e r of t h e p o l l e n s p e c t r a and h e n c e , t o t h e d i s t i n c t i v e n e s s of z o n a t i o n . Tauber ( 1 9 6 5 ) h a s i d e n t i f i e d t h r e e d i s p e r s a l elements i n t h e p o l l e n r a i n of a f o r e s t e d area: t r u n k s p a c e p o l l e n , above c a n o p y p o l l e n , a n d r a i n - o u t p o l l e n . The r e l a t i v e c o n t r i b u t i o n o f e a c h v a r i e s w i t h t h e s i z e o f t h e b a s i n of d e p o s i t i o n , a b o v e c a n o p y a n d r a i n o u t c o m p o n e n t s b e c o m i n g more i m p o r t a n t as b a s i n s i z e i n c r e a s e s . I n l a k e s e d i m e n t s p o l l e n s p e c t r a may show c o n s i d e r a b l e h o r i z o n t a l v a r i atio? ( 2 a v i s e t al., 1 9 7 1 ; D a v i s a n d B r u b a k e r , 1 9 7 3 ) . M i x i n g by b u r r o w e r s n a y h o m o g e n i z e p o l l e n t h r o u g h 15 em o f s e d i m e n t ( D a v i s , 1 9 7 4 ) . T h i s may s u p r e s s f l u c t u a t i o n s i n t h e p o l l e n r e c o r d a n d i n f l u e n c e t h e determination of p o l l e n zones. P o l l e n s p e c t r a may b e m o d i f i e d by t h e e f f e c t s o f d i f f e r e n t i a l dec a y a n d by c o r t a m i n a t i o n f r o m o l d e r s e d i m e n t s . D i m b l e b y ( 1 9 7 6 ) c o n c l u d e d . t h a t a f t e r t w o y e a r s a l m o s t n o p o l l e n r e m a i n e d i n s o i l s w i t h pH > 6 . 0 , b u t B r y a n t ( 1 9 6 9 ) h a s r e c o v e r e d p o l l e n i n s o i l s o f pH 8 . 9 a l t h o u g h i d e n t i f i c a t i o n was d i f f i c u l t b e c a u s e o f c o r r o s i o n . C o r r o s i o n carl s e v e r e l y skew a p o l l e n s p e c t r u m . R e s i s t a n t t y p e s become o v e r r e p r e s e n t e d a n d t h e number o f s p e c i e s r e p r e s e n t e d may b e r e d u c e d t o p r o d u c e a s p e c t r u m t h a t may b e u n l i k e t h a t o r i g i n a l l y d e p o s i t e d . C o n v e r s e l y , p o l l e n s p e c t r a may b e a u g m e n t e d by r e d e p o s i t e d p o l l e n . Sornetimes t h e r e d e p o s i t e d c o m p o n e n t may b e m o r p h o l o g i c a l l y d i s t i n g u i s h a b l e f r o m c o p t e m p o r a r y p o l l e n e . g . p r e - T e r t i a r y or T e r t i a r y p o l l e n i n l a t e Q u a t e r n a r y s e d i m e n t s . The a l i e n g r a i n s may b e c o r r c d e d . However, t h e r e may b e n o d i f f e r e n c e s i n d e g r a d a t i o n . Apparent e c o l o g i c a l i n c o n p a t i b i l i t y may n o t b e a v a l i d c r i t e r i o n . Kumerical C o n s t r a i n t s and O b j e c t i v e Zonation The c h a r a c t e r o f a p o l l e n r e c o r d i s i n p a r t d e t e r m i n e d by n u m e r i c a l c o n s t r a i n t s . The r e l i a b i l i t y o f t h e samr?le a s r e p r e s e n t a t i v e o f t h e p o p u l a t i o n i s d e p e n d e n t on t h e n u m b e r of g r a i n s c o u n t e d . N o r m a l l y , 300 t o 500 g r a i n s i s s u f f i c i e n t , b u t n u m e r i c a l s t a b i l i t y I s i n f l u e n c e d by t h e numbers o f t a x a and t h e f r e q u e n c y o f e a c h t a x o n . Stability is a c h i e v e d r a p i d l y when t h e t a x o n i s common, more s l o w l y i f t h e t a x o n i s rare (Figure 2 ) . D e t e r m i n a t i o n o f c o n f i d e n c e l i m i t s can p r o v i d e a n e x p r e s s i o n of r e l i a b i l i t y . Mosimann (1965) p r o d u c e d f o r m u l a e f o r t h e n e c e s s a r y c a l c u l a t i o n s f o r t a x a ' i n s i d e ' a n d ' o u t s i d e ' t h e p o l l e n sum. The v a l u e s a r e e a s i l y d e t e r m i n e d by c o m p u t e r o r p r o g r a m m a b l e c a l c u l a t o r a l t h o u g h Maher ( 1 9 7 2 ) h a s p r o d u c e d nomograms f o r t h e p u r p o s e . I n t h e same p a p e r , h e i l l u s t r a t e s t h e t e c h n i q u e w i t h a p o l l e n r e c o r d f r o m P'Iolas L a k e , C o l o r a d o . The p r e s e n c e o f c o n f i d e n c e l i m i t s on a p o l l e n d i a g r a m h e l p s t o f i l t e r s i g n a l f r o m n o i s e a n d may b e u s e f u l a i d t o s u b j e c t i v e zonation. The c h a r a c t e r o f a p o l l e n d i a g r a m i s d e p e n d e n t n o t o n l y on t h e s i z e o f t h e p o l l e n sum b u t on t h e c o n s t i t u t i o n o f t h a t sum. Zonation i s a l m o s t a l w a y s b a s e d on p e r c e n t a g e v a l u e s o f c o m p o n e n t t a x a . Those p e r c e n t a g e s a r e d e t e r m i n e d from a p o l l e n sum w h i c h i s u s u a l l y t h e sum o f t h e f r e q u e n c i e s o f l e s s t h a n t h e t o t a l number o f t a x a p r e s e n t . S o m e t i m e s o n l y t r e e a n d s h r u b t a x a w i l l p r o v i d e t h e sum. Commonly, n o n - a r b o r e a l ( h e r b ) t a x a w i l l be i n c l u d e d , b u t a q u a t i c s and s p o r e s are u s u a l l y e x c l u d e d ( W r i g h t a n d P a t t e n , 1 9 6 3 ) . The c h o i c e o f t h e c o n s t i t u e n t s o f t h e p o l l e n sum w i l l d e t e r m i n e t h e p e r c e n t a g e s , t h e s h a p e of t h e p o l l e n diagram, and hence s u b j e c t i v e z o n a t i o n of t h a t diagram. The b i a s l i k e l y i n s u b j e c t i v e z o n a t i o n may b e e l i m i n a t e d by t h e numerical zonation techniques (provided a l l t a x a are included). Several of t h e s e o b j e c t i v e m e t h o d s a r e p r e s e n t l y b e i n % u s e d . They a r e summ a r i z e d i n T a b l e 1 a n d by B i r k s a n d B i r k s ( 1 9 7 9 ) . T h o s e m o s t f r e q u e n t l y u s e d a r e t h e s i n g l e - l i n k c l u s t e r i n g t e c h n i q u e , CONSLINK, d e v e l o p e d by G o r d o n a n d B i r k s ( 1 9 7 : ? ) , a n d t h e i r two d i v i s : i v e t e c h n i q u e s SPLI'TINF, b a s e d o n t o t a l i n f o r m a t i o n c o n t e n t , a n d SPLITLSQ, w h i c h d i v i d e s v i a . t h e sum o f l e a s t s q u a r e s . CONSLINK h a s b e e n p r o p o s e d as t h e s t a n d a r d zona t i o n technique f o r p o l l e n data used i n t h e I n t e r n a t i o n a l Geological The r e s u l t s o f t h e t h r e e t e c h n i q u e s above C o r r e l a t i o n Programme ( I G C P ) .
287
30 25 20 15
10
5
200
Figure 2
400
600
800 1000 1200 TOTAL POLLEN
1400
1600
I800
2000
V a r i a t i o n i n p e r c e n t a g e s i n one sample as t o t a l count increases. S t a b i l i t y is achieved earliest i n t h o s e t a x a t h a t a r e common ( a f t e r B i r k s a n d Birks, 1979).
****** T a b l e 1.
Some a p p r o a c h e s t o o b j e c t i v e z o n a t i o n a n d c o m p a r i s o n of p o l l e n diagrams
ORDINATION
CLASSIFICATION
COMPARISON OF DIAGRAMS
A.
Non-metric multi-dimensional (Gordon & B i r k s , 1 9 7 4 ) .
scaling
B.
P r i n c i p a l components a n a l y s i s 1 9 7 4 ; Adam, 1 9 7 4 ) .
A.
Divisive techniques using t o t a l inf o r m a t i o n c o n t e n t ( S D L I T I N F ) a n d sum of l e a s t s q u a r e s d e v i a t i o n s (SPLITLSQ).
B.
Constrained single-link clustering method, CONSLINK (Gordon & Birks,1972).
c.
Sequential correlation Ritchie, 1972).
A.
Ordination techniques used above, KD S CAL , P CA
B.
Sequence s l o t t i n g , Birks, 1974).
(Birks,
(Yarranton &
.
SLOTSEQ
(Gordon &
288
can be included in a pollen diagram as dendrograms. Figure 3 illustrates the application of these techniques and principal components analysis to the diagram for Scaleby Moss, ERgland (Birks, 1974). Cbjective techniques can be compared to each other, and to the local and standard British zonations.
SPLlTlNF 50
PR I NC I PAL SPLITLSQ COMPONENTS 0 500 4 - 4 0 4 0 4
o mm r t r t l r r l l l r m
....
Percentage residual variation
Figure
I
2
3
R e s u l t s of t h e a p p l i c a t i o n o f some o b j e c t i v e zona t i o n t e c h n i q u e s t o a d i a g r a m from S c a l e b y Moss, England. Note g e n e r a l concordance of boundaries (after Birks, 1974).
Walker and Wilson (1978) have proposed an alternative to these multivariate methods that is founded on the basic statistics (mean, variance, e t c . ) of individual taxa. Their technique involves influx not percentage values and therefore requires reliable absolute dating of the sequence. Objective comparison of diagrams, useful to the recognition of regional zones is possible through principal components analysis, multi-dimensional scaling, and by a' sequence slotting technique, SLOTSEQ (Gordon and Birks, 1974). The latter is suggested as regular procedure f o r IGCP projects (Birks, 1979). Objective zonation generally confirms the schemes derived subjectively. However, the techniques provide a hierarchy of division that allows assessment of the importance of each division an.d the distinctiveness of each zone. They are particularly useful for the identification of sub-zones.
The section above outlines the variables that influence the character of a pollen diagram. The constraints suggest perhaps that zonation is inappropriate as a dating technique. However, where regional pollen assemblage zones are well-established normally with I 4 C control, sequences can be dated v i a their pollen stratigraphies. The comparability is best where some of the site variables can be considered constant. Faegri and Iversen (1975) suggest that lakes of c a . 5000 m 2 are optimum sites for the study of regional pollen inputs. Digerfeldt (1979) has outlined criteria for the selection of lakes. The regional assemblage zones for northwestern Minnesota are well established (McAndrews, 1966) as are those for southern Ontario (McAndrews, 1981) and Quebec (Richard, 1980). In England, the zonation scheme originally proposed by Godwin (1940) has been confirmed by more recent work, despite the recognition of many local variants. It is important, however, that zonation be based entirely on the assemblage present and not manipulated to fit a pre.existing scheme. Objective zonation should allay this temptation. Pollen zones tend to occupy broad time periods. Consequently they provide only crude dating. Boundaries between zones may be well-dated but may be metachronous. The presence of large local producers may mask the regional pollen rain enough to produce assemblages that may be misinterpreted and thus incorrectly dated. The spruce zone at the base of a sequence from Blue Mounds Creek, Wisconsin, is well-defined (Figure 4). Its demise is rapid as it is elsewhere in the Midwest. Eowever, the disappearance of boreal woodland occurred at ca. 11,000 BP in this area, not at 9500 BP as it is dated at Blue Mounds Creek (Bernabo and Webb, 1977; Davis, 1977). The spruce zone here is a product of black spruce growing on the site long after the general northward migration of boreal forest. The explanation is confirmed by macrofossil analysis. Non-climatic controls such as soil development, differential migration and disturbance complicate the pollen spectra and zonation. These controls may be progressive (time-bound?) or eDisodic, local or regional (Iversen, 1973). Brubaker (1975) has demonstrated that substrate differences cause persistent modification of pollen spectra. Local disturbances caused by human activity have been largely responsible for the difficulties in recognizing regional pollen assemblage zones in Europe after 5000 BP (Birks and Birks, 1979). Dating beyond the Range of Radiocarbon Zonation of pollen sequences is important for dating sequences beyond the range of 14C, although it normally provides only a crude relative dating within the glacial/interglacial cycle. In Europe, there has long been recognition of a single Drogressive to retrogressive cycle (Firbas, 1949; Iversen, 1958; Andersen, 1966). More recently the concept has been applied to North American sequences (Wright, 1972; Kapp, 1977). The length of interglacials and glacials based on varve-counts is variable. The full cycle appears to require 5O-lOOy0O0 years. Interglacials, the shorter of the two components, seem to vary in length from 10-20,000 years (Davis, 1976). Iversen (1958) has summarized the placial/interglacial cycle in terms of climate, substrate, vegetation structure, composition and succession (Figure 5). Turner and West (1968) have suggested a four zone interglacial sequence. Zone 1, or Pretemperate phase, is marked by increasing forest closure and by boreal taxa. Zone 11, Earlytemperate or Mesocratic phase, has increasing amounts of deciduous trees. Zone 111, the Late-temperate or Oligocratic phase, has latearriving deciduous trees. In Zone IV, the Post-temperate phase, there is a return to boreal taxa and increasingly open conditions.
In North America, Wright (1972) has applied the model to Pleistocene interglacials and the Holocene. Kapp (1977) has suggested the addition of Zone V, Full glacial phase, for unglaciated areas. Figure 6 illustrates the application of the model to the Sangamon/Wisconsin
290
?GO>
EQ 0
*J.,
[ 0
age sequence from the Pittsburg Basin, Illinois (Griger. 1972). The zonation scheme for the glacial/interglacial sequence is simple when compared to those derived for Holocene sequences. Its simplicity is both advantageous and misleading. Undated and partial sequences may be fslottedf into one of the phases in the cycle and a crude relative date assigned. Assignment to the appropriate cycle requires some absolute or stratigraphic date control. Complications arise from the palynological dissimilarities between successive interglacials. It cannot be assumed that the pollen spectra at any phase of a cycle will be identical to those in the same phase of other cycles. In East Anglia, Zones I1 and I11 of successive interglacials show sometimes pronounced differences in their pollen spectra (Figure 7). With some taxa, the differences are variations in frequency and timing. Some species are absent from some interglacials. The length of interglacials is best approximated by varve-counts, although these provide only floating absolute chronologies. The partially varved sequence at Marks Tey, England, indicates that Zone I1 ar,d I11 in the Hoxnian Interglacial lasted about 4500 years (Turner, 1970).
292
0I
l-
5
HOLOCENE
W
23-
ALTONIAN SUBSTAGE
4-1
S C 0 N S I
5SANGAMON
6-
INTERGLACIAL
7-
0
50%
Figure 6
A p p l i c a t i o n of g l a c i a l / i n t e r g l a c i a l p o l l e n z o n a t i o n t o t h e sequence from P i t t s b u r g Basin, I l l i n o i s ( a f t e r Grfiger, 1 9 7 2 , a n d Kapp, 1 9 7 7 ) .
PASTONIAN
Figure 7
V a r i a t i o n s i n p o l l e n s p e c t r a i n Z o n e s I1 a n d I 1 1 through s u c c e s s i v e i n t e r g l a c i a l s , East Anglia ( a f t e r West, 1 9 7 0 ) .
293
Dabrowski (1971) attempted to use pollen influx to determine the length of the Eemian Interglacial. Modern influx measurements provided surrogates for Eemian rates, His calculations of ca. 18,000 palynochrones (a palynochrone = approx. 1 year) is considerably longer than Miiller's (1974) estimate of 11,000 years based on varves. SHORT DURATION AND RECENT EVENTS Pollen analysis may be useful in dating recent events and those of short duration that fall within the standard deviation of 14C. The recent events may be dated historically or by varve-counting. If the pollen stratigraphies are distinctive such events can provide welldefined dated marker horizons. In North America, settlement by immigrants is most often indicated by a rapid rise in Ambrosia. This increase in ragweed can usually be dated in most areas to an accuracy of a decade. In the varved sequence at Crawford Lake, Ontario, ragweed increases rapidly at about 1850 (Boyko, 1973). Oldfield (1969) has recognized the impact of the dissolution of the monasteries (ca. 1540 AD), intensification of agriculture at the start of the Napoleonic wars ( c a . 1800 AD), and more recent forestation in pollen records from the southern Lake District, England, Disease may quickly decimate an important forest species. Chestnut blight was first reported in New York in 1906. By 1930, chestnut had been almost eliminated from the forests of eastern North America. The demise is rapid and clearly marked in pollen records (Brugam, 1978). Tephra units often provide distinctive marker horizons in peat and lake sediments in the northwestern U.S. and southwestern Canada. Within the ran e of radiocarbon these are usually dated by bracketing each unit with C'' dates. However, the length of time involved in the deposition of the unit cannot be determined in this way. Mehringer e t al. (1977) have illustrated the value of pollen influx in estimating the length of those usually short-term depositional events. At L o s t Trail Pass Bog, Bitterroot Mountains, Montana, Glacier Peak ash (ca. 11,500 BP) and Mazama ash (ca. 6700 BP) are present in the sediments. Radiocarbon dates throughout the sequence allowed determination of sedimentation rates and hence pollen influx. Mean influx rates were compared to those derived from the ash. It was determined that the Glacier Peak ash was made up of two falls separated by 10-25 years. The Mazama ash, 7.3 at Lost Trail Pass Bog, was deposited over several years (Figure 8). The bulk, 4.5 cm (unit A), fell between fall and spying, based on very low pollen content and the lack of pollen from spring and summer pollinating trees. Another 1 cm, unit B y fell the following year, and the remaining 1.7 cm, unit C, the year after, although there were probably light, sporadic falls for up to three years. CONCLUSIONS Al.though ''C dating has become routine on pollen sequences, it has not replaced the use o f pollen stratigraphy for age determination. The two can be considered complementary. Many pollen sequences have only one or two I4C dates on them. Analyses done prior to the development of the radiocarbon dating technique have only the pollen stratigraphy on which assumptions about age can be made. Despite the constraints outlined above, zonation remains important in the determination of a chronology. The recognition of regional zonation schemes and interregional relationships between these schemes is critical. These are probably best established via the objective techniques now available , although subjective, ecologically-based zonation is unlikely to be significantly different The complication evident in Holocene sequences tends to be ignored when zonation of older materials is attempted. The models for the glacial/interglacial cycle are almost invariably more simple than those for the latest, the late Pleistocene/Holocene. Without absolute control, any chronology from the glacial/interglacial cycle is relative. The slotting of partial sequences must be tentative.
294 I
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P o l l e n i n f l u x d a t a can b e u s e f u l t o p r o v i d e f l o a t i n g a b s o l u t e c h r o n o l o g i e s f o r s h o r t d u r a t i o n e v e n t s , o r some e s t i m a t e of a g e w i t h i n t h e r a n g e o f r a d i o c a r b o n i n m a t e r i a l s u n s u i t , a b l e f o r 14C d a t i n g . The i n f l u x s t a n d a r d s c a n b e e i t h e r modern s u r r o g a t e s o r f o s s i l o n e s established w i t h radiocarbon or varves. REFERENCES CITED A d a m , D.P. , 1 9 7 4 , P a l y n o l o g i c a l a p p l i c a t i o n s o f p r i n c i p a l c o m p o n e n t and cluster analysis: J. R e s . U.S. Geol. S u r v e y , 2, p . 727-741. Andersen, S . T . , 1966, Interglacial vegetational succession and lake d e v e l o p m e n t i n D e n m a r k : P a l a e o b o t a n i s t , v. 1 5 , p . 1 1 7 - 1 2 7 . B e r n a b o , J.C. a n d W e b b , T . , 1 1 1 , 1 9 7 7 , C h a n g i n g p a t t e r n s i n t h e H o l o c e n e pollen record of northeastern North America: a mapped summary: Q u a t a r n a r y R e s e a r c h , v. 8, p . 64-96.
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land use:
, 1 9 7 7 , V g g g t a t i o n t a r d i g l a c i a r e a u Qu6bec m i r i d i o n a l e t implications pal6oclimatiques: GGogr. p h y s . Q u a t . , v . 3 1 , p . 1 6 1 176. R i c h a r d , P . , 1 9 8 0 , P a l e o p h y t o g g o g r a p h i e p o s t - w i s c o n s i n i e n n e d e QuCbecN-o t e s e t D o c u m e n t s , 8 0 - 0 1 , Labrador: bilan et perspectives: D 6 p t . G6og. U n i v . M o n t r e a l , 30 p .
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B o t a n i c a l Rev.
S h o t t o n , F . W . , 1 9 7 2 , An e x a m p l e o f h a r d - w a t e r e r r o r i n r a d i o c a r b o n N a t u r e , v . 240, p . 460-461. d a t i n g of v e g e t a b l e matter: Smith, A.G. and P i l c h e r , t a t i o n a l h i s t o r y of 903-914.
J.P,., 1973, Eadiocarbon d a t i n g and t h e vegethe British Isles: New P h y t o l o g i s t , v . 72, p .
T a u b e r , H., 1 9 6 5 , D i f f e r e n t i a l p o l l e n d e p o s i t i o n a n d t h e i n t e r p r e t a t i o n of p o l l e n diagrams: ?_an. G e o l . U n d e r s . , S e v . 2 , 8 9 , 6 9 p . T u r n e r , C . , 1970, The Middle P l e i s t o c e n e d e p o s i t s a t Itarks Tey, P h i l . T r a n s . Royal SOC. London, B , v . 257, p . 373-440.
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T u r n e r , C . a n d West, R . G . , 1 9 6 8 , T h e s u b - d i v i s i o n a n d z o n a t i o n o f i n t e r glacial periods: E i s z e i t a l t e r und Gegenwart, v . 1 9 , p . 93-101. Walker, D. and Wilson, S.R., 1978, A s t a t i s t i c a l a l t e r n a t i v e t o t h e z o n i n g of p o l l e n d i a g r a m s : J. B i o g e o k , v . 5 , p. 1-21. West,
R.G., 1970, P l e i s t o c e n e h i s t o r y of t h e B r i t i s h f l o r a , i n Walker, D . a n d West, R . G . , e d s . , S t u d i e s i n t h e V e g e t a t i o n a l H i s t o r y o f t h e B r i t i s h I s l e s , C a m b r i d g e U n i v . P r e s s , p . 1-11.
W r i g h t , H . E . , J r . , 1 9 7 2 , I n t e r g l a c i a l and p o s t g l a c i a l climates: pollen record: Q u a t e r n a r y R e s e a r c h , v . 2, p . 274-282.
the
, 1 9 7 6 , The dynamic n a t u r e o f H o l o c e n e v e g e t a t i o n . problem i n p a l e o c l i m a t o l o g y , biogeography and s t r a t i g r a p h i c nomenclature: Q u a t e r n a r y R e s e a r c h , v. 6 , p . 581-596. Wright, H . E . , Jr. and P a t t e n , S p o r e s , v . 5 , p . 445-50.
H.C.,
1 9 6 3 , The p o l l e n sum:
W r i g h t , H.E., J r . , W i n t e r , T . C . a n d P a t t e n , diagrams from southeastern Minnesota: V. 74, p . 1371-1396.
H.L., Bull.
_Pollen e t
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Y a r r a n t o n , G.A. and R i t c h i e , J . C . , 1972, Sequential c o r r e l a t i o n as an a i d i n p l a c i n g p o l l e n zone boundaries: P o l l e n e t Spores, v. 1 4 , p . 213-223.
A
299
MAMMOTHS, BISON AND TIME IN NORTH AMERICA
C.R. HARI N G TON
ABSTRACT N o r t h American Land M a m m a l Ages a r e v a l u a b l e i n a p p r o x i m a t i n g geol o g i c a l t i m e i n t h e Q u a t e r n a r y , p a r t i c u l a r l y where o t h e r more p r e c i s e d a t i n g methods ( s u c h as r a d i o c a r b o n , t e p h r a and p a l e o m a g n e t i c ) a r e l a c k i n g . Mammoths and b i s o n a r e D a r t i c u l a r l y im-oortant f o s s i l i n d i c a t o r s , f o r t h e e a r l i e s t mammoth r e m a i n s i n N o r t h America m a r k t h e b e g i n n i n g of t h e I r v i n g t o n i a n Land M a m m a l Age ( a b o u t 1 . 8 m i l l i o n y e a r s a g o ) , and t h e e a r l i e s t b i s o n r e m a i n s m a r k t h e b e g i n n i n g of t h e Ranchola b r e a n Land M a m m a l Age ( a b o u t 1 . 2 m i l l i o n y e a r s a g o ) . I t i s w o r t h n o t i n g t h a t t h e R a n c h o l a b r e a n may h a v e o c c u r r e d e a r l i e r i n n o r t h e r n N o r t h America t h a n i n s o u t h e r n N o r t h America, b e c a u s e b i s o n seem t o have been d e l a y e d i n p e n e t r a t i n g t h e s o u t h e r n p l a i n s . The main m o r p h o l o g i c a l c h a n g e s s e e n i n mammoths and b i s o n as t i m e proceeded were, r e s p e c t i v e l y : ( a ) a n i n c r e a s e i n number and c o m p r e s s i o n of enamel p l a t e s , and a t h i n n i n g o f enamel i n m o l a r t e e t h ; and ( b ) a t l e a s t during t h e l a s t 1 2 , 0 0 0 y e a r s , a tendency t o p r o g r e s s i v e reduction i n s i z e of h o r n c o r e s .
More s p e c i f i c a l l y , s o u t h e r n a n d w o o l l y mammoths seem t o b e u s e f u l i n d i c a t o r s o f e a r l y ( N e b r a s k a n t o Kansan) and l a t e ( m a i n l y W i s c o n s i n ) Pleistocene deposits, respectively. Small-horned b i s o n a r e g e n e r a l l y u s e f u l time-guides f o r t h e l a s t 1 2 , 0 0 0 y e a r s , whereas s t e p p e b i s o n u s u a l l y i n d i c a t e d e p o s i t s o l d e r t h a n 1 2 , 0 0 0 BP. INTRODUCTION
Mammoths a n d b i s o n , two i m m i g r a n t s from E u r a s i a , a r e p a r t i c u l a r l y i m p o r t a n t b e c a u s e t h e y have b e e n u s e d b y Savage ( 1 9 5 1 ) i n e s t a b l i s h i n g I r v i n g t o n i a n and R a n c h o l a b r e a n N o r t h American Land M a m m a l Ages. T h i s i s a m a j o r d i v i s i o n of t h e Q u a t e r n a r y ( a p p r o x i m a t e l y t h e l a s t 2 m i l l i o n y e a r s o f g e o l o g i c a l t i m e ) b a s e d on f o s s i l v e r t e b r a t e a s s e m b l a g e s ( F i g u r e 1 ) . Savage e m p h a s i z e d t h a t t h e terms I r v i n g t o n i a n and R a n c h o l a b r e a n are b a s e d on f a u n a l names and o n a n i m a l s w i t h i n t h o s e f a u n a s from C a l i f o r n i a , and t h a t t h e y a r e e n t i r e l y s e p a r a t e from r o c k f o r m a t i o n a l names. Ke i n d i c a t e d t h a t t h e e a r l i e s t mammoth r e m a i n s mark t h e b e g i n n i n g o f t h e R a n c h o l a b r e a n . I r o n i c a l l y , t h e l a t t e r a g e a l m o s t came t o a n end a b o u t 1890 due t o human o v e r h u n t i n g of b i s o n ( R o e , 1 9 7 0 ; B a n f i e l d , 1 9 7 4 ) . The e x a c t t i m e d e m a r c a t i o n o f t h e s e a g e s m a y , p e r h a p s , o n l y b e e s t a b l i s h e d on a r e g i o n a l b a s i s and i n t h e f u t u r e , when t h e r e i s b e t t e r geochronological c o n t r o l over middle Pleistocene land mammal faunas. P r e s e n t l y , I s u g g e s t t h a t mammoths f i r s t e n t e r e d North America a b o u t 1 . 8 m i l l i o n y e a r s a g o d u r i n g t h e Nebraskan G l a c i a t i o n and a t t h e b e g i n n i n g o f t h e Q u a t e r n a r y , and t h a t b i s o n f i r s t e n t e r e d N o r t h America d u r i n g t h e Kansan G l a c i a t i o n a b o u t 1 . 2 m i l l i o n y e a r s a g o ( F i g u r e 1 ) . Of course, t h e e a r l i e s t t i m e s of e n t r y o f mammoths and b i s o n i n t o n o r t h e r n North America would p r e c e d e t h e t i m e s t h e y became e s t a b l i s h e d i n t h e s o u t h .
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F i g u r e 1 Suggested chronology and r e g i o n a l d i s t r i b u t i o n of Chronomammoths a n d b i s o n i n N o r t h A m e r i c a d u r i n g t h e q u a t e r n a r y . l o g i c a l d a t a i n t h r e e columns on l e f t are approximate, (modified from E r i c s o n a n d W o l l i n ( 1 9 6 8 ) , M a g l i o ( 1 9 7 3 ) a n d K u r t e n a n d A n d e r s o n (1980)), and p h y l o g e n i e s a r e h i g h l y s i m p l i f i e d . For example, I have n o t d e a l t w i t h t h e o r i g i n o f t h e d w a r f mammoths o f S a n t a R o s a I s l a n d , C a l i f o r n i a ( S t o c k and F u r l o n g 1 9 2 8 ) . Also, I have avoided t h e i s s u e of t h e pres e n c e o f s t e p p e - l i k e b i s o n o n t h e p l a i n s u n t i l m o r e i s known a b o u t t h e i r g e o l o g i c a l a g e and a f f i n i t i e s . P e r h a p s s t e p p e b i s o n ( e . g . Bison +rassicornis f r o m M a s s a c h u s e t t s r a d i o c a r b o n d a t e d a t a p p r o x i m a t e l y 2 1 , 2 0 0 BP ( R o m e r 1 9 5 1 ) ) p e n e t r a t e d t h e p l a i n s a s e c o n d t i m e before t h e p e a k of t h e l a s t g l a c i a t i o n . Some o f t h e s e s t e p p e - l i k e f o r m s may a l s o r e p r e s e n t smaller-horned d e r i v a t i v e s of g i a n t b i s o n s t o c k . Brunhes, Jara?fagnetic: Normal p o l a r i t y ( b l a c k ) from t o p t o b o t t o m n i l l o , Olduvai; reversed p o l a r i t y (white). G l a c i a l s (black) and Interg l a c i a l ~ : N (Nebraskan); A (Aftonian); K (Kansan); Y (Yarmouth); I ( I l l i n o i a n ) ; S ( S a n g a m o n ) ; W ( W i s c o n s i n ) ; EI ( H o l o c e n e last 10,000 vears). N o t e b r e a k i n s c a l e a t GJ/H b o u n d a r y . E a c h o f t h e t w o c o l u m n s o n t h e r i g h t d e a l i n g w i t h mammoths a n d b i s o n a r e s u b d i v i d e d i n t o B e r i n g i a ("RER." - u n g l a c i a t e d p a r t s o f e a s t e r n S i b e r i a , B e r i n g I s t h m u s , A l a s k a a n d Yukon) on t h e l e f t , a n d P l a i n s ("PL." h e a r t l a n d o f t h e c o n t i n e n t ) on t h e r i g h t , i n o r d e r t o show p o s s i b l e r e l a t i o n s h i p s b e t w e e n p o p u l a t i o n s i n t h o s e r e g i o n s . Dashed l i n e s i n d i c a t e p o i n t s of Freatest u n c e r t a i n t y . Upright arrowh e a d s mark e v i d e n t times of e x t i n c t i o n s .
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301
MAMMOTES Mammoth remains. particularly complete molar teeth, are sometimes useful biostratigraphic indicators, for they are commonly widespread geographically and they tend to preserve well. The most primitive species known from North America, the southern mammoth (Mummuthus m e r i d i o n a l i s l probably entered this continent from Eurasia near the beginning of the Quaternary, when Repenning recognizes an invasion of rodents including P l i o m y s and S y n a p t o m y s (ReDenninp, 1980; Harington, 1980a). Severalmolars from the northern Yukon resemble those of the most advanced southern mammoth stock (Eacton Stage) or Eurasia, and I suspect that they represent the earliest North American mammoths (Harington 1977; Figure 2). This species also appears to be present in the Wellsch Valley fauna in Saskatchewan, where it may have lived Rearly 1.7 million years ago (A. PTacS. Stalker, .sers. commun., 1976), and at Bruneau, Idaho about 1.36 million years ago (Maglio, 1973). North American steppe mammoths (Mummuthus c o l u m b i and/or Mammuthus a r m e n i a c u s ) are the least valuable biostratigraphically and their origins present problems (Harington 1980b). It is now widely accepted that the Columbian mammoth (Mammuthus c o l u m b i , under which I include the imperial mammoth, Mammuthus i m p e r a t o r , following Kurtgn and Anderson 1980) arose from the southern mammoth in North America ( e . g . MagliO, 1973). As an alternative, I propose that steppe mammoths reached North America from Eurasia, replacing southern mammoths during the Kansan Glaciation about 1.2 million years ago. I think that they invaded with primitive muskoxen like S o e r g e l i a and E u c e r a t h e r i u m and perhaps Phenacomys, C J e t h r i o n o m y s , TJeodon and P i t y m y s (Repenning's (1980) "Irvingtonian II" microtines ) , From the evidence I have seen, I can only stress the similarities of Eurasian and North American steppe mammoths. Some specimens I have referred to Mammuthus c f . a r m e n i a c u s from the northern Yukon may record the earliest North American entry of steDpe mammoths from Eurasia (Harington, 1977). Also, Savage's (1951) description and measurements of Mammuthus c o l u m b i molars from Irvingtonian localities fit well with those of the Eurasian steppe mammoths now included in Mammuthus armeni a c u s . Further, Henry Fairfield Osborn (1942), a paleontologist with great experience in studying elephants, derived Columbian mammoths from Eurasian steppe mammoths now included in Mammuthus a r m e n i a c u s by Maglio (1973). If, indeed, Eurasian and North American steppe mammoths are not synonymous, we need a way of differentiating them -- preferably on molar characteristics. Although Maglio (1473) states that the North American lineage leading from southern to Columbian mammoths has broader molars, most steppe mammoth molars I have measured from the northern Yukon are slightly narrower than the average of a sample from Eurasia (Harington, 1977). Steppe mammoths (Figure 3) seem to have occupied North America for more than 1 million years. The most advanced type (sometimes considered as a separate species, Mammuthus j e f f e r s o n i i ) survived on the olains until about 11,000 years aqo (Kurtgn and Anderson, 1980). Evidently some steppe mammoths lived in eastern Ferinpia ?urine: the ?eak of the liisconsin Glaciation approximately 20,000 years ago (Harington, 1980b). Figure 1
Continued
Mammoths: s o u t h e r n (Mummuthus m e r i d i o n a l i s ) ; s t e p p e ( M , a r m e n i a c u s a n d / o r M . c o l u m b i ) ; w o o l l y IM. p r i m i g e n i u s i .
-
steppe (earliest(?) Bison a l a s k e n s i s , a n d l a t e s t - B . c r a s s { c o r n i s : s o m e t i m e s t h e y a r e g r o u p r d u n d e r t h e name B . p r i s c u s ) ; g i a n t (B. l a t i f r o n s ) ; " o c c i d e n t a l i s - a n t i q u u s ~ o m p l e x ' ' I~B . b i s o n o c c i d e n t a l i s e v i d e n t l y d e r i v e d f r o m s t e p p e b i s o n s t o c k i n B e r i n g i a ; B . bison antiquus e v i d e n t l y derived from g i a n t bison s t o c k i n t h e p l a i n s ) ; wood (B. b i s o n a t h a b a s c a e - e v i d e n t l y d e r i v e d m a i n l y f r o m B . b i s o n o c c i d e n t a l i s s t o c k on t h e n o r t h e r n and w e s t e r n f l a n k s of t h e Canadian p l a i n s ) ; p l a i n s (B. b i s o n b i s o n - e v i d e n t l y d e r i v e d f r o m " o c c i d e n t a l i s antiquus" s t o c k on t h e p l a i n s ) . Bison:
302
Figure 2
llammoth t e e t h a r e o f t e n u s e f u l i n d i c a t o r s o f g e o l o g i c a l age and p a s t environment. The l o w e r t h i r d m o l a r o n t h e l e f t ( g r i n d i n g s u r f a c e shown w i t h e n a m e l p l a t e s i n b l a c k ) i s f r o m a s o u t h e r n mammoth (Mummuthus m e r i d i o n a l i s ) f r o n O l d Crow B a s i n , Yukon T e r r i t o r y , a n d s h o w s t h e " p r i m i t i v e " c o n d i t i o n of r e l a t i v e l y few, t h i c k , widely-spaced enamel plates. The l o w e r t h i r d m o l a r o n t h e r i g h t , from a w o o l l y mammoth (Mammuthus p r i m i g e n i u s ) f r o m t h e same r e g i o n o f t h e Y u k o n , s h o w s t h e " a d v a n c e d " c o n d i t i o n i n v o l v i n g more numerous, t h i n n e r , more compressed enamel p l a t e s . I n k s k e t c h by C h a r l e s Douglas.
However w o o l l y mammoths (Mammuthus p r i m i g e n i u s ) w e r e dominant t h e r e d u r i n g t h a t p e r i o d . C o n c e i v a b l y s t e p p e a n d w o o l l y mammoths may h a v e s h a r e d t h e i r r a n g e b o t h n o r t h and s o u t h o f t h e W i s c o n s i n i c e s h e e t s . Woolly mammoths ( F i g u r e 4 ) m i g r a t e d f r o m E u r a s i a t o N o r t h America i n l a t e I l l i n o i a n or e a r l y W i s c o n s i n t i m e ( H a r i n g t o n , 1 9 8 0 a ) . I n t h e s o u r c e a r e a , n o r t h e a s t e r n S i b e r i a , a n e a r l y tyDe o f w o o l l y mammoth i s known from t h e Utka Beds ( p r o b a b l y l a t e I l l i n o i a n ) , and t h e a d v a n c e d t y p e was e s t a b l i s h e d t h e r e by e a r l y W i s c o n s i n t i m e (Iedoma S u i t e ) ( S h e r , 1 9 7 1 ) . E v i d e n t l y w o o l l y mammoths f i r s t r e a c h e d t h e n o r t h e r n p l a i n s ( M e d i c i n e Hat, A l b e r t a ) d u r i n g t h e e a r l y W i s c o n s i n ( S t a l k e r and C h u r c h e r , 1 9 7 0 ) . The s p e c i e s s p r e a d t o e a s t e r n Canada d u r i n g t h e m i d W i s c o n s i n a b o u t 4 5 , 0 0 0 y e a r s a g o ( C h u r c h e r , 1 9 6 8 a ) . It seems t o h a v e occupied d i s c o n t i n u o u s t u n d r a - l i k e r a n g e s o u t h o f t h e Wisconsin i c e s h e e t , e x t e n d i n g from s o u t h e r n B r i t i s h Columbia t o s o u t h e r n O n t a r i o , a n d p e r h a p s f a r t h e r e a s t w a r d . S c a t t e r e d f o s s i l s of' o t h e r t u n d r a a d a p t e d m a m m a l s i n c l u d i n g lemmings, t u n d r a muskoxen and c a r i b o u a l s o o c c u r i n t h i s zone ( H a r i n g t o n a n d Ashworth, i n p r e p a r a t i o n ) . Woolly mammoths d i e d o u t i n N o r t h America a b o u t 1 3 , 0 0 0 y e a r s a g o . I c o n s i d e r t h i s s p e c i e s t o b e a n i n d i c a t o r of Wisconsin-age d e p o s i t s i n s o u t h e r n Canada and n o r t h e r n U n i t e d S t a t e s , a t l e a s t .
303
Figure 3
a N o r t h A m e r i c a n s t e p p e mammoth T h e y seem t o h a v e b e e n t h e m o s t common a n d w i d e s p r e a d o f N o r t h A m e r i c a n mammoths. I n k s k e t c h by C h a r l e s D o u g l a s .
Figure 4
R e s t o r a t i o n o f a w o o l l y mammoth (Mammuthus p r i m i g e n i u s ) . T h e y w e r e a d a p t e d t o t u n d r a - l i k e r a n g e and were w e l l a d a p t e d t o c o l d c o n d i t i o n s . I n k s k e t c h by Charles Douglas.
Restoration of
(Marnrnuthus c o z u m b i ) .
304
BISON The s t u d y of N o r t h A m e r i c a n f o s s i l b i s o n i s f r a u g h t w i t h p r o b l e m s . E x p e r t s d i s a g r e e on many p o i n t s : n a m e l y , g e o l o g i c a l a g e b e y o n d 4 0 , 0 0 0 BP, phylogeny and d i s t r i b u t i o n a l h i s t o r y . I think that clarification o f many o f t h e s e p r o b l e m s d e p e n d s upon g a i n i n g a b e t t e r p e r s p e c t i v e on t h e h i s t o r y of E u r a s i a n f o s s i l b i s o n . To t h i s e n d , D . M . S h a c k l e t o n a n d M . L . W e s t o n , s p o n s o r e d by t h e N a t i o n a l Museum of N a t u r a l S c i e n c e s , b e g a n a s t u d y i n 1 9 7 9 . They a r e p r o c e s s i n g b i o s t r a t i g r a p h i c a n d n e t r i c d a t a on w e s t e r n E u r o p e a n f o s s i l s . Data f r o m 185 s k u l l s , 1 6 m a n d i b l e s a n d 1 6 8 p o s t c r a n i a l e l e m e n t s were c o l l e c t e d f r o m b i s o n f o s s i l s i n Soviet i n s t i t u t i o n s i n 1982.
B i s o n h a d e n t e r e d c e n t r a l A l a s k a by K a n s a n ( ? ) t i m e a c c o r d i n g t o r e m a i n s from t h e Fox G r a v e l u n i t n e a r F a i r b a n k s . The a g e e s t i m a t e r e s t s on P 6 w 6 ' s ( 1 9 7 5 a , b ) s t r a t i g r a p h i c i n t e r p r e t a t i o n of a n e x p o s u r e a t Eva C r e e k . O t h e r e a r l y s p e c i m e n s of b i s o n f r o m A l a s k a a r e f r o m d e p o s i t s of p o s s i b l e Yarmouth I n t e r g l a c i a l a g e n e a r B a l d w i n P e n i n s u l a ; I l l i n o i a n age n e a r F a i r b a n k s ( a f i s s i o n - t r a c k age of approximately 450,000 y e a r s f r o m t h e E a s t e r Ash Bed s u g g e s t s t h a t B i s o n r e m a i n s f r o m t h i s p a r t of t h e G o l d H i l l L o e s s may b e I l l i n o i a n i n a g e (T.L. P & w & , p e r s . commun., 1 9 8 2 ) ) ; a n d Sangamon I n t e r g l a c i a l a g e n e a r T o f t y . A s n o p u b l i s h e d d e s c r i p t i o n s of t h e s e f o s s i l s a r e a v a i l a b l e , i t i s d i f f i c u l t t o know w h e t h e r t h e y s h o u l d b e t r e a t e d m e r e l y a s B i s o n s p . , or w h e t h e r - a s seems l i k e l y - t h e y r e p r e s e n t s t e p p e b i s o n ( s e n s u Kurt6n and Anderson, 1980). I n t h e Yukon, I h a v e c o l l e c t e d s t e p p e b i s o n s p e c i m e n s w i t h unu s u a l l y l a r g e h o r n c o r e s ( B i s o n a t a s k e n s i s ) , one o f w h i c h y i e l d e d a r a d i o c a r b o n d a t e of >39,000 BP ( 1 - 5 4 0 5 ) . I c o n s i d e r i t t o be a s u i t a b l e a n c e s t o r for l a t e r , s m a l l e r - h o r n e d s t e p p e b i s o n ( B i s o n c r a s s i c o r n i s ) and t h e g i a n t b i s o n ( B i s o n Z a t i f r o n s ) of s o u t h e r n North America, and t o be c l o s e l y r e l a t e d t o , i f not.synonymous w i t h , Bison p r i s c u s gigas ( H a r i n g t o n a n d C l u l o w , 1 9 7 3 ; H a r i n g t o n , 1 9 8 0 a ) . The l a t t e r b i s o n was widespread i n E u r a s i a d u r i n g t h e middle P l e i s t o c e n e ( e q u i v a l e n t i n age f r o m Yarmouth t o Sangamon t i m e i n F l e r o v ' s ( 1 9 7 2 ) t e r m i n o l o g y ) . The e a r l i e s t o c c u r r e n c e s o f b i s o n on t h e p l a i n s a D p e a r t o be o f Yarmouth a g e i n C a n a d a ( S t a l k e r a n d C h u r c h e r , 1 9 7 0 ) a n d of p o s t - Kansan t o e a r l y I l l i n o i a n a g e i n t h e U n i t e d S t a t e s ( S c h u l t z and H i l l e r u d , 1977). Giant b i s o n are f i r s t d e f i n i t e l y r e c o r d e d there i n l a t e I l l i n o i a n time a n d seem t o h a v e s u r v i v e d u n t i l l a t e W i s c o n s i n t i m e ( K u r t 6 n a n d Anderson, 1980). N o r t h e r n s m a l l - h o r n e d b i s o n , or w e s t e r n b i s o n ( B i s o n b i s o n o c c i d e n t a Z i s ) e v i d e n t l y a r o s e from stepDe b i s o n ( B i s o n c r a s s i c o r n i s : F i g u r e 5 ) s t o c k i n B e r i n g i a t o w a r d t h e c l o s e of t h e l a s t g l a c i a t i o n . P r o b a b l y warmer, m o i s t e r c o n d i t i o n s o c c u r r i n g i n B e r i n g i a t h e n , a n d t h e r e s u l t i n g more h e a v i l y - w o o d e d t e r r a i n c o n t r i b u t e d t o t h e d e m i s e of s t e p p e b i s o n a n d t h e r i s e of w e s t e r n b i s o n ( H a r i n g t o n , 1 9 7 7 ) . T h e s e b i s o n s p r e a d r a p i d l y s o u t h w a r d , p o s s i b l y m i x i n g w i t h s o u t h e r n smallhorned b i s o n ( B i s o n b i s o n a n t i q u u s ) t h a t had evolved from l a r g e r - h o r n e d s t o c k s o u t h o f t h e W i s c o n s i n i c e s h e e t s ( G u t h r i e , 1 9 7 0 ) . It i s d i f f i c u l t t o u n t i e t h i s k n o t . For example, I have examined s k u l l s w i t h horncores t h a t r e p r e s e n t n e a r l y every p o s s i b l e combination of c h a r a c t e r s thought t o be d i a g n o s t i c of Bison b i s o n o c c i d e n t a Z i s and Bison b i s o n a n t i q u u s . For u t i l i t a r i a n r e a s o n s , I c a l l t h i s m o s a i c " t h e o c c i d e n t a l i s I n f a c t , such small-horned b i s o n a r e g e n e r a l l y usea n t i q u u s complex". f u l i n d i c a t o r s o f 1 2 , 0 0 0 - 6 , 0 0 0 BP d e p o s i t s . I t i s i n t e r e s t i n g t o s p e c u l a t e on t h e e v o l u t i o n o f modern b i s o n from t h e i r small-horned b i s o n a n c e s t o r s . A s a h y p o t h e s i s f o r t e s t i n g , I suggest t h a t t h e Hypsithermal (about 7,000-5,000 BP) p l a c e d r e l a t i v e l y g r e a t a n d s u d d e n s t r e s s o n s m a l l - h o r n e d b i s o n h e r d s t h a t were b e s t a d a p t e d t o w o o d l a n d o r p a r k l a n d c o n d i t i o n s , a n d t h a t some were a b l e t o a d a p t t o l i f e on t h e a r i d g r a s s l a n d s ( p l a i n s b i s o n , B i s o n b i s o n b i s o n ) w h i l e o t h e r s withdrew northward and westward f o l l o w i n g t h e r e t r e a t i n g m a r g i n s of t h e b o r e a l a n d s u b a l p i n e f o r e s t , o r r e m a i n e d i n s u c h a r e a s (wood b i s o n , B i s o n b i s o n a t h a b a s c a e ) . Adaptation of plains bison
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Figure 5
R e s t o r a t i o n o f s t e p p e b i s o n (Bison crassicornis) bulls fighting. This species w a s most common in u n g l a c i a t e d a r e a s of A l a s k a and t h e Y u k o n d u r i n g the last glaciation. Based o n skeletal material from a late-Wisconsin site near Dawson, Yukon Territory. Ink sketch b y Bonnie Dalzell.
p r o b a b l y i n v o l v e d s u b s t a n t i 2 1 morDhologica1 and b e h a v i o u r a l c h a n g e s , w h e r e a s I s u s p e c t t h a t wood b i s o n d i f f e r l i t t l e ( e x c e p t f o r s m a l l e r h o r n s ) from B i s o n b i s o n o c e i d e n t a Z i s ( H a r i n g t o n , 1 9 7 7 ) . A b i s o n s k u l l from B a n f f , A l b e r t a y i e l d i n g a r a d i o c a r b o n d a t e of 3,240 5 90 B P (1-11654) i n d i c a t e s t h a t some w e s t e r n b i s o n s u r v i v e d t h i s p e r i o d o f c r i s i s i n i n t e r m o n t a n e p o c k e t s (Appendix; F i g u r e s 6 , 7 ) . T h i s i s i n c o n t r a s t t o l a r g e s a m p l e s o f s k u l l s b e s t r e f e r r e d t o modern b i s o n o f s i m i l a r g e o l o g i c a l a g e from s i t e s f a r t h e r e a s t n e a r F o r t S a s k a t c h e w a n , A l b e r t a , where I h a v e b e e n c o l l e c t i n g , and n e a r B o t t r e l l , A l b e r t a where L . V . H i l l s and M . Wilson ( p e r s . commun., 1 9 8 1 ) have b e e n collecting.
CONCLUSION I n summary, d i v i s i o n of t h e N o r t h American Q u a t e r n a r y i n t o I r v i n g t o n i a n and R a n c h o l a b r e a n Land M a m m a l Ages i s g e n e r a l l y u s e f u l n a r t i c u l a r l y where more p r e c i s e p h y s i c a l d a t i n g t e c h n i q u e s ( e . g . r a d i o c a r b o n , t e p h r a , p a l e o m a g n e t i c ) a r e n o t a p p l i c a b l e , and where m i c r o t i n e ( e . 9 . t h e O n d a t r a i d a h o e n s i s + a n n e e t e n s -+ n e b r a c e n s i s + z i b e t h i c u s l i n e a g e e x t e n d i n g from e a r l y I r v i n g t o n i a n t o Recent ( N e l s o n and Semken, 1 9 7 0 ) ) , and o t h e r u s e f u l i n d i c a t o r s , s u c h a s S o e r g e Z i a ( H a r i n g t o n , 1 9 7 7 ) , c o n s t i t u t e s u p p l e m e n t a r y f o s s i l e v i d e n c e o f g e o l o g i c a l a g e . Remains o f s o u t h e r n and w o o l l y mammoths seem to b e u s e f u l i n d i c a t o r s of e a r l y ( N e b r a s k a n t o Kansan) and l a t e ( m a i n l y W i s c o n s i n ) P l e i s t o c e n e d e p o s i t s , respectively. S t e p p e mammoth r e m a i n s , a l o n e , a r e n o t v e r y u s e f u l i n t h i s regard. Small-horned b i s o n a r e g e n e r a l l y u s e f u l time-guides f o r t h e l a s t 1 2 , 0 0 0 y e a r s , whereas s t e p p e b i s o n a r e i n d i c a t i v e o f d e p o s i t s o l d e r t h a n 1 2 , 0 0 0 B P . G f a m t b i s o n a r e n o t as u s e f u l i n t h i s r e s p e c t a s t h e y seemed t o b e a d e c a d e a g o , f o r t h e y e v i d e n t l y s u r v i v e d from t h e Yarmouth I n t e r g l a c i a l t o l a t e W i s c o n s i n t i m e . The main m o r p h o l o g i c a l c h a n g e s s e e n i n m o l a r s o f t h e r e l a t i v e l y " p r i m i t i v e " s o u t h e r n mammoth t o t h e r e l a t i v e l y "advanced" w o o l l y mammoth a r e : a n i n c r e a s e i n number and c o m p r e s s i o n of enamel p l a t e s ; and a t h i n n i n g of enamel a s t i m e p r o c e e d e d . I n bison, at l e a s t during the l a s t 1 2 , 0 0 0 y e a r s , t h e r e i s t e n d e n c y t o p r o g r e s s i v e r e d u c t i o n i n s i z e of horncores (Wilson, 1980).
306
F i n a l l y , I wish t o emphasize t h e p o s s i b i l i t y t h a t t h e Rancholabrean may h a v e o c c u r r e d e a r l i e r i n n o r t h e r n N o r t h A m e r i c a ( K a n s a n ? ) t h a n i n s o u t h e r n N o r t h A m e r i c a ( Y a r m o u t h ? ) , b e c a u s e b i s o n seem t o h a v e b e e n delayed i n penetrating the southern p l a i n s . ACKNOWLEDGEYENTS I a m g r a t e f u l t o : Edward J . Hart ( A r c h i v e s of t h e C a n a d i a n R o c k i e s ) f o r t h e l o a n of t h e b i s o n s k u l l from B a n f f , Alberta; Harry F o s t e r ( N a t i o n a l Museums o f C a n a d a ) f o r p h o t o g r a p h s o f t h a t s k u l l ; C h a r l e s H . D o u g l a s ( N a t i o n a l Museum o f N a t u r a l S c i e n c e s ) , a n d B o n n i e D a l z e l l for t h e i n k s k e t c h e s ; a n d Mrs. G a i l R i c e f o r t y p i n g t h e manuscript.
The B a n f f A r c h i v e s B i s o n I n t h e l a t e s u m m e r o f 1 9 6 7 , Mrs. C a t h a r i n e TAJhyte c o l l e c t e d a damaged p o s t e r i o r c r a n i a l f r a g m e n t of a n a d u l t b i s o n ( F i g u r e s 6 , 7) unc o v e r e d by a b u l l d o z e r o p e r a t o r e x c a v a t i n g t h e f o u n d a t i o n o f t h e Archives of t h e Canadian Rockies b u i l d i n g i n Banff, A l b e r t a . The s p e c i m e n c a m e f r o m a d e p t h o f a p p r o x i m a t e l y s e v e n f e e t ( 2 . 1 m) i n " g l a c i a l s i l t " ( b u f f sandy s i l t m a t r i x was found i n c a v i t i e s i n t h e c r a n i a l bone). R a d i o c a r b o n a n a l y s i s of bone c o l l a g e n from t h e b a s e of t h e c r a n i u m y i e l d e d a d a t e o f 3 , 2 4 0 ? 9 0 BP ( 1 - 1 1 6 3 8 ) , w h i c h a p p r o x i m a t e l y d a t e s t h e s t r e a m t e r r a c e f r o m w h i c h t h e s p e c i m e n was d e r i v e d . It i s noteworthy t h a t N. Rutter considered t h e t e r r a c e s i n t h i s a r e a u n l i k e l y t o b e o l d e r t h a n 3 , 5 0 0 BP ( W i l s o n , 1 9 7 5 ) , b e f o r e t h e s p e c i m e n was d a t e d . T h i s Banff t e r r a c e i s e v i d e n t l y c o r r e l a t i v e w i t h t h e l a t e T 2 t e r r a c e d e p o s i t s i n t h e Bow V a l l e y f a r t h e r e a s t ( M . W i l s o n , p e r s . commun., 1 9 8 1 ) .
On t h e b a s i s o f t h e r e l a t i v e l y l o n g ( c o m p a r e d t o m o d e r n b i s o n ) e s t i m a t e d s p r e a d o f t h e h o r n c o r e s ( a p p r o x i m a t e l y 7 8 0 mm), t h e r a t h e r n a r r o w ( c o m p a r e d t o Bison b i s o n a n t i q u u s ) f r o n t a l r e g i o n b e t w e e n h o r n c o r e s a n d o r b i t s ( 3 0 1 mm), a n d t h e b a c k s w e p t h o r n c o r e s ( F i g u r e 6 ; T a b l e l), I c o n s i d e r t h a t t h e s p e c i m e n i s b e s t r e f e r r e d t o t h e w e s t e r n bison (Bison bison occidentaZis). T h e r e f o r e , some b i s o n w i t h m a r k e d c h a r a c t e r i s t i c s of w e s t e r n b i s o n , s u r v i v e d i n a t l e a s t one p o c k e t n e a r t h e e a s t e r n f l a n k o f t h e Rocky M o u n t a i n s u n t i l a b o u t 3 , 0 0 0 y e a r s a g o , w h e r e a s c o n t e m p o r a n e o u s b i s o n f r o m two s i t e s f a r t h e r e a s t ( s e e t e x t ) i n S i n c e w e s t e r n b i s o n ocA l b e r t a a r e b e s t r e f e r r e d t o modern b i s o n . c u r r e d n e a r C o c h r a n e a s e a r l y a s 1 1 , 0 0 0 BP ( C h u r c h e r , 1 9 6 8 b , 1 9 7 5 ) , t h e s p e c i e s o c c u p i e d t h i s p a r t o f w e s t e r n A l b e r t a f o r some 8 , 0 0 0 y e a r s . P e c u l i a r f e a t u r e s of t h e Banff specimen a r e t h e pronounced dep r e s s i o n s on t h e r i g h t ( m e d i a l t o t h e r i g h t o r b i t ) and l e f t ( m e d i a l t o t h e l e f t hornbase) f r o n t a l s : apparently t h e s e depressions are not due t o e r o s i o n d u r i n g t r a n s p o r t i n a stream b e d , b u t r e s u l t e d f r o m damage (by f i g h t i n g o r p o s s i b l y d i s e a s e ? ) i n l i f e .
***
307
Figure 6
Figure 7
Dorsal Q i e w of a bison s k u l l fragment from a l o w terrace a t Banff, Alberta. Bone f r o m t h i s s p e c i m e n h a s b e e n r a d i o c a r b o n d a t e d a t a b o u t 3 , 2 4 0 BP i n d i c a t i n g t h a t t h e r e were some l a t e - s u r v i v i n g w e s t e r n b i s o n ( B i s o n bison o c c i d e n t a z i s ) i n p o c k e t s n e a r t h e moutain Front.
P o s t e r i o r view of t h e western bison c r a n i a l fragment shown i n F i p u r e 6 .
308
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X a g l i o , V.J., 1 9 7 3 , O r i g i n a n d e v o l u t i o n o f t h e E l e p h a n t i d a e : A m e r . P h i l . S O C . (New S e r i e s ) , v . 6-3, p t . 3 , p . 1 - 1 4 9 .
Trans.
N e l s o n , R . S . and Semken, H . A . , 1 9 7 0 , P a l e o e c o l o g i c a l and s t r a t i g r a p h i c Geol. SOC. s i g n i f i c a n c e of t h e muskrat i n P l e i s t o c e n e d e p o s i t s : A m e r . B u l l . , v . 81, p . 3733-3738. Osborn, H.F., 1942, P r o b o s c i d e a , p . 805-1675. Pgw;,
Vol.
11:
N.Y.,
T . L . , 1975a, Q u a t e r n a r y geology of A l a s k a : P r o f . Paper 835, p . 1-145.
A m e r i c a n Museum P r e s s ,
U.S. G e o l .
Survey
, 197513, Q u a t e r n a r y s t r a t i g r a p h i c n o m e n c l a t u r e i n u n g l a c i a t e d U.S. Geol. Survey P r o f . Paper 862, p. 1-32. c e n t r a l Alaska:
309
Repenning, C.A., 1 9 8 0 , F a u n a l e x c h a n g e s between S i b e r i a and N o r t h America: C a n a d i a n J o u r . A n t h r o p o l o g y , v . 1, n o . 1, p . 3 7 - 4 4 . Roe,
a C r i t i c a l Study of t h e 1 9 7 0 , The N o r t h A m e r i c a n B u f f a l o : i n i t s Wild S t a t e , Second E d i t i o n : University of Toronto P r e s s , 991 ?. F.G.,
S pecies
Romer, A . S . , 1 9 5 1 , Bison erassfcornis i n t h e l a t e P l e i s t o c e n e o f N e w England: J o u r n a l o f Mammalogy, v . 3 2 , p . 2 3 0 - 2 3 1 . Savage, D.E., 1 9 5 1 , L a t e C e n o z o i c v e r t e b r a t e s o f t h e S a n F r a n c i s c o Bay region: U n i v e r s i t y of C a l i f o r n i a P u b l i c a t i o n s , B u l l e t i n Department of G e o l o g i c a l S c i e n c e s , v . 28, no. 1 0 , p . 215-314. 1 9 7 7 , T h e a n t i q u i t y o f B i s o n latifrons S c h u l t z , C . B . and H i l l e r u d , J . M . , ( H a r l a n ) i n t h e Great P l a i n s of N o r t h America: Trans. Nebraska Academy o f i e n c e s , v . 4, p . 103-116. Sher, A.V., 1971, M a m m a l s and S t r a t i g r a p h y of t h e P l e i s t o c e n e o f t h e Extreme N o r t h e a s t of t h e U.S.S.R. and PJorth America: Yoscow, I n t e r n a t i o n a l Geology Nauka. (English translation published i n the Review, v . 1 6 , p . 1 - 2 8 4 , 1 9 7 4 ) . S t a l k e r , A . MacS. a n d C h u r c h e r , C . S . , 1 9 7 0 , D e p o s i t s n e a r M e d i c i n e H a t , A l b e r t a , Canada: C h a r t p u b l i s h e d by t h e S u r v e y s and Mapping Branch, Department of Energy, Mines and R e s o u r c e s , O t t a w a . S t o c k , E . a n d F u r l o n g , E . L . , 1 9 2 8 , The P l e i s t o c e n e e l e p h a n t s o f Rosa I s l a n d , C a l i f o r n i a : S c i e n c e , (New S e r i e s ) , v . 5 8 , p .
Santa 140-141.
W i l s o n , M . , 1 9 7 5 , H o l o c e n e f o s s i l b i s o n f r o m Wyoming a n d a d j a c e n t a r e a s (M.A. T h e s i s ) : L a r a m i e , U n i v e r s i t y o f Wyoming, 276 p .
--
, 1980, Morphological d a t i n g of l a t e Quaternary bison on t h e northern plains: s a n a d i a n J o u r . A n t h r o p o l o g y , v . 1 , n o . 1, p . 81-85. T a b l e 1.
Specimen Banff Archives site, Alberta a
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1. S p r e a d o f h o r n c o r e s ( t i p t o t i p ) , 2 . Greatest spread of horncores (on o u t s i d e c u r v e ) , 3. Horncore l e n g t h on upper c u r v e ( t i p t o b u r r ) , 4. Horncore l e n g t h on lower c u r v e ( t i p t o b u r r ) , 5. V e r t i c a l d i a m e t e r of horncore (at r i g h t a n g l e t o l o n g i t u d i n a l a x i s ) , 6 . T r a n s v e r s e d i ameter of h o r n c o r e ( a t r i g h t a n g l e t o l o n g i t u d i n a l a x i s ) , 7 . H o r n c o r e circumference ( a t r i g h t angle t o longitudinal a x i s ) , 8. Cranial width (between u p p e r c e n t r e s of h o r n c o r e b u r r s ) , 9 . C r a n i a l w i d t h ( a t cons t r i c t i o n between h o r n c o r e s and o r b i t s ) .
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FIELD USE OF MACROFEATURES FOR CORRELATING TILLS AND ESTIMATING THEIR AGES: A REVIEW
A. Ma&. STALKER
ABSTRACT
Many g e o l o g i c a l s e c t i o n s i n a r e a s o f m u l t i p l e g l a c i a t i o n l a c k t h e o r g a n i c m a t e r i a l s n e c e s s a r y f o r a b s o l u t e and r e l a t i v e d a t i n g o f t h e i r t i l l s , and f o r c o r r e l a t i o n of t h e t i l l s t o d e p o s i t s o f known a g e e l s e - . where. However, a t i l l s h e e t i n t h e f i e l d n o r m a l l y e x h i b i t s l a r g e s c a l e f e a t u r e s , beyond t h o s e d e p e n d a n t upon c o m p o s i t i o n , t h a t s u f f i c e t o d i s t i n g u i s h i t from most o t h e r t i l l s h e e t s i n a n a r e a , a l l o w i t s c o r r e l a t i o n w i t h d e p o s i t s of known a g e e l s e w h e r e , and may e v e n , a t times, i n d i c a t e i t s approximate age. I n g e n e r a l , t h e v a l u e of t h e s e f e a t u r e s f o r such p u r p o s e s has been u n d e r e s t i m a t e d o r even d i s r e g a r d e d . Examples u s e d h e r e a r e drawn from t h e w r i t e r ' s e x p e r i e n c e w i t h t h e L a u r e n t i d e d e p o s i t s of t h e s o u t h w e s t C a n a d i a n P r a i r i e s , b u t o n l y a few o f t h e many t y p e s of f e a t u r e s a v a i l a b l e a r e d e s c r i b e d . These m a c r o f e a t u r e s d e r i v e from e n v i r o n m e n t a l c o n d i t i o n s p r e v a i l i n g d u r i n g d e p o s i t i o n and d u r i n g t h e p o s t - d e p o s i t i o n a l h i s t o r y o f t h e t i l l , and t h e y r e m a i n f a i r l y c o n s t a n t o v e r much of s o u t h A l b e r t a . Those a r i s i n g p a r t l y o r c h i e f l y from p o s t - d e p o s i t i o n a l f a c t o r s i n c l u d e o x i d i z a t i o n and w e a t h e r i n g , c o m p a c t i o n , j o i n t i n g , and s t y l e o f b r e a k a g e . O x i d i z a t i o n and w e a t h e r i n g h a v e a l o n g r e c o r d o f u s e f o r e s t i m a t i n g a g e s and f o r c o r r e l a t i o n , and s o a r e n o t d i s c u s s e d h e r e . Compaction i s a v a l u a b l e t o o l , for i t d e p e n d s m a i n l y on t h e number and t h i c k n e s s e s o f l a t e r g l a c i e r s t h a t compressed t h e t i l l ; i t becomes w o r t h l e s s , however, where o l d e r t i l l s s u r f a c e beyond t h e l i m i t s o f y o u n g e r g l a c i e r s and s o a r e l i t t l e i n d u r a t e d . J o i n t i n g and s t y l e o f b r e a k a g e n o r m a l l y a r e s u f f i c i e n t l y d i s t i n c t i v e f o r each t i l l t o permit i t s c o r r e l a t i o n t o d e p o s i t s of known a g e e l s e w h e r e . The p r o m i n e n t columnar s t r u c t u r e s d i s p l a y e d by many o f t h e s o u t h e r n A l b e r t a t i l l s a r e o f combined d e p o s i t i o n a l and p o s t - d e p o s i t i o n a l o r i g i n . They a r e of g r e a t v a l u e f o r r e c o g n i t i o n and c o r r e l a t i o n o f a l l b u t s u r f a c e t i l l s . On t h e o t h e r h a n d , t i l l c o l o u r s f o u n d i n s o u t h A l b e r t a a r e d e t e r m i n e d by b o t h c o m p o s i t i o n and p o s t - d e p o s i t i o n a l h i s t o r y . O t h e r t h i n g s b e i n g e q u a l , t h e d a r k e r a t i l l t h e g r e a t e r i t s a g e . The I l l i n o i a n t i l l s i n t h a t r e g i o n a r e v e r y d a r k brown o r g r e y , delsending u p o n w h e t h e r t h e y were l a i d down by s o u t h e a s t o f s o u t h w e s t f l o w i n g i c e ; t h e P r e c l a s s i c a l W i s c o n s i n t i l l s a r e m o d e r a t e l y d a r k brown or g r e y , w h e r e a s t h e t y p i c a l C l a s s i c a l W i s c o n s i n t i l l s a r e b u f f or l i g h t y e l l o w i s h brown. The o l d e s t I l l i n o i a n t i l l i s n e a r l y b l a c k , and i t m a i n t a i n s t h i s sombre c o l o u r , where n o t w e a t h e r e d , n o t o n l y o v e r a b r o a d s p e c t r u m o f b e d r o c k t y p e s b u t a l s o a t d e p t h i n s e c t i o n , and a t t h e s u r f a c e i n t h o s e p l a c e s where i t r e p r e s e n t s t h e s o l e g l a c i a t i o n . The o t h e r t i l l s s i m i l a r l y r e t a i n t h e i r c h a r a c t e r i s t i c colours over broad a r e a s . This allows these colour values t o be used f o r e s t i m a t i n g till ages. The e f f e c t s of e n v i r o n m e n t d u r i n g d e p o s i t i o n a r e d e m o n s t r a t e d by t h e two " c o n t o r t e d " t i l l s o f s o u t h e r n A l b e r t a . The c o n t o r t i o n i n c l u d e s s t r o n g m i x i n g w i t h u n d e r l y i n g m a t e r i a l , a l o n g w i t h d i a p i r s and f l a m e s t r u c t u r e s from t h e t i l l and u n d e r l y i n g f o r m a t i o n s h o o t i n g i n t o and
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p i e r c i n g e a c h o t h e r , a n d i n p l a c e s c o m p l e t e l y e n v e l o p i n g s e g m e n t s of t h e o t h e r . The i n t e r f i n g e r i n g and m i x i n g may b e s o i n t e n s e t h a t no a c c u r a t e b o u n d a r y c a n b e drawn b e t w e e n t h e two u n i t s . The c o n t o r t i o n , which i s e s p e c i a l l y s t r o n g i n t h e y o u n g e r , d i s t i n g u i s h e s t h e s e two t i l l s from a l l o t h e r s , and s o e n a b l e s t h e i r c o r r e l a t i o n from s e c t i o n t o s e c t i o n and t o d e p o s i t s o f known a g e e l s e w h e r e . A l t o g e t h e r , t h e u s e of t h e f u l l s u i t e o f t h e m a c r o f e a t u r e s found i n any a r e a c a n g i v e a f a i r l y good i n d i c a t i o n o f t h e r e l a t i v e a g e s and p o s i t i o n s i n t h e s t r a t i g r a p h i c s e c t i o n o f t h e t i l l s t h e r e , and of t h e i r r e l a t i o n s h i F t o o t h e r t i l l s found e l s e w h e r e . I NTR O D U CT ION
I n t h e s e d a y s o f more r e f i n e d methods f o r a b s o l u t e d a t i n g o f Q u a t e r n a r y d e p o s i t s , and o f more s o p h i s t i c a t e d means o f t i l l s t u d y , i t becomes d i f f i c u l t t o w r i t e a b o u t some o f t h e r a t h e r mundane, b u t more t r a d i t i o n a l , f i e l d methods. They r e m a i n i m p o r t a n t , however, and s t i l l form t h e b a s i s f o r much o f t h e g e o l o g i c a l work on t h e Q u a t e r n a r y . Und o u b t e d l y many of them a r e u s e d r o u t i n e l y and w i t h o u t much t h o u g h t a b o u t t h e i r n a t u r e . O t h e r s , however, h a v e b e e n l a r g e l y n e g l e c t e d i n r e c e n t y e a r s , and i t i s to t h e s e m e t h o d s , b a s e d on f e a t u r e s t h a t a r e i n d e p e n d e n t of c omposition, t h a t a t t e n t i o n i s h e r e d i r e c t e d . Many o f t h e o u t s t a n d i n g f i e l d c h a r a c t e r i s t i c s o f a t i l l do depend c h i e f l y on c o m p o s i t i o n . O t h e r s , however, a r e d e t e r m i n e d to a f a r g r e a t e r e x t e n t by c o n d i t i o n s t h a t p r e v a i l e d d u r i n g t h e i r d e p o s i t i o n or s u b s e q u e n t h i s t o r y . T h i s d i s c u s s i o n i s b a s e d on t h e p r e m i s e t h a t t h e s e c o n d i t i o n s , i n most c a s e s , were s u f f i c i e n t l y d i f f e r e n t f o r e a c h t i l l to endow i t w i t h s p e c i f i c , i n h e r e n t f e a t u r e s t h a t d i s t i n g u i s h i t s u f f i c i e n t l y from o t h e r r e g i o n a l t i l l s to a l l o w i t s r e a d y i d e n t i f i c a t i o n i n t h e f i e l d , i t s c o r r e l a t i o n t o t i l l s e l s e w h e r e , and o f t e n t o e n a b l e a n e s t i m a t e o f i t s a g e . They f u r t h e r o f f e r a means f o r t e s t i n g t h e r e a s o n a b l e n e s s o f a g e s o b t a i n e d by o t h e r methods, i n c l u d i n g a b s o l u t e d a t i n g . T h e r e a r e numerous t y p e s o f t h e s e l a r g e - s c a l e f e a t u r e s w h i c h , f o r c o n v e n i e n c e i n t h i s p a p e r , a r e h e r e a f t e r c a l l e d m a c r o f e a t u r e s . Only a few o f them - more p r e c i s e l y c o m p a c t i o n , f r a c t u r e p a t t e r n , c o l o u r , def o r m a t i o n , columnar s t r u c t u r e - a r e r e v i e w e d h e r e . Some o f t h e o t h e r s , s u c h a s w e a t h e r i n g and s o i l f o r m a t i o n , have l o n g b e e n u s e d t o e s t a b l i s h r e l a t i v e a g e s o f t i l l s h e e t s and as m a r k e r h o r i z o n s , and f o r e s t i m a t i n g a b s o l u t e a g e s , e v e n t h o u g h t h e i r u s e h a s a l s o d e c r e a s e d of l a t e . Most, however, h a v e r e c e i v e d l i t t l e a t t e n t i o n i n r e c e n t y e a r s , and t h e i r e f f i c a c y f o r c o r r e l a t i o n and d e t e r m i n a t i o n o f a g e h a s commonly b e e n They o b v i o u s l y a r e of g r e a t e s t v a l u e u n d e r v a l u e d or e v e n d i s r e g a r d e d . i n t h e s t u d y o f c o m p l i c a t e d , m u l t i - t i l l s e c t i o n s t h a t , as i s o n l y t o o w e l l known, t y p i c a l l y l a c k t h e f o s s i l s or o t h e r o r g a n i c r e m a i n s n e c e s s a r y f o r c o r r e l a t i o n and r a d i o c a r b o n d a t i n g . D i f f e r e n t s u i t e s o f m a c r o f e a t u r e s undoubtedly can be i d e n t i f i e d a n d u s e d i n v a r i o u s r e g i o n s . However, t h e examples g i v e n h e r e a r e drawn from t h e w r i t e r ' s e x p e r i e n c e w i t h t h e L a u r e n t i d e d e p o s i t s of t h e s o u t h w e s t p o r t i o n o f t h e G r e a t P l a i n s o f Canada. T h a t g e n e r a l a r e a , and p l a c e s m e n t i o n e d and s e c t i o n s d e s c r i b e d , a r e shown i n F i g u r e 1. For a number o f r e a s o n s , t h a t r e g i o n i s Drobably as c l o s e as one can come t o an i d e a l place t o study macrofeatures. F i r s t , t h e southwestern p l a i n s have s e e n a l o n g s e r i e s o f g l a c i e r s , most of which a r e r e p r e s e n t e d b y a t l e a s t l o c a l r e m n a n t s o f t i l l . Second, t h e a p p r o x i m a t e t h i c k n e s s e s , e x t e n t s , and l i m i t s o f many o f t h e s e g l a c i e r s a r e known ( S t a l k e r and H a r r i s o n , 1977). T h i r d , t h e s u c c e s s i v e L a u r e n t i d e g l a c i e r s t h a t i n vaded t h e r e g i o n were o f g e n e r a l l y d e c r e a s i n g s t r e n g t h . T h i s i s most i m p o r t a n t , for i t a l l o w s e x a m i n a t i o n o f t h e t i l l s b o t h a t t h e s u r f a c e beyond t h e l i m i t s o f l a t e r g l a c i e r s and a l s o a t d e p t h i n s e c t i o n . T h i s s u c c e s s i v e l y d e c r e a s i n g s t r e n g t h o f t h e g l a c i e r s was a l s o p o i n t e d o u t by S t a l k e r and H a r r i s o n (1977) i n t h e i r s t u d y o f t h e g l a c i a l h i s t o r y o f t h e southwest Great P l a i n s . In t h a t study they reported t h a t t h e f i r s t g l a c i e r known t o h a v e i n v a d e d t h e r e g i o n , t h e one t h a t l a i d down t h e Labuma T i l l , was by f a r t h e t h i c k e s t and e x t e n d e d w e l l beyond t h e l i m i t s of l a t e r o n e s . The r e l a t i v e s i z e s o f t h e f o l l o w i n p two g l a c i e r s , which
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l a i d down t h e Y a u n s e l l a n d t h e B r o c k e t t i l l s i n t h a t o r d e r , a r e unc e r t a i n , b u t b o t h w e r e much weaker t h a n t h e Labuma g l a c i e r . Af’ter t h e B r o c k e t a d v a n c e e a c h s u c c e s s i v e g l a c i e r was s m a l l e r t h a n t h e p r e c e e d i n g one.
U s i n g t h e r n a c r o f e a t u r e s , i n c l u d i n g some o f t h o s e d e s c r i b e d i n t h i s p a p e r , S t a l k e r a n d H a r r i s o n c o r r e l a t e d t h e i r L a u r e n t i d e t i l l s w i t h dep o s i t s o f b e t t e r known a g e f o u n d f a r t h e r e a s t i n t h e p r o v i n c e , a n d e s p e c i a l l y t h o s e exposed i n t h e r i v e r s e c t i o n s found a l o n g t h e South Saskatchewan R i v e r n e a r Medicine Eat and L e t h b r i d g e ( F i g u r e 1 ) . These c o r r e l a t i o n s e n a b l e d them t o p r o p o s e a g e s r a n g i n g from e a r l y T l l i n o i a n t o l a t e W i s c o n s i n for t h e i r t i l l s . The m a c r o f e a t u r e s a l s o e n a b l e d S t a l k e r ( i n p r e s s ) t o c o r r e l a t e c e r t a i n t i l l s o f t h e Cameron Ranch S e c t i o n ( F i g u r e 1, S e c t i o n 16) w i t h t h e M e d i c i n e Hat s e c t i o n s , a n d t h e r e b y t o d i v i d e t h e L a u r e n t i d e t i l l s o f s o u t h e r n and c e n t r a l Alberta i n t o t h r e e g r o u p s , w h i c h he d e s c r i b e d a s I l l i n o i a n , F r e c l a s s i c a l Wisc o n s i n , and C l a s s i c a l Wisconsin i n a g e .
GSC
Figure 1
I n d e x m a p of t h e s o u t h w e s t e r n P r a i r i e s of C a n a d a . Numbers refer t o described sections mentioned in t e x t . N u m b e r s 1 t o 1 2 w e r e d e s c r i b e d by S t a l k e r , 1 9 6 3 ; 1 3 to 1 5 by S t a l k e r , 1 9 6 9 ; a n d n u m b e r 1 6 ( C a m e r o n R a n c h S e c t i o n ) by S t a l k e r , i n p r e s s .
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The a g e s o f t h e d e p o s i t s n e a r M e d i c i n e Hat, t o w h i c h b o t h S t a l k e r and H a r r i s o n (1977) and S t a l k e r ( i n p r e s s ) c o r r e l a t e d , a r e b a s e d p r i m a r i l y on v e r t e b r a t e p a l e o n t o l o g y ( H a r i n g t o n , 1 9 7 8 ; S t a l k e r , 1 9 7 6 ; S t a l k e r and Churcher, 1970, 1 9 7 2 ) . T h i s i s e s p e c i a l l y t h e c a s e f o r t h o s e beyond t h e r a n g e of r a d i o c a r b o n d a t i n g . T h i s use of v e r t e b r a t e chronology c a u s e s d i f f i c u l t y i n r e l a t i n g t h e s t r a t i g r a D h y of t h e s o u t h west Great P l a i n s t o s t r a t i g r a p h i e s p r o p o s e d f o r o t h e r p a r t s o f t h e c o n t i n e n t f o r , a s S t a l k e r a n d H a r r i s o n ( 1 9 7 7 , p . 881-8811) p o i n t e d o u t : "Even t h o u g h t h e t e r m i n o l o g y e m p l o y e d i s t h e same, a g r e a t d e a l o f t h e v e r t e b r a t e c h r o n o l o g y d e v e l o p e d for N o r t h A m e r i c a may b e o u t of phase w i t h t h e g l a c i a l chronology of t h e mid-continent. In p a r t i c u l a r , t h e pre-Wisconsin p o r t i o n of t h e v e r t e b r a t e chronology may e m b r a c e more t i m e t h a n t h e c o r r e s p o n d i n g s e c t i o n of t h e g l a c i a l c h r o n o l o g y . For t h i s r e a s o n . . . t h e terms o f t h e m i d - c o n t i n e n t c h r o n o l o g y a r e u s e d i n o n l y t h e m o s t g e n e r a l s e n s e to i n d i c a t e approximate ages of e v e n t s . This a p p l i e s p a r t i c u l a r l y t o e v e n t s p r e c e d i n g t h e C l a s s i c a l Wisconsin, o r r o u g h l y b e f o r e 30,000 years a g o , for w h i c h r a d i o c a r b o n d a t i n g i s u n a v a i l a b l e . I ' S t a g e a n d s t a d i a 1 names a r e u s e d h e r e i n t h e m a n n e r t h a t t h e y were employed by S t a l k e r and H a r r i s o n ( 1 9 7 7 ) . A s a r e s u l t , a l t h o u g h t h e t e r m s may b e s i m i l a r t o t h o s e i n t h e s t a n d a r d , m i d - N o r t h A m e r i c a n c h r o n l o g y , t h e y a r e n o t n e c e s s a r i l y i n a l l c a s e s f u l l y t i m e e q u i v a l e n t . They a r e used here r e l u c t a n t l y and s o l e l y f o r want of a b e t t e r a l t e r n a t i v e . T h e i r u s e , however, does g i v e a n i n d i c a t i o n o f t h e a g e s of t h e v a r i o u s units. Happily, s i n c e S t a l k e r and H a r r i s o n ' s 1977 r e p o r t , a growing r e a l i z a t i o n of t h e g r e a t e r l e n g t h o f t h e g l a c i e r r e c o r d h a s b r o u g h t t h e two c h r o n o l o g i e s c l o s e r t o a g r e e m e n t . PAST WORK D e s c r i p t i o n s of t h e L a u r e n t i d e t i l l s found i n t h e southwestern C a n a d i a n P r a i r i e s h a v e shown r e m a r k a b l e c o n s i s t e n c y o v e r t h e y e a r s , e v e n t h o u g h e s t i m a t e s o f t h e number a n d a g e s o f t i l l s h e e t s p r e s e n t have v a r i e d c o n s i d e r a b l y . The f i r s t d e s c r i p t i o n s a r e f o u n d i n Dawson ( 1 8 8 5 , p . 1 4 3 ~ )a n d i n s c a t t e r e d r e f e r e n c e s t h r o u g h o u t Dawson a n d M c C o n n e l l ( 1 8 9 5 ) . The 1 8 8 5 d e s c r i p t i o n e v i d e n t l y e n c o m p a s s e d a l l t h e t i l l s o f t h e L a u r e n t i d e s e q u e n c e , for i t s t a t e d : " c o l o u r v a r i e s cons i d e r a b l y , r a n g i n g f r o m d a r k b l a z k i s h or b l u i s h - g r e y t o l i g h t e r t i n t s o f t h e same, a n d o f t e n b e c o m i n g g r e y or f a w n - c o l o u r e d , e s p e c i a l l y where w e a t h e r e d . " T h e s e c o r r e s p o n d t o t h e c o l o u r s l a t e r d e s c r i b e d b y S t a l k e r ( 1 9 6 0 ) f o r t h e Labuma, M a u n s e l l , a n d B u f f a l o Lake t i l l s r e spectively. Next Horberg (1952) d i v i d e d t h e t i l l sequence n e a r Lethb r i d g e i n t o t h r e e u n i t s , which he d e s c r i b e d i n d e t a i l . He s u g g e s t e d a l l were o f W i s c o n s i n Age. L a t e r S t a l k e r ( 1 9 6 0 ) , w h i l e w o r k i n g n e a r Red Deer, A l b e r t a , s i m i l a r l y s e p a r a t e d t h r e e t i l l u n i t s w h i c h h e named, f r o m o l d e r to y o u n g e r r e s p e c t i v e l y , Labuma, M a u n s e l l , a n d B u f f a l o L a k e . These t i l l s c o r r e s p o n d r o u g h l y t o t h e t h r e e u n i t s of Horberg, b u t S t a l k e r s u g g e s t e d o l d e r a g e s t h a n H o r b e r g for t h e two l o w e r t i l l s . S t a l k e r (1963) next described twelve important Quaternary sections ( F i g u r e 1, Nos. 1 - 1 2 ) , m o s t l y a l o n g t h e S o u t h S a s k a t c h e w a n d r a i n a g e s y s t e m , a n d named t h e B r o c k e t T i l l , ( t h e L a u r e n t i d e t i l l d i r e c t l y o v e r lying the Maunsell). I n 1969 he d e s c r i b e d a f u r t h e r t h r e e s e c t i o n s n e a r M e d i c i n e Hat ( F i g u r e 1, N o s . 1 3 - 1 5 ) . These 1 9 6 0 , 1 9 6 3 a n d 1 9 6 9 p a p e r s , a l o n g w i t h a d e s c r i p t i o n of t h e i r n p o r t a n t Cameron Ranch S e c t i o n ( F i g u r e 1, No. 1 6 ) on Oldman R i v e r n o r t h e a s t o f L e t h b r i d g e ( S t a l k e r , i n p r e s s ) , p r o v i d e t h e b a s i s for t h e p r e s e n t s t u d y . I n t h e s e p a p e r s S t a l k e r s u g g e s t s t i l l a g e s r a n g i n g from e a r l y I l l i n o i a n t o l a t e Wisc o n s i n . I n t h e meantime Horberg ( 1 9 5 4 ) and A l l e y ( 1 9 7 3 ) had d i s c u s s e d t h e p a s t i n t e r r e l a t i o n s h i p s of C o r d i l l e r a r . and L a u r e n t i d e g l a c i e r s . The work o n t h e s e i n t e r r e l a t i o n s h i p s . w h i c h i s b a s e d c h i e f l y or: i n t e r f i n g e r i n g o f t h e two s e t s o f t i l l s h e e t s , was c o n t i n u e d b y S t a l k e r (1972, 1 9 7 6 ) and S t a l k e r and H a r r i s o n ( 1 9 7 7 ) .
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THE FEATURES
General statement
Origin
T h i s paper emphasizes use of t h e macrofeatures, r a t h e r t h a n t h e i r o r i g i n . F o r c o n v e n i e n c e , however, t h e f e a t u r e s a r e d i s c u s s e d a c c o r d i n g t o w h e t h e r t h e y formed m a i n l y d u r i n g d e p o s i t i o n o f t h e t i l l , p o s t 7 d e p o s i t i o n a l l y , o r b o t h combined, a n d s h o r t s k e t c h e s o f t h e p r o b a b l e means o f d e v e l o p m e n t a r e g i v e n . More i m p o r t a n t l y , however, t h e f e a t u r e s must b e shown t o b e s u b s t a n t i a l l y i n d e p e n d e n t o f c o m p o s i t i o n , f o r o t h e r w i s e t h e y c o u l d a r i s e m e r e l y from v a r i a t i o n s i n t h e u n d e r l y i n g b e d r o c k and i n l o c a l d i r e c t i o n o f ice-movement, and r o u g h l y s i m i l a r c h a r a c t e r i s t i c s would l i k e l y b e r e p e a t e d i n s e v e r a l o f t h e t i l l s h e e t s . T h i s would h i n d e r t h e d i f f e r e n t i a t i o n o f t h e t i l l s , and s o l e s s e n t h e v a l u e o f t h e m a c r o f e a t u r e s f o r c o r r e l a t i o n and d e t e r m i n a t i o n o f r e l a t i v e a g e s . The i n f l u e n c e o f c o m p o s i t i o n i s s t u d i e d f i r s t .
Composition I n h i s e a r l y s t u d i e s o f t h e t i l l s i n t h e Red Deer - S t e t t l e r r e g i o n o f A l b e r t a , S t a l k e r ( 1 9 6 0 ) c o n c l u d e d t h a t t h e d i s s i m i l a r i t i e s between t h e Labuma, M a u n s e l l , and B u f f a l o Lake t i l l s o f L a u r e n t i d e p r o v e n a n c e were n o t , i n t h e m a i n , d u e t o d i f f e r e n c e s i n c o m p o s i t i o n . O n p a g e 2 2 he p o i n t e d o u t : "Any d i f f e r e n c e i n t h e m a t e r i a l composing t h e t h r e e t i l l s , or i n i t s r e l a t i v e g r a i n s i z e , i s g e n e r a l l y s m a l l e r t h a n l a t e r a l v a r i a t i o n s w i t h i n e a c h t i l l . " On page 23 h e Y u r t h e r s t a t e d d e s p i t e t h e s e l a t e r a l v a r i a t i o n s : "The t h r e e t i l l s h a v e b e e n d e s c r i b e d e l s e where and a p p a r e n t l y r e t a i n t h e same c h a r a c t e r i s t i c s e v e r y w h e r e i n t h e p r o v i n c e [ A l b e r t a ] . " He was r e f e r r i n g t o f e a t u r e s s u c h as c o l o u r , p e r m e a b i l i t y , c o m p a c t i o n , m a s s i v e n e s s , h a r d n e s s , and r e s i s t a n c e t o water erosion, but the statement applies equally w e l l t o t h e other f e a t u r e s d e s c r i b e d h e r e . H e t h e n l i s t e d p l a c e s where t h o s e t i l l s h a d b e e n r e c o r d e d w h i l e r e t a i n i n g t h e same t r a i t s , commonly o v e r v a s t l y cont r a s t i n g t y p e s o f b e d r o c k . The number o f s u c h r e c o r d s h a s i n c r e a s e d markedly d u r i n g t h e e n s u i n g twenty y e a r s ( e . g . , A l l e y , 1973; S t a l k e r , 1 9 6 3 , 1 9 6 9 , 1 9 7 2 , 1 9 7 6 , i n p r e s s ) . The r e t e n t i o n o f u n i q u e t r a i t s by t h e s e and o t h e r t i l l s h e e t s o v e r v a r i o u s b e d r o c k f o r m a t i o n s , c o n s i s t i n g o f m a t e r i a l s as d i v e r s e as d a r k marine s h a l e , b u f f s i l t s t o n e , sandstones o f v a r i o u s s h a d e s , and a l s o o v e r e x p a n s e s o f c o a r s e g r a v e l , c e r t a i n l y i n d i c a t e s a c e r t a i n independence of t h e f e a t u r e s f r o m composition.
The g e n e r a l i n d e p e n d e n c e of t h e s e f e a t u r e s from c o m p o s i t i o n a l c o n t r o l i s f u r t h e r d e m o n s t r a t e d i n v e r t i c a l s e c t i o n . The o u t c r o p s found a l o n g S o u t h S a s k a t c h e w a n R i v e r i n s o u t h e r n A l b e r t a r e v e a l two b a s i c , o v e r a l l d i r e c t i o n s o f L a u r e n t i d e i c e f l o w , one t o t h e s o u t h w e s t and t h e o t h e r t o t h e s o u t h e a s t . Each d i r e c t i o n i s r e p r e s e n t e d b y s e v e r a l t i l l s h e e t s , a n d e v e n a s i n g l e e x p o s u r e may show two or more t i l l s l a i d down b y g l a c i e r s t h a t a d v a n c e d i n t h e same d i r e c t i o n a n d o v e r t h e same bedr o c k f o r m a t i o n s . A l t h o u g h t h i s would c a u s e one t o e x p e c t r a t h e r similar compositions f o r t h e s e t i l l s , t h e t r a i t s they display a r e , nonetheless, v e r y d i v e r s e . T h i s Dhenomenon i s p a r t i c u l a r l y w e l l shown a t t h e Cameron Ranch S e c t i o n ( F i g u r e , No. 1 6 ) ( S t a l k e r , i n p r e s s ) , b u t i s a l s o e v i d e n t a t numerous o t h e r s i t e s (Dawson, 1 8 8 5 ; H o r b e r g , 1 9 5 2 ; S t a l k e r , 1 9 6 3 , 1969,. 1 9 7 7 ) . I n t h e s e c a s e s i t is d i f f i c u l t t o a s c r i b e t h e d i s t i n c t i v e c h a r a c t e r i s t i c s of t h e s e t i l l s t o compositional controls. Composition d o e s , i n d e e d , l i m i t t h e a b i l i t y o f a t i l l t o a c q u i r e many o f t h e f e a t u r e s r e v i e w e d i n t h i s p a p e r . E x t r e m e l y b o u l d e r y t i l l , f o r e x a m p l e , c o u l d n e v e r d e v e l o p t h e d i s t i n c t i v e columnar s t r u c t u r e s or b r e a k a g e p a t t e r n s f o u n d i n many o f t h e t i l l s . However, b o t h t h e ret e n t i o n b y a t i l l o f i t s d i s t i n c t i v e t r a i t s o v e r m a r k e d l y d i v e r s e bedr o c k f o r m a t i o n s and t h e p r e s e n c e i n s e c t i o n o f t i l l s w i t h v a s t l y d i s s i m i l a r f e a t u r e s , l a i d down b y g l a c i e r s t r a v e l l i n g i n much t h e same d i r e c t i o n o v e r s i m i l a r b e d r o c k f o r m a t i o n s , i n d i c a t e a marked i n d e p e n d e n c e o f t h e s e f e a t u r e s from c o m p o s i t i o n a l c o n t r o l . C e r t a i n l y w i t h i n c o m p o s i t i o n a l l i m i t s t h a t a r e n o t e x c e e d e d by t h e L a u r e n t i d e t i l l s o f t h e western P r a i r i e s , t h e macrofeatures described here are l a r g e l y i n dependent of c o m p o s i t i o n .
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Other origins ?hose macrofeatures that are not primarily controlled by compositior. can only develop during deposition of the tills and during their post-depositional history:, hence the threefold division used here of depositional features, combined depositional and later origin, and postdepositional features. The post-depositional features, which are illustrated by compaction, fracture pattern, and colour, are described first, followed by the depositional features, represented by the deformation shown in two of the ti21 sheets. Lastly those of combiried origin, exeaplified by the spectacular columnar structure found in most of the tills, are described. Fast-Depositional Features
Compaction The weight of overriding glaciers appears to be the chief instrument in compacting a till. Other processes are so insignificant they can be ignored for o u r purposes. ?or instance, subsequent burial by material such as lake or river deposits could, theoretically, cause compaction, but these deposits would seldom reach thicknesses great enough to have much effect. For certain other processes, such as cementation, time available was too short to produce much induration of o u r Quaternary tills. However, although the weight of overlying ice is the main cause of compaction, a glacier is unlikely to have much impact on its own till. This is demonstrated by the minor compaction displayed by those surface tills that were subjected solely to the weight of their own glaciers. Certainly an ice-sheet could not compress the ablation till it laid down f r o m its surface during downmelting. As for its basal till, much of that is laid down as a glacier is retreating and thinning, and thereby losing its ability to compress underlying material. Further, while the ice-sheet still persisted locally its freshly deposited drift would tend to be saturated and so less compressible, particularly if escape routes for excess water were blocked. It appears, therefore, that subsequent glaciers are needed to compact a till significantly, and undoubtedly the magnitude of that compaction depends chiefly upon the thickness of those glaciers, and especially that of the thickest one, if other factors, such as height of water-table, are relatively constant. This suggests the possibility of using their relative compactness for recognizing the various tills in the field. The magnificent river sections of south Alberta display very clearly these differences in compaction. The lowest tills are dense, hard, and resistant to erosion, whereas the top till rarely shows much compaction. Its general weakness is demonstrated on river banks by a marked retreat of the cliff face across it and by the ease with which it becomes overgrown. The intermediate tills are likewise intermediate in compaction and in steepness of outcrop face. The differences in compaction are well developed in the twelve sections described by Stalker (1963). In those twelve sections (Figure 1, Nos. 1-12) the two basal tills are typically described as "indurated" (sections 3, 7) or "well-indurated" (sections 4, ll), "compact" (sections 1 2,3,4,7,10,11), "well-consolidated'' (section 2), or "hard" (sections 2, 10). Where not so described, these two tills generally contain sand or silt stringers and inclusions, or else have been deformed by slumping or other processes. The top till, on the other hand, rates terms such as "unconsolidated" (section l), "Poorly consolidated" (sections 2, 10), "only weakly indurated" (section 2), or "poorly indurated and does not form steep cliffs" (section 7). The inbetween tills are in the intermediate range. For instance, at the Brocket Section (section 5) the two lowest Laurentide tills, the Labuma and Maunsell, are described as "indurated and compact,"the overlying Brocket Till is described as "poorly consolidated, though more compact and better indurated than overlying till," which in turn is described as "unconsolidated". The top till, there misidentified as Buffalo Lake Till, is described as "the most poorly consolidated till in this section and it is much slumped." The situation at the comprehensive Cameron Ranch Section is similar (Stalker, in press).
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The d i f f e r e n c e s i n c o m p a c t i o n have o b v i o u s m e r i t s f o r s e p a r a t i n g t i l l s and f o r c o r r e l a t i n g them f r o m s e c t i o n t o s e c t i o n . They a l s o s u g g e s t a method of d e t e r m i n i n g r e l a t i v e a g e s , f o r a w e l l - c o m p a c t e d t i l l t e n d s t o b e o l d e r t h a n a n o t i c e a b l y l e s s compacted o n e , w h e t h e r a t t h e same or d i f f e r e n t o u t c r o p s . Even where a c o n s o l i d a t e d t i l l l i e s a t t h e s u r f a c e :t p r o b a b l y d o e s n o t r e p r e s e n t t h e l a s t g l a c i a t i o n ; more l i k e l y i t was c o m p r e s s e d by a l a t e r g l a c i e r t h a t f a i l e d t o l e a v e d r i f t t h e r e , or e l s e t h e d r i f t i t d i d l e a v e was s u b s e q u e n t l y swept a w a y . Compaction a l s o can b e t h e means o f d i s t i n g u i s h i n g t i l l s t h a t a r e i n c o n t a c t ; f o r example a p o o r l y compacted t i l l a n d a n o v e r l y i n g , b e t t e r compacted one a r e u n l i k e l y t o b e from t h e same g l a c i e r , even t h o u g h t h e y a r e s i m i l a r i n o t h e r w a y s . An e x c e p t i o n c o u l d b e where t h e l a s t g l a c i e r l e f t a b l a t i o n t i l l o v e r a somewhat b e t t e r c o n s o l i d a t e d b a s a l t i l l . Although i n t h i s c a s e n e i t h e r o f t h e s e t i l l s u b u n i t s i s l i k e l y t o be w e l l cons o l i d a t e d , t h e r e l a t i v e d i f f e r e n c e might be enough t o s u g g e s t t h e p r e s e n c e o f two d i s t i n c t t i l l s h e e t s d e r i v e d from d i f f e r e n t g l a c i e r s . However, b e c a u s e t i l l s t h a t e s c a p e d l a t e r g l a c i a t i o n t e n d t o b e unc o n s o l i d a t e d no m a t t e r w h e t h e r t h e y a r e young or o l d , t h i s c r i t e r i o n i s o f l i t t l e v a l u e beyond t h e l i m i t s o f y o u n g e r g l a c i e r s . For i n s t a n c e , i n s o u t h e r n A l b e r t a and even i n t h e w e s t where p r o g r e s s i v e l y o l d e r t i l l s s u r f a c e , everywhere t h e s u r f a c e t i l l i s about e q u a l l y w e l l c o n s o l i d a t e d . Thus t h e o l d e s t (Labuma) t i l l shows l i t t l e c o n s o l i d a t i o n beyond t h e l i m i t s of t h e s u c c e e d i n g M a u n s e l l and B r o c k e t g l a c i e r s , and t h e y i n t u r n a r e r e l a t i v e l y weak beyond t h e b o u n d a r i e s of t h e s t i l l y o u n g e r , Wisc o n s i n g l a c i e r s . T h i s h a s c a u s e d c o n f u s i o n , f o r example i n s e c t i o n s 4 , 5 , 7 , 8 of F i g u r e 1. I n t h o s e s e c t i o n s t h e s u r f a c e t i l l beyond t h e C l a s s i c a l W i s c o n s i n g l a c i a l l i m i t s o f S t a l k e r ( 1 9 7 7 , 1 9 7 8 ) and o f S t a l k e r and H a r r i s o n ( 1 9 7 7 ) was i n c o r r e c t l y i n t e r p r e t a t e d b y S t a l k e r ( 1 9 6 3 ) a s B u f f a l o Lake T i l l o f C l a s s i c a l W i s c o n s i n Age. P a r t o f t h i s c o n f u s i o n was c a u s e d by a n o v e r e s t i m a t i o n - f a i r l y g e n e r a l a t t h a t t i m e o f t h e s t r e n g t h and e x t e n t of t h e C l a s s i c a l W i s c o n s i n i c e a d v a n c e s , b u t a more i m p o r t a n t c a u s e of t h e e r r o r was t h e d e g r e e o f c o n s o l i d a t i o n o f t h i s s u r f a c e t i l l , which r e s e m b l e d t h a t f o u n d i n t h e B u f f a l o Lake T i l l farther east. S t a l k e r ( i n p r e s s ) has s i n c e r e c o g n i z e d t h a t t h e uncons o l i d a t e d n a t u r e o f t h e w e s t e r n t i l l i s more a c o n s e q u e n c e of l a c k of s u b s e q u e n t , o v e r r i d i n g g l a c i e r s t h a n o f y o u t h , and now a s s i g n s a g r e a t e r age t o i t . Fracture pattern The f r a c t u r e p a t t e r n o f a t i l l c a n b e a v a l u a b l e a i d t o i t s r e c o g n i t i o n and c o r r e l a t i o n . I n s o u t h e r n A l b e r t a a n o l d t i l l t e n d s t o b r e a k i n t o s m a l l e r a n d s h a r p e r p i e c e s t h a n a young o n e . The o l d e s t ( L a b u m a ) t i l l , f o r i n s t a n c e , b r e a k s i n t o n a r r o w c h i p s 1 t o 2 cm l o n g , t h e M a u n s e l l T i l l g i v e s r e c t a n g u l a r p i e c e s 5 t o 1 0 cm t o a s i d e , t h e B r o c k e t T i l l , as a r u l e , g i v e s s i m i l a r - s h a p e d b u t somewhat l a r g e r f r a g m e n t s , w h e r e a s t h e y o u n g e s t t i l l s g i v e l a r g e , i r r e g u l a r lumps or a r e t o o l i t t l e c o n s o l i d a t e d t o p r o d u c e any f r a c t u r e p a t t e r n w h a t s o e v e r . T h i s s e q u e n c e i n d i c a t e s t h e r e l a t i v e a g e of a t i l l a n d a i d s i n c o r r e l a t i o n . S i m i l a r s e q u e n c e s u n d o u b t e d l y e x i s t i n o t h e r r e g i o n s . However, f r a c t u r e p a t t e r n a p p e a r s c l o s e l y l i n k e d t o c o m p a c t i o n and c o n s o l i d a t i o n , a n d l i k e them i s u s e l e s s where o l d t i l l s s u r f a c e beyond t h e r a n g e o f y o u n p e r o n e s . A l s o l i k e c o m p a c t i o n and c o n s o l i d a t i o n , i t a p p e a r s t o b e p r o d u c e d during t h e p o s t - d e p o s i t i o n a l h i s t o r y of a till. B a s i c a l l y , f r a c t u r e p a t t e r n a c t s a s a n a d j u n c t t o c o n f i r m t h e r e s u l t s o b t a i n e d from t h e s t u d y o f compaction. Colour C o l o u r , and e s p e c i a l l y i t s v a l u e or l i g h t n e s s , i s a most i n t r i g u i n g p r o p e r t y o f a t i l l and c a n , i n some r e g i o n s , b e a v a l u a b l e t o o l f o r c o r r e l a t i n g t i l l s h e e t s and d e t e r m i n i n g t h e i r r e l a t i v e a g e s . I n g e n e r a l , on t h e s o u t h w e s t e r n Great P l a i n s , t h e d a r k e r a t i l l t h e g r e a t e r i t s a g e . H o r b e r g ( 1 9 5 2 ) e a r l y drew a t t e n t i o n t o t h e d i f f e r e n t c o l o u r s and i n t e n s i t y v a l u e s shown by t h e d i f f e r e n t t i l l s o f s o u t h e r n A l b e r t a when he d e s c r i b e d h i s b a s a l t i l l a s " d a r k b r o w n i s h g r a y " ( p . 3 1 0 1 , and h i s l o w e r and u p p e r t i l l s a s " d a r k g r a y " ( p . 3 1 1 , 3 1 6 ) . I n 1 9 6 0 S t a l k e r p o i n t e d o u t t h e marked c o n t r a s t s i n c o l o u r o f t h e v a r i o u s L a u r e n t i d e t i l l s i n A l b e r t a , when h e s t a t e d ( p . 2 3 ) :
318
"The Labuma t i l l i s d a r k b l u e o r b l a c k , p e r h a p s due t o a l a r g e c o n t e n t o f c a r b o n a c e o u s m a t e r i a l d e r i v e d from C r e t a c e o u s and T e r t i a r y c o a l seams a n d c a r b o n a c e o u s s h a l e , and t o l o n g b u r i a l below t h e w a t e r - t a b l e . However, i t seems to r e t a i n t h i s c o l o u r e v e n when n e a r t h e s u r f a c e and above t h e w a t e r - t a b l e . The Maunsell t i l l i s l i g h t t o d a r k b l u e , a l s o due t o i t s c o n t e n t of carbonaceous m a t e r i a l and l o n g b u r i a l u n d e r t h e w a t e r - t a b l e , t h o u g h f o r a s h o r t e r t i m e t h a n t h e Labuma t i l l . The B u f f a l o Lake t i l l i s more v a r i a b l e i n c o l o u r , w i t h v a r y i n g b r o w n s , y e l l o w s , g r e y s , and l i g h t b l u e s where u n o x i d i z e d . Upon d r y i n g t h e t h r e e t i l l s become l i g h t i n t i n t , t o various shades of grey." L a t e r on S t a l k e r ( 1 9 6 3 , p . 5 - 8 ) o u t l i n e d t h e r e l a t i v e l i g h t n e s s o r d a r k n e s s o f s h a d e of t h e v a r i o u s t i l l s h e e t s and ( i n p r e s s ) g a v e a g e n e r a l d e s c r i p t i o n o f t h e i r o v e r a l l change i n hue and v a l u e from l o w e s t t o y o u n g e s t . Although e a c h of h i s t h r e e groups o f t i l l s ( I l l i n o i a n , P r e c l a s s i c a l W i s c o n s i n , C l a s s i c a l W i s c o n s i n ) i n c l u d e s b o t h brown and g r a y t i l l s , d e p e n d i n g c h i e f l y upon w h e t h e r t h e g l a c i e r r e s p o n s i b l e f o r e a c h t i l l came from n o r t h w e s t o r n o r t h e a s t , t h e Labuma, M a u n s e l l , and Brocket t i l l s of h i s lowest, o r I l l i n o i a n , group a r e by f a r t h e d a r k e s t , w i t h t h e b a s a l (Labuma) one n e a r l y b l a c k . The P r e c l a s s i c a l W i s c o n s i n t i l l s a r e i n t e r m e d i a t e i n v a l u e , b e i n g medium d a r k brown o r b l u i s h t o g r a y i s h brown, b u t n o t i c e a b l y l i g h t e r i n c o l o u r t h a n t h e t i l l s o f t h e u n d e r l y i n g I l l i n o i a n g r o u p . The C l a s s i c a l W i s c o n s i n ( B u f f a l o L a k e ) t i l l s show a v a r i e t y o f c o l o u r s , b u t a l l a r e l i g h t e r i n v a l u e , t y p i c a l l y b e i n g b u f f , b e i g e , o r l i g h t brown. F u r t h e r , c o l o u r was one o f t h e c h a r a c t e r i s t i c s S t a l k e r ( 1 9 6 0 , p . 2 3 ) was r e f e r r i n g to when he s t a t e d t h a t t h e t i l l s " a p p a r e n t l y r e t a i n t h e same c h a r a c t e r i s t i c s e v e r y w h e r e i n t h e province. I n a q u o t a t i o n given e a r l i e r , S t a l k e r (1960, p . 2 3 ) suggested t h a t t h e c o l o u r s o f h i s t i l l s were c o n t r o l l e d b y b o t h c o m p o s i t i o n and p o s t -
d e p o s i t i o n a l i n f l u e n c e s . Whether o r n o t t h e o r i g i n s h e p o s t u l a t e s f o r t h e d i f f e r e n t c o l o u r s and v a l u e s a r e c o r r e c t i s i m m a t e r i a l h e r e ; w h a t i s important i s t h a t t h e d i f f e r e n c e s a r e p r e s e n t , they are remarkably c o n s i s t e n t , and t h e y c a n b e u s e d t o c o r r e l a t e t h e v a r i o u s t i l l s , d e t e r mine t h e i r r e l a t i v e a g e s , and e v e n , i n s o u t h A l b e r t a , o b t a i n a n e s t i m a t e of t h e i r t r u e ages. Colour has f u r t h e r v a l u e b e c a u s e i t a p p a r e n t l y i s independent of whether t h e t i l l has been o v e r r i d d e n by l a t e r g l a c i e r s , and s o i s u s e f u l r i g h t up t o t h e l i m i t s o f g l a c i a t i o n , t h u s a v o i d i n g one o f t h e c h i e f l i m i t a t i o n s o f b o t h t h e c o m p a c t i o n and f r a c t u r e p a t t e r n c r i t e r i a discussed previously. F u r t h e r , i t i s l a r g e l y independent of w h e t h e r a t i l l i s b u r i e d or l i e s a t t h e s u r f a c e . More a t t e n t i o n t o c o l o u r would h a v e a v o i d e d t h e m i s i d e n t i f i c a t i o n s o f t i l l s i n S t a l k e r ( 1 9 6 3 , s e c t i o n s 4,5,7,8) m e n t i o n e d above u n d e r c o n s o l i d a t i o n . C e r t a i n l y t h e t i l l s t h e r e m i s t a k e n f o r t h e B u f f a l o Lake T i l l of C l a s s i c a l Wisc o n s i n Age a p p r o a c h i n c o l o u r i n g and v a l u e o r l i g h t n e s s t h e P r e c l a s s i c a l Wisconsin t i l l s obser ved i n s e c t i o n s f a r t h e r e a s t ( e . g . , I s l a n d B l u f f S e c t i o n , No. 1 5 , of S t a l k e r , 1 9 6 9 ; s e e F i g u r e 1, No. 1 5 ) . Depositional Features Deformation Two o f t h e L a u r e n t i d e t i l l s h e e t s f o u n d on t h e s o u t h w e s t G r e a t P l a i n s a r e c h a r a c t e r i z e d b y a s t r o n g , d i s t i n c t i v e p a t t e r n of deformation or contortion. Strong deformation appears r e s t r i c t e d t o these two t i l l s , t h o u g h o t h e r t i l l s h e e t s may be a f f e c t e d l o c a l l y a n d t h e r e i s a l w a y s t h e p o s s i b i l i t y t h a t a d d i t i o n a l d e f o r m e d t i l l s may y e t b e d i s c o v e r e d . The d e f o r m a t i o n i n t h e s e t i l l s i s g r e a t e s t n e a r t h e b a s e and d e c r e a s e s n o t i c e a b l y upward. F u r t h e r , i t i s much b e t t e r d e v e l o p e d i n t h e y o u n g e r o f them. S t a l k e r ( 1 9 6 9 , p . 1 0 ) d e s c r i b e d t h i s deforma t i o n as f o l l o w s : " T h i s u n i t i s r e f e r r e d t o as t h e ? c o n t o r t e d t i l l ' b e c a u s e i t s s t r o n g d e f o r m a t i o n , b e s t developed towards t h e b a s e of t h e u n i t , i s a d i s t i n g u i s h i n g t r a i t p r a c t i c a l l y e v e r y w h e r e . None o f t h e o t h e r t i l l s show t h i s d e f o r m a t i o n t o n e a r l y t h e same e x t e n t . It r e v e a l s i t s e l f i n c o n v o l u t i o n s ; i n i n c l u s i o n s o f s a n d , s i l t ,
319
or bedrock completely enclosed by till, or of till completely inlcased in sand, silt, or bedrock; in stringers of till jutting into underlying deposits and lenses of those deposits injected upward into the till; and in strong internal folding and faulting. These features commonly are accentuated by abundant inclusions from the underlying material, whether bedrock or surficial deposits, that contrast vividly with the till itself. In places the deformation is so strong and inclusions so abundant that it is impossible to draw a demarcation between the till and underlying deposits." These characteristics apply in lesser degree to the other (older) contorted till. The numerous inclusions of sand and silt referred to, which at times have a high water content, weaken both these tills enough to inhibit them from forming either the steep cliff faces or the columnar structures described in the next section. The writer considers the deformation to be a phenomenon caused by conditions prevailing at time of deposition. If it were postdepositional, the two most likely causes would be permafrost developed during subsequent glaciations or pressures exerted by overriding glaciers. However, both can be rejected. Post-depositional permafrost should have disturbed the upper parts of the units more strongly than the lower, or the reverse of what is seen. Further, permafrost should have acted nearly as strongly upon any other tills or deposits it found overlying these two units, but only these two till sheets, and neither intervening nor overlying tills, nor any other deposits, are significantly affected. Further, in regard to the possible effect of overriding glaciers, Stalker (1977, p. 402) stated, with regard to his "Unit XXIII" at Medicine Hat: "It is . . . strongly so than any of the other termixed with underlying face west of Lethbridge,
contorted and deformed, noticeably more tills, and in places it is strongly indeposits. This till may form the surwhere it has similar traits."
If this suggestion that the traits occur equally well, both where the till is buried in section and exposed at the surface, is correct, they obviously are not caused by subsequent glacier overriding. Further, other till sheets have suffered such overriding with consequent strong compaction without development of the deformation. It appears, therefore, that both subsequent permafrost and glacier overriding can be eliminated as causes, and that the deformation developed near time of till deposition. The writer considers that the deformation formed under permafrost conditions as the glacier advanced over deeply frozen ground, and that the interaction between the glacier and underlying material, along with thawing of the permafrost, churned together and intermixed the lower part of the till the glacier was depositing and the upper part of the underlying unit. Other factors, such as high ground-water pressure due to the glacier blocking the escape of both surface and ground water, could have been important, particularly when one considers the ubiquity of plastic, bentonitic beds on the Prairies. However, no matter what the cause, suitable conditions must have been rare during ice-advances into the region, because well developed deformation appears restricted to these two till sheets. Probably most of the glacier advances in this region were over ground that was not deeply frozen, if frozen at all. These contorted tills, which the writer considers to represent the earliest glacier advances of both the Preclassical and Classical Wisconsin stages, are of immense value in correlation, f o r obviously they should correlate to deformed tills found in other sections, rather than to any non-deformed units that might be present. Further, their differing intensity of deformation allows these two tills to be readily distinguished from each other in most outcrops. Altogether, they form excellent marker beds. As with colour, this criterion can be used beyond the limits of younger glaciers. It would be useful to determine whether similar deformation or other macrofeatures caused by ice-advance under permafrost conditions
can b e found i n o t h e r r e g i o n s . If t h e y c a n , t h e y m i g h t e n a b l e l o n g r a n g e c o r r e l a t i o n from t h o s e r e g i o n s t o t h e t i l l s o f s o u t h A l b e r t a . T h i s should n o t , of course, l i m i t t h e search f o r o t h e r types o f f e a t u r e s , d e v e l o p e d from c o n d i t i o n s p r e v a i l i n g a t t i m e o f d e p o s i t i o n , t h a t m i g h t p r o v e e q u a l l y v a l u a b l e f o r b o t h c o r r e l a t i o n and d e t e r m i n a t i o n of r e l a t i v e a g e s i n t h o s e r e g i o n s , and f o r long-range c o r r e l a t i o n t o s o u t h Alberta. F e a t u r e s o f Combined D e p o s i t i o n a l and P o s t - D e p o s i t i o n a l O r i g i n Columnar s t r u c t u r e Of f e a t u r e s i n t h e s o u t h w e s t G r e a t P l a i n s t h a t owe t h e i r o r i g i n t o combined d e p o s i t i o n a l a n d p o s t - d e p o s i t i o n a l e f f e c t s , t h e most s p e c t a c u l a r a r e t h e l o n g , v e r t i c a l columns d i s p l a y e d i n s e v e r a l o f t h e t i l l s h e e t s . These a r e c a u s e d b y l o n g , v e r t i c a l j o i n t s , whose e f f e c t S t a l k e r ( 1 9 6 0 , p . 2 2 ) described as follows: "On c l i f f s and r i v e r b a n k s t h e j o i n t i n g o f t h e M a u n s e l l t i l l r e s u l t s i n d i s t i n c t i v e b l u f f s a n d c o l u m n a r f o r m s , as t h e t i l l slumps or b r e a k s a l o n g t h e s e f r a c t u r e s and t e n d s t o g i v e v e r t i c a l f a c e s a s much as 7 0 f e e t h i g h . Dawson ( 1 8 8 5 , p p . 1 4 3 c , 1 4 4 c ) d e s c r i b e d s u c h f e a t u r e s a l o n g t h e Oldman R i v e r , s o u t h o f t h e Red DeerS t e t t l e r a r e a . S i m i l a r c l i f f s a l o n g Bow R i v e r , a l s o s o u t h o f t h i s a r e a , a r e formed b y t h e Labuma and M a u n s e l l t i l l s . The j o i n t i n g i n t h e B u f f a l o Lake t i l l i s i r r e g u l a r a n d n o t v e r y p r o m i n e n t . T h i s t i l l i s weak a n d r a r e l y f o r m s v e r t i c a l b l u f f s . " " I f t h e f r a c t u r e p l a n e s i n t h e s e two t i l l s (Labuma He f u r t h e r n o t e d : and M a u n s e l l ) a r e n o t h o r i z o n t a l or v e r t i c a l i t can be assumed t h a t t h e t i l l has slumped or h a s b e e n c o n t o r t e d by o v e r r i d i n g g l a c i e r s . " Columns t h a t a r e n o t n e a r t o v e r t i c a l a r e r a r e .
The a b i l i t y to form columns i s a t y p i c a l a t t r i b u t e o f most t i l l s s e e n h o n t h e s o u t h w e s t P r a i r i e s , t h e c o n t o r t e d t i l l s b e i n g t h e c h i e f exceptions. Apparently t h e c o n t o r t i o n s , along with t h e i n c l u s i o n s of s a n d , s i l t , c l a y , and b e d r o c k , weaken t h e c o n t o r t e d t i l l s s u f f i c i e n t l y t o i n h i b i t f o r m a t i o n o f c o l u m n s . T h i s c a n a l s o happen i n n o r m a l l y columnar t i l l s , where i n c l u s i o n s o f n o n - t i l l m a t e r i a l have a l s o checked t h e i r l o c a l f o r m a k i o n . The a b s e n c e o f columns i n t h e t i l l s t h a t w e r e c o n t o r t e d n e a r t i m e o f d e p o s i t i o n d o e s i n d i c a t e some c o n t r o l o v e r t h e i r f o r m a t i o n by c o n d i t i o n s p r e v a i l i n g d u r i n g d e p o s i t i o n . E q u a l l y , however, t h e r a r i t y o f w e l l d e v e l o p e d columnar s t r u c t u r e s i n s u r f a c e t i l l s t h a t e s c a p e d o v e r r i d i n g by l a t e r g l a c i e r s i n d i c a t e s t h e i m p o r t a n c e o f p o s t depositional controls. U n d o u b t e d l y o t h e r p o s t - d e p o s i t i o n a l phenomena, s u c h a s s h r i n k a g e c a u s e d by d r y i n g , a r e o f t e n o f s i g n i f i c a n c e . A l t o g e t h e r , i n summary, column f o r m a t i o n a p p e a r s t o b e a f u n c t i o n o f b o t h d e p o s i t i o n a l and p o s t - d e p o s i t i o n a l e v e n t s , and s o t h e columns a r e u s e d h e r e t o i l l u s t r a t e t h e f e a t u r e s o f combined o r i g i n . The b e t t e r d e v e l o p m e n t o f t h e columns i n some t i l l s , s u c h as t h e Labuma and M a u n s e l l , t h a n i n o t h e r s , and t h e i r a b s e n c e i n t h e cont o r t e d t i l l s , i s a n i n v a l u a b l e a i d t o c o r r e l a t i o n . However, a s w e l l d e v e l o p e d columns can b e f o u n d i n b o t h o l d and young t i l l s , e x c e p t where t h e y a r e a t t h e s u r f a c e , t h e columns a r e o f l i t t l e v a l u e f o r d e t e r m i n a t i o n of a g e . CONCLUSIONS
Each t i l l h a s i t s own i n d i v i d u a l , l a r g e s c a l e f e a t u r e s t h a t a r e i n d e p e n d e n t o f c o m p o s i t i o n . These h e l p i n f i e l d r e c o g n i t i o n o f t h e t i l l , and commonly e n a b l e i t s c o r r e l a t i o n f r o m s e c t i o n t o s e c t i o n and to s i t e s where i t s a g e can b e d e t e r m i n e d . They c a n a l s o b e u s e d i n p l a c e s t o d e t e r m i n e r e l a t i v e a g e s o f t i l l s and even t o i n d i c a t e t r u e a g e where o t h e r means o f d a t i n g a r e u n a v a i l a b l e . Even i f s u c h o t h e r means o f d a t i n g a r e a v a i l a b l e , t h e m a c r o f e a t u r e s c a n s t i l l s e r v e a s a check on r e s u l t s o b t a i n e d by t h o s e means, i n c l u d i n g a b s o l u t e d a t e s . Use o f t h e s e m a c r o f e a t u r e s h a s b e e n much n e g l e c t e d o f l a t e , and p a r t i c u l a r l y s i n c e t h e improved means o f a b s o l u t e d a t i n g .
321
T h e r e a r e many t y p e s o f t i l l f e a t u r e s a v a i l a b l e f o r t h e s e p u r p o s e s , a n d u n d o u b t e d l y d i f f e r e n t s u i t e s of them c a n b e i d e n t i f i e d f o r u s e i n different regions. The e x a m p l e s f o r t h i s p a p e r h a v e b e e n drawn f r o m s t u d i e s made o f t h e L a u r e n t i d e t i l l s o f t h e s o u t h w e s t p a r t o f t h e Great P l a i n s o f Canada, which i s a l m o s t a n i d e a l s p o t t o s t u d y them. Corr e l a t i o n a n d d e t e r m i n a t i o n o f t h e r e l a t i v e a g e s o f t h e many t i l l s p r e s e n t i n t h a t r e g i o n a r e b a s e a l a r g e l y on f i e i d u s e oI" s u c h m a c r o f e a t u r e s . L u c k i l y , t h a t r e g i o n also h a s s e c t i o n s w h e r e a g e s c a n b e o b t a i n e d by o t h e r means - p r i m a r i l y v e r t e b r a t e p a l e o n t o l o g y - t o w h i c h t h e t i l l u n i t s c a n b e c o r r e l a t e d . Of t h e a b u n d a n t m a c r o f e a t u r e s a v a i l a b l e t h e r e , o n l y a few have been chosen t o i l l u s t r a t e e a c h o f t h e main t y p e s : t h o s e formed n e a r t i m e o f t i l l d e p o s i t i o n , t h o s e formed p o s t - d e p o s i t i o n a l l y , a n d t h o s e o f c o m b i n e d o r i g i n . However, t h o s e c h o s e n a r e u n d o u b t e d l y t h e most s p e c t a c u l a r f o u n d i n s o u t h A l b e r t a , and t h e most v a l u a b l e f o r t h e purpose. REFERENCES CITED Alley, N.F., 1973, Glacial stratigraphy and the limits of the Rocky Mountain and Laurentide ice sheets in southwestern Alberta, Canada: Canadian Petroleum Geol. Bull., v. 21, p. 153-177. Dawson, G.M. 1885, Report on the region in the vicinity of the Bow and Belly Rivers, Northwest Territory: Geol. Survey Can., Report of Progress 1882-3-4, pt.C., 168 p . Dawson, G.M. and McConnell, R.G., 1895, Glacial deposits of southwestern Alberta in the vicinity of the Rocky Mountains: Geol. Amer. Bull., v. 7, p. 31-66.
*S
Harington, C.R., 1 9 7 8 , Quaternary vertebrate faunas of Canada and Alaska and their suggested chronological sequence: National Museums Can., Museum of Natural Sciences, Ottawa: Syllogeus No. 1 5 , 105 p. Horberg, Leland, 1 9 5 2 , Pleistocene drift sheets in the Lethbridge region, Alberta: Jour. Geol., v. 60 p. 303-330.
, 1 9 5 4 , Rocky Mountain and continental Pleistocene deposits i n the Waterton region, Alberta, Canada: Geol. SOC. Amer. Bull., V. 6 5 , p . 1093-1150. Stalker, A. MacS., 1 9 6 0 , Surficial geology of the Red Deer-Stettler Map-area, Alberta: Geol. Survey Can., Memoir 306, 140 p .
, 1 9 6 3 , Quaternary stratigraphy in southern Alberta: Geol. Survey Can., Paper 62-34, 52 p. -
, 1969, Quaternary stratigraphy i n southern Alberta, Report 1 1 : Sections near Medicine Hat: Geol. Survey Can., Paper 69-26, 28 p. , 1972, Southern Alberta: i n Rutter, N.W. and Christiansen, E.A., eds., Quaternary geology and geomorphology between Winnipeg and the Rocky Mountains: Montreal, Twenty-fourth Internat. Geol. Congress, Guidebook, F'ield Excursion C-22, p. 6779.
, 1976, Quaternary stratigraphy o f the southwestern Canadian Prairies: in Mahaney, Y.C., ed., quaternary Stratigraphy of North America: Dowden, Hutchinson & Ros's Inc., Stroudsburg, Pa., p. 381-407. , 1977, The probable extent of Classical Wisconsin ice in southern and central Alberta: Canadian Jour. Earth Sci., v. 1 4 , p. 2614-2619.
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southern half:
1978, The geology of the ice-free corridor: The Canadian Jour. of Anthropology, v. I, p . 11-13.
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, Quaternary stratigraphy in southern Alberta, Report 111: The Cameron Ranch Section: Geol. Survey Can., Paper (in press). Stalker, A . MacS. and Churcher, C . S . , 1970, Deposits near Medicine H a t , Alberta: Geol. Survey Can., Display chart with marginal notes.
, 1972, Glacial stratigraphy of the southwestern Canadian Prairies; the Laurentide record: Montreal, Twenty-fourth Internat. Geol. Congress, Sect. 1 2 , p . 110-119. Stalker, A. MacS. and Harrison, J.E., 1 9 7 7 , Quaternary glaciation o f the Waterton-Castle River region o f Alberta: Canadian Petroleum Geol., Bull., v. 25, p. 882-906.
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EVAL UAT I 0N 0F RELATIVE PEDOSTRA TIGRAPHIC DATING METHODS, WITH SPECIAL REFERENCE TO QUATERNARY SUCCESSIONS OVERLYING WEATHERED PLATFORM MATERIALS CHARLES W FINKL Jr.
ABSTRACT P a l e o s o l s w h i c h l a c k d a t a b l e m a t e r i a l s may b e a s s i g n e d t o r e l a t i v e a g e s e q u e n c e s by e v a l u a t i n g t h e n a t u r e o f s o i l materials and t h e d e g r e e o f p e d o l o g i c a l o r g a n i z a t i o n . The c h r o n o l o g i c a l o r d e r i n g o f p e d o l o g i c a l e p i s o d e s , w h i c h i s i m p o r t a n t t o r e c o n s t r u c t i o n of p a l e o e n v i r o n m e n t s w h e r e s e q u e n c e s o f s o i l s a n d w e a t h e r i n g z o n e s o c c u r , i s b a s e d on s t r a t i g r a p h i c p r i n c i p l e s and p r a c t i c e s . E p i s o d i c development i n t h e pedos p h e r e may b e e s t a b l i s h e d b y a p p l y i n g c o n c e p t s o f s e p a r a t e i d e n t i t y , l a t e r a l c o n t i n u i t y , ascendancy and descendancy, w e a t h e r i n g d i f f e r e n t i a l s , and pedogenic p e r s i s t e n c e t o s o i l m a n t l e s . These c o n c e p t s a r e used i n t h e s t r a t i g r a p h i c ordering of s o i l s a f t e r proofs of e x i s t e n c e have been e s t a b l i s h e d f o r independent ( b u r i e d ) s o i l l a y e r s . A l t h o u g h p e d o l o g i c a l f e a t u r e s a n d s o i l f a b r i c s p r o v i d e a b a s i s for i d e n t i f y i n g t h e p r e s e n c e of r o c k - and s o i l s t r a t i g r a D h i c u n i t s i n whole s o i l p r o f i l e s , they a l s o help e s t a b l i s h the r e l a t i v e ages of juxtaposed s o i l m a t e r i a l s . The n a t u r e o f s k e l e t o n g r a i n s , e s p e c i a l l y t h o s e w h i c h a r e e t c h e d , embayed or i n h e r i t e d f r o m p r e v i o u s c y c l e s , a r e i m D o r t a n t t o e v a l u a t i o n s of s o i l p r o c e s s and s t a g e o f development. Organization of s o i l p l a s m a i n t o d i s t i n c t f a b r i c s c a n , when c o r r e l a + , e d w i t h s p e c i f i c p e d o s t r a t i g r a p h i c i n t e r v a l s , p r o v i d e a n a d d i t i o n a l means o f e s t a b l i s h i x c h r o n o l o g i c a l s e q u e n c e s b a s e d on g e o g r a p h i c a l l y d i s j u n c t p a l e o s o l s . These methods a r e p a r t i c u l a r l y u s e f u l i n d e e p l y w e a t h e r e d t e r r a i n s , s u c h as t h o s e a s s o c i a t e d w i t h t r o p i c a l a n d s u b t r o p i c a l c r a t o n i c r e g i o n s , where Q u a t e r n a r y s o i l - s t r a t i g r a p h i c u n i t s o v e r l i e T e r t i a r y w e a t h e r i n g z o n e s . M i c r o m o r p h o l o g i c a l t e c h n i q u e s e f f e c t i v e l y d i f f e r e n t i a t e pedog e n i c e p i s o d e s where younger s o i l - s t r a t i g r a p h i c l a y e r s c o n t a i n s o i l p a r e n t m a t e r i a l s d e r i v e d from p r e - e x i s t i n g w e a t h e r i n g m a n t l e s . I N T R O D U C T I0N
S o i l s a r e c o m p l i c a t e d t h r e e - p h a s e ( s o l i d , l i q u i d , and g a s ) n a t u r a l systems t h a t occupy p o r t i o n s o f t h e w e a t h e r e d s u r f a c e of t h e e a r t h . A l t h o u g h s o i l s e x i s t a s d i s c r e t e or i n d i v i d u a l b o d i e s . t h e y c o - m i n g l e w i t h l a r g e r e n t i t i e s f o r m i n g c o n t i n u u m s w h i c h make up s o i l m a n t l e s . These m a n t l e s , a s d e f i n e d by F r i d l a n d (1974), h a v e w i d e g e o g r a p h i c d i s t r i b u t i o n a n d a r e d i s t i n g u i s h e d f r o m o n e a n o t h e r on t h e b a s i s o f i n herent characteristics. S u r f a c e s o i l m a n t l e s a r e c o m p r i s e d o f contemp o r a r y s o i l s , w e a t h e r i n g zones, and exposed g a l e o s o l s . Buried s o i l s , p a r t s of s u b s u r f a c e m a n t l e s , r e p r e s e n t p e d o g e n i c p h a s e s t h a t have been a r r e s t e d by d e p o s i t i o n o f o v e r b u r d e n . Many e x p o s u r e s e x h i b i t m u l t i p l e l a y e r s of weathered materials s u c c e s s i v e l y s t a c k e d i n v e r t i c a l sequence (Figure 1). S u b d i v i s i o n o f t h e s o i l c o n t i n u u m h a s t a k e n many t a c k s b u t n o s i n g l e m e t h o d o f d o i n g s o has y e t b e e n a g r e e d u p o n ( B u t l e r , 1 9 8 0 ) . P a r t of t h e c l a s s i f i c a t o r y dilemma i s u n d o u b t e d l y r e l a t e d t o t h e f a c t t h a t s o i l l a y e r i n g , i.e. t h e s t r a t i g r a p h i c s u c c e s s i o n o f u n r e l a t e d s o i l m a t e r i a l s , i s more p r e v a l e n t t h a n g e n e r a l l y a p p r e c i a t e d .
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Figure 1
S o i l development i n a l l u v i a l t e r r a c e s along t h e A surface Stelly River i n northwestern Australia. Red E a r t h ( R E ) o v e r l i e s a t r u n c a t e d R e d P o d z o l i c (RP) s o i l w h i c h i n t u r n c o v e r s a n o l d e r s o i l d e v e l o p e d i n c a l c a r e o u s a l l u v i u m (CA). (Photo: W i l l i a m M. McArthur)
S e p a r a t i o n o f s o i l s on a c h r o n o s t r a t i g r a p h i c b a s i s r e q u i r e s , c a r e f u l a p p l i c a t i o n o f a b s o l u t e and r e l a t i v e d a t i n g methods. U n d e r s t a n d i n g o f s o i l a g e r e l a t i o n s h i p s i s c r i t i c a l to c o m p r e h e n s i o n of c e r t a i n p h y s i c a l and c h e m i c a l c h a r a c t e r i s t i c s , d u r a t i o n and i n t e n s i t y o f p e d o g e n i c p r o c e s s e s , a n d e v e n management o f s o i l r e s o u r c e s . O l d or s e n i l e s o i l s e x h i b i t p r o p e r t i e s q u i t e d i f f e r e n t f r o m t h o s e a s s o c i a t e d w i t h s o i l s r e c e n t l y formed. Soils containing datable materials c a n b e a s s i g n e d a b s o l u t e t i m e f r a m e s . Many s o i l s a r e p l a c e d i n r e l a t i v e t i m e s e q u e n c e s b e c a u s e t h e y a r e n o t a m e n a b l e to p r e c i s e a g e d e t e r minations. R e l a t i v e a g e s e q u e n c e s , thou,gh n o t as d e f i n i t i v e a s a b s o l u t e methods, o f t e n p r o v i d e a r e a s o n a b l e b a s i s f o r e s t i m a t i n g t h e t i m i n g of pedogenic episodes. Specific soil-forming Drocesses, operating f o r c e r t a i n l e n g t h s o f t i m e , a r e known to p r o d u c e t h e same e f f e c t s o v e r a n d over again. Although s o i l n i t r o g e n and phosphorus l e v e l s markedly f l u c t u a t e o v e r t h e s h o r t t e r m , t h e y may r e a c h q u a s i - e q u i l i b r i u m i n 1 0 2 - 1 0 3 y r ( J e n n y , 1 9 4 1 : C r o c k e r & M a j o r , 1 9 5 5 ; Smeck, 1973). The d e velopment of a r g i l l i c h o r i z o n s o r humus-iron p a n s of t o d z o l s developed i n s a n d s may r e q u i r e i n t e r v a l s on t h e o r d e r o f 1 0 3 - l G y r . The f o r m a t i o n of t h i c k m o t t l e d a n d p a l l i d z o n e s o f d e e p l a t e r i t i c p r o f i l e s may r e q u i r e 106-107 yr ( W h i t e h o u s e , 1 9 4 0 . H a n l o n , 1 9 5 0 ; Idurrn & S e n i o r , 1 9 7 8 ; K r o n b e r g e t al., 1 9 7 9 ) . Some r e s i s t a n t e p i d i a g e n e t i c f e a t u r e s s u c h a s f e r r u g l n o u s d u r i c r u s t s , o f t h e t y p e d e s c r i b e d by M a i g n i e n ( 1 9 5 9 ) , f o r m e d a s l o r , g a g o a s t h e O l i g o c e n e ( Q u i l t y , 1 9 7 7 ) , b u t may h a v e t a k e n o n l y 102-103 yr t o h a r d e n u n d e r d e s i c c a t i n g c o n d i t i o n s ( M g r n e r , 1 9 7 8 ) . Leneuf a n d A u b e r t ( 1 9 6 0 ) e s t i m a t e t h a t i t t a k e s s o m e t h i n g OR t h e o r d e r o f 2 2 , 0 0 0 - 7 7 , 0 0 0 y r f o r 1 m of g n e i s s to become c o m p l e t e l y f e r r a l i t i z e d i n a humid t r o p i c a l e n v i r o n m e n t . Deep w e a t h e r i n g i n N i g e r i a i s t h o u g h t to h a v e t a k e n 6 6 0 , 0 0 0 t o 2 . 3 1 0 , 0 0 0 y r to r e a c h 30 m ( J e j s , 1 9 7 0 ) . S u c h f e a t u r e s t h u s s e r v e as p r o x i e s where s o i l a g e i s i n f e r r e d . S o i l s a s s o c i a t e d w i t h s t a b l e p l a t f o r m s , e s p e c i a l l y t h o s e n o t subj e c t e d t o t h e d i r e c t a f f e c t s o f Q u a t e r n a r y g l a c i a t i o n s and which
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p r e s e n t l y l i e i n t r o p i c a l and s u b t r o p i c a l r e g i o n s , have r a t h e r d i f f e r e n t h i s t o r i e s from s o i l m a n t l e s i n N o r t h e r n H e m i s p h e r e c r a t o n s . The m o r p h o t e c t o n i c s t a b i l i t y of t h e A u s t r a l i a n , B r a z i l i a n , and p a r t s of t h e A f r i c a n c r a t o n s p e r m i t t e d d e e p w e a t h e r i n g m a n t l e s t o d e v e l o p on s u i t a b l e s u r f a c e s o v e r v a s t expanses of t e r r i t o r y . I n t e n s e chemical w e a t h e r i n g c o n t i n u e d on a n d o f f f o r m i l l e n i u m s o n t h e West A u s t r a l i a n c r a t o n u n t i l l a r g e l y i n h i b i t e d by O l i g o c e n e i n d u r a t i o n ( F i n k 1 & F a i r b r i d g e , 1 9 7 9 ) . The g r e a t e x t e n t o f d e e p l y w e a t h e r e d c r a t o n i c r e g i o n s ( t h e West A u s t r a l i a n S h i e l d a l o n e o c c u p i e s some 7 1 8 , 0 0 0 k m 2 ) p o i n t s to t h e n e e d for g r e a t e r u n d e r s t a n d i n g o f s o i l a g e r e l a t i o n s h i p s i n t h e s e "unglaciated" regions. E x a m p l e s of r e l a t i v e a g e d e t e r m i n a t i o n s d i s c u s s e d h e r e a r e d e r i v e d f r o m s t u d i e s on t h e A u s t r a l i a n c r a t o n . The c o n c e p t s a n d m e t h o d s a r e , however, r e l e v a n t t o o t h e r l a n d s c a p e s where Q u a t e r n a r y s o i l m a n t l e s o v e r l i e deeply weathered s u b s t r a t e s . C H R O N O L O G I C A L O R D E R I N G OF PEDOLOGICAL EPISODES
Concepts o f s o i l age r e v o l v e around p r e c i s e measurements o f time
or e s t i m a t e s b a s e d on c o m p a r a t i v e s o i l a n a t o m i e s w h i c h g i v e a r e l a t i v e
t i m e r e f e r e n c e . The s o - c a l l e d a b s o l u t e d a t i n g m e t h o d s f o c u s on i s o t o p i c a g e d a t a , namely z i r c o n - f i s s i o n t r a c k a g e s , r a d i o c a r b o n r e s i d e n c e times, uranium s e r i e s d a t i n g , thermoluminescence (see d i s c u s s i o n i n M o o r b a t h , 1 9 6 0 ; Y a a l o n , 1 9 7 1 ) o r c o r r e l a t i o n of l a n d s e q u e n c e s w i t h "0 s t a g e s i n t h e o c e a n i c r e c o r d ( K u k l a , 1 9 7 7 ) . R e l a t i v e d a t i n g methodo l o g i e s , o n t h e o t h e r h a n d , may employ s t r a t i g r a p h i c p r i n c i p l e s ( F i g u r e 2 ) o r r e l y o n n o t i o n s of r e l a t i v e s o i l a g e u s i n g s o i l - g e o m o r p h i c r e l a t i o n s h i p s and a p p r a i s a l o f i n t e n s i t y of p r o f i l e development. Absolute Dating Vethods A l t h o u g h a b s o l u t e d a t i n g m e t h o d s t h e m s e l v e s do n o t f a l l w i t h i n our p u r v i e w , i t i s r e l e v a n t t o e m p h a s i z e t h e i m p o r t a n c e o f t h e s e d a t u m s to r e l a t i v e chronologies. "S s t a g e s i n t h e o c e a n i c r e c o r d f o r e x a m p l e , no m a t t e r how r e m o t e t h e y may seem f r o m l a n d - b a s e d s e q u e n c e s , h e l p e d e s t a b l i s h t h e t i m i n g of Q u a t e r n a r y s u c c e s s i o n s i n Europe. Using r i v e r t e r r a c e s a s a l i n k , K u k l a ( 1 9 7 7 ) was a b l e t o c o n s t r u c t a n i m p r o v e d c l i m a t o - s t r a t i g r a p h i c m o d e l of t h e P l e i s t o c e n e i n C e n t r a l E u r o p e w h e r e t h e r e were s e q u e n c e s o f l o e s s b a n d s a n d s o i l s . Ages of s o i l m a n t l e s c a n a l s o b e e s t i m a t e d when i n t e r c a l a t e d w i t h l a y e r s of v o l c a n i c a s h . I n t h e A m e r i c a n West, f o r e x a m p l e , u s e f u l s t r a t i g r a p h i c marker b e d s a r e a s s o c i a t e d w i t h Mount S t . H e l e n s , Nount R a i r , i e r , Mazama, G l a c i e r P e a k , a n d P e a r l e t t e a s h f a l l s ( s e e d l s c u s s i o n s i n W i l c o x , 1 9 6 5 ) . T e p h r o c h r o n o l o g i e s may i n d e e d b r a c k e t p e d o g e n i c e p i s o d e s b u t t h e y t h e m s e l v e s a r e c o m p l i c a t e d by a s h f a l l s c l o s e l y s p a c e d i n t i m e (Young & P o w e r s , 1 9 6 0 ) . The G l a c i e r P e a k t e p h r a i n t h e n o r t h e r n C a s c a d e s of W a s h i n g t o n a r e d i v i s i b l e i n t o a t l e a s t n i n e s e p a r a t e l a y e r s ( P o r t e r , 1 9 7 8 ) . The Mazama a s h may a l s o c o n t a i n m u l t i p l e ash f a l l s w i t h i n a p p a r e n t l y w e l l - d e f i n e d marker h o r i z o n s . Mack e t al. (1979) s u g g e s t , on t h e b a s i s o f p e t r o g r a D h i c s t u d i e s and r a d i o m e t r i c d a t e s , t h a t t h i s m a r k e r h o r i z o n r e s u l t e d f r o m a t l e a s t two e r u p t i o n s where t h e u p p e r u n i t f a l l s i n t h e r a n g e 6 , 6 0 0 - 7 , 0 0 0 y r BP a n d t h e l o w e r one a r o u r - d 8,300 i 80 yr EP. S i o t u r b a t i o n , f r o s t a c t i v i t y , a n d c r e e p a l s o sometimes i n h i b i t t e p h r o s t r a t i g r a p h i c i n v e s t i g a t i o n s . R e l a t i v e Ape S e q u e n c e s Whether keyed t o d a t e d d e p o s i t s i n s t r a t i g r a p h i c s e q u e n c e s on l a n d or l i n k e d t o m a r i n e s e d i m e n t s o f f s h o r e , i t i s s t i l l o f t e n n e c e s s a r y or a d v a n t a g e o u s t o d e t e r m i n e r e l a t i v e a p e s of i n d e p e n d e n t s o i l s y s t e m s . S o i l m a n t l e s may, f o r e x a m p l e , b e s t a c k e d b e t w e e n o t h e r t y p e s o f c h r o n o s t r a t i g r a p h i c u n i t s or t h e y may b e a s s o c i a t e d w i t h a p a r t i c u l a r m a r k e r h o r i z o n . Where s u c h r e l a t i o n s h i p s d o r A o t e x i s t , r e l a t i v e d a t i n g m e t h o d s g e n e r a l l y f o c u s on s t r a t i g r a p h i c t e c h n i a u e s w h i c h p l a c e s o i l m a n t l e s i n s e q u e n c e f r o m o l d e s t t o y o u n g e s t a c c o r d i n g t o t h e Law of Superposition. S t r a t i g r a p h y , i n t h e absence of absolute d a t e s , prov i d e s a r e l a t i v e chronology o f c y c l i c r e g o l i t h l a y e r i n g f e a t u r e s .
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Figure
2
T h e s o i l d e v e l o p e d i n t h e l o w e r b a s a l t was i n t u r n c o v e r e d b y a l a t e r f l o w w h i c h f o r m e d a new g r o u n d surface. The c o n t e m p o r a r y s u r f a c e s o i l , a c c o r d i n g t o t h e Law of S u p e r p o s i t i o n , i s younger t h a n t h e buried soil. Both b a s a l t f l o w s n e a r D o r r i g o , New S o u t h Wales, a r e o f T e r t i a r y a g e . (Photo: W i l l i a m M. McArthur)
P e d o - l i t h o l o g i c d i s c o n t i n u i t i e s and w e a t h e r i n g ~ t a t u s ~ i n t e r p r e t e df ,o r example, i n t e r m s of s t r a t i g r a p h i c p o s i t i o n ( L e s l i e , 1 9 7 8 ) , a s s i s t i n t h e e l u c i d a t i o n o f d e p o s i t i o n , s o i l f o r m a t i o n , and p e d o s p h e r e s t r i p ping. Soil-geomorphic r e l a t i o n s h i p s o f t e n p r o v i d e u s e f u l c l u e s as t o t h e a g e o f s o i l m a n t l e s when a n e x u s c a n b e d e m o n s t r a t e d between l a n d f o r m s and s o i l s ( e . g . P l a y f o r d , 1 9 5 4 ; Ruhe, 1 9 5 4 ; W r i g h t , 1 9 6 3 ; W a l k e r , 1 9 6 6 ; P a r s o n s e t a l . , 1 9 7 0 ; Young, 1 9 7 0 ; Eden, 1 9 7 1 ; Churchw a r d , 1977; B r o s h & G e r s o n , 1 9 7 8 ; D a n i e l s & G a m b l e , 1 9 7 8 ; Kumar, 1 9 7 9 ) . Linkages between s o i l m a n t l e s and e r o s i o n a l and/or d e p o s i t i o n a l s u r f a c e s a l s o p r o v i d e h i n t s as t o s o i l a g e (Buurman, 1 9 7 2 ) . The " s o i l s t r a t o l a n d s c a p e " ( v a n D i j k , 1979)., a n e x t e n s i o n o f t h e g e n e r a l c o n c e p t embracing s o i l - l a n d f o r m r e l a t i o n s h i p s , i s a l a n d s c a p e u n i t w i t h a s p e c i f i c s o i l - s t r a t i g r a p h i c a s s e m b l a g e . Such u n i t s c o n t a i n r e m n a n t s o f in situ a n d d e p o s i t i o n a l p a l e o s o l l a y e r s a n d r e l i c t w e a t h e r i n g z o n e s which d i s p l a y a c e r t a i n s u r f a c e m o r p h o l o g y . S o i l s t r a t o l a n d s c a p e s a r e n o r m a l l y d e l i n e a t e d as geomorphic s u b d i v i s i o n s w h i c h o f f e r a d v a n t a g e b y i n c o r p o r a t i n g r e s u l t s o f d e t a i l e d a p p r a i s a l of c y c l i c l a n d s c a p e development i n a s i n g l e u n i t . Pedo-morpholith s t u d i e s ( e . g . van D i j k & Rowe, 1 9 8 0 ) i n a n a n a l o g o u s way d e f i n e r e l a t i o n s h i p s b e t w e e n soils t r a t i g r a p h i c p a t t e r n s and l a n d f o r m s . I n t e r p r e t a t i o n o f r e l a t i v e s o i l a g e on t h i s b a s i s i s t h u s v e r y much t i e d t o s t u d i e s o f s o i l p e r i o d i c i t y , f o l l o w i n g t h e K-cycle c o n c e p t f o r m u l a t e d b y B u t l e r ( 1 9 5 9 ) , l a n d s u r f a c e morphology, a n d s t a g e of l a n d s c a p e development i n t h e b r o a d e s t D a v i s i a n s e n s e . C o n c e p t s o f e f f e c t i v c a g e and s t a g e o f d e v e l o p m e n t h a v e b e e n a p p l i e d w i t h v a r y i n g d e g r e e s o f s u c c e s s b u t may h a v e a p p l i c a b i l i t y where t h e p r o p e r t i e s of b u r i e d s o i l s o c c u r i n s u r f a c e a n a l o g u e s (Simonson, 1 9 5 4 ) . F i e l d morphology r a t i n g s y s t e m s ( B i l z i & C i o l k o s z , 1971; Meixner h S i n g e r , 1 9 8 1 ) h a v e a u s e f u l p o t e n t i a l f o r a s s i s t i n g i n t h e e v a l u a t i o n of s o i l f o r m a t i o n a n d d i s c o n t i n u i t i e s . The m e t h o d , based on t h e d i s t i n c t n e s s o f m o r p h o l o g i c a l f e a t u r e s i n a d j a c e n t h o r i z o n s and c o m p a r i s o n of d i s c r e t e h o r i z o n s i n t h e solum w i t h C h o r i z o n s w i t h i n a p e d o n , may h a v e g e n e r a l u s e a s a n i n d i c a t o r o f r e , l a t i v e s o i l a g e . Micromorphological a n a l y s e s o f f e r scope f o r i n c r e a s e d r e s o l u t i o n of
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many s o i l a g e p r o b l e m s , e s p e c i a l l y i n c a s e s w h e r e f i e l d t e c h n i q u e s a r e t o o c o a r s e f o r d e t a i l e d s t u d y . S o i l f a b r i c a n a l y s i s and the i n v e s t i g a t i o n o f s k e l e t o n g r a i n m o r p h o l o g i e s p r o v i d e a d d i t i o n a l means o f establishing r e l a t i v e ages of paleosols. STRATIGRAPHIC PRINCIPLES A N D PRACTICES The o c c u r r e n c e o f b u r i e d w e a t h e r i n g z o n e s i n Q u a t e r n a r y s u c c e s s i o n s i n t h e A m e r i c a n Midwest f i r s t s u g g e s t e d t o some w o r k e r s ( e . g . W o r t h e n , 1 8 6 6 ; McGee, 1 8 9 1 ; L e v e r e t t , 1 8 9 8 ) t h a t t h e l a y e r s or b a n d s c o u l d b e It was l a t e r f o r m a l l y p r o p o s e d u s e d as s t r a t i g r a p h i c m a r k e r h o r i z o n s . t o employ p a l e o s o l s a n d w e a t h e r i n g p r o f i l e s a s s t r a t i g r a p h i c u n i t s (Richmond & F r y e , 1 9 5 7 ; ACSN, 1 9 6 1 ) . The c o n c e p t p r o v e d t o b e a p p l i c able t o Quaternary investigations not only i n glaciated landscapes of t h e Midwest ( F o l l m e r , 1 9 7 3 ) b u t i n w e s t e r n m o u n t a i n r e g i o n s a s w e l l (Morrison, 1 9 6 4 , 1 9 6 7 ) . I n t e r - r e g i o n a l c o r r e l a t i o n s of w e a t h e r e d m a r k e r h o r i z o n s were a l s o a t t e m p t e d ( M o r r i s o n & F r y e , 1 9 6 5 ) . Recognition of s t r a t i g r a p h i c sequences i n deeply weathered t r o p i c a l a n d s u b t r o p i c a l t e r r a i n e s b r o u g h t s o i l s t r a t i g r a p h i c s t u d i e s f r o m midl a t i t u d e s t o e q u a t o r i a l realms ( e . g . E r h a r t , 1 9 4 8 ; R u h e , 1 9 5 4 ) . S t u d i e s of l a y e r e d s o i l m a t e r i a l s i n t h e A u s t r a l i a n r e g i o n g r e a t l y expanded t h e s c o p e o f t h e f i e l d i n t h e 5 0 ' a n d 60's b y i n c l u d i n g a d i v e r s e a r r a y o f weathered materials r a n g i n g from a n c i e n t deep w e a t h e r i n g m a n t l e s and d u r i c r u s t s ( f e r r i c r e t e and s i l c r e t e ) t o r e l a t i v e l y younger s o i l s ( e . g . F a i r b r i d g e & T e i c h e r t , 1 9 5 3 ; Van D i j k , 1 9 5 8 ; B u t l e r , 1 9 5 9 ; J e s s u p , 1 9 6 0 , C h u r c h w a r d , 1 9 6 1 ; W a l k e r , 1 9 6 2 ; Van D i j k , R i d d l e r & Rowe, 1 9 6 8 ; Brewer & W a l k e r , 1 9 6 9 ) . The p r i n c i p l e s o f s o i l s t r a t i g r a p h y , a s s u m m a r i z e d by M o r r i s o n ( 1 9 6 7 , 1 9 7 8 ) a n d r e c e n t l y r e v i e w e d by F i n k l ( 1 9 8 0 ) , u t i l i z e t h e Law o f S u p e r p o s i t i o n and t h e P r i n c i p l e s of S e p a r a t e I d e n t i t y (Walker, 1958; B u t l e r , 1 9 5 9 ) , L a t e r a l C o n t i n u i t y , Ascendancy and Descendancy (Wa lk e r , 1 9 6 6 ) , and Pedogenic P e r s i s t e n c e . These c o n c e p t s have been a p p l i e d t o w e a t h e r e d p l a t f o r m materials and r e l a t e d d e p o s i t s i n e f f o r t s t o est a b l i s h a r e l a t i v e s o i l - s t r a t i g r a p h i c s e q u e n c e f o r t h e West A u s t r a l i a n c r a t o n . D e t a i l e d i n f o r m a t i o n was m a i n l y d e r i v e d f r o m t h e s o u t h w e s t e r n p o r t i o n of t h e c r a t o n i n a d i s s e c t e d upland r e g i o n o f t h e D a r l i n g P l a t e a u , r e f e r r e d t o h e r e a s t h e Blackwood T a b l e l a n d ( F i g u r e 3 ) . M a n t l e o f Deep C h e m i c a l W e a t h e r i n g on t h e Blackwood T a b l e l a n d The l a t e r i t i c d e e p w e a t h e r i n g m a n t l e i s a p a n c r a t o n i c f e a t u r e w h i c h o c c u r s on a l l m a j o r g e o m o r p h i c s u r f a c e s w h e r e s l o p e s a r e l e s s t h a n 1 0 d e g r e e s ( M u l c a h y , 1 9 6 0 ) . The d e p t h o f w e a t h e r i n g a v e r a g e s a t l e a s t 30 m w i t h m o t t l e d a n d p a l l i d z o n e s d e v e l o p e d i n c r y s t a l l i n e b a s e ment a n d s e d i m e n t a r y r o c k s a l i k e ( B u t t a n d S m i t h , 1 9 8 0 ) . R e f e r r e d t o a s t h e W a r r a n i a n i l a y e r or m a n t l e ( F i n k l & C h u r c h w a r d , 1 9 7 6 ) , t h e w e a t h e r i n g zone c o n t a i n s m o t t l e d and p a l l i d zones ( B a l l i j u p materials) s o c h a r a c t e r i s t i c of l a t e r i t i c p r o f i l e s ( S t e p h e n s , 1 9 7 1 ) a n d d e e p r e g o l i t h (Nannup s a p r o l i t e ) on s t e e p v a l l e y s i d e s l o p e s . L a t e r i t i c f a c i e s i n t h e Warraniani mantle indiscriminantly transgress l i t h o l o g i c boundaries (Figure 4 ) establishing t h e pervasive nature of t h e lateritization process. Some p a r e n t m a t e r i a l s a r e more s u s c e ~ t i b l et o i n t e n s e b i o g e o c h e m i c a l w e a t h e r i n g b u t most a r e s i m i l a r l y a f f e c t e d . Because b o t h w e a t h e r i n g u n i t s , B a l l i j u p m a t e r i a l s a n d Nannup s a p r o l i t e , o c c u p y t h e same s t r a t i g r a p h i c i n t e r v a l , a r e c o t e r m i n o u s , a n d g r a d e i n t o o n e a n o t h e r d e p e n d i n g on s l o p e a n d d r a i n a g e c o n d i t i o n s , t h e y a r e r e g a r d e d a s f a c i e s of t h e W a r r a n i a n i l a y e r a n d a r e t h u s c o e v a l i n a g e ( F i n k l & C h u r c h w a r d , 1 9 7 6 ; F i n k l & G i l k e s , 1 9 7 6 ) . The s a p r o l i t e f a c i e s , l i k e t h e l a t e r i t i c deep w e a t h e r i n g , r o t s most s o r t s of c o u n t r y r o c k ( F i g u r e 5 ) t o d e p t h s o f 1 0 m or m o r e . The d e e p w e a t h e r i n g z o n e b e a r s e v i d e n c e o f w i d e s p r e a d s t r i p p i n g t o p r o d u c e a v a r i e t y o f s u r f a c e s i n t e r p r e t e d as e t c h p l a i n s ( M a b b u t t , 1 9 6 1 ; F i n k l & Churchward, 1973; F a i r b r i d g e & F i n k l , 1979; F i n k l , 1 9 7 9 ) . T h e s e s u r f a c e s a r e c a p p e d by f e r r u g i n o u s d u r i c r u s t ( f e r r i c r e t e ) or b u r i e d by o v e r b u r d e n c o n t a i n i n g y o u n g e r s o i l m a n t l e s .
328
Figure 3
T h e w e s t e r n m a r g i n o f t h e West A u s t r a l i a n c r a t o n i s bounded by t h e D a r l i n g f a u l t s c a r p . The Blackwood T a b l e l a n d i s a d i s s e c t e d s e c t o r of t h e D a r l i n g P l a t e a u w i t h numerous d u r i c r u s t e d p l a t e a u remnants. Soil-stratigraphic relationships described for t h i s r e g i o n were o b s e r v e d t h r o u g h o u t t h e humid s o u t h w e s t with analogues occurring i n the a r i d cratonic interior.
The a g e o f t h e d e e p w e a t h e r i n g i n t h e A u s t r a l i a n r e g i o n h a s i n t r i g u e d g e o s c i e n t i s t s f o r t h e l a s t h a l f c e n t u r y or s o . A s s o c i a t e d w i t h t h e g r e a t a n t i q u i t y a n d s t a b i l i t y o f t h e West A u s t r a l i a n c r a t o n , development o f a d e e p l a y e r of c h e m i c a l w e a t h e r i n g seems t o h a v e t a k e n p l a c e from l a t e Mesozoic t o p o s s i b l y l a t e T e r t i a r y t i m e s (Dury e t al., 1 9 6 8 , G e o l . Surv. West. A u s t . , 1 9 7 5 ) . The c h e m i s t r y o f l a t e r i t i c s o i l s and ground w a t e r s s u g g e s t e d t o Kronberg e t al. ( 1 9 7 9 ) t h a t t h e deep l a t e r i t i c p r o f i l e s on t h e B r a z i l i a n S h i e l d r e q u i r e d t e n s of m i l l i o n s o f y e a r s t o d e v e l o p . A v e r y s t a b l e c o n t i n e n t a l s e t t i n g f r e e from m a j o r e r o s i o n a l e v e n t s was p o s t u l a t e d a s b e i n g e s s e n t i a l t o s u c h i n t e n s i v e c h e m i c a l w e a t h e r i n g . The l a c k o f d a t a b l e m a t e r i a l s i n t h e s o f t l a t e r i t i c and s a p r o l i t i c s o i l s r e q u i r e d r e s e a r c h e r s t o l o o k f o r c l u e s e l s e w h e r e , n o t a b l y i n C e n o z o i c m a r i n e s e d i m e n t s o f f s h o r e and i n t h e d u r i crust itself. I n d u r a t i o n o f l a t e r i t i c and s a p r o l i t i c s o i l s seems t o h a v e o c c u r r e d i n t h e O l i g o c e n e (Schmidt & Embleton, 1973; Q u i l t y , 1 9 7 7 ) a s s o c i a t e d i n p a r t with t h e l o s s of a p r o t e c t i v e f o r e s t cover during a c o o l s e m i - a r i d i n t e r v a l (Kemp, 1 9 7 8 ) a s w e l l a s a p r o b a b l e d r o p i n t h e ground w a t e r t a b l e f o l l o w i n g a f a l l i n s e a l e v e l a t about t h i s time (Galloway, 1 9 7 0 ; L e o n t y ' e v , 1 9 7 0 ) . Subsequent d i f f e r e n t i a l s t r i p p i n g ( t r u n c a t i o n ) of r e g o l i t h m a t e r i a l s and d e g r a d a t i o n o f l a t e r i t i c and s a p r o l i t i c p r o f i l e s t h u s sets t h e s t a g e f o r r e l a t i v e age determinations on t h e c r a t o n . The i m p a c t of p r e w e a t h e r e d m a t e r i a l s on y o u n g e r s o i l s h a s b e e n a p p r e c i a t e d i n many s t u d i e s ( s e e summaries i n Mulcahy, 1 9 6 7 ; Churchward, 1 9 7 0 ) . The i n t e n s e l y w e a t h e r e d c o n d i t i o n o f y o u n g e r s o i l s i s t h u s c l e a r l y r e l a t e d t o d e g r a d a t i o n p r o d u c t s o f t h e W a r r a n i a n i deep w e a t h e r i n g m a n t l e a n d o n l y maximum a g e s o f s u c c e s s i v e s o i l c o v e r s c a n be suggested. Mulcahy e t aZ. ( 1 9 7 2 ) e s t i m a t e d t h a t many d i f f e r e n t i a t e d s o i l s on y o u n g e r s u r f a c e s of t h e c r a t o n may h a v e o r i g i n a t e d i n t h e L a s t I n t e r g l a c i a l or s o o n a f t e r w a r d s , e v e n though t h e s o i l s c o n t a i n m a t e r i a l s associated with pre-Quaternary environments.
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Lateritic deep weathering transgresses lithologic b o u n d a r i e s r o t t i n g a v a r i e t y o f c o u n t r y r o c k s to g r e a t depths. The position of the seasonally fluctuating water t a b l e is a p p r o x i m a t e d by t h e m o t t l e d zone w h i c h overlies bleached rock fabrics in the waterlogged pallid zone.
Clastic Materials The stratigraphic relationships of superficial clastic materials, established on the basis of field observations, are summarized in Figures 6 , 7, and 8. The sequence on the Blackwood Tableland (Figure 6) comprises pisolitic accumulations in valley side slopes and fine.grained alluvium on valley floors. The gravels decrease in particlesize downslope and eventually merge with earthy alluvium. Both units, respectively identified as the Minnijup gravels and Bilbup alluvium (Finkl & Churchward, 1976), are derived from portions of the deep weathering mantle which they now overlie farther downslope. A similar stacking of surficial materials occurs in the shallow Balingup dissection. The ironstone gravels, however, give way to an earthy facies (Cundinup earths) on lower valley side slopes and spurs (Figure 7). Stratigraphic sequences in the deeper Bridgetown dissection are complicated by the interposition of transported Tittibinup clays between the Minnijup and Warraniani layers (Figure 8). Surficial Materials as Stratigraphic Elements The stratigraphic positions of in s i t u and clastic materials establish a relative age sequence. The Warraniani layer is the oldest substratum and also the source of pre-weathered materials occurring in the successively younger Tittibinup and Minnijup layers. Application of the Principles of Separate Identity, Lateral Continuity, and Ascendancy and Descendancy lead to the development of this age sequence as detailed by Finkl & Chruchward (1976). Interpretations of i n s i t u and depositional bodies as soil layers were justified on the basis of properties normally attributed to pedogenesis, i.e. soil structure, soil fabric, illuviated clay, and pedological features. The sedimentary layers thus become the Minnijup and Tittibinup soil'layers. They were formalized in terms of the pedoderm concept (Brewer e t aZ., 1970) and ranked stratigraphically, from oldest to youngest: Warraniani Pedoderm, Tittibinup Pedoderm, and Minnijup Pedoderm.
330
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S a p r o l i t e s , o c c u r r i n g on d i s s e c r r e d v a l l e y s i d e s l o p e s , a r e coterminous with l a t e r i t e p r o f i l e s . The r e s i d u a l n a t u r e o f s a p r o l i t i c w e a t h e r i n g , which c r o s s e s g e o l o g i c a l unconformi t i e s , i s i n d i c a t e d by i n t a c t q u a r t z v e i n s a n d w e a t h e r e d rock fabrics.
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P i s o l i t i c (ironstone) gravels blanket upland valley side s l o p e s b u t become more f i n e l y g r a d e d d o w n s l o p e . These c o l l u v i a l d e p o s i t s merge w i t h v a l l e y f l o o r a l l u v i u m , and overlie ferruginous duricrust, soft l a t e r i t i c materials, and bedrock.
SOIL-GEOMORPHIC RELATIONSHIPS C o n s i d e r a b l e c o n t r o v e r s y h a s c e n t e r e d on s o i l - s u r f a c e a p e r e l a t i o n s h i p s i n t h i s r e g i o n b e c a u s e l a t e r i t e p r o f i l e s are found a t v a r i o u s l e v e l s i n t h e l a n d s c a p e . T h e d e s i g n a t i o n o f h i g h and low l e v e 1 , p r i r ” a r y a n d s e c o n d a r y , and o l d e r and y o u n g e r l a t e r i t e s ( C a m p b e l l , 1 9 1 7 ; Woolnough, 1 9 1 8 ) i n h i b i t e d r e s o l u t i o n o f c o n t e n t i o u s i s s u e s f o r d e c a d e s . Based on i n v e s t i g a t i o n s a l o n g w e s t e r n c r a t o n i c m a r g i n s , P l a y f o r d (1954) s u g g e s t e d t h a t l a t e r i t e s o f m a j o r u p l a n d and l o w l a n d s u r f a c e s were o f t h e same a g e h a v i n g formed a f t e r u p l i f t and p a r t i a l d i s s e c t i o n o f upl a n d s . P r i d e r ( 1 9 6 6 ) and F i n k 1 (1971), w o r k i n g i n d i f f e r e n t a r e a s , concurred w i t h Playford’s f i n d i n g s . Buried under sand p l a i n d e p o s i t s and b r o a d f l o o d w a y s c o n t a i n i n g wash m a t e r i a l s , t h e deex, w e a t h e r i n g m a n t l e i n t h e c r a t o n i c h i n t e r l a n d went l a r g e l y u n n o t i c e d . ? l a y f o r d ’ s
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The s u c c e s s i o n o f c l a s t i c a n d in s i t u m a t e r i a l s i n s h a l l o w dissections is similar t o upland valleys except f o r the o c c u r r e n c e of e a r t h s and s a p r o l i t e s on s i d e s l o p e s , t h e former b e i n g s e p a r a t e d from s u b s t r a t e s by a s t o n e l i n e .
d e d u c t i o n was e s s e n t i a l l y c o n f i r m e d f o r t h e c r a t o n i c i n t e r i o r when Churchward (1977) found m o t t l e d and p a l l i d zones, under g r e a t t h i c k n e s s e s o f s u r f i c i a l d r i f t , c o n t i n u o u s l y l i n k i n g m a j o r geomorDhic s u r ‘aces. Young ( 1 9 7 0 ) , w o r k i n g i n e a s t e r n A u s t r a l i a , a l s o s u g g e s t e d a Fost-uplift age f o r the l a t e r i t i c duricrust. The d e e p w e a t h e r i n g m a n t l e t h u s s e e m s t o b e p o s t - u p l i f t i n aRe. E o c e n e s e d i m e n t s o v e r l a p p i n g t h e downwarped s o u t h e r n m a r g i n of t h e c r a t o n a r e l a t e r i t i z e d a n d c o n s e q u e n t l y t h e W a r r a n i a n i Pedoderm c a n b e n 3 o l d e r . O l i g o c e n e i n d u r a t i o n o f e x p o s e d m o t t l e d a n d p a l l i d z o n e s no doubt i n h i b i t e d subsequent l a t e r i t i z a t i o n . Degradation of deeply w e a t h e r e d s u b s t r a t e s and d u r i c r u s t e d r e s i d u a l s , p a r t i c u l a r l y as i t r e l a t e s p r o d u c t s o f s o i l and c r u s t f o r m a t i o n t o s t r i p p e d s u r f a c e s of v a r i o u s a g e s , has b e e n t h e s u b j e c t o f d e t a i l e d i n v e s t i g a t i o n s a s sumr r - a r i z e d by S t e p h e n s ( 1 9 4 6 , 1 9 7 1 ) ; Mulcahy ( 1 9 6 0 , 1 9 6 7 ) ; M a b b u t t ( 1 9 6 1 ) ; C h u r c h w a r d ( 1 9 7 0 ) ; F i n k l & C h u r c h w a r d (1973); Mulcahy & C h u r c h w a r d ( 1 9 7 3 ) ; and F i n k l ( 1 9 7 9 ) . STONE LINES, SURFACES, AND S O I L S S t o n e l i n e s o c c u r as dominant s u b s u r f a c e f e a t u r e s i n d i s s e c t e d l a n d s c a p e s , t h e i r p r e s e n c e b e i n g v e r i f i e d by o b s e r v a t i o n s i n r o a d c u t s end o l d q u a r r i e s . They a p p e a r a s l e n s e s o f c o a r s e c l a s t i c m a t e r i a l
332
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-
Earths on v a l l e y s i d e s l o p e s i n deep d i s s e c t i o n s a r e s e p a r a t e d by s t o n e l i n e s f r o m d a r k - c o l o r e d c l a y e y l e n s e s w h i c h in t u r n o v e r l i e s t o n e l i n e s s e t i n s a p r o l i t e o r mottled zones. Valley laterites c a n b e t r a c e d t o u p l a n d o c c u r r e n c e s by f o l l o w i n g longitudinal v a l l e y p r o f i l e s upstream. The s t r a t i g r a p h i c s u c c e s s i o n o f i n situ w e a t h e r i n g z o n e s a n d c o - a l l u v i a l b o d i e s e s t a b l i s h e s a r e l a t i v e a g e sequence. S o i l s developed i n each l a y e r are coeval i n a g e a n d a l l f a c i e s , w h i c h c o l l e c t i v e l y make u p pedoderms, are ranked according t o relative ages, from o l d e s t t o y o u n g e s t , from t h e W a r r a n i a n i Pedod e r m t h r o u g h t h e T i t t i b i n u p P e d o d e r m t o t h e Minn i j u p Pedoderm.
( q u a r t z , d o l e r i t e , g n e i s s , a n d a m p h i b o l i t e f r a g m e n t s ) and a r e commonly emplaced on t r u n c a t e d s o i l p r o f i l e s (Chruchward, 1970). B u r i e d s o i l s u s u a l l y l a c k A h o r i z o n s a n d much o f t h e B h o r i z o n f r e q u e n t l y h a s b e e n s t r i p p e d away. Transported m a t e r i a l s , t y p i c a l l y finer-textured strati f i e d c o l l u v i u m f r o m u p s l o p e , s i m i l a r t o t h e g r & z e s l i t 6 e s and 6 b o u l i s a r d o n g e s o f p e r i g l a c i a l r e g i o n s a s d e s c r i b e d b y G u i l l i e n (1954) and T r i c a r t (1970), o v e r l i e t r u n c a t e d s a p r o l i t e . Downslope t h e s t o n e l i n e s d i s t a l l y decrease i n particle-size pinching out t o Piner-textured coll u v i a l toe slope or a l l u v i a l deposits. I n t h e s h a l l o w d i s s e c t i o n s , one n e a r s u r f a c e s t o n e l i n e i s u s u a l l y b u r i e d b y a s i n g l e c o l l u v i a l u n i t which i n t u r n i s a f f e c t e d by cont e m p o r a r y s o i l - f o r m i n g p r o c e s s e s p r o d u c i n g a s t i l l a c t i v e Holocene pedon ( F i g u r e 9 ) . A l t h o u g h m u l t i p l e l i n e s a r e e v i d e n t i n d e e p l y d i s s e c t e d t e r r a i n s , t h e most w i d e s p r e a d example i n c o r p o r a t e s two s t o n e l i n e s where f i n e - g r a i n e d c l a s t i c s a r e i n t e r p o s e d b e t w e e n t h e u p p e r and lower stone l i n e s .
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Figure 9
The H o l o c e n e p e d o n , d e v e l o p e d i n a c o l l u v i a l s l o p e d e p o s i t near Southampton Friclge,is s e p a r a t e d from an underlying weathering p r o f i l e by a s t o n e l i n e . T h e i n situ n a t u r e o f t h e w e a t h e r e d s u b s t r a t e i s i n d i c a t e d b y t h e i n t a c t q u a r t z v e i n s shown o n t h e lower l e f t hand s i d e of t h e photo.
In vertical section the stones appear lined-up in single file as more or less horizontal layers giving rise to their name (Kerr, 1881; Sharpe, 1938). In three dimensions they coalesce to sheets or pavements (Parizek & Woodruff, 1957; Ruhe, 1954). Their occurrence in tropical rainforests and semi-arid regions to mid-latitude and subpolar regions has prompted a variety of explanations. Attempts to explain the evolution of stone lines (see summaries in Young, 1976 and Jejk, 1980) generally incorporate pathways associated with (1) soil creep, a steady-state concept where stone lines indicate the lower limit of colluvial soil layers, (2) residual weathering where resistant quartzose materials are concentrated as a coarse lag deposit, ( 3 ) faunal pedoturbation, the selective removal of fine soil fractions from the subsurface to the surface leaving coarse fragments to form a distinct horizon, and (4) climatic change where coarse pedisediments are deposited during arid unstable phases and later covered by finertextured materials during more stable wetter phases. Examples of the biological hypothesis, advocated by proponents of faunal pedoturbation, commonly refer to the industrious activities of termites (Lee & Wood, 1971). It has been suggested, for example, that termites and a n t s selectively remove fine-textured materials to the ground surface eventu-. ally burying the stone pavements creating stone lines (Nye, 1955; De Villiers, 1965; Williams, 1968). The selective sorting process, as described by Holt e t al. (1980), involves the incorporation of finely graded subsoil materials, brought up from 1-2 m depth, into termite mounds. They further suggest that 20 cm thick A horizons of red and yellow earths in northern Queensland may have accumulated from erosional degradation of termiteria in 8 x lo3 yrs in a landscape where subsoils may be on the order of 2 x l o 6 yrs old. Springer (1958), on the otherhand, identifies an upward movement of stones in self-mulching desert soils containing clayey, montmorillonitic B .horizons. Repeated swelling and shrinking, due to wet and dry phases, apparently pushes stones upward. Essential features of the process have been demonstrated in the laboratory. ,
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Although d i f f e r e n t p r o c e s s e s have been a s c r i b e d t o t h e f o r m a t i o n o f s t o n e l i n e s i n s o i l s ( s e e s u m m a r y i n J e J & , 1980), most e x p l a n a t i o n s , recognizing t h a t stone l i n e s represent ecologically disruptive features i n t h e l a n d s c a p e , r e f e r t o t h e c o l l u v i a l n a t u r e o f t h e d e p o s i t s which a r e a l l o c h t k o n o u s w i t h r e s p e c t t o u n d e r l y i n g m a t e r i a l s ( B i g a r e l l a e t az., 1 9 6 5 ; AbSaber, 1 9 6 7 ; Ojanuga & W i r t h , 1 9 7 7 ) . S t o n e l i n e s a r e commonly r e g a r d e d a s b u r i e d e x a m p l e s o f what w e r e once e l u v i a l d e p o s i t s , s t o n e pavement o r l a g g r a v e l s formed a t t h e g r o u n d s u r f a c e , and a s s u c h a r e e v i d e n c e of s t r a t i f i c a t i o n i n s u r f i c i a l m a t e r i a l s . Buried s t o n e l i n e s on t h e West A u s t r a l i a n c r a t o n a p p e a r t o b e a n a l a g o u s t o t h e t y p e s obs e r v e d by Ruhe e t aZ. ( 1 9 6 7 ) e l s e w h e r e b e c a u s e t h e y c o n t a i n a v a r i e t y of u n r e l a t e d i i t h o r e l i c t s , d e f l e c t i n t a c t q u a r t z v e i n s and s m a l l d o l e r i t e d i k e s downslope e n t r a i n i n g t h e r o c k d e b r i s , d e c r e a s e i n p a r t i c l e s i z e t o w a r d s d i s t a l s e c t i o n s , f o l l o w a d e t a i l e d m i c r o - r e l i e f , cap t r u n c a t e d d e e p w e a t h e r i n g p r o f i l e s , a n d t l ; e m s e l v e s a r e o v e r l a i n by f i n e r - t e x t u r e d t r a n s p o r t e d m a t e r i a l s . These f e a t u r e s t h u s seem t o s i g n a l u n s t a b l e e r o s i v e p h a s e s i n l a n d s c a p e h i s t o r y where s t r i p p i n g e v e n t s a r e f o l l o w e d b y d e p o s i t i o n o f c o l l u v i a l m a t e r i a l s and s o i l development ( F i n k l , 1 9 8 2 ) . The s t o n e l i n e s h e l p e s t a b l i s h a r e l a t i v e a g e s e q u e n c e by d e l i n e a t i n g s t r a t i f i c a t i o n i n s o i l p a r e n t m a t e r i a l s . The f a c t t h a t s t o n e l i n e s and m a t e r i a l s above them o v e r l i e a v a r i e t y of s u b s t r a t e s ( F i n k l & Churchward, 1 9 7 6 ) i s f u r t h e r p r o o f o f c o l l u v i a l o r i g i n and t h a t t h e y mark p r i o r e r o s i o n s u r f a c e s which c o n s t i t u t e t i m e s t r a t i g r a p h i c boundaries. Micromorphological i n v e s t i g a t i o n s corr o b o r a t e many o f t h e s e d e d u c t i o n s ( s e e l a t e r d i s c u s s i o n s ) . GRAVEL/SAND-CLAY INTERFACES L a t e r i t i c g r a v e l s ( F e - p i s o l i t e s ) a r e w i d e l y d i s t . ? i b u t e d on u p l a n d v a l l e y s i d e s l o p e s . T h e s e d e p o s i t s c a n be t r a c e d t o a p r o v e n a n c e on d u r i c r u s t e d r e s i d u a l s and i n t e r f l u v e s where m o t t l e d zone m a t e r i a l s h a v e become i n d u r a t e d (Mulcahy e t al., 1 9 7 2 ) . D e g r a d a t i o n of d u r i c r u s t e d r e s i d u a l s ( b r e a k a w a y s ) and m a s s i v e l a t e r i t i c pavements r e l e a s e s g r a v e l - s i z e d m a t e r i a l s f o r t r a n s p o r t d o w n s l o p e . The c o l l u v i a l n a t u r e o f t h e g r a v e l s h a s b e e n e s t a b l i s h e d f o r w i d e l y s e p a r a t e d r e g i o n s on t h e c r a t o n (Mulcahy, 1 9 6 7 ; Churchward, 1 9 7 0 ; F i n k l , 1 9 7 9 ) where t h e y o v e r l i e m o t t l e d and p a l l i d z o n e s , s a p r o l i t e s , and r o c k . Abrupt wavy bounda r i e s c h a r a c t e r i z e t h e g r a v e l - c l a y i n t e r f a c e where p i s o l i t i c accumulat i o n s o v e r l i e p o r t i o n s of t h e deeD w e a t h e r i n g z o n e . The r e l a t i o n s h i p h a s b e e n i d e n t i f i e d a s l i t h o l o g i c a l i n c o m p a t a b i l i t y between two s o i l s t r a t i g r a p h i c u n i t s ( g r a v e l f a c i e s o f t h e M i n n i j u p Pedoderm o v e r l y i n g B a l l i j u p m a t e r i a l s o f t h e W a r r a n i a n i Pedoderm) and d e m o n s t r a t e d u s i n g micromorphological techniques ( F i n k l & Gilkes, 1 9 7 6 ) . Similar r e l a t i o n s h i p s were o b s e r v e d by Mulcahy ( 1 9 6 4 ) and Brewer & B e t t e n a y ( 1 9 7 3 ) i n t h e i r i n v e s t i g a t i o n s o f W e s t e r n A u s t r a l i a n s p i l l w a y s and some p l a i n s i n the arid cratonic i n t e r i o r . The s u p e r p o s i t i o n o f p i s o l i t i c g r a v e l s or s a n d s p l a y s o v e r d e e p l y weathered m a t e r i a l s i s t h u s r e c o g n i z e d as a l i t h o l o g i c a l d i s c o n t i n u i t y . P e d o g e n e s i s was nowhere o b s e r v e d t o t r a n s g r e s s t h e s e b o u n d a r i e s ( s e e s u b s e q u e n t d i s c u s s i o n ) and as s u c h t h e y a l s o r e p r e s e n t p e d o l o g i c a l d i s c o n t i n u i t i e s . I n t e r v e n i n g sediment s e p a r a t e s t h e v e r t i c a l succession o f s o i l p r o f i l e s , a s i s commonly o b s e r v e d i n l o e s s or a l l u v i a l s e c t i o n s ( F i n k , 1 9 6 9 ; P e E s i , 1 9 7 5 ; M o r r i s o n , 1 9 7 8 ) . I n o t h e r c a s e s where s t r a t i g r a p h i c u n i t s a r e m o d e r a t e l y t h i n and s o i l development c o m p a r a t i v e l y i n t e n s e , p e d o g e n i c p h a s e s may b e p a r t i a l l y imposed one on t o p o f t h e o t h e r . The mergence o f a g r o u n d s u r f a c e solum w i t h a b u r i e d s o i l h a s b e e n r e f e r r e d t o a s p e d o g e n i c o v e r p r i n t i n g ( F i n k l , 1 9 8 0 ) and by Ruhe & Olson ( 1 9 8 0 ) as s o i l w e l d i n g . P e d o - l i t h o l o g i c a l b o u n d a r i e s t h u s h e l p d e l i n e a t e p a r e n t m a t e r i a l s and p h a s e s o f s o i l development p r o v i d i n g a n a d d i t i o n a l basis f o r estimating r e l a t i v e s o i l ages. M A C R O M O R P H O L O G I C A L DEPTH FUNCTIONS
Many s o i l s e c t i o n s i n t h e r e g i o n show m a s s i v e s u r f a c e e a r t h s o v e r l y i n g s t r u c t u r e d s u b s o i l s . The e a r t h s , f i n e - g r a i n e d d i s t a l end members of c o l l u v i a l s e q u e n c e s on v a l l e y s i d e s l o p e s , a r e r e g a r d e d as a f a c i e s o f t h e M i n n i j u p Pedoderm. The n a t u r e o f t h e d i s c o n t i n u i t y s e p a r a t i n g t h e e a r t h s from u n d e r l y i n g s u b s o i l s was p a r t l y e s t a b l i s h e d by a n a l y z i n g
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macromorphological depth f u n c t i o n s . Because t h e s p a t i a l d i s t r i b u t i o n o f s o i l c h a r a c t e r i s t i c s i s depthd e p e n d e n t , i t i s p o s s i b l e t o g r a p h t h e v e c t o r i a l p r o p e r t i e s of s e l e c t e d f e a t u r e s d o w n p r o f i l e . S u b s t a n c e s which e a s i l y m i g r a t e i n t h e s o i l , e . g . organ!.,: m a t t e r , s o l u b l e s a l t s , c o l l o i d a l p a r t i c l e s , s e s q u i o x i d e s , as w e l l a s c h a n g e s i n m a c r o m o r p h o l o g i c a l p r o p e r t i e s , a r e amenable t o a n a l y s e s where d e p t h i s p l o t t e d a l o n g t h e v e r t i c a l a x i s and t h e p r o p o r t i o n , r e l a t i v e f r e q u e n c y , or d e g r e e o f development i s shown a l o n g t h e h o r i z o n t a l a x i s where t h e s c a l e or o r d e r o f m a g n i t u d e i s r e l a t i v e t o e a c h f e a t u r e . P l o t s of s p e c i f i c “ d e p t h f u n c t i o n s , ” as J e n n y (1941) r e f e r r e d t o them, show s i m i l a r t r e n d s w i t h d e p t h i n m o n o g e n e t i c s o i l s . Abrupt c h a n g e s or m u l t i p l e p e a k s i n t h e d i s t r i b u t i o n p a t t e r n s r e q u i r e a d d i t i o n a l e x p l a n a t i o n . C h u r c h w a r d ‘ s ( 1 9 6 1 ) a n a l y s i s of macromorphol o g i c a l d e p t h f u n c t i o n s ( w a x i n g a n d waning t r e n d s i n p e d a l i t y , c l a y s k i n s , c o h e r e n c e , and p o r o s i t y ) f o r V i c t o r i a n soils a t Swan H i l l showed t h a t such f e a t u r e s can be used t o i d e n t i f y pedogenic d i s c o n t i n u i t i e s i n s o i l sections. The t e r m e a r t h , a s a p p l i e d p e d o g e n i c a l l y i n A u s t r a l i a , r e f e r s t o p o r o u s s o i l m a t e r i a l s t h a t c o n t a i n s t a b l e crumbs which r e s i s t c l o d formation. The p l a s m a i s d o m i n a n t l y f l o e i n m a t e r i a l s e l s e w h e r e des c r i b e d by Kubiena (1953) a s r o t e r d g . B o u n d a r i e s s e p a r a t i n g t h e e a r t h s from u n d e r l y i n g w e a t h e r e d m a t e r i a l s a r e d i f f u s e compared t o t h o s e s e p a r a t i n g g r a v e l s and s a n d s from s u b c l a y s . The marked i n c r e a s e i n p e d a l i t y and c o h e r e n c e and d e c r e a s e i n p o r o s i t y i n t h e v i c i n i t y of t h e i n t e r f a c e , which s p a n s a v e r t i c a l d i s t a n c e o f s e v e r a l c e n t i m e t e r s , c l e a r l y s e p a r a t e s u n r e l a t e d s o i l m a t e r i a l s . Macromorphological d e p t h f u n c t i o n s , a s d i s p l a y e d i n F i g u r e 1 0 , a r e s e e n as a n o t h e r p a r a m e t e r f o r d i f f e r e n t i a t i n g s o i l l a y e r s which a r e s e p a r a t e d i n t i m e 2nd s p a c e .
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M a c r o m o r p h o l o g i c a l d e p t h f u n c t i o n s f o r t h e Redshaw soil profile. Abrupt changes o r breaks i n t r e n d s f o r s o i l s t r u c t u r e , c o n s i s t e n c e , and p o r o s i t y a t stone l i n e s suggest t h e presence of unrelated s o i l materiCals i n s t a c k e d p r o f i l e s .
336
REVERSE WEATEERING DIFFERENTIALS The p r o g r e s s i v e d e c r e a s e i n w e a t h e r i n g i n t e n s i t y w i t h d e p t h , t h r o u g h t h e solum t o t h e b a s e of t h e p r o f i l e a t t h e w e a t h e r i n g f r o n t , t y p i f i e s " n o r m a l " s o i l d e v e l o p m e n t ( J e n n y , 1 9 4 1 ; N i k i f o r o f f , 1959). E x c e p t i o n s i n c l u d e , f o r e x a m p l e , (1) w e a k l y d e v e l o p e d s o i l s w i t h l i t t l e c h a n g e i n m i n e r a l o g y or p e d o c h e m i c a l c o m p o s i t i o n w i t h d e p t h a n d (2) l a t e r i t e d e e p w e a t h e r i n g p r o f i l e s where p e d o l o g i c a l w e a t h e r i n g g r a d u a l l y g i v e s way t o g e o c h e m i c a l a n d o t h e r e p i d i a g e n e t i c p r o c e s s e s w h i c h r o t t h e p a r e n t r o c k s t o d e p t h s o f 3 0 , 5 0 , or 1 0 0 m or m o r e ( H o l z e r & Weaver, 1 9 6 5 ) . P a r t i c u l a r l y troublesome i s weathered material which has been t r a n s p o r t e d a n d d e p o s i t e d a g a i n , f o r m i n g a new b u t e n t i r e l y w e a t h e r e d p a r e n t m a t e r i a l , t h e r o c h e f i l l e o f F r e n c h w o r k e r s . The w i d e s p r e a d occurrence o f preweathered p a r e n t materials i n t r o p i c a l and s u b t r o p i c a l r e g i o n s ( B u r i n g h , 1970) c o m p l i c a t e s t h e i n t e r p r e t a t i o n o f many s o i l p r o f i l e s e c t i o n s a s p o l y m o r p h i c or p o l y g e n e t i c . C u m u l a t i v e s o i l s a n d t h o s e where l e s s w e a t h e r e d m a t e r i a l s a t t h e s o i l s u r f a c e t r u n c a t e more s t r o n g l y weathered materials a t depth are a l s o i n c o n s i s t e n t with t h i s model. Continuous a d d i t i o n s of f r e s h g r u s from t o r s t o s u r f a c e s o i l l a y e r s i n u n g l a c i a t e d r e g i o n s of New Brunswick ( C a n a d a ) , f o r example, produce pronounced w e a t h e r i n g d i f f e r e n t i a l s . Colluvial grus with inh e r i t e d g i b b s i t e forms a s u r f a c e C h o r i z o n which o v e r l i e s t r u n c a t e d B h o r i z o n s o f b u r i e d s o i l s (Wang e t aZ., 1 9 8 1 ) . I n t r o p i c a l r e g i o n s , l a y e r s of p i s o l i t i c and/or f r e s h rock fragments overlying t h i c k c l a y h o r i z o n s , o r b r o w n ( 5 Y F 4 / 3 ) a n d r e d (2.5YR 4 / 6 ) c l a y s o v e r l y i n g mott l e d / p a l l i d z o n e s or s a p r o l i t e s s u g g e s t r e v e r s a l s of n o r m a l w e a t h e r i n g t r e n d s . These " r e v e r s e W e a t h e r i n g d i f f e r e n t i a l s " ( F i n k l & Churchward, 1 9 7 6 ) h a v e b e e n e x p l a i n e d i n t e r m s o f s o i l l a y e r i n g , s o i l p r o f i l e s , or s t r a t i g r a p h i c s u c c e s s i o n o f s o i l m a t e r i a l s , where y o u n g e r s o i l m a t e r i a l s a r e d e p o s i t e d on o l d e r w e s a t h e r e d s u b s t r a t e s . S u c h r e l a t i o n ships a r e w i d e s p r e a d i n t h e A u s t r a l i a n r e g i o n ( S t e p n e n s , 1 9 4 6 ; C h u r c h w a r d , 1 9 7 0 ) a n d a r e p a r t i c u l a r l y e v i d e n t on t h e w e a t h e r e d c r a t o n s (Brewer & B e t t e n a y , 1 9 7 3 ) . SOIL FABFIC ANALYSIS The s p a t i a l a r r a n g e m e n t o f d i s c r e t e m i n e r a l g r a i n s , c o m ~ o u n d p a r t i c l e s , and a s s o c i a t e d v o i d s i s d e s c r i b e d as s o i l f a b r i c a c c o r d i n g t o B r e w e r ' s ( 1 9 6 4 ) t e r m i n o l o g y . D i f f e r e n c e s i n t h e s i z e , s h a p e , and a r r a n g e m e n t of s k e l e t o n g r a i n s , s o i l p l a s m a , p e d o l o g i c a l f e a t u r e s , a n d v o i d s c h a r a c t e r i z e t h e n a t u r e o f i n d i v i d u a l s o i l m a t e r i a l s . The a n a l y s i s o f s o i l f a b r i c changes i n a s p a t i a l c o n t e x t , i.e. i n b o t h v e r t i c a l a n d h o r i z o n t a l d i r e c t i o n s , p r o v i d e s a n i n d e p e n d e n t means f o r s u b d i v i d i n g t h e s o i l continuum. Micromorphological t e c h n i q u e s are o f t e n c a p a b l e o f d e t e c t i n g s u b t l e changes n o t a l w a y s a p D a r e n t a t macro- and m e s o s c a l e s , a n d i n some c a s e s a r e t h e o n l y means o f d i f f e r e n t i a t i n g s o i l m a t e r i a l s w h i c h a p p e a r d e c e p t i v e l y sirnila? ( V e r s t r a t e c , 1980). D e t a i l e d s t u d i e s o f complex g r a n i t i c r e g o l i t h s i n n o r t h w e s t S p a i n , f o r e x a m p l e , showed c h a r a c t e r i s t i c p r o f i l e f a b r i c z o n a t i o n s w h i c h h e l p e d d i f f e r e n t i a t e younger s u r f a c e c o l l u v i u m from o l d e r s a p r o l i t i c subs t r a t e s ( B i s d o m , 1 9 6 7 ) . S k e l s e p i c p l a s m i c f a b r i c s were common i n c o l l u v i u m whereas t h e s a p r o l i t e s were c h a r a c t e r i z e d by s p h e r o i d a l w e a t h e r i n g c o n d i t i o n s by m i c r o - c r a c k n e t w o r k s . I n d e e p l y w e a t h e r e d c r a t o n i c r e g i o n s where similar s o i l materials are j u x t a p o s e d , m i c r o m o r p h o l o g i c a l t e c h n i q u e s a r e combined w i t h m a c r o m o r p h o l o g i c a l methods i n e f f o r t s t o unravel r e l a t i v e soil-age relationships. S o i l F a b r i c Zones The m i c r o m o r p h o l o g i c a l i n v e s t i g a t i o n o f some s i x t y s o i l - s t r a t i g r a p h i c s e c t i o n s on t h e West A u s t r a l i a n c r a t o n s e e m e d a t f i r s t t o i n d i c a t e a b e w i l d e r i n g a r r a y o f s o i l f a b r i c s . More d e t a i l e d c o n s i d e r a t i o n s , howe v e r , showed t h a t a b r u p t c h a n g e s i n s o i l f a b r i c c o i n c i d e d w i t h o t h e r s o r t s of s t r a t i g r a p h i c d i s c o n t i n u i t i e s o c c u r r i n g , f o r example, a t s t o n e l i n e s or g r a v e l - c l a y i n t e r f a c e s . When t r e n d s i n s o i l f a b r i c z o n a t i o n were s t u d i e d i n r e l a t i o n t o t h e p o s i t i o n i n g o f s o i l m a n t l e s , i t b e c a m e a p p a r e n t t h a t t h e v e r t i c a l s e q u e n c e s o f s o i l f a b r i c z o n e s were r e p e t a t i v e . L a t e r a l c h a n g e s i n s o i l f a b r i c were f o u n d t o r e f l e c t f a c i e s w i t h i n pedoderms ( F i n k l & G i l k e s , 1 9 7 6 ) .
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S o i l f a b r i c z o n a t i o n i n t h e K u r i n g i a (P579) s o i l p r o f i l e i s t y p i c a l o f many t a b l e l a n d s o i l s e c t i o n s w h e r e p i s o l i t i c g r a v e l s o v e r l i e m o t t l e d and p a l l i d zones ( b o r e h o l e s 2 and 3 i n F i g u r e 6 ) . Micromorphological p r o p e y t l e s o f t h e t y p e s e c t i o n a r e summarized i n t h e K u r i n g i a S o i l Prof i l e D e s c r i p t i o n ( A p p e n d i x ) . M i c r o m o r p h o l o g i c a l d e p t h f u n c t i o n s for a r g i l l a n s , n o d u l e s , a n d p a p u l e s a r e d e t a i l e d i n F i g u r e 11. The c o i n c i d e n c e of a b r u p t c h a n g e s i n t h e o c c u r r e n c e a n d d i s t r i b u t i o n p a t t e r n s o f p e d o l o g i c a l f e a t u r e s a t t h e g r a v e l - c l a y i n t e r f a c e s u p p o r t s t h e cont e n t i o n t h a t s e p a r a t e e p i s o d e s of p e d o g e n i c d e v e l o p m e n t a r e e v i d e n t i n the profile. The S - m a t r i x o f t h e M i n n i j u p Dedoderm ( g r a v e l f a c i e s ) i s c h a r a c t e r i z e d by a d a r k b r o w n u n d u l i c p o r p h y r o s k e l i c f a b r i c w i t h a g glomerated plasma. C o n s i d e r a t i o n of s o i l f a b r i c s a t t h e l e v e l of r e l a t e d d i s t r i b u t i o n of f - m a t r i x ( f i n e r m a t e r i a l ) t o f-members ( c o a r s e r m a t e r i a l s ) ( B r e w e r , 1979) d i s c l o s e s a g r a d a t i o n f r o m m u l l g r a n i c t o mullgranoidic fabrics i n these surface horizons. Abundant o r g a n i c f r a g m e n t s a s w e l l a s f a e c a l p e l l e t s o c c u r to a d e p t h of a b o u t 4 5 cm. The s e c o n d f a b r i c z o n e , c o r r e s p o n d i n g more or l e s s w i t h t h e B2cn h o r i zon, i s a n i s e p i c - i n u n d u l i c p o r p h y r o s k e l i c i n t e r g r a d e which becomes r e d d e r i n c o l o r , d e n s e r , a n d more s e p i c w i t h d e p t h . T h i s f a b r i c zone a b r u p t l y t e r m i n a t e s a t a b o u t 7 2 em, w h i c h m a r k s t h e u p p e r p o r t i o n of a t r u n c a t e d m o t t l e d z o n e . The m o t t l e d y e l l o w to b u f f c o l o r e d s o i l mat e r i a l s show a s t r o n g l y i n s e p i c p o r p h y r o s k e l i c f a b r i c w i t h b r o a d z o n e s of m a s e p i c a n d s m a l l e r p a t c h e s o f u n d u l i c D o r p h y r o s k e l i c f a b r i c s . A r g i l l a n s a r e more a b u n d a n t t h a n i n f a b r i c zone 2 where R 2 0 3 ( s e s q u i o x i d i c ) n o d u l e s show a m a r k e d d e c r e a s e i n f r e q u e n c y . The c o m p l e x i t y of f a b r i c zone 3 i n c r e a s e s w i t h d e p t h as t h e i n t e n s i f i e d m o t t l i n g p r o d u c e s ma-mosepic p a t c h e s . A t a b o u t 1 6 0 cm t h i s z o n e g i v e s way to a p a l e - . c o l o r e d s t r o n g l y w e a t h e r e d rocl. f a b r i c w i t h i n c l u d e d p e d o l o p i c a l f e a t u r e s s u c h as v o i d a r p i l l a n s and p a n u l e s ( F a b r i c zone 4 ) .
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Micromorphological depth functions f o r the Kuringia s o i l p r o f i l e on a t r u n c a t e d margin of t h e Blackwood T a b l e l a n d show m a r k e d c h a n g e s i n d i s t r i b u t i o n p a t t e r n s a t the gravel-clay i n t e r f a c e (72 cm). Rock a n d s o i l n o d u l e s are c o n f i n e d t o t h e u p p e r s o i l p r o f i l e while R203 nodules, which occur i n t h e younger s u r f a c e s o i l and o l d e r buried laterite, are most f r e q u e n t i n s u r f a c e h o r i z o n s b u t show a marked d e c r e a s e a t t h e d i s c o n t i n u i t y . Void a r g i l l a n s show two f r e q u e n c y maxima, o n e i n e a c h p r o file. Dark-brown u n d u l i c p o r p h y r o s k e l i c f a b r i c s w i t h a g g r e g a t e d plasma g r a d e t o d e n s e r , more b i r e f r i n g e n t , r e d colored s o i l materials i n f a b r i c z o n e 2 (B2cn h o r i z o n ) . Strongly insepic yellowcolored porphyroskelic fabrics i n the truncated laterite grade t o weathered rock f a b r i c s i n zone 4 .
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The Redshaw (P577) s o i l p r o f i l e ( t y p e s e c t i o n 1 3 i n F i g u r e 8 ) o f f e r s g r e a t e r complexity w i t h f a c i e s of t h e Minnijup, T i t t i b i n u p , and W a r r a n i a n i Pedoderms o c c u r r i n g i n t h e s e c t i o n . Ma c r o mo rp h o lo g ic a l d e s c r i p t i o n s o f e a c h p e d o d e r m , s e p a r a t e d by s t o n e l i n e s , a r e g i v e n ' i n t h e Redshaw S o i l P r o f i l e D e s c r i p t i o n ( A p p e n d i x ) . The M i n n i j u p Pedoderm c o n t a i n s two f a b r i c zones ( F i g u r e 1 2 ) which c o r r e l a t e w i t h t h e pedog e n i c A a n d B h o r i z o n s . The f o r m e r i s c h a r a c t e r i z e d by a p o r o u s , d a r k colored inundulic porphyroskelic f a b r i c with agglomerated plasma. Organic f r a g m e n t s , f a e c a l p e l l e t s , and o p a l p h y t o l i t h s o c c u r throughout t h e zone. The p l a s m a becomes somewhat more b i r e f r i n g e n t w i t h d e p t h t e n d i n g toward a r g i l l i s e p i c e l e m e n t s . F a b r i c zone 2 i s e s s e n t i a l l y i n s e p i c p o r p h y r o s k e l i c b u t c o n t a i n s weak s k e l - m a m o s e p i c s e p a r a t i o n s a n d broad masepic zones i n t h e B2. I t i s s e p a r a t e d f r o m f a b r i c z o n e 3 by a m a s s i v e a c c u m u l a t i o n o f r o c k n o d u l e s . The l i t h o r e l i c t s a r e p r e dominantly weathered a m p h i b o l i t e and c o n t a i n i n c l u d e d a r g i l l a n s . F a b r i c zone 3 i s d e n s e r , r e d d e r i n c o l o r , and l a c k s o p a l p h y t o l i t h s and o t h e r o r g a n i c r e m a i n s . The z o n e i s d o m i n a t e d by i n s e p i c p o r p h y r o s k e l i c f a b r i c s b u t c o n t a i n s weak m o s e p i c i n c l u s i o n s . A massive b u i l d u p of n o d u l e s o c c u r s a t a b o u t t h e 1 7 0 cm l e v e l f o r m i n g a v e r y d e n s e a n d corn-p a c t l a y e r of l i t h o r e l i c t s . W e a t h e r e d r o c k f r a g m e n t s a n d R203 n o d u l e s o c c u r a t t h e b a s e o f z o n e 3 . Basal f a b r i c s g r a d e f r o m weak m a - i n s e p i c p o r p h y r o s k e l i c i n zone 4 t o r e d and y e l l o w m o t t l e d w e a t h e r e d r o c k f a b r i c s i n zone 5 . M u l t i p l e peaks i n t h e d i s t r i b u t i o n p a t t e r n s of'pedol o g i c a l f e a t u r e s or a b r u p t t e r m i n a t i o n s o f i n d i v i d u a l t r e n d s a t s t o n e l i n e s s u g g e s t t h e p r e s e n c e of u n r e l a t e d s o i l m a t e r i a l s .
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Figure 1 2
M i c r o m o r p h o l o g i c a l d e p t h f u n c t i o n s f o r t h e Redshaw soil profile. Stone l i n e s separate the three pedoderms. Fabric zonation is partly related to i n t e n s i t y o f p r o f i l e development w i t h i n pedoderms and t o t h e p r e s e n c e of u n r e l a t e d s o i l materials. Prominent breaks i n f a b r i c t r e n d s occur a t t h e d i s c o c t i n u i t i e s as do marked c h a n g e s i n f r e q u e n c y distribution patterns for pedolopical features.
S o i l M a t e r i a l s , F a c i e s , a n d R e l a t i v e Age S e q u e n c e s A l t h o u g h t h e K u r i n g i a a n d Redshaw p r o f i l e s a r e b u t two o f many examples d e p i c t i n g v e r t i c a l ch a n g e s i n s o i l f a b r i c , i t s h o u l d b e app r e c i a t e d t h a t f a b r i c zonation c o i n c i d e s w i t h d i s c o n t i n u i t i e s observed i n the field. A d d i t i o n a l s e c t i o n s from d i f f e r e n t landscape p o s i t i o n s
( s e e F i g u r e s 6 , 7 , and 8 ) p r o d u c e d a r a n g e o f f a b r i c s a s s o c i a t e d w i t h e a c h pedoderm. The l a t e r a l c h a n g e s i n s o i l f a b r i c t h u s r e p r e s e n t f a c i e s c h a n g e s by r e f l e c t i n g d i f f e r e n c e s i n t h e n a t u r e o f p r e w e a t h e r e d p a r e n t m a t e r i a l s , a c c u m u l a t i o n of o r g a n i c m a t t e r , and d r a i n a g e c o n d i t i o n s . C o n s i d e r a t i o n of s o i l a s a m a t e r i a l ( H a l l , 1945; P a t o n , 1978), as opposed t o s o i l a s a n i n d i v i d u a l , f a c i l i t a t e s t h e o r g a n i z a t i o n of s o i l f a b r i c s i n d e s c r i p t i v e t e r m s . The term "soma," d e r i v e d from soil m a t e r i a l a s s u g g e s t e d by R . Brewer ( p e r s . comm.), c o u l d b e c o m b i n e d w i t h a g e o g r a p h i c name t o d e s c r i b e a g i v e n r a n g e of s o i l m a t e r i a l s . Grisoma ( a f t e r Grimwade t o w n s h i p ) , f o r e x a m p l e , i d e n t i f i e s s o i l m a t e r i a l ass o c i a t e d w i t h t h e M i n n i j u p Pedoderm ( F i g u r e 1 3 ) . S i x v a r i e t i e s o f Grisoma commonly o c c u r on t h e West A u s t r a l i a n c r a t o n , namely: n o r m a l , p e l l e t e d , a r g i l l a s e p i c , o r g a n i c , g r a v e l l y , and n o d u l a r . Characterist i c s o f t h e s e m a t e r i a l s a r e summarized i n T a b l e 1. P r o p e r t i e s a s s o c i a t e d w i t h B a l i s o m a , summarized i n T a b l e 2 , c h a r a c t e r i z e s o i l f a b r i c s i n t h e T i t t i b i n u p Pedoderm ( F i p u r e 1 1 1 ) .
Figure 13
Normal Grisoma i n t h e M i n n i j u p Pedoderm. The organic-rich agglomerated plasma c h a r a c t e r i z e s surface s o i l materials. N o t e t h e s o i l n o d u l e (SN) d e r i v e d from p a l l i d zone materials * a r t h e r UQs l o p e a n d p l a s m a i n f u s i o n ( P I ) i n embayed q u a r t z grains. (Plane l i g h t 2.5X)
M o t t l e d zone m a t e r i a l s a r e y e l l o w t o b u f f c o l o r e d i n r e f l e c t e d l i g h t . I n s e p i c p o r p h y r o s k e l i c f a b r i c s commonly s u r r o u n d opaque m o t t l e s which c o n t a i n i n c l u d e d a r g i l l a n s . Weak ma-mosepic f a b r i c s a r e i n t e r s p e r s e d w i t h o t h e r complex f a b r i c z o n e s . P a l e - c o l o r e d s o i l m a t e r i a l s i n t h e lower p o r t i o n s of t h e l a t e r i t e p r o f i l e s have p r e d o m i n a n t l y i n s e p i c p o r p h y r o s k e l i c f a b r i c s w i t h weak m o s e p i c z o n e s . B l e a c h e d m i n e r a l g r a i n s , e s p e c i a l l y e x f o l i a t e d m i c a s , and l o c a l i z a t i o n o f s e s q u i o x i d e s a r e a l s o common f e a t u r e s . S a p r o l i t i c f a c i e s c o n t a i n w e a t h e r e d r o c k f a b r i c s sometimes w i t h i n c l u d e d s o i l f a b r i c s and p e d o l o g i c a l f e a t u r e s . A f t e r t h e s u c c e s s i o n o f s o i l m a t e r i a l s h a s been c o r r e l a t e d w i t h pedoderms, s o i l f a b r i c s t h e m s e l v e s c a n be u s e d t o i n d i c a t e s o i l l a y e r i n g which i n t u r n i m p l i e s a g e d i f f e r e n t i a l s . Every a t t e m p t h a s b e e n made t o a v o i d a c i r c u Z u s vitiosus b u t once t h e c o r r e l a t i o n o f macro- and m i c r o - m o r p h o l o g i c a l f e a t u r e s has b e e n made, t h e r e i s no r e a s o n t o i g n o r e i t . Agglomerated, o r g a n i c - r i c h , u n d u l i c - i n u n d u l i c p o r p h y r o s k e l i c
340
Figure 14
Normal B a l i s o m a from t h e b u r i e d T i t t i b i n u p Pedoderm s h o w i n g ma-mosepic p o r p h y r o s k e l i c f a b r i c w i t h The d e n s e r e d - c o l o r e d p l a s m a void a r g i l l a n s (VA). i s p a r t i a l l y masked by s e s q u i o x i d i c a c c u m u l a t i o n s (dark patches). (Plane l i g h t lox)
f a b r i c s a r e always a s s o c i a t e d w i t h t h e Y i n n i j u p Pedoderm. G r a v e l l y or n o d u l a r v a r i e t i e s o f Grisoma a r e a s s o c i a t e d w i t h a l a t e r i t i c p r o v e n a n c e on u p l a n d v a l l e y s i d e s l o p e s and c o l l u v i a l t o e s l o p e s i n d i s s e c t i o n s . Normal, p e l l e t e d , or o r g a n i c v a r i e t i e s o c c u r on d i s s e c t e d s l o p e s whereas a r g i l l a s e p i c v a r i e t i e s i n d i c a t e a t h i n n i n g o f t h e pedoderm w i t h more s e p i c m a t e r i a l s l y i n g n o t f a r below. The s u c c e s s i o n of s o i l m a t e r i a l s i s summarized i n F i g u r e 15 f o r k e y l a n d s c a p e p o s i t i o n s . V a r i e t i e s of Gris0m.a o c c u r e v e r y w h e r e a t t h e g r o u n d - s u r f a c e and r e p r e s e n t t h e y o u n g e s t s o i l m a t e r i a l s . D e n s e r , more b i r e f r i n g e n t s o i l f a b r i c s showing g r e a t e r p e d o l o g i c a l o r g a n i z a t i o n a l w a y s o c c u r a s a s u b s u r f a c e l a y e r below t h e u p p e r s t o n e l i n e s i n d i s sections. I n t e n s e l y weathered s u b s t r a t e s mantle a l l major landscape p o s i t i o n s a n d a r e o v e r l a i n by weak t o m o d e r a t e l y w e l l d e v e l o p e d s o i l fabrics. Age of s o i l m a t e r i a l s i s t h u s i n f e r r e d from m i c r o m o r p h o l o g i c a l d a t a which d e l i n e a t e s o i l f a b r i c z o n e s and t h e n a t u r e o f s o i l m a t e r i a l s . Use o f s o i l f a b r i c a n a l y s i s on a n ad h o e b a s i s i s n o t o n l y f e a s i b l e b u t a n a p p r o p r i a t e means f o r e s t i m a t i n g r e l a t i v e s o i l a g e . SKELETON G R A I N MORPHOL,OGY AND A G E OF SOILS S k e l e t o n g r a i n s of a s o i l m a t e r i a l , a s d e f i n e d by Brewer and Sleeman (1960), i n c l u d e i n d i v i d u a l m i n e r a l g r a i n s and r e s i s t a n t s i l i c e ous or o r g a n i c b o d i e s l a r g e r t h a n c o l l o i d a l s i z e . A l t h o u g h r e l a t i v e l y s t a b l e , most g r a i n s i n humid t r o p i c a l e n v i r o n m e n t s show t h e e f f e c t s o f b i o c h e m i c a l w e a t h e r i n g p r o c e s s e s , sometimes p a s s i n g t h r o u g h a p a p u l a r s t a g e a s i n t h e c a s e o f some f e l d s p a r s ( F i g u r e 16), t o form s o i l plasma. S u r f a c e t e x t u r e s and g r a i n s h a p e t h u s seem t o o f f e r c l u e s t o m i n e r a l p r o v e n a n c e , t h e g e o c h r o n o l o g y o f s t r o n g l y w e a t h e r e d t e r r a i n s , and t h e n a t u r e of w e a t h e r i n g phenomena i n t r o p i c a l p e d o c h e m i c a l m i c r o e n v i r o n ments ( C l e a r y and C o n o l l y , 1 9 7 2 ; e . g . Powers e t a l . , 1 9 7 9 ) . F e a t u r e s s u c h as s t r i a t i o n s , e t c h p a t t e r n s , d i s s o l u t i o n p i t s , q u a r t z overgrowths,
341
B
A
I
UPLAND DIVIDE
I
580
1
579
SIDE SLOPE
1
586
I
I
PLATEAU SURFACE VALLEY
SHALLOW
DISSECTION
I
FLOOR
~
603
I 50-
50-
100
100-
-
C DISSECT I 0N
DEEP CREST
601
Crests
599
& SPUR
Spurs
590
Valley NOSE
584
MIDDLE
577
Side
SLOPE
578
Slopes
576
FOOTSLOPE
600
-150
Figure 15 The succession of soil materials in key landscape positions ( A , B , C ) on the West Australian craton. Discontinuities in the numbered soil-stratigraphic sections are marked by stone lines (small circles) and abrupt changes in patterns of soil fabric zonation. Pisolitic grapels (1) and organic-rich agglomerated plasmas, referred to here as Grisoma ( 2 ) , are always associated with surface soils in the Minnijup Pedoderm. Denser, more strongly developed fabrics occur in buried soils. Sepic plasmic fabrics, characterized by Balisoma ( 3 ) , and "mica fabrics" ( 6 ) dominate the Tittibinup Pedoderm whereas substrates in the Warraniani Pedoderm contain mottled/pallid zone materials (MZ/PZ), pseudogranoblastic fabrics ( 5 ) , and weathered rock fabrics ( 4 ) . Macro- and micromorphological descriptions are given in Fink1 (1971) and are filed at the C . S . I . R . O . PerthLab.
342
Figure 16
P a p u l e f l o o d (F’F) s h o w i n g f e l d s p a r s a n d h o r n b l e n d e grains weathering t o clay. Note plasma i n f u s i o n (PI) i n mineral g r a i n s b u t general l a c k of a plasma groundmass i n t h i s weathered r o c k f a b r i c . (Plane l i g h t 40X)
s c a l i n g , s i l i c a f l o w e r s , p e l i i c i e s , e x f o l i a t i o n , and plasma i n f u s i o n a r e i n t e r p r e t e d a s i n d i c a t o r s n o t o n l y o f p r o v e n a n c e b u t a1’so o f w e a t h e r i n g r e g i m e s and r e l a t i v e s o i l age (Walker, 1969; Wilson, 1970; O j a n u g a , 1 9 7 3 ; D o u g l a s & F ’ l a t t , 1977; E s w a r a n & S t o o p s , 1979). I n s p i t e o f l o n g t e r m i n t e r e s t i n f e a t u r e s c o n t e n d e d t o be diagnost i c of t h e environmental h i s t o r y of t h e d e p o s i t ( e . 9 . Babington, 1 8 2 i ) , r e l a t i v e l y few s t u d i e s h a v e f o c u s e d a t t e n t i o n o n t h e m o r p h o l o g y o f w e a t h e r e d s k e l e t o n g r a i n s p r i o r t o e r o s i o n and t r a n s p o r t . The i n f l u e n c e o f p e d o c h e m i c a l w e a t h e r i n g i s , h o w e v e r , r e c o g n i z e d i n many s t u d i e s which a t t r i b u t e h i g h l y e t c h e d and rounded s o l u t i o n - w e a t h e r e d q u a r t z g r a i n s t o l o n g p e r i o d s of l e a c h i n g ( R a e s i d e , 1959; Crook, 1 9 6 8 : L i t t l e e t aZ., 1978). M i c r o b i a l d e g r a d a t i o n or w e a t h e r i n g by c o m p l e x i n g o r g a n i c a c i d s , a s commonly o c c u r s ir! d e e p r e g o l i t h s u n d e r t r o D i c a l r a i n f o r e s t conditions, produces a d i f f e r e n t sequence of mineral s t a b i l i t y t h a n t r a d i t i o n a l l y q u o t e d for p u r e w a t e r (Huang & K e l l e r , 1972; W i l s o n , 1 9 7 0 , 1 9 7 5 ; B e r t h e l i n & B e l g y , 1979) a n d i s p a r k i c u l a r l y i m p o r t a n t to f e l d s p a r s t a b i l i t y i n s o i l s d u e to p o t a s s i u m c h e l a t i o n by p l a n t s ( E v a n s , 1964). Van d e r Waals ( 1 9 6 7 ) f u r t h e r w a r n s t h a t c o r r o s i o n o f q u a r t z g r a i n s a n d d e p o s i t i o n o f s i l i c a on t h e m may b e m o r e t h e r e s u l t o f t h e micromilieu rather than age. Feldspar dissolution i n natural weathering i s c o n t r o l l e d b y pH ( N i x o n , 1 9 7 9 ) b u t g r a i n s w i t h s o l u t i o n p i t s a p p e a r t o r e p r e s e n t n e a r e n d members o f w e a t h e r i n g e n v i r o n m e n t s . B e r n e r e t al. (1980), o n t h e o t h e r h a n d , r e D o r t t h a t t h e n a t u r e o f e t c h f e a t u r e s on s o i l g r a i n s i s p r i m a r i l y c o n t r o l l e d by c r y s t a l l o g r a p h y a n d n o t t h e t y p e o f e t c h a n t . E t c h p i t s were o b s e r v e d to o c c u r o n t h e g r a i n s w h e r e d i s l o c a t i o n s o u t c r o p a n d , as m i g h t b e e x p e c t e d , r a t e s o f s e l e c t i v e e t c h i n g d i f f e r e d for c o e x i s t i n g g r a i n s i n t h e same s o i l . S e v e r a l e t c h mechanisms a r e t h u s a p p a r e n t l y c a p a b l e of p r o d u c i n g s i m i l a r s u r f a c e g r a i n t e x t u r e , and even though d i f f e r e n t d i s s o l u t i o n r a t e p r o c e s s e s may g i v e r i s e to d i f f e r e n t e q u i l i b r i u m s h a p e s . s e v e r a l (1) p a i e o s o l s may b e t h e s o u r c e o f generalizations s t i l l appear valid: s u p e r - m a t u r e s a n d s , (2) t e x t u r a l l y m a t u r e g r a i n s t e n d t o b e p r o d u c e d by chemical w e a t h e r i n g i n p h y s i o g r a p h i c s e t t i n g s which are t e c t o n i c a l i y
343
q u i e s c e n t , ( 3 ) s t r o n g l y w e a t h e r e d s k e l e t o n g r a i n s a r e p r o d u c t s of ext e n d e d b i o c h e m i c a l l e a c h i n g r e g i m e s , and ( 4 ) e t c h e d o r s o l u t i o n - r o u n d e d g r a i n s a r e p o l y c y c l i c and d i s t i n c t from " f i r s t c y c l e " g r a i n s ( I n g e r s o l l ,
1974).
S e v e r a l o f t h e f e a t u r e s a s s o c i a t e d w i t h t h e c h e m i c a l w e a t h e r i n g of m i n e r a l g r a i n s i n a t r o p i c a l e n v i r o n m e n t , s u c h as d e s c r i b e d b y Crook (1968), Doornkamp & K r i n s l e y (1971), Eswarari RC Sousa (i975), Eswaran & Heng (1976), Eswaran & Bin (1978), and Eswaran & S t o o p s (1979), were o b s e r v e d i n r e g o l i t h s on t h e West A u s t r a l i a n c r a t o n . G r a i n s showing t h e e f f e c t s o f e p i d i a g e n e t i c a l t e r a t i o n w e r e most a b u n d a n t n e a r t h e s o i l s u r f a c e i n t h e M i n n i j u p Pedoderm. I n t e n s e l y w e a t h e r e d g r a i n s were l e s s f r e q u e n t i n t h e T i t t i b i n u p Pedoderm and r a r e i n t h e u n d e r l y i n g a u t o c h t h o n o u s zone. I n t e n s i t y o f w e a t h e r i n g ( e t c h i n g ) , as w e l l as t h e f o r m a t i o n o f s o l u t i o n p i t s and embayments d e c r e a s e d w i t h d e p t h . Embayed s k e l e t o n g r a i n s and " r u n i q u a r t z , " s i m i l a r t o t h a t d e s c r i b e d by Eswaran and S t o o p s (1979), o f t e n o c c u r r e d w i t h i n l a r g e R,O, n o d u l e s , d e r i v e d from p e t r o p l i n t h i t e , b u t w e r e most commonly f r e e - f l o a t i n g and i n v a d e d by p l a s m a m a t e r i a l . O t h e r embayed or s t r e s s e d q u a r t z g r a i n s showed u n d u l o s e e x t i n c t i o n s w i t h neoformed q u a r t z i n expanded s o l u t i o n pits. I t i s i n t e r e s t i n g t o n o t e i n p a s s i n g t h a t many examples of t h e e t c h f e a t u r e s o b s e r v e d i n s o i l s k e l e t o n g r a i n s a l s o commonly o c c u r i n a c h r o n d i t e s ( m e t e o r i t e f r a g m e n t s l a c k i n g c h r o n d u l e s ) . Does t h e amusing p o s s i b i l i t y e x i s t t h a t some of t h e r u n i q u a r t z o b s e r v e d i n t h e M i n n i j u p Pedoderm i s a n i n h e r i t e d f e a t u r e o f i n t e r s t e l l a r o r i g i n , or have t e r r e s t r i a l ( p e d o - b i o g e o c h e m i c a l ) p r o c e s s e s and e x t r a t e r r e s t r i a l e t c h p r o c e s s e s produced s i m i l a r forms?
I n sum, t h e o l d e s t m i n e r a l g r a i n s , i . e . s t r o n g l y w e a t h e r e d s k e l e t o n g r a i n s w i t h m o d i f i e d s u r f a c e t e x t u r e s and embayments f i l l e d w i t h plasma, were p r e d o m i n a n t l y a s s o c i a t e d w i t h t h e y o u n g e s t s o i l m a n t l e ( M i n n i j u p nodules, Pedoderm). T h e o c c u r r e n c e o f t h e s e s t r e s s e d g r a i n s i n R,O, s o i l n o d u l e s , and l i t h o r e l i c t s s u g g e s t s d i v e r s e modes o f f o r m a t i o n and p o l y c y c l i c o r i g i n s . Weathered f r e e - f l o a t i n g g r a i n s ( r u n i q u a r t z ) n e a r t h e s u r f a c e a r e p r o b a b l y younger t h a n t h o s e s e t i n f r a g m e n t s of p e t r o p l i n t h i t e a n d may b e p r o d u c t s of s o i l - w a t e r r e l a t i o n s h i p s , a n a l a g o u s t o t h o s e d e s c r i b e d by Douglas and P l a t t (1977) i n a g l a c i a l t e r r a i n where A horizons dry out f a s t e r than subsoils. Embayed g r a i n s i n l i t h o r e l i c t s m a y r e p r e s e n t t h e r e m a i n s of s m a l l c o r e s t o n e s w h i c h s u r v i v e d i n t e n s e l a t e r i t i c w e a t h e r i n g . They may h a v e r e c e n t l y e n t e r e d t h e c o n t e m p o r a r y w e a t h e r i n g e n v i r o n m e n t a f t e r becoming e n t r a i n e d i n t h e s u r f a c e c o l l u v i a l s o i l m a n t l e . These f e a t u r e s a r e r a r e i n t h e i n s i t u w e a t h e r i n g zone below s t o n e l i n e s b e c a u s e most o f t h e g r a i n s h a v e w e a t h e r e d t o s e c o n d a r y m i n e r a l s or o c c u r a s p a p u l e s o r j u s t b l e a c h e d g r a i n s . DISCUSSION AND C O N C L U S I O N Modern s o i l s i n t h e d e e p l y w e a t h e r e d West A u s t r a l i a n c r a t o n a r e v a r i o u s l y d e v e l o p e d i n reworked c o l l u v i a l a n d a l l u v i a l d e p o s i t s o f d i f f e r e n t a g e s . Q u a t e r n a r y geomorphic a c t i v i t y has i n v o l v e d r e p e a t e d e r o s i o n and d e p o s i t i o n o f p r e w e a t h e r e d m a t e r i a l s t h a t were and p o s t f a c t o remain d e s c r i p t i v e l y similar. Superpositioning o f s o i l p r o f i l e s t o form s o i l - s t r a t i g r a p h i c s e q u e n c e s e s t a b l i s h e s a r e l a t i v e s o i l c h r o n o l o g y for t h e r e g i o n . B r e a k s or d i s c o n t i n u i t i e s i n s o i l s t r a t i g r a p h i c p r o f i l e s s e p a r a t e p e d o g e n i c e v e n t s which a r e c h a r a c t e r i z e d by a d i s t i n c t a r r a y o f s o i l m a t e r i a l s . R e l i a b l e r e l a t i v e a g e s e q u e n c e s depend on r e c o g n i t i o n o f c y c l i c p e d o g e n i c d e v e l o p m e n t s i n t h e s o i l m a n t l e b u t p r o o f s of e x i s t e n c e and s e p a r a t e i d e n t i t y , b o t h o f t e n taken f o r granted, a r e s t i l l b a s i c t o s t u d i e s of s o i l age. Incomplete s e q u e n c e s where p a r t s o f s e c t i o n s ( u s u a l l y u p p e r p o r t i o n s o f s o i l p r o f i l e s ) h a v e b e e n e r o d e d away, p e d o g e n i c o v e r p r i n t i n g ( s o i l w e l d i n g ) , and p r e w e a t h e r e d p a r e n t m a t e r i a l s a r e b u i l t - i n p r o b l e m s which must b e comprehended i n r e l a t i v e s o i l d a t i n g m e t h o d o l o g i e s . S t a g e o f S o i l Development I n t e n s i t y of s o i l p r o f i l e development w i t h i n and between pedoderms
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o f f e r s a d d i t i o n a l s c o p e f o r a s s e s s i n g s o i l a g e , A p p l i c a t i o n of M u i r ' s ( 1 9 6 9 ) P r i n c i p l e o f D e v e l o p m e n t a l S e q u e n c e s , w h i c h o r d e r s dynamic s o i l p r o c e s s e s ( l o s s e s f r o m h o r i z o n s a n d g a i n s t o h o r i z o n s ) i n terms o f t h o s e p a s s i n g f r o m l e a s t t o m o s t d e v e l o p e d , or S i m o n s o n ' s ( 1 9 5 9 ) g e n e r a l t h e o r y of pedogenesis leads t o c o n s i d e r a t i o n of s o i l age r e l a t i o n s h i p s b a s e d on s o i l p r o p e r t i e s . The o c c u r r e n c e o f s p a r i n g l y s o l u b l e s a l t s , iron-humus c o m p l e x e s , F e y A l - s e s q u i o x i d e s n e o f o r m e d q u a r t z , a n d ill u v i a t e d c l a y a r e , i n a g e n e r a l way, r e l a t e d t o i n c r e a s i n g p r o f i l e d e ve lopment . S o i l d e v e l o p m e n t i n H o l o c e n e p e d o n s o f t h e M i n n i j u p Pedoderm i n v o l v e s accumulation of o r g a n i c matter a t t h e s o i l s u r f a c e , a moderate degree of s t r u c t u r a l development, a g g r e g a t i o n of plasma, m o b i l i z a t i o n and minor s e g r e g a t i o n of i r o n and aluminum, and a moderate d e g r e e of c l a y i l l u v i a t i o n . Most o f t h e a r g i l l a n s a r e k a o l i n a n s a n d g o e t h a n s , a l t h o u g h some m o n t m o r i l l a n s a n d v e r m i c u l a n s a l s o o c c u r ( F i n k l , 1 9 7 1 ) . A s a matter of consensus, it has been observed t h a t w i t h i n c r e a s i n g a g e or p r o p o r t i o n o f p l a s m a , s o i l m a t e r i a l s t e n d t o h a v e p o r p h y r o s k e l i c f a b r i c s (Brewer & S l e e m a n , 1 9 6 9 ) . S o i l m a t e r i a l s i n t h e T i t t i b i n u p Pedoderm t h u s c o n t r a s t w i t h t h e u n d u l i c - i n u n d u l i c a g g l o m e r o p l a s m i c f a b r i c s i n t h e s u r f a c e M i n n i j u p P e d o d e r m . Amorphous s e s q u i o x i d i c o r g a n o a r g i l l a n s and secondary s i l i c a i n t h e form of g r a s s o p a l phytol i t h s a r e a l s o commonly a s s o c i a t e d w i t h u p p e r h o r i z o n s o f t h e s e y o u n g e r surface soils. Sesquioxidic glaebules, concretions (concentric f a b r i c ) and n o d u l e s ( u n d i f f e r e n t i a t e d f a b r i c ) , can o c c u r i n any k i n d of p r o f i l e as t h e y a r e i n h e r i t e d from p r e - e x i s t i n g s o i l s and s e d i m e n t s . D e r i v e d f r o m v e r y o l d d e g r a d e d l a t e r i t i c p r o f i l e s , Fe-Mn g l a e b u l e s now f o r m q u i t e l a r g e s u r f a c e a c c u m u l a t i o n s on u p l a n d v a l l e y s i d e s l o p e s a n d on s p u t n o s e s as p a r t o f t h e M i n n i j u p Pedoderm. S o i l s developed i n t h e s e g r a v e l l y d e p o s i t s show m i n i m a l p r o f i l e d e v e l o p m e n t . The m o d e r a t e g r a d e s o f f i n e a n d medium a n g u l a r b l o c k y s t r u c t u r e , degree of s o i l compaction, c l e a r h o r i z o n a t i o n s , s t r o n g l y s e p i c s o i l f a b r i c s , a n d w e l l d e v e l o p e d p e d o l o g i c a l f e a t u r e s i n t h e T i t t i b i n u p Pedoderm s u g g e s t a m o d e r a t e l y s t r o n g p h a s e o f s o i l d e v e l o p m e n t c o m p a r e d t o t h a t a s s o c i a t e d w i t h s u r f a c e Holocene pedons. The e x t e n s i v e o c c u r r e n c e o f c o m p l e x v o i d a r g i l l a n s seems t o s u p p o r t a p r o l o n g e d p h a s e o f w e a t h e r i n g s u g g e s t i n g i n t u r n t h a t t h e T i t t i b i n u p s o i l m a t e r i a l s , exp r e s s e d i n t e r m s o f B a l i s o m a f a b r i c s , a r e more s t r o n g l y d e v e l o p e d ( o l d e r ) t h a n s u r f a c e Grisoma. S t u d i e s of l a t e r i t i c s u b s t r a t e s ( F i n k l , 1 9 7 1 ; F i n k l & G i l k e s , 1976) show t h a t t h e d e e p e r p a l l i d z o n e s a r e m a i n l y b l e a c h e d w e a t h e r e d r o c k f a b r i c s where t h e r e i s a h i g h p r o p o r t i o n o f r o c k n o d u l e s w i t h p a t c h e s o f g r a y - c o l o r e d s e p i c s o i l f a b r i c s . M o t t l e d z o n e s i n c o n t r a s t , cont a i n a n abundance of w e l l d e v e l o p e d p e d o l o g i c a l f e a t u r e s as w e l l as s t r o n g l y s e g r e g a t e d s e s q u i o x i d e s and c l a y accumulations i n d i c a t i n g t h a t t h i s z o n e i s t h e more s t r o n g l y p e d o g e n i c a l l y o r g a n i z e d p o r t i o n of s o f t l a t e r i t e p r o f i l e s . T h e s e s o i l m a t e r i a l s a r e t h u s , by f a r , t h e o l d e s t i n t h e r e g i o n and a l l o t h e r weathered m a t e r i a l s , i n c l u d i n g those i n p a l l i d zones, are r e l a t i v e l y younger.
C l i m a t e , G r o u n d s u r f a c e s , a n d S o i l Age S e q u e n c e s
R e l a t i v e p e d o l o g i c a l c h r o n o l o g i e s on t h e West A u s t r a l i a n c r a t o n d e p e n d , i n t h e f i r s t i n s t a n c e , on i n t e r p r e t a t i o n s o n t h e n a t u r e o f d e e p weathering events. P r o l o n g e d s u b a e r i a l w e a t h e r i n g i n a s t a b l e morp h o t e c t o n i c s e t t i n g which produced l a t e r i t e s c o e v a l i n age h a s been contrasted with i n t e r p r e t a t i o n s of sorcalled older (primary, plateau) a n d y o u n g e r ( d e t r i t a l , v a l l e y ) l a t e r i t e s ( s e e d i s c u s s i o n s i n Woolnough, 1918 and P r i d e r , 1 9 6 6 ) . Although a n e x t e n d e d p h a s e of deep w e a t h e r i n g , p o s s i b l y i n i t i a t e d i n t h e l a t e r M e s o z o i c , i s now p r e f e r r e d ( e . 9 . P r i d e r , 1 9 6 6 ; F a i r b r i d g e & F'inkl, 1979; F i n k l , 1971, 1 9 7 9 ) , i t i s t h e w e l l d e f i n e d Oligocene i n d u r a t i o n e v e n t ( Q u i l t y , 1 9 7 7 ) which s e t s t h e c l o c k f o r t h e p e d o c h r o n o l o g i c a l s e q u e n c e . V a r i a b l e s t r i p p i n g o f t h e deep w e a t h e r i n g m a n t l e p r e c e e d e d i n d u r a t i o n a n d i n t h e Q u a t e r n a r y t h e r e were o t h e r u n s t a b l e p h a s e s , m o s t l y a s s o c i a t e d w i t h i n c r e a s i n g a r i d i t y which a f f e c t e d much o f t h e A u s t r a l i a n c o n t i n e n t ( B o w l e r , 1 9 7 6 ) . Q u a t e r n a r y c l i m a t i c h i s t o r y i n Western A u s t r a l i a , b a s e d i n p a r t on b i o t i c
345
distributions (Wyrwoll, 1979), is characterized by contrasting wet and dry regimes. Relatively dry conditions punctuated by high precipitation events of short duration seem to have prevailed until the end o f the Pleistocene (Wyrwoll & Milton, 1976). Alternation of Quaternary paleoclimates contributed to unstable geomorphic conditions which favored stripping o f weathered materials forming new ground-surfaces with lag gravel or coarse slump deposits which in turn became stone lines when buried by finer-grained colluvium. These deposits, which must be regarded as post-duricrust in age because they contain fragments of the ferricrete, were then pedogenically altered to form pedoderms. A significant increase in precipitation at the onset of the Holocene, associated with a rising sea level and warmer sea surface temperatures, is reflected in surface pedons of the relatively youngest soils on the craton. REFERENCES CITED Ab'Saber, A.N., 1967, Problemas geomorfologicas da Amazonia brasiliera: Atlas do Simposio sobre a biota amazonica, v. 1 , p. 35-67. American Commission o n Stratigraphic Nomenclature, 1961, Code of stratigraphic nomenclature: 4 e r . Assoc. Pet. Geol. Bull., v. 4 5 , p . 645-665. Babington, B., 1821, Decomposition of hornblende and feldspars in laterite formation: Trans. Geol. SOC., v . 5 , p. 328-329. Berner, R.A., Sjgberg, E.L., Velbel, M.A. and K r o m , M . D . , 1980, Dissolution of pyroxenes and amphiboles during weathering: Science, V . 207, p. 1205-1206. Berthelin, J. and Belgy, G . , 1 9 7 9 , Microbial degradation during simulated podzolization: Geoderma, v . 214, p. 297-310. Bigarella, J.J., Mousinho, M.R. and da Silva, J . X . , 1 9 6 5 , Processes and environments of the Brazilian Quaternary: ! . I 1 INQUA Congress (Symposium o n Cold Climate Processes and Environments, Fairbanks, Alaska), p. 3-71. Bilzi, A.F. and Ciolkosz, E.J., 1971, A field morphology-rating scale for evaluating pedological development: Soil Science, v. 1 2 4 , p. 45-49. Bisdom, E.B.A., 1967, Micromorphology of a weathered granite near the Ria de Arosa (N.W. Spain): Leidse Geologische Mededelingen, v. 37, p. 33-67. Bowler, J.M., 1976, Aridity in Australia: a g e , origins and expression i n aeolian landforms and sediments: E a r t h Science Reviews, v. 1 2 , p. 279-310. Brewer, R., 1964, Fabric and Mineral Analysis o f Soils, New York, Wiley, 470 p.
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EVALUATION OF DATING METHODS USED TO ASSIGN AGES IN THE WIND RIVER AND TETON RANGES, WESTERN WYOMING WC. MAHANEY, D.L. HALVORSON, JAMES PIEGAT and K. SANMUGADAS
ASSTRACT G l a c i a l a n d n o n g l a c i a l d e p o s i t s i n t h e Wind R i v e r a n d T e t o n r a n g e s o f w e s t e r n Wyoming h a v e b e e n d a t e d b y r a d i o c a r b o n , t o p o g r a p h i c p o s i t i o n , s u r f a c e morphology, w e a t h e r i n g c h a r a c t e r i s t i c s , l i c h e n o m e t r y and s o i l s t r a t i g r a p h y . O f t h e s e many m e t h o d s r a d i o c a r b o n , l i c h e n o m e t r y , w e a t h e r i n g f e a t u r e s , and s o i l s t r a t i g r a p h y are t h e m o s t u s e f u l i n d i f f e r e n t i a t i n g d e p o s i t s w h i c h f o r m e d m a i n l y d u r i n g p e r i o d s o f g l a c i a t i o n . No l i c h e n g r o w t h - r a t e c u r v e i s a v a i l a b l e f o r t h e m o u n t a i n s o f w e s t e r n Wyoming. However, numerous l i c h e n t r a n s e c t s a c r o s s N e o g l a c i a l d e p o s i t s y i e l d r e l a t i v e d a t a on l i c h e n s i z e a n d p e r c e n t c o v e r t h a t c o r r e l a t e w i t h t h e F r o n t R a n g e , C o l o r a d o , where g r o w t h r a t e s f o r Rhizocarpon g e o g r a p h i c u r n a r e known w i t h p r e c i s i o n . S u r f a c e w e a t h e r i n g f e a t u r e s t h a t a s s i s t i n d e p o s i t d i f f e r e n t i a t i o n and c o r r e l a t i o n i n c l u d e : b o u l d e r f r e q u e n c y r a t i o o f f r e s h t o weathered s t o n e s , w e a t h e r i n g r i n d t h i c k n e s s , and d e p t h o f p i t t i n g on s t o n e s u r f a c e s . S o i l p r o f i l e s from w e l l - d r a i n e d s i t e s i n d e p o s i t s c o n t a i n i n g g r a n i t i c , g n e i s s i c , and g r a n o d i o r i t i c c l a s t s a r e used t o e s t a b l i s h r e l a t i v e a g e . S o i l morphology, p a r t i c l e s i z e , o r g a n i c c o n s t i t u e n t s , s e l e c t e d s o i l - c h e m i c a l p a r a m e t e r s , p r i m a r y m i n e r a l a l t e r a t i o n and c l a y mineral composition are a l l used t o e s t a b l i s h a s o i l chronosequence. S o i l s i n t h e sequence a r e , from youngest t o o l d e s t , post-Gannett Peak, post-Audubon, p o s t - I n d i a n B a s i n , p o s t - P i n e d a l e , p o s t - B u l l L a k e , and I n p a r t i c u l a r , changes i n c l a y m i n e r a l composition w i t h p r e - B u l l Lake. d e p t h and changes i n t h e r a t i o of o x a l a t e - e x t r a c t a b l e t o d i t h i o n i t e e x t r a c t a b l e i r o n oxide are important age i n d i c a t o r s . P e t r o g r a o h i c a n a l y s i s of f i n e and very f i n e sand s e p a r a t e s (250-63pm) r e v e a l s changes i n t h e r a t i o of q u a r t z t o p l a g i o c l a s e f e l d s p a r , which a s s i s t i n a g e differentiation. INTRODUCTION The u s e o f m u l t i p l e g e o m o r p h i c a n d p e d o l o g i c c r i t e r i a t o d i f f e r e n t i a t e d e p o s i t s i n g e o l o g i c s u c c e s s i o n s p r o v i d e s an i m p o r t a n t t o o l f o r Q u a t e r n a r y s t r a t i g r a p h e r s . R e l a t i v e d a t i n g (FID) t e c h n i q u e s t h a t h a v e b e e n u s e d i n t h e T e t o n a n d Wind R i v e r r a n g e s o f w e s t e r n Wyoming i n c l u d e : changes i n d e p o s i t morphology, t o p o g r a p h i c p o s i t i o n , w e a t h e r i n g chara c t e r i s t i c s , l i c h e n o m e t r y and s o i l s t r a t i g r a p h y . Absolute d a t i n g ( A D ) i s c o n f i n e d t o r a d i o c a r b o n methods b e c a u s e d a t a b l e p y r o c l a s t i c l a y e r s a r e a b s e n t , and N e o g l a c i a l s u c c e s s i o n s are above t i m b e r l i n e where dendrochronology cannot b e a p p l i e d . Uncontaminated o r g a n i c materials a r e s c a r c e a n d t h e few d a t e s a v a i l a b l e d o c u m e n t o n l y t h e c l o s e o f P i n e d a l e G l a c i a t i o n a n d t e r m i n a t i o n of t h e I n d i a n B a s i n a d v a n c e .
AD a n d RD m e t h o d s o u t l i n e d a b o v e h a v e b e e n u s e d a t many f i e l d l o c a l i t i e s t o differentiate deposits i n the geologic succession. O f t h e s e many m e t h o d s r a d i o c a r b o n , w e a t h e r i n g f e a t u r e s , l i c h e n o m e t r y , a n d s o i l s t r a t i g r a p h y h a v e t h e g r e a t e s t u t i l i t y . Used i n d i v i d u a l l y e a c h s u f f e r s f r o m l i m i t a t i o n s t h a t h i n d e r r e f i n e m e n t of t h e s t r a t i g r a p h i c
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record. The use of several methods at each locality increases dating precision and helps strengthen the chronology. In this paper the advantages and disadvantages of each method are discussed and evaluated. FIELD AREAS The Teton Range (Figure l), a Tertiary fault block tilted to the west, makes up part of the Middle Rocky Mountain Province as defined by Fenneman (1931). Extending from the Pitchstone Plateau in Yellowstone Park to the Teton Pass Mountains in the south, the range varies from l5 to 2 3 km in width. Upthrusting and erosion at the east side of the fault block has exposed a complex of Precambrian rocks in which granites, gneisses, schists and pegmatites predominate (J. Reed, pers. comm., 1980). Along the backslope of the Range Paleozoic and Mesozoic strata prevail. Jackson Hole, to the east, is a downfaulted block floored with Cretaceous and Tertiary rocks, covered to the north end by glacial till and outwash. Representative basins with nearly complete glacial seauences include Moran. Paintbrush. Cascade. Garnet. and ivalanche canyons. Along both Flanks of the range giaciers are genera 1ly found above 3,000 m. I
Figure 1
L o c a t i o n map s h o w i n g ( a ) t h e l o c a t i o n o f C o t t o n w o o d C r e e k , P a i n t b r u s h Canyon and Jaw C i r q u e i n t h e T e t o n R a n g e , Wyoming, a n d ( b ) t h e l o c a t i o n o f T i t c o m b a n d I n d i a n B a s i n s , a n d F r e m o n t L a k e , Wind R i v e r R a n g e , Wyoming.
The Wind River Range (Figure 1) forms a broad anticlinal uplift, trending northwest-southeast. Extending from South Pass to the Gros Ventre Mountains, the range is 190 km long and 60 km wide. The uplift of Tertiary age exposes a core of Precambrian granite, gneiss, and schist ( W o r l , 1968). Paleozoic and Mesozoic rocks cover the eastern slopes, whereas conglomerates, shales, tuffs and volcanics of Tertiary age crop out on the western slope. Along the crest of the range a complex of igneous and metamorphic rocks with fine, medium, coarse, and porphyroblastic textures prevails. Linearity of major valleys suggests the importance of faults in controlling drainage in the area (Mahaney, 1978). Glacial and periglacial deposits contain clasts of dioritic
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g n e i s s , g a b b r o d i o r i t i c g n e i s s , q u a r t z - m o n z o n i t i c g n e i s s , and g r a n i t i c g n e i s s . R e p r e s e n t a t i v e g l a c i a t e d b a s i n s i n c l u d e Green R i v e r , New F o r k , T i t c o m b , I n d i a n , P i n e C r e e k , F r e m o n t , F a l l s C r e e k , S i l v e r C r e e k , and L i t t l e Pop0 Agie (Mahaney, 1 9 7 8 ) . Along b o t h f l a n k s o f t h e r a n g e g l a c i e r s a r e g e n e r a l l y f o u n d above 3 , 3 0 0 m and i c e i s more common i n c i r q u e b a s i n s w i t h n o r t h or n o r t h e a s t o r i e n t a t i o n s . A s i n t h e T e t o n s , a l l modern g l a c i e r s occupy p o r t i o n s of c i r q u e floors o r i g i n a l l y formed by P l e i s t o c e n e a.nd Holocene i c e . C l i m a t i c d a t a for a l p i n e and s u b a l p i n e z o n e s i n t h e T e t o n s a r e l a c k i n g , b u t a s u m m a r y of a 1 5 - y e a r p e r i o d ( 1 9 5 5 - 7 0 ) i s a v a i l a b l e f o r Moran, Wyoming, l o c a t e d i n J a c k s o n Hole (U.S. D e p t . Commerce, 1 9 7 0 ) . 0 a r e sumT e m p e r a t u r e e x t r a p o l a t i o n s f o r t h e m o u n t a i n b a s i n s ( ~ ~ 3 , 0 0m) m a r i z e d by Mahaney ( 1 9 7 5 , p . 1 4 3 ) . The a v e r a g e a n n u a l t e m p e r a t u r e app r o x i m a t e s -4.7"C a t 3 , 0 0 0 m w i t h a maximum of t 3 . 0 " C a n d a minimum o f -12.5"C. C l i m a t i c d a t a for s t a t i o n s i n i n t e r m o n t a n e b a s i n s a d j a c e n t t o t h e Wind R i v e r Range a r e a v a i l a b l e from t h e U.S. D e p t . Commerce ( 1 9 6 5 ) . T e m p e r a t u r e e x t r a p o l a t i o n s for t h e m o u n t a i n v a l l e y s ( $ 3 , 0 0 0 m) a r e summ a r i z e d by Mahaney ( 1 9 7 8 , p . 2 3 1 ) . The a v e r a g e a n n u a l t e m p e r a t u r e i n t h e a l p i n e b a s i n s a p p r o x i m a t e s -3.5"C. V e g e t a t i o n above t i m b e r l i n e i n b o t h r a n g e s i s d o m i n a t e d by p e r e n n i a l s e d g e s , g r a s s e s , and h e r b a c e o u s p l a n t s c o n s i s t i n g of D r y a s , s e d g e g r a s s , and w i l l o w - s e d g e s t a n d t y p e s . The t i m b e r l i n e c o n t a i n s procumbent w h i t e b a r k p i n e ( P i n u s a l b i c a u l i s ) , l i m b e r p i n e ( P i n u s f l e x i l i s ) , suba l p i n e f i r ( A b i e s Z a s i o c a r p a ) , and Engelmann s p r u c e ( F i c e a e n g e l r n a n n i i ) t h a t merges a t ~ ~ 3 , 0 0m0 i n t o a s u b a l p i n e f o r e s t of Engelmann s p r u c e ( P . e n g e l m a n n i i ) , and s u b a l p i n e f i r ( A . l a s i o c a r p a ) . The l o w e r l i m i t o f t h e s u b a l p i n e f o r e s t merges w i t h a montane f o r e s t o f l o d g e p o l e p i n e ( P i n u s c o n t o r t a l , Douglas fir ( P s e u d o t s u g a r n e n z i e s i i ) and a s p e n ( P o p u l u s t r e m u l o i d e s ) ($2,500 m i n t h e T e t o n s ; 2,750 m i n t h e Wind R i v e r R a n g e ) . S a g e b r u s h v e g e t a t i o n ( A r t e m i s i a t r i d e n t a t a ) i s t h e dominant ground cover below t h e l o w e r l i m i t o f t r e e s . METHODS
S e v e r a l d a t i n g methods a r e r o u t i n e l y u s e d t o a s s i g n r e l a t i v e and a b s o l u t e a g e s t o d e p o s i t s . These i n c l u d e v a r i o u s r o c k - w e a t h e r i n g p a r a m e t e r s , l i c h e n d a t a , s o i l morphology and l o e s s t h i c k n e s s , and r a d i o carbon. Weathering C h a r a c t e r i s t i c s S e v e r a l d i f f e r e n t w e a t h e r i n g p a r a m e t e r s a r e m e a s u r e d on s a m p l e s of 50 o r 1 0 0 stones a t each f i e l d l o c a l i t y . Counts a r e made of b o u l d e r f r e q u e n c y ; t h a t i s , t h e number of b o u l d e r s o v e r 0 . 3 m d i a m e t e r i n u n i t a r e a s o f 10m x 10m (loom2) or 5m x 5m ( 2 5 m 2 ) , f o l l o w i n g t h e p i o n e e r i n g work o f B l a c k w e l d e r ( 1 9 1 5 ) and Nelson ( 1 9 5 4 ) . U s u a l l y t h e a r e a i s p a c e d o u t i n t h e f i e l d and t o t a l c o u n t s a r e made o f a l l s t o n e s w i t h o u t regard t o lithologic differences. The r e s u l t s a l l o w d i f f e r e n t i a t i o n between d e p o s i t s o f g l a c i a t i o n r a n k , b u t a r e n o t g e n e r a l l y u s e f u l f o r d i f f e r e n t i a t i o n a t t h e s u b s t a g e or l e s s e r l e v e l (Mahaney, e t a l . , 1 9 8 1 ) . The r a t i o o f f r e s h t o w e a t h e r e d s t o n e s c a n b e d e t e r m i n e d by r e c o r d i n g t h e sound p r o d u c e d by b o u l d e r s s t r u c k w i t h a hammer. A s h a r p r i n g and s t r o n g r e b o u n d o f t h e hammer i n d i c a t e s a f r e s h b o u l d e r , wherea s a w e a t h e r e d r o c k p r o d u c e s a d u l l sound and w e a k e r hammer r e c o i l . A l t h o u g h t h i s method i s a d m i t t e d l y n o t p a r t i c u l a r l y s e n s i t i v e , i t i s q u i c k l y done i n t h e f i e l d . A l t e r n a t i v e l y , one c o u l d u s e t h e f r e t t i n g r a t i o o f S h a r p ( 1 9 6 9 ) , where a w e a t h e r e d s t o n e i s d e f i n e d as one where more t h a n 5 0 p e r c e n t o f t h e s t o n e s u r f a c e i s w e a t h e r e d t o a d e p t h g r e a t e r t h a n t h e a v e r a g e g r a i n d i a m e t e r . Care s h o u l d b e t a k e n t o s e l e c t s t o n e s o f s i m i l a r r o c k t y p e on a s i n g l e v a l l e y s i d e t o a v o i d l i t h o l o g i c and m i c r o c l i m a t i c e f f e c t s . Data p r o d u c e d as w e a t h e r i n g r a t i o s a l l o w d i f f e r e n t i a t i o n among d e p o s i t s a t t h e g l a c i a l s t a g e r a n k (Pinedale u s . B u l l Lake), but a r e not generally u s e f u l f o r d i f f e r e n t i a t i o n a t t h e s u b s t a g e o r s t a d i a 1 l e v e l (Mahaney, e t al., 1 9 8 1 ) .
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W e a t h e r i n g r i n d s a r e i n d i c a t i v e o f t h e d e g r e e t o which i r o n - b e a r i n g m i n e r a l s o x i d i z e and d i s c o l o r t h e o u t e r p e r i p h e r y of c l a s t s . T h e t h i c k n e s s o f r i n d s measured p e r p e n d i c u l a r t o s t o n e s u r f a c e s i s r e l a t e d t o t h e time s i n c e d e p o s i t i o n . T n i s method has b e e n d e s c r i b e d b y numerous w o r k e r s i n c l u d i n g B i r k e l a n d ( 1 9 7 3 ) , Mahaney (1973, 1978, 1 9 8 1 ) a n d K i v e r (1974). Most w o r k e r s m e a s u r e o n l y t h e maximum t h i c k n e s s o f d i s c o l o r a t i o n , n e g l e c t i n g t h e i r r e g u l a r l y d e v e l o p e d r i n d commonly f o u n d on many s t o n e s . Some i n v e s t i g a t o r s u n d o u b t e d l y m e a s u r e d i s c o l o r a t i o n a l o n g f r a c t u r e f a c e s which may amount t o 2 or even 3 t i m e s t h e r i n d t h i c k n e s s . Our p r a c t i c e i s t o i g n o r e f r a c t u r e f a c e s and t o m e a s u r e b o t h t h e m a x i m u m and minimum r i n d ( T a b l e s 1 and 2 ) . We have a l s o m e a s u r e d t h e d e g r e e o f i n t e r n a l d i s c o l o r a t i o n of s t o n e s , b u t f i n d i t c o n t r o l l e d more by l i t h o l o g y t h a n t i m e , and d i f f i c u l t t o q u a n t i f y . Weathering p i t s i n b o u l d e r s , c o b b l e s o r r o c k o u t c r o p s can be measured w i t h t h e a i d o f a d i a l or v e r n i e r c a l i p e r . Depth and w i d t h o f p i t s r e v e a l d i f f e r e n c e s b e t w e e n s e t s o f d e p o s i t s , p r o v i d e d t h a t some c a r e i s t a k e n t o a v o i d p o r p h y r i t i c r o c k s , and i n c h o o s i n g o n l y r e p r e s e n t a t i v e l i t h o l o g i e s ( e . g . g r a n o d i o r i t e i n t h e Wind R i v e r Range and g r a n i t e and f e l s i c g n e i s s i n t h e T e t o n R a n g e ) . A c c u r a t e w i d t h measurements a r e r e a s o n a b l y e a s y t o o b t a i n , b u t d e t e r m i n i n g d e p t h i s more d i r f i c u l t . Measurements a r e made b y f u l l y e x t e n d i n g t h e c a l i p e r d e p t h r o d and p l a c i n g i t i n t h e b o t t o m of t h e p i t p e r p e n d i c u l a r t o t h e p i t r i m . The main beam i s t h e n b r o u g h t t o a p o s i t i o n l e v e l w i t h t h e t o p o f t h e p i t and a measurement i s made. F i f t y measurements a r e t a k e n a t e a c h s i t e and t h e means c a l c u l a t e d . The d a t a i n T a b l e s 1 and 2 show t h e d e g r e e t o which d e p o s i t s can be d i f f e r e n t i a t e d u s i n g measurements o f b o u l d e r f r e q u e n c y , w e a t h e r i n g r a t i o s , r i n d s , and p i t s . The d a t a a l l o w d i f f e r e n t i a t i o n a t t h e g l a c i a l s t a g e l e v e l , w h i l e d i f f e r e n t i a t i o n b e t w e e n s t a d e s i s n o t a l w a y s poss i b l e . The p r e s e n c e / a b s e n c e o f some minimum r i n d development i s u s e f u l i n d i s t i n g u i s h i n g I n d i a n B a s i n d e p o s i t s from d e p o s i t s o f l a t e - P i n e d a l e age. L i chenome t r g
Maximum d i a m e t e r s and p e r c e n t c o v e r o f t h e f o u r dominant c r u s t o s e l i c h e n s -- R h i z o c a r p o n g e o g r a p h i c u r n , L e c a n o r a t h o r n s o n i i , L e c a n o r a a s p i c i l i a , and L e c i d e a a t r o b r u n n e a - a r e u s e d t o d i f f e r e n t i a t e d e p o s i t s i n t h e N e o g l a c i a l s u c c e s s i o n . The method r e s t s on t h e f o l l o w i n g assumptions:
1
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t h e l a r g e s t l i c h e n t h a l l u s r e p r e s e n t s t h e o l d e s t and f a s t e s t growing l i c h e n on a s u b s t r a t e ,
2
-
l i c h e n growth ( i n c l u d i n g p e r c e n t c o v e r ) i s i n d i c a t i v e of elapsed time s i n c e deposition,
3
-
t h e s u b s t r a t e f a l l s w i t h i n the l i f e span of an i n d i v i d u a l l i c h e n s p e c i e s , and
4 - e f f e c t s o f c l i m a t i c change a r e smoothed o u t o v e r t i m e . D e p o s i t s i n t h e T e t o n Range (Jaw C i r q u e and P a i n t b r u s h Canyon) ( F i g u r e 1) were sampled t o d e t e r m i n e t h e l a r g e s t d i a m e t e r o f e a c h crustose lichen. P e r c e n t c o v e r was e s t i m a t e d from p e r c e n t a g e d i a g r a m s i n Oyama and T a k e h a r a ( 1 9 7 0 ) . Measurements o f i n d i v i d u a l t h a l l i were made t o t h e n e a r e s t m i l l i m e t e r . Only t h e l a r g e s t e l l i p t i c a l - s h a p e d t h a l l i w e r e m e a s u r e d , and o n l y maximum d i a m e t e r s w e r e u s e d t o a s s i g n r e l a t i v e a g e s . T h a l l i w i t h i r r e g u l a r s h a p e s and t h o s e f o u n d i n d e p r e s s i o n s were b y p a s s e d t o a v o i d p r o b l e m s a s s o c i a t e d w i t h “ i n t e r f i n g e r i n g “ by i n d i v i d u a l s o f t h e same s p e c i e s (Mahaney, 1 9 7 3 ) a n d s n o w k i l l ( C u r r y , 1969), respectively. A d d i t i o n a l s a m p l i n g r e s t r i c t i o n s f o l l o w e d by o t h e r w o r k e r s ( B e s c h e l , 1 9 5 7 , 1958; B e n e d i c t , 1 9 6 8 ; and Andrews a n d Webber, 1 9 6 4 ) i n c l u d i n g s t a n d a r d i z a t i o n o f r o c k t y p e , p r e v a i l i n g wind, e x p o s u r e t o i c e c r y s t a l b l a s t i n g , a s p e c t , a v a i l a b l e m o i s t u r e , and s u r f a c e s t a b i l i t y were f o l l o w e d i n t h i s s t u d y .
T a b l e 1. Site
Age
Weathering dataa f o r sites i n Indian Basin ( I B ) Wind R i v e r R a n g e , Wyoming Stone Frequency (stones > 0 . 3m diameter
W e a t h e r i n g R a t i o sb
4 F r e s h %Weathered
n
and I s l a n d Lake
Weathering Rinds
(IL)
areas,
Weathering P i t s
Mean Maximum Imm)
Mean Minimum (mm)
n
2.6 1.6 2.3 2.6 2.4 1.6
0 0 0 0 0 0
25 50 50 50 50 50
-
2.1 3.6 4.1
0 0 0
3.0 3.8
0 0 0 0.16
Mean Depth (mm 1
Mean Width (mm)
n
-
25m2)
IBlOb IBll
Audubon
IB12 IB13b
IB19 IB41 Indian Basin
IB5' IB9
IBlOa
IR8
IB14 IB22
IR43
60 65 58 60 69 57
100 100 100 100 100
-
0 0
50 50 50 50 50
41 37 45 35 32 31
100 100
0 0
50 50
30
IB16
Late Pinedale
IB21 IL1
a
(i)
20 25 15
-
100 100 100
0 0 0
-
0 0 0
-
50 50 50
-
-
-
98
2 2
50 50
98
2.8 4.2
0 6.0
11.8
-
0.04 0.84
4.2 5.3 3.3
23.5 29.3 26.2
50 50 50
50 50 50
6.7 -
60.6 -
50
50
6.2
73.7
5.3
38.0
50
11.6 8.0 12.4
94.8 38.5 76.3
50 50 50
50 50 50
-
50 50
-
-
-
-
50
-
n o t sampled
W e a t h e r e d l f r e s h d i f f e r e n t i a t i o n i s based on t h e a c o u s t i c a l p r o p e r t i e s o f s t o n e s . clang, weathered s t o n e s a d u l l thud. C
-
Fresh stones give a sharp
Samples t a k e n from B horizon i n s o i l p i t . w
m LD
m
M
4
aJ
u
*d i
ccccccc vlvlvlmmmvl
ccccccc
d
d d
cccccc mmcmcc
m C
cc .ccc
cvlcccc d d d
vlNCCClA
w
ccc c m v l d
W
dChl
. . . . . . c. c. c.
dCrnrlUvl
cccccc
...
vlmw
N
W
W
+rid
ccc ccc
w h o \
. . . . . . . dmvlr-cc ...... m N m N “ cccccc cccccc d d d d d d
lAcowdNmw
r l d d d d d
cccccc cccccc
ddhlddCC
ccccccc cccccoc
cccccc
d d r i r l d d m
ccccccc
cccccc cccccc d d r i d d d
cccccoc ccccccc d r i d d d r i d
aJ d
C *d n
aJ
rd
C 0
a
1
e 4
1
-0
361
The d a t a i n T a b l e 3 a r e u s e d t o i d e n t i f y N e o g l a c i a l a d v a n c e s ; i . e . G a n n e t t P e a k , Audubon, a n d I n d i a n B a s i n . ' R . geographicurn a n d L . t h o m s o n i i a r e t h e two m o s t i m p o r t a n t c r u s t o s e l i c h e n s b e c a u s e t h e i r maximum d i a m e t e r s f a l l i n t o t h r e e d i s c r e t e g r o u p s . L . a s p i c i l i a i s a b s e n t on G a n n e t t P e a k d e p o s i t s , b u t g r o w s r a p i d l y on o l d e r s u b s t r a t e s . L . a t r o b r u n n e a , a b s e n t o n m o s t d e p o s i t s of G a n n e t t P e a k a g e , a p p e a r s on Audubon a n d I n d i a n B a s i n d e p o s i t s . Percent l i c h e n cover v a r i e s across t h e N e o g l a c i a l d e p o s i t s , r a n g i n g from n i l on G a n n e t t Peak and 1 0 - 4 0 p e r c e n t o n Audubon s u b s t r a t e s t o 40-75 p e r c e n t on I n d i a n B a s i n s u r f a c e s . Because o f t h e g e o l o g i c a l l y s h o r t l i f e span o f i n d i v i d u a l t h a l l i (% 3 , 0 0 0 y r s . ) t h e u s e of l i c h e n o m e t r y i s r e s t r i c t e d t o d e p o s i t s o f Neog l a c i a l age. L i c h e n o m e t r y a l l o w s t h e s e p a r a t i o n o f G a n n e t t P e a k a n d Audubon dep o s i t s which are i n d i s t i n g u i s h a b l e on t h e b a s i s o f s u r r a c e morphology a n d o n l y m a r g i n a l l y i d e n t i f i a b l e by w e a t h e r i n g c h a r a c t e r i s t i c s . L i c h e n ometry can a l s o be u s e d as a n AD t o o l , b u t i t c a n n o t b e o v e r e m p h a s i z e d t h a t a growth curve must be e s t a b l i s h e d ( u s i n g h i s t o r i c , r a d i o m e t r i c a n d / o r d e n d r o c h r o n o l o g i c c o n t r o l s ) for e a c h g e o g r a D h i c a 1 a r e a s t u d i e d . I f t h e d a t a f o r maximum t h a l l i d i a m e t e r s a n d p e r c e n t c o v e r b e t w e e n c o r r e l a t i v e u n i t s i n t h e T e t o n ( t h i s p a p e r , T a b l e 3 ) a n d Wind R i v e r (Mahaney, 1 9 7 8 , p . 2 3 5 ) r a n g e s a r e c o m p a r e d some d i f f e r e n c e s e m e r g e , t h e a p p a r e n t r e s u l t of c l i m a t i c a n d / o r t e m p o r a l v a r i a t i o n s . T h u s , a s e p a r a t e g r o w t h c u r v e m u s t b e made for e a c h r a n g e i f l i c h e n o m e t r y i s t o b e u s e d as a n AD m e t h o d . Soil Stratigraphy D e p o s i t s i n g e o l o g i c s u c c e s s i o n s a r e s e p a r a t e d u s i n g s o i l morpho l o g y , p a r t i c l e s i z e d i s t r i b u t i o n s , c l a y m i n e r a l o g y , and s e l e c t e d s o i l chemical parameters. I n d i v i d u a l pedons a r e a s s i g n e d t o s o i l s t r a t i g r a p h i c u n i t s on t h e b a s i s of s t r a t i g r a p h i c p o s i t i o n a n d m o r p h o l o g i c a l f e a t u r e s w h i c h may b e c o n t i n u o u s l y r e c o g n i z e d a n d mapped (Mahaney a n d F a h e y , 1 9 7 6 ; Mahaney, 1 9 7 8 , 1 9 8 2 a ) . Such u n i t s f o r m , i n situ, i n d e p o s i t s by t h e a c t i o n of p e d o l o g i c a l p r o c e s s e s w o r k i n g downward on f r e s h , unaltered materials. S o i l s t r a t i g r a p h i c u n i t s are assigned informel names c o i n c i d i n g w i t h t h e a g e of t h e d e p o s i t s i n w h i c h t h e y f o r m ( e . 9 . use of t h e p r e f i x "post" t o avoid a p r o l i f e r a t i o n of geologic names). A f e w e x a m p l e s i l l u s t r a t i n g t h e u s e of s o i l s a n d s o i l p r o p e r t i e s i n d i f f e r e n t i a t i n g Q u a t e r n a r y d e p o s i t s follows. P l e i s t o c e n e Sequences M o r a l n e s e n c l o s i n g F r e m o n t Lake i n t h e Wind R i v e r Range h a v e b e e n d e s c r i b e d by n u m e r o u s w o r k e r s ( B l a c k w e l d e r , 1 9 1 5 ; R i c h m o n d , 1 9 6 5 , 1 9 7 4 ; a n d Mahaney, 1 9 7 8 ) . S i t e s on p r e - B u l l L a k e , 3 u l l L a k e , a n d P i n e d a l e m o r a i n e s ( F i g u r e 2 ) a r e l o c a t e d t o t h e e a s t o f t h e l a k e u n d e r montane f o r e s t . S o i l s a t t h e s e s i t e s were e x a m i n e d t o d e t e r m i n e s p e c i f i c p r o p e r t i e s t h a t w o u l d p r o v e u s e f u l i n d i f f e r e n t i a t i o n . A s shown i n T a b l e 4 s o i l d e p t h and l o e s s t h i c k n e s s g e n e r a l l y i n c r e a s e w i t h a g e , a s i n d i c a t e d by h i g h e r s i l t c o n t e n t i n t h e s o l a a n d u p p e r s u b s o i l o f t h e t h r e e p r o f i l e s . Clay c o n t e n t g e n e r a l l y i n c r e a s e s o v e r time, w i t h t h e h i g h e s t v a l u e s i n t h e p r e - B u l l Lake s o i l . The h i g h v a l u e o f 2 8 p e r c e n t i n t h e p o s t - P i n e d a l e s o i l may r e s u l t f r o m a i r f a l l i n f l u x a s t h i s s i t e i s l o c a t e d o n a h i g h m o r a i n e r i d g e 2 5 0 m a b o v e F r e m o n t L a k e . S o i l pH d i s t r i b u t i o n s r a n g e from s l i g h t l y a c i d i c i n t h e p o s t - P i n e d a l e s o i l , t o m o d e r a t e l y a c i d i c i n t h e p o s t - a n d p r e - B u l l Lake s o i l s . G r e a t e r v a r i a t i o n i n pH w i t h d e p t h w h i c h o c c u r s i n t h e two o l d e r s o i l s , i s a t t r i b u t e d t o v a r i a b l e movement o f Et i o n s . Clay and primary m i n e r a l d i s t r i b u t i o n s (Table 4 ) i n t h e f i n e c l a y s e p a r a t e s ( < 2 p m ) r e v e a l i n f o r m a t i o n r e l a t e d t o s o u r c i n g of t h e i c e ar.d/or s u b s e q u e n t w e a t h e r i n g and i s u s e f u l i n a g e d i f f e r e n t i a t i o n . O f s p e c i a l s i g n i f i c a n c e i s t h e p r e s e n c e of h i g h a m o u n t s o f k a o l i n i t e i n 'This s t r a t i g r a p h i c t e r m replaces "Early Neoglacial". It i s def i n e d f r o m t h e t y p e s e c t i o n i n I n d i a n B a s i n , Wind R i v e r R a n g e ( M a h a n e y , 1982a).
362 Table 3.
Lichen c h a r a c t e r i s t i c s a f o r Neoglacial deposits i n Jaw C i r q u e (JAW) a n d P a i n t b r u s h C a n y o n ( P B ) a r e a s , T e t o n R a n g e , Wyoming ~
Age
S i t e
~
~~
~
Rhizocarpon geographicurn
Lecanora Lecanora Lecidea % thornsonii a s p i c i l i a atrobrunnea c o v e r nil nil
PB7 PB8
Gannett Peak
9 15
10 16
n i l n i l
15 n i l
JAW2 JAW4 PB5 PB9
Audubon
46 55 50 45
67 72 83 61
70 66 55 64
65 61 54 55
nil-30 nil-25 5-40 5-30
JAW3 JAW5 PB1 PB20a PB20b PB21 PB22 PB23
Indian Basin
86 85 123 110 69 85 78 95
105 120 155 112 90 119 132 116
115 97 245 189 146 140 232 129
109 68 225 136 115 110 173 114
5-50 nil-50 50-75 40-70 35-60 40-80 35-75 40-80
a
Lichen measurements of
t h e maximum d i a m e t e r i n m m .
t h e FL9 p r o f i l e , w h i c h i s t h o u g h t t o r e s u l t from w e a t h e r i n g i n a m o i s t environment w i t h a g r e a t e r w e a t h e r i n g i n t e n s i t y . T h i s c o r r e l a t e s c l o s e l y w i t h a p r e - B u l l Lake s o i l d e s c r i b e d i n t h e F r o n t Range o f C o l o r a d o (Mahaney and F a h e y , 1980). The a b s e n c e of c h l o r i t e and s m e c t i t e , and t h e s m a l l amount of m i x e d - l a y e r c l a y s and v e r m i c u l i t e i n t h e p o s t - P i n e d a l e s o i l c o n t r a s t s h a r p l y w i t h t h e p o s t - B u l l Lake and p r e - B u l l Lake s o i l s . The l a r g e amounts o f o r t h o c l a s e and p l a g i o c l a s e f e l d s p a r s i n t h e p r e - B u l l Lake s o i l may r e f l e c t a d i f f e r e n t s o u r c e f o r t h e i c e . O v e r a l l , t h e upward d e c r e a s e i n f e l d s p a r i n t h e p r o f i l e s i s g r e a t e s t i n t h e p r e - B u l l Lake s o i l . The s o i l c h e m i s t r y i n T a b l e 5 s u p p o r t s t h e c l a y m i n e r a l o g y des c r i b e d a b o v e . The h i g h e r Kt, Cat', and Mgt2 i n t h e p r e - B u l l Lake s o i l f o l l o w from t h e g r e a t e r amounts of i l l i t e , s m e c t i t e , m i x e d - l a y e r i l l i t e s m e c t i t e , and v e r m i c u l i t e , r e s p e c t i v e l y . The d a t a i n d i c a t e t h a t t h e Base s a t u r a t i o n i s h i g h e r i n t h e p o s t smectite i s c a l c i c , not sodic. P i n e d a l e s o i l , b u t d e c l i n e s t o 35 p e r c e n t i n t h e u p p e r solum o f t h e p o s t - B u l l Lake s o i l . V a r i a t i o n s i n o r g a n i c m a t t e r a n d n i t r o g e n app a r e n t l y r e f l e c t d i f f e r e n c e s i n b u l k d e n s i t y and v a r i a b l e l e a c h i n g i n t h e p r o f i l e s . The n o t i o n t h a t l e a c h i n g v a r i e s o v e r t i m e i s s u p p o r t e d by t h e o r g a n i c m a t t e r a n d n i t r o g e n d i s t r i b u t i o n s i n t h e t h r e e p r o f i l e s . Organic m a t t e r content decreases slowly with depth i n t h e post-Pinedale s o i l , d e c r e a s e s a b r u p t l y i n t h e p o s t - - B u l l Lake s o i l , and t e n d s t o i n c r e a s e i n t h e l o w e r solum and s u b s o i l o f t h e p r e - B u l l Lake p e d o n . I n t h e T e t o n Range P l e i s t o c e n e d e p o s i t s can b e s e p a r a t e d b y t h e d e g r e e of s o i l e x p r e s s i o n . R e p r e s e n t a t i v e s o i l s i n P i n e d a l e and B u l l Lake d e p o s i t s a r e shown i n T a b l e 6 . A s i n t h e Frernont Lake s e q u e n c e , s o i l d e p t h and l o e s s t h i c k n e s s i n c r e a s e w i t h a g e . The h i g h s i l t cont e n t i n t h e p o s t - B u l l Lake s o i l i s r e p r e s e n t a t i v e of p r e - W i s c o n s i n a n s o i l s i n t h e T e t o n Range t h a t h a v e s u b s t a n t i a l l o e s s c a p s . S o i l r e a c t i o n s ( p H ) , a l t h o u g h s l i g h t l y more a c i d i c i n t h e p o s t - B u l l Lake solum, are not considered age-dependent. Whereas t h e p r i m a r y m i n e r a l d i s t r i b u t i o n s p r o v i d e l i t t l e i n f o r m a t i o n on a g e , t h e c l a y m i n e r a l comp o s i t i o n y i e l d s d a t a t h a t a r e i m p o r t a n t i n a g e d e t e r m i n a t i o n and p a l e o c l i m a t i c r e c o n s t r u c t i o n . W i t h i n t h e 1:l c l a y m i n e r a l s , t h e i n c r e a s e i n k a o l i n i t e w i t h a g e may w e l l r e p r e s e n t i n c r e a s e d w e a t h e r i n g o v e r time. The p r e s e n c e o f h a l l o y s i t e i n t h e p o s t - B u l l Lake s o i l may r e f l e c t h y d r a t i o n of k a o l i n i t e i n a n e n v i r o n m e n t w e t t e r t h a n t o d a y (Mahaney, 1 9 8 1 ) . The i n c r e a s e i n 2.1 c l a y m i n e r a l s , e s p e c i a l l y s m e c t i t e , i l l i t e s m e c t i t e , and v e r m i c u l i t e , i n t h e p o s t - B u l l Lake s o i l r e f l e c t s a s i m i l a r
363
Figure 2
S u r f i c i a l geology and s o i l sites i n late-Quaternary g l a c i a l d e p o s i t s , east of Fremont Lake i n t h e montane f o r e s t zone, Wind R i v e r R a n g e , W y o m i n g .
p a t t e r n t h a t occurs i n t h e P l e i s t o c e n e sequence a t t h e type Pinedale l o c a l i t y (Mahaney, 1 9 7 8 , and t h i s p a p e r ) . A l t h o u g h i t i s e n t i r e l y p o s s i b l e t h a t t h e c l a y m i n e r a l composition of t h e s e p a l e o s o l s o r i g i n - a t e s by some c o m b i n a t i o n o f w e a t h e r i n g o v e r t i m e , a e o l i a n i n p u t , a n d / o r p a l e o c l i m a t e , t h e d i f f e r e n c e s a p p e a r common and w i d e s p r e a d , making them u s e f u l i n r e l a t i v e age determinations. A n a l y s i s o f C u m u l a t i v e P a r t i c l e S i z e Curves P a r t i c l e s i z e d i s t r i b u t i o n s commonly a l l o w d i f f e r e n t i a t i o n o f s o i l s i n geologic successions. I f t h e amount o f c l a y i n t h e p a r e n t m a t e r i a l of e a c h s o i l i s f a i r l y u n i f o r m , t h e n changes i n c l a y c o n t e n t i n B h o r i z o n s o f s o i l - s t r a t i g r a p h i c u n i t s s h o u l d r e f l e c t r e l a t i v e amounts o f w e a t h e r i n g o v e r t i m e a n d / o r a i r f a l l i n f l u x . P o s t - B u l l Lake (MOS1) and P i n e d a l e ( P B 1 8 ) s o i l s i n t h e T e t o n Range show h e a v i e r t e x t u r e s i n B h o r i z o n s b e c a u s e c l a y c o n t e n t i n c r e a s e s w i t h age ( F i g u r e s 3 and 4 ) . The d a t a show t h a t t h e t a i l s (‘3.9 pm) o f t h e B h o r i z o n c u r v e s for s o i l s MOSl and P B 1 8 t e n d t o f l a t t e n w i t h i n c r e a s i n g a g e , a r e s u l t o f
Table 4.
Site
FL 8
b
P h y s i c a l a , c h e m i c a l and m i n e r a l p r o p e r t i e s of p o s t - P i n e d a l e v e g e t a t i o n z o n e s , Wind R i v e r M o u n t a i n s , Wyoming.
Age a n d Elevation Vegetation S o i l Depth Parent (m) Horizon (cm) Material Pinedale till
2450
Montane Forest
A l l A12 B21 IIB22 IICOX
IICn
FL 7
B u l l Lake till
2445
Montane Forest
A2 B21t 822
Clox IIC20x IIC3ox
IICn
FL9
pre-Bull Lake t i l l
2400
Montane Forest
A1 B21t B22t B23t B24t IIB25t IIB26t IIClox
IIC2ox IICnm
x
Sand 2mm-63pm
x,
Silt 63-4pm
x;
Clay pH <4pm ( 1 : l )
0-6 6-22 22-28 28-38 38-102 102+
43.0 52.5 55.9 63.9 65.8 67.9
28.0 13.5 32.1 12.0 27.1 9.0 25.2 9.0 25.1 7.0
6.2 6.4 6.4 6.4 6.3 6.3
0-5 5-23 23-54 54-07 87-119 119-193 193+
55.9 56.4 61.7 58.4 67.2 64.4 68.8
35.6 8.5 32.6 11.0 28.8 9.5 34.6 7.0 7.0 25.8 9.0 26.6 7.0 24.2
5.3 5.9 5.9 6.5 6.6 7.3 7.3
0-4 4-10 10-36 36-60 60-95 95-122 122-158 158-175 175-200 ZOO+
44.6 44.0 43.4 48.2 50.0 63.6 61.9 73.5 77.4 80.2
37.9 34.0 22.1 22.8 25.5 14.4 10.1 10.5 13.6 10.8
5.7 5.5 5.6 5.5 5.7 6.6 5.7 6.0 5.7 6.9
29.0 34.0
11.5 22.0 34.5 29.0 24.5 22.0 28.0 16.0 9.0 9.0
a D a t a a r e g i v e n i n w e i g h t - p e r c e n t a g e s o f s a n d . s i l t a n d c l a y (<2mm). s i e v i n g ; f i n e p a r t i c l e s i z e s (63-1.95pm) d e t e r m i n e d by h y d r o m e t e r .
w
a
soils i n montane and s a g e b r u s h s t e p p e
Primary Minerals
C l a y M i n e r a l s (<2pm) K
x x ii
I
S
Mx
- t r - x x - x x
-
-
'1
-
tr
? ?
- x - t r
tr tr
- - - -
-
-
-
-
-
-
t T
-
-
xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx
tr
?
-
-
-
- - -
x
tr tr
- -
x x x xx x x
-
-
-
x
- -
Q
-
tr
tr
C
-
x
-
V
-
-
-
tr
-
tr
xxx x
x xx x xx x xxx x x
x xxx xxx xxx xxx xxx xx xxx
xxx xxx xxx xxx xxx xxx xxx
x x x x x xxx xx xxx xxx xxx xxx xxx xx xxx
t K
XXX
XXX
XXX
t K
-
-
-
-
X
-
-
-
@
-
-
C o a r s e p a r t i c l e s i z e s (2000-63pm)
xx xxx xxx xxx xxx xxx xxx xxx xxx xxx
0
tr
P tr
x x x
x x x x x
tr tr
x x
x x x x x xx xx
tr tr
tr tr
tr tr
tr
tr tr
-
tr xxx xxx
-
xxx xxx xxx
B
x x xx xxx xx xxx x xxx t r xxx
d e t e r m i n e d by
n i l (-); minor amount ( t r ) ; s m a l l amount ( x ) ; m o d e r a t e amount ( x x ) ; bMineral abundance is based on peak height: abundance (XXX). C l a y m i n e r a l s a r e k a o l i n i t e (K). h a l l o y s i t e ( H ) , i l l i t e ( I ) , s m e c t i t e (S). v e r m i c u l i t e ( V ) , m i x e d - l a y e r i l l i t e - s m e c t i t e (Mx). Primary minerals are q u a r t z ( Q ) . p l a g i o c l a s e f e l d s p a r (P), o r t h o c l a s e ( 0 ) . and b i o t i t e (B).
-
-
Table
5.
S e l e c t e d rhemiral p r o p e r t i e s i n s o i l s of m i d Wyoming.
to
l a t e - P l e i s t o c e n e a g e , Wind R i v e r R a n g e ,
----
S e l e c t e d r h e m i r a l p r o p e r t i e s i n s o i l s o f m i d - t o l a t e - P l e i s t o c e n e a g e , Wind R i v e r R a n g e , E x t r a c t a b l e a C a t i o n s (meq/lOOg) CEC Base Organic Nitrogen Wyoming. meq/ Saturation M a t t e r (%) ---(cm) Na+ K+ Ca+’ Mg” ( X I ( % I l 0 0 g S i t e Horizon Depth E x t r a c t a b l e a C a t i o n s (meq/lOOg) CEC Base Organic Nitrogen meq/ Saturation Matter (%) 19.8 0.875 FL8 A l l 0- 6 ( c m ) 1 . 2 N a + 1 . 2 K+ 2 7 . 8 C a + ’ 4 . 8 Mg” 41.1 85 ( X I ( % I l00g 0.218 5.1 A12 6-22 0.1 0.9 7.7 1.7 14.8 70 0.148 3.7 B21 22-28 0.1 0.8 4.3 1.3 10.2 64 19.8 0.875 FL8 A l l 0- 6 1.2 1.2 27.8 4.8 41.1 85 0.058 1.3 B22 28-38 0.1 0.5 2.9 0.8 5.9 73 0.218 5.1 A12 6-22 0.1 0.9 7.7 1.7 14.8 70 0.029 cox 38-102 0.1 0.2 2.3 0.6 5.0 64 0.6 0.148 3.7 B21 22-28 0.1 0.8 4.3 1.3 10.2 64 0.015 0.3 Cn 102+ 0.1 0.1 1.6 0.5 3.9 59 0.058 1.3 B22 28-38 0.1 0.5 2.9 0.8 5.9 73 0 cox 38-102 0.1 0.2 2.3 0.6 5.0 64 0.6 0 . 1 7 8. 0 2 9 6.4 FL7 A2 0-5 0.1 0.4 3.0 0.7 12.1 35 0.015 0.3 Cn 102+ 0.1 0.1 1.6 0.5 3.9 59 0.024 1.0 B21t 5-23 0.0 0.2 2.8 0.7 5.7 65 0.013 0.4 B22 23-54 0.0 0.1 1.2 0.5 3.4 53 0.178 6.4 FL7 A2 0-5 0.1 0.4 3.0 0.7 12.1 35 0.008 0.1 Clox 54-87 0.1 0.1 1.3 0.6 3.0 70 0.024 1.0 B21t 5-23 0.0 0.2 2.8 0.7 5.7 65 0.009 0.1 c20x 87-119 0.1 0.1 2.3 0.7 4.1 78 0.013 0.4 B22 23-54 0.0 0.1 1.2 0.5 3.4 53 0.016 0.1 1.1 5.7 95 C3ox 119-193 0 . 2 0.1 4.0 0.008 0.1 Clox 54-87 0.1 0.1 1.3 0.6 3.0 70 0.006 0.2 Cn 193+ 0.2 0.2 3.0 1.0 4.6 94 0.009 0.1 c20x 87-119 0.1 0.1 2.3 0.7 4.1 78 1.1 5.7 95 C3ox 119-193 0 . 2 0.1 4.0 0 . 2 400. 0 1 6 14.1 0.1 2.3 46.0 37 0- 4 0.9 13.8 0.03 A1 FL9 0.2 Cn 193+ 0.2 0.2 3.0 1.0 4.6 94 0 . 0 604. 0 0 6 3.3 27.9 26 1.3 4-10 0.08 0.5 5.3 B21t 0.030 0 . 4 2.3 20.0 47 10-36 0.4 6.5 0.16 B22t 0.240 14.1 2.3 46.0 37 0- 4 0.9 13.8 0.03 A1 FL9 0.039 0.5 28.2 4.3 55 0.4 10.7 36-60 0.i7 B23t 0.064 3.3 27.9 26 1.3 4-10 0.08 0.5 5.3 B21t 0.006 0.9 77 5.8 23.0 11.5 0.4 60-95 0.07 B24t 0.030 0.4 2.3 20.0 47 10-36 0.4 6.5 0.16 B22t 0.007 0.7 72 22.4 4.4 95-122 0.3 11.3 IIB25t 0.09 0.039 0.5 28.2 4.3 55 0.4 10.7 36-60 0.i7 B23t 0.010 0.7 7.1 96 29.7 122-158 0.15 0.5 20.8 IIB26t 0.006 0.9 77 5.8 23.0 11.5 0.4 60-95 0.07 B24t 0.008 100 0.7 21.6 15.1 0.4 I I C l O X 6.3 158-175 0.16 0.007 0.7 72 22.4 4.4 95-122 0.3 11.3 IIB25t 0.09 0.029 1.2 20.4 12.1 4.1 89 0.2 IIC20x 175-200 0.06 0.010 0.7 7.1 96 29.7 122-158 0.15 0.5 20.8 IIB26t 0.009 0.7 88 14.1 3.1 200+ 0.17 0.2 8.9 IIC3nm 0.008 100 0.7 21.6 15.1 0.4 I I C l O X 6.3 158-175 0.16 0.029 1.2 20.4 12.1 4.1 89 0.2 IIC20x 175-200 0.06 0.009 0.7 88 14.1 3.1 2 0 0 + 0 . 1 7 0 . 2 8 . 9 IIC3a nm - i n 1 N a m m o n i a a c e t a t e o f pH 7 . 0 S i t e
Horizon
Table
Depth
a
5.
- i n 1 N ammonia a c e t a t e of
pH 7 . 0
T a b l e 6. Site
A g e and
S e l e c t e d physicala, c h e m i c a l and m i n e r a l b p r o p e r t i e s o f late-Pleistocene soils, T e t o n R a n g e , Wyoming. Elevation
Parent M a ter ia 1
(m)
postPinedale
2120
Soil Depth Vegetation Horizon (cm) ~~
PB18
MOSl
postBull Lake
2060
Montane Forest
Montane Forest
2 2 2 Sand Silt Clay P H 2mm-63um 63-4um < 4 U m (1:l)
~~~~
~
~~
Clay M i n e r a l s ( < 2 u m )
~
A1 IIB2 IIClOX IIC~OX IICn
0-9 9-34 34-87 87-116 116+
52.7 69.8 92.8 92.9 75.5
35.3 19.7 1.2 6.1 18.5
12.0 10.5 6.0 1.0 6.0
6.2 6.1 6.3 6.3
A21 A22 B2irh Clox c20x c3 IIC4ox IID
0-14 14-24 24 55 55-92 92-110 110- 14 3 143-170 170+
7.0 5.9 2.7 6.7 14.4 19.4 55.9
78.5 61.1 54.8 54.8 51.6 38.6 23.1
14.5 33.0 42.5 38.5 34.0 42.0 21.0
5.4 6.1 5.8 6.3 6.7 8.3 8.5
6.0
Primary Minerals
cIM x
--
~
x - tr x - tr tr - tr
- tr -
-
tr
xx x xxx xx x xxx xx x xxx tr - xx - -
x xx
-
-
-
-
-
-
-
-
-
-
tr
x
x
-
tr tr
x
-
-
-
-
-
xxx xxx xx xxx xxx xxx -
-
x X
-
-
t
r
xxx xxx xxx xxx xxx
x tr tr tr tr
xxx xxx xxx xxx xxx xxx x
x x x xxx xx x X
'Data a r e g i v e n in weight-percentages of s a n d ; silt and clay (<2mm). C o a r s e p a r t i c l e s i z e s (2000-63um) d e t e r m i n e d by s i e v i n g ; f i n e p a r t i c l e s i z e s (63-1.95~10) determined by hydrometer. b M i n e r a l a b u n d a n c e is based o n peak height: nil ( - ) ; m i n o r amount (tr); s m a l l a m o u n t (x); m o d e r a t e a m o u n t (xx); a b u n d a n t (xxx). Clay m i n e r a l s are: k a o l i n i t e (K). h a l l o y s i t e (H). i l l i t e (I), s m e c t i t e m i x e d l a y e r i l l i t e - s m e c t i t e (Mx). c h l o r i t e (C). Primary m i n e r a l s are: o r t h o c l a s e (0). p l a ~ i o c l a s e f e l d s p a r (P), and q u a r t z (Q).
(s),
X
x xx xx xx X X
X
xxx x
-
tr
367
1 C N T W O I T H G R A D E SCALE I M I C I O N S I
Figure 3
P a r t i c l e s i z e d i s t r i b u t i o n c u r v e f o r PB18 s o i l p r o f i l e (post-Pinedale s o i l ) .
Figure 4
Particle s i z e distribution curve for MOSl s o i l p r o f i l e ( p o s t - B u l l Lake s o i l ) .
368
clay buildup over time. Not a l l o f t h e c l a y a c c u m u l a t e s by i n s i t u w e a t h e r i n g , a s p o s t - B u l l Lake s o i l s i n t h e T e t o n Range h a v e t h i c k l o e s s caps s u g g e s t i n g l a r g e a i r f a l l i n f l u x e s of s i l t during t h e l a t e QuaternF a r y ( p r o b a b l y d u r i n g t h e P i n e d a l e G l a c i a t i o n ) . Both s o i l s d i s p l a y upward f i n i n g s e q u e n c e s i n t h e p r o f i l e s , t h e r e s u l t o f l o e s s b e i n g t r a n s l o c a t e d downward i n t o t h e solum by p e r c o l a t i n g w a t e r . The o l d e r s o i l h a s a t h i c k e r l o e s s cap (1143 cm) and a n o v e r a l l s i l t c o n t e n t n e a r l y f o u r times g r e a t e r t h a n i n t h e younger s o i l . T h i s c o n d i t i o n i s att r i b u t e d t o a l o n g e r t i m e f o r w e a t h e r i n g a n d g r e a t e r amounts o f l o e s s The h i g h e r amount o f blown i n from n e a r b y P i n e d a l e g l a c i a l l i m i t s . c l a y i n t h e p o s t - B u l l Lake s o i l i s a l s o u n d o u b t e d l y p a r t l y a f u n c t i o n of g r e a t e r t i m e f o r w e a t h e r i n g a n d a i r f a l l i n f l u x . The c e n t e r s o f g r a v i t y (mean p h i ) f o r t h e B h o r i z o n s o f f o u r p o s t F i n e d a l e p r o f i l e s i n l o w e r P a i n t b r u s h Canyon a n d one p o s t - B u l l Lake s o i l i n a p r o m i n e n t end m o r a i n e n e a r Cottonwood C r e e k , T e t o n Range, ( F i g u r e 1) a r e c a l c u l a t e d f r c m :
The d a t a ( T a b l e 7 ) c l e a r l y show a c o n s i d e r a b l e d i f f e r e n c e between B 2 h o r i z o n s i n t h e two s o i l s t r a t i g r a p h i c u n i t s , w h i c h i s u s e f u l i n d i f ferentiating these s o i l s . Late-Pinedale-Indian
Basin s o i l sequences
I n many of t h e h i g h a l p i n e v a l l e y s o f t h e Wind R i v e r a n d T e t o n r a n g e s , b e t w e e n 2,800 and 3 , 3 0 9 m e l e v a t i o n , m o r a i n e s o f l a t e - P i n e d a l e and I n d i a n B a s i n a g e a r e p r o m i n e n t l y d i s p l a y e d . D i f f e r e n t i a t i o n o f t h e s e d e p o s i t s i s o f t e n d i f f i c u l t on t h e b a s i s o f s u p e r f i c i a l c h a r a c t e r i s t i c s , and r e q u i r e s d e t a i l e d w e a t h e r i n g , s o i l s t r a t i g r a p h i c , and i s o t o p i c d a t a . The most d e t a i l e d i n f o r m a t i o n c a n be g l e a n e d from morp h o l o g i c a l , c h e m i c a l , and c l a y m i n e r a l a n a l y s i s of i n d i v i d u a l pedons ( s e e Mahaney, 1 9 7 8 ) . A r e p r e s e n t a t i v e s e q u e n c e c a n be f o u n d i n Titcomb B a s i n i n t h e Wind R i v e r Mountains ( s i t e s T B 1 4 a n d T B l ; s e e Mahaney, 1 9 7 8 , p . 2 4 2 - 2 4 3 ; l o c a t i o n p . 2 2 7 - 2 2 8 ) , and t h e u s e o f p a r t i c l e s i z e d i s t r i b u t i o n , c l a y m i n e r a l o g y , and c h e m i c a l p a r a m e t e r s i n d i f f e r e n t i a t i n g two s o i l s t r a t i g r a p h i c u n i t s t h e r e have b e e n d i s c u s s e d by Mahaney ( 1 9 7 8 ) . Major d i f f e r e n c e s i n c l u d e i n c r e a s e d c l a y c o n t e n t i n t h e p o s t P i n e d a l e s o i l , h i g h e r amounts o f 2 : l c l a y m i n e r a l s , s u c h as s m e c t i t e , i n t h e B a n d C h o r i z o n s o f t h e p o s t - I n d i a n B a s i n s o i l , and h i g h e r p e r c e n t a g e s of b a s i c c a t i o n s and s a l t s i n t h e p o s t - I n d i a n B a s i n s o i l . Extractable i r o n oxides Data from t h e s o i l s u n d e r montane f o r e s t n e a r Fremont Lake were u s e d t o d e t e r m i n e t h e d e g r e e t o w h i c h e x t r a c t a b l e Fe o x i d e s ( C o f f i n , 1 9 6 3 ; McKeague a n d D a y , 1 9 6 6 ) a r e u s e f u l a g e i n d i c a t o r s . A s shown i n T a b l e 8 , o r g a n i c a l l y - b o u n d Fe ( p y r o p h o s p h a t e e x t r a c t a b l e F e p ) h a s s i m i l a r v a l u e s i n t h e two y o u n g e s t soils, i n c r e a s i n g i n t h e l o w e r s o l u m o f t h e p r e - B u l l Lake s o i l . O r g a n i c a l l y complexed Fe a p p e a r s t o i n c r e a s e s l i g h t l y w i t h g r e a t e r a g e , and t e n d s t o b e d i s t r i b u t e d t h r o u g h the p r o f i l e s t o considerable depth. O x a l a t e e x t r a c t a b l e Fe ( F e , ) , r e p r e s e n t i n g t h e o r g a n i c p l u s t h e amorphous F e y f o r m s s l o w l y r e a c h i n g t h e h i g h e s t v a l u e s i n t h e p o s t - B u l l Lake s o i l , t h e r e a f t e r d e c r e a s i n g t o low amounts i n t h e s u b s o i l o f t h e o l d e s t p r o f i l e . T h i s t r e n d s u g g e s t s t h a t more amorphous Fe i s conv e r t e d t o c r y s t a l l i n e Fe w i t h g r e a t e r a g e . D i t h i o n i t e e x t r a c t a b l e Fe ( F e d ) d e c r e a s e s from a maximum o f 1 . 3 6 % i n t h e p o s t - P i n e d a l e s o i l t o 1 . 1 8 % i n t h e p o s t - B u l l Lake s o i l , and t h e n r i s e s t o n e a r l y 6 . 0 % i n t h e p r e - B u l l L a k e s o i l . Lower o v e r a l l c l a y v a l u e s ( T a b l e 4) c a u s e t h e low v a l u e i n t h e p o s t - B u l l Lake soil. The Fe r a t i o ( F e o / F e d ) i s i n i t i a l l y q u i t e v a r i a b l e , i n c r e a s i n g i n t h e s u b s o i l a s Feo i n c r e a s e s and Fed d e c r e a s e s w i t h d e p t h . I n t h e p o s t B u l l Lake s o i l t h e r a t i o i s d i s t r i b u t e d more u n i f o r m l y w i t h d e p t h . The r a t i o d r o p s i n t h e pre-Bu11 Lake s o i l , and i n t h e I I B 2 6 t h o r i z o n f a l l s
369 Table 7 .
Mean p h i c a l c u l a t i o n s f o r p o s t - P i n e d a l e L a k e s o i l s , T e t o n R a n g e , Wyoming.
-
Site
Horizon
ll_l_l
Age
post? Pinedale
and post-Bull
_ I -
Percentilea 16
50
84
z4?
1.1
4.2
7.7
4.3
1.1
3.8
7.2
4.0
PB19
B2
PB16
B2
PB17
B2lir
0.8
3.0
6.4
3.4
B22ir
1.7
2.8
6.5
3.7
PB18
B2
0.9
2.6
6.4
3.3
MOSl
32irh
4.9
6.
a
14.0
8.6
postB u l l Lake
.
----a
The p h i v a l u e s f o r t h e 1 6 t h , 5 0 t h , a n d 8 4 t h p e r c e n t i l e s t a k e n from p a r t i c l e s i z e d i s t r i b u t i o n c u r v e s . T h e s a n d s e p a r a t e s were d e t e r m i n e d by s i e v i n g ; f i n e p a r t i c l e s i z e s (<63um) by hydrometer.
to <0.10.
Here the percent of Fed far outweighs the percent of Fe,.
The Fe ratio i's an important age indicator, especially for Pleistocene soil sequences. The data herein support the findings of Alexander (19741, who analyzed variations in the Fe ratio in a sequence of Quaternary soils formed in stream deposits along the east flank of the Sierra Nevada, California. His overall trend of low to high to low ratios parallels 'data described in this study. Quartz/feldspar ratios Weathering intensity can be determined using weathering ratios of light minerals following procedures set forth by Ruhe (1956). In the light mineral suite quartz is more resistant than feldspar. Counts of resistant and weatherable minerals for each size fraction (250-63pm) in each soil horizon are made under a binocular microscope with an electronic point counter. Larger quotients indicate more resistant than weatherable minerals, whereas smaller quotients reflect more feldspar present in the sample. When ratios for the Cox and Cn horizons are similar, we assume that parent materials have similar lithologic compositions. Higher quotients from A and B horizons suggest that weathering processes have removed some feldspars leaving higher ratios of quartz/feldspar. Applying the quartz/feldspar weathering indices to two soil sequences in the Wind River Range provides data useful in interpreting the relative weathering states of individual profiles. In the Fremont Lake area (for profile morphology and location of sites see Mahaney, 1978) five profiles FL1, 2, 3, 4, and 6 were analyzed. In Figure 5, the post-Pinedale soils have quartz/feldspar ratios that range from 1.0 to 1.5; most profiles are uniform with depth. Changes in the post-Bull Lake soils indicate that the quartz/ feldspar ratio increases from 3.0 to 3.5 due mainly to increased weathering over greater time. The data support the hypothesis that the Bull Lake Glaciation is pre-Wisconsinan in age and may be correlative with the Illinoian Glaciation in midwestern and eastern North America. Furthermore, this would make the post-Bull Lake soilcoeval with the Sangamon Interglacial Stage (%100,000 BP). Additional evidence for a pre-Wisconsinan age of the post-Bull Lake soil is found in Mahaney (1381). In Titcomb Basin (Figure 1) changes in quartz/feldspar ratios in a soil sequence formed in glacial deposits show some interesting variations (Figure 6). The soils range in age from ~ 1 0 0yrs BP (TB15; post-Gannett Peak), %lo00 y r s BP ( T B 6 ; post-Audubon), ~ 3 0 0 0yrs BP (TB14; post-Indian Basin) and ~ 8 0 0 0yrs BP ( T B 1 and 3; post-Pinedale). The ratio i s close to 1.0 for all parent materials in all profiles. There is little change between the post-Audubon and post-Indian Basin
T a b l e 8.
Site
Age
S o i l C o l o r and iron in the pyrophosphate, citrate-dithionite, and acid oxalate extracts for sites in the Wind River Range
Horizon
Depth (cm) ~~
FL8
FL7
FL9
postPinedale
postBull Lake
preBull Lake
A1 1 A12 B21 IIB22 IICOX IICn
0-6 6-22 22-28 28-38 28-102 102+
A2 B21t B22 Clox IIC20x IIC 3ox IICn
5-23 23-54 54-87 87-119 119-193 193+
A1 B21t B22t B23t B24t IIB25t IIB26t IIClOX IIC20x IICnm
0-4 4-10 10-36 36-60 60-95 95-122 122-158 158-1 75 175-200 200+
0-5
Hue
~~
valuejchroma moist dry
-
-
Fe ( X ) in extract Pyrophosphate CitrateDithionite (Fep) (Fed)
I _ -
Acid Oxalate
(Fee)
~~
1 OYR
1OYR lOYR lOYR lOYR 2.5Y lOYR lOYR lOYR lOYR lOYR lOYR 2.5Y lOYR lOYR lOYR lOYR lOYR 7.5YR lOYR lOYR 7.5YR lOYR 2.5Y
213 312 413 513 613 513 51 2 414 614,613 5/3,5/4,6/3 513 514,516,614 614 212,213 51 4 614 614 514
414 41 4
514 516,414 811 713
312,412
41 2
413 613 713 813
612 713 812,712 812 813
814
813
412 513 713 714 614 614 514,616 614 6/4,7 14 614,714
0.16 0.24 0.22 0.12 0.09 0.07 0.20 0.16 0.09 0.06 0.05 0.06 0.06 0.42 0.32 0.07 0.08 0.18 0.13 0.12 0.09 0.10 0.08
1.05 1.35 1.36 1.34 1.07
1.04 1.12 1.18 0.97 0.87 0.83 0.91 0.88 1.70 0.81 1.83 2.69 1.86 4.58 5.98 3.18 2.30 3.24
0.52 0.47
0.44
0.63 0.77 0.72 0.82 0.96 0.77 0.69 0.69 0.64 0.66 0.82 0.71 0.43 0.63 0.77 0.70 0.37 0.58 0.45 0.36
0.50 0.35 0.32 0.47 0.72 0.69 0.73 0.81 0.79 0.79 0.83 0.70 0.75 0.48 0.88 0.23 0.23 0.41 0.15 0.06 0.18 0.20 0.11
w
4
0
371
Otz / Feldspar
7
Ratio
10 15 I
05
AI
10
B2t
B22t B23t
B22t
30 40
L
20 30 40
924 t
50
.? 60 :2
90.
r r :n
100. Cn 110
Q
CI
E
'
120.
CI Cn
c2
Cn
130' 140.
c2 Cn I5O1
I post Pinedale soils-
Figure 5
-post-
70 80
-
7
0
3
90 I00
110
120 I30
I40
I50
' FL6
FL4
FL2
FL1
I
' 0 6
Bull Lake soils
Mineral weathering r a t i o s of post-Pinedale and postB u l l L a k e p a l e o s o l s , s o u t h of F r e m o n t L a k e i n t h e Wind R i v e r R a n g e , Wyoming. S e e M a h a n e y , ( 1 9 7 8 ) , p . 230 f o r l o c a t i o n ; p. 262-64 f o r s o i l d e s c r i p t i o n s .
p r o f i l e s except t h a t t h e A 1 horizon contains l e s s feldspar i n the postI n d i a n B a s i n s o i l . The p o s t - P i n e d a l e s o i l s ( T B 1 and T B 3 ) c o n t a i n h o r izons with considerably lower f e l d s p a r counts. Q u a r t z / f e l d s p a r r a t i o s a p p e a r t o have g r e a t u t i l i t y i n s u b d i v i d i n g s o i l s i n a l p i n e g l a c i a l s u c c e s s i o n s . The q u o t i e n t s o b t a i n e d , however, may p r o v i d e v a l i d d i f f e r e n t i a , t i o n c r i t e r i a only f o r i n d i v i d u a l drainages. Additional r e s e a r c h i s n e e d e d to d e t e r m i n e i f t h e m a g n i t u d e of c h a n g e i n r a t i o s among s o i l s i s u n i f o r m i n d i f f e r e n t d r a i n a g e s of one r a n g e . Radiocarbon d a t e s O r g a n i c m a t e r i a l s from t h e b o t t o m o f two s h o r t c o r e s i n bogs on t h e l a t e - P i n e d a l e m o r a i n e s y s t e m i n Titcomb B a s i n y i e l d e d r a d i o c a r b o n a g e s o f 7,380 f 150 y r s BP (Gak-8361; s i t e T B 2 4 ) a n d 7 , 9 4 0 ? 1 9 0 y r s BP (Gak-8216; s i t e T B 2 3 ) (Mahaney, 198213, i n p r e p a r a t i o n ) . These d a t e s D r o v i d e minimum a g e s f o r l a t e P i n e d a l e d r i f t i n Titcomb B a s i n and maximum a g e s for p r o f i l e T B 1 . They i n d i c a t e a n a p p r o x i m a t e t i m e z e r o of ~ 8 , 0 0 0y e a r s BP for t h e b e g i n n i n g o f s o i l d e v e l o p m e n t i n l a t e P i n e d a l e s u b s t r a t e s . No f i n i t e r a d i o c a r b o n d a t e s e x i s t for t h e b e g i n n i n g o f t h e I n d i a n B a s i n a d v a n c e i n t h e Wind R i v e r and T e t o n r a n g e s , b u t u s i n g a r a d i o c a r b o n d a t e o f 5050 f 1 7 0 y r s BP (Gak-8359; s a m p l e NR6-Alb) from a s t o n e - b a n k e d l o b e on Niwot R i d g e i n t h e F r o n t Range, C o l o r a d o , we i n f e r a n aDDroximate d a t e f o r t h e o n s e t o f N e o a l a c i a k i n n o f 5 , 0 0 0 y r s B P . ~
372
I
10
20'
30 40
50
60
-E
.
QTZ/FELDSPAR 10 IS
R A T I O I S 2 0
2s
30
1
1.5
2,0
2,s
I
3,O
3,s
, 0
!O 50
'O 50
0
m
"E
3
:
I
70
70
I
2@J I
:so
I /
w
0
100
80
90
01
I00
110
110
120
I20
I30
1 s
I40
140
150 I
TB15
post-Ganneft
TB6
I
~ o s l -A u d u b o n
P e a l 1011
Figure 6
1011
TB14
TB1
TB3
I50
p o s f - lndran
Basln
sol1
M i n e r a l w e a t h e r i n g r a t i o s of Holocene p a l e o s o l s and s o i l s i n T i t c o m b B a s i n , Wind R i v e r R a n g e , W y o m i n g . S e e Mahaney ( 1 9 7 8 ) p . 227-28 f o r l o c a t i o n , p . 25659 f o r s o i l d e s c r i p t i o n s .
T h i s c o r r e l a t e s c l o s e l y w i t h e s t i m a t e s made by B e n e d i c t ( 1 9 6 8 , 1973). 4 d a t e from a s o i l b u r i e d b e n e a t h l a c u s t r i n f s a n d i n d i c a t e s t h a t t h e I n d i a n B a s i n a d v a n c e ended more t h a n 3,05C t 1 2 0 y r s B F ( G a k - 6 0 2 4 ) . T h i s s e q u e n c e documents a r i s e i n t h e l e v e l o f Lower Titcomb Lake (for l o c a t i o n s e e s i t e TB9, Mahaney, 1 9 7 8 , p . 2 2 8 ) . CONCLUSIONS
V a r i o u s a b s o l u t e ( A D ) and r e l a t i v e (RC) d a t i n g m e t h o d s u s e f u l i n e s t a b l i s h i n g a g e s f o r d e p o s i t s i n t h e m o u n t a i n s of w e s t e r n Wyoming have been d i s c u s s e d . Radiocarbon d a t i n g i s very u s e f u l , b u t because d a t a b l e 3 r g a n i c m a t e r i a l s a r e s c a r c e , most w o r k e r s must r e l y on c e r t a i n r e l a t i v e -nethods t h a t r e q u i r e a b r o a d r a n g e of g e o l o g i c , geomorphic and p e d o l o g i c e x p e r t i s e . Every a t t e m p t s h o u l d b e made t o u s e a number o f d i f f e r e n t -.eth,ods i n e s t a b l i s h i n g g e o l o g l c a l c h r o n o l o g i e s . O f t h e RD m e t h o d s , s o i l s t r a t i g r a p h y h a s g r e a t u t i l i t y i n d e p o s i t d i f f e r e n t i a t i o n a s many i n d i v i d u a l p r o p e r t i e s depend on e l a p s e d t i m e s i n c e d e p o s i t i o n . Because s o i l p r o p e r t i e s aDproach s t e a d y s t a t e a t d i f f e r e n t r a t e s , we h a v e c o n c e n t r a t e d on s p e c i f i c c h a r a c t e r i s t i c s t h a t u n d e r g o change o v e r l o n g t i m e p e r i o d s ( e . g . 0 . 5 x l o 6 y e a r s ) . T h u s , d e p t h o f w e a t h e r i n g , p a r t i c l e s i z e c h a n g e s ( e s p e c i a l l y upward f i n i n g s e q u e n c e s and s h i f t s i n mean p h i ) , r e l a t i v e c l a y m i n e r a l a s s e m b l a g e s , q u a r t z / f e l d s p a r r a t i o s ( w i t h i n t h e f i n e and v e r y f i n e s a n d f r a c t i o n s ) , znd some s o i l c h e m i c a l p a r a m e t e r s p r o v i d e a d e t a i l e d body of d a t a u s e 5 1 i n r e l a t i v e a g e d e t e r m i n a t i o n . A s more a b s o l u t e d a t e s become z v a i l a b l e i t i s i m p e r a t i v e t o e s t a b l i s h l i c h e n growth c u r v e s , weathering r i n d c u r v e s , and i r o n o x i d e c u r v e s f o r i n d i v i d u a l r a n g e s .
373
ACKNOWLEDGEMENTS We thank B.D. Fahey (Guelph University) and W.N. Melhorn (Purdue University) for critical reviews of this paper. N. Stokes assisted in the field in 1972. Students in the York University Mountain Geomorphology Field Camp (1973-75 and 1978-81) assisted with the field work. L.J. Gowland helped with the excavation and analysis of several profiles discussed In the text. Soil and sediment samples were analyzed in the Geomorphology and Pedology Laboratory at York University with the assistance of G. Berssenbrupge. Rock and sand samples were analyzed at the North Dakota Geological Survey, Grand Forks, N.D., and Geomorphology Research Center, Purdue University, W. Lafayette, Indiana. Research was financed by grants from York University, and carried out with the cooperation of the U.S. Forest Service and National Park Service. We are grateful to Mr. Bob Wood (NPS) for permission to carry out the rield work. REFERENCES C I T E D Alexander, E.B., 1 9 7 4 , E x t r a c t a b l e i r o n i n r e l a t i o n t o s o i l a g e on t e r r a c e s a l o n g t h e Truckee R i v e r , Nevada: S o i l Sci. SOC. A m e r . P r o c . , v . 38, p. 1 2 1 - 1 2 4 . Andrews, J . T . and Webber, P . J . , 1964, A l i c h e n o m e t r i c a l s t u d y of t h e n o r t h w e s t e r n m a r g i n of t h e B a r n e s I c e Cap: a geomorphological t e c h n i q u e : G e o g r a p h i c a l B u l l . , O t t a w a , no. 22, p . 80-104. 1968, Recent g l a c i a l h i s t o r y of a n a l p i n e area i n t h e Benedict, J . B . , I1 D a t i n g t h e g l a c i a l d e p o s i t s : J o u r . Colorado F r o n t Range, U.S.A. of G l a c i o l o g v , v . 7 , n o . 4 9 , p . 77-87. Range:
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, 1 9 7 8 , L a t e - Q u a t e r n a ' r y s t r a t i g r a p h y a n d s o i l s i n t h e Wind ed.: R i v e r M o u n t a i n s , w e s t e r n Wyoming: i n M a h a n e y , W . C . , Q u a t e r n a r y S o i l s , Norwich, U . K . , G e o a b s t r a c t s L t d . , p . 223-264. , 1981, P a l e o c l i m a t e r e c o n s t r u c t e d from p a l e o s o l s : evidence f r o m t h e Rocky M o u n t a i n s and E a s t A f r i c a : i n Mahaney, W . C . , ed.: Q u a t e r n a r y P a l e o c l i m a t e , N o r w i c h , U . K . , G e o a b s t r a c t s L t d . , p . 227247. , 1982a, C o r r e l a t i o n of Quaternary g l a c i a l and p e r i g l a c i a l d e p o s i t s o n Mount K e n y a , E a s t A f r i c a w i t h t h e Rocky M o u n t a i n s o f w e s t e r n U . S . : i n A b s t r a c t s V o l . I , X I INQUA Congress, MOSCOW, U.S.S.R., p. 208. , 1 9 8 2 b , S u p e r p o s e d N e o g l a c i a l / l a t e P i n e d a l e t i l l s , Summer Boreas, I c e L a k e - T i t c o m b B a s i n , Wind R i v e r M o u n t a i n s , W y o m i n g : submitted. Mahaney, W . C . and F a h e y , B . D . , 1976, Ouaternary S o i l S t r a t i g r a p h y of t h e F r o n t Range, Colorado: i n Mahaney, W . C . , ed.: Quaternary S t r a t i g r a p h y o f N o r t h A m e r i c a , S t r o u d s b u r g , P a . , Dowden, H u t c h i n s o n and ROSS, p . 319-352.
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Sharp, R.P., 1969, S e m i q u a n t i t a t i v e d i f f e r e n t i a t i o n of g l a c i a l moraines n e a r Convict Lake, S i e r r a Nevada, C a l i f . : J o u r . Geol:-, v. 77, p . 68-91. U.S.
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NOAA,
Climat-
S u b l e t t e County,
375
DATING METHODS APPLICABLE TO LATE GLACIAL DEPOSITS OF THE LAKE AGASSIZ BASIN, MANITOBA
R.W. K LASSE N
ABSTRACT A r a d i o c a r b o n c h r o n o l o g y b a s e d on n i n e t e e n d a t e s from wood, s h e l l s , p e a t , g y t t j a a n d bone from s i t e s w i t h i n a n d a d j a c e n t t o t h e Lake A g a s s i z b a s i n i n Manitoba s u g g e s t s t h i s l a k e began i n s o u t h e r n Manitoba b e f o r e 1 4 , 0 0 0 y e a r s a g o and d r a i n e d i n t o t h e T y r r e l l S e a a b o u t 8 , 0 0 0 y e a r s ago. R e l a t i v e d a t i n g methods i n c l u d i n g r o u n d n e s s e s o f b e a c h p e b b l e s , a v e r a g e r a t e s o f s e d i m e n t a t i o n and i c e r e t r e a t e a c h a c c o u n t f o r a b o u t o n e - h a l f t o t w o - t h i r d s o f t h e 6 , 0 0 0 y e a r s p a n s u g g e s t e d by t h e r a d i o c a r b o n d a t e s .
The d i s c r e p a n c i e s between t h e v a r i o u s d u r a t i o n s i n f e r r e d from e a c h o f t h e d a t i n g methods t o some e x t e n t r e f l e c t t h e l i m i t a t i o n s of t h e methods. However, t h e r e l a t i v e d . a t i n g methods do n o t a c c o u n t f o r t h e d u r a t i o n o f s e v e r a l low w a t e r s t a g e s and s t i l l s t a n d s o f t h e g l a c i e r . C o n s i d e r a t i o n o f t h e s t r a t i g r a p h i c p o s i t i o n s of t h e most r e l i a b l e r a d i o c a r b o n d a t e s and minimal t i m e i n t e r v a l s i n f e r r e d from r e l a t i v e d a t i n g s u g g e s t s t h e o l d e s t r a d i o c a r b o n d a t e may b e a b o u t 1 0 0 0 y e a r s t o o o l d and t h a t Lake A g a s s i z e x i s t e d b e t w e e n a b o u t 1 3 , 0 0 0 and 8 , 0 0 0 y e a r s ago i n Manitoba. INTRODUCTION
The v a r i o u s c h r o n o l o g i e s p r o p o s e d f o r g l a c i a l Lake A g a s s i z d u r i n g t h e l a s t s e v e r a l d e c a d e s a r e b a s e d on r a d i o c a r b o n d a t e s from a v a r i e t y of o r g a n i c m a t e r i a l s . The p u r p o s e o f t h i s p a p e r i s t o a p p r a i s e t h e s e r a d i o c a r b o n c h r o n o l o g i e s o f Lake A g a s s i z i n Manitoba on t h e b a s i s o f t h e s t r a t i g r a p h y of d a t e d s i t e s i n t h e v i c i n i t y o f A s s i n i b o i n e d e l t a and by r e l a t i v e d a t i n g methods i n c l u d i n g s e d i m e n t a t i o n r a t e s b a s e d on v a r v e c o u n t s , r a t e of i c e r e t r e a t and t h e d e g r e e o f r o u n d n e s s of b e a c h pebbles. R e l a t i v e d a t i n g methods p r o v i d e a measure o f t h e a p p r o x i m a t e d u r a t i o n o f c e r t a i n i n t e r v a l s o f l a k e h i s t o r y and a means o f a p p r a i s i n g t h e r a d i o c a r b o n c h r o n o l o g i e s . T h i s a p p r o a c h o f f e r s a way of e v a l u a t i n g t h e r e l i a b i l i t y o f - t h e o l d e s t f i n i t e d a t e s o b t a i n e d from o r g a n i c detritus. BACKGROUND
T h i s p a p e r f o c u s e s on p u b l i s h e d and u n p u b l i s h e d s t u d i e s d e a l i n g p r i m a r i l y w i t h t h e c h r o n o l o g y o f g l a c i a l Lake A g a s s i z i n M a n i t o b a . J o h n s t o n ( 1 9 4 6 , p . 1 7 ) a s s i g n e d a g e s t o t h e main b e a c h e s i n Manitoba by c o r r e l a t i n g w i t h A n t e v ' s ( 1 9 3 9 ) v a r v e c h r o n o l o g y for t h e G r e a t Lakes region. He c o n c l u d e d t h a t t h e o l d e s t (Herman) b e a c h formed i n s o u t h e r n Manitoba a b o u t 2 0 , 0 0 0 y e a r s a g o and t h a t t h e y o u n g e s t b e a c h e s i n t h e n o r t h e r n p a r t o f t h e b a s i n formed a b o u t 4 , 0 0 0 y e a r s a g o . A c c o r d i n g t o t h i s c h r o n o l o g y , t h e v a r i o u s s t a g e s of Lake A g a s s i z i n Manitoba o c c u r r e d o v e r some 1 6 , 0 0 0 y e a r s .
376
The a d v e n t o f r a d i o c a r b o n d a t i n g i n t h e l a t e 1950’s r e s u l t e d i n a s u b s t a n t i a l r e v i s i o n of Johnston‘s chronology. Elson (1957) c i t e d f i v e r a d i o c a r b o n d a t e s between a b o u t 1 2 , 4 0 0 and 8.000 y e a r s ago t h a t spanned t h e s o - c a l l e d “ L a k e A g a s s i z 1-11” i n t e r v a l . T h e s e r a d i o c a r b o n d a t e s w e r e i n c l u d e d i n a s u b s e q u e n t l i s t of t w e n t y - o n e r a d i o c a r b o n d a t e s t h a t f o r m e d t h e b a s i s o f t h e m o s t d e t a i l e d c h r o n o l o f f i c h i s t o r y of L a k e Agassiz i n Manitoba t o d a t e ( E l s o n , 1967, p . 8 8 - 9 4 ) . E l s o n ’ s chrono l o g y p l a c e d t h e d u r a t i o n o f Lake AgasFiz i n Y a n i t o b a between “ p r o b a b l y more t h a n 1 2 , 4 0 0 y e a r s a g o ” a n d a b o u t 7 , 5 0 0 y e a r s a g o . Two a d d i t i o n a l r a d i o c a r b o n d a t e s of a b o u t 1 2 , 8 0 0 y e a r s ( T a b l e 1, 1 - 1 6 8 2 ) a n d 8,500 y e a r s ( T a b l e 1, GSC-896) l e d E l s o n ( 1 9 7 1 , p . 2 8 9 ) t o r e v i s e t h e D r e v i o u s c h r o n o l o g y somewhat.
L a t e r r e v i s i o n s t o t h e c h r o n o l o g y o f t h e b e g i n n i n g of Lake A g a s s i z i n M a n i t o b a a r e i n c l u d e d i n r e p o r t s by K l a s s e n ( 1 9 7 2 , p . 553), C h r i s t i a n s e n ( 1 9 7 9 , p . 9 2 6 ) a n d T e l l e r a n d F e n t o n ( 1 9 8 0 , p . 3 2 ) . Add i t i o n a l r a d i o c a r b o n d a t e s f r o m t h e w e s t e r n r n a r g i c o f Lake P g a s s i z a n d t h e p r a i r i e s t o t h e west l e d K l a s s e n ( 1 9 7 2 , p . 5 5 3 ) a n d C h r i s t i a n s e n ( 1 9 7 9 , p . 9 2 6 ) t o s u g g e s t t h a t A s s i n i b o i n e d e l t a began forming between a b o u t 1 5 , 0 0 0 a n d 1 4 , 0 0 0 y e a r s a g o . A c c o r d i n g t o t h e l a t e s t p r o D o s a 1 by T e l l e r and Fenton (1980, p . 3 2 ) d e l t a b u i l d i n g o c c u r r e d c o n s i d e r a b l y l a t e r . They s u g g e s t t h e l a s t a d v a n c e of a g l a c i e r i n t o N o r t h D a k o t a o c c u r r e d a b o u t 1 2 , 0 0 0 y e a r s a g o , f o l l o w e d by r e t r e a t t o a p o s i t i o n n o r t h of t h e d e l t a by a b o u t 1 1 , 0 0 0 y e a r s a g o . T h i s t i m e frame accommodates t h e e s t a b l i s h e d chronology of l a t e Wisconsin e v e n t s s o u t h o f t h e I n t e r n a t i o n a l Boundary b u t r e q u i r e s t h e r e j e c t i o n o f t h e o l d e s t p o s t g l a c i a l r a d i o c a r b o n d a t e s from s o u t h e r n I ’ l a n i t o b a ( T e l l e r a n d F e n t o n , 1980, p. 3 0 ) . CURIiENT STATE OF R A D I O C A R B O N C H R O N O L O G Y R a d i o c a r b o n d a t e s on a v a r i e t y of o r g a n i c m a t e r i a l s f r o m p o s t g l a c i a l s e d i m e n t s w i t h i n and a d j a c e n t t o t h e Lake A g a s s i z b a s i n i n Manitoba s u g g e s t Lake A g a s s i z began b e f o r e 14,000 y e a r s a g o and ended a b o u t 8 , 0 0 0 y e a r s a g o . T h e s e r a d i o c a r b o n d a t e s i n c l u d e d a new d a t e o f a b o u t 1 3 , 9 0 0 y e a r s ( R i t c h i e , 1 9 7 6 , p . 1 7 9 9 ) f r o m t h e s u r f a c e of A s s i n i b o i n e d e l t a , a d a t e of a b o u t 8300 y e a r s (Lowdon e t aZ., 1 9 7 7 , p . 1 2 ) from a b e a c h n o r t h o f Lake Winnipeg formed d u r i n g a f i n a l s t a g e of L a k e A g a s s i z a n d 5 new r a d i o c a r b o n d a t e s f r o m p o s t g l a c i a l s e d i m e n t s w i t h i n or a d j a c e n t t o t h e n o r t h e r n e n d o f t h e Lake A p a s s i z b a s i n ( Y i g u r e 1, T a b l e 1 ) . S e v e r a l r e c e n t p u b l i c a t i o n s h a v e r a i s e d t h e q u e s t i o n of t h e v a l i d i t y of c e r t a i n r a d i o c a r b o n d a t e s on o r g a n i c d e t r i t u s f r o m l i m n i c s e d i m e n t s ( K a r r o w a n d A n d e r s o n , 1 9 7 5 ; N a m b u d i r i e t aZ., 1980; Mathewes a n d W e s t g a t e , 1 9 8 0 ) . The s o - c a l l e d “ o l d c a r b o n e f f e c t “ ( M a t h e w e s a n d W e s t g a t e , 1 9 8 0 , p . 1 4 5 0 ) i s b e l i e v e d t o h a v e r e s u l t e d i n a n a g e some 4000 y e a r s t o o o l d for a d a t e i n t h e 1 4 , 0 0 0 y e a r r a n g e i n e a s t e r n C a n a d a ( K a r r o w a n d A n d e r s o n , 1 9 7 5 , p . 1 8 0 9 ) a n d i n a g e s some 1 4 0 0 y e a r s t o o o l d for d a t e s i n t h e 4000 y e a r r a n g e i n s o u t h e r n B r i t i s h C o l u m b i a . T e l l e r a n d F e n t o n ( 1 9 8 0 , p . 3 1 ) r e j e c t e d t h e o l d e s t non-wood d a t e s ( T a b l e 1) on t h e b a s i s of c o n c l u s i o n s d r a w n by N a m b u d i r i e t aZ. ( 1 9 8 0 ) r e g a r d i n g r a d i o c a r b o n d a t i n g of p o s t g l a c i a l s e d i m e n t s f r o m L a k e Manitoba. The o l d e s t m a t e r i a l d a t e d was t h o u g h t t o b e c o n t a m i n a t e d b y dead-carbon from “ p r e - Q u a t e r n a r y o r g a n i c matter d e r i v e d from l i g n i t e s a n d s h a l e s ” ( T e l l e r a n d F e n t o n , 1980, p . 3 1 ) . The s t r a t i g r a p h i c p o s i t i o n s a n d r e l a t i v e e l e v a t i o n s o f a number of r a d i o c a r b o n d a t e d s i t e s i n p o s t - A s s i n i b o i n e d e l t a s e d i m e n t s a r e shown i n F i g u r e 2 . The r e l a t i v e e l e v a t i o n s of t h e d a t e d si.tes shown i n F i g u r e 2 a r e c h r o n o l o g i c a l l y s i g n i f i c a n t for t h e y r e l a t e t o a s e q u e n c e o f e v e n t s w i t h i n a s i n g l e f l u v i o - l a c u s t r i n e s y s t e m . The m a i n e v e n t s were t h e i n i t i a l e x c a v a t i o n of A s s i n i b o i n e v a l l e y a c r o s s t h e d e l t a a n d t h e s u b s e q u e n t i n f i l l i n g a n d r e - e x c a v a t i o n of t h e v a l l e y .
The p o s t - d e p o s i t i o n a l c h a n g e s i n r e l a t i v e e l e v a t i o n s o f t h e v a r i o u s s i t e s m u s t b e m i n i m a l a s t h e y a r e w i t h i n or a d j a c e n t t o A s s i n i b o i n e V a l l e y which r o u g h l y p a r a l l e l s J o h n s t o n ’ s ( 1 9 4 6 , p . 2 ) i s o b a s e 5 drawn from Lake A g a s s i z b e a c h e s .
377
3
Figure 1
L o c a t i o n s of s e l e c t e d r a d i o c a r b o n d a t e s of l a t e g l a c i a l and p o s t g l a c i a l a g e s i n Manitoba, and c r o s s s e c t i o n s F i g u r e s 2 a , 2b a n d 2 c .
The o l d e s t d a t e o f a b o u t 13,900 y e a r s BP ( T a b l e 1, No. 1) came from t h e h i g h e s t s i t e d i r e c t l y o v e r l y i n g d e l t a s e d i m e n t s . T h i s a g e i s some 2 0 0 0 y e a r s g r e a t e r t h a n t h e a D p r o x i m a t e a g e s o f 1 2 , 0 0 0 y e a r s from p e a t and wood ( T a b l e 1, Nos. 2 , 3 and 4) i n p o s t - d e l t a , a l l u v i a l f i l l a t s i t e s a t 1 8 m or more l o w e r i n e l e v a t i o n . The o l d e s t d a t e may w e l l be t o o o l d because o f c o n t a m i n a t i o n by dead carbon from t h e u n d e r l y i n g s a n d t h a t h a s a c a r b o n a t e c o n t e n t o f a b o u t 2 5 p e r c e n t ( K l a s s e n , 1971, p . 2 5 3 ) . C o n t a m i n a t i o n from l i g n i t e s and s h a l e , as s u g g e s t e d by T e l l e r and F e n t o n ( 1 9 8 0 , p . 31), a p p e a r s u n l i k e l y a s t h e r e i s n o e v i d e n c e o f t h e p r e - Q u a t e r n a r y p o l l e n ( R i t c h i e , 1 9 8 0 , p e r s . comm.) t h a t was f o u n d s o a b u n d a n t l y i n c o n t a m i n a t e d s a m p l e s s t u d i e d b y Nambudiri e t 0 2 . (1980, p . 1 2 4 ) . The two d a t e s o f a b o u t 1 2 , 0 0 0 y e a r s BP ( T a b l e 1, Nos. 2 a n d 3 ; F i g u r e 1) w e r e on p e a t w i t h i n a l l u v i a l f i l l i n a g u l l y e x c a v a t e d i n t o d e l t a s e d i m e n t s . Contamination of t h e dated material i s p o s s i b l e , though only t o a l e s s e r e x t e n t t h a t i n the m a t e r i a l t h a t y i e l d e d t h e
T ~ b l e1 .
S e l r c t e d Radioc,irhon nates
Pelev.int
to t h e l l i s t o r y o f
T,al
P
Ag,irsiz
i n Hanitob,~.
cu 4
00
Years BP
L a b No.
Material
Lat.
Long.
References
Comments
Dates from d e p o s i t s younger t h a n main p a r t of A s s i n i b o i n e d e l t a b u t o l d e r
1) 1 3 , 9 0 0 5 2 4 0
1-3476
organic detritus
*49"50'N
99'35'W
Ritchie, p. 1 7 9 9
2) 12,400k420
Y-165
3 ) 12,100k160
BSC-1319
4) 11,600k430
GSC-1081 wood d e t r i t u s
peat
49'47"
98'35'14
Prestcn e t az., 1955
peat
49'47"
98'35'W
Lowdon e t a Z . , 1 9 7 1 , p . 282
100'50'Y
Lowdon e t a l . , 1 9 7 1 , p . 286
49'51'E
1976,
**********
t h a n Campbell beach
Brandon; d a t e d sediment w a s from a p o s t g l a c i a l l a k e w i t h i n a swampy f l a t on t h e upper p a r t of Assiniboine d e l t a a t ca. 3 7 5 m a . s . 1 . R o s s e n d a l e ; a l l u v i a l f i l l ca. 4 m below s u r f a c e of abandoned c h a n n e l o n d e l t a a t ca. 3 2 8 m a . s . 1 . D u p l i c a t e o f R o s s e n d a l e s a m p l e Y-165; i d . b y 14. Kuc a s p e a t m o s s ( S c o r p i d i u r n
s c o r p i o i d e s1
Virden; c o r e from b a s a l zone of Assiniboine alluvium (Klassen, 1975, p . 1 8 a n d 5 8 ) a t ca. 1 8 m b e l o w s u r f a c e a t ca. 3 7 5 m a . s . 1 .
Dates from d e p o s i t s r e l a t e d t o Campbell beach
5)
1 0 , 6 0 0 ~ 1 5 0 GSC-902
plant detritus
4 9 '4 5 ' N
9 8 ' 3 9 ' 14
6)
10,3005 200
BGS-617
bone
52"lI'N
7)
10,200580
GSC-1909
49'06"
96'14'W
49'45"
98'45'W
101'26'W
Lowdon a n d B l a k e , 1 9 7 0 , p . 65 Teller,1980,p.6 Lowdon a n d B l a k e , 1976, p. 7 Lowdon a n d B l a k e , 1 9 7 0 , p . 65
8 ) 10,000k150
GSC-870
organic detritus wood
9)
GSC-1428
wood
49'54'11
9,9005160
GSC-391
wood
4 9 '0 0 ' N
9 5 '1 4 ' Id
Lowdon e t aZ., 1967, p. 10
11) 9,7005140
GSC-797
wood
4 3 '4 5 ' N
9 8 '3 9 '9
Lowdon a n d B l a k e , 1970, p . 65
lo)
1 0 , 0 0 0 5 280
100'18'tJ
Lowdon a n d B l a k e , 1 9 7 3 , p . 22
Rossendale; fluvio-lacustrine sediment ca. 1 8 m b e l o w C a m p b e l l t e r r a c e a t 3 2 0 m a.s.1. Swan R i v e r , w a t e r w o r n b i s o n b o n e fragment between upper and lower C a m p b e l l b e a c h a t ca. 3 4 4 m a . s . 1 . S u n d o w n ; ca. 8 m b e l o w s u r f a c e o f C a m p b e l l s t r a n d l i n e a t ca. 3 2 6 m a . s . 1 . R o s s e n d a l e ; (SW s i d e b a s i n ) a l l u v i u m , ca. 8 m b e l o w s u r f a c e o f C a m p b e l l terrace i n Assiniboine v a l l e y across d e l t a a t ca. 3 2 0 m a . s . 1 . A l e x a n d e r : A s s i n i b o i n e a l l u v i u m ca. 9 m d e p t h b e l o w s u r f a c e a t ca. 3 5 4 m (Klassen, 1975, p. 58) B u f f a l o P o i n t ; SE ca. 2 m below C a m p b e l l t e r r a c e s u r f a c e a t ca. 3 2 3 m a.s.1. lower Campbell beach R o s s e n d a l e ; a l l u v i u m , ea. 4 m b e l o w s u r f a c e o f C a m p b e l l t e r r a c e a t ca.
J_
Date f r o m d e p o s i t s r e l a t e d
12) 8,3105180
GSC-1679
freshwater shell
54'10'N
.._
-3. 2- J . .b J. *.> ,I ,~ ,\ ,\ ,\ ,\
-\>
I~
II
I.
32C m a . s . 1 . across d e l t a
in Assiniboine valley
2 ,\
t o l a t e s t a g e i n n o r t h e r n Manitoba
38'50'V
**********
Lowdon e t a l . , 1977, p. 12
beach r i d g e ca. Lake Winnipeg
245 m a . s . l . ,
n o r t h of
Dates f r o m p o s t Lake A g a s s i z d e p o s i t s i n n o r t h e r n M a n i t o b a 13) 8,5305220
GSC-896
marine S h e l l s
54'29"
14) 7,970+150
GSC-1825
gyttja
54"45'N
1Ol041'W
1 5 ) 7,5705140
GSC-878
marine s h e l l
56'02'N
93'17'W
16) 7,0305170
GSC-2294
marine s h e l l
56'31"
94"05'W
17) 6,9205150
GsC-1818
organic debris
56'21"
97'58'W
Lowdon a n d B l a k e , 1975, p. 14
56"52'N
95'47'W
Lowdon e t az., 1977, p. 12 Lowdon a n d B l a k e , 1979, p. 15
59'28'N
90"24'W
101'13'W
* L o c a t i o n i s c o r r e c t e d f r o m 49'35'N
99'15'W
Lowdon a n d B l a k e , 1 9 7 0 , p . 64 Lowdon a n d B l a k e , 1975, p. 14 Lowdon a n d B l a k e , 1 9 7 0 , p . 64 unpublished
marine beach a t ca. 125 m a . s . l . , ca. 1 0 m below marine l i m i t Flin Flon; basal layer i n s m a l l lake a t ca. 305 m a.s.1. Hayes River; s h e l l s i n l i v i n g p o s i t i o n i n s i l t y c l a y a t ca. 114 m a.s.1. G i l l a m ; m a r i n e c l a y a t 90 m a . s . 1 . c a . 30 m below t h e h i g h e s t T y r r e l l Sea beaches Thompson; S e t t e e m o r a i n e , b a s a l o r g a n i c l a y e r i n K e t t l e Lake ca. 305 m a.s.1. Recluse Lake; b a s a l p e a t i n permanentl y f r o z e n bog o v e r t i l l Moorby L a k e ; b a s a l p e a t f r o m p e r m a n e n t l y f r o z e n bog
g i v e n o r i g i n a l l y by R i t c h i e ( 1 9 7 6 , p .
1795)
380 Metres (a E I ) 0 0 n
e 0
0 R
W -1
Metres (a s,l.) 0
0
P
Metres (o.s.1.) 0
0
oldest date. However, the wood date of 11,600 years (Table 1, No. 4; Figure 1) from within Assiniboine alluvium some 40 km upstream from the delta, is another minimal post-delta date as shown by its stratigraphic position and elevation relative to the delta surface (Figures 2 and 2A). This date is corroborated by the 10,000 year wood date (Table 1 , No. 9 ; Figure 1) at a stratigraphically higher position within Assiniboine alluvium further downstream (Figure 2 and 2B). A series of radiocarbon dates from wood and organic detritus (Figure 2C) date the fluctuations of Lake Agassiz associated with a rising phase between about 10,800 and 9,700 years BP. Stream and lake deposits of an earlier valley fill form a terrace within Assiniboine valley where it crosses the outer part of the delta. The fossils
381
Metres (a.s.1,) 0 0
m
0 0
@l
( o s t T a c o d s a n d p o l l e n ) anC clzted m a t e r i a l w i t h i n t h e f l l l l e d K l a s s e n a n d E l s o n (1972, p . 1.3 znd 11) to propose t h e f o i l 3 w i n g s e q u e n c e o f everts: “The s u c c e s s i o n s e e n ir t h i s s e c t i o n r e c o r d s t h r e e i n t e r v a l s cf i n u n d a t i o n of t h e A s s i n l b c i n e v a l l e y by Lake A g a s s i z . D u r i n g t h e e a r l i e s t i n t e r v e l (IJzit 1) t h e l a k e was a t least a t t h e Campbeli l e v e l 1 , 0 5 0 f e e t ( 3 2 0 m) a . s . 1 . a n d a r e l a t i v e l y c o o l c l i m a t e (max. a i r temp. 2 6 ° C ) ? r e v a i l e d . S h o r t l y Der’cre a b o u t 1 0 , 6 0 0 y e a r s BP t h e l a k e fell below 9 8 0 f e e t ( 3 0 0 m) e.s.1. and t h e c l i m a t e became wamer ( i J n i t 2 , max. a i r temp. c a . 3 1 ° C ) . T h i s 1ol.i water p h a s e p r o b a b l y c o r r e s p o n d s t o a n u n c o r + f c r r n i t y r a d i o c a - b o n dzted i n N o r t h D a k s t e and M i n n e s o t a b e t w e e n 9 , 9 0 0 a n d 1 1 , 0 0 0 y e a r s
382
B e f o r e 1 0 , 0 0 0 y e a r s BP t h e c l i m a t e a g a i n became c o o l e r ( U n i t s 3 a n d 4, max. a i r temp. c a . 26OC) and t h e l a k e r o s e t o a b o u t t h e Campbell l e v e l . About 1 0 , 0 0 0 y e a r s BP t h e c l i m a t e was warmer (max. a i r temp. 35°C) and t h e l a k e was below t h e 1 , 0 1 0 f o o t ( 3 1 0 m ) l e v e l ( U n i t 5 ) . A v a r i e t y of p l a n t s ( P i c e a , P i n u s , B e t u Z a , A r t e m e s i a , M y r i o p h y Zum: i d e n t i f i e d b y R .J . Mott , G e o l o g i c a l S u r v e y o f Canada) grew a d j a c e n t t o t h e stream t h a t o c c u p i e d t h e v a l l e y . A warmer and d r i e r i n t e r v a l ( U n i t 6 ) and a m o i s t e r i n t e r v a l ( U n i t 7 ) preceded t h e f i n a l i n u n d a t i o n of t h e v a l l e y by Lake A g a s s i z ( U n i t 6 ) . A f t e r c a . 9 , 7 0 0 y e a r s BP t h e c l i m a t e became c o o l e r ( m a x . a i r temp. 2 6 ° C ) and L a k e A g a s s i z r o s e t o a t l e a s t 1 , 0 7 0 f e e t (325 m ) a . s . 1 . " BP.
The Campbell b e a c h t h a t c a n b e t r a c e d a l o n g much o f t h e w e s t e r n m a r g i n o f t h e b a s i n i n s o u t h e r n Manitoba ( F i g u r e 3 ) formed d u r i n g t h o s e i n t e r v a l s . Some 2 0 0 km n o r t h and n o r t h e a s t o f t h e n o r t h e r n m o s t p a r t of t h e Campbell b e a c h i n M a n i t o b a ( F i g u r e 3 ) r a d i o c a r b o n d a t e s from b o g b o t t o m s and s h e l l s ( T a b l e 1, F i g u r e 1) a r e a b o u t 8 , 0 0 0 y e a r s old and 7 , 0 0 0 t o 6 , 5 0 0 y e a r s o l d n e a r t h e Hudson Bay Lowland. W i t h t h e exc e p t i o n o f a m a r i n e s h e l l d a t e o f a b o u t 8,500 y e a r s ( T a b l e 1, No. 1 3 ) from t h e Lowland, t h e p r o g r e s s i v e l y y o u n g e r d a t e s t o t h e n o r t h r e f l e c t t h e d i r e c t i o n o f d e g l a c i a t i o n , and s u g g e s t t h a t c o n t a m i n a t i o n by o l d c a r b o n i s n o t a problem h e r e , even though t h e s u b s t r a t a i n p l a c e s has a s i g n i f i c a n t c a r b o n a t e c o n t e n t . For e x a m p l e , t h e f i n e f r a c t i o n of t h e t i l l b e n e a t h t h e 6500 y e a r o l d p e a t s a m p l e s a t R e c l u s e Lake ( T a b l e 1, No. 1 8 ) has 2 0 p e r c e n t c a r b o n a t e ( K l a s s e n , i n p r e s s ) . .-
-..__..__.
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Figure 3 Glacial features i n Manitoba associated with s i g n i f i c a n t phases of Lake Agassiz.
LEGEND Marine sediments
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Glacial lake Sediments A . Red River basin B. Grass River basin Delta sediments C. Assiniboine delta D. Saskatchewan delta Campbell beach End moraine E . The Pas F. Burntwood-Etowney G. Sachigo Direction of ice flow features
3 83
GLACfAL LAKE SEDIMENTS AND CHROPIOLOGY Glaciolacustrine silt and clay are widely distributed in the northern (Grass River basin) and southern (Red River basin) parts of the region covered by Lake Agassiz in Manitoba (Figure 3). Elsewhere within the region, glaciolacustrine deposits are thin and patchy and extensive areas consist of eroded till or bedrock. Antevs (1931, p . 47-50) calculated the rate of ice recession in the Grass River basin from studies of varved clay. However, studies of the glaciolacustrine sediments in the Red River basin have focussed primarily on distribution and stratigraphy (Elson, 1967, p. 45-50; Fenton and Anderson, 1971; McPherson e t al.., 1971; Teller, 1976; Teller and Fenton, 1980), and their chronologic implications have not been considered. There are various reasons for this, such as the general abandonment of varve counting as a dating method with the advent of radiocarbon dating, the lack of good exposures, and the discontinuity of sediments north o f the Red River basin. The thick, "laminated" to massive clay and silt (Teller, 1976, p.
31-36) that constitutes the bulk of the offshore sediment in the Red River basin apparently reflects a considerable span of time. A rough
estimate of this time can be made on the basis of the average sedimentation rate indicated by Antev's (1931) varve counts in the Grass River basin. The average thickness of varves measured acrLss some 225 km o f the Grass River basin between Wekusko Lake and Split Lake (Antevs, 1 9 3 1 , p . 57-67) is 1.5 cm. Thicknesses average 1 cm in the distal (western) part of the basin and 2 cm in the proximal (eastern) part. The thickness of offshore sediment in the Red Fiiver basin averages about 40 m thick (Teller, 1976, p. 34-35) suggesting 2600 years f o r deposition. Yurthermore, a significant hiatus occurred between the lower predominantly clayey sediments averaging some 30 m in thickness and the upper 10 m of predominantly silty sediments (Elson, 1967, p. 45-50; McPherson e t aZ., 1971, p . 280). The latter likely correlate with the post-delta sediments within Assiniboine valley (Figure 2C). The average thickness of varved clay at 17 sites measured by Antevs (1931, p. 51) across the Grass River basin was 2.5 m . Borings in the vicinity of Thompson in the western part of the basin penetrated clays up to 9 m thick, but much ground ice was included (Klassen, 1976, p. 36). According to the average thicknesses obtained from Antevs' measurements, the sediment represents about 160 years of deposition. Rate of Ice Recession The study of the varved sediments of the Grass River basin by Antevs (1931, p. 47-70) provided basic data for calculating the average rate of ice retreat in this recion. The average rate of retreat, based on Antevs (1931, p. 50) figures for the annual retreat rate between 40 sites covering some 225 km, was roughly 300 m Der year. This figure compares well with the rate of 200 to 300 m per year proposed by Ritchie (1976, p. 1809) for northward migration of the boreal forest, following deglaciation of the Canadian prairies, and also with the rate o f 275 m per year, during final stages of deglaciation of this region, proposed by Christiansen (1979, p . 934). An average retreat of 250 m per year between the northern margin
o f Assiniboine delta and the western margin of the Hudson Bay Lowland
is likely a maximum figure, because the rate during the early stages of deglaciation was probably somewhat slower than it was during the final stages. Christiansen (1979, p. 934) suggested an initial rate of 60 m per year and final rate of 275 m per year during deglaciation of southern Saskatchewan. Continuous deglaciation, not considering halts in retreat, or ice readvances, across the Lake Agassiz basin, at a rate of 250 m per year would require some 3000 years. Roundness of Beach Pebbles Some 20 abandoned beaches have been identified ,in Manitoba and
384
r e l a t e d t o t h e b r o a d c h r o n o l o g i c f r a m e w o r k o f Lake A g a s s i z h i s t o r y ( J o h n s t o n , 1946; E l s o n , 1 9 6 7 and 1 9 7 1 ) . E l s o n ' s ( 1 9 7 1 ) s t u d y o f b e a c h e s n o r t h and s o u t h o f t h e I n t e r n a t i o n a l Boundary d e m o n s t r a t e d t h a t t h e d e g r e e of roundness of t h e b e a c h p e b b l e s r e f l e c t e d t h e d u r a t i o n o f v a r i o u s w a t e r p l a n e s a n d p r o v i d e d a means o f e s t a b l i s h i n g t h e r e l a t i v e ages of the beaches. The C a m p b e l l b e a c h ( F i g u r e 3 ) r e p r e s e n t s t h e l o n g e s t p e r i o d o f It i s i n t e r e s t i n g t o s t a b i l i t y o f Lake A g a s s i z ( E l s o n , 1 9 7 1 , p . 2 8 8 ) . n o t e t h a t t h e p e b b l e s from t h i s beach are r o u n d e r t h a n t h o s e from p r e s e n t day Lake M a n i t o b a b e a c h e s . The p e b b l e s f r o m t h e o t h e r b e a c h e s a r e s i g n i f i c a n t l y l e s s rounded, p a r t i c u l a r l y t h o s e from t h e youngest b e a c h e s w h i c h e x h i b i t e s s e n t i a l l y t h e same d e g r e e o f r o u n d n e s s a s p e b b l e s from t h e t i l l s o u r c e . E l s o n (1971, p . 2 9 0 ) concluded t h a t roundness s t u d i e s "can a i d i n He c a l c u l a t e d a p p o r t i o n i n g t h e t i m e s p a n n e d by r a d i o c a r b o n d a t e s " . t h a t t h e b e a c h e s b e l o w amd i n c l u d i n g t h e l o w e r C a m p b e l l f o r m e d d u r i n g a 1 4 0 0 y e a r i n t e r v a l . The r e s u l t s o f t h e r o u n d n e s s s t u d i e s a l o n g w i t h r a d i o c a r b o n d a t e s were t h e b a s i s f o r t h e Lake A g a s s i z c h r o n o l o g y shown i n Figure 4 . C H R O N O L O G I C IMPLICATIONS OF R E G I O N A L I C E FLOW PATTERNS AND I C E - M A R G I N A L MORAINES
The m a n n e r i n w h i c h i c e l o b e s were d e p l o y e d a n d t h e n a t u r e o f e n d m o r a i n e s w i t h i n and a d j a c e n t t o t h e Lake A g a s s i z b a s i n have i m p l i c a t i o n s c o n c e r n i n g t h e h i s t o r y o f Lake A g a s s i z . Ice-flow markings u s u a l l y p r o v i d e a means of d e t e r m i n i n g t h e r e l a t i v e s e q u e n c e of a t l e a s t t h e l a s t t w o f l o w d i r e c t i o n s . The m a r g i n a l m o r a i n e s , i n t u r n may p r o v i d e a means o f d e t e r m i n i n g w h e t h e r or n o t t h e f l u c t u a t i o n s o f c e r t a i n i c e l o b e s were s y n c h r o n o u s a n d i n d i c a t e t h e m a n n e r i n w h i c h t h e l o b e s r e treated. A prominent system of i n t e r l o b a t e kane moraines i n t h e n o r t h e r n p a r t o f t h e L a k e A g a s s i z b a s i n ( F i g u r e 3 ) shows s y n c h r o n o u s d e p l o y m e n t o f t w o d i s t i n c t i c e l o b e s i n t h i s r e g i o n ( K l a s s e n , i n p r e s s ) . They r e f l e c t f l o w s from i c e c e n t e r s e a s t ( L a b r a d o r i a n ) a n d n o r t h ( K e e w a t i n ) of t h i s r e g i o n a s p r o p o s e d by T y r r e l l ( 1 9 1 3 ) . S h i l t s e t aZ. (1979, p . 5 3 7 ) p r o p o s e d t h a t o u t f l o w from t h e s e c e n t e r s d u r i n g t h e l a s t g i a c i a l maximum was s y n c h r o n o u s a n d t h a t t h e y met s o m e w h e r e b e t w e e n N e l s o n R i v e r and C h u r c h i l l .
The p a t t e r n o f i c e f l o w m a r k i n g s i n t h e s o u t h e r n p a r t o f t h e Lake A g a s s i z b a s i n ( F i g u r e 3 ) r e f l e c t s t h e c o a l e s c e n c e of L a b r a d o r i a n a n d K e e w a t i n i c e . However, w i t h i n t h e w e s t - c e n t r a l p a r t o f t h e b a s i n , t h e s o u t h e a s t m a r k i n g s ( K e e w a t i n ) a r e m o d i f i e d or m a s k e d by t h e s o u t h w e s t ( L a b r a d o r i a n ) o n e s t h a t a l s o c r o s s The P a s m o r a i n e . The s u p e r i m p o s e d f e a t u r e s r e f l e c t a r e a d v a n c e of L a b r a d o r i a n i c e f o l l o w i n g an i n t e r v a l of g e n e r a l r e t r e a t o f b o t h L a b r a d o r i a n a n d K e e w a t i n i c e f r o m t h e s o u t h e r n p a r t of t h e b a s i n . A d d i t i o n a l evidence f o r a major readvance of L a b r a d o r i a n i c e , f o l l o w i n g t h a t g e n e r a l r e t r e a t , i s a c l a y e y t i l l o v e r l y i n g t h e r e g i o n a l s i l t y t i l l i n t h e a r e a b e t w e e n The Pas m o r a i n e and A s s i n i b o i n e d e l t a ( K l a s s e n , 1965, p . 5 ) . T h i s r e a d v a n c e p r e d a t e s t h e C a m D b e l l b e a c h f o r t h a t b e a c h i s cont i n u o u s a c r o s s t h e a r e a o v e r r i d d e n by t h e r e a d v a n c e ( F i g u r e 3 ) . Most l i k e l y t h e r i s i n g p h a s e o f L a k e A g a s s i z , as r e c o r d e d i n t h e A s s i n i boine v a l l e y fill, r e f l e c t s t h i s readvance. A r e c e n t l y reported ( N i e l s o n e t aZ., 1 9 8 1 , p . A - 4 3 ) r e a d v a n c e o f L a b r a d o r i a n i c e t o t h e Hartman m o r a i n e i n w e s t e r n O n t a r i o (Hughes, 1 9 6 5 ) a f t e r 1 1 , 4 0 0 y e a r s a g o ( f r e s h - w a t e r s h e l l d a t e GSC-3114, u n p u b l i s h e d ) p r o b a b l y c o r r e l a t e s w i t h t h e readvance i n Manitoba. IMPLICATIONS OF ABSOLUTE AND R E L A T I V E D A T I X G METHODS FOR LAKE AGASSIZ C H R O N O L O G Y R a d i o c a r b o n c h r o n o l o g y s u g g e s t s t h a t t h e e a r l i e s t s t a g e o f Lake A g a s s i z i n Manitoba began a b o u t 1 h Y 0 0 O y e a r s &go and t h a t i t s f i n a l
385
THOUSANDS OF YEARS BEFORE PRESENT 9 HERMAN NORCROSS TINTAH
1
CAMPBELL McCAULY VLL, BLANCHARD HILLSBORO EMERADO
OSSOWA
- 1III - II PAS LOWER PAS -! GlMLl GRAND RAPIDSPlPUN u Unnamed
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Figure 4
D u r a t i o n of v a r i o u s l e v e l s of Lake Agassiz i n f e r r e d from r a d i o c a r b o n d a t e s , t h e p o s i t i o n s of which a r e i n d i c a t e d by s h o r t l i n e s a l o n g t h e a b s c i s s a , a n d beach pebble roundnesses ( a f t e r Elson, 1971).
d r a i n a g e i n t o t h e T y r r e l l s e a o c c u r r e d a b o u t 8 , 0 0 0 y e a r s a g o . Uncert a i n t i e s c o n c e r n i n g t h e v a l i d i t y of t h e o l d e s t d a t e s , d i s c u s s e d i n p r e c e d i n g s e c t i o n s , s u g g e s t t h e l a k e may h a v e begun l a t e r b u t t h e r e i s l i t t l e reason t o question t h e time i n f e r r e d f o r i t s f i n a l drainage. F i g u r e 5 g i v e s a c o m p a r i s o n o f t h e t i m e i n t e r v a l s d e d u c e d b y means o f v a r i o u s d a t i n g methods. I t shows t h e d u r a t i o n o f t h e l a k e b a s e d on r a d i o c a r b o n d a t i n g as a c o n t i n u o u s l i n e r e p r e s e n t i n g 6 , 0 0 0 y e a r s , and t i m e i n t e r v a l s d e d u c e d by t h e r e l a t i v e d a t i n g methods a r e shown a t i n f e r r e d p o s i t i o n s w i t h i n t h a t a b s o l u t e t i m e f r a m e . Sums of t h e t o t a l y e a r s o b t a i n e d by e a c h d a t i n g method a r e shown a l o n g t h e r i g h t o r d i n a t e . The 4 , 5 0 0 y e a r sum deduced from i c e r e t r e a t i n c l u d e s t h e 3 0 0 0 years c a l c u l a t e d f o r continuous r e t r e a t a c r o s s t h e b a s i n n o r t h of A s s i n i b o i n e d e l t a a n d t h e 1500 y e a r s a l l o c a t e d t o a m a j o r r e a d v a n c e from j u s t n o r t h o f A s s i n i b o i n e d e l t a . The d i f f e r e n c e s i n t h e sums o b t a i n e d f o r , e a c h o f t h e d a t i n g A methods i n p a r t r e f l e c t t h e d e g r e e s o f i n a c c u r a c i e s o f t h e methods. s i g n i f i c a n t c o n s i d e r a t i o n however, i s t h a t t h e t i m e r e c o r d e d by t h e r e l a t i v e d a t i n g methods d o e s n o t i n c l u d e t h e d u r a t i o n o f low water l e v e l s and t h e s t i l l s t a n d of t h e i c e t h a t formed The Pas m o r a i n e p r i o r t o a major readvance. C o n c l u s i o n s from t h i s a p p r a i s a l o f a b s o l u t e and r e l a t i v e methods o f d a t i n g Lake A g a s s i z d e p o s i t s i n M a n i t o b a a r e as f o l l o w s : 1.
The s t r a t i g r a p h i c p o s i t i o n s and d i s t r i b u t i o n of t h e most
2.
The s e d i m e n t a t i o n r a t e and p e b b l e r o u n d n e s s methods t h a t a c c o u n t f o r some 3000 y e a r s of l a k e h i s t o r y f a i 1 . t ~i n c l u d e
r e l i a b l e r a d i o c a r b o n d a t e d s i t e s e s t a b l i s h t h a t Lake A g a s s i z ended a b o u t 8 , 0 0 0 y e a r s a g o , and b e g a n a t l e a s t 1 2 , 0 0 0 y e a r s ago i n Manitoba. The l a k e l a s t e d a t l e a s t 4 , 0 0 0 y e a r s .
3 86
DATING METHOD
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YEARS B. P. (x 1000) 14
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ROUNDNESS OF BEACH PEBBLES
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Figure 5
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IRED RIVERIRED RIVER1 GRASS 1 BASIN I BASIN 1 RIVER I I I BASIN I I I
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THE PAS I IWORAINE I I I I I I I I_ I
UPPER
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D u r a t i o n of Lake Agassiz i n Manitoba a c c o r d i n g t o r a d i o c a r b o n d a t e s , compared t o t h e d u r a t i o n of v a r i o u s s t a g e s deduced by r e l a t i v e d a t i n g methods.
t h e l e n g t h of t h e low w a t e r s t a g e s . The 4,500 y e a r s d e d u c e d f r o m t h e e s t i m a t e d r a t e o f i c e r e t r e a t s u g g e s t s t h e low water s t a g e s may r e p r e s e n t some 1500 y e a r s o f l a k e h i s t o r y .
3.
The r a d i o c a r b o n c h r o n o l o g y b a s e d o n a v a i l a b l e d a t e s s u g g e s t i n g t h e d u r 2 a t i o n of Lake A g a s s i z was f r o m r o u g h l y 1 4 , 0 0 0 t o 8,OCO y e a r s a g o may r e q u i r e some m o d i f i c a t i o n b a s e d o n t h e p o s s i b l e c o n t a m i n a t i o n o f t h e o l d e s t d a t e s a n d t h e somewhat s h o r t e r d u r a t i o n i n f e r r e d from r e l a t i v e d a t i n g methods. A time frame b e t w e e n a b o u t 1 3 , 0 0 0 a n d 8,000 y e a r s BP i s p r o p o s e d as a r e a l i s t i c a c c o m m o d a t i o n o f t h e time s p a n s a r r i v e d a t by t h e d a t i n g methods d i s c u s s e d i n t h i s p a p e r . REFERENCES C T T E D
Antevs, E r n s t , Manitoba:
1931, L a t e - g l a c i a l c o r r e l a t i o n s and ice r e c e s s i o n i n G e o l o g i c a l S u r v e y of C a n a d a , Memoir 1 6 8 , 7 6 p .
387
America:
,
1939, Late Quaternary upwarpings of n o r t h - e a s t e r n J o u r n a l o f Geology, v . 47, no. 7 , p . 707-720.
North
Christiansen, E.A., 1 9 7 9 , The W i s c o n s i n a n d e g l a c i a t i o n o f s o u t h e r n Saskatchewan and a d j a c e n t areas: Canadian J o u r n a l of E a r t h Sciences, v . 1 6 , no. 4, p. 9 1 3 ~ 9 3 8 1 1957, Lake A g a s s i z and t h e Mankato-Valders Elson, J.A., S c i e n c e , v . 1 2 6 , n o . 3 2 8 1 , p. 9 9 9 - 1 0 0 2 .
, 1967, Geology of g l a c i a W.J., e d . , L i f e , Land a n d Water, e n c e on E n v i r o n m e n t a l S t u d i e s of Winnipeg, U n i v e r s i t y of Manitoba
problem:
l Lake A g a s s i z , i n Mayer O a k e s , Proceedings of t h e 1965 Confert h e G l a c i a l Lake A g a s s i z r e g i o n : P r e s s , 414 p.
, 1 9 7 1 , Roundness of g l a c i a l Lake A g a s s i z b e a c h p e b b l e s , i n Geological Turnock, A.D., ed., Geoscience Studies i n Manitoba: A s s o c i a t i o n of Canada, S p e c i a l P a p e r n o . 9 , p . 285-291. F e n t o n , M.M. Portage science Special
and Anderson, D . T . , 1971, P l e i s t o c e n e s t r a t i g r a p h y of t h e l a P r a i r i e a r e a , M a n i t o b a , i n T u r n o c k , A . D . , e d . , GeoStudies i n Manitoba: G e o l o g i c a l A s s o c i a t i o n of Canada, P a p e r no. 9 , p . 271-276.
1946, G l a c i a l Lake A g a s s i z , w i t h s p e c i a l r e f e r e n c e t o J o h n s t o n , W.A., t h e mode o f d e f o r m a t i o n o f t h e b e a c h e s : G e o l o g i c a l Survey of C a n a d a , B u l l e t i n 7 , 20 p . 1975, Palynolopical study of lake Karrow, P.F. and Anderson, T.W., Discussion: sediment p r o f i l e s from s o u t h w e s t e r n New Brunswick: Canadian J o u r n a l of E a r t h S c i e n c e s , v . 1 2 , p . 1808-1812. Klassen, R.W., 1965, S u r f i c i a l geology of t h e Waterhen-Grand Rapids area, Manitoba: G e o l o g i c a l Survey of Canada, Paper 66-36, 6 p .
, 1 9 7 1 , N a t u r e , t h i c k n e s s and s u b s u r f a c e s t r a t i g r a p h y of t h e d r i f t i n southwestern Manitoba: i n Turnock, A . C . , e d . , G e o s c i e n c e S t u d i e s i n M a n i t o b a , G e o l o g i c a l A s s o c i a t i o n of Canada, S p e c i a l P a p e r n o . 9 , p . 235--261. , 1972, Wisconsin e v e n t s and t h e A s s i n i b o i n e and Qu'Appelle v a l l e y s o f Manitoba and Saskatchewan: Canadian J o u r n a l of E a r t h S c i e n c e s , v. 9 , n o . 5 , p . 544-560. , 1976, Landforms and s u r f a c e materials a t s e l e c t e d s i t e s i n a p a r t of t h e S h i e l d n o r t h - c e n t r a l Manitoba, w i t h appendix of t e s t h o l e d a t a a n d s o i l i n d e x p r o p e r t i e s by J e a n V e i l l e t t e : G e o l o g i c a l Survey of Canada, Paper 75-19, 4 1 p . , i n p r e s s , S u r f i c i a l g e o l o g y of G e o l o g i c a l S u r v e y o f C a n a d a , Memoir.
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Manitoba:
K l a s s e n , R.W. and E l s o n , J . A . , 1 9 7 2 , S o u t h w e s t e r n M a n i t o b a : in Q u a t e r n a r y geology and geomorphology between Winnipeg and t h e Rocky M o u n t a i n s , Guidebook F i e l d E x c u r s i o n C - 2 2 , I n t e r n a t i o n a l G e o l o g i c C o n g r e s s , p . 3-23. Lowden, J . A . a n d B l a k e , W . radiocarbon dates IX: P a r t B, p . 46-86.
J r . , 1 9 7 0 ; G e o l o g i c a l Survey o f Canada G e o l o g i c a l Survey of Canada, P a p e r 70-2,
, 1 9 7 3 , G e o l o g i c a l Survey o f Canada r a d i o c a r b o n d a t e s X I I I : G e o l o g i c a l Survey of Canada, Paper 73-7, 6 1 p .
, 1975, G e o l o g i c a l Survey of Canada r a d i o c a r b o n d a t e s XV: G e o l o g i c a l S u r v e y o f Canada P a p e r 75-7, 32 p . , 1 9 7 6 , G e o l o g i c a l Survey o f Canada r a d i o c a r b o n d a t e s X V I : G e o l o g i c a l Survey of Canada, Paper 76-7, 2 1 p .
388
, 1 9 7 9 , G e o l o g i c a l Survey of Canada r a d i o c a r b o n d a t e s X I X : G e o l o g i c a l S u r v e y o f C a n a d a , P a p e r 79-7, 57 p . Lowden, J . A . , F y l e s , J . G . , B l a k e , W . , : Y . , 1967, Geological Survey of Canada r a d i o c a r b o n d a t e s : G e o l o g i c a l Survey of Canada, P a p e r 672 , P a r t B , 45 p . [.owden, J . A . , R o b e r t s o n , I . M . a n d Blak.e, W . , Survey of Canada r a d i o c a r b o n d a t e s X I : Canada, P a p e r 71-7, p . 255-324.
Jr., 1971, Geological Geological Survey of
, 1 9 7 7 , G e o l o g i c a l Survey of Canada r a d i o c a r b o n d a t e s X V I I : G e o l o g i c a l Survey of Canada, P a p e r 77-7, 25 p . Y a t h e w e s , R.W. and W e s t g a t e , J . A . , 1980, Bridge River Tephra: revised d i s t r i b u t i o n and s i g n i f i c a n c e f o r d e t e c t i n g o l d carbon e r r o r s i n r a d i o c a r b o n d a t e s of l i m n i c s e d i m e n t s i n s o u t h e r n B r i t i s h Columbia: Canadian J o u r n a l of E a r t h Sciences., v . 1 7 , p . 1454-1461. 1971, Pleistocene YcPherson, R.A., LefLh, E I. and Anderson, D.T., i n Geoscience s t r a t i g r a p h y of a p o r t i o n of southeastern Yanitoba: Canada, S p e c i a l S t u d i e s ir; M a n i t o b a , G e o l o g i c a l A s s o c i a t i o n o f P a p e r , No. 9 , p . 2 7 7 - 2 8 3 . Sambudiri, E.M.V., Teller, J . T . and Last, W.M., 1980, Pre-quaternary micro-fossils - A guide t o errors i n radiocarbon dating: Geology, V. 8 , p . 123-126. N i e l s o n , E . , M c K i l l o p , W . B . a n d McCoy, J . P . , 1 9 8 1 , Age o f t h e H a r t m a n moraine and t h e Campbell l e v e l of Lake A g a s s i z . n o r t h w e s t e r n Ontario: A b s t r a c t s , Geological, A s s o c i a t i o n of Canada, Annual Meeting, t h e U n i v e r s i t y of C a l g a r y . P r e s t o n , R . S . , P e r s o n , E . and Deevey, E . J . , 1 9 5 5 , Yale n a t u r a l r a d i o c a r b o n m e a s u r e m e n t s 11: S c i e n c e , v . 1 2 2 , p . 9 5 4 - 9 6 0 . Ritchie, J.C., 1 9 7 6 , The l a t e - Q u a t e r n a r y v e g e t a t i o n a l h i s t o r y of Western I n t e r i o r of Canada: C a n a d i a n J o u r n a l of B o t a n y , v . no. 1 5 , p . 1793-,1818.
the 54,
S h i l t s , W.W., Cunningham, C.M. and K a s z y c k i , C . A . , 1979, Keewatin i c e sheet - Re-evaluation of t h e t r a d i t i o n a l concept of t h e Laurentide ice sheet: Geology, v. 7 , p . 537-541. T e l l e r , J . T . , 1976, Lake A g a s s i z d e p o s i t s i n t h e main o f f s h o r e b a s i n i n southern Manitoba: C a n a d i a n J o u r n a l of E a r t h S c i e n c e s , v . 1 3 , p . 27-43.
, 1980, Radiocarbon d a t e s i n Manitoba: Mineral Resources D i v i s i o n , G e o l o g i c a l R e p o r t GR80-4, M a n i t o b a D e p a r t m e n t o f E n e r g y and Mines, 6 1 p . Teller, J.T. and Fenton, M.M., 1980, Late Wisconsinan g l a c i a l stratigraphy and h i s t o r y of s o u t h e a s t e r n Manitoba: Canadian J o u r n a l of E a r t h S c i e n c e s , v . 1 7 , n o . 1, p . 1 9 - 3 5 . r y r r e l l , J . B . , 1 9 1 3 , Hudson Bay e x p l o r i n g e x p e d i t i o n , B u r e a u o f M i n e s , 22nd Annual R e p o r t , p . 161-209.
1912:
Ontario
389
THE QUATERNARY SUCCESSION IN THE RIO BLANCO BASIN, CORDON DEL PLATA, MENDOZA PROVINCE, ARGENTINA: AN APPLICATION OF MULTIPLE RELATIVEDATING TECHNIQUES W.J. WAYNE
AB STR A C T
I n t h e b a s i n o f R f o B l a n c o , w h i c h f l o w s e a s t w a r d f r o m t h e Cord6n d e l P l a t a w e s t o f Mendoza, A r g e n t i n a , m o r a i n e s a n d f o s s i l r o c k g l a c i e r s r e c o r d s e v e r a l p e r i o d s of a c t i v i t y d u r i n g and s i n c e t h e l a s t major g l a c i a t i o n , t h e V a l l e c t i o s ( = W i s e o n s i n a n ) . F a r t h e r down t h e v a l l e y t h a n t h e moraines of V a l l e c i t o s age a r e t h r e e o l d e r t i l l s , one o f which i s e x p o s e d a l m o s t a s f a r d o w n s t r e a m as t h e j u n c t i o n w i t h Rfo Mendoza, more t h a n 17 km b e y o n d t h e o u t e r V a l l e c i t o s m o r a i n e s . No v e g e t a t i o n b o r d e r s t h e modern g l a c i e r s a n d r o c k g l a c i e r s i n t h i s d r y p a r t o f t h e C e n t r a l Andes, and e f f o r t s t o o b t a i n d a t a b l e o r g a n i c m a t e r i a l proved f r u i t l e s s . A l e n s e of c o a r s e v o l c a n i c ash t h a t o v e r l i e s o n e o f t h e t i l l s was d a t e d by z i r c o n f i s s i o n - t r a c k m e t h o d s a s between 100,000 and 200,000 y e a r s o l d .
R e l a t i v e d a t i n g ( R D ) t e c h n i q u e s were u s e d , t h e r e f o r e , i n o r d e r t o t r y to d e v e l o p a s t r a t i g r a D h i c o r d e r for t h e d e n o s i t s . Granites are r a r e i n t h e Rfo B l a n c o b a s i n a n d , a l t h o u g h t h e q u a r t z i t e s a n d r h y o l i t e s t h a t d o m i n a t e t h e a r e a w e a t h e r , few e f f e c t s o f w e a t h e r i n g on t h e s e For d e p o s i t s o f V a l l e c i t o s a g e a n d b o u l d e r s c o u l d be q u a n t i f i e d . y o u n g e r , t h e m o s t c o n s i s t e n t d a t a came f r o m 1) m o r a i n a l a n d r o c k g l a c i e r p o s i t i o n and o v e r l a p , 2 ) s h a r p n e s s o f topography, 3 ) l o e s s t h i c k n e s s , 4) s o i l p r o f i l e c h a r a c t e r i s t i c s , 5 ) v e g e t a t i o n c o v e r , a n d 6 ) l i c h e n s . For t h e o l d e r ( p r e - l a s t g l a c i a t i o n ) d e p o s i t s , d a t a f o r c o m p a r i s o n was p r o v i d e d b y : 1) t o p o g r a o h i c p o s i t i o n , 2 ) s t r a t i g r a p h i c s e q u e n c e , 3 ) s o i l p r o f i l e c h a r a c t e r i s t i c s a n d d e v e l o p m e n t , a n d 4) b o u l d e r w e a t h e r i n g .
INTFi OD U C T I 0 N ?he C e n t r a l Andes b e t w e e n S a n t i a g o , C h i l e a n d PTendoza, A r g e n t i n a , c o n s i s t of 3 major p a r t s , t h e C o r d i l l e r a PPincipal, C o r d i l l e r a F r o n t a l a n d P r e c o r d i l l e r a a n d i n c l u d e s !!It. A c o n c a g u a , a t a p p r o x i m a t e l y 7000 m y t h e h i g h e s t peak i n t h e Western Hemisphere. The p i e d m o n t D l a i n s on e a c h s i d e o f t h e Andean C h a i n a r e a r i d , a n d t h e w e l l d e v e l o p e d a g r i - . c u l t u r a l economy d e p e n d s h e a v i l y on snow m e l t a n d g l a c i e r m e l t w a t e r runoff. G l a c i e r s a r e t h e Andean t y p e ( P o l a n s k i , 1954) w h i c h h a v e snowf i e l d s f e e d i n g small areas of open g l a c i e r above d e b r i s - c o v e r e d i c e ; i n many, t h e t e r m i n a l p a r t i s a l o n g i c e - c o r e d r o c k g l a c i e r t o n g u e . F a r t h e r down t h e v a l l e y s a r e i n a c t i v e a n d f o s s i l r o c k g l a c i e r d e p o s i t s . F o r t h e p a s t c e n t u r y , m o s t o f t h e g l a c i e r s o f t h e d r y C e n t r a l Andes h a v e b e e n s h r i n k i n g . T h e r m o k a r s t p i t s a r e common i n t h e d e b r i s - c o v e r e d s e g m e n t s o f many, a n d some r o c k g l a c i e r t e r m i n i h a v e become i n a c t i v e .
?he Cord6n d e l P l a t a i s o n e o f t h e r a n g e s o f t h e C o r d i l l e r a F r o n t a l i n t h e n o r t h w e s t e r n p a r t o f t h e p r o v i n c e o f Mendoza, A r g e n t i n a ( F i g u r e 1). From i t s h i g h e s t p e a k , C e r r o d e l P l a t a (6100 m ) , i t e x t e n d s n o r t h w a r d t h r o u g h C e r r o B l a n c o (51190 m ) a n d s o u t h w e s t w a r d to t h e l o w e r C o r d 6 n d e S a n t a C l a r a . The c r e s t o f t h e r a n g e f o r m s t h e d i v i d e b e t w e e n R f o T u p u n g a t o , w h i c h f l o w s n o r t h w a r d to e n t e r Rfo Mendoza a t P u n t a d e
390
Figure 1
Hap s h o w i n g r o c k g l a c i e r s a n d g l a c i a l d e p o s i t s o f t h e u p p e r R f o B l a n c o b a s i n , Cord611 d e l P l a t a , Mendoza P r o v i n c e , A r g e n t i n a .
Vacas and t h e e a s t w a r d - f l o w i n a t r i b u t a r i e s of b o t h Rfo Mendoza and Rfo Tunuysn. Two Rfo B l a n c o s d r a i n t h e n o r t h e r n D a r t o f t h e Cord6n d e l P l a t a ; one f l o w s n o r t h w a r d a n d t h e o t h e r e a s t w a r d . The a r e a s e l e c t e d for t h i s i n v e s t i g a t i o n was Q u e b r a d a d e 10s V a l l e c i t o s , a r e l a t i v e l y a c c e s s i b l e a r e a of t h e CordBn d e l P l a t a , where g l a c i a l m o r a i n e s as w e l l as a l a r g e number o f a c t i v e , i n a c t i v e , a n d f o s s i l r o c k g l a c i e r s c o u l d b e s t u d i e d . The s t u d y was c o n t i n u e d i n t o Q u e b r a d a d e l a A n g o s t u r a d r a i n a g e which j o i n s t h a t f r o m Q . d e 10s V a l l e c i t o s t o form Rfo B l a n c o ( F i g u r e l), a m a j o r t r i b u t a r y o f R f o Mendoza. An a r e a o f 85 km2 i s d r a i n e d by t h e s e two s t r e a m s above t h e i r j u n c t i o n , w h i c h i s t h e s t a r t o f Rfo B l a n c o . These s u b - b a s i n s l i e b e t w e e n 3 3 ' 0 2 ' and 32'52's l a t . a n d r u n f r o m 67'25' t o 67'26'E l o n g . The c l i m a t e of t h i s p a r t o f A r g e n t i n a i s c o n t i n e n t a l , w i t h r e l a t i v e l y low p r e c i p i t a t i o n (400-600 m m ) a n d w i t h l a r g e d i u r n a l t e m p e r a t u r e c h a n g e s . I n c o m p l e t e r e c o r d s f o r a l i t t l e more t h a n 2 y e a r s from t h e
391
meteorological station at Vallecitos (2470 m) suggest a mean annual temperature of approximately 5°C and 450 mm precipitation. An 8-year record at a station near the south edge of the area, Estancion Las Aguaditas (2225 m), indicates a mean annual temperature of 7.6"C and a precipitation o f 294 mm (Estrella e t aZ., 1980). The altitude at which 0°C is the mean annual temperature should be 3400 m in the basin of R f o Blanco, based on a lapse rate of 0.6"C per 100 m. This study was undertaken in an effort to determine whether the fossil rock glaciers in the Central Andes had been active continuously or only spasmodically since disappearance of the last glaciers about 12,000 years ago. If their activity had been spasmodic it might be possible to determine the frequency and timing o f advance. A second curpose was to work out criteria by which active and inactive rock glaciers and debris-covered glaciers, all of which contain ice that represents water in storage for the irrigated Mendozan plains, can be distinguished reliably from fossil rock glaciers by airphoto study. !he project was part of a cooperative program between the National Science Foundation (U.S.) and the Instituto Argentino de Nivologfa y Glaciologia (IANIGLA), a part o f the Consejo FJacional de Investigaclones Cientfficas y Tgcnicas (CONICET). GENERAL GEOLOGY Between Co. del Plata and Co. Blanco, the Cord6n del Plata is composed of quartzites and related metasediments; rocks of volcanic origin, primarily rhyolites and andesites; and a few plutonic rocks. Within these two sub-basins, the southern part is composed almost wholly of dark-colored quartzites, but the northern part is dominated by reddishbrown rhyolites. Caminos (1965) and Polanski (1972) mapped the spine o f the Cord6n del Plata as the El Plata Formation, a Paleozoic unit more than 7000 m thick. Quartzitic rocks o f the El Plata Formation dominate the slopes of Quebrada de la Angostura, & . de 10s Vallecitos, and Q. Rinc6n, and make up some of the southwest side of Q. Stepanek. Along the southwest side o f 0. Stepanek and to the northeast in Q. Colorada, the metasedimentary rocks of the El Plata Formation give way to rhyolitic tuffs and andesitic breccias, which are part of the Variscan Volcanic Association (Caminos, 1965, p. 370-378). Plutonic rocks are not abundant in the basin. Coarse-grained biotite granite o f the stock of Cuchilla de las Minas (Caminos, 1965) crops out between 3400 and 3500 m in & . de la Angostura, and the distribution o f erratic boulders in one moraine suggests that it is buried beneath Pleistocene sediments in Q. Stepanek. Deeply weathered gran-itic rocks also crop out on the slopes near the junction of Vallecitos and Angostura Creeks. The only other ~ l u t o n i crocks recognized are medium- and coarse-grained diabasic gabbros, which occur as dikes. GEOMORPHIC FRAMEWORK
From Co. del Plata northward, most o f the crest of C o r d h del Plata is an arete punctuated by a few horns. The cordon widens at Co. del Plata, though, and includes broad sloDing surfaces of cryoplanation below which glaciers have gouged the valleys. All the valleys show clear evidence of having been enlarged and scoured by glacial ice, but frost shattering and avalanching have cut channels through the oncesmooth valley walls, and the U-shape characteristic of the troughs carved by alpine glaciers is becoming less obvious. Nevertheless, polished and striated rock surfaces can be seen in some places between avalanche chutes and cones and on bedrock surfaces in the valleys above 3500 m. The cirques that head along the main ridge of the range contain small uncovered ice glaciers. The lower parts o f the main valleys o f Angostura and of Vallecitos are largely filled with moraines deposited by moving ice, although small rock glaciers have formed along the north wall in the ursper valleys since the disappearance of the glacier. In the other two valleys, Stepanek and Colorada, the bulk of the sediments consists of active, inactive, and fossil rock-glacier debris. Below about 2700 m , the topography is dominantly produced by running water, but both glacial and fluvial sediments are exposed along
392
t h e v a l l e y and c h a n n e l walls. Both V a l l e c i t o s and Angostura c r e e k s c a r r y n e l t w a t e r from w i n t e r p r e c i p i t a t i o n and t h e small g l a c i e r s and r o c k g l a c i e r i n t h e g l a c i a l l y widened u p p e r p a r t s of t h e i r v a l l e y s . A n g o s t u r r Creek flows t h r o u g h H o l o c e n e o u t w a s h b o r d e r e d a n d o v e r l a i n by r o c k g l a c i e r s on t h e n o r t h s i d e ar.d l o o s e t a l u s on t h e s o u t h t o 3 5 0 0 IP, w h e r e i t f a l l s t h r o u g h a granite outcrop. B e t w e e n 3500 a n d 27CO m i t f l o w s down a s t e e p , IT-shaped t r e n c h between a massive moraine and t h e bedrock v a l l e y s i d e . !he g l a c i a l l y s c o u r e d b e d r o c k w a l l s t h r o u g h t h i s r e a c h o f t h e v a l l e y h a v e b e e n m o d i f i e d by e r o s i o n ar.d mass w a s t i n g . About 3 km d o w n s t r e a m from t h e g r a n i t e f a l l s t h e stream t u r n s r o r t h w a r d t h r o u g h a narrow g o r g e a b o u t 1 0 0 0 m l o n g . The v a l l e y i s l i t t l e more t h a n 1 0 0 m w i d e w h e r e t h e s t r e a m p a s s e s t h r o u g h t h i s “ n a r r o w s i 1 . A t t h e e n d of t h e c o n s t r i c t i o n s , i t t u r n s n o r t h e a s t w a r d a g a i n a n d t h e v a l l e y w i d e n s f r o m 75 n t o 3CO m i n a d i s t a n c e o f 1 kx. I t w i d e n s t o 550 m a t t h e j u n c t i o n w i t h V a l l e c i t o s C r e e k , b u t c o n s t r i c t s a b r u p t l y to a w i d t h o f o n l y 200 m w h e r e t h e t w o c r e e k s j o i n t o become R i o 3 l a n c o . ? o r a l i t t l e more t h a n 1 0 0 m t h e stream p a s s e s t h r o u g h a second “ n a r r o w s ” , t h e n t h e v a l l e y o p e n s a b r u p t l y w h e r e i t l e a v e s t h e e a s t e d g e of t h e C o r d & d e l P l a t a . 2rom t h i s p o i n t t o i t s j u n c t i o n w i t h R f o M e n d o z a , a b o u t 11 km downstream, R i o B l a n c o becomes e n t r e n c h e d b e n e a t h a wide a l l u v i a l s l o p e . G L A C I A L CEOLOGY
The C o r d 6 n d e l P l a t a , a l o n g w i t h o t h e r r a n g e s i n t h e d r y C e n t r a l Andes, s u p p o r t e d c o n s i d e r a b l y l a r g e r g l a c i e r s d u r i n g t h e l a s t m a j o r g l a c i a t i o n w h i c h t e r m i n a t e d , a c c o r d i n g to M e r c e r (1976, p . 155), by 1 2 , 0 0 0 BP i n P a t a g o n i a . I n t h e Cord6c d e l P l a t a , i c e o f t h i s g l a c i a t i o n , named t h e V a l l e c i t o s S t a g e ( C o r t e , 1957, D . 141, e x t e n d e d downv a l l e y t o a b o u t 2 6 0 0 m y where it d e p o s i t e d end moraines a c r o s s t h e v a l l e y s of b o t h V a l l e c i t o s and Angostura Creeks. L i t t l e d i s a g r e e m e n t e x i s t s among g e o l o g i s t s who h a v e s t u d i e d t h e a r e a t h a t g l a c i e r s e x p a n d e d e x t e n s i v e l y i n t h e v a l l e y s o f t h e Mendoza Andes d u r i n g t h e l a s t g l a c i a l a g e . W h e t h e r or n o t g l a c i e r s were a s l a r g e d u r i n g e a r l i e r c o l d p e r i o d s of t h e T l e i s t o c e n e h a s been questioned h o w e v e r . D e s a n t e ( 1 9 4 6 ) , G r o e b e r (1954) a n d o t h e r s d e s c r i b e d d i a m i c t o n s t h a t t h e y r e g a r d e d t o b e e v i d e n c e of a n e a r l i e r g l a c i a l advance o n t o t h e piedmont i n t h e v a l l e y s of R f o s Diamante and A t u e l i n t h e s o u t h e r n p a r t of Nendoza P r o v i n c e . Ccrte ( 1 9 5 7 ) d i s c u s s e d b r i e f l y t h e m o r a i n e s o f t h e l a s t g l a c i a t i o n and d e s c r i b e d s e v e r a l e x p o s u r e s of a d i a m i c t o n t h a t i s e x p o s e d i n t h e s t e e p v a l l e y w a l l s o f Rfo B l a n c o a n d c a n b e t r a c e d f r o m t h e j u n c t i o n o f & . d e l a A n g o s t u r a a n d Q . d e 10s V a l l e c i t o s t o P o t r e r i l l o s , w h e r e Rfo B l a n c o e n t e r s t h e v a l l e y o f Rfo Mendoza. He considered t h i s diamicton t o be till deposited during an earlier glaciation.
P o l a n s k i (1963, 1965) d i s a g r e e d w i t h t h e i d e n t i f i c a t i o n s made by G r o e b e r a n d by C o r t e o f t h e s e d i a m i c t o n s a s t i l l . I n s t e a d , h e r e g a r d e d them t o b e v o l c a n i c mudflows t h a t , a l o n g w i t h t h e b o u l d e r y g r a v e l s accompanying them, r e p r e s e n t t h e s t r a t i g r a p h i c r e c o r d of u p l i f t o f t h e C e n t r a l Andes. P o l a n s k i a r g u e d t h a t t h e r e g i o n of L a t . 32”s i s a n d always h a s been t o o d r y t o have suDported P l e i s t o c e n e g l a c i e r s s u f f i c i e n t l y l a r g e t o h a v e r e a c h e d t h e Diedmont a n d h e a g r e e d w i t h S t a p p e n b e c k (1917) t h a t t h e m o s t e x t e n s i v e g l a c i a t i o n o f t h e Andes was t h e last one. I n t h i s r e p o r t t h e r e l a t i v e d a t i n g methods t h a t proved u s e f u l i n r e c o n s t r u c t i n g t h e P l e i s t o c e n e h i s t o r y o f t h e Rfo B l a n c o v a l l e y a r e described. Also a d d i t i o n a l d a t a on t h e d i a m i c t o n s i n t h e b a s i n o f R f o B l a n c o t h a 5 I b e l i e v e w i l l f i r m l y e s t a b l i s h t h e i r g l a c i a l o r i g i n are discussed. L i t t l e v e g e t a t i o n b o r d e r s t h e modern g l a c i e r s and r o c k g l a c i e r s i n t h i s d r y p a r t o f t h e C e n t r a l Andes, a n d e f f o r t s t o d i g t h r o u g h one r o c k g l a c i e r margin t o o b t a i n d a t a b l e organic material proved f r u i t l e s s . Lenses of c o a r s e v o l c a n i c a s h t h a t o v e r l i e a till exposed a l o n g Rfo B l a n c o were d a t e d by z i r c o n f i s s i o n - t r a c k m e t h o d s ( G l e n I z e t t a n d
393
C h a r l e s N a e s e r , l e t t e r 1 2 , Dee. 1 9 8 0 ) a s b e t w e e n 100,000 and 2 0 0 , 0 0 0 y e a r s o l d , b u t a r e t o o young t o b e d a t e d more p r e c i s e l y . R e l a t i v e d a t i n g ( R D ) t e c h n i q u e s were u s e d , t h e r e f o r e , i n o r d e r t o t r y t o p r o v i d e some s t r a t i g r a p h i c o r d e r for t h e s e q u e n c e s of d e p o s i t s mapped. RELATIVE D A T I N G METHODS
S i n c e 1 9 3 1 , when B l a c k w e l d e r d e m o n s t r a t e d t h e v a l u e o f comparing e r o s i o n a l m o d i f i c a t i o n of m o r a i n e s and t h e w e a t h e r i n g of g r a n i t i c b o u l d e r s a t and j u s t below t h e s u r f a c e t o d i s t i n g u i s h g l a c i a l d e p o s i t s o f d i f f e r e n t s t a g e s i n t h e m o u n t a i n s of w e s t e r n U . S . , N o r t h American g l a c i a l g e o l o g i s t s have r e l i e d on t h e s e t e c h n i q u e s t o work o u r m o r a i n a l s e q u e n c e s . B l a c k w e l d e r ’ s c r i t e r i a w e r e r e f i n e d by S h a r p ( 1 9 3 9 ) , Richmond ( 1 9 6 2 ) , and o t h e r s t u d e n t s of a l p i n e g l a c i a l d e p o s i t s d u r i n g t h e d e c a d e s t h a t f o l l o w e d . Most r e c e n t l y , a more c o m p r e h e n s i v e s e t of c r i t e r i a h a s b e e n worked o u t s o t h a t measurements of s e v e r a l f e a t u r e s c a n b e compared t o e s t a b l i s h t h e p r o b a b l e r e l a t i v e a g e of a p a r t i c u l a r d r i f t and t o c o r r e l a t e i t from one v a l l e y t o a n o t h e r ( B i r k e l a n d , 1 9 7 3 ; C a r r o l l , 1 9 7 4 ; Burke a n d B i r k e l a n d , 1 9 7 9 ; B i r k e l a n d e t az., 1 9 7 9 , 1980; Burke and B i r k e l a n d , 1 9 8 0 ) . W e a t h e r i n g p r o c e s s e s t a k e p l a c e c o n t i n u o u s l y , and t h e d e g r e e t o which w e a t h e r i n g w i l l a f f e c t r o c k d e c o m p o s i t i o n and s o i l p r o f i l e development i s d e p e n d e n t on t i m e , as w e l l a s on t h e f a c t o r s o f c l i m a t e , r o c k t y p e , s l o p e , and b i o t a . Landform morphology a l s o changes w i t h t i m e , and may b e a f f e c t e d by b o t h e r o s i o n and d e p o s i t i o n . Those c h a n g e s a r e s u b j e c t t o measurement and a s - o e c t s of t h e r a t e o f change d e t e r m i n e d . Some o f t h e p a r a m e t e r s t o be m e a s u r e d a r e more u s e f u l f o r s h o r t s p a n s o f t i m e , o t h e r s for l o n g e r p e r i o d s ; t h u s a few f e a t u r e s s h o u l d b e q u a n t i f i a b l e f o r most d e p o s i t s o f P l e i s t o c e n e a g e . Some sub-. j e c t i v i t y i s p r e s e n t i n t h e method, and t h e s p e c i f i c measurements of two or more w o r k e r s may n o t b e i d e n t i c a l . N e v e r t h e l e s s , most g e o l o g i s t s w i l l p i c k m a j o r b r e a k s i n a s e q u e n c e a t t h e same p l a c e . Use of R D t e c h n i q u e s i n s o u - t h e r n S o u t h America h a s b e e n l i m i t e d . F l i n t and F i d a l g o ( 1 9 6 4 , 1 9 6 9 ) examined t h e d e g r e e of w e a t h e r i n g of granitic clasts i n the s o i l profile t o distinguish three t i l l s i n the r e g i o n a r o u n d B a r i l o c h e . They a l s o u s e d t h e h e i g h t of outwash above p r e s e n t d r a i n a g e and s u b j e c t i v e e v a l u a t i o n s o f t h e d e g r e e o f e r o s i o n a l m o d i f i c a t i o n of t h e l a n d f o r m s . They examined s o i l p r o f i l e s b u t were u n a b l e t o d e t e c t s u f f i c i e n t d i f f e r e n c e s i n t h e weakly d e v e l o p e d p r o f i l e s t o u s e them. Caviedes and Paskoff ( 1 9 7 5 ) r e c o g n i z e d d i f f e r e n c e s i n t h e d e g r e e of v a l l e y e r o s i o n s i n c e d e g l a c i a t i o n : t h e y used t h e s e m o r p h o l o g i c c h a n g e s a l o n g w i t h a l t e r a t i o n of heavy m i n e r a l s and s o i l p r o f i l e t h i c k n e s s t o d i s t i n g u i s h d e p o s i t s of t h r e e g l a c i a t i o n s i n t h e Aconcagua and E l q u i v a l l e y s of C h i l e . Mercer ( 1 9 7 6 ) m e n t i o n e d t h e much g r e a t e r w e a t h e r i n g and e r o s i o n o f t h r e e p r e - L l a n q u i h u e t i l l s , t h e Casma, C o l e g u a l , and R f o F r f o , of t h e Lago L l a n q u i h u e a r e a i n C h i l e w e s t of B a r i l o c h e , A r g e n t i n a . He n o t e d t h e p r o g r e s s i v e t h i c k n e s s o f t h e w e a t h e r i n g r i n d s on a n d e s i t i c c l a s t s i n t h e s e t i l l s . Wayne ( 1 9 8 1 ) u s e d many of t h e RD p a r a m e t e r s u s e d c u r r e n t l y i n N o r t h America a l p i n e s t u d i e s i n w o r k i n g o u t a l a t e g l a c i a l - H o l o c e n e g l a c i a l geomorphic h i s t o r y f o r t h e u p p e r p a r t of t h e Rfo B l a n c o b a s i n . The f o l l o w i n g d i s c u s s i o n c o v e r s t h e RD methods t h a t I have found u s e f u l i n e f f o r t s t o i d e n t i f y t h e d e p o s i t s of d i f f e r e n t g l a c i a l advances i n t h e Rfo Blanco b a s i n . The p r i n c i p a l p a r a m e t e r s t h a t were m e a s u r a b l e and i n v a r y i n g d e g r e s s u s e f u l i n t h i s s t u d y i n c l u d e d morphos t r a t i g r a p h i c r e l a t i o n s h i p s , m u l t i D l e t i l l e x p o s u r e s , morphologic c h a n g e s , s o i l p r o f i l e d e v e l o p m e n t , s u r f a c e and s u b s u r f a c e b o u l d e r w e a t h e r i n g , d i a g e n e t i c c h a n g e s on s u r f a c e of q u a r t z s a n d g r a i n s , v a s c u l a r p l a n t cover and l i c h e n c o v e r . Momhostratigraphy Advancing i c e b u i l d s m o r a i n e s t h a t may o v e r l a p p a r t s o f o l d e r m o r a i n e s ; l i k e w i s e a c t i v e r o c k g l a c i e r s may b u r y o l d e r s e d i m e n t s . Such m o r p h o s t r a t i g r a p h i c r e l a t i o n s h i p s a r e most u s e f u l among y o u n g e r dep o s i t s t h a t h a v e u n d e r g o n e l i t t l e e r o s i o n a l change b u t a r e of d e c r e a s i n g
394
v a l u e i n o l d e r m a t e r i a l s . Some of t h e E o l o c e n e s e d i m e n t s show c l e a r l y t h e i r r e l a t i o n s h i n s by r n o r n h o s t r a t i v r a n h i c o v e r l a n (Firnure 2 ) .
Figure 2
q u e h r a d a d e 10s V a l l e c i t o s , s u c c e s s i v e m o r p h o stratigraphic overlap: H o l o c e n e I11 i n b a c k g r o u n d a n d r o c k g l a c i e r o n r i g h t o v e r r i d i n g H o l o c e n e I1 l a t e r a l moraine o v e r r i d i n g t o e of Holocene I rock g l a c i e r , w h i c h l i e s a c r o s s V a l l e c i t o s I1 l a t e r a l moraine.
M u l t i p l e T i l l Exoosures Holocene d e p o s i t s a r e o n l y s l i g h t l y d i s s e c t e d , b u t t h e meltwater s t r e a m s f l o w i n g f r o m t h e g l a c i e r r e m n a n t s h a v e c u t away t h e s i d e s of m o r a i n e s i n a few p l a c e s . I n a d d i t i o n , downstream from t h e outermost e n d m o r a i n e s o f t h e l a s t glaciation, s t r e a m e n t r e n c h m e n t a l o n y Rfo Blanco e x p o s e s , t w o t i l l s i n s e v e r a l p l a c e s . With s u c h e x p o s u r e s a t i l l c a n b e r e c o g n i z e d b y i t s p h y s i c a l c h a r a c t e r i s t i c s a n d t r a c e d from or,e exposure t o t h e next (Figure 3 ) .
Figure 3
E x p o s u r e a l o n g Rfo S l a n c o o f two t i l l s , w h i c h c a n b e d i s t i n g u i s h e d i n t h e f i e l d by c o l o r ( t h e l o w e r t i l l i s brown, p r o b a b l y from o x i d a t i o n , and t h e upper one i s g r e e n i s h g r a y ) a n d by t h e r e l a t i v e a b u n d a n c e o f boulders.
395
A c t i v e r o c k g l a c i e r s a r e a n E u l a r and f r e s h l y d e p o s i t e d m o r a i n e s h a v e s h a r p t o p o g r a p h y , but i n t i m e t h e i r s u r f a c e s become r o u n d e d t h r o u g h e r o s i o n , mass w a s t i n g , a n d d e p o s i t i o n . A D r o m e s s i v e r e d u c t i o n i n s l o p e and i n c r e a s e i n roundness of s u r f a c e f e a t u r e s c h a r a c t e r i z e s t h e i n c r e a s e s i n a g e 'of d e p o s i t s ( ' i g u r e s and 5 ) .
Figure 4
S u r f a c e o f rock g l a c i e r d e p o s i t s i n Cuebrada C o l o r a d o , s h o w i n g p r o g r e s s i v e s o f t e n i n g of t o p o g r a p h i c f o r m , r e d u c t i o n of s l o p e a n g l e s , and i n crease i n vegetation cover with increasing age. Active rock g l a c i e r f r o n t i n background; foreground i s V a l l e c i t o s I1 s u r f a c e .
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C h a r t s h o v i n g i n c r e a s e i n l o e s s t h i c k n e s s and dec r e a s e i n s l o p e a n g l e of r o c k g l a c i e r f r o n t s w i t h increasing ape.
396
Dust a c c u m u l a t e s on u p l a n d s u r f a c e s i n d r y a l p i n e a r e a s , and t h e t h i c k n e s s s l o w l y i n c r e a s e s . The s o u r c e i n many r e g i o n s i s t h e o c c a s i o n a l d u s t s t o r m s on t h e a d j a c e n t a r i d p i e d m o n t r e g i o n . Very young mora i n e s and r o c k g l a c i e r s h a v e l i t t l e or n o n e , b u t a m e a s u r a b l e l a y e r i s p r e s e n t on o l d e r s u r f a c e s . Dust a c c r e t i o n s u r e l y d o e s n o t t a k e p l a c e c o n t i n u o u s l y and u n i f o r m l y on m o r a i n a l s u r f a c e s , n e v e r t h e l e s s , a p l o t o f a v e r a g e l o e s s t h i c k n e s s on Holocene and V a l l e c i t o s s u r f a c e s i n t h e Rfo B l a n c o b a s i n a g a i n s t t i m e i s n e a r l y l i n e a r ( 3 i g u r e 5 ) . The r e l a t i o n s h i p does n o t h o l d f o r o l d e r d e p o s i t s , t h o u g h . S u r f a c e We a t he r i n g Chemical and p h y s i c a l w e a t h e r i n g D r o c e s s e s b e g i n t o a l t e r b o u l d e r s a t t h e s u r f a c e o f m o r a i n e s and r o c k g l a c i e r s a s s o o n as t h e y a r e exp o s e d . G r a n i t e s show t h e most r e a d i l y m e a s u r a b l e c h a n g e s and a r e t h e r o c k t y p e t h a t h a s b e e n u s e d most commonly i n m e a s u r e m e n t s o f b o u l d e r w e a t h e r i n g . U n f o r t u n a t e l y , some r e g i o n s t h a t h a v e e x t e n s i v e m o r a i n e s and r o c k g l a c i e r d e p o s i t s c o n t a i n few or no o u t c r o p s t h a t c o u l d h a v e s u p p l i e d g r a n i t i c c l a s t s t o t h e g l a c i e r s e r o d i n g them. I n t h e Rfo B l a n c o b a s i n o n l y two s m a l l s i t e s w e r e s o u r c e a r e a s f o r g r a n i t e c l a s t s , s o m e a s u r e m e n t s o f t h e d e g r e e of w e a t h e r i n g o f g r a n i t e s was o n l y mini m a l l y useful. One b i o t i t e g r a n i t e o u t c r o p t h o u g h , had b e e n c o v e r e d b y g l a c i a l i c e d u r i n g t h e l a s t m a j o r g l a c i a l a d v a n c e ( V a l l e c i t o s ) , and p a r t o f i t was a g a i n c o v e r e d d u r i n g a Holocene g l a c i a t i o n . The r e c e n t l y g l a c i a t e d s u r f a c e i s g o l i s h e d and s t r i a t e d ; t h e p a r t s c o v e r e d s e v e r a l m i l l e n i a e a r l i e r a r e p i t t e d and t h e s u r f a c e i s g r u s i f i e d . A l l g r a n i t e b o u l d e r s o b s e r v e d , t o t a l l i n c f e w e r t h a n 1 0 0 , were r o u n d e d , had s u r f a c e s e t c h e d t h r o u g h w e a t h e r i n g , and many were p i t t e d . R h y o l i t e p o r p h y r i e s , which s h a t t e r r e a d i l y i n t h e p r e s e n t f r o s t c l i m a t e above 3 5 0 0 m y were t h e most a b u n d a n t r o c k i n two of t h e r o c k g l a c i e r - f i l l e d v a l l e y s s t u d i e d ; q u a r t z i t e s made up Lhe b u l k o f t h e b o u l d e r s i n t h e o t h e r two. When f i r s t b r o k e n from t h e o u t c r o p , t h e e d g e s o f most q u a r t z i t e b o u l d e r s a r e s h a r p l y a n g u l a r . The e d g e s l o s e t h e i r sharpness with exposure t o weathering p r o c e s s e s , though, s o b o u l d e r - e d g e r o u n d i n g p r o v i d e d one' measurement of some v a l u e . I looked f o r c l a s t s t h a t were 30 cm or g r e a t e r i n i n t e r m e d i a t e d i a m e t e r a n d c o u n t e d a t l e a s t 2 5 a t e a c h s i t e , i f t h a t many w e r e a v a i l a b l e , l i s t i n g e a c h i n one of 4 c a t e g o r i e s : a n g u l a r , s u b a n g u l a r , s u b r o u n d e d , and r o u n d e d . L o s s of a n g u l a r i t y seemed t o b e s l o w l y D r o g r e s s i v e i n t h e Andean t u n d r a , b u t r o u n d i n p was much more n o t i c e a b l e where b o u l d e r s l a y i n woody s h r u b - c o v e r e d p a r t s o f t h e m o r a i n e s . F i r e s p a l l i n g may h a v e a c c e l e r a t e d t h e r o u n d i n g on s u r f a c e s where s u f f i c i e n t v e g e t a t i o n exi s t e d t o h a v e s u p p o r t e d b r u s h Yires. A split-to-non-split r a t i o o f b o u l d e r s was d e t e r m i n e d w h e r e v e r s p l i t b o u l d e r s were f o u n d . B o u l d e r s p l i t t i n g however, seemed t o b e
more s t r o n g l y a f f e c t e d by n e a r n e s s t o a l a t e r g l a c i e r f r o n t and t h e p e r i g l a c i a l c l i m a t e c o n d i t i o n s t h a t accompanied i t t h a n to t i m e a l o n e . The a r e a s of most h i g h l y s p l i t b o u l d e r s a l s o contain abundant sorted circles. Where r o c k s t h a t d e v e l o p w e a t h e r i n g r i n d s a r e a b u n d a n t , r i n d - - t o In the n o n - r i n d r a t i o s and t h i c k n e s s o f r i n d s a r e u s e f u l m e a s u r e m e n t s . R f o B l a n c o basin measurable r i n d development seems to h a v e t a k e n p l a c e o n l y on f i n e g r a i n e d d i o r i t i c r o c k s , w h i c h a r e r a r e i n t h e d e p o s i t s . Only a Yew measurements w e r e made, b u t t h e d a t a seemed t o b e c o n s i s t e n t f o r L a t e P l e i s t o c e n e and E o l o c e n e d e p o s i t s .
Subsurface Weathering Boulders t h a t a r e b u r i e d and t h u s kegt moist a r e s u b j e c t e d t o h y d r o l y s i s , which c a u s e s them t o d i s i n t e g r a t e . P l u t o n i c r o c k s i n t h e zone of w e a t h e r i n g a r e most r e a d i l y a f f e c t e d and t h e i r s o u n d n e s s has b e e n u s e d i n t h e S i e r r a Nevada a n d Rocky M o u n t a i n s a s a g u i d e t o t h e l e n g t h of t i m e t h e y h a v e u n d e r g o n e w e a t h e r i n g i n p l a c e ( B i r k e l a n d , 1 9 7 3 ; Burke and B i r k e l a n d , 1 9 7 9 ; C a r r o l l , 1 9 7 4 ) . T h i s t e c h n i q u e h a s l i t t l e v a l u e i n t h e s e p a r a t i o n o f t h e d e p o s i t s of and s i n c e t h e l a s t m a j o r g l a c i a t i o n i n t h e Rfo B l a n c o b a s i n b e c a u s e o f t h e p a u c i t y o f p l u t o n i c
391
rocks. I t h a s some v a l u e , h o w e v e r , f o r comparing o l d e r t i l l s , i n which t h e few p l u t o n i c r o c k s w e r e c o m p l e t e l y g r u s i f i e d and q u a r t z i t e c l a s t s had become f r a g m e n t e d . S o i l p r o f i l e s develop slowly i n t h e a l p i n e environment; neverthel e s s , t h e p r o c e s s i s s u f f i c i e n t l y r a p i d t h a t d i s t i n c t i v e changes w i l l t a k e p l a c e w i t h i n a few t h o u s a n d y e a r s . Even t h e s l i g h t development on young m a t e r i a l s c a n b e r e c o g n i z e d and m e a s u r e d . A l t h o u g h c e r t a i n l y n o t a l i n e a r f u n c t i o n (Colman, 1981), s o i l - p r o f i l e development i s p r o g r e s s i v e w i t h t i m e , s o s t u d y o f t h e p r o f i l e s i s a n i m p o r t a n t method t o h e l p d i s t i n g u i s h d e p o s i t s from t h e v e r y young t o t h o s e o f e a r l y P l e i s t o c e n e a g e . A c c u r a t e and d e t a i l e d f i e l d d e s c r i p t i o n s , u s i n g I l u n s e l l c o l o r d e s i g n a t i o n s , p e r m i t r e c o g n i t i o n o f many g l a c i a l d e p o s i t s . The d a t a from l a b o r a t o r y a n a l y s i s , p a r t i c u l a r l y t h e k i n d and amount o f c l a y m i n e r a l s p r e s e n t throup;h t h e p r o f i l e and t h e d e g r e e o f a l t e r a t i o n of minerals such as hornblende, a i d s i n t h e c h a r a c t e r i z a t i o n . Difficulties were e n c o u n t e r e d i n o b t a i n i n g a d e q u a t e l a b o r a t o r y a n a l y s e s beyond g r a i n s i z e d i s t r i b u t i o n , s o s o i l p r o f i l e s t u d i e s used i n t h i s p r o j e c t a r e All b u t a s m a l l number o f t h e s o i l b a s e d l a r g e l y on f i e l d d e s c r i p t i o n s . p r o f i l e s examined r e q u i r e d t h e e x c a v a t i o n of a p i t from 30 t o 9 0 o r m o r e em i n d e p t h . Quartz Grain Surface Textures Some s e d i m e n t t r a n s p o r t p r o c e s s e s i m p o r t d i s t i n c t i v e s e t s o f f e a t u r e s t o t h e s u r f a c e s o f d e t r i t a l q u a r t z g r a i n s ( K r i n s l e y and Doornkamp, 1 9 7 3 ) . G r a i n s c a r r i e d by moving i c e commonly a r e f r a c t u r e d s o t h a t t h e y h a v e a h i g h r e l i e f and u n i q u e D a t t e r n s u n l i k e t h o s e i m p a r t e d t o q u a r t z g r a i n s i n o t h e r e n v i r o n m e n t s . Because s u r f a c e t e x t u r e s o f g l a c i a l l y t r a n s p o r t e d g r a i n s a r e s o d i s t i n c t i v e , t h e p r e s e n c e of a r e l a t i v e l y h i g h p r o p o r t i o n of g r a i n s w i t h t h e s e t e x t u r e s i n a s e d i m e n t means t h a t t h e g r a i n s were c a r r i e d i n g l a c i a l i c e , a n d . i f no o t h e r p a t t e r n i s o v e r p r i n t e d t h e s e d i m e n t from which t h e y came, p r o b a b l y i s glacially deposited. Sand g r a i n s were s e p a r a t e d from a s m a l l s a m p l e o f e a c h of t h e t i l l s or d i a m i c t o n s t h o u g h t t o b e a t i l l i n t h e Rio B l a n c o v a l l e y and i n two
m a j o r h e a d w a t e r s t r e a m s . A f t e r s e p a r a t i o n , t h e g r a i n s w e r e washed i n d i s t i l l e d w a t e r t o remove c l a y - s i z e d p a r t i c l e s . N o n o c r y s t a l l i n e q u a r t z g r a i n s w e r e p i c k e d from e a c h s a m p l e and 1 5 o f them mounted on s t u b s and coated w i t h gold f o r examination under a scanning e l e c t r o n microscope
(SEM).
N e a r l y a l l g r a i n s from t h e s a m p l e s t a k e n from t h e t i l l s and r o c k g l a c i e r d e p o s i t s o f t h e l a s t ( V a l l e c i t o s ) g l a c i a t i o n s were c o m p l e t e l y and s h a r p l y f r a c t u r e d ( F i g u r e 6 a , b ) . No st g r a i n s from t h e o l d e r t i l l s showed c o m p a r a b l e s u r f a c e t e x t u r e s , b u t some had one or more s u r f a c e s t h a t had n o t b e e n b r o k e n i n t r a n s D o r t . I n a d d i t i o n , g r a i n s from t h e o l d e s t s e d i m e n t t h o u g h t t o b e a t i l l showed a n e t c h i n g o v e r p r i n t on t h e g l a c i a l l y formed f r a c t u r e p a t t e r n ( F i g u r e 6). S i m i l a r e t c h i n g h a s begun b u t i s o n l y s l i g h t l y d e v e l o p e d on g r a i n s from t h e t i l l s of i n t e r m e d i a t e a g e s ( F i g u r e 6 ) . These d i a g e n e t i c c h a n g e s , t h o u g h s u b t l e , e v i d e n t l y a r e p r o g r e s s i v e , s o t h a t i t may b e p o s s i b l e t o u s e them a s one a d d i t i o n a l means o f o r d e r i n g t i l l s w i t h i n a s e q u e n c e r e l a t i v e t o each o t h e r , p r o v i d e d t h e d i f f e r e n c e s a r e of a t l e a s t s t a g e magnitude. The r a t e of d i a g e n e t i c e t c h i n g s u r e l y i s t o o slow f o r i t t o h a v e v a l u e i n s e p a r a t i n g d e p o s i t s of s u b s t a g e s o f a g l a c i a t i o n . B i o t i c Cover L i c h e n s r e q u i r e l i g h t and m o i s t u r e i n o r d e r t o become e s t a b l i s h e d on r o c k s . Thus t h e y a r e r a r e on t h e s u r f a c e s o f a c t i v e r o c k g l a c i e r s and do n o t b e g i n t o grow on m o r a i n a l b o u l d e r s u n t i l g l a c i a l i c e h a s m e l t e d from t h e a r e a . The r a t e of g r o w t h o f R h i z o c a r p o n g e o g r a p h i c u r n s e n s u lato has b e e n u s e d i n many p a r t s o f t h e w o r l d a s a t o o l t o d e t e r m i n e a n a b s o l u t e a g e f o r l a t e Holocene g l a c i a l d e p o s i t s ( B e n e d i c t , 1 9 6 7 , 1 9 6 8 ; B e s c h e l , 1 9 7 3 ) . Because c l i m a t e , and p a r t i c u l a r l y a v a i l a b l e m o i s t u r e , d i r e c t l y a f f e c t t h e r a t e of g r o w t h of t h i s and o t h e r l i c h e n s , a t h a l l u s g r o w t h - r a t e c u r v e must b e develoDed f o r e a c h a r e a
398
Figure 6
Scanning E l e c t r o n Microscope images of q u a r t z grain surfaces. a . and b . 0 . C o l o r a d o , V a l l e c i t o s I1 S t a g e r o c k glacier c . Q . d e 10s V a l l e c i t o s , V a l l e c i t o s S t a g e , t i l l d . 0. d e l a Angostura, R i o Blanco S t a g e , till e. Q. de l a Angostura, Angostura S t a g e , till f . Mes6n d e l P l a t a , L o s M e s o n e s S t a g e , t i l l
where l i c h e n o m e t r y i s t o b e u s e d t o c a l c u l a t e t h e a g e i n y e a r s of a particular deposit. Even t h o u g h a g r o w t h r a t e c u r v e h a s n o t - - o r can not--be worked o u t f o r a p a r t i c u l a r a r e a , two m e a s u r e m e n t s - maximum t h a l l u s d i a m e t e r and p e r c e n t o f l i c h e n c o v e r - a r e u s e f u l i n r e l a t i v e d a t i n g ( s e e Mahaney e t a l . , t h i s v o l u m e ) . I n a l p i n e r e g i o n s , v a s c u l a r p l a n t s a r e more s p a r s e on y o u n g e r d e p o s i t s t h a n t h e y a r e on o l d e r ( a n d l o w e r ) s u r f a c e s . P a r t of t h e d i f f e r e n c e i s a g e , which may b e r e l a t e d t o b o t h l o e s s t h j - c k n e s s and s o i l development. P a r t may a l s o be c l i m a t i c . Nevertheless a d i s t i n c t d i f f e r e n c e i n p l a n t c o v e r was n o t e d on g r o u p s of m o r a i n e s and on f o s s i l r o c k g l a c i e r s i n t h e Rfo B l a n c o b a s i n . These e s t i m a t e s of c o v e r were
399
one a d d i t i o n a l p a r a m e t e r t h a t seemed t o h a v e v a l u e i n t h e Holocene dep o s i t s r a n g i n g f r o m a f e w h u n d r e d t o a few t h o u s a n d y e a r s o l d . A P P L I C A T I O N OF RELATIVE D A T I N G D A T A
Throughout a l l d e p o s i t s , m a t u r i t y and c h a r a c t e r of s o i l p r o f i l e d e v e l o p m e n t p r o v i d e d e f f e c t i v e i n f o r m a t i o n f o r u s e i n c l a s s i f y i n g by a g e . A l t h o u g h t h e s o i l p r o f i l e d e v e l o p m e n t was o n l y o n e o f s e v e r a l k i n d s of d a t a u s e d t o d e t e r m i n e t h e number o f H o l o c e n e a n d L a t e P l e i s t e c e n e g l a c i a l a d v a n c e s , i t was o n e t h a t s e e m e d t o b e p a r t i c u l a r l y r e l i a b l e . For t h e o l d e r d e p o s i t s , i t a n d t i l l s u p e r p o s i t i o n were t h e two u s e d w i t h t h e g r e a t e s t d e g r e e of c o n f i d e n c e . A d e s c r i p t i o n o f a s o i l p r o f i l e t h a t i s c h a r a c t e r i s t i c o f e a c h of t h e d e p o s i t s mapped i s i n t h e appendix. Holocene D e p o s i t s S e v e r a l k i r ? d s o f RD m e a s u r e m e n t s a r e s u f f i c i e n t l y d i s t i n c t i v e t o p e r m i t s e p a r a t i o n of t h e H o l o c e n e g l a c i a l d e p o s i t s i n t o t h r e e g r o u p s ( T a b l e 1 ) . P e r h a p s t h e most i m D o r t a n t i s t h a t t h e m o r a i n e s and r o c k g l a c i e r s o f t h e most r e c e n t i c e a d v a n c e , which a r e t h e e q u i v a l e n t o f t h e r e c e n t l y e n d e d " L i t t l e I c e A g e " , r e m a i n i c e c o r e d , a n d many o f t h e r o c k g l a c i e r s s t i l l a r e a c t i v e . A l l s o i l p r o f i l e s a r e w e a k , b u t show i n c r e a s i n g t h i c k n e s s and i n t e n s i t y o f c o l o r w i t h a g e . Both l i c h e n c o v e r on b o u l d e r s a n d t o t a l v e g e t a t i o n c o v e r o f t h e s u r f a c e i n c r e a s e s w i t h t h e a g e o f t h e s e d e p o s i t s . O n l y t h e o l d e s t of t h e H o l o c e n e r o c k g l a c i e r s has a l o e s s c o v e r , and t h e f r o n t a l a n g l e s o f r o c k g l a c i e r s become l e s s s t e e p w i t h i n c r e a s i n g a g e ( F i g u r e 5 ) . I n some v a l l e y s , s u c c e s s i v e p a r t i a l o v e r l a p makes r e c o g n i t i o n of r e l a t i v e p o s i t i o n s i m p l e , s o t h a t measurable d a t a can be related r e a d i l y and used t o i d e n t i f y d e p o s i t s of c o r r e l a t i v e age i n o t h e r v a l l e y s . V a l l e c i t o s / W i s c o n s i n a n / ~ ~ ~D ~ rem posits
The o l d e s t a n d o u t e r m o s t d e p o s i t s t h a t h a v e u n d i s p u t e d g l a c i a l m o r p h o l o g y a r e t h o s e of t h e l a s t P l e i s t o c e n e g l a c i a t i o n , named by C o r t e (1957) t h e V a l l e c i t o s S t a g e . E a c h o f t h e m e a s u r a b l e p a r a m e t e r s u s e d t o d i s t i n g u i s h and i d e n t i f y t h e moraines and r o c k g l a c i e r s of Holocene g l a c i a l p h a s e s a r e a p p l i c a b l e t o t h o s e of V a l l e c i t o s a g e a s w e l l ( T a b l e 1 ) . Loess t h i c k n e s s i s g r e a t e r , and r o c k g l a c i e r f r o n t a l s l o p e a n g l e s are lower ( F i g u r e 2 ) . L i c h e n c o v e r on b o u l d e r s a n d v e g e t a t i o n c o v e r of m o r a i n a l s u r f a c e s a r e s i g n i f i c a n t l y g r e a t e r t h a n a r e t h o s e on t h e y o u n g e r d e p o s i t s . B o t h s o i l p r o f i l e d e v e l o p m e n t ( T a b l e 1, A p p e n d i x ) a n d l o e s s c o v e r c l e a r l y show t h a t t h e V a l l e c i t o s m o r a i n e s f a l l i n t o two g r o u p s . The y o u n g e r s u r e l y a r e t h e same age a s t h o s e d a t e d i n P a t a g o n i a ( M e r c e r , 1976) a s 1 3 , 0 0 0 t o 1 9 , 0 0 0 or s o y e a r s o l d ; t h e o l d e r g r o u p may m a t c h t h o s e M e r c e r f o u n d t o h a v e b e e n d e p o s i t e d more t h a n 4 0 , 0 0 0 y e a r s ago. Pre-Vallecitos
Deposits :
T i l l s v s Pludflows
Ciamictons g r e a t e r i n age than those i n the moraines of Vallecitos age are p r e s e n t a t lower a l t i t u d e s i n Rfo Blanco v a l l e y . Although C o r t e ( 1 9 5 7 ) d e s c r i b e d a d i a m i c t o n t h a t c a n b e t r a c e d t o t h e v a l l e y of R f o Mendoza, 1 7 km d o w n s t r e a m f r o m t h e o u t e r m o s t V a l l e c i t o s S t a g e moraine, and c a l l e d i t t h e Rfo Blanco t i l l , P o l a n s k i (1953, 1963, 1 9 6 5 ) a r g u e d t h a t t h e l a s t g l a c i a t i o n was t h e most e x t e n s i v e i n t h e d r y Dlendoza A n d e s , a n d t h a t t h e d i a m i c t o n s f a r t h e r down v a l l e y a r e m u d f l o w s t h a t r e c o r d u p l i f t of t h e C o r d i l l e r a . Before t h e s e e x t r a - V a l l e c i t o s d i a m i c t o n s can b e c o n s i d e r e d evidence of e a r l i e r g l a c i a t i o n s , i t w i l l be n e c e s s a r y t o d e t e r m i n e whether t h e y a r e i n f a c t more l i k e l y t i l l s o r mudflow s e d i m e n t s . They f i l l t h e l o w e r p a r t o f t h e Rfo B l a n c o v a l l e y , w h e r e t h e y a r e o v e r l a i n b y b o u l d e r y g r a v e l a n d s a n d a n d , i n o n e p l a c e , by a b e d o f c l e a n , v o l c a n i c a s h . The s o u r c e of t h e a s h has n o t b e e n d e t e r m i n e d , b u t i t may h a v e come f r o m o n e of t h e r e c e n t l y a c t i v e v o l c a n i c c o n e s f a r t h e r s o u t h i n Mendoza Province.
Table 1.
Relative Dating ( R D ) c h a r a c t e r i s t i c s o f t h e l a t e Pleistocene a n d H o l o c e n c g l a c i e r a n L l r o c k g l a c i e r d e p i ~ s l to~f the upper part of the Rio Blanco drainage basin
CHARACTERISTICS
VALLECITOS I
VALLECITOS I1
HOLOCENE I
HOLOCENE 111 ("Little Ice Age")
Clustered moraines extend to 3450 lil Rock glacier tongue to 3100 m
Ice-cored moraines beyond present ice margin Active or recently inactive rock glaciers to 3250m, fronts 33"45", contain ice
Position of moraines or rock glaciers
End moraines low in valley (below 2600m) Outer lateral moraine smoothly rounded
Moraines and fossil rock glaciers fill valleys between 2600 and 3400 m
Modification of construct ional landforms
Loess variable, 35-80 cm; Moraines partly destroyed by stream
Loess averages 20 cm; Loess averages 5cm Moraines trenched but Rock glacier front 25" 32" little valley widening Rock glacier fronts Smoothly rounded 23"-30°
No loess Trenched only where meltwater stream passes through. Rock glacier fronts 27'-30"
Ice-cored moraines starting t o collapse Some rock glaciers inactive, rounded crests, still contain ice
Boulder weathering
Granites etched, rounded, pitted to 3 cm Quartzites subroundek (A) 0-(SA)57- (SR)43(R) 0
No granites; quartzites: (A)20(SA) 73- (SR) 7- ( R ) 0 Rinds 0.2mm o n diorites
All angular
soil profile development
Moderate, Mollisols B-horizon massive to weak blocky structure 5YR3/3-5/4(B) /10YR 4/4(C)
Granites etched, No granites: quartzites: rounded, pitted to (A)29-(SA)-62(SR)S-(R)O 3 cm Rinds 0.5mm o n diorites Non-granitic rocks: (A)lO-(SA)37-(SR)32(R)21 Rinds 0.5 to 1.0 mm Moderate, Mollisols Weak, Inceptisols B-horizon massive Cox horizon loessldebris 7.5YR4/4(B)/lOYR4/4 7.5YR414 to 5YR4/4(B)/ (C) 7.5YR4/4(C)
Weak, Inceptisols Cox 0-5 cm (till) 10YR4/3(B)/2.5Y 5/2(C)
None, Entisols
Lichens
10% to 90% covered Diameters meaninglesq multigenerations
3 0 % to 40% covered 50% to 7 0 % covered Diameters meaningless R . geographicurn 70mm multigenerations
10% to 40% covered R . geographicurn to 60mm
Cover less than 2% No H . g e o g r a p h i c u r n observed
Vegetation
Fully covered (60%90%) Grasses. forbs, woody shrubs
Fully covered (60%About 40% covered 90%) Grasses, forbs, Grasses, forbs. woody prostrate Adesmia shrubs
5 % to 20% covered
Grasses, forbs
Less than 2% covered Succulent forbs
Large, massive Thick loess accumulation moraines beneath boulders at toe Sorted stripes, lobes of slope on slopes above Sorted circles 1-1.5m moraines diam. to 50 m downslope Avalanche cones beside moraines in valleys
Sorted circles 11.5m diam. on adjacent surfaces 750 m downslope
Many rock glaciers expanding now Ice-cored moraine rock-glacierized
19,000-13,000 BP
2700-2000 BP
300 BP-0
Other features
Estimated age
X40.000 BP
Rock glaciers at 3200 m
HOLOCENE I1
-
4600-4000 BP
401
Mudflows commonly o r i g i n a t e f r o m t h e f r o n t a l r e g i o n s o f a c t i v e g l a c i e r s , b u t m u d f l o w s i n t h e e n v i r o n m e n t o f s m a l l v a l l e y g l a c i e r s do not r e s u l t i n massive, t h i c k , several-km-long, v a l l e y - f i l l i n g diam i c t o n s s u c h as t h e one i n R l o B l a n c o v a l l e y . A l p i n e mudflows o f v o l c a n i c o r i g i n , s u c h a s t h o s e t h a t were c a u s e d by t h e r e c e n t e r u n t i o n o f M t . S t . H e l e n s i n t h e C a s c a d e Range o f t h e U n i t e d S t a t e s , d o l e a v e s e d i m e n t s c o m p a r a b l e i n volume t o t h o s e o f R f o B l a n c o . The C o r d 6 n d e l P l a t a c o n t a i n s no v o l c a n o e s , t h o u g h , s o s u c h a s o u r c e would be most u n l i k e l y . A l s o , t h e s e d i m e n t s o f v o l c a n i c mudflows i n v a r i a b l y c o n t a i n some e x t r u s i v e v o l c a n i c m a t e r i a l s : t h e s a n d f r a c t i o n o f t h e F i o B l a n c o d i a m i c t o n s c o n t a i n s n o v o l c a n i c d e b r i s a n d i s d o m i n a t e d by l i t h i c c l a s t s o f f i n e g r a i n e d q u a r t z i t e s . S c a n n i n g e l e c t r o n m i c r o s c o p e (SEY) images of q u a r t z g r a i n s from t h e sand f r a c t i o n s of t h e s e d i a m i c t o n s e x h i b i t s u r f a c e t e x t u r e s c h a r a c t e r i s t i c of t h o s e p r o d u c e d by g l a c i e r t r a n s p o r t , as do, t h e g r a i n s from u n q u e s t i o n e d t i l l s h i g h e r i n t h e v a l l e y ( F i g u r e 6 a ) . For t h e s e r e a s o n s , I b e l i e v e t h e d i a m i c t o n s o f t h e Rfo B l a n c o v a l l e y a r e more l i k e l y t i l l s t h a n mudflow s e d i m e n t s , a l t h o u g h t h e p o s s i b i l i t y c a n n o t y e t b e r u l e d o u t t h a t some o f t h e s e d i m e n t c o u l d h a v e o r i g i n a t e d a s mudflow from t h e f r o n t of a g l a c i e r . They d o r e p r e s e n t t h e d e p o s i t s of p r e - V a l l e c i t o s g l a c i a t i o n s . Downstream from t h e o u t e r m o s t and o l d e s t V a l l e c i t o s m o r a i n e , A n g o s t u r a Creek f l o w s i n a t r e n c h e r o d e d b e l o w a b o u l d e r - s t r e w n , f a n shaped s u r f a c e . T i l l u n d e r l i e s t h i s s u r f a c e , which c o n t a i n s a v e n e e r o f l o e s s a l s i l t a b o u t 3 0 em t h i c k . A s o i l p r o f i l e c o m p a r a b l e t o t h a t on some o f t h e l o w e r V a l l e c i t o s S t a g e moraines h a s d e v e l o p e d t h r o u g h t h e l o e s s a n d i n t o t h e t o p o f t h e t i l l . The s u r f a c e i s V a l l e c i t o s i n a g e , b u t i t was c u t by c u r r e n t s o f V a l l e c i t o s m e l t w a t e r a c r o s s DyeV a l l e c i t o s t i l l s . Two t i l l s , t h e l o w e r o n e o f w h i c h h a s a w e l l d e v e l o p e d b u r i e d p a l e o s o l i n i t , a r e e x n o s e d i n t h e 8-m-high b a n k s a l o n g the creek. A t one p l a c e , t h e c r e e k has swept f r e e t h e v a l l e y s i d e and e x p o s e d a much t h i c k e r s e c t i o n , w i t h a m a t u r e c a r b o n a t e r i c h s o i l p r o f i l e on t h e u p p e r t i l l a n d a b u r i e d p a l e o s o l w i t h a t h i c k a r g i l l i c B h o r i z o n on t h e l o w e r o n e ( T a b l e 2 ; A D p e n d i x ) . C l e a r l y , e x p o s u r e s o f t h e two t i l l s i n s u p e r p o s i t i o n , a s t a n d a r d s t r a t i g r a p h i c a p p r o a c h , h a s c l a r i f i e d t h e a g e s o f t h e s e two r e l a t i v e t o e a c h o t h e r . The w e l l d e v e l o p e d s o i l p r o f i l e a t t h e t o p o f t h e y o u n g e r t i l l shows i t n o t o n l y t o be o l d e r t h a n t h e V a l l e c i t o s t i l l s , which have l e s s m a t u r e l y dev e l o p e d p r o f i l e s , b u t a l s o t o h a v e f o r m e d u n d e r more a r i d c l i m a t i c conditions. Both o f these t i l l s c a n be s e e n i n s u p e r p o s i t i o n i n a n o t h e r exp o s u r e a b o u t 3 km d o w n s t r e a m ( F i g u r e 3 ) , b u t o n l y o n e o f t h e t i l l s s e e m s t o b e p r e s e n t i n t h e l o w e r p a r t o f R i o B l a n c o v a l l e y . The s o i l p r o f i l e on t h e u p p e r o n e c l e a r l y shows t h e e f f e c t o f t h e w a r m d r y c l i mate t h a t e v i d e n t l y e x i s t e d d u r i n g t h e i n t e r g l a c i a l s t a g e p r e c e e d i n g t h e V a l l e c i t o s G l a c i a t i o n . A s mentioned e a r l i e r , a n e f f o r t t o d a t e t h e v o l c a n i c a s h b e d t h a t o v e r l i e s t h e t i l l C o r t e named t h e R f o B l a n c o was n o t w h o l l y s u c c e s s f u l , b u t a z i r c o n f i s s i o n - t r a c k s t u d y showed i t t o b e between 1 0 0 , 0 0 0 and 2 0 0 , 0 0 0 y e a r s o l d (Glen I z e t t , Charles N a e s e r , l e t t e r s , December 1 9 8 0 ) . The t w o t i l l s i n t h e u p p e r v a l l e y e x posures have d i s t i n c t l y d i f f e r e n t bouider/matrix r a t i o s ; t h e lower t i l l i s v e r y b o u l d e r y , and t h e u p p e r one i s s i m i l a r t o t h e Rfo Blanco t i l l d o w n s t r e a m . For t h e s e r e a s o n s , I h a v e i d e n t i f i e d t h e u p p e r t i l l i n t h e s e m u l t i p l e e x p o s u r e s a s t h e R f o B l a n c o t i l l o f C o r t e (1957) a n d named t h e l o w e r o n e t h e A n g o s t u r a t i l l (Wayne, i n p r e s s ) . S t i l l one a d d i t i o n a l d i a m i c t o n i s p r e s e n t i n t h e Rfo Blanco v a l l e y . It c a p s a s l o p i n g f l a t - s u r f a c e d r i d g e t h a t s t a n d s n e a r l y 200 m above p r e s e n t d r a i n a g e a n d i s a l i g n e d w i t h t h e u1sl)er p a r t o f 0,. d e l a Angostura ( F i g u r e 1 ) . It h a s a t h i c k , c a r b o n a t e - r i c h s o i l p r o f i l e , q u a r t z i t e b o u l d e r s e x p o s e d a r e e x t r e m e l y r o u n d e d , a n d many i n t h e s o i l a r e e x t e n s i v e l y w e a t h e r e d . It was i n c l u d e d i n P o l a n s k i ' s ( 1 9 6 3 ) Los M e s o n e s F o r m a t i o n b e c a u s e i t c a p s Mes6n d e l P l a t a i n t h e R f o B l a n c o v a l l e y . I t s t o p o g r a p h i c p o s i t i o n , s o i l p r o f i l e development, and b o u l d e r w e a t h e r i n g c h a r a c t e r i s t i c s show i t t o b e much o l d e r t h a n a n y o f t h e o t h e r glaciogenic sediments (Table 2 ) i n t h e area.
Table 2 .
Relative Dating (RD) characteristics of the Pleistocene glacial deposits, R f o Blanco drainage basin
Position o f constructional landforms
None left. Till present along valley to 1800 m
Modification
None recognized; Surface of till buried to about 1P50 m
Yorainal remnants present about 2200m. 1900m and 1700m. Till present in valley to 1 4 0 0 m
Moraines and fossil rock glaciers in valley between 2 4 0 0 m and 3 4 0 0 m
Fragments o f once Buried beneath younger s l o.~ i .n aplain cap till between 2 5 0 0 m and . mesetas 75 to 2 0 0 ab ut 1950 m. m above present streams Some loess in soil profile
Pediment cut across till surface from 2500 to 2 1 0 0 m: Moraine at 1700 m deeply trenchefi Coarse gravel over till downstream Loess cap 15 to 3 0 cm
Moraines fresh to rounded, outer ones moderately breached Inner moraines trenched Loess 2 0 - 3 5 + c m
Boulder weathering
F e w boulders vis.- No boulders at surface G r nites in buried ible on surface Only quartzites, paleosol grusified a l l well rounded, many split Quartzites in soil completely weathered. Few granites observed, all completely grusified. Boulders exposed only along edges
Surface boulders not corn-mo n Only quartzite fragments to 3 m in s o i l profile at outermost moraine recognized Boulders o n higher moraine fragment (A)15-(SA) 21-(SR25-(R)39% with 70% broken
Granites rounded, etched, pitted to 3 cm Quartzites subround t o subangular Rinds 0 . 5 to 1.0 mm V . I : ( A ) 0 - ( S A ) 57- (SR)4 3 (R)O V . I T : (A)lO-(SA)37-(SK) 1(R) 2 1
Soil Profile Development
Very strong: at Buried paleosol with 3 0 0 0 m argillic B- strongly developed argil. horizon with strong lic B 2 , weak Bca and blocky structure Cca horizons (27cmthick) Aridisol below 2350 m. with thick chalky Bca-Cca horizon Locally strongly cemented (12-80+cm)
Moraine fragments at 1900 5 2 3 0 0 m: Aridisol with Bca 25-35 cm and Cca(Y,) horizons 35-60 cm, platy Outer moraine at 1700 m: silty aridisol, Bca 35-60 cm, weakly cemented
Moderate, Mollisols B-horizon massive to w e . l k blorky Acid
Other Features
Grassland, with Fills bottom of bedrock scattered woody valley shrubs Rests on weathered andesite in 0 . d e la Angostura Till calcareous when exposed
Capped by ash bed in a few Grass with woody shrubs exposures Till not calcareous Till n o t calcareous under pediment but contains secondary C a C 0 3 in other places
Estimated A g e
1.2-1.0
x lo6 y r s
450,000
vrs BP ( ? )
403 C 0 N C LU S I 0 N S
The r e l a t i v e d a t i n g ( F I D ) t e c h n i q u e s t h a t h a v e b e e n d e v e l o p e d b y i n v e s t i g a t o r s o f g l a c i a l s e d i m e n t s t o s u b d i v i d e and c l a s s i f y t h e g l a c i a l d e p o s i t s o f N o r t h America h a v e s e r v e d w e l l i n t h e s t u d y o f g l a c i o g e n i c d e p o s i t s o f t h e C e n t r a l Andes o f Mendoza P r o v i n c e , A r g e n t i n a . S t a n d a r d s t r a t i g r a p h i c p r a c t i c e s of s u p e r p o s i t i o n o f b o t h s t r a t a and m o r p h o l o g i c u n i t s , s u p p l e m e n t e d b y w e a t h e r i n g and e r o s i o n a l m e a s u r e m e n t s , h a v e l e d t o t h e r e c o g n i t i o n o f t h e d e p o s i t s o f f o u r g l a c i a l s t a g e s and o f t h r e e p h a s e s of g l a c i a t i o n d u r i n g Holocene t i m e . ACKNOWLEDGEMENTS The f i e l d work for t h i s r e p o r t was done between J a n u a r y and J u n e 1980 w h i l e I was a t t a c h e d t o t h e I n s t i t u t o A r g e n t i n o d e N i v o l o g f a y G l a c i o l o g f a ( I A N I G L A ) i n Mendoza,Aryentina, t h r o u g h mutual agreement between t h e N a t i o n a l S c i e n c e F o u n d a t i o n o f t h e U.S.A. and t h e C o n s e j o National de I n v e s t i g a c i o n e s Cientif'icas y Tecnicas ( C O N I C E T ) of A r g e n t i n a . I t was s u p p o r t e d i n p a r t b y NSF G r a n t No. I N T -- 7920798. I am g r e a t l y i n d e b t e d t o Dr. A r t u r o E . C o r t e o f I A N I G L A , who s u g g e s t e d t h e p r o b l e m s r e l a t e d to t h e s e d i m e n t s i n t h e Rfo B l a n c o d r a i n a g e b a s i n , i n v e s t i g a t e d i n t h i s p r o j e c t . My u n d e r s t a n d i n g o f t h e d e p o s i t s s t u d i e d was h e l p e d g r e a t l y by d i s c u s s i o n s w i t h h i m b o t h i n and o u t o f t h e f i e l d w h i l e t h e f i e l d work was i n p r o g r e s s . I would a l s o l i k e t o t h a n k t h e s t a f f o f I A N I G L A , f o r p r o v i d i n g a c c e s s to a n i n s t i t u t e v e h i c l e f o r f i e l d work; and t o my w i f e , Naomi kiayne, who s e r v e d a s f i e l d and l a b o r a t o r y a s s i s t a n t throughout t h e p r o j e c t . REFERENCES C I T E D Benedict, J.B., 1967, Recent g l a c i a l h i s t o r y of a n a l p i n e a r e a i n t h e Colorado F r o n t Range, U.S.A. I. E s t a b l i s h i n g a lichen-growth curve: J o u r n a l o f G l a c i o l o g y , v . 6 , no. 48, p . 817-832.
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, 1968, Recent g l a c i a l h i s t o r y of an a l p i n e area i n t h e J o u r n a l of C o l o r a d o F r o n t R a n g e , 11. D a t i n g t h e g l a c i a l d e p o s i t s : G l a c i o l s , v. 7 , no. 49, p . 77-87.
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B e s c h e l , Roland ( t r . by W. B a r r ) , 1973, L i c h e n s as a measure o f of r e c e n t moraines: A r c t i c and Alpine Research, v . 5, no. 303-309.
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C o r t e , A . E . , 1 9 5 7 , S o b r e G e o l o g i a g l a c i a l F ' l e i s t o c e n i c a d e Mendoza: Anales d e l Departamento de I n v e s t i g a c i o n e s C l i e n t i f i c a s , U n i v e r s i d a d N a c i o n a l d e Cuyo, Tomo 2 , n o . 2 , p . 1 - 2 7 . Dessanti,
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R.N.,
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, 1 9 6 9 , G l a c i a l d r i f t i n t h e e a s t e r n A r g e n t i n e Andes b e t w e e n l a t i t u d e 41'10s. a n d l a t i t u d e 43'10's.: Geol. SOC. Amer. Bull., V. 80, p. 1043-1052.
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Iirinsley, D.H. T e x t u r es:
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, 1954, Contribucio'n a 1 conocimj-ento y l a s y s t e m 6 t i c a d e l e n g l a z a m i e n t o a c t u a l d e l a A l t a C o r d i l l e r a d e Mendoza: Rev. A s o c i a c i 6 n G e o l 6 g i c a A r g e n t i n a , T . 9 , no. 4 , p . 232-245. , 1963, E s t r a t i g r a f f a , neotectGnica, y geomorfologia d e l Pleistocene p e d e m o n t a n o e n t r e R i o s D i a m a n t e y M e n d o z a : Rev. A s o c i a ci dn G e o l 6 g i c a A r g e n t i n a , T . 1 7 , n o . 3-4, p . 127-349.
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Humboldt v. 50, p.
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1917, Geolopia de l a f a l d a o r i e n t a l de l a c o r d i l l e r a Anales d e l l l i n i s t e r i o de .Agricultura de la Nacion, T.
1 9 8 1 , La e v o l u c i o n d e g l a c i a r e s d e e s c o m b r o s y m o r r e n a s en
405
l a cuenca d e l Rfo Blanco, Nendoza, Argentino.
VIII C o n g r e s c o G e o l o ' g i c o
, M u l t i p l e g l a c i a t i o n of t h e Cordon d e l P l a t a , Mendoza, Argentina: Palaeogeography, ?alaeoclimatology, Palaeoecology i n press.
,
A P P E P DIX
C h a r a c t e r i s t i c s o i l p r o f i l e s on r o c k g l a c i e r d e b r i s and till descriptions) 1.
S t a b l e r i d g e a l o n g l e f t margin of r o c k g l a c i e r i n Q . a b o u t 6 7 5 m u p s t r e a m f r o m a c t i v e f r o n t H o l o c e n e I11 0 - 5 c m
Rock c h i p s ,
5
Loam w i t h 5 0 % r o c k f r a g m e n t s > 2 m m ,
-
2 0 cm
(field
Colorada,
base abrupt 7.5YR414,
pH 3 . 8
C o l l a p s e d r o c k g l a c i e r , Q. C o l o r a d e , H o l o c e n e 11, 5 0 m d o w n v a l l e y from a c t i v e f a c e , s u r f a c e l e v e l 0 .- 6 cm
6
- 17
-
17
cm
3 0 cm
Rock c h i p s Sand,
( 1 t o 1 0 cm d i a m ) b a s e a b r u p t
silty,
loose,
5YR3/4,
p€I 5 . 2 ,
Sand and r o c k f r a g m e n t s , l o o s e , hole stopped a t boulder
Long r o c k g l a c i e r t o n g u e i n 0 . C o l o r a d a , a c t i v e f a c e , H o l o c e n e 11, v e g e t a t i o n 5 %
base clear
7.5YR414,
200 m d o w n s t r e a m f r o m
0 - 5 c m
Rock c h i p s , b a s e a b r u p t
5 - 9 c m
Sandy loam, loose
407: r o c k f r a g m e n t s > 2 m m ,
9 -. 2 5 cm
S i l t y sand, loose
60% r o c k fragments >2 mm,
Crest of l e f t l a t e r a l m o r a i n e , vegetation 1X
0 - 5 c m 5
-
2 5 cm
Q.
7.5YR4/4, 7.5YR4/4,
pH 5 . 2 , pH 5 . 0 ,
d e 10s V a l l e c i t o s , H o l o c e n e 11,
Loam, 1 0 % r o c k f r a g m e n t s > 2 mm, base clear Sandy loam,
pH 5 . 6 ,
l O Y R 4 1 3 , pH 4 . 2 ,
5 0 % r o c k f r a g m e n t s > 2 mm,
2.5YR4/2,
pH 3 . 8
Crest of f o s s i l g l a c i e r t o n g u e t h a t o v e r r o d e a n o l d e r r o c k g l a c i e r . Vegetation cover 50%, f l a t . H o l o c e n e I. 0.
- 2
cm
2 - 8 c m
8
-
3 0 cm
Rock c h i p s ,
angular,
diam 2
-
10 cm
G r i t t y l o a m , 1 5 % c l a s t s > 2 mm, 4.0, base clear S i l t y sand,
50% c l a s t s > 2 mm,
5YR3/2,
massive,
pH
5 Y R 4 1 4 , l o o s e , pH 5 . 6
Crown o f v e r y b o u l d e r y , s m a l l , f o s s i l r o c k g l a c i e r o n l e f t v a l l e y w a l l of 0. d e 10s V a l l e c i t o s , o v e r l a p s o l d e r l e f t l a t e r a l m o r a i n e Holocene I o r and i s o v e r r i d d e n by younger l e f t l a t e r a l moraine. V e g e t a t i o n c o v e r 40 50% V a l l e c i t o s 11.
-
0 - 2 c m
Rock c h i p s
2 - 8 c m
Loamy s a n d , 2 0 % r o c k f r a g m e n t s > 2 m m , 7 . 5 Y R 3 / 2 , p l a s t i c , non s t i c k y , pF 6 . 4 , b a s e a b r u p t
8
12
-
1 2 cm
-
3 0 cm
Loam, 2 0 % c l a s t s > 2 mm, base clear Loam,
sandy,
5YR4/2,
50% c l a s t s > 2 mm,
sl. plastic, 7.5YR4/4,
non
pH 5 . 0 ,
l o o s e , pH 4 . 6
Crest of l e f t l a t e r a l m o r a i n e a b o u t 50 m u p v a l l e y from m o r a i n a l loop. V a l l e c i t o s 11. V e g e t a t i o n c o v e r 7 0 % ; f l a t : stabilized s o r t e d c i r c l e s 1 m i n diameter on s u r f a c e . 0 - 8 c m
Loam,
15% c l a s t s >2mm,
1 0 Y R 3 1 3 , sl. p l a s t i c ,
pH 3 . 8 ,
406
base clear 8
.-
-
18 8.
Loam, s a n d y , s l . c l a y e y , 3 0 % c l a s t s > 2 m m , s l . p l a s t i c , pH 3 . 8 , b a s e c l e a r
1 8 cm 3 5 cm
Sandy loam,
7 . 5 ~ R 5 / 4 , l o o s e , pH 3 . 8
L e f t l a t e r a l m o r a i n e o f l a r g e m o r a i n a l m a s s , 0. d e 1 0 s V a l l e c i t o s . A t junction with 9 . Colorada. Vegetation cover 100%. V a l l e c i t o s I1
- 1 0 cm 1 0 - 2 2 cm
Loam,
10% c l a s t s > 2 mm,
Loam,
pebbly,
4 0 cm
S i l t ,
sandy, pebbly, pH 6 . 0
0
-
22 ?.
50% c l a s t s >2 mm,
2.5Y414,
loose,
50% > 2 mm,
7.5YR3/2,
DH
7.5YR3/4,
6.4,
base clear
pH 6 . 0 ,
50% c l a s t s > 2 mm,
base clear
10YR4/4,
F l a t a r e a b e t w e e n t w o l a t e r a l m o r a i n a l r i d p e s , 0. d e l a A n g o s t u r a , V a l l e c i t o s 11. V e g e t a t i o n c o v e r l o o % , 1 0 % s l o p e t o E . 0 - 5 c m 5
-
1 2 cm
-
12
Loam,
7.5YR3/2,
pH 5 . 6 - 5 . 8 ,
Loam,
10% c l a s t s > 2 mm,
Loam, 2 5 % c l a s t s > 2 m m , pH 4 . 0
44 c m
base gradual
7.5YR312, 10YR4/3,
s l . p l a s t i c , pH 5 . 2 loose,
sl. plastic,
10. C r e s t o f e n d m o r a i n e i n o f f s e t g o r g e o f 0 . d e l a A n g o s t u r a . Vallecitos I (?). 2570 m. 0
-
11
-
26
-
11 cm
V e g e t a t i o n cover SO%, 5YR3/2,
s l o p e 5 % t o €3,
Loam,
sandy,
26 cm clear
Loam,
5YR5/3,
s l . p l a s t i c , pH 5 . 0 ,
46 c m
Loam, s a n d y , p e b b l y , 5 0 % c l a s t s > 2 m m , 1 0 Y R 4 1 4 , l o o s e , pH 5 . 8 - 6 . 0
sl. plastic,
massive,
altitude
base clear
pH 5 . 0 - 5 . 2 ,
base
some l a r g e ,
11. S e c t i o n e x p o s e d i n s t r e a m b a n k a l o n g 0. d e l a A n g o s t u r a , 1 0 0 0 m above j u n c t i o n w i t h 0 . de 10s V a l l e c i t o s , Rio Blanco till over Angostura till
0 - 7 c m
7
-
1 3 cm
Loam, 5 Y R 3 / 2 , diffuse
loose,
Loam,
l o o s e , pH 7 . 0 ,
5YR4/2,
r o o t s abundant,
pH 6 . 4 ,
base
base abrupt
13
-
3 5 cm
35
-
S i l t , 7.5YR7/2, c h a l k y , c e m e n t a t i o n weak t o s t r o n g , strongly effervescent with HC1, base clear
6.00 m
T i l l , pebbly, out
sandy,
silty.
5Y5/3,
calcareous through-
6.00-6.15
m
Loam, 1 0 Y R 4 / 4 m o t t l e d w i t h 7 . 5 Y R 4 / 4 , base clear
6.15-6.40
m
Loam,
6.40-6.83
m
Loam, 5 Y R 4 / 4 , m o t t l e d w i t h 5YR313, c l a y e y , w e a k b l o c k y s t r u c t u r e , pH 5 . 6 , b a s e c l e a r
6.83-7.58
m
Loam, 7 . 5 Y R 5 1 4 , w i t h v e i n l e t s o f 7 . 5 Y R 7 / 2 . Contains ghost cobbles and g r u s i f i e d g r a n i t i c boulders
7.58-8.20
m
8.20-11.20
7.5YR4/6,
very compact,
very compact, base c l e a r
Covered m Loam,
s i l t y , pebbly
(till),
2.5Y5/4,
pH 6 . 2
407
PAN EL DISCUSSION The f o l l o w i n g r e m a r k s w e r e a i r e d i n t h e d i s c u s s i o n s e s s i o n f o l l o w The d i s c u s s i o n p a n e l i n g t h e Symposium on S a t u r d a y e v e n i n g , May 2 3 . i n c l u d e d Dr. C . S . C h u r c h e r ( R o y a l O n t a r i o Museum), Dr. D . C o a t e s (S.U.N.Y., Binghamton, N . Y . ) , Dr. H . B . S . Cooke ( V a n c o u v e r , B.C.) and Dr. J . Terasmae ( B r o c k U n i v e r s i t y , S t . C a t h a r i n e s , O n t a r i o ) . W .J . Wayne ( U n i v e r s i t y o f N e b r a s k a ) : T h i s c o n f e r e n c e h a s b e e n v e r y i n f o r m a t i v e t o me. I t h i n k we made a good q u i c k r e v i e w o f t h e d a t i n g methods t h a t a r e b e i n g u s e d and a r e working. I l e a r n e d o f some a b s o l u t e d a t i n g t e c h n i q u e s t h a t I h a d n ' t b e e n aware o f b e f o r e . A f t e r l i s t e n i n g t o r e p o r t s on v a r i o u s m e t h o d s , and p a r t i c u l a r l y i n t h e amino a c i d t e c h n i q u e s , I s u s p e c t t h a t I may have b e e n a l i t t l e t o o w i l l i n g t o p u t f a i t h i n r a d i o c a r b o n d a t e s ; p e r h a p s f e w e r of them t h a n I t h o u g h t a r e v a l i d . Under some c i r c u m s t a n c e s , we may b e a b l e t o do as w e l l w i t h e s t i m a t e s on t i m e from r e l a t i v e d a t i n g m e t h o d s . We l o o k e d a t t h e r e l a t i v e d a t i n g methods and r e s u l t s on g l a c i a l s e d i m e n t s i n t h e Brooks Range, c e n t r a l Canada, t h e Wind R i v e r s , and A r g e n t i n a . I'm i n t e r e s t e d i n s e e i n g what might be done f u r t h e r on t e r r a c e d a t i n g . What do f l u v i a l t e r r a c e s i n L o u i s i a n a , N e b r a s k a , t h e R o c k i e s , f o r e x a m p l e , mean w i t h r e s p e c t t o t h e c o n t i n e n t a l and a l p i n e g l a c i a t i o n s on t h e N o r t h American c o n t i n e n t ? H.B.S.
C o e :
The t h i n g t h a t s t r i k e s me most a b o u t any k i n d o f c o n f e r e n c e cons i d e r i n g d a t i n g and t h e Q u a t e r n a r y i s t h e f a c t t h a t Q u a t e r n a r y s t u d i e s a r e one of t h e most i n v o l v e d i n t e r d i s c i p l i n a r y s t u d i e s t h a t can b e i m a g i n e d . I t i s a l m o s t i m p o s s i b l e t o s o l v e Q u a t e r n a r y p r o b l e m s by a n a t t a c k o f any s i n g l e c o n v e n t i o n a l d i s c i p l i n e . We n e e d t o b e d e t e c t i v e s ; w e n e e d t o i n t e r w e a v e t h e l i n e s o f e v i d e n c e we a c h i e v e from d i f f e r e n t s o u r c e s . Some y e a r s ago Kenneth Gakley of t h e B r i t i s h Museum made a very i m p o r t a n t c o n t r i b u t i o n t o t h e q u e s t i o n of d a t i n g i n t h e archaeo l o g i c a l f i e l d which, a f t e r a l l , i s p a r t of t h e Q u a t e r n a r y problem. I n t r y i n g t o r e c o g n i z e v a r i o u s l e v e l s of d a t i n g he d i s t i n c u i s h e d i n p a r t i c u l a r t h e d i f f e r e n c e between c h r o n o m e t r i c (or a b s o l u t e ) d a t i n g and r el at iv e d a t i n g . He a l s o t r i e d - 3 6 l a y down some g r o u n d r u l e s f o r t h e c r i t e r i a which r e p r e s e n t d i f f e r e n t l e v e l s of r e l i a b i l i t y o f a b s o l u t e I n t h i s conference it and r e l a t i v e d a t i n g s o as t o form a h i e r a r c h y . seems t o me t h a t we h a v e d e a l t w i t h b o t h a s p e c t s a b s o l u t e d a t i n g and r e l a t i v e dating. I n t e r m s of a b s o l u t e d a t i n g t h e r e a r e e s s e n t i a l l y o n l y t h r e e t e c h n i q u e s which Kenneth Gakley would p u t a s f i r s t o r d e r d a t i n g t e c h n i q u e s , namely t h e r a d i o m e t r i c m e t h o d s , k a d i o c a r b o n , p o t a s sium a r g o n ) . Amino a c i d d a t i n g b e l o n g s t o a l o w e r o r d e r a s i t d e p e n d s on h a v i n g a c a l i b r a t i o n d e r i v e d from a r a d i o m e t r i c s c a l e , and i t i s n o t of t h e same o r d e r of a b s o l u t e n e s s . Even t h o u g h t h e r a d i o m e t r i c d a t e s may b e wrong, t h e y a r e p h y s i c a l l y a c c u r a t e d e t e r m i n a t i o n s o f t h e sample t h a t h a s b e e n p r e s e n t e d , b u t one s t i l l h a s t o e v a l u a t e t h e meanings of the dates. I w i l l immediately g e t i n t o c o n f l i c t w i t h t h e paleomaqneti s t s by s a y i n g t h a t p a l e o r n a g n e t i s m d o e s n o t r e p r e s e n t a d i r e c t method o f a b s o l u t e d a t i n g . The r e a s o n f o r t h i s i s t h a t p a l e o r n a g n e t i s m o n l y g i v e s you a s i g n a l - i t ' s a y e s or a "0. n o r m a l or r e v e r s e d . You a r e t h e r e f o r e f i n g e r p r i n t i n g y o u r s a m p l e and have t o u s e o t h e r c r i t e r i a t o i d e n t i f y which n o r m a l o r which r e v e r s e p e r i o d you t a k e t o match UD w i t h p a r t of t h e d a t e d p a l e o m a g n e t i c t i m e s c a l e . I n t h e ocean s e d i m e n t s , paleornagnetism works v e r y w e l l b e c a u s e you c a n assume t h a t you s t a r t a t t h e t o p and work down s o t h a t you b e g i n t h e s c a l e from t h e p r e s e n t d a y . T h i s has caused o c c a s i o n a l t r o u b l e . We know t h a t Glomar C h a l l e n g e r , f o r e x a m p l e , p u b l i s h e d some r e s u l t s r e p o r t i n g t h a t t h e y h a d r e a c h e d v e r y i n t e r e s t i n g d e p o s i t s i n t h e l o w e r P l e i s t o c e n e which t u r n e d o u t t o
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b e O l i g o c e n e ! T h e r e was a h i a t u s i n b e t w e e n . So p a l e o m a g n e t i c d a t i n g i s n o t a f i r s t o r d e r m e t h o d . On t h e o t h e r h a n d , i f o n e a l l i e s p a l e o magnetic d a t i n g w i t h o t h e r methods, s o t h a t one can g e t a b a l l p a r k f i g u r e i n terms o f b i o c h r o n o l o g y or a b s o l u t e c h r o n o l o g y or s o m e t h i n g e l s e to p l u g i n a s t a r t i n g p o i n t for t h e m a g n e t i c s c a l e , t h e n t h e p a l e o m a g n e t i c m e t h o d p r o v i d e s a n e x t r e m e l y i m p o r t a n t tool. I t i s t h e o n l y t o o l t h a t we h a v e a t t h e p r e s e n t t i m e w h i c h w i l l i n t e r l i n k o c e a n i c a n d t e r r e s t r i a l e n v i r o n m e n t s o f a l l k i n d s a n d t h i s , I t h i n k , i s somet h i n g t h a t was p e r h a p s u n d e r p l a y e d i n t h e s y m p o s i u m . So I w o u l d l i k e j u s t t o r e i n f o r c e t h i s i d e a o f K e n n e t h O a k l e y l s o f d i f f e r i n g o r d e r s or r a n k s for s c a l e s o f d a t i n g a n d to t h r o w i n t h e s e t h o u g h t s a t t h i s s t a g e .
--C . Kolb ( L o u i s i a n a S t a t e U n i v e r s i t y ) : Dr. Cooke y o u h a v e j u s t s u m m a r i z e d t h r e e m a j o r f a c e t s o f a b s o l u t e d a t i n g . Would y o u do t h a t f o r r e l a t i v e d a t i n g , or d o y o u h a v e t h i s k i n d o f a summary i n y o u r mind f o r r e l a t i v e d a t i n g ?
E.B.S. Cooke: I n r e l a t i v e d a t i n g w e s t a r t w i t h t h e f a c t t h a t t h e key t o any k i n d o f i n t e r p r e t a t i o n o f g e o l o g i c a l h o r i z o n s r u s t b e b a s e d on t h e n o r m a l g e o l o g i c a l p r i n c i p l e s of s t r a t i g r a p h y a n d s u p e r p o s i t i o n . About t h e only thing that I believe absolutely i s i f i n a r e l a t i v e l y undisturbed a r e a , I s e e o n e b e d o v e r l y i n g a n o t h e r ; t h e n I b e l i e v e t h a t t h e o n e on t o p i s younger t h a n t h e one u n d e r n e a t h . R e l a t i v e d a t i n g i s o b v i o u s l y t h e f u n d a m e n t a l m e c h a n i s m by w h i c h o n e d e r i v e s a s e r i e s i n t o w h i c h o n e So w h e t h e r we a r e t a k i n g c o r e s f r o m a wishes t o plug "absolute" ages. b o g or l o o k i n g a t s u c c e s s i o n s o f g e o m o r D h o l o g i c a 1 f e a t u r e s , o n e of w h i c h c u t s a n o t h e r , we a r e d a t i n g t h e s e t h i n g s r e l a t i v e l y b y t h e n o r m a l So I t h i n k t h a t I w o u l d c a l l g e o l o g i c a l and geomorphological c r i t e r i a . s t r a t i g r a p h y a f i r s t o r d e r m e t h o d o f r e l a t i v e d a t i n g . The geomorpho l o g i c a l c r i t e r i a I t h i n k w o u l d h a v e t o b e d e g r a d e d to s e c o n d l e v e l d a t i n g b e c a u s e , w h i l e t h e r e l a t i v e a g e s a r e r i g h t , t h e f a c t t h a t we d o n o t h a v e d i r e c t s u p e r p o s i t i o n d o e s mean t h a t t h e r e a r e o f t e n p o s s i b l e a l t e r n a t i v e i n t e r p r e t a t i o n s . One h a s t o c h o o s e b e t w e e n t h e s e a l t e r n a t i v e s , s o o n e h a s to make s e c o n d o r d e r i n f e r e n c e s , r a t h e r t h a n f i r s t order conclusions. "Bed A i s u n d e r n e a t h B" i s a p o s i t i v e s t a t e m e n t . But s a y i n g t h a t a w a t e r f a l l has r e t r e a t e d as a r e s u l t o f some geomorp h o l o g i c a l o p e r a t i o n i s a s e c o n d o r d e r i n f e r e n c e a n 3 i-s n o t q u i t e o f t h e same v a l u e . I n r e l a t i v e d a t i n g w e employ f u n d a m e n t a l t h i n g s l i k e t h e " l a w o f f a u n a l s u c c e s s i o n " , where we l o c k a t s e q u e n c e s o f f o s s i l m a t e r i a l t h r o u g h known s t r a t i g r a p h i c s e q u e n c e s or c o r r e l a t i v e s t r a t i graphic sequences. Then w e u s e o u r k n o w l e d g e o f t h e d e v e l o p m e n t o f c h a n g i n g l i f e f o r m s t h r o u g h a s e q u e n c e ( a s F a r i n g t o n was d o i n g w i t h t h e Mammoths or t h e B i s o n s ) to t r y t o r e c o g n i z e f r a g m e n t s o f a c o n t i n u o u s t i m e s e r i e s a n d l i n k them b y c o r r e l a t i o n i n t o o t h e r a r e a s . Now, a s s o o n a s y o u i n v o l v e c o r r e l a t i o n you a r e d e a l i n g w i t h a t o t a l l y d i f f e r e n t p r o b l e m i n d a t i n g , a n d I t h i n k t h i s i s s o m e t h i n g t h a t was n o t r e a l l y brought o u t . T h e r e a r e a l s o mechanisms open t o t h e p a l e o b i o l o g i s t w h i c h a r e i n a way d a t i n g t e c h n i q u e s t h a t we d i d n o t d i s c u s s . T h e s e i n v o l v e t h e s t u d y of t h e development of l i f e forms t h r o u g h e s s e n t i a l l y I f one d o e s t h a t , i t i s o f t e n p o s s i b l e continuous sequences of s t r a t a . to f i n d m e t r i c a l c r i t e r i a on v a r i a t i o n i n a b i o l o g i c a l s p e c i e s t h r o u g h t h e s e q u e n c e ; f o r e x a m p l e , we c a n s e e t h e c h a n g e i n t h e number o f p l a t e s a n d t h e t h i c k n e s s o f t h e e n a m e l i n e l e p h a n t t e e t h . We c a n a c t u a l l y p u t n u m b e r s on t h e s e c h a n g e s a n d w e c a n u s e a b s o l u t e or o t h e r f o r m s o f d a t i n g to p u t t i m e i n t o t h e s e q u e n c e . We c a n t h e n p u t i n v a l u e s f o r r a t e s o f change i n t h e b i o l o g i c a l s e r i e s and t h i s e n a b l e s us t o t a k e a n d a n a l y z e t h e m s t a t i s t i c a l l y s o as to p l u g i n a d a t e w h i c h i s a l m o s t as r e l i a b l e a s some o f t h e d a t e s t h a t w e h a v e b e e n g e t t i n g f r o m other criteria. So t h i s i s a n o t h e r t o o l . I d o n ' t t h i n k I have r e a l l y answered your q u e s t i o n . C . Kolb: ~-
You h a v e i n a way. I assume t h a t p e d o l o g i c a l e v i d e n c e would p r o v i d e a breakdown i n a r e l a t i v e d a t i n g s e q u e n c e . Is t h i s c o r r e c t ?
409
H.B.S. Cookg: S u r e . What we need i n t h e s o i l s t u d i e s a r e a i s some k i n d o f a breakthrough t o g e t age values out of s o i l forming p r o c e s s e s . I think i n t h e l o n g r u n t h a t "Be may s o l v e t h i s p r o b l e m , b u t i t i s s t i l l a l o n g way off. C . Kolb: -_--
Why d i d you make a d i s t i n c t i o n b e t w e e n w h a t you C a l l ' c o r r e l a t i o n ' and ' r e l a t i v e d a t i n g ' ? To me i t may be d i f f e r e n t , b u t you end up w i t h For i n s t a n c e , i n my a r e a o f L o u i s i a n a we a r e i n t e r t h e same r e s u l t . e s t e d i n t r y i n g t o f i n d a method t o c o r r e l a t e t e r r a c e l e v e l s . And, o f c o u r s e we a r e l e a n i n g t o w a r d a p e d o l o g i c a l o r c h e m i c a l k e y , or s i g n a t u r e i f you w i l l , t h a t w i l l h o p e f u l l y p e r m i t u s t o s a y , " w e l l , w e ' v e g o t t h r e e t e r r a c e s o v e r i n t h i s v a l l e y and we have t h r e e t e r r a c e s i n a n o t h e r v a l l e y , and t h e t o p t e r r a c e o f t h i s one o v e r h e r e i s i n d e e d t h e same a g e a s a n o t h e r o n e , or i t was l a i d down a t r o u g h l y t h e same t i m e and c o r r e l a t e s w i t h i t . I' 3.B.S.
Cooke:
I t h i n k you have a n s w e r e d y o u r own q u e s t i o n i n t h a t y o u have a l r e a d y drawn a d i s t i n c t i o n b e t w e e n t h e s t r a t i g r a p h i c s u c c e s s i o n t h a t you s e e i n one p l a c e , and t h e s t r a t i g r a p h i c s u c c e s s i o n you s e e i n a n o t h e r p l a c e . Then you must b r i n g i n c o r r e l a t i o n f o r t h e p u r p o s e of i n t e r r e l a t i n g t h e s e two d i f f e r e n t s e q u e n c e s . S o c o r r e l a t i o n i s a s t e p t h a t i n t e r r e l a t e s two o r more s e t s o f s t r a t i g r a p h i c o b s e r v a t i o n . New d a t a may a l t e r t h e c o r r e l a t i o n , b u t n o t t h e i n d i v i d u a l s e q u e n c e s . I f you h a v e a t e r r a c e s e q u e n c e i n w h i c h you can a c t u a l l y w a l k o u t t h e t e r r a c e a l o n g t h e b a n k s of t h e r i v e r f o r many t e n s of k i l o m e t r e s , t h i s i s n o t c o r r e l a t i o n ; t h i s i s g e o l o g i c a l mapping and c o n s i s t s of t r a c i n g a n a c t u a l s t r a t u m c o n t i n u o u s l y . B u t , i f you have a b r e a k o f t e n m i l e s o r t e n k i l o m e t r e s , or 1 0 0 k i l o m e t r e s , t h e n you a r e making a s t r a t i g r a p h i c s e t of o b s e r v a t i o n s i n more t h a n one l o c a t i o n , and you a r e d r a w i n g upon a l l k i n d s of c r i t e r i a t h a t you c a n u s e t o show t h a t t h e r e i s a u n i q u e r e l a t i o n s h i p b e t w e e n t h e s e s e t s of d a t a . The i m p o r t a n c e of t h i s i s t h a t t h e r e a r e many t h i n g s i n g e o l o g y which p r o d u c e , i n f a c t , t h e same e f f e c t s . F o r e x a m p l e , i f you h a v e a c y c l e g o i n g from w e t t e r t o d r i e r , and b a c k t o wet i n a r i v e r s y s t e m , you w i l l p r o d u c e a c e r t a i n p a t t e r n of t e r r a c e s . I f you r e p e a t t h a t p a t t e r n i n a n o t h e r a r e a , b u t a t a n o t h e r t i m e , you w i l l g e t a r e p l i c a o f t h a t s e t o f t e r r a c e s . S o , i f you r e p e a t t h e c o n d i t i o n you w i l l g e t a r e p l i c a o f t h e geomorphic s e t o f e v i d e n c e . A c c o r d i n g l y , mere s i m i l a r i t y o f g e o l o g i c a l , g e o g r a p h i c a l or geomorphic s e t t i n g d o e s n o t p r o v i d e you w i t h t h e b a c k g r o u n d f o r making a r e l i a b l e c o r r e l a t i o n . What you have t o do i s t o s a y , "I b e l i e v e t h i s u n i t i s t h e same a s t h a t one b e c a u s e i t h a s a s o i l which i s s a n d w i c h e d between u n i t s B and C i n one s e c t i o n and a s i m i l a r s o i l i s s a n d w i c h e d between u n i t s P and Q i n my o t h e r s e c t i o n . They b o t h h a v e t h e same chemical f e a t u r e s and p o l l e n s p e c t r a " . A c t u a l l y , I h a v e p i c k e d a bad It i s very d i f example f o r w i t h p o l l e n s p e c t r a we can be i n t r o u b l e . f i c u l t t o i d e n t i f y p a r t i c u l a r i n t e r g l a c i a l s i n a b s o l u t e t e r m s . It i s p o s s i b l e t o r e c o g n i z e t h a t you have a n i n t e r g l a c i a l p o l l e n a s s e m b l a g e , b u t t o s a y w i t h a s s u r a n c e which i n t e r g l a c i a l i s n o t s o e a s y . S o , t h i s i s why I s a y t h a t w e have a v e r y complex d i s c i p l i n e i n w h i c h , i n o r d e r t o make c o r r e l a t i o n s , we n e e d t o draw upon a l l t h e t o o l s we h a v e b e e n I t i s o n l y by combining a s u b s t a n t i a l number o f t h e hearing about. t o o l s t h a t we c a n f i n a l l y a r r i v e a t good and f i r m c o r r e l a t i o n s , b e c a u s e we a r e o p e r a t i n g w i t h i n t h e framework of our r e l a t i v e t i m e s c a l e . C . Beaty ----
(Lethbridge):
One b r i e f comment: I t h i n k t h e most t e l l i n g p i e c e o f e v i d e n c e s u p p o r t i n g t h e p r o p o s i t i o n t h a t H u t t o n - P l a y f a i r t y p e g e o l o g y works i s r e p r e s e n t e d by t h e f a c t t h a t t h e g e o l o g i c column was p u t t o g e t h e r b y : ( A ) f o r m u l a t i o n and t h e n ; (B) a p p l i c a t i o n of t h e b i g r u l e s : original h o r i z o n t a l i t y s u p e r p o s i t i o n , f a u n a l s u c c e s s i o n and a s s e m b l i e s , and c r o s s c u t t i n g r e l a t i o n s h i p s , and i t h a s n ' t b e e n c h a n g e d . The r e l a t i v e p o s i t i o n o f t h e u n i t s i n t h e column has n o t b e e n a l t e r e d w i t h t h e advent
410
and a v a i l a b i l i t y of t h e s o p h i s t i c a t e d t e c h n i q u e s o f t h e s o - c a l l e d new g e o l o g y . It seems t o me t h a t t h a t , i n i t s e l f , i s a l l t h e argument you need t o d e f e n d t h e i d e a t h a t t h e r e s t i l l , even i n t h e l a t t e r p a r t o f t h e 2 0 t h century, i s a l o t t o be said f o r old-fashioned geology based upon f i e l d o b s e r v a t i o n s p r o p e r l y comprehended. H.B.S.
Cooke:
T h i s i s where i t a l l s t a r t s . But I would n o t u n d e r r a t e t h e g r e a t i m p o r t a n c e of some o f t h e new t e c h n i q u e s . For e x a m p l e , i n t h e f i e l d of' t e p h r o c h r o n o l o g y i t i s one t h i n g t o s a y we have a n a s h i n t h e s e q u e n c e , b u t i t i s a n o t h e r t h i n g t o s a y , " t h i s i s t h e same a s h as t h e one t h a t I have 1 0 0 k i l o m e t r e s a w a y " . The o n l y answer t o t h i s h a s b e e n t h e development o f v e r y s o p h i s t i c a t e d t e c h n i q u e s o f t r a c e e l e m e n t a n a l y s i s which have e n a b l e d one t o a s s i g n f i n g e r p r i n t s t o t h e s e a s h e s . R.
K l a s s z ( G e o l o g i c a l S u r v e y of C a n a d a ) :
I would l i k e t o s a y s o m e t h i n g a b o u t b a s i c s t r a t i g r a p h y , i t s i n t e r p r e t a t i o n , a n d what a s t r a t i g r a p h i c u n i t means w i t h i n a g l a c i a l l a k e b a s i n . On a f i e l d t r i p l a s t f a l l n e a r t h e s o u t h end o f G l a c i a l Lake A g a s s i z , I n o t e d t h a t g e o l o g i s t s l o o k i n g a t l a k e b a s i n s e d i m e n t may h a v e q u i t e d i f f e r e n t i n t e r p r e t a t i o n s on t h e g e n e s i s of g l a c i a l l a k e b a s i n d e p o s i t s . What t o one g e o l o g i s t i s s t o n y l a k e c l a y t o a n o t h e r i s t i l l . C e r t a i n l y t h i s d i f f e r e n c e i n i n t e r p r e t a t i o n i s g o i n g t o make q u i t e a d i f f e r e n c e i n d e c i p h e r e d g l a c i a l h i s t o r y . A t t h e n o r t h end of t h e b a s i n a t y p i c a l s u c c e s s i o n c o n s i s t s of varved c l a y beds s e v e r a l i n c h e s t h i c k s e p a r a t e d by similar t h i c k n e s s e s of s t o n y c l a y . T h i s s e q u e n c e g r a d e s downward i n t o s t o n y s a n d s i m i l a r t o sandy t i l l b e d s s e e n a t t h e s o u t h end of t h e b a s i n . I would i n t e r p r e t t h i s t y p e of sequence as e n t i r e l y l a c u s t r i n e i n o r i g i n . C.
Kolb:
Could you e x p e c t ported by streams?
t h e r m o l u m i n e s c e n c e t o work on m a t e r i a l t r a n s -
H.B.S. Cooke: I t h i n k i t l s i n c o n c e i v a b l e t h a t t h e m a t e r i a l could be c a r r i e d by a stream and d e p o s i t e d i n a t e r r a c e w i t h o u t b e i n g e x p o s e d t o s o l a r radiation. C.
Kolb:
Even t h o u g h i t may h a v e b e e n d e p o s i t e d i n s a y a band a f o o t and a half thick?
H.B.S. Cooke: I t h i n k t h a t ' s of no c o n s e q u e n c e . I t has t o g o t h r o u g h t h e p r o c e s s o f s l o p e e r o s i o n from t h e s o u r c e and t r a n s p o r t i n t o t h e stream s y s t e m and w h a t you a r e d a t i n g b y t h e r m o l u m i n e s c e n c e i s when i t c e a s e d t o b e exposed t o s u n l i g h t . So once i t i s b u r i e d you g e t t h e d a t e of b u r i a l . T h i s means t h a t you m i g h t a l s o t a k e s a m p l e s from d i f f e r e n t l e v e l s i n y o u r d e p o s i t and g e t some i d e a of r e l a t i v e a g e s . Another p o s s i b i l i t y i s paleomagnetism. I was c o n c e r n e d a b o u t i t b e c a u s e I b e l i e v e Don C o a t e s a n d I were t a l k i n g a b o u t t h i s a n d t h e p r o b l e m was t h a t t h i s m a t e r i a l i s water l a i d . C u r r e n t s a f f e c t t h e o r i e n t a t i o n of m a g n e t i c p a r t i c l e s b u t i f t h e c u r r e n t s a r e t o o s t r o n g t h e o r i e n t a t i o n of t h e l i t t l e magnets becomes d i s t u r b e d , w h e r e a s i f t h e y s e t t l e down i n t h e w a t e r column t h e m a g n e t s a l i g n t h e m s e l v e s i n t h e E a r t h ' s f i e l d . D i a g e n e s i s a l s o may d e p o s i t m a g n e t i c m a t e r i a l and you t h e n h a v e t h e d a t e of t h e f i x i n g o f t h e c h e m i c a l r e m n a n t s , which may h a v e b e e n c o n s i d e r ably later than the o r i g i n a l depositions. I t h i n k you h a v e more of a p r o b l e m t h a n you i m a g i n e . The l a s t m a j o r p o l a r i t y change was 7OO,OOO y e a r s a g o , s o t h e c h a n c e s of y o u r g e t t i n $ a r e v e r s e t o normal change a r e n o t v e r y g r e a t . What a b o u t t h e l i t t l e e x c u r s i o n s i n b e t w e e n ? Well t h e p r o b l e m t h e r e i s t h a t e x c u r s i o n s a r e p r o b a b l y a g r e a t d e a l more
411
common t h a n most p a l e o m a g n e t i s t s p r e s e n t l y a d m i t .
T h i s means t h a t t h e r e
a r e f r e q u e n t b l i p s of which o n l y a r e l a t i v e l y s m a l l number have a c t u a l l y b e e n s t u d i e d ; s o i f you d o f i n d s u c h a b l i p , you n e e d some o t h e r d a t i n g method t o know w h e r e a b o u t s you a r e w i t h i n t h e p a l e o m a g n e t i c t i m e s c a l e . C.
Kolb: Thermoluminescence?
H.B.S.
Cooke:
Yes, t h e r m o l u m i n e s c e n c e would do a s a c o n t r o l . I t m i g h t n o t p r o v i d e you w i t h a n e x a c t d a t e , b u t i t m i g h t p r o v i d e you w i t h a b a l l p a r k f i g u r e . The g r e a t hope would b e t h a t i n a c o n t i n u o u s s e q u e n c e l i k e t h a t you c o u l d s t u d y t h e d e c l i n a t i o n v a r i a t i o n and you would g e t c u r v e s which c o u l d u l t i m a t e l y b e matched w i t h o t h e r s e q u e n c e s , as h a s b e e n done i n t h e Great Lakes a r e a . But t h e s e q u e n c e s f o r comDarison a r e n o t yet available. C.
Kolb: D r i f t i n g o f t h e p o l e , i s what you mean?
H.B.S.
Cooke:
Yes, b u t w h i l e we c a n do t h i s f o r t h e l a s t 2 0 , 0 0 0 y e a r s w e c a n ' t do i t f o r 3 0 0 , 0 0 0 y e a r s ago b e c a u s e we d o n ' t have s e q u e n c e s which have And i f you d o n ' t h a v e t h e f i n g e r p r i n t been s t u d i e d i n t h e r i g h t way. t o match you c a n n o t c o n n e c t t o t h e t i m e s c a l e . We h a v e r e l a t i v e l y good f i n g e r p r i n t s from t h e n e a r b y a r e a , t h e G u l f of Mexico, o v e r t h e l a s t 2 0 - 2 5 , 0 0 0 y e a r s , b u t beyond t h a t , n o . I t ' s possible that the top level o f y o u r s e q u e n c e i s 7 O O , O O O or more y e a r s o l d and o f c o u r s e i f you f i n d a r e v e r s a l i n i t t h e n you do have s o m e t h i n g p o s i t i v e . W.J.
Wayne:
I n t h e p a s t , many o f t h e g e o l o g i s t s who worked on P l e i s t o c e n e dep o s i t s had n e v e r s e e n a g l a c i e r or a n a c t i v e p e r m a f r o s t r e g i o n , s o t h e y h a v e had t o r e l y on i m a g i n a t i o n r a t h e r t h a n o b s e r v e d e x p e r i e n c e t o t r y t o u n d e r s t a n d g l a c i e r or f r o z e n g r o u n d p r o c e s s e s and i n t e r p r e t t h e d e p o s i t s t h e y s t u d i e d . F o r t u n a t e l y , f o r many o f u s t o d a y , f u n d i n g and modern t r a n s p o r t a t i o n methods h a v e p e r m i t t e d many more g e o l o g i s t s t o v i s i t and work i n t h e e n v i r o n m e n t s t h a t e x i s t e d a r o u n d t h e i c e s h e e t s d u r i n g t h e P l e i s t o c e n e . We s t i l l h a v e p r o b l e m s t h a t a r e s e d i m e n t o l o g i c , t h o u g h ; f o r e x a m p l e , t h e d i f f e r e n c e s between i c e - l a i d t i l l a n d d e b r i s from a l a n d s l i d e or mudflow sometimes seem t o d e f y i d e n t i f i c a t i o n . I ' m t r y i n g t o f i n d c r i t e r i a t o u s e w i t h c o n f i d e n c e . F a b r i c works w i t h some t i l l s ; o t h e r sediments deposited along andbeyondthe i c e margin r e a l l y a r e g l a c i e r - g e n e r a t e d mudflows, y e t - I ' m s u r e I h a v e c a l l e d some o f them tills. C.S. C h ur cher: I ' d l i k e t o s a y a t h i n g or two a b o u t p r e s e n c e a n d a b s e n c e of f o s s i l s . You know when you a r e d e a l i n g w i t h one p a r t i c u l a r a s p e c t you t h i n k you know what y o u ' r e d o i n g a n d r e l y on o t h e r p e o p l e b e c a u s e you t h i n k t h e y know s o m e t h i n g b e t t e r t h a n you d o . But I h a v e f o u n d as a p a l e o n t o l o g i s t t h a t I w i l l p e r h a p s s a y , "I have g o t a mammoth h e r e . " ( a s Dick H a r i n g t o n s a i d e a r l i e r t o d a y ) , and t h e n someone e l s e w i l l s a y , "You a r e i n t h e I r v i n g t o n i a n , " when b e f o r e I t h o u g h t i t was t h e Blancan. But what d o e s i t r e a l l y t e l l you? It t e l l s you o n l y you h a v e a n e l e p h a n t i n t h a t p l a c e and l e v e l . It d o e s n ' t t e l l you t h a t t h e r e w e r e n ' t e l e p h a n t s e a r l i e r o n . And l a t e r on you s a y "Mammoth d i e s o u t 10,500 B P , or whenever.'' But t h a t d o e s n ' t t e l l you t h a t t h e e l e p h a n t s d i d d i e o u t t h e n , i t t e l l s you t h a t ' s when you l a s t h a d t h e l a s t e l e p h a n t , or t h e l a s t b i t o f e v i d e n c e t h a t e l e p h a n t s were e x t a n t . So t h e r e i s t h i s b i n a r y s c h i s m b e t w e e n i n f o r m a t i o n t h a t you have and i n f o r m a t i o n you d o n ' t h a v e . A l l t h e l a n d m a m m a l f a u n a s t h a t h a v e b e e n i d e n t i f i e d (to which Dick H a r i n g t o n a g a i n r e f e r r e d and Don Savage p u t t o g e t h e r
412
o r i g i n a l l y ) , a r e u s e f u l a s c o n c e p t s . But t h e y d o make f o r t r o u b l e s b e c a u s e somebody w i l l s a y , " I t i s o l d e n o u g h , i t h a s t o b e 3 a n c h a l a brean." However, i f y o u ' v e g o t t h i s c e r t a i n a n i m a l i n i t , p e r h a D s i t c a n ' t be R a n c h a l a b r e a n , a n d t h e r e f o r e i t h a s t o b e some o t h e r f a u n a l a g e . We a l s o know t h a t w e ' v e f o u n d l i v i n g f o s s i l s i n d i f f e r e n t p a r t s o f t h e w o r l d a n d o n e h a s t o w a t c h f o r t h i s s o r t o f " r i n g e r " when i t t u r n s up. A v e r y good " r i n g e r " t h a t I l i k e t o c o n s i d e r i s one t h a t i s t i m e transgressive i n a r e a l l y broad sense; it is a zebra called P Z e s i p p u s which l i v e d i n North America a t t h e e n d of t h e P l i o c e n e and i n t o t h e e a r l y P l e i s t o c e n e . P Z e s i p p u s has v a r i o u s s p e c i e s , P . s h o s h o n e n s i s i s o n e o f t h e m , f r o m t h e S h o s h o n e R i v e r i n Wyomina, w h i c h i s o b v i o u s l y a g o o d N o r t h A m e r i c a n s p e c i e s : t h e g e n u s was i s o l a t e d p a l e o n t o l o g i c a l l y f o r y e a r s . Meanwhile i n Europe Milne-Edwards d e s c r i b e d a m o d e r n A f r i c a n a n i m a l a s G r g v y ' s z e b r a (named a f t e r t h e t h e n P r e s i d e n t o f F r a n c e ) when i t f i r s t became known t o m o d e r n E u r o p e a n s , t h a t i s , a f t e r t h e Romans who d i d know i t . T h i s m o d e r n z e b r a h a s now b e e n p u t i n t o t h e s u b - g e n u s G o l i c h o h i p p u s , as h a s P Z e s i p p u s . And t a x o n o m i s t s s a y t h a t e f f e c t i v e l y w e ' v e g o t t h e same z e b r a i n t h e P l i o c e n e o f North America and i n t h e A f r i c a n F l e i s t o c e n e and l i v i n g i n t h e h o r n of A f r i c a t o d a y . Specifically d i f f e r e n t perhaps, but horses a r e very Now we known n o t h i n g a b o u t A f r i c a ' s r e l a t i o n s w i t h hard t o t e l l apart. Asia a t t h i s p o i n t when i t comes t o G r g v y ' s z e b r a . T h e r e a r e some t h i n g s a b o u t w h i c h y o u c a n s p e c u l a t e . B u t somewhere a c r o s s t h a t A s i a t i c l a n d mass, p e r h a p s i n B e r i n g i a , y o u ' r e g o i n g t o f i n d G r 6 v y ' s z e b r a s , or P Z e s i p p u s a k i n t o P . s h o s h o n e n s i s , w h a t e v e r w e may c a l l t h e m . And somewhere y o u ' r e g o i n g t o h a v e t o s a y when i t d i e d o u t a n d somewhere y o u ' r e g o i n g t o s a y when i t came i n . What a r e w e g o i n g t o Dut as d a t e s on t h i s h i s t o r y ? We're g o i n g t o h a v e t o r e l y - I ' m g o i n g t o h a v e t o r e l y - on a l l o f y o u t o t e l l me how old my a n i m a l i s . And t h e n some o f y o u w i l l come a r o u n d a n d s a y . " I ' v e a o t y o u r a n i m a l , how o l d i s i t ? ' ' And I d o n ' t s e e how we There i s then t h i s l c v e l y c i r c u l a r reasoning! a v o i d i t . We've g o t t h e same p r o b l e m w i t h mammoths. D i c k H a r i n g t o n g a v e y o u a v e r y n i c e s e q u e n c e o f mammoth t e e t h a n d B a s i l Cooke h a s j u s t r e f e r r e d t o t h e m . You c a n d o v a r i o u s t h i n g s w i t h t h e t o o t h p l a t e s . You c a n make n i c e l i t t l e s e q u e n c e s , b u t y o u ' v e g o t p a r a l l e l l i n e a g e s , or c o - e v o l u t i o n b e t w e e n l i n e a g e s i f y o u l i k e . One l o t o f N o r t h American mammoths E Z e p h a s coZumbi ( t h e C o l u m b i a n mammoth) s t a r t s o f f w i t h w i d e p l a t e s a n d , a s i t became m o r e " a d v a n c e d " , i t b e c o m e s a n a n i m a l w i t h n a r r o w e r p l a t e s a b l e t o d e a l w i t h t o u g h e r and presumably more f i b r o u s S l i g h t l y l a t e r , d u r i n g t h e Sangamon, v e g e t a t i o n , as w e l i k e t o t h i n k . E l e p h a s p r i m i g e n i u s ( t h e S i b e r i a n or w o o l l y mammoth) e n t e r e d N o r t h A m e r i c a a n d h a s n i c e l y c o m p r e s s e d t e e t h , b u t t h e e a r l i e s t N o r t h American E . p r i m i g e n i u s has t e e t h t h a t a r e l e s s c o m p r e s s e d t h a n t h e l a s t o f t h e C o l u m b i a n mammoths i n t h e P l e i s t o c e n e - H o l o c e n e b o u n d a r y . T h u s we h a v e t h e s e two g r o u p s e v o l v i n g i n p a r a l l e l b u t one l a g s b e h i n d t h e o t h e r . And I ' m s u r e t h a t i f w e c o u l d c h e c k t h e r e c o r d , (I c a n ' t p o i n t t h e f i n g e r o f s c o r n a t anybody p a r t i c u l a r l y ) w e ' r e g o i n g t o f i n d a n i m a l s i d e n t i f i e d as E Z e p h a s coZumbi ' a d v a n c e d f o r m ' w h i c h a r e a c t u a l l y E . primigenius 'primitive form'. T h u s t h e d a t i n g o f f a u n a s or d e p o s i t s by t h e r e l a t i v e c o m p r e s s i o n of t h e t e e t h i s u n s u r e t a x o n o m i c a l l y . There a r e l o t s of p i t f a l l s i n p a l a e o n t o l o g y l i k e t h i s and as a p a l a e o n t o l o I t h i n k i t was B a s i l Cooke who g i s t I n e e d y o u r h e l p t o overcome them. was s a y i n g t h a t w e r e a l l y n e e d t o t a k e c a r e o v e r our c i r c u l a r a r g u m e n t s , and I t h i n k a l o t of o u r arguments circular. I f t h e y k e e p on a s y o u m i g h t s a y , r e v o l v i n g w i t h o u t g r a t i n g . a n d w i t h o u t t o o much o f a wobble, t h e n we've p r o b a b l y g o t a c o n c e p t t h a t i s r e a s o n a b l e t o go along w i t h and one t h a t ' s a p p l i c a b l e t h a t w e can u s e as a working model.
are
R.
B a r e n d r e g t ( U n i v e r s i t y of L e t h b r i d g e ) :
I was d i s c u s s i n g t h i s p o i n t e a r l i e r w i t h D r . C h u r c h e r a n d I t h i n k i t was h e who a c t u a l l y made t h e p o i n t t h a t w e h a v e g e n e t i c d r i f t w h e r e a n i m a l s c o m i n g o u t o f a wooded a r e a o n t o t h e p l a i n s , f o r e x a m p l e , may develop s l i g h t l y d i f f e r e n t c h a r a c t e r i s t i c s . These v a r i a t i o n s a r e n o t n e c e s s a r i l y a c r i t e r i o n f o r d i f f e r e n t i a t i n g t h e two. A s animals disp e r s e , d i v e r g e n t c h a r a c t e r i s t i c s develop and t h e s e are n o t n e c e s s a r i l y I t h i n k you f i n d t h i s i n a l l p l a n t and indicative of d i f f e r e n t ages. a n i m a l kingdoms. S i m i l a r l y i n t h e c a s e of t h e p a l e o m a g n e t i c r e c o r d , we see normally and r e v e r s e l y magnetized sediments, b u t a d e t a i l e d r e c o r d
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may show a s e c u l a r v a r i a t i o n w h i c h c a n b e a v e r y g o o d c o r r e l a t i v e t o o l . T h i s s e c u l a r . v a r i a t i o n may b e t h e r e s u l t o f v e r y l o c a l i z e d p h e n o m e n a , n e v e r t h e l e s s t h e r e c o r d can be u s e f u l f o r c o r r e l a t i o n o f n e a r b y s e c tions. Maybe Dr. C h u r c h e r w o u l d l i k e t o comment f u r t h e r on t h i s i n regards t o the bison. C . S. C h u r c h e r : W i t h t h e b i s o R , y e s , t h e r e i s t h e m a t t e r o f t h e loss o f l a r g e horns. You h e a r d D i c k E a r i n g t o n s a y how l a r g e t h e e a r l y b i s o n w e r e , n o m a t t e r w h a t name y o u u s e f o r t h e m , a n d how t h e m o d e r n b i s o n h a v e s m a l l h o r n s . B u t t h e t r o u b l e i n t h i s i s t h a t t h e e a r l y s a m p l e s u s u a l l y were o b t a i n e d by p e o p l e who w e r e c o n c e r n e d w i t h o n l y p i c k i n g up t h e w e l l p r e s e r v e d m a t e r i a l . F a i r e n o u g h , i t was t h e e a s i e r t h i n g t o d o a n d t h e b e t t e r p r e s e r v e d m a t e r i a l was u s u a l l y t h e b e t t e r o s s i f i e d m a t e r i a l a n d It i s one u s u a l l y t h e b i g g e r t h e bone i s t h e b e t t e r i t i s p r e s e r v e d . o f t h e s t a n d a r d r u l e s o f thumb of p a l a e o n t o l o g y t h a t t h e l i t t l e o n e s d o n ' t g e t p r e s e r v e d a s w e l l a s t h e b i g o n e s a n d , a n y h o w , a mammoth l e a v e s more b i t s a n d p i e c e s a r o u n d t h a n d o e s a m o u s e . S o t h e b i g b i s o n h o r n c o r e s a n d t h e b i g b a c k e n d s o f t h e s k u l l were p i c k e d u p , a n d y o u ' v e g o t t h e h u g e b i s o n . A t t h e same t i m e i t s e e m s now t h e r e were s m a l l e r b i s o n , t h e r e were y o u n g e r b i s o n , or f e m a l e b i s o n , w h a t e v e r t h e b i s o n were t h a t h a d s m a l l e r h o r n s . R e a l l y t h e r e h a s b e e n s e l e c t i v i t y on t h e p a r t o f t h e p a l a e o n t o l o g i s t s who t o o k up t h e b i g g e r s p e c i m e n s o n l y , a n d t h e w h o l e v a r i a t i o n w a s n ' t known. One o f t h e l i t t l e s u s p i c i o n s I h a v e a t t h e moment, f o u n d e d o n t a l k i n g t o Dr. Y i c h a e l W i l s o n i n C a l g a r y , i s t h a t t h e e a r l i e s t b i s o n p o p u l a t i o n s a l s o had s m a l l - h o r n e d i n d i v i d u a l s . T h e s e may h a v e b e e n f e m a l e s , b u t we d o n ' t know. So w h a t h a s h a p p e n e d i s t h a t t h e b i g g e r a n i m a l s became l e s s u s e f u l i n some s p e c i f i c s e n s e , a n d t h e y w e r e n ' t r e q u i r e d t o h a v e b i g h o r n s t o b e s u c c e s s f u l m a l e s , or f o r w h a t e v e r t h e r e a s o n was. a n d t h e r e ' s a p h y s i o l o g i c a l economy i f y o u So, whatever car, b e s u c c e s s f u l a n d h a v e s m a l l e r r a t h e r t h a n b i g h o r n s . t h e r e a s o n , l e t ' s s a y t h e y came o u t o n t o t h e p l a i n s a n d c o u l d b e s e e n more e a s i l y . T h u s , i n s t e a d o f h a v i n g l a r g e h o r n s , w h i c h c o s t a l o t t o make, i t w o u l d b e e a s i e r t o b u i l d a b e a r d a n d a c a p e , or w h a t e v e r y o u c a l l t h e t h i n g s t h e North American b i s o n have and 3 u r o p e a n b i s o n d o n ' t . This i s a possible adaptation t o being a p l a i n s animal a s hair i s ins u l a t i o n , a n d h a i r i s much c h e a p e r t o p r o d u c e t h a n g r e a t b i g h o r n s . Now i t m i g h t b e s a i d t h a t t h i s c o u l d b e g e n e t i c d r i f t a t some r a t h e r low l e v e l a n d w a s n ' t i m p o r t a n t t o t h e a n i m a l e x c e p t i n b r e e d i n g d i s p l a y . However, w e ' v e s e t u p a w h o l e c h r o n o l o g y on t h e h o r n c o r e s a n d w i l l have t o undo t h i s c h r o n o l o g y . The c h r o n o l o g y o f t h e l a r g e b i s o n g o e s t o a b o u t 1 5 , 0 0 0 B P , I t h i n k , f o l l o w e d by a s u d d e n r e d u c t i o n i n s i z e , a n d by a b o u t 1 0 , 0 0 0 B P t h e r e a r e b a s i c a l l y s m a l l h o r n e d b i s o n , w h i c h i s a v e r y q u i c k e v o l u t i o n a r y c h a n g e . Yet b i s o n f i r s t a p p e a r i n t h e I l l i n o i a n of North America, and h u n d r e d s o f t h o u s a n d s o f y e a r s p a s s e d d u r i n g w h i c h t h e b i s o n d i d n ' t change t h e s i z e of t h e i r h o r n s . So t h e r e m u s t h a v e b e e n a c h a n g e i n h a b i t , a n d p r o b a b l y a c h a n g e i n h a b i t a t as w e l l , t o g o a l o n g w i t h t h e impulse t o change h o r n , - s i z e .
R-_ . Barendregt: Yes, j u s t a s t h e y p o s s i b l y c o l l e c t e d o n l y t h e b e s t a n d l a r g e s t specimens of t h e s e h o r n c o r e s s o w e o n l y b u i l d o u r m a g n e t i c c h r o n o l o g y on t h e b e s t r e c o r d s a n d w e ' v e avoided t h e p o o r e r or u n i n t e r e s t i n g r e c o r d s . N e v e r t h e l e s s t h e s e r e c o r d s may c o n t a i n i n f o r m a t i o n t h a t s h o u l d h a v e goRe i n t o t h e c o n s t r u c t i o n o f t h e d e t a i l e d p a l a e o m a g n e t i c t i m e scale
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C.S. Churcher: ----
We've u s e d o n l y t h e e a s y o n e s , I s u s p e c t . R . Barendregt :
Yes. We s t a r t o u t w i t h t h e e a s y o n e s , a n d I g u e s s t h a t ' s n a t u r a l , b u t we s h o u l d a l s o c o n t i n u e t o l o o k a t t h e c o m p l e x r e c o r d s . T h e y ' r e t e l l i n g u s more.
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C. St earns (Tufts University): I s n ' t one o f t h e i l l u s t r a t i o n s of y o u r c o l l e c t i n g b i a s t h a t i t ' s r e a l l y - I w o n ' t be a b l e t o p u t t h i s a s a number, b u t i s n ' t i t s a y t h e l a s t 25 t o 4 0 y e a r s t h a t r o d e n t s h a v e come i n t o t h e i r own? C.S. hurcher: --CYes. I w o u l d n ' t l i k e t o go a l o n g w i t h 25 t o 4 0 , w h a t e v e r i t i s , b u t r e c e n t l y , ''yes". C.
Stearns:
I _ -
I t i s n ' t b e c a u s e t h e r e were no r o d e n t s i n t h e f o s s i l r e c o r d b e f o r e , b u t t h e y were h a r d e r t o c o l l e c t . C.S.
Churcher:
I have h o r r i b l e e x p e r i e n c e s o c c a s i o n a l l y as That's very t r u e . when I r e a d a n a u t h o r i n t h e 1 9 2 0 ' s who s a i d i n e f f e c t , "I c o l l e c t e d a l l t h e s e s p e c i m e n s from Kom Ombo i n E g y p t . T h e r e w e r e many r o d e n t b o n e s , b u t I d i d n ' t b o t h e r w i t h them." If I o n l y had had t h e r o d e n t b o n e s I c o u l d t e l l more t h a n h a v i n g had a l l t h e b i g a n i m a l s ' b o n e s . The r o d e n t s a n d t h e s m a l l e r a n i m a l s a r e c o ~ s i d e r e dt o b e b e t t e r c l i m a t i c i n d i c a t o r s . Of c o u r s e , t h e y c a n ' t g e t away from t h e c l i m a t e ; t h e y ' v e g o t t o a d a p t t o l o c a l c l i m a t e . You know a mouse t h a t g o e s 1 0 0 y a r d s from home h a s gone a l o n g w a y , b u t a n e l e p h a n t can walk a l l t h e way from, l e t ' s s a y , S a s k a t o o n down t o t h e Texas Gulf C o a s t and b a c k a g a i n w i t h o u t s t r a i n i n g i t s e l f , i n a b o u t 4 months. That g i v e s i t 8 m o n t h s , 3 months up n o r t h i n t h e summer and 5 months on t h e c o a s t , w i t h no e f f o r t a t a l l . You g e t p r o b l e m s from t h i s b e c a u s e p e o p l e d o n ' t c o n s i d e r t h e p o s s i b i l i t y o f l a r g e a n i m a l s ' m i g r a t i o n s . They d o n ' t r e a l i z e t h a t i f you had d o z e n s o f m a m m a l s t h a t a r e g o i n g s o u t h , you There m i g h t h a v e E Z e p h a s p r i r n i g e n i u s coming s o u t h i n t h e w i n t e r t o o . would b e t h i s m i g r a t o r y s h i f t g o i n g on a n n u a l l y . And s o you m i g h t f i n d E. p r i r n i g e n i u s i n w i n t e r and E. colurnbi i n summer t u r n i n g up i n t h e same l o c a l d e p o s i t . Then o f c o u r s e i f you d o n ' t c o n s i d e r t h a t t h i s c o u l d h a p p e n , what a r e you g o i n g t o c a l l t h a t mixed p o p u l a t i o n . And who can s a y what t h e mixed s a m p l e t e l l s you a b o u t t h e a g e o f t h i n g s . A mixed mammoth p o p u l a t i o n has t o b e m a r k e d l y d i v e r g e n t or you g e t t h e a g e wrong on t h e b a s i s o f t h e t o o t h p l a t e s . C. I
Stearns: _
_
~
I n t a l k i n g a b o u t d a t i n g a s t r a t i g r a p h i c s u c c e s s i o n we a r e t a l k i n g a b o u t r e c o g n i z i n g e v e n t s i n g e o l o g i c h i s t o r y , a r r a n g i n g them i n t h e p r o p e r o r d e r , and t h e n s e e i n g i f w e can a t t a c h s o m e t h i n g l i k e a t i m e i n y e a r s or on somebody's c a l e n d a r . We c a n s t i l l d i s a g r e e a b o u t t h e n a t u r e o f t h e e v e n t s t h a t we a r e p l a c i n g i n o r d e r a n d r e t a i n c o m p l e t e agreement a b o u t t h e o r d e r i n which t h o s e e v e n t s o c c u r r e d . D i s a g r e e m e n t a b o u t t h e n a t u r e o f e v e n t s may be p a r t i c u l a r l y e m b a r r a s s i n g i n Q u a t e r n a r y s t u d i e s b e c a u s e we t r y t o make s o many c o r r e l a t i o n s i n r e l a t i o n t o our n o t i o n s a b o u t s e q u e n c e s o f c l i m a t i c c h a n g e . The n a t u r e o f t h e e v e n t t h e r e f o r e seems i m p o r t a n t . B u t , i n f a c t , r e l a t i v e a g e and e v e n a b s o l u t e a g e c a n be e s t a b l i s h e d independently of t h e n a t u r e of t h e event. D.Coates: -
Well t h e r e a r e t h r e e q u e s t i o n s t h a t I t h i n k emerge a t a n a f f a i r l i k e t h i s w h i c h a r e - what d i d we a c c o m p l i s h , what s h o u l d we h a v e a c c o m p l i s h e d , and where do we go from h e r e . We c a n ' t s o l v e a l l t h e Is t h e r e a n e x t s t e p or a n e x t symposium or problems of t h e u n i v e r s e . w i l l w e l o s e w h a t e v e r momentum h a s b e e n g a i n e d h e r e . Where d o e s t h i s l e a d u s i n t h e f u t u r e ? What h a v e you l e a r n e d o r what would you s a y h a s been a cc omplished. L e t m e g i v e a l i t t l e p r e a m b l e c o n c e r n i n g t h e symposium. The one I purposely kept t h i s out of item missing i s environmental relevance. my t a l k t o s e e w h e t h e r I c o u l d g i v e a p r e s e n t a t i o n t h a t d i d n o t i n v o l v e
415
e n v i r o n m e n t a l and economic a p p l i c a t i o n s of d i f f e r e n t d a t i n g methods and how s u c h m e t h o d s c a n b e u s e d i n t h e " r e a l " w o r l d . F o r e x a m p l e , I p u r p o s e l y d i d n ' t s a y a n y t h i n g i n my p a p e r a b o u t n u c l e a r s i t e s . I n s i t i n g Nuclear s u c h p l a n t s d a t i n g of Q u a t e r n a r y e v e n t s i s v e r y i m p o r t a n t . R e g u l a t o r y Commission g u i d e l i n e s r e q u i r e t h a t p l a n t s s h o u l d n o t b e c o n s t r u c t e d whel-e t h e r e i s a f a u l t t h a t h a s moved w i t h i n t h e l a s t 3 5 , 0 0 0 y e a r s , or w h e r e t h e r e h a v e b e e n m u l t i p l e o c c u r r e n c e s o f a f a u l t i n t h e l a s t 5 O O , O O G y e a r s , o r where t h e r e i s a f a u l t n e a r b y w h i c h i s o f h i g h magnitude. I ' v e been i n v o l v e d i n s e v e r a l o f t h e s e i n v e s t i g a t i o n s and we h a v e u s e d many o f t h e t y p e s o f d a t i n g t h a t h a v e b e e n m e n t i o n e d . lde've a l s o u s e d o t h e r s t h a t d i d n ' t emerge from t h e s e m e e t i n g s , s u c h as t e m p e r a t u r e d i f f e r e n t i a l s a n d how t h e s e c a n g i v e a p a t t e r n o f e v e n t s t h r o u g h d i f f e r e n t k i n d s o f f l u i d i n c l u s i o n work w h i c h i s v e r y a p p l i c a b l e t o t h e t h i n g s t h a t w e ' r e t a l k i n g a b o u t . The e n g i n e e r s , as Dr. Kolb w e l l k n o w s , when t h e y d e s i g n dams o r f l o o d w a l l s t h e y m u s t know w h e t h e r t h e y ' r e g o i n g t o d e s i g n f o r a 1OG-year f l o o d o r some o t h e r r e c u r r e n c e i n t e r v a l . The p e o p l e a t S t . H e l e n ' s a r e now w o n d e r i n g w h a t i s g o i n g t o be t h e r e c u r r e n c e i n t e r v a l o f e a r t h q u a k e s and v o l c a n i c a c t i v i t y for t h a t r e g i o ' n . S o w e h a v e a l o t t o s a y i n terms o f t h e r e a l w o r l d , i n l a n d u s e p l a n n i n g a n d on e c o n o m i c r e s o u r c e s a n d d e v e l o p m e n t . All o f t h e s e a r e i n l a r g e m e a s u r e d e p e n d e n t u p o n e i t h e r a b s o l u t e or r e l a t i v e d a t i n g i n some way. And t h i s i s p e r h a p s o n e t o p i c t h a t c o u l d b e a d d r e s s e d a t some f u t u r e m e e t i n g .
417
QUATERNARY STRATIGRAPHY OF THE COASTAL BLUFFS OF LAKE ONTARIO EAST OF OSHAWA
1. P. MARTINI, M.E. BROOKFIELD and Q. H.J. G WYN
ABSTRACT The dhore bluffs between Oshawa and Port Hope have good, albeit discontinuous, exposures of Wisconsin glacial and interstadial deposits. The stratigraphic units show strong lateral variations in thickness and lithology. Most o f them have undergone considerable l o c a l erosion. Some are found as isolated remnants. The reconstructed generalized stratigraphic sequence can be correlated on the basis o f g r o s s lithology and stratigranhic position to the classical sequence of Scarborough (to date no sufficient organic material has been found in the eastern sections to be dated). Variations that occur between the two areas relate to different environments and to the closer proximity of the eastern sequences to the Precambrian source and to a calcareous shale substratum.
To facilitate communication, tentative informal names have been given to the major till layers. Accordingly, the Port Hope till is correlated with the Sunnybrook till o f Scarborough;--the Bond Head till with the Seminary (or Meadowcliffe ( ? ) ) , and the Bowmanville with the former Lower Leaside. Local occurrences of a blocky, silty upper till can be tentatively correlated lithologically with the Halton till. Detailed analysis of the waterlaid sequences has revealed well developed interstratifications of waterlaid tills (pebbly sandstones) within other lacustrine deposits, and, most importantly, large valleys probably cut during the Plum Point interstadial and filled by crossbedded, sorted sands, and in part, by Bowmanville till. INTRODUCTION Thick interlayered deposits of till, glacio-fluvial and glaciolacustrine sediments are exposed along the bluffs of the north shore o f Lake Ontario between Toronto and Port Hope. They are eroded locally at a very fast rate ( 3 0 cm/year, Terasmae e t a l . , 1972). Considerable research has been done on the erosional processes and gross stratigraphy of the deposits (Karrow, 1967, 1974; Singer, 1974). The objectives o f this paper and field guide are to establish the stratigraphic framework f o r those materials, extending eastward the scheme Karrow (1967, 1974) established in the Scarborough area, and to reconstruct sedimentologically the events that led to the emplacement and disruption of great lakes during Wisconsin and Holocene times (Figure 1). One major drawback to this work is the absence of organic material for absolute dating from the sections east of Oshawa. Within this area, and between this area and the Scarborough bluffs. only lithostratigraphic correlations are possible. Because of the regional variability in pebbles and boulders and in the calcite/dolomite ratio, locally influenced by the bedrock, some of the correlations may not stand the test o f close scrutiny. However, the resultant stratigraphic scheme is Food enough to allow a sedimentological analysis of the various environments to be made (Figure 2).
418
Figure 1
L o c a t i o n m a p of s e c t i o n s of t h e c o a s t a l b l u f f s . T h e n u m b e r i n g s e q u e n c e of S i n g e r (1974) w a s f o l l o w e d e x t e n d i n g it e a s t w a r d to P o r t t f o g e . S e c t i o n s 121-189 h a v e not b e e n m e a s u r e d .
Of the several sedimentological facies present, some tills and varved lacustrine sequences show consistent characteristics over a wide distance and they can be utilized as key beds. Four major tills are found in the Oshawa-Port Hope area having characteristics similar to layers of the Scarborough sections and are exposed at the correct stratigraphic position to be tentatively correlated. Other tills are present but they are lenses and do not serve correlative purposes. The four best developed tills, and their correlations are as follows. The lower till [proposed name: Port Hope Till (Brookfieid e t a 2 . i ; Lower Glacial Till Unit of Singer, 1 9 N 1 is a moderately compact, dark grey, silty till with approximately 5% pebbles most of which (more than 9 0 % ) are of local carbonate origin. The distribution of these pebbles is patchy, from local virtual absence to clusters of well imbricated stones. The matrix is calcareous (less than 20%) and shows a variable calcite/dolomite ratio averaging approximately 2.9 to 3.0 (Table 1; Singer, 1974; Brookfield e t aZ., in preparation). The concentration of heavy minerals is low (0.25%). This till is best developed in the Port Hope area where it reaches a thickness of 8m. It is present at other localities, but in many other areas it lies below the present day lake In the Raby Head area Singer shows that this till and level (Figure 2). the whole Pleistocene sequence disappear either because of non deposition or most probably because of erosion, approximately 0.5 km offshore at the edge of the shelf of Lake Ontario (Singer, 1974: Figure 5). This till had been correlated by previous authors with the Sunnybrook Till of the Scarborough Bluffs. Main lithological differences between the two tills are that the Port Hope till has less clay (approximately 30%) and a higher carbonate content (Table 2 ) . The middle till (proposed name: Bond Head Till (Brookfield e t aZ.); probably Middle Glacial Unit of Singer, 1974) is a compact, dark grey,
419
s i l t y ( 4 0 t o 7 0 % ) t i l l w i t h less t h a n 5% p e b b l e s . It h a s a n a v e r a g e h i g h c a l c i t e c o n t e n t ( 3 5 % ) and a r e l a t i v e l y h i g h a v e r a g e c a l c i t e / dolomite r a t i o ( 5 . 5 ; Singer, 1 9 7 4 ) . Its s t r a t i g r a p h i c p o s i t i o n l e d S i n g e r t o c o r r e l a t e i t w i t h t h e Meadowcliffe T i l l of t h e Scarborough section. However, i t s t e x t u r e i s much more s i m i l a r t o t h a t o f t h e Seminary T i l l ( T a b l e 1 ) . I t i s a d i s t i n c t c l i f f - f o r m i n g t i l l r e a c h i n g a maximum t h i c k n e s s o f 4 m between Bond Head and B o u c h e t t e P o i n t , b u t on most o t h e r s e c t i o n s a l o n g t h e b l u f f s i t was n o t d e p o s i t e d o r was eroded o f f .
'l'ahlc
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Singer
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lor
Thick. m
1
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.
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from Brookfield
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PG
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Carbonate % Dol. Tot. Carb.
froru
Call
Dol.
Halton T i l l (Leaside T i l l ) Sandy T i l l Meadowclif f e -
1 _I Halton T i l l Upper G l a c i a l U n i t
Upper G l a c i a l U n i t
wz 1iB o n d
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I
Middle G l a c i a l Unit
1
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Lower G l a c i a l U n i t
54
35
11
58
5
18
--
19
24
7-10
45
37
18
45
12
26
-
7
6
1-2
I0
57
33
2-15
12
37
51
6
30
~
Sunnybrook
Bowmanvi l l e T i l
0-14
~
~~
4.0
~
0.9
6.5
12.5
25
5
30
6
39
6
45 34
6.5
31
4 6
43
5.2
25 41
3 5
17
1.3
______
(upper part) (lower
part)
'
7
9
1-20
12
7-4
14
1-13
42 45 39
51
27 27
46 49
32
54
62
24 14
35 32
56 51
27 17
36
7 4 7
19
6
10 21
7
34 90
-
-
-
10
7
7
28
.
5.5 2.9
421
Fhe u p p e r t i l l ( s ) ( p r o p o s e d name: B o w m a n v i l l e T i l l ( s ) ( B r o o k f i e l d e t ~ 2 . ) ; U p p e r G l a c i a l U n i t o f S i n g e r , 1974) i s a s t o n y ( u p t o 15% p e b b l e s ) s a n d y ( 4 0 - 6 0 % ) t i l l . I n most p l a c e s i t i s s p l i t i n t o a n u p p e r a n d a l o w e r s h e e t by i n t e r l a y e r e d s a n d , g r a v e l o r by a s t o n e p a v e m e n t . Although h i g h l y v a r i a b l e from l o c a t i o n t o l o c a t i o n , l i t h o l o g i c a l l y t h e two s h e e t s a r e v e r y s i m i l a r , e x c e p t f o r a l o o s e r s t r u c t u r e and p e r h a p s a g r e a t e r amount o f P r e c a m b r i a n b o u l d e r s i n t h e u p p e r u n i t . G e n e r a l l y t h e s e t i l l s a r e o x i d i z e d and a r e g r a y brown i n c o l o r . O c c a s i o n a l t h i n l e n s e s of sand and a p p a r e n t s t r a t i f i c a t i o n s occur i n t h e uppermost u n i t . The t i l l s a r e r a t h e r c a l c a r e o u s ( i n e x c e s s o f 40%; S i n g e r 1974), h a v e a h i g h c a l c i t e / d o l o m i t e r a t i o ( i n e x c e s s o f 5 ) a n d h a v e a low h e a v y m i n e r a l c o n t e n t (0.54%, T a b l e 1 ) . L o c a l l y , t h e y r e a c h a c o m b i n e d t h i c k n e s s o f up t o l5m. They h a v e a s t r o n g l y u n d u l a t i n g l o w e r a n d Gwyn ( P e r s . comm.) h a s t r a c e d t h e Bowmanu p p e r s u r f a c e ( F i g u r e s 1, 2 ) . v i l l e T i l l ( s ) n o r t h w a r d t o t h e Oak R i d g e M o r a i n e a n d w e s t w a r d t o W h i t b y . This till forms t h e c o r e of t h e n o r t h w e s t - s o u t h e a s t t r e n d i n g drumlins s o u t h o f t h e Oak R i d g e M o r a i n e ( F i g u r e 3 : Chapman a n d P u t n a m , 1966, G r a v e p o r , 1957). T h i s t i l l was c o r r e l a t e d by S i n g e r w i t h t h e L e a s i d e T i l l o f S c a r b o r o u g h ( K a r r o w , 1 9 6 7 ) . Yore r e c e n t l y , t h e L e a s i d e T i l l has b e e n s u b d i v i d e d i n t o two u n i t s ( K a r r o w , 1974; Morgan, 1979). On t h e b a s i s of i t s l i t h o l o g y and s t r a t i g r a p h i c p o s i t i o n , t h e Bowmanville T i l l ( s ) i s c o r r e l a t e d w i t h t h e ' s a n d y u n i t ' , w h i c h was t h e f o r m e r l o w e r Leaside T i l l .
Figure 3
P a l e o z o i c g e o l o g y a n d P l e i s t o c e n e d r u m l i n s of p a r t s of Southern Ontario (after C h a p m a n and P u t n a m , 1966). 1. P r e c a m b r i a n ; 2. T r e n t o n - B l a c k R i v e r ; 3. C o l l i n g w o o d ; 4 . V e a f o r d - D u n d a s , B l u e M t n . ; 5. Q u e e n s t o n ; 6. M e d i n a - C l i n t o n ; 7. L o c k p o r t - G u e l p h ; 8. S a l i n a a n d B a s s I s l a n d ; 9. B o i s B l a n c ; 1 0 . Delaware; 11. Hamilton.
The u p p e r m o s t t i l l was n o t p r e v i o u s l y r e c o g n i z e d i n t h e b l u f f s I t i s t h i n ( i - 2 m) a n d p r e s e n t o n l y i n b e t w e e n Oshawa a n d P o r t E o p e . a f e w s e c t i o n s . Xowever, i t i s s u f f i c i e n t l y d i s t i n c t t o b e c o r r e l a t e d l i t h o l o g i c a l l y w i t h t h e H a l t o n T i l l o f t h e S c a r b o r o u g h a n d Oak R i d g e
422
a r e a s (Gwyn, p e r s . comm.). I t i s a g r e y brown, c l a y e y , s i l t y , e s s e n t i a l l y p e b b l e f r e e t i l l , showing w e l l d e v e l o p e d b l o c k y s t r u c t u r e . It h a s a n a v e r a g e c a r b o n a t e c o n t e n t of a p p r o x i m a t e l y 3 0 % and a c a l c i t e / d o l o m i t e r a t i o of 6 . Where p r e s e n t , i t may e i t h e r d i r e c t l y o v e r l i e t h e Bcwmanville T i l l ( s ) or i t may c a p s a n d y u n i t s (Fii1;ure 2 ) .
MAJOR INTERSTADIAL UNITS Tour t i l l s form t h e s t r a t i g r a p h i c framework of t h e Oshawa-Port Hope b l u f f s ( F i g u r e 2 ) . They r e c o r d m a j o r a d v a n c e s of t h e g l a c i a l l o b e s d u r i n g Middle and L a t e W i s c o n s i n t i m e s . I n t e r s t a d i a l sediments a r e c h a r a c t e r i z e d by r e c u r r i n g s e q u e n c e s o f l a c u s t r i n e and g l a c i o - f l u v i a l d e p o s i t s , l o c a l l y i n t e r s t r a t i f i e d w i t h f l o w t i l l s a n d w a t e r l a i d , uns o r t e d , l e n s i n g m a s s i v e t o weakly s t r a t i f i e d sandy t i l l s b e s t d e s c r i b e d by t h e f i e l d t e r m of ' p e b b l y s a n d s ' . Three major l a c u s t r i n e events have o c c u r r e d , w i t h p o s s i b l e o t h e r s m a l l e r p o n d i n g s o c c u r r i n g a t one t i m e or a n o t h e r i n d i f f e r e n t p a r t s o f t h e s e c t i o n . The Lower L a c u s t r i n e e v e n t ( p a r t o f t h e C l a r k e u n i t o f W i l s o n , 1 9 0 5 ; Coleman, 1909; S i n g e r , 1974) s t a r t s w i t h w e l l d e v e l o p e d d i s t a l v a r v e s ( c l a y e y ) , l o c a l l y i n t e n s e l y d e f o r m e d and s l u m p e d , g r a d i n g upwards i n t o i n c r e a s i n g l y s i l t i e r and s a n d i e r r h y t h m i t e s , i n t o p r o x i m a l v a r v e s ( s a n d y ) w i t h w e l l d e f i n e d r i p p l e - d r i f t c r o s s - l a m i n a t i o n s , and f i n a l l y i n t o m a s s i v e t o s l i g h t l y r i p p l e d and c r o s s b e d d e d s a n d s ( F i g u r e s 4,5). L o c a l l y t h e s e u p p e r s a n d s a r e c u t b y deep c h a n n e l s f i l l e d w i t h g r a v e l and g r a v e l l y s a n d s . I n t h e u p p e r p a r t of t h i s s e q u e n c e , p a r t i c u l a r l y i n t h e a r e a b e t w e e n Bond Head and B o u c h e t t e P o i n t , t h e l a c u s t r i n e s e q u e n c e i s i r r e g u l a r l y i n t e r s t r a t i f i e d w i t h s u b a q u e o u s d e b r i s ( t i l l l i k e u n i t s ) and ' p e b b l y s a n d s ' ( w a t e r l a i d f l o w t i l l ) . G e n e r a l l y t h e s e t i l l s o c c u r and a r e b e s t deThe s i l t y n a t u r e of v e l o p e d i n a s s o c i a t i o n w i t h t h e Bond Head T i l l . t h e Bond Head T i l l r e f l e c t s i t s c a n n i b a l i s t i c b e h a v i o u r i n e n p l o b a t i n g t h e f i n e m a t e r i a l s of t h e u n d e r l y i n g l a c u s t r i n e s e q u e n c e . A f t e r t h e r e t r e a t o f t h e i c e r e s p o n s i b l e f o r t h e s e d i m e n t a t i o n of t h e Bond Head T i l l , a s e c o n d i m p o r t a n t l a k e s t a g e (Upper L a c u s t r i n e ) i s r e c o r d e d d i r e c t l y on t o p o f t h e t i l l , or i t o v e r l i e c - t h e sandy u n i t s o f t h e Lower L a c u s t r i n e e v e n t ( F i g u r e 2 , 4 , 5 ) . T h i s s e c o n d m a j o r l a c u s t r i n e u n i t shows more p r o x i m a l and s h a l l o w e r c h a r a c t e r i s t i c s a s t h e s i l t i e r component of t h e v a r v e d c o u p l e t s p r e v a i l s . I r r e g u l a r s t r a t i f i c a t i o n s of w e l l s o r t e d s a n d s r e c o r d subaqueous c h a n n e l s very w i d e (5m) and s h a l l o w ( 1 0 t o 3 0 e m ) , p o s s i b l y s c o u r e d and f i l l e d by t u r b i d i t e events. S i m i l a r l y t o t h e Lower L a c u s t r i n e d e p o s i t s , t h i s Upper L a c u s t r i n e u n i t shows a w e l l d e f i n e d r e g r e s s i v e s e q u e n c e , t h e uppermost p a r t of w h i c h i s n o t p r e s e n t , h o w e v e r , b e c a u s e i t i s t r u n c a t e d by a major unconformity. The a b s o l u t e t i m e of t h i s u n c o n f o r m i t y c a n n o t be e s t a b l i s h e d t h r o u g h d i r e c t e v i d e n c e from t h e b l u f f s . Circumstantial considerations would s u g g e s t a m a j o r l o w e r i n g o f t h e l a k e l e v e l d u r i n g Plum P o i n t t i m e s ( D r e i m a n i s and Karrow, 1 9 7 2 ) . The u n c o n f o r m i t y i s c h a r a c t e r i z e d by d e e p v a l l e y s c u t i n t o t h e l a c u s t r i n e s e q u e n c e o f t h e post-Bond Head t i l l t i m e s , l o c a l l y t h r o u g h o u t t h e P l e i s t o c e n e s e q u e n c e down t o t h e p r e s e n t day lake l e v e l (Figure 2 ) . These v a l l e y s w e r e formed a t a p a r t i c u l a r l y low s t a g e ( l o w e r t h a n t h e modern Lake O n t a r i o ) o f t h e a n c i e n t g l a c i a l l a k e s , when t h e o p e n i n g of some s o u t h w a r d f l o w i n g o u t l e t s o c c u r r e d d u r i n g t h e Plum P o i n t i n t e r s t a d i a l (Mijrner, 1 9 7 1 ) . The v a l l e y s were s u b s e q u e n t l y f i l l e d by w e l l s o r t e d , c r o s s b e d d e d n e a r s h o r e s a n d s l o c a l l y r e w o r k e d i n t o w e l l d e f i n e d c h a n n e l f i l l s . As t h e g l a c i e r s c o v e r e d t h i s a r e a , p a r t s of t h e d e p o s i t s w e r e e r o d e d t o form t h e s a n d y o v e r l y i n g t i l l s , a n d some v a l l e y s were f i l l e d w i t h t h e Bowmanville T i l l . The i r r e g u l a r s u r f a c e s o f t h e Bowmanville T i l l ( s ) h a v e b e e n r e c o g n i z e d by p r e v i o u s a u t h o r s , however t h e i m p o r t a n c e o f t h e p r e c e d i n g u n c o n f o r m i t y was n o t , n o r w e r e t h e e x i s t e n c e o f v a l l e y s f i l l e d w i t h s a n d , n o t e d . I n d e e d , t h e s e s a n d s have been p r e v i o u s l y c o n s i d e r e d as p a r t o f t h e C l a r k e U n i t ( F i g u r e 2 ) . The s l o p e s of t h e v a l l e y s i d e s and t h e l i t h o l o g y and s t r u c t u r a l c h a r a c t e r i s t i c s o f t h e s a n d s o f t h e v a l l e y s and t h e i r paleocurrent d i r e c t i o n s , v a r i a b l e but predominantly t o t h e west, d i f f e r e n t i a t e them from t h e m a s s i v e and r i p p l e d s a n d s of t h e Lower and Upper L a c u s t r i n e s e q u e n c e s . Whereas t h e l a c u s t r i n e u n i t s t o g e t h e r w i t h
423
0Sand 0Silt
B Clay D Pebbles Cross bedding B Ripples E3 Deformed beds Ea Ripple . drift rn Plant roots B Covered Partially covered
ca
\
Paleocurrent direction (north= top of page)
190
Kl
SOIL
-* - VARVES
SOIL SOIL
UPPER TILL IPC boulders]
UPPER TILL [P€ boulders] RHY THMITES MIDDLE TILL
t
1 CLAY PEBBLES
*
VARVES
1lrn
LOWER TILL - LAKE LEVEL--
Figure 4
Detailed stratigraphic B r i t a i n area.
s e c t i o n s from t h e P o r t
t h e Bond Head T i l l a n d t h e s a n d y w a t e r l a i d t i l l i n t e r l a y e r s may b e c o r r e l a t e d t o t h e complex T h o r n c l i f f e Formation o f S c a r b o r o u g h s u b d i v i d e d l o c a l l y b y t h e S e m i n a r y a n d M e a d o w c l i f f e ( M i d d l e Complex o f C o l e m a n , 1932) t i l l s , t h e sandy v a l l e y f i l l s and t h e l a r g e unconformity have n o t b e e n r e p o r t e d i n t h e S c a r b o r o u g h a r e a ( M E r n e r , 1 9 7 1 : K a r r o w , 1 9 7 4 ) . If t h e y e x i s t t h e r e a t a l l , t h e y s h o u l d be f o u n d i n t h e Upper T h o r n c l i f f e s a n d y u n i t s where i t may b e d i f f i c u l t t o r e c o g n i z e t h e v a l l e y f i l l s from t h e o t h e r r e g u l a r d e l t a i c s a n d s . The l a s t l a c u s t r i n e e v e n t s r e c o r d e d i n t h e e a s t e r n b l u f f s o f t h e n o r t h s h o r e of Lake O n t a r i o h a v e d e v e l o p e d o v e r t h e Bowmanville T i l l , or where p r e s e n t , o v e r t h e H a l t o n T i l l . They r e l a t e t o t h e d e p o s i t s S u b s e q u e n t c h a n g e s t h a t h a v e o c c u r r e d to o f t h e e a r l y Lake I r o q u o i s . t h e d i s t r i b u t i o n o f s e d i m e n t s a n d t o t h e morphology of t h e area a r e due t o t h e l o w e r i n g o f t h e l a k e l e v e l t o L a k e O n t a r i o (Chapman a n d P u t n a m , 1 9 6 6 ; K a r r o w , 1 9 6 7 ) . The Lake I r o q u o i s d e p o s i t s a r e d i s c o n t i n u o u s i n
424
rn?:I1
62
0Sand 0Sllt
Clav E 3 Pebbles m Cross bedding R Ripples Deformed beds E B Ripple . drift Plant roots LXI Covered Partially covered
\
Paleocurrent direction (north= top of page]
K
l UPPER
r + TILL LENSE
UPPER TILLS
MIDDLE TILL VARVES
w
TILL
VARVES TILL VARVES LAKE LEVEL Figure 5
Detailed stratigraphic sections from the Bouchette Pt. - Bond Head area.
d i s t r i b u t i o n a s t h e y have b e e n i n t e n s e l y e r o d e d d u r i n g t h e l o w e r i n g of t h e l a k e l e v e l . L o c a l l y i n small v a l l e y s , good d i a m i c t i c d i s t a l v a r v e s a r e preserved. These g r a d e l a t e r a l l y and p a r t i c u l a r l y landward i n t o s a n d s o f n e a r s h o r e b a r and b e a c h d e p o s i t s . D u r i n g l o w e r i n g o f t h e l a k e l e v e l , v a l l e y s h a v e f o r m e d , and l o c a l f l u v i a l s a n d s and g r a v e l s were deposited. A t t h e f o o t of t h e b l u f f s , t h e p r e s e n t day b e a c h i s c h a r a c t e r i z e d p r i m a r i l y by t h i n , n a r r o w p e b b l y d e p o s i t s . SUMMARY AND CONCLUSIONS The s e r i e s o f e v e n t s r e c o r d e d i n t h e b l u f f s between Oshawa and P o r t Hope i s a s f o l l o w s :
-
Advance o f a g l a c i a l l o b e w h i c h c a n n i b a l i z e s p r e - e x i s t e n t f i n e l a c u s t r i n e d e p o s i t s a n d forms t h e s i l t y c l a y , b a s a l Port Hope Till. L o c a l l y reworked b l o c k s of v a r v e s a r e r e c o g n i z a b l e i n t h e upper
425
p a r t of t h i s t i l l .
- R e t r e a t of t h e g l a c i e r a n d f o r m a t i o n o f a r e l a t i v e l y d e e p l a k e where d i s t a l v a r v e s c o u l d f o r m a n d ; o c a l l y s l u m p o f f t h e s u b m e r g e d t o e of t h e g l a c i e r or l o c a l t i l l h i g h s . The v a r v e s b e c o n e i n c r e a s i n g l y r e g u l a r upward a n d , as t h e g l a c i e r r e t r e a t s f a r t h e r e a s t w a r d , a d r o p i n l a k e l e v e l l e d t o t h e f o r m a t i o n of s i l t i e r a n d s a n d i e r p r o x i m a l v a r v e s of s h a l l o w l a c u s t r i n e d e p o s i t s . L o c a l l y , s u b a e r i a l b r a i d e d s t r e a m c o n d i t i o n s were e s t a b l i s h e d . -
A r a p i d r e a d v a n c e o f t h e g l a c i e r l o b e l e d t o t h e f o r m a t i o n of t i l l s f l o w i n g o f f t h e f r o n t of t h e g l a c i e r i n t o t h e g l a c i a l l a k e , and i n t e r s t r a t i f i c a t i o n o c c u r r e d between t h e w a t e r l a i d t i l l s and v a r v e d s i l t s a r d s a n d y u n i t s , some of w h i c h were p o s s i b l y forrned as s u b a q u e o u s o u t w a s h .
- T h i s g l a c i a l a d v a n c e c u l m i n a t e d i n t h e f o r m a t i o n of t h e c l a y e y
s i l t y 3 o n d Head T i l l w h i c h h a d a d i s c o n t i n u o u s d i s t r i b u t i o n , m o s t l y b e c a u s e i t has b e e n s u b s e q u e n t l y e r o d e d , b u t a l s o b e c a u s e o f i r r e g u l a r d e p o s i t i o n a l t h i c k n e s s . I n p l a c e s , t h e Lower L a c u s t r i n e u n i t i s s e p a r a t e d f r o m a n X p p e r L a c u s t r i n e e v e n t by a few cm (5-10 em) of t i l l r e p r e s e n t i n g a l l t h a t was d e p o s i t e d a t t h a t l o c a l i t y o f t h e Bond Head T i l l .
-
A f t e r t h e d e p o s i t i o n of’ t h e 3 o n d Head T i l l a m a j o r r e t r e a t of T h i s l e d t o t h e r e g r e s s i v e s e q u e n c e of t h e U p p e r L a c u s t r i n e u n i t w h o s e b a s a l l a y e r s a r e made up o f w e l l d e v e l o p e d b u t s i l t i e r v a r v e s t h a n t h o s e o f t h e Lower L a c u s t r i n e , c a p p e d by r i p p l e d s a n d s . the g l a c i e r occurred.
- 3 u r i n g l o w e r s t a g e s o f t h e l a k e , p o s s i b l y d u r i n g ?lum F o i n t
t i m e s , d e e p v a l l e y s were e n t r e n c h e d a n d p a r t i a l l y f i l l e d by w e l l s o r t e d crossbedded sands.
l o b e i n t h i s a r e a l e d t o r e w o r k i n g of some of t h e s a n d s of t h e l a c u s t r i n e a n d s u b a e r i a l e v e n t s . The s a n d y t i l l s o f t h e S o w m a n v i l l e T i l l ( s ) were f o r m e d u n d e r f l u c t u a t i n g l o b e c o n d i t i o n s . The B o w m a n v i l l e T i l l i s s e p a r a t e d i n t o two u n i t s by l o c a l l y w e l l d e f i n e d s h a l l o w p o n d s f i l l e d by s a n d . F i n a l l y the Halton T i l l , possibly r e f l e c t i n g a f u r t h e r f l u c t u a t i o n o f t h e g l a c i e r or s i m p l y a l o c a l t i l l f a c i e s , was d e p o s i t e d i n some a r e a s .
- A readvance of t h e g ; a c i a l
- After t h i s event the glacier retreated, not t o return.
The Lake I r o q u o i s s e q u e n c e , c h a r a c t e r i z e & by l o w e r d i s t a l v a r v e s c a p p e d by more p r o x i m a l n e a r s h o r e s a n d s , was f o r m e d . L a t e r l o w e r i n g o f t h e l a k e l e v e l b e l o w m o d e r n Lake O n t a r i o r e s u l t e d i n t h e e r o s i o n o f p a r t of t h e L a k e I r o q u o i s s e q u e n c e . t h e s c o u r i n g o f v a l l e y s ar,d p o s s i b l y t h e l o c a l f o r m a t i o n of b r a i d e d s a n d y a n d g r a v e l l y s a n d d e p o s i t s . I s o s t a t i c u p l i f t o f t h e e a s t e r n o u t l e t r a i s e d Lake Ontario again t o i t s present level.
- Present a c t i v i t i e s along the lake c o n s i s t of very a c t i v e e r o s i o n
o f t h e b l u f f s , r e w o r k i n g o f t h e f i n e r m a t e r i a l s w e s t w a r d , ai?d f o r m a t i o n of t h i n , narrow, g r a v e l l y beaches a t t h e b a s e of t h e bluffs. ROAD LOG
Due t o t h e i n t e n s e s p r i n g s l u m p i n g o f t h e b l u f f s w h i c h d r a s t i c a l l y m o d i f i e s t h e e x p o s u r e s , a n d b e c a u s e of t h e r e d u c e d a c c e s s i b i l i t y t o t h e o u t c r o p s due t o mudflows and water l e v e l o f Lake O n t a r i o , no s p e c i f i c I n s t e a d , t h e m a j o r f e a t u r e s t o b e obr stop description i s given here. s e r v e d and d i s c u s s e d a t s e l e c t e d l o c a t i o n s a r e i n d i c a t e d .
1.
P o r t B r i t a i n ( S e c t i o n s 190-191; F i g . 4)
-
Lower t i l l c a p p e d by d e f o r m e d v a r v e s . cumbent f o l d s .
Note o r i e n t a t i o n of re-
426
- Deformed varves grading upward into regular varves, then into sandy rhythmites, and into massive to slightly crossbedded sands. - Transition between the Lower Lacustrine Sequence and the Upper Lacustrine Sequence marked by thin clay pebbly layers possibly correlated to the Bond Head Till.
- Upper Lacustrine sequence characterized by silt-rich rhythmites, and turbiditic events. - Upper Till (Bowmanville Till) cutting through the lacustrine sequences.
- To the east of the sections visited a valley (Plum Point ( ? ) ) was partially filled by sandy gravels, later covered by Bowmanville Till. - To the west of section 191 note the undulated lower part of the Bowmanville Till. Are some of the sandy gravels found in cut and fills (Section 190) of Lower Lacustrine age, or are they the precursors of the Bowmanville Till, thus part of the Plum Point ( ? ) valley fill? 2.
Bouchette Point East (Sections 80-81)
-
Thick section of Bowmanville Till, locally subdivided into two units and showing some pseudo-stratifications in the upper unit.
- To the west of this area is a gradational rise of the section and a development of the Bond Head Till.
3.
Bouchette Point West (Sections 74-72; Figure 5)
-
Lower till (Port Hope Till) overlain by regular distal varves, some of which are locally folded.
- Vertical transition of distal varves to more Droximal rhythmites with well developed riDple-drift cross-laminations. - Complex interrelations between "pebbly sands" (waterlaid tills) and sandy gravels of cut and fills. What is the age and significance of these cut and fills? Possible subaqueous outwash? - Good exposures of Bond Head Till. - Development of upper lacustrine sequence over the Bond Head Till.
-
Vertical gradation of the upper lacustrine sequence from diadactic varves t o silty rhythmites, to thick, apparently massive, sandy beds.
4.
Is it Bowmanville Till, Halton Till or both?
Bond Head East (Sections 58-60; Figure 5)
5.
Upper till:
Sandy fill of a ?lum Point ( ? ) valley.
Port Darlington East (Sections 3 9 - b 2 )
- Valley between two till layers filled with sandy, proximal varves showing well developed ripple-drift cross-laminations. The sand is of uncertain age. Singer (1974) interprets it to be a sandy unit separating the lower from the upper till unit of the Bowmanville Till. Brookfield e t al. offer an alternative solution considering the till exposed at lake level at section 42 to be the Port Hope Till, thus interpreting the section as a complex one where the Port Hope Till is separated from the Bowmanville Till by a remnant of the Lower Lacustrine Unit to the west of section 42
427
a n d s a n d o f a Plum P o i n t ( ? ) v a l l e y f i l l to t h e e a s t .
6.
Raby Head E a s t ( S e c t i o n s 27-30]"
- Large v a l l e y f i l l between t h e upper and lower u n i t s of t h e Bowmanville T i l l . The v a l l e y i s f i l l e d w i t h r i p p l e d a n d c r o s s bedded w e l l s o r t e d s a n d s . REFERENCES C I T E D Brookfield, M.E., Gwyn, Q . H . J . a n d M a r t i n i , a l o n g t h e n o r t h s h o r e of Lake O n t a r i o : Preparation).
I.P., Quaternary sequences Oshawa-Port Hope. (In
Chapman, L . J . a n d P u t n a m , D . F . , 1 9 6 6 , T h e P h y s i o g r a p h y o f -~ Ontario: Univ. of T o r o n t o P r e s s , 386 p . Coleman, A . P . , 1 9 0 9 , C l a s s i f i c a t i o n and n o m e n c l a t u r e of O n t a r i o Bureau of E l i s , v . 1 8 , p . 294-297.
Southern
Ontario d r i f t :
-, 1 9 3 2 , T h e P l e i s t o c e n e o f t h e T o r o n t o r e g i o n ; b y Map 4 1 8 : C n t a r i o Dept. Mines, v . 41.
accompanied
Dreimanis, A. and Karrow, P . F . , 1972, G l a c i a l h i s t o r y i n t h e Great Lakes-St. Lawrence Region, t h e c l a . s s i f i c a t i o n o f t h e W i s c o n s i n ( a n ) S t a g e , and i t s c o r r e l a t i v e s : 24th I n t e r n . Geol. Congress, Section 1 2 , p. 8. Gravenor, C.P., 1957, S u r f i c i a l geology of t h e Lindsay-Peterborough area. O n t a r i o , V i c t o r i a , P e t e r b o r o u g h , Durham a n d N o r t h u m b e r l a n d counties, Ontario: seal. S u r v . C a n a d a , M e m . 2 8 8 , O t t a w a . Karrow, P . F . , 1 9 6 7 , P l e i s t o c e n e g e o l o g y of t h e S c a r b o r o u g h a r e a : O n t a r i o Dept. of Mines, Geol. Report 46.
__ ,
Geol.
1974, T i l l s t r a t i g r a p h y i n p a r t s of S O C . A m e r . B u l l . , v . 85, p . 7 6 1 - 7 6 8 .
southwestern Ontario:
Karrow, P.F. and Morgan, A . V . , 1975, Ouaternary s t r a t i g r a p h y of t h e G e o l . A s s o c . C a n . , K i n . A s s o c . Can,, N.C. Geol. Toronto area: SOC. Am., F i e l d T r i p s Guidebook. Univ. of W a t e r l o o , p . 161-179. L i b e r t y , B . A . , 1969, P a l e o z o i c Geology of t h e Lake Simcoe area, O n t a r i o : Geol. S u r v . Canada, Mem. 355, Dept. of Energy, Mines and Resources, Ottawa. Morgan, A . V . , 1 9 7 9 , A f i e l d g u i d e t o t h e Don V a l l e y B r i c k - P i t and t h e ed., S c a r b o r o u g h B l u f f s , T o r o n t o , O n t a r i o i n Mahaney, W . C . , O u a t e r n a r y C l i m a t i c Change Symposium, A b s t r a c t s w i t h Program and F i e l d G u i d e , York U n i v e r s i t y , T o r o n t o , p. 77-99.
-
Morner, N.A., 1 9 7 1 , The Plum P o i n t i n t e r s t a d i a l : Age, climate and subdivision: Canadian J o u r . of E a r t h S c i e n c e s , v . 8 , p. 1423-1431. Singer, S.N., 1974, A hydrogeological study along t h e north shore of Ontario Ministry L a k e O n t a r i o i n t h e B o w m a n v i l l e - N e w c a s t l e A;ea: o f t h e E n v i r o n m e n t , Water R e s o u r c e s R e p o r t 5 d , 7 2 p . ---
T e r a s m a e , J., K a r r o w , P . F . a n d D r e i m a n i s , A . , 1 9 7 2 , Q u a t e r n a r y s t r a t i g r a p h y and geomorphology of t h e e a s t e r n Great Lakes r e g i o n of S o u t h e r n O n t a r i o , Guidebook f o r E x c u r s i o n A 42: TXIV Intern. Geol. Cong., M o n t r e a l , Quebec. 1905, A f o r t y - m i l e s e c t i o n of P l e i s t o c e n e d e p o s i t s Wilson, A.W.G., n o r t h of Lake O n t a r i o : Canadian I n s t . Trans., Toronto, v. 8, p. 11-21. *Subject
t o p e r m i s s i o n from O n t a r i o Hydro t o e n t e r t h e l o c a l i t y .
429
INDEX i nxn- 2 6
A. 1
~1.;
105:~e-129
d a t i r g , 28
da:ing,
. h i r . o a c i k d a t i n g , 171-193, 195-2C9 a n a l y t i c a l ne',hods J / L rz',55sy
195
7
kine',ic
175-175
modeLling,
153-187 r z c e n i z a t i o n , 178-179 s t r a t i g r a p h i c e v a l u a l i o n , 179-181
A.r - L C / Ar- 3 9 me ti?o 5 e l l m i r a t i o n sf we? c h e m i s t r y , 70 d a t i n g voLcanic r o c k s , 7 3 disadvactages, 7 1 irterference reactions,
69
s t e p 'r.eatir.g v a r i a n t , 7 0 Be r y 1li uri- 10
29
ii-Ar d a t i n g
basic p r i n c i p l e , 5 7 - 5 3 Lake Agassiz B a s i n ,
375-383
g l a c i a i l a k e s e d i r e n t s , 383 seDble r o s F d n e s s , 383-384 2adiocarbon chrocology 376-382 r a t e of i c e r e c e s s i o n ,
383
r e g i o n a l i c e f i o w , 334 ;andforrns, a s a n e a s u r e o f time, 247-267
cross c u t t i n g r e l a t i o n s h i p s
258
,
d a t i n g of endogenic p r o c e s s e s ,
263-264
ciatirg, 27 B i s h o p ash b e d ,
l a n d s c a p e change f a c t o r , 2 5 1
95
q u a n t i t a t i v e m o r p h o l o g y , 254
Sluefish 3asin, 202 B o n n e t Plume B a s i n , 2 0 3
r e l a t i v e d a t i 3 g of f l u v i a l landforms,
Cesium-i 37
sluiceways, 262
neasurerrients
, 78
theory, 250
L'ast i n t e r g l a c i a l s e a l e v e l , 57-
C h l o r i n e - 36
58
d a t i n g , 27-28 C o a s t a l B l u f f s o f Lake O n t a r i o 4 1 7 -
427
Bond Head T i l l ,
418-420
Bowmanville T i l l , P o r t Hope T i l l , Dendrochronology,
420
418 211-225
background, 211-212 crossdating, 212-213
L a t e Cenozoic p o l a r i t y t i m e s c a l e ,
106
Lead-210
d a t i n g , 75-84
78
profiles,
L i b b y , 1, 2 , 1 7
227-246
Lichenometry,
environmental considerations,
232-233
growth c u r v e , 235-238
d a t e d d e p o s i t s , 217-220
i n t e r s p e c i f i c r a t i o s , 233-234
d e n s i t o m e t r y , 213-215
measurement
s t a n d a r d i z i n g d a t a , 215-217
problems
Eutyrrhenian,
61
F i s s i o n t r a c k d a t i n g , 87-100 a d v a n t a g e s , 90-94 l i m i t a t i o n s , 90-94 theory,
87-90
G e o m a g n e t i c dynamo theory
,
102-104
p a l e o m a g n e t i c p o l e , 107 Geomagnetic f i e l d v a r i a t i o n s , 1 2 4
purpose
,
,
,
231-232
241-242
227-228
s p e c i e s , 230-231 M a c r o f e a t u r e s , for c o r r e l a t i n g t i l l s , 311-322 c o l o r , 317-318 columnar s t r u c t u r e , 320 c o m p a c t i o n , 316-317 c o m p o s i t i o n , 315 f r a c t u r e p a t t e r n , 317
430
M u l t i p l e d a t i n g methods,
355-374
i r o n o x i d e , 368-369 Late P i n e d a l e - I n d i a n B a s i n s e q u e n c e s , 368 l i c h e n o m e t r y , 3 5 8 , 362 p a r t i c l e s i z e c u r v e s , 363, 367-368 q u a r t z / f e l d s p a r r a t i o s , 369, 371 r a d i o c a r b o n , 371-372 s o i l stratigraphy weathering c h a r a c t e r i s t i c s , 357-358
Obsidian hydration d a t i n g , 141-
151
chemical composition, 147-149
141
h y d r a t i o n m e a s u r e m e n t , 1112-
143
O l d Crow B a s i n ,
200-201
Paleomagnetic c h a r a c t e r i z a t i o n , 101-102 d a t i n g , 124-128 p r o b l e m s , 112-117 remanence, 104-108
s t o n e l i n e s , 332-335 s u r f i c i a l m a t e r i a l s , 329-330 P o l l e n , 283-297 d a t i n g beyond t h e r a n g e o f 14C, 289-290 n u m e r i c a l c o n s t r a i n t s , 286-289 p l u v i a l l a k e s e q u e n c e s , 43 pollen behavior,
285-286
z o n a t i o n a n d d a t i n g , 284-293 Radiocarbon, 2 , 1 7 c o r r e c t i o n of r a d i o c a r b o n d a t e s ,
7
cosmic r a y f l u x , 3, 1 7 d a t i n g , 21, 25 dendrochronology, 4 earthquake d a t i n g , 26 fluctuations, 3 i s o b a r s e l e c t i o n , 20 i s o t o p i c e n r i c h m e n t , 11 i s o t o p i c f r a c t i o n a t i o n , 3 , 6-7 model, 2 n u c l e a r weapons t e s t i n g , 6
s a m p l i n g p r o c e d u r e s , 128-132
p a r t i c l e a c c e l e r a t o r , 17
s t a b i l i t y of magnetization, 107 time r e s o l u t i o n , 134
reference standards, 5
Paleomagnetic p o l a r i t y excurs i o n s , 115, 1 2 7 Paleosols, relative dating of, 269-281 average pedogenic i n d e x , 275276 d a t i n g s o i l p r o p e r t i e s , 274-
275
d a t i n g s u r f a c e s , 273-274 paleopedogenic index, 277 s o i l f o r m i n g f a c t o r s , 270 v a r i a b l e pedogenic index, 276-277 Pedostratigraphy r e l a t i v e a g e s e q u e n c e s , 325 s k e l e t o n g r a i n morphology,
3 41-3 44
s o i l f a b r i c , 337
s o i l f a c i e s , 339-341
,
b e t a r e a d i a t i o n c o u n t i n g , 11
N o r t h e r n Yukon, 196
f o r m a t i o n and h y d r a t i o n ,
s t a g e o f s o i l development 344-345
production, 3 s a m p l e s i z e , 10 stable isotopes, 5
Rfo Blanco Basin g l a c i a l g e o l o g y , 392-393 l o e s s t h i c k n e s s , 395 m o r p h o s t r a t i g r a p h y , 393-394 quartz grain surface textures, 397-39 8 r e l a t i v e d a t i n g methods , 393 s u b s u r f a c e w e a t h e r i n g , 397 s u r f a c e w e a t h e r i n g , 396
t i l l s u s . mudflows
T a t a R i v e r t e r r a c e s , 42 T a y l o r Dry V a l l e y , A n t a r c t i c a , Tephrochronology, 94 T e t o n R a n g e , 356 T h e r m o l u m i n e s c e n c e , 3 4 , 153-170 a g e e q u a t i o n , 156-157
43
431
age l i m i t s , 159 equivalent dose, 160-162 r e l e a s e of e n e r g y ,
155-156 e n e r g y , 155
storage of ravertine ages, 4 1
- s e r i e s d a t i n g of v o l c a n i c rocks, 47 r m i u m i s o t o p e s , 34
d e c a y chains, for u r a n i u m
i s o t o p e s , 36
s p e l e o t h e m , 3 3 , 38-40 uranium t r e n d a n a l y s i s , 46 V e r t e b r a t e chronology, 299-309 mammoths,
301-303
b i s o n , 304-305
a c t i v i t y r a t i o s , 37
Wind i i i v e r R a n g e , 356-357
closed system model, 54
Yoldia c l a y s , 112
