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Lecture Notes in Earth Sciences Editors: S. Bhattacharji, Brooklyn G. M. Friedman, Brooklyn and Troy H. J. Neugebauer, Bonn A. Seilacher, Tuebingen
49
Radiograph of Preboreat laminated sediments from Lake Holzmaar, Eifel, Germany (scale bar = 1 cm).
Jdrg E W. Negendank Bernd Zolitschka (Eds.)
Paleolimnology of European Maar Lakes
Springer-Verlag Berlin Heidelberg NewYork London Paris Tokyo Hong Kong Barcelona Budapest
Editors Prof. Dr. J/3rg F. W. Negendank GFZ Telegrafenberg A 26, 14473 Potsdam, FRG Dr. Bernd Zolitschka FB Geographie/Geowissenschaften Universit~tt Trier 54286 Trier, FRG
"For all Lecture Notes in Earth Sciences published till now please see final pages of the book"
ISBN 3-540-56570-1 Springer-Verlag Berlin Heidelberg New York ISBN 0-387-56570-1 Springer-Verlag New York Berlin Heidelberg
This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, re-use of illustrations, recitation, broadcasting, reproduction on microfilms or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag. Violations are liable for prosecution under the German Copyright Law. @.Springer-Verlag Berlin Heidelberg 1993 Printed in Germany Typesetting: Camera ready by author 32/3140-543210 - Printed on acid-free paper
IX PALAEOBIOLOGY W. Hofmann: Late-Glacial/Holocene changes of the climate and trophic conditions in three Eifel maar lakes, as indicated by faunal remains. I. Cladocera W. Hofmann: Late-Glacial/Holocene changes of the climate and trophic conditions in three Eifel maar lakes, as indicated by faunal remains. II. Chironomidae (Diptera) B.W. Scharf: Ostracoda (Crustacea) and trichoptera (Insecta) from Lateand Postglacial sediments of some European maar lakes H. Weiler: Oligocene dinoflageUate-cysts in Quaternary freshwater sediments of Eifel maars
393
421 435 441
TERTIARY MAAR LAKES G. Btichel & M. Pirrung: Tertiary maars of the Hocheifel Volcanic Field (Germany) W. Zimmerle: Some aspects of Cenozoic maar sediments in Europe: The source rock potential and their exceptionally good fossil preservation B. Zolitschka: Palaeoecological implications from the sedimentary record of a subtropical maax lake (Eocene Eckfelder Maar, Germany) H. Lutz: Arthropods from the Eocene Eckfelder Maar (Eifel, Germany) as a source for paleoecological information H. Frankenh~iuser & V. Wilde: Flowers from the Middle Eocene of Eckfeld (Eifel, Germany) - First results V. Wilde & H. Frankenh/iuser: Initial results on the importance of a flora from the Middle Eocene of Eckfeld (Eifel, Germany)
447 467 477 485 491 499
FUTURE PLANS J.F.W. Negendank & B. Zolitschka: International Maar Deep Drilling Project (MDDP) - A challenge for earth sciences?
505
List of Contributors
511
Preface This book contains papers presented at a symposium held May 21-25, 1991, at Hans Beda, Bitburg, Germany. At the meeting 60 specialists from 8 countries discussed various aspects of palaeolimnology of European maar lakes. Of the more than 50 presentations given at the Bitburg symposium, 31 are published here. Four additional contributions have been excepted. The subject of this book are terrestrial deposifional environments. The time span under discussion is the late Quaternary (the last ca. 250 ka) and the Tertiary, especially the Eocene (ca. 49 Ma). Sediments, recovered from volcanogenic lakes of the Westeifel (Germany), of Massif Central (France) and of the Italian Peninsula were analysed with a multitude of methods. High time resolution is the major advantage of all of these records providing detailed information on changing patterns of sedimentation as a result of palaeoclimatic, volcanogenic and anthropogenic forcing. The biotic response to this influences within the lake and its catchment area is documented as well. Palaeomagnetic investigations make available data on the behaviour of the geomagnetic field and, additionally, serve as a source of palaeoclimafic proxy-data. This volume is organized thematically into 9 groups. After starting with the "Formation of maars" (2 papers) a "Regional over'Jew" i s given on the volcanic areas of the Westeifel, Massif Central and Italy (3 papers). "Seismics" give first information on the structure, tectonics and even palaeoclimatology of a maar lake basin (4 papers). This knowledge is tremendously extended with the study of sediment cores using the methods of "Sedimentology" (9 papers), "Geochemistry" (3 papers), "Palaeomagnetism" (3 papers) and "Palaeobiology"
(4 papers).
Comparable with these late Quaternary
deposifional
environments are lacustrine sediments of "Tertiary maar lakes" (6 papers). A concluding paper is dedicated to "Future plans" demanding an international and interdisciplinary cooperation in this up-to-date field of Quaternary research, contributing to the understanding of past global changes.
J6rg F.W. Negendank Bernd Zolitschka
Table of Contents
Preface
V
FORMATION OF MAARS G. Bfichel: Maars of the Westeifel (Germany) G. Bfichel & V. Lorenz: Syn- and posteruptive mechanisms of the Alaskan Uldnrek Maars in 1977
15
REGIONAL OVERVIEW J.F.W. Negendank & B. Zolitschka: Maars and maar lakes of the Westeifel Volcanic Field E. Juvign6, G. Camus & A. de Go6r de Herve: Maars of northern Auvergne (Massif Central, France): State of knowledge M. Follieri, D. Magri & B. Narcisi: Paleoenvironmental investigations on long sediment cores from volcanic lakes of Lazio (central Italy) - An overview
61 81
95
SEISMICS S. Wende & R. Kirsch: Geophysical mapping of organic sediments A. Stefanon: Preliminary uniboom survey of the Monticchio Lakes (southern Italy) R.B. Hansen: Sonar investigations in the Laghi di Monticchio (Mt. Vtilture, Italy) F. Niessen, A. Lami & P. Guilizzoni: Climatic and tectonic effects on sedimentation in central Italian volcano lakes (Latium) Implications from high resolution seismic profiles -
109 117 119
129
SEDIMENTOLOGY T. Heinz, B. Rein & J.F.W. Negendank: Sediments and basin analysis of Lake Schalkenmehrener Maar
149
VIII B. Rein & J.F.W. Negendank: Organic carbon contents of sediments from Lake Schalkenmehrener Maar: A palaeoclimate indicator F. Wegner & J.F.W. Negendank: Basin analysis for selected time-frames using sedimentation rates in Lake Meerfelder Maar (Westeifel, FRG) D. Drohmann & J.F.W. Negendank: Turbidites in the sediments of Lake Meerfelder Maar (Germany) and the explanation of suspension sediments D. Poth & J.F.W. Negendank: Palaeoclimate reconstruction at the Pleistocene/Holocene transition - a varve dated microstratigraphic record from lake Meerfelder Maar (Westeifel, Germany) A. Brauer & J.F.W. Negendank: Paleoenvironmental reconstruction of the Lateand Postglacial sedimentary record of lake Weinfelder Maar E. Truze & K. Kelts: Sedimentology and paleoenvironment from the maar Lac du Bouchet for the last climatic cycle, 0 - 120,000 years (Massif Central, France) B. Zolitschka & J.F.W. Negendank: Lago Grande di Monticchio (southern Italy) - a high resolution -sedimentaryrecord of the last 70,000 years P. Francus, S. Leroy, I. Mergeai, G. Seret & G. Wansard: A multidisciplinary study of the Vico Maar sequence (Latium, Italy): Part of the last cycle in the Mediterranean area. Preliminary results
163 173 195
209.. 223
237 277
289
GEOCHEMISTRY B.G. Lottermoser, R. Oberhgnsli, B. Zolitschka, J.F.W. Negendank, U. Schtitz & J.Boenecke: Environmental geology and geochemistry of lake sediments (Holzmaar, Eifel, Germany) C. Robinson, G.B. Shimmield & K.M. Creer: Geochemistry of I.ago Grande di Monticchio (southern Italy) A. Newton & A. Dugmore: Tephrochronology of core C from Lago Grande di Monticchio
305 317 333
PALAEOMAGNETISM B. Haverkamp & T. Beuker: A palaeomagnetic study of maar-lake sediments from the Westeifel T. Williams, K.M. Creer & N. Thouveny: Preliminary 50 m palaeomagnetic records from Lac du Bouchet, Haute Loire (France) I. Turton: Palaeomagnetic investigations of Lago Grande di Monticchio (southern Italy)
349 367 377
MAARS OF THE WESTEIFEL, GERMANY
G. Btichel Institut fiir Geowissenschaften, Universititt Mainz, Postfach 3980, 6500 Mainz, Germany
ABSTRACT Within the Westeifel Volcanic Field 27 % of the 250 Quaternary eruptive centers are maars. Maars form as a result of a highly explosive interactive" process between rising melt and groundwater. In the Westeifel, probably thermal water plays an important role for the productive phreatomagmatic interaction process and, con-sequently, the high number of maars. The Westeifel maars show all transitions to scoria cones. Only the youngest maars are filled by a maar lake or a raised bog, and are well preserved. The older maars show a low diameter to depth ratio. Nearly one third of the Westeifel maars were formed during the Weichselian glaciation period. The isostatic movements during the increasing and decreasing glaciation generated tectonic stress in front of the ice cap and, probably, caused the inten-sive volcanic activity during the last glaciation. This assumed to be the reason why for the last 10000 years BP (Ulmen maar activity) no volcanic activity
has
happened.
INTRODUCTION The Quaternary Westeifel Volcanic Field (QWVF) is the type locality of maars. Here more than a quarter of all eruption centers are represented by maars, the youngest filled by maar lakes. The QWVF is located in the. western part of the Hercynian
Rhenish
strong uplift,
Massif.
Especially
which amounted to 300m.
during
the
Pleistocene,
it
experienced
The volcanic field is clearly orientated
NW-SE. Its longitudinal axis is directed towards the western marginal faults of the Lower Rhenish Basin. The analysis of the tectonic features associated with the volcanism
of the
QWVF clearly demonstrates
the relationship
with,
and
depen-
dence on, the recent regional European stress field, characterized by a dominantly NW-SE (N 1300 E) orientated compressional stress (Btichel, 1984). The QWVF can be attributed freely and easily to the Central European Rift System (Fig. 1, inset).
The
Lecture Notes in Earth Sciences, Vol. 49 L F. W. Negendank, B. Zolitschka (Eds.) Paleolimnology of European Maar Lake~ 9 Sp~nger-Verlag Bedln Heidelberg 1993
volcanic field is located on its western margin, within a lithosphere, here only 50 km thick. (Panza et al., 1980; Illies & Baumarm, 1982).
which
is
The QWVF covers an area of approximately 600 km 2, extending 50kin in NW-SEdirection
(Fig.
1).
One
branch
central
part of the Tertiary
centers
about
basanites
were
aberrantly
extends
Hocheifel Volcanic
Field
20 km
northward
(THVF).
At 250
into
eruptive
1.7 km 3 of primitive MgO-rich foidites (leucitites, nephelinites) produced.
Only 3 % of the
magma is highly
the
differentiated
and and
consists of tephrites and phonolites (Mertes, 1982; Mertes & Schmincke, 1985).
Fig. 1: Map of the eruption centers of the Quaternary Westeifel Volcanic Field (triangles and circles). Crosses mark the volcanoes of the Te'rtiary Hocheifel Volcanic Field (after: Btichel & Mertes, 1982). Inset: Location of the working area in the Central European Rift System. Approximate depth to asthenosphere from Panza et al. (1980).
Following the results of the K/Ar, 4~
and 14C
age
determinations,
the
Westeifel volcanism is younger than 1 Ma, presumably even younger than 0.6 Ma. The youngest volcano, the Ulmen maar, 0.01 Ma old, most probably don't represent the termination of the volcanic activity (Btlchel & Lorenz, 1982; Fuhrmann & Lippolt,
1982; Lippolt,
1983; Mertes & Schmincke,
1983; Lottermoser et al., this
volume). In the past, the
recent
maars years,
were generelly recognized only when well-preserved. with
the
help
of
combined
geological/geophysical
tions, many more mostly strongly eroded maars have been identified Mertes,
1982; BOchel,
During
investiga(Biichel &
1984, Bi~chel, unpublished results). A review of the occur-
rences, known so far, and aspects of their origin are presented in this paper.
THE MAAR SYSTEM DEFINITION Maars
are
small
or
large
monogenetic volcanic craters
(up
to
about
2 km
in
diameter), cut into the pre-eruptive country rocks rocks and surrounded by a low ring wall (tephra ring) of pyroclastic material (Fig. 2). They are formed by a polycyclical gravitative collapse of the cover rocks above an eruption chamber, about 200 m deep (maximum hydrostatic pressure 20-30 bar) in the beginning; it may deepen during the sequence of eruptions (Lorenz, 1986). From a rock mechanical
standpoint,
the
maars
can
be
compared
to
sinkholes,
produced
by
subrosion. Maars originate from strong thermal explosions. The eruption clouds partly rise, like cauliflowers, up to several kilometers altitude, partly move radially outwards as surges with high velocities of up to 300 km/h. Ballistic transport is of minor importance. The smaller part of the maar tephra, transported by the eruption clouds, accumulates in a ring wall, several tens of meters high, around the maar. The major part of the material drifts outward by winds up to several hundred kilometers. Originally the term "maar" described a topographic feature, consisting of a crater and a tuff rim. Since the formation of this topographic feature is closely connected with the specific maar explosion process, we suggest the term "maar" for the whole structure and its formation. This term comprises the ring wall (tephra below the tuff rim), the crater sediments, the diatreme, and the feeder dyke system (Fig. 2).
Fig. 2: Schematic plot of a maar. The maar consists of the maar crater, the ring wall (= tephra ring), the crater sediments, the maar diatreme, and the feeder dyke. SYN-ERUPTIVE PROCESSES Maars forme as a result of a highly explosive interactive process between confined groundwater (probably thermal water and/or mineralized deep groundwater) and the rising melt. After initial mechanical mixing of the two phases, the vapour films collapses triggered by some kind of shock. The water" becomes superheated, resulting in an explosive expansion of highly pressurized steam to ambient pressure (Zimanowski et al. 1991). The transfer of the pressure pulse to
the pore and joint water pressure in the surrounding rocks leads to fracture (hydrofrac). The pressure drop .of the local excess pressure results in large-scale explosions. The mixture of disrupted magma, water and fractured host rock jets through a narrow eruption pipe to the Earth's surface and is discharged as an eruption cloud into the atmosphere. This process is repeated eposodically (probably cyclically), until the magma supply is exhausted or no more groundwater is available.
POST-ERUPTIVE PROCESSES The post-eruptive development of the just formed maar is subjected to exodynamic processes. The undercutting of the groundwater level by. the maar crater leads to the formation of a lake. The filling-up of the maar lake is controlled predominantly by five processes:
1. mass movements and mass flows of many types, e.g.
collapse and sliding of obersteep crater walls, blocks rolling down, land slides and lahars; 2. delta deposits at discharging creeks; 3. atmospheric loads, e.g. rain, ash of
nearby
volcanic
eruptions,
wind-transported
sediments;
4.
production
of
organic matter within the maar lake and sedimentation of it; 5. ascendent of groundwater, deep groundwater, and post-volcanic emanations (e.g. CO2) influence the chemistry of the lake water and, consequently, the diagenesis of the crater sediments
and the diatreme filling.
The
is
lake
then
filled
by marginal
and profundal
sediments
(e.g.
bituminous
sediments,
tubidity layers) until invisibility. The filling up of the maar crater is
influenced
by
various
exogenic
processes
which
depend
on
the
paleo-climatic
conditions. In the Eifel, the interaction of stade and interstade of the Pleistocene glaciations played an important role.
THE WESTEIFEL MAARS DISTRIBUTION In the QWVF, there are 250 eruptive centers. 62 % (154) are scoria cones, half of them with lava flows, 27 % (68) are maars. 5 % (13) are tuff rings, 3 % (8) are scoria rings, and 3 % are strongly eroded undefinable pyroclastic vents. The maars are not distributed regularly over the entire volcanic field. Especially in the northwestern and the southeastern parts maar volcanoes often predominate
over scoria cones (Fig. 1). This uneven distribution is strange, because the availability of groundwater, necessary for phreatomagmatic eruptions, should be lowest within the marginal region. Poorly permeable Lower Devonian shales and siltstones occur here, in contrast to the highly permeable Middle Devonian limestones and Triassic Bunter Sandstones in the central part (groundwater aquifer in fissured rocks). A series of factors is assumed to be responsible for the irregular distribution. Large production rates of magma and the magma fractionation occur in the central part of the volcanic field, and they are lower in the marginal zones. Furthermore, in the central zone graben tectonics prevail. Here, the Bunter Sandstone subsided up to 200m (Wienecke, 1984). The marginal zones present horst blocks, which were probably affected more easily by deep-reaching fractures. The rock matrix is here poorly permeable, as mentioned above. Thus, thermal mineral water could form and enough of it should have been available for continuing phreatomagmatic interactions. In the central zone, the fracture systems are more interconnected due to the competent behaviour of the rocks. This is probably the reason why no springs of thermal water has been found here. In contrast, within the southeastern marginal zone of the volcanic system warm springs occur (Bad Bertrich, 3 2 ~ ; Strotzbiisch, 19.2~ Dreis, 18.9~ and Dockweiler, 16.2 0C; Langguth & Plum, 1984). It seems thus to be indicated that phreatomagmatic interactions in fracture systems are caused by thermal
water!
MAARS WITHIN VOLCANIC SYSTEMS Maars often occur within eruption systems. The eruption systems of the QWVF consist of a whole series of linearly arranged scoria cones and/or maars, which were active successively within a short time period. The study of these eruption systems shows that the maars were formed both in the beginning (e.g. Strohner maar) and during (e.g. Boss maars E and W) the volcanic activity, but mostly towards the end (e.g. Meerfeld maar, Sprink maar, Hardt maar). It is concluded that these extremely different shapes of volcanoes were generated by external factors;
probably
the
specific
hydrodynamic role
of the
valleys
is
significant
(Lorenz, 1973): on the plateaus and hillsides the magma rise was not affected, and it was ejected in lava fountains. A scoria cone was formed. In valleys, however, explosive
interaction
between
(thermal)
groundwater
and
rising
magma
mostly
occured and maars were formed. Also, there are a few eruptive systems in the Westeifel, where only maars or only scoria cones occur. The Holzmaar system is an example for the first kind. It consists of the Holzmaar filled with water, the Dtirres Maar filled with a raised bog,
and the
Hitsche, the smallest maar of the Westeifel (present-day diameter:
original
diameter:
maybe
100 m,
70m).
TRANSITIONS BETWEEN MAARS AND SCORIA CONES About h a l f o f the 68 Westeifel maars owe their formation to continuous magmatic
eruptions.
The
other
half,
however,
contains
small
scoria
phreato-
cones
within
the crater or at the margin of the crater bottom, which formed syneruptively (e.g. SchOnfeld maar, Pulvermaar) or towards the end of the volcanic activity (e.g. Gees maar).
At
covered
the
by
present
time,
sediments;
the
(Biichel,
1987).
investigations
most
scoria
cones
interpretation
is
are
based
invisible, on
because
results
of
they
are
geomagnetic
Only one case is known, in which the final scoria cone activity was so intense that a lava flow extruded
at the maar bottom. This is the Gerolstein maar,
characterized by a geomagnetic anomaly of almost 3 0 0 0 n T
(Mertes,
which is
1982).
In exceptional cases, a tuff ring formed at Westeifel maars towards the end of the maar activity (e.g. Laach maar). The Westeifel tuff rings d e v e l o p e d exclusively in valleys were
which
had
generated
plenty
by
water
(creek water).
rings
(Lorenz,
water.
ion phase;
they
of
assumption
interaction
is
manifest
between
that
magma
Such a formation process is generally assumed
be mentioned
any phreatomagmatic
diameter
The
explosive
tuff
and
rings
surface
for most tuff
1986).
Finally, it should without
of
highly
are therefore
several
that in the Q W V F there is h a r d l y
phase. Most scoria cones had
hundred
underlain meters.
by a maar volcano,
After
the
explosive
a scoria cone
an initial maar eruptwhich
initial
may
phase
reach of
a
these
scoria cones, within the crater of the initial maar a scoria cone evolved, filled the maar with
scoria,
Scoria rings (Fig. cases,
on
the
and
finally, grew higher than
the surrounding
1) are maars with intensive final scoria cone
ring
walls
scoria
rings
developed
with
a
plateau. activity. In these
possible
thickness
of
serveral tens of meters (the Briick maar ring is, e.g., 40 m thick).
P O S T - E R U P T M V E PROCESSES Maars sensitive
represent to
unstable
erosion
morphological
processes
(Fig.
3).
features,
which
Immediately
after
post-eruptively their
formation
react maars
exhibit steep crater walls and deep craters (A in Fig. 3). Mass movements and mass flows, both driven by gravitation, decrease the inclination of the crater wails and cause the growth o f talus slopes. The high sedimentation rate loads to fast filling
Fig. 3: Schematic plot of the post-eruptive evolution of a maar volcano in four stages. A: initial stage, B: lake stage, C: post-lake stage, D and E: post-sedimentary erosion stages.
of the craters (B in Fig. 3). If sedimentation continues the maar lake finally filled up. Depending on the climatic situation, a bog phase may begin. The bog is finally covered by sediments of slope debris, slope wash, and solifluction debris (C in Fig. 3). The ring wall is preserved only in small remnants. The crater sediments may be eroded later, e.g. by a lowering of the valley bottom due to uplift (E in Fig. 3). This is the case for some old maars of the Westeifel (e.g. Seiderath maar, Wolfsbeuel E maar). Some day in the future the state of preservation might be realized as shown in E in Fig. 3. Some Tertiary maars of the Hocheifel (Btichel & Pirrung, this volume), present this stage. Here, the denudation under tropical to subtropical conditions was so effective that the resistant diatreme filling was carved out as a montain.
AGE OF THE WESTEIFEL MAA S When the present-day morphological shapes of the Westeifel maars are described by depth and diameter (Fig. 4), most maars yield a small depth to diameter ratio. In the beginning it is close to 1 : 5 (Wood, 1974), as at the Ukinrek maars (Btichel & Lorenz, this volume). The youngest maar of the Westeifel is close to this ratio. The older the maars are, the lower the ratio becomes (e.g. Auel maar) and the more the crater depth decreases as a result of post-eruptive filling. At the same t i m e the diameter increases due to gravitative erosion processes. These interrelations are, however, strongly affected by external factors, as post-eruptive valley erosion and pre-volcanic relief. The different erosion levels of maars, in combination with other data ( 1 4 C - a g e , climatic indicators, relative location to adjacent v o l c a n o e s ) , can be used quantitatively or qualitatively for the age determination (Btichel & Lorenz, 1982; Btichel,
1984).
It is concluded
that about
one third
of all
maars
are
probably
younger than 70 000 BP. This would indicate that during the time period from 70 000 to 1 0 0 0 0 BP, on average, approximately three maars were formed each 10 000 years (Fig. 5). All these maars are located in the eastern part of the volcanic field (Fig. 6). The situation is similar for scoria cones (Mertes, 1982; Mertes & Schmincke, 1983). Why did the volcanic activity increase so strongly during the time from 70 000 to 10000 BP, and for the last 10000 BP not a single volcano has formed? It is obvious that the young volcanoes coincide with the Weichselian glaciation (Fig. 5). The change of lithostatic pressure as a result of the increasing (stade) and decreasing glaciation the
(interstade)
isostatic
approached
rebound
during
the
generated
the volcanic field
glaciation crustal
from the
caused
tension
isostatic
in front
north to a
of
movements. the
ice
Possibly, cap
which
distance of 5 0 0 k m . Fractures
10
Fig. 4: Crater depth versus crater diameter of the Westeifel maars. Additional to the 68 maars, the two Ukinrek maars (Alaska) and the three potentially Tertiary maars, the DOttingen maar, the Jungferweiher, and the Elfenmaar, are plotted (cf. Fig. 1). The ration 1 : 5 (see straight line) is characterized for very young (original) maars. Almost all of the plotted circles, however, are located far away from this straight line as a consequence of intensive erosion. The depth to diameter ratio of the old Elfenmaar is high caused by strong post-eruptive valley erosion.
11
Fig. 5: Atmospheric temperature change of the Eemian, Weichselian, and the Holocene derived from isotopic profile (from Barnola et al., 1987), together with the estimated age data of the young Westeifel maars. Almost one third of all maars was formed during the time period from 70000 to 10 000 BP. So, another maar eruption has been overdue for a long time!
for the available magma opened in increasing numbers. Since about 13 000 BP the ice cap melted away. After reaching a new isostatic equilibrium, consequently, the volcanic
activity
diminished.
In Iceland, too, the high volcanic
activity
of the
Holocene coincides with the time of disappearance of glaciers. After 4500 BP the volcanic activity has decreased remarkably (Sigvaldason et al., 1992). Magmas obviously are still available below the Eifel (Raikes & Bonjer, 1983). What seems to be missing at the moment is a trigger!
12
Fig. 6: Spatial distribution of the Westeifel maars, which formed during the time period from 6 0 0 0 0 to 10000 BP (open circles), compared to older m a a r s (about 600 000 to 60 000 BP) a chronological evolution from west to east can be observed clearly. The three maars of probably Tertiary age are located on a N - S - l i n e at the part o f the volcanic field where the youngest maars occur. It is a s s u m e d that an old N-S-zone o f structural weakness is reactivated by young maar volcanoes.
~ C F _ ~ Barnola, J.M., Raynaud, D., Korotkevich, Y.S. & Lorius, C. (1987): Vostock ice core provides 160,000-year record of atmospheric CO2. Nature, 329: 408-414. Biichel. G. (1984): Die Maare im Vulkanfeld der Westeifel, ihr g e o p h y s i k a l i s c h e r Nachweis, ihr Alter und ihre Beziehung zur Tektonik der Erdkruste. 385 p., dortoral thesis; University of Mainz.
13 Btichei, G. (1984): The Westeifel Volcanic Field - Evidence for active tectonism in the Cenral European Rift System. Terra cognita 4: p. 96. Biichel, G. (1987): Geophysik tier Eifel-Maare. 1: Erkundung neuer Maare im Vulkanfeld der Eifel mit Hilfe geomagnetischer Untersuehungen. Mainzer geowiss. Mitt., 16: 227-274. Biichel, G. & Lorenz, V. (1982): Zum Alter des Maarvulkanismus der Westeifel. N. Jb. Geol. Pal/tont. Abh, 163: 1-22. B~chel, G., Lorenz, V., Schmincke, H.-U. & Zimanowski, B. (1986): Quartlire Vulkanfelder der Eifel. Fortschr. Miner., 64, Beih. 2: 97-141. Btichel, G. Mertes, H. (1982): Die Eruptionzentren des Westeifeler Vulkanfeldes. Z. dt. geol. Ges., 133: 409-429. Fuhrmann, U. & Lippolt, H.J. (1982): Das Alter des jungen Vulkanismus der Westeifel aufgrund yon 4~ Fortschr. Miner., 60, Beih 1: 80-82. Illies, H. & Baumann, H. (1982): Crustal dynamics and morphodynamics of the Western European Rift System. Z. Geomorph. N.F., Suppl., 42: 135-165. Lippolt, H.J. (1983): Distribution of volcanic activity in space and time. In: Fuchs, K., yon Gehlen, K., M~ilzer, H., Murawski, H. & Semmel, A. (eds.). Plateau uplift. The Rhenish Shield - a case history, 112-120, Springer; Berlin. Langguth, H.R. & Plum, H. (1984): Untersuchtmg der Mineral- und Thermalquellen der Eifel auf geothermische Indikatoren. BMFT-Forschungsbericht, T 84-019. Lorenz, V. (1973): On the Formation of maars. Bull. Volcanol., 37-2: 138-204. Lorenz, V. (1986): On the growth of maars and diatremes and its relevance to the formation of tuff rings. Bull. Volcanol, 48: 265-274. Mertes, H. (1982): Aufbau und Genese des Westeifeler Vulkanfeldes. 415 p., doctoral thesis, University of Bochum. Mertes, H. & Schmicke, H.-U. (1983): Age distribution of volcanoes in the West Eifel. N. Jb. Geol. Pal~tont. Abh., 166: 260-293. Mertes, H. & Sehmincke, H.-U. (1985): Mafic potassic lavas of the Quaternary West Eifel volcanic field. Contrib. Mineral. Petrol., 89: 330-345. Pauza, G.F., Mueller, St. & Calcagnile, G. (1980): The gross features of the lithosphere-astenosphere system in Europe from seismic surface waves and body waves. Pure Appl. Geoph., 118:1209-1213 Raikes, S. & Bonjer, K.-P. (1983): Large-scale mantle heterogeneity beneath the Rhenish Massif and its vicinity from teleseismic P-residuals measurements. In: Fuchs, K., yon Gehlen, K., M~ilzer, H., Murawski, H. & Semmel, A. (eds.), Plateau uplift. The Rhenish Shield - a case history, 315-331, Springer; Berlin. Sigvaldason, G.E.; Annertz, K. & Nilsson, M. (1992): Effect of glacier loading/deloading on volcanism: postglacial volcanic production rate of the Dyngjufj011 area, central Iceland. Bull. Volcanol., 54: 385-392. Wienecke, K. (1984: Strukturelle Untersuchungen im Mesozoikum der Eifeler Nord-S~id-Zone. 187 p., doctoral thesis; University of Bonn. Wood, C.A. (1974): Reconnaissance geophysics and geology of the Pinacate Craters, Sonora, Mexico. Bull. Volcanol., 38: 149-172. Zimanowski, B., Fr0hlich, G. & Lorenz, V. (1991): Quantitative experiments on phreatomagmatic explosions. J. Volcanol. Geotherm. Res., 48: 341-358.
SYN- AND P O S T - E R U P T I V E M E C H A N I S M O F T H E A L A S K A N UKINREK
M A A R S IN 1977
G. Biichel* & V. Lorenz# *Institut
ffir Geowissenschaften,
UniversitAt Mainz,
Postfach
3980,
6500 Mainz, Germany #Institut
ftir Geologie, Universit~t Wtirzburg, 8700
Pleicherwall
1,
Wiirzburg, Germany
ABSTRACT The two alkali olivine basaltic Ukinrek Maars (East Maar and West Maar) and one scoria cone within East Maar erupted within eleven days (March 30 - April 9, 1977) on the A l a s k a n Peninsula,
13 km north of Mt. Peulik, an andesite volcano of the
Aleutian Range. The East Maar, with a diameter of 300 m, is located within a small graben
system
350kin.
striking N
110 ~ E, oblique to the Aleutian trench in a distance of
On these tensional faults two eruption centres occur: The East Maar and a
scoria cone active more
on the southeastern margin of its crater bottom. The or less
during the
whole eruption activity of East
scoria cone was Maar.
The
West
Maar, with a diameter of 140m, is located just west of the East Maar. In the West Maar, out.
after the They
maar
first of three eruption cycles, feeder dike fragments
indicate
formation.
water
and
the
that
the
A f t e r the
explosion
eruption
coneshaped
talus
chamber
activity,
fans
both
continued
migrated craters to
were thrown
downwards filled up
grow
on
on
the
the
during
the
with
ground-
crater
floors,
m o d i f i e d by gravity sliding and lahars.
hNTRODUCTION The
two
Ukinrek
Maars
erupted
in March/April
1977
Alaskan
Peninsula.
1.8 km south of Lake Becharof on an E-W-trending hill, which rises up to 100 m above sea level (Figs.
1 to 5). Detailed accounts of the two maars and the eleven
days o f their eruptive activity were already published by Kienle et al. (1980)
and
Self et al. (1980). Only few maars have erupted in this century (Kienle et al. 1980) and very
little has
been
reported on their eruptions.
Therefore,
the
formation
of
t
Lecture Notes in Earth Sciences, Vol. 49 I. F. W. Negendank, B. Zolitschka (Eds.) Paleolimnology of European Maar Lakes 9 Springer-Verlag Berlin Heidelberg 1993
16 the maar
Ukinrek
Maars
genesis.
posteruptive
In
has
been highly
addition,
history of maars
they
informative give
the
in respect
unique
to the
opportunity
principles to
study
of the
from the very beginning.
In August 1981 we examined in some detail the maar ejecta sequences, the country rocks exposed
in the lower crater wails,
and the crater floors.
new
the
strombolian
data
on
phreatomagmatic
and
eruption
Thus
we present
acitivity,
on
the
tectonic setting as well as on the "early" post-eruptive history of the two maars.
COUNTRY ROCKS The
deepest
stratigraphic
levels
penetrated
by
the
diatremes
of the
two
maars
belong to the Upper Jurassic Naknek Formation (Kienle et al., 1980). Ejected clasts, up to block
size, consist of
consolidated
polymict
conglomerates,
sandstones, and
Fig. 1: Index map of Ukinrek Maars, the near-by Pleistocene Gas Rock volcanoes, and Mount Peulik, a stratovolcano of the Aleutian Arc, Alaska.
17
Fig. 2: G e o m o r p h o l o g i c a l map o f the Ukinrek Maars, and their vicinity taken from 1980 aerial photographs. The two maars are located on a moraine rampart, which was cut by a NW-SE trending valley (cf. Fig. 3). The surrounding area consists of debris material coming from a young debris flow of Mt. Peulik. North of the two maars former beach levels remodel the moraine and debris deposits. The drawn boundary o f distribution of the East Maar tephra marks the b o u n d a r y between c o v e r e d and n o n - c o v e r e d tundra v e g e t a t i o n . Outside o f this b o u n d a r y the thickness o f Ukinrek Maar tephra is less than l m, near the beach o f Becharof Lake less than 1 rim. The distribution of scoria relates to one o f the last eruptions of the scoria cone located in the SE of East Maar crater.
18
shales.
The
depth
between
the
pre-eruption
Formation has been assumed to be about 7 0 m
surface
and
the
top
of
the
Naknek
(Self et al. 1980). Judging from the
ejecta, the Naknek Formation is overlain by Quaternary sediments, w h i c h
make up
a considerable portion of the xenoliths of the maar ejecta. The upper p a r t of these deposits is exposed in the crater walls of both maars (East Maar up to 21.5 m thick, West Maar up to 1 0 m thick, in 1981). The lower part is deduced from xenoliths of the
East
containing non-welded
maar: some
From striated
bottom
to
top
the
pebbles, pumiceous
ignimbrites. Individual
Quaternary tuff beds
beds of ignimbrites
sediments (?),
and
consist several
of
till
units
of
change r a p i d l y in thick-
Fig. 3: Photograph o f the Ukinrek Maars taken in 1981 from the Southeast: The East M a a r is l o c a t e d in the centre o f the photograph. The dry v a l l e y in the foreground is located on a normal fault zone. The fault zone crops out in the southeastern and northwestern inner walls o f the East Maar crater. The northwestern continuation of the fault zone is indicated by a N W - S E trending dry valley seen in the background behind the East Maar. 250 m south (left) of this valley the smaller W e s t Maar is located. Notice the trees (near the arrow), which have lost their bark and twigs due to the dynamics of the base surges during the eruption activities; but they d i d n ' t have lost their branches.
19 hess and
a few even die out. They are overlain by a channel f i l l e d with fluvio-
glacial beds and, finally, by a tundra soil with a maximum thickness o f I m (Figs. 6 and 7). In the W e s t Maar no traces of ignimbrites are found, neither in outcrops nor in the ejecta (Figs. 8 and 9). This may suggest that the ignimbrites occupy a N W - S E trending
paleovalley in which the East Maar is located. The
ignimbrites boundary
is
also
favoured
by
subsidence
due
to
a
conservation
graben.
The
of this graben is characterized by a NW-SE-trending n o r m a l
Some of these
faults are
exposed in the
inner crater
of the
northeast fault zone.
wall of the East Maar (Figs. 6
Fig. 4: Photograph of Ukinrek East Maar taken in 1981 from the southsoutheast. The m a x i m u m d i a m e t e r of the maar is 3 4 0 m , the m a x i m u m t h i c k n e s s of the ignimbrites from lake level to the basis of tephra on the north side 21.50 m, and the maximum thickness o f tephra beds on the north side 22 m. Notice the distinct major and some minor concave collapse embayments in the southwest rim of the maar, which were formed during the eruption activity.
20 and
7).
Between
East
and West
Maar
additional
normal
faults
related
to
the
southwestern boundary of the graben are assumed. The Quaternary sediments of the West Maar consist of till deposits. ejecta blocks, up to 4•
In addition,
m in size, from deeper, non-exposed Quaternary deposits
are found. They consist of unconsolidated greenish silt and coarse sands, in part with pebble beds (pebbles up to 20 cm in size). Despite the ejection process and their
present
we assume
unconsolidated state
that
these
Quaternary
their original sediments
bedding
were
is preserved.
in a state of
Therefore,
permafrost at the
Fig. 5: Photograph of Ukinrek West Maar taken in 1981 from the Southeast. The maximum diameter is 170m (N-S), the minimum diameter 105 m (E-W), and the depth is about 35 m including the water depth (1 m) of the small maar lake. The northern crater rim of the West Maar is 180m away from the NW-SE-trending dry valley (cf. Fig. 3). Along the northwestern and eastern rim of the crater semicircular faults indicate collaps processes of the crater floor and the underlying diatreme.
21 time
of
contains
eruption.
According
discontinuous
to
permafrost.
Wasburn
(1979)
Consequently,
the
Alaskan
permafrost
Peninsula
lenses
should
still have
occurred below the West Maar. At the East Maar no permafrost ejecta were found. However, from a photograph taken just after the eruption activity (Kienle et al., 1980:
Fig.
6e)
channel-like
injection
of
groundwater
from
the
peremable
ignimbrites of the crater wall can be identified. This groundwater filled the maar crater.
The
channel-like
injection
from
a homogeneous
looking
aquifer
could
indicate frozen and non- frozen parts within these country rocks. Towards the E and SE, to a small extent also towards the W, the maar craters are surrounded by a hummocky terrain. This particular surface feature was caused by
Fig. 6: East inner crater wall of the East Maar: The almost vertical lower wall exposes faulted, pre-eruptive, unwelded ignimbrites (a). On top of the ignimbrites redeposited and also faulted ignimbrites follow unconformably (b). The also unconformably overlying fluvioglacial deposits (c) and tundra soil (d) are not faulted. On top of the tundra soil East Maar tephra beds (e) follow.
22 a catastrophic Holocene dacite debris flow derived from Mr. Peulik. The site of the younger maars was not reached by the debris flow. 3 km NNE of the Ukinrek maars the rocky promontory of the Gas Rocks (Fig. I) consists of a dacite twin dome volcano in the NW which uplifted conglomeratic sediments of the Naknek Formation. In the SE, the Gas Rocks consist of a tuff-ring volcano as does a rocky cliff along the shore of Lake Becharof 8 km towards the W.
All three
volcanoes in the
vicinity of the
Ukinrek maars
are
affected by
erosion and probably erupted during the Pleistocene. In addition, a number of CO2 springs exist at the SE, E and NE side of the Gas Rocks along the shore of and inside Lake Becharof (Self et al., 1980).
Fig. 7: West inner crater wall of the East Maar: The almost vertical lower wall exposes faulted ignimbrites (a), redeposited faulted ignimbrites (b), thick fluvioglacial channel-like deposits (c), tundra soil (d), and East Maar tephra (e). The upper crater wall shows distinct concave collapse embayments.
23 WEST MAAR
GENERAL
The
West Maar erupted within only 3 days, starting March
April 1,
1977.
30,
1977
On March 30, several eruption clouds rich in steam
and ending
and ash were
observed and related to a maar crater only 30-35 m in diameter (details in Kienle et al., 1980). During the next two days, no eruptions were observed but a few must have occurred because the maar grew to its final size. On the fourth day, when the East Maar had started to erupt, the West Maar was already occupied by a shallow lake
identicating
eastern, southern
the
availability
of
near-surface
groundwater.
Initially,
the
and western wall of the maar were nearly vertical (Kienle et al.,
1980). In 1981 the West Maar had a maximum diameter of 170 m, a minimum diameter of 105 m, and a depth of 35 m. The northern part of the crater floor was occupied by a shallow lake about 80 cm deep.
WEST M A A
Previously,
EJECTA
the
ejecta
c a u l i f l o w e r bombs
and
welded
at the base and an overlying explosive p h r e a t o m a g m a t i c
sequence
was
assumed
to
consist
of
spatter
bed. It
is now clear, however, that up to 6.5 m thick ejecta had been overlooked between the original tundra soil and the base of the spatter ( p h a s e
1). These
ejecta consist
of crudely bedded till and fluvioglacial material and at first glance do not differ very much
from the underlying exposed country rocks (c in Figs. 8, 9, and
10).
Fragments of tundra soil and even ejected wood, up to 1 m in length, also occur in these ejecta. Frequently, the tundra soil fragments are concentrated the
ejecta
which
show
an
inverse
stratigraphy.
This
type
at the base of of
stratification
indicates that some of the ejected fragments, up to 4x5 m in size, were overturned during
the
initial
eruptive
process.
Within the initial ejecta a juvenile fraction occurs consists
of
scoriaceous
smaller juvenile locally
contain
lapilli bedded
cauliflower and
country
lenses
lapilli rock
o f ordinary
and clasts.
and may bombs In
vesicular
be less than
which
addition black
in
turn
these
scoria
up
basal to
1%.
It
contain ejecta several
24 centimeters in size. In some cases the ejected till and fluvioglacial m a t e r i a l lying
these
juvenile
fragments
are
thermally
effected.
Neither
the
over-
cauliflower
lapilli and bombs nor the black scoriaceous material occur in contact to
the intact
tundra
originated
soil fragments.
This means that this juvenile material must have
and was ejected during the eruption process of the initially ejected till and fluvioglacial The
material.
juvenile
intermittantly
material or, less
must
have
been
probable, because
produced of the small
either
from
size of the
the
same
initial
vent
crater, it
Fig. 8: Northwest inner crater wall of the Ukinrek West Maar, showing a lake on the crater floor, the lower crater wall with exposed moraines (a), and o v e r l y i n g tundra soil (b) as well as talus deposits on the lower crater wall. In the crater sector of the photograph the tundra soil is overlain by 1 to 3 m of W e s t and a few centimeters of East Maar tephra (e). The West Maar tephra consist o f moraine xenoliths in the lower part (c) and of juvenile tephra (d) in the u p p e r part (cf. Fig. 10). In the centre of the photograph, a syn-eruptive collapse ring fault due to growth o f the maar crater is seen, cutting the p r e - e m p t i v e rocks from the centre of the photograph towards the lower left. Notice the trees (near the arrow), which have lost their branches due to the dynamics of the base surges d u r i n g the eruption activities.
25 was ejected from a second vent within the present West Maar area, similar to what happened later within the East Maar. The
cauliflower
lapilli
and
bombs
within
these
basal
ejecta
indicate
eruptive process which had thrown out the till and fluvioglacial material
that
the
was only
partly phreatomagmatic, but the main type of eruptive
style was phreatic,
similar
to
water
slowly
a
cauldron
explosion,
where
within
a gas-tight container until
Along
the
considerably.
inner
crater
rim,
the
superheating
the container
the
initial
of
takes
place
explodes.
eruptive
deposits
vary
in
thickness
On the west side (Fig. 8), they show an undulating surface and the
highs seem to extend outward in radial ridges, similar to rays of impact craters. Furthermore,
these
ejecta
seem
to
decrease
in
thickness
outward
rapidly.
The
thickness variation also seems to imply that the vent for these ejecta was located within the southern half of the present crater (Fig. 10).
Fig. 9: East inner crater wall of the Ukinrek West Maar, showing the lower crater wall with exposed moraines (a), overlying tundra soil (b), as well as talus deposits in the foreground. The tundra soil is overlain by tephra consisting o f ejected moraine material (c) and tephra (d) of the West Maar, and several decimeter of East Maar tephra (e). Notice the ejected block of presently unconsolidated (moraine) deposits of possibly permafrost origin in the centre of the photograph, resting on the crater wall.
26 Whereas the lower part of the ejected orientation, and
dips
the
upper
towards
the
part,
in contrast,
interior
of the
Quaternary
sediments
unconformably crater
with
is near-horizontal
overlies
about
the
earlier
300 . Thus,
the
existing briefly prior to the deposition of the upper ejected Quaternary
in
beds crater,
sediments
had about the same size as the present one. The ejected
Quaternary
by scoriaceous
clasts
sediments deposited
and juvenile in
pyroclasts of phase
at least four successive
beds
1 are (phase
overlain 2).
The
latter reach a maximum thickness of about 6 m at the southwest rim and almost die
Fig. 10: The letters at profile I mark the same units as in Figs. 8 and 9. Notice, that in profiles I, II, and VI the ejected tundra soil inversely rests on the pre-eruptive tundra soil. This means that the wall rocks were torn away from their original position by the impuls of the first explosion, overturned and deposited inversely at both sides of the crater. After this violent initial explosion event (phase 1), the crater already had its present size, because the overlying scoria o f phase 2 unconformably overlies the ejecta of phase 1 and dips towards the centre of the crater (see profiles V and VI). Whereas during phase 1 only 1 % of scoriaceous material was added to the wall rock blocks, in total this material represented the main component during phase 2. It is also possible that considerably less walt rock material was added during phase 2.
27 out at the northern rim (Fig. 10). This also demonstrates that the
respective vent
was located in the southern half of the maar crater. In the NE, S, and SW some of the
scoriaceous
material
dips
inward
as
do
ejected Quaternary sediments (Fig. 11). Thus,
the
upper
beds
of
the
underlying
the crater slightly d e c r e a s e d in size.
At the southern wall parts of this material are highly welded and in 1979 had been mistaken
by the
second
author
(V. L.)
for irregular dyke-like basaltic
intrusions
(Kienle et al., 1980: p. 21). Possibly, the reduction of the crater by scoria deposited on the inner
crater wall
might have been much more distinct than it is obvious today. Due
to subsequent
syn-eruptive
these
dipping
to
collapse the
crater
processes
and
centre,
have
post-eruptive been
eroded
mass except
movements for
the
present
scoria, small
remains (cf. chapter "Faults at the West Maar".
Fig. 11: U k i n r e k W e s t Maar, southeast inner crater wall s h o w i n g s c o r i a c e o u s c a u l i f l o w e r bomb deposits with orientation towards the interior of the maar. The upper part o f the underlying Quaternary ejecta shows the same orientation. This means that the crater has reached ist p r e s e n t size already shortly before the deposition of the scoriaceous material.
28 The
majority
of
the
scoriaceous
clasts
of
phase
2
consists
of
scoriaceous
to
vesicular cauliflower lapilli and bombs. At the SE, S, but predominantly
at the SW
rim,
after their
they
are welded
deposition. This
and oxidized
is in accordance
which points to high temperatures
with temperatures of 805 ~
measured
on
April
15, 1977 at a depth of 1.1 m below the surface of the whole ejecta sequence on the SW rim of West Maar (Kienle et al., 1980). The scoriaceous beds contain some lapilli and blocks up to 50 cm in size of Naknek
conglomerates
and shales,
o f slightly
baked and oxidized till, and finally of basalt. The same rock types also occur as xenoliths
within
Formation through
the
prove the
cauliflower
that
the
bombs.
explosions
Quaternary
sediments
The
wall
during and
rock
phase
finally
2
clasts had
into
from
the
penetrated
the
Naknek
downward
underlying
Naknek
Formation. The the
cauliflower bombs Wollmerath
cauliflower strongly
at the West Maar resemble very much
similar d e p o s i t s
K o p f scoria cone/Westeifel (Lorenz & Zimanowski,
bombs
formed
phreatomagmatic
on
phase
Surtsey
in
and
lava
the
a
transitional fountaining
1983).
period phase
Similar
between
(Lorenz,
The cauliflower bomb horizon at the West Maar, its elevated t e m p e r a t u r e , rather weakly in the
small
amount
of
country
phreatomagmatic
and
rock
that
clasts
only
imply
relatively
that
little
these
the
1974). and the
explosions
groundwater
at
were
participated
explosions.
The scoriaceous ejecta of phase 2 are overlain by two beds ( p h a s e
3),
t o g e t h e r up
to 60 cm thick, which consist of ash, lapilli, and blocks (Fig. 12). The first bed is rather blocks
sandy
in
derived
appearance, from
the
the
second
Naknek
and shales; the largest block being Quaternary
sediments,
and
basalt
bed
Formation
is much
coarser
(pebbles,
and
contains
conglomerates,
1.5 m in size). Furthermore, p e b b l e s blocks
occur.
Juvenile
many
sandstones, from
cauliflower
the
lapilli
account for up to 10-20 % and are mostly 1-3 cm in size. The matrix is sandy and derived from These
two
the Quaternary
phreatomagmatic
and Jurassic
sediments.
beds represent the final phase
(3) of the
and are overlain by up to 60 cm thick pyroclastic beds from
the
West
Maar
subsequent
East
Maar activity. At the southwest rim the lower part of the East Maar b e d s is stained red indicating that the cauliflower bomb horizon
was still giving off heat
and after the eruptions of the East Maar. This is in accord with
during
the temperature
measurements
(s.a.). The ejecta of phase 3 are obviously the result o f an eruptive
phase
was
which
even
more
explosive
than
the
previous
ones
but
which
had
29
Fig. 12: Final deposits of the Ukinrek West Maar with country rock clasts derived from t h e Jurassic Naknek F o r m a t i o n , ash, c a u l i f l o w e r b o m b s and l a p i l l i and juvenile clasts from phase 2 eruptions. Also the basalt fragments from the feeder dyke system come from this unit (phase 3).
escaped
observation.
Its
explosion
site
probably
was
located
previous eruptions because of the large number of Naknek
deeper
than
the
ejecta.
FRAGMENTS FROM THE FEEDER DYKE
tn the cauliflower bomb horizon and the subsequent block horizon o f the eruptive phases
2
and
45x40x150cm) olivine
3
a
number
exist.
phenocrysts
of
basalt
blocks
They consist of fresh
and empty
vesicles, the
up
1.5 m
in
alkali olivine basalt latter up
flow texture is distinct. Some blocks are bordered by or reddish
to
to
0.5 m m
diameter
(e.g.
and show
small
in
diameter.
A
contact-metamorphosed pale
siltstone on one side or on two parallel sides (Fig. 13). At these
contacts
30
Fig. 13: Ejected block of the feeder dyke of the Ukinrek W e s t Maar pointing to growth of the maar diatreme by downward penetration of the explosion centre during the eruption processes of phase 1, 2, and 3. The dyke block shows tension gashes at the boundary to slightly baked shales of the Jurassic Naknek Formation. the
basalt
displays
typical tension
gashes up to
1 cm deep
which
are
perpendicular to the flow texture within the basalt and to slickensides siltstone.
One
basalt
block
contains
a
shale
layer,
several
centimeters
indicating two closely spaced dykes. Another block, 60 cm in diameter, tectonic
breccia
consisting of shale
fragments and injected
oriented
within the thick,
shows a
by slightly vesicular
basalt. Obviously, these basalt blocks represent basalt emplaced in one or several dykes, each 45 cm thick or more, within the siltstones of the Naknek Formation. The
slickensides
and the breccia suggest dyke emplacement
along a preexisting
fault. The comparison of the dyke basalt with clasts of the West and East Maar clearly shows
that the
fresh dyke basalt
is identical
with the juvenile
pyroclasts
and,
therefore, its emplacement has to be considered to be part of the eruptive history,
31 i.e. the basalt dyke formed as a part of the plumbing system of the West Maar. This is supported by analyses of the halogens and other volatile phases
of the dyke
basalt and juvenile basaltic elasts of the Ukinrek Maar (Dreibus et al., 1986). The flow texture, the scarcity of vesicles, the marginal tension gashes as well as the chemical analyses (Dreibus et al.,
1986) together suggest that the
alkali
olivine
basalt magma which rose within the early fissure system and got chilled against the
sediments
was
relatively
viscous
and neither
highly
vesicular
nor
strongly
vesiculating close to the Earth's surface. The
basalt
blocks
only
occur
in
the
cauliflower
bomb
and
overlying
block
horizons of phases 2 and 3 to which clasts derived from the Naknek Formation are confined too. It is, therefore, highly conceivable that these blocks from
the
dyke
system
feeding
phase
1.
Consequently,
the
were derived
explosions
had
penetrated successively downwards to deeper levels from phase 1 to phases 2 and 3. The occurrence of the angular dyke blocks also shows that the basalt magma in the dykes had already been solid or quasi-solid when the dykes became disrupted. They completely lack any macroscopic features which indicate that some fluid or at least viscous melt might still have existed within the dykes at the time when the dykes were fra~-nnented and ejected.
FAULTS AT THE WEST MAAR The West Maar displays two prominent and several subordinate semicircular faults along
the
rim
and
entering
the
crater.
The
eastern
prominent
fault
(Fig.
5)
displaces country rocks and the overlying West Maar pyroclastic beds downthrow towards the crater centre of up to 2 m. In the south the fault enters the lower crater wall and cannot be traced any further. Towards the north, it dies out. The northwestern
fault
(Fig.
8) starts
at the
NNW crater
rim
and,
with
increasing
displacement, enters the northwestern crater interior where it has a dip of 54-630 and even displays some slickensides in the Quaternary wall rocks. These and also the subordinate faults were caused by slope instability of the inner crater wail.
This
slope instability
must be
related
to collapse
processes
of the
crater floor and the underlying diatreme because of the ejection Of large amounts of
wall
rocks
from
the
phase 3 (Lorenz, 1986).
depth
during
the
final
phreatomagmatic
explosions
of
32 POST-ERUI~IVE HISTORY OF THE WEST MAAR On the day after the eruptions of the West Maar had ended (April 2, 1977) a lake already existed in the crater indicating the availability of high-level groundwater within
the
disappeared
maar hill.
The lake
reached
a depth of 4 . 7 0 m
in April
during the summer between the end of May and late
1977 but
August
1977
leaving only a hot spring (Kienle et al., 1980). In April 1977 the lake had a pH of about 6. In August 1977 the hot spring had a pH of 6.3 and a temperature of 81 ~ In August 1981 the
lake was cold
and had a pH of 7-8 (Fig. 14). In 1979 the
shallow
Fig. 14: Crater of the Ukinrek West Maar (of 1981) with exposed Quaternary moraines and fluvioglacial deposits in the western wall. On the crater floor of the crater a shallow muddy crater lake shows a series of earlier beach levels. Scree deposits surround the lake and interfinger with the muddy lake deposits. At the lower right crater wall some fallen country rock blocks are still visible. Person on left slope for scale. Note that the eruption centre of the crater is probably located at the upper left side of the photograph.
33 lake
occupied
only
the northern
part
of the
craterfloor
and
showed
rising
CO2
bubbles (Kienle et al., 1980) whereas in 1981 no CO2 was observed anymore. In 1981 the lake was still located in the northern part o f the crater floor 80cm
deep
(Fig.
14). The
lake level was 9 0 c m
terrace due to summer-related The
below the
and
highest
was only
visible lake
low water.
post-eruptive sediments in the lake show a pale yellow c o l o r and
are finely
laminated, with alternating beds of clay, silt, and sand, the coarse beds up to a few millimeters
thick.
The
coarse
beds
probably
represent
the
deposits
of
small
turbidite flows which have spread out on the lake floor. Between the lake and the crater rim in the W, S, and E, several talus fans, partly related to post-eruptive mass movements
at the crater walls, contain small and big blocks,
up to several
meters in size (Fig. 14). These talus fans have been changing the maar t o p o g r a p h y rapidly by flattening the formerly steep to almost vertical crater walls
and filling
up the crater bottom.
EAST MAAR
GENERAL
400 m east of the West Maar the larger of the two Ukinrek maars, the East Maar, is located. It erupted during days 4 to 11 of the whole eruptive activity. The East Maar is located on a hill between two NW-SE trending valleys, which are located on a normal
fault
northwestern represents exposed
zone
(Fig.
inner
wall
the in
the
3).
The
of the
northeastern
fault
East
Maar
boundary
near-vertical
crater
zone
crater
of
wails
crops
a
out
in
the
southeastern
(Figs.
6
and
7)
small
were
graben.
dealt
with
The in
and
and
probably
country
rocks
chapter"Country
rocks". The eruptive activity described by Kienle et al. (1980) was observed to be of two different part
styles. Phreatomagmatic
simultaneously
the crater.
The
there
was
activity predominated. strombolian
strombolian activity built up
activity
To
from
a scoria cone
a lesser extent a
seperate
vent
and
within
(called lava dome
Kienle et al., 1980, and Self et al., 1980) on the southeastern crater floor.
in
by
34 PHREATOMAGMATIC TEPHRA OF THE EAST MAAR The tephra beds of the East Maar are exposed all along the interior wall and thus allow the rather unique study of thickness variations a historic
maar.
Unfortunately,
radial
exposures
and directionaI
are
absent
and,
deposition therefore,
at the
study of the radial facies variation of individual beds from their proximal to distal position
is impossible
at present.
A detailed
description
of the East
Maar
pyro-
clastic beds is given in Fig. 15. The p y r o e l a s t i c
beds of East Maar consist of at least 135 distinct tephra beds of
phreatomagmatic
origin.
Additionally
there
are interbedded
scoria
discussed later (chapter "Scoria cone within the East Maar").
beds
which
are
Distinct phreatomag-
matic beds are 1 to nearly 80 cm, on average 10 cm thick. At the crater rim the total
thickness
of the
beds
varies
between
5.2 m
in the
southwest
and
22.7m
(after Kienle et al. 28 m) in the northeast (Fig. 16). This strong variation is partly the
result
variable
of tephra wind
fans
direction.
directed
by pyroclastic
Furthermore
it
is
surges
probably
and
partly
related
to
due an
to the
excentric
collapse of the maar crater. The
pyroclastic
Within
the
beds
show
light grey bands
predominate.
These
low-ash
a
characteristic
the
light
grey/dark
ash content is low;
tephra
beds
are
lapilli,
unconsolidated
grey
lamination.
blocks, and
easily by wind and rain. Dark grey beds of smaller thickness
and
bombs
be
eroded
can
of high-ash
tephra
are intercalated. Due to their high resistance to weathering they stick out (Figs. 17 and ance.
18). Most of these high-ash layers are poorly bedded and massive They
contain
considerably
more
country
rock
fragments
than
interlayered
low-ash beds rich in lapilli, blocks, and bombs. Their impact craters, vesiculated
tufts,
and
accretionary
lapilli
(cf.
chapter
"Specific
East Maar tephra") indicate that they were wet during deposition. high-ash
layers
represent
high-energy
phreatomagmatic
in appearplasterings,
textures
in
Obviously
explosion
which fragmented the country rocks to ash grain size. The evidence
the these
events,
for moisture
in the tephra at the time of deposition indicates that at time of eruptionplenty of groundwater was present at the locus of explosion. The j u v e n i l e
fraction
of
the
phreatomagmatic
tephra
(without
the
juven-
ile fraction of the scoria cone, cf. chapter "Scoria cone within the East Maar") is estimated to be about 20 % to 80 %. The central part of unit J in sections I and II contains the highest content o f phreatomagmatic
juvenile fraction (Fig.
t9).
Most
35 o f it consists bombs
(Fig.
of cauliflower lapilli. Less common 20).
Some
lapilli and bombs
are c a u l i f l o w e r
represent fragments
and breadcrust
of cauliflower
and
breadcrust bombs. In contrast to the juvenile fraction of the scoria cone (s.b.) the lapilli and bombs are mostly rather compact, nearly devoid o f vesicles and appear rather
glassy.
olivine, larger
Others
many
show
juvenile
cauliflower
some
clasts
lapilli
vesicles.
contain
have
a
The
bombs
xenoliths
dark
of
reddish
contain
country
color
phenocrysts
rocks.
which
Some
implies
of
of the
that
after
clasts
from
deposition they were still rather hot and became oxidized. Furthermore,
the
phreatomagmatic
tephra
contains
angular
basalt
lapilli to block size. In part this basalt is compact, in part slightly vesicular; it is always
porphyric.
bombs
and
clasts
(in
basalt
clasts
represent
either
fragments
lapilli, or fragments of basalt dykes.. A large unit
intermittantly lapilli tuff"
The
J)
may
also
be
derived
from
active intra crater scoria cone.
amount
a potential
These
of
o f these basalt
lava
flow
so-called "blocks
are discussed in chapter "Blocks of indurated
cauliflower
from
the
o f indurated
lapilli tuff of the East
Maar". The
phreatomagmatic
derived
and
tephra
ejected
from
contains
various
large
amounts
of
country
subsurface
horizons
(s.a.).
The
rock
clasts
largest
blocks
were found within units A, C, D, G, L, and N. They consist of Naknek rocks and are up to
lm
in diameter. In some horizons large blocks are remarkably abundant,
as
e.g. at the base of unit D in section I (Fig. 21). The matrix of these block layers is composed mostly of massive ash tuff rich in country rock. These layers most likely indicate
collapse
processes
within
the
diatreme:
Particularly
strong
phreato-
magmatic explosions at the diatreme root result in the collapse o f the surrounding country
rocks
process.
Afterwards
icantly
and
(rocks
of
the
Naknek
Formation
collapsed country
in rocks
finally transported to the Earth's
this are
case)
during
fragmented
surface by
the
explosion
rather
and w i t h i n
insignif-
the
eruption
cloud. The pyroclastic beds at the East Maar lie directly on the t u n d r a
soil - there exist
no W e s t Maar deposits on the tundra soil at East Maar - beginning with fine and coarse
lapilli beds
Quaternary indicates
which
sediments that
the
in
addition to
the juvenile
clasts
already contain clasts from the Naknek
explosion
East Maar were already
sites of
the
and
fragments
Formation.
first phreatomagmatic
This
explosions
located within the Naknek Formation, i.e. at a
from fact
at the
level which
36
37
38
caption for Fig. 15 (the two pages before) Sections I to V mapped along the inner crater wall o f the East M a a r . F o r the locations of the profiles see the location map of the East Maar. The s e c t i o n s are divided into 14 units (A to N). As far as possible, these units were c o r r e l a t e d over the 5 profiles. The correlation was not easy, because the units s t r o n g l y v a r y in thickness and facies even at short distances. Profile V is located e x a c t l y o v e r the soria cone. The high portion of scoria from this scoria cone as w e l l as the p r e d o m i n a n t l y f i n e - g r a i n e d character of the pyroclastics in this p r o f i l e m a d e a correlation i m p o s s i b l e except for unit J. The percentage of scoria w h i c h intercalate with the tephra beds beginning with unit D, is estimated in c o m p a r i s i o n with p h r e a t o m a g m a t i c p y r o c l a s t i c s .
Fig. 16: North and west inner crater wall of the Ukinrek East M a a r s h o w i n g the steep to v e r t i c a l wall in the country rocks o f i g n i m b r i t e s and f l u v i o g l a c i a l deposits and the well bedded sequence of the overlying East M a a r ejecta. In the f o r e g r o u n d of the p h o t o g r a p h roots and branches from the p r e - e r u p t i v e tundra soil at the base o f the maar ejecta are seen. In the western c o r n e r there is a clearly v i s i b l e slump scar formed early in the syn-eruptive history o f the East Maar. The W e s t Maar is located in the background in a distance o f 4 0 0 m away from the western rim of the East Maar. At the lowest spot o f the t e p h r a rim the normal fault zone is localized (cf. Fig. 7).
39 is d e e p e r than that of the initial West Maar explosions and deeper
than the 70 m
Self et al. (1980) assumed. More or less in all tephra beds of the East Maar f r a g m e n t s the ejecta. They ation
at
are derived from branches
the site of the
present crater and,
wood
occur within
or roots of the f o r m e r
tundra veget-
in part,
bark
of
and
twiglets
are
still
preserved. Some wood fragments, as e.g. in horizon J, were c o a l i f i e d in the lower part
only, i.e. they were
coalified
only after they had been
ejected and emplaced
Fig. 17: Closeup o f the southwest inner crater wall of the U k i n r e k E a s t Maar showing 5.5 m of bedded tephra overlying the pre-emptive country rocks. At this point the tephra rim has its smallest thickness. The tephra s e q u e n c e c o r r e s p o n d s to section IV in Fig. 15. In the lower half high-ash layers dominate. The units A to J are condensed in about 3 m thickness. In profile I, h o w e v e r , they are almost 16 m thick, ashes are by far less abundant. The dominance o f the ash layers is the result o f the greater lateral range o f the h i g h - e n e r g y and w a t e r - r i c h e r u p t i o n clouds. The eruption clouds, which produced the coarse-grained lapilti and bombs, have a smaller lateral range. At the base of the tephra sequence no tephra beds of the W e s t M a a r were found. This means that the W e s t M a a r t e p h r a a l m o s t completely dies out within a distance of 400 m. Note that the last eruptive activity is characterized by strombolian eruptions of the small scoria cone in the southeast interior o f the crater.
40 in the tephra layer. Within unit C of section I (Fig. 15) a block of tundra soil, 30 cm in size, was found still containing rootlets. The ejected wood and the fragments of tundra soil can be used as an indicator for a model
favouring
(Lorenz,
a
nearly
continuous
growth
of
the
maar
crater
by
1986). As the crater became enlarged by collapse, the wood
collapse and other
surficial sediments as well as pyroclastic debris from the tephra ring slumped into the crater again and again and were fed into the ejection process. Thus, wood from the
original
surface
was
ejected
nearly
continuously
without
having
been
coalified.
SPECIFIC TEXTURES IN THE EAST MAAR TEPHRA
The phreatomagmatic beds at the East Maar show a number of informative aspects which
give
additional
hints
in
respect to
mode
of deposition
of the
pyroclastic
Fig. 18: Uppermost tephra of the Ukinrek East Maar at the northern inner slope, showing units M and N of section II (Fig. 15). On the northern side of the Ukinrek East Maar the last eruption activity is represented by coarse-grained lapilli tuff with many Naknek blocks and bombs of indurated lapilli tuff (cf. Figs. 26 to 28).
41 material and hence to the origin of the maar. Within unit C in section I (Fig. 15) a 4 cm thick bed of grey v e s i c u l a t e d
tuff
was found containing
vesicles
up to
3
mm in size. In the East Maar deposits just west of the West Maar, v e s i c u l a t e d tufts were found already in 1979 (Kienle et al., 1980) on the east side of blocks ejected from
the
West Maar.
The vesiculated tufts
at these blocks form
aerodynamically
shaped flat mounds on the (east) stoss side of the blocks. Vesiculated tufts at maars usually indicate deposition of a three phase system (solid, liquid, gas) from a base surge (Lorenz, 1974 a, b; see, however, Walker 1983; Rosi, 1992). Unit L in section III at the northwest comer of the East Maar (Fig. one ash bed with a c c r e t i o n a r y contain
small
basalt
clasts
accretionary
lapilli
ends
Accretionary
lapilli
are
available
grainsize
of
l a p i l l i up to 5 mm in diameter. as
with
cores. a
frequently ash
grains
thin
The
fine-grained,
found is
crude
in
small
concentric and
enough
and
when
S o m e o f them
layering
slightly
phreatomagmatic
15) contains
pinkish
deposits there
of
layer.
when is
the
the
enough
Fig. 19: East Maar tephra of the middle part of unit J of section II (Fig. 15) at the northern rim, showing j u v e n i l e - r i c h ejecta. The central and l o w e r part o f the photograph consists at 80 % of cauliflower lapilli and bombs. B r e c c i a tuffs rich in wall rocks overlie this unit.
42 moisture
in the eruption clouds (Fisher & Schmincke,
Moore 1967; Schumacher & Schmincke, Most
of
the
blocks
and
cauliflower
1984;
Lorenz,
1973,
1974;
sags
in the
1991). bombs
do
not
show
impact
underlying beds. This indicates that they were deposited by base surge f l o w s 21).
In a number
largest
blocks
of beds, however,
within
the
crater
wall
there are a few impact sags (Fig. tephra
are
only
slightly
larger
(Fig.
22).
The
than
1m
(e.g. at the base of unit D in section I, Fig. 15). Self et al. (1980) describe blocks 23 m in diameter with impact craters, which lie at a distance of up to 7 0 0 m
from
the centre of the East Maar. In
a
small
pyroelastic
gully
northwest
beds
from
distance o f 900 wall some foreset
of
the East
the
West
Maar
Maar occur
dune
type
cross-bedded
indicating
base
surge
m northwest of the East Maar centre. Within
flows
the n o r t h e r n
at a crater
bedding was observed within unit K in section I and within unit
Fig. 20: B r e a d c r u s t bomb, more than 0.5 m in diameter, within unit J o f the northern crater. This bomb represents an eruption within the p h r e a t o m a g m a t i c cycle, which ejected almost exclusively juvenile material (cf. Fig. 19). Possibly, very little g r o u n d w a t e r was present during this eruption process, s o that the e n e r g y due to the e x p a n d i n g water v a p o u r was low and r e s u l t e d in the fragmentation o f only little wall rock. A bomb like the one shown is hard to distinguish from a cauliflower bomb produced by the scoria cone.
43 D in section II implying base surge flows from SW towards the NE. Thus, the vent for these pyroclastic tephra should have been located in the western part of the maar crater, northwest of the scoria cone. Undulating channnei-like
t e p h r a beds (e.g. units A, C, and E in section III, Fig. 15) as well as structures
(unit E in section II, Fig. 15) indicate an upper flow
regime of base surge flow and deposition.
Fig. 21: Block layer at the base of unit D in section I (northeastern part of the crater wall of the East Maar, Fig. 15). The blocks are situated within a matrix of totally unsorted, mostly unbedded, massive ash tufts. The block layers probably iaadicate increased collapse processes.
44 A number of blocks in the East Maar tephra are c o v e r e d
by
ash
which
pinches
out against the blocks or becomes thinner on top of the blocks c o m p a r e d
to the
sides, or they drape over one side and start on the opposite side at a l o w e r level. This feature in unit K in section I points to a base surge flow again from the SW towards the NE. Plastered mud deposits on boulders were observed by
S e l f et. al
(1980). Summing up, most of the specific textures in the East Maar tephra indicate that the maar
was
formed
magmatic (Btichel, Maar
textures this
were
by phreatomagmatic are rather
volume). not
as
Thus
processes.
rare compared at times
distinct as
in
However,
to other maars,
the phreatomagmatic
other maars.
The
some e.g.
of
to
phreato-
Westeifel
processes
transition
the
of
normal
maars
the
East
magma
fountaining activity becomes quite obvious within some units, e.g. unit J.
Fig. 22: I m p a c t structure below a juvenile block within unit E of s e c t i o n II (Fig. 15). The impact of the block happened immediately before the d e p o s i t i o n o f the ash layer and resulted in the deformation of the underlying lapilli tufts.
45 SCORIA CONE WITHIN THE EAST MAAR Strombolian and sixth
activity had been observed within the East Maar on the day of the maar's
8 days eruptive
activity (Kienle
second, fifth
et al.,
1980).
The
strombolian activity was not very intensive and gave rise to the ejection of scoria rising only to a height of about 30-50m. interbedded
within
the
35 distinct unwelded
phreatomagmatic
beds.
Only
scoria layers
the
most
are
important
scoriaceous beds are shown in Fig. 15. The interbedded scoria beds clearly indicate repeated
strombolian
Maar.
None
of the
found
only
in
activity
during
almost the whole
scoria layers surrounds
specific
wall
sections
active
period
the whole crater.
indicating
directed
of the East
Most
of them
ejection
or
are
ejection
influenced by wind. Most of the scoria beds occur in sections I and V (Fig. 15). The scoria beds consist of scoria which is usually up to 3 cm in size, rarely up to 30cm.
Generally,
phreatomagmatic
the scoria can be distinguished origin
by
its
considerably
from the juvenile
higher
vesicle
fragments
content.
of
Furthermore,
it is mostly black and only rarely contains small wall rock ctasts (cf. Figs. 19 and 23). But some scoria fragments with cauliflower surface texture exist (e.g. unit H in section I). They have a rather compact (low vesicularity) interior.
In this case
the distinction is not always clear (cf. Fig. 20). In several scoria layers the clasts themselves
and even the base
of the overlying phreatomagmatic
tephra
beds
are
partly oxidized which points to elevated temperatures of the scoria clasts still after their
deposition.
At the surface of the East Maar rim two distinct ejecta fans of dark black and in parts only slightly oxidized juvenile clasts extend for about 300 m to the SE and the NW from the crater rim (Fig. 2). In addition to ordinary vesicular scoria lapilli and cauliflower bombs (up to 1.6x0.6x0.5m),
canon ball bombs (up to 0 . 6 x 0 . 5 x 0 . 5 m)
and ribbon bombs are found. The ribbon bombs obviously represent viscous the
flowbanded
ribbon-like
spherical,
others
to the flat shape.
magma, internal
are flat,
where parting.
the
flowbanding
Some
and the latter
controlled
cauliflower
display
bombs
an internal
At the short ends the cauliflower-shaped
fragments of
breaking are
and
relatively
flowbanding surface
also
parallel
cuts the flow
banding. With the ejection
of scoria decribed just
above the eruption
activity
of the two
Ukinrek Maars came to an end. These last scoria horizons had as their centre of origin
the
scoria
cone
and
were
produced
by
an
unobserved,
pobably
rather
46 intensive later.
strombolian
Considering
the
activity during volume
the
of ejected
last day of eruptions scoria
during
the
or possibly whole
scoria
activity (Fig.
15) this last strombolian event was one of the most intensive
cone eruptive
activities within East Maar.
even cone scoria
At the end of the East Maar eruptive activity the scoria cone was about 40 m high (Kienle et al., 1980) and its crater had a diameter of about 80 m. With two projections the scoria cone extended up the near-vertical SE-wall (Fig. 24). In
1979 and
1981 these two projections reached above the lake surface. Depth sounding of the lake floor in July 1981 showed the scoria cone to rise from the 32 m deep flat lake floor to about 19-21 m water depth (Fig. 25). At the two projections reaching above the lake level, welded scoria, in parts of broccoli texture,
and cauliflower bombs
up to I m in diameter were observed by landing on the projections with a boat.
Fig. 23: Scoria lapilli layer within unit G of section I (Fig. 15), interbedded with phreatomagmatic tephra. The light grey sandy tephra between the black scoria fragments originated in phreatomagmatic eruptions which have been active simultaneously.
47
Fig. 24: On the attached one o f part o f (cf. Fig.
View over the East Maar towards Mr. Peulik, visible in a distance of 13 km. right side of the photograph the uppermost extension of the scoria cone is to the vertical southern inner crater wall (cf. Fig. 25). Only the bigger the two projections is shown. To the left of the projection, in the central the" photograph, the normal fault in the Quaternary wall rocks crops out 6).
Kienle et al. (1980) and Self et al. (1980) called this scoria cone a lava dome. Those parts
reaching above the lake level are definitely scoriaceous and depth
sounding
(Fig. 25) and Fig. 6e in K i e n l e et al. (1980) as well as other photographs clearly show the cone to contain a crater.
BLOCKS OF INDURATED LAPILLI TUFF OF THE EAST MAAR
A
highly
comprises
interesting
and
unusual
clast
blocks, up to 1 m in diameter, of
type
within
indurated
the
oxidized
East
Maar
tephra
lapilli t u f f .
These
48
Fig. 25: Post-eruptive subaqueous topography of the East Maar of 1981 and two cross-sections through the crater lake. Note that the crater floor is almost horizontal. This is due to intensive redeposition and accumulation of the sediments coming down the debris slopes along the margin of the lake and their ditribution on the lake floor by turbidity currents.
49 clasts occur from horizon C upwards to the very last phreatomagmatic horizon at the surface where a number of large blocks can be found. The blocks of indurated lapilli country
tuff contain rock
angular
xenoliths),
basalt
mostly
fragments,
thermally
cauliflower
altered
clasts
lapilli of
(with
country
enclosed
rocks,
and
fragments of the crater wall. Many of the larger blocks also contain thin basalt dykes which clearly intruded the lapilli tuff and was the cause for the induration and oxidation (Figs. 26 to 28). A few blocks contain two of these closely spaced basalt dykes (Figs. 27 and 28). One block of indurated lapilli tuff (50 cm in size) is located at the top of horizon K and is overlain by an ash bed, the lowermost 1 cm of which became
oxidized.
Obviously, the
block of lapilli tuff was still hot when it
Fig. 26: Ejected dyke rock from the surface of the northern crater rim of the East Maar. The vesicle-poor basalt intruded into indurated lapilli tuffs mostly consisting of compact subrounded and angular basalt fragments. Subordinately there are baked wall rock fragments.
50
1
FLOW STRUCTURESWITHIN A NON-FRAGMENTED BASALT
2
INCLUSIONS OF BAKED WALL ROCK
3
CGNTACTBETWEEN BASALT AND FRAGMENTED MATERIAL (WALL ROCK AND JUVENILE COMPONENTS)
4 TRASITION ZONE: WELDED PYROCLASTS, JUVENILE COMPONENTS WITH LOW DEGREE OF VESlCULATION 5
BIGINNING OF THE FORMATION OF CAULIFLOWER BOMBS (LAPILLIi
6
CAULIFLOWERBOMBS (LAPILL!)
7 FRAGMENTSOg SLIGHTLY BAKED WALL ROCK
Fig. 27: Sketch of two blocks of iadurated lapilli tuff which contain dykes that intruded the tephra within the lower levels of the East Maar diatreme. The lower part of the block is shown in Fig. 28.
51
Fig. 28: Photograph of an ejected block of indurated lapilli tuff (lower block in Fig. 27). This dyke irregularly intruded the pyroclastic debris in almost subparallel interconnected dykes in the lower levels of the East Maar diatreme, possibly in the diatreme root zone. was emplaced to cause thermal oxidation of the following overlying ash bed. Some of
the
(chapter
angular
basalt
blocks
"Phreatomagmatic
within
tephra
the
tephra,
of the East
which
Maar"),
are
possibly
described are
above
fragments
of
these indurated lapilli tufts (Fig. 22). The
occurrence
within
the
basaltic may
basal
of
these
blocks
suggests
that
part of the East Maar diatreme
phreatomagrnatic was intruded
lapilli
tephra
by rising viscous
magma. This basalt magma may have been related to the scoria cone or
have
intruded
the
lowermost
diatreme
intermittantly:
In favour of the first possibility is the fact that both, the scoria of the scoria cone of the
southeastern
crater
floor
and
the
blocks
of indurated
lapilli
tuff
almost
52 occur together from units C/D upward in the pyroclastic sequence of the East Maar tephra. They are added to the deposits until the end of the eruption activity of the East Maar. In case of the first possibility the following processes might have taken place: Possibly coming from the side, the viscous basaltic magma intruded into the growing diatreme. Here, East Maar tephra and wall rock fragments of the Upper Quaternary
sequence
- failing and sliding back into the crater and sinking
into
the diatreme - were present. Within part of the growing diatreme, the magma rose to
the
crater
process,
parts
successively
floor and of the deeper
phreatomagmatic
formed a scoria cone. Due to the
feeder dykes of the in
eruption
the
diatreme.
process,
continuing
scoria cone were
Finally
fragmented
they and
were
cut
off
collapse and
encorporated
in
sank the
ejected.
The second possibility can have worked only in case of an intermittent eruption process:
During calm phreatomagmatic eruption phases,
with possibly
only
little
groundwater present, magma could have intruded into those parts of the diatreme root,
which
high-energy
did
not
eruption
participate processes,
in
the
when
interaction
plenty
of
process.
During
groundwater
was
subsequent present,
dykes, still hot, were fragmented together with their wall rocks (i.e.
lapilli
the tuff),
and ejected. From the tephra beds of the East Maar an intermittent
eruption process can be
deduced
East
(cf.
chapter
"Phreatomagmatic
tephra
of
the
Maar").
The
high
content of juvenile clasts within numerous units (e.g. unit J) is a reason to believe in
little
groundwater
participation
and,
thus,
weak
phreatomagmatic
interaction
processes. Similar processes took place during the formation of the scoria cone. In contrast there are the ash tuff horizons of the East Maar tephra which are rich in country
rock
explosions
fragments.
They
were
formed
by
high-energy
phreatomagmatic
and are, consequently, indicative of optimal large volumes
of ground-
water. Whatever
are
the
exact
causes
for
the
blocks
of
indurated
lapilli
tuff,
they
represent a rarity in maar deposits world-wide. To study the blocks of indurated lapilli tuff in detail could help to formation of maars.
at least partly - unravel the problem of the
53 TECTONIC SETTING
A c c o r d i n g to Kienle et al. (1980) Bruin
Bay
Naknek
Fault,
along
which
Formation
(SE).
The
(granite, and
granodiorite,
boulders
from
the maars are located on the
a Jurassic many
diorite,
individual
gabbro)
disintegrated
batholith
Naknek
(NW)
ejected
clearly
was
blocks
represent
Formation
NE-SW thrust
of
trending onto
plutonic
well-rounded
conglomerates,
the rocks
pebbles
within
which
they also occur. Thus, the diatremes of the two maars appear not to be located on the Bruin Bay Fault but clearly penetrate only sediments of the Naknek Formation. The
Quaternary
which
almost
movements maximum normal some
of
deposits coincides the
exposed
faults,
the
maximum
faults
are
dislocation amounts indicate
faults
sequence
with
individual
which
antithetic
youngest
in the East Maar display a N W - S E - t r e n d i n g
of
are
present
the
(Figs.
The
tectonics. 7
and
ignimbrites,
diameter
decimeter
to 3 . 5 0 m .
extensional
redeposited
in
maar
faults
Only 29).
which
older ignimbrites. Near the top o f the Quaternary
to
(Fig. meter
29).
The
range.
The
are m o s t l y
within
The
fault zone,
the
faults
western
wall
penetrated
the
unconformably
sedimentary
synthetic
overlie
sequence,
the
the fault
zone is overlain by a channel, filled with fluvioglacial beds and till. The overlying tundra
soil
is
recent
times.
thickest
(1 m)
above
the
channel,
which
implies
solifluction
in
A statistical analysis of the faults and joints within the fault zone (Fig. 29) shows that most of the faults are orientated in E-W to ESE-WNW direction. The maximum direction o f joints
is E S E - W N W
and coincides with the direction
o f most
faults.
Other j o i n t directions, e.g. NE-SW, probably could have originated in the collapse tectonics o f the crater.
The prominent WNW-ESE direction indicates that the fault
zone trends from the east crater wall to the northwest crater wall (Fig. 29). The continuation
is
assumed
below
the
valleys northwest
and
southeast
of
the
East
Maar (Fig. 3). The
analysis o f linear features in aerial photographs
the shade
such
o f grey and vegetation or topographic features,
as such
linear variations
of
as the orientation
of small creeks and the linear orientation of escarpments, is shown in Fig. 30. In the
rose
diagram
directions appear.
o f linears These
taken
from
are the N 70 ~ E,
aerial photographs,
only
N 120 ~ E and N 10 ~ W
three
prominent
directions.
One
maximum very well coincides with the prominent W N W - E S E direction of joints and faults
in the rose
d i a ~ a m of Fig. 29. Also the N-S to
NNW-SSE
direction
coincides
54
Fig. 29: Extensional faults exposed in the northwestern and eastern crater wall of the Ukinrek East Maar (cf. Figs. 6 and 7). Possibly, these faults r e p r e s e n t the northeastern boundary of a graben structure. The southwestern b o u n d a r y o f this graben is unknown. It must b e located between the East and the W e s t Maar, because the ignimbrites, exposed in the crater wall of the East Maar, are present neither in the crater wall nor below the crater o f the W e s t Maar. The rose diagram shows the distribution of the fault and j o i n t orientation, e x p o s e d in the crater walls o f the East Maar.
with that of some joints and, additionally, with the maximum diameter o f the West Maar. No correlation could be found with N 70 ~ E trending linears. O n l y the COz gas vents, which bubble up through the lake east of Gas Rocks ( S e l f et al., 1980) are orientated into two lines almost along the same direction. The
linears
fracture
in
zones.
the These
aerial
photographs
fracture
zones
probably originated
represent in
and
traces are
of
young
controlled
in
orientation by the recent stress field. In recent times the North Pacific Plate drifts relative to the North American Plate towards the N W with a velocity o f about 5 to 8 cm/a
(Minster
&
Jordan,
1978;
Stone
& Wallace,
1987).
The
main
horizontal
55 compressional
normal
stress
trajectories
in this
part
of
the
where the two Ukinrek Maars are located, is N 1 4 0 ~
Aleutian
Peninsular,
E (Estabrook
1991, Fig. 31). This direction intersects the two maxima in the rose
& Jacob, diagram
of
linears (Fig. 30). Therefore, we propose that the two maxima of aerial photograph linears tly,
we
represent propose
fractures also
that
which the
formed normal
by
strike
faults
at
slipe the
inner
originated by right-hand strike slip movements. This means situated on a right-handed
movements. sides
of
Consequenthe
that the East
fault zone which at the same time has
crater
Maar is
an extensional
component due to the extension in NE-SW direction (sigma 3).
Fig. 30: A n a l y s i s o f linears from aerial photographys taken in 1980. The linears represent traces o f young fractures. In the aerial photograph they are visible as dark grey zones, linear inhomogeneities of vegetation, segments of small creeks and escarpments. The most important linear zones are emphasized by a dotted grid. On the right side the rose diagram of the analyzed linears is shown. The W N W - E S E and N N W - S S E directions can probably be interpreted as conjugate shear planes, related to a NW-SE-trending compressional stress field. The N E - S W direction could represent traces of P-shears (Larter & Allison, 1983).
56
Fig. 31: Direction of the maximum horizontal compression derived from seismic focal mechanisms, volcanic indicators, geologic faults, and borehole breakout data (from Estabrook & Jacob, 1991). In the study area of the Ukinrek Maars the stress trajectories have an orientation of about N 140-150~ The plate-motion vectors are taken from DeMets et al., 1990).
Fig. 32: Stress regime related to the different geotectonic positions. In the volcanic zone, where the Ukinrek Maars are located, srike-slip movements predominate (from Nakamura & Uyeda, 1980).
5Y Consequently, the West Maar should be located on a N-S to N N W - S S E trending lefthanded fault zone. This fault zone is not exposed at the crater wails. The faulted wall rock fragments
at the feeder dyke fragments (s.a.) only inicate
that the rise
of magma took place along a fault. The N-S to NNW-SSE orientation is only hinted at by a prominent linear zone NNW of the West Maar crater (Fig. 30). According
to
Nakamura
and
Uyeda
(1980)
the
general
tectonic
regime
in
the
volcanic zone between the trench and the back arc areas is strike slip with sigma 1 perpendicular to the trench and sigma 3 parallel to the trench. The intermediate normal stress component sigma 2 is perpendicular to the Earth's Thus,
surface (Fig. 32).
the p r o p o s e d interpretation fits well into the general pattern.
POST-ERUPTIVE HISTORY OF THE EAST MAAR
On the
17th
of August
1981, the crater lake water had
a rather greenish colour
owing to algae and it had a pH of 9 and a temperature of 14~
The results of depth
sounding of the crater lake are given in Fig. 25. Except for the region of the scoria cone,
the
surrounded
central by
area
of
the
lake
subaqueous
and
has
subaerial
a
nearly
debris
horizontal
slopes
and
floor, the
which
is
near-vertical
crater walls. Accumulation of reworked clastic material in the debris slopes and on the crater floor since the end of the eruptive activity has already covered parts of the scoria cone and, with time, will completely cover it. The flat floor of the crater lake was already established within the first four posteruptive years. Such a flat floor is rather characteristic of maar lakes, as e.g. the majority of the nine maar lakes in the Westeifel Volcanic Field has a flat floor and so has the and Lorenz, by
debris
120 m deep Nanwaksjak maar lake on Nunivak I s l a n d / A l a s k a (Biichel unpublished moving
down
data, from
1981). The the
surrounding
tephra
rim
and
slopes the
most
crater
probably
wall.
The
form mass
transport is initiated by the effect of rain, frost and wind. This way often groovelike depressions form in the crater wall (Fig. 33). The highs between frequently
collapse
and
slide
into
the
lake
with
high
velocities.
the groovs There,
they
produce a high wave, which in turn undercuts the talus slopes. In the subaqueous
debris slope the coarser material accumulates
part as subaqueous
lahars) and the finer material spreads out on the lake floor in
thin
turbidity
currents. An association
of coarse
on
lahars in marginal
the slope (in
subaqueous
58
Fig. 33: The southwestern crater wall of the Ukinrek East Maar (cf. Fig. 17). The near-vertical crater wall, consisting predominantly of unwelded ignimbrites from Mt. Peulik, is strongly eroded in the form of grooves. debris slope beds
and thin turbidite
casts, etc.) interbedded
beds
(with reverse and normal
grading,
with mud- and siltstones deposited on a former maar lake,
is exposed in the Orapa kimberlite mine/Botswana (Lorenz,
1985).
Photographs from the East Maar taken during its eruptive activity in March (Kienle
et
al.,
load
1980: Fig.
3) show
a syn-eruptive
talus
slope
within
the
1977 crater
already on the second day of the existence of the maar. In 1981 about one third of the crater lake shore is on talus slopes. The talus fans having been formed by the accumulation
of
debris
on
pyroclastic
a
subaqueous
contains
reworked
material
collapsed from the Quaternary
debris
from
debris
fan.
the crater
sediments
The rim
subaerial
beds
and,
debris
fan
additionally,
exposed in the crater
wall.
talus is bedded and has a dip of about 320 . Below the lake level the debris
The slope
continues with a dip of 220 . In places where no subaerial talus fan is present at the lake shore, the subaqueous dip is up to 42 ~ On a stormy day (August 26, 1991) the waves of the lake undercut the subaerial slopes
for about 2 m , causing, within
one
day, a removal of more
than 3 0 0 m $ o f
69
Fig. 34: Northeastern crater wall of the Ukinrek East Maar showing unwelded ignimbrites of Mt. Peulik in the very steep lower wall, overlying tundra soil and up to 22.7 m of well-bedded ejecta. The irregular lower crater wall and a number of scars in the tephra walls show that during the growth of the maar slumping was active. On the right Side of the photograph a debris fan formed already very early in post-eruptive time, but still accumulates debris. At the lake level the debris slope is undercut by wave action due to a very stormy day (with eolian erosion of the ejecta) and, consequently, the subaqueous debris slope must have grown and possibly caused turbidite flow moving forward to the almost horizontal lake floor. debris
along
the
total
length
of talus
slopes
(about
300m
in
1981)
and
its
subsequent deposition on the subaqueous slope and the flat floor (Fig. 34). At the same tephra
time rim
the
storm
into
the
transported lake.
a lot of unconsolidated
dry
tephra
These tephra-loaded clouds also erode
wall
from
the
rocks by
sandblasting. The journal "Geology", Vol. 20, No. 3, March 1992, has a cover photograph taken by Cathryn R. Newton, Syracuse University. The date of taking the photo is not given (probably winter
1991/92). The immensly grown subaerial
talus
fans, project out
of the lake in many places, in spite of the water level being
10 m higher than
before. Some day, not too far away, the talus fans will surround all of the lake. Finally, by continued growth, they will move forward to the centre of the lake and interfinger. In the end they will reach up to the tephra rim, fill up the lake and take up the whole maar. The ring wall will be eroded and ultimately disappear, and the maar crater will only be visible as a flat depression.
60 ~ C E S DeMets, C., Gordon, R.G., Argus, D.F. & Stein, S. (1990): Current plate motions. Geophysics J. Int., 101: 425-478. Dreibus, G.; Graup, G.; Lorenz, V. & Wltnke, H. (1986): Int. Volcanol. Congr., Hamilton, New Zealand, abstracts, p. 147. Estabrook, C.H. & Jacob, K.H. (1991): Stress indicators in Alaska. In: Slemmons, D.B., Engdahl, E.R., Zoback, M.D. & Blackwell, D.D. (eds.), Neotectonics of North America, 387-399, Geol. Soc.; Boulder, Colorado. Fisher, R.V. & Schmincke, H.-U., (1984): Pyroclastic rocks. 472 p., Springer, Berlin. Kienle, J., Kyle, P.R., Self, S., Motyka, R.J. & Lorenz, J. (1980): Ukinrek Maars, Alaska, I. April 1977 eruption sequence, petrology and tectonic setting. J. Volcanol. Geotherm. Res., 7: 11-37. Latter, R.C.L. & Allison, I. (1983): An inexpensive device for modelling strike-slip and oblique-slip fault zones. J. Geol. Educ., 31: 200-205. Lorenz, V. (1973): On the formation of maars. Bull. Volcanol., 37: 183-204. Lorenz, V. (1974): Vesiculated turfs and associated features. Sedimentology, 21: 273291. Lorenz, V. (1985): Maars and diatremes of phreatomagmatic origin, a review. Trans. Geol. Soc. S. Afr., 88: 459-470. Lorenz, V. (1986): On the growth of maars and diatremes and its relevance to the formation of tuff rings. Bull. volcanol., 48: 265-274. Lorenz, V. & Zimanowski, B. (1983): Fragmentation of alkali-basaltic magmas and wall-rocks by explosive volcanism. In: Kornprobst, J. (ed.), Kimberlites. III: Documents, 15-25, Ann. Sci. Univ., Clermont-Fd. Minster, J.B. & Jordan, T.H. (1978): Present-day plate motions. J. Geophys. Res., 83: 5331-5354. Moore, J.G., (1967): Base surge in recent volcanic eruption. Bull. Volcanol., 30: 337363. Nakamura, K. & Uyeda, S. (1980): Stress gradient in arc-back regions and plate subduction. J. Geophys. Res., 85: 6419-6428. Rosi, M. (1992): A model for the formation of vesiculated tuffs by the coalesce of accretionary lapilli. Bull. volcanol., 54: 429-435. Schumacher, R. & Schmincke, H.-U. (1991): Internal structure and occurrence of accretionary lapilli - a case study at Laacher See Volcano. Bull. volcanol., 53: 612-634. Self, S., Kienle, J. & Huot, J.-P. (1980): Ukinrek Maars, Alaska, II. Deposits and formation of the 1977 craters. J. Volcanol. Geotherm. Res., 7: 39-65. Stone, D.B. & Wallace, K.W. (1987): A Geological framwork of Alaska. Episodes, 10: 283-289. Walker, G.P.L., (1983): Ignimbrites types and ignimbrite problems. J. Volcanol. Geotherm. Res., 17: 65-68. Wasburn, A.L. (1979): Geocryology. A survey of periglacial processes and environments. 406 p., Edward Arnold; London.
MAARS AND MAAR LAKES OF THE WESTEIFEL VOLCANIC FIELD Jfrg F.W. Negendank* & Bernd Zolitschka** *GeoForschungsZentrum, Telegrafenberg A26, O-1561 Potsdam **Fachbereich VI/Geologie, Universit~t Trier, D-5500 Trier
ABSTRACT This overview gives a "state of the art" of the scientific knowledge concerning the Eifel maar lake sediments. The high resolution deposits, partly even with an annual resolution, provide detailed information on past global changes: climatic, volcanic and human influences are the reason for variations of the mode of sedimentation and for the trophic state of the lakes controlling autochthonous biogenic productivity. Palaeomagnetic investigations make available data on the behaviour of the geomagnetic field and, additionally, serve as a source of palaeoclimatic proxy-data. Time sequence analyses reveal climate as dominating factor controlling sedimentation of the annually laminated sediments due to sun spot cycles and other astronomic periodicities.
INTRODUCTION The investigation of Eifel maar lakes started 14 years ago at the sites of the Eocene Eckfelder Maar (NEGENDANK et al. 1982) and the Late-Quaternary Meerfelder Maar (HANSEN et al. 1980). First results, especially from the sediments of the young maar lake, were promising and lead to the development of a modification of the Livingstone piston corer (USINGER 1991). This "Usinger corer" enabled to recover 15.5 m of lacustrine sediments from 18 m of water depth (IRION & NEGENDANK t984). Lateron, further improvement
allowed
to obtain
45.6
m
of sediments
from
Meerfelder
Maar
(NEGENDANK et al. 1990) and even 52 m of sediments from Lago Grande di Monticchio (Italy) at 6 m water depth (ZOLITSCHKA & NEGENDANK, this vol.). Using this coring technique, Holocene to late Weichselian sediments have been recovered from the lakes of Meerfelder Maar, Holzmaar, Schalkenmehrener Maar, Gemtindener Maar and Weinfelder Maar giving evidence of climatic changes, volcanic activities and anthropogenic perturbations during the last 13,000 to 25,000 years. Thus data on past
Lecture Netes in Earth Sciences, VoL 4,9 J. F. W. Negendank, B. Zolit~chka (Eds.) Paleolimnology of European Maar Lakes 9 Springer-Verlag Berlin Heidelberg 1993
62 global changes, even with an annual resolution, were already available when the first ideas of an "International Geosphere-Biosphere Programme: A Study of Global Change" (IGBP) were put in more concrete terms during the end of the 1980's. Since the beginning investigations of Eifel maar lake sediments were studied with an interdisciplinary approach focussing on sedimentology, valve chronology, palaeomagnetism and palynology, but also including many other fields of Quaternary research and palaeolimnology.
Fig. i: Distribution of eruptive centers of the Quaternary West and ,East Eifel volcanic fields, the recent crustal movements in this area and the 81 axis from in situ stress measurements parallel to the main alignment of the Westeifel volcanic field (according to FUCHS 1983).
63
GEOLOGY In the 19th century first geological investigations have been carried out in the Eifel area (STEININGER 1821, 1853; HUMBOLDT 1858) leading nowadays to a sophisticated knowledge of phreatomagmatism. The internationally accepted geological term "maar" is a traditional name out of this region, probably deriving from the latin word "mare", which means sea or ocean. The Westeifel volcanic field is aligned NW-SE from Ormont to Bad Bertrich and located in the western part of the Rhenish Massif acting as a hinge between shear rifting along the Rhine Graben and extensional rifting at the Lower Rhine Embayment due to the NW-SE compressional regime in this part of central Europe. Therefore, the Rhenish Massif was uplifted during Oligocene times and from Miocene times until today accompanied by especially Pleistocene volcanic activities in the eastern and western part of the Eifel (Fig. 1). One famous cataclysmic event has been the Laacher See eruption within the East Eifel volcanic field (11,000 years BP). Its tephra was distributed over the whole continental central and northern Europe providing a usefull stratigraphic marker in lake and bog sediments, loess deposits and soils. Volcanism started in the Westeifel volcanic field ca. 700 ka ago producing 250 eruptive centers with more than 50 maars, of which 8 are occupied by lakes (B~ICHEL 1984). The ages of most of these maar eruptions are still unknown. 27 maars and 8 maar lakes are concentrated in a small area surrounding the towns of Daun and Manderscheid in the southern part of the Westeifel volcanic field (Fig. 2).
TERTIARY ECKFELDER MAAR The dry maar of Eckfeld (Fig. 2) is the oldest maar investigated so far in the Eifel area (NEGENDANK et al. 1982; NEGENDANK 1983; LUTZ 1991). It was previously dated by pollen analysis (Borkener Pollenspektrum, PFLUG 1959) to middle Eocene age (Lutetium). When remains of a horse (Propaleotherium) have been discovered (TOBIEN 1990), mammal-stratigraphy slightly changed this age estimation to 49 Ma (middle Eocene, Geiseltalium/Lutetium). The Eckfelder Maar is of the same age as the famous German sites of Messel near Darmstadt and Geiseltal near Halle. The oil shales of Eckfeld are wellknown since the beginning of the t9th century (WEBER 1853). Scientific investigations started again more than 100 years later. VON DER BRELIE et al. (1969) dated these sediments to an Eocene age using palynology. In 1980 a 65.5 m
64
Fig. 2: Dry maars (numbered) and maar lakes of the southern part of the Westeifel volcanic field.
65 long sediment sequence was recovered (NEGENDANK et al. 1982) displaying lacustrine deposits (Fig. 3). This profile is divided into 4 subsequences: 0.0 - 9.5 m: clay and silt laminations; 9.5 - 16.0 m: transition zone with less bituminous silts; 16.0 - 50.5 m: laminated lacustrine sediments (oil shales) with diatoms, diatornites and bituminous material; 50.5 - 65.5 m: horizontally bedded reworked pyroclastics with vesicular basalt lapilli and fractionated Devonian rocks.
Fig. 3: Lithology, organic carbon and stable isotope composition of siderite of the sediments from Eckfelder Maar (NEGENDANK et al. 1982; BAHRIG 1989).
66 The stable isotope composition of siderite indicates an anoxic sediment/water interface and strong methanogenesis (Fig. 3). While anoxic conditions extended into the deep water, sapropel formation established (BAHRIG 1989). Anoxic bottom water conditions also favoured the formation of annually laminated (varved) sediments. Examinations of some thin sections proved an organic, diatom dominated varve type from a depth of 43.25 m, typical for eutrophic maar lakes (ZOLITSCHKA, this vol.). The more important scientific results from this site were obtained by excavations for fossils since 1987 (FRANKENHAUSER & WILDE, this vol.; LUTZ 1991, and this vot.; WILDE & FRANKENHAUSER, this vol.). 11,000 leaves, fruits and seeds, 150 flowers, more than 1600 insects, 600 fish, many snails and crayfish, but also frogs, crocodiles and bats as well as mammals were excavated.
LATE-QUATERNARY MAARS Summarized information on location, size, structure, water depth, limnological parameters, seismic structure, recovered types of sediments, number of cores and tephrochronological markers are listed in Tab. 1, Figs. 2 and 4.
Mosenberg-Meerfelder Maar-Complex The volcanic system west of Manderscheid seems to consist of 6 centers of eruption localized on a NW-SE oriented fault. The 5 southernmost vents build up alkali-basaltic cones. The probably youngest and northernmost vent is the largest crater of the Eifel: Meerfelder Maar. The oldest one is formed by several small scotia cones. The eruptive sequence continued with the southern Mosenberg crater discharging a lava flow into the smatl Horngraben valley, which thus was blocked. While estimating the rate o f erosion for the Horngraben cutting through this lava flow, BUCHEL & LORENZ (1984) calculated the age of the eruption to approximately 30,000 to 50,000 years. A piece of schist from this lavaflow was dated to 42 +/- 3 ka using thermoluminescence dating (ZOLLER 1991). Another scotia cone builds up the summit of the Mosenberg (517 m above sea level). Number 4 of the eruptive sequence was the Windsborn crater with walls consisting of scotia, tuff and lava fragments. The smaI1 crater lake was drained by several meters in t840 for digging peat, as well as the adjacent Hinkelsmaar, the second youngest crater of the Mosenberg-complex, which today has a dry crater bottom.
67
Tab. 1: Summarized morphological and limnological parameters of investigated Eifel maar lakes and recovered sediments.
Meerfelder Maar
Lake location
elevation (m a.s.I.) max. depth water surface (1000 m 2)
catchment area
Holzmaar
Pulvermaar
Weinfelder Schalkenm. GemQnMaar Maar dener Maar 6050 ' E 50~ 11' N
6~ E 50 ~ 1G N
6045 ' E 5oo6' N
6053 E 5007 ` N
6o55 ' E 50~ 6' N
336.5
426.1
411.2
18
20
70
39
52
21
246
58
335
75
169
21g
5760
2000
810
430
190
10&.0
406.6
6~ 50 ~
E N
420.5
484.0
(1000 m s) 2.6. 1987
date
3.6.1987
s~ng
secchi depth (rn) ph conductivity (rn S/m) * Ca, Mg (~ N O 3 - N (pg/l) N O 2 - N (p.g/I) N H 4 - N (pg/l) total P ( g / I ) chlorophyll a (pg/I) 0 - 10rn 0- 20rn trophio seismic
spring
spdng
29. 5. 1987
1975/83
1982J84
1979/83 2.5
3.2
,5.5
7.0
2.3
80
7,7
8.D
7.5
7.7
9.5 1,6
4.6
27.5 5.5 42O
1200-5,300
140
3.7
0.7 100-200
6 5 -10
73
18 -41
6 7(1979)
0-300
0
1
'180
i
70
0-100 10 - 17
0-350
20
36.44
6(1979}
t 3.6
19.9 3.1
state
e~,~c
structure
-
-
L
e,,~-,h~=
3.0
0,4
3.8
2.8 ol~ot roppic
o~igotrophic
oligotropI'~c
eutrophic
investigated
recovered sediments (m)
46
32
number of
75
18
cores
Holocene j sedimentation rate (ram/a, siderite varves organic varves Pleistocene turbidites dropstones clay-/silt- laminites Tephrochronological markers (thickness in rnm) Ulmener Maar - Tephra 10.020 v.yBP Laacher See - Tephra 11.300 v.y.BP Basaltic Ash Tuff ca. 25 Ka BP
0,60 - 1.16
0 49 - 1.74
p~-~y
~Jy
yes
yes
yes yes yes
yes yes yes
0.2
1.5
027 - 1.26
.oa~,~ no
0.33- 0.*,~
0.23 - 0.75
partly 13o
partly parW
yes yes yes
t-
coarse fife
c~arse fine
6O
T6
5CO
coarse [ire 67
fine coarse 100
co~'$e ~ne 67
68
Fig. 4: Size, water depth and structure of investigated Eifel maar lakes.
69 ERLENKEUSER et al. (1972) used a 10 cm thick volcanic ash layer from Hinkelsmaar sediments - they related it to the eruption of the Meerfelder Maar - to determine the age of the nearby crater. This connection was apparently not true, because Meerfelder Maar tephra is up to 1 m thick in the surroundings of Hinkelsmaar. Thus it seems unlikely, that only 10 cm of tephra deposited in Hinkelsmaar itself. JUVIGNt~ et al. (1988) connected the same volcanic ash layer to the eruption of the Laacher See matching to their age determination of 11,000 years BP. They also dated the onset of clastic sedimentation within Hinkelsmaar to 28,400 years BP and thus obtained a minimum age for this eruption. The base of lacustrine sedimentation within the youngest feature of the MosenbergMeerfelder Maar-complex has not been reached yet. Therefore dating is restricted to the tephra deposits at the location of Deudesfeld, 1.5 km to the west of the center of the maar. Radiocarbon dating of organic material from below the base o f the Meerfelder Maar tephra obtained a minimum age of 29,000
years BP
(BidCHEL &
LORENZ
1982).
NEGENDANK (1988) suggested, that this eruption might even be older, because the huge explosion left only little tephra in the area (cf. HENK 1984). Sediment cores from Lake Meerfelder Maar, where 45.6 m of lacustrine, laminated sediments
have
been
recovered
(NEGENDANK
1989;
ZOLITSCHKA
1989;
NEGENDANK et al. 1990) contain two marked horizons: I.aacher See Tephra in ca. 8 m sediment depth (NEGENDANK 1984) and a still unknown basaltic ash layer of 50 cm thickness in 38.5 m sediment depth (NEGENDANK 1989). This tuff layer might be an evidence for the eruption of Hinkelsmaar. But looking at the silt dominated grain size distribution it is more likely, that the basaltic ash layer derived from a more distant source like from one eruption of the Dauner Maar Group or from Pulvermaar (NEGENDANK 1989). Additionally, a thin and fine-grained tephra layer from the eruption of Ulmener Maar was found in Meerfelder Maar sediments as well as in Holzmaar, Gemfindener Maa.r, Weinfelder Maar and Schalkenmehrener Maar (ZOLITSCHKA et al. 1991; ZOLITSCHKA & NEGENDANK in prep.; LOTI'ERMOSER et al., this vol.). The uppermost 20 m of sediments from Lake Meeffelder Maar have been examined in detail by sedimentologcial, mineralogical, geochemical, palynological, paleomagnetic, paleobotanical and paleozoological methods (summarized in: IRION & NEGENDANK 1984 and NEGENDANK et at. 1985). Since these publications a lot of new investigations have been done on Holocene and Late Glacial annually laminated organic sediments. After a first seismic survey of the sediments within the maar basin (SCHLOTER 1987) new cores down to a sediment depth of 45.6 m
70 have been recovered and investigated (Tab. 1). MicrostratigraphJc investigation of these sediments ended up with the ftrst varve chronology for the Holocene and the Late-Glaciat in central
Europe
(ZOLITSCHKA
1986,
1987,
1988).
Since
that
early
stage
microstratigraphicat and varve chronological results have been refined (ZOLITSCHKA 1990; POTH & NEGENDANK, this vol.). Detailed sedimentological investigations revealed interesting results concerning mineralogy of the sediments as well as heavy minerat contents and magnetic carriers (NEGENDANK 1989; NEGENDANK et at. 1991), turbidites (DROHMANN 1991; DROHMANN & NEGENDANK, this vol.) and
basin modelling for selected time windows based on
sedimentation rates from a suite of cores (WEGNER 1992; WEGNER & NEGENDANK, this vol.). Thermoluminescense dating provided a rough age estimation for the basaltic ash layer at 38.5 m sediment depth of 25 +/- 2.5 ka (VELDE 1988). This seems to be consistent with palaeomagnetic data (HAVERKAMP this vol.) but not with counting of clay/silt laminations the assumption, that these laminations are true Palaeomagnetic investigations (HAVERKAMP 1991;
1991; HAVERKAMP & BEUKER, yielding an age of only 17.5 ka with varves (BRAUER, unpubl, data). BEUKER 1991; HAVERKAMP &
BEUKER, this vol.) also allowed to establish absolute dated palaeosecular variation curves for the last 13,000 years. Comparison with other European records confirmed the results of varve chronology for the younger part of the sedimentary sequence. Major elements and
susceptibility were determined and related
to climatic
and
anthropogenic influences (THOUVENY 1989). Variations in the carbon isotope composition of organic matter were measured and explained with environmental changes of the past (BROWN et al. 1991). Palaeobiological studies on cladoceran and chironomids (HOFMANN 1990), pigments (MOLLER 1985; M(~LLER & SCHARF 1986, SCHARF & EHLSCHEID 1992), ostracods (SCHARF 1988) and pollen (USINGER 1982, 1984) provided new ideas about the floral and faunal development during the late Weichselian and the Holocene. Holzmaar
Comparable results have been obtained from lacustrine sediments of lake Holzmaar, 7 km to the NE of Lake Meerfelder Maar (Figs. 2 and 4) (ZOLITSCHKA 1989, 1990). The laminated sediments of this site developed into one of the best studied Late-Quaternary sediment sequences. As a whole 18 cores up to a sediment depth of 32 m have been recovered (Tab. 1). A standard profile was given in NEGENDANK et at. (1990). Microstratigraphicat investigations proved the annual character of these finely laminated
71 deposits using sedimentotogical, diatomological and palynological methods (ZOLITSCHKA t989, 1990, 1991). Palaeomagnetic studies reconstructed the magnetic field of the earth during the last 13,000 years (HAVERKAMP 1991; HAVERKAMP & BEUKER, this vol.). High resolution palynological investigations resulted in a detailed history of vegetation and plant succession (USINGER & WOLF 1991 and unpubl, data). All these information enable to reconstruct palaeoclimatic changes (ZOL1TSCHKA 1992a; ZOLITSCHKA et al. 1992) and changes due to influences of human woodland clearance and settlements in the catchment area of Lake Holzmaar (ZOLITSCHKA 1992b). Using all of these information it was possible to establish the first varve year calendar for the last 13,000 years of central Europe (Fig. 5). For palaeoclimatic interpretation variations in thickness and composition of varves were related to glacier advances and retreats in the Alps according to their absolute age (Fig. 6) (ZOLITSCHKA 1990, 1992a). Until 8800 varve years BP glacier advances coincide with sedimentation rate minima, prior to 8800 varve years BP they coincide with sedimentaiton rate maxima. A periodicity of roughly 1000 years has been realized concerning suggested "cold/warm" cycles during the early and middle Holocene. The period of 1000 years is close to a cyclicity, examined in North American plant successions by OVERPECK (1987). This coincidence points to a global mechanism controlling palaeoclimate. A sequence of 512 varve thickness measurements has been subjected to spectral analysis, resulting in the preliminary proof of the l 1-year sun-spot cycle (ZOLITSCHKA 1990, 1992a; SONETF, pers. comm.; VOSS & DREPPER, pers. comm.). This is another evidence for astronomical modulation of climate on earth controlling sedimentation, e.g. controlling autochthonous lacustrine productivity and/or surface runoff with related variations in nutrient flux rates. Sedimentation rates and the contents of charcoal fragments provided a detailed idea of human activities in this region (Fig. 7). Four neolithic periods of colonization (Bandkeramik, Rbssen, Michelsberg, Schnurkeramik) and the following periods of Urnenfelderzeit, Latbne, Roman Empire and the Middle Ages were recognized by comparison of the varve dated sedimentary record with dendrochronologically calibrated radiocarbon dates of the archaeological record (ZOLITSCHKA 1990, 1992b). Two earlier occurrances of charcoal may be related to mesolithic and palaeolithic cultures, but also natural fires have to be considered as source for charcoal during that time. Detailed microstratigraphic studies revealed insights into the Younger Dryas climatic event (ZOL1TSCHKA
et
al.
1992).
Geochemical
studies
distinguished
allochthonous
volcanogenic and anthropogenic influences from autochthonous deposition (WARNECKE
72
~f
O
0
o
L~
73
Fig. 6: Valve dated sedimentation rates from Lake Holzmaar with indicated Late-Glacial and Holocene climatic fluctuations (ZOLITSCHKA 1992a).
74
Fig. 7: Varve dated sedimentation rates from lake Holzmaar with indicated periods of intensified human activities (ZOLITSCHKA 1992b).
75 1991; LOTTERMOSER et al., this vol.). The mineralogy of some sediments of the sequence was determined by NEGENDANK (1989). Palaeobiological investigations were carried out on ostracods (SCHARF, in press), on pigments (MOLLER 1985; MI21LLER & SCHARF 1986), and on cladoceran and diptera (HOFMANN, this vol.). The Glacial (Weichselian) record is currently investigated in detail to understand the processes of sedimentation for silt/clay laminations as well as for different turbidites to prove the annuallity of these laminations. Surprisingly, detailed thickness measurements of ca. 1000 laminations revealed wellknown cyclicities which may be related to sun spot activities or other astronomic periodicities (BRAUER, pers. comm.).
Dauner Maar Group (Gemandener Maar, Weinfelder Maar, Schalkenmehrener Maar) The Dauner Maar Group is a young NW-SE oriented system of three water-filled maars (Schalkenmehrener Maar/West, Gemiindener Maar, Weinfelder Maar), of two dry maars (Schalkenmehrener Maa.r/North-East and South-East) and of one scoria cone (located in Schalkenmehrener Maar/North-East). Bf0CHEL & KRAWCZYK (1986) characterized the Schalkenmehrener Maar as trilobate, which today is believed to be quadrilobate. The magmatic components of all eruptions are similar and consist of SiO2-undersaturated alkalibasalt (foidite) according to MERTES (1983). The eruption caused lapilli tephra with a high amount (> 75%) of lithic fragments (ZIMANOWSKI 1985). Transport by base surges is assumed by presence of antidunes and bombs without impact crater (B'0CHEL & KRAWZCYK 1986). Airfall deposits are subordinated. Alltogether a phreatomagmatic eruption is very likely (LORENZ 1973, 1984, MEYER 1985). For a long time age determinations of these maars relied on palynological investigations from bog and lake sediments. Accordingly, the volcanic activities of the Dauner Maar Group were dated to about 10,500 to ll,000 years BP (STRAKA 1975). BOCHEL & KRAWCZYK (1986) think of a minimum age of 20,000 to 30,000 years. They suggested the following sequence of eruptions: First was the scoria cone and the northeastern Schalkenmehrener Maar. Volcanism continued with southeastern Schalkenmehrener Maar, western Schalkenmehrener Maar and Gemtindener Maar. Weinfelder Maar was the final activity of the Dauner Maar Group. Weinfelder Maar is a N-S oriented double crater with a small volcanic cone on the western side, located where both circles intersect each other. Standart sediment profiles from lakes Schalkemnehrener Maar/West, Weinfelder Maar and Gem[indener Maar were published by NEGENDANK (1989) and NEGENDANK et al. (1990). They all show the ash layer of the Laacher See eruption in 6 to 8 m sediment depth
76 measuring up to 10 cm in thickness. In Lake Weinfelder Maar the Laacher See Tephra was dated to 11,000 years by estimation of sedimentation rates (BRAUER 1988; NEGENDANK et al. 1990). Similar ages were obtained from sediments of lake Schalkenmehrener Maar/West (HE/NZ 1991; HEINZ et al., this vol.) and Lake Gem/indener Maar (ZOLITSCHKA 1990). Therefore the volcanic system is older than previously suggested by STRAKA (1975). Using aerial photographs and profiles made by echo sounder, BRAUER (1988) confirmed the existence of lake level fluctuations. Probably they were caused climatically, occurring not only in lake Weinfelder Maar and Lake Gemfindener Maar, but also in Lake Pulvermaar and Lake Holzmaar. Two marked lake levels were recognized: a lower one (12 m, relative to the present lake level), representing the arid high-Glacial, and a higher lake level (+ 3 m), formed during the most humid Postglacial of the Atlantic (BRAUER 1988; NEGENDANK et al. 1990). Because of the lower, probably high-Glacial, lake level it is very likely, that these lakes and consequently the maars are older than ca. 18 ka. This is in agreement with the age estimation of BOCHEL & KRAWCZYK (1986). The sedimentary sequence recovered from Schalkenmehrener Maar allows to establish a relative varve chronology and gives palaeoclimatic information based on flux rates of organic carbon (REIN 1991; REIN & NEGENDANK, this vol.; HEINZ 1991; HEINZ et al., this vol.). The unique siderite laminations from Lake Weinfelder Maar and Lake Gemfindener Maar give evidence of natural eutrophication during the Holocene climatic optimum (BRAUER 1988; BRAUER & NEGENDANK, this vol.; ZOLITSCHKA 1990). These results are corroborated by faunal remains (HOFMANN, this vol.). Trophic variations are also determined by the contentof photosynthetic pigments (SCHARF & EHLSCHEID 1992).
CONCLUSIONS Further scientific activities will focus on the study of varves to obtain information on factors controlling varve formation under present day conditions as well as under subarctic 0ike during the glacial periods of the Pleistocene) and subtropic conditions (like during the Eocene of Eckfeld). Astronomic forcing of sedimentation within these depositional environments is very likely but still has to be proved. Studies in progress are microstratigraphical and palaeomagnetic investigations of a suite of cores from the lakes of Meerfelder Maar, Holzmaar and Schalkenmehrener Maar covering the Holocene and the Late-Glacial to assess the variation of the palaeorecords within each
77
lake. A further step will be to establish an interlake correlation and develop a varve chronologicaliy controlled and statistically supported stratigraphic master scale for the last 13,000 years of the Eifel. Hopefully, the method of time series analysis will allow to proove the annual nature of laminations from the sterile clastic sediments of the Weichselian, thus providing a tool for calibration of the various proxy-records. For a time period without any reliable age determinations this would be a major step forward towards a better understanding of depositional environments and their palaeoclimatic forcing.
ACKNOWLEDGEMENTS Research programs financed by the Deutsche Forschungsgemeinschaft (DFG-Ne 154/131/4, 154/21-1/2, 154/22-1, 154/24-1) have been the nucleus for initiating the cooperative projects "Geomaar" and "Euromaar" of the European Communities Science Programme. We appreciate their financial support. All of these projects would not have been successfull without the help of colleagues, staff and many students. Thanks to all of them.
REFERENCES Bahrig, B. (1989): Stable isotope composition of siderite as an indicator of the paleoenvironmental history of oil shale lakes. Palaeogeogr., Palaeoclimatol., Palaeoecol., 70: 139-151. Beuker, T. (1991): Gesteinsmagnetische Untersuchungen an Seesedimenten des Meerfelder Maares. Diploma-Thesis, Univ. M~inster, 107 pp. Brauer, A. (1988): Versuch einer Erfassung alter Seespiegelstande an ausgesuchten Eifelmaaren und mikrostratigraphische Untersuchungen an Sedimenten des Weinfelder Maares. Diploma-Thesis, Univ. Trier, 117 pp. Brown, H.A., Eakin, P.A., Fallick, A.E. & Creel K.M. (1991): Variations in the carbon isotopic composition of organic matter in lacustrine sediments of Meerfelder Maar. Paleolimnology of maar lakes: Abstracts & Excursion Guide, Zolitschka, B. & Negendank, J.F.W. (eds): 18-20. Btichel, G. (1984): Die Maare im Vulkanfeld der Westeifel, ihr geophysikalischer Nachweis, ihr Alter und ihre Beziehung zur Tektonik tier Erdkruste. PhD-Thesis, Univ. Mainz, 385 pp. Btichel, G. & Lorenz, V. (1982): Zum Alter des Maarvulkanismus der Westeifel. N. Jb. Geol. PaI~iont. Abh., 163: 1-22. Biiehel, G. & Lorenz, V. (1984): Zum Alter des Meerfelder Maares. Cour. Forsch.-Inst. Senckenberg, 65: 13-15.
78 B/ichel, G. & Krawczyk, E. (1986): Zur Genese der Dauner Maare im Vulkanfeld der Westeifel. Mainzer Geowiss. Mitt., 15: 219-238. Drohmann, D. (1991): Sedimentologische Untersuchungen an sphtglazialen Turbiditen des Meerfelder Maares (Westeifel/Bundesrepublik Deutschland). Diploma-Thesis, Univ. Trier, 113 pp. Erlenkeuser, H., Frechen, J., Straka, H. & Willkomm, H. (1972): Das Alter einiger Eifelmaare nach neuen petrologischen, pollenanalytischen und RadiokarbonUntersuchungen. Dechenania, 125: 113-129. Fuchs, K. (1983): Plateau uplift: the Rhenish Shield; a case study. 411 pp; Berlin. Hansen, R., Irion, G. & Negendank, J.F.W. (1980): Geochemische und sedimentologische Untersuchungen an Sedimentkernen aus dem Meerfelder Maar. Senckenbergiana maritima, 12: 269-280. Haverkamp, B. (1991): Pal~omagnetische Untersuchungen an sp~itquart~ren Maarseesedimenten zur Pal~ios,~tkularvariation im Gebiet der Westeifel w~i.hrend der letzten 20-25.000 Jahre. PhD-Thesis, Univ. Mfinster, 235 pp. Heinz, T. (1991): Pal~iolimnologische und spektralanalytische Untersuchungen an jahreszeitlich geschichteten Sedimenten des Sehalkenmehrener Maares/West. Diploma-Thesis, Univ. Trier, 107 pp. Henk, A. (1984): Zur Geologie und Geophysik des Meerfelder Maares und seiner Umgebung/Westeifel. Diploma-Thesis, Univ. Mainz, 153 pp. Hofmann, W. (1990): Weichselian chironomid and cladoceran assemblages from maar lakes. Hydrobiol., 214:207-211. Humboldt, A. (1958): Kosmos - Entwurf einer physischen Weltbeschreibung, Vol. 4; Stuttgart. Irion, G. & Negendank, J.F.W. (1984): Das Meerfelder Maar. Cour. Forsch.-Inst. Senckenberg, 65: 1-101. Juvignr, E., Boenigk, K., Brunnacker, K., Duchesne, J.C. & Windheuser, H. (1988): Zur Schlotfiillung des Hinkelsmaares (Eifel, Deutschland): Alter und Genese. N. Jb. Geol. Pal~iont. Mh., 9: 544-562. Lorenz, V. (1973): On the formation of maars. Bull. volc., 37: 183-204. Lorenz, V. (1984): Zur Geologie des Meerfelder Maares. Cour. Forsch.-Inst. Senckenberg, 65: 5-15. Lutz, H. (1991): Fossilfundstelle Eckfelder Maar, 51 pp; Mainz. Mertes, H. (1983): Aufbau und Genese des Westeifeler Vulkanfeldes. Bochumer geol. geotechn. Arb., 9:415 pp. Meyer, W. (1985): Zur Entstehung der Maare der Eifel. Z. Dt. Geol. Ges., 136: 141-155. Mrller, W. (1985): Der Chlorophyll-Gehalt im Sediment versch~edener Eifelmaare. Diploma-Thesis, FH Bad Kreuznach, 123 pp. Mrller, W. & Scharf, B. (1986): The content of chlorophyll in the sediment of the volcanic maar lakes in the Eifel region (Germany) as an indicator for eutrophication. Hydrobiol., 143: 327-329. Negendank, J.F.W. (1983): Trier und Umgebung. Sammlung Geol. Ffihrer, 60:195 pp. Negendank, J.F.W. (1984): Die Untersuchung der Schwerminerale der Seesedimente des Meerfelder Maares und des "Laacher Bims-Tuffes" in den Sedimenten des Meerfelder Maares, des Hinkelsmaares und der Hitsche. Cour. Forsch.-Inst. Senckenberg, 65: 41-47. Negendank, J.F.W. (1988): Zur Geologie der Umgebuug von Manderscheid. Schriftenr. Die Schrne Eifel, Ausgabe: Vulkaneifel um Manderscheid: 13-37; Trier. Negendank, J.F.W. (1989): Pleistoz~ne und Holoz~ne Maarseesedimente der Eifel. Z. Dt. Geol. Ges., 140: 13-24. Negendank, J.F.W., Irion, G. & Linden, J. (1982): Ein eoz~es Maar bei Eckfeld nordrstlich Manderscheid (SW-Eifel). Mainzer Geowiss. Mitt., 11: 157-172. Negendank, J.F.W., B/ichel, G., Hansen, R.B., Hofmann, W., Irion, G., Haverkamp, B., Lorenz, V., Scharf, B., Sonne, V., Usinger, H. & Weiler, H. (1985): The Meerfeld Maar deposits. Z. f. Gletscherkunde u. Glazialgeol., 21: 67-70.
79 Negendank, J.F.W., Brauer, A. & Zolitschka, B. (1990): Die Eifelmaare als erdgeschichtliche Fallen und Quellen zur Rekonstruktion des Pal~ioenvironments. Mainzer Geowiss. Mitt., 19: 235-262. Negendank, J.F.W., Hansen, R.B. & Briickner, H.-P. (1991): Mineralogische, sedimentpetrographische und geochemische Untersuehungen an Sedimenten aus dem Lac du Bouchet. Doc. du C.E.R.L.A.T., Mere. 2: 189-205. Overpeck, J.T. (1987): Pollen time series and Holocene climate varability of the Midwest United States. in: Abrupt climatic change, Berger, W.H. & Labeyrie, L.D. (eds): 137-143; Dordrecht. Pflug, H. (1959): Die Deformationsbilder im Tertigx des rheinisch-saxonischen Feldes. Freiberger Forschungs-H., C71, 110 pp; Berlin. Rein, B. (1991): Versuch einer Rekonstruktion des Pal~ioenvironments anhand hochzeitaufl6sender geochemischer und sedimentologischer Untersuchungen an sp/it- und postglazialen Sedimenten des Schalkenmehrener Maarsees (Westeifel/Bundesrepublik Deutschland). Diploma-Thesis, Univ. Trier, 109 pp. Scharf, B.W. (1988): Sp/it- und postglaziale Muschelkrebse (Crustacea, Ostracoda) aus Maarseen der Eifel. Nachr. Dt. Geol. Ges., H. 39: 81. Scharf, B.W. (in press): Ostracoda from eutrophic and oligotrophic maar lakes of the Eifel region (Germany) in Late- and Postglacial periods. Proc. Syrup. Warrnambool, Australia 1991. Scharf, B.W. & Ehlscheid, T. (1992): Limnology of Eifel maar lakes (Germany); 3. Summary of paleolimnological investigations with special reference to LateQuaternary trophic variations. Arch. Hydrobiol. Beih. Ergebn. Limnol., (in press). Schliiter, H.U. (1987): Bericht fiber reflexionsseismische Flachwassermessungen im Meerfelder Maar, Eifel. Nieders~chsisches Landesamt f. Bodenforsch., unpubl. report, 15 pp. Steininger, J. (1821): Neue Beiti~ige zur Geschichte der rheinischen Vulkane; Mainz. Steininger, J. (1853): Geognostische Beschreibung der Eifel; Trier. Straka, H. (1975): Die sp~tquart~e Vegetationsgeschichte der Vulkaneifel. Beitr. z. Landespflege in Rheinl.-Pfalz, Beih. 3, 163 pp. Thouveny, U. (1989): Pal~oklimatische Aussagen anhand chemischer Parameter der Sedimentsequenz des Meerfelder Maares. Diploma-Thesis, Univ. Trier, 70 pp. Tobien, H. (1990): Bemerkungen zu zwei S~iugerresten aus der alttertiaren Fossillagerst~tte Eckfeld (Kr. Manderscheid) SW-Eifel, Deutschland. Mainzer Naturwiss. Archiv, 28: 7-21. Usinger, H. (1982): Pollenanalytische Untersuchungen an sp/itglazialen und pr~iborealen Sedimenten aus dem Meerfelder Maar (Eifel). Flora, 172: 373-409. Usinger, H. (1984): Pollenanalytische Untersuchungen zum Alter des Meerfelder Maares und zur Vegetationsentwicklung in der Westeifel wShrend der ausklingenden Eiszeit. Cour. Forsch.-Inst. Senckenberg, 65: 49-66. Usinger, H. (1991): Ein Stechbohrger~it zum Bergen von Torfen und Seesedimenten f/Jr Einsatz bis zu gr6sseren Tiefen. Paleolimnology of maar lakes: Abstracts & Excursion Guide, Zolitschka, B. & Negendank, J.F.W. (eds): 55. Usinger, H. & Wolf, A. (1991): Pollenanalytische Untersuchungen an jahresgeschichteten sp~it- und postglazialen Sedimenten des Holzmaares/Eifel. Paleolimnology of maar lakes: Abstracts & Excursion Guide, Zolitschka, B. & Negendank, J.F.W. (eds): 56. Velde, C. (1988): Thermolumineszenzbestimmungen an Seesedimenten der Eifelmaare. Diploma-Thesis, Univ. Trier, 70 pp. Von der Brelie, G., Quitzow, H.W. & Stadler, G. (1969): Neue Untersuchungen im Altterti~ir von Eckfeld bei Manderscheid (Eifel). Fortschr. Geol. d. Rheinl. u. WestL, 17: 27-40. Warnecke, H. (1991): Geochemische Untersuchungen an Sedimenten des Holzmaares/Westeifel. Diploma-Thesis, Univ. Trier, 88 pp. Weber, C.O. (1853): Uber das Braunkohlenlager bei Eckfeld in der Eifel. Verh. Naturhist. Ver. Rheinl. Westfalen, 10: 409-415.
80 Wegner, F. (1992): Fazielle Entwicklung und Verteilung der Sedimente im Meerfelder Maar (Westeifet/Bundesrepublik Deutschland) - FAn Beitrag zur holoz~en Seegeschichte. Diploma-Thesis, Univ. Trier, 88 pp. Zimanowski, B. (1985): Fragmentatiosprozesse beim explosiven Westeifelvulkanismus. PhD-Thesis, Univ. Mainz, 329 pp. Zrller, L. (1991): Thermoluminescence dating of upper Pleistocene volcanoes. Paleolimnology of maar lakes: Abstracts & Excursion Guide, Zolitschka, B. & Negendank, J.F.W. (eds): 62. Zolitschka, B. (1986): Warvenchronologie des Meerfelder Maares - Licht- und elektronenmikroskopische Untersuchungen spatglazialer und holoz~er Seesedimente. Diploma-Thesis, Univ. Trier, 119 pp. Zolitschka, B. (1987): Jahreszeitlich geschichtete Sedimente aus dem Meerfelder Maar (Westeifel). Heidelberger Geowiss. Abh., 8: 270-272. Zolitschka, B. (1988): Sp/itquartMe Sedimentationsgeschichte des Meerfelder Maares (Westeifel) - Mikrostratigraphie jahreszeitlich geschichteter Seesedimente. Eiszeitalter u. Gegenwart, 38: 87-93. Zolitschka, B. (1989): Ja~eszeitlich geschichtete Seesedimente aus dem Holzmaar und dem Meerfelder Maar (Westeifel). Z. Dt. Geol. Ges., 140: 25-33. Zolitschka, B. (1990): Sp~itquartXre jahreszeitlich geschichtete Seesedimente ausgew/ihlter Eifelmaare. Documenta naturae, 60:226 pp; M/inchen. Zolitschka, B. (1991): Absolute dating of late-Quaternary lacustrine sediments by high resolution varve chronology. Hydrobiol., 214: 59-61. Zolitschka, B. (1992a): Climatic change evidence and lacustrine varves from maar lakes, Germany. Climate Dynamics, 6: 229-232. Zolitschka, B. (1992b): Human history recorded in the annually laminated sediments of Lake Holzmaar, Eifel Mountains, Germany. Geol. Survey of Finland, Spec. Paper, 14: 17-24. Zolitschka, B., Brauer, A., Haverkamp, B., Heinz, T., Negendank, J.F.W. & Poth, D. (1991): Sedimentologischer Nachweis und Datierung einer frfihholozS.nen Maareruption (Ulmener Maar?) in der Vulkaneifel. Paleolimnology of maar lakes: Abstracts & Excursion Guide, Zolitschka, B. & Negendank, I.F.W. (eds): 63. Zolitschka, B., Haverkamp, B. & Negendank, J.F.W. (1992): Younger Dryas oscillation varve dated microstratigraphic, palynological and palaeomagnetic records from Lake Holzmaar, Germany. in: The last deglaciation: Absolute and radiocarbon chronologies, Bard, E. & Broecker, W.S. (eds), NATO ASI Series, Vol. I 2: 80-101. Zolitschka, B. & Negendank, J.F.W. (in prep.): Sedimentologischer Nachweis und absolute Datierung von Deutschlands jiingstem Vulkan, dem Ulmener Maar (Vulkaneifel). Geol. Rs.
MAARS OF NORTHERN AUVERGNE (MASSIF CENTRAL, FRANCE): STATE OF KNOWLEDGE
E. Juvign@+, G. Camus* & A. de Go@r de Herve * + Fends national de la Recherche scientifique, Laboratoire de G@ologie du Quaternaire, 7, Place du XX Am)t, 4000 Li@ge, Belgique *Universit@ Blaise Pascal, Observatoire de Physique du Globe et Centre de Recherches volcanologiques, 5, rue Kessler, 63038 Clermont-Ferrand Cedex, France
ABSTRACT Hydromagmatic ~3rocesses are very common throughout any Tertiary to Quaternary volcanic field of the French Central Massif. Those processes and their correlative products are described. This is followed by monographies of about twenty Maars of northern Auvergne. TL-dating was applied to three of them (Tazenat, Saint-Hippolyte, Clermont). Investigations of lacustrine sediments from another six maars give minimum ages for each relevant eruption (Saint-Hippolyte, Clermont, Ampoix, Espinasse, Chauvet, Godivelle d'En Haut). A few maars were demonstrated to have a well differenciated magma and very widespread tephra sheets. In the glaciated area (Monts-Dore and C@zafier) no tuff-rings were found associated to any of the maars which were identified by morphology. They are supposed to have erupted either throughout the ice cap or prior to its development.
IMPORTANCE OF HYDROMAGMATISM IN THE FRENCH MASSIF CENTRAL The present paper deals only with maars in a very restricted area of northern Auvergne. Most o f them are very recent (160 ka to 6 ka). They form a roughly N-S oriented series, stretching through the Cha~ne des Puys, the Monts-Dore, and the C@zalier (fig. 1). Altogether they represent only a very small part of hydromagmatic volcanoes which are typical for the Tertiary and Quaternary volcanism in the French Massif Central (=FMC). In all, 774 hydromagmatic structures were recognized by Boivin et al. (1984, unpublished): Chai'ne de la Sioule and Sillon Houiller (60), Chafne des Puys (28), Limagne sensu late (148), Forez (44), Monts Dore (36), C@zalier (39), Bassin du
Lecture Notes in Earth Sciences, Vol, 49 1. F. W. Negendank, B. Zolitschka (Eds,) Paleolimnotogy of European Maar Lakes 9 Sprlnger-Verlag Berlin Heidelberg 1993
82
83 Puy, Dev~s and Velay (79), Bas-Vivarais (6), Cantal (190), Aubrac (53), Coirons (25), Causses, Escandorgue and Bas-Langedoc (66). Some more maars were discovered recently. Different hydromagmatic structures and their relationship with eruptive processes Subaquaous type (Surtsey type) and subaerian type (phreatomagmatic) were distinguished by Lorenz (1973) and Camus et aL (1980). The latter type is very frequent in any part of the FMC. The former type occurred only in previous lakes such as: 1) the basin of Puy-en-Velay; 2) the basin of Langeac/HauteLoire; 3) the Saint-Flour Trough (Cantal); 4) the western part of the Aubrac (Loz6re). Phreatomagmatic and magmatic phases alternate very often during an eruption. On the one hand, a basic magma may be responsible for an exclusively magmatic eruption, giving rise to a strombolian cone. On the other hand an exclusively phreatomagmatic eruption forms a Maar. In most of the cases those processes are combined and different phases can alternate. 1. When the magma gets in iouch with vadose water, phreatomagmatic explosions open the way. Exclusive phreatomagmatic activity gives rise to a maar, which may be occupied by a lake after the volcano is extinct (Gour de Tazenat/ChaTne des Puys). Later on, lacustrine sedimentation begins with essentially organic material (diatomite, peat), chemical precipitation (silicates, carbonates), and detrital deposits (delta, slope material).
Figure 1.- Map of the volcanic areas of France [B], and of the volcanoes of northern Auvergne [C]. B. Volcanic areas: 1, ChaTne de la Sioule; 2, ChaTne des Puys; 3, Limagne, Comte, Livadois; 4, Monts-Dore; 5, C~zalier; 6, Cantal; 7, Aubrac; 8, Causses; 9, Escandorgue; 10, Bas-Languedoc; 11, Bourgogne; 12, Forez; 13, Bassin du Puy and Emblav~s; 14, Dev~s; 15, Eastern Velay; 16, Bas-Vivarais; 17, Coirons. C. Names of maars: 1, Tazenat; 2, Chalard; 3, Rochenoire; 4, Beaunit; 5, St. Hippolyte; 6, Volvic; 7, Ladoux; 8, Anchal; 9, Beauloup; 10,.Chanat; 11, Clerzat; 12, Ceyssat; 13, Montchatre; 14, Viltars; 15, Chamali~res-Clermont; 16, Enval; 17, Ampoix; 18, Espinasse; 19,.Servi~res; 20, Beaune-le-Froid; 21 Pavin; 22, Esfivadoux; 23, Chauvet; 24,.La Fage; 25, Cros du Joran; 26, Grand Joran; 27, Blatte; 28, d'En Haut; 29, Chastelets; 30, Graspet.
84 2. Phreatomagmatic activity may be followed by a magmatic phase especially when water supply stops for any geological or geomorphological reason. With basic magmas, the activity turns to: a) effusive phase in case of gas-poor magmas. These give rise to lava lakes (numerous examples in Limagne sensu lato), and sometimes lava flows, b) strombolian phase in case of gaz-rich magmas. These give rise to scoria cones which fills up the crater partly (Beaunit/ChaTne des Puys), or completely (Puy de TartaretJCha~ne des Puys; Mont Burel/Dev~s); the latter one is the Zuni type (Oilier, 1969). With more diffenciated and viscous magmas extrusive forms may appear such as domes (Sarcouy/ChaTne des Puys) or needles (Chopine/ Cha~ne des Puys). In northern Auvergne trachytic magmas started to erupt with phreatomagmatic activity as showed by cauliflower bombs and initial maar shaped craters. Those volcanoes are: Chopine, Grand Sarcouy, Kilian, Vasset, Pavin. Their tephras are widespread (up to several hundred of km). The maar shaped crater is well preserved at Pavin volcano, but in the central Cha~ne des Puys (the other four volcanoes), posterior formation of domes or needles has-burried the maar morphology. Morphological evolution of Maars. Despite the fact that Menat and Malmouche Maars (Puy de Dome Dpt) formed during the Paleocene (about 60 Ma), their morphology and part of their tephras are well preserved. Moreover, diatomite still occurs at the Menat Maar. If the filling material of the crater is more resistant than the terrains, the erosion is more efficient in the latter ones and this gives rise to inversion of relief: 1) columnar basaltic necks (numerous examples in Limagne sensu lato); 2) silicified limestones (Comt~ d'Auvergne in Limagne); 3) diatomites (Montagne d'Andance/Coirons); 4) peperites (see below) including stratified tuffs (Crouel), stratified tuffs and chaotic brecchia (Saint Roch butte/Langeac, Haute Loire), silicified limestones (Buss~ol), diatomite (Puy de Mur); 5) peperite overlain by columnar basaltic lava-lake (Le Chauffour). Role of the basement. In terrains containing several aquifers phreatomagmatic activity is dominant. This is the case in the sedimentary terrains of Limagne where almost all volcanoes are phreatomagmatic as well as in large stratovolcanoes or basaltic plateaus, where porous layers may also contain aquifers (Cantal and Dev~s). On the contrary, cristalline and crystallophyllian terrains have less phreatic horizons and the hydromagmatic activity occurs only in subaerial environments. Therefore only 25% of the volcanoes in the ChaTne des Puys are hydromagmatic. The peperites. In northern Auvergne two regions may be distinguished. In the western part, from the Gour de Tazenat to the La Godivelle area, the volcanic activity is Pleistocene-Holocene (Cha~ne des Puys volcanic phase), whereas in
85 the eastern part (Limagne) it is as ancient as the Lower and Middle Miocene. In Limagne volcanoes have erupted trough sedimentary terrains (marls) and gave rise to peperites (french sense), which were previously considered to have occurred in lacustrine environment. Peperite (french sense) is a rock characterized by the presence of variable amounts of vitric basaltic granules ("peper grain" type) distributed in a fine sedimentary matrix. Studies of peperites have shown, that they correspond to diatremes (pipes filled up with stratified tuff and chaotic breccia). Those peperites are produced by the interaction of basaltic magma and phreatic water within sedimentary terrains. The magma is quenched, pulverized and mixed with adjacent fine sediments. Those products are ejected with aerosols into the atmosphere and deposited into the crater and around the vent (base surge deposits). The presence of accretionary lapilli is a consequence of the eruption of a water- and dust-rich column. During the eruption circular fractures form through the adjacent basement and cylindric to conic blocks as well as the overlying tephra deposits collapse. This forms the diatreme and its stratified peperite. A coarse chaotic breccia occurs within the pipe being active during the collapsing movement. Sometimes, blocks of sedimentary deposits may be squeezed between the pyroclastic products and the undisturbed surrounding sediments (Crouel, Jussat). Peperites are more resistant than the surrounding marls. Therefore they correspond to buttes (see above).
MONOGRAPHIES OF MAARS OF NORTHERN AUVERGNE In the last two decades desparate efforts were undertaken to investigate the maars of that area (fig.l). The following work is a synthesis of the most important results (see exhaustive reference list). The ChaTne des Puys has essentially Late Pleistocene and Holocene volcanoes (Go~r et al., 1991), while the Monts-Dore and the C~zalier are Lower Pliocene to Middle Pleistocene volcanic regions (Cantagrel and Baubron, 1983; Cantagrel et aL, 1987), in which a few younger volcanoes have erupted during the Late Pleistocene and the Holocene (see Go~r et al., 1991). The different Maars will be presented from N to S. Detailed figures (map, cross sections) have been published for different maars. They are not reproduced in this article. Most of them are available in a recent volume of Go~r et aL (1991). Tazenat-, Puy de Chalard- and Rochenoire Maars These maars are the northernmost maars of the FMC, very likely connected by a
86 single fault-line (Jeambrun, 1983). Tazenat Maar. Most of its features were described by Baudry and Camus (1972): 1) the crater-lake is 600 to 700 m across; 2) the eruption occurred both in a small valley and on a fracture; 3) the tuff-ring represents only about one tenth of the crater in volume; 4) the tephra is stratified and includes about two third of granitic xenoliths; 5).the magmatic component is basaltic, cauliflower bombs are present. Despite the fact that morphology indicates an age which could be as recent as Holocene (Camus et al., 1975), products from the tephra were recently dated by thermoluminescence, and two different ages were obtained: 124+10 ka and 166+14 ka (Pilleyre, 1991). Recently, cores were taken by Juvign~ from the deepest part of the crater-lake (66 m of water). Investigation is still ongoing. According to the pollen content (Bastin/ Louvain-la-Neuve, Belgium), the uppermost 5 m of the lake sediment are younger than Atlantic (<4700 BP). Based upon the rate of sedimentation in the Late Holocene (about 1 m/1000yr) and the shape of the crater, the above reported TL-ages seem to be overestimated .(Juvign6 and Bastin, in preparation). Puy du Chalard. At the foot of-the cone several outcrops show strombolian deposits overlying maar products. Geophysical soundings (Barbaud et aL, 1981) confirmed that the cone has grown into a maar crater which is about 1 km across. The latter is nearly filled up and a small tuff-ring only occurs at its southern rim. A lava flow of the Puy Chalard was dated to 51+8 ka by thermoluminescence dating applied to plagioclases (Gu~rin, 1983). Rochenoire Maar. This is a small (400 m across) depression. There is no outcrop available to describe the products. Two spatter cones are present at the western side of the maar. They should be synchronous. Beaunit Maar The Maar formed in the Amb~ne valley. Its diameter is about 1 km at the outer rim of the crater. A tuff-ring accumulated NE of the crater; its magmatic component is basaltic (Camus, 1975). The action of water ceased during the eruption, likely due to the fact that the Amb~ne river was dammed by the construction of the tuff-ring. Then the activity turned to strombolian, supplying high amount of scoria into the crater (Aubert et al., 1984). The filling was achieved by lacustrine deposits and lava flows erupted from the nearby Puy Thiollet. The strombolian cone is the only one of the ChaTne des Puys sensu stricto, which contains granulite and charnockite from the lower crust, and peridotite from the upper mantle (Brousse and Rudel, 1964). A TLage of 43.9 ka was obtained by Gu6rin (1983) on the lava from the Puy Thiollet. This represents the final volcanic filling of the Beaunit Maar.
87 Saint-Hippolyte Maar The St-Hippolyte Maar formed throughout Oligocene fluvio-deltaic gravelly sands. A small tuff-ring accumulated at the NE of the crater. The crater was filled by deltaic and lacustrine deposits. The lake disappeared and the flat surface of the lacustrine sediments is about 700 to 800 m across. The last interglacial/glacial transition (80 to 60 ka) was identified in the sediments and an age of about 94 ka was obtained on tephra material from the tuff-ring (Raynal et al., 1984, 1985). Maars along the fissure Enval-Volvic Four maars are connected by a fissure running along the eastern margin of the main ChaTne des Puys; they are named from N to S respectively: Volvic Maar, Chanat and Clerzat complex, and Enval Maar. The magmatic component of all those maars is undersaturated (basanite) and characterized by an original mineral suite including magmatic scapolite (Boivin and Camus, 1981). The eruption should be synchronous with that of the St-Hippolyte Maar (90 ka for the Chanat lava flow). The Enval Tephra is welded due to palagonitization. Ceyssat Maar Ceyssat Maar is only known by its tephra which is characterized by a high xenolith content and basaltic cauliflower bombs. Since it was filled up by more recent lava flows from nearby vents, it does not appear anymore in the morphology. Villars Maar The Villars Maar is situated on the granitic eastern plateau of the ChaTne des Puys. Its presence has only been demonstrated by drilling for ground water supply to 96 m depth. The flat surface of lacustrine deposits is about 900 m across (Jeambrun, 1987). No tuff-ring associated with that maar has been found. The Viliars Maar could not be a recent maar. A correlation with the Pliocene Prudelle-Montagne Perc~e lava flow (3.5 Ma) is quite possible. Montchatre Maar The Montchatre Maar (400 m across) is a horse shoe shaped depression located 3 km NW of Villars. After the complete erosion of the pyroclasts, that maar could also be correlated with the Pliocene generation cited above (see Villars M.). Recent magnetic measurements applied to the maar demonstrated the presence of sedimentary deposits overlying an ancient lava lake. Anchal Maar That maar is located at the margin of the Sioule valley, 9 km west of the main axis of the Chafne des Puys. The crater is about 1 km across and the basement
88 consists of gneiss. Due to digging for a hydroelectric construction the tephra of the maar is visible at the bottom of the crater. The Anchal Maar may be correlated with the Pliocene volcanic phase of The ChaTne de la Sioule (2 to 5 Ma), as well as with the strombolian Puy de Moufle located at its northern margin.
Clermont-Chamali&res Maar The hill settled by the older part of Clermont-Ferrand corresponds to a voluminous tuff-ring dispIaying the typical features of maar deposits: xenolith rich stratified products, numerous basaltic cauliflower bombs, accretionary lapilli (Pelletier, 1969; Baudry and Camus, 1972). Moreover, several drillings have demonstrated the presence of a filled crater 1.5 km across at the western foot of the tuff-ring. Recently a 91 m long core was taken in the middle part of the crater filling; that core showed lacustrine sediments containing numerous interbedded tephra beds (Vernet, 1992). A TL-age of about 156 ka was obtained by Miallier (1982) on tephra material from the tuff-ring. Another TL-age of 126 ka was recently obtained by Pilleyre (1991) on a tephra sample taken at a depth of 78 m in the above cited sediment core. Ladoux Maar Ladoux Maar is only known by its typical products of the basaltic phreatomagmatic eruption which contains numerous accretionary lapilli. A TLage of less than 250 ka was obtained (Pilleyre, 1991) for its tephra. The maar itself is totaly hidden by recent swamp deposits covering the Grande Limagne. Its location is uncertain (Camus et al., 1983; Go~r et aL, 1991). The tephras of the Ladou• and Clermont Maars erupted throughout Oligocene marls of the Limagne; they are representative of the peperites (see above) prior to their diagenetic induration. Narse d'Ampoix- and Narse d'Espinasse Maars These two maars formed on a single fissure together with a spatter cone and a strombolian cone named Puy de I'Enfer (Baudry and Camus, 1970; Camus, 1975). The magmatic composition of both maars is basaltic. The tephra from the Narse d'Ampoix accumulated on a small hill at the northern rim of the crater. The tephra from the Narse d'Espinasse is interbedded in the nearby strombolian cone Puy de I'Enfer. The lacustrine sediments of both maars were investigated recently (Juvign~ and Gewelt, 1987; Beaulieu and Goeury, 1987; Juvign~ et al., 1988), and it was concluded that t h e volcanic activity along the fissure Ampoix-Espinasse took place either during the Younger Dryas or somewhat earlier, i.e. around 11,000 BP. The Narse d'Ampoix was proposed as a tephrostratotype of the southern
89 ChaTne des Puys, due to the presence of at least 5 tephra beds interbeded in a well dated peat sequence. Servi~res Maar The Servi~res lake is about 500 m across and less than 27 m deep. The crater formed throughout the previous lava flows constituting the northern part of Monts-Dore stratovolcano. The tephra (basaltic) of the Servi~res Maar was recently discovered (Morel, 1987). The age of the maar has not yet been determined, but according to the absence of an identifiable tuff-ring that volcano should be as old as Chauvet- and La Godivelle d'en Haut Maars (see below), i.e. syn- or pro-glacial. Beaune-le-Froid Maar This maar is located 1.5 km NW of Murol, at an elevation of about 1000 m. A small tuff-ring is present at the southeastern margin of the crater. The products consist of xenoliths (50%) and basaltic juvenile pyroclasts (50%), including a high amount of cauliflower bombs (Besson, 1978). The age of that maar is not well known, but it could be correlated either to the oldest volcanic phase .of the Chai'ne des Puys (>50 ka), or to the formation of the Sancy stratovolcano (between 0.25 and 1 Ma). Pavin Maar The crater is 900 to 1000 m across at the rim. It is occupied by a funnel shaped lake, which is about 750 m across, with a maximum water depth of 93 m. A 1 m thick diatomite covers the bottom of the lake. Pavin volcano has erupted a huge amount of yellowish pumiceous tephra (Camus et al., 1973a), which was studied in details by Bourdier (1980). Bourdier has shown the presence of cauliflower bombs, dense lapilli, and antidune structures in the proximal tephra layer. The magmatic component is benmoreitic in the southern lobe (Bourdier, 1980) to trachytic in the northern lobe (Juvign6, 1991). Tephra erupted by the Pavin volcano was recently found in several peat-bogs as far as 60 km to the South of the vent (Juvign6 and Gilot, 1986). The existence of a northern lobe was demonstrated by the discovery of a trachytic tephra layer in two peat-bogs of the southern Chai'ne des Puys: Narse d'Ampoix (Juvign~ and Gewelt, 1987), and Narse d'Espinasse (Juvign~ et aL, 1988; Juvign~, 1991; Juvign6 1992a). Camus et aL (1973a) proposed a 14 C age of about 3450 BP for the Pavin volcano, but recently different authors discussed ages of about 6000.BP (Guenet, 1986; Juvign6 and Gilot, 1986). The Pavin Maar is the youngest volcano of Chai"ne des Puys.
90 Estivadoux Maar Estivadoux Maar was first named "Costes Maar" (Camus et al., 1973a). The name changed when Bourdier (1980) determined the accurate location of that partly burried crater. Those authors also described the associated stratified basaltic tephra interbedded in a sequence displaying the products of the Besse-en-Chandesse volcano group: Montcineyre Tephra, Estivadoux T., Montchal T., and Pavin T.. Such a stratigraphical position implies that the Estivadoux Maar erupted at about 6000 BP (see Juvign6 and Gilot, 1986). Chauvet Maar The Chauvet lake, which is about 800 m across with a maximum water depth of 80 m, occupies an ancient crater with a roughly funnel shaped bottom. No tephra has been found and no tuff-ring morphology is visible at the margin of the lake. Cores were recently taken from the lacustrine sediment. These cores showed from top to bottom (Juvign6, 1992b): 1) diatomite .including the regional Holocene tephra layers (Pavin T./about 5950 BP, Montcineyre T./about 6050 BP (Juvign6 and Gilot, 1986), La Taphanel T. /8500 BP (Juvign6, 1983, 1987; Juvign6 et al., 1992c); 2) Late Glaciat clay including the Godivetle Tephra 4/10,350 BP (Bastin et aL, 1990), and the Godivelle Tephra 5/10,750 BP (Juvign6, 1987; Bastin et aL, 1990); 3) high glacial moraine which stopped the coring. Those evidences demonstrate that Chauvet Maar is older than deglaciation of the Monts-Dore. Eruption throughout the ice cap or before glaciation could explain the absence of a tuff-ring (Juvign6, 1992). La Fage Maar This maar is a small funnel shaped crater on the western flank of the strombolian cone of Puy de ta Vaisse. The crater has a reservoir for water supply (150 m across) surrounded by a swamp. At the margin of the lake, cores were taken (Juvign6, unpublished) and show Holocene peat containing thick tephra layers from Pavin and Montcineyre volcanoes, respectively. Big boulders (slope deposits) are present in the peat a few decimeters below the Montcineyre Tephra. The Godivelle d'En Haut Maar The crater is occupied by a lake with a size of 375 x 500 m and a maximum water depth of 45 m. That volcano erupted through the southeastern flank of a strombolian cone (Montagne de Janson). A sample of this basaltic material was previously taken at the eastern margin of the lake, and an age of about 113+ 13 ka was obtained by thermoluminescence dating (Gu6rin et al., 1983) Cores were recently taken from the lacustrine sediment (Juvign6, 1992),
9t which showed a sequence similar to that described in the Chauvet lake (see above). A moraine has stopped the coring. The age of the maar can only be estimated within a time range from 113 ka to the WOrmian Pleniglacial. Another few hypothetical maars in the La Godivelle area In the Haut C(~zalier and especially in the surrounding of La Godivelle, some circular to horse shoe shaped peat-bogs are present. Their morphology is close to that of glacial cirques, but their location on a plateau and their different orientations aTlow us to suppose tant they represent scars of maars rather than glacial features. Despite the large size of some of them (up to 1.5 km like Cros du Joran) no tuff-ring was found at the rim of any of those hypothetical maars. Moreover, Late Glacial clay including the Allerod T4 and T5 tephra beds (see above) were found in each of the following depressions (Bastin et al., 1990; Juvign~, unpublished), which are presented as examples of hypothetical maars without tuff-ring. 1) Graspet Bas (1271 m asl; 2 km SW of La Godivelle), circular crater of about 350 m across; 2) the Chastelets peat-bog (750 m across) could occupy an ancient maar, in the middle of which the Chastelets butte could have grown as a scoria cone (compare with the above decribed Beaunit Maar). Graspet Bas, Chastelets, and La Godivelle d'en Haut lake are situated up to 2 km SW of La Godivelle, along a SW-NE lineament, which could correspond to an eruptive fissure. 3) The semi-closed depressions of Cros du Joran, Grand Joran, Blatte, situated 2 to 3 km NW to NE of La Godivelle.
CONCLUSION Phreatomagmatic eruptions have been very frequent in northern Auvergne, especially during the last 200 ka. Most of the maars have basaltic magmas, but some of them erupted in relation with trachytic magmas. There are probably much more ancient maars than those described, but a lot of them might have been burried or heavily disturbed by posterior volcanic features (domes, needles, lava flows, scoria cones, etc). The absence of identifiable tuff-rings seems to be a common feature in the glaciated area (Monts-Dore, C~zalier); this emphasize the hypotheses of sub-glacial or pre-glacial eruptions.
92 REFERENCES Aubert, M., Camus, G. & Fournier, C. (1984): Resistivity and magnetic surveys in groundwater prospecting in volcanic areas- Case history maar of Beaunit, Puy de DSme, France. Geophys. Prospecting, 32: 554-563. Barbaud, J.-Y., Benderitter, Y., Blavoux, B., Camus, G., Martinet, J., Mudry, J. & Nougier, J. (1981): Etude hydrovolcanique d'un satellite de la ChaTne des Puys: le Chalard. Rapport du Laboratoire de G~ologie, Avignon. Bastin, B., Gewelt, M. & Juvign~, E . (1990): A propos de I'&ge et de I'origine des t~phras tardiglaciaires T4 et T5 de Godivelle-Nord (Massif Central, France). Ann. Soc. g6ol. Belg., 113: 165-178. Baudry, D. & Camus, G. (1972): Les projections volcaniques de la ChaTne des Puys et leurs utilisations. Bull. B.R.G.M., (2) II, 1972-2: 1-53. Beaulieu, J.-L. de & Goeury, C. (1987): Zonation automatique appliqu~e & I'analyse pollinique: exemple de la Narse d'Ampoix (Puy de DSme, France). Bull. Ass. fr. Et. Quat., 29: 49-61. Besson, J.-C. (1978): Les formations volcaniques du versant oriental du Massif du Mont-Dore (Massif Central frangais).Th~se de 3e Cycle, Universit6 Clermont-Ferrand II, 188 p. Boivin, P. & Camus, G. (1981): Igneous scapolite bearing association in the Cha~ne des Puys, Massif Central (France) and Atakor, Hoggar (AIg6rie). Contrib. Mineral. Petrolo., 77: 365-375. Boivin, P., Boudon, G., Camus, G., Go6r de Herve, A. de, Gourgaud, A., Kieffer, G., Ly, M.H., Mergoil, J. & Vincent, P.-M., (1984): Inventaire des structures volcaniques de type Maar du Massif Central frangais. Rapport in~dit, 398 p.+ dossier cartographique. Bourdier, J.-L. (1980): Contribution ~. 1'6tude volcanologique de deux secteurs d'int~r~t g~othermique dans le Mont Dore: le groupe du Pavin et le Massif du Sancy. Th~se 3e cycle, UniversitY, de Clermont-Ferrand II, 180 p. Brousse, R., Delibrias, G., Labeyrie, J. & Rudel, A. (1969): Elements de chronologie de la Cha~ne des Puys. Bull. Soc. G6ol. Fr., 11: 770-793. Camus, G. (1975): La Chafne des.Puys (Massif Central frangais ). Etude structurale et volcanologique. Th~se Doc. Etat, Ann. Universit~ de Clermont-Ferrand, S~rie G6ologie et Min~ralogie, 56/28, 322 p. Camus, G., Boivin, P., Go~r de Herve, A. de, Gourgaud, A., Kieffer, G., Mergoil, J. & Vincent, P.-M. (1980): Le phr~atomagmatisme. Bull. du P.I.R.P.S.E.V., CNRS-INAG, Paris, 27, 32p. Camus, G., Go~r de Herve, A. de, Kieffer, G., Mergoil, J. & Vincent, P.-M. (1973a): Mise au point sur le dynamisme et la chronologie des volcans holoc~nes de la r~gion de Besse-en-Chandesse (Massif Central frangais). C.R. Acad. Sci. Paris, 277, S~rie D: 629-632. Camus, G., Go~r de Herve, A., Kieffer, G., Mergoil, J. & Vincent, P.-M. (1973b): Nouvelle interpretation du syst~me Puy Chopine- Puy des Gouttes (ChaTne des Puys, Massif Central fran~ais). C. R. Acad. Sci. Paris, 277, S~rie D: 1121-1124. Camus, G., Go6r de Herve, A. de, Kieffer, G., Mergoil, J. & Vincent, P.-M. (1975): Volcanologie de la Cha~ne des Puys. Parc naturel r~gional des volcans d'Auvergne, 1 Carte & 1/25.000 et une notice explicative, 112 p. Camus, G., Go~r de Herve, A. de, Kieffer, G., Mergoil, J. & Vincent, P.-M. (1983): Volcanologie de la Chafne des Puys. Parc naturel r~gional des volcans
93 d'Auvergne, 8 (2e ~d.), 1 Carte & 1/25.000 et une notice, 112 p. Cantagrel, J.-M. & Baubron, J.-C. (1983): Chronologie des ~ruptions dans le massif volcanique des Monts Dore (M~thode potassium- argon), implications volcanologiques. BRGM, G~ologie de la France, 2/1-2: 123-142. Cantagrel, J.M., Sigmarsson, O., Condomines, M. & Kieffer, G. (1987): Chronologie du volcanisme aux environs de Chassolle. G~ologie de la France, 4: 157-162. Gewelt, M. & Juvign~, E. (1988): T~phrochronologie du Tardiglaciaire et de I'Holoc~ne dans le Cantal, le C6zalier, et los Monts Dore (Massif Central, France): r~sultats nouveaux et synth~se. Bull. Ass. fr. Et. Quat., 1988-1: 2233, Paris. Go~r de Herve, A. de, Camus, G., Boivin, P., Gourgaud, A., Kieffer, G., Mergoil, J. & Vincent, P.-M. (1991): Volcanclogie de la ChaTne des Puys, 3e ~d. Parc naturel r~gional des volcans d'Auvergne, 1 carte & 1/25.000 et une notice, 128 p. Guenet, P. (1986): Datation par I'analyse poNinique de I'explosion des volcans du groupe Pavin (Besse-en-Chandesse, Puy de DSme, France). 11e r~union des Sciences de la Terre, Clermont-Ferrand, Soc. g~ol. France, ~d., p. 86. Guenet, P. (1986): Analyse potlinique de la tourbi~re de Chambedaze et recherches pollenanalytiques dans les Monts Dore et le C~zalier, Massif Central, France. Th~se, Universit~ d'Aix-Marseille Ill, Laboratoire de Palynoiogie et de Botanique historique, 107 p. Guenet, P. & ReiNe, M. (1988): Analyse pollinique du lac-tourbi~re de Chambedaze (Massif Central, France) et datation de I'explosion des plus jeunes volcans d'Auvergne. Bull. Ass. fr. Et. Quat., 36: 175-194. Gu~rin, G. (1983): La thermoluminescence des plagioclases. M~thode de datation du volcanisme. Application au domaine volcanique frans Chafne des Puys, Merit Dore et C~zalier, Bas Vivarais. Th~se Dec. Etat, Universit6 P. et M. Curie, Paris, 253 p. Gu6rin, G., Gillot, P.-Y., Le Garrec, M.-J. & Brousse, R. (1981): Age subactuel des derni~res manifestations ~ruptives du Mont-Dore et du C~zallier. C. R. Acad. Sci. Paris, 292, S6rie I1: 855-857. Jeambrun, M. (1983): Le Puy Chalard et le Maar de Laty, appareils compl6mentaires du syst6me phr6atomagmatique d6velopp~ au nord de la ChaTne des Puys. Bull. Soc. G6ol. Fr., 7, XXV, 2: 273-275. Jeambrun, M. (1987): Le Maar de Villars: un r~servoir d'eau aux portes de Clermont-Ferrand. Rapport B.R.G.M., 87 Auv. 023, 4 p. Juvign~, E. (1983): Un marqueur stratigraphique suppl~mentaire dans les tourbi,~res du Cantal: la retomb~e volcanique de la Taphanel. Bull. Ass. frans Et. Quat., 13: 3-7. Juvign~, E. (1987): Deux retomb6es volcaniques tardiglaciaires dans le C~zallier (Massif Central, France). Bull. Ass. fr. Et. Quat., 32 241-249. Juvign~, E. (1991): Sp~.tglaziale und holoz&ne Tephrostratigraphie im Zentral Massiv (Frankreich). SonderverSffentlichungen, Geologisches Institut der Universit~.t zu KSIn, 82 (Festschrift K. Brunnacker): 163-174. Juvign~, E. (1992a): Distribution of widespread Late Glacial and Holocene tephra beds in the French Central Massif. Quaternary International, 7/8: 8184. Juvign~, E. (1992b): Approche de I'&ge de deux crat~res volcaniques lacustres d'Auvergne (France). C. R. Acad. Sci. Paris, 314, S6rie I1: 401-404.
94 Juvign~, E. & Gewelt, M. (1987): La Narse d'Ampoix comme t~phrostratotype dans la Chafne des Puys m~ridionale (France). Bull. Ass. fr. Et. Quat., 29: 3749. Juvign~, E. & Gilot, E. (1986): Ages et zones de dispersion de t~phras ~mises par les volcans du Montcineyre et du lac Pavin (Massif Central, France). Z. dt. geol. Ges., 137: 613-623. Juvign~, E., Kroonenberg, S., Veldkamp, A., El Arabi, A. & Vernet, G. (1992c): Widespread AIlered and Boreal trachyandesitique to trachytic tephra layers as stratigraphical markers in the French Central Massif. Quaternaire, in press. Juvign~, E., Lousberg, N. & Gewelt, M. (1988): Evolution morpho-s~dimentaire de la Narse d'Espinasse. Rev. Sc. nat. d'Auvergne, 53: 7-14. Juvign6, E., Milcamps, V., Delibrias, G. & Evin, J. (1988): Ages de traits polliniques et chronozonation du Tardiglaciaire et de I'Holoc&,ne dans le Massif Central (France). Med. rijks geol. Dienst, 41/4: 33-50. Lorenz, V., (1973): On the formation of maars. Bull. Volc., 37, 2: 183-204. Miallier, D. (1982): L'usage des d~tecteurs solides de traces dans le cadre de la datation par thermoluminescence. Th~se Doct. 3e cycle, Univ. ClermontFerrand II, 115 p. Morel, J.M. (1987): Volcanologie du Massif de I'Aiguiller (Monts-Dore, Massif Central frangais): ~tude p6trographique, dynamique, structurale, et rh~ologie des coulees bcueuses associ~es. Th~se, Universit~ Blaise Pascal, Clermont-Ferrand, 199 p. et 1 Carte hors texte. Oilier, C. (1969): A volcano introduction to systematic geomorphology. (6). The M.I.T. press, Cambridge, Mass, and London. Pelletier, H. (1969) Clermont est-il b&ti sur un volcan? Auvergne Magazine, 18: 2-8. Pilleyre, T. (1991): Datation par thermoluminescence. Application & la chronologie des retomb~es volcaniques. Th~se, Universit~ de ClermontFerrand II, DU 345, 175 p. Raynal, J.-P., Daugas, J.-P., Paquereau, M.-M., Guadelli, J.-L., Marchianti, D., Miallier, D., Fain, J. & Sanzelle, S. (1984): Le Maar de Saint-Hippolyte (Puy de DSme, France). Datation par thermoluminescence, flores et faunes fossiles, presence humaine, climatologie et dynamique du syst~me pal~o-lacustre. Rev. Sc. Nat. d'Auvergne, 50: 97-114. Raynal, J.-P., Paquereau, M.-M., Daugas, J.-P., Miallier, D., Fain, J. & Sanzelle, S. (1985): Contribution & la datation du volcanisme quaternaire du Massif Central frangais par thermoluminescence des inclusions de quartz. Bull. Ass. fr. Et. Quaternaire, 4: 1983-207. Vernet, G. (1992): Message du volcanisme r~gional dans les formations quaternaires de Limagne occidentale (Massif Central frangais), Min6raux denses et retomb6es. Th~se, Universit~ de Bordeaux, 724, 335 p.
PALAEOENVIRO~R~TAL VOLCANIC
INVESTIGATIONS
LAKES
OF LAZIO
ON LONG
(CENTRAL
SEDTMR~T
ITALY)
- AN
CORES
FROM
OVERVIEW
Maria Follieri + , Donatella Magri + , Biancamaria Narcisi* + Dip. Biologia Vegetale,
Univ. La Sapienza,
* ENEA C.R.E. Casaccia,
C.P. 2400,
00185 Roma,
00100 Roma A.D.,
Italy
Italy
ABSTRACT
Seven
volcanic
Pleistocene
between
considered.
They
(Lagaccione], di
lakes the
belong
Vico
Martignano,
(Lago
features
is
presented,
project. as
the
spite
correlated
outlined. with
not
of
and
the
and
Upper
Apennines
districts:
are
Vulsini
(Lago di Monterosi,
Baccano) survey
and
of
studies
Colli
the
from
investigations
suitable
Lago Albani
results
continuous
in
progress
for palaeoenvironmental
interrupted
floristic
on the basis
the
Middle
site the main geographical A
palynological
is very
were
local
di
the
volcanic
Sabatini
Valle
For each study
together
records
Sea
following
di Vico),
and
The region
of
the
are
lithostratigraphical
during
Tyrrhenian
to
Stracciacappa,
(Va!le di Castiglione). geological
formed
during
the
differences,
the
the general
trends
glacial
pollen of
of
cores or
in
studies,
periods.
records
the main
and
may
In be
vegetation
phases.
INTRODUCTION The Lazio
region
is geologically
extension of volcanic rocks (Accordi & Carbone, The
volcanic
petrographic - the by
the
series
i),
characterized
by a c o n s i d e r a b l e
covering approx.
30% of its area
1988). products
have
been
(Barberi & Innocenti,
"tosco-laziale
Tolfa,
(fig.
Cerite
and
province", Manziana
referred
to
two
distinct
1980):
represented
districts
and
in
the
Lazio
region
by
the
Monti
Cimini
Lecture Notes in Earth Sciences, Vol. 49 1. F. W. Negendank, B. Zolitschka (Eds.) Paleolimnology of European Maar Lakes 9 Spnnger-Verlag Berlin Heidelberg 1993
96
Fig.
! -
The volcanic districts of the Lazio region: a) v o l c a n i c rocks b) study sites: 1 Lagaccione, 2 Lago di rico, 3 Lago di Monterosi, 4 Stracciacappa, 5 Valle di Baccano, 6 Martignano, 7 Valle di Castiglione.
volcanic
district;
the
nature;
their
anatectic (Fornaseri, - the a
"Roman comagmatic
pronounced from
explosions, districts, of
Vico,
acidic
Lazio
character
spans
900
ka
to
generating tectonic
highs
1986).
represented
and Colli
alkaline-potassic 40
maar
Albani
character. ka
from
and
4
MA
are
of
1
MA
to
The
age
craters,
of
the
1985),
occurred
region b y
districts,
in
with
products
Hydromagmatic most
of
the
at the end of their activity,
as a consequence
stress;
the
interactions The
in the Lazio
vol~anic
(Fornaseri,
they
are
related
of the carbonatic basement
magma/water
SDosato,
show
in
province",
Sabatini
particularly
increased
structural the
age
1985);
the Vulsini, ranges
products
lakes
(Accordi
considered
& in
to
functioning Carbone, this
presence
of
as aquifers
in
1988;
paper
De
belong
Rita to
&
this
volcanic province. The
climate
geographic
of
position:
the region
is particularly
complex b e c a u s e
on the one side it is mildened
by the
of the
Tyrrhenian
97
Sea,
on the other side it is influenced by the Apennines which,
to over high
2000 m a.s.l.,
rainfall.
this general ecological
bar
the way
to westerly
air
streams
A wide range of local modifications
trend,
rising
and cause
is s u p e r i m p o s e d
on
producing a remarkable variability of climatic and
conditions,
to the point
that Almagi~
(1966)
stated
that
a
enhances
a
climate of Lazio does not exist. The heterogeneity wealth
of
1984)
and
flora
(about
great
biological
of geography,
vascular
vegetational
entity,
the
complex
of transitions
African
vegetations
longitudinally
3000
substratmm and climate
vegetation between
plants
diversity: of
the
(Montelucci,
Lazio
the opposed
latitudinally,
according rather
Anzalone, a
region
central
and
to
than
unitary
represents
European
montane
a
and North
and
Balkan
1976-77).
VALLE DI CASTIGLIONE Valle di Castiglione district;
complex,
as it is thought
time as Rita,
Alessio
The
Funiciello
volcanic
pyroclastic The
1986;
products
is
of
Fornaseri,
of
this
the
oldest
400 ka,
the central Scherillo
crater
have
of
the
at the same edifice
&
(De
Ventriglia,
been
studied
by
1978.
leucitic
lies about 20 km east of Roma The
catchment
undersaturated
area
(latitude
(60 k m 2)
volcanic
is
rocks,
41"
entirely
mainly
of
type.
lake
bed
Morphological The
of the main ~nits
al.,
44 m a.s.l.).
of
activity
to have occurred around
di Castig!ione
altitude
composed
et
& Parotto,
Valle 53'N,
hydromagmatic
the emplacement
in
1963).
its
is a dried-out maar lake of the Colli A!bani
volcanic
investigations
present-day
temperature
is circular mean
7"C,
in shape, have
annual
mean July
been
its
carried
temperature
temperature
diameter
25"C)
out
is
is
by
15"C
about
Segre
1 km. (1975).
(mean
January
and the average
rainfall
is about 800 mm, with precipitation mainly in autmnm- and winter. The
sediments
of
Valle
di
Castiglione
have
continuously
to a depth
of
88 m. Multidisciplinary
carried
in
10
m
out
geochemistry,
the fauna
lithostratigraphy al. Magri
(1992); &
of
detailed
Sadori
Castiglione
top
is
and microscopic
and
of
pollen
the whole pollen
(1988), discussed charcoal
the
while in
core,
analysis
been
studies
concerning
(Alessio
core has
et
been
al., by
been p u b l i s h e d biostratigraphy
Magri
&
in Magri
Sadori
been
1986);
described
has
Follieri,
have
geochronology,
the pollen
are presented
drilled
the
Narcisi
by
et
Follieri,
of
Valle
(1990);
& Ciuffarella
di
pollen (1991);
98
the
exponential
Magri
(1989a);
have
been
growth
discussed
by
lithostratigraphical entire
sequence
annual
laminations
The sequence in
past
plant
dynamics
Magri
&
populations
Magri
Follieri
(1992);
& Narcisi,
(Magri & Narcisi,
from Valle
Italy,
as
it
palynological and at the
can
be
of the Quaternary two
regarded
and
of the
scale as
environmental
complete
of
changes
by both pollen and sedimentological
with the aim of filling the core gaps of the preceding sediments
a key-
interglacial-glacial
The study of a second drilling at Valle di Castiglione a record of the lacustrine
in
1992).
contains
clearly highlighted
interpreted
both at the scale 1992)
di Castiglione
for an understanding
is
at the end of the interglacials
data have been compared
(Follieri,
record
central
cycles,
of
the vegetation
and the tephra
data.
is planned
core and having
layers
down
to the
bedrock.
LAGACCIONE The Lagaccione activity Latera,
maar belongs
represents 1983).
is reported
Lagaccione
A short
lies Sea.
the last century, and 750 m. The volcanic
dated products
in Alberti
the Tyrrhenian
volcanic
one of the final manifestations
whose youngest
& Vezzoli,
to the Vulsini
district;
of the V o l c a n o
are 145,000 years
description
of
its
old
the products
of
of
(Metzeltin this
crater
et al., 1970.
at 355 m a.s.l., The lake bed,
is elliptical
catchment
35 km
in shape,
area
(latitude
artificially
(approx.
42"
34'
N)
from
dried out at the end of
its axes being approx.
2 km 2) is entirely
900 m
composed
of
rocks.
The
present-day
temperature
7"C,
mean
mean
July
annual
temperature
temperature
23"C);
is
14"C
(mean
the average
January
rainfall
is
about 950 mm. Geophysical assess
the
investigations
morphology
their
thickness.
These
investigations
of
the
Vertical
characterized
by different top,
the
lacustrine volcanic with
sediments, basement.
concentric
substratum
a
(fig.
body
and a resistant
contour
lines.
displays
The a
have
of three
values: in
out
in
lacustrine
soundings
resistivity
resistivity
carried
the
the presence
conductor
The 2b)
been of
electrical
revealed
body
at
have bottom
the
been
a moderately both
body at the bottom, (fig.
map
of
the
substantially
2a) top
shows of
to and
performed.
overlapping
middle,
map
order
deposits
units
conductor formed
that
by
is the
a
structure
the
resistant
symmetrical
and
flat
99
Fig.
2 -
G e o e l e c t r i c a l i n v e s t i g a t i o n s at L a g a c c i o n e : a) r e s i s t i v i t y map, b) map of the top of the r e s i s t a n t s u b s t r a t u m .
100
Shape of the Lagaccione basin as inferred by the geoelectrical investigations across the sections AA' and BB' of fig. 2. Note that the vertical and horizontal scales are different.
Fig. 3
basin,
with the maximum depth of the infilling
centre
of
crossing general
the
the
bed. part
In of
shape of the deposit
Preliminary shown
lake
central
the
pollen
fig.
and the more resistant
of
site
this
sediments,
to be very rich in pollen,
in the
agreement
with
centre the
as
regards
ascribable
(Magri
show
1989b)
to the Holocene,
lake bed
geophysical
sections
they
palaeoeco!ogical
investigations,
of the
at the the
layer on top.
core
both in concentration
identified taxa. Following these preliminary good
interpretative
from a 5.5 m deep
interest
drilled
two
(46 m)
are presented;
analysis
in fact the analysed
was
3,
the basin
sediments
turned out
and in the number a continuous
to a depth
of
investigations,
has
studies;
borehole
49.5
the
of
m.
In
volcanic
basement was reached at about 46 m. Preliminary reported several
in
lithostratigraphical
fig.
lithozones,
4.
The mainly
lacustrine
and
palynological
sequence
distinguished
by
can the
be
studies
schematized
content
of
are in
organic
101
matter
(determined
by
K 2 Cr 2 07
titration
method):
sediments are found from ca. 42.00 m to 39.50 m, m
and
lavic
from 4.70 m
to 1.40
clasts
present
are
m.
Fine
at
organic-rich
from 37.75 m to 37.35
silts with decreasing
the
transition
between
pumices
the
and
volcanic
basement and the typical lacustrine sediments. The pollen record is characterized by alternating forest and nonforest
phases.
higher
than
Three
80%,
organic matter
periods
are
corresponding
content.
found
to
the
with
AP
stretches
(arboreal of
Between 30 m and 20 m there
core are
pollen)
with
high
several w e a k
fluctuations of trees. The characters of flora and vegetation of the L a g a c c i o n e sequence are
generally
Castiglione,
very
similar
to
those
of
the
record
in particular when considering the .most
from
Valle
important
di
forest
phases:
the oldest should correspond to St Germain I, the s e c o n d to St
Germain
If;
the
Ho!ocene
is
found
at
the
top
of
both
records.
sequence of fluctuations of AP between 30 m and 20 m at L a g a c c i o n e
The is
regarded as conten~orary with the peaks of AP between 21 m and 15 m at Valle di Castig!ione, According from
to
this
Lagaccione
corresponding to the plenig!acia!
preliminary
spans
chronological
approximately
interstadials.
framework,
i00,000
years.
the
The
sequence
pollen-based
chronology therefore represents a new contribution towards e v a l u a t i n g the age of the end of the volcanic activity, w h i c h is s u p p o s e d l y not m u c h older than the beginning of the lacustrine sedimentation.
STRACCIACAPPA
The Stracciacappa crater is located in the eastern Sabatini 10'). the
volcanic
Together
last
Detailed
with
explosive
about
several phase
investigations
Rita & Zanetti 1834,
district,
(1986a,
other
of
on
1
km.
The
the
its
km
small
north
products
of
vents,
district,
have
area
(less
(latitude
it was
been
80
active ka
and
carried
during 40
out
42"
by
ka. De
artificially dried out in
its shape is circular,
catchment
sector of the
Roma
between
1986b). The lake bed,
lies at 220 m a.s.l.;
approx.
35
the diameter being
than
2
km 2)
is
14"C
is
entirely
composed of volcanic rocks. The p r e s e n t - d a y m e a n rainfall
is
about
1050
described in Blasi et al. A
continuous
annual mm.
temperature
The
modern
vegetation
and of
the
average
the
area
is
of
32.60
m,
(1981).
borehole
has
been
drilled
reaching the volcanic basement at around 29 m.
to
a
depth
102
Fig. 4 -
Preliminary records of lithostratigraphy, organic matter and pollen from the Lagaccione core; i pedogenized sediments, 2 fine silts, 3 organic-rich fine silts, 4 tephra layer, 5 fine silts with dispersed pumices and lavic clasts, 6 volcanic sands, 7 coarse-grained pyroclasts.
103
Palynological are in progress. significant and
and lithostratigraphical
Preliminary pollen data,
concordances
Lagaccione,
interstadials
there
interrupting & Giardini,
and
with
the
indicate were
records
that
several
the succession
investigations from Valle
during
weak
on the core
spanning approx. the
spreads
60 ka,
di
Castiglione
last
of
pleniglacial
angiosperm
of steppe and grassland
show
trees
formations
(Magri
1992).
VALLE DI BACCANO Valle its
di Baccano
activity
Zanetti,
is a caldera of the Sabatini volcanic
of hydromagmatic
1986b).
origin
Investigations
on
occurred
around
the structure
Baccano have been carried out by De Rita et al. & Zanetti The The
80 ka
and
the
(1983)
(De Rita,
products
of
a n d b y De Rita
(1986b). lake was
lake
district;
bed
is
art%ficially almost
dried
circular
2 km. Its lowest elevation
out
in
for
shape,
is 205 m a.s.l.
the
last
with
a
time
mean
The catchment
in
1838.
diameter area
of
is about
9 km 2 . The present-day
mean
annual
temperature
is
14"C
and
the
average
rainfall is about 1050 r~n. Pedological
and
sedimentological
on deposits of the last 2000 years Pollen Holocene,
analysis
of a 10.70 m
core,
were performed by Bonatti
With
the
aim
lithostratigraphical
of
investigations
(~gelelli
were
carried
& Dowgiallo,
corresponding
out
1989).
to part
of
the
(1963).
carrying
investigations,
a
on new
palynological
continuous
and
borehole
has
been drilled by our te~m to a depth of 88.50 m.
OTHER VOLCANIC LAKES Other volcanic of
lakes
palaeoenviroP~nental
them deserve
mention:
from Lazio
(fig.
investigations Lago
by
di Martignano,
!) have
formed
various
authors;
Lago
the
subject
three
di Monterosi,
of
Lago
di
Vico. Laao
di
volcanic (1986a)
Martianano. district
It
belongs
explosive
its products with
the
last
According
phase
of
to De
the
Rita
Sabatini &
Zanetti
lie on the hydromagmatic unit of V a l l e di Baccano
and are covered by the volcanics Stracciacappa,
to
activity.
which
Lago
from Stracciacappa. di
Martignano
The distance
shares
part
of
from its
104
northern the
depth has
crater rim,
east,
is
1500
is ca.
60 m. A
provided
1991).
Laao
Monterosi.
according during
the
record
This
first (1970).
last
The
mineralogical,
to
(1983)
of
from Valle di Baccano,
207
m
the
the
11,000
Sabatini
about
have
palyno!ogical
and
its
in the centre last
evolution
lies
a.s.l,
of
of
been
The
of
lake
the
lake &
district; developed
Sacrofano-Baccano
is reported
40 k m NNW
5 m.
the
(Kelly
volcanic
the
to
maximum
years
the activity of this vent
is around
years
the
of the products
lake
depth
25,000
at
spanning
belongs
stage
its maximum
lies
6.71 m core collected
A brief description
Ventriglia a.s.l.;
lake
to De Rita et al.
the
caldera.
The
a pollen
Huntley, di
is 500 m; the distance
m.
in M a t t i a s
Roma,
at
sediments
subject
of
and h y d r o b i o l o g i c a l a n a l y s e s
&
237
m
spanning chemical,
(Hutchinson,
1970). Laao di Vico. volcanic
This
lake is located in the central
district.
A
volcano
is reported
surface
area
a.s.l.,
in
published
the
a pollen
of
influences wet
50 m
the
conditions
vegetation record
of
(Lulli
from
a
7.75
(1970).
climate
The
the
lake,
inside
1990).
core,
the Vico
of
spanning
a
510 m
caldera,
both
Frank
Vico
with
of
the
environment,
al., m
of
geology
and an elevation
in the et
caldera
the
& Ventriglia
km 2, a depth
persistently
and
description
in Mattias
12.1
significantly
inducing soils
of
general
in
(1969)
the
the has
Holocene
and part of the last pleniglacial. The lakes of Martignano, with
an interesting
either due (e.g.
at
potential.
to the methods the
present-day
lake
Monterosi
margin),
environment,
However,
applied, or
and Vico have p r o v i d e d these
studies
are p r o b l e m a t i c
or due to the location
to
inadequacies
indispensable
records
of
the core
in k n o w l e d g e
of
the
for correct p a l a e o e n v i r o n m e n t a l
interpretations. As
regards
lake bottom,
Lago
di Vico,
two
new
cores
were
recovered
within the frame of the EUROMAARS project.
studied with a multidisciplinary
approach,
from
the
They a r e being
with the aim of
filling
the
gaps of the record from the lake margin.
CONCLUSIONS The
data
synthetically -
The
so
far
obtained
from
the
volcanic
lakes
of
Lazio
indicate that:
region
is
a
reference
in that the age of the
area
for palaeoenvironmental
formation of the lakes
is much
studies,
older
than
105
the
Holocene,
and
the
records
were
not
interrupted
during
the
glacial periods. The
sediments
from volcanic
palaeoenvironmental deepest
part
of
lakes
the
basins,
showing
relatively
significant
inflows
are absent.
exclusively
composed
reworked pollen grains crater
drainage the
area,
basin)
As
regards
basins
the
significance lithology, 1991).
It
of is
possible, The pollen
diversity
certainly consider steppe
deposition
factors
lake
be
life,
used
the
the
flora
important
the
for
to
Magri
epionthological
of
the
of as
& Spada,
phases local
of
the
such
(Anselmi
many
have
of
climate,
as
et al,
sequences
as
to interpret
sites.
lakes of Lazio
phases in
forests
Campagna 1990),
and
indicators,
complexity
beech
studies
(morphometric,
indicate
genera~_y floristic
that
showed
the
differences
1992).
vegetation
underlimit
formations
holistic
characteristics
study
even though
the
to
a
the
clastic matter
vegetational
and
exclusively
efficiency
indicators
(Magri & Giardini, of
of
for
aspects, etc.)
of
lithology
variations
environmental
mineralogy,
sites,
(e.g.
of
of the
and underground
can be attributed may
from the volcanic
accounts
Follieri,
the
of the main vegetation
in all
present-day
over
to single out helpful
records
trend
rocks,
and to make cross comparison between
can be observed The
size,
clearly
the succession same
volcanic
sedimentological
in order
their meaning
when areas
hydrogeological, variability
grain
continuous
as the catchment
different
the
the
Moreover,
lithostratigraphical
geomorphological, evidenced
and
for
from
provide
and shape of surface
parameters
changes
with
suitable
sedimentation
As the background
constant
sedimentological
environmental interpretation.
generally
undisturbed
of
extension
are
very
collected
older than the age of the formation
can be excluded.
of catchment
proved
when
they
sequences, are
have
investigations;
and
the
the
course
the
it
1961),
understood
time
of
Suffice
(Montelucci,
can be
of
plasticity
region.
(Anzalone,
Romana
which
in
or
to the
1976-77; only
in
terms.
AC ~NOWLEDGEME~fS
This
work
has
been
0008-I and SCI-0295-C).
supported
by
EEC
financing
(contracts
EV4C-
106
REFERENCES Accordi, G. & Carbone, F. (eds) (1988): Carta delle litofacies del Lazio-Abruzzo ed aree limitrofe + Note illustrative. CNR, Quaderni de "La Ricerca Scientifica", 114(5), p 224; Roma. Alberti, A., Bertini, M., Del Buono, G.L., Nappi, G. & Salvati, L. (1970): Note illustrative della Carta Geologica d ' I t a l i a alla scala 1:100.000, Foglio 136 (Tuscania) e Foglio 142 (Civitavecchia), Poligrafica e Cartevalori; Ercolano, Napoli. Alessio, M., Allegri, L., Bella, F., Calderoni, G., Cortesi, C., Dai Pra, G., De Rita, D., Esu, D., Follieri, M., Improta, S., Magri, D., Narcisi, B., Petrone, V. & Sadori, L. (1986): 14C dating, geochemical features, faunistic and pollen analyses of the uppermost 10 m core from Valle di Castiglione (Rome, Italy). Geologica Romana, 2 5 : 2 8 7 - 3 0 8 (issued 1989). Almagi~, R. (1966): Lazio. Le Regioni d'Italia, p 750, U.T.E.T.; Torino. Angelelli, F. & Dowgiallo, M.G. (1989): Studio stratigrafico in un sito archeologico della Valle di Baccano (Lazio centrale). Mem. Soc. Geol. It., 42: 129-138. ~nselmi, B., Catalano, F. & Narcisi, B. (1991): V a r i a b i l i t y and validity of geological palaeoclimatic indicators: examples in Central Italy. Abstracts XIII INQUA Congress, i0; Beijing, China. Anzalone, B. (1961): Sul limite altimetrico inferiore del F a g g i o nella regione laziale. Anna!i di Botanica, 27: 80-109. Anzalone, B. (1984): Elenco preliminare delle piante v a s c o l a r i del Lazio, p 251; Roma. Barberi, F. & Innocenti, F. (1980): Volcanisme N6og6ne et Quaternaire. In: Soc. It. Mineral. e Petrol., Introduction & la G6ologie g~n~rale d'Italie et Guide ~ l'excursion 122 A. XXVI IGC, 99-104; Paris. Blasi, C., Abbate, G., Fascetti, S. & Michetti, L. (1981): La vegetazione del bacino del F. Treia. CNR, AQ/I/237, p 33; Roma. Bonatti, E. (1963): Stratigrafia pollinica dei sedimenti p o s t g l a c i a l i di Baccano, lago craterico del Lazio. Mem. Soc. tosc. Sc. nat., 70: 40-48. De Rita, D., Funiciello, R., Rossi, U. & Sposato, A. (1983): Structure and evolution of the Sacrofano-Baccano caldera, Sabatini volcanic complex, Rome. J. Volcan. and Geotherm. Res., 17: 219-236. De Rita, D. & Sposato, A. (1986): Correlazione tra eventi esplosivi e assetto strutturale del substrato sedimentario nel complesso vulcanico sabatino. Mem. Soc. Geol. It., 35: 727-733. De Rita, D. & Zanetti, G. (1986a): Caratteri vulcanologici e deposizionali delle piroclastiti di Stracciacappe (Sabatini orientali, Roma). Mem. Soc. Geol. It., 35: 667-677. De Rita, D. & Zanetti, G. (1986b): I centri esplosivi di B a c c a n o e di Stracciacappe (Sabatini orientali, Roma): analogie e differenze della modellistica esplosiva in funzione del grado di interazione ac~aa/magTna. Mem. Soc. Geol. It., 35: 689-697. Follieri, M., Magri, D. & Narcisi, B. (1990): A comparison between lithostratigraphy and palynology from the lacustrine sediments of Valle di Castiglione (Roma) over the last 0.25 MA. Mem. Soc. Geol. It., 45, in the press. Follieri, M., Magri, D. & Sadori, L. (1988): 2 5 0 , 0 0 0 - y e a r pollen record from Valle di Castiglione (Roma). Pollen et Spores, 30: 329-356. Follieri, M., Magri, D. & Sadori, L. (1990): Pollen stratigraphical synthesis from Valle di Castiglione (Roma). Quaternary International, 3/4(1989): 81-84.
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Follieri, M., Magri, D. & Spada, F. (1990): Esiste una relazione tra le steppe antropogeniche attuali e le steppe-praterie oloceniche della regione mediotirrenica? Giorn. Bot. It., 124(1): 145. Fornaseri, M. (1985): Geochronology of volcanic rocks from Latium (Italy). Rend. Soc. It. Mineral. e Petrol., 40: 73-106. Fornaseri, M., Scherillo, A. & Ventrig!ia, U. (1963): La regione vulcanica dei Colli Albani. CNR, p 561; Roma. Frank, A.H.E. (1969): Pollen stratigraphy of the Lake of Vico (central Italy). Pa!aeogeography, Palaeoclimatology, Palaeoecology, 6: 6785. Funiciello, R. & Parotto, M. (1978): Ii substrato sedimentario hell'area dei Colli Albani: considerazioni geodinamiche e paleogeografiche sul margine tirrenico dell'Appennino centrale. Geologica Romana, 17: 233-287. Hutchinson, G.E. (ed.) (1970): Ianula: an account of the history and development of the Lago di Monterosi, Latium, Italy. Trans. Am. Philos. Soc., n.s., 60(4): 1-178. Kelly, M.G. & Huntley, B. (1991): An ll000-year record of vegetation and environment from Lago di Martignano, Latium, Italy. Journal of Quaternary Science, 6(3): 209-224. Lulli, L., Bidini, D., Lorenzoni, P., Quantin, P. & Raglione, M. (1990): I suoli caposaldo dell'apparato vulcanico di Vico. Istituto Sperimentale per Io Studio e la Difesa de! Suolo, p 158, tip. Coppini; Firenze. Magri, D. (1989a) : Interpreting long-term exponential growth of plant populations in a 250,000-year pollen record from Valle di Castiglione (Roma). New Phytologist, 112: 123-128. Magri, D. (1989b): Palinologia di sedimenti lacustri olocenici a Lagaccione, presso i! Lago di Bolsena. Giorn. Bot. It., 123(5/6): 297-306. Magri, D. & Ciuffarella, L. (1991): Incendi e vegetazione nella Campagna Romana durante il Quaternario superiore. Giorn. Bot. It., 125(3): 288. Magri, D. & Follieri, M. (1992): Transitions from interglacial to glacial at Valle di Castiglione (Roma). In: Kukla, G. & Went, E. (eds) Start of a Glacial, Proceedings of the Mallorca NATO ARW, NATO ASI Series I, Volume 3: 23-36, Springer Verlag; Heidelberg. Magri, D. & Giardini, M. (1992): Vegetational features of the "lastpleniglacial interstadia!s" in central Italy. Abstracts 8th Palynological Congress, Aix-en-Provence, 95. Magri, D. & Narcisi, B. (1992): Annually laminated sediments at Val!e di Castiglione (Roma, Italy). Journal of the European Study Group on Physical, Chemical, Mathematical and Biological Tec~mi_ques Applied to .Archaeology, PACT, in the press. Mattias, P.P. & Ventriglia, U. (1970): La regione vulcanica dei Monti Sabatini e Cimini. Mem. Soc. Geol. It., 9: 331-384. Me~zeltin, S. & Vezzoli, L. (1983): Contributi alla geologia del Vulcano di Latera (Monti Vulsini, ToscanaMeridionale-Lazio settentrionale). Mem. Soc. Geol. It., 25: 247-271. Montelucci, G. (1976-77): Lineamenti della vegetazione del Lazio. Annali di Botanica, 35-36: 1-107. Narcisi, B., Anselmi, B., Catalano, F., Dai Pra, G. & Magri, G. (1992): Lithostratigraphy of the 250,000 year record of lacustrine sediments from the Valle di Castiglione crater, Roma. Quaternary Science Reviews, Ii: 353-362. Segre, A.G. (1975): Morfologia e Quaternario della zona OsaCastiglione. Bull. Paletnol. It., 81 (1972-74): 259-275.
GEOPHYSICAL
MAPPING
OF ORGANIC SEDIMENTS
Stefan Wende and Reinhard Kirsch Institut ffir Geophysik der Universits K i d Olshausenstr. 40, D-2300 Kiel 1
ABSTRACT Different geophysical methods can be applied to estimate lateral and vertical extensions of organic sediments as found at maar lakes or peat areas as an addition to the methods of drilling. We will demonstrate results of three-component seismic surveys and electromagnetic mapping carried out over org~Ific soils in different re#ons of Northern Germany. High resolution seismics allowed to map the base of organic sediments (e.g. peat) down to depths of at least 12 m with an accuracy of about i m. Vertical se~.smic profiling from borehole shots allowed to resolve internal boundaries of the organic layers due to different degrees of compaction. The mapping of electric conductivity by electromagnetic induction methods proved to be a fast method in detecting lateral boundaries between organic and mineralic soils.
INTRODUCTION Lnformation about laterM and vertical extension of maar lake sediments or peat bodies used to be achieved by drilling. High resolution geophysical exploration methods - - a.s used in the field of near-surface engineering work - - might be applied to enable a better correlation of drilling results. In the following paper seismic arzd electromagnetic measurements at different locations in fens and bogs of Northern Germany are described. Seismic exploration is based on the variation of seismic velocities of underground structures. In contrast to sediments which are mainly composed of minerals, the velocities found in organic soils are extremely slow. Exploration in peat areas is rather difficult due to a high attenuation of wave energy in organic material. Shear waves were found to be less subject to this attenuation than compressional waves. Using short source-receiver offsets up to 50 m, shear-wave reflections from depths of 15 m could be recorded. With three-component seismic data, representing the complete seismic wave field, it was possible to improve the interpretation of seismic arrivals.
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Electromagnetic exploration is based upon contrasts of electric conductivity of different soil types peat shows a significantly higher electric conductivity than mineral sediments free of clay. In electromagnetic exploration, a high frequency EM field of a transmitter coil induces eddy cur-
-
-
rents in the subsurface. These currents induce a secondary EM field which is recorded by a receiver coil. The comparison of primary and secondary EM field leads to a determination o f the average electric conductivity of the uppermost subsoil.
FIELD TECHNIQUES F O R SEISMIC MEASUREMENTS IN P E A T In areas with soils of mainly organic content (e.g. peat), the well established m e t h o d of seismic prospection faces a variety of problems. These include a high absorption of wave energy, highly variable coupling of source and receivers to the ground and limited access to the area of prospection. Field methods and seismic sources had to be modified to be employed in these special regions. Figure 1 shows coupling plates for the generation of shear waves in peat areas: the large vertical plates of the source in the background guarantee a firm coupling to the ground. W h e n struck sidewards with a hammer, shear waves are generated. By turning the source 90 deg., shear waves oscillating in two (perpendicular) horizontal directions will be generated. Hitting the ground vertically will result in vertical oscillations.
Figure 1: Left: Three-component receiver (triphone). Center: S t a n d a r d s h e a r wave harrow. Right: Modified shear wave source for use in soft, organic soils. Large base and sidepia~es allow firm coupIing to the ground.
Thus, seismic waves oscillating in three perpendicular directions can be generated (let X denote inline-, Y crosshne- and Z vertical directions). On the left of fig. 1, a three-component receiver (triphone) is shown. Data traces from eight triphones can be recorded simultaneously by a digital seismic recording unit which was developed at the Institut ffir Geophysik, Universit/tt Kit1. In addition to hammer-impact sources blasting caps were used to generate compressional waves in a borehole. A field crew of three to four was required for seismic surveys.
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PROCESSING AND DISPLAY OF THREE-COMPONENT SEISMIC DATA Recording full three-component seismograms simplifies the discrimination between noise and signal and between different types of seismic waves. The most obvious way of displaying three-component seismic data is a plot of oscillation trajectories. Fig. 2 shows two of these polarization diagrams as pseudo 3-D plots. However, these diagrams are inefficient for a routine interpretation, because only small time windows can be displayed in such a way. Furthermore, information about traze-to-trace correlation of seismic events is completely lost.
Figure 2: Left: Polarization diagram of a ch'rec~ shear wave oscillating cross-line (Meyer, 1989). Right: A direct compressional wave, polarized along in-line X-direction.
Interpreting events from the nine single component (X, Y, Z-source ~ X, Y, Z-recelver) sections also proves to be quite difficult. For this reason, the nine raw data sections might be reduced to three color-coded a~tribute seismograms (for sources X,Y,Z) by the following scheme: For a given source type ( x , v , Z ) the amplitudes of these composite seismic sections are given by the maximum instantaneous amplitudes of the recorded raw X-,Y-,Z-sections. The color-code gives information which instantaneous oscillation direction was found for a given time sample. Light grey denotes an oscillation mainly in vertical direction (Z), dark grey in horizontal in-line direction (X) and black in horizontal cross-line direction (Y). Examples for this type of composite seismograms are shown in figs. 3, 4 and 6. Eigenvalue analysis of three-component seismic data allows the calculation of polarization quality and -direction for a wavefield. Again, these parameters can be used to color-code sections and improve interpretation. All data processing and display routines are implemented on micro-computers and on the recording unit. By this means, first quality controls are possible during field work (Wende et al., 1991).
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ESTIMATION OF THE THICKNESS OF A PEAT LAYER W I T H C O M P R E S S I O N A L AND SHEAR WAVES AT T H E "DOSENMOOR", NORTHERN GERMANY Reflection and refraction seismic measurements were carried out in a central p a r t of the raised bog "Dosenmoor" near Neumfi.uster, Northern Germany. Geological survey drillings found a peat layer extending down to about 8 m, with an internal boundary between near-surface "WeiBtorf" and more compact "Schwarztorf" at depths around 5 m. Below 8 m~ anorganic m u d or sand was found (Schuschan, 1989). Figs. 3 and 4 show the results of in-line and cross-line wave excitation, respectively.
Figure 3: Composite seismic section: instantaneous amplitudes color-coded wi~h instantaneous polarization direction (light grey: Z, dark grey: X, black: Y). Source: in-line (X) ar 69 m (i.e. S V shear wave). Note strong surface-wave noise energy.
The in-line shear blows of fig. 3 produce strong direct compressional waves (P-waves), with velocities Vp of 45 m / s (color-coded in light grey shade). This extremely slow wave velocity is due to a high content of gas in the pore filling (Yaramko and Ses'Kov, 1979). At source-receiver distances of more than 6 m, a refractedP-wave with Vp of 80-150 m / s is color-coded in dark grey shades, i.e. the main direction of polarization is vertical. Analysis of intercept times and wave velocities result in a depth of 6 m for the refracting layer. A strong seismic event at intercept times around 1.2 s is polari~.ed in the Z-direction - - taking into account the small source-receiver offset, it must be interpreted as a steep-angle, vertically polarized (SV) shear wave reflection. Ray tracing modelling yields a reflector depth of 9 m with a SV-velocity of Vs ~"' = 22 m/s. In fig. 3, the direct SV-wave is subdued by strong surface Wave noise with velocities around 15 m/s. Fig. 4 shows the result of cross-line shear wave excitation. The section is dominated by horizontally polarized shear wave energy (SH-waves, color-coded as black). In addition to the prominent reflection arrival at 1.2 s: some weaker near-surface reflections can be recogni~.ed which were hidden hy surface wave noise in fig. 3. Reflector depths axe 5 and 9 m with velocities of 16 and 22 m/s.
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Figure 4: Composite seismic section: instantaneous amplitudes color-coded with instantaneous polarization direction. Source: cross-line (Y) at 69 m (i.e. SH shear wave). Note weak SH-reflection at 0.8 s (compare fig. 3, where shallow reflections are ohscuretd by noise).
The rather low attenuation of compresslonal wave energy at the "Dosenmoor" site allowed the registration of a vertical seismic profile. For this VSP, blasting caps were shot at 1 m intervalls at borehole depths ranging from 2.5 to 8.5 m. Single channel seismic traces with Z-geophones were recorded at an offset of 0.5 m to the well mouth. The resulting VSP of fig. 5 shows downgoing P-waves with Vp = 60 m / s in the near-surface layer down to depths of 4.5 m, and with 120-160 m / s in a layer extending down to 8 m. The two layers are interpreted as "WeiBtorf" and "Sehwarztorf". The differences in velocities are likely to depend on different degrees of water and gas saturation and compaction. Beyond 8 m depth, high P-wave velocities are found due to anorganic mud or sand. The high velocity contrast from peat to sand at this boundary results in an upgoing P-reflection.
Figure 5: P-wave vertical seismic profile. Sources: blasting caps in borehole. Receiver: single channel Z-geophone. Differences in velocities and upgoing reflections indicate two boundaries at
4.5 and 8 m depth.
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MAPPING OF A PEAT-SAND BOUNDARY WITH SHEAR WAVES AT THE "BELAUER SEE" SITE, NORTHERN GERMANY Shear wave reflection seismic measurements at the "Belauer See" site were carried out to map the boundary of fen peat to ancient sea-bottom sediments.
Figure 6: Composite seismic section. Source: cross-fine ( V) at 25 m. No~e the sh/ft of ~he reflection hyperbola apex with increasing travel ~ime due ~o a dipping reflector. The second reflection is a multiple. At the lef~most ~races, onsets of a refracted SH-wave can be seen.
Figure 7: Comparison of results from shear wave re~ec~ion profiling and drilling (drilling resut~s pets. comm. H. Usinger, Botanisches lnsti~ut, Universit~t Kiel). Depths estimated by shear wave seismics are denoted by symbols, drilling results are drawn as solid lines.
An example of a spht-spread reflection section with a Y-shear source situated at 25 m is presented in fig. 6. The obvious shift of the reflection hyperbola apex towards lower X-coordinates is a result
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of dipping layers. Reflection hyperbolas following the first hyperbola are interpreted as multiple seismic events. Ray tracing modelling of the complete set of seismic sections leaxts to a model of near-surface peats with V s ' " = 32 m/s overlaying anorganic sea-bottom sediments with Vs "~" = 150 m/s. The depths of peat-sand boundaries, as established by seismic methods, axe in good accordance with depth values found by drilling (fig. 7). The extremely slow seismic velocities and signal frequencies which are characteristic for organic soils result in low signal wavelengths, and thus allow a high vertical and horizontal resolution.
ELECTROMAGNETIC
MEASUREMENTS
IN PEAT
AREAS
Electromagnetic surveys are more rapidly performed than seismic field work and require only one operator, resulting in less environmental impact. Due to the potential character of EM fields, all depth estimations must remain ambiguous. However, for mapping lateral extensions of peat bodies EM methods showed reasonable results. Using a model EM-31 induction instrument, a maximum depth penetration of 9 m can be expected. The electric conductivity of peat is significantly higher than those of the adjacent anorganic soils. Fig. 8 shows two parallel profiles crossing the outer boundary of the "Dosenmoor". Electric conductivities of 7-10 m S / m for mineral soil and 20 mS/m for peat were found.
Figure 8:
Two paradlel profiles (a, b) across the boundary of mineral soft to peat showing electric conductivity vadues from EM-31 measurements. Peat was found at coordinates 30-100 m. Note large transition zone as a result of increasing thickness of the peat layer.
Profile fig. 9 hes in a central part of the raised bog "Dosenmoor". Here, the electric conductivity of peat is 15-17 mS/re. Between coordinates 60-200 m, a region with low electric conductivity of 6 mS/m was crossed, indicating a large near-surface mineral inclusion into the peat bog.
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Figure 9: EM profile crossing a mineral body in a peat bog. The mineral body with an electric conductivity o[ 6 m S / m was Iocated between coordinates 60 and 200 m.
CONCLUSION
Geophysics/exploration methods such as seismic surveying and electromagnetic mapping can o~er the following information in addition to drilling results: 9 depth extent of the organic layer (as found in peat bogs or maax lake sediments) 9 internal boundaries (e.g. "Wei/]torf"/"Schwarztorf") 9 lateral extension of an organic body Seismic velocities of the organic layer might be used as indicators for gas content, water saturation and compaction. Results of geophysics/surveys can be used for interpolations between drillings.
REFERENCES
Meyer, J.H. (1989): Darstellung und Verarbeitung vektorieller seismischer Wellenfelder am Beispiel yon in-situ Untersuchungen yon Kompressions- und Scherwellen in HolozKn-Torfen. Dissertation, Universitgt Kiel. Schuschan, A. (1989): Pollenans/ytische Untersuchungen an einer postglazis/en Kernfolge aus dem Dosenmoor bei Einfeld. Diplomarbeit am Botanischen Institut der UniversitKt Kiel. Wende, S., Marzahl-Kroekauer, K., Meyer, J.H., Kirsch, R., Stiimpel, H. und Meiner, R. (1991): Scherwellenseismik in organischen B6den. Final report, Deutsche Forsehungsgemeinschaft. Yaramko, V.N. and Ses'Kov, V.E. (1979): Construction properties of soils: Elastic and dissipative properties of peats and organic muds. Soil Mech. Found. Eng., 16, 17-22.
PRELIMINARY UNIBOOM SURVEY OF THE MONTICCHIO LAKES (SOUTHERN ITALY) A. Stefanon Istituto Universitari 0 Navale, Via Acton No. 38 1-80133 Napoli, Italy
During spring 1991 a preliminary survey with an UNIBOOM system was carried out in the Monticchio lakes. Aim of the study was to detect the thickness of the uppermost, heavily polluted sediment layer which should be dredged out to improve the environmental conditions of the lake. The freshwater lakes consist of a couple of small volcanic craters derided by a narrow ridge. The smaller one has a size of about 500 x 350 m with a maximum depth of 38 m, and the bigger one is about 850 x 600 m with a maximum depth of 36 m, according to a detailed bathymetric survey carried out by Prof. Negendank (Univ. of Trier). The larger Lago Grande di Monticchio reaches the maximum depth of 36 m at the bottom of a sharp, elongated, funnel-like depression, located on the north-western side. The remaining part of the lake (about 60 %) is rather flat and less than 10 m deep. The smaller Lago Piccolo di Monticchio has no shallow areas at all. Up to 52 m long cores, collected by Prof. Negendank's team in the shallow part of Lago Grande di Monticchio, show an almost continuous lacustrine sequence of organic and finegrained minerogenic sediments (cf. Zolitschka, this vol.). High gas contents has been detected, especially at the bottom depression. The small size of the water surface did not allow an optimum handling of the towed gears and caused difficulties in running the UNIBOOM survey. The records revealed very peculiar features, in spite of the massive gas diffusions into the sediment, blanking out the sound penetration below a few metres. On the northern sides of both lakes the bottom and the walls reveal some sharp irregularities, with a maximum relief of about two metres. They were interpreted as surface tectonic evidence of deep movements, most likely related to the latest evolution of the local volcanism. Lago Piccolo di Monticchio is characterized by a funnel-like structure, showing regular, steep walls and a rather flat bottom. No sedimentary structures related to the flanks appear on the records. It is hard to say, whether this is due to the steep angle or to the presence of outcropping rocks. The flat Lecture Notes in Earth Sciences, VoL 49 J. F. W. Negendank. B. Zolitschka (Eds.) Paleolirrmology of European Maar Lakes 9 Sprlnger-Verlag Bedin Heidelberg 1993
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bottom reveals a very fine, regular sedimentary sequence with sound penetration of up to 8 m.
Lago Grande di Monticchio reveals a different structure with very thick and laminated sediments, where more than 150 volcanic ash layers were detected (Negendank, pers. comm.). Evidence of horizontal, finely stratified sediments were detected outcropping on the walls of the depression, which seems to be due to a violent eruption, ejecting parts of the former, shallow lake bottom. The author believes that temperature and density anomalies found by some investigators in the lakes water column are due to volcanic vents, correlable to the tectonic structures evidenced by the UNIBOOM survey. Further investigations are planned to verify these assumptions and to date the last volcanic event of the Monticchio lakes.
SONAR INVESTIGATIONS IN THE LAGHI DI M O N T I C C H I O (Mt. VI3LTURE, ITALY)
Ralph B. Hansen*
Dept. of Geology,Universityof Trier, D-5500 Trier
ABSTRACT Sonar profiles across the Lago Grande and Lago Piccolo di Monticchio (two lakes in southern Italy, 20 km S of Melfi) were recorded to get knowledge on the lake basins an their surface prior coring. The combination of echo-graph data with digital landscape modelling was suitable for the detection and interpretation of complex structures of the lake bottom. The interpretation of the model shows the distortion of an old continuos sedimentation by younger tectonic events. The presence of terraces above and below the present-day lake level are interpreted as response to paleoclimatic fluctuations and human activities.
INTRODUCTION In 1990 four sediment sequences were taken out of Lago Grande di Monticchio. The core sequences were drilled from a special raft, using the Usinger-Corer for unconsolidated sediments. The obtained core sequences were described and analysed within the project EUROMAAR,financed by the EC (Zolitschka & Negendank, this volume). Exept the maximum depth of around 40 m, nothing was known about the lake floor-morphology. Therefore detailed echo profiles with the X-16 sonar system have been carried out preceeding the coring activities. The aims cover three fields: 1. Geomorphologic interpretation of the lake bottom and its surroundings. 2. Suitable drilling location for deep coring. 3. Information on properties of sediment and lake water.
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METHODS As sonar equipment we used a compact LOWRANCE echo-graph (Mod. X-16). The model is battery-powered, easy to install even in very different boats and can be used with different frequencies and different sonar-angles depending on water depth. The data for this study refer to 192 kHz sounder with an angle of 20 ~ The obtained data were digitised, recalculated and transferred to a spreatsheet for data processing. It has to be taken into account that the sonar system is not registrating water depth. The system can detect time differences of a given signal, which is reflected from the ground. The emitted signal covers an angle of 8-20", so that reflections coming vertically from the ground are detected first. Furthermore the incoming sonar signals vary depending on physical properies of the bottom. The sonar echoes coming from a smooth, unconsolidated surface were registrated with unsharp contours without the so-called "grey line". In contrast, reflections coming from a hard and flat surface show a thin "grey-line" with a clear upper level. For 3-dimensional modelling of the lake floors we used the SURFER-program (release 4.12). Some smaller inaccuracies occurring in the shallow lake regions between 0 and 1.5 m water depth were corrected manually. To reduce interpolation errors the shoreline as a boundary line was added to the data base as well as some topographic values of the surroundings. These data have been obtained from the topographical map, sheet F'-187 Melfi (scale 1 : 25,000). All profiles stopped approx. 10 m in front of the shoreline, because the litoral vegetation is legally protected. The water depth in those areas is between 1.5 and 2.0 m, so there was no problem to fit the missing parts to the database.
GEOMORPHOLOGY The Laghi di Monticchio are embedded in the lowermost part of the Monte-Vfilture caldera. The Monte-Vfilture complex is build up by K-alkaline series of lava flows and tephra. NE of the lakes, the maximum altitudes are 1326 m a.s.l. The water level of the two lakes nowadays differs by 1 m (Lago Grande: 656 m a.s.l., Lago Piccolo: 657 m a.s.1.). Both are connected by a small artificial channel. The outlet leads to another channel to the Ferriera valley and to the Val d'Ofanto, W of the Monte-Vdlture complex.
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Before canalisation, a common water table of around 4.5 m above the present level existed in the two lakes. Some parts of the lakeside roads around the lakes follow that level on former lake terraces. At the base of this terrace some fine-grained, well sorted sands can be found in stratified layers. These sediments have been also found in the eastern part of the Ferriera valley along the main channel. Here they cover most of the central parts of the valley. Towards the slopes some debris material is situated on top of the sediment sequence. Between the level of the Ofanto river and the lakes level there is a remarkable difference in altitude of 360 m at a distance of only 7 km. Because of the steep gradient and a continous high runoff, coming from the mountains, the erosion rates are very high. Specially the lower drainage basin of the valley is characterised by a rapid downcutting in a ravine-shaped valley since the formation of the drainage pattern. This can be seen near the railway station of Aquilonia. The material which builds up the wall be~'een the basins can be characterised in two small outcrops N to the ruins of the ancient monastery San Ippolito. Here volcanic lapilli and decomposed volcanic bombs cover older pyroclastics. Some parts of the younger pyroclastics are stratified. They resemble the ejecta around maar lakes, indicating a phreatomagmatic volcanic activity. These pyroclastics consists of dark, coarse grained volcanic tephra with some thin lava flows and sills. Underneath the younger volcanic layers older lake sediments are supposed. The unstability of the deeper strata probably has been the reason why the buildings of the old monastary were abandoned. The western part of the buildings are moving downslope towards the Lago Grande under their own load.
122
B A T H Y M E T R Y O F T H E L A G O G R A N D E DI M O N T I C C H I O In the north-western part of the lakes surface the Lago Grande reaches the deepest parts of the basin. This area is clearly outside the morphological centre of the lake. Fig. 1 shows a sonar profile SE-NW. Here the maximum depth of around 33 m is reached in two separate basins, separated by a barrier. This structure can also be seen in different sonar profiles which have been recorded in 10-15 m distance from the SE.NW profile. The barrier reaches 6-8 m height from the lowermost parts of the basins. In consequence we can expect an asymmetric shape of the lake basin. This can be seen in an echogram, showing a S-N-profile (Fig. 2).
According to these profiles the bottoms surface can be divided into tree units: Zone 1:33-24 m water depth, covering the deepest parts of the lake, including the barrier. Zone 2:24-16 m water depth, covering the steep slope areas in the northern area. Zone 3:16-0 m water depth, covering the shallow part in the south of Lago Grande. Zone 1 is characterised by unsharp reflections of the sonar signal, the surface is rather rough, showing big differences in the region near the slopes. These structures may be interpreted as a result of some smaller turbidites or slumpings, which transported finegrained material from the slopes towards the maximum depth. The sonar echoes were registrated with very unsharp contours without the so-called "grey line". The sediments of this area were build up of very soft and unconsolidated material with a high content of poorly decomposed organic matter. This corresponds to the character of the sediment samples taken from a test core coming out of the deepest parts. The sediments did not show any stratification. The gas content was remarkable.
123
Good reflections were detected in the barrier area, indicating a different sediment/ground character. These reflections are very similar to those we found on a hard surface, i.e. compacted sediments and/or hard bedrock. Anyway it can be concluded, that the barrier consists of a harder material, compared to the sediment fill of the two funnel-like structures (vents?) of the deepestmost parts. Zone 2 covers almost all of the slope area in the northern lake basin. The slopes generally are very steep. The thickness of the unconsolidated sediment cover can be interpreted as thin, because most of the reflected signals were strong. Between 22-30 m water depth as well as in the depth between 12-16 m the profiles show a very sharp edge in the bottoms surface. In the S-N-profile (Fig. 2) the obtained reflections are indicating nearly vertical slopes. Because there are almost no reflections there is no information on the material or the surface of these vertical sections. One could argue that it should be a very hard material like a lava flow to maintain a vertical structure for a long time. In the drilled sediment sequence of 51 m (location approx. 500 m S of the slope area) we did not find lava flow-like material. Zone 3 includes more than 2/3 of the lake's bottom surface, which corresponds more or less to the complete southern half. The depth is ranging from 0-2 m to a maximum of around 12 m towards the northern part. Besides a SE-NW oriented valley-shaped structure the surface is poorly structured. The structure could be interpreted as a drainage pattern during times of a lower water table. If this interpretation is correct missing parts of the sediment sequence due to terrestrial erosion should be expected. Figs. 3 and 4 show the isobaths of the Lago Grande di Monticchio. The isolines follow each other in 2 m distance.
Because of the
flatness of this area the Surfer-program
had some
problems to interpolate the data during the gridding.
124
Particularly in the depth-range 0-1m there was no congruence with the lakes border line. These effects could be reduced by the addition of an exclusive border line, digitised form the topographical map. Even than it was necessary to add more data in these areas by a data editor. It should be recognised that in the topographical map the border line of the western lakes border is drawn as uncertain.
BATHYMETRY
OF THE LAGO PICCOLO
DI M O N T I C C H I O
The morphology of the Lago Piccolo basin is less complicated. All the slopes are very steep, the 10 m isobath is reached in approx. 10-12 m distance from the shore (Fig. 5 and 6). In the eastern part of the lake, especially on the slopes, some slumping processes may be the cause for unconformities in the slope angle. Here very shallow parts are alternating with deep, subaquatic ravines. The surface of these areas seems to be rather rough
and irregular. The obtained reflections are very sharp. They are indicating a hard reflector at the bottom surface. As a whole these parts show block-like structures. They can be regarded as bigger rocks originating from the inner crater walls. Due to tectonic ac-
125
tivities in the Mt. V~Iture complex it can be assumed that these blocks rolled into the lake during one of the numerous earthquakes. Fissures and cracks SW of the new monastery building are oriented parallel to the eastern crater wall. The tectonic conditions as well as some missing parts in the eastern slope profile may indicate the former location of those blocks. The structures can be seen in the SW-NE profile (Fig. 5). Here one of the blocks reaches an altitude of = 10 m from the bottom. The Lago Piccolo is almost completely surrounded by the 4.5 m terrace.
The
new
lakeside
road follows that level. At the base of this erosion terrace some fine-grained well sorted
sands
have
been found in stratified layers. They correspond to sediments which are present in the eastern part of the Ferriera valley along the main channel.
GEOMORPHOLOGICAL
CONCLUSIONS
Fig. 7 shows the 3-dimensional interpretation of the lake area, calculated on the base of all available field data. In the model it becomes evident that the shallow ridge between the two lakes appeared when the artificial channel in the Ferriera valley was excavated.
126
This caused a lowering of the lake level of around 4.5 m. So it can be concluded that there was one water level for the two lakes before the 18th century. In consequence the hydrochemical conditions should be very similar in the older parts of the sediment record. It can be assumed that possible differences in the hydrology-timnology during the deposition of these sediments are first controlled by the water depth and not by the affiliation to the smaller or to the larger lake basin.
The deepest parts of the two crater basins can be seen in the model as furmel-shaped holes. In the eastern basin (Lago Piccolo) the very steep crater wall above the lake level fits directly to the lakes subaquatic shape. The Lago Grande shows in the southern part a very smooth and shallow lake bottom's surface. This part is slightly inclined versus the deeper north-west and characterised by a poorly developed drainage pattern. The drainage pattern indicates a (temporary?) lower lake water level of at least 10 m below the
127
present-day level. In addition to the former lake level, which formed the 4.5 m terrace we can expect a total lowering of the water table of more than 15 m.
DRILLING
LOCATIONS
Usually the best drilling position in lakes is the deepest part of the lake situated in the centre of the lake. Here a continous sedimentation of fine grained material can be expected. In Lago Grande the deepest part does not correspond to the central part. The maximum depth is reached in the north-west. Furthermore there are two possible localities in the deepest region. Because these areas indicate a close relationship to the steep slopes of the funnel, we can expect a complete different sediment infill. Based on the obtained echo profiles it can be concluded that the structures in the deepest part of the lake are very young. There is no reason to choose this area in the Lago Grande as a suitable coring location. The better position will be the shallow part (zone 1) outside of the funnel-like structure in the NW.
SEDIMENTS AND LAKE WATER Due to the high frequency of the acoustic waves it is not possible to enter the sediment and to get detailed information on sediment composition as it is known from othe geophysical investigations. The information about the uppermost sediments are restricted to only a few details, concerning the hardness of the reflecting material. "Hard" sediments, i.e. sediments consisting of sand, gravel or fine grained clastic material, give clear reflections of the sonar signals. In the echo graphs of the X-16-System this can be seen from the sharp upper llne, resulting from the first incoming signal and the sharp lower line, limiting the so called "grey-line". If there is no clear border between water and sediment, the first incoming signals are reflected by strong differences in water density. By use of higher amplification rates even very small differences in water density can be observed. Those may be caused by changes in salinity and/or water temperature. The sediments of the Lago Grande generally are build up by fine grained clastic material with a certain amount of organic matter. This is referred to the normal sedimentation process, recorded in the sediment sequence of the southern lake basin. In contrary the
128
sediment sequence in the deepest parts of the northern area seems to be build up in a very short time. Here a large amount of unconsolidated, highly organic material as bad reflectors can be observed. In the zones 1,and 2 (16 m-33 m) there are no echo signals coming from fishes and fish swarms. All of these biota are concentrated in the zone 3, i.e. in the region between water surface and approx. 4 m water depth. Because of the absence of such signals in depths of more than 4 m it can be concluded that most of the lake's sediment surface is exposed to anaerobic conditions. At higher amplification rates another reflector can be seen in the depth of around 10 m in the Lago Grande and 13 m in the Lago Piccolo. This reflector has been interpreted as a sparse thermocline in the water column. Here as well as in the O2-content of the water there are big differences between the two lakes of Monticchio. As shown, the O2-1imit can be found of Lago Grande at 4 m water depth, in Lago Piccolo the 02 undersaturation starts at 12 m depth. This effect is caused by strong eutrophication of Lago Grande resulting from direct waste water disposal into this lake. Due to the growing tourism in the region (on a single Sunday up to 30,000 visitors, using numerous restaurants and other facilities) this effect will influence the Lago Piccolo in a few years.
CLIMATIC AND TECTONIC EFFECTS ON SEDIMENTATION IN CENTRAL ITALIAN VOLCANO LAKES
(LATIUM)
-
LMPLICATIONS FROM HIGH
RESOLUTION SEISMIC PROFILES
F. Niessen*, A. Lami+ & P. Guilizzoni+ *G-eologisches Institut, ETH-Zentrum, CH-8092 Ziidch, Switzerland +C.N.R. Istituto Italiano di Idrobiologia, 1-28048 Verbania-Pallanza, Italy
ABSTRACT Pilot studies have investigated the sedimentological records of Lakes Bolsena, Bracciano and Albano (Latium, central Italy) for their climatic and tectonic histories. Literature has also been reviewed to assess the chronology of lake formation during the Pleistocene (for the above lakes c a . 150 ka, 100 ka, 50 ka BP, respectively) and for evidence that the large Lakes Bolsena and Bracciano were affected by young regional tectonic activity and are not typical caldera lakes. Our field studies, using a high resolution seismic reflection method (3.5 kHz system), focus on the sedimentary evolution of the above lakes. Seismic stratigraphy shows three units with similar characteristics in all three lakes: unit I, interpreted as Holocene deposits, overlies latest Glacial and last Glacial maximum sediments (unit II), over middle- and early-Wtirm deposits (unit III). Truncation of up to 50m-thick sediment packages on one side of both Lakes Bolsena and Bracciano is observed. The truncation was caused by faults which were probably active toward the end of the last Glacial. A shallowing-upward deepening-upward lacustrine sequence subsequently developed in places. Lake level changes appear to have been controlled by tectonic- and/or climatic boundary conditions for Lakes Bolsena and Bracciano. There is no evidence for synsedimentary faulting in the maar Lake Albano.
1. INTRODUCTION The potential of volcano-lake sediment sequences for palaeoenvironmental studies are discussed elsewhere in this volume (e.g. NEGENDANK & ZOLITSCHKA). With respect to palaeocIimatological interpretations, volcanic lakes have indeed several advantages in comparison with other types of lake basins. They are characterized by small catchmentto-lake-surface ratios which result in little spatial integration of climatic signals. Often they have sediment records extending back in time for several tens of millenia that have Lecture Notes in Earth Sciences, Vol. 49 I. F. W. Negendank, B. Zolitschka (Eds.) Paleolimnolc~;y of European Maar Lakes 9 Springer-Verlag Berlin Heidelberg 1993
130
not been interupted by glacial activity. Varves are commonly present, providing high time resolution. On the other hand many volcanic lakes are situated in areas which are still tectonically or volcanically active with the consequent effects on aquatic sedimentation. For example, synsedimentary faulting can lead to characteristic depositional patterns (e.g. VAIL 1987, SANDER & ROSENDAHL 1989) superimposed on climatically induced shifts in sedimentation. Seismic reflection methods are therefore particularly suitable for investigating the past climatic and tectonic effects on subaquatic sedimentation (e.g. BALLY 1987). Both climatic and tectonic imprints on lacustrine sedimentation are expected for many of the Italian volcano lakes on the Latium volcanic terrain (central Italy). The area was affected by both large climatic shifts during the Quarternary (e.g. HUTCttlNSON & COWGILL 1970, FRANK 1969, FOLIERI et al. 1988, KELLY & HUNTLEY 1991) and relatively strong tectonism and volcanism until recently (VAREKAMP 1980, LOCARDI et al. 1976). SurFrisingly little is known about the history of the basin fill, particularly for the deep Latian lakes and their records of tectonic and/or environmental changes. During a cooperative Italian-Swiss campaign in 1990, field studies on six of the Latian lakes investigated the gross character of the basin fills (reported he-re) and the history of eutrophication (MASAFERRO et al. 1992, LAMI et al. 1992, LAMI et al. in prep.). Here we focus on the influence of Pleistocene and Holocene climatic changes and neotectonic/volcanic processes on sedimentation in the two large caldera lakes (L. Bolsena, 114 km 2, maximum depth 151 m; L. Bracciano, 57 km 2, maximum water depth 165 m) and the smaller maar-lake, Lake Albano (6 km 2, maximum water depth 175 m). To provide the reader with necessary background information, a brief literature review on the origin, volcanic and tectonic histories of the lakes is now presented. New evidence is also presented to demonstrate that (i) sediment sequences in all three lakes show comparable cyclic changes on the order of several 10 ka, which are probably due to climatic changes and, (ii), that faulting in the two caldera lakes, probably during late Pleistocene, resulted in vertical displacements of older lake deposits and changed the subsequent sedimentation patterns.
2. AN OVERVIEW OF THE ORIGIN AND CHRONOLOGY OF THE LAKES Volcanism in Latium is related to post-Miocene distensional horst-and-graben tectonics in a belt between areas of crustal growth to the NE (Apennines) and crustal thinning to the SW (Tyrrhenian Basin). For details concerning the link between tectonism and petrogenesis the readers are refered to overviews given by LOCARDI et al. (1976) and PECCERILLO & MANETTI (1985). Two major volcanic phases can be distinguished: a Tertiary series with acidic products and a Quaternary eruption series with a potassic alkaline character which continued until recently (Fig. l). All the volcanic lakes in Latium are located in a widespread Pleistocene volcanic terrain (Fig. 1), mostly of
131
alkaline pyroclastic flow deposits. Four major volcanic areas can be distinguished: Vulsini, Vico, Sabatini and Alban Hills (Fig. l). Radiometric dating and stratigraphic evidence suggest that the volcanism in these four areas was almost contemporary, although each area is characterized by several well differentiated volcanic phases and different types of lake basins formed at various times.
Fig. 1: Geological map of the Latium volcanic district (redrawn after LOCARDI et al. 1976, TORO 1976, NAPPI et al. 1991). Only the formation of Lakes Bolsena (Vulsini), Bracciano (Sabatini)and Albano (Alban Hills) is discussed here. Because of their characteristic circular shapes, the origins of the large lakes Bolsena and Bracciano has often been related to subsidence and caldera formation during the final stages of the Bolsena and Bracciano volcanic activity (e.g. PICHLER I970). However a direct control by large-scale tectonic processes may also play a major role (VAREKAMP 1980, WALKER 1984, NAPPI et al. 1991). VAREKAMP (1980) suggested that the Lake Bolsena basin is largely the relict of a major NW-SE graben structure, similar to the Radicofani and Tiber grabens to the northwest and east respectively
(Fig. l),
rather
than
a simple
caldera.
According
to
his
interpretation, the palaeo Bolsena depression was formed about 400 ka BP; the lake already existed before the formation of the Latera caldera, at about 100 ka BP, to the west of Lake Bolsena (Fig. 1). Furthermore, NAPPI et al. (1991) calculated that the 6.5 km 3 of ignimbrites derived from the palaeo Bolsena volcano do not account for the total
132
volume of the Bolsena caldera, so tectonic subsidence must be invoked.
WALKER
(1984) argued that many Quaternary calderas are not related to catastrophic collapse events at the end of magmatic cycles, but result from long-term sagging, often without ring faults. He used the Bolsena basin as a typical example for a "sag" caldera. A complex volcano-tectonic origin was also suggested for the Lake Bracciano basin by LOCARDI et al. (1976) in the center of the Sabatini volcanic terrain. The area is characterized by a major distension-fracture zone, which developed after 900 ka BP and lead to the displacement of two major concentric volcanic blocks by about 14 km in a NNW-SSE direction (Fig. l). The movement was associated with the formation of strike-slip faults. The shifting of the two blocks appears to have caused both the sinking of the area oceupied by the present Lake Bracciano and intensive phreatomagmatic eruptions along the fracture zone. The latter led to the formation of numerous small craters of which tWO form the present lake basins of Martignano and Monterosi (Fig. 1). The origin of Lake Albano is related to intensive phreato-magmatism during the final stages of volcanic activity in the Alban Hills (LOCARDI et al. 1976, FORNASERI 1985) which was associated with the development of NW-SE oriented tectonic lines (Fig. 1). Deep explosion craters were formed near the center of that eruption zone, two of which are now occupied by Lakes Atbano and Nemi (Fig. l). The precise age of the Latian lakes and the influences of very young volcanic activity on the lacustrine systems is difficult to assess. The geochronology for Latian volcanism is incomplete because of dating problems for very young volcanic rocks. Most of the published radiometric ages for the alkaline potassic series (largely based on K-At dating) include large errors (FORNASERI 1985). The following chronological data is, if not cited otherwise, based on the compilation by FORNASERI (1985) and may be indicative for the ages of the lakes. The activity of the palaeo Bolsena volcano ceased well before 150 ka BP. Taking VAREKAMP'S (1980) tectonic arguments into account, the lake is certainly older than 100 ka and formed probably at about 150 ka BP. Thus during the early stages of the lake history, volcanie activity in the adjacent Latera complex (Fig. 1, ca 310 ka - 100 ka BP) must have had a strong effect on the lacustrine sedimentation in Lake Bolsena. Moreover, very young (late Pleistocene/Holocene?) minor volcanic activity is reported for many places in the Vulsini area. Two islands in Lake Bolsena (Bisentina and Martana; Fig.2) belong to these young local volcanic features. They originated partly as sub-aquatic "Surtseyan-type" eruptions (VAREKAMF 1980, LOCARDI et al. 1976). To our knowledge, however, there is no radiometrie data on exactly when those eruptions occurred. The chronology for the Sabatinian eruptions is still uncertain. The final major volcanic events are dated at between 442 ka and 225 ka BP. We therefore assume that the age of Lake Bracciano is I00 ka or older. The last explosive volcanism in the Alban Hills, which caused the formation of Lake Albano, is more precisely known; fission-trackand 23~ dating give respectively ages of 59 ka and 67 ka BP. Several wood samples
133
from the youngest tuffs all gave "dead" 14C ages (> 40 ka BP). At present it may be reasonably assumed that the last Albano eruptions took place at about 40 - 50 ka BP. Probably the lake formed soon after these eruptions.
Fig. 2: Left: Bathymetric maps of Lakes Bolsena and Bracciano (water-depth in metres, simplified after INSTITUTO ITALIANO DI IDROBIOLOGIA, PALLANZA 1971), location of seismic profiles (labels A to M) and short cores (labels BOL and BRA). Arrows mark locations of major stream inputs and outlets. Right: Bathymetric characterization of "platform" and "basin', location of proposed faults and channel/overbank features associated with tectonic activity.
134 There are, to our k n o w l e d g e , no l o n g s e d i m e n t c o r e s from L a k e s B o l s e n a , B r a c c i a n o or Albano. H o w e v e r cores from other Latian lakes all penetrated into s t r a t a w h i c h were d e p o s i t e d during and/or before the L a s t G l a c i a l (Wilrm, marine o x y g e n s t a g e s 2 to 5). F o r e x a m p l e , b a s a l - c o r e ages of m o r e than 14 k a for Lake Vico (FRANK 1969), 23 ka for the S a b a t i n i a n Lake Monterosi (HUTCHINSON& COWGILL 1970) and e v e n 250 ka for a f o r m e r crater lake in the Valle C a s t i g l i o n e near Rome (FOLIERI et al.
1988) are
r e p o r t e d . The latter belongs to the Alban H i l l s terrain and is located n e a r t h e p e r i p h e r y of that v o l c a n i c complex (Fig. 1). N o n e o f the cores reached b a s a l l a c u s t r i n e s e d i m e n t s . Earlier results from an air-gun s e i s m i c reflection s u r v e y of the s u b - b o t t o m s t r u c t u r e s in Lake B o l s e n a ( C . N . R . - ISMES S . p . A . 1989) r e v e a l e d e v i d e n c e for a s e d i m e n t a r y fill of ca. 650 m t h i c k n e s s , of which the u p p e r part (up to ca. 120 m) is i n t e r p r e t e d as having a l a c u s t r i n e origin.
3. METHODS P r e l i m i n a r y h i g h - r e s o l u t i o n seismic r e f l e c t i o n s u r v e y s were c o m p l e t e d in f i v e Latian lakes in S e p t e m b e r 1990. P o s i t i o n i n g was d o n e by radar. S e d i m e n t s in o n l y the three lakes d e s c r i b e d here allowed adequate s o u n d p e n e t r a t i o n . A total o f 70 kin, 4 0 km and 9 km of v e r t i c a l reflection profiles
were
recorded
respectively for Lakes
Bolsena,
B r a c c i a n o and A l b a n o with a 3.5 k H z F e r r a n t i ORE s e i s m i c t r a n s m i t t e r w i t h a 20 kW output. The received signals were filtered (100 to 7 kHz), a m p l i f i e d and s i m u l t a n e o u s l y printed by an E P C - r e c o r d e r . The maximum u s e f u l reach of our s y s t e m is a b o u t 50 m for u n c o n s o l i d a t e d g a s - f r e e lacustrine muds. The v e r t i c a l r e s o l u t i o n is b e t w e e n 20 and 30 cm s e d i m e n t t h i c k n e s s . All given travel times c o r r e s p o n d to the 2 - w a y travel paths. The i n t e r p r e t a t i o n o f the p r o f i l e s f o l l o w s the a p p r o a c h o f BALLY et al. (1987).
4. R E S U L T S A N D D I S C U S S I O N 4.1 SEISMIC STRATIGRAPHY Strong d i f f r a c t i o n of the seismic s i g n a l in the p r o f u n d a l s e d i m e n t s o f the L a t i a n lakes is c o m m o n l y r e g i s t e r e d from the lake bottom to s e d i m e n t depths e q u i v a l e n t to ca. 20 ms sonic travel time. The diffraction h o r i z o n s are often clearly d i s c o n t i n u o u s to o t h e r w i s e p r e d o m i n a n t l y s u b - p a r a l l e l reflectors (e.g. F i g . 3 ) . T h e r e f o r e w e a s s u m e the diffraction is c a u s e d by a p o s t - d e p o s i t i o n a l e v o l u t i o n o f b i o g e n i c a n d / o r v o l c a n o g e n i c gas (e.g. CO2). As a result, continuous s e i s m i c r e f l e c t o r s c a n n o t be i d e n t i f i e d in p l a c e s or they are v i s i b l e only in the t o p m o s t part o f the b a s i n fill (above 20 ms or a p p r o x i m a t e l y 14 m). M a x i m u m p e n e t r a t i o n and reflection (up to 50 ms, ca. 35 m) was r e a c h e d in only a few p l a c e s a b o v e or c l o s e to p l a t f o r m s , d o m e s or s l o p e s .
135
Fig. 3: Seismic stratigraphy and definition of units BO I - I I I for Lake B o l s e n a (location of profiles in Fig. 2). Where seismic penetration in the sediments is possible, reflectors are seen to generally drape underground topographies in all the lakes; onlaps are rare. Reflectors can usually be traced over long distances, exept for where gas diffracts the signal. There is generally some thinning of sediments over topographic highs and thickening in the topographic lows (Fig.2 to 5). This is more pronounced in Lakes Bolsena and Bracciano (Fig.3, 4) than in Lake Albano (Fig.5). The c r o s s - s e c t i o n (C-D-profile, Fig.5) for Lake Albano shows undisturbed sediments only in the western part of the profile, whereas in the central and eastern areas thick slump deposits are present at depth (chaotic, mostly diffracted signals). On the basis of the seismic records, the primary deposits can be characterized into three main units, each of which has similar character in all the investigated lakes (units I to III in Figures 3 to 5). In deep water areas (water depths below 120 ms or 84 m), units I and III are similar to each other in seismic character. Both s h o w little lateral variation in thickness and the presence of weak sub-parallel reflectors. Unit II, in contrast, is characterized by stronger reflectors and either a significant decrease in thicknesses or onlap terminations on slopes (e.g. in Lake Bracciano, Fig.4). Draping of reflectors in this setting is commonly indicative for fine grained (clay, silt) sediments which characteristically settle vertically from suspension t h r o u g h the water column. Such deposits are present in units I and III, and to a less extent in II. Some
136
s e d i m e n t f o c u s s i n g towards the center of each o f the lakes is indicated by s o m e lateral variation in the s e d i m e n t t h i c k n e s s e s , most p r o m i n e n t l y in unit II. This i n t e r p r e t a t i o n is s u p p o r t e d by e v i d e n c e from s h o r t cores (top metre o f unit I) from central l a k e areas ( F i g . 2 and 5). H o m o g e n e o u s or laminated ( v a r v e d ) s e d i m e n t s o f mixed a u t o c h t h o n o u s and a l l o c h t h o n o u s origin intercalated with a few thin turbidites are c h a r a c t e r i s t i c (Lami et al. in p r e p . ) . Sedimentation rates are calculated at between 1 and 2 mm p e r y e a r for the top parts o f those cores. This is c o n s i s t e n t with s e d i m e n t a t i o n rates c a l c u l a t e d from r a d i o m e t r i c r e s u l t s (137Cs) for Lakes B o l s e n a and Bracciano (SIMPSON et al. in p r e p . )
Fig. 4: S e i s m i c s t r a t i g r a p h y and definition o f units BR I - I I I for L a k e (location of p r o f i l e s in Fig. 2).
Bracciano
The s e i s m i c s t r a t i g r a p h i e s s u g g e s t that the s e d i m e n t a t i o n rates r e m a i n e d r e l a t i v e l y stable with time in all lakes. The rates are p r o b a b l y s i m i l a r for units I and I I I , and s o m e w h a t h i g h e r f o r unit I I in the deeper parts o f the l a k e s as a r e s u l t o f s e d i m e n t f o c u s i n g . That may also i n c l u d e a h i g h e r p r o p o r t i o n o f d e n s i t y - c u r r e n t d e p o s i t s (e.g. t u r b i d i t e s ) . It is interesting to note that the long s e d i m e n t a r y record o f the f o r m e r crater l a k e in the C a s t i g l i o n e Valley (Alban H i l l s , F i g . 1 and 6) s h o w s r e m a r k a b l y c o n s t a n t s e d i m e n t a t i o n
137
138
Fig. 5 (on previous page): B a t h y m e t r i c map of Lake A l b a n o , location o f s e i s m i c p r o f i l e s and cores (labels ALB) as well as seismic s t r a t i g r a p h y and d e f i n i t i o n o f units AL I - I I I for Lake Albano. rates o v e r the last 100 ka (0.32 mm/a for the last Glacial, 0 . 3 0 mm/a f o r the H o l o c e n e ; FOLLIERI et al. 1988) d e s p i t e large changes in climate and c a t c h m e n t v e g e t a t i o n during this p e r i o d . This s u p p o r t s our interpretation for the e m p l a c e m e n t o f units I and III. Precise unit-thicknesses
and ages for the s e i s m i c r e f l e c t o r s
can o n l y
be roughly
a s s e s s e d until long cores from these lakes e v e n t u a l l y p r o v i d e d a t a b l e material. The acoustic penetration, h o w e v e r , s u g g e s t s that the sediments c o n s i s t o f u n c o n s o l i d a t e d lacustrine muds with s e i s m i c velocities t y p i c a l l y between 1.5 and 1.8 kin/s, d e p e n d i n g on s e d i m e n t water contents (HEIM & FINCKH 1984). Units I, II and I I I c o n s e q u e n t l y have calculated t h i c k n e s s e s o f 2.5 - 10 m, 2 - 8 m, and from 12 to m o r e than 30 m, d e p e n d i n g on lake and location ( F i g . 3 to 5). In t h o s e Latian lakes w h e r e core data are available, the t h i c k n e s s of H o l o c e n e d e p o s i t s is b e t w e e n about 2 and 6.5 m (FRANK 1969, HUTCHINSON & COWGILL 1970, KELLY & HUNTLEY 1991). It t h e r e f o r e seems r e a s o n a b l e to correlate the a c o u s t i c b o u n d a r y I/II with the P l e i s t o c e n e / H o l o c e n e transition.
In that case the I / I I b o u n d a r y
marks
a
r e s p o n s e to climatic change. Indeed the similarities of the vertical variation- in s e i s m i c character for the d i f f e r e n t lakes s u g g e s t s at l e a s t a r e g i o n a l scale e n v i r o n m e n t a l control, such as G l a c i a l / I n t e r g l a c i a l climatic shifts
in central Italy o v e r the l a s t
100
ka.
A s s u m i n g r e l a t i v e l y c o n s t a n t s e d i m e n t a t i o n rates with time, unit II w o u l d r e p r e s e n t the last G l a c i a l maximum and unit I I I the e a r l i e r p h a s e s of the l a s t G l a c i a l (Wtlrm). Is this h y p o t h e s i s s u p p o r t e d by other p a l a e o e n v i r o n m e n t a l data from the r e g i o n ? Changes in vegetation and lake levels, as interpreted from p o l l e n , l a c u s t r i n e s e d i m e n t s and p a l a e o - s h o r e l i n e s , a l l o w a major s u b d i v i s i o n o f the l a s t ca.
100 k a into three
p e r i o d s ( F i g . 6 ) . (i) During m o s t o f the WUrm glaciation, the mid Italian r e g i o n was e s s e n t i a l l y treeless, being c o v e r e d by a cold s t e p p e v e g e t a t i o n under a r e l a t i v e l y dry climate (Bonatti 1966). The latter may have lead to r e l a t i v e l y m i n o r e r o s i o n and r u n o f f of s u s p e n s i o n from the lake catchments d e s p i t e the s p a r s e v e g e t a t i o n c o v e r and can thus explain the p r o p o s e d low l a c u s t r i n e s e d i m e n t a t i o n rates for unit III. (ii) T o w a r d the Last G l a c i a l M a x i m u m (40 - 18 ka), there is e v i d e n c e for an i n c r e a s e in lake levels in central I t a l y , and a wetter climate (e.g. GIRAUDI 1989). This c o u l d give one e x p l a n a t i o n for the i n c r e a s e d s e d i m e n t f o c u s i n g in the central parts o f the lakes interpreted for unit II. C a t c h m e n t e r o s i o n
might have been
more
9i n c r e a s e d t r a n s p o r t of detrital s e d i m e n t s to the lakes. C o n s e q u e n t l y
active,
causing
m o r e frequent
d e n s i t y currents w o u l d have o c c u r r e d in the lake, either d i r e c t l y related to f l o o d s or to s l u m p s as a r e s u l t o f o v e r s t e e p e n i n g o f the s m a l l delta fronts in the lakes. This w o u l d in turn have lead to h i g h e r d e p o s i t i o n rates in the d e e p e r parts o f the lake. Thus there are p r o b a b l y more t u r b i d i t e s in unit II than in units I and III. This w o u l d also 9 the s t r o n g e r s e i s m i c r e f l e c t i v i t y a s s o c i a t e d with unit II, b e c a u s e t u r b i d i t e s lead to d i s t i n c t d e n s i t y changes and sonic i m p e d a n c e c o n t r a s t s in the s e d i m e n t a r y s e q u e n c e .
139
Fig.
6:
C o r r e l a t i o n o f s e i s m i c units (this s t u d y ) with c h a n g e s in l a k e levels COWGILL 1970) and vegetation (FOLLIERI et al. 1988). F o r the location o f F u c i n o ( A b r u z z o ) see F i g . 1.
(HUTCHINSON •
(iii) The t r a n s i t i o n from the last G l a c i a l m a x i m u m to the b e g i n n i n g o f t h e H o l o c e n e (18 to 10 ka) was m a r k e d by a s i g n i f i c a n t drop in lake l e v e l s , as has been r e c o n s t r u c t e d for both the Abruzzo and Latium area ( F i g . 6 ) . Also during this p e r i o d (after ca. 14 ka), the tree v e g e t a t i o n i n c r e a s e d with the r a p i d w a r m i n g ( F i g . 6 ) . D e c r e a s e o f w a t e r levels in a lacustrine basin may result in s u b - a e r i a l e x p o s u r e and s u b s e q u e n t r e w o r k i n g of older s e d i m e n t s t o w a r d s the basin centre as has been r e p o r t e d for m a r i n e e n v i r o n m e n t s as a result o f sea level l o w e r i n g
(e.g.
BOUMA et al.
1989,
KINDINGER
1989).
Thus
i n c r e a s e d s e d i m e n t f o c u s s i n g , as d i s c u s s e d for unit II above, can be the r e s u l t of both i n c r e a s e d input of detrital s e d i m e n t during wet p e r i o d s (open b a s i n with a s l i g h t l y higher lake level) and increased s e d i m e n t r e w o r k i n g during p e r i o d s w i t h take level fall ( d r y e r a n d l o r w a r m e r r e s u l t i n g in open basin c o n d i t i o n s with a r e l a t i v e l y low lake level or even c l o s e d basin c o n d i t i o n s with a much l o w e r lake level). Lake level changes are, for e x a m p l e , indicated by the s e i s m i c r e c o r d o f Lake B o l s e n a for d e p o s i t s l y i n g a b o v e 80 m water depth. In p r o f i l e L - M ( F i g . 3 ) , indication that channels
w e r e i n c i s e d into the d r a p e d
sediments
there is some
o f unit II.
The
s e d i m e n t s of unit II, i n c l u d i n g the s u b s e q u e n t channel fills, are u n c o n f o r m a b l y overlain by unit I s e d i m e n t s in an onlap sequence. A c c o r d i n g to VAIL ( 1 9 8 7 ) ,
the o b s e r v e d
pattern can be e x p l a i n e d by water level changes. The t r a n s i t i o n from unit II to unit I w o u l d then mark a major drop in water level, which c a u s e d e r o s i o n and i n c i s i o n of
140
channels along the slope, followed by a rise in lake level leading to onlaps and, finally, drapes on top of the channel fills. We believe that the transition between units I and II corresponds to a phase of lowering lake levels corresponding to those which were reported to have occurred between ca. 18 to 10 ka BP for other lake basins of the region (Fig. 6). Since the basal part of unit II is not affected by erosion ( F i g . 3 , L-M), we assume that those sediments reflect increased detrital sediment input to the lake under a wetter climate and higher lake level, as discussed above (e.g. at ca. 18 ka BP, F i g . 6 ) . There is no evidence for a fourth seismic unit of thickness and reflection character similar to that of unit I (Holocene) underneath unit III, which is interpreted as early/middle Witrm. We therefore assume that the sediments of the last I n t e r g l a c i a l as shown in the pollen record of Castiglione Valley (FOLLIERI et al. 1988; F i g . 6 ) are either not present in the lakes or were not seismically located. However, assuming similar sedimentation rates for units I and III, it becomes obvious that Lake Albano is considerably younger than, for example, Lake Bolsena, because the thickness ratio between the units I and III is up to 1:10 for Bolsena but only 1:3 for Albano. The latter allows a rough estimation of the ages of the proto lakes; Lake Albano about 40 ka and Lake Bolsena about 100 ka BP. That is of the same order as those concluded from radiometric and stratigraphic evidence outside the lake (as reviewed above).
4.2 TECTONIC FEATURES The
bottom
morphologies
of
both
caldera
lakes
Bolsena
and
Bracciano
are
asymmetrical. Large "platforms", which are separated from the p r o f u n d a l basins by relatively steep slopes, exist only off the western-southwestern s h o r e l i n e of Lake Bolsena and off the eastern-southern shoreline of Lake Bracciano ( F i g . 2 ) . F o r both lakes, seismic profiles across the platform-basin transition show truncation of thick sediment beds ( F i g . 7 , 8 ) , whereas similar features are not present on the o p p o s i t e slopes of either of the lakes. Truncation is most remarkable in the profiles from Lake Bolsena, where sediment packages appear cut by an angle of ca. 30 - 50 ~ with a vertical component of up to 90 ms (or ca. 50 m; Fig.7, A-B, C-D). Direct counterparts of the sediment packages below the platforms could not be seismically identified in the deep basins of either L. Bolsena or L. Bracciano, because of signal diffraction. The seismic records are characterized by a typical sequence. F o r Lake Bolsena, this is the case on both sides of the line separating platform and profundal area. F r o m bottom to top, the sequence is built up of seismically transparent, sub-parallel reflectors of unit III type below the platform which are overlain by sub-parallel reflectors of a stronger reflectivity ( p o s s i b l y unit II type), then by sediments forming a more pronounced topography (channeUoverbank complex). The latter show several i n c i s e d channels linked to thin deposits of overbank sediments on the platform and thick, seismically transparent overbank deposits in the basin. Deposits of channel/overbank c o m p l e x e s are extremely variable in lateral distribution and thickness (Fig.7, A-B, C-D). F i n a l l y , the
14t
142
Fig. 7 (on previous page): Seismic profiles "platform" - "basin" (A-D), and "platform" (E-F) showing evidence for neotectonism (truncation, fault) and subsequent imprint on sedimentation patterns (channel- and overbank complexes) for Lake Bolsena (location of profiles in Fig.2). channel/overbank complexes in both platform and basin areas are covered by deposits showing onlaps near the base and more draping of sub-bottom topographies toward the top. Lake Braeciano is characterized by a nearly similar stratigraphy along the "platform* edge. Channels are cut into the top of truncated sediment packages which are separated from the basin by steep slopes. Overbank deposits are not observed for Lake Bracciano and the capping sediment-drape marking the top of the sequence is much thinner than it is in Lake Bolsena. If compared to the seismic stratigraphy for other parts of Lake Bracciano (Fig.4), it is hard to say whether the truncated sediments are of type III, II or even I. In contrast to the situation in Lakes Bolsena and Bracciano, the morphology of the maar Lake Albano is more simple (Fig.5), basically mirroring the kidney shape of the doublecrater of Albano (PICHLER 1970). Unlike in the caldera lakes, large truncation features are not recorded in the seismic profiles of Lake Albano. Some reflectors drape the underground topography from the deeper to the shallower parts of the basin without vertical displacement (Fig.5). There is geometric indication that the sediment truncation observed for Lakes Bolsena and Bracciano is due to tectonic and not to sedimentary processes: If the slope of the platforms was formed by deposition, platform features (e.g. downlaps, progradation) should show a typical sigmoidal cross-section geometry, as is the case for marine platforms (VAIL 1987). In contrast, the sediment packages forming the "platforms" in Lakes Bolsena and Bracciano have a simple geometry in which reflectors thin and pinch out shoreward, as is typical for suspension deposits in deep lacustrine basins. Thus the truncated sediments of the *platform" could not have been built up where they are now unless there was a basinward continuation of those deposits (Fig.9). The present morphology of the basin does not allow such a continuation, not even if a sigmoidal geometry including a depositional basinward slope is assumed as a possible counterpart, because the slope is too steep to let such sediment bodies accumulate. Therefore post-depositional slumps cannot account for the large truncation features either. We suggest that a large part of the lacustrine sediment fill was vertically displaced by normal faulting resulting in an extension of two major sediment blocks (Fig.9). A similar interpretation was already given by NAPPI et al. (1991) who related the platform/slope morphology of the present Lake Bolsena basin to neotectonic movements during the Holocene. The youngest truncated sediments in Lake Bolsena, which are unconformably overlain by a non-truncated
channel
overbank
complex
(Fig.7
and
9),
show
seismic
characteristics similar to those of unit II. Assuming that the above interpretation of the seismic stratigraphy is correct, tectonic activity occurred sometime between the last Glacial maximum and the Holocene (18-10 ka BP). The formation of channel/overbank
143
144 Fig. 8 (on previous page): S e i s m i c p r o f i l e s "platform" - "basin" ( A - F ) , s h o w i n g evidence for n e o t e c t o n i s m (truncation, fault) and s u b s e q u e n t i m p r i n t on s e d i m e n t a t i o n patterns (incised c h a n n e l s ) for Lake Bracciano (location of p r o f i l e s in F i g . 2 ) . c o m p l e x e s on d e e p - w a t e r s e d i m e n t s indicates a m o r e - o r - l e s s direct and s p o n t a n e o u s r e s p o n s e o f the s e d i m e n t a r y e n v i r o n m e n t , s u g g e s t i n g a tectonic e v e n t r a t h e r than a continuous d i s p l a c e m e n t o v e r a long period of time. The d e p o s i t i o n of a s h a l l o w i n g - u p w a r d
sequence
sequence can be b e s t seen in the s e i s m i c r e c o r d s
underlying
a deepening
o f Lake B o l s e n a
upward
(Fig.7).
The
transition of more or less s u b - p a r a l l e l reflectors of t y p e I I I and II ( t y p i c a l d e e p water lacustrine s e d i m e n t s ) to the o v e r l y i n g c h a n n e l / o v e r b a n k c o m p l e x is i n t e r p r e t e d as a r e s p o n s e to a major d r o p in water level over the plat.form. S e v e r a l c h a n n e l s can be traced which eroded unit II s e d i m e n t s and caused m a j o r i n c i s i o n s into the s l o p e plain toward the basin ( F i g . 2 and 7). The latter are still clearly i d e n t i f i a b l e as s c o u r features in the p r e s e n t s l o p e (Fig. 2). In f r o n t o f the s l o p e channels, thick p a c k a g e s o f o v e r b a n k d e p o s i t s and i n d i s t i n c t channels have the c h a r a c t e r i s t i c s of l o w - s t a n d b a s i n - f l o o r fans. They p r o b a b l y resulted from b a s i n w a r d s e d i m e n t t r a n s p o r t o v e r the p l a t f o r m . Despite the large t h i c k n e s s e s f o r o v e r b a n k c o m p l e x e s of, in places, up to 5 0 ms (or ca. 35 m), the d e p o s i t i o n time w a s p r o b a b l y rather s h o r t (perhaps on the o r d e r o f a f e w h u n d r e d s or t h o u s a n d s o f y e a r s ) b e c a u s e no i n t e r f i n g e r i n g with h e m i - p e l a g i e s e d i m e n t s
is
o b s e r v e d . Thus s e d i m e n t a t i o n rates in the c h a n n e l / o v e r b a n k c o m p l e x w e r e in places p r o b a b l y very high, b u t there was little or no effect on the s e d i m e n t a t i o n o f the central plain. The channel o v e r b a n k s y s t e m s on both p l a t f o r m and basin floor, are i n a c t i v e today (below 60 ms or 42 m water depth) as e v i d e n c e d by a modern s e d i m e n t d r a p e o f n e a r l y constant thickness o v e r large areas. This is i n t e r p r e t e d as a r e s p o n s e to a r i s e in lake level, which i n c r e a s i n g l y d i m i n i s h e d lateral s e d i m e n t t r a n s p o r t o v e r the p l a t f o r m , so that d e p o s i t i o n of h e m i - p e l a g i c s e d i m e n t s became relatively m o r e d o m i n a n t for both p l a t f o r m and basin areas ( F i g . 9 ) . The w a t e r - l e v e l rise p r o b a b i y o c c u r r e d d u r i n g the early H o l o c e n e ,
b e c a u s e the onlap s e q u e n c e o f type I s e d i m e n t s
(Holocene)
still
documents the last p h a s e o f the c h a n g e in lake level. A similar d e v e l o p m e n t is s u g g e s t e d for L a k e B r a c c i a n o , where, l i k e a m i r r o r i m a g e of the situation of Lake B o l s e n a , the s o u t h - e a s t e r n part o f the lake was lifted r e l a t i v e to the central basin. The uncertain s t r a t i g r a p h i c level o f the truncated
b e d s makes it
difficult to a s s e s s an age for the tectonic event. Since the channels
are n o t filled
(indicating e r o s i o n until r e c e n t l y ) and the d r a p e o v e r the truncated b e d s is v e r y thin (Fig. 8), the p r o p o s e d d i s p l a c e m e n t o f the t w o b l o c k s appears to be y o u n g e r in Lake Bracciano than in L a k e B o l s e n a . H o w e v e r the s e i s m i c profiles for L a k e B r a c c i a n o are from s h a l l o w e r water, so may d o c u m e n t laterally d i f f e r e n t facies rather than d i f f e r e n t times, in c o m p a r i s o n to L a k e B o l s e n a . Many q u e s t i o n s w h i c h w o u l d be o f particular i n t e r e s t for further i n t e r p r e t a t i o n s remain at the p r e s e n t state o f the s t u d y . (i) As o u t l i n e d above, we c a n n o t d e c i d e w h e t h e r the tectonic movements in both lakes w e r e s y n c h r o n o u s or not. (ii) It is hard to s a y w h e t h e r
145
the b a s i n s were already nearly filled with s e d i m e n t s and then c o n s i d e r a b l y d e e p e n e d after the tectonic event or, vice versa,
deep water s e d i m e n t s w e r e l i f t e d up into
s h a l l o w e r water resulting in a r e d u c t i o n of the volumes of the lakes. This p r o b l e m arises b e c a u s e the m o r p h o l o g i c a l l y higher b l o c k could have been affected by a relative drop in l a k e level, even if it was not moved. F o r example, if the i n c r e a s e o f the basin volume was faster than could be c o m p e n s a t e d by catchment r u n o f f and p r e c i p i t a t i o n , the a b s o l u t e lake level w o u l d have d r o p p e d for s o m e time. The same could h a v e h a p p e n e d if there was a tectonic control on the o u t f l o w . (iii) The tectonieally i n d u c e d c h a n g e s in lake level, in particular in L a k e B o l s e n a , seem to be s y n c h r o n o u s w i t h c l i m a t i c a l l y induced l a k e level changes s u g g e s t e d for Lake B o l s e n a and other lakes o f the r e g i o n , as has been d i s c u s s e d above. H e r e we are facing the p r o b l e m o f a tectonic o v e r p r i n t of o t h e r w i s e climatically driven c h a n g e s in the s e d i m e n t a t i o n of the lakes. W e b e l i e v e that climatic change caused s t r a t i g r a p h i c alteration over large areas of the l a k e s , b e c a u s e d e p o s i t i o n a l changes o f similar c h a r a c t e r are e v i d e n t in all three lakes, w h e r e a s tectonic activity had a very s t r o n g effect on s e d i m e n t a t i o n in the "caldera" lakes b u t r e s t r i c t e d in space only to areas adjacent to the faults.
Fig. 9: Interpretation of the s t r u c t u r a l e v o l u t i o n of Lake B o l s e n a . Left: l o c a t i o n of c r o s s - s e c t i o n and b a s i n fill (units II and I I I ) prior to n e o t e c t o n i s m . Right: tectonic d i s p l a c e m e n t after the d e p o s i t i o n o f unit II and s u b s e q u e n t i m p r i n t on s e d i m e n t a t i o n patterns such as f o r m a t i o n and d e p o s i t i o n o f c h a n n e l / o v e r b a n k c o m p l e x e s (COC). The latter is interpreted as a result o f a r e l a t i v e l o w e r i n g of the lake level o v e r the platform. The final drape of unit I s e d i m e n t s o v e r l y i n g COC is a s s o c i a t e d with a r i s e in l a k e level o v e r the platform. F i n a l l y , some s p e c u l a t i v e c o m m e n t s on the relative ages of m a g m a t i c i n t r u s i o n s or eruptions in the Lake B o l s e n a basin may be p o s s i b l e from this s t u d y . The I s l a n d of B i s e n t i n a ( F i g . 2 ) , a y o u n g v o l c a n i c feature related to " S u r t s e y a n - t y p e " e r u p t i o n s into the lake water (VAREKAMP 1980), is c o n t e m p o r a n e o u s with or o l d e r than the tectonic event d e s c r i b e d above. The c h a n n e l o v e r b a n k c o m p l e x e s , w h i c h d e v e l o p e d after the event, show
a northeastern
basinward
" d e v i a t i o n " around the island (Fig. 2). This
indicates that the island m u s t have been p r e s e n t during the d e p o s i t i o n o f the s l o p e fans and functioned as a b a r r i e r a g a i n s t d i r e c t l y - b a s i n w a r d s u s p e n s i o n f l o w s . F u r t h e r m o r e ,
146
overbank deposits and the overlying hemipelagic deposits pinch c o n f o r m a b l y out islandward onto the bedrock slope (Fig.7, C-D). Consequently the lacustrine deposits, interpreted as being of Late Glacial and Holocene age, must be younger than the island. Another volcanic and/or tectonic feature is observed as a dome-like elevation in the center of lake Bolsena (Fig.2), which probably consists of volcanic bedrock draped by sediments of type III to I (Fig.3, G-H). Therefore the bedrock must have been formed prior to or contemporaneously with the deposition of type III sediments. The latter are interpreted as early to middle Wtirm in age. However, unlike the undisturbed drape of II and I deposits, type III sediments seem to be truncated toward the side of the "dome" (Fig.3, G-H), which may indicate some tectonic uplift of the feature after the deposition of unit III. The uplift was probably older than the large tectonic event in the westsouthwestern part of the lake, because type II deposits show no evidence o f truncation on the "dome" as they do below the "platform".
5. CONCLUSIONS Much of our interpretation remains speculative until long cores with precise chronologies become available. However this study shows the potential of sedimentary investigations in the deep Latian lakes for interpreting the climatic and tectonic history of the region. It may also aid a better understanding of the evolution of y o u n g calderas. The following conclusions can be drawn: 1. The morphologies of the large lakes Bolsena and Braeciano were strongly affected by relatively young (latest Glacial?) tectonic activity. Our study is thus in agreement with other investigations proposing that these basins have not the typical caldera ring faults indicative of catastrophic subsidence but show normal faults more characteristic of horst-and-graben tectonics. In fact, the evidence for large fault systems restricted to one side of the lakes, which show mirror images in Lake Bolsena and Lake Bracciano along the major NW-SE tectonic lines of the region, may suggests half-graben tectonics as observed in recent rift-system lakes (SANDER & ROSENDAHL 1989). In contrast, the lacustrine sediment fill in the double crater of Albano is not affected by such tectonic movements. Therefore, of the three lakes discussed in this paper only Lake Albano has the typical characteristics of a maar lake. 2.
Since the seismic stratigraphies display three similar units in all investigated
lakes, the formation of these units is assumed to be mainly related to climatic changes during the Wtirm Glacial and the Holocene. The geometry of the units s u g g e s t s changes in temporal sedimentation patterns. We suggest that the stronger sediment focusing toward the basin centres, as indicated by unit II deposits (including the transition to unit I), occurred between the Last Glacial maximum and the early Holocene. It is interpreted as being caused by a higher basinward transport of detrital sediments related to increased catchment erosion at ca. 18 ka BP and/or to a drop in lake level between ca. 18 ka and 10 ka BP. Moreover, a basin-wide tectonic overprint on climatically
147
controlled sedimentation, including tectonically caused lake-level changes, cannot be ruled out for the large lakes of Bolsena and Bracciano. In these particular basins, the control of temporal and spatial sedimention is thus extremely complex. Their understanding requests further intensive sedimentological studies which this paper may help to stimulate.
6. ACKNOWLEDGEMENTS The study was financially supported by the Swiss Academy of Natural Sciences (travel grant to F. Niessen) and by the C.N.R. (Istituto Italiano di Idrobiologia, Pallanza). V. Libera (CNR Pallanza) provided a major support to the project by technical preparations of the trip and by radar measurements during the seismic campaign. We thank J. Masaferro (CNR Pallanza) and C. Chondrogianni (Limnological Institute, University of Constance) for their help in the field. Without the efford of K. Ghilardi (ETH Ziirich), who has designed and constructed equipment in order to adapt the ETH seismic system for use on very small boats, this project would not have been possible. We are also particulary grateful to Sig.Ing. Fioravanti (Bolsena) who provided his vessel (MV Vulcano) for the Lake Bolsena survey and gave insight to various private documents about the lake. His interest in our work was highly appreciated. Last but not least, we thank G. Lister (ETH ZiJrieh) for the critical review of the manuscript and his useful comments.
REFERENCES BALLY, A.W. (1987): Atlas of Seismic Stratigraphy.- AAPG Studies in Geology, 27 (Vol. 1). The American Association of Petroleum Geologists, Tulsa, 125p. BONATTI, E. (1966): North Mediterranean Climate During the Last Wiirm Glaciation.Nature, 5, 984 - 985 BOUMA, A.H.; COLEMAN, J.M.; STELTING, C.E. & KOHL, B. (1989): Influence of Relative Sea Level Changes on the Construction of the Mississippi Fan.- GeoMarine Lett., 9, 161 - 170 C.N.R. - ISMES S.p.A. (1989): Progetto finalizzato energetica II ~ sottoprogetto energia geotermica - - Rilievi continui, sismici a riflessione e magnetici nel Lago di Bolsena.-ISMES, Bergamo, 58p FOLLIERI, M.; MAGRI, D. & SADORI, L. (1988): 250,000-Year Pollen Record from Valle di Castiglione (Roma).- Pollen et Spores, Vol. XXX/3-4, 329 - 356 FORNASERI, M. (1985): Geochronology of volcanic rocks from Latium (Italy).- Rend. Soe. Ital. Mineral. Petrolog., 40, 73 - 106 FRANK, A. H. E. (1969): Pollenstratigraphy of the Lake of Vico (Central Italy).Palaeogeog. Palaeoclimatol. Palaeoecol., 6, 67-85 GIRAUDI, C. (1989): Lake Levels and Climate for the Last 30,000 Years in the Fucino Area (Abruzzo - Central Italy) - - A Review.- Pal., Pal., Pal., 70, 249 - 260 HEIM, C. & FINCKH, P. (1984): Sonic velocity measurements on cores from Ztibo.Contr. Sedimentology, 13, 1 2 5 - 134 HUTCHINSON, G.E. & COWGILL, U. (1970): The History of the Lake: A Synthesis.In: HUTCHINSON, G.E. (Ed.): Ianula: An Account of the History and Development of the Lago di Montesori, Latium, Italy:- Trans. Amer. Phil. Soc., Philadelphia, 1 6 3 - 170
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INSTITUTO ITALIANO DI IDROBIOLOGIA, PALLANZA (1971): Limnologia ed ecologia dei Laghi di Bolsena, Bracciano, Trasimeno e Vico: Situazione attuale e prevedibili eonseguenze derivanti da una loro utilizzazione mnltipla.- Verbania Ed., Pallanza, 263p KELLY, M.G. & HUNTLEY, B. (1991): An 11 000-year record of vegetation and environment from Lago di Martignano, Latium, Italy.- J. Quat. Sci., 6 (3), 209 224 KINDINGER, J.L. (1989): Depositional History of the Lagniappe Delta, Northern Gulf of Mexico.- Geo-Marine Lett., 9, 59 - 66 LAMI, A.; NIESSEN, F.; GUILIZZONI, P. & MASAFERRO, J. (1992): Preliminar results on palaeolimnology of crater Lake Albano (Latium, Italy).- Verh. Int. Vet. Limnol., 25 (in press) LAMI, A.; NIESSEN, F.; GUILIZZONI, P. & MASAFERRO, J. (in prep.): Palaeolimnological studies on the volcano Lake Albano (Central Italy).- J. Paleolimnology LOCARDI, E.; LOMBARDI, G.; FUNICIELLO, R. & PAROTTO, M. (1976): The Main Volcanic Groups of Latium (Italy): Relations Between Structural Evolution and Petrogenesis.- Intern. Colloq. Planet. Geol..- Geologica Romana, 15, 279 - 300 MASAFERRO, J.; LAMI, A.; GUILIZZONI, P. & NIESSEN, F. (1992): Record of changes in the fossil chironomids and other parameters in the volcano Lake Nemi (Central Italy).- Verh. Int. Verein. Limnol., 25 (in press) NAPPI, G.; RENZULLI, A & SANTI, P. (1991): Evidence of incremental growth in the Vulsinian calderas (Central Italy).- J. Volcan. Geotherm. Res., 47, 13 - 31 PECCERILLO, A. & MANETTI, P. (1985): The Potassium Alkaline Volcanism of Central-Southern Italy: A Review of the Data Relevant to Petrogenesis and Geodynamic Significance.- Trans. Geol. Soc. S. Afr., 88, 379 - 394 PICHLER, H. (1970): Italienische Vulkan-Gebiete I: Somma-Vesuv, Latium, Toseana.In: LOTZE, F. (Ed.): Sammlung Geologischer Fiihrer, 51, Gebr. Borntdiger Vrl., Berlin, 258p SANDER, S. & ROSENDAHL, B.R. (1989): The geometry of rifting in Lake Tanganyika, East Africa.- J. African Earth Sci., 8 No 2/3/4, 323 - 354 SIMPSON, H . I . ; FRAS CARI, F. ; B ORTOLUZZI, G. ; G UERZONI, S. ; QUARANTOTTO, G. & RAMPAZZO, G. (in prep.): Regional Atmospheric Deposition of Trace Metals Derived from Lake Sediments of Central Italy.unpublished manuscript TORO, B. (1976): Gravimetry and deep structure of the Sabatinian and Alban volcanic areas (Latium).- Intern. Colloq. Planet. Geol..- Geologica Romana, 15, 301 - 310 VAIL, P.R. (1987): Seismic Stratigraphy Interpretation Using Sequence Stratigraphy. Part 1: Seismic stratigraphy interpretation procedure.- I n : BALLY, A.W.(Ed.): Atlas of Seismic Stratigraphy.- AAPG Studies in Geology, 27 (Vol. 1). The American Association of Petroleum Geologists, Tulsa, 1-10 VAREKAMP, J.C. (1980): The Geology of the Vulsinian Area, Lazio, Italy.- Bull. Volcanol., 43-3, 487 - 503 WALKER, G . P . L . (1984): Downsag Calderas, Ring Faults, Caldera Sizes, and Incremental Caldera Growth.- J. Geoph. Res., 89, 8407 - 8416
SEDIMENTS AND BASIN ANALYSIS OF LAKE SCHALKENMEHRENER MAAR
T. Heinz*, B. Rein & J.F.W. Negendank Dept. of Geology, University of Trier, D - 5500 Trier
ABSTRACT After microscopic examination, varve counting and comparison with other maar lake sediments a chronology was elaborated for sediments from Schalkenmehrener Maar. The dating of correlation layers in core sequences AII and AIII, which can be traced in the sediments over the whole lake basin, made it possible to date the remaining nine core sequences. A comparison of sediment thickness between the correlation layers illuminate a chronologically and locally highly variable discharge and deposition of allochthonous and autochthonous sediments.
INTRODUCTION Lake Schalkenmehrener Mzzr (SMM) belongs to the "Dauner Maar Cluster" in the southeastem part of the Quaternary volcanic field of the Westeifel region. SMM is the largest maar of the Dauner Maar Cluster and consists of three eruption centers (BOCHEL & KRAWZCYK 1986). The recent eutrophic maar lake occupies the western crater. Ancient lakes in the eastern craters and parts of the modern lake basin are silted up. The water surface covers 219.000 m2 of the 1.080.000 m2 of the catchment area. The greatest water depth is 21,25 m. About 80% of the lake has a water depth of more than 18 m. Bf.)CHEL & KRAWZCYK (1986) estimate the age of SMM, based on morphologic features, to 20.000 to 30.000 yrs. Peat and sediments from the SMM dry maars were investigated palynologically by STRAKA (1975). *in memory of Thomas Heinz, who died during an aircraft accident in 1991
Lecture Notes in Earth Sciences, Vol. 49 I. F. W. Negendank, B. Zolitschka (Eds.) Paleolirrmology of European Maar Lakes 9 Springer-Verlag Berlin Heidelberg 1993
150
Fig. 1: Coring sites of Lake SMM (numbers indicate waterdepth at the coring sites).
SEDIMENTS Eleven core sequences (Fig. 1), each 8 to 11 m long, were recovered from Lake SMM with the "LrSINGER-SONDE" from water depths between 17.6 and 21.25 m. The core sequences have been correlated macroscopically by prominent layers (turbidites, thick diatom and siderite layers, volcanic tephras) (Fig. 2). The overlapping sequences (All / AIII, Fig. 3) were continuously subsampled for preparation of large thin sections and for geochemical and sedimentological investigations. The correlation layers enabled to date the marginal core sequences indirectly. After macroscopic inspection the sediment columns have been divided into 4 sediment types: Type 1: mostly homogeneous diatomaceous gyttja interbedded with clastic turbidites. Type2: layered diatomaceous gyttja. Type3: homogeneous silts and clays with occasional turbidites. Type4: coarse debris, consisting of lapilli tuff with fragments of Devonian country rock. These sediment types are subdivided by a total of 17 local lithozones (Fig. 3, Tab. 1). All recovered sediment profiles show sediments of types 1 and 2, whereas type 4 only occurs in the marginal sequences NI, NII, SII and OII. Because of insufficient drill-hole depth this basic
151
Fig. 2: Sediments and correlation layers of the N-S and W-E transects.
152
type is not reached at the other coring sites. But from former coring activities it is known (NEGENDANK 1989), that there is at least one 250 cm thick debris layer in the profundal at sediment depths of about 12 m. Silts and clays (type 3) are present in cores AII, AIII and OI with a thickness of more than 350 cm, whereas in SII type 3 sediments are only 50 cm thick and lack completely in NI, NII, OII and WI]. Instead of these pure silt and clay layers, in core OII several debris layers (6-29 cm in thickness) alternate with thin (less than 8 cm) clay/silt/debris layers. The debris layers are slightly graded. The transitions from the debris layers to the clay/silts are blurred. In NII clay/silts and debris form a homogeneous mixture. Downcore follows at least 170 cm of very coarse material. An olivin-bomb (80 x 57 ram) became stuck at the end of the core tube and saved the loose-packed debris of the tube for recovery. Downmost debris contains nearly no fine-grained material. The LST CLaacher See Tephra) as an important time-marker was not recovered in WI, WII and NIL At WII the sediment sequence stopped just below correlation layer "6". This means, in analogue to the other sequences, about 50 cm above I.ST. In WI the sediments consist of 150 cm of sands below correlation layer "6", which made it impossible to extrude the tube. Shaking out the tube, the sediment structure was destroyed and furnishing proof of LST was not possible. In NI the last but one coring tube finished in that sediment depth in which LST was to be expected. In the following tube only sediments of type 4 were recovered. The transects (Fig. 2) show, that the sedimentation over the southern part of the lake was smooth. The thickness of sediments between correlation layers (Fig. 2) in SI and SII is nearly equal. This tendency can be followed into profundal AII, with exception of the section between the downmost markers. In the central lake these sediments are much thicker also in comparison to OI. In the west WI is shortened compared with WII and the profundal sequences. The correlation layers indicate a lower accumulation rate over the whole sediment column in WI. The eastern part of the lake experienced a varying depositional history. There is a trend to decreasing deposition from margin to center, but without any uniform tendency. Internal variations of sediment thickness between correlation layers are considerable. The same has been observed between AI and All as a result of "slumping'.
CHRONOLOGY On the base of the assumed annual character of laminations, HEINZ (1991) elaborated a chronology for the Late- and Postglacial lake sediments from SMM. Because of poor preservation of laminations he abstained from varve counting below LST.
153
Fig. 3: Sediment profile AII / AIII with local lithozones (b) and mean annual increase rotes (a)
154
Fig. 4: Different varvetypes from different sediment lithozones of Lake S MM (HEINZ 199 I)
155
Tab.l: Description of lithozones A - Q in the core sequence AII/AIII. 0 - 187 mm s.d. (A) --- few turbidites, siderite- and diatomlayers; plant rests, pyrite framboids, occasionally viviante-nodes; diatoms: CycloteUa spec. and Nitzschia spec. were relieved downwards by Melosira spec.. 187 - 354 mm s.d. (B) --- turbidites (< 16mm thick), clearly recognizable siderite- and sectionally also diatom- layers are missing, siderite only diffuse distibuted and tied to turbidites; pyrite-framboids are scarce; diatoms: Melosira spec. at the top, later on increased number of species and dominance of Campylodiscus noricus. 354 - 679 mm s.d. (C) --- regular diatom- and siderite- layers with few turbidites; diatoms: layer-forming Cyclotella spec. and Nitzschia spec., Campylodiscus nor. in coarse silt layers of turbidites. 679 - 1944 mm s.d. (D) --- many turbidites (< 55mm); clear lamination by diatom- and especially siderite- layers; pyrite regulary diffuse distributed; diatoms: like "C". 1944 - 2144 mm s.d. (E) --- few tubidites, becoming more frequent in the top; diatoms: Nitzschia spec.- layer-forming, Campylodiscus nor. tied to turbidites. 2144 - 2448 mm s.d. (F) --- clear diatom layers (Cyclotella spec.) with numerous graded turbidites (with campylodiscus nor.), number of diatom species increase downward. 2448 - 2990 mm s.d. (G) --- transition from tarbidite dominated sediments with charcoal and more frequent plant-rests to clearly laminated diatomeceous gyttja with vivianite-nodes; diatoms: number of species decrease downcore and Stephanodiscus spec., Nitzschia spec. and Cyclotella spec. are gaining importance; Campylodiscus nor. fled to turbidites in the top. 2990 - 3525 mm s.d. (H) --- sectionally homogeneous with high contents o f plant-rests and clastic detritus; in the top still thin turbidites; diatoms: Nitzschia spec. disappears downward as layer-forming diatom species and is replaced by Stephanodiscus spec. and Cyclotella spec.. 3525 - 3819 mm s.d. (I) -- turbidites are missing, occasionally vivianite-nodes, diatom layers formed by Nitzschia spee. and Stephanodiscus spec.. 3819 - 4433 mm s.d. (3) --- frequently vivianite in nodes and diffus; diatom-layers by
Nitzschia, Stephanodiscus and Cyclotella spec.. 4433 - 4708 mm s.d. (K) --- Calcite-layers; diatom-layers like under "J". 4708 - 5458 mm s.d. (L) --- frequently pyrite, occasionally vivianite, very thin clay-layers within varves; Ulmen Maar tephra at 5330 mm s.d. (3mm thick) 5458 - 5750 mm s.d. (M) --- poorly layered silty diatomaceous (Nitzschia spec., Campylodiscus nor.) between sediment-dominating ungraded turbidites; large aggregates of siderite, less pyrite. 5750 - 6345 rnm s.d. (N) --- with turbidites and large plant-rests; LST (5850 - 5917 mm s.d.) divided into a younger (coarse-grained) and older(fine-grained) layer, diatoms: Campylodiscus nor. and few Stephanodiscus spec.. 6345 - 6543 mm s.d. (O) --- plant-rests, diffuse distributed siderite and pyrite, calcitesised ostracode shells; diatoms: Campylodiscus nor., also Melosira spec. and Stephanodiscus spec.. 6543 - 7066 mm s.d. (P) --- with many turbidites, few pyrite, nearly no organic material; diatoms: few species, Campylodiscus dominante; high contents of horizontal orientated large plant-rests but turbidites are missing between 6644 - 6724 mm sA.. 7066 - 7229 mm s.d. (Q) --- with turbidites, siderite diffuse and with preference to fine-silt of turbidites, diatoms: only a few species, CampyIodiscus nor. dominante but not any longer layer-forming.
156
There are three different qualities of layering and varying: 1. A homogeneous or poorly layered section from 5750 to 5458 mm s.d. ( sediment depth), for which only 34 varves and 6 turbidites were counted. 2. The section from 2902 mm s.d. to sediment surface consists of silty diatomaceous gyttja with numerous turbidites and siderite layers. This gyttja is only partly the result of a regular sedimentation with seasonal lamination (Fig. 4). A combination of siderite- and diatomlayers with turbidites is assumed to be annual. The annual nature of siderite-laminae precipitated at the sediment-water-interface is described by BRAUER (this vot.). The counting of varves ended up with 986 years. 3. The sediments from 5850 to 5750 and 5458 to 2902 mm s.d. are well layered and unambiguously varved. Inserts of turbidites are scarce and homogeneous sections do not exist. The results of varve counting in these sections can be assumed as confidential within the margin of error of +/- 2% as suggested by SAARNISTO (1985). Counting of all laminations above LST resulted in 8082 varves. Estimated 50 years were added because of the uncountable uppermost sediments. These are 8132 years and not the expected 11.000 varves. Approximately 3000 varves are'missing. To establish a chronology in spite of this difference it is necessary to explain the reasons of missing varves. The seasonal character of layers in the turbidite-free or -poor sections is distinct and allows exact counting. Assuming that all 11.000 varves above LST are present in these laminated sections a deficit in a range of +/- 220 varves can be expected with an estimated error of +]2%. This is a very low value in comparison to 3000 missing varves. Therefore missing varves outside of these varved sections are very likely. Either conditions for the formation of varves (distinct cycles of deposition) or conditions for preservation were not favourable. In case of S M M it may be assumed that turbidites are mostly responsible for missing varves for some sections and periods. The steep subaquatic slopes (up to 15~ acc. to BRAUER 1988) and the exposure to winds are responsible factors for numerous turbidites. Most of the missing varves are probably not generated or preserved in these turbidite-dominated sediments (5750 - 5458 mm s.d and 2902 - 0 mm s.d.). On the base of this assumption the following time-scale was elaborated. The dating based on counting of varves started at the time marker of LST. The turbiditedominated sections were dated by comparing the sedimentation histories from SMM with the sediment records from HZM (Holzmaar), MFM (Meerfelder Maar), WFM (Weinfelder Maar) and GMM (Gemtindener Maar). The most important and helpful features for correlation with these sediments were quality, thickness and structure of varves and turbidites, occurence and assemblages of different diatoms and/or of authigenic minerals. These features enable to transfer pollen zones from MFM and HZM to SMM. A detailed summary of results comparing the mentioned maar lakes in view of climate, vegetation and dating is given by HEINZ (1991).
157
The oldest microscopically investigated sediments from SMM occur at 6550 mm s.d. at the Older Dryas / Aller6d transition. Changing sediment color caused by increasing organic contents (REIN & NEGENDANK, this vol.) and an increased number of diatom species point to a more productive lacustrine environment (cf. ZOLITSCHKA 1986, 1990, BRAUER 1988). From LST to the AllerOd / Younger Dryas boundary 119 varves were counted (Tab. 2). Sedimentation rates and silt contents increased with the onset of Younger Dryas. Nearly 98% of Younger Dryas sediments are homogeneous, not graded turbidites with litoral diatoms, large plant macrofossils and diffuse distributed siderite. Only 34 varves were identified. A dating within this section was not possible. The boundary Younger Dryas / Preboreal at 5460 mm s.d. was fixed to 10.610 v. yrs BP analogically to results from HZM (ZOLITSCHKA 1990). Decreasing sediment increase rates (SIR) (Fig. 3), disappearance of turbidites, decreasing silt contents and enforced appearance of light layers formed by diatom blooms are the characteristic features for discrimination. Separation of Preboreal and Boreal sediments was difficult. The SIR in the well layered Boreal sediments are slightly lower (Fig. 3) and the organic components increase. For these reasons the transition was set to 5020 mm s.d.. This results in a 950 years duration for the Preboreal, a value similar to the 1.t300 years from HZM.
Tab. 2: Valves and turbidites - Their thickness and percentages for different time intervals.
Thickness
Pollenzones
Mean varve thickness with without turbidites tu rbidites
Turbidites
mm
rnm
% 2600 " 938 *~
0.76 2.07
0.39 1.07
7
2742
0.58
0.58
0
0
2987
0.40
' 0.40
309
0
0
1331
0.23
0.23
950
441
5
1.1
950
0.46
0.46
Y. Dryas
471
292
285
97.8
471 " 34 ""
0.62 7.30
0.20 0.21
Alleroed :Jown to LST
119
100
2
2
119
0.84
0.82
Subatlantic
2600
1944
935
48.1
Subboreal
2742
1581
110
Atlantic
2987
1183
Boreal
1331
Pretxxeal
"'" - estimated value, "*'" - counted varves
t58 There is a marked change of the sediment character at 4710 mm s.d.. Thick varves with pronounced diatom layers and first calcite layers make it reasonable to assume here, after 1300 years of duration for the Boreal (HZM = 1500 yrs), the transition from Boreal to Atlantic sediments at 8350 v. yrs BP. After uniform varying up to 3530 mm s.d. slowly reappearing and thickening turbidites increase the silt contents and the SIR. In comparison to other maar lake sediments it seems reasonable to fix the end of the Atlantic after 3000 yrs of duration (HZM = 3050 yr) at 3530 rnm s.d. (ca. 5350 v. yrs BP) The section from 1950 mm s.d. to the top the sediments are turbidite dominated (48% o f sediments consists of turbidites) and confidential varve counting is not possible. Here the transition from Subboreal to Subatlantic is suspected, dated to ca. 2600 v. yrs BP according to the sediments from HZM (ZOLITSCHKA 1990). Dating within Subatlantic sediments o f SMM is based on assumed constant sedimentation rates. Although varve counting is not continuous the elaborated chronology seems to be a good approximation of the time-scale. BASIN ANALYSIS Seven correlation layers were selected to reconstruct the history of sedimentation of recent Lake SMM. Between these layers mean annual sedimentation rates were calculated, excluding the not well defined time-window related to the topmost sediments. The mean SIR for every selected time-window (Tab. 3) was used to construct computer-isoline plots (REIN 1991).
Tab. 3: Sediment increase rates for selected time-windows for 11 coring sites of Lake SMM.
Nt
NII
WII
Ol
0.78
1.22 1.03 1.27 0.62
0.72
0.31 0.80
0 . 7 3 0.60 0.61 0.60
0.89
0 . 6 5 0 . 9 9 0 . 5 8 0 . 6 2 0.63
1 . 8 7 1 . 0 8 0.28 0.29 0.26
0.43
0.39
0.18
0.41 0.40
2250 c,_
4600 6550 8000
:; 10450 11200
SI
SII
0 . 2 3 0.36 0.30 0.68
WI
0.36
0 . 5 4 0.54 1.38 1.37
0t~
AI
All
AIII 0.58
0 . 3 2 0 . 3 5 0 . 3 5 0.32 0.44 0 . 3 2 0.27
0.32
0.41 0 . 5 9 0.55 0 . 9 5 0.53 0.82 0 . 7 4 1.05 1 . 1 3 1.17
118O0
Time - window 1:
11.800 - 11.200 v. yrs BP
The SIR in the south is almost twice as high as in the east and also higher than in the centre. There is no direct information about SIR in the north and west, but concluded from the
159
position of correlation layer "11" in IVII (REIN 1991) one can suppose that SIR in the north must be even higher than in the south. The sediments in OI seem to be strongly influenced by discharge from the S and the N. The enormous sediment input is due to high amounts of minerogenic detritus. Exclusively fine-clastic sediments in Oil contrast to coarse-grained deposits in other marginal cores. This astonishing feature may be explained by a larger lake surface during the Late-Glacial, when the present-day dry maar of the eastern basin was still a part of the lake (cf. STRAKA 1975). Coarse-grained input from the east at that time was trapped in the western basin and did not enter the lake. Time - win~low 2;
11.200 - 10.450 v. yrs BP
There is no dominant direction of discharge recognizable, although input from the N is somewhat higher. The lowest SIR experienced OI in contrast to the close AII, where SIR is by far the highest. Both is probably the result of slumping or redeposition. Tim~ - window 3; 10.450 - 8.000 v. yrs BP SIR is low, especially in the north. The higher sediment thickness in the east is due to an increased.input of diatoms. Pure diatom-layers, typical for this sediment-section, are much thicker in the east than in the rest of the lake basin. The decrease of SIR from OII over OI to A is proportional to thickness of diatom layers. In the west the minerogenie portion is higher. Development of vegetation and warmer climate in combination with the beginning of silting up of the lake in the eastern basin (STRAKA 1975) provide a higher nutrient supply especially for the eastern sites (OI, Oil). Timr window 4; 8.000 - 6.550 v. yrs BP In the north a destabilization of marginal sediments is visible, which dominates deposition with large plant remnants as well as higher minerogenic input for the whole period, whereas nearly pure organic sediments (diatom-maximum) dominate in the rest of the lake. An expansion of the reed belt is insufficient to explain this supply of macrorests. The relatively high minerogenic portion still has to be explained. Since dense woodlands within the crater rim existed (STRAKA 1975), an important input by denudation can be excluded. Erosion of the shoreline and especially redeposition of older sediments from marginal areas seems to be probable. Over the whole section of NI intensive slumping is visible. This points to at least temporal discharge or redeposition events. In the southern and eastern lake basin the sediment accumulation is decreasing compared to the preceding period. In WI accumulation is low. The sediment structure gives no evidence if less deposition occured or if parts of the sediment were remobilized.
160
Time - window 5:
6.550 - 4.600 v. yrs BP
SIR of the whole basin is high with maxima in the east and west. No important differences exist in sediment features macroscopically, except for lots of plant macrorests in the west, which are also found to a lower extent in the north and east. With ongoing silting up of the eastern lake basin OI and OII became more and more marginal. Sediments of the former eastern maar lake could have been remobilized by wave impact and redeposited in OII, thus explaining the very high SIR. Tim~ - window (i: 4.600 - 2.250 v. yrs BP SIR increased strongly. Turbidites dominate sedimentation. Above all in the S and N the deposition is very high. Parts of these sediments represent the period of the "Mehrener Culture". Settlement activities and agriculture reached a first maximum and caused a development of waste- and heathland and lateron depopulation or"the area which was repopulated probably not before Roman Times. The thickness of sediments above LST decrease from the margin to the center. In the Late- and Postglacial sediments no funnel effect is recognizeable. Indications for funnel effects can be found in pre-AllerOd sediments. The silts and clays are much thicker in the central area, a result of remobilized marginal sediments during periods of low lake levels. In contrast to maar lakes with inlet dominated sedimentation like MFM (WEGNER 1992), deposition in SMM developed temporally and locally very differentiated. Periods with temporal allochthonous sediments and high SIR (Lateglacial, Subboreal, Subatlantic) alternate with periods of nearly excusively authochthonous sediments and low SIR (Boreal, Atlantic). The delta in the NE seems to be an old formation, whereas the delta in the S developed not before the Subboreal. As the catchment area of the inlet was small, allochthonous sediments are of some importance only in the south. The silting up of the lake in the eastern basin influenced sedimentation. During the Late Glacial it acted as a sediment-trap for detritus from the eastern crater rim. Lateron it gained more and more importance as sediment- and nutrient-supply. In the east the sediment column above LST is the thickest and measures more than 8 m. This is not the result of regular higher sedimentation rates (Fig. 2). BRAUER (1988) mapped subaquatic terraces at I 1 - 12 m water depth which can be interpreted as High Glacial lake level marks, whereas a terrace at 3 - 4 m water depth is explained by HAAREN (1988) as an organic formation, as breakage of peat at a depth to which the reed belt reached until the turn of the century. There is no evidence for higher lake levels than the present one. Either the existence of the outlet prevented a rise of the lake level or its evidences were destroyed by agriculture (BRAIDER 1988).
161
REFERENCES Brauer, A. (1988): Versuch einer Erfassung alter Seespiegelst~de an ausgesuchten Eifelmaaren und mikrostratigraphische Untersuchungen an Sedimenten des Weinfelder Maares. Diplomarbeit Univ. Trier, 117 pp. Bfichel, G. & Krawzcyk, E. (1986): Zur Genese der Dauner Maare im Vulkanfeld der Westeifel. - Mainzer Geowiss. Mitt., 15,219 - 238, Mainz. Haaren, Chr. v. (1988): Eifelmaare. Landschafts~ikologisch - historische Betrachtung und Naturschutzplanung. Pollichia, XVI,548 pp. Heinz, T (1991): Pal~iolimnologische und spektralanalytische Untersuchungen an jahreszeitlichgeschichteten Sedimenten des Schalkenmehrener Maares/West. - Diplomarbeit Univ. Trier, 107 pp. Negendank, J.F.W. (1989): Pleistoz~ne und holoz~ine Maarsedimente der Eifel. Z.Dt.Geol.Ges., 140:13 - 24, Hannover. Negendank, J.F.W., Brauer~ A. & Zolitschka, B. (1990): Die Eifelmaare als erdgeschichtliche Fallen und Quellen zur Rekonstruktion des Pal~ioenvironments. - Mainzer geowiss. Mitt., 235 - 262. Rein, B. (1991): Versuch einer Rekonstruktion des Paleoenvironments anhand hochzeitaufl~isender geochemischer und sedimentologischer Untersuchungen an sprit- und postglazialen Sedimenten des Schalkenmehrener Maarsees (Westeifel/Deutschland). Diplomarbeit Univ. Trier, 109-pp. Saamisto, M. (1985): Long varve series in Finland. - Boreas, 14:133 - 137. Straka, H. (1975): Die spfitquartiare Vegetationsgeschichte der Vulkaneifel. - Beitrag zur Landespflege in Rheinl.-Pfalz, Beih. 3, 163 pp., Oppenheim. Usinger, H. (1982): Pollenanalytische Untersuchungen an sp~itglazialen und prfiborealen Sedimenten aus dem Meerfelder Mnnr (Eifel), Flora, 172:373 -409. Wegner, F. (1992): Fazielle Entwicklung und Verteilung der Sedimente im Meerfelder Maar Ein Beitrag zur holoz!inen Seengeschichte, Dipl.-arb. Univ. Trier, 88 pp. Zolitschka, B. (1986): Warvenchronologie des Meerfelder maares - licht- und elektronenoptische Untersuchungen sp~itglazialer und holozfiner Seesedimente. Diplomarbeit Univ. Trier. Zolitschka, B. (1990): Sp~itquart~e jahreszeitlich geschichtete Seesedimente ausgew~ihlter Eifelmaare. Diss. Univ. Trier 1990, 241 pp.
ORGANIC CARBON CONTENTS OF SEDIMENTS F R O M LAKE S C H A L K E N M E H R E N E R MAAR: A P A L E O C L I M A T E I N D I C A T O R
B. Rein & J.F.W. Negendank Dept. of Geology, University of Trier, D - 5500 Trier
ABSTRACT Investigations on organic carbon in sediments of Maar lakes reveal a relationship to paleotemperatures as reconstructed from Camp Century ice cores. Of great influence are also anthropogenic activities in the catchment area. Clearing of forest, agriculture and related settlement are related to increased soil erosion. These periods are characterised by high amounts of minerogenic deposition and increased absolute organic carbon accumulation but lower organic carbon weight percentages. A microscopic examination of sediment thin sections provides data of mineral- and diatom assemblages, varve counts and enables to estimate sedimentation- and accumulation rates. Based on this information it seems to be possible to separate climatic and anthropogenic reasons for organic carbon contents in Lake Schalkenmehrener Maar.
INTRODUCTION
In Lake Schalkenmehrener Maar (SMM) the organic carbon contents of the sediments were determined with a high time resolution (average: 30 yrs) within a continuously subsampled lacustrine sediment sequence of 7 m length. The organic carbon ( in %) was determined chromatographically in the combustion tube and loss on ignition by dry ignition in the muffel furnace as weight percentages of dried sediment (dried 24 h at 105~ Ignition temperature for both was 560~ Analysis time in combustion tube was 26 minutes, for loss on ignition 1 h, because with this time and temperature the reproduction of results was best (REIN 1991). Additional investigations on water contents, dry weight and dry density together with varve chronological dating (HEINZ 1991), made it possible to calculate yearly accumulation rates of organic carbon in mg carbon per area and year from Corg percentages (REIN 1991). Lecture Notes in Earth Sciences, Vol. 49 ]. F. W. Negetldank, B. Zolitschka (Eds.) Paleolimnology of European Maar Lakes 9 Springer-Verlag Berlin Heidelberg 1993
164
For the regional settings of Lake Schalkenmehrener Maar and sediment description see HEINZ et al. (this volume).
ORGANIC C A R B O N The Late Glacial is characterized by very low organic carbon (Corg) contents (<1%) (Fig. 1), firstly increasing rapidly during Allerrd and retarding during Younger Dryas. With the beginning of the Preboreal values increase at first rapidly then gradually. Additionally, siderite dissociates during analysis and produces apparently higher organic carbon contents in the deposits of the AllerOd, Younger Dryas and the beginning of Preboreal. From 9.000 to 8.400 v. yrs BP (Boreal) a first very pronounced maximum is obtained. The transition to the Atlantic minimum (7.500 v. yrs BP) took place in several oscillations. During the following 150-200 years Corg contents decreases to low values comparable to early Aller'rd sediments. Afterwards Corg contents rises again very soon to a level already reached in Preboreal sediments. About 6.000 v. yrs BP the values increase again and form a maximum (up to 28%) for the following 600 years. The end of the Atlantic appears as a remarkable caesura. During the Subboreal two maxima (14 and 12%) do not attain the values of former peaks. A slight trend towards lower minima and maxima is noticable until 2.600 v. yrs BP. Since 2200 v. yrs BP the tendency reciprocal. A pronounced minimum from the late Middle Ages to early Modem Times is followed by a rapid increase in organic carbon contents of the sediments. The organic carbon contents (in %) of all Holocene sediments in SMM show a structuring in two era (Fig. 1). On the one hand the two maxima from Preboreai to late Atlantic ( 5.300 v. yrs BP.) with high to very high organic carbon contents, (>I0%), on the other hand Subboreal and Subatlantic sediments with distinct lower values. The organic carbon accumulation rates (CorgAR) (Fig. 2) are not as indicative and seem to be dominated by a Subatlantic maximum (2.100-1.000 yrs BP) preceded by Several oscillations since the Late Glacial. The maxima since 6.000 v. yrs BP are the most pronounced. Since there is no varve dating possible beneath LST (Laacher See Tephra), calculations of organic carbon accumulation rates start at this isochrone. The yearly accumulation rate starts on a medium level (3-8 mg cm-2a-1). During Boreal and Atlantic mean values decrease. Only a few maxima attain Preboreal values, which, because of thermal-dissociated siderite are too high. Since 5.500 v. yrs BP there is an increase to a first larger two-summit maximum, representing the highest values of the whole record. But these extremely high values are not the result of increased primary production, but the consequence of allochthonous plant remnains. At 4.500 v. yrs BP there is a transition to a minimum (4.000 v. yrs BP), followed
165
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.~_ ~
CL
E
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.E
CO
~8 o~ 0 e--
~E
r
.~ LI-
166
only a few centuries by another maximum (13 mg cm2a "1) (3.700-3.200 v. yes BP). Passing a period of low CorgAR (3.200-2.600 v. yrs BP) values increase rapidly from 2.600 v. yrs BP to values of about 11 mg and more, culminating at 22 mg. About 1.000 v. yrs BP the accumulation rate falls to a level of 3 mg. For the youngest sediments the organic carbon influx into the sediment is about 3 mg. Asthonishing seem to be the low values during the Holocene climatic optimum (7.500 v. yrs BP) appearing also in the Corgweight percentages. These are the lowest values of the whole record. HOFMANN (1991, this vol.) describes in the same stratigraphic position a dramatically changing assemblage of cladocerae. There is an important decrease in the number of species. Small species of Aloha reach high relative and absolute ratios up to 96%. The shift to small species seems to point to a rich nutrient supply. Nutrients could be supplied from removed sediments from the eastern peat area (former lake) (HEINZ et al., this vol.). It remains unsolved, if intensive grazing of zooplankton or low primary production is the cause for the minima in both of the curves. I tend to prefer the f'~rstexplanation. Any lack of nutrients due to reduced soil erosion (vegetation climax) can be excluded. Probably Particulate Organic Carbon (POC) is transformed to Dissolved Organic Carbon (DOC) by grazing, autolysis and destruction by microorganisms. DOC leaves the lake via the outlet. A hazard event limiting the primary production of the lake for centuries seems unlikely, because the purely organic diatomaceous sediments do not provide any indication of such an event.
ORGANIC CARBON: PALEOCLIMATIC VERSUS HUMAN INFLUENCES A graphic correlation (Fig. 1 & 2) between the amount of organic carbon in the sediments of Lake SMM and 160/180-isotope paleotemperatures of the Camp Century ice-core record (Greenland, DAANSGAARD et al. 1971) seems to exist. The slight shift of peaks in the curves may be explained by different dating methods. Explanations by different geographic latitudes would point to an asynchronous development of climate in the temperate zones and higher latitudes of the northern hemisphere. In temperate zones ameliorations of climate, as far as detectable from high primary production in the lake, would have established up to 200 years earlier. During Preboreal the time-shift is up to 400 years possibly due to different positions of the deglaciation front. More distinct than weight percentages (Corg%), the mean annual accumulation rate of Corg accentuates the agreement with the paleotemperature curve. The different variations in paleotemperature during the Holocene Thermal Optimum are reflected in both carbon curves as well as the post-Atlantic development of climate. The climatic pessimum of the Piora-oscillation (about 4000 v. yrs BP), the main-pessimum of the
167 Q. rn
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0 e-
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8 ,o t-
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168
Tab. 1: Interpretation of Subboreal and Subatlantic sediments of Lake SMM
years BP
%
about 5.000
-min
min
4.700 4.200
+ max
++ max
4.200 - 3.700
min
min
3.700 - 3.300
+ max
+ max
SIR (++)(about 3.600 yrs BP),t (++), m (++)
Bronze Age climatic optimum, SIR points possibly to recolonisation with a delay to climatic amelioration
3.300 2.600
min
mi~
SIR (- -), C org % decrease slower than C org accumulation rate, m (- -)
development towards Subatlantic climatic deterioration, reduced soil erosion depopulation?
4-
4-
SIR (+), several minor grain-pollen maxima, charcoal
renewed acquisition of land by man
+
+4-
extremely thick turbidites followed by increasing SIR and organic carbon contents
increased productivity of lake by cultural eutrophication? 2.250 Latene 2.050 aquisition of land by Romans 1.650 late Roman intensive land cultivation 1.350 first Carolingian clearing period (increase of non-arboral pollen)
about 1.500
rain
rain
SIR (- -), heath-pollen t (- -)
time of Migration of the Peoples, reduced human activities in the catchment area
1.300 - 900
+ max
+ max
Sir (max then - -)
Medieval Optimum
600 300
-min
SIR (+), t (+)
min
Little Ice Age, growing population density (mentioned in a document)
-
-
from
2.60O (2.400) on about 2.25O, 2.050, 1.650, 1.350
-
Corg I mgC/a
other features
interpretation
m (++), t (++), grain-pollen maximum (STRAKA 1975)
human activities in the catchment area, increased soil erosion, further eutrophication of lake
charcoal, t, grain-pollen maximum (STRAKA 1975)
possibly related to "Schnurkeramik'culture (4.400 - 4.000 yrs BP)
t(-), m (-)
climatic pessimum (Piora - oszillation)
min = minimum values max = maximum values m = minerogen t = turbidites decreasing - - strongly decreasing + increasing ++ strongly increasing
169
Subatlantic climatic deterioration (about 3000-2600 v. yrs BP), as well as the minima during the period of "Migration of the Peoples" and the Little Ice Age (Fig. 3 & 4) subdivide the carbon curves in agreement with the paleotemperatures of Camp Century ice-core. The same can be observed during phases with warmer climatic conditions, like Bronze Age, Roman Times and Medieval Optimum (Fig. 3 & 4). The organic production of Lake SMM therefore, seems to be related to climatic conditions as evidenced by comparison of organic carbon data with the paleotemperature curves. But local anthropogenic influences on sedimentation during the second half of the Holocene, like changes in settlement and landuse in the catchment area, have to be taken into account. These influences may be isolated and calculated by combined examination of Corg percentages and accumulation rates (Tab. 1). In the Eifel area periods of warmer climates correspond to periods of enforced settlement activities, this often results in a reciprocal development of weight percentages and absolute contents of Corg in the lake sediments. Decreasing weight percentages of Corg with at the same time increasing or retarding Corg accumulation rates are the result of higher minerogen input by anthropogenic soil erosion and cultural eutrophication of waters. The eutrophication results either from sewage or from nutrients adsorbed to minerogenic particles washed into the lake. A pronounced example is the Atlantic to Subboreal boundary. With the first grain-pollen maximum about 5.000 v. yrs BP the Corg contents decreased. Although the Corg deposition increased subsequently, weight percentages never again reach to former values of sedimentation era 1 (Fig. 1), but accumulation rates exceed former values. The same effect is very distinct in the second millenium BP. The precipituous drop in temperature during Younger Dryas is connected with a relative minimum deposition of organic carbon. Remnants of plants and siderite in connection with deposition as turbidites explain the relatively high "organic" carbon contents of the sediment. During Younger Dryas minerogenic sediments dominated over the entire lake pointing to an important eolian input (HEINZ et a l . , this vol.). Both, turbidity currents and eolian sediments, are due to an assumed cooler and drier climate causing a more open plant cover. The erodibility of soils increased. Lake level changes due to dryness may have contributed to shoreline degradation and erosion of unconsolidated litoral sediments. The transport of eroded material into the profundal was carried out by turbidity currents. This is indicated by lots of litoral species of diatoms (Nitzschia spec.) found in these deposits. Beginning in the second half of Younger Dryas (-10.700 v. yrs BP) the organic carbon contents increased dramatically. The minerogenic contribution to the sediment got smaller and smaller and the SIR reacted in the same way.
170
Towards the Boreal the rise in organic carbon content continued and reached a first Holocene maximum in the late Boreal (-8.700 v. yrs BP). A comparison of Preboreal and Boreal Corg accumulation rates show a considerable decrease, although the organic portion of the sediment is getting larger. Thin, fine-grained minerogenic layers, characteristic for Preboreal sediments, dissappear in favour of more or less purely organic sediments (diatomaceous gyttja). This indicates the further consolidation of the catchment area under a denser plant cover due to increasing temperatures and soil - forming processes after the Preboreal to Boreal boundary (N9.300 v. yrs BP). Soil erosion and transfer of nutrients into the lake was reduced. There is no climatic explanation for the decrease of organic carbon between -7.800-7.000 v. yrs BP. Gradually thickening diatom- and calcite-layers and lateron vivianite-nodules indicate a rich nutrient supply. The carbon cycle seems to be ~hort-cut by intensive grazing of zooplankton. Thus less is deposited. But also intensive destruction within the sediment may have recycled carbon from the sediment into the water. About 7.000 v. yrs BP organic carbon increased rapidly, reaching maximum-values of up to 27% at 6.000-5.300 v. yrs BP. These are even somewhat higher than those of the Boreal maximum. Remarkable is the strong variation in productivity, most distinctly expressed in the accumulation rate of organic carbon. Like before the curve is controlled by climatic factors. The highest natural controlled productivity occurred in the lake during the end of the Atlantic (-5.300 v. yrs BP), connected with a change in phenology of the sediments. Organic carbon, water contents, dry weights as well as SIR (HEINZ 1991, REIN 1991, HEINZ et al., this vol.) reflect the change in sedimentation. Until then nearly pure organic sediments were deposited. Now the minerogenic components push back the organic carbon to values less than 10%, values of Late Glacial character. Increasing Corgaccumulation rates indicate, that decreasing Corg% values are not due to decreasing productivity of the lake but to increasing minerogenic input. There is only one sensitive explanation: Human influence. First grain-pollen-maxima and the spreading of heath, dated by STRAKA (1975) into the early Subboreal, indicate for this period that human activity is not only regionally but locally a factor controlling sedimentation. Erosive processes carried more and more soil particles from slopes and crater walls into the lake. The Corg percentages of the sediment decreased continuously, while further eutrophication (high absolute amounts of Corg) is induced by nutrients adsorbed to soil particles. The silting up of the lake in the eastern part of the crater, with extensive reed and sedge vegetation (STRAKA 1975), developed to an nutrient source (REIN 1991, HEINZ et al., this vol.). Homogeneous sediments with high amounts of minerogenic deposition and plant-remnants within varved diatomaceous gyttja as well as turbidites are indicators of a more event controlled sedimentation and bioturbation (Tab. 1). In spite of anthropogenic influences
171
climate remains the dominating factor controlling sediments of SMM.
organic
carbon deposition in
the
REFERENCES Brauer, A. (1988): Versuch einer Erfassung alter Seespiegelst~de an ausgesuchten Eifelmaaren und mikrostratigraphische Untersuchungen an Sedimenten des Weinfelder Maares. Diplomarbeit Univ. Trier, 117pp. Daansgaard, W. et al. (1971): Climatic record revealed by the Camp Century ice core. - The late cenozoic glacial ages, Turekian, K. (ed.), 37 - 56, New Haven, Conn. Haaren, Chr. v. (1988): Eifelmaare. LandschaftsSkologisch - historische Betrachtung und Naturschutzplanung. Pollichia, XVI, 548 pp. Haverkamp, B. (1991): Pal~omagnetische Untersuchungen an Seesedimenten aus den Eifelmaaren. Diss. Univ. Mtinster. Heinz, T (1991): Pal~tolimnologische und spektralanalytische Untersuchungen an jahreszeitlich geschichteten Sedimenten des Schalkenmehrener Maares/West. - Diplomarbeit Univ. Trier, 107 pp.. Hofmann, W. (1991): Late- / Postglacial planktonic and littoral cladoceran assemblages from two eutrophic maar lakes. - Abstract vol. Syrup. Paleolimnology of Maar Lakes, May 21 25, Bitburg, Germany. Rein, B. (1991): Versuch einer Rekonstruktion des Paleoenvironments anhand hochzeitauflrsender geochemischer und sedimentologischer Untersuchungen an sp~t- und postglazialen Sedimenten des Schalkenmehrener Maarsees (Westeifel/ Deutschland). Diplomarbeit Univ. Trier, 109 pp. Straka, H. (1975): Die sp~tquartitre Vegetationsgeschichte der Vulkaneifel. - Beitrag zur Landespflege in Rheinl.-Pfalz, Beih. 3, 163 pp., Oppenheim. Usinger, H. (1982): Pollenanalytische Untersuchungen an sp~tglazialen und pr~borealen Sedimenten aus dem Meerfelder M~:ar (Eifel), Flora, 172:373 -409, Zolitschka, B. (1990): Sp~tquart~e jahreszeiflich geschichtete Seesedimente ausgew~ihlter Eifelmaare. -Diss. Univ. Trier 1990, 241 pp.
Basin Analysis For Selected Time-Frames Using Sedimentation Rates In Lake Meerfelder Maar (Westeifel FRG) F. Wegner* & J.F.W. Negendank** *Waste Management (Deutschland) GmbH, W-4300 Essen ** Dept. of Geology, University of Trier, W-5500 Trier Abstract
Twenty-seven dated, high resolution sedimentary sequences in Lake Meerfelder Maar (ZOLITSCHKA 1990; DROHMANN et al. 1989) allow the calculation of the average linear sedimentation-rates (LSR) for different timeframes. Some plots with the LSR as z-axis for these selected times are showing the changes of the maximum sediment increase in the maar-lake history. There is a high variation in the sedimentation process in the Late-Glacial and Holocene history of the maar. It is possible to distinguish these variations by comparing different time-frames (TF) as well as variations within single timeframes.
1. Introduction The work at hand is aimed at depicting the influences and variations of the sedimentary process in reference of the time and, consequently, reconstructing the evolution of the "Meerfelder Maar" lake. The variability of the sedimentary process within the lake (of a specific period of time) shall be presented and discussed as well. This is possible due to the existing twenty-five coring-sites plus approx, the same amount of correlation borings all over the lake,which provide relatively reliable data. This analysis is based partly on computer-interpolated data and already existing studies about the Lake Meerfelder Maar. Lecture Notes in Earth Sciences, Vot. 49 J. F. W. Negendank, B. Zolitschka (Eds.) Paleolimnology of European Maar Lakes 9 Springer-Verlag Berlin Heidelberg 1993
174
2. Methods
Figure 1: Lake Meerfelder Maar (Location of the coring-sites). 2.1 Sampling/Drilling Engineering
From July to September 1988, twenty-seven sediment-cores were extracted from a raft, by the USINGER-sond. The amount of the log's results from seven coringlocations in WSW-ENE direction (I) and four locations in NNW-SSE (11) direction, each with its correlation-sequence. These overlapping sequences are necessary to achieve complete sequences. Within the quadrants, achieved by the lake-profiles, five further sequences were extracted, for which the correlation-sequences were not carried down because they correlated exceptionally with the sequences of the profundal. All sequences were described, photographed and sampled for geochemical, sedimentological and microstratigraphical studies on-site. 2.2 Dating and Division of Time-Frames
Sixteen striking layers (marker) which are detectable almost over the whole lake were of great help for the correlation of the sequences. The markers were fixed in their depth below the sediment-surface (in cm) in all
175 cores. After completion of the works described above, the marker-layers in sequence IIC of the profundal were correlated with the A-sequence (cf. ZOLITSCHKA 1990) and dated with the help of varve-chronology, appropriate to ZOLITSGHKA (1986), by ZOLITSCHKA (pers.comm.1989), in which B.P. is a synonym for "before 1950". This sequence
was the "MASTERSCALE" for the further
sediment-
sequences.
Table 1: Time-frames used for computer-modelling. Era
2
7 8 9 10 11 12 13 14 15 16
Duration (a) 1736 704 554 1232 679 413 1755 1344 603 996 419 714 576 289 355 17
Marker 0 to 1 1 to 2 2 to 3 3 to 4 4 to 5 5 to 6 6to 7 7 to 8 8 to 9 9to 10 10to 11 11 to 12 12to 13 13 to 14 14to 15 15to 16
from to LSR/Era Fluxrate/Era (B.P.) mm/a (rag cm2a -1) 0-1736 1,16 15,92 1736-2440 0,82 13,40 2440-2994 0,89 12,46 2994-4226 0,86 14,22 4226-4905 0,77 12,11 4905-5318 0,77 14,02 5318-7073 0,61 15,64 7073-8417 0,60 15,98 8417-9020 0,87 24,14 9020-10016 0,76 21,84 10016-10435 1,11 51,04 10435-11149! 0,76 24,76 11149-11725 0,95 42,24 11725-12014 1,68 80,31 12014-12369 2,63 162,34 12369-12386 7,64
For the time (Tab.l) between these markers (eras), the average linear sedimentation-rate (LSR) and accumulation-rate (fluxrate) were calculated and shown in isoline plots with the aid of a computer.
2.3 Computer evaluation The entire isoline-plots were composed by the programm "SURFER",
176
released by "Golden Software Inc.". The process was the following: The appropriate RECHTS- and HOCHWERT as X- and Y-value was assigned to every coring-site. The Z-value corresponded to the LSR in mm/a, calculated as mm of sediment-column between two markers, divided by the duration.The flux-rate is the product of the accumulation of sediments (in mm/a) and the bulkdensity (in mg/cm 3) divided by 10. Following limit-values were taken into account for the grid in order to achieve figures of the same size. Table 2: Values of the used grid (Rechts- & Hochwert) GRID-MINIMUM
GRID-MAXIMUM
X
2554025
2554505
Y
5551688
5552075
The method of "Inverse Distance" to the fourth degree was taken for the interpolation (see equation of Fig. 2). With this method every data-point is weighted in a manner that its influence decreases with the distance to the next point. The higher the degree, the weaker is the influence on the next points and therefore differences in sedimentation are better to be analyzed.
Z=
d t h e dist,9~ce n the a m o u n t of Z elements
Figure 2: Equationusedfor interpolationaccording to the methodof "inverse Distance
177
Thereafter grid-smoothing of the calculated grids occurred with a weighted filter-matrix.
3, DQ#~riotion of the sediments 3.1 The "Masterscale" (sequence IIC) Core IIC was chosen as "Masterscale" because it comes from the profundal of the Maar and implies all striking correlation-layers, and its description shall stand for all sequences. Point "zero", in other words the youngest sediments, originates from a depth of 200 cm, the whole profile length is 10,65 m. From the top ofthe profile to a depth of approximately 650 cm the sediment consists of a dark-brown (10 YR 2/2) diatomaceous gyttja with a high content of organic material. Between 150 and 500 cm vivianite emerges which varies in size. From 650 to 700 cm a layer follows that consists of clays and silts of the Younger Dryas and represents the cold relapse. Between 700 and 800 cm begins a region of finely laminated diatomaceous gyttja interbedding with clays and silts; this layer continues between 950 and 1020 cm. The depth from 730 to 770 cm is characterized by finely laminated grey to olive-black (N2 to 54 2/1 )diatomaceous gyttja, together with the Laacher Pumice Tuff (LBT) in 750 cm, and corresponds to a depth of 950 cm below sediment surface. Between 800 and 950 cm slumping structures appear within the sediment. From 1020 cm on Late Glacial clays and silts dominate, as well as a great amount of turbidites.
4. Basin anlzlysi8 for 8elected time window8 4.1 The era 17 (LBT (11.149 B.P.) to recent) As the period from the isochrone of Laacher Pumice Tuff to Zero B.P. contains
178
most cores, this era was created in order to have a comparison sample. This era enclosing 11.149 years describes the entire Holocene plus a part of the Late Glacial. The depth of the sediment column down to the Laacher Pumice Tuff was taken into consideration and divided by the time (amount of years). In order to preserve the chronology until the last period (17 periods were described in the whole work), number 17 was used for the characterization. In the following the time together with its isoline plots will be frequently referred to. Figure 3 depicts the accumulation rate in the lake during these timeperiods. The average sedimentation rate for all cores is 1,06 mm/a, the accumulation rate (flux rate) is 24,84 mg cm 2 a -1. In figure 3 one can clearly notice a subdivision in the lake. The coring sites in the northern part are below average, the southern ones are above, which means that the brook called "Meerbach", which enters the lake in the southern part of the lake, had an impact on the sedimentary process in the last, approximately 12.000 years.
Figure 3" Meerfelder Maar: LSR in mm/a from LBT to recent times (Era 17).
179 This region of below-average sedimentation could be determined during the drilling process as well. The delta-zone of the lake was trespassed for the purpose of core extraction at the coring sites IliA, IIIB and IllE - the water depth was between 4 and 10 m. For reaching the sedimentation surface, water depths of 17 m had to be overcome in the profundal. The pattern of accumulation-rates of figure 4 is very similar to that of figure 3; the flux-rates demonstrate clearly the influence of the brook "Meerbach". The sediment-sequences K/L and A/B handled by ZOLITSCHKA (1986, 1990) are indeed located in the region of smallest sedimentation - this complicates the counting of varves in the thin sections - but a strong effect by means of the "Meerbach" is not to be expected. The "Meerbach"-delta seems to have shifted to the West in comparison to the previously described time frames (WEGNER, 1992).
Figure 4: Meerfelder Maar: mean accumulation rate in mg cm"2 a"1 from LBT to recent times (Era 17).
180
In our opinion this is not the case, as in drilling IliA the LBT is the undermost mark which means that the marks 13-16 could not be drilled and this sequence was consequently not consulted for the interpolation of the plot of for example the era 14. 4,2 The era 14 (12.014 to 11.725 B.R) An average sedimentation rate derived from 10 corings indicates a further decrease of about 35 % in comparison to era 15. These drillings within the profundal area indicate the highest values, whereas in the West and the East exist the lowest values. After a closer study of the profiles, the distribution makes sense. The "Masterscale" IIC contains a slumping area with a thickness of about 1,5 m. A thinner one exists also at location IC.
Figure 5" Meerfelder Maar: LSR in mm/a from 12,014 to 11.725 B.P. (Era 14 = B(311ing). Hence it follows that sediment in marginal positions slides toward the profundal. Origin of this slide was a 20 cm thick, coarse-grained layer (poorly subrounded, coarse-, middle- to fine gravel) in clayish-silty matrix,
181
primarily consisting of clay schist and graywacke that can be found in liD. The base of this fan was found 22 cm above marker 14, the base of the slumping in core IIC is located about 18 cm above marker 14. Assuming that the sedimentation rate in core IIC was (according to its location) lower than in liD, until this fan-event there exists a time correlation. When calculating the coarsely clastic layer - which is to be considered as a special event - drilling liD results in a LSR of 1,18 mm/a. This would mean that the slide occurred approximatety 12.000 B.P.. These thoughts lead into the assumption that probably a portion of the sediment in both, the delta region and the profundal area, had been captured, respectively rearranged, to provide enough material for the partly very thick "slumping-intercept". 4.3 The era 12 ( 11.149 to 10.435 B.P. ) The base of the 714 years enclosing era 12 is formed by the ashes of the disastrous outburst of lake "Laach". The LBT isochrone is found throughout Europe. This isochrone was regarded as the most important one for this examination, as well. As such, an attempt was made to reach down to this marking layer with each coring. In contrast with sample era 17 the plots for this time frame are less structured. Remarkable is the very low LSR as well as a Iowflux-rate in the East at drilling site IA. This is substantiated by a layer of coarsely clastic material, about 18 cm above LBT. This layer consists partly of rock fragments and has very likely captured the sediment in the sequence. Directly above this layer there are located clays and silts of the Younger Dryas period. This leads to the assumPtion of a sediment gap, as era 12 in drilling site llC depicts a sedimentation column with a depth of 54 cm and not, as described above, only 18 cm in IA.The areas of less sedimentation in the East, perceivable (to be seen) in figures 6 and 7, should therefore not be overinterpreted. They simply represent the absence of an unknown amount of sediment.
182
9Figure 6: Meerfelder Maar: LSR in rnm/a frorn 11,149 to 10.435 B.P. (Era 12 = AllerSd / Younger Dryas).
It is difficult to bring the "fan-event" into correct chronological order because it did not occur within the era but rather later near the end of the time period.The only plausible explanation is that 480 years are missing within core I A.This has been calculated by means of LSR-data from the other cores. An average LSR of 0,76 mm/a and an average flux-rate of 24,76 mg cm2a -1 calculated from 18 drilling locations are very good indicators for an improvement of climate during this period of time because low values point to both higher organic sedimentation and production, and, particularly, a low minerogenic supply within the lake. As already mentioned in chapter 3.1, the sediment consists of finely laminated diatomaceous gyttja in interbedding with clays and silts. The rapid, intensive warming during the "Aller6d" led to an expansion of closed woodland (cf. USINGER 1984).
183
Therfore, less clastic material was available and transported into the lake (see ZOLITSCHKA 1986, 1990). Towards the end of the era the temperatures decreased (see ZOLITSCHKA (1986, 1990)) but this influence can not be observed in the sediment. The border with the following era is a sharp change from finely laminated diatomaceous gyttja to clays and silts in era 11, clearly characterized by a change of colour.
Figure 7: Meerfelder Maar: mean
accumulation-rates in mg cm "2 a "1 from 11.149 to
10.435 B.P. (Era 12 = Allerod/Younger Dryas)
Nevertheless it should be emphasized that this change does not definitely describe the boundary AllerSd / Younger Dryas. This boundary lies somewhat earlier because changes of climate become apparent in the sediment with a time lag. 4.4 The era 11 (10.435 to 10.016 B.P.)
This time flame, representing 419 years, mirrors the sedimentary conditions during the cold period of the Younger Dryas. The closed woodland of the AllerSd changed into tundra vegetation (cf.ZOLITSCHKA 1986, 1990 and USINGER 1984).This led to an increase of
184
the mean LSR to 1,11 mm/a (average of 18 corings) and a flux-rate of 51,04 mg cm -2 a -1, in contrast to 0,76 mm/a and 24,14 mg cm -2 a-1 during era 12. Furthermore the relatively high scattering of the values within the lake can be regarded as an indicator for the cold relapse because again clastic material is available which deposits more readily in the delta area. The range of LSR values is 2,17 mm/a (Xmax = 2,98 mm/a (IliA), Xmi n -- 0,81 mm/a (IF)), the range of the flux-rate is 99,43 mg cm "2 a 1 (Xmax = 136,54 mg cm "2 a "1 (IliA), Xmin = 37,11 mg cm "2 a "1 IF)).The values of drilling IA lie somewhat lower than those of drilling IF. Because of the presence of a hiatus, it was not used because it is not the "normal" sedimentation that is reflected in this sequence.
Figure 8: Meerfelder Maar: LSR in mm/a from 10.435 to 10.016 B.P. (Era 11). The junction of the isolines with a maximum in the area of sequence IliA shows clearly the influence of the "Meerbach" on the sedimentary process. A secondary maximum at IIA indicates that at this location a small creek has probably flown into the maar or a slide has occurred. After having a closer look on the map of the "geornorphologic structures of the Meerfelder Maar" by HENK A. (1984, page 66) (see also figure 12), this
185
assumption is made more probable. Exactly in the northern extension of profile II there exists a sharply cut morphologic trench in the crater wall of unknown age. Hence, it would be possible that solifluidal clastic material has been transported through this trench in era 11 and, consequently, a small delta had been formed.
Figure 9: Meerfelder Maar: mean accumulation-rates in mg cm- 2 a "1 from 10.435 to 10.016 B.P. (Era 11).
4.5 The era 6 (5.318 to 4.905 B.P.)
This period consists of 413 years and is located at the transition of pollen zones Atlantic/Subboreal. The average linear sedimentation rate for all drillings is 0,77 mm/a which means an increase of about 30 % in comparison to era 7. Concerning the mass accumulation rate, however, one can state a decrease of more than 10 % in comparison to era 7. This trend, continuing towards era 7 (rising sedimentation rate (LSR) together with decreasing flux-rate) indicates an obviously increased impact on the sedimentary process by organic material, as
186
well as stronger organic sedimentation. According to ZOLITSCHKA (1990) and USI NGER (1984) the vegetation does not change remarkably in comparison to the preceding time period. All that can be noticed is a light change into an oak lime - maximum at the beginning of the Subboreal.
Figure 10; Meerfelder Maar: LSR in mm/a from 5.318 to 4.905 B.P. (Era 6 = Atlantic).
As recognizable in figure 10, the drillings of the central area of the lake provide similar values. Only drilling IIIB, situated in the western delta area of the lake, has a distinctly increased value. In this sequence the layers of coarse organic material are much deeper. There are three possible explanations: - an accumulation of leaves and parts of plants introduced by the "Meerbach", in addition to the fact that at those times the stream-inflow was located more to the West; - the wind, predominantly from the East to the North-East, which leads to the concentration of the materials floating on the surface of the lake; and
187
- anthropogenic activities for which no macroscopic records are found within the sediment. ZOLITSCHKA (1989) estimates the earliest neolithic colonization in the area to about 3.400 to 3.200 B.C. which coincides with the first part of era 6. ZOLITSCHKA (1988) discovered residues of charcoal taken from this time which, in his opinion, mirrors the human influence on the sediment. Therefore the third explanation seems to make more sense. 4.6 The era I (1.736 to 0 B.R) This era lasting 1736 years, describes the sedimentary conditions from 214 A.D. until the present and is therefore to be connected to the pollen zone of the Subatlantic. An increase of the average LSR in comparison to era 2 of more than 40 % to 1,16 mm/a (average value of all drilling sites) and an increase of only 19% of the average flux-rate to 15,92 mg cm -2a-1 in comparison to era 2, indicates a high organic production, respectively sedimentation. The part of allochthonous material has not increased so much, compared with the autochtonously formed part of the sediment. In all probability this stands in relation to the improved agricultural utility (profit) and the appropriate fertilizing (manuring) within the crater in comparison to former times. The corresponding influx of nutrient elements into the lake over the last centuries brought with it both a worsening water quality and a high bioproduction. Anthropogenic influences on the sedimentation are to be regarded as dominant during this specific time frame. Looking at figure 11, a clear influence of the "Meerbach" is evident, though coring IE which has the highest LSR with 1,43 mm/a and a flux-rate of 19,66 mg cm-2a-1 , is located in the delta area of the "Meerbach". According to its values, "Masterscale" IIC has to be considered to be influenced by the Meerbach. All remaining drilling sites indicate values of 1,04 to 1,14 mm/a LSR and 14,03 to 15,68 mg cm -2 a -1 flux-rate.
188
Figure 11: Meerfelder Maar: LSR in mm/a from 1.736 to 0 B.P. (Era 1 = Subatlantic). Comparing the photographs of the cores IE and. e.g., IB, one will immediately recognize the difference in sedimentation. In IB one notices only occasional thin bands of clay enclosed in diatomatious gyttja. In IE the amount and thickness of these bands is higher. The influence of the"Meerbach" is spatially extremely limited to this era.
5. Summary of the results The time sequences described above demonstrate clearly that the sedimentary processes in the Lake Meerfelder Maar during the Holocene period had intensive variations. In times of predominantly clastic sedimentation the isoline plots are easier to understand as they are more clearly structured than during times of dominantly biogenic sedimentation. The influential factors on the latter seem to be substantially more complex and numerous. During earlier periods the isoline plots are oriented to the East/West, later this changes into a North/South orientation.
189
AS the "Meerbach" flows into the lake from the South, the East-West orientation is possibly an indicator for its dominance over the sedimentary conditions. The North-South orientation of the isolines, on the other hand, can be considered as a sign of the influence of the wind direction (intermixture) on organic sedimentation. A coarsely clastic layer in the delta area (drilling liD) could be shown in era 14 as the source of a "slump" existing in various other core sequences from the profundal. This "slumping" can be interpreted as a landslide, induced by the "fan - event". By means of the average linear sedimentation rate (LSR) this accretion of coarsely clastic material can be dated from 12.000 B.P. Another coarse clastic layer in sequence IA in era 12 was identified as beeing the cause for the missing of approximately 480 years in profile IA during that time. As the following period (era 11) is to be regarded as complete (see figures 6 and 7), the moment of entry is calculated as follows: 18 cm of sediment with an average LSR of 0,76 mm/a indicates a period of 236 years. In other words, the coarse clastic material would have been deposited 236 years after the eruption of Lake Laach, i.e. ca. 10.900 B.P.. As mentioned above, 480 years are missing in IA from this time period. These annual accounts, however, can only give an allusion to the true age because microstratigraphic examinations would be necessary to define the exact time. The cold period of theYounger Dryas is reflected in the plot of era 11. The Meerbach again takes a dominant positionas clearly indicated in drilling IliA (delta area). Another maximum is found in sequence IIA. In this time period there must have been at this spot of the lake a heightened influx too, possibly initiated by a streamlet or a slide. This assumption is deepened by HENK (1984) (cf. figure 12) who has charted geomorphologic structures on the crater's edge. A morphologic trench which he mapped is close to drilling location IIA. The material could have been transported through this trench which is responsible
190
for the high sediment increase at this location. Furthermore figure 12 shows clearly that the sharp morphologic trench and the talus fans in the West of the maar cauldron continues until inside of the Maarlake. Drilling IG consists of a sequence of nearly 7 m - coarsely clastic material and is located in direct elongation of this trench, respectively talus fan. As there were no marking layers found in sequence IG, a chronological order is hardly possible. Since point "zero" of this sequence is equivalent to a depth of 655 cm and proceeds from an average LSR in the Postglacial period of 1,06 mm/a the upper limit of the very thick coarsely clastic layer has an estimated age of 6.500 years. The ages mentioned above though, are to be regarded as rough estimate. In the course of the Postglacial the vegetation density can be read clearly. High LSR values together with low flux-rates indicate a predominantly biogenic, autochthonous sediment with, respectively, low allochthonous, clastic material. -
Already in era 6 (5.318 to 4.905 B.P.) the first anthropogenic influences were visible. The thickness of clay layers in comparison to corings in the delta area profundal point out these influences caused by clearings etc.. The existence of a settlement during the Roman Age can be confirmed by the relation of LSR and flux-rate in the sediment, as well. Era 1 (1.736 to 0 B.P.) is evidently influenced by mankind in its sedimentary process. By means of agricultural utilization and, consequently, fertilizing, nutrients got into the lake; this led to a high bioproduction, together with a high LSR. At the same time, the soil utilization supports the increase of the flux-rate, however not in the same relation as the LSR. Figure 13 shows the course of the curves of LSR and flux rates in comparison. The low decrease of LSR from era 13 to era 12 is clearly to identify in comparison to the flux-rate. This indicates a temperature increase and an increase in vegetation density.
191
Figure 1 2: Morphologic structures of the "Meerfelder Maar'.
In era 11 the increase of the flux-rate in comparison to LSR is definitely high, which again is substantiated in the cold period of Younger Dryas. The higher increase of LSR in relation to the flux-rate in era 1 is clear in the following graph.
192
F i g u r e 1 3 : Mean LSR and fluxrates from 12.369 B.P. to the present. (Values for all time frames are meanvalues over all coring sites.)
193
References
BRAUER, A. (1988): Versuch einer Erfassung alter Seespiegelst&nde an ausgesuchten Eifelmaaren und mikrostratigraphische Untersuchungen an Sedimenten des Weinfelder Maares.- UnverSff. Diplom-Arb. Univ. Trier, 117 S. BRAUER, A. (1989): Aufbau der Sedimente und Mikrostrukturen im Sedimentgef~Jge - Erste Ergebnisse der D~)nnschliff-Auswertungen der Meerfelder Maar Tiefbohrungen MFM-A/B.- UnverSff. Arbeitsbericht, 32 S., Trier. BRAUER, A. & NEGENDANK, J.F.W. (1989a): Lake-level changes indicated by lake-level terraces in six maar lakes in the Eifel, FRG. - Terra abstacts, 1 : S. 226; Strasbourg. DROHMANN, D. (1990): Sedimentologische Untersuchungen an sp&tglazialen Turbiditen des Meerfelder Maares (Westeifel/Bundesrepbulik Deutschland). UnverSff. Diplom-Arb. Univ. Trier, 113 S. DROHMANN, D. & NEGENDANK, J.EW. (1991): Distribution, structure and classification of Turbidites of Meerfeld Maar Lake (Westeifel, FRG). - Terra abstracts, 3: 347, Strasbourg. DROHMANN, D. & NEGENDANK, J.EW. (1991 a): Sedimentologische Untersuchungen an sp&tglazialen Turbiditen des Meerfelder Maares (Westeifel). - Progr. Abstr. Exc. Guide Symp. Paleolimnol. Maar Lakes: 25; Trier. HAVERKAMP, B. (1991): Pal&omagnetische Untersuchungen an sp&tquart#,ren Maarseesedimenten zur Pal&osAkularvariation im Gebiet der Westeifel w&hrend der letzten 20-25.000 Jahre.- UnverSff. Diss. Univ. MQnster, 235 S. HENK, A. (1984): Zur Geologie und Geophysik des Meerfelder Maares und seiner Umgebung (Westeifel,- UnverSff. Diplom-Arb. Univ. Mainz, 153 S. NEGENDANK, J.F.W. (1989): Pleistoz&ne und holoz&ne Maarseesedimente der Eifel.- Z. dt. Geol. Ges., 140:13-24. NEGENDANK, J.F.W. (1991 ): Maare als kontinentale erdgeschichtliche Fallen mit hoher ZeitauflSsung. - Progr. Abstr. Exc. Guide Symp. Maar Lakes: 41; Trier. NEGENDANK, J.EW., BRAUER, A. & ZOLITSCHKA, B. (1990): Die Eifelmaare als erdgeschichtliche Fallen und Quellen zur Rekonstruktion des Pal&oenvironments.- Mainzer Geowiss. Mitt., 19: 235-262. NEGENDANK, J.EW., DROHMANN, D., POTH, D., SEUL, C. & WEGNER, F. (1989): Sedimentology of Meerfeld Maar lake sediments
194
WEGNER, F. (1992): Fazielle Entwicklung und Verteilung der Sedimente im Meerfelder Maar (Westeifel/Bundesrepublik Deutschland) - Ein Beitrag zur holoz&nen Seegeschichte -; UnverSff. Diplom-Arb. Univ. Trier, 90 S. ZOLITSCHKA, B. (1986): Warvenchronologie des Meerfelder Maares - Lichtund elektronenmikroskopische Untersuchungen sp&tglazialer und holoz&ner Seesedimente.- UnverSff. Diplom-Arb. Univ. Trier, 119 S. ZOLITSCHKA, B. (1988): Sp&tquart&re Sedimentationsgeschichte des Meerfelder Maares (Westeffel) - Mikrostratigraphie jahreszeitlich geschichteter Seesedimente.- Eiszeitaiter und Gegenwart 38: 87-93, Hannover. ZOLITSCHKA, B. (1989): Jahreszeitlich geschichtete Seesedimente aus dem Holzmaar und dem Meerfelder Maar (Westeifel).- Z. Dt. Geol. Ges., 140: 2533, Hannover. ZOLITSCHKA, B. (1990): Sp&tquart&re jahreszeitlich geschichtete Seesedimente ausgew&hlter Eifelmaare.- Documenta naturae, 60: 226, M0nchen.
TURBIDITES IN THE SEDIMENTS OF LAKE MEERFELDER MAAR (GERMANY) AND THE EXPLANATION OF SUSPENSION SEDIMENTS
D. DROHMANN & J.F.W. NEGENDANK
* Waste Management (Deutschlanc0 GmbH, D-4300 Essen ** Dept. of Geology, University of Trier, D-5500 Tder
ABSTRACT
Detailed grain-size and thin section analyses show the textural and structural variability of an individual lacustrine turbidite along its current path in vertical and lateral variation. These results enable the implication of a facies model of lacustrine turbidity current deposits in maar lakes.
I. INTRODUCTION
In 1988, twenty-seven sediment cores were taken across the whole Lake Meerfelder Maar, belonging to the Quaternary Westeifel Volcanic Field. One profile (I) is located WSW - ENE (680m in lenght) another one (11)is located in NNW - SSE direction with a lenght of 400m (Fig. 1). The sequences show Holocene organic-rich and Late Glacial mineral layers with the Laacher See Tephra as important isochrone. 16 special layers allowed to correlate the different cores of the whole lake. Figure 2 demonstrates the standard-sequence. From this sequence a succession of three Late Glacial turbidites was investigated and correlated between nine coring sites of Lake Meerfelder Maar.
Lecture Notes in Earth Sciences, Vol. 49 J. F. W. Negendank. B. Zolitschka (Eds.) Paleolimnology of European Maar Lakes 9 Springer-Verlag Berlin Heidelberg 1993
t96
Figure 1: Subsurface contour map (altitude above sea-level in m) of Lake Meerfelder Maar including the profiles and the coring sites (NEGENDANK et al., 1990 according to DROHMANN et al., 1989).
Figure 2: Stratigraphy of the standard-sequence (profile II C}. Zero corresponds to an depth of 200 cm be{ow sediment surface (DROHMANN et al. ,1989).
197 Detailed grain-size and thin section analyses show the textural and structural variability of an individual lacustrine turbidite along its current path in vertical and lateral variation. The turbidity currents were probably initiated by spring runoffs (meltwater & rainfall) of the Meerbach (a small river), forming a delta at the southern lake shore. The lack of litoral diatoms indicates that the turbidity currents were river-induced and not the result of lateral resedimentation (e.g. slumping, subaqueous slide). These results enable the implication of a facies model of lacustrine turbidity current deposits in maar lakes.
II. RESULTS 1. Variation of thickness The investigated sequence of the three turbidites varies in thickness from 16 cm at the source in the sOuth to 4 cm at the most distal point in the north, within a distance of 260m. Computer generated isoline-graphic shows a lobe-like trend in distal areas (Fig. 3). This is presumably the effect of the interface of the unstable system turbidity current/ stagnant ambient medium (according to ALLEN, 1985 and SIMPSON, 1969,1972).
Figure 3: Lateral variation of the thickness (in cm) of the entire sequence (three turbidites). The computer generated isoline-graphic demonstrates the lobe like trend in distal areas.
198 2. Granulometry of the turbidites a.) Lithology Lithology changes from thicker beds of proximal sandy turbidites in the south to clayrich, thinner distal facies northwards. The textural classification (after SHEPARD, 1954 and MOLLER, G., 1961,1964) of the turbidites present sand-silts in the proximal parts and clayey-sandy silts in the central parts to clayey silts and clay-silts in the distal zone. Table 1 gives a survey of some grain-size parameters. Table 1: Grain-size parameters of the investigated turbidites. The upper turbidites corresponds to the correlation-layer 16. Distality increases from top to bottom. The grain-size classes refer to the median.
samnle 91
median (um) 100.0
40
77.0
73 55 47 21
23.0 12.3 11.8 3.6
75 45 57 113 128
52.6 37.2 31.3 11.0 10.3
erains-size class veryfine sand veryfine sand medium silt fine silt fine silt veryfine silt very coarse silt coarse silt medium sift fine silt fine silt
mearl (l~m) 96.7 73.3 24.7 16.6 16.5 8.5 75.9 38.2 37.4 16.3 13.9
clay-~;ontent (weight-%) 5.4 9.3 17.0 upper 22.5 turbidites 23.6 39.7 5.2 14.5 6.9 23.0 32.6
lower turbidites
b.) Downslope variation of the grain-size distribution The samples represent unimodal grain-size distributions with decreasing mean grainsize and increasing clay-content downslope. The sinusoidal relationship of mean grain-size and standard deviation indicates mixing of two grain-size populations. Proximal turbidites show positively skewed leptokurtic distributions whereas distal deposits are characterized by negative skewness and platykurtic values. Decrease in streampower implies reduction of transporting capacity and tractive-force.
199 c.) The turbidites in probability plots The cumulative-frequency curves, drawn on probability paper, show the distribution of the samples as straight-line segments. Each segment has a different slope and is separated by a sharp break between the segments. The slope of each straight-line segment and the position of the breaks between the segments reflect the mechanism of deposition and is a function of sorting (FRIEDMAN & SANDERS, 1978). In a sample with three straight-line segments (Fig. 4), the segment at the fine end of the distribution resulted from deposition of particles deposited in suspension, and that at the coarse end from particles deposited by rolling or sliding (bed load). The central segment may represent a subpopulation that moved in the current by saltation (jumping motion).
Figure 4: Sample 75 in probability plot. The pattern of the cumulative curve of particle size is a function of the transportation mode. The 2-phi-break is characteristic for the transition saltation / bed-load. The pattern of cumulative curves of particle sizes on probability plots is a function of the processes that formed the sediment. Therefore we can conclude (figures 5 and 6):
->
The cumulative curves differ in sorting and quantity of the segments and the location in the diagram,
200 Subpopulations of the same transport mode are parallel, but a displacement of the breaks can be observed, from proximal to distal to the finer-grained field. The breaks of the distal samples obtain higher y-axis values from proximal to distal samples the suspension population is increasing whereas the bed load population decreases, ->
The probability plots discriminate the character of a turbidite (proximal or distal).
Figure 5: Cumulative curves (A) and probability plots (B) of the upper turbidites.
201
Figure 6: Cumulative curves (A) and probability plots (B) of the lower turbidites.
d.) The turbidites in a C/M-diagram C/M-diagrams (according to PASSEGA, 1957) in which C is the coarsest one percentile and M is the median of the grain-size distribution, characterize the coarsest fractions of the sample. These parameters are closely related to transport and deposition mechanisms, the grain-size image gives precise information on hydraulic conditions and environment.
202 The diagram demonstrates the position of the investigated samples in the turbidite pattern (Fig. 7), except the most distal sample, which occurs in the segment of pelagic suspension, nevertheless representing a distal turbidite. This result leads to the conclusion that the frequently occuring Glacial and Late Glacial suspension sediments can represent distal turbidites. The most proximal samples (P91, P40) are situated in the QR-segment, the mainchannel deposits. They document a graded suspension, mainly bearing the saltation population and the bed- load population. The remaining samples belonging to the RSrespectively RT-segment representing a uniform suspension or the transition graded suspension/uniform suspension.
Figure 7: The investigated turbidites in a C/M-diagram (accordingto PASSEGA, 1957, 1964). 1. pelagic suspension,
2. turbidity currents,
3. tractive currents.
203 3. Structure and classification of the turbidites Various hydrodynamic conditions caused several sedimentary structures. Proximal parts are documented by bed-load sediments and graded bedded sandlayers with plant and wood fragments at the base, overlain by silt and clay units (Fig. 8), whereas the bedload sediments and the graded bedded sandlayers are lacking at distal sequences (Fig. 9). Intraclasts, lentils and convolute lamination occur in the fine-grained units.
Figure 8: Structural-sketch of a proximalturbidite (thin section of sample 40).
Figure 9; Structural-sketch of a distal turbidite (thin section of sample 55).
204 The particles of the graded bedded sandlayers are orientated parallel to the current direction with their longitudinal axis. The larger ends of the grains tend to point towards the source. Thereby, orientated sampling is possible if the current direction is known. Proximal (medium-grained) turbidites can be better described by using the ideal sequence of BOUMA (1962, 1964) (Fig. 10 ). The classification of mud-turbidites (according to STOW & Piper, 1984) is more useful for the distal (fine-grained) samples (Fig. 11 ). But complete sequences are lacking. The absence of bioturbation indicates an anoxic environment during and after sedimentation.
Figure 10: BOUMA sequence of grain-sizes and sedimentary structures in a turbidite, and its hydraulic interpretation (according to ALLEN, 1985). The thickness varies from a few centimeters up to >lm.
Figure 11: Facies model of the fine- grained turbidites. A. silt turbidite, B. mud turbidite (STOW & PIPER, 1984).
205 The populations of the probability plots and the interpretation of C/M-diagram are reflected in the thin sections. The segment-lengths reflect the approximate thickness of the intervals.
.
Depositional model of river-induced lacustrine turbidity currents in Lake Meerfelder Maar
The results enable the implication of a depositional model and a facies model. Different facies can be related to different parts of the idealized sequences and hence to a particular type of flow. The sediment load was probably transported and deposited in the lake by underflows (undercurrents) depending on the density difference between river and lake water. Sedimentladen stream water flowed downslope the deltafront from S-SE to N-NW direction, resulting from spring runoffs. Figure 12 demonstrates this facies change. The proximal fan is represented by thick silty sands. Bed load population (ST1) and saltation population (ST2) are dominant. Basal sand-layers and graded bedded sand to coarse-silt strata are the result. Generally, the sequence is overlain by uniform suspension sediments (ST3). The transition from the deltafront to the basin plain implies reduction of current velocity, which caused different sedimentary structures. The central fan is documented by two-dimensional sand-/silt-discharge, extended by the pelagic suspension interval (ST4) in the proximal-central parts and possibly reduced to the intervals ST3 (uniform suspension) and ST2 (graded suspension) in the centraldistal zones. Northwards the distal fan is following, characterized by further bed thinning. The structure is more complicated, although the sequence is only based on uniform suspension (ST3) and pelagic suspension (ST4). Intraclasts, lentils and convolute lamination occur. Lithology changes to clayey silts and clay-silts, whereby interlocking to the pelagic environment has taken place. From the proximal to the distal facies the high-density turbidity current (the current which keeps the sand-fraction in suspension) was transformed into a low-density current.
The fine-grained suspension sediments can represent distal turbidites.
206
Figure 12: Structural variability and facies model of Late Glacial turbidites of Lake Meerfelder Maar. The dotted pear shaped lobe corresponds approximately to the distribution of the turbidite sequence (discharge zone). The arrow shows the current direction. The legend of the intervals (ST1-ST4) is the same as in the sketches of the thin sections (Fig.8 - Fig.9). 5. Comparisons with other maar lakes If we compare these results with grain-size analyses of samples from Lac du Bouchet (HASS, 1989), we can conclude that (Fig.13): ->
the "A-samples", which HASS (1989) interpreted as suspension sediments, represent distal turbidites,
->
the "B-samples", which HASS characterized as turbidites, represent central turbidites,
->
the "C-samples", which he interpreted as sandlayers or top-cut-turbidites represent proximal turbidites.
207
Figure 13: Selected sediment types of Lac du Bouchet (Massif Central, France) investigated by HASS (1989) in C/M-diagram. A: distal turbidites,
B: central turbidites,
C: proximal turbidites.
The investigations of NEGENDANK (t989) (sediments of Eifel maar lakes) and NELSON (1967) (sediments of Crater Lake, Oregon) allow similar implications: ->
the field of the clay/s/it sediments of the Eifel maar lakes and the field of the Crater Lake mud layer represent distal to central turbidites,
->
the field of the silt/sand sediments of the Eifel maar lakes and the sand layers of Crater Lake represent central to proximal turbidites.
If this implications are correct we have finally to point out, that turbidity currents are important transport and depositional mechanisms in rnaar lakes and, therefore, maar lakes are suitable for turbidity current investigations.
208 III. REFERENCES ALLEN, J. R.L (1985): Principles of physical sedimentology.- 272 pp, London. BOUMA, A. H. (1962): Sedimentology of some Flysch deposits.- 168 pp, Amsterdam. BOUMA, A.H. (1964): Ancient and recent turbidites.- Geol. en Mijnbouw, 43, 375-379. DROHMANN, D. & POTH, D. & SEUL, CH. & WEGNER, F. & NEGENDANK, J.F.W. (1989): Sedimentoloy of Meerfeld Maar lake sediments (Westeifel, FRG).Terra abstracts, Vol. 1, p 226, Strasbourg. DROHMANN, D. & NEGENDANK, J.F.W (1991): Sedimentologische Untersuchungen an sp&tglazialen Turbiditen des Meerfelder Maares (Westeifel/FRG).Symposium on paleolimnology of maar lakes, p 25, Bitburg. DROHMANN, D. & NEGENDANK, J.F.W. (1991): Distribution, structure and classification of turbidites of Meerfeld Maar Lake (Westeifel, Germany).Terra abstracts,Vol. 3, p 347, Strasbourg. FRIEDMAN, G. M. & SANDERS, J. E. (1978): Principles of sedimentology.- 792 pp, Santa Barbara, New York, Brisbane, Chichester, Toronto. HASS, C. (1989): Sedimentologische und schwermineralogische Untersuchungen an ausgew&hlten Sedimenttypen des Lac du Bouchet (Massif Central, Frankreich).Diploma thesis, University of Trier (Germany) 327 pp - (unpublished). MOLLER, G. (1961): Das Sand-Silt-Ton-Verh<nis in rezenten marinen Sedimenten.N.Jb. Miner., Mh 1961,148-163. MOLLER, G. (1964): Methoden der Sediment-Untersuchung.- 303 pp, Stuttgart. NEGENDANK, J. F. W. (1989): Pleistoz&ne und holoz&ne Maarsedimente der Effel.Z. dt. geol. Ges., 140, 13-24, Hannover. NEGENDANK, J.F.W. & BRAUER, A. & ZOLITSCHKA, B. (1990): Die Eifelmaare als erdgeschichtliche Fallen und Quellen zur Rekonstruktion des Pal&oenvironments.- Mainzer geowiss. Mitt., 19, 235-262, Mainz. NELSON, C.H. (1967): Sediments of Crater Lake, Oregon.- Geol. Soc. Am., Bulk., 78, 833-848. PASSEGA, R. (1957): Texture as characteristic of clastic deposition.- Am. Assoc. Petroleum Geol., Bull., 41, 1952-1984. PASSEGA, R. (1964): Grain-size representation by CM patterns as a geological tool.J. Sed. Petrol., 34, 830-847. SHEPARD, F. P. (1954): Nomenclature based on sand-silt-clay ratios.- J. Sed. Petrol., 24,151-158. SIMPSON, J. E. (1972): Effects of the lower boundary on the head of a gravity current.J. Fluid Mech., 53, 759-768. STOW, D. A. V. & PIPER, D .J.W. (eds., 1984): Fine-grained sediments: Deep-water processes and facies.- 659 pp, Oxford, London, Edinburgh, Boston, Palo Alto, Melbourne.
PALEOCLIMATE RECONSTRUCTION AT THE P L E I S T O C E N E / H O L O C E N E TRANSITION - A VARVE DATED MICROSTRATIGRAPHIC R E C O R D FROM LAKE M E E R F E L D E R MAAR (WESTEIFEL, GERMANY) A contribution to the discussion on the Younger Dryas climatic oscillation
D. Poth & J.F.W. Negendank
Dept. of Geology, University of Trier, D-5500 Trier, Germany
ABSTRACT Along two cross-sections 27 sediment cores were taken from Lake Meerfelder Maar. Microstratigraphic studies have been carried out on one of these annually laminated, high resolution sediment sequences, subdivided by local lithozones. Using sediment increase rates, accumulation rates and the structure of varves it was possible to prove rapid climatic variations at the Pleistocene/Holocene transition. The boundaries of local lithozones and their corresponding biozones are calculated in varve years BP (=v.y. BP, ref. year 1950). The Younger Dryas is a tripartite sequence characterized by lithozones M3a-c. Lithozone M2 characterizes organic deposits of the Late Glacial with Laacher See Tephra (LST) close to its top. The base of Younger Dryas (M2/M3a) is dated to 11.070 v.y. BP. After this period of rapid cooling a dramatic sedimentary change to clay-silt-laminations occurs at 10.850 v.y. BP characterizing the main Younger Dryas climatic deterioration (M3b). From now on soil erosion dominates due to open tundra-like vegetation of the periglacial environment, causing clastic deposition rich in sand and silt. After around 140 years organic sedimentation reappears abruptly and allochthonous minerogenic detritus recedes. This succession characterizes the transition to early Holocene warming and regeneration of soils and vegetation. The top of Younger Dryas (M3c/M4) is dated to 10.610 v.y. BP when first distinct layers of planktonic diatoms occur. Therefore the Pleistocene/Holocene boundary is fixed lithologically by varve counting, beginning at the isochrone of LST (11.323 +L 224 v.y. BP), to 10.610 v.y. BP.
INTRODUCTION Maar lakes of the Quaternary Westeifel Volcanic Field (Fig. 1) reveal continuous sedimentary records for paleoenvironmental and paleoclimatic studies covering at least the last 13.000 years. Lecture Notes in Earth Sciences, Vol. 49 L F. W. Negcndank. B. Zoli~chka (Eds.) Paleolimnology of European Maar Lakes 9 SprJngcr-Verlag Berlin Heidelberg 1993
210
The finely, annually laminated sediments enable to study lacustrine depositional systems with a high degree of time resolution. It is possible to perform absolute dating for each part of the sediment sequence by varve chronology (e.g. HEINZ 1991, POTH and NEGENDANK 1991, ZOL1TSCHKA 1990).
Fig. 1: Maar lakes of the Quaternary Westeifel Volcanic Field, changed according to ZOLITSCHKA (1990). One of the dominating factors controlling sedimentation in lacustrine depositional environments is climate, causing several biotic responses in the lake and in the drainage basin. Therefore, maar lakes are best archives to study climatic and environmental changes during the Late Glacial and the transition to the Holocene. As chronozones cannot be related to events in the bio- and lithostratigraphic record, the Pleistocene/Holocene transition as determined in the chronostratigraphic sense to 10.000 years BP (conventional radiocarbon years) (MANGERUD et al. 1974) is of minor importance for paleoenvironmental investigations. More significant are e.g. paleotemperature reconstructions obtained from ~180 measurements demonstrating a severe temperature setback corresponding to the
211
Younger Dryas climatic deterioration prior to the early Holocene temperature rise (EICHER and SIEGENTHALER 1976, EICHER 1987). As paleoclimatic conditions changed rapidly in the course of centuries or even decades, dramatically changing ecosystems and consequently influencing processes of sedimentation, this drastic incision is a major point of interest of paleoclimatie and paleoenvironmental investigations during the last years (cf. BARD and BROECKER 1992). Still the major problem is to obtain conclusive data for the duration of the Younger Dryas climatic oscillation and the determination of the Pleistocene/Holocene boundary. In fact, any attempt at reconstruction and interpretation of timing and rates of paleoclimatic changes and their responses in biosphere will only be useful on the basis of a reliable chronology. This paper presents a varve-dated microstratigraphic record from Lake Meerfelder Maar, Westeifel/Germany.
METHODS Along two cross-sections 27 sediment cores were taken from eutrophic Lake Meerfelder Maar (6~ 45" E, 50 ~ 6" N) (Figs. 1, 2). The finely laminated deposits, Late Glacial silts and clays and Holocene diatomaceous gyttja (Fig. 3), are in extraordinary good condition.
Fig. 2: Bathymetric map with location of the coring sites, changed according to DROHMANN et al. (1989).
The sediment sequences with an average thickness of 12m, each consisting of several lm and 2m long and 80ram wide core sections, were recovered from a raft using a modified LIVINGSTONE
212
Fig. 3a: Characteristic sediment sequences from lake MFM, according to DROHMANN et al. (1989).
213
Fig. 3b: Characteristic sediment sequences from lake MFM, according to DROHMANN et al. (1989).
214
piston-corer ("USINGER-corer"). The object was to recover the Late Glacial in each sequence, including the pyroclastic layer of Laacher See Tephra (LST) as an important isochrone. For each coring site, two alternating core series were correlated macroscopically according to marker horizons and compiled to one continuous sedimentary record. Additionally, it was possible to correlate all cores from the different coring sites of the whole lake (Fig. 3).
Fig. 4: General view of sediment sequence MFM II Dc/II Dd.
For microstratigraphic investigations one of these high resolution sediment sequences (MFM II Dc/II Dd, Figs. 3b, 4, 6) was subsampled continuously, beginning at the isochrone of LST, to prepare 10 cm long overlapping thin sections. The thin section evaluation comprised: 1. semi-quantitative determination of authigenic minerals, 2. semi-quantitative determination of the ratio between organogenic and minerogenic components,
215
3. determination of the important diatom species, 4. determination of the annual sediment increase rates, 5. absolute dating by varve counting, beginning at the isochrone of Laacber See Tephra (I 1.323 +L 224 v.y. BP, ZOL1TSCHKA et al. 1992), 6. subdivision of the sedimentary record into local lithozones. Diatomological observations proved, that the prevailing organic Late Glacial and Holocene deposits are annually laminated. The varves consist of a spring/summer layer of chrysophytes and planktonic diatoms, an autumn layer of littoral diatoms and organic material and a winter layer of minerogenic detritus (Fig. 5).
Fig. 5: Typical organic varve of Late Glacial and Holocene deposits from Lake Meerfelder Maar.
The dating accuracy of +/_ 1.4% on average, coming up to the error calculation of +/_ 2% given by SAARNISTO (1985), was obtained by triplicate varve counts but depends in detail on the distinctness of varves. The counting accuracy varies from 0.6% for distinct, clear-cut organic varves to 4.3% for Younger Dryas minerogenic deposits. The boundaries of local lithozones and tfieir corresponding biozones are calculated in var,'e years BP (=v.y. BP, ref. year 1950).
216
Taking the whole width of thin sections into consideration and subtracting gaps caused by preparation varve by varve measurements allow to determine the sediment increase rates with annual resolution (Fig. 8). In the following the term "sediment increase rate" will be used synonymously with "sedimentation rate". The commonly accepted term "sedimentation rate" will probably be misleading because of different, depth-related phenomena of compaction and relaxation of sediment. To eleminate the factors water content and compaction of the sediment, dry density data (g cm-3) estimated by ZOLITSCHKA (1990) were used to calculate accumulation rates (nag cm -2 a -1 ), showing the absolute annual sediment input for a defined area of the lake bottom (Fig. 8).
RESULTS The Late Glacial and Holocene sediment sequences from Lake Meerfelder Maar consist of finely, annually laminated lacustrine deposits. Thin section evaluation provides the most accurate counting of annual laminations, and additional information about sedimentological and to some extent even about paleobiological changes can be achieved. Absolute dating by varve chronology (Tab. 1) as well as exact determination of the sediment increase rates and calculation of accumulation rates with annual resolution (Fig. 8) has been carried out, beginning at the isochrone of LST (11.323 +/_ 224 v.y. BP, ZOLITSCHKA et al. 1992) up to the early Boreal, leading to a subdivision of the sedimentary record (Fig. 6) into local lithozones (M2- M5). Sedimentological and diatomological observations show the following results: I. Lithozone M2 characterizes organic deposits of the Late Glacial with Laacher See Tephra close to its top. The boundary of M2/M3 is dated to 11.070 v.y. BP. 2. Minerogenic deposits of M3, supposed to be the imprint of the Younger Dryas climatic oscillation, show a subdivision into 3 phases (M3a - M3c). Duration of M3 is determined to about 460 years. 3. The M3/M4 boundary is dated to 10.610 v.y. BP. Lithozone M4 showing a division into 3 subzones (M4a - M4c) characterizes finely laminated organic deposits (diatomaceous gyttja) of the early Holocene. Therefore the Pleistocene/Holocene transition is fixed lithologically to 10.610 v.y. BP. 4. After a transition period of about 90 years the M4/M5 boundary is dated to 9.790 v.y. BP. Within the lithozone M2 the biogenic deposits mainly contain organic detritus and planktonic diatoms. The top of M2 is indicated by a gradual decrease of organic sedimentation and a slight increase of minerogenic deposition. The sediment increase rates show an average value of 1.1 mm/a. From 11.070 v.y. BP both distinct gradual diminution of biogenic sedimentation in favour of allochthonous clastic deposition and increase of the sediment increase rates up to 1.4 mm/a on average characterize the be~nning of the Younger Dryas climatic oscillation. Vivianite and pyrite diminish and some times cease completely. Marked diatom blooms diminish, but often up to
217
Fig. 6: Detailed sediment description (MFM II Dc/II Dd) based on macroscopic and microscopic data.
218
7.0 mm thick turbidites occur with a basal layer containing silt, littoral diatoms and plant fragments. Campylodiscus noricus and, to some extent, chrysophytes indicate cold climatic conditions. This first phase of Younger Dryas (M3a) is interpreted as transition period while climatic deterioration begins. After this period of rapid cooling a dramatic sedimentary change to clay-silt-laminations which contain few littoral diatoms (e.g. Cyclotella Kii~ingiana, Fragilaria capucina, Melosira arenaria, Surirella linearis, Synedra ulna), Campylodiscus noricus and chrysophytes (Fig. 7) occurs at 10.850 v.y. BP characterizing the main Younger Dryas climatic oscillation (M3b). From now on soil erosion dominates due to open tundra-like vegetation of the periglacial environment, causing clastic deposition rich in sand and silt with average sediment increase rates of 6.0 mm/a.
Fig. 7:
Characteristic clay-silt-laminations of the main Younger Dryas climatic oscillation (M3b) with a tentative seasonal classification.
The end of tripartite Younger Dryas (M3c) is indicated by abrupt reappearance of organic sedimentation at 10.715 v.y. BP. Allochthonous minerogenic detritus recedes and the sediment increase rates decrease to 0.7 mm/a on average. Vivianite reappears as well, plant fragments are pyritized with increasing tendency. This succession characterizes the transition to the early Holocene warming and regeneration of soils and vegetation. At 10.610 v.y. BP first marked layers of planktonic diatoms occur. Anorganic detritus diminishes and the sediment increase rates drop to an average value of 0.5 mm/a within lithozone M4, show-
219
Fig. 8: Sediment increase rates and accumulation rates from Lake Meerfelder Maar (core MFM II Dc/II Dd) at the Pleistocene/Holocene transition. Smoothed data (3 point moving average).
220
ing a division into 3 subzones (M4a - M4c). A continuing development incline autochthonous organic production and decrease of allochthonous minerogenic deposition indicate a relative fast amelioration of climate during the early Holocene. The transition to lithozone M5 is characterized by a distinct decrease of the sediment increase rates to 0.35 mm/a on average at 9.880 v.y. BP. The biogenic production increases. After about 90 years allochthonous clastic sedimentation ceases completly. In lithozone M5 the sediment is composed of pure organic detritus and planktonic diatoms.
Tab. 1: Division of the Late Glacial and the early Holocene. Zonation in (varve*) years BP (reference year 1950). MA NGERUD et al. ( 1 9 7 4 )
STRAKA (197~
radiometric dating Chronozones
palyaological dating
Pollenzones
radiometricdating
* P O T H and N E G E N D A b , q ~ (this vol.)
* ZOL1TSC HKA et al. ( 1 9 9 2 )
varvechronological dating Firbas Pollenzone~s
Lithozones
~ HZM #
8OOO
M F M ##
7800 VII
Boreal
9003 - -
M5
%q 9O3O
transition
1-15 960O--
9790-988O--
C
Prcboreal
V
IV I
_
H4 1(3(3(30
10200
a
10420 - 10530 - 10630
M4
IV
III
10410-. ,
1100(3
M3
II/ 11800
# H Z M = Lake Holzmaar
n
I42
10715b --lO~Oa
111380
11070--
11323 - - L S T A/ler0d
10610--
C
--1O8OO-1-13 b 109OO-a
IIOCO -'
10110--
a
s
Younger DE'as
.., b
~11323--
b,~2
11800 ## .MF-~4 = Lake Meeffelder Maar
CONCLUSIONS The microstratigraphic sedimentary record from Lake Meerfelder Maar suggests a division of Younger Dryas (lithozone M3) into 3 subzones, and duration of the Younger Dryas climatic deterioration is determined to about 460 years. The Pleistocene/Holocene boundary is fixed litholo~cally to 10.610 v.y. BP, comparable with Greenland ice-core dating (10.720 years, HAMMER et al. 1986) and the Swedish varve chronology (10.750 v.y. BP, BJORCK et al. 1987). This is in contradiction to the termination of Younger Dryas which has been dated to 11.300 calibrated conventional radiocarbon years BP, derived from 14C calibration using a mixture of dendroyears and varve years (BECKER and KROMER 1986, STUIVER et al. 1991).
221
The necessity to calibrate radiocarbon years to sidereal or calendar years to obtain a more precise time control of climatic and environmental changes during the kate Glacial and the transition to the Holocene is described in detail by LOTTER (1991) and ZOLITSCHKA et al. (1992). The Swedish varve chronology proposes a duration of Younger Dryas of 260 - 400 varve years (BJ(~RCK et al. 1987). In Greenland ice-cores duration of Younger Dryas is determined to 450 years (HAMMER et al. 1986). This is in the same order of magnitude like Lake Meerfelder Maar (460 varve years) and Lake Holzmaar varve counts (450 varve years, ZOLITSCHKA et al. 1992). Varve counts on Swiss lake sediments (Soppensee) end up with a duration of Younger Do'as of 680 varve years, but duration of the Preboreal is determined to only 290 varve years in this record (LOTTER 1991), which is in contradiction to about 800 varve years in Lake Meerfelder Maar and in Lake Holzmaar (Tab. 1). According to ZOLITSCHKA et al. (1992) these problems are probably due to the long distance between investigated sites and also to different geographical settings making a comparison, based on biozones, impossible. On the other hand the question raises if the duration of biozones is similar all over Europe, but still there is no answer because comparisons between sites and methods are still at the very beginning.
ACKNOWLEDGEMENTS These studies received financial support from the European Communities Commission (ECprojekt "GEOMAAR", No. ST 2J 0128 1).
REFERENCES Bard, E. & Broecker, W. S. (1992): The last deglaciation. Absolute and radiocarbon chronologies. NATO ASI Series, Vol. 1 2: p 358; Berlin, Heidelberg. Becker, B. & Kromer, B. (1986): Extension of the Holocene dendrochronology by the Preboreal pine series, 8.800 to 10.100 BP. Radiocarbon, 28 (2B): 961-967. Bjrrck, S., Sandgren, P. & Holmquist, B. (1987): A magnetostratigraphic comparison between 14C years and varve years during the Late Weichselian, indicating significant differences between the time scales. J. Quat. Sci., 2: 133-140. Drohmann, D., Poth, D., Seul, C., Wegner, F. & Negendank, J.F.W. (1989): Sedimentology of Meerfeld Maar lake sediments (Westeifel, FRG). Terra abstracts, 1: 226; Strasburg. Eicher, U. (1987): Die sp~itglazialen sowie frtihpostglazialen Klimaverh/iltnisse im Bereich der Alpen: Sauerstoffisotopenkurven kalkhahiger Sedimente. Geogr. Helv., 42/2: 99-104. Eicher, U. & Siegenthaler, U. (1976): Palynological and oxygen isotope investigations on Late Glacial sediment cores from Swiss lakes. Boreas, 5:109-117; Oslo. Hammer, C.U., Clausen, H.B. & Tauber, H. (1986): Ice core dating of the Pleistocene/Holocene boundary applied to a Calibration of the 14C time scale. Radiocarbon, 28: 284-291. Heinz, T. (1991): Pal/~olimnologische und spektralanalytische Untersuchungen an jahreszeitlich geschichteten Sedimenten des Schatkenmehrener Maares/West. Dipl.-Arb. Univ. Trier, p 107; Trier (unpubl.). Lotter, A. (1991): Absolute dating of the Late-Glacial period in Switzerland using annually laminated sediments. Quat. Res., 35: 321-330.
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Mangerud, J., Anderson, S.T., Berglund, B.E. & Donner, J.J. (1974): Quaternary stratigraphy of Norden: a proposal for terminology and classification. Boreas, 3: 109-128; Oslo. Poth, D. & Negendank, J.F.W. (1991): Sedimentmikrostratigraphische Untersuchungen sp~itquart~er jahreszeitlich geschichteter Seesedimente des Meerfelder Maares (Westeifel, FRG) Ein Beitrag zur Rekonstruktion des Pal~ioklimas an der Grenze Pleistozian/Holoz~in. In: Zolitschka, B. & Negendank, J.F.W. (eds.): Symposium on Paleolimnology of Maar Lakes. Abstract Volume, 48; Trier. Saamisto, M. (1985): Long varve series in Finland. Boreas, 14: 133-137; Oslo. Straka, H. (1975): Die sp~itquar~re Vegetationsgeschichte der Vulkaneifel. Beitr. Landespflege in Rheinl.-Pfalz, Beih. 3: 1-163; Oppenheim. Stuiver, M., Braziunas, T.F., Becker, B. & Kromer, B. (1991): Climatic, solar, oceanic and geomagnetic influences on Late Glacial and Holocene atmospheric 14C/~C change. Quat. Res., 35: 1-24. Zolitschka, B. (1990): Sp~itquart~ire jahreszeitlich geschichtete Seesedimente ausgew~ihlter Eifelmaare. Pal~iolimnologische Untersuchungen als Beitrag zur sp/it- und postglazialen Klima- und Besiedlungsgeschichte. Documenta naturae, 60: p 241; Milnchen. Zolitschka, B., Brauer, A., Haverkamp, B., Heinz, T., Negendank, J.F.W. & Poth, D. (1991): Sedimentolo~scher Nachweis und Datierung einer frtihholoz~nen Maareruption (Ulmener Maar?) in der Vulkaneifel. In: Zolitschka, B. & Negendank J.F.W. (eds.): Symposium on Paleolimnology of Maar lakes. Abstract Volume, 63; Trier. Zolitschka, B., Haverkamp, B. & Negendank, J.F.W. (1992): Younger Dryas oscillation - Varve dated microstratigraphic, palynological and palaeomagnetic records from Lake Holzmaar, Germany. In: Bard, E. & Broecker, W.S. (eds.), The last degiaciation. Absolute and radiocarbon chronologies, NATO ASI Series, VoL 1 2: 81-101; Berlin, Heidelberg. -
PALEOENVIRONMENTAL RECONSTRUCTION OF THE LATE- AND POSTGLACIAL SEDIMENTARY RECORD OF LAKE W E I N F E L D E R M A A R
A. Brauer & LF.W. Negendank University of Trier, Dept. of Geology, D-5500 Trier, Germany
Abstract Three cores from the center of Lake Weinfelder Maar revealed a sediment column consisting of two different facies. The Late Glacial sequence is dominated by minerogenic material influenced by slumping, while the more organic Holocene sequence is characterized by periods of siderite lamination. Counting of these lamination provides an estimated chronology. Additionally, formation of siderite lamination indicates changes of paleoenvironment. It is assumed that during Holocene the lake changed from holomictic to meromictic conditions and vice versa. Meromixis was initiated by an increase in biological activity. Corresponding changes of trophic state are caused by climatic evolution during early Holocene and by a combination of climatic and anthropogenio influences during historical times.
1. Introduction The Westeifel Volcanic Field is welt known for its maar lakes, containing continuous high resolving sediment sequences, which allow to reconstruct the Late Quaternary environmental history (Negendank et al. 1990). Despite the same genetic origin maar lakes are different in size, depth, morphological setting and hydrological conditions resulting in diverging limnological characteristics and sediment composition. In lakes with a higher trophic state like Lake Holzmaar and Lake Meerfelder Maar organic varves are preserved and allow to establish a precise varve chronology for the last 13,000 years (Zolitschka 1990).
Lecture Notes in Earth Sciences. Vol. 49 J. F. W. Negendank, B. Zolltschka (Eds.) Paleolimnology of European Maar Lakes 9 SprLuger-Ver]ag Berlin Heidelberg 1993
224
This study focuses on the formation of rhythmical siderite lamination of predominatly clastic sediments from oligotrophic Lake Weinfelder Maar. Similar lamination has been described from other recent lakes (Aiapieti & Saarnisto 1981, Anthony 1977, Bahrig 1985) as well as from pre-Holocene lakes (Bahrig 1989, Dickinson 1988; Goth 1986, 1990). This type of lamination is regarded to be of annual origin and indicates the paleoenvironmental state of the lake, because siderite precipitation is restricted to a special thermodynamic stability field: 9
an anoxic and reducing environment (Eh < 100 mV at pH 7)
9 a partial COs pressure > 10"~atm, where source of CO2 is either bacterial decay of organic matter (Bahrig 1989, Dickinson 1988) or of volcanic origin (Bahrig 1985). 9 a Fe/Ca ratio > 0.05 (Bemer 1971) 9 a sulfide concentration < 10.7 tool 1" Hs" (Stumm & Morgan 1981)
2.
Investigation Site
Lake Weinfelder Maar (WFM) belongs to a group of three maar lakes called "Dauner Maargruppe" and is located at 50"11' N and 6"51' E (fig.l). These maars formed 20,000 30,000 years ago (Bfichel I984) as part of the Westeifel Volcanic Field. Geological setting and volcanological evolution are described by Bfichel & Krawczyk (1986).
Fig. 1: Location of investigated site.
225
The lake level of the 52 m deep WFM is 484 m above sea level and thus 77 m resp. 63 m above the lake levels of adjacent Lake GemSndener Maar (600 m to the west) and Lake Schalkenmehrener Maar (450 m to the southeast). The steep-sided and fiat-bottomed, lake basin has no inlet and outflow. The crater rim marks a very small catchment area of 19 ha resulting in a low catchment area/lake surface ratio of 1.2 (Scharf 1987). Under these conditions a typical oligotrophic and holomictie lake formed as first described by Thienemann 1914/15. Recently Scharf (1987) reported a deterioration of water quality caused by an increased use as recreational facility. The saturation with oxygen of the deep water decreased and an oxygen-free zone with a higher concentration of phosphate, ammonium, manganese and iron formed during summer 1984 (Scharf 1987). This indicates cultural eutrophication and a trend to meromictic conditions. 3.
Methods
Three overlapping cores, WFM-A, -B and -C, each of about 8 m in length were recovered from the profundal of Lake WFM with a modified Livingstone piston corer (Usinger corer). From the composite profile a continuous series of large-sized thin sections (120 mm x 35 mm) was accomplished. Thin sections have been analysed in detail with respect to composition and structure of sediments using a polarizing microscope and additionally a projection macroscope. Furtheron X-ray diffraction and SEM investigations have been carried out. Water contents were ascertained when 1 cm thick subsamples were dried at 105"C for 24 hours. From 99 dry samples total organic carbon (TOC) and siderite was determined with a LECO analyser using oxygen as carrier gas. A two-phase analysis was set up to distinguish between these components (fig.2), because tests showed that sedimentary siderite becomes thermally dissociated at about 400"C - 420"C whereas the maximum of thermal dissociation of organic bound carbon in these sediments is at about 250'C. At phase 1 the sample was slowly heated up to 360'C with a ramp fate of 50*C/min. This temperature was kept for 120 seconds before heating up to 600*C with a ramp rate of 150" C/min.
226
Fig.2: Two-phase analysis for determination of TOC and siderite.
4.
Sediments
Correlation of the cores enables to establish a composite profle (fg.3). Two different facies marked by a distinct colour change from grey to brown at 360 cm depth were distinguished. The lower elastic sequence consists of coarse detritie material in sandy-silty matrix, slumped and tilted sections, units of homogeneous silt containing abundant diatoms (Campylodiscus noricus) as well as laminated clay and silt. Marker horizon at 580 cm depth is the 10 cm thick pyroclastie layer of Laacher See Tephra (LST) dated from Lake Holzmaar (Zolitschka 1990). In general, the upper sequence is characterized by higher contents of organic components. Starting at 290 em depth continuous cyclic siderite laminations are discernable until 190 cm (fig.4), upcore increasingly interrupted by unlaminated minerogenie gyttia until its complete cessation at 85 cm depth. On top of a turbidite sequence at about 110 cm depth extremely thick and distinct layers appear. In the upper part siderite also forms as dispersed concretions. Siderite appears as spherical aggregates of 2 - 20 #m diameter with prevailing grain sizes between 5 and 10 gm. "Wheatgrain" shaped crystalls o f 1 to 5 tma are also ascertained.
227
Fig. 3: Composite sediment profile from Lake Weinfelder blaar.
228
As a second autochtonous mineral vivianite is present throughout almost the whole upper sequence (fig.3), but most frequent between 290 cm and 190 cm depth. Unlike siderite vivianite only occurs as patches of granular masses partly as alteration of macrorests by decomposition or finely dispersed in the sediment. It is often but not necessarily associated with siderite (fig. 5).
Fig. 4: Siderite lamination at 255 cm depth (Core WFM-A).
Fig. 5: Vivianite patch (light grey) associated with siderite spheroides (WFM-A).
229
5.
Chronology
Lake WFM sediments are lacking a continuous valve formation, therefore an estimated chronology is established. Lateral extension of the occurring siderite lamination as well as the similarities in shape of siderite crystalls (Goth 1990) proves their synsedimentary formation. Furtheron, they seem to be of annual origin, caused by changes of deep water chemistry within the seasonal cycle. One year is represented in a couple of a dark organic layer and a light yellowish siderite layer. In spring ice-rafted dropstones deformed siderite layers after their deposition and thus point to a siderite precipitation in autumn/winter. During this season leaves and other macrorests as well as autochtonous organic matter are accumulated in the sediment providing a source for bacterial decomposition. CO= concentration derived from decay of organic matter is considered as a controlling factor for siderite precipitation (Bahrig 1989). Thickness of the assumed varves varies between 250 grn and 1000 gm (fig.6).
Fig.6: Varve thickness variations (WFM-A between 280 cm and 235 cm depth).
230
Counting of these laminations serves as basis for a chronology whereas sedimentation rates for unlaminated sections are estimated from calculated sedimentation rates of adjacent counted well-laminated sections. This method of estimation reveals satisfactory results and leads to an age of about 10,500 years for the distinct colour change at 360 cm depth corresponding to its interpretation as Late Glacial/Holocene transition. Consequently, Firbas pollen zones were adapted to WFM sediments (fig. 3). Because of uncertainties in unlaminated sections the ascertained sedimentation rates (tab. 1) should be looked upon as mean values. Especially during Subatlantic fluctuations caused by human influence (history of settlement) have to be considered. These are not distinguished here, but indicated in TOC and siderite contents (fig.7). Tab. 1: Calculated and estimated mean sedimentation rates. Biozone
Mean Sedimentation rate
6.
SA
0,46 mm/a
SB
0,31 mm/a
At
0,35 mm/a
Bo
0,29 mm/a
Pb
0,25 mm/a
Results and Discussion
During Younger Dryas sedimentation in Lake WFM is determined by a large scale mass movement inferred from intercalated coarse detritic sections up to 70 cm in thickness. This event is related to the climatic deterioration, causing a loss or decrease of vegetation cover within the crater and a lower lake level resulting in unstable slopes (Brauer 1988). Compared to other maar lakes of this region climatic deterioration has an even stronger effect at Lake WFM because of its open, unprotected location at high altitude. This local modification can also be deduced from the structure of the LST. Graded bedding infers to an accumulation of the ash fall on the ice covered lake in late spring, when other maar lakes, where LST forms a typical double layer with a fine-grained lower and a coarse upper layer (Zolitschka 1990), already have been ice-free. Stabilization of sedimentation became apparent during Late Younger Dryas with undisturbed clay and silt laminations. The onset of the Holocene is marked by a drastic
231
increase in TOC and water contents, a sudden increase in number of Cladocera (Hofmann, this volume), and a first peak in phaeopigments (Ehlscheid 1990) as well as in occurrence of siderite appearing finely dispersed in the sediment. Laminated siderite for the first time appeared during early Atlantic corresponding to a further increase in siderite contents (fig.7). Since siderite precipitation requires an anoxic environment at the lake bottom this indicates a change to meromictic conditions (Brauer 1988) favoured by the relative depth of lake WFM (fig.8) and initiated by an increased biological activity. During the Atlantic when Holocene climatic evolution reached its optimum with warm and wet conditions (Lamb 1977) lake productivity increased. Additionally, a higher amount of macrorests verifies an enlarged input of allochtonous organic matter from forested crater walls. This is visible by a further increase of TOC (fig.7) and favours bacterial decomposition resulting in depletion of oxygen. Hence, decay of organic matter provides both, sufficient CO 2 for siderite formation as well as favourable geochemical conditions for precipitation of siderite. An anoxic hypolirnnion is stabalized when Fe(III), sufficiently provided by basement rocks, is reduced and enriched in the monimolimnion (Kjensmo 1968). The appearence of a monimolimnion is corroborated by eutrophication at Boreal/Atlantic transition indicated by a succession of Cladocera species (Hofmann, this volume). Consequently, depletion of lake productivity caused by climatic deterioration during SubboreaYSubatlantic induces a reversed development of Cladoceran assemblage (Hofmann, this volume). These changes are also reflected by an increasingly interupted (fig.3, fig.7) formation of siderite laminations inferring to a weakened and at times ceased monimolimnion. The mechanism leading to a disappearing monimolimnion is difficult to ascertain. Anthony (1977) assumes sediment filling of the lake basin favours complete circulation at Lake of the Clouds, where laminated siderite is similarly distributed. This is unlikely at WFM because of its greater depth (fig. 8). Definitely, relative contents of TOC decreased during Subboreal/Subatlantic (fig.7) matching a depletion in vivianite. It seems plausible that less consumption of oxygen through bacterial decay diminishes the monimolimnion allowing complete mixing of lake water during storm events when circulation was strong enough to be transmitted to bottom waters. It has to be considered that during Subboreal/Subatlantic climatic influence on sedimentation is increasingly restrained by human activities in the catchment area as revealed from lakes Holzmaar and Meerfelder Maar (Zolitschka 1990). At WFM this becomes apparent with a sequence of turbidites (fig.3) assumed to reflect building activities on the crater rim during Roman times (Haaren 1988). On top of this horizon a few extremely thick siderite layers formed probably as result of short-termed influx of
232
large amounts of organic matter and nutrients caused by forest clearing within the catchment area. During Subatlantic a conspicuous peak in TOC and water content (fig.7) corroborated by results of loss-on-ignition CEhlscheid 1990, Hofmann, this volume) is ascertained corresponding to a sudden increase of an eutrophic chydorid species (Alona
quadrangularis) as well as a significant decrease in oligotrophic chironomidae (Micropsectra) (Hofmann, this volume). This also matches the reappearance of siderite laminations. Thin section analysis of this horizon re~,eals extremely few allochtonous minerogenic components, but instead a large number of leaves and pieces of small branches. This points to a period with less human activities and less soil erosion but high input of organic matter from a forested catchment. Probably this coincides to FrankishCarolingian times around 1600 to 900 V'T B.P., which is characterized at Lake Holzmaar by a distinctly lower influx of clastic sediments, too (Zolitschka 1990).
Fig.7: Water contents, TOC and siderite from Lake WFM sediments.
233
Fig.8: Relative depth (Hutchinson 1957) versus lake surface from lakes with annually laminated sediments (grid area), modified according to O'Sullivan (1983)
6.
Conclusions
Laminated siderite is a typical facies in some sections of WFM sediments. In combination with further sedimentological analysis it is interpreted as of annual origin and thus allows to establish an estimated chronology for lake WFM. Furtheron, siderite precipitation at the sediment/water interface is an indicator of paleoenvironmental changes with respect to water circulation and trophic state. These changes are controlled by climatic evolution as shown for the onset of the lamination. However, the reason for its cessation remains elusive because of both, climatic changes and human activities affect sedimentation. Due to its morphometrie situation Lake WFM generally tends to react sensitive on changes within the catchment either climatically induced or affected by human activities. This has to be considered when recent trends of changing lake environment are evaluated.
234
Acknowledgement We would like to thank the DFG (Deutsche Forschungsgemeinschaft) for financial support for this study, which is part of a research project on Quaternary geology and paleolimnology ofEifel maar lakes (Ne 154/13-3 and Ne 154/22-1)
7.
References
ALAPIETI, T. & SAARNISTO, M. (1981): Energy dispersive X-ray microanalysis of laminated sediments from Lake Valkiaj~rvi, Finland. - Bulletin Geological Society Finland, 53: 3-9; Helsinki. ANTHONY, R.S. (1977): Iron-rich rhythmically laminated sediments in Lake of the Clouds, northeastern Minnesota. - Limnology Oceanogr., 22: 45-54; Lawrence, Kansas. BAHRIG, B. (1985): Sedimentation und Diagenese im Laacher Seebecken (Osteifel). Bochumer geologische und geotechnische Arbeiten, 19:1-231; Bochum. BAHRIG, B. (1989): Stable isotope composition of siderite as an indicator of the paleoenvironmental history of oil shale lakes. - Palaeogeograpy, Palaeoclimatology, Palaeoecology, 70:139-151; Amsterdam. BERNER, R.A. (1971): Chemical Sedimentology. - 240 p.; New York. BOCHEL, G. (1984): Die Maare im Vulkanfeld Westeifel, ihr geophysikalischer Nachweis, ihr Alter und ihre Beziehungen zur Tektonik der Erdkruste. - Dissertation Universitit Mainz, 385 p.; Mainz. BUCHEL, G. & KRAWCZYK, E. (1986): Zur Genese der Dauner Maare im Vulkanfeld Westeifel. - Mainzer geowissenschafllich. Mitteilungen, 15:219-238; Mainz. BRAUER, A. (1988): Versuch einer Erfassung alter Seespiegelstfinde an ausgesuchten Eifelmaaren und mikrostratigraphische Untersuchungen an Sedimenten des Weinfelder Maares. - Diplomarbeit, 117 p., Universitgt Trier (unver/Sffentlicht). DICKINSON, K.A. (1988): Paleolimnology of Lake Tubutulik, an iron meromictic eocene lake, Eastern Seward Peninsula, Alaska. Sedimentary Geology, 54: 303-320; Amsterdam. EHLSCHEID, T. (1990): Planktotogische und pal~olimnologische Untersuchungen an n~hrstoffarmen Eifelmaaren unter Ber/icksichtigung der ver~nderten fischereilichen Nutzung des Weinfelder Maares. - Dissertation Universit/it. Mainz, 213 p.; Mainz. GOTH, K. (1986): Mikrofazielle Untersuchungen am Messeler 131schiefer. - Cour. Forsch.-Inst. Senckenberg, 85:209-211; FrankfuWMain. GOTH, K. (1990): Der Messeler 131schiefer - ein Algenlaminit. - Cour. Forsch.-Inst. Senckenberg, 131: 1-143; Frankfurt/Main. HAAREN, C.v. (1988): Eifelmaare. Landschafts/Skologisch-historische Betrachtung und Naturschutzplanung. Pollichia Buch Nr. 13,548 p; Bad Diirkheim HOFMANN, W. (this volume): Late-Glacial/Holocene changes of the climatic and trophic conditions in three Eifel maar lakes, as indicated by faunal remains. I. Cladocera. HOFMANN, W. (this volume): Late-Glacial/Holocene changes of the climatic and trophic conditions in three Eifel maar lakes, as indicated by faunal remains. II. Chironomidae (Diptera). HUTCHINSON, G.E. (1957): A treatise on limnotogy. Vol. I Geograpy, Physics and Chemistry. - 1015 p.; New York.
235
KJENSMO, J. (1968): Iron as primary factor rendering lakes meromictic and related problems. - Mitteilungen Internationale Vereinigung f. Theoretische u. Angewandte Limnologie, 14: 83-93; Stuttgart. LAMB, H.H. (1977): Climate - present past and future, Vol. 2: Climatic history and the future. - 835 p.; London. NEGENDANK, LF.W.; BRAUER, A. & ZOLITSCHKA, B. (1990): Die Eifelmaare als erdgeschichtliche Fallen und Quellen zur Rekonstruktion des Pal~ioenvironments. Mainzer geowissenschaftliche Mitteilungen 19: 235-262; Maiuz. O'SULLIVAN, P.E. (1983): Annually laminated lake sediments and the study of Quaternary environmental changes - a review. - Quaternary Science Review, 1: 245313; Oxford. SCHARF, B.W. (1987): Limnologische Beschreibung, Nutzung und Unterhaltung yon Eifelmaaren. - 117 p.; Mainz. STUMM, W & MORGAN, J.J. (1981): Aquatic chemistry - an introduction emphasizing chemical equilibria in natural waters. - 780 p.; New York. THIENEMANN, A. (1914/15): Physikalische und chemische Untersuchungen an den Maaren der Eifel. - Verhandlungen naturhistorischer Verein preul3ische Rheinlande, 70: 249-302 / 71: 273-389. ZOLITSCH-KA, B. (1990): Sp~tquart~ire jahreszeitlich geschichtete Seesedimente ausgew/ihlter Eifelmaare. Pal~olimnologische Untersuchungen als Beitrag zur sp/it- und postglazialen Klima- und Besiedlungsgeschichte. - Documenta naturae, 60:241 p.; Mfinchen.
SEDIMENTOLOGY AND PALEOENVIRONMENT FROM THE MAAR LAC DU BOUCHET FOR THE LAST CLIMATIC CYCLE, 0-120,000 YEARS (MASSIF CENTRAL, FRANCE)
Elisabeth Truze
and
Kerry
Kelts
Limnological Research Center, University of Minnesota, Pillsbury Hall, Minneapolis, Minnesota 55455, USA ABSTRACT This paper summarizes sedimentological and geochemical studies from seven cores taken from the Bouchet crater lake, France. It includes studies of water and soils from the drainage basin. The sedimentary record is subdivided into 14 distinct sedimentary units, which comprise different combinations of 8 recurring lithofacies. The lithostratigraphic results are correlated with prior information from magnetic-, pollen-, diatom-, and chrono-stratigraphy to define the sedimentary dynamics during the last 120,000 years. Chemical composition and stable isotopes (~)180, oqD) indicate dominantly meteoric sources for the lake waters. Low bicarbonate, calcium, and magnesium values reflect weak weathering of the basaltic country rocks and no evidence of hydrothermal influences. There is little evidence of lake level fluctuations. Soil profiles in the drainage basin display a partitioning of clay minerals. Core intervals from humid periods have more smectite and mixed-layer-clays. Periods of intensified erosion also lead to increases in smectite and mixed-layerclay content in intervals with slumps or turbidite deposits whfch are associated with glacial phases Sediments are very-fine grained during intervals with warmer climate when vegetation-cover limited erosion. Nutrient input allowed abundant algal productivity and in some cases led to bottom anoxia. Results of organic matter analyses (%TOC, Pyrolysis, Gq13C,palynofacies) delimit intervals of climate change which are characterized as barren, or else dominated by terrestrial or aqueous sources. Only vivianite and siderite occur as authigenic minerals in the cores, and are found in association with organic matter. Correlating to the Grande Pile time scale, the Bouchet sediments of the glacial maximum were fine clay with pellet-laminae suggesting permenant ice cover. Climate oscillations around 40 kyrs BP show up as levels with organic matter production and rhythmites. Around 75-50 kyrs BP coarser turbidite layers are common. Coarser intervals in general match times with less tree pollen, less organic productivity, and higher magnetic susceptibility. Several unit boundaries are abrupt suggesting threshold behavior. The lithostratigraphy appears to be organized into 4 sedimentary cycles which we believe reflect orbital rhythms of about 20 and 40 kyrs.
Lecture N•tes in Earth Sciences, Vol. 49 L F. W. Negendank, B. Zolitschka (FAs.) Paleo|imnology of European Maar Lakes 9 Springer-Vedag Berlin Heidelberg 1993
238
1.
Introduction
In recent years, efforts to correlate between marine and terrestrial Quaternary paleoclimate records have had difficulties with differences in resolution and dating, and a lack of long continental records. Woillard (1982) first proposed that La Grande Pile pollen spectrum matched the oceanic oxygen-isotope stratigraphy up to 5e These results held up to later comparison with pollen spectra in the Les Echets site (de Beaulieu and Reille, t984). Guiot, et al. (1989) then applied transfer functions to derive a climatic curve for central France over the last 140'000 years. Both of these sites are from small, shallow peat bogs, and doubts linger whether their records are complete. Deep crater lakes, such as Lac de Bouchet, provide the advantages of long continuous, aqueous histories whereby the signatures from defined catchment areas can be matched with changes in lake biogeochemistry. In a sense, they are good paleo-pluviometers which respond quickly to atmospheric changes and collect an integrated sample of regional pollen rain. A lake reacts quicker to climate forcing than regional vegetation. Lake sediment archives a variety of changes in different components of a lake system as well as signatures of weathering and soils in the drainage. In 1981, research was initiated on Lac de Bouchet to collect cores for paleomagnetism, and to reconstruct a climate history defining prehistoric cave sites ( cf. Bonifay et al, 1987). They discovered that the lake contained a continuous Quaternary sequence covering the last glacial within 20 m. The quality and potential length of the Quaternary record led to a joint European coring program (GEOMAAR) for maar lakes in France (Bouchet, Costaros, St Front) and in Germany (Holzmaar, Meerfelder: cf. Zolitschka, 1989). For Bouchet lake, palynological ( Reille et al., 1990 ), diatom ( Pailles, 1989) and magnetic ( Thouveny et al., 1990) stratigraphies are now available. The character of lake sediment changes rapidly with the environment. Sedimentary texture, for example, gives clues to changing sedimentation rates, processes, and unique events. Because of the difficulties with dating Quaternary sediments beyond the radiocarbon scale, it is necessary to understand changes in the sedimentary matrix when making paleoecological or paleoclimate analyses. The goal of this paper therefore is to provide this matrix
239
Fig. 1: Simplified geologic m ap and location of the maar, Lac du 13ouchet, in the Massif Central a r e a o f France.
240
for cores from the Lac de Bouchet and show how the sediment character can be applied to improve our understanding of the paleoenvironment. We also attempt to reconstruct the lake history as a system, and show how the sediment registers each of the climate episodes and events over the last 120 000 years even though there are uncertainties in the dating. The Lac du Bouchet as a system is relatively well-defined. The catchment is small, with uniform volcanic geology. No major rivers enter the lake, and water exits by groundwater. This reduces the amount of detrital input. The lake lies beyond the glaciated terrain of Europe, but close enough to register the climatic extremes.
2.
Study
area
Setting Bouchet crater lake (44.9 ~ N, 3.8 ~ E) is located in the Velay region of The Massif Central in central France at 1207 m altitude (fig.l). It is the only maar on the Plateau du Deves still occupied by a lake. The lake is 800 m in diameter and occupies a clearly defined crater formed by a phreatomagmatic explosion dated as ca. 0.8 Myr by K/Ar analysis of the basaltic flows surrounding the crater (Teulade et ai.,1988). The crater rim, reaching heights between 30 and 70 m above the lake, is sub-circular in outline, with a diameter twice the diameter of the circular lake. The maximum water depth is about 27 m. The lake has a littoral rim 10 m deep and a profile sloping-steeply from 50 m away from the shore, extending to a flat-bottomed, subcircular, central area (Decobert, 1988; fig. 2). The present-day-lake is holomictic and well-oxygenated. The lake water has a low conductivity (36 p.S/cm) characterized by bicarbonate, calcium, and magnesium, with silica
241
Fig. 2 : Map o f Lac d u Bouchet with c a t c h m e n t t o p o g r a p h y ( c o n t o u r interval l m ) . Position o f the core LDB D m~d the sismic profile nO14.
Fig. 3 : I n t e r p r e t a t i o n o f the seismic profile n ~ across the b a s i n a l a r e a a n d the p r o j e c t e d l o c a t i o n o f LDB D. T h e acoustic s t r a t i g r a p h y shows a z o n e of chaotic reflectors as e v i d e n c e o f slump units. Vertical scale is c a l i b r a t e d as w a t e r depth, a s s u m i n g 1500 k m / s Vp.
242
Geology The explosion crater of Bouchet is oriented NW-SE in the southeastern corner of the Plateau du Deves, a basaltic highland horst with a mean altitude of 1000 m and maximum of 1458 m, formed during the Gauss Period. The Deves is considered to be a part of an Oligocene distensional intra-plate zone. The basement is a part of the Paleozoic Hercynian chain and is composed of gneiss and granite. Bouchet Lake is a closed-drainage lake, surrounded by three strombolien cones. A non-active NW-SE fault crosses the basin as the principal structure of the Plateau du Deves. On the ridge of the plain are about 60 dried crater lakes.
Hydrology Lecocq (1987) determin-ated the ground-water flow direction to be from W to WSW, corresponding to the trends of the basaltic lava flows and with the upstream springs and the system of rivers. The Bouchet water chemistry presents a good correlation with that of water circulated through the basaltic lava and the scoria. There is little contribution of potassium and sodium ions from the granite basement, which is about 20 meters beneath the lake surface. The isotopic values all plot (Truze, 1988) below the condensation meteoric mean water line (Craig, 1961). In absence of hydrothermal sources, the stable lake level derives from a balance of groundwater input and precipitation.
Climate Bouchet is situated in the center of France, where precipitation is concentrated in May and October. The rainfall adds up to an annual mean of 870 mm. The mean annual temperature is 6 ~ C, with monthly averages ranging between 25~ in July-August and -10~ in December-January. The Deves region is characterized by an insolation period of 2000 hours per year, and the lake area has a deficit in the water balance in August and September (Fillod, 1986; C.D.M.H.L., 1987). The Bouchet region could b e regarded as in a climatic transition zone of a continental area, with interactions of maritime influences from the Mediterranean sea (from 165 km) and Atlantic Ocean (from 400 km) which generate thunderstorms during summer and irregular snows during the cool winter.
243
A Quaternary Deves ice-sheet? During colder periods of the Quaternary the low mean altitude of the area, the aridity of the climate, the dominant NW winds, and the asymmetrical plateau with gentle eastern slope did not favor formation of a large ice-sheet. From these observations and the investigations of morainal development in the Massif Central, the Plateau du Deves did not contain an ice-field during the Quaternary (Etlicher, 1980; Veyret, 1981). The nearest advance of an ice sheet would have come from the alps over 200 km to the NW.
3.
Methods
Since beginning of the project 1981 (Bonifay et al. 1987) several different sets of Mackereth cores were extracted from Lac de Bouchet. The first series of 6 m cores recovered the sedimentary record to 20 kyrs B.P; then a series of 9 m cores extended the record to 35 kyrs B.P, and finally a series of 12 m cores allowed a further extension to about 50 kyrs B.P. Beginning in1986, the "Geomaar 1" EEC project, enabled us to collect five cores 16 - 20 m long with a modified Livingstone-Wright corer (Wright, 1963) designed by H. Usinger (University of Kiel, F.R.G). The "Livingstone" cores A, B, C, D were collected from five boreholes localized within a central basin area of less then 1000 m 2. Core sediment was extruded in successive sections of either 1 or 2 m and 80, 50, or 30 mm in diameter. After the study of 7 cores to establish correlations (LDB A, B, C, D, and B60, B53, B21), the Core LDB D was chosen as a type core. Core LDB D contains the longest sequence, and was used for multidisciplinary sampling, including palynology (Reille et de Beaulieu, 1988,1990), paleomagnetism (Thouveny et at, 1990), diatom ecology (Pailles, 1989), and sedimentology (Truze, 1990). Radiocarbon dates were obtained from the 6 m core series by the Centre des Faibles Radioactivites CNRS/CEA (Gif sur Yvette, France), using the conventional method of proportional counting. AMS dates were obtained for the 9 m core series from the High Energy Mass Spectrometer Laboratory, Oxford University, U.K. and for the 12 m series from the Accelerator Mass Spectrometer Laboratory at Tuscon University, Arizona. 90 samples were analysed by x-ray diffraction for mineralogy, including the 2 gm fraction for clays (L.G.Q., Marseille). A Rock Eval pyrolysis method was used to quantify organic matter for 40 samples
244
at the Institut Francais du Petrole ( Espitalie et al., 1987). ~13C was determined for the kerogen fraction of 15 selected samples (Lab. Biogeochem. Isotope, Univ. Paris VI) and palynofacies (Lab. Petro. Org., Orleans). Chemical analyses of waters were run with ionic chromatography for 30 samples (6 profiles) taken at every 5 meters depth in summer. Oxygen/Deuterium-isotopic ratios were determined for lake (6 samples) and interstitial waters (60 samples) using a VG Micromass 602 spectromass (L.H.G.I., Orsay). One day of seismic profiling, run by Comp. Gen. Geophy in 1983 used a high resolution Uniboom system (ef.Allison, 1983).
4.
Results
Lithology On freshly cut surfaces, the entire 20 m cored section from Lac de Bouchet appears as fine-grained mud, colored in banded shades of light gray to dark brown. Slumps are conspicuous between 12m14m. The top and bottom sections are darker due to organic matter. Closer examination reveals complex sedimentary patterns. The sedimentary record is thus subdivided into 14 distinct sedimentary units, which comprise different combinations of 8 recurring lithofacies (Truze, 1990). These are discussed individually below. Figure 4 summarizes the essential sedimentological analyses of seven cores from the basinal flat area with LBD D selected as a type core. These results are further correlated with the time scale derived from pollen correlation by Reille and Beaulieu (1988, 1990) and paleomagnetic correlations (Thouveny, 1990).
4.1 Chronology and rate of sedimentation
Radiocarbon,
pollen
and
geomagnetic
chronostratigraphy
Pollen analysis on the cores LDB and B5 were supported with 13 radiocarbon AMS dates on bulk samples on well defined macrophyte samples. These pin reliable chronstratigraphic markers at aboutl3,000 yr B.P. for the beginning of B611ing, and 15,350+350 yrs B.P. for the beginning of Oldest Dryas (Reille and Beaulieu, 1988). Older dates between 30-45 kyrs BP (Creer et al, 1986) appear to consistent with interpolated depths for organic enrichment in the Bouchet Unit F. The half-life of 14C limits the range of radiocarbon chronology to about 10 m depth. Older sediments have been dated by correlation
245
of the climatic oscillations deduced from the pollen assemblages (Reille and de Beaulieu, 1988) according to the correlation established by Woillard and Mook (1982) between the sequence of La Grande Pile (Vosges, France) and the oxygen-isotope (180/160) stages defined in the oceanic record (Shackleton and Opdyke, 1973). The boundary between isotopic stages 4 and 3 correlates with the end of stadial IV identified in core LBD D at 14.3 m and is estimated to be around 61 kyrs B.P. The boundaries at isotopic stages 4/5a and 5c/5d are correlated with the St. Germain II and I interstadials ( 2 mild climatic events), identified at the intervals 15.9 m-16.3 m and 17.7 m-18.9 m, are estimated only for the end of St. Germain II ( 4 / 5 a ) around 73 kyrs B.P. Two chronological estimations are available for the basal part of the record : 1) the coldest phase (Melisey I) of stage 5d, dated at 108 kyrs B.P., is correlated with the stadial maximum at 19.25 m.; 2) the boundary between stage 5d/5e, dated at 115 kyrs B.P., is correlated with the end of the Eemian at 19.6 m. Thouveny (1990) and Creer et al. (1986) maintain that the curve of secular variation from the Lac de Bouchet cores provide the basis for a reliable geomagnetic chronology (fig.5).
Rates
of
sedimentation
Figure 5 summarizes the interpolated sedimentation curve for the chronological information available correlated to Iithologic unit boundaries. The combination indicates 7 zones of distinct high or low bulk-sedimentation rates. The age-versus-depth curve is steep, during the pleniglacial (0.133 mm/y; 0.265 mm/y; 0.185 mm/y) but the highest rates occur during deglaciation (0.39 ram/y). During the Holocene and the end of the Eemian, the mean rate of sedimentation was about 0.06mm/y and 0.02 mm/y with an extreme minimum during the Bolling-Allercd estimated at about 0.007 mm/y.
4.2 Sedimentological Results
Terrigenous
input
Detrital input is composed principally of clay minerals. Sands and silts include quartz, feldspar, mica, and amphibole, pyroxene, and titano-magnetites. Components derive from the basaltic and granite bedrock. There is little evidence of riverine transport. Coarser-
246
grained particles consists of the fraction > 50 i.tm-500 I~m and become abundant (up to 70%) during glacial intervals. Biogenic grains occur in in organic matter-rich layers and comprise Chara oogonia, diatoms frustules, Pinus pollen, plant debris such as leaves, cutin, spores, and insect mandibles. Locally they constitute 80% of the bulk material during warm periods as sensitive ecosystem clues (fig.4b). Grain
Size
Analysis
( l l ~ m - 500 Izm)
Samples of the Lac de Bouchet were compared to the Passega classification of transport mechanisms and energy (Passega,1964). Three types of deposits are distinguished: (1) turbidity currents producing graded beds, (2) low density suspension currents giving homogenous beds, and (3) sediment settling-out, or combinations of these. Grain size distributions were further analysed by the faciesdistinction method of Riviere (1977). This allowed a separation between 1) mud decanted from convecting currents below ice, 2) mud from cloudy suspensions, and 3) mud filtered through a vegetation cover. The first is platykurtic; the second is mesokurtic with a symmetrical distribution; the last is leptokurtic (table 1). Clay
minerals
Illite, kaolinite, chlorite, smectite, and interlayers are present in soil profiles and lacustrine sediment and indicate no evidence of authigenic formation (Chamley, 1989; Singer1984). Soil analyses from the drainage basin show better-developed smectite and interlayers at the bottom of the profile than the top (15% up to 40%) due to a preferential washing-away of the finest particles. It also shows increasing of kaolonite (0 % up to 30%) from East to West, reflecting the presence of a tuff-ring and alluvial cones (fig. 4b). In cores, the dominant clay minerals are illite (30% to 60%), kaolinite (25% to 35%) and chlorite (10% to 25%). Smectite (0 to 25%) and mixed layer clay proportions (0 to 30%) seem to increase in beds with coarser-grained particles (70%) (cf fig.4b4). Characteristic clay mineral zones (fig.4.b4) do n o t match the boundaries of the palynological zones. Soil processes are slow (Millot,1964). Clay mineral patterns are thus interpreted in terms of erosional processes and mechanical segregation of the clay-soil assemblages (fig.6). Maximum peaks for coarse-grained particles
247
Fig. 4a : Summary log of 15 organic matter analyses. TOC (%), Oxygen Index (no unit), Hydrogen Index, 6 13C (PDB per mil)m~d palynofacies (thick line :% of amorphous). Semiqum~titative estimate of transport energy for deposition in relation to age levels.
248
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.--2
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249
coincide with glacial intervals (cf. zone III-d and III-b) and the beginnings of humid phases (cf. zone II-a, II-b, III-a). Organic
Matter
Hydrogen index (IH) and oxygen index (IO) are related to the atomic ratios (H/C and O/C) and help define sources of organic-matter (Talbot et a1.,1989; Tissot et al., 1984; Espitalie et al., 1987). Organicmatter sedimentation in Bouchet (fig.4a) is an interplay of two different sources linked to climatic variations (fig 7) : one derives from lacustrine algae, characterized by amorphous organic matter with high IH (>300 up to 500) and low IO (< 200); the other is rich in lignitic debris with low HI (<100 down to 80) and high IO (>300 up to 2000) and derives from detrital organic matter input from terrestrial sources.
In Bouchet cores, the sequence does not display downhole diagenetic trends for organic matter but rather distinctly recurrent facies. Levels richer in amorphous organic matter reflect either (1) higher phytoplankton productivity and anoxic conditions, (eg. warmer St Germain I, II and Holocene) or (2) mechanisms which inhibit transport of detrital organic-matter from the catchment (eg. stadial Melisey I and II). Some organic matter reflects intermediate conditions and mixing of sources (IH varies between 150 to 300), (eg. end of Eemian and up to the 40 kyrs interstadial ). 513C
In
Organic
Matter
A selection of 14 downhole samples of bulk organic matter revealed 813C variations ranging from -26 per mil to -29 per mil (cf. fig.4a). Warmer intervals tend to show lighter values in concert with higher values for the hydrogen index. This is consistent with an interpretation of ~13 C values controlled by admixtures of amorphous organic matter of algal origin. Authigenic
minerals
:
siderite
and
vivianite
Vivianite is present as a diagenetie mineral throughout the cores and occurs in 4 modes: 1. distinct lamina, 2. spots of powdery,dessiminated microlites, 3. in mm-size spherulitic concretions , or as 4. euhedral crystal grains. (fig.4b). The occurences are related to the form and distribution of organic matter in the sediments.
250
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251
Siderite occurs as microcrystal aggregates, only in the organicsilt facies corresponding to the Interstade around 40 kyrs BP (8.85 m up to 8.70 m). Composition was determined by infrared spectroscopy. Mineral formation is controlled by physical-chemical factors such as oxidation of organic matter producing adequate pCO2 and a pH between 7 and 10.5 (Garrels and Christ, 1965) which would be higher than the modem Lac de Bouchet. In terms of paleoenvironment, this interval corresponds to a period of increased seasonality ( Guiot et al, 1989). The climate must have generated weathering conditions which produced more alkaline waters than today, as is also suggested by the abundance of calcareous Characeae ogoonia in the older sediments. Siderite probably formed at a redox interface, indicating that a portion of the phytoplankton was oxidized, perhaps in bottomwaters which were seasonally oxic. Berner (1980) suggests that a Ca/Fe ratio of less than 5 is conducive to siderite formation. The Bouchet waters are low in calcium, and obtain abundant iron from weathering of the surrounding volcanic rocks (magnetite) and soils (hematite, goethite), not otherwise fixed by vegetation.
Lithofacies On the basis of mineralogy, color, and sedimentary structures we recognize 8 recurring lithological subfacies (cf. Truze, 1990 and Tablel). These strongly reflect the energy of transport and sedimentation (cf.fig.4a). Arranged in order of hydraulic energy these are : 1) laminated organic-rich clays, 2) organic-rich homogeneous clays, 3) organic-rich silty clays, 4) homogeneous silty clays, 5) clay with sandy layers, 6) clay with graded sandy layers, 7) silt with irregular stratification, 8) disturbed or slumped mud and sand layers.
1) Laminated
organic-rich
clays
(LOC)
This subfacies consists of a succession of thin rhythmic lamina, rich in organic matter, with color varying from light olive-black to black. Lamina thickness is mm to cm. Couplets, interpreted as varves, are common in the Holoccne unit, but rare or absent in lower units. The contact between each lamina is gradational. Based on the paleomagnetic secular variation scale and palynological dates, the Holocene displays a 300-yr rhythmicity. Organic-carbon contents range between 4 and 20 % TOC. The palynofacies is amorphous, with high IH and low IO. Clays are
252
Fig. 6 : Schematic process model of clay mineral segregation in soil profiles from Lac du Bouchet catchement :T1) mixed layer clays and smectite are washed o u t d u r i n g temperate climate condition. T2) fine grained smectite a n d mixed layer clays are stored at the base of a profile near the end of a temperate phase. T3) during cold inter~,'als, soil profiles are eroded which then reinjects smectites and mixed layer clays into the lake.Fig.
253
Fig. 7 : Plot of pyrolisis results for selected organic matter samples from Lac du Bouchet cores. Above) Hydrogen Index against TOC showing shaded areas with group samples from characteristic climate intervals. Below) Hydrogen h-idex versus Oxygen Index with symbols keyed to litbologic units correlated with caracteristic climates.
254
abundant (fraction > 50 Ilm is around 95%). The mean arithmeticTrask is very low (14 to 25 l.tm) and poorly sorted (Trask-class indices > 3) with skewness towards coarser grained for the fraction 150 l.tm, giving a Ieptokurtik facies. This LOC facies occurs in the first meter of sediment, then at 19.73 m to 19.61 m, at 18.87 m to 18.75 m, at 18.24 m to 18.2 m, and at 16.3 m to 15.9 m. The LOC subfacies reflects low energy conditions with anoxia at the sediment surface preventing bioturbation. 2) O r g a n i c - r i c h homogeneous clays (OHC) This homogeneous facies is similar to the LOC but without laminations. It contains more detrital organic debris. The formation of this sedimentary facies is not clear, but the abundant clay implies slow deposition of particles in suspension, and mixing by bioturbation or bottom currents. This facies is observed at 20 m to 19.61 m, then at 8.30 m to 7.80 m, and at 1.30 m to 0.68 m. The lake was probably better mixed, with oxic to anoxic conditions and benthic fauna in the OHC sediment.
3) O r g a n i c - r i c h homogeneous silty clay (OHSC) This brown subfacies is clay-rich but intercalated with bands of silty clay.. These are coarser grained (4% > 50 I.tm), and associated with fibrous detrital organic matter characterized by a lower hydrogen index. Vivianite spots are characteristic. The host clay is massive, and implies either large mud flow events or extensive bioturbation. This subfacies reccurs at 19.61 m to 18.87 m, at 18.20 m to 17.70 m, 15.51 m to 14.74 m, at 10.70 m to 6.95 m and finally at 1.18 m to 1.13 m. The texture of organic particles suggests transport under cooler, but humid climate, with little algal productivity.
4) Homogeneous silty clays
(HSC) This subfacies is lighter gray but similar to OHSC, without organic matter . Vivianite is rarer, but better-crystallized. A good part of the mud appears massive. The arithmetic mean grain size is very low (around 10 I.tm). Grain size distribution is hyperbolic, but heterogeneity is shown by the Trask indices between 2 and 4.6, a kurtosis between 0.4 and 1.2 toward fine particles, and a skewness between 0.12 and 0.25. They are mesokurtic. These deposits are related to low energy transport during arid glacial conditions. Textures suggest deposition with f u l l circulation of
255
the lake. Oxidation processes removed some organic matter (TOC < 0.5 %, vivianite) at the sediment water interface. A unique variant of the HSC subfacies occurs only in core D between 4.30 m up to 3.30 m and is referred to here as the C l o t t e d clay-ball facies discussed further below as unit D. It is characterized by silty-clay spheroid structures in a clay matrix, which define discrete cm-thick, yellow-grey layers, although the contacts are diffuse. The subfacies differs from the HSC above by strong skewness toward finer grained (> 0.9), giving a platikurtic distribution.
5) Clay with sandy layers (CSL) This subfacies is light-gray, with a matrix of uniform clay overprinted by discrete beds of silty sand up to several millimeters thick with erosive lower contacts and overlain by unstratified mud. The Trask-arithmetic mean is around 30 Ixm, the kurtosis varies between 0.8 and 0.4, the Trask-index is nearly 3, and the skewness is high > 0.2. The grain-size cumulative curve for silty-sands are sublogarithmic or parabolic suggesting mixtures of several modes. This subfacies reccurs at 18.20 m to 17.70 m, at 15.37 m to 14.74 m, at 10.2 m to 9.20 m, at 6.95 m to 6.10 m, and at 5.10 m to 4.30 m. 6) C l a y w i t h intercalated graded-sand (CGS) This subfacies is very similar to the above CSL, with the exception that the sandy layers are thick enough to r e c o g n i z e a clear grading. The graded basal part is up to a few m i l l i m e t e r s or centimeters. Beds are intercalated with h o m o g e n e o u s terrigenous muds displaying sharp lower contacts. The upper contact is generally gradational and is characterized by a dark silt followed by a distinct clay cap. The Trask mean is greater than 70 ~tm, Trask-index is around 2, and Kurtosis coefficient is around 0.22, implying a single mode of dispersion. We interpret this type of bedded subfacies as the result of turbidity current deposition. Grading is non-uniform due to the mixture of discrete size groups. This subfacies occurs in several units, but is most c o m m o n from unit I and J. The overall low sedimentation rates of the core suggest that the events leading to the CGS subfacies (storms, slumps, floods) rarely affected the Lac de Bouchet.
256
7) Silt with i r r e g u l a r sand laminae (SSL) This maroon-colored subfacies is characterized by a greater content of silty matrix, with intermittent laminae of lobate, festooned sand. Mean-size is 62 I.tm, the Trask-index is very good (1.3) and kurtosis is 0.13, indicating a plurimodal dispersion, but the skewness is exceptionally high (>1.3) being enriched in finer-grained particles. This facies occurs at 9.2 m to 8.6 m and at 12.3 m to 11.5 m. The sand-silt beds were deposited by lacustrine currents with sediment derived from mechanical erosion and runoff, probably sheet flow type.
8) Slumped
beds
with sand
(SBS)
Several horizons of the core show distinct evidence of slumping. These are marked by folds and micro-faults involving mainly subfacies SSL, bedded sand and siltly-clay layers. Slumped beds occur from 13.60 m up to 12.30 m, and 3.40 m up to 2.75 m. The deposits of slumped beds suggest a link to cold intervals. The preservation of bedding shows that transport distances are short, and facies seem in continuity with the substratum. Slumps could be generated by varying lake levels, ice wedging of marginal sediments or even relaxation of permafrost. Seismic profiles (3.5 kHz) from Lac de Bouchet sediments, define the presence of large slumped structures from meter to decameter thicknesses (fig.3).
4.4
Lithological
paleoclimate
(table
paleolimnological 1 and 2)
units,
implications,
Fourteen lithostratigraphic units have been established in core D, named by letters A-O. In the following discussion we interpret each of these sedimentary units in terms of the time scale available. The objective is to evaluate whether sediment character is consistent with the climate parameters implied for the various time windows (eg. Guiot et al, 1989). In some cases the boundaries do not match proposed climate changes. Such cases suggest that there are lags in the system, or that the irregularities of sedimentation rates, and correlations cannot be resolved at present. The interpretations below are thus considered as a set of working hypotheses, awaiting a more precise chronostratigraphy. We are however confident that relative leads and lags between sediment and pollen boundaries in Lac de Bouchet are significant.
257
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259
UNIT 0 (20.0 m to 19.61 m) correlation to warm interglacial E n d o f E e m i a n (5e) 115 kyrs BP This unit does not contain coarser clastic intercalations which interrupt beds of laminated and homogeneous organic-rich clays. This suggests low energy deposition without bioturbation, in concert with a high organic carbon content ranging between 4% and 8%. The leptokurtic distribution for the minor sand fraction (< 2 %) imply a dense vegetation and weak erosion in the catchment. IH and IO indices suggest that algal productivity in the lake was active (60 % amorphous, with mixed input of terrestrial plant debris (613C -27.2 per mil). Productivity would be related to nutrient input from weathering of volcanic rocks. Vivianite, FePO4, is another indicator of organic matter sedimentation and degradatiom The upper limit of Unit O is abrupt, consistent with rapid deterioration of the Eemian, dated at 115 kyrs B.P. The occurrence of Buxus and Taxus flora and Abies forest (Reille and de Beaulieu, 1989) place Unit O in the end of the warmest interval of the last interglacial (Substage 5e). Unit O has two subdivisions: O1 (20.0 m to 19.73 m) comprises homogeneous dark organic clay (subfacies OHC) with a variable organic carbon content and thin laminae of vivianite; 0 2 comprises organic clay with cm-scale laminations (subfacies LOC). The organic carbon content of this unit is lower than for Holocene muds, suggesting that organic productivity or preservation was less. Even grain size is finer than during the Holocene. Clay minerals show (zone Ia) abundant illite (30%) and interlayers illitechlorite (15%), which characterize the soil profile of this warm period in the catchment. Botanical evidence from Les Echets and La Grande Pile, show a drop in temperature and an increase of the precipitation at the end of the Eemian (Guiot et a1.,1989). This is reflected in the sediments as a change to more clastic input; primary productivity declined.
U N I T N (19.61 m to 18.87 m) correlated to cool stadial Melisey I (5d) 108 kyrs BP, 0.10 mm/yr Unit N is characterized by dark to black, homogeneous, silty clay with a clotted texture due to abundant fragments of organic fibers (subfacies OHSC). The detrital silts and organics imply more variable conditions and diminished vegetation, as is hinted by decreases in
260
organic carbon content. Sand forms a general 4% of the sediment although there is a thin, graded turbidite halfway up the unit. The palynofacies has less amorphous organic matter (30%) and more abundant spores (40%) and lignitic fragments (15%). The hydrogen index reaches a minimum (IH 80) and an oxygen index around 300 as a result of higher plant input. Pollen analysis suggests that the forest disappears. During Melisey I, a steppe vegetation (Poaceaes) colonized the catchment and then is replaced by Pinus at the end of the stadial (Reille et al. 1989). This does not match the mineralogy which shows no major change in sedimentation over this interval. The upper contact of Unit N is sharp, and does not coincide with the pollen boundary (19.25 m) for the end of a cold stadial Melisey I at 108 kyrs BP (substage 5d), but it does match an abrupt drop in magnetic susceptibility which reflects a warming trend. 8 13C of the Unit N organic fraction is -28.2 per mil. This is consistent with a drop in the amounts of organic matter and land plant sources. The general character of Unit N, corresponds to the expectations from Guiot et al (1989) which suggests a dimunition of 300 mm/y in precipitation and -7~ temperature for the equivalent interval to 5d. Grain size did not change much, which can be explained if the climate change affected temperature more than hydrology. UNIT M (18.87 m to 18.20 m) correlated with warm interstadial S t germain I (5c) 95 kyrs BP, 0.05 mm/yr Unit M is characterized by mixtures of the subfacies LOC and HSC. These indicate quiet, slow, sedimentation interrupted by clastic events. Unit M has 5 subdivisions: M1 (18.87 m to 18.75 m) is massive, red clay with vivianite (subfacies HSC); M2 (18.75 m to 18.50 m) is homogeneous silty organic clay with vivianite spots (facies 3); M3 (18.50 m to 18.46 m) has white laminae with vivianite (facies1); M4 (18.46 m to 18.24 m) is homogeneous organic clay with coarse vivianite crystals scattered in cm-thick bands (OHSC) ; M5 (18.24 to 18.250 m) has white mm-scale laminae associated with finely deseminated vivianite (facies LOC). The vegetation of the basin was dense, comprising Betula, Quercus, Pinus, and Picea, signifying a warm interval with coarser-
261
grained detrital materials trapped in the catchment. These phosphatic (only HSC subfacies), organic-rich sediments were deposited during conditions favorable for aqueous biota. The unit i s characterized by an increase in organic carbon content of around 4%, and the palynofacies is mostly amorphous (60%) associated with high IH (350). The bulk organic 813C value is -29.5 per mil PDB. These data suggest a significant increase in the organic productivity during St Germain I which was greater than during the end of Eemian. A comparison with Grand Pile suggests that algal productivty appears to have been more sensitive to variations in temperature than to the precipitation which shows less change for this warmer interval (Guiot et al, 1989). The greater variability in climate and lithology during Unit M is also reflected in the curve of magnetic suspetibility. Detrital influx was limited by forest development on the catchment. The clay mineral spectrum reflects the leached soil profile, showing a decrease in interlayers and smectite (zone Ib) in contrast to layers below. The upper limit is eroded and marked by a decrease in organic content, noted by lighter hues, an increase of chlorite. This boundary does not match exactly with the St.Germain I (substage 5c) at 95 kyrs BP which is placed further above at 17.70 m. Botanical evidence from both Les Echets and La Grande Pile indicate that the interstadial St Germian I was climatically complex and interrupted by short, relatively cold and humid episodes. The latter half displays a precipitation maximum following a marked fall in temperature. Pailles (1989) interpreted the diatom spectrum for the Unit M interval as evidence of higher, but fluctuating lake levels. This would imply a period with reduction of the catchment consistent with the finer grained, but variable deposits observed. U N I T L ( 18.20 m to 16.25 m) correlation cool stadial Melisey H (5b) 85 kyrs BP, 0.19 mm/yr Unit L is characterized by a brown to dark organic silty clay interrupted by occasional sandy layers with abundant fibrous plants debris. It is divided into 3 members : L1 (18.20 m to 17.70 m) comprises organic clays intercalated with thin sandy ungraded beds (subfacies CLS); L2 (17.70 m to 16.38 m) comprises graded sand in a brownish-red clay matrix associated with vivianite s p o t s and black organic-enriched, mm-thin layers (subfacies CGS); L3 (16.38 m to 16.25 m) is made of decimeter thick beds of brownish-red uniform clay associated with some Fe-pigmented mottles, pebbles, and organic fibers (facies CLS and HSC).
262
These suggest increasing but variable hydrologic energies. Sand is generally 4% of the sediment. The high concentration of vivianite suggests an important landplant source of organics rather than an increase in lake productivity. The palynofacies contains less amorphous organic matter (30%). The hydrogen index reaches a minimum (IH 80) and the oxygen index is around 300. The 813C value is -28 per mil PDB. Pollen analysis suggests that the forest disappears during Melisey II, and a steppe vegetation (Poaceaes) colonized the catchment due to a drop in precipitation of about 700 mm/y to 300 mm/y ( cf Guiot et al, 1989). The upper contact of Unit L is gradational over several centimeters, and does coincide with a decrease in susceptibility and the pollen boundary marking the end of stadial Melisey II (substage 5b) at 85 kyrs BP. The clay mineral spectrum (zone Ib) shows first a decrease, then an increase and finally a decrease of smectite and interlayers (Illite-vermiculite, illite-smectite, illite-chlorite). The smectite in this case indicates greater erosion of the catchment due a drop in precipitation (- 700 mm/y) by mechanisms of changing vegetation as discussed above. The diagram in figure 5 assumes a sedimentation rate which is higher than would derive from a direct plot against the dating used by Touveny et al (1990). This is based on arguments from the sediment texture, evidence of increased erosion, and loss of forest cover, all of which indicate higher rates of input than in the overlying Unit K. A similar correction is also applied to the Unit N (Melisey II).
UNIT K (16.25 m to 15.85 m) correlated to temperate interstadial S t Germain H (5a), 75 kyrs BP, 0.08 mm/yr Unit K is composed of organic enriched, brown, cm-larninated clay (subfacies LOC), indicating low hydrologic energy and anoxia preserving organic matter. Organic carbon content increases to around 6%, dominated by amorphous organic matter (80%) with high IH (350) and low IO (50), documenting a significant increase in productivity. The 813C value is -28.7 per mil PDB. The sand fraction remains low (< 2%) consistent with a very low sedimentation rate. Clay minerals (zone lc, fig.4.4), suggest active erosion during initial stages, then diminishing as vegetation is developed which binds smectite and mixed-layer clays within soil profiles. A dense forest made of Pinus, Betula, Picea, and Quercus controls the sedimentation and provides a filter barrier to coarser-grained particles. The upper
263
limit is sharp and coincides with a decrease in organic carbon matched by an increase in the sandy fraction, and a sharp break in clay mineral spectrum. Susceptibility increases. The lithological contact roaches the pollen zone boundary which is correlated with the end of St. Germain II, a warm event ranging from 85 to 75 kyrs BP. Guiot et al. (1989) propose a large increase in precipitation up to around -300 mm/yr followed by a drop at the end. Temperature conditions approached modern values, which is consistent with the sediment character and abundant diatom frustules. UNIT J (15.85 m to 14.70 m ) correlated with cool Stadial I V (4a/3) 65 kyrs BP, 0.20 mmlyr
Unit J is characterized by brownish-red silty-sand without welldeveloped bedding. It is subdivided into 3 members : J1 (15.85 m to 15.51 m) is a clay layer with 4% sand fraction and vivianite crystals. The organic content is around 2% derived from landplants (subfacies HSC); J2 (15.51 m to15.37 m) is made of lighter-brown silty clay associated with vivianite crystals and an increase in organic content (3%) (subfacies OHSC); J1 (15.37 m to 14.74 m) has several intercalated sand beds with diffuses near of this member but better graded upward (subfacies CLS). The organic carbon content decreases gradually as grain size increases, in concert with destruction of vegetation on the cachtment. Organic carbon fraction (1.5 % ) has rare amorphous components (15%), low IH (80) and low IO (500), and a ~il3C value of -28.8 per mil PDB, which imply significant decreases in the productivity. Graded sands at the top of this unit indicate destabilization of the landscape. Clay mineral spectrum with high contents of smectite (10%) and interlayers (5%) decrease uniformly, then disappear at the top of Unit J, suggesting that during this interval, the soil profil was denuded down to the zone of clay segregation. Pollen spectra s h o w a minimum of vegetation cover. A short climatic event is suggested by an increase in Pinus and Picea in Core D, which corresponds to a silty homogeneous bed with higher organic carbon contents (J2). Sandy intercalations into the red silty clay near the upper boundary contain abundant Characeae oogonia which were redeposited from the littoral shelf perhaps due to changes in water level. The upper contact of Unit J is sharp and eroded and correlated with the pollen boundary at the end of Stadial IV at 65 kyrs.
264
UNIT I (14.70 m to 14.07 m) Pleniglacial to 58 kyrs BP,
0.21
mmlyr Unit I comprises dark-brown mud associated with several graded-sand beds, which contain fibers of organic-matter, indicating episodic clastic input (subfacies CGS). Detrital materials were transported into the lake by higher energy turbidity currents. These beds are indicated by highly variable magnetic susceptibility. The low organic carbon content (<1%), derives from low primary productivity. The sample contains only 15% amorphous organic matter, and has low IH (90) and low IO (310). The 813C value is -28.9 per mil PDB. The conditions can be explained by a drop in temperature (Guiot et al, 1989, -12~ drop) leading to more circulation and clastic input. The clay mineral zone (beginning of IIa) shows an increase of smectite (5%) and mixed layers (around 10%), associated with coarser-grained particles. These match increased erosion of the basin and a steppe vegetation. Turbidites suggest a modification of the landscape as climate shifted toward aridity and the accompaning larger rates of evaporation. The upper limit is characterized by the appearance of disturbed sand beds which are correlated with 58 kyrs from the pollen chronology. We have reasonable confidience in this correlation because it matches a large drop in the estimate of precipitation and temperature (-700 mm/y and -12~ from today) as shown by Guiot et al, 1989. UNIT H (13.75 m to 10.70 m) S l u m p s
This unit has numerous disturbed layers of dark-brown mud with sand layers, marked by micro-faults and folds creases inter sediments (subfacies SBS), is interpreted as slumps with turbidities. The Unit is well recognized by secondary magnetic fabrics shown by alpha (a >20 ~ indicates reworked sediment) and mu susceptibility parameters (Rees et al, 1968). The slumped beds can be subdivided into 3 groups (cf Truze, 1990), with one including thin layers rich in vegetal fibers. Clay minerals define two groups: the first poor in smectite and mixed layer associated with illite and well-crystallized kaolinite, and the second is rich in smectite and mixed layer clays. The slumping is consistent with an unstable paleoenvironment due to prograding of the slope or else a lowering of lake level due to changing climate. The slumped beds were emplaced as a unit very quickly, perhaps within a few hours. We are however unsure how much,if
265
any, of the section is repeated. Pailles (1989) proposed a lowering of lake level of -15 m based on the diatom assemblage just above the base of the slump. As an alternative interpretation, diatom patterns could merely be an artifact of the redeposition of more littoral sediment and needs reevaluation. For the pleni-glacial period, Guiot (1989) estimated a decrease in temperature as low as -10~ and precipitation around -400 mm/yr less the current climate. These could have indeed led to lower lake levels. Slumping may produce oversteepening leading to a new equilibrium stage for the slope. The top of Unit H is defined by horizontal beds of sand (11.50 m to 10.70 m) and the contact is gradational. These might indicate back filling. UNIT G (10.70 m to 9.17 m ) f r o m
58 to 48 kyrs BP,
0.15 mm/y
Unit G is characterized by dark-brown silty clays (subfacies CLS) and a progressive increase from 15% to 30% of coarser-grained particles (>50 I.tm). The distribution and size patterns of this Unit are unique. These deposits are rich in chara oogonia and minerals are altered. The clay minerals clay show a weak decrease in smectite and poorly crystalline mixed-layer varieties (zone IIb), indicating that the soil profil was denuded down to the zone of clay segregation. This suggests that during this period the climate might have been more humid. Detrital increase is contemporaneous with an end to maximum aridity around 58 kyrs, with a proposed increase from -800 mm/yr to -300 mm/yr ( Guiot et al., 1989) and also noted in the dust record of Vostok (Petit et al., 1990) which shows a large decrease near this time. The upper limit is well characterized by the appearance of thin turbidities, suggesting instable lake levels. A peak in smectite and mixed- layers, indicates an increased erosion for the catchment, just before the colonization by vegetation, monitored by an increase of organic matter. This limit is correlated to the beginning of a forest cycle, marked by Pinus and Picea (Reille et de Beaulieu, 1989). Our age correlation is not in agreement with the radicarbon date of 40 kyrs picked for this lithologic contact (figure 10 in Touveny et al), although it is within potential error limits. We place weight on the higher rate of sedimentation for this unit (cf. figure 5). UNIT F (9.17 m to 6.95 m) Interstadial 48 kyrs to 30 kyrs BP, 0.12 mm/yr Unit F comprises dark and light colored, cm-thick organic clay, associated a few graded sand beds. The overall sand fraction is low (4%), indicating variable hydrologic energies. The organic carbon
266
content increases up to 2% parallel with amorphous organic matter (60% amorphous with 10% recognizable algal), with higher IH (350) and lower IO (50). These show the beginning of lake productivity. The Unit has abundant diatom frustules, and is characterized by higher Tree/Steppe index and variable susceptibility. The /5 13 C organic value is -26.9 per mil PDB, as a reflection of increases in temperature and a forest marked by Pinus and Picea (Reille et de Beaulieu, 1989) which was suitable for trapping the coarser-grained particles. Volcanic grains in the coarse fraction are very altered. Other grains include oogonia, diatoms, mandibles and vegetal fragments. Siderite occurs at the bottom of Unit F, suggesting an increase in pCO2 due to the oxidation of organic matter, and a increase in alkalinity suggesting more weathering. A more humid climate is also supported by the clay minerals (zone IIIa) showing a sharp peak for smectite and mixed layers (5% to 20% together) decreasing above as vegetation cover reduces the c l a y transport. Aspects of siderite and component occurrences imply more variability in evaporation, precipitation and productivity during this interstadial. The upper contact is marked by decrease in amorphous organic matter (20%) and 813C (-28.8 per mil PDB). We are more confident about the radiocarbon date around 30 kyrs BP at the top of this unit. UNIT E (6.95 m to 4.30 m)Middle kyrs BP, 0.33 mm/y
Pleniglacial 30 kyrs to 22
Unit E comprises grey oxidized, massive clay associated with several decimeter-scale, graded sand beds. Unit E has three subdivisions : E1 (6.95 m to 6.10 m) comprises silty clay with thin, graded sands which contain a few organic fibers, mandibules, and mollusc shells. Yellow laminae of subfacies CLS are common at the top of the E1 division; E2 (6.10 m to 5.10 m) comprises several thick turbidites in subfacies CGS, and E3 (5.10 m to 4.30 m) comprises silty clay with few sands (subfacies CLS). These indicate a higher energy of deposition in concert with a larger sedimentation rate. The massive clays contain a lot of Fe-pigmented spots as evidence of organic degradation in the sediment. Yellow mm-lamina imply arid periods, w h e n sedimentation rate is lower. The carbon content is lower (<0.5%), indicating an oxidizing environment with little preservation of organic-matter. Vivianite crystals occurs in the deposit. Around 29
267
kyrs BP, this sedimentation is intercalated with decimeter-thick turbidities suggesting a destabilisation of slopes with weather events. This is consistent with the clay minerals (zone IIIb) showing a peak for smectite and mixed-layers. This period is correlated with the pollen record (Reille and de Beaulieu, 1988) showing a clear steppe vegetation; diatoms are absent or dissolved (Pailles, 1989). The upper contact is sharp and occurs with the appearance of a clotted texture, which coincides with an decrease of magnetic susceptibility estimated at 22 kyrs BP. U N I T D (4.36 rn to 2.50 rn) Maxium glacial 22 kyrs to 16 kyrs BP, 0.17 m m l y Unit D is characterized by the occurrence of cm-scale clay-rich beds with diffuse contacts and which have irregular spheroid clay pellets in shades of green (subfacies HSC). The sequence is interrupted by a slump. A few thin sand layers and scattered vivianite crystals occur at the bottom of the unit. Slightly lower magnetic susceptibility in Unit D would be consistent with slower overall sedimentation rates. This unit is subdivided into 3 : D1 (4.30 m to 3.40 m) is composed of clotted clay ; D2 (3.40 m to 2.75 m) is interpreted as a slump, which is delimited by a sharp lower boundary formed by a 5 c m , graded sand associated with a micro-fault. D2 comprises mainly grey clay with basal intercalations of 5 to 30 cmthick turbidites and deformed sand layers, some with small offsets. Magnetic susceptibility increases abruptly at this level with inclined sandy beds (subfacies SBS); D3 (2.75 m to 2.35 m) is composed of clay with spheroids (subfacies HSC) with a few thin sands. Layer thickness decreases progressively upward, likely as part of the climate trend. The genesis of such spheroid clay pellets is not well-understood for lacustrine environments. Laboratory experiments (Adolphe, 1976) have shown that such structures occur in soil profiles as a result of frost and thaw in homogeneous sandy clays. A complex circular movement of particles during the freezing process-imparts a spheroid structure. In soil profiles, however, those spheric structures should be associated with coarser-grained particles which do not occur in the Unit D layers. During this interval the basin was without the vegetation cover needed to inhibit clastic transport. It is therefore likely that extreme permafrost and ice cover on the lake limited the input of larger particles. The frozen surface of the lake could act as a site of formation of spheroid pellets from thin layers of aeolian particles which settle
268
during melting. Very special conditions are needed to explain the discrete occurrence in only 20-21 thin bands. These are perhaps indicators of the coldest events. The platikurtic grain-size pattern, typical of Unit D is interpreted as settling in a lake with slow full circulation. The bands with rows of clay spheroids provide ideal marker horizons for correlation. All 20 parallel beds can be traced among cores. As a rough estimate we note that the recurrence rate for clay spheroid bands would be about 330 years. The slump is interpreted in terms of a temporary fluctuation in lake level followed anew by cold, dry conditions producing clay spheroids without a sand fraction. Apparently, the extreme conditions led to minimal productivity. Organic carbon content is less than 0.2%. Diatom frustules are absent, (Pailles, 1989). Pollen analysis (Reille et de Beaullieu, 1988) indicates an increase in the tree/steppe index which is more related to wind transport of pollen than forest cover during a period with significant decreases in precipitation and temperature (Guiot et a1.,1989). These sediments record deposition during the extreme cold episodes between 22 kyrs and 16 kyrs when many other lake sites in Europe remain associated with ice or till cover and Antarctic ice was characterized by dust (Lorius, 1985). The upper limit is gradational, marked by the loss of clot clay textures, but with an increase in smectite and mixed-layers. Magnetic susceptibility remains near the same levels. It is correlated with the beginning of deglaciation, generally considered around 16 kyrs BP.
UNIT C (2.50 m to 1.18 m) Final Pleniglacial f r o m l 6 kyrs to 10 kyrs BP, 0.22 mm/yr Unit C is characterized by uniform gray clay, with only rare thin sand laminae with some oogonia. There are 3 subdivisions: C1 (2.50 m to 1.50 m) is characterized by homogeneous silty clay, with intercalated thin silts (subfacies CLS) in concert with an increase of smectite and mixed-layers and an increase in magnetic suceptibility. C2 (1.50 m to 1.18 m ) is characterized by an decrease of smectite and mixed-layers which imply colonisation by vegetaton during a more humid period. C3(1.18 m to 1.13 m ) is a special bed only 5 cm thick w h i c h comprises organic matter-rich clay above a cm-thick sand underlain by 3 yellowish, iron-rich laminae (subfacies OHSC). Sand includes fossil grains from organic fibers, chara oogonia , diatom frustules, Pinus pollen. Organic carbon content reaches nearly 1 % , in a sand layer. Organics derive from a renewed expansion of steppe species. In this one narrow zone, the sedimentation is highly reduced as documented by the compression of pollen zones for B~lling up to
269
Preboreal. This condensation might be related to the Younger Dryas event. The resolution however is inadequate to provide a strong vegetation signal of sudden cooling. The upper limit is gradational over a few centimeters, in concert with major increases in organic carbon correlated with the beginning of the Holocene. The boundary is selected as 10,5 kyrs and also accompanied by a significante decrease in magnetic suceptibility. All the core parameters show the establishment of lake productivity in a temperate,humid environment and a reduction in the hydrologic energy displayed by the sediments.
This Lac du Bouchet stratigraphy avoids the use of term for a Unit B to remain consistent with published articles which include a core description (eg. Creer, et al, 1986, Bonifay et a1.,1987) U N I T A ( 1.13 m to 0.00 m) H o l o c e n e ,
0.11mm/yr
Unit A is a varied combination of dark brown homogeneous or laminated organic-rich diatomaceous clays (facies LOC and HOC). The sandy fraction makes up uniformly less than 2% and grain size is leptokurtic. TOC varies between 20% to 9%. High organic productivity is documented by the dominance of amorphous palynofacies (70%) with the highest IH (300 up to 500) and a low IO (80). ~13C value is -28.3 per mil PDB. X-ray diffraction spectrums for clay minerals show a broad amorphous peak i n the diagrams, leaving them uninterpretable. The pollen stratigraphy for the Holocene sequence (Reille et al., 1988) is well-defined and easy to correlate with other European sites. The following characterizes 4 lithological subdivisions which match closely with these pollen zones with dates from radiocarbon interpolation. Each interval with more arid conditions is accompanied by a lack of diatom frustules. AI (1.13 m to 0.68 m / Preboreal) is a special layer with the character of a Sphagnum moss. It is composed of black massive mud with matted organic fibers, but without diatom frustules, and was deposited between 10.3 kyrs and 9. kyrs; - A2 (0.68 m to 0.28 m / Atlantic optimum,8-4.7 kyrs.) has brown to green cm-laminae with well-defined, rhythmic banding associated with an increase of organic carbon content (9% up to 20%). The dry Boreal event is only represented by a basal zone of this section which comprises light brown mud with a decrease in TOC (9%);
270
A3 (0.28 m to 0.20 m). More arid sedimentation during the Subboreal episode between 4.7 and 2.6 kyrs BP includes a decimeter band of varves with couplets of black, green, or yellow mm-laminae. These do not occur in all of the basinal Lac du Bouchet cores, although this may be a result of coring disturbance. TOC is around 14% ; A4 ( 0.20 m to 0.00 m) is an uniform red brown mud, with very high water content, which covers the anthropogenic period over the last 2000 years or more. Organic carbon content decreased to around 5% and the sand fraction increased up to 15%. The cores do not resolve the major climate fluctuations although conditions were generally cooler, with more precipitation then before. During the Holocene, trends towards less coarse fraction reflect patterns of colonization by various forest patterns, in concert with higher lake productivity. Bottom water anoxia occurred at times. Magnetic susceptibility dropped significantly reflecting lower erosion rates. It is also possible that chemical redox changes altered the magnetic signal. There is some indication from the diatom assemblages of lake level changes (Pailles,1989). Lower levels seem most likely in conjuction with moss-dominated sediments of the preboreal. The diatom method is based on analogies with modem benthic patterns for the current lake morphology. It is unsure how much these hold for no-analogue situations in the past. Organic matter content varies during the Holocene. Highest TOC values occure during Atlantic, the climatic optimum. This corresponds to the interval of highest IC, or maximum planktic productivity. During Boreal and the Subboreal events marked by colder conditions and more arid weathering, values that detrital organic matter diluted the planktic signal. The antroprogenic record could not be deconvolved from these samples and requires further work on undisturbed short gravity -
cores.
CONCLUSIONS The above scenario for lithological correlations is certain to be in error in details of the correlations and timings. It does show however, the close correspondence between sediment characteristics, and the climate history. The sediment picture adds new clues for patterns of seasonal variation in temperature and precipitation which can be tested against aspects of the pollen, or diatom record and interpolations of chronostratigraphic data. Sediment character must at least correspond to relative changes in sedimentation rates. This is
271
particulary the case for sections which are suspected of being slumps, or event deposits. In spite of the uncertainties in both methods, and some circular aspects of the arguments, these results of the lithostratigraphy support general trends proposed by Guiot et al, 1989. We found that assuming that major climate excursions are synchronous, event boundaries could provide a first order interation of the chronology. The Lac d u Bouchet core provides useful new control points, exactly for those barren, cold intervals which are poorly represented in peat bog sites, or for sections beyond the limits of the radiocarbon scale. For example, our arguments for a revision of the age picks for the period 50-60 kyr are buttressed by sedimentological arguments for higher rates during Unit G. They also fit well with interpolated ages and curves linking pollen and Oxygen-18 stratigraphy in the Atlantic ( eg. Lezine and Casanova, 1991). Sedimentology of the cores entails an integration of various types of analyses. By grouping the cored sediments into characteristic lithofacies, it was possible to demonstrate a recurrence pattern that was essentially driven by climatic conditions. Within an interpolated time framework, it became apparent that the recurrence of certain lithological types are consistent with a Milankowitch models of cycles. The lithological patterns help differentiate the interplay among interdependent climatic paramaters; arid to humid versus cold to warm. The glacial maximum and deglaciation for example, had very low levels of coarse clastic input Sediment structures changed, but textures much less. Warming during the interstadial, on the other hand, also led to fine grained deposits but with higher plankton productivity, and low clastic input due to vegetation inhibiting erosion. Long sequences from maar lakes hold tremendous potential for correlating events among continental regions as well as for land-sea correlations. To realize the full potential of lake response signatures in these high-resolution environmental records requires a systematic documentation of the sedimentary matrix from which multiproxy samples are taken. Many measurable parameters should be keyed to exactly the same layers. This will avoid confusion caused by comparing samples at levels only millimeters apart, but which may already represent changed environments.
272
References Adolphe J.P., 1976 - Obtention d'edifices physico-chimiques par congelation experimentale. Comparaison avec ceux dus a la chaleur. Cahiers Geol., 92 p.165-176. Allisson A., 1983 - Interpretation of reflection sismic profils from lac du Bouchet. Projet report. Univ. of Edingbourg, 40p. Berner R.A., 1980 Authigenic mineral formation resulting from organic matter decomposition in modern sediments. Fortschr. Miner. 59(1), 117-135, Bonifay E., Creer K.M., Beaulieu J.L., Casta L., Delibrias L., Perinet G., Pons A., Reille J.L., Servant S., Smith G., Thouveny N., Truze E., and Thucholka P. 1987 - Study of the Holocen and Late Wurmian sediments of Lac du Bouchet (HauteLoire, France) first results. In: Climate, history, periodicity and predictability, M.R. Rampino, J.E. Sanders, W.S. Newman & L.K. Konigsson (Editors) Van Nostran Reinhold Co., New York., NY, 90-116. Butrym J., 1964 - New interpretation of "periglacial structures", Folia Quaternaria,17, p. 1-34. Chamley, H., 1989 - Clay sedimentology. Spring-verlag, Berlin, 623p. Craig H., 1961 - Isotopic variations in meteoric waters. S c i e n c e , 133, 1702-1703. Creer K.M., Smith G., Tucholka P., Bonifay E., Thouveny N., and Truze E. 1986 A preliminary paleomagnetism study of the Holocene and Late Glacial sediments of Lac du Bouchet, (Haute-Loire, France). Geophys. J.R.Astron. Soc., 86, 943964 Decobert M., 1988- Carte bathymetrique du Lac du Bouchet presentee au colloque du Puy-en Velay 4-6 mai !988. Espitalie J.L, Deroo G., Marquis F., 1987 - Rock-Eval pyrolisis and its applications, Rev. IFP, Paris, Internal. Rep., n ~ 33578. Etlichter B., 1980 - Problemes du glaciaire forezien. Bull. Labo. Rhod. Geomorph., 7, 3-27. Fillod A., 1985 - Le climat en Haute-Loire. - Ed. de la Borne, Saint Vidal, 90p. Guiot J, Pons A, de Beaulieu J.L, Reille M., 1989 - A 140,000-year continental climate reconstruction from two European pollen records. Nature, vol. 338, 309-313. Garrels R.M., and Christ C.L.,1965 - Solution, Minerals and Equilibria. Harper et Rowm (Ed.) New York, 450p.
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Lecocq A., 1987 - Hydrogeologie en milieu volcanique. Etude de la partie nord du plateau du Deves (Massi-Central) These, Clermont Ferrand 221p. Lorenz, V., 1986 - On the growth of maars and diatrems and its relevance to the formation of tuff rings. Bull. Volcanolo. 48. 265-274. Lorius C., Jouzel J., Ritz., 1985 - A 150,000-year record from Antartic ice.Nature, 316/6029, 15 August 1985, 591-596. Lezine, A.M., and Casanova, J., 1991- Correlated oceanic and continental records demonstrate past-climate and hydrogeology of North Africa, (0-14 Ka). Geology,19, 307310. Pailles C., 1989 - Les diatomees du lac de maar du Bouchet (Massif central, France) Reconstruction des paleoenvironnements au cours des 120 derniers millenaires, These Universite d'Aix-Marseille II. 274 p. Passega R., 1964 - Grain size representation by C.M. patterns as a geolological tool, J. Sed. Petr. 34, 4, p830. Petit J.R, Mounier L., Jouzel J., Korotkevich Y.S., Kotlyakov V.I., Lorius C., 1990 - Palaeoclimatological and chronological implications of the Vostok core dust record. Nature, Voi.343, 56-58. Reille M. and de Beaulieu J.L., 1988 - History of the Wurm and Holocene vegetation in western Velay (Massif Central, France): a comparison of pollen analysis from three corings at lac du Bouchet. Review of Paleobot. and Palyno, 54 233-248. Reille M., and de Beaulieu J.L., 1990 - La fin de l'Eemien et les interstades du Prewurm mis pour la premiere fois en evidence dans le Massif Central francais par l'annalyse pollinique. C.R. Acad. Sci. Paris, 306 (II) ; 1205-1210. Riviere A.,1977 - Methodes granulometriques:techniques et interpretations Masson 167p. Shackleton N.J, Opdike N.D., 1973- Oxygen isotop and paleomagnetic stratigraphy of equatorial pacific core V28-239: oxygen isotop temperature ice volume on al05 years and 10 6 years scale. Quat. Res., 3,3 39-55. Singer A.,1984 - The paleoclimatic interpretation of clay minerals in sediments : Areview. Earth Science review, 21, 251-293 Talbot M. R., and Livingstone D. A., 1989 - Hydrogen index and carbon isotopes of lacustrine organic
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matter as level indicators. Palaeogeog. Palaeoclimat. Palaeoecol, 70, 121-137 . Teulade A., Mergoil J., Boivin A., 1988 - Etudes geologiques et volcanologiques des environs du Lac du Bouchet. Document CERLAT n ~ 2. Thouveny N.,1990 - Les variatrions du champ magnetique terrestre dans le dernier cycle climatique (0-120000 ans BP) .These d e l'Universite de Aix-Marseille II, 192 p. Thouveny , N. Creer, K. M., & Blunk,I., 1990- Extension of the Lac du Bouchet palaeomagnetic record over the last 120,000 years.Earth and planetary science Letters, 97,140-161 Tissot B.P., Welte D.H., 1984 - Petroleum formation and occurence. Spinger-Verlag, New York, 538p. Truze E., 1990. Etude sedimentologique et geochimique du m a a r du Bouchet (Massif-Central, France). Evolution d'un systeme
lacustre au cours du dernier cycle climatique (0 -120 000 ans). These d'universite d'Aix-Marseille II , 260p. Truze E., 1988 - Chimie et geochimie des eaux actuelles du lac du Bouchet. In Bonifay E. (Editor), International Report 3, EEC program of research Geomaar Marseille, France. Truze E., 1983.- Etude preliminaire de la sedimentation dans les lacs de maars du Deves: le lac du Bouchet. Memoire de DEA, Universite d'Aix-Marseille II, p55 Veyret Y., 1981 - Les modeles et formations d'origine glaciaire dans le Massif central Francais: problemes de distribution et de limites dans un milieu de moyenne montagne. These, Lille III, 753p. Woillard G.M., Mook W.G., 1982 - Carbon dates at grande Pile; correlatin of land and sea chronologies. Science, 309, 103122. Zolitschka B., 1989. - Jahreszeitlich geschichtete See sedimente aus dem Holzmaar und dem Meerfelder Maar. Z. dt. geol. Ges. 140,25-33.
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Acknowledgements This paper represents an English condensation of the Ph.D. thesis of ET. It was written during a tenure as visiting scholar at the Limnological Research Center. Travel support was provided by LRC grant Nr. 91/1 to KRK. The study of Lac de Bouchet was based on cores collected under the auspices of a CNRS grant to E. Bonifay. Cores and samples are archived at LGQ, Marseille. W e greatly appreciate the sharing of ideas, discussions and preliminary data with G.Seret , Reille M. and de Beaulieu, Thouveny N. and Blunk , Teulade A., Mergoil J., and C. Goetz (1990). Seismic profiling was done by Comp. Gen. Geophy. under a grant to E. Bonifay. Various analyses were carried out with the much appreciated help from others: A.Huc with organic matter; A13C with R. LeTolle; palynofacies with Ph. Betrand; water chemistry and isotopes with J.C.Fontes;
LAGO GRANDE DI MONTICCHIO (SOUTHE~RN ITALY) A HIGH RESOLUTION SEDIMENTARY RECORD OF THE LAST 70,000 YEARS Bernd Zolitschka* & JSrg F.W. Negendank** Geologle, Universit~t Trier, D-5500 Trier **GeoForschungsZentrum, Telegrafenberg A26, O-1561 Potsdam
ABSTRACT Lithology, measurements of dry density, total organic carbon and first microstratigraphic investigations were used to calculate a tentative time scale for the lacustrine sediments from Lago Grande di Monticchio. A comparison of these records with dated Italian lake sediments of Valle di Castiglione, Lago di Vico and Lagaccione Maar and with the eolian dust record from Vostok ice core (Antarctica) demonstrates the time scale to be reasonable. According to this tentative time scale the 51.8 m of sediment cover the last 70,000 years.
INTRODUCTION The incentive to study maar lake sediments from Lago Grande di Monticchio in southern Italy was the search for varved organic deposits beyond the Pleistocene/Holocene transition. Such deposits have sedimentation rates of one or two orders of magnitude higher than marine records and therefore make available information of the past with a much higher time resolution. As evidenced by maar lake sediments from the Eifel area, organic deposition may also provide an internal time scale: varve chronology (Zolitschka 1991). Unfortunately, in the Eifel maar lakes there are no organic sediments older than 13,000 years. To obtain a longer record a site south of the Alps was selected hoping to recover sediments which enable to establish a varve chronology with biogenic varves back into the last Glacial. Such sediments would provide a new quality of age control for a period, when high resolution absolute dating is not available.
Lecture Notes in Earth Sciences, Vol. 49 J. F. W. Negendank. B. Zolitschka (Eds.) Paleolimnology of European Maar Lakes 9 Springer-Vedag Berlin Heidelberg 1993
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STUDY AREA Lago Grande di Monficchio (Basilicata, southern Italy) is situated in a highly active volcanic area approximately 280 km southeast of Rome and 120 km east of Naples (Fig. 1). Lago Grande as well as Lago Piccolo di Monticchio formed during phreatomagmatic eruptions in a caldera at the western slope of Monte Vulture, a volcano belonging to the Campanian Igneous Province. Volcanic activities at Monte Vulture began 1 Ma ago and ceased at ca. 40,000 years BP (Guest et al. 1988, Pichler 1989). At least during most of the Holocene the two lakes were combined to one large lake with a common water level of approximately 5 m above the current one as evidenced by lake-shore terraces. Probably related to the foundation of the monastry San Ippolito (Fig. 2) in the year 1059 AD drains for both lakes were dug and the lake levels dropped to their present level. During the 17th century San Ippolito was abandoned because of problems with rising ground water. The new monastry of San Michele was constructed 170 m above Lago Piccolo (Fig. 2). The morphology of both lakes is summarized in comparison with selected Eifel maar lakes (Tab. 1). Although I_ago Grande di Monticchio has a maximum depth of 36 m, two thirds of the lake are less than 12 m deep. Only the northern part of the basin deepens to an intermediate level of 24 m and then drops nearly vertically at two locations from 24 m to 32 m and 36 m, respectively (Fig. 3).
Tab. 1: Morphometric data of Lagi di Monticchio in comparison with data of selected maar lakes of the Eifel (LGM = Lago Grande di Monticchio; LPM = Lago Piccolo di Monticchio; MFM = Meerfelder Maar; HZM = Holzmaar; GMM = Gemfindener Maar). lake location
LGM
LPM
15~ 15~ 40~ 40~ elevation (m asl) 656 658 max. depth (m) ,~ 36 38 lake surface (1000 ~ ) 405 135 catchment (1000 m~) 2370 1050 take catchment/lake surface 5.9 7.8 length of shoreline (m) 2500 1400 shore line development 1.1 1.1 max. elevation in catchment (m) 956 1262 relief energy (m) 300 604 trophic state eu/poly oligo/meso
MFM 6~ 50~ 336.5 18 248 5760 11.5 1975 1.1 531 194 eu
HZM 6~ 50~ 425.1 20 58 2000 34.5 1100 1.3 477 52 meso/eu
GMM 6~ 50~ 406.6 39 75 430 5.7 975 1.0 560 153 oligo
279
Fig. 1: Location map of the site of Monticchio.
Fig. 2: Lagi di Monticchio and their catchment areas with location of the two monastries. Figures indicate the present lake levels and the lowest and highest elevations in the catchment (m as1).
280
METHODS Four sediment cores were recovered from Lago Grande di Monticchio with a modified Livingstone piston corer (Usinger corer) during September 1990 (cf. Fig. 3). The core from the deepest part of the lake (LGM-A) was stuck in coarse pyroclastic material at a sediment depth of only 4 m. Recovered sediments were highly disturbed. The coring sites of the shallow southern lake basin provided three overlapping sediment profiles to a sediment depth of 16 m (LGM-C), 40 m (LGM-B) and 52 m (LGM-D). Based on macroscopic correlation the core series of LGM-B and LGM-D were combined to form one composite profile (Fig. 4). This profile was subsampled continuously with 3 cm increments for determination of dry density. The same samples were analysed for total organic carbon (TOC) by standard LECO combustion on dried samples. CO: release was integrated as TOC within the temperature interval 200~
to 550~
Problems arise with this method because siderite
starts to disintegrate already at 400~ (Brauer & Negendank, this vol.). Therefore TOC values are overestimated if siderite is present. Elemental composition of some of the major pyroclastic layers was analysed by atomic absorption spectrometry. The whole sediment sequence was subsampled continuously to prepare large-sized thin sections for microstratigraphical investigation as well. Until now only 5 % of a total of ca. 700 thin sections have been analysed.
SEDIMENTS The sediments from Lago Grande di Monticchio are laminated and suggest a continuous lacustrine deposition. They consist of an upper highly organic (TOC > > 10%) and a lower minerogenic (TOC <5%) section (Fig. 4). Although considerably low in TOC, the lower minerogenic part also gives evidences for fluctuations in the amount oforganic deposition. These variations are reflected in the colour of the sediments which serve as a first means to identify local lithozones (Tab. 2). The values of dry density corroborate this description (Fig. 4). Dry density is an indicator of the biogenic/minerogenic ratio of the sediments with low values resulting from organogenic and high values resulting from minerogenic deposition.
281
Fig. 3: Bathymetric map of l_ago Grande di Monticchio with indicated coring sites (Hansen 1991, modified). Combined information from lithozone discrimination (Tab. 2), TOC and dry density (Fig. 4) allows to distinguish 4 major sediment stages (A, B, C and D) for the depositional history: (A) is characterized by low mean dry densities of 0.15 g c m -3 and high TOC of > > 10% indicating organic deposition during lithozone I and II. (B) comprises pure minerogenic deposition during lithozone IV with highest dry densities of the whole record ( > I g c m -3) and lowest TOC values (<1% TOC). There are transition zones to sediment stage A (lithozone [II) and C (lithozone V).
282
0
~
9 ~
o~
~E
~8 <.-.. 0
0
~
283
(C) may be characterized as a period of minerogenic and organogenic sedimentation (lithozone VI), indicated by dry densities of 0.5 g cm -3, increased TOC values ( > 3% TOC) and also a shift from dominating grey and brown colours to a black hue. (D) increasing dry densities up to 0.9 g cm -3 and decreasing TOC values point to a second less pronounced maximum of minerogenic deposition during lithozone VII. Intercalated are black periods related to distinct drops in dry density.
Tab. 2: Local lithozones of the sediments from Lago Grande di Monticchio. lithozones
depth (m) 4.9 8.4 19.0 -
4.9 8.4 19.0 23.0
V
23,0
27.6
VI VII
27.6 39.8
39.8 51.8
I
II III IV
0.0
-
sediment description dusky brown to olive brown black olive grey with 4 black periods brownish grey elastic turbidites with greyish yellow calcite layers olive grey and greyish brown with 2 black periods black to olive black dusky brown with several greyish black to olive black periods
An estimated amount of 500 pyroclastic layers, ranging in thickness from a fraction of a millimeter up to 35 cm, interrupt the lacustrine deposition. They are the reason for the spiky nature of the dry density record. The summarized thickness of all pyroclastic layers reaches ca. 3.5 rn. Ash layers of more than 3 cm in thickness are listed in Tab. 4. Their elemental composition varies from basaltic
to andesitic and thrachytic (Tabs. 3, 4; cf.
Newton & Dugmore, this vol.). Another addition to the lacustrine deposition are two slumped horizons at 22 m and 28.5 m sediment depth with a thickness of 101 cm and 122 cm, respectively. Both slumps are related to preceeding ash falls of 11 cm and 2.5 cm in thickness. Microstratigraphic investigation was restricted to a few 10 cm long sections from various depths. From sediment surface to a depth of ca. 9 m and around 49 m annual laminations have been observed (Figs. 5, 6). They consist of diatom-rich layers alternating with layers of organic detritus and are very similar to varves wellknown from Eifel maar lakes (Zolitschka 1991; Poth & Negendank, this vol.; Heinz et al., this vol.). The thickness of varves ranges from 0.25 mm to 1.25 mm for the upper varved section and varies between 0.2 mm and 0.4 mm in the varved section around 49 m sediment depth. The remaining sediments consist of calcite and siderite laminations, possibly of annual nature (cf. Brauer & Negendank, this vol.), but also of turbidites and homogeneous bioturbated deposits.
284
Fig. 5: Micrograph of annually laminated sediments from 2.9 m sediment depth. Thickness of shown section: 2 ram. Pale layers are planktonic diatoms (Pennales), dark layers are organic detritus and litoral organisms.
Zig. 6: Micrograph of annually aminated sediments from 49.2 m ediment depth. Thickness of hown section: 2 ram. Dark layers re made of planktonic diatoms Centrales), pale layers are made ,f minerogenic and organogenic etritus and of litoral organisms.
285
Tab. 3: Geochemical analyses of 6 pyroclastic layers (PL) from sediments of Lago Grande di Monticchio. PL 7
PL 8
PL 9
PL 11
PL 13
PL 15
SiO2 TiO2 A1203 Fe,,O3 FeO MnO MgO CaO Na20 K20 LOI-1050~ P205 Total
50.75 0.68 16.08 3.15 2.77 0.18 3.12 10.43 1.81 4.46 5.40 0.41 99.24 hawaiite
49.35 1.06 13.85 4.77 2.79 0.15 4.53 9.04 0.90 3.38 8.90 0.59 99.31 basalt
59.17 0.48 17.82 2.08 1.85 0.17 0.86 2.71 4.48 6.70 3.80 0.11 100.23 +
59.76 0.56 17.26 1.85 1.98 0.26 0.52 1.14 5.40 5.96 5.30 0.08 100.07 +
49.89 1.40 14.67 6.44 2.97 0.t4 6.16 9.56 0.90 2.02 5.00 0.44 99.59 basalt
55.09 1.12 13.45 2.77 3.42 0.13 4.72 7.41 0.91 2.24 7.60 0.31 99.17 ++
+: trachyte
+ +: basaltic andesite-andesite
Tab. 4: Composition and estimated date of deposition for major pyroclastic layers (PL) of more than 3 cm in thickness. PL
sediment depth (m)
thickness (cm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
3.3 6.6 7.9 10.1 11.1 14.6 I7.2 23.0 25.1 31.8 39.0 45.5 46.2 46.5 48.6
3 11 5 7 18 25 9 11 17 3 35 4 6 7 9
n.d.: not determined according to Newton & Dugmore (this vol.)
composition n.d. n.d. n.d. n.d. trachytic (C33): trachytic (C41) hawaiitic basaltic trachytic n.d. trachytic n.& basaltic n.d. basaltic-andesitic to andesitic
approx, age (ka) 4.4 8.8 10.5 13.5 14.8 19.5 22.9 30.7 33.5 42.4 52.0 60.7 61.6 62.0 64.8
286
CHRONOLOGY The presented tentative chronology is based on lithology, dry density, TOC data and first results of microstratigraphic investigations. Sediment stage A with high organic deposition is related to the Postglacial. This is generally accepted by palynology (Watts 1985, unpubl, data), palaeomagnetics (Turton, this vol.) and geochemistry (Robinson et al., this vol.). Extending the mean sedimentation rate of 0.75 mm a -1 for this stage to the whole record a maximum age of 69.1 ka is achieved for the 51.8 m long record (Tab. 5). Taking into account that pyroclastic layers and two slumps, together comprising a thickness of 5.7 m, should be excluded from this calculation, the maximum age will be reduced to 61.5 ka. Of course, both ages are only preliminary because sedimentation rate certainly was not constant throughout the Weichselian. Nevertheless the dating provides a possibility to compare this record with records from other sites. For this purpose only the first, older age estimation will be considered, because an underestimation of the true age due to erosion, e.g. at the base of the slumped horizons, is more likely than an overestimation.
Tab. 5: Lithozones, sediment stages and their estimated duration. lithozones
I
II III IV V VI VII
sediment stages
estimated duration (ka)
A
0.0
-
A B B B C D
6.5 11.2 25.3 30.7 36.8 53.1
-
6.5 11.2 25.3 30.7 36.8 53.1 69.1
depth (m) 0.0 4.9 8.4 19.0 23.0 27.6 39.8
-
-
4.9 8.4 19.0 23.0 27.6 39.8 51.8
The minerogenic sediment stages B and D are dominated by silt grain sizes and might be caused by eolian dust input, mainly depending on wind speed, circulation patterns and availability of dust sources. Assuming, that the avaibility to transport dust is a global athmosperic characteristic, a comparison with a high resolution dust record from Vostok ice core (Antarctica) may be feasable. This record indicates two periods of increased dust accumulation centered around 20 ka and 60 ka (De Angelis et al. 1987). They correspond to the oxygen isotope stages 2 and 4 (Fig. 7) and corroborate the tentative time scale for Lago Grande di Monticchio with increased clastic deposition during sediment stage B (centered around 24 ka) and D (centered around 61 ka).
287
Turton (this vol.) suggests that the record is as old as 250 ka including two interglacials. But there is no evidence for warmer climates. TOC stays below 5 % (Fig. 4) and steppe-like vegetation with dominating Graminae and Artemisia prevails below 8.4 m sediment depth (Watts, unpubl, data). As evidenced from the Italian sites of Valle di Castiglione (Follieri et al. 1988), Lagaccione Maar (Follieri et al., this vol.) and Lago di Vico (Francus et al., this vol.), the Eemian is very pronounced with high values for arboreal pollen and organic matter, both comparable with the Holocene. Even the warm interstadials of St Germain I and St Germain II correspond to considerable high arboreal pollen sums and distinctly higher organic matter contents. Therefore the last interglacial as well as the Early Glacial interstadials are not present in the record of Lago Grande di Monticchio. Applying the oxygen isotope zonation to the record from Lago Grande di Monticchio, sediment stages A through D correspond to oxygen isotope stages 1 through 4 (Fig. 4). As there is no indication for the warmer oxygen isotope stage 5a, the base of the investigated record should be younger than 73 ka, which is the age of the transition from oxygen isotope stage 5a to 4. Varves from around 49 m sediment depth may be interpreted as a last imprint of the preceeding warmer period of St Germain II.
Fig. 7: Eolian dust record, represented by A1 concentration, and oxygen isotope record from Vostok ice core, Antarctica (De Angelis et al. 1987).
288
Further evidence for the applicability of the presented time scale is given by tephrochronological studies of Newton & Dugmore (this vol.). They analysed two pyroclastic layers (PL 5 = C33 and PL 6 = C41) of trachytic composition which were related to eruptions in the Campi Flegrei during the last 35 ka: the younger of the two ash layers (PL 5) may be the Yellow Neapolitan Tufts (12,000 years BP) and the older one (PL 6) the 35,000 years old Campanian Ignimbrite (cf. Barberi et al. 1991). Using the tentative time scale PL 5 yields an age of 14.8 ka which is nearly identical with the age of the Neapolitan Tufts (Tab. 3). According to the tentative time scale the ash layer of PL 6 corresponds to an age of only 19.5 ka, ca. 15 ka too young for the Campanian Ignimbrite. But it is possible, that another trachytic ash layer (PL 9) with an approximate age of 33.5 ka (Tab. 3) reflects the Campanian Ignimbrite, whereas PL 6 is an additional one. With this modification tephrochronology would also corroborate the suggested time scale.
ACKNOWLEDGEMENTS This study was supported by the European Community Science Programme (EUROMAARS Project) and a grant of the German Science Foundation (DFG Ne 154/21-2).
REFERENCES Barberi, F., Cassano, E., La Ton'e, P. & Sbrana, A. (1991): Structural evolution of Carnpi Flegrei caldera in the light of volcanological and geophysical data. J. Volcanol. Geotherm. Res., 48: 33-49. De Angelis, M., Barkov, N.I. & Petrov, V.N. (1987): Aerosol concentrations over the last climatic cycle (160 kyr) from an Antarctic ice core. Nature, 325:318-321. FoUieri, M., Magri, D. & Sadori, L. (1988): 250,000-year pollen record from Valle di Castiglione (Roma). Pollen et Spores, 30: 329-356. Guest, J.E., Duncan, A.M. & Chester, D.K. (1988): Monte Vulture volcano (Basilicata, Italy): an analysis of morphology and volcanoclastic facies. Bull. Volcanol., 50: 244257. Hansen, R.B. (1991): Zur Bathymetrie der Lagi di Monticchio (Italien). EUROMAAR Report, 6-14; Trier (unpubl.). Pichler, H. (1989): Italienische Vulkan-Gebiete V. Sammlung geol. Ffihrer 83, 271pp. Watts, W.A. (1985): A long pollen record from Laghi di Monticchio, southern Italy; a preliminary account. J. Geol. Soc. London, 142: 491-499. Zolitschka, B. (1991): Absolute dating of late Quaternary lacustrine sediments by high resolution varve chronology. Hydrobiol., 214: 59-61.
A MULTIDISClYIANARY STUDY OF THE V I C O ]~La~AR SEQUENCE (LATIUM, ITALY): PART OF THE LAST CYCLE IN THE MEDITERRANEAN AREA. PRELIMINARY RESULTS.
P. FRANCUS*, S. LEROY*,I. MERGEAI*0, G. SERET*& G. WANSARD* *Pal4ontologie et Pal~og6ographie, UCL, Fl. L. Pasteur, 3, B-1348 Louvain-la-Neuve 0D6partement de G6ologie, Facult6s Notre-Dame de la Paix, B-5000 N a m u r
ABSTRACT Three boreholes have been drilled in the sediments of Vico Maar lake (North of Roma, Italy). A preliminary reconstruction of the climate change between the-last pyroclastic eruption (between 138 and 95 ka), and the end of the Pleniglacial is presented. The pollen diagram shows an interglacial period (Eenfian?), characterized b y a vegetation succession, followed by two steppe and two shorter forest periods. This new pollen diagram adds to the still sparse information on the palaeoenvironment of the Mediterranean area. A m u l t i d i s c i p l i n a r y a p p r o a c h details m o s t of this interglacial. A d d i t i o n a l palaeoenvironmental informations are: sedimentation, local biomass, temperature, water parameters - salinity, p H and lake level. The geochemical method of trace-elements on ostracod shells, recently developed, is tested by comparison with the other data. The Mgcontent in the shells seems to correlate with the indicators of palaeotemperature.
INTRODUCTION The Vico Maar lake is located at approximately 55 km north-northwest of Roma (fig. 1). It belongs to a belt of volcanoes stretching from Vesuvius in the South to Monte Amiata in the North (Laurenzi & Villa, 1987). The volcanism at Vico began 419 ka ago. A hydromagmatic explosion occurred 138 ka ago and formed the caldera. It was followed by an undated pyroclastic event, named "Tuff Finale". At 95 + 10 ka the Monte Venere lava cone grew within the caldera ending the volcanic activity in the area (Bertagnini & Sbrana, 1986; Laurenzi & Villa, 1985 and 1987). The surface of the present day Vico lake (~ 12 krn 2) lies at an altitude of 507 m a.s.1.. Its mean depth is 22 m, with a m a x i m u m of 50 m. The surrounding caldera walls reach an altitude of 800-900 m a.s.l, at its northern side and an altitude of 965 m a.s.1, at its western side. The easthern and the southern part of the wall are lower, at about 650 m a.s.1..
Lecture Notes in Each Sciences, Vol, 49 J. F. W. Negend~nk, B. Zolitschka (Eds.) Paleolimnology of European Maar Lakes Springer-Veriag Berlin Heidelberg 1993
290
The present vegetation on the inner slopes of the caldera is a forest of Quercus cerris. At higher altitude, on Monte Fogliano (965 m a.s.l.) and Monte Venere (838 m a.s.l.), a pure Fagus sylvatica forest, sometimes mixed with Q. cerris, is developed. Frank (1969) has studied a 780 crn long sediment core made on the shore of the present lake reaching back to the Br6rup Interstadial, according to the author. In December 1990 we drilled three adjacent boreholes (Lago di Vico: LVI, LVII and LVIII) located at about 300 m northwest of the present lake shore (fig. 1). They reached 22, 19, and 16 m depth. These cores provide a good opportunity to complete information on the last climatic cycle in the Mediterranean region from which few studies are reported (Wijmstra, 1969; Wijmstra et al., 1976; Watts, 1985; Follieri et al., 1988; Ports et al., 1988; ...).
Figure 1: Topographic map of Vico Maar and location of the drill holes.
291
In the first part of this paper, technical methods are described. In the second part, the preliminary pollen diagram and the organic carbon content curve obtained along the whole LV II borehole are discussed. The study, still in progress, records three steppic and three forested landscapes in the sediment. In a third part, most of the oldest forested landscape is studied in detail in the three holes from multidisciplinary analyses : sediment, pollen, diatoms and trace-elements in ostracods. A strong covariation between these independent parameters strengthens their palaeoclimatic significance.
1. METHODS The best sections of LVI, LVII and LVIII are used to build a composite depth below 12.50 m depth. Above this, depths are given from LVII.
1.1. Sedimentology. 350 organic carbon contents (org. C) are measured by chemical titration. The accuracy of the method is about 1%. Thin sections have been cut from resin hardened samples. The sporopollinic material is extracted according to Dricot & Leroy (1989). In the lower part of the sequence (16.37-12.79 m), samples are analysed every 3 cm. About 300 grains are counted per sample. In the upper part, the study is still in progress. The pollen grains are very well preserved, and the percentages of i n d e t e r m i n a t e d and indeterminable grains are low (mean of 5%). The A P / N A P curves (fig. 2 and 4) are calculated on a sum excluding the indeterminated and indeterminable grains. The individual curves (fig. 3) and the pollen concentration (fig. 4) are calculated on all the grains. The concentration in number of grains of pollen and spores per g of dry sediment is generally above 50,000 g r . / g , very high during the forested phases and lower during the steppe phases (fig. 4). Most of Quercus grains belong to the robur-type and not to the ilextype. The taxon Ulmus-Zelkova is used and probably includes both genera (Follieri et al., 1986). 1.3, Trace-elements in ostracods. Ostracods are small crustaceans occurring in nearly all ldnds of aquatic environments. They have calcitic valves. Ostracods grow incrementally by moulting. Turpen and Angel] (1971) have demonstrated that the calcium used in the lattice of the low-Mg calcite valves is only taken up from the water in which the animals live and only at the time of moulting. This process is fairly rapid, the full calcification being achieved within a few days. Thus, lacustrine environment recorded in the ostracod valves relates to conditions prevailing at the time of formation of a new shell. It is now common knowledge that Mg and Sr among other elements can be incorporated in trace within the calcium carbonate lattice during calcite formation. Recent studies
292
(Chivas et al., 1983, 1985, 1986) have experimentally established the relationship between the Mg and Sr trace-elements in shells and the properties of the host water controlling trace-element contents. These relationships are expressed by coefficients of partitioning, Kd
I1VI~ Kd
(M/Ca)ostracod shell T= ~ w a t ~ r
where M is Sr or Mg, and T is water temperature.
The authors demonstrated that the Sr-content in ostracods of the same genus is virtually temperature-independent and is related to the salinity of the water. In lakes having a well defined and restricted area (crater lakes for example), changes in the salinity of the water should only result from variations in the ratio evaporation/precipitation affecting the lake and its catchment (Chivas et al., 1985). Chivas et al. (1986) also showed in laboratory cultures the strong positive dependence on temperature and on salinity of Mg incorporated in ostracods of the same genus. The knowledge of the salinity (St-content) permits to deduce the contribution of the temperature to the Mg content. Thus geochemical analyses on ostracod shells provide a promising tool to reconstruct palaeosalinity and palaeotemperature changes in lacustrine environments. The whole sequence has been examined every 30 r to detect the presence of ostracods. Shells are extracted front core samples with 10% H202, washed on sieves and dried. The Ca, Sr and Mg analyses are performed on ostracod valves of Candona angulata. Each valve is cleaned in tri-distilled water and dried. Only very clean adult valves are taken, restricting the number of analyses. Valves are dissolved in 10 ml of tri-distilled water with 1% HNO3 Suprapur Merck. The solutions are analysed using a plasma emission spectrometer (D.C.P. Techmation@). Blanks are performed to deduce possible contaminant concentrations. 1.4. Diatoms. For each 1 g sample, the sediment is cleaned with hydrogen peroxyde and hydrochloric acid. The material is then washed in distilled water and mounted on glass microscope slide in Naphrax, a synthetic resin (refraction index 1.74). 500 diatom valves are counted for each sample.
2. PRELIMINARY RESULTS OF THE WHOLE SEQUENCE The lithological description, org. C data and preliminary pollen analyses provide first palaeoenvironmental indications of the whole sequence (fig. 2). The sediment below 16.37 m depth is a more than 6 meter thick pyroclastic formation essentially composed of lapilli and volcanic sands with some interlaying thin clayey layers, poor in pollen.
Figure 2: Lithological description, organic carbon content (smoothed on three points) and preliminary synthetic pollen diagram of the Vico sequence, LVLI depth.
294
Between 16.37 and 15.80 m depth, the sediment is a bedded organic mud. Pollen analyses record the transition of steppic conditions (73% NAP), with high percentages of Artemisia and Gramineae, to a diversified deciduous forest (87% AP without Pinus). This vegetation succession shows the whole transition to a palynological interglacial period: Zone 1, occurring between 16.37 and 14.17 m depth (fig 3). First appear the pioneer shrubs and trees: Hippophae and Betula. The first thermophilous tree, Quercus, indicates a forest starting at 16.10 m with values over 20 %, including some Fraxinus. Tilia develops at 15.85 m. Between 15.80 and 15.I0 m depth, the sediment is a non bedded black organic mud. Between 15.69 and 15.61 m depth, a sandy layer corresponds to a volcanic ash rich in clinopyroxenes, amphiboles, apatites, and euhedral zircons. This volcanic ash layer shows characteristic load casts, indicating a sedimentation on a still very fluid m u d . The alga Botryococcus is abundant. This Chlorophyceae alga forms colonies of plankton on ponds and lakes, often floating on the water. Abundant forms of planktonic and benthic lifes belonging to the photic zone (Pediastrum, Desmidiaceae, C h i r o n o m i d a e , Cladocera, Turbellaria, Rotifera, Porifera, ...) are present, pointing to a shallow portion of the lake. The highest org. C values (up tO 20%) occur at 15.32 m depth. The pollen diagram shows the development of a deciduous forest. Ulmus-Zelkova, then Carpinus, develops at 15.80 m. A diversified mesic forest dominated by Fagus appears at 15.77 m. It soon reaches a plateau from 15.77 m to 14.80 m depth. Well-preserved ostracods are only present in this part of the whole sequence. Between 15.10 and 10 m devth, the sediment becomes a bedded organic mud. Some grainsi~e analyses and thin sections show a very fine sediment with amorphous organic matter, plant remain layers and diatoms. The fraction smaller than 2 m m reaches 50%. The mean org. C is lower, around 6%. The last part of Zone 1 includes the culmination of Fagus percentages at 14.92 m. This vegetation indicates high precipitations without dry summer. The deciduous elements are then replaced by Abies with up to 35%. Thus the end of this palynological interglacial Zone 1 corresponds to a boreal forest. The temperature decreases. In short, th e following succession characterizes this palynological interglacial, Zone I (16.37 to 14.17 m) (fig. 3): Hippophae, Betula, Quercus, Ulmus-Zelkova, Carpinus, Fagus a n d
Abies. Zone 2 (14.17-14.03 m) displays a steppe development with 67% of NAP. The concentration decreases until 150,000 gr/g. Zone 3 (14.03-13.55 m) is a forested period showing a vegetation succession : Betula, Quercus, Ulmus-Zelkova, Picea (4%), Carpinus, Fagus a n d Abies (more than 50%). T h e m a x i m u m of AP, Pinus non included, is 82%. The associations contain Asphodelus. Very
295
Figure 3: Vegetation succession in % between 1650 and 1280 cm, composite depth.
296
t h e r m o p h i l o u s elements (Hedera, Tilia, Ilex a n d Buxus) are present. The pollen concentration is 350,000 gr./g. Zone 4 (13.55-13.32 m) is again a steppe vegetation. The maximum of NAP is 93 % and the concentration is 100,000 gr./g. Zone 5 (13.32 -13.12 m) is the last forested phase, characterized by a third type of succession : Betula, Quercus, Ulmus-Zelkova, Picea (7%), Carpinus, and Fagus. The m a x i m u m of AP without Pinus is 57 %. The most thermophilous taxa are Hedera, Tilia and Ilex. The pollen concentration is 300,000 gr./g. The Pleniglacial starts at the base of Zone 6 (13.12 m) with a steppe d o m i n a t e d by Gramineae, Artemisia, Pinus and Juniperus. Each forested period (Zones 1, 3 and 5) shows a different signature, excluding possibility of repetition. Zone 1 is characterized by the important development of Abies, Zone 3 by Abies and Picea and Zone 5 only by Picea. The successive three forested periods are increasingly cold because the vegetation succession is progressively weaker, with less thermophLlous elements and decreasing AP. The steppic Zones 2, 4 and 6 are increasingly dry, with higher NAP. The relative high altitude and the exposition to humid westerly winds probably give to the Vico forests an aspect comparable to the Middle European area (Le D e v ~ : Pons et al., 1990; Les Echets: Beaulieu et al., 1989; La Grande Pile: Woillard, 1978; Samerberg: Grfiger; 1979). They were dissimilar to the ones in other Mediterranean sites such as Padul in Spain (Pons et al., 1988), and Tenagi Philippon in Greece (Wijmstra, 1969; Wijmstra et al., 1976). Above 10 m depth, it is a clayey sediment with plant remains, mostly Drepanocladus, a moss living in shallow lakes and peat bogs. The sedimentation is mostly biogertic. The org. C is generally low (around 3%) but records a brief increase at 5.5 m depth (7%). Results from X-ray diffraction indicate a very low content in clay minerals, poorly crystallised illite and kaolinite. The pollen data display a very open landscape dominated b y Gramineae, Artemisia and Pinus with some increases of Picea (at 9.68, 8.28 and 7.38 m), Quercus (at 9.87, 7.38 and 5.57 m) and Fagus (at 7.38 m). Above 9.5 m depth, Isoetes is continuously present. The uppermost 1.3. m of the hole is an orange sediment showing some subvertical roots and corresponding to the modern pedogenesis. At 1 m depth, a sample obviously affected by oxydation shows high percentages of Corylus and Tilia.
Tentative comparison to other sequences From the provisory low resolution of the glacial period of our diagram (fig. 2), a correlation to the previous Vico diagram (Frank, 1969) is not available yet. The nearest site to Vico recording the last climate cycle is Valle di Castiglione (Follieri et al., 1988): 65 kin to the South and 44 m a.s.1. The Mediterranean character of the vegetation is well expressed except in Zone VdC-1Z This zone, characterized by a Fagus forest f o l l o w e d b y an Abies one, is the most similar to Zone 1 of Vico.
297
Botanical arguments are not available to correlate Vico to any other diagram. However, it is very tempting to attribute an Eemian age only from the general aspect of the A P / N A P curve : a long palynological interglacial period followed by two forested phases of similar importance showing a progressive deterioration of the climate, suggesting Saint-Germaln 1 (~518O stage 5(:) and Salnt-Germain 2 (8 18 O stage 5a). The pyroclastic layers, underlying Zone 1, belong either to the 138 ka event, or to the "Tuff Finale" (138-95 ka), or to the 95 ka last volcanic event. Zone 1 then correlates with either the Eemian, or the Saint-Germain 1, or the Saint-Germain 2. Further mineralogical data and 4~ / 39.A.a- datings are awaited.
3. MULTIDISCIPLINARY RESULTS FROM 16.37 TO 15.10 M A study of a part of the oldest forested landscape Zone 1, rich in ostracods, is made in details from the three holes.
3.1. Sedimentology The org. C is minimum just above the lapilli (fig. 4) and increases at 16.26 m. Above, org. C remains stable till 15.76 m. A first maximum (11%) occurs at the base of the unbedded black organic mud. Between 15.69 and 15.61 m depth, the dark m u d is interrupted by a sandy volcanic layer affected by load casts. A second m a x i m u m of org. C is reached at 15.32 m depth (20%), after a steady increase. Above 15.32 m, the org. C is decreasing. Therefore, an apparently homogeneous black sediment provides sharp variations in org. C. Thin sections between 15.35 and 15.10 m depth show a diatomite rich in organic matter. Small (about 10 ~ n ) and few (less than 1%) minerals are present. At higher magnification, an horizontal preferential orientation is perceptible as well as some w e a k compaction structures. The thin sections confirm significantly undisturbed biogenic sediment.
3.2. Pollen analyses The progressive increase of the forest cover is interrupted by a short fluctuation between 16.00 and 15.92 m depth with the decrease of Quercus and the correlative increase of Artemisia and Gramineae (fig. 3 and 4). The pollen concentration is not affected. Above 15.92 m Cnigh values of Quercus) the concentration is really increasing. Above 15.74 m, the pollen concentration is higher than 1.106 gr./g, corresponding to the development of the mesic diversified forest with Fagus. Later on, the m a x i m u m of pollen concentration with more than 2. 106 g r . / g is reached at 15.32 m (fig 4). The accurate position of the climatic optimum can not be seen in the pollen diagram because of the relative stability of the mesic forest. The m a x i m u m of 4 very
298
thermophilous taxa (Buxus, Tilia, Ilex and Hedera.) is at 15.42 m depth. They are indicators of a temperature optimum.
Figure 4: Comparison of the various data, between 1637 and 1510 cm, composite depth. Pollen concentration in million gr./g.
299
3.3. Trace-elements in ostracod shells Ostracods are only present b e t w e e n 15.90 and 15.12 m depth (fig. 4). Between 15.90 an d 15.83 m depth, small scarce valves are b a d l y preserved. They can not be identified. Between 15.83 and 15.12 m depth, valves are well preserved.
Candona angulata is l a r g e l y d o m i n a n t . Scarce p r e s e n c e of Darwinula stevensoni a n d Ilyocypris is observed. The occurrence of D. stevensoni is indicative of s h a l l o w w a t e r (0-10 m depth) (Devoto, 1965). The Sr-content (fig. 5 a n d table 2) s h o w s f e w fluctuations, i n d i c a t i n g a n e a r l y c o n s t a n t salinity of the water d u r i n g the analysed interval. The w a t e r ch em i st r y of the p r esen t d a y lake indicates a water w i t h a salinity around 0.15%~ (table 1).
[ p.p~n
24
0.263
22
26
17
<5
Table 1: Water chemistry of the present d a y lake (sampled in D e c e m b e r 1990).
Composite depth (cm) 1514 1534 1534 1537 1537 1544 1545 1545 1545 1552 1552 1559 1559 1571 1571 1571 1573 1573 1573 1573 1573 1573 1573 1579 1581
2 fragments 2 valves 1,3 valves 1 valve 2 fragments 1,5 valves 2 valves 2 valves 2 valves 2 valves 2 valves 2 valves 2 valves 2 valves 1 valve 2 valves 2 valves 2 valves 2 valves 2 valves 2 valves 2 valves 2 valves 2 valves I valve
..Ca~tg 16.27 28.22 21.53 11.43
Srpg 0.077 0.I45 0,093 0.052
Mg~g 0.068 0.135 0.095 0.050
Sr/Ca at. Mg/.Ca at 0.0022 0.0069 0.0023 0.0079 0.0020 0.0073 0.0021 0.(J071
18.44
0.o68
o.o91
o.oo17
o.0081
' "21.32 33.68 28,94 33.78 28.12 30.49 39.76 38.73 27.50 18.23 37.70 ' 60.15 50.57 57.89 40.38 44.29 45.84 44.91 "38.73 11.43
0.I03 0.157 0.135 0.155 0.129 0,142 0.209 0.199 0.124 0.074 0.159 0.270 0.238 0.266 0.167 0.214 0.197 0.219 0.148 0.051
0,037 0.158 0.129 0.126 0.116 0.132 0.142 0.133 0.146 0.088 0.122 0.197 0.176 0.196 0.102 0.135 0.160 0.148 0~132 0.037
0.0022 0.0021 0.0021 0.0021 0.0021 0.0021 0.0024 0.0023 0.0021' 0.0018 0.0019 0.002'1 0.0021 0.0021 0.0019 0.0022 0.0020 0.0022 0/)017 0.0020
0.0068
0.0077 0.0073 0.0061 0.0068 0.0071 0.0059 0.0057 0.0088 0.0080 0.0053 0.0054 0.0057 0.0056 0.0042 0.0050 0.0057 0.0054 0.0056 0.0054
Table 2: Chemical analyses of Candona angulata valves f r o m Vico s e q u e n c e . S r / C a and M g / C a ratios are expressed as atomic ratio. Analyst : A. Iserentant.
300
Using the disizibution coefficient in Candona for Sr (Kd (Sr) = 0.406, according to Engstrom and Nelson, 1991) and a 0.0021 value as mean Sr/Ca ratio in ostracods of the analysed interval, the calculated Sr/Ca ratio in the water is 0.0052, very near to the m e a s u r e d 0.0050 present day lake. So the salinity of this interval seems similar to the modern one. The Mg-content fluctuations (fig. 4 and table 2) can mostly be linked to the temperature of the water due to the constant salinity during the same interval. From 15.81 to 15.73 m depth, M g / C a ratios are low. A first m a x i m u m of Mg-content occurs at 15.71 m. It ~s followed by a m i n i m u m at 15.59 m. Above 15.59 m depth, the M g / C a ratio increases reaching the maximum at 15.37 m. At 15.14 m depth, Mg-content is slightly lower.
3.4. Diatoms
The diatom study of Vico sediments provides three palaeoecological data (fig. 5): the waterlevel, the p H and the salinity of the water.
3.4.1. Waterlevel A clear change in the diatom assemblage appears at 16.10 m: the lowest part of the sequence is dominated by epiphytic diatoms (up to 85%). The encountered species essentially are microcephala (K~tzing) Cleve (up to 83%), Nitzschia hantzschiana Rabenhorst, Navicula cintra (Ehrenberg) Ralfs and Epithemia argus (Ehrenberg) KLitzing. Above 16.10 m depth, the planktonic diatoms Cyclotella kutzingiana (?) Thwaites and Fragilaria brevistriata Grunow dominate with percentages of 80%-90%. This change in the diatom flora dearly indicates a rise of the lake level.
Achnanthes
3.4.2. Water p H The p H curve is calculated from the formula Renberg/Hellberg CRenberg & Hellberg, 1982) that take into account the percentage of neutrophilous, acidophilous, acidobiontic, alkaliphilous and alkalibiontic diatoms: I
pH= 6,40-0,85 Log B index
I
.. % neutroph Ious.,,6 r acidoDhlous~4 0 (=/oacidob bn t~c) w l h B ~oex..-=, ~'oneutrophlous43,5 (Yoalkaliph Iou s y l 0 8 ('/~ a~,alib bntic)
The lowest part of the sequence is characterized by the abundance of neutrophilous diatoms: Achnanthes microcephala (K~itzing) Cleve, Nitzschia hantzschiana Rabenhorst, Navicula cincta (Ehrenberg) Ralfs and Epithemia argus (Ehrenberg) K(itzing. The acidophilous diatoms represent up to 25% of the assemblage. A b o v e 16.10 m, the
301
acidophilous diatoms decrease (0-5%) and the alkaliphilous diatoms Cyclotella kutzingiana (?) Thwaites and Fragilaria brevistriata Grunow dominate (up to 100%) (fig. 5). Below 16.10 m depth, calculated p H values oscillate around 6.5; above 16.10 m depth, the pH values are just below 7. At 15.75 m depth, a p H peak of 8 is.noteworthy. Above 15.60 m depth, the pH becomes more alkaline with values around 9.
Figure 5: Palaeoenvironmental data of the lake water from diatom study and Sr-content in ostracod shells, between 1637 and 1510 cm, composite depth.
302
3.4.3. Water s..ali.n.jty The majority of the species (90% to 100%) in the Vico sediments are oligohalobous diatoms (freshwater diatoms; total salinity of water < 2 ~'~): Achnanthes raicrocephala (K~tzing) Cleve, Nitzschia hantzschiana Rabenhorst, Navicula cincta (Ehrenberg) Ralfs and Epithemia argus (Ehrenberg) Kiitzing, Cyclotella kutzingiana (?) Thwaites and Fragilaria brevistriata Grunow. The salinity is very low and constant during the analysed interval. 3.5. General Discussion
The whole interval (16.37-15.10 m) shows a transition from a steppe to a forested landscape and a climatic optimum. The salinity of the water is permanently low, as indicated by the percentages of oligohalobous diatoms and St-content in ostracods. Between 16.37 and 16.10 m depth the sediment is a bedded organic mud. The landscape is dominated by steppe vegetation (Artemisia and Gramineae) with shrubs and some small trees. The water level is low. First values of org. C above 4% are reached at 16.26 m corresponding to the development of the first trees in the caldera and to the transition from Hippophae to Betula. Above 16.I0 m depth the sediment is still a bedded organic mud. A Quercus forest progressively develops. Planktonic diatoms are replacing the epiphytic ones indicating a relative rise of the waterlevel. However at the drill site, the lake r e m a i n s relatively shallow, not much deeper than 10 m as suggested by the presence of Chara and the ostracod Darwinula stevensoni. After this period, the water level seems to remain constant. .Above 15.82 m depth the sediment becomes a microscopically bedded black mud. The pH becomes alkaline. The ostracods are now well preserved. It seems that the type of the sediment could explain the preservation of ostracods. A deciduous Fagus forest rich in thermophilous taxa develops. An increase of is observed at (Mg/Ca) rises water. Botryoccocus is
the biomass, illustrated by higher values of org. C and pollen concentration, about 15.75 m and corresponds to high p H values. The water temperature above 15.71 m. The biomass seems to increase before the w a r m i n g of the extremely abundant, possibly indicating eutrophication.
Between 15.69 and 15.61 m d e p t h the decrease of org. C and pollen concentration corresponds to a dilution by a coarser sediment, a volcanic ash layer. The p H also decreases. Fagus is receeding, giving place to Pinus and Carpinus. Above 15.60 m depth org. C, pollen concentration, M g / C a ratio and the p H are increasing again up to 15.32 m depth which reflects the climatic o p t i m u m of the whole Zone 1. During the improvement, an interruption occurs at 15.42 m depth in all the data, except for the thermophilous pollen grains. Above 15.30 m depth a general decrease of org. C, pollen concentration and M g / C a ratio Occurs.
303
The optimum of the palynological interglacial Zone 1 therefore appears early, before the .end of its first half.
CONCLUSION Vico lake sediment is fine-grained providing a continuous high resolution record for several disciplines. The new pollen diagram completes a still too sparse climate change information on the Mediterranean area. Three different attempts of stratigraphical correlations are possible and still in progress for the interglacial Zone 1, followed by forested Zones 3 and 5. Zone 1 could be correlated either with the Eemian, or the Saint-Germain 1, or the Saint-Germain 2. The upper part of the sequence reaches the end of the Pleniglacial. In Zone 1, the fluctuations of supraregional and regional vegetation, local biomass, water parameters (temperature, salinity, pH and lake level), and sedimentation are analyzed in details. The covariation of independent parameters allows more accurate climatic change interpretations: minor climate fluctuations are recorded and the climatic optimum, early in the palynological interglacial, is particularly better defined. The geochemical method of trace-elements in ostracod shells is tested by comparison with other data. In a constant low salinit)r water, the Mg-content in shells seems an usefull indicator of temperature.
Acknowledgement We are grateful to the technical team of Laboratoire de Palc~ontologie et Pal6og6ographie and especially to A. Lannoye and M. Bravin for the drilling and to M.-J. Goor-Detinne for laboratory assistance. We thank Prof. M. Follieri for introducing us to the site and for the drilling authorizations. Prof. J.L. De Sloover has determined the moss leaves. A. Iserenta~t has kindly and carefully realized the geochemical measures on ostracod shells. The Belgian state - Prime Minister's Service- Science Policy Office - has supported this research, thanks to "Impulse Programme Global Change".
Bibliography Beaulieu, J.-L. de & Reille, M. (1989): The transition from temperate phases to stadials in the long Upper Pleistocene sequence from Les Echets (France). Palaeog. Palaeocl. Palaeoec., 72: 147-159. Bertagnini, A. & Sbrana, A. (1986): I1 vulcano di Vico : stratigrafia del complesso vulcanico e sequenze eruttive delle formazioni piroclastiche. Mere. Soc. Geol. It., 35: 699-713. Chivas, A.K, De Deckker, P. & Shelley, J.M.G. (1983): Magnesium, strontium and barium in nonmarine ostracode shells and their use in paleoenvironmental reconstruction-a
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preliminary study. Applications of Ostracoda (R.F. Maddocks, ed) Univ. Houston Geosc.: 238-249. Chivas, A.R., De Deckker, P. & Shelley, J.M.G. (1985): Strontium content of ostracods indicates lacustrine palaeosalirdty. Nature, 316: 251-253. Chivas, A.R., De Deckker, P. & Shelley, I.M.G. (1986): Magnesium content of non-marine ostracod shells : a new palaeosalinometer and palaeothermometer. Palaeog. Palaeocl. Palaeoec., 54: 43-61. Devoto, G. (1965): Lacustrine Pleistocene in Lower Liri valley. Geol. Romana, W: 291-368. Dricot, E. & Leroy, S. (1989): Peptization and sieving for palynological purposes. Geobound 2: 114-126. Engstrom, D. IL & Nelson S. R. (1991): Paleosalinity from trace metals in fossil ostracodes compared with observational records at Devils Lake, North Dakota, USA. Palaeog. Palaeocl. Palaeoec., 83: 295-312. Follieri, M., Magri, D. & Sadori, L. (1986): Late Pleistocene Zelkova extinction in Central Italy. New Phytologist, 103:269 - 273. Follieri, M., Magri, D. & Sadori, L. (1988): 250,000-year pollen record f r o m Valle di Castiglione (Roma). Pollen et Spores, 30, 3/4: 329-356. Frank, A. (1969): Pollen stratigraphy of the lake of Vico (Central Italy). Palaeog. Palaeocl. Palaeoec., 6: 67-85. Grfiger, E. (1979): Die Seeablagerungen vom Samerberg/Obb. und ihre Stellung im Jungpleistoz~. Eiszeitalter u. Gegenwart, 27 : 23-34. Laurenzi, M.A. & Villa, I.M. (1985): K/Ar chronology of the Vico volcano (Latium, Italy). Abstracts and time schedule of IAVCEI, 1985 Scientific assembly, Giardini - Naxos, Italy. Laurenzi, M.A. & Villa, I.M. (1987): 40Ar / 39Ar chronostratigraphy of Vico ignimbrites. Per. Mineral., 56: 285-293. Pons, A., Beaulieu, J.-L. de, Guiot, ]. & Reille, M. (1990): Vingt cinq ans de recherche en analyse pollinique. Ecologia Mediterranea, 16: 169-I93. Pons, A. & Reille, M. (1988): The Holocene and Upper Pleistocene pollen record from Padul (Granada, Spain): a new study. Palaeog. Palaeocl. Palaeoec., 66: 243-263. Renberg, I. & Hellberg, T. (1982): The pH history of lakes in south-western Sweden, as calculated from the subfossil Diatoma flora of the sediments. Ambio II, 1: 30-33. Turpen, J.B. & Angell R.W. (1971): Aspects of molting and calcification in the ostracod Heterocypris. Biol. Bull., 140: 331-338. Watts W. (1985): A long pollen record from Laghi Monticchio, s o u t h e r n Italy: a preliminary account. ]. geol. Soc. London, 142: 491-499. Wijmstra, T. (I969): Palynology of the first 30 meters of a 120 m deep section in Northern Greece. Acta Bot. Neerl., 18, 4: 511-527. Wijmstra, T. & Stair, A. (1976): Palynology of the middle part (30-78 metres) of the 120 m deep section in Northern Greece (Macedonia). Acta Bot. Neerl., 25, 4: 297-312. Woillard, G. (1978): Grande Pile peat bog: a continuous pollen record for the last 140,000 years. Quat. Res., 9, 1: 1-21.
ENVIRONMENTAL GEOLOGY AND GEOCHEMISTRY OF LAKE SEDIMENTS (HOLZMAAR, EIFEL, GERMANY)
B.G. Lottermoser*, R. Oberh~insli*, B. Zolitschka #, J.F.W. Negendank #, U. Schfitz* & J. Boenecke* *Institut ffir Geowissenschaften, Universit/it Mainz, Postfach 3980, W-6500 Mainz, Federal Republic of Germany #Fachbereich Geographie/Geowissenschaften, Universit~it Trier, Postfach 3825, W-5500 Trier, Federal Republic of Germany
ABSTRACT Quaternary sediments of the lake Holzmaar comprise a well laminated, undisturbed sequence of diatomaceous gyttja, silt/clay laminites and tuff layers. The annually deposited gyttja has been dated by varve chronology yielding a continuous high-resolution time sequence for the Holocene and early Pleistocene. These organic-rich sediments provide an unique paleolimnological record on climatic changes, forest fires, volcanic eruptions, and anthropogenic influences on the sediment composition. Volcanic activity produced thin tephra layers (Laacher See Tephra 11,200-k_120 radiocarbon yrs BP; Ulmener Maar Tephra 9435+70 radiocarbon yrs BP) in the sedimentary column which possess elevated Ba, Sr, Zr, Y and Nb ~ Rb, Ga) values, and heavy metal contents broadly similar to the enclosing gyttja sediments. In contrast, anthropogenic additions of heavy metals occurred in the upper most varvites deposited since the end of the 18th century. These high metal values are interpreted as disturbances of the natural heavy metal cycles due to fossil fuel combustion and processing of geological ores in the industrial and metropolitan areas of central Europe and associated regional anthropogenic metal pollution of the atmosphere. L~cture Notes in Earth Sciences, VoL 49 J. F, W, Negendank, B. Zolitschka (Eds.) Palr of European Maar Lakes 9 Springer-Verlag Berlin Heidelberg 1993
306
INTRODUCTION Lake sediments provide a paleolimnological record on physical, chemical and biological sedimentation processes including long-term v a r i a t i o n s in sedimentation rates and instantaneous historical events such as forest fires and volcanic eruptions. In addition, geochemical investigations of these sedimentary materials can reveal anthropogenic caused disturbances of natural geochemical cycles. Increases in heavy metal and organic pollutant loads have been detected by several authors in recent lake sediments of North America and Europe. Down core pollutants profiles coupled with age determinations provide detailed information on historical changes in anthropogenic pollutant inputs into the environment (MARC, 1985). However, observed heavy metal increases in sediment samples may not only be caused by anthropogenic pollution, but also by weathering of lithologies with variable chemical compositions, by forest fires and associated metal release, by efficient synsedimentary metal concentration in organic matter, by volcanically derived metal inputs or by postsedimentary mobilisation processes. Lake Holzmaar (West Eifel, Germany) contains a laminated sequence of Holocene and late glacial diatomaceous gyttja and of glacial clay/silt laminations (Negendank, 1989; Zolitschka, 1989; Negendank et al., 1990) (Fig. 1). Holocene sediments consist of annually laminated deposits (organic varves) which were used to establish a high-resolution varve chronology (Zolitschka, 1991). Thus lake Holzmaar provides the essential conditions necessary for the incorporation and fixing of heavy metals in sediments: reducing conditions, a non-turbulent environment, steady deposition, and the presence of suitable fine-grained particles for metal fixation such as clay minerals, Fe/Mn compounds and organic matter. Sediments from lake Holzmaar represent ideal environmental samples for historical monitoring of anthropogenic impacts on ecosystems (cf. Zolitschka, 1992). The presented geological and geochemical data will a d d to the understanding of d o w n core heavy metal and trace element profiles in Quaternary lake sediments. Mineralogically the coarse fraction of Holzmaar sediments is d o m i n a t e d by quartz, feldspar, mica and fragments of country rocks (sand-/siltstones, graywackes, shales, quartzites). Heavy minerals of this fraction are related to the tuff ring of the Holzmaar and to the Laacher See Tephra. Olivine, augite and
307
Fig.1.
Lithology of Late-Glacial and Holocene sediments from Lake Holzmaar with varve-dated time scale.
308
hornblende are the most common heavy minerals. The fine fraction consists of quartz, opal (diatoms), chlorite, illite and authochthonous calcite, siderite and vivianite. INFLUENCE OF VOLCANIC ACTWITIES Thin tephra layers in the basal parts of the Holzmaar section can be assigned to the Laacher See Tephra and the Ulmener Maar Tephra (Fig. 1). Both tephra horizons derive from volcanic eruption centers which are located in the immediate NE vicinity of the Holzmaar: Laacher See 40 km, Ulmener Maar 13 km. The two tephra layers from these late Quaternary volcanoes produced major changes in the trace element profile of the sedimentary column: Ba, Sr, Zr, Y and ~NPo(+ Rb, Ga) values are elevated, yet their heavy metal contents are broadly similar to the values of the enclosing gyttja sediments (Fig. 2). The lower volcanoclastic layer in the Holzmaar section represents the Laacher See Tephra (11,200!-_120 radiocarbon yrs BP). The Laacher See volcano produced various, chemically distinct tephra layers of different regional distributions (W6rner and Schminke, 1984; Bogaard and Schminke, 1985). Trace elements contents of the crystal rich phonolitic pumice in the Holzmaar correspond to the trace element data of the Upper Laacher See Tephra (ULST) (Whrner and Schminke, 1984). Thus despite the relatively short distance (40 km) between Holzmaar and Laacher See only remnants of the last phreatomagmatic eruptions are found in the Holzmaar section. The initial plinian eruption (LLST) shows distinctly different trace element distributions and it is not recorded in the Holzmaar. This is in agreement with paleowind data d o c u m e n t e d by Bogaard and Schminke (1985), which indicate that only products of the latest Laacher See eruption (ULST) were transported in SSW to SW directions. A second volcanoclastic horizon in the sedimentary sequence, the Ulmener Maar Tephra, was radiocarbon dated to 9435+70 conventional radiocarbon years BP and varve dated to 10,020 varve years BP (Zolitschka et al., 1991). These age determinations coincide with earlier radiocarbon datings by Biichel and Lorenz (1982). They obtained a wide time interval for the Ulmener Maar Tephra from 7,000 to 11,000 years BP due to postdepositional contaminations. Thus the wide time span and the elevated trace element contents of Ulmener Maar Tephra layers could be due to postsedimentary mobilisation processes.
309
Fig. 2.
Selected profiles of total trace element contents detected in Holzmaar lake sediments. UMT: Ulmener Maar Tephra; LST: Laacher See Tephra.
310
There is no published information on the chemical composition of the Ulmener Maar volcanoclastics and thus leucitite and nephelinite data (Mertes and Schminke, 1985) compiled for the Westeifel maars are taken to represent the Ulmener Maar Tephra. Trace element contents of the volcanoclastics in the Holzmaar largely correspond to the inferred chemistry of the Ulmener Maar pyroclastics, considering that samples from the Ulmener Maar Tephra horizon are to some extent a mixture of volcanoclastic material and gyttja sediments. Thus major amounts of Ba, Sr, Zr, Y and Nb (+ Rb, Ga) were introduced into the sedimentary column by magmatic activities originating from the Laacher See and Ulmener Maar volcanoes. Concentrations of Cr and Ni in recent sediments are similar to those of the Ulmener Maar Tephra (Fig. 2). Devonian sediments exposed a r o u n d the Holzmaar also possess Cr values similar to the gyttja sediments. In contrast, the Ni contents of the Ulmener Maar Tephra and of the recent sediments are higher than those detected in the Devonian schists. The Zn contents in recent sediments are about two times the values found in Ulmener Maar volcanoclastics and by a factor of six higher than in the Devonian country rocks. Thus an additional, external input of heavy metals into the most recent sediments is indicated by this trace element profile. ANTHROPOGENIC HEAVY METAL POLLUTION Leaching of sediments was performed using a H 2 0 2 - H N O 3 mixture and the leached fractions were subsequently analysed by atomic absorption spectrometry. The H202-HNO3 treatment was intended to extract metals from the sediments which were transported into the lake in a dissolved form and subsequently incorporated into organic material. Metals in the sediments orginating from the physical weathering of surrounding country rocks were to be excluded. Similar H 2 0 2 - H N O 3 extractions were performed by other authors in order to destroy organic material in sediments and to liberate metals attached to these particles for further analysis (cf. Papp et al., 1991). Heavy metal (Cu, Cr, Pb, Ni, Zn) stratigraphies for the upper most Holzmaar sediments are shown in Fig. 3. Results depict similar depth distributions for the heavy metals Cu, Pb, Ni and Zn. The similarities include: (a) significant accumulations and peak concentrations located in the upper most layers; (b)
311
Fig. 3.
Profile of heavy metal contents in the oxidizable sediment fraction and of organic carbon concentrations in the Holzmaar varvites.
312
variable metal values throughout the consolidated sediments; and (c) distinctly low metal contents in sediments deposited during the last glacial. For the elements Cu, Ni, Pb, and Zn which decrease in concentration with depth, reduced element contents generally occur at depths greater than 57 to 81cm. These depths correspond to sediments deposited in the lake about 145 to 205 varve yrs BP according to varve chronology. Significantly lower element concentrations are reached in sediment samples with low organic contents taken from the lower most part of the sequence representing s e d i m e n t deposited during the last glacial. The heavy metal contents correlate positively with the amount of C o r g present within the various samples (Fig. 4). Organic constituents are important agents for metal mobilisation, transport and concentration. Associations of metals with organic phases of soils and terrestrial and marine sediments of recent to ancient age have been recognized for some time and have been reported from algal mats, oil shales, black shales, peat, and coals (e.g., Saxby, 1967; Parnell, 1988; Disnar and Sureau, 1990). High metal concentrations in organic-rich s e d i m e n t s have generally been interpreted as the result of efficient scavenging mechanisms of metals from the water column onto organic matter. This strongly suggests that the a m o u n t of organic matter influences the a m o u n t of metals within the accumulating varvites. It has generally been accepted that heavy metal stratigraphies represent accurate paleolimnological records due to the low solubility of most of their chemical forms and their tendency to enter into complexation and sorption reactions with solids. However, postdepositional mobilisation of metals may especially occur due to bioturbation, distinct changes in the redox conditions, bacterial and h y d r o t h e r m a l activities, and acid lake waters. Some of these mobilisation processes can be excluded for the Holzmaar sediments as the samples do not exhibit bioturbation or features of hydrothermal fluid/sediment interactions and the present lake waters have near neutral pH values. Thus the detected heavy metal profiles represent a paleolimnological record of changing heavy metal inputs into the lake system. The concentrations of metals within the upper sedimentary layers represent substantial accumulations of these materials. The increases in heavy metals loads
313
Fig. 4.
H e a v y metal contents of oxidizable sediment fractions versus organic carbon concentrations.
314
are interpreted as historical loading of metals to the lake system. Local pollution sources such as factories or wastewater releases are absent and therefore cannot account for the observed metal increases. The last peak of iron smelting in the Eifel mountains occurred from the middle of the 18th century until 1815 when Prussians regained this territory from Napoleon's troops and the early industrial iron processing collapsed. During most of the 19th and the 20th centuries more distant sources are responsible for heavy metal pollution of the Lake Holzmaar. Sources of heavy metals very likely include the combustion of fossil fuels and industrial smelting of raw materials originating from the m e t r o p o l i t a n and industrial areas of Luxembourg, Belgium, France (Lorraine) a n d G e r m a n y (Rhine-Ruhr area). Heavy metals were released from raw materials during industrial processing, manufacturing and fossil fuel c o n s u m p t i o n into the atmosphere, transported via aerosols to the Holzmaar and s u b s e q u e n t l y deposited in accumulating varved sediments. Increases in pollutant loads have been detected by other authors in estuarine, river and lake sediments of central Europe. These studies indicate that pollutant fluxes into the geological reservoirs began with the latter half of 19th century (F6rstner and Mfiller, 1974; Miiller et al., 1977; Salomons and De Groot, 1978; Salomons and Mook, 1980; Oldfield et al., 1980; Vernet and Favarger, 1982; Matschullat et al., 1987). In contrast, results of this study show that anthropogenic Pb and Zn release into the atmosphere and associated regional pollution occurred in central Europe since the end of 18th century, whereas anthropogenic Cu and Ni pollution is detectable since the beginning of 19th century.
CONCLUSIONS Quaternary sediments of the Holzmaar comprise a well laminated sequence of diatomaceous gyttja, tuff layers and silt/clay laminites. This sedimentary column provides a paleolimnological record on long-term variations in sedimentation rates and climatic conditions, instantaneous historical events such as forest fires and volcanic eruptions, and anthropogenic and volcanically caused disturbances of natural geochemical cycles. Geochemical analyses of these environmental samples indicate significant inputs of Ba, Sr, Zr, Y and Nb (+ Rb, Ga) from volcanic tephra layers into the
315
sedimentary column and significant inputs of Cu, Pb, Ni and Zn in sediments deposited since the end of the 18th century. The anthropogenic input of heavy metals originates from iron ore smelting in the Eifel and from industrial and metropolitan areas of central Europe (Luxembourg, Belgium, France, Germany). Combustion of fossil fuels and industrial processing and manufacturing led to the release of metals into the atmosphere. The mobilized metals were subsequently transported via paleowinds to the Holzmaar region and introduced into the Holzmaar lake system. The results indicate that regional, anthropogenic heavy metal pollution began in central Europe since the end of the 18th century.
REFERENCES Boogard, P. and Schminke, H.-U., 1985. Laacher See Tephra" A widespread isochronous late Quaternary tephra layer in central and northern Europe. Geol. SOc. Amer. Bull., 96: 1554-1571. Bfichel, G. and Lorenz, V., 1982. Zum Alter des Maarvulkanismus der Westeifel. N. Jb. Geol Pal~iont. Abh., 163: 1-22. Disnar, J.R. and Sureau, J.F., 1989. Organic matter in ore genesis: progress and perspectives. Org. Geochem., 16: 577-599. F6rstner, U. and Mfiller, G., 1974. Schwermetallanreicherungen in datierten Sedimentkernen aus dem Bodensee und aus dem Tegernsee. Tschermaks Min. Petr. Mitt., 21: 145-163. F6rstner, U., 1986. Metal speciation in solid wastes - factors affecting mobility. In: Speciation of Metals in Water, Sediment and Soil Systems, L. Landner (Ed.), Lecture Notes in Earth Sciences, 11, Springer Verlag, Berlin, pp. 13-41. MARC (Monitoring and Assessment Research Centre), 1985. Historical Monitoring. Technical Report No. 31, University of London, London, 313pp. Matschullat, J., Heinrichs, H., Schneider, J. and Sturm, M., 1987. Schwermetallgehalte in Seesedimenten des Westharzes (BRD). Chem. Erde, 47: 181-194. Mertes, H. and Scminke, H.-U., 1985. Mafic potassic lavas of the Quaternary West Eifel volcanic field. I. Major and trace elements. Contrib. Mineral. Petrol., 89: 330-345. Mfiller, G., Grimmer, G. and B6hnke, H., 1977. Sedimentary record of heavy metals and polycyclic aromatic hydrocarbons in Lake Constance. Naturwissenschaften, 64: 427-431. Negendank, J.F.W., 1989. Pleistoz/ine und holoz~ne Maarsedimente der Eifel. Z. dr. geol. Ges., 140: 13-24. Negendank, J.F.W., Brauer, A. and Zolitschka, B., 1990. Die Eifelmaare als e r d g e s c h i c h t l i c h e Fallen u n d Quellen zur R e k o n s t r u k t i o n des Pal~oenvironments. Mainzer geowiss. Mitt., 19: 235-262.
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Oldfield, F., Appleby, P.G. and Petit, D., 1980. A re-evaluation of lead-210 chronology and the history of total lead influx in a small south Belgian pond. Ambio, 9: 97-99. Papp, C.S.E., Filipek, L.H. and Smith, K.S., 1991. Selectivity and effectiveness of extractants used to release metals associated with organic matter. Applied Geochem., 6: 349-353. Parnell, J., 1988. Metal enrichments in solid bitumens: a review. Mineral. Deposita, 23: 191-199. Salomons, W. and De Groot, A.J., 1978. Pollution history of trace metals in sediments, as affected by the R h i n e River. In: E n v i r o n m e n t a l biogeochemistry and Geomicrobiology, Vol. 1. The Aquatic Environment, W.E. Krumbein (Ed.), Ann Arbor Sci. Inc., Michigan, 149-162. Salomons, W. and Mook, W.G., 1980. Biogeochemical processes affecting metal concentrations in lake sediments (Ijsselmeer, The Netherlands). Sci. Total Environ., 16: 217-229. Saxby, J.D., 1967. Metal-organic chemistry of the geochemical cycle. Rev. Pure Appl. Chem., 19: 131-150. Vernet, J.P. and Favargar, P.-Y., 1982. Climatic and anthropogenic effects on the sedimentation and geochemistry of Lakes Bourget, Annecy and L6man. Hydrobiolo~a, 92: 643-650. W6rner, G. and Schminke, H.-U., 1984. Mineralogical and chemical zonation of the Laacher See Tephra sequence (East Eifel, W. Germany). J. Petrol., 25: 805835. Zolitschka, B., I989. Jahreszeitlich geschichtete Seesedimente aus dem Holzmaar und dem Meerfelder Maar. Z. dt. geol. Ges., 140: 25-33. Zolitschka, B., 1991. Absolute dating of late Quaternary lacustrine sediments by high resolution varve chronology. Hydrobiol., 214: 59-61. Zolitschka, B., Brauer, A., Haverkamp, B., Heinz, T., Negendank, J.F.W. and Poth, D., 1991. Sedimentologischer Nachweis und D a t i e r u n g einer frLihholoz/inen Maareruption (Ulmener Maar?) in der Vulkaneifel. In: Symposium on paleolimnology of maar lakes, B. Zolitschka & J.F.W. Negendank, eds., Bitburg, May 1991, abstracts, p. 63. Zolitschka, B., 1992. Human history recorded in the annually laminated sediments of lake Holzmaar, Eifel Mountains, Germany. Geol. Surv. Finnland, Spec. Paper 14: 17-24.
GEOCHEMISTRY OF LAGO GRANDE DI MONTICCHIO, S. ITALY
C. Robinson, G.B. Shimmield & K.M. Creer Dept. of Geology and Geophysics, Edinburgh University, King's Buildngs EDINBURGH EH9 3JW
ABSTRACT This account describes the results of bulk geochemical analysis on the uppermost 15m of a 51m profile obtained from Lago Grande di Monticchio. This section is thought to extend from historic times back into the late Pleistocene, hence including the glacialHolocene transition. One of the clearest changes seen occurs in the organic carbon content of the sediment, rising from modest values in lower parts of the core to values as high as 30wt% at around 8m depth. In this zone, thought to be correlated with the glacial-Holocene transition, other elements vary directly (Br) or inversely (A1, Y) with organic carbon concentration. Multivariate techniques such as principal components analysis have been used to help identify these elemental associations. Such groupings are suggested as reflecting different contributors to the sediments (eg. plant matter, clays, residual minerals.) The stable isotopic composition of organic carbon displays something of a shift towards lighter values across the glacial-Holocene transition zone, but the profile as a whole is difficult to interpret without further information on the nature of the organic matter. Several elements (P, Mn, Mo) exhibit enrichments in discrete temporal zones and it is thought that these are indicative of diagenetic processes occuring in the sediment.
INTRODUCTION Lake sediments can provide a detailed record of environmental change. The physical and chemical properties of the sediment reflect developments in the lake ecosystem and changes in rates of process (eg. weathering) around the lake catchment. Also, biotic material contained within the sediments provides indications of change in the local ecology. Lake sediment geochemical studies concerned with long records of deposition (>100,000 years) are relatively unusual in the literature. Exceptions include work on Lecture Notes in Earth Sciences, Vol. 49 i. F. W. Negendank, B. Zolitsc~',ka (Eds.) Paleolimnology of European Maar Lake~ 9 Spfinger-Verlag Berlin Heidelberg 1993
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Lac du Bouchet in France (Truze, 1990) and on Lake Biwa in Japan (Horie, 19721981). Lago Grande di Monticchio lies at 650m altitude in the Vulture region, east of Naples. Sediment cores covering a 51m deep prof'de were recovered from the site during September 1990. The material obtained is well preserved. It is possible that the sequence from Lago Grande di Monticchio covers the last 250,000 years, almost continuously (Watts & Huntley, unpubl, work), though more lines of evidence would be desirable for confirming this age. This work forms part of an ongoing PhD project, the aim of which is to undertake a comprehensive geochemical study of the whole sequence. In the future this can be integrated with parallel studies (incl. diatoms, paleomagnetism, palynology and sedirnentology) to recreate an overall picture.
GENERAL LITHOLOGY The uppermost 15m broadly consists of laminated muds and gyttjas. A brown diatom gyttja (0-5m) passes dovm into black, highly organic mud (5-8.4m.) The_ latter contains localised patches of vivianite up to lcm in size. Below 8.4m the sequence continues with laminated olive-grey muds. Occasional horizons rich in plant material (mostly mosses) occur here and are up to 2cm in thickness. Tephra layers are found throughout and provide useful stratigraphic markers. They are of varied compositional nature and range from >20era in thickness to microscopic horizons not readily detectable.
SAMPLING AND ANALYSIS Material was taken from Cores C and E which cover the f'trst 15m. Sampling was made at approximately 10cm resolution, providing 159 data points. Owing to the high water content of the top sediments, large continuous quadrants had to be removed from the cores in order to provide sufficient material for all analyses. The sediment was dried for several days at 50"C and ground for 75 seconds in a tungsten carbide mill. This provided a homogeneous fine powder on which subsequent analyses were based. X-ray fluorescence (XRF) was carried out using pressed powder and fused glass discs to measure trace and major element concentrations respectively. Total carbon and nitrogen were determined on a Carlo Erba element analyser. The samples were boiled with phosphoric acid and the amount of carbon dioxide evolved measured pressometdcally. This provided a measure of the inorganic (carbonate) carbon content. By subtracting these values from total carbon a measure of the organic carbon content was found. Biogenic silica was determined using an alkaline wet-chemical leaching technique (Eggimann et al, 1980.) The silica content of the resulting solution was found using a
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modified colorimetric method (based on Eggimann et al, 1980.) Around 30 samples were selected for isotopic analysis of the organic carbon present. A quantity of ground sediment was washed with an excess of 1M HC1 to remove inorganic carbon present. Up to 60mg of a treated sample was placed in a silica tube together with an excess of copper oxide. The tube was evacuated and roasted overnight at 850"C. The carbon dioxide produced was purified on a vacuum line apparatus by means of cryogenic distillation. The purified gas was collected in another tube and transferred to a gas-source mass spectrometer. The ratios measured are relative to the PDB standard.
DATA ANALYSIS With over 30 parameters determined on each sample, it would be convenient to reduce the amount of data viewed while maintaining maximum information on how the complete data set varies. For example, organic carbon and bromine (Fig. 2) show very similar profiles and could perhaps be grouped together into an association reflecting organic matter in the sediments. Multivariate methods of data analysis, principal components analysis (PCA) and correspondence analysis (basic and detrended), were applied in an attempt to eliminate redundancy in the data and to identify a smaller number of associations. The results obtained are best displayed graphically (Fig. 1.) This is an example of R-mode analysis using the PCA method. It shows how the two main axes of variance divide elements measured on Cores C and E into fields and groupings. A number of possible associations can be stated: a) -the tight clustering of C, N and Br is related to organic matter (plants and animal soft parts) b) -a band or arch of elements extending from Mg to Ba may represent incompletely weathered igneous minerals (eg. pyroxene), clays and heavy minerals (eg. zircon) c).-biogenic silica represents contributions from diatom productivity d) -inorganic carbon reflecting the presence of calcite and/or siderite e) -Rb, K and Na may be associated with feldspars/feldspathoids, especially in tephra derived material f) -a broad group of elements (U, Zn, Mn, P, Fe, V, Mo) are largely derived from minerals locally, but show an association with organic matter and may reflect the influence of diagenesis upon the sediment Axis 1 accounts for 54% of the variance exhibited by the dataset and appears to separate biological vs. minerogenic inputs to the sediment. Thus, the largest differences in sediment nature depend on the relative contributions from these two sources. Axis 2 accounts for 12% of total variance and appears to divide elements strongly influenced by diagenesis from more residual or immobile elements.
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Fig. 1. Results of PCA analysis on the data
The three techniques used on the data all produce similar groupings, albeit with some minor variations. In Q-mode it is possible to classify the samples into different facies with moderate success. Without pushing interpretations too far, it is apparent that the elements measured show potential for tracing different sedimentary factors.
RESULTS AND DISCUSSION The sediment geochemistry may be discussed in terms of terrigenous clastie, biological and diagenetic aspects, with some overlap.
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Clastic material is mainly supplied by surface runoff and mass movement within the crater area. This is augmented by sporadic inputs of volcanic material or tephra fallout. There may be some clastic material transported from further afield by aeolian processes, for example during the more arid glacial times. Productivity within the lake (diatoms, algae, macrophytes, ostracods, etc.) and terrestrial inputs of plant debris and pollen contribute to the biological aspect of sedimentation. These primary inputs may be affected by diagenetic processes such as the degradation of organic matter and the release/enrichment of trace metals. Obvious examples of this are the presence of vivianite and siderite.
BIOLOGICAL MATTER Many lake sediment studies have considered organic carbon to be the most important geochemical variable. Mackereth (1965; 1966) was one of the first to measure a wide range of elements on sediments from the English Lake District. In a series of lakes he found that the carbon content rose rapidly after the cessation of glaciation to reach a maximum in the first half of the post-glacial period. Similar patterns have been found in many other settings (Brown, 1991; Truze, 1990.) Organic carbon values (Fig. 2) are relatively low, but increasing gradually, between the oldest sediments at 1500cm and 850cm. Around 850cm values increase sharply, reaching a maximum at 450cm, before decreasing somewhat towards the top (most recent) part of the profile. The negative spikes are samples rich in tephra and are organic poor. These pervasive tephra layers add noise to underlying climatic signals. The rise in carbon around 850cm probably reflects increased productivity or vegetation growth in and around the lake, in response to a more humid and perhaps warmer climate. Terrestrial vegetation development will at the same time stabilise the crater slopes. Reduced erosional activity dilutes the amount of clastic input which will also be seen as an apparent rise in organic content. Thirdly, as phyical sedimentation declines the lake waters will stratify more easily (Truze, 1990.) With nutrient enrichment due to enhanced weathering, high internal productivity could lead to anoxic conditions. This too could cause higher organic contents through favoured preservation of matter. Initial discussions with other groups working on Monticchio suggests that 850cm may be close to the start of the Holocene. This depth is comparable to that found for the Holocene boundary in an earlier palynological study (Watts, 1985) made on a littoral core. It is possible that the actual transition is near 750cm. In this case, the area from 850cm to 790cm is reflecting a late-glacial interstadial, with an intervening period of climatic deterioration between 790cm and 750cm. Such an interpretation requires palynological and dating evidence for support. C/N ratios can assist in defining the nature of the organic matter (Stuermer et al, 1978.) The C/N values (Fig. 3) rise from around 5 at 1500cm to a maximum of 12 at 1350cm, before dropping back to 5 at l l50cm (with some fluctuation.) From 1150cm to 750cm
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Fig. 2. Organic carbon, bromine and bromine/carbon ratio.
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Fig. 3. Carbon/nitrogen ratio, biogenic silica and stable carbon isotopes.
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the ratio rises gradually towards 10. Above this depth, values remain high at between 9 and 12. Again, tephra-rich samples impose negative spikes on the overall trend. The shifts to lower ratios could indicate a change to organic sedimentation dominated by algae and diatoms (Horie, 1972/1977). Lower organisms have ratios of 6-7 as they are rich in proteins. Subsequent rises could signify an increasing influence from higher plant material.This is more cellulose-rich and can have ratios >20. It is likely that the organic matter present is a complex mixture from more than one source. It is not clear why the tephra samples should drag the ratio to such low values. An organic-poor component should dilute organic C and N to similar degrees. Such interpretations are complicated by the possibility of diagenesis preferentially removing nitrogen during organic degradation. This has been observed in marine environments (Stevenson & Cheng, 1972.) Conversely, as total N is being measured, the presence of inorganic nitrogen, can create artificially low ratios (Mackereth, 1966; Muller, 1977.) Therefore if a sediment is organic poor, but contains a small quantity of inorganic N, for example in the form of fixed ammonium, a low ratio would be detected. This could explain the negative tephra spikes although further investigation is needed. Bromine and organic carbon are strongly correlated (r=0.938). It is known that sediments are enriched in Br due to the presence of plant material (Cosgrove, 1970) and that the element is concentrated in humic layers of soils (Vinogradov, 1959.) Since most Br is believed to arrive through ocean-derived aerosols, the Br/C ratio has been thought to reflect "oceanicity" (wind conditions, etc.) of the atmospheric/climatic system by some workers (Mackereth, 1966.) It could also reflect change in the type of organic matter deposited or processes of diagenesis and recycling. Examination of the Br/C ratios on the material under study (Fig.2) shows that marginally higher ratios occur between 850cm and 1450cm. Thus, if the ratio depended only on rate of halogen supply from rainfall, the ocean may have had more influence on local climate during this earlier period. Positive spikes associated with tephra samples show that these layers have excessively high bromine contents in proportion to their low organic carbon values. Actual biogenic silica results correlate well with a normative biogenic silica calculated using XRF major element oxide data: biogenic silica=SiO2-2.8*A1203 This calculation assumes that total silica is composed of a biogenic component and a minerogenic/aluminosilicate component. The factor of 2.8 selected is lower than an average shale silica:alumina ratio of 3.4 (Turekian & Wedepohl, 1961), but fits the sediment type from these cores better. Actual and normative values are plotted on the same axes for comparison (Fig. 3.) As the actual measurement of biogenic silica is time consuming it was decided to continue with a smaller number of selected samples below 970cm in order to verify the accuracy of the XRF estimation. Results bear some comparison with elements reflecting organic matter. From 1500cm to
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750cm there is a rise from 5% to 50% total sediment composed of biogenic silica, with local troughs (tephra layers) and peaks present. Above 750cm values peak briefly at 540cm and are high for a longer phase between 300era and 50cm. It seems likely that diatom productivity has contributed appreciably to the sedimentation throughout most of the time period covered by these cores. Over the upper half of the sequence the sediment is almost entirely made up of amorphous silica and organic matter. Here, the relative dominance of these two components varies with time. This may result from the changing role of diatoms and other plant and algal life in the lake productive system. Fig. 3 shows the results of stable isotopic analysis on bulk organic carbon in the sediments. Since the 1960's possible relationships with environmental changes have been investigated. H/~kansson (1985) reviews a series of factors which could contribute to variation in the isotopic ratio and presents results from Swedish lakes. In these sediments a marked decrease in ]3C/12C coincided with the glacial to post-glacial transition. This shift to isotopically lighter carbon has also been found in Meerfeldermaar, Germany (Brown, 1991) and to some extent in Lac du Bouchet (Tmze, 1990.) On the other hand Nakai (1972) associated more temperate periods with heavier isotopic ratios, as did Stuiver (1975.) Harkness and Walker (1991) observe a superficial correlation between 13C enrichment and climatic change, but identify features which do not correlate with a simple climatic relationship. The carbon isotope values determined on Lago Grande di Monticchio vary between -25.5 and -22%o. This could be said to typify a fairly average mix of lacustrine organic matter. When compared with the element information it is sometimes difficult to relate the fluctuations seen with apparent environmental changes. For example, between 950cm and 750cm there appears to be a pronounced shift, but beyond this values fluctuate without clear explanation. The probable Holocene section contains both high and low points. Further study of the organic matter and palaeobotanical data would help with the interpretation. Ideally it would be more informative to look at isotopic variations within individual organic compounds (Rieley et al., 1991.) The lack of a coherent shift in isotopic values could result from the lake's southerly location. Perhaps the vegetation associated with some of the northern European sites suffered more climatic stress between glacial and interglacial periods. It is also possible that autochthonous organic matter is profoundly influenced by the lake water bicarbonate reservoir. This could modify the expected ratios if a majority of plants are assimilating carbon from this source. These results emphasise the local differences between individual lakes.
TERRIGENOUS CLASTIC MATERIAL Many of the elements measured are associated almost exclusively with the minerogenic sediment fraction (A1, Zr, K, etc.) Fig. 4 shows how A1 and Y contents vary. Between
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Fig. 4. Aluminium, yttrium and zirconium/rubidium ratio.
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1500cm and 850cm concentrations are decreasing gradually, with a slight step downwards at 1200cm. At 850cm values drop sharply, but return a brief maximum over the next 100cm. From 750cm to the top the concentrations are very low, but tephra layers add positive spikes to the general pattern. A minimum seems to be reached at 540cm after which there is a gentle increase towards more recent times. If the amount of clastic material sedimented has fallen this could be interpreted as being due to: (1) reduced erosional and transportational processes and (2) a dilution effect from increased organic sedimentation. The major control on this is believed to be slope binding or stabilisation arising from terrestrial vegetation development. The surrounding volcanic rocks are composed of pyroxenes and feldspathoid minerals with smaller quantities of apatite and oxides present. X-ray diffraction analysis suggests that a certain amount of pyroxene and apatite finds its way into the sediment. Mineralogical analysis is difficult where the material is so dominated by organic matter and amorphous silica. Clay minerals are probably present too, though they are proving difficult to extract for identification. Quartz has been identified in a few samples from the lower parts of the sequence. This mineral may be derived from outside the crater area. The tephra layers tend to include feldspars, such as sanidine, along with some pyroxene and apatite. Ratios such as Zr/Rb have been used to identify change in grain size (silt/clay) of the clastic material sedimented (O'Donnell, 1987.) Results (Fig. 4) show a decrease from Zr/Rb ratios of 3.5 at 1500cm to 1.5 at 700cm. From 700cm upwards the ratio remains steady at close to 1.5. This suggests a gradual coarsening of grain size below the probable Holocene base. Some tephra layers give negative spikes which could reflect their geochemical nature (relatively alkali-rich compared to zircon content.) Since the tephra represent coarse grained layers they might otherwise be expected to add strong positive spikes to the curve.
DIAGENETIC FEATURES Some examples of diagenetic effects are touched on in this section. An immediate feature from visual examination is the presence of vivianite at around 600cm depth in the black gyttja. This has been recognised as a diagenetic precipitate in a wide range of lake sediments (Mackereth, 1966; Nriagu & Dell, 1974; Truze, 1990.) Fig. 5 shows enrichments in P, Fe and Mn between 500cm ~ d 700cm. Thus vivianite appears to be concentrated in a discrete time zone, thought to represent the early Holocene, rather than being found throughout the Holocene. Nriagu & Dell (1974) considered t h e precipitation or dissolution of phosphate to act as a buffer, regulating phosphorus levels in the interstitial waters and release to the overlying lake waters. Formation is often associated with anaerobic decay in organic-rich sediments. Phosphorus might be derived from organic matter releasing nutrient phosphate or from mineral/skeletal apatite decomposition. During the early Holocene conditions within the sediment may have
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Fig. 5. Phosphorus, iron and manganese.
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changed to reducing in nature and this might have converted large quantities of ferric iron to the more soluble ferrous form. Upwards migration of pore waters (aided by sediment compaction) could lead to high concentrations of both Fe2+ and phosphate in near surface sediments and cause vivianite to precipitate. Perhaps after this boundary period later organic accumulations have not had access to sufficient iron to allow more vivianite formation. Other mechanisms are undoubtedly possible. The Mo profile (Fig. 6) shows a pronounced enrichment (<90ppm) between 850cm and 800cm. Above this horizon concentrations decline exponentially towards the 10ppm level. The element is known to accumulate in .living plants owing to its physiological properties, such as biocatalysing nitrogen fixation (Bortels, 1930.) However, the enrichment observed has probably been enhanced through diagenetic processes. Two of
Fig. 6. Molybdenum and carbonate carbon
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the major mechanisms of uptake are coprecipitation with Fe sulphides and fixation by adsorption or reduction in organic rich sediments. The former is possible as sulphide contents are believed to be elevated (from preliminary investigations.) The area of enrichment is also one of high organic carbon content and was earlier suggested to represent a late-glacial interstadial event. A change to reducing conditions in the sediment at this boundary zone (and resulting migration/fixation of Mo from underlying sediments) might be invoked in similar manner to the early Holocene-vivianite explanation. The zones of phosphate and molybdenum enrichment occur in different time planes illustrating how elements may respond differently to the conditions of diagenesis. Carbonate carbon (Fig. 6) is present at certain levels, with XRD analysis identifyng the presence of both calcite and siderite. These are probably of diagenetic origin, although ostracods found below 1500cm (Wansard, pers. com.) show that skeletal bioclastic carbonate occurs lower in the sediment column. XRD results suggest that the area between 10cm and 50cm contains calcite along with some gypsum. Between 100era and ll50cm in the sequence the carbonate occurrences tend to be associated with tephra samples. It is not clear whether carbonate minerals have arrived with the tephra fall or whether the influxes of alkaline volcanic material have increased the pH value of the lake water favouring temporary carbonate precipitation. Towards 1500cm carbonate contents are not associated with tephra inputs and must reflect longer term physicochemical conditions in the lake, such as evaporation and Pco2. Elevated Fe concentrations occur in this lower region, where siderite is more prevalent. Mn is also higher and is probably substituted in the carbonate. This contrasts with the Fe and Mn enrichment associated with phophate at higher levels.
CONCLUSIONS These initial geochemical results reveal a wide variety of trends with much opportunity for further explanation. A 9 division can be made between the upper half of the section (rich in organic matter and biogenic silica) and the lower half (containing modest amounts of the latter two components and an increasing amount of elastic material.) The added presence of diagenetic phases (vivianite, siderite, etc.) in discrete temporal zones could further help in reconstructing environmental conditions. Influxes 9of tephra could have a marked impact on what otherwise may be a climatically controlled regime. It is hoped to continue this characterisation through the whole 51 metres of succession. Investigation of specific components of the sediment, such as organic geochemical studies, will give insight into problems less readily explained by bulk analysis. Dating methods are now required to aid the interpretations made.
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ACKNOWLEDGEMENTS This work is funded by an EC Science Programme Grant No. SC1 0295 C. CR acknowledges support from a NERC studentship award. Dodie James, Godfrey Fitton, Geoff Angell, Tim Brand, Shirley Derrick and Mike Saunders are thanked for assistance With the analytical work. Tony Fallick and Paul Eakin are also thanked for discussions over the stable isotope studies.
REFERENCES Bortels, H. (1930): Molybdan als katalysator bei der biologischen Stickstoffbindung. Arch. Mikrobiol, 1, p333. Brown, H.A. (1991): A palaeomagnetic, geochronological and palaeoenvironmental investigation of late and post glacial maar lake sediments from NW-Europe. PhD thesis, University of Edinburgh. Cosgrove, M.A. (1970): Iodine in the bituminous Kimmeridge shales of the Dorset coast, England. Geochim. Cosmochim. Acta, 34, p830. Eggimann, D.W. et al. (1980): Dissolution and analysis of amorphous silica in marine sediments. J. Sed. Pet., v.50 No.l, p215. HLkansson, S. (1985): A review of various factors influencing the stable carbon isotope ratio of organic lake sediments by the change from glacial to post-glacial enviromnental conditions. Quat. Sci. Reviews, 4, p135. Harkness, D.D. & Walker, M.J.C. (1991): The Devensian Lateglacial carbon isotope record from Llanilid, South Wales. Quaternary Proc. No. 1, p35. Horie, S. (1972-81): Palaeolimnology of Lake Biwa and the Japanese Pleistocene. Proc. Jpn. Acad. Mackereth, F.J.H. (1965): Chemical investigation of lake sediments and their interpretation. Proc. Royal Soc. London, 161B, p295. Mackereth, F.J.H. (1966): Some chemical observations on post-glacial lake sediments. Phil. Trans. Royal Soc. London, 250B, p165. Muller, P.J. (1977): C/N ratio in Pacific deep-sea sediments: Effect of inorganic ammonium and organic nitrogen compounds sorbed by clays. Geochim. Cosmochim. Acta, 41, p765. Nakai, N. (1972): Carbon isotopic variation and the palaeoclimate of sediments from Lake Biwa. Proc. Jpn. Acad., 48, p516. Nriagu, J.O. & Dell, C.I. (1974): Diagenetic formation of iron phosphates in recent lake sediments. Ame. Mineralogist, 59, 934. O'Donnell, D. (1987): Geochemical cycles of trace elements in coastal sediments. PhD thesis, University of Edinburgh. Rieley, G. et al. (1991): Sources of sedimentary lipids deduced from stable carbon -isotope analyses of individual compound. Nature, 352, p425. Stevenson FJ. & Cheng, C.-N. (1972): Organic geochemistry of the Argentine Basin sediments: carbon-nitrogen relationships and Quaternary correlations. Geochim. Cosmochim. Acta, 36, p653. Stuermer, D.H. et al. (1978): Source indicators of humic substances and proto-kerogen. Stable isotope ratios, elemental compositions and electron spin resonance spectra. Geochim. Cosmochim. Acta, 42, p989. Stuiver, M. (1975): Climate versus changes in 13C content of the organic component of
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lake sediments during the late Quaternary. Quaternary Research, 5, p251. Truze, E. (1990): Etude sedimentologique et geochemique des depots du maar du Bouchet (Massif Central, France). Evolution d'un systeme lacustre au tours du demier cycle climatique (0-120 000 ans). These, Universite d'Aix Marseille II. Tureldan, K.K. & Wedepohl, K.H. (1961): Distribution of the elements in some major units of the earth's crust. Bull. Geol. Soc. America, 72, p175. Vinogradov, A.P. (1959): The geochemistry of rare and dispersed chemical elements in soils, 2rid ed. New York: Consultants Bureau. Watts, W.A. (1985): A long pollen record from Laghi di Monticchio, southern Italy: a preliminary account. J. Geol. Soc. London, 142, p491.
TEPHROCHRONOLOGY OF CORE C FROM LAGO GRANDE DI MONTICCHIO:
Anthony J. Newton and Andrew J. Dugmore Department of Geography, University of Edinburgh Drummond Street, Edinburgh, EH8 9XP, U.K.
ABSTRACT
Tephrochronological studies of sediment from Lago Grande di Monticchio have begun with detailed electron microprobe analyses of the two thickest tephra layers found in Core C. A total of 76 analyses of both layers reveals them to be of trachytic composition, of the type erupted from the Campi Flegrei during the last 35,000 years. LGM C41, the lower layer, is a co-ignimbrite ash-fall deposit and is tentatively correlated with the Campanian Ignimbrite which was erupted about 35,000 BP. The Yellow Neapolitan Tuff (12,000 BP) is suggested as a possible correlation for the upper layer, LGM C33. Some geochemical differences exist within units in LGM C33 but there appears to be none within LGM C41.
Conflicting
geochemical and palaeomagnetic work on the lake sediments suggests that these two layers may be 80,000 years older and possible correlations are also made with three trachytic tephra layers ranging in age between 90,000 and 120,000 BP. The 51 metre deep profile obtained in Core D provides the possibility of a tephrochronological record stretching back into the Eemian and the glacial cycle before that.
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AIMS
This paper presents a tephrochronological study of a 15.5 metre long Livingstone core (LGM C) from Lago Grande di Monticchio. Two other cores LGM B (40 metres long) and LGM D (51 metres long) were retrieved at the same time. The aim of this study is to provide accurately dated marker horizons within the cores, so that precise correlation within the sediment sequence of Lago Grande di Monticchio and between the lake and other lacustrine, marine and terrestrial sequences will be possible. As work develops palaeoenvironmental impacts associated with the tephra layers will be assessed. The excellent tephrochronological record represented in the cores will inevitably increase our knowledge of volcanic activity within the region.
G E N E R A L V O L C A N O L O G I C A L SETTING
Lago Grande di Monticchio is the larger of two maar lakes which occupy a caldera on the slopes of Monte Vulture about 120 kilometres east of Naples. They were both formed by phreatomagmatic activity. Monte Vulture has been dormant for at least the last 40,000 years (Guest et al., 1988) but is part of an extensive igneous province and lies within 600 kilometres of several volcanic centres which have been active throughout the Quaternary (Figure 1). The Roman Igneous Province about 150 kilometres to the north of Monte Vulture has been active for much of the last million years. The earliest activity is recorded at Monte Cimini, 920,000 years BP and the last at Monte Vico about 55,000 years BP (Varecamp 1980). Pantellaria, a volcanic island located on the continental rift system of the Sicily Channel, has been active for at least the last 320,000 years and most recently in 1831 and 1891 when submarine eruptions occurred close to the island (Civetta et al., 1988). Europe's largest volcano, Mount Ema, is over 300,000 years old (Chester et al., 1985) and is still active today. Ema is composed of a series of cones, which have been superimposed on one another. Several of these have been destroyed by caldera collapse. Present day activity is divided between mild strombolian activity at the summit crater and periodic flank eruptions,
335 which produce vast amounts of very fluid lava. For example, several villages were threatened by lava flows during a flank eruption in 1991.
Figure 1 l_xxzation map of volcanic areas in southern Italy active during the Quaternary
The Eolian Islands consist of seven major volcanic islands, with the earliest signs of subaeriaJ activity dating from about 500,000 years BP (Crisci et al., 1991). The two currently active islands are Vulcano, which last erupted in 1890 and Strornboli, which is continually erupting at regular intervals of 20-30 minutes. The Campanian Igneous Province, which includes Monte Vulture also contains four other major volcanic centres: Roccamonfina, Ischia, the Campi Flegrei and Vesuvius. Roecamonfina, situated about 40 kilometres to the north of Naples, was at its most active between 1.26 million and 370,000 years ago (Keller et al., 1978). There is also evidence of activity between 30,000 and 40,000 BP (Keller et al., 1978 and Paterne et al., 1988). Some activity in the Roccamonfina area as recently as 276 BC is also reported by Simkin et al. (1981). The island of lschia, 35 kilometres to the east of Naples, is thought to have been active for at least the last 750,000 years (Capaldi et al., 1977). There appears to have been little activity between 300.000 and 100,000 years ago but the last 100,000 years has been dominated by explosive activity, v,ith the most recent eruption being in 1302 AD. The Campi Flegrei, which cover an area of 400 square kilometres, was originally the site of
336
a large stratovolcano which began activity over 50,000 years ago. Between 40,000 and 25,000 years ago a series of large eruptions from this volcano resulted in the formation of the caldera (Paterne et al., 1988). The Campanian ignimbrite, erupted about 35,000 BP (Barbed et al., 1991), covers an area of at least 7,000 km2 (Barberi et al., 1978) and tephra layers associated with this event have been found in the eastern Mediterranean; the total volume of pyroclastic material produced by this eruption has been estimated to have been between 80 and 100 km 3 (Cramp et al., 1989 and Barbed et al., 1991). After about 8,000 years of quiescence, activity resumed and the caldera now contains 19 volcanic craters. The most recently active is Monte Nouvo, which erupted in 1538 AD. Vesuvius, found 12 kilometres to the east of Naples has been active for at least the last 25,000 years (Civetta et al., 1991). Several large cones have been built up and subsequently partially destroyed by large eruptions. There have been at least 8 major Plinian eruptions (Barberi et al., 1983), each one heralding the start of a new phase in Vesuvius's activity.
The last time
this occurred was the infamous eruption of 79 AD and the most recent activity stopped in 1944. Monte Vulture is the eccentrically positioned volcano of the Campanian Igneous Province, being the only Quaternary volcano found east of the Apennines. Activity began about one million years ago and finally ended about 40,000 BP. Guest et al. (1988) divide the activity into 3 stages. The first was the "Basal Stage" which lasted until about 830,000 years ago and produced lots of pyroclastic flow deposits. The next, the "Cone Building Stage", produced a mixture of lavas, pyroclastics and lahars. Plinian eruptions were common and built up layer upon layer of ash-fall deposits. The collapse of the summit formed the Valle dei Grigi about 500,000 years ago. Guest et al. (1988) attribute this to a gravitational sector collapse, not volcanic activity. This explanation, however, is disputed by La Volpe and Principe (1991). The final stage of activity included the formation of the two maar lakes.
The date of
formation of the two maar lakes has yet to be determined, but was probably several hundred thousand years ago. The study area at Monte Vulture and Lago Grande di Monticchio is situated in a highly active volcanic region. Large, widespread pyroclastic deposits have been produced at some stage by virtually all of the volcanic centres discussed above. The incorporation of these layers into lake sediments, such as those in Mondcchio, provide an excellent opportunity for tephrochronological study.
337 SAMPLING AND PHYSICAL DESCRIPTION OF TEPHRA LAYERS
All of the visible tephra layers from Core C were sampled.
49 tephra layers were
preliminarily identified but due to complex structures within particular layers, a total of 65 tephra samples were collected (Figure 2). This initial study focuses on the two thickest tephra layers, LGM C33 and LGM C41, which are found in the lower half of the core. The 23 cm thick tephra layer LGM C41 is found between 14.35 and 14.58 metres, and can be divided into two main units. There is a lower, 2 cm thick, coarse sand grade layer and an upper fine grained deposit. The boundary between these two units is sharp and there is no evidence of grading between the two. There are occasional bands of coarse material within the upper fine grained layer. This type of tephra deposit, containing a coarse lower part and a finer upper part, is typical of co-ignimbrite ash-fall deposits as described by Sparks and Huang (1980). Samples were taken from the lower coarse layer (LGM C41D) and the upper finer one (LGM C41A and B). A sample was also taken of the thickest of the coarse bands within the fine deposit (LGM C41C). A series of alternating coarse and fine layers (5 units), which have sharp boundaries and no grading between them make up the 20 cm thick tephra layer LGM C33, found between 10.95 to 11.15 metres. A tephra layer (LGM C34), immediately below LGM C33, is only separated from the layer above by 3 mm of gyttja and is very similar in appearance to the basal unit of the larger layer. LGM C34 contains coarse (sand grade) black and white grains with slightly larger pieces of white pumice present amongst a slightly finer matrix. The basal unit of the larger layer (LGM C33E) is composed of very similar, though somewhat coarser material, to LGM C34. The layer above this, LGM C33D, is slightly finer than LGM C33E and is again principally composed of black and white grains. LGM C33C is a coarser layer composed mostly of white pumaceaus grains; whilst LGM C33B is fine grained and made up of mostly black grains. The upper most unit, LGM C33A, is a coarse black and white speckled deposit.
338
Figure 2 Log of Core C showing the visible tephra layers and detail of LGM C33, C34 and C41. E L E C T R O N MICROPROBE ANALYSIS OF GLASS SHARDS
Method Glass shards within the tephm layers were analysed in thin section, so that a fresh glass face was presented to the electron probe. The shards were incorporated into a thick resin film on a frosted slide and ground to a thickness of about 75 ~m and polished with 6 ~tm and 1 ~m diamond pastes. After carbon coating, the samples were analysed using a WDS technique with an accelerating voltage of 20 Kv and a beam current of 15nA on a Cambridge Instruments
339
Microscan V. compounds.
Standards used were a mixture of pure elements, oxides and simple silica Corrections were made for counter dead time, atomic number effects,
fluorescence and absorptions using a ZAF correction programme based on Sweatman and Long (1969). To reduce mobilization of sodium, which leads to progressive over-estimation of the abundances of other elements, glass analyses are carried out with a low beam current (15 Na). An andradite standard was analysed as an unknown before, during and after a series of tephra analyses in order to provide a clear indication of instrument stability. Only those analyses with element totals above 95 per cent were considered for comparison (Froggat, 1990).
Geochemical Results
38 analyses were obtained from both LGM C41A, B, C, D and LGM C33A, C, D and E. It was not possible to acquire any satisfactory results from LGM C33B. The results are shown in Tables 1 and 2. Both of the major tephra layers are trachytic in character
(Figure 3).
The detailed sub-
sampling of each layer has enabled the degree of internal variation to assessed. In LGM C33 some significant variations between the sub-units are apparent. LGM C33D has a wider range of total iron, titanium, magnesium and calcium oxides than the rest of the tephra layer. These consistently higher values contrast to the lower total iron, magnesium and calcium oxide values for LGM C33C.
LGM C33C, however, has a higher total alkali figure than C33D,
whilst LGM C33D has a higher K20/Na20 ratio than C33C. There does not appear to be any significant differences between LGM C33A and E (Table 1).
C41 does not show the same degree of variation between the various units within the tephra layer. There does not seem to be any significant variation between the upper finer part and the lower coarser unit (Table 2). The geochemical difference between the two tephra layers is not great. There is considerable overlap in the alkali concentration, with LGM C41 having more higher KzO/Na20 ratios than
340
Figure 3 Geochemical characteristics of LGM C33 and LGM C41 and their correlation ~xith the Trachyte tephras as defined by selected analyses from Paterne et al. (1988). LGM C33, whilst the latter has more lower total alkalis than C41 (Figure 3).
Greater
differences exist in the amounts of total iron, magnesium and calcium oxides, There is a greater range of total iron values for LGM C33 than C41. LGM C33 also has higher calcium o.'dde whilst LGM C41 has higher magnesium oxide.
When total iron, calcium and
magnesium oxides are plotted on a triangular graph (Figure 4) the differences become more apparent with only three points from LGM C41 overlapping with LGM C33.
341
Figure 4 Triangular graph to show the differences in total iron, calcium and magnesium oxides between LGM C33 and I_,GM C41. All the data has been standardised to 100%
The trachytic characteristics of both layers suggest a common source area in the Campanian Igneous Province. Furthermore, they probably belong to the trachyte group of tephras as defined by Paterne et al., 1988 (Figure 3), suggesting that the most probable source is the Campi Flegrei. Trachytes from the Campi Flegrei typically have a higher K20/Na20 ratio than those from Ischia and Vesuvius has produced tephra of mainly tephritic-leucitic-phonolitic composition with few trachytes (Pateme et al., 1988).
342
DISCUSSION
There are considerable problems with the dating of cores from Lago Grande di Monticchio. Radiocarbon dates obtained during palynological work by Watts (1985) appeared to be "too old", probably as a result of the inwashing of older organic material from the crater wall. For this reason no radiocarbon dates have been taken from the recent cores B, C and D and dating evidence is being gathered from current geochemical, palaeomagnetic and sedimentological studies. Pollen analyses can provide evidence of cooling and warming and so date the core by palaeoclimatic correlations, but analyses are at an early stage. The identification of tephra layers within the core and their correlation with parts of the same layer dated elsewhere, could provide a dating framework for the Lago Grande di Monticchio sediments. From palaeomagnetic work carried out by Turton (this volume) the base of the 51 metre Core D may be 300,000 years old. Robinson (this volume) has identified geochemical evidence to suggest that the Holocene boundary (ca 10,000 BP) in Core C is about 7.5 metres in depth. If the older estimate of the age is correct the base of Core C may be some 120,000 years old with a major part of the sediment sequence from the last glaciation missing. The younger estimate is of an age of 30,000 to 40,000 for the base of core C. If this later estimate is correct the base of core probably corresponds to the beginning of a major phase of activity in the Campi Flegrei. The results of analyses of both LGM C33 and C41, correlate in general with the Campanian trachyte tephras defined by Paterne et al., 1988 (Figure 3). Some of the glass shards from LGM C41 with low KzO/Na20 ratios may belong to the peralkalic tephras.
The Eolian
trachytes usually have a higher percentage of Fe203 and lower alkali content than those from Campanian Province. Unfortunately our analyses only include total iron, not Fe203, so direct comparison with iron values of Paterne is not possible. Pateme et al. (1988) further identified two types of Campanian trachyte.
The first type, the peralkalic trachytes, recognised as
coming mainly from Ischia have a lower K20/Na20 ratio compared with the trachytic tephras from the Campi Flegrei.
During the last 80,000 years, the peralkalic tephras are present
throughout the cores obtained by Paterne et al., 1988). But between 80,000 and 40,000 years ago the trachyte tephras are mixed with the peralkalic tephras and only form homogeneous layers during the last 40,000 years. This is interpreted as being the beginning of a major
343
phase of activity in the Campi Flegrei, whilst the decrease in peralkalic tephra during the last 15,000 years is seen as evidence of a decline in the explosive activity of Ischia. If the base of core C is actually 120,000 years old, there are still some possible eruptions to which the tephra layers may be correlated. Keller et al. (1978) identified three trachytic tephra layers X-2, X-5 and X-6, of probable Campanian origin, from two marine cores located east of Sicily. X-2 is dated to about 90,000 BP, whilst X-5 and X-6 are between 110,000 and 120,000 years old. Although they have a similar major element composition to LGM C33 and C41 (Figure 3), only one set of analyses for each layer have been published, which makes correlation difficult. Direct comparison with the data published by Paterne et al. (1988) and Keller et al. (1988) is difficult because only selected results are shown for each tephra layer. It has to be assumed that the results shown are typical of each layer and if this is so, it is not possible to correlate either LGM C33 or LGM C41 with any of the tephra layers from the marine cores. When compared to Paterne et al.'s (1988) data there are distinct differences with L G M C33 and C41 in the concentration of A1203, with any fall to the 17 to 18 per cent level in Paterne et a l ' s results being accompanied by a rise in SiO 2 which is not seen in either LGM C33 or LGM C41. X-2, X-5 and X-6 have varying differences in silica, aluminium, iron and potassium oxides with the two Monticchio layers, but how significant these differences are is hard to tell. Electron microprobe geochemical data is only comparable if the glass shards are analysed under standardized conditions and that standards of known composition are regularly read before, during and after the analyses and any machine drift corrected frequently. Comprehensive data on machine type and analytical conditions should be given as a matter of course (Froggat 1990).
For the benefit of other workers it is also important that full
geochemical results are given, not just averages or representative selections. Publishing only one typical or mean analysis for a tephra layer does not give any indication of the spread of results, or their concentration around certain values.
Similarly characteristic geochemical
trends can not be identified, such as shown on the total iron/magnesium graph. Comparing grain discrete electron microprobe analysis of volcanic glass with bulk XRF studies of tephra, may be misleading because of the presence of lithic fragments and minerals.
344 WIDER IMPLICATIONS
Using the high resolution sediments in the three cores from Lago Grande Monticchio it will be possible to extend the regional tephrochronology back several hundred thousand years. This chronology may be used to correlate terrestrial, lacustrine and marine sequences. A general lack of knowledge currently prevents accurate correlation between LGM C33 and C41 with tephra deposits elsewhere. The sediments of Lago Grande di Monticchio contain an extensive record of volcanic activity in southern Italy, which will provide information not only on the evolution of volcanism in the area, but also a dating framework, which other disciplines will be able to use.
CONCLUSION
At least 47 tephra layers occur in the 15.5 metres of core C from Lago Grande di Monticchio. Most tephra layers are less than a couple of centimetres thick. The two thickest layers are found between 10.95 to 11.15 metres (LGM C33) and 14.35 to 14.58 metres (LGM C41) in depth.
Both LGM C33 and C41 are composed of trachytic tephra from the Campanian
Igneous Province, most probably Campi Flegrei.
LGM C41 also contains tephra most
probably from Ischia. These layers cannot as yet be dated, but were probably erupted sometime between 15,000 and 35,000 years ago or around 100,000 years ago. The coarse layers within the mainly fine grained upper part of LGM C33 may be due to inwashing of the coarser deposits (of the same eruption) from the crater walls, rather than repeated changes in grain size during the eruption. LGM C41 appears to be a co-ignimbrite ash-fall deposit (ie a coarse lower and fine grained upper portion) which may be related to the Campanian Ignimbrite which was erupted about 35,000 BP (Barberi et al., 1991). The Yellow Neapolitan Tufts erupted about 12,000 BP (Barberi et al., 1991), as a result of small scale caldera collapse, may well be associated with
345
LGM C33. This correlation may prove to be wrong if the palaeomagnetic evidence is correct. The three tephra layers X-2, X-5 and X-6 are possible correlations in this case. Unfortunately the older date also requires a large part of the last glacial period to be missing from the core, thus reducing the completeness of the tephrochronological record. The sediments from Lago Grande di Monticchio contain a high resolution tephrochronological record which may be 300,000 thousand years old. This study has shown the importance of multidisciplinary collaborative research in order to fully explain long sediment sequences.
Acknowledgements Microprobe analyses were undertaken in the Department of Geology and Geophysics, University of Edinburgh, with the kind support of Professor G.S. Boulton, Dr. P. Hill and Dr. S. Kearns.
Ian Turton and Christian Robinson also greatly helped with advice on
palaeomagnetic and geochemical characteristics of the sediments.
346
TABLE 1 LGM C33A SiO 2
TiO, AlzO~ FeOr MnO CaO N~O
~o Total
59.36 0.44 17.45 3.32 0.20 0.46 3.41 4.35 7.90 96.89
58.06 0.57 17.58 3.57 0.15 0.73 3.36 3.98 7.97 95.97
61.45 0.29 17.61 2.55 0.18 0.28 2.61 4.81 7.80 97.58
60.56 0.34 17.89 3.08 0.11 0.41 3,28 3.55 8.53 97.75
58.20 0.37 18.0I 4.08 0.18 0.71 4.09 3.88 7.82 97.35
60.01 0.39 18.22 3.14 0.14 0.39 3.14 4.19 8.07 97.70
60.88 0.30 17.57 2.65 0.16 0.25 2.55 4.83 7.20 96.39
57.48 0.41 17.69 3.72 0.18 0.69 3.79 3.69 7.70 95.35
58.70 0.40 18.17 3.33 0.16 0.41 3.11 4.15 8,26 96.71
61.77 0.37 17.84 2.47 0.13 0.22 2.72 4.65 8.54 98.71
62.08 0.42 17,53 2.93 0.16 0.28 2.84 4.10 8.65 99.00
61.3l 0.35 17.77 2.75 0.20 0.29 2.75 4.33 8.31 98.07
59.72 0.32 17.86 2.60 0.15 0.29 2.72 4.60 8.30 96.58
60.81 0.36 17.29 2.62 0.19 0.29 2.68 4,59 7.75 96.59
59.71 0.37 17.66 3.05 0.15 0.38 3.01 4.15 7.73 96.19
59.6t 0.34 17.26 2,60 0.20 0.28 2.61 4.69 7.79 95.39
60.09 0.32 17.58 2.60 0.13 0.26 2.54 4.61 7.89 96.02
60.23 0.34 18.20 2.63 0.17 0.24 2.43 4.41 7.76 96.42
51.58 0.96 19.05 6.29 0.09 3.11 7.66 2.42 5.90 97.07
51.83 0.70 18.30 6.61 0.18 1.92 7.77 2.65 5.45 95.41
58.57 0,35 17.92 3.56 0.15 0.49 3.48 4.16 8.03 96.71
58.03 0.42 17.73 3.57 0.11 0.64 4.17 3.23 8.37 96.26
56.72 0.44 17.94 3.61 0.14 0.72 4.85 3.04 7.89 95.36
55,77 0.42 18.17 4.28 0.12 0,88 4.57 3.58 7.54 95.35
58.74 0.36 18.41 2.81 0.04 0.46 3.63 2.80 9.28 96.53
59.89 0.39 18.13 2.97 0.08 0.44 3.22 4.47 7.94 97.52
58.08 0.37 17.74 3.35 0.I1 0.46 3.11 3.79 8~15 95.16
60.16 0.30 18.35 2.88 0.13 0.36 3,41 3.78 8.39 97.77
60.13 0.31 17.74 3.17 0,15 0.41 3.01 4.43 8.37 97.73
59.44 0.44 18.08 3.61 0.17 0.63 3.84 3.64 7.93 97.79
59.3l 0.41 17.61 3.52 0.17 0.49 3.17 4.35 7.79 96.81
58.62 0.44 17.33 3.57 0.18 0.62 3.70 4.03 7.78 96.27
57.27 0.41 17.86 3.44 0.16 0.51 3.59 4.05 8.02 95.33
56.98 0.57 18.03 4.79 0.19 1.20 5.34 3.67 7.48 98.25
58.86 0.39 17.72 3.35 0.17 0.49 3.41 3.67 7.90 95.96
61.29 0.34 17.66 2.72 0.14 0.27 2.54 4.51 8.09 97.56
LGM C33C
sio, TiO 2
AI20, FeO~ MnO
M~ CaO Na~O
K=O Total
60.31 0.37 17.80 2.54 0.19 0.24 2.71 4.44 8.08 96.68
LGM C33D
sio, TIC), A|~O~ FeOT MnO MgO CaO
Na~O K~O Total
LGM C33E
si~ TiO 2
AI~O~ Fe~ MnO M~
CaO
KO To~
60.93 0.39 17.83 2.63 0.18 0.24 2.57 4.78 7.79 97.35
347
TABLE 2
LGM C41A SiO~ TiO2 AI20J FeOr MnO MgO CaO Na20 KzO Total
59.20 0.40 17.86 3.45 0.10 0.77 2.67 3.30 8.82 96.57
59.21 0,44 17.85 3.56 0.12 0.75 2.66 3.20 9.11 96.89
59.01 0.42 17.79 2.78 0.10 0.54 2.14 4.31 8.46 95.53
59.88 0.38 17.34 2.85 0.12 0.46 2.08 3.89 8.73 95.73
58.58 0.42 17.61 3.57 0.11 0.82 2.73 3.12 9.28 96.23
59.57 0.40 17.69 3.07 0.08 0.65 2.42 4.11 8.57 96.56
59.65 0.37 17.63 3.22 0.13 0.60 2.33 3.88 8.71 96.50
59.50 0.45 17.34 3.16 0.08 0.77 2.53 3.56 9.22 96.62
59.22 0.39 17.78 3.24 0.12 0.75 2.64 3.60 8.74 96.49
57.93 0.44 17.36 3.54 0.10 0.80 2,87 3.13 9.48 95.65
61.81 0.33 17,52 2.76 0.10 0.45 1.99 4.68 8.15 97.80
60.77 0.32 17.61 2.90 0.13 0.45 2.06 4.47 8.33 97.04
58.48 0.41 17.47 3.52 0.08 0.80 2.71 3.48 8.82 95.77
59.29 0.36 17.23 2.84 0.12 0.58 2.29 3.91 8.68 95.30
60.33 0.39 17.65 2.86 0.13 0.48 2.08 4.30 8.57 96.79
59.37 0.43 17.50 3.17 0.15 0.71 2.45 3.32 9.42 96.54
60.19 0.31 18.08 2.25 0.08 0.41 2.38 3.27 10.01 96.99
59.59 0.39 17.90 2.48 0.07 0.45 2.52 2.90 9.64 95.94
59.12 0.42 17.78 3.28 0.13 0.70 2.30 3.51 9.05 96.29
59.60 0.36 17.38 2.64 0.18 0.42 2.09 4.38 8.14 95.19
59.42 0.40 17.57 2.77 0.08 0.43 2.05 4.02 8.45 95.20
59.29 0.38 17.48 3.15 0.12 0.64 2.34 3.67 9.09 96.16
58.42 0.42 17.69 3.58 0.I0 0.83 2,67 3.43 9.18 96.31
58.98 0.37 17.59 3.31 0.10 0.77 2,80 3.54 9.31 96.77
59.98 0.37 17.55 2.60 0.11 0.37 2.60 4.17 7.97 95.21
59.61 0.41 17.82 2.80 0.08 0.47 2.08 4.44 8.68 96.39
59.19 0.42 17.62 3.34 0.1l 0.78 2,69 3.45 9.31 96.91
62.91 0.34 16.99 2.52 0.21 0.26 1.58 5.51 6.94 97.25
61.33 0.37 17.63 2.84 0.11 0.42 2.07 4.65 8.00 97.44
60.52 0.41 17.38 2.60 0.15 0.45 2.14 4.31 8.60 96.57
58.93 0.36 17.59 3.03 0.17 0.56 2.29 3.80 9.18 95.91
57.70 0.49 17.83 3.50 0.09 0.87 2.75 3.19 9.40 95.81
59.28 0.34 17.76 2.89 0.15 0.54 2.29 3.95 8.65 95.85
60.03 0.39 17.46 2.66 0.17 0.34 2.01 4,75 8,18 96.00
59.73 0.38 17.09 2.89 0.14 0.50 2.23 4.08 8.42 95.45
59.94 0.44 17.26 2.83 0.13 0.37 2.07 4.35 8.18 95.57
60.91 0.44 17.47 2.67 0.16 0.45 2.11 4.61 7.95 96.80
60.76 0.43 17.87 3.47 0.12 0.73 2.50 4.10 8.50 98.47
LGM C41B SiO2 TiO: AI~O3 FeOv Mr'~) MgO CaO N~O K20 Total
LGM C41C SiO2 TiO= A1~O3 FeOr MnO MgO CaO N~O K~O Toud
LGM C41D SiOa TiOz AlaO3 FeOr MnO MgO CaO NhO K20 Total
348
REFERENCES Barberi, F., Innocenti, F., Lirer, L., Munno, R., Pascatore, T. & Santacorce, R. (1978): The Campanian Ignimbrite; a major prehistoric eruption in the Neapolitan area (Italy). Bull. Volcanol, 41(1): 10-31. Barberi, F., Cassano, E., La Torre, P. & Sbrana A. (1991): Structural evolution of Campi Flegrei caldera in the light of volcanological and geophysical data. J. Volcanol. Geotherm. Res, 48: 33-49. Capaldi, G., Civetta, L. & Gaparini, P. (1976): Volcanic history of the Island of Ischia (S. Italy). Bull. Volcanol, 40:11-22. Chester, D.K., Duncan, A.M., Guest, J.E. & Kilburn, C.R.J. (1985): Mount Etna, The anatomy of a volcano. Chapman & Hall Ltd.; London. Civetta, L., Cornette, Y., Gillot, P.Y. & Orsi, G. (1988): The eruptive history of Pantellaria (Sicily Channel) in the last 50 ka. Bull. Volcanol., 50: 47-57. Civetta, L., Galati, R. & Santacroce, R. (1991): Magma mixing and convective compositional layering within the Vesuvius magma chamber. Bull. Volcanol., 53: 287-300. Crisci, G.M., De Rosa, R., Esperanqa, S., Mazzuoli, R. & Sonnino, M. (1991): Temporal evolution of a three component system: the island of Lipari (Aeolian Arc, southern Italy). Bull. Volcanol., 53: 207-221. Cramp, A., Vitaliano, C.J. & Collins, M.B. (1989): Identification and dispersion of the Campanian Layer (Y-5) in the sediments of the Eastern Mediterranean. Geo-Marine Letters. 9: 19-25. Froggat, P.C. (1990): INQUA Inter-Congress Committee on tephrochronology field conference and workshop on tephrochronology. Abstracts. Guest, J.E., Duncan, A.M. & Chester, D.K. (1988): Monte Vulture volcano (Basilicata, Italy): an analysis of morphology and volcanoclastic facies. Bull. Volcanol., 50: 244-257. Keller, J. & Ryan, W.B.F. (1978): Explosive volcanic activity in the Mediterranean over the past 200,000 years as recorded in deep-sea sediments. Geol. Soc. Am. Bull., 89(4) :591-604. La Volpe, L. & Principe, C. (1991): Comments on "Monte Vulture volcano (Basilicata, Italy): an analysis of morphology and volcanoclastic facies" by J.E. Guest, A.N. Duncan and D.K. Chester. Bull. Volcanol., 53: 222-227. Paterne, M., Guichard, F. & Labeyrie, J. (1988): Explosive activity of the south Italian volcanoes during the past 80,000 Years as determined by marine tephrochronology. J. Volcanol. Geotherm. Res., 34: 153-157. Sweatman, T.R. & Long, J.V.P. (1969): Quantitative electron probe microanalysis of rock forming minerals. J. Petrol., 11: 53-69. Varekamp, J.C. (1980): The geology of the Vulsinian area, Lazio, Italy. Bull. Volcanol., 43: 487-503. Watts, W.A. (1985): A long pollen record from Laghi di Monticchio, southern Italy; a preliminary account. J. Geol. Soc. London., 142: 491-499.
A PALAEOMAGNETIC
STUDY
OF M A A R - L A K E S E D I M E N T S F R O M
THE WESTEIFEL B. Haverkamp, Th. Beuker Institut ffir Geophysik, WWU Mfinster, Corrensstr.24, D-4400 M/inster
ABSTRACT A record of geomagnetic secular variations (SV) has been obtMned from 13 cores from the maar-lakes Meerfelder Maar (MFM), Holzmaax (HZM) and Sehalkenmehrener Maar (SMM). Logs of initial magnetic susceptibility have been used for intercore correlation. Remanence variations in declination and first of all in inclination from the different lakes have been combined. The Westeifel SV curves as derived so far for the last 10,000 yrs agree well with the SV mastercurves for the U.K. (Turner & Thompson 1979, 1981, 1982), but with differences in age up to 1000 yrs. The time-scale for the post-glacial Westeifel sediments is based on varve-chronology (Zolitschka 1986, 1989, 1990). The glacial sediment sequences were dated by matching our SV records with those from Lac du Bouchet (Thouveny et al. 1990). An ash flow tuff (basaltic ash tuff, MFM 38.5 m) was dated in this way to 25,700 yrs BP. This age agrees well with a thermoluminescence age for this tuff of about 25,000 yrs BP (Velde 1988).
INTROD
UCTION
Magnetic records in lake sediments contribute to our understanding of recent behavior of the geomagnetic field by extending back observatory data to a few 10,000 years. However, not every lake meets the basic requirements for magnetic recording. In the last 20 years more than 100 lakes have been investigated, but only about 20 of these studies led to acceptable SV-records (Thompson & Oldfield 1986). Therefore the magnetization of sediments has to be analyzed carefully in order to see whether the measured variations in declination and inclination correspond to fluctuations of the past geomagnetic field or not. The present study represents a part of a broader project to construct a detailed quaternary stratigraphy. Within this cooperation the main interest has been to use palaeomagnetic methods for correlating multiple cores taken from one lake or from different lakes and to Lecture Notes in Earth Sciences, Vol. 49 3. F. W. Negendank, B. Zolitschka (Eds.) Paleoliranology of European Maar Lakes 9 Springer-Vedag Berlin Heidelberg 1993
350 date sediments. High resolution intercore correlation was carried out by matching logs of initial magnetic susceptibility. Palaeomagnetic dating is possible by comparing measured curves of secular variation with well-dated SV-curves from other sites.
CORING AND SAMPLING From 1980 to 1987 13 cores were taken from three lakes: Holzmaar (HZM), Meerfelder Mawr (MFM) and Schalkenmehrener Maar (SMM) (Figure 1). Spechnens with a volume
Figure 1: Logs of lithology of palaeomagnetic investigated cores from HZM, MFM and SMM (after Negendank 1989) varying from 6.4 cm 3 to 9.6 cm 3 were subsampled at an average distance of 2.3 cm along the cores were used for palaeomagnetic investigations. Since 1980 the coring technique, the length of the subcores (drives), a~d the sampling method improve d step by step. Table 1 shows the different coring parameters changing for the different coring campaigns. Accordingly the palaeomagnetic results from the different cores are of different quality.
STRATICRAPHIC CONTROL Intercore correlation was completed by comparing logs of initial susceptibility (~). The magnitude of ~ is linked to the amount of minerogenic material. For example, turbidites or thin elastic layers result in sharp spikes of ~. The postglaeial organic sediment sequences from MFM A and MFM C (Fi~ure 2) with strong variations in mineral content could
351
u
..m
o
cf~
,.Q
352 Table 1: Coring parameters ; sc= simple ram piston corer, ic= Livingston piston corer, uc=improved piston corer after Usinger, cyl=cylinder, cub=cube. location core year
corer length (m) subdrives: number diameter (cm) specimen: number shape
HZM
q
MFM A 87 uc 40
84 lc 2
C 84 uc 13
K 84 lc 15
L 84 lc 15
23 5
1 5
I0 8
9 8
II
13
18
20
5
5
5
8/5
s/5
8
9 5
10 5
1096 cyl
86 cyl
438 cub
385 cub
656 cyl
638 cyl
1325 cub
1238 cub
322 cub
478 cyl
481 cyl
P 84 lc 30
I i0
126 cyl
B 87 uc 42
C 87 uc 10
SMM N O 84 84 lc lc 13 13
B 84 uc 12
R 83 sc 2
be matched with a precision of 2 - 5 cm depth. The amplitude of the ~-logs from the glacial sequences MFM A and 13 does not vary as much as in the case of the postglacial sediments. However, it can be matched with a maximal error of 10 cm depth (Figure 2). h combining the individual depth-scales of the different cores in order to. get a common depth-scale, a preliminary reference-scale was chosen from one of the cores. Thus it was possible to determine the length of gaps between subcores. A varve chronolog)" was performed for the postglacial sediments of MFM K, L (Zolitschka 1986, 1990). Accordingly the depth-scale from these cores was taken as the reference-scale, which gave a well-determined relationship between depth and time. Besides it was possible to reduce the 7.30 m long twisted and mixed-up slumping zone beginning at a depth of 8.50 m in MFM A and B, to its original undisturbed thickness of 4.30 m in sequence MFM K / L (Figure 3). Similarly the depth-scale was reduced for a clastic layer and for three turbidites which each were of about 30 cm thickness. These fast deposited segments were removed from the depth-scale, because magnetic recording is not possible in such layers and the depth-scale would have been streched too much in relation to the time-scale. In a similar way a 'modified depth-scale' was related to each lake. In contrast to the easily performed intercore correlation by the aid of x-logs, interlake correlation was obviously not possible by this method (Figure 4), although the ~-logs show the same trends especially for HZM and SMM (Figure 4). All logs have high ~-values for the glacial and late glacial sediments (below LBT) and for the uppermost sediments in common, whereas the early and middle HoIocene organic sediments between these periods usually have very low values.
353
STABILITY OF NATURAL REMANENT MAGNETIZATION (NRM) Before interpreting palaeomagnetic data it is important to get information about the stability of the NRM of the samples, because a primary magnetization representing the earth's magnetic-field at the time of deposition can be masked by a secondary magnetization aquired after deposition. Alternating field (AF) demagnetization is a conventional method for stability studies. A detailed step-by-step AF-demagnetization was carried out for 1/4 of all samples. The specimens with high NRM-intensities (> 0.1 m A / m ) showed partly a weak secondary component which was removed in peak demagnetizing fields of 8 10 mT (Figure 5 a, c). Stronger peak-fields resulted in decreasing intensity of the primary magnetization only, indicating that no other secondary magnetization is left. Therefore
Figure 3: Logs of initial susceptibility as a function of common depth-scale for the stacked curves MFM A/B and MFM K/L. The slumping zone in MFM A/B was reduced to its original thickness by comparing with the undisturbed logs from MFM K/L
354
Figure 4: Logs of initial susceptibility from HZM, MFM and SMM versus common depth-scale. The tephra layer LST is marked with an asterix the remaining specimens were demagnetized at 10 mT. For the samples from postglacial sequences with high organic content and weak NRMintensities (0.1 - 1.0 mA/m) AF-demagnetization was difficult to do. This was partly due to the sensitivity of the magnetometer, but mainly to the fact that they were highly susceptible to laboratory remagnetization effects like gyro-remanent-magnetization (GRM) (Stephenson 1980, 1981) which results in a zigzag demagnetization-curve and can prevent the detection of the primary component (Figure 5 b). Viscous remanent magnetization (VRM) was found in weak magnetized organic postglacial segments from the Meeffelder Maar. According to the results of our investigations oa these sediments these viscous effects were mainly caused by superparamagnetic grains of hematite. The coercive force of this fraction is comparable to that of the primary magnetization. Therefore AF-demagnetization is not suitable to separate the two magnetic components. As it is well known, VRM decays in field free space (Creer 1957). Therefore the VRM was reduced by storing our samples in a # - metal cylinder for several weeks. We found that 15 weeks were enough to eliminate the aquired VRM-components to a high degree. Fhrther storing of up to 6 months showed no improvment. Figure 6 shows the distribution of NRM-directions for a group of specimens before storage and after 15 weeks storage in field free space. It is a mixture of specimens with high anct low NRMintensities of about the same age. Thus the directions should be roughly the same. The
355
p.'~
N
0
v
0
0
m
0
~
e
~
m
~k ~
m
cL.~ .0
~2
356 specimens with high intensities and, according to the stability-tests, reliable magnetic directions group very well at about 300 ~ declination and 70 ~ inclination. The samples with low NRM- intensities show different directions before storage and change their direction towards that of the further group after storage. According to isothermal remanent magnetization (IRM) and anhysteretic remanent magnetization (ARM) experiments, the main carriers of the primary magnetic component are assumed to be titanomagnetites with single-domain grain size. Superparamagnetic grains and a high-coercive fraction like hematite were detected in MFM sediments.
CONSTRUCTION OF THE PALAEOMAGNETIC SECULAR VARIATION RECORD Before an acceptable curve of secular variation can be obtained, first spurious data have to be removed from records of remanence inclination and declination. Plots of raw inclination and declination demagnetized at 10 mT for the glacial sequences of the cores MFM A and B are shown in Figure 7. The mean declination of each subcore was set to zero, because there was no core orientation in the horizontal plane. A moving-vector-average of
Figure 6: Distribution of NRM directions (a) before storing in field free space and (b) after storing for 15 weeks 20 points width was used to detect spurious data. A specimen, with magnetic direction varying more than 25 - a precision parameter known from Fisher-statistics - from the mean direction of its group, was determind as an outlier and was removed (Beck 1983, Fisher 1953). A total of 199 data pairs were removed from the cores MFM A, B, and C. 66 pairs were removed by using this method, 70 pairs, because they originate from deformed sediments and 63 were identified as turbidites. The remaining 1462 data pairs
357
"0 0
N
0
bO
E~'-~
358
~
.~
bO
0
~,~
.~ ,.-,, 0
0 ~
r~
359
(inclination and declination) were smoothed with a moving-vector-average of 10 points width. A continuous declination curve was derived by fitting the declination record for the single drives of one core to those from the overlapping drives of the parallel cores (Figure
7). The results from MFM A, B, C were stacked to a mean curve of inclination and declination
Figure 8: Stacked SV-curves derived from MFM A,B,C as a function of the 'modiKed depthscMe', smoothed with a moving average of order 10. The envelope-curves on the left and on the right side represent the a95 angle of confidence.
360 as a function of depth (Figure 8). The 5 m long gap between 8 m and 13 m depth is the previously mentioned slumping zone (Figure 3) which only occurs in MFM A, B. Results from mu early coring campaign in 1982 were used to close the gap although these data are of lower quality. Investigations of cores MFM K, L yielded similar results as cores MFM A,B and C, but the records were of worse quality, due to sampling with cyhndrical boxes. The attempt to get a comparable record of a postglacial SV from HZM and SMM cores failed. This might have been caused by a mixture of difficulties encountered with these sediments: inclined coring directions, insufficient overlapping of the subcores (only 1 m length) and a weak Nt=tM intensity (< 0.1 mA/m).
Figure 9: Comparision of inclination and declination results from MFM with the mastercurve from the U.K. (Turner & Thompson 1979, 1981, 1982). Notations were taken over from Creer (1985). Time scales are Varve time (Westeifel) and uncorrected 14C ages (U.K.). The declination and inclination records of U.K. are shifted to the right to compare the directio, plots easily.
361
AGE CONTROL AND SV RECORDS FROM OTHER STUDIES It is a general problem to compare palaeomag-netic results from diffcrent studies with one another by means of their time scale. If an independent age control exisits, it is usually derived from several radiocarbon datings along the sediment sequence. Radiocarbon dating is susceptible to systematic errors (Hedges 1983) and the necessary interpolation between dated points is in contradiction to the continuous nature of the magnetic record. A m~jor advantage of sediments from MFM and HZM with rcgard to palaeomagnetic work is the annual lamination of the postglacial sediments which allows the application of varve chronology: An assessment of the exactness of palaeomagnetic records can only be made by comparing them with records from other studies. One of the best SV records of the l ~ t 10,000 years is the mastercurve from the U.K. (Turner & Thompson 1979, 1981, 1982). With respect to the higher latitude of the British lakes the inclination for MFM is expected to be 3.5 ~ lower. The curves from U.K. and from MFM agree very well (Figure 9). This good agreement of the ages for the different minima and maxima is suprising as the dating of MFM depends on varve time (calendar years) wtfile the scale for the U.K.-curve is derived from uncorrected 14C-years. A comparision of the varve-scale with the corrected 14Ctimescale shows differences in age of up to 1000 years. Readman et al. (1990) recognized similar discrepancies between their radiocarbon ages from Sot0 SO Denmark and those of Turner & Thompson (1979, 1981, 1982). A shallower inclination of about 8 ~ for the MFM sediments, deposited in the last 4000 yrs, might be due to the well-known inclination error (King 1955). The palaeomagnetic measurements on glacial sediments from the three maar lakes, HZM, MFM and SMM resulted in well comparable curves of inclination (Figure 11). Acceptable curves of declination were not obtained, because of the previously mentioned problems in coring and sampling. In Figure 11 an inclination record from Lac du Bouchet is shown (Thouveny et al. 1990) together with the corresponding results from the Westeifel. Obviously corresponding peaks between the curves were given the same numbers. Varve chronology is not yet available for the glacial Westeifel maar lake sediments. Therefore 14C ages from Lac du Bouchet were transfered. By this way a ash flow tuff, found at 38.5 m depth in MFM A and B was dated palaeomagnetically to 25,700 yrs BP. This corresponds well to a thermoluminescense age of 25,000 yrs (Velde 1988).
362
CONCLUSIONS The patterns of declination and inclination from sediment cores from the Westeifel maar lakes agree reasonably well with those from other European lakes. On that account it can be taken as certain that the secular variation of the geomagnetic field must be the main cause for the magnetic records obtained from the sediment cores discussed in this study. Therefore the SV-record shown in Figure 10, which is a combination of the results from HZM- and MFM-cores, should be a good estimation of the SV for the last 30,000 years. Using the pattern of declination and inclination from the Westeifel it will be possible to transfer the varve chronological time-scale to other European SV-records.
Figure 10: SV record for the Westeifei, Germany. Stacked declination and inclination records from HZM (only for 0 - 800 yrs BP) and MFM. Varve- time for postglacia2 sediments and palaeomag-aetic dating according to Lac du Bouchet for glacial sediments.
363
o~
364
REFERENCES
Beck, M.E. (1983): Comment on: Determination of the angle of a Fisher distribution which will be exceeded with a given probability by P.L. McFadden. Geophys. J., 75: 847-849. Creer, K.M. (1957): Palaeomagnetic investigations in Great-Britain 5, the remanent magnetization of unstable Keuper marls. Phil. Trans. Roy. Soc. London, A250: 130-143. Creer, K.M. (1985): Review of Lake Sediment P'Aaeomagnetic Data. Geophys. Surveys. 7: 125-160. Fisher, R.A. (1953): Dispersion on a sphere. Proc. Roy. Soc. London, A217: 294-305. Hedges, R.E.M. (]983): Radiocarbon Dating of Sediments. in: Creer, K.M., Tucholka, P. & Barton, C.E. (ed.): Geomagnetism of Baked Clays and recent Sediments. Elsevier, Amsterdam: 37-44. King, R.F. (1955): The remanent ma~etiz~don of artificially deposited sediments. Mon. Not. Roy. astro. Soc., Geophys. Suppl., 7: 115- 134. Negendank, J.F.W. (1989): Pleistoz~ine und holoziine Maarsedimente der Eifel. Z. dt. geol. Ges., 140: 13-24. Readman, P.W. (1990): Geomagnetic secular variation from Holocene lake sediments of Sore Sr Denmark. Phys. Earth Planet. Inter., 62: 4-18. Stephenson, A. (1980): Gyromagnetism and the remanence aquired by a rotating rock in an alternating field. Nature, 284: 48-51. Stephenson, A. (1981): Gyroremanent magnetization in an anisotropic rock sample. Phys. Earth Planet. Inter., 25: 163-166. Thompson, R. & Oldfield, F. (1986): Environmental Magnetism. Allen &: Unwin, London. Thouveny, N., Creer K.M. & Blunk, I. (1990): Extension of the Lac du Bouchet palaeomagnetic record over the last 120,000 years. Earth Planet. Sci. Lett., 97: 140-161. Turner, G.M. ~ Thompson, R. (1979): The behavior of the Earth's Magnetic Field as Recorded in the Sediments of Loch Lomond. Earth Planet. Sci. Lett., 42: 412-426. Turner, G.M. & Thompson, R. (1981): Lake sediment record of the geomagnetic secular variation in Britain during Holocene times. Geophys. J., 65: 703-725. Turner, G.M. ~z Thompson, R. (1982): Detransformation of the british geomagnetic secular variation record for Holocene times. Geophys. J., 70: 789-792. Velde, C.(1988): Thermolumineszenzbestimmungen an Seesedimeaten der Eifelmaare. Diplomarbeit, Institut ffir Geographie Univ. Trier.
365
Zolitschka, B. (1986): Wazvenchronologie des Mcerfelder Maares. Diplomarbeit, Institut fiir Geographie Univ. rIYier. Zolitschka~ B. (1989): Jahreszeitlich geschichtete Seesedimente aus dem Holzmaar und dem Meerfelder Maar. Z. dr. geol. Ces., 40: 25-33. Zolitschka, B. (1990): Sp~itquart~re jahreszeitlich gesch[chtete Seesedimente ausgew~hlter Eifelmaare. Dissertation Fachbereich Geowissenschahen Univ. Trier, Documenta naturae, 60.
PRELIMINARY 50m PALAEOMAGNETIC RECORDS FROM LAC DU BOUCHET, HAUTE LOIRE, FRANCE
T. Williams t, K.M.Creer I & N. Thouveny2 1Dept of Geology and Geophysics, Grant Institute, King's Buildings, University of Edinburgh, EH9 3JW, UK. 2Labomtoire de G6ologie du Quatemaire, CNRS, Luminy, Marseille, France
ABSTRACT Following on from earlier studies carried out in successive phases, four much longer cores were collected from Lac du Bouchet, a maar lake in the Velay region of France (44.9~ 3.8~ Prelirninzry palaeomagnetic records from core H to a depth of 46m are presented here. Measurements of natural remanant magnetization (N'RM), both raw and after demas in alternating fields of up to 60rot were carried out on 1382 subsamples. Also anhysteretic [ARM] and isothermal [IRM] remanent magnetizations, and low field magnetic susceptibility were measured to give a picture of the Changes in magnetic mineralogy recorded downcore. These laboratory implanted magnetizations are useful as proxy indicators of climate and for assessing the suitability of the sediment for making estimates of relative geomagnetic palaeointensity. We have, at this stage of the work, normalized N-RM intensity only with susceptibility but argue that this provides a valid record of palaeointensity, except for where the sediment is highly organic. Preliminary pollen analyses suggest that Holsteinian interglacial sediments are present at a depth of about 42m, so it is thought that the records cover at least the last 250000 years. Long wavelength, high amplitude oscillations in palaeointensity are found which are different from the oscillations .in climate over this period.
1. INTRODUCTION Lac du Bouchet is a maar lake, located at 44.9~ 3.8~ in the Velay region of France. It forms a circular shape of 800m diameter. Maar lake basins constitute rather small and isolated catchment areas, resulting in slow and continuous sedimentation under quiet conditions through extended periods of time. Lac du Bouchet has proven especially suitable for the recovery of palaeomagnetic records because the rocks forming the crater
Lecture Notes irt Earth Sciences, Vol. 49 J. F. W. Negendank, B. Zolitschka (Eds.) Paleolimnology of European Maar Lakes 9 Springer-Verlag Berlin Heidelberg 1993
368
rim, which constitutes the entire catchment area, are exclusively basaltic. This provides a source of detritus rich in magnetic grains, resulting in an NRM intensity which is unusually high for sediments, and a fairly consistent grain composition downcore. The bottom sediments have been cored on several occasions during the last 10 years, each time with longer cores being recovered (Bonifay et al., 1986, 87; Smith and Creer, 1986; Creer, 1989). The penultimate phase formed part of the Geomaars project, when a depth of 20m, equivalent to an age of -115 Ka was reached (Creer et al, 1990, Thouveny et al, 1990). Four cores were taken in September 1990, the maximum depth reached being 65m, using a SEDI drill as part of the E.C. funded Euromaars project and a wide range of measurements involving many different disciplines are currently being carried out on them. At the time of writing, sufficient palaeomagnetic results have been obtained to describe preliminary records extending down to a depth of 46m, which we estimate to be ~250 Ka old.
2. CORING AND SAMPLING 2.1 Coring The drilling raft consisted of a 7 by 10m working platform supported by steel flotation chambers. It was held at a chosen point near the centre of the lake b y four steel cables to restrain any possible movement during the extraction of the sections comprising each core. Four cores (named G, H, I and K) were extracted in sections of 1.5m to lm length, at lateral separations of ~10m. The upper 25m of the bottom sediments were soft enough to be cored with the piston corer which took each core in sections of 105cm length and 8cm diameter b y application of a steady vertical force with a hydraulic ram, without any rotation. The sediment cored in each section was held in the coring barrel on retrieval with a Kullenberg piston, in a similar fashion to the well-known %ivingston' method. Immediately after extraction the core sections were extruded into plastic storage tubes. Typically about 3cm were lost from the top of each section in the process. Core sections obtained in this manner were of good quality, with very little internal sediment deformation. At greater depths between -25 and ~50m, where the sediment was more compact, the Mazier method had to be used. The Mazier corer consists of an outer tube running from the raft to the bottom of the hole with a coring head on the lower end, and a short inner core barrel. The latter is dropped down through the outer tube and is fLxed to the bottom of it, but is left free to rotate. To take a core section, the outer tube is rotated and downward pressure applied. The inner barrel is acted upon by the friction of the sediment and should remain rotationaUy stationary while it penetrates the sediment. When the inner barrel is full, it is retrieved by means of a cable. Sections of up to 150cm length and of 5cm diameter were taken in this way, and typically about 10cm were lost from the bottom of each section when the head of the inner barrel was removed to get at the sediment. In practice, it was found impossible to prevent rotation of the inner tube absolutely, as
369
evidenced by a long wavelength a~irnuthal twist in the logs of declination of NRM (see section 4), though otherwise the recovered sediment appeared to the naked eye to be undisturbed, and of good quality. None of the core sections were azimuthally oriented. Below ~50m, the sediment was very coarse grained and contained much gravel. 2.2 Sub-sampling The cores were opened and sub-sampled in Marseille at the CNRS Laboratory of Quaternary Geology, Luminy. First the plastic tubing was cut by pulling the core sections through a pair of rotary buzz-saws, and the sediment core was then carefully split in half down its length with a wire. Sub-samples for palaeomagnetism were taken in clear plastic cubic boxes of 2cm side which were pressed into one of the half-cores every 2.5cm down the central axis, omitting broken sediment where it occurred. The full sample boxes were then removed and sealed to prevent loss of moisture. Below a depth of 46m, the sediment became too hard for boxes to be inserted.
3. MEASUREMENTS 3.1 Weight The samples were weighed soon after sub-sampling in order to be able to normalize all magnetic measurements to unit rna~s and to determine the water content at the end of all the experiments when dry weights will be measured. 3.2 Magnetic susceptibility Susceptibility is defined as the reversible magnetization induced (per unit mass) by a weak field (in these experiments an altemating field of 0.05roT using a Bartington bridge instrument). Its value depends on the concentration of magnetic grains (the dominant effect in our cores), on the grain size (large grains result in high susceptibility) and also on mineral composition. Susceptibility was measured fu'st on the unopened core sections and then on all individual sub-samples. Susceptibility provides a reliable and rapidly obtained proxy record of climatic change, being low in the more organic sediments deposited during warm climatic conditions, and high in detritus rich sediments deposited during cold climates (Creer and Thouveny, 1990). 3.3 Natural remanent magnetisation (NRM) The directions and intensities of /fiRM of all samples were measured on a 2G cryogenic magnetometer with 7.5 cm aperture. The NRM of lake sediments is usually acquired mainly after deposition of the sediments on the lake bottom while the water content remains higher than about 70%. When it falls below this critical value further grain rotation is inhibited and a post depositional remanent magnetization (PDRM)
370
becomes 'frozen in'. Laboratory experiments (Tucker, 1980, 1983) showed that this occurs at a depth of 15 - 20era, corresponding, for Lae du Bouchet deposition rates, to a time lag of a few hundreds of years after sediment deposition. The process is helped by a prealignment of the magnetic grains as they fall through the water to the bottom o f the lake. NRM intensity thus depends partly on the concentration, size and type of magnetic grains, but importantly also upon the degree of alignment of those grains, which has been shown in the laboratory (Tucker, 1980, 83) to be proportional to the geomagnetic applied (and weak) magnetic field. The concentration of magnetic grains is affected by climatic change. Thus, in order to recover the geomagnetic palaeointensity, we first have to remove the climate-induced contribution to the NRM intensity. This problem is discussed in section 4. The direction of NRM is specified by the declination (azimuthal angle) and inclination of the total vector to the horizontal plane (positive downwards). Since the core sections were not oriented, there is no common 'zero' azimuth of declination down the whole core, and an arbitrary zero has to be assigned to each section of core. These 'floating' declinations must be adjusted across section boundaries if a continuous downcore declination record is to be reconstructed. The method applied requires at least two cores with overlapping section ends. For the fn'st core, section-average declinations were calculated and then subtracted from the individual declination measurements through the section. Then declination trends at the base and top of adjacent sections were aligned with the continuous record from the second core, using a modification of a method originally descibed by De~harn (1981). In practice more than two paralleI cores are desirable to reconstruct continuous downcore declination records and since, at the time of writing, only one core has been studied in depth, it has not yet been possible to begin this task. 3.4 Alternating field demagnetization All samples were demagnetized using progressively stronger alternating fields, from 2.5 to 60roT. Direction and intensity were measured after each step. Viscous magnetization was completely removed by 10roT. A stable direction was left through the remaining demagnetization steps. The average median destructive field for core H is about 16mT. 3.5 Laboratory implanted remnant magnetizations Magnetizations can be implanted in the laboratory under different experimental conditions designed to affect a range of different grain and domain sizes. The magnetically softer (larger) grains contribute mainly to the weak-field susceptibilty. Anyhysteretic remauent magnetization (ARM) is implanted by applying a weak direct (bias) field of about 0- lmT in the presence of a much stronger alternating field which is decreased from a high value (-100roT) to zero. Hence the downcore ARM/susceptibility ratio shows variations in the relative importance of small over large grains through time, other parameters remaining constant.
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Fig 1. Low field susceptibility (10 -6 x m3/kg) plotted against ARM (dimensionless, normalized to the applied direct field of 0. linT).
Fig 2. Low field susceptibility (10-6 x m3/kg) plotted against Qk-rat/o (A/m).
372 Isothermal remanem magnetization (IRM) is implanted with a very strong direct field of up to IT and its strength is largely dependent on concentration of the smaller grains. 4. RESULTS AND DISCUSSION 4.1 Constancy of downcore magnetic mineralogy Figure 1 shows a plot of ARM against susceptibility. If only the concentration of the magnetic carrier were changing downcore, we would expect representative points for all of the samples to lie on a straight line through the origin. Samples containing a greater proportion of smaller grains would fall above the line while those with a preponderance of larger grains would fall below the line (King et al., 1982). We conclude that magnetic mineral grain size remains rather constant down the length of the core. Variations in magnetic mineralogy could, in principle, be caused by changes in rates and types of weathering of the source rock, by changes in production of bacterial magnetite or by changes in the flux of wind-blown material from further afield. 4.2 Palaeointensity determination We conclude that the magnitudes of susceptibility, ARM and IRM all reflect primarily a variation in the concentration of magnetic minerals in the sediments. Hence, all three measured quantities may be used in attempts to normalize the measured NRM in order to recover the intensity of the ancient geomagnetic field. The normalization procedure used here is simply to divide the NRM intensity by susceptibility (k) to give the Qk-ratio, by ARM intensity to give the Qa-ratio or by IRM to give the Qi_ratio. The validity of the palaeointensity record from the top 6m of the Lac du Bouchet cores has been assessed by plotting the Q-ratio against the normalising parameter fl'houveny, 1987). This method is applied here to core H and Figure 2 shows the Qk-ratio plotted against susceptibility. Two types of behaviour are to be observed: (i) there is large scatter in Qk-ratio for susceptibilities above 350 • 108kg/m 3, indicating that it is independent of susceptibility; (ii) below this value, the Qk-ratio is always low, and we think that this is due to a different magnetic mineralogy of the more organically rich samples and we should therefore be wary of palaeointensities derived from these horizons. In fact at the Holocene/late glacial boundary (1.2m) a jump in the Qk-ratio occurs where previous palaeointensity studies (Constable and Tauxe, 1987) show a steadily increasing field strength. With this reservation in mind, we think that our Qk-mtio provides a good record of geomagnetic palaeointensity. 4.3 Palaeomagnetic logs Figure 3 shows five palaeomaguetic parameters (declination, inclination, cleaned NRM intensity, susceptibility and Qk-ratio) running downcore on a depth scale constructed
373
Fig 3. Downcore logs of declination (~ inclination (~ NRM intensity (10 .6 x Am2/kg), low field magnetic susceptibility (10 "~ x m3/kg) and Qk-ratio (A/m). Each dot represents one sample measurement. The solid lines on the intensity, susceptibility and Qk-ratio plots trace a 10 point running mean of the data. The horizontal bars on the declination plot locate the show the boundaries of the sections of core H. The plotted values for NRM declination, inclination and intensity are those obtained after partial demagnetization in a 10roT alternating field.
374
from data mainly from core H (1128 samples). Gaps in the core H records have been filled using data from cores B and G (1382 samples in total). Declination measurements have been rotated to plot around each core section average, but without further rotations to match up the derivatives (slopes) across section ends. Below 25m a clockwise twist (looking downcore) of some sections is evident. Inclination has an average of 59.4 ~, slightly less than the value expected for an axial dipole field (63.4 ~ at the site latitude). A detailed (negative) correlation between the % of arboreal pollen and magnetic susceptibility has been established for the top 20m of the Lac du Bouchet record (Creer, 1991). Susceptibility has also previously been used as a proxy indicator of climate in loess (Kukla et al., 1988) and in deep marine sediments (Kent, 1982). Warm climatic periods identified from pollen analysis (Reille and de Beaulieu, 1988) can be seen as low susceptibility intervals in figure 3: the Holocene above 1.2m; the St Germain I and H events around 19m and 16.5m; and the Eemian interglacial at 21m. Below the Fen'dan the only interval to be provisionally identified by palynology so far is the Holsteinian interglacial at 42m (immediately above a --4~0cm tephra layer), and on this evidence we estimate the bottom of the records to have an age of about 250000 years. Differences in pattern of the three right hand records in Figure 3 support our contention that susceptibility and Qk-ratio register different signals, for example the St Germain 1I climatic event (16.5m) is almost completely normalised out of the Qk-ratio record. We interpret susceptibility as a climatic indicator, the Qk-ratio as a geomagnetic indicator, and NRM intensity as having both geomagnetic and climatic inputs. 4.4 Comment on Palaeointensity Record Assuming our Qk-ratio log is a reasonable approximation to past geomagnetic palaeointensity, we can begin to comment on the past behaviour of the Earth's field over the last 250000 years. First, it is seen that the palaeointensity is far from constant over this time, having very large amplitude changes. Second, the Qk-rafio plot also shows these changes to be mostly long wavelength, with shorter wavelengths superimposed on top. Third, for the most part, the palaeointensity and susceptaq~ility (-climate) do not follow each other. Fourth, palaeointensity lows at 7m and 21m correspond to times when 'excursions' of the field (Laschamp and Blake respectively) have been found recorded in volcanic lava flows (Thouveny et al., 1990).
5. CONCLUSIONS Palaeomagnetic and rock-magnetic measurements made on core H from the latest (Euromaars) Lac du Bouchet series, give preliminary records extending to 46m depth. Interpretation of these records is at an early stage. The susceptibility variations provide a rapidly obtained proxy-indication of climatic change and hence the core is provisionally interpreted as spanning more than two full glacial cycles, and together with the provisional
375
palynological identification of the Holsteinian interglacial near to the lower end of the core, the bottom of the record may be dated at around 250 Ka before the present. The Qkratio is interpreted as providing a reliable record of relative geomagnetic palaeointensity. The NRM intensity is thought to have been controlled by both the climate and geomagnetic field strength, with the latter having the stronger effect. Susceptibility and Ok-ratio (geomagnetic palaeointensity) logs exhibit long wavelength trends which show differences suggesting that they are recording different phenomena. The average of inclination values downcore (estimated time coverage about 200Ka) is similar to that expected for an axial dipole field. Declination measurements show that some of the sections cored using the Mazier technique have been subjected to a twisting about the vertical axis. 6. REFERENCES Bonifay, E., Creer, K.M., Smith, G., Thouveny, N., Truze, E., and Tucholka, P. A preliminary palaeomagnetie survey of the Holocene and late Wurmian sediments of Lac du Bouchet (Haute Loire, France). Geophys. I. Roy. astr. Soc., 86, 943 - 964, 1986. Bonifay, E., Creer, K.M., de Beaulieu, LL., Casta, L., Delibrias, G., Perinet, G., Pons, A., ReiUe, M., Servant, S., Smith, G., Thouveny, N., Truze, E. and Tucholka, P. Study of the Holocene and late W~rnian sediments of Lac du Bouchet (I-Iaute-Loire, France): first results, pp 90 - 116 in Climate: History, Periodicity and Predictability, eds: M.R. Rampino, LE. Sanders, W.S. Newman and L.K. K{Snigsson, van Nostrand ReinhoM Co Inc; Stroudsburg. 1987. Constable, C.G, and Tauxe, L Palaeointensity in the pelagic maim: marine sediment data compared with archaeomagnetic and lake sediment records. Geophys. J.R. astr. Soc., 90: 43-59, 1987. Creer, K.M. The Lac du Bouchet palaeomagnetic record: its reliability and some inferences about the character of geomagnetic secular variations through the last 50,000 years, pp 71 - 89 in Geomagnetism and Palaeomagnetism, eds. F.L Lowes, D.W. Collinson, LH. Parry, S.K. Runcom, A.W. Soward and D.C. Tozer, Kluwer Academic Press, Dordrecht, 1989. Creer, K.M. Dating of a maar lake sediment sequence covering the last glacial cycle. Quaternary Proceedings, 1: 75-87. 1991. Creer, K.M., Thouveny, N., and Blunk I. Climatic and geomagnetic influences on the Lac du Bouchet palaeomagnetic SV record through the last 110,000 years. Phys. Earth Planet. Inter., 64: 314-341.1990.
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Denham, C.R. Numerical correlation of recent palaeomagnetie records from two Lake Tahoe cores. Earth planet. Sci. Lett., 54, 48 - 52, 1981. Kent, D. Apparent correlation of palaeomagnetic intensity and climatic records in deepsea sediments. Nature, 299: 538-540. 1982. King, J.W., S.K. Banerjee, L Marvin and O. Ozdemir. A comparison of different magnetic methods for determining the relative grain size of magnetite in natural materials: some results from lake sediments. Earth Planet., Sci. Lett., 59: 404-419. 1982. King, J.W., S.K. Banerjee and J. Marvin. A new rock magnetic approach to selecting sediments for geomagnetic palaeointensity studies: application palaeointensity for the last 4000 years. J. Geophys. Res., 88 : 5911-5921. 1983. Kulda, G., F. Heller, Liu Xiu Ming, Xu Tong Churl, Sheng Liu Tung and An Zhi Sheng. Pleistocene climates in China dated by magnetic susceptibility. Geol., 72: 811-814. 1988. Reille, M. and de Beaulieu, J.L. La fan de LT_~emianet les interstades du Prewurm mis pour la premiere lois en evidence darts le massif central francais par l'analyse pollinique. C. R. Acad. Sci., Paris. 1988. Sorley, R.
Susceptibility and climatic variations down a lake sediment core. Internal
report, Dept of Geophysics, Edinburgh, 34pp. 1989. Smith, G. and Creer, K.M. Analysis of geomagnetic secular variations 1000 to 30,000 years BP, Lac du Bouchet, France. Phys.Earth Planet. Int., 44: 1-14. 1986. Thouveny, N. Variations of the relative palaeointensity of the geomagnetic field in western Europe in the interval 25-10 Ka BP as deduced from analyses of lake sediments. Geophys. J.R. astr. Soc., 91: 123-142. 1987. Thouveny, N., Creer K.M., and Blunk I. Extension of the Lac du Bouchet palaeomagnetic record over the last 120,000 years. Earth Planet. Sci. Lett., 97: 140-161. 1990. Tucker, P. Magnetisation of unconsolidated sediments and theory of DRM. pp 9 - 19 in: K.M.Creer, P.Tucholka and C.E.Barton (eds,), Geomagnetism of Baked Clays and Recent Sediments, Elsevier; Amsterdam. 1983. Tucker, P. A grain mobility model of post-depositional realignment. Geophys. Ja2. astr. Soc. 63: 149-163.1980.
P A L A E O M A G N E T I C I N V E S T I G A T I O N S OF LAGO G R A N D E DI MONTICCHIO, S O U T H E R N ITALY
Inn Turton Department of Geology and Geophysics, Kings Buildings, University of Edinburgh, Edinburgh, Scotland
ABSTRACT Two long sediment cores were collected from Lago Grande di Monticchio in 1990. Whilst not yet firmly dated, it is generally agreed that this record spans the last 250,000 years. An initial time scale has been calculated for the record by using susceptibility as a proxy climate indicator. The depth to time transform used was confirmed by the initial results of the pollen analysis of the cores. A relative palaeointensity record has been calculated for the core. It is concluded provisionally that the pMaeointensity recorded in the sediments is effected by the astronomical frequencies associated with the precession of the Earth, the eccentricity and the obliquity of the Earth's orbit.
~TRODUCTION The Laghi di Monticchio are a pair of lakes situated in the caldera of Monte Vtilture. The lakes form the deepest part of the basin of this caldera. The lake surfaces are at about 656 m above sea level and e~pproximately 100 km east of Naples (see figure 1). In 1990 a series of cores were collected using a Livingstone type corer, modified by H. Usinger of Kid University, FRG. The first of these coring operations in the center of the lake was abandoned due to the extreme hardness of the sediments at a depth of only 4 m below the lake bottom. The raft and coring equipment were then moved to shallower water and a further three cores were obtained. The~e cores were transported
Lccttlre Notes in Earth Sc/.ences, Vol. 49 3. F. W. Negendank, B. Zoli~chka (Eds.)
Paleolimnology of European Maar Lakes 9 Springer-Verlag Berlin Heidelberg 1993
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Figure 1: Location of Laghi di Monticchio to Trier University, FRG, for subsampling for geochemical, micro--sedimentology, pollen and ostracod analysis, as well as palaeomagneticwork. PALAEOMAGNETIC
RESULTS
Of the Livingstone cores collected, the longest (core D) was sampled for palaeomagnetic study. The natural remanent magnetizations (NRM's) of the cores was measured on a 3--axis Superconducting Rock Magnetometer produced by 2-G, California (figure 2). Each sample was also weighed and the NRM's were normalized by the weight of each sample (figures 2).
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Figure 2: NRM measurements of Core D
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Some core sections were not sub-sampled as they had been disturbed in coting or during the extrusion process, or because they contained only slumped sediments which would not provide a reliable palaeomagnetic signal. This disturbance was easily recognised by visual inspection of the cores which were normally horizontally laminated. It can be seen that the NRM of samples from the upper 10 m of the core ere very weak and that both the incl~nation and declination are very scattered. However below this depth the intensity of the NRM increases and the directions are more coherent. The thin peaks in intensity axe identified with tephra layers. The susceptibility of all samples was also measured on a Bartington Bridge (see figure 5). A series of pilot samples (5%) were selected from core D. The tenth and thirtieth samples from each metre section of core (approximately 40 samples) were chosen and alternating field (AF) demagnetization was applied in a series of steps at peak fields of 5, 10, 15, 20, 30 roT, using the demagnetizing coils built into the 2G magnetometer. Some of the results are shown in Figure 3, and it can be seen the samples can be divided into two groups. The first group, from the upper part of the core, have low intensities and give generally scattered directions during demagnetization. The second group, from the lower sections of the core, produce much better demagnetization plots. There is very little scatter and once a small viscous component has been removed they tend to produce straight lines towards the origin on the Zijderveld plots, which represents the direction of stable remanence. The remainder of core D was demagnetized only in 10 and 20 mT and the results of the 10 mT demagnetization are shown in figure 4, which starts at a depth of 10m since the upper weak sections were not me~ured. The results of the 20 mT demagnetization are similar and are not shown. The samples from core D were given an anhysteretic remanent magnetization (ARM) in an AC field of 100mT, biased by a DC field of 0.1mT, to allow the determination of the magnetic mineral concentration and size distribution.
These ARM's were AF
demagnetized with a 10rot peak field to allow the comparison of the stabilities of ARM's and NRM's.
Each sample was given a saturated isothermal remanent magnetization
(SIRM) in a DC field of IT. Climate effects are seen most easily in measurements of parameters which are only
381
Figure 3: Pilot demagnetization plots of Core D
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Figure 4: Results of Core D, alter AF demagnetization in a peak field of 10roT
383
effected by the concentration of magnetic minerals rather than by their alignment. Three magnetic parameters can be used for this - susceptibility, AP~Mand SIRM. As can be seen in figure 5, the downcore variations of these are all similar for the sediments of Monticchio. The crosscorrelation coefficients for these curves are all of the order of .68 which is a highly signific&ut correlation with this many points. These climatic v~ations are likely to be a manifestation of the Milankovitch periodicities which control global climate in the long term. In an attempt to determine more information about the magnetic mineralogy of the core ratios of xo/ARM, ARM/SIRM and xo/SIRM were calculated. A graph of X ~
versus X (as defined King et al. (198"2)) can be used to determine
the magnetic grain size of the sample. The magnitude of susceptibility is controlled by the concentration of large grains of magnetite, whereas ARM and SIRM are a~ected by finer grained magnetic material. They show that if there is a significant susceptibility (X) associated with the non-magnetic matrix there will be an offset of the abscissa, this appears to be absent from figure 6. The ratios xo/ARM and Xo/SIRM plotted down core (figure 8) show generally stra;ght Lines with very Little variation in the sections which show high susceptibility, ARM's and SIRM's and more scatter in the sections where these parameters are low. This is most probably an instrumental effect in that the errors will be proportionally higher in the sections with lower values which will tend to increase the scatter. A1LM/SIRM (figure 8) also shows a similar steady value with no coherent signal being present. These three ratios indicate that there is very little variation in size or type of magnetic mineral down the length of the core. There may be some changes in the section of the core deposited during warm periods (low magnetic parameters), though the quality of the results at such low values leaves this interpretation open to doubt. The uniformity of the mineral type and grain size validates an attempt to determine the relative palaeointensity of the geomagnetic field. This can be done by normalizing the NRM intensity by any of the p~rameters controlled by grain properties. Therefore three more ratios were calculated for the samples down the core, NRM/susceptibility, NRM/ARM, NRM/SIRM (see figure 9). At this point any sample with a susceptibil~ ity/ARM ratio of greater than 5 was removed as it was felt that these samples represented
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Figure 5: The magnetic parameters which are dependent on magnetic grain size and type, and are linked to the climate (see text for more details) a mineral assemblage too far from the straight line plotted in figure 7. This lead to the removal of ,,,250 samples, the majority from the upper ten metres of the core but with some from the lower sections which corresponded to tephra layers, ~ recorded in the sedimentological description of the cores. The NRM intensity and susceptibility are included in figure 9 for comparison. The similarity of the three ratios (correlation coefficients are all greater than .70) should be
385
Figure 6: X.4~M vs. X as defined by King et al (1982) noted first. This is due to the stability of the magnetic minerals down the core. Therefore it is not really necessary to determine which of the three magnetic parameters is the best normalizing factor since they all appear equally good. The second point to note in figure 9 is that while the intensity of the NRM and the susceptibility show the Milankovitch periodicities. The ratios calculated also show a strong periodicity but this is out of phase with the climate record (as shown by susceptibility). They all appear to be offset by about 6 metres. If these normalized values of intensity are really indicating the intensity of the geomagnetic field then it appears that the geodynamo also shows the Milankovitch periodicities but that they are lagging the climatic effects which seems to indicate that there must be a direct linkage between the geodynamo and the astronomical phenomena and that this
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Figure 7: Intensity of Susceptibility vs. ARM of Core D. Crosses indicate tephra layers which were removed from later analysis (see text for more details) linkage can not be through the climate. INITLA,L DATING OF THE CORE The conversion of the depth scale to time scale is
always a
difficult part of any
palaemecular variation study. In the case of these cores the age of the sediment makes the use of radiocarbon dating impractical except in the upper sections, where the palaeomagnetic signal is poor. Also Watts (1985) comments that there were problems with the conventional radiocarbon dates that he obtained from a core from this lake. He indicates that there may be inclusion of old carbon in the sediments of the lake. No radiocarbon age determinations have yet been made on the cores taken in 1990. A second common method for the age determination of cores is the use of pollen analysis to indicate periods of warming and cooling, which can be correlated to similar occurrences at other sites which have been dated. Therefore in an initial attempt to date the long cores from Monticcklo the downcore
387
Figure 8: The three mineral magnetic ratios calculated for the core, all three are indications of the variability of magnetic gzain type and size
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Figure 9: Normalized palaeointensity curves for ~re D, ~'ith the urmormalized NRM intensity and susceptibility shown for comparison
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Figure 10: The SPECMAP 6lso Isotope Curve, Imbrie et al. 1984
Figure 11: Carbon to Nitrogen Ratio of Core C susceptibility profiles were used. Previous work from Lac du Bouchet (Creer 1991) has shown that the percentage of non-arboreal pollen can be correlated well with susceptibility, showing that susceptibility is a good proxy climate indicator. This is advantageous since magnetic susceptibility is a very simple measurement to make and can be carried out on a core relatively quickly. In the case of the Monticchio cores the magnetic susceptibility was compared to the SPECMAP 61so record derived from marine cores by Imbrie et aL (1984) (figure 10). There is evidence from geochemical analysis of the cores that the base of the Holocene is at about 7.5 metres down (Robinson et al, this volume). This is indicated by the change in values of the carbon/nitrogen ratio (figure 11). It can be interpreted as a change in organic input coming from aquatic plants to terrestrial plants, which would accompany the climatic change at the end of the Weichselian glacial period. The remainder of the correlation was completed by comparison of the smoothed susceptibility record with the SPECMAP 61so record. Several possible correlations were tried, assuming that the long period variations seen in the susceptibility record were -,,40
390
Figure 12: A comparison of the depth to time transforms used Kyr and ,~i00 Kyr. Figure 12 shows the two initialestimates of the depth to time transform that were derived. The final line shown on figure 12 is a depth to time transform derived from preliminary pollen analysis (Watts and Allen~ Pers. Com.). As can be seen this line agrees with the older estimate of age except for the location of the end of oxygen isotope stage 2. This depth time transform was selected for the finalconversion of core D to a time.scale. Figure 13 shows the normalized palaeointensity parameters on this time $c~e.
CONCLUSIONS A first attempt to date core D from Lago Grande di Monticchio was m a d e by correlating the variation of susceptibility taken to provide a proxy climate indicator with the marine oxygen isotope record (Imbrie et al., 1984). This method has great potential for
391
Figure 13: The normalized palaeointensity parameters transformed to a time scale, Nl~M intensity and susceptibility are included for reference
392
application to work on long sediment cores because susceptibility can be measured much more rapidly than direct dating by conventional methods such as palynology. The base of core D is thus tentatively dated as at 250 Kyr. This analysis is verified by preliminary pollen analysis results. Palaeointensities have been calculated for all the samples from core D, by normalizing with susceptibility, ARM and SIRM to correct for variations in the quantities of magnetic minerals down-core, and a tong wavelength signal is visually apparent. Inspection of these periodicities would appear to indicate that the geedynamo is modulated by the Milankovitch frequencies. REFERENCES Creer, K. (1991): Dating of a Ma,xr Lake Sediment Sequence Covering the Last Glacial Cycle. Quat. Procs., h 75--87 Imbrie, J., Hays, J., Martinson, D., McIntyre, A., Mix, A., Morley, J., Pisias, N., Prell W. & Shackleton, N. (1984): The Orbital Theory of Pleistocene Climate: Support from a revised Chronology of the Marine 180 Record, In: Berger A., et al. (ed.), Milankovitch and Climate, 269-305, D. Reidel, Hingham, Mass. King, J., Banerjee, S., Marvin, J. & Ozdemir, (). (1982): A Comparison of Different Magnetic Methods for Determining the Relative Grain Size of Magnetite in Natural Materials: Some Results from Lake Sediments. E.P.S.L., 59:405-419 Watts, W., (1985): A Long Pollen Record from Laghi di Monticchio, South Italy, a Preliminary Account. J. Geol. Soc., 142(3): 491-499.
LATE-GLACIAL/HOLOCENE CHANGES OF THE CLIMATIC AND TROPHIC CONDITIONS IN THREE EIFEL MAAR LAKES, AS INDICATED BY FAUNAL REMAINS. I. CLADOCERA
Wolfgang Hofmann
Max-Planek-Institut ftir Limnologie, Abt. Mikrobenrkologie, Postfach 165, D-2320 P16n
Abstract
The oligotrophic Weinfelder Maar, on the one hand, and the eutrophic Schalkenmehrener Maar and Hohmaar on the other, showed distinct differences in their planktonic and benthic cladoceran faunas since the beginning of the Holocene. In the Weinfelder Maar, Bosmina longispina occurred in the Late-Glacial and early Holocene. It was not detected in the other two maar lakes. In the latter, there was a dramatic change in the structure of the chydorid community as indicated by a strong decrease in species diversity and an increase in the abundance of small Aloha species in the Atlantic period induced by eutrophication. The climatic change at the Late-Glacial/Holocene boundary was not reflected by an increase in chydorid diversity.
1. Introduction
Some planktonic and benthic Cladocera leave behind well preserved remains in lake sediments, which permit the analysis of former cladoceran faunas and the reconstruction of their long-term development by sediment analysis. Faunal shifts provide information on
Lecture Notes in Earth Sciences, Vol. 49 J. F. W. Negendank, B. Zolitschka (Eds.) Paleolimnology of European Maar Lakes ,P,,Springer-Verlag Berlin Heidelberg 1"393
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former changes of the environmental conditions (Frey 1986). The aim of this study is to view a basic pattern of long-term development of the cladoceran fauna of an oligotrophic (Weinfelder Maar) and two eutrophic (Schalkenmehrener Maar, Holzmaar) Eifel maar lakes. The analysis is based on a relatively low number of samples and long vertical sample intervals, i. e. low time resolution. Furthermore, an attempt will be made to use the subfossil cladoceran assemblages as indicators of the former ecological conditions of the lakes with special reference to climatic and trophic changes.
2. Material and methods
Limnological characteristics of the Eifel maar lakes under discussion were recently given by Brtick (1985) and Scharf (1987).
The sediment samples analysed were from the following profiles: Holzmaar profile P, Schalkenmehrener Maar profile AI, and Weird'elder Maar profiles A and B. Only the upper 12 m section of the Holzmaar profile is considered here, for faunal remains of the lower section see Hofmann (1990). The profile P has been correlated with the dated cores B/C according to Zolitschka 1989. The profile A I from the Schalkenmehrener Maar was described by Rein (1991) including correlation with profiles AII/III. The samples from the Weinfelder Maar profiles A and B have been combined in accordance with the standard profile arranged by Brauer (1988). No datings exist from this profile so far. However, Brauer proposed chronological zonations based on sedimentological data which are also used in this paper. Lengths of the profiles, dates, and chronology - as far as available - are shown in Fig. 1. The position of the layer of the Laacher See Tephra (LST) indicates that the profiles reach down to the Allerrd period.
The profiles analysed were dated only by correlation with dated profiles or have not been dated at all (Weinfelder Maar). Moreover, sampling distances were rather large. Therefore, association of sediment horizons with chronozones in the text remains vague.
395
Sampling intervals were 30 cm in the Holzmaar and 20 cm in the Schalkenmehrener Maar and the Weinfelder Maar, respectively. In particular sections distances were reduced.
Five to ten g fresh sediment were treated with hot 10 % KOH on a magnetic stirrer. For cladoceran analysis the fractions >100 um and 55-100 um were examined separately. In aliquot subsamples, equivalent to 0.06 to 1.75 g sediment (mostly 0.2 - 0.5 g) cladoceran remains were counted under a microscope at 80x magnification. For identification F16Bner (1972) and Frey (1958, 1959) were used. Taxonomy refers to F16Bner (1972).
The index Hs was calculated as a measure of species diversity (Lloyd & Ghelardi 1964). The chydorid assemblages were grouped by cluster analysis (UPGMA) on the basis of percentage similarities (Southwood 1971, Sheath & Sokal 1973, Hofmann 1986). Ecological grouping of the chydorid taxa refers to Whiteside (1970) and Fl6Bner (1972). Loss on ignition was determined at 550 ~
F I G U R E 1 Holzmaar (HZM), profile P; Schalkenmehrener Maar (SMM), profile AI; Weinfelder Maar (WFM), profiles A/B: length of the profiles, chronozones, and position of the Laacher See tephra layer (LST) (chronozones: AL - AUer6d; D3 - Dryas 3; PB Preboreal; B - Boreal; AT - Atlantic; SB - Subatlantic; SA - Subatlantic) (after Zolitschka 1989, Brauer 1988, Rein 1991).
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3. Results
3.1 Loss on ignition
The long term changes of the organic content of the sediments as indicated by the loss on ignition (% DW) followed the same basic pattern in all three profiles: (1) low values (approximately 5 %) in the Late-Glacial, (2) increase in the Preboreal (> 10 %), (3) maximum in the Atlantic/Subboreal (30 - > 50 %), and (4) a significant decrease in the Subatlantic (down to approximately 10 %) (Fig. 2).
This variation, based on a relatively low number of samples, is in accordance with the much more detailed study of the Schalkenmehrener Maar by Rein (1991) in which also a second peak was found at approximately 550 cm (Boreal) was found. Rein has shown that in these sediments the relationship between loss on ignition and content of organic carbon is not constant but varies considerably within the profile. However, the general course of changes in organic carbon content is also reflected by loss on ignition (Rein & Negendank, this volume).
In the Weinfelder Maar, Ehlscheid (1990) found the same pattem as shown in Fig. 2 including the peak at 60-70 cm sediment depth.
3.2 Total Cladocera
The Weinfelder Maar profile is divided into four sections based on the abundance of Cladocera remains per gram DW. (1) From 782-482 cm sediment depth there were only 9single findings. Zero to 21 specimens per sample were found per sample and mean abundance was 5.2 specimens/g. These samples were from Late-Glacial sediments which were disturbed by allochthonous material. (2) In the 462-382 cm section abundance
397
FIGURE 2 Weinfelder Maar (WFM), Schalkenmehrener Maar (SMM), Holzmaar (HZM): loss on ignition (% DW).
398
increased averaging 140 specimens/g. The sediment consists of Younger Dryas silt and clay layers. (3) From 382-372 cm abundance increased by more than two orders of magnitudes. In the section 372-163 cm abundance ranged from 8,900 to 92,000 (mean value: 37,000) specimens/g. The sharp increase corresponds to the Younger Dryas/Preboreal boundary and this section spans the Preboreal - Atlantic period. (4) In the uppermost section 153-1 cm which more or less corresponds to the Subboreal-Subatlantic (Brauer 1988), the number of specimens decreased again to an average of 5,000 specimens/g (Fig. 3).
In the Schalkenmehrener Maar, abundances increased in the section 790-710 cm from 4 to 33,000 specimens/g. This section spans the Upper Pleniglacial, early Late-Glacial and the beginning of the Aller6d. Maximum values occurred between 610- 450 crn (Boreal Atlantic): mean value 34,000 specimens/g. There was a distinct decline of abundance at 450-410 cm.
In the section 430-210 cm (late Atlantic - Subboreal - early Subatlantic) the average was 13,000 specimens/g and from 190- 50 cm (Subatlantic) the mean value was 7,000 specimens/g. In the uppermost samples there was an abrupt increase again: At 30 cm and 10 cm there were 30,000 and 33,000 specimens/g, respectively. As shown in Fig. 3, abundances varied considerably within the individual sections.
In the deepest section from Holzmaar (1077-843 cm) (AllertSd - Preboreal), several thousand specimens/g (mean value: 3,400) were found. From 811-211 cm values generally ranged between 10,000 and 22,000 specimens/g (mean value: 16,500) (Boreal Subatlantic). In the uppermost 200 cm abundances were distinctly lower averaging 5,800 specimens/g.
With respect to abundance of cladoceran remains, the three maar lakes showed a similar course of development with high values in the middle Holocene and a decrease in the Subatlantic. Furthermore, the values were in the same range of several 104 specimens/g. The Weinfelder Maar differed from the other two lakes by having much lower abundances
399
FIGURE 3 Weinfelder Maar (WFM), Schalkenmehrener Maar (SMM), (HZM): Cladocera abundance (N*g DW-1).
Holzmaar
400
during the Late-Glacial and did not reach 10,000 specimens/g until the Preboreal. Furthermore, the decrease towards the top of the profile was more distinct.
3,3 Planktonic Cladocera
The subfossil Cladocera found in profundal sediments come from two different environments. The Chydoridae live in benthic littoral habitats whereas the planktonic Bosminidae and Daphniidae live in the pelagic zone. With respect to the planktonic elements, only the genera Daphnia and Bosmina are considered here. The abdominal claws of Daphnia and the shells (carapaces) of Bosmina were counted.
In the Weinfelder Maar, the portion of the planktonic elements of total Cladocera changed distinctly within the profile. Percentages for the individual samples cannot be given for the deepest 3 m of the core. However, in this section 102 cladocerans were counted; only 8 were planktonic and all 8 were found in the same sample (522 cm). From 462 cm there was a steady increase in this portion and in the section 362-163 cm about 90 % of the Cladocera were planktonic. In the uppermost 140 cm of the profile they nearly disappeared and accounted for less than 5 % of the Cladocera found (Fig. 4).
Similarly, in the Schalkenmehrener Maar the abundance of planktonic taxa was extremely low in the deepest section (790-730 cm). Their percentage was less than 0.5 % of total Cladocera. From 710-310 cm values were scattering between 40- 90 % with considerable variation over short distances. In the upper 3 m of the profile the planktonic species consistently accounted for 90 % of the total Cladocera.
In the Holzmaar, the portion of pelagic elements in the lower 6.3 m (1077-443 cm) varied between 60-90 %, followed by a decline and a minimum from 411-311 cm of 40-70 %. In the upper 211 cm percentages increased to approximately 90 % (with an exception at 179 cm).
401
FIGURE 4 Weinfelder Maar (WFM), Schalkenmehrener Maar (SMM), Holzmaar (HZM): Bosmina + Daphnia abundance as percentage of total Cladocera.
402
Regarding the contribution of individual planktonic taxa, in the Weinfelder Maar the genus
Daphnia played a minor role. The percentage of Daphnia remains exceeded 5 % of the total planktonic Cladocera in only four samples which were from the 352-262 cm area. Two species of the genus Bosmina, B. longispina and B. longirostris, were the predominating taxa among the planktonic Cladocera which determined the pattern shown in Fig. 4.
This is the first record of the species Bosmina longispina from the Eifel region. It does not exist here today and was not found in the sediment profiles from the Meerfelder Maar (Hofmann 1984a) as well as from the Schalkenmehrener and Holzmaar. The population had a short first antenna (mean length: 170-180 urn) with a low number of segments (mean number: 13). The mucrones exhibited long-term morphological variation: In the Younger Dryas (372 cm) they were significantly shorter (mean length: 54 urn) than in the Preboreal (342 cm, mean length: 88 grn) (t-test; p < 0.001).
Apart from the high abundance of B. longirostris at 362 cm, there was a clear succession from B. longispina predominating at 352-332 cm to high abundance of B. longirostris in the adjacent section 322-153 cm. Above this point only five single findings of this species exist, with exception of the sample taken at 20 cm where 509 specimens/g DW were found (Fig. 5).
In the Schalkenmehrener Maar the dynamics in the planktonic cladocerans were characterized by alternating fluctuations of Daphnia sp. and Bosmina longirostris.
Daphnia predominated in the sections 710-550 cm (AllerSd - Atlantic) and 450-270 cm (Subboreal - early Subatlantic). B. longirostris showed an isolated peak in the Boreal (610 cm), predominated in the middle Atlantic (510-470 cm), where its maximum abundance (90,000 specimens/g DW) was found, and in the middle Subatlantic (250-10 cm). In this species fluctuations were particularly high as shown by extremely large differences between neighbouring samples even if the vertical distances of 20 cm are considered. For instance, the samples from 470 cm and 450 cm include maximum abundance and total absence of B. longirostris.
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FIGURE 5 Weinfelder Maar (WFM): Bosmina longispina and Bosmina longirostris (N*g DW-1); Schalkenmehrener Maar (SMM) and Holzmaar (HZM): Bosmina longirostris and Daphnia sp.(N*g DWI).
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The development in the Holzmaar resembles the situation in the Schalkenmehrener Maar; only Daphnia was found in the deepest section (1077-843 cm) and B. longirostris predominated in upper part of the profile (211-5 cm). From 843-211 cm Daphnia was generally more abundant, but Bosmina however had very high abundances at several individual horizons. Abundance of Daphnia was remarkably low on these occasions thus giving the impression of countercurrent fluctuations in these two planktonic ctadocerans.
3.4 Benthic littoral Cladocera
Twenty-three chydorid species were found in the Schalkenmehrener Maar and 20 species were found in both the Weinfelder Maar and Holzmaar. In the Weinfelder Maar the species Aloha intermedia and Chydorus piger were present but were lacking in the other two maar lakes. However Oxyurella tenuicaudis, Leydigia quadrangularis, and Pleuroxus
uncinatus were found in the Schalkenmehrener Maar and Holzmaar (Fig. 6). In the Weinfelder Maar it was not possible to distinguish between Alona rectangula and
Aloha intermedia carapaces so it was not possible to give accurate abundances during periods of co-occurrence. Abundances were estimated by the portion of head shields of the two taxa. The number of head shields was generally low, so the precision of the percentages as shown in Fig. 6 is limited.
In the lower part of the profile (782-482 cm) abundance of chydorids was so low that samples of two sections had to be combined. The horizon "582 cm" in Fig. 6 refers to the section 782-582 cm the total material from which consisted of only 31 specimens. Likewise, "482 cm" means the section 562-482 cm (63 specimens). Otherwise, the data represent individual horizons in each of which the number of counted specimens was > 100.
During the Late-Glacial/Postglaciat period distinct shifts in the structure of the chydorid assemblages occurred by changes in both the number of species and the percentage composition.
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FIGURE 6 Weinfelder Maar (WFM), Schalkenmehrener Maar (SMM), and Holzmaar (HZM): percentages of chydorid species.
406
In the beginning (782-422 cm) (Older Dryas (?) - Allerrd) the assemblage consisted of only five elements: Alona rectangula, Alona quadrangularis, Alona affinis, Alonella nana,
and Chyclorus sphaericus. In the subsequent period a stepwise increase in the species number occurred. For instance, at 442 cm Camptocercus rectirostris, Acroperus
eleongatus, Graptoleberis testudinaria, Monospilus dispar, and Chydorus piger appeared in very low frequencies. As deep as 352 cm (Preboreal) the assemblages were characterized by six predominating species. Towards the top of the profile, other species were found in fairly high abundance: Monospilus dispar, Disparalona rostrata, Alonella
exigua, and Chydorus piger. Clustering of the assemblages based on their percentage similarities, clearly divided the development into four major phases (Fig. 7). The grouping is determined by the most abundant taxa: (1) 782-392 cm: Chydorus sphaericus, Alonelta nana, Alona rectangula,
Aloha quadrangularis; (2) 382-332 cm: Alonella nana, Alona intermedia, Acroperus harpae; (3) 322-140 cm: Aloha intermedia, Alonella nana, Alona rectangula, Monospilus dispar; (4) 120-1 cm: Chydorus sphaericus, Alonella nana, Alona intermedia. In the latter section, the 60 cm sample was conspicuous due to its high percentage of Alona
quadranguIaris. In the Late-Glacial from Schalkenmehrener Maar, three species were predominating:
Chydorus sphaericus, Alona rectangula, and Acroperus harpae with a shift from Aloha to Acroperus in the AllerSd just below LST. The following chronozones (Younger Dryas, Preboreal, Boreal) were each represented by one sample only. But the situation in the 650610 cm section remained more or less constant. A dranaatic cut is discemible at the beginning of the Atlantic. This period was characterized by extremely high predominance of Atona rectangula; percentages ranged between 39.5-89 % (mean value: 62.7 %). A second species, Chydorus sphaericus, showed considerably high percentages (9.9-41%) and ,a third species, Pleuroxus trigonellus, was found in all the samples from this period. The remaining taxa irregularly occurred in very low abundances. With respect, to the two
407 most abundant species the chydorid assemblage resembled the fauna found in the deepest Atlertid section.
Subsequently, Chydorus sphaericus became the most abundant species (percentages: 37-64 %) during a short section (430-390 cm) which represents the Atlantic/Subboreal boundary.
In the following Subboreal/Subatlantic period a chydorid was established which was characterized by maximum number of species and a rather even percentage distribution. The
most abundant species were Chydorus sphaericus, Acroperus harpae, Alona
quadrangularis, Alonella nana, and Camptocercus rectirostris. As some species became abundant which had already occurred before the Atlantic (Alona affinis, Mona
quadrangularis, Leydigia quadrangularis, Alonella nana) the Subboreal/Subatlantic assemblage had a structure very similar to that from Preboreal/Boreal-times.
This relationship is also reflected by the cluster diagram in which the samples from the 630-690 cm section are situated together with those from 370-10 cm on the same two main branches (Fig. 7). The Atlantic assemblages with extremely high percentages of
Alona rectangula are grouped on the left side of the cluster together with further samples from the sections 750-710 cm and 470-350 cm in which Alonarectangula and Chydorus
sphaericus were also the most abundant taxa. The development of the chydorid fauna in the Holzmaar can be separated into six phases (Figs. 6, 7): In the first section (1077-911 cm) spanning the Late-Glacial six taxa predominated: Acroperus harpae, Chydorus sphaericus, Leydigia quadrangularis, Alona
rectangula, Aloha quadrangularis, and Alona affinis. In the following period (879-779 era; Preboreal/Boreal) Acroperus harpae, Alona affinis, and Leydigia quadrangularis decreased while Alona rectangula and Alonella nana increased their percentages thus becoming the most abundant taxa - together with Chydorus sphaericus.
In the Atlantic period (section 743-541 cm) the same shift to very high percentages in
408
FIGURE 7 Weinfelder Maar (WFM), Schalkenmehrener Maar (SMM), and Holzmaar (HZM): cluster diagram of the percentage similarities between the chydorid assemblages of the individual samples.
409
Alona rectangula occurred as shown in the Schalkenmehrener Maar. Mean percentage of this species for this period was 69 % however on two occasions values were higher than 90 % (611,579 cm). The chydorid fauna differed from Schalkenmehrener Maar due to lower abundances of Chydorus sphaericus varying around 10 %. Graptoleberis
testudinaria exhibited a phase of relatively high percentages in both lakes in the same period from Boreal to middle (?) Atlantic.
In the subsequent period - end of the Subboreal and beginning of the Subatlantic (511-411 cm) - there was a change in the small Alona species from A. rectangula to A. costata. In this case there was the same problem in the estimation of the abundances as mentioned with respect to A. rectanguIa and A. intermedia in the Weinfelder Maar. This period is also characteristic due to exceptionally high abundance of Pleuroxus trigonellus.
In the middle (?) Subatlantic (379-211 cm) the number o~" species and evenness of percentage distribution increased, although on individual horizons Alona rectangula,
Chydorus sphaericus, Aloha quadrangularis, and Graptoleberis testudinaria reached very high percentages. Thus, there were several shifts in the predominating species.
The uppermost section (179-5 cm; late (?) Subatlantic) was more uniform and characterized by predominance of two species: Alonella nana and Chydorus sphaericus. Some species which were frequently found in the section beneath did not appear, i. e.
Pleuroxus trigonellus and Peracantha truncata.
The cluster diagram illustrates the particular chydorid fauna of the Atlantic by an isolated branch on the left which includes the 311 cm horizon due to its extremely high A.
rectangula value. It also shows the Aloha costata section (511-411 cm) in an isolated position. The heterogeneity of the chydorid development during the early Subatlantic (379211 cm) led to the fact the samples from this period are spread over three main branches.
In the Weinfelder Maar, chydorid diversity was low during the Late-Glacial. With the exception of two samples, values varied around 2. In the Preboreal diversity increased,
410
provided that the 352 cm horizon was still Younger Dryas (s. Brauer 1988), and henceforth remained within the range of 3.0 and 3.4, except for the 60 c m sample in which diversity was distinctly lower (2.63) (Fig. 8).
In the Schalkenmehrener Maar diversity increased from the Allert~d to the Younger Dryas starting from a very low level of 1.0 to 2.5. Referring to Rein (1991), the two deepest samples with diversities of only 1.0 and 1.1, respectively, were from t h e Middle Weichselian. In the beginning of the Holocene (Preboreal - Boreal; 630-610 cm) values did not get higher and in the Atlantic diversity dropped again even below the AllerSd level reaching a minimum value of 0.56 at 550 cm. Otherwise, the values ranged mostly between 1.0 and 2.0 during the Atlantic period. In the Subboreal diversity increased and reached a maximum level (3.0-3.5) from the Subatlantic to the top of the profile. Fig. 8 shows that chydorid diversity is heavily influenced by the abundance of Aloha rectangula. In the Atlantic period abundances exceeded 1,000 specimens/ g DW which led to a distinct drop in diversity of the chydorid fauna. The data also indicate that extremely high percentages of Aloha rectangula during this period were not due to low abundances of the remaining species but were attributed to increased abundance of A. rectanguta itself.
In the Holzmaar, diversity dynamics generally corresponded with the pattem observed in the Schalkenmehrener Maar: (1) no difference between Late-Glacial (Allerrd - Younger Dryas) and early Holocene values (Preboreal - Boreal); (2) in the Atlantic period a drop in diversity during the period of maximum abundance of two small Alona species (A.
rectangula, A. costata); (3) increase during the Subboreal and maximum diversity in the upper section of the profile (Subatlantic).
In the Weinfelder Maar, the percentages of chydorid species considered to be typical of "clear water lakes" and those associated with eutrophic conditions or "turbid water lakes" changed with a clear long-term pattern. The term "turbid water lakes" as used here is equivalent to "polluted lakes" in the sense of Whiteside (1970) (Korhola 1990). In the lower section of the profile "ttirbid water species" predominated, in the middle section "clear water taxa" were most abundant, and in the upper 100 cm the portions of both
411
FIGURE 8 Weinfelder Maar (WFM), Schalkenmehrener Maar (SMM), and Holzmaar (HZM): chydorid species diversity (Hs) and abundance (log N * g DW"~) of Aloha rectangula + Aloha intermedia (WFM), Aloha rectangula (SMM), and Aloha rectangula + Aloha costata (HZM).
412
groups were nearly equivalent. It has to be considered that this course of development was determined by one "turbid water species" which was very abundant during the LateGlacial as well as in Subatlantic (Chydorus sphaericus). It should also be mentioned that the evidence of the "clear water" values is limited by the uncertainty in distinguishing
Mona intermedia and Alona rectanguIa which belong to different groups. Likewise, in the Late-Glacial of Schalkenmehrener Maar percentages of "turbid water taxa" were related to the high abundances of Chydorus sphaericus, in this case together with Alona rectanguIa. In the Atlantic period "turbid water species" predominated again reflecting the extremely high abundance of Aloha rectangula. In the subsequent period to the top of the profile no clear pattern was discemible as the values of the two groups varied irregularly in the range between 30-60 %.
Also in the Holzmaar, abundance of "turbid water species" was high due to high percentages of the same species, Chydorus sphaericus, Alona rectangula, and Leydigia
quadrangularis. In the Atlantic period the shift from Mona rectangula to AIona costata distinctly changed the "clear water" /"turbid water species" ratio. During the Subboreal / Subatlantic the situation was rather complex due to considerable variation in the percentages of both groups.
During the Holocene of Weinfelder Maar most of the chydorids were bottom dwellers clearly surpassing the number of specimens from species typically associated with vegetation. The reason is that bottom dwelling species such as Aloha intermedia,
Monospilus dispar, Aloha quadrangularis, and Chydorus piger, very consistently occurred in considerable numbers. Non-specific taxa, Aloha rectangula and Alonella nana, also played a major role. Such non-specific species predominated during the Late-Glacial and made up more than 75 % of the chydorids (Fig. i0).
Contrarily, in the Schalkenmehrener and Holzmaar chydorids living on vegetation generally predominated the Holocene, except for the period of maximum abundance of the non-specific Alona rectangula. After the Atlantic period in the Holzmaar variation of
413
FIGURE 9 Weinfelder Maar (WFM), Schalkenmehrener Maar (SMM), and Holzmaar (HZM): percentages of chydorid species typical of "clear water lakes" and "turbid water lakes".
414
FIGURE 10 Weinfelder Maar (WFM), Schalkenmehrener Maar (SMM), and Holzrnaar (HZM): Chydoridae: percentages of bottom dwellers and species associated with vegetation.
415
percentages was distinctly higher than in Schalkenmehrener Maar. A peak of "vegetation species" (Alona costata) at 511 cm was followed by a minimum of both "vegetation" and "bottom" elements at 311 cm, due to predominance of the non-specific Alona rectangula. There was another maximum of "vegetation" taxa at 242 cm mainly caused by three species: Graptoleberis testudinaria, Acroperus harpae, and Alona costata. The decrease in the "vegetation" elements occu.rring in the upper 200 cm was not compensated by an equivalent increase in bottom dwellers, but was brought about by high numbers of two non-specific taxa (Alonella nana and Chydorus sphaericus).
4. Discussion
The discussion will focus upon the influence of climatic and trophic conditions on the long-term dynamics of the structure of the planktonic and benthic cladoceran fauna.
Late-Glacial chydorid faunas have been found to be composed of a low number of arctic and subarctic species (Harmsworth 1968) and the subsequent climatic amelioration in the Holocene generally led to a distinct increase in species diversity (Hofmann 1987). In contrast, the climatic changes at the Late-Glacial / Holocene boundary were not reflected by the chydorid diversity curve for the Eifel maar lakes under discussion (Fig. 8). This was particularly true for Schalkenmehrener and Holzmaar where diversity in the Younger Dryas was considerably high (2.5) and did not differ from the Boreal values. This is supported by the percentage composition as shown in Fig. 6. In both cases, Late-Glacial assemblages did not exhibit striking differences to Holocene assemblages with respect to species number and evenness.
Leydigia quadrangularis was again found to be a typical constituent of the Late-Glacial chydorid fauna of Eifel maar lakes as it was in the Meerfelder Maar (Hofmann 1984).
In the Weinfelder Maar, Late-Glacial diversity was generally low (2.0-2.5). In two
416
samples, however, the values were in the range of the Holocene assemblages. As many as 14 taxa were present in the 382 cm sample and from the Younger Dryas o f this lake a total of 16 species were recorded. The Younger Dryas / Preboreal boundary was also accentuated by a sudden increase in the number of Cladocera per g DW in the Weinfelder Maar. This was not the case in the two other lakes (Fig. 3).
Whether the particularly high Late-Glacial chydorid diversity in the Eifel lakes - as compared with locations in Denmark, England, North Germany, Switzerland (s. Hofmann 1987) - can be explained by the large distance from the glaciated area, has to be verified by further faunal and climatic data.
In the planktonic Cladocera, the species Bosmina longispina only occurred in the oligotrophic Weinfelder Maar and was totally missing in the two eutrophic maar lakes as well as in the eutrophic Meerfelder Maar (Hofmann 1984). This species is indeed typically associated with oligotrophic conditions (Flt~13ner 1972). However, the occurrence in the Weinfelder Maar was restricted to the end of the Younger Dryas and the early Holocene (Preboreal, Boreal(?)). Then it was replaced by Bosmina longirostris which is typical of eutrophic lakes (FlrBner 1972). Such a succession from B. longispina to B. longirostris is generally considered to be induced by eutrophication (s. Hofmann 1987). Bosmina
longirostris played an important role in the following Atlantic period and rapidly diminished in the Subboreal possibly replaced by Ceriodaphnia and Diaphanosoma which predominate today (Hofmann 1980). Their ephippia were abundantly found in the upper section of the profile, but they have not been counted.
Bosmina longispina is lacking in the Weinfelder Maar at the present time in spite of its oligotrophic state probably because the species became extinct in the Eifel region in the 9early Holocene. Hence, there was no chance of re-colonization in the Subatlantic.
Although the Schalkenmehrener Maar is less than 1 krn away from the Weinfelder Maar
Bosmina longispina was not able to establish a population in the Younger Dryas and the Preboreal. Therefore ecological differences already existed between the lakes during this period.
417
Interpretation of the alternations between Bosmina longirostris and Daphnia sp. during the Holocene of Schalkenmehrener Maar and Holzmaar is limited by the fact that the Daphnia species involved are not known. High abundance of Bosmina longirostris during the Subatlantic of both lakes is in accordance with the recent eutrophic conditions. The B.
longirostris peaks observed in earlier periods might indicate eutrophic conditions for the total Holocene period and the alternations might have been induced by competition with
Daphnia or another element of the zooplankton not preserved in the sediment. Nevertheless, the excessive fluctuations in Daphnia and Bosmina longirostris indicate massive long-term variation of the ecological conditions in both lakes.
In the chydorids one species typical of oligotrophic conditions, Mona intermedia, was found only in the Weinfelder Maar, whereas two species more related to eutrophic lakes,
Leydigia quadrangularis, Pleuroxus uncinatus (FlOl3ner 1972), were present only in the Schalkenmehrener Maar and Holzmaar. In Aloha intermedia and Leydigia quadrangularis this difference between the lakes lasted since the Late-Glacial.
In terms of long-term dynamics, the structure of the chydorid fauna of the Weinfelder Maar showed a higher degree of stability than in the other two lakes. This becomes obvious, if the percentage composition, the abundance of "clear water/turbid water" species and of bottom and macrophyte dwellers (Figs. 6, 9, 10) are compared and shown by species diversity (Fig. 8). Nevertheless, there was also a signal in the Weinfelder Maar which was the sudden increase in Aloha quadrangularis in the 60-70 cm horizon. This increase clearly affected the portion of "turbid water species" and bottom dwellers (Figs. 6, 9, 10).
In both eutrophic lakes there was a dramatic decline in diversity by excessive increase in absolute (number/g DW) and relative (% chydorids) abundance of the same species, Mona
rectangula. A similar event has also been observed in the eutrophic Meerfelder Maar (Hofmann 1984). A correlation with the Daphnia/Bosmina longirostris fluctuations is not discernible; during this period high and low abundances of both species occurred.
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In none of the case studies from the Holocene of lakes in England, Denmark and northern Germany (Goulden 1964, Harmsworth 1968, Whiteside 1970, Hofmarm 1983, 1986, Fltil3ner 1990) was a similar fluctuation of the chydorid fauna reported. However, on the basis of cladoceran remains in surficial sediments Whiteside & Harmsworth (1967) found chydorid diversity to be strongly negatively correlated with lake productivity. Low diversity in polluted Danish lakes was caused by excessive predominance of Chydorus
sphaericus or Alona rectangula (or both) (Whiteside 1970). Hence, the structure of the chydorid fauna during the Atlantic period of Schalkenmehrener Maar and Holzmaar is possibly indicative of strong eutrophication of the lakes. Consequently, increased Subboreal / Subatlantic diversity could be explained by a lowering of lake productivity.
The long-term dynamics in the cladoceran fauna suggest an increase of lake productivity in the early Holocene whereby different compartments were affected under oligotrophic and eutrophic conditions, respectively. In the Weinfelder Maar, a shift from Bosmina
longispina to B. longirostris was induced in the plankton. In the eutrophic Schalkenmehrener Maar and Holzmaar diversity of the benthic littoral fauna decreased dramatically. Vice versa, reduction of lake productivity in the Subboreal / Subatlantic diminished the abundance of Bosmina longirostris in the Weinfelder Maar and raised chydorid diversity in the two eutrophic maar lakes.
The view of decreasing lake productivity in the late Holocene seems to be in accordance with the decline in loss on ignition in the sediments of all three lakes (Fig. 2) and has recently been deducted from more detailed data on sediment chemistry. Rein (1991) reported a distinct decrease in the accumulation rate of organic carbon in the last 1,000 years of Schalkenmehrener Maar. Content of organic carbon and loss on ignition showed maxima in the Boreal and in the middle and late Atlantic, however accumulation of organic carbon was rather low during those times (Rein & Negendank, this volume). In the Weinfelder Maar, both loss on ignition and content of chlorophyll a + phaeopigments
419
increased as early as in the Boreal and declined in the uppermost section of the profile which is attributed to soil deterioration in the catchment area. The horizon of 60-70 cm sediment depth where a peculiar chydorid assemblage was present was also conspicuous due to a distinct peak in loss on ignition and pigment concentration (Ehlscheid 1990, Ehlscheid & Scharf, this volume).
5. References Brauer, A. (1988): Versuch einer Erfassung alter Seespiegelst~nde an ausgesuchten Eifelmaaren und mikrostratigraphische Untersuchungen an Sedimenten des Weinfelder Maares. diploma thesis, Trier. Brtick,H.(1985): Neue physiographische Daten der Eifelmaare. Decheniana, 138: 193-220. Ehlscheid, T. (1990): Auswirkungen tier Fischerei auf das Plankton und die Wasserbeschaffenheit yon Seen. Landesamt ftir Wasserwirtschaft Rheinland-Pfalz, Mainz. Ftrt~ner, D. (t972): Kiemen- und Blattffii3er, Branchipoda; Fischl~use Branchiura. Die Tierwelt Deutschlands, 60: 1-499. F16gner, D. (1990): Die Geschichte der Cladocerenfauna des Kleinen Barsch-Sees, eines sauren, kalkarmen Moorweihers im mitteleurop~ischen Flacttland. Lirnnologica, 21: 125135. Frey, D.G. (1958): The late-glacial cladoceran fauna of a small lake. Arch. Hydrobiol., 54: 209-275. Frey, D.G. (1959): The taxonomic and phylogenetic significance of the head pores of the Chydoridae (Cladocera). Int. Rev. ges. Hydrobiol., 44: 27-50. Frey, D.G. (1986): Cladocera analysis. In: Berglund, B.E. (ed.), Handbook of Holocene Palaeoecology and Palaeohydrology, 667-692, Wiley & Sons; Chichester. Goulden, C.E. (1964): The history of the cladoceran fauna of Esthwaite Water (England) and its limnological significance. Arch. Hydrobiol., 60: 1-52. Harrnsworth, R.V. (1968): The developmental history of Blelham Tam (England) as shown by animal microfossils, with special reference to the Cladocera. Ecol. Monogr., 38: 223-241. Hofmann, W. (1980): Zum Zooplankton der Eifelmaare. Mitt. Pollichia, 68: 166-176. Hofmann, W. (1983): Stratigraphy of Cladocera and Chironomidae in a core from a shallow North German lake. Hydrobiologia, 103: 235-239. Hofmann, W. (1984): Stratigraphie subfossiler Cladocera (Crustacea) und Chironomidae (Diptera) in zwei Sedimentprofilen des Meerfelder Maares. Cour. Forsch. Inst. Senckenberg, 65: 67-80. Hofmann, W. (1986): Developmental history of the Groger P16ner See and the Schrhsee (north Germany): cladoceran analysis, with special reference to eutrophication. Arch. Hydrobiol. Suppl. 74: 259-287. Hofmann, W. (1987): Cladocera in space and time: analysis of lake sediments. Hydrobiologia, 145: 315-321.
420
Hofmann, W. (1990): Weichselian chironomid and cladoceran assemblages from maar lakes. Hydrobiologia, 214: 207-211. Lloyd, M. & Ghelardi, R.J. (1964): A table for calculating the 'equitability' component of species diversity. J.Anim. Ecol., 33: 217-225. Rein, B. (1991): Versuch einer Rekonstmktion des Pal~io environments anhand hochzeitaufl6sender geochemischer und sedimentologischer Untersuchungen an sp~it- und postglazialen Sedimenten des Schalkenmehrener Maares. diploma thesis, Trier. Scharf, B.W. (1987): Limnologische Beschreibung, Nut.zung und Unterhaltung yon Eifelmaaren, p 117, Ministerium ftir Umwelt und Gesundheit Rheinland-Pfalz, Mainz. Sheath, H.A. & Sokal, R.R. (1973): Numerical taxonomy, p 573, Chapman & Hall; San Francisco. Southwood, T.R.E. (1971): Ecological methods, p 391, Chapman & Hall; London. Whiteside, M.C. & Harmsworth, R.V. (1967): Species diversity in chydorid (Cladocera) communities. Ecology, 48: 664-667. Whiteside, M.C. (1970): Danish chydorid Cladocera: modem ecology and core studies. Ecol. Monogr., 40: 79-118. Zolitschka, B. (1989): Jahreszeitlich geschichtete Seesedimente aus dem Holzmaar und dem Meerfelder Maar. Z. dt. geol. Ges., 140: 25-33.
LATE-GLACIAL/HOLOCENE CHANGES OF THE CLIMATIC AND TROPHIC CONDITIONS IN THREE EIFEL MAAR LAKES, AS INDICATED BY FAUNAL REMAINS. II. CHIRONOMIDAE (DIPTERA)
Wolfgang Hofmann
Max-Planck-Institut far Limnologie, Abt. Mikrobenrkologie, Postfach 165, D-2320 P1/Sn
Abstract
Differences in the structure of the chironomid faunas between the oligotrophic Weinfelder Maar and the eutrophic Schalkenmehrener Maar and Holzrnaar existed since the beginning of the Holocene. The eutrophic maar lakes passed through a period of strong eutrophication in the Atlantic period. In the Weinfelder Maar, a decrease of Micropsectra indicated a higher trophic state in the early Subatlantic,
i. Introduction
Thienemann (1913) found the profundal zones of the Weinfelder and the Schalkenmehrener Maar were occupied by distinctly different chironomid faunas. In the former a species of the genus Chironomus occurred whereas the latter was characterized by a species of the tribus Tanytarsini, Micropsecta coracina (Lauterbornia coracina). The author explained this faunal differentiation by the specific oxygen and nutrient budgets of
Lecture Notes in Earth Sciences, Vol. 49 J, F. W. Negendank, B. Zolitschka (Eds,) Faleolimnology of European Maar Lakes 9 Sprluger-Verlag Berlin Heidelberg 1993
422
"Tanytarsus-" and "Chironomus-" lakes representing oligotrophic and eutrophic conditions, respectively. This relationship between the structure of the profundal chironomid fauna and the trophic conditions of stratified temperate lakes was confirmed as a general principle in the following decades by Lundbeck (1936), Brundin (1949, 1956), and Saether (1979).
The aim of this study was to follow the long term development of the chironomid fauna by analysis of the subfossil remains preserved in the sediment and to use these chironomids as indicators of climatic and trophic changes in the lakes during the lateglacial and postglacial period. This is of special interest as Weinfelder Maar and Schalkenmehrener represent oligotrophic and eutrophic systems, respectively, situated only 0.5 km apart (Scharf 1987). Hence, the different trophic situations are determined by local factors rather than by regional characteristics.
2. Material and methods
The sediment profiles used, datings, sampling distances, and sample preparation is described by Hofmann (this volume).
For chironomid analysis the fraction >200 grn was examined under a stereo microscope at about 20x magnification. The larval head capsules were picked out, dehydrated in 96 % alcohol and mounted in euparal (Hofmann 1986). The keys by Hofmann (1971), Wiederholm (1983) and Moiler Pillot (1984) were used for identification.
3. Results
The taxa found and their proportion in the total material are listed in Table 1. The taxon
Paramerina may include specimens of Zavrelimyia and Xenopelopia. "Chironomini A" refers to a Chironomini head capsule pictured by Hofmann (1991), which so far could not
423
TABLE 1 Percentages of the chironomid taxa in the subfossil material from Weinfelder Maar (WFM), Schalkenmehrener Maar (SMM), and Hohmaar (HZM) (++ = < 1%; + = < 0.5 %).
WFM Tanypodinae
Ablabesmyia Guttipelopia gutb'pennis Labrundinia Iong~a~is Proc/adius Paramefina Pentaneurinl indet
Diamesinae
Protanypus
ProdiamesJnae
Prodiamesa olivacea
Orthocladiinae
Bnllia modesta Cricotopus Corynoneura Heterotrissocladius Orthocladius Paracladius Parakiefferfella Psectrocladius
Chiron~m~nae Chironomini
Tanytarsini
TolaJ number
2
SMM ++ +
HZM 4 + +
11
6
12
1 2
+ ++
3
7
5
++
++
+ +
4" +
+ +
1 1 3 +
+ 3 2
§
4
4
2
Chiron~mini A
++
Chironomus gr.anthrac[nus Chironomus gr.p/umosus C/adopelma Ctyptoch[ronomus Demicryptochironomus Dicrotend~o4es Einfeldia Endochironomus a#3[pennis Endochironomus tendens Gtyptotendipes Lauterbomiella agrayloides M~crotendipes Pagastie/la orophila Parachironomus Parac/adope/ma Paratendipes Phaenopsectra Polyl:~dilum sordens Polypedi/um part. Pseudoch[ronomus Stictochironomus Tfibelcs L~textus
+
++
+
2
+
+ + + 3 2
++
2
§
+
3
3
Cladotanytarsus Corynocera ambigua Corynocer~ otiveri Micrepsec~'a Para tanytarsus Sten~ellina Stempe/linella Tanytarsus %o. D Tanytarsus part. Thienemanniola ploenensis
+
+
+ ++ + 6 + + 3 ++ + ++ + + 3 2
3
+
4-
13
14
++
++
13
18
+ +
+4-
+
+
+
2 1
3 1
++
+
4-+
+
+
4--t-
3 1
2 1
12 2
2 ++
1 30
3 + + ++ 15
4-
3 17
16
+
1521
1582
710
424
be associated with a known genus. Tanytarsus sp. D is characterized by a distally rounded spur at the antennal pedestal.
In general, littoral chironomids play a dominant role in the material with respect to both the number of taxa and their abundance in the three lakes. Profundal taxa such as
Chironomus were rare even in the sediments from the two eutrophic maar lakes. In the Weird'elder Maar Micropsectra contributed 30 % of the total chironomids while its abundance was distinctly lower in the two eutrophic lakes. Vice versa Glyptotendipes and
Microtendipes were more abundant in the Schalkenmehrener Maar and Holzmaar. In the genus Tanytarsus a difference between the three lakes was not discernible. Together with
ProcIadius these five most abundant taxa amounted to 61-62 % of the material. The less abundant taxa, Heterotrissocladius, Parakiefferiella, Einfeldia, Paracladopelma, and
Tribelos intextus showed slightly higher frequencies in the Weinfelder Maar. Cricotopus and Polypedilum sordens were more often found in the other two lakes. In the genus Cor'ynocera, C. oliveri was present in the Weinfelder Maar while in the Schalkenmehrener Maar and Holzrnaar only C. ambigua occurred.
With respect to vertical distribution in the profiles, the number of head capsules per sample was too low to quantify the abundance of the individual taxa. Therefore, neighbouring samples were combined to get total numbers of about 100 head capsules. In a few instances smaller units had to be made due to very distinct changes in species composition. The abundance of the predominant chironomid taxa (percentage > 5 %) in the individual sections are summarized in Tables 2-4.
Maximum numbers of head capsules per g sediment dry weight occurred at depths of 630 cm (Preboreal) and 410 cm (Atlantic) in the Schalkenmehrener Maar with 103 and 102 specimens, respectively. In the Weinfelder Maar and Holzmaar the highest values (29-45 specimens/g DW) were found in early/middle Subatlantic.
In the Weinfelder Maar, the number of taxa was low during the Late-Glacial and
425
Subaflantic. However the Preboreal and Boreal values were higher (20-28) (the low number in the section 282-203 cm is obviously related to the limited material of only 89 specimens). In contrast, in the other two lakes a minimum is indicated in the samples from the Atlantic.
With respect to vertical distribution in the Weinfelder Maar profile, four different pattems are discernible. (t) Three taxa were predominant throughout the whole profile or its Holocene section: Micropsectra, Tanytarsus, and Procladius. (2) Some taxa were abundant in the Late-Glacial (Preboreal) period only: Paracladopelma, Corynocera oliveri,
Cricotopus, Heterotrissocladius, and Paratanytarsus. (3) Microtendipes, Parakiefferiella, Glyptotendipes, and Dicrotendipes reached percentages > 5 % somewhere in the period from Dl~'as 3 to early Subatlantic. (4) Predominance of five taxa was restricted to the Subatlantic: Paramerina, Cladotanytarsus, Einfeldia, Tribelos intextus, and Psectrocladius. The distribution of the latter three groups substantiate a long term shift in the chironomid fauna of the lake. TABLE 4 Holzmaar: percentages of dominant chironomid taxa (> 5 %) in 7 sections of the profile (for abbreviations of the chronozones see Tables 2/3). HOLZMAAR section (cm) chronozone total number N/g DW number of taxa % oligotrophic taxa
Tan}tarsus part. Microtendipes Procladius Ablabesmyia Cricotopus Psectrocladius Polypedilum sordens G~/ptotendipes Paramerina Ciadopelma Tribelos intextus Dicrotendipes Ctadotanytarsus
1077 911 AL-D3 210 11 20 3 8 35 16 6 8 7
879 779 PB-B 39 5 14 13
743 611 AT 97 16 15 3
579 479 $8 45 11 18 2
443 411 SA 126 39 19 0
379 311 SA 113 29 18 0
279 79 SA 82 5 23 0
8 8 8
20 5 6
18
18 17
,9 17 20 7
26 7 17 7
26 5 34
16 11 7 7 7
6 17 10
20
6 5
6
426
T A B L E S 2/3 Weinfelder Maar and Schalkenmehrener Maar: percentages o f dominant chironomid taxa (> 5 %) in 13 sections of each profile (chronozones: U P - Upper Pleniglacial; D2 - Dryas 2; A L - Allertid; D3 - Dryas 3; PB - Preboreal; B - Boreal; AT Atlantic; SB - Subboreal; SA -Subatlantic). WEINFELDER MAAR seciion (cm) chmnozone N/g DW numberoftaxa %oligotmphictaxa
Micropsectra Microtendipes Parakiefferiella Tanytarsus Pmcladius Paracladopelma Corynocera olived Cricotopus Heterotrissocladius Paratanytarsus Glyptotendipes Dicrotendipes Paramerina Cladotanytarsus Einfe!dia Tnbelos intextus Psectrocladius
722 582 602 382 D2-ALAL-D3 0.5 2 4 12 94 29 81
17 38
372 D3 12 14 50
362 302 PB 5 28 26
29
14
8 19 13
11 15 5 18 12
19 9 8
282 203 B-AT 10 19 36
183 120 SB 16 24 39
110 80 SB 12 20 54
29 6
32
51
19 9
16 11
70
60
50
40 SA 45 17 53
30 20 SA 18 11 62
10 1 SA 30 12 58
SA 30 19 3
SA 35 19 1
SA 40 13 20 20
52
62
58
18 15
10 9
7
14 12
9 9 8 5
23 21
6 23 27
-
-7 7 5 11 8
8
13 5 6 7 7
12 6
7
7 8
SCHALKENMEHRENER MAAR
section (crn) chmnozone totalnumber N/g DW numberoftaxa %oligotmphictaxa
790 730 UP-AL 41 3 6 93
81 Micropsectra Microtendipes Dicrotendipes Cricotopus Tanytarsus part. Stictochironornus Paratanytarsus Psectrocladius Chironomus plumosus Corynocera ambigua Tanytarsus sp. D G~yptotendipes Procladius Polypedilum sordens Cladotanytarsus Pseudochironomus
710 690 AL 181 35 17 6
670 650 D3 108 35 20 2
630
-
69
28
1,,
7
15 10 6
20 8 16
7
15
37
15
9 9
23 14
56
PB 159 103 10 70
13
610 550 B-AT 108 20 23 0
530 470 AT 69 12 17 0
450 430 AT 106 61 14 0
410 AT 191 102 14 0
390 370 SB 135 58 18 0.7
9
16 8 8 7
19 11 7 7
41 1
21 16 6
350 270 SB-SA 127 15 21 6 6 22
250 210 SA 100 17 18 22
190 130 SA 130 13 19 0.8
110 10 SA 125 11 22 4
11
14
11
22 9 5 14 14
8 39
34
9 6
6 9
9
6
7 6
7
8
8
11 9
12 7 6 8 7
427 In the Schalkenmehrener Maar, four taxa were abundant from the Late-Glacial to the Subatlantic: Microtendipes, Dicrotendipes, Cricotopus, and Tanytarsus part. Percentages > 5 % of Stictochironomus and Paratanytarsus only occurred in the Late-Glacial. From the Preboreal to the Subatlantic a group of predominat taxa were replacing each other:
Corynocera ambigua - Tanytarsus sp. D - Glyptotendipes - Procladius - Polypedilum sordens - Cladotanytarsus - Pseudochironomus. Three taxa showed isolated periods of predominance: Chironomus gr. plumosus in the Aller~d and Boreal/early Atlantic;
Psectrocladius
in the AUertid/Dyas 3 and late Subatlantic. In Micropsectra relative
abundance exceeded 5 % in Upper Pleniglacial/early AllerOd, the Preboreal, and in the late Subboreal/early Subatlantic.
In the Holzmaar, interpretation is limited due to restricted material, in particular by the low numbers obtained from Preboreal/Boreal and Subboreal periods. Tanytarsus part.,
Microtendipes, and Procladius were abundant almost throughout the whole profile. Ablabesmy~a was abundant in the deepest (Late-Glacial) as well as in the uppermost section (Subatlantic). The remaining taxa can be arranged as a succession of predominating chironomids occurring during the Late-Glacial/Holocene period. It should be mentioned that in the case of Micropsectra all the specimens from the 879-779 cm section were from the deepest Preboreal sample.
Regarding the general distribution patterns, there were some basic differences between the Weinfelder Maar on the one hand and Schalkenmehrener Maar and Holzrnaar on the other. (1) In the Weinfelder Maar, Micropsectra played a major role almost throughout the whole profile. (2) In this lake, the chironomid fauna of the Late-Glacial was characterized by some specific elements (besides Micropsectra) such as Paracladopelma, Corynocera
oliveri, and Heterotrissocladius which were found in very low numbers or not at all in the other two lakes. (3) In the Schalkenmehrener Maar and Holzmaar the Late-Glacial assemblages included more chironomid taxa (17-20) than in the Weinfelder Maar (12-14). (4) The genus Glyptotendipes exhibited exceptionally high abundances during the Atlantic period in both Schalkenmehrener Maar (maximum: 56 %) and Holzmaar (34 %). Abundances of this taxon were generally higher than in the Weinfelder Maar.
428
Among the chironomids found in these Eifel maar lakes there is a group of taxa which are generally associated with oligotrophic conditions (Thienemann 1925, Lundbeck 1936, Brundin 1956, Saether 1979): Protanypus, Heterotrissocladius,
Paracladopelma,
Stictochironomus, and Micropsectra. They belong to a community typical of the profundal zone of oligotrophic temperate lakes.
In the Weinfelder Maar, the percentages of the specimens of these taxa were generally > 25 %. The values were particularly high in the uppermost 40 cm of the profile. Since the Boreal period the percentages were almost exclusively determined by the abundance of
Micropsectra. During the Late-Glacial and Preboreal Paracladopelma and Heterotrissocladius also contributed substantial percentages. The most striking event was a significant decrease in the portion of oligotrophi6 taxa, i.e. Micropsectra, in 60-70 cm sediment depth followed by an increase from 40 cm to the sediment surface.
In the Schalkenmehrener Maar, abundance of oligotmphic taxa, mainly Micropsectra, was considerably high during three separate periods, the Upper Pleniglacial/early AllerSd, the Preboreal and again in the late Subboreal/early Subatlantic. In the Atlantic no oligotrophic taxa were found.
In the Holzmaar, oligotrophic elements were almost totally lacking. The percentages given in Table 4 represent single findings, 13 Microspectra and 2 Heterotrissocladius in total, i.e. 2.1% of the material. For the section 779-879 cm, the value of 13 % oligotrophic taxa was based upon only four specimens of Micropsectra from the deepest Preboreal sample.
The profundal zone of eutrophic lakes is typically inhabited by species of the genus
Chironomus. However, this taxon can hardly be used as an indicator with respect to the chironomid material under discussion. Chironomus gr. anthracinus as well as Chironomus gr. plumosus were found in very low numbers even in the eutrophic maar lakes where the assemblages were dominated by littoral elements. Hence, an attempt was made to bring in one of these littoral chironomids.
429
FIGURE 1 Percentages of Micropsectra and Glyptotendipes in 13 sections from the Weinfelder Maar (WFM) and Schalkenmehrener Maar (SMM) and 7 sections from the Holzmaar (HZM) (sediment depths (cm) refer to the lower boundary of each section).
430
Species of the genus Glyptotendipes are frequently found under rather eutrophic conditions i.e. carp ponds (Wunder 1936), shallow "Glyptotendipes-lakes" (Wundsch 1943), polluted flowing waters (MoUer Pillot 1984). They also occur in oligotrophic lakes (Brundin 1949) but are never major constituents of the fauna. A limitation of the discussion of the subfossil material lies in the fact that the species involved are not known.
Nevertheless, if Glyptotendipes is considered here as indicative of eutrophic conditions and is compared with the oligotrophic element Micropsectra, striking differences between the Weinfelder Maar and the two eutrophic maar lakes are evident. The two taxa show adverse distributional patterns; the abundance of Micropsectra decreases in the direction from Weinfelder Maar to Schalkenmehrener Maar and Holzmaar while abundance of
Glyptotendipes increases. They also display altemating fluctuations within the individual profiles. In the eutrophic maar lakes Glyptotendipes was most abundant during the Atlantic period (SMM: sections 450-430cm, 410 crn; HZM: section 611-743 em). In the Weinfelder Maar
a Glyptotendipes signal is discernible during the same period in the section 203-282 em which spans the Boreal/Atlantic. In the following periods there was a drop in the portions of Glyptotendipes in the Schalkenmehrener Maar as well as in the Holzmaar. In the former lake the considerable percentage of Micropsectra at 210-250 cm (middle Subaflantic) clearly indicates the re-establishment of this species, which decreased afterwards.
4. Discussion
The chironomid taxa used here as indicators of oligotrophic conditions, Micropsectra,
Paracladopelma, Heterotrissocladius, Protanypus, are members of the Tanytarsus-lugenscommunity. In the temperate zone these species are restricted to the profundal zone of oligotrophic lakes because they require both high oxygen concentrations and low temperature (Brundin 1956). Therefore they have also been used for paleoclimatic studies
43l
(Walker et al. 1991). However, the differences in the occurrence of these taxa in the Eifel maar lakes under discussion can certainly not be explained by climatic differences. Furthermore, in the Weinfelder Maar the sharp change of the climatic conditions at the Late-Glacial/Holocene boundary is not reflected by Micropsectra because during the postglacial period the profundal zone of the lake still provided suitable temperature conditions for cold-stenothermal organisms. Therefore, the occurrence of these chironomid species in the EifeI lakes depends on the hypolimnetic oxygen conditions and is related to the trophic state of the lake.
Corynocera oliveri possibly represents a species whose distribution in the core from the Weinfelder Maar is indeed determined by the climatic conditions. Its occurrence was restricted to the Late-Glacial and accounted for 15 % of the chironomids i.n the Aller6d/early Dryas 3 section. The species is known from northern Fermoscandia (Lindeberg 1970) and from I3ryas 1 of Lobsigensee (Switzerland) (Hofmarm 1983, 1984). These occurrences suggest dependence on low temperatures. If it would be a true littoral species, like the second species of the genus, Corynocera ambigua, it was not able to invade into the profundal zone but disappeared from the lake when the temperature increased during the beginning of the Holocene.
The results of the chironomid analysis support the view of long term trophic development of the three Eifel maar lakes indicated by the Cladocera (Hofmann, this volume) with respect to two major points. (1) The differences in the trophic conditions between the Weinfelder Maar, on the one hand, and Schalkenmehrener Maar and Holzmaar, on the other, existed since the beginning of the Holocene, probably due to morphometric differences between the lakes (Thienemann 1913, 1925) in combination with peculiar local properties of their drainage areas. (2) The Eifel lakes passed through a period of strong eutrophication during the Atlantic period. The remains of both Cladocera and Chironomidae give the impression that the two eutrophic maar lakes at that time were even "more eutrophic" than today.
In the Weinfelder Maar, both animal groups clearly indicated a higher trophic state in the
432
early Subatlantic. In contrast, the re-appearance of Micropsectra and the decrease of
Glyptotendipes obviously reflected a rather oligotrophic situation during this period in the Schalkenmehremer Maar. The renewed disappearance of the former taxon is related to eutrophication during the late Subatlantic.
The trophic fluctuations derived from the subfossil fauna are in accordance with the results based on sediment parameters such as loss on ignition, chlorophyll content, and occurrence of siderite (Brauer 1988, Rein 1991, Ehlscheid 1990). In the case of the Schalkenrnehrener Maar, the trophic changes of the Subatlantic period have been related to human activities in the drainage area and deterioration of the soils (Rein 1991).
References Brauer, A. (1988): Versuch einer Erfassung alter Seespiegel-st~tnde an ausgesuchten Eifelmaaren und mikrostratigraphische Untersuchungen an Sedimenten des Weinfelder Maares. diploma thesis, Trier. Brundin, L. (1949): Chironomiden und andere Bodentiere der stidschwedischen Urgebirgsseen. Rep. Inst. Freshw. Res. Drottningholm, 30: 1-914. Brtmdin, L. (1956): Die bodenfaunistischen Seetypen und ihre Anwendung auf die Siidhalbkugel. Rep. Inst. Freshw. Res. Drottningholm, 37: 186-235. Ehlscheid, T. (1990): Auswirkungen der Fischerei auf das Plankton und die Wasserbeschaffenheit yon Seen. Landesamt ftir Wasserwirtschaft Rheinland-Pfalz, Mainz. Hofmann, W. (1971): Zur Taxonomie und Paltikologie subfossiler Chironomiden (Dipt.) in Seesedimenten. Arch. Hydrobiol. Beih. Ergebn. Limnol., 6: 1-50. Hofmann, W. (1983): Stratigraphy of subfossil Chironomidae and Ceratopogonidae (Insecta: Diptera) in late glacial littoral sediments from Lobsigensee (Swiss Plateau). Rev. Paleobiol., 2: 205-209. Hofmann, W. (1984): A subfossil record of the presumed larva of Corynocera oliveri Lindeberg from the Lobsigensee (Swiss Plateau). Spixiana, 7: 211-214. Hofmann, W. (1986): Chironomid analysis. In: Berglund, B.E. (ed.), Handbook of Holocene palaeoecology and palaeohydrology, 715-727, Wiley & Sons; Chichester. Hofmann, W. (1988): The significance of chironomid analysis (Insecta: Diptera) for paleolimnological research. Palaeogeogr., Palaeoclimatol., Palaeoecol., 62: 501-509. Hofmann, W. (1991): Stratigraphy of Chironomidae (Insecta: Diptera) and Cladocera (Crustacea) in Holocene and Wurm sediments from Lac du Bouchet (Haute Loire, France). Doc. C.E.R.L.A.T, memoire 2: 363-386. Lindeberg, B. (1970): Tanytarsini (Diptera, Chironomidae) from northern Fermoscandia. Ann. 7.ool. Fenn., 7: 303-312. Lundbeck, J. (1936): Untersuchungen fiber die Bodenbesiedelung der Alpenrandseen.
433
Arch. Hydrobiol. Suppl., 10: 207-358. Moiler Pillot, H.K.M. (1984): De larven der Nederlandse Chironomidae (Diptera). Nederl. Faun. Meded., 1A" 1-277. Rein, B. (1991): Versuch einer Rekonstruktion des Pal~ioenvironments anhand hochzeitauflrsender geochemischer und sedimentologischer Untersuchungen an sp~t- und postglazialen Sedimenten des Schalkenmehrener Maares. diploma thesis, Trier. Saether, O.A. (1979): Chironomid communities as water quality indicators. Holarct. Ecol., 2: 65-74. Scharf, B.W. (1987): Limnologische Beschreibung, Nutzung und Unterhaltung von Eifelmaaren. Ministerium ftir Umwelt und Gesundheit Rheinland-Pfalz (ed.), p 117, Mainz. Thienemann, A. (1913): Der Zusammenhang zwischen dem Sauerstoffgehalt des Tiefenwassers und der Zusammensetzung der Tiefenfauna unserer Seen. Int. Revue ges. Hydrobiol., 6: 243-249. Thienemann, A. (1925): Die Binnengew~isser Mitteleuropas. In: Thienemann, A. (ed.), Die Binnengew~isser, 1, p. 255, Schweizerbart; Stuttgart. Walker, I.R., Smol, J.P., Engstrom, D.R., Birks, H.J.B. (1991): An assessment of Chironomidae as quantitative indicators of past climatic change. Can. J. Fish. Aquat. Sci., 48: 975-987. Wiederholm, T. (ed.) (1983): Chironomidae of the Holarctic region. Keys and diagnoses. Part 1. Larvae. Ent. Scand. Suppl., 19: 1-457. Wunder, W. (1936): Die Chironomidenlarven in der Uferregion und an den weichen Wasserpflanzen im Karpfenteich. Z. Fischerei, 34: 213-224. Wundsch, H.H. (1943): Die Seen der mitfleren Have1 als Glyptotendipes-Gew~isser und die Metamorphose yon Glyptotendipesparipes EDWARDS. Arch. Hydrobiol., 40: 362-380.
OSTRACODA (CRUSTACEA) AND TRICHOPTERA (INSECTA) FROM LATEAND POST-GLACIAL SEDIMENTS OF SOME EUROPEAN MAAR LAKES
Burkhard W. Scharf GKSS-Institut f-firGew~sserforschung Gouvemementsberg 1, O-3010 Magdeburg, Germany
ABSTRACT The investigation of the living and sub-fossil ostracods from the maar lakes in the Eifel region (Germany) and from the Lac du Bouchet (Auvergne, France) is summarized. During the Late-Glacial the number of ostracod species increased e.g. in Lake Meerfelder Maar from 3 to 9. Among the 3 species Cytherissa lacustris dominated, an indice of an oligotrophic state. The later immigrated species are characteristic for an mesotrophic lake with tendency to eutrophic state. In the post-glacial sediment no shells could be found. From actuogeological studies can be concluded that the ostracod shells are decalcified, a consequence of the eutrophic state of the lakes. The ostracod shells are better preserved in lakes with a high content of calcium than in these lakes with a low calcium concentration. In the post-glacial sediment of Lake Meerfelder Maar (Eifel) some cases of the Trichoptera Orthotrichia costalis cf. could be found. Examples are presented by figures. The finding confirms the eutrophic state of Lake Meerfelder Maar since the Atlantic. INTRODUCTION Investigations on the living and subfossil ostracods (Crustacea) from the maar lakes in the Eifel region (Germany) and in the Auvergne (France) are now published in detail (Scharf 1980b, 1983, 1984, in press; Kempf & Scharf 1980; Scharf & Keyser 1991; Wendling & Scharf 1992). Here the results are summarized. In the sediment of Lake Meerfelder Maar some cases of caddis fly larvae (Insecta, Trichoptera) could be found. The finding will be described and discussed.
Lecture Notes in Earth Sciences. Vol. 49 J. F. W, Negendank, B. Zolitschka (Eds.) Paleolimnology of European Maar Lakes 9 Springer-Verlag Berlin Heidelberg 1993
436
METHODS The sediment of the following maar lakes in the Eifel (Germany) and in the Auvergne (France) were investigated (Tab. 1). The methods are described in Scharf (1984). Table 1. List of the investigated maar lakes and their actual trophic state according to Scharf & Oehms (1992) and Eckartz-Nolden & Nolden (1991). lake Meerfelder Maar Schalkenmehrener Maar Holzmaar Gemtindener Maar Weinfelder Maar Lac du Bouchet
region Eifel Eifel Eifel Eifel Eifel Auvergne
actual trophic state 9hypertrophic --> mesotrophic eutrophic mesotrophic - eutrophic oligotrophic - mesotrophic oligotrophic oligotrophic
RESULTS AND DISCUSSION I.p_ early Late-Glacial Period the fauna of the ostrac0ds in the lakes Meerfelder Maar, Schalkenmehrener Maar and Holzmaar was poor of species and individuals. Only 3 species could be found e.g. in Lake Meerfelder Maar. The predominant species was Cytherissa lacustris. At this time these lakes were oligotrophic, later on, during Aller6d they eutrophicated. Submersed macrophytes appeared and formed additional ecological niches. As a consequence, the number of ostracod species increased up to 9 species in Lake Meerfelder Maar. The immigrated species are characteristic for mesotrophic lakes with tendency to eutrophic state. In the cores with post-glacial sediments no shells of ostracods were found. It is possible that in this time no ostracods lived in these lakes. But this is not very probable. Today we find living ostracods in eutrophic and hypertrophic lakes, sometimes also i n the profundal (Wendling & Scharf 1992). But no shells were preserved in the deepest part of these lakes. In Lake Laacher See (Eifel) it could be observed that there are all transitions from intact ostracod shells to completely decalcified ones on the sediment surface at the deepest part (Scharf 1980a). This depends on the high calcite solution in the deeper water (Friebertsh/iuser, MSgling & Bahrig 1992). The lack of ostracod shells in the post-glacial sediments of the hard-water maar lakes have to be valued as an indice of eutrophic conditions which are confirmed by the investigations of other animal groups (Hofmarm, this volume) or plant pigments (Scharf & Ehlscheid 1992).
437 In terms of zoogeography, the finding of Leucocythere mirabilis in late-glacial sediments of Lake Schalkenmehrener Maar is notably to mention. Meantime L. mirabilis has disappeared in Germany. In Lake Weinfelder Maar only two sub-fossil ostracod species were found: Cypria ophtalmica in the youngest sediments and Cyclocypris ovum at the end of Late-Glacial Period. In the sediment of Lake Gemfindener Maar no sub-fossil ostracods could be detected.
Fig. 1: Sub-fossil larval Trichoptera case of Orthotrichia costalis cL; left: total case with a length of 2.9 mm; right: detail of the left case; from Lake Meerfelder Maar, core IIIc, depth of 500-551 cm In the Lac du Bouchet only four living species of ostracods could be observed in small numbers (Cryptocandona reducta, Cypria ophtalmica, Cyclocypris ovum and Terrestricythere ivanovae). It is the first European record of Terrestricythere ivanovae. Cores, which reached glacial sediments, did not contain ostracods (Schaff & Keyser 1991). The conditions for preserving ostracod shells are better in lakes with a high content of calcium (Meerfelder Maar, Holzmaar, Schalkenmehrener Maar) than in lakes with a low one (Weinfelder Maar, Gemfindener Maar, Lac du Bouchet). The cases of the Trichoptera larvae in the sediment of l a k e Meerfelder Maar belong to the genus Orthotricha, probably to the species O. costalis. In the core from the depth of 350-400 cm four examples with a length between 1.7 and 2.9 mm and in the core from the depth of 500-551 cm three cases of the same size could be found (Figs. 1 and 2).
438
O. costalis lives in ponds and lakes, slowly flowing water, often on reeds in the outerpart of reed swamps (Marshall 1978). In 1982 O. costalis was present in Lake Meerfelder Maar (Wendling & Scharf 1992). This record confirms the statement that Lake Meerfelder Maar was eutrophic in Atlantic and later on (Negendank, Brauer & Zolitschka 1990, Ehlscheid & Scharf 1992).
Fig. 2: Sub-fossil larval Trichoptera cases of Orthotrichia costalis cf.; length of both cases: 1.7 ram; from Lake Meerfelder Maar, core lllc, above: depth of 350-400 cm, below: depth of 500-551 cm ACKNOWLEDGEMENTS I thank Dipl.-Geol. D. Drohmann, Geological Institute of the University Trier, for the taking and sieving the cores from Lake Meerfelder Maar, Dipl.-Biol. G. Erpelding, Mainz, for the determination of the Trichoptera cases and Priv.-Doz. Dr. G. Eisenbeis, Zoological Institute of University Mainz, for the SEM-pictures. Priv.-Doz. Dr. C. Wilhelm suggested improvements of the english text. REFERENCES Eckartz-Nolden, G. & Nolden, M. (1991): Lac du Bouchet (France, Massif Central): Results of two investigations: Chemistry, phytoplankton and zooplankton, p. 113123. In: Bonifay, E.: Le Lac du Bouchet (I): Environnement naturel et &ude des sediments du demier cycle climatique (depuis 120 000 ans B.P.) - Documents du Centre d'rtudes et de recherches sur les lacs, anciens lacs et tourbieres du MassifCentrals (C.E.R.L.A.T.) 2:394 pp. Friebertsh~iuser, D., M0gling, A. & Bahrig, B. (1992): Calcite precipitation and solution in Lake Laacher See. Arch. Hydrobiol. Beih. Ergebn. Limnol. 38: 85-102. Stuttgart Hofmann, W.: Late-Glacial/Holocene changes of the climate and trophic conditions in three Eifel maar lakes, as indicated by faunal remains. I. Cladocera. this volume
439 Hofmarm, W.: Late-Glacial/Holocene changes of the climate and trophic conditions in three Eifel maar lakes, as indicated by faunal remains. 1I. Chironomidae (Diptera). this volume Kempf, E.K. & Scharf, B.W. (1980) Lebende und fossile Muschelkrebse (Crusteacea Ostracoda) vom Laacher See. Mitt. PoUichia 68: 205-236. Bad Diirkheim Marshall, J.E. (1978): Trichopera, Hydroptilidae. Handbooks for the Identification of British Insects 1 (14a). (Roy. Ent. Soc. London) Negendank, J.F.W., Brauer, A. & Zolitschka, B. (1990): Die Eifelmaare als erdgeschichtliche Fallen und Quellen zur Rekonstruktion des Paliioenviroments. Mainzer geowiss. Mitt. 19: 235-262. Mainz Scharf, B.W. (1980a): Zur Morphometrie und Hydrodynamik der Eifelmaare. Mitt. Pollichia 68:101-110. Bad Diirkheim Scharf, B.W. (1980b): Zur rezenten Muschelkrebsfauna der Eifelmaare (Crustacea: Ostracoda). Mitt. Pollichia 68:185-204. Bad Diirkheim Scharf, B.W. (1983): Bemerkenswerte Muschelkrebse (Crustacea: Ostracoda) aus den Eifelmaaren. Mitt. PoUichia 69: 262-272. Bad Diirkheim " Scharf, B.W. (1984): Lebende und fossile Muschelkrebse (Crustacea: Ostracoda) im Meerfelder Maar. Courier Forschungsinstitut Senckenberg 65: 81-86. Frankfurt Scharf, B.W. (in press): Ostracoda from eutrophic and oligotrophic maarlakes of the Eifel region (Germany) in Late- and Post-Glacial Periods. 11th Internat. Symposium on Ostracoda in Warmambool, Australia 1991 Scharf, B.W. & Keyser, D. (I99I): Living and subfossil Ostracoda from the Lac du Bouchet (France, Auvergne). p. 387-391 In: Bonifay, E.: Le Lac du Bouchet (I): Envixonnement naturel et 6tude des sediments du demier cycle climatique (depuis 120 000 ans B.P.) - Documents du Centre d'~tudes et de recherches sur les lacs, anciens lacs et tourbieres du Massif-Centrals (C.E.R.L.A.T.) 2:394 pp. Scharf, B.W. &Oehms, M. (1992): Physical and chemical characterisics.- p. 63-83 in Scharf, B.W. & Bjtirk. S.(eds.): Lirnnology of Eifel maar laakes. Arch. Hydrobiol. Beih. Ergebn. Limnol. 38:348 pp. Stuttgart Wendling, K. & Scharf, B.W. (1992): Macrozoobenthos including Ostracoda. p. 239262 in Scharf, B.W. & Bjtirk. S.(eds.): Limnology of Eifel maar lakes. Arch. Hydrobiol. Beih. Ergebn. Limnol. 38:348 pp. Stuttgart
OLIGOCENE DINOFLAGELLATE-CYSTS IN
QUATERNARY FRESHWATER SEDIMENTS OF EIFEL MAARS H. Weiler
Geologisches
Landesamt
Rheinland-Pfalz,
Mainz.
Abstract:
Well preserved age,
dinoflagellate-cysts,
were found
sediments.
in young-quarternary
They are the remnants
transgression
indicating
an oligocene
fresh water maare-lake
of a former wide
from the Mainz-Basin
spread
into the Eifel-area.
Introduction
Well preserved
dinoflagellate-cysts
cores
from the freshwater
obtained
the M e e r f e l d e r indicate
Maar
Mainz-Basin.
1984).
sediments
investigations
sediment
more d i n o f l a g e l l a t e - c y s t s
also Ostracodes, indicating
of
fossils environment.
in the
1982).
Due to extended cores,
in the first
These
age and a brackish-marine
from Oligocene
(WEILER,
found
maar-lake-sediments
(SONNE & WEILER
an Oligocene
They are well known
were
Foraminifera,
the same age-range:
from new M e e r f e l d e r - M a a r
Cirripeds Rupel
were
found,
and Molluscs,
- Chatt
and all
(Oligocene).
Lecture Notes in Earth Sciences, Vol. 49 2".F. W. Negendank. B. Zolitschka (Eds.) Paleolimnology of European Maar [-akcs
9 Springer-Verlag Berlin Heidelberg 1993
442
In t h e y e a r s samples Maar
1981 - 1985 e x a m i n a t i o n
from H o l z m a a r ,
(Fig.
of a d d i t i o n a l
sediment
Schalkenmehrener Maar and from Dehner
i) y i e l d e d m o r e w e l l p r e s e r v e d d i n o f l a g e l l a t e -
c y s t s and for t h e f i r s t t i m e c a l c i s p h e r e s . All f o s s i l s p r o v e u n e q u i v o c a l l y of R u p e l - Chatt.
(Figs.
the same s t r a t i g r a p h i c
range
2 - ii)
Interpretation
The l o c a t i o n s ,
w h e r e the d i n o f l a g e l l a t e - c y s t s
are 37 k m apart. sentative these
T h e c y s t s are n u m e r o u s
f r o m the M a i n z - B a s i n
Other records
of b r a c k i s h - m a r i n e
Eifel support this assumption.
period.
of a f o r m e r w i d e
transgression
fossils
repre-
I believe, spread
into the Eifel
(SONNE,
found,
and t h e y a r e
of a v e r y s m a l l s t r a t i g r a p h i c
f o s s i l s are the r e m n a n t s
were
area.
from H u n s r ~ c k
V.
& WEILER,
and
H.
1984). A consequent
s e a r c h for t e r t i a r y s e d i m e n t s
d i n g a ~ e a of the m a a r s w a s u n s u c c e s s f u l , (1987)
d i s c o v e r e d one d i n o f l a g e l l a t e -
but E. G R A M O W S K I
c y s t and one f o r a m i n i -
fera in a l i t t l e c r e e k n e a r the M e e r f e l d e r However,
in the s u r r o u n -
Maar that
the search
were
their way
into the maar.
sediments
in the u p p e r p a r t of that c r e e k was a l s o w i t h o u t
on
for t e r t i a r y
success.
P r e v i o u s l y the m a a r - l a k e given
indications
sediments
of the y o u n g e s t g e o l o g i c a l
the p r e s e n c e of the t e r t i a r y gression
Shield.
area
history.
have Now
fossils prove a short trans-
from the M a i n z - B a s i n ,
paleogeography
from the Eifel
an i m p o r t a n t r e s u l t
and for t h e u p l i f t - h i s t o r y
for the
of the R h e n i s h
443
Fi~;. 1: Map of maars and other locations (~), where dinoflageUate-cysts_have been found in Ohgocene sediments of the Rhenish-Shield. Arrows indicate the supposed transgression lrom the Mainz-Basin into the Rhenish-Shield (ROTHAUSEN & SONNE 1984, modified).
I
r
8
=o
0~ 0 ~~176~
~ ~ ,~
I
Meerfelder Maar
9
~
~;
._~ ~
~
9
~
9
9
9
~~~ .~.
~
~,
9
~
9
9
9
9
9
~
~ a ~ .g
-~ ~
o
u
-~
o
~-
~
~
o
~
9
9
9
9
~176 Maar
~
9
I
Schalkenmehrener Mast
Holzmaar
~,
~
9
@
i f
9
9
i 9
i
9
9
9
9
9
9
9
9
i
Fig. 2: Dinoflagellate-cysts found in maar lake sediments of the Eifel.
9
444
Explanation of Fig. 3 - 11
Fig.
3.
Cordosphaeridium inodes. Meerfelder Maar. Diameter: 78 jum
Fig.
4.
Oligosphaeridium complex. Meerfelder Maar. Diameter: 80 pm
Fig.
5.
Oligosphaeridium sp. Holzmaar. Diameter: 1 2 0 ~ m
Fig.
6.
Spiniferites ramosus. Holzmaar. Diameter: 56 ~m
Fig.
7.
Thalassiphora ? pelagica. Holzmaar.
Fig.
8.
Hystrichokolpoma sp. Holzmaar. Diameter: 65 ~ m
Fig.
9.
Impletosphaeridium multispinosum. Holzmaar. Diameter: 48 /am
Fig. 10.
Calcisphere, Morphotyp C 3. Holzmaar. Broken specimen. Thewall is built of one layer. Secondary crystal-growth. Some coccoliths.
Fig. 11.
Coccoliths, found in the endocoel of a Calcisphere. Holzmaar.
445
Fig. 3
Fig. 4
Fig. 7
Fig. 6
.~,,
Fig. 9
~
~
Fig. 5
Fig. 8
'
Fig. 10
Fig. 11
446
References: GRAMOWSKI,
E.
(1987): Marine,
oligoz~ne Fossilien
quart~ren Sedimenten der Westeifel. Pal~ogeographie.Universit~t, 50 Abb., ROTHAUSEN,
K.H.
geolog.
(oligoz~nen)
9 Tab., Mainz. (1984): Mainzer Becken.
79, 203 S., 21 Abb.,
H.
Senckenberg
WEILER, H.
24 Taf.,
(1984): Die detritischen
Faunen- und Florenelemente
Sedimenten des Meerfelder Maares.
Inst.
150 S.,
Sammlung
Born-
Berlin.
SONNE, V. & WEILER,
J.F.W.
zur
Diplom-Arbeit der Job. G u t e n b e r g -
& SONNE, V.
FUhrer,
traeger,
Uberlegungen
Institut fur Geowissenschaften.
i0 Ktn.,
in jung-
65; 87-95,
1 Taf.
Senckenberg,
(1982):
In: IRION,
Forsch.
Inst.
G. & NEGENDANK,
Cour.
Forsch.
65, i-i01, Frankfurt/Main.
Bestandsaufnahme
(Dinoflagellaten-Zysten, Calciodinelloideen)
in den
- Cour.
(1984): Das Meerfelder-Maar.-
altterti~ren
des P h y t o p l a n k t o n s
Prasinophyceae und
im "prae-aquitanen"
Mainzer Beckens.- Mainzer geowiss.
Terti~r
Mitt.,
i0,
des
13-17,
1 Tab., Mainz.
WEILER, H.
(1990):
Calcispaeren aus oligoz~nen
Mainzer Beckens und des Oberrheingrabens.geowiss.
Mitt.,
19, 9-48,
89 Abb.,
Schichten Mainzer
1 Tab., Mainz.
des
TERTIARY MAARS OF THE HOCHEIFEL VOLCANIC FIELD, GERMANY
G. Biichel* & M. Pirrung* *Institut ftir Geowissenschaften, Universititt Mainz, Postfach 3980, 6500 Mainz, Germany
ABSTRACT In the Hocheifel only three maars are known. The Eckfeld Maar, filled with Eocene lacustrine sediments, is located at the southern boundary of the volcanic field, where the Tertiary peneplain has been eroded only slightly. The ages of the Jungferweiher Maar in the southeast and the DOttingen Maar in the northeast of the Tertiary Hocheifel Volcanic Field (THVF) are unknown and their timing within the THVF is uncertain. In contrast to the surrounding area, the central part of the THVF is deeply eroded. Here, six large diatremes (> 600 m) were identified in the area of Kelberg with one exception. So far, no relics of crater sediments have been found within the large diatremes. They probably represent deeply eroded maars; it is, however, also possible that two of the large diatremes constitute
small
calderas.
INTRODUCI'ION In volcanic fields of Quaternary to recent age the study of maars is promising. Craters, crater sediments and ring walls are well-preserved (cf. Negendank & Zollitschka, this volume; Btichel, this volume). In Tertiary volcanic fields, on the other hand, this is not the case. From the study of Tertiary diatremes, as e.g. the Swabian
diatremes
(Cloos,
1941;
Lorenz,
1979),
the
Oberpfalz
diatremes
(Altenschmidt, 1991), the diatremes in north-central Montana (Hearn, 1968), and the Ellendale lamproite diatremes in Western Australia (Stachel et al. 1991; Stachel 1992), we know that in many cases denudation and erosion have totally removed the
upper
part of the maar volcanoes. Only a remainder of the
former crater
filling is left, and, with continuing erosion, just a deep section of the underlying diatreme remains. In the THVF, so far the research of maars and maar diatremes
have
not yet
received too much attention. An exception is the Eocene occurrence of Eckfeld at
Lecture Notes in Earth Sciences, VoL 49 J. F. W. Negendank, B. Zolitschka (Eds.) Paleolimnology of European Maar Lakes 9 Springer-Verlag Berlin Heidelberg 1993
44B the southern boundary of the THVF. This structure, filled with lake sediments, has been interpreted as a maar based on tuff discovered below laminated sediments (Negendank et. al., 1982). At this time, additional maars were unknown in the THVF. Recent mapping of the THVF (Huckenholz & Biachel, 1989; Bilchel, 1990, 1992) has totally changed our understanding of this volcanic field, and of its maars.
THE HOCHEIFEL VOLCANIC FIELD The
Hocheifel
Volcanic
Field
was
active
from 48
Ma
(beginning Miocene) (Lippolt 1983; Miiller-Sohnius et al., approx.
1000 km 2
(Eocene)
until
1989). Within
23
Ma
an area of
(40 km N-S x 25 km E-W) 400 relics - of volcanoes have been
mapped (Meyer, 1988; Huckenholz & Biichel, 1989; Biichel 1990, 1992; Btichel & Huckenholz, olivine
basalts,
products The
1993)
(Fig.
nepheline
(hawaiites,
central
mojority
of
1) consisting of alkali basanites,
mugearites,
part
of
the
fractioned
the
olivine
benmoreites,
volcanic
field
volcanic
basalts
is
rocks
nephelinites)
and
trachytes)
located is
(olivine
To
and
alkali-
fractionated
(Huckenholz0
around
found.
basalts,
Kelberg, the
SW
1983).
where
the
adjoins
the
Quaternary Westeifel Volcanic Field, 0.6-0.01 Ma old striking NW-SE. A branch of it extends into the THVF.
UPLIFT AND EROSION According to the results of recent mapping, the central part of the THVF is most deeply
eroded.
The
amount
of
erosion
has
been
deduced
preservation of the Eocene/Miocene volcanic edifices;
e.g.,
from the
the
highest
state
of
elevation
of the Hocheifel, "Hohe Acht" (Fig. 1), 746.9 m a.s.l., is formed by the remains of a small lava lake. It is a nepheline basanite with a K-Ar whole rock age of 39.0 +1.1 Ma (Cantarel & Lippolt, 1977). The present plateau at 6 0 0 m a.s.1, surrounding the hill
is covered by grey plastosols
interpreted
of Tertiary
age.
The
latter
have
to be
as remains of a long-term intensive post-Eocene erosion phase,
which
lowered the plateau by at least 150 m. The amount of erosion could be considerably higher. The intensive erosion in the central part of the THVF has been controlled by uplift processes. The central part of the Hocheifel probably rose above its before
the
beginning
of
the
volcanic
activity.
During
the
surroundings
Tertiary
uplift
449
Fig. 1: Geological map of the Eifel region (after: Ledoux, 1987; Zitzmann & Griinig, I987; Knapp, t979; Kuckelhom & Vorster, I926) with the eruption centers of the Tertiary Hocheifel Volcanic Field (after: Huckenholz 1983; Huckenholz & Bilchel, 1988; Meyer, 1988; Biichel, 1990; Biichel & Huckenholz, 1993). The volcanics of Siebengebirge, Osteifel, and Westeifel as well as the Tertiary sediments of the Neuwied Basin are not shown.
450
continued
with
Kelberg.
Today
present,
respect there
to
its
margins.
is a significant,
The
center
of
uplift
was
located
almost perfectly circular m a g n e t i c
20 km in diameter (Fig. 2). The origin of this anomaly
uplift were interpreted by Btichel (1992)
near
anomaly
and t h e
Tertiary
as effects of a Tertiary m a g m a
chamber.
The center o f uplift is evident even today in the rivers radially r u n n i n g
outwards
(Fig. 2). It is thus
not
surprising
that in the central part of the
Hocheifel
there
are no
remains o f lava flows and primary tephra deposits. Also, no volcanic e d i f i c e s have been
preserved
preserved.
The
elevations
were
primary
and,
therefore,
northern hardly
Tertiary
no
and
maars;
southern
affected
tuff beds with
by
diatremes,
margins
such
plant
maar an
of
the
intensive
fossils occur
however
have
been
Hocheifel
at
lower
erosion.
at the
Here
Buerberg
relics
near
of
Schutz
(5 km W N W o f the Eckfeld maar) and at the Warth near Daun (10 km to the NNW of the Eckfeld maar); their exact age is still unknown
(Kr~iusel & W e y l a n d ,
1942).
The Eckfeld Maar has been preserved at the southern boundary o f the T H V F , example.
In
discovered weiher
the
northeastern
whose
age
Maar in the
is
part
of
the
unknown
so
far.
THVF
the
D0ttingen
Furthermore
southeastern part is uncertain.
the
maar,
age
It is quite
of
for
has
the
possible
been
Jungferthat these
two maars are o f Tertiary age (Fig. 1).
DIATREMES OF THE HOCHEIFEL Apart from the Eocene Eckfeld M a a r at the southern boundary, the D 0 t t i n g e n in the
northeast
and
the Jungferweiher
maar in
the
southeast
of the
Maar
THVF,
no
additional maars have been identified in the Hocheifel. However, in the central part of the THVF six large diatremes have been by
intensive
more
than
geological/geophysical
600 m.
investigations
Additionally, numerous
(Fig.
2).
Their
smaller diatremes have
discovered
diameters been
found
diatremes with diameters between 400 and 250 m; the others smaller than The
small
diatremes
underlie
either
small
former
maars
or
former
are (six
200 m).
scoria
cones
with an initial maar phase. They will not be discussed in this context. Except Kelberg
for
the
(Fig.
Kirsbach 2).
diatreme,
Together
all
with
of
the
trachyte
large and
diatremes
benmoreite
are
located
domes
they
near are
concentrated in a small elliptical area of 4x6 km 2 with the town o f K e l b e r g located at
its
western
margin.
The
area
of
4 x 6 k m 2 is interpreted
as
a small
caldera
451
Fig. 2: After geological and geophysical investigation during recent years (Bilchel, 1990, 1992) a completely new view of the THVF is presented: The large diatremes (dots), many Of then recently discovered, as well as trachyte and benmoreite domes (crosses) are concentrated within an elliptical area of 4x6 km 2, indicating a small caldera. Xenoliths of benmoreite, trachyte and syenite fragments have a considerably wider distribution of the differentiates in the lower and deeper underground, than had been detected from the surface. The caldera is located at the center of the aeromagnetic anomaly of the so-called Kelberg Magnetic High, here presented without local anomalies (BGR, 1976; BiJchel, 1992). It is produced by the magnetic effect of a magma chamber at a depth of about 10 km. The radial drainage pattern (valleys of at least the fourth order according to Strahler, 1952) indicates that the center of uplift of the THVF is also located at Kelberg. The D~3ttingen, Jungferweiher and Eckfeld Maars are situated at the margins of the THVF.
452
(Btlchei,
1992).
The
semicircular arrangement of the diatremes
in
the
southern
part of the caldera might indicate parts of the hypothetic ring fracture Only in the large diatremes, as in the Eckfeld maar, remnants of crater sediments can be expected to have survived the strong erosion. Before some o f
the
diatremes will be described, the well studied Eckfeld maar is presented.
large
A short
description of the DOttingen maar of questionable Tertiary age will complete the presentation of THVF diatremes.
ECKFELD MAAR The
Eckfeld
northeast Eocene
Maar
represents
of Manderscheid sediments
are
an
at the
located
occurrence
of Eocene
lake
southern margin of the
sediments
THVF
in a topographic depression
with
located
(Fig. a
1).
diameter
The of
450x500 m 2. They are completely surrounded by folded lower Devonian sandstones and siltstones. Well preserved middle Eocene fossils are found at this locality (Lutz 1991,
1993,
this
Frankenh~user,
this
volume;
Frankenh~tuser
volume).
The
origin
& of
Wilde,
the
this
volume;
sediments
is
Wilde
under
&
discussion
until today. Von Dechen (1886) postulated a graben in which part of an originally larger sedimentary body was protected from erosion. Von der Brelie et
al. (1969)
proposed that the Tertiary sediments may result from a collapse structure over an emptied
magma
chamber
during
the
Pleistocene.
Negendank
et
al.
(1982)
and
Bahrig (t989) studied the core of a 66.5 m deep borehole, drilled in the central part of Eckfeld Maar in 1980. At a depth of 50.5 m basic (?) pyroclastics are overlain by bituminous
lake
sediments.
This
sequence
was interpreted
by Negendank
et
al.
(1982) as the filling of a maar crater. The surfical distribution of the Eocene lake sediments was documented b y detailled geological
mapping
(Pirrung,
1992a)
(Fig.
3).
In
the
center
of
the
locality
bituminous shales and light pelites occur. In contrast, the marginal facies consists of gravel
and debris
of lower Devonian sandstones
and
siltstones
in
a pelitic
matrix. In Fig. 4 the transition between the proftmdal and the marginal
facies is
presented as based on shallow drill holes at the northern part of the topographic depression. Within
the
coarse-grained
margin
of
the
pumice
have
diameter,
been
contain
sediments,
topographic found. xenoliths
occuring at the
depression,
The pumice of
lower
low-
to
fragments, Devonian
northern
and
high-vesicular ranging sandstones
from and
northwestern fragments
1 to
20 cm
siltstones
of in and
453
Fig. 3: Geological map of Eckfeld maar. The distribution of Eocene lake sediments indicates an originally funnel-shaped crater filling. On the basis of a geomagnetic survey and geological mapping, there may be two more small Tertiary maars in the vicinity of the Eckfeld Maar. Quaternary slope wash is not shown.
454
Fig. 4: Profiles of shallow drill holes at the northern boundary of the topographic depression of Eckfeld showing the transition from marginal to profundal facies of the lake sediments (cf. Fig. 5).
455 rarely magnetites (?) (or other minerals of the spinel group). By means of powder diffraction
analysis
the
main
component was
identified
to
be
montmorillonite.
Additionally microcline was observed (B. Friese, Naturhistorisches Museum Mainz, pers.
comm.). On the base of this preliminary observation the pumice fragments
are thought to be trachytic pyroclasts. As early
as
1853, Weber observed pumice in the loamy cover of the Tertiary
deposits. In the deep drill hole, however, no pumice was discovered (Negendank et al. 1982). In the Eckfeld Maar and its surroundings geomagnetic surveys were carried out (Pirrung,
1992a).
The
observed
geomagnetic
anomalies
of +/-40
small indicating a low susceptibility of the crater sediments
nT
are
rather
and the underlying
filling. The geomagnetic anomalies of the basaltic THVF diatrems are much higher (several 100 to several 1000aT). It is thus unlikely that the crater sediments are underlain by basaltic diatrem tufts. "basic"
pyroclastics
in
the
deep
Consequently, it cannot be excluded that the drill
hole
(Negendank
et
al.,
1982)
represent
strongly altered fragments of pumice. The
geomagnetic
anomalies
similar
data to
in
the
those
surrounding
area
in the Eckfeld
show
Maar,
two
possibly
small
regions
indicating
two
with more
Tertiary maars (Hillscheid and Pellen, Fig. 3). To
prove
(Pirrung,
the
maar
1992a).
The
hypotheses Eckfeld
for
the
structure
Eckfeld is
Maar
gravity
characterized
by
an
was
measured
approximately
concentrical Bouguer anomaly of -2.7 regal. On the basis of average rock densities of samples for the
from the exposures
profundal
sediments,
(1.9g/cm 3 for the marginal sediments,
and 2.5 g/cm3for
the
Lower
Devonian
1.5 g/cm 3 sediments)
and an estimated density of 2.0 g/cm 3 for the assumed underlying diatreme
filling
three-dimensional model bodies were calculated. In Fig. 5 the best fitting model is shown. From the geological point of view, it is plausible that a body of 3 6 0 m maximum, depth represents maar lake sediments underlain by a maar diatreme. Finally, geological and geophysical data were brought together to reconstruct the original
maar
crater
(Pirrung,
1992b).
After
this
reconstruction
the
original
diameter of the crater might have been 800-1000m cut into an old landscape at a level which today would be at about 430-450 m a.s.l. The initial water depth might have been
150-160 m.
Where do the pumices of Eckfeld Maar result from? It cannot be excluded that they were erupted in the Eckfeld Maar itself. At the locality of Hillscheid north of the Eckfeld Maar a vent is located of an eroded scoria cone from the THVF (Huckenholz & Biichel, 1988). It is an alkali
456
basalt
occurence;
south
of it
a tuff body
occurs
rich
in
fragments
of
lower
Devonian rocks, pumice, and some clay (Fig. 3). The pumice fragments contain xenotiths
of Devonian rocks and magnetite (?), similar to the pumice
within the marginal facies of the Eckfeld Maar. Furthermore, amphibole
crystal
was
found
(H.
Lutz,
Naturhistorisches
fragments
a fragment of an
Museum
Mainz,
On
basis
pers.
comm.). The
tuff might be the
pyroclastic filling of a diatreme.
the
of the
geomagnetic anomaly the diameter of the diatreme is about 300 m. This occurence
Fig. 5: Schematic geological profile through Eckfeld Maar based on modelling, shallow drilling and geological mapping. The results of deep (Negendank et al., 1982). are also integrated.
gravity drilling
457 is thought to be represent the deeply eroded vent zone of a maar. Thus, this vent could be an alternative source for the pumice fragments of the Eckfeld Maar. We
prefer
the
interpretation
that
the
pumice
fragments
were
erupted
in
the
Eckfeld Maar itself because the geomagnetic anomalies of the Eckfeld
Maar are
similar to those of the Hillscheid diatreme.
there
three
Tertiary
maars
(including
geochemistry clustered region investigated
at this
the
It is therefore
Pellen
locality.
geomagnetic
It is,
likely that anomaly)
of
however, surprisingly
is located near the southern boundary of the
are
trachytic
because THVF,
the
where
only primitive melts are to be expected (Huckenholz & Btlchel, 1988). Until now, highly differenciated
volcanics like trachytes, which one must postulate
for the
origin of pumice, are known only from the central part of the Hocheifel. Future geochemical investigations might help to solve this phenomenon.
STEINK)kULCHEN DIATREME 4kin
ENE of Kelberg, a N-S elongated tuff occurrence, 1800 m long and 900m
wide, is called Steinkaulcheu diatreme (Fig. 6). Along highway B 410 pyroclastics of this diatreme are exposed. Von Dechen (1886, p. 300) described them as basaltic conglomerate, Devonian
consisting
sandstone
of
and
"basaltic
friable
matrix,
weathered
surrounding
clasts
rounded
clasts
of
of sanidine-oligoclase-trachyte".
This is a appropriate description. Knetsch (In: Frechen et al., no year) and Fuchs (1974) mention this occurrence, without describing its extent. None
of
the
pyroclastics
are
bedded.
They
are
grey
and
dark
brown
tephra
consisting of ash, lapilli, and blocks. In the central part Of the diatreme north of highway
B 410
Although
the
ash
tufts
tufts are
with
variable
weathered,
content of lapilli
sperical
juvenile
lapilli
have and
been
mapped.
Devonian
rock
fragments of up to block size have been identified. In the northern part of the diatreme a 40 cm thick basalt dyke is exposed striking E-W (Fig. 6). The ajacent tufts show only few small trachyte lapilli, similar to those, yon Dechen described in
the
B 410
road
cut.
nepheline-basanitic isolated
from
the
in
Basaltic
intrusives
composition.
diatreme.
At
were
Holzberg,
found
also
at
nepheline
The N-S-elogation indicate
Steink/tulchen,
basanites
a earlier
small
occur volcanic
system. In the exist.
southern The
part
dyke-like
of the hawaiite
diatreme
three
intrusives
stocks of Geisberg
and
of hawaiitic Beilstein,
southern boundary, are topographically curved out. The Beilstein
composition
located
at the
stock is assumed
458
Fig. 6: Simplified geological map of the Steink~tulchen diatreme, the largest diatreme in the THVF (after: BOchel, 1990). There are several small dome-like intrusions of trachytic as well as benmoreitic magma within the massive pyroclastic diatreme filling. Trachytic xenoliths within the diatreme tephra either originated from the younger trachyte domes as reworked material (first possibility) or from older domes, which were present in this area already before the diatreme was active (second possibility). Numerous basaltic dyke intrusions indicate that the diatreme is eroded to a deep level and, support the second possibility.
459
to continue below the surface further to the W and E. This is indicated by two elongate basalt Beilstein
occurrences. The thickening of this intrusion in the
summit
with
rosette-like
basalt
column
pattern
is
area of the
interpreted
as
a
spherical ecplosion chamber, which originally extended into a scoria cone at the Earth's
surface.
In addition to the nephelinitic and hawaiitic intrusives, the southern part of the diatreme
is
penetrated
by
small
benmoreite
and
trachyte
domes.
Alternatively
they could be interpreted as large blocks of the diatreme wall, collapsed into the diatreme, although no benmoreites have been mapped in the close vicinity of the diatreme so far, and only a small trachyte occurrence is exposed at the western boundary of the diatreme. The
results
diatreme diatreme
of
of
magnetic
and
Steink~ulchen
gravity
consists
measurements
of
two
indicate
composite
that
diatremes,
north of highway B 410 and a southern diatreme south
the
large
a - northern of it (HOller,
1988). The reconstruction of the diatreme formation in chronological order is as follows: Prior to the formation of the two diatremes, a trachyte dome was present in the area
of
the
later
diatremes,
probably
an apophysis
of the
Reimerath
north of the location. Two maars then originate. Later nephelinitic magma intrusions,
trachyte
and hawaiitic
preferably located at the periphery of the two maars,
formed
small scoria cones at the Earth's surface (e.g. Beilstein). Sooner or later benmoreitic and trachytic melts penetrated the diatreme as domes. The subsequent erosion removes
all
of
the
upper
volcanic
edifices.
Ealier
crater
sediments
are
not
preserved.
KC)TI'ELBACH DIATRE/vlE 1.5 km SE of Kelberg, a NW-SE elongate diatreme occurs, 1400m long and 800m wide. After mapping and magnetic surveying (by the first author), it was surveyed
in
detail
magnetically
by
Schwank
(1983)
and
gravimetrically
by
Franzreb
(1989) (Fig. 7). A high positive magnatic anomaly of up to 1450nT outlines the poorly exposed diatreme.
A small negative gravity anomaly (-1.4regal)
indicates
that the diatreme is not only light pyroclastics but also compact magmatites. In its northwestern the
southern
part, parts
the
soil profile
domes
contains
of mugearites
weathered and
turfs,
benmoreites
in
the
occur.
central The
and
volcanic
rocks in the village of K6ttelbach, consisting of diatreme tuffs and a benmoreite
460
Fig. 7: Simplified geological map of the KOttelbach diatreme without the Quaternary debris cover (after: Btichel, 1990). The small diatreme in K0ttelbach represents an apophysis of the K0ttelbach diatreme. At the eastern margin of the map part of the Hochkelberg diatreme appears. dome,
are
assumed
to be
an
branch
of the
Ktittelbach
diatreme.
At
Brinken-
k0pfchen H.G. Huckenholz discovered a benmoreite dyke within mugearite,
0.5 m
thick (H.G. Huckenholz, Miinchen, pers. comm). This occurrence is now buried. We conclude that
the benmoreites
are younger than the diatreme
and
interprete
the
benmoreites as small intrusive domes within the diatreme. The
Steink~ulchen
and K0ttelbach diatremes
could be
also
interpreted
as
small
caldera volcanoes, in view of their poly-phase magmatic history. Not too far away, such a caldera is present in the Quaternary Osteifel Volcanic Field near Rieden: The
crater,
1.5x2.5 km
in
size,
contains
5
eruption
centers.
The
crater
and
461
diatreme tephra is intruded by leucite phonolite domes during the final stages of the formation of the caldera volcano. The top of the underlying magma chamber has been estimated to be located at a depth of 4km (Viereck, 1984).
DOTI'INGEN MAAR The DOttingen maar, a 40 m deep and 2 km wide topographic depression, is located in the northeastern part of the THVF (Fig. 1). H. Weiler (Geologisches Landesamt, Mainz, pers. comm.) first assumed that the topographic depression was formed by a maar volcano. This was continued by a geomagnetic survey (Biichel, 1984). The structure is covered by Quaternary slope wash, slope debris, loam, and alluvial sediments.
Additionally,
the
Quaternary
scoria
beds
of
Niveligsberg
the
northernmost scoria cone of the Quaternary Westeifel Volcanic Field - overlie the western part of the depression (Fig. 8). On the eastern side, there are Quaternary tephra
deposits
of unknown origin and a 7 0 m
wide Tertiary
basaltic
vent.
In
Herresbach, NE of the Drttingen maar, a Quaternary diatreme was identified which contains basaltic blocks of Tertiary age. The results of the geophysical surveys show that the underlying diatreme is about 1200 m in diameter (Fig. 8). Modelling of the gravity anomaly suggests a coneshaped diatreme,
extending down to a depth of at least
1500m.
Furthermore, a
small central body was calculated which has a low density of 2.0 g/cm 3 (Fig. 9). In 1989 a trench was dug in order to investigate what material the central model body consists of: It has been found to be mud, 2.40-2.75 m thick, underlain by peat-
and
peat-bog
beds.
interglacial (0.32-0.18Ma)
The
peat-bog
had
formed
during
the
Holsteinian
(H. Usinger, Kiel, pets. comm.).
Up till now, it is not known whether this Quaternary sequence is underlain by a Tertiary
diatreme
(and
crater
sediments
?)
and
it
still
remains
questionable,
wether the D0ttingen maar is a volcano of the THVF or of the Quaternary Westeifel Volcanic Field.
FURTHER EXPLORATION OF MAAR LAKE SEDIMENTS The Eckfeld Maar can be regarded as a sediment trap, yielding information on the Eocene environment in the Eifel and on the late Tertiary and Pleistocene uplift. The origin of the sediments in the Eckfeld Maar as filling of a maar crater is of
462
Fig. 8: Volcanological and topographic map of the DOttingen maar. Also shown are the Quat.emary volcanics of the Niveligsberg scoria cone and the Herresbach diatreme filling. Quaternary tephra of unknown origin and a small Tertiary basaltic vent occur on the eastern side of the maar great
interest
geological, sediments
for
the
geochemical will
paleoenvironmental and
be helpful
area of the Rhenish Massif.
reconstruction
geophysical prospection
of
the
of unknown
in reconstructing the Tertiary
Eifel. Tertiary
history of the
Further crater uplifted
463
Fig. 9: Three-dimensional circular disk models of the D0ttingen maar-diatreme, calculated on the basis of gravity data (after: Stachel & Biichel, 1989).
REFERF_~CES Altenschmidt, H. (1991): Maare in der Oberpfalz. Gedanken zur Entstehung des Parksteins. Aufschluss, 42: 83-93. Bahrig, B. (1989): Stable isotope composition of siderite as an indicator of the paleoenvironmental history of oil shale lakes. Palaeogeogr., Palaeoclimatol., Palaeoecol., 70: 139-151. Brelie, G. von der0 Quitzow, H.W. & Stadler, G. (1969): Neue Untersuchungen im Altterti~r yon Eckfeld bei Manderscheid (Eifel). Fortschr. Geol. Rheinl. Westf., 17: 27-40. Biachel. G. (1984): Die Maare im Vutkanfeld der Westeifel, ihr geophysikalischer Nachweis, ihr Alter und ihre Beziehung zur Tektonik der Erdkruste. 385 p., doctoral thesis; University of Mainz. Bilchel, G. (1990): Das Kelberger Hoch - ein integriertes Modell einer terti~tren Magmakammer. 142 p., post-doctoral thesis, Mainz.
464 Btichel, G. (1992): Das Kelberger Hoch. Tiefenstruktur und G e o d y n a m i k einer magnetischen Anomalie in der Eifel. Die Geowissensch., 5: 132-142. Bundesanstalt fiir Geowissenschaften und Rohstoffe (BGR), (1976): Karte der Anomalien der Totalintensit/it des erdmagnetischen Feldes in der Bundesrepublik Deutschland, 1 : 500 000. Hannover. Btichel, G. & Huckenholz, H.G. (1993): Das terti~ire Vulkanfeld der Hocheifel. Samml. geol. Ftlhrer, Borntriiger; Berlin, Stuttgart, (in prep.). Cloos, H. (1941): Bau und T/itigkeit von Tuffschloten. (Untersuchungen an dem Schw/lbischen Vulkan). Geol. Rundschan, 32: 705-800. Dechen, H. Von (1886): Geognostischer FUhrer zu der Vulkanreihe der Vordereifel nebst einem Anhange tiber die vulkanischen Erscheinungen der HohenEifel. 2. ed., 323 p., Cohen; Bonn. DEKORP Research Group (1991): Results of the deep seismic reflection studies in the western part of the Rhenish Massif. Geophys. J. Int., 106: 203-227. Franzreb, S. (1989): Gravimetrische Untersuchungen eines Gebietes in der Hocheifel bei Kelberg. 112 p., Diploma thesis; University of Frankfurt a. M. Frechen, J., Hopmann, M. & Knetsch, G. : Die vulkanische Eifel. 4. Aufl., 140 p., Stollfus; Bonn. Fuchs, G. (1974): Das Unterdevon am Ostrand der Eifeler Nordsiid-Zone. Beitr. naturk. Forsch. Stidwestdeutschl., Beih. 2: 3-163. Hearn, B.C. (1968): Diatremes with kimberlitic affinities in north-central Montana. Science, 159: 622-625. H611er, I. (1988): Der terti~tre Vulkanismus 6stlich Kelberg (Hocheifel), basierend auf neuen geophysikalischen, vulkanologischen und s t r u k t u r g e o l o g i s c h e n Untersuchungen. 114 p., Diploma thesis; University of Mainz. Huckenholz, H.G. (1983): Tertiary volcanism of the Hocheifel area. In: Fuchs, K., von Gehlen, K., M~ilzer, H., Murawski, H. & Semmel, A. (eds.), Plateau uplift. The Rhenish Shield - a case history, 121-128, Springer; Berlin. Huckenholz, H.G. & Btichel, G. (1988): Das terti~ire Vulkanfeld der Hocheifel. Fortschr. Min., 66, Beih.. 2: 43-82. Knapp, G. (1979): Geologische Karte der n6rdlichen Eifel 1 : 1 0 0 0 0 0 . 3. Aufl., Geol. Landesamt Nordrhein-Westfalen; Krefeld. Kriiusel, R. & Weyland, H. (1942): Terti~re und quart~tre Pflanzenreste aus den vulkanischen Tuffen der Eifel. Abh. senckenb, naturf. Ges., 463: 1-63. Kuckelhorn, L. & Vorster, H. (1929): Das Gebiet der Blankenheimer, Rohrer und Dollendorfer Mukde in der Eifel. Geol. Rundschau, 17: 512-543. Ledoux, H. (1987): Geologische Karte von Nordrhein-Westfalen 1 : 1 0 0 0 0 0 , Blatt C 5506 Bona. Geol. Landesamt Nordrhein-WestL; Krefeld. Lippolt, H.J. (1983): Distribution of volcanic activity in space and time. In: Fuchs, K., yon Gehlen, K., M/ilzer, H., Murawski, H. & Semmel, A. (eds.), Plateau uplift. The Rhenish Shield - a case history, 112-120, Springer; Berlin. Lorenz, V. (1979): Phreatomagmatic origin of the olivine melilite diatremes of the Swabian Alb, Germany. In: Boyd, F.R. & Myer, H.O. (eds.), Kimberlites, diatremes, and diamonds: their geology, petrology, and geochemistry, 354363, A.G.U.; Washington. L6hnertz, W. (1978): Zur Altersstellung der tiefliegenden fluviatilen Tertiarablagerungen der SE-Eifel (Rheinisches Schiefergebirge). N. Jb. Geol. Pal/iont. Abh., 156: 179-206. Lutz, H. (1991): Fossilfundst~tte Eckfelder Maar, 51 p., Landessammlg. Naturk. Rheinl.-Pfalz; Mainz. Meyer, W. (1988): Geologie der Eifel. 2. Aufl., 615 p., Schweizerbart; Stuttgart. M~iller-Sohnius, D., Horn, P. & Huckenholz, H.G. (1989): Kalium-Argon-Datiernngen an tertiaren Vulkaniten der Hocheifel (BRD). Chemie Erde, 49: 119-136.
465 Negendank, J.F.W., Irion, G. & Linden, J. (1982): Ein eoz/ines Maar bei Eckfeld nordOstlich Manderscheid (SW-Eifel, Bundesrepublik Deutschland), Mainzer Geowiss. Mitt., 11: 157-172. Pirrung, B. M. (1992a): Geologische und geophysikalische Untersuchungen am terti~iren Eekfelder Maar, Sildwesteifel. Mainzer Naturwiss. Archiv, 30: 3-21. Pirrung, B. M: (1992b): Zur Frage der Entstehung e o z ~ e r Sedimente im "Eckfelder Maar" bei Manderscheid, Siidwesteifel. Mitt. Pollichia (in press). Schwank, P. (1983): Vergleichende strukturgeologische, photogeologische, vulkanologische und geomagnetische Untersuchungen im Raum Kelberg/Eifel. 101 p., Diploma thesis; University of Mainz. Stachel, T. (1992): The olivine and leucite lamproite pipes of the Ellendale Volcanic Field (Western Australia). Z. dt. geol. Ges., 143: 133-158. Stachel, T. & Blichel, G. (1989): Das DOttinger Maar: Fallstudie eines grol]en terti/iren (?) Tuffschlotes im Vulkanfeld der Hocheifel. Z. dt. geol. Ges., 140: 35-41. Stachel, T., Lorenz, V., Smith, C.B. & Jaques, A.L. (1991): Volcanology and geochemistry of the Ellendale Lamproite Field (Western Australia). CPRM, Spec. Publ., 2/91: 392-394. Strahler, A.N. (1952): Hypsometric (area-altitude) analysis of erosional topography. Geol. Soc. Amer. Bull., 63: 1117-1142. Viereek, L. (1984): Geologische und petrologische Entwicklung des pleistoz~inen Vulkankomplexes Rieden, Ost-Eifel. Bochumer geol. geotechn. Arb., 17: 1-337. Weber, C.O. (1853): Ueber das Braunkohlenlager yon Eckfeld in der Eifel. Verh. Naturhist. Ver. Preuss. Rheinl. Westph., 10: 409-415. Zitzmann, A. & Grtinig, S. (1987): Geologische lJbersichtskarte 1 : 2 0 0 000, Blatt CC 6302 Trier. Bundesanst. Geowiss. Rohst.; Hannover.
SOME ASPECTS OF CENOZOICMAARSEDIMENTS IN EUROPE: THE SOURCE-ROCK POTENTIAL AND THEIR EXCEPTIONALLY GOODFOSSIL PRESERVATION
W. Zimmerle Prinzengarten 6 . D-W-3100 Celle
ABSTRACT
During the l a s t decade, two particular aspects of maar geology have been reported using Cenozoic examples: (1) the formation of sediments rich in organic matter (hydrocarbon source rocks} and (2) the exceptionally good fossil preservation in maar sediments, which invariably contain a certain amount of volcanogenic material. The periodically high sedimentation rate of maar sediments leads to rapid burial of animal or plant remains. The minute particle size of much volcanic ash, t h e i r thixotropic behaviour, and the neoformation of
cryptocrystalline s i l i c a
ensure rapid
and complete iso-
lation of organic matter and fossils. These factors impede exchange between the pore water in the sediment and the overlying water body, thus protecting the organic material against oxidation.
Lecture Notes in Earth Sciences, VoL 49 J. F. W. Negendank, B. Zolitsehka (Eds.) Paleolimnology of European Maar Lakes 9 Springer-Verlag Berlin Heidelberg 1993
468
INTRODUCTION
Sediments in
maar lakes provide a record of deposition
environment,
often
therefore,
under unusual conditions.
in a restricted
Maar-lake sediments can,
prove useful as a geological model for
the interpretation of
specific sedimentary and diagenetic processes. The present paper i s focussed on two of these processes: (1) the formation of sediments rich in organic matter (hydrocarbon source rocks) and (2)
the exceptionally good fossil
preservation in maar-lake deposits.
MAARSEDIMENTSAS SOURCEROCKS.
When discussing with new aspects of the formation of hydrocarbon source rocks, ZIMMERLE (1985) stressed the fact that episodic volcanic a c t i v i t y favours the formation of some organic-rich sediments. The role of volcanism in the production of organic-rich sediments involves: (1) exhalation of SO2 and/or CO2 and (2) generation of anoxic conditions which favour the preservation of organic matter. Apart from this direct influence, subaqueous alteration
of
volcanic
ash produces highly
surface-active clay
minerals
(smectite), which enhance the thixotropic behaviour of the sediments and enrichment of organic material
in them. Among the examples mentioned from
Europe and America, ZI~ERLE referred localities
to Cenozoic maar-lake deposits from
such as the Eckfeld Maar (Eocene), the
Randeck Maar (Upper
Miocene) and the Plio-Pleistocene maar lakes of the Massif Central, France. In the Eocene maar near Eckfeld, Germany, a 66.5 m deep borehole penetrated an alternating
sequence of bituminous laminites and pyroclastic
(NEGENDANK et al.
1982, IRION & NEGENDANK 1984, NEGENDANK 1989).
deposits Initial
deposition of pyroclastics was followed by the formation of diatomites and
469 oil
shales. A 72.0 m deep water well within the circular
(about 1 km in
diameter) Randeck Maar, 35 km SE of Stuttgart, Germany, passes through an alternating
sequence of
Miocene lacustrine
sediments comprising
turfs,
bituminous paper shales (dysodile), dark gray clays, marls, and carbonates (limestone, dolomite) as described by JANKOWSKI (1981). The organic-carbon content of the paper shales ranges between 2 and 8 %. A similar stratigraphic section was observed in the Plio-Pleistocene maar lakes near Velay, Massif Central, France, which show a sequence of smectitic pyroclastics, diatomites, clays, s i l t s and sands (BONIFAY & TRUZE 1984). The total organic-carbon content of the overlying Holocene sediments, mainly of terrestrial provenance, ranges between 40 and 70 %. The sedimentation rates fluctuated considerably as a function of climate. The Middle Eocene lacustrine oil
shales of Messel, 20 km SE of Frankfurt,
Germany, approximately 190 m thick, serve as a well documented case study (KUBANEK et al. 1988). I t was thoroughly studied in the past because of the high organic-matter content and the unusually good preservation of fossils. The Eocene Messel l a k e was postulated by RIETSCHEL (1988), mainly on paleontological considerations, to have been a maar lake. The Messel Oil Shale contains well preserved leaves, seeds, f r u i t s , pollen, s i l i c i f i e d wood, freshwater gastropods, sponges, sharks, crocodiles, turtles, frogs, insects, birds, and mammals. Moreover, recently during the International S~nnposium "Eocene Lake Messel" at Frankfurt, the Messel Oil Shale was demonstrated on petrographic evidence to be of volcanogenic origin. Considerable a~unts of volcanoclastic material in the Messel Oil Shale, such as detritus or thin layers of airborne t u f f provide convincing evidence of this origin. The main clay mineral is smectite; kaolinite and i l l i t e Pyrite,
marcasite, siderite,
vivianite,
are only of minor importance.
messelite,
and other
phosphate
minerals are of early diagenetic origin. Some of the samPles consist of the following
cryptocrystalline mineral components (approximate particle
size
l-2~m): smectite, opal A and opal CT as well as of opaline sponge fragments and zeolites to a minor extent.
S i l i c i c l a s t i c material,
i.e.
quartz and
potash feldspar (with an unusual high barium content) also occurs. Biotite i s rare
and characterized
by
a marked chromium content.
Long prismatic
chlorapatite (~ 4 ~m) is a characteristic accessory within thoroughly altered lithic
clasts,
presumably fragments of basic volcanic
rock.
Disseminated
xenomorphic fine- or cryptocrystalline titanium oxides are abundant.
470
RIETSCHEL (1988) mentioned the following arguments for a maar origin of the Messel deposits: (1) selective preservation of animal ontogenetic stages, (2) reconstruction
of the environmental conditions of the Messel lake, and (3)
geological circumstances of the lake tectonics and sedimentation. Biasing of fauna and flora
is a t t r i b u t e d b y this author to environmental conditions
unfavourable for l i f e in the lake. According to RIETSCHEL (op. c i t . ) ,
the
Messel lake was "small in diameter (l to 1.2 km), with predominantly steep banks, surrounded by a subtropical/tropical forest. The origin of this lake is believed to be of a maar-type and was formed as the product of early Tertiary regional tectonic and volcanic a c t i v i t y " . Possibly the most spectacular occurrence of alginites (oil
shales), associ-
ated with basaltic t u f f maar-type craters, is that from Hungary. SOLTi (1980) and SOLTI et al.
(1991) describe seven maar-like
tuff
craters
from the
Varpalota Basin in Hungary containing alginites with a Corg content varying between 8 and 49 %. The highest grade of oil
shales is observed over a
thickness of a few meters immediately below a "basaltic bentonite" layer. This confirms the previous postulate by ZI~MERLE (1985). The crater lakes were non-agitated and free of wave and current influence. volcanic
material
and carbonate precipitates
Algae, altered
deposited as sapropelic
mud
formed a characteristic type of oil shale, the so-called alginite. In the context with the above case study from Hungary, KEDVES (1983) stressed the
importance of
plant microfossils
in
oil
shales,
especially
of
the
freshwater algae Botryococcus. Freshwater algae have been reported from oil shales in the Eocene Green River Formation (BRADLEY 1931), in the Eocene Messel Oil Shale (GOTH 1990), in the Upper Tertiary of Hungary (KEDVES 1983), in the Tertiary Mae Sot Basin, Thailand (GIBLING et al. 1985) and from many Phanerozoic sediments of other localities (TRAVERSE 1955, BURNS 1982). Ancient oil-bearing rocks probably derive their oil content from the hydrocarbon-producing algae Botryococcus and similar forms. The same genus produces fatty lakes.
sediments in
Botryococcus-like
(TRAVERSE 1955, 345).
sapropels of modern brackish
algae date in
fact
back as far
and freshwater as
Ordovician
471
Several contributions at the present "Symposium on Paleolimnology of Maar Lakes" also focussed on late Pleistocene/Holocene caldera or maar lakes in central
Italy (Latio) and in southern Italy (Lago Grande di Monticchio). No
petrographic details relating to source-rock potential or the exceptionally good fossil preservation, however, were reported, even i f any are available.
M&~R SEDIMENTSAND FOSSIL PRESERVATION
A review of several classical fossil localities by WOLLANKE & ZIMMERLE (1990) showed that, apart from the recently discovered fossils from the Eckfeld Maar, some classical Tertiary fossil
localities are associated with maar-
lake deposits (Table l ) , e.g. the Eocene (Lutetian) Messel Oil Shale and the Miocene Ohningen Limestone (Plattenkalk). RUTTE (1956) depicted the maar-lake setting of the ~hningen fossil deposits quite convincingly (Fig. l ) . The possible association of the famous Eocene Monte Bolca Fish Beds with explosive maar volcanism has not yet been considered or studied. Volcanogenic sediments or admixture of volcanic material in maars appear to favour effective fossilization, especially the sideritization of the soft parts of animals.
ROCK UNIT IAGE )UPPER (]HNINGEN i
SEDIMENTARY ROCK TYPES Light-colored limestones,
LIMESTONE TERTIARY maristones,and ~uffs
(Platten-Kalke)(Miocene)
I~,IESSELOIL L O W E R TERTIARY (Lutetlan) ]IVlONTE LOWER IBOLCAFISH TERTIARY [BEDSCPes- lLutetian}
~
[ci~-a")
Table l
CLAY MINERAL AVE~t~.GE ENVIRONMENT COMPOSITION PAR~ SIZE OF DEPOSITION Mainlysmectite. miner Fine-grained LACUSTRINE illite,kaolinite,and
chlorite
Bituminous claystones with Mainly smeetite (up to < 5 !4m minor tuff intercalations 90",;).minorillitemuseovite and kaolinite Marly limestones, partly Mainly smectite, with Extremely dolomiticwithminor minorilhzeandkaolinite free hyaloclasticintercalations
(MAAR-LAKE
:TYPE)
L~,CUSTRINE (MAAR-LAKE rI~'PE} SHALLOW MARINE
Petrographic parameters of famous Tertiary fossil deposits in central Europe (after WOLLANKE & ZI~ERLE 1990)
472
Fig. 1
Geological model of Ohningen Plattenkalk deposition in a maar setting (after RUTTE 1956) l = Wangen..tuff pipe 2 = Lower Ohningen Beds: Calcareous tuffaceous maar deposits rich in fossils (center and thin marl beds outside the maar proper 3 = Arenaceous TransitionBeds poor in fossils 4 = Tuff cover: with lake deposits (5) in a central depression 5 = Lake deposits: Upper Ohningen Limestones 6 Upper Ohningen Beds: thick and extensive sequence of marls
The excellent state of preservation of
fossils
in
the Messel Oil
Shale
(KUBANEK et al. 1988) includes delicate features such as skin, feathers, hair etc. which were embedded r e l a t i v e l y quickly and subsequently preserved by sideritized bacteria (WUTTKE 1983). Plant fossils, diatoms, insects, frogs, and ostracods are extremely well preserved in marls from the Randeck maar which
are
carbonate
rocks
containing
reworked volcanogenic
material
(JANKOWSKI 1981). Also the Eckfeld maar sediments turned out to be an exceptional l o c a l i t y (LUTZ 1991). I t
fossil
has been argued that some of the best preserved
fossils from the Eckfeld maar have been found within t u r b i d i t e layers in spite of the fact that t u r b i d i t e deposits do not normally contain except-
473
ionally well preserved fossils unless they are reworked. Thus, the mechanism of fossilization in maars seems to be more complex than mere rapid burial. In the Pleistocene and Holocene sediment sequences of several maar lakes in the West Eifel
volcanic province (Fig. 2) volcanogenic material is present in
various forms. Two characteristic tephrochronologic marker horizons have been recognized: the younger Laach pumice t u f f
(Laacher Bimstuff) and an older
basaltic ash t u f f (NEGENDANK 1989). Moreover, concealed tephra horizons are to be expected in the intervening sequence, and volcanogenic material
is
probably present in the other maar sediments. The large amount (I0-50 %) of mineralogically unstable s i l t -
and sand-sized heavy minerals demonstrates a
marked input of volcanic material in the maar sediments (NEGENDANK 1989, 18/19). Clay and sand turbidites are intercalated in the above sequences. The origin of the siderite laminites s t i l l seems to remain a matter of debate.
Fig. 2 Pleistocene and Holocene sediment sequences of several maar lakes, West Eifel volcanic province (after NEGENDANK 1989)
474
The good preservation of fossils appears to be related to the presence of volcanogenic material. Firstly, the h i g h sedimentation rate of ash-fall deposits led to rapid burial of the animal or plant remains. Secondly, the minute particle size of many volcanic ashes and of other smectitic sediments, the mineralogical
i n s t a b i l i t y , the physical, and especially the thixotropic
behaviour of such sediments, and the inconspicuous neoformation of cryptocrystalline silica guaranteed complete isolation of the fossils as a f i r s t step to their unusually good preservation.
As pointed out by to
BOSWELL
(1961) smectitic clays, bentonites and fine- grained lime muds such as the lithographic limestone and lake marls show a high p l a s t i c i t y . In addition, the
presence of
organic substance acts
in
the
same direction.
These
properties lead to the reduction of the vertical permeability increasing thus the preservation potential. As compared with other clay minerals, smectites influence the properties of clay as follows: (1) their minute particle size enhances the thixotropic behaviour of c~ays and their adsorption of organic matter and water, (2) their high water-exchange capacity (strong swelling behaviour) increases thixotropy and (3) the high cation-exchange capacity augments the chemical reaction potential. The exchange reactions between pore water, atmospheric gases, and solid particular case. Though the
sediments are
extremely complex and unique in
examples discussed above strongly
volcanogenic material
support the
idea
each
that
favours the fossil preservation, i t cannot be excluded
that some of the fossil assemblages in maar lake sediments are the result of mass extinction of
animals by sudden outbursts
of C~
similar
to those
recently observed in Lake Nios, Cameroon. In this context LOCKLEY & RICE {1990) stressed the need tO understand the interrelation
between fossils
and their
entombing sediments as
far
as
sedimentology, stratigraphy, taphonomy, and diagenesis are concerned and to encourage research in this particular field.
"There is substantial evidence
that volcanism has played a significant role in shaping the biostratigraphic record" much more than we previously thought.
475
REFERENCES
Bonifay, R. & Truze, E. (1984): Structures et dynamique s~dimentaire dans les lacs de maars: L'exemple du Velay (Massif Central francais) (Abstract). 5th European Regional Meeting of Sedimentology, Marseille, April 9-11, 1984, 68-69; Marseille. Boswell, P.G.H. (1961): Muddy sediments: Some geotechnical studies for geologists, engineers and soil scientists, p 140 (W. Heffer & Sons Ltd.); Cambridge. Bradley, W.H. (1931): Origin and microfossils of the oil shale of the Green River formation of Colorado and Utah. USGS, Prof. Pap. 168, p 58, Washington. Burns, D.A. (1982): A transmission electron microscope comparison of modern Botryococcus braunii with some microfossils previously referred to that species. Rev. esp. Micropaleont., 14: 165-185; Madrid. Gibling, M.R., Tantisukrit, C., Uttamo, W., Thanasuthipitak, T. & Haraluck, M. (1985): Oil shale sedimentology and geochemistry in Cenozoic Mae Sot Basin, Thailand. Amer. Assoc. Petroleum Geologists Bull. 65: 767-780; Tulsa. Goth, K. (19go): Der Messeler Olschiefer, ein Algenlaminit. Cour. Forsch.Inst. Senckenberg, 131: p 43; Frankfurt a. M. Irion, G. & Negendank, J.F.W. (1984): Das Meerfelder Maar. Untersuchungen zur Entwicklungsgeschichte e i n e s Eifelmaares. C o u r . Forsch.-Inst. Senckenberg, 65: p lOl; Frankfurt a. M. Jankowski, B. (1981): Die Geschichte der Sedimentation im N~rdlinger Ries und Randecker Maar. Bochumer geol. u. geotechn. Arb., 6: p 315; Bochum. Kedves, M. (1983): Etude pal~obotanique sur les schistes p~trolif~res du Tertiaire sup~rieur de Hongrie. Rev. Micropal~ontologie, 26: 48-53; Paris. Kubanek, F., N~Itner, T., Weber, J. & Zimmerle, W. (1988): On the l i t h o genesis of the Messel Oil Shale. Cour. Forsch.-Inst. Senckenberg, I07: 13-28; Frankfurt a. M. Lockley, M.G. & Rice, A. (1990): Volcanism and fossil biotas.- Geol. Soc. America, Spec. Paper 244, p 125; Boulder. Lutz, H. (1991): The Middle Eocene "Fossillagerst~tte Eckfelder Maar" (Eifel, Germany) (Abstract). Intern. Conf. Monument Grube Messel Perspectives and Relationships, 6-9 Nov. 1991, Hess. Landesmuseum Darmstadt. Negendan~, J.F.W. (1989): Pleistoz~ne und holoz~ne Maarsedimente der Eifel. Z. dt. geol. Ges., 140: 13-24; Hannover. Negendank, J.F.W., Irion, G. & Linden, J. (1982): Ein eoz~nes Maar bei Eckfeld nord6stlich Manderscheid (SW-Eifel). Mainzer geowiss. M i t t . , l l : 157-172; Mainz. Rietschel, S. (1988): Taphonomic biasing in the Messel fauna and flora. Cour. Forsch.-Inst. Senckenberg, I07: 16g-182; Frankfurt a. M. Rutte, E. (1956): Die Geologie des Schienerberges (Bodensee) und der Ohninger Fundst~tten. N. Jb. Geol. Pal~ont. Abh., I02: 143-282; Stuttgart.
476
Solti, G. (1980): The oil shale deposit of Varpalota. Acta Mineralogica Petrographica, 24, 289-300; Szeged. Solti, G., Ravasz, C. & Csirik, G. (1991): Alginite (oil-shale) and basaltic bentonite deposits in basaltic tuff maar-type craters, Hungary (Abstract). In: Zolitschka, B. & Negendank, J. F. W. (eds.) S~n~posium on Paleolimnology of Maar Lakes, May 21-25, 1991, 52, Bitburg, Germany. Traverse, A. (1955): Occurrence of the oil-forming alga Botryococcus in lignites and other Tertiary sediments. Micropaleontology, I: 343-350; New York. Wollanke, G. & Zimmerle, W. {1990): Petrographic and geochemical aspects of fossil embedding in exceptionally well preserved fossil deposits. Mitt. Geol.-Pal~ont. Inst. Univ. Hamburg, 69: 77-97; Hamburg. Wuttke, M. (1983): "Weichteilerhaltung" durch l i t h i f i z i e r t e Mikroorganismen bei mittel-eoz~nen Vertebraten aus den Olschiefern der "Grube Messel" bei Darmstadt. Senckenbergiana lethaea, 64: 509-527; Frankfurt a. M. Zimmerle, W. (1985): New aspects on the formation of hydrocarbon source rocks. Geol. Rundschau, 74: 385-416; Stuttgart.
PALAEOECOLOGICAL IMPLICATIONS FROM THE SEDIMENTARY R E C O R D OF A SUBTROPICAL MAAR LAKE (EOCENE ECKFELDER MAAR; GERMANY)
Bernd Zolitschka Geologie, Universit~t Trier, D-5500 Trier
ABSTRACT The sediments from Eocene Eckfelder Maar display a cycle from an initial clastic stage via organic oil shale deposition to a final clastic stage. Increasing aridity is related with the onset of oil shale formation, increasing freshwater input with the gradual change from organic back to final clastic depositioia. Eutrophic conditions occurred only during a short period of the early oil shale stage related to maximum organic carbon and oxygen isotope values and to the formation of pyrite and vivianite.
INTRODUCTION Eckfelder Maar is located in the volcanic field of the Eifel part of the Rhenish Massif (Germany). The lacustrine deposition ended when the lake silted up. Today it is forming a dry maar at an elevation of 340 m above sea level. Eckfelder Maa.r is surrounded by Devonian shales, graywacke and sandstones. The surface of the former lake, determined by silty-clayey deposits on top of the Devonian basement, covered an area of approximately 380 x 460 m. The depth of the lake was estimated to 65 to 150 m (Negendank et al. 1982). Assuming the shallower depth of 65 m, the Eocene lake of Eckfelder Maar may be compared with present day Lake Weinfelder Maar, which is of a similar morphology. Eckfelder Maar is the oldest maar of the Eifel area investigated so far. It was previously dated by pollen analysis to the Middle Eocene (Lutetium) (Pflug 1959). When remains of a horse (Propalaeotherium) have been discovered in 1990, mammai-stratigraphy allowed to establish a more precise stratigraphic classification, which now became 49 Ma old (Middle Eocene, Geiseltalium/Lutetium)
(Lutz 1991). There is a good correlation of this
Lecture Notes in Earth Sciences, Vol. 49 J. F. W. Negendank, B. Zolitschka (Eds.) Paleolimnology of European Maar Lakes 9 Spdnger-Verlag Berlin Iieidelberg 1993
478 stratigraphic position with the radiometric dating of the onset of volcanic activities in this area ca. 46 Ma ago related to the uplift of the Rhenish Massif (Cantarell & Lippolt 1977). Eckfelder Maar is of the same age like the famous German sites of Messel near Darmstadt and Geiseltal near Halle. The oil shales of Eckfeld are weUknown since the beginning of the 19th century (Weber 1853). Scientific investigations started more than 100 years later providing first ideas about the time of deposition by means of pollen analysis (Pflug 1959, v.d. Brelie et al. 1969). In 1980 a 66.5 m long sediment core was recovered displaying lacustrine sediments (Negendank et al. 1982). Since 1987 excavations for fossils provided a huge amount of floral and faunal remains: 11,000 leaves, fruits and seeds, 150 flowers, more than 1600 insects, 600 fish, many snails, crayfish, frogs, crocodiles and bats (Lutz 1991, this vol.; Wilde & Frankenh~iuser this vol.; Frankenhgtuser & Wilde this vol.). Flora .and fauna indicate subtropic climatic conditions with a mean annual temperature of at least 25~
in
contrast to present day values of 9~ This study compiles the former sedimentary investigations of Negendank et al. (1982) and Bahrig (1989) and for the first time includes a detailed examination of all the thin sections from oil shales of Eckfeld, which have been prepared in connection with first studies by Negendank et al. (1982).
SEDIMENTS The 66.5 m long sediment sequence from Eckfelder Maar is subdivided into the four lithozones A-D (cf. Negendank et al. 1982): D: 0.0 - 9.5 m
clay and silt laminations.
C: 9.5 - 16.0 m
transition zone with bituminous silts.
B: 16.0 - 50.5 m
laminated lacustrine oil shales with diatoms and bituminous material.
A: 50.5 - 66.5 m
horizontally bedded reworked pyroclastics with vesicular basalt lapilli and fractionated Devonian rocks.
Of major interest is lithozone B related to high contents of organic carbon (Fig. 1) and formation of siderite. Isotopic composition of siderite indicates an anoxic water/sediment interface with strong methanogenesis (Bahrig 1989). Meromictic conditions are also evidenced by preserved sediment layers without any bioturbation. Laminations are not very distinct. They consist of couplets with different sublaminations: (1) minerogenic and fine organic detritus, (2) diatoms and fine organic detritus, and (3) different types of fine organic detritus. Although allochthonous organic debris (leaves, twiglets) is present, most
479
Fig. 1: Lithology and organic carbon of sediments from Eckfelder Maar (Negendank et al. 1982, modified).
480
of the organic detritus is of autochthonous algal origin. Some of this structureless optical isotropic material is of orange colour and may be related to the green algae Tetraedron (cf. Goth 1990), which is usually dominating organic deposition of oil shale lakes. Further proof o f the organic productivity of the lake provide diatoms, chrysophyte cysts and spiculae of freshwater sponges (Tab. 1). Diatoms consist of only one single planktonic species (Melosira granulata), indicating eutrophic conditions. The authigenic minerals pyrite and vivianite occur together with diatom frustules at 43.25 m sediment depth. Diagenetic apatite is restricted to the same period, whereas siderite is common throughout lithozone B (Tab. 1). Tab. 1: Relative amount of minerogenic (rain) and organogenic (org) deposition, of diatoms (dia), chrysophyte cysts (chr) and spiculae of freshwater sponges (spi), and of apatite (apa), pyrite (pyr), siderite (sid) and vivianite (viv) estimated from 10 thin sections o f Eckfelder Maar (- = no; o = scarce; + = common; + + = abundant).
EFM-33.6 EFM-38.25b EFM-38.25a EFM-41.6 EFM-43.25 EFM-43.7 EFM-47.5 EFM-48.8 EFM-49.1 EFM-50.3 EFM-50.5
min
org
dia
chr
spi
o +
++ ++ + + + + ++ + + ++ + + + +
o + + -
o o o + + +
+ + o + o + o
o o o + o + + +
apa
+ +
pyr
sid
o
o + + + + +
+ +
+ -
o
o + o o
viv
o + + o
Siderite does not occur in distinct layers like in Holocene maar lake sediments from Lake Weinfelder Maar (Brauer 1988, this vol.) and Lake Gemiindener Maar (Zolitschka 1990) but forms lenticular bodies, patches or a matrix made up of xenomorphic crystals 1 to 5 #m in diameter. The whole oil shale has a faint lamination with only undistinct layers occurring only when diatoms are abundant. This is the only period of the record where an annual rhythm may be assumed, but still there is no evidence of a seasonal nature. One of these "annual layers" is composed of the following elements: top:
600/~m
of fine organic detritus, many diatoms, some plant macrorests
base:
200/~m
of diatoms
200/zm
of plant macrorests, few diatoms and pyrite
500/~m
of diatoms
481
The thickness of laminations varies considerably. The mean value is 2.27 mm (rain. = 0.3 mm; max. = 25 ram). Such diatom rich layers have been detected in only one thin section at 43.25 m sediment depth (Fig. 2). This corresponds to diatoms found during grain size analyses at 43.5 and 44.5 m sediment depth (Negendank et al. 1982). Based on the available thin sections it is impossible to use the faint laminations of other parts of the sediment sequence for the reconstruction of sedimentation rates.
IMPLICATIONS The sediments from Eckfelder Maar reveal a cycle in the depositional history of a lake which is common to most lake basins but rarely investigated completely. The initial sta~e 0ithozone A) is characterized by mass movements into the early lake basin due to steep, instable and debris covered slopes formed during the maar eruption. A short transition zone (less than 2 m) consists of fine-grained clastic sediments indicating a stabilization of the crater walls, probably due to soil formation and development of a plant cover.
With the onset of lithozone 13 the major stage of sediment history began. It is dominated by organic (oil shale) sedimentation with graded intercalations of siliclastic material, probably related to heavy rain storms causing single discharge events. Most of the oil shale is characterized by isolated sand grains (Fig. 3), which might have been transported into the lake as "tropical dropstones" clinging to the roots of littoral? plants. Pulled out by regular occurring subtropical storms these plants drifted across the lake and lost their elastic load (cf. Goth 1990). Another source of detritic mineral grains may be deposition of volcanic ashes. Syndepositional or early diagenetic formation of siderite is common throughout lithozone B. The oxygene isotope composition of siderite was used to obtain information about some palaeoenvironmental conditions of the former lake (Bahrig 1989). Occurrence of intact laminations, of siderite and the stable isotope composition of siderite indicate meromictic conditions with an anoxic hypolimnion and strong methanogenesis. Figures of the next page: Fig. 2: Micrograph of undistinct diatom-rich laminations from the oil shale stage of sedimentation in Eckfelder Maar (EFM-43.25). Fig. 3: Micrograph of oil shale deposition with turbidite and "tropical dropstones" from the major stage of sedimentation in Eckfelder Maar (EFM-49.1). Fig. 4: Micrograph of laminations from the major oil shale stage of sedimentation in Eckfelder Maar (EFM-38.25a). Dark layers represent organic detritus. Composition of the pale layers is not determinable (opal?).
482
~:. ,..~;.~ .~.~
.~ ',-
Fig. 2
Fig. 3
Fig. 4
483
Such conditions need a considerable deep lake basin and a small catchment area to provide oligotrophic to mesotrophic conditions over a long period of time. Additionally, reduced and dissolved iron in the hypolimnion enhanced the stabilisation of the meromixis forming a chemical stratification (Kjensmo 1968, Dickinson 1988). High lacustrine productivity of an eutrophic lake would provide a larger amount of sulphur for the sediments thus inhibiting the formation of siderite but favouring precipitation of pyrite. Oligotrophic conditions are evidenced by a poor biocenosis of aquatic arthropods (Lutz, this vol.) and by a lack of diatoms throughout most of the record. Chrysophyte cysts and spiculae of freshwater sponges are common in eutrophic as well as in oligotrophic lake sediments and do not provide any indication of the trophic state of a lake (Brauer 1988, Zolitschka 1990). Diatoms are restricted to only two of the investigated thin sections. This might be a result of dissolution of diatom frustules with a later recrystallisation as chert (cf. Goth 1990) not recognizeable by microscopic investigations (Fig. 4). The thick layers of the eutrophic diatom Melosira granulata are associated with pyrite framboids and lenses of vivianite, all pointing to an increase of productivity in the lake. The diagenetic formation of apatite within these layers is probably related to a transformation of autochthonous calcite which would indicate a high organic productivity of the epilimnion as well (Kelts & Hsfi 1978). These results indicate nutrient poor conditions during the early period of the major stage of Eckfelder Maar which turned to eutrophic for only a short time until turning back to me.soor oligotrophic conditions. The eutrophic period is related to a maximum in organic carbon (Fig. 1) and to the most positive oxygen isotope values at the end of a distinct increase from -2 to +5 per mille interpreted as a decreasing precipitation/evaporation ratio (Bahrig 1989). This would cause a drop in lake level reducing the water volume of the lake and slightly enlarging the catchment area. Together this produces a higher nutrient level in the water column. The final stage of the lake started at 16 m sediment depth with a change from oil shales to silt and clay laminations (lithozones C and D). Preceding this change occurred a negative shift of oxygen isotope values by 7 per mille at 21 m depth indicating an increase of the precipitation/evaporation ratio. This has been interpreted as a rise in freshwater input (Bahrig 1989). Probably a river is entering the lake from that time on. At the beginning this caused an increase in organic carbon but after 5 m the discharge of large amounts of siliclastic material initiated the silting up of the lake basin. During the early period of this process sediments are still slightly bituminous (lithozone C) becoming more and more minerogenie within lithozone D. The topmost sediment is probably redeposited debris from surrounding slopes.
484 REFERENCES Bahrig, B. (1989): Stable isotope composition of siderite as an indicator of the paleoenvironmental history of oil shale lakes. Palaeogeogr., Palaeoclimatol., Palaeoecol., 70: 139-151. Bmuer, A. (1988): Versuch einer Erfassung alter Seespiegelst~nde an ausgesuchten Eifelmaaren und mikrostrafigmphische Untersuchungen an Sedimenten des Weinfelder Maares. Diploma-Thesis, Univ. Trier, 117 pp. Cantarell, P. & Lippolt, H.J. (1977): Alter und Abfolge des Vulkanismus der Hocheifel. N. Jb. Geol. Pal/iontol., Mh. 1977: 600-612. Dickinson, K.A. (1988): Paleolimnology of Lake Tubulik, an iron-meromicfic Eocene lake, eastern Seward Peninsula, Alaska. Seal. Geol., 54: 303-320. Goth, K. (1990): Der Messeler Olschiefer - ein Algenlaminit. Courier Forsch.-Inst. Senckenberg, 131: 1-143. Kelts, K. & Hsfi, K.J. (1978): Freshwater carbonate sedimentation. In: Lakes - chemistry, geology, physics, Lerman, A. (ed), 295-323; New York. Kjensmo, J. (1968): Iron as a primary factor rendering lakes meromictic, and related problems. Mitt. Int. Ver. Limnol., 14: 83-93. Lutz, H. (1991): Fossilfundstelle Eckfelder Maar, 51 pp; Mainz. Negendank, J.F.W., Irion, G. & Linden, J. (1982): Ein eoz/haes Maar bei Eckfeld nord6stlich Manderscheid (SW-Eifel). Mainzer Geowiss. Mitt., 11: 157-172. Pflug. H. (1959): Die Deformationsbilder im Tertifir des rheinisch-saxonischen Feldes. Freiberger Forschungs-H., C71, 110 pp; Berlin. Von der Brelie, G., Quitzow, H.W. & Stadler, G. (1969): Neue Untersuchungen im Alttertifir von Eckfeld bei Manderscheid (Eifel). Fortschr. Geol. d. Rheinl. u. Westf., 17: 27-40. Weber, C.O. (1853): 0ber das Braunkohlenlager bei Eckfeld in der Eifel. Verh. Naturhist. Ver. Rheinl. Wesffalen, 10: 409-415. Zolitschka, B. (1990): Sp/itquart~re jahreszeitlich geschichtete Seesedimente ausgewfihlter Eifelmaare. Documenta naturae, 60:226 pp; Mfinchen.
ARTHROPODS FROM THE EOCENE ECKFELDER MAAR (EIFEL, GERMANY} AS A SOURCE FOR PALEOECOLOGICAL INFORMATION
H. Lutz Naturhistorisches Museum Mainz/Landessammlung fdr Naturkunde, Reichklarastr. 10, D-6500 Mainz
ABSTRACT The arthropod-thanatocoenosis from the Middle-Eocene sediments of the Eckfelder Maar (Eifel, Germany)is characterized by a striking cDntrast between a highly diversified assemblage of terrestrial species on one hand and very few aquatic species on the other hand. This most likely does not result from taphonomic biassing, but reflects a poor aquatic arthropod-community. INTRODUCTION A first description of a "browncoal" outcrop near Eckfeld and of plant fossils collected at this site was given by WEBER already in 1853. NEGENDANK, IRION & LINDEN (1982) published results from a core-drilling project. Since that time the locality is interpreted as the oldest known Maar in the Eifel, as pollen-stratigraphy proved these sediments to be of MiddleEocene age. Since 1987 the Museum of Natural History/State-Collection of Natural History of Rhineland-Palatinate is excavating fossils and collecting taphonomical and sedimentological data from the bituminous clay- and siltstones of the Eckfelder Maar. These sediments consist of finely laminated oilshale and turbiditic sequences. Up to now - besides approximately 14.000 botanical objects, 800 fishes and 300 remains of higher vertebrates - about 2.900 insects, 17 spiders and numerous ostracods have been found. Recent finds of mammals confirm the Middle-Eocene age.
Lecture Notes in Earth Sciences, Vol, 49 J. F. W. Negendank, B. ZolJtschka (Eds.) Paleolimnology of European Maar Lakes 9 Springer-Verlag Bct'tin Heidelberg 1993
486
RESULTS Terrestrial insects are represented both rich in species and specimens. Coleoptera (beetles), especially Curculionoidea (wheevils), are dominating. Besides these we know representatives of at least 10 other beetle-families. The orders Blattodea (cockroaches), Isoptera (termites), Homoptera (leaf-hoppers), Heteroptera (bugs), Hymenoptera (wasps and bees), and Diptera (flies and midges) are comparatively rare, partly even represented only by a few specimens. On the contrary, aquatic groups, which should be common in limnic sediments, are almost completely absent. This is not only the case for larvae of Odonata (dragon- and damselflies), Ephemeroptera (mayflies), Diptera, Trichoptera (caddis-flies), and Plecoptera (stone-flies), but also for many representatives of the Heteroptera and Coleoptera (aquatic bugs and water-beetles), which do not only breed in freshwater but also spent at least part of their adult life in ponds and lakes. Up to now, only a few larval cases of two Trichoptera-species and a pupa of a Nematoceran Diptera have been found. One type of Trichopteran larval case is built up of coarse sand while the other is made of silk only. Both types are known from the Messel oilshale pit as welt. The pupa most likely refers to the Chironomidae (midges). More common are small cylindrical objects, which are about 3 - 4 mm long and 1 mm wide. We cannot exclude that these are also larval cases of Trichoptera or Diptera (Chironomidae) but it seems more likely that these are coprolithes of small fish. The two imagines of Odonata do not provide us with any information concerning the Eocene lake. Larvae have not yet been found. There is no aquatic form among the 17 known specimens of Arachnids. Crustacea are represented by Ostracoda. These are known from only seven different laminae within a 4.5 m thick sequence of lake sediments. As in all cases numerous specimens have reached the profundal parts of the lake, huge numbers of individuals must have lived in the littoral. The taxon which tentatively had been identified as a member of the Conchostraca meanwhile turned out to be an undetermined species of the Bivalvia (cf. Unionidae) being preserved as periostraca. They are reaching a length of up to 40 mm. All but one have been found within turbiditic layers that are rich in coarse clastic material and plant detritus. Obviously they have been introduced from the littoral by slumps when the carbonate of their shell already was dissolved (GROH & JUNGBLUTH, pers. comm.).
487
DISCUSSION The imbalance between a diverse sample of terrestrial insects on one hand and an extremely poor one of autochthonous, aquatic species on the other hand is worth being discussed. Concerning this imbalance the Eckfeld lake is strikingly similar to that of Messel (LUTZ 1988, 1990, LUTZ et al. 1991). At a first glance one might assume, that the preponderance of terrestrial taxa is due to the fact that they all reached the surface of the lake as aktively flying objects. On the contrary, aquatic arthropods, which - if they existed in the lake - certainly had been restricted to the well oxygenized epilimnion and the littoral respectively, might have been held back by some effective filter, e.g. a dense belt of submersed plants. Up to now, there is no proof for the existence of such a filter, however. Characeae are the only submersed plants we found and these are only known from two layers. Taphonomic biassing therefore is very unlikely. This is also supported by the abundance of fragments of wood, twiglets, leaves, fruits, seeds and flowers, Only leaves, winged fruits, and flowers may have been blown into the lake even over considerable distances. If the majority of terrestrial material has been washed in, this even more easily should have been the case with aquatic arthropods living in the littoral. Besides Characeae we know a few poorly preserved leaf-fragments referable to the swimming water-hyacinth Eichhornia, a minute fragment that looks like the swimming fern Azofla, and - till now from one single layer (WILDE, pers. comm.) - cysts of the green alga Ovoidites that may have formed floating mats. These species might have covered great parts of the surface, but we do not have any evidence that this regularly was the case. If so, this certainly would much more have influenced the input of flying or windblown material than that of aquatic arthropods. Thus the known thanatocoenosis very likely reflects a poor aquatic arthropod-biocenosis. We d o not yet know, which type of inhibitory factor prevented insects and other arthropods from breeding and living in the lake over long periods. With respect to the contemporaneous Messel lake several abiotic factors, e.g. extreme pH-values, a high contents in dissolved minerals or toxic, organic substances and high temperatures have been discussed (LUTZ 1990). But besides all this there is another, less "spectacular" possibility: The finely laminated sediments being rich in siderite, pyrite and organic matter and showing no signs of bioturbation prove that they have been deposited in stagnant water under anoxic conditions. In other words, the Eckfeld lake was stratified the whole time. In subtropical climates with only slight annual variation of temperature a difference of 1-2" C between epi- and hypolimnion is sufficient to stabilize a thermocline and thus stratification over long periods {PAYNE 1986:
488
38). But not only temperature controls the establishment of lasting discontinuities in a waterbody. Different concentrations of dissolved substances may cause and stabilize stratification as well. Stratified lakes with only a partial annual turnover are called meromictic, their waterbody being separated into a mixolimnion (top) and a monimolimnion (bottom). According to GOTH (1990:44 ff.) the Messel lake was of that type. As the Eckfeld lake was a Maar it should have been meromictic too. If there were sufficient dissolved nutrients available, algae did rapidly bloom, producing considerable amounts of organic material. After the collapse of the population, most of the nutrients, which now were fixed in the organic material, were accumulated and trapped at the bottom of the lake, as extremely low contents or even complete absence of oxygen in the monimolimnion minimized their remineralisation (PAYNE 1986:35 ff.). A diffusion of ions like NO ~- and PO, 3- up into the mixolimnion, was restricted by the chemocline. As long as the resulting deficiencies in nutrients were not compensated, the Eckfeld lake had a low productivity. Additionally, Tetraedron, the dominating alga in the Messel lake, was also a common species in the Eckfeld lake. Because of its small size and the chemistry and morphology of its cell wall this species presumably was of little or even no importance as foodsource for phytoplancton-feeders (GOTH 1990: 50). As a result, the biocenosis of aquatic arthropods was poor both in taxa and specimens; complex foodchains did not establish. Obviously the lakes of Rott (LUTZ 1989), Randeck and Ohningen were completely different ecosystems. In their sediments comparatively diverse thanatocoenosises of aquatic arthropods have been preserved which allow a fairly detailed reconstruction of the different habitats of these lakes. The striking differences between the lakes of Eckfeld and Messel on one hand and those of Rott, Randeck. and Ohningen on the other hand may therefore be due to the following facts: At least the lake of Eckfeld has been very deep as compared to its diameter. The well oxygenized part of the littoral was rather narrow and steep. In contrast, the lakes of Rott, Randeck, and Ohningen presumably have had a rather shallow and broad littoral. Secondly - besides this morphometric aspect - the change from a subtropical to tropical climate during the Middle-Eocene to a moderately warm one in the Oligocene and Miocene (BUCHARDT 1978) presumably played an important role as well. REFERENCES
BUCHARDT. B. (1978): Oxygen isotope palaeotemperatures from the Tertiary period in the North Sea area. - Nature, 275: 121-123, 2 Abb.; Basingstoke.
489
GOTH, K. (1990): Der Messeler Olschiefer - ein Algenlaminit. - Cour. Forsch.-Inst. Senckenberg, 131:143 S., 27 Abb., 20 Taf., 9 Tab.; Frankfurt a. M. LUTZ, H. (1988): Die Arthropoden-ThanatozSnose yore "Eckfelder Maar" Ein erster 0berblick.- Mainzer Naturwiss. Archly, 26: 151-155; Mainz. 9 (1989): Die fossile Insektenfauna yon Rott. - in.. KOENIGSWALD, W. v. [Hr,sg.]: Fossillagerst~tte Rott bei Hennef am Siebengebirge: 33-46, 16 Abb., Rheinlandia; Siegburg. 9 (1990): Systematische und palSkologische Untersuchungen an Insekten aus dem MitteI-Eoz~n der Grube Messel bei Darmstadt. - Cour. Forsch.-Inst. Senckenberg, 124:165 S., 33 Abb., 6 Taf., 16 Tab.; Frankfurt a. M. LUTZ, H. et al. (1991): Fossilfundst~tte Eckfelder Maar. Beitr~ge zur Flora und Fauna des Mitteleoz~ns in der Eifel. - 51 S., 36 Abb.; Mainz. NEGENDANK, J. E W., IRION, G. & LINDEN, J. (1982): Ein eoz~ines Maar bei Eckfeld nord~bstlichManderscheid (SW-Eifel). - Mainzer Geowiss, Mitt., 11: 157-172, 12 Abb., 2 Tab., Mainz. PAYNE, A. I. (1986): The Ecology of Tropical Lakes and Rivers, p. 301, 78 Abb., 10 Tab., John Wiley & Sons; Chichester, New York, Toranto, Brisbane, Singapore. WEBER, C. O. (1853): Ueber das Braunkohlenlager von Eckfeld in der Eifel. - Correspondenzbl. Naturhist. Ver. Preussischen Rheinlande u. Westphalens, 10: 409-415, 1 Tar.; Bonn.
FLOWERS FROM THE MIDDLE EOCENE OF ECKFELD (EIFEL, GERMANY) FIRST RESULTS
H. FrankenMuser" & V. Wilde + *Naturhistorisches Museum Malnz/Landessammlung fOr Naturkunde Rheirdand Pfalz, Reichldarastrafle 10, D-6500 Mainz 1 +Forschungsinsfitut Senckenberg, Botanik/Pal~obotanik, Senckenberganlage 25, D-6000 Frankfurt/M. 1
ABSTRACT
The important Middle Eocene flora of the maar lake sediments of Eckfeld (Eifel, Germany) includes a number of angiosperm flowers. Their great diversity, including highly complex sympetalous taxa, is preliminarily described.
INTRODUCTION
Because the Middle Eocene was a time of optimal climatic conditions and major angiosperm radiation, plant remains of that age are especially interesting. Due to their delicate and highly composite nature angiosperm flowers only rarely entered the fossil record. But because flowers form the base of angiosperm systematics, even single fossil specimens are of interest and may be helpful in the reconstruction of phylogenetic patterns. Apart from some North American localities, up to now the oil shale Of Messel was the most famous source of Middle Eocene flowers. In addition to hundreds of palm flowers (Schaztschmidt & Wilde 1986), other types (Schaarschmidt 1984) are often only represented by single specimens in Messel. Due to taphonomie factors, flowers are more abundant in the maar lake sediments of Eckfeld. This is one of the reasons that make the site extremely important, even after only three years of collecting. In combination with other localities of about the same age, for example the Geiseltal near Halle and the area of Helmstedt, the lake sediments of Eckfeld and Messel represent an essential part of the vegetational puzzle of Middle Eocene Europe (Wilde & Frankenh3user, present volume).
Lecture Notes in Earth Sciences, Vol. 49 L F. W. Negendank, 8, Zolitschka (Eds.) PaleolJmnology of European Maar Lakes 9 St~fin~er-Verla~ Berlin Heidelberg, 1993
492
Some major types of flowers from Eckfeld are presented here prior to detailed investigations. Up to now, more than 200 specimens have been found, about two thirds of them with pollen grains preserved in situ. Future studies of the flowers will clarify their systematic as well as evolutionary position.
DESCRIPTION
The Middle Eocene flowers of Eckfeld include both major groups of angiosperms, dicotyledons and monocotyledons, the latter with only few proven types. Frequently perianth and androecium may be studied, while details of the gynoecium are missing. Trimerous monocotyledonous flowers are common. Their perianth is tepaloid (Fig. 1), and monosulcate pollen grains have been found within the anthers. The majority of the flowers represent dicotyledons. Most of them are apetalous or are provided with free sepals and petals (choripetalous). The Juglandales are represented by some pollen beating catkin-like staminate inflorescences of anemophilous platycaryoid luglandaceae (Fig. 2, compare Manchester 1987). It is interesting to note that in contrast to the relatively rare flowers and inflorescences, numerous leaves ofjuglandaceous affinity occur. Another type of tiny flowers without a perianth, or with a strongly reduced perianth, is inconspicious, but their stamens are striking (Fig. 3, 4). In this taxon, fkm,ers ha anthesis and also flower buds sometimes occur in inflorescence structures (Fig. 5). While the pollen sacs of these anthers opened by a longitudinal slit, another similar flower has anthers with valvate dehiscence (Fig. 6). This type of pollen release is known today only from a few families of the Magnoliidae (Lauraceae) and Hamamelidae (Hamamelidaceae and Berberidaceae) (Endress 1989). There are also some tetramerous (Fig. 7, 8) flowers existing in which it is still difficult to distinguish different types. Pentamerous flowers are as numerous as trimerous flowers. The perianth of the pentamerous chofipetalous flowers is quite variable. Shape of the petals is different, elongated (Fig. 9), broad triangular (Fig. 10), club shaped (Fig. 11), or well rounded (Fig. 12). Great diversity is also expressed in the androecium of these flowers. The filaments of the stamen in Fig. 11 have a substantial base and taper strikingly towards their distal tip, where the anthers join. In this type of stamen, pollen may be released by random movements in response even to the weakest agitation. In contrast, the massive stamens of Fig. 13 are dumpbell shaped. Those in Fig. 14 are arranged in two prominent pentamerous cycles with filaments inflated throughout most of their length and anthers attached perpendicularly. Sympetalous flowers with connate petals are rare but diverse. Such flowers with distinct petiolate corollar tubes, sometimes inflated and bell shaped occur (Fig. 15-17). Even extremely zygomorphic flowers, possibly related to Scrophulariales (including Laminales, sensu Rohweder & Endress 1983) with a tubular and nearIy closed corolla (Fig. t8) have been found.
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.., : ,
496
Explanation of figures (scale bar for all figures is lmm; photographs by Naturhistorisches Museum Mainz): Fig. 1: Trimerous monocotyledonous flower Fig. 2: Fragment of a ?platycaryoid juglandaceous male inflorescence Fig. 3-6: Flowers without or with a strongly reduced perianth Fig. 3 and 4: Two different types of flowers with conspicuous androecium Fig. 5: Part of an inflorescence Fig. 6: ?hamamelidaceous flower with valvate anthers Fig. 7 and 8: Two tetramerous flowers of different size and organization Fig. 9-14: Selected pentamerous flowers Fig. 9: Petiolate flower with narrow petals Fig. 10: Flower with broad triangular petals Fig. 11: Flower with club-shaped petals and filaments conspicuously tapering towards anthers Fig. 12: Petiolate flower with well rounded petals Fig. 13: Flower with massive dumpbeU-shaped stamen Fig. 14: Flower with two prominent pentamerous cycles of stamen with inflated filaments Fig. 15-18: Sympetalous flowers Fig. 15: Petiolate flower with narrow corollar tube at the base Fig. 16: Petiolate flower with inflated coroltar tube Fig. 17: Persistent calyx of a pentamerous flower Fig. 18: D.orsiventral flower with calyx and tubular corolla
CONCLUSIONS
During the initial field seasons, many angiosperm flowers have been found in the Middle Eocene sediments of the Eckfeld locality. The number of taxa as well as their diversity already makes it one of the most important palaeobotanical localities of comparable age. With regard to evolutionary level, most of the major modern types of flowers are represented. The Eckfeld locality shows remarkable diversity, for Eocene times, of complex sympetalous flowers. This is another proof for complex coevolutionary relationships between flowers and pollinators in the Middle Eocene as already indicated from the fossil record by Crepet & Friis (1987) and Friis & Crepet (1987), among others. The coevolutionary level was thus more advanced than suggested theoretically by HeB (1990).
REFERENCES
Crepet, W.L. & Friis, E.M. (1987): The evolution of insect pollination in angiosperms. In: Friis, E.M., Chaloner, W.M. & Crane, P.R. (eds.), The origin of angiosperms and their biological consequences, 181-201, Cambridge Univ. Press; Cambridge. Endress, P.K. (1989): Phylogenetic relationships in the Hamamelidoideae. In: Crane, P.R. & Blackmore, S. (eds.), Evolution, systematics, and fossil history of the Hamamelidae. Systematics Assoc. Spec. Publ., 40A: 227-248; Oxford.
497
Friis, E.M. & Crepet, W.L. (1987): Time of appearance of floral features. In: Friis, E.M., Chaloner, W.G. & Crane. P.R. (eds.), The origins of angiosperms and their biological consequences, 145-179, Cambridge Univ. Press; Cambridge. Hell, D. (1990): Die Blfite. 2nd ed., 458 p., Eugen Ulmer; Stuttgart Manchester, S.R. (1987): The fossil history of the Juglandaceae. Monogr. Syst. Bot., Missouri Pot. Garden, 21: 1- 137; St. Louis. Schaarschmidt, F. (1984): Flowers from the Eocene oil shale of Messel: a preliminary report. Ann. Missouri Bot. Garden, 71: 599-606; St. Louis. Schaarschmidt, F. & Wilde, V. (1986): Palmenblfiten und -blOtter aus dem EozSn yon Messel. Cour. Forsch.-Inst. Senckenberg, 86: 177-202; Frankfurt am Main. Rohweder, O. & Endless, P.K. (1983): Samenpflanzen, 391 p., Georg Thieme; Stuttgart. Wilde, V. & Frankenh.%ser, H. ('m press): Initial results on the importance of a flora from the Middle Eocene of Eckfeld. Lecture Notes Earth Sci., present volume.
INITIAL RESULTS ON THE IM~JORTANCE OF A FLORA FROM THE MIDDLE EOCENE OF ECKFELD (EIFEL, W.-GERMANY)
V. Wilde" & H. Frankenh~iuser + "Forschungsinstitut Senckenberg, Botanik/Palfobotanik, Senckenberganlage 25, D-6000 Frankfurt am Main 1 +Naturhistorisches Museum Mainz/Landessammlung fOr Naturkunde Rheinland Pfalz, Reiehklarastr. 10, D-6500 Mainz 1
ABSTRACT
Numerous plant fossils have been excavated from the Middle Eocene maar lake sediments of Eckfeld (Eifel, Germany). Diverse angiosperms are represented by leaves, fruits/seeds and even flowers. A few remains of conifers and some algae, as well as fragments of bryophytes and ferns have also been found. In combination with data from some contemporaneous floras, a reconstruction of Middle Eocene floral diversity for parts of present Europe may be possible in the future.
INTRODUCTION
Due to a restricted catchment area, maar lake sediments in general may be considered as important sources of palaeobotanical information. The fine grained, mostly laminated deposits are often famous for their well preserved plant remains. When dealing with terrestrial sediments, fossil macrofloras are commonly used as an important tool for the reconstruction of past climates and ecosystems. Because the climatic requirements of their nearest living relatives may be used for comparison, especially the angiosperms of younger earth history have been widely employed in speculations about climatic diversification of the Cenozoic. While most of the angiosperm-dominated macrofloras known from Germany are not older than Eocene, a number of Middle Eocene floras exist. Most of them have recently been described with the help of modem methods such as cuticular analysis, e.g. those from Messel (Wilde 1989), the
Lecture Notes in Earth Sciences, Vol. 49 J. F, W. Negendank, B- Zolitschka (Eds.) Paleolimnology of European Maar Lakes 9 Springer-Verlag Berlin HeideIberg 1993
500
WeiBelster-Basin (Fischer 1990), and the Geiseltal area (Zentrales Geologisches Institut 1976). Additionally some floras of the same age, for example those of Grube Prinz yon Hessen near Darmstadt (Wilde 1989), the Eifel (Lhhnertz 1978) and the area of Helmstedt (Wilde 1989) are known, but detailed work has yet to be done.
THE LOCALITY
An isolated occurrence of Tertiary sediments near Eckfeld had been described for the first time by Weber (1853). He noted several plant remains and compared them with those published by him from the Rhenish Browncoal (Weber 1852). More than a century later Pflug (1959) discovered a single specimen from the locality in the collections of the Geological Institute of the University of Cologne and included it in his palynostrafigraphic comparisons. Due to a characteristic spectrum of pollen and spores ('Borkener Bild") he concluded it to be of Upper Eocene age. Von der Brelie et al. (1969) in their reinvestigations of the locality confirmed the results of Pflug (1959) but they stated that it was of Middle Eocene age according to Tobien (I961), who had described a Middle Eocene -Lophiodon cf.
cuvieti from
the type locality of the "Borkener Bild". A Middle Eocene age of the sediments has been proven recently by mammalian remains (Landessammlung t'fir Naturkunde Rheinland-Pfalz 1991). The sequence o f volcanoclastics and lake sediments in a 60m core from the locality has been interpreted by Negendank et al. (1982) as representing the filling of an Eocene maar. Apart from Weber (1853) plant macrofossils from Eckfeld have been mentioned later only by l_xShnertz (1978). Stimulated by the excavations in the more or tess contemporaneous lake sediments of Messel, there was a rising interest in the superficially similar sediments of Eckfeld by private collectors and scientists. As a consequence, excavations bythe Naturhistorisches Museum Mainz/Landessammlungen f~r Naturkunde Rheinland-Pfalz started in 1987. In addition to interesting vertebrate remains of Middle Eocene age, many invertebrates, especially insects, and plant remains have yet been found. While preliminary results on the locality have been summarized in "/.andessammlung N r Naturkunde Rheinland-Pfalz" (1991), first data on the flora were published by Wilde (1989).
THE FLORA OF THE MIDDLE EOCENE OF ECKFELD
Due to the highly restricted nature of the sedimentary basin, a variety of plant remains have been accumulated in the Middle Eocene sediments of Eckfeld. The taxa recognized up to now are listed below in Tab. 1. While there are only a very few fragments of conifers, the angiosperms are represented by numerous leaves, fruits/seeds, flowers, and wood fragments. A diversity of cryptogams includes not only ferns but also mosses and some algae.
501
Tab. 1: Plant taxa recognized up to now in the Middle Eocene of Eckfeld (except for pollen and spores) Funzi - ep[phyllous fruiting bodies of Ascomyceta Algae dlatoms zygnematacean cysts (Ovoidites of. ligneolus) colonies of Botryococcus - Tetraedron (determined by fluorescence, not yet confh'med by SEM) - non calcareous oosporangia of charophytes Bryophyta (compare WILDE 1990) - Dicranites sp. Muscites sp. div sp. pter~dophyta - Lygodium kaulfussi (Schizaeaceae), sporophyUs and trophophylls cf. Ruffordia subcretacea (Schizaeaceae), fragments of a sterile frond and isolated sterile pinnae Osmunda lignitum (Osmundaceae), sterile pinnae cf. "Blechnum" dentatum (inc. sed.), sterile fragments of pinnules - ?Azolla sp. (Azollaceae, water ferns), sterile foliage -
-
-
-
-
-
-
-
Gymnos~rmar - taxodiaceous twig fragment
taxodiaceous ovuliferous cone scale single needle-leaf (not yet determined) Angiospermar (diCOtyledonous) Betulaceae - 2 types of Carpinus-like bracts, exceptionally with fruits, and isolated fruits Fagaceae - Lithocarpus-like triangular fruits Hamamelidaceae hamamelidaceous flower with valvate anthers Juglandaceae leaves and leaflets of engelhardioid type - ?Engelhardia sp., fruits - eL Paraengelhardia sp., fruits - pterocaryoid fruits - fragments of platycaryoid male inflorescences Lauraceae - cf. Laurophyllum sp., leaves cf. Daphnogene sp., leaves Menispermaceae at least 1 type of fruits Moraceae - ?Ficus sp., leaves Myricaceae - Comptonia sp., leaf fragments Nyssaceae - Nyssa sp., fruits - of. Quercus cruciata (possibly = Nyssa altenburgensis), leaves Papilionaceae (=Leguminosae) isolated leaflets Rosaceae - Rosa vel Rubus, complete compound leaf Rutaceae at least 1 type of seeds Theaceae - cf. Polyspora saxonica, leaves Ternstroemites sp., leaves similar to Ternstroemites dentatus -
-
-
-
-
-
-
-
-
502
Tab. 1 continued: Ulmaceae ulmoid leaves and ?fruits - Z e l k o v a - l i k e leaves Vitaceae vitioid leaves and seeds Angi0sperma~ (monocotyledonous) - ef. Smilax sp. (Srnilacaceae) ef. Eichhornia eocenica (Pontederiaceae), fragments of highly degraded leaves - ef. Sabalites sp. (Arecaceae=Palmae), leaf fragments fragments of palm rhachids with prominent arcuate spines diverse fragments of monocotyledonous linear leaves - at least 2 types of monocotyledonous flowers -
-
-
-
-
ON THE IMPORTANCE OF THE LOCALITY
The most important mechanisms for the incorporation of plant fragments in lake sediments are aerial transport by wind or gravity alorie, and minor water transport via tributaries (Ferguson 1985). Gravity-induced sliding and slumping at the slopes of a maar structure may have been of some additional importance in preserving plant debris from preconcentrations along the shoreline in Eckfeld. With decreasing areal extent of a lake the re.lative amount of plant material incorporated increases. The more isolated the lake, the more local in origin is the flora represented in the sediments. A steep slope of the sedimentary basin may be infered in Eckfeld from several badly sorted layers of slump deposits and from the lack of plant remains that may be assigned to a marginal belt of anchored aquatic to semiaquatie or swamp inhabiting plants. As a consequence, there was a minimum of taphonomic filtering and a maximum of the forest surrounding the lake should be represented. Additionally there are only few remains of free floating or submerged aquatic plants from the macrophytic vegetation of the lake itself. Therefore the importance of the Middle Eocene flora of Eckfeld is not only due to the enormous number of specimens, but also to their derivation from a flora that may have been composed of zonal elements. In contrast, most elements of the more or less autochthonous floras of the Middle Eocene near-coastal coalswamps of the Geiseltal and the Helmstedt area should be interpreted as azonal, while the flora of Messel is of somewhat intermediate composition. This is well mirrored by certain differences between these floras that have been noted for the first time by Wilde (1989a,b) and will later be discussed again in detail. Future investigations of these floras offer a unique opportunity to get an idea of floral diversity within the limited area of a few hundred km 2 at the time of the Middle Eocene climatic optimum.
503
REFERENCES
Brelie, G. yon der, Quitzow, H.W. & Stadler, G. (1969): Neue Untersuchungen im Altterti~r yon Eckfeld bei Manderseheid (Eifel). Fortschr. Geol. Rheinland u. Wesffalen, 17: 27-40; Krefeld. Ferguson, D.K. (1985): The origin of leaf-assemblages -- new light on an old problem. -- Rev. Palaeobot. Palynol., 46: 117-188; Amsterdam. Fischer, O. (1990): Bl~tter-Floren aus mitteleoz~tnen Sedimenten des s~dlichen Weil3elster-Beckens (Profen und Scheiplitz). Dissertation A, Humboldt-Universit~t Berlin, 118 p.; Berlin [unpublished thesis]. Landessammlung ffir Naturkunde Rheinland-Pfalz (ed.)(1991): Fossilfundstelle Eckfeld. Beitr~ge zur Flora und Fauna des Mitteleoz~s in der Eifel. 51 p., Landesslg. f. Naturkde. RheinlandPfalz; Mainz. l_x3hnertz, W. (1978): Zur Altersstellung der tiefliegenden fluviatilen Terti~"ablagerungen der SE-Eifel (Rheinishes Schiefergebirge). N. Jb. Geol. Pal~ont. Abh., 156: 179-206; Stuttgart. Negendartk, J.F.W., /don, G. & Linden, J. (1982): Ein eoz,~nes Maar bei Eckfeld nordSsflieh Manderscheid (SW-Eifel). Mainzer geowiss. Mitt., 11: 157-172; Mainz. Pflug, H.D. (1959): Die Deformafionsbilder im Terti~r des rheinisch-saxonischen Feldes. Freiberger Forsch.-H., (C)71: 1-110; Berlin. Tobien, H. (1961): Ein Lophiodon-Fund (Tapiroidea, Mamm.) aus den niederhessischen Braunkohlen. Notizbl. hess. L.-Amt Bodenforsch., 89: 7-16; Wiesbaden. Weber, C.O. (1852): Die TertiLrflora der Niederrheinischen Braunkohlen formation. P .al.al.al.al.al.al,~ontographica,2(4/5): 115-236; Cassel. Weber, C.O. (1853): Ueber das Braunkohlenlager yon Eckfeld in der Eifel. Verh. naturhist. Ver. d. preul~. Rheinlande u. Westfalens, 10: 409-415; Bonn. Wilde, V. (1989a): Vod~ufige Mitteilungen zur Flora aus dem Altterti~ von Eckfeld -- Ergebnisse einer ersten Durchsicht des Fundmateriales aus den Grabungen yon 1987 und 1988. Mainzer naturwiss. Archly, 27: 23-31; Mainz. Wilde, V. (1989b): Untersuchungen zur Systematik der Blattreste aus dem bli~teleoz;an der Grube Messel bei Darmstadt (I-Iessen, Bundesrepublik Deutschland). Cour. Forsch.-Inst. Senekenberg, 115: 1-213; Frankfurt am Main. Wilde, V. (1990): Moosreste aus dem Altterti~ von Eckfeld bei Manderscheid in der Eifel. Mainzer naturwiss. Archiv, 28: 1-6; Mainz. Zentrales Geologisches Institut (ed.)(1976): E o z ~ e Floren des Geiseltales, 507 p., Akademie-Verlag; Berlin.
INTERNATIONAL MAAR DEEP DRILLING PROJECT (MDDP) A CHALLENGE FOR EARTH SCIENCES?
J6rg F.W. Negendank* & Bernd Zolitschka** *GeoForschungsZentrum, Telegrafenberg A26, O-1561 Potsdam **Fachbereich VI/Geologie, Universi~t Trier, D-5500 Trier
Today the reconstruction of palaeoenvironmetal changes is the most thriving scientific problem of Quaternary research. One of the few outstanding methods to achieve a high resolution age determination, a prerequisite to obtain reliable information on past global changes, is varve chronology. This method provides data with annual resolution and, in addition, data on sediment composition. Until now best results concerning the three possible varve types (clastic, organic, evaporitic) were achieved from organic varves, where seasonal variations of pollen, diatoms and calcite precipitation give an internal proof of the annual nature of these laminations. Maar lakes are ideal sources for recovering long and complete laminated continental sedimentary records. These volcanic craters are circular to lobate with a water depth initially exceeding 50 m. Therefore lake level changes do not affect profundal sedimentary conditions and continuous lacustrine conditions are guaranteed. Influx is restricted to a small catchment area, mostly within the crater rim. According to the prevailing climatic conditions and the vegetational cover, clastic or organic or evaporitic laminations may evolve, often with an annual rhythm. Sedimentation rates vary between 0.1 and 6 mm a-~. Thus maar lake deposits provide high resolution records for the reconstruction of past environmental changes. These sediments are geologic archives for data on: (1) Palaeoclimatic fluctuations; (2) Geomagnetic field variations; (3) Volcanic activities; (4) Plant successions and history of vegetation; (5) Changes in the nutrient (trophic) state of the lake, due to both, natural and human influences; (6) Anthropogenic perturbations, e.g. soil erosion and/or heavy metal accumulation.
Lecture Notes in Earth Sciences, Vol. 49 L F. W. Negenda'~k, B. Zolitschka (Eds.) Paleolimno|ogy of European Maar Lakes 9 Springer-Verlag Berlin Heidelberg 1993
506 Sediments from these deep volcanic craters at sites outside glacier limits extent back in time into the Weichselian (e.g. Lago Grande di Monticchio, Italy; Lake Holzmaar and Lake Meerfelder Maar, both Germany) or even beyond (Lac du Bouchet, France; Lago di Vico, Italy). For the last 13,000 years a varve dated sedimentary calendar for central Europe was established using deposits from two maar lakes of the Westeifel Volcanic Field (Germany). The elaborated varve chronologies from Lake Holzmaar and Lake Meerfelder Maar are the base for calibration of palynological and palaeomagnetic records as well as for calibration of radiocarbon datings. These lacustrine varves provide the highest time resolution of all types of sediments and allow to establish an absolute chronology in addition and extension of dendrochronology. Although recovery, processing and study of unconsolidated organic muds is rather difficult, there is one major advantage compared to the only other absolute dating method of dendrochronology: The whole record stems from only one known site covering tens or even hundreds of thousand of years, whereas dendrochronology in Europe has to rely on short records (only several centuries) of one individual tree from an unknown habitat. Many of these short records have to be statistically combined to form the long Holocene dendrochronologies. Another advantage of varved sediments is the possibility to carry out a huge variety of supplementary geochemical, geological, palaeomagnetic, palaeobotanical and palaeozoological investigations on the same material used for dating. During previous studies the idea was born to trace the earth's history of central Europe back in time using maar lake deposits from maars of different ages. Assuming a continuous volcanic activity in the area there is some chance to establish relative stratigraphies for different sites and different time windows which might be combined to produce a long regional record by statistical matching of some sort of geochemical, palaeomagnetic or palynological data. To take into consideration the spatial variations of the palaeorecord cooperative projects with the European Community have been launched to investigate maar lake sediments from lakes Holzmaar and Meerfelder Maar (both Germany), Lac du Bouchet (France), Lago di Vico and Lago Grande di Monticchio (both Italy). Pleistocene sediments from southern Europe (out of the influence of periglacial conditions) 9 also contain annually laminated sediments (organic varves) with a potential for creating a varve chronology, e.g. Lago Grande di Monticchio (Italy). Deposits of the same age from German maar lakes are of different composition. Instead of organic varves clastic laminations dominate due to the cold periglacial environment. They consist of fine-grained silt/clay laminations and intercalated turbidites. In general, clastic laminations are looked
507 upon as providing only an event stratigraphy, depicting time discontinually in short periods, like hours or days. But clastic varve formation and turbidity currents may be closely related phenomenons. In periglacial environments turbidity currents are strongly seasonal caused by the spring peak of melt water discharge. Proximal locations within a lake give clear evidence of turbidites, but distal locations display suspension fallout deposits, as only the fines reach out to the center of the lake. It will be one crucial task for future studies to prove the annual origin of these elastic deposits.
~.~
~,
o
yr/season
10.O00yr
Lake sediments
yr/season
1.000.000 yr
:~i:~:!~
Palaeosoils
100 yr
Loess
10 yr
Polar ice cores
~
O
0
o
,.
Tree-tings
~::;~::~,~~iljii: !ii~iii:,~:,::!
O
X X a X X XX X X
w X X X X
i:. ii/::~:~i.~::;: .~ :x ~x~:iw:~x~:~i~!i~xl ~:~, x:
100.000 yr
X X s
1.000.t300 yr
X X s
yr
300.000 yr
X X a
Mid-latitude glaciers
yr
10.000 yr
Historical records
yr / day
2.000 yr
Fig. 1 : Sources of terrestrial palaeoclimatic proxy-data.
X X X X
X
X X a X X
X
X X
X X X X X
508
Among marine and continental sedimentary proxy records maar lake sediments reveal the highest density of information from local perturbations to solar and astronomic forcing (Fig. 1). This is the important advantage in comparison with any other archive of palaeoclimatic proxy-data. However, until today the coring technique has set a limit. In an European transect from lake Holzmaar and Lake Meerfelder Maar (Westeifel Volcanic Field, Germany) across the volcanic crater of Lac du Bouchet (Massif Central, France) to Lago Grande di Monticchio (Basilicata, Italy) up to 52 m of lacustrine sediments have been recovered documenting the palaeoenvironmental history of the last 18 ka (Germany), 250 ka (France) and 70 ka (Italy). For this kind of drilling the Usinger corer was used, which is a modified Livingstone piston corer with a piston fixed by an additional inner rod to permit precise coring from a raft. This high precision coting device is necessary for recovery of undistrubed sediments. In future this technique has to be improved to enable further penetration into greater depth. These studies will provide detailed information on local, regional, and continent-wide variations of palaeoenvironmental conditions focussing primarily on the climatic history of the last glacial/interglacial cycles. In order to reconstruct past global changes on an annual base along geotraverses crossing different continental areas as well as oceanic terranes (maars also occur on elevated islands of the mid-oceanic ridges) it is necessary to initiate an
International Maar Deep Drilling Project.
AIMS AND OBJECTIVES Studies have to be undertaken as multi- and interdisciplinary investigations based on hightech core recovery providing complete and undisturbed sediment sequences to a depth of up to 200 m from land- and lake-based coring platforms. Precoring seismic, geological and limnological surveys in maar lakes and/or dry maars (silted up former lakes) have to be carried out to find the most promising site and coting location. These studies will contribute to national and international programs for earth sciences like IGBP, Glopals, Pages and various others. The main objectives of the Maar Deep Drilling Project (MDDP) should be focussed on:
5O9 absolute chronology based on varve counting, calibration of radiocarbon dating, construction of paleosecular variation curves, reconstruction of the vegetational history, study of climatic changes on an annual base, study of solar and astronomic periodicities. The results we have to hand on studies of maar lake sediments, published in this volume and elsewhere, demonstrate the wealth of information stored in this type of depositional environment. Only a group of specialised earth scientists from different disciplines and countries that come together and work together in the long term make these results possible. There is some hope to continue and even improve this co-operation under the auspices of an
International Maar Deep Drilling Project (MDDP).
List of Contributors
Thomas Beuker, H. Rosen Engineering GmbH, Am Seitenkanal 8, D-4450 Lingen (Eros), Germany J. Boeneeke, Institut fiir Geowissenschaften, Universitat Mainz, Posffach 3980, D-6500 Mainz, Germany Achim Brauer, Fachbereich VI/Geologie, Universitat Trier, Postfach 3825, D-5500 Trier, Germany Georg Biiehel, Johann Gutenberg Universit~t, Institut ffir Geowissenschaflen, Postfach 3980, D-6500 Mainz, Germany G. Camus, Universit6 Blaise Pascal, Observatoire de Physique du Globe et Centre de Recherches Volcanologiques, 5 Rue Kessler, F-63038 Clermont-Fen-and Cedex, France Ken M. Creer, Department of Geology and Geophysics, University of Edinburgh, King's Buildings, Edinburgh EH9 3JW, U.K. Dieter Drotlmann, Waste Management (Deutschland) GmbH, D-4300 Essen, Germany Andrew J. Dugmore, Department of Geography, University of Edinburgh, Drummond Street, Edinburgh EH8 9XP, U.K. Maria Follieri, Dipartimento Biologia Vegetale, Universi~ La Sapienza, 1-00185 Roma, Italy Pierre Franeus, Palrontologie et Palrogfographie, UCL, Place Louis Pasteur 3, B-1348 Louvain-la-Neuve, Belgium tIerbert Frankenh~iuser, Naturhistorisches Museum Mainz/Landessammlung ffir Naturkunde Rheinland Pfalz, Reichklarastr. 10, D-6500 Mainz, Germany A. de Goer de tIerve, Universit6 Blaise Pascal, Observatoire de Physique du Globe et Centre de Recherches Volcanologiques, 5 Rue Kessler, F-63038 Clermont-Ferrand Cedex, France P. Guilizzoni, C.N.R. Istituto Italiano di Idrobiologia, 1-28048 Verbania-Pallanza, Italy Ralph B. Hansen, Fachbereich VI/Geologie, UniversitSt Trier, Postfach 3825, D-5500 Trier, Germany Bernt Itaverkamp, Kurt-Schumacher-Str. 46, D-3257 Springe I, Germany Thomas Heinz, Fachbereich VUGeologie, Universit~t Trier, Postfach 3825, D-5500 Trier, Germany Wolfgang Hofmann, Max Planck Institut ffir Limnologie, Abteilung Mikrobenrkologie, Posffach 165, D-2320 P16n, Germany Etienne Juvignr, Laboratoire de Grologie du Quaternaire, 7 Place du XX Aofit, B-4000 Liege, Belgium
512
Kerry KeRs, Limnoiogical Research Center, University of Minnesota, Pillsbury Hall, Minneapolis, MN 55455, USA Re|nhard Kirsch, Institut fiir Geophysik der Universifft Kiel, Olshausenstr. 40, D-2300 Kiel, Germany A. Lanai, C.N.R. Istituto Italiano di Idrobiologia, 1-281N8 Verbania-Pallanza, Italy Suzanne Leroy, Palrontologie et Palrogrogmphie, UCL, Place Louis Pasteur 3, B-1348 Louvain-la-Neuve, Belgium Volker Lorenz, Institut ffir Geologie, Universit~t Wtirzburg, Pleicherwall 1, D-8700 W/irzburg, Germany Bernd G. Lottermoser, Institut ffir Geowissenschaften, Universit~t Mainz, Postfach 3980, D-6500 Mainz, Germany Herbert Lutz, Naturhistorisches Museum Mainz/Landessammlung ffir Naturkunde Rheinland Pfalz, Reichktamstr. 10, D-6500 Mainz, Germany Donatella Magri, Dipartimento Biologia Vegetale, Universi~ La Sapienza, 1-00185 Roma, Italy Isabelle Mergeai, Drpartement de Grologie, Facultrs Notre-Dame de la Paix, B-5000 Namur, Belgium Bianeamaria Narcisi, ENEA C.R.E. Casaccia, C.P. 2400, 1-00100 Roma A.D., Italy Ji~rg F.W. Negendank, GeoForschungsZentrum Potsdam, Telegrafenberg A26, D/O-1561 Potsdam, Germany Anthony J. Newton, Department of Geography, University of Edinburgh, Drummond Street, Edinburgh EH8 9XP, U.K. Frank Niessen, Geologisches Institut, ETH-Zentrum, CH-8092 ZiJrich, Switzerland R. Oberh~insli, Institut f~ir Geowissenschaften, Universit~t Mainz, Postfach 3980, D-6500 Mainz, Germany Michael Pirrung, InstitUt ffir Geowissenschaften, UniversiffttMainz, Postfach 3980, D-6500 Mainz, Germany Detlef Poth, Fachbereich VI/Geologie, Universit~t Trier, Postfach 3825, D-5500 Trier, Germany Bert Rein, Fachbereich VI/Geologie, Universit~t Trier, Postfach 3825, D-5500 Trier, Germany Christian Robinson, Department of Geology and Geophysics, University of Edinburgh, King's Buildings, Edinburgh EH9 3JW, U.K. Burldaard W. Seharf, GKSS-Institut f~ir Gew~sserforschung, Gouvemementsberg 1, D/O-3010 Magdeburg, Germany U. Schlitz, Institut f~ir Geowissenschaften, Universit~t Mainz, Postfach 3980,
513
D-6500 Mainz, Germany Guy Seret, Paldontologie et Palrogdographie, UCL, Place Louis Pasteur 3, B-1348 Louvain-la-Neuve, Belgium G.B. Shimmield, Department of Geology and Geophysics, University of Edinburgh, King's Buildings, Edinburgh EH9 3JW, U.K. Antonio Stefanon, UNESCO, 7 Place de Fontenoy, F-75700 Paris, France Nicolas Thouveny, Laboratoire de Grologie du Quaternaire, CNRS, Luminy, F-1328 Marseille cedex 9, France Elisabeth Truze, Limnological Research Center, University of Minnesota, Pillsbury Hall, Minneapolis, MN 55455, USA Ian Turton, School of Geography, Leeds University, Leeds, U.K. Guy Wansard, Palrontologie et Palrogrographie, UCL, Place Louis Pasteur 3, B-1348 Louvain-la-Neuve, Belgium Frank Wegner, Waste Management (Deutschland) GmbH, Wilhelm-Leuschner-Str. 11-17, D-6520 Worms, Germany Helmut WeUer, Geologisches Landesamt Rheinland-Pfalz, Emmeransstr. 36, D-6500 Mainz, Germany Stefan Wende, Institut fflr Geophysik der Universit~t Kiel, Olshausenstr. 40, D-2300 Kiel, Germany Volker Wilde, Forschungsinstitut Senckenberg, Botanik/Pal~obotanik, Senckenberganlage 25, D-6000 Frankfurt am Main 1, Germany Trevor Williams, Department of Geology and Geophysics, University of Edinburgh, King's Buildings, Edinburgh EH9 3JW, U.K. Winfried Zirm~erle, Prinzengarten 6, D-3100 CeUe, Germany Bernd Zolitschka, Fachbereich VI/Geologie, Universitat Trier, Posffach 3825, D-5500 Trier, Germany
Springer-Verlag and the Environment
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Lecture Notes in Earth Sciences
Vol. 1: Sedimentary and Evolutionary Cycles. Edited by U. Bayer and A. Seilacher. VI, 465 pages. 1985. (out of print).
Vol. 22: I. I. Mueller, S. Zerbini (Eds.), The Interdisciplinary Role of Space Geodesy. XV, 300 pages. 1989.
Vol. 2: U. Bayer, Pattern Recognition Problems in Geology and Paleontology. VII, 229 pages. 1985.
Vol. 23: K. B. FSUmi, Evolution of the Mid-Cretaceous Triad. VII, 153 pages. 1989.
Vol. 3: Th. Aigner, Storm Depositional Systems. VIII, 174 pages. 1985.
Vol. 24: B. Knipping, Basalt Intrusions in Evaporites. VI, 132 pages. 1989.
Vol. 4: Aspects of Fluvial Sedimentation in the Lower Triassic Buntsandstein of Europe. Edited by D. Mader. VIII, 626 pages. 1985.
Vol. 25: F. Sans5, R. Rummel (Eds.), Theory of Satellite Geodesy and Gravity Field Theory. XII, 491 pages. 1989.
Vol. 5: Paleogeothermics. Edited by G. Buntebarth and L. Stegena. II, 234 pages. 1986. Vol. 6: W. Ricken, Diagenetie Bedding. X, 210 pages. 1986. VoI. 7: Mathematical and Numerical Techniques in Physical Geodesy. Edited by H. S~nkel. IX, 548 pages. 1986. Vol. 8: Global Bio-Events. Edited by O. H. Wailiser. IX, 442 pages. 1986. Vol. 9: G. Gerdes, W. E. Krumbein, Biolaminated Deposits. IX, 183 pages. 1987. Vol. 10: T.M. Peryt (Ed.), The Zechstein Facies in Europe. V, 272 pages. 1987. Vol. 11: L. Landner (Ed.), Contamination of the Environment. Proceedings, 1986. VII, 190 pages.1987.
Vol. 26: R. D. Stoll, Sediment Acoustics. V, 155 pages. 1989. Vol. 27: G.-P. Merkler, H. Militzer, H. HStzl, H. Armbruster, J. Brauns (Eds.), Detection o f Subsurface Flow Phenomena. IX, 514 pages. 1989. Vol. 28: V. Mosbrugger, The Tree Habit in Land Plants. V, 161 pages. 1990. Vol. 29: F. K. Brenner, C. Rizos rEds.), Developments in Four-Dimensional Geodesy. X, 264 pages. 1990. Vol. 30: E. G. Kauffman, O.H. WaUiser rEds.), Extinction Events in Earth History. VI, 432 pages. 1990. Vol. 31: K.-R. Koch, Bayesian Inference with Geodetic Applications. IX, 198 pages. 1990. Vol. 32: B. Lehmann, Metallogeny of Tin. VIII, 211 pages. 1990.
Vol. 12: S. Turner (Ed.), Applied Geodesy. VIII, 393 pages. 1987.
Vol. 33: B. Allard, H. Bor8n, A. Grimvail (Eds.), Humic Substances in the Aquatic and Terrestrial Environment. VIII, 514 pages. 1991.
Vol. 13: T. M. Peryt (Ed.), Evaporite Basins. V, 188 pages. 1987.
Vol. 34: R. Stein, Accumulation of Organic Carbon in Marine Sediments. XIII, 217 pages. 1991.
Vol. 14: N. Cristescu, H. I. Ene (Eds.), Rock and Soil Rheology. VIII, 289 pages. 1988.
Vol. 35: L. H~kanson, Ecometric and Dynamic Modelling. VI, 158 pages. 1991.
Vol. 15: V. H. Jacobshagen (Ed.), The Atlas System of Morocco. VI, 499 pages. 1988.
Vol. 36: D. Sbangguan, Cellular Growth of Crystals. XV, 209 pages. 1991.
Vol. 16: H. Wanner, U. Siegenthaler (Eds.), Long and Short Term Variability of Climate. VII, 175 pages. 1988.
Vol. 37: A. Armanini, G. Di Silvio (Etis.), Fluvial Hydraulics of Mountain Regions. X, 468 pages. 1991.
Vol. 17: H. Bahlburg, Ch. Breitkreuz, P. Giese (Eds.), The Southern Central Andes. VIII, 261 pages. 1988.
Vol. 38: W. Smykatz-Kloss, S. St. J. Warne, Thermal Analysis in the Geosciences. XII, 379 pages. 1991.
Vol. 18: N.M.S. Rock, Numerical Geology. XI, 427 pages. 1988.
Vol. 39: S.-E. Hjelt, Pragmatic Inversion of Geophysical Data. IX, 262 pages. 1992.
Vol. 19: E. Groten, R. Straug (Eds.), GPS-Techniques Applied to Geodesy and Surveying. XVII, 532 pages. 1988.
Vol. 40: S. W. Petters, Regional Geology of Africa. XXIII, 722 pages. 1991.
Vol. 20: P. Baccini (Ed.), The Landfill. IX, 439 pages. 1989. Vol. 21: U. F6rstner, Contaminated Sediments. V, 157 pages. 1989.
Vol. 41: R. Pflug, J. W. Harbaugh (Eds.), Computer Graphics in Geology. XVII, 298 pages. 1992. Vol. 42: A. Cendrero, G. Ltittig, F. Chr. Wolff (Eds.), Planning the Use of the Earth's Surface. IX, 556 pages. 1992.
Vol. 43: N. Clauer, S. Chaudhuri (Eds.), Isotopic Signatures and Sedimentary Records. VIII, 529 pages. 1992. Vol. 44: D. A. Edwards, Turbidity Currents: Dynamics, Deposits and Reversals. XIII, 175 pages. 1993. Vol. 45: A. G. Herrmann, B. Knipping, Waste Disposal and Evaporites. XII, 193 pages, t993. Vol. 47: R. L. Littke, Deposition, Diagenesis and Weathering of Organic Matter-Rich Sediments. IX, 216 pages. 1993. Vol. 48: B. R. Roberts, Water Management in Desert Environments. XVII, 337 pages. 1993. Vol. 49: J. F. W. Negendank, B. Zolitschka (Eds.), Paleolimnology of European Maar Lakes. IX, 513 pages. 1993. Vol. 50: R. Rummel, F. Sans6 (Eds.), Satellite Altimetry. in Geodesy and Oceanography. XII, 479 pages. 1993.
