Rock Coatings (Developments in Earth Surface Processes 6)

Developments in Earth Surface Procc!-.ses 6 ROCK COATINGS ELSEVIER SCIENCE ll V Sara Bur~erhan~traat 25 I' 0 Box 211...

Author: R.I. Dorn

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Developments in Earth Surface Procc!-.ses 6

ROCK COATINGS

ELSEVIER SCIENCE ll V Sara Bur~erhan~traat 25 I' 0 Box 211. I000 AE A nl\lcnJam. The Netherland'

Ll br a r y of Congress Catalogtng-tn-Publ lcatton Data

Oorn. Ronald I. Roc~ coatongs I by Ronald I. Oorn. p. c•. -- !Oevelop•ents on earth surface processes . 61 Includes btbllographocal references and tndexes. ISBN 0-444-82919-9 1. Rocks--Surfaces. 2. Coatings. 3. Geoche•tstry. I. T11 le. II. Series. OE431.5.067 1998 552' .06--dc21 98- 10344 CJP

ISBN 0-1-14-82919-9 4) 199~ Ehe' ocr Sncncc B V

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l'octhcrland\. Spccoal regulatums for rcaJcr' on the U.S.A. Thi' puhltcaiiiHt hu, hccn rcgo;tcred wuh the C'OJIYII!,dll Clearance Center In< . ICCCJ. 222 Ru,cwood Dnvc. Dmi\Cf'. MA (11\121 lnformauun c;m he ohtamcd lrom the ('((' ahnut condouon\ under whoch photocopoc, nf pam nl th" publtcn11nn ma) be m.odc on the l .S -\ ,\II uthcr cup)nght que•uon,. onduthn~ photocop)mg nut,or "") tO)UI) andl<>r damage to J'C"''"' nr propcn~ ~- a mancr ot product' hablltt). nej!hgcncc or uthcrv.o\C. or lmm an~ u\C <>r <>pcraunn of an) oncthu
Preface and Acknowledgements This book is dedicated to Denise, with deepest respect and love. I owe her an intellectual and emotional debt I will gladly spend the rest of my life repaying. She started my interest in the wonders of the desert, and her intuition guideA me through the many rough spots on the way to completing this book. This book would not have been written without support from the John Simon Guggenheim Memorial Foundation and without urging from Will Graf. The Guggenheim Foundation provided the platform and time to complete and compile this research. Will Graf taught me much about scope and vision in scientific research. As I write this, I am sitting at sunset at the top of an alluvial fan in the McDowell Mountains in Arizona, and rock coatings are everywhere. Older alluvial-fan segments look like snakes with intricate patterns woven by patches of well-varnished desert pavements that contrast with carbonate crusts exposed by road construction. Rock faces and boulders look like they have had a bad case of sun burn and are peeling; as they spall, they erode and expose different rock coatings that developed in fractures. The petroglyph before me is etched into a 'blackboard' of rock varnish, while the engraving itself has an iron film. In contrast, modem development all around the base of the range is so visible, because people destroy the softer colors of coated rocks in a rush to replace them with the harsher and higher albedo of bare rock, spray paint graffiti and cement. My research since 1977 has been driven by a desire to expand basic knowledge about accretions on rock surfaces. In my first Geology class, I brought the professor specimens from the Berkeley Hills with dark red coatings. I asked him what the coating was and how it formed. He cleared his throat and told me that it was nothing more than a bit of iron oxidation, and then went on to talk about the joy of studying the minerals beneath. I chose not argue about the merits of fresh minerals as opposed to rock scum, but I was not interested in the rocks themselves. I was interested in how they were altered at the surface of the earth. In order to avoid misleading the reader, I have several confessions about the content of this book. First, this book does not emphasize dating techniques or paleoenvironmental methods, because these are topics more appropriate for separate monographs. I did not want to dilute my primary interest in the geography of rock coatings. I cover these 'applications' only from the perspective of how they inform about rock coatings. Second, this book has an uneven focus. Some rock coatings are well understood, whereas others have not been studied in as great a depth. Third, this book is not an enclycopedic literature review. I have endeavored to be thorough in bibliographic scholarship, but the glue that binds this book is my view of rock coatings, organized in the way I think about the subject. Nor is this monograph a patchwork quilt of my previous publications. Almost all of the original research presented here has not been published previously. If you do not like this book, the fault is entirely mine. However, if you do like some of this material, it is probably due to the guidance of so many special individuals that were so important at critical times in the research. Ted Oberlander (TMO) pushed

vi

Preface and acknowledgements

me to look at the big picture and to ask the big questions as an undergraduate. The two senior scholars of "desert varnish" research, Charlie Hunt and Robert Sharp, provided valuable advice and encouragement at the fight time. Debbie Elliott-Fisk, John Dixon, Norman Meek, and most especially Dave Whitley have kept my enthusiasm going, all while asking hard questions. In addition to numerous stimulating arguments, Dave Krinsley kept reminding me of an important lesson in science: never be satisfied with one way of looking at the black box; there is always a new angle or technique that can provide the missing piece of the puzzle. Dave Whitley opened the door to rock art research, and then taught me the need for understanding the philosophy of science. When I was at the University of California at Berkeley, Jake Bendix, Roger Byrne, Tom CahiU, Mary Firestone, Greg Lumpkin, Doug Powell, Larry Price, Dave Quimby, Paul Sypherd, Scott Stine, and Don Sullivan were invaluable in conceptual and technical help. While at UCLA, I benefited by the assistance of Henry Ajie, Doug Bamforth, Rainer Berger, Mike DeNiro, Norman Meek, Tony Orme, Bill Schopf, Louis Scuderi, Vatche Tchakerian, Tom Gill, and Jonathan Sauer. While at Texas Tech University, Otis Templer was unwavering in his support, even to the point of allowing me to modify a projection room into a 'dirt lab'. Since being at ASU, I owe a tremendous amount to Jim Clark, for his patience, his humor, and his skills. I have also been most fortunate to associate with some of the very best scientists in desert research relating to rock varnish: Doug Bamforth, John Bell, Julie Francis, Nick Lancaster, Larry Loendorf, Margaret Nobbs, Fred Peterson, and Alice Tretebas. The students I have been fortunate to work with have taught me much about rock coatings and angles on new techniques: Andy Bach, Sean Campbell, Steve Gordon, Tanzhuo Liu, Alan Overson, Tom Paradise, Greg Pope, Marty Roberge, Lorenzo Vazquez Selem, Steve Stadelman, Donna Tanner, Nicolle Villa and Thad Wasklewicz. Looking out over desert lowlands surrounded by mountain islands, I am struck by just how important rock coatings are in defining the aesthetics of the bare-rock landscapes that I love. Many people have gone to great effort and expense to watch sunlight play off Haleakala Volcano in Hawaii, Half Dome in Yosemite, the Sinai of Egypt, the Nazca lines of Peru, the Grand Canyon in Arizona, Ayers Rock in Australia, the Khumbu of Nepal, and Death Valley. Yet much of the color and texture of these stark landscapes is in large part from rock coatings. As the sun sets, I realize that much of this glorious warm hue is only skin deep.

vii

Contents

Contents

Preface and Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

v

Section 1. General Perspectives ................ ........... . C hapter 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. 1.2. 1.3. 1.4.

2

Focus and Organization of Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Awareness of Rock Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . 1 I Historical Perspective on Major Research Threads . . . . . . . . • . . . . . . . . 16

Chapter 2. Paradigms and Methods in Rock Coating Research . . .

19

2.1. Alternative Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Adopting the Paradigm of Landscape Geochemistry . . . . . . . . . . . . . . . . . 2.2.1. Adjudicating Competing Hypotheses . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Introduction to Landscape Geochemistry . . . . . . . . . . . . . . . . . . . . . 2.2.2. 1. The Development of Landscape Geochemistry in Soviet Geography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2.2 Landscape Geochemistry Outside of Russia/Soviet Union ....... 2.2.2.3. Fundaments of Landscape Geochemistry . . . . . . . . . . . . . . . . . . 2.2.3. Rock Coatings as a Part of the Geochemical Landscape . . . . . . . . . . . 2.3. Methods Used in Original Data Gathering . . . . . . . . . . . . . . . . . . . . . . . 2.3.1. Field Collection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2. Preparation of Polished Cross-sections and Ultrathin Sections . . . . . . . 2.3.3. Secondary Electron Microscopy ............... ............ 2.3.4. Backscattered Electron Microscopy . . . . . . . . . . . • . . . . . . . . . . . . . 2.3.5. X-Ray Spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.5.1. X-Ray Images ............... ............... ..... 2.3.5.2. Energy Dispersive Spectrometry . . . . . . . . . . . . . . . . . . . . . . . 2.3.5.3. Wavelength Dispersive Spectrometry.. . . . . . . . . . . . . . . . . . 2.3.5.4. Detection of Organic Matter . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.6. Admission of Bias ............... ............... ......

19 20 20 20 20 21 22 24 27 27 29 29 30 30 31 31 31 31 32

viii

Coments

Section 2 0 Different Rock Coatings 00000000000000000000000000 33 Chapter 3o Anthropogenic Pigments 0 00 0 00 00 0 00 00 00 0 0 00 0 00 0 00 34 30 10 Introduction 0 0 00 0 0 0 00 0 0 00 0 00 0 00 0 0 0 00 0 00 0 0 00 0 0 00 0 0 0 00 0 0 0 3020 Prehistoric Rock Pigments 0 0 00 0 0 00 0 0 00 0 00 0 00 0 0 00 0 0 00 0 0 00 0 0 0 3o3o Historic Rock Painting 0 00 0 0 0 00 0 00 0 00 0 0 0 00 0 0 00 0 00 0 00 0 0 0 00 0 3.3o loCoatings to Change Appearances 0 00 00 0 00 00 00 00 00 00 0 000 0 00 0 3.3020 Coatings to Preserve Stone 0 00 0 00 0 00 0 00 0 00 0 00 0 00 0 0 00 0 00 0 0 3o3o3o Coatings to Preserve Landscape Aesthetics 0 00 00 00 0000 000 00 000 0

34 34 36 36 38 38

Chap ter 4o Litbobiontic Coa tings 00 0 00 0 0 00 0 0 00 0 00 0 00 0 0 0 0 00 0 41 401. Introduction . 0 0 0 0 00 0 0 0 00 0 00 0 0 00 0 00 0 0 00 0 00 0 0 0 00 0 0 00 0 0 0 00 41 4o2o Different Types of Lithobiontic Coatings 00 00 00 00 00 00 00 00 0 000 0 00 0 43 4020 10 Bacteria 00 0 0 . 0 0 0 00 0 0 00 0 0 00 0 00 0 00 0 0 00 0 00 0 0 00 0 0 00 0 0 00 43 402020 Cyanobacteria 00 0 0 00 0 0 00 00 0 00 0 00 0 0 00 00 0 00 0 00 0 0 . 00 0 0 0 48 4o2.3o Fungi 0 0 0 0 0 0 0 0 0 0 0 00 0 0 0 0 0 00 0 0 0 00 0 0 0 0 0 0 0 00 0 0 0 0 00 0 0 0 0 49 4o2.4o Algae 0 0 0 0 0 00 0 0 0 00 0 0 0 0 00 0 .. 0 0 0 00 0 0 0• 0 0 0 0 00 0 0 0 0 0 00 0 56 402050 Lichens 0 0 0 0 0 0 0 00 0 0 0 00 0 0 00 0 0 0 0 00 0 0 0 0 00 00 0 0 0 0 0 00 0 0 0 0 57 4o2o6o Higher Plants 0 0 0 0 0 0 0 00 0 00 0 0 00 0 0 0 00 0 0 00 0 0 0 0 00 0 00 0 0 0 0 0 60 4o3o Controls on Distributions 00 0 0 00 0 0 00 0 0 00 0 0 0 0 00 0 00 0 0 0 0 00 0 0 00 0 6 1 4040 Impact of Lithobiontic Coatings 0 0 00 0 00 0 00 0 00 00 00 0 00 0 0 00 00 0 0 0 63 4o4o l. On Organic Remains .. 0 0 . 0 0 . 00 00 0 00 0 0 00 0 00 0 00 0 . 0 0 0 00 0 0 63 4.4020 On Rock Weathering 00 0 00 00 0 00 0. 00 0 0 00 00 0 0 00 0 00 00 0 0 00 0 64 4.4030 On Other Rock Coatings 0 0 00 0 00 00 0 0 00 0 00 0 00 0 00 0 00 0 0 0 00 0 0 65 Chapter 5. Carbonate Crusts . 00 . 00 0 0. 00 00 . 00 . 0 00 0 00• 0 00 0 . 0 . 67 5olo Introduction 0 0 0 0••• 0 0• 0 0 0• 0 0 0 00 00 0 00 0 0 0 . 0 0 0 00 0 0 0• 0 0• 0 • 0 67 5o2o Freshwater Deposits 0 • 0 00 0 00 0 00 0 0 00 0 00 0 00 0 00 0 . 00 0 00 0 00 0 0 00 67 5.30 Marine Littoral 0 0 00 0 0 0 00 0• 0 0• 0 0 0 00 0 00 00 0 00 00 0 0 0 00 0 0 0 00 0 0 70 5.4o Pedogenic 0 0 0 0 0 00 0 0 0•• 0 0•• 0 00 0 0• 0 00 0 0 00 0 0 00 0 • 00 0 0 0 00 0 0 72 5o5o Subaerial Rock Faces •• 0• 0 0 0• 00 • 00 0 . 0 0 . 0 . 00 . 00 0 00 0 00 0 00 0 0 0 79 5060 Carbonate Crusts and Greenhouse Warming 0000 0000 000 000• 00 000 00 83 Chapter 60 Case Hardening Agents 0 0 000 0 00 00 0 000 00 0 00 00 00 0 0 85 60 10 Introduction 0 0 00 0 0 0• 0 0 0 00 0 00 0 00 0 . 00 . 0 0 00 . 0 0 0 0 00 0 0 0 00 0 0 0 85 6020 Characteristics 0 . 0 0 00 0 0 00 0 00 0 00 0 00 0 0 . 00 0 00 0 00 0 00 0 0 00 0 0 0 0 86 602010 Environmental Settings 00 0 0 0 00 0 0 00 0 0 0 . 0 0 00 0 0 00 0 0 0 00 0 0 86 6o2o 1o1o Subaerial Desert Exposures 0 00 0 00 0• 0 00 0• 00 0 00 0• 0 0 000 0 0 86 60201020 Subsurface Origins 0 0• 0 0 00 0 00 0 00 0 0 0 00 00 0 0 0• 0 0 00 0 00 0 87 60201.30 Associated with Carved Rock • 0 . 00 00 00 0 000 00 00 0• 00 . • 0 0 88 6020 1.40 Tropics 0 0 0 0 0 0 0 00 0 0 00 0 0 00 . 0 0 . 0• 0 0 . 0 . 0 0 0 0 0 00 0 0 0 0 0 91 6020 105 Temperate Environments 0 0 00 0 00 00 0 . 0 . 0 0 00 00 0 0• 0 00 0 00 94

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Arctic and Alpine Environments Composition Material Added to Weathering Rind Rock Coatings as Case Hardening Agents Fused Rock as a Case Hardening Agent Rates of Formation Origin 6o2ol.6o

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Chapter 8. Heavy Meta l Skins ...... Introduction . .. ..... Manganese Skins Environmental Settings . Composition Mineralogy Chemistry Scavenging of Other Heavy Metals Rates of Formation Morphology Origin Biotic Hypotheses Abiotic Hypotheses Combination of Biotic and Abiotic Formation Heavy metal skins as a mix of natural and anthropogenic factors Introduction Lead-enriched Heavy Metal Skins Copper and Other Heavy Metal Skins Patina, Metal Corrosion and Rock Coatings

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Contents

9.2.1.8. Iron Films Interdigitated With Other Rock Coatings .......... 9.2.2. Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.2.1. Mineralogy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.2.2. Type I Iron Films . . . . • . . . . . • . . . . . . . . . . . . . . . . . . . . . . 9.2.2.3. Type II Iron Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.2.4. Type Ill Iron Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.2.5 Heavy Metal Scavenging. . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.3. Information on Rates of Formation ......................... 9.3. Origin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3.1. Source of the Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3 .2. Abiotic Genesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3.3. Biotic Genesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3.4. General Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

160 163 164 164 169 171 178 178 180 180 181 182 184

C hapter 10. Manga niferous Rock Varnish . . . . . . . . . . . . . . . . . . . . 186 10.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2. Characteristics ........... . ......................... .. .. 10.2.1. Environmental Settings: Desert Varnish or Rock Varnish? . . . . . . . . . 10.2.1. 1. Perspectives Prior to World War II . . . . . . . . . . . . . . . . . . . . . 10.2. 1.2 . Perspectives in the Middle Years ...................... 10.2. 1.3. Notion of an Ideal Climate of Formation . . . . . . . . . . . . . . . . 10.2.1.4. Environmental Context . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.2. Physical-Chemical Characteristics ......................... 10.2.2.1. Thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.2.2. Color . . . . . . . . . . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . 10.2.2.3. Sheen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.2.4. Mineralogy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.2.5. Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.2.6. Micromorphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.2.7. Textures Seen in Cross-Section ....................... 10.2.2.8. Post-Depositional Modification . . . . . . . . . . . . . . . . . . . . . . 10.2.3. Classification of Rock Varnish . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.3.1. Prior Perspectives on Classification . . . . . . . . . . . . . . . . . . . . 10.2.3.2. Why Classify Rock Varnish? . . . . . . . . . . . . . . . . . . . . . . . . 10.2.3.3. Color/Chemistry Differences ......................... 10.2.3.4. Geomorphic Differences . . .......................... 10.2.3.5. Microscopic Distinctions ........................... 10.2.3.6. Dangers of Misidentification ..................... .... I0.2.3.7 . Dangers of Instituting a Bad Classification ................ 10.2.3.8. A Tiered Classification for Rock Varnish ................. 10.2.4. Rates of Formation ................................ . . I0.2.4. 1. Observations Prior to World War II . . . . . . . . . . . . . . . . . . . . 10.2.4.2. Observations from World War II to the First Dissertation ...... 10.2.4.3. Calculating Rates of Formation . . . . . . . . . . . . . . . . . . . . . . . 10.3. Origin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I0.3.1. Framing the Issues Historically . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.1. Debate Prior to World War II .......................... 10.3.1.1. Internal Origin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

186 188 188 188 189 189 19 I 193 193 193 194 195 198 206 209 2 12 2 14 214 215 2 16 216 217 218 221 222 224 224 225 227 23 l 231 231 231

Contents

10.3.1.2. External Origin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.1.3. Biological Origin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.1.4. Polygenetic Origin ....................... ...... 10.3.2. Debate from World War II to the First Dissertation ........... 10.3.2.1. Internal Origin ................................ 10.3.2.2. External Origin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.2.3. Both Internal and External . . . . . . . . . . . . . . . . . . . . . . . . 10.3.2.4. Manganese Enhancement by Chemical Processes. . . . . . . . . 10.3.2.5. Manganese Enhancement by Biotic Processes . . . . . . . . . . . 10.3.2. Source of the Manganese . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.3. New Polygenetic Model of Varnish Formation . . . . . . . . . . . . . . . . 10.3.2.1. Clay Minerals at the Building Block Level . . . . . . . . . . . . . 10.3.2.2. Manganese Enhancement ......................... 10.3.2.3. How Rock Varnish Grows. . . . . . . . . . . . . . . . . . . . . . . C hapter 11.

xi

233 234 235 236 236 237 237 237 237 239 241 24 1 242 246

Nitrates and Other Uncommon Rock Coatings ..... . 248

ll.l. introduction ............. • ........................... 11.2. Phosphate Skins ............ .......... .... • ............ 11.3. Nitrate Crusts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.4. Salt Crusts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.5. Sulphate Crusts ........................................ 11.6 Why these Rock Coatings have a Limited Distribution . . . . . . . . . . . . . .

248 248 254 256 262 266

C hapter 12. Oxalate-rich C rus ts . . . . . . • . . . . . . . . . . . . . . . . . . . . . 268 12.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2. Characteristics ......................................... 12.2.1 . Environmental Settings ................................ 12.2.2. Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2.3. Rates of Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3. Origin ........................ ... ...................

268 269 269 270 272 275

C hapte r 13. Silica G laze .................................. 279 13.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2. Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2.1. Environmental Settings ................................ 13.2.1.1. Silica Accumulation in Geologi<; and Pedogenic Systems ...... 13.2.1.2. Subaerial Surfaces in Deserts ......................... 13.2.1.3. Sub-glacial and pro-glacial environments . . . . . . . . . . . . . . . . . 13.2. 1.4. Hawaiian Silica Glaze . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2.1.5. Silica Glazes in Temperate Environments.. . . . . . . . . . . . . . . 13.2.1.6. Antarctic Silica Glaze . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2.1.7. Silica Glaze on Artifacts ....................... • .... 13.2.1.8. Silica Glaze and Rock Art ........................... 13.2.1.9. Silica Glaze on Stone Monuments ..................... l 3.2.l.IO. Streams ......................................

279 280 280 280 282 283 284 286 287 288 288 289 292

Copyrighted Material

xii

Contents

13.2.1.1 1. Silica Glazes Interdigitated With Other Rock Coatings . . . . . . 13.2.2. Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2.2.1 Type I. Homogeneous Amorphous Silica Glaze. . . . . . . . . . . . 13.2.2.2. Type II. Detrital-rich Silica Glaze . . . . . . . . . . . . . . . . . . . . . 13.2.2.3. Type III. Alumina-Iron-rich Silica Glaze . . . . . . . . . . . . . . . . . 13.2.2.4. Type IV. Alumina-rich Silica Glaze . . . . . . . . . . . . . . . . . . . . 13.2.2.5. Type V. Iron-rich Silica Glaze ........................ 13.2.2.6. Type VI. Alumina Glaze. . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2.3. Rates of Formation .................................. 13.3. Origin .............................................. 13.3.1. Source of the Silica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.3.2. Abiotic Genesis ................................ . .... 13.3.3. Biotic Genesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.3.4. General Models ..................................... 13.4. Speculation on Iron Films on Mars ......................... . .

Section 3.

292 293 294 298 301 306 309 310 313 317 317 3 17 317 318 321

Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323

C h a pter 14. Gen eral Model of R ock Coating Development . . . . . . 324 14.1 Introduction ............................ • .............. 14.2 Landscape Geochemical Hierarchy of Controls . . . . . . . . . . . . . . . . . . . . 14.2.1. First-Order Processes: Geomorphic Controls .................. 14.2.1. 1. Exposure of Bare Rock ............................. 14.2.1.2. Stability of Rock Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 14.2.1.3. The Role of Rock Type ............................ 14.2.2. Second-Order Processes: Inheritance from a Subsurface Position . . . . . 14.2.3. Third-Order Processes: Habitability for Lithobionts . . . . . . . . . . . . . 14.2.4. Fourth-Order Processes: Transport Pathways . . . . . . . . . . . . . . . . . . 14.2.5. Fifth-Order Processes: Biogeochemical Barriers .......... . . . . . . 14.2.6. A consideration for the dynamic .......................... 14.3. The Hierarchical Model as an Interpretive Tool . . . . . . . . . . . . . . . . . . .

326 326 326 326 327 329 329 333 334 336 337 340

C ha pter 15. Ana lyzing Geogr a phical Va riations in Rock Coatings 345 15.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2. Different Approaches to Map Regional Geographical Variability ........ 15.2.1 Generalization of Micron-Scale Analyses ..................... 15.2.2 Generalization of Field Observations ........................ 15.2.3 Remotely Sensed Imagery ............................... 15.3 Case Study in Regional Variability: Himalayan Transect . . . . . . . . . . . . . 15.3.1. Study Arc~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.3.2. Rock Coatings in the Khumbu ........................... 15.3.3. Rock Coatings in Ashikule Basin. West Kunlun Mountains. . . . . . . 15.3.4. Discussion and Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.3.4.1. Asymmetry in Rock Coatings and Landscape Aesthetics ....... Copyrighted Material

346 346 351 352 355 356 356 360 365 372 372

Contents

15.3.4.2. Role of Rock Coatings in the Weathering System ........... 15.3.4.2. I. Iron Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.3.4.2.2. Silica Glaze . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.3.4.2.3. Dust Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.3.4.2.4. Carbonate and Sulfate Crusts .....•............... 15.3.4.2.5. Oxalate-Rich Crusts ........................... 15.3.4.2.7. Phosphate Skins ............................ . 15.3.4.2.8. Rock Varnish . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.3.4.3. Comparison with Rock Coatings in Other Geographic Settings .. 15.3.4.4. Implications for Understanding Geographical Variations in Rock Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.4. Concluding Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xiii

374 374 374 375 375 375 376 376 377 378 378

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380 Geographical Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418 Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422

Section 1.

General Perspectives

"In a geochemical landscape we are inclined to see the solution to the problem of geographic landscape." B.B. Polynov (1877-1952), from Perel'man (1966, p. 15)

There are two published monographs on rock coatings, and both focus on rock varnish. The first was written at the turn of the century (Lucas, 1905) on Nile River and desert coatings in Egypt. The other was written about alterations of rock surfaces in North Africa; it included discussions relating to iron films, weathering rinds and silica glaze, but the main focus was on rock varnish (Haberland, 1975). There are no papers in journals or chapters in books that synthesize the wide variety of rock coatings found at the earth's surface. There are no masters theses or dissertations that consider more than few coatings. Thus, there are no comprehensive treatments on the topic of accretions on rock surfaces. This book is an attempt to fill this void, and this first section raises general issues related to the burgeoning literature on rock coatings. The first chapter in this section introduces the wide variety of terrestrial rock coatings. I introduce some of the basic questions asked by researchers studying rock coatings, and I outline the structure of this book. The second chapter summarizes the conceptual baggage that I bring to the study of rock coatings. It is now widely recognized that science is greatly influenced by the paradigm under which data are acquired and analyzed. Kuhn (1970) presented clear examples where data felt to be impossible to explain under one mental framework were easily integrated with another perspective. Hence, I believe it is incumbent for me to be explicit about the paradigm I operate under. Thus, the second chapter starts with the conceptual framework of landscape geochemistry. There is an important corollary to this point. Scientific findings are also greatly influenced by the tools used, although relatively few scientists recognize this point explicitly. In the second chapter I, therefore, also explain that most of the methods I use to analyze rock coatings yield data at the micron scale. All geographical scales of my interpretation are influenced by my reliance on microscopic imagery and chemical data obtained at this spatial scale.

Chapter I

INTRODUCTION

TO ROCK

COATINGS

"An atom at large in the biota is too free to know freedom; an atom back in the sea has forgotten it. For every atom lost to the sea, the prairie pulls another out of the

decaying rocks. The only certain truth is that its creatures must suck hard, live fast, and die often, lest its losses exceedits gains." (Leopold,1949,p.107)

1.1. Focus and Organization of Book

We travel great distances to ponder the sublime landscapes of the Grand Canyon of Arizona, the Khumbu Himal of Nepal, Petra in Jordan, or Ayers Rock in the center of Australia. We adorn our homes and offices with calendar photographs of natural scenes. Most the grandeur and enthrallment of these awesome visual displays has to do with monumental character of landforms in places of modern pilgrimage. Much like frosting can make or break the taste of a cake, however, these spectacular natural treasures are greatly influenced by the color and texture of natural coatings on the exposed rock faces. Alexander von Humboldt (1812) initiated the scholarly study of rock coatings by isolating the basic research themes of composition, origin, spatial distribution, and environmental relations. The field of rock coatings has since grown into an academic orphan. Without a home in any particular discipline, rock coatings have been the foci of thousands of publications written by scientists in analytical chemistry, archaeology, art history, astronomy, botany, conservation of stone monuments, climatology, construction, engineering, hydrology, geochemistry, geology, geomorphology, landscape architecture, microbiology, mineralogy, pedology, oceanography, remote sensing, weathering, and von Humboldt's discipline of geography. The objective of this book is to present the first systematic treatment of rock coatings. My goal is to synthesize the various threads of research that are now largely unconnected due to the lack of communication among the many disciplines investigating rock coatings. Figure 1.1 presents the organization for this monograph. The first chapter gives the reader a general appreciation for the scope of the field of rock coatings. In the second chapter, I review the different paradigms by which scientists in different fields have defined research questions and acquired new knowledge on rock coatings. I argue that the principles of landscape geochemistry should be applied to the study of rock coatings. The second section, chapters 3 through 13, presents and analyzes biogeochemical characteristics and possible origins of different rock coatings. This section is the 'meat and potatoes' of acquired empirical knowledge on rock coatings. In the third and last section chapters 14 and 15 work towards a synthesis on rock coatings. Chapter 14 presents a general hierarchical model for the development of rock coatings. Chapter 15 applies that model to the interpretation of the geography of rock coatings. At this point, I should express a few caveats regarding the material presented here.

Introduction

3

First, I have been selective. Although I have attempted to be thorough in my scholarship, this book is not an encyclopedia on rock coatings. Separate monographs could be compiled for each rock coating. I, therefore, admit to a bias of presenting information needed to develop a general theory of rock coatings and to interpret their geography.

SECTION 1" GENERAL PERSPECTIVES Chapter 1: Introduction Chapter 2: Paradigms and Methods in Rock Coating Research

SECTION 2: DIFFERENT ROCK COATINGS Chapter III. Anthropogenic Pigments Chapter IV. Lithobiontic Coatings Chapter V. Carbonate Crusts Chapter VI. Case Hardening Agents Chapter VII. Dust Films Chapter VIII. Heavy Metal Skins Chapter IX. Iron Films Chapter X. Manganiferous Rock Varnish Chapter XI. Nitrates and Other Uncommon Crusts Chapter XII. Oxalate-Rich Crusts Chapter XIII. Silica Glaze i

it

ii

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ii

iiii

ii

H

............... SECTION 3: sYNTHESIS ......................... I Chapter XIV. General Model of Rock Coating Development [ Chapter XV. Analyzing Geographical Variation,s in Rock Coatings,,,] Figure 1.1. Organization of the book.

Second, I exclude an entire set of geological phenomena that might be considered rock coatings by some. These are minerals that crystallize within fractures. Epidote is a common example. This hydrous calcium iron aluminum silicate forms green coatings in fractures in metamorphic, pegmatite, and in felsic igneous rocks that contain calcium. These fracture precipitates are perceived as rock coatings upon erosion of the overlying rock. I also exclude rock coatings found in marine or littoral settings (Arthur et al., 1983; Fewkes, 1976). In contrast the focus of this book is on rock coatings that develop in the terrestrial weathering environment, at the earth's surface or in regolith near the earth's surface.

4

Chapter I

Third, there is a substantial literature regarding the coatings that form on minerals in the fields of geology (Nardi et al., 1994; Peryt, 1983), soils (Chadwick et al., 1987; Kaushansky et al., 1984; Singer and Warrington, 1992) and geochemistry (Zachara et al., 1995). Rather than present a separate chapter on grain coatings, I explore the relevant literature in the context of analogous rock coatings. I similarly cover accretions that originate in subsurface settings because some of these coatings are later exposed by erosion. Lastly, I admit to being consumed by a concern for understanding the nature of place. I am a geographer. My motivation for conducting this research has focused on trying to answer the question: why does this particular rock coating grow in this place. Why do rock varnishes dominate in warm deserts, while silica glazes are ubiquitous in Hawai'i, and iron films are commonly associated with acidic Arctic and alpine waters? I am not posing new questions. Although rock coatings have yet to be recognized as an interdisciplinary research arena, people have been aware of rock coatings for millennia. 1.2. Awareness of Rock Coatings

Coatings on rocks are among the earliest phenomena that we know humans studied. Prehistoric artists used rock coatings as a blackboard for engraving motifs (Figure 1.2).

Figure 1.2. Oval Australian petroglyph WH5 from the Wharton Hill site in South Australia (Nobbs, 1984). This petroglyph is carved into black rock vamish, and in turn is coated by the rock varnish.

Rock coatings were mentioned in the Old Testament approximately 3000 years ago (Krumbein and Jens, 1981): "If I inflict a fungus infection upon the house in the land you have occupied, its owner shall come and report to the priest that there appears to be a patch of infection on his house. If on inspection the priest finds the patch on the walls consists of greenish and reddish depressions, apparently going deeper than the surface, he shall go out of the house. On the seventh day he shall come back and inspect the house and if the patch has spread in the wails, it really is an infection." (Levithicus 13,14)

Ancient cultures took advantage of rock coatings in major stone works. Nabateans used visual differences between rock coatings and the underlying Nubian Sandstone to highlight monumental architecture in Roman times in the mid-East (Figure 1.3.).

Introduction

5

Prehistoric Nasca cultures constructed giant ground figures by juxtaposing rocks with dark coatings and rocks without dark coatings in the Peruvian Desert around 2000 years ago (Figure 1.4.). Whenever and wherever humans used stones, they modified rock coatings.

Figure 1.3. Petra, Jordan, where architecture was carved to take advantage of lithologically-constrained weathering features. Photography courtesy of Dr. Thomas Paradise.

Figure 1.4. Ground figures on the pampa of Nazca Peru, where the dark surface is a smooth desert pavement of cobbles coated by rock varnish. The ground figures were made by clearing the pavement, thus exposing the underlying lighter colored silt and unvamished rocks.

Rock coatings are ubiquitous, whether we observe cultural or natural landscapes. Stone faces on older historic buildings are usually altered by the development rock coatings (Figure 1.5) (Baranski, 1993; Blackwelder, 1948; Emery, 1960; Krumbein, 1988; Urmeneta et al., 1993). Modern construction leaves long-term scars because of the disturbance of rock coatings (Figure 1.6).

6

Chapter 1

Figure 1.5. Red Fort at Agra, India, where crusts of calcium oxalate (whewellite) and amorphous silica darken the surface of the sandstone.

Figure 1.6. The building of roads and houses in deserts makes a tremendous visual contrast between lightcolored rocks without rock coatings (those seen in road) with rocks darkened by manganese-rich rock varnish. This alluvial-fan at the southern end of the McDoweLl Mountains has seen a great increase in visual disturbance from 1990 (upper photo) to 1995 (lower photo).

Many of our favorite cross-cultural sites are dominated by rock coatings. Ayers Rock, Australia, is a sacred site for Aborigines and also a favorite vacation destination for Australians of all origins. A favorite past-time of individuals is to take photographs at dawn and dusk, as the low-light emphasizes the already orange-red color of the inselberg (Figure 1.7). However, the true color of the rock is ivory; it is colored orange by iron films.

Introduction

7

Figure 1.7. Ayers Rock, Australia, where individuals travel thousands of kilometers to the center of Australia. Pilgrims culminate their trip with a hike to the top, all the while walking over iron films, silica glazes, oxalate-rich crusts and rock varnish.

My estimate is that rock coatings in semiarid, arid, or hyperarid climates cover more than three percent of the Earth's surface. Three percent is a very conservative planetary estimate because exposed rock surfaces occur in other climates. For example, lichens are dominant biological on over eight percent of the Earth's surface (Koppes, 1990), and lichens are but one type of lithobiontic rock coating. One aspect of the relevance of rock coatings is that we spend millions of dollars to conduct remote sensing investigations with satellites orbiting the Earth (Figure 1.8) and Mars (Figure 1.9) to study planetary surfaces that are coated with rock accretions.

Figure 1.8. SPOT TM satellite image of Death Valley, California, where different degrees of rock vamish create a mosaic of contrast on the alluvial fans (scale bar-- 15 km).

8

Chapter 1

Figure 1.9. Viking Lander view of the surface of Mars (from NASA). Even before the Pathfinder mission there was considerable speculation that rock varnish may be a ubiquitous rock coating on Mars (E1-Baz and Prestel, 1980; King, 1988).

I contend that rock coatings are an important, but unappreciated component of landscape aesthetics. Consider the cases illustrated in Figures 1.10 to 1.15. We go out of our way to visit ancient motifs that are engraved into rock coatings (Figure 1.10). The 'red rock' landscapes of the Colorado Plateau in the western United States are almost completely covered by rock coatings that obscure the true color of the underlying rock (Figure 1.11).

Figure 1.10. The Puuloa petroglyph site (Cox and Stasack, 1970) in Hawai'i Volcanoes National Park is heavily impacted by visitors. The petroglyphs are engraved into a natural coating of silica glaze.

Introduction

9

The phrase bare or naked rock is almost always a misnomer. Even in heavily vegetated Great Britain, concern over the preservation of "naked rock" (Baird, 1994) is a concern for coated rock. Consider Half Dome in Yosemite National Park. This magnificent monolith stands out in photographs in part because of the dark streaks of organisms, oxalates and rock varnish (Figure 1.12). The flanks of the West Kunlun Mountains in Tibet are mostly buffed by a monotonous tan loess; what visual contrast exists is provided by rock coatings (Figure 1.13). The central Sinai Peninsula is a mixture of dark and bright surfaces (Figure 1.14) created by coatings of orange and black rock varnish. The true color of unaltered rock in the Wind River Mountains is darkened by films of iron oxides (Figure 1.15). All these examples exemplify the importance of rock coatings in influencing the appearance of natural landscapes.

Figure 1.11. The dark streaks mnning down the side of the sandstone cliff at Canyon de Chelly, Arizona, are composed of mostly manganiferous rock varnish and calcium oxalate, although there are also coatings of silica glaze and biofilms (lichens, algae, cyanobacteria, fungi).

10

Chapter 1

Figure 1.12. Half Dome in Yosemite National Park. The dark streaks are composed of manganiferous rock varnish, calcium oxalate, silica glaze, but mostly biofilms (lichens, algae, cyanobacteria, fungi).

Figure 1.13. West Kunlun Mountains of Tibet, Figure 1.14. Eastern Sinai Peninsula, where the where the dark streaks on the hillsides are a rocks in the desert pavement are coated by mixture of manganiferous rock varnish, manganiferousrockvamish. phosphate skins, and silica glaze.

Introduction

11

Rock coatings alter the appearance and stability of human-modified stones. In urban settings "soiling" alters the beauty of buildings and can enhanced deterioration (Meierding, 1993a; Meierding, 1993b; Schiavon, 1993; Schiavon et al., 1995; Whalley et al., 1992), eventually leading to the loss of monuments and art (Krumbein, 1988). The field of stone conservation is increasingly using technical tools to understand processes of degradation (Doehne, 1994; Price, 1996) and the role of rock coatings (Krumbein and Dyer, 1985; Krumbein and Urzi, 1993; Schiavon et al., 1995), all in order to better preserve culanal treasures.

Figure 1.15. Iron films darken the flow paths of snow melt streams in the alpine Wind River Mountains, western United States.

It is well beyond the scope of this book to assess the influence of rock coatings on aesthetics, but it my goal to heighten your awareness. Rock coatings are everywhere. They add color and contrast to natural and human landscapes. This was recognized early on in the study of rock coatings in the context of the Great Cataracts of the Orinoco River in South America. "They [rock-encmstations] give the landscape a singularly gloomy aspect; their color being in strong contrast with that of the foam of the river which covers them, and of the vegetation by which they are surrounded." (von Humboldt, 1812)

1.3. Nomenclature

Rock coatings are part of the interdisciplinary field of weathering. Weathering is the breakdown and decay of the lithosphere into products that are in equilibrium with conditions at or near the earth's surface. Biophysical weathering splits rocks into smaller particles through biological and physical processes, and while biochemical weathering decays minerals. Weathering processes also forms new compounds that are more in equilibrium with the environment at and near the earth's surface (Brimhall et

12

Chapter 1

al., 1991; Nahon, 1991; Pope et al., 1995; Yatsu, 1988). I contend that this includes rock coatings. The earth's surface is dominated by the products of weathering (Lukashev, 1970), with clays minerals being the most common (Nahon, 1991). Secondary weathering products such as laterites (Brown et al., 1994), calcretes (Hutton and Dixon, 1981), evaporite minerals (Watson, 1985), and aluminum ores (Brimhall et al., 1988) are analogous to rock coatings in that they represent the concentration of weathering products. The weathering of minerals can change the appearance of a rock through the creation of a weathering find. A case in point can be found in the rocks on top of Mauna Kea, Hawai'i. This now dry volcanic summit (Figure 1.16) hosted an ice cap that last melted back about 16,000 years ago (Dorn et al., 1991). During the last sixteen millennia, the outer skin of the rock has developed a weathering rind (Figure 1.17).

Figure 1.16. The summit area of Mauna Kea was last glaciated about 16,000 years ago (Dom et al., 1991).

Figure 1.17. In the last 16,000 years, the basaltic rocks near the summit of Mauna Kea developed lightercolored rims called weathering rinds. The weathering rinds are about 2 millimeters thick in this photograph.

Weathering finds are sometimes interpreted as rock coatings (Demangeot, 1971; Lucas, 1905; Salemi et al., 1989). There is, however, a clear distinction. Weathering is the breakdown and decay of minerals in place (Figure 1.17 and Figure 1.18). In contrast, rock coatings are accretions on top of the rock. In most cases, rock coatings are not directly derived from the underlying rock. The formation of weathering finds and rock coatings are distinct phenomena, although they are in direct physical juxtaposition.

Introduction

13

Figure 1.18. This backscattered electron microscope image shows a close-up view of the weathering rind in Figure 1.17. The weathering rind shows an increase in porosity of the minerals. The porosity shows up in the backscattered imagery as black void spaces.

The constituents of rock coatings are transported to a rock surface. The transportation may be a distance of a few microns or thousands of kilometers, but rock coatings are accretions added to rock surfaces. Figures 1.19, 1.20, 1.21, 1.22 and 1.23 illustrate this important point.

Figure 1.19. A backscattered electron microscope image of rock varnish resting on top of feldspar minerals at Kitt Peak, Arizona. The very distinct morphological boundary between the brighter varnish and the darker minerals illustrates that the rock varnish is an accretion.

Figure 1.20. A secondary electron microscope image of rock varnish from Death Valley Canyon alluvial fan in Death Valley. The arrow shows a trapped piece of airborne dust.

Figure 1.21. A backscattered electron micrograph of a Vermont oxalate-rich crust, resting on quartz. Note the clean break between the fresh quartz and the oxalate.

14

Chapter I

Rock coatings form at or near the earth's surface. Like soils, they rarely represent a single mineral, but rather a mixture of materials accreted to the rock by physiochemical processes that are often biologically mediated. And like soils, this milieu is sensitive to environmental changes. The terminology of rock coatings has not been standardized. In archaeology, for example, "patina" is the term generally used to designate a rock coating or a surficial alteration to stone. This term originally described alteration of copper (Marani et al., 1995), yet it has been generalized to the point where it has been used by scientists in other fields working with archaeologists (Papamzzo and Moretto, 1995).

Figure 1.22. A backscattered electron micrograph of an aluminum glaze, resting on top of basalt from Haleakala Volcano, Maui. The aluminum glaze is relatively dark, because aluminum has a much lower atomic number than the basaltic minerals,

Figure 1.23. A backscattered electron micrograph of an iron film formed on a feldspar (plagioclase), Conejo volcanics, western Santa Monica Mountains, southern California. Note the clean break between iron film accretion and the underlying rock.

Although more than a half-century old, a general criticism of the use of patina is still valid: "It cannot be too strongly emphasized that there is a special danger involved in the use of the term "patina." It is a single word which designates many different kinds of surface change. A considerable number of writers have been guilty of employing "patina" as though it were a well-understood and precise concept. The reader is often unable to decide whether the term refers to a mechanically induced gloss, a "desert vamish," a corroded surface, a stain of some sort, or ordinary weathering. This word has such a wide indefinite usage at present that it is especially necessary to determine just what kind of surface change is being called "patina" in any particular instance." (Service, 1941, p. 557)

Table 1.1. presents the first nomenclature for rock coatings. I worry that the institution of a nomenclature may encourage future researchers to pigeon-hole their unique rock coatings into a narrow range of options. The sole purpose of the rough classification in Table 1.1. is to make communication more efficient for the rest of this book. In reality coatings listed here are 'end members'. Many coatings are blends and different end-member coatings may grow side-by-side in direct contact or may be interstmtified.

15

Introduction

Table 1.1. Nomenclature of rock coatings, in alphabetical order. Term

Summa~ Description

Related Terms

Carbonate Skin

Coating composed primarily of carbonate, usually calcium carbonate, but could be combined with magnesium or other cations

Caliche, calcrete, patina, travertine, carbonate skin, dolocrete, dolomite

Case Hardening Agents

Addition of cementing agent to rock matrix material; the agent may be manganese, sulfate, carbonate, silica, iron, oxalate, organisms, or anthropogenic

Sometimes called a particular type of rock coating

Dust Film

Light powder of clay- and silt-sized particles attached to rough surfaces and in rock fractures

Gesetz der Wiistenbildung; clay skins; clay films; soiling

Heavy Metal Skins

Coatings of iron, manganese, copper, zinc, nickel, mercury, lead and other heavy metals on rocks in natural and human-altered settings

Described by chemical composition of the film

Iron Film

Composed primarily of iron oxides or oxyhydroxides; unlike orange rock varnish because it does not have clay as a major constituent

Ground patina, ferric oxide coating, red staining, ferric hydroxides, iron staining, iron-rich rock varnish, red-brown coating

Lithobiontic Coatings

Organic remains form the rock coating, for example lichens, moss, fungi, cyanobacteria, algae

Organic mat, biofilms

Nitrate Crust

Potassium and calcium nitrate coatings on rocks, often in caves and rock shelters in limestone areas

saltpeter, niter, icing

Oxalate Crust

Mostly calcium oxalate with variable concentrations of silica, magnesium, aluminum, potassium, phosphorus, sulfur, barium, and manganese; often found forming near or with lichens; usually dark in color, but can be as light as ivory

Oxalate patina, lichenproduced crusts, patina, scialbatura

Phosphate Skin

Various phosphate minerals (e.g. iron phosphates or apatite) that are mixed with clays and sometimes manganese

Organophosphate film; epilithic biofilm

Pigment

Human-manufactured material placed on rock surfaces by people

Pictograph, paint, sometimes described by the nature of the material

Rock Vamish

Clay minerals, Mn and Fe oxides, and minor and trace elements; color ranges from orange to black in color produced by variable concentrations of different manganese and iron oxides.

Desert vamish, desert lacquer, patina, manteau otecteaur, iistenlack, Schutzrinden, cataract films,

Salt Crust

The precipitation of sodium chloride on rock surfaces

Halite crust, efflorescence, salcrete

Silica Glaze

Usually clear white to orange shiny luster, but can be darker in appearance, composed primarily of amorphous silica and aluminum, but often with iron.

Desert glaze, turtle-skin patina, siliceous crusts, silica-alumina coating, silica skins

Sulfate Crust

Composed of sulfates (e.g., barite, gypsum) accreted on rocks

Gypsum crusts; sulfate skin

16

Chapter I

I have tried to use terms that are well established in the literature. Where no term is in favor, I have attempted to blend field appearance with composition. I apologize to scholars who have used different terms. This cannot be helped because dissimilar terms have been adopted for the same phenomenon because rock coatings are studied by scholars in distinct disciplines. It would be far too chaotic, however, to be terminologically inclusive. Consider, for example, the number of different terms that have been used to describe manganiferousclay coatings on rocks: brownish black crust; desert lacquer; desert film; patina, patination; manteau protecteaur; dunkle Rinden; W0stenlack; Schutzrinden; black coatings; black find; desert film; manganese film; and cataract film. While there was no consistent term employed prior to World War II. "Desert varnish" (Merrill, 1898) began to catch on in the English-language geological literature after World War I and became dominant in the geological literature after World War II (Allen, 1978; Bauman, 1976; Bull, 1984; Dora and Oberlander, 1981a; Dragovich, 1986a; Duerden et al., 1986; Eastes et al., 1988; Elvidge, 1979; Engel and Sharp, 1958; Hooke et al., 1969; Hunt, 1954; Iskander, 1952; Knauss and Ku, 1980; Lakin et al., 1963; Laudermilk, 1931; Moore and Elvidge, 1982; Perry and Adams, 1978; Potter and Rossman, 1977; Staley et al., 1991; Watson, 1989; Whalley, 1983; White, 1924). In the archeological literature, "patina" or "patination" describes this same coating (Amsden, 1939; Bard, 1979; Bednarik, 1979; Childers and Minshall, 1980; Clarke, 1977; Cremaschi, 1992; Friedman et al., 1994; Grant, 1967; Hayden, 1976; Heizer and Baumhoff, 1962; Jacobson et al., 1989; Pineda et al., 1989; Pineda et al., 1990; Service, 1941; Whitley et al., 1984). "Rock varnish" has somewhat replaced desert varnish or patina as the preferred term because clay-manganiferous coatings are common outside of deserts (Anderson and Krinsley, 1989; Clayton et al., 1990; Dora and Oberlander, 1981b; Drake et al., 1993; Jones, 1991; Krumbein and Jens, 1981; Nagy et al., 1991; O'Hara et al., 1989; Pineda et al., 1988; Raymond et al., 1992; Reneau, 1993; Whalley et al., 1990; White, 1990; Zhang et al., 1990). Although I suggested the terminological change, I now regret the suggestion. Rock varnish is certainly not the only varnish on rocks and 'manganiferous clay skins' or manganiferous clay varnish' would be a better term. Yet, rock varnish has become widely accepted. Thus, to change the term this point would create unnecessary confusion. I hope that the nomenclature in Table 1.1. facilitates communication. A list of synonyms is also included to help minimize misunderstandings.

1.4. Historical Perspective on Major Research Threads

Researchers interested in rock coatings have had the same basic curiosity about our planet's geography that led Alexander von Humboldt (1812, published 1907: 242-247) to analyze black rock-encrustations and naturalist Charles Darwin to ponder brown rock coatings (Darwin, 1897). These intellectual giants raised the basic questions that have driven research on this topic for the last two centuries, von Humboldt and Darwin were concerned about: composition; origin; distribution; relationship between coating and the underlying rock; relationship between environment and coating formation; and rates of growth.

Introduction

17

Composition" "Dr. Koenig...of the Royal Society of London...says "the black crust is composed, according to the analysis of Mr. Children, of the oxide of iron and manganese." Some experiments made at Mexico, jointly with Sefior del Rio, led me to think that the rocks of Atures, which blacken the paper in which they are wrapped, contain, besides oxide of manganese, carbon, and supercarburetted iron...At the Orinoco, granitic masses of forty or fifty feet thick are uniformly coated with these oxides; and however thin these crusts may appear, they must nevertheless contain pretty considerably quantities of iron and manganese, since they occupy a space of above a league square." von Humboldt (.1812, p. 244) "Here the coating is of a rich brown instead of a black colour, and seems to be composed of ferruginous matter alone." Darwin (1897, p. 13)

Origin: von Humboldt reasoned that coatings on rocks adjacent to rivers must have been deposited from solution, where the oxides originated from ground water: "In reflecting...we are rather inclined to think that this matter is deposited by the Orinoco... Adopting this hypothesis, it may be asked whether the river holds the oxides suspended like sand and other earthy substances, or whether they are found in a state of chemical solution. The first supposition is less admissible, on account of the homogeneil~y of the crusts, which contain neither grains of sand, nor spangles of mica, mixed with the oxides. We must then recur to the idea of a chemical solution...The mud of the Nile...is destitute of manganese...The mud consequently is not the cause of the black crusts on the rocks of Syene...That celebrated chemist [Berzelius] was of the opinion that the rivers do not take up these oxides from the soil over which they flow, but that they derive them from their subterranean sources, and deposit them on the rocks in the manner of cementation." yon Humboldt (1812, p. 245-6)

von Humboldt argued that the dark phenomenon is a coating, and not a product of weathering of the underlying rock. "The colouring matter does not penetrate the stone, which is coarse-grained granite, containing a few solitary crystals of homblende...The black crust is 0.3 of a line in thickness; it is found chiefly on the quartzose parts. The crystals of feldspar sometimes preserve extemaUy their reddish-white colour, and rise above the black crust. On breaking the stone with a hammer, the inside is found to be white, and without any trace of decomposition...Quartz and feldspar scarcely contain five or six thousandths of oxide of iron and of manganese; but in mica and hornblende these oxides, and particularly that of iron, amount, according to Klaproth and Herrmann, to fifteen or twenty parts in a hundred...Now, if these black crusts were formed by a slow decomposition of the granitic rock, under the double influence of humidity and the tropical sun, how is it to be conceived that these oxides are spread so uniformly over the whole surface of the stony masses, and are not more abundant round a crystal of mica or homblende than on the feldspar and milky quartz?" von Humboldt (1812, p. 244-5)

Distribution:

With vast travel experience von Humboldt was able to place the coatings in a more global context: "We must observe, in the first place, that this phenomenon does not belong to the cataracts of the Orinoco alone, but is found in both hemispheres. At my return from Mexico in 1807, when I showed the granites of Atures and Maypures to M. Rozi~re, who had traveled over the valley of Egypt, the coasts of the Red Sea, and Mount Sinai, this learned geologists pointed out to me that the primitive rocks of the little cataracts of Syene display like the rocks of the Orinoco, a glossy surface, of a blackish-grey, or almost leaden colour, and of which some of the fragments seem coated with tar." von Humboldt (1812, p.243-4)

Following von Humboldt's observations, Charles Darwin (1897, p. 12-13) explained that coatings in littoral, as well as riverine contexts may have a similar origin.

18

Chapter I "Bahia, or San Salvador. Brazil, Feb 29th...On a point not far from the city, where a rivulet entered the sea, I observed a fact connected with a subject discussed by Humboldt. At the cataracts of the great riven Orinoco, Nile, and Congo, the syenitic rocks are coated by a black substance, appearing as if they had been polished with plumbago... In the Orinoco it occurs on the rocks periodically washed by the floods, and in those parts alone where the stream is rapid; or, as the Indians say, "the rocks are black where the waters are white."

However, Darwin associated rock coatings with riverine or littoral contexts: "They occur only within the limits of the tidal waves... In like manner, the rise and fall of the tide probably answer to the periodical inundations; and thus the same effects are produced under apparently different but really similar circumstances." Darwin (1897, p. 13)

Environmental illness: von Humboldt explains that these coatings are unlikely to cause ill health attributed to them by natives and Missionaries. "In the Missions of the Orinoco, the neighbourhood of bare rocks, and especially of the masses that have crusts of carbon, oxide of iron, and manganese, are considered injurious to health...many examples are cited of persons, who, after havingpassed the night on these black and naked rocks, have awakened in the morning with a strong paroxysm of fever...Can it be admitted that, under the influence of excessive heat and of constant humidity, the black cmsts of the granitic rocks are capable of acting upon the ambient air...? This I doubt...in an atmosphere renewed every instant by the action of little currents of air..." yon Humboldt (1812, p. 246)

The theme of environmental illness related to rock coatings has not recurred since in the scientific literature since, although a variety of clinical symptoms may be found upon exposure to abundant manganese, especially in manganese miners and steel-plant workers (Griffin et al., 1973). Relationship to Environment: Alexander von Humboldt was also aware that coatings can influenced the environment in turn. "It is probably dangerous to sleep on the laxas negras only because these rocks retain a very elevated temperature during the night. I have found their temperature in the day at 48 ~ the air in the shade being at 29.7~ during the night the thermometer on the rock indicated 36 ~, the air being 26~ they have acquired more in the day they lose at night by radiation...There is a certain maximum which they cannot pass, because they do not change the state of their surface, their density, or their capacity for c~oric..." von Humboldt (1812, p. 247).

Controls on thickness: Both von Humboldt and Darwin were concerned with the appearance of uniform thickness, and they pondered issues of rates of formation. "Cementation seems to explain why the crests augment so little in thickness." von Humboldt (1812, p. 246) "and no reason, I believe, can be assigned for their thickness remaining the same." Darwin (1897, p. 13)

In summary, the sophistication of chemical and physical data has increased over the last two centuries. However, the basic research questions asked about rock coatings can be traced to the questions posed by Charles Darwin and especially by Alexander von Humboldt.

19

Chapter 2

PARADIGMS RESEARCH

AND METHODS

IN R O C K

COATING

Jo of the North Sea said, 'You can't discuss the ocean with a well frog, he's limited by the space he lives in. You can't discuss ice with a summer insect, he's bound to a single season. You can't discuss the Way with a cramped scholar, he's shackled by his doctrines.' Chuiang Tzu from "Autumn Floods"

2.1. Alternative Perspectives We are approaching the bicentennial of the formal study of rock coatings, started by Alexander von Humboldt in his travels to the New World from 1799 to 1804 (von Humboldt, 1812). Rock coatings do not belong to any single academic tradition. Similar questions of genesis and characteristics are asked by investigators with different backgrounds, but frequently using different tools, and often for different reasons. The questions, tools and problems explored by investigators in different disciplines may focus on the same rock coating, but the knowledge gained has often been produced and interpreted with completely different perspectives. Archaeologists focus on prehistoric disturbances to coatings and interaction between material culture and subsequent development of natural accretions. Botanists may analyze the activity of organisms that bioaccumulate chemical constituents into rock coatings. Chemists largely document chemical characteristics with a burgeoning array of analytical tools, and they favor physiochemical origins. Conservators of stone antiquities turn to natural rock coatings as analogs for anthropogenic solutions to deteriorating stone monuments. Economic geologists measure trace elements in oxides in rock coatings as a tool in the search for valuable lodes. Geographers tend to stress how coatings vary in different environments. Geologists typically examine mineralogy, or look for relationships between the coating and the underlying rock. Material engineers have been intrigued by natural phenomena similar to what they have manufactured in clean rooms. Microbial ecologists study past or present organisms interacting with coatings, or work with purely organic biofilms. Pedologists consider rock coatings as an extension of cumulic soils. Hydrologists are concerned with the influence of rock coatings on permeability. Planetary scientists look for Martian analogs. Even popular literary authors have gotten into the picture (Twain, 1869). Twain (1869: 55), for example, uses coatings on bone to emphasize antiquity saying that "as much as ten thousand years before...the flood" has passed. Discipline-specific spectacles have advantages and disadvantages. On the positive side multiple paradigms inhibit the development of research prohibitions; for example, research on continental drift was carried out by the meteorologist Alfred Wegener, all while geologists were firm in the knowledge that continents were immobile. Negative aspects revolve around isolation that occurs in different disciplines. Approximately

20

Chapter 2

two-thirds of the publications on any given rock coating do not cite key advances on that same rock coating when those advances are made in a different academic discipline. I prepared a chapter of case studies on how dead-ends were created by a combination of paradigm blinders and discipline-specific literature-blindness. While it can be useful to be reminded that scientists wear intellectual biases on the sleeves of their white lab coats, I decided to delete the chapter. An important purpose of this book is to rally those interested in rock coatings to recognize their multi-disciplinary subject. Perhaps when the field of rock coatings has matured, this type of a retrospective analysis might be helpful. Finger pointing would be counterproductive for now. Rather, I propose the adoption of an established and inclusive paradigm that permits rock-coating researchers to move forward in a cohesive, yet flexible manner.

2.2. Adopting the Paradigm of Landscape Geochemistry Natural scientists operate under paradigms where interpretation of empirical data is driven by the underlying conceptualizations of the scientist (Kuhn, 1970). It is, therefore, appropriate and important that I present two components of my particular perspective on rock coating research: how I assess competing hypotheses; and my view that rock coatings are best interpreted from the paradigm of landscape geochemistry. 2.2.1. Adjudicating Competing Hylx)theses The establishment of "unequivocal proof" only pertains to theoretical endeavors such as logic and mathematics, and is widely recognized as unobtainable in empirical sciences (Copi, 1982). Consequently and adopting the perspective of numerous philosophers of science (Copi, 1982; Farr, 1983; Hempel, 1966; Newton-Smith, 1981; Popper, 1966), scientific hypotheses are best adjudicated by criteria, including: (1) quantity of data explained; (2) diversity of different kinds of data explained; (3) consistency of a hypothesis with established theoretical frameworks and accepted theories; (4) predictive capabilities; (5) relevance; (6) plausibility; and (7) simplicity. 2.2.2. Introduction to Landscape Geochemistry My interpretation of rock coatings is based on "landscape geochemistry" where coatings are a part of a dynamic landscape of chemicals. Rock coatings are deposited where biogeochemical and geochemical processes act as a barrier to the migration of constituent species. Chemical deposits remain in place only so long as they are biogeochemically stable in that environment. Their occurrence or lack thereof is an indicator of the nature of biogeochemical processes operating at a place (Polynov, 1937). This simple interpretation rests within a paradigm born of traditions in the Russian schools of geography, soil science and geochemistry.

2.2.2.1. The Development of Landscape Geochemistry in Soviet Geography Soviet geography, enriched with the tradition of "landscape science" (Isachenko, 1965; Isachenko, 1968; Kalesnik, 1961; Yefremov, 1961), started the field of landscape

Paradigms and Methods

21

geochemistry (Pererman, 1966). The blend between landscapes and geochemistry stems from geochemistry's early ties to landscape science and the influential work of Vemadskiy and Dokuchayev (Pererman, 1966). V.V. Dokuchayev, the founder of modem soil science, recognized the importance of viewing natural systems as the product of landscape interaction of rock type, slope, plants, animals, time and climate. He believed geography should be "landshaftovedenie" (Landscape Science) and should encompass the study of the natural zones and the transformation by people (Pererman, 1966). V.I. Vernadskiy, the founder of Soviet geochemistry, studied under Mendeleev (who first described the periodic table) and accompanied Dokuchayev on soil expeditions. Vernadskiy approached geochemistry from a holistic landscape science perspective (Pererman, 1966; Vinogradov, 1963). L.S. Berg continued to develop and promote the theme of landscape science and A.E. Fursman and A.P. Vinogradov continued to develop holistic approaches to geochemistry (Kalesnik, 1961; Pererman, 1966). B.B. Polynov (Polynov, 1937; Polynov, 1951) worked in a parallel fashion coming from a physiographic perspective to lay the foundation of landscape geochemistry. Polynov argued that element migration is controlled by the interaction of the biosphere with three basic geochemical landscape types. Eluvial landscapes are where the water table is below the surface. Boggy landscapes are where the water table is more or less coincident with the surface, and aqual landscapes are the solid matter is below water as in lakes or rivers. Each of these landscape types contain different conditions that alter elemental mobility. Through the doctrine of landscape science, Soviet geography became concerned with the intel~retation and interaction between the lithosphere, hydrosphere, atmosphere, biosphere, and humans (Glazovskaya, 1968; Glazovskaya, 1973; Isachenko, 1968; Kalesnik, 1961; Syntko et al., 1980; Yefremov, 1961). The weathered portion of the earth's crust became the initial overlap of interest between Polynov, Vinogradov and other geochemists. The synergism between Pererman and Glazovskaya developed landscape geochemistry into a usable paradigm (Glazovskaya, 1967; Glazovskaya, 1971; Pererman, 1961; Pererman, 1966; Pererman, 1967; Pererman, 1980). Formal training in landscape geochemistry started under Glazovskaya in Geography at Moscow University in 1959 (cf. Pererman, 1966, p. 11). Pererman's initial interests were in mineralogy and geology (Pererman, 1961; Pererman, 1966; Pererman, 1967). Glazovskaya's were in soil geography (Glazovskaya, 1967; Glazovskaya, 1968; Glazovskaya, 1971; Glazovskaya, 1973). By the time Pererman's (1966) monograph outlining the field of landscape geochemistry was published, the subdiscipline was firmly grounded in geography and was favored by leading Soviet physical geographers as a way to study the "spatial differentiation" and "geochemical zonality" of landscapes (Glazovskaya, 1973; Syntko et al., 1980). Landscape geochemistry found a logical home in Soviet geography, rather than another discipline, because of physical geography's holistic approach to landscape science. 2.2.2.2. Landscape Geochemistry Outside of Russia~Soviet Union Landscape science has many similarities to the holistic approach of "landscape synthesis" or "landscape ecology" adopted by ecologists and social scientists (Forman and Godron, 1986; Mazur, 1983; Naveh and Lieberman, 1984; Urban et al., 1987). Many ecologists have lamented the lack of theory to explain the spatial heterogeneity of energy, nutrients, water plants and animals in the landscape, in contrast to the

22

Chapter 2

relative homogenous ecosystems like a marsh or an agricultural field, or even a housing development. Landscape ecology focuses on relationships among landscape elements, flows of energy nutrients and species, and ecological dynamics of the mosaic of landscapes over time (Forman and Godron, 1986). It is in response to a "need to understand the development and dynamics of pattern in ecological phenomena (Urban et al., 1987, p. 119)." However, it is ecologically and not spatially based. The interdisciplinary field of biogeochemical cycling has burgeoned into a major paradigm (Likens et al., 1981; Likens et al., 1977) that has been widely adopted, as reflected by its presence in introductory environmental science textbooks (Cunningham and Saigo, 1992; Miller, 1990; Nebel, 1981). Although somewhat similar to landscape geochemistry, the emphasis of biogeochemical cycling and environmental geochemistry is not on landscape, nor is it focused on place. Research questions address flows between different systems, drawn conceptually as arrows between boxes. In contrast, with its roots within geography, the primary focus of landscape geochemistry is understanding the geochemistry of place. No paradigm of geochemistry in the English literature has the same spatial foci as landscape geochemistry, and Fortescue (1980) recognized the need within "environmental geochemistry" to understand the changing spatial patterns of chemicals in landscapes. He introduced the field of landscape geochemistry into the Englishlanguage literature (Fortescue, 1980).

2.2.2.3. Fundaments of Landscape Geochemistry It is not possible, nor appropriate, to detail field of landscape geochemistry. That has been done eloquently elsewhere (Fortescue, 1980; Perel'man, 1966). The purpose of this section is to give the reader an overview, so that rock coatings can be placed adequately within this conceptual framework. Perel'man (1966, p. 13-4) writes: "One can represent the content of landscape geochemistry as a special scientific discipline a s follows: 1. General landscape geochemistry, which deals with the geochemical features characteristic of all or most landscapes. 2. Classification of geochemical landscapes, which deals with geochemical classification of landscapes and clarifies the geochemical features of particular types. 3. Geography of geochemical landscapes: the laws of spatial location of geochemical landscapes, with the principles of zoning and mapping. 4. Historical landscape geochemistry, the geochemical features of landscapes in past geological periods. 5. Geochemistry of individual elements in landscape, which deals with the history of elements in landscapes and explains the laws of their migration via the properties of atoms."

The purpose of presenting Table 2.1 and Table 2.2 is to give the reader an overview of landscape geochemistry. Table 2.1 lists the basic taxonomic units of geochemical landscape classification from Pererman (1966). Table 2.2. is abstracted from the Table of Contents from Perel'man (1966).

Paradigms and Methods

23

Table 2.1. Basic Taxonomic Units of Geochemical Landscape Classification from Perel'man (1966, p. 203).

Number

Name

Criteria for Distinction

I

Series

Form of motion of matter (physical, chemical, biological) related to element migration in the landscape.

II

Group

Biological circulation of air migrants, relation of total mass of living matter to annual production, organism types involved in biological circulation

III

Type

Decomposition rate of remains of organisms

IV

Family

Living matter production within the type

V

Class

Typical elements and ions in water migration

VI

Genus

Rates of water circulation and physical migration

VII

Species

Secondary aspects of migration

i

Table 2.2. Table of Contents of

Landscape Geochemistry (Perel'man, 1966).

Part 1: GENERAL LANDSCAPE GEOCHEMISTRY 1. Geochemical Landscape 2. Element Migration in a Landscape 3. Biological Element Circulation in a Landscape 4. The Biosphere 5. Aqueous Element Migration in a Landscape 6. Element Migration in the Atmosphere 7. General Features of Element Migration in a Landscape 8. Methods of Landscape Geochemistry 9. Geochemical Activity of Man and Geochemistry of an Inhabited Landscape 10. Landscape Geochemistry and Mineral Prospecting 11. Landscape Geochemistry and Health Part 2: GEOCHEMISTRY OF THE MAIN TYPES OF LANDSCAPE AND CLASSWICATION OF GEOCHEMICAL LANDSCAPES 12. Geochemical Landscape Classification 13. The Wooded Landscape A. Humid Tropics Type B. Landscapes with Broad-Leafed Woods C. Taiga Landscapes 14. Steppe and Desert Landscapes A. Steppe Landscapes B. Desert Landscapes 15. The Tundra Landscape Group A. Tundra Landscape B. Upland Bogs 16. Primitive-Desert Landscape Part 3: GEOGRAPHY OF GEOCHEMICAL LANDSCAPES 17. Principles of the Geography of Geochemical Landscapes 18. Maps of Geochemical Landscapes Part 4: HISTORICAL LANDSCAPE GEOCHEMISTRY 19. Major Stages in the Development of Geochemical Landscapes During Geological Time 20. General Trends in the Development of Geochemical Landscapes Part 5: GEOCHEMISTRY OF PARTICULAR ELEMENTS IN LANDSCAPE 21. Geochemistry of Air Migrants in Landscape 22. Geochemistry of Water Migrants Bibliography

24

Chapter 2

Fortescue (1980) advocates the need for the general paradigm of landscape geochemistry, bringing the Soviet~ussian field of landscape geochemistry into the English literature, with the hope that "these basic concepts and principles should lead to a new paradigm for environmental geochemistry." He emphasizes seven key components of the paradigm: "Concept 1: Elemental abundance: as chemicals circulate in landscapes, concerned with numerical abundance, relative abundance, partial abundance, and geochemical speciation Concept 2: Element migration: concerned with relative mobility, absolute rates of migration, biological accumulation, and role of water geochemistry Concept 3: Geochemical flows: corresponding to the biogeochemical cycling Concept 4: Geochemical gradients: at local, regional, or global scales involving both continuous and discontinuous series Concept 5: Geochemical barriers: where a change in local conditions allow the preferential accumulation of elements. The barriers may be mechanical, physicochemical, or biological. It involves geomorphic elements and epigenetic processes Concept 6: Historical geochemistry of landscapes: where components of today's geochemical landscape can be inherited from previous conditions, where former geochemical landscapes can be fossilized and may reflect past periods of supergene enrichment Concept 7: Geochemical landscapes: where the earth's surface can be classified and mapped according to different chemical landscape"

2.2.3. Rock Coatings as a Part of the Geochemical Landscape The very existence of a rock coating directly implies the presence of a geochemical barrier to the flow of chemical constituents. These geochemical barriers are put in place by a physical geography system (Figure 2.1) that is inherently dynamic. i

Climatology Pedol

ology

Biogeography

Geomorphology Geology

Figure 2.1. Different components the physical geography system of a place all influence the development of rock coatings on that rock surface.

Geochemical barriers to the migration of elements are traditionally considered in terms of Eh and pH, that in turn define the mobility of many inorganic components of the terrestrial weathering environment (Figure 2.2). Acidic regimes are usually

25

Paradigms and Methods

associated with mobility of most elements, but aluminum and iron can become stable in particular environments like laterite duricrusts or acid mine drainages. Alkaline processes are associated with arid areas and the precipitation of carbonates, sulfates and chlorides. In extremely alkaline settings, silica is mobilized and reprecipitated. Reducing environments are rarely found on rock surfaces, because these places are constantly aerated, almost by definition. However, the rock coatings that start in the subsurface may be exposed to reducing environments and may, for example, experience the mobilization and reprecipitation of iron and manganese that is common in reducing environments.

.2 -. .

0.8 ~ 0.6 -

~,

0.4

,.~

0.2-0--0.2 --0.4 --0.6 -- 0 98

.

II

0

2

.

l

4

.

.

il

If

6

8

'II

I

'II

10 12 14

pH Figure 2.2. Inorganic processes that are found in terrestrial weathering environments create three core regimes of chemical processes: acidic, alkaline, and reducing environments; the diagram is based on Chesworth (1992, p. 38).

Pererman (1967) defined two general types of geographical expressions of the geochemical barriers that produce rock coatings. One is an areal barrier that may extend over a wide region; these are usually isotropic and isometric in plan view. The second is a linear barrier, reflecting processes that are anisotrophic and develop at chemical discontinuities. The dynamic nature of rock coatings is expressed when the location of a particular type of coating shifts from place to place. Geochemical barriers usually change locations over times scales varying from 10-3 to 105 years. Rock coatings can, however, remain stable over very periods in some geological contexts, such as paleoland surfaces in Arizona and Antarctica (Dorn and Dickinson, 1989; Marchant et al., 1996; Marchant et al., 1993). Fortescue (1980, p. 304) suggested a general template for how to interpret a geochemical "Entity X" in the landscape. Table 2.3 follows Fortescue's template and provides an generalized overview of the landscape geochemistry of rock coatings, which will be used throughout the book, but applied in the third section of the book.

26

Chapter 2

Table 2.3. General approach for interpreting rock coatings as an entity in a geochemical landscape, from the template proposed by Fortescue (1980, p. 304).

Part I. General environmental geochemistry of rock coatings (a) General statement of environmental problems rr to rock coatings" . The formation of rock coatings on exposed rock surfaces are relevant to a large number of scholarly concerns: aesthetics and human development because rock coatings alter the appearance of landscapes; geomorphology, archaeology, and assessment of sites to 'dump' waste because coatings may record time since a surface stabilized; environmental change because coatings may record past fluctuations; geochemical exploration because coatings can indicate abundance of trace elements; remote sensing of surfaces on Earth and perhaps Mars because coatings can influence the character of reflected solar radiation and emitted terrestrial long-wave radiation at several different wavebands. 0a) State of knowledge of rock coatings at the global, regional, and local levels" Rock coatings occur where chemical compounds adhere to rock surfaces. Rock coatings occur globally on or near the subaerial lithosphere-atmosphere interface, but analogs occur in lacustrine or marine settings. Rock coatings are regionally dommant in the drier areas of the globe where erosion exceeds weathering rates, exposing rock surfaces in weathering-limited landscapes. At local levels, at the 1"1 scale, rock coatings occur m all terrestrial weathering environments. (c) State of knowledge of rock coatings in technological, ecological, oedological, and gr time: The rate of formation varies considerably in different geochemical landscapes_ In some places, coatings form in technological time or within a few years to decades. In other settings (e.g., hyper-arid deserts) formation occurs on pedological time scales, where 104 years may be necessary to completely coat rocks. (d) State of knowledge in relation to methoOs and cff0rt 0~ed to detr its presence in relation to human activity: The measurement of rock coatings, both in situ and separated from the host rock, has been accomplished by a variety of techniques. Since rock coating forms on surfaces exposed by human activity, it may be useful as to understand time and environment since the surface was altered. (e) State of knowledge of rock coating with res_t)ect to chcmir S_ocr The matrix of each rock .coating differs. Some coatings may contain complex assemblages over 30 other minor and trace elements m different minerals, while others are dominated by a single mineral, while others by amorphous material. Part II. The landscape geochemistry of rock coatings (a) The abundance of rock coatings in the environment~

Rock coatings occur in all terrestrial weathering environments, but are most abundant in deserts, glaciated and periglacial environments ebecause of the ubiquitous occurrence of bare rock surfaces. (b) The migration rate of rock coatings in the environment: Coatings migrate mechanically, when the host rock is transported by geomorphic.processes. The rate of mechanical migration is usually at pedological time scales. More commonly, me various elements within coating migrate chemically at rates varying between technological to pedological time scales, depending upon the redox chemistry and biogeochemical environment. (c) The role of rock coatings in landscape flows: Rock coatings may play key roles in regulating geochemical cycles, such slowing rates of carbon dioxide uptake by weathering of silicate minerals. (d) Rock coatings and geochemical gradients' The distribution of different rock coatings record redox chemistry gradients. For some rock coatings, trace elements can vary spatially. For example, the lead concentration in the uppermost layers of rock coatings will decrease with distance from a road, until a general background level is reached. (e) Rock coatings and geochemical barriers; Rock coatings form at geochemical barriers for the migration of the constituents. (t3 Rock coatings and the historical development of landscapes: The pedological time dimension of coating formation makes it useful to understand the history of geochemical landscapes. When a coating is preserved on a buried paleo-surface, its propeaies can indicate the nature of geochemical landscape. i'g~ Rock coatings and the geochemical classification of landsca_ocS: Rock coatings form on surfaces stable with respect to mechanical geomorphic and anthropogenic processes. Therefore, coatings can assist in the classification of landforms and archaeological/historical landscapes.

Paradigms and Methods

27

My reasons for adopting the paradigm of landscape geochemistry (Perel'man, 1966; Fortescue, 1980) reflect the aforementioned criteria for assessing competing perspectives. No other way of looking at rock coatings: explains the quantity or the diversity of data; is consistent with established biogeochemical theory; is a predictive tool; is as relevant to a variety of disciplines; and provides a simple way to understand rock coatings.

2.3. Methods Used in Original Data Gathering Much of this book involves the presentation of original data, gathered through a combination of field sampling and analytical work. Because methods tend to bias the perspectives of the researcher (Hempel, 1966; Salmon, 1982), this chapter is an appropriate place to discuss the analytical tools used to acquire original data. 2.3.1. Field Collection The vast majority of published research on rock coatings has not focused on field characteristics. Instead, most papers concentrate on results from laboratory studies on a very small number of samples. Thus, there is a huge subjective bias in the literature born from the reality that almost all rock coatings are spatially heterogeneous on scales from microns to kilometers. Vastly different results are, therefore obtainable, even a centimeters apart. Researchers rarely go into explicit detail on criteria used to collect the samples that are eventually analyzed. Typical examples wording found in papers are "...a sample of black crust was analyzed with X-ray fluorescence..." or "...fragments of the rock and the patina were placed in an electron microscope..." Sometimes, subjectivity is made slightly more objective through the use of multiple researchers to collect and sort through samples. Only rarely do researchers provide explicit criteria by which they collect material (e.g., Table 2.4). This is not meant as a criticism. On the contrary, rigid adherence to a specific methodology may be completely inappropriate to the nature of the investigation. This is because most of the research conducted on rock coatings has been exploratory. If a researcher said that she used 'intuition' in field sampling, the paper would be rejected by referees. Thus, a minimum of information is usually provided on sampling criteria. Few rock coating researchers are experimentalists, trying to follow a rigid procedure. Most are scientific detectives attempting to understand the nature and origin of a poorly studied earth-surface phenomena. It is only when an area of study moves beyond the exploratory that an attempt is made at replicable sampling criteria (cf. Table 2.4). The lack of specificity in field collection criteria in the vast majority of papers reflects the craft involved in rock-coating research. My sampling criteria are largely intuitive in the cases where I was trying to select "representative samples" of a rock coating. The exception is chapter 10, where sampling criteria in Table 2.4 were used to select subaerial rock varnishes.

28

Chapter 2

Table 2.4. Sampling criteria used in the collection of samples used to analyze microlaminations found within rock varnish (Liu, 1994).

Sampling Criteria

Note

1. Varnish should be collected on stable cobbles that have a "rounded, abraded" appearance.

This is a subjective judgment, but a critical one. Cobbles or boulders that are not rounded have a high likelihood of being from gravel shattering (Amit et al., 1993), hence the varnish on these cobbles may not represent the last geomorphic event that disturbed a geomorphic surface.

2.V amish should be collected on tops of cobbles.

Vamish on tops of cobbles may have the most complete layering patterns.

3. Ground-line band varnish should be avoided.

A ground-line band is created at the soil-varnish-atmosphere interface (Engel and Sharp, 1958), creating a different environment for deposition of varnish layering. As the cobble changes position with respect to the accumulation loess in a desert pavement (Mabbutt, 1979), previous ground-line bands can be placed a few millimeters to centimeters above the soil surface.

4. The best "looking" varnish should be avoided.

The best "looking" vamish is not usually the oldest. Ground-line band varnishes, varnishes collected within a few centimeters of a soil surface always have the best "looking", and varnishes that started in rock fractures look the darkest.

5. All rock coatings that are not subaerial manganiferous varnishes should be avoided.

Only subaerial manganiferous varnishes change their layering patterns with respect to climatic changes. Vamishes formed in rock crevices and too close to the ground surface are influenced by local microenvironment.

6.Varnish should be sampled from positions where varnish first starts to grow.

Ongoing studies of historical rock engravings and faced stones reveal that colonization occurs first in specific types of places on different lithologies. For example, varnish first colonizes vesicles in basalt, impurities in chert, fractures in quartz and silicified dolomite, and grain boundaries in granitic rock and sandstone.

7.Vamish at locales of water collection on cobbles should be avoided

Depressions on rock surfaces can display "bathtub" rings analogous to ground-line bands. These are places where water is retained after precipitation events. These basins of water collection are avoided because they destroy varnish layering patterns due to enhanced leaching effect of longer water contact.

8. Lichens and microcolonial fungi on varnish should be avoided.

Lichen and microcolonial fungi erode hollows in vamish, or erode the surface varnish away by producing acidic materials. This erosion disturbs and, in some case, destroys vamish layering patterns.

My samples were procured in different ways. Field samples for biological study with the SEM were chipped with a rock hammer into small sizes suitable for placement on a SEM stub and placed directly in sterilized test tubes to avoid biological contamination. Some of the biological samples were also prepared by the critical point drying method, which removes the interface between liquid and vapor (source of surface tension forces) before the drying process is begun. Split samples that were and were not subject to critical point drying (Cohen et al., 1968) yielded similar results in terms of appearance. I suspect that this is because the samples have already been partially dehydrated at the naturally hot air-drying temperatures found at rock surfaces. Fast-curing epoxy was applied before sample removal if the rock coating was delicate. For most samples, however, chisels and rock hammers were used to collect fist-sized fragments from the host rock.

Paradigms and Methods

29

2.3.2. Preparation of Polished Cross-sections and Ultrathin Sections Backscattered electron microscopy (BSE) is the technique used to image most of the rock coatings in this book. The proper use of BSE and quantitative chemical analyses by wavelength dispersive (WDS) speclrometry requires a polished surface fReed, 1993). A sample is reduced in size using a hammer or low-speed diamond saw. The rock chip is mounted in epoxy in a greased aluminum mold about the size of a standard microscopy cross section (25 mm in diameter and 15 mm deep), cured at room temperature for at least 48 hours and then removed from the mold. This epoxied rock chip is trimmed down with abrasive papers of 60-grit to the depth at which a specific coating on the rock chip is almost, but not exposed. Then the surface is then polished with abrasive papers of 400- to 600- and 1200-grit. After ultrasonic cleaning, the surface is polished on a microcloth with 5l,tm, ltxm, and 0.3l.tm-sized AI20 3 powder. Ultrathin sections of some rock coatings were made for viewing by light microscopy. Two procedures were used. Originally, a varnished rock chip was polished down from 60-grit to 400-grit. Then, this surface was glued to a glass slide, as in a conventional geological thin section. However, conventional geological thin sections are too thick to "see through" some rock coatings. Therefore, the sample was carefully ground down until a thickness of <101.tm is reached. Liu (1994) developed a new method of making ultrathin varnish sections. Surface 1 is polished until the spot of interest is exposed. The edge of surface 1 is marked to serve as a visible reference. This polished and marked cross section is then covered with epoxy. After curing, the re-epoxied varnish cross-section is polished from its bottom surface until the rock coating is thin enough to see through.

2.3.3. Secondary Electron Microscopy The topography of a rock coming is imaged well with secondary electrons (SE) with a scanning electron microscope (SEM). This section provides only a brief description of SE images obtained by SEM. A more complete discussion of theory and description of scanning electron microscopy is presented elsewhere (Bohor and Hughes, 1971; Goldstein et al., 1981; Krinsley and Doornkamp, 1973; Reimer, 1986; Thornton, 1968). In a SEM, a tungsten filament is heated so that it emits electrons. The image, called a micrograph, is produced when a collector-scintillator-photomultiplier system captures and processes the electrons that leave the surface of the sample. The scintillator emits light in proportion to the number of electrons striking it (per unit time). The number of electrons escaping the sample is related to the topography of the sample. Fewer secondary electrons escape depressions and shadowed regions, so they appear dark. Contrast derives from differences in emitted secondary electrons. The resolution of SE in a SEM in theory is less than 10 nanometers. Samples prepared for topographic study with SE were stored for transport in cushioned bags, and then chipped down so the sample could fit on a SEM specimen stage. Samples were not scrubbed clean (as others have done), because mechanical cleaning tends to destroy environmental relations found on the varnish surface. Instead, unattached surficial debris was gently blown off with 'canned air' commercially available at photographic retail stores. Laboratory biological samples were fixed in 1% glutaraldehyde for 30 minutes, washed 3 times in 0.1M cacodylate buffer (pH 6.7), and progressively dehydrated at

30

Chapter 2

30%-50%-70%-90% at 10 minutes each and 100% EtOH (ethanol) for 2 changes (20 minutes each). In making the changes, a thin film of EtOH was left to prevent desiccation. These laboratory biological samples were then prepared for SEM viewing using a critical point drying method (Cohen et al., 1968). In order to avoid charging non-conducting samples, samples to be images with SE were usually coated with either a -200 angstrom-thick layer of gold-paladium Goldpaladium was found to have a superior resolution, especially for biological material, and charging is less of a problem. However, gold-paladium poses complications for energy dispersive X-ray analyses. Several different types of scanning electron microscopes were used to obtain secondary imagery: an Hitachi S-500, a Cambridge Stereoscan $600, an ISI, and a JEOL JXZ-8600 electron microprobe. The accelerating voltage used varied between 5 and 30 KeV, most typically 15 KeV. Scale is presented for all of the electron microscope images presented here. In most cases, the scale bar is imprinted on the image. In a few cases, the scale of the image is presented in the caption through a description of the height or width of the image. 2.3.4. Backscattered Electron Microscopy Backscatter electron microscopy (BSE) involves the formation of images by backscattered electrons, or electrons that suffer collisions which result in their reemergence from the surface. If the surface of the sample is polished, sample composition can be determined since the backscattered-electron yield is a function of the average atomic number (Z) of the sample (Reed, 1993). The resolution of BSE is an order of magnitude less than SE. The various shades of gray in the image represent varying elemental compositions within the sample, with lower Z regions appearing darker and higher Z regions appearing brighter. The resolution of BSE imagery is lower than SE micrographs, because backscattered electrons are generated from a greater depth. BSE image is sensitive to surface topography, because the detector perceives the specimen from only one direction. Hence, samples must be polished. The quality of the BSE image is dependent upon the quality of the polish. A thin (--200 angstrom) carbon coating will also improve image quality, while not interfering with the image. 2.3.5. X-Ray Spectrometry The electrons that bombard the sample generate X-rays. The elements present in a sample can be identified by either the energy or wavelength, and the concentrations can be measured by the intensity of the lines in the X-ray spectrum. One of the key advantages of X-ray spectrometry is the ability to analyze samples in situ and to analyze extremely small volumes; the diameter of the electron beam is an important control on the area analyzed, but the volume analyzed can be larger than just the size of the spot (Williams, 1987). 2.3.5.1. X-Ray Images X-ray 'dot maps' were made with pulses derived from a spectrometer set to detect the characteristic X-rays of that particular element. Each dot, theoretically, corresponds to

Paradigms and Methods

31

one X-ray photon. This is a qualitative image, where the density of dots is roughly proportional to the concentration of the element. However, dots appear in the image even where the element is absent, because of the 'background' of the continuous X-ray spectrum. X-ray dot maps are not good for distinguishing low concentrations from background (Reed, 1993).

2.3.5.2. Energy Dispersive Spectrometry Energy-dispersive spectrometers (EDS) were used to obtain qualitative and semiquantitative data on sample composition. Since all elements of the X-ray spectrum are collected simultaneously, the technique provides relatively rapid information about the composition of the sample (generally from Na to higher atomic numbers). EDS detectors were most useful in obtaining data on samples with a rough surface (unpolished), because wavelength dispersive spectrometry (see next section) requires a flat surface. EDS units attached to a SEM were not used to obtain quantitative data for reasons that relate to the comparative advantages of wavelength dispersive spectrometers.

2.3.5.3. Wavelength Dispersive Spectrometry Wavelength dispersive (WDS) spectroscopy discriminates the characteristic X-rays given off by the sample by their differing wavelengths. Quantitative measurements were made with WDS using ZAF corrections and a JEOL JXZ-8600 superprobe. Beam size, current, and counting times varied with the purpose and the desired limit of detection. This technique offers two valuable advantages in quantitative analysis with an electron microprobe over the more commonly used EDS technique (Reed, 1993; Williams, 1987). The first of these is the lower limit of detectability; with WDS this is frequently below 100 parts per million for certain elements, which is much lower than the level that can be reliably detected with EDS. Second, the WDS spectral resolution is literally an order of magnitude greater than that of EDS (14eV vs. 142 eV for Mn Ktx FWHM). Peak overlaps that are extremely problematic to EDS are easily revolved by WDS, for example the overlap between Mn-KI3 and Fe-Ktx.

2.3.5.4. Detection of Organic Matter The combination of SE and BSE can be used to determine the presence of organic matter (Watts, 1985), and confirmed with WDS. The contrast between backscattered electrons (BSE) and secondary electrons (SE) is used to determine the stratigraphic position of organic material in cross-sections. In BSE images, for example, rock varnishes typically appear brighter than an underlying rock rich in silica or organic matter, because Mn (Z=25) and Fe (Z=26) have a higher atomic number than Si (Z=14) or C (Z=6). In contrast, SE behave much in the same manner as light rays, and contain information pertinent to the topography of the sample surface. A secondary electron image is quite similar to an optical image, and exhibits both high spatial resolution and considerable depth-of-field. Organic matter is present in a topographic image created by secondary electrons SE, but the C-H-N in organic matter have a low atomic number (carbon, Z=6) and hence

32

Chapter 2

appear dark in BSE images. The relative abundance of carbon is also examined with WDS, to test whether the dark part of the BSE image is truly carbon. The 200 angstrom-thick coating with carbon provides a general 'background signal', but in places where carbon is present in the sample, the WDS signal is much stronger. So WDS was used as a qualitative tool to confirm the presence and abundance of carbon. 2.3.6. Admission of Bias My own perceptions of rock coatings rely heavily upon the use of electron microscopy and X-ray spectrometry. This creates a problem in jumping scales because of the potential for falsely assuming that a sample is representative. When I collect a sample of heavy metal skins under a steal beam of a rusting bridge, for example, valid conclusions can only be drawn for that one small sample. The last section of this book, however, is a search for general principles in order to understand the geography of rock coatings. Since the vast majority of data on rock coatings are based on microscopic studies similar to my own, there is an inherent problem in using microscopic samples to understanding field relations (Pope, 1994). Thus, I have two clear biases. I operate under the paradigm of landscape geochemistry and I rely heavily on results from microscopic t~hniques. The reader is forewarned and is welcome to reinterpret these findings with a different set of spectacles.

33

Section 2.

Different Rock Coatings

Supposing we study the phenomenon of error in itself; it becomes apparent that everyone makes mistakes. This is one of life's realities, and to admit it is already to have taken a great step forward. If we are to tread the narrow path of troth and keep our hold on reality, we have to agree that all of us can err, otherwise, we should all be perfect. So, it is well to cultivate a friendly feeling toward error, to treat it as a companion inseparable from our lives, as something having a purpose, which it truly has . . . . Whichever way we look, a certain "Mr. Error" is always present! If we seek perfection, we must pay attention to our own defects, for it is only by correcting these that we can improve ourselves. (Montessori, 1967, p. 246-247)

Section 2 presents, for the first time, the full variety of rock coatings found at the earth's surface. The purpose of this section is to organize knowledge about the characteristics and origins of different of rock coatings. This empirical knowledge is essential to the development of a more general understanding of the behavior of rock coatings explored in Section 3 of this book. The first three chapters in this section have little original data. Anthropogenic pigments, lithobiontic coatings and carbonate crusts all have extensive literatures. In my review of these rock coatings, I instead try to relate these different topics to rock coatings. For example, most of the literature on lichens takes for granted their presence on rock surfaces. Yet, the growth of lichens as a lithobiontic rock coating frequently inhibits the development of other rock coatings. Thus, my focus in including these three chapters rests in a promoting a change in perspective - - to view these materials as rock coatings at the interface between the atmosphere and lithosphere. The last eight chapters in this section present previously unpublished, original data to supplement the scientific literature. The need for more original data derives from the relative paucity of information on many types of rock coatings such as dust films, heavy metal skins and silica glaze. The general format for these chapters is to detail the environments where these different coatings occur, present the physical and chemical characteristics of these coatings, and lastly to discuss possible modes of coating genesis. With the exception of anthropogenic pigments, there is no consensus on why any given rock coating forms. Biologically-based researchers tend to favor bioaccumulation. Individuals with chemically- and geologically-oriented backgrounds typically favor geochemical explanations. The consequence of the lack of a synthesis has been an everexpanding set of hypotheses to explain the characteristics and genesis of different rock coatings. The chapters in this section are placed, for the most part, in alphabetical order. This organizational tool is employed to de-emphasize the importance of one rock coating over another. In a few cases, however, I have combined different coatings into a single chapter. For example, the wide variety of rock coatings in chapter 3 have in common that they are directly applied by humans, whereas the coatings that are grouped together in chapter 11 are all relatively uncommon, in part because of their greater solubility in water.

34

Chapter 3

ANTHROPOGENIC

PIGMENTS

Not to know what happened before we were bom is to remain perpetually a child. For what is the worth of human life unless it is woven into the life of our ancestors by the records of history? Cicero

3.1. Introduction

The focus of this chapter rests on coatings that are applied to rock surfaces by humans. This chapter is unique in this book in that the origin of anthropogenic pigments is not in question. In the case of historic or modem anthropogenic coatings, the materials are often known and the processes of application are detailed. Instead of thinking about anthropogenic pigments as entities unto themselves, for example urban graffiti or archaeological pictographs, they all represent one end of a continuum of human-nature interaction. The other end of the continuum would be rock coatings that are natural, such as the silica glazes found in Antarctica (see chapter 13). There are also rock coatings whose characteristics reflect a combination of natural and anthropogenic forcings. For example, some heavy metal skins (see chapter 8) are formed by natural processes, but with substantial inputs from human activity. I do an injustice to every type of anthropogenic rock coating in this chapter. Whether your concern is the study rock paintings in South Africa or a sociological analysis of urban graffiti, you will not find your particular focus covered. Each of these topics rightly deserves a monograph. My desire is to be inclusive of all the different types of materials applied by humans to rock surfaces. Thus, by dividing the discussion into prehistoric and historic rock coatings, I purposefully juxtapose topics that have vastly different literatures and thematic concerns. The purpose of this chapter is to promote a change in perspective. 3.2. Prehistoric Rock Pigments

The literature on the composition of rock paintings has exploded in the last decade. This burgeoning research has brought a realization that it is possible to assess local trends in the composition of paint 'recipes' (Clottes, 1993), but also that there are tremendous complexities in the raw ingredients of prehistoric paintings. Most of the literature on prehistoric pictographs is based on chemical and physical characterization. X-ray diffraction is used to determine mineralogy (McKee and Thomas, 1973) and X-ray techniques determine elemental chemistry (Clottes et al., 1990; Whitley and Dorn, 1984). Infrared spectroscopy and organic analyses have also been attempted (Reese et al., 1996).

Anthropogenic Pigments

35

Studies from around the worm have shown considerable variability in minerals that produce color variations in rock paintings (Clarke, 1976; Cole and Watchman, 1993; Cook et al., 1990; Ford et al., 1994; Koski et al., 1973; Mawk et al., 1996; McKee and Thomas, 1973; Scott and Hyder, 1993; Taylor et al., 1974; Zolensky, 1982). Reds, oranges and yellows in paint come from a several types of iron oxides and hydroxides (hematite, goethite, ferrihydrate, magnetite) and even lead oxides. Brown paints usually derive from manganese hydroxides or the iron hydroxide goethite. White can be produced from clays like kaolinite, from carbonate minerals (typically calcite, but also minerals like huntite), muscovite, gypsum, and quartz. Blue and green paint derive from copper hydroxides and carbonates. Black and gray paints may be produced by magnetite, manganese oxides and hydroxides, and charcoal (Farrell and Burton, 1992; McDonald et al., 1990; Valladas et al., 1992; van der Merwe et al., 1987). One theme in the literature is that the colors seen today may not be those applied when the artist worked. In the Australian outback, for example, hematite is more stable than goethite as a red pigment where organic matter is lacking. Goethite, however can be stabilized by the presence of organic matter (Cook and Davidson, 1993). Thus, environmental processes alter pigment mineralogy and appearance. Another complexity in understanding the appearance of rock pigments involves natural rock coatings that often interdigitate with anthropogenic pigments. Calcium oxalates (Russ et al., 1996), silica glaze (Watchman, 1992), and phosphate skins (MacLeod et al., 1995) interdigitate with pictographs. Figure 3.1 illustrates the interdigitation of natural and anthropogenic rock coatings. First, a petroglyph was made. Second, iron-oxide pigment was placed in the petroglyph groove. Third, naturally accreting silica glaze has been mixed with the underlying paint pigment. Lastly, a new layer of silica glaze has been deposited naturally on the surface. The physical and chemical analysis of prehistoric rock paintings is most detailed for French paintings. Not surprisingly, mineralogical and electron microscope-based chemical analyses reveal a complexity of ingredients to the point where particular paint recipes have been discerned (Clottes, 1993; Clottes et al., 1990; Labeau, 1990). For example, the paintings in the caves of Niaux were completed, for the most part, with hematite for the reds and manganese and charcoal for the blacks. A variety of other recipes involve additions and subtractions of ingredients like orthoclase, biotite, and talc (Clottes, 1993). One of the most exciting arenas of research on prehistoric rock paintings is one of the most difficult: organic analyses with such tools as gas chromatography, liquid chromatography, infrared spectroscopy, immunoelectrophoresis, and even DNA analysis. The most detailed review paper on the composition on prehistoric rock paintings concludes that "organic analyses are much rarer and none have been independently confirmed as of this writing; thus the organic analyses must be accepted with caution (Rowe, 1997)." A few examples of exciting, albeit experimental, organic results follow. Three to four millennia-old DNA in west Texas pictographs derived from an ungulate mammal (Reese et al., 1996). Animal urine (Biesele, 1974) and Tassili milk (Lajoux, 1963) were sometimes added in Africa Bees-wax has been used in some places in Australia (Nelson et al., 1995). Paints in the Laura region of Queensland, Australia, contain fibers from orchids, kapok and unidentified plants occur in abundance; they may be binders or remnants of brushes used to apply the art (Cole and Watchman, 1992). Twenty-thousand-year old human blood was found in Laurie Creek art (Loy, 1993; Loy, 1994; Loy et al., 1990). Blood also occurs Chumash Indian pigments in coastal California (Scott et al., 1996).

36

Chapter 3

Figure 3.1. Backscattered electron microscope image of a portion of a snake petroglyph from the Cima Volcanic Field (Whitley and Dom, 1987). The underlying basaltic rock is coated by a very bright layer that is almost ]pure iron oxide. On top of the iron oxide is a mixture of silica glaze and the iron pigment. The surface layer Is just silica glaze.

3.3. Historic Rock Painting 3.3.1. Coatings to Change Appearances Rock painting continues today. Rather than explore compositional variations, that are often already established (Turner, 1988), research efforts have focused on anthropological, art and psychological insights. One of the advantages of the study of historic rock paintings is immobility. Art and writing applied to stone is a unique form of art communication, because the stone rarely moves. It yields information about place-specific local conditions (Reisner, 1971). In Bulgaria, for example, painting (as well as inscriptions) show possession, art, and indicate that perhaps seventy percent of the Bulgarian population in the seventh through the seventeen centuries were literate (Dimitrov, 1996). In another example twelfth through fourteenth century graffiti in Norwegian churches yield information on the religious culture (Blindheim, 1985). Contemporary society has not altered a fundamental desire to coat stone surfaces. Rock paintings occur in cultures as disparate as urban dwellers (Bushnell, 1990; Chalfant and Prigoff, 1987) and Tibetan Buddhists (Figure 3.2). Ironically an destructively, those who study ancient rock art often leave their own anthropogenic residues (Figure 3.3).

Anthropogenic Pigments

37

The greatest concentration of research on anthropogenic pigments rests in the understanding and removal of gang-related graffiti (Clifton, 1983; Weaver, 1995). Most of this literature is local in nature and focuses on community efforts to counteract gang activities. In the past, it would be logistically impossible to access and compile this 'gray literature'. With the Internet, however, access to this material only requires the use of search engines and with the terms 'graffiti' + 'gangs'.

Figure 3.2. Buddhist monastery in Tibet with paint applied to spheroidally weathered granitic boulders.

Figure 3.3. Chalk is often applied to rock engravings to make photography easier. This backscattered electron microscope image snows a layer of chalk that accreted onto a petroglyph at Legend Rock, Wyoming. The host sandstone has a gray appearance. The natural coating of rock varnish has a bright appearance. Then, chalk was applied to the petroglyph. The chalking process abraded some of the rock varnish, then some of the chalk recemented to the surface of the petroglyph in a 5-micron-thick layer.

38

Chapter 3

3.3.2. Coatings to Preserve Stone The purposeful conservation of rock surfaces sometimes involves the application of coatings. The issue of whether these compounds are good or bad for preservation is beyond the scope of this chapter; it is a complex issue involving the status of weathering of the host material, hydraulics of water flow in the host material, and shear stresses. Rather, this section is a brief introduction to the wide variety of different types of material that have been applied as coatings to conserve rock surfaces. Barium hydroxide is sometimes applied to limestone to promote the precipitation of barium carbonate (Lewin and Baer, 1974), since barium carbonate is less soluble than calcium carbonate. In addition, when barium reacts with sulfur pollution, barite crystals may precipitate in place of the more destructive gypsum. Biocides have a long history of application, with the intention to kill organic agents of rock destruction (Caneva and Salvadiori, 1989; Childers, 1994). The removal of a lithobiontic community is a complex issue, because these organisms are sometimes important agents of rock cohesion (Viles, 1995). Lithobiontic coatings are covered in greater detail in chapter 4. Calcium carbonate added to limestone may help in its consolidation. The process typically starts with the application of calcium hydroxide. Reaction with carbonic acid and carbon dioxide results in a diagenesis to calcium carbonate (Ashurst, 1990), usually in the outer few millimeters of the host rock (Price et al., 1988). Water repellents are often added to stone surfaces (Karsa and Davies, 1995). Alkoxysilanes are an example; generally, they are first applied to stone surfaces. Then, reaction with moisture leads to the condensation of a silicone polymer that repels water and made add resistance against shear stresses (Wheeler, 1992). Some current efforts appear to be aimed at tailoring fluoropolymers to rock type in .order to enhance the water-repellent nature of a coating (Guidetti et al., 1992). Colloidal silica has also been used as a water repellent (Kozlowski et al., 1992). A wide variety of other materials have been applied as conservation agents including natural and manufactured organic polymers, acrylics, cyanates, polyurathanes, phosphonates (Auras, 1993; Black et al., 1991; Coffman et al., 1991; Littmann et al., 1993; Sinner, 1991). Emulsions are sometimes applied (Piacenti et al., 1993), as are epoxies (Selwitz, 1992). The aforementioned additives only pertain to efforts at stone conservation in the scholarly literature. Adobe, block, brick, concrete, concrete, flagstone, mortar, native stone, rock walls, stucco, tile, and other materials are widely used in building construction. The general public has a wide variety of commercial products to coat these surfaces from weathering and erosion. It is difficult, however, to estimate extent to which commercial products have been applied to natural and human-made stone surfaces. A study of the sales volume of these commercial products, however, might provide a rough maximum estimate of their usage. 3.3.3. Coatings to Preserve Landscape Aesthetics Human destruction of natural stone surfaces removes the pre-existing rock coating. This leaves an aesthetic scar on the landscape (Figure 1.4 and Figure 1.6). "Artificial varnish" was suggested and tested as a means to reconstructing the natural appearance of surfaces in arid and semi-arid areas (Figure 3.4) (Elvidge and Moore, 1980; Henniger, 1995).

Anthropogenic Pigments

39

The coloring agents of iron and manganese are added together in the mobile divalent state. The relative abundances of manganese and iron are mixed in a ratio to match the color of the surrounding natural rock varnish (Elvidge and Moore, 1980). Then, the solution is oxidized through a reaction with sodium hydroxide. The resultant precipitate is fixed on the rock surface. With permission from the National Park Service in Death Valley, artificial varnish from Skyline Drive (Figure 3.4) was analyzed for its stability over a ten-year period after its application. Over time, there has been very little change in its texture as seen with electron microscopy (Figure 3.5), although its appearance does not resemble natural rock varnish (see chapter 10). In addition, relatively minor chemical changes occurred (Table 3.1).

Figure 3.4. Photographs of artificial "desert varnish" applied to rock surfaces. The upper left is from Petrified Forest National Park, where artificial varnish was applied to mask graffiti (Elvidge and Moore, 1980). The upper fight is of Death Valley at Skyline Drive, where samples were collected to assess changes in artificial varnish (Elvidge and Moore, 1980) over a 10-year period (Figure 3.5 and Table 3.1). The lower two are from the Phoenix metropolitan area in new housing developments; the paint disguises scars associated with road cuts and house-pads. Ironically, the artificial paint in the lower left inverts what is seen naturally; desert streams lack varnish whereas the suburban drainage has been artifically coated,

Table 3.1 Wavelength dispersive electron microprobe analyses of 'artificial varnish' applied at Death Valley National Monument, Skyline Drive. Measurements were on fused beads of ten milligrams of scraped material, and hence represent 'bulk' chemical analyses. ~

Year

Na20 MgO i

1983 1985 1987 1989 1993

A120 3 SiO2 i

0.26 0.13 0.07 0.06 0.06

0.61 0.61 0.57 0.59 0.54

P205 SO3

K20

CaO

TiO2

MnO

Fe20 3 BaO

Total

41.45 32.84 35.14 35.90 35.28

19.28 0.02 20.88 0.02 19.80 0.07 21.44 0.02 20.62 0.02

78.52 74.68 73.63 76.41 75.83

i

2.53 4.80 4.30 4.09 4.16

8.90 9.12 6.11 6.95 7.89

0.07 0.09 0.26 0.13 0.11

4.67 '0.33 0.25 0.16 5.00 0 . 6 3 0 . 3 5 0 . 2 1 6.22 0.39 0 . 5 1 0 . 1 9 6.04 0.46 0 . 5 3 0 . 1 9 5.90 0.54 0 . 4 8 0 . 2 1

40

Chapter 3

Figure 3.5. Backscattered electron microscope image of artificial vamish that was applied to Skyline Drive in Death Valley National Monument (Elvidge and Moore, 1980). The bright material on the top is artificial varnish, while the gray mineral underneath is plagioclase feldspar. This sample was collected in 1993. It looks similar to samples collected in 1983, 1985, 1987, and 1989.

When the artificial varnish was placed in the field, there was a relatively rapid reduction in manganese. Then, concentrations settled down. Presumably, the quick loss of about 15% of the manganese was from leaching of unadsorbed manganese. Concentrations stabilized, however, after four years. Sodium and magnesium appear to be slowly leached, while potassium and calcium slowly builds in concentration. Iron, aluminum, silica, and sulfur remain relatively stable in their abundance. In summary, after ten years of study, the artificial varnish in Death Valley appears to be chemically and texturally stable.

41

Chapter 4

LITHOBIONTIC

COATINGS

"Without the multitude of living reactions as channels for energy and electrons and without the enormous capability of life to transfer with and against chemical gradients, there would be practically no or at least a very reduced geochemical rock cycle on this planet." (Kmmbein and Dyer, 1985, p. 157)

4.1. Introduction

A variety of terms have been applied to groups of organisms that inhabit rock surfaces (Hirsch et al., 1995). Geofungi (Noell, 1973), microbial films (Mack et al., 1975), organic coating (Jones, 1994, p. 435), and attached microbial growths (Characklis, 1973) are only a few of the phrases that have burgeoned in the literature. Consider the term biofilm, which has become popular and is used in a wide variety of ways. In medical microbiology, biofilms are clumps or aggregates of bacteria and other microorganisms that can infect medical equipment (Potera, 1996). Bacterial biofilms interfere with civil water works (Lazarova and Manem, 1995; McCoy et al., 1981). In the realm of rock coatings, biofilm has been used to describe mucus substances surrounding bacteria (Folk, 1993; Westall and Rince, 1994), epilithic microbial communities in streams (Konhauser et al., 1994), agents of stone monument weathering and erosion (Krumbein and Urzi, 1993; Morton, 1996; Seaward, 1988), and keys to mineral precipitation (Fyfe et al., 1989; Pedley, 1992). I adopt consistent classification of life forms that grow on rock surfaces (Golubic et al., 1981). This avoids the terminological confusion associated with single terms applied to multiple topics. Lithobiontic (Golubic et al., 1981) coatings are divided into different niches, depending upon their position (Table 4.1). In harsh dry or cold environments, for example, lithobionts may be driven underneath the surface (Figure 4.1). The most noticeable lithobionts are epilithic organisms that live entirely on the surfaces of a rock in less hostile settings (Figure 4.2). There are, of course, organisms that occupy different positions at the same time. An angiosperm plant, for example, would be mostly epilithic but could have euendolithic and chasmoendolithic roots.

42

Chapter 4

Table 4.1. Classification of lithobiontic organisms (Golubic et al., 1981). Term Epiliths

Niche Organisms living entirely on the surface

Euendoliths

Organisms actively boring into the rock

Chasmoliths

Organisms living in fractures

Cryptoendoliths

Organisms living in pore spaces within the rock

Hypolithic

Organisms living beneath stones

Combination

Organisms occupyin~ different niches at once

Figure 4.1. Zones of cryptoendolithic lichens growing within Antarctic sandstone. The diagram is modified from Friedmann (1982, p. 1047).

Figure 4.2. The darkening in the upper part of the limestone at Absolum's Tomb, Israel, is due to the growth of black fungi and other epilithic organisms in the upper part of the image. The lower, lighter colored rock, was buried prior to exposure and hence protected from epilithic organisms.

Lithobiontic Coatings

43

I also adopt a thickness-based classification for lithobiontic coatings proposed by Viles (1994) (Table 4.2). This classification has the advantage of incorporating most of the previous uses of the term biofilm including organisms, mucal slime, and associated mineral material that are measured in microns, that is to say less than a millimeter in thickness. Viles' (1995) classification also prevents future terminological confusion as thicker organically-based or organically-derived coatings are viewed holisticaUy as a part of lithobiontic phenomena. Table 4.2. Classification of lithobiontic coatings by varying thickness (Viles, 1995). i

iii ill

Term Biofilms Biorinds Biocrust

llll Hllll

llllllll

Descriptio n . . . . . . . . . . . . . . . . . Accumulationsof microorganismsless than ~lmm thick Associationsof organisms 1-5 mm thick Association of organismsgreaterthan 5 mm i

iii

ii

i

ii

Because it is largely an analysis of the literature, this chapter is organized differently than most of the others. The variety of lithobiontic coatings are reviewed section 4.2. Factors controlling distribution are summarized in section 4.3, and interfaces with other rock coatings are outlined in section 4.4.

4.2. Different Types of Lithobiontic Coatings Before I explore the different types of organisms that are lithobionfic coatings, I offer some warnings. First, different types of lithobiontic coatings often exist together. They are separated here to illustrate the diversity of rock-surface organisms. Second, each section could occupy a separate volume in a book series on lithobiontic coatings. Since I cannot be comprehensive, I have chosen to exemplify themes that are most relevant to the development of a general theory on the geography of rock coatings. 4.2.1. Bacteria Bacteria are single-cell prokaryotes and therefore do not have a true nucleus. Bacterial biofilms may include two distinct evolutionary groups, eubacteria and archaebacteria. Bacteria in biofilms may exist as isolated cells, in aggregates, or in association with other organisms (Krumbein, 1969; Krumbein and Jens, 1981; Lee et al., 1995). There has long been an association between bacteria and the decay of minerals (Branner, 1897). Bacteria are able to etch feldspars and quartz surfaces in subsurface environments. Silica and aluminum are mobilized by the secretion of organic acids (Hiebert and Bennett, 1992). Bacterial communities living in deep basalt aquifers of the Columbia Plateau obtain energy from weathering reactions (Stevens and McKinley, 1995). Bacteria able to tolerate saline environments secrete organic acids that weathering rocks that are typically thought to be broken only by salt weathering (Viles, 1995). Microbiologist William Ghiorse of Cornell University was quoted on the results of a 5-year survey of underground microbial life: "I'd say that there are bacteria almost everywhere (Appenzeller, 1992, p. 222)."

44

Chapter 4

The amount of mineral matter deposited by bacteria over geological could cover the entire earth surface (Krumbein and Dyer, 1985; Krumbein and Schellnhuber, 1992). It should not be surprising, therefore, that bacteria are involved in the accretion of minerals in different types of rock coatings (Beveridge, 1989; Ehrlich, 1996; SchultzeLam et al., 1996). There are six categories of data that could be presented as evidence for the role of bacteria in the formation of largely inorganic coatings. These categories are exemplified here for rock varnish, a type of rock coating dominated by anomalous concentrations of manganese hydroxides that are thought to be bioenhanced. First, bacteria can be cultured from rock surfaces and grown in a controlled laboratory setting. Abundant culturing work has shown that manganese oxidizers are found on desert rock surfaces (Dorn and Oberlander, 1981a; Dora and Oberlander, 1982; Drake et al., 1993; Jones, 1991; Krumbein, 1969; Krumbein, 1971; Krumbein and Jens, 1981; Nagy et al., 1991; Palmer et al., 1985). Second, in situ studies can be made on the metabolic activity of bacteria. This approach has explored in only a few papers. However, available data are consistent with the importance of bacteria in the precipitation of manganese on rock surfaces (Drake et al., 1993; Jones, 1991). Third, molecular studies of enzymes and DNA could help elucidate the functioning and evolutionary biology of manganese-concentrating epilithic lithobionts. I do not know of research in rock varnish on this type of evidence. Fourth, electron microscopes can be used to catch the bacteria 'in the act' of growing and concentrating manganese (Dorn and Oberlander, 1981a; Dora and Oberlander, 1982; Krumbein, 1969; Krumbein, 1971; Krumbein and Jens, 1981; Mustoe, 1981; Nagy et al., 1991). In the case of rock varnish, budding bacteria (Hirsch, 1974) with filamentous hyphae can be seen blooming on desert rock surfaces a week or two after a thorough soaking of precipitation (Figure 4.3, Figure 4.4, Figure 4.5, Figure 4.6).

Figure 4.3. Mat of epilithic bacteria growing on a cobble on the high shoreline of the precursor to the Dead Sea, Heistocene Lake Lisan, Israel.

Lithobiontic Coatings

45

Figure 4.4. Mat of Metallogenium-like bacteria growing on the surface of a chert cobble in a Negev Desert reg. The bright spots are budding cells that extend out of partially buried hyphae. The corresDonding energy dispersive analysis of X-rays compares the chemistry of a bacterial hyphae with the overan rock varnish; the oacterial biofilm greatly enhances manganese and iron.

Figure 4.5. Mat of a zoogleal bacterial form (Aristovkaya and Zavarzin, 1971) found on rock varnish from West Texas. The bacterial hyphae are greatly concentrated in Mn.

Figure 4.6. Unidentified rod and cocci bacteria on rock vamish-encrusted granodiorite, from Joshua Tree, eastern California. These forms are associated with enhancements of Mn and Fe.

Fifth, there may be fossils of bacteria preserved within the deposit. In the case of rock varnish, fossils are extremely rare. Fossil bacterial structures can sometimes be seen in high resolution transmission electron microscope imagery (Figure 4.7). They are not normally seen in cross sections of rock varnish with scanning electron microscopes, but etching with hydrofluoric acid can bring out bacterial cells encrusted with manganese and iron (Figure 4.8). In a few cases, cell-wall remnants have been seen on rock surfaces (Figure 4.9).

46

Chapter 4

Figure 4.7. High resolution transmission electron microscope view of manganese precipitates on bacterial cell wall, Mt. Van Valkenburg, Antarctica. The scale bar is 20 nanometers.

Figure 4.8. Backscattered electron microscope image of rod-shaped bacteria in a cross-section of rock varnish from South Australia. The cross-section was etched with HF, partly dissolving the clay minerals. The rod forms are brighter than the matrix because they are encmsted with manganese and iron. The scale bar is 5 microns

Figure 4.9. Secondary electron microscope image of a cell-wall cast, from the same site as Figure 4.4. When the organisms die, they leave behind a hollowed center.

Lithobiontic Coatings

47

Six, bacterial enhancement can lead to a stromatolite-like morphology thought to be biologically created. In the case of rock varnish, the form is created by accretion around bacterial nucleation sites called microbotryoidal (Figure 4.10). The building blocks of the botryoidal form are micron-sized bacterial cocci forms. When bacterially-mediated manganese grows together, larger structures develop. These larger can appear as dark dots on rock surfaces (Figure 4.11) that grow together to form a more continuous coating. When these structures are seen in cross-section, they appear like miniature stromatolites (Figure 4.12). Some individuals believe that these stromatolitic structures are definitive indicators of a biological origin (Nagy et al., 1991; Raymond et al., 1992), but this relationship is only an assumption and is not certain (Grotzinger and Rothman, 1996).

Figure 4.10. Secondary electron image of microbotryoidal rock varnish, collected from near Sunflower, central Arizona. Individual micron-scale cocci cells grow together to produce botryoidal forms.

Figure 4.11. Optical photograph of rock varnish 'colonies' collected from near Sunflower, central Arizona. The scale of the individual dots are up to a millimeter in diameter, yet each one consists of microbotryoidal forms (Figure 4.10).

48

Chapter 4

Figure 4.12. Backscattered electron image of botryoidal forms from Sunflower, central Arizona (cf. Figure 4.10) seen in cross-section. These forms appear similar to traditional calcium carbonate stromatolites (Walter, 1976), only much smaller.

In summary, bacterial biofilms are ubiquitous on natural and anthropogenic rock surfaces. They play a role in weathering and in the genesis of new minerals in the terrestrial weathering environment. A variety of different types of evidence may be employed to indicate that largely inorganic rock coatings can still have a bacterial genesis. 4.2.2. Cyanobacteria Cyanobacteria (or blue-green algae) are prokaryotic organisms. They are distinct from other bacteria, however, in that they are autotrophic and rely on photosynthesis. Cyanobacteria are quite common on rock and soil surfaces. Cyanobacteria are important agents of weathering. In the humid-tropical landscape of Surinam, for example, epilithic biocrusts organic acids weathers removes aluminum and iron from granodiorite, leading to the formation of clay minerals like vermiculite (Bakker, 1970). On limestone, cyanobacteria often have a coccoid form and behave as euendoliths, boring tunnels into the rock through the secretion of acids (Danin, 1983; Danin, 1986; Danin and Garty, 1983). Biomechanical action may also aid weathering through the growth of cyanobacteria in cryptoendolithic positions causing a physical separation and spalling (Vestal, 1993). Like bacteria and all other lithobionts, cyanobacteria are capable of forming new mineral deposits. A case study is Aldabra Atoll where cyanobacteria deposit a calcium carbonate biorind (Viles, 1987b). Beachrock may also be produced by the activity of cyanobacteria (Krumbein, 1979). There is some thought that cyanobacteria may be involved in the oxidation, and hence precipitation, of manganese (Krumbein, 1969; Scheffer et al., 1963).

Lithobiontic Coatings

49

Probably the most widely cited example of cyanobacterial precipitation is the formation of calcium carbonate stromatolites by Archaean cyanobacteria and other primitive organisms (Figure 4.13) (Walter, 1976). These larger stromatolites are biocrusts in the terminology of Viles (Table 4.2).

Figure 4.13. Stromatolites at Shark Bay in Australia are thought to be modem analogs for Archaean (>2.5 billion years) biocmsts.

4.2.3. Fungi Fungi, like algae, higher plants and animals, have cells with a membrane around the nucleus and are called eukaryotic organisms. Fungi play key, if not dominant, roles in the cycling of nutrients in terrestrial ecosystems (Harley, 1971). The focus here rests on fungi that form biofilms in epilithic, and sometimes chasmolithic and euendolithic positions (Urzi et al., 1993). The fungi that coat and invade rock surfaces weather minerals through a variety of processes including the secretion of citric, oxalic and gluconic acids (De la Torre et al., 1993; Eckhardt, 1985; Henderson and Duff, 1963; Urzi et al., 1993). Fungi not only acidify solutions, but they also release complexing agents that aid in the removal of iron (Avakyan et al., 1981; Silverman and Munoz, 1970). There is evidence that some fungi can participate in the precipitation of manganese (Grote and Krumbein, 1992; Krumbein and Jens, 1981), in particular fungi that display hyphae-like filamentous growths (Dorn and Oberlander, 1982) (Figure 4.14). Other fungi may both weather the underlying rock and generate new materials; for example divalent manganese replaces calcium in historic marble structures (Gorbushina et al., 1993). Fungi are an important biofilm, simply from the perspective of coating host rocks. Most of the research on epilithic fungi have been conducted on stone monuments (Diakumaku et al., 1995; Gorbushina et al., 1993; Krumbein, 1988; Krumbein and Urzi, 1993; Urzi et al., 1993; Urzi et al., 1992). Fungi provide dark red, dark brown and black coloration to many different rock surfaces in Mediterranean climates (Urzi et al., 1993) (Figure 4.15). Fungi can grow quite rapidly over fresh rock surfaces; for example, a fresh block of marble "was covered within 6 weeks by a dense film of a black coating produced by black fungi (Knmabein and Urzi, 1993, p. 563)."

50

Chapter 4

Figure 4.14. Fungi from Pisgah Crater, Mojave Desert, California. The upper energy dispersive X-ray spectrum is of the overall varnish; the bottom X-ray analysis is a focused point on a fungal filament, indicating that manganese and iron are concentrated by fungi.

Figure 4.15. Black coatings on marble at Hadrian's Villa, Italy, appear to be Composed of mostly fungi.

Fungi coat rocks in a wide variety of settings (Hunt and DurreU, 1966; Krumbein and Dyer, 1985; Krumbein and Jens, 1981). Black fungi are especially noticeable as epilithic coatings on stone monuments (De la Torre et al., 1993). I have seen fungal hyphae on well over ninety percent of all samples that I have examined with secondary electron microscopy. Examples include Hawaiian basalt flows (Figure 4.16), millimeter-scale depressions on the Bishop Tuff in eastern California (Figure 4.17), artifacts on the Yermo fan in the Mojave Desert, eastern California (Figure 4.18), and on quartz near Sunflower, central Arizona (Figure 4.19). The filamentous form seen in these micrographs is the most common morphology. Although fungi yield a wide variety of colors, the most common coloration imposed by epilithic fungi is black. A filamentous morphology is the most common, but fungi may also grow in clusters diagnostic of "black globular units" or "microcolonial fungi" (Borns et al., 1980; Staley et al., 1991; Staley et al., 1983; Staley et al., 1982; Taylor-George et al., 1983). In some cases, microcolonial fungi secrete acids that weather rock coatings (Dragovich, 1993a; Krinsley et al., 1990). In other circumstances cases, microcolonial fungi are adventitious, simply coating rock surfaces and other rock coatings (Figure 4.20 and Figure 4.21).

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Figure 4.16. Scanning electron microscope image of fungal hyphae growing on a 700-year-old lava flow of Hualalai, Hawai'i, that in turn is being coated by the plate-like accretion of manganiferous rock varnish.

Figure 4.17. Scanning electron image of filamentous fungi growing on the Bishop Tuff of eastern California. Clay particles, manganese and iron can be found inter-layered with the fungL

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Figure 4.18. Chert artifacts often have voids, in which grow black fungi, as seen in this secondary electron image of a corer collected near the Calico Early Man site, Mojave Desert, eastem California.

Figure 4.19. Desiccated filamentous fungi grows on the surface of a quartz cobble, collected near Sunflower, central Arizona.

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Figure 4.20. Microcolonial fungi can extend filaments out of a central colony after precipitation wets the surface of a desert rock. In this case, the microcolonial fungi is growing on top of rock vamish formed on Warm Springs Alluvial Fan, Death Valley.

Figure 4.21. Several microcolonies are found on this sample of rock varnish from South Mountain Park, Arizona. The sample was collected by C. Elvidge. For the most part, the fungi simply rest on the vamish. However, they display euendolithic (boring) behavior at the bottom of the image.

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There is some thought that the microcolonies can serve as a host 'template' to help develop botryoidal morphologies for rock coatings that accretes on top of the fungi, much like bacteria can serve as the core of botryoidal forms (Figure 4.10). Although microcolonial fungi look superficially like the botryoidal textures of rock varnish (Dora, 1986) and oxalate-rich crusts (Russ et al., 1996), the ~5ktm fungal structures are usually much smaller than most of the botryoidal morphologies found on rock coatings (Figure 4.22). A much more common interaction between fungi on host rock is the euendolithic behavior as these organisms bore holes into the host material (Figure 4.23, Figure 4.24, Figure 4.25).

Figure 4.22. Comparison of microcolonial fungi (lower part of image) and botryoidal rock vamish (upper part of image) from near Lake Roosevelt, central Arizona.

Figure 4.23. Microcolonial fungi growing on a petroglyph at Deer Valley, Phoenix, Arizona. The left image (secondary electrons) shows topography, revealing the honeycombed texture of microcolonial fungi The fight image (backscattered electrons) shows chemistry where the quartz rock is dark gray, the rock varnish is bright, and organic matter is black. Fungi release organic acids that dissolves the rock varnish.

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Figure 4.24. Microcolonial fungi boring into the host rock vamish and the underlying rock, near Florence Junction, central Arizona.

Figure 4.25. Euendolithic microcolonial fungi, boring into the rock varnish and the underlying rock. The sample was collected from the Olary Province of South Australia.

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4.2.4. Algae Just as brown algae (Phaeophyta) can dominate shores of rocky coasts in areas proximate to oceanic upwelling, green algae (Chlorophyta) are dominant in freshwater settings (Figure 4.26). Freshwater algal coatings are common lithobionts that can interact with anthropogenic inputs. For example, they are used to help absorb and remove heavy metals in aquatic settings (Sneddon and Pappas, 1991). In moistened contexts, algae can dominate exposed rock surfaces.

Figure 4.26. Epilithic green algae and moss growing on a stream cobble in a tributary of the Little Colorado River, northeastem Arizona.

Non-lichenized, free-living algae are also important lithobionts in terrestrial settings (Hunt and Durrell, 1966; Starks et al., 1981). For example, algal mats encrust limestone quarry surfaces on the island of Thasos, Greece (Doehne, 1994). Lithiobiontic algae can occupy a number of niches (Figure 4.27), including epilithic, hypolithic, chasmolithic, and endolithic positions (Friedmann and Galun, 1974; Friedmann et al., 1957).

Endolithic

Epilithic Hypolithic

Chasmolithic

Figure 4.27. Different lithobiontic positions of algae on a river terrace, desert pavement and bedrock.

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The traditional perception of non-lichenized epilithic algae is that these organisms represent a step in biological succession (Fritsch, 1907; Jackson, 1971). The argument made in introductory biology texts is that algae help prepare the environment for higher plants through weathering (Keeton and Gould, 1986). Algae play a role in rock weathering (Krumbein, 1969; Tiano et al., 1993; Viles, 1995). For example, the expansion of contraction of algae in chasmolithic positions can result in flaking. Algae loosen the connection between the outermost shell of the weathering rind and the host rock. With water and wind-imposed sheer stresses, the resultant flaking can drastically alter the appearance of rock surfaces in cold-wet (Hall and Otte, 1990), cold-dry (Broady, 1981), and warm-wet climates (Fiol et al., 1996). Hypolithic algae take the form a green algal biorind on the undersides of surface stones. They are most common on semi-translucent rocks such as quartz (Figure 4.28), limestone and gypsum (Friedmann et al., 1967). Hylx~lithic algae take advantage of a place of reduced light intensity, an increase of water availability, and a reduced heat stress (Friedmann and Galun, 1974).

Figure 4.28. Hypolithic algae growing on the underside of a quartz cobble, resting on a desert pavement near Baker, Mojave Desert, eastern Califomia.

Chasmolithic algae grow in pre-existing rock fractures, and along the separating grains of granitic boulders (Friedmann et al., 1967). These fractures offer a location of greater moisture availability as the algal biorind can seal off the internal pore space. These algae live in a balanced position where there is enough light penetration for photosynthesis, but a reduction in moisture and temperature stress (Friedmann and Galun, 1974). Endolithic algae live under the surface of rocks, but within the upper few millimeters. They occupy pore spaces within the weathering rind. They are most frequently reported in sandstones (Friedmann et al., 1967). Within warm deserts, they prefer wetter environments (Bell and Sommerfeld, 1987; Dahlquist and Somerfeld, 1991). However, cryptoendolithic algae are found in the very harsh environments Antarctica (Friedmann, 1980; Friedmann, 1982; Wharton, 1993) and nunataks in Alaska (Hall and Otte, 1990). 4.2.5. Lichens Lichens are ubiquitous lithobiontic coatings, able to tolerate the harshest temperature and moisture conditions on the planet (Friedmann, 1982; Friedmann and Galun, 1974). They are dominant on over eight percent of the Earth's surface (Koppes, 1990). Foliose, fruticose, and crustose lichens are all found on rock surfaces. In dry regions, however, most of the lichens are crustose, a form that limits exposure to the atmosphere (Galun, 1963).

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Even epilithic lichens are not entirely on the surface. Foliose and fruticose lichens attach to the rock with bundles of fungal hyphae, whereas crustose lichens are firmly attached. Epilithic lichens are, therefore, also euendoliths in that they bore into the host rock (Gehrmann et al., 1988; Gehrmann and Krumbein, 1994) (Figure 4.29 and Figure 4.30).

Figure 4.29. The epilithic lichen Rhizocarpum geographicum growing on a basalt flow of the SP Crater, San Francisco Peaks volcanic field, Arizona.

Figure 4.30. The epilithic cmstose lichen Stereocaulon vulcani is shown growing on a Hualalai Volcano lava flow. Image (a) shows round algal fruiting bodies and fungal hyphae. The euendolithic effect of this lichen is shown in the cross-section undemeath the lichen in (b). T h e lichen does not show up in the backscattered image (b) because its atomic number is too low, but the increased porosity in the weathering rind (above the white line) is from the boring effect of the lichen. The scale bars are in microns.

Euendolithic lichens are common. They grow on and weather a wide range of lithologies including basalt (Figure 4.30), limestone (Gehrmann and Krumbein, 1994; Viles, 1988), quartz (Cooks and Otto, 1990), gabbro (Cooks and Fourie, 1990), syenite (Barker, 1994) and other lithologies. Endolithic lichens (Figure 4.1) have been studied most extensively in hot and cold deserts. A shortage of moisture allows higher humidity levels within the pore spaces of rock weathering finds (Friedmann, 1980; Friedmann and Ocampo-Friedmann, 1984; Golubic et al., 1981; Palmer and Friedmann, 1990). Endolithic lichens impact rock surfaces by promoting erosion of the surface, thus creating a "characteristic color

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mosaic pattern on the surface, a telltale sign of cryptoendolithic lichen colonization (Friedmann et al., 1988, p. 254)." A substantial literature indicates the importance of lichens as agents of weathering on natural (Cooks and Fourie, 1990; Gehrmann et al., 1988; Jackson and Keller, 1970; Jones and Wilson, 1985; Paradise, 1997; Wilson and Jones, 1983; Wilson et al., 1981) and anthropogenic (Danin and Caneva, 1990; Krumbein, 1988; Romao and Rattazzi, 1996) rock surfaces. Certain lichens on carbonate lithologies produce distinctive weathering patterns (Danin, 1986; Danin et al., 1983; Danin et al., 1982) on the host rock (Figure 4.31). These patterns can be found under actively growing lichens (Figure 4.32), hence the presence of puzzle patterns where these lichens no longer grow can be used to indicate wetter climates in the past (Danin, 1985).

Figure 4.31. 'Puzzle' pattem left behind by ancient lichens that grew in this limestone rock in the Negev Desert of Israel.

Figure 4.32. Lichens growing on limestone in the Judean Hills, near Jerusalem, Israel. They have a puzzle pattern. Sunglasses provide scale.

There is evidence that lichens are able to protect the underlying rock surface from erosion (Benedict, 1993; Danin and Garty, 1983; Fry, 1926; Paradise, 1993b; Viles and Pentacost, 1994). Protection from erosion does not necessarily imply that weathering has stopped, only that the surface has been stabilized. Whether lichens help erode or preserve the host rock appears to be controlled in part by climate. The erosional impact of lichens appears to be less in wetter climates (Krumbein, 1969; Viles, 1987a). Removal of lichens to protect rock art and stone monuments could accelerate erosion (Childers, 1994). Different types of lichens on different types of rock in different climates have different effects. It is, therefore, difficult to predict the effect of lichen removal. In some cases, lichens hold loose material in place; the removal of the 'dam' could greatly increase erosion rates; there is also the danger of the unknown effects of the chemicals used to kill the lichens (Childers, 1994). Lichens can aid in the accretion of largely inorganic rock coatings. The combination of metabolic activity and weathering releases a variety of compounds that can migrate into rock coatings (Friedmann et al., 1993; Garty et al., 1996). For example, lichens appear to play a role in the formation of certain textures of calcretes (Klappa, 1979). Lichens are also agents of oxalate precipitation (Gehrmann and Krumbein, 1994; Russ et al., 1996; Syers et al., 1967).

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4.2.6. Higher Organisms Bryophytes (liverworts, hornworts, mosses) are common lithobionts on moistened rock surfaces such as shelves around springs and rivers and stones exposed by cultivation in fields (van Wesemael et al., 1996). As a moss plant colonizes a surface in its multicellular diploid or halpoid phase, it penetrates into the pore spaces of the host rock. Mosses are, therefore, both epiliths and euendoliths. The largest aerial extent of mosses on stones is probably in the tundra (Jonsdottir et al., 1995; Linskens, 1995; Walton, 1993). Ubiquitous growths in high latitudes have prompted their use as a tool to understand pollution (Ayras et al., 1997; Steinnes, 1995). Cryptogamic crusts of moss, along with cyanobacteria, fungi, lichens, and algae, form over softs in arid and semiarid environments. Cryptogamic crusts are presented in this section, not because of the relative importance of moss, but because the different types of organisms that are a part of a cryptogamic association have been presented earlier and because the word cryptogamic means hidden reproductive cells and relates to the mosses in the crusts. However, cyanobacteria and lichen are the dominant organisms in most of these crusts (Metting, 1991). I do not consider knobby black biocrusts on loose soil to be a rock coating. However, cryptogamic crusts can be found growing over rock faces. In Arctic and alpine areas, for example, cryptogamic mosses and lichens can cover bare rock (Walton, 1993). I have observed the coveting of a gnamma pit with cryptogamic crusts (Figure 4.33). Cryptogamic crusts start on the unconsolidated particles in the center of a metersized gnamma pit, holding them in place. The cryptogamic crust then accelerates the refilling of the gnamma pit through the protective trapping of silt and sand. Eventually, cryptogamic crusts grow over adjacent rock surfaces.

Figure 4.33. Photographs of knobby cryptogamic crust from the Colorado Plateau A hat provides scale for the right image, and a bush provides scale for the left image. The crusts are largely built from cyanobacteria, along with fungi, moss and lichens.

Higher plants cover rock surfaces. Gymnosperms and angiosperms are not normally considered lithobiontic agents. However, they are often found growing directly on top of natural and human-modified rock faces (Alestalo, 1971; Caneva et al., 1993; Cochran and Berner, 1996; Lewin and Charola, 1981; Mishra et al., 1995; Shroder and Jr., 1980; Zhang et al., 1994). It is difficult to assess the aerial significance of plant growth directly on rock surfaces. Higher plants are very important weathering agents (Cochran and Berner, 1993a; Cochran and Berner, 1992; Cochran and Bemer, 1993b; Cochran and Berner, 1996; Danin et al., 1987), although their impact is difficult to measure directly (Stallard, 1985; Stallard, 1995). It is quite possible that the development of angiosperms may have been critical in the depletion of carbon dioxide by rock weathering and in the

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cooling of climate (Schwartzman and Volk, 1989; Schwartzman and Volk, 1991; Schwartzman and Volk, 1992). Thus, the role of lithobiotic plants as weathering agents deserves greater attention. It would be difficult to discuss rock coatings of a biological origin and not mention guano. On tropical atolls, for example, guano occurs as a weak cement, typically gray to brown in color but with an outer white film (Stoddart and Scoffin, 1983). Bat guano coatings rocks in a wide range of cave settings (Hill, 1981; White, 1976). Bird guano may even play a role in preserving rock art (MacLeod et al., 1995).

4.3.

Controls

on

Distribution

The biogeography of lithobionts is an enormous topic, with many controls. An example of the importance of rock type can be found in the discovery of a new type of glacial deposit: Minturn circles. Black epilithic lichens form on syenite, but they are less frequent on the surrounding gneissic rock type. The difference in color created by the lithobiontic coating allows ready recognition of this new circular glacial deposit (Appel, 1996). Moisture is a particularly important control on lithobiontic distributions in subtropical deserts. In contrast to soils that have moisture-holding retention within clays, rock surfaces do not have as much water absorbency. In fog deserts, however, lithobionts have an advantage over rooted plants because there is a much higher ratio of dew deposition to biomass. Lichens are, therefore, able to dominate in the Atacama (Rundel, 1978; Thomson and Iltis, 1968), Baja California (Nebeker, 1977), and Namib (Rundel, 1978) fog deserts. An landmark study on distribution of lithobionts in the Negev Desert revealed that climate is particularly important. The relative importance of epilithic lichens, endolithic lichens, and euendolithic cyanobacteria varies at different scales. Position on a hillslope and host rock type are important at the scale of a landform (Figure 4.34).

Figure 4.34. Generalized distribution of different lithobiontic coatings over a single hill in the Negev Desert of Israel, modified from Danin and Garty (1983, p. 437). Epilithic lichens dominate on the north-facing slope, cyanobacteria on the upper south-facing slopes, and endolithic lichens on the lower south-facing slopes.

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In a m o r e r e g i o n a l p e r s p e c t i v e , D a n i n ( 1 9 8 6 ) a r g u e s that t h e c l i m a t i c g r a d i e n t in I s r a e l is a d o m i n a n t c o n t r o l o v e r l i t h o b i o n t i c c o m m u n i t i e s ( F i g u r e 4.35).

Figure 4.35. Generalized map of the distribution of dominant lithobiontic agents in Israel, modified from Danin (1986, p. 245).

Figure 4.36. Preliminary growth curves for Nigardsbreen, southem Norway, adapted from Matthews (1994, p. 199). Direct curves are based on direct observations on rates of growth, whereas indirect curves are based on measurements at points in time. 'Max-Dir' is the direct growth curve based on maximum growth rates. 'Mean-Dir' is the direct growth curve based on mean growth rate. 'Larg-Ind' is the indirect calibration for the single largest lichen, while 'Mean-Ind' is the indirect calibration for the mean of the five largest lichens.

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Time is a certainly another important control on the occurrence and abundance of lithobionts. It takes time to colonize rock surfaces and for lithobiontic coatings to grow. A classic case in point is the development of lichens. The measurement of lichen size (or other metrics of lichen growth) has been widely used to indicate the length of time a rock surface has been exposed (Figure 4.36) in Arctic and alpine settings (Lock et al., 1979; Mahaney, 1990; Matthews, 1994; McCarroll et al., 1995; McCarthy and Smith, 1995). This chapter is not the place to review basic biogeography theory that includes controlling factors such as colonization and succession (Fritsch, 1907; Jackson, 1971; Keeton and Gould, 1986), coevolution (Maser et al., 1985), availability of space on the substrate (Viles, 1995), predation (Butler, 1995; Sharnoff, 1994), light (Friedmann, 1993; Friedmann and Galun, 1974), wind (Broady, 1981), temperature (Friedmann and Galun, 1974), in addition to lithology, moisture and time (Figure 4.34-4.36). However, the controls on lithobionts is of vital importance to understand other, mostly inorganic coatings. I argue in chapter 14 that the biogeography of lithobionts is a primary control on the distribution of other rock coatings. Because lithobionts grow much faster than most other rock coatings, they successfully outcompete these coatings for space on a rock surface.

4.4. Impact of Lithobiontic Coatings 4.4.1. On Organic Remains The lithobionts that are seen today on rock surfaces may not necessarily be responsible for the organic matter seen in association with them. Rocks interact with organics and store organic matter in pore spaces (Friedmann and Galun, 1974; Krumbein and Dyer, 1985; Viles, 1995) - - even at depths well below the earth's surface (Chapelle and Bradley, 1996; Danin et al., 1987; Fyfe, 1996; Stevens and McKinley, 1995). Consider the following perspectives. "The diversity of bacteria [in the DOE survey of underground microbial life] was as striking as their distribution--so broad that DOE researchers are even talking about the possibility that deep-living bacteria harbor entirely new metabolic activities. That, in turn, could mean new styles of interplay between the bacteria and the rock and water than enclose them (Appenzeller, 1992, p. 222)." "We should never forget that the highly fractured external and internal surfaces of rock materials add up to hundreds of square kilometers of surface area, and that water is held physically and biologically in all these areas and that at least 100,000 and up to 1,000,000,000 (on billion) microorganisms may live within the intemal surface area of one cubic centimeter (2g) of deteriorated rock (Krumbein, 1988, p. 377)."

Fragments of organic remains deposited within rock gradually alter in structure (Hedges et al., 1993). The literature on organic matter diagenesis reveals that the changes can progress to the point where even components of coal (inertinite and vitrinite) form in the terrestrial weathering environment (Chitale, 1986; Karlov, 1961; Moore et al., 1996). For example, a sample was collected fi'om a rock crevice in Portugal that had opened several centimeters, but the overlying block had not yet fallen off. Rock material was pried out from within this crevice. A bulk sample that had a radiocarbon age of

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23,550!-_190 (Beta 82457) was split into shiny, dense particles (vitrinite-like) with an age of 29,990!-_240 (Beta 86633) and less dense, more fibrous material (carbonized woody remains) with an age of 17,460&_70 (Beta 86632) (Dorn, 1997; Welsh and Dorn, 1997). In another example, 30,000 to >50,000 14C year-old organics are emplaced within fractures tends of meters beneath the surface (Danin et al., 1987). In summary, lithobionts can leave fossil remains behind that can be misinterpreted as contemporaries of the overlying rock coating. 4.4.2. On Rock Weathering The focus of this chapter is on lithobionts as rock coatings. Yet, a part of this process must involve their metabolic activities, in particular their influence on the decay of rock material. A holistic perspective is given by Krumbein and Dyer (1985, p. 158) who view the weathering activity of lithobionts as an integral part of remaking the lithosphere in terms of both erosion: "Transfer of the rocks and minerals of the lithosphere through time and space is considerably speeded up and directed by biological processes on all scales and through all the five kingdoms of the living natural bodies (biota)...The transfer of rocks and minerals of the Earth's lithosphere are in the firm grip of the powerful force of life, because after all this planet is alive."

A full treatment of lithobiontic weathering is beyond the scope of this chapter, since there are monographs on just weathering (Oilier, 1969; Oilier, 1984; Robinson and Williams, 1994a; White and Brantley, 1995; Yatsu, 1988). My focus here rests on the impact of rock weathering on rock coatings. Lithobiontic weathering has two key effects on rock coatings. First, lithobiontic weathering exposes uncoated minerals. Chasmolithic algae may create a distinctive flaking pattern on rock surfaces (Hall and Otte, 1990). Lichens may leave a mosaic of patches (Danin et al., 1983; Friedmann et al., 1993) or secrete acids to erode other rock coatings (Figure 4.37).

Figure 4.37. The growth of epilithic lichens (light gray) on sandstone joint faces in eastem Wyoming have come at the expense of manganese-rich rock vamish (black surface in middle) and silica glaze (n upper left and lower right). The lichens secrete acids that dissolve the rock vamish.

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The second key effect is that weathering exposes chasmolithic lithobionts. As a spall erodes, the organisms growing within the rock fractures are exposed. This leads to a gradual succession of lithobiontic organisms on the rock surface. The chasmolithic lithobionts cannot tolerate surface conditions, so there is a succession of epilithic organisms replacing the chasmoliths.

4.4.3. On Other Rock Comings In contrast to weathering, lithobionts may aid in the precipitation of new mineral material in the form of largely inorganic rock coatings. Krumbein and Dyer (1985, p. 158) wrote: "The transfer of activity of the biosphere includes enrichments for practically all elements, purposeful arrangement according to the requirements of the biota, and rate limiting controls."

One school of thought interprets the vast majority of sedimentary mineral accumulations as a product of biological activity (Krumbein and Schellnhuber, 1992). Lithobionts create sinks for manganese (Ghiorse and Ehrlich, 1992; Uren and Leeper, 1978), iron (Nealson, 1983), phosphorus (Konhauser et al., 1994), calcium in different forms (Folk, 1993; Klappa, 1979; Russ et al., 1996) and other materials (Gerdes and Krumbein, 1987; Krumbein, 1983). In this paradigm, the mobilization and fixation of rock coatings are heavily influenced by lithobionts. Laminated or stromatolitic deposits are generally associated with organic processes, for example calcium carbonate stromatolites were produced by Archaen cyanobacteria (Gerdes and Krumbein, 1987). Stromatolitic forms have been observed in association with arsenic (Leblanc et al., 1997), manganese (Figure 4.12), phosphate (Burnett and Riggs, 1991), carbonate (Folk, 1994; Klappa, 1979), oxalate (Russ et al., 1996), and even iridium-enrichment (Dyer et al., 1989). Thus, the occurrence of laminations in rock coatings are often interpreted as evidence of biological activity (Raymond et al., 1992). The role of organisms in the formation of different types of rock coatings will be explored in greater detail in subsequent chapters. There is a dialectic in the roles of lithobionts as agents of both weathering and depositional. The resolution to this contradiction rests in the competition for space. Lithobionts, although they can produce inorganic rock coatings, fight for possession of rock surfaces. Where biological weathering is effective, lithobionts are growing and winning the battle for possession of rock surfaces. Where biological weathering is less effective, slower growing rock coatings develop on rock surfaces sometimes with the assistance of lithobionts. The interpretation of lithobionts as a competition for space may be scale invariant. At the scale of microns, lithobionts slowly dominate space formerly occupied by rock coatings through secreting acids (Dragovich, 1986b; Dragovich, 1993a; Paradise, 1997). This dialectic can also be considered at the scale of a sandstone face in the Black Hills of eastern Wyoming (Figure 4.37). The most extensive inorganic rock comings are rock varnish (Chapter 10) and silica glaze (Chapter 13). Both of these coatings originally formed in rock crevices, all before spalling exposed the face. With exposure to sunlight, epilithic lichens outcompete pre-existing inorganic comings by physically and biochemically eroding them. The distribution of lithobionts at the scale of a small country also highlights the battlefield for possession of rock surfaces (Figure 4.35). Largely inorganic rock varnish

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dominates in the driest sections of Israel. In the more moist sections of the country, different lithobiontic communities dominate rock surfaces. Lithobionts almost always grow much faster than the largely inorganic rock coatings discussed in the remaining chapters in section 2. Thus, if biogeographic conditions foster their growth, lithobionts define the character of the rock coating at any given place. It is only when lithobionts are unable to colonize a surface that other rock coatings are given a chance to grow.

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Chapter 5

CARBONATE

CRUSTS

Perhaps "insightful" hypotheses are only slightly more likely than random ones to be successful, and only a tiny bit of the success of science is to be explained by the insights of scientists. (Amundson, 1989, p. 427)

5.1. Introduction This chapter explores a rock coating with an established topical literature, as in the previous two chapters. Carbonate crusts have not yet been perceived or synthesized as a rock coating, but considerable research has already been conducted on carbonate deposits in freshwater, marine, and pedogenic contexts. Dozens of monographs and review papers have been written on specific topics in this chapter. It would be inappropriate, therefore, to present a comprehensive review of these carbonate literatures here. Instead, my effort here is to explore knowledge on very different carbonate deposits from the perspective that they all form rock coatings. Sections 5.2. through 5.4. summarize well-understood carbonate crusts from the perspective of a focus on rock coatings. Section 5.5. reviews the limited literature on carbonate crusts that form on subaerial rock faces. The last section, 5.6., explores the broader significance of carbonate crusts in terms of the earth's landscape geochemistry. Although carbonate crusts have a wide variety of origins, mineralogy, isotope chemistry and forms, they all form rock coatings at the atmosphere-lithosphere interface.

5.2. Freshwater Carbonate Deposits The deposition of calcium carbonate in freshwater is called tufa, travertine, sinter and other names (Ford and Pedley, 1996; Viles and Goudie, 1990). There is considerable terminological confusion as some authors restrict the use of tufa to just spring deposits, while others limit sinter to abiotic precipitates with a dense mineralogical texture. Regardless of the term that is used, these deposits are all considered carbonate crusts where they accrete on a host rock. Carbonate crusts form today over rocks and boulders in a wide range of freshwater settings. Travertine coats stream boulders (Cole and Batchelder, 1969); tufa entombs talus (Russell, 1889); and carbonate crusts precipitate underneath glaciers (HaUet, 1976; Watts, 1985). Hot springs are frequent locales of carbonate crusting (Sorey, 1997). Carbonate crusts are common precipitates in caves, especially in karst topography (Vardenoe, 1965), but they also encrust basaltic lava tubes (Fyfe, 1996).

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Carbonate crusts are not restricted to contemporary freshwater settings. Thicker crusts survive for tens of thousands of years without dissolving (Szabo, 1990). Consider tufa that cements talus boulders at Pyramid Lake, Nevada (Figure 5.1). Carbonate crusts have stabilized steep talus slopes since the lake level dropped from this position about 16,000 years ago (Benson et al., 1990).

Figure 5.1. Talus boulders have been cemented (see arrow) by tufa at an elevation of around 1308 m at Mullen Pass, Pyramid Lake. This is the elevation of the overflow sill to the Walker Basin when the region formed the interconnected Pleistocene Lake Lahontan (Benson et al., 1990).

Freshwater carbonate can display a wide variety of forms. The most comprehensive analysis of tufa forms was presented by Benson (1994). Some of these forms include the accretion of carbonate crusts on rocks. Figure 5.2. exemplifies dense, lithoid tufa that can be found coating boulders on shorelines of Pleistocene lakes, such as Lake Lahontan in Nevada. Figures 5.3. and Figure 5.4. illustrate how more complex tufa sometimes drapes over host rocks. Figure 5.5. shows a cross-section of a slope of talus boulders that have been coated in tufa.

Figure 5.2. Sketch of tufa deposited over a granodiorite core stone on a shoreline of Lake Lahontan. The boulder is about 1 meter across.

Carbonate Crusts

69

Figure 5.3. Sketch of ten centimeter-thick reef-form tufa deposited over marble, Pyramid Lake, Nevada.

Figure 5.4. Pillow-form tufa draped over andesite, Pyramid Lake, Nevada.

Figure 5.5. Tufa-encmsted talus, Marble Bluff, Pyramid Lake, Nevada.

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There are two general schools of thought, biotic and abiotic, on how tufa and related carbonate crusts may accrete on top of rocks. One perspective is that organisms are critical in the deposition of tufa and related carbonate deposits; bacteria, cyanobacteria, algae, moss, and higher plants all appear to be agents of carbonate deposition (Folk, 1994; Fyfe, 1996; Pentecost, 1985; Robbins and Blackwelder, 1992; Viles, 1984; Viles and Goudie, 1990; Weed, 1888). On the other hand, inorganic explanations are also preferred by many. Abiotic models involve the supersaturation of water with carbonate (Dandurand et al., 1982), evaporation (Dunkerley, 1987), degassing of carbon dioxide (Jacobson and Usdowski, 1975), temperature changes (Bargar, 1978), and changes in water velocity (Vardenoe, 1965) that result in the accretion of carbonate on rocks.

5.3. Marine Littoral Carbonate Crusts

Calcium carbonate is commonly precipitated along marine coasts in tropical settings. Stromatolitic growths and beachrock exemplify these deposits. In the geological literature, stromatolites have long been regarded as contemporary and fossil evidence for biotic precipitation of calcium carbonate (Gerdes and Krumbein, 1987). These biolaminations grow in littoral settings (Figure 4.13), and they also cover rock surfaces (Gerdes and Krumbein, 1987). Beachrock is more common than stromatolitic carbonate. Beachrock grows along marine coastlines, and almost always in tropical and subtropical latitudes (Scoffin and Stoddart, 1983; Stoddart and Cann, 1965). The circumstances where the calcium carbonate cements sands or smaller particles (Figure 5.6) cannot be considered a rock coating, but rather a type of rock type unto itself.

Figure 5.6. Beachrock-cemented coral sands on Green Island, Queensland, Australia, illustrate the formation of a calcareous sandstone, and not a rock coating.

Beachrock can form rock coatings, but only where calcium carbonate encapsulates a pre-existing rock. This can occur along coral coasts, for example, where boulder tracts of massive corals are piled by tropical storms (Scoffin and Stoddart, 1983). Beackrock as carbonate crust also forms where topical beaches interface with rapidly eroding continental settings. Figure 5.7, for example, shows actively forming beachrock at the

Carbonate Crusts

71

interface of an alluvial fan and the Gulf of Aqaba. Figure 5.8 illustrates a coast uplifted by tectonic forces where metamorphic clasts are coated by fossilized beachrock.

Figure 5.7. Beachrock-cemented alluvial-fan cobbles on the east coast of the Sinai Peninsula near Wadi al Hasi. Cyanobacterial casts are found throughout this beachrock and active cyanobacteria give it a dark appearance.

Figure 5.8. Beachrock coats metamorphic rocks near the southern tip of the Sinai Peninsula, where the coastline has been uplifted by neotectonic activity.

Abiotic and biotic explanations have been presented for the formation of tropical beachrock. Abiotic precipitation is generally thought to be from the evaporation of sea water (Stoddart and Cann, 1965). Biotic explanations involve the direct activity of bacteria, cyanobacteria, algae, mangrove and worms (Krumbein, 1979; Scoffin and Stoddart, 1983) or modifications of a littoral microenvironment by organisms (Krumbein, 1979).

72

Chapter 5

5.4. Pedogenic Carbonate Crusts The accumulation of calcium carbonate in soils is widespread in arid and semi-arid regions (Goudie, 1973; Goudie, 1983). Caliche is commonly used to describe these subsurface accumulations (Reeves, 1976), but calcrete is increasingly preferred as a term in the scientific literature (Dixon, 1994). Calcrete is, by definition, a soil phenomenon. Therefore, the vast mass of pedogenic carbonate is beyond the scope of this book, because is not exposed at the surface. Calcrete interfaces with this book only where carbonate crusts are exposed at the earth's surface. Calcrete coats the gravel in soils in stages. At first, thin and discontinuous coatings grow on cobble undersides (Figure 5.9). Then, coatings become more continuous on the upper and lower sides.

Figure 5.9. Gravels of a beach bar (Stockton Bar) of Pleistocene Lake Bonneville in Utah are encrusted on their bottom sides with calcium carbonate at a depth of around 160 cm.

Gradually, pore spaces between the cobbles fill with carbonate. When pores are plugged, a horizon of laminar calcrete precipitates all on top (Figure 5.10) of the impermeable soil (Gile et al., 1966; Machette, 1985). Laminar calcrete can also line the fractures in bedrock where there is low permeability.

Figure 5.10. The upper laminar horizon of a calcrete on the south side of Tempe Butte, near the campus of Arizona State University. The branches of creosote bush (Larrea tridentata), at about 2 cm in diameter, provide scale.

Carbonate Crusts

73

Pedogenic calcrete can reach several meters in thickness (Goudie, 1973; Lattman, 1973; Reeves, 1976; Watts, 1980) (Figure 5.11 and Figure 5.12). These thick accumulations of calcium carbonate are then called calcrete duricrusts, because they provide a durable crust that inhibits erosion of the landscape.

Figure 5.11. The OgaUala formation in west Texas (HoUiday, 1990) is capped by a pedogenic calcrete several meters thick.

Figure 5.12. A meter-thick calcrete (upper, whitish layer) occurs along the coast of Israel near Haifa.

Soil-forming processes produce the vast majority of calcretes (Goudie, 1973; Goudie, 1983), where much of the calcium carbonate derives from dust and is transported down into the soil by percolating waters (Machette, 1985; Reheis et al., 1989). A number of physico-chemical factors may be forcing the carbonate precipitation, including: evaporation, pH, carbon dioxide partial pressure (Schlesinger, 1985); and temperature (Barnes, 1965). Organisms (Chitale, 1986; Klappa, 1979; Phillips et al., 1987) also aid in the precipitation of pedogenic carbonate. Fungal filaments (Figure 5.13) and fungal mats (Figure 5.14), for example, have been seen in pedogenic calcrete in southwestern North America (Stadelman, 1994). These organic remains gradually undergo a diagenesis into macerals of coal, including vitrinite and intertinite (Chitale, 1986).

74

Chapter 5

Figure 5.13 Replacement of a fungal filament with calcium carbonate, from Pyramid Lake, Nevada (see Stadelman, 1994 for site context). The micrograph was taken with secondary electrons.

Figure 5.14 Calcified fungal mat, from Hanaupah Canyon Alluvial Fan, Death Valley (see Stadelman, 1989 for site context). The micrograph was taken with secondary electrons. Not all calcretes form by downward percolation. Calcretes may precipitate through the interaction of different sources of water, around streams or coasts (Jacobson et al., 1988; Mann and Horwitz, 1979; Watts, 1980). In these setting, carbonate is precipitated from groundwater (Gardner, 1972) or through from floodwaters (Goudie, 1983). Figure 5.15 shows a massive calcrete that is likely the result of ground water action.

Carbonate Crusts

75

Figure 5.15 A massive calcrete duricmst that is at least 4 meters thick on the south side of Tempe Butte, central Arizona. Periodic inundation by ground water probably deposited calcrete around talus, plugging all of the pore spaces. Periods of ground water deposition m wetter periods of the Pleistocene were separated by phases of deposition of laminar calcrete (Figure 5.10) during dry periods. The railroad tracks provide scale.

The vast majority of calcrete is never seen at the surface. Calcrete is seen as a subaerial rock coating, a carbonate crust, through the actions of two general types of processes. First, there are some who favor the hypothesis that laminate calcrete coatings are born at the surface. Second, calcrete coatings are exposed by erosion (Figure 5.16).

Figure 5.16. Calcrete crusts have formed around cobbles exposed in a gully-side wall on Hanaupah Canyon alluvial fan, Death Valley, California. Some of carbonate skins may have formed by the evaporation of soil water on the walls of the gully. The rock hammer provides scale.

The genesis of calcrete at the surface is controversial. Although most soil scientists favor subsurface precipitation, there is good evidence that calcrete crusts can form in the

76

Chapter 5

subaerial environment. For example, lichens aid in the precipitation of carbonate in places where water flows over plugged soils (Klappa, 1979). This produces a type of laminar calcrete. The shallow, lateral flow of water in soils can cement the walls of gullies, creating gully-bed calcretes that encrust cobbles (Figure 5.16). When pedogenic calcretes form duricrusts, the eroding ledges expose carbonate rock coatings (Figure 5.11 and 5.12). Subaerial exposure of calcrete occurs through soil erosion. In places where the calcrete forms a resistant layer that impedes erosion (Figure 5.11 and Figure 5.12), calcrete-encrusted boulders can be seen along the edges of a plateau (Figure 5.17). Where hillslopes are composed of bedrock, mass wasting exposes spall faces with carbonate crusts on the walls of rock fractures (Figure 5.18).

Figure 5.17. Aerial view of the eroding white edges OgaUala calcrete in west Texas in the vicinity of the Lake Ransom Canyon housing development, near Lubbock, Texas. The plateau on the upper left side of the image is sheltered from erosion, because of the presence of the Ogallala calcrete duricrust.

Figure 5.18. Eroded joint face, where laminar calcrete formed on the walls of the fracture in the subsurface, Tempe Butte, Arizona. The white spoon provides scale.

One of the more common ways that calcrete crusts are exposed at the surface is through the very slow erosion of landforms. The example explored here is the erosion of alluvial-fan deposits. Pedogenic carbonate forms first in the subsurface. This takes place, for example, under smooth desert pavements (Figure 5.19). Over tens of thousands of years, the foci

Carbonate Crusts

77

of alluvial deposition shifts. Depositional landforms then become erosional landforms, and gullies begin to erode into these ancient deposits.

Figure 5.19. Smooth desert pavement on Hanaupah Canyon alluvial fan, Death Valley, where cobbles are coated with manganiferous rock varnish. No pedogenic carbonate is seen at the surface, because gullies have not yet developed.

As gullies slowly erode into desert pavements (Figure 5.20), pedogenic calcrete and alluvial-fan cobbles mix. The result is a mixing of cobbles and the rubble of the eroding calcrete soil, creating a 'salt and pepper' appearance to cobbles coated with either dark rock varnish or bright carbonate crusts (Figure 5.21). Towards the edges of the gullies, the pedogenic calcrete forms ledges of carbonate crusts on gravel (Figure 5.22).

Figure 5.20. Aerial photograph of older sections of Hanaupah Canyon alluvial fan, Death Valley. Gullying has cut back into the desert pavement (arrows). The lighter colored material surrounding the desert pavement includes calcrete exposed by gully erosion.

78

Chapter 5

Figure 5.21. As desert pavements erode, the calcrete in the soil begins to break up into rubble and calcretecrusted cobbles are exposed at the surface of this eroding desert pavement (see left arrow in Figure 5.20). The image has a salt and pepper appearance, where the white fragments are pieces of calcrete rubble.

Figure 5.22. A calcrete cap about 50 cm thick, exposed by gullying of an old alluvial fan, near Van Hom, west Texas. The exposed cobbles have crusts of calcium carbonate.

Pedogenic carbonate crusts are not permanent features in the subaerial environment. Upon exposure, microorganisms often erode hollows into these exposed carbonate crusts (Verheye, 1986). Cyanobacteria and lichen burrow into exposed carbonate crusts throughout the Basin and Range of Nevada. Carbonic acid in precipitation and organic acids from lithobionts eventually dissolve carbonate crests from the tops of the cobbles and "lime beards" reform on the undersides of surface cobbles (Verheye, 1986). The time scale of dissolution of the exposed carbonate crust is not known. Certainly, the presence of calcrete rubble at the surface of eroded alluvial fans (Figures 5.21 and 5.22) indicates that rates of carbonate crust dissolution are slow. Similarly, the encrustation of Pleistocene tufa (Figure 5.1) indicates that dissolution is very slow in arid lands. Pedogenic carbonate and gypsum exposed in the Nasca lines of Peru (Figure 5.23) have not dissolved in the -2000 years since the geoglyphs were made (Clarkson, 1990) (Figure 5.23). Broken pieces of calcrete are similarly common within the cleared areas of undisturbed geoglyphs along the Colorado River (von Werlhof, 1989; von Werlhof,

Carbonate Crusts

79

1994). All of these examples are anecdotal, but they speak to the stability of carbonate crusts in desert regions.

Figure 5.23. Pedogeniccalcrete crusts and gypcretecoat someof the cobbles exposed in the Nasca lines of Peru. A pen provides scale.

5.5. Subaerial Rock Faces

Carbonate crusts form on rock surfaces in a wide variety of environments. In this section, I explore sodium carbonates as efflorescence, calcium carbonates in deserts, calcium carbonates in areas outside of deserts, dolomite crusts, and lastly carbonate crusts on worked stone. Sodium carbonates are temporary rock coatings because they are quite soluble. These salts are often seen as efflorescence derived from the evaporation of capillary moisture. Although these rock coatings are short-lived, they are very effective agents of rock weathering (Evans, 1970; Goudie, 1989; Smith and McAlister, 1986). Calcium carbonates have long been recognized as a subaerial coatings in deserts. Secondary precipitates of calcite were thought to protect rock surfaces in Egypt (Hume, 1925, p. 147-148). In the Valley of Fire of southern Nevada, in addition to case hardened sandstone, calcite-clay coatings form over rock varnish (Conca and Rossman, 1982). Protective crusts of calcite protect the Towel Creek Tuff, Cottonwood Basin, Arizona (Conca, 1985). Carbonate crusts in deserts can interdigitate with other types of rock coatings, including manganiferous rock varnish in Australia (Dragovich, 1986a) and South Africa (Butzer et al., 1979). Calcite layers are found in oxalate-rich crusts, and they also interlayer with gypsum crusts (Figure 5.24). Subaerial carbonate crusts may originate within rock fractures that are later exposed by erosion (Figure 5.18). In the high Canadian Arctic, carbonate crusts are precipitated in rock fractures under glaciers; then, with subsequent spalling of these fractures, crusts are exposed at the surface. These carbonate crusts do not survive long in the subaerial environment because they are "readily dissolved" (Watts, 1985, p. 169). Carbonate crusts can be found in periglacial regions, and not simply as relicts of past periglacial (Vogt and Del Valle, 1994) or subglacial precipitation (Watts, 1985). Collections from Kiirkevagge, Northern Scandinavia (Dixon et al., 1995) contain carbonatecrusts (Figure 5.25 and Table 5.1). The carbonate is interbedded with silicarich and iron-rich layers.

80

Chapter 5

Figure 5.24. Carbonate crust formed within fracture of lava flow in Akesu volcanic field, West Kunlun Mountains, Tibet. In this backscattered electron image, the darker, inner material is calcium sulfate with abundant pore spaces. The outer layer is a less porous calcium carbonate.

Figure 5.25. Carbonate crust deposited on top of a schist clast from K/irkevagge, Northem Scandinavia (Dixon et al., 1995). The line indicates the location of the electron microprobe transect presented in Table 5.1. The uppermost part of the crust is a silica glaze (Chapter 13), and there is a pocket of iron film (Chapter 9) where the material is brighter. However, most of the rock coating is composed of calcium carbonate.

Precipitates of calcium carbonate are also found limestone in tropical regions, for example Puerto Rico (Monroe, 1976). These carbonate crusts may play a role in the case-hardening of limestone surfaces in the wet tropics (Trudgill, 1976). One of the great mysteries in sedimentary geology is why dolomite (magnesium carbonate) is so abundant in Proterozoic carbonate strata when dolomite/limestone ratios were about 3:1 (Garrels and Mackenzie, 1971; Holland, 1984), while dolomite has only been precipitated from sea water in recent times under very specialized circumstances (Budd, 1997). There is growing recognition that bacteria may be the key to natural massive dolomite formation today and in the distant past (Casconcelos et al., 1995; Folk, 1993).

Carbonate Crusts

81

Dolomite precipitates today as rock coatings in urban settings (Caner and Seeley, 1981; Del Monte and Sabbioni, 1980; Rodriguez-Navarro et al., 1997). According to the "Urban Model" of dolomite formation (Rodriguez-Navarro et al., 1997), fungi and bacteria "alter kinetic barriers and provide active sites for dolomite nucleation (Rodriguez-Navarro et al., 1997, p. 1). Carbonate crusts of dolomite develop in natural, non-urban settings. Calcium and magnesium carbonates are found the walls of lava tube caves in Kauai, Hawai'i (Fyfe, 1996). Magnesium carbonates are found in soils. For example, dolomitic calcretes are found in the less indurated portions of South Australian soil profiles (Hutton and Dixon, 1981). Authigenic dolomite is also found as micron-sized minerals in saline soils in Alberta, Canada (Kohut et al., 1995). Table 5.1.

Carbonate crust deposited on top of a schist clast collected by J. Dixon from Karkevagge, Northern Scandinavia. The line in Figure 5.25 indicates the location of the electron microprobe transect, using a focused beam. See Dixon et al. (1995) for further details. /

'Na20 MgO

9

A120 3 SiO2

P205

SO3

K20

CaO

'TiO 2

Mn0

FeO

Total

0.07

0.07

1 . 9 7 '8'8.29 '0.00

0.02

0.60

0.04

0.07

0.05

0 . 2 3 '91.41

0.03

0.00

0.09

99.35 0.00

0.00

0.04

0.04

0.00

0.01

0.00

0.04

0.17

0.00

0.00

0.00

0.55

0.01

53.32 0 . 0 2

0.14

0.96

55.21

0.00

0.17

0.00

0.04

0.00

0.65

0.01

50.89 0 . 0 2

0.12

2.10

54.00

0.04

0.13

0.13

0.02

0.02

0.62

0.00

48.72 0.00

0.23

2.41

52.32

0.03

0.08

0.32

0.21

0.02

0.25

0.00

46.86 0.00

0.36

5.83

53.%

0.19

0.12

1.02

0.51

0.02

0.32

0.05

36.95 0.00

0.22

17.53 56.93 30.98 62.68

99.56

0.13

0.07

0.28

0.77

0.09

0.50

0.08

29.61 0 . 0 3

0.14

0.07

0.08

9.01

0.39

0.00

0.42

0.05

32.53 0.00

0.21

15.91 58.67

0.03

0.23

0.02

0.00

0.00

0.57

0.00

51.55 0.00

0.23

1.42

54.05

0.00

0.22

0.00

0.04

0.00

0.65

0.00

49.45 0.00

0.19

1.89

52.44

0.03

0.18

0.04

0.43

0.00

0.70

0.00

41.77 0.00

0.19

13.44 56.78

0.00

0.07

0.25

0.47

0.00

0.52

0.00

38.39 0 . 0 7

0.19

18.60 58.56

0.03

0.17

0.13

0.13

0.00

0.75

0.01

46.97 0.00

0.00

4.77

0.04

0.23

0.00

1.71

0.00

0.25

0.01

15.81 0.00

0.17

52.10 70.32

52.%

0.04

0.20

0.02

1.75 0.07

0.32

0.01

12.90 0.00

0.25

5 5 . 5 8 71.14

0.01

0.23

0.00

0.00

0.02

0.62

0.00

48.23 0.00

0.08

1.09

50.28

0.01

0.17

0.00

0.02

0.09

0.70

0.00

48.16 0.00

0.23

1.76

51.14

0.00

0.18

2.00

0.19

0.00

0.25

0.00

44.24 0.00

0.21

5.17

52.24

0.01

0.17

0.09

0.49

0.07

0.37

0.11

41.23 0 . 0 2

0.25

10.97 53.78

.,i

,.

,.

Dolomite forms carbonate crusts on natural subaerial surfaces. Figure 5.26 shows a rock face in the Mojave Desert containing lichens, rock varnish and carbonate crusts (Figure 5.27). Since lichens are suspected to be involved in dolomite formation on rock surfaces (Gehrmann and Krumbein, 1994), it may be that the lichens contribute to the formation of dolomite 'crusts.

82

Chapter 5

Figure 5.26. Rock face on top of Bird Spring Limestone, Providence Mountains, Mojave Desert, eastern California, showing a mixture of three types of rock coatings. The lightest gray coating is a crustose lichen in the genus Xanthoparmelia. The black coating is manganiferous rock varnish on silicified limestone. Whitish areas are carbonate crusts of dolomite and calcium carbonate (see Figure 5.27).

Figure 5.27. Backscattered electron image of dolomite and calcium-carbonate crust formed. The larger pieces of a few microns in length are fragments of the host limestone rock; they yield energy-dispersive Xray spectra with only calcium. The smaller and darker, micron-diameter pieces are magnesium carbonates.

Carbonate crusts grow on a variety of building materials, but most often limestone. Calcite is a component of black crusts in urban settings (Gorbushina et al., 1993; Rodriguez-Navarro et al., 1997; Urzi et al., 1992). In drier areas, calcium carbonate

Carbonate Crusts

83

forms crusts from the evaporation of carbonate-rich irrigation water (Figure 5.28; Table

5.2).

Figure 5.28. Backscattered electron microscope images of granitic tombstone in southeast Phoenix cemetery, where sprinklers watered the cemetery regularly. Note that there are two types of evaporite deposits: massive and finely laminated. At the very top there is a mound of microcolonial fungi that has been encrusted with calcium carbonate. The line indicates the location of electron microprobe measurements in Table 5.2. (Sample courtesy of Thad Waskelewicz)

Table 5.2. Carbonate crust formed on top of a granitic tombstone in southeast Phoenix cemetery. The chemical analyses were made along a transect indicated on Figure 5.28. Low probe totals for the carbonate reflect a combination of porosity, carbon (which is not measured) and the carbonate mineralogy. Na20 MgO A120 3 SiO2

P205

SO3

K20

CaO

TiO2 MnO

Fe20 3 BaO

23.98 0.12 0 . 0 0 0 . 6 1 0 . 0 8 54.34

0.39

2 . 2 1 3.16

21.61 0.30

1.52

0.36

0.46

2.09

31.53 0.37

1.37

0 . 3 9 29.33 0.30 0 . 0 6 0.90

0.57

3.18 0.19

11.53 0 . 9 2

2 . 9 7 0.06

0.38

2.59

0.07

1 . 4 1 0.04

0.01 0.11

2.70

0 . 5 1 5 . 3 9 0.50 0.30 0.00

1.72

41.63 0.00 0 . 0 0

0 . 0 5 47.73 0.00 0 . 0 0

Total

0.00

69.50

0 . 0 0 0.00

61.05

0 . 0 3 0 . 0 9 58.99

0 . 4 7 0 . 0 1 55.06 0.00 0 . 0 0

0 . 0 0 0 . 0 4 57.40

1.48 0.08 0.13 0.09

0 . 0 7 0.00

0 . 1 0 0.06

1.96 0.55 2.08 0.48

0 . 8 5 0 . 0 8 52.02 0.00 0 . 0 3

54.47 0.00 0 . 0 0

56.49

0 . 2 7 0 . 0 1 58.44

5.6. Carbonate Crusts and Greenhouse Warming This section explores connections between carbonate crusts and the greenhouse effect. T h e c a l c i u m a n d m a g n e s i u m c a r b o n a t e 'locked up' in the world's l i m e s t o n e s and d o l o m i t e s are a vivid testament to how the earth has maintains an even temperature for the past 3 billion years ( W a l k e r et al., 1981). W i t h the sun increasing in l u m i n o s i t y by s o m e 25% ( C a l d e i r a and Kastings, 1992), the draw d o w n of a t m o s p h e r i c c a r b o n

84

Chapter 5

dioxide has kept the Earth's temperature fairly even. The key process is chemical weathering of silicate minerals, by the net reactions: CO2+ CaSiO3 --> CaCO3 + SiO2 (CaSiO3 is a Ca-silicate, e.g. plagioclase) CO2+ MgSiO3 --> MgCO3 + SiO2 (MgSiO3 is a Mg-silicate, e.g. olivine). Silicate weathering creates a negative feedback. Temperature increases speed up chemical weathering and the draw down of carbon dioxide; this temperature decreases slows the draw down of carbon dioxide (Berner, 1993; Berner, 1995; Berner and Maasch, 1996; Brady, 1991; Brady, 1996; Brady and Caroll, 1994; Schwartzman and Volk, 1991; Schwartzman and Volk, 1992). Venus did not have this negative feedback operating. The great geological masses of limestone and dolomite represent carbon dioxide locked up over the earth's long history, preventing a runaway greenhouse effect. The speed at which the weathering negative feedback operates is on the order of hundreds of thousands of years (Brady, 1991; Brady and Caroll, 1994) and is not of immediate relevancy to the contemporary debate over fossil-fuel induced greenhouse wanning. Carbonate crusts at or near to the earth's surface represent carbon dioxide that is on its way elsewhere in the hydrologic cycle, whether it is to the oceans or back to the atmosphere. Carbonate crusts are readily deposited in a variety of freshwater, marine, soil, and even subaerial contexts described in this chapter. They are also easily dissolved. Thus, carbonate crusts represent only a temporary storage of carbon dioxide. An issue of relevance in the contemporary global carbon budget, although it has not yet been calculated or incorporated into carbon models, is the role of carbon dioxide stored in carbonate crusts. What is the mass of carbon dioxide stored in carbonate crusts? How fast are carbonate crusts being deposited? How fast are they being dissolved? Since rock coatings represent a direct atmosphere/lithosphere interface, the unknown answers to these questions are of potential importance in the debate over modem global warming.

85

Chapter 6

CASE

HARDENING

AGENTS

"I always take off my hat when I stop to speak to a stone cutter. Why? you ask me. Because I know that his is the only labor which is likely to endure. A score of centuries has not effaced the marks of the Greek's or Roman's chisel." Dr. Oliver Wendell Homes, from a pamphlet World's Columbian Exposition, Exhibit of Barre Granite, 1893 (requoted in 1997 in Focus, volume 44 (1): 34)

6.1. Introduction Case hardening describes rock surfaces that have on outer shell that is more resistant to erosion than interior material. The key element of the research problem is that the interior and the exterior is the same rock type. The following is a general description of how case hardening appears in the field (Figure 6.1 and Figure 6.2): "Case hardening takes the form of red-brown coating concentrated at the surface, but extending several crystals deep beneath it , that is commonly developed on exposed surfaces in arid and semiarid lands. It is associated with projecting lips, is obviously more resistant than the unaltered rock, and for this reason, is widely referred to as case hardening." (Twidale, 1982, p. 297)

Figure 6.1. A sandstone rock shelter in southeastem Colorado is protected from erosion by case hardening produced by coatings of amorphous silica, calcium oxalate and clays. Without this induration, the sandstone would erode much more rapidly. The foot ruler provides scale.

86

Chapter 6

Figure 6.2. A limestone block from the Paran talus flatiron sequence in Israel (Gerson, 1982) has been case hardened by a combination of calcite, clay minerals and silica glaze. Without the protection from the case hardening, the inside of the boulder has eroded.

Two general types of processes create a case-hardened rock: core softening of the interior; and case hardening of the exterior. Lithology appears to be important in whether case hardening or core softening causes the differential weathering. As a generalization, "crystalline rocks tend to core-soften whereas clastic materials caseharden (Conca, 1985, p. 204)." Case hardening, by definition, is not a rock coating; it is an alteration of the host rock. So why is it included as a chapter in this book? There are three reasons. In some contexts, ingredients are added to the host rock from rock coatings. In other contexts, rock coatings themselves act to case harden the host rock. Finally, the study of rock coatings should not be decoupled from the understanding processes involved in the redistribution of materials within the host rock only a few microns beneath the coating. The chapter is divided into two sections. First, I outline the characteristics of casehardened rocks. The characteristics of case-hardened surfaces are subdivided into three sections: environments where they have been noted; composition; and rates of formation. In the second section of the chapter, I discuss possible origins of case hardening. The focus throughout this chapter is distinguishing material added as an accretion from material derived from the host rock.

6.2.

Characteristics

6.2.1. Environmental Settings

6.2.1.1. Subaerial Desert Exposures Case-hardened rocks, which have been called "protecting barks" (Wilhelmy, 1964), are found on exposed surfaces throughout warm deserts. Case hardening has been noted in Australia, Egypt, Pakistan, southwestern United States, northern Africa, and central Asia (Bryan, 1922; Hobbs, 1917; Smith, 1988; Twidale, 1982; Wilhelmy, 1964) and elsewhere.

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87

6.2.1.2. Subsurface Origins Case-hardened rock faces are seen at the surface, but the processes that create a difference in resistance to erosion often originate in the subsurface. This is true near Prawle Point, southern England (Mottershead and Pye, 1994), where three processes are at work. First, greenschist hardens along joint faces within the subsurface. Silica and aluminum comprise the bulk of the case-hardening agent, but iron is also present. Second, erosion of the land surface brings joint faces to the surface. Third, erosion of the underlying rock creates cavities in the rock called tafoni. Tafoni are relevant because they highlight the case-hardened outer visors (Mottershead and Pye, 1994). The formation of case-hardened joints is not limited by climate. In Antarctica for example, iron-stained silica coatings, acting as a case-hardening agents, precipitate along internal joint planes (Conca, 1985). Joint surfaces in welded tufts (Figure 6.3) exemplify case hardening in subsurface positions in semiarid regions (Conca, 1985; Fuller and Sharp, 1992). In another example, granitic joints at Pikes Peak, Colorado, are indurated by a combination of iron oxides and silica (Merrill, 1906). The case hardening is noticeable when the remainder of the grussified grains erode away. Rock fissures in humid climates often experience induration (Wilhelmy, 1964).

Figure 6.3. Differential weathering of Bishop Tuff in the Mono Basin, eastem Califomia. Tafoni caverns eroded more rapidly than the case-hardened outer shells, because of spatial variations in the devitrification of the glassy matrix of the welded tuff (Conca, 1985). Meter-high Artemesia tridentada shrubs provide a rough scale. The tafoni visors are darkened from manganiferous rock varnish, oxalate, silica glaze and biofilm coatings.

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6.2.1.3. Associated with Carved Rock

Petroglyphs are frequently carved into planar joint surfaces (Cremaschi, 1992; Dorn, 1997; Grisafe, 1992; Renaud, 1936). In many cases, these joint surfaces are casehardened. For example, iron-rich material impregnates the upper few millimeters of a petroglyph site in Sweden (Swantesson, 1994). In other cases, the very surfaces of the petroglyph grooves have been case hardened in southern Nevada (Conca and Rossman, 1982) and in Alberta, Canada (Campbell, 1991). Black coatings in southwest Sweden also appear to play a role in hardening the rock surface (Sj0berg, 1994). Many petroglyph sites owe their very existence to the case hardening of joint faces on which the motifs are carved. Figure 6.4 and Figure 6.5 exemplify this for Whoop-up Canyon in Wyoming. The photograph (Figure 6.4) shows a remnant of the casehardened joint face on which engravings occur. The case hardening is from calcium oxalate that has penetrated into the interstices of the sandstone; oxalate also forms the bulk of the rock coating (Figure 6.5). The rock coating has three layers. A manganiferous rock varnish started to grow, but it was out competed by a thick layer of calcium oxalate. Lastly, there is an upper layer (about 100ktm thick) that is a mixture of oxalate, silica glaze and manganiferous rock varnish. This outer layer gives the rock face its dark color (Figure 6.4). Another example of the importance of case hardening in petroglyph preservation comes from Kaho'olawe Island, Hawai'i (Stasack et al., 1996; Stasack and Lee, 1993). Figure 6.6 shows a weathered basalt boulder that has eroded out of a lava flow. The very surface of the boulder is discolored from a combination of silica glaze and biofilms. Some of the silica glaze has migrated into the underlying basalt rock, creating a case-hardened shell (Figure 6.7) Almost all of the petroglyphs that I have encountered on sandstone are carved into case-hardened joint faces. The case-hardening processes are relict in many cases (e.g. Figure 6.8 and Figure 6.9). In these circumstances, it is distinctly possible that case hardening occurred before subaerial exposure. In other places, case hardening is an active process and can be seen on younger erosional spalls (Figure 6.10).

Figure 6.4. Photograph of a panel at Whoop-up Canyon, eastem Wyoming. The view from side to side is a couple of meters. The dark patch in the center is a remnant of the case-hardened joint face, into which petroglyphs have been carved. The darkening agent is a combination of manganiferous rock vamish and silica glaze (Figure 6.5). Lichens and fungi coat the rock on the right side of the photograph.

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Figure 6.5. Backscattered electron micrograph of a sample of a case-hardened joint face at Whoop-up Canyon, eastern Wyoming (Figure 6.4). The dark gray material at the bottom of the image are quartz grains. The quartz grains are surrounded by mostly calcium oxalate, case hardening the sandstone. Then, a rock coating has formed on top of the case-hardened joint face.

Figure 6.6. Photograph of a boulder hosting petroglyph K22 (Stasack et al., 1996) on Kaho'olawe Island, Hawai'i. The darkest areas on the boulder are biofilms. The brown portions of the boulder are coated with silica glaze. Underneath the sunglasses, on the left side, the boulder is eroding because of weathering of the underlying basalt.

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Figure 6.7. Backscattered electron micrograph of a sample from Figure 6.6. The very top of the image shows a Type I silica glaze of largely amorphous silica (see chapter 13). Underneath this rock coating is a mixture of basalt minerals and black pore spaces created by dissolution of these minerals. The middle-left hand portion of the micrograph shows a large patch of silica glaze that has filled in the pore spaces, thereby holding the weathered basalt together.

Figure 6.8. Petroglyph panel at the Medicine Lodge Site in Wyoming. All of the petroglyphs are carved into the joint face protected from erosion by a combination of carbonate crusts, dust films, and rock vamish.

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Figure 6. 9. Petroglyph panel at Nine-Mile-Canyon, Utah on a sandstone joint face that is protected from erosion by calcite, clays and iron creating a case-hardened surface. The surface of the joint face is also coated with rock varnish. The lower part of the image shows the development of weathering pits created when the case hardening is lost.

Carved rock is certainly not limited to petroglyphs. One of the world's most famous sculpted faces can be found at Petra in Jordan (Figure 1.3.). The Nabatean monuments contrast with the natural faces darkened by mostly rock varnish. Some of the hardness in the natural rock face is due to iron oxide that has precipitated within the void spaces in the host sandstone (Figure 6.11) (Paradise, 1993a).

6.2.1.4. Tropics Case hardening occurs within the tropics in a variety of circumstances. Iron-rich coatings harden boulders along streams in West Africa that are characterized by large seasonal differences in stage. One hypothesis is that the dry season promotes evaporation of iron oxides (Tricart, 1972).

Figure 6.10. This joint face in Petrified Forest National Park, Arizona is eroding just above the wavy-line petroglyph. The case hardening is from a combination of clay minerals and calcium carbonate filing the interstices of the sandstone and from manganiferous rock varnish. Note that within the circle of erosion there are two stages: an older scar that has started to be coated with visual accumulations of rock varnish and a younger scar with only micron-scale accretions.

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Figure 6.11. Backscattered electron image, and corresponding map, illustrating case hardening at Petra, Jordan (Figure 1.3). The dark gray pieces are quartz grains. Black areas are pore s p a c e s , and the white regions are iron hydroxides, adapted from Paradise (1993b).

Subaerial rock surfaces in tropical and subtropical locales frequently have protective finds where iron oxides precipitate in and amongst interstices of weathering rinds (Ollier, 1984). Examples can be found in southern Africa (Watson and Pye, 1985), Chad (Mainguet, 1972), East Africa (Wilhelmy, 1964), Nigeria and Mali (Schtilke, 1973). A case study can be found on the drier slopes of Kaho'olawe and Maui islands in Hawai'i, where polygonal cracks and gnamma pits dominate the surfaces of casehardened basalt boulders (Figure 6.12). These boulders are protected by an orangebrown glazes of mostly amorphous silica, as well as patches of manganiferous rock varnish and biofilms of fungi. Gnamma pits grow through a two-stage process. First, the host basalt dissolves, leaving pores in the weathering find. As the basalt dissolves, a silica glaze grows on the surface. The silica glaze helps protect the basalt from erosion (Figure 6.13). However, a point is reached where the inter-granular cohesion is exceeded and the basalt erodes, taking the silica glaze with it (Dora, 1995b). An example of the role of tropical silica glaze in case hardening can be found on the surfaces of even very young lava flows. Silica glaze on the 25-yr-old Mauna Ulu flow, Hawai'i (Figure 13.24) are associated with the lava flow retaining a glassy appearance (Figure 6.14). One explanation for the mutual occurrence of silica glaze and a glassy appearance might be that the silica glaze acts as a case-hardening agent.

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Figure 6.12. Basalt boulder at Pu'u Hipa (--280 m) on the west side of Maui, Hawai'i. The gnamma pits are coated with silica glaze (Figure 6.13), manganiferous rock varnish and biofilms of fungi. Scale is provided by the glasses.

Figure 6.13. Backscattered electron imagery of weathering associated with gnamma pitting of basalt from Maui Island, Hawaii. The host basalt undergoes weathering, as is evidenced by the abundant dissolution pores (black void spaces). The silica glaze on the surface of the rock protects the basalt from erosion until a threshold is reached and the intergranular cohesion is less than erosional stresses created by such agencies as rain splash (Dom, 1995b).

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Figure 6.14. Photograph of the surface of the ~25-yr-old Mauna Ulu flow, Hawai'i, Most of the silica glaze collected at --300 m has a golden yellow hue; however, a blue silica glaze also colors the flow. The electron micrograph in Figure 13.24 is from the same site.

6.2.1.5 Temperate Environments

Case-hardened surfaces have been studied in a wide variety of temperate environments, for example, in England, France and Morocco (Robinson and Williams, 1987; Robinson and Williams, 1992), Australia (Branagan, 1983; Dragovich, 1969), the Mediterranean islands of Corsica and Elba (Wilhelmy, 1964), coastal California, Wisconsin (Merrill, 1906), the southeastern United States (White, 1944), and many other locales. Case hardening tends to be highly localized in temperate environments. Consider Stone Mountain,Georgia (Figure 6.15), a granitic inselberg covered by several different types of rock coatings including biofilms, silica glazes, iron films, manganiferous rock varnishes, oxalate-rich crusts, and phosphate crusts. Iron films (see chapter 9) are found as rock coatings at the surface, but they also penetrate into the weathering rind of the rock (Figure 6.16). Iron films help hold in place loose pieces of the granodiorite.

Figure 6.15. The discoloration due to rock coatings can be seen when Stone Mountain, Georgia, is contrasted with historic stone carvings. In some cases, the rock coatings have migrated into the interior of the rock, providing a case hardening effect (Figure 6.16).

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Figure 6.16. Backscattered electron image of an iron film (see chapter 9) that has migrated into the interior of the weathering rind at Stone Mountain (see Figure 6.15). Quartz (darker gray) and feldspar (lighter gray) minerals are surrounded by the bright-colored iron. Although the total depth of penetration in this image is only --0.3 mm towards the lower left side, iron hydroxides migrate into pore spaces at least 2 mm beneath the surface.

6.2.1.6. Arctic and Alpine Environments

Periglacial regions host case-hardened rocks. Greenland (Washburn, 1969a), Baffin Island (Watts, 1979), Pikes Peak (Merrill, 1906), and Antarctica (Calkin and Cailleux, 1962; Conca, 1985; Conca and Astor, 1987; Selby, 1977; Weed and Ackert, 1986) display case hardening. Case hardening may play a role in the preservation of glacial polish (Washburn, 1969a). Figure 6.17 illustrates glacially-polish quartzite in the Bear River drainage of the Sierra Nevada. Iron films coat the surface, and they also penetrate into the weathering find, perhaps aiding in it resistance to erosion. (Figure 6.18)

Figure 6.17. Glacially-polished quartzite of the Bear River drainage, Sierra Nevada, Califomia. The polish has a slightly reddish hue due to the presence of iron films (Figure 6.18).

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Figure 6. 18. Backscattered electron image of glacially-polished quartzite, Bear River drainage, Sierra Nevada, California (see Figure 6.17). In this backscattered image the darker quartzite is fractured. The brighter material is iron hydroxides that have precipitated on the surface and have migrated into the interior of the weathering find, perhaps acting as a case hardening agent to help preserve the glacial polish.

Case hardening is only infrequently noted areas that are currently glaciated or recently deglaciated. This may be because differential weathering does not have enough time to make case-hardened rocks easy to spot. Tafoni and other clear evidence of differential weathering draw attention to case hardening. Yet, case hardening may still be an active process in glaciated topography. Silica (Hallet, 1975), iron and manganese (Andersen and Sollid, 1971; Whalley et al., 1990) are precipitated as rock coatings underneath glaciers, perhaps as an incipient part of the case hardening of glaciated rock faces.

6.2.2. Composition The study of case hardening is in its infancy. A variety of different substances may protect a rock face from erosion. Single key ingredients are sometimes isolated. In other circumstances, different types of materials are identified. All the while, there is no general theory to explain the variable composition of weathering finds, or why they vary from place to place. 6.2.2.1. Material Added to Weathering Rind

A plethora of different constituents have been isolated as case-hardening agents. This section presents a variety of studies, all pointing to the conclusion that much of the material found within the host rock has an external origin.

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The clay mineral kaolinite is a key ingredient in protecting the sandstones of Valley of Fire, southern Nevada, clays affect the appearance of the sandstone both in the field and in cross-section (Figure 6.19). Kaolinite is five times more abundant in the outer rind than in the sandstone itself. Calcite and the hydrated calcium borate, colemanite may play a role in hardening rock surfaces (Conca and Rossman, 1982).

Figure 6.19. Cross-section of the surface of the case-hardened Aztec Sandstone in Valley of Fire, southern Nevada, based on the observations of Conca (1985).

Calcium carbonate, by itself, has been isolated as an agent of case hardening (Bryan, 1926; Hobbs, 1917; Smith, 1988). Calcium carbonate may also combine with iron oxides (Smith, 1978), silica and iron oxides (Anderson, 1931; Fairbridge, 1968b; Hobbs, 1917; Merrill, 1906), or clay minerals (Conca and Rossman, 1982) to strengthen the outer shell of a rock. Iron-rich material can protect the outermost exposures (Oilier, 1984; Swantesson, 1994). Figure 6.11 illustrates the impregnation of sandstone at Petra, Jordan (Figure 1.3). Figure 6.16 illustrates how iron oxides hold granitic material in place at Stone Mountain, Georgia (Figure 6.15). Surface films of iron have penetrated into quartzite at the Bear River in the Sierra Nevada of California (Figure 6.18), and they may play a role in stabilizing the glacial polish (Figure 6.17). Iron-rich material case hardens clasts exposed to intermittent water flow (Tricart, 1972). Alpine regions frequently have iron films on rocks in melt water streams. Figure 1.15 illustrates iron films in the Wind River Mountains of Wyoming; some of the iron penetrates into the host rock, filling void spaces (Figure 6.20). Case-hardening also occurs along the shores of lakes with seasonal fluctuations. Mirror Lake in Yosemite, for example, dries out in drought periods, leaving behind a bathtub ring of rock coatings (Figure 6.21 and Figure 6.22). There is an important perceptual point. A lack of differential weathering does not necessarily imply that case hardening is lacking. Rates of shoreline erosion or cobble disintegration may be retarded by case-hardened cobbles. Simply because differential erosion is not visible does not mean that the outer skins of the rock are not hardened.

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Figure 6.20. Backscattered electron micrograph of iron that has penetrated into the host rock. The sample came from the Wind River Mountains where there is intermittent water flow (see Figure 1.15). While iron f'flms do also form rock coatings, the iron typically penetrates no more than a millimeter into the host rock.

Figure 6.21. Mirror Lake in Yosemite, Califomia, displays a water line of rock varnish and iron films as rock coatings.

Figure 6.22. Backscattered electron image of a case-hardened cobble on the shores of Mirror Lake (Figure 6.21). An orange iron films forms a very thin (<10 ~tm) coating on the surface. The iron hydroxides, which appear bright because of the higher atomic number, have penetrated to a depth of --0.1 ram. The iron appears to have impregnated the host rock material.

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The aforementioned examples of iron as a case-hardening agent suggest an external source. The penetration of iron into the host rock is usually less than a millimeter. These examples are characterized by the presence of iron films on top of finds casehardened by iron. Another example of a rock coating adding material to the weathering rind can be found with rock varnish. Manganese and iron hydroxides are the agents that cement clay minerals to rock surfaces, ultimately forming rock varnish (Potter and Rossman, 1977; Potter and Rossman, 1979a). At the same time, manganese and iron are often leached out of rock varnish (Dorn and Krinsley, 1991; Krinsley and Dorn, 1991). Figure 6.23 illustrates manganese and iron that has been redeposited within the weathering rind of a petroglyph sample. Thus, even if the original rock varnish has eroded away, manganese and iron can be found within the host rock as case-hardening agents (Kiersch, 1950; Merrill, 1906; Twidale and Bourne, 1975).

Figure 6.23. Backscattered electron image of a sample from a petroglyph in South Australia (Nobbs and Dom, 1993). The upper ~100pm is the rock vamish (chapter 10). Directly underneath the rock coating is the weathering find of the petroglyph; much of the weathering rind consists of black pore spaces and black areas of organic carbon. The bright material found between the dark gray silica is manganese and iron that has been reprecipitated within the pore spaces.

Manganese and iron are case-hardening agents where water flows over cliff faces. Black streaks on the red rocks of Sedona, Arizona (Figure 6.24) are more than just a rock coating. The manganese and iron that precipitates on the surface as rock coating can be found almost a millimeter within the host rock (Figure 6.25).

Figure 6.24. Waterflow rock coatings formed on the Supai Formation near Sedona, Arizona. Some of the black streams are composed of manganiferous rock varnish.

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Chapter 6

,

,i

.

.

.

.

.

.

.

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QuartzCementedby ManganiferousVamish

QuartzLoosely Cemented by Clay Minerals

Figure 6.25. Backscattered electron micrograph and corresponding map of waterflow coatings on the Supai Formation near Sedona, Arizona (Figure 6.24). The brighter manganiferous vamish can be seen on the surface and also penetrating into the sandstone.

Amorphous silica is commonly reported as a precipitate and a case-hardening agent within pore spaces in the outermost shell of rock faces (Branagan, 1983; Conca, 1985; Fairbridge, 1968b; Hobbs, 1917; Merrill, 1906; Robinson and Williams, 1987; Weed and Norton, 1991; Williams and Robinson, 1989). Examples are presented in Figures 6.6, 6.7, 6.12, 6.13 and Figure 6.14. Figure 6.26 illustrates a general model for the case hardening of sandstone with amorphous silica. .. '" """~t ,

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with amorphous silica, adapted from (Weed and Ackert, 1986). The model starts at the top with unaltered sandstone. Then, in the middle frame amorphous silica fills in the pore spaces and casehardens the sandstone. Lastly, a surface coating of silica glaze accretes.

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Calcium oxalate-rich crusts can act as a case-hardening agent. In the examples that I have seen, however, there is usually a combination of an oxalate-rich rock coating with some of the oxalate filling in pore spaces in the weathering rind (Figure 6.5). A cautionary note should be added here on the weathering role of some of these casehardening agents. While calcium carbonate can case harden some lithologies, there is also a literature on the role of calcium carbonate in the mechanical break up of desert clasts (P6w6 and Burt, 1978; P6w6 et al., 1981). Similarly, iron can aid in the physical weathering of rocks (Dixon et al., 1995; Pope et al., 1995). Thus, the precipitates found within the outer shell of a rock may be a double-edged sword with the effect ultimately determined by lithology (Conca, 1985) and perhaps environment.

6.2.2.2. Rock Coatings as Case Hardening Agents A wide variety of rock coatings may aid in case hardening a rock surface. For example, ferruginous coatings are thought to impede fluvial erosion in tropical settings (Tricart, 1972), as may brown stains in Namibia (Goudie, 1972). Glazes of mostly silica and aluminum with some iron, only 20-301am thick, impede erosion of greenschist in southern England (Mottershead and Pye, 1994). Iron-stained silica glaze in Antarctica "act as an induration agent to depths of a few centimeters (Conca, 1985, p. 119)." Dark coatings of charcoal, along with silica and oxides of Fe-Mn, case harden rock faces at Yarwondutta Rock, Australia (Twidale, 1982). Anthropogenic soot may also reduce weathering rates on the walls of a Namibian rock shelter (Selby, 1977). Oxalate crusts play a role in protective rock surfaces. Lichen-generated oxalates in Petra, Jordan, may be responsible for slowing erosion of the Roman Theater (Paradise, 1993b). On the island of Thasos, Greece, oxalate crusts retard the dissolution of carbonate (Doehne, 1994). Phosphate skins derived from bird guano protects rock faces in Australia. A transformation of the calcium carbonate into calcium phosphate and calcium pryophosphate stabilized rock paintings and the underlying host rock (MacLeod et al., 1995). Most of the literature on lithobionts concerns their role as agents of weathering and erosion. There is also a literature on the role of lithobionts as protective agents. In coastal areas, algae protect tafoni and alveoli surfaces (Mustoe, 1982). While lichens are erosional agents in drier climates, lichens may protect the underlying rock from erosion in wetter regions (Aborg et al., 1993; Krumbein, 1969; Viles, 1987a). The largest literature on the role of rock coatings as case-hardening agents can be found for "desert varnish" or rock varnish (chapter 9). Geologists dominated the varnish literature in the earlier part of the twentieth century, and an important concern was the role of varnish as a case-hardening agent (Ball, 1916; Jutson, 1914; Merrill, 1898). They thought that the same capillary waters carrying salts to the surface (to precipitate as desert varnish) resulted in core softening of the host rock. The varnish then acted as a "hard shell" over this weathered rock (Hobbs, 1919). Desert geomorphologists later in the twentieth century maintained the conclusion that rock varnishes harden soft rocks or weathered substrates (Butler and Mount, 1986; Butzer and Hansen, 1968; Daveau, 1966; Demangeot, 1971; Kiersch, 1950; Oberlander, 1977; Peel, 1960; Tricart, 1972; Tricart and Cailleaux, 1964; Wilhelmy, 1964). There is controversy over whether rock coatings less a tenth of a millimeter thick can inhibit erosion. The position has been advocated that "something more than a surface

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coating is needed..." (Bradley et al., 1979, p.121). The question is whether differential erosion is from the rock coating, from an unidentified case-hardening agent, or from core softening. As noted in the previous section, rock coatings can simply rest on a host rock that has already been case hardened. In this circumstances, it is not possible to decide which material is responsible for the greater resistance of erosion. However, evidence is presented here to indicate that silica glaze and rock varnish help inhibit erosion of the underlying rock. Silica glaze helps stabilize several different types of rock surfaces in HawaiT Recent lava flow surfaces are better maintained with silica glaze (Figure 6.14). Petroglyph sites (Figure 6.6 and Figure 6.7) and geomorphic features such as gnamma pits (Figure 6.12 and Figure 6.13) are preserved from erosion by silica glaze. The role of silica glaze can be striking for temperate sandstones (Robinson and Williams, 1994b, p. 382): "One of the most important characteristics of many porous sandstones is their tendency to case-harden owing to the development of a surface crust or find." When the sandstone is not particular well cemented to start with the increase in hardness provided by the silica glaze can result in the development of striking differentially-weathered landforms. Silica glaze in others settings can help stabilize rock surfaces. Figure 6.27 exemplifies a granitic surface in the Palm Springs area of southern California. The granite experienced extensive dissolution and fracturing associated with biotite expansion, leading to grussification. A relatively thin film of ,-50 micrometers is all that provides surface stability.

Figure 6.27. Backscattered electron image of granodiorite in the Palm Springs area. Although the weathering-rind of the granite is porous and fractured, grains are held in place by a -50~tm thick silica glaze. The sample was collected by Dr. Thad Wasklewicz.

The ability of silica glaze to stabilize certain rock surfaces can result in differential weathering. In Antarctica, silica glaze reduces permeability and channels moisture flux towards uncoated rock surfaces. Thus, weathering is concentrated away from the rock coatings. "Antarctica and arid regions in general have a greater variety and complexity of weathering forms because the relative absence of fluvial activity and the lower water budgets produce more complex moisture fluxes that act over longer time spans than in more humid regions." (Conca and Astor, 1987, p. 154)

I used to be suspicious of the idea that rock varnish alone could inhibit erosion. Yet, over the last fifteen years of field and microscope observations, I now believe it can. Figure 6.28 is one of many examples; it shows varnish on a boulder on the oldest glacial moraine at Bishop Creek, Sierra Nevada, eastern California. The host rock is

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extensively weathered, as is evidenced by the abundant porosity in the host rock. The rock varnish that has penetrated into the void spaces in the upper 401am case hardens the boulder. Figure 6.24 and 6.25 provide another example of very friable sandstone held together by rock varnish.

Figure 6.28. Backscattered electron image and map of a case-hardened boulder on the oldest glacial moraine at Bishop Creek, eastem Califomia.

Grussified granitic core stones are very common in the Sonoran and Mojave Deserts of North America. In most places only very thin accumulations of rock varnish are able to develop on these unstable substrates. An issue raised in the literature is whether these extremely thin coatings can have any protective value (Bradley et al., 1979, p.121). Figure 6.29 illustrates one of many examples I have observed of grus stability promoted by manganiferous rock varnish. The case hardening ability of rock varnish is exemplified further in Figure 6.30.

Figure 6.29. Backscattered electron image of a gmnodiorite core stone exposed on a pediment in east Mesa, Arizona (P4w6 and Burr, 1978). The jumble of loose weathered minerals in the weathering rind are held in place by rock varnish.

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Figure 6.30. Electron microscope imagery of rock varnish and the underlying weathering rind from Starvation Canyon alluvial fan, Death Valley. The correspondingmap illustrates that the weathering rind is more than twice as thick as the varnish. Yet, the varnish holds the weathering rind in place.

The weathering rind under rock varnish is often porous, as shown in Figure 6.30. And yet this boulder has not eroded in over a hundred thousand years (Tanzhuo Liu, personal communication, 1995). It is likely that the presence of manganiferous rock varnish stabilizes the surface of a rock.

6.2.2.3. Fused Rock as a Case Hardening Agent Fulgerites are normally found in soil fused by lightning. Yet, lightning can also fuse rock surfaces. These glassy crusts are not true rock coatings, since they are derived from the constituents of the host rock. Rather, they are a type of case hardening. Figure 6.31 illustrates an example of a fused crust from the top of SP Crater in the San Francisco Volcanic Field, northern Arizona. Fusion crusts would be another type of case hardening. They are found on meteorites. Typically brown to black, they are usually only a few microns in thickness. Fusion crusts are often weathered by terrestrial processes. They are one way that meteorites can be identified in the field (Buchwald, 1975; Haag and Haag, 1991).

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Figure 6.31. Scanning electron microscope image of a fused crust on a volcanic bomb on SP Crater of the San Francisco Volcanic Field, northern Arizona. The upper section is probably glass, with unfused minerals beneath.

6.2.3. Rates of Formation There is a considerable variety of opinion on how long it takes case hardening to form. Some argue for a start after only a month of atmospheric exposure (Kiersch, 1950). Early monographs also argue for a very rapid change in rock-surface hardness: "The induration sometimes takes place so rapidly that even an exposure of but a few months is sufficient to produce very marked results on freshly broken surfaces. This peculiarity of certain classes of rocks has long been known to quarrymen and stone workers, who recognize the fact that a well-seasoned stone yields much less readily under the chisel than one that is newly quarried (Merrill, 1906,p. 240)."

On the other hand, there are those who argue for slow rates of formation. For example, the development of case-hardened stream boulders in the tropics "seems to be a slow process (Tricart, 1972, p. 81)." In the Mojave Desert, ancient petroglyphs in the Valley of Fire area have "well-developed case-hardened crusts" (Conca and Rossman, 1982, p. 523). The relationship between hardening of the Bishop Tuff and its age of surface exposure was studied in the Mono Basin of eastern California (Conca, 1985). The evidence indicates that changes in hardness occur on the time scale of thousands to tens of thousands of years (Figure 6.32).

t/

- - rI

Thousands ofYe,ar8 Before l~e,sent

Figure 6.32. Hardening of the surfaces of Bishop Tuff in the Mono Basin over time. The diagram is adapted from Conca (1985, p. 171).

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Chapter 6

There are certainly many factors other than time that influence case hardening. Case hardening is in part dependent upon climate (Smith, 1978). The case hardening that originates in the subsurface is 'born' well developed when erosion first exposes a joint face. Lithology is also important (Conca, 1985), as are the type of case hardening agent (Conca and Rossman, 1982), the role of different rock coatings resting on top of the case-hardened rock, the inherited weathering, and also moisture retention within the weathering find.

6.3. Origin There is little controversy on whether biotic or abiotic processes are responsible for case hardening. There are only a few advocates for the perspective that organisms play an active role in hardening the rock. Algae may protect tafoni (Mustoe, 1982) and lichens protect rock surfaces in humid regions (Aborg et al., 1993; Krumbein, 1969; Viles, 1987a). Other than these perspectives, abiotic processes are felt by most to be the way that case hardening forms. Conca (1985, p. 8) wrote: "I have found no evidence for direct microbial mediation (in the formation of case hardening) in this study." I agree with Conca's observation. With the exception of microbial processes that build rock coatings, biotic agencies tend to soften the rock (Friedmann, 1980; Friedmann and Ocampo-Friedmann, 1984; Gehrmann et al., 1988; Gehrmann and Krumbein, 1994; Krumbein and Dyer, 1985; Krumbein and Urzi, 1993; Viles, 1995). There is controversy, however, on the origin of the materials that harden rock surfaces. There are two distinct schools of thought. Those in the minority contend that materials external to the hold rock art important. Those in the majority posit an internal (underlying rock) origin for case-hardening agents. The predominant perspective in textbooks and in the literature is that solutions of weathered material are mobilized from the rock, drawn out by evaporative stresses, and reprecipitated in the rock's outer shell (Garner, 1974; Merrill, 1906; Watson and Pye, 1985; Wilhelmy, 1964). The following quotes are typical of the literature. "At many sites the cementing agents added to the crust are derived from within the rock...Solution in water in the interior of the rock will be rapid, and deposition from solution will occur within the crust as water is drawn to the surface and evaporated...much of the secondary material deposited in the crust appears to be derived from a subcrustal zone, which becomes weak and loosely cemented (Williams and Robinson, 1989, p. 157)." Iron oxide case hardening is "presumably as a result of the leaching and illuviation of soluble salts from e surface to the weathering front, where they are concentrate... (Twidale, 1982, p. 299-300)"

Those that favor an internal source for the hardening-agents sometimes conclude that hardening occurs in the subsurface. For example, the weathering of iron-bearing alumino-silicate minerals in greenschist generated solutions that precipitated on the walls of joint faces in southern E n g l a n d - all processes taking place in the subsurface (Mottershead and Pye, 1994). The hardening may also be from relict geological processes. The concentration of kaolinite clays during ancient hydrothermal activity, for example, helped harden the faces of the Bishop Tuff (Figure 6.3) in eastern California (Conca, 1985). In another example, calcite added by sedimentary processes millions of years ago plays a key role

Case HardeningAgents

107

in the present-day induration seen in rhyolite in the Cottonwood Basin of Arizona (Conca, 1985). Only a few scientists have advocated an external source of case-hardening agents. The evaporation of fiver solutions (Tricart, 1972) and the eolian addition of kaolinite, calcium carbonate, and calcium borate (Conca and Rossman, 1982) exemplify the minority opinion. I join Tricart (1972) and Conca and Rossman (1982) in favoring an external origin. Virtually every instance where I examine case-hardened surfaces, there is good evidence for an external origin. Manganese (Figure 6.22, Figure 6. 23, Figure 6.25), silica (Figure 6.7, Figure 6.16, Figure 6.18, Figure 6.20), iron (Figure 6.7, Figure 6.16, Figure 6.18, Figure 6.20), and calcium oxalate (Figure 6.5) all migrate into the rock from external rock coatings. In the case of manganese and oxalate, the concentrations in the host rock are minimum. Thus, the simpler hypothesis is an external origin for at least these materials. The direction of future work on case hardening is uncertain. Most of the research is at the stage of analyzing case studies. This is certainly helpful Yet, the literature has reached the point where generalizations can lead to future discoveries and further insights. The difficulty is that there is a general paucity of theory building in the case hardening literature. There is no general theory for the composition of the material, the processes by which case-hardening agents are deposited, or the geography of casehardened rock faces. From the practical perspective of trying to understand the influence of the case-hardened rock on the rock coating, or visa versa, the lack of theory inhibits application of knowledge about case hardening to stone conservation. The only general theory on case hardening comes from Conca (1985, p. viii), who presented a lithologically-based theory to explain the occurrence of case hardening in clastic versus crystalline rocks: "Because of the overall differences in the intergranular bonding character between crystalline materials such as granite and clastic materials such as sandstone, the results of this study indicate that crystalline rocks tend to core-soften whereas clastic materials case-harden. Clastic materials will be affected by redistribution of secondary cements and greater accumulation at an interface can result in case hardening. In clastic rocks therefore, the hardness of different areas can either increase or decrease with time. On the other hand, a crystalline rock in a weathering environment will have its intergranular and intragranular bonds disrupted by chemical alternation. Spatial variations in disruption can result in core softening or case softening, but the hardness of all areas will decrease with time. Accumulation of secondary cements can often enhance differential effects in crystalline rocks but without case hardening the rock."

I agree with Conca's perspective. He illustrates clearly that the field of case hardening would benefit from the further development of theory.

108

Chapter 7

DUST

FILMS

"The answer, my friend, is blowing in the wind..." Joan Baez, Blowin' in the Wind

7.1. Introduction Dust accumulates in ocean sediment, in ice cores, on soils, and even forms entire landforms as loess. Dust is transported and deposited by wind in all global environments (Brimhall et al., 1988; Cahill and Braaten, 1986; Dowdeswell and White, 1995; Gerson and Amit, 1987; Goudie, 1978; P6csi, 1990; P6w6 et al., 1981; Pye, 1987; Pye, 1989). Dust erosion, transport and deposition are readily observable phenomena in the climate literature (Brazel, 1989). Dust accumulates rapidly enough to be a measured (Goudie, 1995; P6w6 et al., 1981). Dust composed of kaolinite clays and calcite coat plants in deserts (Conca, 1985). It is, therefore, surprising that there are relatively few references to or research on dust films on rock surfaces. The purpose of this chapter is to bring together knowledge on dust films on rock surfaces. This is a comparatively short chapter, because little has been written on the topic. What little has been written is focused on deserts. The discussion of characteristics and origins is a combination of literature material and new data. By the end of this chapter, I hope that the reader realizes that the topic is ripe for a more comprehensive study.

7.2.

Characteristics

7.2.1. Environmental Settings Dust films have been recognized in the literature on desert geomorphology (Blake, 1905; Goossens, 1994; Hobbs, 1917; Rivard et al., 1992; Walther, 1891; Walther, 1912; Zhu et al., 1985). However, the literature is dominated by scattered observations of a few sentences simply documenting the occurrence of dust films. They have not been characterized in any detail. I have similarly found dust films in different deserts. Figure 7.1 through Figure 7.4 illustrate light brown dust films from central Australia, the Sinai Peninsula, Tibetan Plateau, and the Atacama Desert in northern Chile. The sampling sites were all

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109

subaerial rock surfaces where the dust film sits directly on top of the host rock; in other words, there are no other types of rock coatings between the dust film and the rock. These films survived transport without any special attachment procedures, such as the application of epoxy in the field. These examples are, however, anecdotal; they only verify prior observations that dust films form in different deserts. In most cases, the particles have a subparallel orientation (Figures 7.1-7.4). The texture of these dust films consists of clay-sized particles deposited as an accretion on the host rock. The most detailed research on dust films has been conducted on accumulations within rock fractures (Coud6-Gaussen, 1989; Coud6-Gaussen et al., 1984; Villa et al., 1995). Villa et al. (1995) presented a general model where the dust within rock fractures is analogous to a cumulic soil and coined the term fissuresol. Figure 7.5 presents a generalized interpretive model of dust films adhering to the walls of a rock fracture in a miniature catena or fissuresol.

Figure 7.1. Backscattered electron microscope image of a dust film on an underhang at Undoolya Gap near Alice Springs, Northern Territory, Australia. Note how the clay particles are deposited with a subparallel orientation on top of quart (dark gray) and plagioclase (bright gray) minerals. The image has a height of 100 microns.

Figure 7.2. Backscattered electron microscope image of a dust film on a vertical rock face of a bedrock hillslope, about 20 km south of Eilat in the Sinai Peninsula. Even though the host rock is highly weathered (porous), there is no transition between the weathered rock and the clay film.

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Figure 7.3. Backscattered electron microscope image of a dust film on a glacial moraine of the West Kunlun Mountains, Tibet. Bits of silt-sized rock material are trapped within a matrix of sub-micron sized clay particles.

Figure 7.4. Backscattered electron microscope image of dust film on a geoglyph in Northem Chile. Note that the clay particles are not oriented with respect to the underlying quartzite rock. The scale bar is 10 lam, or about the thickness of the dust film.

Probably the largest amount of scholarship on dust films relates to the accumulation of aerosols on stones in urban settings. In this literature dust films are sometimes called soiling, and they contribute to the weathering and erosion of building stone (Whalley et al., 1992). For example, airborne particles create black crusts that are heated differentially compared to adjacent lighter-colored uncoated rock; differences in temperature lead to thermal stresses and the erosion of stone (Warke et al., 1996).

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111

Minisol Catena erosion of weathered material Zone E and prior dust accumulation, a 'residual' minisol Zone C accumulation of eolian fines, weathered material eroded from zone E, and some material from rock beneath I cm

Zone M translocation of micelles, mixing with weathering from joint walls, from mostly capillary water Figure 7.5. Generalized model of dust that has accumulated within a rock fracture in the Sonoran Desert. In the zone of erosion (E), dust and weathered material washes down into a rock crevice. In the cumulative zone (C), the fines fill up the fracture. In the narrowest part of the rock fracture, micelles in zone M slowly migrate into the narrow rock crevice and attach to fracture walls.

A case study on the role of dust on stone monuments is the Taj Mahal (Sharma and Gupta, 1993). During the dry season, dust storms from the Thar desert result in the dry deposition of corrosive materials. Added to natural dust sources are coal-based particles, automobile fumes, and air pollution from urbanization in and around Agra. The suspended particle matter includes both soluble and insoluble materials. Sulfur dioxide is particularly harmful, especially in combination with heavy metals (e.g. Pb, Mn, Zn) that act as catalysts for the conversion of SO2 to SO3. One effect of the dust is to yellow the surface of the marble. Dust films may undergo diagenesis over time and may interact with the host rock surface, exemplified by a study of tombs at Beni Hasan, Egypt (Wilson-Yang and Burns, 1987; Wilson-Yang and Bums, 1988). Dust blown into the tombs is mostly silica; yet the dust films in the tomb are composed of a variety of constituents, including calcium carbonate, silica, phosphate and chloride. Carbon dioxide in the atmosphere combines with hygroscopic water, which together acidifies the dust film. The acidified dust film then traps water that decays the host murals. Dust films grow on rocks on ancient geological surfaces, and there is a general perception in the geological record that clay coatings imply aridity (Mathison and McPherson, 1991; Van Couvering and Miller, 1969). However, these films are also found in humid contexts, such as Mt. St. Helens. Thus the occurrence of dust films in the geological record can present a false signal of aridity (Harris and Van Couvering, 1995). The eolian biome is where life is dependent upon wind for both nutrients and colonizers (Swan, 1992). Glaciers exemplify eolian environments. In these settings,

112

Chapter 7

dust films on glacier surfaces provide a fertile environment for the growth of epilithic algae (Tazaki et al., 1994). Rock coatings of dust may also be present in extraterrestrial settings. There was early speculation that ferric oxides dust particles settle on Martian rocks m giving rocks a reddish appearance (Arvidson et al., 1983; Christensen, 1986; Sharp and Malin, 1984). The existence of dust films on Mars was supported by observations from Sojourner (JPL, 1997). On our moon, melted iron glass coats mineral grains, gradually altering the appearance of the lunar surface (Clark and Johnson, 1996). In summary, dust films are most abundant in arid regions. Yet, they are not limited to arid rock surfaces. They also occur in urban, archaeological, geological, glaciological, and extraterrestrial settings. Unfortunately, there is no clear understanding of the frequency of these coatings or their geographic controls.

7.2.2. Composition Dust films vary considerably in their composition. For example, urban airborne pollutants deposited on building surfaces in the United Kingdom contain pollen, fly ash, clay minerals, salts, and microorganisms (Whalley et al., 1992). In southern Nevada, in contrast, dust films are dominated by kaolinite clay and calcite (Conca, 1985). Dust films within the fissures of the Supai Formation from near Sedona, Arizona, contain an abundance of organic detritus, in addition to clays and quartz fragments (Villa et al., 1995). Quartz fragments likely derive from the host sandstone, but Villa et al. (1995) also determined that dust films show a relative enrichment in Ca, Mg, Na, and P as compared to the underlying sandstone. Figure 7.6 illustrates that considerable noise exists in the geochemistry of the Sedona dust films.

Sedona Dust 40.00

-

35.00

-

0

13 N a 2 0

30.00

-

0

A~

O AI203

25.00

-

20.00

(!

15.00 10.o0

.~r

o

X P205

o 0

XSO3

.=

~

..I-

~t-

5.00 o.oo

0

OK20

~

o Fe20a

40

50

60

70

80

90

100

SiO 2 Figure 7.6. Electron microprobe measurements of dust films, collected from rock fissures near Sedona, Arizona with a defocused beam of ~10 microns (Villa et al., 1995). This diagram illustrates the considerable chemical variability found in dust films. Silica in the form of qualaz and clay minerals is the largest component. Aluminum in clay minerals is the second dominant element. Other constituents vary considerably. The oxide weight percentages were normalized to 100%, because there was considerable porosity.

113

Dust Films

The dust of the Phoenix, Arizona, region has been studied intensively (Brazel, 1989; Prw6 et al., 1981). Dust storms are common during the 'monsoon season' and dust accumulates on rock surfaces. Papago Park is an urban inselbergs composed of small hills of fanglomerate. The fanglomerate hosts tafoni and an abundance of dust films (Figure 7.7). Like Sedona, the composition of the dust films varies considerably (Figure 7.8)

Figure 7.7. Photograph of a dust storm in the Phoenix area and a backscattered electron image of dust fragments found on the surface of Papago Park, Phoenix, Arizona. The dust fragments are loosely attached to the rock. They were preserved in sampling through the application of epoxy in the field.

Papago Park Dust 35.00 .................................................................... 30.00 25.00 -- O

0 0

O

A1203

rt

..~ 20.00 --

X

&

K20 CaO

0

0

X -

FeO Na20

O

MgO

r,/3

15.00--

ao

l~176176 5.00

g

0.

0.00 40.0 50.0 60.0 70.0 80.0 90.0 SiO 2

[ 100.0

Figure 7.8. Electron microprobe measurements of dust films, collected from the surfaces of rock faces at Papago Park, Phoenix Arizona. A defocused beam of about 10~tm was used. This diagram illustrates that silica is the dominant constituent, followed by aluminum. Potassium is contributed by dust particles of orthoclase (potassium-feldspar). The remaining elements vary considerably. The oxide weight percentages are normalized to 100%.

Chapter 7

114

Villa et al. (1995) examined the chemistry of dust films in rock fissures at different elevations in Death Valley, California. Dust films were only analyzed from quartzite. The dust within the fissures, in contrast, was composed of a wide variety of minor and trace elements (Table 7.1). Villa et al. (1995) concluded that the dust in these rock fractures did not derive from the host rock, because these elements are not found in the quartzite.

Table 7.1. Electron microprobe measurements of bulk chemistry of fine material in rock fissures in quartzite, Death Valley, California. Rock Fissure

Collection Site

Quartzite

Sea Level, Hanaupah Canyon Fan, Desert Scrub Vegetation Near Playa Margin

AI20 3 0.57 Na20 3.52% MgO SiO2 98.70 A1203 10.44% SiO2 Fe20 3 0.73 P205 2.47% SO3 CaO 4.20% K20 TiO2 0.72% MnO Fe20 3 5.37% BaO

pH of fissure fine material 9.8+0.7

,,,,

~1000 m, Panamint Range, Desert Scrub, on Crest of SiO2 98.56 Na20 2.08% MgO Hill, several kilometers from Playa A1203 25.49% SiO2 CaO 0.24 Fe20 3 1.20 P205 3.80% SO3 pH of fissure fine material 8.5+0.3 CaO 5.18% K20 TiO2 0.90% MnO Fe20 3 7.77% BaO --2000 m, Panamint Range, Juniper dwarf woodland, east-facing slope

pH of fissure fine material 8.0"20.5

4.27% 46.30% 0.38% 3.55% 0.11% 0.17%

SiO2 99.03 Fe20 3 0.97

Na20 1.80% A1203 33.10% P205 5.27% CaO 4.18% TiO2 0.97% Fe20 3 8.17%

MgO SiO2 SO3 K20 MnO BaO

3.87% 39.3% 0.29% 2.67% 0.21% 0.17%

A1203 0.37 SiO2 99.13 Fe20 3 0.50

Na20 1.22% MgO A1203 29.11% SiO2 P205 4.72% SO 3 CaO 3.94% K20 TiO2 0.85% MnO Fe20 3 7.90% BaO

3.52% 44.89% 0.31% 3.17% 0.25% O.12%

pH of fissure fine material 8.2.t.0.7

-3000 m, Panamint Range, Limber Bristle-cone Pine woodland, east-facing slope

3.41% 66.28% 0.27% 2.93% 0.10% 0.29%

,

,

Mineralogical data are lacking for dust films. Conca (1985) determined the kaolinite could cover plants in southern Nevada and that kaolinite was an important constituent of adjacent case hardened rock. Figure 7.9 shows kaolinite as a constituent of a clay film in central Arizona. However, the mineralogy of dust films has not yet been investigated in a systematic fashion.

Dust Films

115

Figure 7.9. Transmission electron microscope image of kaolinite (letter B) deposited on a grain of potassium feldspar (orthoclase) (letter A). The image is modifiedfrom Overson(1993).

7.2.3. Rates of Formation Dust is measured through a variety of means. Annual estimates of dust production for the entire earth range from ~100 million tons up to 5000 million tons. Rates of dust deposition are generally measured in arid and semi-arid areas; in these drylands, annual rates of deposition range from 5 tons to ~100 tons of dust per square kilometer (Goudie, 1995). Analyses of ice cores and marine sediment reveal that dust production has changed considerably over time (Boutron et al., 1994; Thompson et al., 1993; Yung et al., 1996). Unfortunately, no data are available in the literature on rates of dust accretion on rock surfaces. I present here three examples, demonstr/iting that dust films can form rapidly. Example number 1 comes from Tempe in central Arizona. Aboutfifteen minutes before a summer monsoon dust storm, a quarried rock face at Tempe Butte was cleaned with water and allowed to dry. The dust storm then arrived, but without precipitation. A fast-drying epoxy (cyanoacrylate) was placed on the rock surface after passage of the dust storm in order to preserve the material on the surface. Figure 7.10 exemplifies the resultant dust film. Example number 2 comes from a hut wall in the Khumbu region of Nepal. A strong katabatic wind deflated ~lust from a dry fiver bed containing glacial flour. The dust adhered to the wall of the hut that had been cleaned a few hours before by the abrasive action of a backpack. Figure 7.11 exemplifies the resultant dust film. Example number 3 comes from the island of Hawai'i. Wind-deflated dust picked up from the beach face can deposit on rock faces. Figure 7.12 exemplifies dust deposited on an olivine phenocryst in a host basalt rock. The deposition occurred in a single strong wind event.

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Figure 7.10. The backscattered electron microscopy image is of a dust film formed by a single monsoon dust form in central Arizona. The underlying mineral is plagioclase feldspar, and the dust particles range from orthoclase, to quartz, plagioclase, and clay particles.

Figure 7.11. Backscattered electron microscope image of a dust film formed in a single katabatic wind dust storm. The dust film formed on a hut at Dugla, Khumbu, Nepal. The underlying mineral is quartz, and the dust is composed of deflated glacial flour.

Dust Films

117

Figure 7.12. Backscattered electron microscope image of a dust f'dm formed in a single dust storm event at a beach in Hawai'i. The dust film has formed on an olivine phenocryst.

These examples only show that dust films can form rapidly. Also, dust films are easily removed, since they have an abrasive hardness of less than 2 and sometimes less than 1 on Moh's hardness scale. The rapidity of formation and the likelihood of rapid erosion suggests that dust films should not be used to indicate the antiquity of a surface. It also suggests that the characterization of a dust film at any given place may change over historic time scales. Thus, dust films may offer an opportunity to study air pollution.

7.3. Origin There are several hypotheses for the origin of dust films. One is that they are attached to rock surfaces through the physical force of wind. The "light dust powder which is beaten against the rocks by the strong winds, and becomes attached to the rough surfaces. In a short time, the rocks so affected become covered with a light dust coating... (Hume, 1925, p. 145)." A second hypothesis is that the dust films originate in rock crevices, and are exposed with spalling of a joint face. "Wind-blown deposits that have worked into fractures, including incipient detachment surfaces, become exposed when the slabs detach from bedrock (Rivard et al., 1992, p. 296)." There is some thought that brown coatings of dust on carbonate rocks in Israel are thought to have some relationship to microbial activity (Danin, 1985; Danin, 1986). Figure 7.13 presents an electron microscope image of one of these browned surfaces from Maktesh Ramon, Negev Desert, Israel. No microorganisms were seen, but this does not rule out the possible role of microorganisms that bloom periodically and might perhaps produce extracellular polymers to aid mineral adsorbtion. The physical force of the wind, inheritance of dust from a subsurface fracture and the role of epilithic organisms are not mutually exclusive explanations. Dust may adhere to rock surfaces through all of these processes. However, it is likely that other processes may play critical-limiting roles in the formation of clay films.

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Chapter 7

Figure 7.13. Secondary electron microscope image of a dust film formed on limestone at MakteshRamon, Negev Desert. The particles that adhere to the rock surface are largely inorganic, according to energy

dispersive and wavelengthdispersive analyses.

Van der Waals forces of attraction are thought to be responsible for the flocculation of colloidal particles. A problem is that Van der Waals forces have a strong distancedecay function. Yet, the large surface areas of the clay particles might provide enough van der Waals attraction to hold clay films together (van Olphen, 1963) on rock surfaces. Soil crusts may also provide a useful analogy for dust films, because the layered nature of many dust films presented here are similar to the layered nature of depositional soil crusts (Valentin and Bresson, 1992). There may be structural and chemical bonds that foster adherence, in a fashion similar to different types of soil crusts (Singer and Warrington, 1992; Sumner and Stewart, 1992; Valentin and Bresson, 1992). I believe that dust films may represent an incipient stage of development in a variety of other types of rock comings. Dust deposition is certainly important in the genesis of rock varnish (Allen, 1978; Dora and Oberlander, 1982; Moore and Elvidge, 1982; Potter and Rossman, 1977) (see chapter 10). Dust films may also be critical in the formation of silica glazes (see chapter 13; Figure 7.14).

Figure 7.14. Backscatteredelectron microscope image of a dust film formed on top of a 5 micron-thick silica glaze, all on a rock face of Papago Park, Phoenix, Arizona.

Dust Films

119

Figure 7.14 may be showing the incipient phase of the formation of a detrital-rich silica glaze; this is called a Type II silica glaze in chapter 13. There is a ten-micron thick layer of silica glaze, with a dust film that is in turn adsorbed onto the amorphous silica. The dust may contribute silica, or it may be incorporated into the silica glaze as detrital particles. In summary, very little is known about dust films. They are usually found in deserts, but they also occur on urban buildings in a variety of climates. Clay minerals are an important component and are probably critical in the adherence of dust films to rock surfaces, but the precise mode of formation is uncertain. It is ironic that dust films are the least understood rock coating, yet they may play a critical role in the genesis of other, more extensively studied rock coatings.

120

Chapter 8 HEAVY

METAL

SKINS

Adaptation of the a priori to the real world has no more originated from 'experience' than has adaptation of the fin of the fish to the properties o t w a t e r . (Lorenz, 1982, p.125)

8.1. Introduction The definition of a heavy metal varies. Some individuals specify that the metals must have a density five times greater than water (Garbarino et al., 1995). Others place the boundary at four (Connell and Miller, 1984) or specify atomic weights between 63.546 and 200.590 (Kennish, 1992). More broadly, the literature on heavy metals in the environment deals with metals of high atomic number such as manganese, iron, copper, lead, uranium, mercury and other potentially toxic elements. The focus of this chapter is on rock coatings that have heavy metals in concentrations that are measured in terms of element weight percent, not in parts per million. Such high levels are generally toxic (Connell and Miller, 1984; Wood, 1974), and since the mobility of heavy metal skins on rocks is not well understood, their potential to influence biological systems is uncertain. The purpose of this chapter is to synthesize available knowledge on heavy metal skins. The chapter is divided into two sections. The first section explores the dominant heavy metal skin found in natural systems, namely manganese skins. Then, in the second section, I turn to heavy metals skins where the metals derive from anthropogenic activities.

8.2. Manganese Skins The most abundant heavy metal skins are composed of iron and manganese. Iron films are so abundant that they are the focus of the next chapter (chapter 9) and are not considered here. Manganese is also a common constituent of a heavy metal skin, so abundant that it is subdivideA into two general types. One manganese coating is rock varnish; it not dominated by manganese, but by a mixture of clay minerals that are cemented to the rock by manganese and iron. In contrast, the manganese skins explored here have manganese as the dominant element. This section on manganese skins is subdivided into three topics: an analysis of the environments where manganese skins occur; the chemical composition; and possible modes of origin.

Heavy Metal Skins

121

8.2.1. Environmental Settings Manganese skins can be found in association with geological ore deposits (Galanopoulos, 1995; Heubner and Flohr, 1990; King et al., 1944; Spencer, 1991; Strakhov, 1967). In some cases, these veins are found at widths less than 0.1mm (Heubner and Flohr, 1990). With erosion of the overlying rock, these narrow ore deposits can mimic the appearance of manganese skins in terrestrial settings. Although processes responsible for the formation of some manganese ores may relate to the contemporary formation of manganese skins (Roy, 1981), the general topic of manganese ores is beyond the scope of this section. Other type of manganese deposits beyond the scope of this section are nodules in lakes and oceans (Dubinina, 1980) (Fewkes, 1976; Giovanoli et al., 1980; Glasby, 1977; Margolis and Glasby, 1973; Moore, W.S. 1981) and nodules in terrestrial wetlands and soils (Alhonen et al., 1974; Robbins et al., 1992; Taira et al., 1981). Although marine manganese nodules may be related to manganese precipitation on rocks in terrestrial settings (Bauman, 1976), and terrestrial nodules are very likely related through the activity of bacteria, a detailed examination of this literature is beyond the scope of this book. Terrestrial settings where rocks are coated by manganese skins are summarized in Table 8.1. Manganese skins form and are eroded in these environments. Some are natural settings, such as caves and active glacial systems. Others are human features, such as pipelines, that interface with natural processes wgere there are geochemical sinks.

Table 8.1. Terrestrial locations of manganese skins. Environment Caves Cobbles in soils Dendrites Glacial, Active Intertidal Pipelines Raised Beach Regolith, Equatorial and Tropical Regolith, Humid Subtropical

Spring, Cold Spring, Hot Streams, Alpine Streams, Arctic and Subarctic Streams, Glaciofluvial Streams, Humid Subtropical Streams, Humid Temperate Streams, Humid Tropical Streams, Monsoon Asia Streams, Pleistocene

References

(Hill, 1982; Moore, G.W. 1981; Potter and Rossman, 1979b) (Ha-mung, 1968; Khak-mun, 1966) (Chopard et al., 1991; Potter and Rossman, 1979b) (Potter and Rossman, 1979b) (Boul~gue et al., 1978) (Tyler and Marshall, 1967a, 1967b; Zapffe, 1931) (Hosking and Pisarski, 1955-1964) (Miicke and Okujeni, 1984; Nahon et al., 1985; S6a et al., 1994) (Weaver, 1978) (Hariya, 1980; Hariya and Kikuchi, 1964; Mustoe, 1981) (Hariya and Kikuchi, 1964; Savage, 1936) (Dom and Oberlander, 1982) (Koljonen et al., 1976; Ljunggren, 1951) (Carlson et al., 1978; Koljonen et al., 1976) (Ceding and Turner, 1982) (Nowlan, 1976a; Pouer and Rossman, 1979b; Robinson, 1993) (Alexandre and Lequarre, 1978; von Humboldt, 1812) (Bhatt and Bhat, 1980) (Cailleaux, 1965)

Although the focus of this book rests with terrestrial rock coatings, there is good evidence for extraterrestrial heavy metal skins. Ferric-oxide coatings are on Mars (JPL,

Chapter 8

122

1997). Iron-rich films also cover lunar regolith particles; the depositional process may be related to impact-produced vapor (Hapke et al., 1994). On Venus, there may be iron sulfide rock coatings from chemical weathering (Plaut, 1993). As planetary exploration continues, I anticipate that other heavy metal skins will be found on extraterrestrial rock surfaces. At that time, interest will probably increase in the study of terrestrial analogs. 8.2.2. Composition

8.2.2.1. Mineralogy The mineralogy of manganese skins varies considerably, and there may be a connection between manganese mineralogy and environment. Birnessite is favored in streams, beneath glaciers, and within caves (Moore, G.W. 1981; Potter and Rossman, 1979b). Manganese dendrites are composed of romanechite, hollandite or cryptomelane (Potter and Rossman, 1979b). Todorokite forms in intertidal areas where spring water mixes with sea water (Boul~gue et al., 1978). Phyllomanganates were found in a black crust on a stream cobble in an acid drainage stream, Pinal Creek, Arizona (Lind and Hem, 1993).

8.2.2.2. Chemistry Manganese skins are characterized by manganese as the dominant element. This contrasts with rock varnish (chapter 10), where manganese is not dominant but is in a mixture with clay minerals and iron. Table 8.2 and Table 8.3 present chemical analyses of manganese skins on stream cobbles. In both Virginia (Robinson, 1993) and the United Kingdom (Buckley, 1989), manganese comprises more than half of the measured elements. Also note that aluminum and silica are low, indicating that clays are not a major constituent of these manganese-rich coatings.

Table 8.2. Energy dispersive semi-quantitative analyses of manganese skins in streams in Virginia, normalized to 100% (Robinson, 1993). Values are elemental weight percent. While some of the stream coatings had chemistries similar to manganiferous rock varnish (with an abundance of Si, Al indicating the importance of clay minerals), these films are composed of more than half manganese with low concentrations of A1 and Si. i|

' Stream Cub Run Cub Run Goodwin Creek Swift Run Swift Run i

i

Na

Mg

A1

Si

K

Ca

Mn

Fe

0.5 0.1 1.0 2.3 1.8

1.3 0.2 1.1 0.4 0.1

5.7 1.9 4.7 4.5 1.4

6.1 3.2 5.3 2.8 3.4

1.0 0.1 0.6 0.6 0.1

9.3 9.4 5.2 3.5 3.5

66.4 58.4 61.7 78.3 69.9

9.7 26.7 20.3 7.6 19.8

ii

Heavy Metal Skins

123

Table 8.3. Energy dispersive semi-quantitative analyses of manganese skins in streams in the United Kingdom (Buckley, 1989). Values are elemental weight percent. Elements were not normalized to 100% and nd indicates below the limit of detection at-0.1%. ,

,

,

Stream ' North Wales-1 North Wales-3 Scotland-1 Cumbria-1 North Wales-5 North Wales-7

=,

A1 5.70 9.32 2.74 1.66 3.87 0.35

Si 0.35 0.60 0.74 0.23 0.77 0.48 i

,,

S 0.25 0.25 .11 nd 0.20 nd

Ca nd 0.20 1.01 nd nd 0.10

Mn Fe 29.40 4.40 17.83 8.80 27.57 11.30 21.09 14.36 30.64 0.89 47.93 nd

Other Co-1.00;Ce 0.88 Pb-0.30;Zn0.37" Ba 0.20 Zn 0.69; B~t 1.40 Co0.41; Pb3.00 Co 0.82; Cu 0.3~ Zn 0.28; Pb 15.27 Zn 25.67

ii

Francis (1921) is the parent of biological theories of formation of manganese-rich rock coatings. Francis (1921) studied the origin of black coatings of iron and manganese on rocks in and adjacent to tropical rainforest streams in Queensland, Australia (Figure 8.1 and 8.2). Table 8.4 presents analyses of a manganese skin collected in 1990 from Francis' field area. Manganese skins are often found on rocks in snow-melt streams (Figure 8.3 and Figure 8.4). Figure 8.5 presents a backscattered electron image of a manganese skin at the front of the Khumbu Glacier, Nepal. Note the clean boundary between the host rock and the brighter manganese skin; then, a silica glaze (Chapter 13) formed on top of the manganese skin.

Figure 8.1. Manganese skins on rocks in a stream in the Kin Kin District of Queensland, Australia.

Chapter8

124

Table 8.4. Microchemical variability in rock varnishes collected from stream in the Kin Kin District, Queensland, Australia. Analysis is by wavelength dispersive microprobe, using ZAF correction, 10 na, and a 10 micrometer spot size. Low probe totals are from porosity, hygroscopic water, and organic matter. Bid means below the limits of delectability. MgO

A120 3 SiO2

K20

CaO

TiO2

MnO

FeO

BaO

Total

0.09 bid

1 9 . 6 7 0.64 24.55 0.18

0.04 0.80

0.04 bid

bid bid

50.98 44.93

0.08 0.13

3.46 3.10

75.00 73.69

0.05 bid

2 0 . 7 3 0.72 20.57 0.21

0.10 bid

bid bid

bid bid

41.66 50.01

0.62 0.08

2.89 3.34

66.77 74.21

Figure 8.2. Backscattered electron microscope image of manganese skins in the same field area where Francis (1921) observed algae. Note the porous circular features, about 40 ~tm in diameter, that might be manganese casts of algae.

Figure 8.3. Manganese skins on cobbles on the side of the Khumbu Glacier, Nepal, at Dugla.

Heavy Metal Skins

125

Figure 8.4. Manganese skins on cobbles in a anow-melt intermittent stream on the Dana Plateau at -3300 m, Sierra Nevada, California.

Figure 8.5. Backscattered electron micrograph of a manganese skin covering a cobble in a snow-melt stream on the Dana Plateau, Sierra Nevada, Califomia (see Figure 8.4 for photo of the site). The black dots correspond to the chemical analyses in Table 8.5.

Table 8.5. Microchemical variability in a manganese skin collected from a snow-melt stream on the Dana Plateau, Sierra Nevada, Califomia (Figure 8.4). Black dots in Figure 8.5 correspond to these analyses (from top to bottom) by a wavelength dispersive microprobe, using ZAF, 10 na, and a 10 Ixm spot size. Low probe totals are from porosity, hygroscopic water, and organics. Na20 MgO

A1203 SiO2

P205

SO3

K20

CaO

TiO 2 MnO

Fe20 3 BaO

Total

0.15

0.90

1 . 6 2 2.46

0.21

0.00

0.20

0.84

0 . 4 8 35.90 4.85

2.13

49.74"'

0.23

0.95

2.25

3.19

0.21

0.10

0.20

0.85

0.60

38.39 5.79

2.36

55.12

0.16

0.86

2.21

3.49

0.23

0.02

0.19

0.91

0 . 7 2 35.51 7.28

2.08

53.66

126

Chapter 8

Manganese skins are also found in caves. Manganese, brought into the cave by water flow, is precipitated by bacteria (Peck, 1986). In thin skins on speleothems, microprobe analyses reveal that rod-shaped bacteria (Table 8.6) and cocci-shaped bacteria (Table 8.7) residues that are several tens of microns thick. There are also much thicker deposits of manganese in Jewel Cave, South Dakota (Hill, 1982). Table 8.6. Wavelength dispersive microprobe analyses of rod-shaped bacterial casts at Jewel Cave, South Dakota. Each analysis represents a different bacterium. Analyses were made with a focused beam (~21.tm) and at 10na. N a Mg, Si, AI, S, K, Ti, and Fe were below the limit of detection (<0.1%). Low probe totals are from porosity, hygroscopic water, and organic matter. P205

CaO

MnO

BaO

2.41 1.76 0.18 0.71 0.30 0.41 0.18 2.04 2.31 2.66

3.29 3.05 0.41 1.20 0.46 0.62 0.50 3.33 3.64 4.20

50.64 56.22 60.65 53.43 58.92 54.52 55.17 27.26 44.24 37.03

3.66 1.18 1.16 0.74 1.07 1.22 3.05 7.70 3.89 8.42

Table 8.7. Wavelength dispersive electron microprobe analyses of cocci-shaped bacteria in manganese deposits at Jewel Cave, South Dakota. Each analysis represents a different bacterium. Analyses were made with a focused beam (--21.tm) and at 10na. Although Na, Mg, Si, AI, S, K, and Ti were analyzed, they were below the limit of detection (<0.1% for each of these elements). Low probe totals are from porosity, hygroscopic water, organic matter. P205

CaO

MnO

Fe20 3

BaO

0.60

1.16

43.08

2.23

11.72

2.25

3.68

40.74

2.27

11.33

0.11

0.38

42.42

2.24

11.47

0.14

0.41

41.77

2.31

11.67

4.97

8.33

18.53

2.00

3.77

1.58

2.39

43.90

2.29

8.95

1.42

2.53

41.72

2.24

8.64

0.09

0.29

42.83

2.26

10.48

0.09

0.34

45.66

2.30

9.03

0.09

0.35

44.32

2.23

10.66

127

Heavy Metal Skins

Manganese skins are often found in regolith fractures in a variety of settings (Table 8.1). These fractures are seen at the earth's surface after erosion of the overlying material by natural and anthropogenic activity. The manganese skins can have a fairly even mixture of iron and manganese, but these are different from rock varnish in that they have low amounts of clay, as indicated by low amounts of aluminum and silica (Table 8.8). Figure 8.6 illustrates the backscattered-electron texture of one such regolith manganese skin.

Figure 8.6. Backscattered electron micrograph of a manganese skin covering fractures in a railroad cut near the battlefield of Gettysburg, Pennsylvania. The line indicates the location of the electron microprobe measurements in Table 8.8.

Table 8.8. Microchemical variability in a manganese skin collected from regolith near Gettysburg, Pennsylvania. The line in Figure 8.6 correspond with the approximate positions of analyses by a wavelength dispersive microprobe, using ZAF correction, 10 na, and a 5 micrometer spot size. Low probe totals are from porosity, hygroscopic water, and organic matter. Na20 MgO A120 3 SiO2

P205

SO3

K20

0.00

0.02

5 . 3 3 0.96

1.58

0.00

0105.... 0 . 1 7

0.00

0.33

7.65

9.48 2.52

0.00

0.49

1.80 1.79

0.04

0.08

5.95

0.00

0.03

11.28 1.35

0.00

0.03

7.48

0 . 0 2 0.14

CaO

TiO2 MnO 0.08

0.35 0.50

Fe203 BaO

Total

24.75 23.48 0.86

57.28

17.06 25.46

64.15

0.31

0 . 2 5 0 . 3 8 31.64 27.05 0 . 7 8 69.92

2.43

0.10

0 . 0 1 0 . 1 8 0 . 1 2 33.34 21.98 0 . 3 2

71.14

1.37 2.22

0.00

0.06

71.07

,ill ,11

0.21

0 . 0 5 33.46 25.43 0.76 H I

II,,llll

Chapter 8

128

In contrast with the Gettysburg example, manganese skins in regolith sometimes have much less iron than manganese. A road cut on Kauai (Figure 8.7) exposed manganese skins in basaltic regolith. Electron microprobe measurements on the manganese skins (Figure 8.8) also reveal very little alumina or silica (Table 8.9).

Figure 8.7. Photograph of road cut on Kauai, Hawai'i, exposing manganese skins in the basaltic regolith.

Figure 8.8. Backscattered electron micrograph of a manganese skin covering fractures in a roadcut on Kauai, Hawai'i. The dots indicate the location of electron microprobe measurements in Table 8.9.

Table 8.9. Microchemical variability in a manganese skin collected from a road cut in Kauai, Hawai'i. The dots in Figure 8.8 correspond with the approximate positions of analyses by wavelength dispersive microprobe, using ZAF correction, 10 na, and a 10 micrometer spot size. Low probe totals are from porosity, hygroscopic water, and organic matter. Na20 MgO

A120 3 SiO2

P205

SO3

K20

0.18

0.33

3.42

2.08

0.80

0.22

0.03

0.33

4.99

1 . 8 2 0.64

0.10

0.01

0.28

4.35

3.14

0.00

0.62

CaO

TiO2 MnO

Fe20 3 BaO

Total

0.11

1.93

0 . 0 7 23.87

9.75

43.42

0.06

2.17

0 . 0 5 29.54

11.05 0 . 7 7 51.55

0.06

2.20

0 . 0 2 27.84

9.82

0.66 0.82

49.16

Heavy Metal Skins

129

Relatively little is known about manganese deposits underneath glaciers. They may be formed by physicochemical mechanisms (Whalley et al., 1990). They have a birnessite mineralogy, similar to the manganese skins found in streams and caves (Potter and Rossman, 1979b). Manganese skins are frequently found at terminus of glaciers (Figure 8.9). In these contexts; it is difficult to know whether the skins were truly formed in a subglacial context. However, these few observations reveal that they can be dominated by manganese (Table 8.10).

Figure 8.9. Photograph of the terminus of the Khumbu Glacier, Nepal. Manganese skins are found directly adjacent to the glacier (Table 8.10 sample), along the slope of the terminal moraine, and in outwash streams.

Table 8.10. Microchemical variability in a manganese skin collected from the terminus of the Khumbu Glacier, Nepal (see Figure 8.9). Analyses were made by wavelength dispersive microprobe, using ZAF correction, 10 na, and a 5 micrometer spot size. Low probe totals are from porosity, hygroscopic water, and organic matter. MgO

AI20 3 SiO2

P205 SO3

K20

CaO

TiO2

MnO

Fe20 3 BaO

Total

1.09

5.35

1 2 . 5 8 0.82

0.10

1.18 2.63

0.35

61.80 3.37

0.12

89.39

0.33

4.59

3.04

1.81

0.12

0.88

2.97

0 . 1 3 67.65 6.66

0.13

88.31

0.48

5.76

7.62

2.04

0.00

0.92

1.23

0.15

0.14

85.15

0.85 7 . 6 5 0.55 6.86 0.76 8.67

62.82 3.99

11.08 0.64 0.12 0.99 2.17 0.40 54.63 10.26 0.18 88.97 8.73 2.11 0.07 0.76 2.77 0.35 56.23 11.80 0.26 90.49 9.82 2.11 0.45 0.69 3.08 0.33 61.90 8.16 0.41 96.38

Manganese skins are sometimes found interlayered with other rock comings. For example, analyses of manganese skins are presented in Chapter 12, because they interdigitate with oxalate-rich crusts formed on petroglyphs in eastern Wyoming (see Table 12.2 and Figure 12.3 in chapter 12).

130

Chapter 8

8.2.2.3. Scavenging of Other Heavy Metals One of the characteristics of manganese skins is that they have relatively high quantities of other heavy metals, due to the scavenging abilities of manganese oxides and hydroxides (Filipek et al., 1981; Jenne, 1968; Loganathan and Burau, 1973; Robinson, 1981; Saeki et al., 1995). However, the type of heavy metals that are scavenged varies considerably. Different stream systems appear to be able to scavenge different metals. In Maine streams, Mn scavenges a variety of elements, such as Ba, Cd, Co, Ni, TI, and Zn (Nowlan, 1976a). Streams in the Genesee Watershed, New York, scavenge mostly Cd and Ni (Whitney, 1981). In some streams in the U.K., lead was preferentially adsorbed to amounts of 15%; in others over a quarter of the coming was zinc; in still others Co, Ce, Ba, and Cu were present in varying amounts (Table 8.3) (Buckley, 1989). There are also differences when streams are compared with manganese skins in littoral settings. In an intertidal area in France, cobalt abundance reaches 1% in the skins, zinc 0.1 to 1.5% and uranium 100-250 parts per million (Boul~gue et al., 1978).

8.2.2.4. Rates of Formation Little is known about the rates of formation of manganese skins. Available data are largely based on a very few observations from historic contexts. Manganese skins can form in proglacial streams within twenty five years (Dora and Oberlander, 1982) and within a year in humid continental perennial streams (Carpenter and Hayes, 1980; Cerling and Turner, 1982). Because these types of observations are necessarily limited to historic contexts, these cases may provide a very biased perspective on how fast manganese skins form. Direct 4~ dating of K-Mn-oxides formed in regolith reveal that manganese skins can also be geologically ancient, forming over the Pleistocene and Tertiary (Vasconcelos et al., 1992). With such wide ranging estimates, a time signal would be very difficult to interpret just from the presence of manganese skins.

8.2.2.5. Morphology There are only a few observations on the physical forms of manganese skins. Colloform structures occur along streams in United Kingdom (Buckley, 1989). Spherical particles ranging from bacterial size (~ llam) to 701.tm botryoidal forms occur in streams in Virginia (Robinson, 1993). I should note, however, that the coatings with the botryoidal structures and bacterial casts contain more aluminum and silica, with essentially a chemistry similar to rock varnish (Robinson, 1993). My observations reveal that layered or laminar morphologies are common in both stream and regolith environments. Although I have observed botryoidal and colloform structures, Figure 8.10, Figure 8.11 and Figure 8.12 are more typical for the manganese skins that I have seen.

Heavy Metal Skins

131

Figure 8.10. Secondary electron microscope image of a lamellate (smooth) micromorphology for the manganese skin on an intermittent stream on the Dana Plateau, Sierra Nevada, Califomia (see Figure 8.5).

Figure 8.11. Secondary electron image of a lamellate micromorphology for a manganese skin from a Kauai, Hawai'i, regolith fracture exposed in a road cut (see Figure 8.7 and Figure 8.8).

Figure 8.12. Secondary electron image of a lameUate micromorphology for a manganese skin collected from a landslide in the Berkeley Hills, San Francisco Bay Area. The skin was deposited on top of chert.

132

Chapter 8

8.2.3. Origin

8.2.3.1. Biotic Hypotheses The parent of biological hypotheses for the origin of manganese skins is Francis (1921), who advocated the position that different types of organisms may aid in the formation of manganese skins on rocks in streams in Queensland. According to Francis, in the lichen-generated coatings "the iron manganese compounds are deposited in, or partly replace, the substance of the lichen thallus to form the black coating (Francis, 1921, p. 111)." More common, however, are 31am to 4 llxm thick coatings: "that may be the altered remains of an encrusting alga [Hildenbrandtia ] [see Figure 8.2] ... supported by the following facts: (a) correspondence in distribution; (b) comparability of thickness; (c) the presence of the cellular structure of the thallus of the alga in 50 percent of the examples of the black coating rendered transparent by hydrochloric acid (Francis, 1921, p. 112)."

Francis (1921, p. 114) tried to test the production of black stream coatings in fieldbased experiments, but "rocks were lost trace of through the action of floods." Biologic activity is the dominant explanation for the fixation of manganese, at least in places where the pH is neutral to acidic (Figure 8.13). This is because neutral and acidic pH values would inhibit the physico-chemical oxidation of manganese (Dubinina, 1980; Mustoe, 1981; Robinson, 1993; Schweisfurth et al., 1980; Uren and Leeper, 1978; Wolfe, 1964) and leave manganese in the mobile divalent state. Some sort of a biogeochemical barrier is necessary to explain the concentration of manganese on rock surfaces.

Figure 8.13. Eh-pH diagram for manganese oxides and carbonates. Note the extensive shaded area where manganese is divalent and mobile modified from Maynard (1983, p. 128).

Heavy Metal Skins

133

Of the different organisms capable of fixing manganese, bacteria are generally the most favored (Table 8.11). Figure 8.14 exemplifies filamentous bacteria that can be found on some manganese skin deposits. There are others, however, who find evidence for lichens, algae (Francis, 1921), and bryophytes (Ljunggren, 1953) as agents of manganese concentration in perennial streams and wetlands. Table 8.11 Environments and where microbial enhancement of manganese is favored. Environment Caves Ground water Lakes Pipelines Soils Springs, Cold Springs, Hot Streams Wetlands

References ' ,

,i,

(Moore, G.W. 1981" Peck, 1986) (Vandenabeele et al., 1992) (1)ubinina, 1980; Dubinina and Deryugina, 1971) (Meek et al., 1973; Tyler and Marshall, 1967b) (Aristovkaya and Zavarzin, 1971" Bolotina, 1976; Bromfield and David, 1978; Douka, 1980; van Veen, 1973) (Mustoe, 1981; Rheinheimer, 1980) (Hariya and Kikuchi, 1964) (Bhatt and Bhat, 1980; Robinson, 1993) (Ghiorse, 1984; Ghiorse and Ehrlich, 1992)

Figure 8.14. Bacterial filament growing on a manganese skin, from Crooked Creek, near Scuffletown, Kentucky. The upper dispersive X-ray analysis reveals a great enhancement of manganese around the filament, as opposed to the surrounding coating measured in the lower X-ray analysis. Note how the bacteria attaches itself to the manganese skin through the secretions every few tenths of a micron.

134

Chapter 8

8.2.3.2. Abiotic Hypotheses Abiotic explanations for the enhancement of manganese are generally associated with specific environments. For example, manganese skins may be from mixing of a freshwater spring and sea water in an intertidal zone in France (Boul~gue et al., 1978). The most popular abiotic explanation for manganese skins is that fixation occurs in streams, where there is a mixing of solutions with different pH-Eh. A common explanation invokes ground water mixing with overlying stream water (Filipek et al., 1981; Malmqvist et al., 1978; Nowlan, 1976b; Nowlan et al., 1983). Compelling arguments are made for the abiotic enhancement of manganese at pH values above 7.8, based on microscopic and spectroscopic observations at mineral surfaces (Junta and Hochella, 1994; Figure 8.15).

A

B

1

L

Figure 8.15. Two modes of manganese precipitation on mineral surfaces. Mode A is where precipitation starts along fractures and then grows to be fairly continuous over iron oxide surfaces. Mode B is where recipitation occurs only at fractures, for example breaks in plagioclase crystals, adapted from Junta and ochella (1994, p. 4996)

There are two growth styles for abiotic manganese skins, depending upon the substrate and how the initial precipitates form. One path is where a fairly continuous layer of manganese oxide forms on the iron oxides hematite and goethite; these protocystallites grow across the entire mineral surface. A second path is precipitation of manganese on broken step edges. Both styles of precipitation can be seen in the field.

8.2.3.3. Combination of Biotic and Abiotic Formation Some investigators favor a combination of abiotic and biotic mechanisms for manganese skins in streams (Robinson, 1993) and springs (Hariya, 1980). It may also be possible that abiotic and biotic precipitation mixes vertically; for example, iron films in proglacial streams may form first by inorganic precipitation. Then, manganese skins can develop on top of the iron (Carlson et al., 1978).

Heavy Metal Skins

135

8.3. Heavy metal skins as a mix of natural and anthropogenic factors

8.3.1. Introduction Humans are perhaps the most important agent of environmental change. One way to gauge the impact of humans on natural systems is to explore the impact of people on natural systems that are slow to change, such as rock coatings. Thus, an underlying theme of this section is the development of baseline knowledge to better understand the impact of humans on slow-to-change landscape geochemical systems such as rock coatings. Heavy metals in the natural environment are most frequently found as colloids and other particles associated with hydroxides, oxides, sulfides, clay minerals and organic matter (McMartin and Henderson, 1997; Zhou and Kot, 1995). Sediment composed of clay and silt usually has the highest levels of adsorbed metal (Connell and Miller, 1984; Jones and Jarvis, 1981; Lee, 1975; Schulthress and Huang, 1990). The vast majority of the literature on heavy metals in the natural environment examines its movement in soil, water and air natural biogeochemical systems (Ayras et al., 1997; Graf, 1985; Graf, 1996; Graf et al., 1991; Hallberg, 1979; Jones and Jarvis, 1981; Klinkhammer and Bender, 1981; Ledin and Pedersen, 1996; Newsome et al., 1997; Nimick and Moore, 1991). The heavy metals that move through hydrologic, pedogenic or atmospheric pathways are typically found in trace abundances. Health concerns, in contrast, occur where heavy metals are both biologically available and concentrated to levels that are toxic. This section examines heavy metals adsorbed onto rock surfaces that have been released into natural systems by anthropogenic activity. The metals may have come from a variety of sources, such as mine waste (Ledin and Pedersen, 1996), automobile exhausts (Bayard and Ter Haar, 1971; Motto et al., 1970; Patterson, 1980), machinery products, paint and ink, electroplating, textile mills, organic and inorganic chemicals, rubber manufacturing, iron and steel foundries, nonferrous metal foundries, leather processing, petroleum refining, and steam-generation power plants (Garbarino et al., 1995). Consider the decay of steel in automobiles where Cd, Cr, and Ni are commonly because of their excellent corrosion properties and shiny appearance. There are point sources and non-point sources of heavy metals. Point sources may be specific industrial and waste sludge sites, mine tailing, or domestic effluent. Nonpoint sources may be natural releases of heavy metals through weathering (Graf, 1985; Plouffe, 1995) or airborne fallout of volcanics, forest fire smoke, plant exudates or ocean spray (Kennish, 1992). Anthropogenic non-point sources may be airborne fallout of fossil fuel pollution or urban storm water runoff (ConneU and Miller, 1984). Regardless of the source of the heavy metals, the transport pathway immediately before deposition on rock surfaces is almost always water flow. Even if the heavy metal skin is deposited as a dust particle on a subaerial surface, water is necessary to mobilize the heavy metal prior to fixation in a rock coating (Connell and Miller, 1984; IwAin and Pedersen, 1996). In a general sense, metals are fixed onto rock surfaces from decreases in salinity, increases in redox potential, or increases in pH. Decreases in salinity reduce competition between cations like sodium and metals for binding sites. Whereas metals might be driven off into the water with high salinity, they would be adsorbed in low salinity. As redox potential increases, away from anoxic conditions and into aerobic locations, and as pH increases, metals will be in a bound state and will not be released into the water flow (Connell and Miller, 1984).

136

Chapter 8

An underlying theme of this section is that heavy metal skins form as a result of a combination of anthropogenic and natural processes. These mixed-origin skins are exemplified in three different contexts. First, lead-enriched heavy metal skins can form next to heavily-used roadways. Second, copper-enriched heavy metal skins grow on cobbles downstream from mine tailings. Lastly, heavy metal skins are associated with the corrosion of metals. These contexts are certainly not the only places where anthropogenic sources of heavy metals results in rock coatings. However, the literature on this topic is not extensive, and this section is the first systematic treatment of the topic.

8.3.2. Lead-enriched Heavy Metal Skins Lead contamination of the environment can come from many sources. Lead acetate trihydrate is in cotton dyes, metal coatings, paints, varnishes, pigment inks, and hair dyes. It is also in antifouling paints, waterproofing, and insecticides (National-InstituteHealth, 1997). Tetraethyl lead was introduced into gasoline in 1922 as an antiknock ingredient. The resultant global pollution has impacted areas as remote as Antarctica and Greenland (Boutron et al., 1994; Boutron and Patterson, 1986). My focus here is the lead contamination from gasoline exhaust emissions that are known to contaminate soils and sediment adjacent to roadways (Bayard and Ter Haar, 1971; Brinkmann, 1994; Motto et al., 1970; Newsome et al., 1997; Patterson, 1980). My observations along heavily-used roadways reveals that there is an enhancement of lead in some rock coatings. Limited sampling demonslrates that lead enrichment occurs in three types of rock coatings: phosphate skins (see chapter 11); iron skins (see chapter 9); and manganiferous rock varnish (see chapter 10). Figure 8.16 shows a phosphate skin, collected near the Pacific Coast Highway in California. Electron microprobe analyses reveal lead concentrations from 0.24% to 0.88%. Lead is known to combine with phosphates in soils and sediments (Nriagu, 1984). Hence, there may be an attraction between lead and natural phosphate skins.

Figure 8.16. Backscattered electron image of a phosphate skin formed over a plagioclase feldspar and a magnetite crystal. The thin line of bright material is where lead has been greatly enhanced. The enhancement is from below the limit of detection in microprobe measurements (0.09%) in adjacent minerals to values of lead oxide 0.24 to 0.88 in focused beam measurements of the bright line. The sample w a s collected from the Conejo volcanics along the Pacific Coast Highway, Ventura County, California.

Heavy Metal Skins

137

Lead also has an affinity for iron hydroxides. Figure 8.17 reveals an iron film that has been greatly enriched by lead. The source of the lead is probably gasoline exhaust from cars that have traveled on the adjacent Pacific Coast Highway. The lead enhancement was present in 3 of the 20 iron films studied.

Figure 8.17. Backscattered electron image of iron films, still unexposed within a rock fracture. The arrow points to the iron film that has lead concentrations below the limits of detection (0.09%). The brighter iron film had lead oxide concentrations ranging from 0.18 to 0.54 weight percent. The sample was collected from a rock face in the Conejo volcanics along the Pacific Coast Highway, Ventura County, Califomia.

The enhancement of lead in iron films is not limited to coastal areas. A roadcut in Richmond Virginia yielded lead-enriched iron films inside rock fractures (Figure 8.18). Lead-enriched iron films also occur as delicate rock coatings on the surfaces of road cuts (Figure 8.19). The explanation probably relates to the ability of iron hydroxides to preferentially adsorb lead (Benjamin, 1983; Benjamin and Leckie, 1981; Coston et al., 1995; Forbes et al., 1976).

Figure 8.18. Backscattered electron microscope image of an iron film, still within a rock fracture, where the entire bright line is a layer rich in lead. The spot size of the microprobe is too large to not include the surrounding quartz, but minimum PbO values ranged from 0.15 to 0.22%.

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Figure 8.19. Backscattered electron micrograph of lead-enriched iron film on a magnetite crystal. The sample came from a road cut near Williamsburg, Virginia. PbO values were as high as 0.24% for focused beam analyses.

Lead accumulates in rock varnishes and dust films on desert surfaces (see chapter 7). Electron microprobe profiles reveal that lead is a contaminant in the uppermost surfaces of rock varnishes, but these concentrations drop to background levels below the surface (Figure 8.20). One interpretation is that tetraethyl lead has contaminated the very surface of natural rock coatings that have formed since lead additives were introduced into gasoline in 1922. Lead oxide can also be a primary component of heavy-metal skins. The Acritani pillars in Venice, for example, have coatings that contain 57.6% lead (Fassina et al., 1993). Rock coatings with a white color on marble statues in Padua, Italy, contain from 25% to 86% lead (Fassina and Borsella, 1993). 8.3.3. Copper and Other Heavy Metal Skins Mining and mine tailings contribute heavy metals to fluvial systems (Graf et al., 1991). In more humid regions, heavy metal skins are used to assess levels geochemical contamination. In a study of a tributary of the Susquehanna River, Pennsylvania, for example, metal-hydroxide precipitates on rock surfaces show loads of iron, manganese, zinc, cobalt, nickel and copper (Breen and Gavin, 1997). In humid regions, turquoise colored films of copper may be a result of bacterial precipitation of abundant copper in a stream system (Robbins and Hayes, 1997). Copper mining in Arizona has resulted in heavy metals being contributed to stream systems; for the most part, the heavy metals are contained in fine-grained sediment

139

Heavy Metal Skins

(Graf et al., 1991). There are places where copper oxide has precipitated on the surfaces of rocks in combination with silicates (Figure 8.21). In other cases, the copper appears to have concentrated by evaporation (Figure 8.22). Zinc-rich coatings have also been found coating quartzite at Queen Creek, Arizona, downstream from mine tailings (Figure 8.23); the zinc oxide has migrates into the fractures and even into weathering pores within quartz grains. A similar migration into pore spaces has taken place for cadmium, zinc and lead. However, these heavy metals have moved into pore spaces within a rock coating, namely silica glaze (Figure 8.24). Figure 8.24 was collected from a stream system in southern Arizona that may have been contaminated by mining in Mexico and perhaps local tlrban runoff. There are other types of heavy metal skins that are a result of anthropogenic additions and natural fixation processes. For example, arsenic-rich coatings may be produced by bacterial mats (Wakao et al., 1988) or in association with iron hydroxides (Pierce and Moore, 1980). Other heavy metal skins are found throughout metropolitan areas where urban pollution interacts with rock surfaces.

Lead (Weight Percent) 0.1

0.2

0.3

I

#

I

Lead Spike I

.

__

Depth in Microns .

Backgronnd Lead .

i

-- F i s h e ~ ~Fishe~

Control

- - e - - P e r k e r Snake ---o--Parker S ~

Control

Figure 8.20. Electron microprobe analyses of lead in dust films and rock varnish formed on top of geoglyphs along the Colorado River (von Werlhof, 1989). The snake and fisherman geoglyphs only contain lead-enriched dust films, and hence the rock coatings may have only accumulated during the time when tetraethyl lead was added to gasoline. Rock varnishes formed on desert pavements to the side of these geoglyphs (controls) show lead enhancement only in the very surface layer.

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Figure 8.21. Backscattered electron microscope image of copper skins on limestone, collected by S. Gordon downstream of the tailings at Superior, Arizona. Note how the bright (copper oxide) particles are mixed with darker (silicate) components of the rock coating.

Figure 8.22. Backscattered electron microscope image of copper skins on limestone, collected by S. Gordon downstream of the tailings at Superior, Arizona. It is possible that these deposits were concentrated by evaporation of copper-bearing solutions.

Figure 8.23. Backscattered electron microscope image of a zinc heavy-metal skin on quartzite, Queen Creek, Arizona.

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141

Figure 8.24. Backscattered electron microscope image of heavy-metal migration into pore spaces in a silica glaze (see chapter 13). There are two components to this silica glaze. An outer silica glaze is porous, and the bright specks are precipitates of heavy metals. Wavelength dispersive electron microscope measurements reveal abundances of Zn, Cd, Cu and Pb that exceed 0.20%. An inner glaze is massive and the heavy metals have not migrated into or through the relatively dense coating. The sample was collected from the San Pedro River, Arizona.

8.3.4. Patina, Metal Corrosion and Rock Coatings A wide variety of coatings are related to the corrosion of metals. Consider the well used term in the archaeological literature, patina; in dictionaries patina refers to a thin layer of corrosion of copper or bronze, usually with a brown or a green color (Davies, 1993; Editors, 1997). The tarnishing of silver and the rusting of iron are other readily observable cases. A case could be made for corrosion being a coating. The oxidation of metals results in a thin film by surface diffusion of absorbed oxygen, producing films three to four monolayers thick (Chawla and Gupta, 1993). This is perhaps analogous to the formation of "green rusts" that are associated with steel corrosion and with natural iron hydroxides (Schwertmann and Fechter, 1994). The general topic of metal corrosion has an extensive literature that is beyond the scope of this book. There are places, however, where metal corrosion contributes to natural coatings on rocks. As noted earlier, metal corrosion injects heavy metals into aquatic, soil and eolian transport pathways. There are many places where corroded metal forms streaks over stonework, most certainly the result of the transport of heavy metals by runoff.

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The interface between metal corrosion and the subsequent redeposition of heavy metals on stone surfaces is a relatively unexplored topic. What follows are cases documenting that metal corrosion can contribute to the formation of heavy metal skins. The weathering of steel in old bridges produces stains on top of rock surfaces. Streaks of 'rust' found underneath corroded steel of a bridge in downtown Oakland forms skins over worked stone (Figure 8.25). In some cases, the heavy metal skins are in turn coated by other types of rock coatings (Figure 8.26).

Figure 8.25. Backscattered electron microscopy of an iron heavy-metal film formed over orthoclase feldspar. The heavy metal film was deposited on granitic material directly underneath a corroding bridge in Oakland, California. Semi-quantitative energy dispersive analyses reveal that the coating is over 80% iron with some manganese and zinc.

Figure 8.26 Backscattered electron microscopy of a rock coating underneath a -50-year-old bridge in Richmond, Virginia. The bright layer is mostly iron (with some manganese). The iron is a product of corrosion of the bridge steel. On top of the heavy metal skin is a crest of gypsum, a probable result of an interaction between sulfur and limestone.

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143

The aforementioned examples illustrate that heavy metal skins can form rapidly, at least when they are a product of corrosion in fairly moist climates. While the corrosion of metal gas cans may produce heavy metal skins within decades in Egypt (Eastes et al., 1988), time scales of thousands of years are needed for the formation of very thin heavy metal skins in the Negev Desert of Israel (Figure 8.27).

Figure 8.27 Backscattered electron microscopy of a heavy-metal coating that is the product of the longterm corrosion of slag. The slag, from Solomon's Copper Mine at Timna, Negev Desert Israel, was !ncorporated into desert pavements. Corrosion of the slag resulted in this reprecipitation of a mixture of iron and copper on quartz, according to energy dispersive X-ray analyses.

In summary, the first section of this chapter documents that manganese comprises the bulk of natural heavy metal skins, along with the topic of the next chapter, iron films. The second part of this chapter concerns heavy metals released into the environment by human activity. These contaminants accumulate in rock coatings, but through natural processes. These observations open the door to the use rock coatings as a pollution-prospecting tool. The usefulness of rock coatings to assess heavy-metal pollution rests in understanding two sets of processes: the reason for the biogeochemical barrier that created the rock coating; and the processes by which heavy metals are incorporated into the rock coating. Given the fight rock coating, it may be possible to detect an environmental hazard an early stage.

144

Chapter 9 IRON FILMS "It was found that ocherous scums...consisted mainly and in many places entirely of iron-precipitating organisms or their remains." (Harder,1919, p. 7)

9.1. Introduction Introductory chemistry teachers around the world all teach that rust is an example of iron oxidation. Introductory earth-science teachers around the world all teach that the orange-reddish coloration found on rocks is from the weathering of iron-bearing minerals, essentially little more than the oxidation of ferrous iron. My own interest in rock coatings started with iron films. I brought a specimen to the office hours of my introductory geology teacher, a world-renowned mineralogist. I asked about the reddish material on the surface. "Oh that," was his reply. "It's nothing more than a bit of iron oxidation. You need a good clean surface to look at the specimen." Whack! went the rock hammer, exposing a metamorphic fabric and the instructor went into great detail on the minerals. He never understood my question, or perhaps he thought the problem to be so simple as to be obvious. The answer dampened my interest in iron films, and thus I turned to rock varnish ~ the subject of chapter 10. The purpose of this chapter is to show that the reddish coloration on rocks, that provides so much color and beauty to rocky landscapes on Earth, and Mars as well, is not a solved problem. It is a very complex topic, and geography is important to the understanding of iron films that exist in so many different terrestrial settings (Table 9.1). Although iron films are ubiquitous, there is no consensus on their characteristics, on how they form, or even on where they form. There is no nomenclature for iron films, and no review paper has yet synthesized the literature on this ubiquitous phenomenon. An important objectives is for the reader to understand that not all iron films are alike; I will show here that they display a range of compositions, morphologies, and that the environment of formation greatly influences their development. I make no attempt to fie it all of the material together into a neat package. In fact, I hope that I leave you frustrated about the lack of a synthesis at the end of the chapter. This is because I do not think there is enough knowledge to even pretend to pull all of the loose threads together. The present state of knowledge is so poor that we need more well-controlled case studies, more research on specific types of iron films, and a better understanding of the interaction between mineralogy and environmental controls.

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Table 9.1. Summary of iron films found in different terrestrial weathering environments. .

.

.

.

.

.

.

.

.

... Environmental Se Artifacts

g

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

Characteristics ..... Selected References Iron oxides with colors that can vary (Goodwin, 1960) from yellow, to orange, to red, with considerable speculation on whether appearance relates to time ,i,H

Caves

ii

_

ii

Yellowish color

(White, 1976)

in

Orange to red color composed of clay minerals and iron oxides

(Kiersch, 1950)

Fractures of rocks mined for iron and other ores

Iron oxides lack clay minerals in these contexts

(Ostwald, 1992)

Grains in sand dunes

Contains hematite and clays

(Glennie, 1970; Norris, 1969; Walker, 1979)

The term'iron crust' is sometimes used to describe the surficial expression of massive laterites

(Beauvais and Colin, 1993; Brown et al., 1994; Biidel, 1982; Nahon, 1977)

Spring discharge on rocks

Iron oxides are commonly associated with bacteria

(Harder, 1919; Konhauser and Ferris, 1996)

Streams that are seasonally-wetted in tropics

Yellow, to orange, to red, to black colors from iron hydroxides

(Alexandre and Lequarre, 1978; Biidel, 1982)

Streams with a neutral pH

Yellow, to orange, to red color from iron (Chukhrov et al., 1973" hydroxides Ghiorse and Ehrlich, 1992)

Streams with acidic pH (e.g. mine drainage)

Yellow to red color from iron hydroxides

(Mallard, 1981" Singer and Stumm, 1970)

Subglacial bedrock

Rust-colored 'stains'

(Andersen and Sollid, 1971)

Subaerial rock surfaces in cold-dry deserts like Antarctica

Iron oxyhydroxides color the outer rim of (Claridge, 1965; Glasby et al., Fe-bearing host rock, such as dolerite, 1981; Johnston et al., 1984; and can be over a millimeter thick Selby, 1977; Weed and Ackert, 1986)

Subaerial rock surfaces in deserts

An accretion that contains clay minerals, and can be weak red, reddish brown, or dark gray in color

(Butzer and Hansen, 1968; Dragovich,1984; Dragovich, 1988a,b; Dragovich, 1994; Elvidge, 1979; White, 1990; Zhu et al., 1985)

Subaerial rock surfaces in different climates

Red to yellow-brown stains that derive from biotite weathering

(Conca and Rossman, 1985; Selby, 1977)

Subaerial rock surfaces in temperate humid settings

Reddish or yellow-brown accretions, often seen along joints before exposed to subaerial environment

(Smith and Magee, 1990)

Subaerial rocks in tropical settings

Orange to brown-colored iron coatings that may be millimeters up to 5 cm thick

(Biidel, 1982)

Underside deserts

Brown to red color contains iron oxides and clays minerals

(White, 1990)

Fractures deserts

in

rocks

Laterite duricrusts tropical environments

iiiiiii

of rocks ii i

in

in iiii

ii ii

ii

i

_

ii

ii

iii

i

ii

iiiiiii

i iiiiiiii

The purpose of this chapter is to to present the first synthesis on what is known about iron films. I organize the chapter into the characteristics of iron films in section 9.2. Then, I turn to speculation their on genesis in section 9.3.

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9.2.

Chapter 9

Characteristics

9.2.1. Environmental Settings Iron films are found in a wide variety of settings, and this is reported in the literature. Yet, in most cases, the focus of the cited paper is not on iron films. Thus, there is usually not enough information is available to say that certain films are truly different (or similar) to others. Thus, the first subsection on the characteristics of iron films organizes this mostly qualitative literature into the different environmental settings where iron films have been noted.

9.2.1.1. Artifacts and Other Human-Modified Stones Patina or patination is the general term for the surficial alteration of artifacts. Archaeologists have long recognized that this patination can consist of iron-rich films that may be a combination of in situ weathering and also the external precipitation of iron oxides such as goethite (Kelly, 1956; Viereck, 1964). There is also a view that yellow, brown and red patinas on artifacts may be the product of 'staining' by iron-rich material from the adjacent soils (Nadel and Gordon, 1993). Iron films are but one of many different types of surface coatings on stone monuments. Electron microprobe studies of tesserae from limestone mosaics at Paphos, Cyprus reveal coatings of iron hydroxides (Doehne, 1994), as due studies of urban stone monuments in Spain and Portugal (Schiavon, 1993). Iron films are also associated with rock engravings (Campbell, 1991, Dorn, 1997). Petroglyphs are often carved into iron films, and iron films in turn form over motifs (Figure 9.1).

Figure 9.1. Petroglyph in the Mojave Desert, Califomia that is carved into a coating of rock vamish, and in turn the engraving is coated with an iron film.

Iron Films

147

9.2.1.2. Fractures in Rocks

Iron films form within rock fractures, coating the sides of joints. These fracture films are then exposed at the surface by spalling. Iron films originating in fractures are very common and and occur in a variety of environments. As one might expect, iron-rich sediments have iron films. And iron oxides are often reported in rock fractures in weathered ferroginous sediments (Ostwald, 1992). The fact that iron films are common in iron-rich material could have played a role in explaining why nobody has studied iron films in a systematic fashion - - other than to simply note its occurrence. A case study of iron films in a cold, dry Arctic environment comes from Lindsay's Nunatak in Greenland. Within a basaltic boulder there was a zonation of iron films. An outer rim of a reddish-brown coating gave way to an inner section of the crevice, where the film had a greenish color (Koch et al., 1995). Iron oxyhydroxides form within fractures in schist in the cold and wet Arctic environment of Karkevagge, Northern Scandinavia (Dixon et al., 1995). The color of these films are dark brown to black, and they occur where the surface waters are acidic. Electron microscope imagery (Figure 9.2.) reveals that these films have a braided pattern; as the iron oxyhydroxides precipitate, they appear to be aiding in the mechanical weathering of schist (Dixon et al., 1995). Similar to Karkevagge, iron has migrated along fractures within a granite-porphyry in Antarctica (Ishimaru and Yoshikawa, 1995).

Figure 9.2. Iron film deposited in fractures in a schist clast from K/irkevagge, Northern Scandinavia (Dixon et al., 1995). In this backscattered electron microscope image, note how the brighter iron appears to have migrated into the underlying rock, spliuing apart the gray feldspar and quartz minerals. The sample was collected by J. Dixon.

The cobbles in intermittent streams in alpine regions are frequently stained on the surface with iron films. Yet, even in these surficial films, there are tendrils of

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anastomotosing iron films filling fractures the host rock (Figure 9.3). In other cases, iron films appear to be filling pores within the upper millimeter of the clast (Figures 9.4 and 9.5).

Figure 9.3. Iron film formed on and within a granodiorite clast, collected from Middle Creek, White Mountains, Nevada at an elevation of about 3100 m. The color of the film was a dark red (Munsell 2.5YR 3/6). In this backscatter image, the bright fdaments are the iron film and the dark areas are quartz.

Figure 9.4. Iron film on a granodiorite rock in an intermittent stream on the Dana Plateau, --3800 m, Sierra Nevada, California. The color of the film is reddish brown (Munsell 2.5 YR 4/4). In this backscatter image, note how the brighter iron film migrated into pore spaces between the plagioclase feldspars.

Iron films frequently deposit on the walls of fractures within rocks in temperate humid settings (Isherwood and Street, 1976). Throughout the Franciscan Formation in California, for example, films of iron are common (Figure 9.6). Iron films are found in association with manganese-rich accretions (Hein and Koski, 1987). In another example, iron films were found between the pressure-release shells of Stone Mountain, Georgia (Figure 9.7).

Iron Films

149

Figure 9.5. Iron film on quartzite clast in an intermittent stream in the Wind River Mountains, at about 2800 m, Wyoming. The color of the film was dusky red (MunseU 2.5 YR 3/2). One of the pockets illustrates a botryoidal texture suggestive of hematite.

Figure 9.6. Iron film deposited in chert in the Franciscan Formation, Marin Peninsula, central California. The color of the film was red (MunseU 2.5 YR 5/8). In this backscattered image, micron-sized f'daments of iron films appear to be migrating into the chert. The iron films penetrate through both the porous (plagioclase) and less porous (quartz) laost' minerals.

Iron films are ubiquitous in rock crevices (Figure 9.8) in desert regions (Kiersch, 1950). They develop when the rock crevice is still tightly closed (Figure 9.9). These iron films are similar to rock varnish, in that they are composeA of iron oxides and clay minerals, but they are not enriched manganese hydroxides like the rock varnish discussed in chapter 10.

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The iron films that form in rock crevices are quite significant to a variety of fields in many subtle ways. Geomorphologists interested in relating color changes to exposure age will find themselves confounded, because these iron films are inherited from a subsurface position. Stone conservators interested in preserving petroglyph panels or stone monuments may misinterpret an environmental context thinking that what is subaerial today is a phenomenon that is naturally rejuvenating, whereas the iron film may be able to form at the surface. Whereas the usual tendency is to think that coatings form where we see them, in many cases they do not.

Figure 9. 7. Iron film deposited within fractures of Stone Mountain, Georgia. The color of the film was reddish yellow (MunseU 5YR 7/8). In this backscattered image, the darker material is quartz. The brighter iron films have two phases: relatively pure iron oxyhydroxides that is brighter in the image. The slightly darker material is mixture of silica and iron that incorporates fragments of quartz within the iron film.

Figure 9.8. Photograph of a quartzite rock from Daylight Pass, Nevada, that was split apart with a rock hammer. The surface of the rock (to the right) is coated with manganese-rich rock varnish. However, the newly opened fractures have iron films of around 10% iron oxides by weight with the rest constituting elements in clay minerals.

Iron Films

151

Figure 9.9. Backscattered electron image of iron f'dm within a schist clast from the high shoreline of Silver Lake, Mojave Desert. When fractures are closed, iron films can penetrate into the rock, as seen by the bright material sandwiched between feldspar minerals.

9.2.1.3. Grain Coating on Sand

Grain coatings, or accretions on the perimeter of sand, silt and clay-sized particles, is a topic mostly beyond the scope of this monograph. I am concerned with coatings on rocks. However, there are extensive dune systems that are so heavily coated with iron films that the appearance turns the dunes from white to orange or red (White et al., 1997). The reddish coating on sand grains (Figure 9.10) is analogous to the iron films found in desert rock crevices and on the undersides of desert rocks, in that they are accretions of iron and clay minerals (EI-Baz and Prestel, 1980; Mahaney, 1996). There is a literature in desert geomorphology that explores why dunes rubify or redden (Gardner and Pye, 1981; McKee, 1982; Norris, 1969; Walker, 1979). The different hypotheses have been broken into four basic categories (Walden et al., 1996, p. 349): "i) the increasing age of the sands inland allowing greater time for weathering processes to develop the hematite coatings around quartz grains; ii) pre-existing red coatings are lost progressively due to abrasion during sand transport either as materials experience longer periods of abrasion or in zones of greater transport energy; iii) derivation from different sand source materials (either different concentrations of Fe-bearing minerals or residual cement coating); and iv) regional climatic gradients...providing a control on rates of weathering processes which generate the hematite coatings."

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Based on analysis of satellite imagery and mineral magnetic analyses, Walden et al. (1996) believe that dune color may be influenced by source material, climatic gradients, and abrasion during transport.

Figure 9.10. Two different views of iron films on the same orthoclase feldspar sand grain, collected from the Parker Dune Field, western Arizona. The secondary electron image on the left shows the topography of the grain, revealing how individual clay particles are attaching to the outside of the grain. The backscattered electron image on the right shows the same sand grain in cross-section. Because the coating has a lot of clay, it appears darker than the host potassium-rich feldspar. The iron is integrated into the clays, but there are bright specks in the grain coating comes from concentrations of iron oxides.

There are also other hypotheses in the literature on why dunes redden. v) Range fires may accelerate the reddening process, as heading accelerates the loss of water from ferric oxides (Jacobberger, 1990). vi) Fluctuations between arid and semi-arid conditions may redden dune sands (Achyutan and Rajaguru, 1993). vii) The abundance of clay within a dune system is important, at least in Australian continental dunes. Where there is an absence of clay, the precipitation of silica from water in the dunes leads to the formation of precipitated sheets of silica. Plates of clays accrete where clays are common. Interdigitation of these coatings can also occur (Pell and Chivas, 1995). There are relatively few links between the literature on rock coatings with the literature on grain coatings in soils. Yet, the literature on iron grain coatings in soils is extensive (Anderson et al., 1982; Cremaschi and Busacca, 1994; Cremaschi et al., 1990; Greenland and Mott, 1978; Hazel et al., 1949; Scheidegger et al., 1993; White and Walden, 1997; Zachara et al., 1995) and its relevance will be highlighted in section 9.3. I also think that it is important to note that future insights may be obtained by studies of organic macromolecules in stabilizing the iron oxide cements on soil grains (Ryan and Gschwend, 1990) and acidic streams (McKnight et al., 1992). Iron oxide coatings adsorb onto clay minerals (Jefferson et al., 1975; Ohtsubo et al., 1991; Saleh and Jones, 1984) all before iron-clay coatings reach a rock surface. It may also be possible to apply multispectral satellite-based techniques used for iron oxides in dunes (White et al., 1997) to mapping rock faces with digital imagery in the field.

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153

9.2.1.4. Springs Iron oxide films on rocks have long been associated with cold springs (Harder, 1919), which may be related to iron films in caves (Jones, 1994). They also occur on rocks at hot springs (Figure 9.11). Iron oxyhydroxides develop laminated coatings on volcanic clasts in a hydrothermal vent system of Papua New Guinea (Pichler and Dix, 1996).

Figure 9.11. Secondary electron view of an iron film on ignimbrite, from Mammoth Hot Springs, Yellowstone National Park, Wyoming. The smooth structure of the iron film contrasts with the rougher surface of the host rock, seen in the upper left hand comer.

Much of the literature on the genesis of iron films in hot springs has centered around the role of bacteria. In three hydrothermal systems in Iceland, bacteria appear to be intricately linked to the precipitation of amorphous ferric hydroxide and goethite (Konhauser and Ferris, 1996). Iron-rich capsules and gel-like grains are seen encrusting cells in transmission electron microscope imagery. Bacteria in terrestrial hydrothermal systems, such as those found at Crater Lake, Oregon, also create mats, and these bacteria appear to derive energy from the oxidation of ferrous iron (Dymond et al., 1989) m that is they are chemolithotrophic.

9.2.1.5. Streams Iron films (and manganese films) occur as rock coatings in natural streams that are both near-neutral in pH and acidic. While most of these iron films often have a redorange color, they may also appear dark brown to black in color. Iron films may be produced by subglacial water flow. Iron "stains" (Andersen and Sollid, 1971; Drewry, 1986; Whalley et al., 1990), along with silica glaze (HaUet, 1975) and carbonate crusts are associated with water flow under glaciers. Because iron stains were found on either side of manganiferous rock varnishes, Whalley et al. (1990) believe that slight changes in Eh/pH could have mobilized the manganese and not the iron, leading to the observed separation of iron from manganese. Iron films in tropical environments are often found on stream-side rocks that are seasonally wetted (Alexandre and Lequarre, 1978; Tricart, 1972; von Humboldt, 1812). In tropical waters, it is possible that some of the iron films may be a product of a previous environment, from when bedrock may have been covered by alluvium and

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soils. The mobilized iron encounters the less permeable bedrock and flows over rocks and through joints, precipitating in place (Alexandre and Lequarre, 1978). Iron oxyhydroxides are very common in high latitude and high altitude settings where waters are naturally acidic (Dixon et al., 1995). Dark brown to black in color, the iron films occur on the surfaces of rocks in Karkevagge, Sweden (Figure 9.12). Electron microprobe chemical analyses reveal that over 76-79% of Karkevagge coatings are iron, phosphorus, sulfur, aluminum and manganese all contribute less than 5% by weight. In another example, outwash streams from the Dana Glacier, Sierra Nevada, California have iron films that are dark red in color (Munsell 10R 3/6) and often underlain by a coating of mostly amorphous silica (Figure 9.13). Experimental studies have shown that iron oxides (goethite, hematite) have a strong adherence to quartz surfaces, due to the formation of chemical Fe-O-Si bonds (Scheidegger et al., 1993). It may be that the silica glaze helps the iron oxides adhere as a rock coating. A number of orange, red, brown and particularly yellow films also form on cobbles in streams that receive acidic effluent from mining operations (Herbert, 1995). The yellow color of these coatings has fostered the name 'yellow-boy'. Iron bacteria (Nealson, 1983) may be involved in the precipitation of iron in these settings.

Figure 9. 12. Iron film deposited on the surface of a schist clast from K~irkevagge, Northern Scandinavia (Dixon et al., 1995). Note how the iron covers the rough surface of the cobble in the lower magnification view (upper image). In the higher magnification view, subtle differences in texture and chemistry include less iron in the uppermost, slightly darker layer. The sample was collected by J. Dixon.

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155

Figure 9.13. Iron film on outwash of the Dana Glacier, Sierra Nevada, California. In this backscattered image, plagioclase feldspar is at the bottom, ovedain by a coating of silica glaze (see chapter 13), and in tum overlain by an iron film.

In summary, iron films are very common in a wide variety of stream settings. In most cases, there is not enough information provided to assess whether iron films in streams are similar in chemistry, texture or origin. Certainly, the stream conditions associated with acid-mine drainages are very different from subglacial water flows. Thus, future researchers and the reader should be very aware of the potential to inadvertently mix very different types of iron films.

9.2.1.6. Subaerial Rock Surfaces There is a general impression among earth scientists and archaeologists that subaerial iron films are simply and simplistically the product of the weathering of iron-rich minerals. Certainly, geological sources of iron accelerate the reddening of the surrounding rocks by providing a source for iron, that is then redistributed by surficial water flow. Banded iron formations (Cloud, 1973), for example, appear red because of the abundance of reprecipitated iron oxides. A close look at subaerial iron films reveals that these stains are in reality accretions on rock surfaces. Subaerial reddened rocks are common in alpine settings. Iron films are often found on glacially-polished bedrock (Figure 9.14). It is possible, however, the iron films seen today may have originated in a rock crevice (Figure 9.15; see also section 9.2.1.2. Fractures in Rocks).

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Figure 9.14. Iron fdm on glacially-polished quartzite, Bear River drainage, Sierra Nevada, California. In this backscattered image the darker quartzite is fractured, and some of the iron has migrated into the pore spaces. The sample was collected by A. James.

Figure 9.15. Bright iron films on a schist morainal boulder on the side of the Kala Pattar (~5400 m), Khumbu region of Nepal, imaged by backscattered electrons. Sometimes, iron films are seen at the surface, but the same vein of iron film dips under the surface and cuts through the schist.

Reddish brown (Munsell color 10R4/4) coatings are common on the surfaces of Antarctic dolerite rocks; these discolorations are typically given the name 'desert varnish' (Bockheim, 1979; Campbell and Claridge, 1992; Claridge, 1965; Glasby et al., 1981; Johnston et al., 1984; Selby, 1977; Weed and Ackert, 1986). In reality, these discolorations are composed of mostly iron oxides that are integrated into the host weathering finds. Thus, the Antarctic 'desert varnish' literature is, for the most part, not relevant to the rock varnish literature. It is more relevant to the literature on weathering find. There are, however, true iron films present in Antarctica (Glazovskaya, 1958; Glazovskaya, 1971). Antarctic stains occur as external coatings, as stains that appear to lessen from the outside to the inside of the rock, and as millimeter-scale weathering rims around iron-bearing mineral grains within the rock (Selby, 1977; Weed and Ackert, 1986). On granite-porphyry of the Thiel Mountains, inland Antarctica (Ishimaru and Yoshikawa, 1995), for example, the iron in hypersthene, biotite,

Iron Films

157

ilmenite and magnetite is weathered and limonite precipitates on the surface as an iron film. The endolithic microbial community in Antarctica (Friedmann, 1982; Friedmann and Ocampo-Friedmann, 1984) may play a role in the mobilization and enhancement of iron in sandstones. In particular, oxalates released by fungal hyphae may mobilize the iron from minerals and generate enhancements of up to eight times the iron concentrations found in the original sandstone (Weed and Norton, 1991). Lindsay's Nunatak in Greenland provides a cold-dry environment similar to Antarctica. Iron-rich coatings of hematite were found on a basaltic erratic weathered for at least the last 10,000 years (Koch et al., 1995). In contrast to Antarctica, the nature of the subaeral coating clearly differed from material within a protected crack. The iron films that occur in cold deserts can have a significant effect on the remote sensing of rock surfaces (Cloutis, 1992). Iron films are distinguished in studies of near infra-red reflectance spectra, opening the way for mapping of their subaerial extent in large areas of exposed bedrock in Arctic and Antarctic deserts. Iron films are found on subaerial surfaces in temperate settings. Ironically, despite large population centers with major universities, there is less written about temperate iron films than Antarctic iron films. Some literature on temperate iron films concerns their interdigitation with silica glaze (see section 9.2.1.8.) (Dora and Meek, 1995; Robinson and Williams, 1987). Subaerial iron films are common in warm deserts. Rock coatings in the Egyptian desert vary in color, including weak red (7.5-10R 4-5/2-4), dusky red (10R-2.5YR 3/23), red brown (2.5-5YR 2-5/2-4), and dark gray (SYR 3-4/1) (Butzer and Hansen, 1968, p. 204). In southwestern Jordan, iron oxide streaks run down the sides of sandstone inselbergs (Osbom and Duford, 1981). Many gibbers in the stony deserts of Australia have a "veneer of iron and silica rich material" (Twidale, 1970, p. 227), and Gobi in the Gobi desert have iron films of a mixture of iron oxides and clay minerals (Zhu et al., 1985). Iron films are also found on granitic outcrops in South Australia (Bradley et al., 1979). In the Pilbara of Western Australia, the oxidation of iron-beating minerals is also associated with grayish red (2.5YR4/2) and dull reddish brown (5YR5/4) surfaces (Dragovich, 1993b); while these coatings may appear like manganiferous rock varnish, they are composed of around 85% iron oxides attributed to contact with ferruginous sediments (Ostwald, 1992). Iron films in southern Tunisia (Drake et al., 1993), the Namib Desert (Besler, 1979), and the central Sahara (Haberland, 1975; Selby, 1977) are more common than manganiferous rock varnishes. In the Death Valley region of North America, iron-clay orange varnishes are the dominant rock coating that has formed during Holocene (Figure 9.16). Thus, a common types of rock coating formed in hyperarid deserts is a type of an iron film that is rich in clay minerals and is similar to the grain coatings found on sand grains. The subaerial rock surfaces where desert iron films form tend to be less stable than where manganiferous rock varnish grows (White, 1990; White, 1993b). For example, iron films accrete on porous rocks and massive crystalline limestones in Tunisia (Drake et al., 1993). In the Pilbara of Western Australia, iron films also occur on surfaces that are too unstable to form good coatings of manganiferous rock varnish (Dragovich, 1994). The importance of surface stability on the distribution of rock coatings in drylands was emphasized by Drake et al. (1993, p. 39): "It is evident that the main control on the distribution of rock vamish in southern Tunisia is the stability of the substrate over the time period required for varnish formation...The more stable a rock, the greaterthe amount of coating on its surface, both in terms of surface area and thickness. Stabilityis, however,a relativeterm and

158

Chapter 9 depends on the rate of varnish formation. Stability also operates at a number of different scales. The smallest scale is that of the mineral grain and its resistance to weathering. This is the critical factor in southem Tunisia and is controlled by microbial.~rocesses. At a larger scale, stability depends upon rock resistance to spalling, llais is controlled by susceptibility to fracturing, and may be promoted by chasmoendolithic lichens, salts, fires, water and temperature changes. The largest scale is that of the landform. This is controlled by such factors are the degree and rate of stream channel incision, which exposes fresh rock, while previously stable surfaces are abraded in the river channel and the rock coating is removed. Stability also depends on factors that control the frequency of landsliding and slumping. Microorganisms have no role at this scale."

Figure 9.16. Backscattered electron view of an iron-clay coating of orange vamish that has formed on a Holocene alluvial-fan surface on Hanaupah Canyon fan in Death Valley (Hooke and Dom, 1992).

There are conflicting thoughts on the role of desert iron films on the stabilization of rock surfaces. Microbial communities that live within the rock can accelerate erosion of weathering rinds; simply because subaerial iron films form faster than Mn-rich varnishes, they are more noticeable in places where rock surfaces are not stability (Drake et al., 1993). On the Swedish west coast, natural rock surfaces appear red due to iron impregnating the upper few millimeters of the rock, which can in turn destabilize rock surfaces (Swantesson, 1994). On the other hand, there is a perception in the geomorphic literature that iron films can have a case-hardening effect on the underlying rock (Kiersch, 1950; Talbot, 1910; Twidale, 1982). The role of iron oxides in case hardening a rock surface was emphasized by research in Petra, Jordan (Paradise, 1993a; Paradise, 1993b); figure 9.17 illustrates one variation of an iron film at Petra. Laterite duricrusts in tropical regions are sometimes given the name 'iron crust' (Beauvais and Colin, 1993; Brown et al., 1994; Btidel, 1982; Nahon, 1977). These iron accumulations are not rock coatings and are beyond the scope of this work. However, iron films are sometimes found on top of laterite iron crusts.

Iron Films

159

Figure 9.17. Backscattered (left) and secondarr (fight) electron microscope views of iron film on sandstone at Petra, Jordan. The sample was collected by Tom Paradise.

Orange to brown-colored iron films on laterites may only be a few millimeters thick or they may be up to 5 cm thick on top of bedrock. These iron films have been noted in central Australia and in the Sahara on sandstones, quartzites and granitic material. These iron coatings are distinct from other types of iron films in that they are much thicker and that they may be a relict of a past, more humid climate (Btidel, 1982).

9.2.1.7. Underside of Rocks

Iron films are ubiquitous on the undersides of cobbles in desert pavements. In the geographical and geological literatures, these are called orange-bottom varnishes (Engel and Sharp, 1958). 'Ground patina' is the term sometimes found in the archaeological literature (Begole, 1973; Childers and Minshall, 1980; Hayden, 1976). 'Rubification' is used by soil scientists (Helms et al., 1995). These iron films are typically less than 10 I.tm in thickness (Figure 9.18) and they are composed of mostly clay minerals (Potter and Rossman, 1977). The iron hydroxides (-10-15%) only provide the 'glue' to cement the clays to the rock. Climate is an important control on the color of iron films on the undersides of desert pavement cobbles. When time is kept constant by sampling these iron films on the --14,000 to 16,000-yr-old high shorelines of paleolakes of the Basin in Range in the Western U.S., the color reddens in warmer climates. On the shorelines of the colder Lake Lahontan at Pyramid Lake, the color is typically brown (Munsell 7.5YR 5/4 to 3/2). On the shorelines of a slightly warmer paleolake Owens in eastern California, the color is typically reddish yellow to yellowish red (Munsell 5YR 5/8). On the shorelines of Lake Manly in Death Valley and Lake Cochise in the Sonoran Desert, the color turns red (e.g. Munsell 10R page). The increasing red may have to do with a change from the iron oxide goethite to hematite, since higher temperatures and lower temperatures favor hematite over goethite (Torrent et al., 1982).

160

Chapter 9

Figure 9.18. Secondary electron images of orange-bottom varnish on the late Pleistocene high stand of Searles Lake (Smith, 1979). This was the thickest accretion seen. The structure appears flaky because most of the dominance of clay minerals (Potter and Rossman, 1977). The line shows the coating/rock contact as determined by energy-dispersive X-ray analyses.

9.2.1.8. Iron Films lnterdigitated With Other Rock Coatings Iron films frequently interfinger with rock varnish in rock crevices in desert environments. As noted in section 9.2.1.2. (Fractures in Rocks), iron films form within tightly closed rock joints (Figure 9.9). As erosion starts to open a joint fracture, however, iron films in deserts are often covered by rock varnish enriched in manganese (Figures 9.19 and 9.20).

Figure 9.19. Backscattered electron images of iron film within an opened rock crevice in a schist clast from the high shoreline of Silver Lake, Mojave Desert. As the fracture opened slightly: the iron film was coated with manganese-rich rock varnish. The lower left hand comer of the image ts organic matter, which appears black in backscattered electrons. Then, the bottom of the rock coating is the iron film, which has a dull gray appearance, due to the abundance of clay minerals. The overlying manganese-rich varnish has more manganese and less clay, giving it a brighter appearance.

Iron Films

161

Figure 9.20. Optical microscope photograph of a thin section of a rock crevice on a schist clast on the high shoreline of Silver Lake. Note how the black, surface layer (manganese-rich) varnish is coating the undedying iron f'dm that has a gray appearance in this photograph, but orange when seen in color.

In the literature on manganiferous rock varnish, there has been a long-standing observation that iron-rich layers are common underneath manganese-rich varnish (Engel, 1957; Engel and Sharp, 1958; Hobbs, 1917; Hooke et al., 1969). I have observed this most frequently on the planar surfaces that used to be inside a rock crevice. Thus, I conclude that the iron-rich material may have originated in a rock crevice as an iron film. Another common style of interdigitation is to encounter iron films interfaced with silica glazes (Robinson and Williams, 1992). As noted earlier, iron films can rest on top of silica glazes (Figure 9.13). It is common to see thin layers of iron films sandwiched between silica glazes and the underlying rock (Figures 9.21). Iron films can in turn be coated by silica glaze (Figure 9.22).

Figure 9.21. Backscattered electron image of the rock coating on a horse petroglyph at C6a in Portugal (Dom, 1997). The bright band is an iron film composed of --70% Fe20 3. It rests directly on the carved schist rock, and it is overlain by silica glaze.

162

Chapter9

Figure 9.22. Backscattered electron image of rock coating on glacial polish on andesite bedrock of Iztaccihuatl volcano, Mexico. This is one of the thickest iron f'dms I have encountered, at 0.5 millimeters. The iron film is capped by a -.,801am thick silica glaze at the top, with bits of silica glaze scattered throughout. Note the honeycombed pattern within the iron films; it is especially apparent near the base. Although a stratigraphic interpretation of iron film first, followed by silica glaze, is likely. The honeycombed pattern could also be iron replacing woody tissues that were trapped by silica glaze.

Iron films, formed on cobbles within the subsurface, can be coated in turn by other types of accretions. For example, iron films on the undersides of rocks in desert pavements (section 9.2.1.7.) are sometimes overturned when desert pavement cobbles are disturbed. In making earth figures called geoglyphs, ancient cultures rotated rocks, putting the top position in the subsurface and the bottom position exposed to the atmosphere. This means that the iron film forms on top of subaerial manganiferous varnish and visa versa (Figure 9.23). The significance of an inversion of the iron film and rock varnish rests in understanding how giant earthen figures were made and perhaps in learning how old they are by an analysis of the rock coatings. The inversion process also gives clues to how rock coatings develop when the environment changes suddenly. In the case of Figure 9.23, there appears to be a cofitinuous sedimentary sequence even though the rolling of the boulder instantaneously thrust two rock coatings into different geochemical environments. This provides clear evidence that environmental changes can influence the types of rock coatings deposited on rock surfaces.

Iron Films

163

Figure 9.23. Two microscope views of thin sections of chips taken off of a boulder alignment in southem Nevada (boulder collected by T. Hartwell, Desert Research Institute, personal communication, 1995). The collection from the top of the boulder (upper image; coating is about 40gm) shows a black layer of vamish on top of a gray layer of iron film. In other words, the rock surface started out in the subsurface and was coated by an iron film. When the geoglyph was made, rock vamish started to form. The collection from the bottom of the boulder (lower image; coating is about 30p,m) shows the inverse, where the gray layer of iron film rests on top of the black layer of rock vamish. This position on the boulder started out on top with a coating of rock varnish, and when the geoglyph was made, an iron film formed in the subsurface on top of what was once a subaerial coating.

9.2.2. Composition Iron films are characterized by iron as a major element and iron oxides as a coloring agent. Among iron films there is considerable variability in chemistry. This reflects the reality that there are many different types of iron films and that the same iron film sometimes has a heterogeneous composition. A lot more information is available on the chemistry of iron films than on their mineralogy. The section on mineralogy (9.2.2.1.) reveal that there is not enough information to make any conclusions about the iron oxide mineralogy of iron films. There is a lot more information, however, on the elemental composition of iron films. I have observed that iron films may be divided into three broad types, based on their chemistry: Type I) iron is the only major element; Type II) aluminum and silica are major elements, but taken together they are less abundant than iron; and Type III) aluminum and silica are major elements, and taken together they are more abundant than iron. Sections 9.2.2.2 through 9.2.2.4 explore the differences among these different types of iron films.

164

Chapter 9

9.2.2.1. Mineralogy One of the great research needs in the study of iron films is the paucity of information on mineralogy in different environmental settings. Based on anecdotal evidence, however, different types of iron hydroxides seem to occur in different types of iron films. Goethite and lepidocrocite are found in iron f'dms associated with acid mine leachate (Herbert, 1995). In the Namib Desert, iron films of limonite and perhaps goethite form as external coatings on feldspar and quartz crystals (Selby, 1977). In a very different environment, in the dry Arctic environment of Lindsay's Nunatak in Greenland, reddish-brown material in the outer part of a rock crevice contains goethite, magnetite/maghemite, but no hematite. An inner part of the rock crevice has a greenish color, due mainly to nontronite. Since nontronite forms from a silica solution with ferrous iron, the oxidation of iron in the outer rind may have helped maintain a mixture of ferrous and ferric compounds in the greenish rust in the innermost part of the crevice (Koch et al., 1995). At least 90% of the iron hydroxides were made up of hematite in orange coatings on the underside of cobbles in desert pavements from the Mojave Desert. These are Type III iron films (see section 9.2.2.4). Clay minerals, mostly illite-montmorillonite, comprise the bulk of these iron films (Potter and Rossman, 1979c). Coatings on sand grains are thought to resemble orange-bottom varnish (EI-Baz and Prestel, 1980). The sand grain coatings are externally applied, and they are composed of mostly clay minerals (providing AI and Si) and iron oxides (Mahaney, 1996; Windspear and Pye, 1995), particularly hematite (EI-Baz and Prestel, 1980; Glennie, 1970; Norris, 1969; Walker, 1979). The study of iron films is, clearly, ripe for a systematic analysis of the mineralogy of iron films in different environmental settings. I predict, however, that this research will not beconducted until the planetary geology community realizes that iron films are distinct from "desert varnish" and that the study of earth analogs may be useful to the understanding of iron films in extra-terrestrial settings.

9.2.2.2. Type I Iron Films Type I iron films are where iron is the only major element. Type I iron films have been seen in variety of contexts. For example, D. Dragovich analyzed a dark coating in a semi-arid environment west of Port Augusta in south Australia. Electron microprobe measurements reveal that 88.90% of the coating was iron; the next most abundant element was Si at 3.14% (Dora and Dragovich, 1990). These iron films are not limited to semi-arid environments. Figure 9.24 illustrates an iron film formed on granitic bedrock collected from a hydrophilic Vermont environment (T. Wasklewicz, personal communication, 1996). Although only the subaerial iron film was analyzed for its chemical composition (Table 9.2), tendrils of iron f'dm can be seen penetrating into the rock and splitting the underlying quartz (Figure 9.24). In this sample, iron is the dominant constituent (Table 9.2). Figure 9.25 illustrates another Type I iron film, and the iron film has also penetrated into the underlying host rock. In this case, the clast is from a slag pile of the old Kaiser Steel Plant near Fontana in southern California (Dorn and Meek, 1995). Figure 9.26 shows the different colors on the slag pile; these colors are a product of the development of different rock coatings, including iron films (Dorn and Meek, 1995).

Iron Films

165

The sample came from the west slag pile that was constructed between 1952 and 1972. Thus, the age of the slag pile and the iron film would be between 20 and 40 years.

Figure 9.24. Backscattered images showing the location of an electron microprobe transect (Table 9. 2) of an iron film on Westmore Granite in Vermont (T. Wasklewicz, personal communication, 1996). Note how the iron film is penetrating into the underlying quartz.

Table 9.2. Electron microprobe transect of iron film from Vermont. K, Ti, Mn and Ba were measured, but they are all below the limits of detection (bid). Low probe totals are from water and porosity. The analyses were made with a 5~tm diameter beam, and the points are evenly spaced from top to bottom on Figure 9.24. Na20

MgO

A1203

SiO2

CaO

FeO

Total

0.03 bid 0.03

0.02 0.05 0.02

1.23 3.17 0.02

2.48 2.12 2.61

0.38 0.45 0.46

66.55 70.69 77.78 83.57 76.87 80.01

0.03

0.07

1.08

2.18

0.49

78.41 82.26

bld

0.02

0.19

2.57

0.45

75.08 78.31

bld

0.02

0.25

2.29

0.41

76.91 79.88

bld bid 0.03 bid

0.05 0.03 0.07 0.08

0.28 0.19 0.11 0.17

2.57 2.57 2.55 2.78

0.42 0.42 0.43 0.64

75.05 76.29 75.03 76.50

78.37 79.50 78.22 80.17

Iron films accrete on the slag pile where water concentrates and flows over the slag fragments. Like other iron films, there is a distinct contact between the iron film and the host rock (Figure 9.25). This morphological evidence indicates that 'leaching' from the rock immediately underneath the film is probably not important. The constituents of the iron films could be derived from airborne fallout, fog, rain, dissolved constituents

166

Chapter 9

of weathering finds in water flow over slag surfaces, or all of the above. Like other iron films, whatever mechanism precipitates iron is not precipitating manganese.

Figure 9.25. Backscattered images showing the location of an electron microprobe transect (Table 9.3) of an iron film on slag from the steel plant in Fontana, Califomia (Dorn and Meek, 1995).

Figure 9.26. Ground view of the different appearances of rock coatings on the west slag pile of the old Kaiser Steel Plant. The west slag pile was between 20 and 40 years old when sample collection occurred (Dora and Meek, 1995).

Another example of a Type I iron film is found on rock surfaces in the Sonoran Desert. Figure 9.27 presents a view of iron films accreted on inselbergs of Papago Park in Phoenix, Arizona 0~igure 9.28). The textural characteristics of this Type I iron film is similar to several others in that it is found at the surface and in fractures less than a millimeter underneath the rock face. Thus, iron films seen at the surface may simply be inherited from formation in fractures. Also, there is a clear morphological contact between the iron film and the host rock or fracture faces, indicating that constituents were applied to mineral faces.

Iron Films

167

Table 9.3. Electron microprobe transect of iron film formed on slag from the Kaiser steel plant in Fontana, California. Low probe totals are from water and porosity. The analyses were made with a 10ktm diameter beam, and the points are evenly spaced from left to right on Figure 9.25. Na, Mg, K and Ba were all below the limits of detection. A120 3 SiO2

P205

SO3

4.08

1.08

1.44

0.11

5.15

2.19

1.50

0.17

2.99

1.44

1.05

0.23

3.15

1.05

1.49

0.28

3.81

1.06

1.61

0.22

CaO

TiO2

MnO

Fe20 3 Total

0.77

0.15

0.74

81.00

89.37

0.84

0.23

0.99

80.14

91.21

0.85

0.33

0.74

7 9 . 0 5 86.68

0.70

0.42

0.80

76.88

0.74

0.17

1.03

7 9 . 5 7 88.21

84.77

Figure 9.27. Backscattered image of iron film formed on south Bames Butte in Papago Park, Phoenix Arizona. The end points of the dark line indicate the location of the microprobe measurements in Table 9.4.

Figure 9.28. Ground view and inset satellite image of Barnes Butte, Papago Park, Phoenix, Arizona. The arrow in the Landsat Thematic Mapper image (Band 6) points to the location of Bames Butte.

Chapter 9

168

Table 9.4. Electron microprobe analyses of iron film formed on the surface of Barnes Butte in Papago Park, Phoenix, Arizona. Low probe totals are from water and porosity. The analyses were made with a 10l.tm diameter beam, and the data were collected from end points on the line indicated on Figure 9.27. S and Ba were measured, but are below the limits of detection. Na20

MgO

AL203

SiO2

K20

CaO

TiO2

MnO

FeO

Total

0.12 0.20

0.02 0.05

2.44 4.46

0.41 0.51

0.07 0.07

0.13 0.29

0.80 0.53

0.03 0.04

84.11 81.71

88.13 87.86

Dark brown to black Type I iron films occur in K~rkevagge, Northern Scandinavia (Dixon et al., 1995). Some portions of these iron films fall into Type I (Figure 9.29 and Table 9.5), where there are only minor amounts of AI, Si, P, S and sometimes Mn. As will be discussed in the next section, different portions of these same iron films are chemically heterogeneous and can fall into other chemical types. This emphasizes the point that most rock coatings are heterogeneous phenomena.

Figure 9.29. Backscattered image showing the location of electron microprobe transect (Table 9.5A) of a Type I iron film on schist clasts from K~irkevagge,Northern Scandinavia (Dixon et al., 1995). The sample was collected by J. Dixon.

Table 9.5A. Electron microprobe transects of iron films on a schist clast from K~kevagge, Northern Scandinavia (Dixon et al., 1995). The end points of the line on Figure 9.29 are the probe spots. Na, K, Ca, Ti and Ba were measured, but were all below the limits of detection (bid). MgO

A1203

SiO2

P205

SO3

FeO

Total

bid 0.07

0.28 0.62

1.22 0.90

0.73 0.57

0.70 72.48 75.43 0.70 73.11 76.01

Type I iron films form 40-501.tm thick coatings on urban stone monuments in Spain and Portugal. The abundance of iron in these films is more than twenty times other elements (e.g. AI, Si, P, S, K, Ca) (Schiavon, 1993). These iron films contain

169

Iron Films

fragments detached from the underlying rock and soil dust particles, and they can also show some evidence of lamination. 9.2.2.3. Type H Iron Films Type II iron films are characterized by aluminum and silicon as major elements, but when taken together they are still less abundant than iron. In contrast to Type I iron films that are found in a wide variety of environments, Type II films have only been noted in subaerial settings thus far. Type II iron films are often found interfingering with other rock coatings. A thin iron film, associated with silica glaze from C6a in northern Portugal (Figure 9.21), contained-70% Fe203, but A1203 and S iO2 were both found in abundances ranging from 5 to 10%. Type II iron films can similarly interfinger with Type I films. For example, in K~kevagge, Northern Scandinavia (Dixon et al., 1995), abundances of both AI203 and S iO2 can reach over 3%. Figure 9.30 displays a Type II iron film that reaches a thickness of about a millimeter.

Figure 9.30. Backscattered image showing the location of electron microprobe transect (Table 9.5A) of a Type II iron film on schist clasts from K~irkevagge, Northern Scandinavia (Dixon et al., 1995).

Table 9.5B. Electron microprobe transects of iron films on a schist clast from K~kevagge, Northern Scandinavia (Dixon et al., 1995). The microprobe measurements (top to bottom) were made along the small line indicated on Figure 9.30. Bid means below the limit of detection. Na20

MgO

A1203 SiO2

P205

SO3

K20

CaO

TiO2

MnO

FeO

Total

0.08 bid bid bid

0.51" 0.15 0.12 0.45

3.3'6 1.98 1.83 2.80

3.77 2.05 2.12 4.17

0.27 bid 0.18 0.11

0.42 0.30 0.62 0.57

0.39 0.20 0.07 0.39

0.13 0.04 0.01 0.03

0.07 0.02 bid 0.03

0.03" 0.03 0.14 0.03

68.07 73.75 72.15 68.22

77.10 78.52 77.24 76.80

bid bid

0.08 0.07

2.63 3.06

1.58 1.43

0.44 0.27

0.47 0.52

0.01 bid

bid 0.04

0.03 bid

0.09 0.15

70.35 67.77

75.68 73.31

bid

0.07

2.66

1.84

0.50

0.55

0.04

0.01

0.02

0.06

6 9 . 1 1 74.86

Chapter 9

170

An example of a subaerial iron film, from Solatario Canyon alluvial fan in the Crater Flat area of Southern Nevada, illustrates a variety of a Type II iron film where manganese is abundant. Figure 9.31 illustrates the ~10-201.tm thick iron film, along with the location of the electron microprobe transect (Table 9.6). Rather than alumina, silica and manganese together comprise about 20% of these coatings.

Figure 9.31. Backscattered electron image of an iron film on a cobble on Solatario Canyon alluvial fan, Crater Flat, Southem Nevada. The small dark line indicates the location of the electron microprobe transect (Table 9.6) (The sample was collected by D. Krinsley.)

Table 9.6. Electron microprobe transect of an iron film on a subaerial clast on Solatario Canyon alluvial fan, Crater Flat, Southern Nevada. The probe measurements, with a focused beam, were made from top to bottom along the line shown in Figure 9.31. Bid means below the limit of detection. "A1203

sio 2

K20

'0.9'6

5.37

1.00

5.31

1.08 0.96 0.85 ,.

CaO

MnO

Fe20 3

CuO

0.10

1.41

11.67

76.43

0.04

0.09

0.41

0.05

96'53

0.07

1.27

11.30

76.65

0.06

0.09

0.38

0.10

96.23

6.48

0.11

1.62

11.43

74.95

bid

0.14

0.40

bid

96.21

5.52 6.12

0.12 0.11

1.47 1.46

13.79 8.95

73.26 79.22

0.04 0.09

0.02 0.12

0.58 0.29

bid bid

95.76 97.21

.

ZnO

BaO

.,

PI~

Total

,

Type II iron films have been noted in the literature on rock varnish. In a study of a metal gas can abandoned in the Sahara desert for decades, there was an iron-rich zone with a chemistry similar in chemistry to the Solatario Canyon Type II films (Table 9.7). Type II films are, therefore, apparently able to form rapidly.

Table 9.7. Chemistry of iron-rich zone on metal coating on a gas can abandoned in the Sahara Desert for decades, from Eastes et al. (1988). Na20

MgO

SiO2

A1203

SO4

Cl

K20

CaO

3.10

1.40

0.20

0.30

Fe20 3

MnO

85.40

0.20

Total

ii

1.10

0.60 i

4.60 i

2.90 i

ii

i

i

99.80 i

Iron Films

171

9.2.2.4. Type III Iron Films Type III iron films are distinct from Types I and II in that iron is not the dominant element. While the iron provides orange to red coloration, silica and aluminum are more abundant than the iron. Mineralogical analyses of Type III iron films reveal that they are mostly made up of clay minerals (EI-Baz and Prestel, 1980; Potter, 1979; Windspear and Pye, 1995); this mineralogy is certainly consistent with the abundance of Si and AI. Like Type I iron films, these coatings are found in a wide variety of environments. Unlike Type I and Type II iron films, most of the chemical data on iron films has been on Type III coatings (Table 9.8). Data in Table 9.8 indicate that silica is the most common constituent, except where it was not analyzed. The abundance of iron hydroxides rests in the range of 10% to 25%.

Table 9.8. Electron microprobe analyses of Type III iron films in the literature. Where an oxide weight percentage was left blank, no analyses were reported. Where FeO was reported, it was recalculated to Fe20 3 for consistency. Orange bottom Death Valley Hooke et al., 1969 Na20 MgO A1203 SiO2 P205 SO4 K20 CaO TiO2 MnO Fe203

3.8 11.3

Crevice Iceland Douglas 1987

Orange bottom Mojave Desert Potter 1979

0.39 1.18 20.38 27.98 1.04

0.19 4.27 27.09 53.1 0.1

1.4 1.1 2.7 0.6 23.5

0.4 1.28

20.9

52.92

0.27 11.94

BaO

Total

3.57 0.99 0.79 0.14 9.80 0.18 90.24

Iron-richcoating WhaUey, 1984

5.75 63.8 0.55 1.25 1.22 0.91 0.24 16.2 73.72

Type III iron films are found on sand grains in desert regions. Table 9.9 presents analyses of iron films on three different sand grains from the Parker Dune field. Note that iron is not very abundant in these analyses. Yet these sand grains are as red as those found in the dunes of the Simpson Desert and Namibia. Type III iron films interdigitate with other types of rock coatings, and other types of iron films. Consider the case of Iztacc~uatl volcano, Mexico. The dominant rock coating on this mountain is silica glaze. However, Type III iron films also occur in

Chapter 9

172

subaerial positions. Table 9.10 presents three randomly selected chemical analyses of the Type III iron film on a morainal boulder (Figure 9.22).

Table 9.9. Electron microprobe analyses of Type III iron films on 3 different sand grains of the Parker Dune field, Arizona (see Figure 9.10). Bid means below the limit of detection. Na20 MgO AL20 3 SiO2 0.34 0.86 1.08

P205 SO3

K20

CaO

TiO2 MnO Fe203 BaO

0 . 1 3 52.51 11.96 0.21 0 . 0 5 0 . 3 4 7 . 6 3 0 . 0 5 0 . 1 3 3.89 2 . 7 9 11.02 46.77 0.30 0 . 0 7 3 . 1 2 0 . 9 8 0 . 3 5 0 . 1 2 5.43 1.01 8 . 6 4 37.18 0.30 0 . 1 5 1.60 0 . 4 9 0.23 0 . 1 0 3.73

0.06 bid bid

Total 77.30 71.81 54.51

Table 9.10. Electron microprobe analyses of Type III iron film on Iztacc~uatl volcano, Mexico. These are focused beam analyses of the cross-section displayed in Figure 9.22. Na20

MgO

2.18 0.74 1.47

AL20 3 SiO2

P205

SO3

K20

CaO

TiO2

MnO

Fe20 3 Total

0.22

13.08 41.44 1.42

0.35

0.25

9.35

2 9 . 3 5 2.18

0.65

1.47

1.57

0.30

0.81

2 6 . 5 3 89.37

0.33

0.78

0.48

1.21

3 7 . 3 0 82.62

0.61

10.11

4 7 . 3 0 1.03

0.55

0.76

1.72

0.37

0.41

17.66 81.99

Orange-bottom varnishes (Engel and Sharp, 1968; section 9.2.1.7) are Type III iron films. The dominant minerals are clays (Potter, 1979), and therefore the dominant elements are silicon and aluminum. Figure 9.32 illustrates a Type III iron film on the underside of a desert pavement cobble in southern Nevada, where a layered structure is imposed by clays.

Figure 9.32. Backscattered electron image of an orange-bottom varnish on a cobble on Solatario Canyon alluvial fan, Crater Flat, Southem Nevada. The two lines show the location of electron microprobe transects presented in Table 9.11. (The sample was collected by D. Krinsley.)

173

Iron Films

Table 9.11. Electron microprobe transect of an orange-bottom varnish on a clast on Solatario Canyon alluvial fan, Crater Flat, Southern Nevada. The probe measurements were made with a focused beam on transects shown in Figure 9.32. Bid means below the limit of detection.

Left Transect, Top to Bottom 3320 3 SiO2

K20

CaO

TiO2

MnO

Fe20 3 CuO

Z nO

BaO

PbO

Total

24.92 23.36 19.95 21.33 22.56

1.24 1.51 1.52 2.01 1.43

0.31 0.22 1.86 0.74 0.64

0.42 0.52 0.50 0.73 0.63

0.62 0.49 0.24 0.28 0.17

11192 16.23 14.87 13.11 18.87

0.06 0.03 0.04 0.11 0.06

bid 0.04 0.07 0.04 0.05

0.13 0.09 0.17 0.16 0.29

0.15 0.12 0.24 0.19 0.38

92.59 89.62 79.42 78.28 82.63

52.82 47.01 39.96 39.58 37.55

Right Transect, Top to Bottom 33203 SiO2

K20

CaO

TiO2

Mn0'

Fe20 3 CuO

ZnO

Ba()

Pts

Total

23.09 23.59 20.99 22.24 20.97

1.26 2.01 1.06 2.63 1.93

'0.69 0.39 0.55 0.85 0.91

0.62 0.58 0.58 0.87 0.88

0.17 0.65 0.29 0.08 0.18

16.04 12.67 13.54 17.95 16.54

0.07 0.11 0.14 0.21 0.04

0.23 0.31 0.46 0.34 0.35

0.32 bid bid bid 0.32

89.08 80.59 82.73 90.27 79.99

1..

46.50 40.24 44.98 44.96 37.74

.

0.09 0.04 0.14 0.14 0.13

..

..

,

Iron films on the underside of desert pavement cobbles are generally characterized by thicknesses less than 201am. They are also characterized concentrations of around twothird alumina and silica with only 10-20% iron hydroxides (Table 9.11). The chemical analyses reveal that there is considerable variability in the minor and trace elements. Figure 9.33. illustrates a case where Type III iron films interdigitate with ground-line band varnish. Ground-line band varnish is a very shiny, and very thin manganiferous varnish that forms a rim around a desert pavement clast at ground level (Engel and Sharp, 1958).

Figure 9.33. Backscattered electron image of a ground-line band varnish (Engel and Sharp, 1958) that formed on top of orange-bottom varnish on a cobble on Solatario Canyon alluvial fan, Crater Flat, Southem Nevada. The line indicates the location of the electron microprobe transect presented in Table 9.12. (The sample was collected by D. Krinsley.) The rock coating has a total thickness of 15 microns.

The position of a cobble can change in a desert pavement. Sometimes, the soil level drops with respect to the cobble. This was the case in for the cobble in Figure 9.33, and a ground-line band formed on top of the orange-bottom varnish. The brighter band

174

Chapter 9

at the top of the coating in Figure 9.33 is the ground-line band that shows an enrichment in manganese and barium (Table 9.12). Yet, below the ground-line band, the chemistry returns to the range of values found in the Type III iron films (orangebottom varnishes, cf. Engel and Sharp, 1958) of Table 9.11.

Table 9.12. Electron microprobe transect of ground-line band varnish superimposed on top of a Type III iron film on Solatario Canyon alluvial fan, Crater Flat, Southern Nevada. The probe measurements were made with a focused beam on transects shown in Figure 9.33. A1203'Si(J 2

"K20

"CaO

TiO2

MnO

Fe203 Cub

Zn'O "BaO

10'.64 0.2'0 '0.10

PbO

Total

15.62 '22.12 ' 1.5'2

0.85

0.55

36.49

0.70

0.20 ..... 88.99

16.07

23.15

1.79

0.50

0.68

3 0 . 6 3 10.97 0.13

0.14

0.64

0.21

84.91

21.99

42.32

1.00

0.53

0.53

1.98

0.17

0.47

0.10

80.43

16.91

41.92

2.04

0.60

1.40

0.68

1 6 . 1 4 0.13

0.11

0.17

0.56

80.66

21.07

41.53

1.37

1.29

0.62

0.67

1 5 . 8 8 0.08

0.12

0.16

0.47

83.26

,

,,

,

,

,,

,,

,

,

1 1 . 1 8 0.16

,

,,

,,,

Orange to red iron films found in desert rock crevices are also Type III iron films. They display an abundance of aluminum and silica, with iron concentrations in the range of 10-20%. Figure 9.34 illustrates one such iron film from near Shoshone, in eastern California. The electron microprobe analyses of this crack varnish reveal a chemistry that is similar to iron films on the bottoms of desert pavement cobbles (Table 9.13).

Figure 9.34. Backscattered electron images of an iron film formed within a rock crevice on a clast from near Shoshone, eastern California. The image on the left provides an 'overview', while the filled crevice is magnified in the fight image. The black line indicate the location of the electron microprobe transect presented in Table 9.13. (The sample was collected by D. Krinsley.)

175

Iron Films

Table 9.13. Electron microprobe transect of an iron film formed in a rock crevice from near Shoshone, eastem California. The probe measurements were made with a focused beam on the transect (top to bottom) shown in Figure 9.34. A120 3 SiO2

K20

CaO

TiO2 ' M n O

ii

10.98

Fe20 3 CuO

ZnO

BaO

PbO

"I'otal

0.16

70.03

i,i

35.41

4.25

0.90

.,.

0.57

1.97

1 5 . 3 6 0.11

0.09

0.23

15.49

38.96

1.30

0.91

0.47

1.84

1 6 . 6 8 0.09

0.10

0.19

bid

76.03

14.49

23.58

0.86

0.70

0.60

1.52

16.78

bid

0.07

0.17

0.08

58.85

18.52

33.99

2.93

6.38

0.85

1.99

15.71

bid

20.82

38.66

3.51

4.42

0.60

0.86

1 5 . 0 4 0.11

0.10

0.10

bid

80.57

0.07

0.09

bid

84.18

Type III iron films are common in subaerial positions in fog deserts and hyper-arid deserts. Entire mountain ranges in the Atacama Desert are dominated by orange colors due to the formation of iron films. Figure 9.35, for example, illustrates that the appearance of an entire range in southern Peru can be altered by iron films. Normally white quartzite presents an orange color. Much of the Peninsula of Baja California exemplifies the dominance of iron films in a subaerial context. Instead of black rock varnish, the surfaces of rocks are coated by iron films. A sample collected by C. Siebe on basalt illustrates that these orange coatings have chemistries like other Type III iron films (Table 9.14). Type III iron films are also deposited as a layer in classic, black manganese-rich rock varnish. Throughout the western United States, China, Patagonia, and Israel (T. Liu, personal communication, 1997) the uppermost layers in rock varnishes are capped by a layer that is orange in color, when seen in cross-section (Figure 9.36).

Table 9.14. Electron microprobe analyses of Type III iron films in a subaerial position on basalt collected from Baja California by C. Siebe, personal communication. Bid means below the limit of detection. Ya20

MgO

A120 3

SiO2

P205

K20

bid

2.12

0.09

2.24

0.08

CaO

TiO'2

MnO

25.49

34.42

2.61

0.86

22.54

29.74

3.51

1.41

2.21

22.31

31.98

2.64

1.06

0.15

0.13

2.64

21.45

30.42

1.35

1.02

0.05

2.50

23.24

31.34

2.27

1.30

bid

2.50

22.37

30.27

2.82

bid

2.19

23.03

35.36

0.08

2.79

23.90

32.28

0.04

2.93

27.27

bid

2.85

26.21

FeO

Total

0.14

0.50

1.8"f

17.78

85.79

0.14

0.65

1.96

22.62

84.90

0.72

5.46

17.63

84.24

0.11

1.03

4.16

12.71

75.02

0.13

0.77

3.63

17.34

82.57

1.02

0.13

0.73

3.98

21.59

85.41

1.81

1.05

0.15

0.62

4.33

16.21

84.75

1.90

1.22

0.11

0.87

4.17

17.32

84.64

36.54

1.21

1.16

0.07

0.68

2.07

11.78

83.75

33.91

1.47

0.95

0.10

0.80

2.40

13.84

82.53

176

Chapter 9

Figure 9.35. Aerial view of a small range near the town of Nasca, Peru. The scale of the image is about 3 kilometers from side to side. The light color of the upper left image is where rock coatings have been abraded by stream flow. In contrast, the range is darkened by the formation of iron film on the quartzite bedrock.

Figure 9.36. Ultra-thin section of rock varnishes collected from different locations in Death Valley, California (Liu and Dom, 1996). The uppermost layers are yellow-orange in color from clay minerals and iron oxides. In contrast, the black layers are rich in manganese, which gives the varnish its dark color when seen in the field. The thickness of the iron film at the top of the varnish is about 10 microns.

Figure 9.37 presents an optical cross-section of rock varnish, but the uppermost layer is a Type III iron film. The corresponding X-ray map in this figure shows that this top-most layer is dominated by silicon and aluminum, followed by iron. Manganese is not greatly enhanced in these interlayered Type III iron films (Table 9.15). I do not want to give the impression that Type III iron films are limited to dryland contexts. As noted previously, they occur in patches on rock surfaces at Iztacc~uatl volcano, Mexico (Table 9.10). I have also observed grayish brown (Munsell 2.5Y 4/2) films from a roadcut near Cairns in Queensland, Australia. These coatings have a chemistry similar to Type III iron films elsewhere, but with one exception; there are comparatively high concentrations of potassium (Table 9.16).

Iron Films

177

Figure 9. 37. X-ray maps of the chemistry of a cross-section of a rock varnish collected from Desth Valley, California. In these maps, whiter areas reveal higher concentrations. Note that the uppermost layer has the characteristic of Type III iron films: high in A1, Si and Fe. These images are courtesy of T. Liu (personal communication, 1997).

Table 9.15 Electron microprobe analyses of surface layer of two different rock varnishes collected from the same alluvial fan (Galena Canyon) in Death Valley. Note how they are similar in that they have high concentrations of AI and Si (from clay minerals) and iron oxides. Cross-section 1 "Na20 MgO

A120 3 SiO2

P205

0.04

1.06

20.23 27.09

1.08

0.09

1 . 5 5 21.69 28.90

1.29

0.11

2.08

21.45 26.96

1.27

0.21

1 . 2 6 0.60

0.08

2.27

19.94 27.62

1.45

0.19

1 . 2 5 0.68

0.11

2.23

19.81 27.08

1.31

0.21

Cross-section 2 0.15 1 . 5 7 22.52 28.35

1.06

2.12 23.96 29.23 1 . 1 5 2.12 20.64 26.46 1.31 1 . 8 4 18.81 22.25 1 . 5 2

0.15 0.16 0.20 |1

SO3

K20

CaO

0.09

0.51

0.48

0.27

3.97

10.17 0.05

65.04

0.21

0.92

0.45

0.26

3.24

8.77 0.31

67.68

0.37

3.30

9.71

0.42

67.74

0.34

4.30

9.78

0.48

68.38

1 . 2 7 0.70

0.37

4.91

10.02 0.45

68.47

0.19

0.76

0.24

2.40

7.06

0.07

64.79

0.19 0.16 0.24

1 . 0 8 0.60 1 . 0 3 0.67 0.98 0 . 9 1

0.36 0.37 0.34

1.57 1.48 2.86

7.84 9.84 9.24

0.16 0.37 0.25

68.41 64.61 59.44

0.42

TiO 2

MnO

Fe20 3 BaO

Total

Chapter 9

178

Table 9.16. Electron microprobe analyses of an iron film from a roadcut near Cairns, in Queensland, Australia. Note the relatively high concentrations of potassium, relative to type III iron films in deserts. Bid means below the limit of detection. Na20

MgO

A120 3

Si()2

0.09 .. 0.12 bid

1.46 1.36 0.43

27.55 21.84 6.12

4 1 1 5 9 0.05 ........ 7.26 33.69 0.16 5.75 65.04 0.30 1.53

0.08

1.79

24.41

40.41

bid

0.12

1.46

23.13

32.41

0.07

0.11

1.67

27.13

43.64

bid

0.08 bid

2.35 1.51

26.62 4.06

41.76 75.35

0.09 0.11

iii

i

i

P205

iiii

K20

ii

MnO

Fe20 3

Total

0.10 ........ 0.13 0 . 2 1 0.13 0 . 2 5 0.05

0.10 0.19 0.17

7.58 17.47 22.42

85.91 80.92 96.31

7.20

0 . 1 5 0.13

0.08

6.21

80.46

6.83

0.08

0.15

0.10

5.38

69.73

7.49

0 . 1 5 0.17

0.13

9.15

89.64

8.20 0.66

0.10 0.24

0.13 0.19

11.31 13.35

90.82 95.49

iiii

CaO

TiO 2

0.18 0.02 iiiii

iii

ii

ii

i

In summary, Type III iron films are characterized by an orange color, imparted by iron oxides. However, the dominant constituents are clay minerals. These clays concomitantly enforce a texture of subparallel layers visible in both backscattered and secondary electron images.

9.2.2.5 Heavy Metal Scavenging Iron hydroxides are widely recognized as scavenging agents of metals such as lead, cadmium copper, zinc (Benjamin, 1983; Davis and Leckie, 1978; Jenne, 1968; Lee, 1975; Shuman, 1977; Venkataramani et al., 1978). For example, red beds are known to be places where heavy metals accumulate (Zielinksi et al., 1983), and iron-rich accretions in acidic mine waters are also known to accumulate potentially toxic levels of arsenic (Leblanc et al., 1997). Tables 9.11. through 9.13 reveal that copper, zinc and lead adsorb onto iron films in deserts. However, an analysis of variance indicates that the relationships between the abundance of iron and these heavy metals is not clear. Time, the conditions of hydroxide formation and other environmental factors (Lee, 1975) may confound a simple association between iron hydroxides and heavy-metal abundance.

9.2.3. Rates of Formation Some iron films can form very rapidly. Iron films grow on rocks on active rock glaciers in Nepal (Figure 9.38). Twenty to forty-year-old slag piles in southern California (Dora and Meek, 1995), and twentieth century surfaces in the Egypt (Eastes et al., 1988). These isolated examples only suggest that iron films can grow very rapidly. On the other hand, the Type III iron films that cover the very surfaces of rock varnishes in the western United States (Figures 9.36 and 9.37) appear to accrete at rates on the order of microns per thousands of years (Liu, 1994).

Iron Films

179

Figure 9.38. Backscattered electron microscope image of iron film formed on the surface of the Tshola rock glacier (--4500 m), Nepal. Note how the iron film has mobilized from the surface coating and moved into the rock in the form of veins of iron hydroxides reprecipitating within the clast.

A problem in interpreting rates of accretion of subaerial iron films is that many of these coatings start within rock crevices (see section 9.2.1.2). These films are then exposed at the surface by spalling of overlying rock material. Rates of formation or exposure ages are impossible to calculate if the iron film truly formed in a subaerial setting. A case in point is the inselberg of Ayers Rock in Australia. The vast majority of the surface of Ayers Rock is coated by iron films that originated in rock fractures (Figure 9.39). There are also coatings of rock varnish, silica glaze, lithobionts, and oxalaterich crusts. However, iron films are the dominant rock coating --- imparting an orange color to a rock type that is truly ivory. The relevance here is that all of the iron films on Ayers Rock originated in the subsurface. The other rock coatings form on top of the iron films, after the surface of Ayers Rock spalls (Figure 9.39).

Figure 9.39. Photograph of the eroding surface of Ayers rock. The orange color of the surface is due in large part to the erosion exposing Type III iron films that originated in rock fractures. The flaky appearance of the surface is from exfoliation of centimeter-scale spalls.

180

Chapter 9

An iron film that has received attention as a potential dating tool is Type III iron films on the undersides of rocks in desert pavements; these are orange bottom varnishes in the terminology of Engel and Sharp (1958). In contrast to dark, manganiferous varnishes that form patches on rock surfaces, Type III iron films can form reasonably complete coatings within 1400 to 2000 years in Israel (Bowman, 1982). Complete Type III iron films can form on artifacts in the Mojave Desert in less than 2000 years (Bamforth and Dora, 1988). Using a chronosequence of alluvial surfaces in the Mojave Desert, the reddening of these Type III iron films has been placed in an index. The maximum color saturation was a Munsell value of 10R 4/8, and this value was reached by about 10,000 to 20,000 years (Helms et al., 1995). The undersides of the clast experienced diurnal wet/dry cycles for almost a third of the 39 days of monitoring, suggesting that the greater moisture availability on cobble undersides may play a role in the faster rate of coating on the underside of clasts in desert pavements (Helms et al., 1995). An important warning on this research should be that the length of time to develop a Type III iron film does vary with environment (Childers and Minshall, 1980). Furthermore, the appearance of the coating will vary with rock type, with the soil type in which the cobble rests, and with the climate in which the iron hydroxides undergo diagenesis over time. Hence, efforts to estimate time from these Type III iron films would probably be valid only on a very local basis.

9.3.Origin There are no general models to explain the origin of iron films, and I do not believe that there is any single process that can explain this truly heterogeneous rock coating. The purpose of this section is to present a variety of different perspectives on the source of the iron, on abiotic models, on biotic models, and on ways to combine both biotic and abiotic processes to explain the formation and occurrence of different types of iron films. However, the reader should keep in mind that most of these ideas are highly speculative and there is little evidence to warrant general application of any of these models.

9.3.1. Source of the Iron The literature on iron films does not address the source of the iron, perhaps because this is not perceived as a problem. Iron is an ubiquitous element. Iron is found in sufficient quantities in water to support aquatic iron films (Lee, 1975). In subaerial contexts where eolian fallout may play a role and in soils, iron is readily transported by clay minerals (Carroll, 1958). Thus, there is sufficient iron in aquatic, soil and eolian transportation pathways to support iron films. There are general impressions in the literature, however, that should be conveyed. The first is that the iron probably originates from the weathering-find of the underlying rock (Twidale, 1982). In the case of iron films in Namibia, biotite has been specifically named as a source of iron (Selby, 1977). In the urban environment of Rio de Janeiro, Brazeil, yellow-brown films on granite are thought to potentially derive from iron sulfide in fossil fuels (Smith and Magee, 1990). More detailed electron microprobe

Iron Films

181

studies of tesserae from limestone mosaics at Paphos, Cyprus reveal that coatings of iron hydroxides derive from pyrite inclusions in limestone (Doehne, 1994).

9.3.2. Abiotic Genesis In the general terrestrial weathering environment, acidic and anaerobic conditions favor reduced, ferrous (Fe2§ iron. Divalent iron is soluble and hence mobile in water. In contrast, alkaline and aerobic conditions favor oxidized, ferric (Fe3§ iron which has low solubility and low mobility in water. In inorganic laboratory tests, iron kinetics depend strongly on pH and redox potential (Eh) (Collins and Buol, 1970; Marshall, 1977). This means that "above pH 5 the inorganic oxidation of Fe 2§ to Fe 3§ is so rapid, that bacteria play only a minor role in the mediating process (Holland, 1978, p. 47)." There are places where iron oxides and hydroxides precipitate on rocks in aquatic systems with an acidic pH. In these cases it is very difficult to explain the occurrence of ferric iron films by abiotic mechanisms. The expectation is that the iron would be reduced and mobilized. Yet, there are still several inorganic explanations for iron precipitation in acidic systems. The type of iron oxide to precipitate would depend upon more than just pH. Other factors include the concentration of ferrous and ferric iron in solution, the redox potential, partial pressure of carbon dioxide, and abundance of other dissolved materials such as carbonates and sulfates. In general, the precipitation of ferric hydroxides could occur as Fe 3+ + 2H20 -> FeOOH(s) + 3H+ (Schwertmann and Taylor, 1989). It is possible, therefore, that ferric minerals form at the capillary fringe of acidic ground water (Herbert, 1995) and in systems with ferrous sulfate (Stahl et al., 1993). The iron films found in Antarctica, often falsely called 'desert varnish', appear to be formed by the mobilization and reprecipitation of iron from the host rock (Glasby et al., 1981; Johnston and Cardile, 1984; Johnston et al., 1984). In the SOr Rondane Mountains, the iron films are mostly composed of the K-Fe mineral jarosite. The iron and potassium is mobilized by sulfuric acid in the soil. It moves up through fractures within the host rock and then precipitates on the surface with evaporation (Figure 9.40) (Hayashi, 1989). Photoreduction and oxidation of iron may also play a role in the precipitation of iron films in acidic drainages. In field-based studies of a small mountain stream in Colorado, photoreduction of ferric iron was more than three times greater than the nighttime oxidation of ferrous iron (McKnight et al., 1988). This likely means that amorphous iron oxides should be common components of the coatings. It may be possible that clay minerals and aluminum oxides in aquatic systems could play a role in the formation of Type II and Type III iron films. They accelerate the oxygenation of ferrous iron and induce for the formation of ferric oxyhydroxides (Deng and Stumm, 1994). Type III iron films dominate in desert environments. There general impression in the literature is that formation of subaerial iron-rich desert coatings are likely inorganic, simply due to a combination of two factors. First, there is a lack of evidence for a biotic origin for these coatings. Second, ferric iron is stable in these aerobic, high pH (Dorn and Oberlander, 1982; Smith and Whalley, 1988). The problem with biotic hypotheses comes down to a simple question: why is it necessary or possible invoke biotic agents when the iron is already oxidized.

182

Chapter 9

EvaporaUon t t t Jarosite 9

/'t '

.

.

.

Surface/

.

. / i (__ oa~;

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d

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,":

.L."21 '

'

"~.~, . - . < ~

IX"

.

t t K so4 -.H2so ,.,

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.

Figure 9.40. Model for the formation of iron films in Antarctica, modified from Hayashi and Miura (1989, p. 79). Sulfuric acid in the soil mobilized iron and potassium. The solutions migrate through rock crevices, and jarosite precipitates on the surface of the rock.

Another issue in the genesis of desert iron films is their distribution on the scale of millimeters and hundreds of kilometers. Unlike black, manganiferous varnishes that start out in patches that gradually grow together, iron films on the bottom of desert pavement cobbles and iron films in rock crevices tend to form a complete coating. The intuitive implication is that biological films nucleate at discrete points where microbial agents colonize, whereas films produced by physico-chemical processes tend to coat a cobble more uniformly. A last point relates to the distribution of iron films on the scale of hundreds of kilometers. Iron films are most common in hyper-arid deserts. In these settings, the paucity of moisture deters the growth of bacteria that can concentrate manganese and make rock varnish. In these settings, there is enough moisture to permit the physicochemical fixation of iron on rock surfaces.

9.3.3. Biotic Genesis There are three general ways in which biotic agencies may play a role in the genesis of iron films: in the release of iron from host materials; in the oxidation of iron; and in aiding the accretion of the film. Organic acids are important agents of mineral weathering (Berthelin, 1988; McCalroll and Viles, 1995; Viles, 1995). The iron released by organic acids may be important in the genesis of ferromanganese coatings in the Sahara Desert (Scheffer et al., 1963). In more humid climates on carbonate rocks, goethite may be produced through carbonate weathering by lichens (Ascaso et al., 1976). However, aside from these references, the role of organisms in the release of iron has not been explored.

Iron Films

183

There is a large literature, however, on the ability of bacteria to oxidize iron. The largest literature is for aquatic systems (Ghiorse, 1984; Harder, 1919; Kuenen and Gottschal, 1982; Mallard, 1981; Nealson, 1983; Wolfe, 1964). The bacteria in acidic waters often fall within the Thiobacillus genus (Aristovkaya and Zavarzin, 1971; Singer and Stumm, 1970). This genus is acidophilous, and iron bacteria offer an elegant explanation for the fixation of iron in what are extremely acidic conditions. Rhe previous section certainly presented abiotic explanations, but the simplest explanation is the activity of iron bacteria. There are also iron bacteria that oxidize iron (and manganese) in near-neutral waters. These include genera of Gallionella, Leptothrix, Siderocapsa, and Siderococcus (Chukhrov et al., 1973; Ghiorse and Ehrlich, 1992; Mallard, 1981; Robbins et al., 1992), along with physical evidence of bacteria-like structures (Jones, 1994, p. 444) Because iron oxidation can occur by both by bacteria and physico-chemical mechanisms under near neutral pH conditions, proving role of these genera in the fixation of aquatic iron films usually requires good in situ evidence in the form of cell-waU accretions. Some have argued that microorganisms are involved in the accretion of iron films in subaerial contexts in deserts (Adams et al., 1992). Radiolabelling and microscopy indicates the presence of respiting microbes, and fungi and bacteria on iron films in Tunisia (Drake et al., 1993). Although no clear pattern of iron accumulation was found for microcolonial fungi, iron was sometimes found enhanced within these common inhabitants on desert rocks (Dragovich, 1993a). One link between biotic activity and biotic genesis is the occurrence microbial remains on or in iron films (Adams et al., 1992). A linkage can be as simple as the presence of cocci-like forms (Figure 9.41) on the surfaces of iron films in Karkevagge, Northern Scandinavia (Dixon et al., 1995). Figure 9.42 presents fossilized cocci-like forms in an iron film from Antarctica. Microbial casts have been noted previously in Antarctica (Friedmann and Weed, 1987). However, I have only rarely encountered evidence for microbial enhancement of iron in arid contexts (Dora and Oberlander, 1982). The encrustation of cells with iron has been documented in active microbial systems (Caldewell and Caldwell, 1980; Konhauser et al., 1993). Bacterial iron accretions sometimes have a layered or stromatolitic form, suggestive of microbial control on the genesis of the rock coating (Leblanc et al., 1997).

Figure 9.41. Secondary electron image of cocci forms on the surface of iron films in K/irkevagge, Northern Scandinavia. The sample was collected by J. Dixon.

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Chapter9

Figure 9.42. Backscattered electron image of cocci-like forms in cross-section in an iron film on dolerite, collected from an older moraine of upper Beacon Valley, Victoria Land, Antarctica by Mort Turner.

Schiavon (1993, p. 276) advocated a "mainly biological origin" for the iron films on stone monuments in Portugal. The evidence is circumstantial, but suggestive and brings together a number of lines of evidence including: colonization of the surfaces by biological agencies, dissolution features associated with organic acids from lichens, a high percentage of organic matter in black layers that yields pigmentation, and the evidence of lamination textures that "is a feature typical of biological growths."

9.3.4. General Models There are good arguments for both abiotic and biotic mechanisms of iron film formation. I believe that bacterial explanations are far simpler in aquatic settings and abiotic processes are similarly simpler in subaerial dryland contexts. Yet, nobody has yet addressed the origin of the iron films that originate in subsurface fractures. Nobody has yet addressed the genesis of iron films in tropical settings. And the evidence is certainly not conclusive on whether abiotic or biotic mechanisms are more potent in near neutral geochemical settings. Given the multitude of explanations, I think it appropriate to suggest two general models that may be applicable to the formation of iron films in a wide variety of settings. One involves the role of microorganisms. The other draws on the role of chemical iron-oxygen-silicon bonding of the iron film to the host rock. A general microbial model has been proposed to explain accretions of iron films that have a substantial silica component (Beveridge and Fyfe, 1985; Ferris et al., 1986; Fen'is et al., 1987a; Konhauser et al., 1993). The model involves three-steps: i) iron is bound to anionic polymers of bacterial cells walls by electrostatic forces; ii) the iron then acts as a nucleation site for subsequent iron deposition; and then iii) if silica concentrations are high in the associated waters, silica will also be bound to the bacterial cell walls (Urrutia and Beveridge, 1994). Silica is particularly important in the preservation of the iron coating. Because iron reducing bacteria are often found in close association with coatings, the combination of

Iron Films

185

silica with iron may act to preserve the iron by making it unattractive for iron reducing bacteria (Konhauser and Ferris, 1996). An alternative to the general microbial model, but not necessarily a mutually competitive one, comes from the soil science literature. Iron films are attached to the rock by the formation Of chemical Fe-O-Si bonds (Hazel et al., 1949; Scheidegger et al., 1993). The key to this model is the presence of silica, which requires either a quartz substrate or an initial layer of silica glaze. Initially, the iron hydroxides attach to the silica surface from electrostatic attractions. X-ray photoelectron spectroscopy indicates the formation of chemical bonds Fe-O-Si as: -Fe-OH+HO-Si- --> -Fe-O-Si-H20. Then, once they attach, the iron oxides "adhere very strongly and irreversibly on the silica surface (Scheidegger et al., 1993, p. 62)." These general models have not yet been considered in the broader contexts of iron films. The microbial model was proposed for circumstances where microorganisms are abundant and in aquatic systems The abiotic chemical bonding model, being derived from the soil literature, similarly has not been evaluated in a broader context. Thus, I present them here as speculative, but mutually compatible and general explanations in the literature for the occurrence of iron films.

186

Chapter 10 MANGANIFEROUS

ROCK

VARNISH

"Now, if these black cmsts were formed by a slow decomposition of the granitic rock, under the double influence of humidity and the tropical sun, how is it to be conceived that these oxides are spread so uniformly over the whole surface of the stony masses, and are not more abundant round a crystal of mica or hornblende than on the feldspar and milky quartz?" yon Humboldt (1812, p. 245)

I0.I.

Introduction

Rock varnish is a dark coating that is characterized by clay minerals cemented to rock surfaces by oxides and hydroxides of manganese and iron. The term "desert varnish" is also common, because these accretions are particularly noticeable in arid regions. Although paper thin, rock varnish can completely alter the appearance of a landscape (Figure 10.1).

Figure 10.1. These images show how rock vamish can completely alter the color of a landscape. The upper photograph is of the Gebel Zuweira range in the southern Sinai Peninsula, where rock varnish turns light-colored granites and into ebony. The middle view shows how even a patchy cover of ~30~m-thick vamish progressively darkens older debris flows on the Black Mountains in southern Death VaUey. The lower p~cture, taken from Hoover Dam, Arizona/Nevada, reveals visual impact by subtraction; note the ~oathtub ring' where the vamish has been chemically eroded by the waters of Lake Mead.

Rock Varnish

Rock varnish is the most studied of all rock coatings. In the last two hundred years, there have only been two monograph-length treatments of rock coatings, and both have addressed rock varnish. The first analyzed blackened rocks in the Egyptian Desert and along the Nile (Lucas, 1905). The second characterized the interaction between case hardening crusts, manganiferous rock varnishes, polishes from eolian abrasion, and weathering rinds in North Africa (Haberland, 1975). The number of papers written about rock varnish exceeds all other rock coatings combined. Hence, you will find that this chapter is the most detailed. I divide literature discussions in this chapter into three eras of rock varnish research. The first is prior to World War II. The second rests between World War II and the first dissertations written about rock comings, in this case rock varnish (Bard, 1979; Potter, 1979). The third era extends to the present, where an increasing number of students have chosen to conduct thesis and dissertation research on different aspects of rock varnish (Anderson, 1995; Bard, 1979; Best, 1989; Dorn, 1980; Dom, 1985; Elvidge, 1979; Liu, 1994; Perry, 1979; Potter, 1979; Spatz, 1988). Rather than jump straight into the details of the rock coating, the literature on rock varnish is sufficiently extensive to warrant an evaluation of how the field has evolved over time. Research problems in the field of rock varnish have changed little since Alexander von Humboldt (1812) initiated the scholarly study of black coatings on rocks. He was concerned with its composition being dominated by manganese, whether it was extemaUy applied (his preferred hypothesis) or derived from the host rock, spatial distribution (along rivers and in deserts), controls on why it remains the same minimal thickness, relationship to the environment, and its possible role in human illness. Since von Humboldt accurately dismissed native concerns about the role of rock varnish on human sickness, the role of varnish on health is the only topic explored by von Humboldt that has not burgeoned into a substantial literature. In the 'early years' of varnish research prior to World War II, several researchers drew justification for their studies from the unsolved problems isolated by von Humboldt (1804) and reiterated by Charles Darwin (Darwin, 1897). The main debate rested on whether rock varnish derived from the weathering products of the underlying rock, or from externally-applied material. Based on the chemical analyses of Lucas (1905) and the model of Linck (1901), the majority of researchers concluded that varnish derived from capillary solutions drawn from the rock and evaporated on the surface. An emerging focus of research in archaeology, and to a lesser extent geomorphology, was over varnish as an indicator of antiquity. In the 'middle years' of research between World War II and the late 1970's when the field matured to the point where dissertations and theses were written, there was a gradual improvement in the precision of chemical measurements, more concern over the use of varnish as an indicator of exposure age, exploration of a biological origin by culturing, and debates over the optimal climate for varnish formation. However, there were no major shifts in the nature of research problems. "Desert varnish" was thought to have its origins in the underlying rock. The great enhancement of manganese was thought by most, but not all, to occur by the greater mobility of manganese as compared to iron. Substantial shifts in research focus, methodology, and schools of thought occurred in the 'modem period' of research, heralded by dissertations and theses in the latter half of the 1970's. Probably the most important finding was that the bulk of varnish was composed of clay minerals (Haberland, 1975; Potter, 1979; Potter and Rossman, 1977). A consensus was also reached, on the basis of more spatially precise electron microscope and electron microprobe data, that varnish was an external accretion and that

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varnish was found in virtually all terrestrial weathering environments. Although there has been a great expansion of researchers advocating a microbial explanation for Mnenhancement in varnish, a consensus on varnish genesis has not been reached. The trend towards specialization in science in general was felt in rock varnish over the last two decades, as the research problems asked by most investigators narrowed in focus to new applications of different analytical techniques and to very specific issues of varnish characteristics. In addition, there has been an expansion in funding to investigate the dating of rock varnish, which resulted in a wave of publications assessing its usefulness as an indicator of antiquity. The focus of this chapter is like the others in section two of this book: to provide an understanding of the state-of-the-art knowledge on a rock coating in order to set up section three of the book, where I attempt to promote a more general understanding of the geography rock coatings. Section 10.2 of this chapter details the characteristics of this ubiquitous rock coating, and section 10.3 integrates abiotic and biotic explanations to develop a new model to explain the genesis of rock varnish. 10.2. Characteristics 10.2.1. Environmental Settings: Desert Varnish or Rock Varnish? There are two dominant English-language terms for manganese-iron rock coatings with a clay matrix, desert varnish and rock varnish. Desert varnish (Merrill, 1898) is older than rock varnish (Dora and Oberlander, 1981b; Krumbein and Jens, 1981). And while the use of rock varnish has increased in recent years, desert varnish is still in widespread use (e.g., Cremaschi, 1996). A search of the Intemet, for example, will see more hits for desert varnish than rock varnish. The basic question raised in this section is whether the term "desert varnish" is a misnomer, simply because this rock coating is found in a wide variety of settings other than deserts. 10.2.1.1. Perspectives Prior to Worm War H

As in today's literature, there were three general opinions on the distribution of manganese-iron films on rocks. One group simply kept their observations limited to arid regions, often with concomitant opinions about the importance of the harsh sunlight or heat of the desert. These researchers noted the presence of rock varnish in Australian (Basedow, 1914; Moulden, 1905; Talbot, 1910; Woodward, 1914), North American (Blake, 1855; Blake, 1858; Bryan, 1922; Gilbert, 1875; Loew, 1876; Stevenson, 1881; Walther, 1892), and Saharan-Arabian Deserts (Ball, 1916; Blanck et al., 1926; Linck, 1901; National-Geographic-Society, 1924; Walther, 1891). Another group noted that black coatings on rocks had a global distribution in a variety of environments from deserts to glaciers, that these coatings were similar in character next to glaciers, on quartzite boulders in Washington D.C., on the banks of tropical rivers, and other types of Mn-Fe accumulations such as in laterites and deep-sea nodules (Ball, 1903; Boussingault, 1882; Brandes, 1901; Knaust, 1930; Lucas, 1905; Merrill, 1906; Polynov, 1937; Zahn, 1929). For example, the dark coatings found in the Egyptian Desert were felt to be similar to those found at the Cataracts of the Nile (Roziere 1813, cited by Linck, 1930, p. 243; Lucas, 1905). Linck brought this perspective together (1930) and argued with many examples that "Schutzrinden" had a

Rock Varnish

global distribution; it occurred in deserts from around the globe, adjacent to streams, in tropical soils, near glaciers, and on virtually all rock types. There were some, however, that acknowledged the ubiquitous occurrence of manganese-iron coatings in a variety of environments, but considered the black coatings in deserts to be different. Many were persuaded by Walther (1891, 1912) who argued that the black coatings on banks of tropical rivers are not associated with desert films, since they are produced without the aid of tropical climate or flowing water.

10.2.1.2. Perspectives in the Middle Years

After World War II, many geomorphologists considered the "desert varnish" in drylands to be a unique phenomenon (Blackwelder, 1954; Daveau, 1966; Lukashev, 1970; Peel, 1960). These views were promoted in introductory and specialty textbooks (Heizer and Baumhoff, 1962; Longwell et al., 1950; Muller and Oberlander, 1978; Oilier, 1984). Black Mn-Fe coatings called "desert varnish" were also recognized a wide variety of environments, including: alpine (Glazovskaya, 1968; HOllerman, 1963; Hooke et al., 1969; Hunt, 1954; Klute and Krasser, 1940; Krumbein, 1969; Scheffer et al., 1963), riverine (Blackwelder, 1948; Hunt, 1954), Arctic (Btidel, 1960; Cailleux, 1967; Krumbein, 1969; Rapp, 1960; Skarland and Giddings, 1948; Washburn, 1969b), Antarctic (Glazovskaya, 1958; Glazovskaya, 1971; Markov, 1960; Markov et al., 1970; McGraw, 1967; Skarland and Giddings, 1948; Taubert, 1956; Tedrow and Ugolini, 1966; Ugolini, 1970), littoral (Hooke et al., 1969), and humid mid-lattitude settings (Hooke et al., 1969; Hunt, 1954; Kelly, 1956; Krumbein, 1969; Tricart and Cailleaux, 1964). Hunt (1954) and Engel and Sharp (1958) argued that since manganese-iron coatings are best developed in arid regions, the phenomenon deserves the name "desert varnish" (Engel and Sharp, 1958; Hunt, 1954; Hunt and Mabey, 1966). Krumbein (1969), in contrast, suggested the term "iron-manganese crusts" be substituted to reflect its more global distribution. The term rock varnish was suggested later (Dora and Oberlander, 198 lb; Krumbein and Jens, 1981).

10.2.1.3. Notion of an Ideal Climate of Formation

There was a debate over the role of climate on varnish formation, clearly associated with the foregoing question. The debate has largely died out, although the issue has not been resolved. The discussion was polarized between those believing varnish forms best in deserts, and those believing that varnishes seen in deserts are relicts of a former "pluvial" (wetter) climate. The most common opinion was that the optimum climatic conditions for "desert varnish" formation occur in deserts, where varnish is currently thought to be forming: in the Mojave Desert (Denny, 1965; Engel, 1957; Engel and Sharp, 1958; Hildreth, 1976; Hooke et al., 1969), the Great Basin (Hooke et al., 1969; Lakin et al., 1963; Springer, 1958), the Negev Desert (Krumbein, 1969), Thar Desert in India (Allchin et al., 1978), Libya (Glennie, 1970, p. 20), Australian drylands (Hills et al., 1966; Twidale, 1968), Atacama Desert (Klute and Krasser, 1940; McGinnies et al., 1968), the

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Egyptian Pyramids (Blackwelder, 1948; Emery, 1960; Iskander, 1952), and other locales in Africa (Clark, 1954; Daveau, 1966; Glennie, 1970; Scheffer et al., 1963). Sometimes, climatic conditions were specified. Engel (1957) and Engel and Sharp (1958) observed varnished cobbles in a dirt road in the Mojave Desert that was about 25-years-old at the time of study. As a consequence, they proposed that mean annual precipitation of under 4 inches (10 cm), and hot temperatures after wetting, represents the "optimum" conditions for varnish formation. Grant et al. (1968, p. 44) argued that dust storms, followed by rain and high temperatures, favor varnish development. Altithermal climates favored black manganese-rich varnishes (Glennie, 1970, p. 19; Hayden, 1976, p. 277; Allen, 1978). Other investigators believed that frequent moistening increased rates of varnish formation (Goodwin, 1960; Klute and Krasser, 1940; Turner, 1963), and that the black (manganese-rich) varnish now found in deserts formed more rapidly and extensively in prior "pluvial" (wet) periods (Capot-Rey, 1965; Hunt, 1954; Hunt, 1961; Hunt, 1974; Hunt, 1975; Hunt and Mabey, 1966; Passarge, 1955; Tricart and Cailleaux, 1964). Hayden (1976, p. 278) felt that orange-bottom varnish has a pluvial origin. Varnish is thought to be forming currently on: fiver boulders between high and lower water stages (Hunt, 1954; Potter, 1979), cliffs over which seepage flows (Engel and Sharp, 1958; Hunt, 1954; Hunt, 1961; Hunt, 1972; Smith, 1978; Supplee et al., 1971), in more moist microenvironments (Turner, 1963), alpine settings (Engel and Sharp, 1958; Glazovskaya, 1968; H011erman, 1963; Klute and Krasser, 1940; Krumbein, 1969), Arctic and Antarctic environments (Btidel, 1960; Ugolini, 1970; Washburn, 1969b), semi-arid locales such as the Sahel (Tricart and Cailleaux, 1964). The explanation for requiring moisture was that the metals enhanced in varnish required transport by water (Butzer and Hansen, 1968; Hunt, 1954; Hunt, 1961; Supplee et al., 1971). Klute and Krasser (1940) argued that the poor varnish in the Atacama desert, compared with excellent varnish in the Patagonian Pampa and European Alps, indicates the importance of frequent wetting. As long as the rock surface is free of vegetation, frequent wetting was felt to assist in the penetration of water in stones that contained iron and manganese on moraines in the Alps. This explained rapid formation of varnish (coveting 10% of rocks) on moraines deposition ninety years earlier in the 1850's. The English language debate over the climate of varnish formation was probably best formulated in the writings of Charles Hunt (1954, 1961, 1974) and Celeste Engel and Robert Sharp (1958). Whereas Hunt (1954) argued that varnishing was a product of past pluvial periods, Engel and Sharp (1958) noted varnish formation within 25 years in the Mojave Desert; they also pointed to dendritic growth forms and the coalescence of black spots indicative of contemporary varnish formation. Engel and Sharp (1958) advocated a steady-state model, where the state of varnish was a balance between formative and destructive processes. Varnishes are so abundant in deserts, just because varnishforming processes are more important than varnish-destroying processes. Hunt (1961, p. B 194-B195) responded: "Engel and Sharp (1958, p. 515-516) overemphasize such a locality---one where they infer varnish has been deposited in 25 years. But the total record indicates clearly that such deposition is highly localized and exceptional. Were it otherwise, buildings and other surfaces, artificial and natural, that are as old as 25 years should generally be darkly stained."

Engel and Sharp's (1958) conclusion that varnish can form in 25 years has been extensively cited (Cooke and Warren, 1973; Fairbridge, 1968a; Goudie and Wilkinson, 1977; Hem, 1964), and sometimes with a critical eye (Mabbutt, 1977). Later studies (Dorn and Oberlander, 1982; Elvidge, 1982) revealed that the Mojave road construction

Rock Varnish

site was a case of a reformed desert pavement. Some of the cobbles had abrasion marks made by the road construction equipment, and varnish had not reformed in these marks by 1980 (Dora and Oberlander, 1982); however, the cobbles thrust into the soil by road formation did reappear. The up thrusting process, in part responsible for pavement reformation (Springer, 1958), was not recognized when Engel (1957) conducted the original field work. An issue in this debate revolves around the interpretation that past climate could be inferred by studying the health of varnish at a site; the occurrence of flaking varnish, circular blisters, and isolated patches of varnish on a largely unvarnished surface could indicate that varnish was not forming at present and was in a state of deterioriation (Clements and Clements, 1953; Demangeot, 1971; Engel, 1957; Engel and Sharp, 1958; Grant et al., 1968; Hildreth, 1976; Hunt and Mabey, 1966). Daveau (1966) argued that much varnish in North Africa was relict. Still others argued the varnish in northwestern Australia formed in the more arid, last full-glacial period (Clarke, 1977). Some used varnish to interpret paleoclimates on the basis of coatings on buried artifacts (Clark, 1950). Caution was also urged in the use of the occurrence of desert varnish as an indicator of past aridity, because "desert varnish" existed in a variety of environments (Cailleaux, 1969; Krumbein, 1969; Krumbein, 1971; Mabbutt, 1977). Uncertainty over the role of climate was mirrored in a minor disagreement over the role of aspect. Observations in the Mojave Desert (Engel and Sharp, 1958) suggested there was no consistent variation related to aspect; rather the stability of the underlying rock is more important (Engel and Sharp, 1958). Engel (1957) observed that entire desert mountain ranges, miles long and thousands of feet high were "completely" coated with "desert varnish". In contrast, aspect was thought to influence varnish development in southern Africa (Goodwin, 1960). Turner (1963, p. 14-15) concurred for Glen Canyon in the Colorado Plateau. "Petroglyphs that are shaded all year around do not have patina. Petroglyphs of the same style, with some parts shaded part of the time and some sunlit, vary in patina, although almost always the sunlit designs have more patina than the partially shaded designs (Turner, 1963, p. 14-15)."

10.2.1.4. Environmental Context

There is no debate in the current literature over the environment where desert varnish or rock varnish occurs. There now appears to be a widespread acceptance that the same phenomenon can be found in a wide variety of environmental settings (Table 10.1). Concomitantly but unfortunately, dialog over the climate most favorable for varnish growth has also been dropped from the literature in the last two decades. This issue, however, has not been resolved. Rock varnish is known to grow all over rock surfaces within decades in some settings (Dorn and Meek, 1995; Dora and Oberlander, 1982; Klute and Krasser, 1940), while in other places only spot coverage grows after a few thousand years (Blackwelder, 1948; Dorn and Oberlander, 1982). Figure 10.2 presents a tentative model to explain rapid growth in places of abundant moisture, but where competing organisms are unable to grow.

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192

Table 10.1. Environments where rock varnish has been observed. Where Found Artifacts Fractures in rocks Geologic Settings Ground Figures Ground-line Historical Surfaces Littoral Petroglyphs Springs

References (Toplyn, 1988) (Dom and Oberlander, 1982; Engel and Sharp, 1958) (Dom and Dickinson, 1989; Marchant et al., 1996) (Clarkson, 1994; von Wedhof, 1989) (Engel and Sharp, 1958; Hooke et al., 1969) (Iskander, 1952) (Hooke et al., 1969) (Bard, 1979; Bard et al., 1978) (Frugal and Sharp, 1958; Hunt, 1954; Hunt, 1961; Hunt, 1972; Supplee et al., 1971) (Lucas, 1905) see also http://minerals'gps'caltech'edu/files/varnish/river-v'gif (Dora and Obedander, 1982) (Bucldey, 1989; Hunt, 1954) (Boussingault, 1882; Phaup, 1932) (Glazovskaya, 1958; Glazovskaya, 1971; Markov, 1960; Markov et al., 197~ Taubert, 1956) (Glazovskiy, 1985; H611erman, 1963; WhaUey, 1984) (Jahn and Maneck, 1991; Washburn, 1969b) (Walther, 1912; White, 1990) (Klute and Krasser, 1940; Linck, 1930) (Krumbein, 1969; Tricart and Cailleaux, 1964) (Linck, 1930; Whalley et al., 1990)

Streams, exoreic Streams, periglacial Streams, temperate Streams, tropical Subaerial Antarctic Subaerial, alpine Subaerial, Arctic Subaerial, deserts Subaerial, periglacial Subaerial, temperate Subglacial

Yam~h Oop~#lon

l

4-- } .--~ ~hrni~h~ l o n

RaridRate d Gro~h [ e.g. eJI]n e s etlinosl Moderate Rat,

(e.g ~tacemaDosedl y ~

~i~e

RaridGrovth ,,it h ~mis h Erosion I e.g. Centml Australial I ~ . .'--? ; ~

J

Mamdam~

Figure 10.2. Generalized model of rates of rock vamish formation, where moisture abundance promotes growth, but only so long as acid-producing lithobionts are uncommon.

Rock Varnish

The model in Figure 10.2 is based on case studies and a general understanding of biotic processes of manganese fixation and biochemical erosion. The slowest varnishing takes place in the harshest deserts where moisture limits manganese enhancement by biotic agencies. Moderate growth rates are found in deserts like the Negev and Mojave; competing lithobionts can grow, but generally only in the wettest microenvironments. The most rapid varnishing takes place where moisture is readily available but other lithobionts have not yet started to grow, for example proglacial streams or rock fractures. Places of rapid lithobiontic growth, like microcolonial fungi central Australia, are places of rapid varnishing and rapid varnish erosion. I note, however, that this model has not been subject to rigorous attempts at falsification.

10.2.2. Physical-Chemical Characteristics 10.2.2.1. Thickness

The thickness of varnishes found in deserts is generally less than 100 micrometers (Cooke et al., 1993; Daveau, 1959; Daveau, 1966; Engel, 1957; Engel and Sharp, 1958; Hayden, 1976; Hildreth, 1976; Hooke et al., 1969; Hunt and Mabey, 1966; Karlov, 1961; Mabbutt, 1977; Marshall, 1962; Scheffer et al., 1963). However, the reader should keep in mind that these individuals sampled prominent examples of rock varnish. When I instituted a 'random' sampling of 30 rock varnishes along a transect one meter in length at the classic 'Salt Springs' site, Mojave Desert, of Engel and Sharp (1958), varnish thickness averaged 24 ktm with a standard deviation of 14 ~tm. The thickest true rock varnish that I have ever observed reached ~600 micrometers; it developed on a chert of the Providence Mountains, Mojave Desert, eastern California. There are reports of varnishes up to 7 millimeters thick (Klute and Krasser, 1940; Krumbein, 1969; Peel, 1960; Rogers, 1966; Tricart and Cailleaux, 1964). The writing of these papers, however, could be interpreted to indicate that these measurements may be on the combined thickness of weathering finds and the overlying rock varnish. Unlike the uniform thickness of tropical fiver varnishes reported by von Humboldt (1804) and Darwin (1897), the "thickness and character of varnish can change sharply even within one locality...In some deserts virtually all rock surfaces are covered with varnish; in others, varnish is absent (Cooke and Warren, 1973, p. 87-88)." Varnish is thickest in protected places (along rock crevices, hollows in the rock, depressions, pits, etc.), on fine grained igneous rocks, on rough or porous landforms without a soil cover, and on rocks rich in iron and manganese (Blackwelder, 1954; Cooke et al., 1993; Daveau, 1959; Daveau, 1966; Engel, 1957; Engel and Sharp, 1958; Hooke et al., 1969; Hunt and Mabey, 1966; Krumbein, 1969; Mabbutt, 1977; Scheffer et al., 1963; Washburn, 1969b).

10.2.2.2. Color

For the most part, opinions on the source of the color of rock varnish has not changed. There is agreement that "the tapestries that are blue-black in color are formed by the manganese, and those that are red are formed by iron" (Supplee et al., 1971, p. 7).

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Figure 10.3. Typical appearance of rock varnish seen in the field, where the darkest (blackest) varnishes a r e enriched in manganese in the surface layers and orange vamishes are not. A. Black vamish growing on quartzite from Salt Springs, Mojave Dese~ the width of the frame is ~10 cm. B. Dusky-brownvamish, growing on a quartzite boulder of Pleistocene shoreline of Searles Lake, Califomia. C. Black (upper rock surface) and orange (surface that the rock hammer rests on) varnishes. The orange varnish is an iron film from a rock crevice position (see Type III iron film in Chapter 9) growing on a rhyodacite lithology from Rainbow Basin, Mojave Desert. D. Orange varnish (Type III iron film) from the underside of a granodiorite cobble in a desert pavement from Mesa, Arizona the width of the frame is ~14 on.

Turner (1963, p. 14) added concerns over the role of varnish thickness and the influence of the host rock, noting that "color is considered to be a function of patina depth." Studying the northern Karroo in South Africa, Goodwin (1960, p. 308) also added the perspective that other rock coatings could influence the appearance of varnish, in that "droppings from perching birds (there are no trees) affect this dark skin of epidaphic patination, rotting and bleaching the slightly glazed dark varnish to a mat, pale russet brown."

10.2.2.3. She en A noted and sometimes conspicuous characteristic of some black varnish is its sheen (Figure 10.4). The shiny appearance has been attributed to eolian polishing with dust (Garner, 1974; Goudie and Wilkinson, 1977; Grant et al., 1968; Hunt, 1954; Klute and Krasser, 1940) and sand blasting (Begole, 1973; Supplee et al., 1971; Turtle, 1983). Eolian abrasion, however, readily removes rock varnish (Laity, 1995) and is not important in creating its sheen. Other alternative explanations for the sheen of varnish have been noted in the literature. These include a thin coating of goethite (Kelly, 1956), the metabolic products of cyanobacteria (Scheffer et al., 1963), and exposure to the sun because shadowed surfaces and varnishes formed in rock crevices had dull varnishes (Engel and Sharp, 1958). While these factors may be important, I favor the importance of a smooth or lamellate surface micromorphology (Krumbein, 1969) in combination with manganese enrichment at the very surface of the varnish (Dora and Oberlander, 1982).

Rock Varnish

Figure 10.4. Shiny vamish can be found in different locations, but the most common is at the ground line. Shiny ground-line band vamishes can be found on cobbles in desert pavements or where soil covers bedrock, a s in this case on basalt at Dry Falls of the Owens River, eastern Califomia.

10.2.2.4. Mineralogy Varnish minerals were originally reported to be amorphous (Engel, 1957; Engel and Sharp, 1958; Hildreth, 1976; Hooke et al., 1969; Mabbutt, 1977), with a streak that is dark brown (Engel and Sharp, 1958). Some thought that goethite (Kelly, 1956; Scheffer et al., 1963) and ferric chamosite (Washburn, 1969b) were important components. Seminal research conducted with inflared spectroscopy, X-ray diffraction and electron microscopy at the California Institute of Technology revealed that the bulk of rock varnish is composed of clay minerals (Potter, 1979; Potter and Rossman, 1977). Clay minerals typically comprise 60% to 70% of rock varnish by weight. The major clays are illite, montmorillonite, mixed-layer illite-montmorillonite, kaolinite (usually a minor constituent, but in some cases a major), and chlorite. The dominance of clay minerals distinguishes rock varnish from manganese-rich heavy metal skins (see chapter 8). Clays are a very minor part of heavy-metal skins, whereas clays are vital to the structure and formation of rock varnish. Electron microscopy exemplifies that clays dominate the structure of varnish; this can be seen as a new layer of varnish is laid down (Figure 10.5) and it can be seen by layered structure seen in cross-sections (Figure 10.6). Clay minerals are cemented to the host rock by oxides of manganese and iron (Potter, 1979; Potter and Rossman, 1977; Potter and Rossman, 1978; Potter and Rossman, 1979a; Potter and Rossman, 1979c). Birnessite is the dominant manganese mineral in black varnish, and hematite is a major iron oxide in both black and orange varnish. Figure 10.7 presents one of the largest Mn-crystals I have seen on or in rock varnish; they may be the radiating rods of birnessite. Figure 10.8 illustrates a large hematite grain. Most of the iron and manganese oxides that cements clay minerals together (Potter, 1979; Potter and Rossman, 1979a) are much smaller - - on the order of nanometers as seen in High Resolution Transmission Electron Microscopy.

195

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Figure 10.5. Secondary electron image of clay minerals accreting on rock varnish on Hanaupah Canyon Fan, eastem California. Note how the individual clay platelets overlap as they cement onto the surface.

Figure 10.6. High resolution transmission electron microscopy image of the parallel structure imposed by clay minerals. The top of the varnish is to the left. Note that the scale bar is in nanometers, so the image is a cross-section of perhaps ten of platelets in Figure 10.5 The sample is from Hanaupah Canyon Fan, Death Valley and was prepared by A. Overson.

Rock Varnish

Figure 10.7. Secondary electron image of radiating crystals that are rich in manganese, according to energy-dispersive X-ray analyses. The radiating form is from a broken inner surface of rock varnish from Undoolya Gap, central Australia and it could be bimessite (Chukhrov et al., 1980).

Figure 10.8. High resolution transmission electron microscopy image of hematite, identified by electron diffraction. The sample is from Hanaupah Canyon Fan, Death Valley and was prepared by A. Overson.

Submicroscopic fragments of detrital minerals are trapped by the clay minerals. The presence of detrital grains was first inferred, based on anomalous Fe and Mn electron microprobe measurements in the lower "subordinate" layer of the varnish (Hooke et al., 1969). Upon closer examination, detrital minerals are common in subaerial rock varnishes. Quartz (Potter, 1979), calcite, titanomagnetites, feldspars (Dorn, 1986; Dorn

197

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Chapter 10

et al., 1990), and barium sulfate (Krinsley and Manley, 1989) have been noted. Figure 10.9 exemplifies different perspectives on detrital minerals, trapped within rock varnish as the clays are cemented by Mn-Fe oxides.

Figure 10.9. Views of detrital minerals, trapped in rock varnish as clays are cemented by Mn-Fe oxides. The abbreviations are on the imagery for the different type of imagery are HRTEMfor high resolution transmission electron microscopy;SE for secondaryelectrons, and BSE for backscattered electrons.

10.2.2.5. Chemistry The analytical chemist Celeste Engel (1957) completed the first M.S. thesis research on rock varnish, and then teamed up with Robert Sharp to write a seminal paper on the chemistry of rock varnish in the Mojave Desert (Engel and Sharp, 1958). Although Engel and Sharp realized the potential for some contamination by the underlying rock, scraping varnish was done with great care to minimize the influence of the rock. The principle elements and oxides in "desert varnish" were determined to be O, H, Si, A1, Fe, Mn with the major oxides of SiO2, A1203, H20, Fe203, and MnO (Engel, 1957; Engel and Sharp, 1958). Like Lucas (1905), a major effort was spent on comparing the chemistry of the rock and varnish, with the elements most enriched in varnish relative to the rock being Mn>H20>Fe>P. Enrichment ratios (varnish:rock) varied from 66 to 292 for Mn, 2 to 66 for Fe, and 13 to 60 for H20 (Engel, 1957; Engel and Sharp, 1958). The essential "problem" of varnish formation, identified originally by von Humboldt (1812), and reiterated by Engel and Sharp (1958) was how to explain the great enrichment in Mn over Fe, relative to the underlying rock. Varnish Mn:Fe ratios

Rock Varnish

varied from <1 to >20, in contrast with ratios of 1:60 in the earth's crust (Engel, 1957; Engel and Sharp, 1958; Glazovskaya, 1968; Glazovskaya, 1971; Hooke et al., 1969; Hunt and Mabey, 1966; Krumbein, 1969; Lakin et al., 1963; Scheffer et al., 1963). Varnish is a 'kitchen soup' of heterogeneous constituents. The inherent heterogeneity became abundantly clear in the minor elements. In order of decreasing concentration, these are: Ca, Mg, Na, K, Ti, and P (Engel, 1957; Engel and Sharp, 1958; Glazovskaya, 1968; Hooke et al., 1969; Marshall, 1962). Like Mn, minor elements could vary considerably (0.5 - 1.5%) from place to place in bulk samples (Engel and Sharp, 1958), and microprobe analyses showed even greater variation with measurements as high as 16.2% (Hooke et al., 1969, p. 283). Ba, and Sr were the most abundant trace elements, with Cu, Ni, Zr, Pb, V, Co, La, Y, B, Cr, Sc, and Yb found in all varnishes in order of decreasing concentration (Engel and Sharp, 1958). U was enriched in varnish more than twenty times, as compared to the underlying rock (Marshall, 1962). Negev varnishes may be rich in Cu and Co (Evenari et al., 1971). The elements Cd, W, Ag, Nb, Sn, Ga, Mo, and Zn were found in some, but not all varnishes (Engel and Sharp, 1958; Lakin et al., 1963). High ambient levels of Cu, B, Ar, and Sb were reflected in higher concentrations of these elements in varnishes from those areas, suggestive that varnish could be useful as a geochemical prospecting tool (Lakin et al., 1963). An electron microprobe examination (Hooke et al., 1969) provided the first definitive data on the chemistry of varnish, because contamination from the underlying rock could be ruled out by using micron-sized beams. In varnishes from Deep Springs Valley and Death Valley, eastern California, Mn and Fe usually increased outward from the rock into the varnish. CaO and MgO usually decreased outward. When A1203, K20, and FeO were in abundance in the rock, they decreased outward from the rock into the varnish. When these oxides were in low concentrations in the rock, they increased in abundance in the varnish. In all samples, MnO increased in an outward direction from the rock. TiO2 was correlated with FeO. With the acquisition of more and more precise chemical data, there has been the realization that the chemical composition of the varnish varies greatly at all spatial scales: from micron to micron within a single depth profile or chemical transect, between locations on the same rock, between different rocks from the same area and between different areas (Dom et al., 1990; Dora et al., 1992a; Dorn and Krinsley, 1991; Dorn and Oberlander, 1982; Dragovich, 1988a; Dragovich, 1988b; Dragovich, 1993a; Duerden et al., 1986; Engel and Sharp, 1958; Glazovskaya, 1968; Hooke et al., 1969; Krinsley and Anderson, 1989; Kfinsley and Dora, 1991; Krumbein, 1969; Lakin et al., 1963; O'Hara et al., 1989; O'Hara et al., 1990). There is also considerably chemical variability. In some places, trace elements like barium may reach minor element proportions. In other places, barium is below the limit of detection. In some places, fragments of organic matter are commonly mixed in and underneath varnish; in other places, organic matter is lacking altogether. The spatial heterogeneity of varnish chemistry can be illustrated through chemical analyses and visually. Table 10.2 presents the bulk elemental chemistry of rock varnishes in deserts. Although several milligrams were measured together to average internal heterogeneity, there is still a lot of variability in the chemistry of varnish in different places and in different geomorphic positions. The chemical variability is not limited to desert regions. Table 10.3 demonstrates both the intrasite and intersite variability. Manganese and silica show the highest variability, since these are major constituents and they are negatively correlated. However, all constituents show place-to-place and intrasite variability that often exceeds 25%. Consider the chemistry of stream varnish in Virginia (Table 10.4). Intersite

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Chapter 10

variability for manganese ranges from 30% to 50% by weight. Although lower in abundance, the ranges from other elements are greater. Calcium, for example, ranges from 1.6% to 7.9%. The larger conclusion is that non-desert rock varnishes have a chemistry similar to desert rock varnishes, and that they display considerable variability. The variability in the constituents of subaerial varnishes can be considered from the perspective of different places in the world (Table 10.5) and from different places within a localized region (Table 10.6). These results reveal that intrasite variation displayed in Table 10.5 is similar t6 intersite variation displayed in Table 10.6. Furthermore, averaging many different electron microprobe measurements averaged together yields a general compositional picture similar to bulk chemical analyses (Table 10.2) (Engel and Sharp, 1958; Lakin et al., 1963).

Table 10.2. Examples of elemental variation exhibited in bulk chemical analyses of rock varnishes found in desert regions. SITE:

POSITION: Na Mg AI Si P S K Ca Ti Mn Fe Ni Cu Zn Rb Sr Zr Ba Fb

Salt TrailFan, Manix Makanak Sinai Petroglyph Ingenio, Ayers Peru Rock, Springs, D e a t h Lake, a Till, Peninsula, South Mojave Valley Mojave Hawai'i Egypt Australia Desert Australia Desert* Desert Unknown Former Rock Fracture 0.25** bid*** 4.4 0.14 25.84 23.74 37.49 39.09 1.61 0.49 na 0.7 2.35 3.45 0.8 4.87 0.74 1.52 11.77 10.87 14.5 13.47 na 0.13 na 0.12 na 0.27 na bid na bid na 0.29 0.25 0.85 na bid

> lm Above Soil 1.1 3.44 25.77 32.35 1.15 0.3 2.11 1.35 0.84 12.47 18.09 bid 0.22 0.3 0.25 0.21 0.22 0.19 0.74

With Silica Skin 0.62 1.98 21.13 29.77 0.69 0.2 3.3 4.89 0.73 13.6 21.13 bid 0.33 0.49 bid bid bid 0.16 0.98

>lm Above Soil 0.28 1.5 22.94 32.81 bid bid 2.42 2.91 0.68 11.97 22.94 bid 0.25 0.42 bid 0.42 bid 0.18 0.27

> 1m Above Soil 0.17 1.21 22.81 33.34 0.53 bid 2.79 2.18 0.65 21.7 13.26 bid 0.44 0.44 bid bid bid 0.14 0.34

At Soil Surface na**** 2.11 20.45 45.88 0.53 1.13 2.91 6.22 0.85 4.94 12.03 bid 0.04 0.16 bid 0.11 bid 2.42 0.22

From Rock Fracture na 1.58 28.77 35.69 bid bld 2.11 1.45 1.19 11.91 16.57 bid bid bid bid bld bid 0.73 bid i

i

* Results are normalized to 100% ** Measurements by PIXE (Cahill, 1986), except for the Salt Springs sample which were analyzed by electron microprobe (Potter and Rossman, 1979a). *** Below limit of detection **** Not available

201

Rock Varnish

Table 10.3 Analyses of rock varnishes in non-desert environments. Measurements were made with the electron microprobe. The saprolite sample (data from Weaver, 1978) had a water content of 11.95%. The Icelandic varnish came from a subsurface position (data from Douglas, 1987), but the other analyses were on subaerial varnishes or varnishes exposed to water flow. Ave (n) refers to the number of electron microprobe analyses averaged. SD refers to the standard deviation. Table abbreviations are as follows: nm, not measured; nr, not reported; and bid, below limit of detection. Site

Na20 MgO AI20 3 SiO2 P205 SO3 K20 CaO TiO2 MnO Fe20 3 BaO Total

Saprolite

nr

0

21.51 25.64nr

nr

0.67 0

Iceland

0.23

0.66

27.18 25.400.04

nr

0.26

1.73 1.47 9.90

1 . 2 6 14.76 18.67 4.08 0.10

0.92

1.04 0.55 16.34 21.00 1.66 80.37

0.39

Lake Louis Ave bid (reservoir) (5) shoreline, SD bid Wyoming

Mt. Van Ave bid Valkenburg (49) Antarctica SD bid

2.33

622

0.33 1.22 5.41

66.73

10.51 nr

77.39

0.79

0.07

0.34 0.12

1.86 16.36 21.98 2.50

0.24

1.16 1.92 1.09 20.15 11.05 1.90 80.20

0.66

0.38

0.63

1.83

7.04 0.90

0.31

11.58 3.45

1.15

5.37

1.17 3.24

29.309.45

0 . 8 3 1.39 4.22 0.26 31.39 0.98

0.37

82.65

020

2.18

11.15 3.97

029

0.15

2.96

PopoAjie River, Wyoming

Ave 0.11 1 . 0 4 5.27 (10) SD 0.26 0,29 2 2 5

28.92 9.78

1.31 1.39 10.20 0.15 25.00 1.57

0.70

85.42

11.24 3.44

0.55

029

0.54

3.37

Boulder, Sky Lake, TienShan, China

Ave 0.30 1.85 13.62 23.51 nm (211) SD 0 . 2 3 0.39 2 . 6 1 5.02 nm

nm

1.82 1.47 0.73 19.83 10.89 1.76 75.78

nm

0.35 0.36

Ave 0.04 (9) SD 0.13

031

1.62

5.87

2.90

321

3.71

r~g, So.

1.14

0_52 5.79

nr

0.49 8.63

0.83

0.10 10.30 0.84

0.32 5.30

2.09

0.56

12.02

Table 10.4. Energy dispersive semi-quantitative analyses of rock varnish in streams in Virginia, normalized to 100% (Robinson, 1993). Elements below the limit of detection (-0.8%) are listed as nd. Values are elemental weight percent. Stream War Branch Goodwin Creek Elk Run Elk Run

N a i

1.1 0.4 0.7 0.2

Mg 0.4 0.3 3.0 0.3

A1 10.4 4.7 11.6 10.8

Si 14.6 22.2 15.8 23.5

i

K nd 0.7 nd 1.5

Ca 4.4 3.7 1.6 7.9

Mn 43.3 50.2 43.8 30.1

Fe 25.8 17.8 23.5 25.7

Chapter 10

202

Table 10.5. Averages and standard deviations of electron microprobe analyses of rock varnishes from different desert regions: New South Wales in Australia (Dragovich, 1988b); Nasca line in Peru; alluvial fan in Tunisia (White et al., 1996); the Takla Makan desert in western China; and the Sonoran Desert in Arizona (Perry, 1979). Ave (n) refers to the number of electron microprobe analyses averaged. SD refers to the standard deviation; nr means not reported. Site

Na2OMgO A1203 SiO2 P205 SO3 K20 CaO TiO2 MnO Fe20 3 BaO Total

New South Wales

Ave 0.60 1.25 23.38 28.73 2.28 0 . 1 7 1.11 1.09 0.87 15.14 15.01 (~0) SD 0 . 3 5 0.10 1.41 5 . 0 3 0 . 4 3 0 . 0 5 0.31 0.49 0.23 7.75 2.00

Nasca line Peru

Ave 0.12 0.86 17.65 21.08 1.61 0 . 1 2 1.53 1.00 0.50 15.34 21.66 1.68 83.14 (10) SD 0 . 0 5 0.34 1.82 1.89 0.71 0 . 1 0 0.38 0.49 0.33 8.21 12.40 0 . 7 3 3.31

Alluvial Ave 0.22 2.05 12.36 24.83 0.47 1.35 1.40 1.10 0.71 16.05 8.51 fan (10) Tunisia SD 0.20 0.37 1 . 4 0 4 . 3 0 0 . 3 0 0 . 8 6 0.50 0.45 0.23 2.31 0.75 Takla Makan China

0.70 90.57 0.35 2.54

4.88 73.92 1.19 5.17

Ave 0.08 1.99 16.14 23.42 1.55 0 . 0 3 1.42 1.57 0.56 17.39 14.06 1 . 0 0 79.22 (25) SD 0 . 0 5 0.55 1 . 5 4 3 . 1 3 0 . 3 3 0 . 0 4 0.37 0.20 0.13 5.10 1.58 0.27 4.37

Sonoran Ave 0.21 2.48 23.19 30.95 1.25 0 . 1 7 1.76 0.94 0.62 11.55 19.00 Desert (8) Arizona SD 0 . 0 8 0.55 2 . 7 1 5 . 2 4 0 . 1 7 0 . 0 3 0.37 0.25 0.14 9.41 4.67

nr

92.11

nr

2.23 i

9

Table 10.6. Averages and standard deviations of electron microprobe analyses of rock varnishes within four regions. This table is characterized by samples coming from different geomorphic surfaces in a region: for Bishop Creek, samples came from 156 different places from glacial moraines; for Death Valley, 300 locations from alluvial fans; for the Mojave Desert, 100 petroglyphs; for South Australia, 20 alluvial fan boulders. Site

Na20 MgO A1203 SiO2 P205 SO3 K20 CaO TiO2 MnO Fe20 3 BaO Total

Bishop Ave 0 . 0 9 1 . 0 6 13.54 21.45 2.13 0.15 0.96 1.03 0.79 16.99 15.91 0.26 74.37 Creek (156) California SD 0.22 0 . 6 1 2 . 9 6 7 . 8 4 0.96 0.21 0.58 0.34 0.36 14.33 3.26 0 . 2 7 6.18 Death Ave 0.29 Valley (300) California SD 0.23

1.61 16.77 20.81 2.01 0.20 1.34 1.22 0.62 14.44 12.16 0.65 72.12 0 . 6 0 2 . 7 3 4 . 2 5 0.73 0.24 0.34 0.52 0.57 8.30 2 . 7 2

Mojave Ave 0 . 2 3 0 . 8 1 11.26 33.69 1.12 0.17 1.08 1.11 0.50 23.07 7.73 Desert (115) Califomia SD 0.45 0.61 7 . 7 4 23.55 0.89 0.21 0.75 0.66 0.56 18.13 5.87 Olary Ave 0.16 South (100) Australia SD 0.39

0.66

8.68

0 . 6 2 81.38 0 . 5 9 9.57

1.55 16.80 26.35 2.15 0.06 1.15 0.86 0.68 16.18 14.70 0.50

81.15

0 . 5 5 1 . 9 6 7 . 6 6 0.39 0.12 0.35 0.39 0.24 6.87 2 . 8 8 0 . 2 5 3.88

203

Rock Varnish

The most detailed work on the trace element chemistry of varnish was done with the aid of neutron activation (Bard, 1979; Bard et al., 1978). Varnishes were scraped from andesite boulders and petroglyphs on shorelines of Pyramid Lake, Nevada. Table 10.7 exemplifies data obtained in the dissertation of James Bard. Most of the trace elements found in rock varnish are highly correlated with the abundance of manganese. This is likely because of the scavenging ability of manganese oxides (Filipek et al., 1981; Jenne, 1968; Loganathan and Burau, 1973; Robinson, 1981; Saeki et al., 1995). Silicon is not measured by neutron activation, but microprobe measurements indicate that silica is inversely proportional to manganese, and its co-associated elements: Ca, V, As, U, Ba, Sm, La, Lu, W, Mo, Co, Sc, Sb, Th, Zn, Ce, Yb and Nd.

Table 10.7. Neutron activation analyses for a rock varnish at Eetza Mountain, near Fallon, Nevada (Bard, 1979).

Element

Surface Layer 2nd 'i~ayer

3rd Layer

4th Layer

5th Layer

8.34 5.4 2.36 0.65 21751 7.28 250 14.86 52.5 12.72 5621 17.96 111.83 0.82 37.47 6.82 81.62 19.12 2.28 2.08 41.1 28.34 22 18 2.07 186 4.22 342.2 3.38 0.45 6.53 103.8

9.03 7.0 2.38 0.44 18601 7.07 200 11.50 48.8 11.71 5200 16.19 101.48 0.80 29.24 8.15 73.24 19.02 1.99 1.62 43.6 24.45 49 13 1.83 192 3.9 300.1 3.15 0.43 5.95 91.4

8.6 4.9 2.43 0.29 17771 6.99 246 16.82 44.4 11.27 4896 15.23 94.32 0.71 26.15 6.74 70.45 18.75 2.08 1.59 39.5 22.31 46 25 1.78 182 3.68 283.8 3.19 0.41 5.64 86.1

i

AI % Ca % Na % Ti % Mn ppm Fe % V ppm Dy ppm As ppm U ppm Ba ppm Sm plma La ppm Lu ppm W ppm Mo ppm Co ppm Sc ppm Cs ppm Sb ppm Cr ppm Th ppm Ni ppm Rb ppm Tb ppm Zn ppm Eu ppm Ce ppm Hf ppm Ta ppm Yb ppm Nd ppm

8.95 5.7 2.18 0.42 22651 7.53 280 16.04 60.7 13.76 5777 20.34 126.58 1.01 44.1 11.54 94.12 20.59 2.59 2.57 50.1 36.39 53 23 2.29 200 4.67 417.9 3.4 0.44 7.42 116.9

8.7 5.4 2.35 0.44 21751 7.5 276 17.13 64 13.36 5715 19.16 119.28 0.92 38.21 7.5 85.87 19.95 1.84 2.45 42.2 32.32 44 25 2.15 209 4.46 377.6 3.36 0.43 6.91 110

204

Chapter 10

Some generalizations can be made from the above chemical analyses. 1) Silica and aluminum, taken together, comprise the bulk of rock varnish. This is consistent with clays being the dominant mineralogy. 2) Manganese and iron oxides comprise one-quarter to one-third of rock varnish. However, there is a tremendous amount of point-to-point variability in their abundance; clays are high when oxides are in lower abundance. 3) The minor elements display variable patterns. Generally, Mg, K, and Ca are correlated with clays in cation-exchange positions and not with manganese. Ba is often correlated with S in barium sulfates (Cremaschi, 1996; Dorn et al., 1990; O'Hara et al., 1989). In other cases, both Ba and Ca are correlated with Mn (Reneau et al., 1992). P is sometimes correlated with iron and sometimes with Mn. Ti can be correlated with Fe in titanomagnetite detrital grains, but more often Ti is not well correlated with any major element. 4) The trace elements are generally correlated with Mn abundance, and sometimes with Fe. 5) Because varnish is porous, contains water, carbon, and a great number of trace elements, electron microprobe measurements typically have total measurements that are far less than 100%. These generalizations, in summary, do not greatly alter the fundamental findings in the seminal work of Engel and Sharp (1958). An alternative perspective on the chemistry of rock varnish is to examine the larger picture, literally. When rock varnishes are ground down very thin, it is possible to see through normally opaque layers. When viewed in cross-section with an optical microscope, there are orange, yellow and black layers. Orange and yellow layers are not enriched in manganese and black layers are manganese-rich (Perry, 1979; Perry and Adams, 1978). Figure 10.10 is an example of layers that can be found in rock varnish.

Figure 10.10. Optical ultra thin section of rock varnish from Galena Canyon alluvial fan, Death Valley. The rock varnish is about 200 I.tm in thickness. The dark layers appear black under a microscope. The gray layers are mostly orange, but the lightest gray layers are yellow. This thin section was prepared by Liu (1994).

Originally, only two layers were recognized in many varnishes: a bottom layer rich in iron and a top layer rich in manganese (Engel, 1957; Hunt and Mabey, 1966; Krumbein, 1969). I believe that this is a relatively common sequence that is produced

Rock Varnish

by the opening of rock crevices. Iron films grow in closed rock crevices (chapter 9); as the crevice opens, manganese-rich black varnishes grow over orange, iron films - - all in the rock fracture. When the varnish is truly subaerial in its genesis, the type of layering pattern seen is much more complex, like those reported by Liu and Dorn (1996) and seen in Figure 10.10. There is often a much more complex story in the layering of rock varnishes, spearheaded by the research of Tanzhuo Liu (Liu, 1994; Liu et al., 1997; Liu and Dorn, 1996). Liu explains the three colors seen in cross-sections. Yellow colors form during hyper-arid intervals and are the richest in clay minerals, iron, and tend to be enriched in C1. Orange colors form during arid periods; they can have manganese enrichment of a few percent. Black layers accrete during semi-arid periods in regions that are now deserts and are greatly enriched in manganese over one hundred times above adjacent dust, soil and rock material. Liu's visual work reveals that layering patterns are consistent within a particular region (e.g. Death Valley, California, the Dead Sea, western China, Patagonia), and that these layering patterns may also correlate with larger-scale climatic changes. Chemical variability in rock varnish can also be mapped with X-rays (Figure 10.11). X-ray mapping confirms earlier findings that the optical layers correlate with chemistry (Dom, 1990; Perry and Adams, 1978).

Figure 10.11 X-ray mapping of a thin section of rock vamish from Death Valley, courtesy of Tanzhuo Liu. White indicates the highest concentrations. Note, for example, that the uppermost layer (the Holocene) is rich in Si, A1, Mg and CI, but is not enhanced in Mn.

The great chemical variability reported in rock varnishes may be in part from different layers being analyzed. There is also considerable evidence to indicate that great chemical variability occurs from the analysis of different places in varnishes that are not well layered (Dora and Krinsley, 1991; Krinsley and Dora, 1991; Liu, 1994).

205

206

Chapter 10

10.2.2.6. Micromorphology Interest in the form of rock varnish began with the use of scanning electron microscopes (SEM). While optical microscopy had been used to examine varnishes in cross-section (Haberland, 1975; Hunt, 1961; Lucas, 1905; Perry and Adams, 1978), the characterization of varnish morphology began in earnest with SEM. The first observations of varnish indicated a fiat, smooth micromorphology. Krumbein (1969, p. 353) characterized "desert varnish" as having a "lamellate structure...whose particle size is almost extremely small..." Figure 10.12 and 10.13 exemplify this morphology, certainly the most common in arid settings.

Figure 10.12. Secondary electron image of lamellate rock varnish on the high shoreline of Lake Lisan, a paleolake of the Dead Sea, Israel. Note the subparallel accretion of clay particles on the varnish surface.

Figure 10.13. Secondary electron image of lamellate rock varnish on a debris cone of Death Valley, Califomia Note how the clays impose a lameUate structure in cross-section, as first noticed by Potter and Rossman (1977).

Rock Varnish

Subsurface varnish formed in saprolite fractures surfaces in the southeastern United States have a fiat, clay-oriented fabric (Weaver, 1978). Potter (1979, p. 172-173) similarly characterized Mojave Desert varnish as flat with "a coherent fabric due to clay minerals oriented roughly parallel to the rock surface. Perry (1979, p. 15) observed that microlaminations in cross-sections can be "traced laterally for several millimeters" in these smooth varnishes. Whalley and colleagues observed similar textures in Afghanistan, Norway, and Tunisia (Smith and Whalley, 1988; Whalley, 1983; WhaUey, 1984; WhaUey et al., 1990). "Dendrite-type structures" (Perry, 1979, p. 23) were noted on an Idaho varnish; this is essentially a tongue of a newer layer of Krumbein-texture varnish with a longer length than width. Rougher surface micromorphologies also occur. Perry (1979, p. 20-21) documented the occurrence of "botryoidal growth-structures" that are "mound-like" and "enlarge, coalesce, and are succeeded by new mounds as varnish accretes." Botryoids vary in height from "a few micrometers to as much as 40 micrometers (Perry, 1979, p. 20)." Figures 10.14, 10.15, and 10.16 exemplify botryoidal varnishes.

Figure 10.14 Secondary electron image of botryoidal rock vamish at the North Summit of Mt. Ellen, Henry Mountains, Utah. The sample was collected by J. Bendix.

Figure 10.15 Secondary electron image of botryoidal rock vamish at Point of Rocks Picnic Area, Davis Mountains, west Texas.

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Figure 10.16. Botryoidal varnishes from Kitt Peak, Arizona, seen in two perspectives. The view on the left shows the topography by secondary electrons. The view on the right shows these structures from the bottom upwards with backscattered electrons.

Other, less common, morphologies occur. "Looped structures" were seen on varnish from Stoddard Wells in the.Mojave Desert (Perry, 1979, p. 22); this term described sinuous and anastomotosing micron-scale fibers that have the appearance of montmorillonite (Robert and Tessier, 1992). Smoothed-mounds were also noted at Stoddard Wells in the Mojave Desert (Perry, 1979, p. 24): "This sample was collected from a stone pavement where prevalent winds may cause wind-abrasion and subsequent polishing of the varnish surface." A "wrinkled coat" micromorphology was noted by Whalley (1983, p. 208) that was felt to be a primary depositional features, perhaps related to dehydration. Also, micron-scale "protuberances," some of which are enriched in Mn or Fe rich, are on top of the wrinkles. The vast majority of rock varnishes have micromorphologies that range between lamellate and botryoidal. A number of factors can influence micromorphology, including clay content, manganese abundance, epilithic organisms, morphology of the underlying rock, varnish thickness, eolian abrasion, proximity to soil, and other microenvironmental factors (Dorn, 1985; Dorn, 1986). Figure 10.17 exemplifies the importance of local controls on varrtish form that may be only a few microns apart. The microenvironmental effect seen in Figure 10.17 is consistent with what is known about lamellate varnish. The clay minerals in the rock fracture impose more of a layered structure than the botryoidal varnishes seen on a surface where clays can be washed off with precipitation There also appears to be a general regional correlation between micromorphology and vegetation abundance. Abundant vegetation correlates with micromorphologies that are botryoidal, whereas less vegetation cover is associated with lamellate structures (Dorn, 1985; Dorn, 1986). This is probably because dust is less common in humid regions, and manganese-rich botryoidal centers are able to grow in discrete clusters. In contrast, areas of less vegetation contain more dust. According to this interpretation, clays overwhelm and literally bury tendencies towards nucleation of manganese around discrete clusters.

Rock Varnish

Figure 10.17. Secondary electron image of two different side-by-side vamish morphologies collected from the Poverty Hills, Owens Valley, eastem Califomia. The lamellate varnish on the right side of the micrograph formed in a rock crevice. In contrast, the botryoidal forms on the left side grew in a subaerial environment. The scale bar is 4 microns.

10.2.2.7. Textures Seen in Cross-Section

Rock varnish cross-sections have different appearances depending upon the technique used, including light transmitted through thin sections (see section 10.2.2.5; Engel and Sharp, 1958; Haberland, 1975; Hume, 1925; Hunt, 1954; Laudermilk, 1931; Liu, 1994; Lucas, 1905; Perry, 1979), secondary electrons (Dorn, 1984; Dorn, 1986; Perry, 1979), backscattered electrons (Haberland, 1975; Krinsley and Dorn, 1991; Krinsley et al., 1990; Krinsley and Manley, 1989; Nagy et al., 1991; Raymond et al., 1992; Raymond et al., 1989; Reneau et al., 1992), and transmitted electrons (Krinsley et al., 1995; Raymond et al., 1992). This section explores how varnishes appear in cross section when viewed by different types of electron microscopy. Figure 10.18 compares images made by backscattered, secondary, and transmitted electrons, Backscattered electrons show an overall layering pattern, with secondary electrons revealing that the layers are broken up into overlapping subparallel structures. At very high resolution, seen with transmission electron microscopy, these layers are in turn composed of even smaller subparallel units that are mixture of clays and oxides. The clay minerals impose the lamellate structure seen at the resolution of secondary electrons and resolved with high resolution transmission electron microscopy. High resolution transmission electron microscopy, conducted in collaboration with Dr. David Krinsley (Krinsley and Dorn, 1995), has revealed that clay minerals gradually weather in place. Figure 10.19 shows imagery of clay minerals at different scales. The lattice structures are gradually breaking apart as manganese and iron oxides mobilize and reprecipitate between the lattice structures. This creates 'monolayers' at the scale of nanometer (Robert and Tessier, 1992). Figure 10.19 is organized at different spatial scales, from lower resolution in the upper left to very high resolution in the lower

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fight. All o f the images show that the regular lattice spacing of clay minerals is disturbed by weathering within the varnish. A classic question in rock varnish is whether or not rock varnish derives from the underlying rock. This question was resolved in the late 1970s when scanning electron microscope imagery shows distinct boundaries between the rock and the overlying rock varnish (Allen, 1978; Perry, 1979; Perry and Adams, 1978; Potter and Rossman, 1977). Many images presented throughout this chapter reinforce the idea that rock coatings are accretions. Figure 10.20 shows that there is a clear contact for rock varnish even at the scale of n a n o m e l e r ~ _

Figure 10.18. Different types of electron microscope imagery from Death Valley imagery in row A, Antarctica in row B, and Peru in row C. The backscattered electron (left column) and secondary electron (middle column) imagery provides different perspectives on the samples. The HRTEM imagery (right column) yields much more detailed information. The secondary images are of samples mechanically broken, and the varnish/rock contact (arrows) was determined with the aid of energy dispersive X-ray analyses; these varnishes were coUected a few millimeters away from the section imaged by both BSE and HRTEM. The BSE imagery shows the complete section before ion milling prepared the sample for examination with HRTEM; the double arrow in the middle of the BSE image of Antarctic varnish 031) shows the location of an angular unconformity. The scale bar is in micrometers.

Rock Varnish

Figure 10.19. Weathering of clay minerals in samples from Hawai'i (A,G,K,M), Antarctica (B,D,F,L,N), Peru (C,I), and Death Valley (E,H,J,O) as revealed by transmission electron microscopy imagery with scale bars in nanometers. Images A and B illustrate areas where the organized lattices of mixed-layer illite-montmorillonite clays are starting to separate--highlighted by the arrows. Image B shows a transition in an illite grain from more (left) to less (right) organized material. Images C and D illustrate the migration of granular-textured material into clay minerals, highlighted in image C with an arrow. In image D, the granular material runs from upper left to lower fight and the splitting clays rest on top (between arrows). The arrows in images E-L illustrate the separation of clay minerals into mono-layers (Robert and Tessier, 1992). In some cases, mono-layers feather in the middle of a clay pod (E,F,H). In G, the arrow highlights separation at the contact between two less-weathered clays; the darker grain below the arrow is too thick to be electron transparent. The tip of a clay mineral is starting to separate into mono-layers at about the position of the arrow in image I. The arrows in images J and K exemplify the ubiquitous wavy texture from spalling' along the individual 001 planes. Images L-O illustrate regular lattice spacing juxtaposed against more chaotic textures, all within a single pod. In L, the arrows highlight places where laminae separate and develop an amorphous character. Lattice spacing on the left side of M, lower part of N and left side of O is consistent with iUite and textural interstratified clay (Banfield and Eggleton, 1988; Mall and Komameni, 1989; Robert et al., 1990); the opposite sides illustrate a more disorganized pattern of clay weathering and areas of amorphous (less crystallized) textures.

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Figure 10.20. TEM imagery reveals a clean contact between rock and vamish from (A) Nazca, Peru (B) Antarctica (C) Death Valley (D) Kaho'olawe Island, Hawari. Open arrows indicate the contact. The dark arrow in D shows an ambiguous contact, probably because the rock rests overlying the varnish (or visa versa) in the TEM foil. As in all TEM imagery, the scale bars are in nanometers.

10.2.2.8. Post-Depositional Modification Post-depositional modification is where the varnish changes after it is originally laid down. Probably the most important post-depositional modification is the leaching of cations from rock varnish. Cation exchange (CE) in soils occurs with smectites, hydrous mica, chlorite, interstratified minerals, kaolinite, oxides and hydroxides and organic compounds (Talibudeen, 1981; Tucker, 1983); all of these are found in rock varnish. In addition, both Mn- and Fe-oxides can serve as cation-exchange complexes (Jenne, 1968). Hence, it should not be surprising that rock varnish experiences cation exchange. High resolution transmission electron microscopy reveals that the vast majority of varnish consists of submicron-sized grains (Figure 10.18, Figure 10.19, Figure 10.20). Consequently, large surface areas are available for CE. Little is known about the rates and kinetics of adsorption in natural environments, however, especially in environments like rock varnish; it is clear, however, that CE processes go on as solutions pass through submicron-sized and micron-scale pores. Clay minerals develop a negative charge and attract cations to exchange positions because of isomorphous substitution. A1 (III) in octahedral layers and Si (IV) in tetrahedral sheets are replaced by ions of smaller valence, for example Mg (II), Fe (II),

Rock Varnish

Zn (II) and Ni (II) (Goulding and Talibudeen, 1980). In order to keep electronuetrality, exchangeable cations are attracted. These charge-compensating cations are found in clays within interlayer positions (e.g., smectites), in structural holes, on fracture surfaces, where interlayers are disturbed by hydration or a foreign cation and on external surfaces (Talibudeen, 1981). The mixed-layer illite-montmorillonite clays, for example, have a stronger preference for K, Rb and Cs due to the match of cation size and structural holes in minerals (Brown et al., 1978). Visual (Dorn and Krinsley, 1991; Krinsley et al., 1990; Liu, 1994; Liu et al., 1997; Liu and Dora, 1996) and chemical evidence (Dorn, 1989; Dragovich, 1997; Glazovskiy, 1985; Pineda et al., 1990; Whitley and Annegard, 1994; Whitney and Harrington, 1993; Zhang et al., 1990) all point to cation leaching as an important process in rock varnish. However, arguments have been made that cation-leaching does not occur in rock varnish (Reneau et al., 1990; Reneau et al., 1992). Yet, the evidence for postdepositional modification by leaching is too strong to ignore; it includes layers in rock varnish are interrupted by pockets of chaotic structure (Figure 10.21), and the redeposition of the leached material (Figure 10.22) that cannot be explained without cation leaching.

Figure 10. 21. Backscattered electron image of rock varnish growing on Starvation Canyon Fan, Death Valley. Note how even layering is broken in the upper right hand part of the varnish (arrow). In addition, there are pockets of micron-scale leaching in the lower layer.

There has long been an interest in rock varnish as an agent of case hardening of soft rocks or weathered substrates (Butzer and Hansen, 1968; Daveau, 1966; Demangeot, 1971; Karlov, 1961; Kiersch, 1950; Oberlander, 1977; Peel, 1960; Tricart, 1972; Tricart and Cailleaux, 1964; Wilhelmy, 1964). Chapter 6 presented clear evidence that some of the material leached out of the rock varnish reprecipitates within the underlying host rock, promoting case hardening.

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Figure 10.22. Post-depositional modification of varnish from (A,C) Antarctica and (B,D) Death Valley. The upper imagery is by HRTEM with scale bars in manometers. The lower imagery is by BSE (scale bars in micrometers) is of a different section, collected a few millimeters away from the high resolution transmission electron microscope section. The arrows in B shows where the 'tube' of reprecipitated varnish is covered and re-emerges from undemeath the layered varnish. The arrows in C and D highlight tubes of mostly reprecipitated Mn-Fe (measured by energy and wavelength dispersive X-ray spectrometry).

10.2.3. Classification of Rock Varnish 10.2.3.1. Prior Perspectives on Classification Rock varnish (or "desert varnish" or "patina") means different things to different investigators. Most frequently, these terms are applied to dark coatings on rocks, typically <2001.tm thick, that ale composed of clay minerals cemented to the underlying rock by oxides of manganese, iron, or both (Engel and Sharp, 1958; Haberland, 1975; Potter and Rossman, 1977; Potter and Rossman, 1979a). Beyond these basic characteristics, that define the subject of this chapter, varnishes vary considerably within the same biomes, landforms, individual rocks, and within the same varnish. There are no formal classifications of rock varnish in the literature, aside from the preliminary scheme that I have published (Dorn, 1994) and will elaborate on later in this chapter. Previous researchers, however, have noted several different types of varnishes based on differences in environment-climate, color, and geomorphic position. The environment of formation has been used to distinguish varnishes; for example, black varnishes exposed to river flow were thought to be distinct from those in subaerial positions on rocks above water level (Lucas, 1905; von Humboldt, 1812; Walther, 1891). Mean tropospheric climate has also been used to classify varnish; for

Rock Varnish

example, some consider the varnish found in deserts to be distinct (e.g., Glennie, 1970, p. 19; Hayden, 1976, p. 277; Allen, 1978) from manganiferous rock coatings in other settings. The relevant point is that many have used the climate/environment of occurrence as an informal means of classification. Geomorphic position has been important in distinguishing different varnishes. Four types have been noted in a desert pavement: top black, ground-line band, orange bottom, and black dendrites at the interface of ground-line band and orange bottom varnish (Blake, 1905; Engel and Sharp, 1958; Laudermilk, 1931). Crack varnishes have been observed along the walls of rock crevices. The manganese-rich crack varnishes were felt to be virtually indistinguishable from subaerial varnishes (Daveau, 1959; Engel and Sharp, 1958; Hunt, 1954; Krumbein, 1969; Tricart and Cailleaux, 1964). In fact, Tricart and Cailleaux (1964) noted a type of varnish inversion, where weathering and granular erosion destroys the surface varnish and exposes the crack varnishes to the subaerial environment; then the exposed crack varnishes appears far better developed than varnishes on the surfaces of eroding rocks. In summary, the lack of a formal classification of rock varnish should not imply that prior investigators thought the phenomenon was uniform. Substantial differences were noted. I believe that a classification has not been forthcoming, in part, because of perceptions of great intra-sample and intra-sample heterogeneity (Allen, 1978; Dorn and Oberlander, 1982; Dragovich, 1988a; O'Hara et al., 1989; Reneau et al., 1992). Also, the focus of varnish research has been on its origin, fundamental characteristics, uniqueness of varnish in deserts, and use as a dating technique. There has not been a perceived need for a classification scheme. The question then becomes, why bother now?

10.2.3.2. Why Classify Rock Varnish? I have been in the field with experienced geomorphologists and archaeologists who have proudly shown me a prized find, and knowing my interest in rock varnish, they say "look at this wonderful specimen of varnish". Upon inspection, I find often that the sample may be a polished basalt ventifact, silica glaze, or oxalate skin, and not rock varnish. I generally respond with a polite word how beautiful the sample is, and move on to a different topic. Or I try to point out later in the trip truly classic specimens, and illustrate quick field tests like scratching varnish with a knife to see how it powders. While I have been surprised at times, I have not thought that these individuals were ignorant; just the opposite. Prior to this work, there has not bee a systematic treatment of rock coatings to emphasize the simple point that not all rock coatings are alike. An important purpose of instituting a classification scheme is help people avoid inaccurate comparisons of truly dissimilar phenomena. There is an overwhelming tendency in the technical and non-technical literature to treat all varnishes as the same, and then to compare chemistry, structure, or other measured characteristics with other analyses in the literature. The tendency to treat all varnishes as one phenomenon is analogous to grouping together gabbro and olivine basalt; after all, they are both igneous and they are composed of mafic minerals In rock varnish research, however, biogeochemical characteristics are regularly compared with the unstated assumption that similar phenomena are being studied. Not all rock varnishes are alike, even if they look so superficially to the eye that has not been trained to understand differences. A comparison is informative only when the nature of the sample is recognized and stated clearly. My focus here is to introduce a few of the

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basic differences in color, in geomorphic position, and when varnishes analyzed microscopically.

10.2.3.3. Color~Chemistry Differences Varnishes are black because they are greatly enriched in manganese, typically > 10% measured as weight percent MnO. Orange varnishes (Munsell 10R4/8, 2.5YR4/6 to 5/6, 5YR7/6 to 7/8) generally have MnO concentrations <1%. Dusky-brown varnishes (Munsell 10R3/3 to 4/4, to 2.5 YR3/2) are those intermediate in surface chemistry; they have enough manganese to darken the color, but not enough to give the varnish a black appearance. All of these coatings have a clay matrix and are considered rock varnish; the major difference is substantial variations in the bulk concentration of manganese.

10.2.3.4. Geomorphic Differences Rock varnishes are most noticeable in subaerial environments, both on the surfaces exposed only to the atmosphere. Black manganese skins (see chapter 8) and manganiferous-clay accretions (this chapter) occur in a wide variety of terrestrial weathering environments: episodic water flow (Hunt, 1954; Potter and Rossman, 1979b; von Humboldt, 1812), perennial water flow (Boussingault, 1882; Cerling and Turner, 1982; Francis, 1921; Potter and Rossman, 1979b), subglacial (Potter and Rossman, 1979b; Whalley et al., 1990), cave (Moore, G.W. 1981; Peck, 1986; Potter and Rossman, 1979b), spring (Hariya, 1980; Mustoe, 1981), Quaternary sediments (Alhonen et al., 1974; Cailleux, 1967; Carlson et al., 1978), littoral (Boul~gue et al., 1978; Darwin, 1897; Hooke et al., 1969), wetland settings (Carlson et al., 1978; Ghiorse, 1984; Ljunggren, 1953), within the solum of soils (Ha-mung, 1968; Khakmun, 1973; Sullivan and Koppi, 1992), in the subsurface on fossilized paleo-landforms (Dora and Dickinson, 1989), at the soil-rock-atmosphere interface at the ground-band line (Blake, 1905; Engel and Sharp, 1958), in rock fractures and weathered rock found meters below the surface (Douglas, 1987; Nahon et al., 1985; Vasconcelos et al., 1994; Weaver, 1978), and in rock fractures within centimeters to millimeters of the subaerial surface of the rock (Daveau, 1959; Engel and Sharp, 1958; Hunt, 1954; Krumbein, 1969; Potter, 1979; Potter and Rossman, 1979a; Tricart and Cailleaux, 1964). The foregoing geomorphic contexts have different biogeochemical characteristics. Although these coatings are all rock varnish, because they are a mix of clays and manganiferous oxides, they exist at different types of biogeochemical barriers to the migration of manganese and iron. There are also substantial differences in biogeochemical environment on the same boulder in a desert pavement; consider subaerial, crack, and ground-line band settings. Subaerial environments are subject to cycles of dust deposition and rain-fed runoff, while dust in closed rock crevices remain in contact with rock walls (Villa et al., 1995). The ground-band line in a desert pavement is an area of active cyanobacteria activity (Scheffer et al., 1963), whereas subaerial surfaces of pavement rocks are more dominated by fungi and bacteria (Krumbein, 1969). Rock varnishes usually change geomorphic position. Fracture varnish becomes subaerial when a boulder simply spalls along a joint. Figure 10.23 presents an historic

Rock Varnish

example, where the 1990 Lander's Earthquake caused this granodiorite boulder to spall, exposing fracture varnish.

Figure 10.23. The Lander's Earthquake in the Mojave Desert, Califomia, spaUed this boulder, bringing crack varnish into a subaerial position. The photograph is courtesy of Norman Meek.

There are many other examples where the geomorphic position of the varnish can change. Subaerial varnishes become subsurface varnishes with burial. Ground-line band varnish can becomes subaerial varnish with loss of soil; this occurs most frequently in contexts where drainages incise into desert pavements. In addition, ground-line band varnish form tings around rock depressions at the soil-rock-atmosphere interface; these become subaerial if the depression is breached or the influx of dust changes. Many other examples could be given: subglacial varnish becomes subaerial varnish when the glacier recedes; varnish on a boulder avalanches onto a glacier; or varnish located where there is episodic waterflow becomes subaerial when discharge disappears due to climatic change, is rerouted natural drainage piracy, or is influenced by anthropogenic modifications. In summary, the earth surface environment is dynamic; rock varnishes change geomorphic contexts when rocks change geomorphic position.

10.2.3.5. Microscopic Distinctions

A change in geomorphology often entails changes in biogeochemical environment. It should not be surprising, therefore, that characteristics of a coating can also change. While the basic mix of clays and Mn-Fe oxides that characterize rock varnish means that rock varnishes can look quite similar in hand samples, different types of varnishes have substantially different characteristics when measured for chemistry, mineralogy, textures, and other properties. The heterogeneity of varnish chemistry, texture and micromorphology highlighted in previous sections make a case in point that not all varnishes are the same.

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10.2.3.6. Dangers of Misidentification In the previous section I have tried to establish two generalizations: varnishes that look quite similar may not be; and different types of varnish can change geomorphic position. Two corollaries follow. First, it is likely that different varnishes have been unknowingly compared. Second, many varnishes collected from subaerial positions actually started to form in non-subaerial settings. When I was working on my bachelor's and master's theses at the University of California at Berkeley (Dora, 1980; Dora, 1982), my advisor and I agreed we should collect the "best looking" varnishes in our exploration of the potential of rock varnish as a dating technique. His strong belief (Oberlander, 1994) was that these varnishes would be the oldest, because more time should equate to a better-looking varnish. I agreext at the time. In my first efforts to explore the use of rock varnish as a time indicator, I used this simple criteria, modified to avoid such obvious confounding factors such as lichens, getting too close to the soil surface, and evidence of eolian abrasion. In the process, some of the samples I collected were truly subaerial. However, we also collected samples that were truly crack and ground-line band varnish, which resulted in errors described here. In theory, appearance should work as an age clue, only so long as all the varnishes at a site had been exposed to the same varnish-forming factors. In practice, truly subaefial varnishes typically look much worse than recently exposed crack varnishes and groundline band varnishes that have long periods of contact of clays next to rock surfaces; clay minerals form the matrix of varnish, so it should not be surprising that the varnishes formed in protected positions have a more complete cover over the underlying rock. A variety of other conditions help varnish develop better in cracks and at the groundline. Water is retained longer in rock crevices and at the ground surface, which helps varnish-forming bacteria. As a rock crevice opens, alkaline salts are leached away. Similarly, salts are also readily leached from the soil-atmosphere interface. This permits a pH close to neutral that permits the oxidation of manganese by bacteria. Bacteria in these positions are also aided by less exposure to ultraviolet radiation. Especially for crack varnish, planar weathering fractures make a smooth substrate that permits a smooth varnish to form. In these positions, the surface layer is typically enriched in manganese, giving it a darker and better developed appearance, giving the observer a feeling that these varnishes are better developed and hence older During the rest of this section I explain mistakes I have made in three different contexts: calibrating varnish cation ratios; applying varnish dating to archaeological dating; and applying varnish dating to alluvial-fans. The K-Ar dated rhyolite domes and basalt flows of the Coso Range in eastern California (Duffield and Bacon, 1981; Duffield et al., 1980) were used to assess timedependent trends in rock varnish. I observed a systematic lowering in the elemental ratio between (K+Ca) and Ti (Dora, 1983), when the best looking varnishes were sampled from presumably "original" or "constructional" surfaces. However, rhyolitedacite flows are extremely unstable (Anderson and Krinsley, 1989; Anderson et al., 1994; Cerling, 1990). The darkly varnished surfaces I collected from these felsic flows were crack varnishes, exposed by rock spalling along joints. These varnishes had nothing to do with the exposure history, and could not be related to the K-Ar ages. Detailed resampling and reanalysis of varnish was conducted in the Coso Range, rejecting these crack varnishes as valid calibration points (Dora et al., 1990). This time consuming reassessment could have been avoided if I had not been blinded by the incorrect assumption that the "best looking is the oldest".

Rock Varnish

With permission from D. Simpson of the San Bernardino County Museum and F. Budinger, site curator of the Calico Early Man Site, surface artifacts with shiny and dark varnishes on flake scars were collected from desert pavements near the Calico site. Along with adjacent 'control' pavement cobbles, these varnishes were analyzed for their cation-ratio dating(Dorn, 1983). When these samples were collected, they were above the present-day ground-line band. However, subsequent cross-sectional analyses reveal that these varnishes started at the ground-line band varnish were later exposed to the subaerial environment through progressive soil erosion. These samples, therefore, are inappropriate for dating, because leaching conditions at the soil surface are totally dissimilar to the biogeochemical setting on the very surfaces of rocks. In the early 1980's (Dora, 1984) I sampled and resampled alluvial-fan units at the Silver Lake piedmont for cation-ratio and AMS 14C dating, with results used in subsequent research (Reheis et al., 1989; Wells et al., 1987). I have returned to the site several times to resample and reanalyze varnish. On the Qfl unit (see map in Reheis et al., 1989), I laid out ninety four ten-meter line transects and sampled cobbles every half-meter. I observed: (1) All of the examined clasts on the surface of the Qfl unit are angular, produced by weathering of originally larger clasts. The Qfl unit is quite small; I did not see a single cobbles that showed evidence of water transport. Water-transported clasts on younger units, in contrast, are rounded. The complete degradation of rounded surfaces is due, in part, to the foliated metamorphic lithologies. Hence, varnish at the site should not be used to indicate when the fan unit was deposited (McFadden et al., 1989; Reneau, 1993). (2) Because all of the exposed faces started in a joint, it is reasonable that crack varnish formed prior to subaerial exposure. (3) Because all of the cobbles are within a few centimeters of the current ground-line band, and because pavement-forming processes move cobbles (Mabbutt, 1979; McFadden et al., 1987; Springer, 1958), it is reasonable that ground-line band varnishes could have formed in the past and had been jostled a few centimeters ab ove the present soil surface. (4) Microscopic analyses of "the best looking" varnish in each of the 94 transects on the Qfl unit reveal similarities to crack and ground-line band varnishes. They do not reveal a laterally consistent pattern for manganese in the surface layer, nor do they reveal evidence of post-depositional leaching--characteristics of subaerial varnish. In summary, at the Qfl unit at Silver Lake, Mojave Desert, there are no "original" surfaces that were exposed by fluvial processes; and the varnish currently exposed at the site started to form either in a rock crevice or at the ground-line band. Hence, my prior dating results are invalid, as are the results of others who assumed they were working with subaerial varnish (McFadden et al., 1989; Reneau, 1993). These errors made me realize that the current paradigm, that appearance equates to age is in error. A long-standing tradition in varnish research in the early- and middle-years has been to distinguish relative ages of geomorphic units and archaeological features on the basis of varnish darkness. The assumption that darkness equates with age has continued to the present (e.g., (Derbyshire et al., 1984; Dickey et al., 1980; Espizua, 1993; Harrington and Whitney, 1987; Huckleberry, 1994; McFadden et al., 1989; Oberlander, 1994; Raymond et al., 1991, Reneau, 1993; Swadley and Hoover, 1989; Whitney and Harrington, 1993). These perceptions are emplaced early in the minds of future field scientists with words like these in introductory textbooks (Muller and Oberlander, 1978), p. 445:

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"surfaces of different ages are apparent in the color they display because desert rocks tend to become coated with a patina of iron and manganese oxides with the passage of time--desert varnish. Thus new deposits are light in color, and successively older ones are increasingly darker. All sizable alluvial fans are a mozaic of tones as a consequence."

The essence of this conventional perceptual framework (Reneau, 1993, p. 309) is: "As has long been recognized, rock varnish appears to become darker with age, and this darkening accompanies a coalescing of discrete varnish patches and an increase in varnish thickness over time."

I disagree. Age and appearance are not necessarily or clearly related. Non-subaerial varnishes do not darken with age; they can be "born" dark when they are exposed to the atmosphere by geomorphic processes. They are also "born" with a head start from the fracture varnish already on the rock surface. This view is in opposition to the paradigm used today in desert field work. Varnish darkness is used as a mapping and interpretive tool in the field at two very different scales, in hand samples and in examination of aerial photographs/remotely-sensed imagery. The issues involved in interpreting varnish darkness are quite different at these two scales. At the scale of hand samples, a time signal can be extracted if, and only if, time is the only variable. Table 10.7 exemplifies the many factors influencing varnish development other than time. I am not saying that time is impossible to control, only that it is extremely difficult to do so on the basis of field data. The chances that the darkness of any given varnish relates to age actually decreases with time, because weathering destroys more original surfaces as time progresses; with longer exposure times the darkness of any given sample equates more and more with varnish type than varnish age. In other words, as time goes on, more rocks spall and expose more crack varnishes, and the appearance of the surface darkens because the more prevalent crack varnishes are much darker than subaerial varnishes. A related topic is the mapping of Quaternary landforms using varnish darkness in aerial photographs and remotely sensed visible and near-infrared imagery (e.g., Landsat, SPOT). The youngest landforms appear bright with relatively high albedos because the rocks on them are not varnished. Then, on the time scale of a few thousand years in Death Valley, for example, varnish development on boulders will lower the albedo--causing the aerial photograph to show a darker image. With time boulders weather, spall into smaller pieces, which exposes darker crack varnishes. In addition, these smaller clasts make tighter desert pavements that have lower albedo because less soil is exposed. There is a point in the development of the landform, however, where the darkness on the aerial photograph starts to decrease. If albedo is plotted on an X-Y graph where X is time and Y is albedo, a "bell curve" is created. The lowest part (darkest surface) in the curve usually coincides with the point at which the landform starts to erode. The relevant point here is that varnish darkness is a combination of progressive growth over time, and geomorphic processes that expose darker varnish types that started to form in rock crevices and at the ground-band line. In summary, misunderstanding different varnish types has resulted in incorrect comparisons of varnish analyses, incorrect sampling in the dating of landforms and surface artifacts, and incorrect conceptual models for how to interpret desert landform development from aerial photography and remotely-sensed imagery.

Rock Varnish

Table 10.7. Factors that can influence the appearance of rock varnish, other than age. Factor Vamish Chemistry

Explanation Vamishes with higher concentrations of manganese and lower concentrations of clays appear darker than low-Mn and high-clay varnishes

Lithology

The underlying rock influences the observers perception. For example, darkcolored rocks (e.g., basalt, gabbro) can appear completely varnished, when in reality subaerial coverage on a rock can be less than 10%.

Weathering Rind

Weathering finds are typically lighter in color than the same rock that is unweathered. For example when the same petroglyph is engraved into both fresh and weathered rock, varnish on the weathered rock appears "lighter" than vamish on adjacent fresh rock even a millimeter away.

Water Flow

Runoff can enhance manganese concentrations, can promote more complete vamishing processes, and usually results in darker vamishes.

Water Ponding

Locales where water ponds and dust collects (even on the millimeter-scale) are typically darker, because this environment creates a type of ground-line band varnish. Ground-line band vamish is darker than adjacent varnish that fonn~ on rock surfaces.

Other Coatings

Interdigitation with other rock coatings changes the appearance of rock varnish. Silica glaze interfingering can give varnish a more lustrous look. Interbedded oxalate can give the impression that the varnish is thicker. Phosphate crusts that interlayer with varnish can darken appearance.

Epilithic Organisms

Microcolonial fungi, filamentous fungi, cyanobacteria, and even some species of lichens can be mistaken for rock varnish and can influence surface appearance.

Corrasion

When the initial rock is dark, mechanical abrasion (e.g., from overland flow, eolian transport) can give a polished appearance, which has been mistaken for shiny rock vamish.

Surface Roughness

Rough surfaces usually appear less-well varnished than smooth surfaces.

Soil Proximity

Varnishes next to the soil line can be both darker and lighter than adjacent coatings, depending upon the geomorphic context.

Adjacent Coatings

Darker adjacent surfaces can make vamish in a petroglyph look lighter. Alternatively, sometimes engraving can make rock surface more stable, and adjacent natural surfaces can actuall), have less varnish than an engraving.

10.2.3.7. Dangers of Instituting a Bad Classification While the lack of classification has probably led to errors in interpretation, there are inherent disadvantages to presenting a classification. First, whenever a phenomenon is given a name in an organized framework, there is a tendency of individuals to think an issue is resolved. However, when a disorganized milieu exists, humans detect an unsolved puzzle requiring organization; people love mysteries. Science is best served, then, by imposing a minima of classification in order to keep the allure of the many mysteries that require resolution. Second, classifications tend to emphasize end members, just because they are most distinct from one another. However, intergrades dominate the natural landscape. One of the greatest difficulties I encounter in-teaching natural science on field trips is the

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paucity of "textbook examples" in the real world. I blame classifications because they often overemphasize end members. In the case of rock varnish, simply distinguishing subaerial, crack, and ground-line band varnish does little to emphasize the dynamic biogeochemical environment as climates change and as varnishes change geomorphic position. These different varnishes are not truly not distinct. There are periods when dust will completely cover a tiny depression on a rock surface, which will simulate a rock crevice environment. On a smaller spatial scale, microdepressions within a given varnish will also fill up with dust. A biogeochemical setting produces a certain type of rock varnish. However, the act of identifying the 'class' somehow makes the issue static, when in reality the class is a biogeochemical 'target' that shifts in time, space, and scale. When it comes to classifying natural phenomenon, some scientists are 'lumpers' and others are 'splitters'. I dislike classifications in general, because they create artificial differences and boundaries where none truly exist in nature. However, a classification is necessary, for reasons outlined previously. The objective of the next section is to present a tiered classification scheme, which will hopefully alleviate the worst errors associated with misclassified varnish and minimize the inherent dangers to intellectual curiosity imposed by a classification.

10.2.3.8. A Tiered Classification for Rock Varnish

The first tier of issues in this proposed classification scheme is to decide if the rock coatings is truly rock varnish. By definition, rock varnish is a natural accretion on rocks, dominated by clay minerals, and oxyhydroxides of manganese, iron, or both. Rock varnish is only one of many different types of rock coatings discussed in this book, and it interlayers with several of these accretions. The issue must be to distinguish the nature of the rock coating. And if rock varnish is present, is it pure, or is it intercalated with other rock coatings? The second tier of issues surround the abundance of manganese. Since at least a few percent iron is always present, the two end members are varnishes poor in manganese and those greatly rich in manganese. The continuum in manganese abundance ranges from less than 0.1% to over 50%, measured as MnO by weight. The key issue is whether or not there are biogeochemical processes that create a 'sink' for manganese; in other words, is manganese somehow concentrated from the 'raw' ingredients, for example dust. This concentration can be measured by bulk or microchemical methods, or it can be judged visually in the field with color; black varnishes are always enriched in manganese. If the answer is "no", then the varnish usually has some orange color of iron oxyhydroxides (Munsell 10R4/8, 2.5YR4/6 to 5/6, 5YR7/6 to 7/8). However, if the color has a brown hue, a chemical analysis would be necessary, since similar colored coatings can have different iron minerals that can mimic a similar appearance. The third tier of issues involve identifying the environmental settings where there is a barrier to the migration of Mn. It would be ideal to identify processes of Mnenhancement, as opposed to circumstance where process occurs. However, that would require a level of analysis, using microbial culturing and electron microscopy techniques, that would not be used, and hence would be self-defeating. Instead, questions revolve around a best attempt to analyze the location where the varnish occurs.

Rock Varnish

Table 10.8. Tentative interactive classification for rock varnishes. Tier 1: Type of Rock Coating If not rock vamish, then end this classification. If pure rock varnish, then go to Tier 2. If intercalated, then identify type(s) of rock coating(s) varnish is intercalated with: anthropogenic pigment biofilm carbonate skin case hardening iron film nitrate crust oxalate-rich crust phosphate skin polish film silica glaze sulfate skin This classification scheme goes no further on intercalated coatings, other to identify the interdigitation. Tier 2: Presence of Biogeochemical Barrier to Manganese Migration If manganese is concentrated in the rock varnish above ambient levels, then go to Tier 3. If manganese is not concentrated, the iron in the orange varnish may have been fixed in different biogeochemical settings: formed on surface exposed only to the subaerial environment formed on walls of rock crevices formed on undersides of cobble in desert pavement formed in hydrothermal setting formed in acid waters (usually these are iron skins, lacking clays) Is there evidence that the Mn-poor varnish has changed geomorphic settings, for example planar surface coated with orange varnish exposed to atmosphere (from spalling), or black varnish found on underside and orange vamish found on topside of cobble in desert pavement (from tuming cobble over)? This classification scheme goes no further on Mn-poor coatings, although the text explores more fully the iron biogeochemistry of rock varnish. Tier 3: Biogeochemical Position of Manganese Barrier at Present Time What is the geomorphic context of Mn-rich rock vamish? at rock-atmosphere interface (subaerial), at the soil-rock-atmosphere interface (ground-band line), cave, crenitic, episodic water flow in rock fractures meters below the surface, in rock fractures within meter of the, surface, littoral, perennial water flow, subglacial, under water, within sediments , within soil solum What is the biogeochemical context of Mn-rich rock vamish? pH/eH conditions on rock surface exposure to organic acids position with respect to transported particulate matter position with respect to mechanical abrasion Is there evidence that the Mn-rich varnish has changed geomorphic or biogeochemical settings? If so, go to Tier 4. Tier 4: Dynamic Nature of the Biogeochemical Barrier to Mn Migration Is there evidence of change in the geomorphic position of the varnish? rock erosion (spalling along joints, flaking of rock surfaces, granular distintegration) soil erosion or accumulation (exposure of ground-line band, burial of former surface) change in geomorphic processes (eolian, fluvial, wave, glacial, pedoturbation) anthropogenic influences (rock engraving, geoglyph formation, construction) Is there evidence of environmental fluctuations over time that has changed the biogeochemical processes of varnish formation7 layers with changes in Mn-Fe abundance layers with changes in clay abundance layers with changes in mineralogy layers with changes in trace element chemistry interdigitation with other rock coatings stratigraphic unconformities (geochemical or mechanical)

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The fourth tier of issues involve an evaluation of changes in the geomorphic and biogeochemical settings. The objective is not to identify the possible changes, but to evaluate the likelihood that there has or has not been a change. For example, if samples are collected on an alluvial fan, and the investigator suspects the subaerial exposure of a former crack varnish, it would be immensely helpful even to simply notes that the varnish has formed on a smooth, planar surface that has characteristics of the natural jointing patterns of that lithology in the study area. In a second example, if small cobbles (<10 cm) are collected from a smooth desert pavement only a few centimeters above the soil surface, and the investigator even suspects that the varnish has characteristics similar to ground-line band varnish currently at the soil-rockatmosphere interface (e.g., has dust adhering to it, very shiny, smooth coating), it would be very helpful to note that speculation. In a third example, several investigators have noted a persistent lower layer that is poor in manganese (Engel and Sharp, 1958; Hooke et al., 1969), that can sometimes be seen as an orange layer in cross-section with a hand lens; in my research this layer is persistent and turns out to be a former orange varnish formed in a rock crevice~later superimposed by a former black crack and black subaerial varnish. A complete evaluation of environmental and geomorphic changes in rock varnish is both expensive and time consuming. The issue in this loose classification scheme is not to suggest that competent investigators should conduct these analyses to make an accurate classification. On the contrary, that would not be feasible, and would be defeat the purpose of suggesting this classification. My objective is to have the investigator alert the reader that there is a possibility that the nature of the barrier to manganese migration has changed, or if the investigator is confident of a static context, simply explain the reasoning behind the conclusion. The objective of the fourth tier is only to ask the investigator to analyze the potential for a change in context. Hence, I present Table 10.8 as a flow chart of the questions and possible answers that an investigator might want to ask in classifying rock varnish.

10.2.4. Rates of Formation 10.2.4.1. Observations Plqor to Worm War H

The general issue of rates of rock varnish formation was not a concern of von Humboldt or Darwin, nor was it a major issue to most researchers prior to World War II. The basic question was how the coatings could have such a uniformly thin character. Lucas (1905, p. 45) concurred noting that tropical fiver films are "so thin that it cannot be measured, and never seems to vary or to increase in thickness, but wherever found it is apparently always the same." Blake (1905) and Lucas (1905) characterized rock varnish on desert rocks as similarly thin. Lucas (1905) used the anomalously thick character of manganiferous concentrations on the Nubian sandstone to argue that it was a part of the sandstone, only exposed by erosion of less-well-cemented facies. Lucas was suspicious of the contention of Boussingault (1882, p. 289) that tropical fiver films could reach 0.1 mm in thickness. Still, interest in varnish as a time recorder was burgeoning. Well-cited observations were that the Egyptian pyramids had only thin coatings of varnish (Linck, 1901; Linck, 1930; Schweinfurth, 1903; Walther, 1891). Linck (1930) indicated that the marble on the Acropolis in Athens had a slight coating. In archaeology, there was a feeling that varnish formed slowly, and hence could be used as a general indicator of the antiquity of

Rock Varnish

artifacts and rock engravings (Amsden, 1939; Basedow, 1914; Burkitt, 1928; Etheridge, 1890; Flamand, 1921; Howard, 1935; Renaud, 1936; Wulsin, 1941). Basedow (1914) supported this with observations of thick varnishes on petroglyphs that are perhaps tracks of extinct animals. Tumer (1909, p. 230) noted that varnish formed differentially on geomorphic surfaces: "It is found only on the upper exposed surfaces and on slopes that have been exposed to the elements for a long period, not being observable on boulders of the newer washes and alluvial fans." Field-based observations indicated factors other than time could affect the appearance of 'patina' (Movius, 1942; Rogers, 1929). One important variable was, and still is: how rock type affects the rate of varnishing. Walther (1891) recognized that the resistance of the rock to erosion could govern just how black a rock could become; more resistant cherts could become blacker than limestone. Similarly, Lucas (1905) argued that the Nubian sandstone was too friable to develop a good coat, because the rate of erosion was faster than the rate of varnishing. In other cases, some resistant rocks varnish slowly for reasons not clear at the time, for example "glassy quartz" in the Mojave Desert (Laudermilk, 1931) and Yuma (Ray, 1947).

10.2.4.2. Observations from Worm War H to the First Dissertation

Opinions on the rate of varnish formation were connected to the foregoing question over the climatic condition(s) in which varnish forms, and hence were sharply divided between those who believed that varnish in deserts could form within decades and those who believed the varnishing in drylands is a very slow process taking millennia for full development. I note, however, that the concept of antiquity was sometimes decoupled from how fast varnish forms. In other words, an old varnish might be old because it formed rapidly but survived through a period of non-formation. Concomitantly, just because a varnish forms today does not necessarily mean that it is forming rapidly. Specific examples of rapid varnish formation were made in the literature during the middle years. Engel (1957) and Engel and Sharp (1958) reported the formation of varnish in 25 years on rocks disturbed during road construction in the Mojave Desert were similar to the observations of rocks affected by World War II military maneuvers (Scheffer et al., 1963). Engel and Sharp (1958, p. 515) also report varnish formation on alpine moraines "within the last 40 to 80 years". Varnishing was noted within a few years on Spitzbergen (Btidel, 1960, p. 46-47), on moraines in the Alps (HOllerman, 1963; Klute and Krasser, 1940), and on rock carvings within the Negev Desert (Krumbein, 1969). There were only a few other observations on rates of formation at sites with some independent age control. Very dark and well-developed varnish was noted on the high stands of Lake Manly in Death Valley (Clements and Clements, 1953) and Lake Lahontan in Nevada (Carter, 1966). Barely perceptible varnishes were seen on Egyptian Pyramids (Blackwelder, 1948; Blackwelder, 1954; Emery, 1960; Iskander, 1952). In a comment on Engel and Sharp's Mojave road site, in Death Valley Denny (1965) noted no visible varnishing of quartz fragments at a 50-year-old road construction site, and Hooke et al. (1969, p. 288) did not notice varnish on rock surfaces chipped 35 years prior to their field observations. Turner (1963, 1971) tried to estimate the ages of petroglyphs (and the varnish formed on them) based on associated (but not contiguous) geomorphic and cultural phenomenon. Turner (1963, p. 14) first thought that "on the basis of tentative pottery association, the blue-black patina [on petroglyphs] requires around 900 to 1200 years to

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form and the purple-black patina requires even more time." Then, newer relationships and radiocarbon ages suggested that the rock art could be as old as 4000 to 8000 14C years (Turner, 1971). While the aforementioned observations of rapid varnish formation were well cited (e.g., Hem, 1964, p. B 10; Rogers, 1966, p. 136, Fairbridge, 1968, p. 280; Cooke and Warren, 1973; Goudie and Wilkerson, 1977, p. 18), most geomorphologically and archeological-oriented researchers felt that the type of rock varnish found in deserts took a very long time to develop and could indicate at least the relative antiquity of landforms, petroglyphs, artifacts, and geoglyphs (Anati, 1967; Bull, 1977; Bull, 1984; Butzer and Hansen, 1968; Carter, 1964; Carter, 1966; Carter, 1980; Cooke, 1970; Cooper, 1947; Evenari et al., 1971; Grant, 1967; Grant et al., 1968; Hooke, 1972; Hooke et al., 1969; Hunt and Mabey, 1966; Michels, 1973; Nissen, 1975; Oberlander, 1977; Rahm, 1974; Rhotert, 1952; Rusco, 1970; Schaafsma, 1980; Setzler, 1952; Smith and Turner, 1975; von Werlhof, 1965; Warnke, 1968). There was an intuitive acceptance that what the eye saw, in terms of the relative darkness of varnish, was a valid chronometric indicator. And the intuition of these investigators was that the process of varnishing took a long time, probably thousands of years. The lack of independent chronometric data did not stop speculation on rates of varnish formation: 9 Blackwelder (1948, 1954) asserted that it took 20,000 to 50,000 to develop a full "desert lacquer", and these figures were subsequently written into texts (Hills et al., 1966; Hunt, 1972). 9 In examining geoglyphs along the Colorado River, Setzler (1952, p. 401) wrote "I think we can be pretty certain that it [varnish] forms in less than 10,000 years." 9 Working in Death Valley, Alice Hunt estimated that very little varnish formed on artifacts believed to be 2000 years old (Hunt, 1960), a conclusion subsequently brought into the geological literature by Denny (1965). 9 Heizer and Baumhoff (1962, p. 284) estimated the ages of petroglyphs at Grimes Point, Nevada through a process of circular, but admittedly speculative reasoning: "We can only point out that the least possible age for the youngest petroglyphs at the Grimes site is 150 to 200 years and that they appear as fresh as though they were made yesterday. Because the patination appears to have progressed to its ultimate in the conical pits and grooves at site Ch-3 [Grimes site], we are guessing that they may be fifteen or twenty times as old as the obviously recent examples which have a minimum age of 150 to 200 years; that is the oldest petroglyphs here may be from 2,500 to 3,500 years old."

9 Carter (1964, 1966, 1980) used the qualitative development of rock varnish on shorelines of Pleistocene lakes in the western United States to argue that some artifacts are far older than 10,000 years. 9 Grant et al. (1968) argued that past wet periods (called pluvials at the time) would have eroded varnish. That meant: "there is a strong possibility that the existing patina began forming after the Little Pluvial of 3000 to 4000 years ago when warm summers and thundershowers allowed the desert varnish to build up. This would mean that no drawings [petroglyphs in the Coso Range of eastern California] could be much older than 3000 years."

9 With no basis noted for his opinion, Hildreth (1976) wrote that 1500-year-old gravels had only the barest varnish, and 10,000 to 25,000-year-old pavements had thick varnishes.

Rock Varnish

9 Hayden (1976) described Sierra Pinacate, Mexico, artifacts with thin varnishes thought to be 9000-years-old, artifacts with excellent varnishes thought to be about 19,000-years-old, and unvarnished artifacts less than 3(g)0-years-old. 9 Schaafsma (1980, p. xi) simply assumed that the varnished petroglyphs of the Southwestern United States are limited to the last 3000 years. 9 Sometimes very precise estimates were made on how long it takes varnish to form. For example Bull (1984, p. 234) wrotes: "Most mountain ranges [in Arizona] have many bouldery levees that were formed by debris flows on the hillslopes and along smaller valleys. These levees are at least 4 to 6 thousand years old as is indicated by the dark desert varnish coating..." Some of the foregoing estimates were assertions not supported by any data. In other cases, there was some sort of reasoning used, but again not supported by any data. I decided to present this list, not to embarrass these individuals, but to give the reader a flavor of the way rock varnish was (and still is) used. Geomorphologists and archaeologists did not hesitate to 'guess' ages, and then use the appearance of rock varnish as a justification for their opinions. Not all observers in this period were sanguine about even the validity of obtaining a relative age sequence from the appearance of rock varnish, because of local variations in environment and the possibility of climatic change (Borden, 1971; Cooper, 1947; Glennan, 1974; Mabbutt, 1977; Moen, 1969; P6w6, 1954; Rogers, 1966; Silvester, 1963; Simpson, 1961; Tuttle, 1983; Viereck, 1964). In studying the northern Karroo in South Africa, Goodwin (1960, p. 307-308) lists a host of factors other than time that influence the appearance of varnish: "...epidaphic patination, which covers the upper portion of the anchored rock with a shiny film, deep brown to black, composed principally of oxides of iron and manganese...the main agents governingpatination are : (a) uncertain summerrainfall with extreme diurnal evaporation, (b) distribution of water on exposed surfaces, (c) wind and sun incidence, protecting certain aspects, (d) presence of pitting, holding, or canalizing surface moisture, (e) actions of organisms, notably birds. Each one of these may be regarded as acting differentially through time with little definable relationship to the period involved. Time (though necessarily a most potent factor) does not produce patination alone, but employs the differential activity of passivity of the agents listed above." This list and the factors presented in Table 10.7 are still frequently ignored in research (e.g., McFadden et al., 1989; Reneau, 1993).

10.2.4.3. Calculating Rates of Formation Calculating rates of varnish accretion is difficult. There is evidence that full comings of rock varnish can form quite rapidly, for example, in the Alps (Klute and Krasser, 1940), in the Canadian Rockies (Dorn and Oberlander, 1982), on slag in southern California (Dorn and Meek, 1995). A related coating, manganese heavy metal skins, can also form rapidly (see Chapter 8). There is also evidence that rates of accretion are highly variable from place to place. Consider the fact that the same high ~16,000-yr-old shoreline at Searles Lake (Figure 10.24) has varnishes with vastly different thicknesses (Figure 10.25). Even greater place-to-place variability occurs in a more humid setting, the glacial moraines of Bishop Creek in the Owens Valley, California. Figure 10.26 reveals a variation of several hundred of microns from place to place on boulders of the same surface exposure age.

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These are, however, just isolated case studies. When a larger dataset is gathered on morainal boulders, an interesting pattern emerges (Figure 10.27). Older varnishes are a thicker in the deepest microbasins and in intrabasin locations. Thus, it is obviously critical to compare similar microtopographic settings (basin to basin, inter-basin to interbasin). While place-to-place variability in thickness tends to decrease over time, thickness is not a linear function of time. The rate of varnishing also appears to decrease over time. This may be more attributable to weathering. Varnish grows more rapidly in wetter environments (Figure 10.2). Yet, these are also the places where the rock weathers and erodes more rapidly. Therefore, the locations where varnishes are preserved for tens of thousands of years are the drier settings where rates of varnishing are slower. In other words, the fastestgrowing varnishes are not preserved because the rock has weathered and eroded away. Hence, there is a self-selection for locations of slower weathering rates and slower varnishing rates.

Figure 10.24. Shorelines of Pleistocene Searles Lake, eastern Califomia, at Poison Canyon.

Figure 10.25. Secondary electron microscope images of two different rock vamishes collected from waveabraded boulders on the high stand of Searles Lake. Note that the scale bar in both images is 5 micrometers, and vamish thickness varies greatly, despite the same age for the underlying rock surface.

Rock Varnish

Figure 10.26. Two backscattered electron microscope images from two boulders on the same glacial moraine at Bishop Creek, eastern California. Both varnishes display the same pattern of chemical layering, but they differ in thickness on the scale of an order of magnitude. Also note that the thicker varnish displays more evidence of post-depositional modification of leaching and the reprecipitation of leached elements.

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230

rJ2 t-,

0

2

Transect Leneth (cm) 4

6

8

10

5 lO I- 15

o , ~ 20 40 0 ..~ 30 ~ 60 "~ 9 90

Figure 10.27. Variations in vamish thickness along a ten centimeter transect on boulders on three different glacial moraines of Bishop Creek, eastern California. Thickness was measured by backscattered electrons in polished, contiguous cross sections.

The most detailed study on rates of accumulation was conducted by Liu (1994), but he used an entirely different approach. Liu measured the thicknesses of different varnishes with distinct layering patterns, called layering units (Table 10.9). Based on hundreds of measurements on thin sections, the rates of varnish accretion varied between one and five microns per thousands of years. Liu found that glacial times (LU3/4) encouraged much faster rates of varnish accretion. He also determined that rock type does influence the rate of varnish accretion. Among the four tested lithologies for Death Valley varnishes, varnishes on quartzite generally accumulate relatively faster than on both meta-dacite and greenstone.

Table 10.9. Average rate (and standard deviation) of accumulation of rock varnishes on different lithologies in Death Valley (Liu, 1994). Measurements are in micrometers per thousand years. LU-1/2, LU-3/4, and LU-4/5 refers to varnishes with different layering patterns (Liu, 1994). LU-1/2 and LU-4/5 includes a mix of interglacial and glacial climates, whereas LU3/4 layers only formed during glacial climates. Lithology

Quartzite Meta Dacite Meta Diorite Greenstone

Mean

LU- 1/2 SD

N

Mean

LU-3/4 SD

N

Mean

LU-4/5 SD

N

1.5 1.1 1.4

0.9 0.6 0.5

69 150 46

4.9 3.8 4.7

2.7 2.5 2.8

76 162 46

1.0 1.1

0.4 0.5

82 264

0.8

0.5

81

Rock Varnish

An important conclusion of Liu's (1994) dissertation is that thickness cannot be clearly related to age. Tremendous variations in the accumulation rates occur over short distances in a single cross section. Liu (1994) points out that the accumulation rate of the Holocene layer of Death Valley rock varnishes can not be used to correctly estimate the age of the entire layering sequence varnish. Liu concluded that even relative thickness cannot be used to estimate the age of the varnish.

10.3. Origin 10.3.1. Framing the Issues Historically

10.3.1. Debate Prior to Worm War H

Discussions of the origin of rock varnish prior to World War II set the stage for debates in the later half of this century. The favored hypothesis was that varnish constituents had an internal origin derived from the underlying rock. There were proponents of an external origin, and those that favored polygenetic processes involving both internal and external sources. A few investigators advocated biological catalysts. There were even a few attempts at experimental replication to test favored hypotheses. Although no consensus was reached, the position reported in textbooks was that solutions weathered from the rock evaporated and congealed on surfaces under exposure to the harsh desert sun.

10.3.1.1. Internal Origin

The best case for an internal origin was made by Mr. A. Lucas, who authored the first (of only two; the other written by Haberland, 1975) monograph on rock varnish entitled "The Blackened Rocks of the Nile Cataracts and of the Egyptian Deserts" (Lucas, 1905). Lucas divided the subject into "desert film" and "fiver film," although he thought that these manganiferous black coatings were closely related. In his detailed analysis of the literature of the day, that focused on genesis, Lucas concluded there was need for a more "detailed examination of specimens than had hitherto been attempted (sic.)..." (Lucas, 1905, p. 16), because "no figures or data, however, are given to prove that the film even on small pebbles ever contains ingredients in greater quantity than could have been_supplied from the stones themselves (Lucas, 1905, p. 19)." To rectify this deficiency, Lucas, a chief chemist at the Royal Survey Department Laboratory in Cairo at the turn of the century, conducted hundreds of chemical analyses on coatings, rocks, fiver water, and river suspended sediment. A critical issue to Lucas was how to analyze chemical composition in order to test whether the raw ingredients of the desert film were or were not within the underlying rock. Believing that the film could not be mechanically removed, Lucas decided to use hydrochloric acid to dissolve it. Because he saw inherent problems with quantitative analyses (e.g., separating film solute and rock solute), he concentrated on qualitative

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analyses to determine only the presence or absence of different constituents in the film and the "rock underneath". Lucas (1905) analyzed the composition of "several hundreds of films" on rocks that had black coatings (siliceous limestone, flint, fine-grained red granite, silicified wood). In addition to manganese and iron, Lucas determined the presence of several other constituents in the desert films: phosphoric acid, alumina, lime, magnesia, potash, sulfuric acid, water, and organic matter. Choosing P205 as a characteristic ingredient, Lucas determined that phosphoric acid was concentrated approximately 1000 fold in the films as compared to the rock, but that the mass of P205 in the stone was far more than adequate to supply all the mass of P205 in the film. "With regards to the other constituents of the film the case is very similar, and in the hundreds of samples examined there was not a single instance of anything being found in the film that was not also present in the rock below (Lucas, 1905, p. 20)." The next issue that concerned Lucas was whether chemical processes could mobilize these constituents from the rock. Lucas (1905, p. 23) argued that "these constituents are all soluble to at least some slight extent in water, the solubility being increased in many instances by the presence of carbon dioxide, sodium chloride, phosphoric acid, etc." With insight into the general arena of chemical weathering, Lucas (1905, p. 46) noted "a comparatively large amount of purely chemical disintegration of rocks occurs in the Egyptian desert, and that probably this chemical disintegration of rocks in dry and tropical climates is considerably greater than is usually supposed." In an attempt to gain insight into the issue with a different technique that had been used to study laterites, Lucas had geologist W.F. Hume analyze by microscopy thin sections of Egyptian desert films from siliceous limestone, flints, and crystalline rocks. Although some sections suggested alteration of iron-minerals and depth-dependent trends in rock weathering, an internal origin was thought to be neither disproven or proven by examination of thin sections. There was "no definite and regular alteration or decomposition of the rock from the centre towards the outside" (Lucas, 1905, p. 24). Lucas concluded his study of desert films with a general model of their genesis almost identical to Linck (1901). First, dew or rain "gains access to the rocks even in desert regions, and dissolves the various soluble constituents". Then, "the solution thus formed is brought to the surface by capillary attraction, and that the water is there evaporated leaving the salts. Lastly, "some of the salts, such as those of iron and manganese, are subject to further alteration at the surface, whereby insoluble oxides are formed; and "a hot climate and small rainfall are necessary to the formation and preservation of the film (Lucas, 1905, p. 24)." Lucas (1905) then turned to fiver films which, unlike Walther (1891), he thought as closely related to desert films. Desert and fiver coatings were found to have a similar color and similar dominance of manganese and iron oxides, with the presence of similar minor elements. Lucas' method was similar in that he examined the chemistry of the films, underlying rocks (mostly crystalline), dissolved load of the Nile, suspended load of the Nile. Based upon the data, however, he could not decide whether the coatings were applied externally or internally. On listing arguments in favor of an external origin, Lucas concluded that all of the constituents in the coating were found in the Nile, the coatings were similar on different rocks (granite, gneiss, dolerite, sandstone, limestone), that a white film not derived from the rock interbedded in places with the black film, and that microscopic examination of thin sections affords no conclusive evidence of a weathering gradient from outside to inside. On assembling evidence for an internal origin, Lucas (1905, p. 34) wrote:

Rock Varnish

"all the constituents of the film are present in the rocks, and all rocks being more or less porous, water may penetrate not only through cracks and figures and along bedding planes, but also through the capillary interstices between the various minerals...that the various constituents once dissolved would come to the surface by capillary attraction when the time of high Nile was past and that particular part of the rock surface no longer submerged...therefore all the conditions necessary for the formation of the film in a manner analogous to that suggested for the desert films actually exist."

In the end, Lucas acknowledged that the evidence for the origin of fiver films "is not conclusive either way", but "reasoning by analogy, however, the probabilities are that the fiver film is formed in an exactly similar manner to the desert film, namely from inside the rock (Lucas, 1905, p. 45)." Prior to World War II, the most frequently noted opinion was that rock varnish comes from the underlying rock, by capillary rise of moisture and chemicals from rock, with subsequence precipitation when the solution evaporated (e.g., Blackwelder, 1925; Blanck, 1926; Du Bois, 1903; Gautier, 1928; Knaust, 1930; Linck, 1900; Linck, 1901; Linck, 1928; Linck, 1930; Talbot, 1910; Turner, 1909; Woodward, 1914). The following is wording is typifies writing in this time period: "the coloration is due to the formation of a thin coating of ferric iron oxide or of manganese oxide, or of both, or of a mixture of iron oxides, such as magnetite, derived from the interior of the rock by an osmotic flow, a kind of rock-transpiration tending upwards and outward to supply the excessive evaporation under hot arid conditions (Blake, 1905, p. 374)."

A discoloration several millimeters thick, now called a weathering rind (Colman and Pierce, 1981), was frequently recognized underneath the much thinner rock varnish that was "not thicker than ordinary writing paper" (Blake, 1905, p. 373). Merrill (1898, p. 391) argued that the constituents of varnish came from the weathering rind and that "desert varnish" was "a superficial segregation of the metallic contents of the quartzite in a state of higher oxidation..." Following Merrill, Blake (1905, p. 373-4) argued that the "discoloration...appears to have lost ferrous iron oxide by its concentration at the surface where it has been oxidized to ferric iron (Blake, 1905, p. 373-4)." W.H. Hobbs of the University of Michigan conducted research on the nature of surface coatings in Egypt, and like Lucas (1905), he viewed different types of rock coatings as related (Hobbs, 1917; Hobbs, 1918). Because he was an expert on this subject, his well-read textbook (Hobbs, 1919) had a lasting impact on U.S. opinion on this issue. "Certain of the saline constituents of the rocks, as they are thus drawn out by the sun's rays, fuse with the rock at the surface to form a dense brown substance with smooth surface coat, known as desert varnish. (Hobbs, 1919, p. 201)."

10.3.1.2. External Origin

Certain characteristics of rock varnish were felt, at least by some, to be at odds with an internal origin. Similar appearing rock varnishes were found in a wide variety of lithologies with differing amounts of manganese and iron (Blake, 1905; Comstock, 1905; Hume, 1925). Some rock types, such as quartz and limestone, were felt to lack manganese and iron (Blake, 1905; Walther, 1891; White, 1924), although Lucas (1905) argued that this claim was an assertion. Thin sections did not show any clear gradient of weathering into the varnish (Hume, 1925; Lucas, 1905). Others felt that the lack of black varnish below the soil line in a desert pavement argued for the importance of the

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external ingredient of sunlight (Blake, 1905, p. 374), and that "black patina...is very difficult to explain under any hypothesis other than exposure to sun heat, and perhaps we ought to add, light (Sturge, 1911, p. 156)." There were diverging opinions on the source of externally-applied constituents that compose rock varnish. Blake (1858) originally thought that rock varnish was "a coating of organic matter, which, under the burning rays of the sun in that almost rainless region, has become a perfect lacquer (Blake, 1858, p. 231). " Then, manganese was thought to be the desiccated remains from ancient waters that once occupied the Mojave Desert (Loew, 1876). A few held that desert dust could contain enough Mn and Fe to support rock varnish (Basedow, 1914; Linck, 1901; Lucas, 1905; Walther, 1891). Basedow (1914, p. 199) wrote "Mr. A. Lucas favours deposition by water evaporating from the rock. I am inclined towards the same opinion, but imagine that the excessively fine, floating and suspended material in the desert atmosphere may also take part in the formation." However, Hume (1925, p. 153) "would scarcely expect iron and manganese to be present in sufficient quantity [in transported dust] to account for their widespread distribution throughout desert regions." Probably the most common external hypothesis involved solutions derived from the nearby soil, which were brought up to the surface of the rock by capillary action and then evaporated (Blake, 1905; Hume, 1925; Merrill, 1898; Merrill, 1906; Service, 1941; Walther, 1892; Walther, 1912). W.B. Pollard (reported by Hume, 1925, p. 151153) wondered why the rocks which "blacken most are extremely compact and impervious to water." Pollard noted a white sandstone near Gebel Ahmar near Cairo, which is covered with "a dead-black film" of mostly manganese on the upper surface, but in contact with soil: "No manganese being found in the rock, Pollard contends that the dew theory [of Lucas] cannot apply in this case...Nor owing to the impervious nature of the stone...is it likely that dew could penetrate it. The theory suggested is that ground water brings manganese up in solution to the surface of the soil, and from thence the salts creep over the surfaces of the stones and pebbles lying on it. The manganese salts thus become oxidized on exposure to the air, and so produce the characteristic black film with which the exterior of the rock is covered."

10.3.1.3. Biological Origin Biological theories for the external origin of rock varnish probably have their start with Lucas (1905, p. 43), who brought up an alternative "mode of formation of the manganese dioxide that has not yet been mentioned, namely, that by means of organisms." Lucas noted the accumulation of manganese and iron in pipes in Europe and North America, which "are usually attributed to various species of Crenothrix..." Although Lucas notes the "existence of manganiferous organisms in the Nile water," he discards a bacterial hypothesis for the fiver and desert films CLucas, 1905, p. 43-44): "The reasons for this opinion are, first, the film apparently never thickens but always remains the same mere patina on the surface of the rock; secondly that the film contains little or no organic matter, no slime and no remains of organisms; thirdly, that the conditions that favour the growth of various species of Crenothrix are lack of oxygen, absence of light and presence of carbonic acid, none of which conditions hold where the film is formed; and fourthly, a very similar film is found on flints, etc., in the desert, where rain is almost unknown, and water, except for frequent and heavy dew, is entirely absent and where the heat of the sun is sufficient to kill almost all known forms even of bacterial life."

Rock Varnish

Hume (1925, p. 160) later echoed these views, although he did note (Hume, 1925, p. 153) that some wells in Egypt were abandoned because of "an organism which removed the manganese present in the water and deposited it in the pipes." Francis (1921) is usually thought of as the parent of biological theories of varnish formation, because of the clear presentation of field and laboratory data that algae and lichens play an important role in the origin of black coatings of iron and manganese on rocks in and adjacent to tropical rain forest streams in Queensland, Australia (Francis, 1921). Lichen-related coatings occur, and "the iron manganese compounds are deposited in, or partly replace, the substance of the lichen thallus to form the black coating (Francis, 1921, p. 111)." More common, however, are 31.tm to 4 ll.tm thick coatings "that may be the altered remains of an encrusting alga [Hildenbrandtia ] ... supported by the following facts: (a) correspondence in distribution; (b) comparability of thickness; (c) the presence of the cellular structure of the thallus of the alga in 50 percent of the examples of the black coating rendered transparent by hydrochloric acid (Francis, 1921, p. 112)." White (1924) hypothesized that the abundant pollen that falls on desert rocks seasonally could contain sufficient manganese and iron to form rock varnish. Although he admitted varnish is not "chiefly carbonaceous" (White, 1924, p. 415), "iron and manganese have been found in practically all plant tissues examined" (White, 1924, p. 419). White (1924, p. 420) concluded: "If it [pollen] does carry them [manganese and iron] the process of deposition of desert vamish on the rocks of desert regions is not difficult to explain, since the long hot period after the falling of the pollen would slowly bum the combustible part and leave the ash containing the iron and manages. These metals would be converted to insoluble oxides and remain on the rock when the soluble part of the ash would be carried away by the next season's rain."

The importance of airborne organics was carried further with the organicgeochemistry perspective that peptides can influence the fixation of manganese and iron oxides on rock surfaces (Knaust, 1930; Linck, 1928). Polynov (1937) also noted that plants bioaccumulate Mn, which can combine with humus and form manganese precipitates. Lichens have also been implicated as important in the development of subaerial rock varnish (Blanck, 1926; Comstock, 1905; Francis, 1921; Laudermilk, 1931). While Comstock (1905, p. 1016) and Laudermilk (1931) noted that varnish was not found next to growing lichens, Laudermilk (1931) argued that lichens may be important by dissolving oxides to be later redistributed by capillary action. Laudermilk (1931, p 58) also pointed out that the growth characteristic of varnish argues for a biological control, because varnish starts at "definite points and seems to indicate spreading from a common point of origin outward in all directions. Such a condition would be fulfilled by the presence of a living organism limited to the coated areas..."

10.3.1.4. Polygenetic Origin Even though researchers prior to World War II tended to favor an internal or an external origin, some argued for a combination of rock and external constituents (e.g. Basedow, 1914; Linck, 1901; Walther, 1912). For example, even Merrill (1898), a strong advocate of an internal origin, noted carbon as a minor constituent for Utah varnish, which was thought to perhaps derive from the desiccation of Pleistocene Lake

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Bonneville. Later, Merrill (1906, p. 244-5) acknowledged that cobble size and rock type should play a role if there was an internal origin, but desert varnish appears on almost all rocks. The vast majority of papers written on rock varnish have been single contributions. Few investigators have made more than one contribution. Three exceptions in this early period were Blake, Walther, and Linck, who all tended to argue back and forth in their own mind in their writings. Blake (1855, 1858) initially thought that dark coatings were desiccated organics, left behind by ancient waters. Later, however, Blake (1905) debated with himself over the merits of an internal versus an external origin. On the internal side, he felt that varnish derived from "osmotic fluids" weathered from the discoloration underneath the varnish (now recognized as a weathering rind). However, Blake had trouble with the existence of the same coating on different lithologies, especially quartz. In the end, Blake (1905) argued for a combination of material derived from the underlying weathering rind and in the adjacent soil, brought to the surface as capillary water and then evaporated~ depositing iron and manganese oxides. Walther (1891) initially favored an external origin, because of a great variety coated rocks not containing manganese and iron. Instead, Walther (1891) argued that desert dust could contain all the necessary manganese and iron to form the coatings. However, later, perhaps influenced by Linck (1901), Walther (1912) favored an origin by dewbased solutions weathered from the rock, moved up with capillary action, and dried by the intense heat of the sun and the dry desert air; these solutions could then combine with Mn and Fe in the desert dust particles. Linck (1901) initially argued for a four-fold sequence: (1) dew moves into rock pores; (2) high temperatures favor chemical weathering of the rock; (3) oxidation of the solution upon contact with air; and (4) fixing the new combinations with the aid of the intense heat of the sun. Later, Linck (1930) argued for the importance of combinations of organic peptides with manganese and iron on the surface of rocks

10.3.2. Debate from Worm War H to the First Dissertation

In these middle years, the debate over the genesis of rock varnish still centered on whether constituents derive from the rock or external sources. However, there was a greater focus on the processes by which manganese might be enhanced. 10.3.2.1. Internal Origin

The most common opinion in physical geography, geology, and anthropology was that water (from dew or precipitation) penetrates the rock and weathers minerals. Then, iron and manganese in solutions move outward to the surface by capillary flow and evaporate, leaving the basic varnish constituents (Abu-Al-Izz, 1971; B irot, 1969; Demangeot, 1971; Evenari et al., 1971; Garner, 1974; Glazovskaya, 1968; Glennie, 1970; Grant et al., 1968; Heizer and Baumhoff, 1962; Holmes, 1965; Honea, 1964; Iskander, 1952; Kiersch, 1950; Klute and Krasser, 1940; Marshall, 1962; Opdyke, 1961; Peel, 1960; Smith, 1968; Tricart and Cailleaux, 1964; Ugolini, 1970; Wilhelmy, 1964; Willcox, 1963; Wyckoff, 1966). The general belief in an internal

Rock Varnish

origin continued into the 'modem' period of varnish research (Besler, 1979; Goudie and Wilkinson, 1977; Meyer. 1978; Shlemon, 1978; Turtle, 1983).

10.3.2.2. External Origin

Although in a minority, several authors argued firmly that varnish was an accretion and not derived from the underlying rock (Hayden, 1976; Hildreth, 1976). "It now seems clear that it [desert lacquer] is not an exudation from the rock but that it has been slowly applied from the air... (Blackwelder, 1954, p. 14)." This perspective was supported by studies where no enrichment of manganese occurred during incipient rock weathering (Dennen and Anderson, 1962).

10.3.2.3. Both Internal and External

Those most active in varnish research tended to advocate a combination of constituents derived from the underlying rock and different external sources: soil solutions (EI-Baz, 1977; Engel and Sharp, 1958; Hooke et al., 1969; Hunt, 1954; Hunt, 1961; Hunt, 1974; Hunt and Mabey, 1966; Mabbutt, 1977; Rogers, 1966); soil particles (Engel, 1957; Engel and Sharp, 1958) ; and airborne material (Engel and Sharp, 1958; Mabbutt, 1977). The need for externally-derived constituents came, in part, from the trace element analyses; Engel and Sharp (1958, p. 506) felt it unlikely that "trace elements in the varnish have been drawn from the underlying rock by a leaching process." Even those advocating biological processes to incorporate external constituents believed that the underlying rock added manganese and iron (Cailleaux, 1969; Evenari et al., 1971; Krumbein, 1969; Krumbein, 1971; Scheffer et al., 1963).

10.3.2.4. Manganese Enhancement by Chemical Processes

Engel and Sharp (1958) suggested four chemical processes that might explain the amazingly high Mn:Fe ratios (around one) in varnish: (a) arid weathering solutions may be enriched in Mn; (b) the presence of MnO2 may have a catalytic effect on further Mn deposition; (c) iron is leached after varnish is deposited; and (d) iron deposition may occur before the solution reaches locales of Mn deposition. The latter fractionation process, based on the greater mobility of Mn, was favored by Hooke et al. (1969). Both Engel and Sharp (1958) and Hooke et al. (1969) treated varnish as chemical deposit in equilibrium with its environment. Engel and Sharp (1958) reasoned that rock varnish can both form and erode, and that whatever processes favor its formation must exist in deserts because varnish is ubiquitous in arid lands.

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10.3.2.5. Manganese Enhancement by Biotic Processes Although in a minority, there were many who felt that biotic processes were vital to the formation of rock varnish, involving "minute organisms, among which pollen, lichens, algae, and bacteria... (Blackwelder, 1954, p. 14)." Laudermilk's (1931) earlier lichen hypothesis was widely cited (e.g., Turner, 1963) and even adopted in some geological texts (Emmons et al., 1955). Brownish-black dendrites and coatings were attributed to the activity of bacteria, fungi, cyanobacteria, lichens, and algae in France (Billy and Cailleaux, 1968; Cailleaux, 1965), the Negev Desert (Evenari et al., 1971), Morocco in the Sahara Desert (George, 1976), the Mojave Desert (Jaeger, 1965), the Colorado Plateau (Rahm, 1974), Antarctica (Markov et al., 1970), the high altitudes of the Pamirs (Glazovskaya, 1952), Alps (HOllerrnan, 1963), and Tien Shan (Glazovskaya, 1968). Hayden (1976) contended that plants may have helped provide varnish constituents. There was also speculation that manganese nodules and desert varnish were cogeneric in that both were made by bacteria (Bauman, 1976). C.B. Hunt (1954) was an early advocate for biochemical activity, noting that poorly varnished areas had a lack of wetting, and he suspected that "microorganisms play a major role (Hunt, 1961, p. B195)." However, Hunt and Mabey (1966, p. A92) presented thin section observations by Estella Leopold and Richard Scott of the U.S. Geological Survey, who only found a "single hyphae" and felt that the varnish did not penetrate the rock "in a way that would suggest surface weathering of the rock attributable to microorganisms." In the end, Hunt and Mabey (1966, p. A92) concluded that: "selective precipitation of the metals without mixing with large amounts of other salts could readily b~ brought about by the oxidation caused by bacteria, algae, or other microorganisms, but it seems difficult to achieve in such varied environments by physical-chemical processes along."

Engel and Sharp (1958, p. 513) mostly argued against a role for biological agencies. They noted that varnish contained many elements "not favored by plants." Engel and Sharp also cited a seminal paper on low temperature manganese geochemistry (Krauskopf, 1957, p. 71) that bacteria cannot "serve as a general solution to the manganese problem..." and repeated Lucas' (1905) assertion that bacteria cannot survive harsh desert conditions. However, in the end they wrote "Initial deposition of at least Mn may be caused by biochemical (bacterial) action, but subsequent inorganic deposition may become more important owing to the catalytic effect of MnO2." In a review of this debate, Cooke and Warren (1973, p. 89-90) conclude: "the chemical data of Engel and Sharp (1958) and of Hooke et al. (1960) make this explanation [organic theories of varnish precipitation] difficult to accept in its entirety." In an investigation of the "Biologische Ursachen der Wtistenlackbildun" (biological causes of the development of desert varnish), Scheffer et al. (1963) argued that cyanobacteria are vital in the formation of dull brown, reddish brown, to reddish varnishes in the Mojave Desert, western Sahara of Mauritania, Lybian Desert, and the Vintschgau region of the Alps. According to their model, varnish formation takes place through episodic moisture allowing the metabolic products of Cyanophycees to mobilize iron from the underlying silicates and oxides through pH reduction, formation of chelating compounds, or formation of hydro-complexes. Repeated wetting and drying ensures that the iron is freed in small amounts to be moved in a capillary distribution system. The reduction of iron through organo-complexors, according to Scheffer et al., is followed by desiccation. The presence of dried brines of Cyanophycees inhibits oxide crystallization, rendering the varnish mostly amorphous with some goethite formation.

Rock Varnish

Varnish forms on rocks lacking iron (e.g. quartz) only where the capillary network can transport the varnish constituents. Culturing of cyanobacteria produced spots of Fe-Mn spots on ceramic plates after 6-8 weeks of growth exposed to night time capillary dew action and desiccation during the day by infrared light. In summary, Scheffer et al. (1963) argued that varnish genesis was a product of biochemical alteration of the rock (cyanobacteria in deserts; lichens in the Alps). Although this paper was cited frequently, it was not taken seriously by very many, because by the time this paper was published the key problem in varnish formation was how to enhance manganese. Their model did not address this problem. The color change was thought to be a reaction to sunlight. The issue of manganese enhancement by microorganisms was, in contrast, addressed directly by the microbiologist W. Krumbein (1969) in a seminal paper on "microfloral influences on exogenic dynamics (weathering and crust formation)". Although Krumbein (1969, 1971) sampled from a variety of climates in the Old World, he focused on Negev Desert varnishes. In all but two locales, he found lichen, algae, fungi, and/or bacteria growing on, in, passing through, or under varnish (Krumbein, 1969, p. 356-7). Manganese dendrites were found on the fringes of lichens, due to fungal activity (Krumbein, 1969, p. 356). After isolating fungi, bacteria, and algae, they were grown in manganous sulfate media and produced encrustations of manganese that were X-ray amorphous. The rock was important, not because of its manganese or iron content, but because its microenvironment (Krumbein, 1969; Krumbein, 1971). To Krumbein (1971) varnish was a "biological solution front" consisting of bacteria, algae, fungi, and endolithic and epilithic lichens. On silicified limestones from the Negev, dissolution created pores that were occupied by iron and manganese deposits (Krumbein, 1971, p. 345,359). In countering the abiotic alternative, Hunt and Mabey (1966, p. A92), Krumbein (1969, p. 359), and Rahm (1974, p. 106) stressed that varnish occurs in settings that are not compatible with a physical-chemical precipitation of manganese, but microorganisms can explain the global distribution of varnish in both acidic and alkaline settings (Krumbein, 1969, p. 358-9).

10.3.2. Source of the Manganese One of the key issues in the origin of rock varnish, as well as manganese skins (see Chapter 9) is the relative enrichment of manganese. Iron is not concentrated more than two to three times in rock varnish, relative to abundances in soil and rock material. In contrast, manganese is enhanced sometimes more than two orders of magnitude above soils and crustal material. Thus this section will discuss potential sources of manganese and its abundance in relation to iron. The abundance of manganese in potential inorganic sources is in the range of a half percent. Continental crust averages 0.11% (Rudnick, 1995) and desert dust in central Arizona 0.06-0.09% (P6w6 et al., 1981), when measured as MnO. The abundance in soils ranges from 0.1% to 0.3% Mn elemental weight percent in the former U.S.S.R (Peyve, 1963), and from <0.01% to 0.7% (mean 0.0.34%) element weight percent in the U.S.A. (Matrone et al., 1977). Therefore, the elemental abundance of manganese in rock varnish is usually the order of 50x to 300x concentrations found in adjacent dust, soil or rock material. Another way to explore the issue of enrichment is in a ratio with iron. Table 10.10 illustrates Mn:Fe ratios for material that may come in contact with rock surfaces. The

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lowest ratios are for continental crust. Not surprisingly, soils and airborne dust are similar in ratios that are highly variable from place to place, but these are only slightly higher than continental crust, perhaps because of the addition of vegetative material. Precipitation appears to concentrate manganese greatly. However, the greatest enrichment compared with crustal values comes in vegetation.

Table 10.10. Ratios of manganese to iron in continental crust, soils, airborne dust, precipitation and vegetation. Material . . . . . . . . . . . . . . .

Mn to Fe'""Ratio.............. R'eierence.........................................

Continental Crust

1/59 1/50

Soils

,,

,,,,,iH

,llllllll

,

,

,,,,,,

m,H

,ill

,, ,,,,

,,,,,

, ,,H,

,,,,,,Hi,

Typically 1/50

(Lakin, 1979; Peyve, 1963)

Airborne Dust

1/44 ContinentalFlux 1/40 In U.S. 1/39 Over oceans 1/60 Over Oceans 1/55 to 1/80, Tempe 1/85 to 1/230,Sudan

(Lantzy and Mackenzie, 1979) (Thompson, 1979) (Lee and Duffield, 1979) (Anikiyev and ll'ichev, 1984) (P6w6 et al., 1981) (Penkett et al., 1979)

Global Runoff - Solid Load

1/46

(Anikiyev and II'ichev, 1984)

Global Runoff - Dissolved

1/39

(Anikiyev and II'ichev, 1984)

Precipitation

1/3 to 1/9,Westem USSR (Drozdovaand MakhonXo, 1970) 1/5, New Mexico (Poppet al., 1984)

Vegetation

1/9 Living Matter 4.5/1 Land Vegetation 1/1.4 Land Vegetation 1/2

i

li

,

l,

(Turekian and Wedepohl, 1961) (Rudnick, 1995)

,,,

,,,,,,,,, ,,,,

H

i

(Lakin, 1979) (Cannon, 1960) (Bowen, 1966) (T]ml~kanova,,,,,gnd Kon0va, 1986)

......

One explanation for the variability in Mn/Fe ratios for land vegetation in Table 10.10 could be variable concentrations in different types of plants (Table 10.11). This could be because manganese takes on increasing importance in angiosperms, as it used as a catalyst for photosynthesis (Boychenko, 1987). Table 10.11. Ratio of manganese to iron in different organisms in the biosphere. Data for lichens (Lounamaa, 1965; Nieboer et al., 1978) and pollen (Stanley and Linskens, 1974) are from different sources than the rest of the table (Boychenko, 1987). m

_

i

,,,,

,,

Type of Organism Blue-green algae Lichens Red Algae Brown Algae Green Algae Bryophytes Ferns Pollen Angiosperms

,,

,, Mn/F~,,,,,Ratio 1/>20 1/5 to 1/66, 1/12 to 1/20 1/10 to 1/16 1/8 to 1/12 1/4 to 1/6 1/2 to 1/4 1/7 1/lto 1/2 ,

,

Rock Varnish

An early explanation for the origin of rock varnish involved the deposition of pollen (White, 1924). Pollen remains, however, are only infrequently seen directly incorporated into rock varnish (Figure 10.28).

Figure 10.28. Scanning electron micrograph of an oak pollen grain being incorporated into rock varnish. Fungal filaments surround the pollen grain.

It may be that some of the enrichment in rock varnish could be from manganese having a source in biological materials, that are then reworked by heterotrophic organisms. For example, the microcolonial fungi that are abundant on rock surfaces (Dragovich, 1993a; Staley et al., 1983; Staley et al., 1982) are heterotrophs and may obtain nutrients in part from pollen and other organic deposits. In summary, plant remains show the greatest enrichment of manganese of all of the potential sources. Regardless of the source of the manganese, however, mobilization of divalent maganese is needed to move the cation from source to sink. Then, another mechanism(s) is needed to stabilize manganese by oxidation to Mn (IV).

10.3.3. New Polygenetic Model of Varnish Formation This section presents a new model for rock varnish formation, where both biotic and abiotic processes are involved. The three key ingredients in both rock varnish and this model involve clay minerals, biotic enhancement of manganese, and to a lesser extent the enhancement of iron. Hence, this section first explores the nature of clay minerals in rock varnish. Then, I turn to an exploration of manganese enhancement. Lastly, these components are synthesized in this new polygenetic model.

10.3.2.1. Clay Minerals at the Building Block Level

Clay mineral weathering is ubiquitous in rock varnish (Figure 10.19). The clay minerals (illite, smectite, interstratified illite-smectite, chlorite) which compose the

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bulk of varnish (Potter, 1979; Potter and Rossman, 1977) also weather. The clay minerals start out as 'pods' with regular lattice spacings and coatings (Robert and Terce, 1989), Then, clays begin to feather into mono-layers (Robert and Tessier, 1992). These mono-layers are separated by nanometer-sized possible unit-cells that are similar in appearance to some biomineralized iron (Mann et al., 1993; Robert and Terce, 1989) and manganese (Kuznetsov, 1970; Mackenzie et al., 1971; Mulder, 1972; Perfirev et al., 1965; Vojak, 1984). Evidence for clay-mineral weathering can be found in Figure 10.19. At lower magnifications, detrital grains of clays appear to split at the edges; the splitting process is sometimes associated with the migration of darker material--presumably migrating Mn, Fe that should be more resistant to electron beam (arrow in Figure 10.19A). Figure 10.19B shows transition in an illite grain from organized (left) to more disorganized (fight) as the grain weathers; regular lattice spacings split in association with possible bacterial textures (granular areas, discussed later). A typical pattern is found in Figure 10.19C, where there are 5-6 discrete layers that may have started out as different detrital grains (along with granular bacterial texture, discussed later); however, within each layer even lattice spacing is a rarity; wavy mono-layers dominate. At higher magnification in Figure 10.19, within a single 'pod-detritar clay mineral grain (Figure 10.19D-6F), there is evidence of irregular lattice spacing, indicating separation and the development of mono-layers (arrows in Figure 10.19E-I)~ essentially 001 planes of exfoliation (Robert and Tessier, 1992). In many cases, the feathering is associated with granular textures (of possible bacterial origin, discussed later), as in Figure 10.19D. Consider the arrow in Figure 10.19G, which shows the splitting of the upper illite grain associated with nanometer-sized unit-cells, while the material on the fight side is so disorganized lattice spacing cannot be discerned; the darker grain on the lower left is too thick to be electron transparent. The lower part of Figure 10.19J shows regular spacing, while the upper part shows wavy monolayers and abundant nanometer-sized unit-cells. The monolayers (wavy lines) in Figure 10.19K consist almost entirely of connected unit-cells. At the highest magnifications used (Figure 10.19L-O), disorganized wavy layers rest next to regularly spaced lattice fringes (with spacing that is consistent with illite, smectite, chlorite, and interstratified clay~textural interstratification) (Banfield and Eggleton, 1988; Mall and Komarneni, 1989; Robert et al., 1990). The juxtaposition of organized versus wavy layers may be 'born' when varnish is initially deposited, or it may be a product of gradual in situ diagenesis; however, some in situ diagenesis seems probable given the long times involved. However, the feathering of the clay minerals in rock varnish is a new observation and a key element in its growth.

10.3.2.2. Manganese Enhancement

The general consensus over abiotic vs. abiotic mechanisms of manganese enhancement has largely shifted in the last few decades towards microbial models of genesis. This is true for several reasons: (1) the pH conditions on manganese-rich varnishes in deserts (Dora, 1990) are not high enough to oxidize manganese by inorganic processes (Uren and Leeper, 1978); (2) the rock surfaces that do change local pH conditions (e.g., adjacent to lichens) lack varnish (Dora and Oberlander, 1982; Dragovich, 1986b);

Rock Varnish

(3) fixation of Mn by clay minerals (EI-Demerdashe et al., 1982; Reddy, 1973; Reddy and Perkins, 1976) yield Mn concentrations orders of magnitude below values found in rock varnish; (4) the distribution of incipient rock varnish as millimeter-scale patches is suggestive of biological colonization, while a chemical process would yield a more uniform deposit (Dorn and Oberlander, 1982); (5) rock varnish occurs in acidic environments that do not experience high enough pH values to oxidize Mn (Dorn and Oberlander, 1982; Douglas, 1987; Krumbein, 1969); (6) organic matter is common as a trace compound within rock varnish (Dorn and DeNiro, 1985; Merrill, 1898); (7) manganese-rich varnishes are less common in hyper-arid/hyper-alkaline regions (Jones, 1991); (8) varnish grows rapidly in periglacial and riverine environments (Dorn and Oberlander, 1982; Klute and Krasser, 1940) which do not have sufficient alkalinity or high enough pH values to oxidize Mn, but there is a slow rate of varnish growth in deserts that do experience these fluctuations (Dorn and Oberlander, 1982; Whalley, 1983); (9) no laboratory experiment has yet produced Mn-rich varnish with Eh-pH fluctuations; (10) experimentation reveals that Mn-rich varnish is not produced abiofically (Jones, 1991); and (11) a number of studies have concluded that biological agents of manganese enhancement grow on rock surfaces and can explain the great enhancement of manganese in rock varnish (Dora and Oberlander, 1981a; Dora and Obeflander, 1981b; Dora and Oberlander, 1982; Drake et al., 1993; Francis, 1921; Grote and Krumbein, 1992; Grote and Krumbein, 1993; Ha-mung, 1968; Hungate et al., 1987; Jones, 1991; Krumbein, 1969; Krumbein, 1971; Krumbein and Jens, 1981; Nagy et al., 1991; Palmer et al., 1985; Rahm, 1974; Staley et al., 1991; Taylor-George et al., 1983). At the same time, abiotic arguments have not gone away (Elvidge and Collet, 1981; Engel and Sharp, 1958; Hooke et al., 1969; Moore and Elvidge, 1982; Smith and Whalley, 1988). These models rely on the greater mobility of manganese over iron where small pH~h fluctuations dissolve Mn but not Fe (Krauskopf, 1957). The Mn released by slightly acidic precipitation, is then fixed in clays after evaporation or change in pH. I would like to emphasize two key points in favor of abiotic arguments. First, very few examples of fossilized Mn-oxidizing microorganisms have been reportexl (Dora and Meek, 1995; Smith and Whalley, 1988). Fungal remains have been occasionally seen buffed within rock varnish (Dragovich, 1993a; Nobbs and Dora, 1993; Taylor-George et al., 1983), but these are not enriched in manganese. In other biotically-generated coatings, fossilized remains are common (Beveridge and Fyfe, 1985; Ferris et al., 1986; Konhauser et al., 1994; Schultze-Lam et al., 1996). Second, abiotic genesis could produce the forms observed with high resolution transmission electron microscopy (Figure 10.19): nanometer-scale crystals of Mn and Fe exfoliating clay minerals along 001 planes (cf. Robert and Tessier, 1992: 86). This is not to say that microbial fossils or casts of fossils encrusted in manganese are completely lacking. They do occur (Figure 10.29, Figure 10.30 and Figure 10.31), but they are rare (Dora and Meek, 1995). More commonly, there are granular forms that may be the decayed remnants of bacteria.

243

244

Chapter 10

Figure 10.29. Backscattered electron micrograph of Mn-rich rod-shaped bacterial casts in rock varnish from Kaho'olawe, Hawai'i.

Figure 10.30. Backscattered Oeft) and transmission electron (righ0 micrographs of cocci-forms, encmsted with manganese, in a rock varnishes from Marie Byrd Land, Antarctica. The underlying rock is quartz. The bright cell walls in backscatter are likely composed of granular deposits seen in the TEM image. Images were from the same sample, but not the same thin section.

Figure 10.31. Possible bacterial remains visible with transmission electron microscopy, from Antarctica. According to focused-beam (~1-2gin) qualitative EDS analyses the areas around this object shows great enrichments in Mn. Note how the center of the cocci-like object is brighter than the surrounding material, because the electron beam is less transparent where the walls of a sphere begin to turn.

245

Rock Varnish

Granular forms are very common in four samples examined with transmission electron microscopy (Figure 10.32). These granular forms are conformal with clay minerals). Some have an oval shape, but most are linear. The granular texture is not usually found with clay minerals, but similar granular forms are associated with iron coatings on clays (Robert and Terce, 1989) and with biogenic manganese precipitates (Kuznetsov, 1970; Mackenzie et al., 1971; Mulder, 1972; Perfirev et al., 1965; Vojak, 1984). Granular features also line the edges of bacterial casts (Figure 10.30).

............

L

"

Figure 10.32. These micrographs show a transition between granular-textured material to nanometer-sized 'dot clusters', with samples from Death Valley (A,F,I), Hawai'i (B, H,G), Peru (C,E), and Antarctica (D). The scale bars are in manometers. Granular textures (A-C) may be a stage in the re-mobilization of Mn-Fe from cell-waU fragments. It is possible that the next step is a transition towards a mixture of granular textures and "dusters of dots" (F,I), followed by just dot clusters (D,E,G,H).

In the next section I propose a mdoel where these granular forms are the products of the dissolution and reprecipitation of manganese and iron in bacterial cell walls. Some manganese and iron removed from the bacterial casts is leached from the varnish into the host rock to reprecipitate in the weathering rind and aid in case hardening (chapter 6). However, some of the manganese and iron is redistributed within the clay minerals as unit cells, that in turn weather clay minerals into mono-layers.

246

Chapter 10

10.3.2.3. How Rock Varnish Grows

The initial manganese enhancement of manganese, and to a lesser extent iron, in rock varnish is most likely through the activity of bacteria, followed by abiotic processes. The first step is the concentration of manganese (and iron) on the cells walls of bacteria (Figure 10.29, Figure 10.30, Figure 10.31, Figure 10.32). There may also be some enhancement through the deposition and remobilization of organic remains on rock surfaces (Table 10.11). The second step is the mobilization of divalent manganese away from these cell walls. This could occur immediately after Mn-enhancement; or it could occur thousands of years later. Figure 10.32 shows the gradual break-down of biomineralized Mn-Fe from cells that are thoroughly encrusted (Figure 10.30). As these cell encrustations degrade, the oxides are remobilized as nanometer-scale unit cells (dots in Figure 10.19 and Figure 10.32). The granular material has a high surface/volume ratio, which in turn facilitates further dissolution, mobilization and reprecipitation. The third step is the movement of manganese unit cells into clay minerals. Potter and Rossman (1979a: 93) and Potter (1979:174-175) argued for this step without the benefit of supporting HRTEM imagery: "Clay is more than a passive contaminant in vamish. In some instances it may serve as a medium for capillary movement of varnishing solutions. Deposition of the manganese and iron oxides within the clay matrix might then cement the clay layer. The clay may aid in deposition, lnite is known to fix manganese under the pH and Eh conditions at which varnish forms (Reddy and Perkins, 1976)...the hexagonal arrangement of the oxygens in either the tetrahedral or octahedral layers of the clay minerals could form a suitable template for crystallization of the layered structures of bimessite. The average 0-0 distance of the tetrahedral layer is 3.00/~ in iUitemontmoriUonite mixed-layered clays, which differs only 3.4 percent from the 2.90/~ distance of the hexagonally closed-packed oxygens in bimessite..."

Bimessite can "appear as thin laths up to 0.3 I.tm long" (see Figure 10.7) or "as very minute (tens of Angstrom units) laths, which are coiled bent, or twisted so that they look like filaments (Chukhrov et al., 1980, p. 348-9)." Perry (1979) reported that small particles in rock varnish show a hexagonal net of spot reflections in electron diffraction. This would be consistent with the bimessite mineralogy found in infrared studies (Potter and Rossman, 1979a). The weathering of clays, the development of monolayers, and the occurrence of nanometer-sized dots in and amongst the monolayers (Figure 10.19) is fully consistent with Potter and Rossman's (1977) model where clay minerals cement Mn-Fe oxides to rock surfaces. Potter (1979, p. 174-7) wrote: "...the clay and oxide phases may be mutually dependent: the clay depending on the oxides for resistance to erosion; the oxides dependent on the clay for transport and deposition."

The only steps missing in the Cal Tech model involve the processes by which Mn (and Fe) are microbially enhanced, and then released to cement clays to the rock surface. Thus, bacterial remnants in the form of decayed cell walls and redistributed manganese and iron comprise a significant portion of total varnish volume, even if only a few of the bacteria themselves have survived diagenesis. A large uncertainty in this model concerns iron. Iron is not concentrated above crustal levels in many varnishes, while in other the enhancement can be five-fold. It is possible that some enhancement is through the concomitant concentration of iron, that occurs along with manganese (Dorn and Oberlander, 1982), or separately (Adams et al., 1992). It is possible that some of the granular morphologies include iron. Certainly

Rock Varnish

iron is the key cementing agent in Type III iron films (see chapter 9) and iron likely assists in varnish cementation. In summary, I believe that rock varnish is a by product of the weathering of bacterial casts and the weathering of clay minerals. Clay minerals will not form rock varnish by themselves. Dust films, silica glaze and other rock coatings may result from clays, but not rock varnish. Similarly, bacterially-enhanced manganese will not form rock varnish by itself; heavy-metal skins will result instead. Rock varnish grows only where and when the nanometer-scale decayed remnants of bacterial casts maneuver in between the broken and decayed fragments of clay minerals.

247

248

Chapter 11 NITRATES

AND OTHER

UNCOMMON

ROCK

COATINGS

The theory of knowledge which I wish to propose is a largely Darwinian theory of the growth of knowledge. From the arr,oeba to Einstein, the growth of knowledge is always the same: we try to solve our problems, and to obtain, by a process of elimination, something approaching adequacy in our tentative solutions. (Popper, 1979, p. 261)

11.1. Introduction This chapter focuses on rock coatings that are relatively uncommon. Phosphates, nitrates, halite, and sulfates are rarely reported as coatings on rocks at the earth's surface. But before I get into trouble with those who study "duricrusts", it is important distinguish the term "crust" as it is used in the soils and geomorphic literature from rock coatings. Duricrusts refer to salts that are concentrated in meter-thick layers in a near-surface locations. Terms like calcrete, salcrete and gypcrete are used to describe geomorphologicaUy resistant crusts (Watson, 1989), and caliche is used to describe nitrate duricrusts in the Atacama desert (Ericksen, 1981; Ericksen, 1983). Yet, duricrusts and duricretes are not rock coatings. They are subsurface phenomena that can be exposed at the surface under certain circumstances. The literature on phosphates, nitrates, chlorides, and sulfates is extensive. The literature on phosphates is concentrated geological settings. Papers on nitrate, chloride and sulfate salts tend to focus on their occurrence in playas, their role as geomorphic agents, and as components of deserts soils. Despite the extensive literature on these raw materials, the vast majority of the discussion has nothing to do their accretion as coatings on rocks. The organization of this chapter is to discuss in turn phosphate skins, nitrate crusts, salt crusts, and gypsum crusts. In each section, my focus is to synthesize relevant information on rock coatings from the broader literature. In some cases, I have provided supplementary data from original studies. Lastly, I offer an explanation for the limited distribution of these rock coatings.

11.2. Phosphate Skins Phosphate rock coatings occur in a wide variety of geological contexts, for example copper phosphate cornetite in mines (Nriagu, 1984, p. 31), aluminum phosphates in hydrothermal settings (Nriagu, 1984, p. 129), calcium-aluminum phosphates in pegmatites (Nriagu, 1984, p. 129), in coal seams (Ward et al., 1996) and other geological settings (Zanin, 1989). These same coatings can be seen at the earth's surface when they are exposed by natural erosion and anthropogenic activity. The focus

Uncommon Rock Coatings

here, however, rests on rock coatings formed near or at the earth's surface--not in geological deposits. Phosphorus is the tenth most common element in the earth's crust (Parker, 1967). Phosphate most often combines with iron (ferrous or ferric), aluminum, calcium and manganese to form a wide variety of phosphate minerals (Nriagu, 1984). The largest deposits of commercially mined phosphates are marine in origin (Cook, 1984), but both organic and inorganic phosphates are common components of soils (Mulder and van Veen, 1968; Shang et al., 1992). In general, phosphorus accumulates in alkaline and acidic environments and accumulates in neutral environments mainly as apatite (Zanin, 1989). One of the most common types of phosphate skins forming in today's terrestrial weathering environment can be found on relatively dry coral reef islands (Zanin, 1989, p. 341), largely through the interaction of guano with calcium carbonate in the coral (Fosberg, 1957; Hutchinson, 1950). While most of this phosphate exists as 'phosphate rock' and not as rock coatings, recent deposits "consists normally as a thin (0.1mm) fringe of multilaminar phosphatic cement (carbonate-hydroxy-fluor-apatite) enveloping unaltered sand-sized skeletal grains (Stoddart and Scoffin, 1983), p. 396)." In some cases, the guano-derived phosphate skins may also form in crenitic environments (Trueman, 1965). Guano-derived phosphate skins can be found in drylands (MacLeod et al., 1995). Kestral guano started out as iron, magnesium and calcium phosphates. Upon interaction with calcium carbonate, a calcium pyrophosphate evolved that was more stable than the original guano. Figure 11.1 illustrates the appearance of a typical phosphate skin found on desert rocks, derived from guano deposits of an unknown origin. A preliminary radiocarbon ages for a guano collected from a tafoni in Papago Park, Phoenix, Arizona, ran was about 1300 radiocarbon years, making correlations with contemporary bird occupation difficult. Phosphate skins can incorporate eolian detritus (Figure 11.2). My selected observations indicate that phosphate skins can reach thicknesses of at least 0.5 mm. Electron microprobe analyses of the relatively pure sections of the Papago Park phosphate skin reveals that the much of the coating is phosphorus. The second most common measured element is sodium, which may be a sodium nitrate (Table 11.1).

Figure 11.1. Phosphate skin in Joshua Figure 11.2. Backscattered electron microprobe image of Tree National Monument. The white phosphate skin formed in tafoni of Papago Park, Arizona. The skin originated as a guano deposit within darker detrital grains are quartz, indicating that eolian the tafoni hollow. Mobilization and materials can be trapped by phosphate skins. The bright subsequent reprecipitation resulted in the detrital mineral is hornblende. Electron microprobe deposition of this phosphate skin. measurements (Table 11.1) were made in a transect in the area where the phosphate skin lacked the abundant detritus.

249

Chapter 11

250

Not all phosphate skins necessarily derive from guano, and not all are white. Dusky red coatings (Munsell 2.5YR 3/2) are fairly common on the granodiorite pediment boulders in the Apple Valley area of the Mojave Desert, California. These coatings are typically <51.tm to ~501.tm in thickness. Figure 11.3 illustrates one of the thicker phosphate skins on a core stone on the Apple Valley pediment (Oberlander, 1974). Electron microprobe analyses reveal that these phosphate skins vary considerably in the chemistry, but are characterized by a fairly even mixture of aluminum, silica and phosphorus (e.g. Table 11.2).

Table 11.1 Phosphate skins in rock shelters of Papago Park, central Arizona. Low probe totals may be from water, porosity, organic matter, and the presence of nitrates, reflected in the high sodium content. SiO2, TiO2, MnO, Fe203, and BaO were below the limit of detection. Na20

MgO

A1203

P205

SO3

K20

CaO

Total

4.56 5.55

0.18 0.15

0.11 0.13

33.06 30.18

0.49 0.48

5.55 6.04

0.85 0.66

44.80 43.19

3.44

0.18

0.14

25.00

0.59

3.48

0.09

32.92

4.84

0.16

0.19

28.04

1.04

1.13

0.33

35.73

6.59 5.00

0.11 0.16

0.19 0.11

29.11 30.45

1.22 0.95

6.04 1.90

0.41 0.19

43.67 38.76

2.89

0.11

0.13

22.30

0.11

3.50

0.25

29.29

8.77

0.16

0.22

34.18

0.67

4.09

0.28

48.37

6.70

0.17

0.45

27.07

0.30

5.01

0.48

40.18

Figure 11.3. Backscattered electron microscope image of a relatively thick phosphate skin formed on a granodiorite core stone on an Apple Valley pediment, Mojave Desert, California. The line indicates the location of electron microprobe measurements in Table 11.2.

Uncommon Rock Coatings

251

Table 11.2. Phosphate skin formed on a core stone on Apple Valley pediment, Mojave Desert, California. The line in Figure 11.3. indicates the location of the data measured with a focused beam using a wavelength dispersive electron microprobe. Na20 MgO A1203 SiO2

P205

SO3

K20

CaO

TiO2 MnO Fe20 3 BaO

Total

0.00 0.00 0.00 0.57 0.00 0.00 0.16

25.82 16.80 31.28 7.30 15.54 16.36 14.92

0.22 0.07 0.37 0.00 0.24 0.42 0.17

0.34 0.23 0.14 1.83 0.34 0.59 0.52

0.34 0.59 1.12 6.38 0.96 0.81 0.84

0.00 0.03 0.05 0.40 0.10 0.98 0.10

73.33 72.17 75.86 71.98 74.80 72.64 74.71

0.07 0.05 0.07 0.95 0.46 0.45 0.23

28.36 20.63 33.24 29.92 28.88 28.02 19.97

17.16 33.18 8.15 19.34 24.08 21.59 35.68

0.00 0.00 0.03 0.09 0.00 0.00 0.01

0.77 0.36 1.22 5.21 4.00 3.30 1.96

0.25 0.23 0.19 0.00 0.20 0.12 0.15

Calcium phosphate-rich skins may be found on desert pavement cobbles. For example, figure 11.4. illustrates one skin resting on the surface of a metamorphic clast on an alluvial terrace in the central Sinai Peninsula. The skin had a Munsell color that is dusky red (2.5YR 3/2). The correlation of phosphorus and calcium in Table 11.3 suggests that the phosphorus is bound as a calcium phosphate (apatite). The abundance of aluminum and silica may also reflect abundant clay minerals.

Figure 11.4. Backscattered electron microscope image of a phosphate skin on a plagioclase feldspar, Marsa Muqualba, Sinai Peninsula. The abundance of iron within the phosphate skin in Figure 11.4 could also reflect precipitation of iron phosphate by bacteria. A study of dark granitic skins in the Canadian Arctic revealed the presence of iron phosphate, perhaps the mineral strengite ( F e P O 4 - H 2 0 ) (Konhauser et al., 1994). Konhauser et al. (1994) argue for cyanobacterial and bacterial precipitation of iron phosphate around their cell walls. Then, these precipitates may become nucleation sites for inorganic precipitation of the iron phosphates. I have observed iron phosphate skins in a variety of locations. On Stone Mountain, Georgia (Figure 11.5), for example, coatings composed of iron phosphate and detrital particles can be seen in some of the streaks (Figure 11.6). Although most of the

Chapter 11

252

streaked rock coatings at Stone Mountain are lithobionts, oxalate-rich crusts and silica glazes, there are patches of the iron phosphate skins.

Table 11.3. Focused-beam wavelength dispersive electron microprobe analyses of the phosphate skin in Figure 11.4. Low probe totals are from porosity, bid means below limit of detection. Sodium was measured but it was below the limit of detection. MgO

AI203 SiO2

P205

SO3

K20

CaO

TiO2

MnO

Fe20 3 BaO

Total

2.02

10.24 24.20 12.74 0.10

1.65

19.73 0.40

0.12

8.78

0.07

80.05

2.21

10.69 24.47 14.87 0.07

1.60

21.41 0.32

0.00

9.24

0.08

84.96

1.82

8.69

1 9 . 7 9 14.44 0.00

1.49

22.47 0.28

0.10

7.48

bid

76.58

1.77

7.46

17.29 14.18 0.10

1.45

21.97 0.28

0.10

9.48

0.09

74.17

1.87

8.03

1 8 . 9 8 13.59 0.10

1.54

21.14 0.28

0.06

8.44

0.09

74.12

2.14

9.07

2 1 . 5 2 11.00 0.05

1.55

19.11 0.28

0.09

8.74

bid

73.55

2.11

9.90

2 4 . 0 5 10.68 0.17

1.60

17.83 0.35

0.00

12.87 0 . 0 6

2.04

10.18 24.32 11.39 0.12

1.63

18.09 0.42

0.21

13.48

0.07

81.95

2.04

9.84

2 4 . 1 1 11.73 0.12

1.55

19.28 0.45

0.19

11.84 0 . 1 2

81.27

1.96

8.01

19.47

1.52

19.31 0.33

bid

11.17

74.26

12.35 0.12

Figure 11.5. Stone Mountain, Georgia with dark and light streaks of rock coatings. One type of rock coating found at Stone Mountain consists of iron phosphate.

bid

79.62

Figure 11.6. Backscattered electron microscope image of an ironphosphate skin on Stone Mountain Georgia. The bright fragments in the rock coating were analyzed by energy dispersive x-ray analysis, with peaks of iron and phosphorus. The other particles in the matrix are mostly plagioclase grains, likely derived from erosion of the host rock and subsequent transportation to the site of the rock coating.

Dust films on the Tibetan Plateau contain bacterial-sized particles of iron phosphate (Figure 11.7). Iron phosphate skins may also be found on tropical settings such as I z t a c c ~ u a t l Volcano, Mexico (Figure 11.8 and Figure 11.9). It is possible that the p h o s p h a t e was originally deposited with the aid of microorganisms, since iron phosphate can be indicative of microbial processes (Gall et al., 1994; Konhauser et al., 1994). H o w e v e r , these examples lack microbial fossils, indicating that the iron phosphate has at least been redistributed from an original point of fixation.

Uncommon Rock Coatings

Figure 11.7. Backscauered electron microscope image of dust film on a volcanic lava flow of the Ahisan vent, Akesu Volcanic Field, West Kunlun Mountains, Tibet. Most of the film is dust cemented to the rock surface, but the bright grains are iron phosphate and are bacterial in size.

Figure 11.8. Glacial moraine boulder in Milpuco Valley, Iztacc~uatl Volcano. The dark coating next to the tip of the rock hammer is the iron phosphate skin seen in Figure 11.9.

Figure 11.9. Backscattered electron image of iron-phosphate skin on Milpuco Valley, Iztacc~uatl Volcano, Mexico. The coating rests on a plagioclase grain. The highest concentrations of iron are found near the bottom of the coating, as revealed by energy dispersive X-ray analyses.

253

254

Chapter 11

Phosphate-rich coatings can precipitate as dark skins on the walls and floors of caves. Bat guano interactions with the cave speleothems to form a wide number of phosphate minerals (Hill, 1978). However, apatite (calcium phosphate) is the most common mineral (Hill, 1981). Phosphate seepage through sandstone rock shelters can also produce phosphate skins (Hill, 1978). Phosphate skins accrete on historic buildings, perhaps produced through a combination of ancient stone conservation techniques and natural diagenesis (Lazzarini and Salvadori, 1989). Casein (posphoprotein), egg-white, urine and other compounds may have been applied, and with time these products may have transformed on stone surfaces into calcium oxalates, phosphates and other rock coatings (Kouzeli et al., 1988; Urmeneta et al., 1993). Phosphate skins on the stone monument of the Acritani Venetian pillars, for example, appear to be due to 19th century treatments that involved the use of calcium phosphate as a consolidant. (Fassina, 1995; Fassina et al., 1993). In this example, calcium phosphate (Table 11.4) formed a coating less than 2001am in thickness, with micron-sized tendrils penetrating into fractures.

Table 11.4. Four separate energy dispersive analyses of phosphate skins on the Acritani Pillars, Venice (Fassina et al., 1993). Analyses are elemental weight percent, with nr being not reported. A1

Si

P

S

K

Ca

Fe

2.9

1.1

16.2

3.0

1.1

66.0

4.3

6.4

6.4

14.0

4.3

1.8

61.2

3.0

nr

0.6

20.2

5.1

0.5

72.1

0.2

0.7

2.3

19.3

4.0

0.8

69.8

1.1

In summary, phosphorus is one of the most common elements in the earth's crust. It is readily dissolved and occurs in solutions at the earth's surface. However, the literature on phosphate skins is minimal, and available observations indicate that phosphate skins do not form in very many contemporary contexts, for example in lateritic (Schwab et al., 1989) or more specialized (Zanin et al., 1989) environments. When they do occur, they may be associated with guano, with iron perhaps as an iron phosphate bacterial precipitate, with anthropogenic activity, or in circumstances that are not readily explained.

11.3. Nitrate

Crusts

Nitrates are rarely seen on the surfaces of rocks. They are found as surface coatings only in the very driest locations. For example, nitrate is only preserved as a surface coating in eastern Kentucky only in are dry caves that are protected from precipitation (Mansfield and Boardman, 1932). Subaerial nitrates coatings are only preserved where evapotranspiration greatly exceeds the rate of water seepage.

Uncommon Rock Coatings

Nitrates are mostly found at the surface in hyperarid deserts. The most common nitrate deposit found at the earth's surface is sodium nitrate (NaNO3). Nitrates are present in soils Antarctica (Campbell and Claridge, 1992). Nitrates are concentrated in playa sediments, for example in southern Nevada (Leatham et al., 1983). A variety of organisms are able to concentrate nitrate. Nitrifying bacteria are associated with epilithic organisms on historical monuments (Sand et al., 1991; Sikiotis and Kirkitsos, 1995). Cryptoendolithic organisms are important in the fixation of nitrogen in drylands (Friedmann and Kibler, 1980). In Antarctica, for example, they provide nitrogen to adjacent soils, lakes and rivers (Friedmann et al., 1993). Heterotrophic organisms are important in nitrogen fixation in cryptogamic crusts (Klubek and Skujins, 1979). Cyanobacteria can nitrogen in playas (Leatham et al., 1983). The most extensive surficial nitrate deposits are found in the driest desert on the planet, the Atacama Desert in northern Chile (Ericksen, 1981). Concomitantly, the greatest concentration of nitrate coatings on rock surfaces are also in the Atacama. The term 'caliche' is sometimes used to describe them. Nitrates are only rarely noted as rock coatings elsewhere, despite the nitrogen-fixing ability of organisms. A study of case-hardening of the Bishop Tuff (Figure 6.3) in eastern California showed one example where the principal salt of sodium, calcium and potassium cations to be nitrate, forming NaNO 3, KNO3, and Ca(NO3)2 (Conca, 1985, p. 173). In another example, yellow coatings on the stone monument statues of Prato della VaUe in Italy contain nitrates mixed with oxalates (Fassina and Borsella, 1993). Nitrates have also been noted as efflorescence on wall paintings (Arnold and Zehnder, 1991). Potassium-rich nitrates coat rock surfaces in caves. Saltpeter occurs as nitrate efflorescence associated with sandstone rock shelters (Mansfield and Boardman, 1932). Nitrates may form from ground water seepage, or from soil water (Figure 11.10) (Coy et al., 1984). While some (White, 1976) question the use of cave nitrate as a source of gunpowder, there is good historic evidence for this in the United States (Coy et al., 1984).

Figure 11.10 Deposit of potassium nitrate coating sandstone in a rock shelter in Eastem Kentucky (Mansfield and Boardman, 1932). Note how the saltpeter occurs in rock fractures and as efflorescence in the rock shelter. The photograph is courtesy of Fred E. Coy (written communication, June 4, 1994).

255

256

Chapter 11

Nitrate exists as crusts in the Atacama in different surface contexts. The nitrate can be found as duricrusts covering all different parts of the landscape, mixed with alluvium and coUuvium, and as exposed veins and dikes. "Bedrock caliche," as one type of nitrate deposits is called in Chile, can isolate rock fragments. These nitrate deposits can become rock coatings when the coated rock fragments are exposed (Ericksen, 1981; Ericksen, 1983). The Chilean nitrate beds have remained an enigma with some ten alternative hypothesis to explain their occurrence, including the leaching of seabird guano, the decay of ancient vegetation and volcanism in the Andes Mountains (Ericksen, 1983). The nitrogen and oxygen isotope values of the nitrates in northern Chile are only consistent with one of these hypotheses: the long-term accumulation of atmospheric nitrates without soil leaching and without biological recycling (BOhlke et al., 1997). Ongoing atmospheric deposition without removal could only take place in a climate that has been arid for millions of years (BOhlke et al., 1997; Ericksen, 1981).

11.4. Salt Crusts Sodium chloride is a common salt. Because it is soluble, however, it is usually a temporary rock coating. Halite can precipitate at the surface of a rock as powdery efflorescence (Eswaran et al., 1980). Efflorescence is different from evaporite crusts that result from the evaporation of water (Figure 11.11). Efflorescences crystallize in places where saline soil moisture reaches the surface by capillary action or where ground water seeps slowly enough for evaporation to concentrate salts.

Figure 11.11. The evaporation of saline irrigation water results in the precipitation of salt cmsts on building walls, shown here for a red brick wall in Tempe, Arizona.

According to a model of salt-crust efflorescence proposed for porous rock surfaces (Smith, 1994), soluble sodium chloride is drawn to the surface when the rate of evaporation is less than the rate at which salt-bearing solutions can migrate out from the rock (Figure 11.12). Examples of salt crust from efflorescence can be readily found in drylands around the world (Figure 11.13).

Uncommon Rock Coatings

Figure 11.12. A model for the precipitation of soluble sodium chloride, modified from Smith (1994). Salt crests precipitate as efflorescence when the rate of evaporation is less than the rate of outward migration of the salt solution.

Figure 11.13. A pitted boulder of limestone on a lower shoreline of the precursor to the Dead Sea, Lake Lisan Israel. Precipitation of sodium chloride (bright material) shows up as white patches in this photograph. The glasses in the upper fight provide scale.

If the evaporation rate is too high, salt does not precipitate as a coating, but remains within pores in the rock. The sodium chloride precipitated within the subsurface can then reach the surface through scaling of the surface of the rock (Dragovich, 1969). Salts may also precipitate within pre-existing fractures, and when these fractures spaU the salt is then exposed as a surface coating (Figure 11.14).

Figure 11.14. The sandstone near Punta Caballos, south coast of Peru, is mechanically weathered by the precipitation of halite in fractures. Erosion expose the fractures and salt crests are seen at the surface, as shown in this photograph by the white crest, to the right of the pick of the rock hammer.

257

258

Chapter 11

Halite efflorescence can be a dominant rock coating on historic buildings that are plagued by proximity to saline water tables (Baranski, 1993). Halite can also be found as minor components of gypsum crusts on building stones. In Belfast, for example, halite crystals may derive from marine aerosols (Smith et al., 1994). Wall paintings containing halite crystals may aid in the precipitation of gypsum through the storage of hygroscopic water (Zehnder, 1993). Salinization of soils can result in the precipitation of sodium chloride on rocks that are exposed in agricultural fields. The development of natric horizons from shallow ground waters is important in the western U.S. and elsewhere (Nettleton and Peterson, 1983). For example, Figure 11.15 illustrates the concentration of salts in the central Valley of California.

Figure 11.15. Sodium chloride precipitation at the surface of an agricultural field in the Central Valley of California, near Bakersfield. The linear features are plant stems encrusted with salt. The image is about a meter across.

Figure 11.16. Precipitation of sodium chloride and gypsum crust near the Dead Sea, Israel. The upper photograph shows the overall ground view, with a close-up in the bottom where footprints provide scale.

Uncommon Rock Coatings

Salt playas are perhaps the most common locations where sodium chloride can be seen at the earth's surface (Camur and Mutlu, 1996; Goudie and Cooke, 1984; Neal, 1969). Sodium chloride precipitates as relatively smooth surfaces (Figure 11.16) or as complicated and beautiful topographic forms (Figure 11.17). While salt playas are not rock coatings, rocks are transported to the margins of playas. Salt crusts are commonly found on these rocks as evaporites and efflorescence; yet, these salt crusts are ephemeral because the host rocks ale quickly destroyed by salt weathering (Goudie and Day, 1980).

Figure 11.17. Halite and clays form a topography called the 'Devil's Golf Course' in Death Valley, Califomia.

The coastal deserts of Chile and Peru are extremely arid, with very low ratios of precipitation to evapotranspiration (McGinnies et al., 1968; Oberlander, 1979). The driest regions of the Atacama Desert display salt crusts in a number of different geomorphic contexts. Salt accumulates in fractures in rocks, and can then be exposed at the surface with spaUing (Figure 11.14). Halite mixed with gypsum can blanket both steep (Figure 11.18) and low slopes (Figure 11.19 and Figure 11.20).

Figure 11.18. Clasts of quartzite mantle the surface of a marine terrace riser, near Punta Caballos, Peru. Sodium chloride is found immediately underneath and is sometimes seen coating the cobbles. The excavation is about 50 cm into the 24 ~ slope.

Figure 11.19. Halite and gypsum cover quartz cobbles and eolian sands on the lowest marine terrace, near Punta Cabanos on the south Coast of Peru. The thickness of this salt crust is about 0.8 m.

259

260

Chapter 11

Figure 11.20. Surface of the lowest marine terrace (close to image 11.19). Wetting and drying of halite cmsts can produce irregular, polygonal gilgai-like topography where quartzite cobbles, coated with halite crusts, are thrust to the surface.

The development of halite karst (Frumkin, 1994; Lowry, 1967), although unusual, exposes salt crusts in closed depressions where water may have accumulated in rare precipitation events (Figure 11.21). The salt-crusted landscapes of southern Peru and Chile are characterized by only occasional mass wasting (Figure 11.22).

Figure 11.21. Halite doline dissolved into salt crest deposited on fluvial river terrace in southem Peru.

Figure 11.22. Hillslope near Punta Caballos, southem Peru, where the lack of fluvial or mass wasting activity characterizes the steeper slopes. A rock-faU track can be seen blurred by the movement of salt crests on the surface.

Uncommon Rock Coatings

The origin of the salt crusts of the Atacama is controversial. One hypothesis is that the region was wet enough in the Pleistocene to support lake basins and megafauna. The salt then accumulated in the hyperarid Holocene (Ochsenius, 1982). The alternative is that the halite and other soluble salts have accumulated in aridity that has reigned since the middle Miocene (Alpers and Brimhall, 1988; Alpers and Brimhall, 1989; Ericksen, 1981). The source of the salt may derive from sea spray, from cyanobacterial enrichment, or leaching from loess (Figure 11.23) (Ericksen, 1981).

Figure 11.23. Roadcut approximately 8 km east of the town of Nasca, Peru. The lighter colored material is loess, with halite and gypsum material deposited in the gravels beneath.

Salt crusts occur on rocks in other deserts where evaporite deposits are exposed. For example, clasts found in the pediment surrounding the Mt. Sodom diaper (Figure 11.24) are sometimes encrusted with salt.

Figure 11.24. Pediment of Mt. Sodom diapir, near Dead Seal, Israel.

It is possible that salt crusts may also be found on clasts on the piedmonts of mushroom-shaped diapirs in central Iran (Jackon, 1991) and the margins of exposed diapirs in the Tihama Plain, Yemen (Alsop et al., 1995). The residence time of salt crusts in these settings, however, is probably very limited.

261

262

Chapter 11

11.5. Sulfate Crusts Sulfates may be combined with a variety of cations (e.g., MgSO4, Na2SO4, CaSO4, K2SO4) in rock coatings. The most common sulfate crust, however, is gypsum - - a calcium sulfate. The first part of this section explores sulfate crusts in natural settings. Then, I turn to sulfate crusts in anthropogenic settings. The literature on gypsum deposits at the earth's surface is extensive (Chen, 1997; Dixon, 1994; Jacobson et al., 1988; Noller, 1990; Watson, 1985; Watson, 1988; Watson, 1989; Watson, 1992; White, 1976). Gypsum is concentrated in very dry regions by five general processes: ground water evaporation close to playas; erosion of material overlying subsurface gypcrete deposits; burial of gypsum evaporated in dry lake beds; and as speleothems in caves. While gypsum is common in one or more of these forms in most desert regions, sulfate crusts on rock surfaces are relatively unusual because of its high solubility (Arslan and Dutt, 1993). Indurated 'gypcrete' or 'gypsum crusts' are largely subsurface phenomenon (Watson, 1983; Watson, 1988; Watson, 1989). When they are exposed at the surface by erosion, they often exhibit solution forms (Watson, 1983) and even microbial solution fronts (Drake et al., 1993). Gypsum crusts are considered rock coatings where a crust has been exposed by erosion of the overlying soil material. In these circumstances, the gypsum crusts can encapsulate rock fragments (Watson, 1988). Sulfate crusts may also form on rocks transported into the zone of sulfate evaporites in playas (Cooke et al., 1993, p. 37), and as efflorescence around rocks in hydromorphic soils where ground water brings calcium sulfate to the surface. Understanding the surface distribution of gypsum is best approached through a combination of remote sensing and field observations (White, 1993b). A cycle of transport and precipitation of gypsum in evaporite crusts and gypsum-rich sand dunes is exemplified for Tunisia (Figure 11.25). Rock coatings of gypsum come into play in the cycle of gypsum mostly in the rocky piedmont zone. Some precipitation may also to ground water seepage. Where rocks as not exposed to seepage, however, "the fans themselves have no gypsum except for the small amounts of eroded material found in the active channels (White, 1993b, p. 320)." This is because of gypsum is highly soluble and is readily dissolved from rock surfaces.

263

Uncommon Rock Coatings

Rock Coating & Pedogenic Gypsum Crust

iL

Precipitation at/near surface Solu Lion

~mt

-~

Gypsiferous Source Rocks

Gypsiferous Sand Dunes De~tion Evaporite Gypsum Crust

Solution & Fluvial or Groundwater Transport

Precipitation in Playa

Figure 11.25. Model of the gypsum cycle in south-central Tunisia, modified from White (1993b).

Exposed surfaces in the Namib and other deserts can be encrusted with gypsum (Goudie, 1972). In experiments on the precipitation of sodium and magnesium sulfate in porous rocks (Smith, 1994), crusts form on surfaces in the early phases of drying (Figure 11.26).

Figure 11.26 A model for the precipitation of sulfates both within the host rock and at the surface, adapted from Smith (1994). In Smith's model a surface coating can occur from outward migration of sulfate-rich solutions in the early stages of drying.

264

Chapter 11

There are other processes that may work in tandem with evaporation to produce sulfate crusts. The sulfur-oxidizing bacterium Thiobacillus thiooxydans is associated with transformation of calcite into gypsum and can form gypsum crusts (Krumbein and Pochon, 1964). Gypsum precipitation can be caused by plants (Lowenstam, 1981). Gypsum crusts are be found interbedded with oxalates (Russ et al., 1996; Watchman, 1990b) and as "intrusions in the crust" (Russ et al., 1996, p. 31). Whitish gypsum crusts are found on bedrock and glacial erratic surfaces in the Rondane Mountains of Antarctica, precipitated through sublimation rather than evaporation (Hayashi, 1989). Yet even with all these observations, sulfate crusts are not common on subaerial rock surfaces. In the arid Catavina area of Baja California, for example, gypsum is exceedingly rare (Conca and Rossman, 1985). Sulfate crusts are not found on the surfaces of alluvial fans in Death Valley, nor have they been reported on exposed rock surfaces in the western United States. In contrast to the paucity of gypsum crusts on natural rock faces, gypsum crusts are common on urban limestones, even in humid regions (Camuffo et al., 1983; Rodriguez-Navarro et al., 1997). For example, gypsum forms crusts on top of Pennsylvania marble tombstones in the humid continental climate of the northeastern United States (Feddema and Meierding, 1987). In another example, gypsum crusts are surface layers on tesserae of Greek mosaics at Paphos, Cyprus (Doehne, 1994). The combination of protection from raindrop impacts, calcium-rich host rocks, humaninduced sulfate pollution and natural processes combine to produce sulfate crusts, that are typically 0.2 mm in thickness (Figure 11.27) on marble and limestone (Verg~sBelmin et al., 1993) (Figure 11.28)

Figure 11.27. Generalized diagram of gypsum crust formed on urban limestone in Italy, where chemical dissolution of the limestone occurs along with sulfate deposition modified from Camuffo et al. (1983, p. 353).

Figure 11.28. Map of the interaction of gypsum crust formed on limestone of the Cath6drale d'Amiens. Limestone dissolution occurs underneath the gypsum crust, as gypsum replaces some of the limestone, modified from Verg~s-Belmin et al. (1993, p. 537).

Uncommon Rock Coatings

Gypsum crusts can accrete on building stones other than limestone and marble. Alsatian sandstone monuments have externally deposited gypsum crusts (Jeannette, 1981). Whether gypsum forms on granitic building stone has to do with the supply of calcium ions from the host rock, the supply of sulfate ions from pollution, whether the rock face is exposed to rainwater, and the presence of organisms. Calcium can derive from mortar, and microbial activity is sometimes associated with gypsum precipitation (Schiavon, 1993). The growth of black gypsum crusts on the sandstone of Belfast is often constrained to where calcium is abundant enough to combine with sulfate and to places where surfaces are protected from rain drops. Mortar or limestone facies feed gypsum crusts immediately beneath calcium sources. Gypsum crusts then grow over quartz sand grains and incorporates external particles such as fly ash particles (Smith et al., 1994). Gypsum crystallization can take place around carbon nuclei, such as fly ash or charcoal (Figure 11.27). While the role of carbon in gypsum nucleation has been limited to urban environments that experience airborne pollution (Del Monte and Sabbioni, 1984; Del Monte et al., 1984; Del Monte and Vittori, 1985), this same process occurs in natural settings; sulfate crusts can precipitate around weathering-find vitrinite particles (Figure 11.29).

Figure 11.29. Backscattered and secondary electron microscope images of sulfate crust, manganiferous varnish and silica glaze formed on top of a rock surface at the Karolta petroglyph site, South Australia. The host weathering rind contains vltrinite, a by-product of organic matter diagenesis (Dora, 1996). Calcium sulfate and magnesium sulfate has precipitated in and around the vitrinite and aided in the breakup of the silicified dolomite. Then, manganiferous varnish and silica glaze has been deposited on top of the weathering rind.

265

266

Chapter 11

In addition to forming well-cemented crusts, gypsum may also accrete as powdery efflorescence with the appearance of white spots or as a continuous coatings. Gypsum efflorescence has been noted, for example at Petra (T. Paradise, personal communication, 1994), and on building walls and paints in Europe (Zehnder, 1993). Magnesium sulfate can also form crusts from efflorescence (Evans, 1970). The general thought is that sulfate efflorescence results from outward moving capillary water that is then precipitated by evaporation (Zelmder, 1993).

11.6.

Why these Rock Coatings have a Limited Distribution

In each of these chapters, I am building towards the development of a general model to explain the character and geography of rock coatings. Despite the widespread abundance of phosphates, nitrates, chlorides and sulfates in the terrestrial weathering environment, these are not common rock coatings. The obvious problem that derives from this chapter is a geographic one, exlaining why these rock coatings are so rare on subaerial rock faces. The simplest answer invokes a lack of geochemical barriers to the migration of the raw ingredients. From a landscape geochemistry perspective, rock coatings indicate the presence of a barrier to the further movement of those constituents. The coatings in this chapter are usually too soluble (Zen, 1965). Even where potential evapotranspiration greatly exceeds precipitation, there is usually enough rainfall to dissolve these salts and move them into the soil. Consider the following perspectives. "Phosphate rocks do not build up, they build down. They result from an accumulation of carbonate grains being exposed and then capped by a crust of phosphatic material, which when dissolved drains down and reacts with it." (Stoddart and Scoffin, 1983, p. 397) "Thin, saline encrustations and efflorescences are usually dissolved by even small amounts of rainfall..." (Watson, 1989, p. 34) "Chlorides, sulfates and nitrates are washed from the subsurface horizons of all except the soils of very arid areas." (Cooke et al., 1993, p. 53) "Owing to the susceptibility of gypsum to solution, crests are rarely preserved when the climate becomes wetter. In southem Tunisia surface crusts are undoing solution at present in areas where mean annual rainfall is about 150mm." (Watson, 1983, p.

155)

Subaerial rock surfaces can be temporary sinks for phosphates, nitrates, chlorides and sulfates. Yet because these coatings are relatively soluble, they do not remain on the surface for very long. Solubility is certainly not the total answer, however. Consider the case of phosphate skins. Once formed, calcium phosphate (apatite) is relatively insoluble (Stumm and Morgan, 1981; Zanin, 1989). While mobile in near neutral to slightly acidic settings, phosphates have had a tendency over geological time to precipitate in alkaline and extremely wet and acidic contexts (Zanin, 1989). Furthermore, there are both biotic and abiotic transformations of ph.osphorus into forms that should be stable in a terrestrial setting (Cook, 1984; de Graaf et al., 1995; Jehl and Rougerie, 1996; Lowenstam, 1981). For example, fungi that generate oxalates also excrete phosphates (Gehrmann

Uncommon Rock Coatings

and Krumbein, 1994), and bacteria form iron phosphate skins in a harsh Arctic environment with abundant water for dissolution (Konhauser et al., 1994). Thus, considerably more research is needed to answer the geographical conundrum of why these coatings are so rare, despite the abundance of raw ingredients in the terrestrial weathering environment.

267

268

Chapter 12 OXALATE-RICH

CRUSTS

The environment made its first great contribution during the evolution of the species, but it exerts a different kind of effect during the lifetime of the individual, and the combination of the two effects is the behavior we observe at any given time. Any available information about either contribution helps in the prediction and control of human behavior and in its interpretation in daily life. To the extent that either can be changed, behavior can be changed. (Skinner, 1974, p. 17)

12.1. Introduction Oxalate minerals are composed of carbon and oxygen, along with a divalent cation (e.g. Mg 2+, Ca 2+, Mn 2+) and water. For example, the most common oxalate mineral is whewellite (CaC204.H20). Oxalate minerals are traditionally associated with fractures in rocks where there is abundance of organic carbon. The explanation is that crystallization occurs during coal diagenesis, or in low-temperature hydrothermal systems (Z~k and Sk,qla, 1993). The conditions that promote the formation of oxalate minerals, however occur in other places. Oxalate minerals are found in soils (Cromack et al., 1979; Graustein et al., 1977; Lapeyrie, 1988), within lichens (Gehrmann and Krumbein, 1994; Purvis, 1984; Syers et al., 1967; Traquair, 1986; Wadsten and Moberg, 1985), and they are exuded by higher plants (Frey-Wyssling, 1981; Lowenstam, 1981). This chapter focuses on crusts rich in oxalate minerals that accrete on rock surfaces (Figure 12.1). Oxalate-rich crusts vary considerably in appearance. They may range from whitish, to yellow, orange, reddish-brown, brown, or black. They may be shiny or dull, with smooth to botryoidal textures. Oxalate-rich crusts reported in the literature are typically less than 1 millimeter thick, although accretions up to a centimeter have been measured in Kakadu National Park, Australia (Watchman, 1987b). Most of the information available on oxalate-rich crusts comes from the literature on rock art (Russ et al., 1996; Watchman, 1990b) and stone conservation (Del Monte et al., 1987b). Additional insights have also been obtained from the cognate fields of botany and mineralogy. The organization of this chapter is to present first characteristics of oxalate-rich crusts. Then, I review different hypotheses to explain its origin. The material presented in this study is based on a few anecdotal case studies, rather than a systematic sampling in temporal or environmental transects. Thus, the study of oxalate-rich crusts is in its very early stages.

Oxalate-Ric h Crusts

Figure 12.1. Optical thin section of calcium oxalate formed on welded tuff, Superstition Mountains, Arizona. The layers are yellow and brown, similar to oxalates observed in Australia (Watchman, 1990b).

12.2.

Characteristics

12.2.1. Environmental Settings In Europe, oxalate-rich crusts have been mostly reported on stone monuments, buildings and statues composed of carbonate (Del Monte et al., 1987a; Del Monte et al., 1987b; Edwards et al., 1991; Fassina et al., 1993; Parrini, 1993). Calcium oxalate crusts on the Island of Thasos, Greece, appear to retard the dissolution of carbonate (Doehne, 1994). A few studies describe oxalate on silicate rocks like sandstones (Urmeneta et al., 1993; Winkler, 1975). In Australia, oxalate has been found on quartzite, sandstone, granodiorite and limestone. Oxalate-rich crusts have been studied in tropical, temperate, and desert environments of the Northern Territory, Queensland, and New South Wales in Australia (Watchman, 1990a; Watchman, 1990b). Oxalate-rich crusts have also been noted in North America in association with rock art. These crusts occur on silicate and carbonate rock surfaces (Kaluarachchi et al., 1995; Russ et al., 1994; Russ et al., 1996; Russ et al., 1995; Scott and Hyder, 1993). Oxalates have been found in Antarctica. The endolithic microbial community (Friedmann, 1980; Friedmann, 1982) produces calcium oxalates that mimic the morphology of fungal hyphae (Weed and Norton, 1991). It is difficult to make conclusive statements about the occurrence of oxalate-rich crusts, based on this limited literature. The only obvious generalization is that oxalaterich crusts are found in a wide variety of environments including warm arid, temperate humid, and Antarctica (Cromack et al., 1979; Graustein et al., 1977; Haberland, 1975; Watchman, 1990b; Wilson et al., 1980). Based on my own observations, I believe that the story of the geography of oxalaterich crusts is far from complete. For example, many oxalate-rich coatings have been misinterpreted as rock varnishes, especially where water flows slowly over lichens and other lithobionts.

269

270

Chapter 12

12.2.2. Composition The mineralogy of oxalates is determined through Fourier transform infrared spectroscopy (Figure 12.2), X-ray diffractometry, and Raman spectroscopy (Russ et al., 1995; Watchman, 1990b). Oxalates have also been studied in thin sections with light microscopy (Watchman, 1990b) and scanning electron microscopy (Russ et al., 1996).

BlackPaint II ,

Red Paint .LU 0 Z

0

Ca-Oxalate S1andard

J Jl ~

40OO

~T"t~r-~'-,'-.,~.

~000

:--~,-,-,,-,e.u~

211OO

~,~...e~ot --.,

t000

Wavenumbers Figure 12.2. Fourier transform infrared spectroscopy spectra of different types of rock art pigment from West Texas, as compared with a calcium oxalate standard modified from Russ et al. (1995, p. 56). Oxalate is the principal material within these paints.

The most common natural oxalate mineral contains calcium; the monohydrate form is called whewellite (CaC204.H20). Wedellite (CaC204.2H20) may also occur in oxalate-rich crusts and associated lichens (Gehrmann and Krumbein, 1994). The Mgoxalate, glushinskite (MgC204.2H20), is found in nature (Wilson et al., 1980). Glushinskite has been observed as a white coating on serpentinite where the oxalate is found underneath and intermingling with a lichens that released magnesium from the underlying serpentinite rock (Lewin and Charola, 1981). In the context of rock art in Australia, oxalates frequently interdigitate with a variety of materials, including gypsum, anthropogenic pigments, charcoal from burned wood,

271

Oxalate-Rich Crusts

halite, clay minerals, and silica glaze (Watchman, 1990b). In West Texas, gypsum and clays have been seen interdigitated with the wheweUite (Russ et al., 1996). The chemistry of oxalate-rich crusts has been analyzed by X-ray fluorescence (Watchman, 1990b). In these bulk measurements, calcium (from calcite, oxalate), silica (from silica glaze, quartz), and sulfur (from gypsum) are the major elements ranging from 8% to 30%. However, several different materials (e.g. oxalate, gypsum, charcoal layers) were analyzed together. Thus, it is difficult to determine the elemental composition of the oxalate itself with a bulk method like X-ray fluorescence. Micron-scale X-ray analyses of an oxalate-rich crust in Spain reveals a more spatially accurate picture (Table 12.1). Aside from calcium (cation in the oxalate), the elements found in clay minerals (A1 and Si) are important components. Iron, chlorine, sulfur, potassium, magnesium, and phosphorus also contribute minor elements.

Table 12. 1. Analyses of oxalate-rich crust on stone monuments at Zamora, Spain, as measured by energy dispersive X - r a y analysis (Urmeneta et al., 1993); bld is below the limit of detection. Elemental abundance was normalized to 100%. M~

A1

Si

P

S

K

Ca

Fe

CI

bld bid 2.25

9.74 10.69 13.55

29.49 39.17 46.78

11.25 bid bid

1.52 bid 4.41

2.20 1.86 4.87

41.02 40.95 18.81

4.78 3.91 3.76

bid 3.41 5.57

Table 12.2. presents electron microprobe analyses of a few oxalate-rich crusts in North America (Figure 12.3 and 12.4). The analyses from Whoop-up Canyon start in oxalate and move into manganese-rich heavy metal skins. These transects reveal a similar chemistry for the calcium oxalate that reasonably pure with only minimal contribution from S i, AI, Mg or P.

Figure 12.3. Backscattered and secondary electron images of an oxalate-rich crust formed on top of a petroglyph from Whoop-up Canyon, Black Hills, Wyoming. The map on the fight identifies the smaU areas of manganiferous rock varnish that can be found within the oxalate. There is also a considerable amount of fibrous organic matter ('Y' pattern on map) trapped under the oxalate. The scale bar is 100 ~tm. The location of the electron microprobe transect (Table 12.2) is shown by the dark line.

272

Chapter 12

Figure 12.4. Backscattered electron image of an oxalate-rich crust on granite, Westmore, Vermont (collected by T. Wasklewicz). The oxalate is resting on top of quartz. The line indicates the location of the electron microprobe transect (Table 12.2.)

The chemical analyses do change, however, where the transects pass into localized areas of manganese heavy metal film. Note in the transects of Whoop-up 23 and 30 (Table 12.2) a layer that is enriched in Si and AI. Manganese concentrations increase underneath the Si and AI. The manganese shows up as bright spots in the backscatter image from Whoop-up Canyon (Figure 12.3). The chemical analyses of the black oxalate coating from Vermont (Table 12.2) reveal low probe totals; this is from high porosity that can be seen visually in the backscattered image (Figure 12.4). Although aluminum can sometimes reach a few percent, the Vermont oxalate is relatively free of cations other than calcium.

12.2.3. Rates of Formation Oxalate is made up of carbon. Therefore, it can be radiocarbon dated directly (Russ et al., 1996; Watchman, 1991) and average rates of accretion can be deduced, if there is a closed system with respect to the carbon. At present, there is the assumption that the carbon in the oxalate is the same age as the oxalate. The possibility of systematic or non-systematic bias through the incorporation of older carbon has not yet been evaluated. However, the preliminary work on radiocarbon dating oxalate-rich crusts is very promising (Kaluarachchi et al., 1995; Russ et al., 1996; Russ et al., 1995; Watchman, 1991; Watchman, 1993; Watchman, 1987b). An exciting implication of available radiocarbon ages reveals a likely correlation between oxalate accretion and climate. Russ et al. (1996) argue that the presence of oxalate crusts in the Lower Pecos of West Texas could indicate a period of aridity. Because the local whewellite-producing lichens prefer xeric conditions, "whewellite was likely produced primarily during dry periods, specifically when the substrate was dry (Russ et al., 1996, p. 33)."

Oxalate-Rich Crusts

273

Table 12.2. Electron microprobe transects of calcium-oxalate crusts in North America. The analyses from W h o o p - u p Canyon, Black Hills, Wyoming are from two petroglyphs carved into sandstone (Figure 12.3). The third transect was completed on oxalates on Westmore granite, in Vermont (Figure 12.4). Low probe totals are from water, carbon, and porosity; 'na' means not analyzed.

Na20 MgO AI20 3 SiO2 P205

SO 3

K20' CaO

TiO 2

MnO Fe20 3

BaO

Total ||

i

Whoop-up Canyon 23 0.04

0.90

0.36

0.51

0.11

0.00

0.04 52.60 0.00

0.17

0.11

0.00

54.84

0.00

0.56

0.21

0.34

0.21

0.00

0.00

54.46 0.00

0.03

0.00

0.00

55.81

0.00

0.71

0.93

1 . 9 5 0.09

0.10

0.10 52.01 0.00

0.04

0.24

0.00

56.17

0.00

0.58

0.28

0.17

0.05

0.02 53.69 0.00

0.15

0.03

0.00

55.11

0.03

0.55

0.57

1.75 0.11

0.07

0.08 53.49 0.00

0.26

0.04

0.00

56.95

0.13 0.07

0.99

4.82

11.87 0 . 1 1

0.05

0.78 41.75 0 . 0 5

0.31

1.07

0.00

61.93

2.62

4.76

18.68 0.18

2.75

0.97

0.58

61.36

0.09

1.84

1 . 1 1 4.77

0.20

8.37

65.44

0.14

0.00

0.58

0.14

0.00

0.37

30.05 0 . 1 2

1 . 2 5 0 . 0 8 47.22*

Whoop-up Canyon 30 0.07

1.86

1.72

4.02

0.05

0.10

0.34

45.78 0 . 0 5

0.50

1.09

0.06

55.64

0.16

2.67

3.40

8 . 4 7 0.11

0.10

0.64 44.09 0 . 1 2

0.21

1.49

0.00

61.46

0.12

3.43

4.78

12.04 0.00

0.10

0.96

40.65 0 . 1 5

0.12

1.97

0.00

64.32

0.15

3.48

9.43

0.07

0.81

42.77 0 . 3 3

0.18

1.76

0.00

63.25

0.64 29.05 0 . 0 8 11.96 1.83

0.49

56.34

0.45

1.90

2.01

65.48 71.03

0.20

3.37

0.24

3.61

4.18 2.55 2.08

6.01 3.49

0.09 0.09 0.00

0.07

14.51 0.00 37.02*

0.47

4.58

1.51

2.72

0.09

0.17 0.20

0.24

3.50

0 . 0 8 49.71"

3.40

4.53

0.44

3.68

1.36

2.44

0.14

0.15

0.25

3 . 6 2 0 . 0 8 49.33"

3.70

4.49

69.68

0.03

0.00

0.00

na

na

0.06

27.44 0.00

0.10

0.00

27.67 55.33 51.59

Vermont 0.04

0.00

0.00

0.10

0.08

0.19

na

na

0.12 54.69 0.00

0.00

0.14

0.00

0.04

0.07

0.25

0.24

na

na

0.11

50.75 0.00

0.00

0.13

0.00

0.12

0.08

1.68

0.19

na

na

0.10

42.80 0.00

0.00

0.10

0.00

45.07

0.03

0.08

1.15

0.06

na

na

0.10

48.45 0.00

0.00

0.10

0.00

50.00

27.91 0 . 0 5

0.00

0.00

2.10

0.24

na

na

0.05

0.00

0.09

0.00

30.46

0.03

0.07

0.19

0.13

na

na

0.07 49.32 0.00

0.00

0.09

0.06

50.02

0.04

0.12

0.11

0.36

na

na

0.11

0.00

0.00

0.28

0.00

43.59

0.04 0.00

0.08 0.05

0.06

0.15

na

na

0.08 52.12 0.00

0.00

0.18

0.00

52.72

0.09

0.19

na

na

0.13

0.00

0.10

0.00

52.97

42.56

52.41 0.00

,, | l

* The high values of manganese are from oxalate intedayering with a manganese heavy-metal skin

T h e p a u c i t y o f d a t a on local a n d r e g i o n a l variability in rates o f a c c r e t i o n , h o w e v e r , p r e v e n t s w i d e s p r e a d generalizations. Accretion m a y be locally periodic, f o r m i n g d u r i n g c o n d i t i o n s o f w a t e r flow. T h e r e m a y also be p e r i o d s o f e r o s i o n that m a y v a r y from p l a c e to p l a c e on a r o c k surface.

274

Chapter 12

There are different morphologies of accretion, whose rates of deposition are not established. In the literature, surface morphologies can range from fairly smooth to botryoidal (Russ et al., 1994; Watchman, 1990b). Figure 12.5 displays a botryoidal form for an oxalate-rich crust formed in a rock fracture in a basalt flow.

Figure 12.5. Secondary electron image of an oxalate-rich crust in a fracture in basalt in the Cima volcanic field, southern California. The fracture received water that had flowed over lichens. X-ray analyses reveal an abundance of calcium with little manganese, but carbon is found in abundance in wavelength dispersive analyses.

A case study from Nepal illustrates an extreme example of regional variability in rates of oxalate formation. Figure 12.6 displays a lower layer that is laminated and an upper layer that is more massive and porous. This millimeter-scale oxalate accretion formed in 10 years in an alpine environment in Nepal; this requires a rate of accretion of 0.2 mm per year. Rates of accretion in semi-arid environments, in contrast, appear to be several orders of magnitude slower than this Nepal sample (Watchman, 1990b; Watchman, 1991).

Figure 12.6. Backscattered electron image of an oxalate-rich crust formed on gneiss, in the village of Namche Bazar, Nepal. The gneiss surface had been faced in construction of a wall ten years prior to collection.

Oxalate-Rich Crusts

Oxalate-rich crusts also show potential for holding foreign materials. An example is volcanic ash trapped in an oxalate-rich crust. On glacial boulders of Grant Lake in the Mono Basin, an oxalate-rich crust has incorporated shards of what may be volcanic ash erupted from the nearby Mono rhyolite domes (Figure 12.7).

Figure 12.7. Backscattered electron image of different types of rock coatings formed on ventifacts in the Mono Basin of eastern California. A calcium oxalate-rich crust is on top of a quartz crystal. Inside the lighter-colored oxalate are shards of what appear to be tephra, with a chemistry similar to ash from the nearby Mono rhyolite domes. Another possibility is that shards are a type of silica glaze.

Volcanic ash is possible to date with crystal 4 ~ m e a s u r e m e n t s . There may also be other types of foreign materials incorporated into oxalate-rich crusts that may yield environmental or cultural insights.

12.3. Origin There are three components to the oxalate minerals in oxalate-rich crusts: the cation; the oxalate; and water. The water must derive from precipitation or ground water. The cation derives from the adjacent environment, either from the host rock, eolian dust or in solution. The key to the origin of oxalate-rich crusts probably rests with the origin of the oxalate itself, how it mobilizes and reprecipitates Organic agents are very important in the weathering of minerals (Viles, 1995). Oxalates are no exception; they are recognized agents of mineral decay (Mast and Drever, 1987). Oxalates released by lichens, for example, are important agents in the pitting of minerals (Gehrmann et al., 1988; Gehrmann and Krumbein, 1994). The particular cation released by the oxalate weathering influences the oxalate mineral that crystallizes (Wilson and Bayliss, 1987). For example, the calcium in whewellite likely derives from the calcium within limestone and marble (Verrecchia, 1990), and the magnesium in glushinskite like comes from magnesium-rich minerals in the underlying rock (Wilson et al., 1980). In another example, lichens on copper-bearing rocks produce copper-rich oxalates (Purvis, 1984). The most common theory for the origin of the oxalate invokes the metabolic activity of lichens (Del Monte et al., 1987a; Del Monte et al., 1987b; Dragovich, 1987; Edwards et al., 1992; Russ et al., 1995). There are a number of lines of evidence to support a lichenogenic origin. Whewellite can have botryoidal forms (Figure 12.5), similar to adjacent lichens (Russ et al., 1996). The calcium oxalate minerals of

275

276

Chapter 12

whewellite and weddelite occur on and within (Figure 12.8) a wide variety epilithic and endolithic lichens (Gehrmann and Krumbein, 1994; Wadsten and Moberg, 1985). Furthermore, oxalate crusts often occur adjacent to lichens (Russ et al., 1994; Wilson, 1987; Wilson et al., 1980).

Figure 12.8. Backscattered electron image of crustose lichen growing on a wall that was faced ten years prior to collection. The wall is in Namche Bazar, Nepal. The bright material in the lower part of the image is the gneiss rock. The lichen itself is sitting on the rock, but is only dark gray with dots in the middle representing pieces of dust. The patterns of bright dots above the host rock surface are calcium oxalate crystals within the lichen. These are imaged well by backscattered electrons (Traquair, 1986).

A biogenic origin for oxalate-rich crusts can also involve higher plants, since these form whewellite (Lowenstam, 1981). For example, ivy releases calcium oxalates from branches, stems and suckers. The oxalate-rich crust then deposits on supporting rock faces (Lewin and Charola, 1981). Abiotic mechanisms have also been proposed for the origin of oxalate-rich crusts. For example, oxalate may have been purposefully applied as a conservation treatment for monuments and buildings. In other words, some oxalate may have been produced through the interaction of applied substances and natural processes (Lazzarini and Salvadori, 1989). Another abiotic mechanism that is sometimes casually mentioned is efflorescence, but this abiotic process does not appear likely (Kaushansky et al., 1984). One of the leaders in the analyses of oxalate-rich crusts, Alan Watchman, favors polygenesis, or formation by multiple mechanisms: "It can be categorically stated that there is no single process for the formation of oxalates; oxalate-rich patinae and crusts are formed through a range of mechanisms, on different stable substrates, in different climatic regimes over considerable time spans (Watchman, 1990b, p. 49)."

In addition to oxalic acid generated by nearby lichens, A. Watchman also wrote that there is interaction of organic acids with calcium-rich dust and rain water (Watchman, 1991; Watchman, 1987b). The importance of vegetation burning in Australia and the presence of organic in rainwater led Watchman to this theory. In particular, Watchman argued that the isoprene emitted from plants can be broken down into oxalic acid. Whatever model of formation is ultimately adopted, it must be able to explain the tendency for oxalates to interdigitated with other materials (e.g. charcoal, gypsum, iron

Oxalate-Rich Crusts

skins, heavy metal skin). Consider the interface of iron and oxalates (Fassina et al., 1993). Iron hydroxides can aid in oxalate fixation through adsorption (Parfitt et al., 1977). Concomitantly, oxalates can also promote the formation of hematite (Fischer and Schwertmann, 1975). In these contexts, I suspect that humboldtine (FeC204~ may be found in the future. Dark stripes along lines of water flow are common throughout semi-arid regions. Figure 12.9, for example, illuslrates water flow coatings in the Colorado Plateau of the western United States. Figure 12.10 reveals that these water flow streaks can be from the interfingering of rock varnish and oxalate-rich crusts. One way that these coatings could interfinger in the observed fashion is through the reduction of Mn (IV) oxides with oxalate (Stone, 1987). The importance of this intercalation is that there is an interaction between oxalates and the adjacent environment. Consider that manganese oxalates are known to occur in association with lichens (Jones et al., 1981; Wilson, 1987); thus these may be found in future research on water streaks.

Figure 12.9. Photograph of dark streaks where water flows over a sandstone cliff face, northeastern Arizona. A sample of one dark streak reveals an interdigitation of oxalate-rich crust and manganese heavy-metal skins (Figure 12.10).

Figure 12.10. Backscattered electron image of a cross-section of sample of a dark streak. The dark streak formed were water flows over sandstone (see Figure 12.9). The bright material is manganese-rich rock varnish. The manganese varnish started to form first. Then, both varnish and an oxalate-rich crest grew side-by-side. The oxalate is darker, because it has a lower atomic number.

277

278

Chapter 12

I suspect that future studies on the genesis of oxalate-rich crusts will be able to employ carefully-controlled environmental and chronometric studies of development with the aid of anthropogenically-faced surfaces. The only difficulty may be that oxalates can be introduced to the natural environment through industrial processes. For example, strontium oxalates are used to obtain red colors in fireworks (White, 1996). The possible role of industry in the formation of crystalline carbon-bearing compounds is exemplified by the natural crystallization of phenanthrene (C14H10) crusts. In summary, oxalate-rich crusts are very common in the terrestrial weathering environment. They occur in a wide variety of environments, from warm deserts to Antarctica. They can be dated with radiocarbon and may provide clues to past environments. With these inducements, it is ironic that oxalate-rich crusts are one of the poorest understood of all rock coatings.

279

Chapter 13 SILICA

GLAZE

"There is nothing more difficult to carry out, nor more doubtful of success, nor more dangerous to handle, than to initiate new order of things. For the reformer has enemies in all who profit by the old order, and only lukewarm defenders in all those who would profit by the new order. This luke-warmness arises partly from the fear of their adversaries who have the law in their favour, and partly from the incredulity of mankind, who do not truly believe in anything new until they have actual experience of it." (Machiavelli, 1513)

13.1.

Introduction

Silica is the dioxide of the element silicon. The movement of silica in the atmosphere, biosphere, hydrosphere, lithosphere and pedosphere has a tremendous influence on the earth as a whole (Iler, 1979; Wollast and Mackenzie, 1983). The importance of mobile silica is quickly understood after consideration of the terrestrial weathering environment. Weathering separates silica from the lithosphere and injects it into soil, hydrologic and atmospheric systems. The release of silica from rocks destabilizes landforms, allows net erosion and provides material for the buildup of soil (Birkeland, 1984). The silica that moves from the terrestrial weathering environment to rivers as either solid or dissolved load impacts oceanic biota and chemistry (Fortescue, 1980; Holland, 1978). The process of silicate weathering absorbs carbon dioxide and may be responsible for keeping the earth's temperature stable over the timescale of millions of years (Berner et al., 1983; Brady, 1991; Brady and Zachara, 1996). This is just a partial list of the importance of silica released by terrestrial weathering. This chapter focuses on a part of the global cycle of silica that is rarely treated in models, accretions of mostly amorphous silica on rock surfaces. There is no universally accepted term for rock coatings of mostly amorphous silica. "Desert glaze" (Fisk, 1971) was never adopted in the literature, perhaps because these coatings are not limited to arid regions. "Patina" (Smoot, 1995) is too general a term (Service, 1941). "Silica skin" (Watchman, 1985; Watchman, 1990c) is used in the Australian rock-art literature, but not beyond. "Silica-alumina rock coatings" (Curtiss et al., 1985) describes the accretion, but the term fails to evoke the excitement and beauty of these coatings. "Silica glaze" (Butzer et al., 1979; Smith, 1988) is adopted here because it describes the general chemistry of these rock coatings and also communicates its often lustrous appearance. Silica glazes typify the interdisciplinary literature on rock coatings. Scientists in different disciplines study silica glazes that interface with their particular field, and yet they are frequently unaware of the larger literature on the same phenomenon in other academic arenas. The purpose of this chapter is to bring together literature and data on different silica glazes. I start by reviewing the characteristics of silica glazes. Then, I explore

280

Chapter 13

alternative models to explain its genesis. Lastly, I offer a speculative commentary that silica glaze may be a common rock coating on Mars.

13.2.

Characteristics

13.2.1. Environmental Settings

13.2.1.1. Silica Accumulation in Geologic and Pedogenic Systems Extensive geological deposits of silica accumulate at the earth's surface. The processes that promote the formation of such bodies as cherts, flints, and agates reflect geochemical gradients that draw silicic acid, S i(OH)4, molecules over long distances to accumulate in pore spaces (Landmesser, 1995). Geological processes also produce silica coatings on rocks, for example opaline crusts and siliceous sinter. Opaline crusts can be found, for example, in Hawai'i along steaming fissures where the silica may have been deposited from volcanic gases (MacDonald, 1944). Water at higher temperatures can hold more dissolved silica than water at cooler temperatures. Thus, when alkaline hot springs cool below 100*C, amorphous coatings of silica accrete as banded or bedded opaline sinters in hydrothermal systems (White et al., 1964). In addition to the iron-clay coatings on sand grains described in Chapter 9, amorphous silica can frost dune sands. In a study of dune grains in the Northern Territory of Australia, all dune sand grain sizes developed "turtle-skin" coats of silica (Folk, 1978). The silica typically formed over iron-clay skins. Folk (1978) argued that the most likely source of the silica is opaline phytoliths, although some of the silica could derive from the abrasion and dissolution of quartz grains. The deposition of "turtle-skin" coats of silica on dune sand may be analogous to silica overgrowths seen on grains in geological contexts (McBride, 1989; Overson, 1993). Figure 13.1 presents a close-up view of a quartz sand grain in a Tertiary deposit in Arizona. The quartz is coated by a line of dust particles, as found by Folk (1978). In turn, the dust particles are coated by an overgrowth of silica.

Figure 13.1. Formation of silica overgrowth on a sand grain in a geological context, Arizona, modified from Overson (1983). The transmission electron microscope image has a scale bar of 500 nanometers (0.5~tm). The left side (A) shows the original detrital quartz. The fight side (B) shows the silica overgrowth. Other features are the line of dust (C), void spaces (D), and the edge of the sample (E).

Silica Glaze

Soil science has long been concerned with the accumulation of silica (Nettleton, 1991). In some cases, amorphous silica is transitory (Munk and Southard, 1993), while in others the accumulation can be substantial (Milnes et al., 1991). Figure 13.2 illustrates the accumulation of opaline silica on rock surfaces in a semi-arid soil.

Figure 13.2. Two samples of pedogenic opal as seen in backscattered electron microscopy. The pedogenic opal coats granitic sand in the Sierra Nevada Mountains of Califomia. The opal is mixed with detrital fragments that may be volcanic glass in the upper image. In the lower image, it is more homogeneous.

Pedogenic silica may derive in part from eolian dust (Blank and Fosberg, 1990). In dry areas, silica can precipitate with carbonate in the soil profile, while in other places it is not found with carbonate (Chadwick et al., 1987). S ilcretes are massive accumulations of silica in pedogenic or near-pedogenic environments (Butt, 1985; Milnes and Hutton, 1974; Milnes et al., 1991) that may have paleoclimatic implications (Summerfield, 1983). Pedogenic silica, including silcretes, exemplify a comparatively large 'sink' of silica in a terrestrial weathering environment. Their genetic connection to silica glazes that form in a subaerial environment is unclear, but soil erosion can turn pedogenic silica coatings into surficial silica glazes. Silica glaze in Morocco, for example, can originate in the subsurface: "[A nut-brown coating] is often preserved beneath patches of soil or downwashed sand lying on the pavements, but is generally absent from long exposed portions of the pavement, from which it has presumably been worn away...It is believed to form when rain, percolating through the soil, deposits the iron and other minerals at the soil]rock interface, or when groundwater deposits the same minerals whilst seeping

281

282

Chapter 13

along joints or bedding planes deep in the sandstone (Robinson and Williams, 1992, p. 417)."

Silica glazes seen at the surface may have formed in a pedogenic environment, and may be in a state of erosion.

13.2.1.2. Subaerial Surfaces in Deserts "Desert glaze" was proposed as a term to describe "thin, colorless, transparent, highly lustrous coatings" (Fisk, 1971) that are typically less than 0.1 mm thick. These thin opaline coatings have been noted throughout the Sahara Desert near Pyramids in Egypt (Hobbs, 1917) and in southeastern Libya (Haberland, 1975). It is easily removed by eolian abrasion, and it forms best on siliceous substrates like chert. Fisk (1971) argued that dew dissolves silica that is then precipitated by evaporation. Silica glazes are common in the Australian Deserts. Gibbers are often coated (Figure 13.3) with a thin film of silica (Jessup, 1951; Jessup, 1962) and sometimes with a combination of silica and iron (Bourman and Milnes, 1985; Twidale, 1970). The films on gibbers are typically very smooth and glassy in appearance (Fuller, 1974).

Figure 13.3. Backscattered electron microscope image of silica glaze formed on top of a gibber in a desea pavement in the south MacDonneU Ranges, Australia. The darker appearance is because silica has a lower atomic number than the host rock that contains gray feldspar and bright iron minerals.

The coatings of amorphous silica in Australia are also found on rock faces, for example granitic boulders that contain tafoni weathering (Bradley et al., 1979). Silica glaze also forms on a variety of rock types in Kakadu National Park, north Queensland, and coastal New South Wales (Watchman, 1992). The silica glazes on Australian rock faces can have a variety of appearances, ranging from "transparent to opaque, and white to almost black (Watchman, 1992, p. 61)." Investigators have noted that silica glazes are common in other deserts as well. Early surveys of the western United States noted silica glazes (Stevenson, 1881). Silica

Silica Glaze

glazes form on basalts in eastern Oregon and the Snake River Plain (Farr, 1981) and on gobi in the Gobi Desert (Zhu et al., 1985). I have noted silica glazes in the deserts of the Tibetan Plateau (Figure 13.4), southern Peru (Figure 13.5) and the Sinai Peninsula (Figure 13.6).

Figure 13.4. Backscattered electron images of silica glaze from the Akesu volcanic field, West Kunlun Mountains, Tibetan Plateau. The clasts on and under the silica glaze may be pieces of volcanic ash.

Figure 13. 5. Backscattered electron microscope image of silica glaze (darker thin film) on top of an orthoclase feldspar from a trapezoid geoglyph on Pampa San Jose, Nazca, southem Peru.

13.2.1.3. Sub-glacial and pro-glacial environments

Bedrock exposures in front of a glacier often display films of amorphous silica (Bauer, 1961; Hallet, 1975; Watts, 1985; Whalley et al., 1990). Some of the films may precipitate under the glacier (Bauer, 1961; Hallet, 1975; Peterson and Moresby, 1979). In other cases, there is good evidence that silica deposition post-dates glacial recession (Watts, 1985).

283

284

Chapter 13

Figure 13.6. Backscattered electron image of silica glaze on an alluvial-fan surface at Marsa Muqualba, Sinai Peninsula. There are a large number of micron-size dust particles incorporatedinto this coating.

13.2.1.4. Hawaiian Silica Glaze Coatings of mostly amorphous silica and alumina are ubiquitous in the drylands of Hawai'i in the lee of the larger volcanoes of Maui and Hawai'i, along with the entire island of Kaho'olawe. Hawaiian silica glaze has been studied extensively (Curtiss et al., 1985; Farr and Adams, 1984; Farr, 1981; White and Hochella Jr., 1992), with a focus on relevancy for remote sensing in drylands (Abrams et al., 1991; Adams et al., 1982). Hawiian silica glazes display considerable variability. Most of the coatings are clear to translucent white, but wind-deposited material from the adjacent soils can add micrometer-scale clasts that darken the appearance (Farr and Adams, 1984). These tiny clasts are not evenly distributed. Sometimes, the clasts can be found in layers (Figure 13.7). The thickness of the silica glazes can range from a few microns to almost a millimeter (Figure 13.8).

Figure 13.7. Backscattered electron microscope image of silica glaze on glacial polish on top of Mauna Kea, Hawai'i. The underlying mineral is plagioclase feldspar. The bottom layer of silica glaze is darker because ithas a substantial amount of alumina (~10% Al203). In contrast, the upper layer of silica glaze is brighterbecause it has iron mixed in with the alumina-silica matrix. Althoughthe glacial polish has been exposed for approximately 16,000 years (Dom et al., 1991), the silica glaze may not have been stable for that full time period.

Silica Glaze

Figure 13.8. Backscattered electron microscope image of silica glaze, collected from a joint face of a prehistoric lava flow on Maui, in the rainshadow of Haleakala Volcano. Thicknesses range from almost a millimeter to less 50prn. The line indicates the location of an electron microprobe transect presented in table 13.18.

Hawaiian silica glazes can form rapidly. They are well developed on lava flows over a thousand years old 0~arr and Adams, 1984). Silica glazes have been noted on Kilauea flows 50 years old (Curtiss el al., 1985). Figure 13.9, for example, presents a silica glaze formed within a year. Rates of formation above the trade-wind inversion on Mauna Kea can be slower (Curtiss et al., 1985), perhaps because of the drier air at higher elevations. The silica glazes on Hawai'i are particularly important in understanding genesis, because the the concentration of silica in the glazes can exceed the abundance in the basalt by over 150% (Curtiss et al., 1985). While airborne fallout of continental quartz (Beget et al., 1993; Jackson et al., 1971) might supply some silica (Farr and Adams, 1984), it is more likely that adjacent deflated soil provides the raw materials. Thus, the mode of formation involves the concentration silica, most likely by abiotic dissolution and reprecipitation (Curtiss et al., 1985).

Figure 13.9 Backscattered electron microscope image of a thin layer of dark silica glaze formed on top of a clinopyroxene grain. The surface appearance of the silica glaze was clear with a slightly bluish tinge. The lava flow was collected in 1995 from a surface less than a year old.

285

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Chapter 13

13.2.1.5. Silica Glazes in Temperate Environments

Exposed rocks in temperate environments are coated with silica glazes that range in color from clear translucent, to brownish red, and black, depending upon whether the coloring agent is iron, organic matter or other materials (Oilier, 1984; Robinson and Williams, 1987). When silica glazes coat sandstones in England and France, they bridge grains and enhance its strength (Robinson and Williams, 1987; Robinson and Williams, 1994b). Silica glazes in temperate-humid environments often originate within joint fractures and contain abundant iron (Dorn, 1997; Robinson and Williams, 1992). In the High Atlas of Morocco, coatings are "often found on bedding planes and joint faces (Robinson and Williams, 1992, p. 417)." Joint surfaces on coastal Slopes in South Devon, United Kingdom, are coated with silica glazes --20-30~tm thick (Mottershead and Pye, 1994) that also contain iron as a substantial component. Basaltic cobbles near Puerto Madryn in southeastern Argentina have silica coatings on top of an iron-enriched weathering rind (Vogt and Del Valle, 1994). Silica glazes in temperate-humid environments can also form in subaerial contexts. At the inselberg Stone Mountain in Georgia, silica glaze is a common rock coating (Figures 13.10 and 13.11). Silica glazes are found on glacial boulders in the semi-arid foothills of the eastern Sierra Nevada in California (Dora, 1995a). In Figure 13.12, for example, the weathering of a biotite grain has been 'frozen' by the accumulation of a silica glaze. The Conejo Volcanics in the Santa Monica Mountains, southern California, foster silica glazes in the Coastal Sage biome (Figure 13.13).

Figure 13.10. Lines of water flow on Stone Mountain, Georgia, are traced by deposits of silica glaze, as well as oxalate and iron films.

Figure 13.11. Backscattered electron microscope image'of a silica glaze formed on top of plagioclase at Stone Mountain, Georgia (Figure 13.10)

Figure 13.12. Backscattered electron microscope image of the surface of a ventifact on the moraines of Bishop Creek, California. The bright biotite mineral had begun to weather by hydration, until it was coated by sili"ca glaze, essentially freezing the weathering in place.

287

Silica Glaze

Figure 13.13. Backscattered image of a silica glaze formed on a plagioclase mineral in the Conejo volcanics, collected at the far western end of the Santa Monica Mountains, southern California. The abundant clasts are soil particles incorporated into the silica glaze.

13.2.1.6. Antarctic Silica Glaze

Silica glaze accumulates in a variety of contexts in Antarctica. Sometimes, amorphous silica interdigitates with iron films in rock fractures (Hayashi, 1989; Matsuoka, 1995). In other cases, silica precipitates within the grains of the weathering find. Then, after the sandstone is case hardened (Conca and Astor, 1987; Weed and Norton, 1991; Young and Young, 1992), a subaerial silica glaze spreads over the surface (Figure 13.14).

i

Figure 13.14. Idealized sequence of the accumulation of amorphous silica on sandstone in Antarctica, adapted from Weed and Ackert (1986). In the upper diagram, silica first starts to fill pore spaces in the sandstone. Then in the lower diagram, silica glaze grows over the subaerial surface of the rock face.

Antarctic silica glazes formed in a subaerial context are thin, typically less than 100~tm. They form by the mobilization of silica from airborne quartz, clays and iron (Weed and Ackert, 1986; Weed and Norton, 1991). Clast particles are incorporated that

288

Chapter 13

provide minor amounts of A1, Si, Ca and Fe (Mittlefehldt and Lindstrom, 1991; Weed, 1985; Weed and Ackert, 1986; Weed and Norton, 1991). Silica glazes can fossilize ancient microorganisms within the amorphous silica (Friedmann and Weed, 1987), and by forming a siliceous crust over endolithic organisms (Figure 13.15). Silica glazes have even been observed on clasts of a desert pavement that is about 4.3 million years old (Marchant et al., 1996; Marchant et al., 1993). Surface Stain and Silica Glaze J Silica Glaze ? ' ~ F ,"V"~r~- -.-,.,.-,3 ~ j Black Lichen Zone ~,. x'~,--P.,-'~_.,-u..c~_ ~. Leachecl Zone ~LP

From O.5 cm to 2.0 r

~ '~:~'>:"";-)'--'~ . ~. . . . ?"~'--~-~ ~,-~- ~ - --', ~

c

"~

"...

'-.~-".w~t-,- ' v ' . . . . . . .

f " t . . . . j . . . J " "---

Cut Away Cross-Sectional View Figure 13.15. Idealized cross-section view of silica glaze formed on top of endolithic organisms in a sandstone in Antarctica, adapted from Weed and Norton (1991).

13.2.1.7. Silica Glaze on Artifacts

Stone artifacts are covered with a variety of rock coatings, all of which go under the general term 'patina' or sometimes 'patination'. This leads to terminological confusion when the author does not specify the type of patina (Service, 1941). In many cases, however, it is clear that the coatings are amorphous silica, sometimes called 'colloidal silica'. Colors include transparent, white, blue and red (Goodwin, 1960; Honea, 1964; Rottl~inder, 1975; Schmalz, 1960; Sturge, 1911). Most commonly, however, the smooth and shiny silica coatings on artifacts include iron hydroxides, giving the coating a rusted appearance (Clark, 1950; Friedman et al., 1994). There is some controversy on whether amorphous coatings of silica are from 'use-wear polish' or whether they are a natural accretion (Unger-Hamilton, 1984) composed of a variety of materials (Fullagar, 1991).

13.2.1.8. Silica Glaze and Rock Art

Rock engravings are often coated with silica glazes. In Southern Africa "silica or hydrated silica glazes are essentially transparent and found on granite, gneiss, mica schists, diabase, andesite and other silicate rocks (Butzer et al., 1979, p. 1211)". These silica glazes are often found interdigitated with calcite and manganiferous rock varnish. Silica glaze is common on Australian petroglyphs (Figure 13.16) (Dolanski, 1978; Nobbs and Dorn, 1993; Watchman, 1985; Watchman, 1987a; Watchman, 1990c; Watchman, 1992). The Paleolithic engravings at C6a in northern Portugal (Bahn, 1995b; Zilh~o, 1995a) are coated with two different types of silica glaze. One is mostly silica, described optically as "white amorphous silica" (Watchman, 1995; Watchman, 1996). The other contains abundant detrital particles with aluminum

Silica Glaze

(Watchman, 1995; Watchman, 1996). Both types of silica glazes can interdigitate, as seen in Figure 13.17.

Figure 13.16. A relatively thick coating of silica glaze formed on top of a petroglyph at Wharton Hill, in the Olary Province of arid South Australia. The bright areas underneath and on top of the silica glaze are pockets of Mn-Fe rock vamish. The dark areas undemeath the silica glaze contain organic carbon.

Figure 13.17. Silica glaze on an engraving of a goat at Penascosa site in northem Portugal (Dom, 1997). The upper (aluminum-rich) layer contains abundant detritus and the lower (aluminum-poor) layer is more homogeneous. The brighter material is an iron film. The 1501.tm-long line corresponds with electron microprobe data in Table 13.5.

The earth figures or geoglyphs of Peru (Clarkson, 1990; Clarkson, 1994) and southwestern North America (von Werlhof, 1989) are designed by contrasting stones with different degrees of rock coating development. The most common rock coatings are manganiferous rock varnishes (chapter 10) and iron films (chapter 9). However, silica glazes also occur within the geoglyphs and on clasts in the natural pavement adjacent to the geoglyphs (Figure 13.5). Some of the shiniest silica glazes in North America can be seen on the terraces of the Colorado River, near these geoglyphs. Silica glazes coat Hawaiian petroglyphs (Stasack et al., 1996). They form on Hawai'i (Figure 13.18), trapping charcoal (Figure 13.18), much like carbonized wood is trapped by silica glazes in Australia (Watchman, 1992). Figure 13.19 illustrates the interlayering of two different types of silica glazes on Kaho'olawe Island.

13.2.1.9. Silica Glaze on Stone Monuments

Silica glazes are found coating stone buildings and monuments. In some cases, the silica may come from opal present in mortars (Vincente et al., 1993). In other cases, the silica is natural. For example, silica glaze coats rock surfaces at Petra in Jordan, potentially providing protection against erosion (T. Paradise, personal communication, 1996). Because natural coatings of silica glaze can offer some protection against erosion (see chapter 6 on case hardening agents), stone conservationists have sometimes sought to apply silica to protect stone surfaces (Sattler and Snethlage, 1990). Lithobionts are sometimes associated with the precipitation of silica (see chapter 4). I have observed, for example, silica glazes on exposed stone surfaces at Tikal, Guatemala (Figure 13.20). There is a red coloration associated with fungal filaments on the Tikal

289

290

Chapter 13

stone. A sample from a nearby natural stone surface shows precipitation of silica glaze in association with the fungi.

Figure 13.18. Iron-rich silica glaze has encapsulated charcoal in a cupule petroglyph at Pu'u Loa, Hawai'i.

Silica Glaze

Figure 13.19. Microstratigraphic observations of different positions on petroglyph K20 on Kahoqawe Island (Stasack et al., 1996), suggesting either the gradual growth of silica glaze or that the petroglyph was repecked at different times. All images were taken with backscatter electron microscopy. K20A: The coating is much thinner, and inplaces it has not yet formed over the host underlying rock. K20B: A lower layer rich in iron is present. K20C: The silica glaze coating is evenly divided between the lower, more iron-rich layer and the upper, more iron-poor (aluminum-rich) layer.

Figure 13.20. Worked stone at Tikal, Guatemala, is encmsted with red fungi that precipitates silica glaze, as exemplified by the precipitation of silica platelets (as determined by energy dispersive X-rays) on fungi growing on a nearby natural surface. The fungal body, seen with secondary electrons, has a diameter of 10 ~tm.

291

292

Chapter 13

13.2.1.10. Streams The literature on silica glaze on boulders in streams is limited. Transparent hydrated silica occurs on rocks along the sides of tropical rivers (Alexandre and Lequarre, 1978). For example, silica glazes have been found on basalts of the Batoka Gorge in southern Africa (Clark, 1950). The paucity of observations could be from a lack of awareness. A common way of detecting silica glaze in the field is its sheen. The shiny property of fiver-side rocks can also be imparted by water. Figure 13.21 illustrates a silica glaze formed on a point bar of the Rogue River, Oregon. I did not notice the glaze until the rock surface had dried out. Even then, the 50~tm-thick coating was hard to see.

Figure 13.21. Backscattered electron inaage of silica glaze on a point bar of the Rogue River, Oregon, about 9 kilometers up stream from Gold Beach. The silica glaze is darker than the underlying quartz, because it contains ,--15%A1203to go alongwith -70% SiO2.

13.2.1.11. Silica Glazes lnterdigitated With Other Rock Coatings Silica glazes often interfinger with other types of rock comings. The interdigitation can take several different expressions: by providing an initial layer on a rock surface; by interlayering with other rock coatings; and by replacing rock coatings. Butzer et al. (1979, p. 1211) observed that "silica glazes represent the initial stages of varnish or patination..." This occurs in Antarctica, where silica glaze fills in the pore spaces between minerals and then coats over the surface (Figure 13.14). Then, manganiferous rock varnish can form on top of the silica glaze (Figure 13.22). Probably the most common expression of interdigitation is where silica glaze is found between other types of rock coatings. In South Africa, for example, silica glaze mixes with calcite and manganiferous rock varnish (Butzer et al., 1979). Australian silica glazes (Figure 13.16) are found in the same rock coatings with oxalate-rich crusts, sulfates skins, iron films, carbonate skins, and manganiferous rock varnish (Dragovich, 1986a; Nobbs and Dorn, 1993; Watchman, 1985; Watchman, 1990b; Watchman, 1992). In northern Portugal, there can be complex stratigraphic superpositions of aluminum-poor silica glaze, aluminum-rich silica glaze, and iron-rich layers (Figure 13.17). In some cases, silica glaze can replace other rock coatings. As noted in chapter 12, many of the black-water streaks in semi-arid regions include oxalate-rich crusts. This is the case for the matrix in Figure 13.23, an electron microscope image of a dark streak

Silica Glaze

on the Wingate Sandstone of the Colorado Plateau. The medium gray in the image is the calcium oxalate-rich crust. The darker areas are silica glaze and the brighter regions are rock varnish. Thus, either the oxalate has entombed the silica glaze and rock varnish, or been replaced by them.

Figure 13.22. Backscattered electron image of silica glaze forming the foundation for manganiferous rock varnish in Antarctica. The black line separates the host rock (quartz) from the silica glaze. After the silica glaze filled in the depression and began to spread over the surface, it was in turn coated by Mn-Fe-rich rock varnish (Dorn et al., 1992b). The sample was collected from an older moraine of upper Beacon Valley, Victoria Land, Antarctica by Mort Turner.

Figure 13.23. Backscattered electron image of the interdigitation of oxalate-rich crust (the gray matrix), manganiferous rock vamish (bright areas), and silica glaze (darker areas). The sample came from a black 'water' streak on Wingate Sandstone, Utah.

13.2.2. Composition The mineralogy of silica glazes appears to be characterized by x-ray amorphous silica (Curtiss et al., 1985; Watchman, 1992). The alumina within the silica glaze may also be disordered or in a solid solution (Curtiss et al., 1985). There are detrital clasts within

293

294

Chapter 13

the silica glaze that have suite of minerals similar to soil minerals (Curtiss et al., 1985). Previous classification systems for silica glazes are based on descriptions of colors in optical thin sections (Farr and Adams, 1984; Watchman, 1992; Watchman, 1995; Watchman, 1996) and qualitative chemical analyses (Watchman, 1990c, p. 23-25). I attempted to use the prior classification in this chapter, but there were contradictions between chemistry and appearance. Instead, I propose here a classification based largely on quantitative chemical analyses of silica and the more important minor constituents. All classifications have the distinct disadvantage of 'pigeon holing' truly distinct phenomenon into pre-existing classes. At the same time, generalizations can encourage the development of theory. Thus, what follows is a very general classification for silica glazes that is meant to encourage future connectivity between the areas of chemistry, geography of formation, and genesis: General Classification of Silica Glaze Type I. Homogeneous Amorphous Silica Glaze Type II. Detrital-rich Silica Glaze Type III. Alumina-Iron-rich Silica Glaze Type IV. Alumina-rich Silica Glaze Type V. Iron-rich Silica Glaze Type VI. Alumina Glaze

13.2.2.1 Type I. Homogeneous Amorphous Silica Glaze Silica glazes that are described as clear, translucent, and white are the 'purest' type of silica glaze. Although there are minor and trace elements in these coatings, concentrations are a few percent or less. Table 13.1. presents chemical analyses of Type I silica glazes in the literature. Table 13.1. Electron microprobe analyses of Type I silica glazes in the literature, reported as oxide weight percentages. Sample

Na20

MgO

A1203

SiO2

HI-421

nd5

0.04

nd

89.5

HI_242

0.02

0.70

0.93

91.5

K20

CaO

TiO2

MnO

Fe20 3

nd

nd

nd

nd

0.08

0.29

0.77

nd

nd

0.24

H1033

nd

0.2

2.0

85.6

nd

0.8

nd

nd

nd

Gnatalia4

0.1

0.3

2.8

77.2

3.3

0.1

nd

nd

4.4

I from the Ka'u Desert, Hawaii (Curtiss et al., 1985) 2From the summit region of Mauna Kea, Hawaii (Curtiss et al., 1985). 3From 1840-yr-old Mauna Loa flow, Hawaii: (Farr and Adams, 1984). 4Ave of 5 analyses,Gnatalia Creek, New South Wales, Australia (Watchman, 1992), including 0.4% C1, 0.3% P205, and 2.9% SO3. 5nd means no data, either below limit of detection or not listed.

295

Silica Glaze

Figure 13.24 shows an example of silica glaze collected in 1995 on the Mauna Ulu Flow, Hawai'i (1969-1973). This 25-yr-old silica glaze is mostly homogeneous, as indicated by the even texture in backscatter. There is a subtle change, however, at the very bottom of the electron microprobe transect (line in Figure 13.23). The gray is a little bit darker in the lower third, reflecting the relative paucity of iron in the Type I silica glaze (Table 13.2). There are two other types of silica glazes illustrated in Figure 13.24 and Table 13.2. The silica glaze in the upper two-thirds is Type III (alumina-iron-rich silica glaze). The upper layer is brighter due to higher iron concentrations. There are also small pockets of Type VI (alumina glaze); one is indicated by the arrow in Figure 13.24.

Table 13.2 Focused beam electron microprobe transect through silica glaze on the 20-25-year-old Mauna Ulu Flow, Hawai'i. Figure 13.24 shows the location of the transect. Chromium, manganese and barium were measured, but were below the limit of detection (bid), which was less than 0.04%. The italicized points reveal analyzes of the Type I. silica glaze. Na20

MgO

AI20 3

SiO2

P205

SO3

K20

CaO

TiO2

FeO

Total

0.71

1.48

5.63

72.25

0.02

0.35

0.18

0.35

0.42

4.07

85.46

0.12

1.53

3.82

69.40

0.11

0.55

0.19

0.63

0.40

5.39

82.14

0.11

0.99

2.46

58.47

0.16

0.65

0.19

0.34

0.57

6.43

70.37

0.07

0.88

2.21

59.86

0.05

0.17

0.12

0.25

0.22

1.69

65.52

0.12

1.09

6.31

60.91

bid

0.32

0.12

0.22

0.25

1.33

70.67

0.05

0.35

50.45

20.11

bid

0.07

0.05

0.25

0.13

0.37

71.83

0.08

0.86

7.50

62.15

bid

0.17

0.08

0.14

0.23

1.79

73.00

0.09

0.85

1.17

63.11

bid

0.20

0.07

0.14

0.20

0.96

66.79

0.80

3.43

3.85"

67.97

bM

0.17

0.13

2.76

0.47

2.80

82.38

0.05

1.03

2.51

61.36

bid

bid

0.05

0.13

0.22

1.35

66.70

Figure 13.24. Backscattered electron image of silica glaze at an elevation of -300 m on the Mauna Ulu flow of Kileaua, Hawai'i. The line indicates the location of the electron microprobe transect (Table 13.2) The bottom third is Type I silica glaze The upper two thirds is Type III silica glaze. The arrow indicates the top of a pocket of Type VI glaze, with 50% alumina.

Chapter 13

296

Figure 13.25 illustrates a fairly h o m o g e n e o u s coating of Type I silica glaze. There is s o m e layering, as detrital minerals do have some preference for lying parallel to the surface. Electron m i c r o p r o b e analyses of the silica glaze in Figure 13.25 reveal a composition that is d o m i n a t e d by silica (Table 13.3).

Figure 13.25. Silica glaze formed on lava flow f5d p.35 (Moore and Clague, 1991). Although there are micron-sized fragments of detrital minerals, most of the silica glaze is homogeneous type I. The black line is just to the fight of the electron microprobe transect, which can be seen as qaum points' on the backscatter and secondary electron microscope images (see Table 13.3. for data). There is a small pocket of ironalumina-rich silica glaze to the left of the electron microprobe transect.

Table 13.3 Focused beam electron microprobe transect through silica glaze on a 3000-year-old lava flow on Hualalai (Moore and Clague, 1991), sampled at an elevation of around 61 meters. Figure 13.25 shows the location of the transect. Chromium, titanium, and barium were measured, but were below the limit of detection (bid). Low probe totals are from water and porosity. Na20

MgO

A120 3

SiO2

P205

K20

cao

MnO

FeO

Total

0.12 0.08 bid bid 0.06 0.08 0.10 bid 0.09 bid

0.07 0.12 bid bid bid 0.10 bid 0.15 bid 0.11

0.93 0.62 0.28 0.04 0.15 0.16 0.50 0.47 0.11 0.50

88.42 86.77 88.10 89.85 88.45 88.50 88.44 88.55 89.10 90.09

bid 0.05 bid bid bid bid bid bid bid bid

0.04 0.04 bid bid bid bid bid bid bid bid

0.29 0.21 0.06 bid bid bid bid bid bid bid

bld 0.05 bid bid bid bid bid bid bid bid

0.17 0.04 0.13 0.05 0.08 0.16 0.16 0.04 0.06 0.05

90.04 87.98 88.57 89.94 88.74 89.00 89.20 89.21 89.36 90.75

I I

T h e 'desert glaze' described by Fisk (1971) and others as clear and translucent is probably T y p e I silica glaze. These very shiny skins are often very thin, as illustrated by Figure 13.26. W h e n these shiny materials are analyzed with qualitative energydispersive analyses, their peaks are almost entirely silicon.

297

Silica Glaze

Figure 13.26. Silica glaze formed on a cobble on the high terrace of the Colorado River, collected near Ripley on the Arizona side of the river. Very shiny cobbles in hyperarid deserts often have a very thin coating of Type I silica glaze. The measurements in Table 13.4 were taken from the dots in the upper left and middle bottom of the coating.

Table 13.4 Focused beam electron microprobe transect through silica glaze on a cobble on the high terrace of the Colorado River, Arizona. Figure 13.26 shows the location of the transect. Phosphorus, sulfur, chromium, manganese and barium were measured, but were below the limit of detection. l~a20

MgO

bid 0.05

bid 0.02

A1203 SiO2 0.55 0.85

K20

CaO

TiO2

98.15 bid 96.10 0.02

0.03 bid

bid 0.03

Fe203 Total 0.37 0.94

99.10 98.01

Chemistries similar to Type I silica glazes are found in silcretes. On the southern margins of the Atlas Mountains, for example, clear coatings on gravels have chemistries very similar to the 1840-yr-old Mauna Loa lava flow in Hawaii (Farr and Adams, 1984). Silica averages 85.5%, A120 3 averages 3.0%, MgO 1.6%, F e 2 0 3 1.2% and all other elements are less than 1% (Thiry and Ben-Brahim, 1990). Similar abundances are found in silcretes of the Letlhakeng Valley, Botswana (Nash et al., 1994). When the electron microprobe data presented in this section are analyzed together, the characteristic of Type I silica glazes is a dominance of amorphous silica. To produce Figure 13.27, the probe data were first normalized to 100%. Then, only those minor elements whose oxide weight percent averages over 1% were graphed. At lower silica concentrations, aluminum is the most important minor element. As silica concentrations rise, however, iron becomes the dominant minor element.

298

Chapter 13

9.00 .. 8.00 . .

8

7.00

--

6.00

--

5.00

--

4.00

..

3.00

--

E!

Type I Silica Glaze X MgO O

0 X

tP

,,

2.00 1.00

A1203

n

t

0 CaO

A

t/t

A FeO

0.00 80.00

85.00

90.00

95.00

100.00

SiO2 Figure 13.27. The characterization of Type I silica glaze, based on data presented in this section. Although minor elements can reach several percent, silica is by far the dominant component of these silica glazes. As silica concentrations increase, abundances of the other constituents must decrease. Values are normalized to 100%.

13.2.2.2. Type H. Detrital-rich Silica Glaze Some silica glazes are so dominated by detritus that a separate type of silica glaze is warranted to describe their texture. Type II glazes still has silica as the dominant component. Amorphous silica cements detritus together and fills the pore spaces. Figures 13.17. and 13.28 present Type II silica glazes from the Cfa valley in northern Portugal. These detrital-ladened silica glazes are typically found overlying Type I amorphous silica (Dorn, 1997; Watchman, 1995). Table 13.5. reveals that the Type II silica glazes have relatively high alumina concentrations from 10% to 20% with iron less than 5%. When the trace elements of Na, Mg, K, and Ca are considered, it is likely that the particles probably came from adjacent soils (Watchman, 1995).

Sih.G,~fA,-~,.',*I

Image

Image

Or~n~cMat~ i~M s of Wca|hccingRind ~ Undcrlyi[tgSchlstRock ~[r0n ski~

Figure 13.28. Stratigraphic context of Type II silicaglaze on top of Type I silicaglaze from petroglyph sample FC-95-7c, in northern Portugal (Dora, 1997). The line corresponds with the electron microprobe measurements presented in Table 13.5.

299

Silica Glaze

Table 13.5. Electron microprobe measurements on silica glazes formed on petroglyphs in northern Portugal (Dorn, 1997). The probe diameter was set to 5 micrometers. Low probe totals result from porosity, water, and organic matter. Bid indicates below limit of detection. The measurements were taken along the transects in Figures 13.17 and 13.28. 9

,,

i,

Transect in Figure 13.17 Material Na20 MgO A1203 SiO2 P205 SO3

K20

CaO

TiO2 MnO Fe20 3 BaO

Type II

0.22

0.41

18.70 65130 0.15'"0.80 0.85

bid

0.92

0.17' 88.58

Type lI

0.20

0.43

17.39 59.07 0.19

0.32

0 . 6 2 1.11 0.10

0.04

1.62

0.17

81.26

Type lI Type lI Type lI Type lI Type II Iron Skin Type 1 Type 1

0.32 0.14 0.16 0.18 0.18 bid 0.06 0.03

0.50 0.30 0.39 0.22 0.16 bid 0.14 0.15

17.09 16.17 16.70 16.19 6.05 7.15 0.68 0.50

0.10 0.39 0.81 0.57 0.14 0.63 bid bid

0.50 0.74 0.62 0.74 0.34 0.30 0.20 0.22

0.86 0.70 0.60 0.58 0.40 0.28 0.34 0.41

0.05 bid bid bid bid 3.44 bid bid

2.10 1.14 1.62 1.16 3.52 68.10 0.22 0.33

0.22 0.09 0.22 0.09 bid bid bid bid

81.63 82.34 80.87 80.07 79.47 85.42 86.00 90.05

Transect in Fi[ure 13.28 Material Na20 M g ( ) A 1 2 0 3 SiO2 P205 SO3 Type II 0.24 1 . 1 3 13.15 55.57 0.87 bid Type II 0.13 0.93 12.13 56.31 1.01 bid

K20 0.71 0.80

CaO TiO2 MnO Fe20 3 BaO Total 1 . 0 8 0.55 bid 1.16 0.10 74.56 0.94 0 . 4 8 0.04 2 . 7 5 0 . 0 3 75.55

Type Type Type Type

0.90 0.55 0.39 0.44

0.88 0.57 0.50 0.80

II II 1 1 i

bid 0.12 bid bid

0.88 11.80 0 . 9 1 16.54 0.30 8 . 8 2 0.11 0 . 4 1 ,,, ,,

, |

59.67 62.44 59.53 60.10 68.48 5.03 84.07 88.10

60.14 59.88 70.11 84.10

0.10 0.12 0.10 0.15 0.13 0.22 0.19 0.20

1.05 0.82 0.20 0.11

bid bid bid bid 9

,,,,

'i.00 .....0.06

Total

,,

0.12 0.11 0.12 0.09 0.07 0.27 0.10 0.11

0.13 0.22 0.16 0.14 ,,,

0.06 0.12 bid bid

3.80 1.50 0.20 0.49

0.06 bid bid bid

79.70 81.23 80.68 86.60

,,,

Just as Type I silica glaze forms within a few years in Hawai'i (see section 13.2.1.4.), Type II silica glaze can form rapidly. Figure 13.29 illustrates a coating of silica glaze formed on a slag pile. The chemistry of the silica glaze suggests that it has a high component of clay, as well as manganese and iron that may derive from the dissolution of adjacent rock varnishes and iron skins (Dorn and Meek, 1995).

Figure 13.29. Backscattered electron micrograph of Type III silica glaze formed on top of a 20- to 40year-old slag pile in southern California (Dom and Meek, 1995) (Figure 9.26). The underlying material is enriched in iron and manganese; it is therefore much brighter than the overlying silica glaze. The line indicates the location of the electron microprobe transect (Table 13.6).

300

Chapter 13

Table 13.6. Chemistry of silica glaze observed on the west slag pile, Kaiser steel plant, as measured by electron microprobe with a 10 pm spot size. The location of the transect is in Figure 13.19. Low oxide (weight percent) totals are from porosity, water, and organic matter. K20, TiO 2, and BaO were measured but were below the limit of detection. Na20

MgO

A120 3

SiO2

P205

SO3

CaO

MnO

Fe20 3

Total

0.98

1.23

21.15

43.40

0.23

2.05

1.01

2.57

4.34

76.96

1.01

1.05

22.18 41.07

0.30

3.23

1.23

2.63

4.60

77.30

0.74

2.05

2 3 . 5 1 47.24

0.39

1.97

1.44

2.74

4.67

84.75

0.37

1.75

26.30

41.87

0.44

2.44

1.74

2.60

4.80

82.31

0.14

1.11

25.22

42.17

0.35

3.00

1.40

2.64

5.71

81.74

Type II silica glazes can be found in a variety of anthropogenic settings. In a 15year-old planter box at Arizona State University clay particles have been cemented to the host brick substrate by amorphous silica (Figure 13.30). In some analyses silica concentrations are greater than 70% (Table 13.7). The relatively high calcium concentrations are probably from the abundance of calcium carbonate in the irrigation water. The appearance of the planter box coating is similar to opaline silica found in soils (Figure 13.2). Figure 13.31 displays a more generalized perspective on the chemistry of Type II silica glazes. To produce this figure, the values of the chemical analyses in this section were normalized to 100%, and only trace elements with concentrations greater than 1% are plotted. The relatively high values of the aluminum, along with calcium, sodium and magnesium, likely reflect a clay mineralogy component to the detritus.

Figure 13.30. Backscattered electron microscope image of Type II silica glaze formed on bricks of a 15year-old planter at Arizona State University. The line indicates the location of the electron microprobe transect (Table 13.7). The sample was collected and prepared by S. Campbell.

Silica Glaze

301

Table 13.7 Electron microprobe transect through Type II silica glaze formed on bricks of a 15-year-old planter at Arizona State University. A focused beam was used. Figure 13.30 shows the location of the transect. Chromium was measured, but concentrations were below the limit of detection (bid). Na20

MgO A120 3 SiO2

P205

SO 3

K20

CaO

TiO 2

MnO Fe20 3 BaO

Total

1.20

0.36

3.70

75.26

0.02

0.02

0.41

0.62

0.07

0.04

0.75

bid

82.45

3.92

0.12

13.28 72.87

0.02

0.05

0.94

2.59

0.10

bid

0.57

bid

94.46

2.97

0.08

12.19 27.51

0.02

0.07

0.18

5.07

0.10

bid

0.93

0.08

49.20

5.43

0.02

22.11

59.73

0.09

bid

2.52

4.67

0.08

bid

0.44

bid

95.09

4.93

0.08

20.95

58.25

bid

bid

0.65

5.14

0.12

bid

0.53

bid

90.65

5.07

0.05

17.23 62.11

bid

0.07

1.71

2.95

0.48

bid

0.71

bid

90.38

4.60

0.10

14.44 67.43

0.05

0.02

0.98

3.20

0.20

bid

0.48

bid

91.50

2.97

0.12

14.02 75.07

0.09

0.25

2.95

1.57

0.18

bid

0.68

bid

97.90

5.58

0.15

23.43

57.63

bid

bid

1.11

6.14

0.05

bid

0.63

bid

94.72

0.81

5.34

8.16

67.95

bid

0.40

0.86

4.60

0.88

0.10

3.13

0.15

92.38

Type H Silica Glaze 100.00 I n n ml

9N 9 9

10.00

.

&

9

9

o

1.oo 9~

f

o

+

9 A1203

Ul.n muiR

+

+ FeO

:

A CaO O Na20

O 0.10 40.00

-- MgO 50.00

60.00

70.00

80.00

90.00

SiO 2 Figure 13. 31. The characterization of Type II silica glazes, based on data presented in this section.

13.2.2.3. Type III. Alumina-Iron-rich Silica Glaze Type III silica glazes have relatively high concentrations of both iron and aluminum, although silicon is still dominant. Type III silica glazes are commonly noted in the literature for different lithologies in Australia (Dragovich, 1994; Watchman, 1992), on

302

Chapter 13

dolomite in Israel (Danin, 1985; Danin, 1986), on basalt in Hawai'i (Curtiss et al., 1985; Farr and Adams, 1984), on andesite in Japan (Matsukura et al., 1994), and along the southern coast of England (Mottershead and Pye, 1994). Table 13.8 presents quantitative electron microprobe analyses of Type III silica glazes in the literature. Type III silica glazes can form quite rapidly. A clast on the active surface of the Khumbu Glacier, Nepal, has a layer of Type III silica glaze resting on top of an iron film (Figure 13.32). The backscattered image shows very little variability in gray scale, reflecting a fairly homogeneous coating. The homogeneity is reflected in the chemical analyses (Table 13.9). Although silica is the dominant component, with concentrations over 50%, iron and aluminum oxides together comprise -10% by weight. Table 13.8. Compositions of Type III silica glaze in the literature, measured by electron microprobe. Below limit of detection is represented by bld, and nr means not reported. Sites

Na2OMgO A1203 SiO2

P205 SO3 K20 Ca0 TiO2 MnO Fe203 BaO

,

0.02 '1.18 12.62 70.64 0.13 0 . 0 8 0.33 0.24 0.31 0.15 6 . 5 8 0'03

Si-Fe layer 1 Near Dampier, Western Australia Zone III2 Aso Volcano, Japan Coles Creek, 3 New South Wales, Australia Mauna Kea, Hawaii4 ,

,

0.54 0.69 4 . 0 7 0.1

0.3

2.8

bid

0.19 1 7 . 4

72.87 nr

1.74 0.58 1.38 0.48 0.02 4.32

nr

77.2

0.3

2.9

3.3

4.4

nr

67.7

nr

nr

0.15 1.39 0.89 bid

5.28

nr

0.1

bid

bid

,

lfrom (Dragovich, 1994) 2from (Matsukura et al., 1994) 3from (Watchman, 1992) 4from (Curtiss et al., 1985)

Table 13.9 Electron microprobe analyses of silica glaze precipitated on a clast on the surface of the Khumbu Glacier, Nepal. The location of the transect is presented in Figure 13.32. Low probe totals are from porosity, water and organic matter. Bld means below the limit of detection. Na20'MgO A1203 SiO2 P205 0.03

0 . 4 3 6.93 '52.41

bid

0.45

SO3

K20

CaO

TiO2J' Iv]no F e O

1.24 "0.02 0 . 2 5 0 . 0 6 1.57

6 . 7 2 62.33 1 . 3 7

0 . 0 2 0 . 1 2 0.11

1.33

BaO

Total

i~id ' 7.19 0 . 0 2 70.15 0.05

5 . 2 4 0 . 0 6 77.80

0.03

0 . 4 1 4 . 7 5 56.61 1 . 2 6

0.02 0.11 0.03 0.75 0.04

4.17

0.01

0 . 2 8 6 . 0 8 59.84 1 . 2 4

0.07 0.24 0.04 0.73 0.10

4 . 2 3 0 . 0 6 72.92

0.08

0 . 1 8 5.26

0 . 0 5 0 . 2 3 0 . 0 4 0.68

3.56

.

,

56.61 1 . 4 0

.,,

,,

...

bid .

bid

68.18

0.01 68.10 ..

Silica Glaze

Figure 13.32. Backscattered electron image of silica glaze precipitated on a quartzite clast resting on the surface of the Khumbu glacier (~5250m). The bright material directly underneath the silica glaze (on the left side of the image) is an iron film. The dark spaces are pores. The line on the left side of the image identifies the location of the electron microprobe transect in Table 13.9.

Type III silica glazes have been noted on the upper slopes of several volcanoes, including Mauna Kea, Hawai'i (Curtiss et al., 1985). I have observed them on rocks near the summit of Haleakala volcano, Hawai'i. The Type III silica glaze on glacial moraines of Iztacc~uatl Volcano, Mexico, has a layered appearance (Figure 13.33), as does the Type III silica glaze on Aso Volcano, Japan (Matsukura et al., 1994).

|

"It

'

Figure 13.33. Backscattered electron micrograph of type III silica glaze formed on top of a glacial boulder from the Milpuco Valley, Iztacc~uatl Volcano, Mexico. The line indicates the location of the electron microprobe transect in Table 13.10.

Although Type III silica glazes can be homogeneous (Figure 13.32), they frequently show chemical variability. The layering found on boulders of Iztacc~uatl Volcano, Mexico (Figure 13.33) corresponds with the abundance of iron. Dark layers are relatively rich in aluminum (Table 13.10), while brighter layers are relatively rich in iron.

303

304

Chapter 13

Table 13.10. Focused beam electron microprobe transect through Type III silica glaze formed on a glacial boulder from the Milpuco Valley, Iztaccl'huatl Volcano, Mexico. Figure 13.33 shows the location of the transect. Bid means below the limit of detection. Low probe totals may reflect porosity and water. Na20 MgO A120 3 si0 2

P205 ' s o 3

K20

CaO

TiO2 Cr203 MnO Fe20 3 BaO" Total

0.84 3.44 2.18 0.74 1.47 3.80 4.38 4.08 4.95 5.41

0.34 0.22 0.80 0.37 1.42 0 . 3 5 2.18 0.65 1.03 0 . 5 5 bid bid 0.50 0.07 0.48 0 . 0 7 0.11 bid 0 . 3 0 0.10

0.31 0.30 1.47 0.33 0.76 3.17 5.91 3.32 1.29 2.29

0.83 4.25 1.57 0.78 1.72 2.18 0.66 2.71 7.61 4.37

6123 0.33 0.30 0.48 0.37 0.18 0.17 0.23 0.43 0.17

0.30 0.50 0.22 0.25 0.61 0.27 0.02 1.36 2.32 0.15

5.57 15.65 13.08 9.35 10.11 14.76 17.21 14.89 19.12 20.10

71.7~1 52.93 41.44 29.35 47.30 49.27 56.27 52.37 53.01 58.45 i,

,.

,

.hi,

...

bl'd 0.04 bid bid bid 0.04 0.09 0.04 bid bid

0.1~-5.84 b i d " 86101 0.30 10.72 bid 89.63 0.81 26.53 bid 89.37 1.21 37.30 0.07 82.69 0.41 17.66 bid 81.99 0 . 1 3 3 . 6 5 0 . 0 9 77.54 0 . 2 2 8 . 8 6 0 . 1 2 94.48 0 . 3 5 11.53 bid 91.43 0.15 5.81 bid 94.80 0.03 4 . 2 6 0 . 1 2 95.75

,

.

..

9

,,.

T h e p e t r o g l y p h s of K a h o ' o l a w e Island, Hawai'i (Stasack et al., 1996) are coated by T y p e III silica glaze (Figure 13.17). A l t h o u g h the analyses in Table 13.11 are not necessarily representative of the silica glaze over the entire petroglyph, they do give a fairly consistent picture, w h e r e b y silica comprises the vast majority of the silica glaze, and a l u m i n a and iron together c o m p r i s e - 2 0 - 3 0 % by weight (Table 13.11).

Table 13.11. Electron microprobe analyses of silica glaze on petroglyphs of Kaho'olawe Island, Hawai'i (Stasack et al., 1996). Bid means below the limit of detection. Petroglyph Na2OMgO A1203 SiO2 u

P205

SO3 K 2 0 " C a ( ) T i 0 2

.i

MnO Fe20 3 BaO Toial

i

.

.

K10

0.55 0.10 16.19 72.44 0.02

0 . 2 7 2.24 0.18 0.06

bid

K19

0.19 0.17 10.12 67.41 1.88

0.20 1.07 0.40 0 . 2 0

0 . 4 8 16.97 0.03 99.12

4.14

0 . 0 9 96.28

K26

0.34 0.50

14.11 73.55 0.87

0 . 2 5 0.61 0.78 0.97

0 . 2 0 3 . 1 6 0.10 95.44

K16B

0.29 0.96 13.13 80.31 1.19

0.17 0.22 0.54 0.98

0 . 1 4 1 . 7 5 0 . 0 3 99.71

K15A

0.09 0.88 15.49 58.04 1.05

0 . 1 5 0.50 0.38 0.17

0 . 0 8 18.80 0.06 95.69

K16A

0.15 0.60 21.54 62.07 1.42

0.23 0.61 0.47 0.22

0 . 1 5 7.54

bid

95.00

K30

0.09 0.80 14.23 75.59 1.19

0.25 0.47 0.70 0.17

0 . 0 7 5 . 5 6 0 . 0 2 99.14

Kll

0.40 0.81 13.40 74.07 0 . 6 6

0 . 2 9 0.50 1.04 0.25

0 . 0 7 2.23

K12

0.67 0.90 24.16 65.17 0.48

0 . 1 7 0.84 0.66 0 . 7 0

0 . 2 2 4 . 3 2 0.01 98.30

K22

0.60 1.44 30.17 49.84 1.08

0 . 2 5 1.55 1.11 0.07

0 . 4 1 10.11 0.09 96.72

K28

0.14 1.67 19.00 66.90 1.55

0 . 2 5 1.70 0.20 0.15

0 . 1 1 7 . 0 4 0 . 1 2 98.83

K33

0.55 0.80 18.11 71.60 1.37

0 . 2 9 0.90 0.33 0 . 2 0

0 . 0 9 4 . 2 9 0.14 98.67

K23

1.04 0.44 22.70 66.48 1.07

0.20 0.55 0.37 0.19

0 . 0 9 3 . 3 0 0 . 1 9 96.62

,,

.

..

,

,

.

bid

93.72

305

Silica Glaze

The clear silica glaze found on a gibber in central Australia (Figure 13.3) has a chemical signature that could be classified as Type III silica glaze, although the abundance of alumina, silica, and iron are roughly equivalent (Table 13.12).

Table 13.12. Electron microprobe analyses of clear silica glaze found on a gibber in a desert pavement in central Australia (see Figure 13.3). Bid means below the limit of detection. MgO AI20 3 SiO 2

P205

K20

CaO

TiO 2

MnO

FeO

Ct~

BaO

Total

23.62 23.00 23.96 24.52 25.67

bid bid bid bid bid

bid bid bid 0.18 0.24

bid bid bld bid bid

bid bid bid bid 0.08

0.61 0.66 0.70 0.67 0.63

27.47 27.30 27.07 27.09 27.30

bid bid 0.14 bid 0.09

bid bid 0.07 bid bid

84.55 83.97 86.16 86.94 89.77

12.93 22.69 25.52

bid

0.13

bid

0.05

0.71

26.98

0.11

bid

89.12

12.12 12.00 12.49 12.37 12.95

20.73 21.01 21.73 22.11 22.81

When the chemical data presented in this section are reviewed together, groupings emerge (Figure 13.34). The gibber dataset occupies the upper left comer of the plot, where the silica glaze displays a fairly even distribution of S i, A1 and Fe. The other Type III silica glazes have more aluminum than iron. The negative slope is an artifact of the fact that A1 and Fe increase at the expense of S i.

Type HI Silica Glaze 50.00 -40.00 -l

30.00 - =

=O

9

o A1203

20.00 -

.m

" FeO 0

10.00 -0.00

,

20.00

II

,'

40.00

'

I

60.00

'

I

80.00

'

100.00

SiO 2 Figure 13.34. Chemical character of Type III silica glazes, based on data presented in Chapter 13. Values are normalized to 100%.

306

Chapter 13

13.2.2.4. Type IV. Aluminum-rich Silica Glaze Type IV glazes have silica as the dominant component, but alumina concentrations are typically greater than 10%. The remainder of the other constituents rarely reach a combined total of more than a few percent by weight. Table 13.13 presents chemical measurements reported in the literature for Type IV silica glazes.

Table 13.13. Compositions of Type IV silica glaze in the literature, measured by the electron microprobe. Bid is below limit of detection and nr is not reported. iiiiiiiiii

IIIIIIHI

Na20 l~lgO A1203"SiO2 P205 S03" K20 CaO

TiO2 Fe203

Jim Jim1 Kakadu,Northem Territory

0.1

0.2

36.8 49.5

Mauna Kea, Hawaii2

0.04

0.59

11.8 78.1 nr

0.i....

bid

0.2 .... 0.1

0.2

1.1 ..

nr

1.02 2.28 bid

0.25

lfrom (Watchman, 1992) 2from (Curtiss et al., 1985) If chemical analyses are considered by themselves, Type II silica glazes (those rich in detritus) could be considered Type IV (aluminum-rich silica glaze), because they have aluminum as the second most important constituent. There is, however, an important difference. In Type II, the alumina comes largely from the detritus. In contrast, Type IV silica glazes have a fairly homogeneous appearance in backscatter, indicating that the matrix of the glaze is rich in alumina. Figure 13.35. illustrates a Type IV silica glaze on a 2000-year-old lava flow on the rainshadow of Hualalai Volcano, Hawai'i. There are pieces of detritus scattered through the silica glaze, but the electron microprobe transect was placed in an area relatively free of detritus.

Figure 13.35. Backscattered electron microscope image of type IV silica glaze formed on a 2000-year old lava flow, f5d c8.2 (Moore and Clague, 1991) of Hualalai Volcano, Hawar'i. The line indicates the location of the electron microprobe transects in Table 13.9.

307

Silica Glaze

Table 13.14 Electron microprobe analyses of Type IV silica glaze precipitated on a 2000-year old lava flow of Hualalai Volcano, Hawai'i The location of the transect is presented in Figure 13.35. Low probe totals are from porosity, water and organic matter. Bid means below the limit of detection. Na20 MgO A1203 $iO2

K20

CaO

TiO2

0.13

1 . 6 2 12.83 69.53

P205 0.71

0.96

0.52

0.37

0.19

1 . 3 3 12.57 46.87

0.99

0.81

0.32

0.09

1 . 3 9 12.58 56.59

1.10

0.59

0.31

0.12

1 . 5 9 11.39 62.64

0.89

0.36

0.27

0.34

2.30

0.92

0.69

0.12

1 . 0 8 9.64

57.91 0.69

0.45

,,, ,,

13.72 60.05

MnO Fe203

BaO

Total

,,,,,

12.92 62.81 1 . 2 6

bid

2.77

0 . 1 1 89.55

0.28

bid

14.67 0 . 0 7 78.10

0.07

bid

1.08

bid

73.80

0.02

bid

0.26

bid

77.54

0.81

0.23

0.05

2.95

bid

82.06

0.49

0.13

bid

0.81

bid

71.32

0.09

1.91

0.51

0.63

0.13

bid

1.61

0 . 1 8 82.05

0.18

1 . 3 3 7.97

68.31 0.37

0.82

0.41

0.18

bid

2.60

0 . 1 3 82.30

0.40 0.09

1 . 5 3 9.90 1 . 1 8 8.39

63.58 66.56

0 . 7 2 0 . 7 3 0.22 0.60 0.36 0.08

0.05 bid

9.31 1.66

0.04 bid

0.46 0.46

86.94 79.38

0.03

1 . 3 6 12.30 60.35

1.17

0.59

0.36

0.05

bid

0.86

0 . 2 8 77.35

0.04

1 . 5 4 12.55 65.16

0.60

0.48

0.31

0.05

bid

0.90

0 . 6 5 82.28

i

i

Figure 13.36 illustrates a Type IV silica glaze found on Iztacc~uatl Volcano, Mexico (Table 13.15). This silica glaze is from a morainal boulder adjacent to the Type III silica glaze illustrated in Figure 13.33. The only difference in the microenvironment from one boulder to the next is in the way water flows. The boulder sampled for Figure 13.33 has a sloping hemispherical top that sheds water efficiently. In contrast, the boulder sampled for Figure 13.36 has the appearance of a slab (Figure 13.37); water would pond in millimeter-scale depressions.

Figure 13.36. Backscattered electron micrograph image of type IV silica glaze formed on top of a glacial boulder from the Milpuco Valley, Iztacc~uatl Volcano, Mexico. The line indicates the location of the electron microprobe transect in Table 13.15. It is 300 microns long

Figure 13.37. Sampling site of Figure 13.36.

Chapter 13

308

Table 13.15. Focused beam electron microprobe transect through Type IV silica glaze formed on a glacial boulder from the Milpuco Valley, Iztacc~uatl Volcano, Mexico. Figure 13.36 shows the location of the transect. The measurements are evenly distributed along the transect. Bid means below the limit of detection. Na20

MgO

A1203 SiO2

P205

SO3 ......K20

CaO TiO2

Fe20 3

BaO

Total "

0.13 0.46 0.31 0.32 0.04 0.31 0.31 1.16 0.71 0.81

0.45 0.41 0.41 0.36 0.25 0.32 0.40 0.70 0.95 0.78

10.05 12.55 4.87 5.65 4.55 4.42 5.25 9.49 5.65 7.73

0.14 0.07 0.16 0.09 bid bid 0.05 bld 0.09 0.09

0.60 ........0.31 0.37 0 . 4 9 0.27 0 . 2 4 0.82 0 . 4 5 0.32 0 . 1 6 0.27 0 . 2 8 0.22 0.23 0.37 0 . 7 5 0.42 0.31 0.50 0 . 3 0

0.62 0.98 0.63 0.64 0.43 0.59 0.69 2.18 0.99 1.20

2.41 2.05 0.54 0.63 0.19 0.42 0.91 2.29 0.71 1.20

0.28 0.19 0.06 0.08 0.22 0.15 0.10 0.04 0.13 0.11

78163 85.70 91.50 88.70 82.91 90.21 90.57 89.00 90.09 89.95

63.26 67.77 83.91 79.54 76.72 83.33 82.24 71.52 80.01 76.95

0.38 0.33 0.10 0.12 0.03 0.12 0.17 0.50 0.12 0.28

Figure 13.38. presents a Type IV silica glaze that has formed in a temperate h u m i d location, in the southeastern United States. A light brown glaze formed on the exposed surface of a pressure release shell. Although abundant detrital particles can be seen through the silica glaze, the electron microprobe transect was placed in a region that had a fairly homogeneous backscatter texture.

Figure 13.38. Backscattered electron micrograph of type IV silica glaze formed on a pressure-release shell of Stone Mountain, Georgia. The underlying crystal is quartz. The line is about 401am long and indicates the location of the microprobe transect in Table 13.16.

Silica Glaze

309

Table 13.16. Focused beam electron microprobe transect through Type IV silica glaze formed on Stone Mountain, Georgia. Figure 13.38 shows the location of the transect. Bid means below the limit of detection. 'Na20 MgO A1203 SiO2 P205 SO3

K20 CaO TiO2 Fe203 BaO Total

0.28

0.22

12.17 70.45

0.18

0.85

1.48

0.36

0.35

1.89

0.30

0.04

0.02

2.53

96.88

0.07

0.10

0.10

0.04

0.03

0.30

bid

100.11

1.54

0.03

5.29

86.43 bid

0.10

0.40

0.27

bid

0.14

0.10

94.30

0.54

0.08

6.73

63.90 0.09

0.35

0.92

0.15

0.37

3.77

0.26

77.16

0.05

0.25

7.48

67.52

0.11

0.62

0.42

0.29

0.25

1.33

0.13

78.45

0.27

0.91

39.28

50.85

0.02

0.22

2.66

0.18

0.07

1.60

0.13

96.19

0.05

0.12

7.80

57.14 bid

0.42

1.51

0.28

0.08

1.44

bid

68.84

0.24

0.20

8.37

51.24 0.14

0.40

0.22

0.27

0.37

2.63

0.13

64.21

88.53

The different Type IV silica glazes presented here all display a similar chemical pattern, as revealed by Figure 13.39. Aluminum oxide typically hovers around 10%, and iron oxide is usually half that concentration. Of course, the abundance of these minor constituents decreases as silica concentrations increase.

Type IV Silica Glaze "~ 100.00 - i A

10.00 -

A A1203

: om

"O

A

0 FcO

1.00"O

..

om

0.10

0 i

50.00

I

I

!

I |

I

60.00

I

I

I

I |

I

I

I

70.00

l l l l 1

I l i

0 0 a

|

80.00 90.00

0 I.I

a I •

100.00

SiO z Figure 13. 39. The chemical characterization of Type IV silica glazes, based on data presented in this section. Values are normalized to 100%.

13.2.2.5. Type V. Iron-rich Silica Glaze Type V silica glazes are where iron is the second major constituent (behind silica). A few of these have been reported in the literature. A sample from a 1800-yr-old Mauna Loa lava flow contains 77% SiO 2, 6.8% FeO and only 2.1% A120 3 (Farr and Adams,

310

Chapter 13

1984). A sample from Japan has a layer of silica glaze that contains approximately 67% SiO2, 8.3% FeO and only 3.8% A120 3 (Matsukura et al., 1994). Iron-stained silica glazes are also noted in Antarctica (Conca and Astor, 1987). In the High Atlas of Morocco "a smooth nut-brown glaze...completely masks the underlying sand grains (Robinson and Williams, 1992, p. 417)." This silica glaze is often found on bedding planes, joint faces, and at the soil/sandstone interface. These are locations that have not been exposed to the subaerial environment. According to energy-dispersive analyses, the primary composition is silica and iron, and the aluminum peak is quite small. Similar coatings can be seen in the sandstones of the Bighorn Basin, Wyoming, in association with petroglyphs. These Type V silica glazes formed in joint crevices, and were later exposed by spalling of the overlying rock. Type V silica glazes are particularly important because they often host petroglyph panels, for example at the Legend Rock petroglyph site, Wyoming. And as the joint faces undergo granular disintegration, the silica glaze erodes along with the host sandstone. Yet because these silica glazes are not rejuvinated by formation in the subaerial environment, when the protective silica glaze erodes, the petroglyphs are also lost. Centimeter-sized pebbles in the desert pavements along the Colorado River, Death Valley, and southern Peru are often coated with very thin silica glazes (e.g. Figure 13.26). Many of these are Type I silica glazes of homogeneous amorphous silica (Table 13.4), but some of the glazes on pavement cobbles contain iron in much greater abundance that aluminum (Table 13.17). Type III iron films have an abundance of clay minerals with iron oxide at concentrations similar to Type V silica glazes. The differencerests is the abundance of aluminum and clay minerals. Type V silica glazes have much lower aluminum concentrations than Type III iron films. Still, it is possible that there might be a continuum between Type V silica glazes and Type III iron films, and this continuum could be obfuscated by a rigid classification. Table 13.17. Focused beam electron microprobe transect of silica glaze formed on the same cobble as Figure 13.26 and Table 13.4, but from an area with more iron. Na20 MgO

AI20 3 SiO2 ....P205 .. ....... SO 3

0'.'09

0.07 .... 2.12

..67.41 ...

0.01

0.02

79.85 0.44

0.76

1.88 ..... 0.20 0.05

i(20

CaO

Ti62

MnO

Fe20 3 Total ...

(if07 0.43

0.17

0.48

16.97 ....89.89 ....

0.01

0.05

0.09

12.59 93.97

0.10

13.2.2.6. Type VI. Alumina Glaze The concentration of aluminum oxides at the earth's surface is unusual, but it does occur. Alumina-rich globules occur as evaporites on grass stems (McFarlane and Bowden, 1992). These evaporites speak strongly to the importance of aluminum mobility in how it is concentrated In Malawi, McFarlane and Bowden (1992) felt that organometallic complexing by microorganisms fosters AI mobility. The formation of supergene bauxite ores is favored by higher acidity, good drainage and lower silica concentrations (Nahon et al., 1992) and in some cases by a source in dust (Brimhall et al., 1988).

Silica Glaze

311

G i v e n these p h e n o m e n a , it should not be surprising that a l u m i n u m oxides coat rock surfaces. H o w e v e r , a l u m i n a glazes have not been noted previously. Yet coatings that look like silica glazes (Figure 13.40) have concentrations of A1203 greater than 50% by weight. T h e s e a l u m i n a glazes are found within a few microns o f silica glazes. Pockets of a l u m i n a glaze also interdigitate with silica glazes (Figure 13.24).

Figure 13.40. Photograph of a joint face on a lava flow of Haleakala Volcano, Hawaii, sampled at about 100 m. Figure 13.8 presents the backscatter image of this silica glaze and Table 13.18. the corresponding chemical analyses. The color of the joint face is red with Munsell color of 10R 5/8.

Table 13.18. Electron microprobe transect through Type VI alumina glaze, formed on a lava flow of Haleakala Volcano, Maui. The diameter of the electron microprobe beam was 10ktm. Figure 13.8 shows the location of the transect. Low probe totals are from porosity, water and organic matter. Na20 MgO A120 3 SiO2

P205

SO3

K20

CaO

bid

TiO2 MnO bid

Fe20 3 BaO

Total

0.07

0.20

59.59 7 . 0 8

1.12

0.14

0 . 0 4 0.65

3.43

bid

72.32

bid

0.03

56.34 5 . 8 8

1 . 1 0 0.07

bid

0.03

0 . 0 3 0 . 0 6 0.42

bid

63.96

bid

0.03

60.14 4 . 9 6

1.08

bid

bid

0.04

0.08

bid

0.35

bid

66.68

bid

bid

57.78 5.50

1.01

0.10

bid

0.06

0.03

bid

0.45

bid

64.93

bid

0.08

53.87 15.75 1.17

0 . 0 7 0 . 0 8 0 . 0 4 0.13

bid

0.93

bid

72.12

1.50

1 . 5 9 40.66 24.54 0.69

bid

0.48

0.08 0.68 0.06

4 . 3 2 0.06

74.66

0.22

20.79 19.76 25.31 0.41

bid

0.10

0.22

1.23

0.13

14.19 0.06

82.42

bid

0.10

8 . 7 6 0 . 0 3 60.95

0.04

10.99 25.55 13.54 0.55

0.08

0.13

1.18

bid

0.98

51.70 4 . 0 4

1 . 1 5 0 . 0 7 0.04

0.08

1.92 0 . 0 9

10.70 bid

70.77

bid

0.18

54.25 6 . 1 4

1.28

0.14 0.92 0.19

4.41

bid

67.61

0.05

5.29

46.44 11.92 0 . 7 6

0.05 0.23 0.14 0.82 0.13

6.42

bid

72.2s

bid

0.25

18.82 1.52 0.64

bid

0.02

0.11 4.62 0.14

13.29 0 . 0 4 39.45

0.07

0 . 4 3 30.16 3.74

0.71

bid

0.13

1.29 3.90 0.12

10.95 0.06

51.56

0.32

0 . 0 3 48.20 6.93

0.89

bid

0.64

0.11

3.87

62.17

0.11

0 . 4 3 41.30 3 . 0 6

0.66

bid

0.22

0.06 5.50 0.18

bid

0.10

1.02

bid

0.16

15.27 bid

66.79

312

Chapter 13

Another example of alumina glaze can be found on a 2000-year-old lava flow of Hualalai Volcano, Hawaii (Figure 13.41 and Figure 13.42). No other elements are found in abundance greater than a couple of percent by weight (Table 13.19). Aluminum has been fractionated from all other inorganic constituents.

Figure 13.41. Photograph of the 2000year-old lava flow of Hualalai Volcano, Hawaii, where the alumina glaze in Figure 13.42 was collected from the ligher colored-surface in the foreground.

Figure 13.42. Backscattered electron image of alumina glaze. The line indicates the location of the electron microprobe measurements reported in Table 13.19. The underlying rock is much brighter because it has more iron.

I have also encountered alumina glaze on cobbles within a geoglyph from northern Chile in the Atacama Desert (Table 13.20). This glaze was quite porous, and this is reflected in the very low probe totals. Yet, aluminum oxide is the dominant constituent. Table 13.19. Focused beam electron microprobe measurements of Type VI alumina glaze, formed on a 2000-year-old lava flow of Hualalai Volcano, Hawaii. The ends of the line in Figure 13.42 are the locations of the probe analyses. Low probe totals are from porosity, water and organic matter. Na20 MgO A1203 SiO2

P205

K20

CaO

TiO2

MnO Fe20 3 BaO

Total

0.61

0 . 2 7 69.95 1.65 0 . 2 5

0.17 0.59 0.05 0.03 0.67

0 . 0 3 74.27

0.55

0 . 2 7 66.81 1.63 0 . 3 2

0 . 1 6 0 . 7 0 0.05

0 . 0 4 71.23

bid

0.70

Table 13.20. Focused beam electron microprobe measurements of Type VI alumina glaze, formed on a geoglyph in northern Chile (P. Clarkson, personal communication, 1995). Low probe totals are mostly from high porosity. Mn and Ba are below the limit of detection. i

i

i

i

Na20

MgO

A1203

SiO2

P205

SO3

K20

CaO

TiO2

Fe20 3

Total

0.09 0.04 0.13 0.24

0.05 0.17 0.18 0.12

25.02 30.59 21.69 33.27

0.53 1.28 6.01 3.32

0.11 0.05 0.02 0.64

0.07 bid 0.10 0.17

0.05 0.12 0.14 0.31

0.06 1.19 0.59 0.13

0.13 0.03 0.18 0.27

0.93 0.41 0.68 0.40

27.04 33.88 29.72 38.87

313

Silica Glaze

Figure 13.43 summarizes chemical data on alumina glazes, where the probe measurements are normalized to 100% by weight. The general order of abundance after A1203 is SiO2, Fe203 and then TiO2. However, iron abundance can sometimes exceed silica.

T y p e VI A l u m i n a Glaze ,~

100.0 -. m

~

:+ "n

e~ s,.

10.0

+ SiO 2

- -

+

i

e m

'& 1.0

&

+-

-

I::! Fe203

a Tio2

- [ == ==

e m ==

O

A A

0.1

|

20

30

40

50

|

60

70

80

90 100

Ai20 3 Figure 13. 43. The chemical characterization of Type VI alumina glazes, based on data presented in Chapter 13. Values are normalized to 100%.

13.2.3. Rates of Formation Silica glazes can form very rapidly, based on observations in Hawai'i where several different types of silica glaze have accreted on historic lava flows (Curtiss et al., 1985; Farr and Adams, 1984). I have collected Hawaiian silica and alumina glazes that formed in less than a year (Figure 13.9), within 25 years (Figure 13.24), and in less than 200 years (Figure 13.41 and Figure 13.42). Silica glazes can form just as rapidly elsewhere. Silica glaze formed since the Maya abandonment of Tikal (Figure 13.20), within 20-40 years on an abandoned slag pile (Figure 13.29), within 15 years on a planter at Arizona State University (Figure 13.30), and along active fluvial (Figure 13.21) and glacial (Figure 13.32)systems. These observations would support the opinion in the archaeological literature that "patina has little value in ascertaining age (Service, 1941, p. 556)." The variable thickness of silica glaze of different ages would also support the view that the "thickness of a patina layer is no guide to the date of the manufacture of a flint artifact (Rottl~inder, 1975, p. 109)." An important uncertainty in the rate of formation is the mechanical and chemical stability of silica glazes. Some Hawaiian silica glazes may be unstable (Curtiss et al., 1985). This issue takes on additional importance, because charcoal and other organic

314

Chapter 13

matter have been extracted from silica glazes for the purposes of radiocarbon dating the exposure of surfaces (Watchman, 1992; Watchman, 1996). I conducted an independent evaluation of the long-term stability of silica glazes, based on samples and data that I gathered from samples collected in the C6a valley in Portugal, from Mauna Kea and Hualalai in Hawaii, and from Iztaccihuatl Volcano in central M6xico. These sites were selected, because they all have independent age control. The C6a valley petroglyphs, northern Portugal, galvanized archaeologists world wide (Bahn, 1995a). Since their existence was made public in November of 1994, the uproar created by the possible destruction of this Upper Paleolithic-style art (Zilh[io, 1995b) has stopped construction of a dam that would have flooded them. Cosmogenic 36C1 data are used to constrain maximum ages for Cfa valley rock art (Phillips et al., 1997). The Hawai'i samples come from two contexts. On the rainshadow side of Hualalai volcano, control radiocarbon ages come from burned plants that were destroyed by lava flows (Moore and Clague, 1991). Silica glaze was sampled from surfaces of Wa'awa'a Ranch flow (3030!-_200 14C yr B.P.) at 975 m and Pu'u Neneakolu flow (6360-~_100 14C yr B.P.) at 762 m (Moore and Clague, 1991). The Older Makanaka till (Porter, 1979) was sampled on the southeastern side of Mauna Kea. The samples from central M6xico were collected from Iztacc~uatl volcano, that was glaciated several times (VAzquez-Selem, 1990; White, 1986). One sample (IZ-95-01) was collected on the southern side, from glacial polish. A second sample (IZ-95-07) was collected on the western side, from a morainal boulder. Organics were extracted from silica glazes in three contexts: underneath the silica glaze in the underlying weathering find; the interface between the silica glaze and the rock; and within the silica glaze (Figure 13.44 and Figure 13.45). The material was removed with a tungsten-carbide needle in the laboratory under 45x magnification. The pretreatment for the extracted organics is concentrated HC1 and HF. The insoluble residues were then sent to Beta Analytic Inc for AMS 14C measurements.

A 50pm .m.~-~" , a ~ , ~ l - " ~ - - " " l m , ~ ' , .. !.. 't1~"

e o

Figure 13.44. Fragments of carbonized wood trapped by silica glaze on a glacial polish of IztaccSuatl Volcano, Mexico. Image A (secondary electrons) shows organic matter (arrow) charging. In contrast, image B shows backscattered electrons and the organic matter (arrow) is dark.

Silica Glaze

Figure 13.45. Fragments of organic matter within weathering rind of an older Makanaka till boulder from Mauna Kea, Hawai'i. The photograph shows the boulder that was sampled. The images show matching secondary (A) and backscauered (B) electron micrographs with arrows showing the location of organics located in the weathering rind.

Figure 13.46. Examples of how silica glaze can erode. A. Backscattered electron image of silica glaze from Wa'awa'a Ranch flow, Hualalai, Hawai'i. Note the many fractures in the silica glaze. This is common, and is probably not from sample preparation, since glue was added to the surface at the collection site. B. Backscattered electron image of silica glaze being eroded by an epilithic organisms (perhaps microcolonial fungi?) that has created an indentation in the silica glaze. The organism is dark, because organic matter has a low atomic number. However, the brighter ribbons are silica glaze formed on top and through the organism. There is evidence that the silica-entombed organics can yield ages older than independent controls. Contamination can be from ancient episodes of organic weathering (Dorn, 1997). Similarly, the older age for the glacial boulder in Table 13.20 (IZ-95-07) may be from organics emplaced during an earlier weathering episode.

315

316

Chapter 13

Table 13.21 compares the radiocarbon ages for the organic matter entombed by silica glaze with independent age controls. In almost every case, the silica-glaze organics are considerably younger than the control age, regardless of the position of the organic matter (intra-glaze, interface, weathering find] or the location of the study site. The simplest explanation for the relative youth the entombed organics is that silica glaze does not form a closed system. This could be explained by mechanical instability of the silica glaze. The many fractures that occur in silica glazes (Figure 13.46A) could aid detachment. Silica glazes are also susceptible to biochemical erosion (Figure 13.46B). In another example, bacteria can create a microenvironment that fosters complexing of silica from mineral surfaces (Hiebert and Bennett, 1992). In summary, it is very difficult to determine rates of formation for silica glazes for a number of reasons. First, silica glazes can form very rapidly, within a few decades; yet, the microenvironmental conditions that can bring about these rapid rates of accretion are not understood. Second, silica glazes appear to be unstable over a time frame of thousands of years in many environments. Third, there are mechanical and biochemical explanations for the instability of silica glazes. Lastly, for the purposes of radiocarbon dating, older organic matter can be trapped by silica glazes, thus ancient radiocarbon ages may be systematically illusory.

Table 13.21. Radiocarbon ages of organic matter entombed by silica glazes are younger than independent chronometric controls, unless the organic matter is inherited. The independent radiocarbon age control for Hualalai Volcano, Hawai'i comes from radiocarbon ages on charcoal (Moore and Clague, 1991); for Mauna Kea, Mexico and Portugal, the age control comes from 36C1 ages (Phillips et al., 1997; Phillips et al., 1996). |

i i i

lllul

silica Glaze Samples

.....Silica Glaze 14C A~e

Lab. No Nllll,ll,ll

Iztacc~uatl S~rfaces Glacial Polish, IZ-95-1 (4100 m) Within the silica glaze Under silica glaze and in weathering find Glacial Bodder, IZ-95-7 (4160 m) Under silica glaze in weathering rind Hawai'i Surfaces Makanaka Till, Mauna Kea, Hawai'i, MK-95-O2-OM (3360 m) Within the silica glaze Under silica glaze in weathering rind Wa'awa'a Ranch flow, Hualalai, Hawai'i At interface of silica glaze and rock Pu'u Neneakolu flow, Hualalai, Hawai'i At interface of silica glaze and rock Exposed Panel Surfaces. Coa. Portugal Canada do Infemo, Panel 14, FC-95-2b Under silica glaze in weathering find At interface of silica glaze and rock Within the silica glaze Panel Penascosa, FC-95-5a Under silica glaze in weathering find Panel at Ribeira de Piscos, FC-95-8a Under silica glaze in weathering find At interface of silica glaze and rock ii

Independent 14C A~e i i,,,,,

Independent 36C1 Age i

,,,

~70,000 111.6_+0.6% Beta-99292 2870+_.50 Beta-99293 -7000 878~

Beta-83909 ~16,000

107.9_~.6% Beta-99290 600+_50 Beta-99291 1540~0

Beta-19903

3030+_200

1740"~_120 Beta-19892

636~_100 16,2(X)']:1500

94(X)'L60 Beta-82451 18,510"2"_80 Beta-87058 4700"~_70 Beta-82450 136,1X)~_70,000 10,990-2-60 Beta-82456 918~ Beta-82463 17,09(~_70 Beta-87059 i,i,ll

99,300+_20,000

Silica Glaze

13.3. Origin 13.3.1. Source of the Silica Silica glazes are external accretions, like all other rock coatings. The electron micrographs presented in this chapter provide clear evidence for a distinct morphological boundary between silica glaze and rock. The constituents do not derive from the dissolution of the rock directly underneath silica glaze, as had been thought (Fisk, 1971; Merrill, 1906). Certainly, dissolved ions and detrital particles can be contributed by rock material eroded from adjacent positions (Schiavon, 1993). Some of the silica may come from dissolution of rock material a few microns away (Paraguassu, 1972; WhaUey, 1978). For example, microorganisms may play a role in the release of silica and aluminum from silicate minerals (Hiebert and Bennett, 1992) aided by alkaline conditions (Robert and Berthelin, 1986). Yet, the distinct morphological boundary indicates that silica glaze is an external accretion. Also, if silica came from the rock directly underneath the silica glazes, there would have to be substantial porosity in the underlying weathering find. The micrographs presented in this chapter do not show evidence of this loss of mass. A variety of possible sources for the silica have been proposed, besides the underlying rock. Phytoliths (Anderson-Gerfaud, 1980; Farr and Adams, 1984; Folk, 1978; Kamminga, 1979), volcanic ash (Curtiss et al., 1985; Farr and Adams, 1984), silica in soil (Farr and Adams, 1984), and silica in dust, precipitation, and ground water seepage (Watchman, 1992) have different advocates based on the different field sites. However, there is no concensus on the origin of the silica or other constituents found in silica glaze.

13.3.2. Abiotic Genesis The most common opinion in the literature is that silica glaze is a product of the chemical precipitation of monosilicic acid (Si(OH)4) perhaps deposited as a gel (Krauskopf, 1956; Williams and Robinson, 1989). Silica may be precipitated through evaporation (Fisk, 1971; Merrill, 1906; Watchman, 1992), or perhaps through complexing with organic matter (Watchman, 1992). Experiments indicate that silica dissolved from the rock can precipitate as amorphous silica (Paraguassu, 1972; Whalley, 1978). In a model based on Hawaiian silica glaze (Curtiss et al., 1985), nonviolent wetting (e.g., gentle rain, fog, dew) dissolves silica and fixes dust particles to the rock surface when the solution evaporates.

13.3.3. Biotic Genesis The possible role of biotic processes in the genesis of silica glazes starts with how silica is introduced into the accretionary system. Silica coatings on sand grains in the Northern Territories, Australia, may have a source in opaline phytoliths, which are dissolved and reprecipitated over sand grains (Farr and Adams, 1984; Folk, 1978). There is a large literature on the role of opaline phytoliths in the generation of use-wear polish on artifacts (Anderson-Gerfaud, 1980; Kamminga, 1979). There is also a thought that aluminum coatings on fiver cobbles may be a product of bacterial activity (Robbins and Hayes, 1997).

317

318

Chapter 13

Regardless of how silica is introduced, through abiotic weathering or biotic sources, there is a known association between the presence of microoganisms and the accumulation of amorphous silica (Schultze-Lam et al., 1995). Casts of microorganisms are found in Antarctic silica glazes (Friedmann and Weed, 1987) and in a wide variety of environmental contexts (Pelras and Le Ribault, 1981). It is possible, however, that these microbes are purely adventitious. Chemical analyses presented in this chapter suggest a strong association between iron and silica in Type III and Type V silica glazes and Type III iron films. In addition, iron occurs in concentrations of a few percent in all silica glazes except Type I. The importance of iron revolves around the extensive literature on co-precipitation of silica and iron by bacteria (Duhig et al., 1992; Fenis et al., 1986; Konhauser and Ferris, 1996; Urrutia and Beveridge, 1994). Iron and aluminum occur together in abundance in a number of silica glazes, especially in Type II and Type III silica glazes. Again, the potential importance is based on deduction, and not on direct evidence of microbial casts in silica glazes, because microbes are associated with the precipitation of silica with both Fe and A1 (Ascaso et al., 1976; Jones et al., 1981; Konhauser et al., 1993; Robert and Berthelin, 1986). In culturing experiments, biotic precipitation tends to favor lower concentrations of silica and higher concentrations of other elements (Urrutia and Beveridge, 1994). Heterogeneity seen in the chemistry of Type II through Type V silica glazes could suggest a role for bacterial concentration. For example, the P and K are found up to a few percent in silica glazes, and this enhancement could be from organic products (Urrutia and Beveridge, 1994). Also, the amorphous character of silica glazes would be consistent with bacterial interactions (Schultze-Lam et al., 1996). Unfortunately, there are not very many in situ observations of microorganisms either on or fossilized within silica glazes. In a sandstone rock shelter near Laura, Queensland, spherical shapes several microns in diameter are interpreted to be bacterial cocci or spore-like bodies. These, along with fungal hyphae would be consistent with the involvement of microorganisms and organic activity in the accretion of the silica glaze (Watchman, 1990c, p. 27). Fossilized organisms are in Antarctic silica glazes (Friedmann and Weexl, 1987). In my research, however, I have encountered only a few examples of microorganisms encapsulated within silica glaze (Figure 13.20). Thus, although the geochemistry of silica glazes may consistent with a microbial genesis, I do not see fossilized evidence that microbial processes are critical to the formation of most silica glazes. I view rock coatings from the perspective of landscape geochemistry. In the case of rock varnish, microbial activity establishes a barrier to the migration of divalent manganese ions by transforming them into the tetravalent phase. In the case of oxalate-rich crusts, lichens manufacture and secrete oxalates. In the case of silica glaze, geochemical conditions favor the formation of silica glazes abiotically. Thus, although microorganisms may certainly play important roles in contributing silica and other trace elements, biological activity is not needed to establish the barrier to silica migration. Thus, I propose a general model of silica glaze genesis.

13.3.4. General Model Previous abiotic hypotheses of the precipitation of silicic acid gels through evaporation makes sense for Type I silica glazes (Curtiss et al., 1985; Watchman,

Silica Glaze

1992; Williams and Robinson, 1989). The uniform chemistry is consistent with to precipitation from solution. Additional processes, however, are probably involved where aluminum is abundant in Type II through Type VI silica glazes. I propose a model of formation for Type II-VI silica glazes that involves soluble aluminum silicate complexes and the nature of wetting fronts. Soluble AI-Si complexes [AI(OSi(OH)3) 2+] are ubiquitous and are critical in the formation of new metastable minerals at the water-rock interface (Browne and Driscoll, 1992; Lou and Huang, 1988). These may be released first from the weathering of phyllosilicate minerals (Robert and Tessier, 1992, p. 86,78). Soluble A1-Si complexes should occur under the geochemical conditions found where silica glazes occur. A key in the stabilization of soluble AI-Si complexes might be very gentle wetting (e.g., dew deposition), when the silica glaze starts to form. Curtiss et al. (1985) observed that silica glazes in Hawai'i could be disturbed by violent wetting events, for example heavy rains. Rupturing of metastable films on silica surfaces could disturb an incipient glaze. The transition between complete and partial wetting on silica surfaces rests at about 5070 nm (Zorin et al., 1992). When this transition is crossed, the metastable wetting film on the silica surface is ruptured (Zorin et al., 1992). The notion that silica surfaces are unstable is enhanced by new research showing that silicic acid surfaces are not molecularly smooth and can expand or collapse by changes in solutions (Israelachvili and Wennerstr0m, 1996). Violent wetting (intense rain, runoff) on silica surfaces would cross this wetting threshold and disturb an incipient bond between glaze and rock. In contrast, if the wetting is 'gentle' (e.g., dew or short duration, low intensity rain), a bond between the rock and soluble A1-Si could occur, which initiates glaze formation. Once the coating is well established, silicic acid and soluble A1-Si complexes could more readily bond to the silica glaze than the rock. "Silicon need have no residence time in solution as silicic acid before it is incorporated into a solid reaction product at the surface of a mineral (Casey et al., 1993, p. 255)." Other elements can be incorporated through additional processes. For example, iron can be trapped as AI-Fe (lid hydroxy species that are known to play a role in cementing pedogenic amorphous silica (Taylor et al., 1990). In addition, iron oxides can adhere very strongly to silica surfaces through the formation of Fe-O-Si bonds (Scheidegger et al., 1993). Calcium is found in varying abundance in silica glazes. Concentrations are usually below 1% CaO (Tables 13.1,2,3,4,5,9,11,14,15,16,17). In some cases, concentrations exceed 1% CaO (Tables 13.6,7,8,19,13). The calcium that does occur in silica glazes may aid in the coagulation of nanometer-scale silica (Her, 1975). The phosphates found in silica glazes are positively correlated with both aluminum and iron (Figure 13.47 and Figure 13.48). This makes sense, since phosphates are absorbed by aluminum and iron (Parfitt and Atldnson, 1976; Rajan, 1975). The rate of adsorption of both organic and inorganic phosphates is dependent upon pH. In laboratory experiments, as pH increases the amount of phosphate adsorption decreases for both iron and aluminum precipitates (Shang et al., 1992). This is consistent with an increase in aluminum at lower pH values.

319

320

Chapter 13

y---O.008x + 0.43, r=.302, p<0.001 2.5 t

o

2 P~

o

S 1.5

o

1

oO

o

o

o

0.5 II

-r-

0

20

40

60

80

100

A120 3 Figure 13.47. Scattergram of the concentrations of P and AI in the silica glaze dataset in this chapter The concentration of phosphorus is positively correlated with the abundance of aluminum. The relationship is especially strong where aluminum oxide concentrations exceed 20% by weight..

3

y= 0.02 Ix + 0.48; r=.283; p<0.001

2.5 ~ 2

n n

~tm 1.5

~

0

n

0

0

0

1 0.5 0 0

10

20

30

40

50

FeO Figure 13.48. Scattergrarn of the concentrations of P and Fe in the silica glaze dataset in this chapter. The concentration of phosphorus is positively correlated with the abundance of iron.

In summary, the wide range of contexts and chemistries associated with silica glaze are best explained by multiple processes. Where silicic acid is in abundance and conditions are fight for evaporation, Type I silica glazes form. Where dust is abundant, delrital-rich Type II silica glazes accrete. Yet for the rest of the silica glazes (including Type II), aluminum is ubiquitous. This must be explained, and it is logical that aluminum is directly involved in the genesis of most of the silica glazes. Thus, the proposed model involving the accretion

Silica Glaze

of soluble aluminum silicate complexes provides a reasonable explanation for the occurrence of silica-alumina glazes. Iron is the third most important constituent. Its occurrence may be related to microbial processes (Duhig et al., 1992; Ferris et al., 1986; Konhauser and Ferris, 1996; Urrutia and Beveridge, 1994), to the role of AI-Fe (III) hydroxy species in cementing silica (Taylor et al., 1990) or to the formation of Fe-O-Si bonds (Scheidegger et al., 1993). The other minor and trace elements found in silica glaze may be tied to the Si, AI and Fe. There may also be adventitious processes co-occurring on rock surfaces to explain the variable abundance of these minor and trace elements, for example detrital inputs or microbial activity. 13.4. Speculation on Silica Glaze on Mars

Earth-based (McCord et al., 1982; Singer, 1980; Singer and Roush, 1983) and Marsbased (Guinness et al., 1996; Guinness et al., 1997; Levin et al., 1978) observations have indicated that there are rock coatings on Mars. For over a quarter century scientists have postulated that "desert varnish" may be an important feature on the surface of Mars (EI-Baz and Prestel, 1980; Greeley, 1987; King, 1988; Marshall, 1962; Strickland, 1979). Mars has a red color from ferric oxides (Morris et al., 1993), that are likely in the dust (Levin et al., 1978) and perhaps on rock surfaces (Arvidson et al., 1983; Sharp and Malin, 1984). Manipulation of rocks by the Viking lander arm, however, failed to scrape rock surfaces (Arvidson et al., 1989), indicating that coatings are firmly cemented to the rock surface and are not like the weaker dust films on the earth. The Martian rover Sojourner found that the rocks on the surface of Mars may have four different types of rock coatings (JPL, 1997). Wind-blown dust occurs in rocksurface depressions. Soil-like deposits form thin layers with X-ray spectra that cannot be separated from rock signals. Pinkish or whitish crusts are in places where there iron-mineral spectra are minimized. The rock named 'Ginger' also displayed a crust of ferric minerals. Although there is visual evidence of rock coatings, the chemistry is ambiguous, due to the 1-2 cm spatial resolution of the X-ray sampling. Heterogeneities in thin rock coatings would certainly be lost in the 1-2 cm sampling grid. Given prior insights, I argue that silica glazes will probably be found on Mars. I start with the likelihood that thin films of liquid water may have been present on or near the surface of Mars for periods. Today, the atmospheric pressure on the surface of Mars is too low for liquid water on the surface. In earlier times, however, transient liquid water could have occurred on rock surfaces, perhaps allowing a microbial community to grow (McKay et al., 1992). Regardless of the presence of epilithic or endolithic organisms, water is necessary for the mobilization and precipitation of iron films and other constituents (e.g. silica glaze). The requisite conditions for the formation of silica glaze films could have been met during the period of transient liquid water. A global ferric duricrust on Mars may provide some of the raw material for the dust that may also be a rock coating. A surface crust would require a layer of water that would likely derive from ground water fluctuations (Jakosky and Christensen, 1986). For example, sulfate-bearing acidic groundwater mobilizes silica from host minerals (Bums, 1993). There may also be phyllosilicate phases (Burns, 1986). As acidic groundwater reached the surface, it would sublimate. The silica that would precipitate out could persist on the arid surface of Mars (Burns, 1988), to deflate in the active dust

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transport system (Christensen, 1986) and come to rest on Martian boulders (JPL, 1997; Sharp and Malin, 1984). A much more speculative type of silica glaze may involve a combination of silica crusts and ferric minerals that could have involved the biological activity (Imshenetskii et al., 1978) of organisms that are now being called "extremophiles" (Madigan and Marrs, 1997). A considerable literature exists on the survival strategies of organisms that are exposed to extreme desiccation and high radiation levels (Dose, 1994; Mattimore and Battista, 1996; Ward et al., 1995). Based on studies of the Earth's earliest biosphere, evidence of biological activity should include inorganic mineral deposits attributable to biomineralization (Schopf et al., 1984). For example, Mn-concentrating budding bacteria found on rock varnish look very much like like Metallogenium (Bolotina, 1976; Crerar et al., 1980), and these are morphologically similar to the preCambrian microfossil Eoastrian found in association with Mn-deposits early in the earth's history (Schopf et al., 1984). It is this sort of life form, possible chemolithotrophic and adapted to environments with long periods of desiccation, that might be found lithified within Martian rock coatings. Certainly, the fossils preserved within Antarctic silica glazes (Friedmann and Weed, 1987) and iron films offer hope for the fossilization of life on Mars.

323

Section 3.

Synthesis

"Not only is there interaction between cycles, but also between systems. It is unrealistic to consider terrestrial systems or cycles in isolation from adjacent aquatic and atmosphere systems and cycles." G.E. Likens and others, Biogeochemistry of a forested ecosystem, Springer-Vedag, 1977

Clean, bare rock surfaces are rarely seen at the earth's surface. Instead, they are almost always partially masked by rock coatings. Curiosity about the nature, origin, and utility of these encrustations has generated over a thousand of scientific papers in a wide variety of disciplines. Even as there has been a burgeoning interdisciplinary interest in rock coatings, the literature is best characterized as a series of isolated case studies, and I believe that the lack of synthesis and generalization has hindered advances in research. This third section is an attempt to bring together knowledge about rock coatings with systematic and geographical themes. Chapter 14 presents one conceptual model on how to interpret and explain the development of rock coatings. Chapter 15 utilizes that model to analyze the geography of rock coatings. I have no illusion that the theory developed in this section will withstand the test of time. However, an important part of natural science is the development of generalizations to help interpret the complexities of the natural world. Thus, I offer these two chapters as an attempt to aid in the maturation of a new interdisciplinary field. For without the formation of a general body of theory, the infant field of the study of rock coatings is nothing more than a potpourri of isolated scientific knowledge.

324

Chapter 14 HIERARCHICAL DEVELOPMENT

MODEL

OF

ROCK

COATING

Only through the capacity to see all relevant factors, to weigh them fairly, and to place them in relation to each other, can we hope to reach an accurately balanced judement. Sir Basil H. Liddell Hart (1971)

14.1 Introduction There are no explanations of why one rock coating develops in preference to another type, or why a rock coating may be lacking in one place and grows in another. The purpose of this chapter is to start the process of filling this gap through the presentation of a general model of rock coating development, evolved through a combination of systems theory and landscape geochemistry. I also hope that this hierarchical model may useful as an interpretative tool to understand geography, because I believe that the surface of a rock should be able to inform the observer about environmental conditions at that place. Models are artificial constructs to simplify reality. My artificial perspective is that competition among different processes controls the development of coatings on rock surfaces. Thus, not all processes are of equal importance. There is a hierarchy, with certain key liriaiting processes that constrain the influence of less important controls. Consider, for the sake of introducing the approach, just three basic processes involved in the development of rock coatings: transportation of coating constituents to rock surfaces; biogeochemical barriers that stop the movement of constituents; and erosion of the rock surface itself. The transportation of the components of rock coatings may be as simple as a person applying spray paint to a boulder. It may involve eolian processes, for example bringing dust to a rock face to form dust films or contribute iron to iron films. Water is an ubiquitous transportation agent for both particulate and dissolved material. For example, waters transports ingredients over cliff faces in the Colorado Plateau to produce spectacular streaks of mixed rock coatings (Figure 14.1). Once the raw ingredients come in contact with a rock surface, there must be a barrier to their continued flow. In the case of a lithobiontic coating, it may be a condition that promotes growth of the organism. In the case of a largely inorganic coating like silica glaze or iron films, the biological or geochemical conditions must promote deposition. In the case of Figure 14.2, rock varnish cannot grow because the local conditions are too acidic for the manganese and iron to remain in place Even if the raw material is transported and fixed on a rock surface, a rock coating will not occur where erosion rates are higher than the rate of coating formation. Rock coatings rarely occur on shale, for example, because of its instability. Grussified

Hierarchical Model

granodiorite boulders and weak sandstone rock faces lack coatings where granular decay exceeds the rate of coating formation (Figure 14.3).

Figure 14.1 Dark streaks of rock varnish, oxalate-rich crests and silica glaze move down a cliff face at Canyon de CheUy in northem Arizona. The transporting agent for these mixed coatings is clearly water. However, the movement of water and the development of the coating is episodic. The arrows (approximately five meters wide) identify rock spaUs that post-date the growth of the rock coatings.

Figure 14.2. The weathering of pyrite crystals produce sulfuric acid, which in turn dissolves manganese and iron in the surrounding rock vamish. The width of the image is about 7 centimeters.

Figure 14.3. This sandstone butte in northem Arizona displays iron films that formed within rock crevices (arrows). Upon exposure to the surface, the sandstone undergoes granular decay, thus eroding the coating

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These three processes are not of equal importance, implying that there is an implicit hierarchy of controls. Despite an adequate supply of constituents and geochemical stability, erosion of the rock face in Figure 14.1 at the arrows removed the rock coatings. In Figure 14.2, the geochemical conditions were too acidic for the coating to even accrete. In Figure 14.3, the mechanical stability of the rock was too weak to withstand erosional stresses, and the rock coating eroded along with the host substrate. The next section explores the hierarchy of different processes that influence the development of rock coatings.

14.2 Landscape Geochemical Hierarchy of Controls 14.2.1. First-Order Processes: Geomorphic Controls

14.2.1.1. Exposure of Bare Rock In his monograph on the Henry Mountains of Utah, G.K. discerned two fundamental types of land surfaces where hillslope processes fed weathered debris into river systems: soil-covered slopes and bare-rock slopes (Gilbert, 1877). The phrase transport-limited landscape describes places where soil covers bedrock. The fact that you see a soil cover implies that the rate of surface erosion (transportation) is the limiting factor in landscape change. Weathering of rock material is faster than its transportation, so loose particles build up in place and soil forms. Transport-limited landscapes yield very few rock coatings that can be seen at the surface of the earth. In order to expose rock surfaces, the processes have to be different from the ones that covered the slopes in soil in the first place. Rocks are certainly exposed in stream beds. Yet, most soil-covered landscapes consist of slopes, not river beds. The only slopes in transported-limited landscapes where rock coatings occur are locations of mass movement or anthropogenic erosion. This could be a landslide, bringing rocks to the surface. Another example would be tree throw bringing rocks to the surface in roots. Yet another example of rock exposure would be a rail or road cut. The second type of landscape identified by Gilbert is where the rate of landscape erosion is limited by the rate of rock weathering. In other words, as fast as particles are weathered out of a rock, they are transported. This is the classic bare-rock desert landscape (Figure 14.1 or Figure 14.3). There many be pockets of soil development, but these are often locations that are protected by particularly resistant rocks or vegetation. Thus, the geographical focus of much of this book has been on weathering-limited landscapes. There are many other geomorphic processes on the earth that expose bare rock can be exposed. Glacial processes leave behind abundant exposures of bare rock, often in conjunction with landsliding (Figure 14.4). Periglacial environments are dominated by frost or cryogenic processes that expose bare rock (Figure 14.5). Coastal erosion exposes rocks in the zone of wave action, and a promotes mass wasting of cliff faces. In summary, the first control on whether rock coatings develop is exposure by geomorphic processes. If soils cover a rock face, then this book becomes a moot point. In order to see rock coatings, bare rock must be exposed by either natural or anthropogenic processes.

Hierarchical Model

Figure 14.4. This face of schist in the Khumbu region of Nepal hosts oxalate-rich crusts, lithobiontic coatings (fungi, lichens, algae), iron films and rock varnish. The face was exposed by a combination of glacial processes undermining the slope and mass wasting.

Figure 14.5. This outcrop of granodiorite in Yosemite National Park was not exposed by glaciation, but through a combination of mass wasting and frost weathering. Note the dark rock coatings (mostly fungal lithobionts) down slope from the tree lithobiont.

14.2.1.2. Stability of Rock Surfaces The stability of a rock surface is another key geomorphic control. Rock coatings are only seen if rock erosion proceeds at rate slower than a rock-coating accretion. At first glance, this issue may not be visually obvious; consider Figure 14.3. The surface is undergoing constant erosion, but rock comings are still visible because they form in rock joints in the subsurface. The aesthetics of natural and anthropogenic landscapes such the Nasca lines in Peru (Figure 14.6) would be very different if rock surfaces were too unstable to support the formation of rock comings. Consider Figure 14.7, the Roman Theater in Petra, Jordan. Although rock surfaces appear well preserved after two millennia, they are still eroding too fast to preserve most rock coatings (Paradise, 1993b; Paradise, 1995). In summary, geomorphic processes create first-order controls on the development of rock comings. The issues in this section are summarized in Figure 14.8. Of course, this diagram is only valid for natural geomorphic systems. Human activity plays a critical role in the exposure of bare rock, especially in wetter, transport-limited settings. We do this through a variety of mechanisms including mining, road construction, building construction and the enhancement of erosion by encouraging the overgrazing of animals. In essence, human activity tends to push natural systems towards weathering-limited landscapes through the general set of processes usually grouped together as desertification.

327

Chapter 14

328

~J

2~

s

?:il

Figure 14.6. The Nasca lines of Peru are made by the removal of cobbles darkened by rock varnish, thus exposing the underlying lighter-colored soil. The geoglyphs are visible because the natural cobbles erode slower than the rate at which rock varnish accretes.

....... .......,, ..~...:..,~.i~:-:

Figure 14.7. The Roman Theater in Petra, Jordan (Paradise, 1995), where erosion is too fast for the formation of rock coatings other than oxalate-rich crusts in some places (Paradise, 1993b). Photograph is courtesy of Tom Paradise.

Fli~t Order l~ue: Expos t i e of flodcs on the Land s urface

:~-~'~

" Landscapes -dominated by hillslope I and fiuvlal p r o c e s s e s .... J

(~- -L~ndscapesI dominated by l Iother geomorphicj I processes L

T ran sport-L imi ted Landscape Extensive Soil Cover Rocks Exposed in Streams and by Mass Wasting

Landscape

I

Minimal Soil Cover ] Rock Surfaces Exposed I in Many Places I

........

Expose Abundant

8at 0?k

Processesll xpos I Expose l JRock Alon~ Bar R0ck J

Figure 14.8. Diagram illustrating first-order issues in coating development of whether rock surfaces are exposed at the surface.

Hierarchical Model

14.2.1.3. The Role of Rock Type Some rock coatings do appear to be greatly influenced by lithology. Lithobiontic communities, for example, sometimes have discrete preferences (Gehrmann and Krumbein, 1994). The style of case hardening in sandstone and core softening in crystalline rocks is similarly linked to rock type (Conca, 1985; Conca and Rossman, 1982; Conca and Rossman, 1985). Gypsum crusts in urban environments are produced by an interaction of air pollution and the host carbonate rock (Dolske, 1995; Schiavon et al., 1995). The type of oxalate mineral in oxalate-rich crusts depends on the dominant cation in the host rock (Del Monte et al., 1987a; Purvis, 1984; Whitney and Arnott, 1987; Wilson and Bayliss, 1987). Many other rock coatings appear to be independent on the influence of lithology. Several lithobiontic crusts, carbonate crusts, dust films, heavy metal skins, iron films, rock varnishes and silica glaze are not tied to any particular rock type. The largest literature on this potentially contentious topic comes from rock varnish. Most researchers agree that "desert varnish" forms on virtually every lithology, every type of rocky landform in arid regions, and on rock surfaces with little vegetative cover in more humid settings (Blackwelder, 1948; Blackwelder, 1954; Cooke and Warren, 1973; Engel, 1957; Engel and Sharp, 1958; Evenari et al., 1971; HOllerman, 1963; Hunt, 1954; Klute and Krasser, 1940; Tricart and Cailleaux, 1964; Washburn, 1969b; Wilhelmy, 1964). Although many have observed a lack of varnish on limestone (Blackwelder, 1948; Cooke and Warren, 1973; Engel, 1957; Engel and Sharp, 1958; Evenari et al., 1971; Hooke et al., 1969; Hunt, 1954; Scheffer et al., 1963), some noted its occurrence (Iskander, 1952; Karlov, 1961), as others had before (Lucas, 1905) and later (Perry and Adams, 1978). I contend that while many rock coatings may be independent of lithology, rock type can still impose an influence. In the case of rock varnish, for example, rock type influences the rate of accretion (Table 10.9). Many have long recognized that vein quartz was not as well varnished as adjacent silicate minerals (Engel and Sharp, 1958; Hem, 1964), and that microcracks and tiny crevices in quartz grains develop varnish (Engel and Sharp, 1958; Hooke et al., 1969; Hunt, 1954; Scheffer et al., 1963). Engel (1957) argued that this was because smooth quartz surfaces are not as favorably for "physical attachment of varnish." Thus, the effects of substrate, even if subtle, can be felt on most rock coatings.

14.2.2. Second-Order Processes: Inheritance from a Subsurface Position The previous section boils down to a simple point. We see rock coatings only after a rock face is exposed. In brief, first-order geomorphic controls determine whether or not rock faces are available for coating at the earth's surface. In this section the issue is what happens to rock coatings, that originated in the subsurface, all after they are exposed at the earth's surface by erosion. The bottom line is that it is possible for rock coatings to exist in places where erosion rates exceed the rate of coating accretion. This occurs where coatings are "inherited" from formation in the subsurface. For example, silica glazes in the Atlas Mountains in the subsurface under a soil cover, but they gradually erode with the host rock after subaerial exposure (Robinson and Williams, 1992). Similar examples have been presented throughout in this book.

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What happens to the rock coating and the underlying rock face that is inherited from a subsurface position? There are six general possibilities. The first possibility is that there is a rapid rate of rock-face erosion. If there are no rock coatings within crevices, rock faces are free of rock coatings. If there are inherited rock coatings, slab failure may be fast enough to re-expose another crevice coating before any change occurs. This maintains an equilibrium of appearance. Ayers Rock in Australia is an example of maintenance through re-exposure of the subsurface coating. Figure 14.9 illustrates Ayers Rock in Australia; its overall orange color is due largely to the exposure of iron films as the erodes by the spalling of plates (see also Figure 9.39) (Dora and Dragovich, 1990).

"~,

~..,

~'..~

Figure 14.9. Ayers Rock in Australia. While rock varnish is the type of rock coating that grows on subaerial exposures, this monolith is not dark black in color. It owes its orange color to the exposure of iron films that originated within rock crevices.

A second possibility is where erosion occurs by both slab failure and granular disintegration of the surface. Consider Figure 14.3; iron films that developed within joints are exposed at the surface, and then granular decay of the sandstone results in an erosion of the host rock and overlying coating. Granitic rocks are frequently weathered in the subsurface prior to exposure; this leads to the grussification of boulders at the surface (Twidale, 1982). In many cases, granitic rocks will spall along planar joints. This results in the exposure of rock coatings, that gradually erode when the surface breaks apart, groin-by-groin (Figure 14.10).

Figure 14.10. This joint face at Joshua Tree National Monument, California, developed rock coatings of silica glaze and iron films within the rock crevice, along with case hardening by the accumulation of silica glaze. Upon exposure, granular disintegration of the granodiorite is eroding the joint and the rock coatings exposing the lighter-colored felsic minerals. The image is about 4 meters across

A third possibility for rock coatings inherited from the surface is the formation of lithobiontic coatings. Lichens, fungi, algae, moss and higher plants are often found growing over inorganic coatings inherited from a subsurface position, especially in more humid environments. Figure 14.11, for example, illustrates ferns and moss growing over a silica glaze and iron films. In this case, occasional discharge from a side canyon to the Colorado River permits the growth of ferns and moss.

Hierarchical Model

A fourth possibility is where the subsurface coating is replaced by a completely different type of inorganic coating. An example is the "brown glaze" in Morocco that originates on sandstone in the subsurface; upon exposure, it erodes. Then, over a long time, the rock face is recoated with rock varnish (Robinson and Williams, 1992).

Figure 14.11. This photograph of a side canyon to the Colorado River in the Grand Canyon shows ferns (black) and moss (dark gray) growing over iron films (medium gray) and silica glazes 0.ight gray). Even though water flows over this face only during the largest discharges, inorganic rock coatings are unable to out compete the lithobionts. The photograph is about 5 meters across.

A further example of replacement can be found in northern Portugal. Rock varnish forms coatings on schist joints in the subsurface (Figure 14.12). Upon exposure to the atmosphere, typically by landsliding along the Cba River, the rock varnish dissolves. It is replaced by silica glaze and lithobiont coatings (Figure 14.13).

Figure 14.12. Rock vamish grows in the joint faces of weathered schist in northem Poaugal, along the C6a River. This photograph came from a road cut. The dark coating below the watch is rock vamish

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Another example of replacement by a different rock coating would be carbonate crusts that are exposed in a desert when a rock face spalls (Figure 5.18); over time, the carbonate crust dissolves and rock varnish will grow on the surface. This same sequence can also occur in the subsurface before exposure. Laminar calcrete forms within rock fractures; then, erosion of the overlying rock brings the calcrete close enough to the surface to be subject to downward percolating water. The water dissolves the calcrete, allowing iron films and rock varnish to grow within the rock fracture all before exposure at the surface (Villa et al., 1995).

Figure 14.13. A joint face along the Cta River in northem Portugal that is coated by silica glaze and lithobionts (lichens, fungi). The joint surface is also carved by petroglyphs,identified by the hand, that are probably Paleolithic in age (Zilh~o, 1995a). The previous four cases illustrate rock coatings that are out-of-equilibrium with the surface environment. In the first two pathways, the resistance of the host rock to erosion cannot support a stable rock coating. In the second two cases, biological or chemical processes were responsible for the erosion of the subsurface coating and its replacement with a coating more in equilibrium with the subaerial environment. The last two possibilities, in contrast, are what happens when the subsurface coating is stable in the subaerial environment. The fifth case is where a different type of coating is deposited on top of the inherited coating. The interdigitation of different types of rock coatings is due, in some cases, to the subaefial exposure of a rock surface that was formerly unexposed, or visa versa (Dragovich, 1984). For example, the sandstone joints in Figure 14.14 started with an inherited coating of silica glaze. Upon exposure, lithobionts, oxalate-rich crusts, rock varnishes and additional silica glazes all formed over the joint coating. Other examples have already been discussed in earlier chapters. Figure 10.23 illustrates a Type III iron film that originated in the subsurface; with time, the fracture coating will form a subaerial rock varnish. In the Cta River Valley in northern Portugal, silica glazes originating in rock fractures can in turn be recoated by a different variant of silica glaze (Figure 9.21). The sixth possibility is where the same type of rock coming continues to accrete after subaerial exposure. This is the case for silica glaze in Antarctica (Conca, 1985), and also iron films that had penetrated fractures in K~kevagge, Northern Scandinavia

Hierarchical Model

(Dixon et al., 1995) (Figure 9.2 and 9.12). The Type III iron films that originate in rock crevices in deserts can continue to accrete if the environment is alkaline and dry enough. In summary, second-order processes control what happens to rock coatings that are exposed by erosion. These rock coatings form in the subsurface and are brought to the surface by first-order processes such as landscape erosion. The different pathways of an inherited rock coating are summarized in Figure 14.15.

e-exposure of same type of~ coating by more spalling J Granular erosion of the~l ewly exposed coatings.,)

Second Order Processes: Expos tie of SUbsurface

Coatings by Erosion

'Growth of "~ ithobionts I n coatings~

I

rosion of subsurface coating1 and growth of different subaerial coating Preservation of subsurface~ coating and depositionofI different coating on top .J

reservationof subsurface1 coating and deposition of same coating on top Figure 14.15. Diagram illustrating the second-order processes in coating development. This diagram illustrates what can happen to rock coatings that start in the subsurface (in the soil, in rock joints) and are then exposed by erosion.

14.2.3. Third-Order Processes: Habitability for Lithobionts Lithobionts are organisms that grow on rock surfaces (Golubic et al., 1981). As long as the physical limitations of life are met, the rates at which lithobiontic coatings grow almost always exceeds rock coatings that are largely inorganic. "Transfer of the rocks and minerals of the lithosphere through time and space is considerably speeded up and directed by biological processes on all scales and through all the five kingdoms of the living natural bodies (biota) (Krumbein and Dyer, 1985, p. 158)."

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There are many factors that limit lithobionts. Consider hypolithic organisms found on the underside of rocks to reduce temperature, light and moisture stresses. There may be geochemical limits to growth, such as an overabundance of manganese, iron, arsenic. Other pressures may come from biotic factors, such as competition from other lithobionts, predation by snail grazing, or enough organic matter for heterotrophic organisms. Where physical, geochemical and biotic factors do not prevent the growth of lithobiontic organisms, they become the dominant rock coating. The rate of growth of moss (Figure 14.11) and higher plants is much faster than the rate of growth of silica glaze, which is one of the most rapidly-forming inorganic rock coatings. Lichens typically grow fast enough to erode previous rock varnish and the host weathering rind (Figure 14.16). Even the relatively slow-growing microcolonial fungi can dominate the surface of a desert rock (Dragovich, 1993a).

Figure 14.16. Basalt flow in the Coso Volcanic Field, where epilithic lichens erode the host rock and dissolve rock vamish. The dark black areas are rock varnish (facing away from the camera). The large patches of gray are places where the host rock has spalled off. The brightest spots are lichens. The boulder is about 1.5 meters across.

Inorganic coatings are only able to compete effectively with lithobionts certain settings, for example, where they are able to form coatings within years on fiver cobbles (Cerling and Turner, 1982). There are other places where rapidly-forming coatings grow within a few decades and out-compete lithobionts (Curtiss et al., 1985; Dorn and Meek, 1995; Klute and Krasser, 1940). As a general rule, however, inorganic coatings lose the battle for posession of rock surfaces, at least where biogeographic constraints allow for their growth.

14.2.4. Fourth-Order Processes: Transport Pathways First-order processes expose rock surfaces through erosion. Second-order processes control what happens to the rock surfaces once they are exposed and what happens to rock coatings that are "inherited" from the subsurface by erosion. Third-order processes influence whether or not lithobiontic coatings grow. Fourth-order and fifth-order processes come into play only if rock faces are exposed by erosion, if the exposed rock surfaces are stable enough to support the growth of rock coatings, and if lithobionts do not out compete other coatings. Fourth-order processes revolve around issues of the supply of raw ingredients. Rock coatings grow because there is a positive mass balance at specific places on rock surfaces. Material must be transported to the site of accretion.

Hierarchical Model

These are different from fifth-order processes that concern the biogeochemical stability of the rock-surface environment. For rock coatings to form, the coating constituents must remain at the site of accretion. The supply of raw materials is presented first, as a fourth-order process, simply because without the right raw ingredients, there would not be a rock c o a t i n g - even if biogeochemical conditions were favorable for fixation. Transportation of raw ingredients involves two dependent steps. The constituents must be available, and they must be transported to the rock face. Phosphate skins produced from guano (Figure 11..) or potassium nitrate coatings in caves (Figure 11.10) are both natural examples where growth is limited by supply. The release of copper from tailings would be an example where anthropogenic activities control the supply of raw materials, even though natural transportation pathways bring the copper to the host rock surface (Figure 8.21 and Figure 8.22). The supply of clay minerals is the key difference between manganese heavy-metal skins (Chapter 8) and rock varnish (Chapter 10). There does not appear to be a significant difference in the availability of manganese in one place or another. Rather, where clay minerals are abundant, rock varnish forms. One of the reasons why rock varnish is so common in deserts, as opposed to manganese heavy-metal skins, is because of the abundance of dust in deserts (Pye, 1987; Pye, 1989). Places where the rate of manganese accretion is faster than the deposition of clay minerals foster heavymetal skins. Oxalate-rich crusts (Chapter 12) are another example of the importance of fourth-order processes. Oxalates are usually limited to locations near lichens and other oxalateproducing organic agencies. In addition, transportation pathways also limit the growth of oxalate-rich crusts. Oxalates are found down slope from lichens, not upslope, because water flow transports the oxalates. The importance of transportation is also highlighted where a transporting agency is in excess. Rock coatings composed of nitrate, halite, gypsum, and most carbonates are only seen at the surface when conditions are sufficiently arid (Chapter 11). Once precipitation falls on a regular, annual basis, these coatings dissolve and their constituents are transported into the soil. Thus, the lack of a transporting agency is the key to the formation of extremely soluble rock coatings.

14.2.5. Fifth-Order Processes: B iogeochemical Barriers Assuming that raw ingredients are transported to rock surfaces, rock coatings will only form if there are barriers to their continued migration. These fifth-order fixation processes may be biological, geochemical, or both. One way to understand the evolution of ideas on biogeochemical barriers is to turn to the literature on soil formation. Each soil forms differently. Each soil tells a unique story, but the field of soil development was unified by the Russian school of soil science in the nineteen century (Dokuchaev, 1879). Instead of treating each soil rightfully as an individual case, soil development in general is a function of climate, organisms, topography, parent material, time, and other factors (Amundson et al., 1994; Jenny, 1941). This unified theory has been important in promoting research on soil genesis. The stability of any single rock coating can certainly be interpreted on case-by-case basis. In any given case, the accretion may be a result of complex processes that involve different rock types, different types of rock coatings, modification by different

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biotic agents, modification by different erosion agents, as well as in situ rock weathering and a host of other factors. Consider an iron film that forms on stones in an acid drainage. This iron film may seem unrelated to iron films on subaerial surfaces in deserts, yet they both represent locations where biogeochemical barriers form to further movement of iron. The Type I silica glaze on basalt flows in Hawai'i may seem a far cry from silica glazes on Antarctic sandstones, but their remarkable chemical and microscopic similarity cries out for some attempt at a generalized explanation. Landscape geochemistry, partly born of the Russian school of soil science (see Chapter 2), offers a theoretical basis for interpreting the biogeochemical barriers that fix the constituents of rock coatings in one place. In general, barriers to movement of chemicals on rock surfaces may be a function of the following factors barriers -- f (1, j, ic, oc, w, p, wb, pH, Eh, ai, cs, t, i, s, v, d, pdm, mc .... )

(1)

where the conditions of 1 (lithology) and j (jointing) are acted upon by processes related to ic (inorganic coatings), oc (organic coatings), w (weathering processes), p (position, whether subaerial, crevice, exposed to soil), wb (water balance), pH, Eh, ai (input of airborne material), cs (composition of solutions exposed to rock), t (exposure time), i (insolation), s (slope, including role as a dust trap), v (adjacent vegetation), d (natural disturbances that do not erode), pdm (post-depositional modification), mc (microbial community) and other agents such as anthropogenic activities (...). Each of these variables in turn can be broken down into additional controls. An ideal experimental research design would control all variables but one, and measure how that variable influences rock surface character. This might be possible in the laboratory, but is beyond the scope of this study due to the obvious time constraints of the slow rate of rock-coating development. Thus, like most field research, processes are deduced from observations. Three types of rock coatings exemplify, in a general sense, three different types of barriers: physical barriers are a key to the formation of dust films; geochemical barriers are needed for the accretion of iron films; and biological barriers are needed for manganese heavy-metal skins. These examples will be considered in turn. In each case, however, rock faces are first exposed by erosion, raw ingredients are transported to sites of accretion, and lithobiontic coatings are not competitors for possession of these surfaces. Dust films (Chapter 7) form where there is a physical barrier to transportation and a physical attraction of dust particles to rock surfaces. A rock face exerts enough of a roughness factor to reduce wind speed to the point where dust can accrete. Van der Waals forces of attraction then provide a reasonable physical mechanism for the large surface areas of the clay particles to attach to the rock surface or to previously adhered clays (van Olphen, 1963). The key to the formation of dust films, however, is the lack of water flow over the surface, either because of regional hyperaridity or a position in a rock shelter. In other words, the physical barrier of Van der Waals forces is only applicable where there is an absence water to exert a strong enough shear stress to remove the dust films. Growth of iron films (Chapter 9) is not limited by exposure of bare rock, by supply of iron, or by transportation of iron to rock surfaces. Iron is readily available in dust and in water. Yet something forms a barrier to the transportation of iron at these sites. Researchers have turned to biological and geochemical explanations, and both are necessary to explain the occurrence of iron films at sites with a wide set of pH ranges,

Hierarchical Model

from acid drainages, to near-neutral environments, to high pH and alkaline desert surfaces. The type of barrier to iron migration varies with pH. In extremely acidic conditions, iron bacteria offer the simplest explanation for the precipitation of iron on rock surfaces (Ghiorse, 1984; Harder, 1919; Kuenen and Gottschal, 1982; Mallard, 1981; Nealson, 1983; Wolfe, 1964). In slightly acidic to near neutral pH conditions, iron fixation can be explained by inorganic kinetics (Collins and Buol, 1970; Marshall, 1977) or through bacterial activity (Chukhrov et al., 1973; Ghiorse and Ehrlich, 1992; Mallard, 1981; Robbins et al., 1992). The development of iron films in high pH and alkaline surfaces is more difficult to explain, because the iron is already oxidized before it arrives at a rock surface. It is not soluble is only transported mechanically as dust or as suspended matter by turbulent water flow. Its fixation on rock surfaces in deserts, as Type III iron films, may relate to the formation of chemical Fe-O-Si bonds in association with silicate structures in the clays (Hazel et al., 1949; Scheidegger et al., 1993). Once iron ions attach to silica, they "adhere very strongly and irreversibly on the silica surface (Scheidegger et al., 1993, p. 62)." Natural heavy-metal skins (Chapter 8) that are dominated by an abundance of manganese occur because of a biological barrier to the flow of manganese. Unless the pH is alkaline or unless there is mixing of ground water with overlying stream water, manganese will remain in its mobile divalent state. This means that manganese will remain in solution and will not be precipitated on a rock surface. There are many contexts where neutral and acidic pH values occur along with manganese heavy-metal skins; these environments are inconsistent with physico-chemical oxidation (Dubinina, 1980; Mustoe, 1981; Robinson, 1993; Schweisfurth et al., 1980; Uren and Leeper, 1978; Wolfe, 1964). Thus, the simplest explanation is biological oxidation. A variety of biological agencies are capable of oxidizing manganese, thus creating a barrier to its further migration. Bacteria are the most common agents of manganese oxidation (see Table 8.11), but higher life forms such as algae (Francis, 1921) and bryophytes (Ljunggren, 1953) may also immobilize manganese. Most manganese heavy-metal skins exist only where organisms establish a barrier to the migration of manganese. 14.2.6. A consideration for the dynamic The earth's surface is a dynamic environment. As conditions change, so may the type of rock coating. Thus, environmental change is an overall concern in interpreting the role of different processes in different orders in this hierarchy. The wide variety of dynamic changes can be grouped into four general categories: erosion of the host rock; erosion of the rock coating and replacement with a different one; the accretion of a new rock coating on top of the previous accretion; and human-induced changes. I will address each of these changes in turn. Erosion at the earth's surface can drastically alter the appearance of the landscape. Part of this change in appearance is a change in the rock coatings that we see. For example, pedogenic coatings of calcretes, gypcretes and salt are often seen at the surface through the erosion of overlying soil material (Figure 5.21, Figure 5.22); thus, erosion lightens the appearance of the earth's surface through exposing a new rock coating (Figure 5.20). This process is not limited to terrestrial environments, as scouring of the Martian surface also exposes lighter-colored rock coatings (JPL, 1997).

337

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Chapter 14

The importance of erosion in rock coating development is certainly not a new concept. The stability of the rock type and entire landform has long been recognized as a prerequisite for rock varnish development and preservation (Blackwelder, 1948; Blackwelder, 1954; Cooke and Warren, 1973; Daveau, 1966; Denny, 1965; Engel, 1957; Engel and Sharp, 1958; Evenari et al., 1971; Musick, 1975). Deteriorating boulders or sites of active deposition are not well varnished. The erosion of rock coatings may occur from weathering of the rock immediately underneath (Blackwelder, 1954; Goodwin, 1960), eolian abrasion (Borden, 1971; Cooper, 1947; Simpson, 1961; Turner, 1963), organic activity, and environmental change (Blackwelder, 1948; Blackwelder, 1954; Cooke and Warren, 1973; Daveau, 1966; Denny, 1965; Engel, 1957; Engel and Sharp, 1958; Evenari et al., 1971; Musick, 1975). A second type of dynamic change is where there is replacement of a rock coating. Erosion may result in the old rock coating being replaced by an entirely different coating. Lichens are one example. They secrete acids that erode rock varnish (Dragovich, 1986b) (Figure 14.16) and other rock coatings like iron films (Figure 14.17). But these lichens are a lithobiontic coating, and in turn they promote the formation of oxalate-rich crusts through the concentration oxalates in their structures (Figure 14.17). Thus, there can be a succession of different rock coatings.

4

Figure 14.1 7. Backscattered electron microscope image of lichen eroding an iron film on a sandstone rock outcrop in northern Arizona. The gray minerals in the bottom of the image are quartz. Resting on the quartz is a patchy and thin (.--101.tin) bright iron film, best seen on the left side of the image. The dot pattem in the upper two-thirds of the image is made by calcium-oxalate mineral within a living lichen. The lichen itself is not seen in this image, because of the low atomic number of carbon.

The erosion of rock coatings can sometimes be seen along shorelines of a lake or along the high-water mark of a fiver. As water levels rise, they may dissolve or mechanically erode a rock coating at a shoreline (Figure 10.1). In contradistinction, contact with water can also result in the precipitation of a different type of coating, such as lake tufa (Figure 5.1), fiver travertine (Cole and Batchelder, 1969), iron films (Figures 6.21 and 6.22), or manganese heavy-metal skins (Cerling and Turner, 1982). A third type of dynamic change can be seen in the interdigitation of different types of rock coatings. Interfingering of different types of coatings occurs where the environmental change is not extensive enough to result in the erosion of the previous coating, but it is significant enough to result in the deposition of a new type of

339

Hierarchical Model

accretion. Numerous examples of interdigitation are presented throughout this monograph. A few are highlighted here to show how a shift in the type of biogeochemical barrier can alter the type of rock coating. A shift from a high pH and alkaline environment to a near-neutral pH on desert rocks will change the rock coating from a Type III iron film to a rock varnish. The environmental change may be the exposure of the iron film when a rock crevice spaUs; the manganese-rich rock varnish forms on top of the spalled surface. It may be the result of a cobble in a desert pavement being flipped (Figure 9.23). The change may be from change in climate (Liu, 1994; Liu et al., 1997; Liu and Dorn, 1996), where shifts between drier and semi-arid climates causes alternating layers (Figure 10.10). Rock varnish will sometimes alternate with a variety of other rock coatings. Throughout the Colorado Plateau and intermontane Wyoming, varnish interdigitates with oxalate on sandstone rock faces (Figure 7.9 and Figure 8.23). In other cases, rock faces support layers of both rock varnish and silica glaze (Nobbs and Dorn, 1993). Rock varnish even interfingers with carbonate (Dragovich, 1986a). A fourth type of dynamic change is when humans alter rock coatings. Consider the role of acid precipitation in altering the geochemical balance on rock surfaces (Meierding, 1993b). Through increasing the abundance of dust in the atmosphere, we increase the adherence of dust to rock surfaces (Sharma and Gupta, 1993). Air pollution interacts with rock materials, which results in the precipitation of coatings like gypsum (Del Monte and Sabbioni, 1984). Lead pollution appears to be incorporated into rock varnish (Figure 8.16 through Figure 8.20). We alter rock coatings in the landscape, in more ways that the direct application of pigments to rock surfaces (Chapter 3). Construction removes encrusted rocks, revealing scars that lack rock coatings (Figure 1.4). Construction can also expose different rock coatings that would normally not have been exposed at the surface, such as carbonate crusts in deserts (Figure 1.6) or manganese skins in Mediterranean climates (Figure 14.12). Mining activities bring to the surface the coatings associated with ore bodies (Roy, 1981); mining effluent interacts with natural systems to create heavy-metal skins (Figure 8.22 and Figure 8.23) and to overwhelm a natural system with debris (Figure 14.18).

,, ,9 ~ " 9 84

,.

III~T~!I~

....

Figure 14.18. Debris eroded from mine tailings redistribute heavy metals in the streams just east of Nasca, Peru. Arsenic, zinc, and cadmium were found in abundance on some of the iron films on quartzite cobbles.

One way to think about dynamic environmental changes experienced by a rock surface is in terms of competition for space. Competition is certainly a valid concept for lithobiontic coatings. Faster-growing mosses and algae out compete lichens, given suitable moisture requirements. Lichens are similarly able to out compete microcolonial fungi, and almost all inorganic coatings.

340

Chapter 14

Competition is also a valid concept for microorganisms (Fredrickson and Stephanopoulos, 1981; Kuenen and Gottschal, 1982). This is especially relevant for the microorganisms that aid in the precipitation of different types of rock coatings (Beveridge and Fyfe, 1985; Ferris et al., 1987a; Konhauser et al., 1993; Konhauser et al., 1994; Krumbein, 1983; Krumbein and Dyer, 1985; Mustoe, 1981; Peck, 1986; Wolfe, 1964). A microbe may be changing the microenvironment to inhibit competition. Consider the manganese-oxidizing organisms found on rock surfaces in deserts (Dorn and Oberlander, 1981a; Dorn and Oberlander, 1981b; Drake et al., 1993; Jones, 1991; Krumbein and Jens, 1981). Oxidation of manganese turns surfaces black. This decreases the albedo of surfaces and increases surface temperature; varnished rocks in deserts can exceed 80~ (George, 1976). Higher temperatures then increase the moisture stress of lichens and fungi. Hence, the precipitation of manganese fosters a positive feedback that decreases moisture availability and allows bacteria to occupy surfaces that might otherwise be inhabited by lithobiontic films and crusts. 14.3. The Hierarchical Model as an Interpretive Tool

Any general approach to understanding rock coatings must be useful in interpreting what can be seen in a landscape, and it must be valid at a variety of scales. As I admit in chapter 2, my bias in building this model (Figure 14.19) is my reliance on microscopic observations. Thus, testing the interpretive validity of the model with more electron microscope images would pose a bad test of its general applicability.

Hierarchical Model to Interpret Rock Coatings

[1stOrder: Geomorphic Controls on Exposure of Rock ] ,

,.

i|,l

i

1

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~4th Order: Tr,ansport.'Pat.hways. for Coating constituents 1

I5th Order: Biogeochemical Barriers to Fix Coating Constituents Figure 14.19 Simplified diagram to represent a hierarchical model used to understand rock coatings. Rock coatings can be interpreted by examining five different levels of processes.

Hierarchical Model

This section gives examples of the application of thishierarchical model to interpret rock surfaces that an individual might see visiting a field site. Three examples follow. The aesthetically pleasing "wave rock" Australian landscape in Figure 14.20 is a battleground for posession of the surface by rock coatings. Rock varnish, iron skins, silica glaze, oxalate-rich crusts, manganese-rich heavy-metal crusts, and lithobiontic coatings vie for possession of the surface.

Figure 14.20 "Wave Rock" in westem Australia presents many different types of rock coatings. The vertical face is approximately 10 meters in height.

In Figure 14.20, a first-order issue is that rock comings exist at this site because the setting is a weathering-limited landscape; this means that bare rock is exposed because erosion is faster than weathering. A second-order concern is that iron films were initially developed in rock crevices; these inherited coatings are seen today at the surface where the host rock experiences millimeter-scale spalling. Third, lithobiontic crusts out compete the other rock coatings near the base of this slope, because this is where enough water collects to allow their growth. The other rock surfaces are too xeric to permit extensive formation of lithobionts, allowing other rock coatings to grow. A fourth-order concern is shown by oxalate-rich crusts near the lithobionts; these crusts are limited by the supply of oxalate from lichens and decaying organic matter. Where oxalates are in short supply, other rock coatings can grow. A fifth-order concern rests on the locations of biogeochemical barriers for the fixation of other rock coatings; the different biogeochemical barriers vary from place to place. Silica glaze and manganeserich heavy-metal crusts interdigitate in the locations of water flow. Rock varnish, in contrast, dominates on the upper, more xeric, rock surfaces that are not exposed to runoff. Thus, the variety of rock coatings seen in Figure 14.20 can be interpreted from a hierarchical framework where certain processes set in motion a chain of events that control what type of rock coatings grow in different places. A hierarchical interpretation is possible for the rock coatings at El Morro Rock in New Mexico (Figure 14.21). There are at least three distinct environments for the formation of distinct rock coatings: places of recent massive rock spalling; places of

341

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Chapter 14

active water flow; and more xeric locations where the host sandstone hasn't eroded recently and does not receive abundant runoff. " ~,,. ~ m r : '~.~.( - ,, g V ~?_

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Figure 14.21 E1 Morro Rock in New Mexico illustrates different microenvironments that host different rock coatings. Scale is provided by the people standing at the bottom of the cliff face.

At a first order, this photograph exhibits a weathering-limited landscape. The sandstone cliff face erodes at a faster rate than weathering can produce particles. Thus, bare rock is visible. At the second order, iron skins have been inherited from prior development in the subsurface; these are exposed by the large rock spall in the uppercenter part of the image; this is the relatively smooth face. Third-order processes concern the growth of lithobionts. The largest concentrations of lithobionts are found in association with water streaks in the lower part of the image. The rock surfaces that are moistened by intermittent water flow contain lichens, algae, and fungi. There are also epilithic and endolithic lichens and fungi growing on the more stable sandstone surfaces on the right side of the image. Where lithobionts are able to survive, they out compete with other rock coatings for possession of the surface. Fourth-order processes concern the pathways of transport for the raw ingredients of the rock coatings. Oxalate-rich crusts are found in the water streaks in places where the lithobionts have not grown, because the lichens supply the calcium oxalates. Manganese-rich heavy metal skins are also found in patches and interfingering with the oxalate-rich crusts; manganese is brought in solution and is fixed in place by bacteria. Fifth-order processes on the biogeochemical stability of materials decide what rock coatings possess the rest of the surfaces in the photograph, mostly the sandstone on the fight side of the photograph. The sandstone has a scaling pattern created by spalling of millimeter- to centimeter-thick shells. The recently-spalled shells start out with the inherited iron skins and cryptolithic lithobionts; with time, these shells are coated by both rock varnish and silica glaze. The silica glaze is favored in microenvironments where water flow concentrates, but certainly not to the extent of the visible streaks. The rock varnish grows in places where water flow is less concentrated, probably because the dust (clays) that compose rock varnish are more readily washed off by runoff.

Hierarchical Model

In summary, the type of rock coatings found at Morro Rock (Figure 14.21) are interpretable from the perspective of different processes, but where certain controls are more important than others in determining what coating possesses the rock surface. The last example is an interpretation of rock coatings on a glacially-polished granodiorite surface in the Sierra Nevada of California (Figure 14.22). Rock coatings on this surface are mostly coatings that are a combination of iron films and silica glaze.

Figure 14.22. Glacial polish in the Sierra Nevada of Califomia hosts thin coatings of silica glaze and iron films as seen in the field (upper frame). In addition, there are corresponding backscauered (lower right) and secondary (lower left) electron microscope imagery of rock coatings. The coatings are interdigitations of iron film resting over silica glaze.

The rock coatings in Figure 14.22 are interpretable by the hierarchical model. First, the overall pattern of rock-surface exposure is controlled by geomorphic processes. Glacial erosion and subsequent ablation produced the bare rock surface. However, some glacial till was deposited on the sides of this image, which in turn encouraged the development of vegetation. Neither bare rock surfaces nor rock coatings are seen on the tree-covered transport-limited areas. Second-order concerns about inherited rock coatings are not important in the interpretation of the rock coatings in Figure 14.21. There are patches of rock surface erosion, where portions of the glacial polish and overlying rock coating are eroded. In these locations of spalling, however, there is no evidence that coatings are inherited from prior formation in the subsurface along joint fractures. Third-order processes involve the growth of lithobionts. Lichens have grown on some of the rock faces, and lichens out compete the slower-growing silica glazes and iron films. However, lichens are not dominant, despite the positive water balance at the site. The bare rock surfaces, then, are able to host inorganic coatings. At a fourth level, there is no evidence to indicate that there are any special limitations to the supply of iron or silica. Both iron and silica are in the soil material that would

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Chapter 14

supply solutions to the rock surface. Both iron and silica are in the dust that falls on snow and would be concentrated at the end of the melt season. The fifth-order issue, of why there is a biogeochemical barriers fostering the accumulation of iron and silica, is a much more difficult issue to assess. These rock coatings are characterized by a very thin Type I silica glaze underneath a Type I iron film (Figure 14.22). There are, however, several possible interpretations. One possible interpretation is a change in environment; for example, the silica glaze could have been subglacially deposited (Hallet, 1975), followed by a subaerial or subglacial iron skin (WhaUey et al., 1990). A second interpretation is that all coatings could be subaerial in origin. The sequence of silica glaze under iron film would be consistent with the formation of chemical Fe-O-Si bonds, which in turn helps iron oxides adhere as a rock coating (Scheidegger et al., 1993). In summary, the types of rock coatings that develop in natural landscapes (Figure 14.20, Figure 14.21, and 14.22) are interpretable when the processes of rock-coating development are considered sequentially. The next chapter examines whether this hierarchical model can be used to understand geographical variations in rock coatings over larger regions.

345

Chapter 15. ANALYZING COATINGS

GEOGRAPHICAL

VARIATIONS

IN

ROCK

"Lithologic boundaries in the supergene zone where conditions of migration change drastically and concentrations of chemical elements begin to rise are called geochemical barriers." A.I. Perel'man, Geochemistry of Epigenesis, Plenum Press, 1967

15.1 Introduction

Place to place variability in rock coatings has significance for all aspects of geography. In physical geography the patchwork quilt of different coatings informs on the spatial distribution of geomorphic processes (Hunt and Mabey, 1966; White, 1993a). Layering patterns in rock coatings, for example, may hold clues to past climatic variations; "each weathering crust overprints the previous ones in varying rock microclimates within the [Antarctic] field area (McKay et al., 1983, p. 228)." The type of rock coating growing at a particular place informs on the local biogeochemical environment (Krumbein and Dyer, 1985). Inasmuch as physical geography explores the processes responsible the earth's varied environments, rock coatings are a reflection of that variability in bare-rock landscapes. Geographical variability in rock coatings also has importance at the interface between physical and human geography. Early linkages between people and nature involved the character of place (Glacken, 1967), which is in part defined by the color and texture of landscape elements. Landscape painters such as Thomas Moran tried to capture the quintessential essence of spatial variations in rock-surface textures through art. More recently, nature photographers use lighting contexts to bring forth components of spatial variations in rock surfaces of monumental landscapes. The popularity of national parks and ecotourism attest to the significance of the aesthetic nature of landscapes for people living in industrialized societies. What intangible element would be missing from Canyon de Chelly (Figure 1.11) be without dark streaks on the canyon walls? Would Petra in Jordan (Figure 1.3) be the same without streaks of rock coatings? Certainly the popular and mysterious Nasca lines of Peru (Figure 14.6) would not exist without contrast provided by rock coatings. The previous chapter (Chapter 14) presented a hierarchical model to explain geographical variations on the scale of a few square meters. The focus of this chapter, in contrast, rests on the analysis of geographical variations in rock coatings on the scale of square kilometers. While some geographical systems are scale invariant, the scale of study greatly influences the patterns of rock coatings. In this chapter, I first summarize the different approaches that have been used to map regional patterns in rock coatings. Then, I present a case study of regional geographical

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Chapter 15

variability in rock coatings across the Himalayas into the Tibetan Plateau. Lastly, I conclude with some general observations.

15.2. Different Approaches to Map Regional Geographical Variability Three general approaches have been used to map geographical variability in rock coatings over the scale of square kilometers. Each approach involves different strategies to generalize data. Some of the data are truly accurate for the millimeter scale, while other data are not truly appropriate for understanding rock coatings. In other words, there is a trade-off in cartography where accuracy and precision are sacrificed to produce easily interpreted maps. An important question that I am asking is whether the perceived patterns are real, an artifact of correlation with other variables, or an artifact of sampling.

15.2.1 Generalization of Micron-Scale Analyses Tanzhuo Liu developed a new approach to the understanding of geographical variability in rock coatings (Liu, 1994; Liu et al., 1998). Through the exhaustive preparation of ultra-thin sections of rock varnish, Liu recognized vertical patterns in the layering of microlaminations within rock varnishes. Figure 15.1 illustrates thin sections seen in different "microbasins." He identified eight "layering units" in Death Valley and the surrounding area (Figure 15.2). Liu's layering units provide a stratigraphic approach to understanding environmental change through interpreting coatings. However, Liu (1994) went a step further and translated these patterns into a geographical perspective by mapping the spatial variability of the layering units. In particular, he applied his mapping approach to alluvial-fan deposits. Sections of alluvial fans were placed in mapping units based on the oldest varnish layering unit seen in thin sections on that segment of the fan. Thus, rather than relying on more traditional visual clues to map geomorphology, Liu (1994) turned to microscopic analyses of the rock coatings themselves. Figure 15.3, Figure 15.4, and Figure 15.5 exemplify Liu's approach towards the geographical generalizations of microlaminations. The study site is Warm Springs Alluvial Fan in Death Valley, eastern California. The different development of rock varnish can be seen visually for the youngest units, but older surfaces are hard to distinguish without Liu's microlaminations. Using the ultra-thin sections made from varnishes collected from study sites presented in Figure 15.3, Liu analyzed the layering patterns and generalized this information into a map of the development of an alluvial fan (Figure 15.4). Figure 15.5 presents an oblique aerial photograph view of the site. In summary, Liu's (1994) approach translates a certain characteristic of a rock coating into a map, and then that map can be used to interpret the development of landscapes. While this approach has only been applied to the understanding spatial variations in the layering pattern of rock varnishes, it could be applied to other micron-based techniques.

Geographical Variations

347

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Figure 15.1 Optical microlaminations showing the progressive development of layers over time. This figure displays just a few of the >2000 microbasins examined within a 320 km 2 area in Death Valley (Liu and Dom, 1996). The general stratigraphy is shown on Figure 15.2. Mean vamish thickness is 401am (a), 501am (b) 50~tm (c) 50pro- (d), 80~tm (e), 601am (t3, 120lain (g), 80~m (h).

Chapter 15

348

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349

Geographical Variations

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Chapter 15

350

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3.6~ Figure 15.4. Map of Warm Springs Alluvial Fan, southem Death Valley, based on the layering patterns seen in rock varnishes, from Liu (1994). The mapping units are the oldest layering seen in thin section. LU >8 is the oldest, because varnishes on this portion of the alluvial fan have the most complicated layering. LU 1+2 is a combination of varnishes with only the simplest (youngest) layering patterns.

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Figure 15.5. Aerial photograph of Warm Springs Alluvial Fan, southern Death Valley. The view looks to the southwest.

Geographical Variations

15.2.2 Generalization of Field Observations The first regional map of the distribution of a rock coating was presented for Arizona (Elvidge and Moore, 1979). Elvidge (1979) used initial field observations to develop a correlation between the distribution of rock varnish and a combination of soil alkalinity, annual precipitation and vegetation. Then, using maps of these environmental factors, Elvidge field checked the initial predictive relationship and developed a general map to correspond with vegetation patterns (Figure 15.6).

Figure 15.6. Map of the distribution of rock vamish in Arizona, southwestern United States, modified from Elvidge and Collet (1981).

Elvidge (1979) and Elvidge and Collet (1981) concluded that varnish is best developed in the most arid, alkaline and dusty parts of Arizona, in the "Sonoran Zone" of saguaro and other succulent vegetation. They felt that the second best area of varnish development is in the high desert of the Colorado Plateau, the "Plateau Zone" that is dry and has alkaline soils. The third best area of varnish formation is the Eroded Sonoran Zone where rock varnish is patchy in distribution and often eroding. Upland areas of the state were mapped as having no rock varnish because of the abundance of moisture and vegetation. The lithobiontic coatings in Israel were mapped in a similar fashion (Figure 4.35) (Danin, 1986). Relationships between lithobionts, lithology and climate identified seen in the field, in coordination with laboratory observations, were generalized to the entire country using previously mapped environmental parameters. Correlating rock coatings with established environmental parameters to produce a generalized map is certainly not a new approach in cartography. It yields first-order approximations. For example, living in Arizona, I find Elvidge's map reasonable at first glance. I only disagree with the interpretation of varnish erosion; rock varnish forms in all areas of the state and areas identified as experiencing 'varnish erosion' are actually experiencing erosion of the host rock. The flaw in this general approach is that it admittedly misses a tremendous amount of internal variability that exists in an environmental region. A key question I address later in this chapter is whether the internal variability in rock coating distribution in each environmental region is so great to prevent the use of these types of cartographic short-cuts.

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15.2.3 Remotely Sensed Imagery The third strategy used to map rock comings in the literature is satellite or airborne imagery. In remote sensing an attempt is made to learn about the surface characteristics of specific areas. These 'training sites' are then used to understand the reflectance or emission of areas not yet studied on the ground. The Pixel sizes used are at best a few square meters and more typically tens of meters across. Hence, spectra are treated statistically to extract the relevant information (White et al., 1997). Remote sensing has not been used to map rock coatings, with a few exceptions (e.g., White et al., 1997), but rather to map the geology of surfaces. The interest in rock coatings in the geologically-oriented remote-sensing community is because rock coatings influence the reflectance signature of the geology (Abrams et al., 1991; Adams et al., 1982; Anderson, 1995; Best, 1989; Bothorol et al., 1984; Clayton, 1989; Daily et al., 1979; Elvidge and Collet, 1981; Farr, 1981; Gaddis et al., 1990; Lyon, 1990; Rivard et al., 1992; Salisbury and D'Aria, 1992; Schaber et al., 1976; Shipman and Adams, 1987; Spatz et al., 1989; Sultan et al., 1987; White, 1993a; Wood et al., 1989). This effort is to extract the influence of the rock coating, all in order to obtain more accurate information about the underlying geology, not rock comings. Relationships between rock coatings and geology are complex. First, there are a plethora of rock coatings; however, most research on remote sensing only considers one or at most two of these different coatings. Second, spectral changes involve complex interactions between rock coatings and erosion. Consider Figure 15.7, where Hanaupah Canyon Alluvial Fan in Death Valley darkens and lightens with time and with the exposure of calcrete. In other words, there is a 'bell curve' for the reflectance of alluvialfan surfaces. This general model of lightening over time, based on calcrete, cannot even be applied to adjacent alluvial fans. Immediately to the North, Death Valley Canyon Alluvial Fan undergoes the same spectral change of darkening and then lightening, but the lightening on the older units has to do with the instability of the host rock, not with the exposure of calcrete. The oldest surfaces on Death Valley Canyon Alluvial fan have clasts that have undergone granular disintegration, leaving meter-sized circles (Figure 15.8).

Figure 15.7. SPOT image of Hanaupah Canyon alluvial fan in Death Valley, along with a general model of surface evolution. For a few thousand years, the surface has a 'bar and swale' topography that is rough. In comparison with other surfaces, there is high albedo because of the paucity of rock varnish. After about ten thousand years, the topography changes to a smooth desert pavement that darkens as it varnishes. After about a hundred thousand years, gullies erode into the flat pavement and these surfaces round into baUenas. The general albedo does not simply darken with more varnishing; rather, it darken with varnishing and then lightens with the weathering of rocks and with the exposure of calcrete crusts in the ballenas.

BEandChm~el . . . . . . -v~/-r

Smo(ratPavnt~eat Iacisr Pavement

Geographical Variations

llmCr'~,,~

Figure 15.8. Aerial photograph and ground view of an older surface on Death Valley Canyon alluvial fan. The lighter colored surfaces are the oldest, but they are the lightest because of the breakdown of the host rock into grus; the arrow on the aerial photograph indicates the approximate position of the ground photograph.

The most informed use of remote sensing to study rock coatings and geomorphology has been in Tunisia (White, 1990; White, 1993a; White, 1993b; White et al., 1996; White and Walden, 1994; White et al., 1997). An understanding of field characteristics, laboratory work, and a clear sense of geomorphic relationships were combined to interpret the remotely sensed data. In essence, the strategy combines three scales of information: micron-scale insight from laboratory studies; meter-scale insights from field observations; and kilometer-scale integration through remote sensing - - but all the while with a firm understanding on the processes of rock coating development. Consider the following: "Iron coatings have a less significant effect on host rock spectral reflectance characteristics, due largely to the thin, discontinuous coating relative to manganeserich rock vamishes...Iron-rich rock coatings do not preclude recognition of the host rock lithology from remotely sensed spectral reflectance measurements. Manganese-rich rock varnishes do, however, preclude such recognition, by obscuring the underlying host rock lithology...Because of their more pronounced spectral reflectance characteristics, manganese-rich rock varnishes are likely to be more suitable for deriving geomorphological information, such as mapping geomorphological surfaces of different ages from remotely sensed data (White, 1990, p. 32)."

The distinction of White's work does not rest in working with the most expensive and highest resolution imagery, but rather in an awareness of the interplay between geomorphic conditions and different types of rock coatings, including iron films, rock varnish, gypsum crusts, in addition to an awareness of the distinction between these and weathering rinds. Complexities have been similarly appreciated where biotic rock coatings have been the focus of the remote sensing study. Maps of lithobiontic coatings have been made with the aid of remotely-sensed imagery (Kokaly et al., 1997). Again, this research effort is different from the aforementioned geologic studies, because the focus rests on the lithobiontic coatings (see chapter 4), not on the underlying rock. The end product is a lithobiontic map (Figure 15.09) for cryptogamic crusts in a portion of Arches National Park.

353

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Chapter 15

Arches National Park MierobiotieSoilsMap USOSmbandmap 80% "micro" 20% "vr 72% "micro" 10% ",and" 18% "ve~"

~O1%

"micro"

~% "sand" 16% "v~..

40% "micro" 2,1)% "Salad"

40% "veg"

20~& "micro" ~g0~ "sand" 1oo% t|~d04 BB m

Note: "sa~"- sand,or rock

gok.ly, Clark,and Swayzr 1993

Figure 15.9. This figure illustrates the approximate aerial coverage of the cryptogamic crusts and black lichens (labeled as micro for microbiotic) in a portion of Arches National Park (Kokaly et al., 1997). 1992 AVIRIS remotely sensed imagery were calibrated against laboratory reference spectra to create this map to the north of Wolfe Range and Delicate Arch. The Pixels are 17 meters This image is adapted from Kokaly et al. (1997).

Excellent rock coatings maps, such as Figure 15.9, could be made with remote sensing, but only if there is a clear understanding of the wide variety of rock coatings that are integrated into the individual Pixels. Inaccurate maps are easy to make without interactive feedbacks between micron-scale laboratory data, millimeter- to meter-scale field insights, and remote sensing. This is because rock coatings are not scale invariant. The rock coatings seen with the microscope show great variability from place to place on a boulder, over a landform, and throughout a region (Figure 15.10). Although Death Valley is normally thought to be dominated by rock varnish, meter-long transects sampled across bare rock surfaces for all geomorphic units reveal a coverage of rock varnish of 19% (with a standard deviation of 35%). When only alluvial fans units are considered, varnish coverage averages 24+31%, with the most consistent varnish cover on latest Pleistocene alluvial fan units at 35+22%. The aerial coverage of other rock coatings, including silica glaze, carbonate crusts, iron films, oxalate-rich crusts and lithobionts, exceeds rock varnish by a factor of two. The significance of this issue is that Death Valley is frequently used as a field training site for dozens of remote sensing studies by NASA-funded scientists. Literally tens of millions of dollars have been spent investigating Death Valley with remote sensing, where rock varnish has been an integral part of the research; yet, these studies have all ignored the heterogeneity in rock coatings. These studies have blithely assumed that the only coating is rock varnish. My point is simple. Before the influence of rock coatings can be decoupled from the host rock, rock-coating heterogeneity must be understood at all scales. Maps of lithology and rock coatings may be inaccurate simply because of an incorrect assumption of coating homogeneity. Fortunately, this assumption is not made in all research at the interface of rock coatings and remote sensing (e.g., White, 1990; Kokaly et al., 1997).

Geographical Variations

Figure 15.10. Optical view of rock coatings at different scales reveals vastly different perspectives. The lower right optical microscope view of dark rock vamish on granodiorite (~1 mm wide), was sampled from a -70 cm-wlde boulder flower left), on a smooth desert pavement on Hanaupah Canyon alluvial fan (arrow, upper fight), shown by the box in the SPOT satellite image of Death Valley (upper left frame ~100 km wide).

15.3 Case Study in Regional Variability: Himalayan Transect In order to give the reader a feel for the variety of rock coatings that exist side-byside, all within mapping units used to generalize field observations and certainly within the scale of single Pixels, I present a case study of the variety of rock coatings seen in a large region. Regional sampling offer a means evaluating variations at different scales. Samples are analyzed in the laboratory at the micron to millimeter scale. Samples are collected in the field at the millimeter to meter scale, with sites kilometers apart. The purpose of this section of chapter 15 is to analyze the geographical variability of rock coatings across a region. In the process I will illustrate that it would be very easy to produce inaccurate maps by oversimplifying the realities of rock coatings in a region. This section explores in greater detail the nature of rock coatings found on the two different sides of the "Roof of the World" (Bishop, 1962): the moist and glaciated Khumbu of Nepal and drylands of the West Kunlun in the Tibetan Plateau. The Himalayan Ranges and Tibetan Plateau expose one of the world's greatest expanses of what G.K. Gilbert (1877) characterized as weathering-limited landscapes,

355

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Chapter 15

where the rate of land surface lowering is limited by the rate of weathering. In other words, rock is seen at the surface because erosional processes are faster than weathering. Relatively little is known about the nature of rock coatings in the Himalayan Mountains and Tibet. Working in the Karakoram, visual changes in rock varnish was used as a method to estimate landform age (Derbyshire et al., 1984). Cation-ratio dating has been used to date rock varnish in the Pamirs (Glazovskiy, 1985). High concentrations of bacteria, algae, and fungi grow on varnish in the Tien Shan at ~4200 m (Parfenova and Yarilova, 1965). Others have noted a similarity between rock varnishes in the Greater Himal and warm deserts (Dora, 1991; Kalvoda, 1984; Kalvoda, 1992; Whalley, 1983; Whalley, 1984). To the north of the Tibetan Plateau in inland low deserts, Chinese scientists have explored the nature (Zhu et al., 1985) and cationratio (Zhang et al., 1990) of rock varnish.

15.3.1. Study Areas The glaciated Khumbu region of Nepal rests on the south side of Mount Everest (Figures 15.11). The climate of the Khumbu is greatly influenced by monsoonal circulation patterns and mountain climatology; it has a dry winter and wet summer (Inouye, 1976). Precipitation peaks during early afternoon on glaciers and highlands in the monsoon season, and later in the evening in lower valley locales (Higuchi et al., 1982). Valley wind systems are important in the transportation of sensible heat and water vapor, in both the monsoon and winter season (Ohata et al., 1981). The geomorphic, soils, hydrologic, and biogeographic systems of the Khumbu are influenced by the region's present and past climate (Biiumler et al., 1991; Haffner, 1972). During the last glacial maxima, rock glaciers were active (Jakob, 1992) and glaciers extended down the Imja Khola valley at least to Namche Bazar at 3440 m (Heuberger and Weingartner, 1985), and perhaps as far as Lukla at about 2500 m (Fushimi, 1980), in contrast to present glacial termini reaching around 5000 m (Fushimi et al., 1979). The last major advance was in the Little Ice Age during the 16th century (Fushimi, 1980). The proglacial landscape is still adapting to glacially steepened valley slopes with concomitant hazards of mass wasting, fluvial erosion, and jokulhaup processes. Lithologies in the area include gneiss, leucogranites, and the metasedimentary Everest seres above 8200 m (e.g., limestone, phyllite, quartzite, marble, schist) (Rochette et al., 1994; Vuichard, 1986). Rock coatings in the Khumbu were collected in an altitude transect from several different environmental settings (Figure 15.11): gneiss cobbles on the surface of solifluction lobes on Kala Pattar at about 5400 m; supraglacial quartzite clasts on the surface of the Khumbu Glacier at about 5250 m (Figure 15.12 and 15.13); glacially polished gneiss clasts at the terminus of the Khumbu Glacier at about 4900 m; gneiss avalanche boulders beneath Pokalde at about 4600 m; gneiss clasts on the surface of the Tshola rock glacier at about 4500 m (Figure 15.14); leucogranites stream-side cobbles of the Imja Khola River at about 4(X)0 m near Pangboche; and a faced (during construction) gneiss cobble in a 10-year-old wall (when collected) in Namche Bazar at -3400 m. Although these samples do not represent the complete variety of rock coatings in the Khumbu region, they exemplify some of the more noticeable rock coatings.

Geographical Variations

0

I

1

2 km

I

'86*50

I

~s

\

Pumon

~t

~

.,~-'~'v-~,~.,.,,,,,

.".g

~ Keda l ~ r /

fQ Dugla

357

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~

9

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Mount

Sampling Sites 9 PoK~le 1 K~

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2 Khumbu OMcier Surface 3 Khumbu Glacier Terminus 4 TsholaRock Glacier Surface 5 Talus Boulder 6 Imja Khola Stxuam-side Boulder 7 Namche Bazar Faced Well

./ 9 Arna Dabl~an

Figure 15.11. Map of sampling sites in the Khurnbu region, Nepal.

Figure 15.12. Surface of Khumbu Glacier below the Kala Pattar. The Everest base camp is usually located in the upper left hand side of the photograph. The individual seracs in the ice fall are tens of meters high.

Figure 15.13. Clast in supraglacial melt water on Khumbu glacier -5250 m, collected from the far lower left comer of 4.11. The normally white quartzite is coated by rock varnish.

358

Chapter 15

Figure 15.14. Tshola rock glacier. A sample was collected from the surface of the rock glacier about 100 m above the outwash plain of the Khumbu Glacier in the foreground.

The other end of the environmental transect is in the Ashikule Basin, resting in one of the driest sections of the Tibetan Plateau, on the north side of the west Kunlun Mountains (Figure 15.15). Only a little climatological data exist for this area (Derbyshire et al., 1991). Present day precipitation totals probably rest between 300 mm and 800 mm; in the West Kunlun, however, the ratio of precipitation received on the north and south-facing slopes is 1:5, suggesting that the Ashikule Basin is in the lower end of this range. Present-day climatic snowlines in the area are ~5600 m; and mean monthly temperatures during the warmest summer months probably range from -2~ to +2~ (Yafeng et al., 1992). Snowlines lowered during the Pleistocene, but only by about 200-300 m in the West Kunlun Mountains. This relatively small depression is probably due to the extreme aridity of the area (Yafeng et al., 1992). The latest Pleistocene glacier receded from its terminal position about ~14,000 to 15,000 years ago in the West Kunlun Mountains, where it extended to 5300 m, and about 6.5 km away from the present glacial terminus of the Congce Glacier (Yafeng et al., 1992). The geomorphology of the Ashikule Basin is dominated by the Akesu volcanic field, which consists of potassium-rich lava flows and cinder cones that range in age from ~70 ka to ,-540 ka. Two saline lakes are situated in the Basin: Ashikule and Urukele. Periglacial features such as rock streams also mantle the steeper slopes of the surrounding hills. It is difficult to understate the extent to which eolian processes influence the Ashikule Basin. Loess mantles all lava flows, burying much of the topography. The isotope geochemistry of loess in the region suggests an origin in the adjacent Tarim Basin (Liu et al., 1994) There is also widespread evidence for eolian abrasion of the lava flow surfaces. As will be discussed later, ventifact surfaces do display rock coatings, suggesting that eolian abrasion is episodic. Analyses of the loess indicate that there is a high content of both carbonate and sulfate minerals w perhaps derived from some local sources of deflation of the Ashikule and Urukele saline lakes. Rock coatings in the Ashikule Basin were collected by T. Liu from different microenvironmental settings (Figure 15.15), all at about the same altitude of ~47004800 m. They have the sample designation of AKB in figure captions for this section. The collection sites are: bombs on rims of volcanic cones (Figure 15.16), and constructional surfaces of lava flows (Figure 15.17); outcrops that have been subject to eolian abrasion (Figure 15.18); beach ridges of Urukele Lake (Figure 15.19); rock streams on hillsides of the Ashikule Basin (Figure 15.20); Pleistocene moraines of the West Kunlun Mountains; and a fiver terrace adjacent to the Pulu River.

359

Geographical Variations

0

10

20 km

. b"~.'m ,~,,,,,

--" ~ - ~ . , , . ~ h bAtg . ,~,.'?~~huangKaru'~

"36 ~ N

Karata~, . . . . . . . . .

l),thr

Urukele I... ~ ' ~

69~

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Figure 15.15. Sample sites in Ashikule Basin, Tibet.

Figure 15.16. Ahishan cone in the background with the salt playa of Urekele Lake in the foreground. Photo courtesy of Tanzhuo Liu.

Figure 15.17. Surface of lava flow from Ashisan volcano. The lens cap provides scale. Photo courtesy of Tanzhuo Liu.

Figure 15.18. Surface of Ashishan lava flow that has been subjected to loess deposition and eolian abrasion that reset the development of rock coatings. The rock hammer provides scale. Photo courtesy of Tanzhuo Liu.

360

Chapter 15

Figure 15.19. Beach ridges of Urukele Lake. Photo courtesy of Tanzhuo Liu.

Figure 15.20. Rock streams on hiUslopes on the southeast side of the Ashikule Basin. Photo courtesy of Tanzhuo Liu.

15.3.2. Rock Coatings in the Khumbu The first part of this section gives the reader a feel for the micron-scale characteristics and variety of rock coatings found in the Khumbu in a transect from the Kala Pattar at 5400 m to Pangboche at about 4000 m (Figure 15.11). The second part of this section generalizes some o f the field and the laboratory insights on Khumbu rock coatings. Table 15.1 presents the electron microprobe data for all of the Khumbu rock coatings. Solifluction lobes and turf-bank terraces on the Kala Pattar (--5400 m) contain gneiss clasts with dark brown rock coatings. Figure 15.21a reveals that these coatings are mostly iron films (Table 15.1a) that precipitate in weaknesses defined by foliations in the gneiss. Although these iron skins may be found at the surface, they probably develop first in the fractures and are exposed by spalling. These iron films were similar to those found on the gneiss clasts on the surface of the Tshola rock glacier (Figure 15.14) at about 4500m (Figure 15.21f).

Table 15.1. Electron microprobe analyses of rock coatings in the Khumbu of Nepal. Values in oxide weight percent. Totals do not reach 100% due to porosity, water, and content of organic matter. Transects correspond to lines in indicated figures. ,

9

n

,,, i

i

9

,,,

_

Section a. Iron films

Sample from Kala Pattar (see Figure 15.2l a) TRANSECT: Na20

Left to Right

0.22 0.21 0.28

MgO A1203 Si02

P205

S03 K20

CaO Ti02 MnO FeO BaO

Total

0 . 0 4 4 . 0 2 3 . 2 6 4.18 0.79 0.22 0.06 0.00 0.00 63.17 0.00 75.96 0 . 0 2 4 . 3 0 2 . 9 7 3 . 4 0 1.29 0.19 0.10 0.00 0.00 62.99 0.00 77.03 0 . 0 5 4.51 2 . 8 4 4.09 1.90 0.29 0.12 0.00 0.00 59.07 0.00 75.26

Samole from Khumbu dacier, where iron skin is under silica daze (Figure 15.21b~ TRANSECT: Na20

Top to Bottom

0.00 0.00 0.00

MgO A1203 Si02

P205

S03 K20

CaO Ti02 MnO FeO BaO

Total

5.01 5 . 3 0 7 . 5 2 0.22 0.64 0.00 0.16 0.00 0.15 60.57 0.00 79.57 6 . 3 3 8 . 0 9 8.01 0 . 3 7 0.60 0.00 0.20 0.00 0.19 59.00 0.00 82.79 5 . 3 2 6 . 7 7 9 . 0 6 0 . 5 0 0.55 0.00 0.22 0.00 0.28 57.17 0.00 79.87

Sample from Tshola rock glacier (Figure 15.21f) TRANSECT: Na20

Left to Right

0.19 0.15 0.09

MgO A1203 Si02

P205

S03 K20

CaO Ti02 MnO FeO BaO

Total

0 . 1 1 2 . 1 7 4 . 0 9 3.17 1.03 0.30 0.10 0.00 0.00 69.01 0.00 80.17 0 . 1 9 2 . 0 9 4 . 1 9 2 . 4 0 0.99 0.27 0.12 0.00 0.00 68.18 0.00 78.58 0 . 2 4 1.97 3 . 7 7 3.88 1.44 0.19 0.12 0.00 0.00 67.46 0.00 79.16

Geographical Variations

361

Section b: Silica glaze Sample from Khumbu glacier, where silica glaze is above iron skin (Figure 15.13b) TRANSECT: Na20 MgO A1203 Si02 P205 S03 K20 CaO Ti02 MnO Top 0.03 0.43 6.93 52.41 1.24 0.02 0.25 0.06 1.57 0.00 to 0.00 0.45 6.72 62.33 1 . 3 7 0.02 0.12 0.11 1 . 3 3 0.05 Bottom 0.03 0.41 4.75 56.61 1.26 0.02 0.11 0.03 0.75 0.04 0.01 0.28 6.08 59.84 1.24 0.07 0.24 0.04 0.73 0.10 0.08 0.18 5.26 56.61 1.40 0.05 0.23 0.04 0.68 0.00

FeO 7.19 5.24 4.17 4.23 3.56

BaO 0.02 0.06 0.00 0.06 0.01

Total 70.15 77.80 68.18 72.92 68.10

Section c: Phosphate Skin Sample from avalanche boulder beneath Pokalde (Figure TRANSECT: Na20 MgO A1203 Si02 P205 S03 Right 0.00 0.00 2.02 1.04 1.45 0.21 (bright) 0.00 0.00 1.60 1.14 1.30 0.32 through 0.51 0.15 25.17 18.10 25.82 0.21 dark to 0.48 0.19 22.04 13.17 26.70 0.17 Left 0.33 0.22 21.90 20.11 25.07 0.16 (speckled) 0.45 0.15 20.71 18.10 30.62 0.27 0.40 0.25 30.27 12.20 27.40 0.17 0.13 0.36 18.88 14.08 25.54 0.26 0.14 0.37 19.04 11.59 26.36 0.33

15.23c) K20 CaO 0.00 0.38 0.00 0.41 0.44 7.36 0.32 7.52 0.41 6.74 0.29 6.50 0.44 7.39 0.15 3.96 0.50 4.33

Ti02 0.00 0.00 0.00 0.00 0.00 0.00 0.10 0.40 0.98

MnO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

FeO 74.70 79.07 1.77 2.36 1.96 1.22 2.20 19.07 18.71

BaO 0.00 0.00 0.25 0.23 0.15 0.19 0.00 0.20 0.12

Total 79.80 83.84 79.78 73.18 77.05 78.50 80.82 83.03 82.47

Section d: Rock Varnish Samvle from Namche Bazar rock wall (Figure 18,21d) TRANSECT: Na20 MgO AI203 Si02 P205 S03 Top 0.00 0.32 5.71 12.46 2.98 1 . 2 7 to 0.03 0.38 7.67 14.79 3.71 1 . 4 5 Bottom 0.20 0.33 8.75 10.93 3.07 1.20 0.00 0.48 6.97 11.95 3.14 1 . 1 5 0.13 0.48 9.67 10.10 3.53 1.70

K20 2.33 2.45 2.43 2.42 2.42

CaO 1.01 1.08 1.68 1.65 1.41

Ti02 0.42 0.42 0.43 0.42 0.45

MnO 35.53 36.75 40.16 48.19 36.84

FeO 13.98 13.73 13.58 14.12 14.08

BaO 1.28 1.97 1.09 1.30 1.37

Total 77.29 84.43 83.85 91.79 82.05

Sample from Imja Khola stre.aln-sidr boulder ffigure 15.22) TRANSECT: Na20 MgO A1203 Si02 P205 S03 K20 Top 0.15 0.22 9.17 6.00 0.77 0.07 0.64 to 0.13 0.26 6.11 7.17 0.71 0.08 0.55 Bt~Itma 0.12 0.31 9.07 10.60 0.80 0.00 0.50 0.12 0.44 8.18 9.66 0.82 0.00 0.61 0.11 0.51 7.88 9.90 0.88 0.00 0.49

CaO 0.50 0.57 0.49 0.33 0.56

Ti02 0.51 0.52 0.47 0.39 0.40

MnO 42.17 43.09 40.68 37.65 38.38

FeO 8.51 8.37 9.19 8.66 8.10

BaO 1.50 1.44 1.39 1.10 1.22

Total 70.21 69.00 73.62 67.96 68.43

Section e: Oxalate Crusts Sample from Namche Bazar rock wall (Figure 1~.23c) TRANSECT: Na20 MgO A1203 Si02 P205 S03 Top 0.00 1.44 0.13 0.04 0.05 0.27 to 0.00 1.30 0.14 0.05 0.05 0.29 Bottom 0.00 1.50 0.18 0.06 0.08 0.24 0.11 1.60 2.14 3.61 0.06 0.29 0.13 1.81 3.22 5.26 0.07 0.21 0.09 1.55 3.37 4.89 0.09 0.22 0.15 1.90 3.49 6.00 0.10 0.25 0.10 1.55 4.20 5.48 0.06 0.26

CaO 55.21 54.27 50.86 59.17 49.28 50.48 52.22 57.30

Ti02 0.00 0.00 0.00 0.05 0.06 0.07 0.05 0.00

MnO 1.80 2.20 0.10 0.16 0.90 2.00 1.56 1.12

FeO 0.00 0.00 0.05 0.07 0.05 0.06 0.07 0.00

BaO 0.25 0.32 0.00 0.00 0.15 0.33 0.25 0.18

Total 59.19 58.62 53.07 67.37 61.34 63.37 66.31 70.50

K20 0.00 0.00 0.00 0.11 0.20 0.22 0.27 0.25

362

Chapter 15

Quartzite clasts on the surface of the Khumbu Glacier at about 5250 m (Figure 15.13) have coatings of silica glaze, sometimes interbedded with iron films (Figure 15.21b; Table 15.1b). The appearance of the coating was a clear glossy glaze with an orange tint. Although the silica glaze is an accretion on the underlying rock, there was a clear preference of silica glaze for quartzite clasts on the glacier. These same quartzite clasts also have dark coatings of manganiferous rock varnish (Figure 15.22; Table 15.1d). A similar botryoidal micromorphology is found in stream coatings on leucogranite cobbles along the Imja Khola River near Pangboche at -4000 m.

At the terminus of the Khumbu Glacier at about 4900 m, glacially polished gneiss sometimes has an orange hue. Examination by both BSE and secondary electrons reveals that this "polish" is actually a combination of glacial polish and an extremely thin iron-rich silica glaze (Figure 15.21e). Subglacial rock coatings have been noted previously (Whalley et al., 1990). However, these shiny boulders give the appearance of true glacial polish, at least to the naked eye, and the rock coatings are much thinner than any subglacial rock coatings described previously. Gneiss avalanche boulders beneath Pokalde at about 4600 m have dark coatings of phosphate crusts, sometimes capped by iron hydroxides (Figure 15.21c; Table 15.1c). In places the iron and phosphates are mixed together in an iron phosphate, perhaps strengite (FePO4.2H20). Faced rocks in a wall in the town of Namche Bazar at ,-3400 built about 10 years prior to the time of collection, had two distinct types of rock coatings: manganiferous rock varnish (Figure 15.21d; Table 15.1d) and crusts of calcium oxalate (Figure 15.23). The calcium oxalate crusts on the wall illustrate different stages of development. Figure 15.23a shows a live crustose lichen with white dots of calcium oxalate in the outer section. After the lichen dies, the calcium oxalate apparently undergoes a diagenesis to a granular texture that rests on the rock surface, or a previous rock coating (Figure 15.23b). Older calcium oxalates on the wall have two types of textures: granular and porous; or distinctive layering. These two types of oxalate-rich crusts can interlayer at the scale of microns (Table 15.1e; Figures 15.23c and 15.23d). Different Khumbu rock coatings affect the general landscape, but differently at different scales. The most discernible effect is to darken the appearance of a mountain face (Figure 14.4). All of the inorganic rock coatings, except silica glaze, generally darken a rock's appearance. In addition, lithobiontic rock coatings are common on Khumbu rock surfaces, including lichens, fungi, algae, cyanobacteria, moss and bacterial mats. These organic films often have a dark pigment (Fletcher et al., 1985), perhaps to provide protection against ultraviolet radiation (Vincent and Roy, 1993). Iron skins appear to be most prevalent in locally acidic environments. For example, the pH of soil samples in contact with ten cobbles on the Kala Pattar averaged 4.3+ 1.2. Other iron skins were found on rock surfaces exposed to spring water exiting from bog environments with Sphagnum spp.; acidiphilous iron bacteria may be important in iron precipitation (Nealson, 1983) in these environments. Microenvironmental settings appear to control the distribution of rock coatings. The most dominant type of rock coating in the Khumbu are lithobiontic coatings, where the growth of lichens, moss, algae, cyanobacteria, fungi, and bacteria are determined by biogeographic controls. The second most common rock coating are oxalate-rich crusts; these occur most commonly adjacent to crustose lichens. The third and fourth most abundant coating are rock varnish and manganese heavy-metal skins that grow where water flow occurs seasonally over rocks. In fourth place are iron films in places where acidic waters flow over rocks. The distribution of silica glazes and phosphate films are

Geographical Variations

much more limited, and I could not determine even a speculative environmental control on the distribution of these coatings.

Figure 15.21. Different rock coatings seen in cross-section, from the Khumbu of Nepal. Scale bar in microns. All images are taken with backscattered electrons except the right image in Figure 15.21e which shows topography with secondary electrons. a. Stringers of iron hydroxides precipitate within fractures in gneiss collected from the Kala Pattar (-5400 m). The thickest iron films are found at the surface and underneath some gneiss. As the iron mobilizes and reprecipitates in the fractures, it mechanically weathers the gneiss. b. Silica glaze precipitated on a quartzite clast resting on the surface of the Khumbu glacier (-5250 m). The bright material underneath the silica glaze on the left side of the image is an iron film. The dark spaces are pore spaces in the quartzite weathering rind. c. Phosphate crust on an avalanche boulder on the slope beneath Pokalde, at about 4600 m. The brighter material is a pocket of iron oxides that interdigitates with the less bright phosphate. d. Manganiferous rock varnish on a gneiss cobble that was faced about 10 years before collection, from the village of Namche Bazar at -3400 m. e. Backscatter (left) and secondary electron (right) views of a glacially polished boulder at the terminus of the Khumbu Glacier at about 4900 m. The BSE image reveals that the polish is a different material than the underlying gneissic quartz grain. The coating is less than a micron thick, which is less than the spot size of the electron microprobe. Qualitative EDS analysis that includes both the underlying material (quartz) and the 'polish' reveals that it is an iron-rich silica glaze that contains four elements that are below the limit of detectmn in the underlying rock (iron, aluminum, calcium, and manganese). f. An iron film on gneiss clast on the surface of the Tshola rock glacier (-4500 m) is remobilizing and is precipitating 'stringers' within the host clast.

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Figure 15.22. Secondary electron images of manganiferous rock varnish with a botryoidal micromorphology. The left image shows a cross-section of the botryoidal varnish on top of a gneissic rock found on the surface of the Khumbu glacier on quartzite boulders. The fight image shows a granitic stream-side cobble along the Imja Khola River at about 4000 m near Pangboche. The full length of the dots is indicated in microns. See Table 15. ld for the chemical composition measured in a polished cross-section.

Figure 15.23. Oxalate growing on faced gneiss cobbles on 10-year-old wall in Namche Bazar. Scale bar in microns. All images by BSE. a. Live lichen, showing bright oxalate dots in the crustose portion. The oxalate gives the tissue mechanical strength. b. Granular calcium oxalate resting on the gneiss rock, and in this case next to manganiferous rock varnish (brighter). c. Calcium oxalate crust with an outer porous layer and an inner lamellate layer. d. Close-up of the laminae in the box shown in Figure 15.23C.

Geographical Variations

Rock coatings in the Khumbu appear to grow and erode on time scales from years to centuries. Faced stone used to construct house walls have well-developed manganiferous rock varnish and calcium oxalate-rich crusts. Stream boulders that are abraded each melt season develop manganiferous rock varnish the following low-flow period. Boulders on Little Ice Age moraines (Fushimi, 1980) display several cycles of coating development and boulder spaUing. For example, rock varnish forms on polished surfaces that have inset spalls coated by oxalate-rich crusts, that are in turn eroded by inset spalls and the formation of more oxalate-rich crust. Rapid formation of rock coatings is undoubtedly tied to the moist climate, combined with an abundance of bare rock surfaces. In contrast, rapid rates of coating erosion may also be tied to abundant moisture which promotes acidity and concomitant chemical erosion, along with more rapid boulder spalling.

15.3.3. Rock Coatings in Ashikule Basin, West Kunlun Mountains A variety of rock coatings were found in this cold, dry, and dusty portion of the Tibetan Plateau: rock varnish, silica glaze, oxalate-rich crusts, phosphate films, carbonate crusts, dust films, and sulfate crusts (BaSO4, CaSO4). The most common rock coatings are silica glaze, rock varnish, and dust films. The other types of coatings are much less abundant. Several rock coatings play a role in breaking apart clasts, through a process that is often called salt weathering, a term that is in part a misnomer in this case. Sulfates were found on the sides of fractures in all volcanic samples (Figures 15.24a,b). Barite plays a role in the brecciation of grains (Figure 15.24a), while gypsum spreads fractures by precipitating along joint walls (Figure 15.24b). Carbonate is also found along rock fractures by itself (Figure 15.24d), or precipitated on top of gypsum (Figure 15.24b). Deflation of salts from shorelines of saline lakes likely provides the raw ingredients. On the volcanics, rock varnish is best developed in cracks, where it can be seen separating breaking-off pieces from the main rock (Figure 15.24f). Dust along the sides of crevices probably assists in the opening of joints (Figure 15.24c). In a few samples, phosphates films were found in the narrowest part of the rock crevice (Figure 15.24e). The abundance of loess on the landscape is reflected in the nature of the rock coatings. The surfaces of many rocks are coated with a thin film of dust (Figure 15.25a,b). This dust is then cemented to the rock surface in silica glazes (Figure 15.25c). Dust may also be an important source of silica in glazes that are deposited from solution (Figure 15.25d). The vast majority of subaerial rock varnishes are extremely thin (Figure 15.26d), even though they can appear visually well developed to the naked eye. Thin varnishes, however, often encapsulate thicker deposits of loess deposited in rock-surface depressions (Figure 15.26e, 15.26f, 15.26g). Dust pieces are also incorporated into thicker varnishes (Figure 15.26h). One of the most unusual aspects of Tibetan varnish is the interdigitation of varnish and phosphate films, even at the micron scale (e.g., Figure 15.26c). In order to assess the source of the manganese in the rock varnish (Table 15.2f-1), the dust attached to the volcanics in the Ashikule Basin was analyzed in situ (Table 15.2a) and gently scraped from each sample. The scraped material was homogenized in a flux of lithium metaborate, polished, and analyzed with a 1001am beam with the electron

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m i c r o p r o b e . T h e c o n t e n t o f M n O a v e r a g e d 0 . 1 1 % with a s t a n d a r d d e v i a t i o n o f 0 . 0 8 % . T h e c o n t e n t o f F e O w a s m u c h larger, 5.13_+2.52%. T h e p H o f the d u s t w a s 8.8+0.7.

Figure 15.24. Different types of rock coatings originate in rock fractures in the Ashikule basin. All images are by BSE. Scale bars are in microns. a. Barite (bright material at top of image) precipitated on and appears to be causing a brecciation of the underlying plagioclase (darker) minerals on the wall of a fracture. The sample is from AKB-93-1, a volcanic bomb at the rim of Ashishan volcano. b. A fracture waU has a sequence of gypsum (darker inner material) and calcium carbonate (brighter outer material), from sample AKB-93-5, a sloped surface of the west lava flow of Ashishan volcano. c. Dust accumulating on the wall of a fracture, in sample AKB-93-2, the northeast lava flow of the Ashishan volcano. The line indicates the location of an electron microprobe transect (Table 15.2a) that runs from loose dust to particles cemented by crack varnish. d. Calcium carbonate deposited along a joint fracture of sample AKB-93-7, from the south lava flow from the Ulukeshan cone. The line locates a probe transect (Table 15.2b). e. Iron phosphate found at the base of a rock fracture in AKB-93-23, a lava flow of Migongshan volcano. The line locates a probe transect (Table 15.2c). f. Rock varnish can be seen on the surface and penetrating and opening a rock crevice, from sample AKB-93-21, a lava flow from the Daheishan cone.

367

G e o g r a p h i c a l Variations

Table 15.2. Electron microprobe analyses of rock coatings in the Ashikule Basin of the West Kunlun Mountains, Tibet. Values in oxide weight percent. Totals do not reach 100% due to porosity, water, and content of organic matter. Transects correspond to lines in indicated figures. 9

i

9

i

Section a. Adhered dust and crack varnish (Figure 15.24c) TRANSECT: Na20 MgO AI203 Si02 P205 S03 1(20 CaO Ti02 MnO FeO BaO Top to Bottom

0.44 0.47 0.90 0.23 0.94 0.51 0.77 0.22 0.16 0.49

1.47 1.25 1.65 0.49 0.85 0.81 1.57 1.43 0.90 1.32

0.45 0.45 0.35 0.22 0.15 0.13 0.22 0.30 0.22 0.27

0.12 0.17 0.43 0.08 2.19 3.14 6.25 7.48 9.87 9.80

5.43 6.18 8.30 8.07 19.66 15.74 24.48 23.01 27.44 22.98

0.23 0.21 0.23 0.06 0.21 0.42 6.17 3.68 1.70 0.21

Total 77.19 63.71 62.49 89.27 88.48 88.81 94.51 81.19 95.06 95.86

CaO 3.10 0.72 20.60 0.30 1.70 0.10 56.82 3 . 5 3 2.00 19.09 0 . 5 7 1.57 0.17 52.01 0.95 20.71 18.08 0.00 0 . 1 2 1.22 48.04

Ti02 0.00 0.07 0.47

MnO 0.05 0.00 0.00

FeO 2.13 1.36 1.74

BaO 0.00 0.00 0.27

Total 85.68 80.57 98.38

1.64 4.26 1.94 1.11 1.19 1.24 1.76 2.11 2.16 1.86

14.19 17.87 12.79 7.80 9.58 10.18 15.59 12.19 16.84 16.91

50.49 30.51 33.80 69.64 52.46 54.38 31.77 25.74 30.51 36.97

0.21 0.11 0.27 0.00 0.09 0.09 0.16 0.00 0.07 0.14

0.50 0.30 0.22 0.17 0.12 0.27 3.72 2.42 1.17 0.60

2.02 1.93 1.61 1.40 1.04 1.90 2.05 2.61 4.02 4.31

Section b. Calcium carbonate crust (Figure 15.24d) TRANSECT: Na20 MgO AI203 Si02 P205 S03 K20 Top to Bottom

0.16 0.20 6.78

Section c. Phosphate film (Figure 15.24e) TRANSECT: Na20 MgO A1203 Si02 P205 Top to Bottom

0.88 0.80 0.68 0.75 0.67

0.19 0.21 0.29 0.35 0.40

29.44 30.84 30.04 29.09 32.07

S03 0.23 0.23 0.22 0.21 0.23

K20 0.16 0.06 0.14 0.15 0.16

CaO 8.26 8.67 9.08 8.28 7.69

Ti02 0.00 0.00 0.00 0.00 0.10

MnO 0.00 0.00 0.00 0.00 0.00

FeO 2.30 2.02 2.01 1.94 1.85

BaO 0.00 0.00 0.00 0.00 0.00

Total 76.27 76.92 77.31 75.82 80.75

P205 S03 49.88 0 . 1 9 0.71 47.26 0 . 3 1 0.83 45.99 0 . 3 1 1.06 6 2 . 0 7 1 . 5 2 0.33 63.49 1 . 6 2 0 . 2 9 5 9 . 8 3 1 . 4 9 0.18

K20 2.44 2.16 2.16 0.24 0.18 0.25

CaO 3.03 3.14 3.55 2.17 1.96 1.80

Ti02 0.14 0.23 0.21 0.10 0.13 0.09

MnO 0.00 0.00 0.00 0.22 0.24 0.20

FeO 6.45 4.14 7.19 1.51 1.30 1.97

BaO 0.67 0.70 0.83 0.20 0.19 0.15

Total 84.00 79.26 80.49 74.21 76.03 74.62

K20 0.00 0.00 0.00 0.00 0.00 0.00

CaO 1.03 1.22 1.20 1.09 1.55 1.43

Ti02 0.00 0.00 0.00 0.00 0.00 0.00

MnO 0.00 0.00 0.00 0.00 0.130 0.00

FeO 0.24 0.26 0.30 0.67 0.78 1.02

BaO 0.00 0.00 0.00 0.00 0.00 0.00

Total 92.68 93.46 90.54 86.18 90.15 89.70

3.46 2.00 2.17 2.57 2.48

31.35 32.09 32.68 32.48 35.10

Section d. Silica glaze (Figure 15.25c) TRANSECT: Na20 MgO A1203 Si02 Left to Right

0.67 0.55 0.94 0.18 0.33 0,19

3.64 2.94 2.07 0.45 0.60 1.17

16.18 17.00 16.18 5.22 5.70 7.30

Section e. Silica glaze (Figure 15225d) TRANSECT: Na20 MgO A1203 Si02 Top to Bottom

0.13 0.12 0.15 0.20 0.23 0.19

0.00 0.00 0.00 0.00 0.00 0.00

5.91 5.48 6.29 4.94 6.90 4.06

85.24 86.14 82.34 78.90 80.29 82.55

P205 0.13 0.24 0.26 0.38 0.40 0.45

S03 0.00 0.00 0.00 0.00 0.00 0.00

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Chapter 15

Section L Rock varnish and phosphate (Figure 15.26b) TRANSECT: Na20 MgO Al203 Si02 P205 S03 K20 Left to Right

0.00 2.97 0.22 4.39 0.00 0.00 0.07 0.70 2.32 0.36 0.43 1.31 0.24 0.00 0.00

2.55 1.72 2.12 1.16 1.14 2.84 0.23 2.04 0.93 0.20 3.52 1.29 2.95 4.15 0.66

12.38 9.41 11.85 13.96 10.28 12.23 1.23 12.43 9.50 1.72 13.89 11.36 18.48 1.80 3.29

27.38 34.25 28.18 43.94 22.93 26.25 82.94 34.57 53.36 93.47 31.56 47.15 36.99 13.37 56.59

13.22 10.38 14.09 4.90 18.95 11.46 2.52 12.90 9.85 0.62 11.18 9.81 7.88 25.16 11.57

0.57 0.82 0.17 0.75 0.85 0.20 0.35 0.75 0.20 0.00 0.50 0.00 0.27 0.72 1.07

2.11 0.72 1.82 1.33 1.79 2.75 0.14 1.40 1.02 0.19 1.52 1.25 4.30 0.40 1.01

CaO 18.65 15.34 21.18 12.05 23.56 14.58 2.71 18.75 14.89 1.36 16.94 16.69 10.40 34.81 14.12

Ti02 0.90 0.20 0.70 0.38 0.30 0.25 0.05 0.77 0.15 0.03 0.52 0.38 0.23 0.10 0.22

MnO 1.92 1.34 1.32 0.83 1.14 6.35 0.21 1.34 0.28 0.04 0.58 0.54 1.46 1.64 0.49

FeO 9.31 16.23 8.24 5.06 4.89 11.14 0.51 7.28 3.10 0.39 15.68 6.26 6.35 2.44 1.47

BaO 0.60 1.38 0.36 1.22 0.95 1.33 0.16 0.61 0.10 0.00 0.00 0.07 0.30 0.55 1.41

Section g. Rock varnish and phosphate film (Figure 15. 26c) LEFTTr. Na20 MgO A1203 Si02 P205 S03 K20 CaO Ti02 MnO FeO BaO

Total 89.59 94.76 90.25 89.97 86.78 89.38 91.12 93.54 95.70 98.38 96.32 96.11 89.85 85.14 91.90

Top to Bouom

0.00 0.00 0.00 0.00 0.00 0.00 0.00

2.49 1.14 0.48 0.53 4.00 2.29 2.84

9.66 6.82 2.40 2.12 11.36 11.81 2.44

16.84 13.50 5.69 3.87 22.68 21.35 10.91

14.02 21.52 27.41 21.13 17.32 5.16 27.20

0.32 0.27 0.17 0.00 0.15 3.25 1.05

1.41 1.29 0.52 0.47 2.14 1.84 0.45

18.44 36.16 48.02 21.25 21.60 7.09 37.01

0.33 0.40 0.00 22.07 0.20 0.32 0.10

9.80 1.32 0.58 8.78 0.92 11.39 0.99

18.94 4.19 1.59 18.58 11.50 17.64 3.52

0.63 0.21 0.09 0.15 0.09 5.73 0.50

Total 92.88 86.82 86.95 98.95 91.96 87.87 87.01

RIGHTTr. Top to Bottom

0.05 0.00 0.00 0.00 0.00 0.00 0.00

2.52 1.56 2.49 0.41 0.40 2.17 2.75

13.74 6.20 10.01 2.06 2.38 10.60 1.78

26.53 73.94 19.60 4.39 17.71 20.30 8.49

8.11 1.42 19.89 21.72 23.76 6.71 24.98

0.00 0.10 0.20 0.20 0.32 0.00 1.02

2.22 1.65 1.52 0.37 0.51 1.61 0.35

11.77 2.08 26.71 28.02 40.83 8.27 36.38

0.50 2.00 0.08 0.05 0.28 0.35 0.10

6.46 0.19 0.94 8.58 0.43 13.12 1.82

13.25 2.10 6.23 11.40 1.10 18.84 4.96

0.94 0.51 0.13 0.15 0.09 1.94 0.95

86.09 91.75 87.80 77.35 87.81 83.91 83.58

Section h. Rock varnish and phosphate (Figure 15.26d) TRANSECT Na20 MgO AI203 Si02 P205 S03 K20 CaO Ti02 MnO FeO BaO Top to Bottom

0.00 0.00 0.00 0.00 0.23 0.00 1.46 0.11 0.00 0.00 0.00 0.00

3.08 2.92 3.56 1.94 2.19 2.50 1.69 1.33 3.37 2.01 2.21 2.07

14.44 12.43 10.45 10.45 11.51 9.26 11.24 9.18 11.94 11.13 11.71 10.22

26.76 23.98 37.57 23.47 27.88 36.43 34.47 37.35 26.83 19.40 24.13 19.10

5.11 13.34 10.98 11.14 15.01 12.28 13.79 13.11 13.73 14.23 13.24 18.40

0.10 1.95 7.08 3.67 8.26 15.57 0.98 0.12 2.76 19.73 0.83 1.54 10.92 0.20 0.15 1.39 18.57 0.65 1 . 4 2 6.72 0.36 0.00 1.72 17.03 15.40 0.75 5.86 0.58 0.57 1.86 23.45 0.28 0.44 6.49 0.90 0.37 1.54 18.96 0.35 0.46 6.53 0.45 0.30 1.39 20.09 0.25 0.46 5.18 0.23 0.10 2.37 19.60 0.83 0.89 4.83 0.45 0.25 1.71 20.22 0.52 2.49 8.85 0.46 0.17 1.73 20.48 0.47 4.93 12.47 0.71 0.30 2.00 18.81 0.52 4.53 11.81 0.71 0.05 1.43 27.05 0.32 2.81 8.64 0.40

Total 87.00 88.77 91.82 88.34 90.81 89.13 90.55 90.15 90.37 87.73 89.97 90.49

Geographical Variations Section i. Rock varnish (Figure 15.26e) TRANSECT Na20 MgO A1203 Si02 P205 UpperRight 0.20 3.28 11.90 24.67 1.79 to

Lower Left

0.20 0.00 0.16 5.36 2.72 4.11 3.55

3.12 3.02 2.55 1.43 2.57 2.32 3.18

11.79 11.41 11.19 14.27 9.94 11.58 11.41

22.89 22.19 21.80 61.18 41.65 52.69 53.36

CaO 3.65 4.06 4.46 4.45 4.76 11.75 8.68 8.30

Ti02 0.42 0.32 0.70 0.53 1.15 1.90 1.10 0.97

MnO 15.71 16.35 15.70 13.76 0.25 1.54 0.62 0.53

FeO 18.52 18.37 19.64 24.68 6.41 18.64 9.29 10.25

BaO 1.92 2.18 2.39 2.22 0.23 0.36 0.23 0.21

CaO 0.95 1.11 20.37 0.22 1.75 8.70 0.55 1.63 20.61 0.22 0.46 38.24 0.00 0.40 10.58

Ti02 0.27 0.42 0.47 0.08 0.03

MnO 2.16 11.26 4.34 3.20 0.17

FeO 4.53 14.54 11.85 5.52 3.33

BaO Total 1.47 93.36 1.62 86.10 0.60 85.41 0.32 78.46 0.09 92.56

CaO Ti02 MnO 1.70 0.02 1.48 3.65 0.38 18.76 1.92 0.00 1.53 3.79 0.32 20.25 1.74 0.12 1.55 4.04 0.50 19.70 2.41 0.07 1.69 4.58 0.77 17.64 2.61 0.25 1.76 4.86 0.60 13.08 0.69 12.06 0.25 12.88 0.50 2.83 0.00 35.07 0.00 36.78 0.00 0.00 0.00 31.11 0.00 33.11 0.00 0.00

FeO 17.86 17.19 17.67 19.03 21.45 2.82 0.13 0.14

BaO 2.05 2.32 2.37 2.17 1.65 0.33 0.40 0.45

Total 80.46 81.05 81.65 83.28 83.02 82.98 79.88 74.09

FeO 0.04 6.23 0.04 0.06 0.01 0.09

BaO 0.00 0.02 0.00 0.06 0.00 0.00

Total 93.87 81.72 73.76 76.09 84.35 82.43

2.25 2.41 2.98 0.32 2.50 2.20 1.86

S03 0.40 0.45 0.12 0.15 0.00 0.00 0.17 0.00

K20 1.86 1.69 1.66 1.73 4.25 2.16 3.34 3.02

Section J. Rock varnish and phosphate (Figure 15.26g) TRANSECT Na20 MgO A1203 Si02 P205 S03 K20 Top to Bottom

1.11 0.00 0.00 0.00 5.34

1.31 2.69 1.79 0.60 0.23

10.17 33.69 11.98 25.89 10.64 19.00 2.91 5.16 20.67 49.31

369

16.22 7.03 13.93 21.75 2.41

Section k. Rock varnish on sulfate (Figure 15.26i) TRANSECT Na20 MgO A1203 Si02 P205 S03 K20 Top to Bottom

0.05 0.16 0.07 0.12 0.24 0.53 0.94 1.00

3.02 2.84 2.95 3.05 3.40 2.12 0.00 0.00

10.52 10.90 10.32 11.15 10.83 1.51 1.05 2.11

20.97 19.83 20.62 20.60 22.29 46.46 5.51 6.17

Section !. Calcium oxalate and silica glaze (Figure 15.28a) TRANSECT Na20 MgO A1203 Si02 P205 S03 K20 Upper Left 0.84 0.35 3.14 88.40 0.00 0.00 0.23 to

Lower Right

0.31 0.95 0.16 0.07 1.23

5.75 6.01 2.75 5.46 4.74

8.16 9.46 4.33 1.44 7.90

6.49 12.12 12.66 60.53 45.97

0.00 0.02 0.00 0.16 0.09

0.00 0.00 0.72 1.07 1.25

0.40 0.05 0.28 0.05 0.46

CaO 0.87 53.1 45.11 55.07 15.53 20.67

Ti02 0.00 1.18 0.00 0.00 0.03 0.03

MnO 0.00 0.08 0.00 0.00 0.00 0.00

Total 84.32 83.67 83.70 86.20 99.61 95.73 96.33 96.64

The amount of manganese found in the varnish is about 100 times more than in the loess. Therefore, at least 100 times the varnishes weight in loess must come in contact with the rock surface, somehow chemically release the Mn, which is then reprecipitated on the rock surface. Because the lava flows have been exposed to loess deposition for more than 70,000 years (F.M. Phillips, personal communication, 1995), far more loess has come in contact with the rock surfaces than is necessary to provide enough Mn for the varnish. In other words, the source of the Mn is not a problem. Rather the uncertainty rests in the process by which the manganese is concentrated. The pH values of the dust are far too high to have manganese released from the dust (Mulder, 1972; Schweisfurth et al., 1980; van Veen, 1972). In order to release the Mn, it must be transformed to the mobile divalent state, which might occur with the natural acidity associated with rain/snow. The dissolved Mn could then be reprecipitated on the rock surface by either: (1) evaporation of the moisture; (2) a change in pH when the solution comes in contact with another loess particle; or (3) microbial oxidation and fixation of manganese.

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Figure 15.25. Dust and sihca glaze on rock surfaces. All images are by BSE. Scale bars are in microns. a. A top-down view of dust on surface of AKB-93-2, from the northeast Ashisan flow. The pattern of cracking is similar to mud cracks. The dark gray areas may be remnants of desiccated fungal hyphae. b. A cross-sectional view of the same sample as Fig. 25a, where a splitting biotite grain is enveloped by loosely adhered dust. The dust may enhance weathering by storing capillary water. c. Two types of silica glaze on AKB-93-8, the east lava flow from the Yishan cone. The glaze on the left is composed of mostly loess particles cemented by silica glaze, where as the glaze on the right is composed of mostly silica glaze. The reason for the discontinuity is not clear. The line indicates the location of a probe transect (Table 15.2d). d. Silica glaze formed on AKB-93-5, the west flow of the Ashisan cone. This silica glaze is comparatively unusual, because it does not contain pieces of dust detritus. Instead, the sihca glaze appears to be reprecipitated in laminae, probably from solutions. Also, a perpendicular fracture split the silica glaze (and the underlying rock), before the most recent depositional level. The line locates the probe transect (Table 15.2e).

A bacterial hypothesis for the enhancement of manganese from loess is favored by in situ microscopic observations. Manganese-concentrating bacteria occur on Tibetan varnishes (Figure 15.27a), and they are also found within the varnishes (Figure 15.27b). The bright dots in Figure 15.27a are greatly enriched in Mn, when examined by EDS spots ~ll.tm in diameter. They are the fight size for cocci that precipitate manganese on cell walls (cf. (Ferris et al., 1987b; Greene and Madgwick, 1991). Bright rims around the bacteria in Figure 15.27b are smaller than the ~llam spot size of EDS, but analyses centered on the bacteria reveal high peaks in Mn. Since brightness in BSE is from higher atomic number, it is reasonable that the Mn is associated with the cell walls. A careful examination of Figure 15.27b reveals smaller cocci-shaped features, perhaps the desiccated remains of bacterial casts. Although calcium oxalate crusts are far less common in the Ashikule Basin than in Nepal, they do occur on lava flow surfaces in association with silica glaze (Figure 15.28a,d; Table 15.2m) and lichens (Figure 15.28b,c). In one case, a calcium carbonate crust formed over a calcium oxalate-rich crust (Figure 15.28d).

Geographical Variations

Figure 15.26. Examples of rock vamishes found in the Ashikule Basin, imaged by BSE. Scale bars are in microns a. The rock vamish currently on the surface started in a rock fracture; its progressive growth separates grains from the rock (upper left), from AKB-92-23, southeast flow of the Migongshan volcano. b. A thin rock vamish on the surface (left) is much better developed in a rock fracture. The line indicates the probe transect (Table 15.20. The sample is from AKB-93-8, the east flow of Yishan volcano. c. The interdigitation of varnish and silica glaze is seen here on a very fine scale. The lines indicate the electron microprobe transects (Table 15.2g), from AKB-93-2. d. This is one the thickest vamishes found on the subaerial surfaces of the volcanics in the Ashikule Basin. The line indicates probe transect (Table 15.2h), from AKB-93-1, a bomb on rim of Ashishan volcano. e. Rock vamish formed on top of silica glaze, from AKB-93-8, east flow of Yishan volcano. The line indicates a probe transect (Table 15.2i). f. Varnish cementing dust deposits accumulate in a depression in sample from AKB-93-21, south flow of Daheishan volcano. g. Close-up of the rock varnish in the center of Figure 26f. The line indicates the probe transect (Table 15.2j). h. Rock varnish developed on a glacial moraine from the West Kunlun Mountains. The porous zones are areas of cation leaching where capillary water flows through the varnish. Also note the large detrital grains of dust. The sample is from AKB-93-11, Yishan dome rhyolite. i. Rock varnish formed on top of a sulfate crest. The sample came from a crack vamish, from AKB93-8, east flow of Yishan volcano. The line indicates probe transect (Table 15.2k).

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Fi.gure 15.27. Manganese-concentrating bacteria found in the Ashikule Basin, Tibet. Scale bars are in microns. a. BSE image of the surface of varnish on AKB-93-6, the west flow of the Ashishan volcano. The sample was first subjected to etching by HF fumes to remove the very surface layer of dust, very gently washed with deionized water, and carbon coated. b. BSE image of cross-section of varnish from AKB-93-6, the west flow of the Ashishan volcano.

15.3.4. Discussion and Conclusion The rock coatings on the roof of the world can be placed in a broader context in different ways. For the general physical geographer trying to understand spatial variations in the natural landscape, there is an asymmetry in rock coatings that reflects the entire physical geography system. For the geomorphologist with an eye on the weathering system, rock coatings are a secondary weathering product and a positive feedback on rock weathering. For the purposes of this chapter, the transect across the roof of the world illustrates a serious flaw in attempts to map geographical variability in rock coatings.

15.3.4.1 Asymmetry in Rock Coatings and Landscape Aesthetics Rock coatings are ubiquitous in both the cool and wet Khumbu of Nepal and the cold and dry West Kunlun Mountains of Tibet. Coatings are even found on rocks resting on the surfaces of active glaciers. Uncoated 'bare' rock is seen only where the rate of erosion exceeds the rate of coating, for example in Nepal in places of frequent avalanching. Although rock coatings are ubiquitous in both settings, they reflect the

Geographical Variations

very different physical geography systems on opposite sides of the highest topography on Earth.

Figure 15.28. Calcium oxalate crests in Ashikule Basin, Tibet. The scale bars are in microns a. and c. Lichens, oxalate, and silica glaze, from AKB-93-8 Yishan volcano lava flow. Figure 15.28a presents a view of backscattered electrons, where the lichens appear black; in 15.28c, the lichens are visible. The line in 15.28a indicates the location of a probe transect (Table 15.21). The darker areas in BSE are silica glaze. b. Speckled texture of calcium oxalate on AKB-93-1 next to a lichen, from the Ashishan volcano. d. Superposition of calcium carbonate (brighter outside rim), on oxalate crust (mixed with dark threads of silica glaze), all on top of a basal layer of dark silica glaze, now on the surface of AKB-93-21, the south lava flow from the Daheishan volcano.

There are similarities and differences in rock coatings in the two contrasting study areas. Rock varnish, silica glaze, calcium oxalate crusts, and phosphate films occur in both areas. At the same time, these same coatings display differences. For example, Khumbu rock varnish is intermediate between clay-rich varnish (chapter 10) and clayabsent manganese heavy-metal films (chapter 8); it has much more manganese, fewer clay minerals, and is most common where water flows. Tibetan varnish is more geographically widespread, and interfingers with silica glaze and phosphates, but is extremely thin. Silica glaze is also much more widespread in Tibet. Calcium oxalate is much more geographically restricted in Tibet. For example, on the Ashikule Basin lava flows it occurs next to small patches of lichens, and it interdigitates with silica glaze. The wet and geomorphically active Khumbu is a young landscape, with slope and fluvial processes still adjusting to the retreat of the 16th Century Little Ice Age glaciers. Lithobionts (moss, algae, lichens, fungi, cyanobacteria) tend to form first on moistened surfaces. Dark coatings of calcium oxalate (whewellite) form streaks downflow from organic coatings or plants. Dark films of iron hydroxides are found in the Khumbu where acidic waters support acid-loving iron bacteria (cf. Nealson 1983). Manganiferous rock varnishes occur in the most xeric settings and on stream-side boulders. The net aesthetic effect is to darken the landscape by masking rock minerals with low albedo coatings, which increases the visual contrast between ephemeral snow and the rocky landscape.

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In a West Kunlun, Tibetan, landscape almost devoid of vegetation with lower relief, rock coatings play a more obvious role in defining the landscape aesthetics. High albedo tan-colored loess is everywhere, drowning the microtopography. Although the slower-growing dark manganiferous rock varnish is much thinner in Tibet than in Nepal, it is ubiquitous and provides a strong visual contrast to the loess. Dark colors come from rock varnish, and varnish is interdigitates with silica glaze and phosphate films.

15.3.4.2. Comparison with Rock Coatings in Other Geographic Settings The objective of this section is to compare the chemical and textural aspects of Khumbu and Tibetan rock coatings with rock coatings in other geographic settings.

15.3.4.2.1. Iron Films The ability of bacteria genera such as Leptothrix, Thiobacillus, and Gallionella to oxidize iron in acidic to neutral waters is well known (Aristovskaya, 1975; Chukhrov et al., 1973; Mallard, 1981; Nealson, 1983). Although I did not attempt to culture microorganism, bacteria-like forms were seen with the aid of electron microscopy on the surface of iron skins in Nepal. Because iron skins are associated with acidic drainage and iron is geochemically mobile in its divalent state, bacterial oxidation and fixation is required for the presence of iron skins. Iron films are common near nivation patches (e.g., Cailleux 1967), especially where snow melt moves through Sphagnum moss and waters have low pH values. One less appreciated aspect of iron films is that precipitation of iron helps create silt-sized rock fragments, in a fashion similar to the observations of iron skins from Karkevagge, Northern Scandinavia (Dixon et al., 1995) and Antarctica (Hayashi, 1989). Thus, the glacial flour load of alpine systems may be due, in part, to the weathering of rocks by iron films. Iron is present in Khumbu coatings at concentrations greater than 55% (measured as FeO). Aluminum and silica are the next most common elements, and their abundance appears inversely correlated with the amount of phosphorus and sulfur. Unfortunately, there are not enough analyses globally to determine if the iron films in the Khumbu (Table 15.1a) are typical of iron films in similar environmental settings.

15.3.4.2.2. Silica Glaze Deposits of amorphous silica, mixed with aluminum and iron, are found in virtually every terrestrial environment including: coastal sea cliffs (Mottershead and Pye, 1994), tropical rivers (Alexandre and Lequarre, 1978), subaerial tropical rock surfaces (Curtiss et al., 1985), subtropical deserts (Fisk, 1971), Antarctica (Weed and Norton, 1991), and temperate humid settings (Dorn and Meek, 1995; Robinson and Williams, 1987). The silica glazes analyzed from the Khumbu and West Kunlun Mountains are similar to those found elsewhere. The distinctive morphological and chemical break between rock and coating reflects an external origin (Figures 15.21b, 15.25c,d, 15.28d). Amorphous silica is the main constituent (over half SIO2), with lesser amounts of aluminum (5 to -20% A1203) and iron (1 to ~ 10% FeO). Textures range from those rich in detritus (Figure 15.25c, 15.26e) to even compositions indicative of deposition from solutions (Figure 15.21b, 15.25d). The interdigitation of silica glaze and other

Geographical Variations

rock coatings (Figure 15.21b, 15.26e) occurs elsewhere, for example, South Africa (Butzer et al., 1979) and Hawai'i (Dora et al., 1992a).

15.3.4.2.3. Dust Films

Dust particles attach to rock surfaces in deserts (Hobbs, 1917; Rivard et al., 1992). In Tibet, loess that comes to rest on rock surfaces (Figure 15.25a,b) is cemented to more permanently by rock varnish (Figure 15.24c, 15.26f, h), silica glaze (Figure 15.25c, 15.26e), carbonates (Figure 15.24d), sulfates (Figure 15.26i), and probably oxalates (Figure 15.28b).

15.3.4.2.4. Carbonate and Sulfate Crusts

Sulfate (barite, gypsum) and carbonate crusts rock coatings on Tibetan volcanics probably started as deposits in rock crevices with raw material coming from the deflation of sulfate salts along lake margins. However, gypsum crusts in Antarctica are precipitated as snow sublimates (Hayashi, 1989), a process that could also occur in the Ashikule basin. Sulfate and carbonate occur together in Figure 15.24b, which also occurs in coastal deserts such as the Namib (Goudie, 1972) and Baja California (Conca and Rossman, 1985). The origin of exposed carbonate and gypsum is complex, however, and must be considered on a case-by-case basis (Watson, 1985; Watson, 1992; White, 1993b). Hence, these comparisons are speculative.

15.3.4.2.5. Oxalate-Rich Crusts

Oxalates-rich crusts are much more abundant crusts than many had previously thought. Whewellite (CaC204.H20), a calcium oxalate, is probably the most common mineral in rock-surface settings (Lewin and Charola, 1981; Russ et al., 1994; Zfik and Sk~ila, 1993), although oxalates occur with other cations such as iron (humboldtine) and magnesium (glushinskite). Many oxalates derive from lichens (Del Monte et al., 1987a; Wilson et al., 1980), but they may also come from plants (Lewin and Charola, 1981). Oxalate crusts in Nepal and Tibet appear to derive from lichens. Figure 15.23, all from one wall in Namche Bazar, shows a possible sequence of diagenesis to form oxalate-rich crusts: from speckled Ca-oxalate within live crustose lichens (Figure 15.23a); to granular textured crusts immediately adjacent to lichens (Figure 15.23b); and finally to more massive oxalate (Figure 15.23c,d). The few oxalates found in the Ashikule Basin were found only immediately adjacent to lichens. Calcium-oxalate crusts may include other constituents in variable amounts, for example gypsum, clays, and quartz from eolian sources (Russ et al., 1994). The most common co-constituent in the Khumbu and West Kunlun oxalates is silica-aluminum in the laminar section in Figures 15.23c,d and Figure 15.28a; these elements probably derive from clay minerals. The granular section in Figure 15.23c, however, does not contain A1-Si (Table 15.1e) all while the magnesium content remains constant, suggesting the magnesium has an origin different than clays. The Khumbu oxalate

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also contains a few percent of a Mn-Ba mineral - - perhaps romanechite (Table 15. le), a constituent that has not been reported in other oxalate crusts. 15.3.4.2.7. Phosphate Skins Phosphate skins are poorly researched, especially as rock coatings in alpine environments. Those observed here (Figures 15.21c, 15.24e, 15.26c) are unlike those reported previously in alpine regions. One phosphate skin in Tibet (Figure 15.24e; Table 15.2c) appears to be an aluminum-phosphate, with a chemistry consistent with a Ca-millisite 0~licoteaux and Lucas, 1984; Nriagu, 1984). The chemistry of phosphate that interdigitates rock varnish (Figure 15.26b-Table 15.2f; Figure 15.26c-Table 15.2g; Figure 15.26d-Table 15.2h; Figure 15.26g-Table 15.2j ) is consistent with an apatite (Nriagu, 1984). The phosphate skin in Figure 15.21c, from the Khumbu, has three BSE textures: an upper iron skin; a middle darker section; and a speckled bottom texture. The chemistry of the middle texture is similar to the Table 15.2c in Tibet, perhaps a Ca-millisite. The chemistry of the lower left end of the transect (area with bright speckles) could represent a combination of Ca-millisite and a Fe-P mineral, which could be strengite (FePO4.2H20). The Khumbu phosphate accretion (Figure 15.21c) may relate to iron phosphate films seen in the Arctic (Konhauser et al., 1994) that are produced by bacteria. Iron is present in variable amounts (Table 15.1c). Bacteria (not seen in the BSE images) may be involved in Khumbu phosphate precipitation (Lucas and Pr6v6t, 1981), since bacteria are present on the surface and qualitative EDS spot analyses on these bacteria reveal high P peaks.

15.3.4.2.8. Rock Varnish The varnishes in the Khumbu and West Kunlun Mountains are generally similar to varnishes elsewhere in chemistry in that they are characterized by clay minerals that are cemented to the rock by iron and manganese hydroxides. Their texture indicates that they form as an accretion on top of the underlying rock. The Khumbu varnishes have more hydroxides and less clay (Table 15.1d) than Tibetan (Table 15.2a, 2f-j) and other varnishes (Dorn et al., 1992a). The most common type of Khumbu varnish is found adjacent to running water. This is consistent with Mn-rich rock coatings found along water courses in other alpine settings (Cailleux, 1967; Dora and Obedander, 1982; HOllerman, 1963). West Kunlun varnishes are different from most varnishes found elsewhere in that they interdigitate with phosphates, probably an apatite (Figure 15.26b-Table 15.2f; Figure 15.26c-Table 15.2g; Figure 15.26d-Table 15.2h; Figure 15.26g-Table 15.2j), and sulfates (Figure 15.26i-Table 15.2k). The West Kunlun environment is dry enough and has a high enough pH to preserve well-layered varnishes that record climatic changes. The age of the volcanics is old enough to record climatic changes in varnishes elsewhere. However, varnishes with distinctive layering does not occur in the thin subaerial varnishes on the volcanics. In contrast, West Kunlun varnishes are best developed in a crevice positions, where Mn correlates with Ba, suggesting a Ba-Mn m i n e r a l - perhaps romanechite.

Geographical Variations

The lack of layering on the volcanics could be due external or internal factors. The external factors that might preclude the development of layers be: (1) weathering that exposes rock crevices (Figure 15.24); and (2) eolian abrasion that resets the varnish clock. For example, Figure 15.27b displays fossilized bacteria that have been shaved in half by wind abrasion. While evidence for wind abrasion can be found in the field (T. Liu, personal communication, 1996), the wind abrasion has not been enough to completely abrade off the fossilized bacteria or remove pahoehoe surface textures. Therefore, the eolian abrasion appears to be, at the least, episodic. It may also be that intrinsic varnish-forming factors preclude the development of layers on Akesu volcanics. One possibility may be that subaerial varnish formation that is too slow to record climatic fluctuations. The most likely explanation for the lack of layering is interdigitation with other types of coatings that occurs in a discontinuous fashion. Consider the abundance of phosphate within varnish. The phosphate (Figure 15.26b-Table 15.2f; Figure 15.26c-Table 15.2g; Figure 15.26dTable 15.2h; Figure 15.26g-Table 15.2j) occurs in lenses, as does the silica glaze. Hence, a continuous layering pattern that might record climatic changes does not get a chance to develop. The only setting where layered varnishes, similar to those that occur in warm deserts, have been found is on boulders on glacial moraines of the West Kunlun Mountains (Figure 15.26h).

15.3.4.3. Role of Himal-Tibet Rock Coatings in the Weathering System Rock coatings are not viewed as integrated part of the earth's system in any scientific literature. The typical treatment in physical geography or physical geology books is to add an unconnected section on 'rock varnish' in 'desert landforms'. A more appropriate placement would be within 'weathering and soils', for rock coatings are at the same time a secondary weathering product, and an example of a positive feedback in rock weathering. Weathering breaks down rocks formed at higher temperatures and pressures into products more in equilibrium with conditions found at the earth's surface. All types of rock coatings exemplify this concept. Whether a manganiferous rock varnish, a crust of calcium oxalate, or a glaze of amorphous silica, new mineral assemblages have formed. And these assemblages have distinctive spatial patterns that reflect geographical variability at the atmosphere-biosphere-lithosphere-hydrosphere interface on rock surfaces. A relatively unexplored aspect of rock coatings, well illustrated in both the Khumbu and Tibetan plateau, is their ability to aid in rock weathering by enhancing preexisting rock weaknesses. The rock weathering promoted by rock coatings is a combination of both mechanical and chemical processes. Iron skins in Nepal, for example, form on the surface and within a rock. They can range in color from bright red to a dark brownblack. Iron skins that occur within a rock are deposited from solutions along grain boundaries and joints. The precipitation further widens the fracture. When the next remobilization and reprecipitation event occurs, the solutions move further into the cracks, which in turn splits them further. Images from Nepal (Figure 15.21a,15.21f) are similar to those seen in K~il'kevagge, Northern Scandinavia (Dixon et al., 1995). Several different rock coatings in Ashikule Basin of the West Kunlun Mountains exemplify a positive feedback in that new weathering products further accelerate the breakdown of rocks (Figure 15.24). Dust moves into rock crevices by mechanical transport (Villa et al., 1995) (Figure 1.5.24c). The dust may aid in the widening of

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fractures even further by storing capillary water to be subsequently frozen, which in turn increases the dust flux. Other rock coatings appear to be aggressive in breaking off and enveloping pieces of rock from the crevice sides (see carbonate in Figure 15.24d). Sulfates can brecciate rock walls (Figure 15.24a). The growth of rock varnish in cracks can split fractures as well (Figure 15.240. Chapter 6 emphasized the protective role of rock coatings. These observations, in contrast, indicate that rock coatings can exert an erosional influence. This is a particularly important point in rock conservation efforts. The possible erosional influence of rock coatings is not limited to what has been observed in Nepal and Tibet. Because many rock coatings are dark, they have a low albedo and experience increased temperatures. Thus, they may promote enhanced thermal expansion of minerals. The thermal coefficients of expansion and contraction for the rock coatings may be substantially different from the underlying rock. In this way, rock coatings formed in fractures may increase the friability of the weathering find. A case in point is in Petra in Jordan, where Paradise (1993b, 1995) found that the calcite cement in the host rock expands and contracts at a different rate than the surrounding quartz grains, thus explaining more rapid rates of erosion associated. Although Paradise (1993b, 1995) studied erosion of the rock material, carbonate crusts and other rock coatings could potentially have a similar effect.

15.3.4.4. Implications for Understanding Geographical Variations in Rock Coatings I do not think it is possible to produce an accurate map showing the geographical variability in Himal-Tibetan rock coatings. Too much heterogeneity exists at all spatial scales. Coatings interdigitate at the scale of microns and millimeters. Different rock coatings are found side-by-side at the scale of meters. My experience in the transect discussed in this section, and elsewhere in the world, is that heterogeneity is the rule. While it is possible to map variable characteristics of a single rock coating over an area, it is not possible to generate meaningful mapping units of different types of rock coatings where the graphical scale of the map is in kilometers.

15.4. Concluding Perspectives Anytime a specialized monograph in the natural sciences is written, it is necessarily filled with the sort of detail that a specialist desires. And yet, I had hoped to weave into the detail three general themes of broader interest. My first general goal is to alter your perception of the geography of rock coatings. Rock coatings are one of the least appreciated aspects of rocky landscapes, but these skin-deep accretions are one of the most important agents in defining the color and texture of bare-rock landscapes. The color of many of the world's famous natural rock landmarks is not due to the rock, but the rock coatings. Ayers Rock, Australia, is naturally ivory, but it has an orange appearance due to the growth of iron films (Figure 14.9). The Nazca lines of Peru are noticeable from the air because stones with manganiferous varnish have been pushed to the side (Clarkson, 1990; Silverman, 1990). When the world television audience focused on the 1996 Summer Olympic Games, Stone Mountain, Georgia, was viewed in televised 'shorts', and its coloration is controlled by films of iron, crusts of calcium oxalate, and glazes of silica (Figure 6.15). Bare rocks around the world are covered by lichens (Sharnoff, 1994), including the walls of Milford Sound in New Zealand and Easter Island giant sculptures. I hope that the

Geographical Variations

reader realizes by now that, in fact, there are very few rocky landscapes whose appearance is not altered by natural rock coatings. My second goal is to bring together in one place and synthesize the literature on rock coatings. Ever since von Humboldt (1812) initiated the scholarly study of rock coatings, the diaspora in scholarship in the natural sciences has been mirrored in the burgeoning literature on rock coatings. The reality is that researchers interested in rock coatings come from many different disciplines and publish in places that are rarely encountered by scientists in other fields. By writing the first book on rock coatings, and by integrating these different literatures, I hope to encourage a cross-fertilization of ideas in different fields studying the same phenomenon. My third goal is to promote the development of theory. Right now, the vast majority of research on rock coatings has been the gathering of empirical data. Yet much of the history of natural science has been the search for generalizations to order the chaos of data on the natural world. Thus, I advocate the perspective of landscape geochemistry as a general paradigm to understand rock coatings in a cohesive intellectual fashion. Using landscape geochemistry, I present a general model to interpret the geography of rock coatings. The usefulness of landscape geochemistry and the proposed hierarchical model will be measured in the future: by how well it helps us to understand geochemical pollution; how to best preserve stone monuments; how to estimate the ages of rock surfaces; and more generally, how it aids the interpretation of natural biogeochemical systems at the earth's surface. It was disappointing that I could not meet my third goal more completely. I was able to present only a cursory explanation for the geography of rock coatings. The fault may be mine; biogeochemical patterns may be patently clear to a more perceptive scholar. Yet, I have come to realize that understanding the geography of rock coatings will require an integration of information at different scales. Available methods of data acquisition are simply inappropriate for making jumps in scale, between micron-scale electron microscope imagery and remotely sensed Pixels on the order of meters. Existing efforts to map rock coatings are laudable, but only a few examples exist of researchers who base their general maps on micron-scale knowledge. There has been no effort to generalize microscopic information at the milleter scale, which will be necessary to make the leap to meter scale airborne and satellite data. I challenge future researchers to understand the complexity found at microns, millimeters, and meters all before trends are mapped at the scale of kilometers. It is only by studying rock coatings at different scales that researchers will be able to use different spectacles to understand this complex and ubiquitous aspect of our planet's surface.

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~p

418

Geographical Index

Geographical Index

Acropolis, Greece, 224 Afghanistan, 207 Africa, 1, 34, 35, 79, 86, 91, 92, 187, 190, 191,194, 227, 288, 292, 375 Agra, India 5, 111 Akesu volcanic field, Tibet, 80, 283,358 Alaska, U.S.A, 57 Alice Springs, Australia 109 Antarctica, 25, 34, 46, 57, 87, 95, 101,102, 136, 145, 147, 156, 157, 181,182, 183, 184, 201, 210, 211,212, 214, 238, 244, 245,255, 264, 269, 278, 287, 288, 292, 293,310, 332, 374, 375 Apple Valley, Califomia, U.S.A., 250, 251 Arches National Park, Utah, U.S.A. 353,354 Arctic, 3, 60, 63, 79, 95, 121,147, 157, 164, 189, 190, 192, 251, 267,376 Argentina, 286 Arizona State University, Arizona, U.S.A. 72, 300, 301,313 Ashikule Basin, Tibet 358, 359, 360, 365, 366, 370, 371,372, 373,375,377 Ashishan Volcano, Tibet, 359, 366, 371,372, 373 Atacama Desert, 61, 108, 175, 189, 190, 248, 255, 256, 259, 261, 312 Atlas Mountains, Morocco, 297, 329 Ayers Rock, Australia 2, 6, 179, 200, 330, 378 Baffin Island, Canada, 95 Baja Califomia, Mexico 61, 175, 264, 375 Beacon Valley, Antarctica 184, 293 Bear River, Sierra Nevada, Califomia, U.S.A. 95, 96, 97, 156 Belfast, 258, 265 Berkeley, California, U.S.A. 131, 218

Bighorn Basin, Wyoming, U.S.A. 310 Bishop Creek, Califomia, U.S.A. 103,202, 227, 229, 230, 286 Bishop Tuff, Califomia, U.S.A. 50, 51, 87, 105, 106, 107,255 Black Hills, U.S.A. 65, 271,273 Brazil, 18 Calico Early Man Site, Califomia, U.S.A. 219 Canadian Rockies, 227 Canyon de CheUy, Colorado Plateau, U.S.A. 9, 325, 345 central Australia, 108, 159, 193, 197,305 Central Valley, California, U.S.A., 258 Chad, 92 Chile, 108, 110, 255, 256, 259, 260, 312 China, 175, 201,202, 205 Cima Volcanic Field, Mojave Desert, California, U.S.A., 36, 274 C6a, Portugal 161,169, 288,298, 314,331,332 Colorado Plateau, U.S.A., 8, 60, 191,238, 277, 293,324, 339, 351 Colorado River, U.S.A., 56, 78, 139, 226, 289, 297,310, 331 Columbia Plateau, U.S.A. 43 Conejo Volcanics, Califomia, U.S.A. 286 Coso Volcanic Field, California, U.S.A., 218, 226, 334 Crater Flat, southern Nevada, U.S.A. 170, 172, 173, 174 Cyprus, 146, 181,264 Dana Plateau, Sierra Nevada, Califomia, U.S.A. 125, 131, 148 Daylight Pass, Nevada, U.S.A. 150 Dead Sea, Israel, 44, 205, 206, 257, 258, 261 Death Valley National Monument, Califomia-Nevada, U.S.A.,, 7, 13, 39, 40, 53, 74, 75, 77, 104, 114, 157, 158, 159, 171, 176, 177, 186, 196, 197, 199,

Geographical Index 200, 202, 204, 205,206, 210, 211,212, 213,214, 220, 225, 226, 230, 231,245,259, 264, 310, 346, 347,348, 349, 350, 352,353, 354, 355 Egypt, 1, 17, 79, 86, 111, 143, 178, 200, 233, 235, 282 England, 87, 94, 101, 106, 286, 302 Europe, 234, 266, 269 Fontana, Califomia, 166, 167 France, 94, 130, 134, 238,286 Galena Canyon, Death Valley, California, U.S.A. 177, 204 Gettysburg, Pennsylvania, U.S.A., 127, 128 Gold Beach, Oregon, U.S.A. 292 Grand Canyon, Arizona, U.S.A. 2, 331 Great Basin, U.S.A., 189 Great Britain, 8 Green Island, Australia 70 Greenland, 95, 136, 147, 152, 157, 164 Haifa, Israel, 73 Haleakala, Hawai'i, U.S.A., 14, 285,303, 311 Henry Mountains, Utah, U.S.A., 207,326 Hoover Dam, U.S.A. 186 Hualalai, Hawai'i., 51, 58, 296, 306, 307, 312, 314, 315, 316 Iceland, 153, 171,201 Idaho, U.S.A., 207 Israel, 42, 59, 61, 62, 66, 73, 86, 117, 143, 175, 180,206, 257, 258,261,302, 351 Italy, 50, 138,255,264 Iztacc~uatl Volcano, Mexico, 162, 171,172, 176, 252, 253, 303, 304, 307, 308,314, 316 Jewel Cave, South Dakota, U.S.A., 126 Joshua Tree National Monument, U.S.A., 45, 249, 330 Judean Hills, West Bank, 59 Kakadu, Australia, 268, 282 K~irkevagge, Scandinavia, 79, 80, 81,147, 154, 168, 169, 183, 332,374,377 Kentucky, U.S.A., 133,254, 255 Khumbu, Nepal, 2, 115, 116, 123, 124, 129, 156, 302,303, 327, 355,356, 357,358,360, 361, 362,363, 364, 365,372, 373, 374,375,376,377 Kileaua, Hawai'i, 295 Kitt Peak, Arizona, U.S.A., 13,208 Lake Bonneville, Utah, U.S.A., 72, 235 Lake Lisan, Israel-Jordan, 44, 206, 257

419 Lake Ransom Canyon, Texas, U.S.A., 76 Lake Roosevelt, Arizona, U.S.A., 54 Legend Rock, Wyoming, U.S.A. 37, 310 Libya, 189, 282 Maine, U.S.A. 130 Malawi, 310 Mammoth Hot Springs, Yellowstone National Park, U.S.A., 153 Marie Byrd Land, Antarctica, 244 Mars, 7, 8,26, 112, 121,144,280, 321,322 Maui, Hawai'i, 14, 92, 93,284, 285, 311 Mauna Kea, Hawai'i, 12, 284, 285, 294, 302, 303,306, 314, 315, 316 Mauna Ulu, Hawai'i, 92, 94, 295 Mauritania, 238 McDowell Mountains, Arizona, U.S.A., 6 Medicine Lodge, Wyoming, 90 Mediterranean, 49, 94, 339 Mesa, Arizona, U.S.A. 104, 194 Mexico, 17, 139, 162, 171,172, 176, 227, 240, 252, 253, 303, 304, 307, 308, 314, 316, 341, 342 Migongshan Volcano, Tibet, 366, 371 Mintum, Scotland, 61 Mojave Desert, 50, 52, 57, 81, 82, 103, 105, 146, 151,160, 164, 171,180, 189, 190, 191,193, 194, 198, 200, 202, 207, 208, 217, 219, 225,234, 238, 250, 251 Mono Basin, Califomia, U.S.A., 87, 105, 106, 275 Morocco, 94, 238, 281,286, 310, 331 Mt. Van Valkenburg, Antarctica, 46, 201 Namib Desert, 61, 157, 164, 263, 375 Nasca, Peru, 4, 78, 79, 176, 202, 261,327, 328, 339,345 Negev Desert, Israel, 45, 59, 61, 117, 118, 143, 189,225, 238, 239 New Guinea, 153 New South Wales, Australia, 202, 269,282, 294, 302 New York, U.S.A., 130 Norway, 62, 207 Oakland, California, U.S.A., 142 OgaUala Formation, West Texas, U.S.A. 73 Olary Province, Australia, 55, 289 Orinoco, Venezuela, 11, 17, 18 Owens Valley, California, U.S.A. 209,227 Pakistan, 86

420

Palm Springs, Califomia, U.S.A., 102 Papago Park, Phoenix, Arizona, U.S.A., 113, 118, 166, 167, 168, 249, 250 Paran, Israel, 86 Parker Dunes, Arizona, U.S.A., 152, 171,172, 249 Patagonia, Argentina, 175,205 Peru, 5, 78, 79, 175, 176, 200, 202, 210, 211,212, 245,257, 259, 260, 261,283, 289, 310, 327, 328,339, 345, 378 Petra, Jordan, 2, 5, 91, 92, 97, 101, 158, 159, 266, 289, 327, 328, 345,378 Petrified Forest National Park, Arizona, U.S.A.,, 39, 91 Phoenix, Arizona, U.S.A., 39, 54, 83, 113, 166, 167, 168, 249 Pikes Peak, Colorado, U.S.A., 87, 95 Pisgah Crater, Mojave Desert, California, U.S.A. 50 Poverty Hills, Califomia, U.S.A. 209 Providence Mountains, Califomia, U.S.A., 82, 193 Puerto Rico, 80 Punta CabaUos, Peru, 257,259, 260 Pyramid Lake, Nevada, U.S.A., 68, 69, 74, 159, 203 Pyramids, Egypt, 190, 225, 282 Queen Creek, Arizona, U.S.A., 139, 140 Queensland, Australia, U.S.A., 35, 70, 123, 124, 132, 176, 178, 235,269, 282, 318 Rainbow Basin, Califomia, U.S.A., 194 Red Fort, India, 5 Rogue River, Oregon, U.S.A. 292 Russia, 21 Sahara Desert, 157, 159, 170, 182, 238,282 Sahel, 190 Salt Springs, Mojave Desert, Califomia, U.S.A. 193, 194, 200 San Francisco Peaks Volcanic Field, Arizona, U.S.A., 58 San Pedro River, Arizona, U.S.A., 141 Santa Monica Mountains, Califomia, U.S.A., 14, 286, 287 Searles Lake, Califomia, U.S.A., 160, 194, 227,228 Sedona, Arizona, U.S.A., 99, 100, 112,113 Shoshone, California, U.S.A., 174, 175 Sierra Nevada, California, U.S.A., 95, 96, 97, 103,125, 131, 148, 154, 155, 156, 281,286, 343 Sierra Pinacate, Mexico, 227

GeographicalIndex Silver Lake, Mojave Desert, California, U.S.A., 151,160, 161,219 Sinai Peninsula, Egypt,, 8, 10, 17, 71,108, 109, 186, 200, 251, 283,284 Snake River Plain, U.S.A., 283 Sonoran Desert, southwestern North America, 103, 111, 159, 166, 202, 351 South Africa, 34, 79, 194, 227,292, 375 South Australia, 4, 46, 55, 81,99, 157,200, 202, 265,289 South Mountain Park, Arizona, U.S.A., 53 southeastem Colorado, U.S.A., 85 southern Nevada, U.S.A., 79, 88, 97, 112, 114, 163, 172,255 SP Crater, Arizona, U.S.A. 58, 104, 105 Spain, 146, 168, 271 Spitzbergen, 225 Starvation Canyon, California, U.S.A., 104, 213 Stone Mountain, Georgia, U.S.A., 94, 95, 97, 150, 251,252, 286, 308, 309, 378 Sunflower, Arizona, U.S.A., 47, 48, 50, 52 Superior, Arizona, U.S.A., 140 Superstition Mountains, Arizona, U.S.A., 269 Susquehanna River, northeastem U.S.A., 138 Sweden, 88, 154 Taj Mahal, India, 111 Tassili, Sahara Desert, 35 Tempe Butte, Arizona, U.S.A., 72, 75, 76, 115 Thar Desert, India, 111, 189 Thasos, Greece, 56, 101,269 Thiel Mountains, Antarctica, 156 Tibet, 8, 10,37, 80, 110,253,356, 359, 366, 372, 373,374, 375, 376, 377, 378 Tien Shan Mountains, 201,238, 356 Tikal, Guatemala, 289, 291 Tunisia, 157, 183,202, 207,262, 263,266, 353 United Kingdom, 112, 122, 123, 130,286 Urekele Lake, Tibet, 359 Utah, U.S.A., 72, 91,201,207,235, 293,326 Valley of Fire, southern Nevada, U.S.A., 79, 97, 105 Van Horn, west Texas, U.S.A., 78 Venice, Italy, 138, 254 Ventura, California, U.S.A., 136, 137 Venus, 84, 122 Vermont, U.S.A., 13, 164, 165,272, 273 Victoria Land, Antarctica, 184, 293

Geographical Index Virginia, U.S.A. 122, 130, 137, 138, 142, 199, 201 Warm Springs, Death Valley, California, U.S.A., 53,346, 349, 350 West Kunlun Mountains, Tibet, 8, 10, 80, 110, 253, 283,355, 358,365, 366, 371,372, 374, 375,376, 377 West Texas, U.S.A., 45, 270, 271, 272 Wharton HiM, Australia, 4, 289 White Mountains, Nevada, U.S.A. 148 Wind River Mountains, Wyoming, U.S.A., 8, 11, 97, 98, 149 Wyoming, 8, 11, 37, 64, 65, 88, 89, 90, 97, 98, 129, 149, 153, 201,271,273,310, 339 Yemen, 261 Yishan Volcano, Tibet,, 370, 371, 373 Yosemite National Park, Califomia, U.S.A., 8, 10, 97, 98, 327

421

422

Subject Index

Subject Index

Abiotic, 67, 70, 106, 134, 180, 181, 183,184, 185, 188,239,241, 242,243, 246, 266, 276, 285, 318 Abrasion, 151,152, 187, 191,194, 208, 218, 221,223,280, 282, 338, 358, 359, 377 Acid, 24, 38, 45, 78, 122, 132, 155, 164, 181, 182, 192,223,231, 232, 234, 235,276, 280, 317, 319, 320, 325, 336, 337, 339, 373 Acid drainage, 122, 336, 337 Acid precipitation, 339 Adsorb, 137, 152, 178 Aesthetic, 38, 345,373 Age, 64, 105, 150, 151,165, 187, 218,219, 220, 221,225, 226, 227,228, 231,272, 313, 314, 316, 332, 356, 358,376 Algae, 10, 15, 48, 49, 56, 57, 60, 64, 70, 71,101, 112, 124, 133,235, 238, 239, 240, 327, 330, 337, 342, 356, 362, 373 Alkaline, 24, 25, 181,218, 239, 243,249, 266, 280,317, 333, 337,339,351 Alluvial fan, 7, 13, 71, 75, 77, 78, 104, 170, 172, 173,174, 177, 202, 204, 220, 224, 225, 264, 346, 350, 352, 353,354, 355 Alpine, 3, 11, 60, 63,147, 155, 189, 190, 192, 225, 274, 374, 376 Alumina glaze, 14, 295, 311, 312, 313,321 Amorphous, 5, 15, 26, 90, 92, 100, 119, 153, 154, 181,195, 211, 239, 279, 280, 281,282, 283, 284, 287, 288, 293,297, 298, 300, 310, 317, 318,319, 374, 377 Anthropogenic, 15, 19, 26, 33, 34, 35, 37, 48, 56, 59, 120, 127, 135, 136, 139, 217,223, 254, 262,270, 300, 326, 327, 335, 336 Apatite, 15,249, 251,254, 266, 376 Aquatic, 56, 141, 180, 181, 183, 184, 185, 323 Arid, 7, 24, 26, 60, 72, 78, 85, 102, 112, 115, 152, 164, 175, 182, 183,186, 188, 189, 190, 191,

205, 206, 233,237, 243, 256, 259, 264, 266, 269, 274, 277, 279, 281,286, 289, 329, 335, 339,351 Arsenic, 65, 139, 178, 334 Artifact, 305, 313, 346 Artificial, 39, 40, 190, 222, 324 Backscauer, 148, 244, 272, 291, 295,296, 306, 308, 311 Bacteria, 41, 43, 44, 45, 46, 48, 54, 63, 70, 71, 80, 81,121,126, 133, 145, 153, 154, 181,182, 183, 184, 216, 218,238, 239, 246, 251,255,267,316, 318, 322,337, 340, 342,356, 362, 370, 372, 373,374, 376, 377 Barite, 38, 375 Barium, 15, 38, 174, 198, 199,204, 295,296, 297 Basalt, 14, 28, 43, 50, 58, 88, 90, 92, 93, 115, 175, 195, 215, 218,221,274, 285,302, 336 Bauxite, 310 Beachrock, 70, 71 Best looking, 218, 219 Bias, 3, 27,272,340 Biofilm, 15, 41, 43, 45, 49,223 Biogeochemical, 2, 20, 22, 24, 26, 27, 132, 135, 143,215,216, 217,219, 222, 223,224, 324, 335,336, 339, 341,342, 344, 345,379 Biogeochemical barrier, 132, 143, 216, 324, 335, 336, 339, 341, 344 Biogeochemistry, 223 Biological origin, 47, 61,181,184, 187 Biorind, 48, 57 Biotite, 35, 102, 145, 156, 180,286, 370 Bimessite, 129, 195, 197, 246 Black, 4, 8, 13, 15, 16, 17, 18, 27, 42, 49, 50, 52, 60, 64, 82, 90, 93, 99, 100, 110, 122, 123, 125, 132, 145, 147, 153, 154, 160, 161,163, 168, 174, 175, 176, 182, 184, 186, 187, 188, 189, 190, 193, 194, 195, 204, 205,214, 215, 216, 222, 223,

Subject Index 224, 225, 227, 231,232, 234, 235,238, 265,268, 272, 282, 286, 293,296, 330, 331,334, 340, 354, 373,377 Bog, 362 Bronze, 141 Budding bacteria, 44, 45, 322 Building, 6,38, 47, 82, 107, 110, 112, 256, 258,265,266, 327, 340 Cadmium, 139, 178, 339 Calcite, 35, 79, 86, 91, 107, 108, 112, 197, 264, 271,288, 292, 378 Calcium, 3, 5, 9, 10, 15, 38, 40, 48, 49, 65, 67, 70, 72, 73, 74, 78, 79, 80, 82, 83, 88, 89, 91, 97, 101,107, 111,248,249,251, 254, 255, 262, 264, 265, 266, 269,270, 271,272,273, 274, 275,276, 293,300, 319, 338, 342, 362, 363,364, 365, 366, 370, 373, 375,377,378 Calcrete, 15, 72, 73, 74, 75, 76, 77, 78, 79, 248,332, 352 Caliche, 248, 256 Capillary, 79, 101,181,187,232, 233,234, 235,236, 238, 246, 256, 266, 370, 371,378 Carbon dioxide, 26, 38, 61, 70, 73, 84, 181,232, 279 Carbonate, 15, 33, 35, 38, 48, 49, 59, 65, 67, 68, 70, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 90, 91, 97, 101,107, 111,117, 153, 182,223,249, 266, 269, 281,292, 300, 329, 332, 339, 354, 358, 365, 366, 367,373, 375, 378 Carbonic acid, 38, 234 Case hardening, 79, 85, 86, 87, 88, 89, 91, 92, 95, 96, 97, 100, 101,103, 104, 105, 106, 107, 114, 158, 187,213,223, 245, 287,289, 329, 330 Cast, 46 Cataract, 15, 16 Cation exchange, 212 Cave, 61,126, 216, 223,254, 255 Cell, 43, 46, 183, 184, 244, 245, 246, 251,370 Chamosite, 195 Charcoal, 35, 101,265,271,289, 290, 316 Chemolithotrophic, 153,322 Chert, 28, 45, 149, 193,282 Chlorite, 195, 212,242 Clay mineral, 46, 48, 86, 91, 97, 99, 112, 120, 122, 135, 145, 149, 151,152, 157, 159, 160, 164, 171,176, 177, 178, 180, 181, 186, 187, 195, 196, 197, 205, 207,208, 209, 211,214, 218, 222,241,242,243,245, 246, 247,251,271,300, 310, 335, 373,375, 376 Climate, 21, 59, 61, 87, 106, 108, 159, 180, 187, 189, 190, 191,

423

214, 215, 232, 256, 264, 266, 272,335, 339, 351,356, 365 Coast, 71, 73, 158,257, 302 Cobalt, 130, 138 Cocci bacteria, 45, 47, 126, 183, 184, 244, 318, 370 Cold spring, 153 Color, 2, 6, 8, 11, 15, 35, 39, 61, 88, 138, 141,144, 145, 147, 148, 149, 150, 152, 153,154, 156, 157, 159, 161,164, 175, 176, 178, 179, 180, 186, 193, 194, 214, 216, 220, .221,222, 232, 239,251,286, 311,321,330, 345,377, 378 Compete, 331,334, 341,342, 343 Competition, 65,135,324, 334, 339, 340 Complexing, 49, 310, 316, 317 Copper, 14, 15, 35, 120, 136, 138, 139, 140, 141,143,178, 248, 275,335 Coral, 70, 249 Core softening, 86, 101,102, 107, 329 Corrosion, 135,136, 141,142, 143 Crack, 157, 174, 215,216, 217, 218,219, 220, 222,224, 366, 367 Crenitic, 223,249 Crevice, 63, 111, 147, 149, 160, 161,164, 174, 175, 194, 205, 209,218, 219,222,224, 330, 336, 339, 365,366, 376, 378 Cryptogamic, 60, 255, 354 Cultural, 5, 6, 11,225,275 Culturing, 44, 187,222, 318 Cyanobacteria, 10, 15, 48, 49, 60, 61, 65, 70, 71,194, 216, 221, 238,362, 373 Date, 275, 313, 325,356 Dating, 130, 180, 188, 215,218, 219,220, 272, 314, 316, 356 Degradation, 11, 219 Dehydration, 208 Desert, 1, 5, 14, 15, 16, 28, 39, 44, 53, 56, 57, 76, 77, 78, 79, 101,108, 111,138, 143, 149, 151,156, 157, 158, 159, 160, 162, 164, 170, 171,172, 173, 174, 180, 181,182, 183, 186, 188, 189, 190, 191,194, 195, 198, 199, 200, 201,202, 206, 214, 215, 216, 217,219, 220, 223,224, 226, 227, 231,232, 233,234, 235, 236, 237, 238, 239,248, 249, 251,255, 262, 269,282, 288, 296, 305, 310, 321,326, 329, 332,334, 337, 339, 351,352, 355,377 Desert glaze, 296 Desert P7avement,5, 28, 56, 57, 76, ,78, 143, 159, 162, 164, 172, 173, 174, 180, 182, 191, 194, 195, 215,216, 217,219, 220, 223,224, 234, 251,282, 288, 305, 310, 339, 352, 355 Desert varnish, 14, 16, 39, 101, 156, 164, 181,186, 188, 189,

424

191,198, 206, 214, 220, 226, 227,233, 235,236, 238, 321, 329 Diagenesis, 38, 63, 73, 111, 180, 242,246, 254, 265,268, 362, 375 Diapir, 261 Differential, 86, 96, 97, 102, 227 Diffusion, 141 Dissolution, 78, 90, 93, 101,102, 184, 239, 245,246, 264, 267, 269, 280, 285,299, 317 Dissolve, 78, 79, 84, 165, 181,231, 232,233, 243,254, 260, 266, 279, 280, 317, 324, 334, 335, 338, 369 Dolerite, 145, 156, 184, 232 Dolomite, 15, 28, 79, 80, 81, 82, 84, 302 Dune, 151,152, 280 Duricrust, 75, 76, 321 Dust, 13, 33, 73, 90, 108, 109, 11O, 111,112, 113, 114, 115, 116, 117, 118, 119, 135, 138, 139, 169, 190, 194, 205,208, 216, 217,221,222, 224,234, 236, 239, 240, 253,275,276, 280, 281,284, 317,320, 321,324, 329,335, 336, 337,339, 344, 365,366, 367, 369,370, 371, 372,378 Dust film, 33, 90, 108, 109, 110, 111,112, 113, 114, 115, 116, 117, 118, 119, 138, 139,253, 321,324, 329, 336, 365 Earth figure, 162, 289 Ecosystem, 323 Efflorescence, 15, 79, 255, 256, 257,258, 259, 262, 266, 276 Eh, 24, 132, 134, 153, 181,243, 246, 336 Engraving, 4, 221,223,289 Environment, 3, 11, 16, 18, 20, 24, 26, 28, 48, 57, 63, 76, 78, 79, 101,107, 112, 120, 122, 135, 136, 143, 144, 145, 147, 153, 157, 162, 164, 180, 181,187, 191,209, 214, 215,216, 217, 219,221,222,223,227, 237, 249,266, 267,268,274, 275, 277,278, 279, 281,282, 310, 332,333, 335, 337,339, 344, 345,374,376 Environmental change, 14, 26, 135, 337,338, 339, 346 Eolian, 107, 111,141,180, 187, 194, 208, 218, 221,223,249, 259, 275, 281,282,324, 338, 358,359, 375, 377 Epoxy, 28, 29, 109, 113,115 Erosion, 3, 26, 28, 38, 41, 58, 59, 64, 73, 75, 76, 79, 85, 87, 90, 91, 92, 93, 95, 96, 101,102, 106, 108, 110, 111,117, 121, 127, 158, 160, 179, 193, 215, 219, 223, 224, 225,246, 248, 252,262, 273,279, 281,282, 289, 316, 324, 326, 327, 328, 329,330, 332, 333,334, 336,

Subject Index 337,338, 341,343,351,352, 356, 365, 372, 378 Euendolith, 41, 49, 53, 54, 58, 61 Evaporation, 70, 71, 73, 75, 79, 83, 91,107, 139, 140, 181,227, 233,236, 243,256, 257, 262, 264, 266, 282, 317,320, 369 Evaporite, 12, 83, 256, 261,262 Feedback, 84, 340, 372, 377 Feldspar, 13, 14, 17, 40, 95, 113, 115, 116, 136, 142, 147, 151, 152, 155, 164, 186, 251,282, 283,284 Ferrihydrate, 35 Ferruginous, 17, 101,157 Filament, 29, 50, 74, 133 Filamentous, 44, 49, 50, 51, 52, 133, 221 Fissure, 114 Flood, 19 Fluvial, 101,102, 138,219,223, 260,313, 356, 373 Fossil, 64, 70, 84, 135 Fracture, 3, 76, 80, 109, 111, 117, 131,137, 147, 160, 166, 205, 208,213, 217,220, 274, 332, 366, 370, 371,377 Freshwater, 56, 67, 68, 84, 134 Fungal, 50, 51, 54, 58, 73, 74, 239, 269, 291, 318, 327,370 Fungi, 10, 15,28,42, 49,50,51,52, 53, 54, 55, 60, 81, 83, 92, 93, 183,193, 216, 221,238, 239, 241,266, 290, 291, 315, 327, 330, 332, 334, 340, 342, 356, 362,373 Fuse, 104, 233 Gasoline, 136, 137, 138, 139 Genesis, 19,33, 48, 75, 118, 119, 153,182, 183, 184, 188, 205, 231,232, 236, 239,242, 243, 278,280, 285, 294, 317, 318, 335 Geography, 2, 3, 16, 20, 21, 22, 24, 32, 43,107, 144, 188, 236, 266, 269, 294, 323,324, 345, 372, 373, 377, 378,379 Geology, 2, 3, 21, 80, 144, 164, 236, 352, 377 Geomorphic, 24, 26, 28, 102, 158, 199,202, 214, 216, 217, 218, 219, 220, 221,222, 223, 224, 225,248, 259, 326, 327, 329, 343,345, 353,354, 356 Geomorphology, 2, 26, 108, 151, 187,217, 346, 353,358 Gibber, 282, 305 Glacial, 61, 95, 96, 97, 103, 11O, 115, 121,162, 191,202,223, 227,229, 230, 264, 275, 283, 284, 286, 303,304, 307, 308, 313,314, 327, 343,356, 358, 362, 371,374, 377 Glacial polish, 95, 96, 97, 162, 284, 314,343,362 Glaciated, 12, 26, 96, 314, 355,356

Subject Index Glacier, 112, 129, 179, 217, 283, 303,356, 357, 358,360, 362, 363,364 Glass, 29, 105, 112, 281 Gnamma, 60, 92, 93, 102 Gneiss, 232, 274, 276, 288, 356, 360, 362, 363,364 Gobi, 283 Goethite, 35, 134, 146, 153, 154, 159, 164, 182, 194, 195 Graffiti, 34, 36, 37, 39 Grain coating, 3, 152, 157, 164 Green, 3, 35, 48, 56, 57, 141,240 Greenhouse, 83, 84 Ground water, 17, 74, 75, 134, 181, 234, 255, 256, 258,262, 275, 317,321,337 Grus, 103,353 Guano, 61, 101,249,250, 254, 256, 335 Gullies, 75, 76, 77, 78, 352 Gypcrete, 79, 248, 262 Gypsum, 35, 38, 57, 78, 79, 248, 258,259, 261,262,263, 264, 265,266, 270, 271,335, 339, 353,365, 366, 375 Gypsum crust, 79, 248, 258,262, 264, 265, 353,375 Halite, 248, 257,258, 260, 261, 271,335 Hardness, 91, 102, 105, 107, 117 Heat, 18, 57, 188, 234, 236, 356 Heavy metal, 15, 32, 33, 34, 56, 111,120, 121,130, 135, 136, 138, 139, 141,142, 143, 178, 195,227, 271,272,277, 329, 339,342 Heavy metal skin, 32, 33, 34, 120, 121,135, 136, 138, 139, 142, 143,195, 227,271,277, 329, 342 Hematite, 35, 134, 145, 149, 151, 154, 157, 159, 164, 195, 197, 277 Heterogeneity, 21, 199, 215, 217, 354,378 Heterogeneous, 27, 163, 168, 180, 199 Hierarchy, 324, 326, 337 HiUslope, 61, 109, 326 Homblende, 17, 186, 249 Hot spring, 153, 280 Humid, 48, 87, 102, 106, 111,130, 138, 145, 148, 159, 182, 189, 208, 227, 264, 269, 286, 308, 329, 330, 374 Humus, 235 Hydration, 213 Hydrochloric acid, 132, 231,235 Hydrofluoric acid, 45 Hydrothermal, 107, 153,223,248, 268,280 Hydroxide, 35, 38, 39, 138, 153, 178 Hyphae, 44, 45, 49, 50, 51, 58, 238, 269, 318, 370 Ignimbrite, 153

425

IUite, 164, 195,211,213, 241,242, 246 Induration, 87, 101,105, 107 Infrared, 35, 195, 220, 239, 246, 270 Instability, 316, 352 Interdigitate, 35, 79, 129, 171,173, 270, 289, 311, 341,376, 378 Interfinger, 160, 169, 277,292 Intermittent, 97, 98, 125, 131,147, 148, 149, 342 Intertidal, 122, 130, 134 Iron, 1, 3, 6, 8, 14, 15, 17, 18, 24, 35, 36, 39, 45, 46, 48, 49, 50, 51, 65, 80, 87, 88, 91, 92, 94, 95, 96, 97, 98, 99, 101,106, 107, 112, 120, 122, 123, 127, 128, 132, 134, 135, 136, 137, 138, 139, 141,142, 143, 144, 145, 146, 147, 148, 149, 150, 151,152, 153, 154, 155, 156, 157, 158, 159, 160, 161,162, 163, 164, 165, 166, 167, 168, 169, 170, 171,172, 173, 174, 175, 176, 177, 178, 179, 180, 181,182, 183, 184, 185, 186, 187,188, 189, 190, 193, 194, 195,204, 205,209, 214, 216, 220, 222, 223,227,232, 233, 234, 235, 236, 237,238, 239, 240, 241,242, 243,245, 246, 249, 251,252, 253,254, 267, 277,280, 281,282,284, 286, 287,288, 289, 291,292, 295, 296, 298, 299, 301,302, 303, 304, 305, 309, 310, 312, 313, 318,319, 320, 321,322, 324, 325,327, 329, 330, 331,332, 333,334, 336, 337,338, 339, 341,342, 343,344, 353, 354, 360, 361,362, 363,373, 374, 375,376,378 Iron bacteria, 183,337,362, 373 Iron films, 1, 3, 6, 94, 97, 98, 99, 134, 137, 143, 144, 145, 146, 147, 148, 149, 150, 151,152, 153,154, 155, 156, 157, 158, 159, 160, 161,162, 163, 164, 165,166, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181,182, 183, 184, 185, 188, 205,247, 286, 287,289, 292, 310, 318, 321, 322,324, 325, 327,329, 330, 331,332, 333,336, 337, 338, 339,341,343,353,354, 360, 361,362, 363,374, 378 Iron phosphate, 15, 251,252,253, 254, 267, 362,376 Jarosite, 181, 182 Joint, 64, 76, 87, 88, 89, 90, 91, 106, 117, 160, 219, 285,286, 310, 311,330, 331,332,343, 365, 366 Kaolinite, 35, 97, 107, 108, 112, 114, 115, 195,212 Karst, 67,260

426

Lake, 68, 261,262, 338, 375 Lamellate, 131, 194, 206, 208, 209, 364 Laminar, 72, 75, 76, 130, 375 Lamination, 169, 184 Landscape geochemistry, 1, 2, 20, 21, 22, 24, 25, 26, 27, 32, 67, 266, 318, 324, 379 Laterite, 24 Lava, 51, 58, 67, 80, 81, 88, 92, 102, 253, 285,296, 297, 306, 307,309, 311,312,313, 314, 358, 359, 366, 369, 370, 373 Lava tube, 67, 81 Leaching, 28, 40, 106, 165,212, 213,219, 229, 237,256, 261, 371 Lead, 15, 24, 26, 35, 47, 107, 110, 120, 130, 136, 137, 138, 139, 178 Lichen, 15, 58, 59, 60, 62, 63, 78, 82, 132, 235, 238,239, 275, 276, 338, 362, 364, 373 Light, 6, 15, 29, 31, 57, 63, 64, 108, 117, 176, 186,209,220,234, 239,252, 270, 308, 331,334 Limestone, 15, 38, 42, 48, 56, 57, 58, 59, 80, 82, 84, 86, 118, 140, 146, 181,225,232, 233, 257,264, 265,269, 275, 329, 356 Limonite, 156, 164 Lithobiont, 327,331 Littoral, 3, 17, 18, 70, 71,130, 189, 216,223 Loess, 8, 28, 108, 261,358, 359, 365,369, 370, 374, 375 Lunar, 112, 121 Magnesium, 15, 40, 80, 81, 82, 83, 249,263, 265,270, 271,275, 300, 375 Magnetite, 35, 136, 138, 156, 164, 233 Manganese, 6, 15, 16, 17, 18, 24, 35, 39, 40, 44, 45, 46, 47, 48, 49, 50, 51, 64, 65, 96, 99, 107, 120, 121,122, 123, 124, 125, 126, 127, 128, 129, 130, 131,132, 133, 134, 138, 142, 143,148, 150, 153, 154, 160, 161,166, 170, 174, 175, 176, 182, 183, 186, 187, 188, 189, 190, 193, 194, 195, 197, 200, 203,204, 205,208,209, 214, 215,216, 218, 219, 220, 221, 222, 223, 224, 227,232, 233, 234, 235, 236, 237,238, 239, 240, 241,242, 243,244, 245, 246, 247, 249, 271,272, 273, 274, 277, 295, 297,299, 318, 324, 325, 334, 335,336, 337, 338,339, 340, 341,342, 353, 362,363, 365,369,370, 373, 376 Marble, 49, 50, 69, 111,138, 224, 264, 265, 275,356 Marine, 3, 26, 67, 70, 84, 115, 121, 249,258, 259, 260 Mat, 15, 39, 74, 194, 213

Subject Index Mica, 17, 186, 212, 288 Microcolonial, 28, 50, 53, 54, 55, 83,183,193, 241,315,334 Migration, 20, 21, 22, 23, 24, 26, 139, 141,211,216, 222, 224, 242,257, 263,266, 318, 335, 337,345 MiUisite, 376 Mineral, 14, 26, 40, 41, 43, 44, 48, 65, 97, 112, 116, 117, 134, 152, 156, 157, 160, 181, 182, 195, 211,241,242,249, 251, 254, 268, 270, 275,284, 287, 316, 319, 321,322,329, 338, 375,376, 377 Mixing, 77, 134, 238,337 Model, 2, 100, 109, 111,184, 185, 187, 188, 190, 191,192, 193, 232,238, 241,246, 256, 257, 263,266, 276, 317, 319, 323, 324, 340, 341,343,344, 345, 352,379 Montmorillonite, 164, 195,208, 211,213,246 Monument, 41,254, 255 Moraine, 103, 110, 129, 184, 229, 253,293, 371 Morphology, 47, 50, 206, 208, 269 Moss, 15, 56, 60, 70, 330, 331,334, 362,373, 374 Nitrate, 15, 223, 248, 249, 254, 255, 256,335 Nomenclature, 14, 16, 144 Ocean, 19, 108, 135 Olivine, 84, 115, 117,215,311 Opal, 281,289 Orange, 6, 8, 15, 92, 98, 144, 145, 151,153, 154, 157, 158, 159, 160, 161, 164, 171,172, 173, 174, 175, 176, 178, 179, 180, 190, 194, 195, 204, 205, 215, 222,223, 224, 268,330, 362, 378 Organic acid, 43, 48, 78, 182, 184, 223,276 Organic matter, 31, 35, 54, 63, 124, 126, 127, 128, 129, 135, 160, 184, 199, 232, 234, 243, 250, 265, 271,286, 299, 300, 302, 307,311,312, 315,316, 317, 334, 341,360, 367 Origin, 2, 16, 27, 34, 47, 61, 106, 107, 117, 123,132, 136, 155, 180, 181,184, 187, 190, 215, 231,232, 233,234, 235, 236, 239,241,242, 249,261,268, 275,276, 317,323,344, 358, 374,375 Orthoclase, 35, 113, 115, 116, 142, 152,283 Oxalate, 5, 9, 1O, 13, 15, 54, 59, 65, 79, 88, 89, 94, 101, 107, 129, 179,215, 221,223,252, 268, 269,270, 271,272,273,274, 275,276, 277,278,286, 292, 293, 318, 325,327,328, 329, 332,335, 338, 339,341,342,

Subject Index 354, 362, 364, 365,369, 370, 373,375, 377, 378 Oxalic acid, 276 Oxidation, 48, 132, 141,144, 153, 157, 164, 181,182, 183, 218, 233,236, 238, 241,337, 369, 374 Oxide, 15, 17, 18, 35, 36, 91,106, 113, 121,134, 136, 137, 138, 139, 140, 152, 153, 157, 159, 163,171,181,195,233,238, 246, 294, 297, 300, 309, 310, 311,312, 320, 360, 366 Paint, 15, 34, 35, 37, 39, 135,324 Paradigm, 1, 20, 21, 22, 24, 27, 32, 65,219, 220, 379 Patina, 14, 15, 16, 27, 141, 159, 191,194, 214, 220, 225, 226, 234, 288, 313 Pavement, 5, 28, 56, 57, 77, 78, 159, 162, 172, 173, 174, 182, 191,194, 208, 215, 216, 219, 223,224, 234, 251, 281,282, 288,289, 305,310, 339, 352, 355 Pediment, 104, 250, 251, 261 Pedogenic, 67, 72, 73, 76, 77, 135, 281,282, 319, 337 Periglacial, 26, 79, 192, 243 Petroglyph, 4, 8, 35, 36, 37, 54, 88, 89, 91, 99, 150, 161,221, 265, 271,289, 291,298, 304, 310 pH, 24, 29, 73, 114, 132, 134, 135, 145, 153, 181,183,218, 223, 238, 242, 243,246, 319, 336, 337,339, 362, 369, 374, 376 Phosphate, 10, 15, 35, 65, 94, 101, 111,136, 223,248, 249, 250, 251,252, 253,254, 266, 319, 362,363, 365,366, 367, 368, 369, 373, 374, 376, 377 Pigment, 35, 36, 136, 223, 270, 362 Plagioclase, 14, 40, 84, 109, 116, 134, 136, 148, 149, 155, 251, 252,253, 284, 286, 287, 366 Plant, 18, 41, 60, 135, 166, 167, 235, 241,258, 300 Playa, 255, 359 Point source, 135 Polish, 30, 95, 96, 97, 162,223, 284, 288, 314, 317, 343,362, 363 Pore, 42, 57, 58, 60, 63, 72, 75, 80, 90, 92, 95, 99, 100, 101,139, 141, 148, 156, 280, 287, 292, 298,363 Potassium, 15, 40, 113, 115, 152, 178, 181,182,255,271,335, 358 Precipitation, 15, 24, 28, 38, 41, 44, 48, 49, 53, 59, 65, 70, 71, 73, 75, 78, 79, 115, 121,134, 138, 146, 152, 153, 154, 181, 190, 208, 233,236, 238, 239, 240, 243, 251,254, 256, 257, 258,259, 260, 262, 263, 264, 265,266, 275, 290, 291, 317, 318,319, 321,335,337, 338,

427

339, 340, 351,358,362, 374, 376,377 Prehistoric, 19, 34, 35,285 Preservation, 8, 38, 88, 95, 184, 232,338 Pyrite, 181,325 Quartz, 17, 28, 35, 43, 50, 52, 54, 57, 58, 89, 92, 112, 116, 139, 143, 147, 148, 149, 150, 151, 154, 164, 165, 185, 186, 225, 233,236, 239, 244, 249, 259, 265,271,272,275,280, 285, 292,293, 308, 329,338, 363, 375,378 Quartzite, 95, 96, 97, 110, 114, 139, 140, 149, 150, 156, 175, 176, 188, 194, 230, 233,259, 260, 269, 303, 339, 356, 357, 362, 363,364 Rate, 23, 26, 62, 65, 157, 180, 225, 228,230, 231,243,256, 257, 274, 313, 319, 325,326, 327, 328, 329, 330, 334, 335, 336, 342,356, 372, 378 Rate of growth, 334 Red, 6, 8, 15, 35, 49, 85, 99, 145, 146, 148, 149, 151,153, 154, 155, 157, 158, 159, 171,174, 178,232, 250, 251,256, 278, 286,288, 291,311,321,377 Reddening, 152, 155, 180 Redox, 26, 135, 181 Reg, 45 Regolith, 3, 127, 128, 130, 131 Remote sensing, 2, 7, 26, 157, 262, 284, 352, 353,354 Reprecipitate, 209, 245 Reprecipitation, 24, 143, 181,229, 245,246, 249, 285,377 Review, 2, 33, 35, 63, 67, 144, 238, 268 Rhyolite, 107, 218,275,371 River, 11, 17, 56, 107, 115, 158, 190, 192, 193,214, 224, 231, 232,233, 234, 260, 292, 297, 326, 334, 338,358 Road, 6, 26, 190, 225, 326, 327 Road cut, 39, 128, 131,137, 138, 326 Rock art, 36, 59, 61, 106, 226, 268, 269, 270, 314 Rock shelter, 15, 85, 101,250, 254, 255, 318, 336 Rock vamish, 1, 3, 4, 5, 6, 7, 8, 9, 10, 13, 15, 16, 28, 31, 37, 39, 44, 45, 46, 47, 51, 53, 54, 55, 64, 65, 66, 77, 79, 81, 82, 87, 88, 91, 92, 93, 94, 98, 99, 100, 101,102, 103,104, 118, 120, 122, 124, 127, 130, 136, 138, 139, 144, 146, 149, 150, 153, 156, 157, 160, 161,162, 163,170, 175, 176, 177, 178, 179, 182, 186, 187, 188, 189, 191,192, 193, 194, 195, 196, 197, 198, 199, 200, 201,202, 203,204, 205,206, 207, 208, 210, 212, 213,214, 215, 216,

428

217,218,220,221,222,223, 224,226,227,228,230,231, 233,234,235,236,237,238, 239,241,242,243,244,246, 247,269,271,277,288,289, 292,293,299,318,322,324, 325,327,328,329,330,331, 332,334,335,338,339,342, 346,350,351,352,353,354, 355,356,357,362,363,364, 365,371,373,374,375,376, 377,378 Rod bacteria, 45, 46, 126, 244 Rust, 142, 144, 164 Salcrete, 15,248 SMine, 43,81,233,256,258,266 S~inelake, 358,365 SMtcrust, 248,256,257,259,260, 261 SMt weathering, 43,259,365 Samplepmpa~tion, 315 Sand, 17,60,145,151,152,157, 164,171,172,194,249,262, 265,280,281,310,317 Sandstone, 9,28,37,42,64,65,70, 79,85,88,89,91,97,100, 102,103,107,112,157,159, 224,225,232,234,254,255, 257,265,269,273,277,282, 287,288,310,318,325,329, 330,331,332,338,339,342 Saprohte, 201,207 S~dli~,7, 152,167,352,355 Scale, 1, 26, 30, 46, 47, 50, 58, 59, 60, 61, 65, 66, 72, 75, 76, 78, 79, 87, 105, 110, 117, 156, 157,176,182,196,205,208, 209,210,211,212,213,214, 220,221,222,228,229,243, 245,246,247,257,271,274, 280,284,302,307,319,341, 345,346,353,354,355,359, 360,362,365,371,373,378, 379 Scavenging, 130, 178, 203 Schist, 80,81,147,151,154,156, 160,161,168,169,327,331, 356 S.eason, 19,91,111,113,235,344, 356,365 Secondary, 13,29,30,31,50,52, 54,74,106,107,152,178, 198,208,209,210,265,271, 291,296,314,315,343,362, 363,372,377 Sec~te, 43,48,49,50,64,318,338 Sediment, 108,115,136,138,231 Seepage, 190,254,255,262,317 Sheen, 194,292 Silica, 1,3,5,8,9,10,15,24,31, 33,34,35,36,38,40,64,65, 80,86,87,88,89,90,92,93, 94,97,99,100,101,102, 107,111,113,118,119,122, 123,127,128,130,139,141, 150,152,153,154,155,157, 161,162,163,164,169,170, 171,173,174,179,184,185, 199,203,215,223,247,250,

Subject Index 251,252,265,271,275,279, 280,281,282,283,284,285, 286,287,288,289,290,291, 292,293,294,295,296,297, 298,299,300,301,302,303, 304,305,306,307,308,309, 310,311,313,314,315,316, 317,318,319,320,321,322, 324,325,329,330,331,332, 334,336,337,339,341,342, 343,344,354,360,361,362, 363,365,369,370,371,373, 374, 375, 377, 378 Silica glaze, 1, 3, 8, 9, 10, 33, 34, 35, 36, 64, 65, 80, 86, 88, 89, 90,92,93,94,101,102,118, 119,123,139,141,153,154, 155,157,161,162,169,179, 185,215,223,247,252,265, 271,275,279,281,282,283, 284,285,286,287,288,289, 290,291,292,293,294,295, 296,297,298,299,300,301, 302,303,304,305,306,307, 308,309,310,311,313,314, 315,316,317,318,319,320, 321,322,324,325,329,330, 331,332,334,336,339,341, 342,343,344,354,360,361, 362,363,365,369,370,371, 373,374, 375,377 Silicate, 3, 26, 84, 106, 140, 269, 279,288, 317, 319, 321,329, 337 Silicic acid, 280, 319, 320 Silt, 5, 15, 60, 110, 135, 151,374 Slope, 21, 61, 68, 114, 129,259, 305,327, 335,336, 341,363, 373 Smectite, 241,242 Sodium, 15, 39, 79, 135,232,249, 250, 255, 256, 257,258, 259, 263,300 Soil, 17, 20, 21, 28, 48, 60, 72, 73, 75, 76, 77, 78, 81, 84, 104, 109, 118, 135, 141,152, 159, 169, 173, 180, 181,182, 185, 191,193, 195,205,208, 216, 217,218, 219, 220, 221,223, 224, 234, 236, 237,239, 255, 256, 262, 266, 279, 281,285, 287,294, 310, 317,326, 328, 333,335, 336, 337, 351,362 Soiling of buildings, 11, 15, 110 Solution, 1, 17, 39, 106, 164, 181, 232,233, 234, 236, 237, 238, 239,257, 262,266,275, 317, 319,337, 342,365,369 Soot, 101 Spatial, 2, 21, 22, 31, 87, 187, 199, 209,222, 321,345,346, 372, 377,378 Spring, 67, 122, 134, 216, 362 Stability, 11, 39, 79, 102, 103, 157, 158, 191,313,314, 326, 327, 335,338, 342 Stable, 20, 24, 25, 26, 28, 35, 40, 157, 181,221,249,266, 276, 279,332, 334, 342

Subject Index Steel, 18, 135, 141,142, 166, 167, 300 Stone conservation, 11, 38, 107, 254, 268, 289 Streaks, 8, 9, 10, 99, 141,157,252, 277,324, 325,342, 345, 373 Stream, 18, 56, 67, 105, 122, 123, 124, 125, 130, 131,132, 134, 138, 139, 148, 149, 153, 155, 158, 176, 181,199, 292, 326, 337,356, 361,362, 364, 373 Strengite, 251,362, 376 Stromatolite, 47, 65, 70, 183 Subaerial, 26, 28, 67, 75, 76, 78, 79, 81, 84, 88, 109, 135, 145, 150, 155, 157, 158, 162, 163, 164, 169, 170, 172, 175, 179, 180, 181, 183, 184, 197, 200, 201,205, 209, 214, 215, 216, 217,218, 219, 220, 221,222, 223,224, 235,264, 266, 281, 286, 287, 310, 330, 332, 336, 344, 365, 371,374, 376, 377 Sublimation, 264 Subsurface, 3, 24, 43, 72, 75, 76, 87, 106, 117, 150, 162, 163, 179, 184,201,216,217,248, 257,262, 266, 281,327, 329, 330, 331,332, 333,334, 342, 343 Succession, 57, 63, 65,338 Sulfate crust, 262, 264, 265, 365, 371 Sulfuric acid, 181,232, 325 Tafoni, 87, 101,106, 113,249, 282 Tailings, 135 Talus, 67, 68, 69, 75, 86 Temperate, 94, 102, 145, 148, 157, 192,269, 286, 308,374 Thickness, 17, 18,29, 43, 73, 110, 132, 157, 159, 169, 173, 176, 187,193, 194, 204, 208, 220, 224, 228, 229,230, 231,235, 250, 254, 264, 284, 313, 347 Till, 314, 315, 343 Toxic, 120, 135, 178 Transmission, 45, 46, 153, 196, 197, 198, 209, 211,212,214, 243, 244, 245, 280 Transport, 29, 108, 109, 135, 141, 151,152, 190, 219, 221,239, 246, 262, 322, 326, 327, 342, 343,377 Travertine, 15, 67, 338 Tuff, 87, 269 Tundra, 60 Ultrathin, 29 Underside, 57, 164, 172, 173, 180, 194, 223, 334 Unstable, 103, 157,218, 313,316, 319,327 Uranium, 120, 130 Urban, 11, 34, 36, 81, 82, 110, 112, 113, 119, 135, 139, 146, 168, 180, 264, 265, 329

429

Volcanic, 12, 58, 80, 105, 153,253, 274, 275, 280, 281,283, 317, 358,365, 366 Volcanic ash, 275,283, 317 Water flow, 38, 76, 97, 98, 99, 126, 135, 153, 155, 166, 201,216, 223,269, 273,277,286, 307, 331,335, 336, 337, 341,342, 362, 371,373 Water repellent, 38 Weathering, 1, 2, 3, 5, 11, 12, 13, 14, 17, 24, 25, 26, 38, 41, 43, 48, 57, 58, 59, 60, 63, 64, 65, 79, 84, 86, 87, 91, 92, 93, 94, 95, 96, 97, 99, 101,102, 104, 106, 107, 110, 122, 135, 139, 142, 144, 145, 146, 147, 151, 155, 156, 157, 158, 166, 180, 181,182, 187, 188, 193, 210, 211,215, 216, 218,219, 220, 228,232, 233,236, 237, 238, 239, 241,242, 245,246, 247, 249, 259, 265,266, 267, 275, 278,279, 281,282,286, 287, 314, 315, 316, 317,318, 319, 325,326, 327, 334, 336, 338, 341,342, 345, 353,356, 363, 365,370, 372, 374, 377, 378 Weathering find, 1, 12, 13, 57, 58, 92, 94, 95, 96, 99, 101, 104, 106, 156, 158, 166, 187, 193, 233,236, 245,265,286, 287, 314, 315, 316, 317,334, 353, 363,378 Wetland, 216 Zinc, 15, 130, 138, 139, 140, 142, 178,339