Earth Paleoenvironments: Records preserved in Mid- and Low-Latitude Glaciers (Developments in Paleoenvironmental Research)

Earth Paleoenvironments: Records Preserved in Mid- and Low-Latitude Glaciers

Developments in Paleoenvironmental Research VOLUME 9

Earth Paleoenvironments: Records Preserved in Mid- and Low-Latitude Glaciers Edited by

L. DeWayne Cecil U.S. Geological Survey, Idaho Falls, U.S.A.

Jaromy R. Green Garden City Community College, Kansas, U.S.A. and

Lonnie G. Thompson The Ohio State University, Ohio, U.S.A.

KLUWER ACADEMIC PUBLISHERS NEW YORK, BOSTON, DORDRECHT, LONDON, MOSCOW

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Dedication

This book is dedicated to the scientists and explorers who came before us and those that will follow us.

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Contents

Dedication

v

Contributing Authors

xi

Preface by L.D. Cecil

xv

Foreword by E.J. Steig

xix

Acknowledgments

xxiii

List of Acronyms and Abbreviations

xxv

Published and Forthcoming Titles in the Developments in Paleoenvironmental Research Book Series

xxix

Editors and Board of Advisors of Developments in Paleoenvironmental Research Book Series

xxxi

PART I: INTRODUCTION AND METHODS 1. High-Altitude, Mid and Low-Latitude Ice Core Records: Implications for our Future by L.G. Thompson 1. Introduction 2. What we’ve learned from Mid-and Low-Latitude Alpine Ice Cores 3. Important questions still to be answered 4. References 2. Methods of Mid- and Low-Latitude Glacial Record Collection, Analysis, and Interpretation by J.R. Green et al. 1. Introduction 2. Glacial Record Collection 3. Methods (Chemical and Biological) 4. Methods (Physical Characteristics) 5. Glacial Record Interpretation 6. Summary 7. References

1 3 3 5 11

17 17 17 20 28 31 33 34

viii PART II: THE CLIMATE AND ENVIRONMENTAL CHANGE RECORD OVER THE LAST 200 YEARS

37

3. The Influence of Post-Depositional Effects on Ice Core Studies: Examples from the Alps, Andes, and Altai by U. Schotterer, et al. 1. Introduction 2. Swiss Alps 3. Subtropical and Tropical Andes 4. Mongolian and Siberian Altai 5. Conclusions 6. References

39 39 42 48 53 56 57

4. Event to Decadal-Scale Glaciochemical Variability on the Inilchek Glacier, Central Tien Shan by K. Kreutz et al. 61 1. Introduction 61 2. Sample Collection and Analytical Methods 62 3. Results 65 4. Discussion 72 5. Conclusions 76 6. Acknowledgements 77 7. References 77 5. Climatic Interpretation of the Gradient in Glaciochemical Signals Across the Crest of the Himalaya by C.P. Wake et al. 1. Introduction 2. Climatological Setting 3. Methods 4. Results 5. Discussion 6. Conclusions 7. References

81 81 82 83 85 89 92 93

6. Reconstruction of European Air Pollution from Alpine Ice Cores by M. Schwikowski 1. Introduction 2. Suitable Glaciers and Ice Cores Retrieved 3. Dating and Time Period Accessible 4. Reconstructed Air Pollution Records 5. Conclusions 6. References

95 95 96 97 100 115 116

7. Glacier Research in Mainland Scandinavia by W.B. Whalley 1. Introduction

121 121

ix 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Present Day Glaciers – An Overview Early Work on the Glaciers in Scandinavia and Historic Variations Mass Balance Measurements and Glacier Mapping Little Ice Age Glacier Extents Thermal Regimes and Coring Projects Variations in Mass Balance and Continentality Subglacial Observations Conclusions Acknowledgements References

121 124 125 127 128 130 138 138 139 139

PART III: THE CLIMATE AND ENVIRONMENTAL CHANGE RECORD OVER THE LAST 200 – 500 YEARS

144

8. Four Centuries of Climatic Variation Across the Tibetan Plateau from Ice-Core Accumulation and į18O Records by M.E. Davis et al. 1. Introduction 2. Seasonality in the Tibetan Plateau Ice Cores 3. Precipitation Sources and Influences 4. Conclusions 5. Acknowledgments 6. References

145 145 147 154 158 159 160

9. Climatic Changes over the Last 400 Years Recorded in Ice Collected from the Guliya Ice Cap, Tibetan Plateau by Y. Tandong et al. 1. Introduction 2. Calculation of Glacial Accumulation from the Guliya Ice Core 3. A Comparison between the Precipitation Variations of the Guliya Ice Cap and its Vicinity 4. ENSO Events Recorded in the Guliya Ice Core 5. Conclusions 6. References 10. Evidence of Abrupt Climate Change and the Development of an Historic Mercury Deposition Record Using Chronological Refinement of Ice Cores at Upper Fremont Glacier by P.F. Schuster et al. 1. Introduction 2. Methods 3. Results and Discussions 4. Conclusions 5. References

163 165 169 172 177 178

181 181 185 192 211 212

x 11. Variations between į18O in Recently Deposited Snow and On-Site Air Temperature, Upper Fremont Glacier, Wyoming by Naftz et al. 217 1. Introduction 217 2. Background Information and Methodology 220 3. Results and Discussion 223 4. Conclusions 231 5. References 232 Summary by L.D. Cecil

235

Index

243

Contributing Authors

Vladimir B. Aizen University of Idaho Moscow, Idaho, U.S.A. L. DeWayne Cecil (editor) U.S. Geological Survey Idaho Falls, Idaho U.S.A. [email protected] Mary E. Davis Byrd Polar Research Center The Ohio State University Columbus, Ohio U.S.A. Shawn K. Frape University of Waterloo Waterloo, Ontario, Canada Patrick Ginot The Ohio State University Columbus, Ohio U.S.A. Jaromy R. Green (editor) Garden City Community College Garden City, Kansas U.S.A. [email protected]

xii Sichang Kang Lanzhou Institute of Glaciology and Geocryology Academia Sinica Lanzhou, China Karl J. Kreutz Institute for Quaternary and Climate Studies Department of Geological Sciences University of Main Orono, Main U.S.A. Paul A. Mayewski Institute for Quaternary and Climate Studies University of Maine Orono, Maine U.S.A. Yang Meixue Key Laboratory of Ice Core and Cold Region Environment Cold and Arid Regions Environmental and Engineering Research Institute Chinese Academy of Sciences, Lanzhou 730000, China David L. Naftz U.S. Geological Survey Salt Lake City, Utah U.S.A. Ulrich Schotterer University of Bern Switzerland Paul F. Schuster U.S. Geological Survey Boulder, Colorado U.S.A. Margit Schwikowski Paul Scherrer Institute Villigen PSI, Switzerland Willibald Stichler Institute for Hydrology Neuherberg, Germany

xiii David D. Susong U.S. Geological Survey Salt Lake City, Utah U.S.A. Hans-Arno Synal Institute for Particle Physics Zurich, Switzerland Yao Tandong Key Laboratory of Ice Core and Cold Region Environment Cold and Arid Regions Environmental and Engineering Research Institute Chinese Academy of Sciences, Lanzhou 730000, China Lonnie G. Thompson (editor) Department of Geological Sciences The Ohio State University Columbus, Ohio U.S.A. [email protected] Cameron P. Wake Department of Earth Sciences University of New Hampshire Durham, New Hampshire U.S.A. W. Brian Whalley Queen’s University Belfast UK

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Preface

The earth is in a state of constant physical, chemical, and biological change on a global scale. Global environmental alterations have occurred throughout the existence of the earth and will invariably happen in the next millenium and beyond. Global change can have immediate as well as future consequences that could affect all life on earth. As a result, the importance of understanding current and potential global environmental change has radically increased. Numerous global environmental change studies are currently underway. From monitoring ongoing natural events such as earthquakes and volcanoes to delineating potential anthropogenic effects from industrial chemical fallout from the atmosphere, all studies focus on understanding the immediate and potential environmental change and monetary impacts associated with such events. The study of global environmental change caused by anthropogenic influences requires knowing how and when the influences occurred and what effects the environment will suffer. Once these are known, the resultant future climatic and environmental changes can be projected. Additionally, studies of natural climatic and environmental alterations require the knowledge of long-term historical changes in order to predict or understand future shifts. Knowledge of past changes can only be acquired by studying and analyzing preserved environmental records that act as archives of these changes. Preserved archives of past climatic and environmental conditions do exist in nature. For example, glaciers, ice caps, and ice sheets around the world can be repositories of climatic and environmental change. Ice cores from the polar regions have provided the scientific community with an unprecedented picture of past environmental change through chemical, isotopic, and

xvi stratigraphic data. High-resolution ice core records have also been obtained from high altitude sites in the tropics. However, weather patterns and climate changes affect high-latitude regions of the world differently than mid- to low-latitude areas. In addition, the majority of the world’s population, at least 85 percent, lives between 50° N and 50° S. Therefore, understanding potential environmental change in mid- and low-latitude regions is of prime importance and could be accomplished by utilizing ice cores collected from selected alpine areas. Research on temperate ice cores faces the challenge of several commonly held beliefs about ice cores in “warm” environments. First, that the influence of meltwater percolation – which tends to smooth glaciochemical variations in the glacier forming firn and snow– precludes the use of isotopic and chemical tracers. Second, that the high accumulation rates typical for temperate glaciers and ice sheets limit the length of the record to at most, a few centuries. Third, that the availability of other climate proxies, such as pollen and tree-ring records, makes temperate ice cores unnecessary. Research at several mid-latitude sites worldwide has shown that these common beliefs are not warranted. Glacial research has already proven that ice cores collected from mid-latitude glaciers preserve the isotopic record with surprising accuracy and, for some glaciers, represents thousands of years of record. In addition, ice cores archive not only natural variations in climate and the environment but anthropogenic influences introduced over the last two centuries as well. Such additional anthropogenic information can aid in distinguishing between natural and human additions to the environment and thus further refine the understanding of future global, environmental, and climate change. There is now a small army of diverse researchers worldwide turning to the archived environmental record in mid- and low-latitude ice cores to answer diverse questions from natural and anthropogenic influences on climate change to rates of glacial retardation and growth. With the advent of ultra-sensitive analytical methods such as accelerator mass spectrometry and the experiences of diverse research teams, glaciers worldwide, with their environmental records and markers locked in, are becoming accessible. These new scientific tools and their application to understanding our influence on global environmental processes are the focus of this book. In the field of glacial research and the associated global impacts on humans there is no set of handy formulas into which various parameters can be substituted to obtain answers for the complex problems facing the world’s population. This book was designed with that fact in mind. The papers collected here represent some of the leading research and methods development in the growing scientific field of documenting global climatic and environmental changes using records archived at mid- and low-

xvii latitude sites; historically, presently, and in the future. It is hoped that current researchers and students will find the introductory “how-to” methods section useful in their work. Additionally, with a good solid grounding in the methods utilized in bringing ice core records from remote, harsh environments to the laboratory for analyses and interpretation, students will be prepared to appreciate the significance of any glacial research they may find in the literature. L. DeWayne Cecil U.S. Geological Survey Idaho Falls, Idaho U.S.A.

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Foreword

Several compilations of papers on ice-core science have been published over the last twenty-five years, among them Robin’s, “The Climatic Record in Polar Ice Sheets” (Cambridge, 1983), and Oeschger and Langway’s definitive, “The Environmental Record in Glaciers and Ice Sheets” (Wiley, 1989). The focus of these books (and many others) is the subject of ice cores from polar regions, and for good reason. Polar ice cores have yielded spectacular results, including the discovery of abrupt climate change events during the last glacial period, and the record of atmospheric greenhouse gas concentrations that demonstrate unequivocally the human influence on the atmosphere. One of the consequences of the success of polar ice core science has been that the potential of ice cores in mid- and low-latitudes has been largely overlooked for much of the last two decades. This is not to imply that no one was doing research in this area. Indeed, Paul Mayewski’s University of New Hampshire team (Mayewski is now at the University of Maine) were successfully drilling in the Himalaya in the early 1980s, while Lonnie Thompson and his crew from Ohio State University were on Quelccaya ice cap in Peru in the early 1970s (they obtained the first tropical ice core there in 1983), and have drilled dozens of high-altitude, low-latitude cores since then. Yet it was arguably not until 1995, when W. S. Broecker retracted his former skepticism in an editorial piece in Nature praising the success of Thompson’s team on Huascarán (Peru) that the community at large became aware of the importance of these records: “For years he fought not only the cold condition of his field sites, but also the lukewarm reception by many of those in our field (including me, W. S. Broecker). “Obtaining the cores from Huascarán places Lonnie Thompson in

xx the ranks of our great explorers.” (W. S. Broecker, Nature 376, p. 212, 1995). This is the first book that has been devoted entirely to ice cores from mid- and low-latitudes. In the papers compiled here, researchers highlight the work they have done over the past few decades--in parallel with the work done on polar ice cores--at high altitudes on glaciers ranging from China and Tibet to South America and Africa. Why are these records important? Their heuristic value alone ought to be sufficient justification for the modest amount of funding (compared with polar ice-coring projects) that has been devoted to obtaining them. Cores from mid- and low-latitudes allow us to extend our look at the earth’s recent history, in the beautiful, high-resolution detail that only ice cores achieve, across virtually the entire globe: ice cores have now been obtained from all the continents except Australia. Even Africa, a place that rarely conjures up images of ice, has revealed its secrets through the ice cap on Kilimanjaro. Ice cores from these regions contain information that is more directly relevant to human history than are ice cores from the polar regions. Ice cores obtained from the Alps (Schwikowski, this volume) for example, demonstrate both the dramatic increase in air pollution in Europe over the last century and the reduction in such pollution since regulatory measures were put into effect in the 1970s. These records thus provide stark evidence both of our ability to change the composition of the earth’s atmosphere, and of our ability to do something about it. They also illustrate the dependence of human civilization on climate: cores from the Andes, in particular, show evidence that significant droughts coincided with the decline of major South American cultural groups. Second, these records are important for their scientific value. Ice cores from the Andes appear to be remarkably faithful recorders of conditions in the tropical Pacific, offering the potential to document the frequency and magnitude of El Niño events in the past. Such knowledge is in turn an essential component in determining the sensitivity of tropical climate variability - which dominates global climate variability--to increased radiative forcing from anthropogenic greenhouse gases. Ice cores from Asia similarly offer the possibility of better understanding variability in the intensity and timing of monsoon rainfall. Ice cores from both regions, and elsewhere, contribute to our understanding of atmospheric transport of pollutants such as heavy metals, radioactive isotopes, organic pollutants and sulfate and nitrate aerosols (the primary causes of acid rain) (Schuster et al., this volume). On longer timescales, the growing network of cores from the tropics contributes to our understanding of the causes of ice ages and of variability in the climate system on century to millennial timescales. It has been conventional to attribute climatic variability on these timescales to

xxi various factors - such as sea ice-albedo feedbacks - to features of the climatic system that are unique to the polar regions. In part because of information from tropical ice cores, there is increasing interest in the scientific community on the potential role of the tropics in climatic change, both in the past and in the future. It is my hope that readers will view the present volume not so much as a summary of work accomplished, but as inspiration for future work. There is much that remains to be done on ice cores that have already been drilled. Unlike the major drilling programs in Antarctica and Greenland, carried out by large teams of researchers from many different universities (more than 40 institutions worldwide are involved in the recent and ongoing deep drilling efforts in Greenland), the cores from mid- and low- latitudes have been obtained exclusively by small teams from just one or two universities. Consequently, only a handful of the great number of possible measurements that can be done on the ice - ranging from trace gas concentrations to rare cosmic-ray produced isotopes - have been completed on these cores. Fortunately, researchers have been careful to archive sections of ice at their home institutions, so material is still available. There are also many remaining sites to be drilled. This is especially true of temperate glaciers and ice caps - defined not by their latitude but by the presence of ice that reaches the melting point during the summer. (Glaciers that, due to their high altitude, remain below the melting point throughout the year - often called “polar” glaciers, even if they are at not in the polar regions - dominate the list of sites where mid- and low-latitude ice cores have been obtained). It is commonly believed that temperate glaciers are of limited use because meltwater formed during the summer percolates through the summer snow and erases or homogenizes the chemical information contained therein. Yet useful information may in some cases be preserved because the formation of impermeable ice layers at the end of the summer prevents infiltration. Great success has been demonstrated on the Upper Fremont Glacier in Wyoming, where seasonal variations in stable isotope signatures (a widely-used measure of temperature) are preserved (Naftz et al., this volume). Temperate glaciers with ice hundreds or thousands of years old exist throughout the high-precipitation regions of the Canadian and Chilean west coasts, in New Zealand, and elsewhere. Even longer records are preserved beneath rocky debris in numerous additional glaciers on the drier side of these ranges. Obtaining some of these records will be easy; others, particularly those in remote locations and where rock debris is a problem, will be difficult. In some cases, the scientific payoff will not be immediately apparent to all. Yet the achievements so far suggest the effort is worthwhile. And as Lonnie Thompson has warned us, the cost of waiting

xxii may be to lose these precious records entirely, as almost all glaciers in tropical and middle latitudes are disappearing rapidly. Eric J. Steig Quaternary Research Center University of Washington Seattle, Washington U.S.A.

Acknowledgments

Many colleagues reviewed all or part of this book for its technical and editorial content. We are grateful to the following: Dr. Clay Nichols (he reviewed the entire book for us!), Dr. Mitch Plummer, Dr. Tom W.D. Edwards, Dr. Anne Palmer, Dr. Jim LaBaugh, Dr. Kendrick Taylor, Dr. Gary Gill, Dr. Jim Wiener, Dr. Erik Rolan, Dr. Per Holmlind, Dr. Atle Nesje, Dr. Wilfred Theakstone, Mr. Travis McCling, Mr. Flint Hall, Ms. Linda Channel, Ms. Barbara Kemp, Ms. Kristi Moser-McIntire, and the following contributors to this volume who reviewed collected papers other than ones they were coauthors on: Dr. Karl Kreutz, Dr. Uli Schotterer, Dr. Vladimir B. Aizen, Dr. Eric Steig, Dr. Mary E. Davis, and Dr. Margit Schwikowski. Each of the contributors to this volume and the editors thank their respective Institutions and Agencies for their support during the compilation of this book. Special thanks are due the U.S. Department of Energy, Idaho Operations and the U.S. Geological Survey for their support. Special thanks are due Ms. Nola Hartgraves for her excellent job of formatting and collecting the papers into the volume you are reading. Thanks Nola!

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List of Acronyms and Abbreviations

AMS - accelerator mass spectrometry ASRL - accelerated sea-level rise BC - black carbon BPRC - Byrd Polar Research Center CDC - Climate Diagnostics Center CI - continentality index DOE - United States Department of Energy EC - elemental carbon ECM - electrical-conductivity measurements EDA - energy dispersion analysis ELA - equilibrium line altitude ENSO - El Niño/Southern Oscillation (The term ENSO will be used to refer to both a warm and a cold episode; El Niño will be used to specify a warm episode, and La Niña will be used to specify a cold episode.)

xxvi GCM - General Circulation Model GNIP - Global Network for Isotopes in Precipitation GOALS - Global Ocean Atmosphere Land System GOOS - Global Ocean Observing System IAEA - International Atomic Energy Agency IC - ion-exchange chromatography ICPRG - Ice Core Paleoclimatology Research Group IRI - International Research Institute for Climate Prediction LGM - Last Glacial Maximum LIA - Little Ice Age LICCRE - Laboratory of Ice Core and Cold Regions Environment, Cold and Arid Regions Environmental and Engineering Research Institute NAO - North Atlantic Oscillation NASA - National Aeronautics and Space Administration NICL - National Ice Core Laboratory NOAA - National Oceanic and Atmospheric Administration NSF - National Science Foundation NVE - Norwegian Water Resources and Energy Directorate OAR - (Office of) Oceanic and Atmospheric Research OC - organic carbon QC - quality control SEM - Scanning Electron Microscopy

xxvii SLP - sea level pressure SMOW - Standard Mean Ocean Water SOI - Southern Oscillation Index SST - sea surface temperature TC - total carbon UFG - Upper Fremont Glacier UNESCO - United Nations Educational, Scientific, and Cultural Organization USGS - United States Geological Survey VEI - Volcanic Explosivity Index V-SMOW - Vienna Standard Mean Ocean Water WDMRL - Wisconsin District Mercury Research Lab WMO - World Meteorological Organization WS - weather station

Other abbreviated units used in this volume: Bq - becquerel, which is equal to one radioactive transition per second o

C - degrees Celcius

cm - centimeter cm/s - centimeters per second CO2 - carbon dioxide g/cm3 - grams per cubic centimeter

xxviii hPa - hecto Pascals Ka - thousands of years ago km - kilometer km2 - square kilometers km3 - cubic kilometers m - meter masl - meters above sea level mg/L - milligrams per liter mil - one-thousandth of an inch mm - millimeter m/y - meters per year mȍ - microohms ng/L - nanograms per liter ppt - precipitation µg/L - micrograms per liter µm - micrometer weq - water equivalent į18O - delta oxygen-18 (see Sidebar #1, page 23, for an explanation) į2H - delta deuterium (see Sidebar #1, page 23, for an explanation)

DEVELOPMENTS IN PALEOENVIRONMENTAL RESEARCH BOOK SERIES http://www.wkap.nl/prod/s/DPER http://home.cc.umanitoba.ca/~mlast/paleolim/dper.html

Series Editors: John P. Smol, Department of Biology, Queen's University Kingston, Ontario, Canada William M. Last, Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada

Volume 9:

Earth Paleoenvironments: Records Preserved in Mid-and Low-Latitude Glaciers Edited by L. D. Cecil, J. R. Green and L. G. Thompson Hardbound, ISBN 1-4020-2145-3, 2004

Volume 8:

Long-term Environmental Change in Arctic and Antarctic Lakes Edited by R. Pienitz, M. S. V. Douglas and J. P. Smol Hardbound, ISBN 1-4020-2125-9, forthcoming

Volume 7:

Image Analysis, Sediments and Paleoenvironments Edited by P. Francus Hardbound, ISBN 1-4020-2061-9, forthcoming

Volume 6:

Past Climate Variability through Europe and Africa Edited by R. W. Battarbee, F. Gasse and C. E. Stickley Hardbound, ISBN 1-4020-2120-8, forthcoming

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Tracking Environmental Change Using Lake Sediments. Volume 5: Data Handling and Numerical Techniques Edited by H. J. B. Birks et al. Hardbound, forthcoming

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Tracking Environmental Change Using Lake Sediments. Volume 4: Zoological Indicators Edited by J. P. Smol, H. J. B. Birks and W. M. Last Hardbound, ISBN 1-4020-0658-6, June 2001

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Tracking Environmental Change Using Lake Sediments. Volume 3: Terrestrial, Algal, and Siliceous Indicators Edited by J. P. Smol, H. J. B. Birks and W. M. Last Hardbound, ISBN 1-4020-0681-0, June 2001

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xxx Volume 1:

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EDITORS AND BOARD OF ADVISORS OF DEVELOPMENTS IN PALEOENVIRONMENTAL RESEARCH BOOK SERIES Series Editors: John P. Smol Paleoecological Environmental Assessment and Research Lab (PEARL) Department of Biology Queen’s University Kingston, Ontario, K7L 3N6, Canada e-mail: [email protected] William M. Last Department of Geological Sciences University of Manitoba Winnipeg, Manitoba R3T 2N2, Canada e-mail: [email protected]

Advisory Board : Professor Raymond S. Bradley Department of Geosciences University of Massachusetts Amherst, MA 01003-5820 USA e-mail: [email protected] Professor H. John B. Birks Botanical Institute University of Bergen Allégaten 41 N-5007 Bergen Norway e-mail: [email protected] Dr. Keith Alverson Executive Director, IGBP-PAGES PAGES International Project Office Sulgeneckstrasse 38 3007, Bern, Switzerland e-mail: [email protected]

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PART I: INTRODUCTION AND METHODS

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HIGH ALTITUDE, MID- AND LOW-LATITUDE ICE CORE RECORDS: IMPLICATIONS FOR OUR FUTURE

L. G. Thompson

1.

INTRODUCTION

The 20th Century has seen the acceleration of unprecedented global and regional-scale climatic and environmental changes to which humans are vulnerable, and by which we will become increasingly more affected in the coming centuries. One-half of the Earth’s surface area lies in the tropics between 30o N and 30o S, and this area supports about 70 percent of the global population. Thus, temporal and spatial variations in the occurrence and intensity of coupled ocean-atmosphere phenomena such as El Niño and the Monsoons, which are most strongly expressed in the tropics and subtropics, are of world-wide significance. Unfortunately, meteorological observations in these regions are scarce and of short duration, particularly from high elevation sites. However, ice core records are available from lowlatitude, high-altitude glaciers, and when they are combined with highresolution proxy histories such as those from tree rings, lacustrine and marine cores, corals, etc., they provide an unprecedented view of the Earth’s climatic history over several millennia. This paper provides an overview of these unique glacier archives of past climate and environmental changes on millennial to decadal time scales. Unfortunately, these glacier archives of our past climate and environmental history are at risk. This is illustrated by the recent history of one tropical glacier in particular, the Quelccaya ice cap, 3 L. D. Cecil et al. (eds.), Earth Paleoenvironments: Records Preserved in Mid- and Low-Latitude Glaciers, 3-15. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.

4

High Altitude, Mid- and Low-Latitude Ice Core Records

as shown by a photo of its margin in 1977 (Figure 1a) compared with one taken in 2002 (Figure 1b).

Figure -1. The The margin of the Quelccaya ice cap in (1) 1977 and in (b) 2002

Over the last 25 years the principle objective of the Ice Core Paleoclimatology Research Group (ICPRG) at the Byrd Polar Research Center (BPRC) at the Ohio State University has been the acquisition and analysis of a global array of ice cores that can provide high-resolution climatic and environmental histories which contribute to our understanding of the complex interactions within the Earth’s coupled climate system. With the help of new light-weight drilling equipment, we have achieved one of our main scientific objectives by expanding our research from the polar regions to remote ice fields on some of the highest tropical and subtropical mountains. Ice core records from mountains in Africa, South America, and China make it possible to study processes in the subtropical and tropical latitudes where human activities are concentrated. The 15 sites from where the ICPRG has retrieved high-altitude ice cores are shown in Figure 2. We utilize an ever-expanding ice core database of multiple proxy information (i.e. stable isotopes of oxygen, or (δ18 O) and hydrogen, insoluble dust, major and minor ion chemistry, precipitation reconstruction) that spans the globe in spatial coverage and is of the highest possible temporal resolution.

L. G. Thompson

5

Some of the accomplishments and challenges of our ice core research, as well as those of our colleagues in the USGS and other American institutes, along with those of Chinese, Russian, French and Swiss scientists, are highlighted in this volume.

Figure -2. Locations of sites from where ice cores have been taken by the Ice Core Paleoclimate Research Group.

The records contained within the Earth’s alpine ice caps and glaciers provide a wealth of data that contribute to a broad spectrum of critical scientific questions. These range from the reconstruction of high-resolution climate histories to help explore the oscillatory nature of the climate system, to the timing, duration, and severity of abrupt climate events, to the relative magnitude of 20th Century global climate change and its impact on the cryosphere. The information from these ice core studies complements other proxy records that compose the Earth’s climate history, which is the ultimate yardstick by which the significance of present and projected anthropogenic effects will be assessed.

2.

WHAT WE’VE LEARNED FROM MID-AND LOW-LATITUDE ALPINE ICE CORES

The first program to drill a low-latitude mountain core to bedrock was carried out on the Quelccaya ice cap in Southern Peru (14oS, 71oW) in 1983. In 2002, a subarctic alpine site, Bona-Churchill in the Wrangell Mountains in southeast Alaska, was drilled and 623 meters of ice core were recovered, with one core from surface to bedrock measuring 460 meters in length. In

6

High Altitude, Mid- and Low-Latitude Ice Core Records

between these two programs, we have recovered cores from the Tibetan Plateau (Dunde, Guliya, Dasuopu, Puruogangri), from the Andes (Huascarán and Sajama) and from Kilimanjaro in East Africa. With the exception of Puruogangri and Bona-Churchill, all the cores have been analyzed and their overall climate records have been published. Under the Puruogangri project, two ice cores that were obtained during the collaborative expedition conducted by the Laboratory of Ice Core and Cold Regions Environment, Cold and Arid Regions Environmental and Engineering Research Institute (LICCRE, formally the Lanzhou Institute of Glaciology and Geocryology) and BPRC are being analyzed for physical and chemical parameters.

Figure -3. (a) The margin of the Puruogangri ice cap, central Tibetan Plateau demonstrating the distinct seasonal dust layering. (b) An ice core from the puruogangri ice cap.

L. G. Thompson

7

Figure 3a illustrates the distinct annual dust layers recorded in this ice cap located in the center of Tibet, where each spring aeolian sand leaves an identifiable seasonal horizon that can be seen at the margin. At the margin of this ice cap, over 2000 years can be discerned by counting these layers which average about 50 centimeters in thickness. Figure 3b illustrates one of the cores recovered from the summit of the ice cap in which these annual layers can be seen. The marked enrichment in 18O in the ice over the most recent half century at this location (Figure 4), is consistent with findings from glaciers on the northeastern and southern sides of the Plateau (Thompson et al., 1989; Thompson et al., 2000b).

Figure -4. į 18 O record (by depth) from Core 1 from the Puruogangri ice cap, shown as halfmeter averages.

8

High Altitude, Mid- and Low-Latitude Ice Core Records

Low-latitude, high-altitude ice core records have revealed the nature of climate variability over both glacial and interglacial time scales, specifically over the last 25 thousand years since the Last Glacial Maximum (LGM). Two records from the South American Andes (Huascarán in Northern Peru at 9oS, 78oW and Sajama in Bolivia at 18oS, 69oW) and one fom the Tibetan Plateau (Guliya at 35oN, 81oE) extend to or past the LGM and confirm, along with other climate proxy records (eg. Stute et al., 1995; Guilderson et al., 1994), that the LGM was much colder in the tropics and subtropics than previously believed (Thompson et al., 1995; Thompson et al., 1998). The Guliya record covers over 700,000 years and is the oldest low- latitude, high-altitude record recovered as of this writing (Thompson et al., 1997). Although the LGM period was consistently colder, it was not consistently drier through the lower latitudes as it was in the polar regions. For example, the effective moisture along the axis of the Andes Mountains during the end of the last glacial stage was variable, being much drier in the north than in the Altiplano region in the central part of the range (Thompson et al., 1995; Thompson et al., 1998; Davis, 2002). In another example in Western China, the Guliya ice cap is partly affected by the variability and strength of the Southwest Indian Monsoon system, which was much weaker during the last glacial stage than during the Holocene. However, this region of the Tibetan Plateau also receives (and received) moisture generated from the cyclonic activity carried over Eurasia by the prevailing wintertime westerlies. Not only were lake levels in the Western Kunlun Shan higher than tropical lakes during the LGM (Li and Shi, 1992), but the dust concentrations in the Guliya ice core record were consistent with those of the Early Holocene when the summer Asian Monsoons became stronger suggesting that local sources of aerosols were inhibited during this cold period by higher precipitation and soil moisture levels (Davis, 2002). Tropical and subtropical ice core records during the Holocene show evidence of major climatic disruptions, specifically droughts. Major dust events, beginning between 4.2 and 4.5 ka and lasting several hundred years, are observed in the Huascarán and Kilimanjaro ice cores (Thompson, 2000b; Thompson et al., 2002a, respectively), and the timing and character of the dust spike is similar to one seen in a marine core record from the Gulf of Oman (Cullen et al., 2000) and a speleothem δ13 C record from a cave in Israel (Bar-Matthews et al., 1997). This dry period is also documented in several other proxy climate records throughout Asia and Northern Africa (see contributions in Dalfes et al., 1994). More recently, a historically documented drought in India in the 1790’s, which was associated with monsoon failures and a succession of severe El Niños, was recorded in the insoluble and soluble aerosol concentration records in the Dasuopu ice core (Thompson et al., 2000b). Another recorded

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Asian Monsoon failure in the late 1870’s (Lamb, 1982) is noticeable in the Dasuopu dust flux record, which is a parameter that incorporates both the dust concentration and the annual accumulation rate of ice on the glacier surface.

Figure -5. Composite records of decadal averages of į18 O from ice cores from (a) the South American Andes (Huascaran, Quelccaya, Sajama) and (b) the Tibetan Platteau (Dunde, Guliya and Dasuopu) from 1000 A.D. to the present. All six ice-core records are combined (c) to give a total view of variations in į18 O over the last millennium in the tropics, which is compared with the Northern Hemisphere reconstructed temperature record (d).

High-resolution records of Late Holocene variations in temperature are available from low latitude alpine ice cores. Composites of the δ18 O profiles of the South American cores (Huascarán, Quelccaya, and Sajama) and three of the Tibetan Plateau cores (Dunde, Guliya and Dasuopu) show similar trends in decadal averages over the last millennium (Thompson et al., 2003)(Figure 5). When all six of the records from these mountain glaciers are combined, the resulting composite is similar to the Northern Hemisphere temperature records of Mann et al. (1998) and Jones et al. (1998) covering the last 1000 years. As in polar ice cores, the dominant factor controlling mean δ18 O values in Andean snowfall on decadal, centennial and millennial timescales must be temperature, while on seasonal to annual time scales both temperature and precipitation influence the local δ18 O signal (Vuille et al., 2003). Not only do these comparisons argue for the important role of

10

High Altitude, Mid- and Low-Latitude Ice Core Records

temperature in the composition of oxygen isotopic ratios in glacier ice, but they also demonstrate abrupt warming from the late 19th Century through the 20th Century. Indeed, the 20th Century was the warmest period in the last 1000 years, which also encompasses the time of the “Medieval Warming”. The recent warming is recorded in tropical alpine glaciers in other ways, both within the ice core records and by the rapid retreat of many of the ice fields. In the Andes, on the Tibetan Plateau and in the East Africa Rift Valley region this climate change has left its mark. For example, the many ice fields on Kilimanjaro covered an area of 12.1 km2 in 1912, but today only 2.6 km2 remains. If the current rate of retreat continues, the perennial ice on this mountain will likely disappear within the next 20 years (Thompson et al., 2002a). The lower elevation ice caps in the Andes are experiencing damage to their seasonal δ18 O signals from the lifting of the 0o C isotherm (Davis et al., 1995). For example, not only is the seasonal isotope signal on the Quelccaya ice cap at 14o S in Southern Peru being smoothed out as meltwater percolates through the upper layers of the snow (Thompson et al., 1993), but the ice margins are undergoing rapid and accelerating retreat.

Figure -6. The history of the retreat of the Quelccaya outlet glaciewr, Qori Kalis, from 1963 to 2002.

Figure 6 documents the retreat of Quelccaya’s largest outlet glacier, Qori Kalis, which has been studied by terrestrial photogrammetry since 1978 (Thompson et. al. 2000a). The rate of this retreat from 1983 to 1991 (14 m/yr) was almost three times that between 1963 and 1983 (5 m/yr), and in

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the 2000/2001 year reached 205 m/yr. The sequence of ten maps documents the rapid and accelerating retreat whereby the glacier front is now retreating about 40 times faster than it did in the initial measurement period from 1963 to 1978.

3.

IMPORTANT QUESTIONS STILL TO BE ANSWERED

Seasonal and annual resolution of chemical and physical parameters in ice core records from the Andes Mountains have allowed reconstruction of the variability of the ENSO phenomenon over several hundred years (Thompson et al., 1984; 1992; Henderson, 1996; Henderson et al., 1999). Because the effects of El Niño and La Niña events are spatially variable, ice core records from the northernmost (Columbia) and southernmost (Patagonia) reaches of the Andes Mountains will help further resolve the frequency and intensity of ENSO, along with temperature variations long before human documentation. This will aid in placing the modern climate changes and the modern ENSO into a more comprehensive perspective. The opportunity to study ENSO teleconnections in the most recent centuries between the tropics and the northern subarctic latitudes will be provided by the completed high-resolution record from the Bona- Churchill col in the St. Elias Range of southeast Alaska. Variability of the South Asian Monsoon is also of vital importance for a large percentage of the world’s population that lives in the affected areas. The ICPRG has drilled four cores across the Tibetan Plateau that have yielded millennial-scale histories of monsoon variability across this large region and information on the interaction between the Monsoon system and the prevailing westerlies that are traced back to the Atlantic Ocean. Although marine cores from the Arabian Sea show that the intensity of the South Asian Monsoon has increased over the last four centuries (Anderson et al., 2002), the Dasuopu record from the Himalayas demonstrates that since the early 19th Century the amount of precipitation falling on this region has decreased (Thompson et al., 2000b). However, the Dunde record from the north side of the Plateau shows an accumulation history that is opposite to that in the Himalayas (Davis and Thompson, this volume). Like ENSO, therefore, the South Asian Monsoon systems have varying geographical effects. Retrieval of ice core records from the west side of the Himalayas, which is more directly affected by the SW Indian Monsoon than is the east side where Dasuopu is located, will provide a more comprehensive overview of the precipitation and temperature histories of the Himalayas as a whole. The glaciers on these mountains are vital sources of stream water for the

12

High Altitude, Mid- and Low-Latitude Ice Core Records

populations of Nepal and India during the dry seasons, and their recent disappearance should be a source of great concern for these countries. Meteorological data from around the world suggest that the Earth’s globally averaged temperature has increased 0.6oC since 1950. The El Niño year of 1998 saw the highest globally averaged temperatures on record, while 2001 (a La Niña year) was the second warmest, and 2002 (a non-El Niño year) exceeded the 2001 average temperature. The marked warmth of the last two decades has contributed to the widespread melting of low-latitude, highaltitude glaciers. During this time, the ICPRG has been monitoring the accelerating retreat of this tropical ice in conjunction with its global ice core drilling and climate reconstruction program. Some of the clearest evidence for major climate warming underway today comes from the tropical glaciers, recorded in both the ice core į18O records and in the drastic retreats of both total area and total volume. The rapid retreat causes concern for two reasons. First, these glaciers are the world’s “water towers”, and their loss threatens water resources necessary for hydroelectric production, crop irrigation and municipal water supplies for many nations. The ice fields constitute a “bank account” that is drawn upon during dry periods to supply populations downstream. The current melting is cashing in on that account, which was built over thousands of years but is not currently being replenished. As Figure 7 illustrates, all the mountain glaciers in the tropical latitudes are currently retreating, as are most glaciers in middle and subpolar latitudes. The land between 30oN and 30oS, which constitutes 50 percent of the global surface area, is home to 70 percent of the world’s population and 80 percent of the world’s births. However, only 20 percent of the global agricultural production takes place in these climatically sensitive regions.

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Figure -7. Map demonstrating the current condition of the Earth’s cryosphere. Dark shading depicts regions where glacier retreat is underway, while lighter shading depicts where glacier advance is occurring. Shading over land between 30oN and 30oS indicates the tropical regions where most human activity is currently concentrated

The second concern that is brought about by the disappearance of these ice fields is that they contain paleoclimatic histories that are unattainable elsewhere and, as they melt, the records preserved therein are forever lost. These records are needed to discern how climate has changed in the past in these regions and to assist in predicting future changes. For example, climate records from low-latitude ice cores give us a view of widespread abrupt climate events in the tropics that occurred about 4,200 years (discussed above) and about 5,200 and B.P. (Thompson et al., 2002). These climatic “excursions” may have been catastrophic for early civilizations in Europe, Northern Africa, around the Mediterranean and in the Middle East. However, the geographic scale of these changes, their causes, the thresholds that triggered them, and the role of the tropical hydrological system are still mysteries. The multi-proxy analyses of ice cores is proving to be invaluable in the determination of long-term changes in the magnitude and the frequency of ENSO and monsoon variability over the past approximately 20,000 years, and how these systems may be related. However, high priority needs to be given to more precise dating of paleoclimate records, those from ice cores as well as other sources. In addition, paleoclimate data and climate models must be integrated to better understand past processes in climate change. The mechanisms responsible for the current global warming remain a topic of much debate, but the scientific evidence verifies that the Earth’s globally averaged surface temperature is indeed increasing, although at

14

High Altitude, Mid- and Low-Latitude Ice Core Records

varying rates. For example, over the extensive, elevated area of the Tibetan Plateau, the warming trend is amplified and is accelerating with increasing altitude (Thompson et al., 2000a). Although it is important to place this current warming and tropical glacier retreat into a long-term perspective, it is nevertheless undeniable that global water resources are at risk, and mountain glaciers and their unique climate histories are disappearing at an ever increasing rate. In order to preserve these records that are essential for examining how climate changes, we must accelerate the rate at which ice cores are being recovered and focus on those ice fields that are at the greatest risk. Thus, the loss of tropical mountain glaciers and the climate histories they contain presents an urgency to recover these archives.

4.

REFERENCES

Anderson, D.M., Overpeck, J.T., and Gupta, A.K., 2002, Increase in the Asian Southwest Monsoon during the past four centuries. Science 297, 596-599. Bar-Matthews, M., Ayalon, A. Kaufman, A., Wasserburg, G.J., 1999, The Eastern Mediterranean paleoclimate as a reflection of regional events: Soreq Cave, Israel. Earth and Planetary Letters 166, 85-95. Cullen, H.M., deMenocal, P.B., Hemming, S., Hemming, G., Brown, G.H., Guilderson, T., Sirocko, F., 2000, Climate change and the collapse of the Akkadian empire: Evidence from the deep sea. Geology 28, 379-382. Dalfes, H.N., Kukla, G., and Weiss, H., Eds., 1994, Third Millennium BC Climate Change and Old World Collapse. Springer, Berlin, 723 pp. Davis, M.E., Thompson, L.G., Mosley-Thompson, E., Lin, P.N., Mikhalenko, V.N., and Dai, J., 1995, Recent ice-core climate records from the Cordillera Blanca, Peru. Annals of Glaciology 21, 225-230. Davis, M.E., 2002, Climatic interpretations of eolian dust records from low-latitude, highaltitude ice cores. PhD Thesis, The Ohio State University. Davis, M.E. and Thompson, L.G., 2003, Four centuries of climatic variation across the Tibetan Plateau from ice-core accumulation and δ18 O records. In: Earth Paleoenvironments: Records Preserved in Mid and Low Latitude Glaciers (L.D. Cecil, J.R. Green, L.G. Thompson, eds.) Kluwer, New York. Guilderson, T.P., Fairbanks, R.G., and Rubenstone, J.L., 1994, Tropical temperature variations since 22,000 years ago: modulating inter-hemispheric climate change. Science 263, 663-665 Henderson, K.A., 1996, The El Niño-Southern Oscillation and other modes of interannual tropical climate variability as recorded in ice cores from the Nevado Huascarán col, Peru. M.S. Thesis, The Ohio State University. Henderson, K.A., Thompson, L.G., and Lin, P.N., 1999, Recording of El Niño in ice core δ O 18 records from Nevado Huascarán, Peru. Journal of Geophysical Research, D 104, 31,053-31,065. Jones, P.D., Briffa, K.R., Barnett, T.P., and Tett, S.F.B., 1998, High-resolution palaeoclimatic records for the last millennium: interpretation, integration and comparison with general circulation model control-run temperatures. Holocene 8, 455-471.

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Lamb. H.H., 1982, Climate History and the Modern World. Methuen, London, 387 pp. Li, S. and Y. Shi. (1992) Glacial and lake fluctuations in the area of the west Kunlun Mountains during the last 45,000 years. Annals of Glaciology 16, 79-84. Mann, M.E., Bradley, R.S., and Hughes, M.K., 1998, Global-scale temperature patterns and climate forcing over the past six centuries. Nature 392, 779-787. Stute, M., Forster, M., Frischkorn, H., Serejo, A., Clark, J.F., Schlosser, P., Broecker, W.S., and Bonani, G., 1995, Cooling of tropical Brazil (5oC) during the last glacial maximum. Science 269, 379-383. Thompson, L.G., Mosley-Thompson, E., and Arnao, B.M., 1984, El Niño-Southern Oscillation events recorded in the stratigraphy of the tropical Quelccaya ice cap, Peru. Science, 226, 50-52. Thompson, L.G. and 9 others, 1989, Holocene-Late Pleistocene climatic ice core records from Qinghai-Tibetan Plateau. Science 246, 474-477. Thompson, L.G., Mosley-Thompson, E., and Thompson, P.A., 1992, Reconstructing interannual climate variability from tropical and subtropical ice-core records In: El Niño: Historical and Paleoclimatic Aspects of the Southern Oscillation (H.F. Diaz and V. Markgraf, eds.) Cambridge University Press, Cambridge, pp. 295-322. Thompson, L.G. and 6 others. 1993, “Recent warming”: ice core evidence from tropical ice cores with emphasis upon Central Asia. Global and Planetary Change 7, 145-146. Thompson, L.G. and 7 others, 1995, Late Glacial Stage and Holocene tropical ice core records from Huascarán, Peru. Science 269, 47-50. Thompson, L.G. and 9 others, 1997, Tropical climate instability: the last glacial cycle from a Qinghai-Tibetan ice core. Science 276, 1821-1825. Thompson, L.G. and 10 others, 1998, A 25,000 year tropical climate history from Bolivian ice cores. Science 282, 1858-1864. Thompson, L.G., 2000a, Ice-core evidence for climate change in the Tropics: implications for our future. Quaternary Science Reviews 19, 19-36. Thompson, L.G., Yao, T., Mosley-Thompson, E., Davis. M.E., Henderson, K.A., and Lin, P.N., 2000b, A high-resolution millennial record of the South Asian Monsoon from Himalayan ice cores. Science 289, 1916-1919. Thompson, L.G. and 10 others, 2002, Kilimanjaro ice core records: Evidence of Holocene climate change in tropical Africa. Science 298, 589-593. Thompson, L.G., Mosley-Thompson, E., Davis, M.E., Lin, P.N., Henderson, K., and Mashiotta, T.A., 2003, Tropical glacier and ice core evidence of climate change on annual to millennial time scales. Climatic Change 59, 137-155. Vuille, M, Bradley, R.S., Healy, R., Werner, M., Hardy, D.R., Thompson, L.G., and Keiming, F., 2003, Modeling δ18O in precipitation over the tropical Americas, Part II: Simulation of the stable isotope signal in Andean ice cores. Journal of Geophysical Research, 108, 10.1029/2001JD002039.

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METHODS OF MID- AND LOW-LATITUDE GLACIAL RECORD COLLECTION, ANALYSIS, AND INTERPRETATION Jaromy R. Green, L. DeWayne Cecil, and Shaun K. Frape

1.

INTRODUCTION

The credibility of collecting, analyzing, interpreting and “dating” midand low-latitude glacial ice samples depends primarily on the methods used. Due to the wide variety of characteristics among mid- and low-latitude glaciers (such as location, altitude, precipitation, size, movement, and archived history), the methods used to study such glaciers vary from site to site and must be chosen with care. Of benefit to many of today’s researchers is the ability to build upon the knowledge of past studies of mid- and lowlatitude glaciers. In the 1970s and 80s, research was performed on such places as the Quelccaya Ice Cap in the Andes mountains of Peru (Thompson, 1984, 1985, 1986), the Dunde Ice Cap within the Tibetan Plateau (Thompson, 1988), and the Colle Gnifetti in the Alps (Oeschger et al., 1977). Past and present studies of these glacial sites allow current day researchers to more correctly identify and use those methods that are logistically, scientifically, and statistically sound to research additional midand low-latitude glacial sites. The following chapter details some of the methods used in current research.

2.

GLACIAL RECORD COLLECTION

2.1

Selecting a Glacial Site

Collection of glacial samples typically begins with the first and most important method of mid- and low-latitude glacial studies, which is the selection of a suitable glacial site. Initial reconnaissance of a desirable glacial site includes collecting surface snow samples for several consecutive years to determine if the mid- and low-latitude glacier meets the criteria to effectively archive geochemical and isotopic information in the ice. Some of these criteria are location, altitude, thickness, and topography. 17 L. D. Cecil et al. (eds.), Earth Paleoenvironments: Records Preserved in Mid- and Low-Latitude Glaciers, 17-36. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.

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Methods of Mid- and Low-Latitude Glacial Record Collection

The first of these conditions, location, is perhaps the most important. A glacier that is often accessed and traversed tends to suffer from human contamination that could destroy the archived information stored in the ice. In contrast, remote glacial systems that are impacted minimally by human activity provide a more accurate record of environmental changes. In addition to minimal human impact, the preservation of environmental signals in ice at mid- and low-latitudes can only be accomplished if the glacier is at sufficient altitude to minimize meltwater percolation. Percolation, the process of meltwater infiltrating down through the glacier, can dampen or even wipe out the chemical and isotopic signals stored in the ice. Sufficiently high altitude can minimize or prevent problems with percolation, thus preserving the environmental signals in the ice. The preservation of signals in glacial ice is of greatest use when environmental signals, both natural and anthropogenic, can be observed in the ice. Generally, for mid- and low-latitude glaciers, “seeing” natural environmental changes coincides with ice thicknesses of 100 m or more; the greater the ice thickness, the larger the environmental picture of the past that can be reconstructed from the archived data in the ice. Anthropogenic changes to the environment, on the other hand, are typically observed in ice thicknesses of less than 100 meters (m). Slow glacial movement preserves environmental signals in glacial ice much better than fast movement. The ideal condition for slow glacial movement, low-angle topography of the underlying bedrock, prevents ice mixing that occurs with fast moving glaciers underlain by steep topography. The less that the ice is mixed, the easier it is to view stratigraphic layers, measure the chemistry, determine concentrations of isotopes, and to ascertain time markers in the ice. This leads to a more accurate chronology of the ice core, which ultimately increases the confidence that can be placed in reconstructed evidence of environmental change as obtained from the ice core.

2.2

Sampling

Once it is determined that the mid- or low-latitude glacier meets the requisite criteria, then an expedition to obtain deeper cores from the glacier usually follows. Due to the remoteness and high altitudes of most ice-coring sites, the equipment used to penetrate or drill down into the surface of the glacier must be specialized, rugged, and portable. For shallow cores, on the order of 20 to 30 m, lightweight hand-operated augers can be used relatively easily. However, deeper cores ranging from 30 m to thousands of meters require other types of drills. Choosing the correct method of drilling at a

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mid- or low-latitude glacial site can literally mean the difference between success and failure. For warmer mid- or low-latitude glaciers, with ice that is at or just below the freezing point, thermal drills can be employed that melt down through the glacial ice. Such thermal drills can be operated by generator or by solar power and are fairly compact in size. One example of a warm glacier is the Upper Fremont Glacier, Wyoming, U.S.A, where a thermal drill was used in 1991 and again in 1998 to retrieve surface-to-bedrock ice cores (see Naftz et al., this volume). If the glacier is colder, with ice that is well below freezing, then mechanical drills must be used to drill down into the glacier, even though such drills are often large and difficult to transport to the selected site. Like thermal drills, mechanical drills can be operated by solar or generator power. Many mid- or low-latitude glaciers greater than 5000 meters above sea level are cold glaciers simply because of their elevation.

2.3

Transport and Storage

Once the processes of drilling and collection of ice from a glacier are finished, the ice must be transported away from the site. Transportation of ice from extremely remote sites (such as glaciers located in Kyrghyzstan, Nepal, Peru, or Tibet) can only be successful if the logistics are worked out ahead of time: glacial ice provides little information if it melts while it is being transported. To this end, insulated boxes, dry ice, refrigerated trucks, and refrigerated airplanes are arranged before the expedition commences so that the ice can be quickly and efficiently relocated to a storage facility. Several universities in the United States have storage space for ice cores, including the University of Maine, the University of New Hampshire, and The Ohio State University; the location of storage depends on who sponsors the glacial expedition. In addition to ice-core storage at universities, one of the foremost ice-core laboratories where ice cores from glaciers and ice sheets around the world are stored, is the U.S. National Ice Core Laboratory (NICL), located in Lakewood, Colorado, U.S.A. All ice cores are stored at a constant temperature of –30 °C to preserve the physical and chemical information stored in the ice.

2.4

Processing

Before analyses of glacial ice can begin, the ice must be processed. This refers to the steps taken to prepare the ice-core sections for the various types of analyses. Glacial ice is always processed in a cold environment to maintain the frozen state of the ice. For example, NICL maintains a walk-in

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Methods of Mid- and Low-Latitude Glacial Record Collection

processing room at –24 °C. Plastic, powderless gloves are worn at all times when handling or processing ice. Horizontal and vertical stainless steel bandsaws are used to cut 1-m length ice cores into sections as small as 4 cm. These individual sections of ice are then packaged, sealed, carefully labeled, and shipped frozen to various destinations for further processing, and/or analyses. Once the ice has arrived at a destination, additional processing steps are typically performed, depending on the type of analyses to be done; such steps are designed to minimize contamination. All processing begins with wearing powderless gloves and a facemask at all times while handling the sections of ice to prevent contamination by touch or breath. Next, in order to remove any accidental contamination on the outside of the ice, the sections are typically scraped with a stainless steel microtome. However, according to a new procedure that has been developed by researchers at the University of New Hampshire that eliminates the need to scrape the ice samples, a section of ice is placed on top of a heated band of metal. This process melts out the uncontaminated center section of the core directly into a container and the outer “rind” of the core is discarded. This new procedure allows the remaining ice to be melted immediately. In contrast, a scraped sample is rinsed with greater than 18 Mohm deionized water, allowed to melt in a closed container for a short period of time, swished around in its own meltwater to remove any potential contamination still present (meltwater is poured off), and finally the remaining ice is covered again and allowed to fully melt. Chemical processing of the melted glacial samples is often a required additional step, depending on the type of analysis to be performed. When performing accelerator mass spectrometry (AMS) measurements on glacial samples, for example, there is often only a very small amount of the isotope of interest in the sample. In order to have enough mass to create a target for analysis, a chemist must add stable elemental carrier to the sample (Cecil et al., 1999)

3.

METHODS (CHEMICAL AND BIOLOGICAL)

A significant amount of information can be stored in mid- and lowlatitude glaciers. Due to complications at these sites, any single piece of information gleaned from a glacier may not represent an accurate picture of the climate and/or environment of the past. For example, meltwater percolation can dampen isotopic and chemical signals stored in the ice, making it difficult to reconstruct temperature and deposition records. Subsequently, every method of analysis, whether chemical, biological, or

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geophysical, is used in conjunction with all the other methods to correctly identify and apply the various types of information stored in a glacier. The following sections describe many chemical and some biological methods that have been or are being developed to look at and understand the record archived in mid- and low-latitude glaciers.

3.1

Oxygen-18/Oxygen-16 Ratios

The ratios of naturally occurring oxygen isotopes in precipitation samples can provide significant information relative to temperature, altitude, storm track, distance from source water, and evaporation. The ratio of the two isotopes used, oxygen-18 to oxygen-16, is determined by using some type of mass spectrometer, usually either gas or solid source, to analyze the desired sample. The resultant ratio is divided by a standard oxygen ratio maintained by the International Atomic Energy Agency IIAEA), multiplied by 1,000, and abbreviated as į18O. The į18O value can be positive or negative with respect to the standard and is reported in units of permil (or per thousand). See Sidebar 1, page 23. The values of į18O in glacial ice vary according to season. Ice representative of winter has a more negative į18O value than does ice representative of the summer season. An ice core with a well-preserved į18O signal should show the seasonal į18O oscillations (summer to winter), with more negative į18O values representing cooler air temperatures. As a result of the measurable changes in the į18O values, past changes in air temperatures can be reconstructed for the time period spanned by the icecore record (see Naftz et al., this volume).

3.2

Electrical Conductivity (Acidity)

The ultimate purpose of direct current electrical-conductivity measurements (ECM) performed on glacial ice is to assist in the determination of ice-core chronology. ECM accomplishes this by measuring the acidity in the ice. Once the acidity is known, the seasonal/summer dust layers (for layer counting) can be identified. Additionally, volcanic events in the ice that act as time-markers can be identified (see Schuster et al., this volume). The ECM technique uses a pair of electrodes (greater than 2,000 volts of potential difference) spaced 1 centimeter (cm) apart. The electrodes are moved down the continuous ice core at a constant velocity of 5 centimeters per second (cm/s) and the resultant current through the ice core is measured every millimeter (mm), providing a high-resolution profile of the acidity in the core. The ECM profile easily identifies singular volcanic events as

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Methods of Mid- and Low-Latitude Glacial Record Collection

spikes in the log, allowing the chronology of the mid- or low-latitude ice core to be refined (Schuster et al., 2000 and Schuster et al., this volume).

3.3

Major-Ion Chemistry

Major ions are considered to be chloride (Cl-), nitrate (NO3), sulfate (SO4), sodium (Na), magnesium (Mg), calcium (Ca), ammonium (NH4) and potassium (K). Determination of the concentrations of Cl-, NO3, and SO4 is accomplished by ion-exchange chromatography (IC). In the past, concentrations of Na, Mg, and Ca were determined by inductively-coupled plasma emission spectroscopy. But now, due to recent advances in lowlevel detections, concentrations of these three major ions are typically determined by IC. Concentrations of NH4 and K are also determined by IC (see Schwikowski et al., this volume). Measurements of major ions are conducted in order to see how the chemistry of precipitation at glacial sites changes; the chemistry can change due to specific events that are natural or human-induced. Natural events such as volcanic eruptions and large forest fires sharply increase the concentrations of Cl-, NO3, and SO4 in the atmosphere. These constituents spread around the globe and are deposited and preserved at glacial sites. Anthropogenic effects (biomass burning, acid rain, etc.) can also increase concentrations of these same constituents in the atmosphere. These major ions are then deposited on glaciers where they become time markers and can aid in refining the chronology of ice cores.

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Sidebar 1. Delta Notation for Reporting Stable Isotope Data The absolute measurement of isotopic ratios is a difficult analytical task and, as a result, relative isotopic ratios are measured as a matter of convention (Toran, 1982). For example, the oxygen-18/oxygen-16 (18O/16O) ratio (R in equation 1) of a sample is compared to 18O/16O of a laboratory prepared standard using the following equation: į18O = (Rsample/Rstandard – 1) x 1,000,

(1)

where, Rsample = 18O/16O in the sample, Rstandard = 18O/16O in the standard, and į18O = relative difference in concentration, in units of parts per thousand (permil). Delta 18O (į18O in equation 1) is referred to as delta notation and is the value reported by isotopic laboratories for stable isotope analysis. In equation 1, R is generally used to refer to the ratio of the heavy (or less abundant) to the light isotope (e.g. 18O/16O, 2H/1H). Delta 2H (į2H) is analogous to į18O where the ratio 2H/1H replaces 18O/16O in Rsample and Rstandard. The standard used for determining į18O and į2H in water originally was Standard Mean Ocean Water (SMOW) as defined by Craig (1961). The standard used in the chapters in this book is the Vienna Standard Mean Ocean Water (V-SMOW) that has been prepared by the IAEA. If the į18O or į2H in a water (or melted ice/snow) sample contains more of the heavier isotopes (18O or 2H) than the reference or standard material (the delta value of the standard by convention is zero), they have positive permil values and are referred to as heavier than the reference material, or as being enriched in the heavier isotope. Conversely, if the samples contain more of the lighter isotopes (16O or 1H) than the reference material, they have negative permil values and are referred to as lighter than the reference material, or as being depleted in the heavier isotope. For example, a į18O value of -17.2 can be referred to as lighter than V-SMOW or depleted in 18O relative to V-SMOW. Once the reference material has been specified, it is assumed by convention that all values are reported relative to it unless otherwise indicated. Because V-SMOW reflects the average isotopic composition of the ocean, and because of the nature of isotope fractionation processes, ҏį18O and į2H values of precipitation are always negative. The same terminology for discussions of į18O and į2H relative to V-SMOW can be applied to different samples of precipitation that have different values. For example, if two samples of precipitation have į2H values of -132.8 and -149.4, then the sample with the value of -149.4 can be referred to as lighter than the sample with a value of -132.8. In a similar fashion, the sample with the value of -150.5 is depleted in the heavier isotope relative to the other sample. Armed with this information, researchers can determine source areas for precipitation (snow) that has become glacial ice (see Kreutz et al., this volume for an example). This information in turn can be used to better understand local and regional climatic and geochemical fallout patterns.

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Methods of Mid- and Low-Latitude Glacial Record Collection

The other major ions (Na, K, Mg, NH4, and Ca) arrive at glacial sites in increased concentrations by way of: 1) seasonal dust loading to the surface of the glacier, and 2) volcanic eruptions, large forest fires, and other natural events. Such increased concentrations allow for more accurate counting of annual dust layers as well as assisting in identifying singular time markers in the ice in order to refine the chronology of the ice core. Additionally, these major ions also may indicate paleoclimate change, since colder periods, such as the Little Ice Age (LIA, see Sidebar 2, page 25), are typically windier and dryer, leading toward increased deposition of dust.

3.4

Heavy Metals

Heavy metals such as mercury (Hg), lead (Pb), plutonium (Pu), and uranium (U) naturally exist in the Earth’s environment in small concentrations. With the advent of modern-day industrialization as well as nuclear practices, however, these metals have been released in concentrations larger than the natural background levels. Because these metals can accumulate in the environment through biological uptake processes and can be potential health hazards, it is vital to understand the processes by which they are transported through and deposited in mid- and low-latitude environments as well as to identify quantities of these metals that may have been introduced into the environment. Studies of Hg concentrations in mid- and low-latitude ice were not performed until recently. Schuster et al. (2002) selected samples from the 1991 ice core collected from the Upper Fremont Glacier for Hg analysis. Chemical processing was performed on the samples to assure that all of the Hg species were oxidized. The Hg concentrations were measured by dual amalgamation cold vapor atomic fluorescence spectrometry (USEPA Method 1631, 1999) with a method detection limit of 0.025 ng/L (Schuster et al., 2002). Measurements for Pb, radioactive Pu and U (the other heavy metals potentially archived in mid- and low-latitude glacial ice) have not yet begun.

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Sidebar 2. The Little Ice Age Scientists have postulated that in recent history there was a global climatic change known as the Little Ice Age (LIA). During the period from about 1450 A.D. to 1850 A.D. air temperatures around the world were much cooler than they are today, especially in the Northern Hemisphere. There is substantial evidence that a fundamental change in air temperatures occurred during this period (see Naftz et al., Schuster et al., Thompson, and Whalley in this volume for examples). Inferential evidence abounds in the recent glacial record, in tree rings, and in human records such as documentation of an increased incidence of landslides, avalanches, and floods associated with increased precipitation and cooler air temperatures. Additionally, tax records during this period reflect the human consequences of glacier advances and the effect on surrounding landscapes (Whalley, this volume). There are significant disagreements in the scientific community on when the cooling trend started and why. There is evidence that the cooling trend started at different times in different parts of the world and lasted for centuries. Most of the LIA occurred before the Industrial Revolution in the 1800s and therefore, scientists believe that this climatic change was not due to the burning of fossil fuels but that it had natural causes. Among the postulated causes put forth by scientists, the two most often mentioned with some supporting evidence are a very slight decrease in the sun’s output of energy (about a one-quarter of one percent decrease) reaching the surface of the earth during this time and increased volcanic activity injecting a veil of sun-blocking aerosols into the atmosphere and blocking the sun’s rays (Schuster et al., this volume). The debate continues today over the importance of these two causes in climate change and some scientists add the possibility of a shift in the currents in the world’s oceans as a major driver resulting in the LIA. It seems probable that the earth’s climate was influenced by several factors that lead to this global cooling period.

3.5

Cosmogenic/Anthropogenic Isotopes

A variety of natural and anthropogenic radioisotopes exist in the environment. Radioisotopes that are created in the earth’s upper atmosphere are termed cosmogenic. Upon transfer to the hydrologic or geologic environment, these radioisotopes can potentially be used as tracers of natural and anthropogenic processes. Some cosmogenic radioisotopes that are of interest are beryllium-10 (10Be), carbon-14 (14C), silicon-32 (32Si), sulfur-35 (35S), chlorine-36 (36Cl), and iodine-129 (129I).

26

Methods of Mid- and Low-Latitude Glacial Record Collection

Cosmogenic radioisotopes are naturally created in the upper atmosphere in small quantities by the process of primary or secondary cosmic rays interacting with atmospheric atoms. Cosmic rays include, but are not limited to, high-energy protons, neutrons, muons, alpha particles, gamma rays, and neutrinos. The cosmogenic radioisotope formed as a result of this interaction depends on the energy and type of the cosmic particle and the element in the atmosphere that is interacted with. One example is the formation of the radioisotope 36Cl. The dominant production mechanism of 36Cl is by the thermal neutron capture of chlorine35. Alternatively, argon-39 can capture a thermal neutron as well and then release an alpha particle to form 36Cl. These production mechanisms follow the reactions below. 35

Cl(n,Ȗ)36Cl

39

Ar(n,Į)36Cl

(1)

Radioisotopes of anthropogenic origin can be transferred to the atmosphere as a result of nuclear activities. Some of these events include nuclear-weapons testing in the 1950s and 1960s, nuclear accidents such as Chernobyl, and emissions from nuclear power plants. Radioisotopes produced from these activities include 36Cl, 129I, cesium-137 (137Cs), U, Pu, and tritium (3H). Cesium-137 is a radioisotope that is strictly anthropogenic (fission product) and is not created in the upper atmosphere by the collision of cosmic rays with atmospheric particles. Natural and anthropogenic radioisotopes in the upper atmosphere can circulate for days, months, or years. Chlorine-36, as an example, has a residence time in the upper atmosphere of about 2 years (Synal et al, 1997). Radioisotopes are then transferred, by atmospheric circulation processes, to the lower atmosphere where they have a residence time of a few weeks. Wet and dry deposition then proceed to wash the isotopes out of the atmosphere and into the hydrologic and geologic environment. Deposition of cosmogenic radioisotopes on the surface of the earth is not uniform. The Earth’s magnetic field, atmospheric dynamics, and precipitation rate all have significant effects on where concentrations of radioisotopes will be deposited. For example, 36Cl has been shown to have larger deposition in mid- and low-latitudes than in low-or high-latitude regions of the earth (Bentley et al., 1986). Local and regional atmospheric circulation patterns, elevation above sea level, and distance from oceans also affect deposition of cosmogenic radioisotopes. Once isotopes are deposited in the hydrologic or geologic environment, they can be re-located, re-suspended, or archived. When re-suspension and

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re-location of cosmogenic radioisotopes are minimal in select mid- and lowlatitude glacial environments, such glaciers often represent excellent archives of cosmogenic radioisotope deposition (Cecil et al., this volume) Direct counting methods have allowed concentrations of some of the isotopes (3H, 14C, 35S, 137Cs, and 210Pb) to be determined for half a century or more. But it wasn’t until the advent of AMS in the late 1970s that extremely small concentrations of isotopes with long half-lives could be detected and measured with accuracy. Some of these cosmogenic isotopes that are routinely measured now include 10Be, 14C, 36Cl, and 129I. Many additional isotopes are of interest, such as 32Si, Pu and U isotopes, but the necessary AMS techniques to measure small concentrations of these isotopes in environmental samples have not been refined. For a detailed discussion of a typical AMS process see (Sharma et al. 2000).

3.6

Carbon-14

Carbon-14 is an isotope of carbon (half-life of 5,730 years) that is produced naturally in the upper atmosphere of the earth at a constant rate. Every biological organism, in the process of living, intakes a constant amount of 14C until it dies. At death, the 14C present in the organism begins to decay according to the equation A = A0e-Ȝt

(2)

where A is the current specific activity in the sample due to 14C decay, Ȝ is the decay constant for 14C, A0 is the 14C specific activity of the sample at the time of death, and t is the time since death. Occasionally, organic material can become incorporated into glacial ice. But to apply the method of 14C dating to such organic matter requires the assujption that: 1) the organic material was incorporated into the snow and ice at the actual time of death; and that 2) the initial concentration of 14C in the plant or animal material (A0) is well known and is independent of time, geographic location of the sample, and species of plant or animal. Carbon14 dating of organic matter in glacial ice can provide additional timemarker(s) that aid in refining the chronology of the ice core. A new method of measuring 14C concentrations in samples by AMS has been developed in recent years (Currie et al., 1985). The method, as with many other AMS methods, requires much smaller sample size and significantly less counting time than conventional counting methods. Now, laboratories around the world actively use AMS for 14C measurements. One example of recent and ongoing research into using AMS for 14C analysis is by the Paul Scherrer Institute, located in Zurich, Switzerland, that has

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Methods of Mid- and Low-Latitude Glacial Record Collection

developed an AMS setup for 14C analysis that is small enough to fit onto a laboratory bench.

3.7

Microbiology

The Earth’s atmosphere, replete with microbiological organisms, transports such organisms to all parts of the world, including glacial sites, by way of atmospheric circulation. Microbial cells deposited in polar environments have been shown to survive (respire) in a preserved state for extended time periods and to even colonize microhabitats within layers of snow (Karl, et al., 1999). Few microbiological studies, however, have been conducted on mid- and low-latitude glacial ice. Microbial cell populations in glacial ice may actually preserve a record of atmospheric circulation patterns, land use, and biogeographical conditions near the deposition site. This could occur if established populations within the ice adapted to local ice geochemistry that was reflective of atmospheric conditions during or close to the time of deposition. Adaptations by the dominant microbes could favor aerobic conditions if the ice retained a large capacity for dissolved oxygen. Alternatively, if sufficient organic material accumulates, then respiration could cause a depletion of oxygen, which would then favor anaerobic conditions for microbial cells to flourish in (Pedersen, 1993; Phelps at al, 1994). Additional factors, beyond aerobic and anaerobic respiration, can affect growth and development of the microbiological communities. Two of these factors are the intensity of high-altitude sunlight, which allows for radiationtolerant species to prevail, and the deposition of heavy metals, which may promote the development of metal-resistant populations. Conventional culture methods and molecular techniques based on polymerase chain reaction amplification of nucleic acids are the tools that microbiologists use to analyze ice cores for microbiological communities (Karl et al., 1999).

4.

METHODS (PHYSICAL CHARACTERISTICS)

The physical attributes of a glacier that can be studied usually begin with surface techniques. These techniques often require the collection of data representative of large volumes. Repeated surface measurements describe large-scale temporal variations in glacial structure and behavior as well as provide bulk estimates of various ice properties. The other way to study the physical attributes of a glacier is by borehole applications. In contrast to surface studies, borehole investigations focus on smaller-scale, sub-glacial

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parameters that can provide critical snapshots for the glacier as a whole. By looking at various geophysical measurements, whether surface or subsurface, the factors that influence, change, or control the glacial system can be better understood.

4.1

Radio-Echo Sounding

The technique of using radio-echo sounding to determine ice depth in polar regions has been in use for nearly half a century, but has only recently been used on warmer, mid- and low-latitude ice (0°C) due to the required modifications to the apparatus before accurate measurements could be made. A radio-echo sounding system consists of a transmitter that sends out a radio wave and a receiver that acknowledges the return of the same wave. Measurements of the elapsed time it took the wave to travel from the transmitter to the receiver are performed at various locations on the surface of the glacier by moving the apparatus. The elapsed time is then used to determine ice thickness (Welch, 2000). The technique of radio-echo sounding is also critical in mapping bedrock topography beneath the ice, calculating ice volume, and selecting drill sites.

4.2

Acoustic Televiewer Logging

One way to study the sub-surface area of a glacier is through the use of an acoustic televiewer. An acoustic televiewer is designed such that it can only be used in a borehole that is filled with fluid, thus allowing the acoustic pulses to be transmitted. The actual apparatus is a thin cylindrical instrument that is lowered into a fluid-filled borehole. The instrument is equipped with an acoustic transducer that rotates while in the borehole and emits a certain number of pulses per revolution. Pulses are transmitted through the borehole fluid, reflect off the fluid-formation interface, and return to the tool where acoustic amplitude and transit time are recorded (Zemanek et al., 1970; Morin et al., 2000). The resultant amplitude data are then converted into brightness (gray scale) or color and the resulting image appears as a planar representation of a cylindrical surface. Threedimensional cylindrical projections can be constructed by stacking the polar views. Simply put, this geophysical logging tool/technique generates a magnetically oriented image of the borehole wall.

30

4.3

Methods of Mid- and Low-Latitude Glacial Record Collection

Additional Geophysical Techniques

Other geophysical techniques are often used to characterize glaciers. These include water-level variations and video recordings in boreholes that aid in helping to characterize water movement both within and underneath glaciers (Harper and Humphrey, 1995; Fountain, 1994), borehole inclination logs and gravity measurements that help to better understand englacial deformation processes and mass balances (Hooke et al., 1992), and cross-hole electrical resistivity experiments that attempt to image drainage features (Hubbard et al., 1998).

4.4

Energy-Balance Monitoring Methods

Due to temperate locations, many mid- and low-latitude glaciers lose significant amounts of snow cover each year. Identifying the historical amounts of snow cover that remain on a mid- or low-latitude glacier is necessary to properly interpret high-resolution paleoenvironmental ice-core records. The energy balance of the snow cover (ǻQ) can generally be described by the equation: ǻQ = Snet + H + LȣE + G + M

(3)

where Snet, H, LȣE, G, and M are the net radiative, sensible, latent, conductive, and advective energy fluxes, respectively. If the value for ǻQ is negative, then the snow cover cools, preventing the melting process. Conversely, if the value for ǻQ is positive, the entire snow cover warms until it reaches a temperature of 0.0 °C, after which significant melting can occur (Marks et al., 1999). The number of models that have been developed to simulate the energy and mass balance of the seasonal snow are many and varied. One example is the model ISNOBAL, which can simulate the annual accumulation and melt of the seasonal snow cover to provide an estimate of the snow cover that remains on a glacier at the end of the melt season. In order to study the snow-cover energy and mass balance of a glacial surface, detailed measurement and monitoring of the surface climate must occur. These measurements include determination of solar and thermal radiation, air and snow temperature, relative humidity, and wind speed.

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GLACIAL RECORD INTERPRETATION

Interpretation of the records stored in mid- and low-latitude glaciers is dependant upon merging all of the data and knowledge gained from the various analytical tools. In other words, one analytical technique is not enough to interpret a paleoclimatic or paleoenvironmental record. Instead, multiple analyses provide information that can be used in conjunction to provide a more accurate basis from which to draw conclusions, or from which to interpret the records stored in the ice. Understanding variances from glacier to glacier, identifying atmospheric dynamics on local, regional, and global scales, and identifying global environmental changes are some aspects of interpreting glacial records.

5.1

Inter-Glacial Comparison of Data

A single, isolated glacial record (for example, the Upper Fremont Glacier in southern North America) is not representative of mid- and low-latitude areas worldwide. Using the combined records from mid- and low-latitude glaciers around the world is crucial to piecing together the various aspects of local, regional, and global changes that occur. However, comparing data between glacial sites that are thousands of miles apart is not always simple due to the vast differences in such things as location, prevailing wind patterns, elevation, accumulation, and ablation. These differences, in large part, can be negated if the average background precipitation flux is factored into the equation when calculating fluxes for each glacial site. The flux values can then be compared with other mid- and low-latitude sites with greater confidence.

5.2

Atmospheric Dynamics

The dynamic atmosphere is of great concern to everyone who lives on the earth. Understanding how the past atmospheric signals have changed, or shifted, can provide valuable insight toward interpreting or predicting future changes in the atmosphere and what those changes could mean to people living at mid- and low-latitudes (see Thompson, this volume). While significant studies of atmospheric dynamics of the last 100,000 years have been performed on recovered Greenland ice-cores (Mayeski et al., 1997; Meeker et al., 1997), these records represent at best only proxy records of the area of the Earth where the majority of the population live—mid- and low-latitudes. To the people who live at mid- and low-latitudes, understanding the short-term (less than 1,000 years) atmospheric dynamics

32

Methods of Mid- and Low-Latitude Glacial Record Collection

that control seasonal shifts as well as precipitation events in these areas is of paramount concern. Mid- and low-latitude ice cores, of course, can preserve information about the atmosphere and the associated dynamics at the time of deposition. In particular, such cores can preserve records of natural and anthropogenic atmospheric content, storm trajectories and wind patterns, aerosol and contaminant transport and deposition, and particulate loading. These cores can span the time period from present day to a few thousand years ago, depending on the glacial site. In recent years, atmospheric studies based on mid- and low-latitude glacial-ice research in Central Asia, Europe, and North and South America have yielded a wealth of information that is directly applicable to local, regional, and global scale studies. This volume presents a few examples of this research.

5.3

Global Environmental Change

Mid- and low-latitude glaciers are becoming recognized as valuable tools for reconstructing records of global changes to the environment (Cecil et el., 2000; Cecil and Vogt, 1997; Naftz, 1993; Schuster et al., 2000; Steig, 1999; Thompson et al., 1995, 1998). As an example, the end of the LIA is clearly preserved in the 18O isotopic record in mid- and low-latitude ice cores (Naftz et al., 1996; Thompson et al. 1986). Also preserved in the glacial ice is a continuing warming trend (Naftz et al, this volume) that parallels warming trends in alpine and high-latitude areas worldwide (Hileman, 1999; Haeberli and Beniston, 1998; Mikhalenko, 1997; and Sin’kevich, 1991). Results from analyses of mid- and low-latitude glacial ice often must be interpreted before they can be understood. Interpretation is usually based on measuring as many different aspects of the ice core as possible (physical, chemical, biological, etc.) and then using all of the information together to ascertain what (if any) changes have occurred to the environment and how those changes affect the present and future.

5.4

Glacial Record “Dating”

Any information stored in a glacial ice core is virtually useless unless the core can be dated. “Dating” in the hydrological sense is usually accomplished through the use of environmental tracers that aid in delineating flowpaths, determining water ages, allowing groundwater recharge rates to be determined, and so forth. Glacial systems, while much different from ordinary hydrologic systems, also contain environmental tracers that are preserved over time. These tracers aid in establishing chronologies for the

J.R. Green, et al.

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ice cores. An established chronology transforms otherwise meaningless glacial data into useful information about paleoclimatic and environmental changes. Environmental tracers in glacial ice are typically called event markers. Sometimes events occur, whether natural or anthropogenic, that last for a given amount of time or that are out of the ordinary. Such events that end up being preserved in glacial ice are known as event markers because they represent a specific event that happened during a specific period of time, whether a few days or a few years. Perhaps the easiest event markers to see in glacial ice are the annual stratigraphic layers that accumulate as a result of dust loading to the surface of the glacier. In the case of polar or Greenland ice cores, these layers can be counted with relative ease. Mid- or lowlatitude ice cores, in contrast, may or may not preserve annual stratigraphic layers consistently as a result of meltwater percolation that can dampen, smear, or remove the layers during any given year. Establishing a chronology (or method of dating) for mid- and low-latitude ice cores by this method is therefore ineffective and unreliable. As a necessary, alternative method of dating, other event markers preserved in mid- and low-latitude ice cores must be used to properly reconstruct the chronology of the core. Event markers can include radioactive tracers from nuclear-weapons tests, radiocarbon from insects, natural events such as volcanic eruptions, large forest fires, droughts, El Nino processes, and additional anthropogenic events (see Schuster,et al., this volume).

5.5

Collaborative Records

Information gleaned from mid- and low-latitude ice core research does not stand alone. Many other environmental records exist that support and confirm the records stored in ice. These include instrumental data, corals, tree rings, varves (lakes), ocean sediments, etc. When all of these resources are used in conjunction with ice core results does a more accurate picture of the changing environment present itself.

6.

SUMMARY

Because the majority of the word’s population live at mid- and lowlatitudes, it is vital to understand how the environment is changing, on local, regional, and global scales. Mid- and low-latitude glaciers provide a unique opportunity to look at how the environment has changed in the past, how it is changing today, and to project possible changes in the future. The study

34

Methods of Mid- and Low-Latitude Glacial Record Collection

of mid- and low-latitude glacial ice is not simple, but the process of collecting, analyzing, and interpreting such ice has been, and continues to be, refined. New techniques, lower analytical detection levels, and expanded studies have provided a solid foundation on which to base the wealth of data recorded in mid- and low-latitude glaciers, which in turn facilitates application of the information obtained to societal problems and resource evaluation.

7.

REFERENCES

Bentley, H.W., Phillips, F.W., and Davis, S.N., 1986, Chlorine-36 in the terrestrial environment, in: Handbook of Environmental Isotopes, Volume 2 (P. Fritz and J-C. Fontes, eds.), Elsevier, New York, pp. 422-480. Cecil, L.D., Naftz, D.L., and Green, J.R., 2000, Global ice-core research: understanding and applying environmental records of the past, U.S. Geological Survey Fact Sheet FS-003-00. Cecil, L.D., Green, J.R., Vogt, S., Grape, S.K., Davis, S.N., Cottrell, G.L., and Sharma, P., 1999, Chlorine-36 in water, snow, and mid- and low-latitude glacial ice of North America: Meteoric and weapons-tests production in the vicinity of the Idaho National Engineering and Environmental Laboratory, Idaho. U.S. Geological Survey Water Resources Investigations Report 99-4037, 27p. Cecil, L.D. and S. Vogt, 1997, Identification of bomb-produced 36Cl in mid- and low-latitude glacial ice of North America, Nuclear Instruments and Methods in Physics Research B 123:287-289. Craig, Harmon, 1961, Isotopic variations in meteoric waters. Science, 133:1,702-1,703. Currie, L.A., Klouda, G.A., Elmore, D., and Gove, H.E., 1985, Radiocarbon dating of microgram samples: Accelerator mass spectrometry and electromagnetic isotope separation, Nuclear Instruments and Methods in Physics Research B 12:396-401. Fountain, A.G., 1994, Borehole water-level variations and imperfections for the subglacial hydraulics of South Cascade Glacier, Washington State, U.S.A., Journal of Glaciology 40:293-304. Haeberli, W. and Beniston, M., 1998, Climate change and its impacts on glaciers and permafrost in the Alps, Ambio 27:258-265. Harper, J.T. and Humphrey, N.F., 1995, Borehole video analysis of a temperature glacier’s englacial and subglacial structure: Implications for glacier flow models, Geology 23:901904. Hooke, R.LeB., Pohjola, V.A., Jansson, P., and Kohler, J., 1992, Intra-seasonal changes in deformation profiles revealed by borehole studies, Storglaciären, Sweden, Journal of Glaciology 38:348-358. Hubbard, B., Binley, A., Slater, L., Middleton, R., and Kulessa, B., 1998, Inter-borehole electrical resistivity imaging of englacial drainage, Journal of Glaciology 44:429-434. Karl, D.M., Bird, D.F., Bjorkman, K., Houlihan, T., Shakelford, R., and Tupas, L., 1999, Microorganisms in the accreted ice of Lake Vostok, Antarctica, Science 286:2144-2147. Marks, D., Domingo, J., Susong, D.D., Link, T., and Garen, D., 1999, A spatially distributed energy balance snowmelt model, Hydrological Processes 13, 1935-1959.

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Mayeski, P.A., Meeker, L.D., Twickler, M.S., Whitlow, S., Yang, Q., Lyons, W.B., and Prentice, M., 1997, Major features and forcing of high-latitude northern hemisphere atmospheric circulation using a 110,000-year-long glaciochemical series, Journal of Geophysical Research 102(C12):26,345-26,366. Meeker, L.D., Mayewski, P.A., Twickler, M.S., Whitlow, S.I., and Meese, D., 1997, A 110,000-year history of change in continental biogenic emissions and related atmospheric circulation inferred from the Greenland Ice Sheet Project Ice Core, Journal of Geophysical Research 102(C12):26,489-26,504. Mikhalenko, V.N., 1997, Changes in Eurasian glaciation during the past century: Glacier mass balance and ice-core evidence, Annals of Glaciology 24:283-287. Morin, R.H., Descamps, G.E., and Cecil, L.D., 2000, Acoustic televiewer logging in glacier boreholes. Journal of Glaciology, vol 46, no. 155, 695-699. Naftz, D.L., 1993, Doctoral Thesis, Colorado School of Mines, 204 p. (unpublished). Naftz, D.L., Klusman, R.W., Michel, R.L., Schuster, P.F., Reddy, M.M., Taylor, H.E., Yanosky, T.M., and McConnaughey, E.A., 1996, Little Ice Age evidence from a southcentral North American ice core, U.S.A., Arctic and Alpine Research 28:35-41. Oeschger, H., Schotterer, U., Haeberli, W., and Röthlisberger, H., 1977, First results from alpine core drilling projects, Z. Gletscher. Glazial. 13(H1/2): 193-208. Pedersen, K., 1993, The deep subterranean biosphere, Earth-Science Reviews 34:243-260. Phelps, T.J., Murphy, E.M., Pfiffner, S.M., and White, D.C., 1994, Comparison between geochemical and biological estimates of subsurface microbial activities, Microbial Ecology 28: 335-349. Schuster, P.F., Krabbenhoft, D.P., Naftz, D.L., Cecil, L.D., Olson, M.L., Dewild, J.F., Susong, D.D., Green, J.R., and Abbott, M.L., 2002, Atmospheric mercury deposition during the last 270 years: a glacial ice core record of natural and anthropogenic sources. Environmental Science and Technolog, 36, 2303-2310. Schuster, P.F., White, D.E., Naftz, D.L., and Cecil, L.D., 2000, Chronological refinement of an ice core record at Upper Fremont Glacier in south central North America, Journal of Geophysical Research 105:4657-4666. Sharma, P., Bourgeois, M., Elmore, D., Granger, D., Lipschutz, M.E., Ma, X., Miller, T., Mueller, K., Rickey, F., Simms, P., and Vogt, S., 2000, PRIME lab AMS performance, upgrades and research applications, Nuclear Instruments and Methods in Physics Research B, 172:122-123. Sin’kevich, S.A., 1991, Climate warming in the Twentieth century as reflected in Svalbard ice cores: Glaciers-Ocean-Atmosphere: Interactions, International Association of Hydrological Sciences Publication No. 208, 257-267. Steig, D.J., 1999, TITLE, Eos, Trans., Amer. Geo. Union 80:S143. Synal, H-A., Beer, J., Bonani, G., Suter, M., and Woelfli, W., 1991, Atmospheric transport of bomb-produced 36Cl, Nuclear Instruments and Methods in Physics Research B 52:483488. Thompson, L. G., Davis, M. E., Mosely-Thompson, E., Sowers, T. A., Henderson, K. A., Zagorodnov, V. S., Lin, P.-N., Mikhalenko, V. N., Campen, R. K., Bolzan, J. F., Cole-Dai, J., and Francou, B., 1998, A 25,000-year tropical climate history from Bolivian ice cores, Science 282:1858-1864. Thompson, L.G., Xiaoling, W., Mosley-Thomson, E., and Zichu, X., 1988, Climatic records from the Dunde Ice Cap, China, Annals of Glaciology 10:80-84.

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Methods of Mid- and Low-Latitude Glacial Record Collection

Thompson, L.G., Mosley-Thompson, E., Bolzon, J.F., and Koci, B.R., 1986, The little Ice Age as reported in the stratigraphy of the tropical Quelccaya Ice Cap, Science 234:361364. Thompson, L.G., Mosley-Thompson, E., Bolzon, J.F., and Koci, B.R., 1985, A 1500-year record of tropical precipitation in ice cores from the Quelccaya Ice Cap, Peru. Science 229(4714):971-973. Thompson, L.G., Mosley-Thompson, E., and Arnao, B.M., 1984, El Nino-Southern oscillation events recorded in the stratigraphy of the tropical Quelccaya Ice Cap, Peru, Science 226:50-53. Toran, Laura, 1982, Isotopes in ground-water investigations. Ground Water, 20(6):740-745. USEPA Method 1631 Revision B, 1999, Mercury in water by oxidation, purge and trap, and cold vapor atomic fluorescence spectrometry, U.S. Environmental Protection Agency, Office of Water, Office of Science and Technology, Engineering and Analysis Division (4303), Washington, D.C. Welch, B.C., 2000, How does radio-echo sounding work?, University of Wyoming research webpage, (April 29, 2002); http://research.gg.uwyo.edu/iceradar/radworks.html. Zemanek, J., Glenn, E.E., Norton, L.J., and Caldwell, R.L., 1970, Formation evaluation by inspection with the borehole televiewer, Geophysics 35:254-269.

PART II: THE CLIMATE AND ENVIRONMENTAL CHANGE RECORD OVER THE LAST 200 YEARS

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THE INFLUENCE OF POST-DEPOSITIONAL EFFECTS ON ICE CORE STUDIES: EXAMPLES FROM THE ALPS, ANDES, AND ALTAI Ulrich Schotterer, Willibald Stichler, and Patrick Ginot

1.

INTRODUCTION

Glaciers and ice sheets preserve paleo-precipitation in its most direct form. However, the stored sequence of individual precipitation events and their imbedded isotopic and chemical information is influenced after deposition by various processes like wind drift and erosion, melt, sublimation, and diffusion of water vapor. The most important changes occur during the surface snow to firn transition. For instance, smoothing of a stable isotope record essentially stops when the critical density of 55 grams per cubic centimeter (g/cm3) is reached. In solid ice the thinning of the annual layers with increasing depth becomes dominant; however, this process is less important for alpine glaciers than for polar ice sheets because of the limited ice thickness and the resulting shorter time scales involved. Moreover, models are able to account for the influence of diffusion and the thinning of annual layers and to reconstruct the original seasonal variability (Johnsen 1977, 2000). In general, there is a better and more quantitative knowledge about changes after deposition on a glacier surface for stable isotopes than for chemical constituents simply because the input data and driving forces are better known. This is mainly a result of both GNIP, the Global Network for Isotopes in Precipitation (IAEA/WMO, 2001) and the advanced incorporation of water isotopes in atmospheric general circulation models (e.g. Hoffmann et al. 2000). The influence of snow drift and diffusion was first reported from isotope studies on polar ice sheets where dry snow and low accumulation caused inverse altitude effects or irregularities in the stable isotope/temperature relation (e.g. Lorius et al. 1969, Dansgaard et al. 1973). Changes in the įD/į18O relation due to sublimation have been reported recently (Satake and Kawada 1997; Stichler et al. 2001). In contrast to central polar ice sheets, melt can play an important role on mid and low latitude glaciers and ice 39 L. D. Cecil et al. (eds.), Earth Paleoenvironments: Records Preserved in Mid- and Low-Latitude Glaciers, 39-59. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.

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The Influence of Post-Depositional Effects on Ice Core Studies

sheets, many of which are temperate or close to temperate. Evaporation, melt, and rain cause a re-distribution and/or a washout of isotopic and chemical tracers. Regarding the latter, the bulk of the solute in a seasonal snow cover is already removed in the early meltwater fraction (e.g. Davies et al. 1987). For stable isotopes, a fractionation takes place during phase transitions. For instance, meltwater has depleted isotope ratios as compared to the remaining snow cover, which is correspondingly enriched. Coldchamber experiments, theoretical considerations and field studies have demonstrated this effect (e.g. Buason 1972; Hermann et al. 1981; Stichler and Schotterer 2000). To avoid the influence of percolating meltwater through several annual layers, cold glaciers with temperatures well below 0°C throughout the year (in general temperatures at 10m depths are representative) are better suited for ice core studies. For mid and low latitude glaciers, appropriate conditions only exist either at high altitudes and/or high latitudes. For this reason, drilling sites necessarily are often situated in saddle or summit regions. Unfortunately, such sites are frequently exposed to extreme meteorological conditions. Unconsolidated dry winter snow depleted in stable isotopes is easily blown away and accumulates at lower altitudes where accumulation rates increase. Consequently, the snow layers at higher elevations are more enriched in stable isotopes. This may lead to an inverse altitude effect and complicates the isotope thermometry. In the dry regions of the tropical and subtropical Andes, ice core drilling sites are additionally exposed to high solar radiation that favors sublimation. A glacier may undergo substantial loss in both mass and isotopic information under such conditions. In addition, the concentration patterns of the remaining chemical tracers may be changed severely (Stichler et al. 2001; Ginot et al. 2001).

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Figure -1. Examples of post-depositional effects. Wind (left, at Colle Gnifetti, Swiss Alps), sublimation and melt (right, Cerro Tapado, Chilean Andes). Effects of wind and melt are also visible in a firn core from Colle Gnifetti in the middle. See text for an explanation.

Some of these post-depositional effects are illustrated in Figure 1. On the left, strong winds erode the smooth surface after a winter snowfall in the Swiss Alps. During long dry and stormy periods the wind shapes and hardens the surface. The wind formed sastrugi (snow dunes) may be removed as a whole or are covered by the next snowfall. Accumulation rates on the order of 80-100 cm snow (or 30 cm water equivalent [weq]) per year cannot ensure that a loss of 30 cm of snow, as in this example from Colle Gnifetti, may be sufficiently low to keep the signal of seasonal variability. Penitents are the frequent visible sign of advanced loss in snow cover by sublimation. In the dry regions of the subtropical Andes, a distinct enrichment in stable isotopes has been observed on the surface of penitents depending on their exposition to wind and sun (Peña, 1989). The meter-high snow pyramids surrounding a high-altitude camp near the drilling site at Cerro Tapado in Chile (on the right in Figure 1) are remnants of a long dry period following the 1997/98 El Niño. Near the margins of the snow field additional heating of the dark surface leads to melting. Since the meltwater cannot percolate through the frozen ground it floods the penitents and forms small frozen ponds. Due to the additional cooling effect of sublimation, even at noon, firn temperatures remain below 0°C at a depth of a few centimeters. Post-depositional effects may be recognized in the structure of the firn cores. The example from the Alps in the middle of Figure 1 shows clear signs of melt and wind influence. The latter is indicated by a sloping melt

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The Influence of Post-Depositional Effects on Ice Core Studies

horizon (dashed line). Obviously a sastrugi started to melt during a warm period and was covered by a new snowfall. Awareness of such complications, as well as the short periods of time covered by alpine glaciers, and the difficult logistical challenge of successfully recovering ice cores has certainly retarded the development of this research in comparison to that in polar regions. Nevertheless, during the last two decades, an increasing number of results from mid and low latitude glaciers has documented their importance to supplement paleo-climatic and paleo-environmental information obtained from polar ice cores. The reconstruction of atmospheric pollutants in Europe or information on the variability of paleo-precipitation in the Himalayas or the Andes from glaciochemical and isotope ice-core records has demonstrated the increasing value of such studies (e.g. Wagenbach et al. 1988; Schwikowski et al. 1999, Thompson 2000, Schwikowski, this volume). Importantly, regardless of the time scale, the specific record, or the drilling site being considered, a critical assessment of the influence of post-depositional processes on the integrity of the original precipitation sequence is needed to evaluate its continuity and how the isotopic and/or chemical information may have been modified. The following examples from our own experience span a broad spectrum of climatic regimes focused on isotope records from the surface to the firn-ice transition. Where possible, we employ direct isotope-in-precipitation and other meteorological data to quantitatively assess post-depositional effects. A number of ion-chemistry records are also discussed, demonstrating effects on individual concentration profiles. Most of our examples are from the Swiss Alps, supplemented by results from the tropical and subtropical Andes and the Russian and Mongolian Altai.

2.

SWISS ALPS

Most of the glaciers in Switzerland are temperate. Depending on slope and exposure, sufficient cold firn and ice exists only at summit regions above 3800-4000 m (Suter 1995). The first attempts to test and apply nuclear dating techniques for environmental studies started in the 1950s in temperate firn at Jungfraujoch (e.g. Oeschger et al. 1962, Ambach et al. 1969, Schotterer et al. 1977). The first ice core data from the Jungfraujoch Saddle revealed that this site is too wind-exposed for a reliable reconstruction of environmental parameters. Changes in net accumulation of more than a factor of two over a horizontal distance of 200 m demonstrated how crucial the site selection in saddle regions can be. However, the easy access by train, a fully equipped research station, plus the availability of meteorological data and nuclear fallout and pollutant monitoring have greatly facilitated field

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studies related to ice core research. Stable isotopes in precipitation have been sampled on a monthly basis since 1983. Although no official data on the amount of precipitation are published, due to excessive snow drifting, the total monthly net weight of samples collected after a precipitation event may be a fairly good indication of the seasonal precipitation distribution under field conditions. This data set affords a unique possibility to compare stable isotope data from ice cores recovered from neighboring sites and to assess quantitatively the influence of post-depositional effects on δ-values under present-day conditions.

Figure -2. Map section of the Jungfraujoch region with drilling sites and the research station where the precipitation is sampled (P) together with an aerial view of Colle Gnifetti and the uppermost part of Grenzgletscher.

The location of the drilling sites in the Jungfraujoch and Colle Gnifetti region are shown in Figure 2. Fiescherhorn Plateau (3900 m), a flat smoothsloped glacier 6 km distant from Jungfraujoch research station was drilled three times, in 1987, 2000 and 2002. Bedrock under the deepest part of the glacier at 150m depths has been reached in December 2002. Borehole temperatures in the upper part of the firn zone vary between -4°C and - 6°C. Surface melting during warm periods produces ice layers up to 10 cm thick. Due to the high annual net accumulation of 1-2 m weq, negligible percolation of melt seems to occur through underlying annual layers (Schotterer et al. 1997, 2002a). Several ice cores have been obtained from Colle Gnifetti (4500 m, Swiss/Italian border) since 1976, several of which reached bedrock (at 66 m and 124 m in 1982). The saddle, with steep crests dropping down more than 2000 m to the southeast, is also wind-exposed. It lies at the border between the cold infiltration and the infiltrationrecrystallisation zones. The glacier is frozen to bedrock, as indicated by bore

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hole temperature of -14°C. The mean annual net accumulation at the two 1982 drilling sites, approximately 150 m apart, is 32 cm and 22 cm weq respectively (e.g. Oeschger et al. 1977; Alean et al. 1983; Schotterer et al. 1985). The net accumulation increases further downstream Grenzgletscher by a factor of 6-8 at the 1994 drilling site at 4200 m altitude. Bore hole temperature variations in the firn layer from -0.4°C to -3°C indicate the possibility of percolating meltwater at this site. Below the firn/ice transition the temperature decreases to -9°C (Suter 1995)

Figure -3 į18O profiles from drilling sites in the Swiss Alps. The mean į -values of the ice cores are given together with their difference to an extrapolated precipitation value at the respective altitude (expressed by the term į precip/ice core).

The influence of post-depositional effects on the į18O variability in firn and ice cores is summarized in Figure 3. The selected records represent temperate and cold firn and cover the same interval of time. They are accompanied by an extreme example from Plaine Morte, a flat and sheltered glacier at the equilibrium line where the average mean annual net accumulation amounts to 5-10 cm weq of superimposed ice. Tritium peaks from nuclear weapon tests were used to date the records, which have been adjusted to the same vertical axis for better comparison. Two facts are obvious: the “inverse altitude effect” between Colle Gnifetti and Fiescherhorn, Jungfraujoch Saddle and Plaine Morte, and the markedly different degree of smoothing of the į-values. Only at Fiescherhorn is the į18O variability comparable to what is observed from monthly composites of

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precipitation. On Colle Gnifetti a few cycles are preserved, which probably indicate seasonal variations, whereas no trace of seasonal variability is apparent in the other two records (though acknowledging the coarse sampling resolution for Plaine Morte). The offset in mean ice core į18O from δ-values in precipitation (extrapolated for the respective altitude) is substantial. The 3.5‰ in į18O at Colle Gnifetti or Jungfraujoch Saddle, for example, are comparable to the summer/winter difference (3-5‰) in Swiss precipitation. The surprisingly small offset in superimposed ice may be explained by the absence of erosive loss, whereas Jungfraujoch Saddle is smoothed by the combined effect of wind and melt. Conservation of winter snow is responsible for the apparent full seasonal cycling of the į-values on Fiescherhorn Plateau. Probably the fresh snow contains more moisture as compared to Colle Gnifetti, promoting formation of a frozen surface crust that provides shelter against wind scour. A similar mechanism might account for an 8 m increase in altitude of the Mönch summit (Figure 1) between 1986 and 1993 (according to new measurements from the Swiss Topographic Institute), reflecting build-up of winter snow from higher temperatures and warmer precipitation events. In 1998 and 2000 two, shallow cores were drilled on the temperate Jungfraufirn and the cold Fiescherhorn Plateau, respectively. The į-records of the cores display differing reflections of the isotope variability in precipitation collected at the Jungfraujoch research station (Schotterer et al. 2002a). In Figure 4, the precipitation data from 2001 back to 1993 are plotted twice, versus time and versus accumulated weight. The accumulated weight accounts for the monthly precipitation distribution and compares better with the net accumulation given in weq for the ice core from Fiescherhorn Plateau. Arrows indicate the į-peaks in corresponding summer precipitation. As expected, in the temperate Jungfraufirn summer, į-values are sometimes missing, especially in the years 1993 and 1994. This is attributable to enhanced melting of summer layers and to occasional rain from July through September. This rain is part of the monthly composite sample at the research station, but at the glacier site, it percolates through the firn layer and is thus lost from the record. In contrast, all seasonal cycles are preserved in the record from Fiescherhorn Plateau, including much of the short-term į-variability as recorded in the monthly precipitation data. However, the distribution of this pattern with depth (or the amount in accumulated precipitation related to this pattern) is different. For example, going back in time, in the transition from winter 1999 to summer 1998 much more is accumulated (relative to the precipitation record), winter 1998 is not as pronounced as in the precipitation record, and in the transition from summer 1997 to winter 1997 the net accumulation on Fiescherhorn Plateau is again higher.

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The Influence of Post-Depositional Effects on Ice Core Studies

Figure -4. Comparison of of į18O records recovered from temperate firn (left) and cold firn (right) with į18O in composites of monthly precipitation collected at the Jungfraujoch research station.

Similar results are obtained when the variability in net accumulation on Fiescherhorn is compared to the variability in precipitation amount recorded on other high-altitude meteorological stations: The annual deviation from the 1971-1999 mean in net accumulation and precipitation amount are not correlated (Schotterer and Stichler 2002b). Despite the close agreement of the long-term δ-values on Fiescherhorn Plateau and the extrapolated įvalues in precipitation, the difference in seasonal and annual distribution of net accumulation also results in different weighting of the isotopic expression of climate and climate variability (the stable isotope/temperature relation, for example). The influence of post-depositional kinetic isotopic fractionation on the deuterium excess and the related climatic information during evaporation and/or sublimation may be evaluated on a įD-į18O diagram. On the left side in Figure 5, the į-values from the Jungfraufirn core and the monthly precipitation are compared. Neither the slope of the regression lines nor the deuterium excess values (i.e., with slope constrained to 8) differ markedly for the period between 1998 and 1993. Separate consideration of the two

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core sections, which were influenced by differing climatic conditions, yields the same result, i.e., that comparable d-excess values existed in directly sampled precipitation and net accumulation on the glacier during both periods (right side in Figure 5), verifying that the record has not been distorted by sublimation or evaporation effects and may hence be well-suited for stable isotope/climate studies. Closer consideration of the data reveals that caution is still warranted, however, since it is clear that the approximate 3‰ shift in mean δ18O values between the 1993-96 and 1996-99 intervals in the firn core, as opposed to the occurrence of very similar mean values in annual precipitation, reflects loss of summer precipitation rather than a change in temperature.

Figure -5. įD-į18O diagrams for two sections of the Jungfraujoch Firn core separated according to the different influence by evaporation, melt, and rain (left) and the comparison with the respective precipitation data (right).

Additional complications can also occur when percolating meltwater does not remove the seasonal cycle in stable isotopes, yet chemical records seem to be disturbed. In a 10 m-long firn section of an ice core drilled on Grenzgletscher at 4200 m altitude, for example, this post-depositional effect was attributed to the inflow of surface meltwater via a system of crevasses

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The Influence of Post-Depositional Effects on Ice Core Studies

that opened temporarily (Eichler et al. 2001). Figure 6 shows some selected records from this study, revealing that major ion species were leached with varying efficiencies. The known seasonal pattern for ammonium and chlorine, for instance, is more or less well preserved whereas sulfate and sodium (among others) are leached in a very specific elution sequence. The dashed line in the chlorine/sodium record represents the sea salt ratio, indicating preserved stable chlorine content and the loss of sodium.

Figure -6. Section of the Grenzgletscher core (between 1989 and 1985) influenced by meltwater. Although the į18O does not display a major disturbance, the selected records from ammonium, sulfate, and the chlorine/sodium ratio exhibit widely differing degrees of perturbation.

The elution sequence is explained by ion re-arrangement during snow metamorphism. Because no distinct enrichment could be found in the chemical records further downcore, it was assumed that the draining meltwater did not re-freeze but was discharged completely to the glacier groundwater table.

3.

SUBTROPICAL AND TROPICAL ANDES

Glaciers in the Andes are particularly important natural archives of present and past climatic and environmental changes because of the N-S orientation of this topographic barrier and its influence on the atmospheric

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circulation of the Southern Hemisphere. Between the equator and 30°S, the seasonality and the amount of precipitation changes drastically. Large differences in amount east and west of the Andean divide occur as well as a change from tropical summer precipitation to extra-tropical winter precipitation. Several ice cores down to bedrock have been recovered from this area in the last decade. From Huascaran in Peru (Thompson et al. 1995) and from Sajama (Thompson et al. 1998) and Illimani in Bolivia (Ramirez et al. 2003) climatic information has been reported as far back as to the last glacial maximum (about 15,000 years). Work on Chimborazo (Ecuador) and Tapado (Chile) is in progress. Most of this information is based on records of δ18O, dust, and some major ions. Due to the extreme climate conditions at the drilling sites and their assumed changes in the past, it is essential to assess the influence of post-depositional processes today on the recovered ice-core information. For Quelccaya in Peru it was already reported that seasonal changes in evaporation could remarkably amplify seasonal changes in δ18O (Grootes et al., 1989). The French–Swiss drilling sites on Cerrro Tapado, Illimani, and Chimborazo were drilled twice in a one-year interval. The uppermost 10-15 years should allow assessment of possible post-depositional effects over a longer period of time. Additional field experiments during drilling were carried out to study short-term changes in relation to actual meteorological conditions (Stichler et al. 2001; Ginot et al. 2001). The results from Cerro Tapado and Chimborazo document to what extent ice-core information may be altered by melt and sublimation (Schotterer et al. 2003, Ginot et al 2002). The summit glacier of Chimborazo (6250 m), situated on the equator, is cold, (core hole temperature of -4°C at bedrock) but surface melting cannot be excluded. This happened between the two drilling campaigns when a volcanic eruption covered the glacier with a dark ash-layer, sharply altering the albedo. The combined effect of sublimation and melt removed large parts of the annual accumulation. Accumulation on Cerro Tapado (5550 m) at 30°S is also strongly influenced by sublimation. This glacier is situated at the border of a dry axis that divides tropical and extra-tropical precipitation belts. At the summit drilling site the bore hole temperature of -12.5°C indicates that the glacier is frozen to bedrock. Ice cores were recovered during the 1997/98 El Niño period and one year thereafter. From meteorological records at the site it could be concluded that El Niño was followed by an extreme dry period. The snow at the surface was exposed to intense sublimation over several months. In Figure 7, isotopic and chemical results are combined from an experiment that investigated the influence of such sublimation on the accumulated snow. The isotope data plotted on the left are derived from pit samples containing the precipitation left since the El Niño event and from thin slices of hardened

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The Influence of Post-Depositional Effects on Ice Core Studies

surface snow formed by the sublimation/condensation process. The slices were removed twice a day to study changes at the surface with exposure time.

Figure -7. Sublimation experiment on Cerro Tapado. The data presented are from a highresolution snow pit (b), from thin slices of the snow surface removed twice a day (a, d, e, f). They are accompanied in (c) by samples from the first 7 cm below surface.

į18O increased by 3.5‰ within 3 days, while deuterium excess declined by 10‰ (Figure 7a). A įD-ҏį18O plot also confirms this clear sign of isotopic enrichment. In (Figure 7b) all pit samples below 7 cm are plotted and in

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(Figure 7c) the samples from the upper 7 cm together with the slices of surface snow as plotted. The slope of the pit samples matches that of the meteoric water line, while the slope of the surface samples corresponds to an evaporation line. The fact that enriched δ-values do not penetrate substantially deeper than several centimeters is consistent with evaporation pan data and modeling which show that the mass loss due to sublimation during the experiment was in the order of 2 mm weq (or 0.5 cm snow) per day. Continuous removal of the surface snow enriched in isotopes at such high rates during the experiment ensured that į-values in the deeper firn layers remained practically unchanged. However, it is obvious that persistence of excessive sublimation rates (higher than precipitation) could lead to substantial loss of climate information from intervals within an ice core. Today the annual loss by sublimation on Cerro Tapado is of the same order as the annual net accumulation (30 cm weq). Most major ions varied significantly at the surface during the experimental exposure time (Fig. 7, d, e, f). For instance, chloride concentration more than doubled and calcium increased by a factor of 5. A normalized concentration-sublimation factor allows the comparison between the different ionic species. Three groups of ions can be distinguished according to this factor. The highest enrichment is observed for species irreversibly trapped in the snow matrix and originating from wet and dry deposition as well (d), followed by a group where the enrichment in the surface layer is in proportion to the water loss by sublimation (e). Species that are present in a volatile form are released from the snow (f). Chloride has turned out to be the best quantitative chemical indicator for sublimation on Cerro Tapado, leading to efforts to reconstruct the sublimation history (Ginot et al. 2001). Documented volcanic eruptions serve as important dating tools for both polar and alpine ice cores because they may deposit a specific chemical matrix on a glacier. However, if ash layers change the surface albedo and cause surface melting, this matrix and the accumulated chemical species in the underlying firn may be also changed. In Figure 8, some records from Chimborazo before and after the volcanic eruption are plotted together, using a depth scale related to the surface in 2000. The 1999 accumulation with the depleted δ18O- values is nearly completely removed in 2000 by melt and sublimation. Black and gray stars in the upper part of the į18O-plot mark the respective horizons. The loss corresponds to approximately 70 cm weq. Below a depth of 1.2 m weq the į18O profiles are identical for both cores. Even the individual wiggles match perfectly although the two drilling sites are approximately 100 m apart. Neither the weighted mean δ-values differ, nor do the values for deuterium excess. This is a clear indication that below 1.2 m weq no further loss and/or disturbance of the accumulated isotopic

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information occurred. Any melt infiltrated without refreezing. Indeed, part of the drained meltwater evidently reached a depth of 24 m in the saddle between the two Chimborazo summits, leading to suspension of drilling because the cores were soaked with water. For chemical species, the melt process causes a re-distribution within the individual concentration profiles. On the right side of Figure 8, profiles are plotted for ammonium, sulfate, and the chlorine/sodium ratio.

Figure -8. Comparison of the the į18O record and concentration profiles of chemical species from Chimborazo before and after a volcanic eruption. Dark ash layers changed the albedo and caused intensive surface melting.

Ammonium behaves relatively conservatively. This may be explained (according to the wash-out event on Grenzgletscher, Eichler et al. 2001) by the high solubility and the position of this ion in the matrix of the snow grains. Sulfate, and especially cations like sodium or calcium, are situated more at the surface of the snow crystals and may therefore be washed-out more readily. However, since refreezing can be excluded according to the isotope balance on Chimborazo, the double-peak of sulfate (most probably originating from the volcanic eruption) needs further explanation. This is beyond the scope of this volume.

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MONGOLIAN AND SIBERIAN ALTAI

Similar to the Andes, the climate of the Altai region is also characterized by climatic extremes, in this case due to the exceedingly high contrast in seasonal temperature. Mongolia is perhaps the most continental region in the world. Temperatures in January often fall below -40°C, whilst the summer is short and hot with average July temperatures around 20°C. Precipitation is highly variable, most of it occurring during summer, with average values no greater than 15 cm per year. The glaciated area covers approximately 500 km2 and comprises small glaciers and ice caps in the Altai region of western Mongolia. Several shallow cores were drilled at the summit of the Tsast Ula ice cap (4200 m) in June 1991. Bore hole temperatures are stable at -18°C from 5 m downwards. Only a very few, thin ice layers are observed although the summer temperature at the summit sometimes approaches 0°C. This indicates that sublimation exceeds melting under these dry conditions. Meteorological data for the last 30 years are obtained from the station Khovd (1500 m altitude) at a distance of about 100 km. The yearly mean precipitation from 1970-1990 (the time period covered by the ice core) is 12 cm and varies between 6 and 21 cm. Summer precipitation dominates, constituting up to 80 percent of the annual total. The mean annual net accumulation on Tsast Ula is 25 cm weq, which points to an increase with altitude in the amount of precipitation. From the seasonal shift in δ-values and the relation of high to low values in deuterium excess, it is concluded that at least a part of the winter precipitation is normally preserved (Schotterer et al. 1997). Belukha (4500 m), the highest summit of the entire Altai mountain range, is situated in southern Siberia at the border to Kazakhstan. The glacier between the two Belukha summits was drilled at 4060 m altitude to bedrock (140 m) in 2001. Ice lenses up to 30 cm thick indicate that intensive melting can occur during summer; yet the bore hole temperature of -17.2°C suggest that the glacier is frozen to bedrock (Olivier et al. 2003). Prior to this deep drilling, an exploratory study recovered pit samples and a shallow core from Belukha west plateau at 3900 m. Pit samples were also taken near the equilibrium line of the Ak-tru glacier (3150 m) in order to evaluate the influence of post-depositional effects on the isotope records. For the Belukha area the meteorological station at Ak-kem, situated at 2000 m altitude in vicinity of the Belukha north face, reports a mean annual precipitation of 56 cm. The Siberian drilling site lies about 300 km west of the Mongolian site in the same mountain range. The nearly 5 times higher precipitation amount indicates a strong windward-lee effect. In contrast to Tsast Ula, melt has an important post-depositional effect on the glaciers of the Siberian Altai due to

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the higher air humidity. At Ak-kem station, the relative humidity during the summer months is around 70 percent. Summer dominates winter precipitation in both regions and winter temperatures are comparable (Figure 9). It is therefore of interest to examine the influence of post-depositional effects on the isotope records and to what extent the seasonality is preserved. Fortunately, at least one full annual cycle of isotopes in precipitation is available from Ulan Bator in the GNIP database (IAEA/WMO, 2001) for a comparison with the ice core data from Tsast Ula (Figure 9). Despite the restrictions (a distance of nearly 1500 km, different length and period of time), some qualitative conclusions are possible. The same slope in the δD-δ18O diagram as in Ulan Bator and the high deuterium excess exclude a major influence of sublimation on the isotope record at the Tsast Ula drilling site. Despite an altitude difference of nearly 3000 m, the precipitation in winter months in Ulan Bator is more depleted in δ-values. This is an indication that some precipitation may be lost by wind scour on Tsast Ula. However, the mean δ18O values (-15.8‰ and - 10.7‰ for Tsast Ula and Ulan Bator, respectively) translate to a reasonable lapse rate of -0.17‰ in δ18O per 100 m gained in altitude. The isotope record of the Tsast Ula ice core may therefore also represent a reasonable climate record.

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Figure -9. Long-term seasonal temperature and precipitation distribution near the two ice core drilling sites in the Mongolian and the Siberian Altai (left) and a δD-δ18O plot of precipitation and ice-core data (right).

For the Siberian Altai, the situation is more complex. In Figure 10, the isotope records from the exploratory pit studies prior to the deep drilling are plotted. On the left, the δ18O from the two pits are plotted against depth. Although the obtained Ak-tru record near the equilibrium line is smoothed by the intensive melt process, the mean δ18O value is more than 7‰ lower than the respective record from the nearly 800m higher Belukha plateau. At this site the additional wind scour removes a considerable part of the annual precipitation. Although some seasonality seems to be preserved, the average δ18O values at the Belukha saddle are comparable to the Belukha plateau (Olivier et al. 2003). Moreover, the isotope record of the remaining net accumulation in the recovered ice cores might be influenced sometimes by kinetic fractionation during post-depositional alteration. Despite the high deuterium excess, the slopes in the δD-δ18O diagram on the right side in Figure 10 at least indicate such a possibility.

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Figure -10. Isotope records from snow-pit studies in the Siberian Altai. Accumulation is influenced by wind scour as indicated by an inverse altitude effect with more enriched δvalues at the higher glacier site (left) and melt processes that additionally may cause isotopic enrichment (right).

5.

CONCLUSIONS

Post-depositional processes remove, redistribute, and change the isotopic and chemical information about climate and environment that arrives with the snow flakes falling on a glacier's surface. In nature, it is difficult to evaluate the influence of a single process because they interact with varying intensities. Sublimation hardens a fresh snow cover and may counteract wind scour. Refreezing of melt has the same effect, but sublimation and melting can also change the accumulated information. From the examples considered, it must be concluded that wind scour is probably the most important process at high-altitude drilling sites because it may disturb or even effectively remove seasonal cycles. Careful site selection and exploratory process studies prior to deep drilling may prevent unfortunate surprises. Saddle sites with channeling of winds and steep crests are obviously less well-suited than more open summit sites. If a cold glacier site is excessively wind-exposed, a more sheltered temperate glacier nearby might offer a superior record, at least for acquisition of a stable-isotope record. Chemical species are affected by post-depositional processes in a much more complex manner than stable isotopes. The latter are fundamental

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constituents of water molecules and interpretation of isotope records benefits from quantitative, physically based understanding of their behavior during phase transitions. In spite of this knowledge, no fool-proof strategy exists to anticipate or avoid such complications. Glaciers are dynamic and open systems for precipitation. Wind, sun, cloudiness, precipitation rate, temperature distribution and a myriad of other factors change as climate changes, and so does the information on climate and environment left from the precipitation that accumulates on a glacier.

6.

REFERENCES

Alean, J., Haeberli, W., and Schädler, B., 1983, Snow accumulation, firn temperature and solar radiation in the area of the Colle Gnifetti core drilling site (Monte Rosa, Swiss Alps): distribution pattern and interrelationships. Zeitschrift für Gletscherkunde und Glazialgeologie 19 (2) 131-147. Ambach, W., Eisner, H., and Sauzay. G., 1969, Tritium profiles in two firn cores from alpine glaciers and tritium content in precipitation in the alpine area. Arch. Met. Geophys. Bioklim. Serie B, 17, 93-104. Buason, Th., 1972, Evaluations of isotope fractionation between ice and water in a melting snow column with continuous rain and percolation. J. of Glaciology Vol 11, 387-400. Dansgaard, W., Johnsen, S., Clausen, H.B., and Gunderstrup., H.G., 1973, Stable isotope glaciology, Medd. Groenland 197, 1-53. Davies, T.D., Brimblecombe P., Tranter, M., Tsiouris, S.. Vincent C.E., Abrahams P., and Blackwood I.L., 1987, The Removal of Soluble Ions from Melting Snowpacks, Proceedings of the NATO Advanced Study Institute on Seasonal Snowcover Physics, Chemistry, Hydrology, Les Arcs, France, July 13-25,1986. Edited by H.G. Jones and W.J. Orville-Thomas, pp 337-392. Eichler, A., Schwikowski, M., Gäggeler, H.W., Furrer, V., Synal, H.A., Beer, J., Saurer, M., and Funk, M., 2001, Glaciochemical dating of an ice core from upper Grenzgletscher (4200m a.s.l.), J. of Glaciology Vol. 46 No. 154 307-315. Eichler, A., Schwikowski, M., and Gäggeler, H.W., 2001, Meltwater-induced relocation of chemical species in Alpine firn, Tellus 55B, 192 203. Ginot, P., Kull, Ch., Schwikowski, M., Schotterer U., Pouyaud B., and Gäggeler, H.W., 2001, Effects of post-depositional processes on snow composition of a subtropical glacier (Cerro Tapado, Chilean Andes), J. Geophys. Res. Vol.108, 32375-32386. Ginot, P., Schwikowski, M., Schotterer, U., Stichler, W., Gäggeler, H. W., Francou, B., Gallaire, R., and Pouyaud, B., 2002, Climate variability reconstruction from Andean glaciochemical records, Annals of Glaciology 35. 443-450. Grootes, P..M., M. Stuiver, M., Thompson, L.G, and Mosley-Thompson, E., 1989, Oxygen Isotope Changes in Tropical Ice, Quelccaya, Peru. J. Geophys. Res. Vol. 94, 1187-1194. Hermann, A., Lehrer, M., and Stichler, W., 1981, Isotope Input into Runoff Systems from Melting Snow Covers, Nordic Hydrology, 12: 308-318. Hoffmann, G., Jouzel, J., Masson, V., 2000, Stable water isotopes in atmosphric general circulation models, Hydrol. Proc. 14, 1385-1406. IAEA/WMO, 2001, Global Network of Isotopes in Precipitation, the GNIP Database accessible at: http://isohis.iaea.org.

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Johnsen, S.J., 1977, Stable Isotope Homogenisation of Polar Firn and Snow, in: Isotopes and Impurities in Snow and Ice, IAHS Publ. 118, 210-219. Johnsen, S. J., Clausen, H. B., Cuffey, K. M., Hoffmann, G., Schwander J., and Creyts T., 2000, Diffusion of stable isotopes in polar firn and ice; the isotope effect in firn diffusion, in: Physcics of Ice Core Records (T. Hondoh, ed.), Hokkaido University Press, Sapporo, 121-140. Lorius, C., Merlivat, L., Hagemann, R., 1969, Variation in the mean deuterium content of precipitation in Antarctica, J. Geophys. Res. 74, 7027-7031. Oeschger, H., Renaud, A., and Schumacher, E., 1962 Essai de datation par le Tritium des couches de néve du Jungfraufirn et détermination de l’accumulation annuelle, Bull. Soc. Vaud. Sc. Vol. 68, No 306 (Lausanne, Suisse), 49-56. Oeschger, H., Schotterer, U., Stauffer, B., Haeberli, W., and Röthlisberger, H., 1977, First results from Alpine core drilling projects, Zeitschrift für Gletscherkunde und Glazialgeologie, 13 (1/2), 193-208. 1977. Olivier, S., Schwikowski, M., Brutsch, S., Eyrikh, S., Gaggeler, H.W., Luthi, M., Papina, T., Saurer, M., Schotterer, U., Tobler, L., and Vogel, E., 2003, Glaciochemical investigations of an ice core from Belukha glcier, Siberian Altai, Geophys. Res. Lett. (in press). Peña, H., 1989, Mediciones de 18O y 2H en „penitentes“ de nieve, in Proceedings of a meeting on Estudios de Hidrologia Isotopica en America Latina, Mexico City, 1987, IAEATECDOC-502, Vienna, 143-154. Ramirez E., Hoffmann, G., Taupin, J.D., Francou, B., Ribstein, P., Cuillon, N., Landais, A., Petit, J.R., Pouyaud, B., Schotterer, U., and Stievenard, M., 2003, A new Andean deep ice core from the Illimani (6350m), Bolivia, EPSL, 212, 337-350. Satake, H., and Kawada, K., 1997, The quantitative evaluation of sublimation and the estimation of original hydrogen and oxygen isotope ratios of a firn core at East Queen Maud Land, Antarctica, Bulletin of Glacier Research 15 93-97. Stichler, W., Schotterer U., Fröhlich K., Ginot P., Kull C., Gäggeler H. W., and Pouyaud, B., 2001, The influence of sublimation on stable isotope records recovered from high altitude glaciers in the tropical Andes, J. Geophys. Res., Vol. 106, 22613-22620. Stichler, W. and Schotterer, U., 2000, From accumulation to discharge: modification of stable isotopes during glacial and postglacial processes, Hydrol. Process. 14, 1423-1438. Schotterer, U., Finkel, R., Oeschger, H., Siegenthaler, U., Bart, G., Gäggeler, H., and Von Gunten, H.R., 1977, Isotope measurements on firn and ice cores from Alpine glaciers, in: Isotopes and Impurities in Snow and Ice, IAHS Publ. No. 118, 232-236. Schotterer, U., Oeschger, H., Wagenbach, D., and Münnich, K.O., 1985, Information on paleo-precipitation on a high-altitude glacier, Monte Rosa, Switzerland, Zeitschrift für Gletscherkunde und Glazialgeologie 21, 379-388. Schotterer, U., Fröhlich, K., Gäggeler, H.W., Sandjordj, S., and Stichler, W., 1997, Isotope records from Mongolian and Alpine ice cores as climate indicators, Climatic Change Vol. 36, 3-4, 519-530. Schotterer, U., Stichler, W., Graf, W., Bürki, H.U., Gourcy, L, Ginot, P., and Huber, T., 2002a, Stable isotopes in alpine ice cores: do they record climate variability? in: Proceedings of an International Symposium on the Study of Environmental Change using Isotope Techniques, IAEA Vienna, 23-27 April 2001, IAEA, Vienna, 292-300. Schotterer, U., and Stichler, W., 2002b, Extending Isotope in Precipitation Data Beyond Direct Measurements: The Perspective from Glacier Ice-Core Measurements in Switzerland, in: Stable Isotopes (Edwards, T.D., Kull, Ch., Alverson, K., eds.), PAGES NEWS, Vol. 10-2, 6-7.

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Schotterer, U., Grosjean, M., Stichler W., Ginot, P., Kull, C., Bonnaveira, H., Francou, B., Gäggeler, H. W., Gallaire, R., Hoffmann, G., Pouyaud, B., Ramirez, E., Schwikowski, M., and Taupin, J. D., 2003, Glaciers and climate in the Andes between the Equator and 30°S: What is recorded under extreme environmental conditions?, Climatic Change 59, 157-175. Schwikowski, M., Brütsch S., Gäggeler H.W., and Schotterer, U., 1999, A high-resolution air chemistry record from an Alpine ice core: Fiescherhorn glacier, Swiss Alps, J. Geophys. Res. 104 13709-13719. Suter, S., 1995, Die Verbreitung kalter Firn- und Eisregionen im Alpengebiet. Diploma Thesis, ETH Zürich. Thompson, L.G., Mosley-Thompson E., Davis M.E., Lin P-N., Henderson K.A., Cole-Dai J., Bolzan J. F., and Liu, K., 1995, Late Glacial Stage and Holocene Tropical Ice Core Records from Huascaran, Peru. Science, 269, 47-50. Thompson L.G., Davis M.E., Mosley-Thompson E., Sowers T.A., Henderson K.A., Zagoradnov V., Lin P.-N., Mikhalenko V.M., Campen R.K., Bolzan J.F., Cole-Dai J., and Francou, B., 1998, A 25000-year tropical climate history from Bolivian ice cores, Science, 282, 1858-1864. Thompson L. G., 2000, Ice core evidence for climate change in the Tropics: implications for our future, Quarternary Science Reviews 19, 19-35. Wagenbach, D., Münnich, K.O., Schotterer U., and Oeschger, H., 1988, The anthropogenic impact on snow chemistry at Colle Gnifetti, Swiss Alps, Annals of Glaciology, 10, 183187.

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EVENT TO DECADAL-SCALE GLACIOCHEMICAL VARIABILITY ON THE INILCHEK GLACIER, CENTRAL TIEN SHAN

Karl J. Kreutz, Cameron P. Wake, Vladimir B. Aizen, L. DeWayne Cecil, Jaromy R. Green, and Hans-Arno Synal

1.

INTRODUCTION

Glaciochemical records developed from mid- and low-latitude Asian ice cores provide a unique archive of past atmospheric conditions, and can be used for high-resolution reconstructions of climatic and environmental variability (e.g., Mayewski et al., 1984; Kang et al., 2000a; 2000b; 2002a; Qin et al., 2000; Hou et al., 1999; 2002a; Yao et al., 1996; 1997; Thompson et al., 1995; 1997; 2000). To realize the full potential of chemical signals preserved in ice cores, detailed modern proxy calibration studies must be undertaken to understand the effects of local deposition noise and the relationships between meteorological conditions and time-series chemical variability. Previous work in the Tien Shan mountains of Central Asia (Figure 1) has demonstrated the usefulness of stable water isotopes and soluble ions for investigating temperature, moisture flux, atmospheric circulation, and dust loading on different timescales. On a seasonal basis, Yao et al. (1999) demonstrated a good correlation between į18O ratios and site temperature. Alternatively, Aizen et al. (1996) interpreted fresh snow event isotope data in terms of moisture source and transport pathway. For soluble ions, the strong influence of dust derived from surrounding arid regions has been noted in snow, firn core (Wake et al., 1992; Williams et al., 1992; Kattelmann et al., 1995; Kreutz and Sholkovitz, 2000; Kreutz et al., 2001), and aerosol (Sun et al., 1998) studies. Here we present new fresh snow and snowpit results from the Inilchek Glacier, Central Tien Shan (Figure 2) collected during July/August 2000. During the 2000 field season, two deep ice cores were also recovered, and are being used to develop highresolution stable isotope and soluble ion records of the past 200-500 years. Our goal in this chapter is to assess local-scale spatial chemical variability in 61 L. D. Cecil et al. (eds.), Earth Paleoenvironments: Records Preserved in Mid- and Low-Latitude Glaciers, 61-79. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.

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the Inilchek basin and the relation between time series chemical variability and regional meteorological parameters. Such knowledge is critical for the proper interpretation of high-resolution records developed from deep ice cores, particularly at sites such as the Inilchek where relatively high accumulation rates may allow reconstructions on seasonal or sub-seasonal timescales.

Figure -1. Location map for the Inilchek Glacier, Central Tien Shan Mountains, Kyrgyzstan

2.

SAMPLE COLLECTION AND ANALYTICAL METHODS

Snow samples were collected from each fresh snow event that occurred during the 2000-field expedition, and also from four, 4-m snowpits and one 15-m crevasse wall (Figure 2). In addition, two deep cores (Core 1, 167.05

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m, 5100 meters above sea level (masl); and Core 2, 160.48 m, 5120 masl) were drilled using a solar powered ECLIPSE electromechanical auger. Fresh snow samples were collected along an elevational transect, with five samples collected at each of eight stakes immediately following every fresh snow event. Stake elevations range from 5087 m at stake 1 to 5250 m at stake 36. Snowpits were dug to 4 m depth at four stake locations (Figure 2), and sampled continuously in 5 cm intervals. The upper 100 m of Core 1 was processed in a dedicated science trench, using a polycarbonate lathe and saw system to remove the outer portion of the core and cut continuous 4-cm samples. During fresh snow sampling, snowpit sampling, and ice core processing, workers wore clean suits and polyethylene gloves to prevent contamination. All samples were collected into pre-cleaned polyethylene bottles, and returned frozen to the U.S.A. for analysis. Sampling of the crevasse wall was done without clean gear, and thus major ion analyses were not performed on that sample set. In addition to the ice core samples processed from Core 1, core chips from each Core 1 and Core 2 drill run (typically between 0.5 m and 1 m length) were collected into plastic bags and shaken to homogenize the chips. Two, 20-ml plastic vials were filled with chips from each drill run, and 10 vials were filled from selected runs to ensure that samples were representative of the entire run. Because core chips were not collected using clean sampling protocols, soluble ion measurements were not made.

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Figure -2. Location map showing sampling locations within the Inilchek Glacier basin during the 2000 field expedition. All sites were surveyed with a high-recision Trimble GPS unit. Elevation contours are given at the left side of the figure and contour lines run roughly eastwest (not shown). Core 1 is located south of Core 2. The crevasse wall sampled during 2000 is located approximately 200 m south of the southernmost snowpit. Also shown is the location of the 14 m firn core drilled during the 1998 field season (Kreutz and Sholkovitz, 2000; Kreutz et al., 2001)

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Samples were melted and aliquoted for major ion and stable isotope analyses, and refrozen as necessary for transport. Analysis for major ions at the University of New Hampshire was performed via suppressed ion chromatography using Dionex instruments (e.g., Buck et al., 1992). Samples from the July 30 and July 31 fresh snow events were not run due to low sample volume. Prior to major ion analyses, all snowpit samples were filtered through pre-cleaned Millipore 0.22 įm Durapore filters. Filter blanks for all ion species were Mg2+ > Na+ and ∆C > Cl-) argues for an inland desert dust source for northern slope glaciers as opposed to a modern marine source to the south. The sharp division between dust concentrations on the northern and southern slopes of the Himalaya indicate that the main crest of the Himalaya not only defines different climatic zones, but also separates two different air masses with very different dust loads. For the northern slope sites, SO42-, NO3- and NH4+ show spatial variability that is similar to Ca2+, and ¨C. For example, Kangwure glacier shows approximately twice as much Ca2+ , ¨C, SO42-, NO3- and NH4+ compared to the Far East Rongbuk (FER) glacier. The time-series of all major ions from Kangwure and FER also show similar temporal variability, with correlation coefficients among all ions greater than 0.75. This suggests that most of the major ion chemistry for glaciers on the northern slopes has either a common source (i.e., dust generated from the arid and semi arid regions of the Tibetan Plateau) or has separate sources that lie at sufficient distance upwind (i.e., dust combined with anthropogenic emissions from upwind sources such as Europe and Russia) that the major ions from different sources have become well mixed by the time they are deposited on the northern slopes of the eastern Himalaya. In contrast to the situation for dust, the strongest regional source of anthropogenic aerosols (such as SO42-, NO3- and NH4+) lie to the south of the Himalaya. For example, the 1999 Indian Ocean Experiment documented an extensive pollution layer over much of the Indian subcontinent and Indian Ocean (Lelieveld et al., 2001). Analysis of aerosol and precipitation samples over an entire year in the Khumbu Himal (Shrestha et al., 2000a; 2002) show strong seasonal variability in major ion concentrations, with SO42-, NO3- and NH4+ concentrations 5 to 10 times greater in the pre-monsoon

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(spring) season compared to the summer monsoon season. These seasonal variations can be explained by changes in precipitation regimes associated with large scale changes in atmospheric circulation. Precipitation during the summer monsoon plays an important role in cleansing the atmosphere and results in dramatic reduction of aerosol concentrations in the Himalaya. Conversely, a dearth of precipitation during the pre-monsoon season is characterized by a gradual buildup of pollutants in the Himalaya. Snow on the southern slopes of the Himalaya displays similar or lower fluxes of NO3- and SO42-, and similar fluxes of NH4+, compared to snow on the northern slopes. Given the extensive pollution that characterizes the atmosphere over India, fluxes of NO3- and SO42- from the southern slopes of the Himalaya are surprisingly similar to fluxes measured in snow from the South Pole and from pre-1900 A.D. snow from Summit, Greenland (Table 4). Table -4. Comparison of annual fluxes of NO3 and SO42- in Himalayan and polar snow. Species Nangpai Kangchung Chago SouthPole* Summit Gosum Greenland* SO4211 26 31 9 16 NO323 59 48 11 23 100 54 70