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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Serie: Geophysical Monograph Series
- Sprache: Englisch
Published by the American Geophysical Union as part of the Geophysical Monograph Series, Volume 192.
Antarctic Subglacial Aquatic Environments is the first volume on this important and fascinating subject. With its underlying theme of bridging existing knowledge to future research, it is a benchmark in the history of subglacial lake exploration and study, containing up-to-date discussions about the history and background of subglacial aquatic environments and future exploration. The main topics addressed are identification, location, physiography, and hydrology of 387 subglacial lakes; protocols for environmental stewardship and protection of subglacial lake environments; details of three programs aiming to explore Vostok Subglacial Lake, Ellsworth Subglacial Lake, and Whillans Subglacial Lake over the next 3–5 years; assessment of technological requirements for exploration programs based on best practices for environmental stewardship and scientific success; and knowledge of subglacial lakes as habitats for microbial life and as recorders of past climate and ice sheet change. Its uniqueness, breadth, and inclusiveness will appeal to microbiologists and those interested in life in extreme environments, paleoclimatologists and those interested in sedimentary records of past changes, glaciologists striving to understand how water beneath glaciers affects their flow, and those engaged in developing technology to undertake direct measurement and sampling of extreme environments on Earth and in the solar system.
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CONTENTS
PREFACE
Section I: History and Background
Subglacial Aquatic Environments: A Focus of 21st Century Antarctic Science
1. INTRODUCTION
2. EARLY SCIENTIFIC DEVELOPMENTS
3. MOMENTUM BUILDS
4. VOSTOK SUBGLACIAL LAKE
5. LIFE IN SUBGLACIAL AQUATIC ENVIRONMENTS
6. OTHER SUBGLACIAL LAKES
7. NEW FRONTIER
The Identification and Physiographical Setting of Antarctic Subglacial Lakes: An Update Based on Recent Discoveries
1. INTRODUCTION
2. DISCOVERY, IDENTIFICATION, AND CHARACTERIZATION OF SUBGLACIAL LAKES
3. GEOGRAPHICAL DISTRIBUTION, DIMENSIONS, AND RELATION TO TOPOGRAPHIC AND GLACIOLOGICAL SETTING
4. SUBGLACIAL LAKE CLASSIFICATION, BASAL HYDROLOGY, AND ICE FLOW
5. DISCUSSION AND SUMMARY
Antarctic Subglacial Lake Discharges
1. INTRODUCTION
2. OBSERVATIONS OF SUBGLACIAL LAKE DISCHARGE
3. MECHANISMS OF SUBGLACIAL LAKE DISCHARGE
4. EFFECT OF SUBGLACIAL LAKES AND LAKE DISCHARGE ON ICE FLOW
5. STABILITY OF SUBGLACIAL LAKES
6. EVIDENCE OF FORMER SUBGLACIAL OUTBURSTS
7. CONCLUSIONS AND OUTLOOK
Section II: Vostok Subglacial Lake and Recognition of Subglacial Aquatic Environments
Vostok Subglacial Lake: A Review of Geophysical Data Regarding Its Discovery and Topographic Setting
1. INTRODUCTION
2. DISCOVERY OF VOSTOK SUBGLACIAL LAKE, 1960S—1990S
3. RECENT GEOPHYSICAL CAMPAIGNS
4. GEOPHYSICS RESULTS
5. PHYSICAL PROCESSES IN THE LAKE
6. SUMMARY
Microbial Communities in Antarctic Subglacial Aquatic Environments
1. INTRODUCTION
2. MICROBIOLOGY AND BIOGEOCHEMISTRY OF ANTARCTIC SAE
3. UNEXPLORED SAE BENEATH THE ANTARCTIC ICE SHEET
4. SUMMARY
Subglacial Lake Sediments and Sedimentary Processes: Potential Archives of Ice Sheet Evolution, Past Environmental Change, and the Presence of Life
1. INTRODUCTION
2. SUBGLACIAL LAKE SEDIMENTS AS ARCHIVES OF PALEOENVIRONMENTAL CHANGE
3. DIRECT INFORMATION FROM SURFACE EXPOSURES AND SEDIMENT CORES ACQUIRED FROM SUBGLACIAL PALEOLAKES
4. SEDIMENTARY PROCESSES IN SUBGLACIAL LAKES
5. SEDIMENTARY ARCHIVES OF LAKE FORMATION AND HYDROLOGICAL DISCHARGE
6. TECHNOLOGIES FOR EXPLORATION AND ANALYSIS OF SUBGLACIAL LAKE SEDIMENTS
7. CONCLUSIONS
The Geomorphic Signature of Massive Subglacial Floods in Victoria Land, Antarctica
1. INTRODUCTION
2. MELTWATER LANDFORMS
3. CHRONOLOGY
4. INTERPRETATION
5. GLACIOLOGICAL MODELING
6. CONCLUSIONS
Subglacial Environments and the Search for Life Beyond the Earth
1. INTRODUCTION
2. DISTRIBUTION AND BIOLOGICAL POTENTIAL OF SUBGLACIAL HABITATS ON THE EARTH
3. EXTRATERRESTRIAL SUBGLACIAL ENVIRONMENTS
4. TERRESTRIAL ANALOGS: PARALLELS AND LIMITATIONS
5. RISK OF CONTAMINATION
6. PLANS FOR EXPLORATION
7. CONCLUSION
Section III: Future Exploration Missions
Environmental Protection and Stewardship of Subglacial Aquatic Environments
1. INTRODUCTION
2. PROTECTION OF ANTARCTIC SURFACE WATERS
3. NAS COMMITTEE AND REPORT BUILDING PROCESS
4. SCAR ACTION GROUP
5. ONGOING DEVELOPMENTS
6. CONCLUSIONS
Probe Technology for the Direct Measurement and Sampling of Ellsworth Subglacial Lake
1. INTRODUCTION
2. PERFORMANCE REQUIREMENTS FOR ESL PROBE TECHNOLOGIES
3. REVIEW OF TECHNOLOGIES AND TECHNIQUES APPLICABLE TO SUBGLACIAL MEASUREMENT AND SAMPLING
4. THE DEVELOPMENT OF ESL PROBE TECHNOLOGIES
5. OUTLOOK
Vostok Subglacial Lake: Details of Russian Plans/Activities for Drilling and Sampling
1. INTRODUCTION
2. GEOPHYSICAL STUDIES AND LAKE SETTING
3. DRILLING TOWARD THE LAKE AND ENVIRONMENTAL ISSUES
4. BIOLOGICAL FINDINGS
5. RECENT DRILLING PROGRESS
6. PLANS FOR THE FUTURE
Siple Coast Subglacial Aquatic Environments: The Whillans Ice Stream Subglacial Access Research Drilling Project
1. INTRODUCTION
2. BACKGROUND
3. WISSARD SUBPROJECTS
4. REGIONAL SETTING: WHILLANS ICE STREAM SUBGLACIAL LAKE AND GROUNDING ZONE SYSTEM
5. SCIENTIFIC GOALS OF WISSARD
6. IMPLEMENTATION OF WISSARD
7. EDUCATION AND OUTREACH
8. SUMMARY
Ellsworth Subglacial Lake, West Antarctica: A Review of Its History and Recent Field Campaigns
1. INTRODUCTION
2. LAKE ELLSWORTH: DISCOVERY AND REDISCOVERY
3. RECENT GEOPHYSICAL SURVEYS
4. GEOPHYSICAL RESULTS
5. PLANS FOR EXPLORATION
AGU Category Index
Index
Geophysical Monograph Series
Published under the aegis of the AGU Books Board
Kenneth R. Minschwaner, Chair; Gray E. Bebout, Kenneth H. Brink, Jiasong Fang, Ralf R. Haese, Yonggang Liu, W. Berry Lyons, Laurent Montési, Nancy N. Rabalais, Todd C. Rasmussen, A. Surjalal Sharma, David E. Siskind, Rigobert Tibi, and Peter E. van Keken, members.
Library of Congress Cataloging-in-Publication Data
Antarctic subglacial aquatic environments / Martin J. Siegert, Mahlon C. Kennicutt II, Robert A. Bindschadler, editors.
p. cm. — (Geophysical monograph ; 192)
Includes bibliographical references and index.
ISBN 978-0-87590-482-5 (alk. paper)
1. Subglacial lakes—Antarctica—History. 2. Subglacial lakes—Antarctica—Discovery and exploration. 3. Subglacial lakes—Polar regions—History. 4. Subglacial lakes—Polar regions—Discovery and exploration. 5. Aquatic ecology—Antarctica.
I. Siegert, Martin J. II. Kennicutt, Mahlon C. III. Bindschadler, R. A. (Robert A.)
GC461.A596 2011
551.48’2—dc22
2011007605
ISBN: 978-0-87590-482-5
ISSN: 0065-8448
Cover Image: The location (red triangles) of 387 subglacial lakes superimposed on the BEDMAP database depiction of Antarctic sub-ice topography. (top) The ice sheet surface is illustrated, which is used along with basal topography to predict (bottom) hydrological pathways (blue lines). Image credit: Andrew Wright and Martin Siegert.
Copyright 2011 by the American Geophysical Union
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PREFACE
Between 15 and 17 March 2010, 83 scientists from 11 nations gathered in Baltimore, Maryland, United States, for an AGU Chapman Conference to discuss the current status of knowledge about, and future exploration plans for, Antarctic subglacial aquatic environments. This was the fifth in a series of international conferences. In response to recent funding of two new major exploration programs and the continuation of work at Vostok Subglacial Lake, this meeting focused attention on emerging scientific frontier and the attendant environmental stewardship issues. The chapters of this book expand on keynote presentations and are augmented by selected invited authors to produce the first comprehensive summary of research on, and planning for, the exploration of subglacial aquatic environments. The chapters include summaries of the most recent identification, location, and physiography of 387 subglacial lakes; a detailed analysis of the results, from years of study, from Vostok Subglacial Lake; the rationale for subglacial lakes as analogues for extraterrestrial environments; protocols for the protection and stewardship of these unique environments; critiques of the technological issues facing future exploration programs; and, finally, summaries of the three projects that will enter and sample subglacial aquatic environments in the next 3 to 5 years. This book serves as a benchmark in subglacial aquatic environmental research, marking the beginnings of the main phase of exploration for this new frontier in Antarctic science.
Martin J. SiegertUniversity of Edinburgh
Mahlon C. Kennicutt IITexas A&M University
Robert A. BindschadlerNASA Goddard Space Flight Center
Section I
History and Background
Subglacial Aquatic Environments: A Focus of 21st Century Antarctic Science
Mahlon C. Kennicutt II
Department of Oceanography, Texas A&M University, College Station, Texas, USA
Martin J. Siegert
School of GeoSciences, University of Edinburgh, Edinburgh, UK
In 1996, growing evidence suggested a massive lake of liquid water had pooled beneath the East Antarctic Ice Sheet. This feature became known as “Lake Vostok.” Early on, two hypotheses were posed: the lake contained microbial life that had evolved over millions of years in isolation beneath the ice and lake sediments contained records of past climate change obtainable nowhere else in Antarctica. Many subglacial lakes, in a number of locales, have been identified, suggesting that studies at multiple locations will be needed to fully understand the importance of subglacial aquatic environments. As of 2010, more than 300 lakes have been identified; this will increase as surveys improve spatial coverage. Given the likely pristine nature of these environs and the low levels of microbial life expected, exploration must be done in a manner that causes minimal impact or contamination. It has been shown that many of these lakes are part of an active, sub-ice hydrological system that experiences rapid water flow events over time frames of months, weeks, and even days. Microbial life in subglacial environments has been inferred, and is expected, but it has yet to be directly confirmed by in situ sampling. Current understanding of subglacial environments is incomplete and will only be improved when these subglacial environments are entered and sampled, which is projected to occur in the next few years. This book synthesizes current understanding of subglacial environments and the plans for their exploration as a benchmark for future discoveries.
1. INTRODUCTION
The recent study and exploration of subglacial aquatic environments has transformed our understanding of processes operating at the Antarctic ice sheet bed and the role they have played in the evolution of the continental ice mass. In 1996, an article in Nature reported that a giant lake existed beneath ~4 km of ice in East Antarctica [Kapitsa et al., 1996]. The lake was large enough to be visible in satellite altimetry data of the ice sheet surface [Ridley et al., 1993]. The Nature article marked the beginning of modern subglacial aquatic environment research. Even though the original data had been collected in the 1960s and 1970s, these features had gone largely unnoticed by the broader scientific community for more than two decades [e.g., Oswald and Robin, 1973; Robin et al., 1977]. While the existence of liquid bodies of water beneath ice sheets was in itself of interest to glaciologists, attention swiftly turned to whether these environments harbored unusual life forms. Some also conjectured that if sediments were preserved in the lake they would contain otherwise unavailable, valuable records of ice and climate change in the interior of Antarctica. Kapitsa et al. [1996] had identified Vostok Subglacial Lake buried beneath ~4 km of ice. Not only was the lake’s area an order of magnitude greater than any other subglacial lake known at the time, Kapitsa et al. [1996] also discoverd that the lake contained a water column at least 510 m deep, with an estimated volume of 1800 km3.
An international community of scientists became convinced that subglacial lakes represented a new frontier in Antarctic research. Within a decade, this community developed the scientific rationale for the exploration and study of these environments through a series of international meetings. The first was convened in 1994 in expectation of the publication of the Nature article (Scott Polar Research Institute, University of Cambridge, 1994). The dimensions and setting of Vostok Subglacial Lake were discussed, and a preliminary inventory of Subglacial Lakes was presented. By the time of the publication of the Nature article, 77 subglacial lake features had been identified based on analysis of existing radio echo sounding records [Siegert et al., 1996]. Three more workshops were convened in the late 1990s in quick succession: (1) “Lake Vostok Study: Scientific Objectives and Technological Requirements” (St. Petersburg, March 1998); (2) “Lake Vostok: A Curiosity or a Focus for Scientific Research?” (Washington D. C., United States, November 1998 [Bell and Karl, 1998]); and (3) “Subglacial Lake Exploration” (Scientific Committee on Antarctic Research (SCAR), Cambridge, September 1999). The general conclusion of these gatherings was that to adequately explore subglacial environments a major, sustained investment in time, resources, and scientific effort would be needed for at least a decade. In recognition of this emerging frontier, SCAR convened a forum for scientists and technologists to gather, exchange ideas, and plan for the future: the Subglacial Antarctic Lake Environments Group of Specialists (SALEGoS) (2000–2004). In due course, SALEGoS transformed into a major SCAR Scientific Research Program entitled Subglacial Antarctic Lake Environments (SALE) (2004–2010).
2. EARLY SCIENTIFIC DEVELOPMENTS
In the early stages, understanding of subglacial environments was refined by remote sensing studies and theoretical modeling [Ridley et al., 1993; Siegert and Ridley, 1998; Wüest and Carmack, 2000; Mayer and Siegert, 2000]. The interface between the ice sheet and the underlying bed was shown to contain liquid water at many locations. Ongoing speculation about life in these lakes caught the imagination of not only scientists but the public in general. At the time, the only available samples were surrogates of lake water, the so-called “accreted ice” that forms as lake water adfreezes to the underside of the ice sheet [Karl et al., 1999; Jouzel et al., 1999; Priscu et al., 1999; Bell et al., 2002]. This “accreted ice” was unexpectedly encountered and recovered during deep coring of the Vostok ice borehole. The ice was recognized as unique because of its unusually large crystal sizes (feet in length), lack of meteoric gasses, and the purity of the water collected on melting [Jouzel et al., 1999], compared with the meteoric glacier ice above that contained a well-characterized record of climate change [Petit et al., 1999].
As additional geophysical surveys were conducted and integrated with previously collected data [Tabacco et al., 2002; Studinger et al., 2003; Wright and Siegert, this volume], it was established that subglacial lakes were common beneath thick (>2 km) ice sheets. In early inventories [Siegert et al., 1996], the number and distribution of features was limited by the coverage of surveys. However, it was expected that identification of additional features would continue to mount as unexplored areas of Antarctica were surveyed. On the basis of fundamental considerations, subglacial lakes were expected to occur across the Antarctic continent wherever thick accumulations of ice occurred, a hydrological collection basin was accessible, and a source of water was available. Vostok Subglacial Lake dominated early discussions as it was the only lake whose shape and size were known well; it remains the largest known subglacial lake with an area of about 14,000 km2 and water depths reaching >1000 m [Siegert et al., this volume].
As the inventory of lakes expanded, it was apparent that subglacial aquatic features are not randomly distributed across Antarctica. Instead they are located in preferred settings suggesting that a spectrum of lakes exist that might well have differing histories, ages, origins, and possibly living residents [Dowdeswell and Siegert, 1999]. Clusters of lakes were documented in regions that exhibit distinct ice sheet dynamics in settings defined by the underlying basement morphology [Dowdeswell and Siegert, 2002]. In the vicinity of Dome C and Concordia Station, “lake districts” were identified where subglacial features clustered near ice divides and also at the heads of ice streams [Siegert and Ridley, 1998; Siegert and Bamber, 2000]. It was speculated that some lakes were hydrologically connected in a manner analogous to subaerial lake, stream, and wetland systems [Dowdeswell and Siegert, 2002]. The existence of sub-ice hydrological systems transformed ideas about the evolution and functioning of subglacial environments and redefined interests in these settings to include a wide variety of subglacial aquatic environments.
3. MOMENTUM BUILDS
Planning for, and discussions of, subglacial aquatic environment exploration and study gained additional momentum with the formation and approval of the International Polar Year 2007–2008 program “Subglacial Antarctic Lake Environments Unified International Team for Exploration and Discovery (SALE UNITED).” Together SCAR SALE and SALE UNITED served as forums to exchange information among those interested in the study of these environments. In combination, the programs included scientists and technologists from Belgium, Canada, China, France, Germany, Italy, Russia, the United Kingdom, and the United States. Meetings were convened in Austria (2005), France (2006), the United States (2007), Russia (2008), and Belgium (2009) to develop and refine plans for exploration and to share the latest geophysical, microbiological, and modeling information. An international workshop entitled “Subglacial Antarctic Lake Environment in the IPY 2007-2008: Advanced Science and Technology Planning Workshop” was convened in Grenoble, France, in 2006 bringing together 84 participants from 11 countries [Kennicutt and Petit, 2006]. During this period, understanding of subglacial environments took an unexpected turn.
Analyses of changes in ice sheet surface elevations in central East Antarctica, using satellite remote sensing, demonstrated that a lake in the Adventure Subglacial Trench discharged approximately 1.8 km3 of water over a period of 14 months [Wingham et al., 2006]. The water flowed along the axis of a trench and into at least two other lakes about 200 km downstream. The flux of water, ~ 50 m3 s−1, was equivalent to the flow of the River Thames in London. This discovery was particularly interesting as, up until then, the central East Antarctica Ice Sheet was considered to be un-dynamic compared with West Antarctica. If significant flow of water occurred at the center of East Antarctica, flows of subglacial water were thought to be commonplace in Antarctica. Subglacial aquatic features appeared to be linked by a network of hydrological channels that were defined by basal topography and surface ice sheet slope. Siegert et al. [2007] suggested that groups of lakes were likely to be joined in discrete clusters acting as a system. Wright et al. [2008] established that flow channels were sensitive to the ice surface slope, concluding that small changes in surface slope could result in major alterations of basal water flow. Periods of ice sheet changes, such as after the Last Glacial Maximum, or even as a consequence of global warming, might affect the frequency, magnitude, and direction of these flow events. Up topographic slope (uphill) flow could be expected as discharges were predicted to follow the hydrologic potential established by variations in overlying ice thickness interacting with underlying basement elevations.
Further satellite remote sensing analyses illustrated that subglacial discharge and water flow were indeed commonplace in Antarctica [Smith et al., 2009]. It was confirmed that many newly identified lakes and discharge areas were preferentially located at the heads of ice streams [Siegert and Bamber, 2000; Bell et al., 2007]. Smith et al. [2009] further suggested that lakes actively discharge water into ice stream beds in response to varying basal flows. Satellite investigations of the Byrd Glacier established that subglacial lake discharges coincided with variations in flow velocities observed at an outlet glacier that drained East Antarctica [Stearns et al., 2008]. This inferred subglacial dynamics both influenced, and were influenced by, overlying ice sheet dynamics.
As subglacial lakes represent unique habitats, environmental stewardship during their eventual exploration was seen as a critical issue, and, early on (i.e., Cambridge, 1999), guiding principles were developed and adopted by the community. These concerns included the cleanliness of access techniques, contamination by the experiments that might be performed, the introduction of alien chemicals and biota, how to collect unadulterated samples for laboratory analysis (especially microbiological samples), and how best to protect subglacial aquatic environments as sites of scientific and public interest. The U.S. National Academies convened a committee to review aspects of subglacial lake exploration from an environmental protection and conservation perspective [Committee on Principles of Environmental Stewardship for the Exploration and Study of Subglacial Environments, National Research Council, 2007; Doran and Vincent, this volume]. The National Academy findings were introduced at the Antarctic Treaty Consultative Meeting in 2008 in Kiev, Ukraine, and SCAR subsequently provided guidance on these issues as a code of conduct for subglacial lake exploration [Doran and Vincent, this volume]. These deliberations serve as the basis for promulgating standards and procedures for the responsible conduct of subglacial aquatic environment study and exploration.
4. VOSTOK SUBGLACIAL LAKE
Vostok Subglacial Lake has been, and continues to be, a major focus of subglacial lake research [Lukin and Bulat, this volume]. Aconsortium of Russian research institutions led by the Arctic and Antarctic Research Institute of Roshydromet conducted extensive geophysical surveys of the Vostok Subglacial Lake area and its vicinity within the framework of the Polar Marine Geological Research Expedition and the Russian Antarctic Expedition (RAE) [Masolov et al., 2006; Popov et al., 2006, 2007; Popov and Masolov, 2007]. A series of 1:1,000,000 maps of Vostok Subglacial Lake’s extent, ice and water body thicknesses, and bedrock relief were produced as well as maps of the spatial pattern of internal layers in the overlying ice sheet. From this work, the lake’s dimensions were better defined, the inclination of the ice-water interface was confirmed, and it was recognized that Vostok Subglacial Lake lies in a deep trough. Studinger et al. [2003] collected more than 20,000 km of aerogeophysical data producing detailed assessments of the lake and its glaciological setting. The existence of two basins was confirmed by gravity modeling of lake bathymetry, and the southern basin of the lake was determined to be more than 1 km deep [Studinger et al., 2004; Masolov et al., 2001, 2006; Siegert et al., this volume].
Geophysical, geodetic, and glaciological traverses, undertaken by RAE, measured ice flow lines starting at Ridge B and passing through the drilling site at Vostok Station. An Italian/French/Russian partnership also conducted traverses from Talos Dome via Dome C, Vostok Station, Dome B, and Dome A. Thermomechanical ice flow line models were further constrained by this new information [Richter et al., 2008; Salamatin et al., 2008] to yield accurate estimates of the distribution of accreted ice thickness and freezing rates, refined ice depth ages and temperature profiles, and estimated basal melt rates in the northern part of Vostok Subglacial Lake.
Continued deepening of the borehole at Vostok Station extended the ice core isotopic profiles revealing significant spatial and/or temporal variability in physical conditions during accreted ice formation [Ekaykin et al., 2010]. Analysis of accreted ice revealed a distribution of helium isotopes in the lake water that could be explained by hydrothermal activity contributing to the lake water hydrochemistry [Jean-Baptist et al., 2001; Bulat et al., 2004; de Angelis et al., 2004; P. Jean-Baptist, personal communication, 2009]. Although the lake is known to possess small tides [Dietrich et al., 2001], geodetic GPS observations in the southern part of Vostok Subglacial Lake demonstrated that, on a time scale of 5 years, the lake and ice sheet in the vicinity of Vostok Station were in steady state in contrast to other subglacial lakes that were then known to be dynamic [Richter et al., 2008].
5. LIFE IN SUBGLACIAL AQUATIC ENVIRONMENTS
As understanding of the physical conditions in subglacial environments (temperature, pressure, salinity, etc.) was being refined, the existence of life in the lakes remained a focus of great speculation [Skidmore, this volume]. A consensus grew that extremely low nutrient levels were to be expected, suggesting these habitats could be challenging for possible microbial inhabitants. Superoxic conditions caused by clath-rate decomposition and formation, especially at the water-sediment interface on the lake floor, were also speculated, and it was suggested that these conditions would be toxic to organisms other than anaerobes [Siegert et al., 2003].
At this time, the only clues about possible life in Vostok Subglacial Lake came from extrapolations based on the analyses of the accreted ice [Karl et al., 1999; Priscu et al., 1999]. Contamination of accreted ice samples during drilling, recovery, transportation, and analysis called these results into question as these samples were not originally retrieved for microbiological analyses. The effects of partitioning of lake water constituents during ice formation, under subglacial lake conditions, were poorly understood making inferences of lake water chemistry difficult. The outcome was conflicting evidence for life in the lake and ambiguity about the biogeochemistry of lake water. These uncertainties led to differing opinions about whether hydrothermal effluents contributed to Vostok Subglacial Lake waters. A general consensus evolved that these environments would most likely contain life and that organisms more complex than microbes were highly unlikely. The recognition of hydrological connections among these environments meant that water in many subglacial lakes was likely isolated for far fewer years than first speculated, decreasing the possibility of long-term (>1 Ma) isolation. Depending on the method, the turnover times for water in Vostok Subglacial Lake have been calculated to be between 50,000 and 100,000 years but certainly not millions of years [Siegert et al., 2001; Bell et al., 2002].
More recently, additional accretion ice, and samples of snow collected from layers deposited before the beginning of coring at Vostok Station, contributed further to the debate about possible life within the lake [Bulat et al., 2004, 2007b; Alekhina et al., 2007]. These results suggest that extremely low biomass of both atmospheric and lake water origins is present [Bulat et al., 2009]. Similar studies by United States and United Kingdom researchers confirmed the low cell numbers and low microbial diversity in glacial and accreted ice, though a range of cell numbers and greater diversity have been detected by some investigators [Christner et al., 2006]. The few bacterial phylotypes recovered from accreted ice were isolated from ice layers that contain mineral inclusions raising further questions about their origin [Bulat et al., 2009].
Current knowledge of the lake conditions, inferred from the chemistry of accretion ice studies and from modeling, suggests that the Vostok Subglacial Lake may be inhabited by chemoautotrophic psychrophiles that can tolerate high pressures and possibly high oxygen concentrations, though no conclusive evidence of such microorganisms has yet been found because of a lack of direct sampling of lake water [Bulat et al., 2007a]. The presence of a thermophilic, chemoautotrophic bacterium, Hydrogenophilus thermoluteolus (previously identified in other areas influenced by hydrothermal activity remote from Antarctica), has been reported [Bulatet al., 2004; Lavire et al., 2006]. It has been speculated that water in Vostok Subglacial Lake will be an extremely dilute biological solution suggesting that life, if present, may be primarily restricted to lake sediments and the basal water interface [Bulat et al., 2009]. While studies of the Vostok Subglacial Lake accretion ice have improved comprehension of physical, chemical, and biological processes in the lake, considerable debate continues as to the level and type of life expected in these environments. The debate will not be resolved until direct measurement and sampling of these environments has taken place.
6. OTHER SUBGLACIAL LAKES
Amajor set of subglacial lakes was recently identified at the onset of the Recovery Ice Stream a major East Antarctic ice flow unit [Bell et al., 2007]. Three or possibly four large subglacial lakes (smaller than Vostok Subglacial Lake but, nonetheless, far larger than most) are thought to be coincident with the onset of rapid ice flow. The lakes exhibit distinctive ice surface morphologies including extensive, relatively flat featureless regions bounded by upstream troughs and downstream ridges generated by changes in bottom topography. The Recovery subglacial lakes are hypothesized to contain water derived from basal melting routed to the lake from a large upstream catchment area. To study the Recovery lakes region a U.S.-Norway traverse conducted surface geophysical surveys and installed GPS stations. Ice sheet motion was quantified by collecting gravity magnetics, laser, and radar data over the two southernmost Recovery lakes [Block et al., 2009]. Once fully interpreted, these data will clarify the dynamics of the origins of subglacial water in the lakes and the upstream catchment as well as evaluate the geologic setting of these features. All four of the Recovery lakes were crossed by the U.S.-Norwegian traverse in January 2009, and low-frequency radar was used to map the morphology of the subglacial lakes and image the ice sheet bed of the lakes identified by Smith et al. [2009]. In the coming years, as these data sets are processed, the role that subglacial lakes play in controlling the onset of fast ice flow will be better defined.
7. NEW FRONTIER
Significant progress in the study of subglacial aquatic environments is now at hand with the initiation of an important phase with three exploration programs likely to advance understanding of these environments over the next 3 to 5 years. A United Kingdom–led international program has completed a full survey of Ellsworth Subglacial Lake located in West Antarctica, and plans to undertake direct clean measurement sand sampling of the lake in 2012/2013 are in place [Ross et al., this volume]. The United States has launched a major program to survey, enter, instrument, and sample an “actively discharging” subglacial aquatic system beneath Whillans Ice Stream in West Antarctica at around the same time [Fricker et al., this volume]. Russian researchers are developing further strategies for penetration of Vostok Subglacial Lake, and lake entry is expected in the next few field seasons [Lukin and Bulat, this volume].
In the past decade, our understanding of the importance of subglacial aquatic systems as habitats for life, and of their influence on ice sheet dynamics, has been greatly advanced. Subglacial features that contain liquid water are now known to be common beneath the ice sheets of Antarctica. A spectrum of subglacial environments exists as connected subglacial hydrologic systems and water movement beneath ice sheets can and does occur over a range of spatial and temporal scales. The location of subglacial aquatic accumulations and the onset of ice streams have been shown to be linked in some areas, suggesting that ice sheet dynamics can be affected by hydrological systems at the base of the ice sheet.
The exploration and study of subglacial aquatic environments remains at its early stages and if the major advances realized to date are an indication of what is to come, even more fundamental discoveries will be realized in the years ahead. In little more than a decade, findings regarding subglacial aquatic systems have transformed fundamental concepts about Antarctica and its ice sheets. Ice sheet bases are now seen as being highly dynamic at their beds, involving a complex interplay of hydrology, geology, glaciology, tectonics, and ecology now and in the past. Ongoing and planned projects to directly sample these environments will ultimately determine if subglacial waters house unique and specially adapted microbiological assemblages and records of past climate change. The most remarkable advances to be realized from the study of this next frontier in Antarctic science will probably be wholly unexpected as these recently recognized environments are explored.
This volume serves as a benchmark for knowledge about subglacial aquatic environments and as an update on the latest research developments, setting the stage for major new exploration efforts.
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M. C. Kennicutt, Department of Oceanography, Texas A&M University, College Station, TX 77843-3146, USA.
M. J. Siegert, School of GeoSciences, University of Edinburgh, Edinburgh EH9 3JW, UK. ([email protected])
The Identification and Physiographical Setting of Antarctic Subglacial Lakes: An Update Based on Recent Discoveries
Andrew Wright and Martin J. Siegert
Grant Institute, School of GeoSciences, University of Edinburgh, Edinburgh, UK
We investigate the glaciological and topographic setting of known Antarctic subglacial lakes following a previous assessment by Dowdeswell and Siegert (2002) based on the first inventory of 77 lakes. Procedures used to detect subglacial lakes are discussed, including radio echo sounding (RES) (which was first used to demonstrate the presence of subglacial lakes), surface topography, topographical changes, gravity measurements, and seismic investigations. Recent discoveries of subglacial lakes using these techniques are detailed, from which a revised new inventory of subglacial lakes is established, bringing the total number of known subglacial lakes to 387. Using this new inventory, we examine various controls on subglacial lakes, such as overlying ice thickness and position within the ice sheet and formulate frequency distributions for the entire subglacial lake population based on these (variable) controls. We show how the utility of RES in identifying subglacial lakes is spatially affected; lakes away from the ice divide are not easily detected by this technique, probably due to scattering at the ice sheet base. We show that subglacial lakes are widespread in Antarctica, and it is likely that many are connected within well-defined subglacial hydrological systems.
1. INTRODUCTION
A variety of methods have been used in the discovery and characterization of subglacial lakes and the identification of subglacial water movement in Antarctica (Figure 1). The first inventory of subglacial lakes, recording 77 lake locations, used the technique of radio echo sounding [Siegert et al., 1996]. This was later updated to 145 lakes by Siegert et al. [2005]. Several other techniques are available for the detection of subglacial lakes, including surface topography, topographical changes, gravity survey, and seismic investigations. The aim of this paper is to detail all the techniques available for the detection of subglacial lakes, and to pull together recent information regarding lake locations. In doing so, a revised inventory is established, from which an assessment of the dimensions and topographic setting of subglacial lakes is updated [from Dowdeswell and Siegert, 2002].
2. DISCOVERY, IDENTIFICATION, AND CHARACTERIZATION OF SUBGLACIAL LAKES
2.1. Radio Echo Sounding (RES)
2.1.1. Development of the technique.
The technique of RES takes advantage of a window in the radio part of the electromagnetic spectrum within which emitted waves will travel freely through both ice and air. As with all E-M waves, reflections occur at boundaries between materials with different dielectric properties and therefore different speeds of wave propagation. On entering ice, the speed of the radio wave drops by nearly half, from 300 to 168 m μs−1 [Glen and Paren, 1975]. An active transmit/receive radar antenna, mounted either near the surface [e.g., Popov et al., 2003] or on an airborne platform [e.g., Blankenship et al., 2001], can therefore be used to detect reflections originating from both within and at the base of a glacier or ice sheet.
Figure 1. Methods of investigating subglacial lakes: (a) radio echo sounding (RES), a lake reflector is visible in the center; (b) free-air gravitational anomaly (with ice sheet effects removed) detected from the air above Vostok Subglacial Lake [Studinger et al., 2004b]; (c) prominent flat spot in the ice sheet extending north of Vostok Station, 10-m contours from ERS-1 radar altimetry [Siegert and Ridley, 1998b]; and (d) vertical surface elevation changes measured by ICES at used to identify two lakes within the catchment of the Byrd Glacier, East Antarctica. Reprinted by permission from Macmillan Publishers Ltd: Nature Geoscience [Stearns et al., 2008], copyright 2008.
The initial investigations of the base of the Antarctic ice sheet were carried out as part of a joint Scott Polar Research Institute (SPRI), National Science Foundation (NSF), and Technical University of Denmark (TUD) airborne RES campaign. Between 1967 and 1979, this project completed over 400,000 km of line transects, spaced by an average of 50 km, and covering approximately half of the total area of Antarctica [Robin et al., 1977; Drewry, 1983].
The SPRI-NSF-TUD survey was able to penetrate even the thickest ice in Antarctica using a 60-MHz frequency radar. By measuring the time elapsed between transmit and receive, it was shown that an ice thickness greater than 4 km existed over large parts of East Antarctica [Drewry, 1983]. Nearly 40 years later, this is still the greatest aerial coverage of any single aerogeophysical survey in Antarctica. More recent airborne RES surveys have been characterized by smaller spatial coverage but much higher spatial resolution [e.g., Rémy and Tabacco, 2000; Popov et al., 2002; Rippin et al., 2003; Studinger et al., 2003a, 2004a; Holt et al., 2006a; Vaughan et al., 2006]. In several instances, these surveys have targeted areas not covered, or only sparsely covered by the SPRI-NSF-TUD data, with the result being a patchwork of bed information. Despite these efforts, however, a number of large gaps still exist where our knowledge of the ice sheet bed remains poor [e.g., Le Brocq et al., 2008].
Several reviews detail both developments in the techniques of radioglaciology and the resulting enhancements in our understanding of the ice sheets [e.g., Plewes and Hubbard, 2001; Dowdeswell and Evans, 2004; Bingham and Siegert, 2007]. This subject will therefore not be discussed further here.
2.1.2. The discovery of subglacial lakes.
The discovery of the first subglacial lake occurred near the Russian station at Sovetskaya during the 1967/1968 season of SPRI-NSF-TUD radio echo sounding [Robin et al., 1970]. An area of unusually low signal fading and short duration of the returned pulse, indicating a specular reflection, was found to coincide with low attenuation of the transmitted signal and a near-horizontal, flat bed geometry. This was, at first tentatively, best explained as the result of a sub-ice water body [Robin et al., 1970]. During the 1971/1972 season, an extension of the RES survey over the Dome C area identified a further 16 similar locations, indicating that the occurrence of pockets of liquid water, or subglacial lakes, beneath the central regions of the ice sheet might be relatively commonplace [Oswald and Robin, 1973]. Owing to the lack of penetration through water of radio waves at megahertz frequencies, the depths of these newly discovered features could not be determined, only that a sufficient depth (i.e., a few meters or more) must exist to permit the continuous, strong, and flat echo returns observed [Oswald and Robin, 1973].
2.1.3. Characterization of bed reflections.
Radio echo sounding has been used to gather much of what is currently known about the subglacial environment of Antarctica. When the velocity of the radar pulse is known, or can be estimated, the thickness of the ice can be calculated by measuring the time difference between echoes received from the air/surface and ice/bed interfaces. A time series of such echoes recorded as the observer moves over the ice surface can be used to create a pseudo-cross-section of the ice sheet and of the underlying bed. These data combined with measurements of the surface elevation can be used to reconstruct the topography of the underside of the ice sheet [Drewry, 1983; Lythe et al., 2001].
It was soon realized that much more information about the subglacial environment could be obtained from an analysis of the echo returns. In particular, the strength [Neal, 1976] and shape [Berry, 1973, 1975] of the returned pulse is related to the degree of scattering at the interface and therefore to the microtopography of the subglacial surface. Early studies were limited to the use of “incoherent” radar-sounding apparatus. Modern RES equipment can record both amplitude and phase of reflected pulses (“coherent” radar) allowing a moving platform to operate in the Synthetic Aperture (SAR) mode [Gogineni et al., 1998]. Coherent integration both allows the detection of radar reflections where they would otherwise be obscured by scattering from crevasses, etc. and improves the ability to quantify reflection and scattering from a subglacial interface [Peters et al., 2005].
The shape of a smooth ice/bed or ice/water interface is an additional factor which can affect the strength of the returned echo in the same way that a concave or convex mirror acts to focus light [Tabacco et al., 2000]. In tests conducted over a floating ice tongue, isolated geometrical effects have been shown to influence the total received power by ±6–8 dB [Bianchi et al., 2004].
2.1.4. Identifying subglacial lakes by RES.
The strength of the radio echo from the base of the ice sheet has been used by several authors to infer information about basal conditions in various glaciated regions [e.g., Bentley et al., 1998; Gades et al., 2000; Catania et al., 2003; Peters et al., 2005, 2007]. To do this, the proportion of energy reflected at the bed (the basal reflection coefficient) must be distinguished from the many other factors which can affect the strength of the signal received at the antenna. Probably, the most significant of these is the dielectric power loss during transmission through the ice. This depends sensitively on ice temperature and can vary spatially by 15–20 dB km−1 [Peters et al., 2007]. While subglacial water will always produce a bright radar reflection, an “absolute brightness” criteria for lake identification can be misleading. Rather, it is the brightness of a particular feature “relative” to its surroundings, which can be more useful in identifying subglacial lakes [Carter et al., 2007].
Amplitude fading is the fluctuation in radio echo amplitude as the observer moves at a fixed distance from an interface, it is caused by interference from different scattering centers fore and aft of the observers position and can be used to obtain useful information about bed roughness [Oswald, 1975]. Very low fading (or alternatively, a very large “fading distance”) implies a continuous, flat, mirror-like or “specular” reflection. A purely specular reflection can only occur where the interface is smooth on the scale of the radar footprint. A substantial body of water at the bed of an ice sheet will exhibit a smooth ice-water interface that will also satisfy the criteria of hydrostatic equilibrium. This states that due to the different densities of ice and water, and assuming that the water supports the full overburden pressure of the ice, the ice-water interface will have a slope 11 times greater and in the opposite direction to the slope of the ice surface. Calculations of the hydrological potential field can therefore be useful in evaluating subglacial lake candidates from their radar profile [Oswald, 1975; Carter et al., 2007].
The electrical properties of liquid water act to inhibit the transmission of electromagnetic waves. For this reason, RES cannot normally be used to determine the depths of subglacial lakes. An exception to this has been found in shallow regions of some lakes surveyed with the SPRI-NSF-TUD radar, where bottom reflections have been recorded from depths of up to 21 m below the lake’s surface [Gorman and Siegert, 1999]. These observations confirm that these lakes are, in fact, substantial bodies of water, but indicate only their minimum depths.
2.2. Identification of Subglacial Lakes From Ice Surface Topography
Even before subglacial lakes had been firmly identified in RES records, their surface expressions had been noted by pilots traversing the center of the continent. Unusually flat areas of the ice sheet were often referred to as “lakes” and were frequently used as landmarks for navigation before any connectionwasmade to the subglacial environment [Robinson, 1960].
When an ice sheet flows over a localized body of water, the weight of the ice is taken by the incompressible fluid. Provided that there is no outlet channel for the water to escape, this will lead to the establishment of local hydrostatic equilibrium. This has a significant affect on the flow regime of the ice and, for a large enough lake, can result in the morphological expression of an extremely flat and featureless ice surface, similar to that of a floating ice shelf.
Satellite observations with the Seasat radar altimeter identified a prominent flat area, in Terre Adelie, East Antarctica, the position of which was shown to correspond to a subglacial reflector identified in the SPRI-NSF-TUD radar record [Cudlip and McIntyre, 1987]. This technique achieved greater success when several RES lake reflectors in the area to the north of Vostok Station [Robin et al., 1977] were shown to lie beneath a single, continuous flat surface area observed with the ERS-1 satellite [Ridley et al., 1993; Kapitsa et al., 1996]. A finding later confirmed and elaborated on using more sophisticated radar altimetry techniques [Roemer et al., 2007] and laser altimetry [Studinger et al., 2003a].
Subsequent analysis of ERS-1 data identified flat surface features associated with a further 28 subglacial lakes known from RES records in the Dome C and Terre Adelie regions [Siegert and Ridley, 1998a]. Small subglacial lakes (dimensions <4 km) are generally not found to have a corresponding flat surface feature. Furthermore, flat areas of ice, meeting the criteria for identification of a subglacial lake, have also been shown to occur where no lake exists [Siegert and Ridley, 1998a]. Water-saturated sediments can cause a similar reduction in basal stress and, therefore, induce a similar surface expression. For this reason, a surface flat area alone is not normally sufficient evidence for a lake discovery [Siegert and Ridley, 1998a].
In addition, for floating ice, the retarding force of the basal shear stress is reduced to zero. As a result, ice flowing from a solid bed onto a subglacial lake experiences acceleration. The resulting extensional flow has been shown to cause a local thinning of the ice and a lowering of the surface on the upstream side of the lake [Shoemaker, 1990; Gudmundsson, 2003; Pattyn et al., 2004]. Conversely, a thickening of the ice can occur over the downstream lake shore, where the return of basal drag causes compressive flow.
Until recently, it was thought that no other subglacial lakes of a similar scale to Vostok Subglacial Lake (hereinafter referred to as Lake Vostok) existed beneath Antarctica [Siegert,
