Martin J. Siegert

Rapid discharge connects Antarctic subglacial lakes

The existence of many subglacial lakes provides clear evidence for the widespread presence of water beneath the East Antarctic ice sheet, but the hydrology beneath this ice mass is poorly understood. Such knowledge is critical to understanding ice flow, basal water transfer to the ice margin, glacial landform development and subglacial lake habitats. Here we present ice-sheet surface elevation changes in central East Antarctica that we interpret to represent rapid discharge from a subglacial lake. Our observations indicate that during a period of 16 months, 1.8 km3 of water was transferred over 290 km to at least two other subglacial lakes. While viscous deformation of the ice roof above may moderate discharge, the intrinsic instability of such a system suggests that discharge events are a common mode of basal drainage. If large lakes, such as Lake Vostok or Lake Concordia, are pressurizing, it is possible that substantial discharges could reach the coast. Our observations conflict with expectations that subglacial lakes have long residence times and slow circulations, and we suggest that entire subglacial drainage basins may be flushed periodically. The rapid transfer of water between lakes would result in large-scale solute and microbe relocation, and drainage system contamination from in situ exploration is, therefore, a distinct risk.

A revised inventory of Antarctic subglacial lakes

The locations and details of 145 Antarctic subglacial lakes are presented. The inventory is based on a former catalogue of lake-type features, which has been subsequently reanalysed, and on the results from three additional datasets. The first is from Italian radio-echo sounding (RES) of the Dome C region of East Antarctica, from which 14 new lakes are identified. These data also show that, in a number of occasions, multiple lake-type reflectors thought previously to be individual lakes are in fact reflections from the same relatively large lake. This reduces the former total of lake-type reflectors by six, but also adds a significant level of information to these particular lakes. The second dataset is from a Russian survey of the Dome A and Dome F regions of East Antarctica, which provides evidence of 18 new lakes and extends the coverage of the inventory considerably. The third dataset comprises three airborne RES surveys undertaken by the US in East Antarctica over the last five years, from which forty three new lakes have been identified. Reference to information on Lake Vostok, from Italian and US surveys taken in the last few years, is now included.

Physical, chemical and biological processes in Lake Vostok and other Antarctic subglacial lakes

Over 70 lakes have now been identified beneath the Antarctic ice sheet. Although water from none of the lakes has been sampled directly, analysis of lake ice frozen (accreted) to the underside of the ice sheet above Lake Vostok, the largest of these lakes, has allowed inferences to be made on lake water chemistry and has revealed small quantities of microbes. These findings suggest that Lake Vostok is an extreme, yet viable, environment for life. All subglacial lakes are subject to high pressure (∼350 atmospheres), low temperatures (about -3 °C) and permanent darkness. Any microbes present must therefore use chemical sources to power biological processes. Importantly, dissolved oxygen is available at least at the lake surface, from equilibration with air hydrates released from melting basal glacier ice. Microbes found in Lake Vostoks accreted ice are relatively modern, but the probability of ancient lake-floor sediments leads to a possibility of a very old biota at the base of subglacial lakes.

Large-scale sedimentation on the glacier-influenced polar North Atlantic Margins: Long-range side-scan sonar evidence

Long-range side-scan sonar (GLORIA) imagery of over 600,000 km² of the Polar North Atlantic provides a large-scale view of sedimentation patterns on this glacier-influenced continental margin. High-latitude margins are influenced strongly by glacial history and ice dynamics and, linked to this, the rate of sediment supply. Extensive glacial fans (up to 350,000 km³) were built up from stacked series of large debris flows transferring sediment down the continental slope. The fans were linked with high debris inputs from Quaternary glaciers at the mouths of cross-shelf troughs and deep fjords. Where ice was slower-moving, but still extended to the shelf break, large-scale slide deposits are observed. Where ice failed to cross the continental shelf during full glacials, the continental slope was sediment starved and submarine channels and smaller slides developed. A simple model for large-scale sedimentation on the glaciated continental margins of the Polar North Atlantic is presented.

Modelling the Eurasian ice sheet through a full (Weichselian) glacial cycle

Recently acquired glacial geological and oceanographic datasets provide information on the Weichselian glaciations of Scandinavia and the Eurasian Arctic. A numerical ice-sheet model, forced by global sea level and solar insolation changes, was run to reconstruct ice sheets compatible with these data. A ‘maximum’ reconstruction assumes that the modern-type temperature distribution across the Eurasian Arctic is reduced by 10 8C at three stages during the Weichselian, which are related to minimum levels of solar insolation. Conversely, a ‘minimum’ model incorporates a reduction in temperature of only 5 8C in Early and Middle Weichselian time. The ‘maximum’ reconstruction employs the relatively larger sea-level fall suggested by the d 18 O deep-sea record, while the ‘minimum’ run uses the more conservative sea-level estimate from New Guinea coral reef terraces. The maximum model predicts three major glacial advances in the Weichselian. These compare well to geological evidence for ice-sheet growth during the Early, Middle and Late Weichselian. Geological evidence for the Late Weichselian ice sheet is compatible with either reconstruction if ice growth across the Taymyr Peninsula is curtailed. The models show that ice-sheet advance caused by the interaction of sea level and solar insolation changes yields a time-dependent ice volume function similar to that established from the geological record. Periods of seasonally open water within the seas bordering the Eurasian Arctic generally occur prior to glaciation, and may provide a source of precipitation for ice-sheet growth. In contrast, periods of ice-rafted debris deposition and depletion in surface-ocean d 18 O in sea-floor sediments compare well with the model’s determination of ice-sheet decay and melting. q 2001 Elsevier Science B.V. All rights reserved.

Ice-sheet numerical modeling and marine geophysical measurements of glacier-derived sedimentation on the Eurasian Arctic continental margins

Long-range side-scan sonar images of the Barents Sea continental margin have been analyzed in conjunction with results from previous geophysical investigations to determine a qualitative model for sedimentation over the Bear Island and Storfjorden trough mouth fans. These data indicate that gravity-driven debris flows are major processes in the downslope transport of glacial material, delivered to the shelf break when ice sheets advanced across the continental shelf. During late Weichselian time, ∼4000 km 3 of sediments were deposited over the Bear Island fan (280 000 km 2 ) while ∼700 km 3 of sediments were deposited over the Storfjorden fan (40 000 km 2 ). A numerical ice-sheet model, including sediment deformation and transport beneath ice streams, reconstructs the glacial conditions required to transport large volumes of sediment to the late Weichselian Eurasian continental margin. Model results indicate that glaciation of the Eurasian High Arctic occurred after 28 ka, and that ice streams within bathymetric troughs were active by ca. 25 ka. The maximum ice-sheet thickness over the Barents Sea was about 1400 m; there was a secondary dome −1 (0.13 cm yr −1 averaged over the fan); the rate was 6 cm yr −1 (equivalent to 0.6 cm yr −1 over the fan) over the Storfjorden trough mouth. The modeled sediment volume at the continental margin of the Bear Island and Storfjorden troughs agrees well with the volumes of late Weichselian sediment inferred from seismic records from these large prograding submarine fans. Sensitivity experiments show that adjustments to model environmental inputs do not significantly affect the results.

Antarctic subglacial lakes

Abstract Antarctic subglacial lakes were first identified by Robin et al. (1970) after airborne radio-echo sounding (RES) investigations of the ice-sheet interior. Recently, satellite altimetry was used to measure anomalous near-flat regions on the ice-sheet surface that represent a manifestation of the subglacial lake beneath. Using RES and satellite altimetry, the location and extent of Antarctic subglacial lakes can be identified. The largest subglacial lake exists beneath Vostok Station, and is 14,000 km2 in area. The combined area of additional subglacial lakes beneath Dome C is 15,000 km2 and at least 15,000 km2 of lake surface lies beneath the remainder of the ice sheet. The water depth of subglacial lakes can be estimated through seismic investigations (although data exist only for Lake Vostok) and consideration of the bedrock slopes that border subglacial lakes. The depths of many subglacial lakes are of the order of 10s–100s of metres. The total volume of water held beneath the ice sheet is estimated between 4000 and 12,000 km3. To date, there are six known examples of radio-echo reflections from the lake floors (at a depth of no more than 20 m). Since e/m attenuation through water is related to the salinity, these data indicate that subglacial water is very pure and fresh. Some near-flat surface regions that usually occur over lakes have been observed where no lakes exist. Such features are may be caused by water-saturated basal sediments rather than subglacial lakes. Finally, the spatial variation in geothermal heat flux around the central regions of Antarctica can be established estimated by employing a simple thermal model of the ice sheet under an assumption that the basal ice temperature above subglacial lakes is equal to the pressure melting value. Calculations indicate that the geothermal heat flux varies spatially over the Antarctic Plate between 37 and 65 mW m−2.

A dynamic early East Antarctic Ice Sheet suggested by ice-covered fjord landscapes

The first Cenozoic ice sheets initiated in Antarctica from the Gamburtsev Subglacial Mountains and other highlands as a result of rapid global cooling ∼34 million years ago. In the subsequent 20 million years, at a time of declining atmospheric carbon dioxide concentrations and an evolving Antarctic circumpolar current, sedimentary sequence interpretation and numerical modelling suggest that cyclical periods of ice-sheet expansion to the continental margin, followed by retreat to the subglacial highlands, occurred up to thirty times. These fluctuations were paced by orbital changes and were a major influence on global sea levels. Ice-sheet models show that the nature of such oscillations is critically dependent on the pattern and extent of Antarctic topographic lowlands. Here we show that the basal topography of the Aurora Subglacial Basin of East Antarctica, at present overlain by 2–4.5 km of ice, is characterized by a series of well-defined topographic channels within a mountain block landscape. The identification of this fjord landscape, based on new data from ice-penetrating radar, provides an improved understanding of the topography of the Aurora Subglacial Basin and its surroundings, and reveals a complex surface sculpted by a succession of ice-sheet configurations substantially different from today’s. At different stages during its fluctuations, the edge of the East Antarctic Ice Sheet lay pinned along the margins of the Aurora Subglacial Basin, the upland boundaries of which are currently above sea level and the deepest parts of which are more than 1 km below sea level. Although the timing of the channel incision remains uncertain, our results suggest that the fjord landscape was carved by at least two iceflow regimes of different scales and directions, each of which would have over-deepened existing topographic depressions, reversing valley floor slopes.

The Gamburtsev mountains and the origin and early evolution of the Antarctic Ice Sheet

Ice-sheet development in Antarctica was a result of significant and rapid global climate change about 34 million years ago. Ice-sheet and climate modelling suggest reductions in atmospheric carbon dioxide (less than three times the pre-industrial level of 280 parts per million by volume) that, in conjunction with the development of the Antarctic Circumpolar Current, led to cooling and glaciation paced by changes in Earth’s orbit. Based on the present subglacial topography, numerical models point to ice-sheet genesis on mountain massifs of Antarctica, including the Gamburtsev mountains at Dome A, the centre of the present ice sheet. Our lack of knowledge of the present-day topography of the Gamburtsev mountains means, however, that the nature of early glaciation and subsequent development of a continental-sized ice sheet are uncertain. Here we present radar information about the base of the ice at Dome A, revealing classic Alpine topography with pre-existing river valleys overdeepened by valley glaciers formed when the mean summer surface temperature was around 3 °C. This landscape is likely to have developed during the initial phases of Antarctic glaciation. According to Antarctic climate history (estimated from offshore sediment records) the Gamburtsev mountains are probably older than 34 million years and were the main centre for ice-sheet growth. Moreover, the landscape has most probably been preserved beneath the present ice sheet for around 14 million years.

An inventory of Antarctic sub-glacial lakes

An extensive analogue database of 60 MHz radio-echo sounding records of Antarctica (covering 50% of the ice sheet) is held at the Scott Polar Research Institute, University of Cambridge. This database was analysed in order to determine the presence and location of Antarctic sub-glacial lakes. In total, 77 sub- glacial lake-type records were identified, 13 more than detected in previous studies. An inventory of these sub-glacial lakes includes geographical coordinates, minimum length and overlying ice thickness for each lake. Information concerning the location of these lakes indicates that the majority (-70%) are found in the proximity of ice divides at Dome C and Ridge B within East Antarctica. Received 15 November 1995, accepted 10 April 1996