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SCIENCE Is there life in Lake Vostok? Martin J. Siegert, Bristol Glaciological Centre, University of Bristol, on 20 December 2001 The base of the central East Antarctic Ice Sheet is warm, due to the combined effect of geothermal heating and the insulation caused by the overlying ice, despite surface air temperatures commonly less than -60°C. The production and flow of water at the ice-sheet bed leads to its accumulation within topographic hollows and, hence, the formation of subglacial lakes. Around 70 lakes have been identified beneath the Antarctic Ice Sheet, of which Lake Vostok is by far the largest. Lake Vostok is at least 240 km long and 50 km wide, and lies between 3750 m (over the south of the lake) and 4150 m (over the north) beneath the central East Antarctic Ice Sheet. The surface of Lake Vostok slopes downwards from south to north with a gradient of about 0.002, which although small is roughly 11 times greater than the slope at the ice surface, but in the opposite direction. Lake Vostok is at least 1000 m deep in the south, and relatively shallow in the north and extreme southwest. There may be several hundred metres of glacial sediments draped over its floor. Lake Vostok is thought to have existed for as long as the ice sheet has been at a continental scale (~15 million years). This is because the ice thickness and subglacial conditions would not have changed significantly even over glacial-interglacial cycles. Several deep ice cores have been extracted from the ice sheet at Vostok Station (at the southern end of Lake Vostok) since drilling began in the mid-1960s (the first 500 m deep borehole was extracted in 1965). The most recent, and deepest (3623 m), ice core was acquired in 1996, and terminated ~120 m from the base of the ice sheet. The basal 84 m of the ice core, from 3539-3623 m, has a chemistry and crystallography which are distinctly different to the `normal' glacier ice above. Its isotopic composition suggests that it formed by the refreezing of lake water to the underside of the ice sheet. Thus, there is ~210 m of accreted Lake Vostok ice beneath Vostok Station, which has been sampled by the ice coring. In 1996, when the discovery of Lake Vostok made world headlines, scientists did not know when, or even if, the lake would ever be sampled. However, with the disclosure of refrozen lake water at the end of the ice core, aquatic biologists and geochemists were handed their first opportunity to investigate material derived from a subglacial lake. Studies of accreted ice subsampled from two different depths (3590 m and 3603 m) near the base of the Vostok ice core have shown that these samples contain bacteria (Figure 1-for pictures and caption see appendix). These microbes may not, however, be representative of those in the deeper regions of Lake Vostok. To work out why microbes are present in the accreted ice, knowledge of the lake's water circulation is required. Since the upper surface (ice-water interface) of Lake Vostok is inclined, the pressure melting point in the south will be slightly (~0.3°C) less than that in the north. Water will circulate in Lake Vostok because of the differences between the density of meltwater and lakewater. Geothermal heating will warm the bottom water to a temperature higher than that of the upper layers. The water density will decrease with increasing temperature because Lake Vostok is in a high-pressure environment, resulting in an unstable water column. This leads to convective circulation conditions in the lake in which cold meltwater sinks down the water column and water warmed by geothermal heat ascends up the water column. The meltwater appears more intimately linked with the ice sheet than with the underlying lake water mass. Because of this, the accreted ice microbes are essentially glacial melt derived organisms restricted to the meltwater zone and would not be necessarily representative of the microbiota of the major part of Lake Vostok. A more appropriate location to find the lake's "natural" biota may be in the deep-water column or at the water-sediment interface at the lake floor where numerous surfaces and significant chemical gradients are likely to occur. Dissolved oxygen, which acts as a power source for biological processes in the absence of sunlight, will be found in the Lake Vostok water column since gas hydrates are released from the melting glacial ice (Figure 2 - for caption see appendix). Gas hydrates (or clathrates) are crystal lattices formed by water molecules around gas molecules under conditions of low temperatures and high pressures. High pressures result in substantial volumes of gas being compressed and trapped within these lattice structures. Air hydrates are known to be present in the glacial ice above Lake Vostok. This is because gases cannot dissolve in the solid ice, and hence all of the air is subject to the confining pressure of the ice. Some of the gases in the air clathrates that enter the lake can dissolve in water, and hence the air clathrate may completely or partially dissolve, dependant on the concentration of dissolved gases already present in the lake water. What chemical processes should be expected in Lake Vostok to help maintain life? Current experience of extreme environments suggests that a range of redox-related processes utilising inorganic energy sources are likely in Lake Vostok, and these will be rate-limited by the availability of reduced compounds, such as sulphides and organic carbon, and of oxidants. The biogeochemical reactions that The environment of Lake Vostok is characterised by high pressure, low temperature and permanent darkness. Despite this, microbial life is expected in the Lake. As the overlying ice sheet melts it provides the nutrients and dissolved gases required to support the microbes in the absence of light. As the lake is very old, there is a very real possibility that unique microbes will be found in the deeper regions of the lake when it is eventually explored and sampled directly in the next 10-15 years. Martin Siegert Discussion After Martin Siegert's presentation, there were a number of interesting points brought out. As the lake is under great pressure, it was suggested that any attempt to penetrate the overlaying ice by simply drilling would generate the equivalent of a "gusher" as the gas hydrates released their gases. However, this is extremely unlikely due to the small size of the borehole which would release the pressure slowly, and the fact that the temperature of the ice is typically 40C, and so the water would refreeze and seal the hole again. The natural water level would be 300m or so below the present ice surface. In any case, the environmental damage caused by allowing direct drilling will prevent such an action in the foreseeable future. Since there are many smaller subglacial lakes, one of these may be chosen for the initial experimentation and drilling to obtain bottom sediments, which would provide a history of the lake over the past 30 million years, and which can be obtained in no other way.
Fig. 2 The most likely method of entering the lake would be by dropping a probe which melted its own way through the ice, leaving a tether behind it frozen into the ice. The tether would provide power and return any data collected. As it approached the lake surface, it could sterilise itself using hydrogen peroxide, which can be converted into water and oxygen to reduce the pollution. Once in the lake, a separate small robot could be release to investigate. The engineering challenges however should not be underestimated as the lake is under extreme pressure. NASA are interested in using this as a dummy run for an expedition to Europa, one of Jupiter's moons, that is encased in a mantle of ice, probably 10 to 15km thick, and under which there could be significant tectonic activity to generate enough heat and chemical fuel to provide the ingredients for life. Lake Vostok is warmed solely by geothermal sources. There is no evidence of any gas released by tectonic sources as in Lake Baikal, and so the convection of the water is expected to be very slow. In addition, the absence of tectonic activity means that there are no other sources of energy for life to utilise in the lake making microbes the highest form of life that is likely. Andy Pepperdine
Figure 1a, b
Figure 1c, d
Siegert, M.J., Ellis-Evans, J.C., Tranter, M., Mayer, C., Petit, J.-R., Salamatin, A. & Priscu, J.C Physical, chemical and biological processes in Lake Vostok and other Antarctic subglacial lakes. Nature 414, 603-609 (2001).
Figure Captions Figure 1. Images of bacteria frozen into Lake Vostok's accreted ice. (a) Atomic Force Microscope image of a single bacteria, illustrating how easily bacteria can be identified using this device. (b-d) Scanning Electron Microscope images showing the occurrence of rod-shaped bacteria (noted by arrows) within the accreted ice. The ice was sampled from the Vostok ice core at a depth of 3590 m, which is ~151 m above the lake surface and 60 m below the meteoric/accreted ice boundary.
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