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Exploration of Antarctic Subglacial Aquatic Environments: Environmental and Scientific Stewardship
The majority of the lakes are located in subglacial basins within roughly 100 km of ice divides in the ice sheet interior. These basins are typically separated by mountain ranges and their topography can either be relatively subdued, often near the center of subglacial basins, or relatively steep, occupying significant subglacial depressions, often near subglacial basin margins (Dowdeswell and Siegert 2003). Deep subglacial lakes are likely to develop in areas where the topography is subdued. Lake Vostok is the only known lake that occupies a large section of a subglacial trough. Perched lakes are located primarily in the interior of the ice sheet on the flanks of subglacial mountain ranges. These perched lakes are frequently small, measuring less than 10 km in length. Many other lakes occur in areas where the ice flow is enhanced, hundreds of kilometers from the ice sheet crest, such as the fast-flowing Byrd Glacier, which drains a very large interior drainage basin into the Ross Ice Shelf. These lakes are expected to be similar in size and depth to the small and most likely shallow subglacial aquatic environments found in the major subglacial basins in the ice sheet interior (Siegert 2002).
Another of these schemes, based on description of surface morphology and shape, has identified three categories of lakes: basin, relief, and trench (Tabacco et al. 2006). Basin lakes occur at the bottom of large depressions, usually in association with areas of considerable relief in the bedrock. They have irregular shapes, but no preferred elongation directions. This group of lakes is therefore thought of as resulting from glacial scouring. Relief lakes occur in subglacial mountainous regions, at fairly high bedrock elevation. They, therefore, have thinner ice cover than the basin lakes. Trench lakes, as their name implies, have an elongated shape controlled by the nature of the depressions in which they are found. These depressions are long and narrow and typically have steep cross-sectional profiles that may be fault bounded.
Lake Vostok and others in the Dome C region may belong to the trench lakes category. Studinger et al. (2003) have modeled Lake Vostok as the product of overthrust faulting followed by a small amount of extensional faulting during reactivation with the opposite-to-original sense of motion (Figure 2.1). However, this interpretation remains controversial because the majority of Antarctic subglacial lakes are developed on basement rocks that became part of the stable craton in the Precambrian, leaving few opportunities for more recent faulting to create basins in which subglacial lakes can form.
The third classification scheme of subglacial lakes is based on the characteristics of the return signals from each one of these features using ice-penetrating radar as displayed on radargrams. With this approach, three categories of lakes have been identified: great lakes, dim lakes, and fuzzy lakes (e.g., Blankenship et al. 2006).
Great lakes are ones that yield well-defined radar images with flat tops and flat bottoms. Their radar echo is bright, and they all seem to be at least 500 m across. Blankenship et al. (2006) estimate that there are about 30 great lakes among the subset of the 130 lakes that were surveyed systematically using ~51,000 line km of radar data.
Dim lakes are characterized by less clear tops and bottoms on radargrams compared to great lakes. Dim lakes may have started as “great lakes” but have since frozen over. Alternatively, their images may be artifacts of the ice temperature correction applied in the calculation being wrong. The evolution from great lake to dim lake may be caused by flow regime change (e.g., changes in the temperature of the ice).
Fuzzy lakes are distinguished by having hydrologically “flat” radar images that are, however, full of reflections that do not appear to be water. Terms such as mushy swamps or wetlands have been used in descriptions of these features.