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2 Geological and Geophysical Setting
Pages 32-61

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From page 32...
... This classification resulted in three categories of subglacial aquatic environments: (1) lakes within subglacial basins located in the ice sheet interior; (2)
From page 33...
... 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)
From page 34...
... . BASEMENT ROCK CHARACTERISTICS An understanding of the bedrock geology beneath the ice sheets is important to help determine how and when subglacial aquatic environments formed and whether these
From page 35...
... The fact that "young" ages in the range of 1800 million to 600 million years are being recognized from materials recovered in the Vostok core, suggests that faults and sutures exist that are juxtaposing basement rocks of a variety of ages. This is relevant to the study of subglacial aquatic environments because if the interior of Antarctica has fractured rocks as suggested by Figure 2.2, then it is not only likely but inevitable that these subglacial aquatic environments constitute a connected hydrologic system.
From page 36...
... This figure illustrates that the interior of the East Antarctic Craton may not be homogeneous, but rather is faulted and may comprise terranes of different ages and origins. The structural relationships may be a boundary condition for how and where lakes form and the extent to which they might be connected.
From page 37...
... FIGURE 2.3 Cross-sectional profile of the Antarctic ice sheet based on BEDMAP bed topography (Lythe et al.
From page 38...
... The modes of flow outlined above are referred to as sheet flow, stream flow, and shelf flow, where different flow mechanisms dominate for each case. For the most part, shelf flow is relevant only to the floating margins of the Antarctic ice sheet, but it also operates over large subglacial lakes such as Lake Vostok where, as for ice shelves, the ice column is afloat and the lower boundary is free of frictional resistance.
From page 39...
... 2.04 FIGURE 2.5 Map of bed temperature of Antarctic ice sheet (in degrees Celsius relative to the pressure melting temperature) as predicted by the numerical ice dynamics model.
From page 40...
... Nevertheless, to a first approximation, regions where the simulated water depth is large correspond to regions where the estimated melt rate is high. GEOGRAPHICAL LOCATION OF ANTARCTIC SUBGLACIAL LAKES Main Traits of Locations and Settings About 80 percent of subglacial lakes reported to date in Antarctica lie at elevations less than a few hundred meters above sea level, while the majority of the remaining lakes are "perched" at higher elevations (Blankenship et al.
From page 41...
... (2006) have found that 66 percent of all known Antarctic subglacial aquatic environments lie within 50 km of an ice divide, and some 88 percent of them are within 100 km of a local ice divide.
From page 42...
... The vast majority of subglacial aquatic environments identified to date are within 100 km of an ice divide. Major clusters of these environments are located in Dome C (DC)
From page 43...
... What is the transit time for water deposited as snow on the surface of the ice sheet to be melted off the base of the Antarctic ice sheet and delivered to a subglacial lake? This establishes the degree of temporal separation between the surface and the subglacial environment and thus the age of microorganisms that are deposited in the lake by bottom melting.
From page 44...
... Here, the major point is that when water from subglacial lakes flows to the global ocean it is unlikely to be more than several million years older than the snow falling on the ice sheet surface. This time scale is relevant to the maximum degree of genetic divergence between organisms deposited on the ice sheet surface and organisms transferred from the ice sheet to the global ocean.
From page 45...
... (2006) present strong evidence for the episodic discharge from a deeply buried Antarctic subglacial lake into two receiver lakes situated at least 290 km downstream from the source lake.
From page 46...
... Ice flow is known to follow the surface slope so the implication of the above is that subglacial water routing is closely similar to the routing of the ice flow, both being determined by the ice surface topography. With knowledge of both the ice surface topography (e.g., Liu et al.
From page 47...
...  GEOLOGICAL AND GEOPHySICAL SETTING FIGURE 2.11 Calculated routings for subglacial meltwater based on surface and bed topography in the Dome C and Ridge B region of East Antarctica and application of a flow routing algorithm. SOURCE: Siegert et al.
From page 48...
... . Groundwater Routing Groundwater flow is driven by gradients in the hydraulic potential that, in turn, are governed by the surface and bottom geometry of the ice sheet.
From page 49...
... CIRCULATION AND STRATIFICATION Although subglacial lakes are isolated from direct wind-induced mixing, a variety of other hydrodynamic mechanisms can operate within the lakes. Subglacial lake circulation results from endogenous processes that drive water from one part of the lake to another and exogenous processes such as "riverine" fluxes to and from a subglacial hydrologic system that extends beyond the lake margins.
From page 50...
... . Whether Antarctic subglacial aquatic environments are "lake-like" or "ocean-like" in their structure and convective circulation depends strongly on their salinity and depth
From page 51...
... Furthermore, the mean water depth for Lake Vostok is around 360 m (Studinger et al.
From page 52...
... where the inverse temperature gradient is maintained by heat losses from the top of the water column to the overlying ice and atmosphere. A large number of the subglacial lakes detected so far have ice thicknesses less than 3170 m and could be stratified; however many have much thicker ice, including Lake Vostok.
From page 53...
... . Although this variety of hydrodynamic mechanisms is likely to disperse contaminants both vertically and horizontally, full homogeneous mixing throughout the water body seems unlikely for all but the smallest of subglacial lakes, and dilution of any contaminant may be much lower than the mixing ratio calculated for the total lake volume.
From page 54...
... Freeze-on of lake water to the ice ceiling of subglacial lakes results in partitioning of water isotopes and chemical impurities between the solid and liquid phases. The heavy isotopes of hydrogen and oxygen are concentrated in the solid phase, whereas chemical impurities are concentrated in the lake.
From page 55...
... . The length of this delay will vary with internal ice sheet dynamics, and the contributions from the different sediment sources will be on very different time scales.
From page 56...
... Most subglacial lakes are relatively small and fairly shallow, but these shorter sediment records will provide valuable information about subglacial lakes. Longer records, however, may provide the key to understanding the origins of subglacial aquatic environments because this information may be at the bottom of sedimentary sequences (SCAR SALE 2006)
From page 57...
... The implications of potential high gas pressures on access to subglacial aquatic environments are largely dependent on the depth of the water body. Gas pressure cannot exceed hydrostatic pressure in subglacial lakes.
From page 58...
... in the Antarctic ice sheet and sub-ice water at the reference sites M and F located in the zones of subglacial melting and freezing, respectively. Gray regions show fields of air hydrate stability.
From page 59...
... The lake sediments could contain an unparalleled record of Antarctic paleoenvironmental information, extending well beyond the limit of ice core records. Most Antarctic subglacial lakes were detected by airborne radar sounding.
From page 60...
... Most subglacial aquatic environments are likely to be part of an extensive subglacial drainage system rather than being hydrologically isolated. The flow of subglacial water is guided by the topography of the ice sheet surface and, to a lesser extent, the topography of the subglacial bed; together these topographic influences subdivide the Antarctic ice sheet into discrete drainage basins.
From page 61...
... A full discussion of the potential biological environment is presented in Chapter 3. There is considerable uncertainty in the values of the coefficients that govern the partitioning of chemical and biological constituents of subglacial aquatic environments during freeze-on of ice accreting at the bottom of the ice sheet.


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