basal melting exceeds net basal freezing. One result of basal melting is excess meltwater production beneath the ice sheet. Of necessity, a drainage system must form to transfer excess water from the ice sheet interior toward the periphery to regions of water storage or removal. Evidence for subglacial water discharge events comes from satellite detection of rapid (month- to year-scale) changes in the ice surface elevation of West Antarctic ice streams and above a known subglacial lake in central East Antarctica. Streams could form in meltwater channels that occur irregularly at the bottom of the ice sheet or could flow within layers of rocks or fractured bedrock that resemble the hyporheic flow paths beneath the beds of most rivers and streams. 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. Although the subglacial water system is likely to be interconnected, either continuously or sporadically, not every part of the water system is connected to every other part. The most plausible conclusion is that subglacial aquatic environments are simply one component of a subglacial drainage system that is organized into hydrologic catchments, analogous to those that occur subaerially. However, a basin-scale analysis of hydrologic catchments is necessary before lake-to-lake interactions can be critically examined.

The circulation of subglacial lakes is driven by small gradients in the potential energy of lake water that can result from vertical or lateral gradients in water density. This density depends on the pressure, temperature, and salinity of water. Of the three density dependents, the salinity of the subglacial aquatic environments is least constrained. Measurements of Vostok accretion ice and modeling results provide evidence for low-salinity lake water. However, if one temperature extrapolation is correct, higher-salinity water is inferred because increased salinity depresses the temperature of freezing. The low-salinity hypothesis also raises questions about the residence time of lake water and how low salinity can be maintained. The question of the salinity of subglacial lakes has not yet been answered definitively.

An important contribution to the circulation of Lake Vostok is associated with the sloping ice ceiling of the lake. Although the ceiling slope is not great, it would be much less if ice flow were aligned with the long axis of the lake. For many subglacial lakes, the ceiling may be nearly flat and this contribution to lake circulation could become small or negligible.

Glacial ice is formed by compression of surface snowfall. During the transformation from snow to ice, air trapped in the snow pack becomes entombed as bubbles within solid ice. As the depth of burial increases, these bubbles disappear and atmospheric gases are stored in solid form as air hydrates. The concentration of air in Antarctic glacial ice is estimated at 90 cm3 (at standard temperature and pressure [STP]) per kilogram of ice (McKay 2003); this concentration of air far exceeds the concentration of air-saturated water under the same conditions. Thus, subglacial meltwater is more highly concentrated in the dominant air gases (N2 and O2) than subaerial water. Freezing of meltwater concentrates chemical impurities in the liquid phase and, in subglacial lakes, could result in hyperconcentration of gases such as oxygen. It is not clear whether microbes are affected by these high concentrations. Chemically, subglacial aquatic environments can be expected to vary widely from site to site, and the complete

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