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Ice Sheets....The Long Version

Ice sheet dynamicsSubglacial hydrologic reservoirs and interconnecting passageways affect ice sheet and ice stream dynamics. The fundamental interaction between subglacial lakes and the overlying ice sheet is through the melting and freezing process. Additional water contributed to or lost from subglacial environments may be mediated by subglacial water drainage systems. Ice floating on subglacial lakes requires that the surface slope be reflected in a slope corresponding to the basal ice interface. This will lead to the downstream thinning of ice creating pressure gradients in a subglacial lake if it is of sufficient size, for example, Lake Vostok. Differences in the basal pressure melting point of the ice sheet initiates ice melting in the deeper regions tending to reduce the basal slope (and thus also the surface slope) to zero. Continuous ice transport and ice deformation across subglacial lakes counteracts this compensating process.

Water plays a crucial role in the stability of major ice sheets. The largest subglacial lakes are situated in the stable interior of the East Antarctic Ice Sheet. Evidence for water flux between lakes in a matter of years and enhanced ice flow associated with the presence of subglacial lakes suggest that there are important linkages to ice sheet stability. The emerging evidence that Antarctic subglacial lakes catastrophically drain and that subglacial lakes are closely linked to the onset of ice streams suggests an integral role for these subglacial environments in the global cryosphere and climate system. For example, it has been speculated that the draining of subglacial lakes could trigger rapid flow in an overlying ice stream thereby enhancing ice flow into the adjacent ocean.

If subglacial lakes form by accumulating water in bedrock troughs, water must be delivered by a system of passageways to the basins. These drainage networks would also act as conduits for draining a subglacial lake if the lake water level is sufficiently high or hydraulic pressure in the lake is large enough. This implies that isolated subglacial lake systems are unlikely to exist in areas of widespread basal melting. The hydrological consequences of lubrication of the ice sheet bed and stick-slip areas, for example, would not be expected to be restricted to subglacial lake – ice sheet interactions. It is likely that the ice sheet itself would react to these changes. In the case of Lake Vostok, the flow field of the ice sheet is clearly influenced by the subglacial lake beneath (Pattyn et al., 2004; Tikku et al., 2004). Alternatively, subglacial lakes may act as internally closed systems with melting and refreezing balanced within the basis.

The magnitude of mass exchange between the ice sheet and a lake will depend on ice flow, ice thickness distribution, geothermal flux and subglacial water chemistry (see the Limnological and Biogeochemistry section above and Mayer et al., 2003). The degree of melting and freezing will have significant implications for the chemistry of subglacial lake water and the existence of a resident ecosystem (Tikku et al., 2004). A well-developed intra-lake circulation system will increase the total amount of gas in the lake water, barring effective removal processes. In most cases, gas will be partitioned between the dissolved and gas hydrate phases (see the Subglacial Hydrology section above and McKay et al., 2003).

Due to the low intensity of predicted subglacial lake circulation, turn over times for the subglacial lake water are likely to be on the order of millennia or more. Continuous release of minerals into subglacial lakes from melting ice, combined with chemical weathering of sediments and/or the lake floor, make it is likely that most subglacial lakes contain at least slightly saline water. Even low salinities have a strong influence on circulation regimes considerably reducing turn over times (Souchez et al., 2000; Mayer et al., 2003). Melt water discharged into subglacial lakes from surrounding catchments will influence thermohaline processes due to its differing salinity and turbidity. These flows will introduce a siliciclastic sedimentary component to subglacial lake sediment deposits (Royston-Bishop et al., 2004). Water drainage from subglacial lakes may take place in the form of subglacial floods providing a negative feedback that induces transient changes in down-stream ice flow velocities (Gray et al., 2005).

The reason for the proximity of subglacial lakes and ice domes is not known. Subglacial hydrologic reservoirs, their fluxes, and their possible impact on ice sheets and ice stream dynamics are also mostly unknown. Observations of vertical motion and borehole geochemical and physical measurements of glaciers and ice sheet would greatly improve our understanding of these linkages. Ice sheet evolution and the formation of subglacial lakes, including the response of subglacial lakes to changes in ice thickness, require further systematic study. Models can simulate these interactions and the outcomes can be tested by field observations.

Important linkages between ice sheets and underlying subglacial environments can be assessed through: (i) observations of the surface manifestation of hydrologic processes such as vertical motion; (ii) borehole observations of basal water (including geochemical and physical measurements); and (iii) numerical modeling of ice sheet evolution that includes the formation of subglacial lakes. Hydrologic and thermodynamic controls can be tested by model simulations (see Inset). The boundary conditions for thermodynamic models, such as geothermal heat flux, lake water chemistry, morphology, sediment thickness, geology, and ice-water interface conditions must be constrained. Studies will include: (i) identification of the spatial and temporal distribution of free water at the ice-bed; (ii) observation and modeling of the rates of water and sediment transport through subglacial environments; (iii) observation and modeling of ice sheet interactions with subglacial water, and (iv) modeling of the turn over time of subglacial lakes. Regional geophysical site survey data (gravity, magnetics, altimetry, seismics, GPS, radar) should be integrated to assist in developing models of subglacial drainage and sedimentation at a variety of temporal and spatial scales. Critical components of predictive climate models need to be improved to account for subglacial interactions. Research on ice sheet dynamics will increase our understanding of subglacial hydrological systems on local, regional, and global scales; explore the relationship between ice sheet stability and fast glacier flow; and assist in reconstructing the development of the Antarctic ice sheet.

 

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