SPATIAL VARIATIONS IN HEAT AT THE BASE OF THE ANTARCTIC ICE-SHEET FROM ANALYSIS OF THE THERMAL REGIME ABOVE SUBGLACIAL LAKES

Antarctic subglacial lakes provide an important boundary condition for thermal analysis of the ice sheet in that the basal ice temperature o ver lakes may be assumed to be at the pressure-melting point. We have used a one-dimensional vertical heat-transfer equation to determine th eoretical temperature values for the ice-sheet. base above 77 subglaci al lakes identified from airborne radio-echo-sounding data: covering 5 0% of Antarctica. Variations in our temperature results to below the p ressure-melting temperature over lakes are due to either our estimate of the geothermal heat flux or a neglect of heat derived from (a) inte rnal ice deformation and (b) basal sliding, in the thermal model. Our results indicate that, when the geothermal heat nux is set at 54 mW m( -2), the ice-sheet base above 70% of the known Antarctic subglacial la kes is calculated to be at the pressure-melting value. These lakes are located mainly around Dome C, Ridge B and Vostok station. For the ice sheet above subglacial lakes located hundreds of kilometres from the ice divide, using the same thermal model, loss of heat due to vertical advection is calculated to be relatively high. in such regions, if th e ice-sheet base is at the pressure-melting point, heat lost due to ve rtical advection must be. supplemented by heat from other sources. For the three I lakes beneath Terre Adi-lie and George V Land, for instan ce, the basal thermal. gradient calculated to produce pressure melting at the ice-sheet base is equivalent to 1.5-2 times the value obtained when 54 mW m of geothermal heat is used as the sole; basal thermal co mponent. We suggest that, as distance from the ice divide increases, s o too does the amount of heat due to internal ice deformation and basa l sliding.: Moreover, by considering the ice-sheet basal thermal chara cteristics above subglacial lakes which lie on the same ice flowline, we demonstrate empirically that the heat due. to these horizontal ice- motion terms varies pseudo-exponentially with distance from the ice di vide. The location along a flowline where a rapid increase in the basa l heat gradient is calculated may correspond to the onset of large-sca le basal sliding.