Calculating basal thermal zones beneath the Antarctic ice sheet

A procedure is presented for using a simple flowline model to calculate the fraction of the bed that is thawed beneath present-day ice sheets, and the refore for mapping thawed, frozen, melting and freezing basal thermal zones . The procedure is based on the proposition, easily demonstrated, that vari ations in surface slope along ice flowlines are due primarily to variations in bed topography and ice-bed coupling, where ice-bed coupling for sheet f low is represented by the basal thawed fraction. This procedure is then app lied to the central flowlines of flow bands on the Antarctic ice sheet wher e accumulation rates, surface elevations and bed topography are mapped with sufficient accuracy, and where sheet flow rather than stream flow prevails . In East Antarctica, the usual condition is a low thawed fraction in subglac ial highlands, but a high thawed fraction in subglacial basins and where ic e converges on ice streams. This is consistent with a greater depression of the basal melting temperature and a slower rate of conducting basal heat t o the surface where ice is thick, and greater basal frictional heat product ion where ice flow is fast, as expected for steady-state flow. This correla tion is reduced or even reversed where steady-state flow has been disrupted recently, notably where ice-stream surges produced the Dibble and Dalton I ceberg Tongues, both of which are now stagnating. In West Antarctica, for ice draining into the Pine Island Bay polynya of th e Amundsen Sea, the basal thawed fraction is consistent with a prolonged an d ongoing surge of Pine Island Glacier and with a recently initiated surge of Thwaites Glacier. For ice draining into the Ross Ice Shelf, long ice str eams extend nearly to the West Antarctic ice divide. Over the rugged bed to pography near the ice divide, no correlation consistent with steady-state s heet flow exists between ice thickness and the basal thawed fraction. The b ed is wholly thawed beneath ice streams, even where stream flow is slow. Th is is consistent with ongoing gravitational collapse of ice entering the Ro ss Sea embayment and with unstable flow in the ice streams.