Identifying areas of low-profile ice sheet and outcrop damming in the Antarctic ice sheet by ERS-1 satellite altimetry

A digital elevation model (DEM) of the surface of the Antarctic ice sheet is compared with a simple two-dimensional ice-flow model to illuminate gross distortions (>500 m) of the ice-surface elevation. We use a DEM derived from ERS-l satellite altimetry, airborne data and TWERLE balloon data. This is compared with an ice-sheet elevation model generated by applying theoretical surface elevations, calculated for twodimensional ice flow, to isolines of distance from the grounding line (continentality). The model is scaled using only one parameter, to match the measured surface elevation at Dome Argus. The model is far from rigorous, violating continuity conditions, ignoring variations in surface mass balance and temperature, and assuming uniform basal conditions. However, the comparison of model and observed surface elevations is illuminating in terms of the behaviour of the ice sheet at a continental scale. Across the ice sheet the rms difference between modelled elevation and the DEM is around 300 m, but much of this results from isolated areas of much greater disagreement. We ascribe these gross differences to the effects of basal conditions. In four areas, the observed surface is more than 500 m higher than the modelled surface. Most of these are immediately upstream of substantial areas of rock outcrop and are caused by the damming effect of these mountain ranges. In nine areas, the measured surface is more than 500 m lower than predicted. Eight of these areas, in West Antarctica and the L ambert Glacier basin, are associated with suspected areas of basal sliding. The ninth is an area of 250 000 km2 in East Antarctica not previously noted as having unusual flow characteristics, but for which very few data exist. We speculate that this area results from unusual basal conditions resulting in a low-profile ice sheet. A low-profile ice sheet of this size within the East Antarctic ice sheet indicates that basal conditions are perhaps more variable than previously thought.

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