Steep reverse bed slope at the grounding line of the Weddell Sea sector in West Antarctica

The West Antarctic Ice Sheet lies on a bed that is, in places, >1.5 km below sea level. A positive ice-loss feedback is theorised to occur by grounding line retreat across a deepening bed, which accelerates ice floatation and ice-sheet decay. This process could raise global sea level by >3 m. Attention has been given to ice-sheet change in the Siple Coast and Amundsen Sea embayment sectors of West Antarctica. Here, we present radio-echo sounding information from the ice-sheet’s third sector, the Weddell Sea embayment, revealing a major subglacial basin immediately upstream of the grounding line. The reverse bed slope is steep (~400 m over ~40 km) and contains little small-scale topography that would delay retreat, unlike the more complex reverse slopes in the Amundsen Sea embayment. The basin floor is smooth and flat, similar to the Siple Coast, suggesting it has been covered with marine sediment and, therefore, was previously deglaciated. Upstream of the basin well-defined glacially carved fjords, with bars at their mouths, testify to the position of a former ice margin ~200 km inland from present. As the ice sheet is close to floatation over the basin, the region is at a physical threshold of substantial change.