Borehole measurements indicate hard bed conditions, Kangerlussuaq sector, western Greenland Ice Sheet

The hydrological processes and sliding dynamics of the Greenland Ice Sheet are closely linked to the structural framework of its bed. Establishing whether the ice mass rests on bedrock relatively clear of rock debris, a thick deformable till layer, or some intermediate combination, is therefore key to understanding the ice sheet response to changes in meltwater inputs. Here we report on direct observations of the ice-bed interface along a flow line transect in the ablation zone of the western Greenland Ice Sheet. Our measurements are derived from a network of 32 boreholes that vary from ~100 m deep near the ice margin to 830 m deep, 46 km upflow from the terminus. We performed a suite of experiments and sampling techniques in the holes to investigate the bed conditions. In contrast to previous geophysical and drilling studies elsewhere in Greenland which have suggested that the ice rests on a thick layer of sediment, we find no evidence for a thick sediment cover on the bed of the Kangerlussuaq sector of the ice sheet. Our observations imply that this area has a relatively clean and hard bed and that this facilitates a basal hydraulic system governed by ice/bedrock interactions. The lack of sediment cover on the bed could be due to recent deglaciation and advance of this sector of the ice sheet and/or variable rates of erosion and preferential routing of sediment through bedrock troughs.