Ice-Bed Coupling of Sediment-Based Glaciers: a Laboratory Study of Basal Processes

The interaction of basal processes with the subglacial drainage system is a critical issue in understanding glacier dynamics. Since the recognition that many glaciers and ice masses overlie soft sediments rather than hard bedrock, much research has been undertaken to investigate how mechanical and hydrological conditions of a deformable substrate control the coupling at the ice-bed interface and thus affect fast ice flow and glacier surging. While it is widely recognized that the motion at the base of a glacier and the drainage of water beneath the ice are strongly interdependent, the exact nature of the relationship between subglacial hydrological conditions and mechanisms of glacier basal motion is not fully understood.

A rotary-ploughing device is being constructed to explore factors that control the coupling at the ice-bed interface and the resulting distribution of motion at the base of soft-bedded glaciers. Specifically, ploughing experiments will be carried out by dragging instrumented objects through sediment under realistic subglacial conditions. In the first of three sets of experiments, a ploughing tip will be dragged through a heterogeneous, clast-rich sediment to measure the rate of collision between this tip and the clasts of the sediment. The goal is to establish that the clast collision frequency as recorded with an object dragged through a subglacial sediment is directly related to the glacier sliding rate. A second set of experiments is aimed at better understanding the rheology of subglacial sediment. The force on a ploughing tip dragged through a homogeneous, fine-grained sediment will be measured to test the hypothesis that the sediment shear strength is independent of the rate of deformation and thus that subglacial sediment essentially behaves as a frictional plastic material. In the final set of experiments clast ploughing will be studied by measuring the drag on hemispheres pushed through sediment. Pore-water pressure will be measured to test the hypothesis that excess pore pressures in sediment downglacier from ploughing clasts weaken the ice-bed interface. This effect may cause rapidly sliding glaciers on sediment to decouple from their beds.