Deformation of subglacial till near ice-sheet grounding zones: theory and experiments

Abstract:

Large-scale ice-sheet dynamics pivot on the deformation and transport of subglacial sediment through changes in the basal sliding velocities of glaciers. Such unconsolidated, water-saturated glacigenic sediment, or till, is found to accumulate into sedimentary wedges, or till-deltas, in grounding zones separating floating ice shelves from grounded ice streams. In addition to affecting glacial slip, such sedimentation may serve to stabilise ice sheets against grounding-line retreat in response to rising sea levels. We present a fluid-mechanical explanation of the formation of these wedges in terms of the jump in hydrostatic loading and unloading of till across the grounding zone, and we compare our findings with geophysical data of sedimentary wedge formation at the modern-day grounding zone of Whillans Ice Stream, West Antarctica. We develop a theoretical model of wedge formation in which we treat both ice and till as viscous fluids spreading under gravity into an inviscid ocean and find that a similar wedge of underlying fluid accumulates around the grounding line in our series of fluid-mechanical laboratory experiments. The experiments were performed in a confined channel geometry. We extend our theory to unconfined geometries in which till deformation is resisted dominantly by vertical shear stresses and the flow of the overlying ice is resisted dominantly either by vertical shear stresses between the ice and till or by extensional stresses characteristic of floating ice shelves and shelfy streams. The former is relevant to less-lubricated, grounded ice sheets whereas the latter is relevant to well-lubricated ice streams, sliding over soft, deformable till of low viscosity and appreciable thickness. We formulate a local condition relating wedge slopes in each of the three scenarios and find a reasonable agreement with geophysical data.