Hydraulics & sediment deformation beneath an ice stream: a multi-component geophysical AVO investigation

Streams of fast-flowing ice (ice streams) play a major role in the movement of large ice sheets such as those of Greenland and Antarctica. Although they cover only a small part of the total ice sheet area, they discharge over 3/4 of the ice mass that flows into surrounding oceans. If the ice sheets were to decay rapidly under conditions of global warming, much of the net mass loss, and the resultant rise in sea level, would take place through the speeding up of ice streams. Geological evidence shows ice streams to have played similar roles in the ice sheets that covered much of North America and Eurasia during the last glacial period. Ice streams flow quickly because of the low friction surface over which they flow. Recent research has shown in many, possibly the majority, of cases studied that this is because extensive areas of the bed are covered by sediment which deforms readily, and provides a rapidly-deforming, low friction carpet that eases ice stream movement. In order to deform under considerable ice thicknesses, the sediment needs to be unfrozen, with high internal water pressures that almost balance the pressure due to overlying ice. This water is derived from melting at the base of the ice sheet, and sometimes from water from the surface that percolates through the ice sheet. If the ice sheet is not to become buoyant and unstable due to water build up at its base, the water must drain through the ice sheet system and be discharged from its margin. A central unsolved problem in glaciology is to determine the water pressure gradient along this drainage pathway, which will determine the local water pressures and therefore the local friction and flow at the ice/bed interface. Recent work on the Rutford Ice Stream of the West Antarctic Ice Sheet has revealed an excellent site for the study of this problem, where a deforming subglacial sediment carpet has been identified using seismic primary (P) waves that are able to recognize whether or not a subglacial sediment is deforming. We propose to extend this technique by using both P and S (shear) seismic waves that will permit us to increase the power to resolve small differences in deforming layer thickness, and to use a more sophisticated survey technique called AVO and a recent development of poro-elasticity theory to deduce a much wider range of subglacial conditions. Amongst these, the most important will be water pressure in the subglacial sediment, which will permit us to reconstruct the form of the subglacial 'water table' which ultimately controls sediment deformation and basal friction. We will use the inferred hydraulic patterns to test theories of subglacial drainage. Another important discovery at the Rutford has been the existence of 'drumlins' (long ridges, streamlined in the direction of ice flow), which have also been shown to be made of deforming sediment, and which move in the direction of ice flow. Drumlins are widespread in areas of former glaciation, and particularly densely clustered along former ice streams. If drumlins are characteristics of a deforming glacier bed as has been suggested, and as is implied by the Rutford observations, they will provide a powerful means of understanding how the large scale dynamics of the bed of an ice sheet operates by studying the beds of Ice Age ice sheets that can be so readily observed over Eurasia and North America. An important part of our research therefore will be to use a radar system to image the form of the ice stream bed and the distribution of drumlins, to relate these to hydraulic conditions and deforming bed processes, and by studies in two field seasons (added to existing, earlier data from the same area), to monitor how the hydraulic and deforming bed system changes through time. We also hope to estimate the flux of water and sediment through the ice stream system, in order better to understand the role it plays in the rate of geomorphological and sedimentary change in this part of Antarctica