Flow variability in the Scandinavian ice sheet: modelling the coupling between ice sheet flow and hydrology

There is increasing geologic evidence for periodic flow variability within large ice sheets, manifested as spatially and temporally variable areas of fast ice flow, and resulting in the very complex patterns of lineations observed in formerly glaciated areas. However, many ice sheet models do not replicate this behaviour. A possible reason for this is that such models do not include a detailed treatment of basal hydrology. Changes in the character of sub-glacial drainage systems are believed to cause surges in valley glaciers. Recent ice sheet models, which have included basal hydrology or at least a link between basal velocity and the presence of water at the bed, often show flow variability. However, these models have typically assumed a deformable bed, or have made no assumptions about the nature of the bed. Whilst these assumptions seem applicable to areas close to the former margins of Quaternary ice sheets, they are less applicable to interior areas, These areas typically show thin or scanty till cover over eroded bedrock, and the presence of eskers, which are indicative of drainage in sub-glacial tunnels. We have developed a two-dimensional time-dependent ice sheet model that includes hard-bed basal hydrology. This allows calculation of sub-glacial water pressures and the use of a water pressure dependent sliding law to calculate ice sheet velocities. When used to simulate the Weichselian Scandinavian ice sheet, with late Quaternary climate and sea level as forcing functions, this model develops localised areas of fast-flowing ice, which vary in extent and in distance of penetration into the interior of the ice sheet both spatially and temporally. The behaviour of these lobes depends crucially on the influence of the evolving ice sheet topography on the routing of subglacial water flow, due to the resulting variations in the subglacial hydraulic potential which drive the water flow. Bedrock topography also has some influence, but fast flow areas are not confined to obvious topographic troughs. A relatively thin ice sheet with low surface slopes is produced in areas experiencing fast ice flow. Generally, two to four separate areas of fast flow can be recognised, and these are similar in size and shape to the 'lobes' identified in some geologically based reconstructions of the Scandinavian ice sheet. Within the fast-flowing areas, sub-glacial drainage is typically in a cavity-based system. However, tunnel-based drainage is predicted to have extended up to 150 km from the ice sheet margin, particularly during deglaciation. Because of the changes in ice sheet topography associated with fast flow, and the resulting changes in the pattern of sub-glacial water flow, the model predicts that these fast-flowing lobes interacted in complex ways, and exhibited quasi-periodic switching between fast and slow flow. (C) 2002 Elsevier Science Ltd. All rights reserved.