A THEORY OF DRAINAGE BENEATH ICE SHEETS, AND THE ORIGIN OF ESKER AND TUNNEL VALLEY SYSTEMS FROM THE LAST EUROPEAN ICE SHEET

The hydraulic system and the efficiency of drainage beneath an ice sheet determine the subglacial water pressure regime, and many aspects of ice sheet dynamics and stability. Esker systems and tunnel valleys are a reflection of the subglacial hydraulic state, such that a theory of their formation would be a key to understanding this vital part of the ice sheet system. A theory is developed that the ice sheet wide pattern of distribution of major esker and tunnel valley systems is a consequence of dynamic interactions between subglacial tunnel flow and groundwater flow, primarily controlled by the basal melting rate and bed transmissivity. Meltwater is drawn to tunnels by transverse-to-ice-flow groundwater flow, and efficiently discharged from the glacier by the longitudinal tunnels. Tunnel spacings relax towards the maximum spacing that is able to discharge the winter meltwater flux (basal melting), without creating dynamic instability in the ice sheet. Summer meltwater fluxes, involving a major component of surface meltwater, are the major controls on glaciofluvial erosion and sediment flux.

The theory is tested by its ability to simulate a modern glacier meltwater system and to simulate the observed systematic changes in esker and tunnel valley frequency in the area of the last European ice sheet.

The theory predicts that this system also controls the pattern of deep groundwater circulation; downward flows in inter-channel zones, and strong upwards flow beneath channels. This pattern has three important effects: on the geochemistry of subglacial groundwaters; in producing large upward potential gradients beneath channels with a tendency for quicksand conditions beneath channel floors; in generating large amounts of heat in permeable sediments beneath channels. The former process can create carbonate precipitates with distinctive geochemistries, and can be used to test the theory. It is suggested that the latter two processes are of major importance in the development of tunnel valleys.