Subglacial hydrology in north-western germany during the last glaciation: groundwater flow, tunnel valleys and hydrological cycles

Numerical modelling of groundwater dynamics in an overpressured system of subglacial aquifers and aquitards under the marginal portion of the Scandinavian ice sheet in northwestern Germany has been coupled with calculations of basal meltwater production rates and with field evidence of meltwater erosional features. Largely fine-grained subglacial sediments with relatively low hydraulic conductivities had a capacity to drain only about 25% of all basal meltwater produced in the relevant subglacial catchment area. The rest of the meltwater was evacuated through tunnel valleys in spontaneous outburst events. Subglacial drainage cycles are proposed, in which each cycle begins with meltwater drainage through the substratum (groundwater flow), followed by water ponding at the ice/bed interface, and ends in catastrophic meltwater releases through tunnel valleys.