THREE LAND-BASED PALAEO-ICE STREAMS OF THE SOUTHERN SCANDINAVIAN ICE SHEET: SUBGLACIAL PROCESSES, SEDIMENTS AND LANDFORMS

During the Last Glaciation the southern, land-based part of the Scandinavian Ice Sheet contained multiple ice streams. These ice streams were highly dynamic, short-lived and diachronous and played a major role in controlling the behaviour of the ice sheet margin along a distance of over 1000 km. Since there are no modern analogues for past land-based ice streams, their reconstruction is challenging. Here we examine geological data to infer subglacial processes under three major palaeo-ice streams in Poland, Germany and Denmark.

The marginal part of Vistula ice stream in central Poland carries a suit of spectacular ice/bed interface landforms. We show that these landforms (now called glacial curvilineations) are erosional remnants carved into the pre-existing morainic plateau by subglacial meltwater flows at a late stage of ice advance. The eroded sediment was carried down-ice in multiple meltwater channels and re-deposited as thick pebble and gravel units at the ice margin. The Odra ice stream in the German-Polish border area hosts a major drumlin field dissected by a series of subglacial meltwater channels. Numerical modelling of subglacial groundwater flow suggests that only a small fraction of meltwater drained from the ice/bed interface into the bed and the surplus meltwater contributed to sediment deformation and fast flow of ice. Tunnel valleys formed preferentially in areas of low hydraulic conductivity of the bed. The Baltic ice stream in Denmark left a field of drumlins and mega-scale glacial lineations. We show that the ice advance prior to the formation of these landforms was characterised by multiple basal de-coupling events caused by pressurized meltwater during which thick outwash sand sheets were deposited subglacially.

Together, data from these three palaeo-ice streams show that the ice/bed interface is a self-organising system of multiple quasi-steady-states separated by thresholds related to hydrological and geomechanical parameters such as meltwater volume, drainage capacity of the bed, porewater pressure, strength of the bed sediments, etc. At critical states slight variations in these parameters may cause dramatic re-organisation of the nature of subglacial processes such as switches between erosion, deposition and deformation.