GROUNDWATER FLOW DYNAMICS UNDER THE ELSTERIAN ICE SHEET IN POLAND AND ITS RELATION TO ICE SHEET STABILITY

Quaternary ice sheets are known to have profoundly affected groundwater flow in Europe and North America. Local subaerial catchments were replaced by orders-of-magnitude larger catchments controlled by ice sheet topography, and pressurized groundwater flowed much faster and deeper than during the interglacial conditions. Glacially-fed groundwater flushed deep aquifers and penetrated aquitards leaving a characteristic isotopic and chemical signature.

In order to simulate subglacial groundwater drainage under the Elsterian (ca. 500-400 ka PB) ice sheet in Poland, we have designed a complex numerical model using the FD MODFLOW code. The model is a 3D, steady-state simulation performed for different palaeo-glaciological scenarios involving a range of ice thicknesses. The model comprises an area of about 300.000 km2 between the present Baltic Sea in the north and the Karpathian/Sudetian Mountains in the south and it consists of 6 major hydrogeological layers down to the impermeable substratum (Permian salt or bedrock) at a maximum depth of ca. 4.6 km.

The most likely scenario shows a total re-organization of the groundwater flow field under the ice sheet as compared to the modern (interglacial) situation. The groundwater flowed from the Baltic Sea towards the mountains in the south (i.e. in the opposite direction as today) and discharged at the ice sheet margin. Due to generally low hydraulic conductivity of the substratum, only a fraction (ca. 4%) of basal meltwater could have drained as groundwater flow which, in the light of the surprising lack of large subglacial channels (tunnel valleys) strongly suggests a possibility of wide-spread basal de-coupling by pressurized subglacial water. This in turn implies instabilities in ice sheet behaviour leading to surges and ice streaming.