Meltwater discharge through the subglacial bed and its land‐forming consequences from numerical experiments in the Polish lowland during the last glaciation

Abstract

Numerical experiments suggest that the last glaciation severely affected the upper lithosphere groundwater system in NW Poland: primarily its flow pattern, velocities and fluxes. We have simulated subglacial groundwater flow in two and three spatial dimensions using finite difference codes for steady-state and transient conditions. The results show how profoundly the ice sheet modifies groundwater pressure heads beneath and some distance beyond the ice margin. All model runs show water discharge at the ice forefield driven by ice-sheet-thickness-modulated, down-ice-decreasing hydraulic heads. In relation to nonglacial times, the transient 3D model shows significant changes in the groundwater flow directions in a regionally extensive aquifer ca. 90 m below the ice-bed interface and up to 40 km in front of the glacier. Comparison with empirical data suggests that, depending on the model run, only between 5 and 24% of the meltwater formed at the ice sole drained through the bed as groundwater. This is consistent with field observations documenting abundant occurrence of tunnel valleys, indicating that the remaining portion of basal meltwater was evacuated through a channelized subglacial drainage system. Groundwater flow simulation suggests that in areas of very low hydraulic conductivity and adverse subglacial slopes water ponding at the ice sole was likely. In these areas the relief shows distinct palaeo-ice lobes, indicating fast ice flow, possibly triggered by the undrained water at the ice-bed interface. Owing to the abundance of low-permeability strata in the bed, the simulated groundwater flow depth is less than ca. 200 m.

Sedimentary imprints of former ice-sheet margins: Insights from an end-Ordovician archive (SW Libya)

“…Subglacial meltwaters, either through subglacial aquifers within channels or at the glacier sole, are usually ascribed to overpressured flows (Boulton and Hindmarsh, 1987;Boulton and Caban, 1995;Boulton et al, 2009;Piotrowski et al, 2009;Clerc et al, 2012). Though overpressured meltwaters may locally result in active subglacial deposition (Fleisher et al, 2010;Phillips and Lee, 2013), structures related to sediment injection, hydrofracturing and downcutting are commonly recognised in the glacial record.…”

Antarctic subglacial groundwater: a concept paper on its measurement and potential influence on ice flow

“…Where contrasts in acoustic impedance exist between stratified layers beneath the ice-sheet bed, such as, for example, those expected at the subglacial till-sedimentary rock interface or interfaces between stratified units within groundwater aquifers in subglacial sedimentary basins (e.g. Boulton et al 1995;Person et al , 2012Bense & Person 2008;Piotrowski et al 2009), then seismic techniques are readily able to resolve layers thicker than about one-quarter of the seismic wavelengthtypically a few metres -and even thinner layers can be interpreted using diagnostic AVO techniques (Booth et al 2012). Additional deep explosive shots into the snow-streamer would facilitate the generation of high-quality seismic velocity models and possibly AVO analysis of basin structures.…”

Unravelling the complex nature of the Upper Weichselian till section at Gdynia Babie Doły, northern Poland

“…This indicates a large spatial variation of rheological conditions in the ice sheet sole, conditioning the formation of the ice-bed decoupling and short-term subglacial flows leading to the deposition of thin sandy laminae (compare Piotrowski & Tulaczyk, 1999;Jørgensen & Piotrowski, 2003;Piotrowski et al, 2006). Occasionally, the formation of subglacial channels occurred allowing deposition of the sorted sediments of a larger thickness (compare Piotrowski et al, 2009). Some of them were subsequently deformed when ice-bed coupling was restored (Fig.…”

“…1D, 3A, 3B). Such an arrangement favoured the diversity in water pressure in the bed, and thus also the variability of sedimentation conditions and heterogeneous susceptibility of sediments to deformation (compare Piotrowski & Kraus, 1997;Piotrowski et al, 2004Piotrowski et al, , 2009). sampling sites -see Fig.…”

“…The presence of large, irregular sandy bodies of flat-bottom shape and deformed upper surface, recorded in the lower part of the till (Fig. 3H), may indicate depositional episodes in small subglacial channels (compare Piotrowski et al, 2009), the filling of which was subsequently deformed once the hydrogeological conditions changed (compare Piotrowski et al, 2004;Tylmann Where the thickness of the yellowish till (subunit B3a) is greater (section BD-2) the upper part of the section can be delimited, indicating an important phase of ice sheet stagnation, where the passively melting-out debris locally underwent processing by subglacial flow of the water discharged from the bottom of the melting ice ( Fig. 3I; compare Shaw, 1982).…”