Abstract
AbstractModelling paleo-glacier networks in mountain ranges on the millennial timescales requires ice flow approximations. Hybrid models calculating ice flow by combining vertical shearing (shallow ice approximation) and longitudinal stretching (shallow shelf approximation) have been applied to model paleo-glacier networks on steep terrain, yet their validity has not yet been assessed quantitatively. Moreover, hybrid models consistently yield higher ice thicknesses than Last Glacial Maximum geomorphological reconstructions in the European Alps. Here, we compare results based on the hybrid Parallel Ice Sheet Model (PISM) and the Stokes model Elmer/Ice on the Rhine Glacier, a catchment of the former European Alpine Icefield. For PISM, we also test two magnitudes of flux limitation in a scheme that reduces shearing velocities. We find that the flux limitation typically used in PISM yields significantly reduced shearing speeds and increases ice thicknesses by up to 500 m, partly explaining previous overestimations. However, reducing the ice flux limitation allows the hybrid model to minimize this mismatch and captures sliding speeds, ice thicknesses, ice extent and basal temperatures in close agreement with those obtained with the Stokes model.
Highlights
During the last glacial cycle the European Alps were covered by a large glacier network with the ice flow direction confined by the underlying topography, a so-called icefield
Based on terminal moraines and erratic boulders it was possible to reconstruct the former ice extent reached by the Alpine Icefield (AlpIF) at the Last Glacial Maximum (LGM) (Benz-Meier, 2003; Bini and others, 2009; Coutterand, 2010; Ehlers and others, 2011)
The maximum ice thickness reconstructions in the European Alps are mainly inferred from trimline reconstructions which are known within 100 m and only exist at few mountainsides in the valley part of the AlpIF (Florineth, 1998; Florineth and Schlüchter, 1998)
Summary
During the last glacial cycle the European Alps were covered by a large glacier network with the ice flow direction confined by the underlying topography, a so-called icefield. Current geomorphological reconstructions portray the AlpIF as a vast icefield with several large piedmont lobes, abundant nunataks and ice flow directions predominantly constrained by the bedrock topography (Florineth, 1998; Florineth and Schlüchter, 1998; Benz-Meier, 2003; Kelly and others, 2004; Bini and others, 2009; Coutterand, 2010). The maximum ice thickness reconstructions in the European Alps are mainly inferred from trimline reconstructions which are known within 100 m and only exist at few mountainsides in the valley part of the AlpIF (Florineth, 1998; Florineth and Schlüchter, 1998) This range of uncertainty is much smaller than the ice thickness overestimations found by ice flow modelling, making the overestimated ice thicknesses significant. No geomorphological evidence from the Alps supporting this idea has been presented so far
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