Numerical reconstructions of the flow and basal conditions of the Rhine glacier, European Central Alps, at the Last Glacial Maximum

• Main Authors: Cohen, Denis, Gillet-Chaulet, Fabien, Haeberli, Wilfried, Machguth, Horst, Fischer, Urs H
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2018
• Subjects: Institute of Geography; 910 Geography & travel; The Cryosphere
• Online Access: https://www.zora.uzh.ch/id/eprint/142809/; https://www.zora.uzh.ch/id/eprint/142809/1/tc-12-2515-2018.pdf; https://doi.org/10.5167/uzh-142809; https://doi.org/10.5194/tc-12-2515-2018

At the Last Glacial Maximum (LGM), the Rhine glacier in the Swiss Alps covered an area of about 16,000 km2. As part of an integrative study about the safety of repositories for radioactive waste under ice age conditions in Switzerland, we modeled the Rhine glacier using a fully-coupled, three-dimensional, transient, thermo-mechanical Stokes flow model down to a horizontal resolution of about 500 m. The accumulation and ablation gradients that roughly reproduced the geomorphic reconstructions of glacial extent and ice thickness suggested extremely cold (TJuly ~ 0 °C at the glacier terminus) and dry (~ 10 to 20 % of today's precipitation) climatic conditions. Forcing the numerical simulations with warmer and wetter conditions that better matched LGM climate proxy records yielded a glacier on average 500 to 700 m thicker than geomorphic reconstructions. Mass balance gradients also controlled ice velocities, fluxes, and sliding speeds. These gradients, however, had only a small effect on basal conditions. All simulations indicated that basal ice reached the pressure melting point over much of the Rhine and Linth piedmont lobes, and also in the glacial valleys that fed these lobes. Only the outer margin of the lobes, bedrock highs beneath the lobes, and Alpine valleys at high elevations in the accumulation zone remained cold based. The Rhine glacier was thus polythermal. Sliding speed estimated with a linear sliding rule ranged from 20 to 100 m a−1 in the lobes, and 50 to 250 m a−1 in Alpine valleys. Velocity ratios (sliding to surface speeds) were > 80 % (lobes) and ~ 60 % (valleys). Basal shear stress was very low in the lobes (0.03–0.1 MPa), much higher in Alpine valleys (> 0.2 MPa). In these valleys, viscous strain heating was a dominant source of heat, particularly when shear rates in the ice increased due to flow constrictions, confluences, or flow past large bedrock obstacles, contributing locally up to several W m−2 but on average 0.03 to 0.2 W m−2. Basal friction acted as a heat source at the bed of ...