Subglacial hydrology from high-resolution ice-flow simulations of the Rhine Glacier during the Last Glacial Maximum: a proxy for glacial erosion

Subglacial hydrology from high-resolution ice-flow simulations of the Rhine Glacier during the Last Glacial Maximum: a proxy for glacial erosion Denis Cohen1,2 , Guillaume Jouvet3,4 , Thomas Zwinger5 , Angela Landgraf6 , and Urs H. Fischer6 1 Department of Earth and Environmental Science, New Mexico Tech, Socorro, New Mexico, USA LLC, Orlando, Florida, USA 3 Department of Geography, University of Zurich, Zurich, Switzerland 4 Institute of Earth Surface Dynamics, University of Lausanne, Lausanne, Switzerland 5 CSC – IT Center for Science Ltd., Espoo, Finland 6 Nagra, Wettingen, Switzerland 2 CoSci Correspondence: Denis Cohen () Relevant dates: Received: 7 November 2022 – Revised: 11 April 2023 – Accepted: 3 July 2023 – Published: 21 August 2023 How to cite: Cohen, D., Jouvet, G., Zwinger, T., Landgraf, A., and Fischer, U. H.: Subglacial hydrology from high-resolution ice-flow simulations of the Rhine Glacier during the Last Glacial Maximum: a proxy for glacial erosion, E&G Quaternary Sci. J., 72, 189–201, https://doi.org/10.5194/egqsj-72-189-2023, 2023. Abstract: At the Last Glacial Maximum (LGM), the Rhine Glacier complex (Rhine and Linth glaciers) formed large piedmont lobes extending north into the Swiss and German Alpine forelands. Numerous overdeepened valleys there were formed by repeated glaciations. A characteristic of these overdeepened valleys is their location close to the LGM ice margin, away from the Alps. Numerical models of ice flow of the Rhine Glacier indicate a poor fit between the sliding distance, a proxy for glacial erosion, and the location of these overdeepenings. Calculations of the hydraulic potential based on the computed time-dependent ice surface elevations of the Rhine Glacier lobe obtained from a highresolution thermo-mechanically coupled Stokes flow model are used to estimate the location of subglacial water drainage routes. Results indicate that the subglacial water discharge is high and focused along glacial valleys and overdeepenings when water pressure is equal to the ice overburden pressure. These conditions are necessary for subglacial water to remove basal sediments, expose fresh bedrock, and favor further erosion by quarrying and abrasion. Knowledge of the location of paleo-subglacial water drainage routes may be useful to understand patterns of subglacial erosion beneath paleo-ice masses that do not otherwise relate to the sliding of ice. Comparison of the erosion pattern from subglacial meltwater with those from quarrying and abrasion shows the importance of subglacial water flow in the formation of distal overdeepenings in the Swiss lowlands. Kurzfassung: Während des letzteiszeitlichen Maximums (LGM) kam es zur Bildung von grossen Vorlandloben des Rheingletschersystems (Rhein- und Linthgletscher), die sich nordwärts in das Schweizer und deutsche Alpenvorland erstreckten. Durch wiederholte Vergletscherungen wurden dort zahlreiche übertiefte Täler ausgeschürft. Ein Merkmal dieser übertieften Tälern ist deren Lage in der Nähe des LGM- Published by Copernicus Publications on behalf of the Deutsche Quartärvereinigung (DEUQUA) e.V. Research article E&G Quaternary Sci. J., 72, 189–201, 2023 https://doi.org/10.5194/egqsj-72-189-2023 © Author(s) 2023. This work is distributed under the Creative Commons Attribution 4.0 License. 190 D. Cohen et al.: Subglacial hydrology of the Rhine Glacier at the LGM Eisrands weit weg von den Alpen. Jedoch zeigen numerische Modellierungen des Eisfliessens des Rheingletschers eine schlechte Übereinstimmung der Gleitdistanz, Proxyindikator für glaziale Erosion, mit der Lage dieser Übertiefungen. Deshalb werden Berechnungen des hydraulischen Potenzials basierend auf zeitabhängigen Höhen der Eisoberfläche der Rheingletscherlobe, welche von einem hoch aufgelösten thermo-mechanisch gekoppelten Modell für Stoke’sches Fliessen resultieren, benutzt, um die Lage der Entwässerungsrouten unter dem Eis abzuschätzen. Die Resultate deuten darauf hin, dass der subglaziale Wasserabfluss gross ist und entlang glazialen Tälern und Übertiefungen geführt wird, wenn der Wasserdruck dem Eisüberlagerungsdruck entspricht. Dies sind notwendige Bedingungen, unter denen basale Sedimente wegtransportiert werden und frischer Fels freigelegt wird, um weitere glaziale Erosion zu begünstigen. Somit ist die Kenntnis der Lage von subglazialen PaläoEntwässerungsrouten nützlich, um die Erosionsmuster unter Paläo-Gletschern zu verstehen, die nicht mit der Gleitbewegung des Eises in Verbindung gebracht werden können. Ein Vergleich der durch subglaziale Schmelzwässer erzeugten Erosionsmuster mit jenen, die durch direkte Gletschererosion entstanden sind, zeigt die Wichtigkeit der subglazialen Schmelzwasserflüsse für die Entstehung von Übertiefungen im alpenfernen Schweizer Vorland auf. 1 Introduction At the Last Glacial Maximum (LGM), the Rhine Glacier complex (Cohen et al., 2018), combining the Rhine and Linth glaciers, descended well into the Swiss and German Alpine forelands, forming two large piedmont lobes. The Rhine Glacier lobe covered present-day Lake Constance and land to the north in southern Germany, and the Linth Glacier lobe advanced beyond the city of Zurich, Switzerland. Repeated glaciations since the Middle–Late Pleistocene (Preusser et al., 2011; Ellwanger et al., 2011) have carved numerous landforms in the Alps such as deep alpine valleys with prominent horns and ridges. In the forelands, the passage of glaciers left numerous imprints on the landscape such as terminal moraines, outwash deposits, and erratics (e.g., Schlüchter, 1988, 2004; Keller and Krayss, 2005a; Preusser et al., 2007; Beckenbach et al., 2014; Gaar et al., 2019); drumlin fields (Kamleitner, 2022); tunnel valleys (Reber and Schlunegger, 2016); and overdeepened valleys now filled with sediments such as the Thur, the Glatt, and the Aare valleys (e.g., Preusser et al., 2011; Dehnert et al., 2012; Dürst Stucki and Schlunegger, 2013) or with water such as Lake Constance and Lake Zurich. A surprising characteristic of these overdeepenings along these valleys is their proximity to the terminal position of past glacial maxima in a region where one would expect that the erosive power of ice, often associated with the rate of sliding at the base (e.g., Hallet, 1981; Humphrey and Raymond, 1994; MacGregor et al., 2000; Koppes et al., 2015; Herman et al., 2015), would be smaller in comparison to that of large alpine valleys where ice fluxes and sliding speeds are higher. The existence of these overdeepenings in a distal-foreland setting thus remains puzzling. Previous models of ice flow of the Rhine Glacier (Cohen and Jouvet, 2017; Haeberli et al., 2020; Fischer et al., 2021; Seguinot and Delaney, 2021) indicate that the slid- E&G Quaternary Sci. J., 72, 189–201, 2023 ing distance, a proxy for glacial erosion based on the timeintegrated sliding speed, or a power of it, during the advance and retreat of the Rhine Glacier lobe into the foreland, does n (...truncated)