MR017-0007
Testing the temperature dependence of high-alpine erosion with in situ cosmogenic 14C-10Be-3He
Wednesday, 16 December 2020
Poster
Donovan Dennis1, Dirk Scherler1, Samuel Niedermann2, Kristina Hippe3, Hella Wittmann4, Ludovic Ravanel5, Marissa M Tremblay6, Benny Guralnik7 and Maarten Lupker8, (1)GFZ German Research Centre for Geosciences, Earth Surface Geochemistry, Potsdam, Germany, (2)Deutsches GeoForschungsZentrum GFZ, Potsdam, Germany, (3)ETH Zürich, Zürich, Switzerland, (4)Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Potsdam, Germany, (5)EDYTEM Environnements, Dynamiques et Territoires de la Montagne, Le Bourget du Lac, France, (6)Purdue University, Department of Earth, Atmospheric, and Planetary Sciences, West Lafayette, IN, United States, (7)Technical University of Denmark, Department of Physics, Kgs. Lyngby, Denmark, (8)ETH Zurich, Geological Institute, Zurich, Switzerland
Abstract:
The erosion of alpine permafrost hillslopes depends not only on rates of frost weathering and accumulated rock damage, but additionally on the removal of the frost-damaged material from the bedrock surface. In the Mont Blanc massif, steep bedrock faces with exposure ages sometimes much older than 50,000 years sit in close proximity to actively-eroding rockwalls. This finding suggests a more complex relationship between temperature and erosion rates than encompassed by the proposed “frost-cracking window,” and likely reflects the influence of stochastic bedrock failures on long-term erosion rates. Rockfalls and rock avalanches, despite being rare, contribute a non-trivial proportion of the total sediment budget in many alpine permafrost regions, in addition to the contribution from background “steady-state” erosion. Here, we develop a methodology based on triple cosmogenic nuclides of
in situ 3He -
10Be-
14C that tests the temperature-dependence of high-alpine erosion while taking into account erosional stochasticity.
From cosmogenic 10Be concentrations of amalgamated samples collected on the Aiguille du Midi (3842 m a.s.l.) in the Mont Blanc massif, we find an order of magnitude difference in erosion rate across the peak’s surface. Our measured erosion rates, ranging between 0.02 mm yr-1 and 0.5 mm yr-1, correlate neither with modern temperature measurements from borehole thermistors, nor with our current estimates of bedrock cosmogenic 3He-derived paleotemperatures. The corresponding cosmogenic 14C/10Be ratios (between 1.70 and 4.0) for these erosion rates indicate that our measurements are not strongly biased by recent rockfall events. Our current results therefore suggest that on geomorphic timescales, bedrock hillslope erosion rates are not dictated by rates of ambient frost-cracking alone, but rather by a combination of frost-cracking and permafrost thaw-induced rockfalls. These insights are relevant both for short-term monitoring of the alpine cryosphere and associated geohazards under a warming climate, as well as for studies of the various proposed buzzsaws operating on glacial-interglacial timescales.
