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Preprints
https://doi.org/10.5194/tc-2020-209
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/tc-2020-209
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  10 Aug 2020

10 Aug 2020

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This preprint is currently under review for the journal TC.

Deciphering the evolution of the Bleis Marscha rock glacier (Val d'Err, eastern Switzerland) with cosmogenic nuclide exposure dating, aerial image correlation, and finite-element modelling

Dominik Amschwand1,a, Susan Ivy-Ochs1,2, Marcel Frehner1, Olivia Steinemann2, Marcus Christl2, and Christof Vockenhuber2 Dominik Amschwand et al.
  • 1Department of Earth Sciences, ETH Zurich, 8092, Zurich, Switzerland
  • 2Laboratory of Ion Beam Physics, ETH Zurich, 8093, Zurich, Switzerland
  • anow at: Department of Geosciences, University of Fribourg, 1700, Fribourg, Switzerland

Abstract. We constrain the Holocene morphodynamic development of the Bleis Marscha rock glacier (Err-Julier area, eastern Swiss Alps) with fifteen cosmogenic nuclide exposure ages (10Be, 36Cl), 2003/2012 horizontal surface creep rate quantification from orthophoto orientation correlation, and semi-quantitative ice-content estimates from finite-element modelling. The results suggest that the complex Bleis Marscha rock glacier formed during two activity phases, one in the early Holocene and one in the late Holocene, separated by a mid-Holocene period of inactivation. The now transitional-inactive low-elevation lobes (first generation) formed after the retreat of the Egesen cirque glacier in a pulse-like manner at 11.5–9.0 ka. Rock-glacier viscosities inverted with the finite-element model hint at ground ice in these lobes which is possibly as old as its early-Holocene debris cover. In contrast to the debris-conditioned rapid emplacement, the thermally controlled permafrost degradation is still ongoing, likely attenuated by thermal decoupling from the insulating coarse-debris boulder mantle. Nuclide loss from boulder erosion, affecting the nuclide inventory of boulders independently, led to a heterogeneous exposure age distribution on the transitional-inactive lobes. Exposure ages on such disturbed lobes record time elapsed since inactivation and are interpreted as (minimum) stabilisation ages. The inception of the active high-elevation lobes (second generation) at 2.8 ka is related to the late-Holocene cooling recorded at numerous sites across the Alps. Precise exposure ages of the last 1.2 ka correlate with down-stream distance and yield a long-term average surface speed coincident with 2003/2012 measurements. These long-term consistent surface creep rates indicate stable permafrost conditions and continuous rock-glacier growth despite the intermittent late-Holocene glacier cover of the Bleis Marscha cirque. The exposure ages on active, undisturbed lobes record time elapsed since boulder emergence at the rock-glacier root and are interpreted as travel time estimates. This work contributes to deciphering the past to quasi-present climate sensitivity of rock glaciers.

Dominik Amschwand et al.

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Dominik Amschwand et al.

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Short summary
We reconstruct the Holocene history of the Bleis Marscha rock glacier (eastern Swiss Alps) by determining the surface residence time of boulders (exposure to cosmic rays). We find that this stream of frozen debris has developed in two phases over the last 10'000 years with a long inactive phase in between, ultimately controlled by the regional climate history. This work adds to our understanding on how these permafrost landforms react to the man-made climate warming.
We reconstruct the Holocene history of the Bleis Marscha rock glacier (eastern Swiss Alps) by...
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