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 Amschwand1,3, Susan Ivy-Ochs1,2, Marcel Frehner1, Olivia Steinemann2, Marcus Christl2, 5 Christof Vockenhuber2 1Department of Earth Sciences, ETH Zurich, 8092, Zurich, Switzerland 2Laboratory of Ion Beam Physics, ETH Zurich, 8093, Zurich, Switzerland 3Now at: Department of Geosciences, University of Fribourg, 1700, Fribourg, Switzerland Correspondence to: Dominik Amschwand (
[email protected]) ORCID: 0000-0003-2179-1481 10 Abstract. We constrain the Holocene morphodynamic development of the active Bleis Marscha rock glacier (Err-Julier area, eastern Swiss Alps) with fifteen 15 cosmogenic nuclide exposure ages (10Be, 36Cl), 2003/2012 horizontal surface creep rate quantification from by orthophoto orientation correlationcorrelating two orthophotos from 2003 and 2012, and finite element modelling to separate the control exerted by topography and material composition on surface movement. and semi- quantitative ice-content estimates from finite-element modelling. The results suggest that the complex Bleis Marscha rock 15 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 20 decoupling from the insulating coarse-debris boulder mantle.