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

Abstract

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.