Abstract. We constrain the Holocene development of the active Bleis Marscha rock glacier (Err–Julier area, eastern Swiss Alps) with 15 cosmogenic nuclide exposure ages (10Be, 36Cl), horizontal surface creep rate quantification by correlating two orthophotos from 2003 and 2012, and finite element modeling. We used the latter to separate the control on surface movement exerted by topography and material properties. Bleis Marscha is a stack of three overriding lobes whose formation phases are separated by time gaps expressed morphologically as over-steepened terrain steps and kinematically as a sharp downslope decrease in surface movement. The three discrete formation phases appear to be correlated to major Holocene climate shifts: Early Holocene low-elevation lobes (∼8.9–8.0 ka, after the Younger Dryas), Middle Holocene lobe (∼5.2–4.8 ka, after the Middle Holocene warm period), and Late Holocene high-elevation lobes (active since ∼2.8 ka, intermittently coexisting with oscillating Bleis Marscha cirque glacierets). The formation phases appear to be controlled in the source area by the climate-sensitive accumulation of an ice-debris mixture in proportions susceptible to rock glacier creep. The ongoing cohesive movement of the older generations requires ice at a depth which is possibly as old as its Early–Middle Holocene debris mantle. Permafrost degradation is attenuated by “thermal filtering” of the coarse debris boulder mantle and implies that the dynamics of the Bleis Marscha lobes that once formed persisted over millennia are less sensitive to climate. The cosmogenic radionuclide inventories of boulders on a moving rock glacier ideally record time since deposition on the rock glacier root but are stochastically altered by boulder instabilities and erosional processes. This work contributes to deciphering the long-term development and the past to quasi-present climate sensitivity of rock glaciers.

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.

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