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Abstract:
Where rockwalls flank glaciers, changes in debris supply and supraglacial cover will modify ice ablation. Yet, quantifying spatiotemporal supply patterns is not trivial. At five adjacent valley glaciers around Pigne d’Arolla in Switzerland (Glacier du Brenay, de Cheilon, d’Otemma, de Pièce, de Tsijiore Nouve), we obtained 39 apparent rockwall erosion rates using 10Be cosmogenic nuclide concentrations ([10Be]) in medial moraine debris. Systematic downglacier-profile-sampling of six medial moraines that receive debris from rockwalls with differing orientation, slope and deglaciation histories enabled us to assess rockwall erosion through time and to investigate how source rockwall morphology may express itself in medial moraine [10Be] records. For each debris sample, we modelled downglacier debris particle trajectories to approximate the time of debris transport/erosion and correct for post-depositional 10Be accumulation. Our derived apparent rockwall erosion rates range between ~0.6 and 10.0 mm yr-1. Whereas the longest downglacier [10Be] record suggests a systematic increase in rockwall erosion rates over the last ~200 years since the end of the Little Ice Age, the shorter [10Be] records only cover the last ~100 years from the recent deglaciation period and indicate temporally more stable rates. Since ice cover changes across most source rockwalls are relatively small, our records seem to be largely unaffected by the contribution of recently deglaciated bedrock of possibly different [10Be], but a possible impact by subglacially-derived debris cannot be excluded at every site. During deglaciation, apparent rockwall erosion rates are higher where rockwalls are steep and north-facing, suggesting a potential slope and temperature control.
