Spatial and temporal variations in rockwall erosion rates around Pigne d’Arolla, Switzerland, derived from cosmogenic ¹⁰Be in medial moraines at five adjacent valley glaciers

Abstract. Rockwall erosion in high-alpine glacial environments varies both temporally and spatially. Where rockwalls flank glaciers, changes in debris supply and supraglacial cover will modify ice ablation. Yet, quantifying spatiotemporal patterns in erosion across deglaciating rockwalls is not trivial. At five adjacent valley glaciers around Pigne d’Arolla in Switzerland, we derived apparent rockwall erosion rates using ¹⁰Be cosmogenic nuclide concentrations ([¹⁰Be]) in medial moraine debris. Systematic downglacier-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 distinct spatial source rockwall morphology may express itself in medial moraine [¹⁰Be] records. Our dataset combines 24 new samples from medial moraines of Glacier du Brenay, Glacier de Cheilon, Glacier de Pièce, and Glacier de Tsijiore Nouve, with 15 published samples from Glacier d’Otemma. For each sample, we simulated the glacial debris transport using a simple debris particle trajectory model, to approximate the time of debris erosion and to correct the measured [¹⁰Be] 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 presumably reaches back to the end of the Little Ice Age (LIA) and suggests a systematic increase in rockwall erosion rates over the last ~200 years, the shorter records only cover the last ~100 years from the recent deglaciation period and indicate temporally more stable erosion rates. For the estimated time of debris erosion, ice cover changes across most source rockwalls were small, suggesting that our records are largely unaffected by the contribution of recently deglaciated bedrock of possibly different [¹⁰Be]; but admixture of subglacially derived debris cannot be excluded at every site. Comparing our sites suggests that apparent rockwall erosion rates are higher where rockwalls are steep and north-facing, indicating a potential slope and temperature control on rockwall erosion around Pigne d’Arolla.