Headwall erosion rates from cosmogenic 10Be in medial moraine debris of five adjacent Swiss valley glaciers

Rock walls in high-alpine glacial environments are becoming increasingly unstable due to climate warming. This instability increases the erosion of headwalls above glaciers modifying glacial surface debris cover and mass balance and, thus, affecting the response of glaciers to climate change. As debris is deposited on glaciers, it is passively transported downglacier forming medial moraines where two glaciers join.

We assess headwall erosion by systematic downglacier-debris sampling of medial moraines and by computing headwall erosion rates from their ¹⁰Be-cosmogenic nuclide concentrations. Around Pigne d’Arolla in Switzerland, we collected a total of 39 downglacier medial moraine debris samples from five adjacent glaciers. We explicitly chose medial moraines with source headwalls that vary in size, orientation and morphology, to investigate how different debris source area characteristics may express themselves in medial moraine cosmogenic nuclide concentrations. At the same time, the downglacier-debris sampling enables us to derive headwall erosion rate estimates through time, as medial moraine deposits tend to be older downglacier.

Preliminary results reveal systematic differences in ¹⁰Be concentrations for the studied glaciers. At Glacier d’Otemma, Glacier du Brenay, and Glacier de Cheilon ¹⁰Be concentrations average at 17x10³, 31x10³, and 4x10³ atoms g^-1, respectively. Downglacier ¹⁰Be concentrations at Glacier d’Otemma vary systematically and headwall erosion rates tend to increase towards the present. At both Glacier du Brenay and Glacier de Cheilon downglacier ¹⁰Be concentrations are more uniform, suggesting that headwall erosion rates did not evolve significantly through time. Results for Glacier de Tsijiore Nouve and Glacier de Pièce will follow soon. In addition, samples at Glacier d’Otemma were collected along two parallel medial moraines sourced by different but adjacent headwalls. Yet, their downglacier ¹⁰Be concentrations deviate and our analyses suggest that at Glacier d’Otemma both differences in headwall orientation and headwall deglaciation history may account for the deviation of the two medial moraine records. For all five glaciers, we currently explore how lithology, slope angles, exposition, deglaciation, and elevation vary between the debris source areas and how differences therein could result in the observed differences in ¹⁰Be concentrations.