Climate change affects the stability and erosion of high‐alpine rock walls above glaciers (headwalls) that deliver debris to glacier surfaces. Since supraglacial debris in the ablation zone alters the melt behaviour of the underlying ice, the responses of debris‐covered glaciers and of headwalls to climate change may be coupled. In this study, we analyse the beryllium‐10 (10Be)‐cosmogenic nuclide concentration history of glacial headwalls delivering debris to the Glacier d'Otemma in Switzerland. By systematic downglacier‐profile‐sampling of two parallel medial moraines, we assess changes in headwall erosion through time for small, well‐defined debris source areas. We compute apparent headwall erosion rates from 10Be concentrations ([10Be]), measured in 15 amalgamated medial moraine debris samples. To estimate both the additional 10Be production during glacial debris transport and the age of our samples we combine our field‐based data with a simple model that simulates downglacier debris trajectories. Furthermore, we evaluate additional grain size fractions for eight samples to test for stochastic mass wasting effects on [10Be]. Our results indicate that [10Be] along the medial moraines vary systematically with time and consistently for different grain sizes. [10Be] are higher for older debris, closer to the glacier terminus, and lower for younger debris, closer to the glacier head. Computed apparent headwall erosion rates vary between ~0.6 and 10.8 mm yr−1, increasing over a maximum time span of ~200 years towards the present. As ice cover retreats, newly exposed headwall surfaces may become susceptible to enhanced weathering and erosion, expand to lower elevations, and contribute formerly shielded bedrock of likely different [10Be]. Hence, we suggest that recently lower [10Be] reflect the deglaciation of the debris source areas since the end of the Little Ice Age.

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<p>Debris-covered glaciers are fed from steep bedrock hillslopes that tower above the ice. These headwalls are eroded by rockfalls and rock avalanches, mobilizing fractured bedrock, which is subsequently deposited on the ice surface along the sides of valley glaciers and transported downglacier on and in the ice. Where glaciers join, marginal debris merges to form medial moraines. Due to the conveyor-belt-nature of glacier ablation zones, debris tends to be older downglacier and, for typical Alpine glaciers, single deposits may persist on the glacier surface for hundreds to a few thousand years.</p><p>Recent observations in high-alpine glacial environments suggest that rock walls are increasingly destabilized due to climate warming. An increase in headwall erosion and debris deposition onto glacier surfaces will modify glacial mass balances, as surface debris cover alters the rate at which underlying ice melts. Consequently, we expect that the response of debris-covered glaciers to climate change is likely also related to the response of headwalls to climate change.</p><p>In this context, we quantify headwall retreat rates by measuring the concentration of in situ-produced cosmogenic <sup>10</sup>Be in debris samples collected from a partly debris-covered Swiss valley glacier. By systematic downglacier-sampling of two parallel medial moraines, we aim to assess changes in headwall erosion through time for small and delineated source areas. Our results indicate that indeed, nuclide concentrations along the medial moraines vary with time: downglacier and further back in time deposits have higher nuclide concentrations, whereas upglacier and more recently deposits have lower concentrations. Currently, we explore possible processes which could account for <sup>10</sup>Be concentration changes through time, other than changes in erosion rates. These include the sensitivity of <sup>10</sup>Be concentrations to supraglacial transport time and to temporal and spatial changes in nuclide production rates on the deglaciating headwalls. First analyses reveal, however, that neither the additional accumulation of <sup>10</sup>Be during transport nor changes in source area production rates associated with the uncovering of formerly ice covered headwall parts alone can account for the observed trend.</p>

<p>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.</p><p>We assess headwall erosion by systematic downglacier-debris sampling of medial moraines and by computing headwall erosion rates from their <sup>10</sup>Be-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.</p><p>Preliminary results reveal systematic differences in <sup>10</sup>Be concentrations for the studied glaciers. At Glacier d’Otemma, Glacier du Brenay, and Glacier de Cheilon <sup>10</sup>Be concentrations average at 17x10<sup>3</sup>, 31x10<sup>3</sup>, and 4x10<sup>3</sup> atoms g<sup>-1</sup>, respectively. Downglacier <sup>10</sup>Be 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 <sup>10</sup>Be 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 <sup>10</sup>Be 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 <sup>10</sup>Be concentrations.</p>

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