Glacial erosion rates across the Alps derived from in situ 10Be in river sediments

Glaciers are strong agents of erosion and play a key role in the evolution of mountain ranges. In order to improve our understanding of the influence of glacial erosion dynamics on landscape evolution and mountain building, it is essential to quantitatively constrain glacial erosion rates across multiple topographic and climatic settings.

In situ cosmogenic ¹⁰Be concentrations measured in river sediments have been widely used over the last twenty years to infer denudation rates integrated at the catchment scale. This approach was mainly applied to fluvial settings because in this case, the ¹⁰Be concentration of detrital sediments is a simple function of denudation. In regions covered by glaciers, river sediments result from a mixture of material produced in the pure fluvial domain and sediments produced by glacier erosion. The ¹⁰Be concentration measured in such settings thus results from the mixture of these two sources. Here, we use a simple mass conservation approach to estimate pure glacial erosion rates from the ¹⁰Be concentration measured in watersheds combining glacial and fluvial domains. In practice, we first established an empirical power-law linking denudation rates to the mean slope of non-glaciated catchments. For each partially glaciated catchment, this law was used to constrain the pure fluvial ¹⁰Be end-member using slopes derived from a DEM. Finally, this input was used to compute the pure glacial erosion rate required to satisfy the ¹⁰Be concentration measured in rivers. This new approach was applied to 2 different datasets:

• Present-day glacier erosion in the Alps. We apply this approach to determine the erosion of modern glaciers across the entire Alps. We used previously published ¹⁰Be concentration measured in river sediments covering partially glaciated watersheds. The fluvial denudation power law was constrained from 148 fluvial – glacier free catchments. We then selected 11 watersheds with glaciers bigger than 5 km² and a glacial cover of at least 5% of their total surface. The so-obtained glacial erosion rates from these 11 watersheds range from 0.2 to 1.5 mm.yr^-1. Finally, we compare those values to satellite-derived glaciers' sliding velocity which is thought to be the main factor controlling glacial erosion rates.

 

• Paleo-erosion in the Var (Southern Alps) setting over the last 75 ka. We apply the same approach to the Var catchment (Southern French Alps) to estimate past glacial erosion rates over the last 75 ka (Mariotti et al., 2021). This basin has been deglaciated since the Holocene and ¹⁰Be modern denudation rates were estimated across 9 sub-basins (Mariotti et al., 2019) providing the required dataset to estimate the local fluvial denudation power law. ¹⁰Be concentrations were measured in two 75 ka sedimentary cores drilled in the Mediterranean Sea when the Var catchment was previously glaciated (Mariotti et al., 2021). Our findings show that during the LGM, the pure glacial erosion rates were 3 times higher (1.5 +/- 1 mm.yr^-1) than during MIS 3-4 (0.4 +/- 0.5 mm.yr^-1). This suggests a nonlinear forcing of climate on glacial erosion, mainly controlled by the interplay between glacier velocity, climate, and basin topography.