Quantifying erosion in a pre-Alpine catchment at high resolution with concentrations of cosmogenic ¹⁰Be, ²⁶Al, and ¹⁴C

Abstract. Quantifying erosion across spatial and temporal scales is essential for assessing different controlling mechanisms and their contribution to long-term sediment production. However, the episodic supply of material through landsliding complicates quantifying the impact of the individual erosional mechanisms at the catchment scale. To address this, we combine the results of geomorphic mapping with measurements of cosmogenic ¹⁰Be, ²⁶Al, and ¹⁴C concentrations in detrital quartz. The sediments were collected in a dense network of nested sub-catchments within the 12 km²-large Gürbe basin that is situated at the northern margin of the Central European Alps of Switzerland. The goal is to quantify the denudation rates, disentangle the contributions of the different erosional mechanisms (landsliding versus overland flow erosion) to the sedimentary budget of the study basin, and to trace the sedimentary material from source to sink. In the Gürbe basin, spatial erosion patterns derived from ¹⁰Be and ²⁶Al concentrations indicate two distinct zones: headwater zone with moderately steep hillslopes dominated by overland flow erosion, with high nuclide concentrations and low denudation rates (~ 0.1 mm/yr), and a steeper lower zone shaped by deep-seated landslides, where lower concentrations correspond to higher denudation rates (up to 0.3 mm/yr). In addition, ²⁶Al/¹⁰Be ratios in the upper zone align with the surface production ratio of these isotopes (6.75), which is consistent with sediment production through overland flow erosion. In the lower zone, higher ²⁶Al/¹⁰Be ratios of up to 8.8 point towards sediment contribution from greater depths, which characterises the landslide signal. The presence of a knickzone in the river channel at the border between the two zones points to the occurrence of a headward migrating erosional front and supports the interpretation that the basin is undergoing a long-term transient response to post-glacial topographic changes. In this context, erosion rates inferred from ¹⁰Be and ²⁶Al isotopes are consistent, suggesting a near-steady, possibly self-organised sediment production regime over the past several thousand years. In such a regime, individual and stochastically operating landslides are aggregate over time in a specific region of higher erosion with a higher average denudation rate. Although in-situ ¹⁴C measurements were also conducted, the resulting concentrations show a non-conclusive pattern.