Tracing source-to-sink sedimentary signals across scales in the Alpine Rhine Basin (Switzerland)

Mountain landscapes show strong spatial variability in sediment production, with long-term erosion often being concentrated in specific areas. This local variability complicates disentangling the relative contribution of different sediment sources and understanding how their signals evolve downstream through dilution, temporary storage, and deposition. Here, we address these questions by characterizing sediment sources and tracing their sedimentary signals along the river network of the 4,300 km² Alpine Rhine basin (Switzerland).

We use concentrations of in-situ cosmogenic ¹⁰Be from riverine quartz collected at 75 sites - and paired ¹⁰Be-²⁶Al data from 45 of them - to quantify erosion rates and identify the relative importance of landsliding versus overland flow erosion on the generation of sediment. We complement these data with information on the bulk geochemical compositions from the same sand samples. We synthesize the geochemical and cosmogenic dataset into mixing models with the goal to identify sediment sources and quantify their relative contributions to the mixed downstream signal.

The concentrations of ¹⁰Be show that erosion rates range widely from ~0.3 to 2 mm yr⁻¹. In addition, paired cosmogenic nuclides indicate negligible burial and highly efficient sediment evacuation across all spatial scales. Although they additionally show evidence for localized input of material from landslides, the majority of paired cosmogenic nuclide samples reveal that sediment generation through overland flow erosion has been the most important mechanism. Finally, the cosmogenic nuclide concentrations in combination with the geochemistry information reveals that sedimentary signals are generated in basins at small scales (<200 km²). Modelling of sedimentary pathways reveals that the signals are generally well mixed for basins >200 km², where the riverine material reflects mixtures of multiple source signals generated at smaller scales. Yet we also find that locally generated signals can disproportionately influence downstream records. This is particularly the case where stochastic mass-wasting events episodically overprint basin-scale signals through high-magnitude sediment inputs. In contrast, signals generated through overland flow erosion become well mixed for basins >200 km², and a further differentiation between potential source signals is no longer possible.