- 1University Bern, Institute of Geological Sciences, Bern, Switzerland (chantal.schmidt@unibe.ch)
- 2Swiss Federal Research Institute WSL, Birmensdorf, Switzerland
- 3ETH Zurich, Laboratory of Ion Beam Physics, Zurich, Switzerland
Quantifying erosion in a catchment across different spatial and time scales is key to understand landslide hazards and the role in long-term sediment generation. In this context, disentangling the contributions of localized landslides to catchment-wide erosion remains challenging due to their stochastic nature and the occurrence of sediment storage. To address this, we measured cosmogenic 10Be, 26Al and 14C concentrations in detrital quartz across a dense network of nested sub-catchments to quantify denudation rates, assess sediment production variability, and trace the source-to-sink cascade within a 12 km2 basin.
The study area, the Gürbe catchment, is located at the northern margin of the Swiss Alps and comprises two distinct geomorphological zones. The upper zone, (c. 1,800–1,200 m a.s.l.), is characterized by steeply dipping Mesozoic limestone cliffs transitioning into Mesozoic-Cenozoic Flysch hills overlain by till. Mapping indicates that sediment production here is dominated by overland flow and channel erosion, with minimal connectivity between hillslopes and channels. In contrast, the lower zone, starting at an elevation of 1,200 m a.s.l. and extending to the Gürbe fan at c. 800 m a.s.l., is underlain by Flysch bedrock, partially mantled by till and interspersed with Neogene Molasse formations. The boundary between the upper and lower zone is marked by a glacially conditioned knickzone, indicating the onset of intensive channel incision. Mapping shows that this lower zone is characterized by a complex topography with pronounced scarps and depressions indicative of deep-seated landslides, some of which are directly coupled to the Gürbe trunk channel, while others supply material via tributary excavation.
Cosmogenic nuclide concentrations reveal distinct patterns. In the upper zone, 10Be and 26Al concentrations are high, yielding denudation rates of c. 0.1 mm/yr. However, concentrations are lowest in the lower zone tributaries leading to a concentration decrease downstream along the Gürbe trunk channel.Accordingly, 10Be and 26Al-based denudation rates calculated for the tributaries in the lower zone are significantly higher, reaching values up to 0.3 mm/yr.In addition, 26Al/10Be ratios in the upper zone align with the surface production ratio 6.75, consistent with sediment production through overland flow erosion. Contrarily, in the tributary material, 26Al/10Be ratios are up to 8.8, suggesting that a significant proportion of this sediment originates from deep-seated landslides. The 14C derived denudation rates are two to three times higher than the 10Be derived denudation rates ranging from 0.2 mm/yr in the upper zone to 1 mm/yr in the most active tributary of the lower zone. We interpret the 14C data as a combined effect of sediment storage and subsequent stochastic, unpredictable and rapid release of substantial amounts of deep material into the system, leading to apparent 14C-based erosion rates that are much higher than the long-term averages measured with in-situ 10Be.
In summary, this study demonstrates that by combining field-based mapping with the analysis of multiple cosmogenic nuclides, it is possible to (i) identify the origin of the sediment, (ii) determine the corresponding mechanisms of sediment generation, and (iii) estimate the time scale for sediment transfer across a geomorphologically diverse catchment.
How to cite: Schmidt, C., Mair, D., Schlunegger, F., McArdell, B., Christl, M., Haghipour, N., and Akçar, N.: From Overland Flow to Landslides: Deciphering Sediment Flux and Erosion Histories with Cosmogenic 10Be, 26Al, and 14C, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19414, https://doi.org/10.5194/egusphere-egu25-19414, 2025.