Control of valley width on the flank erosion rate: measure of erosion rates of Andean and French fluvial valleys by using 10-Be and 26-Al

Any keen observer has noticed that valleys show a large variability in their shapes, explained by the incision and widening processes. However, valley widening processes and rate are still poorly documented while valley evolution has a key role in geomorphological processes (contribution to the formation of abrasion terraces and establishment of ecosystems) and global geochemical cycles (increase of carbon storage in wide valley and buffering sediment fluxes transported to the oceans). Given these issues, it is becoming truly necessary to better understand valley widening rate and its controls.

For that, we focused on several river valleys (in the Arequipa Province, Peru, and in the plateau of Valensole, France) and we have used and further developed the approach tested in northern Chile by Zavala et al. (2021). We collected samples from valley flanks to measure the millennial erosion rates, by using in-situ produced Beryllium-10 and Aluminium-26, and analysed 10-Be and 26-Al concentrations to estimate the local valley flank erosion rate.

We also extracted different factors that may control widening rate (valley width, slope of flanks and valley floors, incision and drainage area, etc) in order to compare these factors to 10-Be and 26-Al concentrations. Our preliminary results show comparable 10-Be concentrations along a single stretch of valley, except for several outliers, for different valleys in the Andes and France, indicating some robustness in the sampling method. These results are promising and will provide new answers, by integrating more metadata, about what controls the widening rate of valleys and how.