Extreme Mediterranean rainfall impact on sedimentary routing systems: what can we learn from Storm Alex using in situ detrital 10Be?

Understanding how extreme meteorological events influence sediment transport is critical for predicting landscape evolution under a changing climate. Detrital cosmogenic 10 Be can provide insights into sediment dynamics following extreme rainfall, but high-resolution datasets tracking 10 Be variations before and after a storm, alongside long-term records, remain rare. The Var catchment (French Southern Alps) presents a unique case study, as its 10 Be signal was well-documented before the October 2020 Storm Alex (>500 mm of rainfall/24 h), which triggered flash floods, mobilized large sediment volumes, and formed a 10 km-long sediment plume in the Mediterranean Sea. We compare 10 Be concentrations in river sediments collected pre-storm (2016–2018), and at +7 days, +21 days, +4 months, and +7 months post-storm. We also use a historical offshore sample and contextualize these results with a 75 ka-long 10 Be record from deep-sea sediment cores. At the Var outlet, 10 Be concentrations initially increased by ∼25 % at +7 and +21 days, attributed to the mobilization of 10 Be-rich sediments from the upstream Var and Tinée sub-catchments. Concentrations returned to pre-storm levels within four months, primarily due to dilution with 10 Be-poor sediments from the Vésubie sub-catchment fluvioglacial terraces. While short-term 10 Be fluctuations at the Var outlet reflect complex sediment sourcing, our comparison with the 0–75 ka record confirms that major glaciation events and potential anthropic influences remain distinguishable, demonstrating that 10 Be is a robust proxy of denudation changes, even when extreme events are involved. • Extreme rainfall reshaped sediment transport altering 10 Be concentration in river sediments. • Storm-induced floods temporarily raised in situ 10 Be by ∼25 % at the Var outlet. • Post-storm 10 Be trends vary, with sediment sources driving spatially distinct responses. • High-res spatial, temporal sampling critical for constraining storm impact on 10 Be