Special seminar: Apolline MARIOTTI (GFZ - German Research Center for Geosciences in Potsdam (Germany))

Summary Reconstructing the evolution of denudation rates over time is an important task to quantify and understand the impact of climate and its variations on landscape evolution. Today, I will present the results of three studies using in-situ 10Be as a tracer of denudation rates in the Var watershed (Alpes-Maritimes).

In the short term, the impact of extreme weather events on the evolution of landscapes and sediment budgets is difficult to constrain. We therefore need quantitative estimates of the geomorphological changes and sediment fluxes associated with the different events (landslides, sediment remobilisation). The frequency of these events on geological and historical time scales and how they may be induced by climate change are also major concerns, especially for natural hazard management.

In the long term, the consequences of major climatic variations, such as the Quaternary glaciations on denudation rates, are also poorly constrained with sometimes contradictory conclusions.

10Be concentrations measured in river sediments are a powerful tool for tracing the evolution of sediment dynamics in the short and long term.

We define two time periods with different climate parameters to determine which climate forcings impact the sedimentary system.

• Short-term changes (2016 - 2021) :
  • On 2 and 3 October 2020, the Var river basin in the French Alps was hit by an extreme rainfall event linked to the storm "Alex" (> 500 mm / 24h). This event caused flash floods in the Vésubie and Var valleys, mobilising a large volume of sediment and causing a 10 km long sediment plume at the Var outlet. In order to characterise the response of the Var system to the "Alex" event, we compare the 10Be concentrations of samples taken in 2016, 2017 and 2018 with the 10Be concentrations of samples taken at the same locations after the 2020 storm at +7 days, +21 days, +4 months and +7 months.
• Long-term changes (0 - 75 ka) :
  • 21 samples from two high-resolution dated sediment cores collected on the Var sedimentary rift in the Mediterranean. They provide a unique record for reconstructing the denudation history of the Var watershed during the last glacial period, the LGM (26 - 19 ka), the deglaciation (19 -11 ka) and the Holocene (11 ka - 0 ka). This sedimentary context allows us to study the impact of rapid climatic fluctuations on denudation rates in a mountainous glacial catchment on a time scale that is not influenced by tectonics.

The October 2020 extreme rain event and our long-term 10Be dataset provide a unique opportunity to assess the sensitivity of a sedimentary system and its ability to relay extreme events in a source-sink system. It is also a rare opportunity to characterise the 10Be geochemical signature of an extreme event. This step is also important for interpreting paleo-10Be signals in the sedimentary record.