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 for quantifying and understanding the impact of climate and its variations on the evolution of landscapes. Today, I'm going to present the results of three studies using in-situ 10Be as a tracer of denudation rates in the Var catchment (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 flows associated with the various phenomena (landslides, sediment remobilisation). The frequency of these events over geological and historical time scales and the way in which they may be induced by climate change are also major concerns, particularly for natural risk 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.

Concentrations of 10Be measured in river sediments are a powerful tool for tracing the evolution of sedimentary dynamics over 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 catchment area in the French Alps was hit by an extreme rainfall event linked to storm "Alex" (> 500 mm / 24h). This event caused flash floods in the Vésubie and Var valleys, mobilising a large volume of sediment and resulting in 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 catchment during the last glacial period, the LGM (26 - 19 ka), deglaciation (19 -11 ka) and the Holocene (11 ka - 0 ka). This sedimentary context makes it possible to study the impact of rapid climatic fluctuations on denudation rates in a mountainous glacial watershed on a time scale that is not influenced by tectonics.

The extreme rainfall event of October 2020 and our long-term 10Be data set 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 palaeo-10Be signals in sedimentary archives.