Impact of climatic forcings on landscape evolution: long-term (0-75 ka) and short-term (2016-2021) comparison of denudation dynamics in the Var basin (France)
Reconstructing the evolution of denudation rates over time is an important task for quantifying and understanding the impact of climate and its variations on landscape evolution. In-situ 10Be concentrations measured in river sediments are a powerful tool for tracing the evolution of denudation rates over long and short timescales.
Over long timescales (> 1 ka), the consequences of major climatic forcing, such as glaciations during the Quaternary period, on denudation rates are poorly constrained, with sometimes contradictory conclusions.
Over shorter timescales (few months to 1 ka), 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 remobilization). The frequency of these events over geological and historical timescales is a major concern, particularly for natural hazard management in the context of climate change.
The Var catchment (Alpes-Maritimes) was studied over two distinct time periods with different climatic settings to determine which climatic forcings have an impact on the sedimentary system.
- Long-term variations (0 - 75 ka):
We analyzed 21 samples from two well-dated sediment cores collected on a sedimentary ridge in the Mediterranean Sea. They provide a unique record for reconstructing the denudation history of the Var watershed during the last glacial period, the LGM (26 - 19 ka), deglaciation (19 -11 ka) and the Holocene (11 ka - 0 ka). This sedimentary record allows us to study the impact of rapid climatic fluctuations on denudation rates in a mountainous glacial catchment on a timescale that is not influenced by tectonic forcings.
- Short-term variations (2016 - 2021):
On October 2 and 3, 2020, the Var watershed in the French Alps was hit by an extreme rainfall event linked to storm “Alex” (> 500 mm / 24h). This event caused flash flooding in the Vésubie and Var valleys, mobilizing a large volume of sediment and resulting in a 10 km-long sediment plume at the Var outlet. To characterize 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.
The extreme rainfall event of October 2020 and our long-term 10Be dataset offer 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 characterize the 10Be geochemical signature of an extreme event.
