Non-linear forcing of climate on denudation in the Alps over the last 75 ka

Reconstruction of denudation rates through time is an important task to quantify and understand the impact of climate on landscape evolution. Cosmogenic nuclides have been widely used as a tool to infer denudation rates at the watershed scale from both river sediments and past stratigraphic records. Here, we analyze the in-situ ¹⁰Be cosmogenic concentration over the last 75 ka in sediments cores that were collected offshore the Var River (Western Mediterranean Sea).

We present 26 ¹⁰Be paleo denudation rates ranging from 0.15 ± 0.01 and 1.26 ± 0.16 mm yr⁻¹. At the exception of the LGM period, the ¹⁰Be paleo denudation rates are similar to these of today in the Var (0.24 ± 0.04 mm yr⁻¹). However, during the LGM, paleo denudation rates were 2 to 3 times higher than today, suggesting that glaciers may have played a role. To investigate this sharp increase in denudation rates, we use a mass balance approach to differentiate the glacial from the fluvial component of denudation rates. The resulting average glacial erosion rate during the LGM is 1.5 (+0.9 / -1.0) mm yr⁻¹, roughly four times above the value of 0.4 (+0.4 / -0.5) mm yr⁻¹ obtained during MIS 3-4 (29 - 71 ka) Our data suggest that climatic variations may only strongly affect denudation beyond a certain threshold, probably controlled by glacier dynamics, the duration of glacial advances, and temperature-driven processes such as frost cracking. Our study indicates that the denudation response to Quaternary glaciations is complex and nonlinear in glaciated areas.