Post- glacial geomorphic response, erosion dynamics and sediment transfer in the Ariège catchment (Eastern Pyrenees, France)

Mountainous landscapes have been progressively shaped during the Quaternary under oscillating glacial-interglacial conditions. However, in the context of current climate change, quantifying alpine erosion dynamics has remained problematic because geomorphic processes operate at interrelated timescales. Thus, deciphering the interactions between climate change, glacier retreat, and sediment production in alpine catchments has proven challenging. In this context, we aim to further constrain the transient geomorphic response and catchment sediment transfer during glacial/interglacial oscillations, and especially during the transition period since the Last Glacial Maximum (LGM, ca. 20 ka), using terrestrial cosmogenic nuclides as proxy for erosion processes.

Here, we focus on the Ariège catchment (Eastern Pyrenees), a high-relief area crossing various lithologies that has been extensively glaciated and shaped by glacial processes during the Quaternary. We collected modern river sediments samples along the main Ariège river and its tributary basins, covering contrasted lithologies, topo-climatic settings and LGM glacial coverage. We complemented this dataset with Lateglacial sediment archives within the Ariège catchment together with fluvio-glacial terrace sediments in the downstream foreland area. Terrestrial cosmogenic nuclide (TCN) ¹⁰Be and ²⁶Al concentrations were measured on these samples with the overall objectives to (1) identify TCN concentration differences between modern and Lateglacial/fluvio-glacial deposits, (2) assess sediment transfer times and potential recycling (using ²⁶Al/¹⁰Be ratios), and finally (3) derive catchment-averaged denudation rates from TCN concentrations to investigate spatial erosion patterns and discuss the main controlling factors.

The ²⁶Al/¹⁰Be ratios calculated for modern river sediments are in majority between 6 and 6.75, consistent with surface production ratios. This suggests that most modern river sediments samples have a simple sediment transfer dynamics with limited effects of burial and recycling from glacial overdeepenings and low input from high elevation slowly eroding weathered surfaces. Measured ¹⁰Be concentrations in modern river sediments are 2 to 5 times higher than for Lateglacial and fluvio-glacial terrace sediments, showing a clear difference in the ¹⁰Be sediment signature during glacial and interglacial periods. This reveals that (1) glacial erosion has been effective enough to partially reset the TCN signals, and (2) the post-glacial period is marked by a re-adjustment of TCN concentrations towards an “interglacial” signal. Finally, TCN-derived catchment-averaged denudation rates reveal a high spatial variability within the Ariège catchment, both between tributary basins (from 33 to 294 mm/ka) and along the main Ariège stream (from 130 and 278 mm/ka). Our preliminary results point towards a topographic control on the modern erosion pattern, with a statistically-significant correlation between denudation rates and mean catchment slopes while other topo-climatic parameters appear less efficient in tuning the spatial distribution in denudation rates. We will discuss the potential influence of long-term orogeny, litho-tectonic configuration, and glacial inheritance on the modern slope distributions and overall topographic patterns in the Ariège catchment. Our work contributes to a better understanding of the sensitivity of alpine landscapes to climate forcing and associated changes in geomorphic processes over glacial-interglacial oscillations, using TCN approach for quantifying erosional processes, sediment production and transfer in mountainous environments.