Increased erosion rates on high-Alpine rockwalls evidenced by comparison of short-term (terrestrial LiDAR) and long-term (cosmogenic nuclides) approaches

Rockwall erosion due to rockfalls is one of the most efficient erosion processes at high elevations. It is, therefore, important to quantify this erosion to understand the long-term evolution of mountain topography. This is especially crucial since rockfall frequency is increasing in high-Alpine areas, such as in the Mont-Blanc massif (MBM), due to regional scale permafrost degradation (which occurs through thickening of the active layer, the subsurface layer freezing and thawing throughout the year), a consequence of climate warming and the multiplication of heat waves.

To better understand rockfalls as a permafrost-related process, we quantify the erosion rates at different time scales by i) a short-term (  ̴ ten-year scale) quantification of the dynamics of the rock walls based on the diachronic comparison of topographic measurements carried out by terrestrial laser scanning (LiDAR) and ii) a long-term quantification (10²-10⁴ year scale) based on the ¹⁰Be concentration of sediment sampled downglacier on medial moraines. Our analysis considered that once the rockfalls have occurred, clasts are transported within the ice stream and amalgamated by ice melt in the ablation zone, forming medial moraines. The ¹⁰Be concentration is linked to the rockwall erosion rate and the time needed to transport from the glacier equilibrium line to the sampling location.

Scanned rockwalls and rockwall sources vary in elevation, aspect, slope, and area, allowing us to assess whether these factors influence the measured ¹⁰Be concentration and erosion rates. We studied rockwalls located on the French side between 2800 m and 4200 m a.s.l. and between 2500 m and 4600 m a.s.l. on the Italian side. We collected 8 (Géant basin and Vallée Blanche, France) and 10 supraglacial samples (Brouillard and Frêney glaciers, Italy), respectively.  

Our results reveal substantial variations in ¹⁰Be concentrations. On the French side of the MBM, ¹⁰Be concentrations vary from 1.2 ± 0.2 to 6.7 ± 0.4 x 10⁴ atoms g^-1, while they range from 3.0 ± 0.2 to 92.0 ± 3.2 x 10⁴ atoms g^-1 on the Italian side. These results suggest that the long-term erosion rates vary between 0.8-1.7 and 0.1-0.3 mm.yr^-1, respectively. The short-term erosion rates for the French side are 4.3 mm.yr^-1 for 2005-2014 and 39.3 for the period of 2015-2022. On the Italian side, they are 0.8 mm.yr^-1 for 2005-2011 and 6.1 for 2011-2017.

Our results show spatial differences in erosion rates on both sides of the MBM. Short-term erosion rate is lower on the Italian side, and ¹⁰Be concentrations are higher, meaning that the rock walls are more stable in this area. However, on both sides of the MBM, erosion rates have increased significantly recently, with a further acceleration during the last decade. This suggests that high-altitude rockwalls, previously unaffected by global warming, are progressively entering a state of permafrost degradation.