A significant doubling of rockfall rates since the Little Ice Age in the Mont-Blanc massif, inferred from 10Be concentrations and rockfall inventories

Abstract

By combining cosmogenic nuclide data and rockfall inventories, we have employed a rigorous methodology to focus on long-term erosion trends and the increase in rockfall in the Mont-Blanc massif (European Alps) over the last century. To do this, we used mathematical formulations based on power law integration, which enabled us to identify the complex links between rockfall distribution and erosion rates. Our approach was applied to the Mer de Glace basin (Mont-Blanc massif), where we combined analyses of ¹⁰Be concentration in the supraglacial load (based on 8 samples) with Terrestrial Laser Scanning (TLS) data capturing 123 rockfalls ranging from 1 to 566 m³, as well as direct observations of 71 rockfalls ranging from 100 to 20,000 m³. Within the overlapping volume range of both inventories, power law fitting yields a common exponent (b-value) of 0.75 ± 0.18. However, the TLS-derived rockfall rate (ā in m^-2.yr^-1) is approximately 5 times higher than that derived from the observation-based inventory. This difference is probably linked to the current intense permafrost degradation affecting scanned rockwalls at altitudes below 3800 m a.s.l. The 20,000 m³ rockfall documented by the network of observers has a statistical return time estimated at <6 years, which suggests that larger or more significant rockfalls will occur in the future. Based on a two-segment power law, the erosion rate is estimated at > 4.1 mm.yr^-1 for the period 2006–2011.

According to our study of glacial dynamics, the supraglacial clasts sampled aggregate ̴800 rockfalls greater than 1 m³ that occurred diachronically between 1845 and 1987 but whose cumulative total corresponds to <7 years of present rockwall erosion rate in the upper Mer de Glace basin. The mean ¹⁰Be concentration of the 8 supraglacial samples is 2.7 ± 1.3 10⁴ at.g^-1 and was obtained when exposing rock faces subjected to erosion of <1.2 ± 1 mm.yr^-1. The erosion rate would, therefore, have significantly increased between the Little Ice Age (maximum 2.2 mm.yr^-1 from ¹⁰Be result) and the beginning of the 21st century (minimum 4.1 mm.yr^-1 for 2003–2011 surveys). These erosion rates do not consider past volume rockfalls greater than those observed recently and are minimal erosion rate estimates. Nevertheless, they highlight the increase in mass movement hazards linked to global warming via permafrost degradation in high-altitude rockwalls.