Postglacial erosion: new insights from the Mont Blanc massif

Assessing the impact of Quaternary glaciation at the Earth’s surface implies understanding of the long-term evolution of alpine landscapes. In particular, it requires simultaneous quantification of the impact of climate variability on past glacier fluctuations and on bedrock erosion.

Since the Last Glacial Maximum (LGM), about 20 kyr ago, Alpine glaciers retreated rapidly from lowland regions and thinned in their high-elevation source areas. This transition created widespread bare-bedrock surfaces that could then erode by a combination of large-scale debuttressing or local frost cracking and weathering. During this period, paleo-glacier reconstructions are limited because they often lack precise temporal constraint.

In this work, a new approach is used for evaluating post-glacial erosion of "roche moutonnees" over timescales from 10^1 to 10^4 years by combining in terrestrial cosmogenic nuclide (TCN) dating with 10Be and optically stimulated luminescence (OSL) surface exposure dating.

This approach is applied on nine bedrock surfaces sampled over two elevation profiles located on the flanks of the Mer de Glace. My results reveal bedrock surface erosion rates with a clear anti-correlation between erosion rate and elevation. The observed spatial variability in erosion rates and surprisingly high rates might reflect morphometric (elevation and surface slope) and climate (temperature and snow cover) controls. Furthermore, and more importantly, the derived erosion rates can be used to correct the timing of deglaciation based on TCN dating, potentially suggesting very rapid ice thinning post-LGM