| Summary: | International audience Rockfalls and rock avalanches are active processes in the Mont Blanc massif, with infrastructure and alpinists at risk. Thanks to a network of observers (hut keepers, mountain guides, alpinists) set up in 2007 current rockfalls are well surveyed and documented (Ravanel and Deline 2013). Rockfall frequency has been studied over the past 150 years by comparison of historical photographs (Ravanel and Deline 2008), showing that it strongly increased during the three last decades, likely due to permafrost degradation caused by the climate change. In order to understand the possible relationship between rockfall frequency and the warmest periods of the Lateglacial and the Holocene, we study the morphodynamics of some selected high-elevated (>3000 m a.s.l.) rockwalls of the massif on a long timescale.Since rockfall deposits in glacial areas are evacuated by the glaciers, our study focuses on the rockfall scars. 10Be TCN dating of a rockwall surface gives us the rock surface exposure age, interpreted as a rockfall age. Here we present a dating dataset of 80 samples carried out between 2006 and 2016 at nine high-elevated rockwalls in the Mont Blanc massif (Figure 1). The resulting ages vary from present (0.04 ± 0.02 ka) to far beyond the Last Glacial Maximum (c. 100 ka). Three clusters of exposure ages are correlated to i) the Holocene Warm Period, ii) the Roman Warm Period, and iii) the Little Ice Age and post-LIA. Ages of this last one are generally related to small rockfall volumes (< 15000 m3), considered as the normal erosion. A 4th cluster at 4.2-5.0 ka is not associated with any evident global climate period.Furthermore, a relationship between the colour of the Mont Blanc granite and its exposure age has been established: fresh rock surface is light grey (e.g. in recent rockfall scars) whereas weathered rock surface is in the range grey to orange/red: the redder a rock surface, the older its age (Böhlert et al, 2008). Reflectance spectroscopy is used to quantify the granite surface ...
|
|---|