Early Holocene rock glacier stabilisation at col du Lautaret (French Alps): Palaeoclimatic implications
Résumé
Rock glaciers, usually preserved in periglacial environments of cold-climate high-relief regions, are useful (palaeo)climate indicators. In the European Alps, numerous studies have focused on describing these geomor-phological features. However, very few have attempted directly dating them. This lack of measurement is particularly evident in the French Alps. To fill this gap, we performed Cosmic Ray Exposure (CRE) dating through beryllium-10 (Be-10) on 10 granitic rock samples collected from two relict rock glaciers, hereafter referred as RG1 and RG2, located at col du Lautaret in the French Alps. This first CRE dating of rock glaciers in the French Alps yielded mean ages of 11.8 +/- 0.5 kyrs and 11.6 +/- 0.6 kyrs for RG1 and RG2, respectively. While RG2 is clearly a talus-derived rock glacier, the origin of RG1 is less clear. It could potentially be talus-derived, glacier-derived or of mixed origin. The Be-10 ages indicate that both rock glaciers became inactive during the transition between the Younger Dryas and the onset of the Holocene, consistently with other studies in the Northern Hemisphere. Considering surface velocity measurements carried out since the 1980s on neighbouring rock glaciers, we hypothesise that the rock glaciers were formed either during the Younger Dryas or slightly earlier, between the Heinrich Stadial 1 and the Younger Dryas. During this period, we estimated temperatures similar to 3.6 degrees C lower than the 1980s and precipitation up to 30% lower than at present.