Climatic significance of glacier retreat and rockglaciers re-assessed in the light of cosmogenic dating, southern French Alps

Category Geomorphology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Cossart, Etienne۱; Fort, Monique۲; Bourles, Didier۳; Braucher, Regis۳; Carcaillet, Julien۴
Holding Date 08 September 2008

Glacial and periglacial features in mountain area are good geomorphic indicators to better understand past and recent climate changes. In addition, Cosmic Ray Exposure (CRE) ages of exposure surface provide opportunities to chronologically constrain the various sequences of this evolution. We apply this method in the Upper Durance catchment (Southern French Alps), in the Clarée valley, where no absolute chronological benchmark was defined. We find evidence of the existence of a Late-Glacial valley glacier that was never reported so far, a fact that suggests the upper valleys became ice-free at the beginning of the Holocene, more precisely during the Holocene Climate Optimum. During the second half of the Holocene only two sequences of cirque glaciation are identified.
For the same period, we also find evidence of three main generations of rockglaciers, the genesis and growth of which probably occurred between the Subboreal and the Little Ice Age periods. We discuss the palaeo-climatic significance of glaciers and rockglaciers. If the evidence and disappearance of glaciers can clearly be related to global cooling and warming at the scale of a mountain range, deciphering the palaeoclimatic signal of rockglaciers is more complicated, because of their location and altitudes that are also strongly dependant on local factors such as aspect, debris sources and debris availability. Relying only on the altitudinal extent of rockglaciers without any chronological data or appreciation of their entire geomorphic context is therefore likely to be misleading in the reconstruction of past climatic environments.