Post‐Last Glacial Maximum glacier fluctuations in the southern Écrins massif (westernmost Alps): insights from 10 Be cosmic ray exposure dating
Résumé
Only a few chronological constraints on Lateglacial and Early Holocene glacier variability in the westernmost Alps have hitherto been obtained. In this paper, moraines of two palaeoglaciers in the southern ecrins massif were mapped. The chronology of the stabilization of selected moraines was established through the use of Be-10 cosmic ray exposure (CRE) dating. The equilibrium line altitude (ELA) during moraine deposition was reconstructed assuming an accumulation area ratio (AAR) of 0.67. Ten pre-Little Ice Age (LIA) ice-marginal positions of the Rougnoux palaeoglacier were identified and seven of these have been dated. The Be-10 CRE age of a boulder on the lowermost sampled moraine indicates that the landform may have been first formed during a period of stable glaciers at around 16.2 +/- 1.7 ka (kiloyears before AD 2017) or that the sampled boulder experienced pre-exposure to secondary cosmic radiation. The moraine was re-occupied or, alternatively, shaped somewhat before 12.2 +/- 0.6 ka when the ELA was lowered by 230 m relative to the LIA ELA. At least six periods of stable ice margins occurred thereafter when the ELA was 220-160 m lower than during the LIA. The innermost dated moraine stabilized at or before 10.9 +/- 0.7 ka. Three Be-10 CRE ages from a moraine of the Prelles palaeoglacier indicate a period of stationary ice margins at or before 10.9 +/- 0.6 ka when the ELA was lowered by 160 m with respect to the end of the LIA. The presented Be-10 CRE ages are in good agreement with those of moraines that have been attributed to the Egesen stadial. Assuming unchanged precipitation, summer temperature in the southern ecrins massif at ~12 ka must have been at least 2 degrees C lower relative to the LIA.