Glacial history of the upper Drac Blanc catchment (French Alps)

anglais. Glacier advances in the Écrins massif (French Alps) during the Late Glacial and the Early Holocene are poorly constrained. Based on field observations, a high-resolution digital elevation model and an orthophoto, glacial landforms in the upper Drac Blanc catchment were mapped. Thanks to 42 new 10Be exposure ages, the timing of the stabilisation of selected moraines was constrained. The application of the accumulation area ratio method with a ratio of 0.67 enabled the determination of the Equilibrium Line Altitude (ELA) during the corresponding glacier advances and the amount it lowered compared to the end of the Little Ice Age. The lowermost sampled moraine may have been reached by a first glacier advance at around 16.9 ± 1.6 ka. Not unexpectedly, multiple moraines in the upper Drac Blanc catchment stabilised during Greenland stadial 1 and in the Early Holocene. During the furthest glacier advance somewhat before 12.5 ± 0.6 ka, a prominent moraine was shaped or, alternatively, re-occupied. The 10Be exposure ages provide evidence for further glacier advances or halts in glacier recession not later than 12.5 ± 0.6, 11.9 ± 1.0 and 11.2 ± 0.4 ka. According to the ELA reconstructions, these events were associated with ELA depressions between 160 and 220 m with respect to the end of the Little Ice Age. Exposure ages from three boulders indicate a still stand or a glacial readvance not later than 10.7 ± 0.6 ka when the ELA was 150 m lower than at the end of the Little Ice Age. Novel findings of this study are (1) the periods of moraine stabilisation at the latest at 11.9 ± 1.0, 11.2 ± 0.4 and 10.7 ± 0.6 ka which have hitherto not been dated in the Écrins massif and (2) the associated ELA lowering during these periods based on the accumulation area ratio method. The 10Be exposure ages agree well with that of moraines at other sites in the Alps which have been correlated with the Egesen and the Kartell stadials.

anglais. Glaciers shape characteristic landforms during the culmination of advances and during temporary halts in glacier recession. One of these landforms which can commonly be found in actual glacier forefields or formerly glaciated areas, are moraines. The elongated ridges which consist of loose sediments characterised by varying grain sizes up to large boulders, are deposited at the margin of a glacier. Different dating techniques enable the age of boulders on moraines to be determined. Hence, the age of the moraine and, therefore, the position of the glacier margin at a given time can be estimated. Glaciers can be subdivided into two areas, an accumulation area, at a higher elevation where the mass balance is positive on a yearly timescale, and the ablation area at a lower elevation where the mass balance is negative on an annual timescale. Given the situation that the front of a glacier is stationary, the negative mass balance in the ablation area is compensated by ice flow. The transition between the accumulation and ablation areas occurs at the equilibrium line altitude, the elevation at which the mass balance is zero on a yearly timescale. Apart from a few exceptions, its position is believed to be influenced by changes in precipitation and/or temperature. Hence, the determination of the change of the equilibrium line altitude relative to the last period of stationary glaciers enables past changes in temperature and/or precipitation to be detected. This information can, in turn, be applied in future studies to predict the effects of anthropogenic climate change, as such forecasts require a detailed knowledge of past climatic variations. Therefore, the magnitude of past climatic changes in different parts of the world need to be determined. However, very little is known about previous climatic variations in the French Alps. This study aims at filling this gap by investigating past glacier variations as indications for former climatic fluctuations. For the first step of this study, moraines and, therefore, the former position of glacier margins in two adjacent valleys in the southern Écrins massif (French Alps) were mapped. For the second part of this study, boulders on selected moraines in the upper Drac Blanc catchment were dated to reconstruct the changes in the extent of the former glaciers through time. Lastly, the lowering of the equilibrium line altitude relative to AD 1850 during the formation of the moraines in the two valleys in the upper Drac Blanc catchment was reconstructed. Overall, the lowermost moraine from which rock samples were obtained, may have been reached by a glacier at around 16900 ± 1600 years before present. However, it is more likely that the moraine is slightly older than 12500 ± 600 years. Further glacier advances or halts in glacier recession occurred at around 12500 ± 600, 11900 ± 1000, 11200 ± 400 and 10700 ± 600 years before present. Considering that the dated moraines lie up to several kilometres down-valley from the areas which the former glaciers occupied before their final extinction, it can be inferred that the fronts of the glaciers were located at significantly lower elevations between 12500 ± 600 and 10700 ± 600 years before present. Hence, the climate then must have been more favourable for glaciers than today. As the aforementioned ages of glacier advances or halts in glacier recession fall in the cool period between 12900 and 11700 years before present, it can be assumed that the cooling caused the glaciers to advance and to stagnate, thereby leading to the deposition moraines at their margins. The lowering of the equilibrium line altitude between 120 and 220 m with respect to its position at around AD 1850 during the glacier advances and halts in glacier recession between 12500 ± 600 and 10700 ± 600 years before present agrees well with studies of other sites in the Alps. Hence, the climatic differences across the Alps at around 12000 years before present were possibly similar than today.