Études des fluctuations glaciaires dans le massif du Mont- Blanc depuis la fin du Dryas Récent à partir des nucléides cosmogéniques in situ

Abstract : The reconstruction of glacial chronologies during the Holocene (i.e. the past 11.7 ka) provides valuable information for the understanding of the morphologic and climatic evolution of icebearing mountainous regions. Indeed, mountain glaciers are well known to be very sensitive to climate changes (especially temperature and precipitations variations), even to those of low amplitude. Understanding their reaction to past climate variations at local scale allows a better comprehension of their future evolution. Moraines and « roches moutonnées » are geomorphologic indicators of the glaciers’ past extensions, and dating them using the in situ cosmogenic nuclide 10Be provides important information about their past behaviour. Moreover, studying glacially-polished bedrock surfaces, uncovered by recent glacier shrinkage, using the new approach combining in situ cosmogenic 14C and 10Be allows the simultaneous investigation of glacial retreat periods during the Holocene and the quantification of Holocene sub-glacial erosion rates, which are both still poorly known. These two approaches are applied in the Mont-Blanc massif (MBM) where glacial fluctuations during the Holocene are still poorly understood, although it is the most heavily glaciated massif in Europe. In order to achieve this, detailed glacial chronologies have been established from 43 cosmogenic 10Be ages of moraine boulders and glacially polished bedrock samples from Argentière and Talèfre glaciers, documenting the main advances and stagnations between 12 ka (end of the Younger Dryas) and the present. The chronologies obtained for the beginning of the Holocene agree with numerous other chronologies established in the Alps, thus emphasizing the broadly synchronous behavior of the alpine glaciers during the deglaciation between 12 and 10 ka, which could be the sign of a response to a climate forcing on a hemispheric scale. Climate modelling applied to the advanced extents of the Argentière glacier suggests a probable temperature range in the MBM during this period between -5.5 and -3.6°C compared with modern temperatures, assuming precipitation values between 45% lower and 10% higher than today. This temperature range is coherent with paleoclimatic reconstruction based on pollen records in Europe. Moreover, this model showed fluctuations of the glacier due to local precipitations variations during the Little Ice Age (LIA; 1550-1860 CE), superimposed to the dominant role of temperatures. Finally, the study of 21 glacially polished bedrock samples uncovered from ice during the last century in the vicinity of Mer de Glace and Talèfre and Argentière glaciers showed low 10Be concentrations, lower than 2 x104 at.g-1 in most cases. The sub-glacial erosion seems to have been too high to preserve enough cosmogenic nuclides for a combined analysis of in situ 10Be and 14C. However, such an analysis was carried out on one of the least eroded polished bedrock samples, which allowed a preliminary quantification of the shrinkage periods estimated to a cumulated duration of more than 9440 years during the Holocene (compared to the glacial extension of 1939), as well as 0.5 m local erosion during the 960 years of glacial covering of this site, uncovered since 1939. The results of this PhD combined with already existing data show that except for the large extents during the early Holocene deglaciation and the successive re-advances during the late Holocene culminating during the LIA, glaciers in the MBM seem to have been restricted to their early 17th century extensions for at least the 2/3rd of the Holocene.
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Marie Protin. Études des fluctuations glaciaires dans le massif du Mont- Blanc depuis la fin du Dryas Récent à partir des nucléides cosmogéniques in situ. Géochimie. Aix Marseille Université, 2019. Français. ⟨tel-02301654⟩

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