An updated overview of Last Glacial Maximum in the Alps: comparisons and climate considerations

During the last decade, a significant improvement has been produced about the knowledge on timing and extent of the Last Glacial Maximum in the Alps. New available dating methods, such as cosmogenic isotopes, luminescence and the abundance of new radiocarbon datings coupled with detailed field survey and dedicated drillings. These have changed the understanding of the dynamics of the Alpine glaciations including how the global circulation processes affected the Alps. The new chronology belongs from new surveys of the end-moraine systems (i.e. Rhine, Dora Riparia, Reuss, Garda and Tagliamento) mostly located in the southern side of the Alps (Monegato et al., 2007, 2017; Preusser et al., 2011; Ravazzi et al., 2014; Reber et al., 2014; Ivy-Ochs et al., in press), and their fluvioglacial megafans (Fontana et al., 2014). The Alpine glaciers displayed a synchronous advance at about 27 ka and they spread out on the piedmont plain from 26 to 24.5 ka, with a second maximum advance at about 23 ka. Oscillations of the glacier snouts in the morainic amphitheatres occurred during the withdrawal until their final collapse at 18-17.5 ka. The waxing and waning of the Alpine glaciers compared to the development of the boreal ice-sheets highlights the singular synchronicity with the maximum extension of the North American Ice Sheet (NAIS) at 25-24 ka (Stokes et al., 2012). Whereas, the European Ice Sheet (EIS) had its maximum at 22-20 ka (Hughes et al., 2016). According to global circulation models, the topography of the NAIS induced the starvation of the EIS and the southward shift of the Polar Front (Beghin et al., 2015). This shifting enhances the southward circulation across the Mediterranean, which fed the Alps especially in the central-eastern sectors, where also small catchments developed large glaciers. At the initial withdrawal of the NAIS, at the beginning of insolation recovery, the jet stream was driven to the north, leading the final waxing of the EIS while the Alps were fed mostly by westerly circulation. This promoted the downwasting of the Alpine glaciers, especially in the southern and eastern sectors, while in the western and northern sectors they likely keep it up until 18-17.5 ka. At this time all the Alpine glaciers rapidly melted enhancing the peak of freshwater discharge recorded in the LGM Adriatic delta (Pellegrini et al., 2018).