A glacier-based reconstruction of the Last Glacial Maximum climate in the southern European Alps    

Improved records of precipitation and temperature are crucial to understand the evolution of Alpine glaciers during the Last Glacial Maximum (LGM). Palaeoclimate models and proxy data have suggested an increased moisture supply to the southern face of the Alps during the LGM, following a south-ward shift of the North-Atlantic jet stream. Ground control for such models, however, has been lacking for many sectors of the Alps, and regional climatic gradients have therefore remained poorly constrained. Here, we present new insights into the LGM palaeoclimate in the southern Alps, using the Equilibrium Line Altitudes (ELAs) of marginal glaciers as proxy. Marginal glaciers include ice caps, cirque, and valley glaciers that throughout the LGM remained isolated from larger outlet lobes connected to the Alpine ice network. Several sites of marginal glaciation were investigated through a combination of geomorphological mapping, surface exposure dating (both 10Be and 36Cl dating), and numerical reconstructions of palaeoglacier geometries and ELAs. The chronological data indicate that marginal glaciers across the southern Alps reached their maximum extent by ca. 24 ka and that an important readvance occurred at 19 ka, at the end of the LGM. Reconstructed palaeoglacier ELAs show considerable variations, from ca. 1100 m a.s.l. in the Julian and Carnic Prealps (SE-Alps) up to almost 2000 m a.s.l. in the Maritime Alps (SW-Alps). Minor differences between the sites can be attributed to topoclimatic factors (i.e., received solar radiation related to catchment aspect). Spatial trends in ELA, however, primarily reflect regional climatic gradients. More specifically, we recognised: (1) a N-S gradient related to increasing summer temperatures with lower latitudes, and (2) a strong E-W gradient driven by precipitation. For all sites, our data indicate little to no reduction in LGM precipitation compared to the present day, highlighting the importance of substantial precipitation for the build-up of marginal LGM glaciers in the southern Alps.