Seasonality controls on Last Glacial Maximum glacier dynamics in the southern European Alps

Glaciers in mountain ranges across Europe reached their local Last Glacial Maximum (l-LGM) extent at different times throughout the last glacial cycle, yet the causes of this asynchronous behaviour remain incompletely understood. Here, we propose precipitation seasonality as a key control for this phenomenon. The hypothesis is based on an internally coherent reconstruction of 16 l-LGM mountain glaciers and their Equilibrium Line Altitudes (ELAs) along the southern margin of the European Alps. 10Be and 36Cl surface exposure dating in selected catchments demonstrates that these marginal glaciers had advanced to their l-LGM positions by 25 to 24 ka, where they remained until ca. 19 ka. By combining the empirical reconstructions with outputs from a regional palaeoclimate model, we show that glacier mass balance during this period was strongly influenced by snowfall not only in winter but also during autumn and spring to early summer. This specific precipitation seasonality distinguished glaciers in the southern Alps from mountain glaciers elsewhere in southern Europe that were mainly fed by winter precipitation and faced widespread retreat after 25 ka under increasingly arid conditions. Spring and autumn snowfall along the Alpine fringe could buffer the effects of increasing aridity and allowed Alpine glaciers to remain at or close to their maximum extent also after 25 ka.