The onset of the Last Glacial Maximum in the southern side of the Alps

The signals of an incoming spread of glaciers in a mountain range are normally recorded in the frontal alluvial plain in terms of an increase of sediment discharge and clast-size at the valley outlets. The Southern Alps of Italy are dissected by a various system of valleys, many of which interested by alpine glaciers during the Last Glacial Maximum. Some other catchments hosted only small glaciers confined to the highest peaks. The palaeobotanical and glaciological data suggest that rainfall was the main trigger for the culmination of the last glaciation in the Southern Alps. Indeed an increase in precipitation can be inferred by palaeovegetation record (pollen and macroremains). This also fits the palaeoclimatic atmospheric circulation models, indicating a significant rainfall gradient on the eastern Southern Alps during MIS3 and LGM. The humidity persisted in the Southern Alps during the glacial maximum and allowed the growth of open conifer forest even close to the mountains and to the piedmont glacier fronts. The response of major catchments to the advance of the alpine glaciers (i.e., Tagliamento, Piave, Brenta, Adda, Dora Baltea systems) is shown by the increase of sediment accumulation in the lower plains at about 28-27.5 ka cal BP, and by the growth of the alluvial fans in the piedmont plain. The stratigraphy of the plain shows that a fairly developed soil is buried by the LGM alluvial sedimentation at 25-35 m from the present surface. The spread of the alpine glaciers reached the maximum extension at the valley outlets at about 27 ka cal BP. Subsequently, in response to changes in precipitation rates during the LGM the frontal position of the glaciers oscillated at least two times within the end moraine systems before the onset of decay at 19 ka cal BP.