Late Quaternary denudation of the Alps, valley and lake fillings and modern river loads

Erosional denudation of the Alps and their role as sediment source underwent major changes throughout the Quaternary, by repeated glaciation and deglaciation. Tie sediment fluxes of 16 major Alpine drainage basins were quant lied by determining the sediment volumes which have been trapped in valleys and lake basins. These became sedimentologically closed after the last glacier retreat around 17 000 cal. BP. The sediment volumes distributed over their provenance areas yield mean mechanical denudation rates between 250 to 1060 mm ka(-1). In contrast, modem denudation rates, derived from river loads and delta surveys, range from 30 to 360 mm ka(-1). Relief, such as rr-ean elevation and slope, turned out to be the primary control of both modem and Late Glacial mechanical denudation. Rock types seem to be responsible for some scatter of the data, but their role is masked by other factors. Modem denudation rates increase with higher proportions of bare rocks and glaciated area, but decrease with forest cover. An area-weighted extrapolation of the studied drainage basins to the entire Alps on the basis of major morphotectonic zones yields a mean denudation rate of 620 mm ka(-1) over the last 17 000 years. This rate clearly exceeds the modem rate of 125 mm ka(-1). Lake sediments and palaeoclimatic reconstructions confirm that the sediment yield of the Alps reached a maximum during deglaciation when large masses of unconsolidated materials were available, vegetation was scarse, and transport capacities were high. During the early Holocene sediment yield declined to a minimum before some climate deterioration and human activities again accelerated erosional processes. Assuming a denudation rate in the early Holocene half of the modem one, the Late Glacial denudation rates must have been in the order of 1100 to 2900 mm ka(-1). Consequently, denudation rates during a glacial/interglacial cycle probably varied by a factor of 14, which ties well within the range of other studies in central Europe, Scandinavia and North America. From large scale sediment budgets of perialpine sedimentary basins the overall denudation rate of the Alps during the Quaternary has been c. 400 mm ka(-1), i.e. about one third lower than the estimate for the last 17000 years. This can be well explained by the outstanding role which deglaciation played in the time span studied here. (C) 2001 Editions scientifiques et medicales Elsevier SAS.