Matthias Hinderer

Erosional denudation of the Alps underwent major climate-controlled changes throughout the Quaternary, especially by repeated glaciation. In this study, the sediment fluxes of 16 major Alpine drainage basins have been quantified by determining the sediment volumes which have been trapped in valleys and lake basins since they became sedimentologically closed after the last glacier retreat around 17000 BP (here refered to as post-LGM, Fig. 1a, b). Spreading back the sediment volumes to their provenance areas taking into account sediment porosities and accumulation periods leads to mean denudation rates of between 250 and 1060 mmka^-1 since closure. In contrast, modern rates derived from river loads and delta surveys are between 30 and 360 mmka^-1. The specific sediment yield range from 620 to 2650 and 74 to 890 tkm^-2a^-1, respectively, based on a rock density of 2.5 gcm^-3 (Fig. 1b).

In order to analyse the controlling factors of denudation, a regression analysis of relevant variables has been carried out and relief parameteres have been determined using a digital elevation model with 100 m X-Y / 18 m Z resolution. Most relief parameters, such as mean elevation and slope, show a significant correlation with denudation rates (p<0.1), thus, relief turned out to be the primary control of both, modern and late-glacial denudation. Rock types seem to be responsible for some scatter of the data but their role is masked by other factors. Modern denudation rates show a significant negative correlation with the proportion of forest cover in the drainage basin, and insignificant positive correlations with unvegetated areas and glacieral cover.

To extrapolate the denudation rates of individual drainage basins to the whole Alps, six morpho-tectonic units have been distinguished: (1) Northern Helvetian Alps, (2) Northern Calcareous Alps, (3) Western Crystalline Alps, (4) Southern Crystalline Alps, (5) Eastern Crystalline Alps, and (6) Southern Calcareous Alps. According to an area-weighted extrapolation unit (3) reaches by far the highest post-LGM and modern denudation rates of about 220 and 1010 mmka^-1, respectively. The lowest denudation rates result for unit (2) with c. 90 and 400 mmka^-1. All other units show only slight variations from 110 to 130 and 490 to 780 mmka^-1, respectively. According to this estimate, the mean denudation rate of the whole Alps has been 620 mmka^-1 over the last 17000 years, clearly exceeding the modern rate of 125 mmka^-1 by a factor of five.

Cores of lake sediments and palaeoclimatic reconstructions give evidence, that the denudation of the Alps reached a maximum during deglaciation when sediment supply was promoted by large masses of unconsolidated materials, scarce vegetation, and high transport capacities. During the early Holocene denudation reached a minimum before climate deterioration and human activities again accelerated erosional processes. If we accept the modern denudation rate to be representative of the last 5000 years and take half the rate for the early Holocene, effective late glacial denudation rates must have been in the order of 1760 mmka^-1. Consequently, the change of denudation rates during a glacial/interglacial cycle for the Alps can be estimated to be in the order of a factor of 14, which lies 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 seems to be c. 400 mmka^-1, hence, about one third lower than our estimate for the last 17000 years. This can be explained by the prominent role played by deglaciation in the time span studied here.