Item

Abstract

An excellent view on long-term denudation processes is provided by the study of alluvial samples using the cosmogenic nuclide method. In situ-produced 10Be is obtained from quartz and measured by accelerator mass spectrometry. The production of 10Be in quartz by secondary cosmic rays in the Earth's surface is a function of latitude and altitude of the sampling point, and the produced nuclide concentration is inversely proportional to the erosion rate. Therefore, measurements of in-situ produced 10Be are a suitable tool to determine spatially-averaged denudation rates. We first performed a detailed erosional study in the Maggia Valley, southern Switzerland, to design an appropriate sampling strategy and to test the method on different catchment sizes. In the Maggia Valley, sediment from second-order tributaries of the Maggia, ranging in size from 2- 46km2, are generally yielding lower erosion rates (between 380 and 840mm/kyr) than the main stream itself. The trunk stream of the Maggia and large, U-shaped valleys (67- 683km2) are eroding at 600 to 2000mm/kyr. This is probably due to the incorporation of glacial sediment, which has been deposited during LGM and is now being remobilized. This sediment has long been shielded from cosmic rays, resulting in a low nuclide concentration and high erosion rates, respectively. We then proceeded to sample medium-scale catchments from Luzern (Central Switzerland) to Bellinzona (Southern Switzerland) in order to map out large-scale patterns of uplift and erosion. In the Central Alps, some of the highest erosion rates ever measured with cosmogenic nuclides for a presently uplifting mountain range have been recorded. In this area, erosion rates range from 480 - 2110mm/kyr. The sampled catchments feature considerable relief (from 620m in the north Alpine Drainage Basin to 1640m in the Reuss valley) and are partly glaciated (up to 18% in a Rhone tributary). Unlike many other environments, we do not recognize a connection between slope gradient, topographic relief, and erosion rate in the Central Alps of Switzerland. The lack of correlation between topography and denudation points at a possible topographic steady state condition. The observed agreement of denudation rates with rock uplift rates within a factor of two is supportive of this assumption. Recent vertical movements relative to the benchmark at Aarburg easily exceed 1000mm/kyr in the rear of the Central Alps and diminish to about 200mm/kyr in the North Alpine Drainage Basin. In other words, we believe that erosion rates are at present balanced by rock uplift rates in the Central Alps of Switzerland, maintaining steady-state topography. Recent climatic parameters such as temperature and precipitation and even the extent of glaciation do not seem to play a major role with respect to Alpine erosion.