Publications

The Amazon basin, the World´s largest basin in terms of water discharge and drainage area, presently exports a total sediment load of ~1000 Mt/yr to the Atlantic Ocean (e.g. Dunne et al., 1998; Guyot et al., 2005). This sediment load has been measured by gauging and inte-grates over several decades. This flux, however, is thought to in disequilibrium with the flux currently ex-ported from the sediment-producing areas (~900 Mt/yr) to the lowland, as ~40% of this Andean-dominated flux have been estimated to be intercepted and deposited in the floodplains close to the foothills of the Bolivian Andes (Guyot et al., 1996). The question now arising is on what time scales this large-scale sediment sink operates, and to what extent sediment in the floodplain is being prone to weathering during its storage. Cosmogenic nuclides operate over longer time scales than sediment gauging and are well suited for tracing erosion and weathering processes (e.g. (von Blanckenburg, 2005). Catchment-wide denu-dation rates from in situ-produced cosmogenic 10Be concentrations can be calculated for large depositional basins under certain prerequisites. These have been identified recently (Wittmann & von Blanckenburg, 2009), e.g. sediment storage in the basin should operate on time scales shorter than the nuclide´s half life (e.g. 1.4 Myr for 10Be, Chmeleff et al., 2010), so that the cosmogenic nuclide signature of the sediment-producing area is preserved throughout floodplain storage and river transport. In the Amazon basin, an average sediment residence time from the Andes to the Atlantic outlet has been estimated to be ~5-14 kyr (Dosseto et al., 2006; Mertes & Dunne, 2007). Hence, cosmogenic nuclide-derived denudation rates provide a measure of sediment production at the same time scale. Wittmann et al. (2009) tested this approach in large Amazon tributaries, the Beni and Mamoré basins, where the average Andean denudation rate is preserved throughout several hundreds of km of floodplain sto-rage. Applied to the entire Amazon basin, this approach yields a sediment flux of ~610 Mt/yr at the outlet of the basin, which compares to an Andean-derived flux of ~460 Mt/yr; these measures of sediment fluxes inte-grate over a few kyr. Compared to modern sediment flux measurements from gauging, both methods record similar magnitudes in Andean erosion and total basin sediment export (although cosmogenic nuclide-derived fluxes are in general somewhat lower than modern fluxes), and also both methods do not record significant differences between the ingoing and the exported sedi-ment fluxes. This is a surprising finding, given the con-siderable differences in methodology and integration time scale. This similarity shows that the central Ama-zon basin is not one in which significant amounts of sediment are deposited on the longer term.

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Wittmann, H.; von Blanckenburg, F.; Maurice, L.; Guyot, J.-L.; Kubik, P. W. (2010): Sediment in the Amazon basin: how much in and how much out?. GeoDarmstadt 2010 - Geowissenschaften sichern Zukunft (Darmstadt 2010).