Publications

We used cosmogenic nuclides in stream sediment to construct a sediment budget of the Amazon basin. The procedure is based on basin-wide denudation rates derived from cosmogenic Be in quartz. The measured denudation rates integrate over 3 years. When multiplied with the basin´s area, the sediment load can be calculated. Within the large Amazon basin, the areas providing sediment to the central Amazon are the Andes, and the Brazilian and Guyana Shield, respectively; these areas have been systematically analyzed for their cosmogenic nuclide signals. An integrated Andean denudation rate of 0.36±0.06 mm/yr has been determined. The headwaters of the Brazilian and Guyana shields denude at very low rates (~0.015 mm/yr), as is expected for tectonically stable cratons. These headwater denudation rates can now be compared with those derived from the central Amazon. Here, we found that nuclide concentrations are strongly grain size dependant; the fine grain size fraction (125-250μm) contains a nuclide signal that is similar to that of Andean source areas. Therefore, we used this fraction to calculate an integrated denudation rate of 0.22±0.02 mm/yr for the entire Amazon basin. Coarse grain size (>500 μm) record the very low denudation signal of the shields, and correspondingly, they contribute only small amounts of sediment into central Amazon streams. The Andean erosion signal is best represented by the fine sand fraction. The integrated cosmogenic-based denudation rate of the entire Amazon basin converts to a total sediment load of 540 Mt/yr, a signal that records sediment production in the Andes and that is not affected by storage processes within the central Amazon floodplain. From presentday gauging, suspended loads are higher by a factor of 2-3 than our loads. This is a surprising finding, because modern loads integrate over much shorter time scales, where high magnitude low frequency events usually do not occur.

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Wittmann, H.; von Blanckenburg, F.; Guyot, J. L.; Maurice, L.; Kubik, P. W. (2009): A cosmogenic nuclide-based sediment budget of the Amazon basin. GV 99th Annual Meeting (Göttingen 2009).