Sediment production and delivery in the Amazon River basin quantified by in situ-produced cosmogenic nuclides and recent river loads

摘要

We use cosmogenic nuclide-derived denudation rates from in situ-produced ~(10)Be in river sediment to determine sediment production rates for the central Amazon River and its major tributaries. Recent developments have shown that this method allows calculating denudation rates in large depo-sitional basins despite intermediate sediment storage, with the result that fluxes of the sediment-producing hinterland can now be linked to those discharged at the basins' outlet. In rivers of the central Amazonian plain, sediment of finer grain sizes (125-500 urn) yields a weighted cosmogenic nuclide-derived denudation rate of 0.24 ± 0.02 mm/yr that is comparable to the integrated rate of all main Andean-draining rivers (0.37 ± 0.06 mm/yr), which are the Beni, Napo, Mamore, Ucayali, and Maraflon rivers. Coarser-grained sediment (>500 μm) of central Amazonian rivers is indicative of a source from the tectonically stable cratonic headwaters of the Guyana and Brazilian shields, for which the denudation rate is 0.01-0.02 mm/yr. Respective sediment loads can be calculated by converting these cosmogenic nuclide-derived rates using their sediment-producing areas. For the Amazon River at Obidos, a sediment production rate of ~610 Mt/yr results; non-Andean source areas contribute only ~45 Mt/yr. A comparison with published modern sediment fluxes shows similarities within a factor of ~2 with an average gauging-derived sediment load of ~1000 Mt/yr at Obidos, for example. We attribute this similar trend in cosmogenic versus modern sediment loads first to the absence of long-term deposition within the basin and second to the buffering capability of the large Amazon floodplain. The buffering capability dampens short-term, high-amplitude fluctuations (climatic variability in source areas and anthropogenic soil erosion) by the time the denudation rate signal of the hinterland is transmitted to the outlet of the basin.