Abstract

The modern Amazon trunk stream and tributaries contain substantial amounts of sediments that were formerly stored for 1–3 m.y. in floodplains. By reworking of floodplain deposits, this sediment is being incorporated into Amazonian streams. We conclude this from using the ratio of in situ−produced cosmogenic 26Al to 10Be in detrital sediment from rivers draining the sediment-producing Andean, Guyana, and Brazilian shield areas, as well as the largest Amazonian lowland rivers. Samples from the modern Amazon River yield 26Al/10Be ratios between ∼3.8 and 5.5, and thus represent mixing between a non-buried sediment end member and a formerly buried sediment end member. The non-buried end member is sourced in the Andes with a ratio of ∼6.5, reflecting recent erosion of hillslopes exposed to cosmic rays, and the buried end member is potentially represented by deeply stored floodplain sediment as old as Miocene age, with a ratio of ∼3, that was partially shielded from cosmic rays. This simple picture of binary mixing is complicated by grain-size effects. Coarser-grained sediment from non-Andean settings (lowlands, cratonic shields) contains slightly more formerly buried material, whereas finer-grained lowland sediment mostly records non-buried 26Al/10Be ratios and nuclide concentrations that are indicative of an Andean source. Our assessment shows an incorporation of formerly buried, old sediment of central Amazonian floodplains to the fresh, Andean-derived sediment mainly carried in the Amazon trunk stream. We interpret our results to mean that the Amazon regularly avulses into these old deposits that crop out in more distal vicinities to the main river and temporarily leaves its current Holocene channel.

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