Cosmogenic nuclides measured in river sediment can be used to provide basin-wide, spatially-averaged denudation rates over time scales meaningful to rock weathering and erosion itself, thus bridging the gap between long-term denudation rates from thermo-chronology and lake fills and short-term river loads [1-4]. In this study, we assess the effects of sediment deposition and storage on this method, because cosmogenic nuclides may be prone to additional irradiation or decay during long-term storage in floodplains. We have modeled depth- and time-dependant nuclide production and radioactive decay for different settings, depths, and sediment reworking time scales. These examples, which are the Amazon, the Mississippi, the Beni, the Rhine, the Pearl, and the Vermillion rivers, are representative for the evolution of channel-floodplain systems at multi-millennial time scales. We found that nuclide budgets are a function of the mixing ratio of upstream sediment discharge and sediment remobilization from floodplains by bank erosion. At the floodplain surface, nuclide accumulation is proportional to the sediment residence time, while at great depths, it is a function of the exponentially decreasing production rate. For long-lived nuclides such as 10Be and 26Al and for all modeled settings, the cosmogenic nuclide concentration does not change; neither an increase due to irradiation during deposition is observed nor a decrease due to decay. Even long-term deposition on time scales of 10^5 to 10^6 yrs does not induce a significant change in the initial nuclide signal that was inherited from the sediment source area. Nuclide concentrations for the short-lived in situ-produced nuclide 14C are however sensitive to floodplain storage on time scales of <20 kyr. The cosmogenic nuclide composition of old floodplain deposits that have been isolated for periods of Myr from the river that has once deposited them are predicted to change its 10Be and 26Al concentration, depending on the depositional depth. These conditions can be evaluated using the 26Al/10Be ratio that readily provides depth and duration of storage. We illustrate these models with examples from the Amazon basin. As predicted, modern bedload collected from the Beni River (Bolivia), a large tributary to the Amazon River, shows no significant change in nuclide concentrations along 800 km of floodplain. In contrast, in the central Amazon floodplain currently untouched by the modern river system, low 26Al/10Be ratios account for minimum burial depths at 5-10 m at a duration of >5 Myr. The important result of this analysis is that in all likely cases of active floodplains, cosmogenic 10Be and 26Al concentrations remain virtually unchanged over the time spans relevant to floodplain storage and the cosmogenic nuclide concentration of the sediment-providing hinterland is recorded at each point in the basin. Thus, spatially-averaged denudation rates of the sediment-producing area can be inferred, providing the opportunity to constrain sediment budgets that are not decoupled from their sediment-providing areas and record erosion processes over at least the kyr time scale.

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Wittmann, H.; von Blanckenburg, F.; Kubik, P. (2009): Cosmogenic Nuclide Budgeting of Floodplain Sediment Transfer and Examples from the Amazon Basin. Tectonics & Sedimentation - Conference on tectonic processes initiating the erosion and transport of sediments into both continental and marine basins - case studies and modelling (Bonn 2009).