The geological significance of cosmogenic nuclides in large lowland river basins

Measured from a bag of sand taken in large rivers, the concentration of cosmogenic nuclides such as in situ-produced 10Be, 26Al, and 14C can be used to constrain the mean sediment flux of the headwaters and assess the duration of sediment storage from source to sink. We revisit these principles, with examples from the Amazon and Ganga basins. We identify two end member cases controlling the concentration of cosmogenic nuclides in lowland river sediment: 1) if the time scale of floodplain sediment storage is short compared to the half-life of the nuclide, in situ cosmogenic nuclide concentrations are not significantly altered in lowland basins. In this case the concentration of e.g. in situ-produced 10Be in the sediment taken in the lowland basin equals in most cases that exported from the sediment source area, but the variability in nuclide concentrations between headwater streams is significantly averaged-out. Thus to convert the measured river sediments' in situ cosmogenic nuclide concentration into a catchment-wide denudation rate, production rates are scaled to include those of the sediment-producing mountainous areas only, rather than the entire catchment area. Nuclide production in the lowlands, where no sediment is being produced, is hence excluded. This correction is termed “floodplain correction”. 2) If sediment buried for periods of the order of the nuclides´ half-life is episodically re-entrained into the active river, paired nuclides, for example the ratio of 26Al/10Be, through the differential decay of these isotopes, constrain the the storage duration and re-mixing of floodplain sediment. As in-situ cosmogenic nuclides measure denudation rates over longer time scales, typically integrating between 103 and 105years, their combination with modern estimates of sediment fluxes from river load gauging offers a rich potential of deciphering the controls of Earth surface fluxes across large basins. From this combination, we can assess how river systems react to external perturbations such as climate change or human interference, and how such signals are transmitted through the alluvial reaches of lowland basins. The most important results in both basins are that i) lowland Amazon and Ganga nuclide concentrations indeed broadly reflect the spatial average of Andean and Himalayan denudation, respectively, ii) the thus calculated sediment fluxes are within uncertainty of modern fluxes from sediment gauging, indicating complete sediment delivery to the