U-series disequilibria in suspended river sediments and implication for sediment transfer time in alluvial plains: The case of the Himalayan rivers

²³⁸U-²³⁴U-²³⁰Th radioactive disequilibria were analyzed in suspended sediments (collected at different depths) from the Ganges River and one of its main tributaries: the Narayani-Gandak River. Results associated with bedload sediment data suggest that uranium-series (U-series) disequilibria in river sediments of the Ganges basin vary with grain size and sampling location. The range of observed U-series disequilibria is explained by a mixing model between a coarse-grained sediment end-member, represented by bedload and bank sediments, and a fine-grained end-member that both originate from Himalaya but undergo different transfer histories within the plain. The coarse-grained sediment end-member transits slowly (i.e. >several 100’s ky) in the plain whereas the fine-grained sediment end-member is transferred much faster (<20-25 ky), as indicated by the absence of significant variations in Th isotope composition of the fine-grained sediment end-members. These results show that U-series isotopes can be used to quantify the various transfer times of river sediments of different sizes and infer that there can be an order of magnitude of difference, or more, between the transfer time of suspended and bedload sediments. This underlines that a good knowledge of the proportion of suspended vs. bedload sediments transported in the river is required to accurately assess how fast erosion products are transferred in a catchment and how fast a catchment is likely to respond to external forcing factors.