Variation of U-Th disequilibria in the different size fractions of river sediments: Evidence from the Himalayan river system.

U-series disequilibria in river sediments have the potential to bring main constrains on the time scale of erosion processes at the scale of watershed. However such a determination requires to characterize and to understand better the variation of 238U-234U-230Th disequilibria in sediment following their nature and/or their size grain. In order to answer this question, we analysed U-Th disequilibria in sediments collected at different depths of the water column of the the Ganges river and one of its main tributary: the Gandak river. We use the natural mineralogical sorting done by the river on the carried sediments to evaluate the sediments U-Th variations as a function of their grain size. These data highlight the significant variations of U-Th disequilibria with the grain size of sediments, implying a difference in the source and transfer time of sediments with their granulometry. For the Ganges river system, it can be proposed from 238U-234U-230Th data that bedload and river-bank sediments originate from Himalayan range and are marked by long transfer time into the plain. By contrast 238U-234U-230Th disequilibria in suspended sediment are explained by a scenario involving an old U gain followed by a recent U-Th fractionation. The U-gain is related to U vegetation recycling in the plain, which is all the more intense than the sediments are far from the high range. Overall, these results indicate that a large proportion of MES in the Ganges river system come from the erosion of soils horizon in the plain, and that their transit time on the plain is short. The detailed study of U-series disequilibria in the different size fraction of river sediments seems therefore important for a correct determination of the transport laws of sediments in alluvial plain.