Isotopic tracing of the dissolved U fluxes of Himalayan rivers: Implications for present and past U budgets of the Ganges-Brahmaputra system

U activity ratios have been measured in the dissolved loads of selected riv ers from the Himalayan range, in Central Nepal, and from the Bangladesh, as well as in some rain waters. A few European and Asian rivers have also bee n analyzed for their U activity ratios. The data confirm the negligible eff ect of rainwater on the budget of dissolved U in river waters. The results also indicate that rivers on each Himalayan structural unit have homogeneou s and specific U isotope compositions: i) (U-234/U-238) activity ratios sli ghtly lower than unity in the dissolved load of the streams draining the Te thyan Sedimentary Series (TSS); ii) values slightly higher than unity for w aters from the High Himalaya Crystalline (HHC) and the Lesser Himalaya (LH) ; iii) systematically higher (U-234/U-238) activity ratios for waters from the Siwaliks. Thus, U activity ratios, in association with Sr isotopic rati os, can be used to trace the sources of dissolved fluxes carried by these r ivers. Coupling of U with Sr isotope data shows (1) that the U carried by t he dissolved load of the Himalayan rivers mainly originates from U-rich lit hologies of the TSS in the northern formations of the Tibetan plateau; and (2) that the elemental U and Sr fluxes carried by the Himalayan rivers at t he outflow of the highlands are fairly homogeneous at the scale of the Hima layan chain. Rivers flowing on the Indian plain define a different trend fr om that of the Himalayan rivers in the U-Sr isotopic diagram, indicating th e contribution of a specific floodplain component to the U and Sr budgets o f the Ganges and the Brahmaputra. The influence of this component remains l imited to 10 to 15 percent for the U flux, but can contribute 35 to 55% of the Sr flux. The variations of the Sr and U fluxes of the Ganges-Brahmaputr a river system in response to climatic variations have been estimated by as suming a temporary cut off of the chemical fluxes from high-altitude terrai ns during g a Ganges-Brahmaputra glacial episodes. This scenario would sign ificantly decrease the dissolved U flux of the Ganges river system and incr ease its U activity ratio. Such a climatic dependence of the Himalayan U fl ux could induce a periodic variation of the mean U activity ratio of the wo rld rivers on glacial-interglacial timescales Copyright (C) 2001 Elsevier S cience Ltd.