Controls On Short-Term Dissolved Sr-87/Sr-86 Variations In Large Rivers: Evidence From The Ganga-Brahmaputra

Variations in the marine Sr-87/Sr-86 record are largely driven by riverine export of weathering-derived strontium. Among the large river systems, the Ganga and the Brahmaputra deliver a disproportionately large flux of radiogenic Sr (i.e., high Sr-87/Sr-86) to the ocean. Understanding the sources and processes that make these river systems such important contributors of radiogenic Sr to seawater has been the focus of many studies; extremely radiogenic silicate and carbonate rocks of the Lesser Himalayan formations impart river water with high Sr-87/Sr-86 values. Furthermore, dissolved Sr-87/Sr-86 compositions show large temporal variability in both headwater and floodplain catchments; the magnitude of these fluctuations is higher than is observed in any other major river globally. Here, we present dissolved and depth-specific suspended particulate Sr time-series data from the rivers Ganga and Brahmaputra with weekly resolution during the monsoon and monthly resolution during the non-monsoon seasons of 2010-2011. We find that dissolved Sr-87/Sr-86 compositions of both rivers show substantial short-term variability with weekto-week changes as large as 0.0134 Sr-87/Sr-86 units. Dissolved Sr-87/Sr-86 compositions correlate strongly with suspended particulate Sr-87/Sr-86 compositions at all depths. To understand the short-term variability of dissolved Sr-87/Sr-86, we developed a hydro-geochemical mixing model using literature-derived Sr concentration and Sr-87/Sr-86 values of major tributaries combined with 5-day integrated rainfall as a proxy for unavailable discharge data. Results indicate that short-term changes in dissolved Sr-87/Sr-86 are largely driven by spatio-temporal rainfall variability across the Ganga and Brahmaputra watersheds. We also observe strong correlations between the dissolved Sr-87/Sr-86 and the particulate Sr-87/Sr-86 along the depth-profile in both rivers as well as in other large rivers globally. Hence, we corroborate the findings from Tipper et al. (2021) and propose that in-stream interactions with radiogenic sediments during transport further contribute to dissolved Sr-87/Sr-86 variability. (C) 2021 Elsevier B.V. All rights reserved.