Controls on short-term dissolved 87Sr/86Sr variations in large rivers: Evidence from the Ganga–Brahmaputra

Abstract

Variations in the marine 87Sr/86Sr 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 87Sr/86Sr) 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 87Sr/86Sr values. Furthermore, dissolved 87Sr/86Sr 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 87Sr/86Sr compositions of both rivers show substantial short-term variability with week-to-week changes as large as 0.0134 87Sr/86Sr units. Dissolved 87Sr/86Sr compositions correlate strongly with suspended particulate 87Sr/86Sr compositions at all depths. To understand the short-term variability of dissolved 87Sr/86Sr, we developed a hydro-geochemical mixing model using literature-derived Sr concentration and 87Sr/86Sr 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 87Sr/86Sr are largely driven by spatio-temporal rainfall variability across the Ganga and Brahmaputra watersheds. We also observe strong correlations between the dissolved 87Sr/86Sr and the particulate 87Sr/86Sr 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 87Sr/86Sr variability.