Sr and ⁸⁷Sr/⁸⁶Sr in waters and sediments of the Brahmaputra river system: silicate weathering, CO₂ consumption and Sr flux

Abstract

The Brahmaputra is a major river system draining the Himalaya. The concentration of Sr and its ⁸⁷Sr/⁸⁶Sr have been measured in dissolved and particulate phases of the Brahmaputra and its tributaries in India to trace the sources of dissolved Sr. Dissolved Sr ranges from 250 to 1050 nM with ⁸⁷Sr/⁸⁶Sr from 0.71298 to 0.75975. The Sr data along with the available concentrations of major ions in the samples show that major contributors of Sr in the Brahmaputra system are the silicates and carbonates of the Himalaya and the Transhimalaya and carbonates and evaporites of the Tibetan basin. Silicate Sr in the Brahmaputra river system ranges from 20% to 80% with an average of ˜45%. The silicate component of Sr in the Brahmaputra system is similar or marginally higher than that in the Ganga system due to contribution from the Transhimalayan calc-alkaline plutonic rocks which have higher concentration of Sr. Evaporites of the Tibetan sub-basin and hot springs along the Indus Tsangpo Suture could be the other significant contributors to the dissolved Sr budget of the Brahmaputra. Contribution from evaporite dissolution decouples the total dissolved Sr flux from the Tibetan basin from atmospheric CO₂ consumption. Radiogenic Sr of the Brahmaputra system is derived from the Himalayan silicates whereas other Sr sources tend to dilute its radiogenic signature. The ⁸⁷Sr/⁸⁶Sr of the dissolved phase shows significant correlation with indices of silicate weathering, indicating that ⁸⁷Sr/⁸⁶Sr can serve as a proxy of silicate weathering. At their outflow, the Brahmaputra is less radiogenic (˜0.72) compared to the Ganga (˜0.73), however, the flux of ⁸⁷Sr from the Brahmaputra is similar to that of the Ganga.