Chemical weathering and Sr flux from the silicate lithology dominated fluvial system: Insights from major ions, dissolved Sr and 87Sr/86Sr of the Teesta headwaters, Sikkim Himalaya

Abstract

Weathering and erosion in Himalayan river systems have an important role in global geochemical cycles and elemental distribution among various reservoirs. The present study has investigated major ions, Sr concentrations and Sr isotopes (87Sr/86Sr) in dissolved loads of the Teesta River System (TRS), which is located in the lower reaches (Sikkim Himalaya) of the Brahmaputra basin and covers ∼2% of the later. The above mentioned chemical analyses in the TRS have been studied to (i) quantify chemical weathering fluxes, (ii) decipher sources of Sr and 87Sr, (iii) assess lithological controls on dissolved components and (iv) test the fidelity of 87Sr/86Sr as a direct proxy for silicate weathering. The dissolved 87Sr/86Sr in the TRS ranges from 0.73269 to 0.85452 (mean ∼0.76) which is more radiogenic compared to that of the Ganga (∼0.73) and the Brahmaputra (∼0.74) at their outflow, indicating the importance of the TRS in Sr isotopic evolution of global ocean water. A positive correlation between Sr and SO42− concentrations in the TRS water indicates the sulfuric acid (H2SO4) mediated weathering in its catchment. Our study suggests carbonate lithology as a chief source of dissolved Sr to the river, whereas 87Sr is mainly derived from silicates, vein calcites and calc-silicates. Poor correlation between dissolved 87Sr/86Sr and silicate weathering indices ((Na*+K)/TZ+, SiO2/TDS, silicate cationic weathering rate (Xsil)) suggests either weathering of non-silicate rocks (carbonate, vein calcite, calc-silicate) and/or incongruent weathering (preferential 87Sr release) control dissolved 87Sr fluxes. The dissolved Sr and 87Sr flux to the Bay of Bengal (through the Brahmaputra River) have been estimated to be 1 × 107 moles/yr and 7.1 × 105 moles/yr, respectively which are 0.03% of the global riverine supply to the oceans.