Sr fluxes and isotopic compositions in the headwaters of the Yangtze River, Tongtian River and Jinsha River originating from the Qinghai–Tibet Plateau

Abstract

Water and sediment samples were collected from the headwaters (the Tuotuo River and Chumaer River) of the Yangtze River, Tongtian River and Jinsha River (HTJR) originating from the Qinghai–Tibet Plateau (QTP). Sr and 87Sr/ 86Sr have been measured to analyze temporal and spatial variation characteristics and determine relative contributions from atmospheric inputs, anthropogenic activities and rock weathering. These results show that Sr in the HTJR waters ranges from 1.37 to 11.07 µmol L − 1, and 87Sr/ 86Sr from 0.70948 to 0.71409. The headwaters Tuotuo River and Chumaer River receive ~ 95% of their Sr from evaporite dissolution. The influence of evaporite decreases gradually downwards from source area as increase in discharge. Silicate weathering contributes 4.9%–28.2% of dissolved Sr to the Jinsha River mainstream and 11.5%–42.4% to the tributaries. 87Sr/ 86Sr ranges from 0.71034 to 0.71177 in the Jinsha River mainstream, which is higher than those in other reaches of the Yangtze River. In the Jinsha River suture belt, granitoids and metamorphic rocks with relatively high 87Sr/ 86Sr are exposed. Moreover, carbonate weathering is another important source of Sr in the Jinsha River waters. It is estimated that 15.9%–46.8% of Sr in the mainstream and 37%–64.5% of Sr in the tributaries are derived from carbonate weathering. Contribution from atmospheric precipitation to the Sr in the HTJR is from 0.75% to 6.1% and influence of anthropogenic activities is slight. The 87Sr ex flux forcing changes in seawater Sr isotopic composition is 1.13 × 10 6 mol a − 1 for the HTJR at Yibin, being far lower than those in the Ganges and Brahmaputra and merely accounting for 1.36% of the total carried by the global rivers. Therefore, impact of the HTJR on steady increase of seawater 87Sr/ 86Sr ratio since 40 Ma may be very small.