Hydrochemical origins and weathering-controlled CO₂ consumption rates in the mainstream of the Yangtze River

Because river water in the Yangtze River watershed with a huge area is influenced by variable lithologies and large-scale water projects such as the Three Gorges Dam, the hydrochemical origins of the main stream are still controversial. Furthermore, previous estimations on the carbon sink in the watershed caused by mineral weathering are mostly based on the mass-balance calculations of cations, but this method generally involves the selection of parameters of various mineral end-members, which causes the uncertainty of results. In this study, the temporal and spatial evolutions of hydrochemistry of the main stream are determined, and a new method for the determination of CO₂ consumption rates during mineral weathering processes are proposed based on the mass-balance calculations of \rmHCO_3^- of the upper reaches. The results show that dissolution of evaporites, circulating salts, minerals weathering and sulfate dissolution are the main hydrogeochemical reactions controlling the ionic compositions of river water of the mainstream of the Yangtze River, while human activities mainly control the contents of \rmNO_3^- in the river water within 3000 km from the estuary. The CO₂ consumption rates of silicate weathering and carbonate weathering in the upper reaches of the Yangtze River are 1.16×10⁵ mol/(km²·a) and 4.75×10⁵ mol/(km²·a), respectively. This study may provide a better understanding of the major hydrochemical processes, enriches and promotes the theory of carbon cycle.