The geochemical behavior of Cu and its isotopes in the Yangtze River

Copper (Cu) isotopes can be a useful tool to constrain the interaction of water and the environment, but they have not been widely applied to riverine research in the preceding decades. Isotopically heavy Cu in rivers (global average: about +0.7 parts per thousand) compared to rocks (at about 0 parts per thousand) has been attributed to: a) the mobilization of heavy Cu during oxidative weathering, and b) partitioning between an isotopically heavy, organically complexed dissolved pool, and an isotopically light pool adsorbed to particulates. Here, we report Cu concentrations and isotope ratios of the main stream of the Yangtze River and its several tributaries. We find that the Yangtze River exhibits anomalously heavy Cu isotope compositions compared to other rivers: delta Cu-65(NIST 976) of dissolved Cu for the main stream, from Chongqing to Nanjing, ranges from +0.59 to +1.65 parts per thousand, while the tributaries vary from +0.48 to +1.20 parts per thousand. A negative correlation is observed between Cu concentrations and Cu isotope compositions. We attribute the anomalous Cu isotope geochemistry of the Yangtze River to two key features of the basin: first, the influence of the Three Gorges Dam (TGD), and second, the presence of extensive Cu sulphide deposits close to the lower reaches of the river. In the upper reaches, downstream towards the TGD, delta Cu-65 values increase as Cu concentrations decrease, reflecting the preferential adsorption of light Cu by sedimenting particulate phases. delta Cu-65 values continue to increase to a maximum of +1.65 parts per thousand in the middle reaches, at Guangxingzhou. The lower reaches, from Jiujiang to Tongling, are characterized by less positive values of delta Cu-65 (at about +0.60 parts per thousand), due to the oxidative weathering of Cu sulphide deposits. The overall Cu-delta Cu-65 trend in the river reflects mixing of these waters from the lower reaches, influenced by Cu sulphides, with waters from upstream, which have lower Cu concentrations and elevated delta Cu-65 values.