Spatial variation of dissolved copper isotope systematics in the Changjiang (Yangtze) River: Response to weathering processes and anthropogenic impacts

Abstract

The stable isotopes of copper (Cu) in rivers and oceans are receiving increasing interest in their potential to trace Cu biogeochemical cycling and fingerprint Cu sources. Nonetheless, the continental sources and processes that control the Cu flux and isotope composition in large rivers, the main source of Cu to the ocean, remain to be fully understood. Here we present a comprehensive study of Cu and its isotopes in the dissolved load of the Changjiang (Yangtze) mainstream and its major tributaries, from its headwater on the Tibetan Plateau to its estuary, with the aim to quantify how weathering processes and anthropogenic sources influence isotopic values of Cu in rivers.The dissolved phase of Changjiang mainstream and tributaries exhibits large variability in both Cu concentration and δ65Cu: from 5.14 to 26.68 nM and 0.88 to 1.75‰, respectively. These results confirm previous finding that the dissolved riverine pools of Cu are usually isotopically heavier with respect to the upper continental crust, which is consistent with partitioning of the heavy Cu into strong organic complexes in the dissolved pool and/or retention of light isotopes by secondary mineral formation. In addition to this first-order control, the observation of progressive downstream increase in δ65Cu accompanied by an increase in Cu concentrations requires the mixing with an isotopically heavy end-member in the river catchment. The absence of relationships between dissolved δ65Cu (as well as Cu concentration) and Ca/Na ratios or sulfate concentration in the river water indicates that the heavy Cu is unlikely sourced from lithological weathering. We suggest that such high riverine δ65Cu could result from addition of an anthropogenically-sourced heavy Cu. Though the precise source of this isotopically heavy Cu needs to be further quantified, it seems possible that it originates from mining, industrial, and agricultural activities in the Changjiang basin. Overall, it is suggested that anthropogenic Cu input could be an alternative factor other than continental weathering in driving the dissolved load towards heavier isotopic values in rivers, and ultimately the oceans.