Fate of particulate copper and zinc isotopes at the Solimões-Negro river confluence, Amazon Basin, Brazil

Abstract

The behaviour and fate of copper (Cu) and zinc (Zn) at river confluences is poorly understood, although chemical and physical processes during mixing of compositionally different tributaries might condition metal availability and fluxes to the ocean. To identify and quantify the effect of such processes in river mixing zones, particulate Cu and Zn isotope signatures (δ65CuSPM and δ66ZnSPM) were measured along cross sections and vertical profiles at the largest river confluence in the world, the “Encontro das Aguas” mixing zone of the Amazon River, where the organic-rich Negro River meets the sediment-rich Solimões River.The Negro River suspended sediments, with highly variable Cu and Zn concentrations as well as δ65CuSPM and δ66ZnSPM, are mostly influenced by organic matter and by the lateritic cover of the watershed. The Solimões River suspended sediments, more homogeneous in Cu and Zn concentration and isotope composition across the river section, reflect the signature of weathered silicate-rich sediments derived from the Andes. The Solimões River supplies the majority of the suspended Cu and Zn to the Amazon River, and despite important flux losses across the confluence (−35% for Cu and −27% for Zn), δ65CuSPM and δ66ZnSPM show a conservative behaviour during the mixing. In the dissolved load, Cu concentrations and δ65Cudiss, mostly supplied by the Solimões River, behave conservatively whereas Zn, derived mainly from the Negro River, suffers an important loss (−58%) that can be attributed to Zn adsorption onto the suspended sediments from the Solimões River. This transfer does not induce a significant δ66ZnSPM shift in the Amazon River suspended sediments. Therefore, Cu and Zn isotope ratios in the suspended sediments behave conservatively through this confluence, which mixes two very chemically-contrasted rivers. Our findings thus suggest that the riverine isotopic information on the sources of particulate Cu and Zn is preserved during tributary mixing.