The copper isotope geochemistry of rivers and the oceans

Abstract

We present the first extensive dataset for copper (Cu) isotopes in rivers, as well as preliminary data for estuaries and seawater. Rivers exhibit a range in dissolved δ 65Cu (relative to NIST SRM 976) of + 0.02 to + 1.45 per mil (‰). The discharge-weighted average dissolved δ 65Cu and Cu concentration for the studied rivers, representing around 25% of the global riverine water discharge, are + 0.68‰ and 18.8 nmol kg − 1 , respectively. Isotopic data for the dissolved phase in two small estuaries in SE England exhibit relatively minor variation through the salinity gradient (+ 0.42 to + 0.94‰) and are close to both the data for the rivers upstream from the estuaries and a measurement of the coastal sea at their mouths. Analyses of the particulate phase in the estuaries reveal a complementary light pool (− 0.24 to − 1.02‰). The isotopic composition of the total Cu inventory is, however, close to that of the crustal source. These data, along with published constraints, suggest that the isotopically heavy dissolved phase of all the rivers originates in an isotopic partitioning of the weathered pool of Cu between a light fraction adsorbed to particulates and a heavy dissolved fraction dominated by Cu bound to strong organic complexes. Open ocean seawater samples generally exhibit even heavier dissolved Cu isotopic compositions than the riverine input (δ 65Cu = + 0.9 to + 1.5‰). The heavy isotopic composition of seawater is inferred to originate from intra-oceanic processes. The small dataset presented here is most consistent with further isotopic partitioning between an isotopically light pool adsorbed to particulates and a heavy dissolved pool strongly bound to organic ligands. Scavenging appears to have greatest impact in the surface ocean and least in both the oxygen minimum zone and the ocean bottom, perhaps reflecting the relative importance of scavenging, and thus the sequestration of light Cu to particulate material, at different depths.