Biogeochemical cycling of Zn and Cd and their stable isotopes in the Eastern Tropical South Pacific

Abstract

Abstract The distribution of Zn and Cd and their stable isotope ratios (δ66Zn and δ114Cd) were measured on samples from the Eastern Subtropical South Pacific Zonal Transect (EPZT), US GEOTRACES section GP16. The broad trends in both δ66Zn and δ114Cd are similar to those observed in other ocean basins, suggesting global similarities in the biogeochemical processes which cycle Zn and Cd. For example, average deep ocean δ66Zn along this transect (+ 0.46‰) are similar to the ~+0.5‰ values observed in the deep North Atlantic and North Pacific. Also similar to other locations, δ66Zn decreases towards the surface ocean. A plume of hydrothermal Zn emanating from the East Pacific Rise is used to calculate hydrothermal end-member δ66Zn as + 0.24‰, which is similar to crustal values and similar to other sources of Zn to the oceans. Average deep-ocean δ114Cd along this transect is + 0.28‰, which is similar to deep-ocean values between + 0.2 and + 0.3 measured elsewhere in the Atlantic, Pacific, and Southern Oceans. δ114Cd increases towards the surface ocean, as observed in other ocean basins, reflecting the preferential biological uptake of lighter Cd isotopes. Elsewhere it has been hypothesized that Cd can precipitate in sulfidic microenvironments on sinking particles, affecting seawater Cd concentrations and isotope ratios. However, we don't find evidence of particularly strong CdS precipitation along this transect, despite the fact that the Peru OMZ is one of the most reducing ocean environments worldwide.