Importance of Cadmium Sulfides for Biogeochemical Cycling of Cd and Its Isotopes in Oxygen Deficient Zones—A Case Study of the Angola Basin

Understanding oceanic cadmium (Cd) cycling is paramount due to its micronutrient-like behavior in seawater, which has been inferred from its similarity to phosphate (PO4) cycling. Cadmium concentrations follow a nutrient-like consumption-regeneration cycle in the top of the water column and are mainly controlled by water mass mixing and circulation in the deep ocean. However, an additional scavenging mechanism through cadmium sulfide (CdS) precipitates, occurring within sinking biogenic particles in oxygen deficient zones (ODZ), has been proposed. In this study, we report Cd stable isotope and concentration data for seven vertical seawater profiles sampled during GEOTRACES cruise GA08 in the northern Cape and Angola Basins, which feature a significant ODZ along their eastern margins. Outside the ODZ, Cd cycling is similar to that previously reported for the South Atlantic. While water mass mixing largely controls deep ocean Cd isotope signatures, Cd isotope fractionation in surface waters can be modeled as an open system at steady state buffered by organic ligand complexation. In the ODZ, stronger Cd depletion relative to PO4 is associated with a shift in δ114Cd toward heavier values, which is indicative of CdS precipitation. Our interpretation is supported by experimental CdS precipitation data and a size-resolved particle model involving bacterial sulfate reduction as a precursor of CdS. Our estimates of the CdS flux to the seafloor (107 to 109 mol/yr) indicate that CdS precipitation is a significant process of Cd removal and constitutes a nonnegligible Cd sink that needs to be better quantified by Cd isotope analyses of marine sediments