Non-Rayleigh control of upper-ocean Cd isotope fractionation in the western South Atlantic

We present seawater Cd isotopic compositions in five depth profiles and a continuous surface water transect, from 50°S to the Equator, in the western South Atlantic, sampled during GEOTRACES cruise 74JC057 (GA02 section, Leg 3), and investigate the mechanisms governing Cd isotope cycling in the upper and deep ocean. The depth profiles generally display high ε112/110Cd at the surface and decrease with increasing depth toward values typical of Antarctic Bottom Water (AABW). However, at stations north of the Subantarctic Front, the decrease in ε112/110Cd is interrupted by a shift to values intermediate between those of surface and bottom waters, which occurs at depths occupied by North Atlantic Deep Water (NADW). This pattern is associated with variations in Cd concentration from low surface values to a maximum at mid-depths and is attributed to preferential utilization of light Cd by phytoplankton in the surface ocean. Our new results show that in this region Cd-deficient waters do not display the extreme, highly fractionated ε112/110Cd reported in some earlier studies from other oceanic regions. Instead, in the surface and subsurface southwest (SW) Atlantic, when [Cd] drops below 0.1 nmol kg−1, ε112/110Cd are relatively homogeneous and cluster around a value of +3.7, in agreement with the mean value of 3.8±3.3 (2SD, n=164) obtained from a statistical evaluation of the global ocean Cd isotope dataset. We suggest that Cd-deficient surface waters may acquire their Cd isotope signature via sorption of Cd onto organic ligands, colloids or bacterial/picoplankton extracellular functional groups. Alternatively, we show that an open system, steady-state model is in good accord with the observed Cd isotope systematics in the upper ocean north of the Southern Ocean. The distribution of ε112/110Cd in intermediate and deep waters is consistent with the water mass distribution, with the north–south variations reflecting changes in the mixing proportion of NADW and either AABW or AAIW depending on the depth. Overall, the SW Atlantic Cd isotope dataset demonstrates that the large-scale ocean circulation exerts the primary control on ε112/110Cd cycling in the global deep ocean.SCOPUS: ar.j