The Southern Ocean Cadmium Isotope Divide

We report Cd isotope and concentrations data in a transect of surface waters con-tinuously sampled in the Southern Ocean along the Zero Meridian (47$\deg$S-63$\deg$S, ANT24-3 cruise, Cape Town-Punta Arenas, February-March 2008). The objective is to examine 1) the efficacy of Cd isotope fractionation to respond to changes in (i) biological productivity in this High-nutrient-Low-Chlorophyll region and (ii) ocean circulation in the wind-driven upwelling system of the Antarctic Circum-Polar Current (ACC) and 2) how these could relate to the efficiency of the biological carbon pump. Our results show a strong northward gradient in both Cd concentrations and iso-topic compositions interrupted by an abrupt change in the sense of variations at about 56$\deg$S. The remarkable coincidence between the Cd isotope divide and the location of the Southern Boundary of the ACC and, the evidence for advection of the Cd isotope and elemental signal by northward Ekman transport in the surface layer demonstrate that Cd isotopes trace surface ocean circulation regimes. Co-variations between Cd isotopes and concentrations exhibit two negative correlations and separate the waters from the ACC and the Weddell Gyre into distinct Cd biogeochemical provinces. The arrays depict Rayleig