Processes controlling the distributions of Cd and PO 4 in the ocean

Depth profiles of dissolved Cd and PO 4 from a global data compilation were used to derive the Cd/P of particles exported from the surface layer, and the results indicate lowest values in the North Atlantic (0.17 ± 0.05), highest in the Southern (0.56 ± 0.24), and intermediate in the South Indian (0.31 ± 0.14) and North Pacific (0.36 ± 0.08) Ocean basins. The Cd/P of exported particles in high nutrient‐low chlorophyll (HNLC) regions is twice that for particles exported in non‐HNLC regions as is the fractionation effect during biological uptake of Cd and PO 4 , and these trends primarily determine the spatial trends of dissolved Cd/PO 4 observed in the surface ocean. In deep waters the lowest dissolved Cd/PO 4 of 0.23 ± 0.07 is found in the North Atlantic Ocean and the result primarily of low Cd/PO 4 of North Atlantic Deep Water (0.23). In contrast, deep waters in the Southern Ocean have significantly higher dissolved Cd/PO 4 (0.30 ± 0.06), which is a result of the Cd/PO 4 of upwelled deep water from the South Pacific and South Indian (0.28) and the high Cd/P of degrading particles. A multibox model that accounts for the impacts of particle degradation and thermohaline circulation in the deep sea yields dissolved Cd and PO 4 interbasin trends close to observations. Model experiments illustrate the dependence of the dissolved Cd/PO 4 of the deep sea on the extent of HNLC conditions in the Southern Ocean and the impact on reconstructing paleo PO 4 concentrations from a Cd proxy.