Abstract
The correlation between dissolved cadmium (Cd) and phosphate (PO4) in the modern ocean has been extensively studied. However, factors that cause the non-linearity in this global correlation are still largely debated and poorly constrained. In this study, we present vertical and meridional distributions of Cd concentrations along the GEOTRACES GACC Leg 3 transect (53 degrees S to 0 degrees S) in the western South Atlantic. The vertical profiles of dissolved Cd show typical nutrient-like distributions, matching those of the macronutrient PO4. In the surface ocean, low Cd concentrations of 0.34-45 pmol kg(-1) are measured at all stations sampled. Below the fluorescence maxima, Cd concentrations increase with depth, reaching maxima at the depths of, or slightly below, the oxygen minimum. Similarly, surface waters have very low dissolved Cd/PO4 ratios, followed by a sudden decrease at depths corresponding to the fluorescence maxima, below which the Cd/PO4 ratios increase with depth.
These variations in Cd and Cd/PO4 ratios are consistent with phytoplankton uptake in the surface ocean at higher Cd/PO4 ratios compared to deep water Redfield Cd/PO4 ratios, and remineralization of high Cd/PO4 organic particles as they sink to depth. In addition, a comparison of Cd and PO4 distributions in waters within the depth range of Subantarctic Mode Water (SAMW) in our study and those in the source regions suggests that SAMW plays an important role in the non-linearity of the Cd-PO4 correlation in the global ocean. On the other hand, intermediate waters at the equatorial stations display significantly lower Cd compared to the southern stations in this study. We attribute this to the influence of low-Cd waters originating in the oxygen-depleted zone (ODZ) of the Angola Basin that are brought in via the Benguela and Equatorial Currents. In the deep ocean (>1000 m), the meridional distributions of dissolved Cd concentrations and Cd/PO4 ratios reflect mixing between North Atlantic Deep Water (NADW) (low nutrients, low Cd/PO4 ratios) and Antarctic Bottom Water (AABW) (high nutrients, high Cd/PO4 ratios), showing that dissolved Cd and PO4 can reliably trace deep ocean circulation in the modern ocean. (C) 2015 Elsevier B.V. All rights reserved.