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Zusammenfassung:
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.
