Off Congo waters (Angola Basin, Atlantic Ocean): A hot spot for cadmium–phosphate fractionation

Matthieu Waeles, Jean-François Maguer, François Baurand and Ricardo D. Riso

Limnol. Oceanogr., 58(4), 2013, 1481-1490 | DOI: 10.4319/lo.2013.58.4.1481

ABSTRACT: We report the distribution of cadmium at 13 stations along 5.83°S in the Angola Basin. The studied area (which is influenced by the plume of the Congo River, doming, and relatively high primary production) is characterized by relatively low-circulation regime, which results in a strong oxygen minimum. In subsurface, very low cadmium : phosphate (Cd : P) ratios (0.05–0.10 nmol L−1 : µmol L−1) were observed below the euphotic zone (depth range = 50–200 m), indicating that mineralization is the main process explaining the Cd–P fractionation. Cd–P decoupling is not restricted to subsurface waters, but occurs also throughout the oxygen minimum zone. An isopycnal analysis of the Cd : P ratio throughout the Atlantic Ocean shows that upper and intermediate waters in the South-east Atlantic are highly altered in terms of Cd–P properties. Benthic inputs at 4000 m were associated with degradation of organic matter with an important siliceous component, but were not accompanied by substantial changes in the Cd : P ratio.

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