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

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.