The distribution of dissolved molybdenum, vanadium and uranium in the Atlantic Ocean

Imelda Velasquez, Sandra Poehle and Andrea Koschinsky, Jacobs University Bremen, Department of Physics and Earth Sciences, Bremen, Germany
A detailed investigation of Mo, U and V, which are often considered to behave conservatively in the ocean, was done in the southeastern Atlantic Ocean during the GEOTRACES cruise GA11 (M121) to understand their possible supply and removal mechanisms and their biogeochemical cycling in the study region. The cruise track crossed areas of major external input sources e.g. dust from the Namib Desert, exchange with the West African continental margin and with the oxygen-depleted shelf sediments of the Benguela upwelling as well as the influence of the plume of the Congo River. We observed slightly elevated subsurface Mo, V and U concentrations in the offshore station, which traverses the Walvis Ridge compared to other stations along the E-W transect between 0oE to 15oE. Within the subsurface layer (upper 100 m) of the S-N transect we observed marginally higher mean concentrations of Mo, V and U between 6oS and 3oS near the Congo River compared to the area further south between 28oS and 22oS (Namibia). This suggests that the Congo River is a potential source of trace metals to the nearby SE Atlantic Ocean. Molybdenum and U were highly variable within the Benguela upwelling region and vertical profiles of deep stations (>500 m) exhibited some scatter in the upper 200 m and a near homogenous distribution in the deeper water column. The distribution of Mo, V and U in some offshore stations appeared to be influenced by the mixing of different water masses in the area. Our data imply that the trace metals Mo, V, and U do not behave strictly conservative in the Atlantic Ocean and concentrations vary between distinct areas with specific environmental conditions.