Do desert dust input and pronounced oxygen minimum zones act as sources or sinks for dissolved molybdenum and uranium? A GEOTRACES study in the eastern Atlantic Ocean

Molybdenum (Mo) and uranium (U) are usually considered as conservative metals in the oceanic water column though deviations have been observed in e.g. surface waters. The key objective was to study the impact of dust input and pronounced oxygen minimum zones (OMZs) on dissolved Mo and U in comparison with the truly conservative element rubidium (Rb) in seawater samples from two GEOTRACES research cruises: GA11 - NE-Atlantic affected by Saharan dust and the Mauritanian upwelling and GA08 - SE-Atlantic affected by Namibian dust and the Benguela upwelling. We observed slightly higher Mo and U concentrations, 114.8 nmol/kg and 14.5 nmol/kg, respectively, under the Saharan dust plume trajectory (NE-Atlantic) compared to their average seawater concentrations (107 nmol/kg and 13.9 nmol/kg) leading to the assumption of Saharan dust being a source. Slightly lower than average Mo and U concentrations, 102.8 nmol/kg and 11.9 nmol/kg, respectively, above the suboxic Namibian shelf (SE-Atlantic) can be attributed to the formation of reduced solid Mo and U smaller than 0.015 μm. A first hint to a link between sediment-released organic matter and colloidal U requires more research. In contrast, Rb concentrations remained constant with depth, unaffected by regional influences in the NE and SE Atlantic, with approx. 1.3 μmol/kg showing truly conservative characteristics. Our study implies that the expansion of less oxygenated waters with a potential shift towards euxinic conditions and more extensive dust events in the future due to e.g. climate change may affect the distribution and potentially isotopic signatures of Mo and U. • Saharan dust seems to be a source of Mo and U to the Eastern Atlantic Ocean. • Namibian shelf might be a sink for Mo and U under suboxic conditions. • Mo and U are truly dissolved in NE-Atlantic and in most parts of the SE-Atlantic. • Up to 7% of dissolved U belong to colloidal fraction above Namibian shelf. • Truly conservative behaviour of Rb irrespective of oxygen minimum zone or dust.