A large offshore near-surface plume of dissolved trace elements is present in the South Atlantic Ocean. The plume flows westwards from the African shelf, occurring in close proximity to freshwater anomalies associated with the Congo River. This plume, which accounts for ~40% of the dissolved iron (dFe) budget for the South Atlantic, is not captured by biogeochemical models, raising questions about its origin. A long-standing hypothesis is that rivers are an inefficient source of dFe to the open ocean due to the efficient (~90–99%) removal of dFe observed in estuaries. Alternative sources of trace metals, such as rainwater or shallow shelf sediments, could generate element anomalies with a spatial distribution similar to that expected from a river plume. To help understand the potential mechanisms, we use tracer release experiments to investigate the origins of elements in the South Atlantic Congo plume. Tracer experiments suggest that elevated concentrations of dFe in the Congo plume are primarily sourced from river discharge and atmospheric deposition, with minimal influence from shelf sediments. Generally, river discharge is the main source in coastal regions and some offshore regions north of 3°S, while atmospheric deposition dominates the southwest regions, primarily south of 3°S. Further analysis along 3°S suggests a decrease in the contribution of river discharge from 90% to 30% dFe, with a corresponding increase in atmospheric deposition from 10% to 70% moving off-shelf from 9°E. Within the Congo shelf, atmospheric deposition accounts for an average contribution of 20–40% dFe, and could be a major source near the river mouth. Integration of data from cruise GA08 reinforces the moderate-to-substantial impact of wet depositions in augmenting the element concentrations at distances over 1000 km away from the river mouth. Therefore, rainfall patterns may have long-term implications for elements budget and ecologically-dependent processes should be further explored.

trace elements,Congo plume,river discharge,wet deposition,dissolved iron