Tracking the Dispersal of River Water, Atmospheric Deposition, and Shallow Sedimentary Trace Metal Inputs From the Congo Region Into the South Atlantic

Recent work has revealed the presence of an offshore near-surface plume of dissolved trace elements in the South Atlantic Ocean (SAO). Dissolved Fe (dFe) supply from the Congo plume is equivalent to ∼40% of the annual atmospheric dFe supply to the SAO. However this plume is not captured by biogeochemical models, raising questions about its exact sources. To help understand the potential source mechanisms, we use particle tracking experiments to investigate elemental distributions. Results suggest that elevated concentrations of some elements in the Congo plume are primarily sourced from river discharge and wet atmospheric deposition with minimal influence from shelf sediments. River discharge is the main source in shelf regions and some off-shelf regions, whereas atmospheric deposition dominates the area to the southwest of the Congo River outflow. A quantitative analysis along 3$°$S specifically for dFe suggests a decrease in the contribution of river discharge from 90% to 30% moving off-shelf, with a corresponding increase in the contribution of atmospheric deposition. Within the shelf zone, atmospheric deposition accounts for roughly 20%–40% and could be a major source of dFe around the river mouth. Integration of data from cruise GA08 reinforces the finding that wet deposition augments the concentrations of dFe, manganese (dMn), and cobalt (dCo) at distances over 1,000 km from the river mouth. Given present-day patterns of nitrate, Fe, and Co limitation for primary producers in the SAO, changing rainfall patterns may have long-term implications for both regional elemental budgets and ecologically dependent processes sensitive to trace element ratios.