Distinguishing the influence of sediments, the Congo River, and water-mass mixing on the distribution of iron and its isotopes in the Southeast Atlantic Ocean

Iron (Fe) is an essential micronutrient for primary production, and Fe isotopic composition (δ56Fe) has become a widely used oceanographic tool for determining sources and evaluating the biogeochemical cycling of dissolved Fe (dFe) in the oceans. Here, we present dFe concentrations and δ56Fe from three unique oceanographic settings (a river dominated margin, a highly productive coastal upwelling margin, and a meridional open ocean transect) collected during the South Atlantic GEOTRACES cruise GA08 along the Namibian-Congo margin. In the North, the offshore Congo River plume dominates the surface ocean, resulting in elevated surface dFe concentrations up to 1000 km from the river mouth, corresponding to increasing δ56Fe values (+0.33 to +0.95‰) with distance from the river outlet. We attribute this unusual and extensive offshore delivery of heavy Fe to dFe preservation by complexation with organic ligands, coupled with rapid off-shelf advection. In the South, the highly productive Benguela Upwelling System produces oxygen depleted to seasonally anoxic bottom waters on the continental shelf, resulting in extremely high subsurface dFe concentrations (up to 42 nmol kg-1) and remarkably light δ56Fe values (as low as -3.31‰), characteristic of dFe(II) production and mobilization via reductive dissolution of Fe oxyhydroxides in sediments. Away from the continental margins, surface waters carry predictably low dFe concentrations (∼0.1 nmol kg-1), associated with isotopically heavy Fe linked to dust deposition and biological uptake. In subsurface waters, and away from Fe sources, we find a remarkably coherent relationship between water masses and dissolved δ56Fe signatures along the GA08 section. Using δ56Fe data from GA08 and water mass analysis, we assign endmember signatures of -0.12 ± 0.02‰ for AAIW, +0.71 ± 0.09‰ for NADW, and +0.35 ± 0.12‰ for AABW. Overall, we find that the distribution of δ56Fe in the South Atlantic can largely be explained by water mass mixing, with some overprinting by local processes and sources, suggesting that dissolved δ56Fe signatures can be used as tracers of deep ocean Fe transport