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 (delta Fe-56) 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 delta Fe-56 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 delta Fe-56 values (+0.33 to +0.95 parts per thousand) 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 delta Fe-56 values (as low as similar to 3.31 parts per thousand), 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 (similar to 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 delta Fe-56 signatures along the GA08 section. Using delta Fe-56 data from GA08 and water mass analysis, we assign endmember signatures of -0.12 + 0.02 parts per thousand for AAIW, +0.71 +/- 0.01 parts per thousand for NADW, and +0.35 +/- 0.12 parts per thousand for AABW. Overall, we find that the distribution of delta Fe-56 in the South Atlantic can largely be explained by water mass mixing, with some overprinting by local processes and sources, suggesting that dissolved delta Fe-56 signatures can be used as tracers of deep ocean Fe transport.

Automated summary

Iron isotopic composition has become a widely used tool for determining the sources and evaluating the biogeochemical cycling of dissolved iron in the oceans. Studies show that the distribution of dissolved iron in the South Atlantic can largely be explained by water mass mixing, suggesting that dissolved iron isotopic signatures can be used as tracers of deep ocean iron transport.