title: Iron isotopes reveal distinct dissolved iron sources and pathways in the intermediate versus deep Southern Ocean.
abstract: As an essential micronutrient, iron plays a key role in oceanic biogeochemistry. It is therefore linked to the global carbon cycle and climate. Here, we report a dissolved iron (DFe) isotope section in the South Atlantic and Southern Ocean. Throughout the section, a striking DFe isotope minimum (light iron) is observed at intermediate depths (200-1,300 m), contrasting with heavier isotopic composition in deep waters. This unambiguously demonstrates distinct DFe sources and processes dominating the iron cycle in the intermediate and deep layers, a feature impossible to see with only iron concentration data largely used thus far in chemical oceanography. At intermediate depths, the data suggest that the dominant DFe sources are linked to organic matter remineralization, either in the water column or at continental margins. In deeper layers, however, abiotic non-reductive release of Fe (desorption, dissolution) from particulate iron-notably lithogenic-likely dominates. These results go against the common but oversimplified view that remineralization of organic matter is the major pathway releasing DFe throughout the water column in the open ocean. They suggest that the oceanic iron cycle, and therefore oceanic primary production and climate, could be more sensitive than previously thought to continental erosion (providing lithogenic particles to the ocean), particle transport within the ocean, dissolved/particle interactions, and deep water upwelling. These processes could also impact the cycles of other elements, including nutrients.
authors.affiliation: LEGOS, University of Toulouse, CNRS, CNES, IRD, UPS, 31400 Toulouse, France.
authors.forename: Cyril
authors.initials: C
authors.lastname: Abadie
authors.affiliation: LEGOS, University of Toulouse, CNRS, CNES, IRD, UPS, 31400 Toulouse, France; francois.lacan@legos.obs-mip.fr.
authors.forename: Francois
authors.initials: F
authors.lastname: Lacan
authors.affiliation: LEGOS, University of Toulouse, CNRS, CNES, IRD, UPS, 31400 Toulouse, France.
authors.forename: Amandine
authors.initials: A
authors.lastname: Radic
authors.affiliation: LEGOS, University of Toulouse, CNRS, CNES, IRD, UPS, 31400 Toulouse, France.
authors.forename: Catherine
authors.initials: C
authors.lastname: Pradoux
authors.affiliation: Géosciences Environnement Toulouse, UMR 5563 CNRS/UPS/IRD, 14-16 avenue Edouard Belin, 31400 Toulouse, France.
authors.forename: Franck
authors.initials: F
authors.lastname: Poitrasson
keywords: GEOTRACES; South Atlantic; Southern Ocean; iron isotopes; remineralization