Iron isotopes as tracers of the iron cycle in the Atlantic sector of the Southern ocean

• Main Author: Abadie, Cyril
• Other Authors: Laboratoire d'études en Géophysique et océanographie spatiales (LEGOS), Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3), Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire Midi-Pyrénées (OMP), Météo France-Centre National d'Études Spatiales Toulouse (CNES)-Université Fédérale Toulouse Midi-Pyrénées-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Météo France-Centre National d'Études Spatiales Toulouse (CNES)-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Centre National de la Recherche Scientifique (CNRS), Universite Toulouse III Paul Sabatier, Lacan Francois, Poitrasson Franck
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2014
• Subjects: Geochemistry; Isotope; iron cycle; carbon cycle; Phytoplankton; Climate; southern ocean; Geochimie; cycle du fer; cycle du carbonne; Phytoplancton; Climat; ocean austral; GEOTRACES; [SDU.OCEAN]Sciences of the Universe [physics]/Ocean; Atmosphere; Austral; Southern Ocean; Weddell
• Online Access: https://tel.archives-ouvertes.fr/tel-01221871; https://tel.archives-ouvertes.fr/tel-01221871/document; https://tel.archives-ouvertes.fr/tel-01221871/file/Cyril%20Abadie_%20thesis.pdf

In wide parts of the ocean, iron is a limiting element for phytoplankton growth due to its very low dissolved concentrations. It is thus a fundamental element of the global carbon cycle and climate. However, its cycle is still not well known. We thus used iron isotopes in order to better constrain its cycle in the Southern Ocean. Our measurement of iron isotopic compositions allowed us to characterize each iron source in this area, but also to evidence processes that we could not have evidenced using iron concentrations only. We have shown that the non reductive release of dissolved iron from the South African margin sediments is a major source of dissolved iron in the vicinity of this margin. A hydrothermal contribution above the mid-oceanic ridge, and lithogenic inputs of different origins in the Weddell Gyre compared to the rest of the section have been characterized. We have estimated the isotopic fractionation factors associated to key processes of the iron cycle, notably in surface waters for the biological uptake of iron by phytoplankton, at intermediate depths for the organic matter remineralization, and in deeper samples for the sorption processes. Our results suggest that the main process releasing dissolved iron in the deep ocean is iron desorption from inorganic particles, contrary to the commonly accepted hypothesis according to which it would be in situ organic matter remineralization. Dans certaines parties de l’océan, le fer est un élément limitant pour la croissance du phytoplancton en raison de ses très faibles concentrations. C’est donc un élément fondamental pour le cycle global du carbone, et par conséquent le climat. Cependant, son cycle est encore mal connu. Nous avons donc utilisé les isotopes du fer afin de mieux contraindre son cycle dans l’océan Austral. Les compositions isotopiques obtenues nous ont permis de caractériser chaque source de fer dans cette zone, mais également de mettre en évidence des processus que nous n’aurions pas pu repérer en utilisant seulement les ...