Iron isotopes in seawater : a new tracer of the oceanic biogeochemistry

• Main Author: Radic, Amandine
• Other Authors: GEOMAR LEGOS, Laboratoire d'études en Géophysique et océanographie spatiales (LEGOS), Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire Midi-Pyrénées (OMP), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales Toulouse (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France-Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales Toulouse (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales Toulouse (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France-Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales Toulouse (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France-Centre National de la Recherche Scientifique (CNRS), Université Paul Sabatier - Toulouse III, François Lacan
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2011
• Subjects: iron isotopes; seawater; iron cycle; biogeochemical tracer; dissolved iron; particulate iron; equatorial Pacific Ocean; Southern Ocean; isotopes du fer; eau de mer; cycle du fer; traceur biogéochimique; fer dissous; fer particulaire; Pacifique équatorial; Océan Austral; [SDU.OCEAN]Sciences of the Universe [physics]/Ocean; Atmosphere
• Online Access: https://theses.hal.science/tel-00589511; https://theses.hal.science/tel-00589511/document; https://theses.hal.science/tel-00589511/file/These_Radic-corr.pdf

Iron is an essential micro-nutrient for phytoplankton growth in the ocean. In broad areas of the ocean, iron limits primary production and therefore plays a role in the carbon cycle. However many questions remain about its marine cycle. Dusts and sediments are considered as the principal sources of iron to the surface open ocean. Because both sources display distinct iron isotopic compositions, iron isotopes in seawater were suggested as a promising new tracer of theses sources. In addition, iron undergoes numerous exchange processes between the various physical and chemical forms coexisting in the water column. Some of these processes have shown isotopic fractionations through in vitro experiments. Iron isotopes in the water column could also help to clarify these processes. At the beginning of my PhD, no dissolved iron isotopes measurements had been performed in the ocean. Because of the very low iron content and the concentrated salty matrix of a seawater sample, such a measurement represented a real analytical challenge. The recovery had to be high enough, with a blank of only a few ng, the matrix had to be efficiently removed and we needed a precise method to correct for the isotopic fractionation occurring during the procedure. We did develop such a method to measure iron isotopes in Fe-depleted seawater satisfying all of these requirements. The successful GEOTRACES intercalibration exercise contributed to validate our method. This method allowed acquiring the first data of dissolved iron isotopes in ocean. We also measured iron isotopes in suspended particles of the seawater, a measurement never performed either so far. The observed δ56Fe variations are significant and range from -0.71 to +0.58‰ with a precision of ±0.08‰ (2σ), the largest variations being in the dissolved phase. Through several oceanic regional studies, first interpretations of the iron isotope cycle in the ocean are highlighted. Below the surface layer of the water column, the Fe isotopic compositions (IC) seem consistent with i) the ...