Insights into early winter Southern Indian Ocean dissolved iron distributions and remineralisation using excess barium (GEOTRACES GIPr07 cruise)

• Main Author: van Horsten, Natasha René
• Other Authors: Laboratoire des Sciences de l'Environnement Marin (LEMAR) (LEMAR), Institut de Recherche pour le Développement (IRD)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Université de Brest (UBO)-Institut Universitaire Européen de la Mer (IUEM), Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS)-Centre National de la Recherche Scientifique (CNRS), Université de Bretagne occidentale - Brest, Universiteit Stellenbosch (Afrique du Sud), Géraldine Sarthou
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: HAL CCSD 2021
• Subjects: Southern Ocean; Winter barium excess; Winter iron cycling; Remineralisation; Océan Austral; Excès de baryum en hiver; Cyclage du fer en hiver; Reminéralisation; [CHIM.OTHE]Chemical Sciences/Other; Antarctic; Austral; Indian; Pacific; The Antarctic; Antarc*; Antarctique*
• Online Access: https://theses.hal.science/tel-04133212; https://theses.hal.science/tel-04133212/document; https://theses.hal.science/tel-04133212/file/These-2021-SML-Chimie_marine-VAN_HORSTEN_Natasha_Rene.pdf

The Southern Ocean is of central importance to the global ocean, connecting the Atlantic, Pacific and Indian basins via the Antarctic Circumpolar Current, which carries large amounts of heat, carbon and nutrients. Thus, this oceanic province partly regulates the Earth's climate. One of the factors influencing the biological carbon pump of the Southern Ocean is the availability of iron, which is an essential nutrient for phytoplankton. However, observations in this region have mainly been conducted in spring and summer, leaving significant gaps in our knowledge of the biogeochemical cycle of iron, in particular on the seasonality of internal processes such as mesopelagic remineralisation.During this study, the distributions of dissolved iron and excess barium (Baxs), a mesopelagic remineralisation proxy, were measured throughout the water column, at seven stations along 30°E in the Southern Indian Ocean during early austral winter of 2017. Our results suggest that remineralization is maintained at comparable levels as observed during summer, well after the bloom decline, at the onset of winter.Regarding dissolved iron, the winter concentrations were not significantly different to summer observations in the western Southern Indian Ocean, remaining at subnanomolar concentrations, from 0.02 to 0.73 nmol L-1.Concentrations in surface water were depleted, indicating that in July there has not yet been a replenishment of the dissolved iron stock in surface waters.The distributions of dissolved iron and Baxs exhibited peaks in intermediate waters on the water column profiles, indicating iron remineralization. Using the apparent oxygen utilization proxy, we estimated that a large fraction (58 ± 1 to 160 ± 2%) of the measured mesopelagic dissolved iron stock was provided by remineralisation. L'océan Austral est d'une importance centrale pour l'océan mondial, reliant les bassins Atlantique, Pacifique et Indien grâce notamment au courant circumpolaire antarctique, qui transporte de grandes quantités de chaleur, de carbone ...