Tracking the chemical elements derived from sediments to the open ocean using Ra isotopes : the case study of the Crozet and Kerguelen Islands (Southern Ocean)

• Main Author: Sanial, Virginie
• 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 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), Université Paul Sabatier - Toulouse III, Pieter Van Beek
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2015
• Subjects: Kerguelen; KEOPS-2; Radium; Barium; Natural Iron fertilisation; GEOTRACES; Southern Ocean; Crozet; Océan Austral; Baryum; Fertilisation naturelle en fer; [SDU.OCEAN]Sciences of the Universe [physics]/Ocean; Atmosphere; Antarc*; Antarctic; Îles Crozet; Kerguelen Islands
• Online Access: https://theses.hal.science/tel-01344351; https://theses.hal.science/tel-01344351/document; https://theses.hal.science/tel-01344351/file/2015TOU30118.pdf

The Southern Ocean is known to be the largest High-Nutrient, Low-Chlorophyll region of the global ocean. While nutrient concentrations (nitrate, phosphate, silicate) are high, the phytoplankton development is paradoxically limited mostly because of the low dissolved iron concentrations of the Southern Ocean waters (Martin_iron_1990). The Crozet and Kerguelen Archipelagos, located in the Indian sector of the Southern Ocean, constitute two major topographic obstacles to the eastward-flowing Antarctic Circumpolar Current. The interaction of the current with the sediments of the shallow Crozet and Kerguelen plateaus contributes to the supply of iron downstream of these islands, thus leading to large phytoplankton blooms in these regions (Blain et al. 2007, Pollard et al. 2007). These phytoplankton blooms constitute unique open-air laboratories to study the response of the ecosystems and the impact on biogeochemical cycles to natural iron fertilization. This PhD thesis was done in the framework of the KEOPS-2 project. Radium isotopes (223Ra, 224Ra, 226Ra and 228Ra), that are powerful tools to study the ocean circulation and mixing, are the main tools used here. The four natural occurring isotopes display half-lives ranging from a few days to thousands of years and are produced by the decay of particle-bound thorium isotopes in sediments. They are delivered to the open ocean by diffusion and advection processes where they behave as conservative tracers in such a way that the water body keeps the signature of its contact with the sediments modulated by the half-lives of the radium isotopes. Therefore, we used Ra isotopes to (i) investigate the origin and the dispersion of the sediment-derived inputs - including iron - and (ii) to estimate the timescales of the transfer of surface waters between the shelf and offshore waters. We compared the Ra dataset with data acquired using physical tools (surface drifters and Lagrangian model derived from altimetry). Firstly, the use of three independent methods - including ...