Processes controlling the distribution of dissolved silicon isotopes (δ30Si) in the Atlantic and the Southern Ocean

• Main Author: Coffineau, Nathalie
• 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 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, Christina de la Rocha
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: HAL CCSD 2013
• Subjects: Silica cycle; Silicon isotopes; Silicic acid DSi and biogenic silica BSi; Rayleigh distillation; Atlantic and the Southern Ocean; Diatoms; Cycle du silicium; Isotopes du silicium; Acide silicique DSi et silice biogénique BSi; Distillation de Rayleigh; Océan Atlantique et Indien; Diatiomées; [CHIM.OTHE]Chemical Sciences/Other; Indian; Southern Ocean
• Online Access: https://tel.archives-ouvertes.fr/tel-01630072; https://tel.archives-ouvertes.fr/tel-01630072/document; https://tel.archives-ouvertes.fr/tel-01630072/file/These-2013-EDSM-Chimie_marine-COFFINEAU_Nathalie.pdf

Use of silicon isotopes (δ30Si) as a paleoceanographic proxy requires sound knowledge of the distribution and behaviour of silicon isotopes throughout the ocean. Over the past few years considerable effort has been made to map the silicon isotope composition (δ30Si) of silicic acid (dissolved silicon, DSi) and biogenic silica (BSi) throughout the ocean. Diatoms uptake DSi to build up their opal frustules (BSi). During this process, diatoms discriminate against the heavier isotope of silicon (30Si) in favor of the light isotope (28Si). This fractionation leads to BSi that has a lower δ30Si than the DSi source by 1.1 ‰ to 1.5 ‰. In turn, this results in surface waters with low DSi concentrations due to biological removal, and high δ30Si values due to Rayleigh distillation. Conversely, when the BSi dissolves it is with discrimination against the heavier isotope producing dissolved silicon with a δ30Si lower by 0.55 ‰. At the same time, episodes of upwelling occurring throughout the growing season, ocean circulation and mixing, contribute to modify the δ30Si of the dissolved silicon pool in the surface mixed layer, which complicate the use of diatom δ30Si as a proxy for DSi removal during the growing season. This dissertation aims to better understand the processes driving the Si cycle and the δ30Si signature of water masses in different regions of the ocean. New data of δ30Si of dissolved Si are presented and discussed. These data come from 6 CTD profiles from ANTXXIII/9 campaign (Atlantic and Indian sector of the Southern Ocean), 7 CTD profiles from ANTXXIV/3 (Atlantic sector of the Southern Ocean), and 5 CTD profiles from the campaign MSM10/1 (north Subtropical and Tropical Atlantic Ocean). Samples were purified by ion-exchange chromatography following preconcentration via Mg(OH)2 precipitation and extraction of silicon using triethylamine molybdate. Isotopic analyses were carried on a Neptune MC-ICP-MS at medium resolution (Ifremer, Brest). L'utilisation des isotopes du silicium (δ30Si) comme proxy ...