Development and use of Actinium 227 as a tracer of deep ocean mixing

• Main Author: Levier, Martin
• Other Authors: Laboratoire des Sciences du Climat et de l'Environnement Gif-sur-Yvette (LSCE), Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris-Saclay-Centre National de la Recherche Scientifique (CNRS), Université Paris-Saclay, Matthieu Roy-Barman
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2022
• Subjects: Actinium; Deep mixing; Geochemistry; Protactinium; Southern Ocean; Geochemical; Mass spectrometry; Mélange profond; Géochimie; Océan Austral; Spectrométrie de masse; [SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry; [SDU.STU.OC]Sciences of the Universe [physics]/Earth Sciences/Oceanography; Austral; Weddell
• Online Access: https://tel.archives-ouvertes.fr/tel-03726231; https://tel.archives-ouvertes.fr/tel-03726231/document; https://tel.archives-ouvertes.fr/tel-03726231/file/102492_LEVIER_2022_archivage.pdf

The vertical mixing in the deep ocean is an important process for the functioning of thermohaline circulation and also for the global climatic system. L'actinium-227 (227Ac) is a natural radioactive isotope product by the protactinium-231 (231Pa) decay in deep marine sediment. The short half-life of this isotope (22 years) makes it a specific tracer of vertical mixing of deep ocean. 227Ac analysis by nuclear counting used until now required from 50 to several hundred liters of water for a limited accuracy. This limited strongly the use of 227Ac in oceanography until today. During this thesis, we developed a join 231Pa-227Ac purification protocol suitable for 10L seawater sample, to perform analysis by MC-ICPMS. In addition to these two isotopes, this separation protocol provides different fractions that can be used to other tracers (Nd, Ra or Th isotopes) from the same water sample.This protocol was applied to archival samples collected during the Bonus GoodHope cruise, in the Southern Ocean. 231Pa was measured at the same time as 227Ac because its analysis is essential to exploit 227Ac data. These new 231Pa data support the previous hypothesis of a strong affinity toward the opal produced by the diatoms and allow to refine the transport of radionuclides along slopping isopycnal surfaces at the South of the ACC. Results from 227Ac data gave estimates of the vertical eddy diffusion coefficient of 1 cm2/s over the mid-ocean ridge, of 14 cm2/s near the African margin due to an enhanced turbulence and of the order of 30 cm2/s in the Weddell gyre. An application of these values is proposed to describe the concentration and isotopic signature distributions of Nd near the continental margin. Le mélange vertical dans l'océan profond est un processus important pour le fonctionnement de la circulation thermohaline océanique et donc pour le système climatique global. L'actinium-227 (227Ac) est un isotope radioactif naturel produit par désintégration du protactinium-231 (231Pa) dans les sédiments marins profonds. Comme il ...