Rare Earth Elements, tracers of lithogenic inputs ad dissolved-particulate exchanges in the subpolar North Atlantic (GEOVIDE cruise, GEOTRACES GA01)

• Main Author: Lagarde, Marion
• 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, Catherine Jeandel, Pascale Deniaud-Lherminier
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2022
• Subjects: Marine Geochemistry; Meridional Overturning Circulation; Land-ocean transfer; REE concentration; Partition coefficients; GEOVIDE; Géochimie marine; Apports continents-océan; Concentration terres rares; Coefficients de partition; [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; North Atlantic
• Online Access: https://theses.hal.science/tel-04557447; https://theses.hal.science/tel-04557447/document; https://theses.hal.science/tel-04557447/file/2022TOU30338a.pdf

In the ocean, Rare Earths elements (REE) concentrations provides information on the exchanges between the dissolved and particulate phases. They are also tracers of lithogenic inputs from the margins and the bottom of the ocean from the sediments deposited there. However, the oceanic cycle of REE is still not fully understood. This is illustrated by the example of the boundary exchange (BE), a term that encompasses mechanisms at the origin of a decorrelation between dissolved Nd concentrations and its isotopic composition (I.C.) near the margins. This difference, called "Nd paradox", reveals the existence of processes not identified yet. The objective of this thesis is to carry out a complete documentation of dissolved and particulate REE concentrations in the subpolar North Atlantic, along the GEOVIDE transect (N/O Pourquoi Pas?, May-June 2014). This work aims to i) determine the influence of margins on the REE cycle ii) understand the mechanisms underlying the boundary exchange iii) clarify dissolved-particle exchanges. The first part of the thesis focuses on the distributions of particulate REE concentrations over the first 1500 meters. These distributions reveal i) elevated lithogenic contributions close to the Iberian margin, more important in nepheloid layers propagating to about 1000 km off the coast ii) a predominance of absorption at the surface, progressively replaced by a dominant adsorption with depth. The first result led to investigate the mechanism at the origin of the formation and propagation of the observed nepheloid layers. For this purpose, a simulation of the generation and dissipation of internal tides (IT) was performed. Combined with available data on the nature of the margin sediments and a reanalysis of a circulation model output, it showed that IT are a source of lithogenic particles for the ocean, dispersed by the circulation up to 400 km off the margin. This result raises the question of the potential role of particles dissolution on dissolved REE distributions. The subsequent ...