Rare Earth elements: tracers of lithogenic inputs and dissolved-particulate exchanges in the Subpolar North Atlantic (GEOVIDE cruise, GEOTRACES GA03)

– Marion Lagarde, Doctorante, Université Paul Sabatier, Toulouse, France –

Résumé :

The objective of this thesis is to carry out a comprehensive documentation of dissolved and particulate concentrations of Rare Earths (REE) 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 ii) understand the mechanisms underlying the exchange at the margins of trace elements iii) clarify dissolved-particle exchanges. Concerning the influence of the margins, the distributions in particulate REE reveal particularly important lithogenic contributions at the Iberian margin, more concentrated in nepheloid layers propagating at about 1000 km from the coast. The subsequent analysis of dissolved REE concentrations, coupled to the use of an optimized multiparametric analysis that allowed to dissociate the role of mixing from that of other processes on the observed concentrations, showed that particles are indeed at the origin of a contribution by dissolution, marked in the nepheloid layers. This multiparametric analysis also showed that deep water masses are the site of intense scavenging. This confirms that one of the mechanisms of exchange at the margins is the dissolution of particles from these inputs followed by scavenging. It has been shown that internal tidal waves (IT) are responsible for the resuspension of sediments at the margin, the propagation in nepheloid layers being explained by the circulation. This work highlights the importance of lateral inputs from the margins in the oceanic cycle of REE, especially under the impulse of internal waves, whose permanence and ubiquity in the ocean suggest a non-negligible role in the inputs of lithogenic trace elements on a global scale. Concerning the dissolved-particle exchanges, the analysis of the particulate REEs allowed to highlight a predominance of the surface absorption and deep adsorption phenomena. The partition coefficients Kd of the REE were calculated by combining these particulate concentrations with the dissolved concentrations and the mass of the particles in suspension. These Kd’s highlight the variable dissolved-particle exchanges in the first 400 m of the water column, while an increase in affinity for particles with depth is observed below. These data, rare in the literature, can be used in models of the Nd cycle, so far parameterized as fixed due to lack of values. This result contributes to reconcile both simulated surface and deep concentrations with those observed.

Jury :

Mme Catherine JEANDEL (CNRS, LEGOS, Toulouse) Directrice de thèse

Mme Pascale LHERMINIER (IFREMER / LOPS, Brest) Co-directrice de thèse

Mme Kazuyo TACHIKAWA (CNRS, CEREGE, Aix-en-Provence) Rapporteure

Mme Hélène PLANQUETTE (CNRS, LEMAR, Brest) Examinatrice

Mme Géraldine SARTHOU (CNRS, LEMAR, Brest) Examinatrice

Jeroen SONKE (CNRS, GET, Toulouse) Examinateur

Mme Phoebe LAM (University of California, Santa Cruz) Rapporteure

Lien Zoom :

https://zoom.us/j/94999975564?pwd=eGlpOXVqdDhCNnk4dnJ0ejYvQW45Zz09