Soutenance de thèse de Claire Charles

Claire Charles, ingénieure géologue et doctorante en géochimie, soutiendra sa thèse le vendredi 29 avril 2022 à 14h30 à l'IUEM (Amphi A). Son sujet :

Paleoceanographic and paleogeographic reconstructions of the Mozambique Channel (Indian Ocean): contribution of the isotopic study of Fe-Mn crusts

Vous pouvez assister à la soutenance en présentiel ou en visio  avec le lien suivant :

 https://bbb-soutenance.univ-brest.fr/b/cha-444-r6n

Composition du jury :

  • Laurie REISBERG – Rapporteur, Directrice de recherche, CNRS, CRPG, France
  • Martin FRANK – Rapporteur, Professeur, GEOMAR, Allemagne
  • Catherine JEANDEL – Examinatrice, Directrice de recherche, CNRS, LEGOS, France
  • Géraldine SARTHOU – Examinatrice, Directrice de recherche, CNRS, IUEM, France
  • Germain BAYON – Examinateur, Chercheur, Ifremer, France
  • Jean-Alix BARRAT – Directeur de thèse, Professeur, IUEM, France

Invités:

  • Ewan PELLETER – Co-encadrant de thèse, Chercheur, Ifremer, France
  • Sidonie REVILLON – Co-encadrante de thèse, Chercheuse, SEDISOR, France
  • Stephan JORRY, Chercheur, Ifremer, France
  • Jean-Michel KLUSKA, Docteur géologie, TotalEnergies, France

Découvrez ci-dessous le résumé de sa thèse :

The Mozambique Channel represents a complex zone of very intense mixing between the water masses arriving in the south, from the Atlantic Ocean and, the water masses circulating to the north, from the Indian Ocean. It is also known and analysed for its history and its very interesting geological characteristics, linked to its location east of the African craton and west of the continental margin of Madagascar. These oceanographic and geological particularities have always raised questions, about whether and how marine currents have been able to evolve and circulate over time adapting to geodynamics of the channel in constant movement.

Of hydrogenetic origin and of intermediate compositions between Fe-Mn crusts of continental margin and open-ocean, the Fe-Mn crusts of the Mozambique Channel are real archives of the geochemical compositions of the surrounding water masses. Geochemical analysis (major and trace elements; Nd and Pb isotopes) and absolute dating (Be isotopes) of 33 Fe-Mn crusts, distributed from the Agulhas Plateau in the south to the Comoros Basin in the north, allows to establish time series up to 30.7 Ma in order to study the evolution of the regional currents since the Oligocene.

The isotopic signatures (Nd, Pb) obtained on the surface of the Fe-Mn crusts show that for 80,000 years, the North Atlantic Deep Water (NADW) has been circulating with high intensity in the channel, arriving via the Agulhas Plateau. It goes up to the deep basin of Mozambique then to the first Îles Eparses in the centre of the channel where its flow gradually decreases, until reaching the Jeffrey Ridge on the west Madagascar margin. The North Indian Deep Water (NIDW) then in the majority in the Comoros archipelago. However, this oceanographic circulation model has not always worked this way.

Since its establishment and until 11.7 Ma, the proto-NADW had a strong influence up to the Îles Glorieuses. It dominated the deep ocean system of the region. However, between the Miocene and Pliocene (11.7-3.4 Ma), a uplift disrupted the geodynamics of the Mozambique Channel, leading to an elevation of its bathymetry and in particular, the Davie Ridge, formed by underwater reliefs that separate the Comoros Basin from the Mozambique Basin. Faced with this topographical obstacle, the flow of NADW in the north of the channel has decreased, and this current has withdrawn from the Comoros basin, in favour of increase of NIDW. The rise of the Davie Ridge did not, however, stop the evolution of NADW to the north of the channel, with a presence recorded up to the Jeffrey Ridge. Between the Pliocene and Pleistocene (5.1-1.6 Ma), it is a subsidence, registered in the centre of the channel (Hall Bank), which modified bathymetry of the area. In addition to these regional events, deep currents of the channel have been impacted by global phenomena. Their geochemical compositions reveal contributions of dissolved elements, coming from the Tethysian system before its closure (23-14 Ma), transported by deep Mediterranean currents, towards the Indian Ocean, and from the Himalayan system, whose strong erosion in the Miocene (9.0 Ma) made it possible to record it through the Indian currents.

This PhD thesis completes and refines the pre-existing oceanographic models of the Indian Ocean, by providing a high-resolution study of the deep currents of Mozambique Channel since the Oligocene. In addition, the use of Fe-Mn crusts in the identification and characterization of vertical movements is an unprecedented advance for paleogeodynamic reconstructions.

 Keywords: Fe-Mn crusts; Indian Ocean; Mozambique channel; NADW; NIDW; Nd, Pb isotopes; uplift