PhD Defense - Ana Alves

• Isabel Cacho, Professeur, Université de Barcelone - Rapporteure
• Thibault de Garidel-Thoron, Directeur de recherche, Aix-Marseille Université - Rapporteur
• Stéphanie Duchamp-Alphonse, Maîtresse de conférence, Université Paris-Saclay - Examinatrice
• William Gray, Chargé de recherche, Université Paris-Saclay - Examinateur
• Jorge Cubo, Professeur, Muséum national d’Histoire naturelle - Examinateur
• Annachiara Bartolini, Professeur,  Muséum national d’Histoire naturelle - Directrice de thèse
• Eva Moreno, Maître de conférence, Sorbonne Université - Co-encadrante de thèse
• Franck Bassinot, Directeur de recherche, Université Paris-Saclay - Co-encadrant de thèse

Abstract:

Tropical volcanic explosive eruptions, especially if stratospheric in nature, are natural phenomena that can significantly influence climate. Sulfur emissions to the atmosphere play a major role. This aerosol-precursor gas can both modify temperatures by reducing incident solar radiation, and disrupt ocean chemistry. The eruption of the Toba volcano (Younger Toba Tuff, YTT) occurred ~74,000 years ago on the island of Sumatra. It is currently considered the largest eruption of the Quaternary. Numerous uncertainties about the eruption’s key parameters (duration, eruptive dynamics, emissions) persist, making it difficult to estimate its impact on climate. However, some studies have suggested that the Toba eruption may have had an impact on the Indian monsoon. The hypothesis is that the global cooling caused by volcanic aerosols may have altered surface temperature gradients and disrupted atmospheric circulation and monsoon winds. This PhD aims to (1) reconstruct the variability of the Indian monsoon over the last 150,000 years, (2) study at very high resolution the transition between marine isotope stage 5 (interglacial) and stage 4 (glacial) in order to observe the impact of the Toba eruption on the surface ocean and ocean-atmosphere dynamics. To carry out this study, a multi-proxy analysis (boron ( δ¹¹B) and oxygen ( δ¹⁸O) isotopes, Mg/Ca ratio of G. ruber foraminifera, X-ray fluorescence (XRF) and abundance of G. bulloides and G. ruber foraminifera) was carried out on two marine cores from the tropical Indian Ocean. Core BAR94-25 was taken from the Andaman Sea (~600 km from volcano Toba) and core MD00-2355 from the Arabian Sea (~4500 km). These cores are ideally located in two regions strongly influenced by the Indian monsoon. Several tephra levels belonging to the Toba volcano were identified in these two cores, and correspond to several explosive events during the MIS 5-MIS 4 climatic transition. Our results show that over the last 150,000 years, salinity variations in the Andaman Sea are in phase with autumn insolation at 6°N. Comparison with other paleoclimatic records, such as the Borneo stalagmites, revealed a more complex history of hydroclimatic variability in the southern Andaman Sea, whose precipitation regime is influenced, beyond the Indian monsoon, also by “El Niño-Southern Oscillation” (ENSO) and the Indian Ocean Dipole (DOI). In the Arabian Sea, we have observed glacial-interglacial variability of the Indian monsoon, with a weakening of the summer (southwest) monsoon during the glacial period. A detailed study of the MIS 5/4 transition revealed episodes of acidification in the Andaman Sea, coinciding with the eruptive activity of the Toba volcano. We also observed a gradual increase in salinity, reflecting a decrease in rainfall in this region. Thus, Toba’s volcanic activity would have disrupted atmospheric circulation characterized by a mean state similar to El Niño and a DOI-. In the Arabian Sea, our results showed that the eruptive activity of the Toba volcano would have reduced, or even stopped, the summer monsoon (Southwest).