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PhD Defense

Tristan Vadsaria (LSCE)

Title : Identify and simulate the forcing factors contributing to the developemment of the last sapropel in the Mediterranean Sea

Date and time : The 17-12-2018 at 14h00

Type : thèse

Université qui délivre le diplôme : UVSQ

Location : Amphithéâtre Galilée (bâtiment 713 de l'Orme des Merisiers, CEA)
Members of jury :

Mme Anne MOUCHET: Chercheuse Sénior, Université de Liège - Rapporteur

Mme Kazuyo TACHIKAWA: Directrice de recherche, Université Aix-Marseille - Rapporteur

M Uwe MIKOLAJEVICZ: Directeur de Recherche, Max Planck Institute for Meteorology - Examinateur

M Eelco ROHLING: Professeur, Australian National University - Examinateur

M Giuseppe SIANI: Professeur, Université Paris Sud - Examinateur

M Gilles RAMSTEIN: Directeur de recherche LSCE/CEA - Directeur de thèse

M Jean-Claude DUTAY: Directeur de recherche LSCE/CEA - CoDirecteur de thèse

M Laurent LI: Directeur de recherche LMD/CNRS - CoDirecteur de thèse

Summary :

Sapropel events are anoxic crisis occurring quasi-periodically (21000 years) since the Mediterranean Sea became semi enclosed around 14 Ma ago, with only one connection to the global ocean through the Gibraltar strait. The high level of organic content found in sapropels suggests the shutdown of the deep ventilation, and an enhanced biological activity leading to the accumulation of organic material in sediments for thousands of years. Numerous studies highlighted the role of African monsoon enhancement, linked to precession variation, as the trigger of sapropels. The increased precipitation over the Nile catchment provided a huge amount of freshwater through the Nile River into the salty Eastern Mediterranean, leading  at the end to the development of anoxic environment. This hypothesis was robust enough to explain the sapropels due to the correlation between their frequency and precession variation. However, the diversity of these events in strength, duration and cyclicality cannot be entirely explained with this sole hypothesis. This is the case, for instance, of the last sapropel event, “S1”, which occurred between 10.5 and 7 ka BP, at the end of the last glacial episode. Recent works depicted the non-synchronous timing of the precession, monsoon enhancement and S1 deposition, and invoked other factors, such as sea level rise and climate variation to drive the establishment of anoxic environment in Mediterranean Sea. In this thesis, we investigate, with the help of climate modeling, the contribution of the Nile River during S1 but also other factors potentially more important. First, we took advantage of recent εNd data investigation during S1 to include this tracer, which is appropriate to assess the paleo-oceanic circulation, in a coupled ocean-atmosphere model at 1/8° to represent properly the changes in convection. In a second part, to account for other hydrological changes linked with the Early Holocene climate, we developed a global-regional model architecture to represent the Mediterranean region climate and its oceanic circulation. Finally, we use this model to evaluate the deglaciation of the Eurasian ice-sheet through the Black Sea toward the Eastern Mediterranean Sea. The main results show the 1) capability of simulated εNd to capture the convection changes in response to high freshwater input, validated by data. 2) Our model architecture allows the simulation of the Mediterranean Early Holocene climate and associated hydrological changes with a numerical cost lower than the coupled configuration. Finally, 3), the continental deglaciation would be complementary to the sea level rise to destabilize the deep ventilation and lead to favorable condition to the development of S1. In summary, we developed three different approaches and dedicated tools to investigate the hydrological perturbations occurring prior to Early Holocene over the Mediterranean basin, which drastically modified the ocean dynamics to favor anoxia crisis.

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