Multiple sulfur isotopes insights on the biogeochemistry of the Mediterranean during the Messinian Salinity Crisis

Soutenance en anglais. Voici le lien pour y assister à distance : https://u-paris.zoom.us/j/89052434735?pwd=UERGd1M3ZmlPT2FrWFFwTlNsR3E2Zz09 ID de réunion : 890 5243 4735 Code secret : 13122022 Jury: Sasha Turchyn (rapporttrice),Université de Cambridge, Royaume-Uni David Johnston (rapporteur), Université de Harvard, Massachusetts, États Unis Rachel Flecker (examinatrice), Université de Bristol, Royaume-Uni Christophe Thomazo (examinateur), Université de Bourgogne-Franche Comté, France Jérôme Gaillardet (Président du Jury), Université Paris Cité, IPGP, France Giovanni Aloisi (Directeur de thèse), CNRS, IPGP, France The late Miocene (5.97– 5.33 Ma) Messinian Salinity Crisis (MSC) is an exceptional - yet controversial - paleoceanographic and geological event that profoundly affected the Mediterranean Basin, leaving behind an immense volume of evaporites known as the Mediterranean Salt Giant (MSG). The MSC was the result of the progressive restriction of the hydrological exchanges between the Mediterranean and the Atlantic Ocean, eventually leading to the complete isolation of the Mediterranean basin from the global ocean. A large body of evidence suggests that hydrological restriction led to a profound modification of Mediterranean biogeochemistry, already before the onset of evaporite precipitation. Nevertheless, the biogeochemical functioning of the Mediterranean Sea during the deposition of evaporite deposits has been overlooked, mainly because these rocks are devoid of classic proxies used in typical paleo-oceanographic reconstructions. In this manuscript, I tackle this open problem by studying the biogeochemical cycling of sulfur in the calcium sulfate deposits that formed in Mediterranean basins during the MSC. Sulfur is particularly interesting in this context. First, because it is one of the building blocks of the minerals that constitute the MSG. Second, because the sulfur cycle is tightly coupled to the carbon cycle, making it sensitive to carbon cycle perturbations, particularly in the context of hydrological restriction. I used multiple sulfur isotope as a tool to characterize and quantify the redox biogeochemical processes that drove the sulfur cycle in the deep and in the marginal basins of the Mediterranean during the MSC. Supported by existing evidence for fossil sulfide oxidizing bacterial in marginal basin gypsum deposits, my multiple S isotope approach provides what is the first evidence of cryptic sulfur cycling in the geological past, involving microbial sulfate reduction into sulfide, and the almost quantitative re-oxidation of sulfide back to sulfate. This process involved the remineralization of exceptional benthic fluxes of organic carbon (3.8 mmol.cm-2.yr-1), comparable to those observed in the most productive areas of the modern ocean, where biogeochemical cycling is driven by microbial consortia functionally equivalent to those active during the MSC. In the second part of my PhD, I used a similar approach to understand the biogeochemical cycling of sulfur in deep, open Mediterranean basins at the end of the MSC. I show that cryptic sulfur cycling was not restricted to shallow, marginal basins of the Mediterranean during the first stages of the MSC, but was a major feature of the biogeochemical cycling of sulfur also in the deep basins and in the final stages of the MSC. As a spin-off of my biogeochemical investigations, I determined an empirical relationship between the variability in isotopic signature of biogeochemically formed iron sulfide minerals and the thickness of the water column under which they formed. I use this relation, together with calculations of crustal vertical motions performed by a PhD colleague, to determine the maximal bathymetric depth of several Mediterranean sub-basins at the end of the MSC; and from this, the water level fall experienced by the Mediterranean Sea during the Messinian Salinity Crisis. The results of this study suggest a sea-level drawdown of minimum 1100m in the western Mediterranean and 2500m in the eastern Mediterranean, supporting independent geological and geophysical evidence for the disconnection of the two mains basins of the Mediterranean during the Messinian Salinity Crisis. Keywords: sulfur cycle, ocean biogeochemistry, salt giant, gypsum, Mediterranean Sea, Messinian Salinity Crisis, multiple sulfur isotopes, iron sulfide, sulfate