Caractérisation géochimique des changements paléoenvironnementaux au cours de l'Évènement Océanique Anoxique 2 (~94 Ma) dans le Bassin Vocontien (SE France)

Abstract : The Cenomanian-Turonian transition (~94 Ma) is marked by a period of biogeochemical perturbations named Oceanic Anoxic Event 2 (OAE 2). This event is characterized by sedimentary records presenting an elevated organic matter content, causing a large positive δ13C excursion observed on the global-scale. Geochemical and biological clues indicate that these sedimentary records are associated with a decrease in the O2 concentration of the water-column. Like all the OAEs of the Cretaceous, OAE 2 was initiated in a context of increasing CO2 concentrations in the oceans and in the atmosphere, interpreted as volcanic outgassing. The heterogeneity in the responses toward this global-scale perturbation suggests that regional or local environmental mechanisms contributed to the scenario of initiation and evolution of OAE 2. The objective of this thesis is to determine the role of these mechanisms at different spatial scales and their interactions. The approach consists in a comparison of the biogeochemical conditions associated with OAE2 in a relatively small-scale basin, as the Vocontian Basin (SE France), with others basins in the world. For this purpose, a multiproxy geochemical study was conducted on two sections of the Vocontien Basin (Pont d’Issole and Lambruisse sections). The parallel evolution between the organic geochemical data (TOC, HI vs. OI), the enrichments in redox sensitive elements (Mo, U, V, Zn, Mn) and the iron speciation data reflect an oscillation between suboxic and anoxic conditions in the deepest parts of the Vocontian Basin. Low enrichments in elements indicative of primary productivity (P, Ni, Cu, Ba), suggest however that this should have been relatively modest for this basin. Oxygen-depleted conditions were disrupted by a phase of intense reoxygenation, named Plenus Cold Event (PCE), likely corresponding to a drop in the pCO2 due to carbon sequestration in organic matter within the sediments and by an increased consumption of CO2 by continental weathering. Relative enrichment values in Mo and U indicate a lowered rate of deep-waters renewal, confirming the semi-restricted configuration of the Vocontian Basin at that time and suggest a stratification of the water-column. The evolution of the nitrogen isotope signal in the sediments reflects periods of intensified atmospheric N2 fixation by diazotrophic bacteria, synchron with deoxygenation in the water column. Those periods are, thus, characterized by a strong depletion of bioavailable nitrogen in the surface waters, suggesting that the water-column was intermittently stratified as well. The evolution of the sulfur isotope signal of pyrite, marked by a large negative excursion, suppose a development of sulfidic conditions from the sediments toward the sediments - water column interface or eventually within the water column. The microbial sulfate-reduction occurring in an “open” system with the marine sulfate reservoir, we propose that this was fuelled by an input of sulfates derived from volcanism. Those sulfidic conditions could have induced a phosphorus recycling in the water column, favouring the maintenance of the primary productivity and the development of diazotrophic bacteria in the surface waters. The configuration of the Vocontian Basin were likely a preconditioning parameter for the development of oxygen-depleted conditions by favouring the stratification of the water column. Besides, the volcanic emission of CO2 and sulfur in the oceans, inducing a cascade of positive retroactions affecting the biogeochemical cycles, is interpreted as the triggering factor for the development of oxygen-depletion in the Vocontian Basin as worldwide. The basins recording the more severe oxygen-depleted conditions in the water-column, as the Central Atlantic or the deep basins of Western Tethys, are characterized by a low rate of deep-water renewal, but also with strong primary productivity conditions, associated with intense nutrient inputs. Those two factors seems to be the principal reasons explaining the heterogeneities in the global recording of OAE 2.
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Julien Danzelle, Julien Par. Caractérisation géochimique des changements paléoenvironnementaux au cours de l'Évènement Océanique Anoxique 2 (~94 Ma) dans le Bassin Vocontien (SE France). Sciences du Vivant [q-bio]. Sorbonne Universite, 2019. Français. ⟨tel-02189497⟩

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