POPULATION DYNAMICS AND GEOCHEMISTRY OF BENTHIC AND PLANKTONIC FORAMINIFERA TO INTERPRET THE PALEOCEANOGRAPHIC CHANGES ACROSS THE CENOMANIAN-TURONIAN OCEANIC ANOXIC EVENT 2 AT LOW AND HIGH LATITUDES

This PhD research project has investigated benthic and planktonic foraminiferal dynamics across one of the greatest environmental perturbations that occurred throughout the Earth’s history, namely the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE 2). It is globally marked by a positive δ13C excursion due to the deposition and burial of large amounts of organic matter in the ocean. The exact causes of OAE 2 are still studied but it is understood that it might be due to a combination of different factors including the emplacement of the Large Igneous Provinces (LIPs), fluctuations of the sea-level, changes in ocean circulation. For this study, four stratigraphic sections have been selected to document how global changes impacted at low and high latitudes. The stratigraphic sections are Clot Chevalier in the Vocontian Basin (SE France) and Eastbourne in the Anglo-Paris Basin (SE England) at low latitudes and IODP Sites U1513 and U1516 in the Mentelle Basin (SW Australia) at high latitudes. Taxonomy, quantitative and statistical analysis have been performed on foraminifera to reconstruct paleoenvironmental and paleoceanographic changes across the OAE 2. In addition, boron isotopes analysis on foraminiferal tests of DSDP Site 463 and of IODP Sites U1513 and U1516 have been conducted to obtain information about ocean acidification across the OAE 2. Samples at low and high latitudes show a continuous foraminiferal record across the OAE 2 displaying high adaptability to extreme environmental conditions, except for a distinct low carbonate interval at Sites U1513 and U1516 characterized by the absence of calcareous taxa. Foraminiferal abundance patterns exhibit notable variances between low and high latitudes with relatively low species diversity and dominance of opportunistic taxa at Sites U1513 and U1516 and with moderate abundances and high species diversity at Clot Chevalier and Eastbourne. This study has delineated six intervals across the Cenomanian-Turonian boundary interval, each marked by unique foraminiferal assemblages reflecting distinct environmental conditions. - Below the OAE 2: high latitudes show low diversity with dominance of opportunistic taxa in the water column that identify a weak stratification and warm waters. At the seafloor benthic foraminifera exhibit taxa tolerant of high organic fluxes, indicating high marine productivity preceding the OAE 2. At low latitudes, a stratified water column indicating meso-oligotrophic regimes and low organic fluxes at the seafloor have been observed. A decrease in ΔpH in surface waters supports enhanced volcanic activity prior to OAE 2, but without significant impacts on foraminiferal tests. Lower part of OAE 2: at high latitudes, biostratigraphic and geochemical data still indicate high marine productivity, warm water conditions and an increase in terrigenous input. In contrast, low latitude planktonic foraminiferal assemblages show the initial thermal disruptions of the stratified water column. At Clot Chevalier the seafloor becomes depleted in benthic foraminifera adapted to low organic fluxes, whereas at Eastbourne a sea-level lowstand is observed. Plenus Cold Event (PCE): at high latitudes, the positive shift in Zr/Rb ratio, the exclusive occurrence of Stensioeina truncata, then increase of epi-infaunal taxa and the decrease in radiolarians indicate improved environmental conditions that might coincide with the PCE. By contrast, low latitudes display clear evidence of the PCE with the occurrence of Boreal planktonic foraminifera and of benthic foraminifera associated to more oxygenated and deeper waters. In addition, a repopulation event is observed at Clot Chevalier. Middle part of OAE 2: the thermal maximum at high latitudes exhibits a remarkable lack of calcareous foraminifera and the dominance of radiolarians indicating extremely high marine productivity. An intensification of the hydrological cycle with increase in rainfall, weathering intensity, and riverine discharge may have disrupted normal oceanic circulation, expanding the Oxygen Minimum Zone (OMZ). At low latitudes, the extinction of Boreal taxa and the dominance of warm water taxa indicate the emplacement of more eutrophic conditions, and a decrease in oxygen levels is observed at the seafloor. Upper part of OAE 2: at low latitudes, unstable eutrophic conditions still occur, but with the diversification of planktonic foraminifera and the decrease in radiolarians at the top of this interval. A change in the benthic foraminiferal assemblage occurs probably due to a slight deepening of the seafloor. ΔpH values in surface waters slightly decrease compared to ΔpH before the OAE 2, but showing a gradual increasing trend upward that supports the hypothesis of the emplacement of more stable conditions. Comparisons with Eastbourne and Clot Chevalier reveal contrasting environments: Eastbourne shows eutrophic conditions, high productivity and sea-level highstand, while Clot Chevalier shows a stable mesotrophic regime. Above OAE 2: high latitudes show generally a turnover toward more eutrophic conditions with a decrease in ΔpH in surface waters. Low latitudes are characterized by a turnover towards more mesotrophic conditions and the establishment of a stratified water column as also testified by the higher ΔpH values in surface waters, but showing fluctuations toward more eutrophic regimes.