MESOZOIC EPISODES OF C-CYCLE AND CLIMATE PERTURBATIONS: GEOCHEMICAL SIGNATURES OF THE OCEANIC ECOSYSTEM DYNAMICS ACROSS THE TOARCIAN OCEANIC ANOXIC EVENT AND THE VALANGINIAN WEISSERT EVENT.

This Ph.D. research project investigated two paleo-environmental dynamics under extreme Mesozoic carbon cycle and climate perturbations evidenced by major anomalies in the carbon isotope record (δ13C) of the Early Jurassic and Early Cretaceous, namely the Toarcian Ocean Anoxic Event (TOAE) and the Valanginian Weissert Event, respectively. The material selected for the study of the T-OAE is from two new cores (Sogno and Gajum), which represent pelagic deepest records (1000-1500 paleowater depth) of the Lombardy Basin (Southern Alps, Italy). Both cores record an expanded black shale interval (the Fish Level) of 5 m and 16 m at Sogno and Gajum, respectively, representing the lithostratigraphic expression of the T-OAE sensu Jenkyns (1985, 1988). The Fish Level correlates with the early Toarcian negative δ13C anomaly named ‘Jenkyns Event’. The Weissert Event was studied in a unique southern polar location, where a 40 m thick Lower Cretaceous black shale section was recovered at Ocean Drilling Program (ODP) Site 692, in the Weddell Sea, nearshore Antarctica. The two case studies were selected in distinct and very different paleo-settings and latitudes, in order to explore how global climate perturbations translate into local paleo-environmental responses in the sedimentary record, documenting potentially different pathways in the bio-geochemical cycles. A detailed characterization of δ 13C stratigraphy, calibrated with nannofossil biostratigraphy, was the basis for a univocal identification of the T-OAE (including the Jenkyns Event) and the Valanginian Weissert Event anomalies. Multi-proxy based organic and inorganic geochemical analyses were used to reconstruct OC burial and preservation rates, the nature and sources of organic matter, carbonate chemistry, the evolution of paleo-temperatures, the paleodepositional environment and redox conditions. Both case studies confirm that local sedimentary records of strongly restricted and/or deep marine basins register the δ 13C signals of global perturbations and associated environmental variables. This has been demonstrated at both locations by reconstructing proxy variations in ocean temperatures, drastic changes in carbonate chemistry and the biologic response, linked to the T-OAE and the Weissert Event. In contrast, both case studies emphasize that basin physiography and local climatic and environmental conditions play a major role in controlling OC rates even across such extreme carboncycle perturbations. Notably, the aim of this Ph.D. research was to design and apply an integrated approach combining modelling and multi-proxy data to demonstrate an innovative way forward to decouple uncertainties and limitations of individual proxy results and locations from regional to global scale interpretations. New temperature/pCO2 relationships based on multi-proxy data and modelling were reconstructed for the Weissert Event case study. The analytical approach was not identical for the two case studies. Modelling and part of the geochemical analyses were not applied to the T-OAE section due to pandemic restrictions that prevented laboratory work and subsequent elaborations. The outcome suggests that the established strategy for the Weddell Sea case study may now serve as a conceptual framework to advance research on the Italian Jurassic sections, and other OAE examples. This research points out that more emphasis should be made in understanding temperature/pCO2 relationships to evaluate processes, rates, and dimensions of climate and environmental change back in time, setting boundaries either for the study of past carbon-cycle/climate perturbations and for projecting future climate and environmental response.