The Sinemurian-Pliensbachian Boundary Event in the Sichuan Basin (Southwest of China) and comparison to Southern Alps (Italy)

he Sinemurian Pliensbachian Boundary Event (~192.5 Ma, SPBE) in the Early Jurassic is marked by a significant negative carbon isotope excursion in marine and continental sedimentary records, alongside evidence of environmental shifts. The SPBE is linked to substantial isotopic light carbon emissions into the atmosphere-ocean system. However, the mechanisms driving the SPBE and the source of extensive carbon emissions are unclear. Current evidence mainly comes from marine successions in the Western Tethys and Laurasian Seaway, making the global impact uncertain. The processes and dynamics of climate and environmental changes during the SPBE are also yet to be clarified. This thesis addresses gaps through a comparative geochemical investigation in two distinct settings: marine deep-water in the Lombardian Basin (Southern Alps, Italy) and continental lacustrine in the Sichuan Basin (China). Specifically, δ13Corg, mercury (Hg) abundance and isotopic composition, total organic carbon (TOC), and total sulfur (TS) were analyzed and integrated with the analysis of Fe-speciation, main and trace elements, across the Sinemurian - Pliensbachian transition in two stratigraphic successions encompassing rocks formed in the Western and Eastern Tethys. Results reveal that Hg enrichment in both marine and continental sections coincides with a major δ13Corg negative perturbation that can be correlated to the SPBE. Furthermore, in marine rocks, Δ199Hg across the SPBE displays near-zero values and variation patterns that are consistent with inputs of volcanogenic Hg into the atmosphere/ocean system. This evidence allows expanding the so far scattered, but consistent, reports from other sedimentary basins, and indicates that Hg enrichments at the SPBE are not local, pointing to a prolonged volcanism phase as a likely cause of the SPBE carbon cycle perturbation. It is proposed that this volcanism could be related to the formation of thinned ocean-continent transitional crust during rifting that led to the break-up of Pangea during the Sinemurian-Pliensbachian transition. Nutrient and productivity enhancements attributed to intensified terrestrial input are recorded in the Sichuan Basin, evidenced by the strong correlation between CIA and phosphorus (P) and elevated Ni/Ti ratios during the SPBE. Mercury isotopes and increased TOC in the Lombardian Basin suggest elevated weathering and nutrient input rates during the initial phases of the SPBE. Iron-speciation and redox-sensitive trace element results show that the Sichuan Basin experienced two brief episodes of intensified anoxic conditions, while high FeHR/FeT ratios in the Lombardian Basin, potentially influenced by carbonate dilution and low FeT content, along with low MoEF* and UEF* values, indicate predominantly oxic conditions. However, despite generally low proxy values, slight variations in all proxies suggest limited oxygen depletion, aligning with suboxic conditions observed in other studies at the SPBE. These redox variations were likely driven by volcanism-related global warming, which intensified terrestrial inputs, boosting primary productivity via phosphorus influx and leading to oxygen depletion across different depositional settings. Although redox shifts in the Lombardian Basin were subtle, it was still not entirely isolated from the broader environmental impacts of the SPBE. Overall, this thesis highlights the global scope and interconnected nature of carbon emissions, nutrient cycling, and environmental feedbacks during the SPBE, contributing to a broader understanding of Early Jurassic environmental dynamics.