Magmatic Forcing and Carbon Cycle Perturbations across the Sinemurian–Pliensbachian Boundary

The Sinemurian–Pliensbachian boundary (~193 Ma) represents an interval of major environmental and geochemical instability within the Early Jurassic, characterized by a globally recognized negative carbon isotope excursion (CIE). Despite the widespread nature of this perturbation, the fundamental drivers governing this Earth system crisis remain a subject of active debate. In this study, we investigate the evolution of seawater osmium (Os) isotopes and apply Rhenium-Osmium (Re-Os) geochronology to the organic-rich successions of the Prees 2 Borehole (Cheshire Basin, UK), drilled by the International Continental Drilling Program (ICDP) Early Jurassic Earth System and Timescale (JET) project, which provides an expanded and stratigraphically robust record of this boundary event. Our high-resolution chemostratigraphic records reveal coupled negative excursions in initial seawater Os-isotope (187Os/188Osi) and stable carbon isotope (δ13Corg) records across the boundary. The magnitude and abruptness of this unradiogenic Os-isotope shift signify a strong influence of mantle-derived inputs, likely linked to intensified volcanism, rift-related hydrothermal activity, or the opening of marine gateways associated with the breakup of Pangaea. The subsequent isotopic recovery is consistent with a transition towards continental weathering dominance and enhanced organic carbon sequestration. These findings suggest that rifting-related magmatism was a primary driver for the Os-isotopic excursions at this time. Ultimately, this study illustrates the application of the seawater Os-isotope proxy in constraining mantle–crust fluxes and the timing of major tectono-magmatic events, providing a more integrated framework for understanding the evolution of the Early Jurassic Earth system in response to profound environmental forcing.