Fossil Ginkgo Leaves Reveal Coupled Carbon (pCO2)-Mercury (Hg) Variations Across the Early-Middle Jurassic Volcanic Forcing

The Early–Middle Jurassic represents a key greenhouse interval marked by major environmental perturbations associated with the emplacement of the Karoo–Ferrar Large Igneous Province (K–FLIP), particularly across the Pliensbachian–Toarcian (Pl–To) transition and the Toarcian Oceanic Anoxic event. While marine records document coupled carbon and mercury (Hg) cycle disturbances during this interval, their expression in terrestrial ecosystems remains less well constrained. In this study, we present a plant-based, multi-proxy dataset derived from a ~20-million-year fossil ginkgo archive spanning the Lower–Middle Jurassic successions of the Qaidam Basin, China. Fossil leaf specimens from multiple stratigraphic horizons were analyzed for leaf morphology, stomatal characteristics, total organic carbon (TOC), carbon isotope composition (δ13C), reconstructed atmospheric CO2 (pCO2) via leaf gas-exchange model, and Hg concentrations. The results reveal stratigraphically coherent variations in δ13C, Hg concentrations, and reconstructed pCO2across the studied interval. In particular, intervals characterized by more negative δ13C values are commonly associated with elevated Hg concentrations and higher reconstructed pCO2 levels, and vice versa. While alternative processes such as wildfire activities cannot be entirely excluded, the observed coupling between carbon isotopic shifts and Hg enrichment is consistent with a scenario in which large-scale volcanic activity contributed to simultaneous perturbations of the carbon and mercury cycles. These findings contribute to a more integrated understanding of how large-scale perturbations, potentially linked to volcanic activity, were expressed across the terrestrial Earth system, and underscore the value of plant-based proxies for reconstructing coupled biogeochemical cycles during past greenhouse climates.