Vegetation dynamics and carbon isotope responses to the early eocene hyperthermals

Early Eocene hyperthermal events, including the Paleocene-Eocene Thermal Maximum (PETM) and subsequent events (ETM2/H1, H2, and I1), provide critical insights into vegetation and carbon cycle responses to extreme warming during the Early Eocene. Here, we present multiproxy biomarker records and compound-specific δ¹³C data from the Fushun Basin in Northeast China, systematically reconstructing the vegetation dynamics and carbon flux changes during these hyperthermals. We focused on gymnosperm-derived diterpenoids and angiosperm-derived triterpenoids as tracers of floral response. During the PETM, gymnosperms experienced a transient surge in vegetation carbon flux, in contrast to angiosperms, which exhibited a persistent decline across all hyperthermal phases. This challenges the conventional paradigm of gymnosperm vulnerability to warming. Concurrently, Climate change has induced complex alterations in vegetation composition. Compound-specific δ¹³C analysis revealed a significant negative carbon isotope excursion (CIE) in diterpenoids during the PETM, comparable to that of total organic carbon (δ¹³C_TOC), suggesting that climate intensity, rather than vegetation shifts, drove isotopic fractionation. These findings highlight the differential responses of gymnosperms and angiosperms to hyperthermals, with gymnosperms dominating carbon flux during the PETM. Our study emphasizes the role of the magnitude of climate change in modulating vegetation carbon fluxes and isotopic fractionation, highlighting the need to consider plant physiological adaptations under extreme climatic conditions to understand carbon cycle dynamics.