Carbon Cycle Perturbation and Atmospheric pCO2 Reconstruction during the Toarcian Oceanic Anoxic Event

The Toarcian Oceanic Anoxic Event (T-OAE, ~183 Ma) represents one of the most severe hyperthermal events of the Mesozoic, accompanied by elevated extinction rates, accelerated continental weathering, ocean acidification and a prominent negative carbon isotope excursion (CIE). Although various hypotheses have been proposed regarding the causes of the event, its underlying mechanisms remain incompletely elucidated. To better understand the carbon cycle dynamics during this period, we analyzed the compound-specific carbon isotopic composition of the biomarker phytane (δ13Cphy) from the Yorkshire section, UK, thereby reconstructing the evolution of the CIE. Our data reveal a pronounced negative excursion in δ13Cphy, with a maximum magnitude of −6‰. This trend is highly consistent with variations in total organic carbon (δ13Corg). A systematic difference of 2.7±1.5‰ is observed between δ13Cphy and δ13Corg, which aligns with the average biosynthetic carbon isotopic fractionation values reported in previous studies. Furthermore, based on the δ13Cphy data, we preliminarily estimated the evolution of atmospheric partial pressure of CO2(pCO2). The results indicate that atmospheric pCO2 levels during the T-OAE increased to threefold the background baseline, slightly exceeding previous estimates of a twofold increase based on C3 plant proxies. The significant rise in atmospheric pCO2 during the T-OAE is likely linked to volcanic outgassing and the release of thermogenic carbon associated with intrusive magmatism from the Ferrar Large Igneous Province (LIP), which profoundly impacted the contemporaneous marine environment and the survival of calcifying organisms.