Evolution of carbon isotope composition in atmospheric CO2 during the early Toarcian

The large negative carbon isotope excursions during the Toarcian Oceanic Anoxic Event (T-CIE) implicate the injections of ¹³C-depleted CO₂ into the Earth's ocean-atmosphere system, with multiple climate-sensitive carbon reservoirs as possible carbon sources. The carbon isotopic composition of atmospheric CO₂ (${\delta }^{13}{C}_{{\mathrm{co}}_2}$) is an important parameter of the Earth's carbon cycle and is crucial for exploring the causes of carbon-isotope excursions and understanding the carbon cycle-climate feedbacks. Here we compile δ¹³C and δ¹⁸O data of brachiopod shells and belemnite rostra sampled from two well-known sections calibrated with high-resolution biostratigraphy in Peniche (Portugal) and Yorkshire (England). According to the equilibrium between ${\delta }^{13}{C}_{\mathrm{DIC}}$ (carbon isotopic composition of oceanic dissolved inorganic carbon) and ${\delta }^{13}{C}_{{\mathrm{co}}_2}$, we reconstruct the evolution of ${\delta }^{13}{C}_{{\mathrm{co}}_2}$ from brachiopod and belemnite δ¹³C records from the late Pliensbachian to the early Toarcian. Our results indicate that the brachiopod-based ${\delta }^{13}{C}_{{\mathrm{co}}_2}$ trend shows a short negative carbon isotope excursion from −7 ‰ to −8 ‰ at the Pliensbachian–Toarcian boundary, followed by a gradual positive excursion from −8 ‰ to −5 ‰ in the Polymorphum Zone, which is interrupted by a ∼ 3 ‰ negative excursion during the T-OAE. After careful consideration of the vital fractionation and living habitats of the belemnites, the belemnite-based ${\delta }^{13}{C}_{{\mathrm{co}}_2}$ trend shows a similar trend of temporal variations to that from the coeval brachiopods with a clear negative CIE (∼2.2 ± 0.4 ‰) during T-OAE. The low atmospheric ${\delta }^{13}{C}_{{\mathrm{co}}_2}$ values relative to volcanic gas (−3.8 to −4.6 ‰) suggest that volcanogenic CO₂ is unlikely to result in such negative carbon-isotopic excursion directly during the Early Toarcian. Using the carbon isotopic records of plant tissue from Portugal section, combined with our reconstructed ${\delta }^{13}{C}_{{\mathrm{co}}_2}$, we conclude that plant carbon isotopic discrimination (△δ¹³C_plant) varied from 15.7 ‰ to 24.3 ‰ during the early Toarcian, with a maximum occurring during the T-OAE, which should be a combined record of environmental factors.