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

Abstract

Listen

Translate article

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 (δ13Cco2) 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 δ13CDIC (carbon isotopic composition of oceanic dissolved inorganic carbon) and δ13Cco2, we reconstruct the evolution of δ13Cco2 from brachiopod and belemnite δ13C records from the late Pliensbachian to the early Toarcian. Our results indicate that the brachiopod-based δ13Cco2 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 δ13Cco2 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 δ13Cco2 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 δ13Cco2, we conclude that plant carbon isotopic discrimination (△δ13Cplant) 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.