Unravelling the sources of carbon emissions at the onset of Oceanic Anoxic Event (OAE) 1a

The early Aptian Oceanic Anoxic Event (OAE) 1a represents a major perturbation of the Earth's climate system and in particular the carbon cycle, as evidenced by widespread preservation of organic matter in marine settings and a characteristic negative carbon isotopic excursion (CIE) at its onset, followed by a broad positive CIE. The contemporaneous emplacement of a large igneous province (LIP) is invoked as a trigger for OAE 1a (and OAEs in general), but this link and the ultimate source of the carbon perturbation at the onset of OAEs is still debated. In this study, we simultaneously assimilate an atmospheric pCO₂ reconstruction along with a δ¹³C record from the Spanish Cau section in an Earth system model to obtain a novel transient reconstruction of emission rates and identify the primary carbon-emitting sources across the negative CIE interval at the onset of OAE 1a. We reconstruct carbon emissions of 4,300–29,200 Pg from a mixture of carbon sources. This estimate is a lower bound, as contemporaneous organic carbon burial is not accounted for. Carbon was first released at slow rates from a ¹³C-depleted reservoir (e.g. thermo- and/or biogenic methane from sill intrusions). Towards the end of the negative CIE the rate of emissions increased and they became more ¹³C-enriched, likely from a dominantly volcanic source (e.g. LIPs). New osmium isotope (¹⁸⁷Os/¹⁸⁸Os) measurements, from the same section as the pCO₂ reconstruction and δ¹³C data, reveal a shift to less radiogenic values coinciding with the change towards mantle-derived carbon emissions as indicated by our modelling results, lending further support to our interpretation. These results highlight that geologically triggered carbon emissions were likely driving the OAE onset.