Changes in carbon dioxide during an oceanic anoxic event linked to intrusion into Gondwana coals

The marine sedimentary record exhibits evidence for episodes of enhanced organic carbon burial known as ‘oceanic anoxic events’ (OAEs)1,2. They are characterized by carbon-isotope excursions in marine3 and terrestrial4 reservoirs and mass extinction of marine faunas5. Causal mechanisms for the enhancement of organic carbon burial during OAEs are still debated6,7, but it is thought that such events should draw down significant quantities of atmospheric carbon dioxide7. In the case of the Toarcian OAE (∼183 million years ago), a short-lived negative carbon-isotope excursion in oceanic and terrestrial reservoirs has been interpreted to indicate raised atmospheric carbon dioxide4 caused by oxidation of methane catastrophically released from either marine gas hydrates4 or magma-intruded organic-rich rocks8. Here we test these two leading hypotheses4,8 for a negative carbon isotopic excursion marking the initiation of the Toarcian OAE using a high-resolution atmospheric carbon dioxide record obtained from fossil leaf stomatal frequency9,10. We find that coincident with the negative carbon-isotope excursion carbon dioxide is first drawn down by 350 ± 100 p.p.m.v. and then abruptly elevated by 1,200 ± 400 p.p.m.v, and infer a global cooling and greenhouse warming of 2.5 ± 0.1 °C and 6.5 ± 1 °C, respectively. The pattern and magnitude of carbon dioxide change are difficult to reconcile with catastrophic input of isotopically light methane from hydrates5 as the cause of the negative isotopic signal. Our carbon dioxide record better supports a magma-intrusion hypothesis8, and suggests that injection of isotopically light carbon from the release of thermogenic methane occurred owing to the intrusion of Gondwana coals by Toarcian-aged Karoo-Ferrar dolerites.