1. Department of Geology, The Field Museum, 1400 S. Lake Shore Drive, Chicago, Illinois 60605-2496, USA
2. Department of Ecology and Evolution, University of Chicago, 5801 South Ellis Avenue, Chicago, Illinois 60637, USA
3. Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK

Correspondence to: Jennifer C. McElwain¹ Correspondence and requests for materials should be addressed to J.C.M. (Email: jmcelwain@fieldmuseum.org).

Abstract

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 debated^{6, 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 hypotheses^{4, 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 frequency^{9, 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.

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