During the Aptian Oceanic Anoxic Event 1a, about 120 million years ago, black shales were deposited in all the main ocean basins1. The event was also associated with elevated sea surface temperatures2,3 and a calcification crisis in calcareous nannoplankton4. These environmental changes have been attributed to variations in atmospheric CO2 concentrations2,3,5,6, but the evolution of the carbon cycle during this event is poorly constrained. Here we present records of atmospheric CO2 concentrations across Oceanic Anoxic Event 1a derived from bulk and compound-specific δ13C from marine rock outcrops in southern Spain and Tunisia. We find that CO2 concentrations doubled in two steps during the oceanic anoxic event and remained above background values for approximately 1.5–2 million years before declining. The rise of CO2 concentrations occurred over several tens to hundreds of thousand years, and thus was unlikely to have resulted in any prolonged surface ocean acidification, suggesting that CO2 emissions were not the primary cause of the nannoplankton calcification crisis. We find that the period of elevated CO2 concentrations coincides with a shift in the oceanic osmium-isotope inventory7 associated with emplacement of the Ontong Java Plateau flood basalts, and conclude that sustained volcanic outgassing was the primary source of carbon dioxide during Oceanic Anoxic Event 1a.
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B.D.A.N. received funding through a Rubicon fellowship, awarded by the Netherlands Organisation for Scientific Research (NWO). Additional funding came from the advanced ERC grant ‘The greenhouse earth system’ (T-GRES). J.M.C. and M.L.Q. were funded by University of Jaén fellowships. D.N.S. was funded by a Royal Society URF. R.D.P. and D.N.S. acknowledge the Royal Society Wolfson Research Merit Award. We wish to thank the University of Jaén (CICT) for the use of analytical facilities and NERC for partial funding of the mass spectrometry facilities at the University of Bristol (contract no. R8/H10/63; www.lsmsf.co.uk). M. Heldt is thanked for providing the samples from Djebel Serdj. This work is a contribution of the research projects CGL2009-10329 and CGL2014-55274-P (Spanish Ministry of Science and Technology), ‘Episodios de Cambio Climático Global’ (Instituto de Estudios Giennenses) and RNM-200 (Junta de Andalucía).
The authors declare no competing financial interests.
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Naafs, B., Castro, J., De Gea, G. et al. Gradual and sustained carbon dioxide release during Aptian Oceanic Anoxic Event 1a. Nature Geosci 9, 135–139 (2016) doi:10.1038/ngeo2627
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