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Gradual and sustained carbon dioxide release during Aptian Oceanic Anoxic Event 1a

2016; Nature Portfolio; Volume: 9; Issue: 2 Linguagem: Inglês

10.1038/ngeo2627

1752-0908

B. David A. Naafs, José Manuel Castro Jiménez, Ginés A. de Gea, M. L. Quijano, Daniela N. Schmidt, Richard D. Pancost,

Geology and Paleoclimatology Research

A period of ocean anoxia about 120 million years ago coincided with high temperatures. A reconstruction of CO2 concentrations shows that volcanic outgassing from the Ontong Java Plateau caused CO2 levels to double during the anoxic event. 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.