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

During the Aptian Oceanic Anoxic Event 1a, about 120 million years ago, black shales were deposited in all the main ocean basins. The event was also associated with elevated sea surface temperatures and a calcification crisis in calcareous nannoplankton. These environmental changes have been attributed to variations in atmospheric CO₂ concentrations, 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 inventory 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.