Carbon sequestration activated by a volcanic CO2 pulse during Ocean Anoxic Event 2

The Cretaceous Ocean Anoxic Event 2 (about 94 million years ago) is thought to be linked to extensive volcanism, which triggered a biogeochemical chain of events that eventually led to widespread marine anoxia and a remarkable increase in carbon burial in marine sediments(1-3). It has been suggested that the event was accompanied by a substantial decrease in atmospheric CO2 concentrations(4), but the quantification of the drawdown remains controversial(5,6). Here we reconstruct atmospheric CO2 concentrations throughout the ocean anoxic event from counts of the stomata in fossil leaves, and use terrestrial carbon isotopes to link the reconstruction to marine records of the event(7,8). We find that before the onset of ocean anoxia, atmospheric CO2 concentrations increased by 20% over background levels of 370(+100)/(-70) ppm. This was part of a long-term rise in atmospheric CO2 levels, presumably caused by volcanism, which reached a peak of 500(+400)/(-180) ppm. However, two pulses of extensive carbon burial during the ocean anoxic event, as indicated by positive carbon isotope excursions, are associated with decreased atmospheric CO2 concentrations. We conclude that the sequestration of marine organic carbon led to a decrease in atmospheric CO2 concentration of up to 26% during Ocean Anoxic Event 2.