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

Abstract

Ninety-four million years ago, during Ocean Anoxic Event 2, there was a marked increase in the burial of organic carbon in marine sediments. Measurements of stomata in fossil leaves show that the two main pulses of carbon burial were associated with a decline in atmospheric CO2 levels of up to 26%. 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 sediments1,2,3. It has been suggested that the event was accompanied by a substantial decrease in atmospheric CO2 concentrations4, but the quantification of the drawdown remains controversial5,6. 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 event7,8. 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.