Numerical simulation of atmospheric and oceanic biogeochemical cycles to an episodic CO2 release event: Implications for the cause of mid-Cretaceous Ocean Anoxic Event-1a

Response of biogeochemical cycles in the atmosphere and ocean to a major episode of CO2 release event was investigated using numerical simulations. The results are applied to examine a previously proposed scenario for the Ocean Anoxic Event-1a (OAE-1a) caused by an episodic CO2 release due to the Ontong Java Plateau (OJP) formation in the mid-Cretaceous. We developed an atmosphere–ocean biogeochemical cycle model that includes geochemical input to and output from the atmosphere and ocean due to submarine volcanic activity, continental weathering and sedimentation. We made a reference state, and then the state was perturbed in 42 cases of episodic CO2 release with different rates and periods. In all cases, the elevated atmospheric CO2 level results in an increase in phosphate concentration in the ocean caused by intensified weathering. Variations in the marine carbon isotope ratio are characterized by negative-to-positive excursions with a longer timescale for the latter. The deep water becomes anoxic in the cases with the release of a large amount of CO2. These features are also seen in the geological records of OAE-1a. From quantitative comparisons, we found that the amount of CO2 release required to cause OAE-1a is about 8 × 1018 mol, and atmospheric CO2 increases by a factor of three or more (> 3600 ppm) in these cases. The amount can be accounted for by CO2 released due to the OJP formation, if the original magma contained a large CO2 fraction. Therefore, we conclude that the OJP formation possibly caused OAE-1a.