Atmospheric CO2 through Oceanic Anoxic Event II, Late Cretaceous (94Ma)

The paleobotanical record provides a means to establish a link between the terrestrial biosphere and the ocean-atmosphere system during a second order mass extinction event in the Late Cretaceous (94Ma). Marine extinction prior to the Cenomanian-Turonian boundary (CTB) occurred due to marine anoxia, but despite decades of research the causal mechanism remains elusive. This oceanic anoxic event (OAEII) represents a major perturbation to the ocean-atmosphere-terrestrial system that lasted 900 ka. Increased marine nutrient levels and elevated marine primary productivity are hypothesized key driving mechanisms, but the source for higher nutrients is unclear. Increased marine primary production would decrease atmospheric CO₂ (pCO₂), and if terrestrial plants were sensitive to pCO₂ changes there should be a coeval response in the stomatal frequency on fossil leaves.

To provide a rigorous test of the primary productivity hypothesis we are applying the stomatal index method for estimating pCO₂ using plant cuticles from paralic sections of the Dakota Sandstone in Utah. This has been correlated to the CTB stratotype in Colorado using bio-, chemo-, & bentonite-stratigraphy. Terrestrial organic matter coeval with the onset of OAEII in the marine section documents a positive δ¹³C isotopic shift known globally in marine carbon. This validates the correlation and provides evidence for terrestrial and marine carbon cycle linkage via the atmosphere. Stomatal index values rapidly decrease prior to OAEII. If confirmed, this suggests increasing pCO₂ before the positive δ¹³C excursion. It may coincide with a short negative δ¹³C excursion observed in the Bonarelli at Gubbio, corresponding with high sea temperatures in δ¹⁸O and Tex₈₆ proxies in the Atlantic. A pulse of CO₂ before the event could have enhanced hydrologic cycling, delivering nutrients to the ocean and triggering anoxia. Both stomatal index and density increase at the onset of OAEII, supporting increased marine primary production as the driving mechanism.