2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 13
Presentation Time: 11:45 AM

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

BARCLAY, Richard, Earth & Planetary Sciences, Northwestern University, 1850 Campus Drive, Evanston, IL 60208, MCELWAIN, Jennifer C., School of Biology and Environmental Science, University College Dublin, Belfield, Dublin, Dublin 4, Ireland and SAGEMAN, Bradley B., Earth and Planetary Sciences, Northwestern University, 1850 Campus Drive, Evanston, IL 60208-2150, barclay@earth.northwestern.edu

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.