TESTING THE PCO2 DRAWDOWN HYPOTHESIS AT THE CENOMANIAN-TURONIAN BOUNDARY USING FOSSIL PLANT CUTICLE

Global oceanic anoxic events (OAEs) represent major perturbations to the ocean-atmosphere system that occur within geologically short time frames; ~800ka in the case of OAEII. They are believed to be caused by large-scale changes in the carbon cycle, forced by perturbations in volcanism, rates of continental weathering, and oceanic processes such as overturn and nutrient availability. Despite what has been learned about the OAEII interval at the Cenomanian-Turonian (C-T) boundary, the driving mechanism for this event remains elusive. A massive increase in marine organic carbon burial at the onset of OAEII may have caused a dramatic drawdown in pCO₂, initiating a temporary cooling of climate. Two previous estimates have shown that pCO₂ does decrease, but the magnitudes of the estimates differ greatly (150ppm vs. 850ppm). The present study uses the stomatal index method on terrestrial plant cuticle to make a more direct estimate of the magnitude of the pCO₂ drawdown, one that is independent of the biases inherent in the two previous estimates taken from pelagic marine sections. We have collected abundant, well-preserved fossil plant cuticle in southwestern Utah from marginal marine sections of the Dakota Sandstone which formed the western shoreline of the Western Interior Seaway. The sections in Utah have been correlated to the orbitally tuned C-T boundary stratotype in Colorado using sequence stratigraphy, biostratigraphy, bentonite stratigraphy, and carbon isotope stratigraphy. This chronostratigraphic framework allows for a higher resolution analysis of pCO₂ changes through the OAEII interval than for previous studies. Initial results document cuticle from a diverse assemblage of plants, including angiosperms (Lauralean), conifers, bennettites, ferns, and potentially ginkgophytes. A comparative database of plant cuticles is being created for online access by the paleobotanical community, intended to foster collaborative research on modern and fossil plant cuticle. Analysis of coal, charcoal, and cuticle fragments document the positive δ¹³C excursion characteristic of the OAEII interval for the first time in marginal marine sections. Some of the δ¹³C samples taken from terrestrial plant material within the OAEII interval are highly enriched, with values near -19‰ (VPDB).