ECOLOGICAL CHANGE ACROSS THE CENOMANIAN-TURONIAN BOUNDARY

Oceanic anoxic event (OAE II) is hypothesized to represent a major perturbation to the ocean-atmosphere-terrestrial system that lasted only 600 to 900 ka, initiated just prior to the Cenomanian-Turonian (C-T) boundary (94Ma). There is currently consensus in support of increased nutrient levels and elevated primary productivity as a key driving mechanism, but the cause of higher nutrient levels is not clear. Higher productivity may have caused a massive increase in marine organic carbon burial rates during OAE II, producing a rapid drawdown of pCO₂ and initiating a temporary climate cooling in the midst of a global greenhouse. Sustained marine anoxia during OAEII produced a second-order marine mass extinction event, but its effect on terrestrial plant ecosystems is not well understood for OAEII or for any other Cretaceous anoxic event. The objective of this study is to evaluate the evidence for changes in terrestrial plant ecosystems in response to massive marine organic carbon burial. To provide a rigorous test of the pCO₂ drawdown hypothesis we are applying the stomatal index method to fossil plant cuticle from paralic sections of the Dakota Sandstone in southwestern Utah. This stratigraphic sequence has been correlated to the orbitally tuned C-T stratotype in Colorado based on biostratigraphy, chemostratigraphy, and bentonite stratigraphy. Terrestrial organic matter coeval with the onset of OAEII in the pelagic marine section documents the positive δ¹³C isotopic shift known globally in marine C_org and C_carb. This provides validation to the correlation and evidence that the terrestrial and marine carbon cycles were linked via the atmosphere. Preliminary stomatal index results from angiosperm cuticle suggest that pCO₂ was high preceding the event, consistent with expectations for a greenhouse world. Massive extinction in the plant record seems to be lacking based upon previous palynological work, but this needs confirmation. This may indicate that climate change during OAEII was not large enough to produce massive extinction, but instead may have shifted ecological dominance to different plant taxa as occurred at the Triassic-Jurassic boundary. Cuticle morphotype and pollen diversity will be analyzed in detail through the event to test for correspondence between changes in the carbon cycle and plant ecology.