This award supports a multidisciplinary paleobotanical and biogeochemical study that will link marine and terrestrial records of a Mesozoic ocean anoxic event (OAE). These geologically brief (600-800 kyr) episodes of enhanced organic carbon burial took place mainly during a time of peak greenhouse warming in the Cretaceous. It has been suggested that some of them caused perturbations in the carbon cycle large enough to impact atmospheric CO2 at levels comparable to, or greater than glacial scale oscillations. Yet the history of atmospheric CO2 before, during, and after an OAE has never been directly evaluated using a terrestrial pCO2 proxy. This project will investigate the links between changes in the biogeochemical cycling of carbon, regional climatic change, and regional scale changes in plant biodiversity and ecology associated with the Cenomanian-Turonian (C-T, +-94 Ma) OAE II in the Western Interior basin of North America. Three main interrelated hypotheses will be tested: (1) was enhanced marine organic carbon production, burial, and preservation during OAE II initiated as a feedback of the marine system to increased concentrations of atmospheric carbon dioxide for a period of time before onset of the event? (2) Did enhanced marine carbon burial and preservation, as indicated by a significant positive isotopic excursion in marine carbonate and organic carbon, result in a draw down of atmospheric carbon dioxide? and (3) Did fluctuations in atmospheric CO2 before and during OAE II cause global climatic shifts which, in turn, resulted in landscape scale shifts in the dominance/diversity structure of terrestrial plant communities? Did these changes play any role in the rise of the angiosperms?
These hypotheses will be tested by examining the rate and magnitude of change in atmospheric paleo-pCO2 (using the stomatal index-pCO2 proxy) and regional mean annual temperature (using leaf margin analysis) on dispersed plant cuticle and macrofloras respectively, collected mainly from marginal marine sections of the Dakota and Straight Cliffs Formations of southwestern Utah. Timing the changes in pCO2 and temperature relative to marine carbon burial will be possible due to detailed correlation of the terrestrial sections to the C-T stratotype in offshore marine facies of Colorado, where a high resolution temporal framework has been established. An online archive of modern cuticle morphotypes developed as part of this project will be used to distinguish dispersed fossil cuticles throughout the C-T interval. Paleoecological analyses of the dispersed cuticle record will be used to track regional vegetation dynamics in response to CO2 and climatic fluctuations throughout the C-T interval. The broader impacts of the project include scientific contributions to basic research on a subject of great societal relevance (understanding relationships between carbon cycle events and climate change), training of future geoscientists, and dissemination of results through innovative teaching initiatives. Two key innovations that distinguish the project are a contribution to geoinformatics via development of an online digital database for cuticle and macrofloral material, and a substantive effort to engage under-represented groups in geoscience through weekend enrichment classes (called Project Excite).
