FOUNDATION FOR THE NEXT GENERATION OF PALEOCEANOGRAPHIC AND BIOGEOCHEMICAL STUDIES: DEVELOPING A NEW LOWER CRETACEOUS TIME SCALE

During unusually warm greenhouse conditions of the Aptian to Cenomanian, several major perturbations in the global carbon cycle are reflected by organic-rich sedimentary deposits and/or carbon isotope excursions (OAEs). To fully understand the ocean-climate dynamics of this greenhouse world, and the mechanistic drivers that propel ocean anoxia and deterioration of ecosystems, new radioisotopic dating, in parallel with a more geographically dispersed array of high-quality Cretaceous sedimentary records are essential. The Yezo Group (YG) on Hokkaido, Japan comprises Barremian to Paleocene sediment deposited in a high latitude Pacific Ocean-facing fore arc basin. Unlike other well-studied Lower Cretaceous sequences, e.g., the Vocontian Basin, France (VB), the YG contains rhyolitic tuffs amenable to precise U-Pb and ⁴⁰Ar/³⁹Ar dating. Our international collaboration aims to: (1) determine U-Pb and ⁴⁰Ar/³⁹Ar ages of rhyolitic tuffs in Aptian-Cenomanian strata of Japan, (2) Integrate these new age determinations with new and existing Os- and C-isotope chemostratigraphy to quantify the timing and rates of oceanic processes associated with major carbon-cycle perturbations, (3) Compile global geochemical proxy data for OAE1a within a common temporal and sequence stratigraphic framework, and analyze trends and patterns in these data in sections from the Pacific to Europe to evaluate the roles of sea level and ocean circulation, (4) Evaluate the competing volcanic and climatic/orbital hypotheses for initiation of the major carbon-cycle perturbations (OAEs) of the Aptian and Albian, and (5) Map and explore the significance of geography in the timing and magnitude of geochemical signals. New dates from the YG indicate that OAE1a occurred between 119.3 and 118.8 Ma, supporting an age for the Barremian-Aptian stage boundary of ~121 Ma. The emerging chronology and chemostratigraphy of the YG will be exported and correlated to new records from drilled core in the Tethyan VB for which we are also generating new Os- and C-isotope chemostratigraphy and an astrochronologic age model focused on the critical onset interval of OAE1a. Improving the Lower Cretaceous time scale by integrating French and Japanese strata aims to rectify critical time scale inaccuracies, and employ refined time scales and new proxy data to address fundamental questions concerning lithosphere-hydrosphere-biosphere interactions associated with major OAEs.