A calibration of the Triassic-Jurassic (Tr-J) time scale based on high-resolution geochronology is essential to realistic evaluation of the causes of, and interrelationships between, the various observed biotic and paleo-environmental anomalies that characterize this transition. It is as yet unclear which of several competing hypotheses for the cause, and in particular the timing, of the Tr-J mass extinction is most accurate. The main reasons for this controversy are that (1) a well-constrained correlation of marine and terrestrial sedimentary archives is notoriously difficult, and (2) precise and accurate radio-isotopic ages in biostratigraphically calibrated marine sections are lacking. A number of potential scenarios arise from the currently available geochronological data, none of which can be unambiguously accepted or rejected. Among them is the possibility that (1) the extinction on land and in the ocean was diachronous, or (2) the biotic crisis was synchronous in both environments and the geochronology, particularly in marine environments, needs significant improvement. PIs will analyze minerals contained in volcanic ash-layers in several marine Tr-J sections. Specifically we will apply the U/Pb technique to closed system zircons and complementary 40Ar/39Ar dating to K-bearing minerals. In addition they will target CAMP (Central Atlantic Magmatic Province) volcanics within terrestrial sections with both U/Pb and 40Ar/39Ar analyses where possible. Through careful integration of U/Pb and 40Ar/39Ar geochronology with magnetostratigraphy and biostratigraphy, the time scale of the Late Triassic through Early Jurassic will be greatly improved. The anticipated calibration of terrestrial and marine sections will permit detailed comparison of the chronology of events on land as well as in the ocean and contribute to a more realistic evaluation of potential causes for the biotic crisis.
