CURRENT STATUS OF THE MESOZOIC ASTRONOMICAL TIME SCALE

An international effort is underway to assemble the Mesozoic Astronomical Time Scale (ATS) using cyclostratigraphy with calibrations to global biostratigraphy and/or magnetostratigraphy, and with quantitative assessments of the cyclostratigraphic signal with that predicted from astronomical modeling. Throughout the Mesozoic, strong, compound cycling at frequencies indicative of orbital eccentricity, axial obliquity and precession cycling is observed, principally in the marine pelagic and hemi-pelagic realms, but also in some shallow marine and long-lived continental lacustrine systems. Recognition of a nearly ubiquitous, dominant ~400-kyr cycling in formations throughout the era has been particularly striking; in some cases, this cycling is defined by third- or fourth-order sea-level sequences. This frequency is associated with the 405-kyr orbital eccentricity cycle, which provides a basic metronome for a construction of the ATS for the entire Mesozoic Era. Numerous formations spanning 10 to 15 myr intervals provide detailed cyclostratigraphic estimates of the ATS. Although many formations this dataset are not temporally overlapping to provide independent verification of signals nor an extended composite scale, it is expected that ongoing work will remedy this situation. Astronomically forced cyclostratigraphy potentially provides a continuous time scale over the Mesozoic that is only constrained by the precision and accuracy of modern solar system modeling. For the early Mesozoic, this constraint of the astronomical target curve represents a resolution limit that is on the order of 0.1 to 0.4 myr; which is a major improvement over published geological time scale compilations. Examples from the Cretaceous and Jurassic highlight the extraordinary fidelity of recoverable astronomical signal; but the Triassic poses special challenges. In addition, some portions of this ATS are yielding insights and some surprises into long-standing geologic problems of rates of ocean crust production and sea level change.