DEVELOPMENTS IN THE INTERCALIBRATION OF RADIOISOTOPE DATING AND ASTROCHRONOLOGY
The radioisotope dating “gold standards” are U-Pb dating of zircon and 40Ar/39Ar dating of feldspar. Researchers now report full (analytical and systematic) 2σ uncertainties on the order of 0.1-0.3%. That is, the attainable precision of radioisotope dating is at the ~20 kyr scale in the Cenozoic and ~100 kyr scale in the Mesozoic. Astrochronologic precision and accuracy depend on multiple factors. Uncertainties in the climatic forcing of cyclostratigraphy affects precision (+10 kyr); Earth deceleration affects precession and obliquity, which if unaccounted for may involve ~600 kyr inaccuracy by 50 Ma (1%). Computer numerical error and chaotic planetary motion affects accuracy of modeled orbital eccentricity, up to +405 kyr at 250 Ma (0.16%).
The high precision of the astrochronologic and radioisotopic methodologies enabled recognition of a bias in 40Ar/39Ar dating of ashes in Miocene Moroccan stratigraphy with respect to astrochronology. This led to a correction in the 40Ar/39Ar Fish Canyon age standard from 28.02±0.56 Ma to 28.201±0.046 Ma. Three-way (40Ar/39Ar, U-Pb dating and astrochronology) intercalibration supports the corrected standard. In deeper time, Triassic/Jurassic boundary U-Pb dates of flood basalts indicate intraflow durations consistent with astrochronology from associated lake deposits. U-Pb dating of multiple ashes in Late Permian stratigraphy indicates intra-ash durations agree with astrochronology from the same succession. In sum, radioisotope-astrochronologic intercalibration will soon be routine in future geologic time scale development.