DEVELOPMENTS IN THE INTERCALIBRATION OF RADIOISOTOPE DATING AND ASTROCHRONOLOGY

The goal of the intercalibration of radioisotope dating and astrochronology is to reduce geologic age uncertainty to the outcrop scale and ultimately to the stratigraphic bedding scale. Radioisotope dating is the principal source of absolute geologic time, but depends on the availability of datable volcanic ash beds, which occur only sporadically in the stratigraphic column. Thus the precision and accuracy of radioisotope dating is lost in stratigraphic intervals between dated ash beds. Astrochronology interpreted from astronomical calibration of cyclostratigraphy resolves geologic time quasi-continuously at the bedding scale and can restore, validate, and enhance time precision and accuracy within ash-poor intervals.

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.