Precision and Accuracy in Modern Geochronology: Astronomical Timescales
Modern radioisotope dating holds great promise for improving estimates of geologic time to an astonishing 0.05-0.1% level of precision. However, radioisotope dating can be carried out only where datable material occurs in the stratigraphic record; for intervals between dated levels, this hard-won precision is lost. Fortunately, astronomically forced stratigraphy can restore much of this precision by providing continuous time information between radioisotopically dated rocks. Therefore, recovery of high-resolution astronomical timescales throughout the geologic record is an important goal in earth science. Current models of Earth's astronomical parameters (eccentricity, obliquity and precession) are now used routinely to construct astronomical timescales from the cyclic stratigraphic record. There are three major sources of error that limit astronomical timescales. First, application of the wrong insolation model for tuning climate-forced stratigraphy can result in precision error of up to 12 kyr (half-precession cycle period). This error can be minimized through evaluation of the phasing relationship between the recorded obliquity and precession. Second, tidal dissipation causes Earth rotation deceleration, decreased ellipticity and a slowing axial precession. Modeling suggests that by 20-25 Ma the tuning error caused by these geodynamical variables is on order of ~68 kyr for precession-tuned records and ~123 kyr for obliquity-tuned records (Lourens et al., 2004). The specific behavior of these variables beyond this time is unknown, but could be constrained through quantitative study of the Earth’s ancient tidalite record. Third, precision and accuracy in the astronomical solution decline rapidly prior to 50 Ma, with the expectation of chaotic behavior in the solar system casting uncertainty on modeled planetary orbits. Only the Earth’s 405-kyr orbital eccentricity cycle arising from interactions between Venus and Jupiter is deemed stable enough for use in accurate tuning. Model stability studies suggest that the uncertainty of this term is 0.1% at 100 Ma, and 0.2% at 250 Ma (Laskar et al., 2004).
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009