Orbital tuning of geomagnetic polarity time-scales

Milankovitch climate cyclicity and magnetic polarity stratigraphy are being successfully combined as a powerful geochronometer in the astronomical polarity timescale (APTS). The APTS for 0–5.23 Ma has been rapidly accepted as the definitive chronology for the Pliocene and Pleistocene against which even high precision radiometric dating is now calibrated. Extensions of astronomical calibration to the late Miocene (5.23 Ma to ca. 10 Ma) in Mediterranean and Pacific marine sections agree to within about a 100 ka eccentricity, which amounts to an uncertainty of only ca. 1% of the age. Orbital eccentricity periods are thought to remain stable over very long times and thus provide the possibility of precise relative age control in the preNeogene. An APTS, for example, has been developed in a thick lacustrine section of Late Triassic age (ca. 202–233 Ma) on the basis of the 404 ka orbital eccentricity cycle modulating the expression of the precession climate cyclicity. Finally, there has been renewed speculation about an obliquity-modulated precessional geodynamo based on periodicities in relative palaeointensity data from Ocean Drilling Program sediment cores.