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 pol arity timescale (APTS). The APTS for 0-5.23 Ma has been rapidly accepted as the definitive chronology for the Pliocene and Pleistocene against which e ven high precision radiometric dating is now calibrated. Extensions of astr onomical calibration to the late Miocene (5.23 Ma to ca. 10 Ma) in Mediterr anean and Pacific marine sections agree to within about a 100 ka eccentrici ty, which amounts to an uncertainty of only ca. 1% of the age. Orbital ecce ntricity periods are thought to remain stable over very long times and thus provide the possibility of precise relative age control in the pre-Neogene . 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 ecce ntricity cycle modulating the expression of the precession climate cyclicit y. 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.