Astronomical ages for Miocene polarity chrons C4Ar–C5r (9.3–11.2 Ma), and for three excursion chrons within C5n.2n

Ocean Drilling Program (ODP) Site 1092 from the sub-Antarctic South Atlantic produced a clear magnetic stratigraphy for the Late Miocene [Evans H.F., Channell, J.E.T., Upper Miocene Magnetic Stratigraphy from ODP Site 1092 (sub-Antarctic South Atlantic): recognition of cryptochrons in C5n, Geophys. Jour. Int., 153, (2003), 483–496]. Three short intervals of reverse polarity were identified within the long (∼ 1 Myr) normal polarity subchron C5n.2n. These excursion chrons were tentatively correlated to the three “cryptochrons” identified within subchron C5n.2n from NE Pacific marine magnetic anomaly data. New oxygen isotope data from ODP Site 1092 have allowed astronomical calibration of eight polarity chron boundaries in the interval between C4Ar.1n and C5r.1n (9.3–11.2 Ma) as well as calibration of the three excursion chrons identified within C5n.2n. The new polarity chron ages in the C4Ar.1n–C5r.1n interval differ from a current (2004) timescale by up to 48 kyr. The astrochronology yields durations in the 3–4 kyr range for the three excursion chrons in C5n.2n. The relative paleointensity record from Site 1092, and hence the three excursion chrons, can be correlated to the deep-tow magnetic anomaly record from 19°S on the East Pacific Rise (EPR). Based on published correlation of the EPR deep-tow record with the sea-surface magnetic anomaly stack from the NE Pacific, the three excursion chrons do not correspond to the “cryptochrons” recognized in the NE Pacific stack.