Geomagnetic palaeointensities and astrochronological ages for the Matuyama–Brunhes boundary and the boundaries of the Jaramillo Subchron: palaeomagnetic and oxygen isotope records from ODP Site 983

Abstract

We have measured relative geomagnetic palaeointensity proxies, palaeomagnetic directions, and δ 18 O for the 700–1100 ka interval from ODP Site 983 (Gardar Drift, North Atlantic), where mean sedimentation rates are ca .13 cm kyr −1 . The age model was generated by matching the benthic δ 18 O data to the Ice Volume Model and confirmed by tuning the precessional components of both signals. For the Matuyama–Brunhes boundary (MBB) and the boundaries of the Jaramillo Subchronozone, the duration of the polarity reversal process, defined by virtual geomagnetic polar latitudes of less than 45°, is ca .5 kyr. Whereas the generally accepted astrochronological estimates for the boundaries of the Jaramillo Subchronozone lie within the polarity transitions as recorded at Site 983, the astrochronological age for the Matuyama–Brunhes polarity transition (780 ka) is ca .5 kyr older than the onset of this transition at Site 983 (775 ka). The polarity reversals lie within palaeointensity lows, with abrupt recovery of palaeointensity post reversal. There is no progressive (‘sawtooth’) decrease in palaeointensity within the Jaramillo Subchronozone or between the top of the Jaramillo and the MBB, but rather, within polarity chrons, several short intervals of low palaeointensity which sometimes coincide with high–amplitude secular variation. Orbital (100 and 41 kyr) periods are present in the palaeointensity record. As they are not obviously attributable to climate/lithology in these records, they may be a feature of the geomagnetic field itself.