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

We have measured relative geomagnetic palaeointensity proxies, palaeomagnet ic directions, and delta(18)O for the 700-1100 ka interval from ODP Site 98 3 (Gardar Drift, North Atlantic), where mean sedimentation rates are: ca. 1 3 cm kyr(-1). The age model was generated by matching the benthic delta(18) O data to the Ice Volume Model and confirmed by tuning the precessional com ponents of both signals. For the Matuyama-Brunhes boundary (MBB) and the bo undaries of the Jaramillo Subchronozone, the duration of the polarity rever sal process, defined by virtual geomagnetic polar latitudes of less than 45 degrees, is ca. 5 kyr. Whereas the generally accepted astrochronological e stimates for the boundaries of the Jaramillo Subchronozone lie within the p olarity 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 l ie within palaeointensity lows, with abrupt recovery of palaeointensity pos t reversal. There is no progressive ('sawtooth') decrease in palaeointensit y within the Jaramillo Subchronozone or between the top of the Jaramillo an d the MBB, but rather, within polarity chrons, several short intervals of l ow palaeointensity which sometimes coincide with high-amplitude secular var iation. Orbital (100 and 41 kyr) periods are present in the palaeointensity record. As they are not obviously attributable to climate/lithology in the se records, they may be a feature of the geomagnetic field itself.