Olduvai termination: detailed palaeomagnetic analysis of a north central Pacific core

Abstract

SUMMARY A high-resolution record of the termination of the Olduvai (normal to reverse) subchron (1.77 Myr) polarity transition was obtained from a fully azimuthally oriented deep-sea sediment piston core, recovered at a northern mid-latitude site (3722.4'N, 179O36.l'W). The N-R Olduvai reversal was 'continuously' sampled at 3 to 5 mm intervals across 130 cm of section. This 'continuous' sampling was achieved by staggering the 1 cc plastic boxes incrementally, so that offset segments corresponding to sampling windows of 3 to 5 mm were obtained. The reversal is well defined, and at least 10 truly intermediate directions have been recorded, displaying a definite directional change of MOO, with a pronounced decrease in intensity of magnetization. The decrease of magnetization and the directional change coincide closely. The inclination record during the transition is characterized by a brief steepening trend early in the reversal passing through high positive values, followed by a gradual shallowing trend, this time passing through the horizontal. From this point on, the inclinations show a final settling about the reversed polarity mean. This record is characterized by full normal and reverse polarity directions which are in good agreement (f2) with those predicted by the geocentric axial dipole field model for the core site latitude. The estimated duration of the directional change was about 10 OOO yr whereas that of the intensity decrease was between 12 OOO and 16 OOO yr. Anhysteretic Remanent Magnetization (ARM) studies and thermomagnetic analyses indicate that a uniform magnetic mineralogy characterizes the transition zone of the core. This further suggests that the J10/ARM70 ratio may represent a relative palaeointensity estimate and may thus indicate a decrease of the intensity of the geomagnetic field during the reversal. The virtual geomagnetic pole (VGP) path of the record is far-sided. Directions in projected (D', 1') space indicate that the N-R Olduvai reversal transition field geometry had contributions from both non-axisymmetric and axisymmetric terms. The results based on this single record show that: (1) The palaeofield at this locality during the Olduvai transition was characterized by contributions from both zonal and non-zonal terms but the contribution of non-zonal quadrupolar terms was predominant. This is in contrast to the observation by Herrero-Bervera et al. (1991), based on four deep-sea records in which the VGPs show general confinement within the longitude band of the Americas and their antipode, that the transitional field may be dominated by a dipolar component. (2) The field intensity dropped to about 22 per cent of its initial or final recovered value during the transition. It stayed at or close to this minimum for approximately half or less of the overall intensity change.