Olduvai onset polarity transition: two detailed paleomagnetic records from North Central Pacific sediments

Abstract

Paleomagnetic analyses of two azimuthally oriented deep-sea sediment cores from the North-Central Pacific Ocean, which contain a record of the onset of the Olduvai Subchron polarity transition, show: (1) intermediate directions of magnetization with transitional inclinations reaching values up to 70°; (2) up to three sequential 180° declination changes, two of which imply rapid oscillations of the geomagnetic field before the main reversal, and (3) a pronounced drop in intensity of magnetization within the transitional interval. The data further indicate that the intensity decay and recovery cycle took longer than the directional changes of the reversal. Whereas the directional changes lasted about 5000 years, the duration of the recorded intensity cycles suggest that the complete onset of the Olduvai reversal may have lasted as much as 28 000 years. Anhysteretic Remanent Magnetization studies and thermomagnetic analyses indicate that uniform magnetic mineralogy characterizes the transition zones of the cores. This further suggests that the J 10/ARM 70 ratio may represent a relative paleointensity estimate and may thus indicate a decrease of the intensity of the geomagnetic field during the onset of the Olduvai reversal. Virtual Geomagnetic Pole paths of the two records are near-sided. Directions in projected ( D′, I′) space indicate that the Olduvai onset transitional field geometry possessed a contribution from non-axisymmetric terms, in addition to the dominating zonal harmonics. The analyses also suggest that the transitional field during the onset of the Olduvai reversal was dominated by harmonics different from those of the Matuyama-Brunhes boundary, Jaramillo transitions and upper Olduvai reversal.