Geomagnetic reversals are an intriguing phenomenon of the Earth's magnetic field. Records of geomagnetic polarity transitions from sediments led to the suggestion that the virtual geomagnetic pole (VGP) paths run preferentially along two bands of longitude, across the Americas and the antipode (Laj et al., 1991; Clement, 1991). It was suggested that certain geophysical parameters, such as the regions of higher seismic velocities in the lower mantle, have a significant control over the reversal process (Laj et al., 1991). The significance of the preferred longitudinal bands has been questioned. Langereis et al. (1992) conclude that the preferred VGP paths can arise from smoothing of pre- and post-transitional directions. The statistical significance of preferred longitudinal VGP bands has also been questioned (McFadden et al., 1993).
Paleomagnetic transitional records from lavas for the past ten million years tend to cluster in at least two specific restricted regions (Hoffman, 1992). These zones correlate with the near-radial flux concentrations of today's nondipole field. Prévot and Camps (1993) in contrast suggest that there is no evidence for preferred cluster zones or longitudinally confined VGP paths in the volcanic rock record. Love's (1998) analysis of paleomagnetic lava data contradicts that of Prévot and Camps (1993) and does not find support for the hypothesis of clustering of transitional VGPs (Hoffman, 1992). On the contrary, Love (1998) concludes that volcanic rocks give intermediate VGPs that tend to fall along American and Asian longitudes.
The natural remanent magnetization (NRM) of sediments is complicated because of postdepositional processes, which lead to the acquisition of magnetization over an extended time period. In addition to these difficulties in remanence acquisition, the NRM of cored sediments can be overprinted by a coring-induced magnetization (CIM). Vertical and radial CIMs were observed on sediment cores from several Ocean Drilling Program (ODP) legs. Fuller et al. (1998) measured axial fields from 1 up to 15 mT pointing along the core barrels and radial fields around a few millitesla within the advanced hydraulic piston corer (APC) inner core barrels. The properties of coring-induced magnetization and the acquisition mechanism were reviewed by Fuller et al. (1998). Below, we examine the transitional VGP paths from three reversals recorded in sediments cored off the California margin. Our results illustrate the sensitivity of VGP paths to factors unrelated to the geomagnetic field. In particular for this study and likely for other studies that use deep-sea drill cores, magnetic overprints acquired during drilling operations prove to be difficult to fully remove using standard alternating-field (AF) demagnetization methods. The bias caused by the overprint is significant enough in these cases that the accuracy, and hence the geomagnetic significance, of the positions of the transitional VGPs is questionable.