EVIDENCE FOR DIPOLAR FIELDS DURING THE COBB MOUNTAIN GEOMAGNETIC POLARITY REVERSALS

RECENT palaeomagnetic studies1-4 of geomagnetic polarity reversals have presented conflicting interpretations of the behaviour of the Earth's magnetic field during reversals. Some have argued that the grouping of transitional virtual geomagnetic poles (VGPs) suggests the presence of large-scale, perhaps even dipolar, symmetries during some reversals1-3, whereas others have claimed that the available data do not support the interpretation of simple transitional field geometries4. An earlier comparisons of two North Atlantic sedimentary records of reversals bounding the Cobb Mountain subchron (1.1 Myr ago) with a volcanic record from Tahiti6 revealed striking similarities in the sequences of transitional VGPs, suggesting the presence of large-scale symmetries in these fields. Here I augment this comparison with two new records of the Cobb Mountain subchron from the western Pacific7, which greatly extend the geographical and temporal coverage. These records exhibit sequences of VGP positions that are very similar to those observed in the North Atlantic and Tahiti, providing evidence of dipolar transitional fields during the Cobb Mountain reversals.