Confounding influence of magnetic fabric on sedimentary records of a field reversal

RECENT compilations of geomagnetic reversal records^1–8 have generated a controversy as to whether the geomagnetic field is geographically biased during polarity transitions. At present there is general agreement that the virtual geomagnetic poles recorded from Cenozoic sediments preferentially lie over the Americas (or the antipodal longitude), yet such a preference is not statistically established^{7, 9}. However, it is intriguing that the claimed preferred paths lie 90Ŷ away from the site longitude^{4, 9}. Although this may be partly inherent in the very poor geographical distribution of the sites, we prefer not to rely on fortuitous coincidences. Several authors have argued that sedimentary palaeomagnetic records may be modified by artefacts linked to the acquisition of magnetization^10–13. Here we report that in two sedimentary records of the Upper Olduvai reversal from Confidence Hills, California, the declinations of the remanent magnetization recorded during the reversal are similar to the directions of the maximum horizontal axes of the ellipsoids of magnetic anisotropy. This supports the idea that, at times of low geomagnetic intensity (such as during a reversal), factors other than the geomagnetic field influence the orientation of elongated grains.