CONFOUNDING INFLUENCE OF MAGNETIC FABRIC ON SEDIMENTARY RECORDS OF A FIELD REVERSAL

RECENT compilations of geomagnetic reversal records(1-8) have generate d a controversy as to whether the geomagnetic field is geographically biased during polarity transitions. At present there is general agreem ent that the virtual geomagnetic poles recorded from Cenozoic sediment s preferentially lie over the Americas (or the antipodal longitude), y et such a preference is not statistically established(7,9). However, i t is intriguing that the claimed preferred paths lie 90 degrees away f rom the site longitude(4,9). Although this may be partly inherent in t he very poor geographical distribution of the sites, we prefer not to rely on fortuitous coincidences. Several authors have argued that sedi mentary palaeomagnetic records may be modified by artefacts linked to the acquisition of magnetization(10-13). Here we report that in two se dimentary records of the Upper Olduvai reversal from Confidence Hills, California, the declinations of the remanent magnetization recorded d uring the reversal are similar to the directions of the maximum horizo ntal axes of the ellipsoids of magnetic anisotropy. This supports the idea that, at times of low geomagnetic intensity (such as during a rev ersal), factors other than the geomagnetic field influence the orienta tion of elongated grains.