HIGH-RESOLUTION RECORD OF THE UPPER OLDUVAI TRANSITION FROM PO VALLEY (ITALY) SEDIMENTS - SUPPORT FOR DIPOLAR TRANSITION GEOMETRY

A detailed record of the Upper Olduvai polarity transition, composed of > 100 transitional directions, has been obtained from the Crostolo section in northern Italy. A careful examination of the rock magnetic properties of the sediments using standard paleomagnetic techniques, thermomagnetic, scanning electron microscopy, microprobe, X-ray diffraction and Mossbauer analysis, shows that greigite is the main magnetic carrier of the remanence. The correlation of a shift in the magnetic record to a small-scale sedimentary feature observed in the section indicates that the magnetization was acquired at deposition, or very shortly after. The virtual geomagnetic pole (VGP) path is largely confined along a great circle over North and South America approximately 90-degrees west of the site and consists of three stages: first, the VGP moves to southern mid-latitudes, then, after a period of standstill, it comes back to almost true north, and, finally, the south pole is reached in a third step. Although different from a record from the southern Indian Ocean, this path is virtually identical to those obtained for the same transition from North Pacific deep-sea cores, and partly coincides with a North Atlantic record, which suggests that a dipolar component may be present in the transitional field during the Upper Olduvai reversal. A review of the recently obtained records of various transitions shows that in more than two-thirds of the cases the VGP paths are similarly confined along a meridian over the Americas or antipodal to them, irrespective of the sampling site and of the sense of the transition. Although not deterministic, this tendency suggests that a similar dipolar component might be present in the transitional fields of other reversals of different ages.