High-resolution record of the Upper Olduvai transition from Po Valley (Italy) sediments: support for dipolar transition geometry?
Abstract
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
Mdssbauer 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 themagnetization 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 — 90° west of the site and consists ofthree stages: first, the VGP moves to southern mid-latitudes, then,
after a period ofstandstill, 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 Olduvas reversal. A review of the recently obtained
records of various transitions shows that in more than two-thirds ofthe cases the VGP paths are similarly confined along a
meridian over the Americas or antipodal to them, irrespective ofthe sampling site and ofthe 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.