Ultra-high geomagnetic field reversal frequency around the Precambrian-Cambrian transition ?

Magnetostratigraphic investigations carried out in Siberia have shown that the middle Cambrian was marked by an extremely high geomagnetic field reversal frequency of about 7 to 10 rev./Myr. The results available for the Lower Cambrian are more uncertain but they may indicate an even higher reversal frequency, which could thus reveal a very unstable nature of the geomagnetic field at this time. Recent magnetostratigraphic results also suggest that the geomagnetic reversal frequency has been extraordinarily high at the end of the Precambrian, thus in agreement with the Lower Cambrian data. We will present a review of these data, and will further describe new results we have obtained from Late Ediacaran-Nemakit-Daldynian sections of the south-western Siberian platform (Enisey range, Teya and Chapa rivers valleys). All these data provide consistent evidences for an ultra-high geomagnetic field reversal frequency, and thus for the exceptional nature of the geomagnetic field, around the Precambrian-Cambrian transition. We will also discuss a number of hypotheses which could explain a temporary destabilization of the geomagnetic field.