An attempt at reconstructing the geomagnetic field at the core-mantle boundary during the Upper Olduvai polarity transition (1.66 Myear)

Abstract

The palaeomagnetic directions of five published records obtained from sedimentary sections from various parts of the world were used to calculate maps of the radial field at the core-mantle boundary (CMB) during the Upper Olduvai polarity reversal, dated at 1.66 Myear. The five records were synchronized assuming that major changes in field direction occurred simultaneously at the five sites at the beginning and the end of the polarity transition. Low-order coefficients of the field were determined, using the hypothesis that the magnetic energy remained constant during the transition. Maps of the radial field at the CMB were calculated, assuming a perfectly insulating mantle. The results of this mapping exercise show a non-axisymmetric field at the CMB in the first and last stages of the transition, with strong flux moving in the longitudinal bands, evidenced as preferred ones for transitional Virtual Geomagnetic Poles (VGP) paths. Maps calculated at mid-reversal do not show a simple geometry. More palaeomagnetic records and more refined modelling techniques are desirable. However, the results are consistent with the idea that the transitional geodynamo is controlled by the thermal structure of the lower mantle.