Rare palaeomagnetic evidence of long-term mantle control of the geodynamo and possible role of the NAD field in the reversal process

Abstract

SUMMARY The degree to which the lowermost mantle influences behaviour of the geodynamo has been debated over the past quarter century. Our analysis of a comprehensive set of 17 Cenozoic palaeomagnetic transitional field records obtained from lavas in the Southern Hemisphere provides robust evidence of stable mantle control since the Pliocene. The records come from a region where—given a significantly weakened axial dipole—the magnetic field today would be largely controlled by the non-axial dipole (NAD) flux patch currently emanating from Earth's outer core beneath western Australia. The palaeomagnetic recording sites from west to east include the south Indian Ocean, eastern Australia, New Zealand and French Polynesia. The analysed records contain from 2 to 26 sequential transitional virtual geomagnetic poles (VGPs). 10 of the 17 records supply at least one VGP within a narrow longitudinal band 10°-wide between 60°S and the equator, centred along 102.4°E. That is, transitional data from 59 per cent of the Cenozoic recordings are found to reside in a region that encompasses a mere 2.8 per cent of the VGP transitional area on Earth's surface. A robust Monte Carlo approach applied to this data set, one that takes into account the number of transitional VGPs contained in each record, finds this result highly improbable (p-value = 0.0006). The present-day pattern of vertical flux at the core–mantle boundary shows an anomalously strong, thin Southern Hemisphere longitudinal band off the west coast of Australia that strikingly coincides with this unusual palaeomagnetic finding. We conclude with a high degree of confidence that this band of flux has remained virtually unmoved for at least the past 3 Myr. Seemingly independent of the behaviour of the axial dipole, our findings indicate that it has dominated the magnetic field over an area of considerable size during attempts by the geodynamo to reverse polarity.