Rare Paleomagnetic Evidence of Long-term Mantle Control of the Geodynamo and Possible Role of the NAD-field in the Reversal Process

International audienceThe degree to which the lower-most mantle influences behavior of the geodynamo has been debated over the past quarter century. Our analysis of a comprehensive set of 17 Cenozoic paleomagnetic 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 paleomagnetic recording sites from west to east include the south Indian Ocean, eastern Downloaded from https://academic.oup.com/gji/advance-article-abstract/doi/10.1093/gji/ggz480/5602599 by guest on 17 November 2019 Australia, New Zealand, and French Polynesia. The analyzed records contain from 2 to 25 sequential transitional virtual geomagnetic poles (VGPs). Ten of the 17 records supply at least one VGP within a narrow longitudinal band 10°-wide between 60°S and the equator, centered along longitude 102.4°E. That is, transitional data from 59% of the Cenozoic recordings are found to reside in a region that encompasses a mere 2.8% of the VGP transitional area on Earth's surface. A robust Monte Carlo approach applied to this dataset, 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 paleomagnetic 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 behavior of the axial dipole, our findings indicate it has dominated the magnetic field over an area of considerable size during attempts by the geodynamo to reverse polarity