Simon Costin and Bruce Buffett
Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, BC Canada
Transition fields inferred from sedimentary and volcanic data suggest that the position of the Virtual Geomagnetic Pole (VGP) during reversals is confined to longitudes through Americas and Eastern Asia. We consider the possibility that lateral variations in electrical conductivity, due to sediments at the CMB, contribute to the VGP reversal paths. We assume that sediments accumulate in topographic highs of the CMB, producing regions of high electrical conductivity because of the interstitial liquid iron. In regions where sediments are absent we expected lower electrical conductivities. The spatial distribution of conductivity in the sediment layer is represented using a single spherical harmonic function with degree and order 2. The pattern of conductivity is chosen to make the region under the central Pacific anomalously conductive. This area coincides with detections of ultra-low velocity zones and with geodynamic predictions of positive (radial) topography at the core-mantle boundary. Variations in the axial dipole field during a reversal induces electric currents in the sediment layer, which produce a secondary magnetic field. Superposition of the secondary field on the axial dipole influences the declination and inclination of the geomagnetic field at the surface, thereby altering the path of the transitional VGPs. We constrain the conductivity model using the present-day rate of decrease in the dipole field, and compare the predicted secondary field with observations of the nondipole field. The best agreement is obtained when the conductance below the central Pacific is $2\times 10^7$ S. This value is subsequently adopted in our model to predict the longitude of VGP paths during reversals. We find that the predictions depend on the geographical location of the observing site. A histogram of VGP paths derived from a uniform distribution of sites reveals four preferred paths, two of which fall through Americas and Eastern Asia. Prominent sites in the volcanic database, including Western North America, Hawaii and Iceland, predict reversal paths that agree with the paleomagnetic observations.