Statistical assessment of preferred transitional VGP longitudes based on palaeomagnetic lava data

Abstract

Summary We examine the statistical significance of a recent report indicating that palaeomagnetic lava data recording reversals and excursions over the past 20 Myr yield transitional virtual geomagnetic poles (VGPs) tending to fall along American and Asian longitudes. Using a bootstrap analysis of spherical densitygrams, we find that this result is neither a sensitive function of nor heavily dependent upon particular transitional records. Previous analyses used χ2 and Kuiper tests of longitudinal uniformity of transitional VGPs. However, the bootstrap analysis presented here takes such statistical tests one step further, showing specifically that American and Asian longitudes are preferred, with significance at about the 95 per cent confidence level. Furthermore, provided the appropriate polarity convention is used, we confirm that Icelandic transitional VGPs tend to fall along Asian longitudes. These results are indicative of non-dipolar transitional fields whose structure is governed by some form of core–mantle coupling.