Absence of preferred longitude sectors for poles from volcanic records of geomagnetic reversals

Abstract

ACCORDING to a recent compilation of sedimentary records from the past 12 Myr, the reversing geomagnetic field displays a marked long-term longitudinal organization which seems to be correlated with the thermal structure of the lower mantle¹. However, the crucial palaeomagnetic observation that intermediate virtual geomagnetic poles are preferentially confined to longitudes over the Americas or antipodal to this sector^2,3 might be an artefact caused by distortion and smoothing of the geomagnetic signal by sediments^4–6. Here we present a compilation of ∼400 intermediate poles from 121 volcanic records of excursions and reversals less than 16 Myr old, sampled at various longitudes and latitudes, which does not confirm previous inferences¹ and reinforces doubts^4–6 about the ability of sediments to properly record a rapidly varying field. In contrast to virtual poles from excursions, those from reversals are uniformly distributed in longitude, which indicates that the reversing field is statistically axisymmetrical. Thus, we conclude that there is no evidence for control of transitional fields by the temperature distribution in the lowermost mantle.