A test of the geocentric axial dipole hypothesis from an analysis of the skewness of the central marine magnetic anomaly

A new, global set of palaeomagnetic observations was obtained from analysis of the symmetry of the shape of 203 crossings of the Central marine anomaly, the anomaly observed above seafloor, formed during the Brunhes normal polarity chron (0-0.78 Ma). The data indicate that the time-averaged field can be described best by a dominant geocentric axial dipole component, whose position differs insignificantly from the present spin axis, and by a small geocentric axial quadrupole component (6.0%(+5.7%)(-6.7%) the size of the dipole component). If we simply assume that the Brunhes palaeomagnetic axis has been aligned with the present spin axis, the quadrupole component is 6.2% +/- 4.7%, which differs significantly from a purely dipolar field, and is in good agreement with estimates from other palaeomagnetic data. Besides expanding the spatial distribution of palaeomagnetic field observations, an important step in removing biases in prior field estimates caused by poor global coverage, these results illustrate that valuable geomagnetic information as well as accurate palaeomagnetic poles can be obtained from skewness data.