On the low-inclination bias of the Precambrian geomagnetic field

A long tradition has emerged in applying the inclination frequency analysis to estimate the functionality of the Geocentric Axial Dipole (GAD) hypothesis in paleomagnetism. In the present study, the theory has been tested to the database with 3246 records of the Precambrian geomagnetic field. To find the best-fitting inclination distribution, different combinations of geocentric axial dipolar (GAD), quadrupolar (G2) and octupolar (G3) spherical harmonics have been analyzed. The influence of various factors on the inclination distribution has been studied, including the geologic age, rock type, magnetic polarity, quality of data and their spatiotemporal distribution. By two-dimensional chi-square analysis on crystalline rocks only, which avoid problems associated with remanence shallowing among sedimentary rocks, the most plausible estimates for the zonal non-dipolar contributions of the field have been determined as 2% for G2 and 5% for G3, values much lower than in previous estimates using the same method. However, the inherent non-uniqueness of the inclination frequency analysis and the uneven spatiotemporal sampling of the field around the globe during the Precambrian necessitate other independent methods of testing the GAD hypothesis.