On the Representation of the Quaternary and Late Tertiary Geomagnetic Fields in Terms of Dipoles and Quadrupoles

Abstract

Summary Methods of computing relative values of the first eight Gauss coefficients of the geomagnetic field from directional data only are described. The purpose is the analysis of the palaeomagnetic directions for which intensity data are normally lacking and which are characteristically subject to considerable experimental uncertainty of the order of 5°. Four solutions have been derived of the basic equations of degree n≤ 2 involving declination D and inclination I by combining (i) the X and Y, (ii) the Z and Y, (iii) the Z and X, and (iv) the Z and H field components. Various populations of Quaternary and Neogene palaeomagnetic directions have been analysed in this way. Improved fits of computed to observed field directions are obtained by grouping the directions derived from individual palaeomagnetic studies to form regionally averaged control points which are more uniformly distributed geographically. The Z-H and Z-X solutions give notably better fits to the observations than the X - Y solutions. Our results are expressed in terms of a single eccentric dipole, the position of which may be obtained from the values of the first eight Gauss coefficients of degree n≤ 2. Analysis of different populations of Quaternary and Neogene data yield a persistent and systematic displacement of the eccentric dipole into the Pacific and northern hemispheres by some 150 km. Analyses of sub-populations of data corresponding to northern, southern, Pacific and ‘continental’ hemispheres reveal appreciable differences in the position of the eccentric dipole suggesting (i) that the Quaternary field cannot be well represented by a single-point source of dipoles and quadrupoles, and (ii) that harmonics of degree n > 2 have been persistently stronger in the continental than in the Pacific hemisphere throughout the Quaternary, just as for historic epochs.