A geomagnetic secular variation study (31000-19 500 bp) in Western Canada

Abstract

Summary. Palaeomagnetic results from 212 horizons spread evenly through an 18 m sedimentary sequence in southern British Columbia are reported. Radiocarbon ages suggest that the sequence spans the interval from 31 200 to 19 500 yr bp. No evidence for any large geomagnetic excursions (such as the so-called Mono Lake Excursion) is found, but a distinctive pattern of ‘normal’ secular variation is observed with declination and inclination swings of 45° and 25° peak to peak amplitude respectively. For the most part the secular variation consists of low amplitude oscillations about the field vector of a geocentric axial dipole expected at the site latitude, but three relatively large perturbations occur at approximately 4000 yr intervals. These perturbations systematically bias the overall mean to shallow inclinations and easterly declinations in a manner reminiscent of the spatially non-isotropic secular variation model proposed by Cox. The bias involved is about 6° in declination and 3° in inclination (overall mean D= 5.8°E, I= 64.2°, α95 = 0.9°, N= 212 horizons), which leads to a pole which is both ‘far-sided’ and ‘right-handed’. If the horizons involved in the three major perturbations are eliminated the mean direction (D= 1.2°E, I= 67.2°, α95 = 0.8°, N= 125 horizons) does not differ from that of a geocentric axial dipole despite the small cone of confidence.