A palaeomagnetic study of some Pleistocene sediments in northern Canada and its bearing on the secular variation of the geomagnetic field

Abstract

Summary Palaeomagnetic data are reported from two 6-m sequences of sediments exposed in north-western Canada near latitude 68°N. The sequences span the Old Crow Tephra which has recently been dated at 86 000±8000 yr BP. During laboratory treatment a single sample suggests the presence of an underlying reversed magnetization, but no other trace of the Blake Event was found. However, the secular variation pattern observed is of some significance. It consists of a spatially isotropic dispersion due to typical non-dipole field behaviour on which are superimposed linear perturbations due to stationary sources at the core–mantle boundary (CMB). A simple magnetostatic model indicates that a radial dipole just below the CMB need only achieve a maximum amplitude of about 8 per cent that of the geocentric dipole to explain the observations. An earlier study at latitude 50°N revealed evidence of similar behaviour although the sediments involved were much younger than those of the present study. The interesting possibility of ‘standing’ secular variation sources with lifetimes up to 105 yr is therefore raised.