Evidence for Palaeomagnetic Inclination Error in Sediment

Abstract

THE inclination error (δ) in sediment is the difference between the inclination of the ambient field and that of the remanent magnetization acquired by a sediment1. The error is reckoned positive (negative) if the inclination of remanence is less (greater) than that of the field. Inclination errors in nature may be studied by comparing observed inclinations in sedimentary rocks (Io) with that expected (Ie) from studies of contemporaneous isotropic igneous rocks which are not subject to the error. There are two methods. Results from nearby localities may be compared directly (method 1), or results from widely spaced localities may be compared by calculating the expected inclination (Ie) calculated from the palaeomagnetic pole derived from observations of igneous rocks with the observed inclination (Io) in sediments (method 2). Method 2 assumes that the Earth's field is a geocentric dipole. Both methods assume that the secular variation is averaged out so that only averages for formations covering many tens of thousands of years may be used. There are errors (α) in determining Ie and I0 so the inclination error is itself subject to error. Because of the statistical inhomogeneity of the palaeomagnetic data a formal calculation of the error in δ would be unrealistic, but a lower limit is set by the α values. In simple observations of fossil remanence a is usually about 10°, whereas after the application of magnetic or thermal cleaning techniques accuracies of about 5° can sometimes be achieved.