The paleomagnetic record from deep-sea sediment cores

Abstract

The paleomagnetic record of deep-sea sediment cores is compared with that which would be expected from our knowledge of the Earth's magnetic field. It is found that some of the scatter in directions of magnetization obtained from deep-sea cores is removed in cores with very low sedimentation rate, the cause being that the secular variation of the Earth's magnetic field is more completely averaged out over the finite size of each sample, when these samples comprise a longer time span of sedimentation. Corrections have been applied to the results from a series of cores in order to obtain the inclination of the average direction of magnetization from the average inclination and the scatter of inclination. These corrected inclination values confirm the hypothesis that the average Earth's field over the past few million years has been similar to an axial dipole displaced towards the North Pole. The amount of displacement obtained was 168 km. The record of short-period polarity intervals within the Brunhes, the Matuyama and the Gauss epochs was studied. It was shown that these intervals are very scattered in position. It is thought that some hitherto undiscovered short-period polarity intervals may be responsible for part of the scatter, but it is also highly likely that many samples give spurious reversals (i.e., ones not caused by the Earth's magnetic field). The possible correlation between climatic changes and the Earth's magnetic field is examined. It is concluded that the cores which show correlations between direction and/or intensity of magnetization and climatic indicators, thus suggesting the possible correlation between climate and the Earth's magnetic field, are not accurately recording the relevant parameters of the Earth's magnetic field. The correlation must be caused by climatic effects which have a direct influence on the magnetization of the sediments. Very little is known about the mechanisms of magnetization in deep-sea sediment cores, and there are several unexplained phenomena, such as the fact that many cores have maximum susceptibilities which are vertical, and the fact that cores differ widely in their ability to record accurately the Earth's magnetic field.