Normalized remanence in sediments of the western equatorial Pacific: Relative paleointensity of the geomagnetic field?

Piston cores from the Ontong‐Java Plateau collected during the Eurydice expedition have been used to estimate regional geomagnetic paleointensity. Rock magnetic measurements of the sediments show that (1) the remanence is carried by stable pseudo‐single domain magnetite, (2) the grain sizes and concentrations are relatively uniform with respect to the normalization technique and (3) susceptibility is an adequate parameter for normalizing remanent intensity in order to determine relative variations in the intensity of the paleomagnetic field. We therefore have measured the rémanent intensity (both natural and demagnetized to 15 mT) and susceptibility of archive quarters of three cores at centimeter intervals using an automated through‐bore magnetometer. A further test of the normalized intensity records is provided by the coherence function between the normalized remanence and the susceptibility of the cores. One of our cores displays high coherence over a broad range of frequencies, while for two others, coherence is insignificantly different from zero at the 95% level of confidence. Oxygen isotopic records, paleontological datum levels and the Brunhes‐Matuyama boundary provide an independent temporal framework for inter‐core comparison. When the two cores which pass our acceptance criteria are placed on a common time scale, the coherence between the normalized intensity records is significantly different from zero at well above the 95% level of confidence, suggesting some common input signal over the same band of frequencies. Furthermore, there is a great deal of similarity between the records from the Ontong‐Java Plateau and a record from core RC10‐167 taken some 3000 km to the north. The distribution of intensities obtained from these sedimentary records has the same shape as the absolute virtual dipole moment data obtained from lava flows (0–5 Ma), with a slightly smaller standard deviation. The inter‐core agreement within a single region, the similarity of records from 3000 km apart and the general correspondence of statistical character of these sediments with lava flow data all argue that the normalized intensity records presented here reflect variations in the intensity of the ancient geomagnetic field. Spectral analysis of the paleointensity time series shows that there a is significant enhancement of variance concentrated in frequencies around 33 thousand years, over the period 0–350 ka; this feature is however lacking for the period 350–700 ka.