FIELD-STRENGTH DEPENDENCE OF DIRECTIONAL DISPERSION IN MAGNETIZATIONSOF SEDIMENT SAMPLES - A STATISTICAL-MODEL AND PALEOMAGNETIC EVIDENCE

A statistical model is proposed for alignments of magnetic moments in a sediment controlled by the strength of the geomagnetic field. The mo del predicts that both of the mean intensity and directional convergen ce of magnetizations in sediment samples are dependent upon the streng th of the geomagnetic field. This prediction was examined with the pal eomagnetic data from wide-diameter cores of marine and lacustrine sedi ments in Japan. The magnetization of a core from the Inland Sea shows a positive correlation between the depth-dependency changes in mean in tensity and the Fisher's precision parameter, k, which is a measure of the clustering of directions at the same age horizon. For cores from Osaka Bay and Lake Yogo, the similar correlation can be recognized in the high frequency components. The time-dependency variations of k of these three cores are consistent with each other. Besides, these varia tions seem to have tight correlation with the secular variation of the geomagnetic field strength from the archeomagnetism. This suggests th at the parameter of directional convergence degree can be used as an e stimator of relative paleointensity. On the other hand, the data of th ese three cores show no correlation of the time-dependency variation o f natural remanent magnetization intensity even after normalization us ing saturation isothermal remanent magnetization. This disagreement ma y be due to alterations of grains' moments by some effects of diagenes is or to difficulty in normalization of quantity of effective magnetic moments. The relative paleointensity for the last 12000 years estimat ed from the directional convergence variation is prominently character ized by a high peak at 2000-3000 years BP and a minimum at 5500-6000 y ears BP. This estimation of the paleointensity in Japan is similar to the archeomagnetic paleointensity in Europe, suggesting that they shou ld be originated from the nature of dipole field. The MEM spectral ana lysis indicates spectral peaks at periods of 4000, 1100, 770 and 590 y ears per cycle. However, the periodicities of 7000 years, pointed out in a previous work, and about 2000 years of the westward-drift are not found in the present analysis.