ALBERT

Beschreibung: A statistical model is proposed for alignments of magnetic moments in a sediment controlled by the strength of the geomagnetic field. The model predicts that both of the mean intensity and directional convergence of magnetizations in sediment samples are dependent upon the strength of the geomagnetic field. This prediction was examined with the paleomagnetic data from wide-diameter cores of marine and lacustrine sediments in Japan. The magnetization of a core from the Inland Sea shows a positive correlation between the depth-dependency changes in mean intensity 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 variations seem to have tight correlation with the secular variation of the geomagnetic field strength from the archeomagnetism. This suggests that the parameter of directional convergence degree can be used as an estimator of relative paleointensity. On the other hand, the data of these three cores show no correlation of the time-dependency variation of natural remanent magnetization intensity even after normalization using saturation isothermal remanent magnetization. This disagreement may be due to alterations of grains' moments by some effects of diagenesis or to difficulty in normalization of quantity of effective magnetic moments. The relative paleointensity for the last 12000 years estimated from the directional convergence variation is prominently characterized by a high peak at 2000-3000 years BP and a minimum at 5500-6000 years BP. This estimation of the paleointensity in Japan is similar to the archeomagnetic paleointensity in Europe, suggesting that they should be originated from the nature of dipole field. The MEM spectral analysis indicates spectral peaks at periods of 4000, 1100, 770 and 590 years 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. © 1993, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.

Beschreibung: A record of the secular variation of the geomagnetic field for the last 11000 years has been obtained from the remanent magnetization of seven wide-diameter (20 cm) cores of marine and lacustrine sediment from central to south-west Japan. Magnetization directions of five cores, possessing high-amplitude variations, exhibit good correlation. Two cores show low-amplitude variations, but they can be correlated well with other records after deconvolution to remove filtering effects of the post-depositional magnetization process. A composite secular variation curve was constructed by stacking the field direction records. Time constraints were obtained from radiocarbon ages of shell and wood fractions or tephrochronology. The secular variation curve agrees well with the archeomagnetic record after 1400 yrBP, with a slight difference in the time range 1400-2000 yrBP. The secular variation for the last 11000 years shows an elongated distribution of VGP's. The azimuth of the elongation, about 40° E, is consistent with that of VGP's from Japanese volcanic and sedimentary rocks during Brunhes epoch. The angular standard deviation is 14.5° (upper limit = 15.4°, lower limit = 13.7°). This estimate is slightly larger than that observed globally during Brunhes epoch and the position of the average VGP deviates 8.5° from the geographic north pole. These VGP analyses suggest a stationary nondipole source. The secular variation records from five northern hemisphere sites between 135°E and 95°W in longitude possess a single prominent feature: an extreme easterly swing in declination. A plot of age versus longitude of the swing at each site shows a clear westward drift, at a rate of about 0.13°/yr. The field vector around the swing represents clockwise looping at all the sites. This suggests that the swing is caused by a large non-dipole source which drifts at least from 135°E to 95°W in longitude. © 1993, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.

Beschreibung: Fuknma indicates that dispersion (k) ofpaleomagnetic sediment samples from the same age horizon depends on intrinsic directional dispersion (ks) of magnetic moments and experimental error dispersion (ke), where dispersion is expressed by a precision parameter of Fisher distribution. He attempts to quantitatively estimate ks, and concluded that contribution of the intrinsic dispersion ks to the dispersion k obtained by paleomagnetic measurements is negligible and then the error dispersion Kg should make a major contribution. We admit necessity of taking account of experimental error, but disagree with his conclusion. The quantitative estimation for Ks and the subsequent argument are based on very doubtful grounds. © 1994, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.