GEOMAGNETIC SECULAR VARIATION RECONSTRUCTED FROM MAGNETIZATIONS OF WIDE-DIAMETER CORES OF HOLOCENE SEDIMENTS IN JAPAN

A record of the secular variation of the geomagnetic field for the las t 1 1000 years has been obtained from the remanent magnetization of se ven 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 co res show low-amplitude variations, but they can be correlated well wit h other records after deconvolution to remove filtering effects of the post-depositional magnetization process. A composite secular variatio n curve was constructed by stacking the field direction records. Time constraints were obtained from radiocarbon ages of shell and wood frac tions or tephrochronology. The secular variation curve agrees well wit h the archeomagnetic record after 1400 yrBP, with a slight difference in the time range 1400-2000 yrBP. The secular variation for the last 1 1000 years shows an elongated distribution of VGP's. The azimuth of th e elongation, about 40-degrees-E, is consistent with that of VGP's fro m Japanese volcanic and sedimentary rocks during Brunhes epoch. The an gular standard deviation is 14.5-degrees (upper limit = 15.4-degrees, lower limit = 13.7-degrees). This estimate is slightly larger than tha t observed globally during Brunhes epoch and the position of the avera ge VGP deviates 8.5-degrees from the geographic north pole. These VGP analyses suggest a stationary nondipole source. The secular variation records from five northern hemisphere sites between 135-degrees-E and 95-degrees-W in longitude possess a single prominent feature: an extre me easterly swing in declination. A plot of age versus longitude of th e swing at each site shows a clear westward drift, at a rate of about 0.13-degrees/yr. The field vector around the swing represents clockwis e looping at all the sites. This suggests that the swing is caused by a large non-dipole source which drifts at least from 135-degrees-E to 95-degrees-W in longitude.