Geomagnetic Secular Variation Reconstructed from Magnetizations of Wide-Diameter Cores of Holocene Sediments in Japan.

Abstract

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 lowamplitude 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.

Speleothem Palaeosecular Variation Records from China: Their Contribution to the Coverage of Holocene Palaeosecular Variation Data in East Asia.

“…A single cycle of drift has been modelled because we are able to compare all of our records over this period. A non-dipole drift rate of 0.13° yr-1 is the same as that reported by several authors for this latitude (Aitken et al, 1989;Hyodo et al, 1993;Yang et al, 1993) and therefore may provide a guide to the accuracy of the recording process in our speleothems.…”

“…The larger amplitude of swings in both declination and inclination over the period 4.0 ka to 0.25 ka cause the VGP's to lie on an elongated azimuthal trend of 225°-045°. A study in Japan by Hyodo et al (1993) also found VGPs on this azimuth although over the period 7 to 3 ka. After the work of Yukutake and Tachinaka (1968) and Evans (1984) this may be ascribed to the proximity of a stationary non-dipole source, such as the Mongolian anomaly, which grows and decays over time whilst maintaining the same polarity.…”

“…Unfortunately no absolute dating was performed on these speleothems providing little basis for comparison. Alternative records available for comparison are a sedimentary record from the Yellow River mouth, China (37.8°N 119.43°E) by Cong and Wei (1989); archaeomagnetic results from China (Wei et al, 1983); SVJ2, a deconvoluted lake and marine sediment composite record from Japan (35.8°N 136.3°E) by Hyodo et al (1993) and the compiled archaeomagnetic data of Hirooka (1983) from south-west Japan (34°N 136°E). These contemporaneous records are compared to our speleothem records in Figs.…”

Lake Sediments High-resolution Recorder of Environmental Changes. Paleomagnetic Dating Using Geomagnetic Secular Variations and Excursions from Lake Sediments in Japan.

“…The dots in the NRM intensity plot represent an average of five specimens' data in the same horizon. The dots and error bars in the declination and inclination plots show an average direction and limit of 95% confidence circle calculated from five specimens' data in the same horizon (after Hyodo et al, 1993). the core KZ1 from Lake Kizaki.…”

“…Small-diameter (7-8cm) cores are from Lake Kasumigaura (Yamazaki et al, 1985), Lake Suwa (Murata et al, 1986), Lake Suigetsu (Itota et al, 1993), and Lake Biwa (Kawai et al, 1972;Yaskawa et al, 1973). cores are from Lake Kizaki, Lake Yogo, and the Inland Sea of Japan (Hyodo et al, 1993).…”