A Geomagnetic Reversal Recorded in a Stalagmite Collected in Western Japan

抄録

A stalagmite sample (ISA) was collected from an unnamed cave located on the West Akiyoshi Plateau in western Japan. The paleomagnetic results from four core samples covered the same growth period were of high quality and in agreement with each other, satisfying internal consistency. No depositional error was recognized in the results. The geomagnetic reversal episode was recorded in the portion between the whitish center part and the innermost of three clearer growth layers. The AF-cleaned direction variation of outer portion than the innermost clearer growth layer corresponds to other records from West Japan over period of 2.5 to 6.5ka. The estimated growth rate in this portion, therefore, is 1.6 to 1.7cm/kyr. We conclude that the reversal occurred between 350ka (²³⁴U/²³⁰Th age of the center part) and 15ka (by the comparison of the direction variation with other paleomagnetic records from Japan). The duration of the reversal is estimated to be about 2500 years, assuming a growth rate for the outer layer to the inner portion. During the period of the reversal, the virtual geomagnetic pole moved roughly along the longitude of the sampling site (131°E), and the geomagnetic paleointensity was estimated to be relatively low.