Magnetization of a stalagmite in Akiyoshi plateau as a record of the geomagnetic secular variation in West Japan.

The investigation of natural remanent magnetization (NRM) of a stalagmite has been carried out during these several years with the aim of producing a secular variation record of the geomagnetic field for the last several thousands of years. The NRM is relatively weak of the order of 10-5 to 10-7 Am2/Kg but has a stable component. The youngest specimens, at the upper surface of the stalagmite, show a magnetization in almost the same direction as that of the present geomagnetic field at the sampling site (declination=-6.4° and inclination=48.3°). Variation of the declination and inclination of the magnetization is relatively consistent with those obtained from archaeomagnetic studies and palaeomagnetic studies of sediments in Japan. The geomagnetic field intensity during the studied period has apparently decreased gradually to the present as deduced from a decrease of the intensity ratio of the NRM to the saturation isothermal remanent magnetization. The continuous geomagnetic secular variation record obtained from this study is considered to presumably cover the past 4000 years.