古地磁気層序による人類紀の年代対比  [in Japanese] Paleomagnetic dating in the middle and late pleistocene  [in Japanese]

The middle and late Pleistocene is the most difficult period to determine the age because it is too old for the radiometric carbon dating and is too young for the potassium-argon method. Several geomagnetic excursions and events in Brunhes Normal Polarity Epoch are found and those must be good time markers for dating this period. Althought here is still considerable ambiguity on the ages of the excursions and events, these might be dated to be 18 ky BP, 49 ky, 50〜60 ky (Ina I excursion), 70〜80 ky (Ina II excursion), 110 ky (Blake event), 170 ky (Biwa I event), 29 ky (Biwa II event), and 35 ky (Biwa II event). Ina I and II excursions are detected in the tephra sediments at many places in Japan from Shimane Prefecture (Hirose) to Hokkaido (Tokachi), so that these excursions are the geomagnetic phenomena having the size at least covering all over Japan. Characteristics of virtual geomagnetic pole (VGP) paths seem to be useful to distinguish these excursions and events. By the precise study of Omachi Tephra, Nagano Prefecture and tephra sediments distributed in Tokachi Plane, Hokkaido, the VGP paths of Ina I and II excursions are clarified. VGPs of both Ina I and II excursions yield the paths going down to the southern hemisphere through the central Pacific. It is characteristic for Ina I excursion that VGPs stayed for a while in a region of the northwestern Atlantic at the end of the excursion.