Archaeomagnetic Study on Some Remains of the Ancient Times in Yamaguchi Prefecture, Japan
AbstractAt the coastal region of the Seto Inland Sea in Yamaguchi Prefecture, west Japan, many remains of the ancient times have been excavated and exposed under the railway construction of the Sanyo Shin-Kansen (the super express line) of Japan National Railways. The present authors have studied the magnetic properties of baked earth come from these ancient remains newly excavated.And they have endeavoured to estimate the absolute ages of some of these remains (archaeomagnetic chronology). The sampling technique performed in this study is also briefly described. In general, igneous rocks and also baked earths have feeble magnetic moment (natural remanent magnetization ; nrm) because of small amount of magnetic minerals, those are mainly magnetites, Fe_3O_4 and haematites, Fe_2O_3. When these minerals were cooled down from certain temperature above their Curie points, they were magnetized in the same direction of the ambient magnetic field (thermo remanent magnetization ; trm). Therefore the baked earth of the ancient kiln has remained the fossil magnetization with the same direction to th earth's magnetic field at the time of the cooling. On the other hand, it is well known that the geomagnetic field has changed rather periodically in direction, both declination and inclination. This is called as the geomagnetic secular variation,and some authors such as Kawai (1972 ; Kawai et al, 1967), Watanabe(1957), and Yukutake (1961 ; Yukutake et al,1964), have independently proposed the modes of periodical changes of those 2,000 years. These curves do not completely coincide each other but not so much differ, therefore the remanent magnetization of the baked earth may give essential informations of their ages. However, at the present, the exact secular varia tion curves as the standard archaeomagnetic scale for the dating of the kiln are not completely established. The present authors' study may be done rather in order to help the accomplishment of the standard secular variation curves than to presume the absolute age of the remain using the secular variation curves mentioned precedingly.
0513-1693, AN00243950, 研究論叢. 自然科学, 23, 2, 27, 32