[S-EM15] Geomagnetism and paleomagnetism

[SEM15-P05] Preliminary results of archeointensity in 800 years ago estimated from Yayoi pottery of Kokujaku

*Yutaka Yoshimura¹, Hyeon-Seon Ahn^2,3, Yuhji Yamamoto², Chisato Anai², Yoshinori Tajiri¹, Tadahiro Hatakeyama⁴, Masao Ohno¹ (1.Faculty of Social and Cultural Studies, Kyushu University, 2.Center for Advanced Marine Core Research, Kochi University, 3.Korea Institute of Geoscience and Mineral Resources, 4.Institute of Frontier Science and Technology, Okayama University of Science)

Keywords:Yayoi pottery, Archeointensity, Tsunakawa-Shaw method, Geomagnetic secular variation

The Earth's magnetic field is variable in various time scales. Recently, the larger secular variations of geomagnetic field intensity than previously thought have been reported using the latest archeological magnetic field intensity (archeointensity) measurements (Tsunakawa-Shaw method: Yamamoto et al., 2003 and IZZI-Thellier method: Yu et al., 2004). For example, maxima have been found at about 3000-2900 BCE in pottery and remains mainly from Israel and the Middle East (Ben-Yosef et al., 2017; Shaar et al., 2017). New maxima have been found in East Asian archeological artifacts, such as slag, with a minimum of about 4200 BCE and a spike at about 3300 BCE (Cai et al., 2017). In addition, Hawaiian lava suggests that there were large 10-100 year-scale variations in geomagnetic field intensity during the Holocene between 1800-2000 BCE and 3000-3500 BCE (Cromwell et al., 2018; Yamamoto and Yamaoka, 2018). If the geomagnetic field intensity is highly variable, a reference curve of archeointensity secular variations in Japan could provide clues to estimation of the baked date of undated archeological materials. However, highly precise Japanese archeointensities obtained with the latest methods are only available after 450 CE (Kitahara et al., 2018, 2021; Tema et al., 2023) although there are highly precise secular variation curves constructed from Japanese archaeomagnetic directions.

We have been studying the archeointensity of Yayoi pottery fragments excavated at the Gokuden site located on the Chikushi Campus of Kyushu University in Kasuga City, Fukuoka Prefecture. From 250 BCE to 50 CE, the archeointensity may have increased from 22 µT to 67 µT (Yoshimura et al., JpGU 2022). However, there are few archeointensity data in Japan for 50 CE, the period when the strongest archeointensity in the study was obtained.

Therefore, in this study, in order to verify whether the geomagnetic intensity after 50 CE was so strong, we started an experiment to estimate archeointensity using Yayoi pottery fragments made from 50 CE to 200 CE excavated at the Kokujaku site in Onojo City, Fukuoka Prefecture. The Kokujaku site is a complex remain from the end of the Jomon period to the Kamakura period, and is a very close, only about 1 km north of the Gokuden site. 10 pottery fragments about 5 ~ 20 cm in size are provided to us from the Board of Education of Onojo City. We carried out archeointensity experiments on a total of four specimens by cutting out two pieces each from two pottery fragments made from 200 CE to 250 CE. The Tsunakawa-Shaw method was carried out in vacuum and archeointensities of 53.5 and 100.5 µT were obtained from two pottery fragments. These are comparable to or stronger than the archeointensity of 50 CE already obtained from the Gokuden Yayoi potteries.

Although the results in this study are preliminary, we can observe that the geomagnetic intensity increased from 250 BCE to 225 CE and reached a maximum at about 225 CE when our archeointensities from the Kokujaku and Gokuden sites are arranged in chronological order. Its maximum, when converted into a virtual axial dipole moment, is equivalent to about 19 × 10²² Am², which is about 2.5 times the present geomagnetic dipole moment. These results suggest that the geomagnetic intensity increased greatly in a short period of time in the Yayoi period, and that it was a period whose conditions were suitable for determining the age of baked archeological artifacts by the archeointensity.