[S-EM15] Geomagnetism, paleomagnetism and rock magnetism

[SEM15-P07] Rapid increase of the geomagnetic field intensity in 2000 years ago inferred from archeointensity results using Japanese Yayoi pottery

*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 known to fluctuate significantly over various time scales. In particular, the reference curve of the secular variation of the geomagnetic field intensity over the past several thousand years is important because it can be used as a reference to estimate the making age of fired archaeological artifacts, whose ages are unknown. In order to reconstruct the secular variation of geomagnetic field intensity in Japan and to create a reference curve, we conducted experiments to estimate archeointensities using a large number of Yayoi pottery samples from the Chikushi site in Fukuoka Prefecture (Northern Kyushu), whose ages are different. A total of 43 pottery samples were collected, 27 of which were used in this study. Thermomagnetic analysis performed in vacuum indicated that the heating and cooling curves of the induced magnetization are approximately reversible, while first order reversal curve (FORC) analysis frequently confirmed that single domain (SD) and superparamagnetic (SP) particles are mixed. These results indicate the suitability of the data for absolute paleointensity estimation. The Tsunakawa-Shaw method was used to estimate the absolute paleointensity in a vacuum. As a result, 60 out of 89 specimens belonging to 19 out of 26 pottery samples passed the statistical criteria of the Tsunakawa-Shaw method, and the archeointensities were obtained. In order to ensure the reliability of these samples, we set a selection criterion of "archaeomagnetic intensity obtained from at least two specimens per pottery sample and their standard deviation meets 15% or less" and selected the average of 11 pottery sample levels. From these eleven samples, we excluded one pottery sample that was thought to be overestimated due to anisotropy. For the one pottery sample with a standard deviation greater than 15%, the Smirnov-Grubbs test was able to detect an outlier, so we added the pottery sample average calculated after excluding the outlier. When the archeointensity averages at the 11 pottery samples level selected in the way are arranged in chronological order, a large increase in geomagnetic intensity from 29 µT to 65 µT is observed in a short period of time, from about 200 BCE to about 50 CE. When we converted them to the virtual axial dipole moment, they correspond to 5.5−12.3 × 10²² Am². Assuming that the increase over the 250 years is monotonic, the rate of increase is 0.027 × 10²² Am²/year. This is about five times higher than the decrease rate of 0.005 × 10²² Am²/year of the geomagnetic dipole moment in the last 120 years (IGRF-13). The large increase in the geomagnetic intensity over a short period found in this study is thought to be a unique pattern of geomagnetic intensity variation of the Yayoi period, and the Yayoi period may have been a period when high-precision pottery dating by archeointensity is feasible.