[S-EM16] Geomagnetism, paleomagnetism, and rock magnetism

[SEM16-P17] Difference of archeomagnetic field intensity in different heating atmospheres and its reliability

*Yutaka Yoshimura¹, Hyeon-Seon Ahn^2,3, Chie Kato^4,7, Yuhji Yamamoto⁵, Chisato Anai⁶, Yoshinori Tajiri^4,7, Tadahiro Hatakeyama⁸, Masao OHNO^4,7 (1.National Institute of Polar Research, 2.Korea Institute of Geoscience and Mineral Resources, 3.University of Science and Technology, 4.Faculty of Social and Cultural Studies, Kyushu University, 5.Marine Core Research Institute, Kochi University, 6.Aso Volcanological Laboratory, Kyoto University, 7.Advanced Asian Archaeological Research Center, Kyushu University, 8.Institute of Frontier Science and Technology, Okayama University of Science)

Keywords:archeomagnetic field intensity (Archeointensity), heating atmosphere, Tsunakawa-Shaw method, Yayoi pottery

The reference curve of secular variations of geomagnetic intensity over the past few thousand years in Japan, as well as the standard curve of geomagnetic direction, provides fundamental data to understand the Earth's core dynamics. Also, the reference curve over the past several thousand years is important because it can be used as a reference to estimate the making age of fired archeological artifacts, whose ages are unknown. To construct such reference curve requires reliable data. In this study, we conducted experiments to estimate archeointensity using Yayoi pottery samples of about 250 BCE to 50 CE collected at the Gokuden site in Kasuga city, Fukuoka, Japan. We conducted thermomagnetic analysis for 45 pottery samples in vacuum and air. The reversibility of thermomagnetic curves was higher in air than in vacuum. As a result, in the case of heating in vacuum, 60 out of 89 specimens from 19 out of 26 pottery samples passed the acceptance criteria of the Tsunakawa-Shaw method. On the other hand, in the case of heating in air, 17 out of 23 specimens from seven out of eight pottery samples passed the criteria. The acceptance rate is slightly higher for heating in air (73 %) than for heating in vacuum (67 %). There is a difference between heating in vacuum and air in the archeointensity obtained from the sister specimens of the same pottery fragment. The sample 45 and sample 46 are known to have been made in the same period (middle of Middle-Yayoi) in the pottery chronology. However, their sample-level means of the archeointensity do not agree within the standard deviation in the case of heating in vacuum, whereas they agree in the case of heating in air. The ARM0-ARM1 plot, indicating the change of anhysteretic remanence magnetization (ARM) before and after heating, is almost all curved in the case of heating in vacuum, and the one with the largest change seems parabolic. In the case of heating in air, it is classified into two types: curved but slight change and non-curved but relatively large change. The curvatures of the ARM0-ARM1 plot are 0–1.40 (histogram peaks are in 0.5–0.6 and 1.2–1.3) in the case of heating in vacuum, and 0–0.53 (histogram peak is in 0–0.1) in the case of heating in air. Comparing the histograms, the curvature of the ARM0-ARM1 plot is smaller in the case of heating in air than in vacuum. These results indicate that the Tsunakawa-Shaw method with heating in air is more reliable than that with heating in vacuum for Yayoi pottery samples in this study. Although it is problematic that the Tsunakawa-Shaw method with heating in vacuum show a high acceptance rate, unreliable results may be eliminated if we set a new acceptance criterion of the curvature of the ARM0-ARM1 plot.