9:30 AM - 9:45 AM
[SEM15-03] On the changes in geomagnetic field strength during Yayoi era in Japan
Keywords:geomagnetic secular variations, absolute paleointensity, Yayoi era, Yayoi earthenware
Absolute Paleointensity (API) measurements on heat-treated archaeological samples such as earthenware are an effective tool for investigating the magnetic field strength and its changes over the past several thousand years. Archaeological samples, especially earthenware, are often arranged using formal changes in each region and are expected to have high temporal resolution. Most of the data from previous studies in Japan prior to 2000 years ago were obtained using old methods, and were not likely to conform to current sample criteria. On the other hand, for post-Yayoi samples, several case studies have recently been reported using new measurement methods and detailed and careful sample selection (Kitahara et al., 2018; 2021; Tema et al., 2022; Hong et al., 2013; etc.). In this study, API measurements are conducted mainly on Yayoi earthenware samples to expand the Japanese strength data set and to discuss strength variation in the same age range.
The samples used in this study were excavated from the Kagoshima University campus site (Research Center for Archaeology of Kagoshima University, 2019), and the main vessel type was jar. The age of the sample is estimated to be early to mid Yayoi period, based on the form of the pottery, in combination with the co-occurrence of pottery of the same form at other sites and areas, and radiocarbon dates (Nakazono, 2004; Kawaguchi et al., 2020). All samples were examined for the direction changes in the thermal demagnetization curves to evaluate secondary heat exposure, and APIs were measured using the Tsunakawa-Shaw method (Yamamoto et al., 2003) for the single component samples.
The results of the stepwise thermal demagnetization measurements showed that more than half of the samples had demagnetization curves with bending around 400-460 degreeC, which is considered to be due to the effect of secondary heat exposure during earthenware use, and was judged to be unsuitable for the Tsunakawa-Shaw method.
The API measurements were performed on the samples that showed linear thermal demagnetization curves, and acceptable data with N>=3 and stddev/mean<20% were obtained from four samples (C2 (500+-50 BCE), C3 (650+-50 BCE), C8 (175+-225 BCE) and B30 (75+-175 BCE)). The mean intensities of the two samples from the older period agree within a standard deviation (C3: 53.0+-2.7 microT, C2: 53.5+-1.9 microT) and show weaker geomagnetic intensities (C8: 39.9+-2.5 microT, B30: 19.8+-0.8 microT) with each successive period. In other words, the geomagnetic field strength in Japan is estimated to have decreased slowly to abruptly during the Early to Middle Yayoi Period. This trend is consistent with the global trend in the GEOMAGIA50 database (Brown et al., 2015), suggesting that this phenomenon was due to a decrease in the axial dipole magnetic field (g10). Subsequently, the Far East region is thought to have experienced a rapid increase in the geomagnetic field over the second century (Tema, 2022; Yoshimura et al., JpGU2022).
The samples used in this study were excavated from the Kagoshima University campus site (Research Center for Archaeology of Kagoshima University, 2019), and the main vessel type was jar. The age of the sample is estimated to be early to mid Yayoi period, based on the form of the pottery, in combination with the co-occurrence of pottery of the same form at other sites and areas, and radiocarbon dates (Nakazono, 2004; Kawaguchi et al., 2020). All samples were examined for the direction changes in the thermal demagnetization curves to evaluate secondary heat exposure, and APIs were measured using the Tsunakawa-Shaw method (Yamamoto et al., 2003) for the single component samples.
The results of the stepwise thermal demagnetization measurements showed that more than half of the samples had demagnetization curves with bending around 400-460 degreeC, which is considered to be due to the effect of secondary heat exposure during earthenware use, and was judged to be unsuitable for the Tsunakawa-Shaw method.
The API measurements were performed on the samples that showed linear thermal demagnetization curves, and acceptable data with N>=3 and stddev/mean<20% were obtained from four samples (C2 (500+-50 BCE), C3 (650+-50 BCE), C8 (175+-225 BCE) and B30 (75+-175 BCE)). The mean intensities of the two samples from the older period agree within a standard deviation (C3: 53.0+-2.7 microT, C2: 53.5+-1.9 microT) and show weaker geomagnetic intensities (C8: 39.9+-2.5 microT, B30: 19.8+-0.8 microT) with each successive period. In other words, the geomagnetic field strength in Japan is estimated to have decreased slowly to abruptly during the Early to Middle Yayoi Period. This trend is consistent with the global trend in the GEOMAGIA50 database (Brown et al., 2015), suggesting that this phenomenon was due to a decrease in the axial dipole magnetic field (g10). Subsequently, the Far East region is thought to have experienced a rapid increase in the geomagnetic field over the second century (Tema, 2022; Yoshimura et al., JpGU2022).