Absolute Intensity Field Variations in Japan During the First Millennium CE: New Data from Kofun and Late Yayoi Archaeological Artifacts

We present new absolute archaeointensity data from Japan obtained from the study of ceramic fragments from six archaeological sites situated at the Okayama prefecture. All studied materials are well dated based on archaeological evidence, with ages ranging from 160 CE to 675 CE, covering the Late Yayoi and Kofun periods. The collected fragments come from pottery, coffins and haniwa artifacts from the sites of Tatezaka, Tatetsuki, Tenguyama, Nima Ohtsuka, Sada Higashizuka and Sada Nishizuka. Rock magnetic experiments suggest the presence of magnetite and/or Ti-magnetite as the main carrier of the remanence. In some samples from Tatetsuki and Nima Ohtsuka sites the presence of a hard magnetic component is observed, too. This high coercivity component is characterized by low Curie temperature of around 300 ^oC, and could be identified as the High Coercivity, magnetically Stable, Low Temperature (HCSLT) magnetic phase, previously recognized in several archaeomagnetic samples as a product of heating of (iron-rich) clays. After thermal demagnetization experiments, the most magnetically stable samples were selected for archaeointensity analysis, performed following the classical Thellier-Thellier modified by Coe method, including partial thermoremanent magnetization (pTRM) checks and pTRM tail-checks. All measurements were corrected for anisotropy and cooling-rate effects. In representative samples, the Tsunakawa-Shaw archaeointensity protocol was also applied. Successful archaeointensity determinations were selected following strict quality criteria. The new data are compared with previously published archaeointensity records from Japan, nearby countries as well as with global geomagnetic field models predictions. Such comparison shows lower intensities than those previously obtained while a possible fast intensity variation around the second century CE is also observed. Such abrupt intensity change seems to be accompanied by high variation in declination and inclination values, but it still needs to be confirmed by further data.

This study is partially funded by the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement BE-ARCHAEO No 823826.