Regional archeointensity curve from 600 BCE to 1700 CE for East Asia and possible recurrence of the weak field intensity

In recent years, there have been reports suggesting the possibility of the West Pacific Anomaly existing in East Asia during the 16th century. To verify whether the anomaly existed in other ages, reliable archeomagnetic intensity (archeointensity) measurements from fired archeological materials in East Asia are necessary. Therefore, we conducted a study on archeointensity and rock magnetism using several Japanese pottery fragments from the Yayoi period, which were made in three stages. Thermomagnetic analysis revealed that the induced magnetization curve in air was more reversible than that in vacuum. Based on these results, we conducted the Tsunakawa-Shaw method with heating in air. As a result, we obtained archeointensities from 18 out of 20 specimens belonging to 4 out of 6 pottery fragments, which were made in three stages. When the average values of the three stages are arranged in chronological order, from 250 BCE to 50 BCE, the archeointensity remained nearly constant at 39.4 ± 4.2 µT to 38.6 ± 5.0 µT, and from 50 BCE to 50 CE, an increase in archeointensity was observed, from 38.6 ± 5.0 µT to 46.8 ± 2.0 µT. We constructed a reference curve from -530 CE to 1725 CE using a total of 30 archeointensity data from the present study and recent studies in Japan and South Korea. In this reference curve, two minima and two maxima were observed. Among these, we newly discovered a minimum around 150 BCE. The time intervals between the minimum and minimum, as well as between the maximum and maximum, were both approximately 900 years. Besides, this suggests the possibility that the West Pacific Anomaly occurred in East Asia at approximately 900-year intervals. This indicates that the reversed flux patch that causes the West Pacific Anomaly may recur at intervals of ~900 years at the core-mantle boundary near East Asia. The characteristic time-interval of approximately 900 years is similar to the recurrence time-interval of the South Atlantic Anomaly, which implies that the common cause (e.g., planetary gyre) may modulate both anomalies.