Radiocarbon dating of concretions containing iron artifacts excavated from the Takashima Kozaki underwater site in Imari Bay, Nagasaki

垣内田 滉*; 南 雅代*; 門脇 誠二*; 吉田 英一*; 柳田 明進*; 脇谷 草一郎*; 天野 由記

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It is believed that the Mongolian fleet, dispatched to Japan for the 1281 invasion, sank due to a storm around Takashima in Imari Bay, Nagasaki. Numerous artifacts, including anchors, wooden planks, and iron concretions, have been excavated from the underwater site at Takashima, and these are thought to have originated from Mongolian ships. The iron concretions consist of corroded iron, sediments and shells, with hollow spaces where the original iron objects once existed. Studying the process of concretion formation is important to obtain information about the original iron objects. It is inferred that the iron concretions formed through a chemical reaction between the iron objects and substances in seawater and sediments after the objects were submerged. During this process, calcium carbonates derived from shells could be incorporated into the concretions. However, the exact mechanism and rate of concretion formation process are not fully understood. It is thought that the shells embedded in the iron concretions have the carbon isotopic ratio corresponding to the seawater present when the concretion formed. Clues to understanding the concretion formation process can be obtained by C dating and chemical analysis. This research aimed to investigate the concretion formation process by C dating of iron concretions and sediments from the underwater site at Takashima. The date of a wooden plank was determined to be 1053-1268 cal AD, which aligns with the period of Mongolian invasions in Japanese history. Two shells, located in the inner and outer parts of the same iron concretion, showed nearly identical dates. This suggests that the formation of iron concretions occurred rapidly. In this presentation, we will discuss the formation process based on additional results.