Formation rate and process of iron concretions excavated from the underwater Takashima archaeological site, Nagasaki, Japan

Abstract

At the Takashima underwater archaeological site in Nagasaki Prefecture, where remains of Mongolian ships from the Mongol invasions are buried, many lump-shaped iron concretions formed by the corrosion of iron artifacts have been excavated. Although the shapes and compositions of these concretions have been significantly altered by corrosion, cavity-like structures resembling molds of the original iron objects remain inside some concretions. It is thought that these iron concretions formed as buried iron artifacts underwent chemical reactions with seawater and sediments, incorporating surrounding shells and clastic materials. However, their detailed formation process and growth rate remain unclear. In this study, we analyzed the inclusions within the iron concretions and the surrounding sediments from the Takashima site to investigate their formation mechanisms and rates. The results revealed that the formation of iron concretions began forming within a few years after the burial of iron artifacts and were completed in as little as 100 years.