Volcanic native sulphur pebbles excavated from the near-shore deposits of Hakata Bay, Kyushu, Japan: Their formation, preservation and archaeological significance

Several archives written in Japan, China and Korea during medieval times document markedly elevated demand for native sulphur which was an ingredient of gunpowder and a key resource during times of war in east Asia. According to historic documents, numerous macroscopic sulphur pebbles (up to about 20 mm in diameter) with distinct lemon-yellow color were excavated from the coastal deposits of Hakata Bay, northern Kyushu (Japan). Radiocarbon (C-14) dating using high-resolution accelerator mass spectrometry of intercalated charcoal remains indicates deposition during the late 11th to early 12th century CE. The age of these deposits indicates the earliest industrial use of native sulphur in the world. The isotopic composition of the excavated sulphur pebbles is consistent with the values obtained for the native volcanic sulphur from the active geothermal regions within Kyushu. Often and very intimately associated opal-CT, cristobalite/tridymite and tridymite fragments were found with the sulphur pebbles, which reinforce the proposed provenance of high-temperature volcanic fumaroles. A possible mechanism for the long-term preservation of the sulphur pebbles within the sediments is discussed on the basis of the isotopic signatures of the easily oxdizable sulphide-sulphur in the matrix. The reduction of marine sulphates in the brackish pore-waters in association with the decay of organic debris, which act as electron donors for microbial reduction of marine sulphates provides a favorable environment for prolonged reducing conditions. Such conditions limited microbial oxidation of the native sulphur pebbles by sulphur-oxidizing bacteria and fungi. Spillage of the pebbles during cargo handling and their incorporation within the sediments and other debris drifting along the near-shore of Hakata Bay is the most plausible explanation for this unusual occurrence of sulphur pebbles.

Automated summary

Archives from medieval Japan, China, and Korea reveal a significant demand for native sulphur for gunpowder production and war efforts in East Asia. Excavations in Hakata Bay, northern Kyushu, Japan, unearthed sulphur pebbles dating back to the 11th and 12th century CE, marking the earliest industrial use of native sulphur. The sulphur samples share isotopic signatures with volcanic sulphur from Kyushu's active geothermal areas, suggesting provenance. The presence of opal-CT, cristobalite/tridymite, and tridymite fragments support the association of sulphur pebbles with high-temperature volcanic fumaroles. The long-term preservation of the sulphur pebbles within sediments can be attributed to reduced conditions facilitated by the reduction of marine sulphates in pore-waters. This limits microbial oxidation and suggests that the pebbles were likely spilled during cargo handling and incorporated into sediments drifting along the near-shore of Hakata Bay.