Are the coral mounds of the Kushimoto area, Wakayama Prefecture, Japan, submerged Holocene reefs?

Abstract

We investigated coral mounds at 10-15 m water depth in the Kushimoto area of Wakayama Prefecture, Japan. These mounds may be submerged Holocene reefs, but no evidence has yet been presented to support this hypothesis. We collected two cores and three dead corals from one of the mounds. Our results show that Miocene bedrock lies only 0.1 m below the surface of the mound, and in situ corals form a thin veneer above the antecedent substrate. Radiocarbon dating of four coral samples yielded ages of 112 cal. yr BP to modern. These findings indicate that the mounds are not submerged Holocene reefs. The sea-surface temperatures in February are <18°C in the study area and would be insufficient for rapid precipitation of calcium carbonate by coral species. Given that the study area faces the open ocean, it is probable that the corals are frequently dislodged by high wave forces during intense tropical cyclones and/or tsunamis. Consequently, the study area appears to lie beyond the distributional limit of coral reefs during the Holocene.