Geochemical and diatom evidence of transition from freshwater to marine environments in the Aira Caldera and Kagoshima Bay, Japan, during post-glacial sea-level rise

The Aira Caldera in Kyushu, southwest Japan, is a huge Quaternary caldera that formed as a result of an eruption ca. 25 ka. Most of the caldera is now submerged under the innermost part of Kagoshima Bay, which is part of a large graben. The caldera was initially a freshwater lake but eventually became a marine environment. Previous studies demonstrated that the center part of the bay was occupied by fresh water during sea-level lowstand around 18 ka, and was invaded by marine water at ca. 13 ka, during rapid post-glacial sea-level rise. Geochemical and fossil diatom assemblages, however, strongly suggest that seawater was present in the caldera lake during the 18-13 ka interval: sediment from this time contains both elevated sedimentary sulfide content and marine diatom species. This evidence implies that water in the center part of Kagoshima Bay was not entirely fresh for the last 18,000 yrs, and that the Aira Caldera changed abruptly from a freshwater lake to a marine environment at 13 ka. This abrupt change may have resulted from collapse of part of the caldera wall during eruption of Sakurajima Volcano.