2021 Volume 127 Issue 6 Pages 363-376
Latest Pleistocene and Holocene marine sediments that accumulated on the floor of the Aira Caldera are exposed on Shinjima Island, which was uplifted above the sea in 1780 by intrusion of dacite magma. In this study, fossil assemblages of benthic and planktonic foraminifers and ostracods from the marine sediments were analyzed to establish the palaeoenvironmental history of Kagoshima Bay. Initially fresh water lake, Aira Caldera became part of Kagoshima Bay shortly before 13,000 years ago, with a depth of > 100 m as sea level rose with global warming and a warm ocean current flowed into there. The caldera has remained open to the sea ever since, despite the growth and intermittent eruptions of Sakurajima volcano on the southern caldera rim. The population and number of species of calcareous microfossils were kept significantly lower during the period 13,000 to 8,000 years ago, possibly in response to the spreading of acidic water from progressive dissolution of the volcanic gas emitted from the Wakamiko Caldera in the northeastern part of the Aira Caldera. About 8,000 years ago, the warm ocean current strengthened, resulting in penetration of warm seawater to the caldera floor, which has a present water depth of 140 m.
