Post-caldera volcanism and environmental changes in Aira Caldera, inner Kagoshima Bay, SW Japan

<p>This paper describes the sedimentological, petrographic, and paleontological features of core samples recovered when drilling a seismic observation well on Shinjima Island, which emerged from the submarine floor of Aira Caldera in 1780 by magma intrusion. The results provide information on post-caldera volcanic activity and environmental change in Aira Caldera. Shortly after its formation at ~30 cal ka BP, Aira Caldera was filled with freshwater, but seawater suddenly replaced the freshwater at ~14.5 cal ka BP as sea level rose after the last glacial maximum. Sakurajima Volcano, located on the southern rim of the caldera, intermittently ejected dacitic tephras into the freshwater caldera lake, and Wakamiko Volcano, located in the northeastern part of the lake, also erupted explosively, releasing sublacustrine rhyolitic density currents over the lake floor. Lahars and floods carried much larger volumes of epiclastic material into the lake. Shortly before the transition from freshwater to seawater, Sakurajima and Wakamiko Volcanoes temporarily became calm and fine particles accumulated in the lake. The explosive eruptions at Wakamiko Volcano resumed at ~13 cal ka BP, culminating in a large eruption that formed Wakamiko Caldera and a thick accumulation of Shinjima Pumice. The explosive eruptions almost ceased with the minor eruption of the Southern Shinjima Pumice immediately after the caldera-forming event. The Southern Shinjima Pumice contains pumice and ash of the Sakurajima Satsuma tephra, indicating that the largest explosive eruption from Sakurajima Volcano occurred during the period between the eruptions of the Shinjima Pumice and the Southern Shinjima Pumice. Since the caldera-forming eruption, Wakamiko Caldera has become quiescent while Sakurajima Volcano has intermittently erupted, producing fallout tephras and lava flows. The Sakurajima tephras and volcaniclastic materials reworked from surrounding areas have accumulated on the submarine floor of Aira Caldera.</p>