Hypolimnetic Transitions and Sand-bar Development in Aso-kai Lagoon (Central Japan) during the Past 1,200 Years, Inferred from Benthic Foraminifera

Paleoenvironmental changes during the last 1,200 years in Aso-kai Lagoon (central Japan) were inferred from the record of fossil benthic foraminifera in cores ASC2 (4 m length) and Aso4-2 (1.6 m length) taken from the center of the lagoon. Analysis of fossil foraminifera showed an alternation between bay environment and seasonally oxygen-poor brackish environment that was repeated twice in the lower part of these cores (before ∼380 cal BP). Year-round near-anoxia with occasional periods of increased oxygen level was established in the upper part of the cores (after ∼380 cal BP), indicating a restricted lagoon. The temporary increases of the dissolved oxygen level near the lagoon bottom in the upper part of the cores are probably related with sand-bar breaches and channel reopening due to extreme storms since seventeenth century.