京都府久見浜湾における最近６０年間の有孔虫群集の変化が示す海面水位変動の意義

Abstract

Brackish foraminifera were investigated from surface sediments and cores collected in Kumihama Bay, northern Kyoto Prefecture, to clarify the impact on brackish organisms of sea-level changes and recent global warming. Foraminifera increased in the 1950s and the early 1970s, corresponding to periods of higher sea levels around the Japanese Islands. Despite continuous sea-level rises from the 1980s, however, they began to decrease and became eutrophic fauna as represented by Trochammina hadai, preferring highly fertilized water. This change may be related to annual changes in sea level and lake water circulation, which become more intense during periods with wider annual ranges of sea-surface atmospheric pressure. Sea-level changes observed in Maizuru Bay during the 1950s and 1970s are intimately related to long-term changes in the Aleutian Low Pressure system called the Pacific Decadal Oscillation (PDO), but this has not been the case since the late 1980s. Thus, the mechanism of modern sea-level rises since the 1980s is probably not the same as that of the pre-1980s, and is caused by modern lake water conditions with less circulation and an abundance of T. hadai. In addition, the widening and deepening of the channel opening to the Sea of Japan between 1972 and 1975 are also found to contribute to the modification of brackish foraminiferal fauna in the lake.