Water mass conditions during the past 11,000 years in the innermost part of the Kagoshima Bay, South Kyushu, Japan : evidence from the fossil shell assemblages, oxygen and carbon isotopic compositions
Quantitative analysis of fossil shell assemblage was carried out in Shinjima Island located in the innermost part of the Kagoshima Bay, South Kyushu, to confirm the change of mollusks' habitat conditions. In addition, stable carbon and oxygen isotopic ratios of carbonate fossil shells were also measured to clarify the water-mass conditions of the Kagoshima Bay during the past 11,000 years. Both the fossil shell community and carbon isotopic ratios clearly indicate that the water-masses in the innermost part of the Kagoshima Bay were influenced strongly by the pelagic water, which replaced the fresh water mass initially present in the paleo-caldera lake. Thus, the pelagic water mass increased progressively, and became the maximum during the period between 6,000-2,300 yrs BP when the shell bed (Moeshima Shell Bed) was formed. The analysis of fossil shell assemblage and shell size distribution, however, indicates that the Neopycnodonte cochlear, the dominant species in the Moeshima Shell Bed, did not reproduce in the bay, but supplied as the planktonic larvae from the pelagic water mass. After that, it started that the influence of pelagic water-mass became to be weak at 2,300 yrs BP, when the supply of the larvae was stopped. As a result, the dominant pelagic species disappeared from the innermost part of the Kagoshima Bay, and consequently the shell community was replaced by the assemblage comprised of the inner bay species. Such a change in mollusks community appears to be depended on the time-series variation of the Kuroshio Current influence.
九州大学理学部紀要 32(1), 97-112, 2007-02-01