抄録
Seasonal shell microgrowth history of a left valve of the bivalve, Mizuhopecten tokyoensis hokurikuensis from the Pleistocene under the Naruto Strait was analyzed by oxygen isotope analysis and by measuring the distance between adjoining lamellae. The oxygen isotope profile showed two high water temperature periods and two low water temperature periods, indicating that two years of seasonal variations in water temperature were recorded in this specimen. The profile shows an annual variation of 3.77‰ corresponding to 16.0℃ in water temperature, suggesting that the fossil locality under Naruto Strait was an inner bay like the Harimanada Sea at the time when the analyzed specimen inhabited the area. This suggestion is consistent with the fact that the fossil association with M. tokyoensis hokurikuensis includes inner bay and bay mouth bivalves, such as Ostrea denselamelosa, Fulvia mutica, Anadara broughtonii, and others. The results of the analysis showed that the distance between adjoining lamellae increased during the low water temperature period. Based on the result that annual rings were formed when water temperature was highest (mid-September), we estimated that the lamella formation started around mid-April to mid-June by counting the lamellae up to the first annual ring.
