Toshiaki Irizuki

Middle Pleistocene Ostracoda from the Takatsukayama Member of the Meimi Formation, Hyogo Prefecture, western Japan: significance of the occurrence of Sinocytheridea impressa

ABSTRACT Thirty-seven ostracode species are identified from 15 samples from lithological Units II and III at the type locality of the middle Pleistocene (Marine Isotope Stage 11: ca. 0.41 Ma) Takatsukayama Member of the Meimi Formation, Hyogo Prefecture, western Japan. Three ostracode biofacies are distinguished on the basis of Q-mode cluster analysis. The depositional environments of each biofacies can be inferred as bay sandy coast (BC), enclosed inner bay (IB) and middle bay influenced by freshwater inflows from rivers on the Harima Plain (CB). A rapid increase in water depth from bay coast to middle bay occurred at the deposition of the middle part of Unit II of the Takatsukayama Member. Sinocytheridea impressa (Brady), which probably disappeared or rapidly decreased from paleo-Osaka Bay at about 0.3–0.35 Ma, is abundant at the study site. One of the reasons for the disappearance or rapid decrease of this species is likely that paleo-Osaka Bay changed from a low-salinity, enclosed nutrient-rich and highly turbid bay to a moderate salinity, ventilated bay with good circulation, together with the vicariance event suggested by previous studies. This change was probably linked with both widespread inundation in the neighboring paleo-Harima-nada Bay and the development of large-scale tidal currents from the Akashi Strait connecting both bays. Thereafter, Neomonoceratina delicata Ishizaki and Kato replaced S. impressa and became the dominant taxon in paleo-Osaka Bay. Sinocytheridea impressa which lived in paleo-Osaka and Harima-nada Bays has larger valves than any specimens of this species from Recent seas. The cause of this large size in the past might have been an abundant food supply or dissolved chemicals from freshwater inflows into paleo-Osaka and Harima-nada Bays. Alternatively, whether large-valve populations occurred as a result of genetic changes in relation to the geographical isolation or not, natural selection operated in favor of increased valve size in paleo-Osaka and Harima Bays for reasons that are as yet uncertain.

Temporal and spatial variations of paleoenvironments of Paleo-Hamana Bay, central Japan, during the Middle Pleistocene-Analyses of fossil ostracode assemblages, and total organic carbon, total nitrogen and total sulfur contents-

中期更新世の古浜名湾の古環境を詳細に復元するため, 浜松層の佐浜泥部層が露出する4地点において貝形虫化石群集, 全有機炭素, 全窒素および全イオウ含有率 (CNS元素分析) についての高分解能解析を行った. 貝形虫化石に関しては26種が得られたが, このうちの5種の合計で全体の98.3%に達する. 貝形虫化石とCNS元素分析結果に基づくと, 古浜名湾は海進初期には汽水性の浅い閉鎖的内湾で, 底層には貧酸素水塊が発達した. その後, 暖流が広く流入し湾はわずかに酸化的となった. 急速な海水準上昇に伴い, 再び湾は閉鎖的になり10 m前後の水深に達した. 高海水準期初期には湾は極めて閉鎖的になり広い範囲で底層に無酸素水塊が発達した. その後, 湾は今日の浜名湖よりも暖かい水塊のもとで浅海化していった. このように, 中期更新世の古浜名湾はほとんど閉鎖的な内湾であり, 海進期と高海水準期には現在の浜名湖よりも塩分が高かったと推定される.