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Takao Okuyama, Yutaka Kanzaki, Tokiyuki Sato:
Early to Middle Miocene paleoceanography around Japanese Islands based on the analysis of calcareous nannofossil assemblages
(Poster)

We describe, in detail, the Early to Middle Miocene (Nishikurosawa age) nannofossil assemblages of samples collected from the Japanese Islands and ODP sites located in the W Pacific Ocean for a reconstruction of the palaeoceanographic conditions related to the opening of the Japan Sea.

The Nishikurosawa Stage (Lower to Middle Miocene), which contains subtropical faunas such as Vicarya (Gastropoda) and Operculina (larger foraminifera), is characterised by the oldest marine sediments related to the opening of the Sea of Japan. The stage is widely distributed on not only the Pacific and Japan Sea side, but also the central part of Japan. We analysed the calcareous nannofossils in the samples collected from 15 areas (Okushiri Island in Hokkaido Prefecture, Fukaura and Kadonosawa areas in Aomori Prefecture, Haginari, Sekiryo, Sunakobuchi and Sugota areas in Akita Prefecture, Imogawa area in Niigata Prefecture, Yatuo area in Toyama Prefecture, Suzu area in Ishikawa Prefecture, Joban area in Fukushima Prefecture, Bihoku area in Hiroshima Prefecture, Kakegawa area in Shizuoka Prefecture, Tomioka area in Gunma Prefecture, and Boso peninsula in Chiba Prefecture), situated in the northern, central and western parts of Japan from 35° to 43° latitude. Calcareous nannofossils from these areas are well preserved and contain the marker-species of Martini's biozonation from NN4 to NN6, such as Sphenolithus heteromorphus, Cyclicargolithus floridanus and Helicosphaera ampliaperta. For reconstruction of the palaeoceanography during the early stage of the opening of the Japan Sea, we divided the calcareous nannofossil assemblages found in NN4 into three groups: warm assemblages (Sphenolithus, Discoaster); cold assemblage (Coccolithus pelagicus); and others (small reticulofenestrids and others). The assemblages found in the northern part of Japan (Aomori and Akita Prefectures) are characterised by the common to abundant occurrence of the cold-water species, C. pelagicus. The warm-water species, such as Discoaster and Sphenolithus, are also found in these areas, however, the relative abundance of these warm-water species in total assemblages is low compared to those in the central to the southern part of Japan. The areas located in the central part of the Japan Sea side of Japan (Hokuriku, Hiroshima areas), and the Joban area on the Pacific side of the central part of Japan, are indicated by abundant occurrences of small Reticulofenestra with moderately occurring Sphenolithus and Discoaster. Although the Imogawa area is also located on the Japan Sea side of Central Japan (similar to the Hokuriku, Hiroshima areas), the assemblage is characterised by the abundant occurrence of warm-water species, which comprise >50% of the total assemblages. These characteristics are similar to those in the Tomioka, Boso and Kakegawa areas on the Pacific side. This suggests that the palaeo-Japan Sea is divided into three palaeobiogeographical areas, characterised by a C. pelagicus assemblage (N Japan), Reticulofenestra assemblage (Japan Sea side areas, except for the Imogawa area and the Joban area on the Pacific side), and a Sphenolithus-Discoaster assemblage (Boso, Kakegawa, Tomioka areas on the Pacific side, and the Imogawa areas on the Japan Sea side). This palaeobiogeography indicates that the warm-water current (palaeo-Kuroshio) flows from the S part of Japan to the Kakegawa and Boso areas, and a branch-stream of the current invaded northward, from the Kakegawa and Boso to the Tomioka and Imogawa areas, based on the abundant occurrence of warm-water species in these areas on the Japan Sea side during the latest Early Miocene (NN4).

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 [Division of Micropalaeontology] [Department of Geosciences] [Bremen University]

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