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Bivalve limestone and non bivalve limestone, occurs in the lower part of the Middle Formation of the Neogene Taishu Group, Tsushima Island, Nagasaki Prefecture, southwest Japan. The fossil fauna of th...e limestone and surroundings of the limestone are considered to be composed of chemosynthetic fossil ecosystem. This limestone is autochthonous originated from a methane seep, based on the occurrence, their very low carbon isotope ratios (δ13C; per mil PDB, respectively) and sulfur isotope ratios (δ34S; per mil PDB, respectively) of sulfide sulfur contained the limestone, and it is highly possible that the chemosynthetic ecosystem was maintained by hydrogen sulfide occurred anaerobic methane oxidation and/or originated from magma. The limestone indicates the existence of a large methane supply, possibly from methane hydrate. The fossil flora and molluscan fauna indicate the Taishu Group was deposited in a warm-temperate zone (mid-latitude), warmer than the climate of the present-day Tsushima district. The lower part of the Middle Formation was deposited in a deep-sea environment at water depths greater than 500 m, as indicated by the habitat depth of the molluscan fauna, the absence of photosynthesizing faunas, and the stabile condition of methane in mid-latitude regions. Rapid subsidence is considered to have occurred over a wide area during the maximum opening stage of the Japan Sea in the middle Miocene. However, in previous studies information concerning the middle Miocene strata indicating such deep-sea environmental conditions in the southwestern part of the Japan Sea is meager. The present findings from the Taishu Group such as limestone originated from a methane seep and of a fossil chemosynthetic ecosystem indicate the existence of a deep-sea environment over a wide area in the Japan Sea during the middle Miocene. These results suggest the early stage of the opening Japan Sea formed in a very deep and narrow ocean.続きを見る
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