Paleolimnologic and paleoceanographic history of the Sea of Japan - a reconstruction based on diatoms -

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Recent paleomagnetic data suggest that Japanese mainland was originally a part of the Asian continent, and subsequently broke off and spread southeastward with the clockwise rotation of southwest Japan and the counterclockwise rotation of northeast Japan. The "Sea of Japan" thus originated as a lake. The purposes of this study are to described the diatom floras in 20 sediments formed when the sea was a fresh-water lake or shallow sea, and to provide information to reconstruct the paleoliminologic and paleoceanographic history of the sea. In particular, I classified Aulacosira and Actinocyclus species into "forms" by scanning electron microscopy, and showed their geographical and stratigraphical distributions. 1. Twenty-five different "forms" (A-U and X) of the genus Aulacosira and three (A, B and C) of the genus Actinocyclus were identified in the twenty sediment samples. 2. RC12-394: Three different associations of Aulacosira "forms" were distinguished by their abundances. 3. Oshima P … More eninsula, Abura, so-called Futoro Formation: Aulacosira "forms" association was characterized by the predominance of "form" B. 3. Sado Islands, Seki, Masaragawa Formation: Aulacosira "form" I was most abundant. 4. Noto Peninsula, Notonakajima, Yamatoda Mudstone Member: Dominant "form" B and rare Actinocyclus sp. A ("form" C) were observed in the member. 5. Fukui, Demura and Hikime, Itoo Formation: Only recrystallized Aulacosira Sp. ("form" X) was found in the formation. 6. Oki Islands, Nogoe, Yui Formation: scarce occurrence of several Aulacosira "form" (B, H and others) and single Actinocyclus "forni" (C) were observed in the formation. From these results, the following are deduced; 1. The Seki flora consists of exclusively fresh-water diatoms deposited when the sea was a lake. The flora is very different from that observed in "Futoro Formation" and Yamatoda Mudstone Member, and the difference could be caused by colder water of the lake. 2. The diatom flora in "Futoro Formation" has some resemblance to that of Yamatoda 11/2,ludstone Member. Admixture of marine species in the uppermost layer of the latter suggests that these strata may hence occupy the same time frame as the first invasion of warm current to the sea. 3. Three associations of Aulacosira "forms" in RC12-394 are not found in the aforementioned land-exposed fresh-water origin diatomaceous rocks. Less