後期第四紀における日本海の古環境--テフロクロノロジ-,有孔虫群集解析,酸素同位体比法による (最終氷期における日本列島の動植物相と自然環境特集号)

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The purpose of this paper is twofold: to establish a tephrochronological framework for the sediments of the Japan Sea on the basis of the identification of the abyssal tephra layers and to examine the paleo-oceanographic conditions of the Japan Sea during the last 60, 000 years, based on the foraminiferal biostratigraphy and 18O record in two piston cores.Accurate determinations of the refractive indices of volcanic glass shards and minerals, together with other data, have enabled successful characterization of several tephra layers and permitted correlation to be made between cores.Of the five marker-tephras described in this paper, three can be correlated with the dated widespread tephras which originated from eruptions of gigantic caldera volcanoes in Kyushu. They are: the Akahoya ash, the Aira-Tn ash and the Aso-4 ash. The Akahoya ash (Kikai caldera, 6, 300Y.B.P.) occurs in cores from the southern part of the Japan Sea. The Aira-Tn ash (Aira caldera, 21, 000-22, 000Y.B.P.) is the most prominent marker, found in cores from the whole area of the sea. The Aso-4 ash (Aso caldera, approximately 50, 000Y.B.P.) is found in cores from the central and southeastern part of the sea.The other two marker-tephras, the Oki ash and the Yamato ash, are the products of major eruptions of Holocene and Late Pleistocene age, and have probably originated from the Ulreung-do Island in South Korea. This estimate is based on their peculiar petrographic nature and distribution. The Oki ash is found in a stratigraphic horizon between the Akahoya ash and the Aira-Tn ash in cores from a tract extending from the area adjacent to the Ulreung-do to the Kinki district of central Honshu, where three radiocarbon ages of around 9, 300Y.B.P. were obtained for the eruption. The Yamato ash occurs in cores from areas to the east of the Ulreung-do. A reliable age for this tephra has not yet been determined, although its stratigraphic relationship to the overlying Aira-Tn ash and the underlying Aso-4 ash, suggests that it must occur within a range of between 25, 000 and 35, 000Y.B.P.Vertical changes in the lithological, foraminiferal and oxygen isotope characteristics of the two tephrochronoiogically dated cores from the Oki bank in the Japan Sea occur at about 6, 500-9, 500Y.B.P., 13, 000 and 23, 000Y.B.P. The paleo-oceanographic conditions of the Japan Sea between these ages are reconstructed from the data of paleosalinity and paleo-temperature, which were calculated on the basis of the 18O values of the benthonic and planktonic foraminiferal tests in the cores.The water of the Japan Sea seems to have been of relatively constant salinity (33-34‰) and low temperature (8-10°C) in the period between 60, 000Y.B.P. and 23, 000Y.B.P. Relatively minor amounts of Pacific sea water flowed into the Japan Sea during this period. A conspicuous, continuous decrease in the salinity of the sea water took place in the period from 23, 000Y.B.P. to 13, 000Y.B.P., as inferred from the decrease of the 18O values of the planktonic foraminiferal tests. It seems probable that the inflow of water from the open sea was checked by some paleogeographic changes at the straits, caused by lowering of sea level during this glacial stage. The 18O values of the planktonic foraminifera tests increased suddenly at about 13, 000Y.B.P. Subsequently, the benthonic foraminiferal fauna which live today in the shallow water of the Northwest Pacific coast appeared in the cores of the Japan Sea. These facts suggest that a remarkable inflow of the Oyashio current through the Tsugaru Straits into the Japan Sea began about 13, 000Y.B.P. The coiling direction of Globigerina pachyderma changed from sinistral to dextral approximately 8, 000Y.B.P. Then warm water planktonic foraminifera appeared in the cores, and the temperature of the surface water of the sea increased abruptly by 7-8°C.
日本第四紀学会の論文

後期第四紀における日本海の古環境--テフロクロノロジ-,有孔虫群集解析,酸素同位体比法による (最終氷期における日本列島の動植物相と自然環境特集号)

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