Late Quaternary Diatoms of the Sea of Japan

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In order to deduce historical changes of the characters and distributional pattern of surface water masses in the Sea of Japan since the youngest glacial maximum, the study has been made in conformity with the following steps. 1) In explanation of a general feature of the geographical distributions of diatom species and preparation of raw data matrix for floral analysis, diatom remains are examined for 78 samples from the bottom surface of the sea. 2) To elucidate the relationship between the abiotic property of surface water and the diatom flora in bottom surface sediments, floral analysis is performed by principal components analysis employing the Q-mode variance-covariance matrix. 3) To deduce historical changes of the characters and distributional pattern of surface water masses in the sea since the youngest glacial maximum, the results obtained from the examination of bottom surface sediments are applied to the diatom flora preserved in the upper part of the eight piston cores studied. On floral analysis, two species associations, which correspond with the distributions of distinctive surface water masses, are defined by the second principal component; the warm-current region is characterized by the occurrence of Pseudoeunotia doliolus, Melosira sulcata and Thalassiosira oestrupii at a high frequency, and the cold-current region by an abundant occurrence of Denticulopsis seminae, Thalassiosira sp. 1 and Thalassiosira nordenskioeldii. From relative frequencies of these species at a random count of about 200 diatom specimens for each sample, R value is obtained to determine the surface water masses in which diatoms were produced. The value is defined as : R=W/(W+C), where W and C are frequencies of the warm- and cold-current region species associations. The R value are generally constant below certain surface water masses classified by their abiotic property. The interpretation of the third principal component leads to such an inference that the relative frequencies of Pseudoeunotia doliolus and Melosira sulcata provide a useful clue to estimate the degree of the influence of low saline water on the upper surface water of the southern parts of the sea; Pseudoeunotia doliolus is dominant in samples from the high saline area, and Melosira sulcata in those from the low saline area. Therefore, both the R value and the specific composition, especially relative frequencies of Pseudoeunotia doliolus and Melosira sulcata, can be regarded as useful criteria for comprehension of the past diatom records. Historical changes of the characters and distribution of surface water masses in the Sea of Japan since the youngest glacial maximum are deduced on the basis of these criteria with the assistance of key tephra to a time scale. The results are : 1) The frontal zone in the eastern half of the sea began the southward shift at least about 6, 000 years B.P. in the offing of the Tsugaru Peninsula, and its southern boundary reached at least as far as the eastern offing of the Oki Islands during a period, conservatively estimated, from about 15, 000 to 9, 000 years B.P., and the zone at that time was probably formed between the low saline water brought from the East China Sea and the cold water occupied the northern part of the sea. 2) The low saline water probably occupied the surface layer of the southern part of the sea at a period about the youngest glacial maximum.
東北大学の論文
1981-03-30

Late Quaternary Diatoms of the Sea of Japan

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