日本海の後期第四紀けいそうと古海況

Abstract

To delineate the paleoceanography of the Sea of Japan since the last glacial maximum, I first tried to formulate the relationship between the present-day distribution and physico-chemical properties of surface water masses and the diatom flora in the modern sediments collected at 78 stations which are widely distributed in the sea. Then the results from this examination were applied for the upper portion of eight piston cores with the assistance of key tephra and its 14C age for a time scale.The floral analysis of the diatoms in modern sediments is performed by principal components analysis employing the Q-mode variance-covariance matrix. On floral analysis, two species associations are defined by the second principal component; species association in the warm-current region (Pseudoeunotia doliolus, Melosira sulcata and Thalassiosira oestrupii) and one in the cold-current region (Denticulopsis seminae, Thalassiosira sp. 1 and Thalassiosira nordenskioeldii), From relative frequency of these species at a random count of about 200 diatom specimens for each sample, R value was obtained to facilitate the application of the results from the diatoms in modern sediment samples to core samples. The value is defined as:R=(P. doliolus+M. sulcata+T. oestrupii)/(P. doliolus+M. sulcata+T. oestrupii+D. seminae+T. sp. 1+T. nordenskioeldii)The value are generally constant below a certain surface water masses classified by their physico-chemical properties. From interpretation of the third principal component, the relative frequencies of Pseudoeunotia doliolus and Melosira sulcata provide a useful clue to estimate the degree of the influence of low saline water of the upper surface water in the southern part of the sea; Pseudoeunotia doliolus is dominant in samples from the high saline area, and Melosira sulcata in those from the low saline area.The paleoceanography of the Sea of Japan since the last glacial maximum are deduced on the basis of these criteria as follows: 1) The supply of the warm water which have a similar oceanographic structure to the present-day Tsushima Warm Current, began about 7, 000-8, 000y.B.P. 2) The frontal zone in the eastern half of the Sea of Japan was to the south of its present position, and its southern boundary reached at least as far as the eastern offing of the Oki Islands during a period, from about 15, 000 to 9, 000y.B.P. in conservative estimation, following decrease of the sea water supply through the Tsushima Straits. 3) The zone at that time was probably formed between the low saline water brought from the East China Sea and the cold water which occupied the northern part of the sea. 4) The low saline water probably occupied the surface layer of the southern part of the sea at the last glacial maximum.