浮遊性有孔虫からみた黒潮流域における過去2万年間の海洋変動 (黒潮圏の第4紀古環境特集号)

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Planktonic foraminiferal thanatocoenoses have been investigated in 275 surface sediment samples collected from seas off the Pacific coasts of Japan, and altogether 32 species are recognized. The geographical distributions of these species are differentiated into cold (Neogloboquadrina pachyderma) and warm water forms according to their geographical distribution. The cold water species, Neogloboquadrina pachyderma, is restricted to areas of the original Oyashio water and its first branch; there is no occurrence of this species in the domain of the Kuroshio current south of the Kuroshio front. On the other hand, warm water forms, i.e. Globigerinoides ruber, Globigerinoides sacculifer and Pulleniatina obliquiloculata, associated with Globigerinita glutinata, are dominant in the domain of the Kuroshio current and abruptly decrease in their abundance in the mixed water region north of the Kuroshio front. In addition, high frequency distribution of Neogloboquadrina incompta in the domain of the Kuroshio current is restricted to north (inner) areas between the current axis of Kuroshio and the Japanese Archipelago, especially in the vicinity of the Enshu-nada and Kumano-nada, where cold water masses are often formed when Kuroshio meanders wide off southwest Honshu. These results were applied to 4 piston cores (P-4, C-4, C-6 and C-1) recovered from the northwest Pacific off Japan in the present-day domain of the Kuroshio current and its front. Historical changes of the Kuroshio path since the last glacial maximum can be reconstructed on the basis of these criteria, with the assistance of both key tephra and 14C age assignment for a time scale. The results are as follows:A) No influence of Oyashio has been observed during the whole studied interval of cores P-4 (Lat. 32°N) and C-4 (Lat. 33°N). Instead, the presence of a large-scale cold-water mass at both core sites is inferred during the last glacial maximum, due to the southward shift of the Kuroshio main path. However, the influence of a branch of the Kuroshio current is recognized at both core sites, especially at that of core P-4.B) Four distinct changes in the flow patterns of Kuroshio can be shown since the last deglaciation.1. The current axis off the southwest of Honshu meandered southeastward within a large region, and the Kuroshio front was situated in an area close to south of the core C-6 site off the Boso Peninsula during the period from 15, 000yrs BP to 14, 000yrs BP. During the period from 13, 000yrs BP to 11, 000yrs BP, a cold-water mass was still formed at both core sites off the southwest of Honshu, especially at the core C-4 site, although a warming trend is recognized at both core sites.2. After the short cold event recognized at both C-1 and C-6 cores at about 11, 000yrs BP, the current axis of Kuroshio began to shift northwest-ward toward Honshu during the period from 10, 000yrs BP to 9, 000yrs BP, and the Kuroshio front started to shift northward simultaneously.3. From 9, 000yrs BP to about 6, 000yrs BP, the current axis off southwestern Honshu continued to shift close to Honshu, and exhibited a flow parallel to the coast. Meanwhile, the Kuroshio front migrated progressively to reach the area off northeastern Honshu, the northernmost position the front has ever attained at about 6, 000yrs BP during the last 20, 000yrs.4. Since 5, 000yrs BP, Kuroshio off southwest of Honshu has flowed in a way similar to that of the present day. On the other hand, the presence of a cold-water mass is inferred during the period from 4, 500yrs BP to 1, 500yrs BP at core C-1 site and at about 3, 000yrs BP at core C-6 site. Furthermore, two cold events are recognized at about 2, 000yrs BP and 1, 000yrs BP at core C-1 site.
日本第四紀学会の論文

浮遊性有孔虫からみた黒潮流域における過去2万年間の海洋変動 (黒潮圏の第4紀古環境特集号)

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