後期鮮新世から前期更新世の間氷期における対馬海流の動態とその要因―特に下部更新統における浮遊性有孔虫Globoconella inflataの産出の古環境学的意義の再検討

The cause of the temporal changes in the Tsushima Current during interglacial periods in the late Pliocene-early Pleistocene Japan Sea. -reexamination of the early Pleistocene occurrences of the planktonic foraminifer Globoconella inflata

Abstract: The stratigraphic distribution of warm-water molluscs and planktonic foraminifers in Plio-Pleistocene strata of the Sea of Japan coast of Japan is the basis for reconstructing the history of the Tsushima Current. These fossil records show that the current flowed at 3.5, 3.2, 2.9, 2.4 and 1.9 Ma, in the interval from 3.5 to 1.7Ma, whereas the current flowed at every interglacial highstand, except for MIS 25, 23 and 21.3, at 1.71 to 0.8Ma (MIS 59 to 20).

The stratigraphic distribution of the warm-water planktonic foraminifer Globigerinoides ruber shows that its relative abundance in marine isotope stages 47, 43 and 31 was significantly higher than in the other interglacial stages. These three stages correspond to periods when eccentricity-modulated precession extremes were aligned with obliquity maxima. Therefore, it seems evident that the early Pleistocene marine climate in the Japan Sea was influenced by eccentricity cycles as well as obliquity cycles.

I previously thought that the planktonic foraminifer Globoconella inflata could be used as an indicator of the existence of a southern strait with a water depth greater than that at present (ca. 130m) (Kitamura, et al., 2001). However, this species was recently found in surface sediment of the southwest Sea of Japan. In the present study, I reexamined the early Pleistocene occurrences of this species, by comparing the recent distribution of planktonic foraminifers between the northern East China Sea and the southwest Sea of Japan. I conclude that the population of G. inflata was extirpated by exceptional low temperature caused by the cool-water mass of Japan Sea Proper Water (JSPW). If this interpretation is correct, the occurrences of the species imply that the productivity of the JSPW declined in marine isotope stages 57, 47, 45, 43, 41 and 29. I believe that this was caused by reduced salinity of the Tsushima Current, due to a relative high input of fresh water, since the inflow of marine water was strongly restricted by the narrow and shallow southern strait.