Abstract:
About fifty papers have been published by Japanese scientists from 1968 to 1990 on paleoenvironments of Pleistocene sediments in the Japan Sea using four DSDP and approximately 60 piston cores. A brief review of these papers and the main results of several papers are described in this chapter. Pleistocene sediments in the Japan Sea have been collected by both deep-sea drilling at four sites and piston coring at many sites in a broad area (Fig. 1 and Table 1). A relatively continuous Pleistocene sequence was obtained at Site 299, one of the four DSDP sites, in the Yamato Basin. The Pleistocene section (about 160 m thickness below the sea floor) at Site 299 is composed of clayey silt and silty clay with various intercalations of sandy deposits including turbidites (Ingle, et al., 1975). Koizumi (1975) examined the diatom assemblages in the drilling samples at Site 299 and recognized cyclic fluctuations between the warm and cold-water floras during the late Pleistocene, compared to warm-water flora prevailing during the early Pleistocene (Fig. 2). Ingle (1975) also presented the existence of the warm period during the early Pleistocene from the coiling change of Neogloboquadrina pachyderma in the same drilling samples (Fig. 2). Koizumi (1975) concluded that the condition of the Japan Sea was essentially controlled by the global climatic change during the Pleistocene. Paleoenvironmental study based on the analysis of piston cores is restricted to the uppermost part of the Pleistocene sequence in the Japan Sea, because only short cores less than 20 m in length can be obtained by piston coring. One piston core (RC12-376), however, seems to reach the middle part of the late Pleistocene. Ichikura and Ujiie (1976) suggested the existence of some warm periods inferred from the planktonic foraminiferal faunal changes in the core. This core was re-examined by Ujiie and his collaborators by means of planktonic and benthic foraminiferal fauna, oxygen and carbon isotopes of foraminiferal tests, and diatom flora (Ujiie et al., MS). The result indicates that the core includes six warm periods and that the bottom of the core seems to penetrate to the oxygen isotopic stage 13, corresponding to about 0.5Ma (Fig. 3). Table 2 is a list of previous works by Japanese authors on the paleoenvironmental study of the uppermost Pleistocene sequence in the Japan Sea by analyzing piston cores. Figure 1 shows the localities of the cores listed on Table 2. In this chapter, a brief historical review on the paleoenvironmental study of the Japan Sea is introduced and the latest result is presented. Paleoenvironmental study of the Japan Sea based on piston cores began at the end of the 1960's (Miyake et al., 1968) and continued in the 1970's by using several different methods such as microfossil analysis (diatom: Koizumi, 1970, 1973, 1977, 1979; foraminifera: Ujiie, 1975, 1977, 1979; Ujiie and Ichikura, 1973; Ichikura and Ujiie; 1976, Maiya et al., 1976; Kato, 1978, 1979), phase change of iron minerals (Kobayashi and Nomura, 1972), elemental analysis of interstitial water (Masuzawa and Kitano, 1977, Masuzawa et al., 1979) and clay minerals (Oinuma and Aoki, 1977).