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doi:10.2204/iodp.proc.346.202.2018

Results

Datum levels

Four datum levels of radiolarian marker species were recognized at Site U1427 (Table T2). The last occurrences (LOs) of Lychnocanomma sakaii (Morley and Nigrini) and Amphimelissa setosa (Cleve) are well known biohorizons in the subarctic Pacific Ocean and its marginal seas (e.g., Kruglikova, 1976; Matul et al., 2002; Okazaki et al., 2005; Tanaka and Takahashi, 2005; Itaki et al., 2009), and their ages have been estimated in the Japan Sea as 0.054 and 0.085 Ma, respectively (Itaki et al., 2007), according to an age model based on oxygen isotope stratigraphy and widespread tephra layers (Kido et al., 2007).

The LO of Schizodiscus japonicus Matsuzaki et al. (2014) is the same as the reported LO of Spongodiscus sp. in the North Pacific Ocean (Ling 1975; Sakai, 1980), the Bering Sea (Ling, 1973; Ikenoue et al., 2016), and the Sea of Okhotsk (Matul et al., 2002). In the Sea of Okhotsk, this datum has been found in MIS 9 (0.29 Ma) (Matul et al., 2002) or MIS 8.5 (0.27 Ma) (Matul et al., 2009).

The first occurrence (FO) of Amphimelissa setosa was recorded at 544 m CCSF-A at Site U1427. Using an astronomically tuned age model, the first common occurrence (FCO) of this species at IODP Site U1341 in the Bering Sea was dated at ~1 Ma (Ikenoue et al., 2016). Most recently, Matsuzaki and Suzuki (2018) report that the FO of this species occurred at 1.48 Ma in the Gulf of Alaska, the eastern subarctic Pacific.

Subtropical water species

The Tsushima Warm Current (TWC) flows into the Japan Sea through the Tsushima Strait (sill depth = 130 m) from the East China Sea, which is the only source of oceanic water in this sea. It is known that inflow of the TWC was strongly restricted during middle to late Pleistocene glacial periods due to a eustatic fall in sea level causing the closure of the Tsushima Strait (e.g., Oba et al., 1991; Kido et al., 2007). The radiolarian taxon Tetrapyle octacantha group, which is a subtropical species restricted to the TWC in this region, was common during interglacial periods but rare during glacial periods over the past 0.64 My (Itaki et al., 2007; Itaki, 2007), suggesting that occurrences of such subtropical species can be related to the sea level fluctuations and TWC flow into the southern Japan Sea.

Figure F2 illustrates the stratigraphic changes in the abundance of subtropical radiolarians together with the depths of biostratigraphic datum levels. Subtropical radiolarians include Tetrapyle octacantha group, Dictyocoryne spp., Didymocyrtis tetrathalamus, Euchitonia spp., and Spongaster tetras, all of which are common in the Kuroshio Current area through the tropical–subtropical Pacific (e.g., Lombari and Boden, 1985) and the East China Sea (Chang et al., 2003). In the Japan Sea, their distributions show a close relation to the TWC (Motoyama et al., 2016).

Total abundance of subtropical radiolarians shows cyclic fluctuation ranging between 0 and 600 individuals/g through the succession, and their abundance maxima are most likely correlated to the interglacial periods during the early Pleistocene to Holocene (Fig. F2). Although correlations of abundance peaks to MIS 1, 5, 7, and 9 are consistent with interpolation among biostratigraphic horizons such as the LO of L. sakaii, A. setosa, and S. japonicus for the upper part of the cored record (Fig. F2), the more numerous peaks below 290 m CCSF-A are sometimes difficult to assign correctly due to few available tie points in the age model. To determine if this correlation could be improved, we reviewed the onboard results of the planktonic foraminiferal biostratigraphy, especially the coiling ratio of Neogloboquadrina pachyderma and the #2 Globorotalia inflata bed, which are well-known biostratigraphic markers in the Japan Sea (e.g., Maiya et al., 1976).

In Figure F3, biohorizons of the N. pachyderma coiling ratio and the position of the top #2 G. inflata bed at Site U1427 are correlated with Ocean Drilling Program (ODP) Leg 128 Site 798 in the southern Japan Sea (Kheradyar, 1992) and Core V20-199 in the northwestern Pacific Ocean (Maiya et al., 1976).

Maiya et al. (1976) describes three datums based on the N. pachyderma coiling ratio as the younger peak (Datum A), older peak (Datum B), and significant drop of the ratio (Datum C) and estimates their ages in Core V20-119 based on interpolation between paleomagnetic datums. According to the most updated GTS 2012, the ages of Datums A, B, and C of the N. pachyderma coiling ratio in Core V20-119/Site 798 could be dated to 1.12–1.13 Ma, 1.21–1.23 Ma, and 1.26–1.29 Ma, respectively, based on interpolation between the base of Chron C1r.1n (1.072 Ma) and the top of Chron C2n (1.778 Ma). Similarly, top #2 G. inflata bed could be placed at 1.30 Ma at Site 798. Kitamura and Kimoto (2004) note that the top #2 G. inflata bed is identified during MIS 47 to MIS 41 from the Omma Formation in the Hokuriku District of central Japan.

Based on this revised correlation for the lower part of the succession (below 450 m CCSF-A), occurrence peaks of subtropical radiolarians could be assigned to MIS 35, 39, 41, 45, 47, and 49 based on the LR04 curve (Lisiecki and Raymo, 2005), as shown in Figure F2. Although the subtropical peak corresponding to MIS 37 was not recognized, Datum B of the N. pachyderma coiling ratio (right coiling dominant), which indicates a warmer climate, could be correlated to this stage. MIS 43 might be missing due to sampling frequency.

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