Isao Motoyama

Radiolarian Assemblages in Surface Sediments of the Japan Sea

Abstract. Radiolarian assemblages in 69 surface sediment samples from the Japan Sea were moderately diversified, consisting of about 100 taxa in total, although only a few species accounted for a large proportion of most assemblages. First, the assemblages were often dominated by deep-dwelling species: Cycladophora davisiana, Actinomma leptodermum, A. boreale, A. langii, and adult forms of Larcopyle buetschlii. These species were restricted to great depths and were characteristic of the cold, oxygen-rich deep water that fills deep basins of this marginal sea, the so-called Japan Sea Proper Water. Second, although the observed Japan Sea assemblages included some subtropical elements, such as the Dictyocoryne and Euchitonia groups, Didymocyrtis tetrathalamus and the Tetrapyle octacantha group, many of the major temperate and subarctic elements of the North Pacific were essentially excluded. The semi-enclosed topography of the Japan Sea is most likely responsible for the dominance of certain subtropical surface dwellers as well as for the near-absence of transitional and cool water species from corresponding latitudes of the North Pacific. Q-mode cluster analyses of the relative abundance data of the radiolarian species distinguished three faunal provinces that reflect the modern surface water circulation and the distributions of the upper water masses, including the Tsushima warm current and the Liman cold current. These results suggest that the radiolarian assemblages are strongly related to the present hydrography of the Japan Sea and can therefore be used as environmental proxies in this region.

Radiolarian Biostratigraphy from Middle Miocene to Late Pleistocene in the Japan Sea

Abstract. In the Integrated Ocean Drilling Program (IODP) Exp. 346, sampling by drilling was conducted at seven sites (U1422–U1427 and U1430) in the Japan Sea. Radiolarians in moderately well preserved states were found in most samples throughout the sequence in varying abundance. Forty-one radiolarian datum events were identified in this study, and the radiolarian zonation that best divides the middle Miocene to Pleistocene sequences with updated ages of radiolarian datum events (estimates based on the geomagnetic polarity time scale (GTS) 2012) was applied to the sedimentary sequences in the Japan Sea. Here, four new radiolarian zones are proposed for the Quaternary of the Japan Sea, and one zone is slightly revised to adjust for differences among other zones. The sequences collected at the sites extended from the Pleistocene Ceratospyris borealis Zone to progressively deeper zones as follows: Site U1427, four zones to the Pleistocene Schizodiscus japonicus; Site U1422, six zones to the late Pliocene Hexacontium parviakitaense Zone; Sites U1423, U1424 and U1426, eight zones to the early Pliocene Larcopyle pylomaticus Zone; and Sites U1425 and U1430, fourteen zones to the middle Miocene Eucyrtidium inflatum Zone. The absence or extremely rare occurrence of Stylatractus universus and E. matuyamai indicates that S. universus lived in the deep water of the northwestern Pacific and had not been able to migrate into the Japan Sea across the Tsugaru Strait since the Pliocene.