Calcareous Nannofossil Biostratigraphy from the Japan Sea, Sites 798 and 799: Evidence for an Oscillating Pleistocene Oceanographic Frontal Boundary

Abstract

Preliminary results of the biostratigraphic analysis of calcareous nannofossils recovered from Ocean Drilling Program Leg 128, Sites 798 and 799, provide clues to the Quaternary oceanography of the Japan Sea. The distribution of calcareous nannofossils from the Quaternary sediments at Site 798 (903 m water depth) may record the position of an Oceanographic frontal boundary between warm water derived from a branch of the Kuroshio Current as it entered the Japan Sea through the Tsushima Straits to the south, and colder water introduced into the western portion of the Japan Sea derived from the winter chilling of northern Japan Sea surface waters. This Oceanographic front probably oscillated north-south over Site 798 in response to glacial/interglacial cycles, or perhaps to some other climatic event or combination of events unique to the Japan Sea. During the last 1.5 m.y., six major intervals are recognized when the Oceanographic front may have been north of Site 798 separated by five major intervals when the frontal boundary may have been south of the site. These migrations were centered around approximately 0.125, 0.29, 0.56, 0.62, 0.85, 0.91, 0.98, 1.0, 1.11, and 1.5 Ma, which correspond to the boundaries separating nannofossil-rich sediments from barren or nearly barren, low-carbonate intervals. Nannofossil-rich intervals may represent times when the frontal boundary was north of Site 798, and the site was above the CCD. Barren or nearly barren intervals represent times when the frontal boundary may have been south of Site 798 and the CCD was probably higher. The distribution of calcareous nannofossils at Site 799 (2073 m water depth) appears to be controlled more by the depth of the CCD than by any climatic effects.The FOD (first occurrence datum) of Emiliania huxleyi, the LOD (last occurrence datum) of Psuedoemiliania lacunosa, Helicosphaera sellii, Calcidiscus macintyrei (10 µm), and the FOD and LOD of Reticulofenestra asanoi are recognized from Site 798 cores. The LOD of P. lacunosa is observed in sediments from Site 799.Only in the sediments younger than 1.5 Ma are the nannofossils from Sites 798 and 799 preserved well enough and sufficiently numerous for age dating and paleoceanographic conjecture. In-situ dissolution in older sediments at both sites precludes any dating or paleoenvironmental interpretations.

Site U1425

“…Similar distribution, abundance, preservation, and diversity patterns were documented by Muza (1992) for Site 799 (Leg 127) drilled near Site U1425 on the Yamato Rise. At Site 799, calcareous nannofossils occur intermittently in the upper 81 m of the section.…”

“…The sporadic occurrence of nannofossils in this interval is attributed to CCD fluctuations. The lack of nannofossils and other calcareous microfossils deeper than this depth is attributed by Muza (1992) to in situ dissolution and production of authigenic carbonates based on the presence of monospecific assemblages comprising overgrown specimens of the highly dissolution resistant species C. pelagicus ( Fig. F32) concurrent with high percentages of small (1-12 µm) carbonate grains.…”

Site U1427

“…Below the B/M boundary, the Bc of R. asanoi and the LO of Gephyrocapsa (>5.5 µm) narrowed a possible depth-age line. Although the LO of H. sellii was an apparent outlier, the age of disappearance of this species was diachronous and might be younger by ~1.2 Ma in this marginal sea (Muza, 1992). The resulting age of the bottom of the hole is 1.4 Ma (Table T23).…”

The Pliocene to recent history of the Kuroshio and Tsushima Currents: a multi-proxy approach

“…(Chinzei et al 1987;Su and Wei 2005). Key warm Tsushima Current indices in the Japan Sea include: G. oceanica, C. leptoporous, and H. carteri (Muza 1992). Ikeya and Cronin (1993) analyzed modern ostracod assemblages around Japan and identified biogeographically significant taxa (Table 1).…”