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Jay P. Muza:
Oceanographic history of the Japan Sea during the last one million years based on the calcareous nannofossil record derived from ODP Leg 128, Hole 798A cores
(Talk)

Results of the biostratigraphic analysis of calcareous nannofossils recovered from ODP Leg 128, Site 798, provides clues to the Quaternary oceanography of the Japan Sea for the last one million years. Fluctuations in the distribution of calcareous nannofossils from the Quaternary sediments at Site 798 (water-depth = 903m) represent changes in phytoplankton productivity and calcium carbonate preservation resulting from glacial/interglacial cycles.

The calcareous nannofossil biostratigraphic analysis of Hole 798A sediment chronicles the history of five major glacial events and five major interglacial periods, which had an impact on phytoplankton productivity and carbonate preservation during the last one million years. The biogenic silica, terrigenous quartz and feldspar, Neogloboquadrina pachyderma coiling ratios, planktonic foraminifera assemblage characteristics, and carbonate oxygen and carbon isotope data derived from the Site 798 sedimentary record are correlated with the calcareous nannofossil record and the Pleistocene standard marine oxygen isotope stages. Results of this correlation lend further evidence that the distribution of calcareous nannofossils from the Site 798 sediments records a history of glacial/interglacial cycles, carbonate preservation events, and calcareous phytoplankton productivity.

The sediment data indicates that, during global glaciation, the Japan Sea was variably restricted to totally cut off from the influence of warm Pacific water inflow. During interglacial periods, the degree of Pacific warm-water inflow into the Japan Sea was greater due to higher sea-levels.

As interglacial conditions cycled into glacial conditions, the Kuroshio Current was progressively restricted from entering the Japan Sea through the Tsushima Strait, and the influence of the colder water derived from the north progressively increased. The strength of the Tsushima Current progressively diminished as sea-level dropped and warm-water inflow from the south, through the Tsushima Strait into the Japan Sea, was progressively cut off. As climatic conditions continued to turn more glacial, the N Pacific Polar Front migrated southward, which allowed cold Oyashio Current water to flow into the northern Japan Sea through the Tsugaru Strait. At the same time, sea-ice formation in the northern Japan Sea enhanced the production of cold bottom-waters. These events led to a filling of the Japan Sea with cold, polar waters that eventually covered the entire surface of the Japan Sea during glacial maxima. Sea-level during glacial maxima was presumably low enough to restrict all inflow of warm Kuroshio Current water from the S and cold Oyashio Current water from the N into the Japan Sea.

However, as climate ameliorated, warmer water once again progressively entered the Japan Sea through the deepening Tsushima Strait, as colder water from the N Pacific Oyashio Current flowed through the Tsugaru Strait. An oceanographic front formed, became established, and then strengthened as sea-level rose, and a warmer surface water-mass was created over the colder water, culminating in present day Japan Sea conditions.

The climatically induced variations in the nannofossil, foraminifer, biogenic silica, carbonate, and terrigenous quartz and feldspar record from Site 798 show significant correlation with the primary Milankovitch orbital periodicities. Spectral analysis of these records generally shows significant spectral variance concentrated at periodicities of 100, 41, 23, and 19ky.

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 [Division of Micropalaeontology] [Department of Geosciences] [Bremen University]

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