| The results of the biostratigraphic analysis of calcareous nannofossils recovered from sediments cored in the Japan Sea and the western North Atlantic Ocean provide clues to the Neogene paleoceanography of those regions. The calcareous nannofossil biostratigraphic analysis of Ocean Drilling Program Site 798 sediments chronicles the history of ten major glacial-eustatic events which had an impact on phytoplankton productivity and carbonate preservation during the last one million years in the Japan Sea. Fluctuations in the distribution of calcareous nannofossils from the upper Pleistocene sediments signal changes in phytoplankton productivity and calcium carbonate preservation resulting from glacial/interglacial cycles. Furthermore, the presence and relative location of a dynamic oceanographic frontal boundary over the last one million years in the Japan Sea can be inferred from the calcareous nannofossil record.;During global glaciation, the Japan Sea is variably restricted to totally cut off from the influence of warm Pacific water inflow. During interglacial periods, the degree of Pacific warm water inflow is greater due to higher sea level. As interglacial conditions cycled into glacial conditions, Kuroshio water 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 into the Japan Sea was progressively cut off. At the same time, sea ice formation enhanced the production of cold bottom waters, which eventually filled the entire Japan Sea. Then, as climate ameliorated, progressively warmer water once again entered the Japan Sea through the deepening Tsushima Strait. An oceanographic front formed, strengthened, and became established as sea level rose, and a warmer surface water mass was created over the colder water, much like the present day sea.;Variations in the nannofossil, foraminifer, biogenic silica, carbonate, and terrigenous sediment components all show significant correlation with the primary Milankovitch orbital periodicities. Spectral analysis of these records generally exhibit significant spectral variance concentrated at periodicities of 100, 41, 23, and 19 k.y.;At a similar northern latitude, but in the western North Atlantic, sediments cored on a Neogene sediment drift at Deep Sea Drilling Project Site 603 on the lower continental rise just east of the crest of the Hatteras Outer Ridge are early Pleistocene in age and older. The more recent sediments have either been eroded or were never deposited due to a more vigorous bottom circulation associated with Northern Hemisphere glaciation. Calcareous nannofossils are sufficiently numerous in the upper 900 m of Site 603 to permit delineation of zones, correlation with the paleomagnetic data, and the detection and age determination of major Miocene hiatuses and their correlation with seismic stratigraphy. Correlation of these hiatuses to local seismic reflectors dates them at this site between 8.5 and 10.4 Ma. I suggest that these hiatuses, which can be correlated with regional Reflector Merlin, are linked closely to intensification of bottom water circulation caused by the buildup of the West Antarctic Ice Sheet. These disconformities help delineate the Vail cycle TM3.1 eustatic event, which has been characterized as the sharpest and most profound sea level drop of the late Miocene. |
