ABSTRACT: Late Neogene Paleoceanography of the Sea of Japan Based upon Diatoms
Lloyd H. Burkle
We identified several time intervals apparent late Neogene stepwise paleoenvironmental change in the southern Sea of Japan. The paleoenvironment of the middle Pliocene and older sediments cannot be characterized because the diatom assemblage is made up of a near monospecific ooze (Coscinodiscus marginatus). The middle Pliocene, prior to 2.4 Ma but after the deposition of the C. marginatus ooze, is characterized by warm water diatoms while the late Pliocene (between approximately 2.4 my and the Pliocene/Pleistocene boundary) is characterized by cool water diatoms belonging to an upwelling assemblage. The Tsushima Strait became emergent at approximately 2.4 Ma owing to sea level drop caused by initiation of continental ice build-up on Greenland and by ongoing tectonic uplif across the southern part of Honshu. Diatoms deposited above the Pliocene/Pleistocene boundary suggest that the Tsushima Strait was again submergent and that a branch of the Kuroshio Current flowed into the Sea of Japan. The early Quaternary is characterized by variable preservation and abundance of diatoms with warm water species predominating. We observed changes in both diatom abundance and preservation in upper Quaternary sediments, probably reflecting changes in productivity induced by major glacial/interglacial oscillations. We found no evidence of the complete isolation of the Sea of Japan from the Pacific but our data do suggest, at least, constriction of the Tsushima Strait during maximum glacial episodes. The late Pliocene-Pleistocene interval is characterized by alternating in ervals of laminated/homogenous sediments. There are some 30 laminated intervals within the last 300 kyr suggesting that this signal may be the second harmonic of precession. The laminated layers may reflect salinity changes in the southern Sea of Japan. During lowered sea level stands, fresh water from the west may mix with waters of the Tsushima Current reducing summer time salinity. If the salinity is reduced enough, winter cooling may not be sufficient to raise the density of surface waters and cause it to sink thus closing down the formation of oxygen-enriched deep waters.
AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990