MUZA, JAY P., Florida State University, Tallahassee, FL; and ATIUR RAHMAN, University of Utah, Salt Lake City, UT

ABSTRACT: Japan Sea Paleoceanography and Geologic History Based on Calcareous Nannofossils

Calcareous nannofossils recovered from Ocean Drilling Program Leg 127 (Sites 794-797), and Leg 128 (Sites 798 and 799) provide clues to the

paleoceanography and geologic history of the Japan Sea. Quaternary, Pliocene, and Miocene nannofossils are present in sediments cored from both Legs. The Quaternary is best represented at Site 798, with a nearly complete nannofossil record for the last 1 Ma. Leg 127 recovered lower and middle Miocene nannofossils, which provide important age controls in constraining the age of the opening of the Japan Sea.

The distribution of nannofossils from the Quaternary sediments cored at Site 798 (903 meter depth) may record the migrations of an oceanographic front separating cooler surface water in the western and northern Japan Sea from warmer surface water brought in by the a branch of the Kuroshio Current (Tsushima Current) through the Tsushima Strait. Comparison of the nannofossil record to other paleoenvironmental indicators (oxygen isotope, foraminifer coiling ratios, silica and carbonate sediment components) suggests that changes in the nannofossil assemblage at Site 798 during the last 1.5 Ma are primarily due to global glacial/interglacial cycles.

Comparison of the nannofossils recovered during Leg 127 with those of the Japanese Islands and northwestern Pacific Ocean suggests a limited influx of warm water in the Late Pliocene to Holocene, and a greater influx during the latest early Miocene to early-middle Miocene. The oldest nannofossil bearing sediment from the Japan Sea (Site 797 in the Yamato basin) implies that marine water invaded the Japan Sea no later than 17-18 Ma. This evidence argues against the paleomagnetic inclination record which support a 12-15 Ma basin opening.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.