동해 한국대지 심해퇴적물의 고환경 고해양 변화

Abstract

Paleoenvironmental and paleoceanographic changes in the East Sea were investigated from the two piston core sediments collected at the Korea Plateau in the East Sea. The core sediments can be classified into 7 sedimentary facies, and the lowermost part was composed of the storm-influenced nearshore to shallow marine sediments of middle Miocene in age (12.8 to 13.4 Ma). The age dating of the sediments was carried out from the 87Sr/86Sr initial ratio (0.70881 to 0.70886), and this is the first report of the middle Miocene shallow water carbonates from the deep-sea sediments in the East Sea. The presence of these sediments implies that the Korea Plateau has been subsequently subsided into the present deep-sea position since the middle (the top of the Korea Plateau is deeper than 600 m). In core M04-PC1A, organic carbon and biogenic carbonate contents show big differences between the upper and lower part. Based on the C/N ratio of organic matter, the organic matter was supplied from the neighboring continent for the M04-PC1A core, whereas the organic matter from the M04-PC2 core (which is located at the central part of Ulleung Basin) was not supplied from the continent but from biogenic source. Oxygen isotopic records of the planktonic foraminifera (N. pachyderma) indicate that the pelagic sediments represent the Marine Isotope Stage 1 to 8 (~250 ka). Unlike the previously reported global paleoceanographic data, the oxygen isotope data during the glacial periods consistently show depleted signatures, indicative of surface-water freshening. This should result from the isolation of the East Sea basin due to the closure of the shallow straits around the East Sea during the Marine Isotope Stages (MIS) of 2.2 and 6.2. Also, the paleotemperature calculated from unsaturated alkenons index of the M04-PC1A core sediment were much lower (about 10 oC) during the MIS 6 compared to the present-day temperature..