동해 한국대지 심해 탄산염퇴적물의 산상

Abstract

East Sea is a semi-isolated marginal sea with an average depth of 1,350m (maximum depth of 3,700m), and comprises three basins such as the Japan, Yamato, and Ulleung Basins.The Korea Plateau is located north of the Ulleung Basin and is less than 1,000m deep.One piston core from the top of the Korea Plateau was obtained at the depth of 758.7m.The core is 587cm long, and is mostly composed of sandy mud and muddy sand with abundant foraminifers.The sediments also contain a few layers of tuffaceous sands and often include granule-sized volcanic fragments.Especially notable are the lowermost part of the core (431~587cm), which consists of carbonate sediments containing sand- to granule-sized bioclastic sands. Radiocarbon dating of some selected foraminifers (G. bulloides & N. pachyderma) shows 6,00235 BP and 45,2001700 BP at the core interval of 7~9cm and 83~85cm, respectively.In contrast, 87Sr/86Sr initial ratio of the lower carbonate sands ranges from 0.70881 to 0.70883 (at the core intervals of 506~507, 526~527, 542~543, 558~559, 571~572, and 586~587cm) and the data indicate the sediment age of 12.8~13.4 Ma (Middle Miocene). The lowermost part of the core (431~587cm), composed of calcareous bioclasts, can be divided into two units (431~490cm and 490~587cm) according to lithology and composition of the sediments. The sediments of the lower unit ranges from -1 to 1 in size with minor contribution of the sediments smaller than 2.This sediment is mostly composed of bioclasts of mollusks and barnacles along with planktonic and benthic foraminifers, bryozoans, echinoids, ostracodes and volcanic rock fragments. In this unit, mean grain size tend to decrease upwards, and this coincides with the abundance of the grains (>-1).The upper unit shows the overall fining-upward sequence.The upper part of the upper unit (431~490cm) is mostly composed of planktonic foraminifers, however the lower part of the upper unit contains more mollusks, benthic foraminifers, bryozoans, echi