--> --> Geochemical Study on the Origin of Organic Matter and Depositional Environment of Late Quaternary Sediments in the Ulleung Basin of the East Sea

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Geochemical Study on the Origin of Organic Matter and Depositional Environment of Late Quaternary Sediments in the Ulleung Basin of the East Sea

Abstract

According to radiocarbon dating of planktonic foraminifera samples from the Ulleung Basin of the East Sea, the cores cover the time interval from the middle stage of marine isotope stage (MIS) III through last glacial maximum (LGM) to Holocene. In the cores, hemipelagic muds (bioturbated and slightly laminated mud facies) were mainly deposited during interglacial periods (MIS I/III), whereas non-hemipelagic muds (laminated and homogenous mud facies) were formed during glacial period (MIS II). During the cold time, sedimentation rates clearly increased more than the other times. Such characteristic changes in sedimentary facies and sedimentation rate evidently reflect paleo-environmental variations on the Ulleung Basin during Late Quaternary. Also, the cores contain several lapilli tephra layers, rhyolitic ash layers and dark laminated muds. Among these, previously well-known tephra layers (Ulleung-Oki, Aira-Tanzawa and Ulleung-Yamato) were identified and used to stratigraphically correlate the cores. Particularly, our sedimentary geochemical studies (Rock-Eval pyrolysis, elemental and stable isotope analyses) allow us to discuss the origin of organic matter and the depositional environment of sediments. Down-core profiles of TOC (Total Organic Carbon), TN (Total Nitrogen) and δ13Corg show the lowest values during MIS II and the highest during MIS I. The relationship between TOC/TN and δ13Corg suggests that the organic matter was predominantly produced by a marine source rather than by a terrestrial source. However, redox-sensitive trace elements have not significantly varied during MIS II as well as during MIS I/III. The ratios of these trace elements also indicate a predominant oxic or suboxic environment, indicating that the Late Quaternary depositional condition of the Ulleung Basin was less dynamically changed than we expected before.