Available online at http://link.springer.com Ocean Sci. J. (2017) http://dx.doi.org/10.1007/s12601-017-0021-8 pISSN 1738-5261 eISSN 2005-7172 Article Fossil Diatom Assemblages as Paleoecological Indicators of Paleo-water Environmental Change in the Ulleung Basin, East Sea, Republic of Korea Suk Min Yun 1,4,5 , Taehee Lee 2 , Seung Won Jung 1 , Joon Sang Park 3 , and Jin Hwan Lee 4 * 1 Library of Marine Samples, South Sea Research Institute, KIOST, Geoje 53201, Korea 2 Jeju Environment Research Section, Jeju International Marine Science Research & Logistics Center, KIOST, Jeju 63349, Korea 3 Marine Ecosystem and Biological Research Center, KIOST, Ansan 15627, Korea 4 Department of Biology, Sangmyung University, Seoul 03015, Korea 5 Protist Resources Research Division, Nakdonggang National Institute of Biological Resources, Sangju 37242, Korea Received 18 February 2016; Revised 28 September 2016; Accepted 28 February 2017  KSO, KIOST and Springer 2017 Abstract  The fossil diatom assemblage record from two sediment cores obtained from the Ulleung Basin, East Sea, Republic of Korea, revealed changes in the diatom assemblage zones in PG1 and PD3 core samples. The two sediment cores were δC 14 dated and approximately represented the late Pleistocene–Holocene. The analysis of age zones in the PG1 core and PD3 core was assessed based on the frequency of variations, and occurrences of biostratigraphical fossil diatom species. During the Last Glacial Maximum (LGM), the sea level was lower than that at present and the Ulleung Basin became isolated from the Pacific Ocean. As a result, there would have been a limited Tsushima Warm Current (TWC) influence, and salinity would have decreased resulting in increased freshwater and coastal diatoms. The distribution pattern of diatoms presented in the cores was associated with changes in water temperature and salinity and the adding of terrigenous material brought about by the input of freshwater. Changes in the abundance of a tychopelagic diatom, Paralia sulcata, reflected the effect of the water currents. Diatom temperature (Td) values and the ratio of centric/pennate diatoms provided evidence of limited influences of the TWC and freshwater inflow. It is thought that all assemblage zones were influenced by the TWC, which had an important effect on the distribution and composition of fossil diatoms. Key words  freshwater, fossil diatom, Last Glacial Maximum, Tsushima Warm Current, Ulleung Basin 1. Introduction Marine sediment cores are a fundamental source of data regarding geological history and paleoclimatic changes (Rothwell and Rack 2006); therefore, it is necessary to recover useful paleoenvironmental data (Haschke 2006). Diatoms are the dominant marine primary producers and play an important role in the carbon, silica, and nutrient budgets of the ocean (Gebühr et al. 2009). In particular, they are sensitive to changes in physical and chemical conditions. Fossil diatoms are created through a process of sedimentation and fossilization of planktonic diatoms. They are particularly suitable for paleontological studies and have been used widely as paleoenvironmental indicators, including species compositions and relative abundance of the main indicator taxa (Reid et al. 1995). Thus, fossil diatoms provide a valuable tool for studying water quality and reconstructing past environments both in freshwater (e.g., Marciniak 1981; Flower et al. 1997; Witoñ and Witkowski 2003) and marine (e.g., Witkowski 1994; Andreen et al. 2000) ecosystems. The East Sea is located in the Pacific Northwest, between the Asian continent and the waters around Japan. The East Sea surrounding Korea is more than 2,000 m deep, owing to the topography of the seabed North of the Japan Basin, East of the Yamato Basin, and in the western Ulleung Basin. The Ulleung Basin is a deep, bowl-shaped, back-arc basin bound by the steep continental slope of the eastern Korean Peninsula (Lee and Suk 1998), and it was formed by an extension of the continental crust, accompanied by a progressive southward drift of the Japanese Arc during the late Oligocene to early Miocene (Yoon and Chough 1995). The modern oceanography of the Ulleung Basin is largely affected by the Tsushima Warm Current (TWC) in the Kuroshio Current (western boundary current). The warm-water and high salinity of the TWC is a *Corresponding author. E-mail: jhlee@smu.ac.kr