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The western coast of the Korean Peninsula has some of the highest tidal ranges in the world. Gomso Bay, located along the western coast of Korea, is a semi-enclosed embayment. Tidal deposits occur under most lowland rural areas along Gomso Bay. In this study, the diatom assemblage and grain size distributions of surface sediments along a tributary of Gomso Bay called Jujin Stream were examined to identify their relationships with tidal levels. Consequently, riverside sediments of Jujin Stream could be separated into four tidal level zones based on their diatom assemblage and grain size distribution. Clear diatom and grain size distribution gaps were found between the tidal (zones 1 and 2), fluvio-tidal transition (zone 3), and freshwater (zone 4) zones. The tidal zone was characterized by allochthonous marine diatom taxa. These marine taxa, and the sand content, gradually decreased in the upstream direction. Sediment in the fluvio-tidal transition zone has particularly distinctive features. Extremely high numbers of diatoms are present, which are dominated by freshwater taxa, with rare marine and marine-brackish taxa. Grain sizes in this zone increase sharply in the upstream direction. Combining diatom and grain size data can provide accurate paleo sea-level information for strongly tidal areas. • Diatom and grain size distributions in an area of large tidal range along Jujin Stream, which flows into Gomso Bay. • Limit of sea-water incursion is appear by the fluvio-tidal transition zone, which has extremely high diatom abundance with a majority of freshwater diatoms and rare marine diatom. • The elevation of the fluvio-tidal transition zone is between the extreme high-water level of spring tide and the maximum tidal level of neap tide, although the distance from the river mouth is stronger factor. • In an area of large tidal range, appearance of marine diatom in sediment suggests the elevation was under extreme high-water level of spring tide. [ABSTRACT FROM AUTHOR]

Fossil benthic and planktonic foraminifera from a part of core ND-02 and the published data of core ND-01 from the Nakdong River delta, southeast Korea were analyzed to evaluate the impact of the Tsushima Warm Current on coastal environments in the Korea/Tsushima Strait between Korea and Japan during the early to middle Holocene. Based on cluster analysis, four sample clusters (A, B, C, and D), suggestive of a landward–seaward gradient of environmental conditions, can be defined. According to the spatio-temporal and cluster distributions of benthic foraminifera in faunal compositions, the landward occurrence of benthic foraminifera was delimited in the inner part of the Nakdong River delta at ∼8–7 ka. The common occurrence of warm-water planktonic foraminifera suggests that this maximum landward expansion of benthic foraminifera within the delta was caused by the intensification of the Tsushima Warm Current in addition to sea level rise during the early to middle Holocene. Changes in both marine biota and sedimentation were reported in the coastal areas of southwestern Japan in that period. They were also caused by a precursor of the enhanced Tsushima Warm Current at ∼8 ka. Thus, the evolution of the Tsushima Warm Current during the early to middle Holocene is important for understanding various phenomena in the coastal environments of the Korea/Tsushima Strait. [ABSTRACT FROM AUTHOR]