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Abstract: A detailed facies analysis of core sediments from the Ulleung Interplain Gap (UIG) reveals ten sedimentary facies which reflect a variety of sedimentary processes: volcaniclastic high-concentration sediment gravity flows, low-density turbidity currents, pelagic settling under either well or poorly oxygenated bottom-water conditions, marine fallout of tephra, hampered sedimentation by bottom currents, top-truncation of turbidites by bottom-current reworking, and Mn-carbonate precipitation under changing bottom-water oxygenation states. Distribution of the facies in the cores, integrated with echo characters of high-resolution subbottom profiles, delineates spatial and temporal changes in sedimentary processes, influenced by variations in bottom-current activity during the late Quaternary. The glacial (> ∼15 ka) sediment units from the Ulleung Interplain Channel (UIC), an erosional axial channel system in the UIG, consist mainly of manganiferous and muddy contourites, interbedded with coarse-grained volcaniclastic turbidites or debrites and non-bioturbated pelagites. This indicates prevalent activity of bottom currents, rarely interrupted by volcaniclastic high-concentration sediment gravity flows and deep-water stagnations which resulted in anoxic bottom-water conditions. On the other hand, the glacial units of the southeastern margin of the UIC, where upslope migrating sediment waves developed, are generally dominated by alternating fine-grained turbidites and bioturbated or non-bioturbated pelagites. This suggests that frequent turbidity currents derived from upslope mass wasting were responsible for the generation of the sediment waves. The post-glacial (< ∼15 ka) units from both the UIC and its southeastern margin consist mostly of muddy contourites or bioturbated pelagites with basal manganiferous contourites. They correspond to an acoustically transparent layer which occurs as an elongate mound parallel to the UIC in the southeastern margin. These features are suggestive of bottom-current-controlled sedimentation, hampered sedimentation in the UIC axis by stronger bottom-current activity and focused accumulation of resuspended sediments in the southeastern margin. The post-glacial bottom-current activity was initiated by renewed circulation with open ocean at ∼ 15 ka, probably more vigorous than that of the present. The bottom-current activity seems to have been augmented by enhanced bottom-water formation by colder sea-surface temperature. [Copyright &y& Elsevier]

The Ulleung Interplain Gap (UIG) is a deep (2300–2700 m) passage which has served as a conduit for deep-water circulation between the Ulleung and Japan basins. A detailed analysis of Chirp (2–7 kHz) subbottom profiles (ca. 6270 line-km) and nine sediment cores (8.6–11.4 m long) together with age data of tephra layers and four AMS 14C from the UIG and the adjacent areas reveals complex sedimentation caused by an interaction between bottom currents and mass flows during the last- and post-glacial periods. From high-resolution subbottom data, rock basement, slide/slump/rock-fall deposits, mass-flow chutes/channels, mass-flow deposits, bottom-current deposits, and a large-scale bottom-current channel system are recognized. Core sediments consist of various deposits of turbidites, muddy contourites, manganiferous contourites, and pelagic/hemipelagic sediments. Based on vertical distribution of sedimentary facies together with a chronostratigraphic framework, core sediments can be divided into Units I (<∼15 ka) and II (>∼15 ka).The extensive mass-flow deposits with slope failures on the entire slopes of topographic highs around the UIG and the dominant turbidites in Unit II (>∼15 ka) suggest that a relatively large amount of sediment was delivered into the UIG by frequent mass flows (recurrence intervals of ca. 250–500 years in the upper Unit II) during the last-glacial period. Erosion or hampered sedimentation by bottom currents is indicated by the truncated reflectors of channel walls and muddy/manganiferous contourites in the Ulleung Interplain Channel (UIC) along the UIG. Interbedded turbidites in the UIC floor reflect that some large-scale mass flows intermittently entered into the UIC.The UIC has an asymmetric channel-flank geometry. The southeastern flank shows a gentle, wide mound morphology of mass-flow deposits derived from large-scale slope failures on the slopes of the Oki Bank, reflecting a dominance of downslope gravitational processes over alongslope bottom currents. In contrast, the northwestern flank is characterized by a narrow, steep geometry of mass-flow deposits, where a relatively small amount of sediment derived from the slopes of the South Korea Plateau could not overcome bottom-current activity.The dominant muddy and manganiferous contourites with rare turbidites in Unit I (<∼15 ka) reflect intensified bottom currents and infrequent slope failures (recurrence intervals of ca. 1700–5000 years) during the post-glacial period. These conditions facilitated the formation of a thin, elongate mound of bottom-current drifts overlying mass-flow deposits on the southeastern UIC flank, and sustained erosion or hampered sedimentation in the UIC. [Copyright &y& Elsevier]