Late Quaternary sedimentary processes and variations in bottom-current activity in the Ulleung Interplain Gap, East Sea (Korea)

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.