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A distinctive lithofacies sequence in marine sediment cores related to eustatic sea-level changes made it possible to determine the eruption age of the Aso-3 tephra, which was identified in marine cores taken from the Ulleung Basin, East Sea/Japan Sea, by its characteristic major element and mineral compositions and grain-size distribution. Ten tephra layers belonging to four tephra groups (Ulleung, SKP, Aira, and Aso groups), based on the source volcanoes, were identified. The Aso-3 tephra occurred only a few (e. g., 95PC-6, 95PC-9, and 95PC-10) among 48 marine cores. These three cores, which were obtained from sites adjacent to the Ulleung deep-sea channel in the northeastern Ulleung Basin, were characterized by relatively low, constant sedimentation rates. The Aso-3 tephra was distinguished from Aira group tephras and the Aso-4 tephra by its grain morphology (prevailing pumiceous glass fragments), grain size (median diameter 3.5∅), and relatively low silica (67.2-68.6wt%) and alkali (8.0-8.3wt%) contents. The lithofacies containing the Aso-3 tephra is a thick-bedded gray (5Y4/1) bioturbated mud (BM-b lithofacies) with extensive pyrite filaments characteristic of deglaciation sediments. This lithofacies overlies thick-bedded fine-grained turbidites deposited by turbidity flows during sea-level low-stands in the Ulleung Basin and underlies thick-bedded grayish olive bioturbated mud with sediment-filled burrows (BM-a lithofacies), which reflects ventilation changes associated with sea-level highstands. This distinctive lithofacies sequence was deposited during the Termination II penultimate deglaciation to interglacial transition in the Ulleung Basin sediments and is repeated during the Termination I period. Therefore, the eruption age of the Aso-3 tephra is inferred to have been about 133ka with lithofacies sequence, assuming constant sedimentation rates in the three cores of the Ulleung Basin.

Evidence in Ulleung Basin Sediment Cores for a Termination II (Penultimate Deglaciation) Eruption of the Aso-3 Tephra