Sediment fabrics, oxygenation history, and circulation modes of Japan Sea during the Late Quaternary

The Quaternary hemipelagic sediments of the Japan Sea are characterized by centimeter-to decimeter-scale alternations of heavily bioturbated, organic-poor light layers and finely laminated, organic-rich dark layers. These dark and light layers, which can be correlated throughout the deeper part of the basin, are considered to reflect changes in bottom-water oxygenation. However, depositional mechanisms responsible for individual dark layers and causes of oxygen depletion are not fully understood. Two possible settings for deposition of organic-rich dark layers are recognized. One is a stratified ocean (like the Black Sea) where limited vertical advection results in euxinic conditions in the deeper parts of the basin. The other is an upwelling region (like the Arabian Sea) where the oxygen minimum zone is intensified by enhanced nutrient upwelling and surface productivity. In this study, fabrics of hemipelagic sediments deposited in the Japan Sea during the last 160 ka were studied in three cores collected from different latitudes and water depths. Fabrics were classified into 4 classes based on corresponding bottom-water oxygenation level. Five dark layer types were defined based on the distribution patterns of sediment fabric classes within each dark layer. Comparison of dark layer types at shallower and deeper sites in a chronologic framework provided evidence for five different modes of deep-water circulation. These circulation modes, which reflect relative intensities of upwelling and density stratification, appear to be related to global sea-level changes as well as climate impact on the nature of water influx through the Tsushima Strait.