Two piston core samples were collected from the northern basin in the Japan Sea. The J-2 core showed sediment accumulated stratifically without disturbances. The J-3 core collected 60 km south of the J-2 site showed two thick turbidite layers. From the vertical profiles of the major composition, pronounced concentration peaks of manganese were found in both the core samples. One of the layers was enriched with only manganese in the J-2 core. This manganese rich layer may be formed during the early diagenesis processes. The other type of manganese rich layers was associated with the concentration peaks of calcium. This type of layers seemed to be characterized by the formation of the authigenic manganese oxides and the biological scavenging processes in the water column, while the environment of the Japan Sea changed from an anoxic to an oxic condition during the period from 10000 to 8000 yrs ago. The manganese concentration of seawater at the time before the transition period was estimated to be 110 μg/l, and the anoxic condition lasted over 15000 years by that period.

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