The sea off Sanriku, northeast Japan, north of ~38°N, is a thermal frontal area between the Oyashio Front and the Kuroshio Front (Fig. F1). Flowing along the coast of Japan and reaching ~35°N, the Kuroshio Current turns east and mixes with the cold, low-salinity Oyashio Current coming down from the Bering and Okhotsk Seas. At the northern margin of the frontal area, Oyashio water intrudes southward along the Sanriku Coast. The Tsugaru Warm Current, flowing from the Japan Sea into the Pacific Ocean through the Tsugaru Strait, flows southward, closer to the Sanriku Coast, and occasionally comes in contact with the Oyashio Intrusion, which elongates, forming a tonguelike shape ~160 km in width (Kawai, 1972).
The sea off northeast Japan is a crucial area for determining the hydrographic relationship between the equatorial Pacific and subarctic North Pacific Ocean over time. However, the northwest Pacific almost entirely lacks carbonate pelagic sediments containing foraminifers and coccolithophorids because its seafloor lies at depths largely below the calcium carbonate compensation depth. Diatom paleoceanographic analyses of the late Neogene on a north-south track across the subarctic front were unravelled during Deep Sea Drilling Project (DSDP) Leg 86 (Koizumi, 1985).
The high-resolution paleoceanographic analyses, however, off the coast of northeast Japan have not been performed because the hydraulic piston corer was not used in previous ocean drilling during DSDP Legs 56, 57, and 87 in the forearc area. Ocean Drilling Program (ODP) Leg 186 provided one opportunity for establishing a high-resolution profile of paleoceanographic history during the Quaternary off northeast Japan based on diatom flora.