Millennial-scale fluctuations in seasonal sea-ice and deep-water formation in the Japan Sea during the late Quaternary

The East Asian winter monsoon is characterized by cold, dry air blowing from the Siberian high to the surrounding lows. This cold wind cools the surface water along the Russian coast of the Japan Sea. Modern oceanographic observations indicate that sea-ice formation along this coast during an extremely cold winter is an important process for deep-water formation. Thus, formation of both seasonal sea ice and deep-water masses in the Japan Sea are closely related to the East Asian winter monsoon. Two sediment cores were analyzed to reconstruct the history of sea-ice expansion and deep-water formation. Seasonal sea-ice history was reconstructed from the occurrence of ice-rafted debris (IRD) in a core from the northeastern Japan Sea, whereas the deep-water ventilation history was estimated on the basis of records of a deep-dwelling radiolarian species, Cycladophora davisiana, in a core from the southwestern Japan Sea. In the records of sea-ice expansion and deep-water ventilation for the last 160 ky, clear fluctuations on a millennial timescale were observed during marine oxygen isotope stages 3–5. Some of the peaks in sea-ice formation and C. davisiana abundance occurred contemporaneously, indicating strong surface cooling along the Russian coast by a strong winter monsoon. Decoupling of the two records might have occurred because of surface-water stratification caused by the freshening of surface water in relation to high precipitation in summer. Synchronicity of high IRD peaks with North Atlantic Heinrich events suggests that high-latitude climate changes related to the Bond cycle also occurred in the Asian monsoon region. On shorter timescales, IRD occurrence was not clearly related to Dansgaard–Oeschger cycles, which are revealed in the alternating dark and light layers in the late Quaternary Japan Sea sediments. Therefore, during the studied period, strong East Asian winter monsoons did not have a simple see-saw-like relationship with strong summer monsoons.