Orbital- and millennial-scale changes of radiolarian assemblages during the last 220 kyrs in the Japan Sea

Significant abundance changes of seven radiolarian species during the last 220 kyrs in the Japan Sea are reported herein based on three sediment cores. During the interglacial periods, warm water species (the Tetrapyle octacantha group) and warmer/high nutrient water species (Lithomelissa setosa) occurred commonly, which may indicate inflows of the Tsushima Warm Current. On the other hand, cold-water species such as Ceratospyris borealis and Stylochlamydium venustum dominated the assemblage during the glacial periods, when the Japan Sea was mostly isolated from the adjacent seas due to lower eustatic sea level. However, at glacial maxima, the habitat of S. venustum was strongly limited due to low salinity surface waters. The abundance of the deep-water species Cycladophora davisiana seems to have changed significantly, possibly related to the intrusion of oxygen and organic materials into deeper waters. Higher abundances of this species in MIS (marine isotope stage)-1, MIS-3 and MIS-5c resulted from well-ventilated deep water and high organic flux, while its low abundance during the glacial maxima and MIS-5e was probably caused by oxygen-poor deep-water. Millennial-scale variations of C. davisiana abundance show high abundance at the transition intervals from interstadial to stadial periods of the Dansgaard-Oeschger Cycles. The disappearance events of Lychnocanoma nipponica sakaii and Amphimelissa setosa were recognized at 54 ka and 85 ka, respectively. Both of these events in the Japan Sea can be correlated with those in the North Pacific, Bering Sea and Sea of Okhotsk.