Abstract:
The Japan Sea has its own deep-circulation system, with its deeper parts occupied by cold and highly oxygenated water formed by winter convection in its northwestern reaches. We elucidate the modern depth distribution of radiolarian species and their relation to water masses, from the study of plankton tows and surface sediments. Cycladophora davisiana occurs in a depth interval between 1000 m and 2000 m (deep layer of JSPW=Japan Sea Proper Water), and Actinomma boreale/leptoderma Group in depths below 2000 m (bottom layer of JSPW). The study of seven sediment cores located in water depths ranging from 807 to 3613 m show that the radiolarian assemblages have varied since 30 cal ka BP, indicating changes in water-ventilation strength in this marginal sea. During the interval from 30 to 17 cal ka BP, ventilation was restricted to an intermediate layer beneath the low-salinity surface water, while the deeper zone was filled with static, anoxic water. Deep ventilation began abruptly at 14 cal ka BP. Cold and oxygen-rich water began to sink into the bottom layer, as a result of cold oceanic inflow from the northwestern Pacific through the Tsugaru Strait in the north. Deep convection activity increased from 13 to 12 cal ka BP, which coincides with the Younger Dryas-like cooling event, and then stopped at 12–11.5 cal ka BP. Bottom-water ventilation during the Holocene has been dependent on high-salinity inflow through the Tsushima Strait in the south and winter cooling in the northwestern part of the Japan Sea. Deep water was being actively formed in the early Holocene, in contrast to the static bottom water. This bottom-water formation has resulted in relatively constant water composition since 9 cal ka BP, with the overall increase in high-salinity oceanic-water inflow, although the latter decreased transiently from 7 to 5 cal ka BP in concert with climatic warming.
