Origin of deep water in the Japan Sea over the last 145kyr
Abstract We examined the paleoceanographic origin of the deep water in the Japan Sea over the last 145 kyr by using stable isotope data from foraminifera and paleontological data related to diatoms and planktonic foraminiferal coiling direction from core KT05-9p2 recovered from the Yamato Rise in the center of the Japan Sea. Carbon and oxygen isotope records of benthic and planktonic foraminiferal tests reveal that not only the Tsushima Current but also the Oyashio Current have been the major sources of Japan Sea water during the past 145 kyr and that changes in deep water properties correspond with eustatic sea-level fluctuations. Seasonal cooling of surface water mainly derived from the Tsushima Current is known to have produced deep water during high sea-level stands. Baroclinic forcing that prevailed during surface-water freshening resulted in the inflow of the Oyashio Current through the Tsugaru Strait at the northeast end of the Japan Sea. After entering the Japan Sea, the Oyashio water sank to the bottom and contributed to deep water formation. Oxygen isotope values of benthic foraminiferal tests from the horizons corresponding to MIS 3 and 4 are similar to the calculated values for Oyashio Current water from periods during which sea level was ~ 80 m lower than at present. The occurrence of authigenic branching pyrite from weakly disturbed layers indicates the onset of oxygen deficiency immediately below the sediment–water interface during stages of low sea-level stands, suggesting the prevalence of deep water stagnation from the weakening of overturning circulation during Japan Sea isolation. Glacio-eustatic sea-level changes have greatly influenced the oceanic regime of the Japan Sea.