Rapid Paleoceanographic Changes in the Okhotsk Sea during Deglaciation, the Holocene and its Implications for North Pacific Intermediate Water

Today's knowledge about the past variability of North Pacific Intermediate Water (NPIW) rests to a large extent on high-quality paleoceanographic time series from the eastern North Pacific, located far away from the water masses' formation regions. Though some datasets also exist for the NW-Pacific, spatial and temporal coverage are to date still insufficient to gain a detailed understanding into past variations, especially on shorter timescales. Okhotsk Sea Intermediate Water (OSIW) is presumed being an important precursor for the formation of NPIW and is also the main contributor to mid-depth ventilation. Formation of OSIW at the Sakhalin margin is in turn closely coupled to the amount of riverine freshwater discharge from the Amur and the dynamics of the regional sea-ice cover during winter. We report results from a set of sediment cores from the Okhotsk Sea that disclose short-term paleoceanographic changes during the last ca. 20,000 years. Gravity cores off the Sakhalin margin from intermediate water depths (600-2000 m) were used to study OSIW ventilation, mixed layer characteristics and primary productivity changes in the eastern Okhotsk Sea. Further cores from the Kamchatka margin, proximal to the inflow of NW-Pacific water masses, complete our set for detection of possible intra-basin gradients. Maximum sedimentation rates range from 20 to 130 cm/kyr, giving us an average sample spacing between 50 and 500 years. Planktic foraminiferal oxygen isotope data from the Sakhalin margin suggest a highly variable mixed layer, caused by large fluctuations in the discharged Amur's freshwater volume, influencing the stratification of local surface water masses. According to benthic carbon isotopes of Cibicidoides spp., OSIW ventilation is variable during the entire Holocene, being generally poorly ventilated down to dysoxic conditions during Termination Ib and the earliest part of the Holocene while e.g. better ventilation is observed during the past 2,000 years. Spectral analyses performed on selected time series reveal millenial to centennial-scale cycles in surface and intermediate water characteristics, in particular we observe spectral powers around 1,200-1,050 and 550 years, known from solar cycles and also from low-latitudinal monsoon records. Amongst other forcings, we assume a transmission of monsoon forcing to the Okhotsk Sea via the amount of precipitation in the Amur watershed and the discharge as freshwater into the Okhotsk Sea; especially during the early Holocene climatic optimum.