Rapid Changes in Okhotsk Sea Intermediate Water during the last 20,000 Years – Causes and Relationship with North Pacific Biogeochemistry

The modern North Pacific is the largest natural oceanic sink of CO2 and comprises some of the most biologically productive, and least oxygenated, regions of the World Ocean. Only the mid-depth water column is weakly ventilated today by Okhotsk Sea Intermediate Water (OSIW). The paleo-hydrography and ventilation of this water mass and the larger North Pacific region varied profoundly over the last 20,000 years, but has remained largely undocumented so far. I present reconstructions of OSIW ventilation over the last glacial termination and the Holocene based on a set of high-resolution radiocarbon-dated sediment cores directly from the source region of modern OSIW formation. During the last glacial termination decreases in OSIW oxygenation intensified during the Bølling-Allerød and Preboreal deglacial warm phases. Correlation of ventilation minima with maxima in nutrient utilization in the open North Pacific implies increased OSIW-sourced supply of micronutrients, silicate and organic matter into distant oceanic regions, in analogy to modern ocean conditions. By such a relief from modern nutrient-limiting conditions, the subarctic North Pacific became a more efficient, transient deglacial CO2 sink. Analysis of Holocene stable carbon istope-based paleo-ventilation timeseries provides evidence for rapid millennial-scale changes in mid-depth ocean ventilation and nutrient supply. I suggest that the modern pattern of OSIW ventilation is a relatively recent and unstable phenomenon that only persisted for the last c. 2,000 years of the Holocene. During the warmer than present early-mid-Holocene, active ventilation was significantly decreased. Resulting estimated oxygen concentrations of OSIW reached only 50-60 % of modern values, and comparison between OSIW in- and outflow sites show that the Okhotsk Sea turned from an early Holocene mid-depth water oxygen sink into the modern oxygen source from around 6,000 years ago.