Description: The Pacific hosts the largest oxygen minimum zones (OMZ) in the world ocean, likely to intensify and expand under future climate warming, with consequences for ecosystems, biogeochemical cycles and living resources. Today, better-oxygenated subsurface North Pacific Intermediate Water mitigates OMZ development, but on instrumental time scales, data indicate decreasing NPIW ventilation, induced by surface freshening and increased stratification of seasonal thermocline water. However, longer variations in oceanographic boundary conditions were potentially large and hinder assessment of anthropogenic influences against natural background shifts. We previously provided evidence that modern well-ventilated waters underwent significant millennial-scale variations over the last ca. 12 ka, with a tipping point ca. 4.5 ka before present. Crossing this mid-Holocene threshold led to the Okhotsk Sea losing its modern ventilation source characteristics, although underlying forcing and physical boundary conditions remain largely enigmatic. A combination of sea ice loss, water temperatures, and remineralization rates may have conceivably induced a nonlinear switch into a different mean state in this region. To constrain these factors, we present surface ocean proxy records from Okhotsk Sea key study sites with multi-decadal resolution to assess changes in upper ocean stratification, nutrient characteristics and resulting mid-depth water ventilation. Our results imply that under assumed past warmer- than-present conditions, regional surface temperatures and upper ocean stratification were increased and changed in a nonlinear mode during the last ca. 6,000 years, associated with changing primary productivity patterns and biogeochemical feedback mechanisms. Complementary results from model simulations corroborate our results and provide evidence for close coupling the Okhotsk Sea and the North Pacific Subarctic Gyre, thus exporting marginal sea signals into large oceanic regions.