<p>Southern Ocean biogeochemistry impacts global nutrient distributions, carbon cycling, and climate, motivating study of its underlying controls across different climate states. Today, poorly-ventilated North Pacific waters supply the majority of carbon and nutrients upwelling in the Southern Ocean, outpacing biological carbon uptake and fueling CO₂ outgassing. Reducing this supply is both central to glacial CO₂ theories involving reduced outgassing and well-supported by paleo-proxy reconstructions. While past studies emphasize physical processes (reduced upwelling, enhanced stratification), we propose a complementary mechanism where the carbon/nutrient load of waters feeding the Southern Ocean surface is reduced remotely, prior to being upwelled. Comparing glacial North Pacific and Southern Ocean proxy records, alongside Earth System Model simulations, we show that ventilating the glacial North Pacific reduces the carbon/nutrient content of waters supplying the Southern Ocean surface and Subantarctic CO₂ outgassing. This highlights an interhemispheric influence on Southern Ocean biogeochemical conditions that could modulate glacial-interglacial CO₂ variability.</p>