Polar Twins: Glacial CO2 outgassing reduced in the Southern Ocean by upwelling of well-ventilated waters from the North Pacific

The Southern Ocean is widely thought to have played a driving role in the atmospheric CO₂ fluctuations of the ice ages, ventilating carbon-rich deep waters to the atmosphere during interglacial periods and limiting this CO₂ leakage during glacial periods. A more efficient Southern Ocean biological pump during glacial periods is one of the leading hypotheses for how this “leak” might have been stemmed, but the exact dynamics responsible are still debated. Previous hypotheses have invoked reduced upwelling and/or enhanced stratification in reducing the carbon and nutrient supply to the glacial Southern Ocean surface, thus enhancing the net efficiency of its biological pump. Here we consider an alternative, complementary scenario in which the nutrient and carbon content of the upwelled water itself is reduced. Noting the striking similarity between proxy records from the North Pacific and Southern Ocean over the Last Glacial Cycle and given that carbon-rich waters upwelling in the Southern Ocean today are largely fed by the North Pacific, we propose that low-carbon/nutrient glacial Southern Ocean surface waters could have been sourced from a well-ventilated, low-carbon/nutrient glacial North Pacific. We then show in intermediate-complexity Earth system model simulations how a well-ventilated North Pacific can directly reduce the outgassing potential of waters upwelled in the Southern Ocean. While not precluding the possibility of changes to upwelling or mixing, our results demonstrate the ability of changes in the upwelled waters’ carbon content – outside of any changes to Southern Ocean physical dynamics (e.g., upwelling rate) – to change Southern Ocean carbon content and outgassing. This provides a novel mechanism linking Northern Hemisphere climate to Southern Ocean carbon cycling and may thus help explain the cyclic CO₂ variations of the ice ages.