Varied contribution of the Southern Ocean to deglacial atmospheric CO2 rise

Glacial–interglacial changes in atmospheric CO₂ are generally attributed to changes in seawater carbon chemistry in response to large-scale shifts in the ocean’s biogeochemistry and general circulation. The Southern Ocean currently takes up more CO₂ than any other and it is likely to have played a crucial role in regulating past atmospheric CO₂. However, the physical, biological and chemical variables that control ocean–atmosphere CO₂ exchange during glacial–interglacial cycles are not completely understood. Here we use boron isotopes and carbon isotopes in planktonic foraminifera and an alkenone-based proxy of temperature to reconstruct seawater pH and CO₂ partial pressure in sub-Antarctic surface waters south of Tasmania over the past 25,000 years, and investigate the mechanisms that regulate seawater CO₂. The new record shows that surface waters in this region were a sink for atmospheric CO₂ during the Last Glacial Maximum. Our reconstruction suggests changes in the strength of the biological pump and the release of deep-ocean CO₂ to surface waters contributed to the last deglacial rise in atmospheric CO₂. These findings demonstrate that variations in upwelling intensity and the distribution of Southern Ocean water masses in this sector played a key role in regulating atmospheric CO₂ during the last glacial–interglacial cycle.