Southern Ocean mechanisms of glacial CO2 drawdown and their links to global climate

The Southern Ocean is a critical player in regulating Earth’s carbon cycle and climate, yet its role under future climate change remains uncertain. Studying the Southern Ocean under past climate states can help address this knowledge gap. For example, changes in the Southern Ocean, through modulating deep ocean carbon content, are widely thought to have played a driving role in the atmospheric carbon dioxide (CO₂) fluctuations of Earth’s past ice ages. Here we present three novel findings that advance our understanding of processes linking Southern Ocean changes to glacial CO₂ drawdown. First, a new proxy record reveals a tight coupling between atmospheric CO₂ levels and deep Southern Ocean carbon storage over the Last Glacial Cycle, providing clear evidence of the systematic transfer of carbon into the deep ocean during glaciation and its release during deglaciation. These results are used to quantify the deep Indo-Pacific’s remineralized carbon content at the Last Glacial Maximum and are found to explain a significant proportion of observed glacial CO₂ drawdown. Second, we demonstrate how improved ventilation of North Pacific mid-depths (evidenced by glacial proxy data) directly impacts Southern Ocean biogeochemistry by reducing the carbon and nutrient load of waters upwelling in the Southern Ocean. This process enhances biological pump efficiency, curtailing Southern Ocean CO₂ outgassing and highlighting a critical interhemispheric connection in glacial nutrient cycling. Finally, idealized numerical modelling experiments demonstrate cooling of the high northern latitudes associated with a large Northern Hemisphere ice sheet can, in isolation of any other forcing, remotely induce glacial-like changes in Southern Ocean sea ice, circulation, and biogeochemistry that work to enhance ocean carbon content. These changes include expanded southern sea ice and cooler, saltier, better-stratified bottom water, which together increase oceanic carbon storage via solubility- and disequilibrium-driven effects. Collectively, these findings underscore the Southern Ocean’s central role in mediating interhemispheric and glacial climate feedbacks, offering new insights into the processes that drove the Earth’s into low-CO₂ glacial periods.