Vertical and lateral exchanges of heat and carbon make Southern Ocean circulation a key regulator of global climate, yet its role in future climate change remains uncertain. To address this knowledge gap, paleoceanographers study the state of the Southern Ocean under past climate states to better understand the processes governing its role in global climate. For instance, the Southern Ocean is widely thought to play 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 deep-surface exchange during glacial periods. However, direct evidence of these dynamics, of glacial deep-ocean carbon storage, and of the Southern Ocean’s overall role in glacial CO2 draw-down remains limited.

Here we present a suite of geochemical data providing new insights into Southern Ocean circulation and carbon cycling, evidencing deep-ocean carbon storage as a mechanism of atmospheric CO2 draw-down over the last glacial cycle. Trace element and stable isotope (d18O) compositions of foraminiferal calcite from the high-latitude Indian Ocean demonstrate how carbon was sequestered in the deep ocean during glacial intensification and subsequently released to surface waters during deglaciation. These dynamics are captured by geochemical records reflecting temperature, pH, circulation, and productivity changes in the Southern Ocean over the last glacial cycle, providing key insights into the processes responsible for this carbon cycling. This data provides the foundation for developing a better mechanistic understanding of the Southern Ocean’s role in past and future climate change, including processes such as advection and mixing, ocean-ice interactions, and productivity dynamics.