<p>Past atmospheric CO₂ fluctuations are thought to be intricately tied to ocean circulation changes involving Southern Ocean and North Atlantic dynamics. The ocean’s capability to store carbon has been linked to the expansion and contraction of southern-sourced waters, but their provenance and structure remain poorly characterized in the past. Here we present neodymium isotope data from the Weddell–Enderby Basin, placing constraints on the spatiotemporal distribution of Antarctic Bottom Water in the Atlantic and Indian sectors of the Southern Ocean over the past 32,000 years. Our data reveal that glacial Antarctic Bottom Water was substantially contracted, with large volumes of the deep Southern Ocean occupied by carbon-rich Circumpolar Deep Waters sourced from the Pacific Ocean, conducive for lowering atmospheric CO₂. During the last deglaciation, Antarctic Bottom Water expanded in two steps coinciding with Antarctic warming. This expansion drove Southern Ocean destratification, which possibly contributed to contemporaneous atmospheric CO₂ rises. Different from the view that the North Atlantic processes dominated deglacial deep South Atlantic water-mass changes, our results indicate only limited influence from northern-sourced waters. Instead, Antarctic Bottom Water dynamics played a critical role in regulating deep ocean circulation and thereby carbon exchange between the deep Southern Ocean and the atmosphere.</p>