The deep ocean naturally releases large amounts of old, pre-industrial carbon dioxide (CO2) to the atmosphere through upwelling in the Southern Ocean, closing the global ocean carbon cycle. This Southern Ocean CO2 release is relevant to the global climate, because its changes could alter atmospheric CO2 levels on long time scales and the present-day potential of the Southern Ocean to take up anthropogenic CO2. Here, based on observational data, we show that this CO2 release arises from a zonal band of subsurface waters between the Subantarctic Front and wintertime sea-ice edge with a potential partial pressure of CO2 exceeding current atmospheric CO2 levels (∆PCO2) by 175 µatm. This band of high ∆PCO2 subsurface water conincides with the outcropping of the 27.8 kg m−3 isoneutral density surface that marks the upwelling of Indo-Pacific Deep Water (IPDW). Vertically, the IPDW layer exhibits a distinct ∆PCO2 maximum in the deep ocean, which is set by remineralization of organic carbon and originates from the northern Pacific and Indian Ocean basins. Below this IPDW layer, the carbon content increases downwards, whereas ∆PCO2 decreases. Most of this vertical ∆PCO2 decline results from decreasing temperatures and increasing alkalinity due to an increased fraction of calcium carbonate dissolution. These two factors limit the CO2 outgassing from the high-carbon content deep waters on more southerly surface outcrops. Our results imply that the response of Southern Ocean CO2 fluxes to possible future changes in upwelling are sensitive to the subsurface carbon chemistry set by the vertical remineralization and dissolution profiles.