The stable carbon isotope ratio of atmospheric CO2 (delta C-13(atm)) is a key parameter in deciphering past carbon cycle changes. Here we present delta C-13(atm) data for the past 24,000 years derived from three independent records from two Antarctic ice cores. We conclude that a pronounced 0.3 per mil decrease in delta C-13(atm) during the early deglaciation can be best explained by upwelling of old, carbon-enriched waters in the Southern Ocean. Later in the deglaciation, regrowth of the terrestrial biosphere, changes in sea surface temperature, and ocean circulation governed the delta C-13(atm) evolution. During the Last Glacial Maximum, delta C-13(atm) and atmospheric CO2 concentration were essentially constant, which suggests that the carbon cycle was in dynamic equilibrium and that the net transfer of carbon to the deep ocean had occurred before then.