It is widely assumed that the ventilation of the Southern Ocean played a crucial role in driving glacial‐interglacial atmospheric CO2‐levels. So far however, ventilation records from the Indian sector of the Southern Ocean, are widely missing. Here we present reconstructions of water residence times (depicted as ΔΔ14C and Δδ13C) for the last 32,000 years on sediment records from the Kerguelen Plateau and the Conrad Rise (~570‐2500 m water depth), along with simulated changes in ocean stratification from a transient climate model experiment. Our data indicate that Circumpolar Deep Waters in the Indian Ocean were part of the glacial carbon pool. At our sites, close to or bathed by upwelling deep‐waters, we find two pulses of decreasing ΔΔ14C and δ13C values (~21‐17ka; ~15‐12ka). Both transient pulses precede a similar pattern in downstream intermediate waters in the tropical Indian Ocean as well as rising atmospheric CO2 values. These findings suggest that 14C‐depleted, CO2‐rich Circumpolar Deep Water from the Indian Ocean contributed to the rise in atmospheric CO2 during HS1 and also the Younger Dryas, and that the southern Indian Ocean acted as a gateway for sequestered carbon to the atmosphere and tropical intermediate waters.

Plain Language Summary

By analyzing air bubbles trapped in glacial ice from Antarctica, we know the pattern of atmospheric CO2 for roughly the last 800,000 years. This record shows a distinctive pattern of warm interglacials with high values of atmospheric CO2 (~280 ppm) and cold glacials with CO2 as low as ~180 ppm. A leading hypothesis assumes that the CO2 that went “missing” from the atmosphere during the glacials was stored in the deep global ocean. Several studies suggest that during glacials, the main connection between the deep ocean and the surface/atmosphere – the Southern Ocean – was significantly interrupted or at least reduced. Until now, it was shown that the deglacial South Pacific, the Drake Passage, and the South Atlantic played a vital role in the release of the stored oceanic CO2 back to the atmosphere. With our study, we want to shed new light on the role, the southernmost Indian Ocean played in this system. Our data from the Kerguelen Plateau and the Conrad Rise indicate that the Indian Ocean also stored CO2 during the last glacial, and released it back to the atmosphere in two pulses during the last deglacial transition.