Boron Isotope Evidence for Oceanic CO2 Leakage During the Last Deglaciation

Atmospheric CO₂ fluctuations over glacial-interglacial cycles remain a major challenge to our understanding of the carbon cycle and the climate system. Leading hypotheses invoke changes in deep ocean carbon storage, likely modulated by processes in the Southern Ocean where much of the deep ocean is ventilated. A central aspect of these Southern Ocean-centric models is that, during deglaciations, an isolated glacial deep ocean carbon reservoir is re-connected with the atmosphere, driving the atmospheric CO₂ rise observed in ice core records. However, direct documentation of changes in oceanic carbon content and CO₂ outgassing has so far been impossible because of the lack of proxies that unambiguously record the ocean carbonate system. ¹⁴C tracks changes in ocean ventilation, but not in its carbon content, whereas proxies that record increased deglacial upwelling do not constrain the proportion of upwelled carbon that is degassed versus that consumed by the biological pump. Here we use the boron isotope-pH proxy in planktic foraminifera to show that surface waters in the Subantarctic Atlantic and the Eastern Equatorial Pacific were acidified during the last deglaciation, contemporaneous with atmospheric CO₂ rise. This indicates that each location became a significant source of CO₂ to the atmosphere, providing the first direct evidence for the role of the ocean as a CO₂ source and supporting the view that a deep ocean carbon reservoir was upwelled via the Southern Ocean during the last deglaciation. Our study therefore highlights the vital role of the Southern Ocean and the low latitude upwelling regions in glacial-interglacial atmospheric CO₂ variations.