J. R. Southon

The Southern Ocean regulates atmospheric CO 2 and Earth's climate as a critical region for air‐sea gas exchange, delicately poised between being a CO 2 source and sink. Here, we estimate how long a water mass has remained isolated from the atmosphere and utilize 14 C/ 12 C ratios (Δ 14 C) to trace the pathway and escape route of carbon sequestered in the deep ocean through the mixed layer to the atmosphere. The position of our core at the northern margin of the Southern Indian Ocean, tracks latitudinal shifts of the Southern Ocean frontal zones across the deglaciation. Our results suggest an expanded glacial Antarctic region trapped CO 2 , whereas deglacial expansion of the subantarctic permitted ventilation of the trapped CO 2 , contributing to a rapid atmospheric CO 2 rise. We identify frontal positions as a key factor balancing CO 2 outgassing versus sequestration in a region currently responsible for nearly half of global ocean CO 2 uptake.