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Abstract The Southern Ocean regulates atmospheric CO₂and Earth's climate as a critical region for air‐sea gas exchange, delicately poised between being a CO₂source and sink. Here, we estimate how long a water mass has remained isolated from the atmosphere and utilize¹⁴C/¹²C ratios (Δ¹⁴C) 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 CO₂, whereas deglacial expansion of the subantarctic permitted ventilation of the trapped CO₂, contributing to a rapid atmospheric CO₂rise. We identify frontal positions as a key factor balancing CO₂outgassing versus sequestration in a region currently responsible for nearly half of global ocean CO₂uptake. 

Abstract The area density proxy of foraminiferal shell thickness and calcification intensity has the potential to provide information about past ocean CO₂content and has the benefit of small sample requirements, simple analytical techniques, and the ability to re‐use the analyzed foraminifera for other paleo‐proxies. Using a series of multicore core‐tops collected from the southeastern Indian Ocean (1.8–3.8 km water depth), we evaluate the reliability of utilizing area density values ofGlobigerina bulloidesfrom sediment cores to estimate surface ocean carbonate parameters. Because foraminifera in marine sediments can rarely be considered “pristine” (or “glassy”), we grouped area density measurements of shells to designate various stages of diagenesis. Visual signs of alteration were apparent at area density values as low as ∼0.122 × 10⁴ µg/µm², with deviations from the “pristine” endmember beginning at area density values of ∼0.087 × 10⁴ µg/µm². We find that increases in area density overprint the surface ocean carbonate signature in thicker (>0.122 × 10⁴ µg/µm²shells), but small increases associated with marine sedimentary burial and diagenesis can be accounted for, allowing this proxy to be applied back in time. Reconstructing the distribution of area density values in a given sample has the potential to provide valuable information on overall sample preservation by estimating the percent of well‐preserved shells (<0.122 × 10⁴ µg/µm²; %wp) in a given sample. Our %wp metric has the potential for use as a proxy for lysocline variability in addition to assessing the suitability of marine sediment samples for surface ocean reconstructions.