Identifying processes within the Earth System that have modulated atmospheric pCO₂ during each glacial cycle of the late Pleistocene stands as one of the grand challenges in climate science. The growing array of surface ocean pH estimates from the boron isotope proxy across the last glacial termination may reveal regions of the ocean that influenced the timing and magnitude of pCO₂ rise. Here we present two new boron isotope records from the subtropical‐subpolar transition zone of the Southwest Pacific that span the last 20 kyr, as well as new radiocarbon data from the same cores. The new data suggest this region was a source of carbon to the atmosphere rather than a moderate sink as it is today. Significantly higher outgassing is observed between ~16.5‐14 kyrBP, associated with increasing δ¹³C and [CO₃]^2‐ at depth, suggesting loss of carbon from the intermediate ocean to the atmosphere. We use these new boron isotope records together with existing records to build a composite pH/pCO₂ curve for the surface oceans. pH disequilibrium/CO₂ outgassing was widespread throughout the last deglaciation, likely explained by upwelling of CO₂ from the deep/intermediate ocean. During the Holocene, a smaller outgassing peak is observed at a time of relatively stable atmospheric CO₂, which may be explained by regrowth of the terrestrial biosphere countering ocean CO₂ release. Our stack is likely biased toward upwelling/CO₂ source regions. Nevertheless, the composite pCO₂ curve provides robust evidence that various parts of the ocean were releasing CO₂ to the atmosphere over the last 25 kyr.