Radiocarbon serves as a tracer that provides unique insights into the ocean’s ability to sequester CO2 from the atmosphere. By applying a Bayesian interpolation method to compiled ocean-atmosphere radiocarbon age offsets (B-Atm), we provide global data fields and mean ocean B-Atm estimates for a suite of time-slices across the last deglaciation. These reveal a stepwise and spatially heterogeneous ‘rejuvenation’ of the deep ocean, and confirm that carbon was incrementally released to the atmosphere through two ‘swings’ of a ventilation seesaw, operating between the North Atlantic and Southern Ocean/North Pacific. A suite of numerical model sensitivity tests further demonstrate that the reconstructed changes could account for two thirds of deglacial atmospheric CO2 rise, depending on the mix of processes driving marine and atmospheric radiocarbon change. Our model sensitivity tests also serve to constrain non-ventilation biases that could affect deglacial B-Atm offsets, under the (extreme) hypothesis of a completely passive ocean response to atmospheric radiocarbon variability driven by radiocarbon production or other non-marine processes. By placing quantitative constraints on the closure of the global radiocarbon budget, our findings help to constrain the contribution of ocean ventilation to observed B-atm changes, and to atmospheric CO2 change, and further suggest that glacial radiocarbon production levels are likely underestimated on average by existing reconstructions.