Ice core records demonstrate a glacial–interglacial atmospheric CO₂ increase of ~ 100 ppm, while ¹⁴C calibration efforts document a strong decrease in atmospheric ¹⁴C concentration during this period. A calculated transfer of ~ 530 Gt of ¹⁴C-depleted carbon is required to produce the deglacial coeval rise of carbon in the atmosphere and terrestrial biosphere. This amount is usually ascribed to oceanic carbon release, although the actual mechanisms remained elusive, since an adequately old and carbon-enriched deep-ocean reservoir seemed unlikely. Here we present a new, though still fragmentary, ocean-wide Δ¹⁴C data set showing that during the Last Glacial Maximum (LGM) and Heinrich Stadial 1 (HS-1) the maximum ¹⁴C age difference between ocean deep waters and the atmosphere exceeded the modern values by up to 1500 ¹⁴C yr, in the extreme reaching 5100 ¹⁴C yr. Below 2000 m depth the ¹⁴C ventilation age of modern ocean waters is directly linked to the concentration of dissolved inorganic carbon (DIC). We propose as a working hypothesis that the modern regression of DIC vs. Δ¹⁴C also applies for LGM times, which implies that a mean LGM aging of ~ 600 ¹⁴C yr corresponded to a global rise of ~ 85–115 μmol DIC kg⁻¹ in the deep ocean. Thus, the prolonged residence time of ocean deep waters may indeed have made it possible to absorb an additional ~ 730–980 Gt DIC, one third of which possibly originated from intermediate waters. We also infer that LGM deep-water O₂ dropped to suboxic values of < 10 μmol kg⁻¹ in the Atlantic sector of the Southern Ocean, possibly also in the subpolar North Pacific. The deglacial transfer of the extra-aged, deep-ocean carbon to the atmosphere via the dynamic ocean–atmosphere carbon exchange would be sufficient to account for two trends observed, (1) for the increase in atmospheric CO₂ and (2) for the 190&permil; drop in atmospheric Δ¹⁴C during the so-called HS-1 "Mystery Interval", when atmospheric ¹⁴C production rates were largely constant.