It has been proposed that the ventilation of the deep Pacific carbon pool was not significantly reduced during the last glacial period, posing a problem for canonical theories of glacial–interglacial CO2 change. However, using radiocarbon dates of marine tephra deposited off New Zealand, we show that deep- (>2000 m>2000 m) and shallow sub-surface ocean–atmosphere 14C age offsets (i.e. ‘reservoir-’ or ‘ventilation’ ages) in the southwest Pacific increased by ∼1089 and 337 yrs respectively, reaching ∼2689 and ∼1037 yrs during the late glacial. A comparison with other radiocarbon data from the southern high-latitudes suggests that broadly similar changes were experienced right across the Southern Ocean. If, like today, the Southern Ocean was the main source of water to the glacial ocean interior, these observations would imply a significant change in the global radiocarbon inventory during the last glacial period, possibly equivalent to an increase in the average radiocarbon age >2 km>2 km of ∼700 yrs∼700 yrs. Simple mass balance arguments and numerical model sensitivity tests suggest that such a change in the ocean's mean radiocarbon age would have had a major impact on the marine carbon inventory and atmospheric CO2, possibly accounting for nearly half of the glacial–interglacial CO2 change. If confirmed, these findings would underline the special role of high latitude shallow sub-surface mixing and air–sea gas exchange in regulating atmospheric CO2 during the late Pleistocene.