Enhanced ocean transport despite reduced radiocarbon ventilation at the Last Glacial Maximum: were the models right all along?

Globally distributed data from the Last Glacial Maximum (LGM) indicate a significant depletion of radiocarbon in the ocean, equivalent to ~800 ¹⁴Cyrs.  Some interpretations of these data have emphasized a slow-down of the North Atlantic overturning, as well as a reduction or even ‘reversal’ of overturning in the North Pacific.  While many model simulations have been able to produce a shoaled and weakened circulation in the Atlantic under glacial conditions, many others (and many of the same) produce a stronger overturning overall and in the Pacific.  If the glacial ocean circulation was indeed stronger, despite reduced radiocarbon ventilation, it would constrain the balance of contributions from marine ‘respired’ and ‘disequilibrium’ carbon pools to glacial atmospheric CO2 drawdown.  Here we show that global marine radiocarbon fields from the LGM and deglaciation are not consistent with the modern transport when taking into account past air-sea equilibration changes at the sea surface.  Rather, they imply a reduced and/or shoaled transport in the North Atlantic (consistent with most interpretations to date), and an enhanced transport throughout the Pacific.  Although the latter conflicts with some previous interpretations of LGM North Pacific radiocarbon data, it coheres with several key model simulations in suggesting an overall ‘faster’ glacial mass turnover despite weaker exchange of CO₂ between the ocean and atmosphere.  This would emphasize the role of the disequilibrium carbon pool (and therefore ocean-atmosphere gas-exchange, influenced by upper ocean mixing, sea ice etc.) in determining the overall ocean’s overall sequestered carbon inventory during the last glacial period.