High-Resolution Radiocarbon Evidence for 1 kyr Older Deep Pacific Ventilation Ages during the Last Glacial Maximum

Lower atmospheric CO2 levels during the Last Glacial Maximum (19-27 kyr BP) were likely caused by carbon storage in the deep ocean, while rising CO2 during the subsequent deglaciation (~8-18 kyr BP) has been linked to oceanic carbon release. Such a scenario implies the deep Pacific circulation was more sluggish during the LGM, followed by progressively enhanced circulation during the deglaciation. Here we use foraminiferal stable isotopes and radiocarbon to reconstruct a high-resolution (average 300-year time increment) ventilation age history for the deep Northeast Pacific from 8 to 32 kyr BP, extending the existing record by more than 10 kyr. We find that 14C ventilation ages were ~1 kyr older during the LGM, due to either higher preformed ages or a slower deep Pacific circulation. We also find four distinct intervals of declining ventilation age, two during the LGM (at ~21 kyr BP and 19 kyr BP) and two during the deglaciation (at ~15.5 kyr BP and 13 kyr BP). The LGM events lack contemporaneous changes in benthic 18O and 13C, so they likely reflect decreasing preformed age, perhaps due to sea ice retreat in the Southern Ocean. In contrast, the deglacial events are characterized by synchronous changes in 14C, 18O and 13C, pointing to enhanced circulation at these times. Our results imply the deep NE Pacific sequestered carbon during the LGM and released it during the deglaciation, but after the initial rise in atmospheric CO2.