Poorly ventilated deep ocean at the Last Glacial Maximum inferred from carbon isotopes: A data‐model comparison study

Atmospheric CO2 was similar to 90 ppmv lower at the Last Glacial Maximum (LGM) compared to the late Holocene, but the mechanisms responsible for this change remain elusive. Here we employ a carbon isotope-enabled Earth System Model to investigate the role of ocean circulation in setting the LGM oceanic delta C-13 distribution, thereby improving our understanding of glacial/interglacial atmospheric CO2 variations. We find that the mean ocean delta C-13 change can be explained by a 378 +/- 88 Gt C (2 sigma) smaller LGM terrestrial carbon reservoir compared to the Holocene. Critically, in this model, differences in the oceanic delta C-13 spatial pattern can only be reconciled with a LGM ocean circulation state characterized by a weak (10-15 Sv) and relatively shallow (2000-2500 m) North Atlantic Deep Water cell, reduced Antarctic Bottom Water transport (<= 10 Sv globally integrated), and relatively weak (6-8 Sv) and shallow (1000-1500 m) North Pacific Intermediate Water formation. This oceanic circulation state is corroborated by results from the isotope-enabled Bern3D ocean model and further confirmed by high LGM ventilation ages in the deep ocean, particularly in the deep South Atlantic and South Pacific. This suggests a poorly ventilated glacial deep ocean which would have facilitated the sequestration of carbon lost from the terrestrial biosphere and atmosphere.