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

Atmospheric CO₂ was ∼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 δ¹³C distribution, thereby improving our understanding of glacial/interglacial atmospheric CO₂ variations. We find that the mean ocean δ¹³C change can be explained by a 378 ± 88 Gt C(2σ) smaller LGM terrestrial carbon reservoir compared to the Holocene. Critically, in this model, differences in the oceanic δ¹³C 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.