Disequilibrium and Soft Tissue Pump Contributions to Glacial CO 2 Drawdown

Abstract A more “efficient” biological pump is thought to have played a key role in glacial CO 2 drawdown, with two principal mechanisms invoked to stem the CO 2 “leak” from the modern Southern Ocean. The first sees a strengthened soft tissue pump, associated with a reduction in the ocean's “preformed” nutrient inventory. The second sees an increase in air‐sea CO 2 disequilibrium (termed the disequilibrium pump). We use an Earth system model (CM2Mc) to show the tracers radiocarbon (Δ 14 C) and oxygen (O 2 ) exhibit distinct sensitivities to these two pumps within the deep ocean: Δ 14 C is more sensitive to disequilibrium pump changes, whereas O 2 is more sensitive to soft tissue pump changes, as expected from the underlying processes. We apply these pump‐specific tracer stoichiometries to available deep ocean Δ 14 C and O 2 proxy data from the Last Glacial Maximum (LGM). Despite the sparsity of O 2 data, the results show a consistent increase in soft tissue and disequilibrium DIC within the deep ocean of ∼135 μmol/kg, with broadly comparable contributions from the two pumps. Our results imply a reduction in air‐sea gas exchange (likely linked to expansion and/or increased isolation of Antarctic Bottom Water), as well as a slowdown of deep ocean overturning and Southern Ocean upwelling at the LGM. Notably, current ocean models struggle to simulate both of these changes simultaneously under glacial forcings. The combined changes in the soft tissue and disequilibrium DIC are of sufficient magnitude to explain the glacial reduction in atmospheric CO 2 once a whole ocean alkalinity increase is accounted for.