Enhanced carbon sequestration by the North Atlantic Ocean during the Last Glacial Maximum

According to Redfield stoichiometry, marine organisms incorporate and release PO₄ and Dissolved Inorganic Carbon (DIC) in a relatively fixed proportion. Additionally, PO₄ in the ocean is not affected by air-sea exchange. Therefore, PO₄ and DIC in the ocean can be used to estimate biology-driven versus air-sea flux-driven oceanic DIC redistributions. Applying this method to sediment core data, a new paper published in Nature Communications calculated air–sea exchange signature of DIC (DIC_as) in the Atlantic Ocean during the Last Glacial Maximum (LGM; ~20,000 years ago). The study demonstrates that North Atlantic carbon pump efficiency during the LGM was almost doubled relative to the Holocene. This new finding stresses the important role of the Atlantic Meridional Overturning Circulation  (AMOC) in the large amount of carbon sequestration that occurred during the LGM, in contrast to the prevailing explanation that suppressed CO₂ outgassing from the Southern Ocean is the main cause of the enhanced carbon sequestration during the LGM.

Figure 1 from Yu et al. (2019): Concepts to distinguish DIC_as. For simplicity, only CO₂ invasion associated with organic matter cycling is considered. In the ocean box, vertical solid and dashed lines (a–d) represent mean PO₄ (blue) and DIC (red) in an abiotic ocean (a). Biology redistributes DIC and PO₄ following Redfield stoichiometry (curves; b). This decreases surface-ocean DIC and pCO₂, and hence causes air-to-sea CO₂ transfer (c). Through mixing and ocean circulation, CO₂ invasion raises water-column DIC, i.e., shifting dashed curve (equals the red-solid curve in b) to red-solid curve (c). The shaded region in c represents air–sea exchange DICas signatures. After removing carbon redistribution by biology based on PO₄-related curvature of the profiles (b), DIC_as can be revealed by the shaded region in d.

Yu, J., Menviel, L., Jin, Z. D., Thornalley, D. J. R., Foster, G. L., Rohling, E. J., … & He, F. (2019). More efficient North Atlantic carbon pump during the Last Glacial Maximum. Nature communications, 10(1), 2170. https://doi.org/10.1038/s41467-019-10028-z. https://www.nature.com/articles/s41467-019-10028-z