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 (DICas) 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 DICas. 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), DICas 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