Changes in deep ocean carbonate chemistry have profound implications for glacial-interglacial atmospheric CO2 changes. Here, we review deep ocean carbonate ion concentration ([CO32–]) changes based on the benthic foraminiferal boron-to-calcium ratio (B/Ca) and their links to global carbon reorganization since the last ice age. Existing deep ocean [CO32–] reconstructions are consistent with changes in the biological pump, in ocean stratification, and in the associated oceanic alkalinity inventory as key mechanisms for modulating atmospheric CO2 on glacial-interglacial time scales. We find that the global mean deep ocean [CO32–] was roughly similar between the Last Glacial Maximum (LGM; 18,000–22,000 years ago) and the Late Holocene (0–5,000 years ago). In view of elevated glacial surface [CO32–], this indicates enhanced storage of respiratory carbon in a more alkaline deep ocean during the LGM. During early deglaciation, rising [CO32–] at three locations in the deep ocean suggests a release of deep-sea CO2 to the atmosphere, probably via the Southern Ocean. Both increased late deglacial carbonate burial in deep-sea sediments due to elevated [CO32–] and Holocene expansion of coral reefs on newly flooded continental shelves depleted global ocean alkalinity, which reduced CO2 solubility in seawater and contributed to atmospheric CO2 rises at these times.

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