Deep Ocean Carbonate Chemistry and Glacial-Interglacial Atmospheric CO2 Changes

Abstract

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.

Keywords

• carbonate chemistry
• deep ocean
• benthic forams