UC Santa Cruz

Abstract All else equal, if the ocean's “biological [carbon] pump” strengthens, the dissolved oxygen (O 2 ) content of the ocean interior declines. Confidence is now high that the ocean interior as a whole contained less oxygen during the ice ages. This is strong evidence that the ocean's biological pump stored more carbon in the ocean interior during the ice ages, providing the core of an explanation for the lower atmospheric carbon dioxide (CO 2 ) concentrations of the ice ages. Vollmer et al. (2022, https://doi.org/10.1029/2021PA004339 ) combine proxies for the oxygen and nutrient content of bottom waters to show that the ocean nutrient reservoir was more completely harnessed by the biological pump during the Last Glacial Maximum, with an increase in the proportion of dissolved nutrients in the ocean interior that were “regenerated” (transported as sinking organic matter from the ocean surface to the interior) rather than “preformed” (transported to the interior as dissolved nutrients by ocean circulation). This points to changes in the Southern Ocean, the dominant source of preformed nutrients in the modern ocean, with an apparent additional contribution from a decline in the preformed nutrient content of North Atlantic‐formed interior water. Vollmer et al. also find a lack of LGM‐to‐Holocene difference in the preformed 13 C/ 12 C ratio of dissolved inorganic carbon. This finding may allow future studies to resolve which of the proposed Southern Ocean mechanisms was most responsible for enhanced ocean CO 2 storage during the ice ages: (a) coupled changes in ocean circulation and biological productivity, or (b) physical limitations on air‐sea gas exchange. Plain Language Summary Recent studies have sealed the case that the concentration of oxygen was reduced in the deep ocean during the ice ages. Vollmer et al. (2022, https://doi.org/10.1029/2021PA004339 ) combine the oxygen results with data on deep ocean nutrient concentrations. They find that, relative to modern, more of the ice age deep ocean's nutrient reservoir arrived as sinking organic matter from surface waters, leading to more storage of carbon dioxide in the deep ocean. Moreover, they calculate that the entire observed drawdown in atmospheric carbon dioxide levels during ice ages can be explained by this strengthening of the ocean's “biological carbon pump.” The findings bring the scientific community an important step closer to explaining why and how ice ages occur and end. The Southern Ocean, the ocean region around Antarctica, must have played a major role. However, it remains unclear whether the carbon dioxide was trapped in the Southern Ocean by changes in its circulation and biology or by limitations on air‐sea gas exchange across the Southern Ocean surface, such as might have occurred due to sea ice cover. Vollmer et al.’s reconstruction of the carbon isotopes in Southern Ocean surface waters may help to answer this question. Key Points Proxies indicate less dissolved O 2 in the ocean interior during ice ages than during interglacials This suggests an increase in CO 2 storage by the ocean's biological pump adequate to explain the lower atmospheric CO 2 of the ice ages Reconstructed nutrient use implicates the Southern Ocean but does not yet resolve the roles of circulation, biology, and gas exchange