Carbon isotope budget indicates biological disequilibrium dominated ocean carbon storage at the Last Glacial Maximum

Understanding the causes of the  ~90 ppmv atmospheric CO₂ swings between glacial and interglacial climates is an important open challenge in paleoclimate research. Although the regularity of the glacial-interglacial cycles hints at a single driving mechanism, Earth System models require many independent physical and biological processes to explain the full observed CO₂ signal. Here we show that biologically sequestered carbon in the ocean can explain an atmospheric CO₂ change of 75 ± 40 ppmv, based on a mass balance calculation using published carbon isotopic measurements. An analysis of the carbon isotopic signatures of different water masses indicates similar regenerated carbon inventories at the Last Glacial Maximum and during the Holocene, requiring that the change in carbon storage was dominated by disequilibrium. We attribute the inferred change in carbon disequilibrium to expansion of sea-ice or change in the overturning circulation.