Biological oxygen productivity during the last 60,000 years from triple oxygen isotope measurements

The oxygen isotope signature of atmospheric O2 is linked to the isotopic signature of seawater (H2O) through photosynthesis and respiration. Fractionation during these processes is mass dependent, affecting δ17O about half as much as δ18O. An “anomalous” fractionation process, which changes δ17O and δ18O of O2 about equally, takes place during isotope exchange between O2 and CO2 in the stratosphere. The relative rates of biologic O2 production and stratospheric processing determine the relationship between δ17O and δ18O of O2 in the atmosphere. Variations of this relationship thus allow us to estimate changes in the rate of mass‐dependent O2 production by photosynthesis versus the rate of O2‐CO2 exchange in the stratosphere with about equal fractionations of δ17O and δ18O. In this study we reconstruct total oxygen productivity for the last glacial, the last glacial termination, and the early Holocene from the triple isotope composition of atmospheric oxygen trapped in ice cores. With a box model we estimate that total biogenic productivity was only ∼76–83% of today for the glacial and was probably lower than today during the glacial‐interglacial transition and the early Holocene. Depending on how reduced the oxygen flux from the land biosphere was during the glacial, the oxygen flux from the glacial ocean biosphere was 88–140% of its present value.