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

Abstract

The oxygen isotope signature of atmospheric O 2 is linked to the isotopic signature of seawater (H 2 O) through photosynthesis and respiration. Fractionation during these processes is mass dependent, affecting δ 17 O about half as much as δ 18 O. An “anomalous” fractionation process, which changes δ 17 O and δ 18 O of O 2 about equally, takes place during isotope exchange between O 2 and CO 2 in the stratosphere. The relative rates of biologic O 2 production and stratospheric processing determine the relationship between δ 17 O and δ 18 O of O 2 in the atmosphere. Variations of this relationship thus allow us to estimate changes in the rate of mass‐dependent O 2 production by photosynthesis versus the rate of O 2 ‐CO 2 exchange in the stratosphere with about equal fractionations of δ 17 O and δ 18 O. 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.