The isotopic composition of atmospheric oxygen

Atmospheric O₂ is almost 24‰ more enriched in ¹⁸O than seawater, and this enrichment is known as the Dole effect. For a long time it has been accepted that there is no oxygen isotope fractionation in photosynthesis, and thus the Dole effect should be the result of preferential terrestrial and marine respiratory consumption of ¹⁶O over ¹⁸O, and also several permil enrichment of leaf water from which terrestrial photosynthesis produces ¹⁸O enriched O₂. This concept has led to the understanding that the record of past Dole effect variations was strongly affected by changes in the ratio of photosynthetic production between land and sea. However, recent studies in our lab have led to two major new observations: (1) O₂ produced by certain marine phytoplankton, representing important groups of primary producers, is significantly enriched (up to 6‰) in ¹⁸O with respect to the substrate seawater and (2) effective oxygen isotope fractionation in soil respiration is considerably smaller than the intrinsic respiratory fractionation. Here we take these two observations into account and show that the magnitudes of the terrestrial and marine components of the Dole effect are close, and both are close to the measured Dole effect. As a result, the magnitude of the global Dole effect should not be sensitive to past changes in the ratio of land-to-sea photosynthetic rates. Instead of the land-sea control, variations in low-latitude hydrology, and possibly changes of fractionations in the marine biosphere, are more important in regulating the magnitude of the Dole effect and its past variations.