One of the very early discoveries in istope geochemistry was that atmospheric oxygen is enriched in 18O with respect to seawater. This enrichment was named Dole effect. By the mid 50' it became clear that the main driving force causing the Dole effect is respiration that favors 16O over 18O. It was also demonstrated that photosynthesis produces oxygen close in isotopic composition to the substrate water. Yet, the Dole effect, whose magnitue is almost 24 ‰, cannot be explained exclusively by respiratory fractionation. Over the past four decades the consensus has been that evapo-transpiration that enriches leaf water in heavy isotopes, enhaces the Dole effect through the production of 18O enriched O2 in leaves of land vegetation. In this concensus, the respiratory fractionation on land has been taken to be about 18 ‰. This assumption, however, was proven wrong by our group. We showed that slow oxygen-gas diffusion in roots and soils reduces the effective respiratory fractionation, and globally, fractionation by soil respiration, which consumes most of the land production of O2, is about 14 ‰. This relatively small fractionation compensates for the evapotranspiration enhancement, and overall, the magnitude of the terrestrial component of the Dole effects remains relatively small, thus leaving the global effect unexplained. Our recent experiments, using 17O/16O and 18O/16O ratios, show that marine photosynthesis produces O2 that is enriched by several ‰ with respect to seawater. This new discovery solves the long standing question about the magnitude of the global Dole effect.
