The influence of vegetation activity on the Dole effect and its implications for changes in biospheric productivity in the mid-Holocene

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Abstract
The Dole effect is defined as the difference between the oxygen isotope composition of atmospheric oxygen and seawater (currently 23.5 parts per thousand) and reflects the balance between processes and fractionations associated with O2 consumption and production by the terrestrial and marine biospheres. Isotopic records from ice cores and ocean sediments provide a means of assessing variations in the Dole effect during the late Quaternary but the biogeochemical interpretation of these changes is limited because we are currently unable to account adequately for vegetation effects on the global isotopic balance of atmospheric O2. Here, I show that the previously unquantified influence of canopy transpiration on the isotopic composition of atmospheric water vapour now closes the mass balance budget for the isotopes of atmospheric O2 under the current climate. Using this new finding, the effects of vegetation on the Dole effect have been assessed at the global scale for the mid-Holocene (6000 years ago). The results indicate that the small reduction in the Dole effect in the mid-Holocene represented a fall in the ratio of terrestrial to marine gross primary production from 1.8 to 1.0. Improved understanding of the environmental and physiological processes controlling the oxygen isotopic composition of plants and their feedback on the isotopes of atmospheric O2 offers considerable promise in quantitatively accounting for the changes in biospheric productivity associated with the Dole effect over glacial–interglacial cycles. In addition, such work should provide an as yet unexploited basis for testing the results of climate models against the oxygen isotope composition of Quaternary plant fossils.