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

The Dole effect is defined as the difference between the oxygen isotope com position of atmospheric oxygen and seawater (currently 23.5 parts per thous and) and reflects the balance between processes and fractionations associat ed with O-2 consumption and production by the terrestrial and marine biosph eres. Isotopic records from ice cores and ocean sediments provide a means o f assessing variations in the Dole effect during the late Quaternary but th e 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 O-2. Here, I show that the previously unqu antified influence of canopy transpiration on the isotopic composition of a tmospheric water vapour now closes the mass balance budget for the isotopes of atmospheric O-2 under the current climate. Using this new finding, the effects of vegetation on che Dole effect have been assessed at the global s cale for the mid-Holocene (6000 years ago). The results indicate that the s mall 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 O-2 offers considerable promise in quantitativ ely accounting for the changes in biospheric productivity associated with t he Dole effect over glacial-interglacial cycles. In addition, such work sho uld provide an as yet unexploited basis for testing the results of climate models against the oxygen isotope composition of Quaternary plant fossils.