This award supports a two year project to study the oxygen-isotopic composition of air in the Vostok and GISP2 ice cores. Photochemical reactions in the stratosphere lead to isotopic exchange between O2 and CO2. They introduce an anomalous "mass independent" signal in the isotopic composition of atmospheric O2 in which d17O varies as much as d18O. Photosynthesis and respiration consume anomalous O2 and replace it with isotopically normal O2 in which d17O varies half as much as d18O. At particular times, there is a small mass-independent signal in the isotope composition of atmospheric O2. Its magnitude depends on the relative rates of photochemical reactions in the stratosphere and photosynthesis and respiration by the global biosphere. Analysis of samples from the Vostok and GISP2 ice cores will give a record of how gross production of the biosphere varied during major climate events associated with glacial/interglacial climate cycles. This record will contribute to our evolving understanding of the nature of interactions between climate and the land and ocean biospheres.
