Glacial-Interglacial Variations in d17O of O2 in Ice Cores: Implications for Interactions Between Climate and the Biosphere

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0087999 BenderThis award supports a project to continue studies of the triple isotope composition (O-17 and O-18) of oxygen in fossil air trapped in ice cores. This property is of interest because photochemical reactions in the stratosphere induce isotope exchange between oxygen and carbon dioxide that lowers the O-17 of oxygen by the same amount as the O-18 ("mass independent fractionation"). This isotopic tag is anomalous in the sense that other factors influencing the isotopic composition of oxygen change O-17 by 0.5 times as much as they change O-18. Respiration consumes ambient (anomalous) oxygen , and photosynthesis replaces it with oxygen which is normally fractionated. Therefore, the triple isotope composition of oxygen give a measure of the gross rate of photosynthesis on the planet. Preliminary results focussing on the last 60,000 years indicate that the production of the global biosphere during the Last Glacial Maximum was about 90% of that today. Because land production was much less than today, the O-isotope data imply that oceanic gross production was much greater than today. This increase has been attributed to a higher glacial flux of dust (and iron) to the oceans. This project will extend current measurements of the triple isotope composition of oxygen back to 400,000 years in order to derive the curve of the relative rate of photosynthetic oxygen production vs. time. The results, at 1 kyr resolution, will constrain changes in production associated with glacial maxima, interglacials and glacial terminations; with long interstadial events; and with orbital insolation cycles unaccompanied by large changes in ice volume. This work will lead to a better understanding of ways in which the biosphere responds to and influences climate.