A record of global photosynthesis to 150ka derived from the triple isotopes of O2 in polar ice cores
Abstract
Records of the global rate of photosynthesis have the potential to reveal the nature of large scale interactions between biology, biogeochemistry and climate. Following Blunier et al [Global Biogeochemical Cycles 16(3) 2002] we have developed a 150ka record of the global rate of photosynthesis using the 16O, 17O, and 18O content of O2 in air bubbles in Greenland and Antarctic ice sheets. Our approach takes advantage of mass independent effects in the isotopic composition of O2. These effects arise from stratospheric exchange reactions between O3, CO2 and O2 that cause the δ 17O and δ 18O of O2 to decrease equally, rather than by the common, "mass dependent" factor of 0.5. The magnitude of the mass-independent anomaly (17Δ ∼ δ 17O - 0.5* δ 18O) depends on the relative rates of stratospheric reactions and biological cycling by photosynthesis and respiration. We have made a preliminary calculation of past rates of photosynthesis by correcting for past atmospheric CO2 concentrations (higher CO2 promotes stratospheric exchange and enhances the anomaly) and for changes of biological isotope effects associated with shifts in the C3/C4 composition of plants. The paleoproductivity record reveals that global photosynthesis was within ± 5% of the modern rate during glacial periods and that it was higher than Holocene levels by as much as 10% during the Eemian (last interglacial). We found no statistically significant variability in paleoproductivity at orbital frequencies of precession or tilt during our study period. These conclusions are provisional and may be modified as more is learned about triple isotope fractionation during evapotranspiration and the secondary processes of O2 consumption.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSM.B41B..01V
- Keywords:
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- 0315 Biosphere/atmosphere interactions;
- 1615 Biogeochemical processes (4805);
- 3344 Paleoclimatology;
- 4267 Paleoceanography;
- 4870 Stable isotopes