TRIPLE ISOTOPE COMPOSITION OF O2 AND THE RECORD OF GLOBAL PHOTOSYNTHESIS DURING THE PAST 150 KA

Records of the global rate of photosynthesis have the potential to reveal the nature of large scale interactions between biology, biogeochemistry and climate. We have developed a 150 ka 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 d¹⁷O of O₂ to decrease by a factor of 1.7 times the decrease in d¹⁸O, rather than the common, “mass dependent” factor of 0.5. The magnitude of the mass-independent anomaly (¹⁷D ~ d¹⁷O – 0.5* d¹⁸O) 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 fell to ca. ~90% of the modern rate during glacial periods and that it was near Holocene levels during the last interglacial. There is no statistically significant variability in paleoproductivity at orbital frequencies of precession or tilt.

The existing paleoproductivity record provides a target for models of the biosphere during the Holocene and Pleistocene. Calculated values of paleoproductivity will evolve and become more accurate as we gain a better understanding of isotope fractionation associated with hydrology, O₂ consumption and stratospheric exchange.