81 Changes in Atmospheric Trace Gases and Aerosols on Glacial-interglacial Time Scales: A Key Challenge for Earth System Models

Wednesday, 30 July 2003
8:00 AM - 12:00 PM

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

Paper number: 81-10
Program: SYMPOSIA
Time: 10:50 AM – 11:10 AM

Authors

Abstract

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.