Oxygen-18 of Atmospheric CO2: Decadal Trends and Climate Variability
E.J. Zakem and J.W.C. White
Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO 80309; 734-751-4037, E-mail: Emily.Zakem@colorado.edu
The stable oxygen isotope 18O is unique to isotope ecology in that it links the hydrosphere to the carbon cycle. The two gross land biosphere fluxes, photosynthesis and ecosystem respiration, are the dominant influences on the δ18O of atmospheric CO2 on decadal timescales. Since these fluxes also dominate the interannual variability of atmospheric CO2 itself, analysis of atmospheric δ18O trends could provide useful insight into the terrestrial carbon cycle. The reasons for the interannual variability of atmospheric δ18O remain unclear, particularly because the mechanisms of the interannual variability of the terrestrial biosphere carbon flux are not fully understood. Data from numerous global sites shows a global decadal oscillation in δ18O, suggesting a climatological forcing. We compare trends in δ18O with climate records, examining correlations and proposing associated mechanisms. Significant correlation is found with the Niño indices. Significant anti-correlation is found with tropical precipitation and tropical humidity. Possible mechanisms include strong effects on δ18O by relative humidity, the 18O of precipitation, and the magnitudes of global photosynthesis and ecosystem respiration. Simple modeling of the δ18O in atmospheric CO2 supports the plausibility of these mechanisms, but does not result in the pattern observed in the data. Results suggest errors in the methods used to calculate isotopic values of the terrestrial biosphere CO2 fluxes, since data trends show the likelihood of global climate influence on δ18O communicated through the terrestrial carbon cycle.
Figure 1. Standardized δ18O of atmospheric CO2 data from multiple sites and the resulting assimilation-weighted global composite.