Oxygen-18 of Atmospheric Carbon Dioxide: Interannual Variability and the Terrestrial Carbon Cycle

The stable oxygen isotope 18O is unique to isotope ecology in that it links the hydrosphere to the carbon cycle. Since the two gross land biosphere fluxes- photosynthesis and ecosystem respiration- are the dominant influences on the δ18O of atmospheric CO2, particularly on decadal timescales, analysis of atmospheric δ18O trends could provide useful insight into the terrestrial carbon cycle. Data from numerous global sites shows a global decadal oscillation in atmospheric δ18O, but reasons for this variability of remain unclear. We compare trends in δ18O with climate records, examining correlations and proposing associated mechanisms. Significant positive correlation is found with the Niño indices. Significant negative 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 influence of surface solar radiation on stomatal conductance. Simple modeling of the δ18O in atmospheric CO2 supports the plausibility of these mechanisms, but none demonstrate the same magnitude of variance observed in the data. Results suggest either an inaccuracy in the methods used to calculate isotopic values of the terrestrial biosphere CO2 fluxes, an alternate climatological influence on the terrestrial hydrosphere or biosphere, or both, since results show the likelihood of global climate influence on δ18O communicated through the terrestrial carbon cycle