• carbon dioxide;
  • oxygen triple isotopes;
  • El Niño–Southern Oscillation


For quantification and prediction of global warming due to anthropogenic carbon dioxide (CO2) emissions, high-resolution determination of carbon cycle reservoir size and rates of exchange is vital. Isotope ratio measurements of 13C and 18O in CO2 have proven their utility in resolving the carbon cycle. Here we report a long-term record of triple oxygen isotopic composition of tropospheric CO2. A presence of a steady state stratospheric component (Δ17O = ln[(δ17O + 1) − 0.522 ln(δ18O + 1)] = 0.08 ± 0.04‰) is observed in tropospheric CO2 after modeling via a long-term spline fit of the record, adding an additional technique for quantification of CO2 fluxes. The δ18O of CO2 (measured as O2 for oxygen triple isotopic composition) from the present study is compared to the measurements of the Scripps Institute of Oceanography CO2 network (measured as CO2 for δ13C and δ18O) and found to be comparable. We note that the triple oxygen isotopic signal (Δ17O) significantly decreased to 0.02 ± 0.02‰ from the steady state value of 0.08 ± 0.04‰ during the 1997–1999 time period. The possible causes for this depletion are evaluated and discussed. An enhanced role of global primary productivity, facilitating water-CO2 isotope exchange is possible, and future measurements and modeling strategies may develop this further.