For quantification and prediction of global warming due to anthropogenic carbon dioxide (CO_2) emissions, high-resolution determination of carbon cycle reservoir size and rates of exchange is vital. Isotope ratio measurements of ~(13)C and ~(18)O in CO_2 have proven their utility in resolving the carbon cycle. Here we report a long-term record of triple oxygen isotopic composition of tropospheric CO_2. A presence of a steady state stratospheric component (Δ~(17)O= ln(δ~(17)O+1) - 0.522 ln(δ~(18)O+ 1) = 0.08 ± 0.04‰) is observed in tropospheric CO_2 after modeling via a long-term spline fit of the record, adding an additional technique for quantification of CO_2 fluxes. The δ~(18)O of CO_2 (measured as O_2 for oxygen triple isotopic composition) from the present study is compared to the measurements of the Scripps Institute of Oceanography CO_2 network (measured as CO_2 for δ~(13)C and δ~(18)O) and found to be comparable. We note that the triple oxygen isotopic signal (Δ~(17)O) 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-CO_2 isotope exchange is possible, and future measurements and modeling strategies may develop this further.
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