Oxygen isotope anomaly in tropospheric CO_2 and implications for CO_2 residence time in the atmosphere and gross primary productivity

The abundance variations of near surface atmospheric CO2isotopologues (primarily ^(16)O^(12)C^(16)O, ^(16)O^(13)C^(16)O, ^(17)O^(12)C^(16)O, and ^(18)O^(12)C^(16)O) represent an integrated signal from anthropogenic/biogeochemical processes, including fossil fuel burning, biospheric photosynthesis and respiration, hydrospheric isotope exchange with water, and stratospheric photochemistry. Oxygen isotopes, in particular, are affected by the carbon and water cycles. Being a useful tracer that directly probes governing processes in CO_2 biogeochemical cycles, Δ^(17)O (=ln(1 + δ^(17)O) − 0.516 × ln(1 + δ^(18)O)) provides an alternative constraint on the strengths of the associated cycles involving CO_2. Here, we analyze Δ^(17)O data from four places (Taipei, Taiwan; South China Sea; La Jolla, United States; Jerusalem, Israel) in the northern hemisphere (with a total of 455 measurements) and find a rather narrow range (0.326 ± 0.005‰). A conservative estimate places a lower limit of 345 ± 70 PgC year^(−1) on the cycling flux between the terrestrial biosphere and atmosphere and infers a residence time of CO_2 of 1.9 ± 0.3 years (upper limit) in the atmosphere. A Monte Carlo simulation that takes various plant uptake scenarios into account yields a terrestrial gross primary productivity of 120 ± 30 PgC year^(−1) and soil invasion of 110 ± 30 PgC year^(−1), providing a quantitative assessment utilizing the oxygen isotope anomaly for quantifying CO_2 cycling