Measurement and significance of ^(13)C^(18)O^(16)O in thermodynamically equilibrated and environmental CO_2

Precise measurements of the abundances of mass 44 45 46, and 47 CO_2 molecules have led to the discovery of non- random mixing of the stable isotopes of carbon and oxygen in CO_2 extracted from ambient air as well as CO_2 equilibrated with H2_O in the laboratory. Mass-47 excesses (dominantly ^(13)C^(16)O^(18)O) of ca. 1‰ are evident in samples that have experienced isotope exchange at low temperatures (i.e., human breath, CO_2 residual of photosynthesis, CO_2 equilibrated with liquid H_2O), relative to samples exchanged at higher temperatures (hydrocarbon combustion products, CO_2 heated to 1000°C). These results agree with theoretical calculations of equilibrium isotopic mixing, suggesting that mass-47 CO_2 may gauge the relative importance of high-temperature and low-temperature sources to atmospheric CO_2. CO_2 from ambient air is intermediate between the high-T and low-T extremes