Unexpected variations in the triple oxygen isotope composition of stratospheric carbon dioxide

• Published in: Proceedings of the National Academy of Sciences
• Other Authors: Wiegel, Aaron (author), Cole, Amanda (author), Hoag, Katherine (author), Atlas, Elliot (author), Schauffler, Sue (author), Boering, Kristie (author)
• Format: Article in Journal/Newspaper
• Language: English
• Published: National Academy of Sciences 2013
• Subjects: Arctic
• Online Access: http://nldr.library.ucar.edu/repository/collections/OSGC-000-000-020-109; https://doi.org/10.1073/pnas.1213082110

We report observations of stratospheric CO₂ that reveal surprisingly large anomalous enrichments in ¹⁷O that vary systematically with latitude, altitude, and season. The triple isotope slopes reached 1.95 ± 0.05(1σ) in the middle stratosphere and 2.22 ± 0.07 in the Arctic vortex versus 1.71 ± 0.03 from previous observations and a remarkable factor of 4 larger than the mass-dependent value of 0.52. Kinetics modeling of laboratory measurements of photochemical ozone–CO₂ isotope exchange demonstrates that non–mass-dependent isotope effects in ozone formation alone quantitatively account for the ¹⁷O anomaly in CO₂ in the laboratory, resolving long-standing discrepancies between models and laboratory measurements. Model sensitivities to hypothetical mass-dependent isotope effects in reactions involving O₃, O(¹D), or CO₂ and to an empirically derived temperature dependence of the anomalous kinetic isotope effects in ozone formation then provide a conceptual framework for understanding the differences in the isotopic composition and the triple isotope slopes between the laboratory and the stratosphere and between different regions of the stratosphere. This understanding in turn provides a firmer foundation for the diverse biogeochemical and paleoclimate applications of ¹⁷O anomalies in tropospheric CO₂, O₂, mineral sulfates, and fossil bones and teeth, which all derive from stratospheric CO₂.