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

Abstract

We report observations of stratospheric CO2 that reveal surprisingly large anomalous enrichments in (17)O 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 (17)O 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((1)D), 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 (17)O anomalies in tropospheric CO2, O2, mineral sulfates, and fossil bones and teeth, which all derive from stratospheric CO2.

Fig. 1.

Stratospheric CO₂ observations. Three isotope plot for the balloon (34°N) and “SOLVE” aircraft (24–83°N) samples, with previous observations: Thiemens (19) and Zipf and Erdman (20) are rocket samples from ∼34°N. Lämmerzahl (22) are balloon samples from 44° and 68°N. Alexander (21) are balloon samples from 68°N. Kawagucci (24) are balloon samples from 39° and 68°N. Data from Boering et al. (23) are not shown because of an earlier analytical mass-dependent artifact that affected ln¹⁷O and ln¹⁸O but not Δ¹⁷O. The mass-dependent fractionation line with slope 0.528 (red) and a hypothetical end member mixing line (black) with slope 1.7 (SI Appendix, Table S6) are also shown. The overall 1σ uncertainties for the SOLVE and Balloon 2004 data including both external precision and accuracy are ±0.1% for ln¹⁸O and ±0.5% for ln¹⁷O.

Fig. 2.

Experimental versus model results. Time evolution of the CO₂ isotopic composition for the 50 torr (A) and 100 torr (B) UV irradiation experiments (symbols) and predictions from a photochemical kinetics model (lines). Shaded area shows uncertainty in the base model predictions, dominated by a conservative estimate of the uncertainty in k_asymmetric for ¹⁷O¹⁶O¹⁶O formation. (C and D): Same as A and B in a three-isotope plot. Also included in different model scenarios (SI Appendix, Table S10) shown here are theoretical mass-dependent (“MD”) isotope effects in O₃ photolysis at 254 nm (48); and large, hypothetical “normal” and “inverse” MD O₃ photolysis isotope effects to illustrate how the three-isotope slope for CO₂ is increased (normal) or decreased (inverse) along a mass-dependent line of slope 0.528 (red dotted line) as the MD isotope effects change the isotopic composition of O₃ and O(¹D), while leaving Δ¹⁷O (Table 2) essentially unchanged (to within small differences in the MD coefficients, λ, in Δ¹⁷O = ln¹⁷O−λln¹⁸O, which can range from 0.500 to 0.529; ref. 2). Under these laboratory conditions, there is only one O(¹D) isotopic composition, so the CO₂ isotopic composition evolves along a straight line connecting the O(¹D) isotopic composition with that of the initial CO₂.

Fig. 3.

Variations in Δln¹⁷O/Δln¹⁸O of CO₂. (A) Δln¹⁷O/Δln¹⁸O of CO₂ versus N₂O mixing ratio and (B) Δln¹⁷O/Δln¹⁸O of CO₂ versus Δ¹⁷O of CO₂; Δln¹⁷O/Δln¹⁸O = 1.7 is shown (dashed line) for reference. In general, the Δln¹⁷O/Δln¹⁸O values increase from a tropospheric, near–mass-dependent value to >1.6 as (A) N₂O decreases and (B) Δ¹⁷O of CO₂ increases, explaining at least part of the larger observed variability in Δln¹⁷O/Δln¹⁸O in the lower stratosphere where “younger,” high N₂O air mixes with “older,” lower N₂O air. Note that, for these samples, the trends in Δln¹⁷O/Δln¹⁸O are still apparent (even though the values change) even if we assume that the entry value for ln¹⁸O of CO₂ entering the stratosphere from the troposphere can vary by ±0.5, either by applying the same offset for every point or by mimicking a seasonal variation within the dataset, and even though the overall 1σ uncertainty in the ln¹⁷O measurements including both accuracy and precision is ±0.5%.