Teresa L. Jackson

Carbon Dioxide and Oxygen Isotope Anomalies in the Mesosphere and Stratosphere

Isotopic (δ17O and δ18O) measurements of stratospheric and mesospheric carbon dioxide (CO2) and oxygen (O2), along with trace species concentrations (N2O, CO, and CO2), were made in samples collected from a rocket-borne cryogenic whole air sampler. A large mass-independent isotopic anomaly was observed in CO2, which may in part derive from photochemical coupling to ozone (O3). The data also require an additional isotopic fractionation process, which is presently unidentified. Mesospheric O2 isotope ratios differed from those in the troposphere and stratosphere. The cause of this isotopic variation in O2 is presently unknown. The inability to account for these observations represents a fundamental gap in the understanding of the O2 chemistry in the stratosphere and mesosphere.

Oxygen isotope fractionation in stratospheric CO2

A new cryogenic collection system has been flown on board a balloon gondola to obtain separate samples of ozone and carbon dioxide without entrapping major atmospheric gases. Precision laboratory isotopic analysis of CO2 samples collected between 26 and 35.5km show a mass-independent enrichment in both 17O and 18O of about 11 per mil (‰) above tropospheric values. Ozone enrichment in its heavy isotopes was 9 to 16% in 50O3 and 8 to 11% in 49O3, respectively [Schueler et al., 1990]. A mechanism to explain the isotope enrichment in CO2 has been recently proposed by Yung et al. [1991]. Their model is based on the isotope exchange between CO2 and O3 via O(1D), resulting in a transfer of the ozone isotope enrichment to carbon dioxide. Predicted enrichments and measured values agree well.

Carbonates in CM2 chondrites: constraints on alteration conditions from oxygen isotopic compositions and petrographic observations

High-precision measurements of the oxygen isotopic compositions of carbonates (calcite and dolomite) from five CM2 chondrites are presented and put into context of the previously determined mineralogic alteration index (MAI), which places these meteorites into an alteration sequence. The carbonate oxygen isotopic compositions range from +20.0 to +35.7‰ for δ18O, +8.0 to +17.7‰ for δ17O, and −0.7 to −2.7‰ for Δ17O. Carbonate Δ17O values are inversely correlated with MAI and track the evolution of fluid composition from higher to lower Δ17O values with increasing alteration on the CM parent body. Similar Δ17O values for calcite and dolomite fractions from the same splits of the same meteorites indicate that calcite and dolomite in each split precipitated from a single fluid reservoir. However, reversed calcite dolomite fractionations (δ18Odol − δ18Occ) indicate that the fluid was subject to processes, such as freeze–thaw or evaporation, that fractionated isotopes in a mass-dependent way. Consideration of the carbonate isotopic data in the context of previously proposed models for aqueous alteration of carbonaceous chondrites has provided important insights into both the evolving alteration conditions and the utility of the models themselves. The data as a whole indicate that the isotopic evolution of the fluid was similar to that predicted by the closed-system, two-reservoir models, but that a slightly larger matrix–water fractionation factor may apply. In the context of this model, more altered samples largely reflect greater reaction progress and thus probably indicate more extended times of fluid exposure. Petrographic observations of carbonates reveal a trend of variable carbonate morphology correlated with alteration that is also consistent with changes in the duration of fluid–rock interaction. The data can also be reconciled with fluid-flow models in a restricted region of the parent body, which is consistent with assertions that the different types of carbonaceous chondrites derive from different regions of their parent bodies. In this case, the model results for a 9-km-radius body, and our data place the location of the CM chondrite formation in a 100-m-thick zone 1 km from the surface. The size of this zone could be increased if the model parameters were adjusted.

Observation of a mass independent oxygen isotopic composition in terrestrial stratospheric CO2, the link to ozone chemistry, and the possible occurrence in the Martian atmosphere

Multi-isotope (δ17O and δ18O) measurements of stratospheric CO2 and 14CO are reported. Samples were acquired from altitudes in excess of 11.5 km from Christchurch, New Zealand to the South Pole. A mass independent isotopic variation is observed in CO2, the magnitude of which correlates well with 14CO concentration, indicating a stratospheric source of the effect. The component arises from isotopic exchange between the product of O3 photolysis, O(¹D) and CO2 thus providing a unique measure of ozone photolysis and turnover. A similar process may occur in the Martian atmosphere, as suggested by water isotopic measurements from SNC meteorites.

Observations of the anomalous oxygen isotopic composition of carbon dioxide in the lower stratosphere and the flux of the anomaly to the troposphere

[1] Measurements of the triple oxygen isotopic composition of stratospheric CO 2 in whole air samples from the NASA ER-2 aircraft show anomalous enrichments in 17 O and 18 O. The compact correlation of the isotope anomaly (defined as Δ 17 O = δ 17 O - 0.516 x δ 18 O) with simultaneous N 2 O measurements demonstrates that Δ 17 O CO2 is a long-lived tracer with a stratospheric source. These characteristics, and an isotopic link to O 3 production, make Δ 17 O CO2 potentially useful as a tracer of integrated stratospheric chemistry and transport. The Δ 17 O CO2 :N 2 O correlation is also used to estimate a net Δ 17 O CO2 flux to the troposphere of 3.6 ± 0.9 x 10 15 ‰ mol CO 2 yr -1 . This flux is required to predict and understand the CO 2 and O 2 isotope anomalies in the troposphere and their use as tracers of gross carbon exchanges between the atmosphere and biosphere on interannual to glacial-interglacial time scales.

A 33S enrichment in ureilite meteorites: evidence for a nebular sulfur component

Abstract Acid volatile sulfur extracted from ureilite meteorites carries a small 33 S enrichment relative to carbonaceous chondrites, enstatite chondrites, ordinary chondrites, and troilite from iron meteorites: Δ 33 S (=δ 33 S − 1,000 × (1 δ 34 S/1,000) 0.515 − 1) = 0.042‰ ± 0.007‰ (standard error of 22 analyses). In situ production of sulfur by cosmic-ray spallation reactions involving Fe is unlikely to cause the enrichment because the ureilites have short cosmic-ray exposure ages, low Fe/S relative to the only documented phases that contain spallogenic sulfur (the metal phase in iron meteorites), and no corresponding 36 S enrichment. Sulfur derived from cosmic-ray spallation has been documented in the metal phase in iron meteorites, and it is characterized by Δ 36 S/Δ 33 S ∼ 8, inconsistent with present observations. We argue that this enrichment derives from heterogeneity in the presolar nebula. A 33 S enrichment in the presolar reservoir may derive from mixing among diverse nucleosynthetic sources or from mass-independent fractionations caused by gas-phase chemistry. In addition, several gas-phase reactions have been shown to produce mass-independent compositions for sulfur isotopes. One that both matches fractionations for all sulfur isotopes and is relevant to the presolar nebula has yet to be identified. An appropriate additive nucleosynthetic component has also not been identified.

Mass-independent fractionation of oxygen isotopes during thermal decomposition of carbonates

Nearly all chemical processes fractionate 17O and 18O in a mass-dependent way relative to 16O, a major exception being the formation of ozone from diatomic oxygen in the presence of UV radiation or electrical discharge. Investigation of oxygen three-isotope behavior during thermal decomposition of naturally occurring carbonates of calcium and magnesium in vacuo has revealed that, surprisingly, anomalous isotopic compositions are also generated during this process. High-precision measurements of the attendant three-isotope fractionation line, and consequently the magnitude of the isotopic anomaly (Δ17O), demonstrate that the slope of the line is independent of the nature of the carbonate but is controlled by empirical factors relating to the decomposition procedure. For a slope identical to that describing terrestrial silicates and waters (0.5247 ± 0.0007 at the 95% confidence level), solid oxides formed during carbonate pyrolysis fit a parallel line offset by −0.241 ± 0.042‰. The corresponding CO2 is characterized by a positive offset of half this magnitude, confirming the mass-independent nature of the fractionation. Slow, protracted thermolysis produces a fractionation line of shallower slope (0.5198 ± 0.0007). These findings of a 17O anomaly being generated from a solid, and solely by thermal means, provide a further challenge to current understanding of the nature of mass-independent isotopic fractionation.

Experimental Test of Self-Shielding in Vacuum Ultraviolet Photodissociation of CO

Self-shielding of carbon monoxide (CO) within the nebular disk has been proposed as the source of isotopically anomalous oxygen in the solar reservoir and the source of meteoritic oxygen isotopic compositions. A series of CO photodissociation experiments at the Advanced Light Source show that vacuum ultraviolet (VUV) photodissociation of CO produces large wavelength-dependent isotopic fractionation. An anomalously enriched atomic oxygen reservoir can thus be generated through CO photodissociation without self-shielding. In the presence of optical self-shielding of VUV light, the fractionation associated with CO dissociation dominates over self-shielding. These results indicate the potential role of photochemistry in early solar system formation and may help in the understanding of oxygen isotopic variations in Genesis solar-wind samples.

Stratospheric CO2 isotopic anomalies and SF6 and CFC Tracer Concentrations in the Arctic Polar Vortex

Isotopic measurements (δ17O and δ18O) of CO2 along with concentration measurements of SF6, CCl3F (CFC-11), CCl2F2 (CFC-12) and CCl2FCClF2 (CFC-113) in stratospheric samples collected within the Arctic polar vortex are reported. These are the first simultaneous measurements of the concentration of fluorinated compounds and the complete oxygen isotopic composition of CO2 in the middle atmosphere. A mass-independent anomaly in the oxygen isotopic composition of CO2 is observed that arises from isotopic exchange with stratospheric O(¹D) derived from O3 photolysis. The data exhibit a strong anti-correlation between the Δ17O (the degree of the mass-independent anomaly) and molecular tracer concentrations. The potential ability of this isotopic proxy to trace mesospheric and stratospheric transport is discussed.

Massive isotopic effect in vacuum UV photodissociation of N2 and implications for meteorite data

Significance In this paper, we account for the wide range (approximately a few thousand permil) of nitrogen isotopic composition measured in solar system materials. Several theoretical models have been proposed to explain the nitrogen isotopic enrichments measured in meteorites (especially in organic matter) and in cometary ice (NH3 and/or HCN). These models include ion−molecular isotope exchange reactions and isotope self-shielding in the disk. However, a major limit is that there are no experiments to substantiate any model. We measured and found massive N-isotopic fractionations during vacuum UV photodissociation of N2, perhaps one of the largest isotope effects ever measured, and present mechanistic evidence for the wide distribution in nitrogen isotopic compositions. Nitrogen isotopic distributions in the solar system extend across an enormous range, from −400‰, in the solar wind and Jovian atmosphere, to about 5,000‰ in organic matter in carbonaceous chondrites. Distributions such as these require complex processing of nitrogen reservoirs and extraordinary isotope effects. While theoretical models invoke ion-neutral exchange reactions outside the protoplanetary disk and photochemical self-shielding on the disk surface to explain the variations, there are no experiments to substantiate these models. Experimental results of N2 photolysis at vacuum UV wavelengths in the presence of hydrogen are presented here, which show a wide range of enriched δ15N values from 648‰ to 13,412‰ in product NH3, depending upon photodissociation wavelength. The measured enrichment range in photodissociation of N2, plausibly explains the range of δ15N in extraterrestrial materials. This study suggests the importance of photochemical processing of the nitrogen reservoirs within the solar nebula.