Carbon dioxide in the atmosphere: isotopic exchange with ozone and its use as a tracer in the middle atmosphere

Triple oxygen isotope constraints on atmospheric O2 and biological productivity during the mid-Proterozoic

Reconstructing the history of biological productivity and atmospheric oxygen partial pressure (pO₂) is a fundamental goal of geobiology. Recently, the mass-independent fractionation of oxygen isotopes (O-MIF) has been used as a tool for estimating pO₂ and productivity during the Proterozoic. O-MIF, reported as Δ'¹⁷O, is produced during the formation of ozone and destroyed by isotopic exchange with water by biological and chemical processes. Atmospheric O-MIF can be preserved in the geologic record when pyrite (FeS₂) is oxidized during weathering, and the sulfur is redeposited as sulfate. Here, sedimentary sulfates from the ∼1.4-Ga Sibley Formation are reanalyzed using a detailed one-dimensional photochemical model that includes physical constraints on air-sea gas exchange. Previous analyses of these data concluded that pO₂ at that time was <1% PAL (times the present atmospheric level). Our model shows that the upper limit on pO₂ is essentially unconstrained by these data. Indeed, pO₂ levels below 0.8% PAL are possible only if atmospheric methane was more abundant than today (so that pCO₂ could have been lower) or if the Sibley O-MIF data were diluted by reprocessing before the sulfates were deposited. Our model also shows that, contrary to previous assertions, marine productivity cannot be reliably constrained by the O-MIF data because the exchange of molecular oxygen (O₂) between the atmosphere and surface ocean is controlled more by air-sea gas transfer rates than by biological productivity. Improved estimates of pCO₂ and/or improved proxies for Δ'¹⁷O of atmospheric O₂ would allow tighter constraints to be placed on mid-Proterozoic pO₂.

Triple oxygen isotope insight into terrestrial pyrite oxidation

The mass-independent minor oxygen isotope compositions (Δ'¹⁷O) of atmospheric O₂ and [Formula: see text] are primarily regulated by their relative partial pressures, [Formula: see text]/[Formula: see text] Pyrite oxidation during chemical weathering on land consumes [Formula: see text] and generates sulfate that is carried to the ocean by rivers. The Δ'¹⁷O values of marine sulfate deposits have thus been proposed to quantitatively track ancient atmospheric conditions. This proxy assumes direct [Formula: see text] incorporation into terrestrial pyrite oxidation-derived sulfate, but a mechanistic understanding of pyrite oxidation-including oxygen sources-in weathering environments remains elusive. To address this issue, we present sulfate source estimates and Δ'¹⁷O measurements from modern rivers transecting the Annapurna Himalaya, Nepal. Sulfate in high-elevation headwaters is quantitatively sourced by pyrite oxidation, but resulting Δ'¹⁷O values imply no direct tropospheric [Formula: see text] incorporation. Rather, our results necessitate incorporation of oxygen atoms from alternative, ¹⁷O-enriched sources such as reactive oxygen species. Sulfate Δ'¹⁷O decreases significantly when moving into warm, low-elevation tributaries draining the same bedrock lithology. We interpret this to reflect overprinting of the pyrite oxidation-derived Δ'¹⁷O anomaly by microbial sulfate reduction and reoxidation, consistent with previously described major sulfur and oxygen isotope relationships. The geologic application of sulfate Δ'¹⁷O as a proxy for past [Formula: see text]/[Formula: see text] should consider both 1) alternative oxygen sources during pyrite oxidation and 2) secondary overprinting by microbial recycling.

Development of a potential energy surface for the O3–Ar system: rovibrational states of the complex

Collisional stabilization is an important step in the process of atmospheric formation of ozone.

Identification of Anthropogenic CO2 Using Triple Oxygen and Clumped Isotopes

Quantification of contributions from various sources of CO₂ is important for understanding the atmospheric CO₂ budget. Considering the number and diversity of sources and sinks, the widely used proxies such as concentration and conventional isotopic compositions (δ¹³C and δ¹⁸O) are not always sufficient to fully constrain the CO₂ budget. Additional constraints may help in understanding the mechanisms of CO₂ production and consumption. The anomaly in triple oxygen isotopes or ¹⁷O excess (denoted by Δ¹⁷O) and molecules containing two rare isotopes, called clumped isotopes, are two recently developed tracers with potentials to independently constrain some important processes that regulate CO₂ in the atmosphere. The clumped isotope for CO₂, denoted by Δ₄₇, is the excess of ¹³C¹⁶O¹⁸O over a random distribution of isotopes in a CO₂ molecule. We measured the concentrations of δ¹³C, δ¹⁸O, Δ¹⁷O, and Δ₄₇ in air CO₂ samples collected from the Hsuehshan tunnel (length: 12.9 km), and applied linear and polynomial regressions to obtain the fossil fuel end-members for all these isotope proxies. The other end-members, the values of all these proxies for background air CO₂, are either assumed or taken as the values obtained over the tunnel and ocean. The fossil fuel (anthropogenic) CO₂ end-member values for δ¹³C, δ¹⁸O, Δ¹⁷O, and Δ₄₇ are estimated using the two component mixing approach: the derived values are -26.76 ± 0.25‰, 24.57 ± 0.33‰, -0.219 ± 0.021‰, and 0.267 ± 0.036‰, respectively. These four major CO₂ isotope tracers along with the concentration were used to estimate the anthropogenic contribution in the atmospheric CO₂ in urban and suburban locations. We demonstrate that Δ¹⁷O and Δ₄₇ have the potential to independently estimate anthropogenic contribution, and the advantages of these two over the conventional isotope proxies are discussed.

Oxygen anomaly in near surface carbon dioxide reveals deep stratospheric intrusion

Stratosphere-troposphere exchange could be enhanced by tropopause folding, linked to variability in the subtropical jet stream. Relevant to tropospheric biogeochemistry is irreversible transport from the stratosphere, associated with deep intrusions. Here, oxygen anomalies in near surface air CO₂ are used to study the irreversible transport from the stratosphere, where the triple oxygen isotopes of CO₂ are distinct from those originating from the Earth’s surface. We show that the oxygen anomaly in CO₂ is observable at sea level and the magnitude of the signal increases during the course of our sampling period (September 2013-February 2014), concordant with the strengthening of the subtropical jet system and the East Asia winter monsoon. The trend of the anomaly is found to be 0.1‰/month (R² = 0.6) during the jet development period in October. Implications for utilizing the oxygen anomaly in CO₂ for CO₂ biogeochemical cycle study and stratospheric intrusion flux at the surface are discussed.

Monocyclic and bicyclic CO4: how stable are they?

For the first time the barriers for the CO₂-elimination from 11 and 12 energy-rich CO₄ were located, they amount to 28.7 and 14.7 kcal mol⁻¹ at the CASPT2(18e,12o)/CBS level of theory, and 23.5 and 21.1 kcal mol⁻¹ at the UCCSD(T)/CBS level of theory.

Sedimentary constraints on the duration of the Marinoan Oxygen-17 Depletion (MOSD) event

The ~635 Ma Marinoan glaciation is marked by dramatic Earth system perturbations. Deposition of nonmass-dependently (17)O-depleted sulfate (SO4(2-)) in worldwide postglacial sediments is, thus far, unique to this glaciation. It is proposed that an extremely high-pCO2 atmosphere can result in highly (17)O-depleted atmospheric O2, or the Marinoan Oxygen-17 Depletion (MOSD) event. This anomalous (17)O signal was imparted to sulfate of oxidative weathering origin. However, (17)O-depleted sulfate occurs in limited sedimentary intervals, suggesting that Earth surface conditions conducive to the MOSD had a finite duration. An MOSD duration can, therefore, provide much needed constraint on modeling Earth system responses at that time. Unfortunately, the sulfate (17)O record is often sparse or lacks radiometric dates. Here, we report 11 barite layers from a post-Marinoan dolostone sequence at Wushanhu in the South China Block. The (17)O depletion fluctuates in magnitude in lower layers but is persistently absent up section, providing the most confident first and last sedimentary appearance of the anomaly. δ(13)C chemostratigraphy is used to correlate the Wushanhu section to two proximal sections on the same shallow platform that lack barite layers but have published U-Pb dates that occur in dolostone and shale. Assuming a similar pattern and rate for carbonate and shale deposition among the different sections, we estimate the MOSD duration at 0-0.99 My. This number can be further constrained by new radiometric dates from equivalent sequences worldwide, thus underpinning models on the nonsteady-state Earth system response in the immediate aftermath of the Marinoan meltdown.

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

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.

Dynamic model constraints on oxygen-17 depletion in atmospheric O2 after a snowball Earth

A large perturbation in atmospheric CO2 and O2 or bioproductivity will result in a drastic pulse of (17)O change in atmospheric O2, as seen in the Marinoan Oxygen-17 Depletion (MOSD) event in the immediate aftermath of a global deglaciation 635 Mya. The exact nature of the perturbation, however, is debated. Here we constructed a coupled, four-box, and quick-response biosphere-atmosphere model to examine both the steady state and dynamics of the MOSD event. Our model shows that the ultra-high CO2 concentrations proposed by the "snowball' Earth hypothesis produce a typical MOSD duration of less than 10(6) y and a magnitude of (17)O depletion reaching approximately -35‰. Both numbers are in remarkable agreement with geological constraints from South China and Svalbard. Moderate CO2 and low O2 concentration (e.g., 3,200 parts per million by volume and 0.01 bar, respectively) could produce distinct sulfate (17)O depletion only if postglacial marine bioproductivity was impossibly low. Our dynamic model also suggests that a snowball in which the ocean is isolated from the atmosphere by a continuous ice cover may be distinguished from one in which cracks in the ice permit ocean-atmosphere exchange only if partial pressure of atmospheric O2 is larger than 0.02 bar during the snowball period and records of weathering-derived sulfate are available for the very first few tens of thousands of years after the onset of the meltdown. In any case, a snowball Earth is a precondition for the observed MOSD event.

An improved CeO2 method for high-precision measurements of 17O/16O ratios for atmospheric carbon dioxide

RATIONALE

The oxygen isotopic composition of carbon dioxide originating at the Earth's surface is modified in the stratosphere by interaction with ozone which has anomalous oxygen isotope ratio (Δ(17)O = 1000 * ln(1 + δ(17)O/1000) - 0.522 * 1000 * ln (1 + δ(18)O/1000) >0). The inherited anomaly provides a powerful tracer for studying biogeochemical cycles involving CO(2). However, the existing methods are either too imprecise or have difficulty in determining the small Δ(17)O variations found in the tropospheric CO(2). In this study an earlier published CeO(2) and CO(2) exchange method has been modified and improved for measuring the Δ(17)O values of atmospheric carbon dioxide with high precision.

METHODS

The CO(2) fraction from air samples was separated by cryogenic means and purified using gas chromatography. This CO(2) was first analyzed in an isotope ratio mass spectrometer, then artificially equilibrated with hot CeO(2) to alter its oxygen isotopes mass-dependently and re-analyzed. From these data the (17)O/(16)O and (18)O/(16)O ratios were calculated and the Δ(17)O value was determined.

RESULTS

The validity of the method was established in several tests by using artificially prepared CO(2) with zero and non-zero Δ(17)O values. The published value of the CO(2)-H(2) O equilibrium slope was also reproduced.

CONCLUSIONS

The CO(2)-CeO(2) equilibration method has been improved to measure the oxygen isotope anomaly (Δ(17)O value) of atmospheric CO(2) with an analytical precision of ±0.12‰ (2σ).

Large and unexpected enrichment in stratospheric 16O13C18O and its meridional variation

The stratospheric CO(2) oxygen isotope budget is thought to be governed primarily by the O((1)D)+CO(2) isotope exchange reaction. However, there is increasing evidence that other important physical processes may be occurring that standard isotopic tools have been unable to identify. Measuring the distribution of the exceedingly rare CO(2) isotopologue (16)O(13)C(18)O, in concert with (18)O and (17)O abundances, provides sensitivities to these additional processes and, thus, is a valuable test of current models. We identify a large and unexpected meridional variation in stratospheric (16)O(13)C(18)O, observed as proportions in the polar vortex that are higher than in any naturally derived CO(2) sample to date. We show, through photochemical experiments, that lower (16)O(13)C(18)O proportions observed in the midlatitudes are determined primarily by the O((1)D)+CO(2) isotope exchange reaction, which promotes a stochastic isotopologue distribution. In contrast, higher (16)O(13)C(18)O proportions in the polar vortex show correlations with long-lived stratospheric tracer and bulk isotope abundances opposite to those observed at midlatitudes and, thus, opposite to those easily explained by O((1)D)+CO(2). We believe the most plausible explanation for this meridional variation is either an unrecognized isotopic fractionation associated with the mesospheric photochemistry of CO(2) or temperature-dependent isotopic exchange on polar stratospheric clouds. Unraveling the ultimate source of stratospheric (16)O(13)C(18)O enrichments may impose additional isotopic constraints on biosphere-atmosphere carbon exchange, biosphere productivity, and their respective responses to climate change.

Oxygen isotopic composition of carbon dioxide in the middle atmosphere

The isotopic composition of long-lived trace molecules provides a window into atmospheric transport and chemistry. Carbon dioxide is a particularly powerful tracer, because its abundance remains >100 parts per million by volume (ppmv) in the mesosphere. Here, we successfully reproduce the isotopic composition of CO(2) in the middle atmosphere, which has not been previously reported. The mass-independent fractionation of oxygen in CO(2) can be satisfactorily explained by the exchange reaction with O((1)D). In the stratosphere, the major source of O((1)D) is O(3) photolysis. Higher in the mesosphere, we discover that the photolysis of (16)O(17)O and (16)O(18)O by solar Lyman-alpha radiation yields O((1)D) 10-100 times more enriched in (17)O and (18)O than that from ozone photodissociation at lower altitudes. This latter source of heavy O((1)D) has not been considered in atmospheric simulations, yet it may potentially affect the "anomalous" oxygen signature in tropospheric CO(2) that should reflect the gross carbon fluxes between the atmosphere and terrestrial biosphere. Additional laboratory and atmospheric measurements are therefore proposed to test our model and validate the use of CO(2) isotopic fractionation as a tracer of atmospheric chemical and dynamical processes.

Mass-dependent and non-mass-dependent isotope effects in ozone photolysis: Resolving theory and experiments

In addition to the anomalous (17)O and (18)O isotope effects in the three-body ozone formation reaction O+O(2)+M, isotope effects in the destruction of ozone by photolysis may also play a role in determining the isotopic composition of ozone and other trace gases in the atmosphere. While previous experiments on ozone photolysis at 254 nm were interpreted as evidence for preferential loss of light ozone that is anomalous (or "non-mass-dependent"), recent semiempirical theoretical calculations predicted a preferential loss of heavy ozone at that wavelength that is mass dependent. Through photochemical modeling results presented here, we resolve this apparent contradiction between experiment and theory. Specifically, we show that the formation of ozone during the UV photolysis experiments is not negligible, as had been assumed, and that the well-known non-mass-dependent isotope effects in ozone formation can account for the non-mass-dependent enrichment of the heavy isotopologs of ozone observed in the experiment. Thus, no unusual non-mass-dependent fractionation in ozone photolysis must be invoked to explain the experimental results. Furthermore, we show that theoretical predictions of a mass-dependent preferential loss of the heavy isotopologs of ozone during UV photolysis are not inconsistent with the experimental data, particularly if mass-dependent isotope effects in the chemical loss reactions of ozone during the photolysis experiments or experimental artifacts enrich the remaining ozone in (17)O and (18)O. Before the calculated fractionation factors can be quantitatively evaluated, however, further investigation of possible mass-dependent isotope effects in the reactions of ozone with O((1)D), O((3)P), O(2)((1)Delta), and O(2)((1)Sigma) is needed through experiments we suggest here.

Identification of the D3h Isomer of Carbon Trioxide (CO3) and Its Implications for Atmospheric Chemistry

The CO3 molecule is considered an important reaction intermediate in the atmospheres of Earth and Mars for quenching electronically excited oxygen atoms and in contributing to the anomalous 18O isotope enrichment. The geometry of the CO3 intermediate plays an important role in explaining these effects; however, only the cyclic (C(2v)) isomer has been experimentally confirmed so far. Here, we report on the first spectroscopic detection of the acyclic (D(3h)) isomer of carbon trioxide (12C16O3) via its nu1 and nu2 vibrational modes centered around 1165 cm(-1) under matrix isolation conditions; the identification of the 12C18O3, 13C16O3, 13C18O3, 16O12C18O2, and 18O12C16O2 isotopomers of the acyclic isomer confirms the assignments.

Unambiguous identification of (17)O containing ozone isotopomers for symmetry selective detection

Symmetry selective detection of the (17)O containing ozone isotopomers (16)O(16)O(17)O and (16)O(17)O(16)O requires the unambiguous identification of absorption lines. We report high resolution tuneable diode laser spectrometer measurements of (17)O containing ozone isotopomers in the R-branch of the nu3 band and present a purely experimental technique that discriminates between (16)O(16)O(17)O and (16)O(17)O(16)O. Around 1040 cm(-1), differences in line positions of (16)O(17)O(16)O upto 4 x 10(-3) cm(-1) between our measurements and present spectroscopic database records (HITRAN 2004) are found.

Atmospheric Trace Molecule Spectroscopy (ATMOS) Experiment Version 3 data retrievals

Version 3 of the Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment data set for some 30 trace and minor gas profiles is available. From the IR solar-absorption spectra measured during four Space Shuttle missions (in 1985, 1992, 1993, and 1994), profiles from more than 350 occultations were retrieved from the upper troposphere to the lower mesosphere. Previous results were unreliable for tropospheric retrievals, but with a new global-fitting algorithm profiles are reliably returned down to altitudes as low as 6.5 km (clouds permitting) and include notably improved retrievals of H2O, CO, and other species. Results for stratospheric water are more consistent across the ATMOS spectral filters and do not indicate a net consumption of H2 in the upper stratosphere. A new sulfuric-acid aerosol product is described. An overview of ATMOS Version 3 processing is presented with a discussion of estimated uncertainties. Differences between these Version 3 and previously reported Version 2 ATMOS results are discussed. Retrievals are available at http://atmos.jpl.nasa.gov/atmos.

Tracer from the Sky

The rate of marine photosynthesis is of great importance for the global carbon cycle but is difficult to measure from environmental properties or in vitro. In this Perspective, Bender highlights a study by Luz and Barkan, who exploit the anomalous isotopic composition of molecular oxygen to determine the gross rate of photosynthesis in seawater. The method allows much longer time scales and larger spatial scales to be covered than with traditional oceanographic techniques.

Ozone isotope enrichment: isotopomer-specific rate coefficients

Six rate coefficients of relative ozone formation contradict the role of molecular symmetry in the process that results in the enrichment of heavy ozone isotopomers. The results show that collisions between light atoms, such as 16O, and heavy molecules, such as 34O2 and 36O2, have a rate coefficient advantage of about 25 and 50 percent, respectively, over collisions involving heavy atoms and light molecules. These results suggest that the observed isotope effect for each isotopomer may be caused by the preponderance of a single reaction channel and not through molecular symmetry selection.

Mass-independent oxygen isotope fractionation in atmospheric CO as a result of the reaction CO + OH

Atmospheric carbon monoxide (CO) exhibits mass-independent fractionation in the oxygen isotopes. An 17O excess up to 7.5 per mil was observed in summer at high northern latitudes. The major source of this puzzling fractionation in this important trace gas is its dominant atmospheric removal reaction, CO + OH --> CO2 + H, in which the surviving CO gains excess 17O. The occurrence of mass-independent fractionation in the reaction of CO with OH raises fundamental questions about kinetic processes. At the same time the effect is a useful marker for the degree to which CO in the atmosphere has been reacting with OH.