thiemans 2011 isotop..
Mass Independent Isotopic Composition of Terrestrial and Extraterrestrial
Materials
M.H. Thiemens and Robina Shaheen
Department of Chemistry and Biochemistry 0356
University of California, San Diego, CA, USA
4.06.1 General Introduction
Stable isotope ratio measurements have been utilized for more than 60 years as a tool in an resolving extended range of chemical and natural processes. This includes, for example, igneous and sedimentary thermometry, paleothermometry, hydrological cycle events (evaporation, condensation), biogeochemical processes, atmospheric and planetary chemistry and solar system evolution. The theoretical basis for calculating the position of equilibrium in isotopic exchange between two molecules was first developed from calculation of isotopic reduced partition
function ratios (Bigeleisen and Mayer, 1947; Urey, 1947)
. It was shown that the position of isotope exchange equilibrium between two differing isotopically substituted molecules (such as H
2
18 O and C 16 O
2
) may be calculated as a function of temperature at very high precision from knowledge of the isotopic reduced partition functions. The difference in chemical behavior for isotopically substituted molecules in this specific instance arises from quantum and statistical mechanical effects and the characteristics of the vibrational frequencies of the isotopically substituted molecules. It is well established that a vibrating molecule is energetically represented as a near classical harmonic oscillator. Thus, in the case of isotopically substituted molecules, the quantized energy levels vary, with the heavier isotopic species possessing lower vibrational frequencies. As a consequence, heavy isotopically substituted molecules possess slightly stronger bond strengths, and the relative reactivities of the isotopically substituted species may be precisely determined. The predominant parameter is the vibrational frequency, which are measured spectroscopically at high resolution facilitating calculation of the isotope exchange values typically in the tenths of per mil level. As a result, knowledge of the partition function temperature dependency and, measurement of the isotopic compositions of the two molecular species precisely fixes the temperature of equilibrium basic physical
could not be measured at a sufficient precision to apply to natural systems because of the small range of isotope ratios. The isotope ratio mass spectrometer permitted isotope ratio measurements at a precision and accuracy of natural samples allowed development of paleothermometry and the use of oceanic planktonic foraminifera to measure ocean temperature changes over long time periods. Subsequently, a broad diversity of isotopic fractionation processes have been resolved in chemical reaction kinetics, molecular atmospheric diffusion (Earth and Mars), evaporation–condensation (e.g. the hydrological cycle), mineral crystallization temperatures, and biologic processes
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(e.g., photosynthesis, respiration, nitrogen fixation, sulfate reduction, and transpiration). The application of relevant isotopic measurements requires basic understanding of physico-chemical principles associated with these transformations. In this Chapter new varieties of isotopic phenomena and how their chemistry is applied towards understanding a variety of natural processes, terrestrial and extra-terrestrial, present and past, are discussed.
First Applications of Mass Independent Isotopic Observations
Although, conventional isotope effects vary widely in the physicochemical basis origin, they all are ultimately dependent upon relative isotopic mass differences for the isotopically substituted molecules. It was shown
1965b). For example, the changes in
33 S and
34 S (or the changes in the 33 S/ 32 S, 34 S/ 32 S ratios respectively, expressed in parts per thousand variation with respect to a standard, which for sulfur is Canon Diablo troilite) are shown to be highly correlated such that the change in
33 S is half that of the concomitant
34 S value. The mass range in the
33 S value is 1 amu (33 – 32 amu) and for
34 S is 2 amu (34 – 32 amu). Thus the relation:
δ 33 S = 0.5δ 34 S (1) is observed in terrestrial geological samples. Equation (1) arises as a consequence of the reliance of chemical and physical processes known at that time, from the relative isotopic mass differences of the sulfur bearing molecules.
The actual coefficient varies slightly vary from 0.516 to 0.524, depending upon the molecular mass and the
atmospheric, geological, and biological processes not achievable otherwise. Development of the concept of mass dependence as an isotopic marker was originally conceived to resolve nuclear (nucleosynthetic or spallogenic) from
composition (
17 O
18 O) was observed in the high-temperature calcium–aluminum inclusions (CAIs) present in
physicochemical processes produced expected mass-dependent
17 O
0.5
18 O. Figure 1 displays the slope one line
(termed Allende CAI Line) in a three isotope plot of oxygen. The slope ½ line represents the solid terrestrial oxygen isotopic reservoir of the bulk earth and moon, with the origin at Standard Mean Ocean Water (SMOW) and which expresses the normal mass dependent isotopic fractionation processes.
A three isotope plot displaying the oxygen isotopic composition of ozone produced from molecular oxygen
thus reproducing the slope of the CAI line depicted in Figure 1, thought to be a nuclear process. The starting molecular isotopic composition is at δ 18 O = δ 17 O = 0.
2
-50
50
Stratospheric Ozone
(70-110, 70-90)
25
Mass-Fractionation Line
(Slope = 0.5)
Sedimentary Rocks
-30
10
H Chondrites
Enstatite Chondrites
Meteoric Water
-10
-10
L and LL
Chondrites
Atmospheric Oxygen
Basaltic / Lunar Rocks
CM Chondrites
10
SMOW
C3 Chondrites
30
18 O (‰)
50
A lle nde
C
AI
L ine
(-40, -40)
-25
-50
Figure 1. Three isotope oxygen isotopic composition of meteoritic and some terrestrial reservoirs. The point at (-40,-40) at the terminus of the slope one line reflects the general calcium-aluminum inclusion (CAI) line first observed by Clayton et al (1973).
It was suggested that this anomalous isotopic composition must derive from a nuclear, rather than chemical process as physical or chemical processes would move along the slope of ½ line, in either direction depending upon the relevant fractionating process. Arithmetically, a
17 O =
18 O relation may arise in two ways, either by alteration of 17 O and 18 O by equal amounts or, by addition/subtraction of pure 16 O. Existing models for supernova processes at that time demonstrated that under certain nuclear reaction conditions and within certain stellar zones, nearly pure
16
O is produced. It was suggested (Clayton et al., 1973) that it is unlikely that
17 O and 18 O would be equally altered in any nuclear reaction sequence , and a 16 O supernova event was deemed the most plausible process to account for the Allende isotopic observations. However, once the basic assumption that chemical reactions must be mass dependent was found to be incorrect when mass independent oxygen isotope fractionation processes was
produced was equally enriched in 17 O and 18 O (
17 O =
18 O) with respect to its precursor molecular oxygen. Figure 2 schematically displays the results of those experiments and this lead to the reconsideration of the existing enigmatic theories of the possible source of oxygen isotopic anomaly.
3
60
Product Ozone
Molecular oxygen
Slope = 1.0
40
20 Slope = 0.52
-80 -60 -40 -20
0
0
-20
20 40
18 O (‰)
60 80
-40
-60
-80
Figure 2. A three isotope plot displaying the oxygen isotopic composition of ozone produced from
thus reproducing the slope of the CAI line depicted in Figure 1. The starting molecular isotopic composition is at
δ 18 O = δ 17 O = 0.
The observations displayed in Figure 2 have several consequences. First, the assumption that only a nuclear process may produce mass-independent isotopic compositions is invalid. Though nucleosynthetic components are present in meteorites, the distinction between chemical and nuclear is not straightforward.Secondly, the
17 O =
18 O relation is identical to that for Allende CAIs. Whether this feature accounts for the observed meteoritic oxygen
been abandoned and most likely derive from chemical processes (Thiemens and Heidenreich, 1983)
.
The source of
Recent advances in the ability to measure isotopic distributions within single mineral grains is possible due to the development of secondary ion mass spectroscopy (SIMs), nano-SiMS, and continuous-flow isotope-ratio mass spectrometry and have facilitated deeper understanding of nebular processes associated with production and secondary alteration of oxygen isotopic compositions. The ability to measure isotopic distributions on tens of nano meter scale lengths requires that enhanced theoretical understanding of the physical chemistry of how isotopic
4
distributions result at these sizes be achieved. An outcome of the development of oxygen isotopic analytical capabiliitiesfor single grain oxygen isotopic measurement was a key factor in the abandonment of the nuclear model for the meteorite oxygen-isotopic compositions. The prediction of 16 O carrier grains or, a direct correlation with
resolution scales. It is well documented that there are grains of highly anomalous oxygen present in meteorites that
the observed bulk level meteoritic oxygen isotopic compositions, or the required isotopic composition. As a consequence, chemical and/or photochemical processes are the preferred mechanism for production of meteoritic oxygen isotopic compositions.
The first theory for chemical production of meteoritic oxygen isotopic components by photochemical
describe the mechanism by which the bulk oxygen isotopic composition of meteorites is acquired as well as the CAI features. Several recent models have been developed to describe the evolution of oxygen isotopes in the solar nebula
that lie above and below the bulk Earth, Mars and the moon fractionation line. The size of the isotopic anomaly and the fact that oxygen is the most abundant element of stony object in the solar system requires that the responsible process is dominant in the evolution of the solar system.
5
Figure 3. A three isotope plot of the oxygen isotopic composition of meteoritic material. The symbols denote:
Terrestrial Fractionation Line (TFL), CV3,CO3,CM, CR are classes of carbonaceous chondrites, enstatite chondrites
(E), ordinary chondrites (H,L,LL), Rumaruti Chondrites (R), and calcium aluminum inclusions (CAI).
chemical processes such as photodissociation molecules. Furthermore, such analysis are required to model many natural processes, not only nebular but also planetary atmospheric. To perform self shielding experiments relevant to the pre solar nebula; CO photodissociation must be done at short wavelengths (< 110 nm) and, at relevant CO columnar densities. Shielding requires isotopically discrete spectral lines in the region between 90 and 110 nm and under such restrictions, and in spectral regions where molecular hydrogen does not absorb. As such, 105.17 nm is the wavelength where it must occur. The results of these experiments are shown in Figure 4.
Fig. 4. Three isotope plot showing anomalous oxygen isotopic composition of CO
2
produced during CO photolysis at different wavelengths. CAI line is shown for comparison. Adapted from Chakraborty et al, (2008)
The data in Fig. 4 show that there is a massive mass independent isotope effect in the process of CO photodissociation. The similarity in slope of wavelengths where self shielding should occur and where it may not was considered demonstration that self shielding does not occur and rather, there is a large isotope effect associated
effect in photolysis and as a result, a highly sensitive wavelength dependency resulting during photodissociation.
Such considerations are not taken into account in self shielding models and it is assumed that there is no isotope effect during photolysis. The interpretation of the CO work is a matter of active discussion at present and further
6
CAI two models, self shielding and chemically produced mass independent chemical fractionation processes, were
would provide a test of self shielding and the oxygen isotopic composition should resemble CAI’s. The self shielding model predict a
17 O of -25 per mil and at first, apparently consistent with the early return solar wind samples from the Genesis space craft, which had a
17 O=
18 O=-56-57 per mill, as predicted by models. It is shown that the Genesis solar wind, however, has a value at
17 O =-79,
18
O = -99 per mil (McKeegan et al., 2011), far from
developed on the basis that the solar wind may not necessarily be relevant to understanding meteoritic oxygen
isotopes (Ozima et al., 2007). From a statistical analysis of the oxygen isotopic (
17 O) composition of approximately 400 meteorites, a systematic trend was demonstrated revealing that the values statically scatter about zero per mil (terrestrial). The scatter for the mean of
17 O = O decreases with representative parent body size. The trend captures the effect of accretion of smaller bodies and the values consequently should approach the implied solar value. From their analysis it was concluded that solar oxygen is the major reservoir of oxygen and isotopically is the same as terrestrial, but different from CAI. This implies then that the solar wind samples do not reflect bulk solar and, that isotopic self-shielding did not produce meteoritc anomalies.
An important aspect of the early solar system oxygen issue is that they are concerned with the basic isotopic effect associated with photodissociation of CO. Investigations of such basic processes are relevant to the present atmosphere as understanding them is needed for interpretation of terrestrial and Martian atmospheric isotopic compositional measurements. Quantum numeric simulations have indicated that a strong and selective isotope effect from UV excitation of molecular nitrogen occurs and these selective isotope effects may be observed
energy surfaces are known at high spectral resolution, an additional isotope effect due to the coupling of diabatic electronic states of different bonding nature occurs. The effect arise where there are two or more excited states of the same, or nearly same energy. Numerical solutions of the time dependent Schrödinger equation of both the electronic and nuclear modes are included in the calculations. Population of excited valence and Rydberg states prior to dissociation were observed with these numeric simulations, and the process is highly wavelength dependent. When the isotope effect associated with the wavelength dependent population of crossing states is convolved with the structured solar spectrum, large isotope effects are apparent, especially at the CIII solar emission line (97.03 nm).
Similar calculations are needed for CO to provide greater details of nebular photochemistry.
Aside from photophysical isotopic effects, understanding new physical phenomena that alter isotopic compositions are presently being developed to understand both meteoritic and atmospheric processes. In accounting for the observed meteoritic oxygen isotopic anomalies non photochemical models have been suggested. MIF in the formation of ozone was observed and attributed to the symmetry of the oxygen molecule. Interestingly the experimentally produced O-isotope fractionation line of the ozone and left over oxygen reservoir was equal in slope
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that on the surface of pre solar metal oxide grains, reactions of oxygen atoms with metallic oxides (e.g. SiO, CaO) lead to symmetry driven isotope effects. Due to a high entropic concentration factor on mineral surfaces, the enrichment leads to a reaction rate increase of many orders of magnitude over gas phase volumetric reactions. On grain surfaces, reactions such as MO
(ads)
+ O
(ads)
→MO
2
*, with MO
2
* being a vibrationally excited molecule and M may be atomic species such as Si, Al, Ca, or Ti, readily occur. The MO
2
* is equally enriched in 17 O, 18 O as a consequence of the symmetry of the molecules and associated mass independent isotopic factors considered in the reactions. The excited molecule evaporates into the gas phase enriching the residual precursor MO and O species in
16 O leading to growth of solid grains of high 16 O enrichment, thus resembling CAI. This model has the advantage in that it produces the slope one configuration in a three isotope plot and accounts for the observation that CAI possess isotopic compositions clustering at about
17 O=
18 O around -50 to -60 per mil. In this consideration only one reservoir is required, a significant advantage over the requirement for multiple reservoir mixing in self shielding models. The present limitation is that there are no relevant experiments to explore the fractionations associated with metal oxide reactions under nebular conditions.
Recently, a heterogeneous chemical reaction model was reported as a new mechanism for production of the origin of the various 16
O reservoirs of the solar system such as those displayed in Figure 5 (Dominguez, 2010). In this
model, heterogeneous chemical reactions on the surfaces of interstellar (IS) dust grains in dense molecular clouds, such as our solar systems parent molecular cloud, are capable of producing oxygen reservoirs from reactions involving water ice. In such environments, accretion of atomic species (H, C, N, and O) on cold IS grain surfaces react with water to produce a variety of molecular species, particularly on 10 5 -10 6 year time scales. A surprising outcome of the ice model is that though the process occurs in interstellar molecular cloud environments of high molecular hydrogen mixing levels, surface grain reactions lead to an ozone intermediary with a large mass independent isotopic anomaly and
17 O=
18 O which is subsequently transferred to different reservoirs. In the model with ozone assisted ice reactions, 16 O enriched nebular gas reservoir and H
2
0-ice, 16 O depleted reservoirs are created along the slope one fractionation line over time periods of 10 6 -10 7 years. Figure 5 schematically represents this process in the parent molecular cloud and the interaction between residual gas and ice grains.
8
Fig. 5. Oxygen in the solar system on a triple-oxygen isotope plot (δ 17 O vs. δ 18 O). Calcium-aluminum-rich inclusions are shown in green, chondrules in white, and SMOW is at the origin by definition. The TFL with the slope ½ line is shown (dashed). The oxygen isotopic composition of Mars (Δ 17 O 0.3‰) is shown in red, asteroidal H
2
O, as inferred by Choi et al. ( 1998
) from studies of magnetite (Δ 17 O
5‰), is shown in blue and molecular cloud is indicated by the gray region.
This model also has the advantage that only one initial reservoir is required and, is consistent with astronomical observations of IS grains and their spectral features. Future models and relevant ice experiments will be of importance in determining the role of such chemistry in the early solar system. In addition, the role of surface chemistry such as this is of relevance in the Earth’s present day atmosphere since of the most poorly quantified
underscores the role of ices, which bears similarity to the well-known polar stratospheric clouds that exert a strong mediating factor in the chemistry of the winter polar atmospheres. As will be discussed, recent atmospheric and potentially Martian meteoritic measurements have shown the importance of grain surface nano film layer chemical
reactions in heterogeneous chemical transformational processes (Shaheen et al., 2010).
4.0. Isotopic anomalies in Extra Terrestrial Atmospheres and Environments
Nebular sulfur, like oxygen has the possibility of production of symmetry, photochemical, and self shielding isotope effects and consequently bears many similarities to oxygen. Sulfur however possesses multiple valence states and has a high volatility rendering it less well suited to maintain its original nebular isotopic signature as a result of secondary exchange reactions. It has the advantage though of possession of four stable isotopes. The only meteoritic class that has the best opportunity to record nebular sulfur isotopic signatures are the ureilites. These
9
meteorites have a combined low sulfur content that prevent secondary alterations and are isotopically anomalous at the bulk level. These are achondritic meteorites, composed mostly of olivines and pyroxenes and possess graphite,
these meteorites has remained enigmatic, and their bulk isotopic composition possess highly unusual oxygen
compositions of these meteorites, with slight 33
S excesses has also been reported (Farquhar et al., 2000a). The
33 S enrichment may potentially derive from a combination of chemical, photochemical, and nuclear processes and stepwise chemical extractions have shown that several achondritic meteorites possess mass independent sulfur
isotopic compositions. Figure 6 is a four isotope sulfur plot of their data and capital delta is a measure of the deviation from mass dependence, and a zero value denotes purely mass dependent compositions. Consequently, the figure displays mass independent correlations of multi isotopes.and associated isotope fractionation processes. The precision and accuracy of measurement of sulfur isotopes is significantly better than for oxygen because of the use of sulfur hexafluoride as the analyte gas. The source of the isotopic anomalies in the meteorite was suggested as
0.15
0.10
0.05
KrF laser
(CS
2
) x
(248
nm)
-Solar
HED
A rF
SO la ser
(1
93
2
nm)
ph ot ol ys is
(Xe la mp)
(CS
2
) x
-UV
0.00
Oldhamite
Ureilites
Aca-Lod
(Norton County)
CDT
Aubrite
-0.05
H
2
S photolysis
-0.3
-0.2
-0.1
0.0
0.1
0.2
36
S
CDT
(‰)
Figure 6. A plot of the deviation from mass independence for the four stable isotopes of sulfur. In this plot, mass dependent isotopic compositions have
S values of zero. The reference lines indicate isotopic fractionation trajectories for laboratory based photochemistry experiments, as discussed in the text.
10
There are no other chemical processes known in laboratory experiments or nature that produce this array. It was
sun, such as an X-wind. Figure 7 displays a potential model of an embedded early UV active sun. In this model, the early phase of solar evolution produces copious amounts of short wavelength UV light as the sun undergoes ignition and proceeds towards the main sequence. During this time, there is convective mixing in the solar nebula which may result in the photochemistry of sulfur bearing molecules and produce the observed mass independent sulfur isotopic anomalies. The sulfur anomaly is produced in a reducing environment, consistent with its observation in the more reduced meteoritic classes. It is also possible that during this photolytic event, oxygen was also photochemically reacted, though at present there is no evidence for a linkage between the two elements.
Figure 7. An illustration indicating how gas phase photolysis by the protosun may produce mass independent sulfur isotopic anomalies.
Short wavelength UV photolysis may produce the sulfur isotopic anomalies observed in meteorites and, indicates that the sun was active prior to planet body condensation. The results are the first direct evidence of photochemistry in the proto-nebular environment and has implications for the synthesis of organic molecules. Mass independent sulfur isotopic compositions in organic molecules extracted and isolated from the Murchison meteorite (methyl,
that these molecular species were created by photochemistry in the nebula, rather than on planetary bodies. This implies that the organic photochemical process was prior to formation of the planetary bodies. If true, then the
11
delivery of organics to the pre biotic Earth would be at least partially governed by photochemical processes and future studies of meteoritic organic molecules will further resolve processes of the origin and evolution of the
Earth’s atmosphere.
be reviewed here. The link between short-lived nuclei and their potential nucleosynthetic sources, such as AGB stars has been discussed in details
(Wasserburg et al., 2006). The analysis establishes chronologies and the recognition of
intervention of specific astrophysical processes in the earliest history of the solar system. In the case of potential oxygen nucleosynthetic contributions there are two key points. First, it is no longer considered likely that the source
nucleosynthetic preserved grains and the concentration of these grains is typically parts per million or less, which is insufficient anomalous oxygen isotopic atoms to produce the observed bulk level meteorite compositions.
4.06.1.1 The Physical Chemistry of Mass-independent Isotope Effects
The first observations of a chemically produced mass independent isotopic fractionation (MIF) process
(Thiemens and Heidenreich, 1983)
revealed that there existed no known physical–chemical mechanism that can account for the ozone isotopic enrichments. Since the recognition of the ozone isotopic anomaly in laboratory experiments and in the atmosphere, there have been extensive studies directed towards understanding the MIF
sulfur) in various solid reservoirs of Earth and Mars, including theoretical development of the MIF processes have been reviewed
In the first observation of a chemically produced oxygen anomaly, a mechanism
chemistry for asymmetric 16 O 16 O 17 O, 16 O 16 O 18 O versus symmetric 16 O 16 O 16 O species for the observed mass-
independent effect was suggested (Heidenreich and Thiemens, 1986). In this mechanism, an equal
17 O,
18 O fractionation occurs as a result of the identical chemistry for the asymmetric isotopomeric species with respect to symmetric 16 O 16 O 16 O, leading to heavy isotope enrichment in the product ozone. It was the rotational symmetry properties that originally led to the discovery of oxygen isotopes. With the sun as the background irradiance source, it was observed that there are extra rotational absorption lines for 16 O 18 O and 16 O 17 O as compared to 16 O 16 O
those papers, besides the importance of the discovery of oxygen isotopes, the work was also a significant
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rotational isotopic symmetry and the associated properties may produce a longer lifetime for the asymmetric isotopomer and consequently a greater probability of stabilization to the ground state ozone molecule. This was based upon the following. 1) The probability of an excited O
*
3
intermediate energetically quenching via collisions is determined by the product of its lifetime (
) and its collisional frequency. 2). The collisional frequencies derive from mass-dependent effects, such as velocity, and the source of the mass independence may not arise from the collisional terms for stabilization. 3).The lifetime of the 17 O and 18 O species ( 16 O 16 O 17 O, 16 O 16 O 18 O) is equal and longer than symmetric 16 O 16 O 16 O leading to a product equally enriched in 17 O and 18 O, i.e.,
17 O =
18 O.
The exact mechanism arises in the process of reaction from the intermediate excited state. Descriptions of such
the reactants interact with one another subject to the relevant potential energy surface. The lifetime of the excited intermediate is on the order of molecular vibrational periods, or ~10
–13 s. The lifetime is a complex function of the chemical reaction dynamics, which in turn depends on the number of available rotational or vibrational states. In this specific instance, there is dependence for the isotopically substituted species. Ozone of pure 16 O has a C
2v
symmetry and half the rotational complement of the asymmetric isotopomers. As a result, it was suggested that the extended lifetime for the asymmetric species leads to a greater probability of stabilization. While these assumptions are valid for a gas phase molecular reaction, they do not account for the totality of the experimental ozone isotopic observations.
theory is employed in its development, which quantitatively describes the energetics of gas phase atom–molecular encounters and the relevant parameters leading to either stabilization and product formation or re-dissociation to atomic and molecular species, with the energetic intermediate being a critical species. These models determine individual rate constants for isotopically and positionally substituted species of ozone. In a theoretical RRKM approach, all parameters are mass dependent, such as collisional frequency, bond strength, vibrational, and rotational energies and symmetry dependence. During the stabilization process of the excited intermediate (
*
O ), there is a
3 partial dependence upon the rate at which excess energy is dispersed. If this does not occur on a sufficiently short time scale stabilization does not occur and the excited
*
O
re-dissociates to the original O and O
3
2
reactants. Part of this energy dispersal process depends upon symmetry factors though the largest contributions are the massdependent processes. It is this symmetry factor that is the source of the mass independence. In the model, there is an inclusion of a “
effect,” which is a modest deviation from the statistical density of states for symmetric versus
from very slight differences in zero-point energies for the exit channels for dissociation of the asymmetric ozone
advancement has been the utilization of isotopically enriched reactants to determine the explicit rate constants of
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individual isotopomeric reactions. The relative rate for 16 O + 16 O 16 O is observed to be slower by a factor 50% than reaction of 16 O + 18 O 18
O, consistent with models(Anderson et al., 1997; Gao and Marcus, 2001) that predict a greater
rate for the asymmetric species. Incorporation of the “
effect” quantitatively accounts for the difference in
Janssen et al., 2001; Mauersberger et al., 1999)
reported the rate differences for several isotopically substituted reactions and have resolved the effect of pressure and third-body composition on the isotopic fractionation step
metastable ozone states formed subsequent to the oxygen atom-molecule collision calculated. In this approach the metastable states are treated as independent species and their stabilization steps are treated separately. The buildup in the population of the states and their ultimate leakage through the different stabilization exit channels are computationally followed. It is found that the stabilization process and the density of these states below the
ZPE are highly dependent upon the isotopic structure of the ozone metastable state. The actual calculation employs different metastable O
3
*(E i
) individual states and their lifetimes obtained utilizing a full dimensional quantum scattering study and a coupled-channel approach with hyperspherical coordinates. Ab initio techniques were
proper sign and approximate magnitude for the isotopomeric formation rates were calculated from this model. Later, treatment of the intermediate metastable ozone states was simplified, reducing potential energy surfaces to two dimensions while maintaining the ability to account for the anomalous isotopic behavior associated with ozone
receive theoretical and experimental consideration. For example, the temperature dependence for the individual isotopomeric reactions needs to be determined to treat the details of the stabilization process at the highest resolution.
The general issue of mass independent chemistry is a rapidly expanding area and there are many
2
and evaluate their zero point energies and the effect of symmetry. Isotopic fractionation during photodissociation of ozone has
2
O has been
provide the best interpretation of atmospheric ozone isotopic analysis. As will be discussed, atmospheric ozone serves as the initiator in a number of atmospheric reactions and its use as a tracer relies upon firm understanding of its formation process.
14
4.06.2 Atmospheric Observations of Mass Independent Isotopic Compositions
There is extensive observational basis of mass-independent isotopic compositions in nature and especially in the atmosphere of earth. When the first laboratory measurements of the mass-independent isotope effect were reported
It is now known that most oxygen-bearing molecules in the atmosphere (except water) possess mass-independent
isotopic compositions (Thiemens, 2006). These molecules include O
2
, O
3
, CO
2
, CO, N
2
O, H
2
O
2
, ClO
4
, and aerosol carbonate, nitrate and sulfate. Figure 8 displays the mass independent isotopic composition of most of the molecules that have mass independent isotopic compositions associated with them.
Figure 8. Three isotope plot depicting mass independent oxygen isotopic compositions of various oxygen carrying molecules in terrestrial atmosphere.
Mass-independent sulfur isotopic compositions are also observed in aerosol (solid) sulfates and nitrates and sulfide and sulfate minerals from the Precambrian, Miocene and present day aerosols, volcanic sulfates, Antarctica dry valley sulfates, Namibian Gypretes, and Chilean nitrates. In addition, Martian (SNC meteorites) carbonates and sulfates possess both mass-independent sulfur and oxygen isotopic compositions. In each example cited above, insights into terrestrial atmospheric or Martian cycles there are features that could not have been detected by concentration or single isotope ratio measurements
4.06.2.1 Stratospheric and Tropospheric Ozone
The importance of ozone in the Earth’s atmosphere is well established. Its presence in the stratosphere serves as a shield of UV light, vital for sustenance of life, particularly land-based. Tropospheric ozone serves as a source of electronically excited atomic oxygen O ( 1 D), which occurs via:
15
H O
2
2OH
The reaction is responsible for creation of tropospheric hydroxyl radicals (OH) which serves as the lifetime regulator of most reduced molecular species in the troposphere, and consequently may be regarded, in part, as a controlling agent of the oxidative capacity of the atmosphere.
A large enrichment of 18 O in stratospheric ozone were observed using in situ mass spectrometry
18 O was measured and it was not recognized that ozone was mass independently
measurements demonstrated that stratospheric ozone was mass-independently fractionated for the first time though the magnitude of the
18
O was significantly less than the original measurements (Mauersberger, 1987). Return ozone
isotopic analysis further demonstrated that stratospheric ozone possessed an isotopic composition consistent with
flights in the 22–33 km range and the details of the sampling device and procedure for collection have been
reported (Krankowsky et al., 2000; Stehr et al., 1996).
The Smithsonian astrophysical far-infrared spectrometer was employed on seven balloon flights to independently measure stratospheric ozone isotopic compositions and it was reported that they are consistent with other in situ ,
structural information, namely, the asymmetric to symmetric ratio: 50 O
3
(or 18 O 16 O 16 ) / 49 O
3
( 17 O 16 O 16 O). The IR instrument (FIRS-2) is a remote sensing Fourier transform spectrometer that detects molecular thermal emission in the atmosphere. The spectrometer resolved wavelength at 0.004 cm
–1
, permitting resolution of 16 O 16 O 16 O,
17 O 16 O 16 O, 16 O 17 O 16 O, 18 O 16 O 16 O, and 16 O 18 O 16 O species. Over the altitudinal range 25–35 km, the average enhancements for symmetric, asymmetric, and total 50 O
3
are 61 ± 18‰, 122 ± 10‰, and 102 ± 9‰, respectively.
The corresponding 17 O enrichment ( 49 O
3
) values are 16 ± 76‰, 80 ± 52‰, and 73 ± 43‰, respectively, in agreement with measurements by other techniques. A solar occultation spectrometer was also utilized to provide vertical profiles for both 16 O 18 O 16 O and 16 O 16 O 18 O and reported a significant difference between symmetric and asymmetric stratospheric species. The average enhancement for the asymmetric over symmetric species is a factor
Thiemens and Heidenreich, 1983)
Tropospheric ozone measurements from various locations, rural, urban, and maritime dominated regions
enrichment for both 17 O and 18 O (70–90‰ with respect to air molecular oxygen). The tropospheric observations have shown that these measurements may provide a mechanism to resolve the complex tropospheric ozone cycle, particularly the complex NO x
interactions. Recent measurements of tropospheric sulfate and nitrate aerosols demonstrate that the complexities of the NO x
–SO x
–O
3
cycle (chemical transformation and transport) are significantly better understood from
17 O,
18 O measurements and that has opened up the possibility that the isotopic composition of polar ice nitrate and sulfate could provide a unique means by which paleo-ozone and
Ozone isotopic
16
measurements have been reviewed which indicated heavy-isotope enrichment extends between 70‰ and 110‰ for
advancements in measurement techniques though details of global ozone vertical and seasonal isotopic variability is needed. The ozone enrichment process cascades through other atmospheric molecular species and this unique feature of the ozone isotopic signatures makes it an ideal tracer of numerous atmospheric physical, chemical, and photochemical processes and will be presented in the ensuing sections.
4.06.2.2 Stratospheric Carbon Dioxide
Prior to 1991, numerous isotopic measurements of stratospheric carbon dioxide were made, though only for
18 O and
13 C. A mass-independent oxygen isotopic composition was first observed in CO
2
collected from 26 km to
between electronically excited atomic oxygen and CO
2
:
Q( 1 D) + CO
2
↔ CO
2
Q* ↔ COQ+ O( 3 P)
Here Q denotes heavy isotope of ozone. In this process, the 17 O, 18 O enrichment of atomic oxygen, inherited from stratospheric ozone is passed on to CO
2
via exchange with excited oxygen atom (Thiemens et al., 1991; Thiemens et
short-lived transition state,
*
CO .
and this process has been studied extensively in various laboratories (Chakraborty
3
isotope equilibrium experiments with a mixture of CO
2
-O
3
and CO
2
-O
2
in the presence of UV light indicated transfer of ozone isotopic anomaly from O
3
to CO
2
depends on the pressure and O
2
/CO
2
2007). These laboratory experiments yielded
17 O/
18 O ~ 1 unlike observation with
17 O/
18 O ranging from 1.5 to
1.7 (Lammerzahl et al., 2002; Thiemens et al., 1995). Cross molecular beam studies with labeled C
16 O
2 and O
3 provided first experimental evidence that isotope exchange can occur through a long-lived CO *
3
intermediate
existing observations and models, it is still not possible to totally account for all of the important aspects of the exchange at present though the major features are understood.
Laboratory and field observations have provided a new, quantitative measure of atomic oxygen, a driving species of upper atmospheric chemistry. Methane and nitrous oxide are both primarily removed from the stratosphere by reaction with O ( 1 D). Recent measurements of carbon dioxide from the Tibetan plateau have shown that there is large isotopic heavy isotope enrichment in CO
2
which appears to coincide with stratospheric down
modeled stratosphere-troposphere exchange using the O-isotope anomaly in CO
2
and predicted that at steady state, a mass independent tropospheric CO
2
component should be observable (Hoag et al., 2005). If verified by triple oxygen
isotope measurements it would provide an independent measure of carbon reservoir interactions and their time scales, an independent means by which the kinetics of biogeochemical cycle interactions may be obtained.
17
Other measurements have shown that CO
2
triple isotope measurements are a new tracer of stratosphere– troposphere mixing. Stratospheric CO
2
is mass-independently fractionated due to its coupling with ozone and at higher altitudes, Lyman-
photolysis of O
2
(Liang et al., 2004) that provide another source of isotopically
anomalous oxygen atoms. Tropospheric CO
2
is mass dependent because of its equilibrium with water (Luz and
mesospheric) in origin. Oxygen triple isotopic composition of the stratospheric CO
2
and concentrations of SF
6
,
CCl
3
F (CFC-11), CCl
2
F
2
(CFC-12) and Cl
2
FCClF
2
(CFC-113) were measured within the Arctic polar vortex in an
aspect is that a CO
2
oxygen isotopic anomaly correlation with SF
6
number density was observed. Sulfur hexafluoride has a lifetime in excess of a thousand years and is only removed in the mesosphere and above by highly energetic processes such as photodecomposition and/or electron attachment. This long lifetime renders SF
6
as an ideal conservative tracer of stratospheric air mass movement. The observation of an inverse relation between the magnitude of mass independence and SF
6
concentration provides a measure of air mass age plus the integrated odd oxygen chemical activity. Combined measurement of fluorocarbons and CO
2
isotopes have been measured on samples acquired by the NASA ER-2 aircraft in the lower stratosphere and measured for 17 ΔO of CO
2
and N
2
O mixing ratios. The measurements display a tight correlation between the 17 O isotopic anomaly in carbon dioxide and
Such measures are important parameters for determination of the gross carbon exchange between the atmospheric reservoirs.
Simultaneous Measurements of CO
2
and O
3
isotope ratios from eight balloon flights from Kiruna, Sweden and Aire-sur-l’Adour, France provided the first simultaneous correlation between
17 O and
18 O of stratospheric
CO
2
and ozone (Mauersberger et al., 2003). The observed
17 O/
18 O ratio in carbon dioxide is 1.71 ± 0.03, independent of altitudinal range. The observations include both CO
2
and O
3
isotopes and aid in providing a quantitative model for the isotopic relation between stratospheric CO
2
and O
3
. In the stratosphere the oxygen isotopic composition of the CO
2
can be understood by the exchange of O 1 D from ozone photolysis. Higher in the atmosphere, photolysis of 16 O 17 O and 16 O 18 O by solar Lyman-
becomes a significant source of heavy isotopic enrichments in CO
2
not accounted for in previous models (Liang et al., 2007; Liang et al., 2008a). Laboratory and
mesospheric measurements are needed to further resolve this model and to amplify understanding of upper atmospheric oxygen chemistry
The multi oxygen isotopic measurements have been shown to be a powerful tool in atmospheric chemistry and there is a major need for global measurements, including observation of temporal and altitudinal variations. A significant limitation is the actual measurement techniques which are difficult and time consumptive. Original and
2
to
CF
4
and O
2.
In this technique CO
2
is reacted with BrF
5
at 800 °C for 48 h in nickel tubes that catalyze the reaction of fluorine with CO
2
. At the termination of the reaction, O
2
is separated from CF
4
by gas chromatographic techniques to remove trace amounts of CF
4
that produce error in the
17 O measurements. Since the level of precision required is at the 0.1 per mil level, the purification step is vital. Conversion to O
2
is required because measurement of CO
2
has
18
isobaric and uncorrectable interferences from the contribution at mass 45 ( 13 C 16 O 16 O or 12 C 16 O 17 O) from 13 C in carbon dioxide. A second technique converts CO
2
to methane and water, with subsequent chemical conversion of water to HF and O
2
by reaction with fluorine (Brenninkmeijer and Rockmann, 1998). Another technique based
solely on CO
2
13 C,
18 O measurement, CO
2
is exchanged with solid CeO
2
thus exchanging the oxygen isotopes with the adsorbed oxygen on the CeO
2
while carbon isotopic composition remaining constant. Since the 13 C/ 12 C ratio does not alter, the
17 O may be determined by calculation of the difference in the two measurements. This technique has several advantages: (i) the mass 44, 45, 46 ratios are commonly measured using standard isotope ratio mass spectrometers; (ii) it is fast and safe, not requiring Br
F
5. In all measurement protocols, the CO
2
isotopic measurements are less time consuming compared to single isotope ratio measurements.
The use of multi oxygen isotope ratio measurements of atmospheric CO
2
has provided higher resolution details of a variety of process, including stratosphere-troposphere mixing, stratospheric-mesospheric oxidative chemical processes and, is also linked to its interaction with the molecular oxygen cycle. Future atmospheric and laboratory measurements along with modeling efforts will provide greater resolution into these atmospheric phenomena and as will be discussed, of gross primary productivity measurements and Martian atmospheric processes.
4.06.3 Atmospheric Aerosol Sulfate: Present Earth’s Atmosphere
The importance of sulfur in the Earth's atmosphere and environment is well established. Aerosol sulfate is known to alter atmospheric radiative processes due to its role in increasing the Earth’s albedo and as a cloud condensation nucleus (CCN) and is a component of the sulfur cycle and environmental acidification. While these important roles are recognized, there remain significant gaps in understanding the role of aerosols in these processes.
An important role of isotopic analysis is for source identification, transport and mechanistic definition of in situ chemical transformational processes. Sulfate and nitrate aerosols are known agents in increasing the incidence of cardiovascular disease and definition of surface characteristics may be of relevance to such studies but, are notoriously difficult to measure. Nitrate is an agent in alteration of terrestrial biodiversity when sufficient quantities are added to soils and streams. Apportionment of nitrate sources is difficult because of the multitude of vectors; consequently, new techniques to aid in source recognition are welcome. Single isotope ratio measurements have been of limited utility in addressing these issues in the past due to limited specificity of single isotope ratio measurements such as
15 N,
34 S, or
18 O.
It is known that 70–80% of atmospheric sulfur species in the northern hemisphere are anthropogenic
to adequately define the oxidation processes that govern sulfur dioxide oxidation and chemical transformational
concentrations in northern latitudes, and it is plausible that heterogeneous chemical reactions are of importance, but inadequately recognized. Measurements of oxygen and sulfur mass-independent isotopic compositions of sulfate
19
aerosols have been developed to provide independent means to study issues of atmospheric sulfur. Mass-
Lee and Thiemens, 2001; Savarino et al., 2003). The measurements of the
17 O,
18 O isotopic composition of aerosol sulfate provide a new means to identify homogeneous vs. heterogeneous oxidative pathways, especially coupled with isotopic measurements of atmospheric H
2
O
2
and ozone (Alexander et al., 2004a; Alexander et al.,
and heterogeneous reaction pathways to be quantified (purely gas phase vs. liquid phase oxidation). The foundation for the model is the recognition that the exclusive gas phase oxygenation pathway for SO
2
oxidation is by reaction with OH radical, known to be strictly mass dependent. Liquid phase oxidation proceeds via reaction with ozone and hydrogen peroxide, both mass independent. If the isotopic composition of oxidizing molecules are known and associated atmospheric variables such as pH, temperature and ambient concentration of the oxidants, the relative
using GEOS-CHEM global models have permitted more specific reaction features to be identified, such as the role of aerosol particle surfaces above the Indian Ocean. This work identified a new aerosol generation process previously not considered in aerosol models. Using multi oxygen isotopic measurements of sulfate collected at
Trinidad Head, California, specific reaction characteristics, such as ozone driven oxidation of SO
2
in sea-spray and
Measurements of
17 O,
18 O, and
34 S of atmospheric sulfates in Baton Rouge, LA for a near two year period have
coastal California, the Baton Rouge region is dominated by H
2
O
2 oxidation rather than OH. The work confirmed model predictions for atmospheric chemical reactions among liquid water, gas, and the role of temperature, pH and aerosols in a region such as Baton Rouge.
The use of multi oxygen isotopes has been applied towards reaction resolution in chemically anomalous regions.
Measurements of sulfate collected in Alert, Canada in the Arctic have shown that unlike other geographic regions studied, Arctic oxidation processes are dominated by SO
2
(S(IV)) reaction with O
2
transition metal catalysis, rather than OH and H
2
O
2
(Alexander et al., 2009). A global chemical network transport model was used to interpret the
data and quantify the role of Fe (III) and Mn (II) catalyzed oxidation of S (IV) by O
2
. The inclusion of these reactions and isotope measurements revealed that the solubility and oxidation state of the metals is set by cloud water content, chemical source and sunlight. Inclusion of the metals characteristic of the Arctic in the sampling time period was able to account for the isotope measurements and define the unusual role of O
2
catalyzed reactions in the
Arctic.
Besides model considerations of oxidative mechanisms, single source characterization has been achievable.
Both sulfur and oxygen isotopic measurements of sulfates produced in controlled combustion processes provided
20
2002). These experimental observations were conducted at the Centre de Recherches Atmospheriques in
Lannemezan, France in a dark combustion chamber to isotopically characterize aerosol and gas emissions from fuel combustion and emission. The chamber has a volume of ~160 m 3 , which effectively minimizes wall effects and maximizes mixing. The collections were done in the dark to reduce potential photochemical effects. Fossil fuel and vegetation burns were performed, including Savanna grasses, Lamto grass, rice grass, hay, diesel fuel, and charcoal.
All of these processes produce sulfur and oxygen isotopic compositions that are strictly mass dependent Recent atmospheric oxygen isotopic sulfate measurements at a coastal La Jolla, California location and were able to define
this work the ship point sources of primary ship sulfate contributions were measured and revealed it to be mass dependent and equal to tropospheric O
2
(
18 O of ~23 per mil). Based upon the precise three isotope source characterization and back trajectory calculations it was demonstrated that ships contribute between 10% and 44% of the non-sea-salt sulfate found in the fine diameter (< 1.5 micron) particulate material in the southern coastal
California region. As concluded, these findings are relevant for public health issues, especially with the predicted increase in ship traffic.
In sum, multi oxygen isotope ratio analysis of present day aerosol sulfate samples have revealed source and chemical mechanistic details in many cases, such as oxidative pathway quantification that are new and quantitative and supplement existing concentration and modeling efforts. Similarly, source apportionment methodologies are also further amplified with the new multi isotopic measurements.
4.06.3.1 Mass-independent Oxygen Isotopic Composition of Non Present Day
Sulfates
It is well established that sulfate oxygen isotopic compositions are stable over long time periods due to the extreme non lability of the S-O bond within sulfate preventing secondary isotopic exchange reactions. For example, sulfates extracted from carbonaceous chondritic meteorites preserve the sulfur and oxygen isotopic record of
ice core samples to document the change of atmospheric processes, especially the alteration of oxidants over time such as O
3
, OH, H
2
O
2
using model and measurements of
17
O anomaly (Alexander et al., 2002; Alexander et al.,
(2004) who measured the past three centuries of sulfate from samples obtained from Greenland. In this record, the effect of elevated biomass burning in the Northern American continent during the industrial revolution time period of the late 19 th early 20 th century was isotopically characterized.
17 O values of sulfate from Greenland and
Antarctica reveal that OH gas phase oxidation dominated over O
3
, H
2
O
2
liquid phase reactions during the last glacial
water vapor content during glacial time periods, though other parameters must be considered, such as perturbations in the OH and O
3
levels as well as NOx, CO, and CH
4 chemistry. Recent
17 O,
34 S,
33 S, and
36 S measurements
21
industrial to industrial time period increases of as much as 50% and decreases in OH by ≈ 20% if accompanied by
H
2
O
2
increases of 50%. The work has demonstrated that the isotopic measurements combined with global transport models are capable of high resolution of global atmospheric chemical changes, such as oxidation chemistry.
Measurements of the oxygen isotopic composition of sulfates have provided atmospheric records for time periods
beyond those obtainable from the ice core record (Bao et al., 2000a; Bao et al., 2000b).
17 O anomalies in continental sulfate deposits, desert varnishes and massive (3 × 10 4 km 2 ) gypsum (CaSO
4
·2H
2
O) deposits of the Namibian desert
atmospheric photo-oxidation of dimethyl sulfide from nearby oceanic upwelling and inland transport. The measurements have established a new paleo-oceanic record of biologic activity variation on tens of millions of year time scales. The oxygen isotopic composition of sulfates extracted from vertical profiles of Antarctic dry valley
Variation of the
17 O values reflects the source of the anomalous sulfate to the Dry Valleys. The source, particle size dependence, and relative amounts of sulfate from sea salt aerosols and biogenic sources were characterized with the mass independent compositional measurements. Other applications included defining sulfate sources to the Atacama
A most striking observation was that of the first negative
17 O values, observed in evaporates and barites
was proposed that these anomalies track the isotopic composition of atmospheric oxygen.
Figure 9. The
17 O of evaporite and barite sulphate over the past 750 million years (adapted from Bao et al., 2008).
22
to the stratospheric ozone-carbon dioxide cycle it was suggested that the negative values displayed in Figure 8 result from dramatic excursions in atmospheric pCO2 levels, which concomitantly depress the molecular oxygen
17 O value, the source of oxidation oxygen to sulfate (along with water derived oxygen). A peak in pCO
2
occurred in the past 750 million years at a time (~650 million years ago) consistent with the melting of the snowball earth and massive CO
2
and methane release. Depending upon model parameters, the CO
2
levels may have exceeded 25,000 ppm. Measurement of sulfate extracted from carbonate lenses within a Neoproterozoic glacial diamictite suite from
Svalbard (635 million year age) also apparently indicates exceptionally high atmospheric carbon dioxide levels or a severely perturbed oxygen cycle. The mass independent observation of sulfate has allowed constraints of the carbon system during this time period to be determined.
Measurement of atmospheric sulfate oxygen isotopic compositions on time scales from the present to the deep past have provided new insights and records of global and local processes that would escape detection by any other measurement. Coupled with modeling efforts, future measurements will develop our recognition of processes including atmospheric sulfur transformation, transport, source identification, changes in the earth’s atmospheric oxidative capacity, and the snowball earth chemistry. Though not discussed in this chapter, sulfate oxygen isotopic measurements in meteorites have provided a mechanism by which hydrothermal processes on parent bodies may be
2003; Farquhar et al., 2000b).
4.06.4 Atmospheric Mass Independent Molecular Oxygen
It has been known for decades that photosynthesis and respiration predominantly establish the steady-state levels of O
2
and CO
2
in the Earth’s atmosphere. From this standpoint, it might be argued that these processes are among the most important of all global biogeochemical cycles and precise establishment of the rates of photosynthetic and primary productivity in the world's oceans is essential to full characterization of global geochemical cycling. This rate is intimately linked to the primary productivity of the oceans, carbon cycling, and
CO
2
due to the magnitude of the oxygen reservoir differences and in the lifetimes of different reservoirs. The detection of an atmospheric O
2
17
anomaly
provided a new and direct technique to evaluate global primary productivity
creation of anomalous oxygen isotopic reservoirs in ozone, CO
2
, and O
2
. This negative anomaly in molecular oxygen arises due to the isotope exchange between CO
2
and O
3
via excited oxygen atom in the stratosphere thus enriching stratospheric CO
2
with concomitant depletion of heavy isotopes from the atmospheric oxygen. The magnitude of the O-isotopic anomaly is in the per mil level in stratospheric CO
2
(~400 ppm), but is not quantifiable immediately for O
2
(~21%) as a steady-state effect on the
17 O,
18 O of atmospheric O
2
is not measurable because of the large difference in reservoir sizes and extended lifetime of O
2
compared to CO
2
(a factor of hundreds). The Oisotopic anomaly of CO
2 is lost by photosynthesis and respiration in the upper ocean and leads to an accrual of a
23
small and measurable negative isotopic anomaly in O
2 over extended time period The magnitude of the anomaly is a direct measure of global primary productivity and measurement of the
17 O composition in oceanic O
2
vertical profiles provides a new, quantitative means by which primary productivity may be evaluated. The rate the O
2 anomaly is removed is dependent upon photosynthetic and respiration activity in the oceans and its rate of disappearance is thereby quantitatively linked to primary productivity. Measurement of
17 O,
18 O of O
2
trapped in polar ice is also used as a means of determining global biospheric primary productivity over time. Oxygen triple isotopic measurements of dissolved oxygen in sea water allowed the estimation of integrated oceanic productivity
by two major processes, liquid vapor equilibrium isotope fractionation and diffusivity of water vapor into air, a kinetic isotope fractionation. The abundance of 17 O in precipitation has been assumed to carry no additional information to that of 18 O. In contrast to the deuterium excess, the 17 Δ of precipitation is controlled primarily by kinetic effects during evaporation of the initial vapor and is dependent on the temperature at the evaporation (and condensation) site. This makes 17 Δ a unique tracer that complements 18 O and deuterium, and may allow for a decoupling of changes in the temperature of the ocean, that serves as the vapor source, from changes in the relative
humidity above it (Angert et al., 2004).
The 18 O enhancement of atmospheric O
2
is utilized to determine paleo-variations in the relative O
2
oxygen cycle is mandatory to resolve the tightly coupled carbon cycle and to quantitatively evaluate the components
Global primary productivity has been modeled using
values for the most important processes (photosynthesis and soil respiration) that determine the isotopic composition of molecular oxygen:
ln(
17
) ln(
18
)
which is a previously defined relation (Mook, 1971; Mook and Vogel, 1968). In this equation,
17
and 18
are single stage isotopic fractionation factors for photosynthetic processes. The term expresses the contribution of the various processes that effect the isotopic composition of atmospheric O
2
. The value of
has been determined for the dark respiration factor, i.e., the cytochrome pathway (0.516 ± 0.001) and its mechanistic alternative (0.514 ± 0.001)
determination of these
-factors leads to the ability to determine the steady-state value for diffusion and respiration processes and their contributions to atmospheric O
2 levels. The isotopic composition of O
2
is established primarily by the relative rates of photorespiration and dark respiration. This conclusion was further strengthened by examining its relation to changes in ice core
17
O measurements (Blunier et al., 2002). This work modeled oxygen changes to
understand the variability in primary productivity for the last glacial, interglacial, early Holocene and demonstrated the utility of the high precision multi oxygen isotope approach in resolving the global oxygen biogeochemical cycle.
24
4.06.5
The Atmospheric Aerosol Nitrate and the Nitrogen Cycle
The nitrogen cycle is a cornerstone of all living processes extending from the air we breathe to the food we eat and is an essential participant in synthesis of amino acids and nucleic acids. Anthropogenic activities have significantly altered the nitrogen cycle by addition of nitrates in various chemical forms especially fertilizers and fossil fuel burning. The nitrogen footprint also expresses its importance in a number of vital biogeochemical cycles. These include, for example its role as a major sink for nearly all NO x
species, which produces its impact as an acid agent in the environment. Nitrate also participates as a heterogeneous reactant in polar stratosphere cloud (PSC) chemistry and associated with the destruction of ozone in the Antarctic polar vortex. Nitrogen is a source of fixed nitrogen to key ecosystems, as photosynthetic CO
2
and N
2
fixation are fundamentally important in mediating production
recognition that the terrestrial nitrogen footprint needs to be better understood has led to the development of new methodologies to study its behavior, including multi isotopic measurements. Some of the facets requiring further resolution includes i) depositional rates; ii) chemical tropospheric chemical transformation mechanisms; iii) source variability and quantitative identification; and iv) significance of long range transport of nitrates .
It has been shown that atmospheric aerosol nitrate possess one of the largest mass-independent oxygen isotopic
ability to perform
17 O,
18 O measurements in aerosol nitrates. In the process of conversion of nitrate to O
2
for mass spectrometer isotope ratio analysis, high purity is essential. Ion chromatography is utilized to simultaneously isolate and concentrate nitrate. The nitrate is subsequently reacted under a proper pH regime to convert to HNO
3
. Reaction with Ag
2
O quantitatively converts nitric acid to AgNO
3
, which is filtered and dried. AgNO
3
is thermally decomposed to O
2
, NO
2
, and Ag at a constant chemical and isotopic branching ratio. Following a final purification process, the O
2
is measured for the
17 O,
18 O composition. A variety of standards have been analyzed, including
USGS-35, a sample of NaNO
3
from the Atacama desert of Chile and IAEA-N3, an internationally distributed standard sample KNO
3
, with a range of reported
18
O values (Revesz et al., 1997). Another method now routinely
employed for measurement of both N and O isotopic composition of nitrate at the natural-abundance level. This procedure is based on the isotopic analysis of nitrous oxide (N
2
O) generated from nitrate by denitrifying bacteria that lack N
2
O-reductase activity (Boehlke et al., 2007; Sigman et al., 2001). However, proper correction methods
with international references (USGS32, USGS34 and USGS35) are needed. As a consequence, it is important to realize that the corrected isotope values are derived from a combination of several other measurements with
associated uncertainties (Casciotti et al., 2007; Chmura et al., 2009; Xue et al., 2010)
Atmospheric nitrates (HNO
3
+ particulate NO
3
) are predominantly formed through oxidation of NO x
(NO +
NO
2
), which originate from soils, lightening and fossil fuel combustion. The oxidation of NO x
occurs via O
3
, OH,
HO
2
/ RO
2
in the atmosphere and nitrate is subsequently deposited to the surface through wet and dry deposition.
Nitrate possesses an extraordinarily large mass-independent isotopic composition, second only to ozone in
25
3
and the
NO x
cycle. The relative significance of these oxidation pathways in the current atmosphere has been investigated using the
17 O of NO
3
since oxidants transfer different
17 O to nitrates (
17 O (O
3
) ~30‰; (
17 O (OH) ~0‰; (
17 O
(HO
2
) ~ 1‰) in aerosols samples first collected at La Jolla, CA. The
17 O (NO
3
) has also been used to infer the role
experiments suggest post depositional nitrate loss from snow packs due to photolysis may affect the
17 O of NO
3
.
However, post depositional renoxification (conversion of HNO
3
back to photochemically active forms that can regenerate ozone such as HONO and NO
2
) and reoxidation involves local oxidants thus producing a mixed signal of locally produced nitrate (~5-10%) and transported NO
3
(McCabe et al., 2005; McCabe et al., 2007). Size resolved
present-day aerosol nitrate isotopic measurements have added additional insight into the resolution of sources and
size-fractionated collection procedures, it is possible to provide mechanistic details of renoxification of nitrate aerosol and their fate in the atmosphere. Oxygen isotope anomalies in atmospheric nitrate has provided a new means to elucidate source and chemical transformation processes to understand nitrate biogeochemical cycles. For example, the massive mass-independent isotopic signature observed in Chilean desert nitrates uniquely reveals atmospherically derived nitrate deposition since all other sources (by measurement) possess mass-dependent isotopic compositions. Resolution of the source of nitrate to these regions has historically been argued and the isotopic measurements have provided a straight forward answer to source regions. In addition, these measurements, coupled with contemporary aerosol nitrate measurements reveal that the oxygen isotopic signatures are stable on
particularly valuable, as this permits measurement of nitrate in polar ice samples to examine paleo-variations in nitrate and NO x
chemistry (Kunasek et al., 2008). As discussed in the next section, when linked to sulfate oxygen
isotopic measurements, an entirely new mechanism to study paleo-atmospheric oxidative capacity variation is available.
4.06.7 Mass-Independent Oxygen Isotopic Compositions in Solids to Reflect Atmospheric
Change: Earth and Mars
It is argued that the terrestrial planets, especially Mars and Earth had roughly occupied comparable starting
had a CO
2
rich atmosphere, therefore, models of Martian climate history includes carbonate precipitation as a potential sink of atmospheric CO
2
. Based on understanding of the link between atmospheric CO
2
levels and greenhouse warming, high levels of CO
2
could support abundant liquid water thus facilitating rock weathering by carbonic acid to release Ca, Mg and Fe ions to water and to promote the precipitation of solid carbonates. However, massive carbonate deposits are missing on Mars as inferred from rovers and orbital based spectroscopic studies
(Bandfield et al., 2003; Bullock and Moore, 2007; Palomba et al., 2009). Like CO
2
, atmospheric SO
2
is another greenhouse gas whose complex geochemical cycle includes dissolution into surface waters, acid weathering of
26
2010). Spectroscopic observations indicated the presence of Mg and CaSO
4
, coexisting with coarse grained
with terrestrial geology, this suggests the presence of liquid water on the surface of Mars for a significant period of time. On earth, massive carbonate sediments representing its early atmosphere are found in association with
(Shergottite-Nakhlite-Chassignite), Zagami and Lafayatte are igneous rocks with trace quantities of secondary minerals. The multi oxygen isotopic composition of the secondary minerals (sulfate and carbonate) in Martian meteorites as well as geochemical observations have provided a basic understanding of the aqueous geochemistry
of Mars (Bandfield, 2002b; Farquhar and Thiemens, 2000; Farquhar et al., 1998; Jull et al., 1995).
In a study of secondary minerals in Martian (SNC) meteorites Thiemens and coworkers (Farquhar and
isotopic compositions (Fig. 10), thought to arise from the interaction of anomalous atmospheric carbon dioxide with water in the Martian regolith. Martian atmospheric CO
2
acquires it isotopic anomaly from the atmospheric O
3
-O
2 cycle, the same as in the Earth’s present atmosphere and are ultimately removed as carbonates. These measurements afford a new means by which Martian atmospheric-surface interactions could be studied. The O-isotopic composition of carbonates, sulfate and water indicate a range of value and do not lie along terrestrial fractionation line passing through the bulk silicate rock, indicating that the Martian meteorites are formed from distinct water reservoirs and are not in equilibrium with the host rock.
27
Fig. 10. Plot of
17 O versus
18 O of SNC meteorite carbonates, sulfates and silicates. Data for water of Karlsson et al., 1992 is included to illustrate the relationship between
17 O of water and carbonates and Farquhar et al
(2000,1998)
of the CO
2
released upon acid digestion was measured using reaction with BrF
5
(Farquhar et al., 1998). Control
experiments were conducted to ensure CO
2
released is exclusively from inorganic carbonates. Excess 17 O (
17 O) in atmospheric carbonates ranged from 0.4 to 3.9‰ (Fig. 11) as compared to soil carbonates (
17 O ≈ 0) which also make up a significant fraction of normal aerosol carbonate. These observations indicate heterogeneous chemical transformation occurs on aerosol surfaces and can preserve isotopic signature of the trace gases that interact with particle surfaces.
Fig. 11. Oxygen isotopic anomaly in terrestrial atmospheric carbonates. For comparison soil carbonates are shown.
’ i O = 1000 ln (1+
i O/1000), i O = 17 O and 18 O. 1
SD = 0.1 ‰.
(data and Figure adapted from Shaheen et al,
2010)
Laboratory experiments have been performed to resolve and quantify the mechanism for the production of the observed isotopically anomalous carbonates. It was shown that transfer of the oxygen isotopic anomaly of ozone to carbonates occurs via two different, previously unrecognized pathways (Fig. 12). Mechanism A involves oxygen isotopic exchange reactions on pre-existing carbonates whereas mechanism B involves in-situ formation of
28
carbonates on mineral particles. In both cases, the interaction of ozone with surface adsorbed water is required to produce the carbonate O-isotopic anomaly. Mechanism (A) initiates with the dissociative adsorption reaction of water on mineral surfaces and the formation of a surface complex (M(OH)(HCO
3
)) sc
. The presence of carbonic acid has been confirmed in numerous other studies on CaCO
3
surfaces in the presence of H
2
is a multi- step process consisting of i) gas phase diffusion of the O
3
molecules to the liquid interface, ii) transfer across the interface, iii) diffusion and reaction with water in the condensed phase, and iv) diffusion out and desorption of the residual reaction products. Isotopically anomalous hydrogen peroxide formation via dissociation of
O
3
transfers the isotopic signature in the hydration layer which in turn transfers the isotopic anomaly to the
measured and O-isotopic anomaly was identified for the first time in surface adsorbed peroxide. Mechanism B) involves in-situ formation of carbonates in the presence of CO
2
and water. The oxygen-bearing mineral aerosols
(CaO, MgO, Fe
2
O
3
, ZnO, SiO
2
, and Cu
2
O) are derived from crustal materials, sea spray and anthropogenic sources and may serve as nucleating sites for in-situ carbonate formation. It is known that alkaline metals in solution enhance the uptake of CO
2
. Similarly, the presence of basic oxide and hydroxide aerosols facilitates the interaction between chemisorbed water and CO
2
and promotes in-situ formation of bicarbonates. The surface layer upon drying becomes supersaturated, leading to secondary carbonate formation on mineral surfaces via heterogeneous chemistry.
Fig. 11. The molecular mechanisms proposed to explain the origin of the oxygen isotopic anomaly in atmospheric carbonates. A). Ozone isotope exchange on existing carbonate aerosols with dissociative adsorption of water and peroxide formation. B). In-situ formation of carbonates and interaction with ozone on particle surfaces. The red circles indicate ozone isotope exchange reaction and probable hydrogen peroxide formation sites. The abbreviation on the side bar are S= solid (MO and MCO
3
) such as CaO, MgO and Fe
2
O
3
, CaCO
3
, MgCO
3
, L= liquid or adsorbed water film, G = gas phase (adapted from Shaheen et al., 2010).
Mars exploration for extinct life is associated with location of bio signatures in secondary minerals, however, simple morphology of microfossils can be mimicked by abiological mineral structures, rendering it difficult to distinguish between true microbial fossils and microscopic pseudofossils. The multi oxygen isotopic values can be a valuable tool to discriminate carbonate fossils from their abiotic counterparts. By analogy with
29
terrestrial carbonate minerals, the O-isotopic signature of fossilized carbonate minerals should follow a standard mass dependent fractionation process (
17 O ≈ 0) if the water is known. The presence of an oxygen isotopic anomaly
(
17 O = 0.4-0.7 ‰) in carbonate minerals in Martian meteorites indicate otherwise, especially since it does not equilibrate with Martian water (Fig. 10). Recent laboratory experiments indicate that heterogeneous chemical transformations on carbonate minerals in the presence of ozone produce excess 17 O in carbonates in the presence of adsorbed water. The reaction intermediate hydrogen peroxide also shown to possesses oxygen isotope anomaly.
These observations collectively suggest that future measurements could be another means by which water levels on
Mars might be quantified. Stepwise extraction of water and hydrogen peroxide with concomitant oxygen isotopic analysis of carbonates and water may help to distinguish surface transformation from biotic and abiotic signatures of carbonate minerals and resolve water levels.
Sulfur is a particularly interesting element on the Martian surface because it is abundant, exists in the atmosphere and regolith, but has a poorly described origin and evolution. Establishing the connection between the reservoirs as well as determining the mechanism of transfer is of interest in the evaluation of potential life on Mars and other planets. It is observed that the SNC meteorites possess a mass independent sulfur isotopic composition and experimental by photochemical studies suggest that the anomalous isotopic compositions are synthesized by UV
Farquhar and Thiemens, 2000; Farquhar and Wing, 2003). To produce the sulfur isotope anomalies, SO
2 photolysis must occur at short wavelengths (186-251 nm) producing an excited electronic state of SO
2
(SO
2
*) which subsequently undergoes a reaction sequence leading to production of a stable sulfate product. Present SNC data rule
out hydrothermal oxidative effects (Farquhar et al., 2007a) . The relation of the sulfate
33 S and
36 S apparently requires photochemistry as the origin and definition of the wavelength dependent source of the photolytic isotope effect can allow better understanding of Martian surficial processes. The SNC observations are significant though, in that they provide a clear mechanistic link between the Martian atmosphere and the surface, however, with the availability of an expanding data base of in-situ Martian mineralogical composition and multi isotopic measurements, these results may aid in interpretation of both surface and subsurface environments.
4.06.8
Sulfur in the Earth’s Earliest Atmosphere: The Rise of Oxygen
In the present day terrestrial atmosphere, UV photochemistry of SO
2
does not occur due to removal of the required spectral UV component t by stratospheric ozone absorption and its short tropospheric life time restricting admixture to the stratosphere. Mass independent sulfur isotopic compositions of sulfide and sulfate samples from the
Earth’s pre-Cambrian period were first reported by Farquhar et al (2000). In that work it was shown (Figure 13) that between 2,090 and 2,450 million years ago the Earths atmospheric composition was dramatically different from today’s, particularly with respect to lowered oxygen levels. Without a stratospheric UV filtration layer, ground level
SO
2
photolysis is possible. As for Mars, the Precambrian isotopic anomalies result from ground level, or at least from tropospheric UV photolysis of sulfur dioxide. As shown in the figure 13, both positive and negative mass
30
independent sulfur anomalies are present, with positive
17 O primarily associated with reduced sulfur species and negative with oxidized.
Fig. 12.
A pictogram of Δ 33 S vs. age, with the blue area representing data from various geological samples. The record divides Earth’s history into three stages: Stage I extends from before 3.8 Ga to 2.45 Ga and is characterized by Δ 33 S of variable sign and magnitude; Stage II extends from 2.45 to 2 Ga and displays more subdued Δ 33 S variability; and Stage III extends from 2 Ga until present and is characterized by Δ 33 S variability much less than
±0.2‰. The large filled circle represents hundreds of analyses of samples younger than 2.0 Ga. Stage III is defined on the basis of these measurements, not on the representative samples shown in the figure. The change from Stage I to Stage II is attributed to a change in the Earth’s atmospheric chemistry. Photolysis reactions involving SO
2
and SO in Earth’s early atmosphere, coupled with an efficient transfer of the signature to the Earth’s surface, produced the
Stage I record. The smaller Stage II record may reflect the onset of oxidative weathering or it may reflect stabilization of atmospheric oxygen to intermediate levels (10
−5 –10 −2
PAL).
Tropospheric sulfur dioxide photochemistry by UV light requires low levels of oxygen and ozone present to allow tropospheric light penetration as the lifetime of SO
2
is too short to allow transport to the stratosphere for
UV photochemical reaction. The isotopic wavelength sensitivity of SO
2
was experimentally studied (184.9, 193, 248 nm) and shown that the 193nm photochemistry produces a fractionation factor most similar to that observed in terrestrial sedimentary rock samples older than 2450Ma. This spectral region overlaps with the Schumann-Runge
31
bands of oxygen and the Hartley ozone bands and it is these features that lead to the suggestion that the absence of oxygen and ozone at these time periods leads to the photochemistry of SO
2
and production of the positive and negative anomalies observed in sulfur isotopes in ancient sediments. Based upon the photo absorption crosssection of O
2
, O
3
, and CO
2
and SO
2
lifetime of, it was suggested that the oxygen levels must have been at least a couple of orders of magnitude lower than present values. The observation that the sulfur anomaly disappears at approximately
2459 Ma years ago is interpreted as indicating the rise of oxygen levels towards levels comparable to present. There are at least three stages involved in the change of the oxygen levels and the details of these processes are reviewed
models and theoretical considerations that expand understanding of how these processes of atmospheric -chemical
documented from perturbations in sulfur isotopes simultaneously occurring in Mount McRae Shale of northwestern
It is apparent from these measurements that globally controlled processes of the biogeochemical cycle of sulfur are involved in producing controlling the observed isotopic anomalies. The Archean isotopic record of sulfates was
and sulfur SO
2
and H
2
S volcanic emissions were defined. These events are particularly significant in accounting for spikes in the MIF isotopic fractionations over time. The use of the MIF compositions of Post-Archean times has also
the isotope effects associated with specific reaction steps and for example, the ability to measure multiple sulfur isotopes at the highest precisions and accuracies. The sulfur isotopic (
34 S and
33 S) record of seawater sulfate were utilized utilizing to place limits on pyrite burial and quantify the geological organic sulfur cycle with resolution of
short term organic burial processes (Wu et al., 2010).
Other possible scenarios for generation of the sulfur mass independent signal have been suggested where the anomalous fractionations of sulfur are produced by non photochemical processes, such as thermo chemical
reactions (Watanabe et al., 2009)
. The suggestion is based upon observed experimental production of a very small sulfur isotopic anomaly when mixtures of sulfate and amino acids are thermolyzed. There are, however, numerous issues with this suggestion and experimental interpretations. There also is no chemical mechanism available for the experiments which render extrapolation to a natural environment limited. The observed variation in
33 S is very small and insufficient to account for the sulfur data of the Archean (Figure 12) and most of the
36 S data is normal within experimental error. There is also no likely mechanism by which volcanoes may produce this anomaly in nature to explain the observed isotopic anomalies. Finally, the sulfur isotope anomalies in the ice core record associated with large volcanic events and the time evolution is consistent with photochemistry of an SO
2
plume and,
32
that fractionation pattern is also consistent with the Archean data. At present, without a clear definition of how this anomaly is chemically produced this mechanism is restricted in probable occurrence.
Another model for the observed fluctuations of sulfur isotopes in the Archean has been proposed
lower atmosphere from UV fluxes responsible for the production of the sulfur anomaly and, simultaneously trigger an anti-greenhouse effect that initiates glacial activity. In this photochemical-geochemical model CH
4
, CO
2
levels mediate the haze level as well as modulates the thickness thereby controlling the evolution of the climate, chemistry, and biology of the Archean. It has also been suggested that based upon the geological record that impact events may be associated with the disappearance of ozone and increase of UV radiation that produce the sulfur anomalies
.
The use of the atmospherically generated mass independent sulfur signal has proven to be a tracer of biological processes.
Microscopic isotopic sulfides in 3490-million-year old marine sulfates suggest that the early
Microscopic pyrites with low 34 S/ 32 S ratios in sedimentary barites (3.47 Gyr old) from North Pole, Australia
reducing bacteria had evolved by ~3.47 billion years ago, contrary to the hypothesis of Philippot et al (2007).
Quadruple sulfur isotope studies of the 3500 million year old Dressler Formation, Western Australia, also suggested
present, the use of mass independent sulfur isotopic compositions have allowed new insights into the earths earliest life and such studies are expanding, allowing details of biogeochemical processes and specific microbial activities
such as phototrophic oxidation of sulfur species by green sulfur bacterium (Zerkle et al., 2008).
The storage of ancient atmospheric samples has proven to be surprisingly robust. Sulfur inclusions from
in diamonds from the Orpa kimberlite pipe in the Kaapvaal-Zimbabwe craton of Botswana possess mass independent sulfur isotopic compositions. The anomalous isotopic fractionation pattern was produced via Archean atmospheric photochemistry and subsequent transfer to the Earth’s surface and sequestration into subducting material. The process initiated with volcanic emissions of sulfur bearing molecules in the reducing, low oxygen atmosphere and subsequent UV photochemistry that produce mass independent sulfur isotopic compositions. This also partitions the mass independent sulfur isotopic anomaly between elemental and oxidized redox states with opposing isotopic sign. The photolytic products are transferred to the surface of the Earth and enter the geological reservoir where they eventually convert to either sulfate (oceanic) or reduced sedimentary sulfides, possibly occurring in marginal seas and estuaries. These reservoirs become buried, metamorphose and are added to mantle material. The measured data imply that the material is mixed from the atmosphere to the mantle in the Archean based upon the mass independent isotopic composition of diamonds greater than 2450Ma. Younger diamonds are isotopically normal consistent with photochemistry of the anomalous oxygen in an older, low oxygen environment.
33
Aside from the preservation of an ancient atmospheric record, the diamond measurements offer an independent insight into mantle convective processes.
Sulfur isotopes in other sulfates are reflective of a variety processes. The pre Cambrian results have been a catalyst in developing understanding of isotope effects associated with sulfur gas phase photochemistry. Sulfur
dioxide as the plume of volcanic emissions passes polewards (Greenland and Antarctica). The sign change in
33 S is also consistent with model predictions. It has been suggested that the anomalous composition of early earth sulfur
involves optical shielding by OCS and SO
2
photolysis that may account for the observed early earth samples based upon their high resolution UV spectroscopic studies of SO
2
(Ueno et al., 2009). Cumulatively, the sulfur (and
oxygen) photochemical studies demonstrate that high resolution interpretation of natural isotopic data require fundamental physical chemical understanding of the relevant chemical processes, but some of the most fascinating problems in geochemistry may be subsequently attacked with these measurements.
4.06.9
Sulfur Isotopic Fractionation Processes in Other Solar System Objects
There are significant examples of mass-independent isotopic fractionations associated with photochemically
initiated gas-solid formation. (Cook et al., 2007). Irradiation (313 nm) of gas phase CS
2
produces a solid aerosol
(CS
2
) x
that is highly mass-independently fractionated, with a 33 S excess of 5–10‰, and 36 S deficit of 61–84‰. An interesting aspect of these observations was that the optical properties of this solid are similar to those observed for aerosols produced by comet Shoemaker–Levy 9(SL9) during collision with the outer atmosphere of Jupiter. It was suggested that the mechanism responsible for the observed sulfur isotopic fractionation process involves the differing rates of nonradiative transfer from the initial absorbing rate to the final reactive state of the electronically excited CS
2
molecule (Colman et al., 1996). The rate of such processes is predominantly dependent upon Franck–
Condon factors that are partially mass dependent and the work experimentally clarified the mechanism for production of the photopolymeric (CS
2
) x
solid from CS
2
photolysis. The sulfur isotopic composition of products from photolysis of 12 CS
2
and 13 CS
2
differ significantly which rules out involvement of a symmetry-dependent fractionation effect. Franck–Condon and vibronic coupling effects were suggested as the source of the observed effect, specifically the nonradiative decay and intersystem crossing rates for the lowest excited states which give rise
This work suggested that atmospheres of planets with CS
2
will give rise to anomalous compositions of sulfur, e.g. the Jupiter produced aerosols. It has also been shown that there exists a new class of mass independent isotopic fractionation process that may occur in extra terrestrial environments.
34
The processes of CO
2
photolysis may be of importance in the Martial upper atmosphere, and definition of relevant fractionation processes are needed to interpret SNC meteorite carbonate and sulfate isotopic data.
Photodecomposition of CO
2
by UV light at (185 nm) and the isotopes of the CO and O
2
products were observed to be highly enriched in 17
O (>100‰) (Bhattacharya et al., 2000). The dissociation arises via a spin-forbidden process
during the singlet to triplet transition. The mass independence derives from the reliance of the reaction rate on essentially resonant spin–orbit coupling of the low-energy vibrational states of the 16 O 12 C 17 O molecule of the singlet with the triplet state. Later studies indicated wavelength dependency (184.9, 123.6, 116.5 nm) and proposed involvement of a hyperfine interaction between nuclear spin and electron spins or orbital motion resulting in preferential 17 O and 13
C photolysis (Mahata and Bhattacharya, 2009). At present, the role of photochemistry of CO
2 in the Martian atmosphere is being evaluated as this photolysis may occur to produce high 17 O enrichments.
4.07 Concluding Comments
Resolution of the actual physical–chemical mechanism has advanced understanding of gas-phase transition states and chemical reactions and fundamental photochemistry. Mass-independent oxygen isotopic compositions have been observed in essentially all atmospheric molecules, except water. This includes: O
2
, O
3
, CO
2
, N
2
O, H
2
O
2
, and aerosol carbonate, sulfate and nitrate. In each specific instance, our understanding of biogeochemical cycle has been enhanced as a result of its specific isotopic signature. Measurements of sulfate and nitrate in ice core samples have provided new details of the past oxidative capacity of the Earth’s atmosphere. Oxygen isotopic compositions of ancient sulfates (e.g. Miocene) have provided new paleo-atmospheric information on the desert and nearby ocean in
Namibia, volcanogenic processes, Antarctic dry valley sources, and desert varnish formation processes. Sulfur massindependent isotopic anomalies in the Earth's oldest rocks (3.8 × 10 9 yr) have provided a new means by which the evolution of oxygen-ozone, and consequently life has evolved until ~2.2 × 10 9 years before present, a parameter sought for decades but only attained from the mass independent sulfur isotopic measurements.
There continue to be new and novel applications of the mass independent isotopic fractionation processes in nature such as the role and potential applicability of 17
O effects in the hydrologic cycle (Angert et al., 2004). The
multi isotopic fractionation associated with precipitation and the temperature independency may render high precision measurements of water an important new and unique tracer of the hydrologic cycle. This application may also extend to measurements of the three isotope composition of ice at high precision may provide important new insight into the relative contributions of kinetic isotope effects, evaporation/condensation, and relative humidity on the hydrologic cycle. This work is an important advancement and is likely to advance understanding of both the hydrologic cycle and climate change.
Measurements of sulfur and oxygen isotopic anomalies in secondary minerals from Martian meteorites have provided new insights into crust–atmosphere interactions on Mars, especially with the observations of a new
35
surface reaction mechanism involving ozone-peroxide-carbonates. The observation of mass independent sulfur isotopic anomalies in the Earth’s Pre Cambrian period has opened an entirely new field for the study of the origin and evolution of oxygen. Finally, chemical mass-independent process appears to be responsible for the production of the anomalous oxygen isotopic compositions observed in meteorites and thus was a major process in the formation of the solar system, though the exact mechanism remains presently unresolved.
There remain many new horizons in mass-independent chemistry and its applications. This includes theory, laboratory experiments, and new applications in nature, extending from Earth throughout the solar system, and in time, from the present to 4.55Ga.
Acknowledgments
The generous support of the NSF (Atmospheric Chemistry and Polar Programs) and NASA (Cosmochemistry and
Origins of Solar Systems) have allowed and facilitated many of the avenues of research discussed in this Chapter.
Without this support a major portion of this chapters work would not have been accomplished. Drs. S. Chakraborty,
J. Farquhar, H. Bao, G. Dominguez are acknowledged for their help with creation of the figures for this text.
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