SPECIAL FEATURE: INTRODUCTION

Introduction to Chemistry and Applicationsin Nature of Mass Independent IsotopeEffects Special FeatureMark H. Thiemens1

Department of Chemistry and Biochemistry, University of California at San Diego, La Jolla, CA 92093-0356

Stable isotope ratio variations are regulated by physical and chemical laws. These rules depend on a relation with mass differences betweenisotopes. New classes of isotope variation effects that deviate from mass dependent laws, termed mass independent isotope effects, werediscovered in 1983 and have a wide range of applications in basic chemistry and nature. In this special edition, new applications of theseeffects to physical chemistry, solar system origin models, terrestrial atmospheric and biogenic evolution, polar paleo climatology, snowballearth geology, and present day atmospheric sciences are presented.

anomalous isotopes | terrestrial fractionation | oxygen isotopes | sulfur isotopes | Archean

The use of stable isotope measurements hasa long history of applications in physics andchemistry dating back to the discovery ofisotopes themselves. Applications includeprocesses that occur on Earth and in space,present and past, and their interpretationhas been enriched by new theories andmeasurements of the fundamental physicalchemistry. Application of relevant physicalchemical laws to high precision isotoperatio measurements of controlled labora-tory chemical and photochemical experi-ments develops a basis for interpretingmeasurements of a wide range of naturalsamples and enhances interpretations. His-torically, development of thermodynamictheory for quantifying stable isotope varia-tions (1, 2) and the ability to measure thesevariations in natural samples via isotoperatio mass spectrometry (3) lead to deeperunderstanding of natural phenomena.Measurement of oxygen isotope ratios invarious geological materials has providedthe ability to follow temperature variationsof the world’s oceans over million years’time scales, determine igneous rock crystal-lization temperatures, and track the earth’shydrologic cycle, present and past (4–6).Isotope ratio measurements of ice corewater has been the primary means by whichtemperature variations are determined.Quantifying the transfer of carbon betweenthe Earth’s atmospheric and geological, bio-logical, and hydrological systems is followedusing carbon isotopes. Greenhouse gas sour-ces and transport are recognized by isotopicmeasurements. Meteorite isotope measure-ments provide details of the origin and evo-lution of the solar system. The observationof isotope ratios of terrestrial sulfur isotopes

in early earth minerals allows recognition ofthe origin and evolution of biological pro-cesses on Earth.In this special issue, isotope ratio measure-

ments are reported for a range of moleculesin experiments and natural systems. Byconvention, the delta notation, is used, whichfor oxygen is

δ18Oðper milÞ=��O18=O16�sample=

�O18=O16

�

standard� 1�× 1; 000:

For oxygen, the standard is standard meanocean water (SMOW). Stable isotope ratiomeasurements are typically reported in thismanner, and a similar isotope ratio is re-ported for δ17O.In general, isotope ratio alterations are

attributable to conventional thermodynamic,kinetic, translational, and gravitational phe-nomena (4–6). When changes are expressedin the delta notation, it is found that thesedifferent processes share the common featurethat they are all ultimately related to differ-ences in mass and in general:

Δ17O= δ17O− 0:5δ18O:

This relation arises from the mass depen-dence of isotope alteration processes, e.g.,a process that varies the δ18O by 10 permil (mass difference of two); the associatedδ17O varies by a factor of approximately halfthat (5 per mil, for the mass difference of 1).A mass-dependent process by definition hasΔ17O = 0, and mass independent is nonzeroand is referred to as a mass-independent frac-tionation (MIF). The coefficient varies be-tween 0.50 and 0.53 and is dependent onthe specific chemical process and the mass of

the relevant molecular species. The variationin the coefficient is used to evaluate variousprocesses and is discussed in several of thecontributed papers.The first application of multi-isotope mea-

surements to detect mass-independent mea-surements was for sulfur (7) to identifynuclear processes, such as cosmic ray spall-ation effects in meteorites. The applicationshave subsequently expanded, particularlyto oxygen isotopes, and in this special issue,many unique uses of both oxygen and sul-fur isotopes are presented (8–18).To identify mass-independent processes,

two or more stable isotope ratios are re-quired. It was assumed by ref. 7 that formeteorites, this deviation must reflect a nu-clear process because no chemical processalters isotope ratios in a manner indepen-dent of mass. It was shown that cosmic rayspallation of iron and nickel by high energygalactic cosmic rays over billion year timescales produces excess 33S and 36S. Oxygenisotopic measurements of calcium-alu-minum rich inclusions (CAIs) from theAllende meteorite revealed a deviationfrom mass dependence (19). Fig. 1 sche-matically displays a collection of meteoritemeasurements including the relation ofδ17O = δ18O for the CAIs. Based on theassumption that chemical processes can-not produce an isotopic composition thatdoes obey mass relations, it was concludedthat the data must be from a nuclear source,specifically addition of pure 16O fromsupernovae (19).

Author contributions: M.H.T. wrote the paper.

The author declares no conflict of interest.

1E-mail: mthiemens@ucsd.edu.

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The basic assumption that a chemicalprocess may not produce a mass-indepen-dent isotopic composition was later experi-mentally demonstrated to be incorrect (20)and initiated the field of mass-independentisotopic chemistry and its applications. Itwas shown that the identical δ17O/δ18O ratioobserved in CAI is produced during the for-mation of ozone from dissociation of molec-ular oxygen. Fig. 2 shows that ozone isproduced with equal 17O, 18O enrichments.Thiemens and Heidenreich (20) interpretedthis as arising from isotopic self-shielding onO2, creating a fractionation pattern depen-dent on abundance rather than mass. Isoto-pic shielding by CO in the solar nebula mightwas suggested as a means to produce theobserved meteoritic oxygen isotopic anoma-lies. Early measurements of the 18O/16Oratios in stratospheric ozone (21) suggestedlarge heavy isotopic enrichment; however,they only measured a single isotope ratioand at least two are required to demonstratea mass-independent process. A later reanal-ysis of the data led to the conclusion thatthese data are unreliable and should be dis-regarded, and subsequent return samples

demonstrated the presence of mass-indepen-dent atmospheric ozone that were consistentwith experimental observations (22).Despite the simple isotopic fractionation

pattern in the ozone formation processshown in Fig. 2, defining the basic physicalchemical mechanism responsible for theprocess remains elusive (5). An early modelattributed the isotope effect to the metasta-ble transition state that arises immediatelyfollowing the oxygen atom-molecule colli-sion and its subsequent stabilization lead-ing to stable ozone (23). In general, thisstabilization process is determined by thelifetime of the metastable species, which isa function of a wide variety of parameters.It was suggested that the isotopic selectivefactor arises from the different numberof states for asymmetric 16O16O18O,16O16O17O, compared with the symmetric16O16O16O species resulting in an increasedprobability of stabilization for the asym-metric species. There have been numerousexperiments directed toward identifyingthe source of the effect, with isotopicallylabeled ozone being particularly insightful(24–30) in defining the role of the isotopic

structure of the ozone molecule on theozone rate of formation, as well as pressure(27) and temperature (28). A full treatmentof the chemical physics of the ozone forma-tion process has been developed in refs.29–33, using a modified statistical Rice–Ramsperger–Kassel–Marcus (RRKM) modelfor the recombination process. A key as-pect was adoption of a new nonstatisticalfactor, termed the “η” effect that incorpo-rates non-RRKM factors associated withthe symmetry of the isotopically substitutedisotopic species. In this special issue, Mar-cus (34) extends this work to fold in theeffect of low pressure phenomena. At lowpressures (less than the Lindemann fall-offregion), the isotope enrichment decreaseswell ahead of the expected O + O2 dropoff and is directly treated (34). A potentialsource of this behavior is attributed to iso-topomeric symmetry of overlapping res-onances leading to a localized chaoticbehavior, and potential testable experimentsare suggested.Alternate mechanisms have been devel-

oped (35–37) based on detailed inclusion ofthe potential energy surfaces of ozone,including the energy barrier region. A fullquantum level treatment of the molecule-atom scattering process is used with a cou-pled channel model that allows incorpora-tion of all states and their coordinates.The model includes resonances among themetastable states and the role of isotopicsymmetry. In a contribution to this issue(38), the formation and stabilization stepsare treated using mixed quantum/classicaltheory incurred during collisional energytransfer and passage of ro-vibrational en-ergy. The involvement of scattering reso-nances and lifetime dependencies on ro-tational excitation, isotopic asymmetry,and the connection between differing reac-tion channels are included in the model.After the unsuccessful first attempts to

measure the isotopic composition of strato-spheric ozone via in situ mass spectrometry,subsequent return sample measurements (39)revealed ozone isotopic compositions consis-tent with experimental observations of ozoneformation (40–42). There now exists a sub-stantial database of stratospheric ozone meas-urements (42, 43). An important applicationof atmospheric ozone isotopes takes advan-tage of the unique, identifiable isotopic com-position of ozone and the ability to trace itthrough differing chemical reaction channels.It was suggested that the product of ozonedissociation, electronically excited atomic ox-ygen (O1D), may interact with carbon diox-ide forming an excited CO3 transition statethat dissociates to ground state O (3P) and

Fig. 1. Oxygen three-isotope plot of meteoritic and lunar materials. The slope 1 line was originally proposed to benucleosynthetic and is now regarded as a mass independent chemical process. Figure from ref. 76.

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CO2 (44, 45). During exchange, the ozoneisotopic signature is transferred to CO2 andserves as a measure of stratospheric ozonelevels and degree of chemical interaction withelectronically excited atomic oxygen (44, 45).From theoretical and experimental studies,CO2 isotopic measurements have providedinsight into the dynamics of the strato-sphere–troposphere exchange (45–53) anda measure of upper atmospheric oxygenradical processes (54–56). Using balloonsand National Aeronautics and Space Ad-ministration (NASA) ER-2 aircraft, CO2

samples were collected in the winter of1999–2000, as well as a 2004 balloon flightacross an altitudinal and latitudinal rangeof the Arctic Polar vortex (15). As reportedin this special issue, laboratory measure-ments coupled with atmospheric modelingmay account for the observed CO2 isotopiccomposition (15). Fig. 3 reveals the widerange in global variations in upper atmo-spheric CO2 oxygen isotopes. The modeland measurements reveal that the compo-sitions are unlikely to be due to artifacts inmeasurement and resolve the complexity ofupper atmospheric oxygen photochemistryand dynamics. A range in isotopic massindependence is observed that varies withlatitude, altitude, and time (15) and ispartly attributed to the position withinand outside of the polar vortex, enhancingunderstanding of polar upper atmosphericoxidative processes. The work suggests thatfurther measurements in tropical regions

would further amplify understanding ofthe contributing factors. In this issue (17),a study of CO2 photolysis in the actinicregion 150–210 nm has documented thephoto physical dynamics occurring in theEarth’s mesosphere that influence overallthe upper atmospheric (stratosphere andmesosphere) isotopic cycle of CO2-O3-O2,suggested as being important (17, 56).The O2-CO2-O3 isotopic interaction pro-

vides a means by which gross primary

productivity may be measured from the verysmall, but significant, variation from massdependence in atmospheric O2 (57, 58). Inthis issue, measurements of the triple oxygenisotopic composition of barites from a post-Minoan (635 Ma) dolostone sequence atWushanhu, in Southern China (16), havebeen shown to define the global biogeochem-ical system during a time period after thesnowball earth period where this oxygen-ozone-carbon dioxide coupling is particularlyperturbed. During this time period, globalglaciation occurred, extending to low latitudeequatorial regions and creating a snowballearth due to the drawdown of atmosphericcarbon dioxide during intense geochemicalweathering processes and producing a globaltemperature decrease. In a remarkable obser-vation (59), a negative Δ17O anomaly in bar-ite has been found in post-Minoan glacialdiamictites at the time of the global melt-down of the global glacial ice cover (16, 59,60). The existence of a negative Δ17O value isinterpreted as deriving from enhanced CO2

levels with elevated interaction with ozonephotochemistry, which amplifies the negativeeffect on O2. It is suggested that the CO2

levels may have exceeded 25,000 ppm to ac-count for the large barite negative isotopicanomaly. This observation represents one ofthe largest historical excursions in CO2 levels,and confirmation of the process and under-standing of its extent in amount and time arevital, particularly because this time period isassociated with a period of major biodiversityexpansion. A key aspect of this unique con-tribution is defining the time of the event

Slope = 1.0

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OZONE

OXYGEN

Fig. 2. Experimental results of Thiemens and Heidenreich (20) demonstrating a mass process in the formation ofozone along a slope one line, identically mimicking CAI in meteorites as shown in Fig. 1.

Fig. 3. A three-isotope oxygen isotopic plot of a wide range of stratospheric carbon dioxide samples. The sampleshave been obtained by balloon, stratospheric aircraft, and rocket borne whole air sampling. Plot taken from Wiegelet al. (15). The paper discusses the utility of understanding both the oxygen chemistry of the upper atmosphere as wellas stratosphere-troposphere dynamics.

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duration and (16) estimate it to be 0–0.99million years using stable and radiometricisotopes. Consequences for global geochem-istry as a consequence of this duration arediscussed.Mass-independent isotopic compositions

are observed in a range of molecules, terres-trial and extraterrestrial. Mass-independentcomposition of water is reported in samplescollected at Vostok, Antarctica (14). Thissignature captures stratosphere–tropospheremixing and a stratospheric water source. Theanomaly derives from methane oxidation aspart of the stratospheric ozone cycle (14).Normally this magnitude would be too lowto be recognized; however, the Vostok areais characterized by extremely low atmo-spheric water content and accumulation rate,allowing the stratospheric water to be distin-guished. The measurements permits a mea-sure of stratosphere–troposphere mixing inAntarctica and enhanced understanding ofthe role of anthropogenic methane sourceson the global atmospheric water cycle.Unique ice core sulfate measurements are

reported in this issue (12) for samples re-trieved from a high-resolution 22-y (1980–2002) snow pit at the South Pole. The oxygenisotopic variations record specific effects

on the environment, including perturba-tions from volcanoes and El-Nino South-ern Oscillations (ENSOs). From measure-ments of the variation in polar sulfate Δ17Ovalues, tropical ozone variations in the uppertroposphere/lower stratosphere have beendetected at South Pole and are observed tobe linked to the three largest ENSO events ofthe past 20 y. Fig. 4 schematically depicts theoverall process, initiating with atmosphericupwelling driven at the equatorial Intertrop-ical Convergence Zone (ITCZ) regions dur-ing ENSO events. It is observed that thesulfate anomaly coherently tracks with theOzone ENSO Index (OEI) obtained fromtropical latitude ozone satellite measure-ments. The El Nino OEI is thought to arisefrom variation of the tropopause height, itselfa consequence of deviations of tropical deepconvection and Brewer-Dobson circulation(12). As a consequence, an enhanced en-trance to the stratosphere within the ITCZis created, and tropospheric air enters thestratosphere and migrates pole ward asshown in Fig. 4, allowing the El Nino pertur-bation to be recognized in Antarctic ice coresamples. These measurements show that thesulfate oxygen isotopic measurements may beused to hemispherically track aerosol and

trace gases along with their oxidative chemi-cal processing. Along with the unique ENSOhemispheric record, the snow pit sulfate dataalso detected the El Chichon, Pinatubo, andCerro Hudson volcanic eruptions.With the special isotopic character of ozone

and its insertion into most atmosphericoxidation processes, it has developed intoa useful probe of atmospheric chemicalprocesses. An important aspect is the needto resolve the role of chemically reactivespecies. A significant amount of the ozonedriven chemistry occurs within the tropicalMarine Boundary Layer (MBL), where theprocess is driven by ozone sources [NOxand volatile organic compounds (VOCs)]and inadequately quantified ozone sinks.The total budget of NOx is inadequatelyknown and limits resolution and quantifi-cation of the nitrogen cycle. Contributionsfrom halogens (especially bromine), surfacechemistry, and nighttime chemistry areconfounding contributors to this complex-ity. From isotopic measurements of a yearlyrecord of nitrates collected at the CapeVerde Atmospheric Observatory, a uniqueinsight into MBL chemistry has beenobtained (13). The measured nitrate oxygenisotopic variations are shown to fit nitratechemistry, with bromine chemistry in-cluded. With application of two varietiesof atmospheric models and inclusion ofBrO, NO, NO2, O3,OH, HO2, and dimethylsulfide (DMS) with the Δ17O values, thecomplex chemistry was modeled. The workrecognizes that aerosol transport time is ap-proximately several days. More impor-tantly, it was recognized that the role ofN2O5 and its reaction products (ClNO2)are likely insignificant in this environment,and high HNO3 production rates fromN2O5 hydrolysis is unlikely to be signifi-cant. The role of halogens in the MBL isconsequently better established with theunique isotopic measurements facilitat-ing diagnosis. Bromine nitrate is a sig-nificant sink, at a level of ∼20% of totalnitrate formation. The work illustrates thathigh precision multi-isotope ratio meas-urements coupled with appropriate con-centration measurements and modelingprovide a deeper understanding of com-plex atmospheric processes, particularlythose that involve short lived, low concen-tration species.Perhaps one of the most interesting appli-

cations of mass independent chemistry hasbeen its amplification of understanding anddetecting the rise and evolution of oxygen inthe Earth’s atmosphere and the involvementof biological processes. For more than a halfcentury, a quantitative record of the rise of

Fig. 4. A pictorial representation of the equatorial uplift of sulfur species, with ozone oxidation and transport to theSouth Pole as shown in Shaheen el al, (12). The multi-oxygen isotopic composition of sulfates from a high temporalresolution snow pit at the South Pole captures equatorial upper tropospheric-lower stratospheric ozone oxidationassociated with the El Nino Southern Hemispheric Oscillation. This climate record is faithfully captured in the massindependent isotopes and could not have been recognized by any other technique. Details of this record and volcanicperturbations in the same time period are discussed in detail in ref. 12.

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oxygen in the Earth’s atmosphere has beensought. Ironically, the best record of oxygenin the Earth’s earliest environment was cap-tured in the mass-independent isotopic com-position of sulfur isotopes (61). It was shownthat mass-independent sulfur isotopic com-positions are recorded in both sulfate andsulfide in a suite of the oldest rocks on earthuntil ∼2.1 × 109 y ago. Laboratory experi-ments and modeling efforts (61–65) demon-strated that the effect is reproduced in thephotolysis of SO2 in the UV region. Nor-mally, this does not occur in the presentearth’s atmosphere because UV light is re-quired to produce the laboratory observedMIF isotope effect, and it is not available inthe troposphere due to stratospheric ozoneabsorption of UV light. The observation ofsulfur isotopic anomalies in the geologic re-cord is due to the lowered O2-O3 levels thatallow penetration of UV light to the tropo-sphere permitting SO2 photolysis. There nowexists a massive dataset of observations ofboth negative and positive sulfur isotopicanomalies in the Archean, which is displayedin Fig. 5. This record has facilitated studies onthe origin and evolution of oxygen on Earthand provided information sought for nearlya half century. With the extended studies us-ing mass-independent sulfur isotopic compo-sition observations of the geochemical record

(oxidized and reduced sulfur species), de-tailed theoretical and experiment inves-tigations and linkage to biological studieshave facilitated advances in evolutionary bio-geochemical systems. The photochemistry ofSO2 is complex, and modeling of the isotopeeffects, especially in the atmosphere, is chal-lenging. In this issue, a unique photochemicalphotoexcitation mechanism that occurs in thestratosphere is presented (11). In Whitehillet al. (18), a creative series of experimentsand trapping the photoexcited sulfur dioxidea new mass independent isotopic signaturepattern was observed. The results imply thatthe effect is not from the excitation processbut rather from isotope selective spin-orbitselection. The experiments demonstrate thecomplexity of photo processes and the possi-ble importance in the early earth. The effectof oxidation reaction on the photochemistryis discussed. From the measurements and as-sociated analysis, deeper insight into ice coreisotopically anomalous sulfate records thatare used to track massive volcanic eruptions(65) and during the Archean (66–68) hasbeen provided. The role of OH oxidation isshown to be particularly important in con-trolling the magnitude of the sulfur isotopicanomaly, especially during volcanic events.The enhanced understanding of the com-bined oxidation mechanisms provides a test

of the sensitivities of parameters, includingphotolysis altitude. The linkage between bi-ology and the production and preservation ofthe mass independent sulfur isotopic compo-sition is mediated by coupling of the geologicand oceanic processes. Quantifying this asso-ciation is difficult; however, it is vital in inter-preting the global mass-independent sulfurisotopic record. In this special issue, ref. 9reports a model that quantifies linkages ofthe biogeochemical reservoirs. From analysisof the sulfur and iron cycles, constraints onthe atmospheric sulfur photochemical cycleare placed, and the magnitude of the MIFsignal following sulfur delivery to the oceansis better interpreted, particularly the frac-tionation associated with sedimentary pres-ervation. The model results suggest thatcloser high-resolution isotopic studies ofpyrites from shallow water environmentswould be fruitful and further understandingof the sulfur cycle in the early earth.One of the most striking aspects of the

Neoarchean is the observation that the at-mospheric record has survived for billion yeartimes scales and, despite mantle recycling, ispreserved. The issue of preservation is ad-dressed in ref. 8, specifically how the MIFsignal may be maintained with the interven-tion of biological sulfate reduction over theoriginal atmospheric mass-independent sig-nature. The key to preservation is the needfor at least two simultaneous oceanic basinsfor sedimentary pyrite. In one case, solublesulfate permits uptake by biological organ-isms, sulfate reduction, and synthesis ofisotopically normal pyrite. In a separatereservoir, insoluble sulfate is inorganicallyconverted to pyrite with preservation of theatmospheric MIF signal (8). Mineral levelsulfur isotopic analysis using a secondaryion MS (SIMS) has allowed data to be ac-quired on single grains of pyrite to provideidentification of different populations of pyritecomposition. The measurement of individualmineral phase sulfur isotopic compositionhas provided enriched understanding of bio-geochemical evolution.The field of chemical mass-independent

chemistry began with the observation that inthe production of ozone, an identical mass-independent isotopic composition to mete-oritic high-temperature calcium aluminum-rich inclusions is included. It is commonlyheld that chemical processes, photochemicalor chemical reactions, or both may be re-sponsible for meteoritic oxygen isotopicanomalies. A deeper understanding of therelevant physical chemistry of photochem-ical processes enhances resolution of neb-ular processes. Meteoritic sulfur possessesmass-independent isotopic compositions,

-4

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33S

Sample Age (Millions of years)

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Fig. 5. A plot of the mass independent sulfur isotopic composition of sulfides and sulfate as a function of time as firstdiscovered by Farquhar et al. (61). The positive values are nearly always associated with politic and psammitic rocks, andthe negative with barite (61–64).The presence of nonzero values is due to reduced atmospheric O2-O3 levels, which allowsfor tropospheric UV photolysis of SO2, which has been shown in laboratory experiments to be mass independent. Furthersupporting evidence is the disappearance of the isotopic anomaly at ∼2 billion years ago during the time associated witha global oxygenation event and development of the protective ozone layer. Various aspects of the physical chemistry ofthe process, biological interactions, and the preservation and new interpretations are provided in contributions to thisspecial issue (8, 9, 11). The data are a collection of published data kindly provided by James Farquhar.

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including in organic molecules (70), and aresuggested as arising from nebular photo-chemistry. There is a large database for SO2

UV photolysis; due to experimental limi-tations, there are no short (vacuum) UVphotolysis experiments relevant in the earlysolar system. To address the lack of photo-chemical data and explore the role of specificelectronic states on photodecompositionand isotopic fractionationprocess, synchro-tron experiments have been performed atnarrow short UV wavelength bands, andthe products have been isotopically analyzed.In ref. 10, vacuum UV photolytic decompo-sition of H2S, a dominant sulfur species in theearly solar system was performed using theAdvanced Light Source synchrotron facility(Lawrence Berkeley Laboratory). Wavelength-dependent mass-independent isotopic frac-tionations are observed and are presumablyassociated with resonance-assisted curvecrossing dynamics. The results are discussedin the context of the photochemistry and ap-plication to meteoritic isotopic anomalies.These unique results emphasize that thefield of mass-independent chemistry ad-vances from concomitant understanding

of fundamental physical chemistry andoccurrence in nature.Mass-independent isotope compositions

occur in heavy elements, including Ba, Ca,Sr, Ti, Cr, and Hg (5). A special type of mass-independent isotope effect observed in heavyelement chemistry known as the nuclear fieldeffect (71, 72) has been suggested as a poten-tial source of isotopic anomalies in meteoriticmaterials generally considered to be nucleo-synthetic (71). This effect arises from an oddmass isotope effect observed in uranium (73,74) and is suggested as being important ingeo- and cosmochemical environments . Thechemical basis is that in heavy element ther-modynamic processes, nuclear field shifts be-come important as a consequence of effectson the electronic shape and size due toslight distortions from the interaction of theelectron with the nucleus and the finiteprobability of a nonzero electron densitywithin the nucleus (contact density). This

effect occurs in odd number nuclei wherea nuclear magnetic moment exists and pro-duces a smaller electronic size. In ref. 75,the basic chemical physics of the nuclearvolume effect, specifically for crystals, hasbeen treated using density functional theoryand applying a projector augmented wavemethod (DFT-PAW). This technique hasthe advantage in its computational ability,and the results are contrasted against othertechniques that estimate nuclear volume. Us-ing this computational technique, the nuclearvolume effect is applied toward under-standing the vapor-crystal isotope fraction-ation for cadmium and mercury wherethere exists a high-quality dataset. The re-sults provide a unique and powerful com-putational technique for detailing isotopefractionations that derive from nuclear fieldshift–associated phenomena. The work hasallowed comparison with a variety of othermeasurements, including Mossbauer andmass spectrometry.

ConclusionsSince the discovery of chemically producedmass independent isotope effects, a widerange of applications have emerged, rangingfrom basic physical chemistry studies, toobservations in the atmosphere, planetaryformational processes, the origin and evolu-tion of oxygen on Earth, and paleo climatol-ogy. Fig. 6 is a plot of atmospheric species onEarth illustrating the range of oxygen isotopiccompositions is large and that all oxygen-bearing atmospheric molecules are mass in-dependent. In each case, specific insight intonatural processes has been provided withobservation of the mass independent isotopiccompositions. Contributions to this specialvolume present a wide array of new applica-tions. There will be new measurements madeon other molecular species, terrestrial andextraterrestrial, and coupled with develop-ments in basic physical chemistry (theory andexperiment) and modeling, will continue toexpand our understanding of nature.

ACKNOWLEDGMENTS. The National Science Foundation(Atmospheric Chemistry and Polar Programs) is gratefullyacknowledged for their support that allowed the initiation ofmany new measurements the Earth’s atmosphere, presentand past, that have facilitated the opening of many newapplications. The National Aeronautics and Space Adminis-tration (Cosmochemistry and Origins of Solar Systems) sup-ported the meteorite and synchrotron experiments that havedeepened our understanding of the origin of the solar sys-tem and planetary atmospheres.

1 Urey H (1947) The thermodynamics of isotopic substances. J Chem

Soc Lond 47(1):562–5681.2 Bigelesien J, Mayer M (1947) Calculation of equilibrium constants

for isotopic exchange reactions. J Chem Phys 15(5):261–267.

3 Nier AO (1947) A mass spectrometer for isotope and gas analysis.

Rev Sci Instrum 18(6):398–411.4 Thiemens MH (2006) History and applications of mass-independent

isotope effects. Annu Rev Earth Planet Sci 34:217–262.

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Fig. 6. A plot of the oxygen isotopic composition of a variety of atmospheric species, including gaseous, aerosol sulfateand nitrate, and rain water peroxide, The small, but significantm negative mass independent isotopic composition ofmolecular oxygen is not observable in this expanded scale, but by mass, is the largest terrestrial mass independent isotopicreservoir. All oxygen atmospheric species measured to date are mass independent in composition. The ozone isotopiccomposition has been normalized to air oxygen at the intersection of the slope 0.5 and 1.0 slopes.

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5 Thiemens MH, Chakraborty S, Dominguez G (2012) The physicalchemistry of mass-independent isotope effects and their observationin nature. Annu Rev Phys Chem 63:155–177.6 Thiemens MH, Shaheen R (2013) Mass independent isotopiccomposition of terrestrial and extraterrestrial materials. Treatise onGeochemistry. The Atmosphere (Elsevier, New York).7 Hulston JR, Thode HG (1965) Variations in S33, S34 and S36

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53 Mauersberger K, Krankowsky D, Janssen C (2003) Oxygen

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