Introduction to Chemistry and Applications in Nature of Mass Independent Isotope Effects Special Feature

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SPECIAL FEATURE: INTRODUCTION INTRODUCTION SPECIAL FEATURE: Introduction to Chemistry and Applications in Nature of Mass Independent Isotope Effects Special Feature

Mark 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 between isotopes. New classes of isotope variation effects that deviate from mass dependent laws, termed mass independent isotope effects, were discovered in 1983 and have a wide range of applications in basic chemistry and nature. In this special edition, new applications of these effects to physical chemistry, solar system origin models, terrestrial atmospheric and biogenic evolution, polar paleo climatology, snowball earth geology, and present day atmospheric sciences are presented.

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

The use of stable isotope measurements has in early earth minerals allows recognition of the relevant molecular species. The variation a long history of applications in physics and theoriginandevolutionofbiologicalpro- in the coefficient is used to evaluate various chemistry dating back to the discovery of cesses on Earth. processes and is discussed in several of the isotopes themselves. Applications include In this special issue, isotope ratio measure- contributed papers. processes that occur on Earth and in space, ments are reported for a range of molecules The first application of multi-isotope mea- present and past, and their interpretation in experiments and natural systems. By surements to detect mass-independent mea- has been enriched by new theories and convention, the delta notation, is used, which surements was for sulfur (7) to identify measurements of the fundamental physical for oxygen is nuclear processes, such as cosmic ray spall- chemistry. Application of relevant physical ÂÀ Á À Á ation effects in meteorites. The applications δ18Oðper milÞ= O18=O16 sample= O18=O16 have subsequently expanded, particularly chemical laws to high precision isotope Ã ratio measurements of controlled labora- standard 1 × 1; 000: to oxygen isotopes, and in this special issue, tory chemical and photochemical experi- many unique uses of both oxygen and sul- ments develops a basis for interpreting fur isotopes are presented (8–18). For oxygen, the standard is standard mean measurements of a wide range of natural To identify mass-independent processes, ocean water (SMOW). Stable isotope ratio samples and enhances interpretations. His- two or more stable isotope ratios are re- measurements are typically reported in this torically, development of thermodynamic quired. It was assumed by ref. 7 that for manner, and a similar isotope ratio is re- fl theory for quantifying stable isotope varia- meteorites, this deviation must re ect a nu- ported for δ17O. tions (1, 2) and the ability to measure these clear process because no chemical process In general, isotope ratio alterations are variations in natural samples via isotope alters isotope ratios in a manner indepen- attributable to conventional thermodynamic, ratio mass spectrometry (3) lead to deeper dent of mass. It was shown that cosmic ray kinetic, translational, and gravitational phe- spallation of iron and nickel by high energy understanding of natural phenomena. nomena (4–6). When changes are expressed Measurement of oxygen isotope ratios in galactic cosmic rays over billion year time in the delta notation, it is found that these 33 36 various geological materials has provided scales produces excess S and S. Oxygen different processes share the common feature isotopic measurements of calcium-alu- the ability to follow temperature variations that they are all ultimately related to differ- ’ ’ minum rich inclusions (CAIs) from the of the world s oceans over million years ences in mass and in general: time scales, determine igneous rock crystal- Allende meteorite revealed a deviation ’ from mass dependence (19). Fig. 1 sche- lization temperatures, and track the earth s Δ17O = δ17O − 0:5δ18O: hydrologic cycle, present and past (4–6). matically displays a collection of meteorite measurements including the relation of Isotope ratio measurements of ice core This relation arises from the mass depen- 17 18 δ O = δ O for the CAIs. Based on the water has been the primary means by which dence of isotope alteration processes, e.g., assumption that chemical processes can- temperature variations are determined. a process that varies the δ18Oby10per not produce an isotopic composition that Quantifying the transfer of carbon between mil (mass difference of two); the associated does obey mass relations, it was concluded the Earth’s atmospheric and geological, bio- δ17O varies by a factor of approximately half that the data must be from a nuclear source, logical, and hydrological systems is followed that (5 per mil, for the mass difference of 1). 16 specifically addition of pure Ofrom using carbon isotopes. Greenhouse gas sour- A mass-dependent process by definition has supernovae (19). ces and transport are recognized by isotopic Δ17O = 0, and mass independent is nonzero measurements. Meteorite isotope measure- and is referred to as a mass-independent frac- ments provide details of the origin and evo- tionation (MIF). The coefficient varies be- Author contributions: M.H.T. wrote the paper. lution of the solar system. The observation tween 0.50 and 0.53 and is dependent on The author declares no conflict of interest. of isotope ratios of terrestrial sulfur isotopes the specific chemical process and the mass of 1E-mail: [email protected].

www.pnas.org/cgi/doi/10.1073/pnas.1312926110 PNAS | October 29, 2013 | vol. 110 | no. 44 | 17631–17637 Downloaded by guest on September 27, 2021 structure of the ozone molecule on the ozone rate of formation, as well as pressure (27) and temperature (28). A full treatment of the chemical physics of the ozone formation process has been developed in refs. 29–33, using a modified statistical Rice– Ramsperger–Kassel–Marcus (RRKM) model for the recombination process. A key aspect was adoption of a new nonstatistical factor, termed the “η” effect that incorpo- rates non-RRKM factors associated with the symmetry of the isotopically substituted isotopic species. In this special issue, Mar- cus (34) extends this work to fold in the effect of low pressure phenomena. At low pressures (less than the Lindemann fall-off region), the isotope enrichment decreases + well ahead of the expected O O2 drop off and is directly treated (34). A potential source of this behavior is attributed to iso- topomeric symmetry of overlapping resonances leading to a localized chaotic behavior, and potential testable experiments are suggested. Alternate mechanisms have been developed (35–37) based on detailed inclusion of the potential energy surfaces of ozone, including the energy barrier region. A full quantum level treatment of the molecule- atom scattering process is used with a coupled channel model that allows incorpora- Fig. 1. Oxygen three-isotope plot of meteoritic and lunar materials. The slope 1 line was originally proposed to be tion of all states and their coordinates. nucleosynthetic and is now regarded as a mass independent chemical process. Figure from ref. 76. The model includes resonances among the metastable states and the role of isotopic symmetry. In a contribution to this issue The basic assumption that a chemical demonstrated the presence of mass-indepen- (38), the formation and stabilization steps process may not produce a mass-independent atmospheric ozone that were consistent are treated using mixed quantum/classical dent isotopic composition was later experi- with experimental observations (22). theory incurred during collisional energy mentally demonstrated to be incorrect (20) Despite the simple isotopic fractionation transfer and passage of ro-vibrational en- and initiated the field of mass-independent pattern in the ozone formation process ergy. The involvement of scattering reso- isotopic chemistry and its applications. It shown in Fig. 2, defining the basic physical nances and lifetime dependencies on ro- 17 18 was shown that the identical δ O/δ Oratio chemical mechanism responsible for the tational excitation, isotopic asymmetry, observed in CAI is produced during the for- process remains elusive (5). An early model and the connection between differing reac- mation of ozone from dissociation of molec- attributed the isotope effect to the metasta- tion channels are included in the model. ular oxygen. Fig. 2 shows that ozone is ble transition state that arises immediately After the unsuccessful first attempts to 17 18 produced with equal O, Oenrichments. following the oxygen atom-molecule colli- measure the isotopic composition of strato- Thiemens and Heidenreich (20) interpreted sion and its subsequent stabilization lead- spheric ozone via in situ mass spectrometry, this as arising from isotopic self-shielding on ing to stable ozone (23). In general, this subsequent return sample measurements (39) O2, creating a fractionation pattern depen- stabilization process is determined by the revealed ozone isotopic compositions consis- dent on abundance rather than mass. Isoto- lifetime of the metastable species, which is tent with experimental observations of ozone picshieldingbyCOinthesolarnebulamight a function of a wide variety of parameters. formation (40–42). There now exists a sub- was suggested as a means to produce the It was suggested that the isotopic selective stantial database of stratospheric ozone meas- observed meteoritic oxygen isotopic anoma- factor arises from the different number urements (42, 43). An important application lies. Early measurements of the 18O/16O of states for asymmetric 16O16O18O, of atmospheric ozone isotopes takes advan- ratios in stratospheric ozone (21) suggested 16O16O17O, compared with the symmetric tage of the unique, identifiable isotopic com- large heavy isotopic enrichment; however, 16O16O16O species resulting in an increased position of ozone and the ability to trace it they only measured a single isotope ratio probability of stabilization for the asym- through differing chemical reaction channels. and at least two are required to demonstrate metric species. There have been numerous It was suggested that the product of ozone a mass-independent process. A later reanal- experiments directed toward identifying dissociation, electronically excited atomic ox- ysis of the data led to the conclusion that the source of the effect, with isotopically ygen (O1D), may interact with carbon diox- these data are unreliable and should be dis- labeled ozone being particularly insightful ide forming an excited CO3 transition state regarded, and subsequent return samples (24–30) in defining the role of the isotopic that dissociates to ground state O (3P) and

17632 | www.pnas.org/cgi/doi/10.1073/pnas.1312926110 Thiemens Downloaded by guest on September 27, 2021 80 productivity may be measured from the very INTRODUCTION small, but signiﬁcant, variation from mass SPECIAL FEATURE: dependence in atmospheric O (57, 58). In 60 2 OZONE this issue, measurements of the triple oxygen isotopic composition of barites from a post- 40 Minoan (635 Ma) dolostone sequence at Slope = 1.0

O (‰) Wushanhu,inSouthernChina(16),have

17 been shown to deﬁne the global biogeochem- 20 ical system during a time period after the TFL: Slope = 0.5 snowball earth period where this oxygen- 0 ozone-carbon dioxide coupling is particularly -80 -40 0 40 80 perturbed. During this time period, global 18O (‰) glaciation occurred, extending to low latitude -20 equatorial regions and creating a snowball earth due to the drawdown of atmospheric carbon dioxide during intense geochemical -40 weathering processes and producing a global temperature decrease. In a remarkable obser- OXYGEN -60 vation (59), a negative Δ17Oanomalyinbar- ite has been found in post-Minoan glacial diamictites at the time of the global melt- -80 down of the global glacial ice cover (16, 59, Δ17 Fig. 2. Experimental results of Thiemens and Heidenreich (20) demonstrating a mass process in the formation of 60). The existence of a negative Ovalueis ozone along a slope one line, identically mimicking CAI in meteorites as shown in Fig. 1. interpreted as deriving from enhanced CO2 levels with elevated interaction with ozone photochemistry, which ampliﬁes the negative

CO2 (44,45).Duringexchange,theozone would further amplify understanding of effect on O2. It is suggested that the CO2 isotopic signature is transferred to CO2 and the contributing factors. In this issue (17), levels may have exceeded 25,000 ppm to ac- serves as a measure of stratospheric ozone a study of CO2 photolysis in the actinic count for the large barite negative isotopic levels and degree of chemical interaction with region 150–210 nm has documented the anomaly. This observation represents one of electronically excited atomic oxygen (44, 45). photo physical dynamics occurring in the the largest historical excursions in CO2 levels, From theoretical and experimental studies, Earth’s mesosphere that influence overall and confirmation of the process and under- CO2 isotopic measurements have provided the upper atmospheric (stratosphere and standingofitsextentinamountandtimeare insight into the dynamics of the strato- mesosphere) isotopic cycle of CO2-O3-O2, vital, particularly because this time period is sphere–troposphere exchange (45–53) and suggested as being important (17, 56). associated with a period of major biodiversity a measure of upper atmospheric oxygen The O2-CO2-O3 isotopic interaction pro- expansion. A key aspect of this unique con- radical processes (54–56). Using balloons videsameansbywhichgrossprimary tribution is defining the time of the event and National Aeronautics and Space Ad- ministration (NASA) ER-2 aircraft, CO2 samples were collected in the winter of 1999–2000, as well as a 2004 balloon flight across an altitudinal and latitudinal range of the Arctic Polar vortex (15). As reported in this special issue, laboratory measurements coupled with atmospheric modeling may account for the observed CO2 isotopic composition (15). Fig. 3 reveals the wide range in global variations in upper atmospheric CO2 oxygen isotopes. The model and measurements reveal that the compositions are unlikely to be due to artifacts in measurement and resolve the complexity of upper atmospheric oxygen photochemistry and dynamics. A range in isotopic mass independence is observed that varies with latitude, altitude, and time (15) and is partly attributed to the position within and outside of the polar vortex, enhancing Fig. 3. A three-isotope oxygen isotopic plot of a wide range of stratospheric carbon dioxide samples. The samples understanding of polar upper atmospheric have been obtained by balloon, stratospheric aircraft, and rocket borne whole air sampling. Plot taken from Wiegel oxidative processes. The work suggests that et al. (15). The paper discusses the utility of understanding both the oxygen chemistry of the upper atmosphere as well further measurements in tropical regions as stratosphere-troposphere dynamics.

Thiemens PNAS | October 29, 2013 | vol. 110 | no. 44 | 17633 Downloaded by guest on September 27, 2021 trace gases along with their oxidative chemical processing. Along with the unique ENSO hemispheric record, the snow pit sulfate data also detected the El Chichon, Pinatubo, and Cerro Hudson volcanic eruptions. With the special isotopic character of ozone and its insertion into most atmospheric oxidation processes, it has developed into a useful probe of atmospheric chemical processes. An important aspect is the need to resolve the role of chemically reactive species. A significant amount of the ozone driven chemistry occurs within the tropical Marine Boundary Layer (MBL), where the process is driven by ozone sources [NOx and volatile organic compounds (VOCs)] and inadequately quantified ozone sinks. The total budget of NOx is inadequately known and limits resolution and quantifi- cation of the nitrogen cycle. Contributions from halogens (especially bromine), surface chemistry, and nighttime chemistry are confounding contributors to this complexity. From isotopic measurements of a yearly record of nitrates collected at the Cape Verde Atmospheric Observatory, a unique Fig. 4. A pictorial representation of the equatorial uplift of sulfur species, with ozone oxidation and transport to the insight into MBL chemistry has been South Pole as shown in Shaheen el al, (12). The multi-oxygen isotopic composition of sulfates from a high temporal resolution snow pit at the South Pole captures equatorial upper tropospheric-lower stratospheric ozone oxidation obtained (13). The measured nitrate oxygen associated with the El Nino Southern Hemispheric Oscillation. This climate record is faithfully captured in the mass isotopic variations are shown to fit nitrate independent isotopes and could not have been recognized by any other technique. Details of this record and volcanic chemistry, with bromine chemistry in- perturbations in the same time period are discussed in detail in ref. 12. cluded. With application of two varieties of atmospheric models and inclusion of BrO, NO, NO2,O3,OH, HO2, and dimethyl duration and (16) estimate it to be 0–0.99 on the environment, including perturba- sulfide (DMS) with the Δ17O values, the million years using stable and radiometric tions from volcanoes and El-Nino South- complex chemistry was modeled. The work isotopes. Consequences for global geochem- ern Oscillations (ENSOs). From measure- recognizes that aerosol transport time is ap- istry as a consequence of this duration are mentsofthevariationinpolarsulfateΔ17O proximately several days. More impor- discussed. values, tropical ozone variations in the upper tantly, it was recognized that the role of Mass-independent isotopic compositions troposphere/lower stratosphere have been N2O5 and its reaction products (ClNO2) are observed in a range of molecules, terres- detected at South Pole and are observed to are likely insignificant in this environment, trial and extraterrestrial. Mass-independent be linked to the three largest ENSO events of and high HNO3 production rates from fi composition of water is reported in samples the past 20 y. Fig. 4 schematically depicts the N2O5 hydrolysis is unlikely to be signi - collected at Vostok, Antarctica (14). This overall process, initiating with atmospheric cant. The role of halogens in the MBL is signature captures stratosphere–troposphere upwelling driven at the equatorial Intertrop- consequently better established with the mixing and a stratospheric water source. The ical Convergence Zone (ITCZ) regions dur- unique isotopic measurements facilitat- anomaly derives from methane oxidation as ing ENSO events. It is observed that the ing diagnosis. Bromine nitrate is a sig- part of the stratospheric ozone cycle (14). sulfate anomaly coherently tracks with the nificant sink, at a level of ∼20% of total Normally this magnitude would be too low Ozone ENSO Index (OEI) obtained from nitrate formation. The work illustrates that to be recognized; however, the Vostok area tropical latitude ozone satellite measure- high precision multi-isotope ratio meas- is characterized by extremely low atmo- ments. The El Nino OEI is thought to arise urements coupled with appropriate con- spheric water content and accumulation rate, from variation of the tropopause height, itself centration measurements and modeling allowing the stratospheric water to be distin- a consequence of deviations of tropical deep provide a deeper understanding of com- guished. The measurements permits a mea- convection and Brewer-Dobson circulation plex atmospheric processes, particularly sure of stratosphere–troposphere mixing in (12). As a consequence, an enhanced en- those that involve short lived, low concen- Antarctica and enhanced understanding of trance to the stratosphere within the ITCZ tration species. the role of anthropogenic methane sources is created, and tropospheric air enters the Perhaps one of the most interesting appli- on the global atmospheric water cycle. stratosphere and migrates pole ward as cations of mass independent chemistry has Unique ice core sulfate measurements are showninFig.4,allowingtheElNinopertur- been its amplification of understanding and reported in this issue (12) for samples re- bation to be recognized in Antarctic ice core detecting the rise and evolution of oxygen in trieved from a high-resolution 22-y (1980– samples. These measurements show that the the Earth’s atmosphere and the involvement 2002) snow pit at the South Pole. The oxygen sulfate oxygen isotopic measurements may be of biological processes. For more than a half isotopic variations record specific effects used to hemispherically track aerosol and century, a quantitative record of the rise of

17634 | www.pnas.org/cgi/doi/10.1073/pnas.1312926110 Thiemens Downloaded by guest on September 27, 2021 12 of the sensitivities of parameters, including INTRODUCTION SPECIAL FEATURE: Younger than ~2.4 Ga photolysis altitude. The linkage between bi- Older than ~2.4 Ga 10 ology and the production and preservation of the mass independent sulfur isotopic composition is mediated by coupling of the geologic 8 and oceanic processes. Quantifying this asso- ciation is difﬁcult; however, it is vital in inter- 6 preting the global mass-independent sulfur isotopic record. In this special issue, ref. 9 S ﬁ

33 4 reports a model that quanti es linkages of the biogeochemical reservoirs. From analysis 2 of the sulfur and iron cycles, constraints on the atmospheric sulfur photochemical cycle are placed, and the magnitude of the MIF 0 signal following sulfur delivery to the oceans is better interpreted, particularly the frac- -2 tionation associated with sedimentary preservation. The model results suggest that -4 closer high-resolution isotopic studies of 0 1000 2000 3000 4000 pyrites from shallow water environments would be fruitful and further understanding Sample Age (Millions of years) ofthesulfurcycleintheearlyearth. Fig. 5. A plot of the mass independent sulfur isotopic composition of sulfides and sulfate as a function of time as first One of the most striking aspects of the discovered by Farquhar et al. (61). The positive values are nearly always associated with politic and psammitic rocks, and Neoarchean is the observation that the at- – the negative with barite (61 64).The presence of nonzero values is due to reduced atmospheric O2-O3 levels, which allows mospheric record has survived for billion year for tropospheric UV photolysis of SO2, which has been shown in laboratory experiments to be mass independent. Further times scales and, despite mantle recycling, is supporting evidence is the disappearance of the isotopic anomaly at ∼2 billion years ago during the time associated with a global oxygenation event and development of the protective ozone layer. Various aspects of the physical chemistry of preserved. The issue of preservation is ad- the process, biological interactions, and the preservation and new interpretations are provided in contributions to this dressed in ref. 8, specifically how the MIF special issue (8, 9, 11). The data are a collection of published data kindly provided by James Farquhar. signal may be maintained with the interven- tion of biological sulfate reduction over the oxygen in the Earth’s atmosphere has been (oxidized and reduced sulfur species), de- original atmospheric mass-independent sig- sought. Ironically, the best record of oxygen tailed theoretical and experiment inves- nature. The key to preservation is the need in the Earth’s earliest environment was cap- tigations and linkage to biological studies for at least two simultaneous oceanic basins tured in the mass-independent isotopic com- have facilitated advances in evolutionary bio- for sedimentary pyrite. In one case, soluble position of sulfur isotopes (61). It was shown geochemical systems. The photochemistry of sulfate permits uptake by biological organ- isms, sulfate reduction, and synthesis of that mass-independent sulfur isotopic com- SO2 is complex, and modeling of the isotope positions are recorded in both sulfate and effects, especially in the atmosphere, is chal- isotopically normal pyrite. In a separate sulfide in a suite of the oldest rocks on earth lenging. In this issue, a unique photochemical reservoir, insoluble sulfate is inorganically 9 converted to pyrite with preservation of the until ∼2.1 × 10 y ago. Laboratory experi- photoexcitation mechanism that occurs in the atmospheric MIF signal (8). Mineral level ments and modeling efforts (61–65) demon- stratosphere is presented (11). In Whitehill sulfur isotopic analysis using a secondary strated that the effect is reproduced in the et al. (18), a creative series of experiments ion MS (SIMS) has allowed data to be ac- photolysis of SO in the UV region. Nor- and trapping the photoexcited sulfur dioxide 2 quired on single grains of pyrite to provide mally, this does not occur in the present a new mass independent isotopic signature fi ’ identi cation of different populations of pyrite earth s atmosphere because UV light is re- pattern was observed. The results imply that composition. The measurement of individual quired to produce the laboratory observed the effect is not from the excitation process mineral phase sulfur isotopic composition MIF isotope effect, and it is not available in but rather from isotope selective spin-orbit has provided enriched understanding of bio- the troposphere due to stratospheric ozone selection. The experiments demonstrate the geochemical evolution. absorption of UV light. The observation of complexity of photo processes and the possi- The field of chemical mass-independent sulfur isotopic anomalies in the geologic re- bleimportanceintheearlyearth.Theeffect chemistry began with the observation that in cord is due to the lowered O2-O3 levels that of oxidation reaction on the photochemistry theproductionofozone,anidenticalmass- allow penetration of UV light to the tropo- is discussed. From the measurements and as- independent isotopic composition to mete- sphere permitting SO2 photolysis. There now sociated analysis, deeper insight into ice core oritic high-temperature calcium aluminum- exists a massive dataset of observations of isotopically anomalous sulfate records that rich inclusions is included. It is commonly both negative and positive sulfur isotopic are used to track massive volcanic eruptions held that chemical processes, photochemical anomalies in the Archean, which is displayed (65) and during the Archean (66–68) has or chemical reactions, or both may be re- in Fig. 5. This record has facilitated studies on been provided. The role of OH oxidation is sponsible for meteoritic oxygen isotopic the origin and evolution of oxygen on Earth shown to be particularly important in con- anomalies. A deeper understanding of the andprovidedinformationsoughtfornearly trolling the magnitude of the sulfur isotopic relevant physical chemistry of photochem- a half century. With the extended studies us- anomaly, especially during volcanic events. ical processes enhances resolution of neb- ing mass-independent sulfur isotopic compo- The enhanced understanding of the com- ular processes. Meteoritic sulfur possesses sition observations of the geochemical record bined oxidation mechanisms provides a test mass-independent isotopic compositions,

Thiemens PNAS | October 29, 2013 | vol. 110 | no. 44 | 17635 Downloaded by guest on September 27, 2021 120 effect occurs in odd number nuclei where Atmospheric oxygen Slope = 1.0 a nuclear magnetic moment exists and produces a smaller electronic size. In ref. 75, Tropo-Ozone the basic chemical physics of the nuclear 100 Strato-Ozone volume effect, speciﬁcally for crystals, has Tropo-CO2 been treated using density functional theory and applying a projector augmented wave Strato-CO2 80 method (DFT-PAW). This technique has Nitrate Aerosols the advantage in its computational ability, Sulfate Aerosols and the results are contrasted against other techniques that estimate nuclear volume. Us- Strato-N2O 60 ing this computational technique, the nuclear Atmos-H2O2 volume effect is applied toward under- Atmospheric-CO standing the vapor-crystal isotope fraction-

O (‰, SMOW) O (‰, ation for cadmium and mercury where

17 40 Carbonate Aerosols

δ there exists a high-quality dataset. The re- TFL: Slope = 0.5 sults provide a unique and powerful computational technique for detailing isotope fi 20 fractionations that derive from nuclear eld shift–associated phenomena. The work has allowed comparison with a variety of other measurements, including Mossbauer and 0 mass spectrometry. 0 20406080100120Conclusions Since the discovery of chemically produced δ18O (‰, SMOW) mass independent isotope effects, a wide range of applications have emerged, ranging Fig. 6. A plot of the oxygen isotopic composition of a variety of atmospheric species, including gaseous, aerosol sulfate from basic physical chemistry studies, to and nitrate, and rain water peroxide, The small, but significantm negative mass independent isotopic composition of observations in the atmosphere, planetary molecular oxygen is not observable in this expanded scale, but by mass, is the largest terrestrial mass independent isotopic reservoir. All oxygen atmospheric species measured to date are mass independent in composition. The ozone isotopic formational processes, the origin and evolu- composition has been normalized to air oxygen at the intersection of the slope 0.5 and 1.0 slopes. tion of oxygen on Earth, and paleo climatology. Fig. 6 is a plot of atmospheric species on Earth illustrating the range of oxygen isotopic including in organic molecules (70), and are of fundamental physical chemistry and compositions is large and that all oxygen- suggested as arising from nebular photo- occurrence in nature. bearing atmospheric molecules are mass in- chemistry. There is a large database for SO2 Mass-independent isotope compositions dependent. In each case, specific insight into UV photolysis; due to experimental limi- occur in heavy elements, including Ba, Ca, natural processes has been provided with tations, there are no short (vacuum) UV Sr, Ti, Cr, and Hg (5). A special type of mass- observation of the mass independent isotopic photolysis experiments relevant in the early independent isotope effect observed in heavy compositions. Contributions to this special solar system. To address the lack of photo- element chemistry known as the nuclear field volume present a wide array of new applica- chemical data and explore the role of specific effect (71, 72) has been suggested as a poten- tions. There will be new measurements made electronic states on photodecomposition tial source of isotopic anomalies in meteoritic and isotopic fractionationprocess, synchro- materials generally considered to be nucleo- on other molecular species, terrestrial and tron experiments have been performed at synthetic (71). This effect arises from an odd extraterrestrial, and coupled with develop- narrow short UV wavelength bands, and mass isotope effect observed in uranium (73, ments in basic physical chemistry (theory and the products have been isotopically analyzed. 74) and is suggested as being important in experiment) and modeling, will continue to In ref. 10, vacuum UV photolytic decompo- geo- and cosmochemical environments . The expand our understanding of nature. sition of H2S, a dominant sulfur species in the chemical basis is that in heavy element ther- fi ACKNOWLEDGMENTS. The National Science Foundation early solar system was performed using the modynamic processes, nuclear eld shifts be- (Atmospheric Chemistry and Polar Programs) is gratefully Advanced Light Source synchrotron facility come important as a consequence of effects acknowledged for their support that allowed the initiation of many new measurements the Earth’s atmosphere, present (Lawrence Berkeley Laboratory). Wavelength- on the electronic shape and size due to and past, that have facilitated the opening of many new dependent mass-independent isotopic frac- slight distortions from the interaction of the applications. The National Aeronautics and Space Adminis- tionations are observed and are presumably electron with the nucleus and the finite tration (Cosmochemistry and Origins of Solar Systems) sup- ported the meteorite and synchrotron experiments that have associated with resonance-assisted curve probability of a nonzero electron density deepened our understanding of the origin of the solar sys- crossing dynamics. The results are discussed within the nucleus (contact density). This tem and planetary atmospheres. in the context of the photochemistry and application to meteoritic isotopic anomalies. These unique results emphasize that the 1 Urey H (1947) The thermodynamics of isotopic substances. J Chem 3 Nier AO (1947) A mass spectrometer for isotope and gas analysis. fi Soc Lond 47(1):562–5681. Rev Sci Instrum 18(6):398–411. eld of mass-independent chemistry ad- 2 Bigelesien J, Mayer M (1947) Calculation of equilibrium constants 4 Thiemens MH (2006) History and applications of mass-independent vances from concomitant understanding for isotopic exchange reactions. J Chem Phys 15(5):261–267. isotope effects. Annu Rev Earth Planet Sci 34:217–262.

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