Carbonate formation events in ALH 84001 trace theevolution of the Martian atmosphereRobina Shaheena, Paul B. Nilesb,1, Kenneth Chonga,c, Catherine M. Corrigand, and Mark H. Thiemensa

aDepartment of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92122; bAstromaterials Research and Exploration Science, NASAJohnson Space Center, Houston, TX 77058; cDepartment of Chemistry, California State Polytechnic University, Pomona, CA 91768; and dSmithsonian Institution,Washington, DC 20004

Edited by David J. Stevenson, California Institute of Technology, Pasadena, CA, and approved November 21, 2014 (received for review August 16, 2013)

Carbonate minerals provide critical information for defining atmo-sphere–hydrosphere interactions. Carbonate minerals in the Mar-tian meteorite ALH 84001 have been dated to ∼3.9 Ga, and both Cand O-triple isotopes can be used to decipher the planet’s climatehistory. Here we report Δ17O, δ18O, and δ13C data of ALH 84001 ofat least two varieties of carbonates, using a stepped acid dissolu-tion technique paired with ion microprobe analyses to specificallytarget carbonates from distinct formation events and constrain theMartian atmosphere–hydrosphere–geosphere interactions andsurficial aqueous alterations. These results indicate the presence ofa Ca-rich carbonate phase enriched in 18O that formed sometimeafter the primary aqueous event at 3.9 Ga. The phases showedexcess 17O (0.7‰) that captured the atmosphere–regolith chemicalreservoir transfer, as well as CO2, O3, and H2O isotopic interactionsat the time of formation of each specific carbonate. The carbonisotopes preserved in the Ca-rich carbonate phase indicate that theNoachian atmosphere of Mars was substantially depleted in 13Ccompared with the modern atmosphere.

Martian meteorite | oxygen isotope anomaly | aqueous interaction |carbon isotope | photochemistry

Geological evidence suggests that early Mars was sufficientlywarm for liquid water to flow on the surface for at least brief

periods, if not longer (1). Identifying the nature and duration ofwarmer conditions on theMartian surface is one of the key pieces ofinformation for understanding atmosphere–hydrosphere–geosphereinteractions, the evolution of the atmosphere, and potential pasthabitability. A better understanding of the evolution of the Martianatmosphere and, in particular, the behavior of its primary compo-nent, CO2, provides a means for characterizing the nature of theancient Martian environment. The amount of CO2 present in theatmosphere should provide critical insight into the characteristics ofthe Martian climate, with a denser atmosphere being more likely tobe able to support prolonged warmer temperatures (2, 3).The Martian meteorite ALH 84001 is a critical source for

understanding the history of the Martian atmosphere, as it is theoldest known rock (crystallographic age ∼4.09 ± 0.03 Ga) (4),and its carbonate fractions (<1% wt/wt) are considered to havepreserved the carbon isotope signature of the ancient atmosphere∼3.9 Ga ago (5). These carbonates are chemically (Mg-, Ca-, and Fe-Mn rich) and isotopically (δ13CVPDB = 27–64, where VPDB standsfor Vienna Pee Dee Belemnite, and δ18OSMOW = −10–27‰, whereSMOW stands for Standard Mean Ocean Water) heterogeneous onmicrometer scales; carbon and oxygen isotopes show a covariantrelationship that is correlated with Mg content of the mineral (6–8).The exact process responsible for their formation is not clear, al-though low-temperature aqueous precipitation, biogenic production,evaporation, and high-temperature reactions are all candidate pro-cesses (9–13). Decoding the fingerprints of various oxygen-carryingreservoirs on Mars (atmosphere–hydrosphere–geosphere) and howthey interact from δ18O alone is nearly impossible because of thelack of direct information on the isotopic composition of the pri-mary O-carrying reservoirs in the carbonate system (CO2–H2O) andthe extreme variability observed in chemical and isotopic

composition of carbonate minerals in ALH 84001. The O-iso-topic anomaly (Δ17O = δ17O − 0.52 × δ18O) observed in O3, SO4,NO3, CO3, and H2O2 has been successfully used to investigatephysicochemical and photochemical processes in terrestrial andextraterrestrial materials (14–18). In this study, we used five stableisotopes of carbonates (12C, 13C, 16O, 17O, and 18O) on Ca- andFe-rich phases to decipher atmosphere–hydrosphere interactionsand Martian CO2/CO3 geochemical cycling. This high-precisionmulti-O-isotope analysis of secondary minerals was coordinatedwith detailed petrographic and ion microprobe analyses.The primary goal of this study was to specifically identify

carbonate phases from distinct formation events to providebetter understanding of oxygen and carbon reservoirs on Mars.There have been no previous measurements of both carbonisotope and O-triple isotope compositions of the same CO2sample from ALH 84001, and previous measurements of O-tri-ple isotopes did not attempt to use stepped extraction to sepa-rate different carbonate phases (19). To accomplish this goal,a stepped acid dissolution technique was performed to extractCO2 from several portions of ALH 84001. The O-isotopevalues (Δ17O, δ18O) are reported with respect to SMOW, andδ13C of the CO2 gas evolved with respect to V-PDB standard.We also report oxygen isotope SIMS (secondary ion massspectrometer or ion microprobe) analyses coupled with SEMimages of petrographically unusual carbonate phases in the mete-orite, which provide a link between ion microprobe data, pet-rographic relationships, and the multiisotopic high-precision bulkanalyses, allowing placement of further constraints on the alter-ation history of the meteorite.

Significance

Martian meteorite ALH 84001 serves as a witness plate to thehistory of the Martian climate ∼4 Ga ago. This study describesion microprobe δ18O analyses coupled with δ13C, δ18O, and Δ17Oanalyses from stepped acid dissolution of the meteorite thatidentifies a new carbonate phase with distinct isotope com-positions. These new measurements of the oxygen isotopecomposition of carbonates within this meteorite reveal severalepisodes of aqueous activity that were strongly influenced byatmospheric chemistry. When paired with carbon isotopemeasurements, these data suggest that the ancient atmo-sphere of Mars was significantly depleted in 13C compared tothe present day. This implies substantial enrichment in the δ13Cof the atmosphere since the Noachian which may have oc-curred through extensive atmospheric loss.

Author contributions: R.S., P.B.N., C.M.C., and M.H.T. designed research; R.S., K.C., andC.M.C. performed research; R.S. and P.B.N. analyzed data; and R.S., P.B.N., C.M.C., andM.H.T. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.1To whom correspondence should be addressed. Email: paul.b.niles@nasa.gov.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1315615112/-/DCSupplemental.

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ResultsCarbonates in ALH 84001 samples exhibit two striking features:both the Ca-rich and Fe-rich phases are highly enriched in δ13C,and the magnitude of the oxygen isotope anomaly (Δ17O = 0.7‰)is identical in both phases (Fig. 1). The sequential acid extractiontechnique and ion microprobe analyses of ALH 84001 reveal thepresence of distinct carbonate populations of different chemicaland isotopic compositions (Fig. 2 and Table 1). The first set ofmeasurements using the acid extraction technique on ALH84001Bindicated a significant difference in C and O-triple isotopes of CO2released after 12 h at 25 °C (δ13C = 12‰, Δ17O = 0.3‰) and CO2released after 3 h at 150 °C (δ13C = 38‰, Δ17O = 0.6‰). Theseresults suggest a potential mixture between terrestrial andMartian phases; therefore, a 1-h acid dissolution step at 25 °Cwas performed similar to that in ref. 20 on a larger sample of themeteorite (2.012 g; ALH84001C) to remove traces of terrestrialcontamination (0.0079% CO3 by mass) that may have formedduring its residence in Antarctica (∼13,000 y; SI Appendix, SIDiscussion). The fraction of CO2 released from this 1-h reactionshowed a Δ17O = 0.03 ± 0.06‰ that is indistinguishable fromother terrestrial/marine carbonates (20). A further 12-h disso-lution step at 25 °C for sample ALH84001C (0.01% CO3)revealed a chemically and isotopically distinct carbonate phasethat possessed a Δ17O = 0.73 ± 0.05‰ similar to the Δ17Ovalues of the Mg- and Fe-rich carbonates that dissolve at highertemperatures (Δ17O = 0.75 ± 0.05‰). Because this phase wasdissolved after 12 h at only 25 °C, it is certainly dominated by Ca-rich carbonate (21), which possesses a distinct δ13C = 20‰ andδ18O = 25‰ (Fig. 1). The 3-h acid extraction step at 150 °C yieldedCO2 with δ13C = 38‰ and δ18O = 21‰, similar to what has beenpreviously reported for the Fe- and Mg-rich carbonate rosettes (9,22) (Fig. 1).A highly Ca-rich carbonate phase was also identified in thin

section, and ion microprobe measurements revealed a distinctδ18O enrichment that can be linked to the stepped acid extrac-tion results (Fig. 2 and Table 1). In thin section, these Ca-richcarbonates occur as isolated domains interstitial to both silicaglass and a Mg-rich carbonate matrix (Fig. 2B). Neither of thesecarbonate occurrences exhibits typical rosette morphology. TheMg-rich matrix is found adjacent to carbonate slabs, with a sim-ilar compositional range to the classic rosettes, but with an ex-tended zone of Ca-rich carbonates (Fig. 2B). Corrigan andHarvey (23) suggested that these Mg-rich and Ca-rich carbonatespostdate formation of the carbonate slabs and rosettes, based onpetrographic data. In their proposed sequence of events, the slabcarbonates formed first, followed by shock events that resulted inlimited carbonate decomposition. Subsequently, the Mg-rich andCa-rich carbonates formed in a distinct aqueous alteration event.

DiscussionThe combination of high-precision multiisotope analyses, pet-rographic interpretation, and ion microprobe analyses suggestthat the Ca-rich phase measured in the 12-h dissolution ofALH84001C is part of the same population as the Ca-rich phaseidentified in thin section (23) and likely formed on Mars. Chemi-cally, these carbonates are the most Ca-rich of the ALH 84001carbonates measured by ion microprobe (Fig. 2A). Fe and Mgcarbonates are not typically reactive at 25 °C (21); therefore,Ca-rich carbonate is expected to be the main product of a 12-h aciddissolution step (SI Appendix, SI Methods) (21). However, thestepped extraction technique does not totally separate each phase;therefore, we expect there is some minimal mixing with less re-active, more Mg-rich phases. Nevertheless, the oxygen isotopicdifference between the SIMS and dual-inlet isotope ratio massspectrometer (IRMS) measurements of the Ca-rich phase is withinthe variability observed in the previous ion probe measurements ofthe meteorite for a particular carbonate composition (∼10‰), but

clearly distinguishes this particular Ca-rich phase (with higher δ18O)from all other Ca-rich carbonates in the meteorite (Fig. 2A).Multiple ion microprobe studies of the more common car-

bonate rosettes have shown a strong positive correlation betweenMg content and δ18O (7, 8, 24, 25), with all the Ca-rich phaseshaving δ18O values below 10‰ (Fig. 2A). The Ca-rich carbonatephase identified in the 12-h dissolution of sample ALH84001Cand in the ion microprobe analyses (Table 1) does not obey thisrelationship, indicating formation from an aqueous event distinctfrom the event that formed the much more common carbonaterosettes, and thus represents a carbonate phase that has not beendocumented before to our knowledge. The carbon isotopecomposition (δ13C = 20‰) of the Ca-rich phase identified herealso differs from the average composition of the Fe-Mg car-bonate rosettes (δ13C = 38‰) measured during this study andprevious studies (6, 9), although it is closer to some of the lowestδ13C values measured by ion microprobe (Fig. 3) (6). Thecombined carbon and oxygen isotope data show that our steppedacid extraction technique was successful in separating a distinctcarbonate phase from the more common rosettes (Fig. 3). If theCO2 obtained after 12 h acid extraction of sample ALH84001Cwas derived from a mixture of the Fe- and Mg-rich carbonaterosettes and some other phase, it would lie on the end ofa straight line drawn through the red points in Fig. 3. This wouldhave an even lower δ13C composition and higher δ18O values,making it even more distinctive from the rosettes.The significant difference in Δ17O composition between ter-

restrial (Δ17O ∼ 0‰) and Martian carbonates observed in thisstudy (Δ17O ∼ 0.7‰), along with the δ13C, shows the use ofmultiple-isotope analysis for distinguishing carbonate formed onMars and Earth and defining processes that led to their for-mation (Fig. 1). Despite evidence that this CaCO3 phase is distinctfrom the other carbonates in ALH 84001, it contains an iden-tical Δ17O anomaly (within error) to that observed in themore common Fe- and Mg-rich carbonate rosettes (Fig. 1). TheO-isotopic anomaly in carbonate (Δ17O = 0.7‰) is higher

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Fig. 1. Multi C(VPDB)-O(SMOW) triple-isotope analysis of multiple phasesof carbonates in Martian meteorite ALH 84001. Blue points are analysesfrom this study that are interpreted to be Martian on the basis of their highΔ17O values. The red point is the terrestrial contamination from the 1-h acidextraction at 25 °C. The purple point is from a 12-h extraction at 25 °C thatwas not preceded by a 1-h extraction, and thus this is likely a mixture be-tween terrestrial contamination and Martian Ca-rich carbonate (Table 1).The O-isotope anomaly (Δ17O) of earlier carbonate measurements onNakhlites and ALH 84001 Martian meteorites is also shown (26). However,carbon isotopic composition was not measured in these samples, andtherefore these data are shown as horizontal lines. The average bulk silicateof Mars and Earth is also shown for reference.

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compared with the isotopic composition of the silicate (Δ17O =0.36‰) measured on the same slice of ALH84001C, using CO2laser fluorination (26) (SI Appendix, Figs. S1 and S2). The higherO-isotope anomaly in the ALH 84001 carbonate compared withhost rock (Δ17O= 0.4‰) (Table 1) is likely a result of the in-teraction of an anomalous fluid with the anomalous atmosphericCO2 that derived its signal from the atmospheric CO2-O(1D) cycleand, ultimately, ozone, the source of the O-isotope anomaly (19,26) (SI Appendix, SI Discussion). Comparison of the O-triple iso-tope composition of carbonates from ALH 84001 with other car-bonates in Nakhla and Lafayette show a similar O-triple isotopiccomposition that is significantly different from the host silicates(26). The present findings are also similar to other measurementsof secondary minerals in Martian meteorites (SNC Δ17Ocarbonates =0.6–0.9‰ and Δ17Osilicates = 0.2–0.4‰) (26), which containsulfates and water with higher Δ17O anomalies compared withthe host silicate rock (SI Appendix, Fig. S1B). NWA7034 providesan exception to this trend, where Δ17Osilicates = 0.6‰ of the host

rock is higher than that of the secondary minerals Δ17Ocarbonates =0‰ and Δ17Osulfate = 0‰ for presently unknown reasons (20). Ifthe carbonates precipitated from liquid water, it is likely that theancient water reservoir on Mars possessed a significant oxygenisotope anomaly.The isotopic composition of the Ca-rich phase measured in

this study indicates that the Ca-rich phase is likely distinct fromthe more common carbonate rosettes. However, they may stillhave formed from the same carbon reservoir, despite havinga different oxygen isotope composition (Fig. 3). Petrographicrelationships show that the Ca-rich phase postdates the moreabundant Mg- and Fe-rich rosettes and formed in a distinct al-teration event (5, 23, 27). The absolute timing of this later al-teration event is difficult to constrain and may represent eitheran event that occurred soon after the primary population ofcarbonate rosettes formed (∼3.9 Ga) or an even later event,perhaps the one that ejected the meteorite from Mars (∼15 Ma)(28). Recrystallization of carbonate during/after impact (12, 29)

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Fig. 2. (A) Blue points show relationship between the Mg content of carbonates in ALH 84001 and δ18O composition from the collection of published ionmicroprobe analyses from ALH 84001 carbonates (7, 8, 24, 25, 35). Red points show ion microprobe analyses of Ca-rich carbonate identified in this study. Theseare the most Ca-rich carbonates ever studied by ion microprobe in ALH 84001 and are much more enriched in δ18O than other Ca-rich phases. The black lineindicates our best guess at where the 12-h extraction at 25 °C of ALH84001C would plot for comparison with the SIMS data. (B) Carbonate assemblage fromALH 84001,302, shown in a backscattered electron image (taken on the FEI NOVA NanoSEM 600 at the Smithsonian Institution). High-Ca carbonates analyzedfor oxygen isotopes are shown in the enlargement in the upper right corner of B. Ovate features within carbonates are ion microprobe pits. Slab, slab-typecarbonates described in Corrigan and Harvey (23); Fs, feldspathic glass; Cb, mixed carbonate (mostly magnesite); Opx, orthopyroxene.

Table 1. C and O-triple isotope data of carbonate extracted from ALH 84001, using step-wise acid etching technique and ionmicroprobe (SIMS) analyses

Sample Treatment % CO3 by mass CO2 μmole O2 μmole δ13C δ17O′* δ 18O′* Δ17O

ALH84001C 1 h at 25 °C 0.0079 2.69 2.58 6.09 19.16 36.51 0.03ALH84001C** 12 h at 25 °C 0.01 3.45 3.36 20.27 14.14 25.60 0.73ALH84001C† 3 h at 150 °C 0.145 14.56 13.8 38.77 11.99 21.46 0.75ALH84001C‡ Laser fluorination 2.54 4.17 0.36ALH84001A 3 h at 150 °C 0.13 6.08 5.15 37.37 12.59 22.93 0.57ALH84001B** 12 h at 25 °C 0.014 0.735 0.65 11.94 18.33 34.42 0.30ALH84001B† 3 h at 150 °C 0.12 6.09 4.15 38.14 12.17 22.16 0.56ALH_302_9 SIMS — — — — — 17.4 ± 2.0 —

ALH_302_15 SIMS — — — — — 20.3 ± 2.2 —

Three different extractions were performed, labeled ALH84001A, ALH84001B, and ALH84001C. Sample A did not contain sufficient CO2 after 12 h at 25 °C,and the 1-h extraction was not performed. The 1-h extraction was also not performed for sample B, but sufficient sample was recovered at the 12-h time step.The overall uncertainty for the complete procedure (CO2 acid extraction, gas chromatography, fluorination, and isotope analysis), based on repeated samplessimilar in size and composition to the Martian rock samples, is ±0.1‰ for δ13C and ±0.2‰ for 17O and 18O. Statistical variations (1σ SD) using laboratory CO2

standards on IRMS were δ13C = ±0.02‰, δ17O, and δ18O = ±0.1‰. Chemical composition of SIMS spots were measured by Eprobe (SI Appendix, SI Methods):ALH_302_9, Ca90Mg7Fe3; ALH_302_15, Ca99Mg0Fe1.*δiO′= 1000 lnð1+ δiO=1000Þ where iO  =   17O and 18O.**25 °C acid dissolution, which mostly releases CO2 from the Ca-rich phase.†150 °C acid dissolution, which releases CO2 from the Fe-Mg-Mn-rich phase.‡O-triple isotopic composition of the whole rock, using laser fluorination (26).

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would allow it to preserve its interaction with the modern at-mospheric CO2 through a transition stage at higher temperature,as suggested in previous studies, following the reaction sequenceoutlined in Fig. 4. This scheme involves replacement of all C andtwo-thirds of the O atoms in carbonates with modern atmo-spheric CO2 during the recrystallization process (30). Because ofthe large sample required for geochronological dating, onlypetrographic and C-O triple isotope compositions are availableto interpret the origin of the Ca-rich phase that constitutesa minor component of the rock (∼0.01%). The textural andisotopic analysis suggest that the Ca-rich phase postdates theMg-rich phase, based on the presence of the Ca-rich phase in thecataclastic zones and a ∼18‰ shift in δ13C.The carbon isotope geochemistry of the carbonates in ALH

84001 is complex and remains difficult to interpret with only twoisotopes. In particular, it would be useful to be able to constrainthe carbon isotope composition of the Martian atmosphere ∼3.9Gya, using the carbonates preserved in this meteorite. As dis-cussed, the Δ17O composition of these carbonates suggests astrong influence of an atmospheric component, and the similarityin Δ17O between the Ca-rich phase and the more commonrosettes suggests a close relationship. The variation in δ13C isvery large, with the results from this study extending it even farther.Niles and colleagues (6) reported a range of δ13C values between28‰ and 64‰, with the most Ca-rich phases having the lowestδ13C values. This study extends that range down to 20‰, probablyby sampling a much more Ca-rich phase. Lower δ13C values thanthis have been reported as well, although the amount of terrestrialcontamination in these analyses is not clear (31, 32).Overall, the carbon isotopic composition of the Ca-rich phase

agrees with the trend of lower δ13C with lower Mg content foundin previous work (6). The problem is that the δ18O composition ismuch higher than would be expected for the Ca-rich compositionof the carbonate and is completely out of the field of previousanalyses (Figs. 2A and 3). If the Ca-rich phase measured hereformed as part of the aqueous event that formed the rest of the

ALH 84001 carbonates, it has some interesting implications fordifferent models of carbonate formation that have been proposed.Several different ALH 84001 carbonate formation models for

the slabs/rosettes have implications for the ancient Martian en-vironment. If this Ca-rich phase formed in close association withthe slab/rosettes, then these models are tested. However, if theCa-rich phase formed in a separate aqueous event altogether,then different conclusions apply (see following). For example, ithas been proposed that the carbonate slab/rosettes formed froma high-pH fluid (6) that initially contained no CO2 and rapidlyprecipitated carbonates on exposure to a CO2-rich atmosphere.In this case, there is a strong kinetic fractionation favoring thelight isotopes, resulting in carbonates depleted in δ13C comparedwith the atmosphere. According to this model, the ancient at-mosphere would have to have had a δ13C value between 30‰and 40‰, and the δ18O covaries with the carbon as a result ofthe inclusion of OH− (depleted in δ18O) during initial carbonateformation. Thus, assuming the Ca-rich phase formed undersimilar conditions as the slab/rosettes, it would be very difficult toobtain a carbonate with low δ13C and high δ18O under theseconditions, as both isotope compositions are pH-dependent andcovary. Although the rosettes may have formed via this mecha-nism, there is no clear way to explain the existence of the Ca-richphase, which has a relatively low δ13C and high δ18O, withoutmajor changes to the atmospheric δ13C.Other models have been based on observations of other Ca-

rich carbonates with relatively low δ18O (25) in the meteorite.

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Fig. 3. Data from this study and other stepped dissolution measurements of ALH 84001 showing the variation in δ13C observed in this study for Martianphases (9, 22). Gray crosses indicate the range of values from SIMS studies correlated by Mg contents. Red points are from acid dissolution measurementsconducted in this study. The CaCO3 phase is distinct from the covariant trend in other ALH 84001 carbonates, indicating that it formed from a distinct aqueousevent. However, the covariant trend indicated by the rest of the ALH 84001 data potentially points to an initial atmospheric composition near 15‰, whichwould also be consistent with the Ca-rich carbonates measured in this study.

Fig. 4. Schematic showing interaction of ancient carbonates with modernCO2 via transition state and recrystallization of carbonates. Red letters withprime symbol indicate modern CO2; dotted line indicates formation of newbonds during the transition stage of recarbonation.

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These studies have suggested that the Ca-rich carbonates formedfrom high-temperature hydrothermal fluids (13) that possessedlower δ13C and δ18O as a result of interaction with silicates andmagmatic reservoirs. If the Ca-rich hydrothermal fluid was in-deed a product of subsurface hydrothermal interaction, it shouldpossess a Δ17O similar to the silicates, instead of a Δ17O anomalyderived from extensive atmospheric interactions. The Ca-richphases identified in this study have a higher δ18O composition,indicating a likely low-temperature origin, and they also havea large Δ17O anomaly, indicating they did not likely form fromhydrothermal fluids. However, it remains possible that the δ18O-poor, Ca-rich carbonates observed in ref. 25 do not have anoxygen isotope anomaly, and were therefore formed from hy-drothermal fluids. A clear measurement of the Δ17O in the δ18O-poor, Ca-rich cores of the slab/rosettes has not been performedand is needed to evaluate this model better.Finally, it has also been suggested that the carbonates formed

from a single CO2-rich fluid that began degassing and evapo-rating as it was exposed near the surface (22, 33). This wouldsuggest an ancient atmosphere that possessed a δ13C of 15–20‰, whereas the carbonates obtained their heavier and co-variant carbon and oxygen isotopic compositions through Rayleighdistillation effects during evaporation and degassing. This pro-vides a compelling model to understand the slab/rosettes, and ifthe Ca-rich phase identified in this study formed under similarconditions, it may have formed from the fluid after the initiallyCO2-rich fluid had degassed, evaporated, and precipitated thebulk of the carbonates. The resulting fluid could have thenreequilibrated with the atmosphere, lowering the δ13C back to itsstarting levels (∼20‰). In this case, the δ18O would remainelevated because the fluid had been enriched by evaporation,and the oxygen would not have been affected by the CO2reequilibration because of mass balance. The main drawback tothis model is that the evaporated fluid should be Mg-rich be-cause the Mg carbonates are much more soluble than the Ca-richcarbonates. It is difficult to explain how the carbonates mightbecome Ca-rich again, as the evaporation process should havedepleted the fluid in Ca2+.All of the previous discussion has assumed that the Ca-rich

phase measured in this study formed in close relation to theother carbonates in the meteorite, and therefore could be un-derstood in their context. It is certainly possible that this is notthe case, and that the Ca-rich phase observed here formed ina completely unrelated event that substantially postdated theformation of the bulk of the carbonate within the rock. In thiscase, the variation in the oxygen isotopic composition could be

understood as being a result of a different aqueous fluid withdifferent δ18O composition. This is quite possible, as δ18O var-iations can be caused by different precipitation patterns, changesin water source, or any number of other things. The δ13C of theCa-rich phase may just represent the composition of the atmo-sphere at the time of formation, especially as the Δ17O contentindicates a close atmospheric relationship. If the Ca-rich phaseprecipitated at room temperature similar to the rosettes (22), itsuggests that the atmosphere δ13C was near 13‰. This atmo-spheric composition is very similar to what is predicted by thedegassing/evaporation models (22, 33) outlined in the case wherethe Ca-rich carbonate formed in close association with the slab/rosettes, presented earlier (Fig. 3).Importantly, it does not matter whether the Ca-rich phase

formed in association with the slab/rosettes or not. In both cases,the most feasible models predict a δ13C value of the ancientatmosphere that is between 10‰ and 20‰. This δ13C value forthe ancient atmosphere is significantly different from the δ13C ofthe modern atmosphere recently measured by the Mars ScienceLaboratory (46‰) (34). There is a distinct possibility, given ourlack of knowledge of the oxygen budget on Mars, that the Ca-rich phase does not record the atmospheric composition, despitepossessing a Δ17O anomaly. However, given the strong atmo-spheric signature recorded in this phase and its similarity to theother carbonates in the meteorite, it suggests that the carbonisotope composition of the Noachian atmosphere had a sub-stantially lower δ13C value at 4 Ga. This implies substantial en-richment in the δ13C of the atmosphere since 4 Ga, which mayhave occurred through extensive atmospheric loss.

MethodsMartian rock ALH 84001–214 was treated with concentrated phosphoric acid,and CO2 extracted was used to measure C and O-triple isotopes (SI Appendix,SI Methods). Ion microprobe oxygen isotope measurements were obtainedfrom two separate thin sections of ALH 84001 (302 and 303), using theCAMECA ims 6f at Arizona State University and the CAMECA ims 1270 at theUniversity of California, Los Angeles. Samples and standards were coatedwith ∼20–30 nm of carbon (SI Appendix, SI Methods).

ACKNOWLEDGMENTS. We thank the reviewers for their critical evaluation,which helped improve the manuscript. H. Bao from Louisiana StateUniversity is greatly acknowledged for providing carbonate crust fromAntarctic Dry valley. P.B.N. acknowledges funding from National Aero-nautics and Space Administration Mars Fundamental Research. M.H.T.and R.S. thank National Science Foundation-Atmospheric Chemistry Divisionfor the partial support Award no. AGS1259305 (to R.S.). C.M.C. would liketo thank Zonta International Foundation and the Ohio Space GrantConsortium.

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Cosmochim Acta 69(5):1359–1370.8. Leshin LA, McKeegan KD, Carpenter PK, Harvey RP (1998) Oxygen isotopic constraints

on the genesis of carbonates fromMartianmeteorite ALH84001.Geochim CosmochimActa

62(1):3–13.9. Romanek CS, et al. (1994) Record of fluid-rock interactions on Mars from the mete-

orite ALH84001. Nature 372(6507):655–657.10. McKay DS, et al. (1996) Search for past life on Mars: Possible relic biogenic activity in

martian meteorite ALH84001. Science 273(5277):924–930.11. Warren PH (1998) Petrologic evidence for low-temperature, possibly flood evaporitic

origin of carbonates in the ALH84001 meteorite. J Geophys Res 103(E7):16759–16773.

12. McSween HY, Harvey RP (1998) An evaporation model for formation of carbonates in

the ALH84001 Martian meteorite. Int Geol Rev 40(9):774–783.13. Harvey RP, McSween HY, Jr (1996) A possible high-temperature origin for the car-

bonates in the martian meteorite ALH84001. Nature 382(6586):49–51.14. Thiemens MH, Chakraborty S, Dominguez G (2012) The physical chemistry of mass-

independent isotope effects and their observation in nature. Annu Rev Phys Chem

63(1):155–177.15. Thiemens MH, Shaheen R (2014) 5.6 - Mass-Independent Isotopic Composition of

Terrestrial and Extraterrestrial Materials. Treatise on Geochemistry, eds Holland HD,

Turekian KK (Elsevier, Oxford), 2nd Ed, pp 151–177.16. Shaheen R, et al. (2010) Detection of oxygen isotopic anomaly in atmospheric car-

bonates and its implications to Mars. Proc Natl Acad Sci 107(47):20213–20218.17. Shaheen R, Janssen C, Röckmann T (2007) Investigation of the photochemical isotope

equilibrium between O2, CO2 and O3. Atmos Chem Phys 7:495–509.18. Shaheen R, et al. (2013) Tales of volcanoes and El-nino southern oscillations with the

oxygen isotope anomaly of sulfate aerosol. Proc Natl Acad Sci USA 110(44):

17662–17667.19. Farquhar J, Thiemens MH, Jackson T (1998) Atmosphere-surface interactions on

Mars: Delta 17O measurements of carbonate from ALH 84001. Science 280(5369):

1580–1582.20. Agee CB, et al. (2013) Unique meteorite from early Amazonian Mars: Water-rich

basaltic breccia Northwest Africa 7034. Science 339(6121):780–785.21. Al-Aasm IS, Taylor BE, South B (1990) Stable isotope analysis of multiple carbonate

samples using selective acid-extraction. Chem Geol 80(2):119–125.

Shaheen et al. PNAS Early Edition | 5 of 6

EART

H,A

TMOSP

HER

IC,

ANDPL

ANET

ARY

SCIENCE

S

22. Halevy I, Fischer WW, Eiler JM (2011) Carbonates in the Martian meteorite Allan Hills

84001 formed at 18 +/- 4 ° C in a near-surface aqueous environment. Proc Natl Acad

Sci USA 108(41):16895–16899.23. Corrigan CM, Harvey RP (2004) Multi-generational carbonate assemblages in martian

meteorite Allan Hills 84001: Implications for nucleation, growth, and alteration.

Meteorit Planet Sci 39(1):17–30.24. Saxton JM, Lyon IC, Turner G (1998) Correlated chemical and isotopic zoning in

carbonates in the Martian meteorite ALH84001. Earth Planet Sci Lett 160(3-4):

811–822.25. Eiler JM, Valley JW, Graham CM, Fournelle J (2002) Two populations of carbonate in

ALH84001: Geochemical evidence for discrimination and genesis. Geochim Cosmo-

chim Acta 66(7):1285–1303.26. Farquhar J, Thiemens MH (2000) Oxygen cycle of the Martian atmosphere-regolith

system: Delta O-17 of secondary phases in Nakhla and Lafayette. J Geophys Res

Planets 105(E5):11991–11997.27. Corrigan CM, Harvey RP (2002) Unique carbonates in Martian meteorite Allan Hills

84001. Lunar and Planetary Science XXXIII Conference [CD-ROM]. Houston, TC: Lunar

and Planetary Institute; 2002.

28. Eugster O, Busemann H, Lorenzetti S, Terrebilini D (2002) Ejection ages from krypton-81-krypton-83 dating and pre- atmospheric sizes of martian meteorites. MeteoritPlanet Sci 37(10):1345–1360.

29. Greenwood JP, McSween HY (2001) Petrogenesis of Allan Hills 84001: Constraintsfrom impact-melted feldspathic and silica glasses. Meteorit Planet Sci 36(1):43–61.

30. Chacko T, Mayeda TK, Clayton RN, Goldsmith JR (1991) Oxygen and carbon isotopefractionations between CO2 and calcite. Geochim Cosmochim Acta 55(10):2867–2882.

31. Jull AJT, Eastoe CJ, Cloudt S (1997) Isotopic composition of carbonates in the SNCmeteorites, Allan Hills 84001 and Zagami. J Geophys Res Planets 102(E1):1663–1669.

32. Jull AJT, Eastoe CJ, Xue S, Herzog GF (1995) Isotopic composition of carbonates in theSNC meteorites Allan Hills 84001 and Nakhla. Meteoritics 30(3):311–318.

33. Niles PB, Zolotov MY, Leshin LA (2009) Insights into the formation of Fe- and Mg-richaqueous solutions on early Mars provided by the ALH 84001 carbonates. Earth PlanetSci Lett 286(1-2):122–130.

34. Webster CR, et al.; MSL Science Team (2013) Isotope ratios of H, C, and O in CO2 andH2O of the martian atmosphere. Science 341(6143):260–263.

35. Valley JW, et al. (1997) Low-temperature carbonate concretions in the Martian me-teorite ALH84001: Evidence from stable isotopes and mineralogy. Science 275(5306):1633–1638.

6 of 6 | www.pnas.org/cgi/doi/10.1073/pnas.1315615112 Shaheen et al.

1

Supplementary Information: 1

SI- Methods 2

A slice (~1g) of ALH84001-214 was crushed (0.1- 100 μm) and divided into two subsections. The two portions of 3

comminuted samples were reacted separately with concentrated H3PO4 in the side arm of reaction vessels that were 4

evacuated for 4 days to ~10-6 Torr. Owing to small amount of CO2 released (~0.5 μmole) at 25 +1oC, a third cut of 5

ALH84001C (~1g) was processed as above and surface impurities were removed by allowing acid to react with the 6

sample for 1 hour at ambient temperature (25 +1oC). Gas released was collected at -196oC after passing through two 7

traps (ethanol slush at -60oC to remove traces of water). The 1-hr extraction was not performed on the first two samples 8

(ALH84001A and ALH84001B) only on the third sample (ALH84001C) (Table 1). CO2 was extracted from all of 9

the samples after acid reaction for 12 h at 25 + 1oC. Finally, the samples were heated for 3h at 150 + 1 oC and CO2 10

collected following the above procedure. The sequential extraction technique at two temperatures allowed us to 11

separate Ca-rich phase from the Fe-Mg rich phase. Laboratory experiments with various minerals and grain sizes, Al-12

asam et al .,(19) noted that < 10% CO2 is released from dolomite at 25o C after 12h which resulted in <1 ‰ shift in 13

C and O isotopes of calcite mineral. The contribution of CO2 from siderite and magnesite can be ruled out at 25o C as 14

no CO2 was released upon heating these mineral to 50o C with phosphoric acid even after 10h (36). CO2 was purified 15

using gas chromatography and C-isotopes were measured using Mat 253 Isotope Ratio Mass spectrometer. For O-16

triple isotope measurement, CO2 gas was reacted with BrF5 at 780 + 10 oC for 45h and O2 gas purified 17

chromatographically to allow multi-oxygen isotopic measurements(37). Carbon and O triple isotopic compositions 18

were measured using isotope ratio mass spectrometer. For replicate mass spectrometric analysis 1 SD standard 19

deviations is 0.01‰ for δ18O and δ17O. Overall analytical error (Stot) of the procedure is determined as (Stot)2 = (S1)2 20

+ (S2)2 + (S3)2 + (S4)2 where S1= acid digestion, S2= gas chromatography, S3 = fluorination step and S4 = isotope ratio 21

measurement for 17O and 18O= 0.5‰. All these processes follow mass dependent fractionation, therefore, uncertainty 22

on Δ17O = + 0.06‰ based on laboratory standards (n = 5). 23

Ion microprobe oxygen isotope measurements were obtained from two separate thin sections of ALH 84001 24

(302 and 303) using the CAMECA ims 6f at Arizona State University and the CAMECA ims 1270 at the University 25

of California at Los Angeles. Samples and standards were coated with ~20-30 nm of carbon. 26

Section ALH 84001,302 was analyzed at Arizona State University. During analyses, negative secondary 27

ions were sputtered by a Cs+ primary beam with a beam current of 25 nA focused to a spot size of ~20 m diameter. 28

The analysis area was flooded with low energy electrons for charge compensation (as in Leshin et al. (8)). Samples 29

were pre-sputtered for 210 seconds to remove the effects of coating. Each measurement was comprised of 55 cycles 30

of counting for ~1.5 seconds on mass16 and ~5.5 seconds on mass 18. Typical count rates for 16O in the calcite 31

standard were ~ 50 million counts per second. 16O was measured using a Faraday Cup (FC) and 18O was measured 32

using an electron multiplier (EM). Peak intensities were corrected for background (FC) and deadtime (EM). 33

The IMF for each unknown was calculated from measurements of calcite, magnesite and siderite standards. 34

Standards were mounted together on one section, separate from the unknown sample. These standards included Joplin 35

calcite, ZS magnesite, MS-siderite, DM dolomite and 2594 Breunnerite. Instrumental mass fractionation (IMF) varied 36

from ~-18‰ to ~+2‰. Uncertainties for individual analyses are ~2‰. 37

2

Section ALH 84001,303 was analyzed on the UCLA ims 1270. During this run, a Cs+ beam with a beam 38

current of ~2 nA was focused to a spot size of ~20 m by ~30 m. Secondary ions were analyzed in multi-collector 39

mode using two Faraday cups. Samples were once again pre-sputtered for 210 seconds. The IMF for each unknown 40

was again calculated from measurements of the same calcite, magnesite and siderite standards listed above. 41

Instrumental mass fractionation (IMF) varied from ~-12‰ to ~-4‰. Uncertainties for individual analyses are again 42

~2‰. 43

The carbonates measured above were described in the main text, as well as thoroughly discussed in Corrigan 44

and Harvey (21). Similar occurrences were found by 49. Petrographic relationships amongst these carbonates were 45

described in Corrigan and Harvey (21) as well. Specifically, in regard to the carbonates shown in Figure 3, Corrigan 46

and Harvey (21) interpreted that a number of steps were required to form the assemblages seen in the thin section. 47

First, the slab/rosette carbonates (“slab”) were deposited by fluids supersaturated in carbonate components. Rosettes 48

would only have had one nucleation point, while slabs would likely have formed from the coalescence of carbonates 49

growing from multiple nucleation sites in rare, large fractures. The Ca-rich portions of the slab and rosette carbonates 50

were the first to form, with compositions becoming more Mg- and Fe-rich over time. Visible zones were formed as 51

the occasional recharge changed the composition of fluids slightly. Magnesite and siderite rims likely formed on 52

rosette/slab carbonates by high temperature thermal decomposition of carbonate during an impact event(21). The Mg-53

rich carbonates (termed “post-slab magnesites”(21) but represented in Figure 3 by the label “Cb”) were formed next, 54

intruding into spaces not filled with slab carbonate(21). Feldspathic glasses (“Fs”) were intruded, and, finally, 55

carbonate and silica glasses were formed interstitial to the Mg-rich carbonate and feldspathic glasses produced the 56

final assemblage seen in the meteorite today. 57

58

59

SI-DISCUSSION: 60

We propose three different pathways to generate the O-isotopic anomaly in Martian CO3 61

i) CO2–O3 isotope exchange via excited oxygen atom O(1D) (R1-R2) (38-41). 62

ii) Catalytic reaction of hydroxyl radicals (HOx) with CO to produce CO2 (R3-R5)(42) 63

iii) Interaction of CO2 with surface adsorbed water on the regolith and aerosol dust particles 64

in the presence of ozone (R6-R8) as suggested by (37). 65

OOQ+ h →Q(1D) + O2 (R1) 66

Q(1D) + CO2 ↔ CO2Q* ↔ COQ + O(3P) (R2) 67

Here Q denote the heavy isotopes of oxygen (17O, 18O) in ozone and the enrichment is transferred 68

to CO2 via short live CO2Q*. 69

Q(1D) + H2O →OH + QH (R3) 70

CO+ QH → COQ + H (R4) 71

CO+ O → CO2 (R5) 72

3

73

OOQ+ (H2O)ads → (H2OQ)ads + O2 (R6) 74

X(SiO3) + (H2OQ)ads → X(QH)(OH) + SiO3 (R7) 75

CO2+ X(HQ)(HO) → X(QH)---COQ----OH →XCOQO + H2O (R8a) 76

COQ+ X(HQ)(HO) → X(QH)---COQ----OH →XCOQ + H2Q (R8b) 77

Here X= Mg, Fe, Mn and Ca rich silicates such as enstatite, ferrosilite, rhodonite,wellostonite etc. 78

The generation of O-isotope anomaly in the CO2 via excite oxygen atoms (route 1) has 79

been extensively studied (40). On Earth oxygen isotope anomaly produced in the stratospheric 80

CO2 (R1-R3) is removed in the troposphere due to the O-isotope exchange between water and CO2 81

by the hydrosphere and biosphere (43). The interaction of anomalous CO2 generated via R1-R2 82

and its precipitation as carbonate to preserve the O-isotope anomaly would depend on the ratio of 83

CO2 /H2O reservoirs. The lack of data on the O-isotopic composition and magnitude of paleo 84

hydrosphere and atmosphere on Mars, however, does not allow us to define this ratio. During CO2-85

H2O equilibration processes HCO3- acquires O-isotopic composition of water and hence 18O 86

values are dictated by the equilibration temperature. The O-triple isotope measurements on both 87

carbonate phases in ALH84001 suggests that source water from which carbonates were 88

precipitated may possess higher oxygen isotope anomaly, provided they were formed after CO2- 89

H2O equilibration processes as CO2 acquires the O-isotopic composition of water. 90

Photolysis of CO2 to yield CO and its reaction with hydroxyl radicals (R3-R5) also 91

produce O-isotope anomaly in product CO2 (44, 45). Numerous pathways have been proposed for 92

the production of hydro peroxy radicals (OHx= H2O2, OH, H O2) on Mars, such as electric 93

discharge on dust devils, photolysis of water and reaction of O(1D) with water vapor (46). Peroxy 94

radicals (OHx= H2O2, OH, H O2) produced by the interaction of ozone with water vapor via O(1D) 95

has shown to inherit ozone isotopic signature (R3) (47). An anti-correlation of O3 with H2O at the 96

equator and summer pole of Mars suggests the role of O(1D) in the production of hydro peroxy 97

radicals (48). Recombination reaction of CO with O atoms is known to produce enrichments in 98

CO2 comparable to ozone (R5) (49, 50). The reaction of OH +CO is also known to produce mass 99

independent fractionation with remaining CO progressively enriched in 17O (51). On Earth, the 100

Δ17O of OHx is preserved in the stratosphere but the original ozone signal is erased due to rapid 101

isotope exchange of the hydroxyl radical with tropospheric water vapor (52). 102

4

The combined mass independent O-isotopic anomaly produced via pathways i) and ii) will 103

result in a steady state isotopically anomalous CO2 reservoir which upon equilibration with water 104

would yield CO3 with a component of the ∆17O of water. Laboratory experiments have 105

demonstrated that CO3 acquires the O-isotopic composition of H2O during CO2-H2O equilibration 106

process (53). Additionally, there would be no microscopic heterogeneity in the δ18O of carbonates 107

precipitated after equilibration with surface water reservoirs and the value of the isotopic anomaly 108

would be constant and the small δ18O value differences would simply reflect differential 109

temperature and reactivity chemistry. Pathway iii) involves interaction of CO2 with surface 110

adsorbed water on the regolith and/or dust particles in a CO2-O3-H2O-H2O2 reaction system that 111

generates microscopic heterogenity (spatial and mineral specific) due to the kinetic isotope effects 112

in the processes of adsorption and sublimation of gas-liquid layers (39, 40, 54, 55). 113

Thermodynamic equilibrium and kinetic processes such as condensation and sublimation of CO2 114

and H2O, however, fractionate O-isotopes in a mass dependent fashion with 17O ~ 0.5 18O (56). 115

The O-isotopic anomaly (Δ17O > 0 ‰) is only generated in processes involving interaction with 116

ozone and consequently are a measure of odd oxygen cycling (O, O3) in the atmosphere of Mars, 117

especially the ozone/water ratio. At present, the oxygen isotopic composition of Martian 118

molecular oxygen and bulk surface water mostly stored as CO2-H2O at the poles or subsurface 119

water is unknown. If the molecular oxygen 17O value is somewhere near bulk oxygen of the 120

silicate, then the observed carbonate and water values in the SNC meteorites reflect change in the 121

ozone isotopic composition and water levels. The observation of similar 17O values in both Ca-122

rich and Fe-Mg rich carbonate phases in the present measurements reflect no significant change in 123

odd oxygen cycle (O, O3) and hydroxy radical reactions. These measurements begin to show that 124

the multi isotope approach on different carbonate phases can advance recognition of atmospheric 125

and surficial changes, allowing for full atmospheric modelling efforts in the future. 126

127

128

5

129

130

Fig. S1a. Oxygen triple isotopic composition (a) O- carrying reservoirs, CO3 (ALH84001 this study), SNC CO3 and 131

silicate (16), mineral water from SNC(57). The insert shows slight offset of water O-isotopic composition from 132

terrestrial fractionation line with δ17O ~ 0.52 δ18O. Here red circles denote CO3, blue square = silicate, open triangle= 133

mineral water released during pyrolysis at 600oC, filled triangle = mineral water released at 1000 oC. Here 17O= 103 134

ln(1+ 17O/103) and 18O= 103 ln(1+ 18O/103). 135

136

137

6

Fig. S1b. (b) Excess 17O (Δ17O) versus 18O of Martian O-carrying compounds as in Fig. 1a. 138

139

140

Fig. S2: The oxygen triple isotopic composition of carbonates in martian meteorites, closed blue symbols= Fe-Mg 141

rich phase in ALH84001 (CO2 released at 150 oC). Open blue square= Ca-rich phase in AH84001 (CO2 released at 142

25oC after removal of terrestrial contamination (green cross). Magenta open triangle= Fe-Mg rich phase in Lafayatte, 143

magenta closed symbol = Fe-Mg rich phase in Nakhla (16). Brown open circle = Oxygen triple isotopic composition 144

of the rock in ALH84001, closed red square = Atmospheric CO2 measured by MSL (webester et al., 2013). 145

146

147

TERRESTRIAL CONTAMINATION: 148

Prolonged residence time of meteorites on ice resulted in surface contamination, possibly due to partial melting of ice 149

and seepage of water to the rock over the 13,000 yrs the rocks laid in Antarctica. To determine and compare the effect 150

of surface weathering on the O-isotopic composition of the ALH84001 Martian meteorite during its residence time in 151

Antarctic, surface crust from an igneous rock in the Dry Valley (DVC) rock was also analyzed. Water in equilibrium 152

with atmospheric CO2 produces mildly acidic conditions (pH <5) whereby causing mineral weathering and release of 153

cations to increase the pH of the solution to less acidic values (>6) and causing CO3 precipitation (Fig. S3). Carbonates 154

formed by surface weathering are enriched in both C and O-isotopes (13C = 11‰, 17O =14‰ and 18O = 28‰), 155

however, no excess 17O (∆ 17O ≈ 0) is observed. By using the measured fractionation factors for pure CO2-H2O system 156

(53) at a range of temperature (0-20oC), the equilibrium values ((13C = +3-1‰ , 18O = -20 to -23‰) is obtained 157

using isotopic composition of preindustrial CO2 (13C ~ -7‰, 18O ~ 41‰) (58) and Standard Light Arctic 158

Precipitation (SLAP 18O = -55.5‰). These values are much lower than measured C and O values for the ADV 159

carbonate crust. Isotopic fractionation of DIC due to CO2 outgassing or multiple freeze thaw cycles may be the primary 160

7

cause of enrichment of precipitated carbonates at low temperature. Terrestrial contamination in the CaCO3 fraction of 161

the carbonates based on the 14C activity of CO2 extracted from EET79001 has also been reported (59). Previous studies 162

of SNC meteorites have not measured O-triple isotopic composition of the calcite fraction owing to small sample 163

size(16). We have reported these values for ALH84001 for the first time after isolating surface contaminants. 164

165

166

Fig. S3: The carbonate crust formed on an igneous rock obtained from Antractic Dry valley. (Courtesy of Prof. H. 167

Bao, Louisiana State University, USA). 168

169

170

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286(5437):90-94. 179

6. Niles PB, Leshin LA, & Guan Y (2005) Microscale carbon isotope variability in 180

ALH84001 carbonates and a discussion of possible formation environments. Geochimica 181

Et Cosmochimica Acta 69(11):2931-2944. 182

7. Holland G, Saxton JM, Lyon IC, & Turner G (2005) Negative delta O-18 values in Allan 183

Hills 84001 carbonate: Possible evidence for water precipitation on Mars. Geochimica Et 184

Cosmochimica Acta 69(5):1359-1370. 185

8. Leshin LA, McKeegan KD, Carpenter PK, & Harvey RP (1998) Oxygen isotopic 186

constraints on the genesis of carbonates from Martian meteorite ALH84001. Geochimica 187

Et Cosmochimica Acta 62(1):3-13. 188

9. Romanek CS, et al. (1994) Record of fluid-rock interactions on Mars from the meteorite 189

ALH84001. Nature 372(6507):655-657. 190

10. McKay DS, et al. (1996) Search for past life on Mars: Possible relic biogenic activity in 191

Martian meteorite ALH84001. Science 273(5277):924-930. 192

11. Warren PH (1998) Petrologic evidence for low-temperature, possibly flood evaporitic 193

origin of carbonates in the ALH84001 meteorite. Journal of Geophysical Research-Planets 194

103(E7):16759-16773. 195

8

12. McSween HY & Harvey RP (1998) An evaporation model for formation of carbonates in 196

the ALH84001 Martian meteorite. International Geology Review 40(9):774-783. 197

13. Harvey RP & McSween HY (1996) A possible high-temperature origin for the carbonates 198

in the Martian meteorite ALH84001. Nature 382(6586):49-51. 199

14. Thiemens MH, Chakraborty S, & Dominguez G (2012) The Physical Chemistry of Mass-200

Independent Isotope Effects and Their Observation in Nature. Annual Review of Physical 201

Chemistry 63(1):155-177. 202

15. Thiemens MH & Shaheen R (2014) 5.6 - Mass-Independent Isotopic Composition of 203

Terrestrial and Extraterrestrial Materials. Treatise on Geochemistry (Second Edition), eds 204

Holland HD & Turekian KK (Elsevier, Oxford), pp 151-177. 205

16. Farquhar J & Thiemens MH (2000) Oxygen cycle of the Martian atmosphere-regolith 206

system: Delta O-17 of secondary phases in Nakhla and Lafayette. Journal of Geophysical 207

Research-Planets 105(E5):11991-11997. 208

17. Farquhar J, Thiemens MH, & Jackson T (1998) Atmosphere-surface interactions on Mars: 209

Delta O-17 measurements of carbonate from ALH 84001. Science 280(5369):1580-1582. 210

18. Agee CB, et al. (2013) Unique Meteorite from Early Amazonian Mars: Water-Rich 211

Basaltic Breccia Northwest Africa 7034. Science 339(6121):780-785. 212

19. Al-Aasm IS, Taylor BE, & South B (1990) Stable isotope analysis of multiple carbonate 213

samples using selective acid-extraction. Chemical Geology 80(2):119-125. 214

20. Halevy I, Fischer WW, & Eiler JM (2011) Carbonates in the Martian meteorite Allan Hills 215

84001 formed at 18 ± 4°C in a near-surface aqueous environment. Proceedings of the 216

National Academy of Sciences 108(41):16895-16899. 217

21. Corrigan CM & Harvey RP (2004) Multi-generational carbonate assemblages in martian 218

meteorite Allan Hills 84001: Implications for nucleation, growth, and alteration. 219

Meteoritics & Planetary Science 39(1):17-30. 220

22. Holland G, Lyon IC, Saxton JM, & Turner G (2000) Very low oxygen-isotopic ratios in 221

Allan Hills 84001 carbonates: A possible meteoric component? Meteoritics & Planetary 222

Science 35:A76-A77. 223

23. Saxton JM, Lyon IC, & Turner G (1998) Correlated chemical and isotopic zoning in 224

carbonates in the Martian meteorite ALH84001. Earth and Planetary Science Letters 225

160(3-4):811-822. 226

24. Eiler JM, Valley JW, Graham CM, & Fournelle J (2002) Two populations of carbonate in 227

ALH84001: Geochemical evidence for discrimination and genesis. Geochimica et 228

Cosmochimica Acta 66(7):1285-1303. 229

25. Corrigan C. M. aHRP (2002) Unique carbonates in Martian meteorite Allan Hills 84001 230

in XXXIII Lunar and Planetary science meeting (Lunar and Planetary Institute, Houston, 231

USA). 232

26. Borg LE, et al. (1999) The Age of the Carbonates in Martian Meteorite ALH84001. Science 233

286(5437):90-94. 234

27. Eugster O, Busemann H, Lorenzetti S, & Terrebilini D (2002) Ejection ages from krypton-235

81-krypton-83 dating and pre- atmospheric sizes of martian meteorites. Meteoritics & 236

Planetary Science 37(10):1345-1360. 237

28. Greenwood JP & McSween HY (2001) Petrogenesis of Allan Hills 84001: Constraints 238

from impact-melted feldspathic and silica glasses. Meteoritics & Planetary Science 239

36(1):43-61. 240

9

29. Chacko T, Mayeda TK, Clayton RN, & Goldsmith JR (1991) OXYGEN AND CARBON 241

ISOTOPE FRACTIONATIONS BETWEEN CO2 AND CALCITE. Geochimica Et 242

Cosmochimica Acta 55(10):2867-2882. 243

30. Jull AJT, Eastoe CJ, & Cloudt S (1997) Isotopic composition of carbonates in the SNC 244

meteorites, Allan Hills 84001 and Zagami. Journal of Geophysical Research-Planets 245

102(E1):1663-1669. 246

31. Jull AJT, Eastoe CJ, Xue S, & Herzog GF (1995) Isotopic composition of carbonates in 247

the SNC meteorites Allan Hills 84001 and Nakhla. Meteoritics 30(3):311-318. 248

32. Niles PB, Zolotov MY, & Leshin LA (2009) Insights into the formation of Fe- and Mg-249

rich aqueous solutions on early Mars provided by the ALH 84001 carbonates. Earth and 250

Planetary Science Letters 286(1-2):122-130. 251

33. Webster CR, et al. (2013) Isotope Ratios of H, C, and O in CO2 and H2O of the Martian 252

Atmosphere. Science 341(6143):260-263. 253

34. Valley JW, et al. (1997) Low-temperature carbonate concretions in the Martian meteorite 254

ALH84001: Evidence from stable isotopes and mineralogy. Science 275(5306):1633-1638. 255

35. Niles PB, Boynton WV, Hoffman JH, Ming DW, & Hamara D (2010) Stable Isotope 256

Measurements of Martian Atmospheric CO2 at the Phoenix Landing Site. Science 257

329(5997):1334-1337. 258

36. Alaasm IS, Taylor BE, & South B (1990) STABLE ISOTOPE ANALYSIS OF 259

MULTIPLE CARBONATE SAMPLES USING SELECTIVE ACID-EXTRACTION. 260

Chemical Geology 80(2):119-125. 261

37. Shaheen R, et al. (2010) Detection of oxygen isotopic anomaly in terrestrial atmospheric 262

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