1. Carbonate formation events in ALH 84001 trace the
evolution of the Martian atmosphere
Robina Shaheena
, Paul B. Nilesb,1
, Kenneth Chonga,c
, Catherine M. Corrigand
, and Mark H. Thiemensa
a
Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92122; b
Astromaterials Research and Exploration Science, NASA
Johnson Space Center, Houston, TX 77058; c
Department of Chemistry, California State Polytechnic University, Pomona, CA 91768; and d
Smithsonian 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 C
and O-triple isotopes can be used to decipher the planet’s climate
history. Here we report Δ17
O, δ18
O, and δ13
C data of ALH 84001 of
at least two varieties of carbonates, using a stepped acid dissolu-
tion technique paired with ion microprobe analyses to specifically
target carbonates from distinct formation events and constrain the
Martian atmosphere–hydrosphere–geosphere interactions and
surficial aqueous alterations. These results indicate the presence of
a Ca-rich carbonate phase enriched in 18
O that formed sometime
after the primary aqueous event at 3.9 Ga. The phases showed
excess 17
O (0.7‰) that captured the atmosphere–regolith chemical
reservoir transfer, as well as CO2, O3, and H2O isotopic interactions
at the time of formation of each specific carbonate. The carbon
isotopes preserved in the Ca-rich carbonate phase indicate that the
Noachian atmosphere of Mars was substantially depleted in 13
C
compared with the modern atmosphere.
Martian meteorite | oxygen isotope anomaly | aqueous interaction |
carbon isotope | photochemistry
Geological evidence suggests that early Mars was sufficiently
warm for liquid water to flow on the surface for at least brief
periods, if not longer (1). Identifying the nature and duration of
warmer conditions on the Martian surface is one of the key pieces of
information for understanding atmosphere–hydrosphere–geosphere
interactions, the evolution of the atmosphere, and potential past
habitability. A better understanding of the evolution of the Martian
atmosphere and, in particular, the behavior of its primary compo-
nent, CO2, provides a means for characterizing the nature of the
ancient Martian environment. The amount of CO2 present in the
atmosphere should provide critical insight into the characteristics of
the Martian climate, with a denser atmosphere being more likely to
be 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 the
oldest known rock (crystallographic age ∼4.09 ± 0.03 Ga) (4),
and its carbonate fractions (<1% wt/wt) are considered to have
preserved 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 (δ13
CVPDB = 27–64, where VPDB stands
for Vienna Pee Dee Belemnite, and δ18
OSMOW = −10–27‰, where
SMOW stands for Standard Mean Ocean Water) heterogeneous on
micrometer scales; carbon and oxygen isotopes show a covariant
relationship 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-carrying
reservoirs on Mars (atmosphere–hydrosphere–geosphere) and how
they interact from δ18
O alone is nearly impossible because of the
lack of direct information on the isotopic composition of the pri-
mary O-carrying reservoirs in the carbonate system (CO2–H2O) and
the extreme variability observed in chemical and isotopic
composition of carbonate minerals in ALH 84001. The O-iso-
topic anomaly (Δ17
O = δ17
O − 0.52 × δ18
O) observed in O3, SO4,
NO3, CO3, and H2O2 has been successfully used to investigate
physicochemical and photochemical processes in terrestrial and
extraterrestrial materials (14–18). In this study, we used five stable
isotopes of carbonates (12
C, 13
C, 16
O, 17
O, and 18
O) on Ca- and
Fe-rich phases to decipher atmosphere–hydrosphere interactions
and Martian CO2/CO3 geochemical cycling. This high-precision
multi-O-isotope analysis of secondary minerals was coordinated
with detailed petrographic and ion microprobe analyses.
The primary goal of this study was to specifically identify
carbonate phases from distinct formation events to provide
better understanding of oxygen and carbon reservoirs on Mars.
There have been no previous measurements of both carbon
isotope and O-triple isotope compositions of the same CO2
sample 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 extract
CO2 from several portions of ALH 84001. The O-isotope
values (Δ17
O, δ18
O) are reported with respect to SMOW, and
δ13
C of the CO2 gas evolved with respect to V-PDB standard.
We also report oxygen isotope SIMS (secondary ion mass
spectrometer or ion microprobe) analyses coupled with SEM
images 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 bulk
analyses, allowing placement of further constraints on the alter-
ation history of the meteorite.
Significance
Martian meteorite ALH 84001 serves as a witness plate to the
history of the Martian climate ∼4 Ga ago. This study describes
ion microprobe δ18
O analyses coupled with δ13
C, δ18
O, and Δ17
O
analyses from stepped acid dissolution of the meteorite that
identifies a new carbonate phase with distinct isotope com-
positions. These new measurements of the oxygen isotope
composition of carbonates within this meteorite reveal several
episodes of aqueous activity that were strongly influenced by
atmospheric chemistry. When paired with carbon isotope
measurements, these data suggest that the ancient atmo-
sphere of Mars was significantly depleted in 13
C compared to
the present day. This implies substantial enrichment in the δ13
C
of 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., and
C.M.C. performed research; R.S. and P.B.N. analyzed data; and R.S., P.B.N., C.M.C., and
M.H.T. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
1
To 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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2. Results
Carbonates in ALH 84001 samples exhibit two striking features:
both the Ca-rich and Fe-rich phases are highly enriched in δ13
C,
and the magnitude of the oxygen isotope anomaly (Δ17
O = 0.7 ‰)
is identical in both phases (Fig. 1). The sequential acid extraction
technique and ion microprobe analyses of ALH 84001 reveal the
presence of distinct carbonate populations of different chemical
and isotopic compositions (Fig. 2 and Table 1). The first set of
measurements using the acid extraction technique on ALH84001B
indicated a significant difference in C and O-triple isotopes of CO2
released after 12 h at 25 °C (δ13
C = 12‰, Δ17
O = 0.3‰) and CO2
released after 3 h at 150 °C (δ13
C = 38‰, Δ17
O = 0.6‰). These
results suggest a potential mixture between terrestrial and
Martian phases; therefore, a 1-h acid dissolution step at 25 °C
was performed similar to that in ref. 20 on a larger sample of the
meteorite (2.012 g; ALH84001C) to remove traces of terrestrial
contamination (0.0079% CO3 by mass) that may have formed
during its residence in Antarctica (∼13,000 y; SI Appendix, SI
Discussion). The fraction of CO2 released from this 1-h reaction
showed a Δ17
O = 0.03 ± 0.06‰ that is indistinguishable from
other 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 phase
that possessed a Δ17
O = 0.73 ± 0.05‰ similar to the Δ17
O
values of the Mg- and Fe-rich carbonates that dissolve at higher
temperatures (Δ17
O = 0.75 ± 0.05‰). Because this phase was
dissolved after 12 h at only 25 °C, it is certainly dominated by Ca-
rich carbonate (21), which possesses a distinct δ13
C = 20‰ and
δ18
O = 25‰ (Fig. 1). The 3-h acid extraction step at 150 °C yielded
CO2 with δ13
C = 38‰ and δ18
O = 21‰, similar to what has been
previously 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 distinct
δ18
O enrichment that can be linked to the stepped acid extrac-
tion results (Fig. 2 and Table 1). In thin section, these Ca-rich
carbonates occur as isolated domains interstitial to both silica
glass and a Mg-rich carbonate matrix (Fig. 2B). Neither of these
carbonate occurrences exhibits typical rosette morphology. The
Mg-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 and
Harvey (23) suggested that these Mg-rich and Ca-rich carbonates
postdate formation of the carbonate slabs and rosettes, based on
petrographic data. In their proposed sequence of events, the slab
carbonates formed first, followed by shock events that resulted in
limited carbonate decomposition. Subsequently, the Mg-rich and
Ca-rich carbonates formed in a distinct aqueous alteration event.
Discussion
The combination of high-precision multiisotope analyses, pet-
rographic interpretation, and ion microprobe analyses suggest
that the Ca-rich phase measured in the 12-h dissolution of
ALH84001C is part of the same population as the Ca-rich phase
identified in thin section (23) and likely formed on Mars. Chemi-
cally, these carbonates are the most Ca-rich of the ALH 84001
carbonates measured by ion microprobe (Fig. 2A). Fe and Mg
carbonates are not typically reactive at 25 °C (21); therefore,
Ca-rich carbonate is expected to be the main product of a 12-h acid
dissolution step (SI Appendix, SI Methods) (21). However, the
stepped 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 isotopic
difference between the SIMS and dual-inlet isotope ratio mass
spectrometer (IRMS) measurements of the Ca-rich phase is within
the variability observed in the previous ion probe measurements of
the meteorite for a particular carbonate composition (∼10‰), but
clearly distinguishes this particular Ca-rich phase (with higher δ18
O)
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 between
Mg content and δ18
O (7, 8, 24, 25), with all the Ca-rich phases
having δ18
O values below 10‰ (Fig. 2A). The Ca-rich carbonate
phase identified in the 12-h dissolution of sample ALH84001C
and in the ion microprobe analyses (Table 1) does not obey this
relationship, indicating formation from an aqueous event distinct
from the event that formed the much more common carbonate
rosettes, and thus represents a carbonate phase that has not been
documented before to our knowledge. The carbon isotope
composition (δ13
C = 20‰) of the Ca-rich phase identified here
also differs from the average composition of the Fe-Mg car-
bonate rosettes (δ13
C = 38‰) measured during this study and
previous studies (6, 9), although it is closer to some of the lowest
δ13
C values measured by ion microprobe (Fig. 3) (6). The
combined carbon and oxygen isotope data show that our stepped
acid extraction technique was successful in separating a distinct
carbonate phase from the more common rosettes (Fig. 3). If the
CO2 obtained after 12 h acid extraction of sample ALH84001C
was derived from a mixture of the Fe- and Mg-rich carbonate
rosettes and some other phase, it would lie on the end of
a straight line drawn through the red points in Fig. 3. This would
have an even lower δ13
C composition and higher δ18
O values,
making it even more distinctive from the rosettes.
The significant difference in Δ17
O composition between ter-
restrial (Δ17
O ∼ 0‰) and Martian carbonates observed in this
study (Δ17
O ∼ 0.7‰), along with the δ13
C, shows the use of
multiple-isotope analysis for distinguishing carbonate formed on
Mars and Earth and defining processes that led to their for-
mation (Fig. 1). Despite evidence that this CaCO3 phase is distinct
from the other carbonates in ALH 84001, it contains an iden-
tical Δ17
O anomaly (within error) to that observed in the
more common Fe- and Mg-rich carbonate rosettes (Fig. 1). The
O-isotopic anomaly in carbonate (Δ17
O = 0.7‰) is higher
-0.1
0.1
0.3
0.5
0.7
0.9
1.1
0 10 20 30 40 50
Δ17O
Carbon Isotope Composi on (δ13C)
ALH 84001
Lafaye e
Nakhla
Mars Silicate
Terrestrial Frac ona on Line
Fig. 1. Multi C(VPDB)-O(SMOW) triple-isotope analysis of multiple phases
of carbonates in Martian meteorite ALH 84001. Blue points are analyses
from this study that are interpreted to be Martian on the basis of their high
Δ17
O values. The red point is the terrestrial contamination from the 1-h acid
extraction at 25 °C. The purple point is from a 12-h extraction at 25 °C that
was 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 (Δ17
O) of earlier carbonate measurements on
Nakhlites and ALH 84001 Martian meteorites is also shown (26). However,
carbon isotopic composition was not measured in these samples, and
therefore these data are shown as horizontal lines. The average bulk silicate
of Mars and Earth is also shown for reference.
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3. compared with the isotopic composition of the silicate (Δ17
O =
0.36‰) measured on the same slice of ALH84001C, using CO2
laser fluorination (26) (SI Appendix, Figs. S1 and S2). The higher
O-isotope anomaly in the ALH 84001 carbonate compared with
host rock (Δ17
O= 0.4‰) (Table 1) is likely a result of the in-
teraction of an anomalous fluid with the anomalous atmospheric
CO2 that derived its signal from the atmospheric CO2-O(1
D) cycle
and, 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 isotopic
composition that is significantly different from the host silicates
(26). The present findings are also similar to other measurements
of secondary minerals in Martian meteorites (SNC Δ17
Ocarbonates =
0.6–0.9‰ and Δ17
Osilicates = 0.2–0.4‰) (26), which contain
sulfates and water with higher Δ17
O anomalies compared with
the host silicate rock (SI Appendix, Fig. S1B). NWA7034 provides
an exception to this trend, where Δ17
Osilicates = 0.6‰ of the host
rock is higher than that of the secondary minerals Δ17
Ocarbonates =
0‰ and Δ17
Osulfate = 0‰ for presently unknown reasons (20). If
the carbonates precipitated from liquid water, it is likely that the
ancient water reservoir on Mars possessed a significant oxygen
isotope anomaly.
The isotopic composition of the Ca-rich phase measured in
this study indicates that the Ca-rich phase is likely distinct from
the more common carbonate rosettes. However, they may still
have formed from the same carbon reservoir, despite having
a different oxygen isotope composition (Fig. 3). Petrographic
relationships show that the Ca-rich phase postdates the more
abundant 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 either
an event that occurred soon after the primary population of
carbonate 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)
-15
-10
-5
0
5
10
15
20
25
30
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
δ18OSMOW
XMg
Previous SIM S data
This study
A B
Fig. 2. (A) Blue points show relationship between the Mg content of carbonates in ALH 84001 and δ18
O composition from the collection of published ion
microprobe analyses from ALH 84001 carbonates (7, 8, 24, 25, 35). Red points show ion microprobe analyses of Ca-rich carbonate identified in this study. These
are the most Ca-rich carbonates ever studied by ion microprobe in ALH 84001 and are much more enriched in δ18
O than other Ca-rich phases. The black line
indicates 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 from
ALH 84001,302, shown in a backscattered electron image (taken on the FEI NOVA NanoSEM 600 at the Smithsonian Institution). High-Ca carbonates analyzed
for oxygen isotopes are shown in the enlargement in the upper right corner of B. Ovate features within carbonates are ion microprobe pits. Slab, slab-type
carbonates 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 ion
microprobe (SIMS) analyses
Sample Treatment % CO3 by mass CO2 μmole O2 μmole δ13
C δ17
O′* δ 18
O′* Δ17
O
ALH84001C 1 h at 25 °C 0.0079 2.69 2.58 6.09 19.16 36.51 0.03
ALH84001C**
12 h at 25 °C 0.01 3.45 3.36 20.27 14.14 25.60 0.73
ALH84001C†
3 h at 150 °C 0.145 14.56 13.8 38.77 11.99 21.46 0.75
ALH84001C‡
Laser fluorination 2.54 4.17 0.36
ALH84001A 3 h at 150 °C 0.13 6.08 5.15 37.37 12.59 22.93 0.57
ALH84001B**
12 h at 25 °C 0.014 0.735 0.65 11.94 18.33 34.42 0.30
ALH84001B†
3 h at 150 °C 0.12 6.09 4.15 38.14 12.17 22.16 0.56
ALH_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 samples
similar in size and composition to the Martian rock samples, is ±0.1‰ for δ13
C and ±0.2‰ for 17
O and 18
O. Statistical variations (1σ SD) using laboratory CO2
standards on IRMS were δ13
C = ±0.02‰, δ17
O, and δ18
O = ±0.1‰. Chemical composition of SIMS spots were measured by Eprobe (SI Appendix, SI Methods):
ALH_302_9, Ca90Mg7Fe3; ALH_302_15, Ca99Mg0Fe1.
*δi
O′ = 1000 lnð1 + δi
O=1000Þ where i
O = 17
O and 18
O.
**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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4. 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 sequence
outlined in Fig. 4. This scheme involves replacement of all C and
two-thirds of the O atoms in carbonates with modern atmo-
spheric CO2 during the recrystallization process (30). Because of
the large sample required for geochronological dating, only
petrographic and C-O triple isotope compositions are available
to interpret the origin of the Ca-rich phase that constitutes
a minor component of the rock (∼0.01%). The textural and
isotopic analysis suggest that the Ca-rich phase postdates the
Mg-rich phase, based on the presence of the Ca-rich phase in the
cataclastic zones and a ∼18‰ shift in δ13
C.
The carbon isotope geochemistry of the carbonates in ALH
84001 is complex and remains difficult to interpret with only two
isotopes. In particular, it would be useful to be able to constrain
the carbon isotope composition of the Martian atmosphere ∼3.9
Gya, using the carbonates preserved in this meteorite. As dis-
cussed, the Δ17
O composition of these carbonates suggests a
strong influence of an atmospheric component, and the similarity
in Δ17
O between the Ca-rich phase and the more common
rosettes suggests a close relationship. The variation in δ13
C is
very large, with the results from this study extending it even farther.
Niles and colleagues (6) reported a range of δ13
C values between
28‰ and 64‰, with the most Ca-rich phases having the lowest
δ13
C values. This study extends that range down to 20‰, probably
by sampling a much more Ca-rich phase. Lower δ13
C values than
this have been reported as well, although the amount of terrestrial
contamination in these analyses is not clear (31, 32).
Overall, the carbon isotopic composition of the Ca-rich phase
agrees with the trend of lower δ13
C with lower Mg content found
in previous work (6). The problem is that the δ18
O composition is
much higher than would be expected for the Ca-rich composition
of the carbonate and is completely out of the field of previous
analyses (Figs. 2A and 3). If the Ca-rich phase measured here
formed as part of the aqueous event that formed the rest of the
ALH 84001 carbonates, it has some interesting implications for
different 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 with
the slab/rosettes, then these models are tested. However, if the
Ca-rich phase formed in a separate aqueous event altogether,
then different conclusions apply (see following). For example, it
has been proposed that the carbonate slab/rosettes formed from
a high-pH fluid (6) that initially contained no CO2 and rapidly
precipitated carbonates on exposure to a CO2-rich atmosphere.
In this case, there is a strong kinetic fractionation favoring the
light isotopes, resulting in carbonates depleted in δ13
C compared
with the atmosphere. According to this model, the ancient at-
mosphere would have to have had a δ13
C value between 30‰
and 40‰, and the δ18
O covaries with the carbon as a result of
the inclusion of OH−
(depleted in δ18
O) during initial carbonate
formation. Thus, assuming the Ca-rich phase formed under
similar conditions as the slab/rosettes, it would be very difficult to
obtain a carbonate with low δ13
C and high δ18
O under these
conditions, as both isotope compositions are pH-dependent and
covary. Although the rosettes may have formed via this mecha-
nism, there is no clear way to explain the existence of the Ca-rich
phase, which has a relatively low δ13
C and high δ18
O, without
major changes to the atmospheric δ13
C.
Other models have been based on observations of other Ca-
rich carbonates with relatively low δ18
O (25) in the meteorite.
-10
0
10
20
30
40
50
60
70
80
90
0 10 20 30 40 50
δ13CVPDB
δ18OSMOW
ALH84001
Webster et al. (2013)
This Study
SIMS
Poten al Noachian Atmospheric CO2
Composi on
Ancient
Carbonates
Modern
Atmosphere
Fig. 3. Data from this study and other stepped dissolution measurements of ALH 84001 showing the variation in δ13
C observed in this study for Martian
phases (9, 22). Gray crosses indicate the range of values from SIMS studies correlated by Mg contents. Red points are from acid dissolution measurements
conducted in this study. The CaCO3 phase is distinct from the covariant trend in other ALH 84001 carbonates, indicating that it formed from a distinct aqueous
event. However, the covariant trend indicated by the rest of the ALH 84001 data potentially points to an initial atmospheric composition near 15‰, which
would also be consistent with the Ca-rich carbonates measured in this study.
Fig. 4. Schematic showing interaction of ancient carbonates with modern
CO2 via transition state and recrystallization of carbonates. Red letters with
prime symbol indicate modern CO2; dotted line indicates formation of new
bonds during the transition stage of recarbonation.
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5. These studies have suggested that the Ca-rich carbonates formed
from high-temperature hydrothermal fluids (13) that possessed
lower δ13
C and δ18
O as a result of interaction with silicates and
magmatic reservoirs. If the Ca-rich hydrothermal fluid was in-
deed a product of subsurface hydrothermal interaction, it should
possess a Δ17
O similar to the silicates, instead of a Δ17
O anomaly
derived from extensive atmospheric interactions. The Ca-rich
phases identified in this study have a higher δ18
O composition,
indicating a likely low-temperature origin, and they also have
a large Δ17
O anomaly, indicating they did not likely form from
hydrothermal fluids. However, it remains possible that the δ18
O-
poor, Ca-rich carbonates observed in ref. 25 do not have an
oxygen isotope anomaly, and were therefore formed from hy-
drothermal fluids. A clear measurement of the Δ17
O in the δ18
O-
poor, Ca-rich cores of the slab/rosettes has not been performed
and 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 would
suggest an ancient atmosphere that possessed a δ13
C of 15–
20‰, whereas the carbonates obtained their heavier and co-
variant carbon and oxygen isotopic compositions through Rayleigh
distillation effects during evaporation and degassing. This pro-
vides a compelling model to understand the slab/rosettes, and if
the Ca-rich phase identified in this study formed under similar
conditions, it may have formed from the fluid after the initially
CO2-rich fluid had degassed, evaporated, and precipitated the
bulk of the carbonates. The resulting fluid could have then
reequilibrated with the atmosphere, lowering the δ13
C back to its
starting levels (∼20‰). In this case, the δ18
O would remain
elevated because the fluid had been enriched by evaporation,
and the oxygen would not have been affected by the CO2
reequilibration because of mass balance. The main drawback to
this model is that the evaporated fluid should be Mg-rich be-
cause the Mg carbonates are much more soluble than the Ca-rich
carbonates. It is difficult to explain how the carbonates might
become Ca-rich again, as the evaporation process should have
depleted 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 the
other carbonates in the meteorite, and therefore could be un-
derstood in their context. It is certainly possible that this is not
the case, and that the Ca-rich phase observed here formed in
a completely unrelated event that substantially postdated the
formation of the bulk of the carbonate within the rock. In this
case, the variation in the oxygen isotopic composition could be
understood as being a result of a different aqueous fluid with
different δ18
O composition. This is quite possible, as δ18
O var-
iations can be caused by different precipitation patterns, changes
in water source, or any number of other things. The δ13
C of the
Ca-rich phase may just represent the composition of the atmo-
sphere at the time of formation, especially as the Δ17
O content
indicates a close atmospheric relationship. If the Ca-rich phase
precipitated at room temperature similar to the rosettes (22), it
suggests that the atmosphere δ13
C was near 13‰. This atmo-
spheric composition is very similar to what is predicted by the
degassing/evaporation models (22, 33) outlined in the case where
the 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 δ13
C value of the ancient
atmosphere that is between 10‰ and 20‰. This δ13
C value for
the ancient atmosphere is significantly different from the δ13
C of
the modern atmosphere recently measured by the Mars Science
Laboratory (46‰) (34). There is a distinct possibility, given our
lack of knowledge of the oxygen budget on Mars, that the Ca-
rich phase does not record the atmospheric composition, despite
possessing a Δ17
O anomaly. However, given the strong atmo-
spheric signature recorded in this phase and its similarity to the
other carbonates in the meteorite, it suggests that the carbon
isotope composition of the Noachian atmosphere had a sub-
stantially lower δ13
C value at 4 Ga. This implies substantial en-
richment in the δ13
C of the atmosphere since 4 Ga, which may
have occurred through extensive atmospheric loss.
Methods
Martian 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 obtained
from two separate thin sections of ALH 84001 (302 and 303), using the
CAMECA ims 6f at Arizona State University and the CAMECA ims 1270 at the
University of California, Los Angeles. Samples and standards were coated
with ∼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 State
University is greatly acknowledged for providing carbonate crust from
Antarctic 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 Division
for the partial support Award no. AGS1259305 (to R.S.). C.M.C. would like
to thank Zonta International Foundation and the Ohio Space Grant
Consortium.
1. Sagan C, Toon OB, Gierasch PJ (1973) Climatic change on Mars. Science 181(4104):
1045–1049.
2. Pollack JB (1979) Climatic change on the terrestrial planets. Icarus 37(3):479–553.
3. Cess RD, Ramanathan V, Owen T (1980) The Martian paleoclimate and enhanced
atmospheric carbon dioxide. Icarus 41(1):159–165.
4. Lapen TJ, et al. (2010) A younger age for ALH84001 and its geochemical link to
shergottite sources in Mars. Science 328(5976):347–351.
5. Borg LE, et al. (1999) The age of the carbonates in martian meteorite ALH84001.
Science 286(5437):90–94.
6. Niles PB, Leshin LA, Guan Y (2005) Microscale carbon isotope variability in ALH84001
carbonates and a discussion of possible formation environments. Geochim Cosmo-
chim Acta 69(11):2931–2944.
7. Holland G, Saxton JM, Lyon IC, Turner G (2005) Negative delta O-18 values in Allan
Hills 84001 carbonate: Possible evidence for water precipitation on Mars. Geochim
Cosmochim Acta 69(5):1359–1370.
8. Leshin LA, McKeegan KD, Carpenter PK, Harvey RP (1998) Oxygen isotopic constraints
on the genesis of carbonates from Martian meteorite ALH84001. Geochim Cosmochim Acta
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.
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84001 formed at 18 +/- 4 ° C in a near-surface aqueous environment. Proc Natl Acad
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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):
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system: Delta O-17 of secondary phases in Nakhla and Lafayette. J Geophys Res
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84001. Lunar and Planetary Science XXXIII Conference [CD-ROM]. Houston, TC: Lunar
and Planetary Institute; 2002.
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Planet Sci 37(10):1345–1360.
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meteorites, 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 the
SNC 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-rich
aqueous solutions on early Mars provided by the ALH 84001 carbonates. Earth Planet
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34. Webster CR, et al.; MSL Science Team (2013) Isotope ratios of H, C, and O in CO2 and
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6 of 6 | www.pnas.org/cgi/doi/10.1073/pnas.1315615112 Shaheen et al.

7. 1
Supplementary Information:1
SI- Methods2
A slice (~1g) of ALH84001-214 was crushed (0.1- 100 μm) and divided into two subsections. The two portions of3
comminuted samples were reacted separately with concentrated H3PO4 in the side arm of reaction vessels that were4
evacuated for 4 days to ~10-6
Torr. Owing to small amount of CO2 released (~0.5 μmole) at 25 +1o
C, a third cut of5
ALH84001C (~1g) was processed as above and surface impurities were removed by allowing acid to react with the6
sample for 1 hour at ambient temperature (25 +1o
C). Gas released was collected at -196o
C after passing through two7
traps (ethanol slush at -60o
C to remove traces of water). The 1-hr extraction was not performed on the first two samples8
(ALH84001A and ALH84001B) only on the third sample (ALH84001C) (Table 1). CO2 was extracted from all of9
the samples after acid reaction for 12 h at 25 + 1o
C. Finally, the samples were heated for 3h at 150 + 1 o
C and CO210
collected following the above procedure. The sequential extraction technique at two temperatures allowed us to11
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 in13
C and O isotopes of calcite mineral. The contribution of CO2 from siderite and magnesite can be ruled out at 25o
C as14
no CO2 was released upon heating these mineral to 50o
C with phosphoric acid even after 10h (36). CO2 was purified15
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 o
C for 45h and O2 gas purified17
chromatographically to allow multi-oxygen isotopic measurements(37). Carbon and O triple isotopic compositions18
were measured using isotope ratio mass spectrometer. For replicate mass spectrometric analysis 1 SD standard19
deviations is 0.01‰ for δ18
O and δ17
O. 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 ratio21
measurement for 17
O and 18
O= 0.5‰. All these processes follow mass dependent fractionation, therefore, uncertainty22
on Δ17
O = + 0.06‰ based on laboratory standards (n = 5).23
Ion microprobe oxygen isotope measurements were obtained from two separate thin sections of ALH 8400124
(302 and 303) using the CAMECA ims 6f at Arizona State University and the CAMECA ims 1270 at the University25
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 secondary27
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)). Samples29
were pre-sputtered for 210 seconds to remove the effects of coating. Each measurement was comprised of 55 cycles30
of counting for ~1.5 seconds on mass16 and ~5.5 seconds on mass 18. Typical count rates for 16
O in the calcite31
standard were ~ 50 million counts per second. 16
O was measured using a Faraday Cup (FC) and 18
O was measured32
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 Joplin35
calcite, ZS magnesite, MS-siderite, DM dolomite and 2594 Breunnerite. Instrumental mass fractionation (IMF) varied36
from ~-18‰ to ~+2‰. Uncertainties for individual analyses are ~2‰.37

8. 2
Section ALH 84001,303 was analyzed on the UCLA ims 1270. During this run, a Cs+
beam with a beam38
current of ~2 nA was focused to a spot size of ~20 m by ~30 m. Secondary ions were analyzed in multi-collector39
mode using two Faraday cups. Samples were once again pre-sputtered for 210 seconds. The IMF for each unknown40
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 again42
~2‰.43
The carbonates measured above were described in the main text, as well as thoroughly discussed in Corrigan44
and Harvey (21). Similar occurrences were found by 49. Petrographic relationships amongst these carbonates were45
described in Corrigan and Harvey (21) as well. Specifically, in regard to the carbonates shown in Figure 3, Corrigan46
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. Rosettes48
would only have had one nucleation point, while slabs would likely have formed from the coalescence of carbonates49
growing from multiple nucleation sites in rare, large fractures. The Ca-rich portions of the slab and rosette carbonates50
were the first to form, with compositions becoming more Mg- and Fe-rich over time. Visible zones were formed as51
the occasional recharge changed the composition of fluids slightly. Magnesite and siderite rims likely formed on52
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 the56
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 CO361
i) CO2–O3 isotope exchange via excited oxygen atom O(1
D) (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 particles64
in the presence of ozone (R6-R8) as suggested by (37).65
OOQ+ h →Q(1
D) + O2 (R1)66
Q(1
D) + CO2 ↔ CO2Q* ↔ COQ + O(3
P) (R2)67
Here Q denote the heavy isotopes of oxygen (17
O, 18
O) in ozone and the enrichment is transferred68
to CO2 via short live CO2Q*.69
Q(1
D) + H2O →OH + QH (R3)70
CO+ QH → COQ + H (R4)71
CO+ O → CO2 (R5)72

9. 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) has79
been extensively studied (40). On Earth oxygen isotope anomaly produced in the stratospheric80
CO2 (R1-R3) is removed in the troposphere due to the O-isotope exchange between water and CO281
by the hydrosphere and biosphere (43). The interaction of anomalous CO2 generated via R1-R282
and its precipitation as carbonate to preserve the O-isotope anomaly would depend on the ratio of83
CO2 /H2O reservoirs. The lack of data on the O-isotopic composition and magnitude of paleo84
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 18
O86
values are dictated by the equilibration temperature. The O-triple isotope measurements on both87
carbonate phases in ALH84001 suggests that source water from which carbonates were88
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) also91
produce O-isotope anomaly in product CO2 (44, 45). Numerous pathways have been proposed for92
the production of hydro peroxy radicals (OHx= H2O2, OH, H O2) on Mars, such as electric93
discharge on dust devils, photolysis of water and reaction of O(1
D) with water vapor (46). Peroxy94
radicals (OHx= H2O2, OH, H O2) produced by the interaction of ozone with water vapor via O(1
D)95
has shown to inherit ozone isotopic signature (R3) (47). An anti-correlation of O3 with H2O at the96
equator and summer pole of Mars suggests the role of O(1
D) in the production of hydro peroxy97
radicals (48). Recombination reaction of CO with O atoms is known to produce enrichments in98
CO2 comparable to ozone (R5) (49, 50). The reaction of OH +CO is also known to produce mass99
independent fractionation with remaining CO progressively enriched in 17
O (51). On Earth, the100
Δ17
O of OHx is preserved in the stratosphere but the original ozone signal is erased due to rapid101
isotope exchange of the hydroxyl radical with tropospheric water vapor (52).102

10. 4
The combined mass independent O-isotopic anomaly produced via pathways i) and ii) will103
result in a steady state isotopically anomalous CO2 reservoir which upon equilibration with water104
would yield CO3 with a component of the ∆17
O of water. Laboratory experiments have105
demonstrated that CO3 acquires the O-isotopic composition of H2O during CO2-H2O equilibration106
process (53). Additionally, there would be no microscopic heterogeneity in the δ18
O of carbonates107
precipitated after equilibration with surface water reservoirs and the value of the isotopic anomaly108
would be constant and the small δ18
O value differences would simply reflect differential109
temperature and reactivity chemistry. Pathway iii) involves interaction of CO2 with surface110
adsorbed water on the regolith and/or dust particles in a CO2-O3-H2O-H2O2 reaction system that111
generates microscopic heterogenity (spatial and mineral specific) due to the kinetic isotope effects112
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 CO2114
and H2O, however, fractionate O-isotopes in a mass dependent fashion with 17
O ~ 0.5 18
O (56).115
The O-isotopic anomaly (Δ17
O > 0 ‰) is only generated in processes involving interaction with116
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 Martian118
molecular oxygen and bulk surface water mostly stored as CO2-H2O at the poles or subsurface119
water is unknown. If the molecular oxygen 17
O value is somewhere near bulk oxygen of the120
silicate, then the observed carbonate and water values in the SNC meteorites reflect change in the121
ozone isotopic composition and water levels. The observation of similar 17
O values in both Ca-122
rich and Fe-Mg rich carbonate phases in the present measurements reflect no significant change in123
odd oxygen cycle (O, O3) and hydroxy radical reactions. These measurements begin to show that124
the multi isotope approach on different carbonate phases can advance recognition of atmospheric125
and surficial changes, allowing for full atmospheric modelling efforts in the future.126
127
128

11. 5
129
130
Fig. S1a. Oxygen triple isotopic composition (a) O- carrying reservoirs, CO3 (ALH84001 this study), SNC CO3 and131
silicate (16), mineral water from SNC(57). The insert shows slight offset of water O-isotopic composition from132
terrestrial fractionation line with δ17
O ~ 0.52 δ18
O. Here red circles denote CO3, blue square = silicate, open triangle=133
mineral water released during pyrolysis at 600o
C, filled triangle = mineral water released at 1000 o
C. Here 17
O= 103
134
ln(1+ 17
O/103
) and 18
O= 103
ln(1+ 18
O/103
).135
136
137

12. 6
Fig. S1b. (b) Excess 17
O (Δ17
O) versus 18
O 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-Mg141
rich phase in ALH84001 (CO2 released at 150 o
C). Open blue square= Ca-rich phase in AH84001 (CO2 released at142
25o
C 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 composition144
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 ice149
and seepage of water to the rock over the 13,000 yrs the rocks laid in Antarctica. To determine and compare the effect150
of surface weathering on the O-isotopic composition of the ALH84001 Martian meteorite during its residence time in151
Antarctic, surface crust from an igneous rock in the Dry Valley (DVC) rock was also analyzed. Water in equilibrium152
with atmospheric CO2 produces mildly acidic conditions (pH <5) whereby causing mineral weathering and release of153
cations to increase the pH of the solution to less acidic values (>6) and causing CO3 precipitation (Fig. S3). Carbonates154
formed by surface weathering are enriched in both C and O-isotopes (13
C = 11‰, 17
O =14‰ and 18
O = 28‰),155
however, no excess 17
O (∆ 17
O ≈ 0) is observed. By using the measured fractionation factors for pure CO2-H2O system156
(53) at a range of temperature (0-20o
C), the equilibrium values ((13
C = +3-1‰ , 18
O = -20 to -23‰) is obtained157
using isotopic composition of preindustrial CO2 (13
C ~ -7‰, 18
O ~ 41‰) (58) and Standard Light Arctic158
Precipitation (SLAP 18
O = -55.5‰). These values are much lower than measured C and O values for the ADV159
carbonate crust. Isotopic fractionation of DIC due to CO2 outgassing or multiple freeze thaw cycles may be the primary160

13. 7
cause of enrichment of precipitated carbonates at low temperature. Terrestrial contamination in the CaCO3 fraction of161
the carbonates based on the 14
C activity of CO2 extracted from EET79001 has also been reported (59). Previous studies162
of SNC meteorites have not measured O-triple isotopic composition of the calcite fraction owing to small sample163
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
1. Sagan C, Toon OB, & Gierasch PJ (1973) Climatic Change on Mars. Science171
181(4104):1045-1049.172
2. Pollack JB (1979) Climatic change on the terrestrial planets. Icarus 37(3):479-553.173
3. Cess RD, Ramanathan V, & Owen T (1980) The Martian paleoclimate and enhanced174
atmospheric carbon dioxide. Icarus 41(1):159-165.175
4. Lapen TJ, et al. (2010) A Younger Age for ALH84001 and Its Geochemical Link to176
Shergottite Sources in Mars. Science 328(5976):347-351.177
5. Borg LE, et al. (1999) The age of the carbonates in martian meteorite ALH84001. Science178
286(5437):90-94.179
6. Niles PB, Leshin LA, & Guan Y (2005) Microscale carbon isotope variability in180
ALH84001 carbonates and a discussion of possible formation environments. Geochimica181
Et Cosmochimica Acta 69(11):2931-2944.182
7. Holland G, Saxton JM, Lyon IC, & Turner G (2005) Negative delta O-18 values in Allan183
Hills 84001 carbonate: Possible evidence for water precipitation on Mars. Geochimica Et184
Cosmochimica Acta 69(5):1359-1370.185
8. Leshin LA, McKeegan KD, Carpenter PK, & Harvey RP (1998) Oxygen isotopic186
constraints on the genesis of carbonates from Martian meteorite ALH84001. Geochimica187
Et Cosmochimica Acta 62(1):3-13.188
9. Romanek CS, et al. (1994) Record of fluid-rock interactions on Mars from the meteorite189
ALH84001. Nature 372(6507):655-657.190
10. McKay DS, et al. (1996) Search for past life on Mars: Possible relic biogenic activity in191
Martian meteorite ALH84001. Science 273(5277):924-930.192
11. Warren PH (1998) Petrologic evidence for low-temperature, possibly flood evaporitic193
origin of carbonates in the ALH84001 meteorite. Journal of Geophysical Research-Planets194
103(E7):16759-16773.195

14. 8
12. McSween HY & Harvey RP (1998) An evaporation model for formation of carbonates in196
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 carbonates198
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 Physical201
Chemistry 63(1):155-177.202
15. Thiemens MH & Shaheen R (2014) 5.6 - Mass-Independent Isotopic Composition of203
Terrestrial and Extraterrestrial Materials. Treatise on Geochemistry (Second Edition), eds204
Holland HD & Turekian KK (Elsevier, Oxford), pp 151-177.205
16. Farquhar J & Thiemens MH (2000) Oxygen cycle of the Martian atmosphere-regolith206
system: Delta O-17 of secondary phases in Nakhla and Lafayette. Journal of Geophysical207
Research-Planets 105(E5):11991-11997.208
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