Isotopic analysis of soluble organic matter in meteorites

Isotopic analysis of soluble organic matter in meteoritesIain Gilmour

Lorentz Center, University of Leiden, December 2011

• Definition and Astrochemical significance

• Analysis

• Classes of compounds

• Isotopic studies

• Relationship to IOM

Soluble organic matter in meteorites

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50 years of publications related to meteoritic organics

Fall

of M

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ison

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GCMS developmentirm-GCMS development

SIMS development

Publ

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Inorganic Organic IOM SOM

Organic matter in Murchison

Meteorite

Solvent ExtractOrganic solvents/water

based extraction

GCxGC-TOF/MSESI-FTICR/MS

PreparativeChromatography

Polar

LCMS Derivitisation

GCMS GCIRMS

Apolar

GCMS GCIRMS

Aromatic

GCMS GCIRMS

...

Residue isolationAcid treatment/CsF

Pyrolysis

Isolation and analysis of SOM

compounds identified in this and other extracts are pre-sented in the Appendix. The compounds extracted weredominated by C10 and C15 compounds, as reported bySephton et al. (2001b). In this study we unambiguouslyidentify several of these compounds containing ten car-bons including camphor (II), borneol (III), terpineol (V)and pinenone (VI); whereas other (C10) compounds aretentatively identified. Of these compounds camphor haspreviously been identified in the Orgueil carbonaceouschondrite (Studier et al., 1968). A series of C15 compoundspreviously unidentified in Orgueil are also present andinclude cadalene (XIII; 4-isopropyl-1,6-dimethylnaphtha-lene), calamenene (IX; 4-isopropyl-1,6-dimethyl-1,2,3,4-tetrahydronaphthalene), 5,6,7,8-tetrahydrocadalene (XI)and curcumene (XII) along with other unknown com-pounds. Tolylethanone, (IV) a C9 compound, is alsopresent and relatively abundant. n-Alkanes (C13–C20)are also present along with the isoprenoid hydro-carbons pristane (2,6,10,14-tetramethylpentadecane)and phytane (2,6,10,14-tetramethylhexadecane). Anunknown compound (A) with a retention time ofapproximately 28 min could not be identified but has amolecular weight of 222 and from the mass spectra (seelater) may have a molecular formula C15H26O orC16H30. Examples of such include methyldrimane oralbicanol.

3.2. Solvent extraction

The dichloromethane/methanol (95:5) solvent extract(Fig. 2) was observed to have the same type of com-pounds to those present in the SFE extract. These com-

pounds include cadalene (XIII; Fig. 3); calamenene (IX;Fig. 3), 5,6,7,8-tetrahydrocadalene (XI; Fig. 3) and acurcumene (XII; Fig. 3), a series of n-alkanes (C13–C20)and pristane and phytane. The more volatile com-pounds such as tolylethanone (IV), camphor (II), bor-neol (III), terpineol (V) and pinenone (VI) are evident atlower relative abundance possibly due to losses duringreduction of the solvent volume prior to GC injection. Adominant peak observed in the chromatogram couldnot be positively identified (retention time !28 min;unknown B, see Fig. 4), but could possibly be a com-pound such as T-muurolol (and another co-elutingcompound). This peak is not present in either the ther-mally-desorbed or the SFE extract. A peak eluting at 21min (unknown A, which is also present in the SFEsample at !28 min; see Fig. 4) could also not be iden-tified. Other minor compounds included phthalate estersand diisopropylnaphthalenes both of which are com-mon laboratory contaminants (Middleditch, 1989). Thetotal GC detectable extract was approximately 2300mg/g of the whole meteorite (calculated from the TICtotal area of the extract minus the procedural blank) orapproximately 5% of the organic carbon present in themeteorite. Cadalene which is the most abundant com-ponent present has a concentration of approximately200 mg/g of the whole meteorite. There were no detect-able peaks in the procedural blank (Fig. 2).

3.3. Thermal desorption

The thermally-desorbed Orgueil sample (Fig. 5)displays a distribution of compounds which differs

Fig. 1. Total current ion chromatogram of the supercritical fluid extract (SFE) of a fragment of the Orgueil meteorite. Structures for Ito XIII are given in the Appendix; ^=n-alkane (n-C13"20), A=unknown A.

J.S. Watson et al. / Organic Geochemistry 34 (2003) 37–47 39

References

Anders, E., Dufresne, E.R., Hayatsu, R., Cavaille, A.,Dufresne, A., Fitch, F.W., 1964. Contaminated meteorite.Science 146, 1157–1161.

Bandurski, E.L., Nagy, B., 1976. The polymer-like material inthe Orgueil meteorite. Geochimica et Cosmochimica Acta 40,1397–1406.

Bastow, T.P., Alexander, R., Kagi, R.I., 1997. Identification

and analysis of dihydro-ar-curcumene enantiomers and rela-ted compounds in petroleum. Organic Geochemistry 26, 79–83.

Becker, R.H., Epstein, S., 1982. Carbon, hydrogen and nitro-gen isotopes in solvent-extractable organic-matter from car-bonaceous chondrites. Geochimica et Cosmochimica Acta46, 97–103.

Claus, G., Nagy, B., 1961. A microbiological examination ofsome carbonaceous chondrites. Nature 192, 594–596.

Appendix

46 J.S. Watson et al. / Organic Geochemistry 34 (2003) 37–47

References







Appendix


References







Appendix


References







Appendix


Contaminants in Orgueil (Watson et al. 2003)

compounds identified in this and other extracts are pre-sented in the Appendix. The compounds extracted weredominated by C10 and C15 compounds, as reported bySephton et al. (2001b). In this study we unambiguouslyidentify several of these compounds containing ten car-bons including camphor (II), borneol (III), terpineol (V)and pinenone (VI); whereas other (C10) compounds aretentatively identified. Of these compounds camphor haspreviously been identified in the Orgueil carbonaceouschondrite (Studier et al., 1968). A series of C15 compoundspreviously unidentified in Orgueil are also present andinclude cadalene (XIII; 4-isopropyl-1,6-dimethylnaphtha-lene), calamenene (IX; 4-isopropyl-1,6-dimethyl-1,2,3,4-tetrahydronaphthalene), 5,6,7,8-tetrahydrocadalene (XI)and curcumene (XII) along with other unknown com-pounds. Tolylethanone, (IV) a C9 compound, is alsopresent and relatively abundant. n-Alkanes (C13–C20)are also present along with the isoprenoid hydro-carbons pristane (2,6,10,14-tetramethylpentadecane)and phytane (2,6,10,14-tetramethylhexadecane). Anunknown compound (A) with a retention time ofapproximately 28 min could not be identified but has amolecular weight of 222 and from the mass spectra (seelater) may have a molecular formula C15H26O orC16H30. Examples of such include methyldrimane oralbicanol.

3.2. Solvent extraction

The dichloromethane/methanol (95:5) solvent extract(Fig. 2) was observed to have the same type of com-pounds to those present in the SFE extract. These com-

pounds include cadalene (XIII; Fig. 3); calamenene (IX;Fig. 3), 5,6,7,8-tetrahydrocadalene (XI; Fig. 3) and acurcumene (XII; Fig. 3), a series of n-alkanes (C13–C20)and pristane and phytane. The more volatile com-pounds such as tolylethanone (IV), camphor (II), bor-neol (III), terpineol (V) and pinenone (VI) are evident atlower relative abundance possibly due to losses duringreduction of the solvent volume prior to GC injection. Adominant peak observed in the chromatogram couldnot be positively identified (retention time !28 min;unknown B, see Fig. 4), but could possibly be a com-pound such as T-muurolol (and another co-elutingcompound). This peak is not present in either the ther-mally-desorbed or the SFE extract. A peak eluting at 21min (unknown A, which is also present in the SFEsample at !28 min; see Fig. 4) could also not be iden-tified. Other minor compounds included phthalate estersand diisopropylnaphthalenes both of which are com-mon laboratory contaminants (Middleditch, 1989). Thetotal GC detectable extract was approximately 2300mg/g of the whole meteorite (calculated from the TICtotal area of the extract minus the procedural blank) orapproximately 5% of the organic carbon present in themeteorite. Cadalene which is the most abundant com-ponent present has a concentration of approximately200 mg/g of the whole meteorite. There were no detect-able peaks in the procedural blank (Fig. 2).

3.3. Thermal desorption

The thermally-desorbed Orgueil sample (Fig. 5)displays a distribution of compounds which differs

Fig. 1. Total current ion chromatogram of the supercritical fluid extract (SFE) of a fragment of the Orgueil meteorite. Structures for Ito XIII are given in the Appendix; ^=n-alkane (n-C13"20), A=unknown A.

J.S. Watson et al. / Organic Geochemistry 34 (2003) 37–47 39

Contaminants in Orgueil (Watson et al. 2003)

Origin of organic matter - contamination issues (from Hayatsu et al., 1973)

bons, from benzene through alkylben-

zenes and alkylnaphthalenes to polynuclear hydrocarbons of up to six

fused benzene rings (18, 22, 23, 25,

36, 37). At higher carbon numbers,

aromatics tend to be less abundant

than normal alkanes, but below about

C1l, the reverse is true (Fig. 4A). In

fact, virtually no normal alkanes at all

are found between C2 and C8, their

place having been taken largely by

benzene, toluene, xylene, and various

alkenes or branched alkanes (18, 36).

A pattern of this sort does not form

directly in the primary Fischer-Tropsch

80 0 c

o o

, .60 o 0

- 40

c o

o20

o

350 I I I l I

2C/13C Fractior

Fischer-Tropsch r

LC2

I--

- C02-Wax

i C02-Wax, C2 and heavier hydrocar

I - i , I U -

3.0 2.8 2.6 2.4 2.2 1000/T (?K-')

C i i

reaction. It does, however, develop when a primary Fischer-Tropsch mix-

ture remains in contact with the cata-

lyst, for a day or so at 350? to 400?C

(Fig. 4B), or longer times at lower

temperatures (18, 38). Under such

conditions, a metastable equilibrium is

approached, with methane and aro-

matic hydrocarbons forming at the

expense of ethane and heavier alkanes

(39). The kinetics and mechanism of

such aromatization on the catalyst sur-

face has been discussed by Galwey

(38). Of the 61 hydrocarbons in the

meteorite, 42 (underlined in Fig. 4)

500 550 ?K :.i ' Th, Fi;. rh^r- r ig. D. . iL lu r -oi.,i-

Tropsch reaction iation In shows a kinetic iso-

eaction tope fractionation between organic and carbonate carbon of the same sign and

magnitude as that in meteorites (black bars on ordinate). Observed fractiona- tions in Cl and C2 chondrites correspond to temperatures of about 360?

bons and 400?K (35). [Fractionation factor

_-_-__ - a is defined as (3C/ 2 0 18 o),o2/(13C/12) ..1

30 min 20 10 40 30 20

Fig. 4. Hydrocarbons from Murchison meteorite and Fischer-Tropsch synthesis; BP, branched paraffin; BO, branched olefin; ,I, phenyl radical. For additional peak iden-

tifications, see (18). Of the 61 hydrocarbons in the meteorite, 42 (underlined) are

also present in the Fischer-Tropsch sample, though often not in comparable amount.

(Solid area) Aromatic hydrocarbons, (stippled area) aliphatic hydrocarbons, (blank

area) compounds containing Cl or S.

784

are also seen in the synthetic sample,

though often not in the same amount.

It remains to be seen whether the

match can be made more quantitative

by changes in the reheating conditions.

When the heating is prolonged or

carried out at higher temperatures.

polynuclear aromatic hydrocarbons with up to seven rings are obtained

(18). This reaction, with CH4 as the

starting material, was discovered by Berthelot over a century ago, and has

lately been reinvestigated by several

authors (40).

Opportunities for such secondary re-

actions certainly existed in the history of meteorites. Temperatures in the

nebula (360? to 400?K, Table 1) may alone have been high enough for sec-

ondary reactions in the time available, on the order of 104 years. Kinetic

studies of a similar reaction [formation of benzene from alcohols, amines, or

fatty acids on Fe2O, or iron-rich peat

catalysts (38)] indicate a benzene for-

mation rate of 5 X 1016 molecules per

gram per year at 360?K. At this rate

it would take only 5000 years to trans-

form all the meteoritic carbon to ben-

zene. Further opportunities were pro- vided by brief thermal pulses during chondrule formation, impact, or tran-

sient shocks (1). Of course, any high-

temperature episodes must have hap-

pened early or on a local scale, to per- mit survival of other, more temperature-sensitive compounds.

Isoprenoid alkanes. At one time it

appeared (26) that nearly all carbona-

ceous chondrites contain the isoprenoid alkanes pristane and phytane (2,6,10,14-

tetramethylpentadecane and 2,6,10,14-

tetramethylhexadecane). These two hy-

drocarbons, which may formally be

regarded as tetramers of isoprene, CH : C(CHa3)CH: CH2, serve as bio-

logical markers on the earth, being derived mainly from the phytol side

chain of the chlorophyll molecule.

Their presence in meteorites thus sug-

gested either extraterrestrial life or an

abiotic process that strongly favored iso-

prenoids over other types of branched

hydrocarbons (13, 41). It seems, however, that these re-

sults reflected terrestrial contamination.

Studier et al. (18) found no tetrameric

isoprenoids in Orgueil and Murray,

only small amounts of dimeric isoprenoids from C9 to CI4. These com-

pounds can, however, be produced in

FTT syntheses (18); in fact, they are

more prominent in synthetic material

than in meteorites (Fig. 4). The

SCIENCE, VOL. 182

\ I t

J

M.A. Sephton et al. / Precambrian Research 106 (2001) 47–58 51

Fig. 1. Histograms illustrating the internal distributions of n-alkanes, pristane (Pr) and phytane (Ph) for the SFE extracts of theseven meteorite samples.

4.4. Vigarano (CV3)

The Vigarano SFE extract contained n-alkanes with a range of n-C12 to n-C17 and amode at n-C15 (Fig. 1). Pristane and phytanecould not be detected. !13C values for n-C13 ton-C15 ranged from −27.9 to −28.9‰ (Table 2,Fig. 2).

4.5. Allende (CV3)

GC-MS analyses of the SFE extract of Allendedetected no n-alkanes (Fig. 1).

4.6. Ornans (CO3)

The Ornans SFE extract contained n-alkanes

n-alkanes in chondrites (Sephton, Pillinger & Gilmour 2001)

Phytane - 2, 6, 10, 14 tetramethyl pentadecane


Fig. 1. Histograms illustrating the internal distributions of n-alkanes, pristane (Pr) and phytane (Ph) for the SFE extracts of theseven meteorite samples.

4.4. Vigarano (CV3)

The Vigarano SFE extract contained n-alkanes with a range of n-C12 to n-C17 and amode at n-C15 (Fig. 1). Pristane and phytanecould not be detected. !13C values for n-C13 ton-C15 ranged from −27.9 to −28.9‰ (Table 2,Fig. 2).

4.5. Allende (CV3)

GC-MS analyses of the SFE extract of Allendedetected no n-alkanes (Fig. 1).

4.6. Ornans (CO3)

The Ornans SFE extract contained n-alkanes

n-alkanes in chondrites (Sephton, Pillinger & Gilmour 2001)

Phytane - 2, 6, 10, 14 tetramethyl pentadecane

J. R. Cronin and S. Pizzarello

-2.01 IO 20 I)0 40 20

4.0

2.0

2.0

1.0

0

-1.0

-2.0

-2.0

-4.0 00 70 20 20 IOC

a.0 84

2.0.

1.0

0

- 1.0 4

loo I40 170 I.0 I*0 200 210

TINE (nln.)

FIG. 2. Total ion chromatograms of whole benzene-methanol extracts of interior (inverted) and exterior samples of Murchison specimen 2. The numbered compounds were tentatively identified as

sample. A large decrease in abundance is observed for each n-alkane from C number 14 to 28. In a few cases where the decrease does not appear to be as pronounced, a coeluting component contributes substantially to the peak in the interior extract trace: a phthalate residue at C,v, phenanthrene and anthracene at Cz,, and an unknown compound with base peak at m/z 208 in the case of Cz5. Whether the small amounts of n-alkanes in the interior extract are indigenous or represent contamination that has diffused into the interior of the stone is not known. However, it is clear that if the IZ- alkanes are indigenous, they are not the predominant compounds, as has been previously claimed (STUDIER et al. 1972).

GC-MS Analyses of Hydrocarbon Extracts of the Murray Meteorite

The Murray meteorite is very similar to the Murchison meteorite but has had a terrestrial exposure period about twice as long. (The Murray fall was in 1950, Murchison in 1969.) Exterior and interior samples of this meteorite were obtained by drilling, respectively, at the surface and into the interior of a fragment that had been obtained some time ago when a larger stone was broken. Benzene-methanol extracts of these samples were prepared and analyzed as was done with Murchison specimen 2. Both extracts contained abundant n-alkanes along with clear indications of terrestrial contamination in the form of isoprenoid alkanes and phthalate esters. The distribution of the n-alkanes in the extract of the interior sample is illustrated by the m/z 57 single ion chromatogram shown in Fig. 3. The maximum abundance of this ion, which is very intense in the mass spectra of acyclic alkanes, occurs at the retention time of n-heptadecane and declines continuously through n-octacosane. It is interesting to note that this distribution is very similar to that observed for the n-alkanes of the gas-phase fraction of residential city air (CAUTREELS and VAN CAUWENBERGHE, 1978).

In an attempt to obtain samples at a greater distance from the surface, a second drilling experiment was carried out with a larger Murray stone. In this experiment, surface material, much of which was fusion crust, was initially ground off. Next, material drilled out ofthe interior was collected between the fresh surface and 1.2 cm (interior 1), and then from 1.2 cm to 2.5 cm (interior 2) into the stone. Each of these samples was obtained and worked up in a protected environment as described above. The content of n-alkanes in these samples was lower by about a factor of 10 in comparison with the

follows: (1) branched alkane, (2) naphthalene, (3) branched alkane, (4) methyl naphthalene, (5) acenaphthene, (6) 2,6-t-butyl-1,4-ben- zoquinone, (7) branched alkane, (8) branched alkane, (9) ?, (10) ?, (11) branched alkane, (I 2) tributyl phosphate, (13) diethyl phthalate, (14) 4-hydroxyl-3,5-di-t-butyl benzaldehyde. (15) methylbutyl phthalate, (16) methyl hexadecanoate, (17) fluoren-9-one, (18) phenanthrene, (19) anthracene, (20) anthracene-9-one, (21) and (22) methyl dibenzothiophenes, (23), (24), (26), and (27) methylphen- anthrenes/methylanthracenes, (25) methyl octadecanoate, (28) dimethyl naphthothiophene, (29) dibutyl/diisobutyl phthalate, (30) dimethyl naphthothiophene, (3 1) ?, (32) fluoroanthene, (33) pyrene, (34) methyl pyrene, (35) butoxyethylbutyl phthalate, (36) di-set-octyl phthalate, (37) naphthacene/chrysene/triphenylene, (38) ?, (39) bis(2- ethylhexyl)/dioctyl sebacate, (40) ?, (4 I) erucamide ?.

Exterior

Exterior

Murchison aliphatic hydrocarbons (Cronin & Pizzarello, 1990)

J. R. Cronin and S. Pizzarello

-2.01 IO 20 I)0 40 20

4.0

2.0

2.0

1.0

0

-1.0

-2.0

-2.0

-4.0 00 70 20 20 IOC

a.0 84

2.0.

1.0

0

- 1.0 4

loo I40 170 I.0 I*0 200 210

TINE (nln.)

FIG. 2. Total ion chromatograms of whole benzene-methanol extracts of interior (inverted) and exterior samples of Murchison specimen 2. The numbered compounds were tentatively identified as

sample. A large decrease in abundance is observed for each n-alkane from C number 14 to 28. In a few cases where the decrease does not appear to be as pronounced, a coeluting component contributes substantially to the peak in the interior extract trace: a phthalate residue at C,v, phenanthrene and anthracene at Cz,, and an unknown compound with base peak at m/z 208 in the case of Cz5. Whether the small amounts of n-alkanes in the interior extract are indigenous or represent contamination that has diffused into the interior of the stone is not known. However, it is clear that if the IZ- alkanes are indigenous, they are not the predominant compounds, as has been previously claimed (STUDIER et al. 1972).

GC-MS Analyses of Hydrocarbon Extracts of the Murray Meteorite

The Murray meteorite is very similar to the Murchison meteorite but has had a terrestrial exposure period about twice as long. (The Murray fall was in 1950, Murchison in 1969.) Exterior and interior samples of this meteorite were obtained by drilling, respectively, at the surface and into the interior of a fragment that had been obtained some time ago when a larger stone was broken. Benzene-methanol extracts of these samples were prepared and analyzed as was done with Murchison specimen 2. Both extracts contained abundant n-alkanes along with clear indications of terrestrial contamination in the form of isoprenoid alkanes and phthalate esters. The distribution of the n-alkanes in the extract of the interior sample is illustrated by the m/z 57 single ion chromatogram shown in Fig. 3. The maximum abundance of this ion, which is very intense in the mass spectra of acyclic alkanes, occurs at the retention time of n-heptadecane and declines continuously through n-octacosane. It is interesting to note that this distribution is very similar to that observed for the n-alkanes of the gas-phase fraction of residential city air (CAUTREELS and VAN CAUWENBERGHE, 1978).

In an attempt to obtain samples at a greater distance from the surface, a second drilling experiment was carried out with a larger Murray stone. In this experiment, surface material, much of which was fusion crust, was initially ground off. Next, material drilled out ofthe interior was collected between the fresh surface and 1.2 cm (interior 1), and then from 1.2 cm to 2.5 cm (interior 2) into the stone. Each of these samples was obtained and worked up in a protected environment as described above. The content of n-alkanes in these samples was lower by about a factor of 10 in comparison with the

follows: (1) branched alkane, (2) naphthalene, (3) branched alkane, (4) methyl naphthalene, (5) acenaphthene, (6) 2,6-t-butyl-1,4-ben- zoquinone, (7) branched alkane, (8) branched alkane, (9) ?, (10) ?, (11) branched alkane, (I 2) tributyl phosphate, (13) diethyl phthalate, (14) 4-hydroxyl-3,5-di-t-butyl benzaldehyde. (15) methylbutyl phthalate, (16) methyl hexadecanoate, (17) fluoren-9-one, (18) phenanthrene, (19) anthracene, (20) anthracene-9-one, (21) and (22) methyl dibenzothiophenes, (23), (24), (26), and (27) methylphen- anthrenes/methylanthracenes, (25) methyl octadecanoate, (28) dimethyl naphthothiophene, (29) dibutyl/diisobutyl phthalate, (30) dimethyl naphthothiophene, (3 1) ?, (32) fluoroanthene, (33) pyrene, (34) methyl pyrene, (35) butoxyethylbutyl phthalate, (36) di-set-octyl phthalate, (37) naphthacene/chrysene/triphenylene, (38) ?, (39) bis(2- ethylhexyl)/dioctyl sebacate, (40) ?, (4 I) erucamide ?.

Exterior

Exterior

Murchison aliphatic hydrocarbons (Cronin & Pizzarello, 1990)

Interior

Interior

Organic compounds in Murchison 2863

50 100 150 200

TIME hln.t

FIG. 3. Single ion chromatogram (m/z 57) of a whole benzene- methanol extract of an interior Murray sample. ‘The series of intense peaks represent the n-alkanes; the intensity maximum corresponds to n-heptadecane.

previous Murray analyses. Interestingly, the distribution of n-alkanes varied significantly among the surface and the two interior samples. The intensity of the n-alkane peaks in the m/z 57 mass fragmentograms is plotted in Fig. 4. A feature unique to the n-alkanes of these samples is a component having a distribution with a maximum at n-tetradecane. In each case this dist~bution is superimposed on a suite of heavier alkanes having distribution maxima at 2CZ8 (exterior sample), CZI (interior l), and Cl7 (interior 2). When the in- tensities of the n-alkane peaks of these three samples are summed, the total intensity dist~bution is similar to that observed for the first Murray samples analyzed, except for the addition of the light fraction peaking at Cr4. Ignoring for the moment the light fraction, the shift of the distribution

3s

30

25

s ’ 3 20

I. L 5 ; is

IO

5

0 IC

I.... I . . . . I . . . . I4

IS 20 25 30

Carbon t(umb~r

FIG. 4. Norma1 alkane abundances in whole benzene-methanol extracts of the Murray meteorite: triangles, exterior sample, mainly fusion crust; circles, interior sample 1 taken from surface (after re- moval of fusion crust) to 1.2 cm; squares, interior sample 2 taken from 1.2 cm to 2.5 cm below surface; hexagons, sum of exterior and two interior samples.

maxima of the remainder of the alkanes toward lighter members of the series as sampling proceeded inward suggests the possibility that airborne hydrocarbons, which accumulated on the surface of the stone, diffused inward, with the lighter, more mobile components reaching the greatest depth.

The presence of phthalates in the sample taken at greatest distance from the surface shows that, even at a depth of 1.2 to 2.5 cm below the surface, the Murray specimen is not free of con~minants. The phthalates in this sample are made evident by a m/z 149 (phthalate base peak) single ion chromatogram, as displayed in Fig. 5a. Moreover, the more abundant isoprenoids, phytane, pristane. etc., are also present, suggesting a substantial terrestrial contribution to the alkanes, in general (see Fig. Sb).

The behavior of the phthalate diester contaminants in these samples is similar to that of the n-alkanes. All three samples contain the principal members of this class of compounds, diethyl-, dibutyl-, and di-2-ethylhexyl phthalate. The total concentration of these three compounds drops off smoothly as one proceeds from the surface to the interior, and in the innermost sample is about 40% of that at the surface. How- ever, the concentration of diethyl phthalate, the lightest of these compounds, is nearly the same in all three samples, whereas the 2-ethylhexyl diester drops sharply from the exterior to the first interior sample-a result that may reflect the effects of diffusion.

2.3.

i j a

x

c 1.0

g E 0.5 f

0 I II ( 50 IO0 150 LOO

nC13

4

TIYE bIlla.)

Eii

m5

n IL 6

b

TIME (min.)

FIG. 5. Single ion chromatograms of the whole benzene-methanol extract of an interior (1.2 to 2.5 cm depth) Murray sample. (a) m/z 149; (b) m/z 57. The isoprenoid alkanes are numbered as in Ta- ble 1.

Murray n-alkanes (Cronin and Pizzarello, 1990)


50 100 150 200

TIME hln.t



3s

30

25

s ’ 3 20

I. L 5 ; is

IO

5

0 IC

I.... I . . . . I . . . . I4

IS 20 25 30

Carbon t(umb~r





2.3.

i j a

x

c 1.0

g E 0.5 f

0 I II ( 50 IO0 150 LOO

nC13

4

TIYE bIlla.)

Eii

m5

n IL 6

b

TIME (min.)


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Fig. 2. Plot of !13C values vs carbon number for individual n-alkanes from meteorite SFE extracts. Assignments are as follows:(open circle) Orgueil, (filled circle) Murchison, (filled diamond) Cold Bokkeveld, (filled square) Vigarano, (open diamond) Ornans,(open square) Bishunpur. The n-C13 to n-C16 measurements for Murchison are from Gilmour and Pillinger (1993). Values ofMurchison aliphatic fractions are included for comparisons (Krishnamurthy et al., 1992), as are ranges of bulk petroleum (Stahl,1979)) and petroleum n-alkanes (Bjorøy et al., 1991).

in general agreement with the majority of theprevious analyses performed on this meteorite.

The aliphatic hydrocarbons in Murchison wereinvestigated on a number of occasions shortly afterthis meteorite fell in 1969. The dominant aliphaticcomponents were reported as cycloalkanes andbranched alkanes (Kvenvolden et al., 1970; Oro etal., 1971) and n-alkanes (Studier et al., 1972).Studier et al. (1972) also detected pristane andphytane in a solvent extract of Murchison. Croninand Pizzarello (1990) analysed this meteorite anddetected cycloalkanes, n-alkanes and isoprenoidalhydrocarbons including pristane and phytane. Theranges of n-alkanes found in these studies ex-tended from n-C8 to n-C18 (Studier et al., 1972)and n-C12 to n-C25 (Cronin and Pizzarello, 1990)and both studies revealed a mode at n-C17. Thecompound distribution for Murchison observed inthis study has strong similarities to the previouswork of Studier et al. (1972) and Cronin andPizzarello (1990). These authors detected slightlydifferent ranges of n-alkanes but the presence ofpristane and phytane and the modes at n-C17 areidentical to that seen in this study.

Earlier studies on the aliphatic hydrocarbons inCold Bokkeveld have detected n-alkane distribu-tions which ranged from n-C14 to n-C26 and dis-played consistent modes at n-C17 (Oro et al., 1966;Nooner and Oro, 1967; Smith and Kaplan, 1970).The results obtained in this study reveal a nar-rower range for these compounds biased towardsthe higher molecular weight range of theprevious analyses. This is reflected in the n-alkanemode of n-C21 which is a higher molecular weightmode than those found in most of the previouswork.

Previous studies on Allende detected n-alkanesas the major extractable components and revealedthat these compounds were concentrated near thesurfaces of the meteorite (Han et al., 1969; Levy etal., 1970, 1973; Cronin and Pizzarello, 1990). Thecomplete absence of n-alkanes in Allende in thisstudy is in conflict with the results obtained bythese previous authors.

Earlier work on the aliphatic hydrocarbons inVigarano detected n-alkane distributions whichranged from n-C14 to n-C25 and displayed modes

Isotopic compositions of aliphatic hydrocarbons (data from Sephton et al., 2001; Gilmour & Pillinger, 1993)









Murchison Orgueil

Cold BokkeveldVigarano

Ornans









Murchison Orgueil

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using least-squares linear regression. Figure 4 illustrates thisdata and compares it to similar data from the CM2 Murch-ison meteorite and the Antarctic CR2 meteorites GRA 95229,EET 92042, and LAP 02342.

The linear regression through the fungal peptide data inFig. 4 illustrates that potentially diagnostic relationships doindeed exist between pairs of fungal amino acids. In thesefits, a negative y intercept indicates that the amino acid onthe y axis is depleted in the heavier isotope compared to theamino acid on the x axis; a positive y intercept indicatesenrichment. The data show that a-AIB from the fungalpeptaibiotics is consistently depleted in 13C compared toboth Gly and L-Ala. a-AIB is also slightly enriched in 15Ncompared to Gly and slightly depleted in d15N compared toL-Ala. L-Ala and Gly have similar d13C values, but L-Ala isenriched in d15N compared to Gly. D-Iva is not plotted be-cause of the scarcity of data from both fungal peptaibioticsand meteorites, but the d13C value of D-Iva is indistin-guishable from a-AIB within experimental error in the twopeptides in which it was detected. All the analyzed pairsshow strong linear correlations (r2 values between 0.87 and0.96), and the slopes of these lines are within one 95% con-fidence interval of 1.0.

The observed correlations may speak to the biosyntheticorigins of the amino acids. If, for example, a-AIB as a

building block for peptaibols is produced by enzymaticmethyl addition via adenosyl-methionine to L-Ala (Kubiceket al., 2007), our measurements suggest that this processcauses enrichment in the a-AIB relative to the L-Ala, withd13C values of a-AIB being enriched by 5–8%. Presumably,this correlation would hold true regardless of the absoluted13C value; that is, a-AIB would be enriched relative to L-Alaeven in peptaibiotics produced from fungi feeding on al-ready enriched meteoritic organics.

If this hypothesis is valid, and if the correlations observedin the linear regressions from these four peptaibiotics can beextended to other fungal peptides, then linear regressioncorrelation of amino acid isotopic values may provide adiagnostic tool for ruling out fungal contamination as asource of certain amino acids in meteorites. Meteoriticamino acids that deviate from the correlation observed inthe fungi are presumed to originate from other syntheticprocesses. From Fig. 4A and 4C, it appears that the a-AIB inthe Murchison meteorite has a different isotopic relation-ship with both Gly and L-Ala than that seen from the fungalpeptides; Murchison a-AIB is more enriched in 13C com-pared to Gly and to L-Ala than the corresponding fungalpeptide amino acids. Figure 4B also shows that L-Ala ismore enriched in 13C compared to Gly in Murchison than inthe fungal peptides. The d15N relationships between

FIG. 4. Isotopic ratios and linear regression fits for d13C for the pairs of amino acids (A) a-AIB vs. Gly, (B) L-Ala vs. Gly, and(C) a-AIB vs. L-Ala, and for d15N for (D) a-AIB vs. Gly, (E) L-Ala vs. Gly, and (F) a-AIB vs. L-Ala. The data from the fourfungal peptides analyzed in this work are represented by open squares (,). Meteoritic data from the CM2 Murchisonmeteorite (!) and the Antarctic CR2 meteorites GRA 95229 and LAP 02342 (~) are also shown (see Fig. 3 caption forreferences). Panel B also includes data from the CR2 meteorite EET 92042 (~) (Martins et al., 2007a).

130 ELSILA ET AL.

Isotopic identification of potential contaminants (Elsila et al., 2011)







130 ELSILA ET AL.


fungal peptaibiotics







130 ELSILA ET AL.


fungal peptaibiotics

Murchison

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Fig. S2. (A–D) Reconstructed mass spectra showing the mass distributions and (E–H) van Krevelen diagrams specific for CHO, CHOS, CHNO, and CHNOSelemental compositions of Murchison methanolic extract from ESI(−) FTICR mass spectra. The most intense signals protruding from the regular distributionpatterns correspond to impurities such as contaminations and/or degradation end-products. (I and J) van Krevelen diagrams obtained under the hypothesis ofSO3 added to (I, E) CHO and (J, F) CHNO series (conversion of C-OH into C-OSO3H).

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Structural diversity of Murchison methanolic extract (Schmitt-Kopplin et al. 2010)

such as Michael addition of ammonia to cyanoacetylene(Miller 1957). Based on this amino acid evidence it wasconcluded that the Orgueil and Ivuna meteorites musthave originated on a chemically distinct parent bodyfrom the CMs, possibly an extinct comet (Ehrenfreundet al. 2001).

A subsequent comparison of the relative abundancesof b-alanine and a-AIB in Orgueil with four CM2meteorites showed that the relative abundances of bothb-alanine and a-AIB tracked with the degree of aqueousalteration of the meteorites, with the more alteredOrgueil meteorite having the highest b-alanine andlowest a-AIB relative abundances (Glavin et al. 2006).Although a similar trend was also observed for the CR1chondrite GRO 95577 and the less altered CR2s EET92042 and GRA 955229 (Martins et al. 2007), there wasno clear trend with aqueous alteration found in relativea-AIB and b-alanine abundances in a previous study ofCM1 and CM2 chondrites (Botta et al. 2007). Onepotential problem with comparing amino acid abundancedata from these different studies is that the meteoriteswere analyzed in different laboratories at different timesusing different instrumentation and different peakintegration procedures. In addition, sample heterogeneity

must also be accounted for when drawing comparisonsbetween laboratories. Here for the first time we comparethe relative abundances of amino acids collected on asuite of CI, CM, and CR carbonaceous chondritesthat cover the entire range of aqueous alteration(petrographic types 1–3) using the identical extraction,analytical, and quantitation techniques in the samelaboratory.

Relative Abundance of b-alanine as an Indicator forAqueous Alteration

A comparison of the relative molar abundances(glycine = 1.0) of a-alanine (d+l enantiomers), b-alanine, a-AIB, and isovaline (d+l enantiomers)measured in nine different carbonaceous chondrites isshown in Fig. 4. The order of carbonaceous chondrites inFig. 4 are based on the approximate degree of aqueousalteration inferred from mineralogical and isotopicevidence with the most altered CI1 Orgueil and othertype 1 chondrites on the left and the most primitive, leastaltered CR chondrites on the right (Zolensky andMcSween 1988; Kallemeyn et al. 1994; Zolensky andBrowning 1994). The relative degree of aqueousalteration among carbonaceous chondrites within the

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Fig. 4. A comparison of the relative molar abundances (glycine = 1.0) of alanine, b-alanine, a-aminoisobutyric acid, andisovaline in the 6M HCl-hydrolyzed, hot-water extracts of the carbonaceous meteorites investigated in this study. The relativeabundances were calculated from the data in Table 2 after correcting for the molecular weights of each amino acid. Theuncertainties were calculated by standard error propagation of the absolute errors in Table 2. The amino acid data forMurchison (USNM 6650) and LEW 90500 were taken from Glavin et al. (2006).

Amino acids in carbonaceous chondrites 1963

Evidence for parent body processing of amino acids (Glavin et al., 2011)

more primitivemore altered

eous chondrites (Murchison, LEW 90500, and LON 94102) par-ticularly stood out because they had the most diverse and abun-dant set of purines measured (Fig. 1 and SI Text). Total purineabundances were 4 to 12 times higher in CM2 carbonaceouschondrites compared to the other meteorites examined. As thedegree of aqueous alteration increases in CM carbonaceouschondrites, the overall abundance and diversity of nucleobasesdecreases as seen in ALH 83100 (CM1/2), SCO 06043 (CM1),and MET 01070 (CM1). Meteorites of other groups (CI andCR) with varying degrees of aqueous alteration (types 1, 2,and 3) also had comparatively less total purine abundance andstructurally diversity compared to CM2 carbonaceous chondrites.Furthermore, there is an apparent correlation between purinediversity and the diversity of other small organic species, suchas amino acids (5), with the maximum diversity found in CM2carbonaceous chondrites. Interestingly, this correlation doesnot hold for the abundances of such compounds; for example,CR2 and CR3 meteorites are rich in amino acids and smallamines (5, 20, 21) but poor in purines. Based on our measure-ments, the CM2 carbonaceous chondrites appear to have pro-vided a more favorable environment for the formation and/orpreservation of purines compared to other types of carbonaceouschondrites.

Establishing an unambiguous extraterrestrial origin for anybiological nucleobase in carbonaceous chondrites is challenging.Unlike meteoritic amino acids, nucleobases are achiral so enan-tiomeric ratios (i.e., D/L ratios) cannot be used to help distinguishbetween abiotic and biotic origins. With sample-limited meteor-ites, compound-specific stable isotope analysis may be impracti-cal or impossible due to the large amount of meteorite materialrequired. Additionally, other meteoritic organics may prevent anunambiguous stable isotope ratio measurement despite extensivesample cleanup and chromatography (19). Indigenous organiccompounds in meteorites are usually present in structurallyhomologous series (2); therefore, finding nucleobase analogsnot typically found in terrestrial biochemistry would strongly sup-port an extraterrestrial origin for these canonical nucleobases be-cause they are often produced concurrently in abiotic syntheses.

Purine, 2,6-diaminopurine, and 6,8-diaminopurine were unam-biguously identified in LON 94102 (Fig. 2) and a larger extract ofMurchison by their chromatographic retention time, accuratemass spectrum (including accurate mass measurements on multi-ple fragmentation products), and coinjection with standards (re-sulting in the detection of a single peak) (22). Additionally, twodifferent formic acid extracts of LON 94102 were analyzed onthree different liquid chromatography–mass spectrometry instru-ments (one triple quadrupole and two Orbitraps) in two separatelaboratories [National Aeronautics and Space Administration(NASA) Goddard Space Flight Center and Thermo Scientific],which all produced similar results. Purine and 6,8-diaminopurinewere identified (by their chromatographic retention time and anaccurate mass measurement for the parent mass) in several othermeteorites as well (Fig. 1 and SI Text). This demonstrates thatboth purine and 6,8-diaminopurine are widely distributed in car-bonaceous chondrites, particularly in CM2 and CR2 meteorites,and provides additional support that purines found in these me-teorites are indigenous and not terrestrial contaminants. Asidefrom one report of 2,6-diaminopurine occurring in cyanophageS-2L (23), these three purines are rare or absent in terrestrialbiology. Studies of 8-aminoadenosine, as a potential cancer ther-apeutic, have shown that this compound is known to inhibit tran-scription by multiple mechanisms (24) so that the presence of 6,8-diaminopurine (8-aminoadenine) in meteorites is highly unlikelyto be the result of terrestrial biological contamination.

All of the purines observed in Murchison and LON 94102(i.e., adenine, guanine, hypoxanthine, xanthine, purine, 2,6-dia-minopurine, and 6,8-diaminopurine) were also generated fromaqueous reactions of NH4CN (see SI Text). Adenine (normalizedto 1) was the most abundant nucleobase in the formic acid ex-tracted NH4CN samples followed by purine (0.79), hypoxanthine(0.23), 6,8-diaminopurine (0.07, assuming the same response fac-tor as 2,6-diaminopurine), 2,6-diaminopurine (0.05), guanine(0.02), and xanthine (0.01). Although the relative abundancesof these purines are different than those detected in carbonac-eous chondrites, this may be attributable to the extensive aqueousand energetic processing the asteroid parent bodies have under-gone during their approximately 4.5 billion-year history (25) com-pared to the NH4CN reactions. The presence of hydrogencyanide and ammonia as synthetic precursor molecules has beendeduced in hydrated carbonaceous chondrites based on the pre-sence of α-amino acids, α-hydroxy acids, and iminodicarboxylicacids reported in the Murchison meteorite, which supports aStrecker-type synthesis requiring hydrogen cyanide and ammonia(13, 26–29). Additionally, abundant ammonia has been detectedin the CR2 carbonaceous chondrite GRA 95229 after hydrother-mal treatment (30), which was one of the meteorites in our study.

Purine, 2,6-diaminopurine, and 6,8-diaminopurine were notdetected (above our parts-per-billion detection limits) in the pro-cedural blanks, nucleobase procedural samples, serpentine con-trol samples, Murchison soil sample (see SI Text), or Antarctic icesample (see SI Text), which strongly suggests that these com-pounds are indigenous to the meteorites. Because adenine, gua-nine, hypoxanthine, and xanthine were observed in both the soiland Antarctic ice samples (though at different ratios and lowerabundances than observed in meteorites; see SI Text), it couldstill be argued that these nucleobases are the result of terrestrialcontamination. On the other hand, these same nucleobases arealso synthesized concurrently with purine, 2,6-diaminopurine,and 6,8-diaminopurine in reactions of NH4CN. Furthermore,the distributions of purines measured in the nine Antarctic me-teorites appear to correlate with meteorite petrology and theextent of parent body alteration rather than with the contentof the terrestrial environments from which they were recovered,arguing against terrestrial contamination from the ice. Based onthe elevated abundances of these compounds in the meteoritescompared with terrestrial sources, we propose that the adenine,

Fig. 1. Distribution of guanine, hypoxanthine, xanthine, adenine, purine,and 2,6-diaminopurine in 11 carbonaceous chondrites and one ureilite.The three CM2 carbonaceous chondrites in this study (Murchison, LEW90500, and LON 94102) contained significantly higher (approximately 4×to 12×) abundances of purine nucleobases as well as greater structurally di-versity. The * represents a tentative assignment. The meteorites are roughlyordered by increasing aqueous alteration (Right to Left) as determined usingmineralogical and isotopic evidence (38–41). The relative degree of aqueousalteration among carbonaceous chondrites within the same group and of thesame petrologic type is less certain, although some ordering can be made.

13996 ∣ www.pnas.org/cgi/doi/10.1073/pnas.1106493108 Callahan et al.

Nucleobases in meteorites (Callahan et al., 2011)

Fig. S2. (A–D) Reconstructed mass spectra showing the mass distributions and (E–H) van Krevelen diagrams specific for CHO, CHOS, CHNO, and CHNOSelemental compositions of Murchison methanolic extract from ESI(−) FTICR mass spectra. The most intense signals protruding from the regular distributionpatterns correspond to impurities such as contaminations and/or degradation end-products. (I and J) van Krevelen diagrams obtained under the hypothesis ofSO3 added to (I, E) CHO and (J, F) CHNO series (conversion of C-OH into C-OSO3H).

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Structural diversity of Murchison methanolic extract (Schmitt-Kopplin et al. 2010)

Deuterium enrichments in amino acids (Data from Pizzarello & Huang, 2005; Pizzarello et al., 2008)

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more 13C-enriched than typical terrestrial PAHsconfirming the indigenous nature of these com-pounds. The predominant aromatic trend isconsistent that observed for C1–C5 compoundsfrom Murchison and indicates an origin by asynthetic process progressively adding 12C to thecarbon skeleton with kinetic isotopic fraction-ation determining the distribution of carbonisotopes between compounds rather thanthermodynamic equilibrium (Gilmour andPillinger, 1994; Naraoka et al., 2000). The d13C

values obtained for extractable aromatic hydro-carbons in CM2 meteorites display a significantamount of isotopic heterogeneity with a range ind13C values of over 20‰. Compounds, withrelatively high molecular weights, but whichdiffer by only one or two carbon atoms alsodisplay significant differences in their d13Cvalues. This has led to the suggestion that duringthe synthetic processes that led to bond for-mation, isotopic fractionation was at its mostextreme, implying that synthesis took place in alow-temperature environment such as interstellarspace (Sephton and Gilmour, 2000). The isotopicheterogeneity displayed by aromatic compoundsin Murchison and Asuka-881458 may alsocontain evidence for different synthetic pathways.There is a 7.5‰ difference in d13C valuesbetween PAHs isomers containing a five-carbonring (e.g., fluoranthene) and those without (e.g.,pyrene), which has been interpreted as evidenceof two possible pathways for the formation ofPAHs (Gilmour and Pillinger, 1994; Naraokaet al., 2000).

The d13C values for the C12–C26 n-alkanesfrom six chondrites are shown in Figure 4(Sephton et al., 2001). None of the n-alkanesexhibit either the 13C-enrichments or systematicisotopic trends that apparently characterize indi-genous organic matter in meteorites. Most of thed13C values are similar both in value and in thetrends shown within homologous series to d13Cvariations observed for terrestrial petroleumproducts or other terrestrial fossil hydrocarbons.These features confirm the long-held suspicionthat these molecules are contaminants from the

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acid, glycine, etc. (source Yuen et al., 1984).

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Figure 4 Carbon stable-isotope compositions ofsolvent extractable n-alkanes from the Orgueil (CI),Cold Bokkeveld (CM2), Murchison (CM2), Vigarano(CV3), Ornans (CO), and Tagish Lake carbonaceouschondrites plotted against carbon number (sources

Sephton et al., 2001; Pizzarello et al., 2001).

Extractable Organic Matter 279

Molecular level isotope analysis - LMW compounds Murchison (Yuen et al., 1984)




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(2). The MCA pattern for 11h shows a trend ofdecreasing d13C with increasing C number, com-parable to results for Murchison (30). Whereasthis trend has been attributed to the preservationof the signature of kinetically controlled C ad-dition in MCA synthesis, which takes place incold, interstellar, or nebular environments (31),our results, which suggest that specimen 11h ismore altered than 5b, imply that such a patternmay be a secondary signature. One possibleexplanation for the pattern in this case is thepreferential exchange of MCA carboxyl C withinorganic C during hydrothermal processing,analogous to the process that occurs in oil-pronesource rocks on Earth (32). In the Tagish Lakemeteorite, the presence of carbonate d13C ~ 67‰(17) may provide a source of isotopically en-riched carbonate for such exchange. Notably,formic acid concentration and C isotopic compo-sition remain relatively constant among the speci-mens (13), which suggests that they are relativelyunaffected by aqueous alteration (10) and may beinherited from preaccretionary material.

Amino acid concentrations and enantiomericexcesses in the Tagish Lake specimens providefurther evidence of the influence of parent bodyaqueous alteration on SOM. We determinedthe distribution and enantiomeric abundances ofthe one- to six-C aliphatic amino acids found inextracts of specimens, 5b, 11h, and 11i by ultra-performance liquid chromatography fluorescencedetection and time-of-flight mass spectrometry(33). We measured stable C isotope analyses ofthe most abundant amino acids in 11h with gaschromatography coupled with quadrupole massspectrometry and isotope ratio mass spectrome-try. The total abundances of amino acids decreasein the order 11h (5.6 ppm) > 5b (0.9 ppm) > 11i(0.04 ppm). The abundances of many amino acidsin 11i were below the analytical detection limit(<1 part per billion), which is consistent with amuch higher degree of alteration experienced by11i as compared to 11h and 5b. The abundance ofthe nonprotein amino acid a-aminoisobutyricacid in specimen 11hwas 0.2 ppm, approximate-ly 200 times higher than previously measuredin two different Tagish Lake meteorite samples(18, 19). Glycine is the most abundant aminoacid in 11h and has a C isotope value of d13C =+19‰, which falls well outside the range forterrestrial organic C of –6 to –40‰ (34) and isconsistent with an extraterrestrial origin.

The enantiomeric ratios of alanine, b-amino-n-butyric acid, and isovaline in 11h were racemicwithin uncertainties (D/L = 1), providing addi-tional evidence of an extraterrestrial origin forthese amino acids. In contrast to specimen 11h,nonracemic isovaline was detected in 5b, with anL-enantiomeric excess of ~7%, and no isovalinewas identified in 11i above the detection limit.Although the mechanism of enrichment re-mains unclear, it has been previously shown thatL-isovaline enantiomeric excesses (ee’s) and theratio of b-alanine to glycine both increase relativeto the degree of aqueous alteration for many

carbonaceous chondrite groups (33, 35). Althoughthe data for specimen 11i relative to 11h or 5bfit this trend (Fig. 3), in detail the sequence ofalteration for 5b and 11h based on these criteriasuggests that 5b is more altered than 11h, incontrast to the result from petrography and IOM.This result suggests that other factors may in-fluence ee’s and the b-alanine/glycine ratio thatare apparent in the Tagish Lake meteorite. Thehigher ratio of b-alanine to glycine in 5b (~0.6)as compared to 11h (~0.2) may be due to en-hanced production of glycine during aqueousalteration of 11h via reactions involving hy-droxy acids known to be present in SOM (36, 37).A study of L-isovaline ee’s in Murchison speci-mens showed a range of ee values from 0 to 15%,roughly correlative with the abundance of hydrated

minerals in the samples, indicating the role ofmultiple, complex, parent body synthetic processesin amino acid formation (38). The amino acids inTagish Lake 11h, including ee’s and overall abun-dance, may therefore be interpreted as reflecting asecondary pulse of amino acid formation resultingfrom hydrothermal alteration on the Tagish Lakeparent body, which overprinted any original ee’swith a racemic mixture.

Substantial heterogeneity is preserved withinthe Tagish Lake meteorite, especially in terms oforganic matter. The correlation between differ-ences in organic matter properties and indicatorsof hydrothermal alteration indicates that the pro-cesses were active after accretion onto the parentbody. In this scenario, chondritic components,including D- and 15N-rich IOM that is best pre-

Fig. 2. C isotopic com-position of monocarbox-ylic acids in the TagishLake meteorite. Uncertain-ties represent the standarddeviation of three injec-tions for each sample. Formeasurements with lowamplitude (such as thoseof nonanoic or decanoicacid) we used a value of4‰, which is based onthe accuracy achievedfor standards run withlow concentrations. Alsoshown are the resultsfrom (31) for Murchisonmonocarboxylic acids.Symbol size reflects relative concentration (13).

Fig. 3. L-isovaline ee’s (bars) and b-alanine/glycine ratios (circles) in Tagish Lake meteorite specimens11h, 5b, and 11i (shown in yellow), compared with results from CI (red), CM (green), and CR (blue)chondrites of differing degrees of aqueous alteration [data from (33)]. The percentage of L excess isdefined as Lee = L% – D%, with a negative value corresponding to a D excess. LEW, Lewis Cliff; LON,Lonewolf Nunataks; QUE, Queen Alexandra Range; EET, Elephant Moraine.

10 JUNE 2011 VOL 332 SCIENCE www.sciencemag.org1306

REPORTS

on

July

6, 2

011

ww

w.s

cien

cem

ag.o

rgD

ownl

oade

d fro

m

Tagish Lake monocarboxylic acids (Herd et al., 2011)

Pizzarello et al.: Organic Material in Carbonaceous Chondrites and IDPs 641

zenes, and naphthalene can also be abundant in the Murchi-son meteorite (Sephton et al., 1998). µL2MS analyses ofCM (Murchison, Murray, Mighei, and Harripura) (Hahn etal., 1988) and CV (Allende) (Hahn et al., 1988; Zenobi etal., 1989) meteorites gave results that, although showingdifferences in the relative distribution of individual polycy-clic compounds at m/z higher than 178, were qualitativelysimilar to the meteorites’ solvent extract. By contrast, µL2MSanalysis of the Ivuna (CI) meteorite showed only three majormasses for naphthalene, phenanthrene/anthracene, and py-rene/fluoranthene. Extraction with supercritical fluid CO2 ex-tracts of the CI Orgueil meteorite also showed a different dis-tribution of only lower molecular weight species with a pre-dominance of hydrated naphthalenes (Sephton et al., 2001).In the Tagish Lake meteorite many lower molecular weightaromatic compounds, such as alkyl and di-alkyl benzenes,were observed directly in the solvent extracts (Gilmour etal., 2001), a finding likely due to the prompt recovery ofthis meteorite. Naphthalene, alkyl- and phenylnaphthalenes,phenanthrene, anthracene, pyrene, and fluoranthene werealso observed in the extracts (Pizzarello et al., 2001, andunpublished data).

The average isotopic values obtained for preparations ofMurchison aromatic hydrocarbons are δD = 353‰ and δ13C =5.5‰ (Krishnamurthy et al., 1992). All C-isotopic valuesestablished for individual aromatic compounds in the mete-orite have been lower than the above average; with the less-volatile larger aromatic clusters giving values from –5.9‰for fluoranthene to –22.3‰ for benzopyrene (Gilmour andPillinger, 1992, 1994). The only trend apparent in these datawas that the addition of a ring to a given cluster structureappeared to decrease the 13C content of the product com-pound, a finding that would be suggestive of condensationof larger ring clusters from lower homologs. Naraoka et al.(2000) confirmed and expanded this dataset by analysis ofa CM2 Asuka Antarctic meteorite. The authors evaluatedthe molecular and isotopic distributions of the meteoriticPAHs to assess any indication of possible thermodynamicequilibration during their formation. They found none andobserved instead a significant difference in C-isotopic com-position between PAHs having very similar thermodynamiccharacteristics, such as pyrene and fluoranthene. In viewof this, they proposed that the compounds could haveformed through two reaction pathways. As shown in Fig. 4,

Fig. 4. Possible reaction pathways in the formation of meteoritic PAHs. (1) naphthalene, (2) biphenyl, (3) acenaphthene, (4) anthra-cene, (5) 1-phenyl naphthalene, (6) phenanthrene, (7) fluoranthene, (8) benz(a)anthracene, (9) benzo(ghi)fluoranthene, (10) triphenylene,(11,13,15) benzofluoranthenes, (12) crysene, (14) pyrene, (16,17) benzopyrenes, (18) perylene, (19) benzo(ghi)perylene. Re-drawn fromNaraoka et al., 2000.

Pathways of condensation (Naraoka et al. 2000)

C. Lecluse et al.: Carbon isotopes of cometary analogues 1177

RP

RPDP

RP

G G

U-trap

mercury manometer

Oxygen supply

KEY : DP : diffusion pump RP : rotary pump G : gauge S+EC : solenoïd + electric control

Sample tubeQuartz reaction tube

S+ECFig. 1. Oxidation apparatus for organic solidresidues. Oxygen supply is ensured by the leftpart of the line. The oxidation occurred in thequartz reaction tube under oxygen atmosphereat temperature above 1000 K. U-trap allowedthe separation of CO2 from H2O and sampletube received pure CO2 for analysis on the massspectrometer (not represented there). See text forcomments.

Carbon isotopic composition for the organic matter are re-ported in Table 1 and expressed in � units defined as follows:

�13C =✓Rsample

Rstd� 1

◆⇥ 1000 (1)

with Rsample and Rstd as 13C/12C ratios for the sample and thePDB (Pee Dee Belemnite Standard) respectively (13C/12CPDB =0.0112372). The isotopic composition of the CH4 before irra-diation was �39.9 ± 0.2 . The carbon isotopic compositionof the samples varied from �25 to �35 . Several sampleswere analysed in triplicate and give a reproducibility on �13Cof ±0.2 (2 standard deviations).

The blank contribution was determined to lie between 0.32and 0.76µmoles, with carbon isotopic compositions of between�27.95 and�27.87 , respectively. The two blanks correspondto analyses of the platinum foil and of the silicon wafer sealedin the platinum.

4. Discussion

In Fig. 2, the isotopic compositions (in � units) of the residuesare reported as a function of 1/Q (Q standing for the carboncontent expressed in µmole). The isotopic composition of theinitial methane is also shown (�39.9 ). All organic residuesexhibit an enrichment in 13C relatively to the initial methane andtheir �13C values vary linearly as a function of 1/Q. Accordingto this correlation, residues appear to reach a fractionation limitof �24 for sample size greater than 10µmoles deposited.

The correlation between the �13C and 1/Q cannot resultfrom a two component mixing process with the low �13C end-member standing for the blank contribution. Indeed, severalsamples exhibit � 13C �29.9 clearly lower than thosefound for blanks (� 13C = �27.9 ).

The correlation between the � 13C and the carbon contentcan be interpreted as a two step process for the formation of

0.0 1.41.21.00.80.60.40.2

-31

-30

-29

-28

-27

-26

-25

-24

1/ quantity (1/µmoles)

δ 1

3

C (‰

)

Blank

-40

-39

METHANE

Fig. 2. � 13C (in ) versus 1/Q (Q in 1/µmoles) in organic residues re-sulting from irradiation of methane ices. Error bars are due to the blankcontribution. The initial isotopic composition of methane is also shown.The decrease in � 13C with sample size is interpreted as a progressivematuration by sputtering of initial polymers having � 13C values closeto �24 (i.e. for 1/Q = 0).

the organic polymers. In the first step, methane is polymerised(probably under the form of aliphatic compounds) and the result-ing organic polymers (see flow chart in Fig. 3) are isotopicallyfractionated relative to methane by 16 with a mean value of⇡ �24 (i.e. 39.9� 24 = 16 ). In the second step, aliphaticcompounds are sputtered by the incoming ion beam (H+ or He+).As a consequence, an additional isotopic fractionation occursand the isotopically heavier species are lost preferentially. Evap-oration by sputtering is not supposed to cause any isotopic frac-

1178 C. Lecluse et al.: Carbon isotopes of cometary analogues

(CH )4 n

CH3 CH3CH2

-40‰

-24‰

δ C

13

C Hi j < -40‰

C Hi j > -24‰

-35‰

p

p

Fig. 3. Flow chart depicting the irradiation model described by Eqs. (1)to (7) (see text). The carbon isotopic composition of each species isindicated in � 13C . The species cij are outgassed form the solidduring irradiation. Methane and the other organic compounds remainas solid phases during irradiation.

tionation between the gas and the remaining solids if no chemi-cal reaction takes place between the two phases. Therefore, theobserved isotopic evolution must be linked to the loss of car-bonaceous fragments, different in isotopic composition fromthe sputtered solid, implying in turn, that organic matter is re-arranged during this second step. Thus one can suppose that thistype of isotopic fractionation is caused by the progressive poly-merisation of aliphatics into aromatic refractory carbon phases.Simple mass balance equations illustrate this second step:

Qi�i = Qaliph.�aliph. + Qlost�lost + Qarom.�arom. (2)

with

�i = �aliph. (3)

and

Qi = Qaliph. + Qlost + Qarom. (4)

Qmes.�mes = Qaliph.�aliph. + Qarom.�arom. (5)

with:

Qmes. = Qaliph. + Qarom. (6)

The subscripts ‘aliph.’, ‘lost’ and ‘arom.’ stand for the aliphaticcarbon produced by the polymerisation of methane, for the car-bon lost during irradiation and for the aromatic carbon producedduring irradiation, respectively. The subscript ‘i’ designates theinitial carbon phases formed by the polymerisation of methane(that is according to Fig. 2, Qi ⇡ 10µmole and �i = �24 ).

The subscript ‘mes.’ stands for the measured values reported inFig. 2. Since the isotopic fractionation occurs in a solid phase,aliphatic compounds which are not sputtered by the incomingbeam are not fractionated relative to their initial values; hence�i = �aliph. in Eq. 2. The conversion yield for aromatic com-pounds can be defined as:

Qarom. = kQlost (7)

k in Eq. (6) represents the number of carbon atoms combinedinto an aromatic structure for 1 carbon atom lost by sputtering.

Eqs. (1) to (6) give:

�mes =⇥Qarom./kQmes.

⇤[�i � �lost] + �i (8)

In Eq. (7) the measured isotopic composition of the residues(i.e. �mes) is a linear function of 1/Qmes. Numerical simu-lations of Eq. (7) that fit the results reported in Fig. 2, showthat: 1) Assuming that the more 13C depleted samples (�mes =�32 ; 1/Qmes. = 1.2µmole; see Fig. 2) represent almost purealiphatic free residues, the carbon isotopic fractionation be-tween aliphatic and aromatic is 8 (i.e. = 32 � 24) and theconversion yield k cannot be higher than 9%; 2) If the isotopicfractionation is somewhat higher than 8 , the conversion yieldk must be lower than 9%. For example, an isotopic fractiona-tion of 15 (i.e. an aromatic polymer with a �arom. = �39 )would correspond to a conversion yield k of 5%.

It has been shown by hydrogen nuclear magnetic resonance,that the formation of complex polycyclic aromatic hydrocarbonsin solid CH4 such as coronene (C24H12) already takes place atlow radiation doses within one collision cascade (Kaiser 1991;Kaiser 1993; Kaiser et al. 1992a,b; Kaiser and Roessler 1992;Patnaik et al. 1990). It is rather a function of linear energytransfer than of the dose. The mechanism discussed here is amulticentre reaction of hot carbon and hydrogen atoms, theirintermediate reaction products and free thermal radicals lost inthe gas phase being located within a zone of 10 A radius fromthe surface (Roessler 1992; Kaiser 1993). As microscopicallyobserved on the wafers (Kaiser et al. 1992; Kaiser and Roessler1992), the successive transformations of CH4 into longer andlonger aliphatic chains, polycyclic structures and finally amor-phous carbon are likely related to the irradiation dose. But evenhere, multicentre processes will minimise the number of reac-tion steps. The small isotopic enrichments of 13C in the residuescan be considered as an additional evidence for the co-ordinatedand concomitant multicentre mechanism.

These observations bear also interesting consequences as faras the origin and evolution of organic material in carbonaceousmeteorites is concerned. Gilmour et al. (1991) and Gilmour &Pillinger (1992) detected organic molecule under the form ofPoly-Aromatic Hydrocarbons (PAHs) in Murchison and Orgueilmeteorites. These authors found that the carbon isotopic compo-sition of PAHs increased with the molecular weight and with thedegree of aromatisation. If the present interpretation is correct,isotopic fractionation of carbon linked to irradiation results in adecrease in the � 13C values associated with an increase in thedegree of aromatisation. This conclusion is opposite to observa-

Carbon isotopic fractionation in methane ice irradiation (Lecluse et al. 1998)

1994MNRAS.269..235G

1994MNRAS.269..235G

Pathways of condensation (Gilmour & Pillinger, 1994)

-500

0

500

1000

1500

2000

2500

-50 -25 0 25 50 75 100

δ13CVPDB / ‰

δ2H

VSM

OW

/ ‰

amino acids

sulfonic acids

hydrocarbons

polar hydrocarbons

carboxylic acids

volatile hydrocarbons

somewhat longer average chain length in water-solublefractions than in IOM-derived compounds. Most of thebranched structures found in the water-soluble can alsobe found in the IOM oxidation products. Exceptionsinclude the absence of formic acid in IOM productssince a methyl substitution on aromatic core will yieldacetic acid (C2), and that branched aliphatic structuresdecline in abundance more rapidly in IOM sample thanin water-soluble compounds (Fig. 3). The concentration ofMCAs inwater extractable ranged from0.002μmol g−1 to14.82 μmol g−1 of meteorite, whereas in the IOM theconcentrations are almost one order of magnitude higher,ranging from 0.05 μmol g−1 to 100.0 1 μmol g−1 (Fig. 4).Overall, however, the strong structural similarity betweenIOM-derived and -soluble monoacids suggests thatthese compounds may have the same origins, i.e., acommon precursor may be responsible for their for-mation. We will further discuss this point based onisotope data in Section 3.5.

3.4. Comparison with IOM structure revealed by NMRand other thermal and chemical degradation methods

Many studies have demonstrated that the IOM ofcarbonaceous chondrites is composed of both aromaticand aliphatic units. Direct observation of IOM usingsolid state 1H and 13C NMR indicates the presence ofvariably condensed aromatic core structure and aliphaticlinkages (Cronin et al., 1987). Gardinier et al. (2000)found 8 different types of carbons, including aromati-cally and aliphatically linked CH3, CH2, aliphatic C-linked to heteroelements, protonated and non-protonat-ed aromatic C, carboxyl and carbonyls. Using solid stateCP/MAS 13C NMR spectroscopy, they also quantifiedthe percentage of aromatic carbon in Murchison (61 to67%) and Orgueil (69–78%) (Gardinier et al., 2000).Their data also suggest a high level of branching in thealiphatic side chains, especially for Murchison. Codyand co-workers using combined 1H and 13NMR onmeteorites with different classifications (CM2, CI1,CR2) further found a much wider range of functionalgroups, and showed that aromatic core units are highlysubstituted by aliphatic side chains with high level ofbranching (Cody et al., 2002; Cody and Alexander,2005). Cody and Alexander suggest that alteration bylow temperature chemical oxidation is responsible forthe variations in chemical compositions of IOM such asaromatic to aliphatic ratios and the degrees of aromaticcondensations (Cody et al., 2002).

Our procedure does not allow quantification of di-and tri-carboxylic acids because these compounds arenot volatile and cannot be revealed by SPME-GC-FID.The previous study (Remusat et al., 2005a) did not

Fig. 4. The amount of monocarboxylic acids released by RuO4

oxidation of Murchison IOM, showing a rapid decline in abundance ofthese compounds with increasing carbon number. The abundances ofall branched compounds (marked as “Bn”, where n is the carbonnumbers) are combined. Cn refers to straight chain monoacids with ncarbon numbers.

Fig. 3. The GC-FID traces of the monocarboxylic acids derived fromRuO4 oxidation of IOM (A), and direct water extraction ofcarbonaceous chondrite Murchison (B). Note that the relatively highsignal response for longer chain acids is due mainly to the high affinityof SPME fiber to these compounds. The acetic acid peak is truncatedsince its peak is ∼4 times higher than propanoic acid. Cn and Bn,where n refers to the carbon number, indicate the straight and branchedchain monocarboxylic acids, respectively.

522 Y. Huang et al. / Earth and Planetary Science Letters 259 (2007) 517–525

Monocarboxylic acids released by RuO4 oxidation of IOM (Huang et al., 2007)

δ13CVPDB / ‰

δ2H

VSM

OW

/ ‰

Monocarboxylic acids released by RuO4 oxidation of IOM (Huang et al., 2007)

-450

0

450

900

1350

1800

-60 -45 -30 -15 0 15

SOM IOM

1997). PAH are observed ubiquitously in our Galaxyas the carriers of the near- and mid-infrared (IR) bands(Tielens et al. 1999), and they are probably by far the mostabundant molecular species in the diffuse interstellarmedium. Moreover, multiple PAH emission features have

been detected recently in other galaxies (e.g. Yan et al. 2005).PAH are thought be mainly produced in space in thehigh-temperature, high-density ejecta of asymptotic giantbranch (AGB) stars (Cherchneff et al. 1992) and are themolecular intermediaries in the soot formation process.

Fig. 4. Murchison PAH, the most abundant type of extraterrestrial organic compound in both meteorites and space.

2500

Murchison organic matter volatilebases

carboxylicacids

sulfonic acids

macromolecularmaterial

hydrocarbons

Terrestrial organic matter

coal andpetroleum

marine organisms

nonmarineorganisms

methaneto

−110

polarhydrocarbons

volatilehydrocarbons

aminoacids2000

1500

1000

500

−500

−1000−70 −60 −40 −20

δ13C (‰)

δD(‰)

0 20 40

0

Fig. 5. The distinction between stable carbon and hydrogen isotope ratios in Murchison and life. The difference allows abiogenic

extraterrestrial organic matter to be distinguished from its terrestrial biological counterpart. The abundances of stable isotopes are expressed

using the d notation. These indicate the difference, in per mil (ø), between the relevant ratio in the sample and the same ratio in an

international standard as follows: dø = ((RsamplexRstandard)/Rstandard)r1000. Where R=D (2H)/1H for hydrogen and 13C/12C for carbon.

Data from Sephton (2002) ; Butterworth et al. (2004) and references therein.

Recognizing organic life 273

Naphthalene alkylation vs. petrologic type (Elsila et al., 2005)appears between extent of aqueous alteration and naphthalenealkylation pattern in our measurements, at least for those me-teorites for which we have information on aqueous exposure;the most aqueously altered CM2 meteorites have the highestamount of naphthalene alkylation.

Figure 2C presents the degree of naphthalene alkylation forfour non-CM2 carbonaceous chondrites that also underwentaqueous alteration. The meteorite GRA 95229 shows a degreeof naphthalene alkylation similar to that of the meteoritesdisplayed in Figure 2A, with an inverse correlation betweenabundance and alkylation. This CR2 meteorite has seen mod-erate aqueous alteration (Weisberg et al., 1993). The ungroupedC2 meteorite MAC 88107 has high abundances of C1- andC2-naphthalene, similar to the meteorites in Figure 2B, but lowlevels of C3-naphthalene, similar to those in Figure 2A. TheTagish Lake meteorite, also an ungrouped C2 chondrite (Zo-lensky et al., 2002), has relatively high amounts of alkylation,as does the Orgueil (CI1) meteorite sample, which has beenextensively aqueously altered (Tomeoka and Buseck, 1988). Aswith the meteorites presented in Figure 2A and 2B, increasingdegree of naphthalene alkylation appears to correlate withincreasing aqueous alteration.

Figure 2D displays the naphthalene alkylation distributionfor three meteorites that have undergone less alteration thanthose in Figures 2A–C. The Allende and Mokoia meteorites areboth CV3 carbonaceous chondrites. Studies suggest thatMokoia has seen some amount of aqueous and anhydrousalteration (Tomeoka and Buseck, 1990; Kimura and Ikeda,1998), while Allende may have experienced slight thermal andhydrothermal alteration (Keller and Buseck, 1991; Guimon et

al., 1995), and both meteorites show evidence of oxidation(McSween, 1977). These two meteorites have high amounts ofalkylation, with alkylated naphthalene compounds being moreabundant than unalkylated naphthalene. The Allende meteoritecontains higher levels of naphthalene alkylation than any othermeteorite studied in this work. The third meteorite displayed,ALH 83108, is classified CO3, and reveals moderate amountsof alkylated naphthalene compounds that decrease in concen-tration with increasing alkylation.

Four analyzed meteorites are not displayed in Figure 2because they had undetectable levels of naphthalene or itsderivatives. The !L2MS spectra of the three CK meteoritesanalyzed (ALH 85002, EET 92002, and Karoonda) are strik-ingly different from all other carbonaceous chondrites studied,but show similarities to each other (Fig. 3). The PAHs com-monly detected in meteorites (e.g., naphthalene, phenanthrene,pyrene) were not observed, although other unidentified peakswere seen at 135 Da, 179 Da, and 250 Da. The presence of oddmasses suggests the possible incorporation of nitrogen intothese hydrocarbons, either as side chains or as part of the parentskeleton (see Section 3.4 for more discussion). Overall signalintensity was also much lower than in most other analyzedchondrites. The CK meteorites have all undergone thermalmetamorphism (Kallemeyn et al., 1991); it is possible that thismetamorphism caused the volatilization and loss of PAHs fromthe free organic material. It is also possible that the CK mete-orites, for which there are no reported studies of organic con-tent, originated from a parent body whose organic content ismarkedly different from the parent bodies of the other carbo-naceous chondrites. The Coolidge meteorite, an ungrouped C4

Fig. 2. Naphthalene alkylation distributions for four sets of carbonaceous chondrites. The symbols C1, C2, and C3represent mono-, di-, and trialkylated naphthalene.

1352 J. E. Elsila et al.

appears between extent of aqueous alteration and naphthalenealkylation pattern in our measurements, at least for those me-teorites for which we have information on aqueous exposure;the most aqueously altered CM2 meteorites have the highestamount of naphthalene alkylation.

Figure 2C presents the degree of naphthalene alkylation forfour non-CM2 carbonaceous chondrites that also underwentaqueous alteration. The meteorite GRA 95229 shows a degreeof naphthalene alkylation similar to that of the meteoritesdisplayed in Figure 2A, with an inverse correlation betweenabundance and alkylation. This CR2 meteorite has seen mod-erate aqueous alteration (Weisberg et al., 1993). The ungroupedC2 meteorite MAC 88107 has high abundances of C1- andC2-naphthalene, similar to the meteorites in Figure 2B, but lowlevels of C3-naphthalene, similar to those in Figure 2A. TheTagish Lake meteorite, also an ungrouped C2 chondrite (Zo-lensky et al., 2002), has relatively high amounts of alkylation,as does the Orgueil (CI1) meteorite sample, which has beenextensively aqueously altered (Tomeoka and Buseck, 1988). Aswith the meteorites presented in Figure 2A and 2B, increasingdegree of naphthalene alkylation appears to correlate withincreasing aqueous alteration.

Figure 2D displays the naphthalene alkylation distributionfor three meteorites that have undergone less alteration thanthose in Figures 2A–C. The Allende and Mokoia meteorites areboth CV3 carbonaceous chondrites. Studies suggest thatMokoia has seen some amount of aqueous and anhydrousalteration (Tomeoka and Buseck, 1990; Kimura and Ikeda,1998), while Allende may have experienced slight thermal andhydrothermal alteration (Keller and Buseck, 1991; Guimon et

al., 1995), and both meteorites show evidence of oxidation(McSween, 1977). These two meteorites have high amounts ofalkylation, with alkylated naphthalene compounds being moreabundant than unalkylated naphthalene. The Allende meteoritecontains higher levels of naphthalene alkylation than any othermeteorite studied in this work. The third meteorite displayed,ALH 83108, is classified CO3, and reveals moderate amountsof alkylated naphthalene compounds that decrease in concen-tration with increasing alkylation.

Four analyzed meteorites are not displayed in Figure 2because they had undetectable levels of naphthalene or itsderivatives. The !L2MS spectra of the three CK meteoritesanalyzed (ALH 85002, EET 92002, and Karoonda) are strik-ingly different from all other carbonaceous chondrites studied,but show similarities to each other (Fig. 3). The PAHs com-monly detected in meteorites (e.g., naphthalene, phenanthrene,pyrene) were not observed, although other unidentified peakswere seen at 135 Da, 179 Da, and 250 Da. The presence of oddmasses suggests the possible incorporation of nitrogen intothese hydrocarbons, either as side chains or as part of the parentskeleton (see Section 3.4 for more discussion). Overall signalintensity was also much lower than in most other analyzedchondrites. The CK meteorites have all undergone thermalmetamorphism (Kallemeyn et al., 1991); it is possible that thismetamorphism caused the volatilization and loss of PAHs fromthe free organic material. It is also possible that the CK mete-orites, for which there are no reported studies of organic con-tent, originated from a parent body whose organic content ismarkedly different from the parent bodies of the other carbo-naceous chondrites. The Coolidge meteorite, an ungrouped C4

Fig. 2. Naphthalene alkylation distributions for four sets of carbonaceous chondrites. The symbols C1, C2, and C3represent mono-, di-, and trialkylated naphthalene.

1352 J. E. Elsila et al.

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Isotopic analysis of soluble organic matter in meteorites

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