Earth and Planetary Science Letters 426 (2015) 101–108
Contents lists available at ScienceDirect
Earth and Planetary Science Letters
www.elsevier.com/locate/epsl
Hydrogen and carbon isotopic ratios of polycyclic aromatic compounds
in two CM2 carbonaceous chondrites and implications for prebiotic
organic synthesis
Yongsong Huang a,∗ , José C. Aponte a , Jiaju Zhao a , Rafael Tarozo a , Christian Hallmann b
a
b
Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI 02912, USA
Max Planck Institute for Biogeochemistry and MARUM, University of Bremen, Leobener Strasse, D-28359, Germany
a r t i c l e
i n f o
Article history:
Received 7 August 2014
Received in revised form 21 April 2015
Accepted 16 June 2015
Available online xxxx
Editor: T. Elliott
Keywords:
carbonaceous chondrites
Murchison
hydrogen isotopic ratios
PAHs
astrobiology
a b s t r a c t
Study of meteoritic organic compounds offers a unique opportunity to understand the origins of the
organic matter in the early Solar System. Meteoritic polycyclic aromatic hydrocarbons (PAHs) and
heteropolycyclic aromatic compounds (HACs) have been studied for over fifty years, however; their
hydrogen stable isotopic ratios (δ D) have never been reported. Compound-specific δ D measurements of
PAHs and HACs are important, in part because the carbon isotopic ratios (δ 13 C) of various meteoritic PAHs
cannot be readily distinguished from their terrestrial counterparts and it is difficult to rule out terrestrial
contamination based on carbon isotopic ratios alone. In this study, we have extracted and identified
more than sixty PAHs and HACs present in two CM2 carbonaceous chondrites Murchison and LON 94101.
Their carbon and hydrogen stable isotopic ratios (δ 13 C and δ D) were measured and used to discuss about
their synthetic environments and formation mechanisms. The concentration of aromatic compounds is
∼30% higher in Murchison than in the Antarctic meteorite LON 94101, but both samples contained
similar suites of PAHs and HACs. All PAHs and HACs found exhibited positive δ D values (up to 1100h)
consistent with an extraterrestrial origin, indicating the relatively low δ 13 C values are indeed an inherent
feature of the meteoritic aromatic compounds. The hydrogen isotopic data suggest aromatic compounds
in carbonaceous chondrites were mainly formed in the cold interstellar environments. Molecular level
variations in hydrogen and carbon isotopic values offer new insights to the formation pathways for the
aromatic compounds in carbonaceous chondrites.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
Carbonaceous chondrites are among the most primitive materials in the Solar System. This type of meteorites can contain
up to 14 wt% of water and 5 wt% of organic carbon that survived planet formation processes by being incorporated into an
asteroid parent body (Cronin and Chang, 1993; Sephton, 2002).
In CM2 carbonaceous chondrites, up to 30% of the total amount
of organic material can be in the form of solvent-soluble compounds (Pizzarello et al., 2006), with the rest being macromolecular organic matter rich in polynuclear aromatic structures linked
by aliphatic moieties (Cody and Alexander, 2005; Remusat et al.,
2005; Huang et al., 2007). Polycyclic aromatic hydrocarbons (PAHs)
are some of the most ubiquitous organic compounds found in
the universe, and have been detected in interplanetary dust par-
*
Corresponding author. Tel.: +1 401 863 3822; fax: +1 401 863 2058.
E-mail address: [email protected] (Y. Huang).
http://dx.doi.org/10.1016/j.epsl.2015.06.027
0012-821X/© 2015 Elsevier B.V. All rights reserved.
ticles, the interstellar medium (ISM) and asteroidal meteorite parent bodies (e.g., Allamandola et al., 1987; Clemett et al., 1993;
Basile et al., 1984). PAHs and HACs may serve as membrane stabilizers for primitive cells on the prebiotic Earth and have special
astrobiological significance (Groen et al., 2012). However, these
molecules are also abundant in organic materials formed on Earth
through natural and anthropogenic combustion processes (e.g.,
Denis et al., 2012 and references therein). The potential complex
sources for PAHs place their primordial origin in carbonaceous
chondrites under debate.
The analysis of aromatic hydrocarbons in carbonaceous chondrites using liquid chromatography and spectrophotometric methods date back to the early 1960s, when compounds such as
phenanthrenes and pyrenes were identified from aromatic hydrocarbon fractions of Orgueil, Murray, Cold Bokkeveld and Allende
meteorites (Studier et al., 1972; Commins and Harington, 1966).
However; because of the complex origins for PAHs, without pristine samples or obtaining isotopic information, it was difficult to
exclude terrestrial contamination as the sources of PAHs and HACs
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Y. Huang et al. / Earth and Planetary Science Letters 426 (2015) 101–108
in carbonaceous chondrites. Shortly after its fall in 1969, studies of
Murchison, an observed fall, greatly strengthened the notion that
PAHs might be indigenous in carbonaceous meteorites (Oró et al.,
1971; Kvenvolden et al., 1970; Pering and Ponnamperuma, 1971;
Studier et al., 1972; Basile et al., 1984). One long standing, puzzling
observation, however, is that the carbon isotopic (δ 13 C) values of
PAHs such as benzene, toluene, naphthalene, biphenyl, phenanthrene and pyrenes in Murchison display significant overlap with
those of terrestrial compounds (Yuen et al., 1984; Sephton et al.,
1998; Sephton and Gilmour, 2000, 2001; Naraoka et al., 2000). The
situation is exacerbated by the identification of terrestrial aliphatic
compounds such as isoprenoidal hydrocarons in the Murchison
carbonaceous chondrite (Hayes, 1967), making it difficult to completely rule out terrestrial contamination of PAHs in carbonaceous
chondrites.
Hydrogen isotopic ratios (δ D) of PAHs in carbonaceous chondrites have not previously been reported, probably because these
compounds contain significantly fewer hydrogen atoms per molecule, making the measurement of the compound specific hydrogen isotopic ratios using GC-IRMS more challenging than aliphatic
compounds. The only previous attempt to determine hydrogen
isotopic ratios of aromatic hydrocarbons was to measure bulk
fractions from chromatographic separations (Krishnamurthy et al.,
1992). However, these bulk fractions may contain various kinds of
unidentified compounds (hence the resulting isotopic data cannot
be attributed specifically to PAHs). Compound specific δ D values
of PAHs could provide more definitive evidence for the extraterrestrial origins and synthetic environments of these meteoritic organic compounds than carbon isotopic values, as exemplified by
more polar compounds such as amino acids (Epstein et al., 1987;
Pizzarello et al., 1991; Pizzarello and Huang, 2005; Elsila et al.,
2012), and carboxylic acids (Huang et al., 2005, 2007; Aponte et
al., 2011, 2014), which are highly enriched in D relatively to their
terrestrial counterparts, indicating they originated in cold interstellar environments.
The objectives of this study are two folds: 1) to determine the
hydrogen isotopic ratios of individual PAHs (and in cases of partial chromatographic coelution, combined hydrogen isotopic values
of multiple PAHs or HACs) in two CM2 carbonaceous chondrites
(Murchison and LON94101) and use the data to infer PAH origins
and synthetic pathways; 2) to compare the carbon and hydrogen
isotopic differences among PAHs and HACs to probe into their synthetic mechanisms. Our study fills an important gap in the analysis
of PAHs from the solvent soluble fraction of carbonaceous chondrites, since the δ D values for meteoritic PAHs and HACs have not
been reported before.
2. Methods and meterials
2.1. Meteorite extraction
PAHs are ubiquitously present on the Earth biosphere, hence
the possibility of terrestrial contamination of meteorite samples
is not trivial. To best differentiate indigenous PAHs and other organic compounds from possible contaminants, we also analyzed
Lonewolf Nunataks (LON) 94101 (an Antarctic meteorite) in addition to Murchison (Aponte et al., 2014). LON 94101.59 (5.17 g)
were obtained from the Antarctic Meteorite Collection (NASA),
and Murchison (catalog number: USNM 6650; CM2; 4.68 g) from
the Smithsonian Institution. Both belong to CM2 carbonaceous
chondrites that experienced substantial aqueous alteration. However, LON 94101 belongs to subtype CM2.2–2.3, which experienced greater extent of aqueous alteration than Murchison (CM2.5)
(Lee et al., 2014). The meteorites’ outer-most layers were removed
with a solvent-cleaned metal file at Brown University. The remaining inner samples were powdered on a clean agate mortar
and suspended in 10 mL of an aqueous 1N NaOH solution. Upon
sonication for 30 min and stirring for 2 h at room temperature, the suspension was centrifuged and the aqueous layer was
separated and kept for previously reported analyses of monocarboxylic acids (Aponte et al., 2014). The remaining powders were
extracted with methanol first (three times, 10 mL each), followed
by dichloromethane (two times, 10 mL each) using ultrasonication
for 30 min at 50 ◦ C. The solvent extracts were combined and dried
over anhydrous sodium sulfate. Freshly prepared copper beads (i.e.,
surface CuO is first removed by rinsing in 0.1M HCl, followed by
water and solvent cleaning) were added to these combined solvent extracts to remove sulfur and then the total volume was reduced to 0.5 mL, and then subjected to Gas Chromatography-Flame
Ionization Detector (GC-FID), GC-Mass Spectrometry (MS) and GCIsotope Ratio Mass Spectrometer (IRMS) analysis. GCMS analyses
reveal majority (>90%) of the compounds in the combined solvent
extract were PAHs and HACs in both meteorite samples (Fig. 1). To
minimize loss of compounds and possibility of contamination, no
liquid chromatographic separation was carried out prior to compound specific isotopic analysis.
2.2. GC-MS and GC-IRMS analyses
Identification of compounds was achieved using an HP6890+
GC interfaced to 5973 N mass detector (MSD). PAHs and HACs in
our samples were identified using GC-MS and by running authentic standards when even possible (compounds 6, 8, 15, 31, 32, 45,
47, 56, 57, and 60 to 64). Quantification was done by GC-MS using calibration lines of different PAHs and HACs. For those we did
not have standards, we used calibration of standard compounds
with the closest structures (i.e., assuming the same response factors). Compound specific isotope ratios were determined with an
HP6890+ GC connected to a Finnigan MAT Delta+-XL mass spectrometer. In all analyses, a DB-5MS 60 m × 0.25 × 0.1 μm capillary column (Agilent) was used. Helium carrier gas flow was
1.1 mL/min. The injection port was set at 320 ◦ C. The oven temperature was programmed to rise from 60 ◦ C (hold for 1 min) to
310 ◦ C at 6 ◦ C min−1 (hold for 20 min). The δ 13 C and δ D values
are reported relative to the Vienna Pee Dee Belemnite (VPDB) and
Vienna Standard Mean Ocean Water (VSMOW) standards, respectively. For δ 13 C isotope ratio measurements, compounds separated
by GC column were converted to CO2 and H2 O in a combustion
furnace (0.5 mm ID × 1.5 mm OD × 34 cm) operated at 940 ◦ C
and loaded with CuO and Pt wires as oxidant and catalyst, respectively (Huang et al., 2005). A small stream of 1% O2 in He was
added right in front of the reactor to maintain the oxidation capacity of the CuO. Six pulses of CO2 reference gas of known δ 13 C
values were injected via the interface to the IRMS, for the computation of δ 13 C values of sample compounds. After compound
identification by GC-MS and compound specific carbon isotopic
analysis, hydrogen isotope analyses of PAHs were performed using a high temperature conversion reactor. The same GC system
and temperature programming as for the carbon analyses were
used, except for replacing the oxidation reactor with a pyrolysis reactor. Compounds separated by GC were converted to H2
through a pyrolysis furnace operated at 1445 ◦ C (Burgoyne and
Hayes, 1998). Six pulses of hydrogen reference gas with known δ D
values were injected via the interface to the IRMS, for the computation of δ D values of sample compounds (Huang et al., 2005;
Huang et al., 2007). The accuracy of the isotopic measurements
was routinely established by measuring lab standards (mixture of
C16 , C18 , C22 , C24 , C26 and C28 fatty acid methyl esters and mixture of C23 , C27 and C29 n-alkanes) with known H and C isotopic
values. The lab standards were established by repeated measurements (∼50 times) after verifying the machine performance using
the standard compounds (C16 , C18 , C20 and C30 n-alkanes acquired
Y. Huang et al. / Earth and Planetary Science Letters 426 (2015) 101–108
103
Fig. 1. GC-MS chromatograms for the solvent extracts from Murchison and LON 94101. Peaks are numbered according to the identification in Table S2. F: fatty acid; U: unable
to identify; P: phthalate (contaminant).
from Indiana University) with off-line measured δ 2 H values. For
hydrogen isotopic ratios, a set of PAHs were analyzed three times
prior to analyzing meteorite samples (Table S1).
3. Results
3.1. Molecular distribution and abundances of polycyclic aromatic
organic molecules
Majority of the early studies reporting solvent-soluble meteoritic PAHs are of a qualitative nature and focused on meteorites such as Orgueil, Murray, Cold Bokkeveld and Allende using spectrophotometric methods and gas chromatography (Studier
et al., 1972; Commins and Harington, 1966; Oró et al., 1971;
Kvenvolden et al., 1970). Studies addressing the quantification
of these molecules in an astrochemical context came after analyzing Murchison, Yamato-74662 and Yamato-793321 meteorites
by GC-MS (Pering and Ponnamperuma, 1971; Basile et al., 1984;
Shimoyama et al., 1989). More recent reports show the abundance of free meteoritic PAHs in Bells, Ivuna and five Antarctic
meteorites, however; these results are presented as supplementary
information only, lacking isotopic data for discussing their astrobiological significance and extraterrestrial indigeneity (Pizzarello et
al., 2008, 2012; Monroe and Pizzarello, 2011).
Fig. 1 shows the gas chromatograms of PAHs and HACs extracted from Murchison and LON 94101 respectively (the chromatograms are partially expanded vertically for easy visualization
of small peaks, see Fig. S1 for more information). Fig. 2 shows
the relative abundance of PAHs and HACs grouped according to
the number of rings on their structure. The total concentrations of
identified PAHs and HACs for Murchison and LON 94101 are 2.58
mmol/g and 1.57 mmol/g respectively. The abundances of PAHs
and HACs in Murchison are 1.82 mmol/g and 0.76 mmol/g; while
in LON 94101 are 1.19 mmol/g and 0.38 mmol/g respectively. The
abundances of PAHs and HACs are higher in Murchison than in
LON 94101 meteorite (differences are generally larger for lower
molecular weight compounds; Table S2; Fig. 2). The most abundant
PAHs were those having 4 fused rings, while the most abundant
HACs were those having 3 fused rings (Table S2). PAHs were more
abundant than HACs in both meteorites, regardless of their ring
size (Fig. 2); as also previously found in Murchison (Basile et al.,
1984).
Fig. 2. Ring number distributions of PAHs and HACs extracted from Murchison and
LON 94101.
3.2. δ D values of aromatic compounds
PAHs contain much fewer hydrogen atoms per carbon than
other aliphatic compounds (for example, naphthalene has a molecular formula of C10 H10 and C/H ratio of 1, compared with carboxylic acid C10 H20 O2 and C/H ratio of 0.5). As a result, greater
amounts of samples are required to generate a measurable signal on gas chromatography isotope ratio mass spectrometry (GCIRMS). Compounds often have to be overloaded in order to generate sufficient peak sizes for D/H isotopic measurements, resulting
in deterioration in chromatographic resolution. Additionally there
are a large suite of PAHs with diverse structures, adding to the
difficulty of base line chromatographic resolution on GC-IRMS. For
these reasons, some δ D data of the PAHs and HACs are reported
as combined values of two or even three peaks, and some compounds were only in sufficient quantity for one single GC-IRMS
analysis (Tables S2 and S3). The δ D values for both meteorite sam-
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Y. Huang et al. / Earth and Planetary Science Letters 426 (2015) 101–108
Fig. 3. The relationship between molecular weights or carbon numbers and hydrogen stable isotope values (h VSMOW) of PAHs (red circle), coeluting of PAHs/HACs (orange
square), and HACs (blue triangle) from Murchison and LON 94101.
ples show a broad range, but most of the compounds fall into the
+120 to +900h range for Murchison, and +180 to +1200h for
LON 94101 (Fig. 3).
3.3. Carbon isotopic values of aromatic compounds
The results of carbon stable isotopic ratios (δ 13 C) of PAHs and
HACs from Murchison and LON 94101 are presented on Table S2
and Fig. 4. The δ 13 C values for Murchison range mainly from −5
to −23h, while for LON 94101, these values range mainly from
−1 to −18h. There is a tendency for PAHs with higher molecular
weight and increasing degree of aromatization (number of rings) to
have lower δ 13 C values (LON 94101 is more obvious than Murchison), similar to those previously found for Murchison, Orgueil,
Cold Bokkeveld, and Antarctic meteorite A-881458 (Gilmour and
Pillinger, 1994; Sephton et al., 1998; Naraoka et al., 2000; Sephton
and Gilmour, 2000). An example is phenanthrene (31), which is 5
and 10h enriched in δ 13 C relative to pyrene (47) in Murchison
and LON 94101, respectively (Table S2). Interestingly, δ 13 C values
of individual PAHs and HACs from LON 94101 often have higher
values than those from Murchison.
4. Discussion
4.1. Variability of PAH distributions and abundances
The most abundant PAHs previously found in the Murchison
meteorite were naphthalene, biphenyl, acenaphthene, phenanthrene, fluoranthene and pyrene (Pering and Ponnamperuma,
1971; Basile et al., 1984). In contrast, the most abundant PAHs we
found were phenanthrene, fluoranthene and pyrene, all of them
found above 200 nmol/g in Murchison and about 160 nmol/g in
LON 94101, these three compounds were 10 to 20 times more
abundant than the rest of compounds and represent about 30%
of the total amount of PAH and HAC compounds extracted from
each meteorite sample (Table S2). We did not find naphthalene,
and only found biphenyl and acenaphthene at relatively low concentrations (<20 nmol/g). The high abundance of phenanthrene,
fluoranthene and pyrene relative to other PAHs in Murchison
and LON 94101 meteorites are in agreement with that of previous reports in various samples (Pering and Ponnamperuma, 1971;
Basile et al., 1984; Shimoyama et al., 1989; Naraoka et al., 2000).
However, our measured concentrations of phenanthrene, fluoranthene and pyrene were about 10 times larger than those previously reported in the Murchison meteorite (Pering and Ponnamperuma, 1971). The most abundant HACs previously found in the
Murchison meteorite were 9-fluorenone, carbazole, bezanthracen7-one and benzonaphthothiophene (Basile et al., 1984), whereas
the HACs we found most abundant in Murchison were diphenylamine (63 nmol/g), 9-fluorenone (105 nmol/g) and carbazole
(57 nmol/g). In LON 94101, the most abundant HACs were benzophenone (19 nmol/g), 9-fluorenone (55 nmol/g) and dibenzothiophene (40 nmol/g). These differences in abundance between
our sample and those reported previously may be attributed to:
1) loss by evaporation of lower molecular weight PAHs during
sample storage and processing; and 2) in the case of Murchison,
the well-documented sample inhomogeneity (e.g., Krishnamurthy
et al., 1992; Pizzarello et al., 2003).
4.2. PAH hydrogen isotopic values: comparison to bulk fractions and
other organic components
Individual PAHs and HACs in carbonaceous chondrites have not
been measured for D/H ratios before. An estimation of their δ D
values was from Krishnamurthy et al. (1992), who reported hydrogen isotopic values of three bulk fractions (i.e., fractions collected from silica gel column chromatography eluted with hexane, benzene and methanol, respectively) of solvent extracts of
Murchison. In that study, the δ D values for the hexane, benzene
and methanol fractions were +264h, +357h and +946h re-
Y. Huang et al. / Earth and Planetary Science Letters 426 (2015) 101–108
105
Fig. 4. Variations in δ D values (h VSMOW) of PAHs and HACs as a function of ring numbers of the aromatic compounds. Weighted mean δ D values are plotted for each ring
number.
spectively. PAHs and HACs should be mostly eluted from silica
gel using benzene. The polar fraction eluted using methanol from
Murchison (Krishnamurthy et al., 1992) should contain more polar organic compounds (compounds containing hydroxyl, carbonyl,
carboxyl and amine functional groups) which are well known to
be more enriched in deuterium (Pizzarello et al., 2006). Thus, the
higher δ D values for the methanol eluted fraction are not surprising and cannot be attributed to specific compound groups. In our
experimental procedure, however, the samples were first subjected
to extraction with aqueous solution to obtain monocarboxylic acids
(Aponte et al., 2014), and hence much of the low molecular weight,
water-soluble polar organic compounds (those with the highest
deuterium contents) were already removed (see experimental procedure). Additionally, compound specific isotopic analysis further
eliminates unwanted polar compounds from our target PAHs and
HACs. From our data, the weighted average δ D values for measured PAHs and HACs in Murchison and LON 94101 were +260,
+312h and +436, +903h respectively. The values for Murchison
are hence in good agreement with the hydrogen isotopic values of
those previously reported for bulk hexane and benzene fractions
(Krishnamurthy et al., 1992). However, our new results remove the
ambiguity of bulk fraction isotopic measurements and allow us to
further explore the underlying causes for hydrogen isotopic differences between individual PAHs and HACs.
In general, hydrogen isotopic values of PAHs decrease with increasing molecular weights or carbon numbers (Fig. 3), although
there is considerable scattering for any given molecular weights
or carbon numbers. We previously observed decreasing δ D values with increasing PAH conjugation in the pyrolysis products
of Murchison macromolecules: benzaldehyde (1285h) > indene
(1067h) > naphthalene (215h) (Wang et al., 2005). As the size
of PAHs increases, the number of H atoms relative to C atoms
decrease. Our observation of less deuteration on the larger, more
condensed PAHs suggests one mechanism may be particularly important in the PAH deuterium enrichment processes in the inter-
stellar environments: gas phase photodissociation reactions (Sandford et al., 2000; Sandford, 2002). There are more H atoms per
unit mass (H/C ratios higher) in smaller, less condensed PAHs,
hence these compounds are more likely be subjected to UV induced H-D exchanges. The larger, more condensed PAHs, on the
other hand, are more likely accommodate the UV photons without
photodissociation occurring, leading to less deuterium enrichment
than smaller PAHs in a deuterium-rich interstellar environment
(Sandford, 2002). Another explanation for the hydrogen isotopic
variations for different sized PAHs is the changes in the relative
contributions from interstellar versus protosolar nebular sources.
Remusat et al. (2010) have proposed that the extremely wide range
of deuterium enrichments in fine organic particles in carbonaceous
chondrites are best explained by multiple populations of organic
precursors derived from diverse interstellar and solar nebular environments. If this is the case, larger PAHs in our samples may
contain greater contributions from protosolar nebular sources than
the smaller PAHs.
The range and variability of the hydrogen isotopic ratios of
PAHs and HACs can be compared to amino acids and monocarboxylic acids. Monocarboxylic acids (MCAs) extracted from Murchison and five other Antarctic carbonaceous chondrites showed δ D
values ranging from +19h to +2024h (Huang et al., 2005;
Aponte et al., 2011); while the δ D values of amino acids found
in various CM2 carbonaceous chondrites range from +303 to
+3027h (Pizzarello and Huang, 2005; Elsila et al., 2012). Clearly,
hydrogen isotopic values of PAHs and HACs are lower than those
of branched MCAs and amino acids (Fig. S2). This is consistent
with the general polarity trend observed for carbonaceous chondrites, i.e., hydrogen isotopic contents increase with increasing polarity for different compound classes (Cronin and Chang, 1993).
Within all the PAHs and HACs we measured in this study, the
weighted mean δ D values of more polar HACs are also slightly
higher than non-substituted PAHs. Polar organic compounds as a
result of addition of oxygen and nitrogen atoms into the carbon
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Y. Huang et al. / Earth and Planetary Science Letters 426 (2015) 101–108
Fig. 5. The relationship between molecular weights or carbon numbers and carbon stable isotope values (h VPDB) of PAHs and HACs extracted from Murchison and LON
94101.
skeletons would likely increase the activity of the adjacent hydrogen atoms, allowing more efficient replacement of hydrogen
by deuterium atoms in the interstellar molecular clouds. In comparison, D/H exchange during the parent body aqueous alteration
may be less important in determining hydrogen isotopic values of
different compound classes in carbonaceous chondrites. Hydrogen
isotopic ratios of water infiltrated CM chondrites have relatively
low δ D values (−158h as estimated by Eiler and Kitchen, 2004;
Alexander et al., 2012). If extensive hydrogen isotopic exchange occur during the parent body aqueous alteration, we would expect
more polar compounds and smaller PAHs to attain lower hydrogen
isotopic values than the non-polar compounds and larger PAHs, because polar and smaller aromatic compounds would be more likely
to contain readily exchangeable hydrogen atoms during aqueous
exchange reactions.
Notably, our samples for PAH isotopic analysis were the residues
from previous aqueous extraction for the analysis of monocarboxylic acids (Aponte et al., 2014). However, there is no evidence
to suggest that aqueous extraction procedure could lead to hydrogen isotopic exchange between water and organic molecules.
Polar organic compounds are more likely to exchange with (isotopically light) water during aqueous extraction: the observation
of higher δ D values in the more polar organic compounds in carbonaceous chondrites are inconsistent with significant isotopic
exchange during experimental extraction procedures. Hydrogen
isotopic exchange between water and meteoritic dicarboxylic acids
during aqueous extraction was found to be negligible under vigorous experimental conditions (Fuller and Huang, 2003).
We acknowledge, however, that there is large variability in the
hydrogen isotopic values of individual PAHs/HACs, even for a given
carbon number (Fig. 3). The observation suggests that PAHs/HACs
in carbonaceous chondrites may have multiple formation pathways. Majority of the compounds are likely formed in the interstellar environments by gas phase reactions, but parent body thermal
alteration processes could also lead to the formation/structural alteration of these compounds.
4.3. PAH carbon isotopic values: variability and relationship to
hydrogen isotopic values
The observation that larger PAHs tend to have lower δ 13 C values
(Fig. 5) suggests a synthetic pathway that higher molecular weight
aromatic compounds are predominantly synthesized from lower
molecular weight compounds under kinetic control by adding the
more reactive 12 C atoms, as previously proposed for short chain
alkanes and acids by Yuen et al. (1984). However, Sephton et al.
(1998) found such a carbon isotopic trend is reversed for lower
molecular weight aromatic compounds (mostly alkyl substituted
benzenes) obtained using supercritical fluid extraction (SFE) from
Murchison, and suggested that small aromatic organic molecules
can also be produced by cracking of larger molecules. In our
data, we see very large scattering of carbon isotopic values for
any given carbon number (Fig. 5), which is superimposed on the
overall trend of carbon isotopic depletion with increasing molecular weights. It is thus possible that cracking of macromolecules
also occurs after PAHs’ initial formation in the interstellar environments (for example, during parent body thermal alteration) and
contributes to the final aromatic compound pool in Murchison and
LON 94101. However, the former process (i.e., synthesis of higher
molecular weight compounds from low molecular weight compounds) must be predominating over the macromolecular cracking
because cracking would lead to greater hydrogen isotope exchange
between smaller molecules and water (hence lower δ D values for
smaller PAHs than larger PAHs which is opposite to our observation) (Schimmelmann et al., 2001).
Although carbon isotopic values of aromatic compounds in
Murchison and LON 94101 have negative values and mostly overlap with terrestrial aromatic compounds, we observe an overall
Y. Huang et al. / Earth and Planetary Science Letters 426 (2015) 101–108
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Fig. 6. Correlations between carbon and hydrogen isotopic values (weighted mean values for compounds with 2 to 4 rings) of PAHs and HACs in Murchison and LON 94101.
correlation between the δ 13 C and δ D values of these compounds
(Fig. 6). This further supports that the negative carbon isotopic
values of aromatic compounds in carbonaceous chondrites are
an inherent feature of carbonaceous chondrites, rather than indicative of terrestrial contamination. Comparison of δ 13 C values
of these aromatic compounds with other compound classes in
Murchison offer additional clues on the organic synthetic pathways. For example, carbon isotopic values of small C1 to C5 alkanes and alkenes in Murchison range from +2.4 to 9.2h (Yuen et
al., 1984), which are significantly more enriched in 13 C than the
aromatic compounds in our data (Table S3; Fig. 5). In addition,
both carbon and hydrogen isotopic ratios of aromatics are substantially lower than other more polar organic compound classes
in the carbonaceous chondrites, such as amino acids and carboxylic acids in Murchison (Sephton, 2002; Huang et al., 2005;
Pizzarello et al., 2006).
Two possible scenarios could contribute to the relatively low
δ 13 C (and to a less extent, δ D) values in PAHs and HACs when
compared to short chain alkanes and polar compound classes. First,
most PAHs and HACs contain 10 to 22 carbon atoms, much more
than those in small alkanes, monocarboxylic acids and amino acids
which have much higher carbon and isotopic values. Therefore,
as proposed by Yuen et al. (1984), a kinetically controlled isotopic fractionation during carbon addition reactions to form larger
molecules from smaller ones can lead to the resulting PAHs and
HACs containing more 12 C. Secondly, assuming most of the organic
compound classes with high deuterium contents were synthesized
in interstellar environments via gas phase reactions (Cronin and
Chang, 1993), our data imply that PAHs and HACs might have been
produced preferentially over compound classes such as branched
carboxylic acids and amino acids and, as a result, acquired more
reactive lighter carbon and hydrogen isotopes (i.e., 12 C and H as
opposed to 13 C and D; Fig. S2), because lighter isotope species react faster in chemical reactions.
hydrogen isotopic trend. A similar pattern of lower carbon isotopic values for higher molecular weight PAHs is consistent with a
kinetically controlled isotopic fractionation processes during stepwise carbon addition to form larger molecules from smaller ones.
PAHs and HACs may have been formed preferentially in the interstellar environments relative to other, more isotopically enriched
compound classes such as branched carboxylic acids and amino
acids, resulting in PAHs and HACs containing greater amounts of
lighter isotope species (12 C and H as opposed to 13 C and D). Despite of the general isotopic patterns (i.e., larger PAHs and HACs
have generally lower δ D and δ 13 C values), there are considerable
scattering in isotopic data for any given molecular weights or carbon numbers. Such scattering suggests other processes, such as
cracking of macromolecules, thermal alteration and condensation
of small compounds during parent body processes, could all have
contributed to the final PAH and HAC pool in the carbonaceous
chondrites.
5. Conclusions
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Acknowledgements
This research was supported by NASA Astrobiology and Exobiology grant NNX09AM82G to YH. JCA is grateful to the NASA
Postdoctoral Program Administered by Oak Ridge Associated Universities. The authors would like to thank T. McCoy, L. Welzenbach,
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Appendix A. Supplementary material
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