H. Naraoka et al., Isotopic evidence from an Antarctic carbonaceous chondrite for two reaction pathways of extraterrestrial PAH formation, EARTH PLAN, 184(1), 2000, pp. 1-7
Polycyclic aromatic hydrocarbons (PAHs) are one of the most ubiquitous orga
nic compounds in the universe. PAHs are sometimes used as a molecular marke
r for biological activity, however, they are also formed by abiogenic proce
sses. Carbon isotopic compositions of individual PAHs have important clues
to clarify their origins and formation mechanisms for the better understand
ing in organic cosmogeochemistry of PAHs. In the Asuka-881458 carbonaceous
chondrite which was recovered from Antarctica in 1989, more than 70 PAHs we
re identified from naphthalene to benzo(ghi)perylene, where fluoranthene an
d pyrene are the most abundant. Carbon isotopic compositions of individual
PAHs range from -26 to 8 parts per thousand (relative to PDB). More condens
ed PAHs are more depleted in C-13 as the H/C ratio decreases. The carbon is
otope distribution of PAHs containing more than three rings is similar to t
hat from the Murchison meteorite, but clearly different from that of the te
rrestrial PAHs. The isotope distribution suggests that the PAHs in carbonac
eous chondrites are formed under kinetic control rather than by thermodynam
ic equilibrium. In particular, two reaction pathways ('pyrene series' and '
fluoranthene series') can be distinguished assuming kinetic control. The re
latively large isotopic fractionation could occur during cyclization and/or
carbon addition in the interstellar or meteorite parent body environment.
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