Isotopic evidence from an Antarctic carbonaceous chondrite for two reaction pathways of extraterrestrial PAH formation

Citation
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
Citations number
28
Categorie Soggetti
Earth Sciences
Journal title
EARTH AND PLANETARY SCIENCE LETTERS
ISSN journal
0012821X → ACNP
Volume
184
Issue
1
Year of publication
2000
Pages
1 - 7
Database
ISI
SICI code
0012-821X(200012)184:1<1:IEFAAC>2.0.ZU;2-E
Abstract
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. (C) 2000 Elsevier Science B.V. All rights reserved.