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

Polycyclic aromatic hydrocarbons (PAHs) are one of the most ubiquitous organic compounds in the universe. PAHs are sometimes used as a molecular marker for biological activity, however, they are also formed by abiogenic processes. Carbon isotopic compositions of individual PAHs have important clues to clarify their origins and formation mechanisms for the better understanding in organic cosmogeochemistry of PAHs. In the Asuka-881458 carbonaceous chondrite which was recovered from Antarctica in 1989, more than 70 PAHs were identified from naphthalene to benzo(ghi)perylene, where fluoranthene and pyrene are the most abundant. Carbon isotopic compositions of individual PAHs range from −26 to 8‰ (relative to PDB). More condensed PAHs are more depleted in 13C as the H/C ratio decreases. The carbon isotope distribution of PAHs containing more than three rings is similar to that from the Murchison meteorite, but clearly different from that of the terrestrial PAHs. The isotope distribution suggests that the PAHs in carbonaceous chondrites are formed under kinetic control rather than by thermodynamic equilibrium. In particular, two reaction pathways (‘pyrene series’ and ‘fluoranthene series’) can be distinguished assuming kinetic control. The relatively large isotopic fractionation could occur during cyclization and/or carbon addition in the interstellar or meteorite parent body environment.