Ancient polycyclic aromatic hydrocarbons in modern soils: 13C, 14C and biomarker evidence

Polycyclic aromatic hydrocarbons (PAHs) of soils from two maize growing fields were analyzed for molecular structure by gas chromatography-mass spectrometry, for 13C/12C ratios by isotope ratio mass spectrometry and for 14C age by accelerator mass spectrometry. These investigations indicate that the aromatic fractions are dominated by pyrolytic inputs. Calculation of molecular maturity parameters based on hopanes and PAHs gives values typical of high thermal stress such as exhibited by high rank coals and petroleums with equivalent vitrinite reflectances of 0.6–1.5%. The interpretations based on molecular compositions are corroborated by isotopic data. Specifically, both the absence of 13C/12C variations in the aromatic fractions during 23 years of maize cropping, and a 14C age greater than 9820 years demonstrate that these substances are not derived by the aromatization of modern plant material or soil biomass. Together, these results show that soil PAHs are mainly of pyrolytic origin, e.g. exogenous (airborne) contributions of fossil fuel combustion products and (old) vegetation fires, with a small contribution of uncombusted fossil fuels. This approach using molecular and isotope information represents a promising way to study the fate of ancient, reworked, fossil organic matter in modern media such as soils, sediments, plants, air and food.