Source and significance of selected polycyclic aromatic hydrocarbons in sediments (Hovea-3 well, Perth Basin, Western Australia) spanning the Permian–Triassic boundary

Abstract

Stable carbon isotopic data for polycyclic aromatic hydrocarbons (PAHs; chrysene and triphenylene, benzo( e)pyrene, 9-methylphenanthrene) in sediments of comparable thermal maturity from a petroleum drill core (Hovea-3 well, Perth Basin, Western Australia) spanning the largest mass extinction event in the past 500 million years (Permian–Triassic boundary) are consistent with an algal kerogen source in the basal Triassic as opposed to a combustion and/or higher-plant source in the underlying Permian sediments. The δ 13C values of extractable PAHs are less negative in the Permian inertinitic interval (−24.7‰ to −29.2‰) than the Triassic sapropelic interval (−30‰ to −33.3‰). A strong correlation is evident between δ 13C of individual PAHs, δ 13C of kerogen and Rock-Eval pyrolysis hydrogen index providing unequivocal evidence that PAH δ 13C is mainly controlled by organic matter type. Abundant algal-derived PAHs in basal Triassic sediments may be a global feature; the lack of coal globally in the basal Triassic sediments is also consistent with this observation. The Early Triassic coal gap appears to coincide with the global extinction of peat-forming plants at the end of the Permian, coal did not reappear until the Middle Triassic and Permian levels of plant diversity and thick peat deposits did not recover until the Late Triassic. The PAH isotopic data support the contention that a global anoxic event at the end of the Permian period contributed to the gradual worldwide asphyxiation and ultimate extinction of many marine and terrigenous organisms.