Aromatic hydrocarbon signatures of the late Miocene-early Pliocene in the Yinggehai Basin, South China Sea: Implications for climate variations

Aromatic hydrocarbons in 36 shales from the late Miocene-early Pliocene strata of the Yinggehai Basin in the South China Sea were investigated. Various higher plant-derived aromatic hydrocarbons were identified, including cadalene, retene, 1,2,7-trimethylnaphthalene, 1-methylphenanthrene (MP), 1,7-dimethylphenanthrene (DMP), 6-isopropyl-1-isohexyl-2-methylnaphthalene, and other polycyclic aromatic hydrocarbons (PAHs). Higher plant parameters suggest that the relative abundance of terrigenous organic matter in the early Pliocene was lower than in the angiosperm-dominated late Miocene Huangliu Formation. Various plant parameters including retene/(retene + cadalene), 1-MP/9-MP, 1,7-DMP/(1,3- + 3,9- + 2,10 + 3,10-DMP), and the higher plant index, indicate that gymnosperms made a larger contribution to the organic matter when the lower Yinggehai Formation was deposited during the early Pliocene. Combustion-derived PAHs, including fluoranthene, benzo[a]anthracene, benzofluoranthenes and coronene, predominate over petrogenic PAHs in the lower Yinggehai Formation and the upper Meishan Formation, indicating that there were more wildfires during deposition of these units compared to the Huangliu Formation. In contrast, there is a mixed input of combustion-derived and petrogenic PAHs in the Huangliu Formation. As also supported by higher abundances of Pinus in palynological records, the combustion-derived PAHs and the gymnosperm/angiosperm parameters provide unequivocal evidence for a cooling climate from the late Miocene to the early Pliocene in the South China Sea.