Reconstruction of early Eocene paleoclimate and paleo-environment in Guchengzi Formation, Fushun Basin, NE China, using biomarkers and aromatic compounds

Abstract

One hundred and twenty-four coal and coaly shale samples were collected from a 13-meter-thick coal open mining section of the early Eocene Guchengzi Formation in the Fushun Basin, NE China. The coals and coaly shales from the Eocene Guchengzi Formation were deposited in an oxic environment and are classified within the subbituminous A rank. Rock-Eval hydrogen index values indicate the presence of Type II2-III kerogens. Molecular compositions were determined using gas chromatography-mass spectrometry (GC-MS) to diagnose source inputs and assess depositional conditions. The n-alkane distribution in these samples is characterized by a dominance of long-chain odd-numbered n-alkanes, exhibiting high odd-to-even preference. The isoprenoid distribution is marked by a high pristane/phytane (Pr/Ph) ratio, which is indicative of oxic conditions during organic matter formation. The samples show high concentrations of bicyclic sesquiterpanes, tricyclic diterpanes, tetracyclic diterpanes, and hopane homologs, along with low concentrations of steranes and the absence of oleanane. This suggests a dominance of gymnosperm biomass input with significant bacterial activity. Aromatic hydrocarbons in the samples are dominated by naphthalene, phenanthrene, and their alkylated homologs, with a low abundance of four- and five-ring aromatic hydrocarbons. Various higher plant-derived aromatic hydrocarbons were identified, including 1,2,5-trimethylnaphthalene, 1,2,5,6-tetramethylnaphthalene, cadalene, 6-isopropyl-1-isohexyl-2-methylnaphthalene, 1,7-dimethylphenanthrene, simonellite, and retene. The distribution of combustion-derived polycyclic aromatic hydrocarbons such as fluoranthene, pyrene, benzo[a]anthracene, and various benzofluoranthenes and benzopyrenes suggests that forest fires occurred in the study area during the early Eocene period. Molecular constituents provide valuable insights into the paleoenvironmental conditions and the type of vegetation contributing to the organic matter in the Fushun Basin during the early Eocene. The geochemical data highlight the complex interplay between terrestrial and aquatic inputs, bacterial activity, and environmental factors such as forest fires, all of which played a significant role in shaping the depositional environment of the Guchengzi Formation coals and coaly shales.