Molecular and carbon isotopic evidence for the possible origin of 13α(n-alkyl)-tricyclic terpanes in Mesoproterozoic bitumens from the Yanliao Rift, North China

The Mesoproterozoic Jixian System in the Yanliao Rift of the North China Craton contains organic-rich shales within the Hongshuizhuang, Tieling, and Xiamaling formations. The biomarker composition of these formations is characterized by the dominance of 13α(n-alkyl)-tricyclic terpanes. The distinctive structure and main occurrence of 13α(n-alkyl)-tricyclic terpanes in the Mesoproterozoic Jixian System in the Yanliao Rift have intrigued researchers, raising questions about their origin and formation mechanisms. This study found that δ¹³C values of 13α(n-alkyl)-tricyclic terpanes ranged from −32.7‰ to −26.2‰, similar to those of kerogen and n-alkanes, but significantly more depleted in ¹³C compared to 13β(H),14α(H)-tricyclic terpanes, which ranged from −28.5‰ to −25.7‰. The difference in δ¹³C values suggested that 13α(n-alkyl)-tricyclic terpanes were not formed by the same mechanism or precursors as 13β(H),14α(H)-tricyclic terpanes. No sterane biomarkers indicative of eukaryotic source input were detected. Instead, the comparable concentrations of 13α(n-alkyl)-tricyclic terpanes and n-alkanes in the samples implied a possible origin from primary organic matter producers, notably cyanobacteria. Moreover, the biomarkers obtained from distinct formations within the Mesoproterozoic Jixian System exhibited an impressive degree of simplicity, similarity and mutual correlation. These findings collectively suggest the prevalence of a cyanobacteria-dominated primitive ecosystem during the Mesoproterozoic Era within the confines of the Yanliao Rift.