Tetracyclic terpanes enriched in Devonian cuticle humic coals

Abstract

Extracts of three samples of Devonian cuticle humic coal from Luquan, Yunnan Province of China have been analysed to characterize their biomarker compositions. The aliphatic fractions contain low amounts of n-alkanes and lack significant amounts of the common biomarkers, i.e. acyclic isoprenoids, steranes and hopanes. Except for one tricyclic terpane, the dominant hydrocarbons of these samples are tetracyclic diterpanes (C n H 2 n − 6 ) and are isomers of kaurane and beyerane. These tetracyclane series extend to at least C 37. In addition, some A-ring monoaromatic tetracyclic terpanes have also been found in these samples. Paleobotanic investigation of Luquan cuticle humic coals revealed that the plants which evolved in Yunnan Province during the Devonian period are mainly small such as Psilophytopsida and Lycopsida of the Pteridophyta. The tetracyclic terpanes cannot be derived from conifers because the Gymnospermeae had not yet evolved. Therefore, the precursors of the tetracyclic terpanes may be of a microbial, fungal or lower plant origin. It is worth noting that the two most abundant tetracyclic terpane homologues in Luquan coals are C 19H 32 and C 20H 34, which have the same carbon number as pristane and phytane. We suggest that there may be a relationship for precarboniferous coals between high contents of tetracyclic terpanes and low concentrations of isoprenoids.