Organic geochemistry and coal petrology of tertiary brown coal in the Zhoujing mine, Baise Basin, South China—4. Biomarker sources inferred from stable carbon isotope compositions of individual compounds

Abstract

Carbon isotopic compositions of extractable compounds derived from higher land plants are within ± 2 0 00 of the bulk coal (δ 13 C = −27.0 0 00 ) from Baise Basin. This data suggests that either similar species contributed these compounds or that the plants from which this coal was formed imposed similar biosynthetic fractionations. The terpenoid hydrocarbons form two isotopically distinct groups: the diterpenoids (δ 13 C = −25.0 ± 1.4 0 00 ) and the sesquiterpenoids (δ 13 C = −25.9 ± 1.5 0 00 ) are on 1–2 0 00 enriched in 13C compared to the bulk coal, whereas the oleanane-ursane-lupane derivatives ( δ 13 C = −29.0 ± 0.8 0 00 ) are on average 1–2 0 00 depleted in 13C compared to the bulk coal. The n-alkanes ranging from C 15 to C 35, have a mean δ 13C value of −32.4 ± 0.6 0 00 , whereby averages for the C 17–C 22 and C 23–C 33 n-alkanes do not significantly differ from the overall average. Hopane derivatives in this coal extract are depleted in 13C compared to the bulk coal by 8–34 0 00 suggesting that bacterial cycling of methane played a role during formation of this coal.