Biomarker signatures of Permian Gondwana coals from India and their palaeobotanical significance

Coal samples belonging to the Lower Permian Barakar Formation and the Upper Permian Raniganj Formation, collected from Damodar Valley Basin of eastern India, have been analysed for understanding their organic geochemical characteristics using Rock-Eval pyrolysis, gas chromatography-mass spectrometry (GC-MS) and comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GC×GC-TOFMS). The Rock-Eval pyrolysis parameters suggest that the coals from both the formations contain a mixture of Type-II and Type-III kerogen. The Rock-Eval Tmax values of the analysed coals vary from 417 to 451 °C. n-Alkanes ranging from C12 to C31 predominate the aliphatic hydrocarbons present in the coal extracts. The carbon preference indices (CPI) as well as the odd-to-even preference (OEP) values of the n-alkanes are > 1, suggesting input from higher vascular land plants. C29 Steranes predominate over C27 and C28 steranes, which corroborates contribution from terrestrial higher plants. Aliphatic diterpanes like ent-beyerane, isopimarane, abietane, phyllocladanes and ent-kaurane as well as aromatic diterpanes like simonellite, bisnorsimonellite, retene, methylretenes and tetrahydroretene which are mainly known to originate from conifers are detected in the studied coals. The Lower Gondwana floral assemblage of Peninsular India is mainly represented by Glossopteris flora (Pteridospermatophyta). However, some primitive conifers have also been reported from this succession. We propose that the diterpanes detected in the studied coals were possibly derived from the primitive conifers. This implies that the biosynthetic pathways to produce these diterpanes had evolved in the Gondwana flora during the Permian period.