Biomarker signatures in miocene lignite, neyveli, southern india

Abstract

Lignite forms upon partial decay and compression of organic matter typically in terrestrial wetland ecosystem. The present study focusses on the hydrocarbon biomarker investigation using gas chromatography mass spectrometry (GC-MS) from lignite deposits collected from the Neyveli lignite mine in Tamil Nadu. We have attempted to understand the organic matter input into formation of the lignite layers and the palaeoenvironment by studying in detail the saturated and aromatic hydrocarbons acquired through silica gel chromatography of the total extract extracted from the sediments. We found the presence of normal alkanes, acyclic isoprenoids such as pristane, phytane, hopanoids, triterpenoids with oleanoid and ursanoid skeletons in the saturated hydrocarbon fractions of the samples. Both completely saturated as well as unsaturated compounds were detected for the hopanoids, oleanoids and ursanoids. The predominance of the mid-chain and longer chain alkanes suggest the input of aquatic biota and terrestrial plant contribution into the lignite formation. The oleanoid and ursanoid compounds confirm the presence of angiosperm flora, whereas the hopanoids reflect prokaryotic microbial lipid contribution. Good preservation of the organic matter is depicted by the carbon preference index values and preservation has been facilitated by the anoxic condition as reflected by the Pr/Ph ratio values. A variety of aromatized angiosperm-derived triterpenoids are detected that provide information on the diagenetic alterations of the hydrocarbon biomarkers occurring in the sediment matrix upon progressive burial of the lignite.