A distinctive biomarker assemblage in an Infracambrian oil and source rock from western India: Molecular signatures of eukaryotic sterols and prokaryotic carotenoids

Abstract

Molecular fossils record aspects of Neoproterozoic-Early Cambrian palaeobiology in the Bikaner-Nagaur Basin, western India. Biomarker hydrocarbons from crude oils and sedimentary rock extracts belonging to the Marwar Supergroup, India, were analyzed using gas chromatography–mass spectrometry, including molecular specific metastable reaction monitoring (MRM) transitions. The distinctive biomarker assemblage indicates significant contribution from both eukaryotes and bacteria. The steranes record contributions from algae as well as demosponges. Dominance of C29 sterane isomers signifies that green algae were important primary producers while detection of traces of dinosteranes suggests input from dinoflagellates or their ancestors. The high C30 sterane ratio (24-isopropylcholestane/24-n-propylcholestane; ∼5) indicates substantial proliferation of Demospongiae which are widely considered as basal metazoa. The community of algae and other taxa, including sponges which require oxygen, thrived in a thin oxygenated layer in the upper water column. However, the presence of an underlying euxinic water column is demonstrated by the presence of gammacerane, extremely low values of pristane/phytane (around 0.4) and carotenoids derived from both green (Chlorobi) and purple (Chromatiaceae) sulfur bacteria. Chlorobactane, isorenieratane and β-isorenieratane indicate both green- and brown-pigmented Chlorobi, whereas the detection of okenane and renierapurpurane are diagnostic for the Chromatiaceae. Paleorenieratane, a carotenoid pigment from an apparently ‘extinct’ Chlorobi taxon is present in high abundance particularly in the sedimentary rock extract. Excellent preservation of the biomarkers and an extremely mild thermal history is indicated by sterane and triterpane molecular ratios with preferences for biological stereoisomers. The entire biomarker assemblage is indicative of a restricted, salinity-stratified depositional environment possibly including the presence of extensive, anoxygenic microbial mats. Some of the observed characteristics mirror those of other coeval successions in Oman, Eastern Siberia and Australia but the particular combination of features appears to be unique to this setting.