Molecular records of microbialites following the end-Permian mass extinction in Chongyang, Hubei Province, South China

By using gas chromatograph (GC) and GC–mass spectrometry, a series of biomarkers were identified in the aliphatic fractions of extracts from microbialites following the end-Permian faunal mass extinction in Chongyang section, Hubei Province, South China. The dominance of lower-molecular-weight n-alkanes in the samples from the section suggests that algae and bacteria were the dominant contributors to the organic fraction preserved in this marine section, with bacteria particularly enhanced within the stromatolite. The end-Permian biotic crisis coincided with a short-term suppression of autotrophs, indicated by the ratio of pristane (Pr) and phytane (Ph) to C17 and C18 n-alkanes, and proliferation of heterotrophs, shown by the ratio of branched-to-normal C17 alkanes. The biotic crisis also may have been associated with a transient oxygenation event in the shallow marine, indicated by elevated Pr/Ph ratios and lowered C27 18α(H)-22,29,30-Trisnorneohopane (Ts) to C27 17α(H)-22,29,30-Trisnorhopane (Tm) and gammacerane (γ) to C31 homohopane (C31HP) ratios. Subsequently, three short-term episodes of intensified anoxia or enhanced salinity at Chongyang are recorded by decreases in Pr/Ph ratios in conjunction with lithological boundaries in the section.