C27-C30 neohop-13(18)-enes and their saturated and aromatic derivatives in sediments: Indicators for diagenesis and water column stratification

Abstract

A limited suite of C27, C29 and C30 rearranged hopenes identified as neohop-13(18)-enes have been reported in immature Recent and ancient marine/lacustrine sediments and their presence has been explained by dehydration and isomerisation of ubiquitous hopanols or hopenes. Here we investigated the source and fate of neohop-13(18)-enes in a range of Recent and ancient sediments. The analysis of δ13C values of hop-17(21)-ene and neohop-13(18)-ene in Arabian Sea surface sediments, in the Monterey Formation and in immature Cenomanian black shales show that they differ by 2–3‰, suggesting that the C30 neohop-13(18)-ene has a source different from those of the non-rearranged C30 hopenes. A new member of the family of neohop-13(18)-enes, the C28 hopene 28,30-dinorhop-13(18)-ene, was identified based on comparison of its mass spectral data with that of other members of the family of neohopenes. Its occurrence explains the formation of a series of orphan aromatic hopanoids bearing an ethyl group at C-21, known to occur in high concentrations in some organic-rich ancient sediments. Circumstantial evidence for this formation pathway is provided by identical δ13C values for the C28 28,30-dinorhop-13(18)-ene and two aromatic hopanoids in two Cretaceous black shales. Relatively abundant C28 28,30-dinorhopene and related aromatic derivatives were present in ancient sediments where the distribution of other biomarkers (i.e. isorenieratene derivatives) indicated a stratified palaeo water column. Therefore, it is suggested that these compounds are derived from bacteria dwelling at or below the chemocline and may be used as indicators of stratified water bodies in the past. 28,30-Dinorhop-13(18)-ene may also be a precursor of the unusual C28 desmethylhopane 28,30-dinorhopane found in high concentrations in anoxic sediments and a limited suite of crude oils, which is consistent with the proposal that it too ultimately derives from bacteria living at the oxic–anoxic interface.