Early diagenesis of bacteriohopanetetrol derivatives: Formation of fossil homohopanoids

Abstract

Diagenetic pathways of bacteriohopanepolyol derivatives are proposed based on the concentrations and 13C contents of homohopanes, homohop-17(21)-enes, benzohopanes, hopanoid thiophenes and sulphides, and macromolecularly S-bound homohopanes present in the extracts of twelve composite one metre samples from a 120 m core recovered from the Upper Cretaceous Jurf ed Darawish Oil Shale (Jordan). A large part (>80–95%) of the pentakishomohopane skeleton occurs in a S-bound form. This reveals the selective preservation of the C35 hopane skeleton by sulphur sequestration and provides a theoretical basis for the homohopane index as an indicator of anoxia in past depositional environments. A smaller part (>50–80%) of the total extended hopane skeletons (C31–C35) occurs in a S-bound form. Of the non-sulphur-containing hopanoids the homohop-17(21)-enes dominate. These latter components show a gradual increase of 22S epimers with depth (45–52%) towards the thermodynamic equilibrium (52–53%) as calculated by molecular mechanics. Molecular mechanic calculations indicate that this increase can be explained by either isomerisation of 22R hop-17(21)-enes or by isomerisation of double bonds of homohopenes formed by dehydration of bacteriohopanepolyols 'en passant' isomerising the chiral centre at C-22. A combination of these two pathways is also possible and provides an explanation for different 13C values of pairs of 22R and 22S epimers. Isomerisation of 17,21(H)-homohopane to 17,21,(H)-homohopane carbon skeletons occurs for all compound classes in a very narrow depth span (ca. 20 m) and is probably induced by small differences in thermal history. Compound-specific carbon isotope analyses indicated that the series of homohop-17(21)-enes have in some cases significant differences in 13C content, indicating that at least two different sources have contributed to this series of components. Differences with macromolecularly S-bound C35 hopane skeletons and free C31 hopanes 13C contents are in some cases even larger. These data show that the diagenetic pathways of bacteriohopanepolyol derivatives are more complex than previously recognized and reveal that multiple precursor bacteriohopanepolyol derivatives prone to different diagenetic pathways have to be envisaged to account for the differences observed.