The effect of aromatization on the isotopic compositions of hydrocarbons during early diagenesis

Katherine H. Freeman*, Christopher J. Boreham, Roger E. Summons, J. M. Hayes

*Corresponding author for this work

    Research output: Contribution to journalArticle

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    Polycyclic aromatic hydrocarbons with varying degrees of aromatization were isolated from the Eocene Messel Shale (Rheingraben, Germany). The high abundances of these compounds and their structural resemblances to cyclic triterpenoid lipids are consistent with derivation from microbial rather than thermal processes. Compounds structurally related to oleanane contain from five to nine double bonds; those within a series of aromatized hopanoids contain from three to nine. All are products of diagenetic reactions that remove hydrogen or methyl groups, and, in several cases, break carbon-carbon bonds to open rings. Aromatized products are on average depleted in 13C relative to possible precursors by 1.2‰ (range: 1.5‰ enrichment to 4‰ depletion, n = 9). The dependence of 13C content on the number of double bonds is not, however, statistically significant and it must be concluded that there is no strong evidence for isotopic fractionation accompanying diagenetic aromatization. Isotopic differences between series (structures related to ursane, des-A-ursane, des-A-lupane, des-A-arborane, and possibly, des-A-gammacerane are present) are much greater, indicating that 13C contents are controlled primarily by source effects. Fractionations due to chromatographic isotope effects during HPLC ranged from 0.1 to 2.8‰.

    Original languageEnglish
    Pages (from-to)1037-1049
    Number of pages13
    JournalOrganic Geochemistry
    Issue number10-11
    Publication statusPublished - 1994


    • carbon isotope fractionation
    • compound-specific isotope analyses
    • Messel shale
    • microbial transformation of triterpenoids
    • polycyclic aromatic hydrocarbons

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