Abstract:
The alteration products of cholesterol subjected to hydrothermal conditions have been analyzed by gas chromatography-mass spectrometry. Four sets of experiments were conducted at temperatures ranging between 150 and 300 °C for 24 h. The reaction mixtures of the first set included only cholesterol and water, whereas those of the second, third and fourth sets also contained oxalic acid, natural sediment and montmorillonite, respectively. The alteration rate and the number of observed alteration products increased with higher temperature and acidity of the reaction mixtures. At lower temperatures cholestenes and cholestenones were major compounds. Cholestene concentrations increased at higher temperatures in all sets and were highest at 200 °C in acidic medium, at 250 °C in the presence of sediment and at 300 °C with montmorillonite. Cholestane concentrations also increased at elevated temperatures, being greatest in the absence of both sediment and montmorillonite. Diacholestenes were detected in an acidic medium at all temperatures and with montmorillonite at >200 °C. Monoaromatic steroid hydrocarbons were found above 200 °C. Thus, backbone rearrangements were the major alteration processes and bond cleavage (cracking) was predominant in an acidic medium, whereas aromatization was enhanced in the presence of both sediment and montmorillonite. These products confirm that reductive biomarker alteration in hydrothermal systems occurs rapidly and at high temperatures (>250 °C).
