Organic compounds in geological hydrothermal systems: A critical review of molecular transformation and distribution

There has been a surge in the number of studies on submarine hydrothermal vents and smokers, which has enhanced our understanding of the evolution of organic hydrocarbons and compounds with heat and various inorganics released from the inner earth. There is great variability in the organic compounds produced in these regions due to the complex and varied synthesis and alteration conditions present on seabeds. This invokes potential influences to interpret hydrocarbon biomarkers and compounds as toolkits to reconstruct paleoenvironments, past organism diversity and catastrophic events and to decipher ore or petroleum systems, which have been associated with hydrothermal activities or events. The degree to which these organic molecules (e.g., n-alkanes, steranes, and dibenzothiophene) could be hydrothermally (secondarily) impacted remains incompletely understood. This review compares the organic geochemistry between submarine and on-land hydrothermal environments, highlight the importance of organic–inorganic interactions, and reappraise hydrothermally altered hydrocarbons, biomarkers and compounds that are extractable by organic solvents. Hydrothermal alteration significantly compromises the use of organic geochemical proxies due to changes in molecular composition, e.g., unresolved complex mixtures (UCMs), biomarkers, unsubstituted/alkyl/phenyl polycyclic aromatic hydrocarbons (PAHs) and heterocyclic PAHs (e.g., S-containing PAHs). These features are recorded to provide pseudomathematical correlations between various parameters and proxies based on varied numerators and denominators of ratios, which may yield misinterpretations. Based on the molecular transformations and distributions of three groups of compounds, i.e., unsubstituted PAHs, phenyl derivatives, and S-containing PAHs, we propose a set of concise molecular marker-based indices and ternary diagrams for identifying and gauging the relative influence of hydrothermal effects on potentially affected geological samples. Finally, we discuss the future outlook for assessing the contribution of hydrothermal alteration to different research fields related to hydrothermal environments or events.