New insight into the contributions of thermogenic processes and biogenic sources to the generation of organic compounds in hydrothermal fluids

Experiments on hydrothermal degradation of Pyrococcus abyssi biomass were conducted at elevated pressure (40 MPa) over a 200–450 °C temperature range in sapphire reaction cells. Few organic compounds could be detected in the 200 °C experiment. This lack was attributed to an incomplete degradation of P. abyssi cells. On the contrary, a wide range of soluble organic molecules were generated at temperatures ≥350 °C including toluene, styrene, C₈–C₁₆ alkyl‐benzenes, naphthalene, C₁₁–C₁₆ alkyl‐naphthalenes, even carbon number C₁₂–C₁₈ polycyclic aromatic hydrocarbons, C₁₅–C₁₈ alkyl‐phenanthrenes and C_8:0–C_16:0 n‐carboxylic acids. The effect of time on the final organic composition of the degraded P. abyssi solutions at 350 °C was also investigated. For that purpose the biomass was exposed for 10, 20, 60, 90, 270 and 720 min at 350 °C. We observed a similar effect of temperature and time on the chemical diversity obtained. In addition, temperature and time increased the degree of alkylation of alkyl‐benzenes. This study offers additional evidence that a portion of the aliphatic hydrocarbons present in the fluids from the Rainbow ultramafic‐hosted hydrothermal field may be abiogenic whereas a portion of the aromatic hydrocarbons and n‐carboxylic acids may have a biogenic origin. We suggest that aromatic hydrocarbons and linear fatty acids at the Rainbow site may be derived directly from thermogenic alteration of material from the sub‐seafloor biosphere. Yet we infer that the formation and dissolution of carboxylic acids in hydrothermal fluids may be controlled by other processes than in our experiments.