Hydrothermal circulation of seawater within the oceanic crust creates conditions suitable for chemosynthesis-based microbial life. Synthesis of abiotic organic compounds takes place during seawater-basement rock interaction at elevated temperatures. Low temperature circulation in the recharge zone allows chemolithoautotrophs to gain energy by oxidizing or reduction of minerals, while deeper and hotter regions (reaction zone) are dominated by rock alteration and produce the reduced conditions which are required in abiotic organic synthesis. In this thesis thermodynamic reaction path models have been used to evaluate the potential of abiotic organosynthesis in sub-seafloor environments as well as to determine the Gibbs energy available for metabolic reaction along given mixing paths for hydrothermal fluid and seawater. By merging model results with geochemical field data and microbiological observations it is shown that thermodynamic calculations can be used to enhance our understanding of hydrothermal processes
