Organic compounds in hydrothermal fluids: techniques, role and advances

The story of organic compounds in hydrothermal fluids dates back from the late 1970s with the idea that hydrothermal conditions are favourable to abiotic organic synthesis (Ingmanson and Dowler, 1977). Notably, oil (hydrocarbons) and the first building blocks of life (prebiotic molecules) could be generated. Both thermodynamics and laboratory work have been supporting these hypotheses. More recently, the discovery of substantial amounts of abiogenic methane in fluids from ultramafic-hosted hydrothermal systems has constituted the first line of evidence of the actual occurrence of abiotic synthesis in natural environment (Charlou et al., 1998). Simply, in the presence of iron bearing minerals, serpentinisation reactions generate H2 which in turn reacts with CO2 to produce CH4 via the Sabatier reaction (4H2 + CO2 ? CH4 + 2H2O). Besides, the presence of larger organic compounds in hydrothermal systems may be due to different processes (Konn et al., 2009). On the one hand, catalytic and abiotic Fisher-Tropsh Type (FTT) reactions are likely to produce larger organic molecules. On the other hand, biogenic and thermogenic processes may also be involved. For examples: CH4 synthesized by methanogens bacteria may polymerize under hydrothermal conditions; Thermal degradation of biomass living in aquifers or of biological debris entrained into hydrothermal circulation would produce organic fragments soluble in hydrothermal fluids. The contribution of each process and of the various carbon sources (mantle CO2, biogenic C) are still very poorly documented and understood. Compound specific measurements are insufficient in discriminating. The fairly intermediate ?13C value (-20 to -30‰) is probably the signature of a mixed-contribution. Investigation of thermogenic processes shed some lights on the problem (Konn et al., 2011). However additional lines of evidence are needed to come to a conclusion. Current and promising approaches that could give clues in understanding geochemical processes involved in the generation of organic compounds in hydrothermal fluids are the use of Raman spectroscopy and quantification of these compounds. Upon the origin of organic compounds, they basically constitute a source of energy and carbon to living organism present in hydrothermal plumes. They also are of major importance to metal transportation and bioavailability via the formation of organo-metallic complexes. This is likely to impact the global metals cycle and speciation in hydrothermal plumes and in the ocean, as well as to influence the microscopic life in hydrothermal plumes (species, larvae development and transport). The purpose of the seminar is to give an overview of the organic chemistry in hydrothermal fluids from sampling to interpretation. Références: Charlou, J.L., Fouquet, Y., Bougault, H., Donval, J.P., Etoubleau, J., Jean-Baptiste, P., Dapoigny, A., Appriou, P., Rona, P.A., 1998. Intense CH4 plumes generated by serpentinization of ultramafic rocks at the intersection of the 15°20'N fracture zone and the Mid-Atlantic Ridge. Geochimica et Cosmochimica Acta 62, 2323-2333. Ingmanson, D.E., Dowler, M.J., 1977. Chemical evolution and the evolution of Earth's crust. Origins of life 8, 221-224. Konn, C., Charlou, J.L., Donval, J.P., Holm, N.G., Dehairs, F., Bouillon, S., 2009. Hydrocarbons and oxidized organic compounds in hydrothermal fluids from Rainbow and Lost City ultramafic-hosted vents. Chemical Geology 258, 299-314. Konn, C., Testemale, D., Querellou, J., Holm, N.G., Charlou, J.L., 2011. New insight into the contributions of thermogenic processes and biogenic sources to the generation of organic compounds in hydrothermal fluids. Geobiology 9, 79-93.