Characterisation of dissolved organic compounds in hydrothermal fluids by stir bar sorptive extraction - gas chomatography - mass spectrometry. Case study: the Rainbow field (36°N, Mid-Atlantic Ridge)

Organic geochemistry is of major importance in both geosciences and life sciences [1]. Investigation, identification and quantification of organic compounds (e.g., biomarkers, prebiotic molecules, hydrocarbons) help in understanding the evolution of the Earth and constraining biogeochemical processes that occurred or are still occurring on Earth. The organic geochemistry of rivers, lakes, estuaries, sedimentary basins, terrestrial rocks and oil reservoirs is being extensively studied. Despite the likely significant impact of hydrothermal circulation on the ocean global energy and matter fluxes and the implication of hydrothermal systems in major issues such as the origin of life [2,3], publications on the organic geochemistry of hydrothermal systems are rare. The literature comprises a few studies devoted to the organic contents of hydrothermal sulphide deposits [4], serpentinites [5], carbonate chimneys [6,7] and sediments [8]. In terms of fluids, the abiotic synthesis of dissolved hydrocarbon gases [9-11] as well as the presence of larger dissolved hydrocarbons and other organic molecules [12-14] in fluids from ultramafic-hosted systems at slow spreading ridges has been reported.Hydrothermal vents are found both on land (e.g. geysers, hot springs) and on the seafloor at Mid-Ocean Ridges (MOR), back arc basins and subduction zones. During hydrothermal circulation, seawater heats up and interacts with rocks in the hot Earth’s crust and mantle. Hydrothermal systems are the places where this modified seawater is expelled as hydrothermal fluids. Water-rock interactions generate gases (CH4, H2, H2S), whereas, major and minor elements such as Fe, Mn, Ca, Li, K, Na, Cl, Si are exchanged between rocks and water. These chemical entities may dissolve in the fluids or precipitate (metal oxide particle). Concentrations in the aqueous phase vary depending on lithologies (rock assemblages), processes that occur during hydrothermal circulation and physico-chemical conditions. For