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Both hydrogen and methane are consistently discharged in large quantities in hydrothermal fluids issued from ultramafic-hosted hydrothermal fields discovered along the Mid-Atlantic Ridge. Considering the vast number of these fields discovered or inferred, hydrothermal fluxes represent a significant input of H-2 and CH4 to the ocean. Although there are lines of evidence of their abiogenic formation from stable C and H isotope results, laboratory experiments, and thermodynamic data, neither their origin nor the reaction pathways generating these gases have been fully constrained yet. Organic compounds detected in the fluids may also be derived from abiotic reactions. Although thermodynamics are favorable and extensive experimental work has been done on Fischer-Tropsch-type reactions, for instance, nothing is clear yet about their origin and formation mechanism from actual data. Since chemolithotrophic microbial communities commonly colonize hydrothermal vents, biogenic and thermogenic processes are likely to contribute to the production of H-2, CH4, and other organic compounds. There seems to be a consensus toward a mixed origin (both sources and processes) that is consistent with the ambiguous nature of the isotopic data. But the question that remains is, to what proportions? More systematic experiments as well as integrated geochemical approaches are needed to disentangle hydrothermal geochemistry. This understanding is of prime importance considering the implications of hydrothermal H-2, CH4, and organic compounds for the ocean global budget, global cycles, and the origin of life.

As part of a Public Private consortium, explorations to locate sulfide mineralization in the French EEZ of Wallis and Futuna allowed to discover new hydrothermal fields. Three cruises organized between 2010 and 2012 have explored this area. The first phase focused on a regional mapping to find areas of recent volcanism using multibeam echosounders and acoustic imaging. Hydrothermal plumes produced by high temperature hydrothermal activity were located using chemical and physical anomalies in the water column. Using a CTD/rosette we collected water samples and measured physical parameters. Onboard methane and manganese concentrations and physical parameters studies helped to locate active fields with a precision of about 5 km. Physical anomalies caused by the hydrothermal plumes were also recorded by analyzing the multibeam backscattered signals of the water column. In a second phase, an AUV, operated at 70m above the seafloor, helped to establish high-resolution (1 m) bathymetric maps on targets identified during Phase 1. Near seafloor, magnetic anomalies were recorded using a deep magnetometer on the AUV. Analysis of AUV acoustic signals in the water column allowed to find the vertical part of the hydrothermal plumes and to pinpoint areas of fluids discharge with a precision of a few meters. During the third phase, manned submersible dives were conducted on the hydrothermal fields to study the hydrothermal fluids, the different types of mineralization and associated ecosystems. This multi-scale strategy allows gaining considerable time in exploration, and led to the discovery of several hundred of active and inactive fields and associated sulfides mineralization on an area not previously mapped. The AUV high-resolution mapping and detection of vertical plumes and inactive chimneys can thus be considered as a missing link of the exploration strategy to locate hydrothermal deposits.

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 C8:0–C16:0 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., MOMARNET