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The first building blocks of life could be produced in ultramafic-hosted hydrothermal systems considering the large amounts of hydrogen and methane generated by serpentinisation and Fischer-Tropsch-Type synthesis, respectively, in those systems. The purpose of this study was to detect and characterise organic molecules in hydrothermal fluids from ultramafic-hosted hydrothermal systems in the Mid-Atlantic Ridge (MAR) region. During the EXOMAR cruise 2005, fluids from the Rainbow (36°14′N) and the Lost City (30°N) hydrothermal fields were collected and treated by Stir Bar Sorptive Extraction (SBSE) and Solid Phase Extraction (SPE). The extracts were analysed by Thermal Desorption-Gas Chromatography-Mass Spectrometry (TD-GC-MS) and GC-MS, respectively. Compared to nearby deep seawater, hydrothermal fluids were clearly enriched in organic compounds, with a more diverse spectrum of molecules. We observed a very similar range of organic compounds in fluids from both sites, with a dominance of aliphatic hydrocarbons (C9-C14), aromatic compounds (C6-C16) and carboxylic acids (C8-C18). The occurrence of these compounds is supported by other field studies on serpentinites and sulfide deposits. Literature on thermodynamic data and experimental work has suggested the possible abiogenic origin of hydrocarbons and organic acids. In addition, it has been shown elsewhere that catalytic reactions producing hydrocarbons likely occur at both Lost City and Rainbow hydrothermal fields as suggested by the evolution of δ 13 C with increasing C number for methane, ethane, propane and butane. In order to investigate the origin of the organic molecules in the fluids, compound-specific carbon isotope ratio measurements were performed on n-alkanes and carboxylic acids, for which the δ 13 C values were in the range of − 46 to − 20‰ (vs. V-PDB). These preliminary data did not allow conclusive support or rejection of an abiogenic origin of the compounds. Indeed, predicting δ 13 C signatures in hydrothermal systems is likely to be complicated, due to differences in source δ 13 C signatures (i.e., of the C building blocks), and a variety of, mostly unknown, fractionation steps which may occur along the synthesis pathways. In addition, even though a fraction of the compounds detected in the fluids is likely abiotically produced, a dominance of biogenic sources and/or processes might hide their characteristic signature

Volcanic eruptions are foundational events that shape the Earth's surface and provide a window into deep Earth processes. How the primary asthenospheric melts form, pond and ascend through the lithosphere is, however, still poorly understood. We document an on-going magmatic event offshore Mayotte Island (North Mozambique channel), associated with large surface displacements, very low frequency earthquakes and exceptionally deep (25-50 km) seismicity swarms. We present data from the May 2019 MAYOBS1 cruise, which reveal that this event gave birth to a 820m tall, ~ 5 km 3 deepsea volcanic edifice. This is the largest active submarine eruption ever documented. The data indicate that deep magma reservoirs were rapidly drained through dykes that intruded the entire lithosphere and that pre-existing subvertical faults in the mantle were reactivated beneath an ancient caldera structure.

Intact natural gas hydrates recovered on the West African margin in the South Atlantic Ocean (ZaiAngo and Neris II projects) and from the Norwegian Sea (Hakon Mosby Mud Volcano) are investigated by micro-Raman spectroscopy at ambient pressure and low temperature. The gas hydrates collected at different geological sites contain a high methane concentration relative to other minor components that are slightly dispersed in the samples. They crystallize in a type I cubic lattice structure as also confirmed by our preliminary synchrotron diffraction results obtained on the ZaiAngo specimen. However, detailed analysis of selected microscopic areas reveals a variation in the gas distribution among the different specimens. Trace amounts of CO 2 and H 2 S can be identified by their characteristic vibrational signatures in the 1000-3800 cm − 1 spectral range. They are found to be coclathrated with methane. Their presence produces a compositional effect on the relative cage occupancy of CH 4 , as determined from the integrated band intensity ratio corresponding to the molecular stretching modes of methane in the hydrate. The comparative Raman analysis of synthetic hydrates of H 2 S, CH 4 and CH 4-deuterohydrates allows the unambiguous assignment of weak band overtones of trapped methane and co-clathrated H 2 S molecular vibrations.

A giant, 800-m wide pockmark, called Regab, was discovered along the Equatorial African margin at 3160-m water depth and was explored by remote operated vehicle (ROV) as part of the Zaiango (1998Zaiango ( -2000 and Biozaire (2001Biozaire ( -2003 projects carried out conjointly by TOTAL and a number of French research institutes. A microbathymetric map obtained using the ROV sensors shows that the pockmark actually consists of a cluster of smaller pockmarks aligned N70 along a 15-m deep depression. Methane was recorded all over the pockmark, the highest values along the axis of the depression where massive carbonate crusts and dense seep communities were also found. Several faunal species belong to the Vesicomyidae and Mytilidae bivalve families, as well as to Siboglinidae (Vestimentifera) tubeworms. Preliminary analyses confirm their association with symbiotic bacteria, thus documenting their dependence on fluid seeps. The pockmark appears to be related to an infilled channel, visible on the seismic data 300 m below the seafloor, which may act as a reservoir for biogenic fluids supplied to the trap from the surrounding sediments. AbstractA giant, 800-m-wide pockmark, called Regab, was discovered along the Equatorial African margin at 3,160-m water depth and was explored by remote operated vehicle (ROV) as part of the Zaiango (1998--2000) and Biozaire (2001Biozaire ( --2003 projects carried out conjointly by TOTAL and a number of French research institutes. A microbathymetric map obtained using the ROV sensors shows that the pockmark actually consists of a cluster of smaller pockmarks aligned N70° along a 15-m-deep depression. Methane was recorded all over the pockmark, the highest values along the axis of the depression where massive carbonate crusts and dense seep communities were also found. Several faunal species belong to the Vesicomyidae and Mytilidae bivalve families, as well as to Siboglinidae (Vestimentifera) tubeworms. Preliminary analyses confirm their association with symbiotic bacteria, thus documenting their dependence on fluid seeps. The pockmark appears to be 2 related to an infilled channel, visible on the seismic data 300 m below the seafloor, which may act as a reservoir for biogenic fluids supplied to the trap from the surrounding sediments.

Hydrographic surveys along the Mid‐Atlantic Ridge (MAR) between 12°N and 26°N, carried out from 1984 to 1990, show a variable pattern of CH4‐rich water column plumes. The vertical distribution of CH4 at stations located every 20–40 km is presented along this 1200‐km‐long section of the MAR. CH4 venting is clearly demonstrated. CH4‐enriched fluids rise from vents as plumes; spreading is confined to the axial valley due to the topography of the MAR. CH4 contents from 45 nmol to 675 nmol/kg are measured in the buoyant plumes above the two active hydrothermal sites (MARK 23°N; TAG 26°N) known at present, whereas CH4 anomalies up to 3.6 nmol/kg are typically observed in plumes emitted either on the inner floor, on the walls, and/or at the top of the rift mountains along the studied ridge section. CH4 concentrations (45 μmol to 144 μmol/kg) in MARK and TAG vent fluids are of the same order of magnitude as those found in the East Pacific Rise fluids. Even though CH4 is known to be unstable with respect to oxidation by dissolved oxygen, and in spite of its microbial oxidation in plumes, these results confirm CH4 to be a good indicator to track hydrothermal plumes and to map the variation of hydrothermal activity along mid oceanic ridges. Moreover, between 12° and 26°N along the MAR, CH4 results show that while hydrothermal activity is present everywhere along the ridge, it is predominant near fracture zones (FZ) (Kane FZ, 15°20′N FZ). Comparison of CH4 tracer with total dissolvable manganese (TDM) tracer in plumes allows us to differentiate subseafloor hydrothermal processes. The high TDM/CH4 found above TAG and MARK areas is indicative of basalt‐seawater interaction, while at 15°N the low TDM/CH4 provides evidence of fluid circulation in ultrabasic rocks. CH4 data confirm the association between mantle degassing, hydrothermal activity, and serpentinization along this 12°–26°N section of the Mid‐Atlantic Ridge.

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