Extending the dataset of fluid geochemistry of the Menez Gwen, Lucky Strike, Rainbow, TAG and Snake Pit hydrothermal vent fields: Investigation of temporal stability and organic contribution

The Menez Gwen, Lucky Strike, Rainbow, TAG (Transatlantic Geotraverse) and Snake Pit hydrothermal vent fields on the Mid-Atlantic Ridge were revisited and resampled for geochemical investigation during the BIOBAZ 2013 and BICOSE 2014 research cruises. Geochemical analysis of the major and minor elements of the hydrothermal fluid and concentrations of gases extends and complements the existing dataset. Our results are consistent with values previously reported and fall within the range of the analytical error. This indicates that the hydrothermal vent field system has remained relatively stable over the last few decades. However, some differences were observed and (i) suggested a recent eruption at Menez Gwen, (ii) supported the occurrence of low-temperature serpentinisation in this same site, (iii) supported a change in the reaction zone or axial magma chamber (AMC) depth at Lucky Strike, (iv) an increase of the temperature at depth at Snake Pit and (v) supported the hypothesis of large seawater entrainment through the TAG hydrothermal mound. Besides, it is possible that small temporal and spatial scale processes may control a significant part of the geochemistry, owing to the fact that some variations in the data could not be interpreted. However, our investigation of the organic geochemistry represents a pioneering addition to research for Menez Gwen, Snake Pit and TAG and a much more comprehensive study for Lucky Strike and Rainbow. Concentrations for a wide variety of semi volatile organic compounds (SVOCs) were obtained for the first time at all sites. Our results showed that a great part of the total organic carbon (TOC) could not be allocated by the total SVOCs studied here, suggesting that other processes/sources of organic carbon remain to be identified. The TAG organic geochemistry seemed entirely based on thermogenic processes whereas mixed processes may occur at the other vent field. The presence of n-alkanes suggested the contribution of a low-temperature fluid at all sites. An additional high-temperature organic matter degradation component was likely present at Menez Gwen and Lucky Strike. Our results also indicated that both abiogenic and biogenic processes produced organic compounds. Therefore, we suggest that a portion of the fatty acids at Menez Gwen and polyaromatic hydrocarbons (PAHs) at Rainbow may be derived from abiogenic processes, whereas biogenic processes could be responsible for the presence of n-fatty acids (n-FAs) at Lucky Strike and Rainbow. Moreover, organic geochemistry data appeared to be helpful in understanding some inorganic processes.