An update on dissolved methane distribution in the subtropical North Atlantic Ocean

Methane (CH4) is a potent greenhouse gas and plays a significant role in recent increasing global temperatures. The oceans are a natural source of methane contributing to atmospheric methane concentrations, yet our understanding of the oceanic methane cycle is poorly constrained. Accumulating evidence indicates that a significant part of oceanic CH4 is produced in oxygenated surface waters as a by-product of phytoplanktonic activity. This study focused on the subtropical North Atlantic Ocean (26∘ N, 80′ W and 26∘ N, 18′ W) where the distribution of dissolved CH4 concentrations and associated air–sea fluxes during winter 2020 were investigated. Water samples from 64 stations were collected from the upper water column up to depths of 400 m. The upper oxic mixed layer was oversaturated in dissolved CH4 with concentrations ranging 3–7 nmol L−1, with the highest concentrations of 7–10 nmol L−1 found to the east of the transect, consistent with other subtropical regions of the world's oceans. The high anomalies of dissolved CH4 were found to be associated with phosphate-depleted waters and regions where the abundance of the ubiquitous picocyanobacteria Synechococcus and Prochlorococcus were elevated. Although other phytoplanktonic phyla cannot be excluded, this suggests that cyanobacteria contribute to the release of CH4 in this region. The calculation of air–sea fluxes further confirmed the subtropical North Atlantic Ocean as a source of CH4. This study provides evidence to corroborate the key role that picocyanobacteria play in helping to explain the oversaturation of CH4 found in surface mixed layer of the open ocean, otherwise known as the “ocean methane paradox”

Methane in deep sea hydrothermal plumes. Development of a new in-situ methane sensing technology

Information on the concentration and distribution of dissolved methane (CH4), together with other geochemical tracers, in real time is of great value in detecting, monitoring, and understanding the functioning of hydrothermal plumes. Water column anomalies of light transmission, dissolved CH4, manganese (Mn), and iron (Fe) were located over segments 15 and 16 of the Central Indian Ridge (CIR 20ºS), in December 2006. Along segment 15, a hydrothermal plume was present at 19°33’S/65°50’E. The source might be located north of that position and dispersed along the western flank by NW-SE currents. Methane to manganese ratios suggest that methane is produced by magmatic processes. On Segment 16, evidence for 1 or 2 hydrothermal plumes were detected over a lava plain (18°20’S/65°18’E). These data suffered from uncertainties due to sampling issues, which demonstrate the need for a reliable in-situ methane sensing technology. Current in-situ methane sensing technology is based on gas partitioning across gas permeable membranes, which are poorly characterised and variable in terms of permeability and environmental pressures. Two optical techniques were laboratory tested for the measurement of dissolved methane; Near Infrared Fibre-optic Evanescent Wave Spectroscopy (FEWS) and Surface Plasmon Resonance (SPR). No detection (at the µM level) was possible with FEWS, but the second technique using SPR sensors associated with a methane specific binding chemically showed great promise. A limit of detection of 0.2 nM and a linear concentration range from 1 to 300 nM was demonstrated, under a range of temperature and salinity. In-situ deployments confirmed the suitability of the method for in-situ measurements. Values given by the sensor correlated well with the concentrations measured by traditional techniques. Future work is needed to decrease instrumental noise and to reduce the response time, and associated hysteresis effect

Differences in gas venting from ultramafic-hosted warm springs: the example of Oman and Voltri Ophiolites

Serpentinisation of mantle rocks, leading to natural venting of hydrogen and methane, has been reported to occur at the global scale, wherever fluids percolate in ultramafic formations. Here we compare gas composition from two on-land, low-temperature, and hyper-alkaline springs hosted on ultramafic rocks in the ophiolite massifs of the Sultanate of Oman and the Ligurian Alps (Voltri Group, Genoa region, Northern Italy). These two settings exhibit similar chemical and mineralogical features but show diverse styles of gas venting. Commonly to all hyper-alkaline springs, gases are characterised by relatively high N-2, very low O-2 and CO2 concentrations, and a strong enrichment in H-2 and CH4. The comparison between Oman and Liguria highlights a high variability of the H-2/CH4 ratios whereby the gas phase of the Oman Ophiolite is enriched in H-2 whereas being CH4 enriched in the Voltri Ophiolite. These results combined with literature data define three groups that may reflect different stages of serpentinisation producing fluids from hydrogen-dominated to methane-dominated. The origin of these distinct groups might lie in the difference of the mineralogical composition of the rocks within which the fluids circulate, on the degree of alteration of the rocks and finally on the geological/metamorphic history of the ophiolite

Reproductive biology and population structure of three hydrothermal gastropods (Lepetodrilus schrolli, L. fijiensis and Shinkailepas tollmanni) from the South West Pacific back-arc basins

Hydrothermal vents host fragmented habitats and are increasingly becoming the target of deep-sea mining projects for their mineral resources. Managing a future sustainable exploitation requires a good understanding of the resilience of biological populations to natural and anthropogenic disturbances, hence a better knowledge of species life history traits and their capacity to replenish local populations or colonise distant sites. In this context, we studied the reproductive biology and recruitment patterns of three main representative hydrothermal vent limpets of the South West Pacific back-arc basins, Lepetodrilus schrolli, Lepetodrilus fijiensis and Shinkailepas tollmanni, in relation to habitats and environmental conditions. Limpets were collected in Bathymodiolus and Ifremeria nautilei habitats at several vent sites in the Manus, North Fiji and Lau back-arc basins, and the Futuna Volcanic Arc during the CHUBACARC cruise in 2019. Population structure, gonad morphology, and gametogenesis were analysed for each species, and fecundity was analysed for the two Lepetodrilus species. Both Lepetodrilus spp. were gonochoric and displayed a sexual size dimorphism with females larger than males. Gametogenesis was continuous or quasi-continuous with all stages of oocyte development present in the gonad and a maximum oocyte size of 124 µm for L. schrolli and 126 µm for L. fijiensis. Fecundity varied between 52 and 205 with a mean of 119 ± 74 (SD) matured oocytes per female in L. schrolli and between 80 and 605 with a mean of 366 ± 183 (SD) matured oocytes per female in L. fijiensis, and was independent of the limpet size for both species. Shinkailepas tollmanni is also a gonochoric gastropod with continuous gametogenesis and a maximum oocyte size of 153 µm. For each species, size-frequency distributions were consistent with a continuous recruitment although episodic larval supply could blur the signal. There was no evidence of an influence of the habitat type nor environmental conditions on population structures

Reproductive biology and population structure of three hydrothermal gastropods (Lepetodrilus schrolli, L. fijiensis and Shinkailepas tollmanni) from the South West Pacific back-arc basins

Hydrothermal vents host fragmented habitats and are increasingly becoming the target of deep-sea mining projects for their mineral resources. Managing a future sustainable exploitation requires a good understanding of the resilience of biological populations to natural and anthropogenic disturbances, hence a better knowledge of species life history traits and their capacity to replenish local populations or colonise distant sites. In this context, we studied the reproductive biology and recruitment patterns of three main representative hydrothermal vent limpets of the South West Pacific back-arc basins, Lepetodrilus schrolli, Lepetodrilus fijiensis and Shinkailepas tollmanni, in relation to habitats and environmental conditions. Limpets were collected in Bathymodiolus and Ifremeria nautilei habitats at several vent sites in the Manus, North Fiji and Lau back-arc basins, and the Futuna Volcanic Arc during the CHUBACARC cruise in 2019. Population structure, gonad morphology, and gametogenesis were analysed for each species, and fecundity was analysed for the two Lepetodrilus species. Both Lepetodrilus spp. were gonochoric and displayed a sexual size dimorphism with females larger than males. Gametogenesis was continuous or quasi-continuous with all stages of oocyte development present in the gonad and a maximum oocyte size of 124 µm for L. schrolli and 126 µm for L. fijiensis. Fecundity varied between 52 and 205 with a mean of 119 ± 74 (SD) matured oocytes per female in L. schrolli and between 80 and 605 with a mean of 366 ± 183 (SD) matured oocytes per female in L. fijiensis, and was independent of the limpet size for both species. Shinkailepas tollmanni is also a gonochoric gastropod with continuous gametogenesis and a maximum oocyte size of 153 µm. For each species, size-frequency distributions were consistent with a continuous recruitment although episodic larval supply could blur the signal. There was no evidence of an influence of the habitat type nor environmental conditions on population structures

Active hydrothermal vents in the Woodlark Basin may act as dispersing centres for hydrothermal fauna

Here we report the discovery of a high-temperature hydrothermal vent field on the Woodlark Ridge, using ship-borne multibeam echosounding and Remotely Operated Vehicle (ROV) exploration. La Scala Vent Field comprises two main active areas and several inactive zones dominated by variably altered basaltic rocks, indicating that an active and stable hydrothermal circulation has been maintained over a long period of time. The Pandora Site, at a depth of 3380 m, is mainly composed of diffuse vents. The Corto site, at a depth of 3360 m, is characterized by vigorous black smokers (temperature above 360 °C). The striking features of this new vent field are the profusion of stalked barnacles Vulcanolepas sp. nov., the absence of mussels and the scarcity of the gastropod symbiotic fauna. We suggest that La Scala Vent Field may act as a dispersing centre for hydrothermal fauna towards the nearby North Fiji, Lau and Manus basins