Gas hydrates, a solid established by water and gas molecules, are widespread along the continental margins of the world. Their dynamics have mainly been regarded through the lens of ...temperature-pressure conditions. A fluctuation in one of these parameters may cause destabilization of gas hydrate-bearing sediments below the seafloor with implications in ocean acidification and eventually in global warming. Here we show throughout an example of the Black Sea, the world's most isolated sea, evidence that extensive gas hydrate dissociation may occur in the future due to recent salinity changes of the sea water. Recent and forthcoming salt diffusion within the sediment will destabilize gas hydrates by reducing the extension and thickness of their thermodynamic stability zone in a region covering at least 2800 square kilometers which focus seepages at the observed sites. We suspect this process to occur in other world regions (e.g., Caspian Sea, Sea of Marmara).
A regional survey of alkaline springs in Oman and Ligurian ophiolites shows that the alkaline water compositions significantly vary from one ophiolite to the other and within the same ophiolite. The ...first‐order correlation between the Na (and K) and Cl concentrations points to fluid compositions only partly due to evaporation. The scatter around the evaporation line implies that Na and Cl may not be conservative during the alteration of the ultramafic rocks. Mg is almost entirely depleted at pH > 10.5 as a result of serpentine formation within the ultramafic body and of brucite (and minor hydrotalcite) precipitation at the springs. Ca accumulates in the high‐pH fluids and is consumed by Ca‐carbonate formation at the springs, by mixing with river waters or by the CO2 supply from the atmosphere. Thermodynamic calculations show that brucite saturation is reached at pH values around 10.5 which triggers major changes in the water composition. The waters evolve from a quartz‐saturated low‐pH continental environment to a brucite‐dominated high‐pH serpentinizing system at low temperature. The highest water salinities are found in springs located along the basal thrust plane of the ophiolite. The highest Al concentrations are found in some springs located on the crustal side of the mantle/crust boundary. This poses the question of the hydrologic pathways and of the role of the mineralogical composition of the altered formations.
Key Points
New temperature and composition of the alkaline waters
Brucite formation at pH 10.5 triggers major chemical and mineralogical changes
Relationship between the water compositions and the hydrologic pathways
We report on the mineralogical assemblages found in the hyperalkaline springs hosted on Liguria and Oman ophiolites based on exhaustive X‐ray diffraction and scanning electron microprobe analyses. In ...Liguria, hyperalkaline springs produce a thin brownish calcite precipitate that covers the bedrock due to the concomitant atmospheric CO2 uptake and neutralization of the hyperalkaline waters. No brucite and portlandite minerals are observed. The discharge of alkaline waters in Oman ophiolite forms white‐orange precipitates. Calcium carbonate minerals (calcite and/or aragonite) are the most abundant and ubiquitous precipitates and are produced by the same mechanism as in Liguria. This process is observed as a thin surface crust made of rhombohedral calcite. Morphological features of aragonite vary from needle‐, bouquet‐, dumbbell‐, spheroidal‐like habitus according to the origin of carbon, temperature, and ionic composition of the hyperalkaline springs, and the biochemical and organic compounds. Brucite is observed both at hyperalkaline springs located at the thrust plane and at the paleo‐Moho. The varying mixing proportions between the surface runoff waters and the hyperalkaline ones control brucite precipitation. The layered double hydroxide minerals occur solely in the vicinity of hyperalkaline springs emerging within the bedded gabbros. Finally, the dominant mineralogical associations we found in Oman (Ca‐bearing carbonates and brucite) in a serpentinizing environment driven by the meteoric waters are surprisingly the same as those observed at the Lost City hydrothermal site in a totally marine environment.
Key Points
Distinct mineralogical association between Oman and liguria alkaline springs
There is no Mg‐bearing carbonate
similar mineral assemblage in marine and surface serpentinizing environment
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.
Iron (Fe) and manganese (Mn) are crucial micronutrients that limit oceanic primary productivity in the Southern Ocean. It has been recently suggested that hydrothermal activity may be an important ...source of oceanic dissolved iron, yet, this contribution is still not fully understood and only one active hydrothermal site has been reported on the Southwest Indian Ridge (SWIR), south of 40°S.
Using a multi-proxy approach, this study demonstrates the occurrence of hydrothermal venting on the SWIR in the near vicinity of the location 44°51.690 S, 36°10.460 E, which is likely to be a low or moderately high temperature fluid. Indeed, we report high values of dissolved methane to manganese ratios (up to 11.1 ± 1.2 mol mol−1), low particulate iron (pFe) and manganese (pMn) concentrations (with maximum values of 0.7 nmol L−1 and 0.06 nmol L−1, respectively) associated with the presence of few oxyhydroxides, as well as high 223Radium (Ra) and 224Ra activities near the seafloor. The Fe and Mn data revealed a significant enrichment at depths influenced by hydrothermal circulation on the seafloor, within the Upper Circumpolar Deep Water. Dissolved Fe (dFe) and dissolved Mn (dMn) concentrations were enriched by 3- and 7-fold, respectively, and pFe and pMn by 2- and 1.5-fold, respectively, compared to a reference station located outside the SWIR. They were however lower than concentrations reported so far near high temperature vents, suggesting a weaker influence of this hydrothermal system on deep Fe and Mn reservoirs. We show that a large fraction of the dFe could be stabilized by organic complexation with humic substances (eHS, estimated 27–60% of dFe). High prokaryotic abundance related to the proximity of the hydrothermal vent suggests that other Fe-complexing ligands of biological origin might also stabilize Fe in its dissolved form. Collectively, these measurements integrated within the concept of a “multi-proxy approach”, helped painting a more detailed picture of the complex interactions and processes in this region of the SWIR. Although the system is a source of both dFe and dMn to the deep ocean, the low current velocities and the bathymetry likely limit the fertilization of surface water by dFe and dMn along this section of the SWIR.
•Occurrence of a clear fluid hydrothermal venting on the SWIR detected by a multi-proxy approach.•Dissolved Fe could be stabilized in the plume by the combined presence of colloidal and organic complexation with humic substances.•Deep hydrothermal Fe and Mn enrichment within Upper Circumpolar Deep Water.
A new sensor for in situ, real time methane (CH4) measurements in aqueous environments is based on the refractive index (RI) modulation of a sensitive film composed of a polydimethylsiloxane (PDMS) ...layer incorporating molecules of cryptophane-A. The RI varies according to the amount of CH4 bound to the cryptophane-A in the film and is determined using surface plasmon resonance (SPR). Tests of the sensor in the summer of 2012 reveal the expansive range of conditions of the Central Baltic Sea with CH4 concentrations varying from 5 nM up to a few hundred nanomolar. The sensor showed detection limits down to 3 nM, sensitivity of 6 to 7 × 10–6 RIU/nM, and response times of 1 to 2 min. Best responses were obtained for concentrations up to 200 nM. Side effects (temperature, cross-sensitivity) are reviewed for future improvements to the sensor design. CH4 values are highest in the Landsort Deep up to 1.2 μM at 400 m depth and lowest in the Gotland Deep with 900 nM at 220 m depth. However, variable values in the upper layers indicate higher mixing rates due to currents and wind driven forces in the Gotland Basin compared with almost constant CH4 values in the Landsort Deep.
Iron is an essential nutrient that regulates productivity in ~30% of the ocean. Compared with deep (>2000 meter) hydrothermal activity at mid-ocean ridges that provide iron to the ocean's interior, ...shallow (<500 meter) hydrothermal fluids are likely to influence the surface's ecosystem. However, their effect is unknown. In this work, we show that fluids emitted along the Tonga volcanic arc (South Pacific) have a substantial impact on iron concentrations in the photic layer through vertical diffusion. This enrichment stimulates biological activity, resulting in an extensive patch of chlorophyll (360,000 square kilometers). Diazotroph activity is two to eight times higher and carbon export fluxes are two to three times higher in iron-enriched waters than in adjacent unfertilized waters. Such findings reveal a previously undescribed mechanism of natural iron fertilization in the ocean that fuels regional hotspot sinks for atmospheric CO
.
Abstract
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.
The use of electrodeposited PdNi–Si Schottky barriers as low power Hydrogen sensors is investigated. The Palladium content of the film causes the Hydrogen molecules to dissociate and be absorbed by ...the film, changing the metal work function and Schottky barrier current. In this work we show that electrodeposited Pd(Ni)–Si Schottky barriers exhibit very low reverse bias currents compared to evaporated Schottky diodes. The Schottky diodes were fabricated on 0.5–1.5Ωcm (100) n-type Si by electrodeposition of PdNi followed by evaporation of aluminium contact pads. Electrical measurements at different Hydrogen pressures were performed on back-to-back Schottky diodes in a vacuum chamber using pure nitrogen and a 5% hydrogen–nitrogen mixture. Very low currents of ∼1nA were measured in the absence of hydrogen. Large increases in the currents, up to a factor of 100, were observed upon exposure to different hydrogen partial pressures. A back-to-back configuration forms a device that draws extremely low power when idle. The low idle current, simplicity of the fabrication process and ability to easily integrate with conventional electronics proves the suitability of electrodeposited PdNi–Si Schottky barriers as low power hydrogen sensors.
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”.
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