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
Zetaproteobacteria have been reported in different marine and terrestrial environments all over the globe. They play an essential role in marine iron-rich microbial mats, as one of their autotrophic ...primary producers, oxidizing Fe(II) and producing Fe-oxyhydroxides with different morphologies. Here, we study and compare the Zetaproteobacterial communities of iron-rich microbial mats from six different sites of the Lucky Strike Hydrothermal Field through the use of the Zetaproteobacterial operational taxonomic unit (ZetaOTU) classification. We report for the first time the Zetaproteobacterial core microbiome of these iron-rich microbial mats, which is composed of four ZetaOTUs that are cosmopolitan and essential for the development of the mats. The study of the presence and abundance of different ZetaOTUs among sites reveals two clusters, which are related to the lithology and permeability of the substratum on which they develop. The Zetaproteobacterial communities of cluster 1 are characteristic of poorly permeable substrata, with little evidence of diffuse venting, while those of cluster 2 develop on hydrothermal slabs or deposits that allow the percolation and outflow of diffuse hydrothermal fluids. In addition, two NewZetaOTUs 1 and 2 were identified, which could be characteristic of anthropic iron and unsedimented basalt, respectively. We also report significant correlations between the abundance of certain ZetaOTUs and that of iron oxide morphologies, indicating that their formation could be taxonomically and/or environmentally driven. We identified a new morphology of Fe(III)-oxyhydroxides that we named "corals." Overall, our work contributes to the knowledge of the biogeography of this bacterial class by providing additional data from the Atlantic Ocean, a lesser-studied ocean in terms of Zetaproteobacterial diversity.IMPORTANCEUp until now, Zetaproteobacterial diversity studies have revealed possible links between Zetaproteobacteria taxa, habitats, and niches. Here, we report for the first time the Zetaproteobacterial core microbiome of iron-rich mats from the Lucky Strike Hydrothermal Field (LSHF), as well as two new Zetaproteobacterial operational taxonomic units (NewZetaOTUs) that could be substratum specific. We highlight that the substratum on which iron-rich microbial mats develop, especially because of its permeability to diffuse hydrothermal venting, has an influence on their Zetaproteobacterial communities. Moreover, our work adds to the knowledge of the biogeography of this bacterial class by providing additional data from the hydrothermal vent sites along the Mid-Atlantic Ridge. In addition to the already described iron oxide morphologies, we identify in our iron-rich mats a new morphology that we named corals. Finally, we argue for significant correlations between the relative abundance of certain ZetaOTUs and that of iron oxide morphologies, contributing to the understanding of the drivers of iron oxide production in iron-oxidizing bacteria.
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
In hydrothermal environments, diffuse fluids emanations provide optimal conditions for the development of iron-rich microbial mats. Here, we present a unique spatial and temporal study of ...phylogenetic and chemical data from this type of mats and their associated hydrothermal fluids from two sites of the Lucky Strike Hydrothermal Field (EMSO-Azores deep-sea observatory), collected annually from 2016 to 2020. Our metabarcoding analyses reveal a completely different microbial community at each site, linked to the distinctive chemical composition of the diffuse fluids nourishing the mats. Capelinhos site is dominated by microorganisms with metabolisms related to iron, methane, and reduced sulphur compounds, coming from hydrothermal fluids, while North Tour Eiffel site presents higher abundances of microorganisms with metabolisms related to nitrogen, organic and oxidized sulphur compounds, coming from seawater. We present for the first time the yearly evolution of these mats over a five-year period. This analysis reveals similar variations of the microbial communities over time at both sites, indicating a regional Lucky Strike influence on the temporal scale. We also highlight more diversified microbial communities at both sites in 2016, pointing out the occurrence of a geological event that could have affected them during this specific year. Except for this year, our study shows that the communities of iron-rich microbial mats remain stable over time at both sites.
We propose and develop a comprehensive model for estimating the refractive index (RI) response over three potential sensing zones in a multimode fiber. The model has been developed based on a ...combined ray optics, Gaussian beam, and wave optics analysis coupled to the consideration of the injected interrogating lightwave characteristics and validated experimentally through the realization of three sensors with different lengths of stripped cladding sections as the sensing region. The experimental results highly corroborate and validate the simulation output from the model for the three RI sensing zones. The sensors can be employed over a very wide dynamic RI range from 1.316 to over 1.608 at a wavelength of 1550 nm, with the best resolution of 2.2447 × 10
RI unit (RIU) obtained in Zone II for a 1-cm sensor length.
An experimental study was carried out to assess the abiotic natural reactions associated with carbonate precipitation leading to the formation of Neoproterozoic cap carbonate sequences. For this ...purpose, synthetic seawater solutions were prepared reflecting the presumed composition of Neoproterozoic seawater. These solutions were heated to 40°C, undergoing degassing and/or evaporation, while monitoring their chemical composition (sulphate concentration, Mg/Ca molar ratio) and identifying the precipitated minerals. The experimental results are in good agreement with previous studies, allowing us to constrain the type of carbonate precipitates formed in the ocean during the Snowball Earth aftermath. Among the phases precipitated during heating of the solutions, Mg-bearing carbonate minerals (i.e. Mg-calcite, magnesite and dolomite) remain of minor importance compared to Ca-carbonates (especially aragonite). Consequently, the basal dolostone units of the cap carbonates (i.e. cap carbonates s.s.) were originally in the form of CaCO3, and evolved spontaneously toward dolomite.
Cap carbonates s.l. (i.e. cap limestones) containing aragonitic cement could result from the degassing of the whole ocean and the concomitant precipitation of aragonite. The calculated thicknesses of carbonates formed in this way are of the order of several metres, in line with the field observations. Although this process of abiotic carbonate precipitation does not rule out biologically-mediated carbonate formation, it may represent an alternate route to produce carbonate, particularly in environments which are not yet colonized by bacteria after the global ice melting.
•Analogical experiments related to Neoproterozoic cap carbonate precipitation•New synthetic model of the cap carbonate formation based on an inorganic phenomenon•Aragonite is the precursor of dolomite forming the basal unit of the cap carbonates.•Seawater equilibrated with atmosphere and sediments is the main source of carbonates.
In order to better constrain the chronology of the Messinian Salinity Crisis (MSC) onland and to provide new observations on the stratigraphic position of the Messinian erosional surface as well as ...evidence for continentalization, we have studied the Mio-Pleistocene succession of the island of Ibiza, ideally located in a strategic position between the peripheral Messinian basins of south-eastern Spain and the MSC-related gypsum of the Palma Basin (Mallorca).
Five lithological units are identified and compared with those known from the island of Mallorca. The equivalent of the “reef” unit, mainly Tortonian in age, is composed of median to inner ramp facies. It is overlain by coastal alluvial fans set up under extensive tectonic control, and then by a carbonate unit rich in oolites and microbialites attributed to the Terminal Carbonate Complex (TCC) of Messinian age. The overlying red clays fossilizing paleosols express a continentalization phase at the end of the Miocene. This regressive episode is evidenced by karstification and erosion incising the pre-crisis units and shaping valleys connected to submarine canyons. The erosion surface is linked to the acme of the MSC, a major sea-level fall, and is then sealed by Pliocene coastal calcarenites or by Pleistocene eolianites. The emptying of the basin led to a global collapse, which is recorded on all the slope domains of the Mediterranean margins. We document this collapse for the first time onland by syn-MSC gravity events such as mudflows, upwelling of geothermal fluids under pressure and soft deformation of the TCC. The interweaving of these phenomena of multiple origin (baselevel decrease, climate change, geothermal activity, sediment transfer) contributes to the onland/offshore reshaping of the Mediterranean margins during the major environmental changes of the Messinian salinity crisis.
•Two main regressions and transgressions control Upper Miocene sedimentation.•Extensional tectonic influence is well expressed during Tortono-Messinian interval.•The major regression with generalized emersion is recorded after TCC deposits.•The emersion is contemporaneous with gravitational instabilities and major erosion.•Major erosion is related to the MSC Margin Erosion Surface.
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.
The Voltri Massif is an ophiolitic complex located in the Ligurian Alps close to the city of Genova (Northern Italy) where several springs discharge high pH (up to 11.7), low salinity waters produced ...by the active serpentinization of the ultramafic basement. Mixing of these hyperalkaline waters with the river waters along with the uptake of atmospheric carbon dioxide forms brownish carbonate precipitates covering the bedrock at the springs. Diverse archaeal and bacterial communities were detected in these carbonate precipitates using 454 pyrosequencing analyses of 16S ribosomal RNA (rRNA) genes. Archaeal communities were dominated by members of potential methane-producing and/or methane-oxidizing
Methanobacteriales
and
Methanosarcinale
s (
Euryarchaeota
) together with ammonia-oxidizing
Nitrososphaerales
(
Thaumarchaeota
) similar to those found in other serpentinization-driven submarine and terrestrial ecosystems. Bacterial communities consisted of members of the
Proteobacteria
,
Actinobacteria
,
Planctomycetes
,
Bacteroidetes
,
Chloroflexi
, and
Verrucomicrobia
phyla, altogether accounting for 92.2 % of total retrieved bacterial 16S rRNA gene sequences. Amongst
Bacteria
, potential chemolithotrophy was mainly associated with
Alpha-
and
Betaproteobacteria
classes, including nitrogen-fixing, methane-oxidizing or hydrogen-oxidizing representatives of the genera
Azospirillum
,
Methylosinus
, and
Hydrogenophaga/
‘
Serpentinomonas
’, respectively. Besides, potential chemoorganotrophy was attributed mainly to representatives of
Actinobacteria
and
Planctomycetales
phyla. The reported 16S rRNA gene data strongly suggested that hydrogen, methane, and nitrogen-based chemolithotrophy can sustain growth of the microbial communities inhabiting the carbonate precipitates in the hyperalkaline springs of the Voltri Massif, similarly to what was previously observed in other serpentinite-hosted ecosystems.
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