Carbonate Chimneys Research Articles

A new type of submarine chimneys built of halite

In contrast to the subaquatic sulphide and carbonate chimneys, which are known from Mid Ocean Ridges and abyssal submarine volcanoes, chimneys built of salts have not been described yet. Here we present such halite chimneys as a new form of cold-water smokers in hypersaline environments. The here described structures rise up from the bottom of the Dead Sea and result from the submarine discharge of saturated halite-dissolution brines into the salt lake, which is at halite saturation and holds remarkable chloride excess. At the interface with the lake brine, halite precipitates instantaneously, forming chimneys up to several meters in height. The brines leading to the formation of these chimneys vary in composition, while their generation processes are similar. Fresh groundwater from surrounding aquifers enters the saline lake sediments and considerably leaches halite in the adjacencies of the lake. Simultaneously, it mixes with ancient brines before it emerges from the lake floor. The distinct differences in composition between the Dead Sea and the emerging chimney brines lead to the instantaneous crystallisation of halite and few other mineral phases. The chimney structure result from the buoyancy flow of the chimney brines, which are less dense then the ambient Dead Sea.The chimneys indicate intense cavitation of massive halite bodies in the subsurface of the Dead Sea environment, a process that leads to increasing formation of hazardous sinkholes. Since chimneys are proven in shallow water but may be expected in deeper parts too, they are comfortably mappable by echo-sounding or aerial imaging. They thus provide in the Dead Sea as in any likewise setting a potent predictive tool to locate dangerous subsurface cavitation and hence areas that are at risk of collapse in the near future.

Carbonate chimneys at the highly productive point Dume methane seep: Fine-scale mineralogical, geochemical, and microbiological heterogeneity reflects dynamic and long-lived methane-metabolizing habitats.

Methane is a potent greenhouse gas that enters the marine system in large quantities at seafloor methane seeps. At a newly discovered seep site off the coast of Point Dume, CA, ~ meter-scale carbonate chimneys host microbial communities that exhibit the highest methane-oxidizing potential recorded to date. Here, we provide a detailed assessment of chimney geobiology through correlative mineralogical, geochemical, and microbiological studies of seven chimney samples in order to clarify the longevity and heterogeneity of these highly productive systems. U-Th dating indicated that a methane-driven carbonate precipitating system at Point Dume has existed for ~20 Kyr, while millimeter-scale variations in carbon and calcium isotopic values, elemental abundances, and carbonate polymorphs revealed changes in carbon source, precipitation rates, and diagenetic processes throughout the chimneys' lifespan. Microbial community analyses revealed diverse modern communities with prominent anaerobic methanotrophs, sulfate-reducing bacteria, and Anaerolineaceae; communities were more similar within a given chimney wall transect than in similar horizons of distinct structures. The chimneys represent long-lived repositories of methane-oxidizing communities and provide a window into how carbon can be transformed, sequestered, and altered over millennia at the Point Dume methane seep.

Occurrence and Genesis of Cold‐Seep Authigenic Carbonates from the South‐Eastern Mediterranean Sea

AbstractMethane‐derived authigenic seep carbonates occur globally along continental margins. These carbonates are important archives to identify seep dynamics, the source of the ascending methane‐enriched fluids together with their timing, and are an important carbon sequestration mechanism. Recently, seep carbonates were discovered in the Levant Basin in the south‐eastern Mediterranean Sea. To elucidate past seepage activity and dynamics across the basin, different seep carbonate morphologies (chimneys, crusts and pavements) retrieved from the Levant Basin were mapped based on remotely operating vehicle data and analysed using standard sediment petrographic techniques, X‐ray diffraction and stable carbon and oxygen isotope analyses. Carbonate chimneys consist of micrite (δ13CVPDB of −10‰ to +5‰) with dispersed baryte and dolomite crystals, fan‐shaped aragonite (δ13CVPDB of −52‰ to −30‰) and high‐magnesium calcite cements, with the latter often growing from low‐magnesium calcite spherules. Botryoidal low‐magnesium calcite cements are forming in small cavities. Carbonate crusts consist of micrite with low‐magnesium calcite breccias, high‐magnesium calcite nodules (δ13CVPDB of −35‰ to −20‰) and cements, and partially replaced fan‐shaped aragonite cements. Carbonate pavements consist of low‐magnesium calcite microsparite, micritic dolomite and high‐magnesium calcite. Fan‐shaped aragonite is locally present as pore‐lining cement. Iron oxides are often seen coating the low‐magnesium calcite, high‐magnesium calcite and dolomite cements. Chimneys and crusts, characterised by high amounts of high‐magnesium calcite and aragonite, are interpreted to have formed through advective methane fluxes. Pavements, with high quantities of dolomite, are explained as the product of diffusive methane flux. Sediment petrographic and geochemical analysis of the different carbonate morphologies and cement phases therein witness distinct modes of ascending fluid fluxes and their mixing with marine pore water and/or sea water during precipitation of the individual phases.

Publisher Correction: Formation of carbonate chimneys in the Mediterranean Sea linked to deep-water oxygen depletion

Publisher Correction: Formation of carbonate chimneys in the Mediterranean Sea linked to deep-water oxygen depletion

New and known species of the genus Desmodora De Man, 1889 (Nematoda: Desmodoridae) from the hydrothermal vent communities of the Piip volcano (south-west Bering Sea)

Species of the marine nematode of the genus Desmodora have been found to dominate (up to 78%) in the nematode fauna from the hydrothermal vent communities of submarine Piip volcano, Bering Sea. The morphological characteristics and molecular genetic data of Desmodora specimens from different habitats of the volcano were studied and three new and one known species have been described: Desmodora hydrothermica sp.nov., Desmodora marci, Desmodora spongiophila sp.nov., Desmodora spongiocola sp.nov. The species often lived together, but a pronounced spatial specialization was observed. Desmodora spongiophila sp.nov. and Desmodora spongiocola sp.nov. were abundant inhabitants of the vulcanellids and some hexactenellids. Desmodora hydrothermica sp.nov. dominated in microbial mats on the carbonate chimneys from the South Summit, while Desmodora marci was found on stones near vents and in bottom sediments with Calyptogena pacifica (Bivalvia: Vesicomyidae). The last two species were already known in deep-sea reduced environments of the Pacific and Atlantic oceans, in similar habitats. The δ13C and δ15N values and fatty acids composition of Desmodora samples from hydrothermal habitats confirmed the consumption of chemosynthetically derived organic matter. SSU and D2-D3 of LSU phylogenetic trees largely agree with those of previous analyses indicating that Desmodora is not monophyletic. Moreover, phylogenetic relationships within the subfamily Desmodorinae remained unresolved.

Mixed carbonate-siliceous hydrothermal chimneys ahead of the Asal propagating rift (SE Afar Rift, Republic of Djibouti)

Numerous active and fossil hydrothermal carbonate chimneys have been reported at Lake Asal and Lake Abhe, in the southeastern part of the Afar Rift (Republic of Djibouti): a tectonically and volcanically active region for the last 30 million years. In this study, we report on a detailed mineralogical and geochemical investigation of unusual hydrothermal mixed carbonate-siliceous chimneys recently discovered in the Allols area (Harralol, Haralé Awda and Sakallol), northwest of the Asal Rift. These inactive chimneys, up to 20 m high, consist of tall spires partly stained by Fe-oxides. They are located along major faults and were formed sub-aqueously when the area was occupied by a lake. Hydrothermal carbonate structures were also found in the same area. The oxygen (δ18O) and carbon (δ13C) isotopic compositions of the chimneys range from −4.5 to 25.3‰ and from −9.44 to 8.92‰, respectively, indicating mixing between magmatic and surficial waters. Their radiogenic Sr isotopic compositions (87Sr/86Sr = 0.7049–0.7056) show values typical of altered basaltic rocks. Chimneys also exhibit negative Ce anomaly suggestive of a genetic link with basalt alteration. Overall, this study suggests that lake water chemistry in the Allols area experienced major changes in the past, reflecting various contributions of mixing fluids (lake water and hydrothermal fluids), leading alternatively to deposition of carbonate chimneys and precipitation of (and/or replacement with) silica.

Lacustrine carbonate towers of Lake Abhe, Djibouti: Interplay of hydrologic and microbial processes

Lake Abhe, located at the triple junction between the Red Sea, Gulf of Aden, and Main Ethiopian rift trends, is a hypersaline and alkaline closed lake that is known for its exposures of massive carbonate chimneys on the flat-lying sediments of the lake's eastern margin. This study describes the morphological, textural, and petrographic characteristics of these chimneys at the basin- to microscopic-scale in order to constrain the carbonate depositional system in the context of basin hydroclimate history. Chimneys occur in three main fields and are assessed according to 1) large-scale (>5 m) morphological variations, 2) meso-scale (cm to <5 m) textures, and 3) micro-scale (<1 cm) fabrics. The dominant chimney fabric is a porous crystalline framework of trigonal prismatic and dendritic calcite crystals that locally contain spherulites, sickle-cell calcite fabrics, and entombed microbial cocci. In addition to the crystalline chimneys, other carbonate sediments include stromatolitic crusts composed of microdigitate laminated columns dominated by crystalline fan microfabrics, as well as carbonate-rich mudstones and relatively rare, localized, carbonate-rich diatomites. Chimneys are interpreted to form primarily as products of mixing between hydrothermal sublacustrine springs and lake waters during lake highstand intervals, while stromatolitic crusts are interpreted to form during lowstand lake levels. Our interpretation of mixing processes is supported by the δ18O composition of chimney calcite, which is representative of lake water and area hot spring endmembers. The observation of stromatolitic crusts with more positive values of δ13C and δ18O than crystalline chimneys indicates that crusts formed during periods of high evaporation and low lake level. Crusts also contain Mg-silicate minerals, which are not present in crystalline chimneys, and further support the interpretation of lowstand depositional conditions. Although the age of chimney formation is not well constrained, evidence from seismic reflection data suggests a pattern of chimney formation during lake level rise and highstand times, followed by lake level fall, subaerial exposure and weathering that occurred at several times throughout the Late Pleistocene and Holocene.

Northern Mid-Atlantic Ridge Hydrothermal Habitats: A Systematic Review of Knowledge Status for Environmental Management

Highly specialised biota occurring at hydrothermally active vents on the northern Mid-Atlantic Ridge (nMAR: from south of Iceland to the Equator) have been the subject of numerous research projects over the 36-year period since these habitats were first discovered in the region. When hydrothermal activity ceases, biota endemic to hydrothermally active habitats are lost, and a new biota colonise these sites. Little is known about the biota colonising hydrothermally inactive sulphide habitats on the nMAR, although these sites may be the target of deep-sea mining within the next decade. In this review, we seek to clarify the current knowledge of biological communities colonising hydrothermally active habitats and inactive sulphide habitats on the nMAR. To achieve this, we (1) used a systematic review process to update the species list of benthic invertebrates associated with hydrothermally active habitats, (2) conducted a regional biogeographic analysis of hydrothermally active vent fields on the nMAR, (3) undertook a comprehensive literature review to provide a descriptive account of biological communities, and (4) identified key knowledge gaps in the current understanding of nMAR hydrothermally active and inactive ecosystems. Our updated species list increases the number of benthic invertebrates recorded from hydrothermally active habitats on the nMAR to 158 taxa. Our regional biogeographic analysis separates nMAR hydrothermal vent fields into distinct clusters based on depth/latitude and chimney composition. Vent fields close to the Azores (Menez Gwen, Lucky Strike, Rainbow) formed a separate cluster from those at greater depths south of the Azores (Broken Spur, TAG, Snake Pit, Logatchev, and Ashadze-1). Moytirra, located north of the Azores, clustered separately, as did Lost City with its unique carbonate chimneys. We present detailed information on the biological communities at hydrothermally active and inactive habitats in this region, and discuss the information available on the diversity, ecosystem function, trophic relationships, connectivity, temporal variability, and resilience and recovery of these communities. Many knowledge gaps still exist, with detailed information needed on all aspects of the biological communities at hydrothermally active habitats and inactive sulphide habitats on the nMAR to understand and predict impacts from natural and human-induced disturbances in the region.

A new method for the U–Th dating of a carbonate chimney deposited during the last glaciation in the northern Okinawa Trough, East China Sea

Authigenic carbonates from cold seep systems, such as chimney and crust carbonate deposits, are important archives of biogeochemical processes associated with the migration and seepage of methane-rich fluids and seepages on the seafloor. Here we use a new U–Th dating method to precisely determine the ages of the authigenic carbonates of a chimney in the Okinawa Trough (OT), East China Sea. The method uses a series of heavy liquids (HL, a mixture of tribromomethane and alcohol with a density of 2.71–2.80 g/cm 3 ) to separate subsamples of carbonate contaminated by detrital material. The contents of relevant elements (Al, Rb, Zr, Nb, and rare earth elements) of the subsamples reveal that the separation of the mineral phase was successful. Five samples from along the radius of a well-structured chimney (PES-1) from station D2-5 in the OT were analyzed and yielded U–Th ages of 28.8 ± 0.4 ka, 26.1 ± 0.5 ka, 24.4 ± 0.7 ka, 22.5 ± 0.6 ka and 20.1 ± 0.6 ka, from the exterior to the interior of the chimney section, and AMS 14 C dating of the same samples produced the similar results. The agreement between the 230 Th/U and AMS 14 C ages suggests that the application of heavy liquid methods in U–Th dating can be very successful, and it reduces the influence of detrital contamination. Therefore, the approach has a great potential space for studying the evolutionary history of authigenic carbonates in cold seep systems. Moreover, the new 230 Th/U ages show that the carbonate chimney was formed during the Last Glacial Maximum (LGM), which corresponding to the large sea-level fall in the East China Sea. Together with information on biogeochemical processes, the results will provide valuable geochronological information for the future study of active fluid venting on continental margins. • Carbonates chimney from metals-rich sediments can be precisely dated with U-series. • Heavy liquids were used to separate subsamples of seep impure carbonate. • Multiple-point HL regressions provide the more reliable chronology. • U–Th ages between 28.8 and 20.1 ka point to sea-level low stands during the LGM.

Shallow-marine serpentinization-derived fluid seepage in the Upper Cretaceous Qahlah Formation, United Arab Emirates

AbstractSerpentinization of ultramafic rocks in the sea and on land leads to the generation of alkaline fluids rich in molecular hydrogen (H2) and methane (CH4) that favour the formation of carbonate mineralization, such as veins in the sub-seafloor, seafloor carbonate chimneys and terrestrial hyperalkaline spring deposits. Examples of this type of seawater–rock interaction and the formation of serpentinization-derived carbonates in a shallow-marine environment are scarce, and almost entirely lacking in the geological record. Here we present evidence for serpentinization-induced fluid seepage in shallow-marine sedimentary rocks from the Upper Cretaceous (upper Campanian to lower Maastrichtian) Qahlah Formation at Jebel Huwayyah, United Arab Emirates. The research object is a metre-scale structure (the Jebel Huwayyah Mound) formed of calcite-cemented sand grains, which formed a positive seafloor feature. The Jebel Huwayyah Mound contains numerous vertically orientated fluid conduits containing two main phases of calcite cement. We use C and O stable isotopes and elemental composition to reconstruct the fluids from which these cements precipitated and infer that the fluids consisted of variable mixtures of seawater and fluids derived from serpentinization of the underlying Semail Ophiolite. Based on their negative δ13C values, hardgrounds in the same section as the Jebel Huwayyah Mound may also have had a similar origin. The Jebel Huwayyah Mound shows that serpentinization of the Semail Ophiolite by seawater occurred very soon after obduction and marine transgression, a process that continued through to the Miocene, and, with interaction of meteoric water, up to the present day.

Multidisciplinary Scientific Cruise to the Northern Mid-Atlantic Ridge and Azores Archipelago

This work presents the preliminary result of the multidisciplinary cruise EXPLOSEA2 surveying the northern Mid-Atlantic Ridge and Azores Archipelago from 46° 30′ N to 38° 30′ N aboard the R/V Sarmiento de Gamboa and ROV Luso over 54 days (June 11 to July 27, 2019). In this cruise report, we detail the geophysical, hydrographic, geological, oceanographic, ecological, and microbiological data acquired and a brief of main findings. The cruise addressed the exploration and comprehensive characterization of venting sites, including the water column, the sediments and rocks that host the hydrothermal activity, and the associated mineralizations, biology, and microbiology. Deep hydrothermal chimneys and massive sulfide deposits (up 3,000 m in depth) within the Moytirra hydrothermal active field were identified on slopes that had not been explored previously. Another striking finding made during the EXPLOSEA2 cruise was the field of carbonate chimneys named the “Magallanes-Elcano” field, a potentially relict ultramafic-hosted hydrothermal site sourced by abiotic methane. This field is related to a serpentinite and gabbro rock outcropping on a dome-shaped massif named the “Iberian Massif.” An outstanding finding of the EXPLOSEA2 survey was the identification of the first garden of soft corals growing after active submarine eruptions were reported in the Azores Archipelago composed by a high density of soft corals the suborder Alcyoniina at the summit and flanks of a recent volcanic cone at 160 m water depth developed during the 1957–1958 eruption of Capelinhos. Several cold-water coral habitats formed by colonial scleractinians (e.g., Lophelia pertusa and Madrepora oculata), coral gardens composed of mixed assemblages of black corals (Leiopathes sp.), and octocorals and dense aggregations of the glass sponge Pheronema carpenteri that may be classified as vulnerable marine ecosystems (VMEs) have been discovered during the EXPLOSEA2 cruise along the northern Mid-Atlantic Ridge. This work reveals the importance of multidisciplinary surveys to the knowledge of deep-sea environments.

Influence of the Precipitation Rate on the Isotope (δ18O, δ13C and δ88Sr) Composition of Carbonate Chimneys of the Lost City Hydrothermal Field (30° N, Mid-Atlantic Ridge)

The behavior of four isotope systems (δ18О, δ13С, δ88Sr and 87Sr/86Sr) during submarine precipitation of inorganic carbonates is considered using the Lost City hydrothermal field as an example of “natural laboratory”. During the carbonate precipitation, the isotope composition, T, and pH of hydrothermal solution change due to mixing of the “end member” Lost City fluid with ocean water. The composition of DIC and carbonates (Cc, Arag) equilibrated with mixed hydrothermal solution was calculated in the isotopic coordinates 87Sr/86Sr–103(Sr/Ca), 87Sr/86Sr–δ18О, 87Sr/86Sr–δ13С, and 87Sr/86Sr–δ88Sr. The observed isotope composition of the Lost City field carbonates is compared with the calculated equilibrium lines. The disequilibrium values of δ18О, δ13С, and δ88Sr in the Cс(Arag) are result of rapid precipitation of carbonates from a hydrothermal fluid under T and pH gradient. The δ18О values of most chimney samples vary around the “DIC–water” equilibrium curve with a slight shift to the Сс(Arag)–water equilibrium. The values of δ13С of chimney carbonates fall between the calculated curves of δ13С (DIC) and equilibrium Сс and Arag. The kinetic isotopic shift Δ88Sr established in chimney carbonates is close to available experimental data on the synthesis of calcite and aragonite from aqueous solutions.

Can Primary Ferroan Dolomite and Ankerite Be Precipitated? Its Implications for Formation of Submarine Methane-Derived Authigenic Carbonate (MDAC) Chimney

Microbes can mediate the precipitation of primary dolomite under surface conditions. Meanwhile, primary dolomite mediated by microbes often contains more Fe2+ than standard dolomite in modern microbial culture experiments. Ferroan dolomite and ankerite have been regarded as secondary products. This paper reviews the process and possible mechanisms of microbial mediated precipitation of primary ferroan dolomite and/or ankerite. In the microbial geochemical Fe cycle, many dissimilatory iron-reducing bacteria (DIRB), sulfate-reducing bacteria (SRB), and methanogens can reduce Fe3+ to Fe2+, while SRB and methanogens can also promote the precipitation of primary dolomite. There are an oxygen respiration zone (ORZ), an iron reduction zone (IRZ), a sulfate reduction zone (SRZ), and a methanogenesis zone (MZ) from top to bottom in the muddy sediment diagenesis zone. DIRB in IRZ provide the lower section with Fe2+, which composes many enzymes and proteins to participate in metabolic processes of SRB and methanogens. Lastly, heterogeneous nucleation of ferroan dolomite on extracellular polymeric substances (EPS) and cell surfaces is mediated by SRB and methanogens. Exploring the origin of microbial ferroan dolomite may help to solve the “dolomite problem”.

Stable isotopes and rare earth element compositions of ancient cold seep carbonates from Enza River, northern Apennines (Italy): Implications for fluids sources and carbonate chimney growth

Abstract Cold seep carbonates are remarkable fossil records of hydrocarbon seepage and related conduits. In the present research, we analysed the carbon and oxygen isotopes, rare earth element (REE) and trace element content of carbonate chimneys and concretions in the Pleistocene Argille Azzurre Formation, northern Apennines, Italy. Strong 13 C depletions (values as low as −40.3‰) observed in both carbonate concretions and chimneys indicate that anaerobic oxidation of biogenic methane played a major role in the formation of these carbonates. Slightly higher δ 13 C values mostly found in the outer layers of the carbonate chimneys reflect a contribution of inorganic carbon from seawater. REE patterns reveal the carbonates formed under mostly anoxic pore water conditions. Combined REE and trace element data reveal the growth of chimneys occurred from the outside-in, with the outer chimney walls experiencing rapid growth. Excellent correlations between Ce/Ce*, REE contents, and La N /Sm N and Y/Ho ratios observed in the chimney walls provide additional evidence for the influence of seawater in the formation of the chimneys. Meanwhile, O isotopic compositions of the samples suggest an 18 O-depleted fluid was involved in carbonate precipitation. This study has major implications for tracking fluid sources of seep carbonates and patterns of carbonate chimney growth.

A microbial role in the construction of Mono Lake carbonate chimneys?

Lacustrine carbonate chimneys are striking, metre-scale constructions. If these were bioinfluenced constructions, they could be priority targets in the search for early and extraterrestrial microbial life. However, there are questions over whether such chimneys are built on a geobiological framework or are solely abiotic geomorphological features produced by mixing of lake and spring waters. Here, we use correlative microscopy to show that microbes were living around Pleistocene Mono Lake carbonate chimneys during their growth. A plausible interpretation, in line with some recent works by others on other lacustrine carbonates, is that benthic cyanobacteria and their associated extracellular organic material (EOM) formed tubular biofilms around rising sublacustrine spring vent waters, binding calcium ions and trapping and binding detrital silicate sediment. Decay of these biofilms would locally have increased calcium and carbonate ion activity, inducing calcite precipitation on and around the biofilms. Early manganese carbonate mineralisation was directly associated with cell walls, potentially related to microbial activity though the precise mechanism remains to be elucidated. Much of the calcite crystal growth was likely abiotic, and no strong evidence for either authigenic silicate growth or a clay mineral precursor framework was observed. Nevertheless, it seems likely that the biofilms provided initial sites for calcite nucleation and encouraged the primary organised crystal growth. We suggest that the nano-, micro- and macroscale fabrics of these Pleistocene Mono Lake chimneys were affected by the presence of centimetre-thick tubular and vertically stacked calcifying microbial mats. Such carbonate chimneys represent a promising macroscale target in the exploration for ancient or extraterrestrial life.

Formation of methane-derived carbonates during the last glacial period on the northern slope of the South China Sea

Methane-derived carbonates with blocky and chimney morphologies were collected on the northern slope of the South China Sea. The studies of petrography, geochemistry and chronology show that the blocky carbonate has a microcrystalline structure with numerous of biogenic tests, whereas the carbonate chimney has a cryptocrystalline structure with scarce biogenic tests. Aragonite can be detected in the carbonate chimney, which indicates that it formed at the shallow sediments or on the seafloor. The δ13C and δ18O values of the blocky carbonate range from −46.8‰ to −28.8‰ VPDB and 2.2‰ to 3.5‰ VPDB, respectively. The δ13C and δ18O values of the carbonate chimney range from −45.0‰ to −22.9‰ VPDB and −2.8‰ to 3.1‰ VPDB, respectively. The carbon isotopic values indicate that these carbonates are related to the anaerobic oxidation of methane (AOM). The oxygen isotopic values show that the gas hydrate dissociation contributed the 18O-enrich fluid to a certain degree during the authigenic carbonate formation. According to the rare earth element results, the carbonate chimney precipitated in stable anoxic conditions. Because of the La-enriched influence, the formation environment of the blocky carbonate is ambiguous. The U/Th chronology results indicate that these authigenic carbonates precipitated during the last glacial period (primarily Marine Isotopic Stage 3) of low sea level stand.

Hydrothermal vent fields discovered in the southern Gulf of California clarify role of habitat in augmenting regional diversity.

Hydrothermal vent communities are distributed along mid-ocean spreading ridges as isolated patches. While distance is a key factor influencing connectivity among sites, habitat characteristics are also critical. The Pescadero Basin (PB) and Alarcón Rise (AR) vent fields, recently discovered in the southern Gulf of California, are bounded by previously known vent localities (e.g. Guaymas Basin and 21° N East Pacific Rise); yet, the newly discovered vents differ markedly in substrata and vent fluid attributes. Out of 116 macrofaunal species observed or collected, only three species are shared among all four vent fields, while 73 occur at only one locality. Foundation species at basalt-hosted sulfide chimneys on the AR differ from the functional equivalents inhabiting sediment-hosted carbonate chimneys in the PB, only 75 km away. The dominant species of symbiont-hosting tubeworms and clams, and peripheral suspension-feeding taxa, differ between the sites. Notably, the PB vents host a limited and specialized fauna in which 17 of 26 species are unknown at other regional vents and many are new species. Rare sightings and captured larvae of the ‘missing’ species revealed that dispersal limitation is not responsible for differences in community composition at the neighbouring vent localities. Instead, larval recruitment-limiting habitat suitability probably favours species differentially. As scenarios develop to design conservation strategies around mining of seafloor sulfide deposits, these results illustrate that models encompassing habitat characteristics are needed to predict metacommunity structure.

Carbon, oxygen and strontium isotopic constraints on fluid sources, temperatures and biogeochemical processes during the formation of seep carbonates - Secchia River site, Northern Apennines

Understanding authigenic seep carbonate formation provides clues for hydrocarbon exploration and insights into contributions to gas budgets of marine environments and the atmosphere. Seep carbonates discovered in the outcropping succession along the Secchia riverbanks (near Modena, Italy) belong to the Argille Azzurre Formation of Early Pleistocene age deposited in an upper shelf environment overlying the Miocene foredeep successions, which include hydrocarbon fields. The fluid migration from the hydrocarbon fields, up to the surface, is presently active on land and started in the marine succession during the Late Miocene.Authigenic globular carbonate concretions and carbonate chimneys are interspersed along the strata throughout the section. A comprehensive geochemical characterisation of the carbonates has been carried out to understand the processes leading to their formation. The carbonate concretions are the record of past hydrocarbon vents linked to the Miocene petroleum system of the Northern Apennines. The samples are composed of >50% microcrystalline dolomite. The δ13C signatures identify two groups in the samples according to different type of formation processes. Globular concretions have positive values that suggest an influence of CO2 associated to secondary methanogenesis due to microbial degradation of higher hydrocarbons. The analysed chimney, with negative δ13C values, is interpreted as former conduit where carbonate precipitation is promoted by Anaerobic Oxidation of Methane coupled with Sulfate Reduction. The δ18O range, coupled with 87/86Sr signatures, indicate that the contribution of deep connate water from the Miocene reservoirs is up to 23% during the formation of the globular concretions. The connate water occurrence is also documented by higher ambient temperatures. The different isotope signatures in seep carbonates result from the relative contribution of the recognised gas and water components, linked to different plumbing systems and fluid supply from a well-defined hydrocarbon field. The seep carbonate characteristics have enlightened variations in biogeochemical processes, which can be rarely quantified in ancient and present-day marine environments.

A fault‐driven circulation model for the Lost City Hydrothermal Field

AbstractThe Lost City Hydrothermal Field on the Atlantis Massif is an off‐axis hydrothermal system, hosted in serpentinized ultramafic and metagabbroic rocks, that discharges through high‐permeability faults and fractures. High‐pH fluids vent through carbonate chimneys with temperatures ranging between 24°C and 91°C, and vent fluid geochemistry indicates quantitative removal of magnesium and water‐rock reactions consistent with heating to between 250°C and 300°C, conductive cooling, and mixing with seawater. This paper develops a relatively simple, steady state, fault‐controlled fluid circulation and crustal heat transfer model that is consistent with these observations. The model results predict that the (1) heat output at Lost City is ~4 × 104 W, (2) circulation depth is approximately 2.7 km, and (3) permeability of the fault zone is ~10−14 m2. The model assumes that the hydrothermal system mines heat from the adjacent country rock, and it does not appear that heat from serpentinization reactions or cooling intrusions is needed to drive the system. Because it takes ~105a for the current model to reach steady state, earlier episodes of hydrothermal activity at LCHF may have operated differently and vent temperatures may have been higher.

Mineralizing Filamentous Bacteria from the Prony Bay Hydrothermal Field Give New Insights into the Functioning of Serpentinization-Based Subseafloor Ecosystems

Despite their potential importance as analogs of primitive microbial metabolisms, the knowledge of the structure and functioning of the deep ecosystems associated with serpentinizing environments is hampered by the lack of accessibility to relevant systems. These hyperalkaline environments are depleted in dissolved inorganic carbon (DIC), making the carbon sources and assimilation pathways in the associated ecosystems highly enigmatic. The Prony Bay Hydrothermal Field (PHF) is an active serpentinization site where, similar to Lost City (Mid-Atlantic Ridge), high-pH fluids rich in H2 and CH4 are discharged from carbonate chimneys at the seafloor, but in a shallower lagoonal environment. This study aimed to characterize the subsurface microbial ecology of this environment by focusing on the earliest stages of chimney construction, dominated by the discharge of hydrothermal fluids of subseafloor origin. By jointly examining the mineralogy and the microbial diversity of the conduits of juvenile edifices at the micrometric scale, we find a central role of uncultivated bacteria belonging to the Firmicutes in the ecology of the PHF. These bacteria, along with members of the phyla Acetothermia and Omnitrophica, are identified as the first chimneys inhabitants before archaeal Methanosarcinales. They are involved in the construction and early consolidation of the carbonate structures via organomineralization processes. Their predominance in the most juvenile and nascent hydrothermal chimneys, and their affiliation with environmental subsurface microorganisms, indicate that they are likely discharged with hydrothermal fluids from the subseafloor. They may thus be representative of endolithic serpentinization-based ecosystems, in an environment where DIC is limited. In contrast, heterotrophic and fermentative microorganisms may consume organic compounds from the abiotic by-products of serpentinization processes and/or from life in the deeper subsurface. We thus propose that the Firmicutes identified at PHF may have a versatile metabolism with the capability to use diverse organic compounds from biological or abiotic origin. From that perspective, this study sheds new light on the structure of deep microbial communities living at the energetic edge in serpentinites and may provide an alternative model of the earliest metabolisms.