Search

The conditions ofmethane (CH4) formation in olivine-hosted secondary fluid inclusions and their prevalence in peridotite and gabbroic rocks from a wide range of geological settings were assessed using confocal Raman spectroscopy, optical and scanning electron microscopy, electron microprobe analysis, and thermodynamic modeling. Detailed examination of 160 samples from ultraslow- to fast-spreading midocean ridges, subduction zones, and ophiolites revealed that hydrogen (H2) and CH4 formation linked to serpentinization within olivine-hosted secondary fluid inclusions is a widespread process. Fluid inclusion contents are dominated by serpentine, brucite, and magnetite, as well as CH4(g) and H2(g) in varying proportions, consistent with serpentinization under strongly reducing, closed-system conditions. Thermodynamic constraints indicate that aqueous fluids entering the upper mantle or lower oceanic crust are trapped in olivine as secondary fluid inclusions at temperatures higher than ~400 °C. When temperatures decrease below ~340 °C, serpentinization of olivine lining the walls of the fluid inclusions leads to a near-quantitative consumption of trapped liquid H2O. The generation of molecular H2 through precipitation of Fe(III)-rich daughter minerals results in conditions that are conducive to the reduction of inorganic carbon and the formation of CH4. Once formed, CH4(g) and H2(g) can be stored over geological timescales until extracted by dissolution or fracturing of the olivine host. Fluid inclusions represent a widespread and significant source of abiotic CH4 and H2 in submarine and subaerial vent systems on Earth, and possibly elsewhere in the solar system. [ABSTRACT FROM AUTHOR]

Most of the geologic CO2 entering Earth's atmosphere and oceans is emitted along plate margins. While C-cycling at mid-ocean ridges and subduction zones has been studied for decades, little attention has been paid to degassing of magmatic CO2 and mineral carbonation of mantle rocks in oceanic transform faults. We studied the formation of soapstone (magnesite-talc rock) and other magnesite-bearing assemblages during mineral carbonation of mantle peridotite in the St. Paul's transform fault, equatorial Atlantic. Clumped carbonate thermometry of soapstone yields a formation (or equilibration) temperature of 147 ± 13 °C which, based on thermodynamic constraints, suggests that CO2(aq) concentrations of the hydrothermal fluid were at least an order of magnitude higher than in seawater. The association of magnesite with apatite in veins, magnesite with a d13C of -3.40 ± 0.04°, and the enrichment of CO2 in hydrothermal fluids point to magmatic degassing and melt-impregnation as the main source of CO2. Melt-rock interaction related to gas-rich alkali olivine basalt volcanism near the St. Paul's Rocks archipelago is manifested in systematic changes in peridotite compositions, notably a strong enrichment in incompatible elements with decreasing MgO/SiO2. These findings reveal a previously undocumented aspect of the geologic carbon cycle in one of the largest oceanic transform faults: Fueled by magmatism in or below the root zone of the transform fault and subsequent degassing, the fault constitutes a conduit for CO2-rich hydrothermal fluids, while carbonation of peridotite represents a vast sink for the emitted CO2. [ABSTRACT FROM AUTHOR]

Single‐celled microbial eukaryotes inhabit deep‐sea hydrothermal vent environments and play critical ecological roles in the vent‐associated microbial food web. 18S rRNA amplicon sequencing of diffuse venting fluids from four geographically‐ and geochemically‐distinct hydrothermal vent fields was applied to investigate community diversity patterns among protistan assemblages. The four vent fields include Axial Seamount at the Juan de Fuca Ridge, Sea Cliff and Apollo at the Gorda Ridge, all in the NE Pacific Ocean, and Piccard and Von Damm at the Mid‐Cayman Rise in the Caribbean Sea. We describe species diversity patterns with respect to hydrothermal vent field and sample type, identify putative vent endemic microbial eukaryotes, and test how vent fluid geochemistry may influence microbial community diversity. At a semi‐global scale, microbial eukaryotic communities at deep‐sea vents were composed of similar proportions of dinoflagellates, ciliates, Rhizaria, and stramenopiles. Individual vent fields supported distinct and highly diverse assemblages of protists that included potentially endemic or novel vent‐associated strains. These findings represent a census of deep‐sea hydrothermal vent protistan communities. Protistan diversity, which is shaped by the hydrothermal vent environment at a local scale, ultimately influences the vent‐associated microbial food web and the broader deep‐sea carbon cycle. [ABSTRACT FROM AUTHOR]

This study investigates the morphology, mineralogy, and geochemistry of seafloor massive sulfide (SMS) deposits from six back-arc hydrothermal vent fields along the Eastern Lau Spreading Center (ELSC) and Valu Fa Ridge (VFR) in the context of endmember vent fluid chemistry and proximity to the Tonga Subduction Zone. To complement deposit geochemistry, vent fluid analyses of Cu, Zn, Ba, Pb and H 2,(aq) were completed to supplement existing data and enable thermodynamic calculations of mineral saturation states at in situ conditions. Results document southward increases in the abundance of mantle-incompatible elements in hydrothermal fluids (Ba and Pb) and SMS deposits (Ba, Pb, As, and Sb), which is also expressed in the abundance of barite (BaSO 4 ) and galena (PbS) in SMS deposits. These increases correspond to a decrease in distance between the ELSC/VFR and the Tonga Subduction Zone that correlates with a change in crustal lithology from back-arc basin basalt in the north to mixed andesite, rhyolite, and dacite in the south. Barite influences deposit morphology, contributing to the formation of horizontal flanges and squat terraces. Results are also consistent with a regional-scale lowering of hydrothermal reaction zone temperatures from north to south (except at the southernmost Mariner vent field) that leads to lower-temperature, higher-pH vent fluids relative to mid-ocean ridges of similar spreading rates (Mottl et al., 2011). These fluids are Cu- and Zn-poor and the deposits formed from these fluids are Cu-poor but Zn-rich. In contrast, at the Mariner vent field, higher-temperature and lower pH vent fluids are hypothesized to result from higher reaction zone temperatures and the localized addition of acidic magmatic volatiles (Mottl et al., 2011). The Mariner fluids are Cu- and Zn-rich and vent from SMS deposits that are rich in Cu but poor in Zn with moderate amounts of Pb. Thermodynamic calculations indicate that the contrasting metal contents of vent fluids and SMS deposits can be accounted for by vent fluid pH. Wurtzite/sphalerite ((Zn, Fe)S) and galena (PbS) are saturated at higher temperatures in higher-pH, Zn-, Cu-, and Pb-poor ELSC/VFR vent fluids, but are undersaturated at similar temperatures in low-pH, Zn-, Cu-, and Pb-rich vent fluids from the Mariner vent field. Indicators of pH in the ELSC and VFR SMS deposits include the presence of co-precipitated wurtzite and chalcopyrite along conduit linings in deposits formed from higher pH fluids, and different correlations between concentrations of Zn and Ag in bulk geochemical analyses. Significant positive bulk geochemical Zn:Ag correlations occur for deposits at vent fields where hydrothermal fluids have a minimum pH (at 25 °C) < 3.3, while correlations of Zn:Ag are weak or negative for deposits at vent fields where the minimum vent fluid pH (at 25 °C) > 3.6. Data show that the compositions of the mineral linings of open conduit chimneys (minerals present, mol% FeS in (Zn,Fe)S) that precipitate directly from hydrothermal fluids closely reflect the temperature and sulfur fugacity of sampled hydrothermal fluids. These mineral lining compositions thus can be used as indicators of hydrothermal fluid temperature and composition (pH, metal content, sulfur fugacity). [ABSTRACT FROM AUTHOR]

Interactions of CO 2 -rich aqueous fluids with mantle peridotite have major implications for geochemical budgets and microbial life in the shallow oceanic lithosphere through the formation of carbonate minerals and reduced carbon species. However, the underlying mechanisms controlling the transformation of CO 2 to carbonates in ultramafic-hosted hydrothermal systems remain incompletely understood. A long-term laboratory experiment was conducted at 300 °C and 35 MPa to investigate serpentinization and carbonate formation pathways during hydrothermal alteration of peridotite. Powdered harzburgite was initially reacted with a Ca-rich aqueous fluid for 14,592 h (608 days) and changes in fluid composition were monitored with time. Once the system reached a steady state, a CO 2( aq ) -rich fluid was injected and allowed to react with the system for 5907 h (246 days). Fluid speciation and mineral analyses suggest that serpentinization of harzburgite in the CO 2 -poor system led to the precipitation of serpentine, brucite, magnetite, and minor calcite, in addition to other minor phases including chlorite and sulfur-poor Ni sulfides. The addition of the CO 2( aq ) -rich fluid caused dolomite, Ca-rich dolomite, and high-Mg calcite to form at the expense of olivine, calcite, and brucite, while serpentine remained unreactive. Replacement textures and mineral assemblages mimic those documented in carbonate-altered seafloor serpentinites, particularly those from the Mid-Atlantic Ridge and the Iberia Margin. In contrast to thermodynamic predictions, magnesite did not form in the experiment because the dissolution of clinopyroxene, in combination with the lack of serpentine reactivity, maintained low Mg/Ca ratios in solution. Clinopyroxene dissolution and unreactive serpentine may similarly maintain low Mg/Ca ratios in submarine serpentinization systems and limit magnesite formation in subseafloor environments. Results of this study suggest that the formation of Ca–Mg carbonates by mineral carbonation is favorable in subseafloor serpentinization systems and likely represents a significant, but poorly quantified, carbon sink in hydrothermally altered oceanic lithosphere at slow-spreading mid-ocean ridges. [ABSTRACT FROM AUTHOR]

• We have tracked Earth's northernmost hydrothermal system to its seafloor source. • Polaris discharges metal-poor, H 2 - and CH 4 -enriched fluids into the Arctic Ocean. • Polaris extends the known diversity of ultramafic-influenced hydrothermal systems. • Hybrid mafic–ultramafic vent sites may be common along ultraslow-spreading ridges. • Our study challenges the ability of established approaches to vent exploration. The nature of deep-sea hydrothermal systems is commonly inferred from physicochemical plume characteristics and seafloor observations, as was the case for the 'Polaris' site on the ultraslow-spreading Gakkel Ridge, Earth's northernmost hydrothermal system. Initial reports showing temperature and turbidity anomalies in its hydrothermal plume combined with its location on a neovolcanic axial seamount suggested a volcanically-hosted 'black smoker'-type system. That interpretation, however, is inconsistent with our more complete data set derived from extensive water column sampling and seafloor surveys. The buoyant plume exhibits minor turbidity anomalies and low metal concentrations (dissolved Mn ≤ 3.1 nM), but contains substantial concentrations of H 2 (275 nM) and 13C-enriched CH 4 (365 nM, δ13C = –13.2). Instead of a 'black smoker' vent field, we observed small-scale chimney structures at the seafloor. Together, these data imply intermediate-temperature reaction of hydrothermal fluids with ultramafic rock in the subseafloor before discharge through pillow basalt outcrops at the seafloor. Our study challenges the ability of established approaches to vent exploration, reliant exclusively on in situ sensing to reveal the full geodiversity of subseafloor hydrothermal venting. Ultramafic-influenced systems, releasing H 2 and CH 4 into the ocean, may be a recurring feature along the entire 25% of the global ridge system that is ultraslow-spreading. [ABSTRACT FROM AUTHOR]

Simple alkyl thiols such as methanethiol (CH3SH) are widely speculated to form in seafloor hot spring fluids. Putative CH3SH synthesis by abiotic (nonbiological) reduction of inorganic carbon (CO2 or CO) has been invoked as an initiation reaction for the emergence of protometabolism and microbial life in primordial hydrothermal settings. Thiols are also presumptive ligands for hydrothermal trace metals and potential fuels for associated microbial communities. In an effort to constrain sources and sinks of CH3SH in seafloor hydrothermal systems, we determined for the first time its abundance in diverse hydrothermal fluids emanating from ultramafic, mafic, and sediment-covered midocean ridge settings. Our data demonstrate that the distribution of CH3SH is inconsistent with metastable equilibrium with inorganic carbon, indicating that production by abiotic carbon reduction is more limited than previously proposed. CH3SH concentrations are uniformly low (~10-8 M) in high-temperature fluids (>200 °C) from all unsedimented systems and, in many cases, suggestive of metastable equilibrium with CH4 instead. Associated low-temperature fluids (<200 °C) formed by admixing of seawater, however, are invariably enriched in CH3SH (up to ~10-6 M) along with Graphic and low-molecular-weight hydrocarbons relative to high-temperature source fluids, resembling our observations from a sediment-hosted system. This strongly implicates thermogenic interactions between upwelling fluids and microbial biomass or associated dissolved organic matter during subsurface mixing in crustal aquifers. Widespread thermal degradation of subsurface organic matter may be an important source of organic production in unsedimented hydrothermal systems and may influence microbial metabolic strategies in cooler near-seafloor and plume habitats. [ABSTRACT FROM AUTHOR]

Ocean worlds provide fascinating opportunities for future ocean research. They allow us to test our understanding of processes we consider fundamental to Earth’s ocean and simultaneously provide motivation to explore our ocean further and develop new technologies to do so. In parallel, ocean worlds research offers opportunities for ocean scientists to provide meaningful contributions to novel investigations in the coming decades that will search for life beyond Earth. Key to the contributions that oceanographers can make to this field is that studies of all other ocean worlds remain extremely data limited. Here, we describe an approach based on ocean systems science in which theoretical modeling can be used, in concert with targeted laboratory experimentation and direct observations in Earth’s ocean, to predict what processes (including those essential to support life) might be occurring on other ocean worlds. In turn, such an approach would help identify new technologies that might be required for future space missions as well as appropriate analog studies that could be conducted on Earth to develop and validate such technologies. Our approach is both integrative and interdisciplinary and considers multiple domains, from processes active in the subseafloor to those associated with ocean-ice feedbacks. [ABSTRACT FROM AUTHOR]

Symbioses between invertebrate animals and chemosynthetic bacteria form the basis of hydrothermal vent ecosystems worldwide. In the Lau Basin, deep-sea vent snails of the genus Alviniconcha associate with either Gammaproteobacteria (A. kojimai, A. strummeri) or Campylobacteria (A. boucheti) that use sulfide and/or hydrogen as energy sources. While the A. boucheti host–symbiont combination (holobiont) dominates at vents with higher concentrations of sulfide and hydrogen, the A. kojimai and A. strummeri holobionts are more abundant at sites with lower concentrations of these reductants. We posit that adaptive differences in symbiont physiology and gene regulation might influence the observed niche partitioning between host taxa. To test this hypothesis, we used high-pressure respirometers to measure symbiont metabolic rates and examine changes in gene expression among holobionts exposed to in situ concentrations of hydrogen (H2: ~25 µM) or hydrogen sulfide (H2S: ~120 µM). The campylobacterial symbiont exhibited the lowest rate of H2S oxidation but the highest rate of H2 oxidation, with fewer transcriptional changes and less carbon fixation relative to the gammaproteobacterial symbionts under each experimental condition. These data reveal potential physiological adaptations among symbiont types, which may account for the observed net differences in metabolic activity and contribute to the observed niche segregation among holobionts. [ABSTRACT FROM AUTHOR]

Subseafloor mixing of high-temperature hot-spring fluids with cold seawater creates intermediate-temperature diffuse fluids that are replete with potential chemical energy. This energy can be harnessed by a chemosynthetic biosphere that permeates hydrothermal regions on Earth. Shifts in the abundance of redoxreactive species in diffuse fluids are often interpreted to reflect the direct influence of subseafloor microbial activity on fluid geochemical budgets. Here, we examine hydrothermal fluids venting at 44 to 149 °C at the Piccard hydrothermal field that span the canonical 122 °C limit to life, and thus provide a rare opportunity to study the transition between habitable and uninhabitable environments. In contrast with previous studies, we show that hydrocarbons are contributed by biomass pyrolysis, while abiotic sulfate (SO42−) reduction produces large depletions in H2. The latter process consumes energy that could otherwise support key metabolic strategies employed by the subseafloor biosphere. Available Gibbs free energy is reduced by 71 to 86% across the habitable temperature range for both hydrogenotrophic SO42− reduction to hydrogen sulfide (H2S) and carbon dioxide (CO2) reduction to methane (CH4). The abiotic H2 sink we identify has implications for the productivity of subseafloor microbial ecosystems and is an important process to consider within models of H2 production and consumption in young oceanic crust. [ABSTRACT FROM AUTHOR]

Sampling of paired black smoker chimney linings and seafloor hydrothermal vent fluids supports the development of trace element proxies for sulfide mineral deposition environments by facilitating analyses of trace element partitioning between mineral and fluid phases under well-constrained physiochemical conditions. Here, concentrations of Co, Ni, Ga, Ag, and In in chalcopyrite lining 22 black smoker chimneys (29 for Co, Ag, and In) are measured using secondary ion mass spectrometry (SIMS) calibrated against inductively coupled plasma mass spectrometry (ICP-MS) and NIST-traceable reference solutions. To provide additional data on the trace element concentrations of vent fluid pairs for 19 of the 29 black smoker chimney linings investigated, this paper also presents new ICP-MS data for 33 hydrothermal vent fluids collected from the Tahi Moana-1, ABE, Tu'i Malila, and Mariner vent fields on the Eastern Lau Spreading Center and Valu Fa Ridge. The chalcopyrite black smoker chimney linings investigated represent a variety of temperature (269–395 °C), chemical (e.g., pH (at 25 °C) = 2.3–4.4), and geologic conditions. Electron microprobe results indicate that mineral stoichiometry ranges from stoichiometric chalcopyrite to mol Cu : mol Fe = 0.65. Trace element concentrations obtained by SIMS are: Co (<2 ng/g–760 μg/g), Ni (<17 ng/g–454 μg/g), Ga (<0.9 ng/g–48 μg/g), Ag (60 μg/g–3800 μg/g), In (<0.5 ng/g–270 μg/g). Concentrations of Ag in chalcopyrite strongly correlate with the free ion activity ratio of {Ag+}:{Cu+} in paired vent fluids, with high Ag concentrations in chalcopyrite indicating formation from near neutral vent fluids containing low Cu concentrations or low-pH vent fluids with high Ag concentrations attributable to subsurface Ag remobilization. Chalcopyrite with low Ag precipitates from low-pH Cu-rich fluids unaffected by extensive Ag remobilization. Concentrations of Ga and In in chalcopyrite exhibit a negative trend with vent fluid pH, possibly reflecting the strength of Ga and In OH− complexes. Thus, Ga and In concentrations differentiate Ag-rich chalcopyrite formed from near-neutral Cu-poor vent fluids or that formed from Ag-rich low-pH vent fluids. In contrast, Co and Ni exhibit no trend with fluid data, but correlate with mineral Cu:Fe ratios, possibly reflecting the greater availability of Fe(II) lattice sites or paired substitution of 2+ ions. Overall, this study demonstrates the potential of paired vent fluid and black smoker chimney samples to provide insight into the partitioning of trace elements in sulfide mineral deposition environments and related proxies of important fluid parameters such as pH and metal concentrations. This study also demonstrates the utility of SIMS to precisely analyze trace elements in chalcopyrite at high spatial resolutions and low detection limits. [ABSTRACT FROM AUTHOR]

Highlights • Dissolved organic matter from hydrothermal vents is chemically distinct from seawater. • The highest vitamin levels were measured in low temperature vent fluids. • Fluids can contain benzoic acid derivatives, amino acids, and organic sulfur. Abstract Deep-sea hydrothermal vents are unique ecosystems that may release chemically distinct dissolved organic matter to the deep ocean. Here, we describe the composition and concentrations of polar dissolved organic compounds observed in low and high temperature hydrothermal vent fluids at 9°50′N on the East Pacific Rise. The concentration of dissolved organic carbon was 46 µM in the low temperature hydrothermal fluids and 14 µM in the high temperature hydrothermal fluids. In the low temperature vent fluids, quantifiable dissolved organic compounds were dominated by water-soluble vitamins and amino acids. Derivatives of benzoic acid and the organic sulfur compound 2,3-dihydroxypropane-1-sulfonate (DHPS) were also present in low and high temperature hydrothermal fluids. The low temperature vent fluids contain organic compounds that are central to biological processes, suggesting that they are a by-product of biological activity in the subseafloor. These compounds may fuel heterotrophic and other metabolic processes at deep-sea hydrothermal vents and beyond. [ABSTRACT FROM AUTHOR]

The composition of submarine hydrothermal vent fluids is affected by a variety of processes, such as interaction of heated seawater with rocks and sediments, addition of magmatic fluids, as well as phase separation and segregation. How these processes specifically affect the vent fluid composition is still poorly understood. In particular, the relative role of phase separation and magmatic degassing, which is common in arc/back-arc hydrothermal systems, is not well known. To provide new insights into these processes, we analysed B contents and isotope ratios in hydrothermal vent fluids and volcanic rocks from the Manus Basin, Papua New Guinea, and Nifonea volcano, New Hebrides back-arc. These fluids show a range of salinities, gas contents, acidities, and host rock compositions; many of them are influenced by phase separation and by addition of magmatic volatiles (both CO 2 and SO 2 ). Previous studies of hydrothermal vents in arc/back-arc settings suggest that B contents and isotopic composition of vent fluids are controlled by interactions between seawater, basement and sediments, and propose that phase separation and magmatic fluids play only a subordinate role. In our study, we demonstrate that vent fluids with minor magmatic input indeed reflect the interaction between seawater and oceanic crust. In contrast, the low-salinity Nifonea fluids and some of the acid-sulphate fluids from the Manus Basin have higher B contents as expected, whereas other volatile-rich fluids from the Manus Basin show B depletions. The lack of correlation between B contents and the intensity of magmatic fluid influx (CO 2 and SO 2 ) may indicate that magma degassing is not responsible for the B enrichments or depletions in these vent fluids. B enrichments might be related to preferential partitioning of B into the vapour phase during phase separation under PT-conditions well above the two-phase curve and critical line (i.e. T >> T critical , P >> P critical ). However, this cannot explain the low B concentrations in the vapour-rich vent fluids from the Manus Basin and the low B isotope ratios in the Nifonea fluids. Instead, we propose that B concentrations and isotope ratios in submarine vent fluids largely depend on the residence and reaction time of the vent fluid in the subsurface. In general, all vent fluids are still influenced by water-rock interaction during hydrothermal circulation. However, vent fluids with short residence times define a trend towards lower B concentrations and isotope ratios, which can be explained by mixing between hydrothermal and magmatic fluid, which is similar to the composition of the host rock. In contrast, the B signature of the magmatic fluid can be overprinted due to preferential mobilisation of B from the oceanic crust into vapour-rich fluids at longer reaction times. Thus, B may provide a tool for estimating the extent of B leaching and hence hydrothermal alteration in the subseafloor. [ABSTRACT FROM AUTHOR]

Below the seafloor at deep-sea hot springs, mixing of geothermal fluids with seawater supports a potentially vast microbial ecosystem. Although the identity of subseafloor microorganisms is largely known, their effect on deep-ocean biogeochemical cycles cannot be predicted without quantitative measurements of their metabolic rates and growth efficiency. Here, we report on incubations of subseafloor fluids under in situ conditions that quantitatively constrain subseafloor primary productivity, biomass standing stock, and turnover time. Single-cell-based activity measurements and 16S rRNA-gene analysis showed that Campylobacteria dominated carbon fixation and that oxygen concentration and temperature drove niche partitioning of closely related phylotypes. Our data reveal a very active subseafloor biosphere that fixes carbon at a rate of up to 321 µg C⋅L-1⋅d-1, turns over rapidly within tens of hours, rivals the productivity of chemosynthetic symbioses above the seafloor, and significantly influences deep-ocean biogeochemical cycling. [ABSTRACT FROM AUTHOR]

Hosted in basaltic substrate on the ultra-slow spreading Mid-Cayman Rise, the Piccard hydrothermal field is the deepest currently known seafloor hot-spring (4957–4987 m). Due to its great depth, the Piccard site is an excellent natural system for investigating the influence of extreme pressure on the formation of submarine vent fluids. To investigate the role of rock composition and deep circulation conditions on fluid chemistry, the abundance and isotopic composition of organic, inorganic, and dissolved volatile species in high temperature vent fluids at Piccard were examined in samples collected in 2012 and 2013. Fluids from the Beebe Vents and Beebe Woods black smokers vent at a maximum temperature of 398 °C at the seafloor, however several lines of evidence derived from inorganic chemistry (Cl, SiO 2 , Ca, Br, Fe, Cu, Mn) support fluid formation at much higher temperatures in the subsurface. These high temperatures, potentially in excess of 500 °C, are attainable due to the great depth of the system. Our data indicate that a single deep-rooted source fluid feeds high temperature vents across the entire Piccard field. High temperature Piccard fluid H 2 abundances (19.9 mM) are even higher than those observed in many ultramafic-influenced systems, such as the Rainbow (16 mM) and the Von Damm hydrothermal fields (18.2 mM). In the case of Piccard, however, these extremely high H 2 abundances can be generated from fluid-basalt reaction occurring at very high temperatures. Magmatic and thermogenic sources of carbon in the high temperature black smoker vents are described. Dissolved ΣCO 2 is likely of magmatic origin, CH 4 may originate from a combination of thermogenic sources and leaching of abiotic CH 4 from mineral-hosted fluid inclusions, and CO abundances are at equilibrium with the water–gas shift reaction. Longer-chained n-alkanes (C 2 H 6 , C 3 H 8 , n- C 4 H 10 , i- C 4 H 10 ) may derive from thermal alteration of dissolved and particulate organic carbon sourced from the original seawater source, entrainment of microbial ecosystems peripheral to high temperature venting, and/or abiotic mantle sources. Dissolved ΣHCOOH in the Beebe Woods fluid is consistent with thermodynamic equilibrium for abiotic production via ΣCO 2 reduction with H 2 at 354 °C measured temperature. A lack of ΣHCOOH in the relatively higher temperature 398 °C Beebe Vent fluids demonstrates the temperature sensitivity of this equilibrium. Abundant basaltic seafloor outcrops and the axial location of the vent field, along with multiple lines of geochemical evidence, support extremely high temperature fluid-rock reaction with mafic substrate as the dominant control on Piccard fluid chemistry. These results expand the known diversity of vent fluid composition, with implications for supporting microbiological life in both the modern and ancient ocean. [ABSTRACT FROM AUTHOR]

Hot-spring fluids emanating from deep-sea vents hosted in unsedimented ultramafic and mafic rock commonly contain high concentrations of methane. Multiple hypotheses have been proposed for the origin(s) of this methane, ranging from synthesis via reduction of aqueous inorganic carbon (∑CO 2 ) during active fluid circulation to leaching of methane-rich fluid inclusions from plutonic rocks of the oceanic crust. To further resolve the process(es) responsible for methane generation in these systems, we determined the relative abundances of several methane isotopologues (including 13 CH 3 D, a “clumped” isotopologue containing two rare isotope substitutions) in hot-spring source fluids sampled from four geochemically-distinct hydrothermal vent fields (Rainbow, Von Damm, Lost City, and Lucky Strike). Apparent equilibrium temperatures retrieved from methane clumped isotopologue analyses average 310 - 42 + 53  °C, with no apparent relation to the wide range of fluid temperatures (96–370 °C) and chemical compositions (pH, [H 2 ], [∑CO 2 ], [CH 4 ]) represented. Combined with very similar bulk stable isotope ratios ( 13 C/ 12 C and D/H) of methane across the suite of hydrothermal fluids, all available geochemical and isotopic data suggest a common mechanism of methane generation at depth that is disconnected from active fluid circulation. Attainment of equilibrium amongst methane isotopologues at temperatures of ca. 270–360 °C is compatible with the thermodynamically-favorable reduction of CO 2 to CH 4 at temperatures at or below ca. 400 °C under redox conditions characterizing intrusive rocks derived from sub-ridge melts. Collectively, the observations support a model where methane-rich aqueous fluids, known to be trapped in rocks of the oceanic lithosphere, are liberated from host rocks during hydrothermal circulation and perhaps represent the major source of methane venting with thermal waters at unsedimented hydrothermal fields. The results also provide further evidence that water-rock reactions occurring at temperatures lower than 200 °C do not contribute significantly to the quantities of methane venting at mid-ocean ridge hot springs. [ABSTRACT FROM AUTHOR]

At deep-sea hydrothermal vents, a large source of potential chemical energy is created when reducing vent fluid and oxidizing seawater mix. In this environment, chemolithoautotrophic microbes catalyze exergonic redox reactions which in turn provide the energy needed to fuel their growth and the fixation of CO 2 into biomass. In addition to producing new organic matter, this process also consumes compounds contained both in vent fluid and entrained seawater ( e.g. H 2 , NO 3 − ). Despite their biogeochemical importance, such reactions have remained difficult to quantify due to methodological limitations. To address this knowledge gap, this study reports a novel application of isobaric gas-tight fluid samplers for conducting incubations of hydrothermal vent fluids at in situ temperature and pressure. Eighteen ~24 h incubations were carried out, representing seven distinct conditions that examine amendments consisting of different electron donors and acceptors. Microbial activity was observed in all treatments, and time series chemical measurements showed that activity was limited by electron acceptor supply, confirming predictions based on geochemical data. Also consistent with these predictions, the presence of nitrate increased rates of hydrogen consumption and yielded ammonium as a product of nitrate respiration. The stoichiometry of predicted redox reactions was also determined, revealing that the sulfur and nitrogen cycles are incompletely understood at deep-sea vents, and likely involve unknown intermediate redox species. Finally, the measured rates of redox processes were either equal to or far greater than what has been reported in previous studies where in situ conditions were not maintained. In addition to providing insights into deep-sea hydrothermal vent biogeochemistry, the methods described herein also offer a practical approach for the incubation of any deep-sea pelagic sample under in situ conditions. [ABSTRACT FROM AUTHOR]

Multiple sulfur isotopes were measured in metal sulfide deposits, elemental sulfur, and aqueous hydrogen sulfide to constrain sulfur sources and the isotopic systematics of precipitation in seafloor hydrothermal vents. Areas studied include the Eastern Manus Basin and Lau Basin back-arc spreading centers and the unsedimented basalt-hosted Southern East Pacific Rise (SEPR) and sediment-hosted Guaymas Basin mid-ocean ridge spreading centers. Chalcopyrite and dissolved hydrogen sulfide (H 2 S) δ 34 S values range from −5.5‰ to +5.6‰ in Manus Basin samples, +2.4‰ to +6.1‰ in Lau Basin samples, and +3.7‰ to +5.7‰ in SEPR samples. Values of δ 34 S for cubic cubanite and H 2 S range from −1.4‰ to +4.7‰ in Guaymas Basin samples. Multiple sulfur isotope systematics in fluid-mineral pairs from the SEPR and Lau Basin show that crustal host rock and thermochemical reduction of seawater-derived dissolved sulfate (SO 4 ) are the primary sources of sulfur in mid-ocean ridge and some back-arc systems. At PACMANUS and SuSu Knolls hydrothermal systems in the Eastern Manus Basin, a significant contribution of sulfur is derived from disproportionation of magmatic sulfur dioxide (SO 2 ), while the remaining sulfur is derived from crustal host rocks and SO 4 reduction. At the sedimented Guaymas Basin hydrothermal system, sulfur sources include crustal host rock, reduced seawater SO 4 , and biogenic sulfide. Vent fluid flow through fresher, less-mature sediment supplies an increased quantity of reactant organic compounds that may reduce 34 S-enriched SO 4 , while fluid interaction with more highly-altered sediments results in H 2 S characterized by a small, but isotopically-significant input of 34 S-depleted biogenic sulfides. Near-zero Δ 33 S values in all samples implicate the abiotic processes of SO 4 reduction and leaching of host rock as the major contributors to sulfur content at a high temperature unsedimented mid-ocean ridge and at a back-arc system. Δ 33 S values indicate that SO 2 disproportionation is an additional process that contributes sulfur to a different back-arc system and to acid spring-type hydrothermal fluid circulation. At the sedimented Guaymus Basin, near-zero Δ 33 S values are also observed, despite negative δ 34 S values that indicate inputs of biogenic pyrite for some samples. In contrast with previous studies reporting isotope disequilibrium between H 2 S and chalcopyrite, the δ 34 S values of chalcopyrite sampled from the inner 1–2 mm of a chimney wall are within ±1‰ of δ 34 S values for H 2 S in the paired vent fluid, suggesting equilibrium fluid-mineral sulfur isotope exchange at 300–400 °C. Isotopic equilibrium between hydrothermal fluid H 2 S and precipitating chalcopyrite implies that sulfur isotopes in the chalcopyrite lining across a chimney wall may accurately record past hydrothermal activity. [ABSTRACT FROM AUTHOR]

Methane is a key component in the global carbon cycle, with a wide range of anthropogenic and natural sources. Although isotopic compositions of methane have traditionally aided source identification, the abundance of its multiply substituted "clumped" isotopologues (for example, 13CH3D) has recently emerged as a proxy for determining methane-formation temperatures. However, the effect of biological processes on methane's clumped isotopologue signature is poorly constrained. We show that methanogenesis proceeding at relatively high rates in cattle, surface environments, and laboratory cultures exerts kinetic control on 13CH3D abundances and results in anomalously elevated formation-temperature estimates. We demonstrate quantitatively that H2 availability accounts for this effect. Clumped methane thermometry can therefore provide constraints on the generation of methane in diverse settings, including continental serpentinization sites and ancient, deep groundwaters. [ABSTRACT FROM AUTHOR]

• GDGTs and archaeols are heavily degraded in a hydrothermal vent at Guaymas Basin. • The rate is affected by vent temperatures and exceeds that of sedimentary diagenesis. • Archaeal intact polar lipids were detected in sediments exposed to 145 °C porewaters. • The degradation of GDGTs may help explain a preservation gap in the biomarker record. Archaea are among the earliest evolved organisms. Today, they exist in nearly every habitable environment and their highly recalcitrant membrane lipids are both abundant and easily detected in modern sediments. Yet, unlike bacteria and eukaryotes, the lipid biomarker signatures of archaea are almost entirely absent in the ancient rock record (>145 Ma). We present a comprehensive study of archaeal lipids from the Cathedral Hill hydrothermal vent complex in Guaymas Basin, Gulf of California. Here, porewaters reach 155 °C by 21 cm below the sea floor (cmbsf), which enables the near-complete tracking of sedimentary organic matter (SOM) accumulation, diagenetic alteration and thermochemical transformation into petroleum-like hydrocarbons within and beyond the habitable zone of life. Identified intact polar lipids (IPLs) of living archaea included mono- and di-glycosidic archaeols (1G-AR, 2G-AR) and glycerol dialkyl glycerol tetraethers (1G- and 2G-GDGTs). The 1G- and 2G-ARs, potentially derive from methane-oxidizing archaeal groups ANME-1 and ANME-2 and anaerobic thermophilic methanogens, reach sediment depths of ∼ 135 cm and 50 °C temperatures. The 1G-and 2G-GDGT lipids reach a depth of ~180 cm where vent fluid temperatures are ~145 ± 15 °C. Core lipids (CLs) include archaeols (AR) and isoprenoidal and branched GDGTs (i GDGTs, br GDGTs). The core GDGTs (c GDGTs) closest to the vent center have high rates of lipid loss. Up to 95% of all identified archaeal biomarkers, including lipids supplied from the upper water column, are degraded or recycled in the surface sediments and do not reach conditions of late diagenesis and catagenesis. Only ∼ 0.11%, proportional to the lipid bound component in the protokerogen, marks thermochemically cracked biphytane – a prominent archaeal hydrocarbon biomarker. This extreme reduction indicates archaeal lipids do not necessarily become incorporated into kerogen thereby helping to explain the deep time archaeal lipid preservation paradox. [ABSTRACT FROM AUTHOR]

• 34 hydrothermal oil samples along a push core transect were analyzed by GC × GC. • Hydrocarbon matrices were compared using MPCA and HCA of whole oil chromatograms. • Sediments were shown to contain migrated and in situ generated hydrocarbons. • The molecular results show a high degree of spatial complexity at shallow depths. Guaymas Basin is a submarine depression in the Gulf of California marking the northern end of the East Pacific Rise mid-oceanic spreading ridge. The basin receives high input of sedimentary organic matter (SOM) from elevated productivity in the overlying surface waters and runoff from the surrounding continent. This, coupled with high sedimentation rates, produces near-uniform compositions of SOM. Various hydrothermal vent complexes occur along this margin. One of these is Cathedral Hill, a hydrothermal mound with sulfide chimneys surrounded by mats dominated by sulfide-oxidizing Beggiatoa , covering sediments stained by locally produced oil. We collected four push cores along a transect at this site (4 m × 0.21 m) extending from near the vent center to the ambient sediment just outside the microbial mat cover. Porewater temperatures near the mound center were projected to reach 155 °C by 21 cm below the sea floor (cmbsf). Within these conditions, a kinetic model based on vitrinite reflectance equivalence (%R e) predicts petroleum formation as shallow as 15–18 cmbsf, with metagenesis commencing at <60 cmbsf. Bulk extract data (total lipid extracts as well as polar and apolar fractions), solvent-extracted sediment TOC (herein referred to as protokerogen TOC), and molecular thermal maturation parameters support these generation estimates. In recent years, the application of chemometric techniques to comprehensive two-dimensional gas chromatographic (GC × GC) analyses has allowed comparison of thousands of unique hydrocarbons within oils. Here we reconstruct the shallow subsurface petroleum system by applying multiway principal component analysis (MPCA) and hierarchical cluster analysis (HCA) directly to GC × GC chromatograms. We then compare the resulting multivariate models to a systematic survey of subtracted GC × GC chromatograms, a transect heat map of sample hydrocarbon compound diversities, and profiles of various thermal maturation parameters to elucidate how these hydrocarbon matrices are attenuated by production, migration, and/or thermochemical oxidation. Sample matrices have up to 5800 unique compounds spanning a range of normal and branched alkanes, saturated and unsaturated biomarkers, substituted and unsubstituted polycyclic aromatic hydrocarbons (PAHs), perhydro-PAHs, and benzothiophenes with up to six ring-cycles (i.e., benzoperylenes, dibenzopyrenes, dibenzochrysenes, and indenopyrenes). These matrices display systematic temperature-dependent trends. Generation likely begins in 6–10 and 15–18 cmbsf where sediments are exposed to ∼115 °C vent porewater temperatures, which is shallower than predictions based on our kinetic model. Independent of these sites of generation is ubiquitous staining of the sediments from advected oil that is heavily dominated by PAHs as well as two stratigraphic bands of migrated oil that extend horizontally across the transect at 0–2 and ∼6–10 cmbsf, respectively. The MPCA models along with non-statistical validation techniques show evidence of decreasing diversities and concentrations of alkylated aromatic hydrocarbons concomitant with elevated abundances of dealkylated PAHs and/or the migration of unsubstituted PAHs from deeper basin depths as sediments become exposed to more severe hydrothermal conditions. These results indicate that even at relatively small spatial scales, the petroliferous sediments at hydrothermal vent sites can be highly complex. [ABSTRACT FROM AUTHOR]

Microbial eukaryotes (or protists) in marine ecosystems are a link between primary producers and all higher trophic levels, and the rate at which heterotrophic protistan grazers consume microbial prey is a key mechanism for carbon transport and recycling in microbial food webs. At deep-sea hydrothermal vents, chemosynthetic bacteria and archaea form the base of a food web that functions in the absence of sunlight, but the role of protistan grazers in these highly productive ecosystems is largely unexplored. Here, we pair grazing experiments with a molecular survey to quantify protistan grazing and to characterize the composition of vent-associated protists in low-temperature diffuse venting fluids from Gorda Ridge in the northeast Pacific Ocean. Results reveal protists exert higher predation pressure at vents compared to the surrounding deep seawater environment and may account for consuming 28 to 62% of the daily stock of prokaryotic biomass within discharging hydrothermal vent fluids. The vent-associated protistan community was more species rich relative to the background deep sea, and patterns in the distribution and co-occurrence of vent microbes provide additional insights into potential predator-prey interactions. Ciliates, followed by dinoflagellates, Syndiniales, rhizaria, and stramenopiles, dominated the vent protistan community and included bacterivorous species, species known to host symbionts, and parasites. Our findings provide an estimate of protistan grazing pressure within hydrothermal vent food webs, highlighting the important role that diverse protistan communities play in deep-sea carbon cycling., Competing Interests: The authors declare no competing interest.

Arguments for an abiotic origin of low-molecular weight organic compounds in deep-sea hot springs are compelling owing to implications for the sustenance of deep biosphere microbial communities and their potential role in the origin of life. Theory predicts that warm H2-rich fluids, like those emanating from serpentinizing hydrothermal systems, create a favorable thermodynamic drive for the abiotic generation of organic compounds from inorganic precursors. Here, we constrain two distinct reaction pathways for abiotic organic synthesis in the natural environment at the Von Damm hydrothermal field and delineate spatially where inorganic carbon is converted into bioavailable reduced carbon. We reveal that carbon transformation reactions in a single system can progress over hours, days, and up to thousands of years. Previous studies have suggested that CH4 and higher hydrocarbons in ultramafic hydrothermal systems were dependent on H2 generation during active serpentinization. Rather, our results indicate that CH4 found in vent fluids is formed in H2-rich fluid inclusions, and higher n-alkanes may likely be derived from the same source. This finding implies that, in contrast with current paradigms, these compounds may form independently of actively circulating serpentinizing fluids in ultramafic-influenced systems. Conversely, widespread production of formate by ΣCO2 reduction at Von Damm occurs rapidly during shallow subsurface mixing of the same fluids, which may support anaerobic methanogenesis. Our finding of abiogenic formate in deep-sea hot springs has significant implications for microbial life strategies in the present-day deep biosphere as well as early life on Earth and beyond.