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Hydrothermal circulation and alteration at mid-ocean ridges and ridge flanks have a key role in regulating seawater chemistry and global chemical fluxes, and support diverse ecosystems in the absence of light. In this Review, we outline tectonic, magmatic and hydrothermal processes that govern crustal architecture, alteration and biogeochemical cycles along mid-ocean ridges with different spreading rates. In general, hydrothermal systems vary from those that are magmatic-dominated with low-pH fluids >300 degrees C to serpentinizing systems with alkaline fluids

Serpentinization-influenced hydrothermal systems, such as the Lost City Hydrothermal Field (LCHF), are considered as potential sites for the origin of life. Despite an abundance of reducing power in this system (H2 and CH4), microbial habitability may be limited by high pH, elevated temperatures, and/or low concentrations of bioavailable carbon. At the LCHF, the relative contribution of biotic and abiotic processes to the vent fluid composition, especially in the lower temperature vents, remain poorly constrained. We present fluid chemistry and isotope data that suggest that all LCHF fluids are derived from a single endmember produced in the hotter, deeper subsurface essentially in the absence of microbial activity. The strontium isotope composition (87Sr/86Sr) of this fluid records the influence of underlying mantle and/or gabbroic rocks, whereas sulfur isotope composition indicates closed-system thermochemical sulfate reduction. Conductive cooling and transport is accompanied by continued sulfate reduction, likely microbial, and mixing with unaltered seawater, which produce second-order vents characterized by higher δ34Ssulfide and lower δ34Ssulfate values. Third-order vent fluids are produced by varying degrees of subsurface mixing between the first- and second-order fluids and a seawater-dominated fluid. Additional biotic and abiotic processes along different flow paths are needed to explain the spatial variability among the vents. Relationships between sulfur geochemistry and hydrogen concentrations dominantly reflect variations in temperature and/or distance from the primary outflow path. Methane concentrations are constant across the field which point to an origin independent of flow path and venting temperature. At Lost City, not all vent fluids appear to have zero Mg concentrations. Thus, we propose an extrapolation to a Sr isotope-endmember composition as an alternative method to estimate endmember fluid compositions at least in similar systems where a two-component mixing with respect to Sr isotopes between seawater and endmember fluids can be established., Geochimica et Cosmochimica Acta, 332, ISSN:0016-7037, ISSN:1872-9533

Carbonate-brucite chimneys are a characteristic of low- to moderate-temperature, ultramafic-hosted alkaline hydrothermal systems, such as the Lost City hydrothermal field located on the Atlantis Massif at 30°N near the Mid-Atlantic Ridge. These chimneys form as a result of mixing between warm, serpentinization-derived vent fluids and cold seawater. Previous work has documented the evolution in mineralogy and geochemistry associated with the aging of the chimneys as hydrothermal activity wanes. However, little is known about spatial heterogeneities within and among actively venting chimneys. New mineralogical and geochemical data (87Sr/86Sr and stable C, O, and clumped isotope) indicate that brucite and calcite precipitate at elevated temperatures in vent fluid-dominated domains in the interior of chimneys. Exterior zones dominated by seawater are brucite-poor and aragonite is the main carbonate mineral. Carbonates form mostly out of oxygen and clumped isotope equilibrium due to rapid precipitation upon vent fluid-seawater mixing. In contrast, the carbonates precipitate close to carbon isotope equilibrium, with dissolved inorganic carbon in seawater as the dominant carbon source, and have δ13C values within the range of marine carbonates. Our data suggest that calcite is a primary mineral in the active hydrothermal chimneys and does not exclusively form as a replacement of aragonite during later alteration with seawater. Elevated formation temperatures and lower 87Sr/86Sr relative to aragonite in the same sample suggest that calcite may be the first carbonate mineral to precipitate.

Hydrogen is an important energy source for subsurface microbial communities, but its availability beyond the flow focused through hydrothermal chimneys is largely unknown. We report the widespread export of H2 across the Atlantis Massif oceanic core complex (30°N, Mid-Atlantic Ridge; up to 44 nM), which is distinct from the circulation system feeding the Lost City Hydrothermal Field (LCHF) on the massif’s southern wall. Methane (CH4) abundances are generally low to undetectable (< 3 nM) in fluids that are not derived from the LCHF. Reducing fluids exit the seafloor over a wide geographical area and depth range, including the summit of the massif and along steep areas of mass wasting east of the field. The depth of the fluids in the water column and their H22/CH4 ratios indicate that some are sourced separately from the LCHF. We argue that extensive H2 export is the natural consequence of fluid flow pathways strongly influenced by tectonic features and the volume and density changes that occur when ultramafic rocks react to form serpentinites, producing H2 as a by-product. Furthermore, the circulation of H2-rich fluids through uplifted mantle rocks at moderate temperatures provides geographically expansive and stable environmental conditions for the early evolution of biochemical pathways. These results provide insight into the spatial extent of H2- and CH4-bearing fluids associated with serpentinization, independent of the focused flow emanating from the LCHF., Geology, 49 (7), ISSN:0091-7613, ISSN:1943-2682

Carbon isotope exchange between carbon-bearing high temperature phases records the carbon (re-) processing in the Earth's interior, where the vast majority of global carbon is stored. Redox reactions between carbonate phases and elemental carbon govern the mobility of carbon, which then can be traced by its isotopes. We determined the carbon isotope fractionation factor between graphite and a Na2CO3-CaCO3 melt at 900–1500 °C and 1 GPa; The failure to isotopically equilibrate preexisting graphite led us to synthesize graphite anew from organic material during the melting of the carbonate mixture. Graphite growth proceeds by (1) decomposition of organic material into globular amorphous carbon, (2) restructuring into nano-crystalline graphite, and (3) recrystallization into hexagonal graphite flakes. Each transition is accompanied by carbon isotope exchange with the carbonate melt. High-temperature (1200–1500 °C) equilibrium isotope fractionation with type (3) graphite can be described by Δ 13 C c a r b o n a t e - g r a p h i t e = 3.17 ( ± 0.07 ) · 10 6 T 2 (temperature T in K). As the experiments do not yield equilibrated bulk graphite at lower temperatures, we combined the ≥1200 °C experimental data with those derived from upper amphibolite and lower granulite facies carbonate-graphite pairs (Kitchen and Valley, 1995; Valley and O'Neil, 1981). This yields the general fractionation function Δ 13 C c a r b o n a t e - g r a p h i t e = 3.37 ± 0.04 · 10 6 T 2 usable as a geothermometer for solid or liquid carbonate at ≥600 °C. Similar to previous observations, lower-temperature experiments (≤1100 °C) deviate from equilibrium. By comparing our results to diffusion and growth rates in graphite, we show that at ≤1100 °C carbon diffusion is slower than graphite growth, hence equilibrium surface isotope effects govern isotope fractionation between graphite and carbonate melt and determine the isotopic composition of newly formed graphite. The competition between diffusive isotope exchange and growth rates requires a more careful interpretation of isotope zoning in graphite and diamond. Based on graphite crystallization rates and bulk isotope equilibration, a minimum diffusivity of Dgraphite = 2 × 10−17 m2s−1 for T > 1150 °C is required. This value is significantly higher than calculated from experimental carbon self-diffusion constants (∼1.6 × 10−29 m2 s−1) but in good agreement with the value calculated for mono-vacancy migration (∼2.8 × 10−16 m2 s−1).

Deep-sea hydrothermal plumes are of major importance in the biogeochemical ocean cycles and in this study we focus on plumes emitted from the Jan Mayen vent fields in the Norwegian-Greenland Sea. These vent fields are of interest because of the high CO2 concentrations and also due to the different styles of venting occurring here. Venting at these sites occurs between 550 and 700 m depth and is characterized by the release of hydrate coated bubbles as well as focused flow venting. This study aims to enhance our current understanding of the impact of CO2 rich hydrate coated bubbles on the water column as well as the interaction between hydrothermally derived gases and metals in the water column. Three water column surveys were conducted in this area in between 2012 and 2014, in which the non-buoyant plume (NBP) produced by focused flow venting from both the Troll Wall and the Perle & Bruse vent sites was identified by primordial helium (3He), methane (CH4), carbon dioxide (CO2) and dissolved manganese (Mn) enrichments close to 500 m water depth. Our results show that venting of hydrate coated CO2 rich bubbles increases bubble rise height, which results in shallow acidification locally above the vent sites. A polymetallic anomaly in the mid-depth water column above the NBP is also hypothesized to be a result of the hydrate coated bubbles. We argue that nanoparticles get sequestered to the hydrate lattice and travel with the bubbles until the lattice becomes unstable due to gas expansion upon depressurization during ascent. This process could fuel the primary production in the pelagic water column.

The majority of undiscovered hydrothermal vent fields are thought to be located on slow and ultraslow spreading ridges. Locating new vent sites on these ridges is important due to their tendency to host large seafloor massive sulfide deposits. It is also important as it can increase our understanding of the tectonics at ultraslow spreading ridges as well as giving us an understanding of how hydrothermal venting in these regions affect and interact with the surrounding water column. To assess the hydrothermal productivity of the slow to ultraslow spreading Mohns and Knipovich ridge segments of the Arctic Mid-Ocean Ridge we used the primordial isotope 3He. Due to the conservative nature of 3He, this isotope can function as a tracer both for hydrothermal activity and on the ocean current pathways. In addition to assessing the hydrothermal productivity of the ridges we also studied the impact the ridges have on ocean circulation. Between 2006 and 2016, 400 water samples were collected along the Mohns Ridge and the Knipovich Ridge and 117 samples were collected in two transects crossing each ridge. The results show that the surface mixed layer (500–0 m depth) directly above each ridge has higher than equilibrium values of δ3He, which we interpret as an effect of the bottom topography on the vertical mixing. We also observe direct indications of at least two undiscovered vent sites along the Mohns Ridge, making the total count of 7 vent fields along this 550 km ridge segment, which gives a vent field frequency (Fs) of 1.27 sites/100 km. This number is comparable to the vent field frequency on other ultraslow ridges. By comparing the current speed to non-buoyant plume measurements obtained at the previously discovered Loki's Castle and the Jan Mayen vent fields (JMVF) we made an estimate of the 3He flux at these vent fields. This estimate gave a flux of 0.22 mmol/km/mm/yr from Loki's Castle, which is well below the world's average of 0.33 mmol/km/mm/yr. However, the flux rate estimated here is comparable to estimates previously calculated from the ultraslow Gakkel ridge. The northeastern termination of the Mohns Ridge, where Loki's Castle is located, is also the thinnest part of the ridge segment indicating a low magmatic influence. The flux rate of the JMVF was 0.40 mmol/km/mm/yr and thus higher than the global average. This could indicate an influence from the Jan Mayen hot spot as similar flux rates has been found at other hot spot influenced vent sites. This study not only illuminates the hydrothermal productivity of ultraslow spreading ridges, but also adds a considerable dataset of helium isotopes in Nordic Seas.

Carbon isotope fractionation in the CO2–CO–CH4–C system was investigated at 300–1200 °C at near-atmospheric pressures by thermally decomposing a variety of organic materials in sealed quartz tubes. Measured gas speciations correspond well to the expected range from thermodynamic calculations. We show that chemical and isotopic equilibrium among gas species is obtained when applying a nickel catalyst for CO2/CH4, CH4/CO, and CO2/CO at ≤600 °C or without a catalyzing agent for CO2/CO at ≥800 °C. The experiments define carbon isotope fractionation factors for the CO2/CH4, CO2/CO and CH4/CO pairs as (i) 10 3 ln ⁡ α CO 2 / CH 4 = 8.9 ( ± 0.6 ) ⋅ 10 5 ⋅ ( 1 T 2 ) 0.825 ( ± 0.005 ) (200–1200 °C) (ii) 10 3 ln ⁡ α CO 2 / CO = 1.07 ( ± 0.05 ) ⋅ 10 6 ⋅ ( 1 T 2 ) 0.830 ( ± 0.003 ) (300–1200 °C) (iii) 10 3 ln ⁡ α CH 4 / CO = 1.1 ( ± 0.2 ) ⋅ 10 3 ⋅ ( 1 T 2 ) 0.462 ( ± 0.001 ) (300–1200 °C), which reproduce the experimental values within 0.2‰ for CO2/CH4 and CO2/CO and within 0.12‰ for CH4/CO (T in K, 1σ fit uncertainties in brackets, CO2/CH4 includes the ≤600 °C experimental data of Horita, 2001 ). Carbon isotope fractionation factors at 1000 °C are still large for CO2/CH4 and CO2/CO (6.6 and 7.5‰, respectively) but only 1.5‰ for CH4/CO. Elemental carbon precipitated through thermal decomposition of the organic starting materials yields δ 13 C values that depend on the X(O) = O/(O + H) of the organic starting material, i.e. the initial oxidation state of carbon in the organics. We further observe a catalytic effect of the quartz walls on chemical and isotopic exchange in the CO2/CO system, probably due to the activation of the silicate surface by H+ and OH− ions at >650 °C. Our experimental results yield improved calibrations of the CO2/CH4 equilibria and the first experimental calibration of CO2/CO and CH4/CO carbon isotope fractionation. Applications are in the tracing of magmatic hydrothermal gas emissions, in carbon-precipitating COH-fluids, and in monitoring of coal-seam fires, but our results may also be applied for quality control during steel-making processes.

Deep-sea hydrothermal fluids are often enriched in carbon dioxide, methane, and hydrogen. Methane effuses from metal-rich black smokers such as the Rainbow hydrothermal field, at temperatures higher than 200 °C. At the Lost City field, CH4 emanates from alkaline fluids at < 100 °C. The abundance of the rare, mass-18 CH4 isotopologues, 13CH3D and 12CH2D2, can mitigate degeneracies in the conventional isotopic signatures of methane. We studied the isotopologue compositions of methane from the Rainbow, Lucky Strike, Von Damm, and Lost City hydrothermal fields. At Rainbow, where the vented fluids are at ∼360°C, our coupled Δ12CH2D2 - Δ13CH3D data establish that methane is in internal equilibrium at °C. This may track the formation temperature of abiotic methane, or it may be the result of equilibration of methane isotopologues within the carrier fluid. Lucky Strike and Von Damm have fluid temperatures < 300°C and although Δ13CH3D values are indistinguishable from those at Rainbow, 12CH2D2 abundances are marginally higher. At Lost City, Δ13CH3D data show a range of values, which at face value correspond to apparent temperatures of between °C and °C, far hotter than fluid temperatures. A unique aspect of the Lost City data is the range of large 12CH2D2 excesses. The Δ12CH2D2 data correspond to temperatures of between °C and °C, showing a near-perfect match with fluid temperatures. We find that mixing scenarios involving microbial methane may not account for all of the isotope data. We suggest that Δ12CH2D2 values, unlike Δ13CH3D values, are prone to near-complete re-equilibration at host fluid temperatures. We suggest that 13CH3D isotopologue data are consistent with abiotic methane being synthesized at ∼350 °C. On the other hand, 12CH2D2 isotopologue ordering records post formation residence temperatures. We explore a possible mechanism decoupling the re-equilibration systematics of the doubly-substituted isotopologues.

We document the discovery of an active, shallow, seafloor hydrothermal system (known as the Seven Sisters Vent Field) hosted in mafic volcaniclasts at a mid-ocean ridge setting. The vent field is located at the southern part of the Arctic mid-ocean ridge where it lies on top of a flat-topped volcano at ~130 m depth. Up to 200 &deg, C phase-separating fluids vent from summit depressions in the volcano, and from pinnacle-like edifices on top of large hydrothermal mounds. The hydrothermal mineralization at Seven Sisters manifests as a replacement of mafic volcaniclasts, as direct intraclast precipitation from the hydrothermal fluid, and as elemental sulfur deposition within orifices. Barite is ubiquitous, and is sequentially replaced by pyrite, which is the first sulfide to form, followed by Zn-Cu-Pb-Ag bearing sulfides, sulfosalts, and silica. The mineralized rocks at Seven Sisters contain highly anomalous concentrations of &lsquo, epithermal suite&rsquo, elements such as Tl, As, Sb and Hg, with secondary alteration assemblages including silica and dickite. Vent fluids have a pH of ~5 and are Ba and metal depleted. Relatively high dissolved Si (~7.6 mmol/L Si) combined with low (0.2&ndash, 0.4) Fe/Mn suggest high-temperature reactions at ~150 bar. A 13C value of &minus, 5.4&permil, in CO2 dominated fluids denotes magmatic degassing from a relatively undegassed reservoir. Furthermore, low CH4 and H2 (&lt, 0.026 mmol/kg and &lt, 0.009 mmol/kg, respectively) and 3He/4He of ~8.3 R/Racorr support a MORB-like, sediment-free fluid signature from an upper mantle source. Sulfide and secondary alteration mineralogy, fluid and gas chemistry, as well as 34S and 87Sr/86Sr values in barite and pyrite indicate that mineralization at Seven Sisters is sustained by the input of magmatic fluids with minimal seawater contribution. 226Ra/Ba radiometric dating of the barite suggests that this hydrothermal system has been active for at least 4670 &plusmn, 60 yr.

At high temperatures, isotope partitioning is often assumed to proceed under equilibrium and trends in the carbon isotope composition within graphite and diamond are used to deduce the redox state of their fluid source. However, kinetic isotope fractionation modifies fluid- or melt-precipitated mineral compositions when growth rates exceed rates of diffusive mixing. As carbon self-diffusion in graphite and diamond is exceptionally slow, this fractionation should be preserved. We have hence performed time series experiments that precipitate graphitic carbon through progressive oxidization of an initially CH4-dominated fluid. Stearic acid was thermally decomposed at 800 °C and 2 kbar, yielding a reduced COH-fluid together with elemental carbon. Progressive hydrogen loss from the capsule caused CH4 to dissociate with time and elemental carbon to continuously precipitate. The newly formed C0, aggregating in globules, is constantly depleted by -6.5±0.3‰ in 13C relative to the methane, which defines a temperature dependent kinetic graphite-methane 13C/12C fractionation factor. Equilibrium fractionation would instead yield graphite heavier than the methane. In dynamic environments, kinetic isotope fractionation may control the carbon isotope composition of graphite or diamond, and, extended to nitrogen, could explain the positive correlation of δ13C and δ15N sometimes observed in coherent diamond growth zones. 13C enrichment trends in diamonds are then consistent with reduced deep fluids oxidizing upon their rise into the subcontinental lithosphere, methane constituting the main source of carbon., Earth and Planetary Science Letters, 529, ISSN:0012-821X, ISSN:1385-013X

The Longqi hydrothermal field at 49.6°E on the Southwest Indian Ridge was the first active hydrothermal field found at a bare-rock ultra-slow spreading mid-ocean ridge. Here we report the chemistry of the hydrothermal fluids, for the first time, that were collected from the S zone and the M zone of the Longqi field by gas-tight isobaric samplers by the HOV “Jiaolong” diving cruise in January 2015. According to H 2 , CH 4 and other chemical data of the vent fluid, we suggest that the basement rock at the Longqi field is dominantly mafic. This is consistent with the observation that the host rock of the active Longqi Hydrothermal field is dominated by extensively distributed basaltic rock. It was very interesting to detect simultaneously discharging brine and vapor caused by phase separation at vents DFF6, DFF20, and DFF5 respectively, in a distance of about 400 m. Based on the end-member fluid chemistry and distance between the vents, we propose that there is a single fluid source at the Longqi field. The fluid branches while rising to the seafloor, and two of the branches reach S zone and M zone and phase separate at similar conditions of about 28–30.2 MPa and 400.6–408.3 °C before they discharge from the vents. The end-member fluid compositions of these vents are comparable with or within the range of variation of known global seafloor hydrothermal fluid chemical data from fast, intermediate and slow spreading ridges, which confirms that the spreading rate is not the key factor that directly controls hydrothermal fluid chemistry. The composition of basement rock, water-rock interaction and phase separation are the major factors that control the composition of the vent fluids in the Longqi field.

Tidal oscillations of venting temperature and chlorinity have been observed in the long-term time series data recorded by the Benthic and Resistivity Sensors (BARS) at the Grotto mound on the Juan de Fuca Ridge. In this study, we use a one-dimensional two-layer poroelastic model to conduct a preliminary investigation of three hypothetical scenarios in which seafloor tidal loading can modulate the venting temperature and chlorinity at Grotto through the mechanisms of subsurface tidal mixing and/or subsurface tidal pumping. For the first scenario, our results demonstrate that it is unlikely for subsurface tidal mixing to cause coupled tidal oscillations in venting temperature and chlorinity of the observed amplitudes. For the second scenario, the model results suggest that it is plausible that the tidal oscillations in venting temperature and chlorinity are decoupled with the former caused by subsurface tidal pumping and the latter caused by subsurface tidal mixing, although the mixing depth is not well constrained. For the third scenario, our results suggest that it is plausible for subsurface tidal pumping to cause coupled tidal oscillations in venting temperature and chlorinity. In this case, the observed tidal phase lag between venting temperature and chlorinity is close to the poroelastic model prediction if brine storage occurs throughout the upflow zone under the premise that layers 2A and 2B have similar crustal permeabilities. However, the predicted phase lag is poorly constrained if brine storage is limited to layer 2B as would be expected when its crustal permeability is much smaller than that of layer 2A.

Hydrothermal fluids (341°C and 19°C) were collected

The degassing of primordial gases from Earth's interior is evidenced by the high He 3 / 4 He ratios in submarine hydrothermal plumes, vent fluids, and rock samples with mantle origin from active hydrothermal systems in mid-ocean ridges (MOR) and subduction zone volcanism. As the largest aquifer on Earth, the uppermost 40-500 m of permeable submarine ridge flank basement (1-65 million-years-old, Myr) holds ∼2% of the ocean volume and accounts for 70% of the seafloor hydrothermal heat flux. However, the degassing of primordial gases through the oceanic ridge flank crust has not yet been directly quantified. Here, we show that high integrity hydrothermal (65 °C) fluids from the sediment-buried 3.5 Myr basaltic crust from the eastern flank of the Juan de Fuca Ridge (JdFR) contain elevated 3He. The He 3 / 4 He for the basaltic fluid is 4.5 ± 0.1 R a (relative to the air ratio), which is greatly elevated when compared to deep seawater (1.05 R a ), but is half of that observed for high-temperature vent fluids (∼8 R a ) emitting from MOR. Only a small fraction of the 3He in ridge flank fluids is derived from the entrainment of high-temperature ridge-axis fluids and is better explained by degassing of the mantle through the mantle-crust boundary. The lower than MOR He 3 / 4 He ratios indicate that radiogenic 4He originates from aged uranium and thorium decay within the mantle as well as from the ridge-flank basalts. The 3He outgassing through warm ridge flanks (4.9 to 36 mol/yr) accounts for 0.7-6% of the global 3He outgassing, exceeded only by degassing through mid-ocean ridges and subduction volcanism. The presence of mantle 3He suggests that the abiogenic methane present in the ridge flank fluids might be mantle-derived. Based on the 3He outgassing flux, a possibly mantle-derived abiotic methane production rate at the ridge flank is estimated to be 0.3 – 35 × 10 8 mol / yr .

IODP Expedition 357 utilized seabed drills for the first time in the history of the ocean drilling program, with the aim of collecting intact sequences of shallow mantle core from the Atlantis Massif to examine serpentinization processes and the deep biosphere. This novel drilling approach required the development of a new remote seafloor system for delivering synthetic tracers during drilling to assess for possible sample contamination. Here, we describe this new tracer delivery system, assess the performance of the system during the expedition, provide an overview of the quality of the core samples collected for deep biosphere investigations based on tracer concentrations, and make recommendations for future applications of the system., Scientific Drilling, 23

Chemical input to the deep sea from hydrothermal systems is a globally distributed phenomenon. Hydrothermal discharge is one of the primary mechanisms by which the Earth’s interior processes manifest themselves at the Earth’s surface, and it provides a source of energy for autotrophic processes by microbes that are too deep to capitalize on sunlight. Much is known about the water-column signature of this discharge from high-temperature mid-ocean Ridge (MOR) environments and their neighboring low-temperature counterparts. Hydrothermal discharge farther away from the ridge, however, has garnered less attention, owing in part to the difficulty in finding this style of venting, which eludes methods of detection that work well for high-temperature ‘black smoker’-type venting. Here we present a case study of the plume from one such ‘invisible’ off-axis environment, The Lost City, with an emphasis on the dissolved volatile content of the hydrothermal plume. Serpentinization and abiotic organic synthesis generate significant concentrations of H2 and CH4 in vent fluid, but these species are unevenly transported to the overlying plume, which itself appears to be a composite of two different sources. A concentrated vent cluster on the talus slope channels fluid through at least eight chimneys, producing a water-column plume with the highest observed concentrations of CH4 in the field. In contrast, a saddle in the topography leading up to a carbonate cap hosts broadly distributed, nearly invisible venting apparent only in its water-column signals of redox potential and dissolved gas content, including the highest observed plume H2. After normalizing H2 and CH4 to the 3He background-corrected anomaly (3HeΔ) to account for mixing and relative amount of mantle input, it appears that, while a minimum of 60% of CH4 is transported out of the system, greater than 90% of the H2 is consumed in the subsurface prior to venting. The exception to this pattern occurs in the plume originating from the area dubbed Chaff Beach, in which somewhat more than 10% of the original H2 remains, indicating that this otherwise unremarkable plume, and others like it, may represent a significant source of H2 to the deep sea.

A robust, simple and efficient gas chromatography (GC) method is presented for determining H2 and CH4 concentrations simultaneously in water samples. The GC system consists of one detector (pulsed discharge helium ionization detector—PDHID), two columns in two ovens, respectively, and two 2-position-6-port purge valves. In this method, H2 and CH4 in water samples were first extracted into the headspace of syringes, then injected into the GC system. Both H2 and CH4 were detected by a PDHID. The separation of CH4 from O2 and N2 in seawater is achieved by switching the O2 and N2 out of the carrier gas flow at the appropriate time window. H2 and CH4 concentrations in water samples collected from hydrothermal plumes at the Southwest Indian Ridge were determined successfully using this method.

Subsurface fluid flow in oceans and lakes affects bathymetric morphology, sediment distribution, and water composition. We present newly discovered giant lacustrine pockmarks in Lake Neuchatel (up to 160m diameter and 30m deep) that rank among the largest known pockmarks in lakes. Our multidisciplinary study reveals 60m of suspended sediment inside a pockmark. The sediment suspension is 2.6 degrees warmer and isotopically lighter in O-18(H2O) by 1.5 than the ambient lake water, documenting currently active fluid flow by karstic groundwater discharge from the Jura Mountain front into the Swiss Plateau hydrological system. Strikingly, the levees of the pockmarks comprise subsurface sediment mobilization deposits representing episodic phases of sediment expulsion during the past. They strongly resemble subsurface fluid flow features in the marine realm. Comparable processes are expected to also be relevant for other carbonate-dominated mountain front ranges, where karstic groundwater discharges into lacustrine or marine settings.

The northern Lau Basin hosts a complicated pattern of volcanism, including Tofua Arc volcanoes, several back-arc spreading centers, and individual “rear-arc” volcanoes not associated with these structures. Elevated 3He/4He ratios in lavas of the NW Lau Spreading Center suggest the influence of a mantle plume, possibly from Samoa. We show that lavas from mid-ocean ridges, volcanic arcs, and hotspots occupy distinct, nonoverlapping fields in a 3He/4He versus C/3He plot. Applied to the northern Lau Basin, this approach shows that most of Lau back-arc spreading systems have mid-ocean ridge 3He/4He-C/3He characteristics, except the NW Lau spreading center, which has 3He/4He-C/3He similar to “high 3He” hotspots such as Loihi, Kilauea, and Yellowstone, but with slightly lower C/3He. Niua seamount, on the northern extension of the Tofua Arc, falls squarely in the arc field. All the NE Lau rear-arc volcanoes, including the recently erupting West Mata, also have arc-like 3He/4He-C/3He characteristics. Ba-Nb-Ti contents of the lavas, which are more traditional trace element indicators of mantle source enrichment, depletion, and subduction input, likewise indicate arc and hot spot influences in the lavas of the northern Lau Basin, but in a more ambiguous fashion because of a complex prior history. This verifies that 3He/4He-C/3He systematics are useful for differentiating between mid-ocean ridge, arc, and hotspot affinities in submarine volcanic systems, that all three of these affinities are expressed in the northern Lau Basin, and provides additional support for the Samoan plume influence in the region.

The oceanic basaltic crust is the largest aquifer on Earth and has the potential to harbor substantial subsurface microbial ecosystems, which hitherto remains largely uncharacterized and is analogous to extraterrestrial subsurface habitats. Within the sediment-buried 3.5 Myr old basaltic crust of the eastern Juan de Fuca Ridge flank, the circulating basement fluids have moderate temperature (∼65 °C) and low to undetectable dissolved oxygen and nitrate concentrations. Sulfate, present in high concentrations, is therefore expected to serve as the major electron acceptor in this subsurface environment. This study focused on the availability and potential sources of two important electron donors, methane (CH4) and hydrogen (H2), for the subseafloor biosphere. High integrity basement fluids were collected via fluid delivery lines associated with Integrated Ocean Drilling Program (IODP) Circulation Obviation Retrofit Kits (CORKs) that extend from basement depths to outlet ports at the seafloor. Two new CORKs installed during IODP 327 in 2010, 1362A and 1362B, were sampled in 2011 and 2013. The two CORKs are superior than earlier style CORKs in that they are equipped with coated casing and polytetrafluoroethylene fluid delivery lines, reducing the interaction between casing materials with the environment. Additional samples were collected from an earlier style CORK at Borehole 1301A. The basement fluids are enriched in H2 (0.05–1.8 μmol/kg), suggesting that the ocean basaltic aquifer can support H2-driven metabolism. The basement fluids also contain significant amount of CH4 (5–32 μmol/kg), revealing CH4 as an available substrate for subseafloor basaltic habitats. The δ13C values of CH4 from the three boreholes ranged from −22.5 to − 58 ‰ , while the δ2H values ranged from −316 to 57‰. The isotopic compositions of CH4 and the molecular compositions of hydrocarbons suggest that CH4 in the basement fluids is of both biogenic and abiotic origins, varying among sites and sampling times. The δ2H values of CH4 in CORK 1301A fluid samples are much more positive than found in all other marine environments investigated to date and are best explained by the partial microbial oxidation of biogenic CH4. In conclusion, our study shows that CH4 and H2 are persistently available to fuel the deep biosphere and that CH4 is both produced and potentially consumed by microorganisms in the oceanic basement.

We present multiple lines of evidence for years to decade-long changes in the location and character of volcanic activity at West Mata seamount in the NE Lau basin over a 16 year period, and a hiatus in summit eruptions from early 2011 to at least September 2012. Boninite lava and pyroclasts were observed erupting from its summit in 2009, and hydroacoustic data from a succession of hydrophones moored nearby show near-continuous eruptive activity from January 2009 to early 2011. Successive differencing of seven multibeam bathymetric surveys of the volcano made in the 1996–2012 period reveals a pattern of extended constructional volcanism on the summit and northwest flank punctuated by eruptions along the volcano's WSW rift zone (WSWRZ). Away from the summit, the volumetrically largest eruption during the observational period occurred between May 2010 and November 2011 at ∼2920 m depth near the base of the WSWRZ. The (nearly) equally long ENE rift zone did not experience any volcanic activity during the 1996–2012 period. The cessation of summit volcanism recorded on the moored hydrophone was accompanied or followed by the formation of a small summit crater and a landslide on the eastern flank. Water column sensors, analysis of gas samples in the overlying hydrothermal plume and dives with a remotely operated vehicle in September 2012 confirmed that the summit eruption had ceased. Based on the historical eruption rates calculated using the bathymetric differencing technique, the volcano could be as young as several thousand years.

Four cruises between 2008 and 2012 monitored the continuing eruption of West Mata volcano in the NE Lau Basin as it produced plumes of chemically altered water above its summit. Although large enrichments in 3He, CO2, Fe, and Mn were observed in the plumes, the most notable enrichment was that of H2, which reached concentrations as high as 14,843 nM. Strongly enriched H2 concentrations in the water column result from reactions between seawater or magmatic water and extremely hot rocks. In 2008, the observation of elevated H2 concentrations in the water column above West Mata pointed to vigorous ongoing eruptions at the volcano's summit. The eruption was confirmed by visual observations made by the ROV Jason 2 in 2009 and demonstrated that H2 measurements are a vital instrument to detect ongoing volcanic eruptions at the seafloor. Elevated H2 in 2010 showed that the eruption was ongoing, although at a reduced level given a maximum H2 concentration of 4410 nM. In 2012, H2 levels in the water column declined significantly, to a maximum of only 7 nM, consistent with visual observations from the Quest-4000 ROV that found no evidence of an ongoing volcanic eruption. Methane behaved independently of other measured gases and its concentrations in the hydrothermal plume were very low. We attribute its minimal enrichments to a mixture of mantle carbon reduced to CH4 and biological CH4 from diffuse flow sites. This study demonstrates that ongoing submarine volcanic eruptions are characterized by high dissolved H2 concentrations present in the overlying water column.

Discharge of volcanic gases in the marine environment can lead to local perturbations in ocean acidity, with consequences for biological communities and the potential for hazards related to depressurization and release of gases at the surface. Numerous hydrothermal vents in the crater of Kolumbo submarine volcano (Aegean Sea) are discharging virtually pure gaseous CO 2 together with clear fluids at temperatures up to 220 °C. Acoustic imaging of the ascending bubbles suggests that the gas is being dissolved into seawater within ∼10 m above the crater floor (500 m below sea level). Dissolution of the gas likely causes local increases in water density that result in sequestration of CO 2 within the enclosed crater, and the accumulation of acidic seawater. Lack of macrofauna at the Kolumbo hydrothermal vents, occurrence of carbonate-poor sediment in the crater, and pH values as low as 5.0 in recovered water samples point to acidic conditions within the crater. Buildup of CO 2 -rich water in the bowl-shaped crater of Kolumbo may be producing conditions analogous to some African volcanic lakes (Lake Monoun and Lake Nyos, Cameroon) where overturn of gas-rich bottom waters led to abrupt releases of CO 2 at the surface. A minimum estimate of 2.0 × 10 5 m 3 (STP) of excess CO 2 may currently exist in the bottom 10 m of the Kolumbo crater.

Active serpentinization systems are abundant on present-day Earth and are of increasing interest because water–rock reactions lead to alkaline, Ca–OH fluids that have the potential to sequester CO 2 and form reduced chemical species (e.g. CH 4 and H 2 ) that can support chemolithoautotrophy. We present a study of alkaline (pH 10–12) springs in the Voltri Massif (Italy) that focuses on the sources and cycling of inorganic carbon in the basement rocks, the interacting fluids, and the resulting surface carbonate deposits. Most springs are located in mantle rocks that underwent varying degrees of ocean-floor serpentinization and incorporation of carbon (as carbonate and organic carbon) in the Jurassic, which is preserved during subsequent subduction and uplift onto the continent. The springs are fed by meteoric water that evolves into alkaline, Ca-rich spring waters that have 2–3 times higher Ca concentrations than the adjacent rivers. Our study is consistent with previous reaction path modeling and identifies the formation of clay minerals in serpentinites that have been altered by alkaline fluids. These strongly altered basement rocks contain up to 2 wt.% C in the shallow subsurface, also documented by the presence of late calcite veins. Concentrations of dissolved inorganic carbon (DIC) of the alkaline fluids are generally low ( 13 C DIC is between − 24.2‰ and + 1.3‰. We argue that the concentrations and isotopic composition of the DIC in the alkaline waters provide evidence for 1) the precipitation of calcium carbonate under closed-system conditions with respect to atmospheric CO 2 and 2) removal of DIC by microbial activity in the subsurface. Late-stage uptake of atmospheric CO 2 in the shallow subsurface or at the exit sites subsequent to water–rock–microbe interactions in the basement result in fluids with lower pH and Ca, and enrichment in DIC and 13 C. At the surface, interaction of the high pH, Ca–OH fluids with atmospheric CO 2 causes precipitation of carbonates as travertines or crusts on the basement rocks, storing CO 2 as calcium carbonate. Carbonate precipitation at the exit sites is strongly dominated by kinetic processes leading to carbon and oxygen isotope compositions as low as − 27.2‰ and − 18.7‰, respectively. With increasing distance from the springs, the 13 C and 18 O content of the carbonate increases and the fluid pH changes towards neutral. Our study shows that surficial carbonate precipitation plays a subordinate role in carbon sequestration, but that the evolution from Mg–HCO 3 spring waters to Ca–OH waters removes significant amounts of carbon, presumably in the subsurface. We calculate that the serpentinites have the capacity to sequester up to 0.50 to 2.05 × 10 9 mol carbon per year and conclude that they can take up significantly more CO 2 than they currently contain.

Mantle rocks exposed on the seafloor constitute a highly reactive chemical and thermal system, in which interaction with seawater to produce serpentinite has major consequences for lithospheric cooling, global geochemical cycles, and microbial activity. Serpentinite-hosted hydrothermal activity is exemplified by the Lost City Hydrothermal Field (30°N, Mid-Atlantic Ridge) where fluid–rock reactions in the underlying ultramafic rocks result in high concentrations of abiotic hydrogen, methane, C 2 + alkanes, and formate . Such systems have been proposed as possible analogs to the Early Earth environments that gave rise to the first biochemical pathways. Thus, characterizing the local microbial communities and their potential link with abiogenic compounds is of particular significance. Here we demonstrate that in active carbonate chimneys where microbial sulfate reduction is important, up to 50% of the microbial biomass is synthesized from mantle carbon. Conversely, mantle carbon contributes only ∼10% of the biomass in areas with minimal sulfate reduction. We attribute this difference to greater incorporation of formate or methane by the dominant microbial species, the Lost City Methanosarcinales , in locations where sulfate reducers are able to facilitate this assimilation. The ability of autotrophic communities at Lost City to capitalize on the steady stream of chemical products resulting from serpentinization reactions and to utilize abiogenic mantle carbon lend credence to the hypothesis that early biosynthetic pathways could have developed in similar environments.

Microbial productivity at hydrothermal vents is among the highest found anywhere in the deep ocean, but constraints on microbial growth and metabolism at vents are lacking. We used a combination of cultivation, molecular, and geochemical tools to verify pure culture H 2 threshold measurements for hyperthermophilic methanogenesis in low-temperature hydrothermal fluids from Axial Volcano and Endeavour Segment in the northeastern Pacific Ocean. Two Methanocaldococcus strains from Axial and Methanocaldococcus jannaschii showed similar Monod growth kinetics when grown in a bioreactor at varying H 2 concentrations. Their H 2 half-saturation value was 66 μM, and growth ceased below 17–23 μM H 2 , 10-fold lower than previously predicted. By comparison, measured H 2 and CH 4 concentrations in fluids suggest that there was generally sufficient H 2 for Methanocaldococcus growth at Axial but not at Endeavour. Fluids from one vent at Axial (Marker 113) had anomalously high CH 4 concentrations and contained various thermal classes of methanogens based on cultivation and mcrA / mrtA analyses. At Endeavour, methanogens were largely undetectable in fluid samples based on cultivation and molecular screens, although abundances of hyperthermophilic heterotrophs were relatively high. Where present, Methanocaldococcus genes were the predominant mcrA / mrtA sequences recovered and comprised ∼0.2–6% of the total archaeal community. Field and coculture data suggest that H 2 limitation may be partly ameliorated by H 2 syntrophy with hyperthermophilic heterotrophs. These data support our estimated H 2 threshold for hyperthermophilic methanogenesis at vents and highlight the need for coupled laboratory and field measurements to constrain microbial distribution and biogeochemical impacts in the deep sea.

The Axial Seamount submarine volcano exhibits an inflation–deflation cycle comparable to similar volcanoes on land. Measurements of ocean bottom pressure document the entire inflation–deflation cycle between eruptions at Axial Seamount in 1998 and 2011, and imply that the timing of submarine eruptions could be more predictable than that of their subaerial counterparts.

The permeable upper oceanic basement serves as a plausible habitat for a variety of microbial communities. There is growing evidence suggesting a substantial subseafloor biosphere. Here new time series data are presented on key inorganic species, methane, hydrogen and dissolved organic carbon (DOC) in ridge flank fluids obtained from subseafloor observatory CORKs (Circulation Obviation Retrofit Kits) at Integrated Ocean Drilling Program (IODP) boreholes 1301A and 1026B. These data show that the new sampling methods ( Cowen et al., 2012 ) employed at 1301A result in lower contamination than earlier studies. Furthermore, sample collection methods permitted most chemical analyses to be performed from aliquots of single large volume samples, thereby allowing more direct comparison of the data. The low phosphate concentrations (0.06–0.2 μM) suggest that relative to carbon and nitrogen, phosphorus could be a limiting nutrient in the basement biosphere. Coexisting sulfate (17–18 mM), hydrogen sulfide (∼0.1 μM), hydrogen (0.3–0.7 μM) and methane (1.5–2 μM) indicates that the basement aquifer at 1301A either draws fluids from multiple flow paths with different redox histories or is a complex environment that is not thermodynamically controlled and may allow co-occurring metabolic pathways including sulfate reduction and methanogenesis. The low DOC concentrations (11–18 μM) confirm that ridge flank basement is a net DOC sink and ultimately a net carbon sink. Based on the net amounts of DOC, oxygen, nitrate and sulfate removed (∼30 μM, ∼80 μM, ∼40 μM and ∼10 mM, respectively) from entrained bottom seawater, organic carbon may be aerobically or anaerobically oxidized in biotic and/or abiotic processes.

The Southwest Indian Ridge (SWIR) is one of the world's slowest spreading ridges with a full spreading rate of ~ 14 mm a− 1. Due to its low thermal budget, high-temperature hydrothermal activity along the SWIR was once considered to be impossible. The Chinese cruise DY115-19 on board R/V Dayang Yihao successfully discovered the first SWIR active hydrothermal field at 37°47′S 49°39′E, located at a magmatically robust spreading segment. Here, the geochemical characteristics of hydrothermal plumes from the hydrothermal field are first reported, and water column anomalies of light transmission, Fe, Mn, Al, both dissolved and particulate, are discussed. The total Fe and dissolved Mn concentrations in the plumes varied from 13.7 to 277.4 nM and 0.47 to 10.41 nM, respectively. The composition of Fe-Mn-Al implied that particles in the plumes were mainly hydrothermal in origin, also included small contributions of resuspended sediments or background particles. Dissolved Fe constituted a considerable fraction of the total Fe, more than 80% in plume samples from station CTD 4. High Fe concentrations might be sustained in the dissolved phase because of the existence of organic complexes and nanoparticles. On board incubation experiments verified the Fe(II) oxidation half-lives for plumes of CTD 4 and CTD 13 were 1.8 and 1.6 h, respectively, which are much longer than the calculated value of ~ 0.5 h based on the deep water pH and oxygen concentration.

The East Pacific Rise from ~ 9–10°N is an archetype for a fast-spreading mid-ocean ridge. In particular, the segment near 9°50'N has been the focus of multidisciplinary research for over two decades, making it one of the best-studied areas of the global ridge system. It is also one of only two sites along the global ridge where two historical volcanic eruptions have been observed. This volcanically active segment has thus offered unparalleled opportunities to investigate a range of complex interactions among magmatic, volcanic, hydrothermal, and biological processes associated with crustal accretion over a full magmatic cycle. At this 9°50'N site, comprehensive physical oceanographic measurements and modeling have also shed light on linkages between hydrodynamic transport of larvae and other materials and biological dynamics influenced by magmatic processes. Integrated results of high-resolution mapping, and both in situ and laboratory-based geophysical, oceanographic, geochemical, and biological observations and sampling, reveal how magmatic events perturb the hydrothermal system and the biological communities it hosts.

Endeavour Segment of the Juan de Fuca Ridge is one of three Integrated Study Sites for the Ridge 2000 Program. It is a remarkable, dynamic environment hosting five major hydrothermal fields, numerous smaller fields, and myriad diffuse-flow sites; magma chambers underlie all fields. Over 800 individual extinct and active chimneys have been documented within the central ~ 15 km portion of the ridge, with some edifices reaching 50 m across and up to 45 m tall. Fluid flow is focused along faults within the rift zone, and seismically active faults along the western axial valley wall have been used by both magmas and upwelling hydrothermal fluids. There is significant chemical heterogeneity in basalt compositions within the axial rift valley, with the greatest diversity occurring near the base of the western axial valley wall where normal, transitional, and enriched type mid-ocean ridge basalts occur within tens of meters of each other. Endeavour is the only site where seismic intensity has been linked directly to heat flux at the individual vent field scale. Installation of the world's first high-power and high-bandwidth cabled observatory at Endeavour via NEPTUNE Canada ensures that new discoveries along the Juan de Fuca Ridge will continue into the future.

Strombolian-style volcanic activity has persisted for six years at the NW Rota-1 submarine volcano in the southern Mariana Arc, allowing direct observation and sampling of gas-rich fluids produced by actively degassing lavas, and permitting study of the magma-hydrothermal transition zone. Fluids sampled centimeters above erupting lava and percolating through volcaniclastic sediments around an active vent have dissolved sulfite >100 mmol/kg, total dissolved sulfide 1 mmol/kg. If NW Rota is representative of submarine arc eruptions, then volcanic vent fluids from seawater-lava interaction on submarine arcs have a significant impact on the global hydrothermal flux of sulfur and Al to the oceans, but a minimal impact on Mg removal. Gas ratios (SO 2 , CO 2 , H 2 , and He) are variable on small spatial and temporal scales, indicative of solubility fractionation and gas scrubbing. Elemental sulfur (S e ) is abundant in solid and molten form, produced primarily by disproportionation of magmatic SO 2 injected into seawater. S e accumulates within the porous rock surrounding the lava conduit connecting the magma source to the seafloor. Accumulated S e can be heated, melted, and pushed upward by rising magma to produce molten S e flows and lavas saturated with S e . Molten S e near the top of the lava conduit may be ejected up into the water column by escaping gases or boiling water. This mechanism of S e accumulation and refluxing may underlie the relatively widespread occurrence of S e deposits of many sizes found on submarine arc volcanoes.

Hydrothermal vents along mid-ocean systems host unique, highly productive biological communities, based on microbial chemoautotrophy, that thrive on the sulphur, metals, nitrogen and carbon emitted from the vents into the deep ocean. Geochemical studies of vents have centred on analyses of high-temperature, focused hydrothermal vents, which exhibit very high flow rates and are generally considered too hot for microbial life. Geochemical fluxes and metabolic activity associated with habitable, lower temperature diffuse fluids remain poorly constrained. As a result, little is known about the extent to which microbial communities, particularly in the subsurface, influence geochemical flux from more diffuse flows. Here, we estimate the net flux of methane, carbon dioxide and hydrogen from diffuse and focused hydrothermal vents along the Juan de Fuca ridge, using an in situ mass spectrometer and flowmeter. We show that geochemical flux from diffuse vents can equal or exceed that emanating from hot, focused vents. Notably, hydrogen concentrations in fluids emerging from diffuse vents are 50% to 80% lower than predicted. We attribute the loss of hydrogen in diffuse vent fluids to microbial consumption in the subsurface, and suggest that subsurface microbial communities can significantly influence hydrothermal geochemical fluxes to the deep ocean.

Fluids from the ultramafic-hosted Lost City hydrothermal field were analyzed for total dissolved organic carbon and dissolved organic acids. Formate (36–158 μmol/kg) and acetate (1–35 μmol/kg) concentrations are higher than in other fluids from unsedimented hydrothermal vents, and are a higher ratio of the total dissolved organic carbon than has been found in most marine geothermal systems. Isotopic evidence is consistent with an abiotic formation mechanism for formate, perhaps during serpentinization processes in the sub-surface. Further support comes from previous studies where the abiological formation of low molecular weight organic acids has been shown to be thermodynamically favorable during hydrothermal alteration of olivine, and laboratory studies in which the reduction of carbon dioxide to formate has been confirmed. As the second most prevalent carbon species after methane, formate may be an important substrate to microbial communities in an environment where dissolved inorganic carbon is limited. Acetate is found in locations where sulfate reduction is believed to be important and is likely to be a microbial by-product, formed either directly by autotrophic metabolic activity or indirectly during the fermentative degradation of larger organic molecules. Given the common occurrence of exposed ultramafic rocks and active serpentinization within the worlds ocean basins, the abiotic formation of formate may be an important process supporting life in these high pH environments and may have critical implications to understanding the organic precursors from which life evolved.

A unique dataset from paired low- and high-temperature vents at 9°50′N East Pacific Rise provides insight into the microbiological activity in low-temperature diffuse fluids. The stable carbon isotopic composition of CH4 and CO2 in 9°50′N hydrothermal fluids indicates microbial methane production, perhaps coupled with microbial methane consumption. Diffuse fluids are depleted in 13C by ∼10‰ in values of δ13C of CH4, and by ∼0.55‰ in values of δ13C of CO2, relative to the values of the high-temperature source fluid (δ13C of CH4 =−20.1 ± 1.2‰, δ13C of CO2 =−4.08 ± 0.15‰). Mixing of seawater or thermogenic sources cannot account for the depletions in 13C of both CH4 and CO2 at diffuse vents relative to adjacent high-temperature vents. The substrate utilization and 13C fractionation associated with the microbiological processes of methanogenesis and methane oxidation can explain observed steady-state CH4 and CO2 concentrations and carbon isotopic compositions. A mass-isotope numerical box model of these paired vent systems is consistent with the hypothesis that microbial methane cycling is active at diffuse vents at 9°50′N. The detectable 13C modification of fluid geochemistry by microbial metabolisms may provide a useful tool for detecting active methanogenesis.

The ability to continuously monitor chemical properties of hydrothermal vent effluents for extended periods of time is essential to understanding dynamic processes responsible for the temporally variable nature of mid-ocean ridge hydrothermal systems. Although instruments do exist for some parameters, there has been no sensor capable of measuring the chloride concentration, an indicator of possible phase separation, on a real-time and long-term basis. In this article, we discuss the construction of a novel instrument which measures solution resistance as a proxy for chloride concentration. The sensor consists of four gold electrodes embedded in a cylindrical ZrO 2 ceramic housing. It has been successfully deployed in several high temperature vents at the Main Endeavour Field (MEF) on the Juan de Fuca ridge in the NE Pacific, and calibrated under simulated hydrothermal conditions ranging up to 380 °C and 300 bar. The in situ data clearly demonstrate a tidal influence on the effluent from some high temperature vents possibly relating to a subsurface mixing process involving non-seawater end-members. Non-tidal changes are used to constrain the sequence and type of controls operating on fluids circulating within the subsurface.

The stable isotopes of dissolved organic carbon (DOC) are a powerful tool for distinguishing sources and inputs of organic matter in aquatic systems. While several methods exist to perform these analyses, no labs routinely utilize a high temperature combustion (HTC) instrument. Advantages of HTC instruments include rapid analysis, small sample volumes and minimal sample preparation, making them the favored devices for most routine oceanic DOC concentration measurements. We developed a stable carbon DOC method based around an HTC system. This method has the benefit of a simple setup, requiring neither vacuum nor high pressures. The main drawback of the method is a significant blank, requiring careful accounting of all blank sources for accurate isotopic and concentration values. We present here a series of experiments to determine the magnitude, source and isotopic composition of the HTC blank. Over time, the blank is very stable at ∼ 20 ng of carbon with a δ 13 C of − 18.1‰ vs. VPDB. The similarity of the isotopic composition of the blank and seawater samples makes corrections relatively minor. The precision of the method was determined by oxidizing organic standards with a wide isotopic and concentration range (− 9‰ to − 39‰; 18 μM to 124 μM). Analysis of seawater samples demonstrates the accuracy for low concentration, high salinity samples. The overall error on the measurement is approximately ± 0.8‰.

Hydrothermal fluids (341°C and 19°C) were collected 1 m apart from a black smoker chimney and a tubeworm mound on the Boardwalk edifice at the Endeavour Segment in the northeastern Pacific Ocean to study anaerobic microbial growth in hydrothermal mineral deposits. Geochemical modelling of mixed vent fluid and seawater suggests the mixture was anoxic above 55°C and that low H2 concentrations (79 μmol kg(-1) in end-member hydrothermal fluid) limit anaerobic hydrogenotrophic growth above this temperature. A thermophilic, hydrogenotrophic sulfur reducer, Desulfurobacterium strain HR11, was isolated from the 19°C fluid raising questions about its H2 -dependent growth kinetics. Strain HR11 grew at 40-77°C (Topt 72-75°C), pH 5-8.5 (pHopt 6-7) and 1-5% (wt vol(-1) ) NaCl (NaClopt 3-4%). The highest growth rates occurred when S2 O3 (2-) and S° were reduced to H2 S. Modest growth occurred by NO3 (-) reduction. Monod constants for its growth were Ks of 30 μM for H2 and Ks of 20 μM for S2 O3 (2-) with a μmax of 2.0 h(-1) . The minimum H2 and S2 O3 (2-) concentrations for growth were 3 μM and 5 μM respectively. Possible sources of S2 O3 (2-) and S° are from abiotic dissolved sulfide and pyrite oxidation by O2 .

The circulation of hydrothermal fluid through the upper oceanic crustal reservoir has a large impact on the chemistry of seawater, yet the impact on dissolved organic carbon (DOC) in the ocean has received almost no attention. To determine whether hydrothermal circulation is a source or a sink for DOC in the oceans, we measured DOC concentrations in hydrothermal fluids from several environments. Hydrothermal fluids were collected from high-temperature vents and diffuse, low-temperature vents on the basalt-hosted Juan de Fuca Ridge axis and also from low-temperature vents on the sedimented eastern flanks. High-temperature fluids from Main Endeavour Field (MEF) and Axial Volcano (AV) contain very low DOC concentrations (average = 15 and 17 μM, respectively) compared to background seawater (36 μM). At MEF and AV, average DOC concentrations in diffuse fluids (47 and 48 μM, respectively) were elevated over background seawater, and high DOC is correlated with high microbial cell counts in diffuse fluids. Fluids from off-axis hydrothermal systems located on 3.5-Ma-old crust at Baby Bare Seamount and Ocean Drilling Program (ODP) Hole 1026B had average DOC concentrations of 11 and 13 μM, respectively, and lowered DOC was correlated with low cell counts. The relative importance of heterotrophic uptake, abiotic sorption to mineral surfaces, thermal decomposition, and microbial production in fixing the DOC concentration in vent fluids remains uncertain. We calculated the potential effect of hydrothermal circulation on the deep-sea DOC cycle using our concentration data and published water flux estimates. Maximum calculated fluxes of DOC are minor compared to most oceanic DOC source and sink terms.

Although commonly utilized in continental geothermal work, the water–hydrogen and methane–hydrogen isotope geothermometers have been neglected in hydrothermal studies. Here we report δD-CH4 and δD-H2 values from high-temperature, black smoker-type hydrothermal vents and low-temperature carbonate-hosted samples from the recently discovered Lost City Hydrothermal Field. Methane deuterium content is uniform across the dataset at −120±12‰. Hydrogen δD values vary from −420‰ to −330‰ at high-temperature vents to −700‰ to −600‰ at Lost City. The application of several geothermometer equations to a suite of hydrothermal vent volatile samples reveals that predicted temperatures are similar to measured vent temperatures at high-temperature vents, and 20–60 °C higher than those measured at the Lost City vents. We conclude that the overestimation of temperature at Lost City reflects 1) that methane and hydrogen are produced by serpentinization at N110 °C, and 2) that isotopic equilibrium at temperatures b70 °C is mediated by microbial sulfate reduction. The successful application of hydrogen isotope geothermometers to low-temperature Lost City hydrothermal samples encourages its employment with lowtemperature diffuse hydrothermal fluids. © 2005 Elsevier B.V. All rights reserved.