Diffuse Hydrothermal Venting Research Articles

Shallow Seafloor Gas emissions Near Heard and McDonald Islands on the Kerguelen Plateau, Southern Indian Ocean

AbstractBubble emission mechanisms from submerged large igneous provinces remains enigmatic. The Kerguelen Plateau, a large igneous province in the southern Indian Ocean, has a long sustained history of active volcanism and glacial/interglacial cycles of sedimentation, both of which may cause seafloor bubble production. We present the results of hydroacoustic flare observations around the underexplored volcanically active Heard Island and McDonald Islands on the Central Kerguelen Plateau. Flares were observed with a split‐beam echosounder and characterized using multifrequency decibel differencing. Deep‐tow camera footage, water properties, water column δ3He, subbottom profile, and sediment δ13C and δ34S data were analyzed to consider flare mechanisms. Excess δ3He near McDonald Islands seeps, indicating mantle‐derived input, suggests proximal hydrothermal activity; McDonald Islands flares may thus indicate CO2, methane, and other minor gas bubbles associated with shallow diffuse hydrothermal venting. The Heard Island seep environment, with subbottom acoustic blanking in thick sediment, muted 3He signal, and δ13C and δ34S fractionation factors, suggest that Heard Island seeps may either be methane gas (possibly both shallow biogenic methane and deeper‐sourced thermogenic methane related to geothermal heat from onshore volcanism) or a combination of methane and CO2, such as seen in sediment‐hosted geothermal systems. These data provide the first evidence of submarine gas escape on the Central Kerguelen Plateau and expand our understanding of seafloor processes and carbon cycling in the data‐poor southern Indian Ocean. Extensive sedimentation of the Kerguelen Plateau and additional zones of submarine volcanic activity mean additional seeps or vents may lie outside the small survey area proximal to the islands.

Diffuse Hydrothermal Venting: A Hidden Source of Iron to the Oceans

Iron (Fe) limits or co-limits primary productivity and nitrogen fixation in large regions of the world’s oceans. Hydrothermal supply of Fe to the deep ocean is extensive; however most of this work has focused on examining high temperature, focused flow on ridge axes that create “black smoker” plumes. The contribution of other types of venting to the global ocean Fe cycle has received little attention. To thoroughly understand hydrothermal Fe sources to the ocean, different types of vent site must be compared. To examine the role of more diffuse, higher pH sources of venting, a hydrothermal plume above the Von Damm vent field (VDVF) was sampled for Total dissolvable Fe (unfiltered, TDFe) dissolved Fe (<0.2 µm, dFe) and soluble Fe (<0.02 µm, sFe). Plume particles sampled in-situ were characterised using scanning electron microscopy and soft x-ray spectromicroscopy. The VDVF vents emit visibly clear fluids with particulate Fe (TDFe-dFe) concentrations up to 196 nmol kg-1 comparable to concentrations measured in black smoker plumes on the Mid-Atlantic Ridge. Colloidal Fe (cFe) and sFe increased as a fraction of TDFe with decreasing TDFe concentration. This increase in the percentage of sFe and cFe within the plume cannot be explained by settling of particulates or mixing with background seawater. The creation of new cFe and sFe within the plume from the breakdown of pFe is required to close the Fe budget. We suggest that the proportional increase in cFe and sFe reflects the entrainment, breakdown and recycling of Fe bearing organic particulates near the vents. Fe plume profiles from the VDVF differ significantly from previous studies of “black smoker” vents where formation of new pFe in the plume decreases the amount of cFe. Formation and removal of Fe-rich colloids and particles will control the amount and physico-chemical composition of dFe supplied to the deep ocean from hydrothermal systems. This study highlights the differences in the stabilization of hydrothermal Fe from an off-axis diffuse source compared to black smokers. Off-axis diffuse venting represent a potentially significant and previously overlooked Fe source to the ocean due to the difficulties in detecting and locating such sites.

Pyrite-walled tube structures in a Mesoproterozoic sediment-hosted metal sulfide deposit

Unusual decimeter-scale structures occur in the sediment-hosted Black Butte Copper Mine Project deposit within lower Mesoproterozoic strata of the Belt Supergroup, Montana. These low domal and stratiform lenses are made up of millimeter-scale, hollow or mineral-filled tubes bounded by pyrite walls. X-ray micro−computed tomography (micro-CT) shows that the tube structures are similar to the porous fabric of modern diffuse hydrothermal vents, and they do not resemble textures associated with the mineralization of known microbial communities. We determined the sulfur isotopic composition of sulfide minerals with in situ secondary ion mass spectrometry (SIMS) and of texture-specific sulfate phases with multicollector−inductively coupled plasma−mass spectrometry (MC-ICP-MS). The sedimentological setting, ore paragenesis, sulfur isotope systematics, and porosity structure of these porous precipitates constrain the site of their formation to above the sediment-water interface where metalliferous hydrothermal fluids vented into the overlying water column. These data constrain the geochemistry of the Mesoproterozoic sediment-water interface and the site of deposition for copper-cobalt-silver mineralization. Metals in the hydrothermal fluids titrated sulfide in seawater to create tortuous fluid-flow conduits. Pyrite that precipitated at the vent sites exhibits large sulfur isotope fractionation (>50‰), which indicates a close association between the vents and sulfate-reducing microbiota. In the subsurface, base metal sulfides precipitated from sulfide formed during the reduction of early diagenetic barite, also ultimately derived from seawater. This model suggests dynamic bottom-water redox conditions at the vent site driven by the interplay between sulfate-reducing organisms and metalliferous fluid effluence.

Hydrothermal nontronite formation associated with microbes from low‐temperature diffuse hydrothermal vents at the South Mid‐Atlantic Ridge

AbstractOceanic nontronite deposits have been identified to be closely related to low‐temperature hydrothermal activities. However, their formation mechanisms associated with microbes in diffuse hydrothermal vents still remain largely unknown. The friable deposits, collected from the low‐temperature diffuse flow at the Southern Atlantic Ridge, display a layered structure. Scanning electron microscope and transmission electron microscope analyses reveal that abundant filamentous, spherical, and rod‐shaped mineralized forms are preserved in the yellowish‐green layer of the deposits. These mineralized forms primarily consist of Si and Fe. Selected area electron diffraction patterns of the mineralized forms indicate that they are composed of nontronite. High intensities of 12C and 12C14N signals derived from cellular structures determined by nanosecondary ion mass spectrometry suggest the intimate relationship between nontronite and microbes. The results of 454 pyrosequencing analyses provide insights into the microbial communities involved in the biologically induced mineralization in the yellowish‐green layer. We propose an evolutionary model for establishing paragenetic sequences among nontronite, Mn oxide, and Fe oxyhydroxide in the deposits. This paragenetic sequence could be widespread in modern and ancient low‐temperature hydrothermal fields.

Detection of diffuse seafloor venting using a structured light laser sensor: 1. Development of a classification based detection method

Systematic and remote detection of diffuse hydrothermal venting is complex yet necessary to establish a comprehensive understanding the distribution, contribution, and context of fluid flow within hydrothermal systems. Diffuse flow is characterized by both low temperature and low flow rates and cannot be consistently distinguished during extensive systematic surveys using current vehicle‐mounted visual, acoustic, or environmental sensors. The remotely operated vehicle‐mounted structured light laser sensor is sensitive to fluid density anomalies and seafloor characteristics. Presented advancements to the detection algorithm include the development and implementation of intensity normalization routines and a support vector machine classification algorithm. The resulting comprehensive maps establish the spatial distribution of potential hydrothermal vents and associated bacteria within a vent field.

Detection of diffuse seafloor venting using a structured light laser sensor: 2. Evaluation of detection sensitivity and limitations

Algorithms to detect diffuse hydrothermal vent fluids using the structured light laser sensor have proven to be successful. However, potential limitations due to survey parameters, including altitude and vehicle heading, have not been explored and are the focus of this study. Data from 12 surveys conducted over a single hydrothermal vent at three different altitudes (2.7 m, 4.2 m, and 6.2 m) and four different headings are analyzed. Increasing survey altitude decreases the resolution and intensity of the imaged laser line; therefore, range‐dependent processing considerations to maintain detection sensitivity are presented. Analysis of these surveys suggests minimal degradation of vent detection capabilities from survey altitudes up to 6 m. Additionally, as the shape and location of the detected distortion patterns vary with survey heading and altitude they are presented as potential indications of plume shape and rise height. The effects of range and survey heading are important for future applications of this sensor which may include surveying from higher, faster flying autonomous underwater vehicles.

In situ estimates of iron-oxidation and accretion rates for iron-oxidizing bacterial mats at Lō’ihi Seamount

It is increasingly recognized that diffuse, hydrothermal venting is an important source of iron to the deep-sea that can influence oceanic iron dynamics and abundance. Lithotrophic Fe-oxidizing bacteria (FeOB) are dominant at diffuse hydrothermal vent sites, producing microbial iron mats that are often centimeters or more thick. At present, little is known about in situ growth rates of FeOB, specific Fe-oxidation rates, or accretion rates for iron mats. An in situ productivity chamber was developed that took advantage of the unique mineral morphotypes produced by FeOB to estimate rates of Fe-oxidation and accretion. Chambers were placed at two diffuse vents (1179 and 1300mbsl) at Lō’ihi Seamount where they were colonized by FeOB for different amounts of time. From this analysis, it was estimated that Fe-oxidation rates could range from 8.2 to 51.9 × 10−6molh−1, and that iron mats could accrete at around 2.2cm. yr−1. Molecular analysis indicated that the relative abundance of Zetaproteobacteria, a group of known FeOB, accounted for 80–90% of the bacteria colonizing the chambers. There was a distinct difference between populations at the 1179m site (Pohaku), and the 1300m site (North Hiolo Ridge). Microscope slides placed within the productivity chambers were colonized by different morphotypes of FeOB. The cells responsible for one common morphotype that produces a Y-shaped filament were identified as Zetaproteobacteria by use of a small subunit rRNA probe. This work confirms the importance of FeOB in the formation of chemosynthetic iron mats, and provides the first estimates for in situ Fe-oxidation rates and mat accretion rates.

Hidden diversity revealed by genome-resolved metagenomics of iron-oxidizing microbial mats from L\u014d\u2019ihi Seamount, Hawai\u2019i

The Zetaproteobacteria are ubiquitous in marine environments, yet this class of Proteobacteria is only represented by a few closely-related cultured isolates. In high-iron environments, such as diffuse hydrothermal vents, the Zetaproteobacteria are important members of the community driving its structure. Biogeography of Zetaproteobacteria has shown two ubiquitous operational taxonomic units (OTUs), yet much is unknown about their genomic diversity. Genome-resolved metagenomics allows for the specific binning of microbial genomes based on genomic signatures present in composite metagenome assemblies. This resulted in the recovery of 93 genome bins, of which 34 were classified as Zetaproteobacteria. Form II ribulose 1,5-bisphosphate carboxylase genes were recovered from nearly all the Zetaproteobacteria genome bins. In addition, the Zetaproteobacteria genome bins contain genes for uptake and utilization of bioavailable nitrogen, detoxification of arsenic, and a terminal electron acceptor adapted for low oxygen concentration. Our results also support the hypothesis of a Cyc2-like protein as the site for iron oxidation, now detected across a majority of the Zetaproteobacteria genome bins. Whole genome comparisons showed a high genomic diversity across the Zetaproteobacteria OTUs and genome bins that were previously unidentified by SSU rRNA gene analysis. A single lineage of cosmopolitan Zetaproteobacteria (zOTU 2) was found to be monophyletic, based on cluster analysis of average nucleotide identity and average amino acid identity comparisons. From these data, we can begin to pinpoint genomic adaptations of the more ecologically ubiquitous Zetaproteobacteria, and further understand their environmental constraints and metabolic potential.

Formation of hydrothermal pits and the role of seamounts in the Guatemala Basin (Equatorial East Pacific) from heat flow, seismic, and core studies

AbstractWe acquired seismic and heat flow data and collected sediment cores in three areas in the Guatemala Basin (Cocos Plate, Eastern Pacific) to investigate the process by which depressions (pits) in the sedimentary cover on young oceanic crust were formed. Median heat flow of 55 mW/m2 for the three areas is about half of the expected conductive cooling value. The heat deficit is caused by massive recharge of cold seawater into the upper crust through seamounts which is inferred from depressed heat flow in the vicinity of seamounts. Heat flow inside of pits is always elevated, in some cases up to three times (max. 300 mW/m2) relative to background. None of the geochemical pore water profiles from cores inside and outside of the pits show any evidence of active fluid flow inside the pits. All three areas originated within the high productivity equatorial zone and moved northwest over the past 15 to 18 Ma. Pits found in the working areas are likely relict dissolution structures formed by diffuse hydrothermal venting in a zone of high biogenic carbonate production which were sealed when they moved north. It is likely that these pits were discharge sites of “hydrothermal siphons” where recharging seamounts could feed cold seawater via the upper crust to several discharging pits. Probably pit density on the whole Cocos Plate is similar to the three working areas and which may explain the huge heat deficit of the Cocos Plate.

Hydrogen Limitation and Syntrophic Growth among Natural Assemblages of Thermophilic Methanogens at Deep-sea Hydrothermal Vents.

Thermophilic methanogens are common autotrophs at hydrothermal vents, but their growth constraints and dependence on H2 syntrophy in situ are poorly understood. Between 2012 and 2015, methanogens and H2-producing heterotrophs were detected by growth at 80°C and 55°C at most diffuse (7–40°C) hydrothermal vent sites at Axial Seamount. Microcosm incubations of diffuse hydrothermal fluids at 80°C and 55°C demonstrated that growth of thermophilic and hyperthermophilic methanogens is primarily limited by H2 availability. Amendment of microcosms with NH4+ generally had no effect on CH4 production. However, annual variations in abundance and CH4 production were observed in relation to the eruption cycle of the seamount. Microcosm incubations of hydrothermal fluids at 80°C and 55°C supplemented with tryptone and no added H2 showed CH4 production indicating the capacity in situ for methanogenic H2 syntrophy. 16S rRNA genes were found in 80°C microcosms from H2-producing archaea and H2-consuming methanogens, but not for any bacteria. In 55°C microcosms, sequences were found from H2-producing bacteria and H2-consuming methanogens and sulfate-reducing bacteria. A co-culture of representative organisms showed that Thermococcus paralvinellae supported the syntrophic growth of Methanocaldococcus bathoardescens at 82°C and Methanothermococcus sp. strain BW11 at 60°C. The results demonstrate that modeling of subseafloor methanogenesis should focus primarily on H2 availability and temperature, and that thermophilic H2 syntrophy can support methanogenesis within natural microbial assemblages and may be an important energy source for thermophilic autotrophs in marine geothermal environments.

Diffuse venting at the ASHES hydrothermal field: Heat flux and tidally modulated flow variability derived from in situ time‐series measurements

AbstractTime‐series measurements of diffuse exit‐fluid temperature and velocity collected with a new, deep‐sea camera, and temperature measurement system, the Diffuse Effluent Measurement System (DEMS), were examined from a fracture network within the ASHES hydrothermal field located in the caldera of Axial Seamount, Juan de Fuca Ridge. The DEMS was installed using the HOV Alvin above a fracture near the Phoenix vent. The system collected 20 s of 20 Hz video imagery and 24 s of 1 Hz temperature measurements each hour between 22 July and 2 August 2014. Fluid velocities were calculated using the Diffuse Fluid Velocimetry (DFV) technique. Over the ∼12 day deployment, median upwelling rates and mean fluid temperature anomalies ranged from 0.5 to 6 cm/s and 0°C to ∼6.5°C above ambient, yielding a heat flux of 0.29 ± 0.22 MW m−2 and heat output of 3.1± 2.5 kW. Using a photo mosaic to measure fracture dimensions, the total diffuse heat output from cracks across ASHES field is estimated to be 2.05 ± 1.95 MW. Variability in temperatures and velocities are strongest at semidiurnal periods and show significant coherence with tidal height variations. These data indicate that periodic variability near Phoenix vent is modulated both by tidally controlled bottom currents and seafloor pressure, with seafloor pressures being the dominant influence. These results emphasize the importance of local permeability on diffuse hydrothermal venting at mid‐ocean ridges and the need to better quantify heat flux associated with young oceanic crust.

Hydrothermal venting and mineralization in the crater of Kick'em Jenny submarine volcano, Grenada (Lesser Antilles)

AbstractKick'em Jenny is a frequently erupting, shallow submarine volcano located 7.5 km off the northern coast of Grenada in the Lesser Antilles subduction zone. Focused and diffuse hydrothermal venting is taking place mainly within a small (∼70 × 110 m) depression within the 300 m diameter crater of the volcano at depths of about 265 m. Much of the crater is blanketed with a layer of fine‐grained tephra that has undergone hydrothermal alteration. Clear fluids and gas are being discharged near the center of the depression from mound‐like vents at a maximum temperature of 180°C. The gas consists of 93–96% CO2 with trace amounts of methane and hydrogen. Gas flux measurements of individual bubble streams range from 10 to 100 kg of CO2 per day. Diffuse venting with temperatures 5–35°C above ambient occurs throughout the depression and over large areas of the main crater. These zones are colonized by reddish‐yellow bacteria with the production of Fe‐oxyhydroxides as surface coatings, fragile spires up to several meters in height, and elongated mounds up to tens of centimeters thick. A high‐resolution photomosaic of the inner crater depression shows fluid flow patterns descending the sides of the depression toward the crater floor. We suggest that the negatively buoyant fluid flow is the result of phase separation of hydrothermal fluids at Kick'em Jenny generating a dense saline component that does not rise despite its elevated temperature.

Tectonic focusing of voluminous basaltic eruptions in magma-deficient backarc rifts

The Coriolis Troughs of the New Hebrides subduction zone are among the youngest backarc rifts in the world. They reach depths of >3 km, despite their small size (<100 km in length and only 25–45 km wide) and their proximity to the arc front (∼50 km). The narrow, deep graben morphology is characteristic of magma-deficient arc rifts in the early stages of backarc extension, where the rate of extension and subsidence exceeds the magmatic input. Unexpectedly, the youngest graben, the Vate Trough, contains a centrally-located 1000-m tall and 14-km wide shield volcano with a large, 5×8 km breached summit caldera. The Nifonea axial volcano has a volume of ∼126 km3, reflecting unusually high extrusion rates, given its young age (<3 Ma), and the summit caldera hosts the remnants of a large lava lake, the first described from a submarine backarc setting. Extensive diffuse hydrothermal venting and several clusters of black smoker chimneys, with the highest recorded fluid temperatures (368 °C) in the SW Pacific, occur on the youngest lava flows. Comparison with similar axial volcanoes on the mid-ocean ridges suggests that the 46×106 m3 of sheet flows in the caldera could have been erupted in <30 hours. The focusing of voluminous basaltic eruptions into an otherwise magma-deficient backarc has been linked to strong left-lateral transtension caused by clockwise rotation and segmentation of the southern portion of the arc after collision with d'Entrecasteaux ridge. This study shows that the upper plate stresses can result in dramatic variability in magma supply and hydrothermal activity at the earliest stages of arc rifting and could explain the wide range of melt compositions, volcanic styles and mineral deposit types found in nascent backarc rifts.

Biogenic Iron-Rich Filaments in the Quartz Veins in the Uppermost Ediacaran Qigebulake Formation, Aksu Area, Northwestern Tarim Basin, China: Implications for Iron Oxidizers in Subseafloor Hydrothermal Systems.

Fe-(oxyhydr)oxide-encrusted filamentous microstructures produced by microorganisms have been widely reported in various modern and ancient extreme environments; however, the iron-dependent microorganisms preserved in hydrothermal quartz veins have not been explored in detail because of limited materials available. In this study, abundant well-preserved filamentous microstructures were observed in the hydrothermal quartz veins of the uppermost dolostones of the terminal-Ediacaran Qigebulake Formation in the Aksu area, northwestern Tarim Basin, China. These filamentous microstructures were permineralized by goethite and hematite as revealed by Raman spectroscopy and completely entombed in chalcedony and quartz cements. Microscopically, they are characterized by biogenic filamentous morphologies (commonly 20-200 μm in length and 1-5 μm in diameter) and structures (curved, tubular sheath-like, segmented, and mat-like filaments), similar to the Fe-oxidizing bacteria (FeOB) living in modern and ancient hydrothermal vent fields. A previous study revealed that quartz-barite vein swarms were subseafloor channels of low-temperature, silica-rich, diffusive hydrothermal vents in the earliest Cambrian, which contributed silica to the deposition of the overlying bedded chert of the Yurtus Formation. In this context, this study suggests that the putative filamentous FeOB preserved in the quartz veins might have thrived in the low-temperature, silica- and Fe(II)-rich hydrothermal vent channels in subseafloor mixing zones and were rapidly fossilized by subsequent higher-temperature, silica-rich hydrothermal fluids in response to waning and waxing fluctuations of diffuse hydrothermal venting. In view of the occurrence in a relatively stable passive continental margin shelf environment in Tarim Block, the silica-rich submarine hydrothermal vent system may represent a new and important geological niche favorable for FeOB colonization, which is different from their traditional habitats reported in hydrothermal vent systems at oceanic spreading centers or volcanic seamounts. Thus, these newly recognized microfossils offer a new clue to explore the biological signatures and habitat diversity of microorganisms on Earth and beyond.

Quantitative PCR analysis of functional genes in iron-rich microbial mats at an active hydrothermal vent system (Lō'ihi Seamount, Hawai'i).

The chemolithotrophic Zetaproteobacteria represent a novel class of Proteobacteria which oxidize Fe(II) to Fe(III) and are the dominant bacterial population in iron-rich microbial mats. Zetaproteobacteria were first discovered at Lō'ihi Seamount, located 35 km southeast off the big island of Hawai'i, which is characterized by low-temperature diffuse hydrothermal venting. Novel nondegenerate quantitative PCR (qPCR) assays for genes associated with microbial nitrogen fixation, denitrification, arsenic detoxification, Calvin-Benson-Bassham (CBB), and reductive tricarboxylic acid (rTCA) cycles were developed using selected microbial mat community-derived metagenomes. Nitrogen fixation genes were not detected, but all other functional genes were present. This suggests that arsenic detoxification and denitrification processes are likely cooccurring in addition to two modes of carbon fixation. Two groups of microbial mat community types were identified by terminal restriction fragment length polymorphism (T-RFLP) and were further described based on qPCR data for zetaproteobacterial abundance and carbon fixation mode preference. qPCR variance was associated with mat morphology but not with temperature or sample site. Geochemistry data were significantly associated with sample site and mat morphology. Together, these qPCR assays constitute a functional gene signature for iron microbial mat communities across a broad array of temperatures, mat types, chemistries, and sampling sites at Lō'ihi Seamount.

Microbial community structure and functioning in marine sediments associated with diffuse hydrothermal venting assessed by integrated meta‐omics

Deep-sea hydrothermal vents are unique environments on Earth, as they host chemosynthetic ecosystems fuelled by geochemical energy with chemolithoautotrophic microorganisms at the basis of the food webs. Whereas discrete high-temperature venting systems have been studied extensively, the microbiotas associated with low-temperature diffuse venting are not well understood. We analysed the structure and functioning of microbial communities in two diffuse venting sediments from the Jan Mayen vent fields in the Norwegian-Greenland Sea, applying an integrated 'omics' approach combining metatranscriptomics, metaproteomics and metagenomics. Polymerase chain reaction-independent three-domain community profiling showed that the two sediments hosted highly similar communities dominated by Epsilonproteobacteria, Deltaproteobacteria and Gammaproteobacteria, besides ciliates, nematodes and various archaeal taxa. Active metabolic pathways were identified through transcripts and peptides, with genes of sulphur and methane oxidation, and carbon fixation pathways highly expressed, in addition to genes of aerobic and anaerobic (nitrate and sulphate) respiratory chains. High expression of chemotaxis and flagella genes reflected a lifestyle in a dynamic habitat rich in physico-chemical gradients. The major metabolic pathways could be assigned to distinct taxonomic groups, thus enabling hypotheses about the function of the different prokaryotic and eukaryotic taxa. This study advances our understanding of the functioning of microbial communities in diffuse hydrothermal venting sediments.

Activity and abundance of denitrifying bacteria in the subsurface biosphere of diffuse hydrothermal vents of the Juan de Fuca Ridge

Abstract. Little is known about fixed nitrogen (N) transformation and elimination at diffuse hydrothermal vents where anoxic fluids are mixed with oxygenated crustal seawater prior to discharge. Oceanic N sinks that remove bio-available N ultimately affect chemosynthetic primary productivity in these ecosystems. Using 15N paired isotope techniques, we determined potential rates of fixed N loss pathways (denitrification, anammox) and dissimilatory nitrate reduction to ammonium (DNRA) in sulfidic hydrothermal vent fluids discharging from the subsurface at several sites at Axial Volcano and the Endeavour Segment on the Juan de Fuca Ridge. We also measured physico-chemical parameters (i.e., temperature, pH, nutrients, H2S and N2O concentrations) as well as the biodiversity and abundance of chemolithoautotrophic nitrate-reducing, sulfur-oxidizing γ-proteobacteria (SUP05 cluster) using sequence analysis of amplified small subunit ribosomal RNA (16S rRNA) genes in combination with taxon-specific quantitative polymerase chain reaction (qPCR) assays. Denitrification was the dominant N loss pathway in the subsurface biosphere of the Juan de Fuca Ridge, with rates of up to ~1000 nmol N l−1 day−1. In comparison, anammox rates were always &lt; 5 nmol N l−1 day−1 and below the detection limit at most of the sites. DNRA rates were up to ~150 nmol N l−1 day−1. These results suggest that bacterial denitrification out-competes anammox in sulfidic hydrothermal vent waters. Taxon-specific qPCR revealed that γ-proteobacteria of the SUP05 cluster sometimes dominated the microbial community (SUP05/total bacteria up to 38%). Significant correlations were found between fixed N loss (i.e., denitrification, anammox) rates and in situ nitrate and dissolved inorganic nitrogen (DIN) deficits in the fluids, indicating that DIN availability may ultimately regulate N loss in the subsurface. Based on our rate measurements, and on published data on hydrothermal fluid fluxes and residence times, we estimated that up to ~10 Tg N yr−1 could globally be removed in the subsurface biosphere of hydrothermal vents systems, thus, representing a small fraction of the total marine N loss (~275 to &gt; 400 Tg N yr−1).

Arctic Bay Formation, Borden Basin, Nunavut (Canada): Basin evolution, black shale, and dissolved metal systematics in the Mesoproterozoic ocean

The Arctic Bay Formation (Nunavut, Canada) represents a late Mesoproterozoic muddy terrigenous ramp and contains >200m of black shale. The formation was studied in order to decipher the tectonostratigraphic and geochemical evolution of the basin, address the origin of metal enrichment, and determine whether this frontier basin has the potential to host sedimentary-exhalative or polymetallic black shale deposits. Samples were analysed in the laboratory for major and trace elements, total organic carbon (TOC), 4-step loss-on-ignition (LOI), and Pb isotopes. Non-calcareous black shale exhibits neither Ce nor Y anomalies, reflecting euxinia in the lower water column, whereas slightly dolomitic black shale has both Ce and Y anomalies, reflecting the dolomite's probable origin as a precipitate in the upper water column. The stratigraphic distribution of the rare earth elements (REEs) indicates an evolving sediment provenance, and Pb isotopic data indicate that the source of clay in the black shale was dominated by weathered, juvenile, mantle-derived material. Base metals and redox-sensitive metals, expressed as enrichment ratios relative to conservative lithophile elements, are elevated and exhibit coherent covariations in the black shale. Enrichment in the redox-sensitive elements, such as Mo and U, correlates with dolomite content of the shale, rather than with organic C or Fepy. From a deep-time ocean evolution perspective, this important observation suggests that enrichment in these metals cannot necessarily be attributed to metal incorporation at an interface between sediment and euxinic water. Instead, in Arctic Bay Formation black shale, the metals were either scavenged onto dolomite as it precipitated in the water column, or secondarily re-distributed within the sediment according to its dolomite content. The base metals that are concentrated in the black shale (e.g., Zn) were probably sourced from diffuse hydrothermal venting, and although there is no evidence at the studied location for a nearby point source of metals (vent), persistent bottom-water euxinia would have ensured the effective scavenging of any dissolved metals supplied, and so the basin has at the very minimum a hypothetical potential for SEDEX and polymetallic mineralisation. Whole-rock U–Th–Pb isotope analysis of black shale yielded a date of 1092±59Ma, which is considered to be the Arctic Bay Formation's depositional age.

Subseafloor nitrogen transformations in diffuse hydrothermal vent fluids of the Juan de Fuca Ridge evidenced by the isotopic composition of nitrate and ammonium

Little is known about dissolved inorganic nitrogen (DIN) transformations in hydrothermal vent (HV) fluids. Here, we present the first isotopic measurements of nitrate (δ15N and δ18O) and ammonium (δ15N) from three HV fields on the Juan de Fuca ridge (NE‐Pacific). The dominant process that drives DIN concentration variations in low‐T diffuse fluids is water mass mixing below the seafloor, with no effect on the DIN isotope ratios. Strong inter‐site variations in the concentration andδ15N of NH4+in high‐T fluids suggest different subsurface nitrogen (N) sources (deep‐sea nitrate versus organic sediments) for hydrothermally discharged ammonium. Low NH4+ community N isotope effects (&lt;3‰) for net NH4+consumption suggest an important contribution from gross ammonium regeneration in low‐T fluids. Elevation of HV nitrate15N/14N and 18O/16O over deep‐sea mean isotope values at some sites, concomitant with decreased nitrate concentrations, indicate assimilatory or dissimilatory nitrate consumption by bacteria in the subsurface, with relatively low community N isotope effects (15εk &lt; 3‰). The low N isotope effects suggest that nitrate assimilation or denitrification occur in bacterial mats, and/or in situ production of low δ15N nitrate. A significantly stronger relative increase for nitrate δ18O than for δ15N was observed at many sites, resulting in marked deviations from the 1:1 relationship for nitrate δ15N versus δ18O that is expected for nitrate reduction in marine settings. Simple box‐model calculation show that the observed un‐coupling of N and O nitrate isotope ratios is consistent with nitrate regeneration by either nitrite reoxidation and/or partial nitrification of hydrothermal ammonium (possibly originating from N2 fixation). Our isotope data confirm the role of subsurface microbial communities in modulating hydrothermal fluxes to the deep ocean.

A dual sensor device to estimate fluid flow velocity at diffuse hydrothermal vents

Numerous attempts have been made over the last thirty years to estimate fluid flow rates at hydrothermal vents, either at the exit of black smoker chimneys or within diffuse flow areas. In this study, we combine two methods to accurately estimate fluid flow velocities at diffuse flow areas. While the first method uses a hot film anemometer that performs high-frequency measurements, the second allows a relatively rapid assessment of fluid flow velocity through video imagery and provides in situ data to calibrate the sensor. Measurements of flow velocities on hydrothermal diffuse flow areas were obtained on the Mid-Atlantic Ridge (MAR). They range from 1.1 to 4.9 mm/s at the substratum level, in low-temperature (4.5–16.4 °C) diffuse flow areas from the Tour Eiffel sulfide edifice. A strong correlation was observed between fluid flow velocities and temperature, supporting the possible use of temperature as a proxy to estimate the flow rates in diffuse flow areas where such a simple linear flow/temperature relation is shown to dominate.