David A. Butterfield

An off-axis hydrothermal vent field near the Mid-Atlantic Ridge at 30° N

Evidence is growing that hydrothermal venting occurs not only along mid-ocean ridges but also on old regions of the oceanic crust away from spreading centres. Here we report the discovery of an extensive hydrothermal field at 30° N near the eastern intersection of the Mid-Atlantic Ridge and the Atlantis fracture zone. The vent field—named ‘Lost City’—is distinctly different from all other known sea-floor hydrothermal fields in that it is located on 1.5-Myr-old crust, nearly 15 km from the spreading axis, and may be driven by the heat of exothermic serpentinization reactions between sea water and mantle rocks. It is located on a dome-like massif and is dominated by steep-sided carbonate chimneys, rather than the sulphide structures typical of ‘black smoker’ hydrothermal fields. We found that vent fluids are relatively cool (40–75 °C) and alkaline (pH 9.0–9.8), supporting dense microbial communities that include anaerobic thermophiles. Because the geological characteristics of the Atlantis massif are similar to numerous areas of old crust along the Mid-Atlantic, Indian and Arctic ridges, these results indicate that a much larger portion of the oceanic crust may support hydrothermal activity and microbial life than previously thought.

Gradients in the composition of hydrothermal fluids from the Endeavour segment vent field: Phase separation and brine loss

Hydrothermal fluid samples collected in 1984, 1987, and 1988 from a large vent field near 47°57′N on the Endeavour segment of the Juan de Fuca Ridge (JFR) have been analyzed for major and minor elements and gases. There are of the order of 100 individual smoker vents on ∼10 large sulfide structures, which are localized along faults and fault intersections across the vent field. Each sulfide structure has a characteristic fluid composition, which varies very little from one vent orifice to the next, or from year to year, on a given structure. However, there are large gradients in fluid composition across the vent field, with endmember chlorinity increasing from ∼255 mmol/kg in the SW to 505 mmol/kg in the NE. End-member concentrations of major elements are well correlated with chlorinity, and endmember volatile concentrations in the lowest chlorinity fluids are approximately twice as high as in the highest chlorinity fluids. The gradients in composition across the vent field and measured vent fluid temperatures >400°C are consistent with supercritical phase separation and loss of brine phase below the seafloor. The factor-of-2 variation in CO2 (and H2S) is larger than expected for loss of a very high-chlorinity brine. Concentrations of iron and manganese are not positively correlated with chlorinity, suggesting that temperature and pH are more important in controlling metal solubility. Elevated ammonia and bromide/chloride ratios indicate that there has been subseafloor interaction between the hydrothermal fluids and organic matter, and high boron concentrations point to a sedimentary source.

Temporal changes in archaeal diversity and chemistry in a mid-ocean ridge subseafloor habitat.

ABSTRACT The temporal variation in archaeal diversity in vent fluids from a midocean ridge subseafloor habitat was examined using PCR-amplified 16S rRNA gene sequence analysis and most-probable-number (MPN) cultivation techniques targeting hyperthermophiles. To determine how variations in temperature and chemical characteristics of subseafloor fluids affect the microbial communities, we performed molecular phylogenetic and chemical analyses on diffuse-flow vent fluids from one site shortly after a volcanic eruption in 1998 and again in 1999 and 2000. The archaeal population was divided into particle-attached (>3-μm-diameter cells) and free-living fractions to test the hypothesis that subseafloor microorganisms associated with active hydrothermal systems are adapted for a lifestyle that involves attachment to solid surfaces and formation of biofilms. To delineate between entrained seawater archaea and the indigenous subseafloor microbial community, a background seawater sample was also examined and found to consist only of Group I Crenarchaeota and Group II Euryarchaeota, both of which were also present in vent fluids. The indigenous subseafloor archaeal community consisted of clones related to both mesophilic and hyperthermophilic Methanococcales, as well as many uncultured Euryarchaeota, some of which have been identified in other vent environments. The particle-attached fraction consistently showed greater diversity than the free-living fraction. The fluid and MPN counts indicate that while culturable hyperthermophiles represent less than 1% of the total microbial community, the subseafloor at new eruption sites does support a hyperthermophilic microbial community. The temperature and chemical indicators of the degree of subseafloor mixing appear to be the most important environmental parameters affecting community diversity, and it is apparent that decreasing fluid temperatures correlated with increased entrainment of seawater, decreased concentrations of hydrothermal chemical species, and increased incidence of seawater archaeal sequences.

Bacterial diversity in a subseafloor habitat following a deep-sea volcanic eruption

Abstract The bacterial diversity in a diffuse flow hydrothermal vent habitat at Axial Volcano, Juan de Fuca Ridge was examined shortly after an eruptive event in 1998 and again in 1999 and 2000 using PCR-amplified 16S rRNA gene sequence analyses. While considerable overlap with deep-sea background seawater was found within the alpha- and gamma-proteobacteria, unique subseafloor phylotypes were distinguishable. These included diverse members of the epsilon-proteobacteria, high temperature groups such as Desulfurobacterium, Gram-positive bacteria, and members of novel candidate divisions WS6 and ABY1. Phylotype richness was highest in the particle-attached populations from all three sampling periods, and diversity appeared to increase over that time, particularly among the epsilon-proteobacteria. A preliminary model of the subseafloor is presented that relates microbial diversity to temperature, chemical characteristics of diffuse flow fluids and the degree of mixing with seawater.

Geochemistry of north Cleft segment vent fluids: Temporal changes in chlorinity and their possible relation to recent volcanism

Hydrothermal vent fluids from the North Cleft segment of the Juan de Fuca Ridge between 44°54′ and 45°00′N were sampled in 1988, 1990, 1991, and 1992. In 1988, chloride-depleted (relative to seawater) diffuse fluids with low dissolved metal concentrations (relative to other mid-ocean ridge (MOR) fluids) were sampled over a distance of ∼10 km along axis. In 1990 and 1991, both high-temperature (>200°C) and diffuse vents were consistently metal- and chloride-enriched. The end-member compositions of high-temperature vents and nearby diffuse vents were very similar. There were small but significant correlated changes in diffuse and high-temperature composition from 1990 to 1991. The large-scale but temporary venting of low-chlorinity fluids over the entire North Cleft segment provides a resolution to the apparent mass imbalance implied by observations of continuous venting of chloride-enriched fluids. It is possible that a volcanic event along the North Cleft segment prior to 1987, for which there is firm geologic evidence, initially caused a boiling event which resulted in the preferential venting of vapor-enriched fluids through 1988, followed by a transition to brine-enriched fluids by 1990. High iron, low sodium, and low Sr/Ca ratios in the high-chlorinity fluids suggest that the brine phase has continued to react and approach reequilibration with an alteration mineral assemblage after the phase separation event. The absence of chloride-depleted fluids from 1990 onward, and the systematics of lithium, boron, and manganese with chloride in the high-temperature fluids from North Cleft suggest that the evolution toward lower chlorinity at Monolith vent from 1990 to 1992 is caused by progressive dilution of a brine with hydrothermal seawater.

Phylogenetic Diversity of Nitrogenase (nifH) Genes in Deep-Sea and Hydrothermal Vent Environments of the Juan de Fuca Ridge

ABSTRACT The subseafloor microbial habitat associated with typical unsedimented mid-ocean-ridge hydrothermal vent ecosystems may be limited by the availability of fixed nitrogen, inferred by the low ammonium and nitrate concentrations measured in diffuse hydrothermal fluid. Dissolved N2 gas, the largest reservoir of nitrogen in the ocean, is abundant in deep-sea and hydrothermal vent fluid. In order to test the hypothesis that biological nitrogen fixation plays an important role in nitrogen cycling in the subseafloor associated with unsedimented hydrothermal vents, degenerate PCR primers were designed to amplify the nitrogenase iron protein gene nifH from hydrothermal vent fluid. A total of 120 nifH sequences were obtained from four samples: a nitrogen-poor diffuse vent named marker 33 on Axial Volcano, sampled twice over a period of 1 year as its temperature decreased; a nitrogen-rich diffuse vent near Puffer on Endeavour Segment; and deep seawater with no detectable hydrothermal plume signal. Subseafloor nifH genes from marker 33 and Puffer are related to anaerobic clostridia and sulfate reducers. Other nifH genes unique to the vent samples include proteobacteria and divergent Archaea. All of the nifH genes from the deep-seawater sample are most closely related to the thermophilic, anaerobic archaeon Methanococcus thermolithotrophicus (77 to 83% amino acid similarity). These results provide the first genetic evidence of potential nitrogen fixers in hydrothermal vent environments and indicate that at least two sources contribute to the diverse assemblage of nifH genes detected in hydrothermal vent fluid: nifH genes from an anaerobic, hot subseafloor and nifH genes from cold, oxygenated deep seawater.

Long-term eruptive activity at a submarine arc volcano

Three-quarters of the Earths volcanic activity is submarine, located mostly along the mid-ocean ridges, with the remainder along intraoceanic arcs and hotspots at depths varying from greater than 4,000 m to near the sea surface. Most observations and sampling of submarine eruptions have been indirect, made from surface vessels or made after the fact. We describe here direct observations and sampling of an eruption at a submarine arc volcano named NW Rota-1, located 60 km northwest of the island of Rota (Commonwealth of the Northern Mariana Islands). We observed a pulsating plume permeated with droplets of molten sulphur disgorging volcanic ash and lapilli from a 15-m diameter pit in March 2004 and again in October 2005 near the summit of the volcano at a water depth of 555 m (depth in 2004). A turbid layer found on the flanks of the volcano (in 2004) at depths from 700 m to more than 1,400 m was probably formed by mass-wasting events related to the eruption. Long-term eruptive activity has produced an unusual chemical environment and a very unstable benthic habitat exploited by only a few mobile decapod species. Such conditions are perhaps distinctive of active arc and hotspot volcanoes.

Submarine venting of liquid carbon dioxide on a Mariana Arc volcano

Although CO2 is generally the most abundant dissolved gas found in submarine hydrothermal fluids, it is rarely found in the form of CO2 liquid. Here we report the discovery of an unusual CO2-rich hydrothermal system at 1600-m depth near the summit of NW Eifuku, a small submarine volcano in the northern Mariana Arc. The site, named Champagne, was found to be discharging two distinct fluids from the same vent field: a 103°C gas-rich hydrothermal fluid and cold (<4°C) droplets composed mainly of liquid CO2. The hot vent fluid contained up to 2.7 moles/kg CO2, the highest ever reported for submarine hydrothermal fluids. The liquid droplets were composed of ∼98% CO2, ∼1% H2S, with only trace amounts of CH4 and H2. Surveys of the overlying water column plumes indicated that the vent fluid and buoyant CO2 droplets ascended <200 m before dispersing into the ocean. Submarine venting of liquid CO2 has been previously observed at only one other locality, in the Okinawa Trough back-arc basin (Sakai et al., 1990a), a geologic setting much different from NW Eifuku, which is a young arc volcano. The discovery of such a high CO2 flux at the Champagne site, estimated to be about 0.1% of the global MOR carbon flux, suggests that submarine arc volcanoes may play a larger role in oceanic carbon cycling than previously realized. The Champagne field may also prove to be a valuable natural laboratory for studying the effects of high CO2 concentrations on marine ecosystems.

Temporal and spatial variability of hydrothermal manganese and iron at Cleft segment, Juan de Fuca Ridge

A unique data set for hydrothermal Mn and Fe was collected at Cleft segment on the Juan de Fuca Ridge between 1983 and 1991. The data set includes observations of focused and diffuse venting fluids and neutrally buoyant plumes formed by chronic and episodic venting. Manganese/heat and iron/heat ratios for plumes from the north end of the Cleft segment were combined with independently determined estimates of plume heat flux to yield annually averaged chronic venting fluxes for Mn of 0.36±0.17 mol s−1 and for Fe of 0.61±0.34 mol s−1. Over 6 years of plume measurements at North Cleft segment, observed episodic hydrothermal discharge accounted for −15% of the total vented Mn and −35% of vented Fe. The chronic fluxes for Mn and Fe at a second venting center located at the south end of the Cleft segment were estimated to be approximately equal to the fluxes at North Cleft segment. Chronic plumes at North Cleft segment are mixtures of focused and diffuse discharge that contribute heat, Mn, and Fe in variable proportions. Similar examination of South Cleft segment data strongly suggests the presence of an as yet unobserved venting source relatively depleted in Mn and Fe but contributing substantially to the overall heat. Temporal and spatial variations in the concentrations of Mn and Fe and in Mn/heat and Fe/heat ratios for focused seafloor vents were difficult to resolve within complex chronic plumes. Manganese/heat and iron/heat ratios of megaplumes suggest they may have derived from reservoirs of diffuse fluids while smaller event plumes may have formed by different processes and have properties similar to chronic plumes. The accurate assessment of segment-scale hydrothermal fluxes of Mn and Fe requires coordinated measurements of representative seafloor sources and the neutrally buoyant plume that integrates all seafloor discharge.

Isolated communities of Epsilonproteobacteria in hydrothermal vent fluids of the Mariana Arc seamounts.

Low-temperature hydrothermal vent fluids represent access points to diverse microbial communities living in oceanic crust. This study examined the distribution, relative abundance, and diversity of Epsilonproteobacteria in 14 low-temperature vent fluids from five volcanically active seamounts of the Mariana Arc using a 454 tag sequencing approach. Most vent fluids were enriched in cell concentrations compared with background seawater, and quantitative PCR results indicated that all fluids were dominated by bacteria. Operational taxonomic unit-based statistical tools applied to 454 data show that all vents from the northern end of the Mariana Arc grouped together, to the exclusion of southern arc seamounts, which were as distinct from one another as they were from northern seamounts. Statistical analysis also showed a significant relationship between seamount and individual vent groupings, suggesting that community membership may be linked to geographical isolation and not geochemical parameters. However, while there may be large-scale geographic differences, distance is not the distinguishing factor in the microbial community composition. At the local scale, most vents host a distinct population of Epsilonproteobacteria, regardless of seamount location. This suggests that there may be barriers to exchange and dispersal for these vent endemic microorganisms at hydrothermal seamounts of the Mariana Arc.