Kevin K. Roe

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.

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.

Mixing, Reaction and Microbial Activity in the Sub‐Seafloor Revealed by Temporal and Spatial Variation in Diffuse Flow Vents at Axial Volcano

To begin to understand the relationship between microbial communities and the geochemical environment, we have conducted systematic sampling and in situ analysis of a range of seafloor vents on or near the January 1998 lava flow at the summit of Axial Volcano on the Juan de Fuca ridge. The systematics of the chemical composition indicate that low-temperature diffuse fluids (3°C-78°C) at Axial Volcano have a high-temperature (>350°C) reaction-zone component overprinted by lower-temperature reactions. The low-temperature reactions include production of methane, ammonia and particulate elemental sulfur, oxidation of hydrogen sulfide, nitrate reduction, stripping of metals from seawater, and reactions that dissolve iron and produce alkalinity. High concentrations of CO 2 from magmatic degassing maintain acidic pH conditions and may be important in promoting low-temperature hydrolysis reactions. H 2 S oxidation is the dominant chemical energy source for microbial metabolism at Axial Volcano, and the energy available from either methanogenesis or iron oxidation is ∼100 times less. Chemical evidence, genetic signatures of thermophilic, non-seawater organisms, presence of culturable thermophiles, and cell counts elevated above backgound seawater in low-temperature fluids indicate microbial activity below the seafloor. Metabolic activity of organisms identified in venting fluids matches the chemical processes occurring in low-temperature sub-seafloor reservoirs.

Hydrogen-limited growth of hyperthermophilic methanogens at deep-sea hydrothermal vents

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 H2 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 H2 concentrations. Their H2 half-saturation value was 66 μM, and growth ceased below 17–23 μM H2, 10-fold lower than previously predicted. By comparison, measured H2 and CH4 concentrations in fluids suggest that there was generally sufficient H2 for Methanocaldococcus growth at Axial but not at Endeavour. Fluids from one vent at Axial (Marker 113) had anomalously high CH4 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 H2 limitation may be partly ameliorated by H2 syntrophy with hyperthermophilic heterotrophs. These data support our estimated H2 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.

Evidence for basaltic Sr in midocean ridge-flank hydrothermal systems and implications for the global oceanic Sr isotope balance

Previous models and calculations of the global mass balance of Sr in the oceans have shown that the input of unradiogenic basaltic Sr from on-axis midocean ridge hydrothermal systems is much less than needed to balance the input of radiogenic Sr delivered to the oceans by rivers. The implication is that either the oceans are far from steady state with respect to Sr isotope balance (and that the 87Sr/86Sr ratio of seawater is increasing at unprecedented rates) or that there is a significant missing source of basaltic Sr. It has long been recognized that off-axis hydrothermal fluxes might significantly affect the mass and isotopic balance of Sr and other elements in the oceans, but nearly all previous work has concluded that the 87Sr/86Sr ratio of pore fluids in ridge-flank hydrothermal areas is virtually indistinguishable from the seawater ratio or is dominated by authigenic carbonates. In contrast, we report here the 87Sr/86Sr ratios of warm springs, sediment pore fluids, and basement reservoir fluid with a clear basaltic signature from the eastern flank of the Juan de Fuca ridge (JFR). Fluids venting from Ocean Drilling Program Hole 1026B on the Juan de Fuca east flank have relatively stable Sr isotope and major element composition for the 3 yr following drilling. These results and similar results recently reported by Elderfield et al. (1999) indicate that low-temperature ridge-flank hydrothermal circulation has an important effect on the Sr isotope balance in the oceans. If published values for the other major sources of Sr input to the oceans (rivers and axial hydrothermal flux) are accurate, then the rate of increase of the 87Sr/86Sr ratio in seawater (∼0.000054 per million years) can be accommodated if ridge flanks on a global scale deliver fluids to the ocean with Δ (87Sr/86Sr)/heat ratios one third to one half of the ratio found in warm JFR basement fluids. Based on published Sr and O isotope signatures of calcite veins in the uppermost basaltic ocean crust, the average Δ (87Sr/86Sr)/heat ratio of low-temperature fluids is in the range required to balance the oceanic Sr isotope budget. Although the 87Sr/86Sr ratios of the JFR flank fluids in this study overlap with fluid properties inferred from some calcite veins in the upper oceanic crust, the magnitudes of the Δ (87Sr/86Sr)/heat ratios of nearly all of the JFR flank fluids are too large to be representative of the average global flank fluid flux; the same has been argued on the basis of the extremely high implied Mg flux.