Margaret K. Tivey

Geology of a vigorous hydrothermal system on the Endeavour Segment, Juan de Fuca Ridge

A high-precision, high-resolution geologic map explicitly documents relationships between tectonic features and large steep-sided, sulfide-sulfate-silica deposits in the vigorously venting Endeavour hydrothermal field near the northern end of the Juan de Fuca Ridge. Water depth in the vent field varies from 2220 to 2200 m. Location of the most massive sulfide structures appears to be controlled by intersections of ridge-parallel normal faults and other fracture-fissure sets that trend oblique to, and perpendicular to the overall structural fabric of the axial valley. The fractured basaltic substrate is primarily composed of well-weathered pillow and lobate flows. As presently mapped, the field is about 200 by 400 m on a side and contains at least 15 large (> 1000 m3) sulfide edifices and many tens of smaller, commonly inactive, sulfide structures. The larger sulfide structures are also the most vigorously venting features in the field; they are commonly more than 30 m in diameter and up to 20 m in height. Actively venting sulfide structures in the northern portion of the field stand higher and are more massive than active structures in the southern portion of the field which tend to be slightly to distinctly smaller. Maximum venting temperatures of 375°C are associated with the smaller structures in the southeastern portion of the field; highest-temperature venting fluids from the more massive structures in the northern portion of the field are consistently 20°–30°C lower. Hydrothermal output from individual active sulfide features varies from no flow in the lower third of the edifice to vigorous output from fracture-controlled black smoker activity near the top of the structures. A different type of high temperature venting takes place from the upper sides of the structures in the form of “overflow” from fully exposed, quiescent pools of buoyant 350°C vent water trapped beneath overhanging sulfide-sulfate-silica ledges, or flanges. These flanges are attached to the upper, outer walls of the large sulfide edifices. Two types of diffuse venting in the Endeavour field include a lower temperature 8°–15°C output through colonies of large tubeworms and 25°–50°C vent fluid that seems to percolate through the tops of overhanging flanges. The large size and steep-walled nature of the these structures evidently results from sustained venting in a “mature” hydrothermal system, coupled with dual mineral depositional mechanisms involving vertical growth by accumulation of chimney sulfide debris and lateral growth by means of flange development.

Deducing patterns of fluid flow and mixing within the TAG active hydrothermal mound using mineralogical and geochemical data

The TAG active hydrothermal mound, located 2.4 km east of the neovolcanic zone at 26°N, Mid-Atlantic Ridge, is ∼200 m in diameter, exhibits 50 m of relief, and is covered entirely by hydrothermal precipitates. Eight different types of vent solids were recovered from the mound by the submersibles Alvin and Mir in 1986, 1990, and 1991. Detailed petrographic and geochemical studies of samples and their distribution are used to deduce patterns of fluid flow and seawater/hydrothermal fluid interaction. Geochemical modeling calculations using fluid composition data corroborate these interpretations. Current activity includes highly focused flow of 363°C fluid from a chimney cluster on the top of the mound and deposition of a high fS 2 -fO 2 mineral assemblage that reflects low concentrations of H 2 S in black smoker fluid. Slow percolation of black smoker fluid pooled beneath the black smoker cluster and entrainment of seawater result in formation of massive sulfide crusts and massive anhydrite. These three sample types are enriched in Co and Se. Blocks of sulfide and white smoker chimneys, enriched in Zn, Au, Ag, Sb, Cd, and Pb, are forming on the surface of the mound from black smoker fluid that has been modified by mixing with entrained seawater, precipitation of sulfides and anhydrite, and dissolution of sphalerite within the mound. This is the first time that on-going remobilization, zone refinement, and significant modification of high-temperature fluid in the near surface has been documented in a seafloor hydrothermal system. Deposits of ocherous material and massive sulfide with outer oxidized layers that formed during previous hydrothermal episodes are exposed on the steep outer walls of the mound. Studies of the full range of samples demonstrate that highly focused fluid flow, consequent seawater entrainment, and mixing within the mound can result in formation of a large seafloor hydrothermal deposit exhibiting sample types similar to those observed in Cyprus-type ore bodies.

Hydrothermal vents near a mantle hot spot: the Lucky Strike vent field at 37'N on the Mid-Atlantic Ridge

The Lucky Strike hydrothermal field occurs in the summit basin of a large seamount that forms the shallow center of a 65 km long ridge segment near 37°N on the Mid-Atlantic Ridge. The depth and chemistry of the ridge segment are influenced by the Azores hot spot, and this hydrothermal field is the first Atlantic site found on crust that is dominated by a hot spot signature. Multiple hydrothermal vents occur over an area of at least 300 m by 700 m. Vent morphologies range from flanges and chimneys with temperatures of 200–212°C, to black smoker chimneys with temperatures up to 333°C. Cooler fluids from northern vents have higher chlorinities and lower gas volumes, while hotter, southern fluids have chlorinities 20% below seawater with higher gas volumes, suggesting phase separation has influenced their compositions. All gas volumes in fluids are higher than those at TAG and Snake Pit hydrothermal fields. Black smokers exhibit their typical mineralogy, except that barite is a major mineral, particularly at lower-temperature sites, which contrasts with previously investigated Atlantic sites. The fluid chemistry, distribution of the relict sulfide deposits on the seamount summit in the areas investigated using DSV Alvin, and contact relationships between active vent sites and surrounding basaltic and sulfide substrate suggest that the hydrothermal system has a long history and may have recently been rejuvenated. Fauna at the Lucky Strike vent sites are dominated by a new species of mussel, and include the first reported sea urchins. The Lucky Strike biological community differs considerably from other vent fauna at the species level and appears to be a new biogeographic province. The Lucky Strike field helps to constrain how variations in the basaltic substrate influence the composition of hydrothermal fluids and solids, because basalt compositions at Lucky Strike are 10–30 times enriched in incompatible elements compared to other Atlantic hydrothermal sites such as TAG, Snake Pit and Broken Spur. The incompatible element

A ubiquitous thermoacidophilic archaeon from deep-sea hydrothermal vents

Deep-sea hydrothermal vents are important in global biogeochemical cycles, providing biological oases at the sea floor that are supported by the thermal and chemical flux from the Earths interior. As hot, acidic and reduced hydrothermal fluids mix with cold, alkaline and oxygenated sea water, minerals precipitate to form porous sulphide–sulphate deposits. These structures provide microhabitats for a diversity of prokaryotes that exploit the geochemical and physical gradients in this dynamic ecosystem. It has been proposed that fluid pH in the actively venting sulphide structures is generally low (pH < 4.5), yet no extreme thermoacidophile has been isolated from vent deposits. Culture-independent surveys based on ribosomal RNA genes from deep-sea hydrothermal deposits have identified a widespread euryarchaeotal lineage, DHVE2 (deep-sea hydrothermal vent euryarchaeotic 2). Despite the ubiquity and apparent deep-sea endemism of DHVE2, cultivation of this group has been unsuccessful and thus its metabolism remains a mystery. Here we report the isolation and cultivation of a member of the DHVE2 group, which is an obligate thermoacidophilic sulphur- or iron-reducing heterotroph capable of growing from pH 3.3 to 5.8 and between 55 and 75 °C. In addition, we demonstrate that this isolate constitutes up to 15% of the archaeal population, providing evidence that thermoacidophiles may be key players in the sulphur and iron cycling at deep-sea vents.

Growth of large sulfide structures on the endeavour segment of the Juan de Fuca ridge

Abstract Mapping and sampling with DSRV “Alvin” has established that sulfide blocks 0.5 m across, dredged from the axial valley of the Endeavour Segment at 47°57′N, are samples of unusually large sulfide structures. The steep-sided structures, up to 30 m in length, 20 m in height, and 10–15 m across, are localized by venting along normal faults at the base of the western axial valley wall, and are distributed for about 200 m along strike paralleling the 020 trend of the ridge crest. High-temperature fluids (350 to more than 400°C) pass through the massive sulfide structures and enter seawater through small, concentric “nozzle-like” features projecting from the top or the sides of the larger vent structures. Diffuse, low-temperature flow is pervasive in the vicinity of the active sulfide structures, exiting from basalt and sulfide surfaces alike. Evidence of recent volcanic activity is sparse. The two largest samples taken with the dredge would not have been recoverable using the submersible. These samples represent massive, complex portions of the sulfide structures which were not closely associated with rapid high-temperature fluid flow at the time of sampling; they contain textural evidence of sealed hydrothermal fluid exit channels. Mineralogy is dominated by Fe sulfides nnd amorphous silica. Pyrite, marcasite, wurtzite, chalcopyrite, and iss are the most common sulfide phases. Pyrrhotite, galena, and sphalerite are present in trace amounts. Barite, amorphous silica, and chalcedony are the only non-sulfide phases; anhydrite is not observed in any of the dredge samples, although it is common in the chimney-like samples recovered by “Alvin”. Specific mineralogical-textural zones within the dredge samples are anaoogous to individual layers in East Pacific Rise at 21°N and southern Juan de Fuca Ridge samples, with two exceptions: a coarse-grained, highly porous Fe sulfide-rich interior containing sulfidized tubeworm casts, and a 2–5 cm thick zone near the outer margin of the samples dominated by late stage amorphous silica. The porous interior may have formed by dendritic crystal growth from a slowly circulating fluid within a large enclosed chamber. The amorphous silica deposited from a seawater/hydrothermal fluid mixture percolating slowly through the walls of the enclosed chamber; conductive cooling of the fluid as it traversed the walls allowed amorphous silica to precipitate. These silica-rich zones are the densest, most durable portions of the structures and may be responsible for the lasting stability of the large sulfide features. Observations in these samples are consistent with two distinct phases of development. Phase 1 is analogous to chimney growth and construction at 21°N and ends when flow channels become sealed to rapid flow of through-going fluid. The flow is evidently redirected within the structure. Phase 2 includes dissolution of anhydrite and precipitation of amorphous silica during conductive cooling of sluggishly circulating hydrothermal fluid or seawater/hydrothermal fluid mixtures. Evolution of vent structures through phase 2 allows lateral and vertical growth of unusually large structures.

An integrated, underwater optical /acoustic communications system

Communication underwater is severely limited when compared to communications in air because water is essentially opaque to electromagnetic radiation except in the visible band. Even in the visible band, light penetrates only a few hundred meters in the clearest waters and much less in waters made turbid by suspended sediment or high concentrations of marine life. Consequently, acoustic techniques have been developed for underwater communication systems and now represent a relatively mature and robust technology. Acoustic systems are capable of long range communication, but offer limited data rates and significant latency (due to the speed of sound in water). We are developing an optical communication system that complements and integrates with existing acoustic systems resulting in an underwater communications capability offering high data rates and low latency when within optical range combined with long range and robustness of acoustics when outside of optical range. Amongst a wide array of applications, this combination of capabilities will make it possible to operate self-powered ROVs from support vessels or platforms without requiring a physical connection to the ROV. Such a capability will help simplify operations and potentially reduce costs through the use of less capable surface vessels. New deployment strategies may offer game-changing opportunities within all areas of undersea activities. For example, rapid event response will be enhanced and repair and maintenance of the emerging ocean observatory infrastructure will become more cost effective. Such through-water communications will likewise enable exchange of large data files from fixed sensors using AUVs (or ROVs) as data mules, shuttling real-time video from untethered vehicles for inspection, identification, and other related operations. Interconnectivity for dense arrays of underwater sensors without the need for expensive and difficult to install undersea cables is also possible. An unmanned battery operated vehicle, dedicated to a subsea node, that can be wirelessly operated though a combination of acoustic and optical communications, will be an important asset for both scientific exploration and commercial applications.

The influence of hydrothermal fluid composition and advection rates on black smoker chimney mineralogy: Insights from modeling transport and reaction

Abstract A detailed study of black smoker chimneys was carried out by drawing together mineralogical observations and measured fluid compositions. The modeling technique, which establishes a link between vent fluid chemistry and vent deposit mineralogy, is used to examine the influence of the physical environment on transport across short length scales (centimeters) which are subject to steep thermal ( ∼350°C) and chemical gradients, and to predict saturation states of minerals as functions of position within chimney walls. Owing to short length scales, steep gradients, and constancy of bounding fluid compositions, concentration profiles are controlled by transport; the effect of reaction rates on profiles is assumed to be negligible. Precipitation at points within the chimney wall is thus kinetically controlled; local equilibrium does not apply, and precipitation must be suppressed when calculating distributions of species and saturation states of minerals. Model calculations are performed in a series of four steps. The first is speciation of measured fluid compositions at in situ temperature and pressure conditions. Results of calculations are plotted on stability diagrams of log fO2 - log f S2 and log fO2 - log [ (a Fe 2+ ) (a cu + ) 2 ] to examine how closely the predicted saturation states fit with observations of minerals coexisting with the hydrothermal fluid at the vent sites. The second step is calculation of heat and mass transport across the chimney wall, the third is speciation of the pore fluid compositions calculated to exist in the wall subsequent to transport, and the fourth is comparison of calculated mineral saturation states to observations of naturally occurring chimney samples. Application of the modeling technique to five vent sites which exhibit distinct fluid compositions allows reproduction of observed mineralogy. Model results demonstrate the sensitivity of geochemical processes to the physical environment (porosity, tortuosity, permeability, mineral composition), to rates of advection of seawater inward or hydrothermal fluid outward across chimney walls, and to endmember fluid composition.

Insights into tide-related variability at seafloor hydrothermal vents from time-series temperature measurements

Abstract Thermocouple/thermistor array packages and an in situ gamma detector were deployed in 1994 at two vent sites on the northern Cleft Segment of the Juan de Fuca Ridge. Continuous records of fluid temperatures were obtained in four separate locations over a period of 5.5 months, and these data were supplemented by current meter observations made ∼2.5 km to the south within the axial valley. Temperatures measured at a location of focused high temperature flow showed that: (1) the maximum temperature in the chimney was stable and did not exhibit tide-related variability; (2) temperatures within the chimney wall were variable on time-scales of minutes, indicating rapid shifts in amounts of cold seawater or hot vent fluid flowing across chimney walls; and (3) the stable maximum temperature within the chimney conduit was ∼9°C less than the maximum fluid temperature recorded at the vent site during the same time interval, and thus stable high temperatures within chimneys are not necessarily indicative of the maximum temperature within the vent structure or the hydrothermal system. Time-series records from areas of diffuse flow indicate modulation of temperature and total radioactivity by tidally induced changes in bottom currents. Spectra of the current meter record and temperature records are similar, with spectral peaks observed at 12.4 h, 16–17 h (inertial peak) and 4–5 days. Phases between maxima in current, tide and temperature records are consistent with temperature changes resulting from periodic shifts in currents from north to south, and the subsequent northward or southward advection of warm fluids venting from multiple local sources. Periodic (12.4 h) variability of temperature was also recorded by a thermocouple buried ∼1 cm within one of the deposits and is likely a result of periodic variations in the temperature at the boundary of the highly conductive sulfide deposit. The time-series results presented demonstrate the need for measuring and considering the effects of local currents when investigating causes of temporal variability within seafloor hydrothermal systems.

A model for growth of steep-sided vent structures on the Endeavour Segment of the Juan de Fuca Ridge: Results of a petrologic and geochemical study

Petrologic and geochemical studies of vent solids from the Main Endeavour Field (MEF) and the High Rise Field (HRF), Juan de Fuca Ridge, demonstrate that the steep-sided vent structures characteristic of these sites form dominantly by flange growth, combined with diffuse flow through sealed portions of structures, and incorporation of flanges into structures. Geochemical calculations suggest that the prevalence of amorphous silica and flanges in Endeavour deposits is the result of conductive cooling of vent fluids that have high concentrations of ammonia. At Endeavour, as the temperature of vent fluid decreases, ammonia-ammonium equilibrium buffers pH and allows more efficient deposition of sulfide minerals and silica from fluids that have a higher pH than conductively cooled ammonia-poor fluids present at most other unsedimented mid-ocean ridge vent sites. Deposition of silica stabilizes flanges and allows structures to attain large size. It also leads to diffuse flow and further conductive cooling by reducing the permeability and porosity of the structures and of feeder zones, thus decreasing entrainment of seawater. Most inactive vent samples recovered from areas peripheral to the HRF and MEF are similar to barite + silica rich samples from the Explorer Ridge and Axial Seamount and likely formed from precipitation of silica and barite on a biological substrate. Active white smoker chimneys from the Clam Bed Field, located south of the HRF, are pyrrhotite rich and likely formed from vent fluids that are depleted in Zn and Cd and enriched in Pb and Ba relative to fluids exiting trans-Atlantic geotraverse (TAG) and Cleft Segment white smoker chimneys.

In situ measurement of dissolved H2 and H2S in high-temperature hydrothermal vent fluids at the Main Endeavour Field, Juan de Fuca Ridge

The first in situ measurements of dissolved H2 and H2S in high-temperature vent fluids were made at the Main Endeavour Field (Juan de Fuca Ridge) using the submersible Alvin and a newly developed electrochemical sensor. The measurements were successfully conducted in chimneys at sites of venting fluid and in pools of more quiescent hydrothermal fluid that underlie flanges on chimney structures at a depth of 2200 m below the sea surface. Fluid temperatures measured simultaneously with dissolved gas concentrations were up to 370°C. At the highest temperatures, dissolved H2 and H2S concentrations were 0.72 and 17.3 mmol/kg, respectively, which are consistent with data obtained at the same sites through conventional sampling methods. The relatively high concentration of dissolved gases measured by both techniques, however, may be linked to recent tectonic and volcanic activity. The ability to measure in situ dissolved gas concentrations simultaneously with fluid temperature in real time represents a major advance in the approaches available to study the origin and temporal evolution of seafloor hydrothermal systems at mid-ocean ridges. Although the present investigation is primarily based on sensor deployment for relatively short-term measurement of vent fluids, long-term monitoring of vent fluid holds great promise for further applications.