Hydrothermal Vent Waters Research Articles

New boron isotopic evidence for sedimentary and magmatic fluid influence in the shallow hydrothermal vent system of Milos Island (Aegean Sea, Greece)

Magmatic sources may contribute a significant amount of volatiles in geothermal springs; however, their role is poorly understood in submarine hydrothermal systems worldwide. In this study, new results of B and δ11B in 41 hydrothermal vent waters collected from the shallow hydrothermal system of Milos island in the Aegean Sea were combined with previously published data from other tectonic settings and laboratory experiments to quantify the effects of phase separation, fluid/sediment interaction and magmatic contribution. Two Cl-extreme solutions were identified, high-Cl waters (Cl as high as 2000mM) and low-Cl waters (Cl <80mM). Both sets of waters were characterized by high B/Cl (~1.2–5.3×10−3mol/mol) and extremely low δ11B (1.4–6.3‰), except for the waters with Mg content of near the seawater value and δ11B=10.3–17.4‰. These high-Cl waters with high B/Cl and low δ11B plot close to the vent waters in sediment-hosted hydrothermal system (i.e., Okinawa Trough) or fumarole condensates from on-land volcanoes, implying B addition from sediment or magmatic fluids plays an important role. This is in agreement with fluid/sediment interactions resulting in the observed B and δ11B, as well as previously reported Br/I/Cl ratios, supporting a scenario of slab-derived fluid addition with elevated B, 11B-rich, and low Br/Cl and I/Cl, which is derived from the dehydration of subducted-sediments. The slab fluid becomes subsequently mixed with the parent magma of Milos. The deep brine reservoir is partially affected by injections of magmatic fluid/gases during degassing. The results presented here are crucial for deciphering the evolution of the brine reservoirs involved in phase separation, fluid/sediment interaction and magmatic contribution in the deep reaction zone of the Milos hydrothermal system; they also have implications in the understanding of the formation of metallic vein mineralization.

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).

Lysogenic virus–host interactions predominate at deep-sea diffuse-flow hydrothermal vents

The consequences of viral infection within microbial communities are dependent on the nature of the viral life cycle. Among the possible outcomes is the substantial influence of temperate viruses on the phenotypes of lysogenic prokaryotes through various forms of genetic exchange. To date, no marine microbial ecosystem has consistently shown a predisposition for containing significant numbers of inducible temperate viruses. Here, we show that deep-sea diffuse-flow hydrothermal vent waters display a consistently high incidence of lysogenic hosts and harbor substantial populations of temperate viruses. Genetic fingerprinting and initial metagenomic analyses indicate that temperate viruses in vent waters appear to be a less diverse subset of the larger virioplankton community and that these viral populations contain an extraordinarily high frequency of novel genes. Thus, it appears likely that temperate viruses are key players in the ecology of prokaryotes within the extreme geothermal ecosystems of the deep sea.

Genome sequence of the deep-sea γ-proteobacterium Idiomarina loihiensis reveals amino acid fermentation as a source of carbon and energy

We report the complete genome sequence of the deep-sea gamma-proteobacterium, Idiomarina loihiensis, isolated recently from a hydrothermal vent at 1,300-m depth on the Loihi submarine volcano, Hawaii. The I. loihiensis genome comprises a single chromosome of 2,839,318 base pairs, encoding 2,640 proteins, four rRNA operons, and 56 tRNA genes. A comparison of I. loihiensis to the genomes of other gamma-proteobacteria reveals abundance of amino acid transport and degradation enzymes, but a loss of sugar transport systems and certain enzymes of sugar metabolism. This finding suggests that I. loihiensis relies primarily on amino acid catabolism, rather than on sugar fermentation, for carbon and energy. Enzymes for biosynthesis of purines, pyrimidines, the majority of amino acids, and coenzymes are encoded in the genome, but biosynthetic pathways for Leu, Ile, Val, Thr, and Met are incomplete. Auxotrophy for Val and Thr was confirmed by in vivo experiments. The I. loihiensis genome contains a cluster of 32 genes encoding enzymes for exopolysaccharide and capsular polysaccharide synthesis. It also encodes diverse peptidases, a variety of peptide and amino acid uptake systems, and versatile signal transduction machinery. We propose that the source of amino acids for I. loihiensis growth are the proteinaceous particles present in the deep sea hydrothermal vent waters. I. loihiensis would colonize these particles by using the secreted exopolysaccharide, digest these proteins, and metabolize the resulting peptides and amino acids. In summary, the I. loihiensis genome reveals an integrated mechanism of metabolic adaptation to the constantly changing deep-sea hydrothermal ecosystem.

Sulfur speciation monitored in situ with solid state gold amalgam voltammetric microelectrodes: polysulfides as a special case in sediments, microbial mats and hydrothermal vent waters.

Sulfur speciation was determined in real time in salt marsh microbial mats, subtidal sediments and hydrothermal vent diffuse flow waters using solid state gold-amalgam voltammetric microelectrodes. Chemical species were measured in situ without any sample manipulation or processing. The partially oxidized sulfur species detected were polysulfides, thiosulfate, elemental sulfur and tetrathionate. Fe(III) oxidation of hydrogen sulfide does not occur within the mats where microbially mediated processes are responsible for oxidation of H2S. In sediments and diffuse flow vent waters, Fe(III) phases are the direct oxidant of H2S. Sulfur speciation determined in this work is due to in situ biogeochemical processes and is not due to artefacts of sample manipulation. The voltammetric data show that polysulfides are the first detectable intermediate during sulfide oxidation which is consistent with previous laboratory studies.

Quantifying elemental sulfur (S 0), bisulfide (HS −) and polysulfides (S x2−) using a voltammetric method

Free polysulfides (S 4 2−), elemental sulfur (S 0) and bisulfide (HS −) can be separated at discrete peak potentials using either cyclic voltammetry (CV) or linear sweep voltammetry (LS) with extremely fast scan rates. A scan rate of 1000 mV/s was found to be optimal for peak separation. Differences in the nucleophilic nature of S x 2− versus HS − and the electrochemical irreversibility of the S 0 in S x 2− allowed for peak separation. For 10 μM solutions at a scan rate of 1000 mV/s, peak potentials were found to occur at −0.66 V for HS − and −0.69 V for S 0. S 4 2− was found to give two discrete peaks at −0.70 and −0.81 V, representing a two step mechanism. In mixed laboratory standards, all the three sulfur species were uniquely identified. Flectrochemical analyses revealed similar peak separation patterns under different environmental conditions, ranging from estuarine pore waters to hydrothermal vents. In estuarine sediments, the sulfur speciation was found to change throughout a core profile, with S 0 dominant in the top layers (0–6 cm), S x 2− dominant in the transition zone (6–7 cm), and HS − dominant in the deeper sediment (>7 cm). In hydrothermal vent waters, different regions were observed to have different sulfur speciation. In diffuse flow regions away from chimneys containing trace O 2 concentrations, polysulfides, elemental sulfur and bisulfide were discretely identified.

The Geochemistry of Hydrothermal Vent Waters from Milos Island, Hellenic Volcanic Arc

The Hellenic Volcanic Arc is a tectonically and volcanically active part of the earth's crust, situated above the subduction zone of the African plate under the Aegean. The arc extends from the Methana peninsula (NE Peloponese) through the greek islands Milos, Santorini, Kos, Nisiros and Yali to the Bodrum peninsula, Turkey (e.g. Fytikas et at., 1986). Hydrothermal venting has been reported from Nisiros, Yali (Varnavas and Cronan, 1991), Santorini (Varnavas and Cronan, 1988) and Milos (Fytikas, 1989). The island of Milos, at the central west part of the volcanic arc, is dominated by volcanic and voIcaniclastic rocks, with the exception of minor occurrences of metamorphic basement rocks and Neogene sediments outcropping at the south part of the island, underneath the volcanics (Fytikas et al., 1986). Submarine and, later, sub-aerial extrusive sequences formed during a prolonged period of volcanic activity (3.5 to 0.1 Ma, Fytikas, 1989). Geochemically, the Milos volcanics show a calcalkaline character and comprise andesites, dacites and ryolites (Fytikas et al., 1986). Hot water springs and fumaroles occur at several localities around the island. In submarine settings, hot fluids exit through the sandy seafloor, often in the form of a mixed liquid/ gas phase; as a result, venting can be located due to the visually impressive presence of streams of gas bubbles. In the most hydrothermally active south-east part of the island (Palaeohori bay), extensive areas of bacteria mats are associated with submarine hydrothermal brine seeps (Fitzsimons et al., 1997). Field observations and sampling

Are undiscovered hydrothermal vents in the southern Red Sea the main source of silica and helium 3 for the Red Sea Deep Water?

Geochemical Ocean Sections Study (GEOSECS) Indian Ocean Expedition data show that silica and 3He in the Red Sea Deep Water are linearly correlated, suggesting a common hydrothermal source. Silica sections for the Red Sea from the GEOSECS and Mer Rouge (MEROU) expeditions show a strong vertical silica maximum centered at 500 to 600 m depth. The MEROU sections, which have higher horizontal resolution than the GEOSECS sections, suggest that the primary silica source lies in the south at that depth. The interpretation is that the large‐scale silica and 3He distributions are due to the dispersal in the Red Sea Deep Water of warm or hot hydrothermal vent waters associated with the seafloor spreading center of the southern Red Sea. The helium‐ and silica‐rich hot brines of the central Red Sea probably cannot provide the large fluxes of silica or 3He required to maintain the inventory of either of these deep water properties in steady state. The hypothesis of a common hydrothermal source for silica and 3He can most readily be tested by measurements of the germanium‐silicon ratio in Red Sea Deep Water.

Benthic foraminiferal distribution at Middle Valley, Juan de Fuca Ridge, a northeast Pacific hydrothermal venting site

Fourteen benthic foraminiferal bearing surface samples collected from the Area of Active Venting (AAV) in Middle Valley at a depth of 2430 m at the northern end of Juan de Fuca Ridge, northeast Pacific Ocean, yielded 156 identifiable taxa. Sample sites were selected near active or recently active vents, where temperatures up to 274 °C have been measured. The sea floor beyond hydrothermal mounds is characterized by unaltered soft hemipelagic mud and is dominated by a calcareous foraminiferal assemblage which includes numerous allochthonous species derived from the shelf via turbidite flows into the valley. Within the AAV, agglutinated foraminifera dominate with proximity to hydrothermal vent activity. Species of the family Hormosinidae, subfamily Reophacinae, dominate in the hydrothermal habitat because of their success at colonization. Attached agglutinated foraminifera densely colonize the indurated sediment in close proximity to vent sites. However, like the macrofauna observed near hydrothermal vents, foraminifera do not venture into the extreme habitats where temperatures are greater than 20 °C. Where clam beds are found in association with active hydrogen sulphide venting, benthic foraminifera occur rarely. These beds occur in loose pyritic mud that is toxic to all but species (e.g. the clam Calyptogena) that co-exist symbiotically with sulphur-oxidizing bacteria. The sparse distribution of calcareous benthic foraminifera in the vent areas is caused by enhanced carbonate dissolution at pH values of 5.2. Low foraminiferal abundance and species diversity at the immediate site of active venting is attributed to rapid changes in physical and chemical conditions caused by circulating hydrothermal vent waters.

Rare earth element evidence of origin and depositional environment of Late Proterozoic ironstone beds and manganese-oxide deposits, SW Brazil and SE Bolivia

Ironstone beds and high grade Mn-oxide deposits (layers or concretions) of Late Precambrian age occur in SW Brazil and SE Bolivia. These deposits, referred to here as Jacadigo-Boqui Fe-Mn deposits after the stratigraphic units hosting most of them, share many characteristics with other siliceous, Precambrian iron formations. Rare earth element (REE) distributions in the Jacadigo-Boqui Fe-Mn deposits were determined by instrumental neutron activation analysis (INAA) and compared with published data for other iron formations and for various water types. REE patterns of both ironstone beds and Mn-oxide deposits are light REE (LREE) enriched on chondrite-normalized plots. Average REE ratios of all samples are: La/Sm = 5.66, La/Ce = 0.63, Eu/Sm = 0.19, Sm/Lu = 8.09. Sm/Lu values for all samples range from 2 to 16, and Sm/Lu values also vary among samples of ironstone beds and Mn-oxide deposits. REE ratios are generally not related to other chemical parameters. Comparison to REE distributions in river, estuarine, and oceanic waters and to particulate matter forming in the oceans and anoxic basins suggests that the Jacadigo-Boqui Fe-Mn deposits formed in waters most similar to those of estuaries or coastal oceans. Formation in river or lake water appears unlikely. The lower Sm/Lu values of oxide layers suggest that these layers formed in a more marine environment than the jasper layers. The balance of continental and marine inputs may have been changed by glacial cycles or sea level changes, either eustatic or tidal. REE patterns of the Jacadigo-Boqui Fe-Mn deposits are roughly similar to those for most other iron formations. However, La/Ce and Eu/Sm values are much lower. A glaciogenic model suggests that the ironstone beds formed in a basin under an ice shelf as a result of freezing. Nodular, detritus-rich Mn-oxide deposits probably formed off the edge of the ice shelf. Layered Mn-oxide deposits most probably formed rapidly during glacial melting and retreat of the shelf as a result of quantitative oxidation of basin water. Glacial melting or lower sea level should result in higher Sm/Lu and lower La/Ce and La/Sm values of chemical sediments (i.e., values more like those of rivers). However, the similarity of REE ratios of layered Mn-oxides and ironstone beds suggest that overturn and oxidation dominated over mixing of marine and river waters during deposition of layered Mn-oxides. The negative Eu anomalies of the Jacadigo-Boqui Fe-Mn deposits distinguish them from most natural waters and other iron formations and are surprising in light of the hypothesis that banded iron formations derive their Fe and Mn from sea-floor, hydrothermal vent waters. It is unlikely that these anomalies were produced during precipitation of Fe- and Mn-oxides from waters with oceanic or mid-ocean and ridge hydrothermal REE patterns. Mixing of REE from a continental water with a negative Eu anomaly and REE from oceanic and ridge hydrothermal waters is believed unlikely because it can not explain all the REE ratios and presents problems regarding the source of Fe and Mn. Alternatives that must be evaluated are derivation of substantial Fe, Mn, and REE from either a continental source or a hydrothermal fluid that had a REE pattern different from that of mid-ocean ridge hydrothermal fluids.

Controls over the chloride concentration of submarine hydrothermal vent fluids: evidence from Sr/Ca and 87Sr/ 86Sr ratios

Strontium and calcium are transported as chloride complexes in submarine hydrothermal vent waters with the Sr and Ca concentrations being controlled by equilibration with secondary mineral phases (mainly epidote and anorthite-albite). The relative proportions of albite and anorthite precipitated from hydrothermal fluids are also dependent on the Cl level of the fluids due to sodium metasomatism of the oceanic crust. The S/Ca ratio of albite is higher than coexisting anorthite, hence the Sr/Ca ratios of the vent waters are also a function Cl concentrations. The Sr isotope composition of hydrothermal fluids is determined by the mixture of seawater Sr passing through the anhydrite precipitation zone and Sr leached from basalt. The proportion and absolute amount of seawater Sr in the hydrothermal waters can be calculated from the 87Sr/ 86Sr ratio of seawater, local basalt and the vent fluids themselves, and is also dependent on the Cl concentrations of the fluids. These two observations have been used to examine the factors controlling chloride concentration of submarine hydrothermal fluids. The data support precipitation of a Cl-rich mineral phase and hydration of oceanic crust as the major processes controlling Cl concentrations in intermediate-Cl vents, such as those at 21°N (East Pacific Rise) and the TAG and MARK sites (Mid-Atlantic Ridge). The Cl levels of vent fields with extreme-Cl concentrations (Juan de Fuca and 11–13°N EPR) appear to be controlled by supercritical brine condensation followed by mixing of the brine and vapor phases.

A laser-based fluorometry system for investigations of seawater and porewater fluorescence

A highly sensitive laser-induced fluorescence (LIF) system has been developed to study the fluorescence of dissolved organic carbon (DOC) in the marine environment. The LIF detector has a detection limit of ∼ 10 attomoles (10 × 10 −18 moles) of pterin and eliminates internal quenching in highly fluorescent samples such as anoxic porewaters encountered when using conventional fluorometry. LIF analysis is rapid, reproducible, and uses only 100 μl of a sample. This small size requirement permits fluorescence analyses of samples often available only in limited amounts, such as porewaters, hydrothermal vent waters, and rainwaters. In addition, the LIF detection system may greatly simplify extraction and separation procedures required to characterize the fluorescent components of DOC.

Petroleum generation, an easy and widespread process in hydrothermal systems: an overview

The conversion of organic matter to petroleum products by hydrothermal activity is an easy process, occurring in nature in many types of environments. Geologically immature organic matter of marine sediments has been shown to be altered to petroleum products by this process in the Guaymas Basin, Escanaba Trough, Bransfield Strait and Atlantis II Deep and this process probably occurs elsewhere in active sedimented rift systems. Contemporary organic detritus and/or viable microorganisms are also converted in part to petroleum-like products when they become entrained by turbulent mixing into hydrothermal vent waters, resulting in their instantaneous hydrous pyrolysis. This latter case occurs in hydrothermal vent fields without a sedimentary cover emanating directly from oceanic ridges, as for example on the East Pacific Rise at 13°N and 21°N and on the Mid-Atlantic Ridge at 26°N. This process is probably also ubiquitous for all actively venting sediment-starved oceanic ridges. The hydrocarbon products generated in these areas have been compared in terms of composition, organic matter sources and analogy to reservoir petroleum. They represent a major carbon source for chemosynthetic assimilation by vent biota and are compositionally similar to normal petroleum.

A laser-based fluorometry system for investigations of seawater and porewater fluorescence

A highly sensitive laser-induced fluorescence (LIF) system has been developed to study the fluorescence of dissolved organic carbon (DOC) in the marine environment. The LIF detector has a detection limit of ∼ 10 attomoles (10 × 10−18 moles) of pterin and eliminates internal quenching in highly fluorescent samples such as anoxic porewaters encountered when using conventional fluorometry. LIF analysis is rapid, reproducible, and uses only 100 μl of a sample. This small size requirement permits fluorescence analyses of samples often available only in limited amounts, such as porewaters, hydrothermal vent waters, and rainwaters. In addition, the LIF detection system may greatly simplify extraction and separation procedures required to characterize the fluorescent components of DOC.

Hydrocarbons associated with hydrothermal minerals, vent waters and talus on the East Pacific Rise and Mid-Atlantic Ridge

Two active hydrothermal vent sites on the East Pacific Rise (EPR) at 13°N and 21°N have similar biogeochemicals. They are characterized by large animal communities surrounding massive porous mixed mineral (e.g. sulfide) edifices venting hot water (200–350°C) as black smokers. An analogous vent system on the Mid-Atlantic Ridge (MAR) at 26°N has also been studied. Sediment (talus), mound mineral and water samples from these areas were analyzed in terms of biological marker lipids such as normal and isoprenoid alkanes, polynuclear aromatic hydrocarbons, triterpanes, steranes and fatty acids to describe their sources, bacterially-mediated transformation products and alteration by hydrothermal fluids. The mixed interior/exterior of a chimney at 21°N on the EPR shows a series of iso- and Anteiso-alkanes which are known constituents of high wax crude oils. These alkanes, coupled with the absence of triterpanes and steranes seem to indicate a higher temperature, water-soluble fraction from hydrothermal pyrolysis which is unfavorable for the preservation of the biomarkers. Chimney fragments with tube worm casts at 21°N contain products that are characteristic of immature biodegraded organic matter as indicated by microbial markers (branched C 15 and C 17 fatty acids) and low thermal alteration which enhanced formation of cholestene isomers and diacholestenes. In comparisons, a metalliferous sediment and sediment trap particles at 13°N on the EPR contain a biomaker mixture, namely recent biomarkers derived from early diagenetic transformations of biogenic compounds (sterols, squalene) and more mature components [5α(H), 14β(H), 17β(H) steranes (20 R + 20 S), dominant 17α(H)-hopane series], both having the same origin. Hydrocarbons isolated from hydrothermal anhydrite, chalcopyrite and sphalerite-enriched samples from 26°N on the MAR exhibit a high degree of thermal maturation, compared to an Fe-oxide-enriched sample, as evidenced by the high abundance of low molecular weight n-alkanes ranging from C 14 to C 25 with a broad distribution of naphthenes. All these trace hydrothermally derived petroleums contain polycyclic aromatic hydrocarbons (PAH). The Fe-oxide-enriched sample has a lower content of PAH than the other mineral samples; however, benzopyrenes and coronene are present at significant levels and a series of mature triterpanes is also detectable. These trace levels of petroleum with the associated biomarkers and PAH strongly suggest generation from contemporary organic detritus by hydrothermal activity and subsequent association with the minerals, talus, vent fluids, and ambient water.

The strontium isotope budget of the modern ocean

We have measured the Sr concentration and isotopic composition of most of the world's major rivers. These data greatly extend the range of geologic, tectonic and climatic drainage basin environments that have been studied and, overall, constitute a doubling of the fluvial Sr isotope data base. This has produced a more accurate estimate for the 87Sr/ 86Sr ratio of global runoff of 0.7119; this is distinctly higher than obtained from previous studies. We have also determined the Sr concentration and 87Sr/ 86Sr ratio of oceanic hydrothermal vent waters from diverse tectonic and volcanogenic environments that have allowed us to further constraint the Sr isotope systematic of this source. These new data have been combined with previously published riverine data and values for the diagenetic benthic flux of Sr from deep-sea sediments to obtain a contemporary oceanic Sr isotope budget. The results of steady-state calculations give a best estimate for the flux of seawater through high-temperature ridge crest hydrothermal system of1.2 ± 0.3 × 10 14 kg/yr; this is substantially higher than recent estimates.

A microbiological study of Guaymas Basin high temperature hydrothermal vents

Water samples and suspended particulate matter were collected from three high temperature (156–319°C) hydrothermal vents in the Guaymas Basin during July 1985, and were analysed for a variety of chemical constituents and for the presence of viable microorganisms. Our results indicate that black smoker fluids (> 150°C) are devoid of recognizable bacteria and contain negligible concentrations of ATP (<10 ng 1 -1) and low concentrations of particulate organic matter. In contrast, vent water samples collected in the hydrothermal plume at a distance of only 25 cm from the point of hot fluid discharge contain a diverse, metabolically active bacterial assemblage, high ATP concentrations (up to 372 ng 1 -1) and high concentrations of particulate organic carbon and nitrogen (up to 999 and 163 μg 1 -1, respectively). The hottest waters (> 150°C) displayed a low, but measurable level of metabolic activity (incorporation of 3H-adenine and 14C-glutamate) at temperatures ranging from 25 to 80°C, 1 atm with maximal activity at 45°C. The maximum rate of incorporation, however, was only ∼ 1% of the activity measured in the samples collected in the hydrothermal plume. The strong metabolic preference for mesophilic growth temperatures (45°C) argues against a high temperature origin. Thermophilic sulfur-respiring bacteria were isolated from a variety of source materials including black smoker vent waters (112–319°C). All positive enrichments grew at 80°C, but none survived at temperatures in excess of 93°C. We conclude that these bacterial cells did not originate from the high temperature hydrothermal vent waters. Two different sample devices (reffered to as vent cap and smoker poker) were designed and employed during in situ collection-incubation experiments. Neither the vent cap attachment-colonization experiment nor the smoker poker collection device revealed the existence of bacterial cells in the black smoker fluids, despite evidence for a significant bacterial population in the hydrothermal plume. Our field and laboratory results indicate that thermophilic and mesophilic bacteria observed in hydrothermal vent plumes are not derived from the hot hydrothermal fluids but must originate in peripheral habitats.

Hydrothermal vent waters at 13°N on the East Pacific Rise: isotopic composition and gas concentration

Gas concentrations and isotopic compositions of water have been measured in hydrothermal waters from 13°N on the East Pacific Rise. In the most Mg-depleted samples (≌ 5 × 10 −3 moles/kg) the gas concentrations are: 3–4.5 × 10 −5 cm 3 STP/kg helium, 0.62–1.24 cm 3 STP/kg CH 4, 10.80–16.71 × 10 −3 moles/kg CO 2. The samples contain large quantities (95–126 cm 3/kg) of H 2 and some carbon monoxide (0.26–0.36 cm 3/kg) which result from reaction with the titanium sampling bottles. δ 13C in methane and CO 2 (−16.6 to −19.5 and −4.1 to −5.5 respectively) indicate temperatures between 475 and 550°C, whereas δ 13C CO is compatible with formation by reduction of CO 2 on Ti at 350°C close to the sampling temperature. 3He/ 4He are very homogeneous at (7.5 ± 0.1)R A( 3He/ 4He = 1.0 × 10 −5) and very similar to already published data as well as CH 4/ 3He ratios between 1.4 and 2.1 × 10 6. 18O and D in water show enrichments from 0.39 to 0.69‰ and from 0.62 to 1.49‰ respectively. These values correspond to W/R ratios of 0.4–7. The distinct 18O enrichments indicate that the isotopic composition of the oceans is not completely buffered by the hydrothermal circulations. The 3He-enthalpy relationship is discussed in terms of both hydrothermal heat flux and 3He mantle flux.

Ocean ridge origin and tectonic setting of mesozoic sulphide and oxide deposits of the Argolis Peninsula of the Peloponnesus, Greece

Metal-oxide and sulphide deposits, chemically similar to the hydrothermal deposits of modern and ancient oceanic spreading axes, occur in two areas of the Argolis Peninsula of the Peloponnesus, Greece. In the first, in northeastern and central Argolis, manganese ores immediately overlie MORB-type extrusives of mid-Jurassic age. After Triassic rifting this oceanic crust formed during Jurassic time and was then tectonically emplaced over the adjacent continental margin in the Late Jurassic. The manganese ores (Palea Epidavros) were precipitated from low-temperature hydrothermal vent waters on the axis of a relatively unrifted ocean ridge, which was possibly slow-spreading. The deep-sea sediment cover comprises pelagic carbonate accumulated on the ridge flanks, then ribbon radiolarites deposited on the abyssal plain below the CCD, located close enough to a continental margin to allow deposition of shallow water-derived calciturbidites. In the second area, Ermioni, in the southern Argolis Peninsula (Adheres), massive sulphides and metalliferous oxide sediments occur with mafic extrusives and terrigenous turbidites. Although previously regarded as a single stratigraphical succession (“Shale-sandstone Formation”), the mafic lavas and metal deposits are interpreted here as ophiolitic slices that were incorporated into the turbidites by thrusting, possibly as a trench-accretionary complex. The massive sulphides were precipitated from high-temperature vents sited near the axis of a relatively unrifted spreading ridge which was possibly fast-spreading. Ferruginous (ochres) and ferromanganiferous (umbers) oxide sediments were dispersed around these vents, whereas strongly fractionated manganese ores precipitated from separate low-temperature hydrothermal sites. Alternative tectonic scenarios are discussed.

Ocean ridge origin and tectonic setting of mesozoic sulphide and oxide deposits of the Argolis Peninsula of the Peloponnesus, Greece

Metal-oxide and sulphide deposits, chemically similar to the hydrothermal deposits of modern and ancient oceanic spreading axes, occur in two areas of the Argolis Peninsula of the Peloponnesus, Greece. In the first, in northeastern and central Argolis, manganese ores immediately overlie MORB-type extrusives of mid-Jurassic age. After Triassic rifting this oceanic crust formed during Jurassic time and was then tectonically emplaced over the adjacent continental margin in the Late Jurassic. The manganese ores (Palea Epidavros) were precipitated from low-temperature hydrothermal vent waters on the axis of a relatively unrifted ocean ridge, which was possibly slow-spreading. The deep-sea sediment cover comprises pelagic carbonate accumulated on the ridge flanks, then ribbon radiolarites deposited on the abyssal plain below the CCD, located close enough to a continental margin to allow deposition of shallow water-derived calciturbidites. In the second area, Ermioni, in the southern Argolis Peninsula (Adheres), massive sulphides and metalliferous oxide sediments occur with mafic extrusives and terrigenous turbidites. Although previously regarded as a single stratigraphical succession (“Shale-sandstone Formation”), the mafic lavas and metal deposits are interpreted here as ophiolitic slices that were incorporated into the turbidites by thrusting, possibly as a trench-accretionary complex. The massive sulphides were precipitated from high-temperature vents sited near the axis of a relatively unrifted spreading ridge which was possibly fast-spreading. Ferruginous (ochres) and ferromanganiferous (umbers) oxide sediments were dispersed around these vents, whereas strongly fractionated manganese ores precipitated from separate low-temperature hydrothermal sites. Alternative tectonic scenarios are discussed.