Hydrothermal Settings Research Articles

Inorganic and biogenic As-sulfide precipitation at seafloor hydrothermal fields

Highlights: • As-sulfides are minor minerals at volcanic arc and back-arc hydrothermal settings. • There are two modes of As-sulfide precipitation: inorganic and biogenic. • The most stable As-sulfide at low-T seafloor hydrothermal conditions is orpiment. • Biogenic type of orpiment represents completely mineralized fungal hyphae. • Fungal orpiment precipitation is a mechanism of bio-sequestration of hydrothermal As. We investigated As-sulfides (orpiment, As2S3 and realgar, As4S4) from four seafloor hydrothermal fields: three from back-arc and one from mid-ocean ridge settings. Our studies suggest two different modes of As-sulfide precipitation at the seafloor hydrothermal sites: inorganic and biogenic. The Eh–pH modeling shows that the most stable As-sulfide at low-temperature (T < 100 °C) seafloor hydrothermal conditions is orpiment, whereas realgar appears as a stable As-sulfide phase at T = 25 °C and under a narrow range of Eh–pH values. Inorganically precipitated realgar forms euhedral crystals whereas the orpiment is mostly colloform. The biogenic type of orpiment (no realgar of this type was found) represents completely mineralized fungal hyphae. The proposed scenario of biogenic orpiment formation assumes that crack-dwelling fungal filaments in the sub-seafloor were killed by an As-rich hydrothermal fluid moving along the same crack network. The fungal organic matter served as a geochemical trap for hydrothermal As which reacted with S (leached from the basement or reduced from seawater sulfate) and mineralized the fungal hyphae as As2S3. This process of fungal orpiment precipitation is a mechanism of bio-sequestration of hydrothermal As and another mode of soft-tissue fossilization not described so far.

Biogeochemical implications of the ubiquitous colonization of marine habitats and redox gradients by Marinobacter species.

The Marinobacter genus comprises widespread marine bacteria, found in localities as diverse as the deep ocean, coastal seawater and sediment, hydrothermal settings, oceanic basalt, sea-ice, sand, solar salterns, and oil fields. Terrestrial sources include saline soil and wine-barrel-decalcification wastewater. The genus was designated in 1992 for the Gram-negative, hydrocarbon-degrading bacterium Marinobacter hydrocarbonoclasticus. Since then, a further 31 type strains have been designated. Nonetheless, the metabolic range of many Marinobacter species remains largely unexplored. Most species have been classified as aerobic heterotrophs, and assessed for limited anaerobic pathways (fermentation or nitrate reduction), whereas studies of low-temperature hydrothermal sediments, basalt at oceanic spreading centers, and phytoplankton have identified species that possess a respiratory repertoire with significant biogeochemical implications. Notable physiological traits include nitrate-dependent Fe(II)-oxidation, arsenic and fumarate redox cycling, and Mn(II) oxidation. There is also evidence for Fe(III) reduction, and metal(loid) detoxification. Considering the ubiquity and metabolic capabilities of the genus, Marinobacter species may perform an important and underestimated role in the biogeochemical cycling of organics and metals in varied marine habitats, and spanning aerobic-to-anoxic redox gradients.

Silver in geological fluids from in situ X-ray absorption spectroscopy and first-principles molecular dynamics

The molecular structure and stability of species formed by silver in aqueous saline solutions typical of hydrothermal settings were quantified using in situ X-ray absorption spectroscopy (XAS) measurements, quantum-chemical modeling of near-edge absorption spectra (XANES) and extended fine structure spectra (EXAFS), and first-principles molecular dynamics (FPMD). Results show that in nitrate-bearing acidic solutions to at least 200°C, silver speciation is dominated by the hydrated Ag+ cation surrounded by 4–6 water molecules in its nearest coordination shell with mean Ag–O distances of 2.32±0.02Å. In NaCl-bearing acidic aqueous solutions of total Cl concentration from 0.7 to 5.9mol/kg H2O (m) at temperatures from 200 to 450°C and pressures to 750bar, the dominant species are the di-chloride complex AgCl2− with Ag–Cl distances of 2.40±0.02Å and Cl–Ag–Cl angle of 160±10°, and the tri-chloride complex AgCl32− of a triangular structure and mean Ag–Cl distances of 2.60±0.05Å. With increasing temperature, the contribution of the tri-chloride species decreases from ∼50% of total dissolved Ag in the most concentrated solution (5.9m Cl) at 200°C to less than 10–20% at supercritical temperatures for all investigated solutions, so that AgCl2− becomes by far the dominant Ag-bearing species at conditions typical of hydrothermal–magmatic fluids. Both di- and tri-chloride species exhibit outer-sphere interactions with the solvent as shown by the detection, using FPMD modeling, of H2O, Cl−, and Na+ at distances of 3–4Å from the silver atom. The species fractions derived from XAS and FPMD analyses, and total AgCl(s) solubilities, measured in situ in this work from the absorption edge height of XAS spectra, are in accord with thermodynamic predictions using the stability constants of AgCl2− and AgCl32− from Akinfiev and Zotov (2001) and Zotov et al. (1995), respectively, which are based on extensive previous AgCl(s) solubility measurements. These data are thus recommended for chemical equilibrium calculations in mineral–fluid systems above 200°C. In contrast, our data disagree with SUPCRT-based datasets for Ag–Cl species, which predict large fractions of high-order chloride species, AgCl32− and AgCl43− in high-temperature saline fluids. Comparisons of the structural and stability data of Ag–Cl species derived in this study with those of their Au and Cu analogs suggest that molecular-level differences amongst the chloride complexes such as geometry, dipole moment, distances, and resulting outer-sphere interactions with the solvent may account, at least partly, for the observed partitioning of Au, Ag and Cu in vapor–brine and fluid–melt systems. In hydrothermal environments dominated by fluid–rock interactions, the contrasting affinity of these metals for sulfur ligands and the differences both in chemistry and stability of their main solid phases (Ag sulfides, Cu–Fe sulfides, and native Au) largely control the concentration and distribution of these metals in their economic deposits.

Neoarchaean seawater sulphate concentrations from sulphur isotopes in massive sulphide ore

The concentration of sulphate in today’s oceans—approximately 28,000 μmol l−1—is maintained by a balance between removal by pyrite burial and evaporite deposition and supply by oxidative weathering and the erosion of sulphate minerals from evaporites1. Oceanic sulphate concentrations were much lower before the rise of atmospheric oxygen about 2.4 Gyr ago2. The limited spread of δ34S values in sedimentary sulphides from 3.85 to 2.5 Gyr ago suggests that microbial sulphate reduction, if it played an important role in the Archaean marine sulphur cycle, must have occurred at sulphate concentrations of 200 μmol l−1 or less3. Here we use sulphur isotope systematics of the 2.7 Gyr old volcanogenic massive sulphide ore deposits from Kidd Creek, Ontario, to provide constraints on seawater sulphate concentrations independent of biological considerations. By comparing these values with metal and sulphur budgets from modern hydrothermal settings, we estimate that seawater sulphate concentrations 2.7 Gyr ago were roughly 80 μmol l−1. At these levels, the residence time of sulphate was on the order of 200,000 years, sufficiently long to make sulphate a conservative compound in the open ocean, but still short enough to suggest that hydrothermal sulphur fluxes were accompanied by a globally significant sink associated with microbial sulphate reduction.

Mineralogical variation of silica induced by Al and Na in hydrothermal solutions

Various silica minerals form in hydrothermal settings; however, the main controls on mineralogical variations in such environments remain poorly understood. In this study, we investigate the effects of Al and Na on the mineralogy of silica precipitates by hydrothermal flow-through experiments for silica precipitation at 430 °C and 31 MPa. We used solutions with elevated Si concentrations, exceeding the solubility of amorphous silica. As the Al and Na concentrations in the input solution were raised from 0 to 7 ppm, the dominant silica minerals systematically changed from amorphous silica to cristobalite to quartz. The positive correlation between Al and Na contents in precipitated quartz indicates that Al coupled with Na substitutes for Si. Our results suggest a possible mechanism for quartz vein formation in which quartz nucleates directly without precursor amorphous silica or cristobalite due to the presence of feldspar-derived Al and Na in hydrothermal fluids.

Effects of tides and weather on sedimentation of iron‐oxyhydroxides in a shallow‐marine hydrothermal environment at Nagahama Bay, Satsuma Iwo‐Jima Island, Kagoshima, southwest Japan

AbstractNagahama Bay of Satsuma Iwo‐Jima Island, southwest Japan, is an excellent place for studying sedimentation of iron‐oxyhydroxides by shallow‐marine low‐temperature hydrothermal activity. Its fishing port has a narrow entrance that limits exchange of seawater between the bay and open ocean, allowing the accumulation of fine‐grained precipitates of iron‐bearing materials (Fe‐oxyhydroxides) on the seafloor. The fishing port is usually filled with orange‐ to brown‐colored Fe‐rich water, and deposits &gt;1.5 m thick Fe‐rich sediments. To elucidate the movement and depositional processes of the Fe‐rich precipitates in the bay, we conducted continuous profiling of turbidity throughout the tidal cycle and monitoring of surface water. The results showed that clear seawater entered the bay during the rising tide, and turbid colored water flowed into the ocean during the ebb tide. Neap tide was found to be an optimal condition for sedimentation of Fe‐oxyhydroxides due to weak tidal currents. Storms and heavy rains were also found to have influenced precipitation of Fe‐oxyhydroxides. Storm waves disturbed the bottom sediments, resulting in increased turbidity and rapid re‐deposition of Fe‐oxyhydroxides with an upward‐fining sequence. Heavy rain carried Fe‐oxyhydroxides originally accumulated in nearby beach sands to the bay. Our findings provide information on optimal conditions for the accumulation of Fe‐rich sediments in shallow‐marine hydrothermal settings.

Earliest evidence for life on Earth: an Australian perspective

Australia is home to some of Earth's best-preserved early Archean rocks. Of particular interest in relation to the origin and evolution of life is the Pilbara Craton of Western Australia, containing one of Earth's oldest and least metamorphosed volcano-sedimentary successions. Over the past decade a number of new claims for Earth's earliest life have come out of these rocks, aided by significant advances in stratigraphic correlation and radiometric dating within this craton. This contribution provides an up-to-date summary of the evidence for life in Pilbara rocks >3000 Ma in age. Potential indicators of life in these rocks include microfossils, trace fossils, stromatolites, biofilms (aggregates of cells and associated extra-cellular substances that have enveloped or colonised a substrate) and microscopic sulfide minerals with distinctive sulfur isotope signatures from normal marine, marginal marine and hydrothermal settings. This shows that Australia contains the most diverse and arguably the most critically evaluated evidence for life on our planet prior to 3000 Ma.

Review of the Pilbara Craton and Fortescue Basin, Western Australia: Crustal evolution providing environments for early life

AbstractThe oldest part of the Pilbara Craton is 3.80–3.55 Ga crust. Between 3.53 and 3.22 Ga, mantle plume activity resulted in eight successive volcanic cycles forming the Pilbara Supergroup. Large volumes of granitic magma were intruded during the same period. By 3.22 Ga, a thick continental crust, the East Pilbara Terrane, had been established. Between 3.22 and 3.16 Ga, rifting of the East Pilbara Terrane separated off two additional terranes (Karratha and Kurrana), with intervening basins of oceanic crust. After 3.16 Ga, the three terranes began to converge, resulting in both obduction of oceanic crust (Regal Terrane) and, in another area, subduction to form a 3.13 Ga island arc (Sholl Terrane). At 3.07 Ga, the Karratha, Regal, and Sholl Terranes collided to form the West Pilbara Superterrane, and this collided with the East Pilbara Terrane. The 3.05–2.93 Ga De Grey Superbasin was deposited as a succession of basins: Gorge Creek, Whim Creek, Mallina, and Mosquito Creek. Eventual closure of the basins, between 2.94 and 2.93 Ga, formed two separate orogenic belts on either side of the East Pilbara Terrane. Post‐orogenic granites were intruded between 2.89 and 2.83 Ga. The 2.78–2.63 Ga Fortescue Basin developed in four stages: (i) rifting of the Pilbara Craton; (ii) folding and erosion; (iii) large igneous province (LIP) volcanism; and (iv) marine sedimentation on a passive margin. A review of all known evidence for early life in the Pilbara Craton is provided. In hydrothermal settings, most of the evidence occurs as filamentous and spheroidal microfossils, organic carbon, microbial mats, and rare stromatolites. By contrast, shallow‐water marine sedimentary rocks contain a diverse range of stromatolites, and microbial mats. Lacustrine and shallow‐water marine carbonate rocks in the Fortescue Basin contain abundant and morphologically diverse stromatolites, widespread microbial mats, and organic carbon.

Softening the “Crystal Scaffold” for Life’s Emergence

Del Giudice’s group studies how water can organize on hydrophilic surfaces forming coherent domains (loaning energy from the quantum vacuum), plus quasifree electrons, whose excitations produce cold vortices, aligning to ambient fields. Their electric and magnetic dipolar modes can couple to oscillatory (electric-organic dipoles) and/or rotary (magnetic-mineral dipoles), besides responding to magnetic potentials. Thus, imprinted electromagnetic patterns of catalytic colloids—compared with Cairns-Smith’s “crystal scaffold”—on their structured water partners could have equipped the latter with a selection basis for “choosing” their context-based “soft-matter” (de Gennes) replacements. We consider the potential of the scenario of an external control on magnetic colloids forming in the Hadean hydrothermal setting (of Russell and coworkers)—via a magnetic rock field—conceptually enabling self-assembly, induction of asymmetries, response effects towards close-to-equilibrium dynamics, and associative networks, besides providing a coherent environment for stabilizing associated symmetry-broken quanta and their feedback interactions with those of coherent water domains, to address the emergence of metabolism and replication.

Tourmaline as a Petrologic Forensic Mineral: A Unique Recorder of Its Geologic Past

Tourmaline is nature's perfect forensic mineral. From a single grain, the full geological past of its host rock can be reconstructed, including the pressure–temperature path it has taken through the Earth and the changing fluid compositions it has encountered. Tourmaline is able to provide this record owing to its compositional and textural sensitivity to the environment in which it grows, and is able to preserve this record because element diffusion in its structure is negligible. Furthermore, tourmaline has an exceptionally broad stability range, allowing it to record conditions in igneous, sedimentary, metamorphic, and hydrothermal settings. As our mineralogical and geochemical tools advance, we are able to interrogate tourmaline's memory with increasing precision, making tourmaline a truly powerful indicator of conditions in the Earth.

Origin of ferroan alabandite and manganoan sphalerite from the Tisovec skarn, Slovakia

Sulphidic skarn in an exo-contact zone between diorite and Triassic carbonates contains the Mn-rich minerals ferroan alabandite (Mn0.84-0.89Fe0.11-0.16S), manganoferroan sphalerite (Zn0.37-0.76Mn0.01-0..36Fe0.22-0.28S), hubnerite, manganoan magnesio horn- blende, and Mn-carbonates. The Mn-content in sphalerite (up to 21.4 wt. %) is the highest ever recorded in hydrothermal settings. Magnetite and sulphides like galena, pyrite, pyrrhotite, arsenopyrite are also present. Anhydrite is ubiquitous as inclusions in sulphide minerals and crystals in drusy cavities, postdating hydrothermal Ca-Mn-Fe carbonates. Primary aqueous inclusions with variable salinity (2.9-44.8 wt. % NaCl eq.) occur in sphalerite and Mn-calcite. The fl uid inclusions locally exhibit textural evidence for heterogeneous entrapment of boiling fl uids. Crystallization temperatures of 300-350 °C were obtained from various mineral geothermometers, δ 34 S values in coexisting galena and sphalerite, and infrared fl uid inclusion microthermometry. The observed anhydrite-chalcopyrite-magnetite-pyrrhotite-arsenopyrite-alabandite assemblage constrains fO2 and fS2values to -34.5 ± 1.5 and -12.5 ± 0.5 log units, respectively, at 300 °C and atmospheric pressure. Together with an assemblage of silicate minerals (chlorite, sericite, K-feldspar, hyalophane) and high-salinity fl uids, the calculated oxygen and sulphur fugacities are consistent with an inter- mediate sulphidation hydrothermal system. The increased δ 34 S values of the ore-forming fl uids, 2-2.5 ‰, result from the disequi- librium isotopic exchange between mantle-derived H2S and marine sulphate.

A demonstration of an affinity between pyrite and organic matter in a hydrothermal setting.

One of the key-principles of the iron-sulphur world theory is to bring organic molecules close enough to interact with each other, using the surface of pyrite as a substrate in a hydrothermal setting. The present paper explores the relationship of pyrite and organic matter in a hydrothermal setting from the geological record; in hydrothermal calcite veins from Carboniferous limestones in central Ireland. Here, the organic matter is accumulated as coatings around, and through, pyrite grains. Most of the pyrite grains are euhedral-subhedral crystals, ranging in size from ca 0.1-0.5 mm in diameter, and they are scattered throughout the matrix of the vein calcite. The organic matter was deposited from a hydrothermal fluid at a temperature of at least 200°C, and gives a Raman signature of disordered carbon. This study points to an example from a hydrothermal setting in the geological record, demonstrating that pyrite can have a high potential for the concentration and accumulation of organic materials.

Geothermal energy in sedimentary basins: What we can learn from regional numerical models

Understanding the interactions between the different processes that control the geothermal and fluid flow fields in sedimentary basins is crucial for exploitation of geothermal energy. Numerical models provide predictive and feasible information for a correct assessment of geothermal resources especially in areas where data acquisition is demanding. Here, we present results from numerical efforts to characterize the thermal structure and its interaction with the fluid system for the area of the North East German Basin (NEGB). The relative impact of the different (diffusive and advective) processes affecting the hydrothermal setting of the basin are investigated by means of three-dimensional numerical simulations. Lithospheric-scale numerical models are evaluated to understand the specific thermal signature of the relevant factors influencing the present-day conductive geothermal field in the NEGB. Shallow and deep structural controls on the thermal configuration of the basin are addressed and quantified. Interaction between the resulting thermal field and the active fluid system is investigated by means of three-dimensional simulations of coupled fluid flow and heat transport. Factors influencing stability and reliability of modeling predictions are discussed. The main effort is to build a physically consistent model for the basin which integrates the impacts of thermal gradients on the regional fluid regime and their coupling with the main geological units defining the basin.

Correction to “Parameters of subsurface brines and hydrothermal processes 12–15 months after the 1999 magmatic event at the Main Endeavor Field as inferred from in situ time series measurements of chloride and temperature”

[1] Phase separation at mid-ocean ridge hydrothermal systems is a widely distributed frequently occurring process. It is highly sensitive to dynamic subsurface pressure and temperature conditions and has a significant impact on chloride concentrations of hydrothermal fluid. Although vapor phases produced by phase separation are observed in a variety of hydrothermal settings, observation of conjugate brines that must be produced concurrently are more rare and the fate of subsurface brines is still not well understood. Here we use an array of in situ chloride sensors deployed at the Main Endeavor Field on the Juan de Fuca Ridge in the wake of a magmatic event to monitor the behavior of hydrothermal vent effluent with high temporal resolution. Our results provide evidence of near-critical chloride depleted fluids within the crust up to 486 m beneath the seafloor. We estimate that a brine reservoir stored beneath the Main Endeavor Field is constrained to temperatures between 131°C and 326°C for chloride concentrations ranging from 6 to 25 Wt.% NaCl, respectively. Our data suggest that subsurface fluids and circulation pathways vary widely over spatial scales on the order of meters. Under these circumstances, simultaneous measurements of multiple fluid parameters at a high temporal resolution are essential to understanding subsurface hydrothermal processes associated with magmatic events.

A model for Ischia hydrothermal system: Evidences from the chemistry of thermal groundwaters

Ischia volcano, in Central Italy, has long been known for its copious surface hydrothermal manifestations, signs of a pervasive circulation of hot fluids in the subsurface. Because of the significant chemical heterogeneity of fumarolic gas discharges and hot spring discharges, evidences of a complex hydrothermal setting, a definite model of fluid circulation at depth is currently unavailable, in spite of the several previous efforts. Here, we report on the chemical and isotopic composition of 120 groundwater samples, collected during several sampling surveys from 2002 to 2007. The acquired data suggest that the composition of surface manifestations reflect contributions from meteoric water, sea water, and thermal fluids rising from two distinct hydrothermal reservoir, with equilibrium temperatures of respectively ~ 150 °C and ~ 270 °C, and depths of 150–300 m and > 300 m (but possibly > 1000 m). We also make use of an isotopic characterization of the dissolved gas phase in thermal waters to demonstrate that the Ischia hydrothermal system is currently supplied by a deep-rising gas component (DGC), characterized by CO 2 ~ 97.7 ± 1.2 vol.% (on a water-free basis), δ 13C CO2 = − 3.51 ± 0.9‰, and helium isotopic ratio of about 3.5 Ra ( 3He/ 4He ratio normalized to the air ratio, R a), likely magmatic in origin. An assessment of the thermal budget for Ischia hydrothermal system is also presented, in the attempt to derive a first estimate of the size and rate of degassing of the magmatic reservoir feeding the gas emissions. We calculate that a heat flow of about 153–222 MW presently drives hydrothermal circulation on the island, which we suggest is supplied in convective form (e.g., by the ascent of a high- T magmatic vapour phase) by complete degassing of 2.2–3.3 · 10 7 m 3 yr − 1 of trachytic magma (with ~ 2.1 wt.% dissolved H 2O content). If extrapolated to entire period of quiescence lasting since the Arso eruption in 1302 A.D., this volume corresponds to 1.6–2.3·10 10 m 3 of magma degassed in about 700 years of quiescent activity.

Survival of mussels in extremely acidic waters on a submarine volcano

Increasing atmospheric carbon dioxide levels are causing ocean acidification, compromising the ability of some marine organisms to build and maintain support structures. An analysis of mussels from a submarine volcano setting with natural low-pH conditions shows low shell thicknesses and growth rates, but survival over up to four decades. Increasing atmospheric carbon dioxide levels are causing ocean acidification1,2, compromising the ability of some marine organisms to build and maintain support structures3 as the equilibrium state of inorganic carbon moves away from calcium carbonate4. Few marine organisms tolerate conditions where ocean pH falls significantly below today’s value of about 8.1 and aragonite and calcite saturation values below 1 (refs 5, 6). Here we report dense clusters of the vent mussel Bathymodiolus brevior in natural conditions of pH values between 5.36 and 7.29 on northwest Eifuku volcano, Mariana arc, where liquid carbon dioxide and hydrogen sulphide emerge in a hydrothermal setting. We find that both shell thickness and daily growth increments in shells from northwest Eifuku are only about half those recorded from mussels living in water with pH>7.8. Low pH may therefore also be implicated in metabolic impairment7. We identify four-decade-old mussels, but suggest that the mussels can survive for so long only if their protective shell covering remains intact: crabs that could expose the underlying calcium carbonate to dissolution are absent from this setting. The mussels’ ability to precipitate shells in such low-pH conditions is remarkable. Nevertheless, the vulnerability of molluscs to predators is likely to increase in a future ocean with low pH.

Parameters of subsurface brines and hydrothermal processes 12–15 months after the 1999 magmatic event at the Main Endeavor Field as inferred from in situ time series measurements of chloride and temperature

Phase separation at mid‐ocean ridge hydrothermal systems is a widely distributed frequently occurring process. It is highly sensitive to dynamic subsurface pressure and temperature conditions and has a significant impact on chloride concentrations of hydrothermal fluid. Although vapor phases produced by phase separation are observed in a variety of hydrothermal settings, observation of conjugate brines that must be produced concurrently are more rare and the fate of subsurface brines is still not well understood. Here we use an array of in situ chloride sensors deployed at the Main Endeavor Field on the Juan de Fuca Ridge in the wake of a magmatic event to monitor the behavior of hydrothermal vent effluent with high temporal resolution. Our results provide evidence of near‐critical chloride depleted fluids within the crust up to 486 m beneath the seafloor. We estimate that a brine reservoir stored beneath the Main Endeavor Field is constrained to temperatures between 131°C and 326°C for chloride concentrations ranging from 6 to 25 Wt.% NaCl, respectively. Our data suggest that subsurface fluids and circulation pathways vary widely over spatial scales on the order of meters. Under these circumstances, simultaneous measurements of multiple fluid parameters at a high temporal resolution are essential to understanding subsurface hydrothermal processes associated with magmatic events.

In‐situ grown silica sinters in Icelandic geothermal areas

Field in-situ sinter growth studies have been carried out in five geochemically very different Icelandic geothermal areas with the aim to quantify the effects of water chemistry, (e.g. silica content (250 to 695 p.p.m. SiO(2)), salinity (meteoric to seawater), pH (7.5 to 10)), temperature (42-96 degrees C) and microbial abundance (prevalence, density) on the growth rates, textures and structures of sinters forming within and around geothermal waters. At each location, sinter growth was monitored over time periods between 30 min and 25 months using glass slides that acted as precipitation substrates from which sinter growth rates were derived. In geothermal areas like Svartsengi and Reykjanes, subaqueous sinters developed rapidly with growth rates of 10 and 304 kg year(-1 )m(-2), respectively, and this was attributed primarily to the near neutral pH, high salinity and medium to high silica content within these geothermal waters. The porous and homogeneous precipitates that formed at these sites were dominated by aggregates of amorphous silica and they contained few if any microorganisms. At Hveragerdi and Geysir, the geothermal waters were characterized by slightly alkaline pH, low salinity and moderate silica contents, resulting in substantially lower rates of sinter growth (0.2-1.4 kg year(-1 )m(-2)). At these sites sinter formation was restricted to the vicinity of the air-water interface (AWI) where evaporation and condensation processes predominated, with sinter textures being governed by the formation of dense and heterogeneous crusts with well-defined spicules and silica terraces. In contrast, the subaqueous sinters at these sites were characterized by extensive biofilms, which, with time, became fully silicified and thus well preserved within the sinter edifices. Finally, at Krafla, the geothermal waters exhibited high sinter growth rates (19.5 kg year(-1 )m(-2)) despite being considerably undersaturated with respect to amorphous silica. However, the bulk of the sinter textures and structure were made up of thick silicified biofilms and this indicated that silica precipitation, i.e. sinter growth, was aided by the surfaces provided by the thick biofilms. These results further suggest that the interplay between purely abiotic processes and the ubiquitous presence of mesophilic and thermophilic microorganisms in modern silica rich terrestrial hydrothermal settings provides an excellent analogue for processes in Earth's and possibly Mars's ancient past.

The effects of volcanic eruptions on atmospheric chemistry

Volcanoes are very strong sources of sulphur, acids and other gases, as well as particles, that are of atmospheric relevance. Some gases only behave as passive tracers, others affect the formation, growth or chemical characteristics of aerosol particles and many lead to adverse effects on vegetation and human health when deposited in the vicinity of volcanoes. In this article the main effects of volcanic emissions on atmospheric chemistry are discussed, with a focus on sulphur and halogen compounds, and to a smaller extent on climate. We primarily focus on quiescent degassing but the main effects of explosive eruptions on the troposphere and stratosphere are covered as well. The key distinction between chemistry in magmatic and hydrothermal settings and the atmosphere is that the atmosphere is oxidising whereas the chemistry is typically reducing in the former cases due to very low oxygen concentrations. Rapid catalytic cycles involving radicals are a further characteristic of atmospheric chemistry. Most reaction cycles involve the photolysis of molecules as a key part of the reaction chains. Recent measurements of halogen radicals in volcanic plumes showed that volcanic plumes are chemically very active. We explain the formation mechanism of halogen oxides in plumes as well as their relevance for the atmosphere.

Presence and activity of anaerobic ammonium-oxidizing bacteria at deep-sea hydrothermal vents

Recent studies indicate that ammonia is an important electron donor for the oxidation of fixed nitrogen, both in the marine water column and sediments. This process, known as anammox, has so far only been observed in a large range of temperature habitats. The present study investigated the role of anammox in hydrothermal settings. During three oceanographic expeditions to the Mid-Atlantic Ridge, hydrothermal samples were collected from five vent sites, at depths ranging from 750 to 3650 m from cold to hot habitats. Evidence for the occurrence of anammox in these particular habitats was demonstrated by concurrent surveys, including the amplification of 16S rRNA gene sequences related to known anammox bacteria, ladderanes lipids analysis and measurement of a (14)N(15)N dinitrogen production in isotope-pairing experiments at 60 and 85 degrees C. Together these results indicate that new deep-branching anammox bacteria may be active in these hot habitats.