Area Of Hydrothermal Activity Research Articles

Submarine hydrothermal fluids facilitated the accumulation of cadmium in Lower Cambrian black shales, South China

The effects of Cd-rich venting fluids on the Cd contents and isotopic compositions of continental margin sediments remain poorly understood. Characterization of the Cd isotopic composition of sediment overprinted by venting fluids is critical, as this may provide insights into how mixing between the venting fluids and bottom waters affects the Cd isotopic composition. Our study determined the Cd isotopic compositions of Lower Cambrian metalliferous sediment deposited on the passive continental margin of the southeastern Yangtze Block, South China. The Cd is predominantly hosted in sphalerite of sulfide-bearing barite sediments, V deposits, and Ni-Mo−platinum group element (PGE) sulfide layers. The Cd isotopic compositions of samples from these three units indicate that the distribution of Cd at this type of continental margin was controlled by submarine hydrothermal activity. Following the venting of hydrothermal fluids onto the seafloor, Cd2+ reacted with a limited amount of H2S generated mainly by thermochemical sulfate reduction near the vents. This resulted in isotopically lighter Cd in the sulfide-bearing barite sediments and V deposits, which have δ114/110Cd values of 0.00‰ ± 0.13‰ and 0.00‰ ± 0.14‰, respectively. The residual dissolved Cd in the fluids migrated with the upwelling seawater and ultimately precipitated almost entirely as sphalerite in the sulfidic wedge, which resulted in Ni-Mo−PGE sulfide layers with a narrow range of high δ114/110Cd values (0.35‰ ± 0.03‰). The presence of Cd-rich venting fluids can alter the Cd contents and isotopic compositions of passive margin sediments, particularly those located within long-term stratified seawater and fault depressions. Previous studies may have underestimated the migration distance and affected areas of hydrothermal activity involving Cd-rich fluids.

New sponge species of the family Vulcanellidae (Demospongiae: Tetractinellida) from the Piip submarine volcano and adjacent areas (Bering Sea, NW Pacific)

Four new species of vulcanellid sponges (Demospongiae: Tetractinellida), Poecillastra malyutini sp. nov., Vulcanella koltuni sp. nov., V. commander sp. nov. and V. grebelnyi sp. nov., are described from the Piip submarine volcano and adjacent area (the Bering Sea, North Pacific). This work includes materials sampled using the ROV Comanche 18 during the cruises of the RV Akademik M.A. Lavrentyev in 2016 and 2018. These specimens were studied and illustrated using light and electron (SEM) microscopy and micro-computed tomography (micro-CT). The distribution of species is characterized by pronounced vertical zoning. In areas of hydrothermal vents and on their periphery, Demospongiae are abundant with a pronounced dominance of a single species, V. koltuni sp. nov. The δ13C value for V. koltuni sp. nov. and P. malyutini sp. nov. from the volcano summits were almost 2-fold lower than for V. commander sp. nov. collected outside the areas of hydrothermal activity. These isotopic signatures indicate the organic matter of chemosynthetic origin in the diet of V. koltuni sp. nov. and P. malyutini sp. nov. Phylogenetic analysis based on 18S rDNA does not confirm the phylogenetic relationship of Lamellomorpha Bergquist, 1968 with the family Vulcanellidae Cárdenas, Xavier, Reveillaud, Schander & Rapp, 2011.

Near-bottom survey of hydrothermal sulfide on an extremely magmatic accretion segment along an ultraslow-spreading ridge

An extremely magmatic accretion could exist along an ultraslow-spreading ridge that has been approved and surveyed, but less attention has been paid to its associated hydrothermal system, as it is supposed to be similar to those of fast/medium-spreading ridges. Here using an autonomous underwater vehicle and a human-occupied vehicle, a systematic investigation was conducted on the hydrothermal system of an extremely magmatic accretion segment along the Southwest Indian Ridge (50°28′ E). A collapsed chimney, microbial mat, and diffused fluid were observed in the old Duanqiao hydrothermal field (DQF), which was earlier a weakly active hydrothermal area. Notably, evident hydrothermal anomalies, including oxidation–reduction potential (ORP), turbidity, and methane, were detected above the new DQF, which lies southwest of the old DQF. Based on this investigation, we concluded that ORP is one of the most effective parameters available to identify vent location by analyzing each type of detected hydrothermal anomaly and the near-bottom magnetic inversion can image the suspected hydrothermal fluid circulation pathways. In general, we believe that the new DQF is controlled by a magmatic chamber different from those found on fast/medium-spreading ridges, and could be a large hydrothermal system.

Hydrothermal clays and their influence on slope stability in residual soils of Southern Brazil

Clayey veins may develop in areas of hydrothermal activity by seepage of hot water through the soil and rock. In a slope located in Southern Brazil, such veins occur along the slip surface of some landslides. This paper aims to characterize this kind of clay and investigate its influence over the stability of that slope. An electro resistivity survey had been conducted to characterize the soil profile, and SPT boreholes were used to confirm geophysical data. An experimental laboratory campaign included the investigation of chemical, mineralogical, and geotechnical properties of these clay veins and the embedding residual soil. Drained shear strength of residual soil was measured through triaxial tests, and residual strength was measured for both materials using a ring shear apparatus. A series of limit equilibrium analyses were conducted to assess the role of clay veins on the slope stability. The geometrical discretization of veins is a difficult task, and an approach based on the assumption of overall shear strength is required to establish the operational shear strength parameters for this slope. A series of back analyses showed that failure surfaces consistent with those observed in the field were only reproduced when the adopted shear strength parameters of the soil mass were c’ = 5 kPa and φ’ = 20°. Considering the measured shear strength parameters of the hydrothermal clay c’ = (0 and φ’ = 8.5°) and the pattern in which the clay veins occur, it seems that they do not fully control the slope stability, but they significantly reduce the global cohesion and the average mass friction angle.

Classical and computed tomographic anatomical analyses in a not-so-cryptic Alviniconcha species complex from hydrothermal vents in the SW Pacific

The chemosymbiotic gastropod Alviniconcha (Provannidae), first described in 1988, is one of the most emblematic hydrothermal-vent taxa described from the Central Indian Ridge and the Southwest (SW) Pacific. Symbiotic bacteria found in the gill of Alviniconcha are thought to be their principal source of nutrition. In the SW Pacific, species distributions for A. kojimai, A. boucheti – and to a lesser extent A. strummeri – overlap. While Alviniconcha species do not appear to truly co-exist in these highly energetic but spatially limited habitats, certain species regularly co-occur within a single vent field and in rare instances, the same edifice. Past research suggests that SW-Pacific Alviniconcha species might aggregate around fluids with distinct geothermal profiles. These small-scale distribution patterns have been attributed to differences in their symbiont assemblages or host physiologies. However, little is known about the anatomy of most Alviniconcha species, beyond that detailed for the type species Alviniconcha hessleri, whose geographic range does not overlap with other congeners. In fact, species within this genus are currently described as cryptic, despite the absence of any comparative morphological studies to assess this. To test whether the genus is genuinely cryptic and identify any functional differences in host anatomy that might also mediate habitat partitioning in SW Pacific species, the current study examined the morphoanatomy of A. kojimai, A. boucheti and A. strummeri from the Fatu Kapa vent field, an area of hydrothermal activity recently discovered north of the Lau Basin near the Wallis and Futuna Islands and the only known example where all three species occur within adjacent vent fields. A combination of detailed dissections, histology and X-ray computed tomography demonstrate that A. kojimai, A. strummeri and A. boucheti are readily identifiable based on shell morphology and ornamentation alone, and therefore not truly cryptic. These traits provide a rapid and reliable means for species identification. However, aside from some subtle differences in radular morphology, these species of Alviniconcha exhibit conserved anatomical features, providing no evidence that functional host anatomy is implicated in habitat partitioning. This provides support for the current belief that host-species distributions are probably governed by symbiont-mediated physiological factors.

Eualus amandae (Decapoda: Caridea: Thoridae) is an indicator of active venting sites in the Southern Ocean

We report in situ observations that reveal the presence of the thorid genus Eualus at hydrothermally active sites. The shrimp Eualus amandae Nye, Copley & Linse, 2013 was first collected in non-venting sites but near areas of hydrothermal activity, on the East Scotia Ridge segment E9 and in the Kemp Caldera, South Sandwich Arc. During a recent expedition of RV Polarstern, specimens of Eualus amandae were observed via a remotely operated vehicle (ROV) at the East Scotia Ridge segment E2 and Kemp Caldera. The animals were seen in shimmering water sites on pillow basalts and at active hydrothermal orifices, both unambiguously hydrothermally active. These sites were also characterised by other vent marginal fauna, such as deep-water comatulid crinoids or stoloniferean cnidarians. The shrimp family Thoridae is more diverse in Antarctic waters than other shrimp families and these records suggest two independent origins of hydrothermal-related habitats in Lebbeus, and now Eualus. These records expand the understanding of the contribution of geothermal activity to larger patterns of Antarctic deep-sea biodiversity.

Seismic and Geodetic Evidences of a Hydrothermal Source in the Md 4.0, 2017, Ischia Earthquake (Italy)

AbstractSeismic events characterize active hydrothermal and volcanic areas and may be due to magma/fluid migration, hydrothermal pressurization, gravitational instability, and local tectonics. On 21 August 2017, an Md 4.0 earthquake occurred at Ischia volcanic island (Italy), within an active hydrothermal system. We analyze seismic, Global Positioning System, and interferometric synthetic aperture radar data to shed light on the source mechanism of such an event. The low‐frequency content (2 Hz), the low stress drop (0.01 MPa), and a low S/P spectral ratio suggest the involvement of fluids in the source mechanism. The focal mechanism suggests a mixed shear‐tensile (opening) rupture with the P first arrivals showing up movements in the nearest stations. Geodetic data describe an E‐W elongated area of coseismic subsidence overlapping a WSW‐ENE fault bounding the hydrothermal reservoir at depth. The modeled deformation field is consistent with a two‐source model consisting of a WSW‐ESE striking, north dipping normal fault, and a closing subhorizontal crack. This closure immediately followed an initial opening related to a fluid pressurization event responsible for the earthquake. We show that moderate magnitude earthquakes in active hydrothermal areas may be associated with the pressurization/depressurization cycles of a hydrothermal reservoir due to self‐sealing processes and not to the arrival of new fluids from depth. Other events like that recorded at Ischia, which have affected the island in historical times, are not necessarily associated with ‘volcanic unrest’ episodes and imply the occurrence of fault‐valve mechanisms. Therefore, the dynamics of hydrothermal systems must be taken into account in the seismic hazard evaluation.

Mineralogical, crystal morphological, and isotopic characteristics of smooth slope travertine deposits at Reshuitang, Tengchong, China

Reshuitang is an important active hydrothermal area in the Tengchong geothermal field and is characterized by many hot springs and their associated deposits. Detailed mineralogical, crystal morphological, and stable isotopic investigations of the smooth slope fossil travertines at Reshuitang and hydrochemical studies of two active springs around the fossil travertines were conducted to study their genesis. The results show that 1) the modern springs discharge waters with high temperature (>70 °C), near-neutral pH (6.3–8.34), and high saturation indices of calcite and aragonite; 2) the fossil travertines contain calcite and small amounts of Mn-Fe oxides, halite, thenardite, and non‑carbonate debris; 3) the calcite include ray crystals, dendrite crystals, platy crystals, and spike crystals; and 4) δ13C and δ18O of the fossil travertines at Reshuitang range from −1.59‰ to −0.87‰ and 6.48‰ to 7.80‰, respectively. According to comparison between the travertines at Reshuitang and other travertines in Tengchong, all these travertines are thermogene travertines, with a large contribution of mantle degassing to their parent CO2, but are formed at different temperatures. The fossil travertines at Reshuitang, similar to those at Shuzhishi, are formed at high temperatures (near 100 °C), which results in weak microbial activities during the formation of the fossil travertines. Most of the calcite crystals at Reshuitang were non-equilibrium products precipitated directly from the spring waters during the deposition process, while the spike crystals originated from diagenetic dissolution of large calcite crystals. Comparison between the fossil travertines at Reshuitang and other smooth slope travertines from other areas shows these smooth slope travertines have different lithofacies and fabrics. In smooth slope environments, temperature may control the distribution and development of microbes and the intensity of CO2 degassing, and consequently cause the precipitation of contrasting fabrics and lithofacies.

Evidence of a shallow water submarine hydrothermal field off Zannone Island from morphological and geochemical characterization: Implications for Tyrrhenian Sea Quaternary volcanism

AbstractDiscoveries from multibeam bathymetry and geochemical surveys performed off Zannone Island (western Pontine Archipelago, Tyrrhenian Sea) provide evidence of an undocumented hydrothermal field characterized by ongoing fluid emissions and morphologically complex giant depressions located in shallow water (<150 m water depth). Based on a detailed morpho‐bathymetric study we identify the seabed morphologies produced by hydrothermal fluid emission activity. We recognize five giant depressions (length >250 m) that host pockmarks, mounds, small cones, and active fluid vents, which are interpreted as complex fluid‐escape features developed both through vigorous‐explosive events and steady seepage. Their spatial distribution suggests that the NE‐SW trending faults bounding the Ponza‐Zannone structural high and the shallow fractured basement are favorable conditions for the upward migration of hydrothermal fluids. Moreover, we performed a detailed geochemical study to investigate the source of the hydrothermal fluids. The geochemical signature of the collected fluids provides information of active CO2‐dominated degassing with a significant contribution of mantle volatiles, with measured 3He/4He values > 3.0 Ra that are similar to those recorded at Stromboli and Panarea volcanoes. The hydrothermal system produces volatiles that may originate from residual magma batches, similar to the Pleistocene trachytes cropping out in the SE sector of Ponza Island that were probably intruded in the shallow crustal levels and never erupted. The discovery of the Zannone hydrothermal field updates the record of active hydrothermal areas of the Mediterranean Sea. Moreover, the recognition of several giant hydrothermal depressions characterized by a complex morphology is peculiar for the Mediterranean Sea.

Reconstruction of the geology and structure of Lake Rotomahana and its hydrothermal systems from high-resolution multibeam mapping and seismic surveys: Effects of the 1886 Tarawera Rift eruption

Present-day Lake Rotomahana is one of the two focal points of the most destructive eruption in New Zealand's historical record, i.e., that of Mt. Tarawera on 10 June 1886, with devastating loss of life and presumed destruction of the iconic Pink and White Terraces that adorned the margins of the lake. Basaltic dikes are considered to have ascended near surface in the area, intruding into hydrothermally altered and water-saturated ground beneath the existing lake. The consequential hydrothermal and phreatomagmatic eruptions ejected 0.5325km3 of material from the lakefloor and below, plastering the nearby landscape for several kilometers with mud and other debris. The eruption buried the natural outlet of the lake, with the bottom of the craters becoming filled by water within months and completely concealed from view within years; today Lake Rotomahana has depths up to 118m.High-resolution (0.5m) bathymetric mapping, when combined with a 2-D seismic reflection survey, enables us to ‘see’ details of the maar craters on the lakefloor, including those parts subsequently buried by sediment. The large Rotomahana Crater described by workers immediately after the eruption measures ~2.5km in diameter near its southwestern end, and excavated ground to 155m below present-day lake level. The vent system, as revealed by the present study, forms an array of right-stepping (dextral) craters, with the main crater being host to two sub-craters Rotomahana West Crater and Rotomahana East Crater today buried beneath the lakefloor, and which are in-filled by 36 and 37m of sediment, respectively. Subordinate craters along the same 057° Tarawera Rift trace include Hochstetter Crater (11m of infill), Waingongongongo Crater (14m) and Rotomakariri Crater (26m). These craters host a total 0.0268km3 of sediment. Other features highlighted by the bathymetric data include; craters not filled by sediment, sediment fan deltas, volcanic ridges and dikes, submerged wave-cut terraces formed during times of lower lake levels and gas pockmarks, all either related to the 1886 eruptive episode or post-eruption hydrothermal and erosional processes.Application of results from bubble plume, CO2 flux, magnetic and heat flux surveys of Lake Rotomahana to this study, when combined with regional earthquake relocation analysis and broadband magnetotelluric data, suggest an explanation in terms of a magmatic heat source located south of Waimangu and a corresponding convective water/heat transport system extending thence to beneath the western end of the lake. A holistic approach has provided a coherent context for the eruption and its effect on the historical Pink and White Terraces hydrothermal system that appears to have been the eastern-most extension of a larger system that lay beneath the Waimangu area before the eruption. The newly named Pink Terraces hydrothermal system (~1.5km2) is a continuation of the historical hydrothermal activity that was concentrated on the western shores of the old lake, and together with the formation of the new, post-1886 Patiti hydrothermal system (~1km2) located SW of Patiti Island, mark the two distinct areas of hydrothermal activity in the lake today.

First hydrothermal discoveries on the Australian‐Antarctic Ridge: Discharge sites, plume chemistry, and vent organisms

AbstractThe Australian‐Antarctic Ridge (AAR) is one of the largest unexplored regions of the global mid‐ocean ridge system. Here, we report a multiyear effort to locate and characterize hydrothermal activity on two first‐order segments of the AAR: KR1 and KR2. To locate vent sites on each segment, we used profiles collected by Miniature Autonomous Plume Recorders on rock corers during R/V Araon cruises in March and December of 2011. Optical and oxidation‐reduction‐potential anomalies indicate multiple active sites on both segments. Seven profiles on KR2 found 3 sites, each separated by ∼25 km. Forty profiles on KR1 identified 17 sites, some within a few kilometer of each other. The spatial density of hydrothermal activity along KR1 and KR2 (plume incidence of 0.34) is consistent with the global trend for a spreading rate of ∼70 mm/yr. The densest area of hydrothermal activity, named “Mujin,” occurred along the 20 km‐long inflated section near the segment center of KR1. Continuous plume surveys conducted in January–February of 2013 on R/V Araon found CH4/3He (1 − 15 × 106) and CH4/Mn (0.01–0.5) ratios in the plume samples, consistent with a basaltic‐hosted system and typical of ridges with intermediate spreading rates. Additionally, some of the plume samples exhibited slightly higher ratios of H2/3He and Fe/Mn than others, suggesting that those plumes are supported by a younger hydrothermal system that may have experienced a recent eruption. The Mujin‐field was populated by Kiwa crabs and seven‐armed Paulasterias starfish previously recorded on the East Scotia Ridge, raising the possibility of circum‐Antarctic biogeographic connections of vent fauna.

He, Ar, N and C isotope compositions in Tatun Volcanic Group (TVG), Taiwan: Evidence for an important contribution of pelagic carbonates in the magmatic source

The Tatun Volcanic Group (TVG), Northeastern Taiwan, is considered to be the extension of the Ryukyu arc, and belongs to the post-collisional collapse Okinawa Trough. Strong hydrothermal activity is concentrated along the Chinshan fault, and Da-you-keng (DYK) represents the main fumarolic area where the most primitive isotopic and chemical composition is observed. In this study, we present chemical and He, Ar, C and N isotopic compositions of fumaroles, bubbling gas and water from hot springs sampled in 2012 and 2013. High 3He/4He ratios from DYK fumaroles (≈6.5Ra) show a typical arc-like setting, whereas other sampling areas show a strong dependence of 3He/4He and CH4/3He ratios with the distance from the main active hydrothermal area (DYK). This could mean strong crustal contamination and thermal decomposition of organic matter from local sediments. Carbon isotope compositions of DYK range from −6.67‰ to −5.85‰, and indicate that carbon contribution comes mainly from pelagic carbonates from the slab (limestone, mantle and sediment contributions are 63%, 19% and 18%, respectively). This is consistent with the negative δ15N values (−1.4±0.5‰) observed for DYK, implying a strong nitrogen-mantle contribution, and an absence of contribution from nitrogen-pelagic carbonates. These results have important consequences related to the Ryukyu subducted slab. In fact, the Ryukyu margin presents little in off scraping the sedimentary cover to the subducting plate that does not permit any nitrogen contribution in magma from TVG.

Geologic setting of PACManus hydrothermal area — High resolution mapping and in situ observations

This study presents a systematic analysis and interpretation of autonomous underwater vehicle-based microbathymetry combined with remotely operated vehicle (ROV) video recordings, rock analyses and temperature measurements within the PACManus hydrothermal area located on Pual Ridge in the Bismarck Sea of eastern Manus Basin. The data obtained during research cruise Magellan-06 and So-216 provides a framework for understanding the relationship between the volcanism, tectonism and hydrothermal activity. PACManus is a submarine felsic vocanically-hosted hydrothermal area that hosts multiple vent fields located within several hundred meters of one another but with different fluid chemistries, vent temperatures and morphologies. The total area of hydrothermal activity is estimated to be 20,279 m2. The microbathymetry maps combined with the ROV video observations allow for precise high-resolution mapping estimates of the areal extents of hydrothermal activity. We find the distribution of hydrothermal fields in the PACManus area is primarily controlled by volcanic features that include lava domes, thick and massive blocky lava flows, breccias and feeder dykes. Spatial variation in the permeability of local volcanic facies appears to control the distribution of venting within a field. We define a three-stage chronological sequence for the volcanic evolution of the PACManus based on lava flow morphology, sediment cover and lava SiO2 concentration. In Stage-1, sparsely to moderately porphyritic dacite lavas (68–69.8 wt.% SiO2) erupted to form domes or cryptodomes. In Stage-2, aphyric lava with slightly lower SiO2 concentrations (67.2–67.9 wt.% SiO2) formed jumbled and pillowed lava flows. In the most recent phase Stage-3, massive blocky lavas with 69 to 72.5 wt.% SiO2 were erupted through multiple vents constructing a volcanic ridge identified as the PACManus neovolcanic zone. The transition between these stages may be gradual and related to progressive heating of a silicic magma following a recharge event of hot, mantle-derived melts.

Neutrophilic lithotrophic iron-oxidizing prokaryotes and their role in the biogeochemical processes of the iron cycle

Biology of lithotrophic neutrophilic iron-oxidizing prokaryotes and their role in the processes of the biogeochemical cycle of iron are discussed. This group of microorganisms is phylogenetically, taxonomically, and physiologically heterogeneous, comprising three metabolically different groups: aerobes, nitrate-dependent anaerobes, and phototrophs; the latter two groups have been revealed relatively recently. Their taxonomy and metabolism are described. Materials on the structure and functioning of the electron transport chain in the course of Fe(II) oxidation by members of various physiological groups are discussed. Occurrence of iron oxidizers in freshwater and marine ecosystems, thermal springs, areas of hydrothermal activity, and underwater volcanic areas are considered. Molecular genetic techniques were used to determine the structure of iron-oxidizing microbial communities in various natural ecosystems. Analysis of stable isotope fractioning of 56/54Fe in pure cultures and model experiments revealed predominance of biological oxidation over abiotic ones in shallow aquatic habitats and mineral springs, which was especially pronounced under microaerobic conditions at the redox zone boundary. Discovery of anaerobic bacterial Fe(II) oxidation resulted in development of new hypotheses concerning the possible role of microorganisms and the mechanisms of formation of the major iron ore deposits in Precambrian and early Proterozoic epoch. Paleobiological data are presented on the microfossils and specific biomarkers retrieved from ancient ore samples and confirming involvement of anaerobic biogenic processes in their formation.

Surficial permeability of the axial valley seafloor: Endeavour Segment, Juan de Fuca Ridge

Hydrothermal systems at mid‐ocean spreading centers play a fundamental role in Earth's geothermal budget. One underexamined facet of marine hydrothermal systems is the role that permeability of the uppermost seafloor veneer plays in the distribution of hydrothermal fluid. As both the initial and final vertical gateway for subsurface fluid circulation, uppermost seafloor permeability may influence the local spatial distribution of hydrothermal flow. A method of deriving a photomosaic from seafloor video was developed and utilized to estimate relative surface permeability in an active hydrothermal area on the Endeavour Segment of the Juan de Fuca Ridge. The mosaic resolves seafloor geology of the axial valley seafloor at submeter resolution over an area greater than 1 km2. Results indicate that the valley walls and basal talus slope are topographically rugged and unsedimented, providing minimal resistance to fluid transmission. Elsewhere, the axial valley floor is capped by an unbroken blanket of low‐permeability sediment, resisting fluid exchange with the subsurface reservoir. Active fluid emission sites were restricted to the high‐permeability zone at the base of the western wall. A series of inactive fossil hydrothermal structures form a linear trend along the western bounding wall, oriented orthogonal to the spreading axis. High‐temperature vent locations appear to have migrated over 100 m along‐ridge‐strike over the decade between surveys. While initially an expression of subsurface faulting, this spatial pattern suggests that increases in seafloor permeability from sedimentation may be at least a secondary contributing factor in regulating fluid flow across the seafloor interface.

Deriving microstructure and fluid state within the Icelandic crust from the inversion of tomography data

The inversion of seismic data to infer rock microstructural properties and fluid flow patterns in the crust is a challenging issue. In this paper, we develop an effective medium model for estimating velocities in porous media including both pores and cracks and use it to derive the distribution of crack density beneath the Reykjanes Peninsula from accurate tomography data. Outside the active hydrothermal areas, crack density is shown to decrease with depth. There are two main reasons for this: the closure of cracks because of the increasing overburden and the secondary filling of cracks because of hydrothermal flows. However, crack density may locally increase with depth beneath the southwestern part of the Kleifarvatn lake. This is consistent with the presence of a deep reservoir with supercritical fluids under pressure, which may activate hydrofracturing processes. We recognize that capturing the link between seismic data and the physical properties of crust is very difficult. This study shows that a combination of mechanical concepts and effective medium theory contributes to improve our understanding of the phenomena occurring within the Icelandic crust

Some geological and hydrogeochemical characteristics of geothermal fields of Turkey

Presently, active hydrothermal areas of Turkey can be divided into three major provinces which correlate particular volcano-tectonic features. These fields are essentially found at western, central and eastern part of the country. There has been no particular geothermal energy fields in the southern part of Turkey since this region is mostly covered by ~2 km thick carbonate sequences and has suffered little recent rifting. The western Turkish geothermal provinces, where horst and graben systems are well developed, have the hottest fluids among others. In this region, normal faults penetrate into the crust and thus the meteoric waters are heated to greater temperatures than in the east where a compressional tectonic regime dominates. In the western Turkish geothermal fields, the fluids have a distinctly higher pH (>7) values than those in the east. This resulted that the Si content of the western Turkish hydrothermal fluids is considerably higher than the others. The geothermal field at Tuzla, situated on the Aegean shore in NW Turkey, has ionic component twice that of conventional seawater. Because of this, the area provides a good opportunity to investigate the interactions between seawater and hot dry rock. Key words: Geothermal fields, seawater, hot dry rock.

Impact of submarine hydrothermal vents on the metal composition of krill and its excretion products

We have measured the metal composition and estimated the excretion rate of trace elements (Ag, Cd, Co, Cu, Ni, Pb, V and Zn) by Antarctic krill ( Euphausia superba) in three locations (two located within a submarine hydrothermal vent field and one away from it) along the Antarctic Peninsula region of the Southern Ocean. Results indicated that krill excreted large amounts of Ag, Cu, Pb and Zn, (range: 1.9–41.2 pmol Ag g DW − 1 h − 1 , 15.3–26.8 nmol Cu g DW − 1 h − 1 , 308.7–1118.3 pmol Pb g DW − 1 h − 1 and 24.4–76.5 nmol Zn g DW − 1 h − 1 ), compared with the non-significant or undetectable release rates of Cd, Co, Ni and V. The metal composition of the excreted material from krill collected in the area of hydrothermal activity was similar to the metal composition reported for suspended particles emitted from those vents. Our results suggest that krill recycling of Ag, Cu, Pb and Zn could potentially influence trace metal concentrations in the water column of the Bransfield region of the Southern Ocean, and that the original source of metals to these waters may be hydrothermal vents.

Structure of the Nemrut caldera (Eastern Anatolia, Turkey) and associated hydrothermal fluid circulation

Plio-Quaternary volcanism played an important role in the present physical state of Eastern Anatolia. Mount Nemrut, situated to the west of Lake Van is one of the main volcanic centers in the region, with a spectacular summit caldera 8.5 × 7 km in diameter. The most recent eruptions of the volcano were in 1441, 1597 and 1692. Nemrut Lake covers the western half of the caldera; it is a deep, half-bowl-shaped lake with a maximum depth of 176 m. Numerous eruption centers are exposed within the caldera as a consequence of magma–water interaction. Current activity of Nemrut caldera is revealed as hot springs, fumaroles and a small, hot lake. Self-potential and bathymetric surveys carried out in the caldera were used to characterize the structure of the caldera and the associated hydrothermal fluid circulation. In addition, analyses based on digital elevation models and satellite imagery were used to improve our knowledge about the structure of the caldera. According to SP results, the flanks of the volcano represent “the hydrogeologic zone”, whereas the intra-caldera region is an “active hydrothermal area” where the fluid circulation is controlled by structural discontinuities. There is also a northern fissure zone which exhibits hydrothermal signatures. Nemrut caldera collapsed piecemeal, with three main blocks. Stress controlling the collapse mechanism seems to be highly affected by the regional neotectonic regime. In addition to the historical activity, current hydrothermal and hydrogeologic conditions in the caldera, in which there is a large lake and shallow water table, increase the risk of the quiescent volcano.

Seismicity characteristics of a potentially active Quaternary volcano: The Tatun Volcano Group, northern Taiwan

The Tatun Volcano Group (TVG) is located at the northern tip of Taiwan, near the capital Taipei and close to two nuclear power plants. Because of lack of any activity in historical times it has been classified as an extinct volcano, even though more recent studies suggest that TVG might have been active during the last 20 ka. In May 2003 a seismic monitoring project at the TVG area was initiated by deploying eight three-component seismic stations some of them equipped with both short-period and broadband sensors. During the 18 months observation period local seismicity mainly consisted of high frequency earthquakes either occurring as isolated events, or as a continuous sequence in the form of spasmodic bursts. Mixed and low frequency events were also present during the same period, even though they occurred only rarely. Arrival times from events with clear P-/S-wave phases were inverted in order to obtain a minimum 1D velocity model with station corrections. Probabilistic nonlinear earthquake locations were calculated for all these events using the newly derived velocity model. Most high frequency seismicity appeared to be concentrated near the areas of hydrothermal activity, forming tight clusters at depths shallower than 4 km. Relative locations, calculated using the double-difference method and utilising catalogue and cross-correlation differential traveltimes, showed insignificant differences when compared to the nonlinear probabilistic locations. In general, seismicity in the TVG area seems to be primarily driven by circulation of hydrothermal fluids as indicated by the occurrence of spasmodic bursts, mixed/low frequency events and a b-value (1.17 ± 0.1) higher than in any other part of Taiwan. These observations, that are similar to those reported in other dormant Quaternary volcanoes, indicate that a magma chamber may still exist beneath TVG and that a future eruption or period of unrest should not be considered unlikely.