Active Hydrothermal Vent Fields Research Articles

Identifying Probable Submarine Hydrothermal Spots in North Santorini Caldera Using the Generalized Moments Method

The presence of active hydrothermal vent fields near residential areas and their possible link to volcanic activity poses a potential hazard to the environment, society, and the economy. By capitalizing on Autonomous Underwater Vehicle sampling methodologies and applying the Generalized Moments Method model for geological and physical processes in these environments, we shed light on the underlying dynamics shaping the physicochemical characteristics of the vents. In this study, we focus on the Northern Caldera of Santorini and, more specifically, on the recorded CTD data (Conductivity, Temperature, Depth). The data sets were collected in 2017 in Santorini using an Autonomous Underwater Vehicle during the GEOMAR POS510 mission. Our research shows that the active vent field within the caldera probably follows a multifractal behavior and exhibits a weak memory effect. Depth Profiles and Time Series show similar behavior among conductivity and temperature. The variance and moments of both parameters underline the existence of two different mechanisms governing the behavior of the vent field. Finally, the structure function shows that changes in the time series are described by a Cauchy–Lorentz distribution.

Effects of abiotic processes on the correlation between pH and pO2 in the Norwegian Sea: Implications for GCS monitoring

In the absence of abiotic sources of CO2, variation in pCO2 and pO2 is expected to be inversely correlated in the water column due to biogenic processes. It has previously been suggested to use this correlation for leakage monitoring of offshore geological carbon storage (GCS) sites. In this study the aim is to investigate the extent of this correlation in ocean water masses with different origin and history in the Norwegian Sea, as well as in water masses in the vicinity of an active hydrothermal vent field at Mohn's Ridge, where pH is used as a proxy for pCO2. Over a hydrothermal vent site, a strong correlation between pH and pO2 is observed from 0 to 1700 m, whereas at depths >1700 m there is no correlation, likely due to CO2 emissions from the hydrothermal vents. However, at a reference site nearly 200 km from the hydrothermal vents, the intermediate Arctic water masses (700 – 1600 m depth) also show pH-pO2 correlations that are inconsistent with biogenic processes, but less pronounced compared to the hydrothermal vent site. These findings show that the suitability of this monitoring strategy will depend on a thorough site-specific evaluation of pH/CO2 and O2 relationships of relevant water masses.

Discovery of active off-axis hydrothermal vents at 9° 54′N East Pacific Rise

Comprehensive knowledge of the distribution of active hydrothermal vent fields along midocean ridges is essential to understanding global chemical and heat fluxes and endemic faunal distributions. However, current knowledge is biased by a historical preference for on-axis surveys. A scarcity of high-resolution bathymetric surveys in off-axis regions limits vent identification, which implies that the number of vents may be underestimated. Here, we present the discovery of an active, high-temperature, off-axis hydrothermal field on a fast-spreading ridge. The vent field is located 750 m east of the East Pacific Rise axis and ∼7 km north of on-axis vents at 9° 50'N, which are situated in a 50- to 100-m-wide trough. This site is currently the largest vent field known on the East Pacific Rise between 9 and 10° N. Its proximity to a normal fault suggests that hydrothermal fluid pathways are tectonically controlled. Geochemical evidence reveals deep fluid circulation to depths only 160 m above the axial magma lens. Relative to on-axis vents at 9° 50'N, these off-axis fluids attain higher temperatures and pressures. This tectonically controlled vent field may therefore exhibit greater stability in fluid composition, in contrast to more dynamic, dike-controlled, on-axis vents. The location of this site indicates that high-temperature convective circulation cells extend to greater distances off axis than previously realized. Thorough high-resolution mapping is necessary to understand the distribution, frequency, and physical controls on active off-axis vent fields so that their contribution to global heat and chemical fluxes and role in metacommunity dynamics can be determined.

SANTORY: SANTORini’s Seafloor Volcanic ObservatorY

Submarine hydrothermal systems along active volcanic ridges and arcs are highly dynamic, responding to both oceanographic (e.g., currents, tides) and deep-seated geological forcing (e.g., magma eruption, seismicity, hydrothermalism, and crustal deformation, etc.). In particular, volcanic and hydrothermal activity may also pose profoundly negative societal impacts (tsunamis, the release of climate-relevant gases and toxic metal(loid)s). These risks are particularly significant in shallow (<1000m) coastal environments, as demonstrated by the January 2022 submarine paroxysmal eruption by the Hunga Tonga-Hunga Ha’apai Volcano that destroyed part of the island, and the October 2011 submarine eruption of El Hierro (Canary Islands) that caused vigorous upwelling, floating lava bombs, and natural seawater acidification. Volcanic hazards may be posed by the Kolumbo submarine volcano, which is part of the subduction-related Hellenic Volcanic Arc at the intersection between the Eurasian and African tectonic plates. There, the Kolumbo submarine volcano, 7 km NE of Santorini and part of Santorini’s volcanic complex, hosts an active hydrothermal vent field (HVF) on its crater floor (~500m b.s.l.), which degasses boiling CO2–dominated fluids at high temperatures (~265°C) with a clear mantle signature. Kolumbo’s HVF hosts actively forming seafloor massive sulfide deposits with high contents of potentially toxic, volatile metal(loid)s (As, Sb, Pb, Ag, Hg, and Tl). The proximity to highly populated/tourist areas at Santorini poses significant risks. However, we have limited knowledge of the potential impacts of this type of magmatic and hydrothermal activity, including those from magmatic gases and seismicity. To better evaluate such risks the activity of the submarine system must be continuously monitored with multidisciplinary and high resolution instrumentation as part of an in-situ observatory supported by discrete sampling and measurements. This paper is a design study that describes a new long-term seafloor observatory that will be installed within the Kolumbo volcano, including cutting-edge and innovative marine-technology that integrates hyperspectral imaging, temperature sensors, a radiation spectrometer, fluid/gas samplers, and pressure gauges. These instruments will be integrated into a hazard monitoring platform aimed at identifying the precursors of potentially disastrous explosive volcanic eruptions, earthquakes, landslides of the hydrothermally weakened volcanic edifice and the release of potentially toxic elements into the water column.

Structure and Connectivity of Hydrothermal Vent Communities Along the Mid-Ocean Ridges in the West Indian Ocean: A Review

To date, 13 biologically active hydrothermal vent (HTV) fields have been described on the West Indian Ocean ridges. Knowledge of benthic communities of these vent ecosystems serves as scientific bases for assessing the resilience of these ecosystems under the global effort to strike an elegant balance between future deep-sea mining and biodiversity conservation. This review aims to summarize our up-to-date knowledge of the benthic community structure and connectivity of these Indian vents and to identify knowledge gaps and key research questions to be prioritized in order to assess the resilience of these communities. The HTVs in the West Indian Ocean are home to many unique invertebrate species such as the remarkable scaly-foot snail. While distinct in composition, the macrofaunal communities of the Indian HTVs share many characteristics with those of other HTVs, including high endemism, strong zonation at the local scale, and a simple food web structure. Furthermore, Indian vent benthic communities are mosaic compositions of Atlantic, Pacific, and Antarctic HTV fauna possibly owning to multiple waves of past colonization. Phylogeographic studies have shed new light into these migratory routes. Current animal connectivity across vent fields appears to be highly influenced by distance and topological barriers. However, contrasting differences in gene flow have been documented across species. Thus, a better understanding of the reproductive biology of the Indian vent animals and the structure of their population at the local scale is crucial for conservation purposes. In addition, increased effort should be given to characterizing the vents’ missing diversity (at both the meio and micro-scale) and elucidating the functional ecology of these vents.

Formation of Fe-Mn coatings on foraminifera from the ultraslow-spreading southwest Indian Ridge: Implications for hydrothermal and diagenetic overprints in sediments

Detailed fine-scale morphological, mineralogical, and geochemical analyses were conducted on the Si-(Fe) hydroxide formations, planktonic foraminifera (PF) and the Fe-Mn coatings within a typical sediment sample collected from near the newly-discovered active hydrothermal vent field at the ultraslow-spreading Southwest Indian Ridge (SWIR). The analyses were conducted with a combination of different techniques, including Laser Scanning Confocal Microscope (LSCM), Laser Raman, high-resolution XRD, SEM-EDS, C-O isotope measurement, EMPA, and in-situ LA-ICP-MS. The mixed-type hydrothermal-hydrogenetic Si-(Fe) hydroxide formations (commonly occur as cavity-fillings inside the PF chambers) contain a considerable amount of organic matter, and are mainly composed of nontronite, goethite, tridymite, and opal-A with the bulk compositions of 8.13–40.91 wt% SiO2, 6.85–54.50 wt% FeO, and 0.43–23.99 wt% Al2O3. Such high Al content can be mainly attributed to the precipitation and accumulation of hydrothermal-derived Al-rich materials, and the Si-Fe-Al assemblage reveals that it is mainly sourced from plagioclase dissolution/alteration during subseafloor fluid-rock interaction. Meanwhile, the relative enrichments of high field strength elements (HFSEs), heavy rare earth elements (HREE), and Cr-Ni-Sc also imply that these Si-(Fe) hydroxide formations have another source from pyroxene alteration. In addition, their distinct microtextures (filamentous, orbicular, or beaded-like) are indicative of bacterial-related precipitation. The hydrogenetic Fe-Mn coatings have a layered texture and exhibit no apparent fluorescence, and are mainly composed of vernadite and goethite with the bulk compositions of 23.98–30.06 wt% FeO and 8.55– 21.52 wt% MnO. Their average Fe/Mn ratios, growth rates, and Zn-As contents are slightly higher than those of the open-ocean SWIR Fe-Mn deposits, which suggests an additional Fe input from submarine hydrothermal activity. Moreover, the altered PF shell is mainly composed of calcite and have three color types (white/yellowish/yellow). The contents of Si, Al, Fe, Cr, Ni, Cu, Zn, Ba, HFSEs, and REE increase as the yellowish color deepens, which indicates the diagenetic contamination from basalt alteration. Consequently, a new diagenetic Fe-carbonate phase (ankerite) was formed in the inner layer of the altered PF shells, and close correlations were also found between their C-O isotope ratios. Both the δ13C and δ18O values tend to drop from white- to yellow-type PF shells, accompanied with the decreasing contents of Ca, Na, Mg, and Sr. All these parameters and signatures would help to unravel the hydrothermal influence and diagenetic overprinting, which could cause confusion when using foraminifera shell geochemistry (e.g., Sr/Ca and Mg/Ca ratios) as a marine tracer in paleoclimate reconstruction.

The Hydrothermal Vent Field at the Eastern Edge of the Hellenic Volcanic Arc: The Avyssos Caldera (Nisyros)

Almost three-quarters of known volcanic activity on Earth occurs in underwater locations. The presence of active hydrothermal vent fields in such environments is a potential natural hazard for the environment, society, and economy. Despite its importance for risk assessment and risk mitigation, the monitoring of volcanic activity is impeded by the remoteness and the extreme conditions of many underwater volcanoes. The morphology and the activity of the submarine caldera, Avyssos, at the northern part of Nisyros volcano in the South Aegean Sea (Greece), were studied using a remotely operated underwater vehicle. The recorded time series of temperature and conductivity over the submarine volcano have been analyzed in terms of the Generalized Moments Method. This type of analysis can be used as an indicator for the state of activity of a submarine volcano. Here, we expand the work conducted for the first time in 2018. We present the findings of the geological exploration and the mathematical analysis, obtained from the data collected in October 2010. The temperature and conductivity time series show minor fluctuations in a rather stable environment. Based on these results, the impact of developing appropriate mechanisms and policies to avoid the associated natural hazard is expected to be important.

Massive Southern Ocean phytoplankton bloom fed by iron of possible hydrothermal origin

Primary production in the Southern Ocean (SO) is limited by iron availability. Hydrothermal vents have been identified as a potentially important source of iron to SO surface waters. Here we identify a recurring phytoplankton bloom in the high-nutrient, low-chlorophyll waters of the Antarctic Circumpolar Current in the Pacific sector of the SO, that we argue is fed by iron of hydrothermal origin. In January 2014 the bloom covered an area of ~266,000 km2 with depth-integrated chlorophyll a > 300 mg m−2, primary production rates >1 g C m−2 d−1, and a mean CO2 flux of −0.38 g C m−2 d−1. The elevated iron supporting this bloom is likely of hydrothermal origin based on the recurrent position of the bloom relative to two active hydrothermal vent fields along the Australian Antarctic Ridge and the association of the elevated iron with a distinct water mass characteristic of a nonbuoyant hydrothermal vent plume.

Influence of Regional Oceanography and Hydrothermal Activity on Protist Diversity and Community Structure in the Okinawa Trough.

Microbial eukaryotes (protists) contribute substantially to ecological functioning in marine ecosystems, but the relative importance of factors shaping protist diversity, such as environmental selection and dispersal, remains difficult to parse. Water masses of a back-arc basin with hydrothermal activity provide a unique opportunity for studying the effects of dispersal and environmental selection on protist communities. In this study, we used metabarcoding to characterize protist communities in the Okinawa Trough, a back-arc spreading basin containing at least twenty-five active hydrothermal vent fields. Water was sampled from four depths at fourteen stations spanning the length of the Okinawa Trough, including three sites influenced by nearby hydrothermal vent sites. While significant differences in community structure reflecting water depth were present, protist communities were mostly homogeneous horizontally. Protist communities in thebottom waters affected by hydrothermal activity were significantly different from communities in other bottom waters, suggesting that environmental factors can be especially important in shaping community composition under specific conditions. Amplicon sequence variants that were enriched in hydrothermally influenced bottom waters largely derived from cosmopolitan protists that were present, but rare, in other near-bottom samples, thus highlighting the importance of the rare biosphere.

CTD data profiling to assess the natural hazard of active submarine vent fields: the case of Santorini Island

Almost three quarters of known volcanic activity on Earth occurs in underwater locations. The presence of active hydrothermal vent fields in such environments is a potential natural hazard for the environment, the society, and the economy. Despite its importance for risk assessment and risk mitigation, monitoring of the activity is impeded by the remoteness and the extreme conditions of underwater volcanoes. The large difference of population present on Santorini between the winter and summer seasons, all within a partially enclosed system, make the Santorini-Kolumbo volcanic field, an ideal place for detailed exploration. In 2017, GEOMAR in collaboration with the National and Kapodistrian University of Athens (mission: POS-510 ANYDROS), used an Autonomous Underwater Vehicle (AUV) to map the NE–trending Santorini–Kolumbo line, where it also collected CTD data. Here we present the preliminary results from the 15-hour survey held on the 25th March 2017, during the POS-510 expedition targeting the vent field which is located in the North Basin of Santorini Caldera. Detailed CTD 3D profiles have been reconstructed from the raw data of Santorini’s vent field. An anomaly emerges at the depth of 350 m in the Conductivity and Salinity depth profiles, as the CTD sensor is placed directly above the vent sources. Anomalies were evident in the 3D maps reconstructed, showing for the first time a rather weak, but underlying hydrothermal vent activity at various locations. As the present results are the first ones produced from this expedition, further investigation is required incorporating the full dataset. Based on those results, the impact of developing appropriate mechanisms and policies to avoid the associated natural hazard is expected to be immense.

Visual and Autonomous Survey of Hydrothermal Vents Using a Hovering‐Type AUV: Launching Hobalin Into the Western Offshore of Kumejima Island

AbstractAutonomous underwater vehicles (AUVs) have become promising tools for marine geological surveys to collect information such as the topography of the seafloor and the location of hydrothermal plumes. Visual surveys and sampling are mainly conducted using human‐occupied vehicles or remotely operated vehicles. To obtain detailed visual data more efficiently, a hovering‐type AUV (Hobalin), which has the novel ability to autonomously navigate within a vent field, was developed to explore hydrothermal deposits. This vehicle was deployed in an active hydrothermal vent field located in the western offshore of Kumejima Island at a depth of approximately 1,400 m. Visual observations were performed with submillimeter image resolution via low‐altitude navigation using still cameras. Moreover, a height estimation method for chimney structures was proposed utilizing an obstacle avoidance system composed of a sheet laser and a forward‐looking camera with hovering maneuver capability. It was determined that hovering‐type AUVs can facilitate the effective survey of hydrothermal vents and the acquired data can be utilized for subsequent sampling by human‐occupied vehicles and remotely operated vehicles.

Detailed Mapping of Hydrothermal Vent Fauna: A 3D Reconstruction Approach Based on Video Imagery

Active hydrothermal vent fields are complex, small-scale habitats hosting endemic fauna that changes at scales of centimeters, influenced by topographical variables. In previous studies, it has been shown that the distance to hydrothermal fluids is also a major structuring factor. Imagery analysis based on two dimensional photo stitching revealed insights to the vent field zonation around fluid exits and a basic knowledge of faunal assemblages within hydrothermal vent fields. However, complex three dimensional surfaces could not be adequately replicated in those studies, and the assemblage structure, as well as their relation to abiotic terrain variables, is often only descriptive. In this study we use ROV video imagery of a hydrothermal vent field on the southeastern Indian Ridge in the Indian Ocean. Structure from Motion photogrammetry was applied to build a high resolution 3D reconstruction model of one side of a newly discovered active hydrothermal chimney complex, allowing for the quantification of abundances. Likewise, the reconstruction was used to infer terrain variables at a scale important for megabenthic specimens, which were related to the abundances of the faunal assemblages. Based on the terrain variables, applied random forest model predicted the faunal assemblage distribution with an accuracy of 84.97 %. The most important structuring variables were the distances to diffuse- and black fluid exits, as well as the height of the chimney complex. This novel approach enabled us to classify quantified abundances of megabenthic taxa to distinct faunal assemblages and relate terrain variables to their distribution. The successful prediction of faunal assemblage occurrences further supports the importance of abiotic terrain variables as key structuring factors in hydrothermal systems and offers the possibility to detect suitable areas for Marine Protected areas on larger spatial scales. This technique works for any kind of video imagery, regardless of its initial purpose and can be implemented in marine monitoring and management.

Geochemical impacts of hydrothermal activity on surface deposits at the Southwest Indian Ridge

Submarine hydrothermal circulation has attracted much scientific interest since seafloor hydrothermal activity was first observed in the 1970s; an area of particular interest is the impact of exported inorganic and organic materials from hydrothermal vent systems into the open ocean. In 2007, the first active hydrothermal vent field, with vent fluid temperatures up to 379 °C, was discovered at the ultraslow spreading Southwest Indian Ridge (SWIR), where active vents are much less abundant than fast spreading ridges, and the effect of hydrothermal extrusion on surface sediments is not fully understood. To explore how geochemical proxy signatures respond to hydrothermal activity, we investigated the distributions of elements, minerals and lipids in surficial normal marine sediments, metalliferous sediments and low-temperature hydrothermal deposits collected from the SWIR. The results showed different effects of hydrothermal activity on the surface deposits. The normal marine sediments were predominantly calcium carbonate characterized by > 42% CaO and > 90% calcite, with a significant autochthonous marine contribution to organic matter (OM) and a predominance of lower molecular weight alkanols and fatty acids; they were uninfluenced by hydrothermal activity but received some terrigenous input represented by abundant high molecular weight n-alkanes with an odd-over-even predominance. The near-field metalliferous sediments and hydrothermal deposits were very different. Some near-field metalliferous sediments were influenced by low-temperature hydrothermal activity, and their distributions of elements and minerals were similar to those of hydrothermal deposits, which were characterized by abundant Fe/Si and opal/nontronite. Other near-field metalliferous sediments were evidently influenced by mixing of high-temperature hydrothermal sulfides typically containing abundant Cu/Zn. With respect to the organic matter assemblages, near-field deposits contained little evidence for thermal maturation of organic matter and all were characterized by a strong microbial signature, including hopanoids, isoprenoidal and non-isoprenoidal dialkyl glycerol ether lipids, and low molecular weight n-alkanes with an even carbon number predominance. The far-field metalliferous sediments, despite the influence of non-buoyant plumes and slightly higher concentrations of hydrothermal-derived metals (e.g., Fe, Cu and Zn), had the same distribution of organic lipids and major mineral composition (> 90% calcite) as did normal marine sediments. Thus, the influence of non-buoyant plume inputs appears to have been minimal possibly due to the dilution of in situ microorganisms by normal marine organisms in sediment and seawater. Furthermore, these characteristics indicate inorganic indices based on abundant metal elements derived from the hydrothermal systems (such as Fe/Cu/Zn content, ∑REE/Fe, the ternary diagram of Fe, Cu×100 and Ca) are more sensitive, serving as better proxies than organic matter assemblages to differentiate the effects of diverse hydrothermal activity on surface deposits.

Geology of the Alarcon Rise, Southern Gulf of California

AbstractMeter‐scale AUV bathymetric mapping and ROV sampling of the entire 47 km‐long Alarcon Rise between the Pescadero and Tamayo transforms show that the shallowest inflated portion of the segment hosts all four active hydrothermal vent fields and the youngest, hottest, and highest effusion rate lava flows. This shallowest inflated part is located ∼1/3 of the way between the Tamayo and Pescadero transforms and is paved by a 16 km2 channelized flow that erupted from 9 km of en echelon fissures and is larger than historic flows on the East Pacific Rise or on the Gorda and Juan de Fuca Ridges. Starting ∼5 km south of the Pescadero transform, 6.5 km of the Alarcon Rise is characterized by faulted ridges and domes of fractionated lavas ranging from basaltic andesite to rhyolite with up to 77.3 wt % SiO2. These are the first known rhyolites from the submarine global mid‐ocean ridge system. Silicic lavas range from >11.7 ka, to as young as 1.1 ka. A basalt‐to‐basaltic andesite sequence and an andesite‐to‐dacite‐to‐rhyolite sequence are consistent with crystal fractionation but some intermediate basaltic andesite and andesite formed by mixing basalt with dacite or rhyolite. Magmatism occurred along the bounding Tamayo and Pescadero transforms as extensive channelized flows. The flows erupted from ring faults surrounding uplifted sediment hills inferred to overlie sills. The transforms are transtensional to accommodate magma migration from the adjacent Alarcon Rise.

Efficient extraction of copper and zinc from seafloor massive sulphide rock samples from the Loki’s Castle area at the Arctic Mid-Ocean Ridge

Seafloor massive sulphide (SMS) deposits have been identified as important marine metal resources for the future. However, literature on the recovery/extraction of metals from SMS is currently limited, and to date, no research has been published on the processing of SMS from the active hydrothermal vent field at the Arctic Mid-Ocean Ridge. In this paper extraction of copper and zinc, as economically important metals, from the seafloor massive sulphide rock samples from the Loki’s Castle area at the Arctic Mid-Ocean Ridge was investigated during nitric acid leaching. The results presented are of the various leaching experiments conducted under different conditions to optimise the extraction of copper and zinc. The mineralogical analysis indicated that the main copper and zinc bearing minerals were chalcopyrite and sphalerite, respectively. It was shown that the leaching efficiency and extraction of copper and zinc can be controlled mainly by temperature and acid concentration. The elemental composition and mineralogical data indicated that 95% of copper and zinc bearing minerals were leached out after 3 h, at the solid-to-liquid ratio of 1:10, temperature of 90 °C and acid concentration of 10%.

Time-resolved interaction of seawater with gabbro: An experimental study of rare-earth element behavior up to 475 °C, 100 MPa

High metal and rare-earth element (REE) concentrations with unusual (‘atypical’) normalized REE patterns are documented in fluids from active hydrothermal vent fields on the Mid-Atlantic Ridge, 5°S and the East Scotia Ridge. Those fluids show relative enrichment of middle heavy REEs and almost no Eu anomalies in chondrite-normalized patterns. To understand the processes that produce such atypical REE patterns we ran a series of experiments, in which natural bottom seawater or aqueous solutions (NaCl, NaCl–MgCl2, or NaCl–CaCl2) were reacted with gabbro and gabbro mineral assemblages from 300 to 475°C and 40 and 100MPa. These P–T conditions are representative for water–rock interactions in hydrothermal root and discharge zones. Fluid flux variability and kinetics were addressed in the experiments by varying the water-to-rock mass ratio (w/r) from 0.5–10 and using different run durations from 3–720h.Only seawater and synthetic MgCl2-bearing fluid mobilized significant amounts of REEs, Si, Ca, Fe, and Mn from gabbro, from clinopyroxene, and from plagioclase. At 425°C and 40MPa, fluids were initially acidic with pH (25°C) of ∼2 increasing to values between ∼4 and 7 upon progressing reactions. Rare earth element and Fe contents peaked within 3–6h after interaction with gabbroic mineral grains (125–500μm) at w/r of 5 (REEs) and 2–5 (Fe) but decreased with continuing reaction without strong REE fractionation. Most of the REEs that were leached from primary minerals and dissolved in the fluids early became redeposited into solid reaction products after 720h.Contents of dissolved SiO2 were pressure-dependent, being about twofold higher at 100MPa than at 40MPa (425°C) and were below quartz saturation with gabbro and clinopyroxene as solid starting material and close to quartz saturation with plagioclase reactant. However, Si in fluids from the rock-dominated experiments at 100MPa with gabbro (w/r 0.5–1) dropped to very low contents. A concomitant decrease in chlorinity suggests that these changes may be due to the breakdown of olivine and the formation of serpentine and Fe-hydroxy chlorides.Regardless of the starting solid reactants, fluid REE patterns were dominantly controlled by w/r. Atypical fluid REE patterns and high fluid REE contents were obtained at high w/r (⩾5). Whereas typical REE patterns known from many mid-ocean ridge vent fluids, showing relative enrichments of light REEs and a positive Eu anomaly, were obtained at low w/r of 0.5–1. Our results hence clearly show that REE contents and patterns of vent fluids are sensitive to variations in the w/r.

A new species of Pachycara Zugmayer, 1911 (Teleostei: Zoarcidae) from deep-sea chemosynthetic environments in the Caribbean Sea.

The 28^th species of the eelpout genus Pachycara Zugmayer, 1911, is described from specimens collected from an active hydrothermal vent field at a depth of about 2300 m at the Mid-Cayman Spreading Centre of the Caribbean Sea. A tentatively identified early juvenile is recorded at a methane seep at a depth of 1049 m near Tobago. The new species is distinguished from its congeners mainly by its few pectoral fin rays, low vertebral counts, single, mediolateral branch of the lateral line system and presence of scales on the nape and cheeks.

Influence of hydrothermal venting on water column properties in the crater of the Kolumbo submarine volcano, Santorini volcanic field (Greece)

The Kolumbo submarine volcano, located 7 km northeast of the island of Santorini, is part of Santorini’s volcanic complex in the south Aegean Sea, Greece. Kolumbo’s last eruption was in 1650 AD. However, a unique and active hydrothermal vent field has been revealed in the northern part of its crater floor during an oceanographic survey by remotely operated vehicles (ROVs) in 2006. In the present study, conductivity-temperature-depth (CTD) data collected by ROV Hercules during three oceanographic surveys onboard E/V Nautilus in 2010 and 2011 have served to investigate the distribution of physicochemical properties in the water column, as well as their behavior directly over the hydrothermal field. Additional CTD measurements were carried out in volcanic cone 3 (VC3) along the same volcanic chain but located 3 km northeast of Kolumbo where no hydrothermal activity has been detected to date. CTD profiles exhibit pronounced anomalies directly above the active vents on Kolumbo’s crater floor. In contrast, VC3 data revealed no such anomalies, essentially resembling open-sea (background) conditions. Steep increases of temperature (e.g., from 16 to 19 °C) and conductivity near the maximum depth (504 m) inside Kolumbo’s cone show marked spatiotemporal correlation. Vertical distributions of CTD signatures suggest a strong connection to Kolumbo’s morphology, with four distinct zones identified (open sea, turbid flow, invariable state, hydrothermal vent field). Additionally, overlaying the near-seafloor temperature measurements on an X–Y coordinate grid generates a detailed 2D distribution of the hydrothermal vent field and clarifies the influence of fluid discharges in its formation.

Development of a deep-sea laser-induced breakdown spectrometer for in situ multi-element chemical analysis

Spectroscopy is emerging as a technique that can expand the envelope of modern oceanographic sensors. The selectivity of spectroscopic techniques enables a single instrument to measure multiple components of the marine environment and can form the basis for versatile tools to perform in situ geochemical analysis. We have developed a deep-sea laser-induced breakdown spectrometer (ChemiCam) and successfully deployed the instrument from a remotely operated vehicle (ROV) to perform in situ multi-element analysis of both seawater and mineral deposits at depths of over 1000m. The instrument consists of a long-nanosecond duration pulse-laser, a spectrometer and a high-speed camera. Power supply, instrument control and signal telemetry are provided through a ROV tether. The instrument has two modes of operation. In the first mode, the laser is focused directly into seawater and spectroscopic measurements of seawater composition are performed. In the second mode, a fiber-optic cable assembly is used to make spectroscopic measurements of mineral deposits. In this mode the laser is fired through a 4m long fiber-optic cable and is focused onto the target’s surface using an optical head and a linear stage that can be held by a ROV manipulator. In this paper, we describe the instrument and the methods developed to process its measurements. Exemplary measurements of both seawater and mineral deposits made during deployments of the device at an active hydrothermal vent field in the Okinawa trough are presented. Through integration with platforms such as underwater vehicles, drilling systems and subsea observatories, it is hoped that this technology can contribute to more efficient scientific surveys of the deep-sea environment.

A reduced crustal magnetization zone near the first observed active hydrothermal vent field on the Southwest Indian Ridge

Inversion of near‐bottom magnetic data reveals a well‐defined low crustal magnetization zone (LMZ) near a local topographic high (37°47′S, 49°39′E) on the ultraslow‐spreading Southwest Indian Ridge (SWIR). The magnetic data were collected by the autonomous underwater vehicle ABE on board R/V DaYangYiHao in February‐March 2007. The first active hydrothermal vent field observed on the SWIR is located in Area A within and adjacent to the LMZ at the local topographic high, implying that this LMZ may be the result of hydrothermal alteration of magnetic minerals. The maximum reduction in crustal magnetization is 3 A/M. The spatial extent of the LMZ is estimated to be at least 6.7 × 104 m2, which is larger than that of the LMZs at the TAG vent field on the Mid‐Atlantic Ridge (MAR), as well as the Relict Field, Bastille, Dante‐Grotto, and New Field vent‐sites on the Juan de Fuca Ridge (JdF). The calculated magnetic moment, i.e., the product of the spatial extent and amplitude of crustal magnetization reduction is at least −3 × 107 Am2 for the LMZ on the SWIR, while that for the TAG field on the MAR is −8 × 107 Am2 and that for the four individual vent fields on the JdF range from −5 × 107 to −3 × 107 Am2. Together these results indicate that crustal demagnetization is a common feature of basalt‐hosted hydrothermal vent fields at mid‐ocean ridges of all spreading rates. Furthermore, the crustal demagnetization of the Area A on the ultraslow‐spreading SWIR is comparable in strength to that of the TAG area on the slow‐spreading MAR.