Search

We measured the oxygen and hydrogen isotope ratios of smectite, chlorite, and illite in hydrothermally active sediments at the Iheya North Knoll, Okinawa Trough, obtained by seafloor drilling of active hydrothermal fields during the Integrated Ocean Drilling Program (IODP) Expedition 331. Formation temperatures calculated from oxygen isotope data were compared to the downhole temperatures that were measured during the same expedition. The δ18O values ranged from +9.9‰ to +13.3‰ for smectite, +1.3 to +3.0‰ for chlorite or chlorite/smectite mixed-layer mineral, and +2.1‰ to +5.1‰ for illite or illite/smectite mixed-layer mineral with minor chlorite. For samples of illite that contained minor chlorite, δ18O values for illite or illite/smectite mixed-layer mineral of +4.4‰ to +6.1‰ were calculated, based on the proportion of the two clay minerals calculated from chemical composition of the individual mineral and the mixture determined by TEM-EDS and EPMA. We estimated the formation temperatures of the minerals using oxygen isotope fractionation data and assumed values of 0 to +1.2‰ for the fluid that was equilibrated with the minerals, based on the reported values of the seawater and vent fluid measured in previous studies. The temperature range calculated was 111–175 °C for a shallow layer less than about 25 m below seafloor (mbsf) that contained smectite and a minor kaolinite component, 177–297 °C for a deeper layer between about 25 and 40 mbsf that was dominated by chlorite or chlorite/smectite mixed-layer mineral, and 220–314 °C for the deepest layer between about 40 and 120 mbsf that contained illite or illite/smectite mixed-layer mineral with less chlorite. The temperatures of mineral formation calculated from the oxygen stable isotope data were slightly higher at some depths, but both calculated and measured temperatures demonstrated a steep, step-wise thermal gradient below the active hydrothermal field, where temperatures of >200 °C were determined at as little as 50 mbsf. Hydrogen isotope values mostly ranged between −40 and −55‰ and did not vary significantly or systematically between the layers, except in the clay fractions containing smectite above 25 mbsf that had relatively low values between −65 and −72‰. Hydrogen isotope results of the clay minerals were problematic to interpret and not consistent with interpretation of the oxygen data. Calculated temperatures from illite and smectite were mostly negative, whereas temperatures calculated from chlorite were more positive (average 145 °C), but lower than those from the oxygen isotope data. Additional work is still required to determine the hydrogen isotope fractionation between clay minerals and water with respect to temperature.

The Pb isotope compositions of galena in hydrothermal deposits obtained by drillings from two hydrothermal fields in the middle Okinawa Trough were studied. One of the study fields was the Iheya North field located on a volcanic complex and the other was the Izena field located in the sediment-filled caldera structure. LA-MC-ICP-MS was applied to directly measure Pb isotope compositions in individual galena grains which highlighted variations not only in regional scale, but also in microscopic scale so that changes of Pb isotope compositions within a galena grain can be tracked. Homogeneous Pb isotope compositions were found within the same hydrothermal site, irrespective of the mineral assemblage, texture and sampling depth beneath the seafloor. In contrast, the isotope compositions varied significantly between the two hydrothermal fields. The Pb isotope composition from the Iheya North field was isotopically close to the volcanic rocks of the Okinawa Trough, whereas that from the Izena field was more radiogenic with values intermediate between sediments and volcanic rocks of the Okinawa Trough. Within the Iheya North field, intra-field variation of Pb isotope compositions was recognized between two active sites 2.5 km apart from each other. The intra-field variation was recognized also in the Izena field. The sub-seafloor massive sulfide layer has a more sediment-like Pb isotope composition, compared to the inactive sulfide mound. These results illustrate that the Pb-mineralizing hydrothermal fluids originate from their local host rocks with/without sediment and that the isotope compositions of the galena grains reflect their metal sources; either the volcanic rocks and/or the sediments via water-rock interactions.

Three strains of Euglena mutabilis were isolated from sediments in acidic inland water systems (pH = 3.4–4.7), in Southwestern Japan—acid mine drainage in Sensui (Fukuoka), cold sulfidic spring in Bougatsuru (Oita), and a temporal pool in the Ebinokogen volcanic area (Miyazaki). All strains grew well in acidic media at pH 3.07. Phylogenetic analysis among these three strains showed high similarities to plastid SSU and nuclear SSU rRNA gene sequences (99.86% and 99.76%, respectively). They were closely related to the cultured isolates from other highly acidic habitats (pH = 2.0–5.9). Concentration of sulfate, aluminum, calcium, and iron had 7–70 fold of differences among the three studied habitats. Our results imply that the rRNA genes of E. mutabilis have compensated for their low genetic diversity by adapting to a wide pH range, as well as various water chemistry of habitats.

A novel deep-branching archaeal lineage was discovered at high-temperature hot springs around Oyasukyo Gorge in Akita Prefecture, Japan. Actively boiling hot spring water contained >1×104 microbes mL–1. The microbial community composition assessed by analyzing 16S rRNA gene amplicons revealed that the dominant bacterial phyla were Proteobacteria and Aquificae (>50% of the microbial composition) in samples collected in 2016 and 2019, respectively. Approximately 10% of the reads obtained in both years were not assigned to any taxonomy. The more detailed phylogenetic positions of the unassigned sequences identified using a clone library and phylogenetic tree showed that they formed a clade that was independent, distantly related to known phyla, and had low similarity (

Artificial hydrothermal vents, created by boreholes that discharge hydrothermal fluids and useful for observing secular changes in mineral precipitates and the chemical compositions of hydrothermal fluids, are periodically cleaned of scale deposits. Here, we report petrographic and geochemical features of hydrothermal scale with a concentric structure and extreme enrichment in Zn, recovered as an intact plug from an artificial hydrothermal vent pipe in the Okinawa Trough. The scale consists of sphalerite with accessory galena and chalcopyrite, and minor cotunnite (PbCl2), barite, an unidentified Zn sulfate, and Bi-rich minerals. It comprises at least five concentric layers alternating between thin, reddish-brown porous layers composed of relatively Fe-rich sphalerite accompanied by galena and chalcopyrite, and coarse-grained, dark gray layers dominated by relatively Fe-poor sphalerite. Cotunnite occurs only in the innermost reddish-brown layer, and barite occurs only in the uppermost and innermost layers. The scale is composed of > 50 wt% Zn, several wt% Fe and Pb, and

The commercial use of metals such as copper, lead, and zinc has markedly increased in recent years, resulting in increased interest in deep-sea mining of seafloor hydrothermal sulfide deposits. However, the full extent of the impact of deep-sea mining at hydrothermal field deposits on the environment remains unclear. In addition to impacting the deep sea, the leaching of heavy metals from extracted sulfide mineral may also affect the upper ocean zones as the sulfide rock is retrieved from the seafloor. Here, we used a delayed fluorescence-based bioassay using the marine cyanobacterium Cyanobium sp. NIES-981 to evaluate the toxicity of three sulfide core samples obtained from three drill holes at the Izena Hole, middle Okinawa Trough, East China Sea. Leaches from two of the cores contained high concentrations of zinc and lead, and they markedly inhibited delayed fluorescence in Cyanobium sp. NIES-981 compared with control. By examining the toxicity of artificial mixed-metal solutions with metal compositions similar to those of the leaches, we confirmed that this inhibition was a result of high zinc and lead concentrations into the leaches. In addition, we conclude that this delayed fluorescence-based bioassay is a viable method for use by deep-sea mining operations because it is quicker and requires less laboratory space and equipment than the standard assay.

Many samples of hydrothermal sulfide minerals from the Okinawa Trough yield 234U–230Th radioactive disequilibrium ages that are inconsistent with the ages of coexistent barite obtained using electron spin resonance (ESR) method. These results contrast strongly against results obtained for the Southern Mariana Trough, for which the U–Th radioactive disequilibrium ages are roughly consistent with the ESR ages. Sulfide minerals from the Okinawa Trough show large variation in U and Th abundances, suggesting open-system behavior. We infer that the interaction of hydrothermal fluids with thick terrigenous sediment caused large-scale migration of U and Th in the Okinawa Trough. The (234U/230Th) – (232Th/230Th) plots for the sulfide samples from the Hatoma Knoll and the Izena Hole show two mixing trends for each site. The presence of Th-rich phase with high (230Th/234U) ratio seems to cause incorrect age estimates of the sulfide deposits.

Effect of silver (Ag) catalyst in bioleaching of enargite (Cu_3AsS_4) concentrate was studied using mixed cultures of moderately thermophilic acidophilic microorganisms at 45ºC. Addition of Ag_2S enabled selective Cu dissolution from enargite while suppressing pyrite oxidation: At the highest Ag_2S concentration of 0.04%, Cu recovery reached 96% while Fe dissolution was suppressed to reach only 29% by day 72. Overall results from thermodynamic calculation, liquid/solid analyses and kinetic study suggested that Ag-catalyzed bioleaching of enargite concentrate proceeds via formation of at least two types of secondary products (chalcocite, Cu_2S; trisilver arsenic sulfide, Ag_3AsS_4): Addition of Ag_2S as Ag catalyst thermodynamically and microbiologically contributed to lowering solution redox potentials during bioleaching, consequently satisfying Eox (Cu_2S) < E_h < E_c (Ag+) to enhance enargite dissolution via formation of chalcocite intermediate. Formation of trisilver arsenic sulfide and its intermediate layer (Cu,Ag)_3AsS_4 indicated that Cu ion in the enargite lattice is gradually substituted with Ag. Such secondary products did not impose a rate-limiting step, since the Ag-catalyzed bioleaching was shown to be controlled by a chemical surface reaction, rather than diffusion through product film which was the case in the absence of Ag_2S.

Molybdenum (Mo) and tungsten (W) are redox-sensitive elements that exhibit contrasting geochemical behaviors under different redox and/or sulfidic conditions despite belonging to the same group in the periodic table. In this study, the geochemistry of Mo and W in a sediment–porewater system was investigated using a core sample collected from sediment containing hydrothermal minerals (Izena Hole, Okinawa, Japan). The Mo contents in the sediment and the porewater were quantified to calculate the partition coefficient Kd (concentration ratio of the solid phase to the aqueous phase) for Mo. However, the W concentration in the porewater was exceedingly low to be detected. Thus, we also conducted laboratory experiments simulating the sediment–porewater system to clarify the partitioning behaviors of Mo and W under redox conditions. X-ray absorption near-edge structure (XANES) spectra were obtained to determine the chemical species of Mo and W in sediment at different depths to understand the chemical processes of Mo and W. In particular, high-sensitivity XANES spectroscopy using wavelength-dispersive fluorescence mode was applied to W L3 edge XANES spectra to reduce the interference signals from coexistent elements (Zn and Ni). This step thereby facilitating the identification of oxygen- and sulfur-coordinated species for W in the sediments. The collected core sample covered a broad range of redox conditions under various hydrogen sulfide (H2S) concentrations depending on the depth. The presence of iron oxides in the upper layer suggested an oxic condition above within 4 cm depth below the seafloor (cmbsf). Iron and Zn sulfide minerals were found in deeper layers (16–24 cmbsf) and indicated the reductive conditions formed in such layer. Hydrogen sulfide was also observed in the porewater of the deeper layer. Analyses for Mo in the natural sediments, as well as XANES analyses, revealed that the Kd was higher in the deeper layer, with high pyrite and H2S contents, than in the upper layer. These results implied that Mo was removed from the porewater under reductive and/or sulfidic conditions. The chemical species of Mo was also an oxygen-coordinated species in the upper layer (0–8 cmbsf). Meanwhile, the detection of sulfide in the deeper layer suggests that Mo sulfidation is an important reaction in Mo enrichment in sediments. On the other hand, at all the depths, W formed oxygen-coordinated species in the sediment. The subtle change of Kd for W with depth suggested that H2S did not affect the W adsorption in our samples. Hence, the Mo/W ratio in the sediments increased with the development of reducing conditions and vice versa in the coexistent porewater. Speciation analysis revealed that the high stability of oxygen-coordinated species of W was responsible for the variations of the Mo/W ratio under various redox conditions.

Meteorite impacts have caused catastrophic perturbations to the global environment and mass extinctions throughout the Earth’s history. Here, we present petrographic and geochemical evidence of a possible impact ejecta layer, dating from about 11 Ma, in deep-sea clayey sediment in the Northwest Pacific. This clay layer has high platinum group element (PGE) concentrations and features a conspicuous negative Os isotope anomaly (187Os/188Os as low as ~0.2), indicating an influx of extraterrestrial material. It also contains abundant spherules that include pseudomorphs suggestive of porphyritic olivine as well as spinel grains with euhedral, dendritic and spherical forms and NiO contents as great as 23.3 wt%, consistent with impact ejecta. Osmium isotope stratigraphy suggests a most plausible depositional age of ~11 Ma (Miocene) for this layer, as determined by fitting with the seawater evolution curve. No large impact crater of this age is known on land, even within the relatively large uncertainty range of the relative Os age. Thus, we suggest that an unrecognised impact event in the middle or late Miocene produced the impact ejecta layer of the Northwest Pacific.

We used micro-Raman spectroscopy to analyze volcanic ejecta from the 7 March 2012 phreatic eruption at Old Crater (Million Dollar Hole) on Ioto Island, Izu-Bonin arc, Japan, to assess the ability of this technique to identify clay and other alteration minerals in volcanic ejecta. We also analyzed five reference clay minerals (kaolinite, dickite, pyrophyllite, montmorillonite, and saponite) using the same technique. Dickite, montmorillonite, gypsum, pyrite, marcasite, quartz (α-quartz), and anatase were identified by comparison with spectra of the reference materials and published databases. These minerals occur in close spatial association with each other. Marcasite, quartz, and anatase in aliquots of the sample were not observed by a previous powder X-ray diffraction (XRD) study. The presence of dickite, montmorillonite, gypsum, quartz, marcasite, and anatase indicates that these alteration minerals were likely derived from the acidic alteration zone (150–190 °C) below the crater on Ioto Island. Our results demonstrate that Raman microspectroscopy, compared with conventional XRD, is a more sensitive technique for identifying and characterizing alteration minerals present in the volcanic ejecta. However, owing to the strong fluorescence from clay minerals during Raman analysis, the presence of other minerals in the sample may be overlooked. Therefore, the use of the two techniques in combination can be a powerful tool for obtaining a full understanding of mineral phases in volcanic ejecta from phreatic eruptions, thereby providing more accurate information on the hydrothermal conditions below volcanic craters.

Taketomi submarine hot spring lies off Ishigaki Island in the southern Ryukyu Islands and vents hot spring waters at temperatures up to ~ 50 °C from the seafloor at a depth of 20 m. We investigated the chemical and isotopic composition of gases discharging from Taketomi hot spring. The gases were composed mainly of methane, with secondary nitrogen at higher than atmospheric concentration. Carbon and hydrogen isotope data suggest that the methane in the discharging gases was derived mainly from thermal decomposition of organic matter. Helium isotopes were enriched in 3 He relative to the atmosphere, suggesting a supply of mantle-derived helium to the discharging gases. The mantle-derived gases transfer the deep-originated thermal energy to the hot spring and thermogenesis of organic matter. The hydrocarbons in the venting gas could be sourced from sedimentary rocks of the Yaeyama or Shimajiri Groups, or Yaeyama metamorphic rocks, and added to the ascending gases as they pass through those source rocks on their way to the surface. Because the Pleistocene rocks of the Ryukyu Group beneath the hot spring have been altered by the spring activity, the Taketomi hot spring began venting after the Pleistocene.

We here report on specific chemical and microbial compositions observed at the Archaean hydrothermal site in the Southern Mariana backarc spreading center, which produces two remarkably different hydrothermal fluids. One was black smoker hydrothermal fluid at 340 °C containing a low concentration of methane with a relatively high carbon isotope ratio (δ13C of methane relative to VPDB), −7.8‰, indicating that methane originated from a magmatic source; thus, this is a fairly ordinary hydrothermal fluid for basalt-hosted hydrothermal activity. In contrast, the other fluid was clear smoker hydrothermal fluid at 117 °C containing a high concentration of methane with a very low carbon isotope ratio (−49.7‰). The host rock of the Archaean hydrothermal site is basalt, and therefore Fischer-Tropsch-type reactions resulting from serpentinization of mantle rocks are not feasible as a source for the “light” carbon in methane. Additionally, the carbon isotope ratio of carbon dioxide demonstrated that the sources of CO2 and CH4 were not organic materials. The remaining possibility, biogenic methane, was confirmed by cultivation of hyperthermophilic hydrogen-oxidizing methanogens from both rocks covering the vent of clear smoker hydrothermal fluids and in situ cultivation systems. Although the dominance and abundance of methanogens were very low in this ecosystem, the number was consistent with the relationship between hydrogen concentration and methanogen abundance (Takai et al., 2015). These results suggest that phase separation led to concentration of hydrogen and subsequent persistence of a hyperthermophilic subsurface lithoautotrophic microbial ecosystem in both serpentine-hosted and basalt-hosted hydrothermal systems. This type of ecosystem may occur in similar settings elsewhere on modern earth; in addition, similar communities may have existed in other types of deep-sea hydrothermal systems in the geological past.

Temperature is an important factor affecting the distributions and life-history traits of marine animals. Deep-sea hydrothermal vents are suitable environments to examine ecologic differences related to temperature, due to the steep temperature gradients around the vents. Rearing experiments under various temperature conditions (5–30 °C) at atmospheric pressure demonstrated a difference in thermal effects on egg hatching and larva in two co-occurring, vent-associated alvinocaridid shrimps – the peripherally distributed Alvinocaris longirostris and the centrally distributed Shinkaicaris leurokolos. The duration before hatching became shorter as temperature increased, while the maximum hatching rate occurred at higher temperatures in S. leurokolos (10–20 °C) than in A. longirostris (10 °C). Hatched larvae of both species were negatively buoyant, and larva with normal abdominal length could actively swim and stay suspended in the mid- or surface water layers of the culture plates under our experimental conditions. However, no larvae settled or metamorphosed into juveniles under the rearing conditions used in this study. Larvae with shortened abdomens occurred under most of the experimental conditions, although they were less frequent at 10 °C in A. longirostris and 20 °C in S. leurokolos. The maximum survival periods at these temperatures were 88 days in A. longirostris and 30 days in S. leurokolos. These characteristics may cause differences in the distributional ranges of the two species. The present results indicate that temperature is an important factor controlling life-history traits of vent shrimps.

Subseafloor microbes beneath active hydrothermal vents are thought to live near the upper temperature limit for life on Earth. We drilled and cored the Iheya North hydrothermal field in the Mid-Okinawa Trough, and examined the phylogenetic compositions and the products of metabolic functions of sub-vent microbial communities. We detected microbial cells, metabolic activities and molecular signatures only in the shallow sediments down to 15.8 m below the seafloor at a moderately distant drilling site from the active hydrothermal vents (450 m). At the drilling site, the profiles of methane and sulfate concentrations and the δ13C and δD isotopic compositions of methane suggested the laterally flowing hydrothermal fluids and the in situ microbial anaerobic methane oxidation. In situ measurements during the drilling constrain the current bottom temperature of the microbially habitable zone to ~45 °C. However, in the past, higher temperatures of 106–198 °C were possible at the depth, as estimated from geochemical thermometry on hydrothermally altered clay minerals. The 16S rRNA gene phylotypes found in the deepest habitable zone are related to those of thermophiles, although sequences typical of known hyperthermophilic microbes were absent from the entire core. Overall our results shed new light on the distribution and composition of the boundary microbial community close to the high-temperature limit for habitability in the subseafloor environment of a hydrothermal field.

The abundance and isotopic compositions of volatile elements trapped in fluid inclusions of submarine hydrothermal mineral deposits in Western Pacific subduction zones (Okinawa Trough, Izu-Bonin arc, Mariana Trough, and Lau Basin) and in Kuroko ores in northeastern Japan are presented. The helium isotopic compositions corrected for air contribution of the Okinawa and Mariana troughs, ranging 4.49–7.68 Ra are lower than those of the Izu-Bonin and Lau Basin, 7.62–8.91 Ra. This characteristic might reflect the differences in regional tectonic setting. The Okinawa and Mariana troughs are related to back-arc spreading with strong graben sedimentary signature, whereas the Izu-Bonin arc is associated with island arc magmatism. The arc contribution to the Lau Basin volcanism is significantly strong, even though it is assigned to back-arc spreading. Nitrogen isotopes can also be explained by a similar hypothesis, whereas argon and carbon isotopes cannot be used to discriminate tectonic setting. δ13C–CO2/3He and δ15N–N2/36Ar diagrams elucidate the source of carbon and nitrogen. The MOR-type mantle contributions to carbon are mostly smaller in the Okinawa and Mariana troughs (ranging 0.06–8.9% with the average of 2.4%) than in the Izu-Bonin and Lau Basin (2.1–25% with the average of 7.7%). The sedimentary contributions to nitrogen are larger in the Okinawa and Mariana troughs (11–65% with the average of 35%) than in the Izu-Bonin and Lau Basin (4–24% with the average of 15%), and the Kuroko samples agree well with the latter. Carbon and nitrogen fluxes are again higher in Okinawa trough than in Izu-Bonin arc.

Ground-water samples were collected from 10 wells in Wadi El-Natrun, Western Desert, Egypt, over four consecutive years. The combination of isotopic composition and hydrochemical analyses was used to identify the origin of the groundwater and determine recharging sources as well as to discuss the mechanism controlling hydrochemical variation among wells. Two water sources, a relatively recent/shallow aquifer recharged from Nile River water and a deep/old aquifer (i.e. the Nubian sandstone aquifer system [NSAS]), are identified from isotopic and geographical relationships and the correlation between isotopic data and bicarbonate concentration. It was found that Nile Delta aquifer water was likely supplied as groundwater flows from the southeast and that old water (paleowater) was supplied upward from deeper layers. Some wells show relatively greater variation in concentration as well as in hydrogen isotope composition during the four-year study. Based on the relationship between concentration and isotope composition the variation was not caused by seawater intrusion or salt dissolution but by intensive water loss. The water loss is not directly due to evaporation or suction but is considered to proceed indirectly within the soil.

Seafloor hydrothermal sulfides, which are expected to be a future resource for metals, could be a potential source for metal contamination in the seawater around mining sites. In this chapter, we illustrate the potential for metal leaching of both non-oxidized (non-exposed to atmosphere; before and during exploitation) and oxidized (exposed to atmosphere; after lifting and recovery) hydrothermal sulfides to seawater under different temperatures and redox conditions. One of the crucial findings was that metal dissolution behaviors differed significantly according to the specific areas and/or the initial oxidation states of the sulfide surfaces. Once the non-oxidized sulfide chips were ground to particulates and mixed with seawater, Zn and Pb were preferentially released even though these metals were included as minor components of the sulfides. For Zn, the dissolution rate increased under the oxic and higher temperature (20 °C) conditions when compared to the anoxic and lower temperature (5 °C) conditions, but the absolute rate was relatively moderate. These findings suggest that instantaneous metal release from sulfides into seawater will not occur before or during the crushing and lifting processes of seafloor mining. In contrast to the non-oxidized sulfides, the oxidized sulfides rapidly released large amounts of various metals and metalloids (e.g., Mn, Fe, Zn, Cu, As, Sb, and Pb) into seawater. The different metal dissolution behaviors between the non-oxidized and oxidized hydrothermal sulfides suggest the importance of the implementation of appropriate environmental measures to prevent leakage of the lifted sulfides to the marine surface.

BackgroundGastropods of the genusProvannaare abundant and widely distributed in deep-sea chemosynthetic environments with seven extant species described in the northwestern Pacific.MethodsWe investigated the population history and connectivity of fiveProvannaspecies in the northwestern Pacific through population genetic analyses using partial sequences of the cytochromecoxidase subunit I gene.ResultsWe found thatP. subglabra, the most abundant and genetically diverse species, is genetically segregated by depth. Among the five species, the three comparatively shallower species (P. lucida, P. kuroshimensis, P. glabra) had a more constant demographic history compared to the deeper species (P. subglabra,P. clathrata).DiscussionEnvironmental differences, especially depth, appears to have a role in the segregation ofProvannasnails. The population ofP. clathratain the Irabu Knoll appears to have expanded afterP. subglabrapopulation. The remaining three species,P. lucida,P. kuroshimensis, andP. glabra, are only known from a single site each, all of which were shallower than 1,000 m. These data indicate thatProvannagastropods are vertically segregated, and that their population characteristics likely depend on hydrothermal activities.