We measured helium isotopic ratios of 84 seawater samples from various depths collected in the western North Pacific Ocean and the western Philippine Sea. The ³He/⁴He ratios varied significantly from δ³He of 0.1% to 22.9%, where δ³He is defined as the percent deviation of the helium isotopic ratio relative to the atmospheric standard. Maximum δ³He > 20% was observed at mid-depth (2000-2500 m) in the western Philippine Sea and in the southern part (∼10°N) of the western North Pacific, though not in the northern part (∼30°N) at the same depth. Contour maps of the lateral δ³He distribution at mid-depth suggest that the helium-3 plume derived from the East Pacific Rise does not flow northward along the Izu-Ogasawara-Mariana Ridge but westward through the Caroline Basin or the Yap-Mariana Junction into the Philippine Sea. It then flows northward in the western Philippine Sea to a region adjacent to the Japanese Islands. Although these flows inferred from the δ³He distribution are roughly similar to those estimated from water properties such as isopycnal distributions, the δ³He distribution could reveal that deep-water circulation seems to be different at each depth (2000, 2500, 3000 m).

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Seafloor seeping methane bubbles were successfully sampled on the summit area of the Kuroshima Knoll (depth of ca. 640 m) using the gas-tight sampler WHATS attached to the Shinkai 2000. To evaluate the origin of the bubbles and verify that the dissociation of methane hydrate was actually in progress, the chemical and isotopic composition of the samples were analyzed. The major component of the gas bubbles was methane (C₁) with traces of CO₂ (67 ± 16 ppmv) and helium (11 ± 1 ppmv; ⁴He/²⁰Ne = 320) having a moderate ³He/⁴He ratio (0.44R_a). C₁ was enriched relative to other hydrocarbons (C₁/(C₂+C₃) > 3000). The δ¹³C values for C₁ (-40.1‰_VPDB), C₂ (-28.3‰_VPDB), and C₅ (-28.0‰_VPDB) were similar to those of hydrocarbons produced by thermal decomposition of organic matter. The contribution of the mantle-derived ³He-enriched component in coexisting helium also supports thermogenic generation. On the other hand, the other light hydrocarbons showed an unusual ¹³C-enrichment in C₃ (-19.1‰_VPDB), iso-C₄ (-22.4‰_VPDB), and n-C₄ (-19.9‰_VPDB). C₃ and C₄, had been fractionated both chemically and isotopically through subsequent microbial destruction during the long storage from Miocene in the gas reservoir. In addition, the anaerobic oxidation of CH₄ within shallow sediments removed about 20% of CH₄, until seepage into ocean water column. Regarding the contribution of gases originating from hydrate dissociation to the bubbles, observed helium amount in the bubbles suggests that methane hydrate is considered to be a minor contributor to the bubbles, at least at present. Direct leakage of gases from deep reservoirs is a more plausible for the source. Regarding the fate of the hydrocarbons in the bubbles in the water column, all rising bubbles at Kuroshima Knoll dissolved within 140 m of the seafloor. After the dissolution, the plume spreads horizontally along with the surface of equal density in the water column, while the concentrations decrease through dilution by eddy diffusion, rather than by oxidation.

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Within neutrally buoyant hydrothermal plumes derived from the Yonaguni Knoll IV hydrothermal field (24°51´ N, 122°42´ E, D = 1,370-1,385 m) in the southwestern Okinawa Trough back-arc basin, we obtained profiles of dissolved manganese (Mn), helium isotopes (³He/⁴He), methane (CH₄) and its stable carbon isotope ratio (δ¹³C_PDB) in young to old plume waters. We first mapped the spatial distribution of hydrothermal plumes by towing a fixed array of optical sensors (Miniature Autonomous Plume Recorders: MAPRs) above the field. We then made water column observations and samplings using a CTD (Conductivity, Temperature, and Depth sensors)-Carousel package with a transmissometer and Niskin-X bottles at five locations, with distances between 0.6 and 6.1 km from the hydrothermally active center so far discovered. Vertical profiles of light transmission and chemical tracers indicated triple-layered plumes, the centers of which were at depths of 700-800 m, ∼1,050 m and ∼1,200 m. The CH₄ concentrations and δ¹³C values for the 1,200-m plume ranged between 1,026 and 10 nmol/kg, and between -22.4 and +40.4‰ (the highest δ¹³C value yet reported for oceanic CH₄), respectively, indicating active microbial CH₄ oxidation accompanied by the δ¹³C increase for residual CH₄. The δ¹³C(CH₄) was shown to be useful for tracing such a “microbial plume” whose CH₄ concentration has already fallen to almost the background level. By applying the Rayleigh distillation equation for a closed system, we estimated the kinetic isotope fractionation factor of 1.012 for the CH₄ oxidation process occurring within the hydrothermal plumes deeper than 1,000 m.

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To constrain the origin of fluoride and arsenic contamination of groundwater in Kalalanwala, Punjab, Pakistan, the geochemical and mineralogical characteristics of the aquifer were studied. Subsurface geology of the area was studied using data from drilled cores. The residential and industrial areas including Kalalanwala are covered with Pleistocene aeolian deposits. These sediments cover two aquifers intercalated by a less permeable layer. Most polluted groundwater appears in the first aquifer at 15 to 25 m depth beneath the center of the village of Kalalanwala. A considerable amount of anthropogenic fluoride contaminates the aquifer along an ancient river channel. Arsenic might derive from anthropogenic sources such as industrial waste, or detrital chlorite.

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Hydrothermal plumes above the HAKUREI and JADE sites, two high-temperature hydrothermal vent sites in the Izena Cauldron at the mid-Okinawa Trough, were investigated in order to gain a preliminary understanding of gas geochemical characteristics at underlying hydrothermal vent sites. Three geochemical tracers, H₂, CH₄ and δ³He, covary with each other above the HAKUREI site but only CH₄ and δ³He are correlated above the JADE site. The carbon isotope ratio of methane within the Izena Cauldron can be accounted by a combination of the fluid dilution by ambient seawater and microbial consumption with the kinetic carbon isotope effect (KIE) of 1.007. An estimated endmember δ¹³C value of -32‰ in the HAKUREI fluid was obtained. Both the plumes above the HAKUREI and JADE sites showed C₁/C₂ ratios between 10³∼10⁴. Only the bottom water around the HAKUREI site showed significant N₂O excess with isotopically light δ¹⁵N and δ¹⁸O, suggesting N₂O input from microbial activity in the sediment. A linear correlation between H₂ and CH₄ in the HAKUREI plume gives a H₂/CH₄ ratio of the HAKUREI fluid of more than 0.022. The estimated H₂/CH₄ ratio in the HAKUREI fluid is significantly higher than that of the JADE fluid, comparable with those of fluids venting at other sediment-related hydrothermal systems, and also comparable with those of thermogenic gases produced by hydrothermal sediment experiments. These facts suggest that fluid-sediment interaction during fluid upwelling appears to modify gas geochemical characteristics at the HAKUREI site but have little influence at the JADE site. This study demonstrates the availability of the Izena Cauldron hydrothermal field and the HAKUREI and JADE sites as a natural laboratory for investigating the fluid-sediment interaction during fluid upwelling.

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He, Ne and Ar isotopic ratios and concentrations of noble gases in different depths of Lakes Nyos and Monoun in Cameroon, West Africa, were investigated. Samples were collected in November 1999, January 2001 and December 2001. Improved sampling method applied in 2001 enabled us to detect detailed isotopic distribution in the lakes. Concentrations of Ne and Xe in the lake waters were low by factors of ∼10 and ∼3, respectively, compared with those of air saturated water (ASW), whereas in CO₂ gas they were depleted by 2 orders of magnitude compared with those in air. In contrast, He was strongly enriched in lake water by more than three orders of magnitude compared to ASW. ³He/⁴He ratios of 8 × 10^-6 (Lake Nyos) and 5 × 10^-6 (Lake Monoun) indicated recharge of magmatic gases of mantle origin to the lakes, though the ratios were lowered by the addition of radiogenic ⁴He from crustal rocks. A small contribution (<2%) of MORB-like Ne was detected in both lake waters. A simple mixing between two end members, i.e., atmosphere and gases from a deep-seated reservoir, can explain the noble gas signatures. Depth profile of He concentration in Lake Nyos showed a clear layered structure with a maximum at 190 m, 20 m above the lake bottom. Below this depth, ³He/⁴He ratio decreased slightly, suggesting that the fluid from the deep-seated reservoir is supplied to the lake mainly at ca. 190 m. C/³He ratios were 1.5 × 10¹⁰ and 0.6 × 10¹⁰ in deep and shallow layers of Lake Nyos, respectively. The C/³He ratio was 1.7 × 10¹⁰ at Lake Monoun. A gas recharge model to explain the observed layered structure in Lake Nyos is presented.

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Thermal and groundwaters are distributed along the west coast and in the inland region of Northwestern Hokkaido, Japan. The waters in the latter group have relatively high temperatures. Stable (D, O) and noble gas isotopic compositions of 23 thermal and groundwaters were analyzed to investigate their origins. The Cl concentrations vary from 45 to 19,300 mg/L and the δD values are in the range of -90 to -8‰. The δD-δ¹⁸O plots show a linear relationship which can be explained by a simple mixing of local meteoric waters and altered sea water, with oxygen isotopic shift of 5‰. However, a δD-Cl plot shows that there is also a contribution of an additional water component of different origin. This 3rd component has a δD value of -20‰ and Cl concentration of 6,000 mg/L. Multivariate statistical analysis of the observed chemical and isotopic compositions using the code M3 (M; multivariate mixing and mass balance) also supports the view that the Toyotomi thermal water has the composition closest to that of the 3rd groundwater component. This component is regarded as a geopressured fluid. The ³He/⁴He ratios in these waters are in the range of (0.1-5) × 10^-6, which indicate contributions of both mantle and crustal He to the thermal and groundwaters in the studied area. The Toyotomi thermal water has a ³He/⁴He ratio of 5.5 × 10^-7, and a high ³He/²⁰Ne ratio (∼700). These results indicate a crustal He input with a small amount of mantle-derived He and imply that the 3rd groundwater component is of deep origin. On the basis of isotopic and chemical compositions, 5 of 23 samples studied groundwater and thermal waters are contributed by a geopressured fluid component. The results show that geopressured fluids with a δD value of -20‰ and Cl concentration of 6,000 mg/L are widely distributed in northwestern Hokkaido.

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Pore waters at the depths of 0-590 cm below seafloor (bsf) were collected from four core samples at four different sites in a mud volcano off Tanegashima Island between Ryukyu trench and Ryukyu arc of Japan. Concentrations of Cl^-, SO₄^2-, CH₄, C₂H₆ and stable isotopic composition of δ¹³C_CH4, δ¹⁸O_H2O, δD_H2O in the pore waters vary as a function of distance from seafloor. This paper reports and discusses the pore waters collected at the summit (CV) site. The concentrations of Cl^- decrease from 540 at the seafloor to 375 mmol/kg at a depth of ∼200 cm and remain constant at around 350 mmol/kg (64% of the concentration of seawater) below the depth. The concentrations of CH₄ are two to three orders of magnitude higher than those at other sites and have a maximum value of 715 μmol/kg at around 120-140 cm bsf. Core samples collected at depths deeper than 180 cm bsf show collapsing gas bubbles and empty voids when they were split open. It was also observed that liquid seeped out from the surface of the split core. Considering the physical condition is favorable for the formation of methane hydrate, the observations suggest the existence of methane hydrates. High concentration of C₂H₆, which had similar depth profile to that of CH₄, was also observed. C₂H₆/CH₄ ratio remained larger than 10^-3 and δ¹³C_CH4 also remained around -45‰ below 180 cm bsf. The data suggest presence of thermogenic methane in the CV site. δ¹⁸O_H2O and δD_H2O profiles exhibited an opposite depth dependence, and only δD_H2O showed a decreasing depth profile similar to the concentration profile of Cl^-. They were inversely correlated with the concentration of Cl^-. The data of these two isotope compositions suggest a dilute fluid originates mainly from clay mineral dehydration but meteoric water. A simple mixing model of fluids from three sources (ambient seawater, water dissociated from methane hydrates, and diagenetic water ascending from deeper depth) with isotopic fraction during methane hydrate dissociation was applied for the observation result below 280 cm bsf to constrain ranges of δ¹⁸O_H2O and δD_H2O of diagenetic water. Using the observed depth profile of Cl^- as a conservative component of ambient seawater, contribution of ambient seawater is estimated to be 64% whereas 36% from other two sources. Considering an isotopic fractionation during methane hydrate dissociation and using the estimated source fractions and observed isotopic composition of pore water, δ¹⁸O_H2O and δD_H2O of the diagenetic water were estimated to range from +15 to +22‰ and from -103 to -43‰, respectively, which are in good agreement with isotopic compositions of water formed from clay minerals during their dehydration but quite different from those of meteoric water, supporting negligible contribution of meteoric water in the Tanegashima mud volcano fluid.

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Atmospheric aerosol samples were collected from 1993 to 2003 at Tsuruoka, Japan, a coastal area on the Sea of Japan. Concentration of water-soluble chemical components and sulfur isotopic ratios of sulfate in aerosol samples were measured to evaluate the temporal variation and identify the source. The _samSO₄^2- and _nssSO₄^2- concentrations range from 7.6 to 354.4 and 3.7 to 335.6 neq m^-3, respectively, and there was no clear seasonal variation. Sulfur isotope ratios of total sulfate (_samδ³⁴S) and non-sea salt sulfate (_nssδ³⁴S) in aerosols are +1.0 to +15.1 with a mean value of +5.7‰ and -4.4 to +14.5 with a mean value of +4.1‰, respectively. The values of _samδ³⁴S and _nssδ³⁴S are relatively low in summer and high in the other three seasons. The _nssδ³⁴S values of aerosols in fall to spring agree with the average sulfur isotope ratios of coal used in northern China, considering the isotope fractionation. These results indicate the possibility of long-range transport of sulfate from the Asian continent to Japan. The variations of atmospheric _samSO₄^2- and _nssSO₄^2- concentrations decreased gradually and reach a minimum in 1999. The decline of SO₂ emission in China due to reduction in industrial coal use, a slowdown of the Chinese economy, and the closure of small and inefficient plants would be among the causes of this decreasing observed trend. On the other hands, the variation ranges of _samδ³⁴S and _nssδ³⁴S values abruptly changed after 2000. The introduction of fuel-gas desulfurization technologies to industrial plants of coke production and thermal power stations would be one of the most important causes of this change.

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We present erosion rates of granitic soil surfaces in the western Abukuma upland, Japan using depth profiles of in-situ produced cosmogenic ¹⁰Be and ²⁶Al based on physical parameters for both neutron and muon interactions. Samples were obtained below the severely weathered zone, from 30 to 190 cm depth below surface (bs). We confirmed that, in this environment, deeper layers from at least 80 cm bs must be analyzed to achieve highly accurate measurement of erosion rate because near-surface layers are potentially influenced by pedogenic processes. The depth profiles obtained suggest a surface-lowering rate of 49-74 m/Myr for a mountain ridge composed of granitic soil. This newly obtained erosion rate is much higher than previously reported granitic or metamorphic bedrock, as well as quartz vein, erosion rates from several climatic environments including humid regions, suggesting that granitic soil surface is more susceptible to erosion.

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