Geochemical monitoring of groundwater in seismically-active regions has been carried out since 1970s. Precursors were well documented, but often criticized for anecdotal or fragmentary signals, and ...for lacking a clear physico-chemical explanation for these anomalies. Here we report - as potential seismic precursor - oxygen isotopic ratio anomalies of +0.24‰ relative to the local background measured in groundwater, a few months before the Tottori earthquake (M 6.6) in Southwest Japan. Samples were deep groundwater located 5 km west of the epicenter, packed in bottles and distributed as drinking water between September 2015 and July 2017, a time frame which covers the pre- and post-event. Small but substantial increase of 0.07‰ was observed soon after the earthquake. Laboratory crushing experiments of aquifer rock aimed to simulating rock deformation under strain and tensile stresses were carried out. Measured helium degassing from the rock and
O-shift suggest that the co-seismic oxygen anomalies are directly related to volumetric strain changes. The findings provide a plausible physico-chemical basis to explain geochemical anomalies in water and may be useful in future earthquake prediction research.
Methane emission from the geosphere is generally characterized by a radiocarbon-free signature and might preserve information on the deep carbon cycle on Earth. Here we report a clear relationship ...between the origin of methane-rich natural gases and the geodynamic setting of the West Pacific convergent plate boundary. Natural gases in the frontal arc basin (South Kanto gas fields, Northeast Japan) show a typical microbial signature with light carbon isotopes, high CH
/C
H
and CH
/
He ratios. In the Akita-Niigata region - which corresponds to the slope stretching from the volcanic-arc to the back-arc -a thermogenic signature characterize the gases, with prevalence of heavy carbon isotopes, low CH
/C
H
and CH
/
He ratios. Natural gases from mud volcanoes in South Taiwan at the collision zone show heavy carbon isotopes, middle CH
/C
H
ratios and low CH
/
He ratios. On the other hand, those from the Tokara Islands situated on the volcanic front of Southwest Japan show the heaviest carbon isotopes, middle CH
/C
H
ratios and the lowest CH
/
He ratios. The observed geochemical signatures of natural gases are clearly explained by a mixing of microbial, thermogenic and abiotic methane. An increasing contribution of abiotic methane towards more tectonically active regions of the plate boundary is suggested.
Helium has two natural isotopes which have contrasted, and variable sources and sinks in the atmosphere (
3
He/
4
He
air
= 1.382 ± 0.005 × 10
−6
). Variations in the atmospheric helium isotopic ...composition may exist below typical measurement precision thresholds (0.2 to 0.5%, 2
σ
). In order to investigate this possibility, it is necessary to be able to consistently measure helium isotopes in air with high precision (below 0.2% 2
σ
). We have created an air purification and measurement system that improves the helium isotope measurement precision. By purifying a large quantity of air at the start of a measurement cycle we can make rapid standard-bracketed measurements. Controlling the amount of helium in each measured aliquot minimizes pressure effects. With this method we improve the standard errors by 2× over measuring the same amount of gas in a single step. Individual measurements have standard errors of 0.2 to 0.3% (2
σ
), with three repeat samples needed to reach 0.1% or better errors. The long-term reproducibility of our calibration sample is 0.033% (2
σ
).
This paper details a method to make high precision (less than 0.2%) helium isotope measurements in air samples.
This report describes spatial and temporal variations of helium, carbon, and nitrogen isotopes measured in CO2-rich bubbling gases from natural springs associated with Mt. Ontake (central Honshu, ...Japan) from November 1981 through September 2015. During the entire period, the 3He/4He ratio decreased concomitantly with increasing distance of the sampling site from the central cone. In contrast, the CO2/3He ratios and the δ13C values of CO2 increased with distance. These spatial trends became more pronounced after the September 27, 2014 Ontake eruption, suggesting reactivation of the volcano plumbing system with enhanced emission of magmatic volatiles, although the spatial trend of nitrogen isotopes disappeared. The 3He/4He ratios of the most proximal site to the central cone remained constant until 2000, apparently increasing from June 2003 through October 2014. They became constant soon after the eruption until September 2015. The ten-year 3He enhancement might have been a precursor of the 2014 Ontake eruption. In contrast, δ13C values of CO2 at the same site remained constant through this period. The lack of δ13C anomaly might be attributable to 1) negligibly small amounts of magmatic CO2 introduced into the source of hot springs compared to ambient CO2 in the air-saturated aquifer, or 2) close resemblance of the carbon signature of magmatic CO2 to that of ambient CO2. Therefore its addition cannot change the overall δ13C value of the sampled hot springs. At distal sampling sites, no measurable change of helium, carbon, or nitrogen isotopes was observed in relation to the 2014 Ontake eruption, suggesting that the effect of this eruption on the Ontake hydrothermal system was geographically localized.
•Spatial variations of helium and carbon isotopes were observed in Ontake volcanic hydrothermal system in central Japan.•3He/4He ratios decrease with the distance from the central cone, while δ13C values and CO2/3He ratios increase.•Temporal variations of helium and carbon isotopes are observed from November 1981 to September 2015.•Enhancement of helium isotopes is observed prior to the 2014 eruption, while there are no variations of δ13C values.•Magmatic carbon may have been masked because small amounts were introduced, or its δ13C value is similar to ambient CO2.
CO2 released from subduction zones plays a vital role in the tectonic carbon cycle. However, the contribution of submarine backarc components to carbon emissions remains poorly understood compared to ...subaerial arc volcanos. This study utilized a combination of geochemical and hydrodynamic approaches to investigate the sources and flux of dissolved inorganic carbon (DIC) in the deep basin of the southwestern Okinawa Trough. Most deep‐water (depth ≥1,000 m) samples, even those retrieved from sites distant from active venting, exhibited mild geochemical anomalies of elevated 3He/4He ratios and higher DIC content than reference sites. Hydrodynamic observations using bottom‐mounted acoustic instruments in the deep basin revealed strong tidal currents and enhanced turbulent mixing. These findings suggest that active solute dispersal and mixing are responsible for the prevalence of mild geochemical anomalies in the basin. Mixing models indicated that hydrothermal vent fluids are the primary source of excess 3He and DIC above background levels in the deep basin, whereas both CO2 and mineral acid in hydrothermal fluids cause excess acidity. Based on hydrodynamic and geochemical data, a box model estimated a hydrothermal DIC flux of 0.62 ± 1.40 × 1010 mol y−1, equivalent to 40% of CO2 emissions from persistently degassing volcanos in the Ryukyu Arc. Our results suggest that submarine carbon emissions are quantitatively important in subduction systems with subaqueous backarc components.
Plain Language Summary
The CO2 released from subduction zones, where tectonic plates collide, is important for the Earth's carbon cycle. However, we know little about CO2 released from submarine backarc basins compared to volcanoes on land. This study examined dissolved inorganic carbon (DIC) sources and quantities released into the deep basin of the southwestern Okinawa Trough. Even far from active hydrothermal vents, deep‐water samples had higher DIC levels than expected. Using instruments placed on the seabed, we discovered strong tidal currents and increased water mixing in the basin. These findings suggest that water movement and mixing contribute to the widespread occurrence of elevated DIC levels. Our calculations show that the primary source of the extra DIC in the deep basin is fluids released from hydrothermal vents, and both CO2 and mineral acid cause excess acidity in the water. Based on our data, we estimate that hydrothermal vents release approximately 6 billion moles of DIC per year, equivalent to 40% of the CO2 emissions from continuously active volcanoes in the Ryukyu Arc. These results demonstrate the potential significance of underwater carbon emissions in submerged backarc basins within subduction zones.
Key Points
Mild geochemical anomalies in 3He and dissolved inorganic carbon were widespread in the southwestern Okinawa Trough deep water
Mixing models confirmed that hot vent fluids are the primary source of excess dissolved inorganic carbon in the deep water
The carbon outgassing flux of the Ryukyu subduction system increases by at least 27% when the backarc contribution is considered
Volatiles are lost from the Earth's mantle to the atmosphere, hydrosphere and the crust through subaerial and submarine volcanism. Quantifying the volatile sources bears fundamental information on a ...number of issues in Earth sciences, from the evolution of the atmosphere and oceans to the nature of chemical heterogeneity of the Earth's mantle. The primordial noble gas isotope
3He provides an unambiguous measure of the volatile flux from the mantle, yet so far in the ocean region; it has been only measured at a mid-ocean ridge. Here, we present original measurements of the
3He flux at the Mid-Okinawa Trough back-arc basin. The
3He flux was estimated from
3He/
20Ne vertical profiles measured in deep-sea sediment pore water. Diffusive
3He fluxes vary from 1.6
3He atoms
cm
−2
s
−1 at the hydrothermally active Izena Cauldron to 0.57
3He atoms
cm
−2
s
−1 at the background site, 13
km away. These values are about 20% of the
3He flux measured at the East Pacific Rise, supporting the never−proven hypothesis that
3He mantle flux from subduction zones is a quarter of that at MOR. Measured ocean−floor
4He flux ranges from 3.3
×
10
5 to 4.8
×
10
5
4He atoms
cm
−2
s
−1, higher than that measured worldwide, suggesting that
4He flux at subduction zones might have been previously underestimated.
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.
Key Points:
Gas geochemistry of submarine hydrothermal ores in Western Pacific
Helium isotopes reflect geotectonic setting of samples
Origin and flux of carbon and nitrogen based on the isotopic and elemental ratios
With large topographic gradients, rifted basins serve as efficient traps for particulate matter from adjacent lands and the ocean surface. However, the fate of organic carbon in the sediment, mostly ...unaltered by the hydrothermal activities known to occur in young rifts, remains poorly understood. In this study, we present an examination of diagenetic activities and carbon remineralization based on the first complete suite of pore-water data of sediment marginally affected by hydrothermal activities in the Southern Okinawa Trough (SOT). The sediment showed an oxygen penetration depth of 1 cm, consumption of NO_3^- in the top 1 cm, smeared profiles of Mn^(2+) and Fe^(2+) with the latter reaching up to 450 μmol L^(-1), and relatively unchanged SO_4^(2-) concentrations with depth. Net production rates of dissolved species resolved from pore-water profiles provide an estimate of 1.68 ± 0.21 mmol C m^(-2) d^(-1) as the total carbon remineralization rate in the upper 30 cm sediment column, with aerobic carbon oxidation being the major pathway. The rate, one order of magnitude lower than that of the adjacent East China Sea, is attributed to the lower bottom-water temperature and carbon flux in the trough. The high carbon burial efficiency of SOT (68% of carbon reaching the seafloor and processed thereunder) reflects the combined effects of small mountain rivers and rifting-induced particle trapping.
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.
We measured soil CO2 flux in the representative hydrothermal areas of the Tatun Volcano Group (TVG), to better understand the volcano's dynamic nature, and to estimate its soil CO2 degassing output. ...Results show that the average soil CO2 fluxes obtained at Da-You-Keng (DYK), Geng-Tze-Ping (GTP), She-Haung-Ping (SHP), and Tatun Natural Park (TNP) were 128gm−2d−1, 518gm−2d−1, 420gm−2d−1, and 25gm−2d−1, respectively. The range is comparable to other active volcanic/hydrothermal areas in the world. Along with Liu-Huang-Ku (LHK), where the soil CO2 flux is known, the total soil CO2 output from measured areas is evaluated at 82td−1. Furthermore, a first total soil CO2 output from the whole hydrothermal areas of the TVG is roughly estimated at 113td−1, which includes 15td−1 mantle contribution. Considering the mantle-derived CO2 flux and H2O/CO2 ratio of fumarolic gas, thermal energy associated with the diffuse degassing at the TVG hydrothermal area is estimated at 8.2MW. Carbon (δ13C) and helium (3He/4He) isotopic ratios of soil samples of the studied areas ranged from −4.4 to −6.7‰, and 2.45 to 6.98 RA, respectively. The extent of air involvement in the soil-degassing system, as constrained by the helium and carbon isotopic compositions, provides essential information for depicting regional degassing features of the hydrothermal areas.
•Soil CO2 flux is measured in the Tatun Volcano Group, Taiwan.•The flux spatial distribution is related to degassing pattern.•Soil CO2 output from the TVG hydrothermal areas is estimated to be 113td−1.•Helium and carbon isotopes of soil gas reveal shallow degassing features.
PDF
