Spring Gases Research Articles

Greenhouse gases (CO2, CH4 and N2O) in lowland springs within an agricultural impacted watershed (Po River Plain, northern Italy)

In the Po River Plain, nitrogen surplus in permeable soils results in elevated downward nitrogen fluxes, mostly as nitrate. Lowland springs, aligned along interfaces between gravel and sandy soils, recycle part of this nitrogen to the surface and we hypothesised that they may be hot spots of N2O and other greenhouse gases, due to incomplete denitrification in the suboxic environment. In early and late summer 2009, water flow was measured and water samples were collected at the outlet and ∼1 km downstream at 14 springs; physico-chemical parameters [temperature, pH, dissolved inorganic nitrogen (DIN) and dissolved gases (O2, N2O, CH4, CO2)] were analysed. All springs were characterised by elevated nitrate concentrations (154–1411 μ M) and recycled to the surface inorganic nitrogen (∼180 kg N on average). Spring waters were suboxic (40–60% of O2 saturation) and CO2, CH4 and N2O supersaturated (26.6–2399.0, 0.002–1.02 and 0.02–1.02 μ M, respectively). CO2 and N2O underwent a significant degassing process from the supersaturated waters to the atmosphere. Calculated N2O emissions (up to 0.646 g N 2O·m −2·d −1, among the highest reported for aquatic environments) highlight the role of lowland springs as hotspots of N2O. We conclude that lowland springs located in heavily impacted watersheds recycle groundwater nitrate and have an extremely elevated potential as greenhouse gas emitters.

Long range gas-geochemical anomalies of a remote earthquake recorded simultaneously at distant monitoring stations in India

Continuous He, 222Rn concentrations and gamma dose rate were simultaneously monitored for pre-seismic signatures at two thermal springs in India that are separated at a distance of approximately 1612 km. The recordings of six month monitoring period (September 15, 2007–March 15, 2008) are presented here. In this paper, we make a cross correlation study of the simultaneously recorded multi-component gas-geochemical data from the two thermal springs. In the monitoring period a number of prominent fluctuations beyond the regular diurnal variations in the gas composition were recorded. We attempt to find out the linkage between the recorded distinct variations with local and remote seismic activities. Anomalous fluctuations in the spring gases were observed during the period December 24–27, 2007 at both the thermal springs and these anomalies may be correlated to the distant (~1000 km) China earthquake of magnitude M = 6.3 that occurred on January 09, 2008. Based on the obtained sequence of data points a time series analysis to relate earthquake magnitude, epicentral distance and precursor time through statistical methods and empirical equations related to the zone of influence is made. Concurrent monitoring of multi-component gas-geochemical anomalies recorded at a number of distant sites appears to be a potential tool to deal with the commonly debated question of earthquake precursors.

Recent tectonomagmatic reactivation of the Kolyuchino-Mechigmen zone of the Chukchi Peninsula from data on the composition of gases in hydrothermal springs

Thirty-three groups of thermal mineral springs in the Chukchi Peninsula are studied. The thermal fluids of the Kolyuchino-Mechigmen Zone (KMZ) show specific characteristics. The zone corresponds to recent depression with occurrences of basic volcanism and high seismicity. Characteristic features of the gases in the springs of the KMZ include the predominance of carbon dioxide, high percentage of mantle-derived helium, enrichment of CO2 in 13C, admixture of nonatmospheric nitrogen, excess in 40Ar and 15N relative to the atmosphere, maximal depletion of thermal water in deuterium and heavy oxygen isotope, and the highest estimates of deep temperature in reservoirs of hydrothermal fluid. All these features indicate the contemporary ascent of a mantle diapir that supplied helium enriched in 3He and thermal energy to the crust, thus provoking thermal metamorphism of rocks and degradation of permafrost.

Isotopically light carbon dioxide in nitrogen rich gases: the gas distribution pattern in the French Massif Central, the Eifel and the western Eger Rift

Based on characteristics of the distribution pattern of the western Eger Rift spring gases, a distribution pattern is presented for the gases of the French Massif Central. The central parts of these areas with ascending magmatic CO2 are characterised by high gas fluxes, high CO2 contents of up to 99.99 vol% and isotopially heavy CO2. In the peripheries, the decrease of δ 13 C values of CO2 and CO2 contents in the gas phase is compensated by a rise in N2 contents. It can be demonstrated that gas fractionation in contrary to mixtures with isotopically light biogenic or crustal CO2 controls the distribution pattern of gas composition and isotopic composition of CO2 in these spring gases. Dissolution of CO2 results in formation of HCO3 ‐ causing isotope fractionation of CO2 and an enrichment of N2 in the gas phase. With multiple equilibrations, values of about ‐17 ‰ or lower are obtained. The scale of gas alteration depends on the gas flux and the gas-water ratios respectively and can result in N2-rich gases. Essential for the interpretation are gas flux measurements with mass balances derived for most of the springs. Without such mass balances it is not possible to discriminate between mixture and fractionation. The processes of isotopic and chemical solubility fractionations evidently control the gas distribution pattern in other regions as well.

Geochemical evidence of gas sources of CO2-rich cold springs from Wudalianchi, Northeast China

CO2-rich cold springs occur near the active volcanoes at Wudalianchi (五大连池), Northeast China. The springs are rich in CO2, with HCO3− as the predominant anion and have elevated contents of total dissolved solid (TDS) (>1 000 mg/L), Fe2+ (>20 mg/L), Sr (>1 mg/L), and dissolved Si (>20 mg/L). The compositions of escaped and dissolved gases of the springs are similar. The δ13C values of escaped gases and dissolved gases in mineral springs at Wudalianchi vary from −8.77‰ to −4.53‰ and −8.24‰ to −5.26‰, while δ18O values vary from −10.68‰ to −7.65‰ and −10.30‰ to −8.84‰, respectively, indicating the same upper mantle origin of CO2 of escaped gases and dissolved gases in the springs. Carbon and oxygen isotope fractionations and water-CO2 exchange were weak in the process of groundwater flow. The 4He content exceeds 5 000×10−6 cm3·STP/mL in escaped gases of the mineral springs, and the 3He/4He ratios of the escaped and dissolved gases vary from 2.64Ra to 3.87Ra and 1.18Ra to 3.30Ra, respectively. It can be postulated that the CO2 of mineral springs deriving from the magma chamber of the upper mantle moves upward to the surface, to increase the content of 4He in the mineral springs and decrease the ratio of 3He/4He. The helium origin of escaped gases in springs can be calculated with the MORB-crust mixing model, but that in the north spring can be better explained with the MORB-crust-air mixing model due to the effect of mixing with surface water. However, dissolved helium in springs, except the north spring, is better explained with the MORB-crust-ASW mixing model.

High 3He emanations from the source regions of recent large earthquakes, central Japan

Recently, two shallow MW 6.6 inland earthquakes, the 2004 mid‐Niigata prefecture earthquake and the 2007 Niigataken Chuetsu‐oki earthquake, occurred in the Niigata sedimentary basin near the boundary between the Eurasian and the North American Plates. In order to elucidate the geographic distribution of the 3He/4He ratios in and around the seismic source regions, new helium isotope data from hot spring gases and water samples from much denser sampling than in previous studies were determined. Although temporal changes in the 3He/4He ratios obtained from each well were unknown before and after the earthquakes, higher 3He/4He ratios were observed in the earthquake source regions compared to other regions. Geochemical evidence for the presence of hot spring gases and natural gases with anomalously high 3He/4He ratios (>3.8 RA) indicates infiltration of mantle fluids derived from the subcrustal lithosphere. A plausible source of mantle helium can be attributed to upward mobilization of aqueous fluids generated by dehydration of the subducting Pacific Plate slab, resulting in the acquisition of primordial helium from a mantle wedge as the fluids into the crust. In addition, aqueous fluids may raise the Earth's surface along permeable conduits leading to the reactivation of high‐angle inverted normal faults and the triggering of recent earthquakes.

On the change of 3He/4He ratios in hot spring gases after The Iwate-Miyagi Nairiku Earthquake in 2008

On the change of 3He/4He ratios in hot spring gases after The Iwate-Miyagi Nairiku Earthquake in 2008

Helium isotopes as geochemical indicators of a serpentinized fore‐arc mantle wedge

New helium isotope data from hot springs on eastern Kyushu Island were obtained in order to elucidate the geographic distribution of 3He/4He ratios in the fore‐arc regions of southwest Japan. Systematic, regional variations in 3He/4He ratios are clearly distinguished across the fore arc from Kyushu Island (0.093–0.79 RA, where RA denotes the atmospheric 3He/4He ratio of 1.4 × 10−6), to Shikoku Island (0.17–3.6 RA) and to the Kii Peninsula (0.24–7.4 RA). The distribution of mantle helium relative to total helium reveals a close association with the geophysical evidence of a serpentinized fore‐arc mantle wedge including the occurrence of deep, nonvolcanic, low‐frequency tremors (LFTs), believed to be caused by upwardly mobile aqueous fluids derived from slab dehydration. Flux of mantle helium carried by aqueous fluids via a non serpentinized fore‐arc mantle wedge is very low on Kyushu Island where LFTs have not been detected and 3He/4He ratios are lower than the atmospheric ratio. On the other hand, hot springs above the regions where LFTs occur in the Kii Peninsula and northern Shikoku Island have high 3He/4He ratios, indicating a significant contribution of mantle helium carried by aqueous fluids via a serpentinized fore‐arc mantle wedge. The helium isotopes in hot and natural spring gases may be a useful geochemical indicator of a serpentinized fore‐arc mantle wedge, especially in areas with sparse seismic station coverage.

Thermochronology of non-volcanic hydrothermal activity in the Kii Peninsula, Southwest Japan: Evidence from fission track dating and helium isotopes in paleo-hydrothermal fluids

Although there is no indication of volcanism during the Pliocene and Quaternary in the Kii Peninsula, Southwest Japan, the peninsula has long been recognized to be unusual and atypical of non-volcanic regions, with anomalous high-temperature hot springs and high 3He content in hot spring gases. Recently, it has been suggested that aqueous fluids generated by dehydration of the subducting Philippine Sea plate carry mantle helium from the subcrustal lithosphere to terrestrial water, resulting in the emanation of high 3He gases at high-temperature hot springs [e.g., Matsumoto T., Kawabata T., Matsuda J., Yamamoto K. and Mimura K. 2003. 3He/ 4He ratios in well gases in the Kinki district, SW Japan surface appearance of slab-derived fluids in a non-volcanic area in Kii Peninsula. Earth Planet. Sci. Lett. 216, 221–230]. Fission track dating of zircon and apatite samples from the altered host rocks around three hot springs (Yunomine Hot Spring, Kamiyu Hot Spring and Tosenji Hot spring) was carried out in order to elucidate the thermochronology of the hydrothermal system in this non-volcanic region. Additionally, new helium isotope data from paleo-fluids trapped in fluid inclusions in hydrothermal minerals were obtained for comparison with the present-day hydrothermal fluids from the above hot springs. The apatite FT ages from the altered host rocks around the hot springs ranged from 2.5 Ma to about 5 Ma, which are significantly younger than the 12 Ma age of the non-altered host rocks far from the hot springs. In the southern Kii Peninsula, therefore, it is obvious that the non-volcanic hydrothermal activity already has been ongoing since Pliocene or early Pleistocene time. The 3He/ 4He ratios of quartz veins and pyrite disseminated in the altered host rock around the hot springs ranged from 1.1 to 3.6 R A , generally in agreement with those of the present-day hydrothermal fluids. Almost constant values of 3He/ 4He ratios over time indicate that no important changes have occurred in deep source gases, at least during the last several million years.

Multiple lines of evidence for crustal magma storage beneath the Mesozoic crystalline Iide Mountains, northeast Japan

It has been recognized that the Iide Mountains, consisting of Mesozoic sedimentary rocks and Late Cretaceous to Paleogene granitic rocks, northeast Japan, are unique. Although they occur in a nonvolcanic region, hot springs in the Iide Mountains have anomalously high heat discharge values similar to those from hot springs in volcanic regions. In order to provide geochemical constraints on the heat source for the hydrothermal activity, the chemical and isotopic compositions of 10 gas and water samples were determined. The 3He/4He ratios determined range from 0.22 to 7.9 Ra, and the highest ratio is similar to MORB‐type helium, indicating a significant contribution of primordial mantle helium. Considering the relationship between the magnitude of the 3He/4He ratios and the distance of the sample locations from the Kitamata‐dake, the peak in the Iide Mountains where the largest ratio occurs, it is apparent that there is a sharp decrease in 3He/4He ratios laterally away from the peak. Furthermore, the peak is also the location where geophysical anomalies such as an anomalous conductive body and zones with low Vp and low Vs have been detected in the middle‐lower crust. These high helium ratios are considered to indicate the likelihood of mantle‐derived materials supplying MORB‐type helium beneath the Kitamata‐dake and the possibility that mantle‐derived helium has been diluted by atmospheric and/or crustal components with lower helium ratios away from the peak. In order to examine whether or not ancient magma of Middle Miocene age is a possible source of the high 3He/4He ratios of the hot spring gases, we calculated the evolution in 3He/4He ratios of the ancient magma with time. As a result, it is concluded that the anomaly beneath the Iide Mountains is due to newly ascending magmas in the present‐day subduction system rather than hydrothermal fluids related to late remnant magmatism of Middle Miocene age.

Continuous monitoring of 222Rn and its progeny at a remote station for seismic hazard surveillance

An experimental station to monitor variations in radon and radon progeny concentrations in the gases from the thermal springs at Bakreswar, West Bengal, India has been installed. This is essentially a part of a continuous, automated, multi-parametric geochemical monitoring system (GMS) installed with a view to detect probable earthquake-related precursory changes in the spring gas emanations. The entire data acquisition system is computer driven and fully automated. Instrument readings acquired over a 24-h interval are transmitted through a tele-link facility from the field laboratory located at the thermal spring site situated about 250 km away, once a day, to a workstation at our Centre in Kolkata for analysis and storage. After the system was installed during mid-December, 2004, concurrent anomalous changes in concentration profiles of 222Rn, 218Po and 214Bi occurred beyond 2 σ level four times during the period of present observation duration up to February 04, 2005. The monitored anomalies may be attributed to precursory dilatation responses arising due to earthquakes that had occurred subsequently in Indonesia and the Andaman and Nicobar Islands. The observed variations reveal evidence of pre-seismic radioactive heterogeneities in escaping subsurface fluids from the monitored thermal spring gases.

Geographical Distribution of 3He/4He Ratios in the Chugoku District, Southwestern Japan

We have collected 34 hot spring and mineral spring gases and waters in the Chugoku and Kansai districts, Southwestern Japan and measured the 3He/4He and 4He/20Ne ratios by using a noble gas mass spectrometer. Observed 3He/4He and 4He/20Ne ratios range from 0.054 Ratm to 5.04 Ratm (where Ratm is the atmospheric 3He/4He ratio of 1.39 × 10−6) and from 0.25 to 36.8, respectively. They are well explained by a mixing of three components, mantle-derived, radiogenic, and atmospheric helium dissolved in water. The 3He/4He ratios corrected for air contamination are low in the frontal arc and high in the volcanic arc regions, which are consistent with data of subduction zones in the literature. The geographical contrast may provide a constraint on the position of the volcanic front in the Chugoku district where it was not well defined by previous works. Taking into account the magma aging effect, we cannot explain the high 3He/4He ratios of the volcanic arc region by the slab melting of the subducting Philippine Sea plate. The other source with pristine mantle material may be required. More precisely, the highest and average 3He/4He ratios of 5.88 Ratm and 3.8±1.6 Ratm, respectively, in the narrow regions near the volcanic front of the Chugoku district are lower than those in Kyushu and Kinki Spot in Southwestern Japan, but close to those in NE Japan. This suggests that the magma source of the former may be related to the subduction of the Pacific plate, in addition to a slight component of melting of the Philippine Sea slab.

Helium and carbon isotopic compositions of thermal springs in the earthquake zone of Sichuan, Southwestern China

The origins of gases from hot springs and earthquake activity in the western part of Sichuan Province, Southwestern China, are discussed in terms of helium and carbon isotopes and energy released by earthquakes. δ 13C values of CO 2 in free and dissolved gases range from −3.34 to −17.09‰, and 3He/ 4He ratios vary from 1.5×10 −8 to 3.63×10 −6. The isotopic compositions indicate that carbon dioxide from the hot springs is a mixture of crustal and mantle CO 2, and helium is derived from the crust, mantle and atmosphere. The thermal springs in different earthquake and/or fault zones, as well as in different parts of the same zone, have different values of δ 13C and 3He/ 4He. The more tectonically active the district is, the larger the δ 13C and 3He/ 4He values and the greater the frequency of earthquakes. This means that more heat energy is derived from the deep earth with upward migration of anatectic fluids in the active-tectonic district, such as in the Kangding district. Therefore, hot spring gases (He, CO 2 etc.) may be important geochemical markers for determination of seismological and tectonic activity.

Dissected hydrologic system at the Grand Canyon: Interaction between deeply derived fluids and plateau aquifer waters in modern springs and travertine

Geochemical study of water and gas discharging from the deeply incised aquifer system at the Grand Canyon, Arizona, provides a paradigm for understanding complex groundwater mixing phenomena, and Quaternary travertines deposited from cool springs provide a paleohydrologic record of this mixing. Geochemical data show that springs have marked compositional variability: those associated with active travertine accumulations (deeply derived endogenic waters) are more saline, richer in CO2, and elevated in 87Sr/86Sr relative to springs derived dominantly from surface recharge of plateau aquifers (epigenic waters). Endogenic waters and associated travertine are preferentially located along basement-penetrating faults. We propose a model whereby deeply derived fluids are conveyed upward via both magmatism and seismicity. Our model is supported by: (1) gas analyses from spring waters with high He/Ar and He/N2 and 3He/4He ratios indicating the presence of mantle-derived He; (2) large volumes of travertine and CO2-rich gases in springs recording high CO2 fluxes; and (3) 87Sr/86Sr in these springs that indicate circulation of waters through Precambrian basement. Geochemical trends are explained by mixing of epigenic waters of the Colorado Plateau aquifers with different endogenic end-member waters in different tectonic subprovinces. Endogenic waters are volumetrically minor but have significant effects on water chemistry. They are an important and largely unrecognized component of the hydrogeochemistry and neotectonics of the southwestern United States.

Abiotic Synthesis of Organic Compounds from Carbon Disulfide Under Hydrothermal Conditions

Abiotic formation of organic compounds under hydrothermal conditions is of interest to bio, geo-, and cosmochemists. Oceanic sulfur-rich hydrothermal systems have been proposed as settings for the abiotic synthesis of organic compounds. Carbon disulfide is a common component of magmatic and hot spring gases, and is present in marine and terrestrial hydrothermal systems. Thus, its reactivity should be considered as another carbon source in addition to carbon dioxide in reductive aqueous thermosynthesis. We have examined the formation of organic compounds in aqueous solutions of carbon disulfide and oxalic acid at 175 degrees C for 5 and 72 h. The synthesis products from carbon disulfide in acidic aqueous solutions yielded a series of organic sulfur compounds. The major compounds after 5 h of reaction included dimethyl polysulfides (54.5%), methyl perthioacetate (27.6%), dimethyl trithiocarbonate (6.8%), trithianes (2.7%), hexathiepane (1.4%), trithiolanes (0.8%), and trithiacycloheptanes (0.3%). The main compounds after 72 h of reaction consisted of trithiacycloheptanes (39.4%), pentathiepane (11.6%), tetrathiocyclooctanes (11.5%), trithiolanes (10.6%), tetrathianes (4.4%), trithianes (1.2%), dimethyl trisulfide (1.1%), and numerous minor compounds. It is concluded that the abiotic formation of aliphatic straight-chain and cyclic polysulfides is possible under hydrothermal conditions and warrants further studies.

Aqueous fluids derived from a subducting slab: Observed high 3He emanation and conductive anomaly in a non-volcanic region, Kii Peninsula southwest Japan

It has long been recognized that the Kii Peninsula in the southwest Japan arc is peculiar in a non-volcanic region, indicated by the presence of high temperature hot springs, high terrestrial heat flow and high 3He content in hot spring gases. Geophysical and geochemical studies were carried out to understand the geotectonic environment in the southern part of the Kii Peninsula. Most of the measured 3He / 4He ratios are similar or higher than air, indicating wide spread incorporation of mantle-derived helium into meteoric water. A region with rather high 3He / 4He ratios (> 4 R A) on the west side of the Omine Mountains coincides with the occurrence of high temperature hot springs. A deep crustal resistivity structure across the Omine Mountains was imaged by wide-band magnetotelluric soundings. A 2-D inversion with N–S strike using both TM and TE modes reveals two conductors, one in the upper (3–7 km depth) and the other in lower crust (25–35 km depth) to the west of the Omine Mountains. The distribution of microearthquakes and low-frequency tremors, and the existence of seismic reflectors indicate that the large conductor in the lower crust is related to aqueous fluids derived from the Philippine Sea plate. The upper-crustal conductive zone may also reflect the aqueous fluids trapped in the upper crust, which are presumably derived from the subducting slab. Considering the occurrence of seismic events in the subducting slab beneath the southern Kii Peninsula, the aqueous fluids generated by dehydration of the slab mantle could plausibly include MORB-type helium derived from the residual lithospheric mantle. Therefore, the high temperature hot springs and high 3He emanations in hot spring gases and other geotectonic events in the southern Kii Peninsula may be due to heat flux and mantle-derived helium discharged from aqueous fluid in the upper crust.

Spatial distribution and temporal variation of 3He/4He in hot spring gas released from Unzen volcanic area, Japan

Following the first phreatic explosion on 17 November 1990, hot spring gases were collected periodically over the next 10years for 3He/4He isotopic ratio and chemical analyses from three hot springs (Obanma, Unzen and Shimabara) located around Unzen volcano, Japan. The 3He/4He ratios, although showing some scatter at each site, show an increase from west to east (Obama<Unzen<Shimabara). This trend corresponds to a decrease in crustal gas contribution as shown by the CH4 content. The 3He/4He ratios at Shimabara and Unzen after 1990 were significantly higher than the single values for the 1983 sample (Marty et al., 1989), possibly due to an additional supply of magmatic helium related to the recent post-1990 volcanic activity. The 3He/4He ratio at Shimabara hot spring increased slightly after November 1990, reaching a maximum value in July 1992, and decreasing later. This suggests that magmatic helium with relatively high 3He/4He ratios took about one year to travel 5km from beneath Fugendake volcanic cone to Shimabara hot spring site, because the magma effusion rate (and magma degassing rate) reached a maximum in June 1991.

Noble gas isotopic compositions and water/gas chemistry of soda springs from the islands of Bioko, São Tomé and Annobon, along with Cameroon Volcanic Line, West Africa

Chemical and isotopic compositions are reported for water, and CO 2 and noble gases in groundwater and soda springs from Bioko, Principé, São Tomé and Annobon, all islands located in the off-shore part of the Cameroon Volcanic Line in West Africa. The soda spring waters are of Ca–Mg–HCO 3 type, with δD and δ 18O values that range from −20 to −8‰ and −5.4 to −2.7‰ respectively, indicative of a meteoric origin. CO 2 is the main gas species in the springs. δ 13C–CO 2 values vary from −2.8 to −5.0‰, overlapping the observed mantle C range (−3 to −8‰). CO 2/ 3He ratios (3–9×10 9) suggest that most C (∼90%) in the samples is derived from the mantle. Neon has atmospheric isotopic compositions, while Ar is slightly enriched in radiogenic 40Ar. 3He/ 4He ratios (3.0 to 10.1×10 −6 or 2.1 to 7.2 R a , where R a is the atmospheric ratio of 1.4×10 −6) are much higher than those for typical crustal fluids (∼10 −8) but lower than those expected for fluids derived from ‘high- 3He/ 4He’ hotspots like Loihi and Iceland. This precludes significant contributions of such fluids in the source regions of the gases, and by inference, in the magmatism of these oceanic islands. Alternatively, approximately 90% of the He in São Tomé gases is inferred to be derived from a source similar to the MORB source. The 3He/ 4He ratio for the Bioko gas (6.6×10 −6) may be derived from a source with a higher time integrated (U+Th)/ 3He ratio than the MORB source.

Geochemical Characteristics of Gases from the Wudalianchi Volcanic Area, Northeastern China

Abstract The origins of gases in springs, pools and wells from the Wudalianchi (WDLC) volcanic area are discussed based upon molecular and isotope compositions of the gases. Nine gas and water samples were collected from bubbles and water of the springs and pools in the WDLC volcanic area, Northeastern China, in August 1997. The molecular components were measured with a MAT‐271 mass spectrometer (MS), helium isotope ratios with a VG‐5400 MS, and δ13C with a MAT‐251 MS in the Lanzhou Institute of Geology. The gases are enriched in CO2, and most of the CO2 concentrations are over 80% (V). The helium and methane concentrations have relatively wide ranges of 0.7 to 380×10−6 and 4 to 180×10−6, respectively. The 3He/4He ratios are between 1.05 Ra and 3.1 Ra (Ra = 1.4×10−6); the 4He/20Ne values are between 0.45 and 1011, larger than the atmospheric value (0.32). The δ13C (PDB) values of carbon dioxide range from −9.6 to −4.2%%. These geochemical data demonstrate that the spring water is from aquifers at different depths, and that helium and carbon dioxide are derived from the mantle, and are contaminated by crust gases during deep fluid migration. Also, there are larger fluxes of deep‐earth matter and energy in the WDLC volcanic area.

Gas geochemistry and seismotectonics: a review

Publications on soil and spring gases have been examined regarding their relationships with both tectonic and seismic activities. The main sources, behaviours and uses of species detected in soils and springs are displayed, and their mode of sampling and analysing briefly described. The main patterns of degassing in soils are described and we outline the wide range of geochemical signatures as the result of both permeability and mineralogical contrasts. Because thermomineral waters have been in contact with great volumes of crustal rocks at various depths, spring gases might be more representative of the local environment than soil gases. Moreover, gas signature comparisons show that spring gases are much more enriched with deep gases and slightly contaminated by atmospheric gases. Therefore, they can be considered as better samples for identifying precursors of earthquakes. Environmental perturbations are examined, and it is shown from divergent cases that pressure, temperature, soil moisture or earth tides may generate very high perturbations of the degassing process. Such effects demonstrate that no systematic correction law can be proposed and that removing external contributions from gas concentrations must be performed case by case. This demonstrates therefore the need for the simultaneous measurement of external parameters during gas monitoring. A qualitative examination of about 150 claimed precursors proposed in the literature has been reviewed. As noted by previous authors, anomalies appear at distances sometimes much greater than typical source dimensions, and occur in the field of strain higher than 10 −9, most of them being in the field of strain higher than 10 −8. Taking into account the very high heterogeneity of such a set of data, we can suggest that amplitudes of gas anomalies are independent of both magnitudes and epicentral distances of related earthquakes, suggesting local conditions to control amplitudes. On the contrary, precursory time and duration of anomalies seem to increase both with magnitudes and epicentral distances. Abundant evidence demonstrates the major role of crustal fluids in the earthquake cycle. Many works have outlined the fact that crustal instabilities can appear as the result of low stress/strain perturbations during loading. It has been suggested that motion of fluids may occur at various scales, from microcrack fluid transfer up to changes of hydraulic levels of water tables. The study of subsequent anomalies is expected to supply a tool for earthquake prediction. Following previous authors, we outline the need for further methodological improvements, including the setting up of multiparameter station networks and the simultaneous recording of the main external parameters (atmospheric pressure, water and air temperature, soil moisture) for signal processing.