δ13C Values Of CO2 Research Articles

Helium and carbon isotope compositions of thermal fluids in the northeastern Anatolia: Implications for the heat source and volatile flux

In this study, we make a quantitative assessment on the volatile flux of mantle-derived fluids and source of volatiles to explain the geologic controls on the transport of volatiles within thermal fluids of northeastern Anatolia. In line with this objective, we collected 22 samples (gas and water phase) from 16 geothermal fields in NE Turkey covering an area of nearly 100,000 km2 that extends from the eastern termination of North Anatolian Fault towards the Georgian and Armenian borders. The 3He/4He ratios of the samples (R) normalized to the atmospheric 3He/4He ratio (Ra = 1.4 × 10−6) vary from 0.31 to 7.15, and are considerably higher than the crustal value of 0.02Ra. Regarding spatial distribution of helium isotope composition, samples collected from areas of tectonic unrest around the Erzincan and Erzurum pull-apart basins in the southern part are represented by a higher range of 3He/4He ratios (>5 Ra) than those in volcanic areas to the east and northeast of the region. δ13CCO2 values of the gas samples varying from −20.76 to 5.43 ‰ are about 4–5 ‰ lower than δ13CDIC values of the water samples that range from −16.90 to 8.85 ‰, indicating CO2 removal from waters by degassing. CO2/3He ratios of gas samples falling in the range of 3.81 × 109 to 2.83 × 1012 imply that carbon is derived from the mixing between the crustal lithologies and mantle, the latter having a contribution up to 40 %. The maximum flux of mantle-derived fluids is found 88 mm/a at Erzincan (eastern part of the North Anatolian Fault Zone). This value is higher by a factor of about 10 than the western part of the fault zone. Calculations show that degassing from magmatic bodies is the sole mechanism to explain the high helium isotope compositions in the region. Therefore, we suggest that mantle-derived He contents might be due to extensive melting associated with active tectonism. The 3He/Heat ratios of thermal waters in northeastern Turkey were calculated in range of 0.25 to 29.7 × 10−15 cm3(STP)/J, consistent with the data reported for waters along the North Anatolian Fault, indicating a crustal heat source for the geothermal systems in northeastern Turkey.

Variations and driving factors for concentrations and carbon isotopes of dissolved CO2 in lake water across different Chinese lakes

Carbon dioxide (CO2) stands as the primary driver of Earth's greenhouse effect, and it's suggested that the global contribution of CO2 emissions from lakes cannot be ignored. Despite the numerous estimations of CO2 fluxes from lakes, limited focus has been directed towards the carbon isotopes (δ13C) of dissolved CO2 in lake water. Particularly, the potential use of δ13C values in tracing the CO2 concentrations in lake water remains as an understudied area, warranting further exploration and investigation. In this study, we conducted an analysis of the concentrations and δ13C values of dissolved CO2 in 33 lakes located at the Tibetan Plateau, Chinese Loess Plateau, and Yangtze Plain (among which high-resolution spatial investigations were performed in 6 lakes through in-situ continuous monitoring). Our findings revealed spatial variations in both the CO2 concentrations and δ13C values in lakes. Additionally, notable differences are observed among lakes in different regions of China, with lakes in the Yangtze Plain exhibiting considerably higher CO2 concentrations, and the overall CO2 δ13C values in lakes on the Tibetan Plateau tend to be more positive, while those in lakes on the Chinese Loess Plateau tend to be more negative. The pH values, dissolved oxygen, and dissolved organic carbon are likely crucial factors influencing the CO2 concentrations and δ13C values in the lakes. Furthermore, lake water CO2 concentrations are negatively correlated with δ13C values of CO2 and dissolved inorganic carbon (DIC) both within a single lake with high spatial resolutions or in lake groups across different regions. These results highlight that the CO2/DIC δ13C values can be applied to trace the concentration variations of dissolved CO2 in lakes.

Groundwater and Dissolved Gases Geochemistry in the Pesaro-Urbino Province (Northern Marche, Central Italy) as a Tool for Seismic Surveillance and Sustainability

The Pesaro-Urbino province (PUP) (northern Marche, central Italy) is one of the most seismically active areas in Italy, with the most recent earthquakes (5.2 and 5.5 Mw) having occurred on 9 November 2022 with an epicenter located in the Adriatic Sea. A detailed geochemical and isotopic characterization of 87 groundwaters (and dissolved gases) circulating in the PUP was carried out to (i) unravel the geochemical processes controlling the water circulation, (ii) investigate the interplay between deep originated fluids and shallow aquifers, (iii) evaluate the reliability of specific geochemical parameters as tracers for seismic activity, and (iv) select the most suitable sampling sites to deploy a monitoring network to highlight possible compositional changes related to the regional and local tectonic activity. The geochemical dataset includes waters showing five different hydrochemical compositional facies: (i) calcium bicarbonate with low Total Dissolved Solids (TDS); (ii) calcium bicarbonate with relatively high concentrations of sulfate (>200 mg/L); (iii) sodium bicarbonate with pH > 8.8; (iv) calcium sulfate; (v) sodium chlorine. Two distinct groups of dissolved gases can be recognized: (a) N2-dominated gases with N2/Ar ratios similar to those of Air-Saturated Water (ASW); (b) CO2- and CH4-rich gases associated with high TDS and springs rich in S-bearing reduced species. The isotopic values of δ13C-CO2 and δ13C-CH4 suggest a predominant biogenic origin for both species with a negligible contribution from deep-seated fluids. The Ca-HCO3(SO4), Ca(Na)-SO4(Cl), and Na-HCO3 waters, being likely related to deep hydrological pathways, are the best candidates to be included in the monitoring network in the Pesaro-Urbino province. This will be of paramount importance in addressing the challenge of unravelling fluid geochemical precursors of earthquakes, thus increasing and improving seismic surveillance practices and hazard mitigation.

Can we see differences between the 14C activities of urban (Zagreb) and rural (Cvetković) sites (central Croatia)?

Radiocarbon (Δ14C) was measured for four years (2019–2022) in Zagreb (Croatia) and in Cvetković village near Jastrebarsko (Zagreb County, Croatia) to see whether there are differences between the city site and the rural one because of the fossil fuel combustion. The δ13CCO2 was measured as grab samples once in a month in period December 2020–November 2022. The bomb-produced 14C has been globally distributed across the planet, but the combustion of fossil fuels that do not contain 14C causes a local effect of lowering Δ14C. Zagreb is considered to be a location with heavy fossil fuel combustion as compared to the Cvetković (rural site). Monthly 14C activity at Zagreb is constantly below the 14C activity at Cvetković. Mean 14C activity at Zagreb (Δ14CZagreb = −18.4 ± 2.6 ‰) is lower than that in Cvetković (Δ14CCve = −2.9 ± 2.1 ‰) due to fossil fuel combustion in the city of Zagreb. This is especially pronounced during winter when the mean value in Zagreb is Δ14CZagreb = −26.0 ± 4.3 ‰ and in Cvetković Δ14CCve = −5.9 ± 3.4 ‰.Natural gas consumption was used as the proxy for fossil fuel combustion, and it shows better correlation with Δ14C in Zagreb than in Cvetković. The Δ14C difference, Δ14CCve ‒ Δ14CZagreb, becomes statistically negligible when natural gas consumption is small.No difference is observed on δ13CCO2; in Zagreb mean δ13CCO2 is −11.0 ± 1.3 ‰, and in Cvetković −11.4 ± 1.4 ‰. Lower δ13CCO2 values are observed in winter (Zagreb −11.9 ± 1.1 ‰, −12.2 ± 1.5 ‰ Cvetković) than in summer (−10.1 ± 0.8 ‰ vs. −10.4 ± 1.0 ‰) at both locations. Together with higher Δ14C in Cvetković, it indicates that at the area of Cvetković biogenic samples of modern origin (biomass, wood) as energy source for heating is more pronounced.

Geochemistry and accumulation of the ultra-deep Ordovician oils in the Shunbei oilfield, Tarim Basin: Coupling of reservoir secondary processes and filling events

Ultra-deep (7300 m–8900 m) condensed and volatile oil reservoirs have been discovered in the Ordovician reservoirs of the Shunbei oilfield, in the Tarim Basin. This study describes the petroleum accumulation process in the Shunbei oilfield by combining an analysis of its geochemical characteristics with seismic section observations and an analysis of petroleum filling events. Geochemical techniques are used to determine the sources, maturity, secondary processes, accumulation periods, and timing of petroleum filling. Analysis of fluid inclusion homogenization temperatures and burial-thermal histories indicates distinct petroleum filling events. Specifically, the north sub-section of the No. 5 fault zone experienced two petroleum filling periods during the late Caledonian and Indosinian epochs while the No. 1 fault zones and the mid-south sub-sections of the No. 5 fault zones experienced three distinct filling periods, encompassing the late Caledonian, Indosinian, and Himalayan periods. The CO2 concentrations and δ13CCO2 values suggest the presence of secondary microbial gases, primarily thermogenic gases. Gas component analysis reveals a transition from kerogen degradation gas to oil cracking gas from north to south. The detection of intact n-alkanes and compounds of the 25-norhopanes series in oil samples suggests a mixture of early biodegradable oil with late fresh oil in the area. Seismic sections reveal diabase intrusions with strong axial reflections. Local hydrothermal activities during the Permian era rapidly cracked saturated hydrocarbon biomarkers, forming polycyclic aromatic hydrocarbons in the oil. Hydrothermal activity in the south sub-section of the No. 5 fault zone depleted the hydrocarbon-generating capacity of the underlying source rocks. Petroleum charged during the Indosinian period likely originated from potential petroleum reservoirs in the Cambrian system. This explains the comparatively low temperature of the SHB57X reservoir (163.32 °C) and the lack of strong hydrothermal activity in the late stage, but no biomarkers of the second-stage charged oil have been detected. Biomarker detection and gas genesis in the mid-south sub-section of the No. 5 fault zone and in the No. 1 fault zone suggest weak or no hydrothermal activity, with Cambrian source rocks maintaining their hydrocarbon generation capacity. Gas filling in the Himalayan period caused evaporative fractionation in the original reservoirs, resulting in a disproportionate (2-MH+2,3-DMP)/(3-MH+2,4-DMP), producing high K1 values in the light hydrocarbons in the present petroleum reservoirs. Differences in gas charging events in the Himalayan period led to the present distribution of petroleum phases.

Isotopic composition of carbon (δ13C) and nitrogen (δ15N) of petrologically different Tertiary lignites and coals

This study investigates the carbon (δ13Corg) and nitrogen (δ15N) isotopic composition of tertiary lignites and coals from six sedimentary basins: Velenje, Mura-Zala, and Zasavje in Slovenia; Sokolov in Czech Republic, Barito in Indonesia; and Istria in Croatia. The aim is to investigate the correlation between the fine detrital (fD) component and δ13C and δ15N in Velenje lignite samples. Additionally, we aim to evaluate the biogeochemical processes of organic substances during their deposition in all analyzed samples, calculate their δ13CCO2 values and compare the analyzed values of δ13C and δ15N to those reported in the literature. Thirty-two samples were analyzed, predominantly from the Velenje ortho-lignite (Pliocene), with additional lignites and coals from the Pannonian to Paleocene epochs for comparison. Carbon isotopic composition (δ13Corg) ranged from -27.9 to -23.6 ‰, and nitrogen isotopic composition (δ15N) ranged from 1.8 to 7.4 ‰. The fine-detrital lithotypes of the Velenje ortho-lignite exhibited the most negative δ13Corg values due to anaerobic bacterial activity in an intramontane alkaline lake environment influenced by the carbonate hinterland. Moreover, gelification processes affected fine-detrital organic matter more than larger wooden pieces. Terbegovci, Hrastnik meta-lignites, and Barito sub-bituminous coal also displayed low δ13Corg values, indicating limited gelification, while variations in the δ15N values suggested differences in mineralization. The Velenje xylitic lithotypes have higher δ15N values, indicating a more intense mineralization under aerobic conditions. Raša ortho-bituminous coal, deposited in a brackish environment, displayed the highest δ13Corg values and a wide range of δ15N values due to fluctuating water tables in a paralic carbonate platform environment. The lowest δ15N value was observed in the Sokolov Basin lignite coal, indicating minimal mineralization and low bacterial activity. The isotopic composition of CO2 in air (δ13Cair), which was calculated using the δ13C values in lignites and coal, ranged from -8.4 to -3.4 ‰, with Velenje lignite displaying the minimum value and Raša coal showing the maximum value. The determined δ13C and δ15N values of the coal and lignite samples in this research fall within the typical range of world coals.

Geochemistry of Geothermal Fluids in the Three Rivers Lateral Collision Zone in Northwest Yunnan, China: Relevance for Tectonic Structure and Seismic Activity

The Three Rivers Lateral Collision Zone (TRLCZ), situated at the southeastern margin of the Tibetan Plateau, is a crucial frontier where materials from the plateau flow southeastward. This study extensively investigated the hydrochemical characteristics and origin of helium and carbon isotopes in 73 thermal springs within the TRLCZ. The analysis revealed dominant processes, including carbonate and silicate interactions, resulting in elevated concentrations of HCO3− and Na+. The impact of Ca/Mg-rich minerals, particularly dolomite, influenced the cation composition. Additionally, gypsum dissolution, notably in the Lancangjiang Fault and Weixi–Qiaohou Fault, was highlighted through Ca/SO4 ratios. The positive correlation between SO42− and Cl− indicated dilution by shallow cold water, explaining the lower SO42− content in the Jingshajiang–Zhongdian Fault and Nujiang Fault compared to the Weixi–Qiaohou Fault and Lancangjiang Fault. The circulation depth of thermal spring water varied, with the northern Weixi–Qiaohou Fault exhibiting the shallowest circulation depth (~3 km), while the Jingshajiang–Zhongdian Fault and southern segments of the Nujiang Fault displayed deeper depths—ranging from 4 to 7 km. A positive correlation between the circulation depth and fault activity was also observed. The Rc/Ra ratios of free gas samples, predominantly indicating crustal origin, varied from 0.01 Ra to 0.53 Ra. Elevated Rc/Ra ratios in the research area suggested potential minor additions of mantle helium through faults and fractures. Crustal limestone was identified as the primary source of CO2-rich samples, with δ13CCO2 values ranging from −1.6‰ to −7.2‰, while trace amounts of mantle CO2 were found. The spatial distribution of the H2 concentration, CO2 concentration, He concentration, and mantle He proportions in gases indicated that higher values of He concentration and mantle He% always occur near sampling points with deeper circulation depths. However, no similar correlation was observed for H2 and CO2. Most earthquakes of magnitude 5 or greater occurred near the regions with high values of mantle source He release, highlighting the critical role of mantle fluids in the occurrence of earthquakes in the region. In this study, a fluid circulation model was developed to describe the process of fluid (water and gas) circulation migration and earthquake generation in the TRLCZ.

Soil CO2 fluxes measured in the Acoculco Geothermal System, Mexico: Baseline emissions from a long-term prospection programme

The Acoculco Caldera Complex is considered a promissory hidden high-temperature geothermal system in Mexico. To support the geothermal prospection of this anomalous area, a comprehensive programme of soil CO2 flux measurements was performed. A long-term measurement programme was conducted to determine the baseline of natural soil CO2 effluxes. Significant efforts were devoted both to measuring the CO2 fluxes between 2015 and 2022 and interpreting their origin. Eighteen soil gas surveys of CO2 were carried out by using the accumulation chamber method. >1200 diffuse CO2 fluxes were measured in six different areas of the Acoculco Caldera. Two areas (Los Azufres and Alcaparrosa) exhibited cold degassing sites, acid-sulphate springs, and gas bubbling in surface water bodies. The soil CO2 fluxes ranged from 1 to 26,000 g m−2 d−1, whereas lower fluxes <29 g m−2 d−1 were determined as the degassing baseline. A total CO2 output of 492 t d−1 km−1 was estimated using an integrated SGS-GSA approach, where the highest total soil CO2 fluxes were obtained for Alcaparrosa (299 t d−1 km−2) compared with Los Azufres (164 t d−1 km−2), and Surroundings (29 t d−1 km−2). Such results agree well with those values measured in other worldwide volcanic and active geothermal ecosystems. The range of CO2 isotopic composition values from −28.83 ‰ to −3.11 ‰, together with their statistical distribution, suggests multiple CO2 production sources feeding soil degassing. The combined interpretation of flux and isotopic data allowed us to identify two distinct gas sources: endogenous and biogenic. The present study highlights the importance of using soil CO2 monitoring to determine baseline emissions at the early exploration stage of geothermal systems.

Geochemical evidence for biodegradation in high-rank coals from Qinshui Basin, North China

Understanding the generation of secondary microbial methane (SMM) is important for the evaluation of natural gas resources and instructive for the stimulation of methane production. Coal seams are popular targets for extracting in situ preserved methane and studying microbe-stimulated methane yield. However, few studies have been done on overmature coals. Here we collected gas samples from coals varying from bituminous to meta-anthracite in the Qinshui Basin, North China, and analysed the molecular and stable isotopic compositions to systematically evaluate the influence of biodegradation in high-rank coals in geological settings. The stable isotope signatures (δ13C and δD) of methane are dominated by the thermal decomposition of organic matter in deep coals but inconsistent with the maturity rank of shallow burial coals. The decoupling of coal maturity with C1/C2+ ratios and δ13C-CH4 values, and positive δ13C-CO2 values (−9.2 to +24.4 ‰) suggest biodegradation of light wet gases (C2+ components) and CO2 reduction. Negative trends between δ13C-CO2 and CH4/CO2 in shallow burial coal seams reveal the mutual conversion of CH4 and CO2 and carbon isotope exchange, driven by microorganisms. The calculated isotopic temperatures (33–328 °C) based on the carbon isotope fractionation factors between CH4 and CO2 (1.024–1.069) demonstrate that carbon isotope exchange is prevalent in high-rank coals. It also reveals that the burial depth is an imperative factor in controlling microbial environments and thus the biodegradation process. This study implicates the potential of high-rank coals as the target for microbial-enhanced methane recovery and also implies that microorganisms are widely involved in reservoir carbon cycling.

Geochemical characteristics of natural springs within the Lenglongling fault zone related to the Menyuan Ms 6.9 earthquake on January 8, 2022, NW China

Geofluids from natural springs connect with the crust and/or mantle in many cases, and their geochemical anomalies could be significant for the study on faults activity and even earthquakes. Several natural springs are distributed along the Lenglongling fault zone (LLLFZ) in the northeastern margin of the Tibetan Plateau, where the Ms 6.9 Menyuan earthquake occurred on January 8th, 2022. Based on chemical and isotopic compositions (δD, δ18O, δ13C, and 3He/4He) of water and gas samples, the origin of geofluids and their potential correlation with fault activity even including earthquakes are preliminarily assessed in this paper. The δ13CCO2 values and 3He/4He ratios showed that the gas originating from the crust was associated with the metamorphism of carbonate rocks, whereas the δ18O and δD values of water samples indicated that the natural springs were predominantly infiltrated with precipitations from local mountains ranging 3.7 - 5.5 km in height. Obvious changes of Ca2+ and HCO3− concentrations in SZK spring waters in the surface rupture zones were observed in a short period (about three months) after the main shock, in contrast to those of the GSK springs far from the surface rupture zones. Such variations might be correlated with the stress increase prior to the 2022 Menyuan Ms 6.9 earthquake. The mechanical fracturing of surrounding limestone rocks during the slipping movement of LLLF could facilitate the water-rock interactions. Compared to three-month observations after the main shock, relatively higher concentrations of HCO3− and heavier δ18OH2O values of the LHG springs were also observed in the short-term period. The shallow stored formation water might be squeezed along the cracks and rose to the surface during earthquake tremors, causing a sandblasting water phenomenon.

Syn-tectonic fluids decoding effects of tectono-metamorphic cycles on regional metallogenic evolution of the Chinese Altai, central Asia

Despite their close temporal and spatial relationships, the effects of tectono-thermal events on ore formation remain obscure. To better understand this process, a comprehensive geochemical investigation on paleofluids from syn-tectonic felsic and quartz veins associated with the Devonian subduction and Permian collision of the Chinese Altai was conducted. We found that the Devonian fluids were organic alkanes-CO2-S-Ca-Mg-rich saline fluids with variable CO2/CH4 (0.09−5.03) and lower F−/SO42− (0.02−0.14) and Al3+/Mg2+ (0−0.11) ratios, whereas the Permian fluids were immiscible fluids including CO2-C4H10-CO-rich oxidized gas bubbles and CH4-C3H8-C2H6-Ca-Na-K-Al-S-Cl-F-rich reduced saline fluids with lower CO2/CH4 (0−1.31, mostly &amp;lt;1) and higher F−/SO42− (0.21−0.76) and Al3+/Mg2+ (0.10−2.56) ratios. The Devonian and Permian fluids also have similar δ13C-CO2 values of −23.8‰ to −3.5‰ and −16.5‰ to −3.7‰, respectively. These data suggest that both fluids derived mainly from devolatilization and dehydration melting of metasediments; the Permian fluids likely involve more biotite melting in the deeper crust and more mantle-derived components, whereas the Devonian fluids contain more meteoric components. Base metal-dominated Devonian mineralization occurred as deep-sourced organic matter- and S-rich fluids promoted base metal migration, whereas the relatively oxidized fluid conditions inhibited the mineralization of many other metals. By contrast, the more reduced and F-rich Permian fluids with more mantle contributions facilitated the extraction of Au and uptakes of rare metals from reworked metasediments and promoted their mineralization. These findings provide a more complete picture of how tectono-thermal events fertilize the crust and demonstrate that syn-tectonic fluids can serve as proxies for metallogenic processes during orogenic cycles in general.

Temporal dynamics and controlling factors of CO2 and CH4 variability in the urban atmosphere of Wroclaw, Poland

Temporal and spatial distribution of both biogenic and anthropogenic components of atmospheric carbon dioxide (CO2) and methane (CH4) is crucial for understanding the environmental impacts of climate change over urban areas. This research focuses on applying stable isotope source-partitioning studies to determine the interactions between biogenic and anthropogenic CO2 and CH4 emissions in an average-sized city environment. Study signifies the weight of instantaneous variability and diurnal averaging as compared with seasonal records of variations of the atmospheric CO2 and CH4 at a variety of typical urban sites in the city of Wroclaw, conducted during a one-year period from June 2017 to August 2018.The findings reveal distinct temporal variations in atmospheric CO2 and CH4 mole fractions and their isotopic composition. The average atmospheric CO2 and CH4 mole fractions during the study period were 416.4 ± 20.5 ppm, and 1.95 ± 0.09 ppm, respectively. The study highlights the high variability of driving forces, including current energy use patterns, natural carbon reservoirs, planetary boundary layer dynamics, and atmospheric transport. Additionally, the relationship between the evolution of the convective boundary layer depth and the CO2 budget was analyzed using the CLASS model with input parameters based on field observations, resulting in insights such as an increase in the range of 25–65 ppm of CO2 during stable nocturnal boundary layers.The observed changes in stable isotopic signatures of air samples allowed for the identification of two main source categories in the city area: fuel combustion and biogenic processes. The δ13C-CO2 values of collected samples suggest that biogenic emissions dominate (up to 60 % of CO2 excess mole fraction) during the growing season, but are reduced by plant photosynthesis during summer afternoons. In contrast, local fossil-fuel CO2 contribution (up to 90 % of excess CO2 mole fraction) from domestic heating, vehicle emissions, and heat and power plants predominantly influence the urban GHG budget during winter. The δ13C-CH4 values indicate anthropogenic sources related to fossil fuel combustion during winter, with values ranging from −44.2 ‰ to −51.4 ‰, while slightly more depleted values, between −47.1 ‰ and −54.2 ‰, reflect a larger input of biological processes in the methane urban budget during summer.Overall, instantaneous and hourly variability of the above-mentioned readings of gas mole fraction and isotopic composition, have shown higher variability than seasonal amplitudes. Hence, respecting this granularity is the key to alignment and understanding significance of such localized atmospheric pollution studies. Additionally, the changing overprint of the system's framework, such as variability of wind and atmospheric layering patterns, weather events, provides context of sampling and data analysis at different frequencies.

Constraining the Geochemical Fingerprints of Gases from the UK Carboniferous Coal Measures at the Glasgow Geoenergy Observatories Field Site, Scotland

Usage of thermal energy contained in abandoned, flooded, coal mines has the potential to contribute to low carbon heating or cooling supply and assist in meeting net-zero carbon emission targets. However, hazardous ground gases, such as CH4 and CO2, can be found naturally in superficial deposits, coal bearing strata and abandoned mines. Determining the presence, magnitude, and origin of subsurface gases, and how their geochemical fingerprints evolve within the shallow subsurface is vital to developing an understanding of how to manage the risk posed by ground gases in geoenergy technology development. Here, we present the first CH4 and CO2 concentration-depth profiles and stable isotope (δ13CCH4, δ13CCO2, and δDCH4) profiles obtained from UK mine workings, through analysis of headspace gas samples degassed from cores and chippings collected during construction of the Glasgow Observatory. These are used to investigate the variability of gas fingerprints with depth within unmined Carboniferous coal measures and Glasgow coal mine workings. Stable isotope compositions of CH4 (δ13CCH4 = −73.4‰ to −14.3‰; δ13CCO2 = −29‰ to −6.1‰; δDCH4 = −277‰ to −88‰) provide evidence of a biogenic source, with carbonate reduction being the primary pathway of CH4 production. Gas samples collected at depths of 63–79 m exhibit enrichments in 13CCH4 and 2H, indicating the oxidative consumption of CH4. This correlates with their proximity to the Glasgow Ell mine workings, which will have increased exposure to O2 from the atmosphere as a result of mining activities. CO2 gas is more abundant than CH4 throughout the succession in all three boreholes, exhibiting high δ13CCO2 values relative to the CH4 present. Gases from unmined bedrock exhibit the highest δ13CCO2 values, with samples from near-surface superficial deposits having the lowest δ13CCO2 values. δ13CCO2 values become progressively lower at shallower depths (above 90 m), which can be explained by the increasing influence of shallow groundwaters containing a mixture of dissolved marine carbonate minerals (∼0‰) and soil gas CO2 (−26‰) as depth decreases. Our findings provide an insight into the variability of mine derived gases within 200 m of the surface, providing an important ‘time-zero’ record of the site, which is required in the design of monitoring approaches.

Quantification of biodegradation rate of hydrocarbons in a contaminated aquifer by CO2 δ13C monitoring at ground surface

Ground surface analysis of CO2 emissions with δ13C determination is experimentally demonstrated to be a potential methodology to monitor, on line, the dynamics of petroleum-hydrocarbon biodegradation in soil aquifers, thanks to the improvement of the Isotopic Ratio Infra Red Spectroscopy technique. Biodegradation rate of remaining hydrocarbon substrates in groundwater can be quantified using basic application of the Rayleigh equations, by δ13CCO2 analysis released at ground surface above the pollution plume instead of usual approaches based on groundwater hydrocarbons δ13C analysis, when physical and chemical properties for the contaminated site meet appropriate conditions.The validation approach for that gasoline contaminated specific site is discussed and verified by comparison of first order attenuation rate constant determined from δ13CCO2 analysis emitted at ground surface and from δ13CTOLUENE analysis in ground water. A kinetic fractionation factor α of 0.9979 (or ε value of −2.1 ± 0.5‰) is estimated for the biodegradation of the most reactive hydrocarbon substrates (TEX). The treatment of this Rayleigh equations by linear regression of δ13CCO2 values along the predominant direction of groundwater flow leads to the following results and conclusions for that site: (i) first order biodegradation rate constants (and annual variation) are maximum after the activation of a Permeable Reactive Barrier (PRB) in May 2014: 0.92(+0.29–0.17) year−1, and during July and October: 0.46(+0.14–0.09) year−1 and minimum in mid-winter in February 2015: 0.17(+0.05–0.03) year−1, given by the estimation range for ε. These results are in the lower range with reported in literature for similar contaminated sites (1.6–18 year−1) considering natural attenuation under sulfate reducing conditions and (ii) the seasonal variation of the first order biodegradation rate constant is mainly correlated with the seasonal variation of the CO2 flux, where maximum values are in summers and minimum values in winters. Both seasonal variations are mainly due to the annual cycle of the natural biodegradation activity at the scale of the pollution plume, rather than the activation of the PRB.This work demonstrates that δ13CCO2 analysis released at ground surface from biodegradation of groundwater hydrocarbons could provide, under characterized and appropriate conditions, a non-intrusive (without soil samplings), fast, and low-cost online method to monitor and therefore to optimize soil remediation processes in real time. (Monitored Natural Attenuation or Enhanced Bioremediation).

Influence of Sedimentary Environment Evolution on Fingerprint Characteristics of Methane Isotopes: A Case Study From Hangzhou Bay

AbstractTo better understand the depositional constraints on the fingerprint characteristics of methane isotopes, we present a set of carbon/hydrogen isotopic data for CH4, CO2, pore water, carbonate, and total organic carbon along a 70‐m sedimentary core. The sedimentary facies (Units I, II, and III from upper to bottom) suggested depositional environments of the present estuary, shallow marine, and floodplain‐estuary. Calculations suggested that around 86% of methane was produced through the CO2 reduction pathway. In this pathway, the hydrogen in CH4 is from ambient water, while the carbon is from dissolved inorganic carbon. Correspondingly, our study showed that the values of δDCH4 displayed similar trends with those of δDH2O and Cl− concentrations along the depth profiles. The low δDCH4 below 44.5 m corresponded to low δDH2O and low salinity during the cold and low‐sea‐level period. The values of δ13CCH4 generally synchronously changed with those of δ13CCO2. The variation trends of δ13CCH4 and δ13CCO2 were the same with δ13Ccarbonate from 10 to 70 m depth but decoupled above 10 m. The values of δ13CCH4 in Units II and III were correlated with the δ13Ccarbonates, which is related to the sedimentary processes. But decoupling of low values of δ13CCH4 and δ13CCO2 from δ13Ccarbonates in Unit I may be related to preferential microbial consumption of labile compounds with light carbon isotopic compositions, such as lipids. In short, the variations of δ13CCH4 and δDCH4 were largely related to the sedimentary process, which controlled the isotopic compositions of both water and carbonate.

Tracing the origin and formation mechanisms of coalbed gas from the Fuxin Basin in China using geochemical and isotopic signatures of the gas and coproduced water

Coalbed gas (CBG) is an unconventional natural gas with a large resource potential. In order to determine its origins, formation pathways and mechanisms, studies on the geochemistry of CBG, coalbed water and coal have been carried out for years. However, the relationship between geochemistry of CBG and coalbed water is still not clear, especially with respect to the CO2 dissolution process. Here, a comprehensive study on the geochemistry of CBG and coproduction water in samples from the Fuxin Basin, China, is presented. Twenty-four gas and water samples were collected directly from the CBG producing well heads. C1/(C2 + C3) values are far >1000, δ13C1 values range between −62.8 and −57.6‰, δDCH4 values are from −252 to −225‰, ΔDH2O-CH4 values are from 148 to 178‰, and αCO2-C1 values are from 1.04 to 1.05. The composition characteristics, associated with the genetic diagrams of δ13C1 vs. δDCH4 and δ13C1 vs. δ13CCO2, and low coal rank (0.4–0.6% Ro), suggest that the CBG in the Fuxi Basin is mainly microbial gas. The Na-HCO3-Cl type of water in the coalbed is favorable for methanogenesis. δDH2O and δ18OH2O plot along the global meteoric water line (GMWL) and to the left of the local meteoric water line (LMWL), suggesting that the coalbed water is mainly from meteoric water recharge. However, isotope values in water may have been modified by methanogenesis and water-rock interaction. ΔDH2O-CH4 values of 148 to 178‰ suggest that methanogenic pathway is mainly CO2 reduction. However, αCO2-C1 values of 1.04 to 1.05 suggest that methanogenesis pathway might be acetoclastic or methylotrophic. This inconsistency may be caused by low δ13CCO2 values (−19.2 to −14.2‰) and thus low αCO2-C1 values due to high dissolution effect of CO2 and relative strong hydrodynamic activity in coal aquifers in the Fuxin Basin. Groundwater flow can carry away 13C-enriched CO2 dissolved in water, thus, the residual CO2 and DIC are depleted in 13C. Consequently, CO2 reduction is likely the main methanogenic pathway in the basin. Although CH4 and H2O may be close to isotope equilibrium for some samples, the CH4, CO2 and HCO3− are isotopically in disequilibrium at present reservoir conditions. Overall, kinetic processes largely control isotopic composition and distribution of the CBG and coalbed water in the Fuxin Basin. The CBG in the Fuxin Basin has been continuously generated, probably since the deposition of the coal-bearing formation. Although part of the CBG may have been lost during coalbed uplift stage, uplift to near surface allow re-inoculation of coalbeds with methanogenic microbial consortia via meteoric water recharge, which can accelerate the formation and accumulation of the microbial CBG. Consequently, most CBG in the present coalbed has been generated likely after the coal strata uplift.

Volatile characteristics and fluxes of He-CO2 systematics in the southeastern Tibetan Plateau: Constraints on regional seismic activities

The spatial and temporal evolutionary mechanisms of volatile components at plate boundaries and their potential as earthquake precursors are not well understood. Based on observations of He–CO2 from 19 hot-spring gases in the Lancang River fault zone, Yunnan Province, the results show that the volatiles mainly come from the crust. The combined He isotope, He–CO2 relationships, and δ13C–CO2 values of the hot-spring volatiles indicate that it is a mixture of mantle and crustal fluids. These are related to the radiogenic He production caused by crustal U and Th elements with <5% mantle-derived He, whereas C is mainly from the metamorphic decarbonization of Tethyan marine carbonate rocks. Based on the He–CO2 isotopic relationship, crustal volatiles flux with a significant proportion were determined within a non-volcanic area, which was estimated to be 9–132 mol a−1, and CO2 degassing was estimated to be 2–120 × 109 mol a−1. These results are comparable to those of some volcanic areas and large seismically-active fault zones (such as the San Andreas Fault zone). The compressional stress field of the rigid crust near the study area leads to the low permeability of large faults and CO2 degassing from the hydrothermal system, resulting in chemical precipitation that further blocks the shallow crust conduits and deduced overpressure below the hydrothermal reservoir, which may be the cause of the presence of earthquakes with magnitudes greater than 7 in the Lancang River fault zone. The results are significant for elements cycling of converging plate boundaries and earthquake hazards monitoring.

Radon-rich waters of the Tulinka aquifers, Novosibirsk, Russia

The first integrated isotope and chemical data on radon-rich waters of the Tulinka aquifers (Novosibirsk, West Siberia) are presented. The Tulinka radon waters are fresh, with neutral to moderately alkaline рН (7.3–7.7), SO4–HCO3 Na–Mg–Ca major-ion chemistry. The amount of total dissolved solids is 720–910 mg/L, Si content 6.41–9.02 mg/L. Eh is within +169.1 – +250.0 mV, corresponding to oxidized conditions. The concentration of dissolved oxygen is 2.86–7.37 mg/L. Radon activity (222Rn) varies from 173 to 276 Bq/L. The concentrations of U, Th, and Ra isotopes are 0.015–0.017 mg/L238U, 9.59∙10−7 to 1.58∙10−5232Th, and up to 4.93∙10−10226Ra; the 232Th/238U ratio is in the range 5.81∙10−5 to 9.42∙10−4. Total α activity is not higher than 891 mBq/L, and β activity is 80 mBq/L. The compositions of stable isotopes are −15.4 to −15.1‰ δ18O, −114.2 to −112.8‰ δD, and −13.9 to −9.9‰ δ13C. The isotope signatures and recharge patterns of the Tulinka waters are insensitive to seasonal effects, which suggests the slow water exchange. Substantial seasonal variations of the content of dissolved inorganic carbon (DIC) are detected. The range of δ13CDIC in the analyzed samples (from −14.3 to −9.0‰) corresponds to a mixed CO2 origin: diffusion of soil CO2 approaching the atmospheric δ13CCO2 values, and biogenic CO2 released by decaying plant residues. The measured isotope characteristics provide evidence that the waters recharge by infiltration from meteoric sources. The approximate radiocarbon age of DIC in the Tulinka waters, determined for the first time, is 2663 ± 144 years. However, this age may be underestimated due to groundwater mixing with the present-day surface waters.

Gas Generation and Its Carbon Isotopic Composition during Pyrite-Catalyzed Pyrolysis of Shale with Different Maturities

In this study, two shale samples with different maturities, from Geniai, Lithuania (Ro = 0.7%), and Wenjiaba, China (Ro = 2.7%), were selected for open-system pyrolysis experiments at 400 °C and 500 °C, respectively. The generation of isotopic gases from the shales with different maturities was investigated, and the effects of pyrite catalysis on the carbon isotopic compositions were also studied. It was found that CO2, CH4 and their isotopic gases were the main gaseous products of the pyrolysis of both shales, and more hydrocarbon gases were generated from the low-maturity Geniai shale. The δ13C1 values fluctuated from −40‰ to −38‰, and δ13C2 showed higher values (−38‰~−34‰) for the Geniai shale. In addition, its δ13CCO2 values ranged from −28‰ to −26‰. Compared with the Geniai shale, lower δ13C1 values (−43‰~−42‰) and higher δ13CCO2 values (−19‰~−14‰) were detected for the Wenjiaba shale. As temperature increased, CH4 became isotopically lighter and C2H6 became isotopically heavier, which changes were due to the mass-induced different reaction rates of 12C and 13C radicals. Furthermore, the pyrite made the kinetic isotope effect stronger and thus made the CH4 isotopically lighter for both shales, especially at the lower temperature of 400 °C.

The effect of thermochemical sulfate reduction of n-pentane and condensate on the chemical and isotopic compositions of gases: Implications for the organic reactant and reaction extent

Evaluating the composition of organic reactants initially involved in thermochemical sulfate reduction (TSR) reaction and better understanding the chemical and isotopic compositions of gases are helpful to minimize the risks involved in exploration of natural sour gas reservoirs. Variations in the chemical and carbon isotopic compositions of the gas produced during high pressure (50Mpa), isothermal (360 °C) hydro-pyrolysis experiments of the TSR reaction and thermal cracking involving n-pentane (n-C5) and condensate were investigated using a high-pressure gold tube system. The results show that the yields of CH4, CO2 and H2S increased with reaction time, and that the yields of C2–C4 firstly increased and then decreased. The δ13C value of alkane gases increased with reaction time. The δ13C value of CO2 produced from the TSR reaction involving n-C5 became less negative. The yields of alkane gas, CO2 and H2S from the TSR reaction of n-C5 were up to two times higher than that those from the TSR reaction of condensate. This indicates that lighter liquid hydrocarbons with higher hydrogen content produce more hydrocarbon and non-hydrocarbon gases. The relationship between the dryness, δ13C values of alkane gases and sourness could help to estimate whether the organic reactant was liquid or gaseous hydrocarbons. The value of ln(C2/C3) at the early stage of TSR reaction can be used to roughly estimate the composition of the liquid hydrocarbons involved in the TSR reaction. Our analysis also shows that the value of ln(C2/C3) firstly increased and then decreased with increasing gas dryness during the TSR reaction, which could be an effective index to assess the TSR reaction extent and provide new clues for identifying gas origin. The δ13C values of methane and ethane produced in the TSR reaction involving n-C5 reversed to δ13C1 > δ13C2 during the 48–60h period, which could be related to the oxidation of alkanes by HSO4−. This could help explain the δ13C1 > δ13C2 in TSR influenced gas reservoirs with low H2S content.