CO₂/³He ratios reveal the presence of mantle gas in the CO₂-rich groundwaters of the Ardenne massif (Spa, Belgium)

Natural CO2-rich groundwaters of eastern Belgium have been known for centuries although the exact origin of the gas they contained was still unclear. This paper presents the results of a sampling campaign in the area (Spa, Stoumont, Malmedy) where 30 samples of both carbogazeous and non-carbogazeous groundwaters have been analyzed for major elements, CO2 content and carbon isotopic composition. Among them, 13 samples were analyzed for 3He / 4He and 4He / 20Ne ratios. The combination of δ13C and 3He / CO2 ratios have shown with a high level of confidence that the CO2 in groundwater from springs and boreholes has a mantellic origin, and can very likely be attributed to the degassing of the neighboring and still buoyant Eifel mantle plume, located at a distance of 100 km eastwards. The identity and nature of the deep-rooted fractures that act as CO2 transport pathway to the surface are still to be clarified, but several major thrust faults exist in the Rhenish Massif and could connect the Eifel volcanic field with the studied area.

Isotope-geochemical characteristics of Wenlock carbonate and terrigenous-carbonate deposits of the Subpolar Urals and the southern part of the Chernyshev ridge

Research subject. Wenlock deposits in the Subpolar Urals and southern part of the Chernyshev Ridge. Materials and methods. Carbonate and terrigenous-carbonate rock samples from sections of the Subpolar Urals (outcrops 212, 217) and the southern part of the Chernyshev Ridge (outcrop 479) were studied by isotope analysis of carbonate carbon and oxygen.Results. The isotopic composition of the studied sections varies across a wide range of δ13С (–6.4…–0.05‰) and δ18O (20.0–26.9‰). Therefore, three time intervals characterized by distinctive isotopic characteristics can be distinguished, roughly corresponding to early Sheinwoodian (I), late Sheinwoodian (II) and Homerian (III). Interval I is characterized exclusively by the rocks of outcrop 479 with δ13С (–3.6…–3.0‰) and δ18O (22.4–23.6‰). In Interval II, the average values of isotopic characteristics of outcrop 479 indicate a tendency to weighting carbon (–5.5…–3.5‰) and somewhat heavier oxygen (23.2–25.2‰) isotopes. In outcrop 212, the average isotopic values for carbon and oxygen vary from –2.9 to –1.3‰ and from 21.9 to 24.3‰, respectively. In outcrop 217, the average values are δ13C (–1.8…–0.8‰) and δ18O (22.4–25.4‰). In Interval III, the average values of carbon isotopic composition in outcrop 479 are becoming heavier from –2.5 to –0.7‰. In outcrop 212, the isotopic values of oxygen (21.9–23.1‰) and carbon (–4.9…–2.5‰) tend to become weighting; however, in outcrop 217, the average isotopic values of δ13C (–1.9…–0.5‰) and δ18O (22.3–24.5‰) remain constant. The conducted litho-facial studies showed that the weighting carbon isotopic composition ranging from –6.4…to –3.0‰ is associated with an increase in microbial activity in sediments, the manifestation of vadose-phreatic conditions, and the intake of light carbon dioxide with a flow of terrigenous material from the earth. In the latter case, oxygen isotopic values are also the most lightweight (20.0–23.0‰).Conclusions. The obtained isotopic characteristics of the Wenlock rocks under study indicate the expedience of identifying three time intervals and their correlation with paleogeographic reconstructions of Wenlockian sedimentation in the Timan-northern Ural region.

Manifestation of the Late Famennian Dasberg Event in the shelf-batial transition (Pai-Khoi sequences)

Research subject. Regional manifestations of the Dasberg eustatic event in the shelf and bathyal Pai-Khoi successions. The event appears in the Lower-Middle expansa zones interval (Upper Devonian, Famennian).Aim. To evaluate the manifestations of the event in the realm of transition from the shallow-water shoal succession of the Pai-Khoi carbonate parautokhtone towards the deep-water (bathyal) successions of the Kara shale allokhtone.Materials and methods. A number of successions comprising different facies and located in different parts of Pai-Khoi were studied: the Silova-Yakha River section and Tal’beyshor Creek section (south-western Pai-Khoi), the Lymbad’yakha section and the Peschanaya River section (northern Pai-Khoi). The interpretation of facies and the reconstruction of transgression-regression couplets were conducted based on the previously developed models of shoal and bathyal sedimentation. The stratigraphic framework comprised data on conodonts, transgression acmes, and carbonate carbon isotopic record.Results. The Pai-Khoi successions comprising Lower–Middle expansa zones demonstrate four transgression-regression cycles. The transgression acme of the third cycle marks the Dasberg eustatic event. The absence of anoxia is characteristic of this event in the region under consideration. The carbonate carbon isotope record of the Silova-Yakha River section shows a structure similar to that of North American successions. Variations in δ13Cкарб were likely to be caused by climate changes and perturbations of the global oceanic circulation.Conclusions. The stratigraphic interval comprising Lower–Middle expansa or Lower–Upper expansa (expansa s. l.) zones is detected clearly in different facies. A more detailed subdivision and correlation in the region under consideration is possible on the basis of evaluating manifestations of the Dasberg eustatic event: a characteristic eustatic succession and variations of the carbonate carbon isotopic composition.

Variations in δ13CDIC and influencing factors in a shallow macrophytic lake on the Qinghai-Tibetan Plateau: implications for the lake carbon cycle

The primary sources of dissolved inorganic carbon (DIC) in water are carbonate materials and CO produced during the biological processing of organic matter. The application of carbon isotope techniques to terrestrial and aquatic ecosystems can accurately elucidate carbon fluxes and other carbon cycle processes in these systems. Lake ecosystems on the Qinghai-Tibetan Plateau are fragile and sensitive to changes in climate and environment. This study explored the relationship between the carbon isotopic composition (δC) of the DIC (δC) in the Genggahai Lake, the lake environment, and the climate of the watershed based on the observed physicochemical parameters of water in areas with different types of submerged macrophyte communities, combined with changes in the temperature and precipitation during the same period. Overall, the δC of the Genggahai Basin exhibited a large range of values, with an average δC for inflowing spring water (δC) of –11.1 ‰, which was the most negative, followed by an average δC value of –10.8 ‰ for that from the Shazhuyu River (δC) and an averageδC value of –6.91 ‰ for lake water (δC). Variations in the photosynthetic activity intensity of different aquatic plants yield significantly changing δC values in areas with varied aquatic plant communities. Hydrochemical observations revealed that δC and aquatic plant photosynthesis primarily affected the differences in the δC values of the Genggahai Lake, thereby identifying them as the key components of the lake carbon cycle.

Mechanism of anomalous manifestations δ13C CO2 underground water on Tashkent geodynamic range

The results of long-term isotope and gas measurements of various groundwater sources in the Tashkent artesian basin are presented. In addition to areal studies, the results of regime observations of selected wells at the Tashkent geodynamic test site are also presented to identify prognostic features as a precursor to strong earthquakes. The first results were obtained related to the manifestation of the Nazarbek earthquake on December 11, 1980, with a magnitude of 5.3, a source depth of 10 km. On the basis of this earthquake, the behavior of isotopic and gas precursors at the Ulugbek well, Tashkent geodynamic test site, was analyzed. A quantitative analysis was made of the anomalous manifestation of the gas and isotopic composition of groundwater in the Ulugbek borehole during the preparation and execution of the Nazarbek earthquake. Determination of CO2 release from rock samples recovered from deep wells of the landfill made it possible to quantify the ratio of “background” CO2 mixing from carbonate decomposition to anomalous. Judging by the obtained ratio, the rocks during the preparation of the earthquake should emit 2 times more carbon dioxide than the background CO2. This was confirmed by observations of the CO2 content in the well, when an increase in its relative concentration of 250% was recorded. The analysis of the δ13C time series, obtained as a result of long-term regime observations of the carbon isotopic composition of CO2 in the groundwater of the Tashkent geodynamic test site, made it possible to identify significant anomalies in the carbon content.

Methane in Lakes: Variability in Stable Carbon Isotopic Composition and the Potential Importance of Groundwater Input

Methane (CH4) is an important component of the carbon (C) cycling in lakes. CH4 production enables carbon in sediments to be either reintroduced to the food web via CH4 oxidation or emitted as a greenhouse gas making lakes one of the largest natural sources of atmospheric CH4. Large stable carbon isotopic fractionation during CH4 oxidation makes changes in 13C:12C ratio (δ13C) a powerful and widely used tool to determine the extent to which lake CH4 is oxidized, rather than emitted. This relies on correct δ13C values of original CH4 sources, the variability of which has rarely been investigated systematically in lakes. In this study, we measured δ13C in CH4 bubbles in littoral sediments and in CH4 dissolved in the anoxic hypolimnion of six boreal lakes with different characteristics. The results indicate that δ13C of CH4 sources is consistently higher (less 13C depletion) in littoral sediments than in deep waters across boreal and subarctic lakes. Variability in organic matter substrates across depths is a potential explanation. In one of the studied lakes available data from nearby soils showed correspondence between δ13C-CH4 in groundwater and deep lake water, and input from the catchment of CH4via groundwater exceeded atmospheric CH4 emissions tenfold over a period of 1 month. It indicates that lateral hydrological transport of CH4 can explain the observed δ13C-CH4 patterns and be important for lake CH4 cycling. Our results have important consequences for modelling and process assessments relative to lake CH4 using δ13C, including for CH4 oxidation, which is a key regulator of lake CH4 emissions.