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Abstract

[1] Over the last 30 years, geochemical research has demonstrated that abiotic methane (CH4), formed by chemical reactions which do not directly involve organic matter, occurs on Earth in several specific geologic environments. It can be produced by either high-temperature magmatic processes in volcanic and geothermal areas, or via low-temperature (<100°C) gas-water-rock reactions in continental settings, even at shallow depths. The isotopic composition of C and H is a first step in distinguishing abiotic from biotic (including either microbial or thermogenic) CH4. Herein we demonstrate that integrated geochemical diagnostic techniques, based on molecular composition of associated gases, noble gas isotopes, mixing models, and a detailed knowledge of the geologic and hydrogeologic context are necessary to confirm the occurrence of abiotic CH4 in natural gases, which are frequently mixtures of multiple sources. Although it has been traditionally assumed that abiotic CH4 is mainly related to mantle-derived or magmatic processes, a new generation of data is showing that low-temperature synthesis related to gas-water-rock reactions is more common than previously thought. This paper reviews the major sources of abiotic CH4 and the primary approaches for differentiating abiotic from biotic CH4, including novel potential tools such as clumped isotope geochemistry. A diagnostic approach for differentiation is proposed.

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... On Earth, life sustains large methane surface fluxes, and so methane has long been regarded as a potential biosignature gas for terrestrial exoplanets. Previous studies have considered abiotic methane production (6)(7)(8)(9)(10)(11), methane biosignatures in the context of chemical disequilibrium (12)(13)(14)(15), and prospects for remote detection of methane in terrestrial atmospheres (6,9,(15)(16)(17). During the Archean eon (4 to 2.5 Ga), Earth's atmosphere likely had high methane abundances (∼10 2 to 10 4 times modern) due to life (i.e., methanogenesis) (8,18,19). ...
... Volcanoes on Earth today do not outgas significant methane. Most subaerial volcanoes produce less than ∼10 −6 Tmol CH 4 per year (10,58), and given the ∼1,500 active volcanoes on Earth today, the estimated global CH 4 flux is <10 −3 Tmol/y, much less than the current biogenic flux of 30 Tmol/y. Similarly, Schindler and Kasting (6) estimated the CH 4 flux from submarine volcanism to be ∼10 −2 Tmol/y. ...
... Under oxidizing planetary conditions conducive to CO 2 degassing, lowtemperature CH 4 production is ultimately limited by the supply of reducing power in the form of ferrous iron (Fe 2+ ) in newly produced crust. One of the most frequently discussed processes for methane production is serpentinization, through which ironbearing minerals are altered by hydration to produce H 2 via the oxidation of Fe 2+ by water (10,69,70): ...
Article
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Significance Astronomers will soon begin searching for biosignatures, atmospheric gases or surface features produced by life, on potentially habitable planets. Since methane is the only biosignature that the James Webb Space Telescope could readily detect in terrestrial atmospheres, it is imperative to understand methane biosignatures to contextualize these upcoming observations. We explore the necessary planetary context for methane to be a persuasive biosignature and assess whether, and in what planetary environments, abiotic sources of methane could result in false-positive scenarios. With these results, we provide a tentative framework for assessing methane biosignatures. If life is abundant in the universe, then with the correct planetary context, atmospheric methane may be the first detectable indication of life beyond Earth.
... It is therefore clear that further data are needed to better constrain the origin of methane in the Dziani Dzaha Lake, and we are aware that the debate on methane origin is open within the scientific community, in particular in order to justify its possible abiotic origin assuming a "magmatic" or "late magmatic" origin [Etiope and Sherwood Lollar, 2013]. However, answering this question is not trivial, considering that the recent submarine eruption only 50 km off-shore from Petite Terre represents by far the largest known submarine eruption until now and that intense seismicity occurs at variable distance from the island , Berthod et al., 2021a,b, Foix et al., 2021. ...
... In this context, a further clue is given by the isotopic signature of helium involved in the outgassing process at Petite Terre, which can be useful in discerning and assessing the deep origin of the gas. Therefore, we considered the isotopic variability of methane by comparing it with the percentage of mantle-related helium, following the approach indicated in Etiope and Sherwood Lollar [2013] (Figure 8). In our case, the Petite Terre gases fall within the area where magmatic CH 4 inputs are clearly recognized (EPR-East Pacific Rise, Socorro, Lost City [Proskurowski et al., 2008, Taran et al., 2010a, Welhan and Craig, 1983). ...
... The results are shown in Figure 9, which allows to extend the preliminary study made by Liuzzo et al. [2021], taking into account recent gas sampling in 2020 at both BAS and Dziani Dzaha Lake areas of Petite Terre. The data considered are those in which there are no obvious secondary variations and/or The Petite Terre data are compared with a larger dataset from various geodynamic origins from Etiope and Sherwood Lollar [2013], and appear consistent with environments where methane of magmatic origin is clearly recognized. ...
... The type of methane present in hydrothermal vent fluid may be mixed in terms of origin and source. This area includes H 2 and CH 4 originating from seep-related microbial (Konn et al. 2015) with additional CH 4 from non-organic (i.e., abiotic) sources (Etiope and Sherwood Lollar 2013). ...
... Abiotic methane differs from microbial and thermogenic methane in that it forms without the input of organic matter, while abiotic methane forms inorganically. There are varieties of methods for producing methane inorganically, including high temperature reactions in the mantle, CO 2 evolution into methane as magma cools and gas-water-rock interactions (Etiope and Sherwood Lollar 2013). One example of methane derived from gas-water-rock interaction begins with serpentinization processes in peridotite hosted hydrothermal systems (e.g., Lost City Hydrothermal Field) (Früh-Green et al. 2004;Kelley et al. 2005;Proskurowski et al. 2008). ...
... The process of abiotic methane generation appears to occur without macroscale or pervasive serpentinization of exhumed mantle material (Etiope and Whiticar 2019; Klein et al. 2019). Although abiotic methane production in laboratory settings (Etiope and Sherwood Lollar 2013), in , the implications of abiotic methane to global carbon inventories are unknown. Etiope and Whiticar (2019) and Klein et al. (2019) have only recently hypothesized that amounts may be far greater than previously presumed. ...
Chapter
Svyatogor Ridge is a gas hydrate-bearing sediment drift on the flank of an ultra-slow spreading mid-ocean ridge. Svyatogor Ridge hosts shallow gas accumulations, a strong bottom simulating reflection and fluid flow pathways (predominantly chimneys and faults) to the seafloor, culminating in pockmarks. Large offset detachment faults underlying Svyatogor Ridge provide access to deeper crustal and mantle ultramafic rocks, likely acting as conduits for warm fluid (and possible abiotic methane produced via serpentinization) to reach the shallow subsurface. This environment is distinct compared to other Arctic gas hydrate systems as it rests on the flank of an active mid-oceanic spreading ridge. It is the only known gas hydrate-bearing sediment drift in the Arctic where crustal-scale processes (mid-ocean ridge spreading) directly control the pressure and temperature regime for gas hydrate formation as well as fluid flow dynamics at the site.
... Electrochemical and photochemical reduction for CO 2 hydrogenation have shown favorable results in overcoming this issue [9]. However, the high costs and low yields of these techniques [10] have led to the study of other alternatives such as the hydrothermal treatment in which CO 2 reduction takes places in water media at high pressures and temperatures [11][12][13]. In this process, water acts as hydrogen donor instead of H 2 , which is flammable and complex to store [14]. ...
... In the case of AC, not only HCO 3− is formed. There is another step in which AC is also decomposed because the H + protons of the ion NH 4 + are being donated to other compounds, and then the yield to FA is reduced because there is a competition between two reactions: the reduction of AC and the thermal decomposition of AC [13,16]. It was observed that the experiments held at 200 • C showed higher yields of FA than the reactions at 250 • C. The reduction of CO 2 is favored by the reaction in alkaline media; when the temperature rises, NH 4 + dissociates into NH 3 and H + , which are species that reduce the alkalinity and might reduce the solubility of CO 2 in water [10,38]. ...
... To do so, experiments with an isotope of sodium bicarbonate (NaH 13 CO 3 ; SB-13 C) were performed with the different catalysts. 13 C-NMR analyses were carried out to identify the fraction of formic acid that possesses 13 C, which comes from the reduction of the carbon source, and the fraction that comes from glucose. The experiments were conducted at 250 • C and 2 h. ...
Article
Full-text available
High-temperature water reactions to reduce carbon dioxide were carried out by using an organic reductant and a series of metals and metal oxides as catalysts, as well as activated carbon (C). As CO2 source, sodium bicarbonate and ammonium carbamate were used. Glucose was the reductant. Cu, Ni, Pd/C 5%, Ru/C 5%, C, Fe2O3 and Fe3O4 were the catalysts tested. The products of CO2 reduction were formic acid and other subproducts from sugar hydrolysis such as acetic acid and lactic acid. Reactions with sodium bicarbonate reached higher yields of formic acid in comparison to ammonium carbamate reactions. Higher yields of formic acid (53% and 52%) were obtained by using C and Fe3O4 as catalysts and sodium bicarbonate as carbon source. Reactions with ammonium carbamate achieved a yield of formic acid up to 25% by using Fe3O4 as catalyst. The origin of the carbon that forms formic acid was investigated by using NaH13CO3 as carbon source. Depending on the catalyst, the fraction of formic acid coming from the reduction of the isotope of sodium bicarbonate varied from 32 to 81%. This fraction decreased in the following order: Pd/C 5% > Ru/C 5% > Ni > Cu > C ≈ Fe2O3 > Fe3O4.
... On Earth, life sustains large methane surface fluxes, and so methane has long been regarded as a potential biosignature gas for terrestrial exoplanets. Previous studies have considered abiotic methane production (6)(7)(8)(9)(10)(11), methane biosignatures in the context of chemical disequilibrium (12)(13)(14)(15), and prospects for remote detection of methane in terrestrial atmospheres (6,9,(15)(16)(17). During the Archean eon (4 to 2.5 Ga), Earth's atmosphere likely had high methane abundances (∼10 2 to 10 4 times modern) due to life (i.e., methanogenesis) (8,18,19). ...
... Volcanoes on Earth today do not outgas significant methane. Most subaerial volcanoes produce less than ∼10 −6 Tmol CH 4 per year (10,58), and given the ∼1,500 active volcanoes on Earth today, the estimated global CH 4 flux is <10 −3 Tmol/y, much less than the current biogenic flux of 30 Tmol/y. Similarly, Schindler and Kasting (6) estimated the CH 4 flux from submarine volcanism to be ∼10 −2 Tmol/y. ...
... Under oxidizing planetary conditions conducive to CO 2 degassing, lowtemperature CH 4 production is ultimately limited by the supply of reducing power in the form of ferrous iron (Fe 2+ ) in newly produced crust. One of the most frequently discussed processes for methane production is serpentinization, through which ironbearing minerals are altered by hydration to produce H 2 via the oxidation of Fe 2+ by water (10,69,70): ...
Preprint
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Methane has been proposed as an exoplanet biosignature. Imminent observations with the James Webb Space Telescope may enable methane detections on potentially habitable exoplanets, so it is essential to assess in what planetary contexts methane is a compelling biosignature. Methane's short photochemical lifetime in terrestrial planet atmospheres implies that abundant methane requires large replenishment fluxes. While methane can be produced by a variety of abiotic mechanisms such as outgassing, serpentinizing reactions, and impacts, we argue that, in contrast to an Earth-like biosphere, known abiotic processes cannot easily generate atmospheres rich in CH$_4$ and CO$_2$ with limited CO due to the strong redox disequilibrium between CH$_4$ and CO$_2$. Methane is thus more likely to be biogenic for planets with 1) a terrestrial bulk density, high mean-molecular-weight and anoxic atmosphere, and an old host star; 2) an abundance of CH$_4$ that implies surface fluxes exceeding what could be supplied by abiotic processes; and 3) atmospheric CO$_2$ with comparatively little CO.
... Abiotic methane (Potter and Konnerup-Madsen, 2003;Etiope and Sherwood Lollar, 2013;Etiope and Whiticar, 2019) forms from H 2 via Fischer-Tropsch Type reactions (FTT, Fischer and Tropsch, 1926). The FTT chemical reactions have been widely studied and applied in industrial catalysis for synthetic fuel and chemicals production and CO 2 consumption (Anderson, 1984;Schulz, 1999;Wang et al., 2011). ...
... Methane in gas samples (all geological settings) with >10% H 2 has average δ 13 C value -21.8‰ (n = 172), while methane in gas samples with H 2 content >0% and ≤10% has average δ 13 C value -38.0‰ (n = 1509). The presence of very high H 2 concentrations together with 13 C-enriched methane likely indicates common abiotic processes in which these gases are co-generated (Etiope and Sherwood Lollar, 2013;Etiope and Schoell, 2014;Etiope and Whiticar, 2019). ...
Article
Geologic molecular hydrogen (H2) occurs in the subsurface and vents and seeps at the surface. However, this valuable natural resource is under-utilized in the economy because the distribution, abundance and origins of H2 are poorly understood. I studied a global dataset of 6246 natural gases with reported H2 concentrations from 16 different geological habitats. The average H2 concentration in all gas samples is 3.5%, but the median concentration is only 0.01%. Gases sampled in Mid-Ocean Ridges and in serpentinites have the highest average concentrations of H2 (~24% and ~21%, respectively). More than 30 different processes may produce H2 observed in natural gases. Hydrogen isotopic composition (expressed as δ²H-H2 values) may indicate crustal (<-650‰) or mantle and primordial (from -650‰ to -100‰) sources of H2, or may result from temperature-dependent equilibration of H2 with water. Much of crustal H2 may be sourced by the reactions of serpentinization, while the quantitative significance of other H2-generating processes such as radiolytic decomposition of water and hydrocarbons, fracture-induced reduction of water, petroleum cracking and coal metamorphism remain speculative. Primordial H2 perhaps vents in some volcanic settings. Provided better understanding of H2 abundance and origins in different geological settings should enable the purposeful exploration for geologic H2 and the assessment of its economic resources.
... T he process of abiotic hydrocarbon formation in the deep Earth is still contested, despite being central in geobiological processes and potential natural energy sources 1,2 . Light hydrocarbons of abiotic origin have been identified in an increasing number of geological fluids in the Earth's lithosphere [3][4][5][6] . Methane has also been detected within deep diamonds, suggesting the presence of abiotic hydrocarbons at mantle depths [7][8][9] . ...
... We conducted several heating runs with different gasket materials such as rhenium (Re) and tungsten (W), and gasket liners, e.g., gold (Au) and Al 2 O 3 (see Supplementary Table 1 for a list of the materials used). We observed that, regardless of the gasket and gasket insert materials, if the diamonds are not protected by Al 2 O 3 , CH 4 and/or C 2 H 6 are always produced. ...
Article
Full-text available
Diamond and graphite are fundamental sources of carbon in the upper mantle, and their reactivity with H 2 -rich fluids present at these depths may represent the key to unravelling deep abiotic hydrocarbon formation. We demonstrate an unexpected high reactivity between carbons’ most common allotropes, diamond and graphite, with hydrogen at conditions comparable with those in the Earth’s upper mantle along subduction zone thermal gradients. Between 0.5-3 GPa and at temperatures as low as 300 °C, carbon reacts readily with H 2 yielding methane (CH 4 ), whilst at higher temperatures (500 °C and above), additional light hydrocarbons such as ethane (C 2 H 6 ) emerge. These results suggest that the interaction between deep H 2 -rich fluids and reduced carbon minerals may be an efficient mechanism for producing abiotic hydrocarbons at the upper mantle.
... For example, hydrothermal circulation of fluids have been postulated to liberate CH 4 -rich fluids from intrusive rocks in the oceanic lithosphere (Labidi et al., 2020;Wang et al., 2018), and redox reactions during the formation of serpentine from mantle olivine and pyroxene generate substantial amounts of H 2 (McCollom & Bach, 2009). Production of H 2 by serpentinization has been identified at mid-ocean ridges (Cannat et al., 2010;Kelley et al., 2005;Konn et al., 2015), as well as on-land (Etiope & Sherwood Lollar, 2013) and in the shallow forearc of subduction zones (Mottl et al., 2003;Ohara et al., 2012). Metagenomic studies of serpentinization-fueled hydrothermal deep-sea vents and continental fluid seeps provide evidence for microbial H 2 and CH 4 utilization in these harsh environments where nutrient and electron acceptor availability may be limited by high pH, which in turn depends on the temperature of the system (Brazelton et al., 2012;Curtis et al., 2013;Ohara et al., 2012;Schrenk et al., 2013). ...
... Biologically mediated respiration of OM can occur ex situ, and 13 C-depleted carbon can be transported via the percolating fluid to the system and in situ by organisms inhabiting the ultramafic rocks. Recent microbiological and metagenomic studies showed that microbial life can be sustained within the lithosphere and that microbial utilization of H 2 , CH 4 , and formate may be common in different serpentinization systems (Brazelton et al., 2012;Curtis et al., 2013;Daae et al., 2013;Etiope & Sherwood Lollar, 2013;Lang et al., 2018;Ohara et al., 2012;Schrenk et al., 2013). Furthermore, recent studies on drillcores from Site BA1B and BA3A showed cell counts ranging from as high as 10 7 cells g −1 in heavily veined areas down to 10 1 cells g −1 in the interior of some peridotites . ...
Article
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A large part of the hydrated oceanic lithosphere consists of serpentinites exposed in ophiolites. Serpentinites constitute reactive chemical and thermal systems and potentially represent an effective sink for CO2. Understanding carbonation mechanisms within ophiolites are almost exclusively based on studies of outcrops, which can limit the interpretation of fossil hydrothermal systems. We present stable and radiogenic carbon isotope data that provide insights into the isotopic trends and fluid evolution of peridotite carbonation in ICDP Oman Drilling Project drill holes BA1B (400‐m deep) and BA3A (300‐m deep). Geochemical investigations of the carbonates in serpentinites indicate formation in the last 50 kyr, implying a distinctly different phase of alteration than the initial oceanic hydration and serpentinization of the Samail Ophiolite. The oldest carbonates (∼31 to >50 kyr) are localized calcite, dolomite, and aragonite veins, formed between 26°C and 43°C and related to focused fluid flow. Subsequent pervasive small amounts of dispersed carbonate precipitated in the last 1,000 years. Macroscopic brecciation and veining of the peridotite indicate that carbonation is influenced by tectonic features allowing infiltration of fluids over extended periods and at different structural levels such as along fracture planes and micro‐fractures and grain boundaries, causing large‐scale hydration of the ophiolite. The formation of dispersed carbonate is related to percolating fluids with δ¹⁸O lower than modern ground and meteoric water. Our study shows that radiocarbon investigations are an essential tool to interpret the carbonation history and that stable oxygen and carbon isotopes alone can result in ambiguous interpretations.
... The presence of CH 4 -pure fluid inclusions is particularly interesting and indicates that such fluids are generated under reduced conditions, coherent to the fluids sustaining gold transport in bisulfide complexes (Goldfarb et al., 2005;Phillips and Powell, 2010;Pokrovski et al., 2014, Goldfarb andGroves, 2015). Several geological events are prone to contribute with CH 4 to the ore-forming fluids, from biogenic to abiotic sources (Etiope and Sherwood Lollar, 2013). One commonly considered source of CH 4 is the thermal degradation of organic matter by the reactions between metamorphic-hydrothermal fluids and organic-rich and carbonate-rich rocks, during regional metamorphism (Fan et al., 2004;Etiope and Sherwood Lollar, 2013;Gaboury, 2013;Zhang et al., 2019). ...
... Several geological events are prone to contribute with CH 4 to the ore-forming fluids, from biogenic to abiotic sources (Etiope and Sherwood Lollar, 2013). One commonly considered source of CH 4 is the thermal degradation of organic matter by the reactions between metamorphic-hydrothermal fluids and organic-rich and carbonate-rich rocks, during regional metamorphism (Fan et al., 2004;Etiope and Sherwood Lollar, 2013;Gaboury, 2013;Zhang et al., 2019). Such reactions can be the main CH 4 supplier at the Monfurado gold prospect, owing to the metamorphism and metasomatism that affected the Escoural Fm. metasediments, and the Monfurado Fm. carbonate-rich units. ...
Article
Full-text available
The Escoural gold district belongs to the Montemor-Ficalho metallogenic belt which is part of the Portuguese section of Ossa-Morena Zone (OMZ), at the SW of Iberia. The Escoural gold district includes twelve gold prospects and/or deposits largely controlled by the NW-SE Montemor-o-Novo Shear Zone (MNSZ) and associated fault zones, extending for approximately 30 km. Ubiquitously, gold-arsenopyrite-loellingite assemblages hosted in quartz-sericite-chlorite veins are found in most deposits, although, in the Monfurado prospect, the gold-bearing assemblages are more complex. This prospect is located in the vicinity of a Cambrian SEDEX-VMS iron deposit, from which massive and disseminated iron-ores hosted in marbles and calcsilicate rocks, were exploited. The interplay of the gold mineralizing processes with the iron-rich host rocks has favored gold deposition at the Monfurado prospect. Selected samples from six drill cores allowed to define two mineralizing events: the pre-ore and ore stages. Two gold mineralization styles characterize the ore-stage: i) massive sulfide horizons in which gold (Au= 85.6 - 86.3 wt. %; Ag= 13.1 - 13.6 wt.%) is hosted in arsenopyrite and pyrite or, seldomly, gold particles (Au= 91.8 wt.%; Ag= 7.1 wt.%) found in an arsenopyrite-rich layer; and ii) quartz-chlorite-pyrite veins crosscutting acid metavolcanic rocks with rhyolite-rhyodacite affinities, in which gold (Au= 80.5 - 82.9 wt.%; Ag= 16.8 - 18.7 wt.%) is found as fracture filling in pyrite, sometimes accompanied by Bi-Te phases. Arsenopyrite geothermometer suggests that for type i the overall deposition temperature falls within the range of 188°C to 372°C. Type ii mineralization lacks arsenopyrite, and for this reason, thermodynamic constraints were gathered from fluid inclusions and chlorite geothermometer. CH4-rich fluid inclusions are ubiquitous in transgranular fluid inclusion planes, suggesting that reduced fluids percolated the rocks that host type ii mineralization. The reduced fluids support the transport of gold in sulfide complexes, such as AuHS- and Au(HS)-2. Furthermore, secondary H2O-NaCl fluid inclusions (Lw2) were found, with mean salinities of 6.0 eq. w(NaCl) and mean homogenization temperature of 226°C, with corresponding pressures of 3.0 MPa, thus suggesting late hydrostatic regimes. Chlorite geothermometer results are in the range of 229 °C and 309 °C, agreeing with the fluid inclusion homogenization temperatures for Lw2 fluids. Sulfur isotope (δ34S) analysis of representative sulfide phases collected from both mineralization types, revealed signatures ranging from 8.5 ‰ and 10.6 ‰, indicating a single sulfur source. The gathered results suggest that although fluid transport is structurally controlled by MNSZ activity, the sulfidation reactions promoted by fluid-rock interactions are the main control on gold deposition from type i mineralization. It is further suggested that a coeval gold-event can lead to the deposition of two different types of mineralization, related to distinct gold deposition mechanisms. The tectonic and geodynamic settings in which the Escoural gold district developed correlate it to worldwide Palaeozoic orogenic gold deposits, with the Monfurado prospect being an example of the complexity of such geological settings.
... (Brovarone et al., 2020). A carbonate methanation reaction produce CH 4 by the reduction of carbonate minerals with H 2 at a temperature range 250−800 • C (Etiope and Sherwood Lollar, 2013), and may also be a potential formation mechanism of abiotic CH 4 during deep serpentinization. Abiotic CH 4 synthesis during deep serpentinization at temperatures >200 • C is compatible with our observations and therefore can be the potential candidates for dominant formation process of Hakuba Happo CH 4 . ...
... Several processes at temperatures over 500 • C have been proposed for abiotic CH 4 synthesis in the mantle, including the hydrolysis of metal carbides (Etiope and Sherwood Lollar, 2013). Here, we define 'mantle-derived CH 4 ' as methane formed in the mantle. ...
Article
Methane (CH4) associated with marine and terrestrial sites of serpentinization has been proposed to be abiotic in origin. However, the source of carbon and the depth and temperature of CH4 synthesis often remain inconclusive. We measured the radiocarbon (¹⁴C), bulk stable isotope ratios (δ13C and δD) and isotopologue abundance (Δ13CH3D) of CH4, and noble gas isotope composition of gas samples from two hyperalkaline geothermal wells (Happo #1 and Happo #3) at Hakuba Happo, Japan, to constrain the source of carbon and the CH4 generation processes. The CH4 samples from both sites were nearly ¹⁴C-free, whereas the carbonate precipitates inside the Happo #1 well contained ¹⁴C corresponding to 51 to 62 percent modern carbon, indicating that the majority of CH4 was not generated from the reduction of dissolved carbonate in the hot spring water. CH4 samples from Happo #1 and Happo #3 yielded Δ13CH3D-based apparent temperatures of 206−40+52 °C and 323−85+143 °C, respectively, which are much higher than the measured well water temperatures (∼50 °C). Therefore, ¹⁴C and clumped isotopologue temperatures suggest that Hakuba Happo CH4 was generated below the depth where the shallow meteoric water circulated. The ³He/⁴He ratios were 4.10 and 4.47 Ratm for Happo #1 and Happo #3, respectively. The ³He/⁴He and ⁴He/²⁰Ne ratios revealed that approximately 50% of He was of mantle origin, suggesting that the Hakuba Happo hot spring received volatiles, including CO2, from the mantle. However, the observed δ13CCH4 values (approximately −35‰) were significantly lower than those of typical mantle-derived carbon (–5‰), implying that mantle-derived CO2 is not the major carbon source for CH4 formation. High CH4 concentrations in the Hakuba Happo fluids, compared to those in hot springs in the area not associated with serpentinization, suggest that CH4 was generated from ¹³C-depleted fossil carbon sources and serpentinization-derived H2 at high temperatures (>200 °C), and subsequently entrained into the cooler circulating meteoric water system.
... δ 2 H and δ 13 C values of CH 4 sampled from each site in this study along with other sites of terrestrial serpentinization are plotted on the DC plot ( Figure 7) along with the empirically derived microbial, thermogenic, and abiogenic fields. The relative positions of these fields have been derived from previous studies (Cumming et al., 2019;Etiope & Sherwood Lollar, 2013). This figure illustrates that while there is some overlap between the fields (specifically the abiotic field and thermogenic field), non-microbial and microbial methane can still be differentiated (Cumming et al., 2019). ...
... Corresponding values for δ 2 H versus δ 13 C for methane from the Chimaera seep in Turkey (Etiope et al., 2011), the Samail Ophiolite in Oman (Miller et al., 2016), the Zambales ophiolite in the Philippines, Hakuba Happo in Japan (Suda et al., 2014), and CROMO in CA, USA (Wang et al., 2015) are also plotted. Empirically derived fields for microbial, thermogenic, and abiogenic methane production, as proposed by Etiope and Sherwood Lollar (2013) and references therein, are included for comparison. ...
Article
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Although the Earth's subsurface hosts an abundance of microbial life, the influence of geochemistry on these communities remains poorly constrained. Ophiolites, sites where oceanic ultramafic minerals can be hydrated to serpentine minerals and metal oxides, create unique conditions capable of sustaining life. The fluid geochemistry of the Tablelands (NL, CAN), The Cedars (CA, USA), and Aqua de Ney (CA, USA), were studied to better characterize the range of fluid compositions observed at terrestrial sites of serpentinization. Fluids from these sites shared many commonalities including being ultra‐basic and reducing as well as having elevated levels of Cl⁻, Na⁺, K⁺, and Br⁻ and depleted concentrations of Mg²⁺. They also exhibited a wide range of geochemistry. Isotopic and compositional data suggested the CH4 from The Cedars was a mixture of microbial and non‐microbial sources while the CH4 from the Tablelands was non‐microbial in origin. Aqua de Ney was the only site where the CH4 plotted in the abiogenic field. Despite being a known product of serpentinization, no H2 was detected at Aqua de Ney, likely due to the formation of abiogenic CH4 as well as the reaction of H2 and SO4²⁻ in the system to produce H2S. These unique sites of terrestrial serpentinization help to better understand the range of geochemistry at sites of serpentinization and its influence on the microbial communities in the subsurface.
... The results show that the bacterial community functions with the highest abundance are chemoheterotrophy, aerobic_chemoheterotrophy, animal_parasites_or_symbionts, hu-man_pathogens and aromatic_compound_degradation ( Figure 4). The main function is to degrade macromolecular organic matters, i.e., organic matter is broken down by fermentation bacteria into substrates available to methanogens (e.g., hydrogen, carbon dioxide, Acetic acid, methanol and dimethyl sulfide), which is also identified by the negative correlation between archaeal richness and 13 δC DIC , as 13 δC DIC is depleted due to fractionation during the decomposition of organic matters [43]. ...
Article
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Geothermal systems represent discrete and relatively homogenous habitats for extremophiles; investigation into the microbial community is key to revealing the geochemical environment and the geochemical evolution of fluids in geothermal reservoirs. The reservoir of the Lindian geothermal field in Northeast China, is highly reducing and rich in methane, but the pathways of methane generation and the related microbial community structure are still unclear. In this research, five thermal water samples were collected and tested, and the microbial community structure and diversity were analyzed. The results show that in the sandstone reservoir belonging to the low-temperature (reservoir temperature < 90°C) brackish water (total dissolved solids concentration between 1000 and 10,000 mg/L) environment, the richness of the microbial community is relatively high. The microbial community structure is different from other geothermal systems reported but similar to that of oilfields, which may be related to the highly reducing geochemical environment with abundant organic matter. According to the analysis of archaeal function, the biogas production in the Lindian geothermal field is dominated by hydrogen nutrition type methane production, while the H2 reducing methylamine type methane production is secondary, and results of Pearson correlation show that the archaeal communities are more strongly correlated to physicochemical factors than the bacterial communities.
... Oils generated through thermal cracking can also undergo microbial degradation, resulting in secondary hydrocarbon gases (Head et al., 2003;Etiope et al., 2009;Milkov, 2011). In addition, CH 4 can be of abiogenic origin and is produced in different geologic environments under a wide range of temperature and pressure during magmatic, volcanic, and high-temperature hydrothermal processes (Giggenbach, 1997;Etiope and Sherwood, 2013;Wen et al., 2016); however, in these cases methane is often not the primary gas present; CO 2 is much more abundant. ...
Article
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The continental margins of the East Siberian Sea and Arctic Ocean are among the Earth’s most inaccessible marine environments for hydrocarbon research due to the almost year-round presence of ice cover. Despite this, limited preliminary assessments which have been carried out to date have all yielded some indication of high oil and gas production potential in these regions. This article presents the results of gas-geochemical studies of seafloor sediments of the East Siberian Sea, obtained in three expeditions onboard the R/V “Akademik Lavrentiev” in 2008 (LV45), 2016 (LV77), and 2020 (LV90). The composition of sorbed hydrocarbon gases in seafloor sediments was analyzed. In addition, the stable isotopic composition of carbon was determined for CH4 , C2H6 , and CO2 in gases, which were desorbed from marine sediments. The sediments were also analyzed for organic matter content. Despite the absence of observable gas seepage directly into the water column, at some stations, increased concentrations of methane and hydrocarbon gases were encountered, indicating the widespread predominance of thermogenically derived gases. We present a hydrocarbon classification system which delineates eight identifiable sources of regional gas occurrences (coal gas, igneous rocks, solid bitumen, condensate-gas, gas-condensate, oil gas, gas oil, and oil gases). A stable isotopic analysis of carbon in CH4 , C2H6 , and CO2 indicates varying degrees of mixing between a shallow, early-kerogen gas source and a deeper mantle carbon source in some areas of the study.
... Such biological conversion of hydrogen to CH4 has been observed in both in subsurface 'town gas' storage sites (Buzek et al. 1994) and in deep mines where drilling introduces microbes that convert geologically produced hydrogen into methane (Warr et al. 2021). Hydrogen may also be consumed during abiotic polymerisation reactions to produce methane and higher alkanes via processes such as Fischer-Tropsch type reactions (Etiope and Sherwood Lollar 2013). Temporally these reaction rates will vary, with microbial consumption of hydrogen likely to be faster compared to larger-scale geological process reactions. ...
... While the abiotic production of these gases is disfavored on terrestrial worlds with temperate surfaces (e.g., see Seager et al. 2013;Sousa-Silva et al. 2020), mass-radius degeneracies (e.g., Guimond & Cowan 2018) and uncertainties concerning the location and thermodynamic state of an unconstrained surface layer may challenge biosignature interpretations for H 2 -rich super-Earths. Even on temperate terrestrial planets, there are substantial abiotic sources of CH 4 such as serpentization reactions (Etiope & Sherwood-Lollar 2013), and the atmospheric mixing ratio of even smaller abiotic fluxes may be enhanced when considering the photochemical impact of M dwarf stars' lower NUV fluxes (Segura et al. 2005;Seager et al. 2013;Schwieterman et al. 2019). Confirming the biogenicity of these potential biosignature gases will rely on additional contextual information, such as characterizing the surface state of the planet or detecting biosignature pairs like CO 2 -CH 4 to confirm a chemical disequilibrium consistent with biology (Krissansen-Totton et al. 2018;Wogan & Catling 2020). ...
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The first potential exoplanet biosignature detections are likely to be ambiguous due to the potential for false positives: abiotic planetary processes that produce observables similar to those anticipated from a global biosphere. Here we propose a class of methylated gases as corroborative `capstone' biosignatures. Capstone biosignatures are metabolic products that may be less immediately detectable, but have substantially lower false positive potential, and can thus serve as confirmation for a primary biosignature such as O$_2$. CH$_3$Cl has previously been established as a biosignature candidate, and other halomethane gases such as CH$_3$Br and CH$_3$I have similar potential. These gases absorb in the mid infrared at wavelengths that are likely to be captured while observing primary biosignatures such as O$_3$ or CH$_4$. We quantitatively explore CH$_3$Br as a new capstone biosignature through photochemical and spectral modeling of Earth-like planets orbiting FGKM stellar hosts. We also re-examine the biosignature potential of CH$_3$Cl over the same set of parameters using our updated model. We show that CH$_3$Cl and CH$_3$Br can build up to relatively high levels in M dwarf environments and analyze synthetic spectra of TRAPPIST-1e. Our results suggest that there is a co-additive spectral effect from multiple CH$_3$X gases in an atmosphere, leading to increased signal-to-noise and greater ability to detect a methylated gas feature. These capstone biosignatures are plausibly detectable in exoplanetary atmospheres, have low false positive potential, and would provide strong evidence for life in conjunction with other well established biosignature candidates.
... Isotopic data relating to CH4 are in the limit between thermogenic and volcanic-geothermal systems area [104]. However, as proposed by [105]), post-magmatic reactions could form CH4 by successive reactions with CO2, H2O, and metal oxides at temperatures of 200-500 °C. ...
Article
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Dihydrogen (H2) is generated by fluid–rock interactions along mid-ocean ridges (MORs) and was not, until recently, considered as a resource. However, in the context of worldwide efforts to decarbonize the energy mix, clean hydrogen is now highly sought after, and the production of natural H2 is considered to be a powerful alternative to electrolysis. The Afar Rift System has many geological features in common with MORs and offers potential in terms of natural H2 resources. Here, we present data acquired during initial exploration in this region. H2 contents in soil and within fumaroles were measured along a 200 km section across the Asal–Ghoubbet rift and the various intervening grabens, extending from Obock to Lake Abhe. These newly acquired data have been synthesized with existing data, including those from the geothermal prospect area of the Asal–Ghoubbet rift zone. Our results demonstrate that basalt alteration with oxidation of iron-rich facies and simultaneous reduction in water is the likely the source of the hydrogen, although H2S reduction cannot be ruled out. However, H2 volumes at the surface within fumaroles were found to be low, reaching only a few percent. These values are considerably lower than those found in MORs. This discrepancy may be attributed to bias introduced by surface sampling; for example, microorganisms may be preferentially consuming H2 near the surface in this environment. However, the low H2 generation rates found in the study area could also be due to a lack of reactants, such as fayalite (i.e., owing to the presence of low-olivine basalts with predominantly magnesian olivines), or to the limited volume and slow circulation of water. In future, access to additional subsurface data acquired through the ongoing geothermal drilling campaign will bring new insight to help answer these questions.
... However, CO 2 and H 2 are produced in much larger quantities (up to 96% of all radiolytic gases) than hydrocarbons. Future work should also determine the isotopic signatures of these gases including δ 13 C CO2 , δD H2 , and δD CH4 , which could provide additional parameters for understanding the genesis of hydrocarbon gases (3). Clumped isotopes (Δ18 values of methane, Δ47 of CO 2 , and ΔDD of H 2 ) and position-specific isotope analyses may be used to calculate formation temperatures (59)(60)(61)(62) and might help to elucidate gas formation mechanisms in future laboratory experiments on radiolytic hydrocarbons. ...
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This is an open access article. You can read it free of charge here https://www.pnas.org/doi/epdf/10.1073/pnas.2114720119 Significance: Natural gas is a key fossil fuel as the world transitions away from coal toward less polluting energy sources in an attempt to minimize the impact of global climate change. Historically, the origin of natural gas produced from conventional reservoirs has been determined based on gas compositional data and stable isotope fingerprints of methane, ethane, and higher n -alkanes, revealing three dominant sources of natural gas: microbial, thermogenic, and abiotic. In our detailed synthesis of published natural gas data from a variety of unconventional hydrocarbon reservoirs worldwide, we demonstrate that there is a previously overlooked source of natural gas that is generated by radiolysis of organic matter in shales.
... curiosity-detects-unusually-high-methane-levels). This is an extraordinary discovery considering there are only two natural sources to the production of biogas: 1) microbial communities and 2) the interaction of some rocks and water (in this case, frozen water) (Etiope and Sherwood Lollar, 2013;Dean et al., 2018). If we hypothesize that methane gas production is due to the presence of microbial communities, the only known microorganisms capable of withstanding Martian conditions are Archaea. ...
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Archaea are a unique system for investigating the diversity of life. There are the most diverse group of organisms with the longest evolutionary history of life on Earth. Phylogenomic investigations reveal the complex evolutionary history of Archaea, overturning longstanding views of the history of life. They exist in the harshest environments and benign conditions, providing a system to investigate the basis for living in extreme environments. They are frequently members of microbial communities, albeit generally rare. Archaea were central in the evolution of Eukaryotes and can be used as a proxy for studying life on other planets. Future advances will depend not only upon phylogenomic studies but also on a better understanding of isolation and cultivation techniques.
... However, sulfate and/or sulfide minerals were not reported in the bedrock (see section 2.1) that can be a source of H 2 S, and the concentration of sulfate in groundwater was low (Table 1). In addition, there was no plume with a high biomass content or petroleum to form biotic CH 4 , and ultramafic rocks were not rich enough to produce abiotic CH 4 in the study area (Etiope and Sherwood Lollar, 2013;Kietäväinen and Purkamo, 2015). ...
Article
Unusual episodic fluctuations of electrical conductivity (EC) were observed twice a year in a national groundwater monitoring network well in South Korea where EC was automatically monitored at a depth of 20 m below ground level (bgl). To address the causes of the observed EC fluctuations, this study examined the depth profile of wellbore water in the 70-meter-deep monitoring well screened between 50 and 70 m bgl and cased down to 50 m bgl. The results of well logging, borehole video recording, and hydrochemical analysis of wellbore water indicated that the CO2-rich groundwater entering through the screened zones below 50 m bgl was physicochemically stratified into three layers with distinct EC that were separated by two transition zones in the well: a bottom layer (70−43 m bgl) with an EC of ∼3900 μS/cm, intermediate layer (35−24 m bgl) of ∼1800 μS/cm, and top layer (16−3 m bgl) of ∼300 μS/cm. The first transition zone at depths of 43−35 m bgl was attributed to CO2 exsolution in the open system and the subsequent physicochemical changes of wellbore water, while the second transition zone at depths of 24−16 m bgl was formed by the precipitation of hydrous ferric oxides with consequent sorption of remaining ions due to a sudden change toward the oxidizing environment. The monitoring probe installed at a depth of 20 m bgl was found to be located within the upper transition zone, which caused EC peaks when the well was purged at a depth of 25 m bgl for well maintenance twice a year. This study shows that automated groundwater monitoring systems may misguide one about the groundwater quality if an unexpected physicochemical variation (such as stratification) occurs in a monitoring well. Therefore, the presence of interface zones caused by abrupt changes in EC must be carefully considered when an automated monitoring well is designed. The screened zone is a suitable location for installing an automated monitoring probe to measure the representative water quality (i.e., EC), in particular, in an aquifer that is under the influence of inputs of low-pH and high-TDS fluids (e.g., CO2-rich groundwater and acid mine drainage).
... As granite rocks are enriched with uranium (Langmuir, 1997), H 2 production by the reaction of water with radiation from radionuclides, such as 238 U or 40 K, is associated with radiolysis (Lin et al., 2005). In granitic rocks, abiotic methane from magmatic processes is ubiquitous long after formation (Etiope and Sherwood Lollar, 2013;Kietäväinen and Purkamo, 2015). However, it remains unknown whether methane can serve as a major energy source in the deep granitic environment. ...
Article
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Rocks that react with liquid water are widespread but spatiotemporally limited throughout the solar system, except for Earth. Rock-forming minerals with high iron content and accessory minerals with high amounts of radioactive elements are essential to support rock-hosted microbial life by supplying organics, molecular hydrogen, and/or oxidants. Recent technological advances have broadened our understanding of the rocky biosphere, where microbial inhabitation appears to be difficult without nutrient and energy inputs from minerals. In particular, microbial proliferation in igneous rock basements has been revealed using innovative geomicrobiological techniques. These recent findings have dramatically changed our perspective on the nature and the extent of microbial life in the rocky biosphere, microbial interactions with minerals, and the influence of external factors on habitability. This study aimed to gather information from scientific and/or technological innovations, such as omics-based and single-cell level characterizations, targeting deep rocky habitats of organisms with minimal dependence on photosynthesis. By synthesizing pieces of rock-hosted life, we can explore the evo-phylogeny and ecophysiology of microbial life on Earth and the life’s potential on other planetary bodies.
... Methane origin is generally biotic (Hunt, 1996;Clayton, 2005), including (1) thermal degradation of organic matter in sedimentary rocks (thermogenic gas), (2) metabolic reactions by certain microorganisms (microbial gas), and (3) biomass burning (pyrogenic gas). In some geological environments, methane can also be generated by chemical reactions in the absence of organic matter (abiotic gas; Etiope and Sherwood Lollar, 2013;Etiope, 2017). Microbial gas generated in sedimentary rocks in petroleum systems is fossil (radiocarbon-free), so as thermogenic gas, and can also be categorized as "geological methane." ...
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Methane (CH4) emissions to the atmosphere from the oil and gas sector in Romania remain highly uncertain despite their relevance for the European Union's goals to reduce greenhouse gas emissions. Measurements of CH4 isotopic composition can be used for source attribution, which is important in top-down studies of emissions from extended areas. We performed isotope measurements of CH4 in atmospheric air samples collected from an aircraft (24 locations) and ground vehicles (83 locations), around oil and gas production sites in Romania, with focus on the Romanian Plain. Ethane to methane ratios were derived at 412 locations of the same fossil fuel activity clusters.The resulting isotopic signals (d13C and d2H in CH 4) covered a wide range of values, indicating mainly thermogenic gas sources (associated with oil production) in the Romanian Plain, mostly in Prahova county (d13C from -67.8 ± 1.2 to -22.4 ± 0.04 ‰ Vienna Pee Dee Belmnite; d2H from -255 ± 12 to -138 ± 11 ‰ Vienna Standard Mean Ocean Water) but also the presence of some natural gas reservoirs of microbial origin in Dolj, Ialomiţa, Prahova, and likely Teleorman counties.The classification based on ethane data was generally in agreement with the one based on CH4 isotopic composition and confirmed the interpretation of the gas origin. In several cases, CH4 enhancements sampled from the aircraft could directly be linked to the underlying production clusters using wind data. The combination of d13C and d2H signals in these samples confirms that the oil and gas production sector is the main source of CH4 emissions in the target areas. We found that average CH4 isotopic signatures in Romania are significantly lower than commonly used values for the global fossil fuel emissions. Our results emphasize the importance of regional variations in CH4 isotopes, with implications for global inversion modeling studies.
... This points to the presence of an active biological system below the ice, but its importance for modifying CH 4 emission to the atmosphere is still unknown. The slope is smaller than what has earlier been attributed to oxidation of dCH 4 (a = 8.6-9; Burns et al., 2018;Etiope & Sherwood Lollar, 2013) and while this indicates that oxidation of subglacial CH 4 takes place, the lower slope for gCH 4 we find suggests additional isotope fractionation processes could impact the isotopic signature of gCH 4 . Future research will focus on understanding what drives the deviation between the isotopic signature of gCH 4 and dCH 4 as it has implications for interpretation of the origin of subglacial CH 4 . ...
Article
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Direct gaseous emissions of methane (CH4) and carbon dioxide (CO2) from the subglacial environment under Greenland Ice Sheet (GrIS) were only recently discovered and it is yet to be determined how important it is for the panarctic carbon budget. We measured in situ net gaseous emissions of subglacial CH4 and CO2, dissolved concentrations and isotopic composition of gases (¹³C and ²H) at the onset, near maximum, and at the end of the melt season in 2018 and 2019. We found a tight relation between gaseous and dissolved CH4 and CO2, respectively, indicating that degassing from the subglacial meltwater is the main source of these gases in the subglacial air. The diurnal variability of in situ mole fractions of CH4 and CO2 in subglacial air was related to meltwater runoff showing that the net emission magnitude is directly related to glacial hydrology. We observed that maximum in situ mole fractions of CH4 and CO2 appeared at the onset of the melt season and decreased over the melt season. The isotopic signature of CH4 in the subglacial air indicated that it likely originated from microbial methanogenesis which remained constant during the season. Isotopic signatures of subglacial CO2 indicate mixed sources from microbial oxidation of CH4, remineralization of sedimentary organic carbon, and possibly influenced by removal of CO2 by weathering. Our study indicate large emissions of both CO2 and CH4, but continuous studies over entire melt seasons are needed to determine the origin and emission magnitudes and their relation to the glacial dynamics.
... Gutsalo and Plotnikov (1981) proposed the first genetic diagram based on the carbon isotopic compositions of methane and carbon dioxide (d 13 C -CO2 vs. d 13 C -CH4 ), which grouped the gases into abiogenic gas, microbial gas and thermogenic gas. Milkov 2011 added to the diagram the genetic region of secondary microbial gases from oil biodegradation, and Etiope et al added new region of abiogenic CH 4 to this genetic diagram (Etiope and Lollar, 2013;Etiope and Schoell, 2014;Etiope, 2017). Milkov and Etiope (2018) revised the three plots based on the data from more than 20,000 natural gas samples published in recent decades, updating a more detailed classification of gas origins ( Figure 5). ...
Article
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With more natural gas hydrate samples recovered and more research approaches applied to hydrate-associated gas studies, data concerning the geochemical characteristics of hydrate-associated gases have been increased significantly in the past decades. Although systematic reviews of hydrocarbons are available, fewer studies have focused on the systematic classification of gas hydrates, yet. In this study, the primary origins and secondary processes that affect the geochemical characteristics of the gases are discussed. The primary origins are affected mainly by the type and /or maturity of the organic matter, which determine the main signature of the gas is microbial gas or thermogenic gas in a broad scheme. Apart from primary origins, secondary processes after gas generation such as migration, mixing, biodegradation and oxidation occur during the migration and/or storage of gases can significantly alter their primary features. Traditional methods such as stable isotope and molecular ratios are basic proxies, which have been widely adopted to identify these primary origins and secondary processes. Isotopic compositions of C2+ gases have been employed to identify the precursor of the gases or source rocks in recent years. Data from novel techniques such as methane clumped isotope and noble gases bring additional insights into the gas origins and sources by providing information about the formation temperature of methane or proxies of mantle contribution. A combination of these multiple geochemical approaches can help to elucidate an accurate delineation of the generation and accumulation processes of gases in a gas hydrate reservoir.
... Large amounts of methane (i.e., and higher order hydrocarbons) are present at continental margins either as gas hydrates or in the gas phase within the sedimentary cover. Methane is particularly abundant in the Earth's lithosphere because it is generated by both organic (i.e., decomposition of organic matter either by microbes or by high temperatures in buried formations) and inorganic processes related to chemical transformations at ultra-high temperatures within the lithosphere (e.g., Etiope & Sherwood Lollar, 2013). The release of these greenhouse gases into the ocean and the atmosphere is part of the Earth carbon budget (e.g., Etiope et al., 2008;Ruppel & Kessler, 2017). ...
Article
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Strong compressive and shear stresses generated by glacial loading and unloading have a direct impact on near‐surface geological processes. Glacial stresses are constantly evolving, creating stress perturbations in the lithosphere that extend significant distances away from the ice. In the Arctic, periodic methane seepage and faulting have been recurrently associated with glacial cycles. However, the evolution of the Arctic glacial stress field and its impact on the upper lithosphere have not been investigated. Here, we compute the evolution in space and time of the glacial stresses induced in the Arctic lithosphere by the North American, Eurasian and Greenland ice sheets during the latest glaciation. We use glacial isostatic adjustment (GIA) methodology to investigate the response of spherical, viscoelastic Earth models with varying lithospheric thickness to the ice loads. We find that the GIA‐induced maximum horizontal stress (σH) is compressive in regions characterized by thick ice cover, with magnitudes of 20–25 MPa in Fennoscandia and 35–40 MPa in Greenland at the last glacial maximum. Simultaneously, a tensile regime with σH magnitude down to −16 MPa dominates across the forebulges with a mean of −4 MPa in the Fram Strait. At present time, σH in the Fram Strait remains tensile with an East‐West orientation. The evolution of GIA‐induced stresses from the last glaciation to present could destabilize faults along tensile forebulges, for example, the west‐coast of Svalbard. A more tensile stress regime as during the Last Glacial Maximum would have more impact on pre‐existing faults that favor gas seepage from gas reservoirs.
... Isotope ratios of carbon ( 13 C/ 12 C) and hydrogen (D/H) have been used to distinguish methane source pathways (Etiope and Sherwood Lollar, 2013;Schoell, 1988;Whiticar, 1999Whiticar, , 2020, as well as sink mechanisms (Barker and Fritz, 1981;Coleman et al., 1981;Whiticar, 1999Whiticar, , 2020. Microbially-produced methane is strongly depleted in the heavy carbon and hydrogen isotopes (Claypool and Kaplan, 1974;Rayleigh, 1896;Whiticar, 2020). ...
Article
Aerobic oxidation of methane (MOx) is an important biologically mediated process that consumes methane in a wide range of environments. Here we report results of culture experiments with the aerobic methane-oxidizing bacterium Methylosinus trichosporium (OB3b) that are used to characterize the mass-18 isotopologue (Δ13CH3D and Δ12CH2D2) signatures of MOx in residual methane gases. MOx activity was confirmed by simultaneous decrease of methane and oxygen in the bulk gas headspace. Bulk carbon (¹³C/¹²C) and hydrogen (D/H) isotope ratios of the methane gas increased while both Δ13CH3D and Δ12CH2D2 decreased as the oxidation proceeded. The corresponding fractionation factors (α) calculated from our experimental results are 0.98485 ± 0.00006 for ¹³C/¹²C, 0.7265 ± 0.0010 for D/H, 0.7141 ± 0.0011 for ¹³CH3D/¹²CH4, and 0.4757 ± 0.0023 for ¹²CH2D2/¹²CH4. Deviations of the mass-18 fractionation factors from the Rule of the Geometric Mean (RGM) expressed as γ values are 0.9981 ± 0.0017 for ¹³CH3D/¹²CH4 and 0.9013 ± 0.0045 for ¹²CH2D2/¹²CH4. Our α and γ values suggest that while MOx fractionates ¹³CH3D within error of the RGM, the Δ13CH3D and Δ12CH2D2 trajectories are very sensitive to even small deviations in ¹³CH3D/¹²CH4 from the RGM. Fractionation of ¹²CH2D2 deviates considerably from RGM, and this causes dramatic and robust effects on the trajectories of residual methane in Δ13CH3D vs. Δ12CH2D2 space. Our models suggest that Δ13CH3D and Δ12CH2D2 could potentially mimic microbial methanogenesis signatures in an environment that exhibits a strong Rayleigh Distillation process with little to no replenishment of methane during oxidation. However, in closed or open systems where oxidation is attended by simultaneous methane production, we find that modest increases in Δ13CH3D and dramatic increases in Δ12CH2D2 are to be expected, thus resulting in isotopologue signatures distinct from microbial methanogenesis. The overall trend in these conditions suggest that methane altered by MOx is distinguishable from other methane sources in Δ13CH3D and Δ12CH2D2 space.
... While both bulk and clumped isotopic compositions of methane can help identify the source(s) of methane, some factors complicate the interpretation of the isotopic signatures. For example, overlapping isotopic signatures in d 13 C and dD often lead to ambiguous source identifications (e.g., Schoell, 1988;Whiticar, 1990Whiticar, , 1999Pohlman et al., 2009;Etiope and Sherwood, 2013), and some microbial methane samples from surface environments (e.g., freshwater lakes, swamps, and cow rumen) have yielded unreasonably high temperature estimates for clumped isotopologue equilibrium (e.g., Stolper et al., 2015;Wang et al., 2015;Douglas et al., 2017;Young et al., 2017). In particular, there are significant discrepancies between the bulk and clumped isotopic signatures observed in natural samples of microbial methane and those produced by laboratory cultures that presumably use the same metabolic pathway (Stolper et al., 2015;Wang et al., 2015;Douglas et al., 2016;Okumura et al., 2016;Young et al., 2017;Gruen et al., 2018;Giunta et al., 2019;Douglas et al., 2020). ...
Article
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Stable isotope analysis has been widely used to aid the source identification of methane. However, the isotopic (¹³C/¹²C and D/H) and isotopologue (¹³CH3D and ¹²CH2D2) signatures of microbial methane in natural environments are often different from those in laboratory cultures in which methanogens are typically grown under optimal conditions. Growth phase and hydrogen (H2) concentration have been proposed as factors controlling the isotopic compositions of methane produced via hydrogenotrophic methanogenesis, but their effects on the relationship among carbon, hydrogen and doubly-substituted “clumped” isotopologue systems have not been assessed in a quantitative framework. Here we experimentally investigate the bulk (δ¹³C and δD) and clumped (Δ¹³CH3D) isotopologue compositions of methane produced by hyperthermophilic hydrogenotrophic (CO2-reducing) methanogens using batch and fed-batch systems at different growth phases and H2 mixing ratios (Methanocaldococcus bathoardescens at 82 or 60 °C and on 80 or 25% H2; Methanothermobacter thermautotrophicus ΔH at 65 °C and on 20, 5 or 1.6% H2). We observed a large range (18 to 63‰) of carbon isotope fractionations, with larger values observed during later growth phase, consistent with previous observations. In contrast, hydrogen isotope fractionations remained relatively constant at –317 ± 25‰. Linear growth was observed for experiments with M. bathoardescens, suggesting that dissolution of gaseous H2 into liquid media became the rate limit as cell density increased. Accordingly, the low (and undersaturated) dissolved H2 concentrations can explain the increased carbon isotope fractionations during the later growth phase. The δD and Δ¹³CH3D values indicated departure from equilibrium throughout experiments. As the cell density increased and dissolved H2 decreased, Δ¹³CH3D decreased (further departure from equilibrium), contrary to expectations from previous models. Our isotopologue flow network model reproduced the observed trends when the last H-addition step is less reversible relative to the first three H-addition steps (up to CH3-CoM). In this differential reversibility model, carbon, hydrogen and clumped isotopologue fractionations are largely controlled by the reversibility of the first three H-addition steps under high H2 concentrations; the last H-addition step becomes important under low H2. The magnitude of depletion and decreasing trend in Δ¹³CH3D values were reproduced when a large (≥6‰) secondary clumped kinetic isotope effect was considered in the model. This study highlights the advantage of combined bulk and clumped isotope analyses and the importance of physiological factors (growth phase) and energy availability (dissolved H2 concentration) when using isotope analyses to better understand methanogenic metabolisms and methane cycling processes.
Article
It is hard to determine secondary microbial gas from deep burial reservoirs with multiple generations of petroleum charge. Herein, oils at depths from 5000 m to 6500 m in the Ordovician carbonate rocks, Tarim basin, are found to have been biodegraded to secondary microbial gas. These oils experienced severe biodegradation and thus contain abundant 25-norhopanes and 17-nortricyclic terpanes. The associated gases have methane δ¹³C1 from −51.9‰ to −47.3‰ and δ²H1 from −327.8‰ to −192.4‰, and CO2 δ¹³CCO2 from −0.7‰ to +15.3‰. These features suggest that the gases are secondary microbial gas, generated predominantly via CO2 reduction with preferential reduction of ¹³C-depleted CO2 and contribution of methane hydrogen from formation water in closed environments. The secondary microbial gas may have generated from biodegradation of oils at reservoir temperatures of about <75°Cduring the Late Permian, and has subsequently mixed with a later charge of non-biodegraded oils and wet gas during the Late Cretaceous. Consequently, the present gas shows relatively low dryness (C1/∑C1–4 < 0.87) and has varied δ¹³C1 and δ²H1 values in methane. The study implies that the signatures of secondary microbial gas can easily be masked by thermogenic gas and thus more secondary microbial gas has yet to be identified.
Thesis
L’existence de fluides géologiques riches en hydrogène (H2) doit nécessairement faire l’objet de travaux d’exploration afin de statuer sur le potentiel énergétique de cette éventuelle ressource décarbonée. Depuis plus d’un siècle de nombreuses exhalations naturelles d’H2 ont été mis en évidence. Or à ce jour il n’existe aucun guide d’exploration basé sur une méthodologie et sur des indicateurs robustes. La détection d’occurrence gazeuse en surface correspond bien évidemment à l’approche la plus efficace et la plus rapide à mettre en œuvre pour identifier des flux. Il n’en reste pas moins qu’un flux ne constitue pas une ressource pour autant, puisqu’à ce jour, l’homme n’exploite que les stocks de ressources énergétiques fossiles. Il sera donc important de développer un guide d’exploration non pas orienté uniquement sur une problématique de surface, mais aussi sur des considérations géologiques profondes intégrant le système hydrogène dans son entier de la source au piège ou à la fuite dans l’atmosphère.Au cours de ce travail de thèse nous proposons d’utiliser le cadre géologique du piémont nord Pyrénéen pour élaborer un guide d’exploration. La compilation des données bibliographiques a révélé un contexte prometteur pour un système H2 du fait d’un lien entre sources profondes, chemins de migration crustale, dynamique de circulation de fluides, et pièges sédimentaires. En effet le nord-ouest des Pyrénées et plus particulièrement le Bassin Mauléon est caractérisé par la présence i) d’un corps mantellique (<10 km) où les conditions pression-température sont favorables à la serpentinisation ; ii) d’accidents structuraux majeurs tels que le Chevauchement Frontal Nord Pyrénéens (CFNP) constituant des drains collecteurs de grande ampleur, iii) des gradients hydrauliques, conjugués à des gradients de température et de pressions qui permettent la mise en mouvement des fluides ; iv) des formations sédimentaires imperméables ou de couvertures comme les évaporites ou les argiles consitutant des pièges pour accumuler l’H2.Suite à cette étude préalable, nous avons mis en place une campagne d’analyses des gaz du sol (H2, CO2, 222Rn, O2, CH4) à l’échelle régionales. Cette campagne d’analyse réalisée sur plus de 7500 km2 a très vite permis de mettre en évidence une zone à très fortes anomalies en H2, CO2, et 222Rn sur le pourtour du Bassin de Mauléon. Cette découverte nous a permis de resserrer rapidement le maillage de prospection sur la partie nord du bassin de Mauléon. Une campagne d’analyses géochimiques et géophysiques a été réalisée à Sauveterre-de-Béarn afin de déterminer l’origine et le parcours des gaz à l’origine de cette anomalie. Sur la base de l'analyse des gaz du sol et des levés électromagnétiques, nous avons confirmé l'existence d'une faille drainant les fluides profonds. De plus, l’étude des données historiques des forages entrepris dans la région il y a plus de 50 ans, conjugué à une mise en perspective des dernières connaissances géologiques et géophysiques de la région, nous a permis de mettre en évidence des zones où l’H2 pourrait s’accumuler.Enfin une partie expérimentale de broyage de quartz et de roches de la région a été menée afin d’explorer de nouveaux mécanismes de production d’H2 le long des failles. Nous avons mis en évidence une très forte influence du rapport eau/roche (W/R) et du pH sur la production d’H2. Ces découvertes apportent un éclairage nouveau sur les mécanismes mécano-radicalaires de production d’H2 où la spéciation des sites des surface des minéraux sont des paramètres clés contrôlant la production d’H2. Nous révélons pour la première fois que le broyage du quartz en présence de solutions carbonatés induit la formation d’espèces carboxylates (formate, acétate, oxalate). En plus de produire de l’H2, les mécanismes mécano-radicalaires permettent donc de produire des espèces réduites du carbones pouvant constituer une source d’énergie pour les écosystèmes microbiens lithotrophe de subsurface.
Chapter
Most reservoir engineering tools, in terms of theory, were developed over 50 years ago. At the same time, many petroleum engineering practices have been deemed unsustainable, making it difficult for today's engineers to develop or design sustainable technologies. To make the situation more challenging, even the term ‘sustainable petroleum development’ was considered to be an oxymoron. Even today, the way the term ‘sustainability’ is used, it makes it difficult to discuss petroleum operations and sustainability in the same vein. The notion of ‘carbon’ is the source of ‘climate emergency’ is not helpful nor is it science based (The science of climate change, Wiley and Scrivener, 2019). In this chapter, a comprehensive sustainability criterion is presented, which is followed for developing fluid and rock characterization. This sets the stage for sustainable development of petroleum reservoirs.
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Carbon capture and storage (CCS) is a key technology to mitigate the environmental impact of carbon dioxide (CO 2 ) emissions. An understanding of the potential trapping and storage mechanisms is required to provide confidence in safe and secure CO 2 geological sequestration 1,2 . Depleted hydrocarbon reservoirs have substantial CO 2 storage potential ¹ , ³ , and numerous hydrocarbon reservoirs have undergone CO 2 injection as a means of enhanced oil recovery (CO 2 -EOR), providing an opportunity to evaluate the (bio)geochemical behaviour of injected carbon. Here we present noble gas, stable isotope, clumped isotope and gene-sequencing analyses from a CO 2 -EOR project in the Olla Field (Louisiana, USA). We show that microbial methanogenesis converted as much as 13–19% of the injected CO 2 to methane (CH 4 ) and up to an additional 74% of CO 2 was dissolved in the groundwater. We calculate an in situ microbial methanogenesis rate from within a natural system of 73–109 millimoles of CH 4 per cubic metre (standard temperature and pressure) per year for the Olla Field. Similar geochemical trends in both injected and natural CO 2 fields suggest that microbial methanogenesis may be an important subsurface sink of CO 2 globally. For CO 2 sequestration sites within the environmental window for microbial methanogenesis, conversion to CH 4 should be considered in site selection.
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In this study, the composition of fluid inclusions in minerals from polymetallic sulfide deposits Ashadze-1, Semenov-2, Krasnov, Rainbow and from the Lost City carbonate structures of hydrothermal fields in the Atlantic Ocean was determined using pyrolysis-free gas chromatography–mass spectrometry. It was found out that sulfides and anhydrite from the hydrothermal ore crystallized in reducing conditions with the active participation of hydrocarbons, including high-molecular, sulfonated, nitrogenated and halogenated compounds as well as water and carbon dioxide. On the other hand, carbonate structures formed in considerably more oxidizing conditions, for which the main components of volatiles were СО2 and Н2О. The amount of detected hydrocarbons, including sulfonated, nitrogenated and halogenated compounds is significantly smaller compared to sulfides of ore vent structures.
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Experiments were conducted to distinguish oil cracking in reservoir rock versus source rock. Oil and source rock samples were heated in pressure vessels at 380 °C for 72 hours, which resulted in oil cracking to gas and pyrobitumen with some residual liquid. The oil samples were heated with different minerals (fine-grained quartz, calcium carbonate, montmorillonite, kaolinite and illite) employing different ratios of the oil to the mineral in each case. Heating oil with quartz or calcium carbonate was used to simulate oil cracking in reservoirs, while heating oil with clay minerals was used to simulate oil cracking within source rocks. Based on the experiments, oil cracking in reservoirs versus source rocks can be differentiated by relative concentrations of the prominent C7 hydrocarbons: n-heptane, methylcyclohexane and methylbenzene (toluene) in the liquid products. The light hydrocarbon distribution in the final cracking products is affected by the “matrix effect” from clay minerals and the surrounding medium in reservoirs and source rocks during oil cracking. No relationship between the types of marine source rock or total organic content (TOC) and the distribution of light liquid hydrocarbons generated by catalysis on clay minerals at high temperature was observed. Future studies are needed to evaluate different types of source rock (e.g., terrigenous versus marine, clastic versus calcareous), and the relationship between light liquid hydrocarbon yield and the quantity of each clay mineral.
Chapter
The glaciated Greenland continental margins contain favorable conditions for hydrate formation if gas is present. No gas hydrates have been encountered in the drilling of offshore wells, however, and only a limited focus has been placed on academic-led hydrate research to date. Nevertheless, analyses of 2D and 3D seismic reflection data have revealed the occurrence of BSRs, DHIs, chimneys and pockmarks. These seismic features all suggest the presence of gas and gas hydrates within three different sections of the Greenland margin. Seismic amplitude observations in Melville Bay, offshore northwest Greenland, indicate the existence of a ~220 m thick gas hydrate deposit over a 50 m high gas column. It is suggested that the paleo-topography of the area has forced the migration of fluid into the overlying stratigraphy. In the Disco area, offshore central West Greenland, seismic observations together with heatflow measurements and sediment core samples suggest that gas and gas hydrates exist in regions with sub-cropping Cretaceous to Paleocene strata and in areas covered by thick postglacial sediments. Finally, 2D seismic reflection data indicate gas and gas hydrate deposits of potentially abiotic origin within the northeast Greenland margin and Molloy Basin, adjacent to the ocean spreading systems in the Fram Strait.
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Molecular dynamics simulation of the pressure-density-temperature properties of supercritical methane (CH4) are made with the COMPASS II force field model in the range of 200–3000 K, 0.1–3.0 GPa, and 0.22–0.668 g·cm⁻³, where 710 states are simulated using NPT ensemble, and 212 states are simulated using NVT ensemble. These results are in good agreement with experimental data and the calculated results from highly accurate reference model of Setzmann and Wagner (J Phys Chem Ref Data 20:1061–1155, 1991) and its extrapolation in the region where the reference model can be validated. The simulation results are calibrated with the reference model. The calibrated simulations results and the reference model are used simultaneously to develop an accurate cubic equation of state for supercritical CH4 in the range of about 300–3000 K and 0–3 GPa (0–0.53 g·cm⁻³). The equation are tested against experimental and simulated data at high temperatures and pressures. Compared with the overwhelming majority of experimental results, the volume deviations are within 0.4 % to 1.1 %, with averages of about 0.1 % to 0.4 %; Compared with the molecular simulation results in literature and this work, the volume deviations are within 0.6 % to 3.7 %, with averages of about 0.1 % to 1.2 %. The equation can accurately predict the fugacity coefficients, residual enthalpies, and entropies and other thermodynamic properties.
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Objectives: This study was designed to investigate the frequency of computed tomography features indicating progression of portal hypertension and their clinical relevance in patients who experienced acute cellular rejection after liver transplantation. Materials and methods: This retrospective study included 141 patients with pathologically diagnosed acute cellular rejection following liver transplant. Patients were divided into early and late rejection groups according to the time of diagnosis. Two radiologists analyzed the interval changes in spleen size and variceal engorgement on computed tomography images obtained at the times of surgery and biopsy. Aggravation of splenomegaly and variceal engorgement were considered computed tomography features associated with the progression of portal hypertension. Clinical outcomes, including responses to treatment and graft survival, were compared between patients with and without these features. Results: The frequency of progression of portal hypertension was 31.9% and did not differ significantly in patients who experienced early (30.8% [28/91]) and late (34.0% [17/50]) rejection (P = .694). In the late rejection group, computed tomography features indicating progression of portal hypertension were significantly associated with poor response to treatment (P = .033). Graft survival in both the early and late rejection groups did not differ significantly in patients with and without progression of portal hypertension. Conclusions: Computed tomography features suggesting the progression of portal hypertension were encountered in about one-third of patients who experienced acute cellular rejection after liver transplant. Progression of portal hypertension was significantly related to poor response to treatment in the late rejection group.
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The search for life on exoplanets is motivated by the universal ways in which life could modify its planetary environment. Atmospheric gases such as oxygen and methane are promising candidates for such environmental modification due to the evolutionary benefits their production would confer. However, confirming that these gases are produced by life, rather than by geochemical or astrophysical processes, will require a thorough understanding of planetary context, including the expected counterfactual atmospheric evolution for lifeless planets. Here, we evaluate current understanding of planetary context for several candidate biosignatures and their upcoming observability. We review the contextual framework for oxygen and describe how conjectured abiotic oxygen scenarios may be testable. In contrast to oxygen, current understanding of how planetary context controls non-biological methane (CH4) production is limited, even though CH4 biosignatures in anoxic atmospheres may be readily detectable with the James Webb Space Telescope. We assess environmental context for CH4 biosignatures and conclude that abundant atmospheric CH4 coexisting with CO2, and CO:CH4 ≪ 1 is suggestive of biological production, although precise thresholds are dependent on stellar context and sparsely characterized abiotic CH4 scenarios. A planetary context framework is also considered for alternative or agnostic biosignatures. Whatever the distribution of life in the Universe, observations of terrestrial exoplanets in coming decades will provide a quantitative understanding of the atmospheric evolution of lifeless worlds. This knowledge will inform future instrument requirements to either corroborate the presence of life elsewhere or confirm its apparent absence. Any detection of potential biosignature molecules like oxygen and methane needs to be put into the planetary environmental context to understand its actual importance. Such a contextual approach is also essential when considering alternative or agnostic biosignatures on planets and exoplanets.
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The search for life on exoplanets is motivated by the universal ways in which life could modify its planetary environment. Atmospheric gases such as oxygen and methane are promising candidates for such environmental modification due to the evolutionary benefits their production would confer. However, confirming that these gases are produced by life, rather than by geochemical or astrophysical processes, will require a thorough understanding of planetary context, including the expected counterfactual atmospheric evolution for lifeless planets. Here, we evaluate current understanding of planetary context for several candidate biosignatures and their upcoming observability. We review the contextual framework for oxygen and describe how conjectured abiotic oxygen scenarios may be testable. In contrast to oxygen, current understanding of how planetary context controls non-biological methane (CH$_4$) production is limited, even though CH$_4$ biosignatures in anoxic atmospheres may be readily detectable with the James Webb Space Telescope. We assess environmental context for CH$_4$ biosignatures and conclude that abundant atmospheric CH$_4$ coexisting with CO$_2$, and CO:CH$_4$ << 1 is suggestive of biological production, although precise thresholds are dependent on stellar context and sparsely characterized abiotic CH$_4$ scenarios. A planetary context framework is also considered for alternative or agnostic biosignatures. Whatever the distribution of life in the Universe, observations of terrestrial exoplanets in coming decades will provide a quantitative understanding of the atmospheric evolution of lifeless worlds. This knowledge will inform future instrument requirements to either corroborate the presence of life elsewhere or confirm its apparent absence.
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The existence of geological fluids rich in natural hydrogen (H2) raises the question about the energy potential of this carbon-free resource. However, to date there is no exploration strategy based on robust methodologies and pathfinders. Therefore, it is important to develop an exploration guide that is not only focused on surface gas monitoring, but that also considers the local deep geological setting integrating the entire hydrogen system from source to trap or leakage into the atmosphere. The northwestern Pyrenees, and particularly the Mauléon Basin, represent a promising geological environment for natural H2 exploration for at least four reasons. First, an ultramafic mantle body is emplaced at shallow depth below the basin under pressure-temperature conditions favorable to serpentinization. Second, major faults such as the North Pyrenean Frontal Thrust constitute large-scale fluid flow convergence and drainage. Third, hydraulic gradients imposed by sharp reliefs and combined with temperature and pressure gradients trigger fluid migration. Fourth, impermeable sedimentary formations or caprocks such as evaporites or claystones overly porous reservoir rocks that could constitute traps for accumulating H2. To investigate H2 migration at the fault scale, we present new geochemical and geophysical data recorded along the North Pyrenean Frontal Thrust. Based on both soil gas and electromagnetic transects, we reveal the presence of a gas-draining fault. Soil gas concentration (H2, CO2, CH4 and ²²²Rn) recorded at 1 m depth increases when approaching the North Pyrenean Frontal Thrust. The maximum H2, CO2 and ²²²Rn concentrations recorded in the fault zone are 822 ppmv, 10.3 vol% and 57 kBq.m⁻³, respectively - whereas their local background concentrations are by 1–2 orders of magnitude lower: 10 ppmv, 0.2 vol% and 0.3 kBq.m⁻³ respectively. Our geochemical and geophysical data support the concept of a deep-fluid migration along the detected fault plane. In addition, the study of historical well data combined with the most recent geological and geophysical surveys carried out in the region, highlights zones where H2 could accumulate at depth. The Triassic salt formations, located at 2800 m–4000 m deep beneath the Mauléon Basin, represent the most promising trap for H2 in the northwestern Pyrenees.
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Abiotic H2 produced in the Precambrian lithospheric crust is a key substrate at the base of the metabolic chain of chemosynthetic and photosynthesis-independent microbial communities, significant to our understanding of life on early Earth and other planets. H2 cycling processes are also relevant to recent hydrogen exploration efforts and engineered subsurface environments such as radioactive waste disposal sites. In the lithospheric crust, H2 is produced through water-rock reactions (serpentinisation) and radiolysis; the latter directly linked to He through radioelement decay (U, Th). The Witwatersrand Basin in South Africa is an ideal place to study the radiolytic production pathway in particular, because of the low abundance of ultramafic and mafic minerals and therefore low potential for serpentinisation reactions. Gas samples and gas flow rate data (n = 12) were collected from the surface of exploration boreholes tapping the Witwatersrand and Ventersdorp Supergroups. The samples were predominantly composed of CH4 (65–99%), N2 (3–27%), He (0.1–15%), and trace amounts of C2+ hydrocarbons. Notably, H2 in these samples was below detection limit, despite the presence of He - providing a critical indicator of processes removing H2 from the system. Using a Bayesian modelling approach, we test the hypothesis that the observed fluids are generated in-situ, driven by radioelement decay and subsequent microbial methanogenesis, and controlled by porosity of the host rock. The observed data is consistent with this hypothesis, and can be accounted for by a variation in porosity between 0.3 and 2.2% (typical values to Precambrian basement) across the different sampling sites. These He-rich hydrocarbon gases observed at the surface originate from a hydrogeological system that is porosity-constrained and isolated from externally-sourced fluids. Radioelement decay is the primary process driving the generation of H2 and therefore energy production in this subsurface system, utilised by hydrogenotrophic methanogens at the base of the deep carbon cycle. Microbial utilisation is the key mechanism for H2 consumptions and, conversely, preservation, suggesting that conditions favourable to commercial H2 discoveries are likely constrained to hypersaline environments where microbial activity is inhibited.
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Stable isotope analysis has been widely used to aid the source identification of methane. However, the isotopic (13C/12C and D/H) and isotopologue (13CH3D and 12CH2D2) signatures of microbial methane in natural environments are often different from those in laboratory cultures in which methanogens are typically grown under optimal conditions. Growth phase and hydrogen (H2) concentration have been proposed as factors controlling the isotopic compositions of methane, but their effects on the relationship among carbon, hydrogen and doubly-substituted “clumped” isotopologue systems have not been assessed in a quantitative framework. Here we experimentally investigate the bulk (δ13C and δD) and clumped (∆13CH3D) isotopologue compositions of methane produced by hyperthermophilic hydrogenotrophic (CO2-reducing) methanogens using batch and fed-batch systems at different growth phases and H2 mixing ratios (Methanocaldococcus bathoardescens at 82 or 60 °C and on 80 or 25% H2; Methanothermobacter thermautotrophicus ∆H at 65 °C and on 20, 5 or 1.6% H2). We observed a large range (18 to 63‰) of carbon isotope fractionations, with larger values observed during later growth phase, consistent with previous observations. In contrast, hydrogen isotope fractionations remained relatively constant at –317 ± 25‰. Linear growth was observed for experiments with M. bathoardescens, suggesting that dissolution of gaseous H2 into liquid media became the rate limit as cell density increased. Accordingly, the low (and undersaturated) dissolved H2 concentrations can explain the increased carbon isotope fractionations during the later growth phase. The δD and Δ13CH3D values indicated departure from equilibrium throughout experiments. As the cell density increased and dissolved H2 decreased, Δ13CH3D decreased (further departure from equilibrium), contrary to expectations from previous models. Our isotopologue flow network model reproduced the observed trends when the last H-addition step is less reversible relative to the first three H-addition steps (up to CH3-CoM). In this differential reversibility model, carbon, hydrogen and clumped isotopologue fractionations are largely controlled by the reversibility of the first three H-addition steps under high H2 concentrations; the last H-addition step becomes important under low H2. The magnitude of depletion and decreasing trend in Δ13CH3D values were reproduced when a large (≥6‰) secondary clumped kinetic isotope effect was considered in the model. This study highlights the advantage of combined bulk and clumped isotope analyses and the importance of physiological factors (growth phase) and energy availability (dissolved H2 concentration) when using isotope analyses to aid the source identification of methane.
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Finding evidence of life beyond Earth is the aim of future space missions to icy moons. Icy worlds with an ocean underlying the icy crust and in contact with a rocky subsurface have great astrobiological interest due to the potential for water-rock interactions that may provide a source of nutrients necessary to sustain life. Such water-rock interactions in icy moons can be indirectly investigated using analogous environments on the deep seafloor on Earth. Here, we investigate the presence of molecular and isotopic biomarkers in two submarine cold seep systems with intense rock-fluid interactions and carbon sink as carbonates with the aim of gaining understanding of potential carbon cycles in the icy worlds' oceans. Authigenic carbonates associated to cold seeps (a chimney from the Gulf of Cádiz and a clathrite from the Pacific Hydrate Ridge) were investigated for their mineralogical composition and lipid biomarker distribution. Molecular and compound-specific isotopic composition of lipid biomarkers allowed us to infer different carbonate origins in both carbonate scenarios: biogenic methane (clathrite) versus thermogenic methane together with allochthonous carbon (chimney). In the Pacific cold seep, carbonate precipitation of the clathrite was deduced to result from the anaerobic oxidation of methane by syntrophic action of methanotrophic archaea with sulfate-reducing bacteria. The distinct carbon sources (thermogenic methane, pelagic biomass, etc.) and sinks (gas clathrates, clathrite, chimney carbonates) were discussed in the light of potentially similar carbon cycling pathways in analogous icy-moon oceans. We show how the isotopic analysis of carbon may be crucial for detecting biosignatures in icy-world carbon sinks. These considerations may affect the strategy of searching for biosignatures in future space missions to the icy worlds.
Article
A conceptual model of the thermal and water recharge of the Ketkinsky geothermal field as a product of magma and water injection from the Koryaksky volcano located 24 km apart was proposed. A digital hydrogeological model of the Ketkinsky geothermal field was developed in the volume of 7 km x 5 km x 2.5 km (from the topographic surface), it includes the space drilled by exploration and production wells. The model is based on an analysis of 3D distributions of temperature, pressure, salinity and CH4 content, geometrization of productive faults and well productivity characteristics. The geofiltration space was zoned in the model with separation of deep and shallow productive geothermal reservoirs, the area of deep thermal fluid upflow in the SSE part of the model base and the area of hidden thermal water discharge at the ground surface. A natural state inversion iTOUGH2-EWASG simulation was performed to estimate the deep thermal water upflow and permeability of productive geothermal reservoirs. The deep thermal water upflow is estimated to be about 10 kg/s, the permeability is estimated to be 190 mD (shallow productive reservoir) and 35 mD (deep productive reservoir). Inverse iTOUGH2-EWASG modeling of the hydrodynamic operating history of 1989–2020 was used to estimate the compressibility of the productive geothermal reservoirs: the compressibility of the deep reservoir is estimated at 7.16E-10 Pa⁻¹, the shallow reservoir at 4.14E-07 Pa⁻¹. Direct iTOUGH2-EWASG modeling with the above parameters reproduces the history of salinity and temperature changes in production wells. Forecast modeling of existing producing wells #23, K6, K01, K5 operation for 25 years with application of submersible pumps at immersion depth of 70 m confirms the possibility of their sustainable operation with total flow rate not less than 14.2 kg/s, adding four producing wells may yield to 54.3 kg/s with retaining of produced water quality (temperature, gas content of CH4, salinity). The use of submersible pumps and reinjection can significantly increase the reserves of Ketkinsky field to 165–175 kg/s of 70–80 °C and 60–70 g/s of CH4. Additional increase in reserves may be obtained by drilling the already known thermal anomaly in the SSE sector of the field and in the SWW foothills of Koryaksky volcano.
Preprint
Orbital missions have shown that some regions of the ancient Martian crust contain hundreds of discrete terrains covered by chloride evaporites. In terrestrial evaporitic systems, aqueous precipitation typically begins with the deposition of carbonates, followed by sulfates, and finally chlorides, a depositional sequence that has not yet been found on Mars. Instead, sulfide deposits are always separated spatially from chlorides and appear younger. This temporal and spatial separation suggests two different depositional regimes. Here, we present a model driven by the Martian chlorine geochemical cycle that allows the formation of chlorides whilst simultaneously inhibiting sulfate and carbonate precipitation. In this model, the chlorides are formed under reducing and acidic conditions in two contrasting settings the hydrothermal alteration of the Martian crust associated with the formation of faults: by direct evaporation of water bodies fed by meteoric solutions (type 1) and by precipitation from ascending saline solutions along the tectonic conduits (type 2). Such processes were sustained under a relatively thick and reducing atmosphere (1-0.1 bar) similar to the predicted conditions for the early Earth. The crustal circulation of chloride-precipitating fluids may have been driven by tectonic suction and pumping processes.
Chapter
With the increasing world population and huge demand for fossil fuels, the emission of CO2 into the atmosphere has reached a level of about 417 ppm. Such an enormous amount of greenhouse gas on our planet has been the primary source to cause climate change and environmental devastations, including droughts, floods, wildfires, hurricanes, etc. The depletion of petroleum resources and stringent ecological regulation at present must inspire the whole world to work jointly for the development of an alternative and green fuel to replace fossil fuels. The top agenda to overcome this global concern is the sequestration of carbon from pre-and post-combustion as well as oxyfuel combustion. Thus, this chapter comprehensively reviews the current progress and advancements of CO2 conversion into valuable fuels, including methane, dimethyl ether, methanol, gasoline, and others. However, further intensive research is needed to obtain economic and commercially viable conversion operations.
Article
Abiotic methane (CH4) generation under subduction zone conditions has been experimentally investigated through aqueous reduction of pure C-bearing materials (e.g. carbonate minerals and organic matter). However, quantitative assessments of CH4 production in these experiments, as well as the potential effects of other components such as silica (Si) on the reduction processes, have not yet been well established. Here, we performed experiments to quantitatively evaluate the time-resolved Ca-carbonate aqueous reduction into CH4 at P = 1 and 2 GPa, and T = 550 oC in the CaO + COH, CaO + SiO2 + COH, and CaO + SiO2 + MgO + COH systems, employing calcite + water ± quartz ± serpentine (synthetic chlorine (Cl)-bearing chrysotile and natural Fe−Al-bearing antigorite) as starting materials. Redox conditions of the experiments were buffered by iron−wüstite (IW) using a double capsule setting, corresponding to oxygen fugacity (fO2) values (expressed as log units relative to the fayalite–magnetite–quartz buffer, ΔFMQ) in the inner capsule of ΔFMQ ≈ −5.5 at 1 GPa and ΔFMQ ≈ −6.0 at 2 GPa. The solid products are mainly composed of portlandite ± larnite ± wollastonite ± brucite, while Ca-carbonate and/or silicate reactants commonly occur as relicts. Quadrupole mass spectrometric analysis shows that CH4 and H2O are the major COH molecular species in the fluid products, with molar ratios between CH4 and starting calcite representing the reaction progress ranging from ~0.13 to ~1.00. Comparisons of experimental run products with thermodynamically predicted phase assemblages, together with time-series experiments, indicate that the reduction processes are primarily controlled by reaction kinetics. At 1 GPa and 550 oC, rate constants of 4.0 × 10-6 s-1, 7.4 × 10-6 s-1, and 2.6 × 10-6 s-1 were retrieved for reactions starting with calcite + quartz + water, calcite + synthetic Cl-bearing chrysotile + water, and calcite + natural Fe−Al-bearing antigorite + water, respectively, significantly higher than the constant of 0.8 × 10-6 s-1 for the silicate-absent reaction. Besides, an increase in pressures can also enhance the reduction efficiency of Ca-carbonates until reaching equilibrium with the fluids. Our data provide experimental evidence for kinetics-controlled Ca-carbonate aqueous reduction into CH4 in subduction zones, indicating that silicate involvement and/or pressure increase can accelerate the reaction rates through short-lived fluid−rock interactions, which may have important implications for deep C mobility.
Article
Continental ultramafic rock systems, through the process of serpentinization, provide chemical and biochemical pathways that lead to the production of methane. The extent to which rock-water-gas reactions and organisms supply methane in these systems is a matter of considerable discussion and debate. Deciphering the interplay of abiotic and microbial methane observed at the surface requires several lines of reasoning as well as a variety of analyses. Despite using multiple models and interpretative tools, conclusions for the origin of methane at a particular site may vary or diverge from regional or global observations. Here, we critically address how possible conclusions of microbial versus abiotic methane in continental serpentinization systems may be interpreted and reinterpreted. We review fundamental concepts, advantages and limits, for three major methane origin models: (a) abiotic CO2 hydrogenation supplying gas reservoirs, (b) derivation from fluid inclusions in olivine-rich rocks, and (c) microbialgenesis in aquifers. We use the case of methane in the Samail ophiolite of Oman as an emblematic example of multiple interpretations; we identify ambiguous information offered by methane clumped isotopes and molecular gas compositions (e.g., the meaning of gaseous hydrocarbons heavier than methane), and suggest key tools, such as radiocarbon (¹⁴C) in methane, which may solve interpretative issues. The major constraint in any model of methane origin is the capability to sustain continuous gas flows, in terms of methane emission intensity, longevity and spatial extension, such as in natural gas sedimentary systems. Overall, this review suggests that any site interpretation can benefit from a holistic approach, integrating geochemical, geological and biological data with gas flow dynamics, as well as including regional and global contextualization.
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We have developed a model of geospatially estimating carbon accumulation and methanogenesis in seabed sediments that uses more accurate and sophisticated inputs to models than used in previous estimates. Using this hybrid stochastic and deterministic model, we estimate the maximum carbon available for methanogenesis in the global seabed, and subsequent microbial methane generated as a function of location and depth (including the gas hydrate stability zone). Global integration over present and previously microbially reactive sediments column yields total carbon and methane to be ∼0.8–2.2 × 10⁶ and 1.1–3.0 × 10⁶ Pg C and CH4, respectively. Our improvements to accuracy include using geospatially machine learned estimates of seafloor inputs to which the methanogenesis modeling is most sensitive (e.g., total organic carbon, heat flux, porosity). Our improvements to model sophistication include geospatially dependent modeling (on a 5 × 5 arc‐minute grid), a new model of sediment compaction (allowing for non‐linear geothermal gradients), and variable age versus depth at each grid cell. A carbon reservoir of the magnitude we estimate here is consistent with the recent IPCC suggestion that long‐term carbon sinks could explain imbalances in reduction of atmospheric CO2 over the last 50 million years. Our technique provides a foundation of using globally updateable machine learning parameters as the input to geologic and geochemical models, allowing for new observations to update global budgets of carbon available for methane, and subsequent total estimates of seabed methane.
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Distinguishing biotic compounds from abiotic ones is important in resource geology, biogeochemistry, and the search for life in the universe. Stable isotopes have traditionally been used to discriminate the origins of organic materials, with particular focus on hydrocarbons. However, despite extensive efforts, unequivocal distinction of abiotic hydrocarbons remains challenging. Recent development of clumped-isotope analysis provides more robust information because it is independent of the stable isotopic composition of the starting material. Here, we report data from a 13C-13C clumped-isotope analysis of ethane and demonstrate that the abiotically-synthesized ethane shows distinctively low 13C-13C abundances compared to thermogenic ethane. A collision frequency model predicts the observed low 13C-13C abundances (anti-clumping) in ethane produced from methyl radical recombination. In contrast, thermogenic ethane presumably exhibits near stochastic 13C-13C distribution inherited from the biological precursor, which undergoes C-C bond cleavage/recombination during metabolism. Further, we find an exceptionally high 13C-13C signature in ethane remaining after microbial oxidation. In summary, the approach distinguishes between thermogenic, microbially altered, and abiotic hydrocarbons. The 13C-13C signature can provide an important step forward for discrimination of the origin of organic molecules on Earth and in extra-terrestrial environments. Distinguishing biotic compounds from abiotic ones is critical to the search for life in the universe. Here, the authors demonstrate that the abiotic ethane has distinctively low 13C-13C abundances compared to biotic ethane.
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From a geological perspective, hydrogen has been neglected. It is not as common as biogenic or thermogenic methane, which are ubiquitous in hydrocarbon basins, or carbon dioxide, which is common in geologically active areas of the world. Nevertheless, small flows of hydrogen naturally reach the Earth’s surface, occur in some metal mines and emerge beneath the oceans in a number of places worldwide. These occurrences of hydrogen are associated with abiogenic and biogenic methane, nitrogen and helium. Five geological environments are theoretically promising for exploration based on field, palaeofluid and theoretical evidence: ophiolites (Alpine, Variscan and Caledonian in order of decreasing prospectivity), thinned-crust basins (failed-arm rifts, aulacogens), potash-bearing basins, basement in cratonic areas and the Mid-Atlantic ridge and its fracture zones. The subsurface areas of these environments are relatively poorly known, compared to hydrocarbon basins. Hydrogen shows may indicate larger reserves in the subsurface, in a similar way to the beginnings of hydrocarbon exploration in the 19th century. The main source of hydrogen is ultramafic rocks, which have experienced serpentinization, although other generation processes have been identified, including biogenic production of hydrogen during very early stages of maturation and radiolysis. There are two main tectonic settings where serpentinization has operated. The main accessible onshore areas are where ophiolites are found tectonically emplaced within fold belts. Potentially much larger investigation areas lie in the subsurface of some ophiolites. These areas generally lie outside hydrocarbon provinces. However, where thrusting has emplaced ophiolites over a hydrocarbon-bearing foreland basin, tests involving sub-thrust conventional hydrocarbon exploration plays could also be employed to search for hydrogen. The other main tectonic setting is in highly extended basins, for example failed rifts or aulacogens, where thick sediments overlie thinned or absent crust above probably serpentinized mantle. These structures occur offshore on continental margins and extend onshore into long-lived rifts which have been reactivated and rejuvenated repeatedly. Conventional seismic reflection data are already available in these areas, but deep subsurface resolution is poor where there are extensive volcanic rocks. Analogues of these occurrences are also found in the deep oceans, along the mid-ocean ridges and offsetting transform faults. Here, thin crust and faulting may facilitate serpentinization of the mantle rocks by seawater ingress. Further research should aim to identify the extent of the hydrogen flux and its probable dominant role in the abiogenic production of hydrocarbons in Precambrian times, a natural process now largely replaced by biogenic participation. A similar industrial process replicates serpentinization, producing hydrogen and ultimately liquid hydrocarbons on a commercial scale in some countries. It remains to be proved whether a contribution from exploration can be made to any future hydrogen economy.
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Iron carbides in association with native iron, graphite, and magnetite were identified in a crystal of diamond from the Juina area, Brazil, that contains a series of other, deep-mantle mineral inclusions. Among the iron carbides, Fe3C, Fe2C ("chalypite"), and Fe23C6 (haxonite) are present; the two latter phases are identified in the terrestrial environment for the first time. Some of the analyzed iron carbide grains contain 7.3-9.1 at.% N and are, in fact, nitrocarbide. We suggest, on the basis of the high-pressure mineral parageneses previously observed in the diamond and experimental data on the system Fe-C, that "chalypite" crystallized within a pressure interval of 50-130 GPa from an iron-carbon melt rich in nitrogen. Following crystallization, iron carbides and native iron were partially oxidized to magnetite, and encapsulated in diamond along with other high-pressure minerals. The finds of various iron carbides, some of which are rich in nitrogen, in lower-mantle diamond confirm a significant role of carbides and nitrogen in the Earth's interior.
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The occurrence and origin of methane (CH4) generated by serpentinization of ultramafic rocks is of current timely interest in planetary geology, astrobiology and energy resource exploration, as it may contribute, in particular, to decipher the source of methane on Mars, the origin of life and the potential of abiotic hydrocarbon synthesis. Methane of dominant abiotic origin in serpentinized peridotites on continents (ophiolites or igneous intrusions) has been documented so far, with complete carbon and hydrogen isotope composition, in six countries, in the Philippines, Turkey, Oman, New Zealand, Japan and Italy. We report the discovery of two new sites in Greece, at Archani and Ekkara, located in the Othrys ophiolite massif. Portable sensors based on Fourier Transform InfraRed spectrometry (FTIR) and Tunable Diode Laser Absorption Spectroscopy (TDLAS) allowed to realize that out of 21 ophiolitic springs, methane is released only by four hyperalkaline (pH from 10.7 to 11.3) and calcium hydroxide (Ca–OH) type waters; all other 17 springs with pH < 8.7 and magnesium-rich waters in the Pindos, Vourinos and Veria ophiolites, do not show methane. This correlation between gas occurrence and water type seems to occur worldwide; accordingly, CH4 production appears to be intimately related to the depth and residence time of the circulating meteoric waters. Methane is emitted into the atmosphere also from the soil surrounding the hyperalkaline springs, with fluxes of the same order of magnitude (~ 102–103 mg m− 2 day− 1) of seepage typically observed over conventional petroleum systems. Othrys CH4 has an isotopic composition (δ13C from − 27‰ to − 37.3‰ VPDB, δ2H from − 250‰ to − 311‰ VSMOW) similar to that reported in ultramafic rocks in New Zealand and Japan, and in Precambrian crystalline shields, which were considered dominantly abiotic and probably derived from Fischer–Tropsch Type reactions. The paucity of CO2, which is the norm in hyperalkaline waters, and of other hydrocarbons prevents from evaluating possible mixing of gas of different sources, including microbial methanogenesis. Also the H2 content is trivial, notwithstanding it being a typical product of serpentinization; this could be due to complete H2 consumption by CO2 reduction in a limited or decreased H2 production system due, for example, to a late stage of increased silica activity, as suggested by preliminary petrographic observations. The low geothermal gradient of the area and the present-day serpentinization imply that, whatever the CH4 production mechanism, it took place at temperatures below those traditionally considered for the origin of abiotic methane in hydrothermal systems.
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Light hydrocarbon gases (methane through pentane) in high-temperature hydrothermal fluids of the Guaymas basin, Gulf of California, are predominantly derived from thermocatalysis of organic carbon in sediments intruded by mid-ocean ridge volcanic rocks. The spectrum of Câ-Câ gases in Guaymas basin hydrothermal fluids is characterized by a preponderance of alkanes, with essentially no alkenes present. Comparison with high-temperature fluids from 21°N on the East Pacific Rise, where geochemical evidence indicates a distinct lack of thermogenic gas and where methane is derived abiogenically from the basalts, shows that the 21°N hydrocarbon gases are characterized by a prominent ethylene signature. Stable isotope compositions of Guaymas basin methane differ from 21°N gas; Guaymas basin has δ¹³C values of -43 to -51 {per thousand} vs. PDB, compared to 21°N where values as high as -15 {per thousand} have been measured. These differences reflect the different origins of light hydrocarbon gases in the two systems, with Guaymas methane being overwhelmingly of an organic, thermogenic derivation and showing no evidence of the abiogenic, basalt-derived gas that characterizes the 21°N environment. Carbon isotope analyses of dissolved inorganic carbon in Guaymas basin hydrothermal fluids indicate that high-temperature hydrocarbon oxidation may be an important process acting in conjunction with high-temperature thermocatalytic hydrocarbon generation. 4 figs., 6 tabs.
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Mars long has been considered a cold, dead planet. However, recent reports of methane in the Martian atmosphere suggest that methane currently is being produced, since its calculated atmospheric lifetime of 400 years or less requires a constant resupply. Possible subsurface sources for this resupply are geological, or even microbiological, in nature. So the question is: Is Mars alive, biologically or geologically speaking?
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Fischer–Tropsch synthesis (FTS) is the core technology for the sustainable transformation of nonpetroleum carbon resources into liquid fuels and valuable chemicals. FTS has received intensive interest and achieved great advances in recent years. Herein, we concisely describe the reaction mechanism, reactor, and catalyst relating to FTS, with special emphasis on the catalyst because of its critical role in FTS. This outline presents the basic feature of FTS and affords the basis for the rational design and development of new FTS technology.
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The chemistry and budgets of atmospheric gases are constrained by their bulk stable isotope compositions (e.g., δ^(13)C values), which are based on mixing ratios of isotopologues containing one rare isotope (e.g., 16O13C16O). Atmospheric gases also have isotopologues containing two or more rare isotopes (e.g., ^(18)O^(13)C^(16)O). These species have unique physical and chemical properties and could help constrain origins of atmospheric gases and expand the scope of stable isotope geochemistry generally. We present the first measurements of the abundance of ^(18)O^(13)C^(16)O from natural and synthetic sources, discuss the factors influencing its natural distribution and, as an example of its applied use, demonstrate how its abundance constrains the sources of CO_2 in the Los Angeles basin. The concentration of ^(18_O^(13)C^(16)O in air can be explained as a combination of ca. 1‰ enrichment (relative to the abundance expected if C and O isotopes are randomly distributed among all possible isotopologues) due to enhanced thermodynamic stability of this isotopologue during isotopic exchange with leaf and surface waters, ca. 0.1‰ depletion due to diffusion through leaf stomata, and subtle (ca. 0.05‰) dilution by ^(18)O^(13)C^(16)O-poor anthropogenic CO_2. Some air samples are slightly (ca. 0.05‰) lower in ^(18)O^(13)C^(16)O than can be explained by these factors alone. Our results suggest that ^(18)O^(13)C^(16)O abundances should vary by up to ca. 0.2‰ with latitude and season, and might have measurable sensitivities to stomatal conductances of land plants. We suggest the greatest use of Δ_(47) measurements will be to “leverage” interpretation of the δ^(18)O of atmospheric CO_2.
Chapter
Methane in 350°C hydrothermal fluids at 21°N on the East Pacific Rise occurs in concentrations greater than 1.1 cc (STP)/kg. Hydrogen concentrations vary from 8 to 38 cc(STP)/kg, showing a considerable range between different vent fields. Helium concentrations exceed 0.021 cc(STP)/kg. The injection rates of methane and hydrogen into the deep ocean indicate replacement times of the order of 30 years, implying that consumption of methane and hydrogen in the water column must be very rapid. Variations of end-member concentrations of methane, hydrogen and possibly helium, as well as δ13C(CH4), among vent fields suggests either chemical control of reactive gas abundances and/or variations in gas contents of ridge crest basalts. Measurements of methane and helium in basalt glass from the EPR show CH4/3He ratios of 2.5 × 106, compared to 3.5 × 106 in hydrothermal fluid from the same area. Carbon isotope evidence, CO2 CH4 isotope geothermometry, the lack of suitable thermocatalytic sources of organic carbon, and the similarity between CH4/3He ratios in these hydrothermal fluids and mid-ocean ridge basalts, point to an abiogenic origin of hydrothermal methane, extracted directly from basalt by circulating seawater.
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Raman spectroscopy shows that CH4, H2 and N2 are the main components in gas inclusions in eudialyte from the Lovozero massif of agpaitic nepheline syenites. Thermodynamic calculations show that gases of such composition can be obtained from aqueous fluids in equilibrium with magma at approx 400oC and 0.5-1 kbar. The loss of water may be due to hydration of rock-forming minerals and decomposition of the fluid into gas and liquid as the T is reduced. (Journal summary)-A.W.H.
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The origins of the hydrogen and hydrocarbon gases in alkaline complexes cannot be determined without researching the isotopic compositions of the main gas-forming elements, hydrogen and carbon. In the gas phase of the Lovozero rocks (Kola peninsula) the dominant methane and molecular hydrogen are accompanied by other saturated hydrocarbons, nitrogen and helium, and sometimes carbon monoxide and dioxide. The best suggestion appears to be that the gas phase was formed as such in the postmagmatic stage, e.g., during changes in the composition, structural state, and purity of the rock-forming minerals, which involved the mobilization of lattice-dissolved hydrocarbons in the micropores or else the splitting of the initial homogeneous fluid into two phases: an essentially aqueous (liquid) one and a reduced gaseous one at temperatures below 400°C. -from Journal translation
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The major components of geothermal gases are steam and carbon dioxide. At high temperatures, chemical and isotopic equilibrium will be established, but as the gases cool, equilibrium conditions are not maintained. Interaction of gases with wet sediments will result in a gas mixture consisting of methane, hydrogen, nitrogen and rare gases.
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Analyses of fluid inclusions in plutonic rocks recovered from the slow-spreading Southwest Indian Ridge (SWIR) record CH4 concentrations of 15-40 times those of hydrothermal vent fluids and of basalt-hosted volcanic gases and provide the first direct sampling of CO2-CH4-H2O-H2-C-bearing fluids in the oceanic crust. Compositional, thermal, and spatial analyses of these fluids are used to model the evolution of volatiles during the crystallization and cooling of mid-ocean ridge magma chambers and to assess the potential importance of carbon-bearing fluids in geochemical processes in the lower crustal component of hydrothermal systems. Results from these analyses show that the earliest fluids to be exsolved from the melts are dominated by CO2-rich vapors, which with progressive fractionation evolved to more H2O-rich compositions. These later fluids were most likely exsolved under immiscible conditions and involved the development of CO2-H2O-rich vapors and CO2-H2O-NaCl brines that were trapped during mineral growth, as well as during later high-temperature fracturing events. CO2+CH4±H2O-rich fluids in olivine and plagioclase minerals that contain up to 30-50 mol % CO2 and 33 mol % CH4 may reflect respeciation of magmatic CO2 during cooling and attendant graphite precipitation. Phase equilibria suggest that these fluids reequilibrated at ∼500°-600°C and at fO2's ∼ -3 log units below, to close to QFM conditions. Alternatively, the inclusions may record respeciation of magmatic fluids attendant with the inward diffusion of H2 into the inclusions and reduction of entrapped CO2 during degassing and cooling of the gabbros throughout the subsolidus regime. Subsequent seawater reaction, at minimum temperatures of ∼400°C, with mafic-rich layers within the gabbroic rocks or with ultramafic material that underlies the plutonic sequence resulted in the formation of CH4-H2O fluids that contain up to 40 mol % CH4, molecular H2, and graphite(?) daughter minerals. These data provide strong evidence that the CH4-H2O-rich fluids produced during serpentinization reactions were trapped under equilibrium conditions in the presence of graphite at very near to QFM conditions. The ubiquitous occurrence of CH4-rich fluids in oceanic crustal layer 3 rocks from the SWIR indicates that these fluids may be a significant and previously unrecognized source for these volatile species in some hydrothermal systems venting on the seafloor and that the deep-seated fluids most likely play an important role in the transfer of carbon from the lithosphere to the hydrosphere.
Article
A meteorite impact structure of Late-Proterozoic age has recently been recognized at Gardnos in the Hallingdal area, South Norway. The structure is situated in pre-1100 Ma gneisses and metasediments, and consists of a zone of autochtonous breccia, a suevite (impact melt) breccia and a series of unbrecciated crater fill sediments. In the late Silurian, the impact structure was overrun by Caledonian nappes, leading to low-grade metamorphic recrystallization (Tmax ≤ 400°C, Pmax ≥ 2.5 kbar). Shocked quartzite from the autochtonous breccia is impregnant by fine-grained carbonaceous material, giving it a nearly black colour in hand specimens. Micro-Raman spectroscopy shows this material to be poorly crystalline. Planar fractures, typical of shocked quartz, are outlined by graphite inclusions and by trails of secondary fluid inclusions. The fluid inclusions comprise methane inclusions, with minor carbon dioxide and no visible water (XCH4 ≥ 96 mol%) and water inclusions with moderate salinity. The methane inclusions show H1, H2 and S2 types of microthermometric behaviour; H1 inclusions show a peak of homogenization temperatures to the liquid at -84 to -112°C. At T ≤ 400°C, this corresponds to isochore pressures of 1 to 2.5 kbar, which is compatible with a Caledonian cooling and uplift path, but not with the extreme pressure at the moment of the impact, nor the low pressures (P ≤ 0.2 kbar) encountered in the shallow parts of the impact structure during the final stages of post-impact cooling. Methane was formed in-situ by reaction between solid carbonaceous material and aqueous metamorphic fluid, and was trapped as the partly open planar fractures healed. The methane-rich fluid inclusions in shocked quartz from the Gardnos impact structure are thus only indirectly related to the meteorite impact and not at all to hydrocarbons of a deep (mantle) origin.
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The concept of the organic origin of petroleum has been dominant for the last 70 years. Generation of petroleum hydrocarbons has been explained by transformation of dissipated organic matter in clay-carbonate sediments under the influence of temperature and pressure increase with depth, separation of the hydrocarbons freshly generated, and their migration into porous reservoir beds. However the stream of new information coming from new exploration areas and from new ideas in the fields of geology, geochemistry, geophysics, astrophysics, and so on has contributed to appearance of new knowledge which categorically rejects all aspects and consequences of the organic theory. The aim of the present survey is to bring to the attention of a wide circle of geologists the new ideas and a picture of the ppresent state of the problem of petroleum formation from the viewpoint of inorganic-origin theory.
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In recent years, methane and other light hydrocarbons with an apparently abiotic origin have been identified in an increasing number of geologic fluids on Earth. These compounds have been found in a variety of geologic settings, including seafloor hydrothermal systems, fracture networks in crystalline rocks from continental and oceanic crust, volcanic gases, and gas seeps from serpentinized rocks (e.g., Abrajano et al. 1990; Kelley 1996; Sherwood Lollar 2002, 2008; Fiebig et al. 2007, 2009; Proskurowski et al. 2008; Taran et al. 2010b). Understanding the origin of these compounds has significant implications for range of topics that includes the global carbon cycle, the distribution of life in the deep subsurface (Gold 1992), and the origin of life (Martin et al. 2008). There are even claims that abiotic sources are major contributors to global hydrocarbon reservoirs (Gold 1993; Glasby 2006; Kutcherov and Krayushkin 2010; Sephton and Hazen 2013). While most experts are highly skeptical of such broad claims, it seems possible that at least some petroleum and gas reservoirs could contain hydrocarbons with an abiotic origin. Conceptually, there are two potential major sources of abiotic hydrocarbons to fluids in Earth’s crust. First, abiotic hydrocarbons could migrate to the crust from deeper sources within Earth, through processes such as convective transport, grain boundary diffusion, or release of magmatic volatiles. Second, abiotic hydrocarbons could form in situ within the crust through reduction of inorganic carbon sources. Potential substrates for carbon reduction include CO2 and CO in circulating fluids, and carbon-bearing solids such as carbonate minerals and graphite. In either case, the ultimate source of the inorganic carbon may be primordial (i.e., from the mantle) or recycled from Earth’s surface. This paper summarizes some of the recent laboratory experimental studies conducted to investigate potential …
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Production and Accumulation of Organic Matter: A Geological Perspective.- Production and Accumulation of Organic Matter The Organic Carbon Cycle.- Evolution of the Biosphere.- Biological Productivity of Modern Aquatic Environments.- Chemical Composition of the Biomass: Bacteria, Phytoplankton, Zooplankton, Higher Plants.- Sedimentary Processes and the Accumulation of Organic Matter.- The Fate of Organic Matter in Sedimentary Basins: Generation of Oil and Gas.- Diagenesis, Catagenesis and Metagenesis of Organic Matter.- Early Transformation of Organic Matter: The Diagenetic Pathway from Organisms to Geochemical Fossils and Kerogen.- Geochemical Fossils and Their Significance in Petroleum Formation.- Kerogen: Composition and Classification.- From Kerogen to Petroleum.- Formation of Gas.- Formation of Petroleum in Relation to Geological Processes. Timing of Oil and Gas Generation.- Coal and its Relation to Oil and Gas.- Oil Shales: A Kerogen-Rich Sediment with Potential Economic Value.- The Migration and Accumulation of Oil and Gas.- An Introduction to Migration and Accumulation of Oil and Gas.- Physicochemical Aspects of Primary Migration.- Geological and Geochemical Aspects of Primary Migration.- Secondary Migration and Accumulation.- Reservoir Rocks and Traps, the Sites of Oil and Gas Pools.- The Composition and Classification of Crude Oils and the Influence of Geological Factors.- Composition of Crude Oils.- Classification of Crude Oils.- Geochemical Fossils in Crude Oils and Sediments as Indicators of Depositional Environment and Geological History.- Geological Control of Petroleum Type.- Petroleum Alteration.- Heavy Oils and Tar Sands.- Oil and Gas Exploration: Application of the Principles of Petroleum Generation and Migration.- Identification of Source Rocks.- Oil and Source Rock Correlation.- Locating Petroleum Prospects: Application of Principle of Petroleum Generation and Migration - Geological Modeling.- Geochemical Modeling: A Quantitative Approach to the Evaluation of Oil and Gas Prospects.- Habitat of Petroleum.- The Distribution of World Oil and Gas Reserves and Geological-Geochemical Implications.
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Application of techniques developed for the interpretation of fluid compositions in high-temperature hydrothermal or geothermal systems to lower-temperature natural gas discharges in New Zealand and the Gulf of Thailand showed that redox potentials, as reflected in relative CH4 and CO2 contents, are closely controlled by interaction with divalent and trivalent Fe in rock contacted by the fluids. In contrast to geothermal systems, where CO2 partial pressures approach equilibrium with respect to the conversion of Ca-Al-silicates to calcite and clay, absolute gas pressures in hydrocarbon reservoirs are governed by reservoir pressure. In high-heatflow, >80mW/m2, sedimentary basins, CH4, and CO2 approach both chemical and 13C-isotopic equilibrium, with the rate of attainment of isotopic equilibrium estimated to be some 400 times slower than that of chemical equilibration. Relative CH4, C2H6, and C3H8 concentrations are controlled by a kinetic processes corresponding to random breakage of CC bonds of more complex molecules. Short-distance variations in relative CH4 and CO2 contents of well discharges are largely due to underground vapor-liquid separation processes. The composition of gases from high-heatflow sedimentary basins suggests that they form in systems open to the escape of earlier formed products and to interaction with redox-active components of the host rock. In addition to oxidative degradation of organic material in contact with comparatively oxidising sedimentary material, interaction with highly reduced mafic rocks may give rise to reductive degradation or even synthesis of reduced C species.
Article
We describe a formation scenario of Enceladus constrained by the deuterium-to-hydrogen ratio in the gas plumes as measured by the Cassini Ion and Neutral Mass Spectrometer (INMS). We propose that, similarly to Titan, Enceladus formed from icy planetesimals that were partly devolatilized during their migration within the Kronian subnebula. In our scenario, at least primordial Ar, CO and N_2 were devolatilized from planetesimals during their drift within the subnebula, due to the increasing temperature and pressure conditions of the gas phase. The origin of methane is still uncertain since it might have been either trapped in the planetesimals of Enceladus during their formation in the solar nebula or produced via serpentinization reactions in the satellite's interior. If the methane of Enceladus originates from the solar nebula, then its D/H ratio should range between ˜ 4.7 × 10^{-5} and 1.5 × 10^{-4}. On the other hand, if the methane of Enceladus results from serpentinization reactions, then its D/H ratio should range between ˜ 2.1 × 10^{-4} and 4.5 × 10^{-4}.
Article
Serpentinite-hosted hydrothermal systems have attracted considerable attention as sites of abiotic organic synthesis and as habitats for the earliest microbial communities. Here, we report a systematic isotopic study of a new serpentinite-hosted system: the Hakuba Happo hot spring in the Shiroumadake area, Japan ( , ). We collected water directly from the hot spring from two drilling wells more than 500 m deep; all water samples were strongly alkaline ( ) and rich in H2 (201-664 μmol/L) and CH4 (124-201 μmol/L). Despite the relatively low temperatures (50-60 °C), thermodynamic calculations suggest that the H2 was likely derived from serpentinization reactions. Hydrogen isotope compositions for Happo #1 (Happo #3) were found to be as follows: ( ), ( ), and ( ). The carbon isotope compositions of methane from Happo #1 and #3 were found to be and , respectively. The CH4-H2-H2O hydrogen isotope systematics indicate that at least two different mechanisms were responsible for methane formation. Happo #1 has a similar hydrogen isotope compositions to other serpentinite-hosted systems reported previously. The elevated (with respect to the equilibrium relationship) suggests that the hydrogen of the Happo #1 methane was not sourced from molecular hydrogen but was derived directly from water. This implies that the methane may not have been produced via the Fischer-Tropsch-type (FTT) synthesis but possibly by the hydration of olivine. Conversely, the depleted (with respect to the equilibrium relationship) in Happo #3 suggests the incorporation of biological methane. Based on a comparison of the hydrogen isotope systematics of our results with those of other serpentinite-hosted hydrothermal systems, we suggest that abiotic CH4 production directly from H2O (without mediation by H2) may be more common in serpentinite-hosted systems. Hydration of olivine may play a more significant role in abiotic methane production than previously thought.
Article
Natural metal carbonates, in particular the abundant alkaline-earth metal carbonates MgCO3 (magnesite), MgCa(CO3)2 (dolomite) and CaCO3 (calcite), are formed by various carbonatization processes with atmospheric carbon dioxide. They represent not only an important buffer system within the ecological carbon cycle, but also the most important feedstocks for the production of MgO (magnesia), CaO (calcia) and related chemicals. The thermal decomposition of these naturally occurring carbonates is well documented: in vacuum, inert or oxidizing atmosphere the main gaseous product is carbon dioxide, and as solid products the pure or mixed metal oxides are formed. Here we report the results of an investigation of the thermal reactivity of pure alkaline-earth metal carbonates and mixed alkaline-earth metal transition metal carbonates in a hydrogen atmosphere. We found that, compared with the analogous degradations in inert or oxidizing atmospheres, the reaction temperatures are lowered by at least 150 K. Depending on the transition metal present in the initial mixed carbonates, CO or CH4 is formed as the main gaseous carbon compound.
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Methane emissions from natural gas reservoirs have long been largely overlooked. The discovery of abundant geological gas seeps in areas of cryosphere degradation highlights the relevance of these emissions to the greenhouse gas budget.