Etiope, Giuseppe

In serpentinised peridotite and ultramafic rock systems, methane (CH4) origin is frequently considered abiotic, but variable microbial and thermogenic components can also exist. Typically, the origin of CH4 is studied using bulk, 13C/12C and 2H/H isotopic composition, molecular gas composition, occasionally radiocarbon (14C), microbiology and geological context. Recent advances in CH4-clumped isotope methods have yielded novel insights into the formation of CH4: nonetheless, their interpretation in natural gas samples is often uncertain and requires additional research. Here, we study the origin of the gas released in hyperalkaline (pH > 10) springs in the Ronda Peridotite Massifs (southern Spain), combining bulk and clumped CH4 isotopes with molecular gas composition, hydrochemical (Total Organic Carbon and Platinum Group Elements in water), geothermal and geo-structural data. Five springs analysed in 2014 have been re-examined for changes in gas chemistry over time, and three newly discovered gas-bearing springs are analysed for the first time. Regardless of whether springs have microbial or abiotic isotopic fingerprints, we find that bulk CH4 isotopes are fairly stable over a seven-year period. This suggests that the CH4 source(s) or postgenetic processes (such as oxidation and diffusion) have not undergone significant temporal changes. Major variations in H2 and CH4 concentrations in certain springs may be the result of changes in gas pressure and migration intensity. Paired CH4 clumped isotopes (Δ12CH2D2 - Δ13CH3D) were analysed in two bubbling springs, where the presence of CH4 can be interpreted as non-microbial based on 13C enrichment, absence of 14C, and the presence of ethane and propane. However, these isotopes are in disequilibrium, which prevents the quantification of the gas formation temperature. Within the Δ12CH2D2 - Δ13CH3D diagram, the data lie within both the microbialgenic zone, suggested by previous authors, and the abiotic zone that results combining data from laboratory gas synthesis and other natural gas samples. Therefore, attributing a microbial origin to CH4 based only on clumped isotopes is less definite than previously assumed. The amount of Total Organic Carbon appears to be correlated with the origin of CH4, as it is higher in 13C-depleted CH4 samples and lower in 13C-enriched samples. Palladium (Pd) and Rhodium (Rh) dissolved in water (the more soluble Platinum Group Elements) can be a proxy for the chromitite ore deposits contained in plagioclase tectonite layers throughout the investigated area, which may act as catalysts for abiotic CO2 hydrogenation. Clumped isotope disequilibrium and the reported absence of diffuse CH4-bearing fluid inclusions in the peridotites appear to rule out high temperature gas genesis in post-magmatic inclusions. These observations, along with the moderate temperatures at the base of the peridotite massifs and the consistent occurrence of gas along tectonic contacts between serpentinised (H2-bearing) peridotite and carbon-bearing rocks, are compatible with the theory of low-temperature CO2 hydrogenation.

Subsurface geological reservoirs of natural hydrogen gas (H2), a clean fuel and energy vector, are currently a target for energy resource exploration. Such reservoirs can be revealed by the presence of H2 within soil, analogous to hydrocarbon seepage in petroleum systems. Nevertheless, defining the level of soil H2 that can indicate a potentially economic resource is currently impossible, and identifying geological H2 within soil-gas is challenging because H2 concentrations and the isotopic composition (δ2H) may overlap with the in-situ biological signature. In spite of these limitations, analogies to conventional hydrocarbon systems suggest that the presence of surface advective gas flows can reveal (unlike diffusion) a subsoil source and even pressurised gas accumulations of H2. Here, a massive release of H2 is reported from a CH4–H2 rich seep in Turkey, known as Chimaera, an emblematic example of H2 advection. The site represents the first case where a closed-chamber flux method was applied for H2 seepage. H2 advection at the site was clearly indicated by numerous gas vents and flames, and by the heterogeneous spatial distribution of pervasive, invisible exhalation (miniseepage), inducing rapid H2 concentration build-up within the chamber. H2 emission (∼10 ± 3 kg day−1, with the highest H2 emission factor reported, thus far, of ∼5000 kg km−2 day−1) is continuous and long lasting (flames have been documented for millennia) and, using an analogy for hydrocarbon seeps, may stem from pressurised accumulations. The Chimaera case is illustrative of how detecting soil H2 advection may help unravel surface (biological) vs. subsoil (geological) gas origins in cases where, in the absence of significant gas seepage, soil H2 concentrations are within the range of biological production (100-103 ppmv, e.g., as for “fairy circles” observed in several countries). Interpretations must, however, be supported by additional geochemical data and evaluations of potential biological H2 production within the surface ecosystem.

Submarine methane-rich gas hydrates in ocean sediments are a potential atmospheric greenhouse gas and energy source. It is considered that microbial methane is generally autochthonous, produced in situ within the gas hydrate stability zone with low gas flux and pressure, while thermogenic gas is allochthonous, migrated from a deeper petroleum system, with higher gas flux and pressure and therefore potentially higher energy resource and environmental impact. Here, we report on the allochthonous nature of large microbial gas hydrate deposits in the Rakhine Basin, Bay of Bengal. An innovative and automatic tool, developed to analyze high-resolution three-dimensional seismic data, allowed to detect hundreds of thousands gas occurrences throughout a 2 km thick Pliocene-Pleistocene sedimentary sequence extending below the gas hydrate stability zone. A supercharged section matching the present-day optimum temperature for microbial methanogenesis was identified. Combining seismic and geochemical data of the Rakhine Basin gas system points to a dominant microbial nature of the gas. Stacked amplitude anomalies and vertical anomaly clusters demonstrate active free-phase gas migration towards the shallow gas hydrate stability zone. The Rakhine Basin gas hydrates are the ultimate seal for the entire petroleum system and represent a case of “frozen seepage” of microbial gas with relatively high flux and pressure.

The Patras Gulf Pockmark field is located in shallow waters offshore Patras City (Greece) and is considered one of the most spectacular and best-documented fluid seepage activities in the Ionian Sea. The field has been under investigation since 1996, though surveying was partially sparse and fragmentary. This paper provides a complete mapping of the field and generates new knowledge regarding the fluid escape structures, the fluid pathways, their origin and the link with seismic activity. For this, data sets were acquired utilising high-resolution marine remote sensing techniques, including multibeam echosounders, side-scan sonars, sub-bottom profilers and remotely operated vehicles, and laboratory techniques focusing on the chemical composition of the escaping fluids. The examined morphometric parameters and spatial distribution patterns of the pockmarks are directly linked to tectonic structures. Acoustic anomalies related to the presence of gas in sediments and in the water column document the activity of the field at present and in the past. Methane is the main component of the fluids and is of microbial origin. Regional and local tectonism, together with the Holocene sedimentary deposits, appear to be the main contributors to the growth of the field. The field preserves evidence that earthquake activity prompts the activation of the field.

Knowledge of the spatial distribution of the fluxes of greenhouse gases (GHGs) and their temporal variability as well as flux attribution to natural and anthropogenic processes is essential to monitoring the progress in mitigating anthropogenic emissions under the Paris Agreement and to inform its global stocktake. This study provides a consolidated synthesis of CH4 and N2O emissions using bottom-up (BU) and top-down (TD) approaches for the European Union and UK (EU27 + UK) and updates earlier syntheses (Petrescu et al., 2020, 2021). The work integrates updated emission inventory data, process-based model results, data-driven sector model results and inverse modeling estimates, and it extends the previous period of 1990–2017 to 2019. BU and TD products are compared with European national greenhouse gas inventories (NGHGIs) reported by parties under the United Nations Framework Convention on Climate Change (UNFCCC) in 2021. Uncertainties in NGHGIs, as reported to the UNFCCC by the EU and its member states, are also included in the synthesis. Variations in estimates produced with other methods, such as atmospheric inversion models (TD) or spatially disaggregated inventory datasets (BU), arise from diverse sources including within-model uncertainty related to parameterization as well as structural differences between models. By comparing NGHGIs with other approaches, the activities included are a key source of bias between estimates, e.g., anthropogenic and natural fluxes, which in atmospheric inversions are sensitive to the prior geospatial distribution of emissions. For CH4 emissions, over the updated 2015–2019 period, which covers a sufficiently robust number of overlapping estimates, and most importantly the NGHGIs, the anthropogenic BU approaches are directly comparable, accounting for mean emissions of 20.5 Tg CH4 yr−1 (EDGARv6.0, last year 2018) and 18.4 Tg CH4 yr−1 (GAINS, last year 2015), close to the NGHGI estimates of 17.5±2.1 Tg CH4 yr−1. TD inversion estimates give higher emission estimates, as they also detect natural emissions. Over the same period, high-resolution regional TD inversions report a mean emission of 34 Tg CH4 yr−1. Coarser-resolution global-scale TD inversions result in emission estimates of 23 and 24 Tg CH4 yr−1 inferred from GOSAT and surface (SURF) network atmospheric measurements, respectively. The magnitude of natural peatland and mineral soil emissions from the JSBACH–HIMMELI model, natural rivers, lake and reservoir emissions, geological sources, and biomass burning together could account for the gap between NGHGI and inversions and account for 8 Tg CH4 yr−1. For N2O emissions, over the 2015–2019 period, both BU products (EDGARv6.0 and GAINS) report a mean value of anthropogenic emissions of 0.9 Tg N2O yr−1, close to the NGHGI data (0.8±55 % Tg N2O yr−1). Over the same period, the mean of TD global and regional inversions was 1.4 Tg N2O yr−1 (excluding TOMCAT, which reported no data). The TD and BU comparison method defined in this study can be operationalized for future annual updates for the calculation of CH4 and N2O budgets at the national and EU27 + UK scales. Future comparability will be enhanced with further steps involving analysis at finer temporal resolutions and estimation of emissions over intra-annual timescales, which is of great importance for CH4 and N2O, and may help identify sector contributions to divergence between prior and posterior estimates at the annual and/or inter-annual scale. Even if currently comparison between CH4 and N2O inversion estimates and NGHGIs is highly uncertain because of the large spread in the inversion results, TD inversions inferred from atmospheric observations represent the most independent data against which inventory totals can be compared. With anticipated improvements in atmospheric modeling and observations, as well as modeling of natural fluxes, TD inversions may arguably emerge as the most powerful tool for verifying emission inventories for CH4, N2O and other GHGs.

Extensive fields of sub-kilometre- to kilometre-scale mounds, cones, domes, shields, and flow-like edifices cover large parts of the martian lowlands. These features have been compared to structures on Earth produced by sedimentary volcanism – a process that involves subsurface sediment/fluid mobilisation and commonly releases methane to the atmosphere. It was proposed that such processes might help to explain the presence of methane in the martian atmosphere and may also have produced habitable, subsurface settings of potential astrobiological relevance. However, it remains unclear if sedimentary volcanism on Earth and Mars share genetic similarities and hence if methane or other gases were released on Mars during this process. The aim of this review is to summarise the current knowledge about mud-volcano-like structures on Mars, address the critical aspects of this process, identify key open questions, and point to areas where further research is needed to understand this phenomenon and its importance for the Red Planet's geological evolution. We show here that after several decades of exploration, the amount of evidence supporting martian sedimentary volcanism has increased significantly, but as the critical ground truth is still lacking, alternative explanations cannot be ruled out. We also highlight that the lower gravity and temperatures on Mars compared to Earth control the dynamics of clastic eruptions and surface emplacement mechanisms and the resulting morphologies of erupted material. This implies that shapes and triggering mechanisms of mud-volcano-like structures may be different from those observed on Earth. Therefore, comparative studies should be done with caution. To provide a better understanding of the significance of these abundant features on Mars, we argue for follow-up studies targeting putative sedimentary volcanic features identified on the planet's surface and, if possible, for in situ investigations by landed missions such as that by the Zhurong rover.

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.

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 (δ13C and δ2H in CH4) covered a wide range of values, indicating mainly thermogenic gas sources (associated with oil production) in the Romanian Plain, mostly in Prahova county (δ13C from –67.8 ± 1.2 to –22.4 ± 0.04 ‰ Vienna Pee Dee Belmnite; δ2H 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 δ13C and δ2H 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. Keywords:

We study the drivers behind the global atmospheric methane (CH4) increase observed after 2006. Candidate emission and sink scenarios are constructed based on proposed hypotheses in the literature. These scenarios are simulated in the TM5 tracer transport model for 1984-2016 to produce three-dimensional fields of CH4 and δ 13C-CH4, which are compared with observations to test the competing hypotheses in the literature in one common model framework. We find that the fossil fuel (FF) CH4 emission trend from the Emissions Database for Global Atmospheric Research 4.3.2 inventory does not agree with observed δ 13C-CH4. Increased FF CH4 emissions are unlikely to be the dominant driver for the post-2006 global CH4 increase despite the possibility for a small FF emission increase. We also find that a significant decrease in the abundance of hydroxyl radicals (OH) cannot explain the post-2006 global CH4 increase since it does not track the observed decrease in global mean δ 13C-CH4. Different CH4 sinks have different fractionation factors for δ 13C-CH4, thus we can investigate the uncertainty introduced by the reaction of CH4 with tropospheric chlorine (Cl), a CH4 sink whose abundance, spatial distribution, and temporal changes remain uncertain. Our results show that including or excluding tropospheric Cl as a 13 Tg/year CH4 sink in our model changes the magnitude of estimated fossil emissions by ∼20%. We also found that by using different wetland emissions based on a static versus a dynamic wetland area map, the partitioning between FF and microbial sources differs by 20 Tg/year, ∼12% of estimated fossil emissions.

Quantifying natural geological sources of methane (CH4) allows to improve the assessment of anthropogenic emissions to the atmosphere from fossil fuel industries. The global CH4 flux of geological gas is, however, an object of debate. Recent fossil (14C-free) CH4 measurements in preindustrial-era ice cores suggest very low global geological emissions (~ 1.6 Tg year-1), implying a larger fossil fuel industry source. This is however in contrast with previously published bottom-up and top-down geo-emission estimates (~ 45 Tg year-1) and even regional-scale emissions of ~ 1-2 Tg year-1. Here we report on significant geological CH4 emissions from the Lusi hydrothermal system (Indonesia), measured by ground-based and satellite (TROPOMI) techniques. Both techniques indicate a total CH4 output of ~ 0.1 Tg year-1, equivalent to the minimum value of global geo-emission derived by ice core 14CH4 estimates. Our results are consistent with the order of magnitude of the emission factors of large seeps used in global bottom-up estimates, and endorse a substantial contribution from natural Earth's CH4 degassing. The preindustrial ice core assessments of geological CH4 release may be underestimated and require further study. Satellite measurements can help to test geological CH4 emission factors and explain the gap between the contrasting estimates.