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Earth’s hydrocarbon degassing through gas-oil seeps, mud volcanoes and diffuse microseepage is a major natural source of methane (CH 4 ) to the atmosphere. While carbon dioxide degassing is typically associated with extensional tectonics, volcanoes, and geothermal areas, CH 4 seepage mostly occurs in petroleum-bearing sedimentary basins, but the role of tectonics in degassing is known only for some case studies at local scale. Here, we perform a global scale geospatial analysis to assess how the presence of hydrocarbon fields, basin geodynamics and the type of faults control CH 4 seepage. Combining georeferenced data of global inventories of onshore seeps, faults, sedimentary basins, petroleum fields and heat flow, we find that hydrocarbon seeps prevail in petroleum fields within convergent basins with heat flow ≤ 98 mW m −2 , and along any type of brittle tectonic structure, mostly in reverse fault settings. Areas potentially hosting additional seeps and microseepage are identified through a global seepage favourability model. CH4 seepage mostly occurs in petroleum-bearing sedimentary basins, but the role of tectonics in degassing is mostly only known at a local scale. Here, the authors conduct a global scale analysis of seeps, faults, sedimentary basins, petroleum fields and heat flow, and find that geological CH 4 seepage preferably develops in convergent basins, while gas seeps can occur along any brittle tectonic structure.

A wide body of literature suggests that geological gas emissions from Earth’s degassing are a major methane (CH4) source to the atmosphere. These emissions are from gas-oil seeps, mud volcanoes, microseepage and submarine seepage in sedimentary (petroleum-bearing) basins, and geothermal and volcanic manifestations. Global bottom-up emission estimates, ranging from 30 to 76 Tg CH4 yr–1, evolved in the last twenty years thanks to the increasing number of flux measurements, and improved knowledge of emission factors and area distribution (activity). Based on recent global grid maps and updated evaluations of mud volcano and microseepage emissions, the global geo-CH4 source is now (bottom-up) estimated to be 45 (27–63) Tg yr–1, i.e., ~8% of total CH4 sources. Top-down verifications, based on independent approaches (including ethane and isotopic observations) from different authors, are consistent with the range of the bottom-up estimate. However, a recent top-down study, based on radiocarbon analyses in polar ice cores, suggests that geological, fossil (14C-free) CH4 emissions about 11,600 years ago were much lower (<15 Tg yr–1, 95% CI) and that this source strength could also be valid today. Here, we show that (i) this geo-CH4 downward revision implies a fossil fuel industry CH4 upward revision of at least 24–35%. (ii) The 95% CI estimates of the recent radiocarbon analysis do not overlap with those of 5 out of 6 other bottom-up and top-down studies (no overlap for the 90% CI estimates). (iii) The contrasting lines of evidence require further discussion, and research opportunities exist to help explain this gap.

Current emission inventories require an additional \"unknown\" source to balance the global atmospheric budgets of ethane (C₂H₆). Here, we provide evidence that a substantial part of the missing source can be attributed to natural gas seepage from petroliferous, geothermal, and volcanic areas. Such geologic sources also inject propane (C₃H₈) into the atmosphere. The analysis of a large data set of methane (CH₄), ethane, and propane concentrations in surface gas emissions of 238 sites from different geographic and geologic areas, coupled with published estimates of geomethane emissions, suggests that Earth's degassing accounts for at least 17% and 10% of total ethane and propane emissions, respectively.

The accelerated increase in global methane (CH 4 ) in the atmosphere, accompanied by a decrease in its 13 C/ 12 C isotopic ratio ( δ 13 C CH4 ) from −47.1‰ to −47.3‰ observed since 2008, has been attributed to increased emissions from wetlands and cattle, as well as from shale gas and shale oil developments. To date both explanations have relied on poorly constrained δ 13 C CH4 source signatures. We use a dataset of δ 13 C CH4 from >1600 produced shale gas samples from regions that account for >97% of global shale gas production to constrain the contribution of shale gas emissions to observed atmospheric increases in the global methane burden. We find that US shale gas extracted since 2008 has volume-weighted-average δ 13 C CH4 of −39.6‰. The average δ 13 C CH4 weighted by US basin-level measured emissions in 2015 was −41.8‰. Therefore, emission increases from shale gas would contribute to an opposite atmospheric δ 13 C CH4 signal in the observed decrease since 2008 (while noting that the global isotopic trend is the net of all dynamic source and sink processes). This observation strongly suggests that changing emissions of other (isotopically-lighter) CH 4 source terms is dominating the increase in global CH 4 emissions. Although production of shale gas has increased rapidly since 2008, and CH 4 emissions associated with this increased production are expected to have increased overall in that timeframe, the simultaneously-observed increase in global atmospheric CH 4 is not dominated by emissions from shale gas and shale oil developments.

Quantifying natural geological sources of methane (CH 4 ) allows to improve the assessment of anthropogenic emissions to the atmosphere from fossil fuel industries. The global CH 4 flux of geological gas is, however, an object of debate. Recent fossil ( 14 C-free) CH 4 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 CH 4 emissions from the Lusi hydrothermal system (Indonesia), measured by ground-based and satellite (TROPOMI) techniques. Both techniques indicate a total CH 4 output of ~ 0.1 Tg year −1 , equivalent to the minimum value of global geo-emission derived by ice core 14 CH 4 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 CH 4 degassing. The preindustrial ice core assessments of geological CH 4 release may be underestimated and require further study. Satellite measurements can help to test geological CH 4 emission factors and explain the gap between the contrasting estimates.

Reports of methane detection in the Martian atmosphere have been intensely debated. The presence of methane could enhance habitability and may even be a signature of life. However, no detection has been confirmed with independent measurements. Here, we report a firm detection of 15.5 ± 2.5 ppb by volume of methane in the Martian atmosphere above Gale Crater on 16 June 2013, by the Planetary Fourier Spectrometer onboard Mars Express, one day after the in situ observation of a methane spike by the Curiosity rover. Methane was not detected in other orbital passages. The detection uses improved observational geometry, as well as more sophisticated data treatment and analysis, and constitutes a contemporaneous, independent detection of methane. We perform ensemble simulations of the Martian atmosphere, using stochastic gas release scenarios to identify a potential source region east of Gale Crater. Our independent geological analysis also points to a source in this region, where faults of Aeolis Mensae may extend into proposed shallow ice of the Medusae Fossae Formation and episodically release gas trapped below or within the ice. Our identification of a probable release location will provide focus for future investigations into the origin of methane on Mars.A methane spike 15.5 ± 2.5 parts per billion by volume was detected in the Martian atmosphere above Gale Crater on 16 June 2013 by Mars Express, independently confirming the debated in situ observation by the Curiosity rover a day earlier.

Revisions in isotopic source signatures reveal that global total fossil fuel methane emissions from industry plus natural geological seepage are much larger than thought. Global fossil fuel methane emissions revised Stefan Schwietzke et al . re-evaluate the global methane budget and the contribution of the fossil fuel industry to methane emissions on the basis of long-term global methane and methane carbon isotope records. They find that total fossil fuel methane emissions (fossil fuel industry plus natural geological methane seepage) are not increasing over time, but are 60–110 per cent greater than was previously thought. They also conclude that methane emissions from natural gas, oil and coal production and their usage are 20–60 per cent greater than inventories and that methane emissions from natural gas as a fraction of production have declined from about 8 per cent to 2 per cent over the past three decades. Methane has the second-largest global radiative forcing impact of anthropogenic greenhouse gases after carbon dioxide, but our understanding of the global atmospheric methane budget is incomplete. The global fossil fuel industry (production and usage of natural gas, oil and coal) is thought to contribute 15 to 22 per cent of methane emissions 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 to the total atmospheric methane budget 11 . However, questions remain regarding methane emission trends as a result of fossil fuel industrial activity and the contribution to total methane emissions of sources from the fossil fuel industry and from natural geological seepage 12 , 13 , which are often co-located. Here we re-evaluate the global methane budget and the contribution of the fossil fuel industry to methane emissions based on long-term global methane and methane carbon isotope records. We compile the largest isotopic methane source signature database so far, including fossil fuel, microbial and biomass-burning methane emission sources. We find that total fossil fuel methane emissions (fossil fuel industry plus natural geological seepage) are not increasing over time, but are 60 to 110 per cent greater than current estimates 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 owing to large revisions in isotope source signatures. We show that this is consistent with the observed global latitudinal methane gradient. After accounting for natural geological methane seepage 12 , 13 , we find that methane emissions from natural gas, oil and coal production and their usage are 20 to 60 per cent greater than inventories 1 , 2 . Our findings imply a greater potential for the fossil fuel industry to mitigate anthropogenic climate forcing, but we also find that methane emissions from natural gas as a fraction of production have declined from approximately 8 per cent to approximately 2 per cent over the past three decades.

The concentration of atmospheric methane (CH4) has more than doubled over the industrial era. To help constrain global and regional CH4 budgets, inverse (top-down) models incorporate data on the concentration and stable carbon (δ13C) and hydrogen (δ2H) isotopic ratios of atmospheric CH4. These models depend on accurate δ13C and δ2H end-member source signatures for each of the main emissions categories. Compared with meticulous measurement and calibration of isotopic CH4 in the atmosphere, there has been relatively less effort to characterize globally representative isotopic source signatures, particularly for fossil fuel sources. Most global CH4 budget models have so far relied on outdated source signature values derived from globally nonrepresentative data. To correct this deficiency, we present a comprehensive, globally representative end-member database of the δ13C and δ2H of CH4 from fossil fuel (conventional natural gas, shale gas, and coal), modern microbial (wetlands, rice paddies, ruminants, termites, and landfills and/or waste) and biomass burning sources. Gas molecular compositional data for fossil fuel categories are also included with the database. The database comprises 10 706 samples (8734 fossil fuel, 1972 non-fossil) from 190 published references. Mean (unweighted) δ13C signatures for fossil fuel CH4 are significantly lighter than values commonly used in CH4 budget models, thus highlighting potential underestimation of fossil fuel CH4 emissions in previous CH4 budget models. This living database will be updated every 2–3 years to provide the atmospheric modeling community with the most complete CH4 source signature data possible. Database digital object identifier (DOI): https://doi.org/10.15138/G3201T.

Hypersaline meromictic lakes are extreme environments in which water stratification is associated with powerful physicochemical gradients and high salt concentrations. Furthermore, their physical stability coupled with vertical water column partitioning makes them important research model systems in microbial niche differentiation and biogeochemical cycling. Here, we compare the prokaryotic assemblages from Ursu and Fara Fund hypersaline meromictic lakes (Transylvanian Basin, Romania) in relation to their limnological factors and infer their role in elemental cycling by matching taxa to known taxon-specific biogeochemical functions. To assess the composition and structure of prokaryotic communities and the environmental factors that structure them, deep-coverage small subunit (SSU) ribosomal RNA (rDNA) amplicon sequencing, community domain-specific quantitative PCR and physicochemical analyses were performed on samples collected along depth profiles. The analyses showed that the lakes harbored multiple and diverse prokaryotic communities whose distribution mirrored the water stratification patterns. Ursu Lake was found to be dominated by Bacteria and to have a greater prokaryotic diversity than Fara Fund Lake that harbored an increased cell density and was populated mostly by Archaea within oxic strata. In spite of their contrasting diversity, the microbial populations indigenous to each lake pointed to similar physiological functions within carbon degradation and sulfate reduction. Furthermore, the taxonomy results coupled with methane detection and its stable C isotope composition indicated the presence of a yet-undescribed methanogenic group in the lakes’ hypersaline monimolimnion. In addition, ultrasmall uncultivated archaeal lineages were detected in the chemocline of Fara Fund Lake, where the recently proposed Nanohaloarchaeota phylum was found to thrive.

Natural hydrocarbon seepages constitute a substantial influx of oil and gas into hydrosphere and atmosphere, impacting the environment and greenhouse gas budgets. The Lusi seepage system (Indonesia) is one of the Earth’s largest surface hydrocarbon manifestations, and is intrinsically associated with the interaction of organic-rich sedimentary rocks and recent magmatism. Previous ground-based and satellite measurements revealed that Lusi is a mega emitter of thermogenic methane, releasing ~0.1 Mt methane per year. Here, linking the quantification of mud flow rates with oil concentration data, we estimate that Lusi released ~0.22-0.28 Mt of oil over 13 years of continuous fluid discharge. This emission is of the same order of magnitude as the Deepwater Horizon spill. The oil discharge into the Porong River and Madura Strait poses an environmental threat to aquatic ecosystems of north-east Java. Combining flux and concentration data, we have also estimated a release into the atmosphere of ~8-10 kt of ethane and ~6-8 kt of propane per year. Over 18 years, Lusi has already injected into the atmosphere more than 20 times the amount of ethane released by the 2015 Aliso Canyon gas storage accident (California), considered as one of the largest gas leaks in U.S. history. The Lusi mega seepage system in Indonesia released around 0.3 Mt of oil over 13 years of continuous fluid discharge, emitting 10 Kt of ethane and 8 Kt of propane annually, according to geochemical analysis of mud and gas samples collected at the site since 2006.