A new estimate of global methane flux from onshore and shallow submarine mud volcanoes to the atmosphere

A new estimate of global methane emission into the atmosphere from mud volcanoes (MVs) on land and shallow seafloor is presented. The estimate, considered a lower limit, is based on 1) new direct measurements of flux, including both venting of methane and diffuse microseepage around craters and vents, and 2) a classification of MV sizes in terms of area (km²) based on a compilation of data from 120 MVs. The methane flux to the atmosphere is conservatively estimated between 6 and 9 Mt y^–1. This emission from MVs is 3–6% of the natural methane sources and is comparable with ocean and hydrate sources, officially considered in the atmospheric methane budget. The total geologic source, including MVs, seepage from seafloor, microseepage in hydrocarbon-prone areas and geothermal sources, would amount to 35–45 Mt y^–1. The authors believe it is time to add this parameter in the Intergovernmental Panel on Climate Change official tables of atmospheric methane sources.GEM     

Methane flux from miniseepage in mud volcanoes of SW Taiwan: Comparison with the data from Italy,Romania, and Azerbaijan

Mud volcanoes (MVs) are considered important methane (CH₄) sources for the atmosphere; gas is not only released from macro-seepage, i.e., from craters and visible gas bubbling manifestations, but also from invisible and pervasive exhalation from the ground, named miniseepage. CH₄ flux related to miniseepage was measured only in a few MVs, in Azerbaijan, Italy, Japan, Romania and Taiwan. This study examines in detail the flux data acquired in 5 MVs and 1 “dry” seep in SW Taiwan, and further compares with other 23 MVs in Italy, Romania and Azerbaijan. Miniseepage from the six manifestations in SW Taiwan MVs and seeps annually contribute at least 110 tons of methane directly to the atmosphere, and represents about ∼80% of total degassing during a quiescent period. Combining miniseepage flux and geo-electrical data from the Wu-shan-ding MV revealed a possible link between gas flux and electrical resistivity of the vadose zone. This suggests that unsaturated subsoil is a preferential zone for shallow gas accumulation and seepage to the atmosphere. Besides, miniseepage flux in Chu-huo everlasting fire decreases by increasing the distance from the main gas channeling zone and molecular fractionation (methane/ethane ratio) is higher for lower flux seepage, consistently with what observed in other MVs worldwide. Measurements from Azerbaijan, Italy, Romania, and Taiwan converge to indicate that miniseepage is directly proportional to the vent output and it is a significant component of the total methane budget of a MV. A miniseepage vs. macro-seepage flux equation has been statistically assessed and it can be used to estimate theoretically at least the order of magnitude of the flux of miniseepage for MVs of which only the flux from vents was evaluated, or will be evaluated in future. This will allow a more complete and objective quantification of gas emission in MVs, thus also refining the estimate of the global methane emission from geological sources.     

地质成因的甲烷释放对大气的影响

地质成因自然源的甲烷释放在整个大气甲烷估算中起着非常重要的作用,它既是不含放射性14C甲烷源(死碳源)缺失部分的重要代表,也是甲烷重碳源的重要部分.概述了国内外关于地质成因甲烷释放对大气甲烷源与汇影响的研究进展,详述了来自地质成因化石燃料泄漏的人为甲烷释放以及来自沉积盆地(含油气盆地)、泥火山、地热区、海洋和甲烷水合物的地质自然源甲烷释放对大气甲烷源与汇的贡献及其影响因素;说明由于地质成因甲烷分布的区域性、不均匀性和时空的高度变化性,以及目前地质成因甲烷的通量估算仅建立在区域性的少量甲烷通量测试基础上,造成了地质成因甲烷释放通量估算的高度不确定性;指出研究中国西北地区油气田集聚区的甲烷释放通量,对油气田地质成因甲烷释放通量的估算具有重要意义.     

Spatial variations in methane emissions from natural wetlands in China

Natural wetlands are thought to be one of the largest natural sources of atmospheric methane concentrations. Although numerous studies referred to the rate of methane fluxes in different geophysical regions, only a few had estimates of the overall geographical methane emissions in China. This study estimated the spatial variations of annual methane emissions with the pixel size of 1 km × 1 km from natural wetlands, excluding water surface, in China. The natural wetland areas were extracted from the database of the 2000 land covers, and geophysical divisions were used to represent different climate conditions. Methane emission in every geophysical region was calculated based on methane release factors obtained from an extensive overview of published literature and the data of elevation and vegetation proportion. The estimated annual methane emissions ranged from 0 to 5,702.8 kg per pixel within the area of 1 km², and the spatial variation in methane emissions was strongly correlated with proportion of wetlands in the area. The total methane emission from natural wetland in China ranged from 3.48 to 7.16 Tg (terrogram, unit of weight) per year, with the mean value of 4.94 Tg per year, based on the area 133,000 km² of natural wetlands. Specifically, the wetland in Northeast China had the highest contribution in China (39 %). Inner Mongolia and Qinghai-Tibet highland represented for about 25 and 21 %, respectively. The other 15 % of the measured methane was released in Northwest, North, Central, and South China.     

Spatial variation in fluid flow and geochemical fluxes across the sediment-seawater interface at the Carlos Ribeiro mud volcano (Gulf of Cadiz)

Submarine mud volcanism is an important pathway for transfer of deep-sourced fluids enriched in hydrocarbons and other elements into the ocean. Numerous mud volcanoes (MVs) have been discovered along oceanic plate margins, and integrated elemental fluxes are potentially significant for oceanic chemical budgets. Here, we present the first detailed study of the spatial variation in fluid and chemical fluxes at the Carlos Ribeiro MV in the Gulf of Cadiz. To this end, we combine analyses of the chemical composition of pore fluids with a 1-D transport-reaction model to quantify fluid fluxes, and fluxes of boron, lithium and methane, across the sediment-seawater interface. The pore fluids are significantly depleted in chloride, but enriched in lithium, boron and hydrocarbons, relative to seawater. Pore water profiles of sulphate, hydrogen sulphide and total alkalinity indicate that anaerobic oxidation of methane occurs at 34-180 cm depth below seafloor. Clay mineral dehydration, and in particular the transformation of smectite to illite, produces pore fluids that are depleted in chloride and potassium. Profiles of boron, lithium and potassium are closely related, which suggests that lithium and boron are released from the sediments during this transformation. Pore fluids are expelled into the water column by advection; fluid flow velocities are 4 cm yr⁻¹ at the apex of the MV but they rapidly decrease to 0.4 cm yr⁻¹ at the periphery. The associated fluxes of boron, lithium and methane vary between 7-301, 0.5-6 and 0-806 mmol m⁻² yr⁻¹, respectively. We demonstrate that fluxes of Li and B due to mud volcanism may be important on a global scale, however, release of methane into the overlying water column is suppressed by microbial methanotrophy.     

冻土区甲烷排放研究进展

冻土区土牡表面和活动层土的ＣＨ_４排放和吸收表现出强烈的时空变化性。根据多年冻土中ＣＨ_４含量的模拟结果表明，全球尺度上，平均每米厚度多年冻土含有ＣＨ_４６５Ｔｇ。在未来的２００年间，多年冻土融化所导致的大气ＣＨ_４附加年源强变化于２～２５Ｔｇ。     

Temporal Changes in Seawater Carbonate Chemistry and Carbon Export from a Southern California Estuary

Estuaries are important subcomponents of the coastal ocean, but knowledge about the temporal and spatial variability of their carbonate chemistry, as well as their contribution to coastal and global carbon fluxes, are limited. In the present study, we measured the temporal and spatial variability of biogeochemical parameters in a saltmarsh estuary in Southern California, the San Dieguito Lagoon (SDL). We also estimated the flux of dissolved inorganic carbon (DIC) and total organic carbon (TOC) to the adjacent coastal ocean over diel and seasonal timescales. The combined net flux of DIC and TOC (F_DIC?+?TOC) to the ocean during outgoing tides ranged from ??1.8±0.5?×?10³ to 9.5±0.7?×?10³?mol C h^?1 during baseline conditions. Based on these fluxes, a rough estimate of the net annual export of DIC and TOC totaled 10±4?×?10⁶?mol C year^?1. Following a major rain event (36 mm rain in 3 days), F_DIC?+?TOC increased and reached values as high as 29.0 ±?0.7?×?10³?mol C h^?1. Assuming a hypothetical scenario of three similar storm events in a year, our annual net flux estimate more than doubled to 25 ±?4?×?10⁶?mol C year^?1. These findings highlight the importance of assessing coastal carbon fluxes on different timescales and incorporating event scale variations in these assessments. Furthermore, for most of the observations elevated levels of total alkalinity (TA) and pH were observed at the estuary mouth relative to the coastal ocean. This suggests that SDL partly buffers against acidification of adjacent coastal surface waters, although the spatial extent of this buffering is likely small.     

Quantification of methane emission from bacterial mat sites at Quepos Slide offshore Costa Rica

Seafloor methane emission from the Quepos Slide on the submarine segment of the Costa Rica fore-arc margin was estimated by extrapolating flux measurements from individual seeps to the total area covered by bacterial mats. This approach is based on the combination of detailed mapping to determine the abundance of seeps and the application of a numerical model to estimate the amount of benthic methane fluxes. Model results suggest that the majority of the studied seeps transport rather limited amount of methane (on average: ~177 μmol cm^?2 a^?1) into the water column due to moderate upward advection, allowing for intense anaerobic oxidation of methane (AOM; on average: 53 % of the methane flux is consumed). Depth-integrated AOM rates (56–1,538 μmol CH₄ cm^?2 a^?1) are comparable with values reported from other active seep sites. The overall amount of dissolved methane released into the water column from the entire area covered by bacterial mats on the Quepos Slide is estimated to be about 0.28 × 10⁶ mol a^?1. This conservative estimate which relies on rather accurate determinations of seafloor methane fluxes emphasizes the potential importance of submarine slides as sites of natural methane seepage; however, at present the global extent of methane seepage from submarine slides is largely unknown.     

Origin of light volatile hydrocarbon gases in mud volcano fluids, Gulf of Cadiz — Evidence for multiple sources and transport mechanisms in active sedimentary wedges

Widespread mud volcanism across the thick (≤ 14 km) seismically active sedimentary prism of the Gulf of Cadiz is driven by tectonic activity along extensive strike–slip faults and thrusts associated with the accommodation of the Africa–Eurasia convergence and building of the Arc of Gibraltar, respectively. An investigation of eleven active sites located on the Moroccan Margin and in deeper waters across the wedge showed that light volatile hydrocarbon gases vented at the mud volcanoes (MVs) have distinct, mainly thermogenic, origins. Gases of higher and lower thermal maturities are mixed at Ginsburg and Mercator MVs on the Moroccan Margin, probably because high maturity gases that are trapped beneath evaporite deposits are transported upwards at the MVs and mixed with shallower, less mature, thermogenic gases during migration. At all other sites except for the westernmost Porto MV, δ¹³C–CH₄ and δ²H–CH₄ values of ~ − 50‰ and − 200‰, respectively, suggest a common origin for methane; however, the ratio of CH₄/(C₂H₆ + C₃H₈) varies from ~ 10 to > 7000 between sites. Mixing of shallow biogenic and deep thermogenic gases cannot account for the observed compositions which instead result mainly from extensive migration of thermogenic gases in the deeply-buried sediments, possibly associated with biodegradation of C₂₊ homologues and secondary methane production at Captain Arutyunov and Carlos Ribeiro MVs. At the deep-water Bonjardim, Olenin and Carlos Ribeiro MVs, generation of C₂₊-enriched gases is probably promoted by high heat flux anomalies which have been measured in the western area of the wedge. At Porto MV, gases are highly enriched in CH₄ having δ¹³C–CH₄ ~ − 50‰, as at most sites, but markedly lower δ²H–CH₄ values < − 250‰, suggesting that it is not generated by thermal cracking of n-alkanes but rather that it has a deep Archaeal origin. The presence of petroleum-type hydrocarbons is consistent with a thermogenic origin, and at sites where CH₄ is predominant support the suggestion that gases have experienced extensive transport during which they mobilized oil from sediments ~ 2–4 km deep. These fluids then migrate into shallower, thermally immature muds, driving their mobilization and extrusion at the seafloor. At Porto MV, the limited presence of petroleum in mud breccia sediments further supports the hypothesis of a predominantly deep microbial origin of CH₄.     

青藏高原土壤碳排放研究进展

青藏高原土壤碳排放研究是评估国家区域碳排放量和预测气候变化所可能导致影响的关键. 首先对青藏高原土壤碳排放的关键性影响因子进行探讨, 并分析了土壤碳排放的时空分布格局变化. 目前青藏高原土壤碳排放研究主要是针对高寒草甸及高寒草地生态系统, 较少涉及高寒荒漠, 研究区域较为分散; 土壤碳排放受到气候环境因素、生物因素及人为因素等多重因素的影响, 其中温度、土壤湿度、土壤区系生物、人为因素及多年冻土退化是最关键的影响因素; 土壤碳排放具有明显的时空变异性, 空间变异性在生物群丛、景观、区域和生物群系四个尺度体现, 时间变异性在日、季、年上体现. 总体而言, 青藏高原土壤碳排放的研究较少, 尤其关于大尺度、长时间序列的研究以及土壤碳排放的机理等方面的研究十分缺乏, 有待于后续加强研究.     

海洋中溶解甲烷的原位检测技术研究进展

海水中溶解甲烷气体不但对全球变暖和海洋环境变化有着重要影响,而且也是发现渗漏型天然气水合物赋存区的依据之一,海水溶解甲烷原位监测的新技术和新方法是获取海水甲烷通量变化过程的主要手段。原位甲烷传感器具有原位、实时、便于多时空尺度定量观测等特点,在海洋环境变化和全球气候变化研究,以及海底资源开发利用中具有广泛的应用前景。介...     

Estimation of methane emission from whole waste landfill site using correlation between flux and ground temperature

A methodology to estimate a methane emission in a waste landfill site was developed. The methane flux at a waste landfill site in summer, autumn, and winter was within the following ranges: from −1.3×10⁻² to 16, from −6.4×10⁻² to 7.5, and from −1.6×10⁻³ to 1.5×10⁻² g-CH₄ m⁻² h⁻¹, respectively. In those seasons, the mean methane emission rate and coefficient of variation were 1.1 g-CH₄ m⁻² h⁻¹ ±290%, 0.57 g-CH₄ m⁻² h⁻¹ ±347%, and 5.4×10⁻² g-CH₄ m⁻² h⁻¹ ±370%, respectively. These results simultaneously showed that fluctuations of methane emission from the landfill surface were both of spatial and temporal variability. In each season, an exponential relationship was observed between the methane flux density and the ground temperature. Total methane emissions were estimated to be 5.7×10⁻², 7.1×10⁻³, and 1.7×10⁻³ g-CH₄ m⁻² h⁻¹ in the summer, autumn, and winter surveys, respectively, using a temperature surrogated-kriging method. The results of this study would improve upon the labor-intensive closed-chamber method, and could be a more practical way to estimate methane emissions from waste landfills.     

Evaluation of methane generation rate and potential from selected landfills in Malaysia

Methane emissions and oxidation were measured during the wet and dry seasons at the Air Hitam, Jeram, and Sungai Sedu landfills in Malaysia. The resulting levels of methane emissions and oxidation were then modeled using the Inter-governmental Panel on Climate Change 1996 first order decay (FOD) model to obtain methane generation rate and potential values. Emissions measurements were performed using a fabricated static flux chamber. A combination of gas concentrations in soil profiles and surface methane and carbon dioxide emissions at four monitoring locations in each landfill was used to estimate the methane oxidation capacity. The methane potential value was 151.7 m³ t^?1 for the Air Hitam and Jeram sanitary landfills and 75.9 m³ t^?1 for the Sungai Sedu open dumping landfill. The methane generation rate value of the Jeram and Air Hitam sanitary landfills during the wet season was 0.136 year^?1, while that of Jeram during the dry season was 0.072 year^?1. The methane generation rate values of the Sungai Sedu open dumping landfill during the wet and dry seasons were 0.008 and 0.0049 year^?1, respectively. The observed values of methane generation rate and potential assist to accurately estimate total methane emissions from Malaysian landfills using the Inter-governmental Panel on Climate Change FOD model.     

1990-2014年西藏季节冻土最大冻结深度的时空变化

最大冻结深度是季节冻土变化的主要指标,也是季节冻土地区工程设计、建设、运营的重要参数。通过斯蒂芬（Stefan）方法计算了1990-2014年西藏地区季节冻土的最大冻结深度,分析了其时空变化特征,结果表明：近25 a西藏地区季节冻土最大冻结深度在空间分布具有垂直分带性、纬度地带性和区域性等规律,基本上呈自西北向东南方向递减的空间分布特征;时间上,在全球气候变暖的背景下,最大冻结深度基本呈逐年减薄的特征。西藏地区季节冻土最大冻结深度与年平均气温和年降水量呈现负相关,随着年平均气温和年降水量的上升,最大冻结深度呈减小的趋势,且最大冻结深度对年平均气温的响应比对年降水量的响应显著。     

含油气盆地地质甲烷释放研究综述

目前地质甲烷在国际上引起了广泛的关注。地质甲烷的释放在整个大气甲烷的源与汇的研究具有重要的意义。它既为不含放射性~(14)C甲烷源缺失部分提供了可能的解释,也为全球气候变暖的研究提供了科学依据。含油气盆地的甲烷是地质甲烷中的重要组成部分。本文概述了国内外含油气盆地甲烷释放的研究,详述了含油气盆地甲烷的源以及运移机制;阐述了目前对于地质甲烷研究的常用监测手段及其优缺点;并对全球含油气盆地的甲烷通量估算进行了总结。但是由于含油气盆地甲烷的通量估算是建立在区域性的部分甲烷通量测试基础上,仍然具有较大的不确定性。有效地结合甲烷的各种监测手段,在全球更多更广区域开展含油气盆地CH_4通量测量,是未来的发展方向和热点。我国是油气生产的大国之一,但相关的探究很少,因此,在我国开展含油气盆地甲烷释放通量的研究将有助于进一步完善全球含油气盆地甲烷数据库,对全球甲烷的源和汇的精确估算具有重要作用和意义。     

Eddy covariance based methane flux in Sundarbans mangroves,India

We report the initial results of the methane flux measured using eddy covariance method during summer months from the world’s largest mangrove ecosystem, Sundarbans of India. Mangrove ecosystems are known sources for methane (CH₄) having very high global warming potential. In order to quantify the methane flux in mangroves, an eddy covariance flux tower was recently erected in the largest unpolluted and undisturbed mangrove ecosystem in Sundarbans (India). The tower is equipped with eddy covariance flux tower instruments to continuously measure methane fluxes besides the mass and energy fluxes. This paper presents the preliminary results of methane flux variations during summer months (i.e., April and May 2012) in Sundarbans mangrove ecosystem. The mean concentrations of CH₄ emission over the study period was 1682 ± 956 ppb. The measured CH₄ fluxes computed from eddy covariance technique showed that the study area acts as a net source for CH₄ with daily mean flux of 150.22 ± 248.87 mg m^?2 day^?1. The methane emission as well as its flux showed very high variability diurnally. Though the environmental conditions controlling methane emission is not yet fully understood, an attempt has been made in the present study to analyse the relationships of methane efflux with tidal activity. This present study is part of Indian Space Research Organisation–Geosphere Biosphere Program (ISRO–GBP) initiative under ‘National Carbon Project’.     

Hydraulic fracturing in unconventional gas reservoirs: risks in the geological system, part 2

Hydraulic fracturing is a method used for the production of unconventional gas resources. Huge amounts of so-called fracturing fluid (10,000–20,000 m³) are injected into a gas reservoir to create fractures in solid rock formations, upon which mobilised methane fills the pore space and the fracturing fluid is withdrawn. Hydraulic fracturing may pose a threat to groundwater resources if fracturing fluid or brine can migrate through fault zones into shallow aquifers. Diffuse methane emissions from the gas reservoir may not only contaminate shallow groundwater aquifers, but also escape into the atmosphere where methane acts as a greenhouse gas. The working group “Risks in the Geological System” as part of ExxonMobil’s hydrofracking dialogue and information dissemination processes was tasked with the assessment of possible hazards posed by migrating fluids as a result of hydraulic fracturing activities. In this work, several flow paths for fracturing fluid, brine and methane are identified and scenarios are set up to qualitatively estimate under what circumstances these fluids would leak into shallower layers. The parametrisation for potential hydraulic fracturing sites in North Rhine-Westphalia and Lower Saxony (both in Germany) is derived from literature using upper and lower bounds of hydraulic parameters. The results show that a significant fluid migration is only possible if a combination of several conservative assumptions is met by a scenario.     

Groundwater and surface-water interactions and impacts of human activities in the Hailiutu catchment,northwest China

The interactions between groundwater and surface water have been significantly affected by human activities in the semi-arid Hailiutu catchment, northwest China. Several methods were used to investigate the spatial and temporal interactions between groundwater and surface water. Isotopic and chemical analyses of water samples determined that groundwater discharges to the Hailiutu River, and mass balance equations were employed to estimate groundwater seepage rates along the river using chemical profiles. The hydrograph separation method was used to estimate temporal variations of groundwater discharges to the river. A numerical groundwater model was constructed to simulate groundwater discharges along the river and to analyze effects of water use in the catchment. The simulated seepage rates along the river compare reasonably well with the seepage estimates derived from a chemical profile in 2012. The impacts of human activities (river-water diversion and groundwater abstraction) on the river discharge were analyzed by calculating the differences between the simulated natural groundwater discharge and the measured river discharge. Water use associated with the Hailiutu River increased from 1986 to 1991, reached its highest level from 1992 to 2000, and decreased from 2001 onwards. The reduction of river discharge might have negative impacts on the riparian ecosystem and the water availability for downstream users. The interactions between groundwater and surface water as well as the consequences of human activities should be taken into account when implementing sustainable water resources management in the Hailiutu catchment.     

Ground-based optical measurements of daytime auroral emissions: A new means of investigating space-weather related processes

The first optical signatures of plasmapause associated processes were gleaned from the daytime auroral emissions monitored from Maitri, the Indian station in Antarctica, using the newly built, ground-based, Multiwavelength Daytime Photometer (MWDPM). The plasmapause boundary inferred from these measurements conforms well with the known empirical relationship with the geomagnetic indexKp. The measurements made at 391.4, 486.1, 557.7 and 630.0 nm emissions revealed significant day-to-day variations in their spatial and temporal extents. These preliminary results are presented and discussed.     

Transient discharges from marine hydrocarbon seeps: spatial and temporal variability

Marine hydrocarbon gas emissions at an intense, 20-m-deep seep in the Santa Barbara Channel, California were studied with a network of three turbine seep-tents and repeated seabed mapping. The tents observed two gas ejection events that are interpreted as due to blockage of constrictions in fractures and subsequent blow-through. Seabed mapping suggests that very large transient emission events occur, are related to tar, and are temporally and spatially variable. Transient emissions have the potential to more efficiently transport methane to the atmosphere than steady-state emissions. We present an electrical model analog of subsurface seepage useful for seepage flux interpretation. The model predicts that variations in resistance at one vent shifts some of its flux to other connected vents, and that the shift is not zero-sum, i.e., a resistance change at one vent causes a flow change for the overall fracture system.