Methane Dynamics Research Articles

Positron-emitting radiotracers spatially resolve unexpected biogeochemical relationships linked with methane oxidation in Arctic soils.

Arctic soils are marked by cryoturbic features, which impact soil-atmosphere methane (CH4 ) dynamics vital to global climate regulation. Cryoturbic diapirism alters C/N chemistry within frost boils by introducing soluble organic carbon and nutrients, potentially influencing microbial CH4 oxidation. CH4 oxidation in soils, however, requires a spatio-temporal convergence of ecological factors to occur. Spatial delineation of microbial activity with respect to these key microbial and biogeochemical factors at relevant scales is experimentally challenging in inherently complex and heterogeneous natural soil matrices. This work aims to overcome this barrier by spatially linking microbial CH4 oxidation with C/N chemistry and metagenomic characteristics. This is achieved by using positron-emitting radiotracers to visualize millimeter-scale active CH4 uptake areas in Arctic soils with and without diapirism. X-ray absorption spectroscopic speciation of active and inactive areas shows CH4 uptake spatially associates with greater proportions of inorganic N in diapiric frost boils. Metagenomic analyses reveal Ralstonia pickettii associates with CH4 uptake across soils along with pertinent CH4 and inorganic N metabolism associated genes. This study highlights the critical relationship between CH4 and N cycles in Arctic soils, with potential implications for better understanding future climate. Furthermore, our experimental framework presents a novel, widely applicable strategy for unraveling ecological relationships underlying greenhouse gas dynamics under global change.

FESDIA: An early diagenesis model including Iron (Fe), Sulfur (S) and Methane (CH4) dynamics

FESDIA: An early diagenesis model including Iron (Fe), Sulfur (S) and Methane (CH4) dynamics

Microsecond molecular dynamics of methane\u2013carbon dioxide swapping in pure and saline water environment

This work aims at proposing the nondestructive methane-carbon dioxide (CH4–CO2) replacement mechanism as an ecofriendly energy production technique from the natural gas hydrate reserves in seafloor and permanently frozen grounds. Although the experimental data is widely available in literature, this replacement mechanism has not been elucidated at molecular level. In this contribution, we perform the microsecond level molecular dynamic simulations to evaluate two different CH4–CO2 replacement mechanisms: (i) direct CH4 displacement from hydrate structure, and (ii) dissociation of existing methane hydrate followed by a reformation of mixed CH4–CO2 hydrates. For this, we analyze CH4–CO2 replacement in three different modes i.e., CO2 as a replacing agent in (i) absence of free water molecules, (ii) presence of free water molecules, and (iii) presence of salt ions and free water molecules. Despite slow kinetics in the first mode, pure CO2 is observed to replace the methane more efficiently, while in the second mode, CO2 forms a new mixed hydrate layer on the existing seed crystal. However, in the third mode, salt ions help in destabilizing the methane hydrate and allow CO2 to form the hydrates. This proves that salt ions are favorable for CH4–CO2 replacement.

Accounting for Methane Dynamics in the Upper Yangtze River Valley Dammed Reservoir in China: A Hierarchical Bayesian Modeling Approach

Accounting for Methane Dynamics in the Upper Yangtze River Valley Dammed Reservoir in China: A Hierarchical Bayesian Modeling Approach

Pond methane dynamics, from microbial communities to ecosystem budget, during summer in Alaska

AbstractPonds play a larger role in the global freshwater methane (CH4) budget than predicted from surface area alone. To improve our understanding of pond CH4 dynamics, we measured summer CH4 production, concentrations, and emissions to the atmosphere in nine Alaskan wetland ponds along with potential physical, chemical, and biological regulators. Pond CH4 production (0.64, 0.086–1.3 mmol m−2 d−1; median, interquartile range), as assessed with slurry incubations, was positively related to water‐column temperature and chlorophyll a (Chl a), negatively influenced by oxygen levels, and varied with microbial community structure. Average water‐column CH4 concentrations (0.39, 0.21–0.87 μmol L−1) were lower in deeper ponds and at higher oxygen levels, and as expected, they were correlated with diffusive emissions (0.055, 0.024–0.20 mmol m−2 d−1) assessed with flux chambers. Based on a mass balance approach, 39–99% of CH4 produced in ponds was oxidized. Pond ebullition (3.7, 0.60–24 mmol m−2 d−1) was higher and more variable than diffusive emissions. Additionally, pond ebullition rates were better correlated with production rates from the previous month. We also systematically compared the ratio of ebullition to diffusive CH4 emissions in our ponds and other northern lakes, which was negatively related to water depth (n = 71), but positively related to Chl a (n = 28). Our study sheds light on the factors that influence pond CH4 dynamics and demonstrates that pond ebullition is a significant CH4 source worthy of continued study.

On the Flow Structure and Dynamics of Methane and Syngas Lean Flames in a Model Gas-Turbine Combustor

The present paper compares the flow structure and flame dynamics during combustion of methane and syngas in a model gas-turbine swirl burner. The burner is based on a design by Turbomeca. The fuel is supplied through injection holes between the swirler blades to provide well-premixed combustion, or fed as a central jet from the swirler’s centerbody to increase flame stability via a pilot flame. The measurements of flow structure and flame front are performed by using the stereo particle image velocimetry and OH planar laser-induced fluorescence methods. The measurements are performed for the atmospheric pressure without preheating and for 2 atm with the air preheated up to 500 K. The flow Reynolds numbers for the non-reacting flows at these two conditions are 1.5 × 103 and 1.0 × 103, respectively. The flame dynamics are analyzed based on a high-speed OH* chemiluminescence imaging. It is found that the flame dynamics at elevated conditions are related with frequent events of flame lift-off and global extinction, followed by re-ignition. The analysis of flow structure via the proper orthogonal decomposition reveals the presence of two different types of coherent flow fluctuations, namely, longitudinal and transverse instability modes. The same procedure is applied to the chemiluminescence images for visualization of bulk movement of the flame front and similar spatial structures are observed. Thus, the longitudinal and transverse instability modes are found in all cases, but for the syngas at the elevated pressure and temperature the longitudinal mode is related to strong thermoacoustic fluctuations. Therefore, the present study demonstrates that a lean syngas flame can become unstable at elevated pressure and temperature conditions due to a greater flame propagation speed, which results in periodic events of flame flash-back, extinction and re-ignition. The reported data is also useful for the validation of numerical simulation codes for syngas flames.

Dynamics of methane and carbon dioxide emissions in the reclaimed islands of Sundarban mangrove ecosystem, India

AbstractThe loss and deterioration of mangroves would lead to potentially important greenhouse gases (GHG) from the mangrove ecosystem. Seasonal variations of methane (CH4) and carbon dioxide (CO2) fluxes are studied in the Indian part of the Sundarban mangrove ecosystem. Considering the importance of CH4 and CO2 fluxes, a dynamic model is proposed here with CH4 and CO2 as state variables. The data used in the model are collected and analysed from the field over two years. The parameters that are taken from the literature sources are calibrated in the model. Sensitivity analysis is conducted to assess the valuable contribution of various parameters to the dynamics of CH4 and CO2 emissions from the mangrove beds. Model simulation results are appropriately validated with the observed data. Results showed that CH4 and CO2 fluxes were higher during monsoon. Pearson correlation was done to assess the relationships between emissions and all the physical–chemical factors involved in the model. Sensitivity analysis indicated that the temperature sensitivity (Q10) was the system‐sensitive parameter for CH4 and CO2 emissions. The soil moisture and relative light intensity were the sensitive parameters for CH4 and CO2 emissions, respectively. Model showed that the increase in the anthropogenic emissions including emissions from livestock and fossil fuel caused an increase of 2% of CH4 and 3% of CO2 in the system, respectively. The proposed model could be a tool for environmental managers to assess the emission process at different global locations.

Pelagic Methane Sink Enhanced by Benthic Methanotrophs Ejected From a Gas Seep

AbstractCold seeps represent hot spots of seabed‐derived methane emissions to the water column, where physical and biological barriers regulate transport of methane to the atmosphere. In our study, we used a methane point source in the North Sea to investigate water‐column methane dynamics within the dispersing plumes. The study is based on methane concentration, distribution and activity of methane‐oxidizing bacteria (MOB), and oceanographic observations. Our findings suggest the ejection of benthic MOB into the water column by sediment resuspension and gas‐bubble‐mediated transport. An applied particle‐tracking model demonstrated that the ejection of 4.29 ± 1.9 × 1012 MOB cells s−1 from the seep site would produce the MOB abundance detected in the down‐current water body. The benthic MOB inoculant accelerated methane oxidation rates by a factor of five. Our study highlights the importance of the bentho‐pelagic transport of microorganisms at seep sites and their feedback on the pelagic methane sink.

Subsurface methane dynamics of a paddy field under long-term fertilization: 13C-evidence from in-situ belowground labeling

Rice paddies are a significant source and sink of methane (CH4). The field surface emission and laboratory turnover potential of CH4 have been extensively studied. However, there is a knowledge gap in underlying mechanisms linking in-situ belowground CH4 production and oxidation potentials affected by rice growth and fertilization. Here, we implemented an improved soil silicone tube approach and 13C-labeled CH4 to measure the in-situ production and oxidation of CH4 in the subsoil (20–30 cm) during rice growth under four long-term (7 years) fertilization treatments. Urea N (uN) fertilization decreased 3-folds CH4 production as compared to non-fertilized control (Control). In combination with uN, biochar application (uN + Biochar) resulted in similarly low [CH4], but manure fertilization (uN + Manure) stimulated CH4 production which increased with rice growth and reached 0.6 mmol L−1 of soil tube at mature stage relative to uN. During the rice growth, δ13C–CH4 values under all fertilizations progressively decreased as compared with the start of the experiment but remained in the range of acetoclastic methanogenesis (−75‰). Pronounced δ13C–CO2 enrichment under uN + Manure indicated the increasing contribution of CO2-reduction pathway. The CH4 oxidation was highest (20.1 μmol L−1) under uN + Manure at tillering stage, followed by uN (14.6 μmol L−1) at seedling stage. The lowest oxidation potential was under uN + Biochar along with the weakest CH4 concentration dynamics. Water-locked conditions, low measured O2 concentration in silicon tubes, as well as the positive relationship between CH4 oxidation and NO3− and DOC suggested a possibility of anaerobic oxidation of CH4 (AOM). In conclusion, the intensive rice growth during seedling and tillering stages controls subsoil CH4 concentrations via methanotrophic activity (anaerobic oxidation pathway). Manure fertilization offset the inhibiting effect of mineral N fertilization on CH4 production but stimulated CH4 oxidation. Among all fertilization treatments biochar strongly suppressed the CH4 turnover and can therefore be recommended for greenhouse gas mitigation strategy in cleaner rice production.

Assessing Spatio‐Temporal Variability of Free Gas in Surficial Cohesive Sediments Using Tidal Pressure Fluctuations

AbstractFrom a geohazard assessment perspective, the distribution, content, and dynamics of free gas in surficial sediment were addressed by imaging and monitoring the upper 15 m beneath the shelf offshore Nice, France. Based on high‐resolution seismic data covering three sites where pore pressure was recorded over three and a half years, the presence of free gas was determined in the upper 2.75–14.75 m of cohesive, silty clay. Seismic velocity changes delineate two layers with gas volume fractions ranging from 0.12% to 1.89%. By considering the tidal response recorded by eight pore pressure sensors, estimates of gas volume fractions vary from 0.26% to more than 9.4% on a spatio‐temporal scale which cannot be achieved with seismic data. To depict spatio‐temporal patterns three types of free gas occurrence (FGO) were distinguished. Type 3 FGO uniquely showed sawtooth fluctuations in overpressure of 27%–45% of the hydrostatic effective stress as evidence of bubble growth and rise. The other two types showed long‐term overpressure trends indicative of a situation whereby bubble growth has ceased. Type 1 FGOs are distinguished from type 2 by their gas volume fraction lower than 9.4% and ratios of overpressure to hydrostatic effective stress lower than 0.3. Values higher than this threshold are considered sufficient for shear failure to initiate from the steep shelf edge (>20°). Beyond site‐specific insights, the distinction of FGO from their overpressure levels yields testable implications for the dynamics of methane in sediments.

Elevated levels of radiocarbon in methane dissolved in seawater reveal likely local contamination from nuclear powered vessels

Measurements of the natural radiocarbon content of methane (14C-CH4) dissolved in seawater and freshwater have been used to investigate sources and dynamics of methane. However, during investigations along the Atlantic, Pacific, and Arctic Ocean Margins of the United States, as well as in the North American Great Lakes, some samples revealed highly elevated 14C-CH4 values, as much as 4–5 times above contemporary atmospheric 14C-CH4 levels. Natural production of the 14CH4 isotopologue is too low to cause these observations nor can it explain the variations in location and depth. Numerous lab and field validation tests and blanks, as well as the relatively small number of samples that display these elevated values, all suggest that these signals are not derived from an unknown procedural issue. Here we suggest that the byproducts of nuclear power generation include localized discharges of the 14CH4 isotopologue into marine and aquatic environments, severely altering the measured 14C-CH4 isotopic signals. Since several of our sample sites are distant from on-land nuclear powerplants, we conduct further calculations concluding that the most elevated anomalies in 14C-CH4 likely originate with discharge from nuclear-powered vessels.

Dynamics of Methane in Mangrove Forest: Will It Worsen with Decreasing Mangrove Forests?

Mangrove forests sequester a significant amount of organic matter in their sediment and are recognized as an important carbon storage source (i.e., blue carbon, including in seagrass ecosystems and other coastal wetlands). The methane-producing archaea in anaerobic sediments releases methane, a greenhouse gas species. The contribution to total greenhouse gas emissions from mangrove ecosystems remains controversial. However, the intensity CH4 emissions from anaerobic mangrove sediment is known to be sensitive to environmental changes, and the sediment is exposed to oxygen by methanotrophic (CH4-oxidizing) bacteria as well as to anthropogenic impacts and climate change in mangrove forests. This review discusses the major factors decreasing the effect of mangroves on CH4 emissions from sediment, the significance of ecosystem protection regarding forest biomass and the hydrosphere/soil environment, and how to evaluate emission status geospatially. An innovative “digital-twin” system overcoming the difficulty of field observation is required for suggesting sustainable mitigation in mangrove ecosystems, such as a locally/regionally/globally heterogenous environment with various random factors.

Hot spot of CH4 production and diffusive flux in rivers with high urbanization

Rivers and streams play a central role in global carbon budget, but our knowledge is limited on the magnitude and extent of urbanization influence on riverine methane (CH4) dynamics. In this study, we investigated dissolved CH4 (dCH4) concentration and CH4 diffusive fluxes in 27 river segments of two 4th-order and three 3rd-order tributary rivers to the Yangtze River in China, which drained land areas with varied urbanization intensities. We found that urban development was the key factor responsible for high fluvial dCH4 concentration and diffusive flux, exceeding the influence of agricultural farming, and these headwater rivers were over-saturated in CH4 with respect to atmospheric equilibrium. dCH4 concentration (3546 ± 6770 nmol L−1) in the river segments draining higher urban area (20% ≤ urban land proportion ≤ 46%) was 5-6 times higher than those (615 ± 627 nmol L−1 and 764 ± 708 nmol L−1) in the river segments draining less urban area (0.1% ≤ urban land proportion < 2% and 2 ≤ urban land proportion < 20%). River segments draining higher urban area also acted as important sources of CH4 to the atmosphere (8.93 ± 14.29 mmol m−2 d−1). Total nitrogen (TN) concentration in river water showed the best prediction capacity when compared to other water parameters. Based on urban land use grouping, nutrient elements could predict dCH4 well in rivers draining higher urban areas (urban ≥ 2%), which also reflected the lateral input of pollutants (TN, ammonia nitrogen, and total phosphorus). River bottom sediment fraction contributed to trapping organic matter and nutrients as well as to oxic and anoxic conditions, thereby determining reach-scale spatial patterns of dCH4 concentration. These findings highlight that combining distal geomorphic and hydrologic drivers can be effective in determining the relationship between riverine CH4 and the proximal controls (e.g., nutrients, dissolved oxygen, dissolved organic carbon), as well as in identifying their key drivers. Being rapid urbanization a common feature of catchments worldwide, our results suggest riverine CH4 emissions will increase into the future.

Carbonate-hosted microbial communities are prolific and pervasive methane oxidizers at geologically diverse marine methane seep sites

At marine methane seeps, vast quantities of methane move through the shallow subseafloor, where it is largely consumed by microbial communities. This process plays an important role in global methane dynamics, but we have yet to identify all of the methane sinks in the deep sea. Here, we conducted a continental-scale survey of seven geologically diverse seafloor seeps and found that carbonate rocks from all sites host methane-oxidizing microbial communities with substantial methanotrophic potential. In laboratory-based mesocosm incubations, chimney-like carbonates from the newly described Point Dume seep off the coast of Southern California exhibited the highest rates of anaerobic methane oxidation measured to date. After a thorough analysis of physicochemical, electrical, and biological factors, we attribute this substantial metabolic activity largely to higher cell density, mineral composition, kinetic parameters including an elevated Vmax, and the presence of specific microbial lineages. Our data also suggest that other features, such as electrical conductance, rock particle size, and microbial community alpha diversity, may influence a sample's methanotrophic potential, but these factors did not demonstrate clear patterns with respect to methane oxidation rates. Based on the apparent pervasiveness within seep carbonates of microbial communities capable of performing anaerobic oxidation of methane, as well as the frequent occurrence of carbonates at seeps, we suggest that rock-hosted methanotrophy may be an important contributor to marine methane consumption.

Methane dynamics of high-elevation lakes in the Sierra Nevada California: the role of elevation, temperature, and inorganic nutrients

ABSTRACT Freshwater lakes are important but poorly constrained sources of methane (CH4) to the atmosphere, reflecting high but variable rates of CH4 production as well as limited and inconsistent measurements worldwide. High-elevation lakes have been particularly overlooked given their high abundance in mountain ranges around the world, and the potential for CH4 dynamics at high elevations is expected to be disproportionally impacted by increases in temperature due to climate change. We examined variations in surface CH4 concentrations and diffusive fluxes, temperature, and inorganic nutrients in 5 montane lakes spanning multiple elevations in the Sierra Nevada of California. Over 2 years, we found strong and consistent seasonality in CH4 concentrations in lakes; higher concentrations were typically observed in the warmest months and lower concentrations right before winter. Changes in CH4 concentrations were significantly related to temperature in the majority of the individual lakes (r 2 = 0.43–0.81) and related to elevation (r 2 = 0.39) across lakes. Methane concentrations in lakes at elevations <3000 m were strongly related to temperature, nitrite concentrations, and elevation (r 2 = 0.90), whereas at elevations >3000 m, CH4 was related to dissolved inorganic nitrogen to dissolved inorganic phosphorus ratios and elevation (r 2 = 0.48). Our results expand our understanding of temporal variations in CH4 and demonstrate substantial seasonality in CH4 concentrations and diffusive fluxes in freshwater lakes. We suggest that temporal variation should be considered in large-scale estimates of CH4 emissions, and these fluxes may be a predictable function of elevation, temperature, and nutrients.

Carbon dioxide and methane dynamics in a peatland headwater stream: origin, processes and implications

Carbon dioxide and methane dynamics in a peatland headwater stream: origin, processes and implications

Tropical Lakes May Emit More Methane

Fresh waters are one of the primary sources of natural methane emissions, but methane dynamics in tropical lakes are not as well understood as those in more temperate regions.

How Are Greenhouse Gases Coupled Across Seasons in a Large Temperate River with Differential Land Use?

Rivers are known to emit large amounts of greenhouse gases globally, however, few studies have evaluated the interacting influence of land use, within river features (for example, sites of major confluence, changes in shape), and hydrology on the coupled dynamics of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). Here, we measured CO2, CH4 and N2O concentrations and fluxes at 15 sites along a 146.6 km stretch of the main stem of a large north temperate river: the Riviere du Nord, which experiences an abrupt change in geology and land use and winter ice cover. Each site along the river was sampled once per season, at targeted low flow and high flow moments, for three consecutive years. Overall, highest concentrations of all gases were found in the lower reaches where urban and agricultural activity are most intensive. CO2 and N2O were highest overall under ice during winter, whereas CH4 was highest during summer. The river was always supersaturated in CH4 whereas surface water was occasionally undersaturated in N2O and CO2, particularly during the summer low flow period in pristine upper reaches. Spatial variability was, however, minimized during periods of high flow. Emissions varied along the continuum, with peaks occurring at turbulent sites of major confluence. Although land use influenced the spatial variability in concentrations along the river, seasonal changes in temperature influenced the relative importance of the different gases to global warming potential, and hydrology mediated where they were produced as well as their overall concentrations and emissions.

Controlling Factors of Methane in Tropical Lakes of Different Depths

AbstractTo understand methane (CH4) dynamics in tropical lakes and describe their importance in overall CH4 emissions, we observed temporal trends of CH4 stored in the water columns of Philippine lakes of different depths (shallow, moderately deep, and deep). We detected higher CH4 stored in profundal anoxic layers of deeper lakes. The average surface and profundal CH4 concentrations throughout our monthly monitoring were 0.27 ± 0.07 and 421 ± 189 μmol L–1 in the shallow lake, 0.48 ± 0.37 and 1,121 ± 125 μmol L–1 in the moderately deep lake, and 0.34 ± 0.23 and 943 ± 119 μmol L–1 in the deep lake. The fate of CH4 stored in the water column was related to depth, owing to different stratification and mixing regimes. Analyses of temporal relationships of CH4 with daily weather and physicochemical parameters revealed their impact on CH4 dynamics by controlling the frequency and mixing intensity throughout the year. Only shallow lake exhibited increase in CH4 concentrations with lowering atmospheric pressure. This allows vertical mixing and transport of CH4 from deeper to shallower layers. In moderately deep, and deep lakes, effects of daily weather parameters were not apparent on CH4 concentrations in the deepest layers (below 30 m depth). This suggests that lake depth and stratification should be considered for precise estimation of tropical lakes' CH4 emissions. Comparison with lakes under different climate conditions revealed the importance of tropical lakes for surface CH4 emission due to higher surface CH4 concentrations. Changes in water temperature is one of the factors controlling profundal CH4 production, storage, and its transport to shallower layer.

Methane dynamics in three different Siberian water bodies under winter and summer conditions

Abstract. Arctic regions and their water bodies are affected by a rapidly warming climate. Arctic lakes and small ponds are known to act as an important source of atmospheric methane. However, not much is known about other types of water bodies in permafrost regions, which include major rivers and coastal bays as a transition type between freshwater and marine environments. We monitored dissolved methane concentrations in three different water bodies (Lena River, Tiksi Bay, and Lake Golzovoye, Siberia, Russia) over a period of 2 years. Sampling was carried out under ice cover (April) and in open water (July–August). The methane oxidation (MOX) rate and the fractional turnover rate (k′) in water and melted ice samples from the late winter of 2017 was determined with the radiotracer method. In the Lena River winter methane concentrations were a quarter of the summer concentrations (8 nmol L−1 vs. 31 nmol L−1), and mean winter MOX rate was low (0.023 nmol L−1 d−1). In contrast, Tiksi Bay winter methane concentrations were 10 times higher than in summer (103 nmol L−1 vs. 13 nmol L−1). Winter MOX rates showed a median of 0.305 nmol L−1 d−1. In Lake Golzovoye, median methane concentrations in winter were 40 times higher than in summer (1957 nmol L−1 vs. 49 nmol L−1). However, MOX was much higher in the lake (2.95 nmol L−1 d−1) than in either the river or bay. The temperature had a strong influence on the MOX (Q10=2.72±0.69). In summer water temperatures ranged from 7–14 ∘C and in winter from −0.7 to 1.3 ∘C. In the ice cores a median methane concentration of 9 nM was observed, with no gradient between the ice surface and the bottom layer at the ice–water interface. MOX in the (melted) ice cores was mostly below the detection limit. Comparing methane concentrations in the ice with the underlaying water column revealed methane concentration in the water column 100–1000 times higher. The winter situation seemed to favor a methane accumulation under ice, especially in the lake with a stagnant water body. While on the other hand, in the Lena River with its flowing water, no methane accumulation under ice was observed. In a changing, warming Arctic, a shorter ice cover period is predicted. With respect to our study this would imply a shortened time for methane to accumulate below the ice and a shorter time for the less efficient winter MOX. Especially for lakes, an extended time of ice-free conditions could reduce the methane flux from the Arctic water bodies.