Methane Dynamics Research Articles

Methane dynamics in the Baltic Sea: investigating concentration, flux, and isotopic composition patterns using the coupled physical–biogeochemical model BALTSEM-CH4 v1.0

Abstract. Methane (CH4) cycling in the Baltic Sea is studied through model simulations that incorporate the stable isotopes of CH4 (12C–CH4 and 13C–CH4) in a physical–biogeochemical model. A major uncertainty is that spatial and temporal variations in the sediment source are not well known. Furthermore, the coarse spatial resolution prevents the model from resolving shallow-water near-shore areas for which measurements indicate occurrences of considerably higher CH4 concentrations and emissions compared with the open Baltic Sea. A preliminary CH4 budget for the central Baltic Sea (the Baltic Proper) identifies benthic release as the dominant CH4 source, which is largely balanced by oxidation in the water column and to a smaller degree by outgassing. The contributions from river loads and lateral exchange with adjacent areas are of marginal importance. Simulated total CH4 emissions from the Baltic Proper correspond to an average ∼1.5 mmol CH4 m−2 yr−1, which can be compared to a fitted sediment source of ∼18 mmol CH4 m−2 yr−1. A large-scale approach is used in this study, but the parameterizations and parameters presented here could also be implemented in models of near-shore areas where CH4 concentrations and fluxes are typically substantially larger and more variable. Currently, it is not known how important local shallow-water CH4 hotspots are compared with the open water outgassing in the Baltic Sea.

Ebullition drives high methane emissions from a eutrophic coastal basin

The production of methane in coastal sediments and its release to the water column is intensified by anthropogenic eutrophication and bottom water hypoxia, and it is still uncertain whether methane emissions to the atmosphere will be enhanced. Here, we assess seasonal variations in methane dynamics in a eutrophic, seasonally euxinic coastal basin (Scharendijke, Lake Grevelingen). In-situ benthic chamber incubations reveal high rates of methane release from the sediment to the water column (74–163 mmol m−2 d−1) during monthly measurements between March and October 2021. Comparison of these in-situ total benthic methane fluxes and calculated diffusive fluxes point towards a major role for ebullition. In spring and fall, when the water column was oxic, microbial removal of dissolved methane occurred aerobically in the bottom water. In summer, in contrast, dissolved methane accumulated to concentrations of up to 67 μmol L−1 below the oxycline. Shifts in δ13C–CH4 and δD-CH4 towards higher values and the abundant presence of methane oxidizing bacteria point towards removal of methane around the oxycline, likely through both aerobic and anaerobic pathways, with the latter possibly linked to iron oxide reduction. Shifts in δ13C–CH4 and δD-CH4 to lower values above the oxycline indicate that bubble dissolution contributed to dissolved methane. Methane emissions to the atmosphere were observed in all seasons with the highest, in-situ measured diffusive fluxes (1.2 mmol m−2 d−1) upon the onset of water column mixing at the end of summer. Methane release events during the measurement of in-situ water-air fluxes and model calculations point towards a flux of methane to the atmosphere in the form of bubbles, which bypass the microbial methane filter. The model calculations suggest a potential year-round ebullitive methane flux between 30 and 120 mmol m−2 d−1. We conclude that methane emissions from eutrophic coastal systems may be much higher than previously thought because of ebullition.

Relationship between fish traits and the methane‐derived carbon contribution to fish in a shallow lake

Abstract Although the transfer of methane‐derived carbon (MDC) to benthic macroinvertebrates such as chironomid larvae and oligochaetes has been well documented, knowledge of the transfer of MDC to fish is limited. We investigated the MDC contribution to 28 fish species through their growth stages and examined the relationship between fish traits including body size, and MDC contribution to fish within species in Lake Kasumigaura, Japan. We used carbon stable isotope ratios and a two‐source mixing model (methane‐oxidation bacteria [MOB] and particulate organic matter) to estimate the MDC contribution to fish. The MDC contribution to individual fish ranged from 0% to 21.5%. Anguilla japonica showed the highest MDC contribution (mean 3.8%). Fish species were classified into four groups according to two criteria: (a) whether the correlation between the MDC contribution and fish size (within species) was significantly negative (negative vs. not significant; ns); and (b) whether the 90th percentile of MDC contribution was 0% or not. The mean MDC contribution was highest (2.18% ± 2.93%, 12 species) for the group with (a)ns/(b)>0%, including the high contribution species such as Abbottina rivularis (mean 5.5%) and Biwia zezera (mean 9.8%). This group (12 species) included relatively high numbers of benthic habitat use (eight species) and omnivores (nine species). The mean MDC contributions were almost zero for the group with (a)ns/(b)=0% (seven species), which included only two species of benthic habitat use and one species of omnivores. The mean MDC contributions were intermediate for the other groups with (a)negative/(b)>0% (1.30% ± 1.17%, seven species) and (a)negative/(b)=0% (0.10%, two species). The negative correlation between fish size within species and MDC contribution in groups with (a)negative/(b)>0% and (a)negative/(b)=0% indicated that ontogenetic habitat and dietary shifts induced changes in MDC transfer. Our results suggest that 21 out of 28 fish species were involved in the MDC transfer in a shallow lake, but variation in MDC transfer could be explained by fish traits (trophic guild and habitat use). Although the mean MDC contribution to all fish species was low (0.61% ± 1.97%), the roughly estimated annual consumption rate of fish on MOB corresponded to approximately 10% of the MOB production in the sediment, suggesting that fish contribute to methane dynamics from the perspective of biogeochemical cycles.

Impact of Crop Residue, Nutrients, and Soil Moisture on Methane Emissions from Soil under Long-Term Conservation Tillage

Greenhouse gas emissions from agricultural production systems are a major area of concern in mitigating climate change. Therefore, a study was conducted to investigate the effects of crop residue, nutrient management, and soil moisture on methane (CH4) emissions from maize, rice, soybean, and wheat production systems. In this study, incubation experiments were conducted with four residue types (maize, rice, soybean, wheat), seven nutrient management treatments {N0P0K0 (no nutrients), N0PK, N100PK, N150PK, N100PK + manure@ 5 Mg ha−1, N100PK + biochar@ 5 Mg ha−1, N150PK+ biochar@ 5 Mg ha−1}, and two soil moisture levels (80% FC, and 60% FC). The results of this study indicated that interactive effects of residue type, nutrient management, and soil moisture significantly affected methane (CH4) fluxes. After 87 days of incubation, the treatment receiving rice residue with N100PK at 60% FC had the highest cumulative CH4 mitigation of −19.4 µg C kg−1 soil, and the highest emission of CH4 was observed in wheat residue application with N0PK at 80% FC (+12.93 µg C kg−1 soil). Nutrient management had mixed effects on CH4 emissions across residue and soil moisture levels in the following order: N150PK > N0PK > N150PK + biochar > N0P0K0 > N100PK + manure > N100PK + biochar > N100PK. Decreasing soil moisture from 80% FC to 60% FC reduced methane emissions across all residue types and nutrient treatments. Wheat and maize residues exhibited the highest carbon mineralization rates, followed by rice and soybean residues. Nutrient inputs generally decreased residue carbon mineralization. The regression analysis indicated that soil moisture and residue C mineralization were the two dominant predictor variables that estimated 31% of soil methane fluxes in Vertisols. The results of this study show the complexity of methane dynamics and emphasize the importance of integrated crop, nutrient, and soil moisture (irrigation) management strategies that need to be developed to minimize methane emissions from agricultural production systems to mitigate climate change.

Methane dynamics altered by reservoir operations in a typical tributary of the Three Gorges Reservoir

Substantial nutrient inputs from reservoir impoundment typically increase sedimentation rate and primary production. This can greatly enhance methane (CH4) production, making reservoirs potentially significant sources of atmospheric CH4. Consequently, elucidating CH4 emissions from reservoirs is crucial for assessing their role in the global methane budget. Reservoir operations can also influence hydrodynamic and biogeochemical processes, potentially leading to pronounced spatiotemporal heterogeneity, especially in reservoirs with complex tributaries, such as the Three Gorges Reservoir (TGR). Although several studies have investigated the spatial and temporal variations in CH4 emissions in the TGR and its tributaries, considerable uncertainties remain regarding the impact of reservoir operations on CH4 dynamics. These uncertainties primarily arise from the limited spatial and temporal resolutions of previous measurements and the complex underlying mechanisms of CH4 dynamics in reservoirs. In this study, we employed a fast-response automated gas equilibrator to measure the spatial distribution and seasonal variations of dissolved CH4 concentrations in XXB, a representative area significantly impacted by TGR operations and known for severe algal blooms. Additionally, we measured CH4 production rates in sediments and diffusive CH4 flux in the surface water. Our multiple campaigns suggest substantial spatial and temporal variability in CH4 concentrations across XXB. Specifically, dissolved CH4 concentrations were generally higher upstream than downstream and exhibited a vertical stratification, with greater concentrations in bottom water compared to surface water. The peak dissolved CH4 concentration was observed in May during the drained period. Our results suggest that the interplay between aquatic organic matter, which promotes CH4 production, and the dilution process caused by intrusion flows from the mainstream primarily drives this spatiotemporal variability. Importantly, our study indicates the feasibility of using strategic reservoir operations to regulate these factors and mitigate CH4 emissions. This eco-environmental approach could also be a pivotal management strategy to reduce greenhouse gas emissions from other reservoirs.

Influence of seasons and environmental variables on methane dynamics in the Muthukuda Mangrove sediments of Tamil Nadu

Aim: In order to understand the methane biochemistry in the Muthukuda mangrove ecosystem, this study aimed to report on methane production, oxidation, flux, and also to investigate the effect of seasons and environmental variables on methane dynamics. Methodology: Sediment analysis was carried out monthly for a year (2021-2022) from the Muthukuda mangrove, south-east coast of India, which is an unexplored region in terms of methane dynamics. Methane parameters methane concentration, methane oxidation, methane production and methanotrophic enumeration were analysed from the sediment samples collected by a PVC corer from the study site. The methane parameters were analysed using standard methods by gas chromatography. The physiological parameters were analysed on the site using portable probes. Results: The physio-chemical parameters varied with the changing seasons at different depths. The concentrations of organic parameters were higher for carbohydrates, followed by lipids and proteins. LOM was higher in the surface and mid sediments in all the seasons. The concentration of methane was higher during summer in surface sediments. Generally, higher methane oxidation was observed in the mid and bottom sediments during all the seasons. Methanotrophic enumeration ranged from 0.4 ±0.7 to 4.8±2.4 x108 g-1. Interpretation: The results suggest that the bacterial community was involved in both methane oxidation and production, though oxidation of methane was lesser than production contributing to the atmospheric methane, methanotrophic bacteria which oxidizes methane significantly playing a major role in the regulation of methane flux. Key words: Methane oxidation, Methanotrophs, Muthukuda mangrove

Dyspersja metanu w zamkniętych obudowach

The increase of the population also involve the increase of the consumption of raw materials. This requires the diversification and development of industrial processes in semi-enclosed or open spaces. The carrying out of human activities of an industrial nature involves the accidental use, handling or presence of explosive substances such as methane. The presence of this gas in closed or semi-closed spaces can generate explosion phenomena. The accumulation of methane in narrow spaces is well studied and known, but the dispersion and especially the dispersion dynamics of methane released from a source considered infinite is less known. Knowing how methane disperses into the air is very important for establishing preventive measures. The paper presents the experiment on the dynamics of methane dispersion in a closed enclosure.

Deflagration Dynamics of Methane–Air Mixtures in Closed Vessels at Elevated Temperatures

In this paper, we explore the deflagration combustion of methane–air mixtures through both experimental and numerical analyses. The key parameters defining deflagration combustion dynamics include maximum explosion pressure (Pmax), maximum rate of explosion pressure rise (dP/dt)max, deflagration index (KG), and laminar burning velocity (SU). Understanding these parameters enhances the process of safety design across the energy sector, where light-emissive fuels play a crucial role in energy transformation. However, most knowledge on these parameters comes from experiments under standard conditions (P = 1 bar, T = 293.15 K), with limited data on light hydrocarbon fuels at elevated temperatures. Our study provides new insights into methane–air mixture deflagration dynamics at temperatures ranging from 293 to 348 K, addressing a gap in the current process industry knowledge, especially in gas and chemical engineering. We also conduct a comparative analysis of predictive models for the laminar burning velocity of methane mixtures in air, including the Manton, Lewis, and von Elbe, Bradley and Mitcheson, and Dahoe models, alongside various chemical kinetic mechanisms based on experimental findings. Notably, despite their simplicity, the Bradley and Dahoe models exhibit a satisfactory predictive accuracy when compared with numerical simulations from three chemical kinetic models using Cantera v. 3.0.0 code. The findings of this study enrich the fundamental combustion data for methane mixtures at elevated temperatures, vital for advancing research on natural gas as an efficient “bridge fuel” in energy transition.

Eutrophication and Deoxygenation Drive High Methane Emissions from a Brackish Coastal System.

Coastal environments are a major source of marine methane in the atmosphere. Eutrophication and deoxygenation have the potential to amplify the coastal methane emissions. Here, we investigate methane dynamics in the eutrophic Stockholm Archipelago. We cover a range of sites with contrasting water column redox conditions and rates of organic matter degradation, with the latter reflected by the depth of the sulfate-methane transition zone (SMTZ) in the sediment. We find the highest benthic release of methane (2.2-8.6 mmol m-2 d-1) at sites where the SMTZ is located close to the sediment-water interface (2-10 cm). A large proportion of methane is removed in the water column via aerobic or anaerobic microbial pathways. At many locations, water column methane is highly depleted in 13C, pointing toward substantial bubble dissolution. Calculated and measured rates of methane release to the atmosphere range from 0.03 to 0.4 mmol m-2 d-1 and from 0.1 to 1.7 mmol m-2 d-1, respectively, with the highest fluxes at locations with a shallow SMTZ and anoxic and sulfidic bottom waters. Taken together, our results show that sites suffering most from both eutrophication and deoxygenation are hotspots of coastal marine methane emissions.

Contrasting Methane Seepage Dynamics in the Hola Trough Offshore Norway: Insights From Two Different Summers

AbstractThis study investigates the temporal variations in methane concentration and flare activity in the Hola trough (offshore Norway) during May 2018 and June 2022. Between these time periods, methane seep activity exhibits 3.5 times increase, as evidenced by hydroacoustic measurements. As the seep area in the Hola trough is constantly within the hydrate stability zone, the observed increase cannot be attributed to migration of its shallow boundary due to temperature increase. However, a combination of low tide conditions resulting in a lower sediment pore pressure and a bottom water temperature increase resulting in a lower methane solubility is likely to explain the increase in the number of seeps observed in June 2022. The hypothesis of tide influence is supported by data collected from a piezometer deployed and recovered during the cruise showing that the tidal effect was observed 3 m below the seafloor. Despite the numerous methane seeps detected, methane concentration and gas flow rates near the seafloor were low (<19 nM and <70 mL min−1, respectively) compared to other areas with methane seep activity. This is likely due to strong currents rapidly dispersing methane in the water column. Sub‐seafloor investigations identified pathways for gas migration in methane seep areas, influenced by topography. This study provides valuable insights into the temporal dynamics of methane concentrations, flare activity, and gas distribution in the Hola trough, contributing to our understanding of offshore methane dynamics in the region.

Unraveling the dynamics of atmospheric methane: the impact of anthropogenic and natural emissions

The reduction in methane concentration is crucial for achieving the goals of the Paris agreement. However, its annual growth rate is unstable, and understanding the reasons for changes in methane growth is essential for climate policy-making. Currently, there is considerable uncertainty regarding its attribution. Here, we utilize multi-source data and optimal fingerprinting methods to detect the contributions of several key drivers to the methane trend and interannual variability. We find that the methane growth trend is primarily influenced by anthropogenic emissions, while interannual variability is predominantly determined by wetland and biomass burning emissions. This result underscores the central role of anthropogenic emissions in methane dynamics, providing confidence in the effectiveness of human efforts to control methane atmospheric concentrations through emission reductions. It also helps alleviate concerns about the recent surge in atmospheric methane concentration, as it may be a short-term peak caused by increased wetland emissions rather than a long-term change.

Organic soil greenhouse gas flux rates in hemiboreal old-growth Scots pine forests at different groundwater levels

Tree biomass and soils (especially organic soils) are significant carbon pools in forest ecosystems, therefore forest management practices, in order to ensure carbon storage in these pools and to mitigate climate change, are essential in reaching climate neutrality goals set by the European Union. Overall studies have focused on diverse aspects of forest carbon storage and greenhouse gas (GHG) fluxes from mineral soils, and recently also from organic soils. However, the information about old-growth forests and the long-term effects of drainage on GHG fluxes of organic soils is missing. Additionally, a large proportion of Scots pine (Pinus sylvestris L.) forests on organic soils in the hemiboreal region are drained to regulate groundwater level and to improve above-ground carbon storage. The study aims to assess the intra-annual dynamics of soil carbon dioxide (CO2) and methane (CH4) fluxes in hemiboreal old-growth Scots pine stands on organic soils with diverse groundwater levels. Six old-growth stands (130–180 years old) were evaluated. In old-growth forests, the main source of soil CO2 emissions is ground vegetation and tree roots (autotrophic respiration), while heterotrophic respiration contributes to almost half (41%) of the total forest floor ecosystem (soil) respiration. The total forest floor respiration and soil heterotrophic respiration are mainly affected by soil temperature, with minor but statistically significant contribution of groundwater level (model R2 = 0.78 and R2 = 0.56, respectively). The CO2 fluxes have a significant, yet weak positive relationship with groundwater level (RtCO2 R2 = 0.06 RhCO2 R2 = 0.08). In contrast, total soil CH4 uptake or release depends primarily on groundwater level fluctuations, with a minor but significant contribution of soil temperature (model R2 = 0.67). CH4 flux has high variability between stands.

Dynamics of methane emissions from northwestern Gulf of Mexico subtropical seagrass meadows

While seagrass meadows are perceived to be pertinent blue carbon reservoirs, they also potentially release methane (CH4) into the atmosphere. Seasonal and diurnal variations in CH4 emissions from a subtropical hypersaline lagoon dominated by Halodule wrightii in southern Texas, USA, on the northwest coast of the Gulf of Mexico were investigated. Dissolved CH4 concentrations decreased in the daytime and increased overnight during the diel observation period, which could be explained by photosynthesis and respiration of seagrasses. Photosynthetic oxygen was found to significantly reduce CH4 emissions from seagrass sediment. Diffusive transport contributed slightly to the release of CH4 from the sediment to the water column, while plant mediation might be the primary mechanism. The diffusive CH4 flux at the sea-air interface was 12.3–816.2 µmol/m2 d, over the range of the sea-air fluxes previously reported from other seagrass meadows. This was related to relatively higher dissolved CH4 concentrations (11.6–258.2 nmol/L) in a mostly closed lagoon with restricted water exchange. This study emphasizes seagrass meadows in the subtropical hypersaline lagoon as a source of atmospheric CH4, providing insights into the interactions between seagrass ecosystems and methane dynamics, with potential implications for seagrass meadow management and conservation efforts.

A Carbon Source in a Carbon Sink: Carbon Dioxide and Methane Dynamics in Open‐Water Peatland Pools

AbstractPeatlands store organic carbon available for decomposition and transfer to neighboring water bodies, which can ultimately generate carbon dioxide (CO2) and methane (CH4) emissions. The objective of this study was to clarify the biogeochemical functioning of open‐water peatland pools and their influence on carbon budgets at the ecosystem and global scale. Continuously operated automated equipment and monthly manual measurements were used to describe the CO2 and CH4 dynamics in boreal ombrotrophic peatland pools and porewater (Québec, Canada) over the growing seasons 2019 and 2020. The peat porewater stable carbon isotope ratios (δ13C) for both CO2 (median δ13C‐CO2: −3.8‰) and CH4 (median δ13C‐CH4: −64.30‰) suggested that hydrogenotrophic methanogenesis was the predominant degradation pathway in peat. Open‐water pools were supersaturated in CO2 and CH4 and received most of these dissolved carbon greenhouse gases (C‐GHG) from peat porewater input. Throughout the growing season, higher CO2 concentrations and fluxes in pools were measured when the water table was low—suggesting a steady release of CO2 from deep peat porewater. Higher CH4 ebullition and diffusion occurred in August when bottom water and peat temperatures were the highest. While this study demonstrates that peatland pools are chimneys of CO2 and CH4 stored in peat, it also shows that the C‐GHG concentrations and flux rates in peat pools are comparable to other aquatic systems of the same size. Although peatlands are often considered uniform entities, our study highlights their biogeochemical heterogeneity, which, if considered, substantially influences their net carbon balance with the atmosphere.

Behaviors of trace elements under varying methane seepage intensity: Insight from tubular seep carbonates in the South China Sea

Seep carbonates are commonly used to study environmental conditions based on trace element patterns. However, the relationship between methane flux and trace element enrichments is not well understood. Tubular seep carbonates, which are interpreted as former fluid conduits, offer an opportunity to investigate how trace element distributions in these structures respond to changes in methane flux. Here, we examined the distribution of trace elements in three tubular seep carbonates collected from Jiulong Methane Reef in the northern South China Sea. The carbonates exhibit low δ13C values (−57.0‰ to −40.4‰ V-PDB). All tubes show a gradual increasing trend in the carbonate contents and a decreasing trend in δ13C values from the outer to the inner portions, indicating an overall increase in methane flux over time. The two tubular carbonates with lower carbon isotopic values likely precipitated in a more reducing environment, as evidenced by relatively higher enrichment factors of Molybdenum (MoEF) and higher Molybdenum/Uranium (Mo/U) ratios. The MoEF values show an increasing trend for the outer to the inner portions for the two samples with lower δ13C values, while the UEF values show an increasing trend for the two samples with higher δ13C values. This suggests that the enrichments of Mo and U are influenced by methane flux, which regulates the redox conditions and the rate of elements supplied from seawater. Only Arsenic (As) and antimony (Sb) show identifiable enrichment among the other trace elements, which is unrelated to methane dynamics. This study provides insights into the behavior of redox-sensitive trace elements under changing methane flux, laying a groundwork for using trace element enrichments to reconstruct paleoenvironmental information, particularly in dynamic environments.

CO2 and CH4 dynamics in a eutrophic tropical Andean reservoir.

We studied the dynamics of methane (CH4) and carbon dioxide (CO2) in a eutrophic tropical reservoir located in the Colombian Andes. Temporal and spatial dynamics were addressed through sampling during six field campaigns conducted throughout a two-year period. We monitored fluxes at the air-water interface, dissolved gas concentrations, physical and chemical properties of the water column, microstructure profiles of turbulence, and meteorological conditions. Throughout the study period, the reservoir was a persistent source of CH4 to the atmosphere with higher emissions occurring in the near inflow region. During periods of low water levels, both the emissions and surface concentrations of CH4 were higher and more spatially heterogeneous. The measured CO2 fluxes at the air-water interface changed direction depending on the time and location, showing alternating uptake and emissions by the water surface. Mass balances of dissolved CH4 in the surface mixed layer revealed that biochemical reactions and gas evasion were the most significant processes influencing the dynamics of dissolved CH4, and provided new evidence of possible oxic methane production. Our results also suggest that surface CH4 concentrations are higher under more eutrophic conditions, which varied both spatially and temporally.

Limnological dynamics of methane (CH4) and carbon dioxide (CO2) emissions from a tropical hypertrophic reservoir lake

ABSTRACT Methane (CH4) and carbon dioxide (CO2) emissions from tropical freshwater ecosystems have been understudied, particularly in terms of their interaction with limnological dynamics, their cycling, and the emission mechanisms of CH4. To help reduce that knowledge gap, this study addressed these processes in Valle de Bravo (VB), a tropical (19° 11. 65′ N) reservoir lake, that provides water supply to Mexico City metropolitan area. CH4 and CO2 concentrations and emissions from VB were measured during four field campaigns distributed along the annual limnological cycle of the reservoir. Dissolved CH4 concentration varied over four orders of magnitude (0.015–176.808 μmol L−1), and dissolved CO2 varied from below atmospheric saturation (15.062 μmol L−1) to 10 times that concentration (219.505 μmol L−1). CH4 fluxes ranged from 23.25 to 1220.80 μmol m−2 day−1, while CO2 fluxes ranged from −60.11 to 254.99 mmol m−2 day−1. Seasonal monitoring also allowed the assessment of the annual emissions as well as the greenhouse gas (GHG) storage during thermal stratification, which accounted for >58% of the total GHG annual emissions from VB. Overall, VB is a source of GHG, and its major contribution is the CH4 released during the autumn overturn.

Biogeochemical investigations of methane seepage at the ultraslow-spreading Arctic mid-ocean ridge: Svyatogor ridge, Fram Strait

The Arctic continental shelves are important reservoirs of methane stored in gas hydrates and deeper geological formations. However, little is known about methane dynamics in deeper oceanic settings such as mid-ocean ridges, mainly due to operational challenges related to their remoteness. This study investigates a recently discovered methane seepage environment at Svyatogor Ridge, a sediment-covered transform fault on the western flank of the Arctic mid-ocean ridge west of Svalbard. Svyatogor Ridge was previously hypothesized to host deep gas hydrates and active fluid flow systems, which is a unique combination for this setting and requires geochemical evidence. Based on sediment and foraminiferal geochemistry, we demonstrate that Svyatogor Ridge hosts a shallow methane cycle with anaerobic oxidation of microbial methane, sustaining chemosynthetic communities at the seafloor. Our geochemical datasets also suggest that methane fluxes were higher in the past than today and long-lasting, with episodes of intense methane oxidation recorded in pre-Holocene sediments. Methane seepage is still ongoing at this location, although no evidence exists that methane is reaching the sea surface. The results provide the first evidence of a methane seepage environment ever reported from the ultra-slow spreading Arctic mid-ocean ridge, thus calling for a reevaluation of the role of this type of ridge in the ocean methane cycle.

Shallow and deep groundwater moderate methane dynamics in a high Arctic glacial catchment

Glacial groundwater can mobilize deep-seated methane from beneath glaciers and permafrost in the Arctic, leading to atmospheric emissions of this greenhouse gas. We present a temporal, hydro-chemical dataset of methane-rich groundwater collected during two melt seasons from a high Arctic glacial forefield to explore the seasonal dynamics of methane emissions. We use methane and ion concentrations and the isotopic composition of water and methane to investigate the sources of groundwater and the origin of the methane that the groundwater transports to the surface. Our results suggest two sources of groundwater, one shallow and one deep, which mix, and moderate methane dynamics. During summer, deep methane-rich groundwater is diluted by shallow oxygenated groundwater, leading to some microbial methane oxidation prior to its emergence at the surface. Characterization of the microbial compositions in the groundwater shows that microbial activity is an important seasonal methane sink along this flow-path. In the groundwater pool studied, we found that potential methane emissions were reduced by an average of 29% (±14%) throughout the summer due to microbial oxidation. During winter, deep groundwater remains active while many shallow systems shut down due to freezing, reducing subsurface methane oxidation, and potentially permitting larger methane emissions. Our results suggest that ratios of the different groundwater sources will change in the future as aquifer capacities and recharge volumes increase in a warming climate.

Methane dynamics in vegetated habitats in inland waters: quantification, regulation, and global significance

Freshwater ecosystems, including lakes, wetlands, and running waters, are estimated to contribute over half the natural emissions of methane (CH4) globally, yet large uncertainties remain in the inland water CH4 budget. These are related to the highly heterogeneous nature and the complex regulation of the CH4 emission pathways, which involve diffusion, ebullition, and plant-associated transport. The latter, in particular, represents a major source of uncertainty in our understanding of inland water CH4 dynamics. Many freshwater ecosystems harbor habitats colonized by submerged and emergent plants, which transport highly variable amounts of CH4 to the atmosphere but whose presence may also profoundly influence local CH4 dynamics. Yet, CH4 dynamics of vegetated habitats and their potential contribution to emission budgets of inland waters remain understudied and poorly quantified. Here we present a synthesis of literature pertaining CH4 dynamics in vegetated habitats, and we (i) provide an overview of the different ways the presence of aquatic vegetation can influence CH4 dynamics (i.e., production, oxidation, and transport) in freshwater ecosystems, (ii) summarize the methods applied to study CH4 fluxes from vegetated habitats, and (iii) summarize the existing data on CH4 fluxes associated to different types of aquatic vegetation and vegetated habitats in inland waters. Finally, we discuss the implications of CH4 fluxes associated with aquatic vegetated habitats for current estimates of aquatic CH4 emissions at the global scale. The fluxes associated to different plant types and from vegetated areas varied widely, ranging from−8.6 to over 2835.8 mg CH4 m−2 d−1, but were on average high relative to fluxes in non-vegetated habitats. We conclude that, based on average vegetation coverage and average flux intensities of plant-associated fluxes, the exclusion of these habitats in lake CH4 balances may lead to a major underestimation of global lake CH4 emissions. This synthesis highlights the need to incorporate vegetated habitats into CH4 emission budgets from natural freshwater ecosystems and further identifies understudied research aspects and relevant future research directions.