Methane Budget Research Articles

Bubble emissions from thermokarst lakes in the Qinghai–Xizang Plateau

It is important to understand the role of Qinghai–Xizang Plateau thermokarst lakes in the global atmospheric methane (CH4) and carbon dioxide (CO2) budget. This study investigated the gas components and isotopic characteristics of bubble gas collected from six thermokarst lakes. The major gas component of the bubbles varied greatly among lakes and bubble sources. Nitrogen (N2), oxygen (O2) and carbon dioxide (CO2) were the predominant constituents of bubbles, but argon (Ar) and methane (CH4) were also present. The N2 was primarily atmospheric in origin, although in part likely originated in sediment organic matter. Isotopic analysis of CO2 and CH4 suggested that CO2 in the bubbles was a mixture of CO2 from decomposed lacustrine carbonate and oxidized organic mass, except for CO2 from organic mass oxidized in Bucha Lake. CH4 in bubbles primarily originated from thermogenic sources and old sediment organic matter.

Constraints on the Late Holocene Anthropogenic Contribution to the Atmospheric Methane Budget

The origin of the late preindustrial Holocene (LPIH) increase in atmospheric methane concentrations has been much debated. Hypotheses invoking changes in solely anthropogenic sources or solely natural sources have been proposed to explain the increase in concentrations. Here two high-resolution, high-precision ice core methane concentration records from Greenland and Antarctica are presented and are used to construct a high-resolution record of the methane inter-polar difference (IPD). The IPD record constrains the latitudinal distribution of emissions and shows that LPIH emissions increased primarily in the tropics, with secondary increases in the subtropical Northern Hemisphere. Anthropogenic and natural sources have different latitudinal characteristics, which are exploited to demonstrate that both anthropogenic and natural sources are needed to explain LPIH changes in methane concentration.

Microbial Communities of Deep-Sea Methane Seeps at Hikurangi Continental Margin (New Zealand)

The methane-emitting cold seeps of Hikurangi margin (New Zealand) are among the few deep-sea chemosynthetic ecosystems of the Southern Hemisphere known to date. Here we compared the biogeochemistry and microbial communities of a variety of Hikurangi cold seep ecosystems. These included highly reduced seep habitats dominated by bacterial mats, partially oxidized habitats populated by heterotrophic ampharetid polychaetes and deeply oxidized habitats dominated by chemosynthetic frenulate tubeworms. The ampharetid habitats were characterized by a thick oxic sediment layer that hosted a diverse and biomass-rich community of aerobic methanotrophic Gammaproteobacteria. These bacteria consumed up to 25% of the emanating methane and clustered within three deep-branching groups named Marine Methylotrophic Group (MMG) 1-3. MMG1 and MMG2 methylotrophs belong to the order Methylococcales, whereas MMG3 methylotrophs are related to the Methylophaga . Organisms of the groups MMG1 and MMG3 are close relatives of chemosynthetic endosymbionts of marine invertebrates. The anoxic sediment layers of all investigated seeps were dominated by anaerobic methanotrophic archaea (ANME) of the ANME-2 clade and sulfate-reducing Deltaproteobacteria. Microbial community analysis using Automated Ribosomal Intergenic Spacer Analysis (ARISA) showed that the different seep habitats hosted distinct microbial communities, which were strongly influenced by the seep-associated fauna and the geographic location. Despite outstanding features of Hikurangi seep communities, the organisms responsible for key ecosystem functions were similar to those found at seeps worldwide. This suggests that similar types of biogeochemical settings select for similar community composition regardless of geographic distance. Because ampharetid polychaetes are widespread at cold seeps the role of aerobic methanotrophy may have been underestimated in seafloor methane budgets.

Termites Facilitate Methane Oxidation and Shape the Methanotrophic Community

Termite-derived methane contributes 3 to 4% to the total methane budget globally. Termites are not known to harbor methane-oxidizing microorganisms (methanotrophs). However, a considerable fraction of the methane produced can be consumed by methanotrophs that inhabit the mound material, yet the methanotroph ecology in these environments is virtually unknown. The potential for methane oxidation was determined using slurry incubations under conditions with high (12%) and in situ (∼0.004%) methane concentrations through a vertical profile of a termite (Macrotermes falciger) mound and a reference soil. Interestingly, the mound material showed higher methanotrophic activity. The methanotroph community structure was determined by means of a pmoA-based diagnostic microarray. Although the methanotrophs in the mound were derived from populations in the reference soil, it appears that termite activity selected for a distinct community. Applying an indicator species analysis revealed that putative atmospheric methane oxidizers (high-indicator-value probes specific for the JR3 cluster) were indicative of the active nest area, whereas methanotrophs belonging to both type I and type II were indicative of the reference soil. We conclude that termites modify their environment, resulting in higher methane oxidation and selecting and/or enriching for a distinct methanotroph population.

Temporal variation of dissolved methane in a subtropical mesoscale eddy during a phytoplankton bloom in the southwest Pacific Ocean

A multi-disciplinary examination of the drivers of dissolved methane was carried out during a phytoplankton bloom located in a subtropical mesoscale eddy. This investigation related temporal signals in methane concentrations with other biophysical and biogeochemical parameters in the upper waters (<300m) of the southwest Pacific Ocean. In the surface mixed layer, methane supersaturation increased and δ13CCH4 became more depleted coincident with increases in particulate dimethylsulfoniopropionate (DMSPp) and succession from the diatom Asterionellopsis glacialis to the nanoflagellate Phaeocystis globosa and the cyanobacterium Synechococcus sp. In situ methane production was calculated in a surface mixed layer methane budget that incorporated sea-to-air exchange and vertical diffusion. Methane concentrations increased in and below the mixed layer when the export of biogenic particles increased. Increased grazing of microbes by microzooplankton may have contributed to particle recycling (rich in organic carbon and DMSP) and increased the potential for methanogenesis. Phytoplankton species composition and biomass in different bloom phases, and eddy dynamics, were important determinants of methane saturation and emission, and the potential implications for methane are considered for the future surface ocean.

Relating sulfate and methane dynamics to geology: Accretionary prism offshore SW Taiwan

Geochemical data (CH4, SO42−, I−, Cl−, particulate organic carbon (POC), δ13C‐CH4, and δ13C‐CO2) are presented from the upper 30 m of marine sediment on a tectonic submarine accretionary wedge offshore southwest Taiwan. The sampling stations covered three ridges (Tai‐Nan, Yung‐An, and Good Weather), each characterized by bottom simulating reflectors, acoustic turbidity, and different types of faulting and anticlines. Sulfate and iodide concentrations varied little from seawater‐like values in the upper 1–3 m of sediment at all stations; a feature that is consistent with irrigation of seawater by gas bubbles rising through the soft surface sediments. Below this depth, sulfate was rapidly consumed within 5–10 m by anaerobic oxidation of methane (AOM) at the sulfate‐methane transition. Carbon isotopic data imply a mainly biogenic methane source. A numerical transport‐reaction model was used to identify the supply pathways of methane and estimate depth‐integrated turnover rates at the three ridges. Methane gas ascending from deep layers, facilitated by thrusts and faults, was by far the dominant term in the methane budget at all sites. Differences in the proximity of the sampling sites to the faults and anticlines mainly accounted for the variability in gas fluxes and depth‐integrated AOM rates. By comparison, methane produced in situ by POC degradation within the modeled sediment column was unimportant. This study demonstrates that the geochemical trends in the continental margins offshore SW Taiwan are closely related to the different geological settings.

Contribution of tundra lakes in western Siberia to the atmospheric methane budget

The emission of methane from tundra lakes in western Siberia is estimated on the basis of experimental data. According to these estimates, over the warm season, the methane flux from the surfaces of lakes to the atmosphere amounts to 0.5 KtCH4 for Arctic tundra, 2.9 KtCH4 for typical tundra, and 8.5 KtCH4 for southern tundra. The short-term spring emission of methane as ice melts in tundra lakes of the region under consideration is determined with consideration for published estimates of the rate of generation of methane in northern-latitude lakes during the cold season. The contribution made by tundra lakes in western Siberia to the atmospheric methane budget is estimated at 20 KtCH4/year.

Spatiotemporal distribution patterns of dissolved methane in lakes: How accurate are the current estimations of the diffusive flux path?

AbstractLakes have been identified as an important source of atmospheric methane. Here the spatiotemporal distribution of dissolved methane was measured in a medium‐sized freshwater lake. The data reveal that littoral zones (nearshore, shallow) are the predominant source of methane. Offshore‐directed gradients of dissolved methane suggest the transport of methane from the nearshore zone to the pelagic epilimnion. The distribution patterns of epilimnetic methane were highly heterogeneous, independent of the mean lake‐wide methane concentration. Consequently, the diffusive flux of methane to the atmosphere strongly varies with location in the lake. A comparison of the diffusive methane flux from different offshore sampling stations indicates that single‐point measurements are not necessarily sufficient to estimate lake‐wide emissions to the atmosphere accurately. Thus, spatially resolved measurements of methane emissions are needed to improve the reliability of estimates of the methane that lakes contribute to the global methane budget.

A method for the calculation of anaerobic oxidation of methane rates across regional scales: an example from the Belt Seas and The Sound (North Sea–Baltic Sea transition)

Estimating the amount of methane in the seafloor globally as well as the flux of methane from sediments toward the ocean–atmosphere system are important considerations in both geological and climate sciences. Nevertheless, global estimates of methane inventories and rates of methane production and consumption through anaerobic oxidation in marine sediments are very poorly constrained. Tools for regionally assessing methane formation and consumption rates would greatly increase our understanding of the spatial heterogeneity of the methane cycle as well as help constrain the global methane budget. In this article, an algorithm for calculating methane consumption rates in the inner shelf is applied to the gas-rich sediments of the Belt Seas and The Sound (North Sea–Baltic Sea transition). It is based on the depth of free gas determined by hydroacoustic techniques and the local methane solubility concentration. Due to the continuous nature of shipboard hydroacoustic measurements, this algorithm captures spatial heterogeneities in methane fluxes better than geochemical analyses of point sources such as observational/sampling stations. The sensibility of the algorithm with respect to the resolution of the free gas depth measurements (2 m vs. 50 cm) is proven of minor importance (a discrepancy of <10%) for a small part of the study area. The algorithm-derived anaerobic methane oxidation rates compare well with previous measured and modeling studies. Finally, regional results reveal that contemporary anaerobic methane oxidation in worldwide inner-shelf sediments may be an order of magnitude lower (ca. 0.24 Tmol year–1) than previous estimates (4.6 Tmol year–1). These algorithms ultimately help improve regional estimates of anaerobic oxidation of methane rates.

Budget of methane emissions from soils, livestock and the river network at the regional scale of the Seine basin (France)

We used various approaches to establish a comprehensive budget of methane (CH4) emissions from the Seine basin, including direct emissions from livestock and soils as well as emissions from the drainage network. For the direct emissions from livestock, we used official livestock census numbers and emission factors (CH4 emitted by each animal species per head per year) available in the literature. For the emissions from soils, we based our estimates on experimental measurements in closed chambers installed on different agricultural plots, forest, and grasslands in 2008 and 2009. The results were extrapolated to the whole Seine basin, including grassland, cropland, and forest soil distributions in the Seine basin. The CH4 emissions from the Seine drainage network were also based on measurements of sampled waters in various rivers and streams (from headwaters to estuary) during different seasons in 2007, 2008, and 2010. After chemical analysis of CH4 concentrations in the water samples using a gas chromatographic technique and calculation of the CH4 supersaturation by stream order in rivers of the Seine basin (from 1 to 8) and by season we could estimate the CH4 emissions for the whole water surface area of the Seine drainage network. The livestock of the Seine basin produce CH4 emissions amounting to 166 × 106 kg C year−1, among which cattle are responsible for 85 %. The total CH4 emission from the Seine drainage network was estimated at 0.3 × 106 kg C year−1, large rivers being responsible for the largest proportion. Ebullition could account for an additional 0.2 × 106 kg C year−1. Soils of the Seine basin are a net sink for CH4 (9.4 × 106 kg C year−1). The water and soils fluxes are low with regard to emissions by livestock, but domestic waste, through landfills, could contribute an additional 40 × 106 kg C year−1.

Global sensitivity of aviation NO&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt; effects to the HNO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;-forming channel of the HO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; + NO reaction

Abstract. The impact of a recently proposed HNO3-forming channel of the HO2 + NO reaction on atmospheric ozone, methane and their precursors is assessed with the aim to investigate its effects on aviation NOx induced radiative forcing. The first part of the study addresses the differences in stratospheric and tropospheric HOx-NOx chemistry in general, by comparing a global climate simulation without the above reaction to two simulations with different rate coefficient parameterizations for HO2 + NO → HNO3. A possible enhancement of the reaction by humidity, as found by a laboratory study, particularly reduces the oxidation capacity of the atmosphere, increasing methane lifetime significantly. Since methane lifetime is an important parameter for determining global methane budgets, this might affect estimates of the anthropogenic greenhouse effect. In the second part aviation NOx effects are isolated independently for each of the three above simulations. Warming and cooling effects of aircraft NOx emissions are both enhanced when considering the HNO3-forming channel, but the sum is shifted towards negative radiative forcing. Uncertainties associated with the inclusion of the HO2 + NO → HNO3 reaction and with its corresponding rate coefficient propagate a considerable additional uncertainty on estimates of the climate impact of aviation and on NOx-related mitigation strategies.

Interannual, seasonal, and retrospective analysis of the methane and carbon dioxide budgets of a temperate peatland

AbstractThree years (2009–2011) of near‐continuous methane (CH4) and carbon dioxide (CO2) fluxes were measured with the eddy covariance (EC) technique at a temperate peatland located within the Marcell Experimental Forest, in northern Minnesota, USA. The peatland was a net source of CH4 and a net sink of CO2 in each year with annual carbon budgets of −26.8 (±18.7), −15.5 (±14.8), and −14.6 (±21.5) g C m−2 yr−1 for 2009–2011, respectively. Differences in the seasonal hydrometeorological conditions among the three study years were most pronounced during 2011, which was considerably warmer (+1.3°C) and wetter (+40 mm) than the 30 year average. The annual CH4 budget was +11.8 (±3.1), +12.2 (±3.0), and +24.9 (±5.6) g C m−2 yr−1 for the respective years and accounted for 23%–39% of the annual carbon budget. The larger CH4 emission in 2011 is attributed to significant warming of the peat column coupled with a high water table position throughout the entire growing season. Historical (1991–2011) CH4 emissions were estimated based on long‐term hydrometeorological records and functional relationships derived from our 3 year field study. The predicted historical annual CH4 budget ranged from +7.8 to +15.2 (±2.7) g CH4‐C m−2 yr−1. Recent (2007–2011) increases in temperature, precipitation, and rising water table at this site suggest that CH4 emissions have been increasing, but were generally greater from 1991 to 1999 when average soil temperature and precipitation were higher than in recent years. The global warming potential (considering CO2 and CH4) for this peatland was calculated based on a 100 year time horizon. In all three study years, the peatland had a net positive radiative forcing on climate and was greatest (+187 g C m−2) in 2011. The interannual variability in CH4 exchange at this site suggests high sensitivity to variations in hydrometeorological conditions.

Contribution of Tundra Lakes in Western Siberia to the Atmospheric Methane Budget

Contribution of Tundra Lakes in Western Siberia to the Atmospheric Methane Budget

Methane emissions associated with the conversion of marshland to cropland and climate change on the Sanjiang Plain of northeast China from 1950 to 2100

Abstract. Wetland loss and climate change are known to alter regional and global methane (CH4) budgets. Over the last six decades, an extensive area of marshland has been converted to cropland on the Sanjiang Plain in northeast China, and a significant increase in air temperature has also been observed there, while the impacts on regional CH4 budgets remain uncertain. Through model simulation, we estimated the changes in CH4 emissions associated with the conversion of marshland to cropland and climate change in this area. Model simulations indicated a significant reduction of 1.1 Tg yr−1 (0.7–1.8 Tg yr−1) from the 1950s to the 2000s in regional CH4 emissions. The cumulative reduction of CH4 from 1960 to 2009 was estimated to be ~36 Tg (24–57 Tg) relative to the 1950s, and marshland conversion and the climate contributed 86% and 14% of this change, respectively. Interannual variation in precipitation (linear trend with P &gt; 0.2) contributed to yearly fluctuations in CH4 emissions, but the relatively lower amount of precipitation over the period 1960–2009 (47 mm yr−1 lower on average than in the 1950s) contributed ~91% of the reduction in the area-weighted CH4 flux. Global warming at a rate of 0.3 ° per decade (P &lt; 0.001) has increased CH4 emissions significantly since the 1990s. Relative to the mean of the 1950s, the warming-induced increase in the CH4 flux has averaged 19 kg ha−1 yr−1 over the last two decades. In the RCP (Representative Concentration Pathway) 2.6, RCP 4.5, RCP 6.0 and RCP 8.5 scenarios of the fifth IPCC assessment report (AR5), the CH4 fluxes are predicted to increase by 36%, 52%, 78% and 95%, respectively, by the 2080s compared to 1961–1990 in response to climate warming and wetting.

Trees are major conduits for methane egress from tropical forested wetlands

Wetlands are the largest source of methane to the atmosphere, with tropical wetlands comprising the most significant global wetland source component. The stems of some wetland-adapted tree species are known to facilitate egress of methane from anoxic soil, but current ground-based flux chamber methods for determining methane inventories in forested wetlands neglect this emission pathway, and consequently, the contribution of tree-mediated emissions to total ecosystem methane flux remains unknown. In this study, we quantify in situ methane emissions from tree stems, peatland surfaces (ponded hollows and hummocks) and root-aerating pneumatophores in a tropical forested peatland in Southeast Asia. We show that tree stems emit substantially more methane than peat surfaces, accounting for 62-87% of total ecosystem methane flux. Tree stem flux strength was controlled by the stem diameter, wood specific density and the amount of methane dissolved in pore water. Our findings highlight the need to integrate this emission pathway in both field studies and models if wetland methane fluxes are to be characterized accurately in global methane budgets, and the discrepancies that exist between field-based flux inventories and top-down estimates of methane emissions from tropical areas are to be reconciled.

Atmospheric methane removal by boreal plants

Several studies have proposed aerobic methane (CH4) emissions by plants. If confirmed, these findings would further increase the imbalance in the global CH4 budget which today underestimates CH4sinks. Oxidation by OH‐radicals in the troposphere is the major identified sink followed by smaller contribution from stratospheric loss and oxidation by methano‐ and methylotrophic bacteria in soils. This study directly investigated CH4 exchange by plants in their natural environment. At a forest site in central Sweden, in situ branch chamber measurements were used to study plant ambient CH4 exchange by spruce (Picea abies), birch (Betula pubescens), rowan (Sorbus aucuparia) and pine (Pinus sylvestris). The results show a net uptake of CH4 by all the studied plants, which might be of importance for the methane budget.

Greenhouse gas emissions from Indian livestock

Livestock constitutes an integral component of Indian agriculture sector and also a major source of GHGs emissions. The study presents a detailed inventory of GHG emissions at district/state level from different age-groups, indigenous and exotic breed of different Indian livestock categories estimated using the recent census 2003 and country-specific emission coefficients based on IPCC guidelines. The total methane emission including enteric fermentation and manure management of livestock was estimated at 11.75 Tg/year for the year 2003. Enteric fermentation constitutes ~91 % of the total methane emissions from Indian livestock. Dairy buffalo and indigenous dairy cattle together contribute 60 % of the methane emissions. The total nitrous oxide emission from Indian livestock for the year 2003 is estimated at 1.42 Gg/year, with 86.1 % contribution from poultry. The total GHGs emission from Indian livestock is estimated at 247.2 Mt in terms of CO2 equivalent emissions. Although the Indian livestock contributes substantially to the methane budget, the per capita emission is only 24.23 kgCH4/animal/year. Using the remote sensing derived potential feed/fodder area available for livestock, the average methane flux was calculated as 74.4 kg/ha. The spatial patterns derived in GIS environment indicated the regions with high GHGs emissions that need to be focused subsequently for mitigation measures. The projected estimates indicate a likely increase of 40 % in methane emissions from buffalo population.

Potential methane reservoirs beneath Antarctica

Once thought to be devoid of life, the ice-covered parts of Antarctica are now known to be a reservoir of metabolically active microbial cells and organic carbon. The potential for methanogenic archaea to support the degradation of organic carbon to methane beneath the ice, however, has not yet been evaluated. Large sedimentary basins containing marine sequences up to 14 kilometres thick and an estimated 21,000 petagrams (1 Pg equals 10(15) g) of organic carbon are buried beneath the Antarctic Ice Sheet. No data exist for rates of methanogenesis in sub-Antarctic marine sediments. Here we present experimental data from other subglacial environments that demonstrate the potential for overridden organic matter beneath glacial systems to produce methane. We also numerically simulate the accumulation of methane in Antarctic sedimentary basins using an established one-dimensional hydrate model and show that pressure/temperature conditions favour methane hydrate formation down to sediment depths of about 300 metres in West Antarctica and 700 metres in East Antarctica. Our results demonstrate the potential for methane hydrate accumulation in Antarctic sedimentary basins, where the total inventory depends on rates of organic carbon degradation and conditions at the ice-sheet bed. We calculate that the sub-Antarctic hydrate inventory could be of the same order of magnitude as that of recent estimates made for Arctic permafrost. Our findings suggest that the Antarctic Ice Sheet may be a neglected but important component of the global methane budget, with the potential to act as a positive feedback on climate warming during ice-sheet wastage.

Geological control and magnitude of methane ebullition from a high-flux seep area in the Black Sea—the Kerch seep area

We investigated gas bubble emissions at the Don–Kuban paleo-fan in the northeastern Black Sea regarding their geological setting, quantities as well as spatial and temporal variabilities during three ship expeditions between 2007 and 2011. About 600 bubble-induced hydroacoustic anomalies in the water column (flares) originating from the seafloor above the gas hydrate stability zone (GHSZ) at ~700m water depth were found. At about 890m water depth a hydrocarbon seep area named “Kerch seep area” was newly discovered within the GHSZ. We propose locally domed sediments (“mounds”) discovered during ultra-high resolution bathymetric mapping with an autonomous underwater vehicle (AUV) to result from gas hydrate accumulation at shallow depths. In situ measurements indicated spatially limited temperature elevations in the shallow sediment likely induced by upward fluid flow which may confine the local GHSZ to a few meters below the seafloor. As a result, gas bubbles are suspected to migrate into near-surface sediments and to escape the seafloor through small-scale faults. Hydroacoustic surveys revealed that several flares originated from a seafloor area of about 1km2 in size. The highest flare disappeared in about 350m water depth, suggesting that the released methane remains in the water column. A methane flux estimate, combining data from visual quantifications during dives with a remotely operated vehicle (ROV) with results from ship-based hydroacoustic surveys and gas analysis revealed that between 2 and 87×106mol CH4 yr−1 escaped into the water column above the Kerch seep area. Our results show that the finding of the Kerch seep area represents a so far underestimated type of hydrocarbon seep, which has to be considered in methane budget calculations.

Could methane produced by sauropod dinosaurs have helped drive Mesozoic climate warmth?

Summary Mesozoic sauropods, like many modern herbivores, are likely to have hosted microbial methanogenic symbionts for the fermentative digestion of their plant food [1]. Today methane from livestock is a significant component of the global methane budget [2]. Sauropod methane emission would probably also have been considerable. Here, we use a simple quantitative approach to estimate the magnitude of such methane production and show that the production of the ‘greenhouse' gas methane by sauropods could have been an important factor in warm Mesozoic climates.