Proportion Of CH4 Research Articles

Temporal and spatial variation in methane emissions from a flooded transgression shore of a boreal lake

Variation of CH4 emissions over a three-year period was studied in a reed-dominated (Phragmites australis) littoral transect of a boreal lake undergoing shoreline displacement due to postglacial rebound. The seasonal variation in plant-mediated CH4 emissions during open-water periods was significantly correlated with sediment temperature. The highest plant-mediated emission rates (up to 2050 mg CH4 m−2 d−1) were found in the outermost reed zone, where culms of the previous growing seasons had accumulated and free-floating plants grew on the decomposing culms. In reed zones closer to the shoreline as well as in mixed stands of reed and cattail, the maximum daily rates were usually > 500 mg CH4 m−2 d−1. The total plant-mediated CH4 emission during the open-water period was significantly correlated with the seasonal maximum of green shoot biomass. This relationship was strongest in the continuously flooded (water depth > 25 cm) outermost zones. In this area, emissions through ebullition were of greatest importance and could exceed plant-mediated emissions. In general, total emissions of the open-water periods varied from ca. 20 to 50 g CH4 m−2 a−1, but in the outermost reed zone, the plant-mediated emissions could be as high as 123 g CH4 m−2 a−1; ebullition emissions from this zone reached > 100 g CH4 m−2 a−1. The proportion of CH4 released in winter was usually < 10% of annual emissions. Emissions of CH4 were higher in this flooded transgression shore the than those measured in boreal peatlands, but the role of ancient carbon stores as a substrate supply compared with recent anthropogenic eutrophication is unknown.

Methane oxidation in a peatland core

We made an experiment on a 30 cm diameter core of Sphagnum‐dominated vegetation and peat to estimate the parameters controlling methane oxidation during movement to the ambient air: 13CH4 was added at the water table, and excess 13CO2 appeared in the gas space above the core. At 20°C in otherwise undisturbed conditions, ∼22% of CH4 was oxidized to CO2 during passage up through the overlying 10‐cm thick unsaturated peat and plants. We simulated the experiment, with seven parameters: transfer coefficients in water, in the gas phase, and through the container wall; the rate of CH4 and of CO2 generation; and the two parameters of a hyperbolic relation between CH4 concentration and the rate of CH4 oxidation. We optimized these parameters to fit the experimental results, and then were able to generalize to any temperature (0°−25°C) and any depth (0‐55 cm) of water table. Changing temperature has important effects on the proportion of CH4 oxidized.

Monitoring Anaerobic Natural Attenuation of Petroleum Using a Novel In Situ Respiration Method in Low-Permeability Sediment

Assessing petroleum biodegradation rates is an important part of predicting natural attenuation in subsurface sediments. Monitoring carbon dioxide (CO2) and methane (CH4) produced in situ, and their radiocarbon 14C), stable carbon (13C) and deuterium (D). signature provide a novel method to assess anaerobic microbial processes. Our objectives were to: (1) estimate the rate of anaerobic petroleum hydrocarbon (PH) mineralization by monitoring the production of soil gas CH4 and CO2 in the vadose zone of low-permeability sediment, (2) evaluate the dominant microbial processes using δ13C and δD, and (3) determine the proportion of CH4 and CO2 attributable to anaerobic mineralization of PH using 14C analysis. Argon was sparged into the subsurface to dilute existing CO2 and CH4 concentrations. Vadose zone CO2, CH4, oxygen, total combustible hydrocarbons, and argon concentrations were measured for 75 days. CO2 and CH4 samples were collected on day 86 and analyzed for 14C, δ13C, and δD. Based on CH4 soil gas production, the anaerobic biodegradation rate was estimated between 0.017 to 0.055 mg/kg soil-d. CH4 14C (2.6 pMC), δ13C (-45.64‰), and δD (-316‰) values indicated that fermentation of PH was the sale source of CH4 in the vadose zone. CO2 14C (62 pMC) indicated that approximately 47% of the total CO2 was from PH mineralization and 53% from plant root respiration. Although low-permeability sediment increases the difficulty of completely replacing in situ soil gas and assuring anaerobic conditions, this novel respiration method distinguished between anaerobic processes responsible for PH degradation.

The isotopic composition of atmospheric methane

Measurements of the 13C/12C, D/H and 14C composition of atmospheric methane (CH4) between 1988 and 1995 are presented. The 13C/12C measurements represent the first global data set with time series records presented for Point Barrow, Alaska (71°N), Olympic Peninsula, Washington (48°N), Mauna Loa, Hawaii (20°N), American Samoa (14°S), Cape Grim, Australia (41°S), and Baring Head, New Zealand (41°S). North‐south trends of the 13C/12C and D/H of atmospheric CH4 from air samples collected during oceanographic research cruises in the Pacific Ocean are also presented. The mean annual δ13C increased southward from about −47.7 ‰ at 71°N to −41.2 ‰ at 41°S. The amplitude of the seasonal cycle in δ13C ranged from about 0.4 ‰ at 71°N to 0.1 ‰ at 14°S. The seasonal δ13C cycle at sites in tropical latitudes could be explained by CH4 loss via reaction with OH radical, whereas at temperate and polar latitudes in the northern hemisphere seasonal changes in the δ13C of the CH4 source were needed to explain the seasonal cycle. The higher δ13C value in the southern (−47.2 ‰) versus northern (−47.4 ‰) hemisphere was a result of interhemispheric transport of CH4. A slight interannual δ13C increase of 0.02±0.005 ‰ yr−1 was measured at all sites between 1990 and 1995. The mean δD of atmospheric CH4 was −86±3 ‰ between 1989 and 1995 with a 10 ‰ depletion in the northern versus southern hemisphere. The 14C content of CH4 measured at 48°N increased from 122 to 128 percent modern between 1987 and 1995. The proportion of CH4 released from fossil sources was 18±9% in the early 1990s as derived from the 14C content of CH4.

Methane oxidation by termite mounds estimated by the carbon isotopic composition of methane

Emission rates and carbon isotope ratios of CH4, emitted by workers of termites, and of CH4, emitted from their mounds, were observed in a dry evergreen forest in Thailand to estimate the proportion of CH4 oxidized during emission through the mound. The δ13C of CH4 emitted from a termite mound (−70.9 to −82.4‰) was higher than that of CH4 emitted by workers in the mound (−85.4 to −97. l‰). Using a fractionation factor (a = 0.987) for oxidation of CH4 which was obtained in the incubation experiment, an emission factor defined as (CH4 emitted from a termite mound/CH4 produced by termites) was calculated. The emission factor obtained in each termite mound was nearly zero for Macrotermes (fungus‐growing termites), of which the nest has a thick soil wall and subterrannean termites, and 0.17 to 0.47 for Termitinae (small‐mound‐making termites). Global CH4 emission by termites was estimated on the basis of the CH4 emission rates by workers and termite biomass with the emission factors. The calculated result was 1.5 to 7.4 Tg/y (0.3 to 1.3% of total source), which is considerably smaller than the estimate by the IPCC [1994].

Raman microprobe analysis on gaseous inclusions from the diagenetically altered Terres Noires (S-E, France)

Raman microprobe determinations have been made on primary gaseous inclusions in quartz crystals found in concretions from diagenetically altered Jurassic sediments from S-E France. The gas compositions vary as a general function of the diagenetic grade of the enclosing sediments. Two major types of gas inclusions were found, a wet gas in the western part of the basin and a dry gas in the eastern part. The proportions of CH4, C2H6, C3H8, CO2, H2S and N2 were determined to within one percent of the amount present. The gas mixtures and fluid phase assemblages indicate that these materials were entrapped during the evolution of the organic and mineral rock assemblages induced by increases of temperature and pressure. There is no evidence for change of the organic material posterior to its entrapment as a fluid inclusion. UV fluorescence optical observations indicate that organic liquids are present in the primary inclusions and also in secondary inclusions situated in healed cracks. These secondary inclusions are found in the whole area. UV fluorescence measurements indicate that this trapped material could be an extract from a more evolved organic material than that found in primary inclusions. These secondary inclusions could have been formed during tectonic event of the Alpine orogenies.

Determination of the rate constant for HO2+ CH4→ H2O2+ CH3at 443 °C

Studies have been made of the addition of CH4 to mixtures of tetramethylbutane (TMB)+ O2 at 443 °C in KCl-coated Pyrex vessels. From measurements of isobutene and formaldehyde, formed virtually as sole products from TMB and CH4, respectively, a value of k10/k1/27= 1.00×10–2(dm3 mol–1 s–1)1/2 has been obtained. HO2+ CH4→ H2O2+ CH3(10), HO2+ HO2→ H2O2+ O2(7). Computer analysis shows that this value is very sensitive to the value of k18/k9 which controls the proportion of CH4 removed by OH attack, and the optimum conditions for studying reaction (10) were carefully selected. Use of A7= 109.48 dm3 mol–1 s–1 and E7= 5.8 kJ mol–1, obtained recently, gives k10=(3.39 ± 0.78)×102 dm3mol–1s–1 at 443 °C. No other experimental determinations of this rate constant have been reported. Based on the reasonable assumption of equal A factors per C—H bond for HO2+ CH4 and HO2+ C2H6, then Arrhenius parameters of A10= 10.05 ± 0.3 dm3mol–1s–1 and E10=103.1 ± 3.5 kJ mol–1 are suggested for the temperature range 350–1000 °C. OH + TMB → H2O + C8H17(9), OH + CH4→ H2O + CH3(18)

Hydrogen metabolism and sulfate-dependent inhibition of methanogenesis in a eutrophic lake sediment (Lake Mendota)

Hydrogen metabolism was studied in anoxic sediments of the stratified Lake Mendota; using a method which allowed the measurement of in situ H2 concentrations and the headspace-free analysis of turnover of dissolved H2. Addition of sulfate resulted in partial but immediate inhibition of H2-dependent methanogenesis. Sulfate addition did not result in an immediate decrease in the steady state concentration of dissolved H2, nor did it significantly stimulate the rate constant of H2 turnover. Sulfate-induced decrease in dissolved H2 was only observed after prolonged incubation or when endogenous H2 production was stimulated by added glucose. The turnover of the in situ H2 accounted for only 14% of the H2-dependent methanogenesis from bicarbonate. While rates of methanogenesis increased during the season, rates of H2 turnover decreased, accounting for only 2% of the H2-dependent methanogenesis at the end of summer stratification. These observations indicate that increasing proportions of CH4 were formed from H2 being directly transferred in syntrophic methanogenic associations. The rapid inhibition of H2-dependent methanogenesis by exogenous sulfate may be explained at least partially by assuming methanogenic associations in which syntrophic sulfate reducers change their metabolism from fermentative H2 production to sulfate reduction.

Factors affecting the high‐temperature carburisation behaviour of chromium‐nickel alloys in gaseous environments

AbstractThe paper is concerned with the corrosion behaviour of a range of commercial and simpler “model” high chromium‐nickel alloys exposed to gaseous carburising environments of low oxygen activity, at temperatures within the range 825°C to 1000°C. H2‐CH4 mixtures have been used and carbon activities of 0.3 and 0.8 studied by varying the proportion of CH4 in the mixture. The surface condition of the specimens and its effect upon carburisation behaviour has been investigated as a variable by the use of standardised surface preparation techniques.The kinetics of the corrosion process have been studied by the use of conventional gravimetric techniques and cross‐sectional microstructural examinations have aided in establishing the factors of primary importance in the degradation of materials by carburisation. The contributions made by the temperature and carbon activity of the gaseous environment have been highlighted and the important material variables such as metallurgical form, composition and surface condition have been investigated in order to evaluate their significance.