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Groupe de Emilien Véret

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AVG @ Methane

Methane in the atmosphere is generated by many different sources, such as fossil fuel development and use, decay of organic matter in wetlands, and as a byproduct of livestock farming. Determining which specific sources are responsible for variations in methane annual increase is difficult. Preliminary analysis of carbon isotopic composition of methane in the NOAA air samples done by the Institute of Arctic and Alpine Research at the University of Colorado, indicates that it is likely that a primary driver of the increased methane burden comes from biological sources of methane such as wetlands or livestock rather than thermogenic sources like oil and gas production and use.

AVG @ methane

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"Although increased fossil emissions may not be fully responsible for the recent growth in methane levels, reducing fossil methane emissions are an important step toward mitigating climate change," said GML research chemist Ed Dlugokencky.

While carbon dioxide remains in the atmosphere for much longer than methane, methane is roughly 25 times more powerful at trapping heat in the atmosphere, and has an important short-term influence on the rate of climate change.

Methane in the atmosphere is generated by many different sources, such as fossil fuel production, transport and use, from the decay of organic matter in wetlands, and as a byproduct of digestion by ruminant animals such as cows. Determining which specific sources are responsible for variations in annual increases of methane is complex, but scientists estimate that fossil fuel production and use contributes roughly 30% of the total methane emissions. These industrial sources of methane are relatively simple to pinpoint and control using current technology.

The graphs show globally-averaged, monthly mean atmospheric methane abundance determined from marine surface sites. The first graph shows monthly means for the last four years plus the current year, and the second graph shows the full NOAA time-series starting in 1983.Values for the last year are preliminary, pending recalibrations of standard gases and other quality control steps. Other impacts on the latest few months of data are described below.

Abstract. Methane is a significant atmospheric trace gas in the context of greenhouse warming and climate change. The dominant sink of atmospheric methane is the hydroxyl radical (OH). Recently, a mechanism for production of chlorine radicals (Cl) in the marine boundary layer (MBL) via bromine autocatalysis has been proposed. The importance of this mechanism in producing a methane sink is not clear at present because of the difficulty of in-situ direct measurement of Cl. However, the large kinetic isotope effect of Cl compared with OH produces a large fractionation of 13C compared with 12C in atmospheric methane. This property can be used to estimate the likely minimum size of the methane sink attributable to MBL Cl. By taking account of the mixing of MBL air into the free troposphere, we estimate that the global methane sink due to reaction with Cl atoms in the MBL could be as large as 19Tgyr-1, or about 3.3% of the total CH4 sink. However, its impact on the methane stable carbon isotope budget is large and warrants further attention.

While studying the sources and formation pathways for methane in LAB groundwater,7 the implications for the high methane concentrations and large volumes of annual groundwater pumping led to the estimation of methane emissions from the LAB, and a comparison with the Marcellus shale region of NE Pennsylvania. Further compilation of recent groundwater methane data,13 and annual groundwater pumping,14 enabled estimation of methane emissions from groundwater pumping in the Principal aquifers of the USA (Fig. 1).

Methane emissions resulting from pumping groundwater in the Principal aquifers of the USA. See Table 5 for aquifer names and values. Not shown but included in the total US groundwater pumping methane emissions are Alaska, Puerto Rico, Hawaii, and US Virgin Islands; *extrapolated estimate

While small compared with total global emissions,11 methane emissions resulting from groundwater abstraction in the USA represent an important source to be quantified and should be included in the global methane budget. Emissions may be significant locally, where groundwater methane concentrations are high. Also, of local significance, methane, as a precursor to ozone formation, could represent an unaccounted for and potentially significant contribution to low-level ozone formation. More detailed analyses of correlations between methane concentrations and groundwater age, carbon system indicators, aquifer redox conditions, Eh potentials, as well as lithology and sediment age may help to better estimate methane occurrence and concentrations in groundwater.13,37 Within a given lithology group, methane concentrations are typically higher in younger sediments than in older sediments,13 and higher in older groundwater (pre 1950s) than recently recharged groundwater.7,13 Previous studies noted that sediment age can be important with respect to redox processes due to the preferential consumption of the most reactive components of sedimentary organic carbon by microbes over time.38,39,40 In support of this hypothesis, McMahon et al.13 found that for aquifers composed of similar lithologies, concentrations of methane, dissolved organic carbon, and ammonium were higher in the aquifers with younger sediments than in aquifers with older sediments.

Emissions from 377 gas actuated (pneumatic) controllers were measured at natural gas production sites and a small number of oil production sites, throughout the United States. A small subset of the devices (19%), with whole gas emission rates in excess of 6 standard cubic feet per hour (scf/h), accounted for 95% of emissions. More than half of the controllers recorded emissions of 0.001 scf/h or less during 15 min of measurement. Pneumatic controllers in level control applications on separators and in compressor applications had higher emission rates than controllers in other types of applications. Regional differences in emissions were observed, with the lowest emissions measured in the Rocky Mountains and the highest emissions in the Gulf Coast. Average methane emissions per controller reported in this work are 17% higher than the average emissions per controller in the 2012 EPA greenhouse gas national emission inventory (2012 GHG NEI, released in 2014); the average of 2.7 controllers per well observed in this work is higher than the 1.0 controllers per well reported in the 2012 GHG NEI.

While most global productivity is driven by modern photosynthesis, river ecosystems are supplied by locally fixed and imported carbon that spans a range of ages. Alluvial aquifers of gravel-bedded river floodplains present a conundrum: despite no possibility for photosynthesis in groundwater and extreme paucity of labile organic carbon, they support diverse and abundant large-bodied consumers (stoneflies, Insecta: Plecoptera). Here we show that up to a majority of the biomass carbon composition of these top consumers in four floodplain aquifers of Montana and Washington is methane-derived. The methane carbon ranges in age from modern to up to >50,000 years old and is mostly derived from biogenic sources, although a thermogenic contribution could not be excluded. We document one of the most expansive ecosystems to contain site-wide macroinvertebrate biomass comprised of methane-derived carbon and thereby advance contemporary understanding of basal resources supporting riverine productivity.

Thus, we investigated the source and role of methane as a potential subsidy to floodplain aquifer food webs, mainly at Nyack but also at three other locations: the Kalispell floodplain on the main stem of the Flathead River in Northwest Montana, the Jocko River floodplain in Southwest Montana and the Methow River floodplain in Washington. At each of these sites a grid of slotted, but not screened, groundwater monitoring wells was available for sampling. Of this suite of aquifers only the Nyack is underlain by a hydrocarbon-containing shale formation. We posited: (1) what is the source of the methane, (2) what are the contributions of various methane sources to stonefly biomass and (3) is a methane subsidy in alluvial aquifers a widespread phenomenon? To identify methane sources, we measured the carbon and deuterium stable isotope ratios of dissolved methane, the radiocarbon ages of dissolved methane, and methane, ethane and propane concentrations. In order to understand the contributions of various methane-derived carbon sources to biomass (question 2), we measured carbon stable isotope ratios and radiocarbon ages of stonefly biomass and organic matter, and then incorporated these values into Bayesian mixing models that were parameterized using a suite of scenarios to give a range of reasonable and conservative estimates of source contributions to biomass. We addressed question 3 by comparing results among study sites.

(a). The Schoell plot (22) of deuterium isotopic signatures vs. carbon isotopic signatures in individual samples. Symbols represent well; colours represent depth. Most samples cluster at a methanogenic origin, while others at HA10 deep and HA17 shallow suggest a thermogenic contribution and/or microbial oxidation. However, samples from the same day at other depths still cluster with methanogenesis. (b). A Bernard plot (24) displays the ratio of methane concentration to summed concentrations of higher chain hydrocarbons (ethane and propane) versus the δ13C of methane.

Radiocarbon ages of stonefly tissue (each point is one individual) were strongly correlated with calculated levels of methane-derived carbon in biomass (R2=0.56, P=60%; (b) non-methane-derived carbon was modern because low levels of methane-derived carbon in biomass approaching 0% corresponded with younger radiocarbon ages; and (c) the maximum methane age could be much older than 6900 years, because all stonefly tissue measured was a mixture of various organic carbon sources. 350c69d7ab

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