Methane atmospheric, isotopic

Concerns over atmospheric methane as a greenhouse gas and the large contribution of biomethanogenesis as a source of this gas make it important to determine the relative significance of various components of this activity. A recent paper (8) summarized estimates (28-30) of source fluxes of atmospheric methane based on several carbon isotopic studies and presented new data on natural sources and biomass burning. These data (Table III) show that of a total flux of 594 million tons (Tg) per year, 83% is produced via biomethanogenesis from a combination of natural (42%) and anthropogenic (41%) sources. 

Indexes o and s define the ratio of the carbon isotopes in the sample and in the standard. A lithospheric carbonate material was accepted as standard. The closest to this zero point value belongs to standard sample NBS-19 (1.95%c). There are some other standard samples NBS-22 oil (—29.74%c), NBS-18 calcium carbonate (— 5.01%c). Usually 813C values for plants are in the range ( 15%o) to (— 30%c), and for oil (— 20%c) to ( 36%c). Atmospheric methane has the lowest content of 13C. Its 813C value is approximately —47%o. 

Atmospheric methane has a mean 5 C-value of around —47%c (Stevens 1988). Quay et al. (1999) presented global time series records between 1988 and 1995 on the carbon and hydrogen isotope composition of atmospheric methane. They measured spatial and temporal variation in and D with a slight emichment observed for the southern hemisphere (—47.2%c) relative to the northern hemisphere (—47.4%o). The mean 5D was —86 3%c with a 10%o depletion in the northern relative to the southern hemisphere. 

Steuber T, Buhl D (2006) Calcium-isotope fractionation in selected modern and ancient marine carbonates. Geochim Cosmochim Acta 70 5507-5521 Stevens CM (1988) Atmospheric methane. Chem Geol 71 11-21... 

Moriizumi, J., K. Nagamine, T. Iida, and Y. Ikebe, Carbon Isotopic Analysis of Atmospheric Methane in Urban and Suburban Areas Fossil and Non-Fossil Methane from Local Sources, Atmos. Environ., 32, 2947-2955 (1998). 

Lassey K. R., Lowe D. C., and Manning M. R. (2000) The trend in atmospheric methane S C and implications for isotopic constraints on the global methane budget. Global Biogeochem. Cycles 14, 41—49. 

Merritt D. A., Hayes J. M., and Des Marais D. M. (1995) Carbon isotopic analysis of atmospheric methane by isotope ratio monitoring gas chromatography-mass spectrometry. J. Geophys. Res. 100, 1317-1326. 

Quay P. D., Stutsman J., Wilbur D., Snover A., Dlugokencky E., and Brown T. (1999) The isotopic composition of atmospheric methane. Global Biogeochem. Cycles 13, 445-461. 

Sugawara S., Nakazawa T., Inoue G., Machida T., Mukai H., Vinnichenko N. K., and Khattatov V. U. (1996) Aircraft measurements of the stable carbon isotopic ratio of atmospheric methane over Siberia. Global Biogeochem. Cycles. 10, 223-231. 

Tans P. P. (1997) A note on isotopic ratios and the global atmospheric methane budget. Global Biogeochem. Cycles 11, 77-81. 

D. R., and Dlugokencky E. J. (1999) Carbon isotope composition of atmospheric methane a comparison of surface level and upper tropospheric air. J. Geophys. Res. 104, 13895-13910. 

Recent measurements (1980) Indicate that the atmospheric methane is about -47.0 0.3 /oo (48). The average isotopic fractionation associated with the sink process is -2.5 1.5°/oo, and there is a +0.3°/oo Isotope effect resulting from the nonsteady state increasing methane concentrations (48). This implies that the average for all sources is about... 

The total annual input of methane from all sources to the atmosphere shown in Table 6.4 is 540 Mt, while the estimated output from atmosphere to sinks is 500 Mt. The potential inaccuracies in flux data can be seen by comparing the observed carbon isotopic signature of atmospheric methane of —47%o with that calculated from the data in Table 6.4 of c— 54%o (the latter is actually equivalent to —58%o upon correcting for the kinetic isotope effect (see Box 1.3) that operates during the hydroxyl abstraction reaction). There are clearly major gaps in our understanding of the pathways of methane into and out of the atmosphere and the fluxes involved, as there are for many anthropogenic substances (see Chapter 7). 

The origin of atmospheric methane can be estimated by determining its, 4C isotope abundance. This abundance should be the same as the l4C content of living plants, if methane is of biological origin or is provided by recently dead organisms. CH4 from fossil fuels or volcanic activity is practically free of radiocarbon, since fuel deposits are very old and their 14C content has already decayed. Measurement of... 

Lassey, K.R., D.C. Lowe, C.A.M. Brenninkmeijer, and A.J. Gomez, Atmospheric methane and its carbon isotopes in the southern hemisphere Their time series and an instructive model. Chemosphere-Global Change Science 26, 95, 1993. 

Chen JS, Shao MR, Huo WG, Yao YY (1984) Carbon isotopes of carbonate strata at Permian-Triassic boundary in Changxing, Zhejiang. Scient Geol Sinica 1984 88-93 Cicerone RJ, Oremland RS (1988) Biogeochemcal aspects of atmospheric methane. Global Biogeochem Cycles 2 299-327... 

The interstitial air trapped during this process preserves a largely unaltered record of the composition of past atmospheres on time scales as short as decades and as long as several hundred thousand years. Such records have provided critical information about past variations in carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), carbon monoxide (CO), and the isotopic composition of some of these trace species. In addition, studies of the major elements of air nitrogen, oxygen, and argon, and their isotopic composition, have contributed... 

The next most likely possibility is cometary delivery of the atmosphere but again there are some problems with the isotope ratios, this time with D/H. The cometary D/H ratios measured in methane from Halley are 31 3 x 10-5 and 29 10 x 10-5 in Hayuatake and 33 8 x 10-5 in Hale-Bopp, whereas methane measurements from Earth of the Titan atmosphere suggest a methane D/H ratio of 10 5 x 10-5, which is considerably smaller than the ratio in the comets. The methane at least in Titan s atmosphere is not exclusively from cometary sources. Degassing of the rocks from which Titan was formed could be a useful source of methane, especially as the subnebula temperature around Saturn (100 K) is somewhat cooler than that around Jupiter. This would allow volatiles to be more easily trapped on Titan and contribute to the formation of a denser atmosphere. This mechanism would, however, apply to all of Saturn s moons equally and this is not the case. 

The volatile-trapping mechanism has a further problem associated with the temperature. Very volatile molecules such as N2, CO and CH4 are not easily trapped in laboratory ice simulation experiments unless the ice temperature is 75 K, which is somewhat lower than the estimated Saturnian subnebula temperature. This has led to the suggestion that the primary source of nitrogen within the Titan surface ices was NH3, which became rapidly photolysed to produce H2 and N2 upon release from the ice. The surface gravity is insufficient to trap the H2 formed and this would be lost to space. However, the origin of methane on Titan is an interesting question. Methane is a minor component of comets, with a CH4/CO ratio of clCT1 compared with the present atmospheric ratio of > 102. The D/H ratio is also intermediate between that of comets and the solar nebula, so there must be an alternative source of methane that maintains the carbon isotope ratio and the D/H isotope ratio and explains the abundance on Titan. 

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