Methane isotopic composition

Dissolved compounds. In an ideal situation the one-time investigation should address the entire list of drinking water standards. In any case, the analyzed ions should include the major ions (Na, K, Ca, Mg, Cl, HC03, S04, N03, Br) dissolved oxygen, C02, and methane isotopic composition (e.g., deuterium, lsO, 13C) and age indicators (e.g., tritium, 14C, 36C1 and 4He the last should be included for waters that are estimated to be old). 

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 naturally occurring variations in carbon isotope composition are greater than 120%c, neglecting extraterrestrial materials. Heavy carbonates with 5 C-values > + 20%c and light methane of < - 100%c have been reported in the literature. 

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. 

Rather than using the isotopic composition of methane alone James (1983, 1990) and others have demonstrated that carbon isotope fractionations between the hydrocarbon components (particularly propane, iso-butane and normal butane) within a natural gas can be used with distinct advantages to determine maturity, gas-source rock and gas-gas correlations. With increasing molecular weight, from Ci to C4, a enrichment is observed which approaches the carbon isotope composition of the source. 

Chung HM, Sackett WM (1979) Use of stable carbon isotope compositions of pyrolyticaUy derived methane as maturity indices for carbonaceous materials. Geochim Cosmochim Acta 43 1979-1988... 

Schoell M (1980) The hydrogen and carbon isotopic composition of methane from natural gases of various origins, Geochim Cosmochim Acta 44 649-661 Schoell M (1983) Genetic characterization of natural gases. Bull Am Ass Petrol Geol 67 2225-... 

Schoell M (1984) Recent advances in petroleum isotope geochemistry. Org Geochem 6 645-663 Schoell M (1988) Multiple origins of methane in the Earth. Chem Geol 71 1-10 Schoell M, McCaffrey MA, Fago FJ, Moldovan JM (1992) Carbon isotope compositions of 28,30-bisnorhopanes and other biological markers in a Monterey crude oil, Geochim Cosmochim Acta 56 1391-1399... 

Alperin, M.J., Blair, N.E., Albert, D.B., Hoehler, T.H., and Martens, C.S. (1992) Factors that control isotopic composition of methane produced in an anaerobic marine sediment. Global Biogeochem. Cycles 6, 271-329. 

Chanton, J.P., and Dacey, J.W.H. (1991) Effects of vegetation on methane flux, and carbon isotopic composition. In Trace Gas Emissions by Plants (Mooney, H.A., and Sharkey, T. eds.), pp. 65-92, Academic Press, San Diego, CA. 

Figure 2 Carbon stable-isotope compositions of low-molecular-weight hydrocarbons, amino acids, and monocarboxylic acids from the Murchison meteorite plotted against carhon number. Carbon number 1 denotes methane and CO2, 2 denotes ethane, ethanoic acid, glycine, etc. (source Yuen et al., 1984).

Happen J. D., Chanton J. P., and Showers W. S. (1994) The influence of methane oxidation on the stable isotopic composition of methane emitted from Florida swamp forests. Geochim. Cosmochim. Acta 58, 4377-4388. 

Popp B. N., Sansone F. J., Rust T. M., and Merritt D. A. (1995) Determination of concentration and carbon isotopic composition of dissolved methane in sediments and nearshore waters. Anal. Chem. 67, 405-411. 

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. 

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