The results of δ13C measurements of several types of major sources of atmospheric methane are as follows: rice paddies, −67‰; the peat bogs of the Lake Agassiz region of northern Minnesota, −67±5‰; swamps of the Florida Everglades, −55±3‰ and biomass burning, −24 to −32‰. In addition, results are presented of a study of the δ13C of CH4 released from a slough, compared to the CH4 in the bottom sediment. These isotopic values are used, together with previously published data, to make up a tentative budget of the fluxes of the major sources for atmospheric methane with an average isotopic composition matching the measured value for atmospheric CH4, taking into account the fractionation effect of the sink processes. This budget requires the existence of a significant flux from an anthropogenic source of heavy CH4, calculated to be 45±15 Tg yr−1 if attributed to CH4 from biomass burning, with δC = −25‰.
Air bubbles in polar ice cores indicate that about 300 years ago the atmospheric mixing ratio of methane began to increase rapidly. Today the mixing ratio is about 1.7 parts per million by volume, and, having doubled once in the past several hundred years, it will double again in the next 60 years if current rates continue. Carbon isotope ratios in methane up to 350 years in age have been measured with as little as 25 kilograms of polar ice recovered in 4-meter-long ice-core segments. The data show that (i) in situ microbiology or chemistry has not altered the ice-core methane concentrations, and (ii) that the carbon-13 to carbon-12 ratio of atmospheric CH(4) in ice from 100 years and 300 years ago was about 2 per mil lower than at present. Atmospheric methane has a rich spectrum of isotopic sources: the ice-core data indicate that anthropogenic burning of the earth's biomass is the principal cause of the recent (13)CH(4) enrichment, although other factors may also contribute.
The sulfur compound most commonly used in mass spectrometric studies of sulfur isotopes is S02. Sulfates in aqueous solutions are usually precipitated as BaS04 and converted to S02 through a series of chemical reactions. Rafter (/) reduced BaS04 to BaS in a platinum Gooch crucible, dissolved the BaS, and precipitated Ag2S. The Ag.2S was filtered, dried, and burned in oxygen to produce S02. Gavelin et al. ( 2) converted BaS04 to the sulfide by heating (750-950 °C) with iron carbonyl and zinc. They treated the sulfide with HC1 and conducted the resulting H2S into a cadmium-acetate absorbing solution to precipitate CdS. The CdS was washed, dried at 400 °C, mixed with V205, and heated at 600 °C to produce S02 in a container attached directly to the mass spectrometer. Ricke (3) used a variation of Gavelin's procedure by sealing off the dried mixture of CdS and V205 in an evacuated quartz tube and heating in an oven for 1.5 hours at 1000 °C. The tube was cooled rapidly so that the S02, which was to be retained in the sealed vessel for subsequent mass spectrometric analysis, represented the equilibrated system at 1000 °C. Thode et al. ( 4) reduced BaS04 to H2S by boiling in a mixture of HI, H3P04, and HC1 in a 200-ml flask with a reflux condenser. CdS was precipitated and converted to Ag2S, which was filtered, washed, and dried. The Ag2S was placed in a quartz boat and burned in 02 at 1350 °C to form so2.
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