PAUL QUAY, JOHN STUTSMAN, AND DAVID WILBUR
School of Oceanography, University of Washington, Seattle 98195
INTRODUCTION
Since 1989 we have been measuring the 13C/12C of atmospheric CH4 on air samples collected at three CMDL sites (Pt. Barrow at 71°N, 156°W, Mauna Loa at 19°N, 155°W and Samoa at 14°S, 170°W) and on the Washington coast at 48°N, 126°W.
The 13C/12C of atmospheric CH4 is a tracer that can distinguish between CH4 input from bacterial and nonbacterial CH4 sources. Bacterial CH4 is microbially produced in anoxic environments likes swamps, bogs, rice paddies, and the rumens of cows. Non-bacterial CH4 sources include thermogenically produced natural gas and CH4 produced during biomass burning. Bacterial CH4 has a d13C of about -60‰ (versus PDB) whereas the d13C of natural gas and CH4 from biomass burning are about -40 and -24‰, respectively [Quay et al., 1991].
The spatial and temporal variations in the 13C/12C of atmospheric CH4 depend on the variations of the relative strength and 13C/12C of the CH4 sources and sinks. Over interannual time scales the trends in the 13C/12C of atmospheric CH4 indicates changes in the source composition, i.e., the relative strength of bacterial versus non-bacterial CH4 sources. Changes in the loss rate of CH4 have little effect on the 13C/12C. Because CH4 will likely contribute about 15% of the radiative forcing during the next century [Wigley and Raper, 1992], it is important to quantify the strength of the individual CH4 sources and to determine whether the CH4 source strengths are changing with time. This latter point has been underscored by the recently observed slowdown in the rate of CH4 increase in the atmosphere [Dlugokencky et al., 1994].
METHODS
The air samples are collected at approximately 2-week intervals using pre-evacuated stainless steel flasks either 15 or 30 liters in volume. The CH4 is extracted from air in our laboratory using the procedure developed by Stevens and Rust [1982]. Briefly, the air is metered into a high vacuum extraction line through a series of liquid nitrogen traps to remove H2O, CO2, and N2O. The air then passes through a bed of Schutze's reagent, I2O5 on silica, to oxidize CO to CO2 which is trapped cryogenically. Then the CH4 in the air is combusted over platinized silica at 800°C to CO2 which is then trapped cryogenically. The yield of the procedure determined from standards is 100 ± 2% (n = 114). The 13C/12C of the CO2 derived from CH4 is measured on a Finnigan MAT 251 gas ratio isotope mass spectrometer. The overall measurement precision is about ±0.1‰. We obtain a d13C of -41.73‰ (versus PDB) for NBS-16.
RESULTS AND DISCUSSION
The seasonal cycle in the d13C of CH4 is greatest at 71°N (Barrow) with
an amplitude of ~0.6‰, and decreases southward to 14°S (Samoa) where the
seasonal trend is at about our measurement precision, i.e., 0.1‰ (Figure
1). The seasonal trends at 71°N and 48°N can be approximated roughly by a single
harmonic with an annual period. Episodes of high CH4 concentrations
associated with very depleted d13C values
occur at these two sites in September and October of each year and are due to
input of bacterial methane. The trend toward higher summertime d13C values is expected as a result of increased CH4
oxidation by OH because the 12CH4 molecules react at a
slightly faster rate (1.0054x) than the 13CH4 molecules
[Cantrell et al., 1990]. It is also likely that there is seasonal change
in the average d13C of
the CH4 sources. The annual mean d13 C values increase southward from about -47.7‰
at 71°N to -47.2‰ at 14°S.
Fig. 1. The time series of the d13C of atmospheric CH4 measured at Pt. Barrow, Olympic Peninsula, Mauna Loa, and Samoa since 1988.
We calculate a global average d13C of CH4 of approximately -47.3‰. The mean global d13C value, when combined with the 14C content of atmospheric CH4, yields estimates of the proportion of bacterial, nonbacterial, and fossil CH4 source strengths [e.g., Quay et al., 1991]. We estimate that bacterial CH4 sources contribute about 75%, fossil CH4 sources about 15%, and biomass burning about 10% of the total CH4 input.
Although the seasonal cycle in d13C dominates the time series measurements in the northern hemisphere, there is evidence for a slight interannual increase. Measurements at all four time-series locations indicate an increase in d13C and, when combined and area weighted, yield a global rate of approximately 0.02‰ per year since 1989. A d13C increase indicates that the ratio of nonbacterial to bacterial CH4 source strength is increasing slightly. The interhemispheric gradient and interannual trend in the isotopic composition of methane yields useful constraints of the magnitude of CH4 sources and the rate at which the source strengths are changing with time.
REFERENCES
Dlugokencky, E.J., L.P. Steele, P.M. Lang, and K.A. Masarie, The growth rate and distribution of atmospheric methane, J. Geosphys. Res., 99, 17,021-17,043, 1994.
Cantrell, C.A., R.E. Shetter, A.H. McDaniel, J.G Calvert, J.A. Davidson, D.C. Lowe, S.C. Tyler, R.J. Cicerone and J.P. Greenberg, Carbon kinetic isotope effect in the oxidation of methane by the hydroxyl radical, J. Geophys. Res., 95, 22,455-22,462, 1990.
Quay, P.D., S.L. King, J. Stutsman, D.O. Wilbur, L.P. Steele, I. Fung, R.H. Gammon, T.A. Brown, G.W. Farwell, P.M. Grootes, and F.H. Schmidt, Carbon isotopic composition of atmospheric methane: Fossil and biomass burning source strengths, Global Biogeochem. Cycles., 5, 25-47, 1991.
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Wigley, T.M.L., and S.C.B. Raper, Implications for climate and sea level of revised IPCC emission scenarios, Nature, 357, 293-300, 1992.
