ABSTRACT

This chapter focuses on how stable (13C, 12C, 2H, and 1H) and radiogenic (14C and 3H) isotopes can be used to investigate the origin, transport, and alteration of methane (CH4) in terrestrial environments. Methane is the most abundant organic gas in the troposphere and plays an important role in both the energy balance and chemistry of the Earth’s atmosphere, participating in a range of chemical reactions that directly or indirectly involve CO, CO2, H2O, H2, CH2O, NOx, O3, Cl, F, and OH* (Cicerone and Oremland 1988). The atmospheric lifetime of CH4 ranges between eight and 14 years with a mean turnover time of ~10 years (Rasmussen and Khalil 1981; Mayer et al. 1982). Destruction of CH4 in the troposphere and stratosphere is complex but ultimately results in the end products CO2 and H2O. The atmospheric mixing ratio

6.1 Introduction .................................................................................................. 167 6.2 Origins of Methane (CH4)............................................................................. 168

6.2.1 Abiogenic Methane (CH4) Production .............................................. 169 6.2.2 Microbial Methane (CH4) Production ............................................... 169

6.3 Stable Isotopes as a Proxy for Methanogenic Pathways ............................... 170 6.3.1 C and H Isotopes of Methane ........................................................... 174

6.4 Radiocarbon and Methane (CH4).................................................................. 177 6.5 Secondary KIEs: Methane Oxidation and Transport Effects ....................... 177 6.6 Methane in Environmental Systems ............................................................. 178

6.6.1 Methane in Groundwater .................................................................. 178 6.6.2 Methane in Coalbeds ........................................................................ 180 6.6.3 Methane in Landlls ......................................................................... 181 6.6.4 Methane in Wetlands ........................................................................ 185

6.7 Summary ...................................................................................................... 189 Acknowledgment ................................................................................................... 189 References .............................................................................................................. 190

of CH4 has increased from ~700 parts per billion by volume (ppbv) to 1750 ppbv during the period 1750 to 2000 (Etheridge et al. 1998). Methane directly inuences the temperature of the lower troposphere through absorption of infrared radiation (IR) and has been estimated to account for ~15% of enhanced radiative forcing of climate as a result of anthropogenic greenhouse gas emissions (Rodhe 1990). The commonly cited ability of CH4 to absorb thermal radiation at a rate ~25 times that of CO2 on a molecule-per-molecule basis is a consequence of the relatively low atmospheric abundance of CH4, and the lack of overlap of absorption bands with CO2 and H2O that absorb IR primarily outside the thermal emission window (Senum and Gaffney 1985).