Seasonal variation in methane fluxes from sedimentary basins to the atmosphere: Results from chamber measurements and modeling of transport from deepsources

Results from modeling of microseepage are presented that suggest that under proper conditions some thermogenic CH4 is transported to the atmosphere wi thout complete consumption by methanotrophic bacteria in soils. These compu ter simulations are supported by direct measurement of CH4 fluxes in season ally dry and/or cold climate sedimentary basins where losses to the atmosph ere were postulated to occur. Measurements in four semiarid sedimentary bas ins where a thermogenic CH4 source is expected found small positive fluxes during cooler/drier seasons and negative fluxes during warmer/wetter season s. Small positive fluxes up to 43 mg CH4 m(-2) d(-1) were found. A similar result was found for two geothermal areas in a semiarid climate where CH4 d erived from the Fischer-Tropsch reaction operating in the reservoir provide s a deep source of CH4. The results of the computer simulations, and direct field measurement of CH4 fluxes in dry and/or cold terrestrial environment s suggest that a portion of the global atmospheric budget of CH4 may be fro m microseepage.