Tuesday, November 14, 2006 - 9:35 AM

Methane Ebullition Associated with Falling Atmospheric Pressure from a Natural Wetland.

Takeshi Tokida, Dept. Of Biological & Env. Eng., Tokyo, 113-8657, Japan, Tsuyoshi Miyazaki, University of Tokyo, Univ. of Tokyo Grad. Sch. Ag. & Life Sci., Dep.ag.eng.yayoi 1-1-1bunkyo, Tokyo, 113-8657, JAPAN, and Masaru Mizoguchi, Bunkyo-ku, University of Tokyo, University of Tokyo, Yayoi 1-1-1, Tokyo, 113-8657, JAPAN.

Natural wetlands, with more than half of their geographical area covered with peat-rich ecosystems, are likely to be the single largest source of atmospheric methane (CH4), a potent greenhouse gas. At present, most of the estimates of CH4 emissions from peatlands are based on infrequent, temporally discontinuous ground-based flux measurements. Efforts have been made to extrapolate measured emission rates to establish seasonal amounts using relevant biogeochemical factors, such as water table positions and peat temperatures, by assuming that the flux was stationary during the substantial non-sampling period. We investigated short-time variations in CH4 flux by conducting a mesocosm experiment, and a 90-hour field study, wherein intensive measurement scheme was adopted. In the laboratory experiment, an intact peat core was subjected to natural air pressure fluctuations while the temperature and water table were controlled and kept constant. The experiment indicated that a reduction in barometric pressure causeed a sudden release of CH4 bubbles from the peat core. Moreover, it demonstrated that ebullition can be the main transport mechanism during the pressure-falling phase. In the field study, we found that the CH4 flux can change by two orders of magnitude in a matter of tens of minutes as a result of the episodic release of accumulated CH4 bubbles. These episodic events represent a very significant contribution to the total flux (probably more than half). Theoretical calculations confirmed our hypothesis that fluctuations in the atmospheric pressure play a dominant role in determining the timing and magnitude of the ebullition events. These results clearly indicated that field campaigns must be designed to cover this rapid temporal variability caused by ebullition, which may be especially important under intemperate weather. Process-based CH4 emission models should also be modified to include air pressure as a key factor for the control of ebullient CH4 release from peatland.