Methane fluxes from wetlands: Competition between Ebullition, Advection, and Diffusion

Methane is transported from wetlands by a variety of physical processes through a variety of pathways: ebullition (the formation of bubbles that rise to the surface), flushing (advection by pore water flowing into streams), and diffusion (both into plant roots and out through the surface).  We have formulated a theory that couples ebullition, diffusion, and flushing, and predicts how the competition between these processes leads to different porewater concentrations of dissolved gases and their isotopes. We apply the theory to explain why ebullition creates much higher concentrations of carbon dioxide and much different ratios of carbon-13 in both methane and carbon dioxide and then we use these results to explain the oberserved differences in carbon dioxide and methane concentrations and their isotope ratios between northern and southern wetlands. Implications of the theory also include: (1) Carbon isotope ratios in methane and carbon dioxide depend not only on the fractionation factor in methanogenesis but also on the magnitude of ebullition relative to advection. (2) Counterintuitively, higher methane concentrations in pore water occur at lower rates of methanogenesis, for all else held constant. (3) Pore water gas concentrations can be used to infer historical rates of ebullition.