Methane emission through ebullition from a non-vegetated estuarine mudflat: The mechanics of tide-driven water level changes

Abstract:

Ebullition is an important pathway for methane (CH₄) to the atmosphere in wetlands. Water level changes have been suggested to trigger ebullition, especially in tidally flooding areas. Bubble transport in submerged sediments results from a multi-phase, dynamic interaction between gaseous and solid phases under the modulation of a liquid phase. To improve understanding of how cyclic ebbing and flooding tides trigger ebullition events, a continuous dynamic dual-chamber system was designed and installed in a non-vegetated mudflat site of an estuarine temperate marsh. Episodic sharp increases in methane concentration signaling ebullition events were primarily observed during ebbing tides (15 events of total 19 events) and occasionally during flooding tides (four events). Laboratory chamber measurements on a mud monolith from the site confirmed that the flooding tide could trigger ebullition releases of gas bubbles. We developed a conceptual sediment fracturing model associated with bubble expansion to unify these observations, arguing that decreases in water level lower the effective stress surrounding isolated gas bubbles and enable trapped bubbles to move upwards via bubble expansion and fracturing of overlying sediments. Increases in relative permittivity measured on the monolith with ground penetrating radar suggest that more water may invade macropores during the initial stage of flooding; subsequent matrix expansion under lowered effective stress then leads to fracture propagation and bubble release. Melting of the surface frozen layer during the spring thaw resulted in increases of methane concentration, comparable in strength to the ebullition fluxes which were associated with large fluctuations in water level around spring tides. Our findings demonstrate the importance of water level changes in triggering ebullition from non-vegetated mudflat areas in tidal wetlands, modulated by the mechanical properties of shallow, soft sediments.