Methane emission through ebullition transport in a temperate estuarine marsh: the role of tide-driven hydraulic pressure

English Abstract:

Ebullition in northern peatlands is considered a dominant pathway for biogenic methane (CH₄) emission. Field observations of ebullition of CH₄ in other wetlands are lacking. A field experiment was conducted in a temperate estuarine marsh in the New Jersey Meadowlands (NJ, USA) to identity episodic releases of CH₄ gas and to determine the primary environmental factors driving such releases. Ebullition events were captured by direct in-situ continuous measurements of CH₄ concentration in the headspace of a flow-through dynamic chamber installed in an unvegetated microsite. Time series of erupted sediment-air CH₄ flux showed a significant correlation with water level drops during daily outgoing tide events, indicating that tide-driven pressure decreases could effectively drive gas bubbles upwards through soil pores by increasing the buoyancy force. Rapid tide-driven hydraulic pressure changes appear to be more effective at triggering ebullition events than longer duration atmospheric pressure changes with an approximately weekly period. A theoretical model based on a balance between the capillary force and buoyancy force of gas bubbles supports the argument that pressure decreases cause a continuous net force on upward transport of gas bubbles through a series of soil pore throats larger than that caused by pressure increases occurring over a similar time period. However, upward gas bubble transport driven by overall pressure drops could be delayed by accumulation below resisting layers, resulting in larger episodic ebullition when these layers are breached. Very few ebullition events were observed when an ice layer formed at the mud flat surface during the winter season, and large numbers of gas bubbles trapped in the ice were clearly observed. This suggests that the length of time the surface is frozen may play an important role in regulating CH₄ emissions and also influence the calculation of yearly average methane emission rates from tidal wetlands.