Methane emission through ebullition from an estuarine mudflat: 2. Field observations and modeling of occurrence probability

Abstract

Ebullition can transport methane (CH4) at a much faster rate than other pathways, albeit over limited time and area, in wetland soils and sediments. However, field observations present large uncertainties in ebullition occurrences and statistic models are needed to describe the function relationship between probability of ebullition occurrence and water level changes. A flow‐through chamber was designed and installed in a mudflat of an estuarine temperate marsh. Episodic increases in CH4 concentration signaling ebullition events were observed during ebbing tides (15 events over 456 ebbing tides) and occasionally during flooding tides (4 events over 455 flooding tides). Ebullition occurrence functions were defined using logistic regression as the relative initial and end water levels, as well as tidal amplitudes were found to be the key functional variables related to ebullition events. Ebullition of methane was restricted by a surface frozen layer during winter; melting of this layer during spring thaw caused increases in CH4 concentration, with ebullition fluxes similar to those associated with large fluctuations in water level around spring tides. Our findings suggest that initial and end relative water levels, in addition to tidal amplitude, partly regulate ebullition events in tidal wetlands, modulated by the lunar cycle, storage of gas bubbles at different depths and seasonal changes in the surface frozen layer. Maximum tidal strength over a few days, rather than hourly water level, may be more closely associated with the possibility of ebullition occurrence as it represents a trade‐off time scale in between hourly and lunar periods.

Identifying dominant environmental predictors of freshwater wetland methane fluxes across diurnal to seasonal time scales

“…This result is similar to other studies which have also found FCH4 pulses during water table drawdown (Bansal et al, 2020;Hatala, Detto, Sonnentag, et al, 2012;Knox et al, 2016;Moore & Dalva, 1993;Sturtevant et al, 2016). These interactions are consistent with the release of stored CH 4 as hydrostatic pressure drops, with peak release occurring as the water table crosses the soil surface (Chen et al, 2017;Knox et al, 2016;Ueyama, Yazaki, et al, 2020). As illustrated in Figure 6f, different magnitudes of FCH4 pulses are therefore likely dependent on the current CH 4 pool in porewater and CH 4 production rates (Bansal et al, 2020;Sturtevant et al, 2016).…”

Mechanism of Faster CH4 Bubble Growth Under Surface Waves in Muddy Aquatic Sediments: Effects of Wave Amplitude, Period, and Water Depth

“…This is coupled with the process of CH 4 diffusion to a growing bubble from within the ambient sediments, responsible for bubble growth (Algar and Boudreau, 2009). Field studies often discern a correlation between episodic ebullition from aquatic sediments with variations in hydrostatic pressure (Martens and Klump, 1980;Miller and Oremland, 1988;Chanton et al, 1989;Mattson and Likens, 1990;Keller and Stallard, 1994;Scandella et al, 2011;Chen and Slater, 2016;Scandella et al, 2016;Chen et al, 2017), which was also recently confirmed by lab observations (Scandella et al, 2017) and numerical studies (Algar et al, 2011b;Katsman, 2019).…”

A Modeling Approach for Addressing Sensitivity and Uncertainty of Estuarine Greenhouse Gas (CO2 and CH4) Dynamics

“…Water‐air transfer rates are further complicated for CH 4 , as its relatively low solubility means that nondiffusive ebullition fluxes can contribute a substantial fraction to CH 4 emissions in some aquatic environments (e.g., Baron et al., 2022; Hoffmann et al., 2017; Schmid et al., 2017). However, our understanding of ebullition's drivers remains incomplete and empirical data on its spatial and temporal variability within estuaries is also too limited (Chen et al., 2017) to meaningfully incorporate it here. The disentanglement of diffusive and ebullition transport of CO 2 and CH 4 through aquatic ecosystems to the atmosphere is therefore a key area for future model development and could be added to uncertainty assessment approach presented here once established.…”