Methane Dynamics in Peat: Importance of Shallow Peats and a Novel Reduced-Complexity Approach for Modeling Ebullition

Abstract

Northern peatlands are one of the largest natural sources of atmospheric methane (CH 4), and it is important to understand the mechanisms of CH 4 loss from these peatlands so that future rates of CH 4 emission can be predicted. CH 4 is lost to the atmosphere from peatlands by diffusion, by plant transport, and as bubbles (ebullition). We argue that ebullition has not been accounted for properly in many previous studies, both in terms of measurement and the conceptualization of the mechanisms involved. We present a new conceptual model of bubble buildup and release that emphasizes the importance of near-surface peat as a source of atmospheric CH 4. We review two possible approaches to modeling bubble buildup and loss within peat soils: the recently proposed bubble threshold approach and a fully computational-fluid-dynamics approach. We suggest that neither satisfies the needs of peatland CH 4 models, and we propose a new reduced-complexity approach that conceptualizes bubble buildup and release as broadly similar to an upside down sandpile. Unlike the threshold approach, our model allows bubbles to accumulate at different depths within the peat profile according to peat structure, yet it retains the simplicity of many cellular (including cellular automata) models. Comparison of the results from one prototype of our model with data from a laboratory experiment suggests that the model captures some of the key dynamics of ebullition in that it reproduces well observed frequency-magnitude relationships. We outline ways in which the model may be further developed to improve its predictive capabilities.

Using hydrologic measurements to investigate free-phase gas ebullition in a Maine peatland, USA

“…CH 4 production likely continues through the winter in the deeper peat, resulting in high CH 4 concentrations (Dise, 1993;Tokida et al, 2007a), whereas CH 4 production in the shallow peat layers will decrease or stop in the cold upper peat layers. It is also possible that the CH 4 concentrations in upper peat are not elevated because FPG is mobilized easily and undergoes diffusion in upper peat near the vadose zone (Joabsson and Christensen, 2001;Glaser et al, 2004;Coulthard et al, 2009). The deeper FPG is not mobilized as easily and may go into and out of the gas phase during changes in atmospheric pressure.…”

“…The bubbles do not escape to the atmosphere immediately but must reach a pressure threshold that triggers an ebullition event. Ebullition events have been known to occur in response to a rising water table as the buoyancy of formed bubbles causes them to propagate upwards with the rising water table (Coulthard et al, 2009). Decreases in atmospheric pressure were thought to cause an increase in pressure difference between pore fluids and the atmosphere causing freephase gas bubbles to release to the atmosphere (Tokida et al, 2007b).…”

“…Left: "deep peat" ebullition model modified from Glaser et al (2004). Right: "shallow peat" ebullition model modified from Coulthard et al (2009). Notice the unknown upward flux of CH 4 from deeper peat that was a "call for further research".…”

“…The deep production model includes diffusion of shallow peat CH 4 to the atmosphere and production at depth due to a downward transport of labile carbon. The second model has been called the "shallow peat model" and was proposed after numerous laboratory studies (Coulthard et al, 2009). The shallow peat model showed steady ebullition, diffusion, and episodic ebullition occurring from the upper layers of peat (Fig.…”

“…These studies did include an unknown upward flux from deeper peat (> 2 m). Little was known about FPG production and flux from deeper peat, and this was to "act as stimulus for further research" (Coulthard et al, 2009).…”