Dynamics of methane ebullition from a peat monolith revealed from a dynamic flux chamber system

Methane (CH4) ebullition in northern peatlands is poorly quantified in part due to its high spatiotemporal variability. In this study, we employed a continuous measurement approach to better understand the coupling between CH4 ebullition fluxes and subsurface gas bubble dynamics and to examine potential triggering effects of atmospheric pressure and temperature on ebullitive CH4 releases. A dynamic flux chamber system (DFC), consisting of a flow-through gas chamber and a fast methane analyzer (LI-COR 7700), was used to continuously measure CH4 fluxes from a monolith of near-surface Sphagnum peat over eight weeks. By using a graphical separation method, episodic ebullition fluxes were extracted from total flux recorded, and the timing and CH4 content of individual ebullition events identified. Coincident transmission ground penetrating radar (GPR) measurements of gas content and dissolved CH4 concentrations in pore water were also acquired at three depths (upper, middle, and lower) within the monolith. Estimated episodic ebullition fluxes were not sensitive to the uncertainties in steady flux quantification associated with the graphical model and the application of the DFC had minimal disturbance on air-peat CH4 exchange. Episodic and steady ebullition fluxes, constrained by modeled diffusion fluxes using Fick’s law and the bulk CH4 concentrations in peat, were estimated as on average 38% and 36% of the total fluxes over the entire study period, respectively. The observations of gas content variations within the three layers along with the timing of episodic ebullition fluxes support the existence of an ebullition threshold regulating CH4 ebullition. However, a larger threshold (gas content of 0.14 m3•m-3) was found for the middle and lower layers, suggesting that multiple mechanisms related to the depth variation of peat structure were responsible for the complex behavior of episodic CH4 ebullition. Temperature variation (23 ̊C to 27 ̊C) was likely only responsible for small episodic ebullition events from the upper peat layer, while large ebullition events from the deeper layers were most likely driven by drops in atmospheric pressure.M.S.Includes bibliographical referencesIncludes vitaby Zhongjie Y