Episodic release of methane bubbles from peatland during spring thaw

Methane (CH4) flux into the atmosphere during spring thaw was investigated in a small ombrotrophic peatland (141°48′E, 43°19′N, Japan) using the conventional chamber method. More than 50 chamber deployments on top of the snow cover were carried out and continued for more than 165 h until the surface snow and underlying ice cover on top of the peat layer had thawed completely. Methane emissions were almost absent in the presence of snow cover. At the very moment the surface ice cover thawed, a large CH4 flush (>10 mg CH4 m−2 h−1) was recorded, which was on the same order of magnitude as episodic ebullition previously observed in the high-summer. Gas bubbles trapped in the ice layer on top of the waterlogged peat were preliminarily analyzed for the volumetric percentage in the total ice volume and their gas species compositions. Results showed that the bubbles occupied about 3.2% volume and that the mixing ratio of CH4 in the bubbles was about 20%. The abundance of the bubble-form CH4 was sufficient to explain the observed episodic CH4 release during the thaw. Results of this study show that CH4 emissions during the thaw season have great temporal variability; emission occurs as an episodic release of bubble-form CH4 stored in the frozen layer. The results also imply the possibility that gas-phase CH4 plays an important role, not only during the growing season but also in cold-season CH4 dynamics in northern peatlands.