THE INFLUENCE OF PRESSURE ON FREE PHASE GAS DYNAMICS IN A NORTHERN PEATLAND FROM FIELD-SCALE RESISTIVITY IMAGING

Peatlands are important in the cycling of greenhouse gases like methane and carbon dioxide. Spatiotemporal variability of these gases, especially in deep (> 1 m) peat, is difficult to assess. In particular, the timing, amplitude, and location of large ebullition (bubbling) fluxes are highly debated. Some have even argued that episodic ebullition events might be responsible for more methane gas flux to the atmosphere than all other modes of gas transport combined. Time-lapse electrical resistivity imaging (ERI) provides an autonomous system for imaging relative changes in the free phase gas (FPG) content of peat at cm to meter resolution throughout a 3D volume. We employed an ERI system to collect over one hundred datasets during July and August of 2013 in Caribou Bog, Maine. Water levels, soil temperature, atmospheric pressure, and limited methane flux data were also logged. Water levels exhibit negative correlations with resistivity at all depths indicating hydrostatic pressure may be regulating bubble volume. However, water level variations do not appear to cause ebullition at this site. Atmospheric pressure drops, on the other hand, appear to often trigger large FPG decreases in the shallow (< 1 m) peat, which we attribute to buoyancy driven ebullition events. In one case, we observe a relatively large loss of FPG from the deep peat during an atmospheric pressure drop that may reflect a ‘rupture’ event where built up gas escaped through a confining layer. These observations support a ‘shallow peat model’ where most ebullition originates in shallow peat.