Geophysical evidence for the lateral distribution of free phase gas at the peat basin scale in a large northern peatland

Northern peatlands contribute to the global carbon cycle by way of gas emissions in the form of biogenic carbon dioxide and methane originating in the subsurface. Determining how much gas is present and where it is trapped is essential to understanding the role of northern peatlands on the global carbon cycle, particularly in regard to methane cycling. In this study, spatial variability in free phase gas content was estimated using ground penetrating radar (GPR) along a 1.4 km transect crossing distinct peat landforms including a bog crest, midslope lawn, and fen. Variations in gas content and distribution were observed in landforms crossed by this transect. Estimated gas content up to 25% by volume was observed in landforms dominated by woody surface vegetation. In areas of the midslope lawn, the estimated gas content was greater than 15%, while estimated gas contents between 0% and 7% were found in the fen. Changes in estimated gas content of up to 20% were observed over distances of 100 m across a transition between a stand of 10 m tall trees and an adjacent fen. The mean of the estimated errors in gas content is 1.3%, suggesting that these large changes in estimated gas content between landforms can be clearly determined using geophysical methods. A geophysical image of the distribution of free phase across this transect reveals evidence for gas storage in deep peat in intervals as thin as 0.35 m. These data support conceptual models based on accumulation and storage of free phase gas in deep peat.