Variations in Biogenic Gas Content Profiles Along a Crest-Lawn-Fen Transect of an Ombrotrophic Minnesota Peatland from Non-Invasive Geophysical Measurements

Northern peatlands contain free-phase carbon gas biogenically produced below the surface under anaerobic conditions. Quantification of the spatial variability of free-phase gas is needed to improve understanding of carbon gas dynamics in peatlands. In an effort to better understand the spatial variability in free-phase gas accumulations between different peat landforms, we completed a series of ground-penetrating radar (GPR) surveys across a crest - lawn - fen transition in a large peatland in Minnesota, USA. Common offset profiles were employed to detect changes in the peat stratigraphy and basal mineral reflector, while common mid-point (CMP) surveys were utilized to estimate gas content within the peat at regular intervals along the survey line. We obtained estimates gas content profiles at 14 locations along a 1.4 km transect using multi-frequency CMPs. Geophysical data was supplemented with direct gas sampling at selected locations. One-dimensional vertical electromagnetic wave velocity models were developed using the Dix relation to calculate velocities for discrete intervals. The Complex Refractive Index Model (CRIM) was applied to convert these velocities into gas content estimates. Clear variations in subsurface gas accumulation are apparent between landforms with fen sites showing homogeneous gas profiles and bog crest locations exhibiting evidence for extensive trapping of free phase gas at depth.