Estimates of biogenic gas dynamics in a northern raised bog inferred from hydrogeophysics

While patterned pools along the crest of northern raised bogs are surface features, their underlying geologic structure has been shown to influence local hydrology and, by extension, biogeochemistry. Beneath these pools, pore spaces accumulate biogenic gases, for which production depends on the availability of labile carbon. Some of these gases are released via ebullition, a bubble-transport mechanism by which gases migrate through the peat column and into the atmosphere. Our team targeted open-water pools in Caribou Bog, Maine (U.S.A.), to capture differences in ebullition and identify contrasts in dissolved methane and carbon dioxide. Distinct sites were selected for comparison: [1] on-esker sites, underlain by permeable glacial deposits, that locally form near-surface ridges, and [2] off-esker sites, underlain by a hydraulically confining glaciomarine clay, that blankets much of the peatland basin. Ebullition recorded in custom-built floating gas traps showed a fivefold increase in collection at the on-esker site. Gas chromatography analysis of sampled ebullition revealed methane concentrations ≥4000 ppm at sites proximal and distal to the esker transect. At both locations, headspace-equilibrated concentrations of dissolved methane in pools surged by two orders of magnitude, coinciding with drops in atmospheric pressure below 101.25 kPa. This suggests that falling surface pressure triggers discrete pulses of gas migration from over-pressured pore spaces. Further, dissolved concentrations of methane in water from nested wells confirm active methanogenesis in the catotelm, down to nine meters depth. Within the peat column, gas accumulation and migration were investigated over a 12-day period in August using repeated common-midpoint ground-penetrating radar (GPR) surveys. Coupling electromagnetic wave velocities with the complex refractive index model, 1D models of gas content were produced for off- and on-esker locations. These gas content estimates indicate that biogenic accumulation is, on average, 5–7% greater at sites along the beaded esker transect. Combined, these results suggest that underlying esker structures act as localized hot spots for biogenic gas production, storage, and enhanced ebullition.