Evolution of biogenic gases in peat blocks inferred from noninvasive dielectric permittivity measurements

Abstract

Peatlands contain methanogenic archea responsible for generating significant amounts of free‐phase biogenic gases (for example, methane and carbon dioxide), but considerable uncertainty still exists regarding the mechanisms of formation and spatial distribution of these gases within the soil matrix. We demonstrate the effectiveness of a new method to record noninvasively the evolution, spatial distribution, and emission patterns of biogenic gases in a peat soil. A peat block (0.022 m3) was extracted from a large freshwater peatland in Maine. The bulk dielectric permittivity (at 1.2 GHz) for multiple slices of the block was measured noninvasively (1) as temperature was increased 2°C d−1 from 5°C to 21°C, and (2) for a subsequent 2‐month period during which temperature was held constant at 21 ± 1°C. Methane emissions at the surface and peat surface deformation were monitored concurrently using a portable methane detector and a grid of surface elevation rods, respectively. Our results demonstrate that (1) the measurement of electromagnetic wave traveltimes across a peat block offers a unique (and more accurate when compared to surface deformation measurements) way to monitor gasdynamics and spatial gas distribution within a peat block without any disturbance to the natural gas regime; (2) the ebullition under our experimental conditions seems to preferentially occur from the near‐surface peat and shows some correspondence with changes in atmospheric pressure; and (3) the ebullition flux exhibits periodicity, suggesting that it may be predictable and quantifiable, which could assist climate modeling efforts. Our findings are consistent with previous studies based on gasdynamics in peat soils (including gas volumes and fluxes associated with biogenic gas ebullition).