Variations in free‐phase gases in peat landforms determined by ground‐penetrating radar

Abstract

[1] The spatial variability of biogenic gas produced by methanogenic archaea is difficult to assess within saturated peat soils and is often poorly quantified. This study uses ground-penetrating radar to noninvasively estimate the vertical distribution of biogenic free-phase gas (FPG) in two distinct peat landform types in the Glacial Lake Agassiz Peatland, Minnesota: a near-crest bog and a midslope lawn (i.e., both as defined by surface vegetation communities). Ground-penetrating radar velocities retrieved from common midpoint surveys were modeled using the Complex Refractive Index Model to estimate free-phase gas volumes along one-dimensional vertical profiles. Near-crest bog landforms are characterized by vertical variability in gas content with accumulations along the peat column up to 24% by volume of FPG localized within the deep peat (e.g., 2-4 m depth). Midslope lawn sites show lower total volumes (up to 12% free-phase gas) and less variability that result in a more even free-phase gas distribution throughout the vertical profile. High-resolution data also suggests the presence of thinner layers (<1 m) containing up to 40% free-phase gas at a near-crest site. Our results suggest that spatial distribution of free-phase gas along the peat column may vary in the Glacial Lake Agassiz Peatlands depending on peat landform. Statistical analysis supports vertical variability of ground-penetrating radar velocity data with a 95% confidence limit. Our results could potentially be upscaled using surface vegetation communities to estimate landscape-scale gas storage potential in peat landforms.

Seasonal changes in peatland surface elevation recorded at GPS stations in the Red Lake Peatlands, northern Minnesota, USA

“…The broad patterns in surface deformation observed at the FEN0 site are captured in a one‐dimensional Kelvin‐Voigt model of soil deformation (Figure ). Assigning a gas content of 10% to the peat blocks, similar to gas content previously measured in the study area using a variety of methods [ Rosenberry et al , ; Glaser et al , ; Parsekian et al , , ], coupled with the removal of 100 kg·m −2 loading of snow, resulted in surface deformation similar to what was measured at the FEN0 site. While there is uncertainty in the elasticity (Young's modulus) assigned to the peat, the similarity in the amount of surface change and pattern of movement lends credibility to the range of values used to simulate peat elasticity, from 5 kPa when under high tension to 100 kPa when highly compressed.…”

Presence of rapidly degrading permafrost plateaus in south-central Alaska

“…10b) survey. The center portion of both images is characterized by moderately continuous and chaotic reflectors (Neal, 2004), as expected for records in unfrozen peat sequences (Parsekian et al, 2010) associated with the collapse-scar bog. The areas underlain by permafrost (i.e., 0-30, 60-90 m) show subdued reflection events deeper than the permafrost table; however, we were unable to image the permafrost base.…”

Evaluating spatial variability of free‐phase gas in peat using ground‐penetrating radar and direct measurement

“…Because measured gas content was significantly related to CH 4 content, it is possible that not only could GPR be used to map FPG volume, but also to estimate subsurface CH 4 stock once some direct measurements are available. However, while this technique shows excellent promise for mapping variability of gas content in two dimensions [see also Parsekian et al , 2010], it could not easily be used to reconstruct variability with depth at high resolution. In some cases researchers may be interested in variability in CH 4 stocks at centimeter scales in order to calculate diffusive gradients [e.g., Laing et al , 2008].…”