Biogenic gas bubble dynamics in a Northern peatland observed using electrical geophysics and other methods

The ability of peatlands to produce, sequester, and release large volumes of greenhouse gases (particularly methane) has resulted in a large body of research dedicated toward understanding how these soils respond to climactic variations. Yet, there still exist many uncertainties regarding variation in the mechanisms that drive the production, storage, and release of methane in peatlands, in part due to limited information at relevant spatiotemporal scales. This work harnesses the power of geophysical methods to bridge this information gap and offer new insights on the mechanics of biogenic gas dynamics in peatlands. The first portion of this work develops a methodological basis for monitoring field-scale biogenic gas variations in peatlands from electrical resistivity imaging (ERI). This approach is used to provide insight on the controlling factors that drive methane ebullition, particularly from deep peat regimes (i.e., below 1 m) that are difficult to sample with traditional methods. Using this technique, anomalous variations in resistivity are observed that correspond to decreases in atmospheric pressure and suggest that biogenic gas releases occur throughout the peat profile. Furthermore, smaller changes in resistivity seem to correspond well with expected variations in gas bubble size due to changing pressure and temperature conditions. A follow up study examines two sites in Caribou Bog, Maine in an effort to understand how peat structural and hydrogeological properties influence gas releases. This study employs a suite of direct methods (i.e., direct coring, methane flux monitoring, and gas sample collection), geochemical and environmental data, and indirect geophysical methods (ERI, ground penetrating radar, and peat deformation) to show that peat structure and underlying hydrogeology likely play a critical role in controlling methane production and release. It is observed that a highly decomposed peat site overlying an esker deposit exhibits larger methane content, larger magnitude resistivity variations (particularly in deep peat), and larger individual methane flux measurements compared to a less well decomposed peat site nearby. The concluding piece of this research develops a quantitative technique to estimate biogenic gas bubble size from frequency information derived from ground penetrating radar signals. Bubble size may be a critical parameter governing methane ebullition given the increased buoyancy and reduced oxidation potential of larger bubbles. Additionally, previous research has demonstrated a disproportionately large percentage of the total methane flux may originate from large bubbles. The approach is used to estimate bubble size variations in two laboratory studies as well as from field data, and suggests that bubbles with radii of greater than 0.04 m may exist in natural peat deposits. Employing this estimation approach in the future may help to understand the connection between bubble size, peat type/structure, and fluxes observed at the surface.Ph.D.Includes bibliographical referencesby Neil Terr