USING NEAR SURFACE GEOPHYSICS TO CHARACTERIZE PEATLAND SYSTEMS

Peatland processes are typically characterized based on atmospheric fluxes, decomposition rates of organic matter and peat surface topography. The significance of subsurface controls on peatland formation has been largely overlooked, perhaps due to the logistical complexity of probing beneath the organic soil layer. Geophysical investigations conducted in Caribou Bog, a multi-unit raised bog (with peat thickness exceeding 11 m) in central Maine, have identified striking correlations between the subsurface stratigraphy, vegetation and hydrology observed at the surface. Electrical resistivity imaging has been applied to model the variability in the thickness of the glaciomarine sediment that underlies Caribou Bog. This thickness shows a clear correlation with the primary vegetation patterning in Caribou Bog, provides evidence that the bog evolved from an original double-basin system and shows that the location of pools within the bog coincides with the protrusion of resistive mineral soil in direct contact with the base of the organic sediment. Ground penetrating radar surveys within Caribou Bog suggest that this resistive mineral soil is an extensive beaded esker deposit that exerts a fundamental control on the location and sustenance of these pools. Ground penetrating radar surveys have also illuminated extensive zones within the peat characterized by distinctive chaotic reflections and loss of signal amplitude. Hydrological data, combined with gas chromatography, support the interpretation of these geophysical signals as zones of extensive free phase gas accumulation, that coincide with localized overpressuring in the peat close to the apex of the bog. The results obtained over the last six years conclusively demonstrate that near surface geophysical methods are invaluable tools for understanding how the subsurface regulates peatland processes traditionally observed primarily at the surface.