[1] Electrical resistivity/induced polarization (0.1-1000 Hz) and vertical hydraulic conductivity (K v ) measurements of peat samples extracted from different depths (0-11 m) in a peatland in Maine were obtained as a function of pore fluid conductivity (s w ) between 0.001 and 2 S/m. Hydraulic conductivity increased with s w (K v / s w 0.3 between 0.001 and 2 S/m), indicating that pore dilation occurs due to the reaction of NaCl with organic functional groups as postulated by previous workers. Electrical measurements were modeled by assuming that ''bulk'' electrolytic conduction through the interconnected pore space and surface conduction in the electrical double layer (EDL) at the organic sediment-fluid interface act in parallel. This analysis suggests that pore space dilation causes a nonlinear relationship between the ''bulk'' electrolytic conductivity (s el ) and s w (s el / s w 1.3 ). The Archie equation predicts a linear dependence of s el on s w and thus appears inappropriate for organic sediments. Induced polarization (IP) measurements of the imaginary part (s 00 surf ) of the surface conductivity (s* surf ) show that s 00 surf is greater and more strongly s w -dependent (s 00 surf / s w 0.5 between 0.001 and 2 S/m) than observed for inorganic sediments. By assuming a linear relationship between the real (s 0 surf ) and the imaginary part (s 00 surf ) of the surface conductivity, we develop an empirical model relating the resistivity and induced polarization measurements to s w in peat. We demonstrate the use of this model to predict (a) s w and (b) the change in K v due to an incremental change in s w from resistivity and induced polarization measurements on organic sediments. Our study has implications for noninvasive geophysical characterization of s w and K v with potential to benefit studies of carbon cycling and greenhouse gas fluxes as well as nutrient supply dynamics in peatlands.

[1] 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 m 3 ) 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 À1 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).Citation: Comas, X., and L. Slater (2007), Evolution of biogenic gases in peat blocks inferred from noninvasive dielectric permittivity measurements, Water Resour. Res., 43, W05424,

[1] A set of high resolution surface ground penetrating radar (GPR) surveys, combined with elevation rod (to monitor surface deformation) and gas flux measurements, were used to investigate in situ biogenic gas dynamics within a northern peatland (Caribou Bog, Maine). Gas production rates were directly estimated from the time series of GPR measurements. Spatial variability in gas production was also investigated by comparing two sites with different geological and ecological attributes, showing differences and/or similarities depending on season. One site characterized by thick highly humified peat deposits (5-6 m), wooded heath vegetation and open pools showed large ebullition events during the summer season, with estimated emissions (based on an assumed range of CH 4 concentration) between 100 and 172 g CH 4 mÀ2 during a single event. The other site characterized by thinner less humified peat deposits (2-3 m) and shrub vegetation showed much smaller ebullition events during the same season (between 13 and 23 g CH 4 m À2 ). A consistent period of free-phase gas (FPG) accumulation during the fall and winter, enhanced by the frozen surficial peat acting as a confining layer, was followed by a decrease in FPG after the snow/ice melt that released estimated fluxes between 100 and 200 g CH 4 mÀ2 from both sites. Estimated FPG production rates during periods of biogenic gas accumulation ranged between 0.22 and 2.00 g CH 4 m 3 d À1 and reflected strong seasonal and spatial variability associated with differences in temperature, peat soil properties, and/or depositional attributes (e.g., stratigraphy). Periods of decreased atmospheric pressure coincided with short-period increases in biogenic gas flux, including a very rapid decrease in FPG content associated with an ebullition event that released an estimated 39 and 67 g CH 4 m À2 in less than 3.5 hours. These results provide insights into the spatial and seasonal variability in production and emission of biogenic gases from northern peatlands.Citation: Comas, X., L. Slater, and A. Reeve (2008), Seasonal geophysical monitoring of biogenic gases in a northern peatland: Implications for temporal and spatial variability in free phase gas production rates,

[1] Atmospheric pressure (P atm ) is known to regulate methane emissions from northern peatlands. However, recent conceptual models differ in how gas production and release occurs in shallow (defined here as less than 1 m depth) versus intermediate to deep peat soils (i.e., more than 1 m depth). We used ground penetrating radar (GPR) measurements to non-invasively estimate the vertical distribution of free-phase gas and the dependence of this distribution on atmospheric pressure in a northern peatland. Variations in the travel time of the electromagnetic wave to three interfaces in the peat column were used in conjunction with deformation rod data and a petrophysical model relating electromagnetic wave velocity to free-phase gas content to model changes in the vertical distribution of free-phase gas over time. We found a negative linear relation between changes in free-phase gas content and changes in P atm for shallow peat soils and a positive linear relation for deeper soils. Our results suggest that (1) free-phase gas content confined in deep peat soils is larger and less variable to changes in P atm than gas in shallow/intermediate peat soils; (2) increases in P atm result in gas release from shallow peat soils into the atmosphere (i.e., rapid ebullition); and (3) decreases in P atm result in upward gas movement from intermediate layers to replenish shallow layers. Our results suggest that changes in P atm drive changes in the vertical distribution of free phase gas in peat soil and regulate methane ebullition from peat soils to the atmosphere. Our data shows a relationship between free phase gas and depth that may be due to changes in peat properties or increasing water pressure with depth.Citation: Comas, X., L. Slater, and A. S. Reeve (2011), Atmospheric pressure drives changes in the vertical distribution of biogenic free-phase gas in a northern peatland,