Effects of atmospheric pressure and temperature on entrapped gas and ebullition in peat

Abstract

<p> Entrapped gas (V g) greatly affects peatland biogeochemistry and hydrology by altering volumetric water content, buoyancy, hydraulic conductivity and generating overpressure zones. These over pressure zones affect hydraulic gradients which influence water and nutrient flow direction and rate. The loss of this entrapped gas to the atmosphere via ebullition (bubbling) has been proposed as the dominant transport mechanisms for CH4 from peatlands, releasing significant amounts of CH4 to the atmosphere in a single event. Atmospheric pressure has been linked to ebullition events and is known to affect gas volumes; similarly, temperature affects gas production and volume. This thesis investigates the relationship between these environmental factors (atmospheric pressure and temperature) on both V g and ebullition processes. </p> <p> An incubation experiment using six peat cores at three incubation temperatures ( 4 °C, 11 °C, 20°C) was conducted in 2004 where each core was incubated in a sealed PVC cylinder and instrumented to measure Vg, pore-water C~ concentrations, and ebullition (volume and C~concentrations). Temperature data for each incubation group and atmospheric pressure were measured within the laboratory setting. </p> <p> Increasing bulk density was associated with decreased frequency of ebullition events and higher average ebullition volumes, indicating a relationship between bulk density ebullition characteristics. Future work will be needed to identify the direct relationship between V g, bulk density and ebullition. </p> <p> Evaluation of ebullition and atmospheric pressure data revealed a strong relationship between periods of falling pressure and ebullition events where 71% of measured events (n = 391) occurred during periods of decreasing pressure. Investigation of falling pressure characteristics revealed that drop duration (days) had a more significant effect on total ebullition volumes than did magnitude (kPa). As such, long periods of decreasing pressure trigger greater gas releases via ebullition than short decreases of large magnitude. This has implications for the prediction and modelling of ebullition events in natural systems, and for the estimation of CH4 fluxes and carbon budgets of peatlands. </p> The V g variability model accounted for changes in V g caused by gas transfer between aqueous and gaseous phases (Henry's law) and thermal and pressure induced volume changes (Ideal gas law) using measured temperature and atmospheric pressure data. Gas loss via ebullition and CH4 production were also accounted for. Good agreement was found between measured and modeled V g values where gas contents were greater than 10% (average r2 value of 0.78). Accuracy of the model indicates a general understanding of the processes, however it also suggests that further factors are influencing internal gas dynamics that require further investigation. </p?