COUPLING PEATLAND SURFACE MOVEMENT TO BIOGENIC GAS ACCUMULATION AND HYDROLOGY USING SIMPLE 1-D DYNAMIC COMPUTER MODELS

Peatlands are important carbon reservoirs within the critical zone, both releasing and sequestering atmospheric carbon-based greenhouse gasses. Biogenic gas trapped within peat likely has an important influence on surface oscillations measured in peatland systems. Various hypotheses invoked to explain these surface oscillations including: expanding/contracting gas pockets within the peat, gas compression/expansion due to changing surface load, and buoyancy effects. GPS and hydro-meteorologic monitoring stations established in the Red Lake Peatland of northern Minnesota provide a year-long, high frequency record of the peat surface position and hydraulic head. GPS antennas mounted to small trees were used to monitor the movement of the surfical peat, revealing seasonal changes in surface elevation synchronous with hydraulic head data. Most of the observed surface elevation change occurred during the spring, when the water table rose rapidly. Rapid changes in surface elevation were also recorded following precipitation events and sporadically throughout the summer. To evaluate the importance of buoyancy on peatland surface movement, a simple 1-D viscoelastic (Kelvin)model of a peat column has been constructed. The peat column is conceptualized as a series of blocks with varying effective mass, connected to each other and the underlying sediment. Simulations of the fen site, a location where both surface elevation and hydraulic head were measured, were performed by varying the water-table position and changing the loading due to snow, over a one year period. Peat elasticity, viscosity, and gas content were varied in these simulations to assess the sensitivity of the models to these poorly constrained parameters. Varying the peat's elasticity from 0.5 to 1.5 GPa (gas content of 10%) resulted in annual surface fluctuations ranging from a few centimeters to about 20 cm, consistent with observed patterns. This model neglects the impact of gas pressure within the peat, which may increase surface elevation variability and produce rapid changes observed in our data sets. Our numerical modeling suggests that buoyancy effects contribute to observed seasonal changes in peat deformation and that changes in gas content within peat deposits strongly influence the magnitude of seasonal surface oscillations.