SEASONAL CHANGES IN PEATLAND SURFACE ELEVATION RECORDED WITH GPS STATIONS IN THE RED LAKE PEATLANDS, NORTHERN MINNESOTA, USA

The elastic nature of peat allows it to expand or contract due to changes in fluid (eg. biogenic gas and water) content. Global Positioning Systems (GPS), deployed in the northern Minnesota within the Glacial Lake Agassiz Peatlands (GLAP), are being used to monitor temporal changes in the peat surface. In previous work, two GPS stations deployed in the GLAP identified short term (less than two hour) decimeter-scale changes in the peat surface elevation hypothesized to be related to biogenic gas storage and release. In 2008 and 2009, nine GPS and three hydrologic and meteorologic stations were established in the GLAP study area to monitor the surface deformation and associated processes within this peatland system. GPS stations were positioned in a nested set of triangular arrays to record the spatio-temporal pattern of surface movement within the peatland. At each station, GPS antennae were fixed to the trunk of a small tree, after removing the top of the tree at chest height. Hydro-meteorologic stations were positioned in a north-south transect bisecting the GPS array and oriented along the regional topographic gradient. Meteorological measurements include air temperature, barometric pressure, wind speed, and precipitation. Hydraulic head and water temperature were measured in nested piezometers with screens placed at 50 cm depth increments through the peat column.

In this presentation we report results from eight hour GPS surveys performed at selected stations located at a bog crest, Sphagnum lawn, and Fen site from January through December, 2009. Surface elevation at the Bog site varied by about 4 cm throughout the year. These surface elevation changes are focused from January through March and in December. At the Fen Site, surface elevation changed by about 10 cm with a rapid increase in surface elevation in March followed by a gradual decrease in elevation throughout the year. Visual comparison of surface elevation and water-table position strongly suggest a poro-elastic response due to changing pore water pressure. Large surface elevation changes also occur when air temperatures pass across the freezing point, suggesting that surface freeze and thaw or loading from snow influences the peat surface elevation.