Time-series analysis for the episodic production and transport of methane from the Glacial Lake Agassiz peatlands, northern Minnesota. Final report

摘要

The large peat basins of North America are an important reservoir in the global carbon cycle and a significant source of atmospheric methane. The authors investigated carbon cycling in the Glacial Lake Agassiz peatlands (GLAP) of Minnesota. Initially in 1990, they identified a dramatic change in the concentration of methane in the pore-waters of the raised bogs in the GLAP during an extreme drought. This methane dissipated when the drought broke in 1991 and the occurrence of deep methane is related to changes in the direction of groundwater flow in the peat column. The production of methane and its diffusive loss to the atmosphere was modeled and was about 10 times less than that measured directly in chambers at the land surface. It is clear from the reversals in hydraulic heat, changes in pore-water chemical composition over time, and paleostratigraphic markers, that regional ground water flow systems that are controlled by climate change are unexpectedly a major control over methanogenesis and carbon cycling in GLAP. Seismic profiles made showed that buried bedrock ridges particularly deflect regional groundwater flow upwards towards the land surface and towards raised bog landforms. In addition, high-resolution GPS measurements from data stations funded by this DOE project have shown this year that the peakland land surface elevation changes daily on a scale of cms, and seasonally on a scale of 10s of cm. This most recent observation is exciting because it may reflect episodic degassing of free phase methane from the peat column to the atmosphere, a source for methane previously unaccounted for by methane researchers.