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Hydraulic reversals and episodic methane emissions during drought cycles in mires

¹ Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244-1070
² Limnological Research Center, University of Minnesota, Minneapolis, Minnesota 55455

Reversals in hydraulic head and extensive methane losses were identified during a drought cycle from changes in hydraulic-head and dissolved-methane profiles in peat, Glacial Lake Agassiz Peatlands, northern Minnesota, United States. During the summer 1990 drought, regional ground water discharged from the mineral soil underlying the peat to raised bogs. Concentrations of dissolved CH₄ in pore water showed that much of the peat column was supersaturated with respect to a reference standard of I atm partial pressure CH₄. By the summer of 1991, the severity of the drought lessened, and the upper peat column was resaturated with precipitation-derived water. Ground-water flow was then controlled by local, precipitation-driven, recharge flow systems, and the regional ground water that was discharged affected only the peat immediately above the mineral soil-peat interface. In contrast, during the summer of 1991, concentrations of dissolved CH₄ in pore water were all undersaturated with respect to 1 atm partial pressure CH₄. The decrease in the amount of dissolved CH₄, in the pore water from the summer of 1990 to the summer of 1991 probably was caused by changes in the CH₄ flux and degassing to the unsaturated zone and to a lesser extent by changes in the partial pressure of gaseous CH₄ in the peat as the peat volume increased with expansion as it resaturated.

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