Temporal variations in dissolved methane deep in the Lake Agassiz Peatlands, Minnesota

A study (August 1990 to July 1991) of profiles of dissolved CH₄ concentrations, diffusive flux of CH₄, and CH₄ production rates of 45 sites in the Lake Agassiz Peatlands in northern Minnesota shows that dissolved CH₄ deep in the peat (> 1 m depth) mobilized easily to the vadose zone. During August 1990 the dissolved CH₄ concentrations at some depths at some sites were supersaturated with respect to one atmosphere partial pressure of CH₄. At one site (2.5 m depth) the concentration of dissolved CH₄ in the peat pore‐water was 140 mg L⁻¹. In July 1991, at no site did the concentration of dissolved CH₄ in the peat pore water exceed 40 mg L⁻¹ in the peat profile. The average calculated diffusive flux of CH₄ decreased from 95 to 45 mg m⁻² d⁻¹ between 1990 and 1991. Gaseous CH₄ was more in evidence in 1990 than in 1991. In 1990, CH₄ at many depths bubbled vigorously when peat pore water was sampled. At some sites there was sufficient pore pressure to eject slugs of water forcibly from piezometers. Similarly, dissolved inorganic carbon (DIC) consisting of H₂CO₃, CO₂, HCO₃⁻ and CO₃²⁻ decreased between the sampling times from an average for both bogs and fens in 1990 of 62 mg C L⁻¹ to 38 mg C L⁻¹ in 1991. A dynamic mechanism must exist which traps CH₄ deep in the peat column allowing gaseous CH₄ to build up, increasing dissolved CH₄. Other times, CH₄ passes freely from deep peat to the vadose zone. We suggest as a hypothesis that a confining layer of trapped CH₄ bubbles forms at depth in the peat, trapping gaseous CH₄. The duration of the “bubble confining layer” is uncertain. We propose two hypotheses. (1) The confining layer is usually present and deteriorates after a major climatic event such as a drought, or (2) the confining layer forms and collapses seasonally with seasonal variations in the water table elevation.