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Temporal variations in dissolved methane deep in the Lake Agassiz Peatlands, Minnesota
Author(s) -
Romanowicz Edwin A.,
Siegel Donald I.,
Chanton Jeffrey P.,
Glaser Paul H.
Publication year - 1995
Publication title -
global biogeochemical cycles
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.512
H-Index - 187
eISSN - 1944-9224
pISSN - 0886-6236
DOI - 10.1029/95gb00634
Subject(s) - peat , dissolved organic carbon , pore water pressure , bog , piezometer , flux (metallurgy) , environmental chemistry , vadose zone , hydrology (agriculture) , methane , atmosphere (unit) , geology , chemistry , groundwater , aquifer , soil water , soil science , ecology , geotechnical engineering , organic chemistry , biology , physics , thermodynamics
A study (August 1990 to July 1991) of profiles of dissolved CH 4 concentrations, diffusive flux of CH 4 , and CH 4 production rates of 45 sites in the Lake Agassiz Peatlands in northern Minnesota shows that dissolved CH 4 deep in the peat (> 1 m depth) mobilized easily to the vadose zone. During August 1990 the dissolved CH 4 concentrations at some depths at some sites were supersaturated with respect to one atmosphere partial pressure of CH 4 . At one site (2.5 m depth) the concentration of dissolved CH 4 in the peat pore‐water was 140 mg L −1 . In July 1991, at no site did the concentration of dissolved CH 4 in the peat pore water exceed 40 mg L −1 in the peat profile. The average calculated diffusive flux of CH 4 decreased from 95 to 45 mg m −2 d −1 between 1990 and 1991. Gaseous CH 4 was more in evidence in 1990 than in 1991. In 1990, CH 4 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 H 2 CO 3 , CO 2 , HCO 3 − and CO 3 2− 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 CH 4 deep in the peat column allowing gaseous CH 4 to build up, increasing dissolved CH 4 . Other times, CH 4 passes freely from deep peat to the vadose zone. We suggest as a hypothesis that a confining layer of trapped CH 4 bubbles forms at depth in the peat, trapping gaseous CH 4 . 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.