METHANE PRODUCTION AND THE HYDROGEOLOGY OF PEATLANDS

Bog peat is an elastic porous medium, capable of expanding and contracting as a function of degree of saturation and changes in pore water chemistry. We have discovered that the transient production of over pressured gaseous methane in the middle of humified bog peat (Glacial Lake Agassiz Peatlands, MN) profiles reverses natural hydraulic gradients and causes peat to expand. This expansion in turn increases the land surface elevation up to 20 cm. The large volumes of produced methane are stored in semi-elastic peat compartments semi-confined by wood layers. After the peat expands, methane episodically degasses to the atmosphere and the peat compresses while depressurizing. We calculated that three such episodes of methane loss released 136 g CH₄/m², exceeding the annual average chamber fluxes measured at the site by ~10 times. After degassing, the landscape "deflates" until the peat re-saturates with water and re-expands to pre-event hydraulic conditions.

Relatively depleted d¹³C and enriched d D values of bog methane indicate that this gas is mostly formed by carbonate reduction. Methanogenesis also deuterates pore waters, similar to that found in landfill leachate. An isotopic mass balance suggests about 1.2 g CH₄/m³-d is produced in the deep bog peat. Finally, ¹⁴C profiles of methane, compared to that of the peat, indicate that modern labile carbon, transported downward by advection, is a major carbon substrate driving the methanogenesis.