Interplay of Climate Change with Mire Hydrology and the Methane Cycle.

The methane cycle of circumboreal peatlands integrates closely with their hydrology and climate. Circumboreal peatlands evolved first as expansive fens on flat, exhumed sea or lake beds. Bogs succeeded fens when either portions of peatlands became raised because of isostatic rebound coupled to river incision (e.g. Hudson Bay Lowlands, Ontario, Canada), or when fen topographies and associated water tables became raised and mounded because of preferential oxidation during drought cycles (e.g. Glacial Lake Agassiz Peatlands, Minnesota, USA). A combination of hydrologic and geochemical measurements show that groundwater recharge under many bogs forces labile carbon from the active peat layer to the middle of decomposed peat profiles. There, methanogens generate methane that episodically degasses when the water table seasonally drops. Isotopic values of deuterium in water, and carbon in methane and dissolved inorganic carbon indicate that as much as 50% of the methane derives from modern carbon sources and that methane is generated in deep peat at a rate of approximately 1 g-CH₄/m³/day. We conservatively estimate that deep peat contains approximately 34 teragrams of methane, an amount approximately equal to one third of the global annual emission of methane to the atmosphere, not counting episodic ebullition events. However, the production of deep methane should diminish over the long term if groundwater mounds no longer develop under the bogs to deliver labile carbon to deep methanogens.