HYDROGEOLOGIC DRIVERS OF MIRE ECOLOGY AND GEOCHEMISTRY

Mires are vast circumboreal peatlands that sequester and cycle a large fraction of terrestrial carbon. These ecosystems, formed on old lake beds, marine deposits, or other terrain are characterized by complex patterns and interactions between acid- tolerant bog and circumneutral-fen plant communities. The ecological succession of these communities, their unique patterns, and their geochemical cycles all appear to be largely controlled by hydrogeology at different scales. Where bogs form over sandy substrate, local recharge from their water table mounds delivers nutrients and labile dissolved organic carbon deep into the peat column, while at intermediate to regional scales, flow systems deliver solutes to the base of the peat. Methanogens in this hydrogeologic setting, typified by the Glacial Lake Agassiz Peatlands (MN), seasonally convert dissolved labile carbon in the center of peat profiles to methane, which as free gas phase can become over pressured, reversing the directions of porewater flow and solute transport. The methane degasses episodically in vents located along the flanks of the bogs, conceivably leading to an ecological succession to fen vegetation. In contrast, vertical gradients under raised bogs over clayey substrate, typified by parts of the Hudson Bay Lowlands, are comparatively small and the delivery of carbon and nutrients to bacteria deep in the peat column is probably less. There, hydrodynamic dispersion of solutes from the underlying mineral soils into the organic peat may be the critical geochemical transport process., which if operating over long distances, is theoretically capable of transporting solutes almost to the land surface. This transport process may account for the maintenance of cover and acidifying effect of organic acids.