PEAT CHARACTERISTICS AND GROUNDWATER GEOCHEMISTRY OF CALCAREOUS FENS IN THE MINNESOTA RIVER BASIN, USA

Calcareous fens in Minnesota are spring-seepage peatlands with a disti nctive flora of rare calciphilic species. Feat characteristics and gro undwater geochemistry were determined for six calcareous fens in the M innesota River Basin to better understand the physical structure and c hemical processes associated with stands of rare vegetation. Onset of peat accumulation in three of the fens ranged from about 4,700 to 11,0 00 C-14 yrs BP and probably resulted from a combination of climate cha nge and local hydrogeologic conditions. Most peat cores had a carbonat e-bearing surface zone with greater than 10% carbonates (average 27%, dry wt basis), an underlying carbonate-depleted zone with 10% or less carbonates (average 4%), and a carbonate-bearing lower zone again with greater than 10% carbonates (average 42%). This carbonate zonation wa s hypothesized to result from the effect of water-table level on carbo nate equilibria: carbonate precipitation occurs when the water table i s above a critical level, and carbonate dissolution occurs when the wa ter table is lower. Other processes that changed the major ion concent rations in upwelling groundwater include dilution by rain water, sulfa te reduction or sulfide oxidation, and ion adsorption or exchange. Geo chemical modeling indicated that average shallow water in the calcareo us fens during the study period was groundwater mixed with about 6 to 13% rain water. Carbonate precipitation in the surface zone of calcare ous fens could be decreased by a number of human activities, especiall y those that lower the water table. Such changes in shallow water geoc hemistry could alter the growing conditions that apparently sustain ra re fen vegetation.