PALAEO-REVERSALS IN GROUNDWATER-FLOW AND PEATLAND DEVELOPMENT AT LOSTRIVER, MINNESOTA, USA

Raised bogs represent end-products of peatland development that are ge nerally assumed to be decoupled from the regional groundwater system. However, the development of two peat mounds in northern Minnesota was driven by three major reversals in its groundwater-flow regime. A 4000 -year record of groundwater flow was reconstructed by stratigraphic st udies that utilized both botanical indicators of the surface-water che mistry, and mineral indicators of the pore-water chemistry and chemica l weathering regime. The mineral stratigraphy indicated that the peat mounds were initially formed under a discharge (upward flow) regime. D uring the succeeding moist period, these peat mounds developed into ra ised bogs as their water-table mounds drove local recharge (downward) flow into the mineral substratum. A major reversal to a prevailing dis charge regime at 1200 BP, however, converted the smaller peat mound in to a spring fen and weathered the silicate minerals within the peat pr ofile of the larger mound. This reversal was apparently caused by the onset of extended droughts, which dissipate the water table mound with in these peat landforms.