Fens: Soil Formation, Processes, and Function.

Fens occur at permanent lake edge and slope discharge points along late Wisconsin moraines in eastern South Dakota. The fens contain carbonate-rich Histosols with organic thicknesses of one to five meter or more. The Histosols classify as Wassists (positive pore pressure to the surface). The Histosols display two unique intermittent surface crusts. The uppermost crust is a carbonate precipitate that mimics and encrusts organic debris. Below upper carbonate crust is an intermittent iron-rich or iron-stained carbonate crust. Both crusts likely form via CO2 degassing of cold, carbonate laden groundwater at or near the soil surface. The crusts are underlain by fibric material that becomes sapric and then hemic with depth. Soil pH is 7 or above throughout. Fen soils are a function of landscape hydrology, permanent saturation, and carbonate laden discharge.