CARBONATE-RICH DISCHARGE HISTOSOLS: FORMATION PROCESSES AND CLASSIFICATION

In the upper glaciated Midcontinent USA (IA, MN, ND, SD, WI), fens with calcareous organic soils (Histosols) form where carbonate-laden water discharges at permanent springs or seeps. Such discharge sites occur along late Wisconsin moraines in eastern SD. The hydrology (upward vector) and geochemistry (redox) drive two main soil-forming processes - organic matter accumulation and mineral precipitation, which yields Histosols of unique character. The organics (fibric and hemic material) form a low bulk density matrix that engulfs a mineral fraction dominated by precipitated carbonates, Fe-mn oxides, and other trace metals (e.g., arsenic and selenium). Soil pH is 6.8 or above throughout, well above the 4.5 euic-dysic limit. The Histosols display two unique intermittent surface crusts. The uppermost crust is a carbonate precipitate that mimics and encrusts organic debris. Below the upper, carbonate crust is an intermittent iron-rich or iron-stained crust. Both crusts likely form via CO2 degassing of cold, carbonate laden groundwater at or near the discharge point. In these soils, silicate minerals are a minor constituent. Traditional soil genesis models and present classification systems do not consider water flow vector or mineral fractions dominated by precipitated, authigenic minerals. We propose three modifications for classifying carbonate-rich Histosols: 1). Revise the “subaqueous soil” criteria from water potential to surface water inundation, which is a directly observable property. 2) Create a “Kalkic” family reaction class for Histosols with the criteria of CaCl₂ pH > 6.8 and presence of secondary carbonates. And, 3) Exclude authigenic carbonate from the mineral fraction that is considered in the organic versus mineral soil criteria.