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 metal oxides. In theses 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 specific Soil Taxonomy 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. This set of revisions removes terrestrial discharge soils from classifying as Wassists. The proposed Kalkic reaction class fills a void, as the only existing pH criteria is the euic-dysic limit (CaCl2 pH 4.5). Lastly, authigenic carbonate in these soils, which has distinctly different properties (e.g., CEC, water holding) than the silicate minerals, would not outweigh organic carbon content in Histosol classification.