REACTION FRONTS IN SOILS DEVELOPED ON PEORIA LOESS DOCUMENT MINERAL WEATHERING KINETICS

Through physical and chemical weathering, primary minerals are transformed to soil minerals in the Critical Zone. Using a published dataset, we investigated reaction fronts occurring in modern soils developed on Peoria Loess in the Mississippi River Valley. After normalization to Zr, elemental concentrations document several different characteristic reaction fronts. Na, Ca, and Mg vary with depth and show depletion-only profiles while Al, Fe, and Si show dissolution-precipitation profiles. We define a depletion-only profile as one depicting continual decrease in concentration from parent to surface, while dissolution-precipitation profiles are represented by surface depletion grading into subsurface enrichment relative to the parent composition. In contrast, the macronutrients P and K show fronts that reveal the effect of biogenic uptake. However, both elements also show enrichment at depth probably related to precipitation of Fe-oxides or clays respectively. Finally, the elemental profile for Mn is classified as an addition-only profile which reflects a relative increase in concentration from parent to surface. This Mn profile is attributed to contaminated dust input. These elemental profiles can be interpreted with respect to reaction kinetics as a function of time, temperature, and rainfall along a transect from north to south.