Loess Contribution to Soils Forming on Dolostone in the Driftless Area of Wisconsin

Abstract

Soils of the southern Driftless Area of Wisconsin are derived from loess mantles and dissolving carbonate from dolostone bedrock found in formations of the Sinnippi Group. Contributions of the two parent components can be estimated by utilizing volumetric compositions of the relatively immobile elements Ti and Zr. The dolostone bedrock is depleted in both elements (3.22 and 0.11 μmol cm−3, respectively) relative to loess derived from sediments of the St. Croix and Upper Mississippi river systems, which contains 184.54 and 7.44 μmol cm−3, respectively. This strong contrast between materials allows contribution indices based on Ti and Zr to be determined using a simple algebraic relationship based on the end‐member concentrations. Contribution indices were determined from Ti and Zr concentrations of horizons from 11 pedons in Iowa County in southwestern Wisconsin. These pedons showed typical morphology for the southern Driftless Area, with loess‐derived silt loam to silty clay loam Ap and Bt horizons and clay‐enriched 2Bt subsoils overlying dolomite residuum (3BC/3C). Mean contribution estimates for the pedons show maximum loess contribution indices (0.802–0.854 kg kgTi basis−1) in the argillic horizons, with lower contribution indices (0.718 kg kgTi basis−1) in the surface horizons due to organic matter dilution and increased porosity (lower bulk density). Mean contribution indices for loess in the horizons just above the dolomitic bedrock are predictably low (3.82 and 1.62 g kgZr basis−1) and not significantly different (P < 0.0005). Contribution indices based on the two elements showed differences in trend with depth due to the individual behavior of the elements, i.e., the affinity of Ti for Fe oxides and clay minerals, and physical transport of small durable zircons moving into the solum through pores. The value of these indices is that they can be utilized to discern not only how much material the weathering loess actually contributed to soil formed on the bedrock surface, but also the relative intensity of argillic horizon development.