EVALUATING THE GEOCHEMICAL CONTRIBUTIONS OF LOESS TO SOIL FORMATION OF THE SOUTHERN DRIFTLESS AREA OF WISCONSIN

Soils of the southern Driftless Area of Wisconsin are derived from a multi-generational loess mantle overlying Galena Fm. dolomitic bedrock. Although loess dominates in the formation of these soils, the dissolution of the carbonate bedrock also contributes to pedogenic processes at depth. A strong contrast between geochemical characteristics of the parent materials allows for a contribution index based on the end-member concentrations of the relatively immobile elements titanium (Ti) and zirconium (Zr). The dolomitic bedrock is depleted in Ti and Zr (3.22 µmol cm^-3 and 0.11µmol cm^-3, respectively) relative to loess derived from sediments of the St. Croix and upper Mississippi River system, which contains 184.54 µmol cm^-3 Ti and 7.44 µmol cm^-3 Zr. By utilizing volumetric compositions, contributions of each component can be estimated using simple algebraic relationships. To test this idea, contribution indices were calculated from Ti and Zr data from 11 pedons from Iowa County in southwestern Wisconsin. These pedons showed typical morphology for the southern Driftless Area, with loess-derived silt loam to silty clay loam textured Ap and Bt horizons and clay-enriched 2Bt subsoils overlying dolomite residuum (3BC/3C). Mean contribution estimates for the pedons showed maximum loess contribution indices (0.8020 – 0.8536 g g^-1 _{Ti basis}) in the upper argillic horizons, with lower mean contribution indices (0.7181 g g^-1 _{Ti basis}) 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 x 10-3 g g^-1 _{Ti basis} and 1.62 x 10-3 g g^-1 _{Zr basis}) 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 iron oxides and clay minerals, and physical transport of small durable zircons moving into the solum through pores. Determining contribution indices from known parent material sources allows for more accurate quantification of geochemical pedogenic processes through application of the indices as proportional constraints in mass balance evaluations of soil evolution in binary parent material systems.