Christoph E. Geiss¹, C. William Zanner², James Bisbee¹

1Tinity College, 300 Summit St., Hartford, CT 06106, christoph.geiss@trincoll.edu
²University of Nebraska, Lincoln, NE

We sampled 17 modern soils along a SW-NE trending transect through Nebraska and western Iowa for pedologic and sediment magnetic analyses. Selecting soils from stable upland positions that developed in loessic parent material allowed us to isolate modern climate as the main influence on soil properties. These sites were complemented by four soils developed in sandy material and seven soils developed in stronglydissectred terrain in order to test the sensitivity of our approach to non-climatic influences.

All sites contain magnetically enhanced A-horizons, characterized by higher values of magnetic susceptibility (χ), anhysteretic remanent magnetization (ARM) and isothermal remanent magnetization (IRM) as compared to the unaltered parent material. The correlation between magnetic enhancement and modern precipitation is best when using grain-size dependent parameters such as ARM (r² = 0.76) or ARM/IRM (r² = 0.77) to describe changes in magnetic properties. Changes in magnetic susceptibility, which have been used successfully in Chinese loess-paleosol sequences to reconstruct changes in paleoprecipitation, do follow the modern precipitation gradient to a lesser degree (r² = 0.56). The better performance of ARM or ARM/IRM as a predictor of modern precipitation is due to the high sensitivity of these parameters to small (d = 0.01 - 0.1 μm), single domain (SD) grains, which are likely of pedogenic origin.