SEDIMENT MAGNETIC SIGNATURE OF CLIMATE IN MODERN LOESSIC SOILS FROM THE GREAT PLAINS

We sampled 17 modern soils along a SW-NE trending transect through Nebraska and western Iowa for pedologic and sediment magnetic analyses. Selecting soils developed in loess on stable upland summits 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 strongly dissected terrain in order to test the sensitivity of our approach to non-climatic influences. All loessic sites contain magnetically enhanced A-horizons, characterized by higher values of magnetic susceptibility, anhysteretic remanent magnetization (ARM) and isothermal remanent magnetization (IRM) as compared to the unaltered parent material. For sites developed in stable upland positions the correlation between magnetic enhancement and modern precipitation is best when using grain-size dependent parameters such as ARM (r²=0.7) or ARM/IRM (r²=0.8) to describe changes in magnetic properties. Enhancement in magnetic susceptibility, which has been used successfully in Chinese loess-paleosol sequences to reconstruct changes in paleoprecipitation, follows the modern precipitation gradient to a lesser degree (r²=0.3). 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 of pedogenic origin. Soils from erosion prone locations show a lower degree of magnetic enhancement due to the continuous loss of magnetically enhanced A-horizon material, while sandy soils show generally very little enhancement regardless of landscape position.