Paleoprecipitation Reconstructions for the Brady Soil Based on Rock-Magnetic Analyses

Magnetic properties of buried soils, especially magnetic susceptibility, have long been used to characterize soil development and quantify past rainfall regimes. Previous analyses of modern loessic soils from the Midwestern United States showed that magnetic remanence parameters such as the ratio between anhysteretic remanent magnetization (ARM) and isothermal remanent magnetization (IRM) yields a reliable rainfall proxy over a wide range of climatic conditions (400 mm/a - 1000 mm/a). To extend our rainfall proxy reconstructions into the central Great Plains, we measured ARM/IRM ratios for the Brady soil in several locations in southwestern Nebraska. Our analyses yielded mean annual precipitation values of 450±50 mm/a (Wauneta, 40.4983°N, -101.404°W), 550±50 mm/a (Devil's Den, 41.4561°N, -100.192°W) and 575±50 mm/a (Harlan County Lake, 40.0709°N, -99.2737°W), which is slightly lower than present-day precipitation at these sites and correlates well with isotope-derived reconstructions of paleoprecipitation. Our reconstructions assume that the magnetic signal is acquired and reaches equilibrium over a reasonably short time (millennia) and the magnetic component survives the burial process. Given these limitations, our results establish a lower limit on precipitation amounts during the formation of the Brady soil, ~14.5 -10 ka ago.