Sediment magnetic signature of climate in modern loessic soils from the Great Plains

Our magnetic analysis of 40 modern loessic soil profiles from the midwestern United States shows that the upper soil horizons of all sites are characterized by an increase in the concentration of fine grained (d<0.1 μm) ferrimagnetic minerals (magnetite and/or maghemite). Our sites were selected from stable upland positions along a precipitation gradient extending from SW Nebraska (mean ann. precip. <500 mm/yr) to central Missouri (mean ann. precip. >1000 mm/yr). Changes in magnetic remanence parameters (IRM, ARM) and magnetic susceptibility (χ) between soil and parent material vary systematically along the transect and are likely to reflect changes in source area and transport distance. Magnetic enhancement is calculated for several magnetic parameters by taking the ratio Menhanced/Mparent material, where M stands for either ARM, IRM or χ. Our analyses show that magnetic enhancement of modern soils based on IRM and χ correlates with changes in mean annual precipitation. The best correlation, however, is observed when ARM is used as a proxy of magnetic enhancement, which might reflect the bias of ARM towards small, single-domain grains of likely pedogenic origin. Our study shows that magnetic soil properties, in combination with non-magnetic proxies, such as solum depth, B-horizon color, depth to carbonates or other weathering indices have the potential to yield reliable reconstructions of paleoclimate in parts of the Great Plains where well preserved loessic paleosols exist.