Direct estimates of pedogenic magnetite as a tool to reconstruct past climates from buried soils

Variations in magnetic properties of buried soils can be used to reconstruct past climatic conditions during paleosol formation. Most methods, however, are based on comparisons between the magnetically enriched upper soil horizons and the magnetically unaltered parent material. In thin loess-paleosol sequences such a comparison can be problematic because all horizons, soil and underlying loess, may be affected to varying degrees by pedogenesis. We propose two direct estimates of pedogenic magnetite based on the analysis of anhysteretic remanent magnetization ratios (chi(ARM)/isothermal remanent magnetization) and coercivity distributions. These estimates are independent of any information regarding the parent material and are possible if pedogenic minerals have similar magnetic properties throughout the study region. This condition seems to be met throughout the Midwestern United States and a few loessic soils elsewhere. The remanence-carrying part of pedogenic magnetite is composed of single-domain particles with consistent, well-constrained magnetic properties. These particles are extremely well dispersed in the soil matrix as indicated by the absence of noticeable magnetostatic interaction effects. Our analyses of over 70 modern loessic soil profiles demonstrate that the abundance of pedogenic magnetite correlates well with modern climate and that the method is suited for reconstruction of past climates from paleosols.