USING THE SEDIMENT-MAGNETIC PROPERTIES OF SOILS AS INDICATORS OF PAST CLIMATES

We have analyzed the physical properties of modern loessic soils along a rainfall gradient from western Nebraska to central Missouri and correlated the systematic variation in magnetic and non-magnetic properties to changes in present-day rainfall. Soil cores were taken from well drained sites in stable upland positions. The cores were described in the field and subsampled for further analyses. All sites are magnetically enhanced, showing a distinctive increase in the abundance of fine-grained ferrimagnetic minerals in the upper soil horizons with respect to the unaltered parent material. The magnetic enhancement is evident in a number of magnetic properties, such as magnetic susceptibility (MS), several remanence parameters (anhysteretic remanent magnetization (ARM), isothermal remanent magnetization(IRM)), and magnetic grain-size proxies (hysteresis loops, time dependence of IRM acquisition, ARM/IRM ratios). The degree of magnetic enhancement was quantified by calculating the ratios between the magnetically enhanced horizons and the parent material for several magnetic properties. In general, the magnetically enhanced horizons are characterized by high values of MS, ARM and IRM, and the magnetic component is shifted towards fine-to ultrafine grained (d < 0.1 μm) ferrimagnetic particles. Strong correlations (r² = 0.7 - 0.8) exist between magnetic enhancement parameters and present-day rainfall, which suggests the application of our detailed sediment magnetic analyses to paleosols and its use as a possible paleo-precipitation proxy. The robustness of our technique is likely to be improved by combining our magnetic study with non-magnetic measurements, such as soil color and depth to carbonates.