References

1. GP13B-0783 The Effect of Prairie Fires on the Magnetic Properties of Modern Soils at Konza Prairie, Kansas Giuliani Lopez1, William C Johnson2, Christoph E Geiss1 1. Environmental Science Program, Trinity College, Hartford, CT, United States. 2. Department of Geography, University of Kansas, Lawrence, KS, United States. Results Abstract Prairie fires have been proposed as a possible mechanism that influences the magnetic enhancement of modern and buried soils because they can create a partially reducing environment that may transform weakly magnetic minerals into strongly magnetic minerals in the upper soil horizons. We analyzed the magnetic properties of shallow loessic soils from Konza Prairie, Kansas to examine the possibility of magnetic enhancement due to prairie fires. Konza Prairie is a Long Term Ecological Research Site (LTER) and has a long record (20+ years) of controlled burns. This makes it an ideal location for isolating the role of repeated prairie fires on the magnetic properties of soils. We sampled 46 sites that have been subjected to fire treatments ranging from annual burns to burn frequencies a low as once every 20 years. Our analyses include measurements of magnetic susceptibility (χ), anhystereticremanent magnetization (ARM), isothermal remanent magnetization (IRM), S-ratios, and coercivity distributions of IRM. Plotting several magnetic enhancement parameters against burn frequency showed that no evidence that repeated prairie fires cause magnetic enhancement in modern soils. Our results suggest that prairie fires do not burn hot enough to significantly alter the magnetic properties of soils and are an unlikely explanation for the magnetic enhancement of paleosols. Methods We used a manual soil probe to extract 46 cores of about 25cm in length. We then used the following sediment magnetic analyses to quantify magnetic enhancement. Figure 5. Magnetic Susceptibility for all burn treatments Figure 9. Average Magnetic Susceptibility Figure 3. Packaging samples Introduction One possible cause of magnetic enhancement is fire. LeBorgne (1960) showed that forest fires can lead to changes in the magnetic properties of soil profiles. Kletetschka and Banerjee (1995) proposed that prairie fires have a similar effect on the magnetic properties of soils despite lower fuel availability in prairies and the resulting lower fire temperatures and shorter fire duration. For the past 20 years soils at the KonzaPrairie Biological Station have been subjected to a regimen of controlled burns, and if prairie fires do cause magnetic enhancement, we would see their effects it in the soils at Konza Prairie. Figure 6. ARM for all burn treatments Figure 10. Average ARM for all burn treatments Figure 4. Plastic tubes hold samples Figure 2. Manual Corer • Low-field Magnetic Susceptibility (χ) reflects the ease in which sediments are magnetized when exposed to a magnetic field and depends on the concentration of all magnetic minerals in the sample. Χ was measured with an AGICO KLY-4s kappa-bridge. • Anhysteretic Remanent Magnetization (ARM) is heavily biased towards small single-domain grains. Samples were magnetized once in a peak AF field of 100 mT and 50 μT bias field using a Magnon International AFD300 alternating field demagnetizer. • Isothermal Remanent Magnetization (IRM) reflects the presence of all remanence-carrying grains. Samples were magnetized three times in a pulsed magnetic field of 100 mT (IRM). • All remanence parameters were measured using an AGICO JR-6 spinner magnetometer Figure 7. IRM for all burn treatments Figure 11. Average IRM for all burn treatments Discussion None of the studied soil profiles showed significant magnetic horizination or enhancement in the upper soil horizons. This may be because there is either no profile development or because magnetic enhancement processes affect the entire soil profile. However, if prairie fires have a significant impact on soil-magnetic properties, yearly burns should still yield a higher magnetic signal than burns that occur at lower frequencies. We did not observe such differences. The uniform magnetic development of all soil profiles, independent of fire treatment could be due to the relatively recent implementation of these treatments, or due to the fact that prairie fires do have negligible effect on the magnetic properties of soils. Our results agree with the findings of Dearing et al. (1996)who showed that fire has little or no influence on the magnetic properties of soils even after centuries of controlled crop burning. Therefore, our study suggests that grassland fires do not burn hot or long enough to have a noticeable effect on the magnetic mineralogy of the upper soil profiles, and prairie fires are an unlikely cause of the observed magnetic enhancement of many prairie soils. Figure 8. ARM/IRM for all burn treatments Figure 12. Average ARM/IRM for all burn treatments Figure 1. Location of sample sites within the KonzaPraire Biological Station south of Manhattan, KS. Flame symbols represent the locations of soil profiles that were analyzed for this study. Burn frequencies ranged from yearly burns, a burn every 2 years, a burn every 4 years, a burn every 10 years to a burn every 20 years. References Dearing, J.A., Hay, K.L., Baban, S.M.J., Huddleston, A.S., Wellington, E.M.H. & Loveland, P.J., 1996. Magneticsusceptibility of soil: an evaluation of conflictingtheoriesusing a national data set, Geophys. J. Int., 127, 728–734. Le Borgne, E. (1960), Influence du feu sur les proprietesmagnetiques du sol et sur celles du schiste et du granite, Annales de Geophysique, 16, 159-195. Kletetschka, G. and Banerjee, S., Magnetic stratigraphy of Chinese loess as a record of natural fires. Rummery, T. A., J. A. Dearing, F. Oldfield and R. Thompson, The persistence of fire-induced magnetic oxides in soils and sediments. Ann. Geophys., 35, 103-107, 1979.