T277 Soils as Archives of Past Climates

Wednesday, 12 October 2011
1:30 PM - 5:30 PM
Room 200FG

EFFECT OF FIRE ON THE MAGNETIC PROPERTIES OF LOESSIC SOILS

Paper number: T277-14
Program: Technical Sessions
Time: 4:50 PM – 5:05 PM

Authors

Abstract

Fires have long been recognized as a possible cause for magnetic enhancement observed in soils. Under sufficiently high temperatures and generally reducing conditions weakly magnetic iron minerals are reduced to strongly magnetic magnetite, which may later be reoxidized to (still strongly magnetic) maghemite. This process has been described and observed for forest soils (Le Borgne 1955), but has been proposed as a possible process of magnetic enhancement for loessic soils of the Chinese Loess Plateau (Kletetschka and Banerjee 1995). Nearly all loessic modern soils in the Midwestern United States are also magnetically enhanced, however, it is questionable whether prairie fires can generate and sustain the necessary high temperatures necessary to create significant amounts of ferrimagnetic material in the topsoil.

Here we present results from two prescribed burn experiments conducted at Hitchcock Nature Area in western Iowa. Weakly developed soils are located near the top and bottom of loess ridges and were originally covered by forest and grassland vegetation. Our results from forest soils confirm Le Borgne’s original findings, but show that grass fires rarely produce conditions hot enough for the formation of ferrimagnetic magnetite or maghemite. Therefore prairie fires seem an unlikely mechanism to explain the widespread magnetic enhancement of loess soils in the Midwestern United States.

Kletetschka, G. and S. K. Banerjee (1995). "Magnetic stratigraphy of Chinese loess as a record of natural fires." Geophysical Research Letters 22 (11): 1241-1343.

Le Borgne, E. (1955). "Susceptibilite magnetique anormale du sol superficiel." Annales de Geophysique 11: 399-419.