It has been proposed that grass fires affect the magnetic properties of soils by combining generally reducing soil conditions with elevated temperatures. To explore this supposition, we analysed surface and subsurface samples from loessic soils and compared their differences in magnetic properties as a function of fire intensity. Fire intensity was established based on types of burnt vegetation, which ranged from low-intensity fires in short-grass areas to high-intensity fires in tall-grass and forested areas. We measured low-field magnetic susceptibility (χ), a common proxy for the abundance of magnetic minerals, frequency-dependent susceptibility (χFD), a proxy for the presence of ultrafine-grained superparamagnetic minerals, and susceptibility of anhysteretic remanent magnetization (χARM), a magnetic parameter highly dependent on the presence of fine, single-domain magnetic particles. Although intense fires led to an increase in frequency-dependent susceptibility and low-field magnetic susceptibility, moderately intense fires did not produce significant changes in magnetic properties. Observed magnetic changes are limited to sites that were very heavily burnt in forest areas. Grass fires are therefore an unlikely mechanism to explain a measurable component of the magnetic enhancement in prairie soils.
This article has been accepted for publication in Geophysical Journal Inernational © 2013 Published by Oxford University Press. All rights reserved.
The published version may be accessed at http://gji.oxfordjournals.org/content/early/2013/09/25/gji.ggt349.full.pdf+html