The impact of fire and abrasion on iron availability in aeolian sediments

Remote sensing and field surveys have demonstrated that firescars can be important dust sources in arid and semi-arid regions. Resulting dust storms are the product of aeolian abrasion and fine particle entrainment which are enhanced when burning removes vegetation, destroys soil aggregates and increases soil erodibility. Wind-eroded particles are transported away from source and their impact in distal terrestrial and marine systems depends on the particle characteristics including size and geochemistry, notably iron content. The form of iron can change during transport, but here we present evidence that it may also change as a result of both heating during the fire, and the abrasion process. Using laboratory experiments on 18 sediment samples with different initial levels of total iron content, it was found that flash heating sediments with an initial spectral redness of ≥0.44 to 250 deg. C and 350 deg. C - representing summer and winter burns respectively - typically caused a decrease in the redness of 1-2% compared with non-treated sediments. Sediments with an initial spectral redness less than 0.44 were found to become slightly redder when heated. Abrasion of non-heat treated sediments caused a decrease in sediment spectral redness of up to 1%. Heat treatment had no impact on the susceptibility of sediments to abrasion i.e. the total volume of dust production during 72 hours of sustained abrasion was not significantly different in the heat-treated samples compared with the untreated ones. Detailed analysis of the spectral characteristics of the sediments shows that the decrease in sediment redness is cause by a decrease in reflectance at wavelengths shorter than 550 nm (resulting from the Fe3+ charge transfer band in the ultraviolet). The hematite crystal field transition absorption feature at 850 nm also decreases following heating and abrasion. Previous studies have suggested that heating increases the redness of samples, but this is not the case for most samples tested here. The impact of heat and abrasion may affect the form of the Fe-oxides in the sediments and is currently being tested. Improved understanding of what the spectral properties of sediments can reveal about the geochemical properties and how this relates to earth surface processes may assist in wider scale mapping of potential sources of iron-rich dust.