Quantification of soil erosion rates within the Indian mounds area in Illinois, the United States

The method of a technogenic magnetic tracer has been applied to assess the rates of soil erosion in the area of ancient (1000 BP) Indian mounds on the Mississippi River terrace with predominant vertic meadow soils (Vertic Haplaquolls). The content and distribution patterns of fly ash (technogenic magnetic spherules) in the soil profiles and the magnitudes of magnetic susceptibility have been analyzed together with data on the topsoil thickness and the organic carbon content. All studied soils contain from 30 to 150 g/m² of magnetic spherules with sizes varying from 1 to 50 μm. From 90 to 99% of these spherules are concentrated in the uppermost 5 cm of the soil profile. The content of fly ash in forested soils is two to four times higher than that in meadow soils. Eroded soils of the mounds are characterized by a lower content of the spherules and a smaller depth of their penetration into the soil profile in comparison with noneroded soils. The average rate of erosion for unplowed mounds is estimated at several hundred g/m ² per year.