Using magnetic fly ash to identify post-settlement alluvium and its record of atmospheric pollution, central USA

Fly ash has been atmospherically dispersed from coal burning in the central USA since the 1860s and subsequently incorporated into post-European settlement alluvial records. Regionally, the onset of significant coal burning approximately coincides with widespread land clearing for agriculture and accelerated erosion. Field identification of post-settlement alluvium (PSA) is difficult in many areas where the record lacks apparent contrast with older alluvium. Here, we show the PSA basal contact can be reliably identified based on the occurrence of magnetic fly ash. Fly ash methodology is relatively simple, effective, and inexpensive. The proportion of spheroidal fly ash grains in silt-size magnetic fractions was evaluated from 17 sites in the Upper Sangamon River Basin, central Illinois. Data indicate the PSA is 25-95 cm thick, increasing linearly with valley width (r2 = 0.74). Yet, the proportion of PSA within Holocene alluvium is inversely and linearly correlated with valley width (r2 = 0.87), implying significant storage of recent sediment in tributary valleys. A peak in fly ash content (∼15-50 cm in Sangamon Valley), circa 1945-1980, is coeval with peaks in 137Cs (cesium), Pb (lead) concentrations, magnetic susceptibility, and silt content during a time of maximal atmospheric pollution and accelerated topsoil erosion. Sharp decreases in fly ash in the upper 10-20 cm (post-1980) of some Sangamon Valley sites coincide with the implementation of particulate pollution controls and reductions in 137Cs and Pb concentrations; more sandy alluvium also suggests proportionally increased stream bank erosion relative to upland erosion. PSA sedimentation averages ∼5 mm/yr, an order of magnitude faster than pre-settlement rates, reflecting impacts of agriculture and other land-use changes.