RAILROAD FLY ASH AS A RELATIVE DATING TOOL IN CENTRAL WISCONSIN LAKES

We determine the feasibility of using railroad fly-ash as a relative dating tool for lakes in central Wisconsin, USA. Fly-ash is a by-product of high temperature coal combustion and is comprised of spherical carbonaceous particles and inorganic ash spheres (IAS), the latter of which are rich in hematite, magnetite and heavy metals, and reflect their mineral origins. Strongly magnetic grains in fly ash are composed of highly substituted magnetite-maghemite formed by re-crystallization pyrite upon combustion. This material is associated with industrial activity including steam powered farm equipment, power plants, and locomotives, and is known to accumulate in lake sediments from atmospheric deposition throughout the 19th - 20th centuries. In central Wisconsin, fly ash should appear in lake sediments beginning when railroads came through the area during the logging era (late 1800’s). Ambrosia spp. pollen, indicative of European land clearing, can also be used as a relative dating tool, but the magnetic separation and identification of fly ash is potentially more efficient. Thirteen lakes in central Wisconsin with existing Ambrosia spp. rise data are used as controls. Magnetic particles are separated from sediment using a magnetic stirrer and differentiated under light microscopy. Magnetic IAS are characteristically round and reflective unlike other fine-grained magnetic fractions and a ratio of IAS to other grains can be established. Additional samples from the same cores are measured for magnetic susceptibility to determine the relative importance of fly ash to total susceptibility of the sediment. In central Wisconsin, railroad spurs were installed about ten years before agricultural land clearing and subsequent Ambrosia peaks. Run-off and terrigenous inputs of soil-fraction magnetic minerals coincides with land clearing and slightly postdates incipient fly ash accumulation in the lakes.