INVESTIGATION OF ANOMALOUSLY MAGNETIC SOILS ON THE OAK RIDGE RESERVATION

Oak Ridge National Laboratory researchers collected high-resolution airborne geophysical data in the winter of 1992-93 on the Oak Ridge Reservation, Oak Ridge, Tennessee to search for possible undocumented waste areas. Instead, they found numerous localized "bulls-eye" magnetic anomalies apparently of natural origin. Susceptibility measurements made on soil cores and drill core collected from the underlying Copper Ridge Dolomite found that the bedrock is not magnetic, but that the local soils contain maghemite throughout the soil profile, with the highest susceptibilities in the near surface. They suggested that the airborne magnetic anomalies may be due to thickened soil profiles associated with colluvial infilling of the regional karst topography, but offered no explanation for the genesis of the maghemite. We hypothesize that the maghemite was created by anaerobic microbial iron reduction followed by formation of single-domain maghemite, or by abiological weathering and reduction of an iron-bearing mineral followed by autoxidation. We are comparing thin sections made from soil cores collected inside and outside one of the magnetic anomalies to look for correlation between pedogenesis and magnetic susceptibility, and using transmission electron microscopy to distinguish biologic from abiologic maghemite.