Investigation of Magnetic Soils on the Oak Ridge Reservation, TN

Abstract

A 1993, airborne geophysical survey found numerous localized "bull's­-eye" magnetic anomalies, apparently of natural origin, associated with dolines on the Oak Ridge Reservation (ORR), Oak Ridge, Tennessee. Susceptibility measurements made on a soil core extracted from one such doline located on Copper Ridge (Anomaly B) showed the soil to be magnetic throughout the soil profile (average susceptibility >200 x 10^-5 S.I. units), while a soil core removed from outside the doline showed an average susceptibility of less than 20 x 10^-5 S.I. units. Pedologic characterization of the soil cores both inside and outside of the doline showed a series of stacked soil profiles, all in the early stage of pedogenesis, with A-horizons lying directly on C-horizons that were transforming to Bw-horizons, indicating periodic burial by fresh colluvium or alluvium. Based on mottling seen throughout the soil cores, which is evidence of hydromorphy, I identified of the soils as aquepts using the USDA soil classification. In the majority of the soil profiles the magnetic susceptibility peaked at the base of the A-horizons. I investigated nine different mechanisms discussed in the literature for the formation of magnetic soil. I used scanning electron microscopy (SEM), energy dispersive X-ray (EDX), and X-ray diffractometry (XRD), to supplement pedologic characterization. Based on the XRD results, the source of the soil magnetism was identified as the ferromagnetic mineral, maghemite. SEM measurements showed that the magnetic particles extracted from the soil cores were too large and rounded to be biogenic maghemite. Apparently the maghemite formed by anaerobic microbial iron reduction followed by the formation of maghemite, or by abiological weathering and reduction of an iron-bearing mineral followed by oxidation. Maghemite occurs in the soil both inside and outside of the magnetic anomaly, but was found in higher concentrations inside the anomaly. EDX analysis showed that the soil core extracted from inside the magnetic anomaly contained more iron than the core extracted outside the anomaly, suggesting a different parent material for the soil inside the doline. ORR reports and soil maps indicate that the parent material is an ancient alluvium. Ancient alluviums found on the ORR have more iron as a result of the weathering of iron rich sediment deposited by an ancient river. Through topographic inversion these ancient alluviums are now found on top of Copper Ridge. I found evidence to support an alluvial source for the soil inside the doline, including rounded metaquartzite and illite (weathered biotite). I conclude that soils derived from ancient alluviums have accumulated in dolines on the ORR and that hydromorphy within the dolines has increased the maghemite concentration. The magnetic anomalies detected by the airborne geophysical survey resulted from a combination of increased soil thickness in the dolines, and the higher maghemite concentration that developed in these iron rich soils.