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DIVISION S-9—SOIL MINERALOGY

Investigation into the Origin of Magnetic Soils on the Oak Ridge Reservation, Tennessee

^a Environmental Sciences Division, Oak Ridge National Lab., P.O. Box 2008, Oak Ridge, TN 37831-6038
^b Jr., Geology Dep., Temple Univ., 1901 N. 13th St., Beury Hall, Philadelphia, PA 19122-6081

^* Corresponding author (rivers{at}geoladm.geol.queensu.ca)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
In 1993–1994, researchers at Oak Ridge National Laboratory collected high-resolution airborne geophysical data on the Oak Ridge Reservation, Oak Ridge, TN. The data were collected in part to address concerns about possible undocumented hazardous waste sites. Interpretation of the aeromagnetic data was complicated, however, by the discovery in remote areas of numerous small magnetic anomalies of natural origin. Magnetic susceptibility measurements of core showed that the underlying Copper Ridge Dolomite was non-magnetic. We attribute the magnetic anomalies to the presence of the ferromagnetic mineral maghemite (-Fe₂O₃), which formed during pedogenesis of Fe-enriched colluvium that had infilled low-lying areas, including dolines. We discuss explanations offered in the literature for the formation of magnetic soils, and present evidence based on profile descriptions, thin sections, x-ray diffraction (XRD), and scanning electron microscopy (SEM), that in this case, maghemite formed either by anaerobic microbial Fe reduction followed by the formation of single-domain maghemite, or by abiological weathering and reduction of an Fe-bearing mineral followed by oxidation. Naturally occurring magnetic soils may produce magnetic anomalies similar to those characteristics of anthropogenic objects, such as buried waste drums, and complicate interpretation of airborne geophysical surveys.

Abbreviations: EDX, energy dispersive x-ray analysis • ICPMS, inductively coupled plasma-mass spectrometry • ORNL, Oak Ridge National Laboratory • SEM, scanning electron microscopy • XRD, x-ray diffraction

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
IN 1993–1994, the U. S. Department of Energy (DOE) commissioned an airborne magnetic and electromagnetic survey of the 14300 ha (36000 acre) Oak Ridge Reservation, located near Oak Ridge, TN. The purpose of the airborne geophysical survey (including radiation, magnetic, and electromagnetic sensors, Doll et al. [2000]) was to gather geologic information to support hydrologic modeling, and to ensure that no waste areas were overlooked during cleanup operations. The resulting aeromagnetic data set revealed large magnetic anomalies over known waste disposal areas and industrial facilities (Nyquist et al., 1996), and numerous localized magnetic anomalies over smaller metallic targets, such as one-lane bridges, abandoned sheds, power line towers, cars, and trucks (Nyquist and Beard, 1996).

Some of the localized anomalies were of natural origin. All three groups of the Oak Ridge Reservation bedrock (Conasauga, Knox and Chickamauga Groups) have weak volume magnetic susceptibility, averaging below 50 x 10^–5 (all susceptibilities in S.I.). The Copper Ridge dolomite, which occurs on the ridge tops where many anomalies were located, has an average volume magnetic susceptibility of only 5.0 x 10^–5. Nonetheless, follow-up field investigations led to the discovery of pockets of highly magnetic soil on Copper Ridge (Doll et al., 1995).

In 1996, Oak Ridge National Laboratory (ORNL) researchers chose a large soil anomaly from the airborne data set for further study. They collected two soil cores divided into 61-cm (2-ft) sections and extending to depths of 15 and 4.6 m, inside, and outside the magnetic anomaly, respectively. The magnetic susceptibility of the cores was measured using a Bartington MS2 meter with an 80-mm diam. MS2C core sensor, which uses a measuring field on the order of 80 A/m (Bartington Instruments Ltd., personal communication, 2003). The ORNL researchers found that the soil within the area of the magnetic anomaly had a volume magnetic susceptibility as high as 400 x 10^–5 in the upper 60 cm and a susceptibility greater than 100 x 10^–5 to a depth of more than 15 m. The shorter, 4.6-m core taken outside of the anomaly had a magnetic susceptibility <50 x 10^–5 over its entire length.

Previous research revealed that some soils on the Oak Ridge Reservation contain maghemite (Lee et al., 1984; Hatcher et al., 1992; Kopp and Lee, 1987). These soils, termed paleudults (old, humid climate ultisols) and referred to as ancient alluviums (late Tertiary), contain highly weathered and Fe-oxide-impregnated chert fragments and metaquartzite cobbles. They are found on the top of Copper Ridge and Chestnut Ridge as a result of topographic inversion (Lietzke, 1994).

Here we will show that the soil from the core collected within the magnetic anomaly is rich in Fe relative to surrounding nonmagnetic soil. The Fe-rich soils are thought to be, in part, a product of the weathering of minerals deposited by the ancestral Tennessee River. Aeromagnetic mapping of the ancient river deposits is possible due to the magnetic transformation of these minerals. Magnetic mapping of the Fe-rich soils can be used in tandem with surface soil maps to better understand their distribution.

Several authors have previously discussed the origin of maghemite in soils. Much of the discussion centers on oxidation of lithologic magnetic parent material (Resende et al., 1986), biogenic production or magnetic material (Fassbinder et al., 1990), and reduction of magnetically weak ferrous materials by heating followed by oxidation to a magnetic form (Le Borgne, 1960). In this case, evidence shows that these mechanisms do not apply to the magnetic soils on the Oak Ridge Reservation but that the soil was magnetized by transformation of nonmagnetic Fe bearing minerals to magnetic forms through redox cycles during pedogenesis.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
We photographed the top five 61-cm sections of the soil core from within the anomaly and top three sections of core from outside the anomaly. We documented horizon changes based on ped structure, wet/dry color, mineralogy, and roots. Magnetic measurements on the cores were verified using the original instrumentation and documented with other core data (Fig. 1). Samples were chosen for thin section, SEM, and XRD based on the nature of the horizon changes and the magnetic susceptibility data. We also extracted magnetic components from both soils using a horseshoe magnet, determined the magnetic fraction of each soil by volume, and characterized the mineralogy of the magnetic fractions by XRD using a Scintag XDS 2000 diffractometer (Scintag Inc., Cupertino, CA) operating at a scan rate of 2° 2 min^–1 with cobalt K- radiation ( = 1.79026Å).

View larger version (42K): [in this window] [in a new window]   Fig. 1. A photograph of the third core tube extracted from the area within the magnetic anomaly with soil horizon characteristics and volume magnetic susceptibility. A horizons were often readily identifiable by a poorly consolidated clay matrix with grass roots, high chert content (probably due to deflation) and angular peds. A well-consolidated clay matrix with subangular or more often columnar peds characterized CBw-horizons, which generally lacked roots and chert.

For XRD analysis of the magnetic minerals in the sand fraction from the magnetic soil cores, some of the magnetic minerals were crushed, and the fine fractions (<5 µm) were separated by centrifugation. The magnetic fractions were separated further using a laboratory magnetic stir bar. Randomly oriented powder mounts were prepared using petrographic glass slides to determine mineralogy of the magnetic minerals (Jackson, 1975). For differentiation of maghemite and magnetite using XRD analysis, a powdered magnetite specimen (Fisher Scientific, Fair Lawn, NJ) was mixed with the fine magnetic fractions (<5 µm) in a 1:2 ratio before the XRD analysis (Kopp and Lee, 1987).

Finally, we measured the Fe content of the underlying Copper Ridge Dolomite by total dissolution in HF/HNO₃ using a Finnigan MAT Element(I) HR-ICPMS.

	RESULTS

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The magnetic fraction of both sand- and silt-sized particles was determined using a hand magnet after particle-size separation. Magnetic grains were found to be a major constituent of the magnetic soil cores, ranging from 5 to 25% of the specimen volume. Lesser amounts of magnetic grains were detected with a hand magnet in the non-magnetic soil cores, usually only in trace amounts (<3%) in sand and silt fractions.

The XRD measurements performed on the sand and silt fractions indicate that maghemite and hematite are the major Fe minerals in the Oak Ridge Reservation soils (Fig. 2). These soils are rich in quartz and kaolinite and less frequently include illite, a diagenetic product of biotite, implying a source other than the underlying dolomite. Hematite (-Fe₂O₃) was identified in the magnetic fractions, which suggest that the magnetic grains are aggregates of both maghemite and hematite (Fig. 3). The strongest peak, at 0.252 nm, of the magnetic sample is characteristic of both maghemite and hematite. Therefore, the 0.296-, 0.208-, and 0.161-nm peaks were used for maghemite identification. It is difficult to differentiate between maghemite and magnetite by XRD because both minerals have similar crystal structures (Schwertmann and Cornell, 2000). The XRD pattern of the mixture (2:1 ratio) of magnetite powder (Fisher Scientific, Fair Lawn, NJ) and magnetic fraction (separated from sand fraction from magnetic soils) shows a differentiation between maghemite and magnetite peaks: 0.296 vs. 0.297 nm, 0.208 vs. 0.210 nm, and 0.161 vs. 0.162 nm (data not shown). The d-spacing value differences along with the line broadening indicate that the magnetic mineral in the Oak Ridge Reservation soils is maghemite rather than magnetite (Kopp and Lee, 1987). This result indicates that maghemite is responsible for the high magnetic susceptibility of some Oak Ridge Reservation soils.

View larger version (47K): [in this window] [in a new window]   Fig. 2. X-ray diffraction analysis of the (A) sand fraction and the (B) silt fraction from the magnetic and nonmagnetic soils. Hematite peaks can be seen in all samples while maghemite peaks are only present in the magnetic samples. Both the magnetic and nonmagnetic samples are rich in quartz and kaolinite.

View larger version (15K): [in this window] [in a new window]   Fig. 3. X-ray diffraction analysis of magnetically separated grains from the sand fraction of the magnetic soil (vertical axis is intensity [CPS]). While difficult to distinguish, this analysis shows peaks more characteristic of maghemite than magnetite. The magnetic grains are aggregates of both maghemite and hematite. (D-spacing labeled on peaks [nm].)

View larger version (19K): [in this window] [in a new window]   Fig. 4. Composite diagram of the top five core sections extracted from within a magnetic anomaly showing horizon changes and magnetic susceptibility. Magnetic susceptibility tends to peak near the A/CBw boundaries, where redox conditions most favored the formation of maghemite.

View larger version (66K): [in this window] [in a new window]   Fig. 5. (A) Scanning electron microscopy image and (B) energy dispersive x-ray analysis of a magnetic particle separated from the soil core, which was typical of others studied. Energy dispersive x-ray data showed this particle by an iron oxide, not greigite. This particle is too large to be fly ash or to be the product of iron biomineralization by magnetotactic or dissimalatory bacteria. The shape is inconsistent with magnetotactic crystals, which are usually cubic or octahedral, and more rarely prismatic, tooth, arrowhead, and bullet-shaped. Vertical axis is in CPS.

Evaluating the Theories of Magnetic Soil Formation
Numerous mechanisms have been proposed in the literature for the genesis of magnetite (which readily oxidizes to maghemite) in soils. On the basis of our work, most can be ruled out for the magnetic soils on the Oak Ridge Reservation. The following is based primarily on a list from Dearing et al. (1996).

1) Long-term weathering and pedogenesis that concentrates primary residual ferrimagnetic minerals. The underlying rock on the Oak Ridge Reservation is nonmagnetic, so it is unlikely that Fe in the dolomite is in a highly magnetic form. Moukarika et al. (1991) also studied magnetic soils formed on dolomite. They determined that the dolomite supplied the Fe to the magnetic soil in the form of hematite and goethite, but that the soil magnetism was due to later transformation of these oxides into maghemite. Although we believe the primary source of Fe in the Oak Ridge Reservation magnetic soils to be other than the underlying dolomite, our study indicates a similar transformation of nonmagnetic oxides to maghemite. The possibility of magnetic enhancement of the soils due to the weathering of a magnetic parent material can be rejected.

2) Accumulation of relatively coarse (>1 µm) airborne magnetic particulates mainly from pollution sources. Typical fly ash has a diameter on the order of 1 µm (Tompson and Oldfield, 1986; King et al., 1999) but can be as large as 190 µm (King et al., 1999). The pure Fe-oxide particles imaged from Oak Ridge soils, one of which can be seen in Fig. 5, are 200 to 2500 µm, which is too large to be fly ash.

3) Strictly anaerobic dissimilatory Fe(III)-reducing bacteria that produce single domain magnetite (Fe₃O₄) grains with diameters <100 nm. The pure Fe-oxide particles located using SEM were far too large to be produced by this mechanism (Fig. 5). Dissimilatory Fe(III)-reducing bacteria only rarely form magnetic particles as large as 200 nm in subsurface environments. The roundness and size of the particles we found does not fit the description of magnetite produced by dissimilatory Fe(III)-reducing bacteria, which usually form cubic-, octahedral-, or elongated prismatic-shaped magnetite with crystal size ranging from 10 to 200 nm (Zhang et al., 1998; Roh et al., 2002, 2003).

4) Anaerobic formation of greigite (Fe₃S₄) linked to microbial reduction. Stanjek et al. (1994) showed that greigite formed at a depth of 75 cm in a gley (reduced) soil. They interpreted its presence was a result of sulfate reduction due to anaerobic respiration. The S was supplied by a pyrite-bearing parent rock of the soil. Two observations make this mechanism unlikely on Oak Ridge Reservation. First, the magnetic enhancement on the Oak Ridge Reservation is associated with oxidized horizons. Second, we have shown that maghemite, not greigite, is the principal magnetic mineral.

5) Microaerophilic assimilatory bacteria (magnetotactic bacteria) that produce chains of single-domain magnetite magnetosomes with diameters 100 to 220 nm. The magnetic Fe-oxide particles located using SEM (Fig. 5) are too large and too rounded to support this hypothesis. The roundness of these particles does not fit the description of magnetite produced by magnetotactic bacteria, which are usually cubic or octahedral, and more rarely prismatic, tooth, arrowhead, and bullet-shaped. Rounded forms are rare and associated with smaller than average magnetotactic crystals (Devouard et al., 1998). Furthermore, these bacteria thrive in reducing conditions, whereas the highest magnetic signal in these soils is associated with oxidized horizons.

6) Thermal transformation of weakly magnetic Fe oxides and hydroxides to ferrimagnetic magnetite or maghemite by natural fires or crop burning in the presence of organic matter. The depth and distribution of the magnetic signal does not seem to support this process. If fire caused the magnetism we would expect magnetic soils to be ubiquitous, not confined to the localized pattern seen on the Oak Ridge Reservation. The depth of the magnetic soils (>15 m) and the repeated occurrence of susceptibility peaks in the lower not the upper portion of A horizons also argues against this possibility.

7) Fulgurites (formations caused by lightning strikes). We would expect the fulgurite (lightning) mechanism to create localized anomalies, but we would not expect any magnetic transformation caused by a single strike to extend more than 15 m deep. To explain the susceptibility peaks in the soil core, lightning would have to strike the low-lying dolines before each subsequent horizon was buried. Furthermore, we found no vesicular glass relicts, created by the fusing of silica during lightning strikes, in thin section.

Two hypotheses remain viable after our analysis.

8) Anaerobic microbial Fe reduction followed by formation of single-domain magnetite or maghemite (-Fe₂O₃) grains with diameters <100 nm.

9) Abiological weathering of the Fe(III) bearing minerals followed by oxidation leading to magnetite or maghemite, as demonstrated in synthetic experiments (Taylor et al., 1987), and documented by Moukarika et al. (1991). One or both of these mechanisms is probably responsible for the magnetic enhancement of soils on the Oak Ridge Reservation. Magnetite contains ferric (oxidized) and ferrous (reduced) iron. Both mechanisms can create magnetite through cycles of reduction followed by oxidation. The only difference between the mechanisms is the reducing agent. In one case the reducing agent is microbial, and in the other it is abiological (water table fluctuations). Although Mullins (1977) reported that microbial iron reduction (Mechanism 8) produced maghemite particles smaller than 100 nm, and similarly Taylor et al. (1987) synthesized particles smaller than 100 nm abiologically (Mechanism 9), in principle either mechanism could produce much larger maghemite particles, concretions or coatings given sufficient time, and repeated redox cycles. Because the morphology and size of the magnetic particles created by these two mechanisms are similar, we could not ascertain which of these two mechanisms was most responsible for the magnetic enhancement of the soil on the Oak Ridge Reservation.

Out of 14 profiles identified in the two soil cores, 10 showed increased magnetic susceptibility at the bottom of the A horizon. This susceptibility pattern is a reflection of the cyclic redox environment optimal for the creation of maghemite. Portions of soil profiles enriched in maghemite are relict redox boundaries of now-buried soils.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Hatcher et al. (1992) indicate that the parent material of the magnetic soil is ancient alluvium now found at the crest of the Copper Ridge as a result of topographic inversion. Crownover et al. (1994) dated the alluvial sediments as early Pleistocene using Appalachian denudation rates. Support for ancient alluvium as a parent material of the soil within the magnetic anomaly is found in the allochthonous quartzite grains located in thin sections taken from the soil. According to Phillips et al. (1998), cobbles of metaquartzite are evidence of ancient alluvium that was deposited by rivers flowing from the Unaka Mountains or Clinch Mountain. It is unclear how much of the magnetic soil in the core is original alluvium as opposed to Copper Ridge residuum, but undoubtedly the two mixed during colluvial deposition.

The alluvium-associated magnetic soil is richer in Fe than the soil from outside of the anomaly. This can be evidenced by EDX data, which showed Fe to be the second most abundant element after silicon in the silt and clay fraction of the magnetic soil and the third (after Si and Al) in the nonmagnetic soil. The major difference between the magnetic and the nonmagnetic soils seems to be the influence of the ancient alluvium, probably originally enriched in Fe by the weathering of minerals carried by a river from the mountains. If underlying dolomite were the primary source of Fe in the magnetic soils on the Oak Ridge Reservation, as was the case in the Moukarika et al. (1991) study of magnetic soils in Brazil, we would expect the magnetic soils to correlate well with the distribution of the dolomite bedrock. Instead the anomalies detected by the airborne survey are small and distinct, and are often found in areas where ancient alluviums and soils derived from them have been mapped. Finally, the average formula of the ferroan dolomite underlying magnetic soils reported by Moukarika et al. (1991) contains 3.25% Fe by weight. The Copper Ridge dolomite underlying the Oak Ridge Reservation magnetic soils averaged <0.6% Fe, making it an unlikely primary Fe source.

A conjugate system of joints on Copper Ridge is oriented perpendicular and parallel to the strike of the ridge (Hatcher et al., 1992). Low-lying areas on Copper Ridge, some of which are dolines, also strike perpendicular and parallel to ridge (Fig. 6), reflecting the orientation of the joint set from which they may have formed. The magnetically anomalous regions reflect the deposition of Fe-rich colluvium or more modern alluvium (derived from ancient alluviums) into low-lying dolines, which may have acted as sediment traps and possibly protected these soils from denudation over their long histories. The transformation of the Fe in the soil to maghemite through redox cycles initiated (or continued) after deposition as evidenced by the soil profiles that show susceptibility spikes at relict A/CBw horizon boundaries. Magnetization of the soil might also be a function of hydromorphy (water fluctuation) in dolines, helping to explain magnetic soil associations with dolines. Subsidence within the dolines allowed the packages of magnetic soils to become thick enough to be detected by airborne magnetometers.

View larger version (83K): [in this window] [in a new window]   Fig. 6. Airborne magnetic anomalies (magenta) overlain on topographic data. The magnetic anomalies are found in depressions on Copper Ridge. These depressions reflect dolines (sinkholes) on the ridge in which Fe rich sediments have collected. The dolines are often oriented perpendicular or parallel to the strike of Copper Ridge suggesting an association with joints in the bedrock. Data in the white area (center) was omitted due to magnetic interference from buildings. The black dot marks the magnetically anomalous area from which the more magnetic soil core was extracted. The less magnetic core was extracted outside the same anomaly.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

Magnetic soils can develop on nonmagnetic bedrock and complicate interpretation of airborne magnetic surveys. The geomorphic history of the Oak Ridge Reservation has led to a complex soil stratigraphy where magnetic soils, developed in depressions and dolines, create localized magnetic anomalies. Using only data from the magnetic survey these anomalies are indistinguishable from those created by anthropogenic objects such as buried waste drums. Understanding the regional magnetic properties and distribution of soils is important when interpreting data collected by high-resolution airborne magnetic surveys.

	ACKNOWLEDGMENTS

 
Thanks to Don Chipley for the ICPMS analysis at Queens University, Kingston, ON. Oak Ridge National Laboratory is managed by UT-Battelle, LLC for the U. S. Department of Energy under contract DE-AC05-00OR22725. The submitted manuscript has been authored by a contractor of the U.S. Government. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so for U.S. Government purposes.

Received for publication March 3, 2003.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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