Observations of magnetite dissolution in poorly drained soils

Dissolution of strongly magnetic minerals is a common and relatively rapid phenomenon in poorly drained soils of the central United States, resulting in low magnetic susceptibility (MS). Low Eh reducing conditions are primarily responsible for magnetic mineral dissolution; a process likely mediated by iron-reducing bacteria in the presence of soil organic matter. Based on transects across drainage sequences from nine sites, natural magnetic minerals (>5 μm) extracted from surface soil consist of 54% ± 18% magnetite, 21% ± 11% titanomagnetite, and 17% ± 14% ilmenite. Magnetite and titanomagnetite dissolution, assessed by scanning electron microscopy on a 0-to-3 scale, inversely correlates with surface soil MS (r = 0.53), a proxy for soil drainage at studied transects. Altered magnetite typically displays etch pits <1-μm diameter, whereas titanomagnetite exhibits a latticework pattern of alteration along planes of crystallographic weakness (<1-μm spacing). Ilmenite, containing solely ferrous iron, is unaffected by reducing conditions, portraying mainly smooth-textured grains. Magnetic fractions (>5 μm) include 26% ± 18% anthropogenic fly ash that also exhibits greater dissolution in low MS soils (r = 0.38), indicating detectable alteration can occur within 150 years in low Eh soils. Laboratory induced reduction of magnetite, titanomagnetite, and magnetic fly ash, with a citrate-bicarbonate- dithionite solution, resulted in dissolution textures similar to those of in situ soil particles. Although experiments indicate that reductive dissolution of magnetite can occur abiotically under extreme conditions, bacteria likely play an important role in the natural environment.