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Iron mineral transformations occurring in hydrocarbon-contaminated sites are linked to the biodegradation of the hydrocarbons. At a hydrocarbon-contaminated site near Bemidji, Minnesota, USA, measurements of magnetic susceptibility (MS) are useful for monitoring the natural attenuation of hydrocarbons related to iron cycling. However, a transient MS, previously observed at the site, remains poorly understood and the iron mineral phases acting as reactants and products associated with this MS perturbation remain largely unknown. To address these unknowns, we acquired mineral magnetism measurements, including hysteresis loops, backfield curves, and isothermal remanent magnetizations on sediment core samples retrieved from the site and magnetite-filled mineral packets installed within the aquifer. Our data show that the core samples and magnetite packs display decreasing magnetization with time and that this loss in magnetization is accompanied by increasing bulk coercivity consistent with decreased average grain size and/or partial oxidation. Low-temperature magnetometry on all samples displayed behavior consistent with magnetite, but samples within the plume also show evidence of maghemitization. This interpretation is supported by the occurrence of shrinkage cracks on the surface of the grains imaged via scanning electron microscopy. Magnetite transformation to maghemite typically occurs under oxic conditions, here, we propose that maghemitization occurs within the anoxic portions of the plume via microbially mediated anaerobic oxidation. Mineral dissolution also occurs within the plume. Microorganisms capable of such anaerobic oxidation have been identified within other areas at the Bemidji site, but additional microbiological studies are needed to link specific anaerobic iron oxidizers with this loss of magnetization. National Science FoundationNational Science Foundation (NSF) [1742938, 1742959]; U.S. Geological Survey Environmental Health Program Published version This material is based upon work supported by the National Science Foundation under Grant Nos. #1742938 and #1742959. We thank Dr. Nicholas Seaton from the Characterization Facility, College of Science and Engineering, the University of Minnesota for his help with the SEM imaging. We thank Selcen Yokus for her tremendous help with the sampling of the magnetite packs and assistance in helping to understand the mineral packets. We thank Doug Kent, U.S. Geological Survey, for his helpful review comments. We thank the three anonymous reviewers for their helpful comments, which significantly improved this manuscript. This project was supported by the U.S. Geological Survey Environmental Health Program. Public domain – authored by a U.S. government employee |
