Microbially Induced Anaerobic Oxidation of Magnetite to Maghemite in a Hydrocarbon‐Contaminated Aquifer. Issue 4 (11th April 2022)

• Record Type: Journal Article
• Authors: Ohenhen, Leonard O.; Feinberg, Joshua M.; Slater, Lee D.; Ntarlagiannis, Dimitrios; Cozzarelli, Isabelle M.; Rios‐Sanchez, Miriam; Isaacson, Carl W.; Stricker, Alexis; Atekwana, Estella A.
• Is Part Of: Journal of geophysical research. Volume 127:Issue 4(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-04-11
• Subjects: hydrocarbon contamination -- magnetite -- magnetization -- maghemitization -- anaerobic oxidation -- microorganisms; Geobiology -- Periodicals; Biogeochemistry -- Periodicals; Biotic communities -- Periodicals; Geophysics -- Periodicals; 577.14
• DOI: 10.1029/2021JG006560
• Languages: English
• ISSNs: 2169-8953

Abstract

Abstract: 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. Plain Language Summary: Iron is the fourth most abundant element in the Earth and can be cycled by microorganisms from one form to another often accompanied by the precipitation of different iron mineral phases. Some of these minerals have magnetic properties resulting in the magnetization of their host rocks. Some studies have documented that the magnetic minerals can be cycled by microorganisms resulting in the increase or loss of the magnetization. The mechanism by which this happens is not well understood. In this study, we conducted an experiment at a hydrocarbon‐contaminated site where iron cycling is occurring to better understand the cause of this loss of magnetization and the accompanying mineral changes associated with this process. Our results show the loss of magnetization is caused by the oxidation of magnetite (a highly magnetic mineral) to maghemite (a less magnetic mineral). This oxidation occurs under oxygen‐limiting conditions caused by microorganisms. However, the oxidation process does not account for all the loss in magnetization, leaving us to suggest dissolution may also be contributing to the loss of magnetization. Our results support that microorganisms may have the potential to mediate changes in the magnetic properties of Earth materials. Key Points: Fluctuations in magnetization may occur due to microbially induced iron mineral transformation in hydrocarbon‐contaminated aquifers Loss of magnetization is due to maghemitization (oxidation) and dissolution of magnetite grains Within the anoxic portions of the hydrocarbon plume, maghemitization occurs through anaerobic oxidation induced by microorganisms