Geophysical signatures of disseminated iron minerals: A proxy for understanding subsurface biophysicochemical processes

• Published in: Journal of geophysical research. Biogeosciences 2014-09, Vol.119 (9), p.1831-1849
• Main Authors: Abdel Aal, Gamal Z., Atekwana, Estella A., Revil, A.
• Format: Article
• Language: English
• Subjects: Biogeochemistry; biophysicochemical processes; Biotransformation; complex conductivity; Conductivity; Geophysics; Grain size; Iron; Magnetic permeability; Magnetic susceptibility; Magnetism; Magnetite; Microorganisms; Mineralogy; Minerals; Particle size; Porous media; Pyrite; Quadratures; Signatures
• ISSN: 2169-8953; 2169-8961
• DOI: 10.1002/2014JG002659

Previous studies have linked biogeophysical signatures to the presence of iron minerals resulting from distinct biophysicochemical processes. Utilizing geophysical methods as a proxy of such biophysicochemical processes requires an understanding of the geophysical signature of the different iron minerals. Laboratory experiments were conducted to investigate the complex conductivity and magnetic susceptibility signatures of five iron minerals disseminated in saturated porous media under variable iron mineral content and grain size. Both pyrite and magnetite show high quadrature and inphase conductivities compared to hematite, goethite, and siderite, whereas magnetite was the highly magnetic mineral dominating the magnetic susceptibility measurements. The quadrature conductivity spectra of both pyrite and magnetite exhibit a well‐defined characteristic relaxation peak below 10 kHz, not observed with the other iron minerals. The quadrature conductivity and magnetic susceptibility of individual and a mixture of iron minerals are dominated and linearly proportional to the mass fraction of the highly conductive (pyrite and magnetite) and magnetic (magnetite) iron minerals, respectively. The quadrature conductivity magnitude increased with decreasing grain size diameter of magnetite and pyrite with a progressive shift of the characteristic relaxation peak toward higher frequencies. The quadrature conductivity response of a mixture of different grain sizes of iron minerals is shown to be additive, whereas magnetic susceptibility measurements were insensitive to the variation in grain size diameters (1–0.075 mm). The integration of complex conductivity and magnetic susceptibility measurements can therefore provide a complimentary tool for the successful investigation of in situ biophysicochemical processes resulting in biotransformation or secondary iron mineral precipitation. Key Points Iron minerals displayed different electrical and magnetic signaturesGeophysical signatures are proxy for biophysicochemical processesGeophysical signatures can locate the marine sulfate‐methane interface