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Published July 30, 2004 | Supplemental Material
Journal Article Open

Ferromagnetic resonance and low-temperature magnetic tests for biogenic magnetite


Magnetite is both a common inorganic rock-forming mineral and a biogenic product formed by a diversity of organisms. Magnetotactic bacteria produce intracellular magnetites of high purity and crystallinity (magnetosomes) arranged in linear chains of crystals. Magnetosomes and their fossils (magnetofossils) have been identified using transmission electron microscopy (TEM) in sediments dating back to ∼510–570 Ma, and possibly in 4 Ga carbonates in Martian meteorite ALH84001. We present the results from two rock magnetic analyses—the low-temperature Moskowitz test and ferromagnetic resonance (FMR)—applied to dozens of samples of magnetite and other materials. The magnetites in these samples are of diverse composition, size, shape, and origin: biologically induced (extracellular), biologically controlled (magnetosomes and chiton teeth), magnetofossil, synthetic, and natural inorganic. We confirm that the Moskowitz test is a distinctive indicator for magnetotactic bacteria and provide the first direct experimental evidence that this is accomplished via sensitivity to the magnetosome chain structure. We also demonstrate that the FMR spectra of four different strains of magnetotactic bacteria and a magnetofossil-bearing carbonate have a form distinct from all other samples measured in this study. We suggest that this signature also results from the magnetosomes' unique arrangement in chains. Because FMR can rapidly identify samples with large fractions of intact, isolated magnetosome chains, it could be a powerful tool for identifying magnetofossils in sediments.

Additional Information

© 2004 Elsevier B.V. Received 19 March 2004; received in revised form 15 April 2004; accepted 22 April 2004. We thank H. Vali, Y. Roh, B. Bertani, D. Newman, M. Dubiel, B. Moskowitz, D. Lovley, G. Rossman, and A. Kappler for providing many of the samples used in this study. We also thank T. Bosak for comments on an earlier draft of this manuscript, and D. Griscom and B. Moskowitz for their thoughtful reviews. S.S.K. was supported by the Mars Instrument Development Project Program, A.K. by a Beckman Senior Research Fellowship, R.E.K. by an NSF Graduate Research Fellowship, and J.L.K. and B.P.W. by the NASA Exobiology Program and the NASA Astrobiology Institute. [KF]

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