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Published July 15, 2006 | Accepted Version
Journal Article Open

Chains, clumps, and strings: Magnetofossil taphonomy with ferromagnetic resonance spectroscopy


Magnetotactic bacteria produce intracellular crystals of magnetite or greigite, the properties of which have been shaped by evolution to maximize the magnetic moment per atom of iron. Intracellular bacterial magnetite therefore possesses traits amenable to detection by physical techniques: typically, narrow size and shape distributions, single-domain size and arrangement in linear chains, and often crystal elongation. Past strategies for searching for bacterial magnetofossils using physical techniques have focused on identifying samples containing significant amounts of single domain magnetite or with narrow coercivity distributions. Searching for additional of traits would, however, increase the likelihood that candidate magnetofossils are truly of biological origin. Ferromagnetic resonance spectroscopy (FMR) is in theory capable of detecting the distinctive magnetic anisotropy produced by chain arrangement and crystal elongation. Here we present analyses of intact and lysed magnetotactic bacteria, dilutions of synthetic magnetite, and sedimentary samples of modern carbonates from the Great Bahama Bank, Oligocene–Miocene deep-sea muds from the South Atlantic, and Pleistocene lacustrine deposits from Mono Basin, California. We demonstrate that FMR can distinguish between intact bacterial magnetite chains, collapsed chains, and linear strings of magnetite formed by physical processes. We also show that sediments in which the magnetization is likely carried by bacterial magnetite have FMR spectra resembling those of intact or altered bacterial magnetite chains.

Additional Information

Author postprint. Published version -- Copyright © 2006 Elsevier B.V. Received 15 February 2006; revised 26 April 2006; accepted 1 May 2006. Editor: S. King. Available online 12 June 2006. We thank J. Grotzinger for access to the Bahamian samples, J. Ewing for field assistance in the Bahamas, G. Rossman, S. Kim, T. Raub, and two anonymous reviewers for helpful discussion, B. Brunschweig and A. Di Bilio for technical assistance with the MPMS and EPR spectrometer, respectively, and the Beckman Institute for funding the use of the MPMS. REK was supported by a NSF Graduate Research Fellowship and a Moore Foundation Fellowship, BPW by the NASA Mars Fundamental Research and NSF Geophysics Programs, ACM and CZN by the Agouron Institute, HV by the Natural Sciences and Engineering Research Council of Canada, and JLK by NASA Exobiology grant NAG5-10618 and the NASA Astrobiology Institute.

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