HR: 16:30h
AN: B24A-03    [Abstracts]
TI: Identification of Magnetosome Chains by Ferromagnetic Resonance Spectroscopy
AU: * Kopp, R E
EM: rkopp@caltech.edu
AF: California Institute of Technology, 1200 E California Blvd., Pasadena, CA 91125 United States
AU: Kirschvink, J L
EM: kirschvink@caltech.edu
AF: California Institute of Technology, 1200 E California Blvd., Pasadena, CA 91125 United States
AU: Weiss, B P
EM: bpweiss@mit.edu
AF: Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139 United States
AB: Magnetotactic microbes require both the geomagnetic field and environmental redox gradients to survive. Magnetofossils, the fossilized remains of the chains of magnetic minerals produced by magnetotactic microbes, are therefore a potential archive of information about ancient environmental conditions and microbial ecology. These minerals have attributes, such as size, shape, purity, and arrangement, that have been optimized by evolution and are therefore distinguishable from magnetic minerals produced by other biotic and abiotic processes. Ferromagnetic resonance spectroscopy (FMR) is a promising tool for screening for sediments likely to contain intact magnetofossil chains. Ferromagnetic resonance is a behavior observed in the microwave absorption spectrum of magnetic materials. In a magnetic field, the Zeeman effect leads to a splitting of the energy level of spin states, which causes the material to absorb microwaves with an energy equal to the splitting energy. Inside a ferromagnetic crystal, strong magnetic fields are produced by three sources: (1) the crystallographic structure (magnetocrystalline anisotropy), (2) shape asymmetry (shape anisotropy), and (3) stress resulting from crystal lattice defects (magnetoelastic anisotropy). Materials with strong magnetic anisotropy therefore have distinctive FMR spectra. To maximize their magnetic stability in the Earth's field, magnetosomes have high anisotropies. In particular, magnetosomes are often elongated along the magnetocrystalline easy axis, which causes the shape anisotropy produced by the elongation to complement the magnetocrystalline anisotropy. Moreover, they are aligned in chains. The magnetic moments of the crystals in a chain sum together, so that in certain respects the crystals act as a single needle-like crystal with extremely high shape anisotropy. Previous work has shown that these distinctive traits are likely responsible for the distinctive FMR spectra of magnetotactic bacteria [1]. Analysis of diluted and mixed samples of magnetosomes and artificial magnetite indicate that FMR is sensitive to the presence of pure magnetosome chains in a sample with a saturation magnetization of 10^-9 Am². Preliminary results from deep sea sediments suggest that FMR is capable of identifying the presence of magnetosomes in natural sediments. We will report additional results from natural sediments and experimental mixtures. 1. B.P. Weiss et al (2004), EPSL 224: 73-89.
DE: 1505 Biogenic magnetic minerals
DE: 1512 Environmental magnetism
DE: 1540 Rock and mineral magnetism
SC: Biogeosciences [B]
MN: Fall Meeting 2005