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

Abstract

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.

Chemical signature of magnetotactic bacteria

“…Magnetosome chains have remarkable magnetic properties (5,6,9), which have been used to identify bacterial magnetofossils in sediments. Although previous studies demonstrated that observations by electron microscopy and/or magnetic measurements could detect bacterial magnetofossils in natural samples (9)(10)(11)(12)(13), the chain structure is generally lost during sediment aging owing to degradation of organic matter assembling magnetosomes (9,14). This strongly complicates the identification of the bacterial magnetofossils.…”

Magnetofossil spike during the Paleocene‐Eocene thermal maximum: Ferromagnetic resonance, rock magnetic, and electron microscopy evidence from Ancora, New Jersey, United States

“…We reanalyzed sediments from one of their drill cores, from Ancora, New Jersey, using ferromagnetic resonance (FMR) spectroscopy, a technique recently adapted for the identification of fossil magnetotactic bacteria [Kopp et al, 2006]. The results, confirmed by electron microscopy, indicate that the magnetic properties of the clay are produced by magnetofossils, not the impact fallout condensate proposed by Kent et al This finding is the first identification of magnetofossils in ancient sediments by FMR and validates the use of the technique for this purpose.…”

“…[17] FMR spectroscopy is an electron spin resonance technique capable of assessing the magnetic anisotropy, magnetic interactions, and morphological heterogeneity of magnetic particles in bulk samples [Kopp et al, 2006]. It is thus ideally suited for distinguishing among (1) detrital magnetite, (2) isolated particles of equidimensional magnetite, and (3) magnetofossils, which are frequently elongate and often preserved in chains.…”

“…[14] We quantify FMR spectra using four empirical parameters: g eff , A, DB FWHM , and a [Kopp et al, 2006]. The effective g factor g eff is the g factor at which peak absorption occurs.…”

“…[13] FMR measurements were made using an X band Bruker ESP 300E EPR spectrometer [Kopp et al, 2006]. About 200-1000 mg of each freeze-dried sediment sample were loaded into quartz glass EPR tubes.…”