S-band ferromagnetic resonance spectroscopy and the detection of magnetofossils
Abstract
Life on Earth is strongly associated with microbes and earliest evidence for their presence has been hypothesized from putative morphological microfossils in Archaean rocks of about 3.5 Ga. Geological records of microbial biota are sparse, because soft-bodied organisms that are expected to dominate natural environments do not preserve well. Magnetotactic bacteria (MTB) and their chemically stable magnetic remains, known as magnetofossils, have attracted considerable interest as proxy to infer microbial ecology during Earth's history. MTB form intracellularly ferrimagnetic particles encapsulated in membranes termed magnetosomes. These biominerals are organized along their [111] magnetic easy axes in chains that are stabilized by cytoskeletal protein filaments. The alignment of the easy axes causes pronounced magnetic interaction-induced shape anisotropy. Although the magnetic properties of MTB are well known, the detection of magnetofossils in geological samples remains ambiguous due to the decay of organic matter during diagenesis, which can critically effect the chain configuration and thus the anisotropy properties. We report the use of S-band ferromagnetic resonance spectroscopy (FMR) to compare the anisotropic properties of magnetite in chains of cultured intact MTB between 300 and 15 K with those of sediment samples of Holocene age in order to infer magnetofossils in a geological time frame. The spectrum of intact MTB at 300 K exhibits distinct uniaxial anisotropy, which becomes less pronounced upon cooling. Below the Verwey transition this anisotropy is nearly vanished mainly due to the change of direction of the easy axes from [111] to [100]. Magnetofossils in natural samples were detected by uniaxial anisotropy traits similar to those obtained from MTB above Verwey transition, which are indicative of chain configurations of the magnetite particles, generally aligned along the easy axes. Our comparative study emphasizes that essential information can be obtained from S-band FMR spectra, which offer even better performance than X-band for discovering magnetofossils, and this in turn can contribute towards strengthening our relatively sparse database for deciphering the microbial ecology during Earth's history.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMGP41A1101G
- Keywords:
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- 1505 GEOMAGNETISM AND PALEOMAGNETISM / Biogenic magnetic minerals;
- 1512 GEOMAGNETISM AND PALEOMAGNETISM / Environmental magnetism;
- 1518 GEOMAGNETISM AND PALEOMAGNETISM / Magnetic fabrics and anisotropy;
- 1594 GEOMAGNETISM AND PALEOMAGNETISM / Instruments and techniques