Combined use of magnetometry and spectroscopy for identifying magnetofossils in sediments

Identification of the mineral remains of magnetotactic bacteria (MTB), known as magnetofossils, is of particular interest because their occurrence can be used for environmental and climatic reconstructions. Single‐domain magnetite particles, which are biomineralized in the cell body of MTB, have characteristic properties that can be used to detect their fossil remains. Acquisition of anhysteretic and isothermal remanent magnetization (ARM and IRM), first‐order reversal curve (FORC) diagrams, and ferromagnetic resonance (FMR) spectra were used to detect the magnetic mineral inventory in Holocene lake sediments. A comparative analysis in terms of the discriminatory power of these methods is presented. The FORC diagrams contain two distinct features: a sharp horizontal ridge centered on the horizontal axis Bc and a feature with symmetric spread along the vertical Bb axis. The coercivity spectra derived from the central ridge coincides with that derived from ARM and IRM acquisition curves and is compatible with the presence of noninteracting linear chains of single‐domain magnetite. The second feature on FORC diagrams is indicative of interacting particles in clusters. In the FMR spectra from bulk sediment, two populations are separated empirically based on the FORC information. An asymmetric signal is taken to describe the population, which contains single‐domain particles in clusters. Empirical spectral separation of this contribution results in FMR spectra that are similar to those of intact MTB, which strongly suggests that a fraction of linear magnetosome chains is present. Combination of FMR and FORC results demonstrates the strong potential of these methods for identifying magnetofossils, based on alignment and interaction patterns of magnetic particles. Key Points Switching field and anisotropy properties of magnetofossils Comparison of magnetometry and magnetic spectroscopy