Combined use of magnetometry and spectroscopy for identifying magnetofossils in sediments

Abstract

[1] 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 B c and a feature with symmetric spread along the vertical B b 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.

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.…”

Searching for single domain magnetite in the “pseudo‐single‐domain” sedimentary haystack: Implications of biogenic magnetite preservation for sediment magnetism and relative paleointensity determinations

“… Egli et al [2010] also argued that the selectivity of FORC diagrams for magnetosome detection is unmatched by other magnetic techniques. Several studies have used FORC diagrams to argue for a dominance of sediment magnetic properties by magnetofossils [e.g., Abrajevitch and Kodama , 2009, 2011; Yamazaki , 2008, 2009, 2012; Kind et al , 2011; Roberts et al , 2011a; Larrasoaña et al , 2012], although only the studies of Roberts et al [2011a], Larrasoaña et al [2012], and Yamazaki [2012] have verified this interpretation with TEM observations of magnetic mineral extracts from the analyzed sediments. If magnetosome chains are disrupted and magnetite particles interact magnetostatically, the central ridge in the FORC distributions will be vertically spread, as is evident in the broader outer contours at ∼20 mT in Figure 4.…”

“…If magnetosome chains are disrupted and magnetite particles interact magnetostatically, the central ridge in the FORC distributions will be vertically spread, as is evident in the broader outer contours at ∼20 mT in Figure 4. Regardless, the presence of SD particles will still be evident in FORC diagrams [e.g., Kind et al , 2011]. Importantly, FORC diagrams enable discrimination of the presence of magnetofossils even when other magnetic minerals are present.…”

“…The broad spectra have two clear maxima at low applied fields and extend asymmetrically to low fields. The shapes of the measured spectra are associated with magnetically ordered minerals; the asymmetry and the fact that they are shifted to lower fields compared to non‐biogenic magnetite makes them distinctive of intact magnetite magnetosome chains [ Weiss et al , 2004; Kopp et al , 2006; Fischer et al , 2008; Charilaou et al , 2011; Kind et al , 2011; Gehring et al , 2011; Chang et al , 2012]. The parameters derived from the spectra [ Weiss et al , 2004; Kopp et al , 2006] (Figure 6f), are all indicative of intact magnetite magnetosome chains (Table 1; FMR parameters for the studied portions of holes 738B and 738C are provided by Roberts et al [2011a], Larrasoaña et al [2012], and Chang et al [2012]).…”

S-band ferromagnetic resonance spectroscopy and the detection of magnetofossils

“…asymmetry of the absorption spectra) permit a direct way to detect the anisotropy of magnetosome chains [11,[27][28][29][30][31][32]. Mastrogiacomo et al [33] showed that FMR experiments of MTB using microwave frequency of 4 GHz, i.e.…”