Magnetic detection and characterization of biogenic magnetic minerals: A comparison of ferromagnetic resonance and first-order reversal curve diagrams

Biogenic magnetic minerals produced by magnetotactic bacteria occur ubiquitously in natural aquatic environments. Their identification and characterization are important for interpretation of paleomagnetic and environmental magnetic records. We compare two magnetic methods for their identification and characterization in a diverse set of sedimentary environments: ferromagnetic resonance (FMR) spectroscopy and first-order reversal curve (FORC) diagrams, constrained by transmission electron microscope observations. The advantages and limitations of each method are evaluated. FMR analysis provides a strong diagnostic indicator because of its ability to detect the strong shape anisotropy that arises from the biogenic chain architecture, but it can be obscured in mixed magnetic mineral assemblages. We develop a new FMR fitting approach that enables separation and characterization of biogenic components in natural samples. FMR spectral fitting on magnetofossil-bearing samples does not always reveal a strong signature of biogenic magnetite with <111>-aligned chains, in contrast to whole magnetotactic bacteria cells. This indicates that strictly <111>-aligned chains are not as common in magnetofossil assemblages, due to either chain collapse or different crystallographic axis orientations. FORC analysis provides an excellent tool for isolating the biogenic component as a "central ridge" signature with peak switching field distribution between 20 and 60 mT. We also analyzed tuff samples with similar FMR characteristics to biogenic magnetite chains, which can cause ambiguity. We propose a magnetic protocol to improve the robustness and efficiency of biogenic magnetite identification and past microbial activity in a wide range of environments.