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

Abstract

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.

Discrimination of biogenic and detrital magnetite through a double Verwey transition temperature

“…FORC diagrams for samples from zone 1 contain two major components: a strong FORC central ridge [ Newell , ; Egli et al ., ] and a FORC distribution with lower coercivity and large vertical spread (Figures g and h). The central ridge component is indicative of single domain (SD) biogenic magnetite in chains [ Egli et al ., ; Roberts et al ., ; Larrasoaña et al ., ; Heslop et al ., ; Chang et al ., , ]. The FORC component with large vertical spread is characteristic of pseudo–single domain (PSD) and multidomain (MD) magnetic particle assemblages [ Roberts et al ., , ].…”

“…This type of FORC diagram is characteristic of SD particle assemblages [ Roberts et al ., , ; Newell , ]. The peak coercivity of this SD component is ~10 mT, which is lower than that of typical biogenic magnetite samples [ Egli et al ., ; Roberts et al ., ; Chang et al ., ; Heslop et al ., ]. The other type of FORC diagram from zone 3 has two major components: a SD distribution and a large positive distribution in the lower left‐hand part (but not in the upper part; dashed box in Figure l).…”

Time‐Asymmetric FORC Diagrams: A New Protocol for Visualizing Thermal Fluctuations and Distinguishing Magnetic Mineral Mixtures

“…They have been described and characterized well by many studies (e.g., Berndt et al, 2015;Berndt, Paterson, et al, 2017;Chang et al, 2014;Jackson et al, 2006;Schlinger et al, 1988;Till et al, 2011). They have been described and characterized well by many studies (e.g., Berndt et al, 2015;Berndt, Paterson, et al, 2017;Chang et al, 2014;Jackson et al, 2006;Schlinger et al, 1988;Till et al, 2011).…”

The Blake Event recorded near the Eemian type locality – A diachronic onset of the Eemian in Europe

“…These samples, therefore, contain a mixture of non-interacting and interacting SD particles. With the non-interacting SD particles having a biogenic origin (Egli et al, 2010;Chang et al, 2014aChang et al, , 2014b. Both types of magnetic minerals are present throughout the stratigraphic column; they are not restricted to a specific zone of magnetic polarity.…”