Mind the gap: Towards a biogenic magnetite palaeoenvironmental proxy through an extensive finite-element micromagnetic simulation

In situ magnetic identification of giant, needle-shaped magnetofossils in Paleocene–Eocene Thermal Maximum sediments

“…We note that TEM observations and the sedimentology of WL-A sediments (5) indicate that these alternative sources do not contribute to the magnetic signal in our specimens. Micromagnetic simulations of conventional magnetosome chains show that some chain configurations (i.e., >10 magnetosomes or with interparticle spacings ∼1 nm) may have coercivity tails that reach ∼180 mT (19,47), but these contributions are smaller than those of the needles described here. The uniaxial single-domain nature of this needle component is also identified in the FORC diagram by the larger field range of coherent single-domain rotation contributions (lower half plane) and by a small contribution (upper half plane) caused by flux closure annihilation (Fig.…”

Diversification of Iron‐Biomineralizing Organisms During the Paleocene‐Eocene Thermal Maximum: Evidence From Quantitative Unmixing of Magnetic Signatures of Conventional and Giant Magnetofossils

“…The physical interpretation of FORC‐PCA endmembers necessarily relies on the assumption that the properties of magnetic components within these endmembers remain relatively constant throughout the CIE event. For example, the coercivity of magnetosome chains depends on geometrical parameters such as particle spacing and the degree of chain bending (Berndt et al., 2020; Chang, Harrison, et al., 2018), which could vary with stratigraphic depth. Large variations of individual coercivity distribution components, however, are not expected: EM2 and EM3 have distributions and features that resemble coercivity components attributed to conventional magnetofossils in a variety of freshwater and marine sediments, which we explain below.…”

Influence of Magnetofossils on Paleointensity Estimations Inferred From Principal Component Analyses of First‐Order Reversal Curve Diagrams for Sediments From the Western Equatorial Pacific

“…This yields a large vertical spread in the direction of the B u axis on FORC diagrams, and hence terrigenous magnetic grains can be discriminated from magnetofossils (Channell et al., 2016; Roberts et al., 2012; Yamazaki & Ikehara, 2012; Yamazaki et al., 2020). Combined with micromagnetic simulations, it was recently revealed that the conditions of biogenic magnetite chains including chain bending and collapse, distance between biogenic magnetites in a chain, and the number of magnetite in a chain significantly influence the distributions of coercivity and magnetostatic interactions (Berndt et al., 2020; Chang et al., 2019).…”

“…It is considered that the FORC-PCA method can reduce the ambiguity of magnetofossil proportion estimations compared with the k ARM /SIRM ratio. Furthermore, information on bending and/or collapse of biogenic magnetite chains in sediments and differences in the morphology of biogenic magnetite can also be obtained from FORC diagrams (Berndt et al, 2020;Chang et al, 2019;Usui & Yamazaki, 2021;Wagner et al, 2021;Yamazaki et al, 2020), which may also influence RPI estimations. In this study, we use three sediment cores taken from the western equatorial Pacific, in which large differences in the proportion of magnetofossils to terrigenous magnetic minerals are expected.…”