Micromagnetic simulation of magnetofossils with realistic size and shape distributions: Linking magnetic proxies with nanoscale observations and implications for magnetofossil identification

Abstract

We build micromagnetic models to investigate the magnetic properties of biogenic magnetite – a common type of magnetic mineral that is responsible for recording a wide range of biological, geophysical and geological processes on earth. The geometry of modelled particles is based on realistic size and shape distributions from nanoscale observations. Systematic changes in microstructures of biogenic magnetite ensembles are built and their magnetic properties are calculated, which enables a quantitative and separate assessment of the effect of crystal morphology and chain structures. Although the same particle size and shape distributions are used in all calculations, simulation results document large variations in magnetic properties, i.e., wide distributions of coercivity ( B c = ∼ 10 –60 mT), coercivity of remanence ( B cr = ∼ 14 –81 mT), dispersion parameter ( DP = ∼ 0.1 –0.5), and skewness values ( S = ∼ 0.7 –1.1) due to variable degree of anisotropy and magnetostatic interactions. Previously, the commonly observed “biogenic soft” and “biogenic hard” components on biogenic magnetite-bearing samples were often interpreted to reflect crystal morphologies, and that the small DP values of coercivity distributions were an indication of narrow particle size distributions. Our simulations suggest that these speculations are not always the case and that magnetosome microstructures likely exert a dominant control over their magnetic properties. Our modelling results provide a new theoretical perspective on the magnetic properties of biogenic magnetite, which is important for understanding magnetic proxy signals from magnetofossils in a wide range of environmental and geological settings, and for the search for biogenic magnetite in terrestrial rocks and in extra-terrestrial materials.