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

Abstract

Magnetotactic bacteria (MTB) preserved in sediments (magnetofossils) are highly sensitive to environmental and climatic forcing and can therefore be used as an easy-to-measure palaeo-environment proxy. It is known that magnetofossils occur with various morphologies – each with its own particular magnetic signature. We present the first systematic extensive micromagnetic study relating magnetosome chain morphology to their hysteresis parameters. We modelled more than 450 different intact magnetosome chains in the finite-element micromagnetic modelling software MERRILL, relating the morphology (magnetosome size, elongation, intra-chain spacing, and chain length) to the magnetic parameters of coercivity, coercivity of remanence, and saturation magnetization, and analyzing domain-states and switching modes. It is shown that magnetic properties are far dominated by the intra-chain spacing. However, analysis of a large set of published TEM images of different MTB strains shows a strong correlation between intra-chain spacing and grain size. This correlation can in principle be used to infer MTB morphology purely from magnetic measurements, providing a framework to use biogenic magnetite as a sensitive palaeo-environment proxy. Additionally, it is found that MTB carefully control intra-chain spacing during magnetosome synthesis to maximize saturation remanence as well as magnetic stability.