Relating the physical characteristics of magnetofossils to their magnetic properties through micromagnetic modelling

Magnetosome chains produced by magnetotactic bacteria (MTB) are abundant magnetic minerals in sediments. They are not only excellent remanence recorders, but are also potentially important proxies for tracing environmental conditions. Not only is there a wide variety of MTB species, each with their particular physical characteristics, but it has also been observed that the physical characteristics of one particular species may evolve in response to environmental conditions such as temperature and oxygenation, etc. This evolution can easily be observed as a change in magnetic signals caused by the MTB, which in principle, provides a powerful tool for paleoenvironmental studies. Often such proxies can be used as a "black-box" relating magnetic signals (such as hysteresis parameters) to environmental parameters through empirical observations. The more appropriate way, however, is to establish (1) how environmental conditions affect the physical characteristics of the magnetosome chains such as particle size, geometry, chain length, inter-chain spacing, etc., and (2) how these give rise to the measured magnetic signals. The second question can be answered by using a micromagnetic model (MERRILL) to calculate hysteresis parameters for a large set of physically different magnetofossils. We present results that show the effect of physical parameters on the magnetic signature of magnetofossils, and how they can possibly be estimated from magnetic measurements. The model further allows to explain the origin of variations in magnetic parameters in terms of domain structure (which are not necessarily uniform for large magnetofossils) and switching modes. It is shown that switching modes, i.e. the paths taken by magnetic moment vectors during switching, depend critically on the physical parameters of the magnetosome chains, which in some cases might help to clearly distinguish different magnetosomes and their chain structures through magnetic measurements.