Magnetic anisotropy, magnetostatic interactions and identification of magnetofossils

Abstract

[1] Single-domain magnetite particles produced by magnetotactic bacteria (MTB) and aligned in chains, called magnetosomes, are potentially important recorders of paleomagnetic, paleoenvironmental and paleolife signals. Rock magnetic properties related to the anisotropy of magnetosome chains have been widely used to identify fossilized magnetosomes (magnetofossils) preserved in geological materials. However, ambiguities exist when linking magnetic properties to the chain structure because of the complexity of chain integrity and magnetostatic interactions among magnetofossils that results from chain collapse during post-depositional diagenesis. In this paper, magnetic properties of three sets of samples containing extracted magnetosomes of the cultured Magnetospirillum magneticum strain AMB-1 were analyzed to determine how chain integrity and particle concentration influence magnetic properties. Intact MTB and well-dispersed magnetosome chains are characterized by strong magnetic anisotropy and weak magnetostatic interactions, but progressive chain breakup and particle clumping significantly increase the degree of magnetostatic interaction. This results in a change of the magnetic signature toward properties typical of interacting, single-domain particles, i.e., a decrease of the ratio of anhysteretic remanent magnetization to the saturation isothermal remanent magnetization, decreasing in the crossing point of the Wohlfarth-Cisowski test and in the delta ratio between losses of field and zero-field cooled remanent magnetization across the Verwey transition, as well as vertical broadening of the first-order reversal curve distribution. We propose a new diagram that summarizes the Verwey transition properties, with diagnostic limits for intact and collapsed chains of magnetosomes. This diagram can be used, in conjunction with other parameters, to identify unoxidized magnetofossils in sediments and rocks.

Controlled cobalt doping in the spinel structure of magnetosome magnetite: new evidences from element- and site-specific X-ray magnetic circular dichroism analyses

“…As shown in figure 2b, the abrupt decrease in remanence around 100-110 K on the thermal decays of saturation remanence magnetization acquired at 2.5 T field at 10 K (SIRM 2.5T_10K ) after cooling in a field of 2.5 T (FC) or zero (ZFC) indicates the Verwey transition of magnetosome magnetite, as observed in previous studies [41]. The difference between the remanence losses of FC (d FC ) and ZFC (d ZFC ) curves upon warming across the Verwey transition, quantitatively described as d FC /d ZFC , is related to the chain configuration of magnetosomes [31,35,41]. As expected, the Verwey transition behaviours of the magnetosomes are distinctly different among the three cultures ( figure 2b,e,h).…”

Chemical signature of magnetotactic bacteria

“…It displays narrow size distributions and is in the magnetic stable single-domain range. Magnetosome chains have remarkable magnetic properties (5,6,9), which have been used to identify bacterial magnetofossils in sediments. Although previous studies demonstrated that observations by electron microscopy and/or magnetic measurements could detect bacterial magnetofossils in natural samples (9)(10)(11)(12)(13), the chain structure is generally lost during sediment aging owing to degradation of organic matter assembling magnetosomes (9,14).…”

Magnetically induced behaviour of ferritin corpuscles in avian ears: can cuticulosomes function as magnetosomes?

“…Derived magnetic susceptibility is generally x ¼ k 3 /(BcVT) and equals (2.5 + 0. The superparamagnetic magnetite exhibits a relatively strong magnetic moment, so we derived its low-field susceptibility from magnetization of dry magnetite sample at five temperature points (figure 4) above the Verwey transition, which is typically less than 123 K [31,32]. The low-field susceptibility is x ¼ q 4 /(m 1 T ) and equals (3 [33] and their packing in corpuscles characterized by factor P ¼ 0.7.…”