Pengfei Xue1,*, Liao Chang2,3,**, Zhaowen Pei2 , and Richard J. Harrison4
1State Key Laboratory of Marine Geology, Tongji University, Shanghai, China
2Laboratory of Orogenic Belts and CrustalEvolution, School of Earth and Space Sciences, Peking University,
Beijing, China
3Laboratory for Marine Geology,Qingdao National Laboratory for Marine Science and Technology, Qingdao, China
4Department of Earth Sciences,University of Cambridge, Cambridge, UK
*Corresponding author.
**Corresponding author.
E-mail addresses: pengfeixue@tongji.edu.cn(P. Xue), liao.chang@pku.edu.cn (L. Chang)
Abstract
Biogenic magnetite crystals produced by magnetotactic bacteria (MTB) and associatedmagnetofossils in sediments are characterized by variable morphologies, grain sizes, and chain structures.Magnetofossils are widely used in paleomagnetic and paleoenvironmental studies, but the complexmagnetofossil shapes and particle arrangements significantly affect magnetic properties, hampering theirmagnetic detection and proxy interpretation. Here we perform coupled experimental and micromagneticmodeling analyses of typical magnetofossil‐rich sediments, where the effects of magnetofossil crystal forms andmicrostructures on magnetic properties can be quantitatively separated. Since the in situ magnetofossil chainstructures in sediments remain poorly known, we compare results from magnetic measurements andmicromagnetic simulations based on realistic magnetofossil shapes and grain size distributions. Our resultssuggest that bullet‐shaped magnetofossils certainly contribute to the biogenic hard (BH) coercivity componentwith a minor contribution from elongated prismatic particles, and collapsed equidimensional grains to thebiogenic soft (BS) component. Micromagnetic simulations with different collapse models of bullet‐shapedmagnetofossils produce variable FORC (first‐order reversal curve) central‐ridge contributions with similarcoercivity distributions. Sensitivity test suggests that samples containing different forms of magnetofossils canproduce the BH coercivity component if the proportion of the bullet‐shaped particles is more than ∼2%.Magnetofossil assemblages with a higher proportion of bullet‐shaped particles have higher coercivities,squareness ratios, and larger BH contents. Our data shed new light on understanding the origin of magnetofossilcoercivity components and the in situ magnetofossil microstructures in sediments, which is widely useful forinterpreting magnetofossil proxy signals in geological records.
Full Airticle: https://doi.org/10.1029/2023JB028501
Fig. (a) Schematic illustration of the workflow in this study to investigate the linkage between magnetofossil features and bulk magnetic properties. The example of coercivity component analysis is modified from Egli (2004). BS: biogenic soft; BH: biogenic hard. (b–d) Schematic diagrams for three types of microstructures in our simulations: (b) a straight magnetofossil chain (c = 0), (c) a fully collapsed magnetofossil chain (c = 1), and (d) isolated magnetofossil particles. In (b–d), modeling parameters are defined as chain bending (c), the number of particles in a chain (n), and particle gap (d). (e–f) An example of micromagnetic simulation of FORC diagrams using a modified version (Chang et al., 2019) of FORCulator (Harrison & Lascu, 2014): (e) geometry, (f) simulated FORCs (black curves) and IRM distribution (red curve), and (g) processed FORC diagram with the following modeling parameters c = 0, n = 10, and d = 5 nm.