Yoichi Usui: Inverse Magnetic Susceptibility Fabrics in Pelagic Sediment
Channel: Institute for Rock Magnetism
Date: 2021-06-02

12th Institute for Rock Magnetism Conference 

Lightning Talk

Title: Inverse Magnetic Susceptibility Fabrics in Pelagic Sediment - Implications for Alignments of Magnetofossils and Sedimentary Particles

Author: Yoichi Usui

Institution:
Japan Agency for Marine-Earth Science and Technology

Abstract:
Anisotropy of magnetic susceptibility (AMS) is widely used to characterize rock and sediment fabrics. AMS studies usually assume that maximum susceptibility coincides with the long axis of ferromagnetic particles. Stable single‐domain (SSD) particles are well-known exceptions for this assumption, which exhibit minimum susceptibility along with their elongation (inverse fabric). Nevertheless, SSD particles are considered to be rare, and inverse AMS fabrics have also been regarded as exceptions. In this contribution, we report inverse AMS fabrics in pelagic clay from the western North Pacific (Usui et al., 2019). Rock magnetic and microscopy data identified magnetofossils as the dominant ferrimagnetic grains in the sediment. We found the inverse AMS fabrics a ∼2 m zone in piston cores of ~15 m in lengths. We modeled the stratigraphic change of AMS fabrics as mixing between an inverse fabric due to magnetofossils and a normal fabric corresponding to terrigenous minerals. The corrected degree of AMS carried by magnetofossils was estimated to be Pj∼1.01. Terrigenous minerals in the sediment were estimated to have a higher degree of anisotropy (Pj ~ 1.1) than the typical bulk AMS of pelagic sediment. We conclude that the fabric intensity of bulk sediment is significantly affected by biogenic magnetite even when the AMS is "normal." The widespread occurrence of biogenic magnetite warns that their inverse fabric contribution may complicate the quantitative interpretation of AMS. Our data also constrain the detectability of magnetofossil alignment in sediments.


Usui, Y., Yamazaki, T., Oka, T., & Kumagai, Y. (2019). Inverse magnetic susceptibility fabrics in pelagic sediment: Implications for magnetofossil abundance and alignment. Journal of Geophysical Research: Solid Earth, 124, 10672– 10686. https://doi.org/10.1029/2019JB018128