Classification of a Complexly Mixed Magnetic Mineral Assemblage in Pacific Ocean Surface Sediment by Electron Microscopy and Supervised Magnetic Unmixing

Abstract

Unambiguous magnetic mineral identification in sediments is a prerequisite for reconstructing paleomagnetic and paleoenvironmental information from environmental magnetic parameters. We studied a deep-sea surface sediment sample from the Clarion Fracture Zone region, central Pacific Ocean, by combining magnetic measurements and scanning and transmission electron microscopic analyses. Eight titanomagnetite and magnetite particle types are recognized based on comprehensive documentation of crystal morphology, size, spatial arrangements, and compositions, which are indicative of their corresponding origins. Type-1 particles are detrital titanomagnetites with micron- and submicron sizes and irregular and angular shapes. Type-2 and -3 particles are well-defined octahedral titanomagnetites with submicron and nanometer sizes, respectively, which are likely related to local hydrothermal and volcanic activity. Type-4 particles are nanometer-sized titanomagnetites hosted within silicates, while type-5 particles are typical dendrite-like titanomagnetites that likely resulted from exsolution within host silicates. Type-6 particles are single domain magnetite magnetofossils related to local magnetotactic bacterial activity. Type-7 particles are superparamagnetic magnetite aggregates, while Type-8 particles are defect-rich single crystals composed of many small regions. Electron microscopy and supervised magnetic unmixing reveal that type-1 to -5 titanomagnetite and magnetite particles are the dominant magnetic minerals. In contrast, the magnetic contribution of magnetite magnetofossils appears to be small. Our work demonstrates that incorporating electron microscopic data removes much of the ambiguity associated with magnetic mineralogical interpretations in traditional rock magnetic measurements.

Diversification of Iron‐Biomineralizing Organisms During the Paleocene‐Eocene Thermal Maximum: Evidence From Quantitative Unmixing of Magnetic Signatures of Conventional and Giant Magnetofossils

“…We also identified rounded, sometimes teardrop‐shaped crystals in each extract (Figure 3). These crystals are scarce (<5 crystals observed per extract) and do not fit any of our criteria for magnetofossil identification, therefore, they are unlikely to be significant magnetic carriers (Li, Liu, Liu, et al., 2020). We note that similar particles were described as small spearhead magnetofossils in one study (Kopp, Schumann, et al., 2009) and larger (>63 µm), similar‐looking particles were described as impact spherules in another study (Schaller et al., 2016).…”

Low‐Temperature Magnetic Properties of Marine Sediments—Quantifying Magnetofossils, Superparamagnetism, and Maghemitization: Eastern Mediterranean Examples

“…Extraction and characterization of magnetic minerals from sediments were performed following Liu, et al. (2020). TEM analysis was conducted at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGG‐CAS) using a JEM‐2100HR TEM with a LaB 6 gun and 200 kV accelerating voltage.…”

Non‐Chained, Non‐Interacting, Stable Single‐Domain Magnetite Octahedra in Deep‐Sea Red Clay: A New Type of Magnetofossil?

“…The contribution of C3 does not show a major change with depth. A component with low B h and large DP similar to C3 has often been observed in pelagic sediment, including red clay, siliceous ooze, and carbonate (e.g., Li, Liu, et al., 2020; Roberts et al., 2013; Yamazaki, 2012; Yamazaki & Shimono, 2013). The origin of this component is not clear, although it is generally attributed to detrital minerals.…”