Understanding the Role of Biogenic Magnetite in Geomagnetic Paleointensity Recording: Insights From Ontong Java Plateau Sediments. Issue 6 (29th May 2022)

• Record Type: Journal Article
• Authors: Li, Jiaxi; Yamazaki, Toshitsugu; Usui, Yoichi; Sagawa, Takuya; Kubota, Yoshimi; Kuroda, Junichiro
• Is Part Of: Journal of geophysical research. Volume 127:Issue 6(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-05-29
• Subjects: geomagnetic paleointensity -- silicate‐hosted magnetic‐mineral inclusion -- biogenic magnetite -- Ontong Java Plateau; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2022JB024387
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Marine sediments can preserve continuous paleomagnetic intensity records. Different magnetic minerals may acquire remanent magnetizations differently, so that compositional variations of magnetic mineral assemblages in sediments may hinder extraction of reliable relative paleointensity (RPI) records. To better understand this issue, we conducted a paleo‐ and rock magnetic study of a sediment core from the Ontong Java Plateau, western equatorial Pacific Ocean. RPI estimated by normalizing natural remanent magnetization (NRM) with anhysteretic remanent magnetization (ARM) decreases downcore with an inverse correlation with the ratio of ARM susceptibility to saturation isothermal remanent magnetization. This relationship indicates that the RPI signal weakens as the proportion of biogenic magnetite increases. The NRM–ARM demagnetization diagrams obtained have concave‐down curvature. These observations indicate that the RPI recording efficiency of the biogenic component is lower than that of the terrigenous component if we assume that the magnetizations of the high‐ and low‐coercivity windows are carried dominantly by biogenic and terrigenous components, respectively. This assumption is supported by first‐order reversal curve measurements, transmission electron microscope observations, low‐temperature measurements, and extraction of silicate‐hosted magnetic inclusions from the sediments. Previous studies have suggested that the RPI recording efficiency of biogenic magnetite is higher than that of the terrigenous component, which disagrees with our results. Different concentrations of silicate‐hosted magnetic inclusions in different sedimentary environments might explain this contradiction. We conclude that biogenic magnetite contributes to RPI records with lower efficiency than unprotected terrigenous magnetic minerals in the studied sediments. Changing biogenic magnetite proportions distort ARM‐normalized RPI estimations. Plain Language Summary: Magnetic minerals in marine sediments can preserve variations in the intensity of Earth's magnetic field (paleointensity) over geologic time, which is important for understanding how the field is generated within Earth's core. Paleointensity can be recorded by magnetic minerals with different recording efficiencies, which complicates recovery of reliable paleointensity records from sediments that contain different magnetic minerals. Our study of a sediment core from the western equatorial Pacific Ocean indicates that it contains two main types of magnetic minerals: biogenic magnetite produced by bacteria that align themselves with the Earth's magnetic field, and magnetic minerals produced by weathering of terrigenous sediments. The paleointensity record obtained from the sediment core weakens with increasing biogenic magnetite abundance in the sediments. We conclude that this biogenic magnetite records paleomagnetic intensity less efficiently than terrigenous magnetic minerals, which conflicts with the results of previous studies. This contradiction might be explained by differences in the concentrations of magnetic minerals embedded in nonmagnetic silicate minerals in our study area compared to those in previous studies. Key Points: ARM‐normalized relative paleointensity is inversely correlated with the proportion of biogenic magnetite in the studied sediments The estimated relative paleointensity recording efficiency of biogenic magnetite is lower than that of terrigenous magnetic minerals Varying abundances of magnetic inclusions might cause contrasting recording efficiencies for biogenic magnetite between different studies