Reductive dissolution of biogenic magnetite

Abstract

Reductive dissolution of magnetite is known to occur below the Fe-redox boundary in sediments. In this study detailed processes associated with biogenic magnetite dissolution are documented. A sediment core from the Japan Sea was used for this purpose, in which reductive dissolution of magnetic minerals is known to start at depths of about 1.15 m and is mostly complete within a depth interval of about 0.35 m. Using first-order reversal curve diagrams, preferential dissolution of biogenic magnetite within this interval is estimated from the observation that a narrow peak that extends along the coercivity axis (central ridge), which is indicative of biogenic magnetite, diminishes downcore. Transmission electron microscopy is used to demonstrate that the sediments contain three magnetofossil morpho-types: octahedra, hexagonal prisms, and bullet-shaped forms. Within the reductive dissolution zone, partially etched crystals are commonly observed. With progressive dissolution, the proportion of bullet-shaped magnetofossils decreases, whereas hexagonal prisms become more dominant. This observation can be explained by the differences in resistance to dissolution among crystal planes of magnetite and the differences in surface area to volume ratios. Magnetofossil morphology may reflect the preference of magnetotactic bacterial lineages for inhabiting specific chemical environments in sediments. However, it could also reflect alteration of the original morphological compositions during reductive diagenesis, which should be considered when using magnetofossil morphology as a paleoenvironmental proxy.

Magnetotactic Bacterial Activity in the North Pacific Ocean and Its Relationship to Asian Dust Inputs and Primary Productivity Since 8.0 Ma

“…(1995) proposed that magnetic particles underwent reductive dissolution because of higher organic carbon fluxes in the weakly magnetic zone. Geochemical results support the inference of reductive dissolution (Dickens & Owen, 1996), which occurs in high‐productivity Pacific pelagic sediment intervals due to increased organic carbon deposition (Korff et al., 2016; Shin et al., 2018; Yamazaki, 2020).…”

Understanding the Role of Biogenic Magnetite in Geomagnetic Paleointensity Recording: Insights From Ontong Java Plateau Sediments

“…In total, more than 2,380 magnetofossils were identified in hundreds of TEM images. We recognized magnetofossils of three distinct morphologies: equant (cubo‐octahedra and hexagonal prisms where length‐to‐width ratios are close to one); elongated (hexagonal prisms and elongated octahedra); and bullet‐shaped magnetofossils (Akai et al., 1991; Bazylinski et al., 1994; Li et al., 2020; Yamazaki, 2020) (Figure 11). The bullet‐shaped morphology is prismatic with one end tapered and curved sides and is easily distinguished from the other magnetofossil types.…”