Reductive dissolution of magnetites is known to occur below the Fe-redox boundary in sediment columns. This study aims to document the detailed processes of biogenic magnetite dissolution. A sediment core taken from the Japan Sea was used for this purpose, in which reductive dissolution of magnetic minerals are known to start at about 1.3 m in depth and mostly complete within an interval of about 0.3 m. Using first-order reversal curve diagrams, preferential dissolution of biogenic magnetites

more »

... thin this interval is estimated from the observation that a narrow peak extending along the coercivity axis (the central ridge), which is indicative of biogenic magnetite, diminishes downcore. Transmission electron microscopy shows that the sediments contain the three morpho-types of magnetofossils: octahedron, hexagonal prism, and bullet shaped. With the progress of reductive dissolution, the proportion of bullet-shaped magnetofossils decreases, whereas that of hexagonal prisms increases. For hexagonal prisms, {111} caps are often etched while {110} side faces are almost intact. These observations can be explained by the differences in resistivity against dissolution among crystal planes of magnetite. A previous study reported that the dissolution rate of (111) planes is higher than that of (110) planes. Hexagonal prisms elongate in the [111] direction and are wrapped with {110} side faces, whereas octahedral and bullet-shaped magnetofossils have larger proportions of surface areas with {111} faces. Magnetosome morphology may reflect preference of inhabiting magnetotactic bacterial lineage for chemical conditions in sediments. One should, however, be cautious for possible alteration of original morphological composition during reductive diagenesis when magnetofossil morphology is used as a paleoenvironmental proxy.