Magnetofossils, the Magnetization of Sediments, and the Evolution of Magnetite Biomineralization

Magnetite Fe3O4 is one of the most stable carriers of natural remanent magnetization (NRM) in sedimentary rocks, and paleomagnetic studies of magnetite-bearing sediments, such as deep-sea cores and pelagic limestones, have provided a detailed calibration between the biostratigraphic and magnetic polarity time scales. Despite this important role, there is as yet a very poor understanding of how ultrafine-grained (< 0.1 um) magnetite is formed, transported, and preserved in marine environments. A major conceptual advance in our understanding of these processes is the recent discovery that biogenic magnetite, formed by magnetotactic bacteria and/or other magnetite-precipitating organisms, is responsible for much of the stable magnetic remanence in many marine sediments and sedimentary rocks. Since these magnetite particles are of biogenic origin, they are termed properly magnetofossils (Kirschvink & Chang 1984). Magnetite biomineralization is now known in three of the five kingdoms of living organisms [Monera (Bacteria), Proctista (Protozoa), and Animalia]. To date, the known functions of biogenic magnetite and its min