The Magnetization of Marine Sediments: A Search for Fossil Biogenic Magnetite

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In the past two decades, natural remanent magnetization carried by marine sediments and sedimentary rocks has been used extensively to monitor the history of the geomagnetic field and to constrain motion of crustal plates. But the origin of the magnetic minerals contributing to observed remanent magnetization is only now being resolved. Recent work suggests that crystals of magnetite precipitated by magnetotactic bacteria and perhaps other magnetotactic microorganisms contribute a large fraction to magnetization of marine sediments. Techniques now exist to extract these particles and identify them with transmission electron microscopy. Intact 'magnetofossil' chains are found in some DSDP sediments, and other biogenic magnetites have been extracted from Early Cambrian limestone. Continued study of bacterial magnetofossils may contribute to a better understanding of the process of fine-particle diagenesis as well as direct measurements of rock-magnetic effects of interparticle interaction. Bacterial magnetofossils may also provide an independent method of measuring the paleointensity of the geomagnetic field as recorded by marine sediments, and might also be useful indicators of primary magnetic remanence. Precipitation of magnetite in bacteria appears to be an oxygen-requiring process, so knowledge of their distribution in time and space may place constraints on the history of free oxygen at the Earth's surface.