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Magnetofossils are the fossil remains of magnetic particles produced by magnetotactic bacteria (magnetobacteria) and preserved in the geologic record. The oldest definitive magnetofossils formed of the mineral magnetite come from the Cretaceous chalk beds of southern England, while magneto fossil reports, not considered to be robust, extend on Earth to the 1.9-billion-year-old Gunflint Chert; they include the four-billion-year-old Martian meteorite ALH84001. Magnetotactic organisms are prokaryotic, with only one example of giant-magnetofossils, likely produced by eukaryotic organisms, having been reported. Magnetotactic bacteria, the source of the magneto fossils, are magnetite (Fe3O4) or greigite (Fe3S4) producing bacteria found in both freshwater and marine environments. These magnetotatic bacteria are found in the oxic-anoxic transition zone where conditions are such that oxygen levels are less than that found in the atmosphere (microaerophilic). Compared to the magnetite producing magnetotactic bacteria and subsequent magnetofossils, little is known about the environments in which greigite magnetofossils are created and the magnetic properties of the preserved greigite particles. Magnetotactic bacteria was first suggested in the 1960s, when Salvatore Bellini of the University of Pavia discovered a bacteria in a bog which appeared to align itself with the magnetic field lines of the Earth. It was after this that researchers began to think of the effect of magnetotactic bacteria on the fossil record and magnetization of sedimentary layers. These sedimentary layers are derived predominantly from marine environments. Although it has been suggested that these magnetofossils can be found in terrigenous sediments, sediments derived from terrestrial (as opposed to marine) environment. Episodes of high sedimentation, not correlating with an increase in magnetobacterial and thus magnetofossil production, can decrease magnetofossil concentrations vastly, although this is not always the case, as an increase in sedimentation normally coincides with an increase of land erosion, and therefore an increase in iron abundance and nutrient supply. == Magnetization == Within the magnetotactic bacteria, magnetite and greigite crystals are biosynthesized (biomineralized) within organelles called magnetosomes. These magnetosomes form chains within the bacterial cell and in doing so, provide the organism with a permanent magnetic dipole. The organism uses this resulting magnetization for geomagnetic navigation, to align itself with the Earth’s geomagnetic field (magnetotaxis) and to reach the optimal position along vertical chemical gradients.〔 When the organism dies the magnetosomes become trapped in sediments. Under the right conditions, notably if the redox conditions are correct, the magnetite can then be fossilized and therefore stored in the sedimentary record. The fossilization of the magnetite within the sediments contributes largely (depending on the concentration of magnetofossils) to the natural remanent magnetization of the sediment layer. The natural remanent magnetization is the permanent magnetism remaining in a rock or sediment after it has formed. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「magnetofossils」の詳細全文を読む スポンサード リンク
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