Wikipedia
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 magnetofossil 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 magnetofossils, are magnetite (FeO) or greigite (FeS) producing bacteria found in both freshwater and marine environments. These magnetite bearing magnetotatic bacteria are found in the oxic-anoxic transition zone where conditions are such that oxygen levels are less than those 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.
Existence of magnetotactic bacteria was first suggested in the 1960s, when Salvatore Bellini of the University of Pavia discovered bacteria in a bog that appeared to align themselves with the magnetic field lines of the Earth. Following this discovery researchers began to think of the effect of magnetotactic bacteria on the fossil record and magnetization of sedimentary layers.
Most of the research concentrated on marine environments, although it has been suggested that these magnetofossils can be found in terrestrial sediments (derived from terrestrial sources). These magnetofossils can be found throughout the sedimentary record, and therefore are influenced by deposition rate. 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. An increase in sedimentation normally coincides with an increase of land erosion, and therefore an increase in iron abundance and nutrient supply.