Magnetofossils

Magnetofossils are the fossil remains of magnetic particles produced by magnetotactic bacteria 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.

Though some magnetotactic bacteria produce greigite instead of or in addition to magnetite, no greigite magnetofossils have been reported. Because most magnetotactic bacteria live under specific redox conditions, several authors have proposed that a better-developed magnetofossil record could serve as a tracer for changing oxygen levels, both locally within sediments and at the planetary level.

References

Kopp, R. E. and Kirschvink, J. L. (2008). "The identification and biogeochemical interpretation of fossil magnetotactic bacteria". Earth Science Reviews 86: 42–61. doi:10.1016/j.earscirev.2007.08.001. http://resolver.caltech.edu/CaltechAUTHORS:KOPesr08. 

Chang, S. R. and Kirschvink, J. L. (1989). "Magnetofossils, the magnetization of sediments, and the evolution of biomineralization". Annual Review of Earth and Planetary Sciences 17: 169–195. doi:10.1146/annurev.ea.17.050189.001125.