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

Magnetotactic bacteria

Magnetotactic bacteria are a polyphyletic group of bacteria discovered by Richard P. Blakemore in 1975, that orient along the magnetic field lines of Earth's magnetic field. To perform this task, these bacteria have organelles called magnetosomes that contain magnetic crystals...

 and preserved in the geologic record. The oldest definitive magnetofossils formed of the mineral magnetite

Magnetite

Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...

 come from the Cretaceous

Cretaceous

The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 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

Gunflint Chert

The Gunflint chert is a sequence of banded iron formation rocks that are exposed in the Gunflint Range of northern Minnesota and northwestern Ontario along the north shore of Lake Superior. The black layers in the sequence contain microfossils that are 1.9 to 2.3 billion years in age. Stromatolite...

; they include the four-billion-year-old Martian meteorite ALH84001

ALH84001

Allan Hills 84001 is a meteorite that was found in Allan Hills, Antarctica on December 27, 1984 by a team of U.S. meteorite hunters from the ANSMET project. Like other members of the group of SNCs , ALH 84001 is thought to be from Mars. However, it does not fit into any of the previously...

.

Though some magnetotactic bacteria produce greigite

Greigite

Greigite is an iron sulfide mineral with formula Fe3S4. It is the sulfur equivalent of the iron oxide magnetite . It was first described in 1964 for an occurrence in San Bernardino County, California, and named after the mineralogist and physical chemist Joseph W...

 instead of or in addition to magnetite, no greigite magnetofossils have been reported. Because most magnetotactic bacteria live under specific redox

Redox

Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

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.