Geochemistry, Geophysics, Geosystems

Cover image for Vol. 16 Issue 12

Impact Factor: 2.923

ISI Journal Citation Reports © Ranking: 2014: 19/79 (Geochemistry & Geophysics)

Online ISSN: 1525-2027

Remanent magnetism of sediment governs magnetofossil alignment

Most bacteria navigate by reacting to different chemical signals in their surroundings, but some bacteria have another navigational tool in their arsenal—the Earth’s magnetic field. Nestled inside these magnetotactic bacteria (MTB) are organelles called magnetosomes, filled with tiny magnetic crystals and arranged in chains, which form nano-sized compass needles. When MTB die and degrade, these tiny crystals can remain in sediment and eventually become magnetic fossils called magnetofossils.

In water, MTB demonstrate the ability to navigate using the Earth’s magnetic field, but little is known about how MTB navigate in the upper layers of marine and freshwater sediment. Without knowing how they navigate, it is difficult to determine how MTB thrive in sediment and how resulting magnetofossils would accurately record the Earth’s past magnetic fields.

Egli et al. present the first observations of sediment-dwelling MTB—they took sediment samples from lakes and ponds in Germany, exposed the MTB to different magnetic fields, and observed their behavior. They found that MTB have only a very weak alignment to the Earth’s magnetic field in their natural environment of sediment. However, the authors used mathematical simulations to show that MTB could still use the magnetic field to navigate to preferred living depths within sediment.

The authors also simulated the process by which magnetofossil orientations align with the Earth’s magnetic field in the uppermost layer of sediment, where they form. This magnetization reflects the direction and inclination of the Earth’s magnetic field lines at the time of magnetofossil formation and eventually becomes “frozen” as magnetofossils diffuse into the “historical layer” of sediment, where little mixing occurs. The authors note that this is a first step in understanding how fossilized MTB could potentially be used in paleomagnetism research.

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