Did marine life in the palaeocene use a compass?

A few years ago, mysterious particles of magnetite were discovered in marine sediments. These magnetite particles were exceptionally large – about 10-20 times larger than the ‘conventional magnetite magnetofossils’ that are made by magnetotactic bacteria for the purposes of passive orientation in the Earth’s field. Giant magnetofossils are observed in a variety of shapes, including needles, spindles, bullets and spearheads. To date, it is unknown which organisms were able to form these gigantic magnetite particles and for what purpose they were used. Although some giant magnetofossils resemble conventional magnetofossils in shape, their unusually large size was though to make them poorly optimised for the purpose of magnetic alignment alone. Instead, an accepted option is that some living beings used such magnetite particles primarily as a protective shield against predators due to their mechanical hardness, meaning that their magnetic properties did not play a major role. However, not all researchers are convinced by this theory.

A compass for sea creatures?

Sergio Valencia, a physicist at HZB, and palaeomagnetism researcher Richard J. Harrison from the University of Cambridge, UK, have now investigated an alternative hypothesis. They suspect that these creatures did indeed use the magnetic properties of these particles to help them navigate using the Earth's magnetic field by detecting small variations in intensity and direction of the field – a kind of in-built magnetic GPS system. To test this idea, it was necessary to map the three dimensional magnetic structure of the magnetofossils, enabling the magnetic energy and associated forces on the particle in the local Earth's magnetic field to be estimated.

Non-destructive investigation 

Harrison and Valencia examined a particle shaped like a spearhead, with a diameter of 1.1 µm and a length of 2.25 µm. It came from the team of Liao Chang, University Beijing, and was found in a marine sediment in the North Atlantic that is around 56 million years old. A major challenge was to examine the internal magnetic structure of this rather thick sample without slicing and destroying it as this modifies the magnetic domain structure. This could be achieved at the Diamond X-ray source in Oxford, UK, using a newly developed technique devised by Claire Donnelly at the Max Planck Institute for the Chemical Physics of Solids (MPI CPfS) in Dresden, Germany. The so-called pre-edge phase X-ray magnetic circular dichroism (XMCD) ptychography allowed them to visualise the magnetic domain structure within the full volume of the sample in a non-destructive way. "This was a truly international collaboration involving experts from different fields, all working together to shed light on the possible functionality of these magnetofossils," says Sergio Valencia, the principal investigator who initiated and coordinated the effort.

3D mapping of the sample

The team was able to map the entire sample volume in three dimensions and at high resolution. ‘With the help of magnetic vector tomography, all three components of the magnetisation could be reconstructed and spatially resolved throughout the entire volume of the grain with a resolution of a few 10 nm,’ says Valencia, emphasising: ‘Once we have set up the successor source BESSY III, such measurements could also be carried out in Berlin.’

Magnetic vortex detected

The results show that the magnetite particle contains a single magnetic vortex that reacts to spatial fluctuations in the Earth's magnetic field with strong forces and could thus provide an organism with an accurate way to map the Earth’s magnetic field intensity, enabling magnetic navigation.

‘Marine organisms, for example a fish, may have used this property for magnetic navigation,’ says Harrison. Even if the particles studied here originally developed as protective armour, it is possible that, in the course of evolution, its descendants also used these particles as a navigational tool.

Use of the Earth's magnetic field to navigate is a widespread phenomenon today, observed in molluscs, amphibians, fish, reptiles, birds and mammals. Giant magnetofossils have been found in sediments dating back as far as 97 Million years, providing fossil evidence that magnetoreceptive navigation developed at least that long ago.

Outlook on particles from Mars 

‘Iron-oxide particles resembling those made by some bacteria on Earth have been discovered on the Martian meteorite ALH84001, although their biological origin is strongly disputed. As we continue the search for evidence of life on Mars through sample return missions, we now have a method to investigate any new potential magnetofossils found and provide evidence to support or refute their biological origin,’ says Richard Harrison. ‘It would be very exciting to use this experimental technique to evaluate the morphological and magnetic fingerprints of those iron oxide particles. This could help in the search for evidence of past life,’ says Valencia.