Mysterious magnetic fossils provide climate clues from the past

There are fossils, found in ancient marine sediments and consisting of just a few magnetic nanoparticles, that can tell us a lot about the climate of the past, especially episodes of sudden global warming. Now researchers, including PhD student Courtney Wagner and associate professor Peter Lippert at the University of Utah, have found a way to collect the valuable information in those fossils without crushing the scarce samples into a fine powder. Their results are published in Proceedings of the National Academy of Sciences.

“It’s so nice to be part of a discovery like this, something that can be used by other researchers studying magnetofossils and intervals of planetary change,” Wagner says. “This work can be used by many other scientists, within and outside our specialized community. This is very exciting and rewarding.”

The name “magnetofossil” may be reminiscent of images of the X-Men, but the reality is that magnetofossils are microscopic bacterial iron fossils. Some bacteria make magnetic particles that are 1/1000 the width of a hair, which, when joined together in a cell in a chain, act like a nanoscale compass. The bacteria, called “magnetotactic bacteria,” can then use this compass to tune in to the Earth’s magnetic field and travel efficiently to their preferred chemical conditions in the water.

For a few periods in Earth’s past, at the beginning and middle of the Eocene from 56 to 34 million years ago, some of these biologically produced magnets grew to ‘gigantic’ sizes, about 20 times larger than typical magnetofossils, and in exotic shapes such as needles, spindles, spearheads and giant bullets. Because the bacteria used their magnetic supersense to find their favorite levels of nutrients and oxygen in the ocean water, and because the giant magnetofossils are associated with periods of rapid climate change and increased temperature on Earth, they can tell us a lot about ocean conditions during that rapid warming, and especially how those conditions changed over time.

Previously, extraction and analysis of these fossils required crushing the samples into a fine powder for imaging by electron microscopy. “The extraction process can be time-consuming and unsuccessful, electron microscopy can be costly, and the destruction of samples means they are no longer useful for most other experiments,” Wagner says. “Collecting and storing these samples requires specialized personnel, equipment and planning, so we want to preserve as much material as possible for additional studies.”

So Wagner, Lippert and colleagues, including Ramon Egli of the Central Institute for Meteorology and Geodynamics and Ioan Lascu of the National Museum of Natural History, found a different way. Using sediment samples collected in New Jersey, they designed a new way to perform an analysis called FORC (first order reversal curve) measurements. With these high-resolution magnetic measurements, they found that the magnetic signature of giant magnetofossils was distinctive – enough to use the technique in other samples to identify the fossils’ presence. “FORC measurements examine the response of magnetic particles to externally applied magnetic fields, allowing different types of iron oxide particles to be distinguished without actually seeing them,” says Egli.

“The ability to quickly find gigantic assemblages of magnetofossils in the geological record will help identify the origins of these unusual magnetofossils,” the researchers write, as well as the ecology of the organisms that formed them. This is important, Wagner says, because no known living organisms form giant magnetofossils today, and we still don’t know which organisms formed them in the past. “The organisms that produced these gigantic magnetofossils are utterly mysterious, but this leaves exciting avenues of exploration open for the future,” added Lascu.

But that aside, the information in magnetofossils helps scientists understand how oceans reacted to past climate changes – and how our current ocean might respond to ongoing warming.