Magnetic bacteria create compasses out of microscopic chains of orientated iron oxide so they can ride the Earth's magnetic field to better feeding grounds. They've been doing this for hundreds of millions of years, but at certain points in Earth's history, chains many times the size of modern versions appear – like finding the occasional titanosaur among lizards.
Although still tiny by our standards, the larger versions are called giant magnetofossils by geologists and can help us reconstruct the Earth's climatic history. A new technique allows us to read this story more cheaply, and without having to destroy the magnetic fossils in the process.
The early Eocene Epoch 56 million years ago is marked by magnetofossils some 20 times larger than those of other periods, although still just a 50th of the width of a human hair. The larger size is accompanied by compasses with shapes far more complex than the simple bars seen at other times. This sudden gigantism coincides with the last great surge in global temperatures (although still 170 times slower than modern times) the Paleocene Eocene Thermal Maximum. Similarly large magnetofossils also appeared during a less dramatic – but still substantial – subsequent warm spell, before disappearing again from the fossil record.
Although we don't know why hot conditions allow magnetic fossils to so drastically increase in size, the relationship offers an opportunity for paleoclimatologists to spot other warm eras we may have missed in the geologic record. Unfortunately, to do so it has been necessary to extract the fossils from the sediments in which they are found. "The extraction process can be time-consuming and unsuccessful, electron microscopy can be costly, and the destruction of samples means that they are no longer useful for most other experiments," said University of Utah PhD student Courtney Wagner in a statement. "Collection and storage of these samples require specialized personnel, equipment and planning, so we want to preserve as much material for additional studies as we can."
Wagner is part of a team that has announced a new method for studying magnetofossils in Proceedings of the National Academy of Sciences. Known as first order reversal curve (FORC) measurements, this allows them to distinguish giant magnetofossils from the more common sizes without impeding future research.
Wagner and co-authors applied FORC to layers of sediment taken from Wilson Lake, New Jersey, confirming the presence of giant magnetofossils coinciding with global hothouse events. The organisms that make them are still unknown, their cells not fossilizing like the metal they deposit, but the authors hope FORC may help us solve this mystery too.
The same edition of the journal contains another paper describing a way to read far more ancient climate records, dating back to before the first shelled animals. The ratio of oxygen isotopes in carbonate mud reveals the temperatures at the time it was deposited, they show, just as more recent temperature fluctuations are revealed from the isotopes in marine shells. From this, the researchers established that during the Cambrian Era – when animals were first truly establishing themselves – cooler periods were associated with the flourishing of animals, while warmer interludes saw microbial life strike back.