Iron atoms spat out by a distant stellar explosion have been found in magnetic crystals produced by bacteria two million years ago – and may have played a role in a mass extinction at the time.
The work, published in the Proceedings of the National Academy of Sciences by Peter Ludwig from the Technical University of Munich and colleagues in Germany and Austria, adds support to two studies published earlier this year that found stellar remnants in ancient Earth and moon rocks from the same period.
When stars more than 10 times the mass of the sun die, they collapse then explode in a cataclysmic blast that spews heavy atoms into the galaxy.
Among them are iron-60 atoms. While the common, stable form of iron has 26 protons and 30 neutrons in its nucleus – so is called iron-56 – a supernova can also produce a form with four extra neutrons into the nucleus, iron-60.
Iron-60 is rare on Earth. It decays with a half-life of 2.6 million years into lighter elements, so any iron-60 leftover from the Earth’s formation 4.5 billion years ago has long disappeared. And there’s no mechanism for it to be produced on the planet today, so any found in the fossil record must have come from outside the solar system.
This doesn’t necessarily mean iron-60 rained on Earth – it could well have arrived on micrometeorites and mixed with rocks. So Ludwig and his colleagues set about finding these supernova signatures in biogenic sources to try to clear this up.
Some bacteria called magnetotactic bacteria sequester iron to produce chains of magnetite nanocrystals called magentosomes. Their preferred habitat is a few centimetres into sediment underwater.
As more sediment is laid on the lake or ocean floor, the bacteria are forced to move up too. And when they die, the cell decays and the nanocrystals are left behind, embedded like a column in the sediment.
Over time, the sediment and nanocrystals fossilise. Ludwig and his colleagues realised they would also lock away any iron-60 supernova signals at the time too.
They picked two sediment cores drilled from the equatorial Pacific Ocean floor. Powerful microscopes confirmed magnetofossils were present in the cores and the layers were dated.
Iron-60 was leached from the rock using mild chemicals, but in a way that only dissolved iron in nanocrystals. It left any larger micrometeorite grains that may have been in the samples intact.
The concentrations of iron-60 were so tiny they had to use an ultrasensitive accelerator mass spectrometer, which fires atoms at super high speeds and counts them.
From eight to around 2.6 million years ago, iron-60 levels were consistently low. But then came a distinct spike which peaked around two million years ago, then dropped over the next 500,000 years to pre-spike levels all the way to the present day.
They attribute this jump in iron-60 to exploding stars, not micrometeorites, because magnetotactic bacteria harvest iron from hydroxides – not from silicate or magentite grains found in micrometeorites. And their protocol for extracting the iron-60 from the drill cores only targeted nanocrystals, not micrometeorite grains, so that spike in iron-60 must have been non-meteoric in origin.
How could distant stars affect life on Earth? No one knows for sure, but studies show around 20 supernovae have exploded within our part of the galaxy in that past 10 million years.
The time span covered by this most recent work coincides with a mass extinction of marine life.
Perhaps a shower of cosmic rays from nearby supernovae bumped and jostled molecules in our atmosphere, depleting the ozone layer to allow damaging UV-B radiation from the sun to wash over the planet and causing global cooling.