April 29th, 2015
Supernovae, or exploding stars, happen many light years away from earth. But the debris from these spectacular stellar explosions often make it to our planet’s surface. And now, for the first time, supernova debris has been found in the fossil record, where it had been taken up by bacteria.
Although iron has been on the earth’s floor for more than 2 billion years, scientists first discovered an isotope of iron, Iron-60, about 10 years ago. This type of iron comes only from space, and could only have been deposited by a supernova. At the annual meeting of the American Physical Society, Shawn Bishop, a geologist at the Munich Technical University, showed that iron-60 had been taken up about 2 million years ago by iron-consuming bacteria. When the bacteria died, the iron remained in the bacteria and both life form and metal became part of the fossil record.
Bishop’s findings are only preliminary; if verified, his work would be the first demonstration of a biological evidence of an exploding star. While nobody is exactly sure what star would have made this iron-rich contribution to earth, there is one suspect—stars in the Scorpio-Centaurus group, which lie about 424 light years from the Sun.
The iron-60 from the supernova likely met up with bacteria that had evolved to incorporate iron that had already been deposited on the ocean floor. About 3.5 to 2 billion years ago, as the oceans started covering the earth, iron oozed from cracks and fissures on the ocean floor. These pure iron ions were dissolved in seawater, and met with oxygen on the water’s surface. Thus oxygenated, they deposited on the floor as hematite or magnetite (literally, rust). Today, these iron deposited are routinely mined for ore, in order to manufacture steel.
As bacteria appeared, some species found that aligning themselves along the bands of rusted iron that had formed on the ocean floor provided the bacteria with more oxygen. The bacteria studied by Bishop were able to ingest magnetite, which literally provides a compass need for the bacteria. This needle points the bacteria to the optimal sources of oxygen.
It was these bacteria, not discriminating between isotopes of iron, that ingested the new iron-60 from the supernova. Bishop and his colleagues wondered if the bacteria had ever ingested the new iron. Since iron-60 has a half-life of 2.8 million years, the metal isotope was a natural for trying to match the supernova explosion with any earthly evidence. Bishop and his colleagues took sediment cores from the bottom of the Pacific Ocean, dating between 1.7 and 3.3 million years ago. By looking at samples every 100,000 years, they could see if any iron-60 was present. It was, but only in core samples from 2.2 million years ago.
While still preliminary and the first to show interstellar interactions with earth’s biology, the research shows how parts of the universe, even light years away, can make contact with the biological world.
Source: American Physical Society, Shawn Bishop
fossil trace, fossil paleontology