By Michael Lucibella
At this year’s April Meeting, researchers reported seeing hints of an ancient supernova embedded in the remains of 2.2-million-year-old bacteria. Scientists in Germany extracted a radioactive element that only forms in a supernova from magnetic crystals that now-fossilized bacteria once used to orient themselves. This is the first time that researchers have been able to pull the rare isotope from the ancient magnetic crystals, an achievement that could turn into a useful tool for astro-archeologists.
The team found radioactive iron-60 in the fossilized bacteria under the deep ocean’s surface. Iron-60 is a rare and unstable element that naturally forms only in the fiery crucible of a supernova. With a half-life of about 2.7 million years, just about all traces of the isotope on Earth should have long decayed away. The fact that there is any at all indicates that the bacteria likely consumed the metal when Earth passed through a supernova’s debris cloud about 2.2 million years ago.
“That we’re here talking about it indicates that the supernova wasn’t too close,” said Shawn Bishop of the Technische Universität München, a member of the team that identified the iron.
The radioactive particles settled on the surface of the planet, and were subsequently absorbed by species of “magnetotactic” bacteria. They consume iron, concentrate it and forge microscopic chains of tiny magnetite crystals, which they use to keep themselves directionally oriented. If there is iron-60 in the environment, the bacteria absorb it along with other more common iron isotopes.
After the bacteria die, their crystals can be preserved in the mud at the bottom of the ocean. Over time layers of the dead bacteria pile on top of each other and solidify, leaving behind a fossil record with their magnetite crystals.
“If there’s iron-60 in these crystallized fossils from the time of the supernova, the fossils are still there and the iron-60 should still be in them,” Bishop said.
To test for the exotic iron, the team dissolved the fossilized bacteria in a chemical bath, and then bombarded the samples with a cesium ion beam from the tandem accelerator at the Maier-Leibnitz-Laboratory in Munich. The cesium bonds with the iron, and is carried along to a particle counter.
“We’re literally counting individual atoms of iron-60 that come out of the sample material,” Bishop said.
He said that it takes about 40 grams of sediment to get a single usable 3-milligram sample of iron. Within that iron sample, the concentration of iron-60 is minuscule, only about one in one quadrillion, hence the need to count individual atoms.
Though the data are promising, Bishop emphasized that the team was presenting raw data, and was not yet definitively claiming a discovery.
“The data are preliminary, they have not been published, and the work has not gone through any peer review. It is hot off the press, so to speak,” Bishop said. “Some fraction of the data was taken in January, and another fraction… was done just ten days ago.”
The sample that Bishop’s team has focused on so far came from four kilometers below the surface of the ocean, a few hundred miles off the coast of Ecuador. They are now looking at a much bigger core sample, and hope to improve the confidence level of their results.
Their findings fit with other indications of a possible ancient supernova at about the same time. According to astronomical evidence, there was likely a supernova in the Scorpius–Centaurus star cluster about 2.2 to 2.4 billion years ago. This fits with a previous set of iron-60 tests from 2004, in which researchers found a similar unusual concentration of the radioactive element in a naturally iron-rich layer of sediment in the Pacific Ocean.
Bishop said he was excited about the possibility of being able to extract the iron from bacteria fossils because iron-rich sediments are rare.
“The bacteria fossils are everywhere in the oceans,” Bishop said. “With this new avenue, especially if the second sediment core confirms what we see here, there’s a chance now to do some sampling in the Pacific and the Atlantic Oceans, looking to see if in fact the deposition of this material on the planet was uniform, or if it was concentrated in one area.”
He added that with some refinement of their technique, and using more samples, they may be able to glean some information about where supernovas were located.
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Staff Science Writer: Michael Lucibella