Chuck Bednar for redOrbit.com – @BednarChuck
Iron, one of the most common elements in the Earth’s crust, was at least partially metabolized by ancient bacteria living along the continental shelves billions of years ago, a team of University of Wisconsin-Madison researchers claim in a new study published this week.
In the paper, which appears in the journal Proceedings of the National Academy of Sciences, UW geosciences professor Clark Johnson and former postdoctoral researcher Weiqiang Li looked at a group of samples from the banded iron formation in Western Australia. Banded iron, they said, is the iron-rich rock that can be found in ore deposits located all over the world.
These deposits can reach depths of up to 150 meters, the researchers said in a statement, and had long been believed to have originally entered the ocean from hot, mineral-rich water released a mid-ocean vents that then precipitated to the ocean floor. Now, however, the authors have found that half of it was actually metabolized by microbes living in the vicinity.
Banded iron formations are the primary source of iron ore worldwide. These rocks, at Soudan Underground Mine State Park, Minnesota, shows banding caused by layers of different minerals in a sample 2.7 billion years old. The study by Johnson and Li at UW-Madison showed that half of the iron in such rocks was metabolized by microbes before being deposited in rocks. (Credit: Courtesy of Clark Johnson)
While it had previously been thought that the banding of this iron represented seasonal changes of some type, Johnson and Li found long-term shifts in the composition, but no evidence of short term variations over periods such as decades of centuries. The team said that their discovery may impact a wide variety of different fields, ranging from mining to the search for alien life.
Research could help find life on other planets
The UW researchers started their analysis by conducting precise measurements of isotopes of iron and neodymium using lasers from the university’s geosciences department. These isotopes, which are atoms that differ only by weight, are often used by researchers to identify the source of various different samples, they explained.
They perfected the operation of the laser and the related mass spectrometry instruments over the course of three years, Li explained, and used light bursts less than one-trillionth of a second long to vaporize thin portions of the sample without actually causing it to heat up.
Previously, studies like this focused solely on iron isotopes, but Li said that he and Johnson opted to add neodymium to obtain more precise measurements of the amount that originated from “shallow continental waters that carried an isotopic signature of life.” They found that, in the absence of oxygen during the early Earth, some organisms metabolized iron for energy.
“These ancient microbes were respiring iron just like we respire oxygen,” Johnson said. “It’s a hard thing to wrap your head around, I admit.” He added that the research reveals the source of some of the planet’s minerals, and helps clarify some of the details of the evolution of life on Earth. It could also help scientists in the hunt for life on other planets, they added.
“It’s no accident that iron is an important part of life, that early biological molecules may have been iron-based,” he continued, adding that the findings reinforce the importance of microbes in geology. “This represents a huge change. In my introductory geochemistry textbook from 1980, there is no mention of biology, and so every diagram showing what minerals are stable at what conditions on the surface of the Earth is absolutely wrong.”
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