Fossil worm dung shows traces of microscopic 'miners' in elemental silver
When one organism's trash contains traces of human treasure, look for ancient microbial activity, according to researchers who found elemental silver in fossilized worm dung.
The researchers found the silver in coprolites, or fossilized feces, collected from the Ravens Throat River Lagerstätte in Canada's Northwest Territories. A lagerstätte is a deposit of exceptionally preserved fossils that sometimes includes fossilized soft tissues, or in this case fossilized worm dung. The fossils date to the Cambrian geologic period more than 500 million years ago. Today the site sits in a cold, mountainous area, but in the Cambrian period it was located near the equator and submerged under the ocean.
"North America looks different now than it did in the Cambrian period," said Julien Kimmig, assistant research professor in Penn State's Earth and Environmental Systems Institute. "Continental plates have moved a lot since then. Nowadays the Ravens Throat River Lagerstätte is in the middle of the Mackenzie Mountains, but during the period we're studying, we're looking at a deeper shelf environment completely underwater."
At first, Kimmig and his colleague, Brian Pratt, professor of geological sciences at the University of Saskatchewan, were unsure of the producer of the feces, until they split and sliced through their rock samples and came across fossilized worms and worm burrows. They realized that the flaky black patches they had were worm feces that had been preserved in burrows made deep under the ocean.
The pair analyzed the coprolites under a scanning electron microscope and found what Kimmig calls "the usual culprits"—carbon, pyrite and aluminum silicates.
"And then something really shiny popped up in the middle of the screen of our instrument, and when we looked at it, it turned out to be elemental silver," he said.
The researchers examined the surrounding rock for elevated amounts of silver and found some, but not enough to account for the big blobs of silver found in the coprolites.
"If you look at silver deposits, usually you find other elements associated with silver, like lead and zinc," Kimmig said. "We didn't see elevated amounts of these elements at our site, so there were different mechanisms at work behind the creation of this deposit compared to ore deposits. The Mackenzie Mountains have some rich ore deposits, and there are several mines in the region, but none has a composition of elevated silver without elevated levels of another metallic element."
To make sense of their findings, Kimmig began looking at studies of how bacteria can extract gold and silver from mine drainage as well as from natural habitats. He found that silver formation has also been linked to bacteria, fungi and algae. The researchers soon realized that microbial activity likely played a large part in the accumulation of silver in the coprolites.
"We likely had the poop first, then we had some bacteria or algae growing on the poop, and some of those were likely leaching silver out of the water column," said Kimmig. "To form the biggest piece of silver we found, which measures 300 micrometers, the microbial colony must have been a relatively decent size."
For comparison, the width of a human hair is roughly 17 to 180 micrometers. It may not sound like much, but 300 micrometers of silver seen under a microscope stands out, especially given the low amounts of silver in the surrounding rock.
The researchers think the silver either came from the water column or, more likely, a brine from the bottom of the ocean.
Some modern organisms—like certain bacteria, fish and oysters—can live with a degree of elevated silver levels in the environment, but it's still extremely toxic, and the metabolic systems dealing with this are poorly understood, according to Kimmig. Some modern microorganisms are useful in extracting noble elements like silver and gold, and scientists have observed similar behavior in the geologic past with bacteria and iron deposits, but they had yet to observe these processes through the geologic past in relation to silver.
"This is an evolutionary story for bacteria and microorganisms," said Kimmig. "Seeing in the Cambrian period that microorganisms were somehow able to accumulate silver suggests that it's a much older trait than what we might have thought beforehand when we just looked at modern microorganisms that do it. It might also indicate that while fluid flow plays a big role in the formation of ore deposits, some ore deposits might have had bacterial help, and these microorganisms could have played a major part in creating some of our bigger silver or gold deposits in the geologic past."
The researchers reported their findings in the Canadian Journal of Earth Sciences.
More information: Julien Kimmig et al, Evidence for microbially mediated silver enrichment in a middle Cambrian Burgess Shale-type deposit, Mackenzie Mountains, northwestern Canada, Canadian Journal of Earth Sciences (2021). DOI: 10.1139/cjes-2021-0035
Journal information: Canadian Journal of Earth Sciences
Provided by Pennsylvania State University