Fossilized Fish Could Be Indicators of Rare-Earth Metal Deposits

Researchers have pinpointed how and why certain rare-earth metals are in specific locations, and it's all down to fish fossils.

Fossilized Fish Could Be Indicators of Rare-Earth Metal Deposits
Fossilized fish tooth Ohta et al./The University of Tokyo

Certain rare metals that are crucial to green industries turn out to have an intriguing origin. 

Scientists at the University of Tokyo in Japan have discovered that a large deposit of rare-earth metal near the island of Minamitorishima has a link with fossilized fish. 

As the fish fossilized, they accumulated crucial elements and these fossil beds turned into rich deposits of these metals. 

The findings were published in the journal Scientific Reports last Thursday.

Why are fossilized fish and rare-earth metals linked?

In their paper, the team of researchers explains that these ancient fish were drawn to specific locations due to climate change and specific kinds of undersea geology. As these ancient fish turned into fossils, an accumulation of valuable elements was gathered around them, and ultimately, their fossil beds became concentrated deposits of rare-earth elements and yttrium (REY).

This did not happen overnight, however. 

Fossilized Fish Could Be Indicators of Rare-Earth Metal Deposits
Depiction of what happened during the Eocen epoch as the fish fossilized, Source: Ohta et al./the University of Tokyo

"That story begins back in time in the Eocene epoch 34.5 million years ago, about halfway between now and the time of the dinosaurs," Junichiro Ohta, lead author of the study, said.

"At that time, several things happened that led to the REY deposit. Firstly, vast amounts of nutrients accumulated in the deep ocean. Secondly, the planet underwent cooling which altered sea currents, stirring up these nutrient deposits. The seamounts then caused upwellings of nutrients delivering them to the fish, which thrived as a result."

Typically, these REY deposits would remain diffuse. However, in the instance where fossillizing fish are present, these instead become incorporated into the fossil. 

The question of these fossils and these REY deposits being linked wasn't new information for the team. What the researchers were trying to figure out was exactly how and when these fossil deposits formed. Something they've now successfully uncovered. 



"I’m really pleased we made this discovery by looking at fragments of bones and teeth," Ohta said.

"It was a difficult but satisfying task dating the deposits by comparing fossils we uncovered against a database of fossils with known ages. Equally so was another way we dated the deposits, by measuring the ratio of osmium isotopes in seawater trapped in REY-rich mud and comparing those to established records."

According to Ohta, the Minamitorishima island deposits could be enough to satisfy the current global demand for hundreds of years to come. That said, it's not just a matter of digging a little and picking them up. The deposits lie over five kilometers (three miles) below sea level at the moment, and no current resource is able to dig that deep, unfortunately. 

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