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Fossilized fish linked to rare earths deposits

Fossil fish tooth. (Image by Junichiro Ohta, courtesy of The University of Tokyo).

Scientists at the University of Tokyo discovered that a large deposit of rare earths near the Japanese island of Minamitorishima is linked to the existence of fossilized fish.

In a paper published in the journal Scientific Reports, the researchers explain that ancient global climate change and certain kinds of undersea geology drove fish populations to specific locations. As remains of the fish fossilized, they accumulated valuable elements and these fossil beds became concentrated deposits of rare-earth elements and yttrium (REY).

As the fish died and underwent fossilization, REY metals in the environment accumulated inside the fossils

“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 in a media statement. “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.”

Similar to the origins of crude oil deposits on land, the fossilized remains around Minamitorishima are the components responsible for the REY deposits.

As the fish died and underwent fossilization, REY metals in the environment, which would otherwise remain diffuse, accumulated inside the fossils.

The research group had previously made this fish-to-REY deposit connection, but how and when the fossil deposits formed was an open question until now.

“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 the researcher, the rare earth elements sources by Minamitorishima could sufficiently satisfy the current global demand for hundreds of years. However, getting to them may be extremely difficult as the deposit is over 5 kilometres below sea level and, at present, no resource has ever been commercially mined from such a depth.