Could rock lumps on the ocean floor be a potential source of valuable metals?

The bottom of oceans, seas, and even some lakes around the world are littered with rock-like lumps shaped like thin crusts, flat discs, and round balls.

These curious deposits – called mineral concretions – appear to play a role in marine ecosystems by providing a hard surface for clams and other creatures to anchor to. They may also be a rich source of in-demand materials, such as rare earth elements. However, very little is known about what these lumps are made of, how they form, and how they change over time.

Researchers from the Geological Survey of Finland set out to change that. Geologist Joonas Wasiljeff and colleagues used the Canadian Light Source (CLS) at the University of Saskatchewan to study the structure and mineral content of lumps collected from the bottom of the Baltic Sea.

The team found that the different shapes and mineral compositions were heavily influenced by things like water movement, how much mud settles on the seafloor, and how much oxygen is in the water.

“They act like a sponge, absorbing anything that is in the water column,” he says.

The iron-rich crusts tend to form in shallow, turbulent water closer to shore and contain more terrestrial minerals like clays and micas, as well as Vanadium and some rare earth elements. The more manganese-rich discs and spheres form in calmer, deeper water and contain zinc, cobalt and an abundance of rare earth elements.

Wasiljeff and colleagues found that microbial processes play a bigger role in the formation of the disc- and sphere-shaped concretions than in the crust-shaped lumps, as the stronger currents in shallow water limit the availability of organic matter, which microbes feed on.

This study helps us understand how these metallic lumps form over time, and how long they may take to recover if they are harvested or disturbed, says Wasiljeff. It also provides insights into the importance of these formations to life on the sea floor, such as clams, and the integrity of the wider food web. That, he says, can help inform decisions on how and where to extract this resource through sea-bed mining.

“We yet don’t know enough that we can just go collect everything from the seafloor,” says Wasiljeff. “If we remove them that may have drastic impacts on the ecosystem that may not ever recover.”

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Wasiljeff, Joonas, Changxun Yu, Pasi Heikkilä, Yann Lahaye, Matti Kurhila, Wei‐Li Hong, Aivo Lepland, Sten Suuroja, Volker Liebetrau, and Joonas J. Virtasalo. "Mineral phases and growth conditions of morphologically diverse shelfal ferromanganese concretions." Geochimica et Cosmochimica Acta 400 (2025): 227-247. https://doi.org/10.1016/j.gca.2025.05.012

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