Mystery of peculiar metal lumps on the ocean floor finally solved

14 Jan 2020

One of the deep-sea nodules. Image: Dietmar Müller/The University of Sydney

Deep-sea nodules that always stay on the top of the ocean floor have long perplexed geologists, until now.

With constantly shifting sands and sediments, ocean floors are always evolving in appearance. Yet one thing that has mystified geologists for decades has been deep-sea nodules – rare metallic elements found in clumps on the ocean floor.

No matter what happens around them, their tops always remain uncovered, which makes them quite a sight for those lucky enough to see them. Now, researchers have published a study to Geology explaining what they believe is behind this strange ability.

The deep-sea nodules are made from manganese, iron and other metals found in all the major ocean basins. They are among the slowest known geological processes, with the ringed concretions growing on average between 10mm and 20mm every million years.

Given their rate of growth, they should be quickly covered by moving sands. However, using machine learning to determine which factors control the nodules’ location, the researchers made a surprising discovery. It showed that, globally, the nodules occur in regions where the bottom current speeds are far too slow to move sediment. Instead, the nodules are associated with seafloor fauna.

Race to harvest them

“Organisms such as star fish, octopods and molluscs seem to keep the nodules at the seafloor surface by foraging, burrowing and ingesting sediment on and around them,” said lead author of the study, Dr Adriana Dutkiewicz of The University of Sydney.

“Although these organisms occur in relatively low concentrations on the abyssal seafloor, they are still abundant enough to locally affect sediment accumulation.”

This was supported by direct seafloor observations of nodule fields by independent studies. “Our conclusion is that deep-sea ecosystems and nodules are inextricably connected,” Dutkiewicz added.

The polymetallic lumps have been earmarked as a future source of rare-earth and other critical elements for mining in the future.

The elements found in the nodules are seen as essential for the development of technologies for low-carbon economies including solar cells, efficient wind turbine and rechargeable batteries.

“It is important that any mining of these resources is done in a way that preserves the fragile deep-sea environments in which they are found,” Dutkiewicz said.

Colm Gorey was a senior journalist with Silicon Republic