How Nodules Stay on Top at the Bottom of the Sea
Boulder, Colo., USA: Rare metallic elements found in clumps on the
deep-ocean floor mysteriously remain uncovered despite the shifting sands
and sediment many leagues under the sea. Scientists now think they know
why, and it could have important implications for mining these metals while
preserving the strange fauna at the bottom of the ocean.
The growth of these deep-sea nodules—metallic lumps of manganese, iron, and
other metals found in all the major ocean basins—is one of the slowest
known geological processes. These ringed concretions, which are potential
sources of rare-earth and other critical elements, grow on average just 10
to 20 millimeters every million years. Yet in one of earth science’s most
enduring mysteries, they somehow manage to avoid being buried by sediment
despite their locations in areas where clay accumulates at least 100 times
faster than the nodules grow.
Understanding how these agglomerations of metals remain on the open sea
floor could help geoscientists provide advice on accessing them for
industrial use. A new study published this month in Geology will
help scientists understand this process better.
“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,” said
, an ARC Future Fellow in the School of Geosciences at The University of
Rare-earth and other critical elements are essential for the development of
technologies needed for low-carbon economies. They will play an
increasingly important role for next-generation solar cells, efficient wind
turbines, and rechargeable batteries that will power the renewables
Solving the Enigma
From scouring bottom currents to burrowing animals, researchers have
proposed a number of mechanisms to account for this enigma. But solving it
depends on a better understanding of where the nodules are situated and the
environmental conditions that prevail there. Now a global study published
in Geology uses predictive machine learning to investigate which
factors control the location of polymetallic nodules. The results offer new
insights to inform deep-sea mineral exploration as well as its regulation.
“The International Seabed Authority
is currently preparing new environmental regulations to govern deep-sea
mining,” Dr. Dutkiewicz said. “Our analysis represents a global,
data-driven synthesis to impartially inform these policies and deep-ocean
Dr. Dutkiewicz and co-authors
Dr. Alexander Judge
Professor Dietmar Müller
combined open-access data for thousands of polymetallic nodules with global
datasets of key environmental parameters to create a machine-learning model
that ranks the factors controlling nodule location. The resulting
predicts where polymetallic nodules are most likely to occur.
The authors were surprised to find that globally the nodules occur in
regions where the bottom current speeds are far too slow to remove
sediment. Instead, the nodules are associated with seafloor fauna.
“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,” Dr. Dutkiewicz said. “Although these
organisms occur in relatively low concentrations on the abyssal seafloor,
they are still abundant enough to locally affect sediment accumulation.”
This insight is supported by direct seafloor observations of nodule fields
by independent studies. “Our conclusion is that deep-sea ecosystems and
nodules are inextricably connected,” Dr. Dutkiewicz said.
The study results also suggest that the regions where nodules are most
likely to occur are more extensive than what has previously been assumed
and include vast areas that are yet to be explored—findings with important
industrial as well as conservation implications.
“Our map highlights regions that may be important economically,” Dr.
Dutkiewicz said, “but at the same time draws attention to areas of the
seafloor that may be hotspots for diverse deep-sea organisms that we know
Because the techniques used by the researchers can be modified to
investigate other seafloor features, this approach—as well as the
conclusion that there is an apparent “symbiosis” between deep-sea fauna and
the nodules—could also have implications for future biodiversity research.
“Vast regions of the deep sea are unexplored,” Dr. Dutkiewicz said, “so the
consequences of nodule mining for deep-sea ecosystems need to be carefully
Environmental predictors of deep-sea polymetallic nodule occurrence in
the global ocean
Adriana Dutkiewicz et al.,
GEOLOGY articles are online at
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