Evidence of deep-sea mining still visible over four decades later

The impact of deep-sea mining on the animals that live on the seafloor is likely to be complicated.

Researchers have revisited a remote site in the Pacific Ocean that was used as a test for deep-sea mining in the 1970s. They have found that the tracks of the 100-tonne machine that was sent down to the seabed are still visible, and that the impact of mining on some of the wildlife is ongoing.

But they have also found signs of recovery. Some life has returned to the region, with sediment-living animals having recolonised the seabed to similar levels seen in untouched regions. This has provided the first evidence for how this ecosystem might respond to future industry.

The study now published in Nature was a collaboration, including Dr Adrian Glover from the Natural History Museum and colleagues at the National Oceanography Centre.

“To tackle the crucial question of recovery from deep-sea mining, we need first to look to the past and use old mining tests to help understand long-term impacts,” says Professor Daniel Jones from the National Oceanography Centre, who with Adrian co-led the expedition which assessed the mining site.

“Forty-four years later, the mining tracks themselves look very similar to when they were first made, with an 8-metre-wide strip of seabed cleared of nodules and two large furrows in the seafloor where the machine passed.”

The impact that this has had on the life that lives there has been mixed. For the more mobile sediment-dwelling creatures, they have largely recolonised the old mining site. But the bigger animals, which are reliant on the nodules, have not returned because the scientists think there are simply no nodules left for them to live on.

This paints a complex but not unexpected picture of the potential impacts that mining will have on these fragile ecosystems.

Mining the deep sea

The Clarion-Clipperton Zone (CCZ) is a huge area of open ocean that stretches between Mexico and Hawaii. At first glance it might not be obvious why this region has gained so much attention in recent years, but it’s what lies on the seabed that is of interest.

The seafloor of the CCZ is covered with potato-sized lumps of rocks known as polymetallic nodules. Formed over tens of millions of years, these nodules contain a wealth of minerals such as cobalt and nickel, which makes them an appealing prospect for mining companies.

The main issue is they are located some 5,000 metres underwater. This makes the logistics of retrieving these nodules incredible difficult. But as sources of these minerals on land have become rarer and more contested, and the technology to go down to these depths and mine them cheaper and more reliable, interest in regions like the CCZ has grown.

One of the biggest things holding the industry back though is the unknown of the environment in which the nodules are found.

Trying to fill this void is Adrian and his colleagues. They have been going out on research vessels to the CCZ to explore, map and document the deep sea to better understand this environment and what lives down there. And they have made some startling discoveries.

What was initially thought to be a lifeless muddy plain is teeming with life. Curious deep-sea fish and crustaceans swim through the frigid waters, tiny alien-looking worms burrow through the sediment as vibrant yellow and purple sea cucumbers rove over the surface, while the nodules themselves provide a solid substrate for extraordinary sea lilies, sponges and corals to grow.

How this life will be impacted by any mining is one of the key questions needing an answer. This is where the 1979 mining test comes in.

“We wanted to survey the same site and see the impact of a multi-decade break since the mining test, because that had never been done before,” explains Adrian. “And we knew we had really quite a good chance of finding it.”

A slow, mixed recovery

In 2023 Adrian and his colleagues set out on a research cruise to the CCZ to find the site. It was within about seven or eight hours of the first dive they managed to find the tracks from the machine that was sent down to mine which “looked like they were made yesterday”.

“There are many ways to interpret this” says Adrian. “They look to some extent like someone’s just driven the machine down there, and that’s striking but also not entirely unexpected.”

“The processes in the deep sea are very slow. The abundances of animals are low, the bioturbation and movement of sediments is slow. So it’s not going be somewhere that recovers quickly.”

Even so, there are still two clear lines where the machine cut through the sea floor scooping up the precious nodules. This has negatively impacted the animals that relied on those nodules to live and grow, but it has also had some other intriguing influences.

For example, the furrows have seemingly collected more organic matter than the surrounding flat plains. This in turn has attracted animals which feed on this material which now occur in greater numbers in these depressions. These are the sort of impacts that need to be considered when deciding on whether to go ahead with mining or not.

“We’ve been working in a vacuum of knowledge,” explains Adrian. “A lot of the debate is about what the mining impact could be, and the argument doesn’t get advanced.”

“Whereas if you actually have some information then people can make a fair judgment of how to move forward. So this is just one step in that process, and we are better informed because of it.”