The Damage From Deep Sea Mining Is Slow To Heal

These manganese nodules are a potential mining target, but the sediments around them play a vital role in recyling organic material and are vulnerable to disturbance. ROV-Team/GEOMAR

As important metals become increasingly hard to find on land, attention has turned to the oceans. Deep-sea mining would be immensely expensive, but deposits around old volcanic vents are often so rich, mining companies are seeking the technologies to make it happen. Unfortunately, it's also disruptive to sea-bed ecosystems, and new research suggests the damage lasts.

In 1989, researchers tested the effects of deep-sea mining by plowing up a patch of the Pacific Ocean 4,000 meters (2.5 miles) below sea level in the Peru Basin, returning three times since to observe the progress. The area chosen was exactly the sort of place where mining might one day occur, a patch of oceanic crust covered in manganese nodules laced with other, even more valuable metals.

Decades later, Max Plank Institute for Marine Microbiology PhD student Tobias Vonnahme was part of a team that returned to the site to see how the ecology had recovered. "Even 26 years after this disturbance, the plough tracks on the seabed were still clearly visible," Vonnahme said in a statement. "And the bacterial inhabitants were also clearly affected."

The oldest tracks had about two-thirds the bacteria of undisturbed areas and more recent ones had only half. Since bacteria are the base of the deep ocean food chain, their absence ripples through everything else.

"Our calculations have shown that it takes at least 50 years for the microbes to fully resume their normal function," Vonnahme said

Close to the surface, currents would remove visible damage, and the faster pace of life might allow ecosystems to recover, but these things are slow in the still energy-poor deep ocean. The upper layer of sediment around the nodules does most of the work to process organic material that settles on the ocean floor, but it's too fragile to survive disturbance. The findings have been reported in Science Advances.

"So far, only few studies have dealt with the disturbance of the biogeochemical function of deep-sea floors caused by mining," said senior author Professor Antje Boetius of the Alfred Wegener Institute. Instead, the focus has been on the effects on animals.

Boetius believes the study will assist in developing environmental standards for deep-sea mining, adding that “ecologically sustainable technologies should definitely avoid removing the densely populated and bioactive surface layer of the seabed." Whether that will be possible, while still giving access to the minerals themselves, remains to be seen.

The areas affected by any large-scale mining would be hundreds or thousands of times larger than those impacted in the trial. On the other hand, these seafloor sites are rich in cobalt, among other metals, which has proven particularly difficult to access on land in an ethical and sustainable manner.

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