The surprising discovery of ‘dark oxygen’ could complicate deep-sea mining | Explained

An unknown process is producing oxygen deep in the world’s oceans, where it is too dark for photosynthesis, scientists reported July 22 in the journal Nature GeoscienceThe discovery has important implications because oxygen helps sustain life and the discovery implies the existence of previously unknown ecosystems.

Many governments should also take note, as one explanation for the presence of oxygen is that polymetallic nodules carry electrical charges that split water molecules around them, releasing oxygen. Polymetallic nodules are chunks of iron, manganese hydroxides and rocks partially submerged in many parts of the ocean floor. If their concentration exceeds 10 kg per square metre, their extraction is considered economically viable, and many countries are planning to do so as a new resource.

On July 22, Reuters According to a “senior government scientist,” whose name has not been released, India is planning to “apply for licenses to explore for deep-sea minerals in the Pacific Ocean.” India’s Ministry of Earth Sciences is also currently building a submersible vehicle that will search for and extract similar resources in the Indian Ocean as part of its “Deep Ocean Mission.”

Where was the study conducted?

The discovery of oxygen raises questions about how deep-sea mining for polymetallic nodules will affect marine ecosystems.

The scientists behind the study, from Germany, the United Kingdom and the United States, were studying the Clarion-Clipperton Zone, a section of ocean floor off the west coast of Mexico. With an area larger than India, the Zone is believed to have the largest concentration of polymetallic nodules in the world, including 6 billion tons of manganese and more than 200 million tons of copper and nickel.

When the scientists conducted experiments at a depth of 4 km, they noticed that the oxygen concentration in some places was rapidly increasing rather than decreasing. They conducted follow-up studies in 2020 and 2021. In each case, they launched a device from the surface that would land on the ocean floor, where it would isolate a small volume of the bottom along with some seawater and measure oxygen levels.

This underwater region is called the abyssal zone, and it receives too little sunlight for photosynthesis to be possible. Instead, the life forms here get oxygen from water that comes through a global circulation called the “Great Conveyor Belt.” Even so, the amount of oxygen is low, and without any local production, the device should have measured oxygen levels falling as small animals consumed it. But the scientists found the opposite: it rose, sometimes tripling in just two days.

They re-verified the finding by recreating ocean floor conditions in their lab and found that oxygen levels rose to a certain point before declining.

What is the source of oxygen?

When they measured the physical characteristics of the nodules, they found that their surfaces had a voltage of up to 0.95 V. Splitting a water molecule requires 1.5 V, but the researchers suspected the voltage could build up if many nodules are close together, like the cells of a battery.

Andrew Sweetman, an ecologist at the Scottish Marine Science Association in the UK and co-author of the study, said Nature“We have another source of oxygen on the planet besides photosynthesis.” His team calls it “dark oxygen.”

Oxygen sources are valuable because they allow life to survive. But, as the laboratory experiment indicated, the nodules could only produce oxygen as long as they could generate sufficient voltage. The nodules’ power source is also unclear.

What is deep sea mining?

Given the amount of metals contained in polymetallic nodules on the ocean floor, deep-sea mining is expected to be a major marine resource extraction activity in the coming decades. The International Seabed Authority has set up 15-year contracts with at least 22 contractors, including the Government of India, to search for polymetallic nodules, polymetallic sulfides, and cobalt-rich ferromanganese crusts on the deep seafloor. China alone is expected to mine 17% of the Clarion-Clipperton Zone.

The new finding raises the possibility that this type of mining could harm ecosystems that need “dark oxygen” to survive. Experts have found that deep-sea mining could be harmful to the marine environment, with or without “dark oxygen.”

Between 1989 and 1996, German scientists conducted the Disturbance and Recolonization Experiment (DISCOL) in the Peru Basin, the world’s “first large-scale impact assessment” to evaluate “environmental impacts from polymetallic nodule mining.” They built a device that disturbed the seafloor as a deep-sea mining operation would, and collected data on how the disturbances changed local oceanographic and sedimentological profiles, among other things.

A 2019 study in the journal Scientific reports He reported that “the effects of the simulated mining impacts induced during the DISCOL [Experiment] were still evident in the megabenthos of the Peru Basin after 26 years.”

How will deep sea mining be affected?

The same study also reported “significantly lower diversity heterogeneity in disturbed areas” and added that “if the results of this experiment… can be extrapolated to the Clarion-Clipperton Zone, the impacts of polymetallic nodule mining there may be greater than expected and could potentially lead to irreversible loss of some ecosystem functions.”

In November 2023, Nature It was reported, based on an article published at the time, that deep-sea mining “for minerals could harm deep-sea jellyfish, according to the first study of the impacts of mining on animals that live in the water column.”

Scientists also know less about deep-sea ecosystems than they do about many of those above ground, meaning that the models scientists use to predict their fate and role in global climate processes may be unreliable. Given these and other concerns, on July 20, three major European insurance companies announced that they would exclude deep-sea mining from their underwriting portfolios.

Dark oxygen adds to these challenges. If deep-sea mining does not find sustainable ways to respond to them, it may become completely unviable.