Could a discovery of ‘dark oxygen’ hold answers to key scientific questions? Photo credit: Michael Benz via Unsplash
Oxygen is key for most life as we know it today. On both land and at the ocean’s surface oxygen is produced via photosynthesis—plants and phytoplankton using sunlight to split water into its constituent parts, producing oxygen as a byproduct. However, a new study published in Nature by Professor Andrew Sweetman and his colleagues suggests that there is another source of oxygen at the deepest depths of the ocean: polymetallic nodules. This so-called “dark oxygen” could hold answers for early life on Earth and may be the final straw in the case against deep-sea mining.
This so-called “dark oxygen” could hold answers for early life on Earth and may be the final straw in the case against deep-sea mining.
Polymetallic nodules are concretions of iron- and magnesium-bearing oxides on the sea floor that have built up over millions of years. Nodule growth is one of the slowest geological processes on Earth, with average rates of 10–20mm per million years. Nodules on the seafloor today were probably also around when our earliest-known ancestors were only just making the transition to walking on two legs. Sweetman and his team suggest that the energy differences between metal ions in different layers of the nodules may be enough to spark a reaction on the nodule’s surface, splitting water into hydrogen and oxygen, analogous to what happens when you put a battery in seawater. Testing this hypothesis in a lab with nodule samples supported the idea, with voltages generated across the nodule surfaces being almost equal to that of a typical AA battery, and more importantly, sufficient to split water.
Nodules on the seafloor today were probably also around when our earliest-known ancestors were only just making the transition to walking on two legs.
Many headlines related to deep-sea nodules describe them as a key resource in the green transition. Clean technologies, such as solar power plants, wind farms, and electric vehicles all require “critical minerals”, including nickel, copper, manganese, cobalt, and molybdenum , all of which can be found bound to the surface of nodules as they form. The increase in demand for critical minerals is estimated to reach as much as 400–600% in the coming decades, with the World Bank estimating that we will need more than three billion tonnes of minerals and metals to deploy the wind, solar, and geothermal power needed to avoid 2°C of global warming, the target set by the Paris Agreement. Although supplies on land could meet these targets, upscaling of terrestrial mining and potential human rights breaches resulting from such upscaling means sticking to land-based mining is unlikely to be sufficient. Mining industries are therefore turning to deep-sea abyssal plains for the solution.
The current status of deep-sea mining is exploratory, both in national and international waters.
The current status of deep-sea mining is exploratory, both in national and international waters. In 2024, Norway announced that they are opening their continental shelf for exploratory deep-sea mining. The International Seabed Authority (ISA), who oversee all mineral resource-related activities on the deep seabed, have granted 30 exploration contracts, opening up 1.4 million square kilometres of the seafloor to prospective miners. 17 of these are for sites in the Clarion-Clipperton Zone (CCZ), an area that spans around 4.5 million square kilometres of the northern Pacific seafloor, where nodule abundances have been found to be the greatest.
Amongst other metals, an estimated 340 million tonnes of nickel—enough for 54 years supply according to the forecasted 2040 demand—could be mined from the nodules across the CCZ alone, making it a key site for deep-sea mining companies.
The CCZ is also the site of Sweetman et al.’s study, alongside many other recent discoveries relating to life on the seafloor. With immense pressures and zero light combined, deep-sea abyssal plains do not seem a likely place for organisms to thrive. However, the seafloor has been shown to host an incredibly diverse ecosystem: in addition to the 438 named and 5,142 unnamed species, there is estimated to be between 6,000–8,000 more unknown animal species thriving down at depths of around 4,000–5,000m. The link between oxygen production by the polymetallic nodules and the prevalence of life in the CCZ requires further exploration, but the researchers have suggested that the same water-splitting process could be have potentially been a fundamental ingredient in the origin of aerobic life. The discovery also opens up the possibility of oxygen production, via polymetallic nodules, occurring on other planets and moons, creating oxygen-rich environments where aerobic life may survive.
The abundance of life on the abyssal plains of the deep ocean, and the multitude of unknowns, puts into question whether looking to nodules as a mineral resource is a good idea.
The abundance of life on the abyssal plains of the deep ocean, and the multitude of unknowns, puts into question whether looking to nodules as a mineral resource is a good idea. In the past, commercial efforts for seabed mining have often failed due to a variety of reasons, including the large up-front costs, lack of regulation, and historically low value of deep-sea ores, prior to the current increase in demand. Advancements in technology and policies within the deep-sea mining industry may aid in confronting these challenges. But one factor remains a sticking point: environmental destruction.
Current mining practices harvest nodules using a mechanical fork-like tool or hydraulic mechanism, picking up sediment alongside their bounty, with the sediment then being emptied back into the ocean at the extraction site once it is separated from the nodules. In 1989 the largest experiment on the potential impacts of commercial deep-sea mining (Disturbance and Recolonization Experiment, or DISCOL) showed that the disturbed sediment buried most of the study area at the time of simulated mining. More worryingly, later visits to the site in 2015 showed that the area had not recovered. Ploughed areas remained as visible as they were when first mined, producing an almost apocalyptic scene. Life did not return.
Sediment plumes may also hinder dark oxygen production. In Sweetman et al.’s experiments, oxygen production was proportional to the surface area of the nodules, with larger nodules producing the most oxygen. As nodule surfaces become covered in sediment redistributed from mining, oxygen production is likely to wane, as less surface is exposed to the surrounding seawater. Decreased oxygen production has the potential to severely impact on the surrounding deep-sea ecosystems; in an interview with Nature, Sweetman suggests that with the large volumes of dark oxygen being produced it is likely that surrounding lifeforms are dependent upon it for survival.
The discovery of dark oxygen is contrary to all previous ideas about how oxygen is produced and is yet again highlights the lack of knowledge we have about life in the deepest ocean. More than 800 marine scientists from 44 countries have already signed a petition calling for a pause on mining activity until we have sufficient and robust scientific information to make more evidence-based decisions on if (and how) commercial deep-sea mining should go ahead. If Sweetman et al.’s study is one of the final nails in the coffin for deep-sea mining, dark oxygen may not only provide answers for how complex life came to be on Earth, but also may redirect our transition into the future.
