Researchers at Boston University have made a surprising discovery: In an area currently being explored for deep-sea mining, rocks are producing “dark oxygen.” More than 12,000 feet below the surface of the Pacific Ocean, in the Clarion-Clipperton Zone (CCZ), the seafloor is covered in ancient rocks millions of years old. Although these rocks appear dead, nestled in the nooks and crannies of their surfaces are home to tiny marine creatures and microbes, many of which are uniquely adapted to life in the dark.

These deep-sea rocks, called polymetallic nodules, not only contain a surprising number of marine animals, but a team of scientists including experts from Boston University discovered that they also produce oxygen on the seabed.

The discovery is surprising, considering that oxygen is usually produced by plants and organisms with the help of the sun, not by rocks on the ocean floor. About half of the oxygen we breathe is produced near the ocean surface by phytoplankton, which photosynthesize just like land plants. Since photosynthesis requires the sun to occur, the search for oxygen production on the seabed, where there is no light, challenges conventional wisdom. It was so unexpected that the scientists involved in the study initially thought it was a mistake.

Discovery of a new phenomenon

“This was really surprising because no one had seen this before,” says Jeffrey Marlowe, a biology professor in BU’s College of Arts and Sciences and co-author of the study. Nature Geology.

Marlow, who specializes in microbes that live in some of the most extreme habitats on Earth, such as solidified lava and deep-sea hydrothermal vents, initially suspected that microbial activity might be responsible for oxygen production. The team used deep-sea chambers that sink to the seafloor and contain seawater, sediment, polymetallic nodules, and living organisms. They then measured how oxygen levels in the chambers changed over 48 hours. If there are many oxygen-breathing organisms, levels will usually decrease due to the activity of the animals in the chamber. But in this case, the oxygen increased.

“We did a lot of research and saw that the oxygen level had increased many times since the first measurement,” Marlowe says. “We are now convinced that this is a real signal.”

Investigating the source of “dark oxygen”

He and his colleagues were on a research vessel tasked with learning more about the ecology of the CCZ, a 1.7 million-square-mile area between Hawaii and Mexico, for an environmental study sponsored by The Metals Company, a deep-sea mining company that mines rock for metals on a massive scale. After conducting experiments on the ship, a team led by Marlowe and Andrew Sweetman of the Scottish Marine Science Association concluded that despite the abundance of different types of microbes both on and inside the ship, the phenomenon was not primarily caused by microbial activity.

Because the polymetallic nodules are composed of rare metals such as copper, nickel, cobalt, iron and manganese, companies are interested in mining them. According to the research, these tightly packed metals are likely to cause “seawater electrolysis”. This is when metal ions in rock layers are unevenly distributed and their electrical charges separate, like inside a battery. This phenomenon creates enough energy to split water molecules into oxygen and hydrogen. They called it “dark oxygen” because it is oxygen that forms without sunlight. The exact mechanism of how this happens when oxygen levels change across the CCZ, and whether oxygen plays a significant role in maintaining the local ecosystem, remains unclear.

Deep sea mining and implications for ecosystems

The Metals Company calls the polymetallic nodules “batteries in the rock” and says on its website that mining them could accelerate the transition to battery-powered electric vehicles and that onshore mining will no longer be necessary. Until now, mining in the CCZ has been for exploration purposes, but the UN’s International Seabed Authority, which governs the area, could start making mining decisions as early as next year. Metals is working with Pacific nations Nauru, Tonga and Kiribati to gain access to mining licences, but many other countries in the South Pacific, including Palau, Fiji and Tuvalu, have publicly backed a moratorium or suspension of mining. Environmental activist groups such as Greenpeace and Ocean Conservancy have called for a complete ban on the operation, while opponents fear it could cause irreversible damage to the seabed.

Meanwhile, scientists have begun to investigate the potential consequences of disrupting a largely unstudied ecosystem. This article Nature Geology It provides an idea of ​​the basic conditions of the area before any large-scale mining begins.

“We don’t know all the implications, but to me this finding suggests that we need to take a hard look at what changing these systems could do to the animal community,” Marlow says, because all animals need oxygen to survive.

Astrobiology and the search for extraterrestrial life

The CCZ is also an ideal environment for studying the planet’s smallest organisms, such as bacteria and archaea (single-celled organisms) found in sediments and nodules. Marlow and co-author Peter Schradl (GRS’25), a graduate student in BU’s Ecology, Behavior and Evolution Program, are focused on using microbes found in extreme environments, especially those in the CCZ, as templates for searching for single-celled life on other planets. The planets and their moons are most similar to the many moons of Mars and Saturn, with deserts, volcanoes and seabed springs. The field, which aims to inform the search for extraterrestrial life by studying Earth systems, is called astrobiology.

“Living in environments like the CCZ provides an opportunity to study ecosystems that have evolved under specific evolutionary pressures and constraints,” says Schradl, who works in Marlowe’s lab. Those conditions—depth, pressure, and aquatic environment—“are similar to what we measure or expect to find on icy moons,” he says.

For example, Jupiter’s moon Enceladus and Saturn’s moon Europa are covered in ice sheets, and sunlight doesn’t reach the water beneath them. “Who knows, if these kinds of rocks are under the oxygen-producing ice, that could allow for a more productive biosphere,” Marlowe says. “If photosynthesis isn’t necessary for oxygen production, then other planets with oceans and metal-rich rocks like these nodules could support more advanced biospheres than we thought possible in the past.”

Marlowe says there are many questions about what the discovery of dark oxygen means for extraterrestrial oceans and for us.

“We usually think of the deep sea as a place where decaying material sinks to the bottom and animals eat the remains. But this discovery recalibrates that dynamic,” he says. “It helps us think of the deep ocean as a production area, similar to what we find in methane seeps and hydrothermal vents that create oases for marine animals and microbes. I think it’s a fun reversal of how we normally think of the deep sea.”