Scientists have made a mysterious discovery in the depths of the Pacific Ocean, where sunlight can’t reach and the seafloor lies in total darkness. Something down there is producing oxygen – one of the building blocks of life – undermining our understanding of the gas as something that can only be created by living things, like plants and algae, interacting with sunlight. The findings could even rewrite what we know about the origins of life on our planet, and even on alien worlds.

So what’s giving off this so-called ‘dark oxygen’? Well, the researchers have traced the surprise discovery back to... some rocks. Specifically, a bunch of plum-sized mineral deposits called polymetallic nodules. These are often known as “batteries in a rock” because of their electrical charge, and are rich in metals that we use to create real batteries back on land, as well as smartphones, solar panels, and other bits of important tech.

How do the “batteries” create oxygen? We’re not sure yet. In fact, the scientists were so confused by the phenomenon that they initially thought their underwater equipment was malfunctioning. When they hauled some nodules onto their ship to repeat the tests, however, they got the same result: dark oxygen.

According to Nicholas Owens, director of the Scottish Association for Marine Science (SAMS), this is “one of the most exciting findings in ocean science in recent times” and “requires us to rethink how the evolution of complex life on the planet might have originated”.

Below, we explore what makes the discovery of dark oxygen such a big deal.

HOW DID SCIENTISTS DISCOVER ‘DARK OXYGEN’?

Andrew Sweetman, a sea-floor ecologist at SAMS in Oban, Scotland, and a co-author of the new Nature Geoscience study on dark oxygen, first noticed something unusual going on in the deep, dark ocean back in 2013, while researching marine ecosystems in the Clarion-Clipperton Zone, an “abyssal plain” bigger than India that stretches between Hawaii and Mexico, and contains an abundance of ocean life. 

To conduct his experiments, Sweetman and his team sent a ‘benthic chamber’ – a device used to measure the exchange of substances between ocean water and sediment – to the bed of the Pacific. This allowed them to isolate “an enclosed microcosm of the seafloor” made up of seawater, sediment, and, in this case, some polymetallic nodules for good measure.

AND WHAT DID THEY FIND?

Normally, the amount of oxygen trapped in the chamber decreases over time, as it’s used up by tiny organisms in the enclosed ecosystem and can’t be replaced by processes like photosynthesis. Sweetman had seen this happen during experiments in the Atlantic Ocean, as well as parts of the Indian, Arctic, and Antarctic Oceans. 

This time, though, the opposite thing happened: the water in the chambers became richer in oxygen over time. For several years, Sweetman was suspicious about the result, assuming it was caused by a sensor malfunction. It was only on subsequent trips in 2021 and 2022 that he confirmed oxygen was actually being created without sunlight. “I suddenly realised that for eight years I’d been ignoring this potentially amazing new process,” he says, “4,000 metres down on the ocean floor.”

HOW ARE THE ROCKS PRODUCING OXYGEN, IF THEY’RE NOT ALIVE?

Until now, photosynthesis was thought to be the only way to naturally produce oxygen – at least on Earth – and it’s carried out by plants, algae, and bacteria using energy from sunlight. In the absence of sunlight, how are some rocks on the bottom of the ocean producing the gas? To put it simply: we’re not quite sure yet.

One theory is that the electrical charge of polymetallic nodules actually split seawater – essentially H2O – into hydrogen and oxygen, in a process called ‘seawater electrolysis’. Technically, they don’t have quite a high enough voltage to trigger this reaction, but they could if they’re ‘chained’ together like two batteries in a series.

WHAT DOES THIS HAVE TO DO WITH LIFE ON EARTH?

Well, oxygen is a vital component of most living things. “The conventional view is that oxygen was first produced around 3 billion years ago by ancient microbes called cyanobacteria and there was a gradual development of complex life thereafter,” Owens explains. However, the new study raises the possibility that oxygen might have been produced elsewhere – and triggered the development of complex life – even earlier. If this is confirmed, it could mean we need to reconsider how all subsequent life developed on our planet.

The study’s authors also suggest that dark oxygen continues to support living organisms at the bottom of the ocean today. This means that we need to think twice about removing the nodules, an act that could cause whole ecosystems to collapse. Unfortunately, the Clarion-Clipperton Zone is a coveted target for deep-sea mining, with companies rushing to extract the valuable resources contained in the mineral deposits. The study itself was partly funded by The Metals Company, a Canadian company that wants to start mining nodules in the CCZ as early as next year.

AND WHAT ABOUT ALIENS?

There are a few objects in our solar system that are covered in oceans, like Europa (one of Jupiter’s moons) and Enceladus (an icy moon of Saturn). These are thought to be some of the best places to look for life beyond Earth, and Sweetman suggests that his recent findings only add to the likelihood that alien biology has evolved there. If dark oxygen is supporting ecosystems on Earth, he says, “could it be helping to generate oxygenated habitats on other ocean worlds such as Enceladus and Europa and providing the opportunity for life to exist?”

On the other hand, finding oxygen in a distant planet’s atmosphere – using technology like the Webb Space Telescope – often earmarks it as a potential host of living organisms, since we don’t know of many other things that can produce oxygen. Now we know about dark oxygen, we’re going to have to be a bit more careful. After all, we wouldn’t want to set out on a decades-long mission to meet an alien race, and arrive to find it’s actually just a bunch of rocks.