Discovery in the deep ocean defies current human knowledge

Far below the waves, between about 1.2 and 3.1 miles (two to five kilometers) down, sunlight never reaches and the water sits just above freezing.

In that darkness, animals and microbes should primarily consume oxygen because photosynthesis – the usual way Earth makes oxygen – needs light and usually happens near the surface.

Photosynthetic organisms, like plants and algae, use energy from sunlight to create the planet’s oxygen.

New evidence, however, shows that oxygen is also produced in complete darkness on the seafloor – 13,000 feet (4,000 meters) below the ocean surface.

Discovering dark oxygen

In a patch of the Pacific’s abyssal plain littered with potato‑sized “polymetallic nodules”, instruments recorded behavior that did not match textbook expectations.

A team led by Prof. Andrew Sweetman of the Scottish Association for Marine Science (SAMS) in Oban, a partner of University of the Highlands and Islands (UHI), made the “dark oxygen” discovery during ship-based fieldwork in the Pacific Ocean.

In a paper published in Nature Geoscience, the researchers describe “dark oxygen production” as a challenge to a basic assumption about where oxygen can come from in the ocean.

“The discovery of oxygen production by a non-photosynthetic process requires us to rethink how the evolution of complex life on the planet might have originated,” Sweetman explained.

“The conventional view is that oxygen was first produced around three billion years ago by ancient microbes called cyanobacteria and there was a gradual development of complex life thereafter.”

Studying dark oxygen

The experiments took place in a licensed area known as NORI‑D, within the Clarion–Clipperton Zone (CCZ) of the equatorial Pacific.

This region holds metal‑rich nodules, hard, layered balls mainly composed of manganese and iron oxides, with traces of nickel, copper, and cobalt.

These “polymetallic nodules” rest on a broad abyssal plain and have attracted interest as potential mining targets.

To test how much dark oxygen the seafloor community uses there, the team sent a robotic lander to the bottom with clear, box‑shaped “chambers.”

The lander pressed each chamber gently into the mud to enclose a small “micro‑ecosystem” of sediment, water, and nodules.

Instruments called optodes tracked dissolved oxygen every few seconds for nearly two days, giving a detailed record of how much oxygen the enclosed patch of seafloor gained or lost.

Oxygen in darkness?

Across 25 tests, the researchers saw that dark oxygen started at about 185 micromoles per liter and climbed over about 47 hours to as high as 819 micromoles per liter in some chambers. That’s more than triple the starting amount.

A micromole per liter is just a way to count how many molecules are in water – basically a very tiny “bucket” of molecules.

When the team calculated how much oxygen was being produced over the seafloor, they found it added up to about 1.7 to 18 millimoles of oxygen per square meter per day. In simple terms, that’s more oxygen being produced than used up during the tests.

To double-check the finding, the researchers used a classic chemical test called the Winkler method. It showed the same pattern, so the results weren’t caused by a faulty sensor.

Chambers that had more nodule surface area (more rock for reactions to happen on) also produced more oxygen.

Checking every possible glitch

Because such a surprising result needs careful checks and rechecks, the scientists looked hard for any simple explanation. Could an air bubble trapped during deployment have slowly leaked oxygen into the water?

At a depth of 2.5 miles (4,000 meters), any bubble would dissolve almost instantly, so that explanation doesn’t match the slow, steady increases of dark oxygen recorded over many hours.

The chambers were designed to purge air as they sank and were built from inert plastic, and control runs with no additions still showed the same pattern of oxygen increase.

To make sure nothing about recovery on deck created the pattern, they ran sealed “ex situ” incubations back on the ship (including poisoned sediment to suppress biology) and still measured increases in oxygen.

Natural radiation can split water molecules (a process called radiolysis), but modeling suggested it would yield far too little oxygen – orders of magnitude less than they observed in two days.

Each of these steps makes it less likely that the “extra” oxygen came from leaks, contamination, or living cells working in the usual ways.

Mud, mining, and nodules

Sediment cover appears to matter for the production of dark oxygen. If a thin layer of mud blankets a nodule, it could block seawater from reaching reactive sites or alter the tiny electrical fields at the surface.

The authors speculate that when the lander arrived, the small “bow wave” it created might have swept sediment off the nodules, briefly exposing fresh, more active surfaces.

This detail has direct ties to human plans for deep‑sea mining in the CCZ. Future mining systems would churn and resettle large amounts of seafloor mud around these nodules.

This oxygen production could be patchy, intermittent, and sensitive to the local seafloor’s “wiring” and chemistry, and the authors explicitly caution against scaling these bursts to the whole CCZ or to long time scales.

“Through this discovery, we have generated many unanswered questions and I think we have a lot to think about in terms of how we mine these nodules, which are effectively batteries in a rock,” Sweetman warned.

Lessons from this dark oxygen study

To sum it all up, this fascinating study adds a plausible, non‑photosynthetic way to generate oxygen in the deep, dark depths of the ocean, at least locally.

If proven by further study, the discover of dark oxygen won’t dethrone photosynthesis as Earth’s oxygen generator – algae and plants still supply almost all of Earth’s oxygen – but it widens the view of how rock, water, and electricity can interact to shape seafloor chemistry.

It also hints at connections across time. If metal‑oxide surfaces can catalyze oxygen release in the dark today, similar processes might have operated in the ancient ocean, when biology and geology co‑evolved and Earth’s atmosphere gradually oxygenated.

Even in well‑studied regions like the CCZ, basic budgets of oxygen in and out can behave in ways we didn’t predict.

This process adds one more piece to the puzzle of how Earth keeps its oxygen cycle running in places scientists are only beginning to understand.

The full study was published in the journal Nature Geoscience.

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