Signs of oxygen production on the ocean floor in a remote part of the Pacific first observed in 2013 convinced ocean scientist Andrew Sweetman that his monitoring equipment was faulty. That’s impossible, the scientist thought. Oxygen-producing species simply do not live at this depth, where there’s no light to allow photosynthesis.

However, the same readings were observed on three subsequent voyages to the Clarion-Clipperton Zone, an abyssal plain as wide as the continental United States and punctuated by seamounts.

Initially skeptical, Sweetman and colleagues couldn’t dismiss the data as errors any longer. The scientists performed experiments on the ocean floor using autonomous submersibles. These experiments revealed unexpected oxygen production close to polymetallic nodules covering the seafloor.

Over two days, oxygen concentrations increased more than threefold, suggesting a process independent of photosynthesis, termed “dark oxygen production” (DOP).

A Paradigm Shift in Ocean Science

Sweetman first noticed the “dark” oxygen while assessing marine biodiversity in a region earmarked for mining polymetallic nodules, CNN reports. These nodules contain metals like cobalt, nickel, copper, lithium, and manganese, all in high demand for batteries and green technologies. However, critics argue that deep-sea mining could irreparably damage underwater ecosystems and disrupt carbon storage in the ocean.

Photosynthetic organisms like plants and algae use sunlight to produce oxygen, which cycles into the ocean depths. Previous studies showed that deep-sea organisms like angler fish only consumed oxygen. Sweetman’s findings suggest this is not entirely true and a mysterious oxygen source lies somewhere on the ocean floor.

“When we first got this data, we thought the sensors were faulty because every study ever done in the deep sea has only seen oxygen being consumed rather than produced,” Sweetman said. “We would come home and recalibrate the sensors, but, over the course of 10 years, these strange oxygen readings kept showing up.”

Sweetman’s team, including scientists at the Scottish Association for Marine Science and Heriot-Watt University, conducted in situ benthic chamber experiments in the Pacific Ocean’s Clarion-Clipperton Zone. These experiments involve deploying autonomous devices, known as benthic chambers, which create enclosed microenvironments on the seabed. By isolating a section of the seafloor, researchers can precisely measure changes in variables like oxygen concentration, nutrient fluxes, and biological activity within this controlled setting.

The researchers expected that their sensors would detect a slow decline in oxygen levels as microscopic animals breathed it in. Instead, the experiment showed oxygen production. It wasn’t until 2021, using a different method, that the scientists accepted the unexpected finding.

“We decided to take a back-up method that worked differently to the optode sensors we were using. When both methods came back with the same result, we knew we were onto something ground-breaking and unthought-of.”

Geobatteries

Consistent net oxygen production was observed, contradicting the common notion that deep-sea sediments are merely oxygen consumers. Statistical analyses ruled out experimental biases, confirming that DOP was a significant factor.

The study proposes that high voltage potentials on nodule surfaces may drive seawater electrolysis, splitting water into hydrogen and oxygen — essentially like natural “geobatteries”. The recorded potentials reached up to 0.95V, supporting this hypothesis. This electrochemical process could be powered by the internal redistribution of electrons within the metal layers of the nodules, which are rich in transition metals like manganese and nickel.

The study’s implications are vast. There’s still so much we don’t know about ocean depths, some areas of which are less explored than Mars. The findings also reveal what’s at stake in exploiting the ocean floor for rare metals and minerals.

“Several large-scale mining companies now aim to extract these precious elements from the seafloor at depths of 10,000 to 20,000 feet below the surface. We need to rethink how to mine these materials, so that we do not deplete the oxygen source for deep-sea life,” said Franz Geiger from Northwestern University, who led the electrochemistry experiments.

This discovery also has implications for understanding the origins of life. One theory is that life evolved on deep-sea hydrothermal vents. Discovering seawater electrolysis forming oxygen in the deep could inspire new ways to think about how life began on Earth.

“For aerobic life to begin on the planet, there had to be oxygen, and our understanding has been that Earth’s oxygen supply began with photosynthetic organisms,” said Sweetman. “But we now know that there is oxygen produced in the deep sea, where there is no light. I think we, therefore, need to revisit questions like: Where could aerobic life have begun?”

With many questions still unanswered, Sweetman emphasizes the need for more research. “I hope it’s the start of something amazing,” he said.

The findings appeared in the journal Nature Geoscience.