Rocks Making Oxygen in Total Darkness

Four thousand meters below the Pacific Ocean surface, where sunlight never reaches, Andrew Sweetman's sensors showed something impossible. Oxygen levels were rising. Fast.

The year was 2013, and Sweetman assumed his equipment had malfunctioned. He sent the sensors back to the manufacturer for recalibration. They told him the instruments worked perfectly. Over the next decade, he returned to the same seafloor repeatedly. The readings held.

Turns out, the deep ocean floor isn't just consuming oxygen—in some places, it's producing it. Without any light. Without any photosynthesis. The discovery, published in Nature Geoscience in July 2024, challenges fundamental assumptions about how Earth's oxygen cycle works. It also complicates a burgeoning industry rush to mine the very rocks that appear to be making this "dark oxygen."

The Clarion-Clipperton Zone Discovery

The Clarion-Clipperton Zone stretches 4.5 million square kilometers between Hawaii and Mexico. At depths exceeding 4,000 meters, the abyssal seafloor is carpeted with potato-sized lumps called polymetallic nodules. These rocks contain manganese, iron, nickel, cobalt, and copper—metals essential for batteries and electronics.

Sweetman's team sealed off small sections of seafloor using benthic chambers to measure oxygen consumption by deep-sea organisms. Standard procedure. Expected result: oxygen levels drop as creatures breathe. Actual result: in some chambers, oxygen concentrations tripled within two days, reaching levels higher than oxygen-rich surface waters.

The finding defied conventional wisdom. Deep-ocean oxygen was thought to come exclusively from surface waters carried down by currents. Abiotic production—oxygen created without living organisms—had never been documented at these depths.

The Geobattery Hypothesis

The team eventually traced the oxygen production to the nodules themselves. Lab experiments confirmed the pattern: chambers containing only seawater and nodules showed rising oxygen levels, even after killing all microorganisms with mercury chloride.

The leading explanation involves a process called seawater electrolysis. Polymetallic nodules grow in irregular layers over millions of years. Different metals deposited at different rates create an uneven distribution of electrical charge—essentially forming a natural battery. Individual nodules measured voltages up to 0.95 volts. Clustered together at densities of roughly 1,170 nodules per square meter, they can generate 1.5 volts or more.

That voltage threshold matters. Splitting a water molecule into hydrogen and oxygen requires about 1.5 volts—comparable to a standard AA battery. The nodules may be running a slow, continuous electrolysis reaction at the ocean floor, producing oxygen in complete darkness.

Measured oxygen production rates in the study area ranged from 1.7 to 18 millimoles per square meter per day. Not enormous, but measurable. And potentially significant for local ecosystems.

Scientific Caution Flags

Several researchers have raised methodological concerns. When benthic landers descend to the seafloor, they can create a small "bow wave" that disturbs sediment and exposes fresh nodule surfaces. Some scientists question whether this disturbance might temporarily boost oxygen readings, creating an artifact rather than revealing a continuous process.

Additional complexity: other measurements from the Clarion-Clipperton Zone show net oxygen consumption, not production. The dark oxygen phenomenon appears patchy or context-dependent. We don't yet know why some areas produce oxygen while others consume it, or whether production rates vary seasonally.

Independent replication studies are underway. Sweetman's team is currently investigating whether microbes play a role, particularly whether hydrogen released during electrolysis serves as an energy source for deep-sea microbial communities. Until these questions resolve, the exact mechanism and ecological significance remain open.

Uncertainty doesn't diminish the finding's importance. Even localized, intermittent oxygen production at depths previously thought incapable of generating it rewrites our understanding of deep-ocean chemistry and life support systems.

The Mining Question

Polymetallic nodules are now prime targets for deep-sea mining operations. Companies market them as essential for the green energy transition—the metals inside power electric vehicle batteries and renewable energy infrastructure. The Metals Company, a leading firm exploring the Clarion-Clipperton Zone, calls nodules "a battery in a rock."

The critique practically writes itself: mining natural geobatteries to build artificial ones. But the stakes run deeper than irony.

Commercial extraction would involve dredging vast sections of seafloor, removing nodules and churning sediment. Since nodules form over millions of years and support diverse microhabitats, recovery timescales extend beyond human relevance. If nodules contribute meaningfully to local oxygen budgets and support microbial ecosystems, their removal could fundamentally alter deep-sea life support conditions.

The knowledge gap is staggering. Researchers estimate that up to 92 percent of Clarion-Clipperton Zone species remain undescribed by science. Mining corporations are pushing to exploit an ecosystem we've barely catalogued, targeting rocks that may perform functions we're only beginning to measure.

Regulatory Crossroads

The discovery has intensified debates within the UN Ocean Decade initiative (2021-2030), which prioritizes deep-sea research and conservation. The International Seabed Authority oversees exploitation regulations for areas beyond national jurisdiction. As of July 2025, member states have not finalized commercial mining rules despite a decade of negotiations. No commercial mining has commenced.

Pressure is mounting. In April 2025, a U.S. Executive Order advanced seabed mining initiatives. In July 2025, NOAA proposed revisions to deep-seabed exploration and recovery permit regulations. The Metals Company has signaled intentions to begin mining by 2026.

Industry representatives have questioned the dark oxygen study's methodology and conclusions, which is predictable when new science threatens a business model. Some conservation groups and scientists are calling for mining moratoriums until knowledge gaps close. Others argue that delaying resource extraction circumvents constructive dialogue about sustainable practices.

The tension reflects a fundamental mismatch: policy decisions moving faster than understanding.

What We're Really Asking

The dark oxygen discovery reminds us that Earth still holds surprises in places we thought thoroughly mapped. It demonstrates why scientific persistence matters—Sweetman spent over a decade confirming readings that initially seemed like equipment failure. It highlights why humility serves us better than certainty when venturing into poorly understood systems.

If the deep ocean floor can generate oxygen through natural electrochemical processes, what else are we missing? And if we're still cataloguing basic functions in the Clarion-Clipperton Zone, on what grounds are we confident we understand the consequences of wholesale disruption?

The deep sea represents a commons with unknown services. Treating mystery like empty real estate rarely ends well.