Deep-sea mining of rare metals produces 'dark oxygen,' new study finds

Mining of rare metals on deep sea and ocean floors miles below the surface could create "dark oxygen" and also cause potentially harmful changes to the marine ecosystem, according to new research.

Dark oxygen – defined in this context as oxygen that's produced without using sunlight, as it is on the Earth's surface via photosynthesis – is produced when nodules comprised of several rare metals and that lie on the deep, dark seafloor react with salt water, according to a paper published in Nature Geoscience. The dark oxygen is produced because the metallic nodules act like batteries when they are disturbed, similar to the way a conventional battery fizzes when placed into seawater. The nodules themselves form over millions of years when metals suspended in seawater collect around assorted fragments of material on the seabed.

Many of the rare-earth elements comprising these polymetallic nodules are essential components of advanced and low-carbon energy technologies, which has made the deep seafloor an attractive target of mining companies, according to the study.

The polymetallic nodules are common on the deep seafloors, or abyssal plains, of oceans worldwide, which are covered in sediment. They are primarily composed of oxides of iron and manganese, but they can also contain metals such as cobalt, lithium and copper, as well as rare elements and minerals.

Mining the seafloor for these elements can disturb the nodules, causing them to potentially produce more dark oxygen, which in turn could negatively impact the ecology of the deep seafloor, the researchers said.

Researchers used chambers placed on the seafloor at depths of about 2.6 miles in areas where the polymetallic nodules were present and measured the oxygen concentration and its consumption by the organisms there. The chambers were placed at multiple locations over thousands of miles across the Clarion–Clipperton Zone, located in the central Pacific Ocean between Hawaii and Mexico. Each metallic nodule also was measured for voltage at the ocean surface.

Almost all of the experiments showed the oxygen concentration steadily increased over two days, according to the paper, indicating that more oxygen was being produced than was being consumed. After follow-up analyses, the researchers hypothesized that the cause of the net oxygen generation was the electrical properties of the nodules, which produced roughly the same amount of voltage as that of a AA battery, leading researchers to dub the findings the "geo-battery" hypothesis.

It is difficult to estimate how much oxygen polymetallic nodules produce on a broader scale, but the ability for this oxygen source to support deep-seafloor ecosystems could be impacted if these nodules were to be mined, according to the study.

About half of oxygen production on Earth comes from the ocean, produced mostly by plankton and other sea-dwelling organisms, such as plants, algae and some bacteria, that can photosynthesize, according to the National Oceanic and Atmospheric Administration.

More than 800 marine scientists have signed a petition calling for a pause on deep-sea mining activity while more research is done on its potential impact, stating that deep-sea ecosystems are already subject to existing human-created environmental stressors, such as climate change, bottom trawling, and pollution.

Mining the polymetallic nodules in the Clarion-Clipperton Zone, where the discovery was made, "could result in the destruction of life and the seabed habitat in the mined areas," according to NOAA.