An international team made headlines in 2024 after they discovered that the metallic nodules now sought by deep-sea mining firms were producing ‘dark oxygen’ in the depths of the Clarion Clipperton Zone (CCZ) in the Pacific. Such nodules contain high levels of metals like nickel, cobalt and lithium, which find uses in green technologies such as rechargeable batteries.
The origin of ‘dark oxygen’ in the ocean’s depths is largely unknown. The nodules themselves may be producing oxygen, with the team previously suggesting that the nodule’s charged surface may create the right conditions for water electrolysis.
The Nippon Foundation – a Japanese organisation with an interest in the world’s oceans – has just given a $5.2 million (£3.7 million) grant for a three-year long project to better understand this phenomenon. This includes deploying three purpose-built landers in the CCZ up to 6km below sea level later this year.
Mason Wakley spoke to Andrew Sweetman, a biogeochemist at the Scottish Association for Marine Science, who is leading the project.
What are the goals of this new project?
The first goal is to see if we can document the presence of dark oxygen again using a whole new set of deep-sea instrumentation.
The second goal is to figure out what is causing this phenomenon. Is it a microbial process? Or is it somehow linked to the chemistry of the sea floor, the nodules or manganese oxide particles in the sediments?
And then the third goal is to try and do some more simulation experiments whereby we see what the effect is of, for example, sediment burial on the process. Sediments may be resuspended during deep-sea mining activities, potentially uncovering electrochemically active sites on the deposits. We think these sites may be involved in the oxygen producing process.
What will the experiments tell us?
Hopefully where the oxygen is coming from and the dominant process responsible for what we’re seeing down at the sea floor. We can then start to understand the process better and put it into an environmental context. For example, if we see that oxygen and hydrogen production are linked, then the process is probably important to the ecosystem because there’ll be microbes that are able to take that hydrogen and synthesise it into living cells.
What experiments are you planning?
During the cruises, we will be measuring things like oxygen and hydrogen production, changes in pH and various components of the carbonate system. And then we’re going to be adding a variety of different chemical tracers to the seafloor and nodules, such as isotopically labelled water, to see if we can detect it in the dissolved oxygen signal. We’ll also be looking at which microbes are active and whether they are taking up any of the chemical tracers. If they are, that should give us an indication as to whether they are actively engaged in oxygen production.
Why is it important to understand dark oxygen?
It’s important to advance our knowledge about the environment that we inhabit. There’s also the deep-sea mining angle – we just don’t know if this process is ecologically important or what’s causing it. We need to understand the process better to figure out if there’s going to be any harm caused during deep-sea mining. If there is, then we need to design the mining process s0o that the effects are limited.
The presence of dark oxygen has not been accepted by everyone in the scientific community. What do you say to doubters?
The suspicion is understandable – I was suspicious of the data for eight or nine years. There’s an enormous amount of data that we’ve circulated to various mining companies and they’re saying that the instrumentation generated the oxygen by trapping a bubble. These instruments have been built over the last 30 years and we’ve used them in a variety of different habitats. We don’t see the oxygen production anywhere else other than when we deploy the lander in the Clarion Clipperton Zone, where there is an abundance of manganese oxide deposits.
Just do a back of an envelope calculation and assume a 500ml air bubble got trapped. Then ask would any of the oxygen still be present after the turbulence generated by 176,000l of water flowing past it as the lander descends to the seafloor. We find that the oxygen would be gone by the time the lander reaches a depth of 2km above the seafloor.
The interesting thing is that there is less than 20ml of space present in the chambers once they fill with water at the ocean surface. The oxygen in a 20ml bubble would be gone when the lander reaches a depth of 3.5km above the seafloor. As such, there doesn’t seem to be any way a trapped air bubble would arrive intact at the seafloor.
We also ran several experiments where we did a pre-incubation in which the chambers didn’t penetrate the seafloor and found no oxygen production during this time. However, once the chambers were pushed into the sediment, the oxygen started rising. We also have reams of other data to show the oxygen production was not an artefact of the experiments.
When can we expect results from the project?
We’ll generate a lot of samples during the cruises to the CCZ that will then go to the lab. We should have data by the end of the year, confirming that this process is going on. Maybe we’ll also have some indications as to what mechanism is taking place. I would say that by the middle of next year we’ll be in a much better position to know what’s going on. We’ll be putting out some data on the website in the meantime.
Could this project influence the prospects for deep-sea mining?
A lot of people think that this project is anti-mining. We don’t have an agenda; we’re just trying to gather more information about this phenomenon. Depending on what we find, we’re going to have to have some discussions with the International Seabed Authority about mining regulations. Not to stop mining, but to make sure that if mining goes ahead, then we have all the information we need to make sure it’s done as safely as possible.
This article has been edited for clarity and brevity.
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