From headlines to criticism of study on deep-sea oxygen production
“We have thought about all this already,” replies the head of the study.
Published
A study made headlines in the Norwegian and international media this summer.
Scientists reported that they had discovered oxygen production on the seafloor, in the journal Nature Geoscience.
The finding might suggest another source of oxygen production on the planet than photosynthesis.
The researchers used an instrument that was lowered to the seafloor and encapsulated the seabed and seawater. Usually, the oxygen level in such experiments decreases over time as microorganisms consume it.
But the opposite happened when the researchers took measurements in the Clarion-Clipperton Zone in the Pacific Ocean, and the oxygen level rose.
The area contains what are called polymetallic nodules. These are lumps that are rich in metals and cover the seabed.
Some companies are interested in harvesting these mineral concretions and extracting the metals from them, which can be used in batteries, for example.
But perhaps the nodules themselves act as natural batteries.
The hypothesis in the study is that the nodules are able to create enough electricity to split water molecules via electrolysis, forming hydrogen and oxygen.
Not convinced
Lars-Kristian Trellevik is CSOO at the Norwegian company Adepth Minerals, which focuses on the extraction of seafloor minerals. He read the article with interest this summer.
“When this paper came out, it was exciting summer reading. But we were surprised by quite a few things,” says Trellevik, who has a PhD in deep-sea mineral exploration.
Trellevik and two colleagues have now published a critical article on the study.
“We are professionals in various disciplines who are interested in understanding the deep sea and the resources there. We’re focused on the Norwegian continental shelf. This study article is about nodules, a resource that we don’t have in the Norwegian ocean areas, so we have no vested interest in the topic in any way,” he says.
Why was oxygen created?
Trellevik and his colleagues point out several things that they believe weaken the study's credibility.
One of the things they reacted to is a numerical value in the study paper indicating that oxygen production occurred evenly.
Trellevik points out that the increase in oxygen might have been caused by the calm seabed being disturbed or by oxygen in the equipment when the measuring instrument was set down. The oxygen level then went down, as one would expect.
“The data indicate a disturbance of the system that is visible in all the measurements. After an increase in oxygen at the start, the level returns to the normally observed consumption of oxygen as reported in all other similar studies we are aware of.”
“We can’t read from these data that oxygen production is what’s going on, and not some other form of introducing or making oxygen available,” says Trellevik.
Concurs that oxygen level dropped again
Andrew Sweetman is a professor at the Scottish Association for Marine Science and led the dark oxygen study.
He does not normally respond to articles that have not been peer-reviewed but is making an exception in this case.
Sweetman concurs that the oxygen production was not linear and that there was an increase only in the first 24 hours and then less in the next 24 hours.
That is why he and his colleagues warned against upscaling the results in the article, for example from a square meter to a square kilometre, says Sweetman. The study researchers wrote that production did not increase steadily and linearly and might not have been continuous.
One of the peer-reviewers of the paper asked Sweetman’s team to calculate the numerical production value, says Sweetman.
He points out that the actual measurements did not start until three to 24 hours after the instrument had landed on the seabed and disturbed the ground. But it is possible that oxygen production started when the lander swirled away mud that had covered some of the nodule areas, according to Sweetman.
Hydrogen not measured
Adepth Minerals also criticized the researchers for not measuring whether hydrogen was created. Hydrogen would also be formed if electrolysis were behind the increase in oxygen.
“A natural control mechanism would be to measure hydrogen in the same way as you measure oxygen,” says Trellevik.
“We find the fact that this measurement was not taken somewhat unusual. In research and statistics, we want to have control data for what we measure – at least if we are striving to prove such a concrete hypothesis as this study sets out to do.”
Sweetman replies that they have not measured hydrogen.
“Last December I was in Chicago and took electrochemical measurements. It was only then that I realized that these nodules were potentially electrochemical oxygen producers because of the voltages we measured,” he says.
“At that point there weren’t any more chances to go out to sea to test whether hydrogen was generated.”
Sweetman hopes to measure hydrogen in further experiments.
He adds that oxygen production is the main finding in the paper.
“The idea that oxygen is mainly formed via electrolysis is only a hypothesis, and it requires further testing.”
The study article was submitted as a "brief communication," which is one of the Nature Geoscience paper formats.
“We didn’t write a full article, because then we would have needed solid evidence for seawater electrolysis,” says Sweetman.
Was the voltage high enough?
Trellevik and his colleagues also point out that the researchers state 0.95 volts as the highest value for one of the nodules. However, they believe the highest measurement appears to be far lower.
“Even if 0.95 volts were measured as stated in the article, that is still not enough current for electrolysis to take place,” says Trellevik.
Electrolysis can occur at 1.23 volts, according to article on dark oxygen.
Andrew Sweetman responds that scientists measured 0.95 volts – or 952 millivolts – and that it was not an unusually high reading. This information is attached to the article as source data. The researchers also measured 722, 485 and 600 millivolts.
“It may be that several nodules clustered together can generate sufficiently high voltage,” says Sweetman.
“It's like putting a battery in a flashlight, but it doesn’t work. You put another battery in, and then the flashlight works.”
“We didn’t reach the 1.23 volts required for seawater electrolysis. But if you put ordinary stones in seawater and test the voltage, there’s nothing there. You put these nodules in seawater, and you test almost one volt. There’s something really interesting about these nodules that needs to be explored further.”
But Trellevik points out that the problem with referring to the table of extended data is that, according to the description, this is uncorrected data that has not been corrected against background voltage. He maintains that the data that was corrected shows lower values and a voltage level far below what is required for electrolysis.
Missing something to compare it to
Trellevik also points to a lack of control data, such as data from a corresponding seabed location that does not have nodules.
Adepth Minerals questions whether the current could have come from somewhere other than the nodules.
“For example, ground faults are relatively common on electrical underwater instruments,” says Trellevik.
“The oxygen production data would look different if an electrical fault created the electrolysis,” says Sweetman.
An electrical fault would produce the same amount of oxygen in each trial, and the increase would have been constant for 48 hours, which is as long as this instrument was turned on. Oxygen production would not have levelled off.
Sweetman notes that they also saw oxygen production in laboratory experiments where nodules in fresh seawater laid in the dark for 48 hours.
“No electrical fault occurred there,” he says.
Sweetman shows a video of bubbles flowing out from a black nodule lying in water as described above.
Rejected the data for ten years
Sweetman says he has been working on these types of experiments for several years and also has 30 years of data from other researchers' experiments.
“These studies showed that the oxygen level drops in areas where no nodules are present.”
Sweetman saw an increase in oxygen concentration in a seabed area with nodules for the first time in 2013. He has seen the phenomenon several times since: in 2015, 2018, 2021 and 2022.
“I rejected the data for ten years. I’ve gone through these thought processes again and again and have come to the conclusion that something is going on. This is not due to errors with the experiments or the instruments. We don’t know exactly what the cause is, but the hypothesis is seawater electrolysis,” he says.
Claims study researchers are wrong about early Earth oxygen
Adepth Minerals is critical of the concluding sentence of the study, which suggests that the study result might shed light on biological evolution and the supply of oxygen to the atmosphere.
According to Adepth Minerals, it is unlikely that oxides, which make up the polymetallic nodules, could have formed without oxygen. Dark oxygen could thus not have contributed to early Earth oxygen production.
The statement in the study "indicates either a profound misunderstanding or intentional misrepresentation of the geological and chemical context," writes Adepth Minerals.
“If no metal oxides were deposited at that time, but a similar process was possible through other types of deposits, it would be interesting to think about how our results could relate to that, that's all I'm saying,” says Sweetman.
Thought about all this before
Trellevik emphasizes that he and his colleagues have only looked at the data presented in Sweetman’s paper and find that what is presented is weak.
“This type of revolutionary discovery opens everyone's eyes and is very exciting. But the discrepancy between the paper’s sensational headline and what it actually says is where we believe it fails.”
Sweetman says Trellevik and his colleagues have shared some interesting thoughts.
“But,” Sweetman says, “we have thought about all this already.”
References:
Andrew K. Sweetman, et.al.: Evidence of dark oxygen production at the abyssal seafloor. Nature Geoscience, 2024. Doi.org/10.1038/s41561-024-01480-8
Alden Denny, Werner Svellingen & Lars-Kristian Trellevik: Critical Examination of Evidence of Dark Oxygen Production at the Abyssal Seafloor. by Sweetman et al. in Nat. Geosci. 1–3 (2024).
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Translated by Ingrid P. Nuse
Read the Norwegian version of this article at forskning.no
