Title: Evidence of dark oxygen production at the abyssal seafloor

Authors: Sweetman et al.

Journal & Year: Nature Geoscience, 2024

BLUF: Scientists have discovered unexpected oxygen production in total darkness at the deep seafloor, possibly driven by electrochemical reactions on polymetallic nodules. In Nature Geoscience (2024), Sweetman et al. report benthic chamber experiments in the Clarion–Clipperton Zone (CCZ) that consistently showed oxygen concentrations increasing over time — contrary to all prior deep-sea respiration studies. The researchers hypothesize that the metal-rich nodules act like natural "geo-batteries," generating oxygen through seawater electrolysis. While the mechanism remains speculative, this discovery highlights the active role nodules may play in seafloor chemistry. For the deep-sea mining industry, it reinforces the importance of continued environmental research and suggests that nodule fields are not only mineral assets but also biogeochemical hotspots that merit detailed study.

Oxygen is generally thought to enter the deep ocean via surface production and slow physical transport.

Yet this study offers direct experimental evidence of oxygen being produced in the absence of light — in situ and ex situ — in seafloor regions rich in polymetallic nodules.

These nodules are found scattered across the sedimented abyssal plains of the Pacific, including the Clarion–Clipperton Zone, a prime area for deep-seabed mining exploration.

Over a series of cruises between 2021 and 2022, researchers deployed benthic chamber landers to the NORI-D exploration zone, operated by Nauru Ocean Resources Inc. (a subsidiary of The Metals Company).

These instruments sealed off patches of seafloor and measured oxygen concentrations over 47-hour periods.

Against expectations, they observed not a decline in oxygen — common as benthic organisms consume it — but a significant increase, in some cases up to threefold relative to background levels.

The rise in oxygen concentration, termed dark oxygen production (DOP), was observed consistently across multiple deployments and treatments, including those without added nutrients or biological inoculants.

Control experiments ruled out contamination from air, oxygen diffusion from plastic materials, or mechanical artifacts.

Additional ex situ experiments in dark, sealed lab settings produced similar results, further supporting the conclusion that this oxygen generation was intrinsic to the system and not externally introduced.

Importantly, DOP was correlated with the surface area of polymetallic nodules in each chamber.

Nodules with higher exposed surface area tended to produce more oxygen, suggesting that the physical and chemical properties of the nodules themselves were a key driver of the effect.

The authors propose that polymetallic nodules can behave as natural electrochemical structures — essentially a “geo-battery.”

Figure 2 (p. 739) shows that most background-corrected surface voltages fall between roughly 0.05 V and 0.30 V , yet spot measurements climbed as high as 0.95 V (reported in the text and Extended Data Table 4), values that approach the threshold needed to electrolyse seawater under deep-sea conditions.

Box and whisker plots of background-corrected voltage potentials on nodule surfaces

This suggests that internal redox gradients within the nodules could provide enough energy to split water molecules, generating oxygen and hydrogen.

This mechanism is striking not only for its novelty but for its implications. If validated by future studies, it would mark the first known abiotic oxygen-producing process in deep-sea sediment systems involving polymetallic nodules.

While the presence of DOP introduces a new dynamic to benthic ecosystems, the study’s authors emphasize that the phenomenon appears spatially variable and episodic.

One interpretation is that sediment removal or disturbance — such as from mining collector tracks — could transiently expose reactive nodule surfaces, temporarily boosting DOP.

This suggests that, rather than always being disruptive, controlled sediment disturbance might occasionally enhance certain oxygenation processes.

However, the durability and ecological significance of this effect remain open questions. The oxygen spikes were not permanent, and declines over time may be linked to degradation of catalytic surfaces or reductions in electrochemical potential. 

For stakeholders in the deep-sea mining industry, this paper offers more than a surprising geochemical finding.

It showcases the kind of cutting-edge science that becomes possible only through collaboration with exploration companies and access to operational sites like NORI-D.

The findings also underscore the importance of supporting comprehensive environmental monitoring and research programs within exploration areas.

Moreover, the fact that this work was conducted under the sponsorship of The Metals Company, with contributions from public research institutions and academic scientists, illustrates a model of responsible, data-driven collaboration between industry and science.