Dark oxygen: A step towards increased climate and nature alignment?

The oxygen cycle in oceans has been evidenced for many years, from the production of oxygen by photosynthetic organisms such as algae or plankton using sunlight, to the consumption of oxygen by deep-sea organisms.

The study published in July in the journal Nature Science¹  revealed that oxygen is not only consumed but also produced on the Pacific Ocean’s abyssal seafloor. Its findings showed this might be attributed to polymetallic nodules – which appear to break down water and create dark oxygen - laying along the seabed.

This scientific research is striking as it challenges the long-held view that oxygen can only be produced through sunlight via photosynthesis - hence the notion of dark oxygen being produced without sunlight far below the ocean surface. Importantly, it illustrates the potential role of polymetallic nodules in this process, as a kind of ‘geo-battery’ that would electrolyse seawater in hydrogen and oxygen thanks to their high voltage potential.²

There are different types of deep-sea mining currently envisaged, although still in an experimental stage: on the abyssal plains targeting polymetallic nodules; stripping seamounts (underwater mountains) of their outer layer of crust; and mining hydrothermal vents (cracks and fissures on the seabed which emits fluids seeming like black smoke, that are surrounded by rich biodiversity ecosystems).

Deep-sea mining targeting polymetallic nodules involves - depending on the processes used - removing or disturbing the top six-to-20 centimetres of the seafloor sediment, leading to the potential extinction of species living on or within it. Following exploration studies, scientists have warned that in these mined areas, biodiversity systems have not replenished, even more than 40 years after the tests¹ . As for the nodules themselves, they support complex ecosystems that would be entirely lost, causing species to become extinct² . 

Scientists have also warned about the indirect consequences that might be of wider impact, from the dispersion of sediment plumes created by the mining processes to the discharge of wastewater back into the ocean³ . Their indirect effects on ecosystems, on carbon cycling and storage into marine sediments are still unknown.

The oceans’ role as a major carbon sink, absorbing up to 30% of human-induced carbon dioxide emissions is well documented¹ . Accordingly, oceans have been the focus of various global agreements, from the United Nations Convention on the Law of the Sea (UNCLOS) in 1982 – a multilateral treaty codifying the standards and principles of international maritime law - to the adoption of the High Seas Treaty in 2023. The latter, also known as the agreement on Biodiversity Beyond National Jurisdictions (BBNJ), is an international agreement on the conservation and sustainable use of marine biological diversity of areas beyond national jurisdictions² .

One of the BBJN’s aims is to establish large-scale marine protected areas (MPAs) - all the more at stake as only about 1% of the high seas is currently protected³  - setting up rules to conduct environmental impact assessments and establishing the sharing of benefits from marine genetic resources⁴ .

The focus on the high seas outside of national jurisdictions is key, as they cover two-thirds of the ocean surface and 95% of its volume⁵ . Quite crucially, given the reported fracture between the Global North and the Global South, it entails the notion of fairness and equity. This aspect was tackled in the United Nations General Assembly as early as 1967, when the exploitation of polymetallic nodules seemed to promise substantial economic benefits that were proclaimed “the common heritage of mankind”⁶ . The High Seas Treaty now formalises this aspect⁷ , in a context of increasing demand for marine resources, whether food, minerals or biotechnologies. Treaties and agreements require ratifications though, and some are still needed before it can enter into force.

As seen above, and somehow surprisingly, the focus on deep-sea polymetallic nodules is not recent. However, it was many years later, in 1994, that the International Seabed Authority (ISA) was established, as an organisation through which states that are parties to UNCLOS organise and control all mineral resources-related activities in the area⁸ . The ISA has accordingly the mandate to ensure the effective protection of the marine environment from harmful effects that may arise from deep seabed-related activities, and to advance scientific knowledge and understanding of deep-sea ecosystems⁹ . Through its council, it seeks to draft a mining code to regulate the exploitation and extraction of polymetallic nodules on the ocean floor, before mining activities begin.