At a time when industrial companies are interested in the metals and rare earths found in the deep sea, Ifremer scientists are heading for the Clarion-Clipperton Zone (CCZ) to discover the biodiversity that lives on and around polymetallic nodules.

Aboard L'Atalante, a vessel of the French Oceanographic Fleet operated by Ifremer and its armament subsidiary Genavir, the scientists of the EDEN campaign are studying the abyssal plains of this Pacific region from November 2024 to January 2025 to gain a better understanding of the biodiversity associated with polymetallic nodules and how these ecosystems function.

The precipitation of metals dissolved in seawater can lead to the formation of polymetallic nodules on certain abyssal plains at depths of between 4,000 and 6,000 m. These pebbles, 5–10 cm in diameter, grow at a rate of around ten mm per million years, forming "nodule fields" that are home to a wide variety of as-yet little-known fauna. At a time when some industrialists were considering harvesting these metal-rich rocks, Ifremer scientists were visiting the French exploration contract zone to gain a better understanding of the ecosystems that depend on these nodules. From November 19, 2024, to January 2, 2025, aboard L'Atalante, they will study the fauna of these abyssal plains and the links between biodiversity and nodule abundance and distribution.

This is the 5th oceanographic campaign conducted by Ifremer since 2004, as part of the nodule exploration contract awarded to the institute by the International Seabed Authority (ISA) on behalf of France. The aim is to identify the baseline state of nodule plains ecosystems, a prerequisite for assessing the potential impact of mining operations. This research is also in line with the objective of acquiring knowledge of the deep seabed as part of the "deep seabed" program of France 2030, which is funding the campaign.

Nodules Provide a Base

Polymetallic nodules are the only solid support available on the unconsolidated soils of the abyssal plains, making them an ideal base for fixed organisms such as cold-water corals, sponges, and cnidarians. Around the nodules, buried in the sediment, crustaceans, echinoderms (including sea stars), marine worms and microbial communities cohabit.

"Species diversity can be very high, but most are very rare, so only a few individuals of each species are observed on each mission. Since the 1980s, several hundred species have been observed, nearly 90% of which were new. The challenge is not only to inventory them but also to understand the role each plays in the ecosystem and their life cycles,” comments Pierre-Antoine Dessandier, Ifremer Benthic Ecology Researcher and Mission Leader.

During the EDEN campaign, Ifremer teams will be taking nearly 120 sediment samples from the Clarion-Clipperton zone. Their objective is to use the latest taxonomic methods (environmental DNA, imaging, and artificial intelligence) to draw up a complete inventory of the species present in these ecosystems and identify the links between the different communities of organisms. Scientists will also complete estimates of the abundance and distribution of nodules in the area, begun during a previous campaign in March 2024. The aim is to better understand the links between biodiversity and polymetallic nodules, both because they represent a habitat for fixed organisms and because of their role in bottom currents and the deposition of organic matter.

Can Biodiversity Return?

The EDEN campaign will also quantify the impacts of polymetallic nodule mining. As part of the European MiningImpact research project of the Joint Programming Initiative Oceans, scientists will also visit the Belgian contract area, where a prototype nodule collector was deployed in 2021 by the company GSR.

"After several years, we can still see the tracks of the caterpillars on the bottom. Our objective on site is to check whether the fauna has been able to return and develop on the site since the collector passed through. This is one of the great unknowns about abyssal plains species: what is their capacity to recolonize an area disturbed by human activity? To find out, we are seeking to better understand their life cycle, their short- and long-term reactions to anthropogenic stress, identify their breeding grounds, and measure the distance they can travel,” adds Pierre-Antoine Dessandier.

In the area covered by the French exploration contract, almost 50% of the seabed consists of slopes with an inclination of more than five degrees. This will be particularly studied during the EDEN campaign. Indeed, in the event of nodule exploitation being authorized by a country, prototype collectors are not capable of operating on these slopes, and they could, therefore, constitute refuge areas for biodiversity.