Discovery of CO₂ hydrates off the coast of Mayotte: a unique site for studying carbon storage in the ocean

The data collected on these CO₂ hydrates discovered in the Indian Ocean have been analysed by an international team comprising Ifremer, the Paris Institute of Earth Physics (IPGP), the French Alternative Energies and Atomic Energy Commission (CEA), the French National Centre for Scientific Research (CNRS), the National Oceanic and Atmospheric Administration (NOAA) and the University of Milan.

Clusters of solid CO₂ on the ocean floor

Hydrates are solid compounds similar to ice, consisting of water and gas molecules. In the natural environment, hydrates are usually composed of methane, and it is extremely rare to find carbon dioxide hydrates on the ocean floor.

This is the first time that clusters of CO₂ hydrates have been observed on the ocean floor that remain stable over several years, of this size and in such quantities. Composed of agglomerated CO₂ droplets, these domes range in height from a few centimetres to 2 metres. This discovery raises new questions about the natural mechanisms of temporary CO₂ storage in the ocean. It could also inform discussions on certain geoengineering approaches aimed at mitigating climate change,” says Cécile Cathalot, a researcher in marine geochemistry at Ifremer and lead author of the study.

These hydrates have been observed within the active volcanic structure known as the Fer à cheval, located 10 km east of the island of Mayotte. Surrounded by cliffs reaching 250 metres in height, this 6 km² underwater feature is one of the many structures in the underwater volcanic chain that stretches eastwards from Mayotte to the Fani Maore underwater volcano. It forms a semi-enclosed space within which CO₂ released from the seabed accumulates periodically in time with the tides.

Furthermore, this site offers the conditions necessary for the formation of hydrates: a combination of cold water—here at 4 degrees—and sufficient pressure exerted by the water column at a depth of 1,400 metres.

“At the Fer à Cheval site, CO₂ hydrates form when droplets of liquid CO₂ come into contact with cold water under high pressure. A solid film then develops on their surface, the growth of which depends closely on temperature, salinity and emission rate. What is remarkable here is that, despite the ocean currents, these hydrates have been able to grow and form large, relatively stable structures,” explains Olivia Fandino, a researcher in the physical chemistry of gas hydrates at Ifremer.

Structures associated with the Fani Maore volcano

It is likely that the emergence of these magmatic sources of liquid CO₂ in the Fer à Cheval area is linked to the seismic-volcanic crisis affecting the island of Mayotte, which manifested itself notably in the formation of the new Fani Maore volcano discovered in 2019. This activity has probably destabilised the volcanic structure of the Fer à cheval, which was formed well before the eruption of Fani Maoré.

Unlike Fani Maoré, which has shown no activity since 2021, the Fer à cheval site remains highly active in terms of seismic activity and fluid emissions, particularly CO2.

A joint campaign led by Ifremer and OceanX has enabled a return to this site of interest four years later.

“Thanks to the Argus ROV deployed from the OceanXplorer vessel, we found that the hydrate cluster field appeared to have remained stable since 2021.” This formation of hydrates depends on the inflow and outflow of carbon dioxide over time: this is an initial insight into the ability of hydrate deposits to store carbon dioxide over several years,” explains Carla Scalabrin, a researcher in water column acoustics at Ifremer.

Studying how biodiversity adapts to ocean acidification

The behaviour of these domes, which sequester liquid CO₂ and then release it as they dissolve, will be monitored over the long term to gain a better understanding of the mechanisms involved and to assess their viability in the medium to long term.

This monitoring, carried out during the MAYOBS missions (IPGP, IPGS, BRGM, IFREMER) and as part of the Mayotte Volcanological and Seismological Monitoring Network (REVOSIMA, IPGP), could also help to determine the consequences of marine acidification for biodiversity.

“We have identified around twenty species related to the coral group in the Fer à Cheval area and observed increased mortality near the clusters, likely linked to the acidity of the environment caused by these CO₂ leaks. To gain a better understanding of local biodiversity and its response to ocean acidification, we need to take this further with more systematic and in-depth sampling,” emphasises Marjolaine Matabos, a researcher in benthic ecology at Ifremer.

This discovery will therefore enable researchers to study the ability of the surrounding biodiversity to thrive and adapt to changes in the acidity of its environment.

Read the publication here: ‘Significant CO₂ seeps and hydrate deposits on the seabed off the coast of Mayotte’

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