Organic carbon preferentially recycled in the deep Earth

On the geological time scale, terrestrial carbon undergoes a long cycle during which it is degassed into the atmosphere via various processes (volcanism, etc.), then trapped in the oceanic lithosphere and finally recycled into the planet’s interior at the level of subduction zones.

Although the time scales involved in this long carbon cycle do not allow it to play a role in the problems of increasing anthropogenic CO2 in the atmosphere, this deep carbon cycle plays a vital role on a global scale. Indeed, deep carbon sequestration helps maintain conditions conducive to life on Earth, by preventing all the carbon from being degassed into the atmosphere (as is the case for Venus, for example).

But as direct sampling of the planet’s depths is impossible, this deep cycle is still very poorly understood. Particular attention is currently being paid to the processes at work at the time of subduction, since some of the carbon present in the oceanic plate could be remobilized during the dehydration of serpentinites, while the rest of the carbon remains trapped in the plate and sinks to the depths of the mantle, feeding the deep carbon cycle.

In a study published at the end of January in the journal Nature Communications, a team of researchers from the Centre de recherches pétrographiques et géochimiques (CNRS, Université de Lorraine), Institut de physique du globe de Paris, Université de Paris and Durham University (UK) set out to determine the mobility of inorganic and organic carbon during the recycling of oceanic lithosphere into the deep mantle. The carbon contained in oceanic lithosphere comes in a variety of forms, including organic and inorganic (e.g. carbonates).

In this study, the international team sampled subduction remnants outcropping in the Swiss Alps (Zermatt Zaas massif). Through petrographic and isotopic (Sr-C-O isotopes) studies, they demonstrated the decoupled behavior of inorganic and organic carbon at high pressure. During dehydration of these serpentinites at the time of subduction, inorganic carbon is preferentially mobilized in the fluid phase in the form of CO2, while organic carbon remains trapped in the rock in the form of inclusions in high-pressure minerals. These observations suggest a preferential recycling of organic carbon towards the Earth’s interior during subduction. Inorganic carbon, on the other hand, is strongly mobilized in the fluid phase and remains in the planet’s surface crust.

Ref:  Decoupling of inorganic and organic carbon during slab mantle devolatilisation.
Nature Communications – Bouilhol, P., Debret, B., Inglis, E.C. et al. DOI : https://doi.org/10.1038/s41467-022-27970-0