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First observations of a major organic carbon reservoir in the deep Earth

The study of the deep carbon cycle enables us to estimate the natural flow of carbon between the Earth's surface and deep interior, ultimately regulating our planet's climate. This cycle remains poorly understood, as the Earth's interior is not directly accessible. By studying samples expelled by mud volcanoes near the Mariana Trench, a team of researchers from the Institut de Physique du Globe de Paris (Université Paris Cité/IPGP/CNRS) has demonstrated the high-pressure formation and trapping of solid organic carbon in the lithosphere. This discovery, published in Science Advances, lifts the veil on a major reservoir of deep organic carbon.

First observations of a major organic carbon reservoir in the deep Earth

Publication date: 22/09/2022

Press, Research

The recycling of carbon to the Earth’s depths is far less constrained than the external carbon cycle, and the duration of these processes is a priori too long to counteract the increase in anthropogenic CO2 in the atmosphere. However, this study is fundamental, as deep carbon sequestration prevents it from being completely released back into the atmosphere (as is the case on Venus). It therefore maintains conditions conducive to the existence of life.

At the Marianas subduction zone, the Pacific oceanic lithosphere sinks beneath the Philippine plate. This subduction zone is unique in the world, as it is home to mud volcanoes that bring up chunks of hydrous mantle from depths of over 15 km. Samples of this deep mantle were collected between 2016 and 2017 in the vicinity of the Mariana Trench, during international expedition 366 of the IODP (International Ocean Discovery Program) ocean drilling program.

Photograph of a core from the Yinazao mud volcano. The mud contains centimetric chunks of hydrated mantle that have been torn off as the mud rises to the surface (© IODP Expedition 366 Closeup images).

Thanks to a combination of new micro-imaging and isotope geochemical approaches developed by the Geomicrobiology team at the Institut de Physique du Globe de Paris (Université Paris Cité, CNRS), the Centre de Recherches Pétrorographiques et Géochimiques (CRPG, CNRS/Université de Lorraine) and the GeoRessources laboratory (CNRS/Université de Lorraine), a team of international researchers has demonstrated the formation of a type of carbon never before observed in high-pressure mantle rocks: abiotic solid organic carbon (i.e. formed without the action of living organisms). It is thought to have been formed by the reduction of carbonates volatilized during the dehydration of the Pacific plunging plate, i.e. at depths of 25 km below the ocean floor. This discovery highlights a new carbon reservoir in the deep Earth.

The quantities of solid organic carbon abiotically formed at high pressure and stored in the mantle could represent 0.05 to 0.39 megatons of carbon per year. Thus, up to 54% of the carbon shipped to the planet’s depths in the Mariana subduction zone would be stored as organic carbon. These estimates show that the deep carbon cycle could be dominated by solid organic compounds of abiotic origin that were previously only observed near the surface in hydrothermal systems. The future of this new carbon reservoir remains to be determined. Its stability could be influenced by plate tectonics. In particular, the formation of mountain ranges, where two continental plates meet when an ocean closes via subduction, would facilitate the volatilization of the solid organic carbon contained in this reservoir and favor the development of life, not in the oceans, but on the continents.

 

Ref : High-pressure synthesis and storage of solid organic compounds in active subduction zones, B. Debret, B. Ménez, B. Walter, H. Bouquerel, P. Bouilhol, N. Mattielli, C. Pisapia, T. Rigaudier and H.M. Williams, Science Advances, 16 Sep 2022, Vol 8, Issue 37 – DOI: 10.1126/sciadv.abo2397

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