🌍 'Non-biological' organic carbon in the depths of the Earth

A study published in Nature Communications highlights a major and previously underestimated source of deep carbon: organic compounds formed without biological intervention. These results challenge the classical interpretation of carbon isotopic signatures in the mantle and shed new light on the deep carbon cycle.

Organic carbon is traditionally associated with biological activity. However, a significant fraction of this carbon can form abiotically during hydrothermal alteration of oceanic lithosphere. When seawater infiltrates deep rocks, it triggers chemical reactions that produce solid organic compounds, independently of any biological activity.


These compounds are then buried in subduction zones, carried to great depths where they are subjected to extreme pressure and temperature conditions.

Thanks to a combination of advanced spectroscopic analyses and isotopic measurements, researchers studied Alpine metamorphic rocks, witnesses of deep burial. Their observations show that these abiotic compounds are remarkably preserved during metamorphism, undergo few chemical transformations, and retain a light isotopic signature.

Until now, this type of signature was widely interpreted as an indicator of biological origin. These results show that it can also result from purely metamorphic processes.

A key role in the deep carbon cycle

The study highlights that these abiotic compounds constitute the main source of light carbon in rocks subjected to high pressure and high temperature during subduction. This carbon can then be recycled into the Earth's mantle, contributing notably to the isotopic diversity observed in some diamonds formed at great depth.

This work leads to reconsidering a fundamental paradigm: the light isotopic signature of carbon in the mantle does not constitute proof of biological origin, quite the contrary.

Beyond understanding the deep carbon cycle, this discovery opens up important perspectives on the formation of organic carbon in extreme environments, on exchanges between the Earth's surface and interior, and on mechanisms that could produce organic carbon on other planets.