Non-biological organic carbon recycled in Earth’s depths

Organic carbon is traditionally associated with biological activity. However, a significant fraction of this carbon can also form abiotically, during the hydrothermal alteration of the oceanic lithosphere. When seawater infiltrates rocks at depth, it triggers chemical reactions that produce solid organic compounds, independently of any living processes. These compounds are then buried in subduction zones and transported to great depths, where they are subjected to extreme pressure and temperature conditions.

Reinterpreting isotopic markers

Using a combination of advanced spectroscopic techniques and isotopic analyses, the researchers studied Alpine metamorphic rocks that record deep burial conditions. Their observations show that these abiotic compounds are remarkably well-preserved during metamorphism, undergo limited chemical transformation, and retain a light isotopic signature.

Until now, such signatures have largely been interpreted as indicators of biological origin. These results demonstrate that they can also arise from purely metamorphic processes.

A key role in the deep carbon cycle

The study shows that these abiotic compounds likely represent the main source of light carbon in rocks subjected to high-pressure, high-temperature conditions during subduction. This carbon can then be recycled into the deep mantle, contributing to the isotopic diversity observed in some diamonds formed at great depths.

These findings call into question a fundamental assumption: that light carbon isotopic signatures necessarily indicate a biological origin.

New perspectives for Earth sciences

Beyond improving our understanding of the deep carbon cycle, this discovery opens new perspectives on the formation of organic carbon in extreme environments, on interactions between Earth’s surface and interior, and on the processes that may generate organic carbon on other planets.

Source : 

Organic carbon recycling in subduction zones, Nature Communications, 2026