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Deep beneath the ocean's surface lies Earth's largest carbon reservoir: marine sediments that have accumulated organic matter over millions of years. Long assumed to be permanently "locked away," this vast carbon pool is far more dynamic than scientists previously believed, according to a new international study.

A research team co-led by Prof. Wang Faming from the South China Botanical Garden of the Chinese Academy of Sciences, along with collaborators from the University of Bremen and Harvard University, has discovered that heat can "awaken" this ancient carbon, converting it into a food source for microbes living far below the seafloor.

Published in , the findings reshape scientific understanding of both life in extreme environments and the deep Earth carbon cycle.

Marine sediments heated to above 40 °C make up nearly half of the global marine sediment volume. Yet until now, the processes sustaining microbial life under such high-temperature conditions remained unclear.

Previous research has documented microbial populations surviving in a geothermally heated sediment sequence roughly 1.2 kilometers deep; however, the source of the energy-rich substrates on which those microbes rely had yet to be identified. This new study fills that gap by detailing the mechanisms that supply organic matter to these hidden subsurface ecosystems.

To conduct their research, the team analyzed 7.8-million-year-old sediment samples collected from the Shikoku Basin in the Pacific Ocean. Using advanced analytical techniques, the researchers developed a model that illustrates how the interaction of biological and abiotic (non-living) processes transforms refractory organic matter—material resistant to decomposition—into forms that microbes can use.

Notably, this model operates opposite to the carbon sequestration mechanisms observed at Earth's surface: A "reversal" of the mineral carbon pump occurs at temperatures above 35 °C, while an inversion of the microbial carbon pump emerges at temperatures exceeding 55 °C.

"These slow processes act like a reverse microbial carbon pump—normally a mechanism that stabilizes carbon at Earth's surface—yet here the heat reactivates carbon that is millions of years old," explained Prof. Gan Shuchai, the study's first author.

At 85 °C, the team found that these carbon-reactivation processes accelerate, producing simple compounds that fuel microbial life. At the same time, biological degradation chains tend to break down and abiotic processes take over.

They restructure the common organic matter-based microbial communities found in anaerobic (oxygen-free) environments and facilitate the final steps of mineralization—the process by which organic substances are broken down into inorganic forms.

Even though only about 0.25% of the organic carbon in these sediments becomes bioavailable, the sheer size of the reservoir makes this a significant energy source. The marine sediment carbon pool holds an estimated 15 million gigatons (Gt) of carbon—compared to 39,000 Gt in the entire oceanic water column—providing enough energy to sustain extensive subsurface ecosystems known as the "deep biosphere."

This study uncovers a key mechanism of deep Earth carbon cycling and reshapes our understanding of planetary processes.

These findings not only resolve a long-standing uncertainty about the energy sources sustaining the deep biosphere in high-temperature marine sediments but also add a critical new dimension to models of Earth's carbon cycle.