Congo Basin’s ‘blackwater’ lakes and rivers are now releasing ancient carbon into the atmosphere

Blackwater lakes and rivers in the Congo Basin are releasing ancient carbon into the atmosphere, a new study has found. Previously, scientists thought this carbon was safely stored in the surrounding peatlands, but this study shows that this is not the case.

The finding contradicts the long-held assumption that old peat carbon remains locked underground and suggests that some tropical peatlands may switch from carbon sinks to major carbon sources.

“We are now faced with a problem of 30 million tonnes, and we need to determine whether this is a small natural leak of ancient carbon or the beginning of a large-scale destabilization,” study lead author Travis Drake, a carbon biogeochemist at the Swiss Federal Institute of Technology Zurich (ETH Zurich), told Live Science in an email.

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Drake and his colleagues have conducted three research trips to the Congo Basin over the past four years. Specifically, the team visited the Cuvette Central region, a 56,000 square mile (145,000 square kilometer) forest and swamp area in the Democratic Republic of the Congo. It contains the largest known tropical peatland complex on Earth. Located in the heart and south of central Cuvette are two large blackwater lakes: Lake Mai Ndombe and Lake Tumba. Meanwhile, the Ruki River, a major blackwater lake, flows west-northwest and joins the Congo River.

Blackwater lakes and rivers contain large amounts of decaying plant debris and dissolved organic carbon, which gives them their black color. This dissolved organic matter, combined with direct inflows of carbon dioxide (CO2) from surrounding marshes and forests, creates supersaturated concentrations of CO2 in Lakes Mai Ndombe and Tumba and the Ruki River. As a result, these bodies of water release large amounts of carbon dioxide into the atmosphere.

Importantly, however, these deposits were previously not thought to have originated from the ancient peat in the center of the cuvette because they were protected from decomposition by the oxygen-deficient, waterlogged environment and were thought to be very stable.

But in a paper published February 23 in the journal Nature Geoscience, Drake and his colleagues found that this is not the case. Their results showed that a significant portion of the CO2 escaping from the black waters of central Cuvette comes from peat carbon that is 2,170 to 3,500 years old.

“We were very surprised because we fully expected carbon dioxide to be modern,” Drake said.

The researchers drew their conclusions from measurements taken at Lake Mai Ndombe in 2022 and 2024, and at Lake Tumba and the Ruki River in 2025. Researchers accessed Lake Mai Ndombe in a small boat, but it was difficult because strong winds nearly capsized the boat, Drake said.

“The ecosystem remains relatively pristine,” he says. “There are some small hamlets and villages scattered around Lake Mai Ndombe, but they are far apart and few and far between.”

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The team measured sediment, greenhouse gases, dissolved organic carbon, and dissolved inorganic carbon, including dissolved CO2, bicarbonate ions (HCO3-), and carbonate ions (CO32-). Then in the lab, the researchers analyzed the samples with a high-precision spectrometer to separate modern carbon from plants and old carbon from soil.

“Because the organic carbon in the lake was modern, we assumed that the inorganic carbon was also modern, and initially we only analyzed a single sample to confirm,” Drake said. But after discovering that about 40% of the inorganic carbon in the sample was thousands of years old, the researchers decided to test the remaining samples.

The results were consistent across Lake Mai Ndombe, so the researchers returned to the Cuvette Central Laboratory and collected samples from Lake Tumba and the Ruki River. Both contain high levels of inorganic carbon derived from ancient peat, suggesting that local microorganisms break down peat carbon into CO2 and methane, which seep into lakes and rivers before becoming airborne.

The center of the cuvette is estimated to hold one-third of the carbon stored in the world’s tropical peatlands, or about 33 billion tonnes (30 billion tonnes). The recent loss of ancient peat carbon may be linked to the formation of new peat deposits, in which case the phenomenon could be nature returning to equilibrium, the study says. But the Congo Basin’s peatlands may be nearing a tipping point, as climate change destabilizes long-buried mineral deposits.

“This pathway highlights significant vulnerabilities,” Drake said. “If the region experiences future droughts, this export mechanism could accelerate, tipping these huge carbon reservoirs from sinks to major sources of supply to the atmosphere.”

Next, the researchers will analyze water trapped in peat in the Congo Basin to see if and how microbes are releasing ancient carbon.

“Ultimately, we aim to confirm whether this process is occurring throughout the center of the cuvette and quantify the oxidation rate to determine whether this leakage is a natural baseline or a sign of instability in this large carbon reservoir,” Drake said.