Blackwater lakes and rivers in the Congo Basin are releasing old carbon into the atmosphere, a new study shows. Previously, scientists believed that this carbon was safely stored in the surrounding peatlands, but the research shows that this is not the case.
The finding contradicts the long-held assumption that ancient peat carbon remains trapped underground, suggesting that some tropical peatlands may be transitioning from being carbon sinks to major carbon sources.
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Drake and his colleagues have completed three research trips to the Congo Basin in the past four years. In particular, the team traveled to the Cuvette Centrale, a 56,000 square kilometer region of forests and swamps in the Democratic Republic of Congo that has the largest known tropical peatland complex on Earth. Located at the heart and south of the Cuvette Centrale are two large blackwater lakes – Lake Mai Ndombe and Lake Tumba – while a large blackwater river, the Ruki River, flows west-northwest across it to meet Congo River.
Blackwater lakes and rivers contain high levels of decaying plant waste, or dissolved organic carbon, which gives them their black color. This dissolved organic matter, together with the direct input of carbon dioxide (CO2) from the surrounding swamps and forests, creates supersaturated concentrations of CO2 in Lakes Mai Ndombe and Tumba and in the Ruki River. As a result, these waters release huge amounts of CO2 into the atmosphere.
Crucially, however, none of the CO2 was previously thought to originate from Cuvette Centrale’s ancient peat, as these deposits, protected from degradation by their oxygen-poor, water-filled environment, were thought to be highly stable.
But in an article published on February 23 in the journal Nature GeosciencesDrake and his colleagues found something else. Their results showed that a significant proportion of the CO2 escaping from Cuvette Centrale’s blackwater deposits is from peat carbon between 2170 and 3500 years old.
“We were very surprised because we fully expected the carbon dioxide to be modern,” Drake said.
The researchers drew their conclusions from measurements they took at Lake Mai Ndombe in 2022 and 2024, and at Lake Tumba and the Ruki River in 2025. They got to Lake Mai Ndombe in small boats, which was difficult because of strong winds that nearly capsized them, Drake said.
“The ecosystems remain in a relatively pristine state,” he said. “There are some small settlements and villages scattered around Lake Mai Ndombe, but they are far and few between.”
The team measured sediments, greenhouse gases, dissolved organic carbon and dissolved inorganic carbon, which includes dissolved CO2, bicarbonate ions (HCO3–) and carbonate ions (CO32-). Later, in the laboratory, the researchers analyzed their samples with high-precision spectrometry to separate modern carbon from plants and older carbon from soil.
“Because the organic carbon in the lake was modern, we assumed the inorganic carbon would be as well, so we first analyzed a single sample to confirm,” Drake said. But when about 40% of the inorganic carbon in that sample turned out to be millennia old, the team decided to test the remaining samples.
The results were consistent across Lake Mai Ndombe, so the researchers returned to Cuvette Centrale to sample Lake Tumba and the Ruki River. Both contained high levels of inorganic carbon derived from old peat, suggesting that microbes in the region break down peat carbon into CO2 and methane, which then seep into lakes and rivers before escaping into the atmosphere.
Cuvette Centrale is estimated to contain a third of the carbon stored in tropical peatlands globally, equivalent to around 33 billion tonnes (30 billion tonnes). It is possible that recent losses of old peat carbon are linked to the formation of new peat deposits, in which case the phenomenon could be nature returning to a state of equilibrium, according to the study. But it is also possible climate change destabilizing long-buried deposits and that the peatlands of the Congo Basin are approaching a tipping point.
“This road highlights a critical vulnerability,” Drake said. “If the region experiences future drought, this export mechanism could accelerate, potentially tipping these massive carbon reservoirs from a sink to a major source of the atmosphere.”
Next, the researchers will analyze water trapped in the Congo Basin’s peat to explore whether and how microbes release ancient carbon.
“Ultimately, we aim to confirm whether this process occurs across the entire Cuvette Centrale and quantify oxidation rates to determine whether this leakage is a natural baseline or a sign of instability in this large carbon reservoir,” Drake said.
Drake, TW, Hemingway, JD, Barthel, M., De Clippele, A., Haghipour, N., Wabakanghanzi, JN, Van Oost, K., & Six, J. (2026). Millennial peat carbon outgassed by large humus lakes in the Congo Basin. Nature Geosciences. https://doi.org/10.1038/s41561-026-01924-3
