‘Blackwater’ lakes and rivers in the Congo Basin are now emitting ancient carbon into the atmosphere
The Congo Basin’s Blackwater lakes and rivers are releasing ancient carbon into the atmosphere, a new study suggests. Previously, scientists thought this carbon was safely stored in surrounding peatlands, but research reveals this is not the case.
The finding contradicts the long-held hypothesis that old carbon in peat remains trapped underground, suggesting that some tropical peatlands could transition from carbon sinks to important carbon sources.
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Drake and his colleagues have made three research trips to the Congo Basin over the past four years. Specifically, the team traveled to the Cuvette Centrale, a 56,000-square-mile (145,000-square-kilometer) region of forests and swamps in the Democratic Republic of Congo that is home to the largest known tropical peatland complex in the world. At the heart and south of the Central Basin are two large blackwater lakes — Lake Mai Ndombe and Lake Tumba — while a major blackwater river, the Ruki River, crosses it from west to northwest to join the river. Congo River.
Blackwater lakes and rivers contain high levels of decaying plant debris or dissolved organic carbon, which gives them their black color. This dissolved organic matter, combined with direct inputs of carbon dioxide (CO2) from 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.
Importantly, it was previously thought that no CO2 came from the ancient Central Basin peat, because these deposits, protected from decomposition by their waterlogged, oxygen-depleted environment, were considered very stable.
But in an article published on February 23 in the journal Natural geosciencesDrake and his colleagues found the opposite. Their results showed that a significant proportion of the CO2 escaping from the black water bodies of the Central Basin comes from carbon from peatlands aged 2,170 to 3,500 years.
“We were very surprised because we expected carbon dioxide to be modern,” Drake said.
The researchers drew their conclusions from measurements they took at Lake Mai Ndombe in 2022 and 2024, as well as at Lake Tumba and the Ruki River in 2025. They accessed Lake Mai Ndombe with small boats, which was difficult due to strong winds that almost capsized them, Drake said.
“Ecosystems remain in a relatively intact state,” he said. “There are some small settlements and villages scattered around Lake Mai Ndombe, but they are very rare.”
The team measured sediment, 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 soils.
“Since the organic carbon in the lake was modern, we assumed the inorganic carbon would be modern as well, so we initially analyzed a single sample to confirm,” Drake said. But when about 40 percent of the inorganic carbon in that sample was found to be millennia old, the team decided to test the remaining samples.
As the results were consistent across Lake Mai Ndombe, the researchers returned to the Cuvette Centrale to sample Lake Tumba and the Ruki River. Both contained high levels of inorganic carbon derived from ancient peat, suggesting that microbes in the region break down peat carbon into CO2 and methane, which then seep into lakes and rivers before spreading into the atmosphere.
The Central Basin is estimated to hold a third of the carbon stored in tropical peatlands globally, the equivalent of around 33 billion tonnes (30 billion metric tonnes). It is possible that recent carbon losses from old peat are linked to the formation of new peat deposits, in which case the phenomenon could be a return of nature to a state of equilibrium, according to the study. But it is also possible that climate change is destabilizing long-buried deposits and the Congo Basin peatlands are approaching a tipping point.
“This pathway highlights a critical vulnerability,” Drake said. “If the region experiences a future drought, this export mechanism could accelerate, potentially switching these enormous carbon reservoirs from being a sink to being a major source in the atmosphere.”
Next, researchers will analyze water trapped in Congo Basin peat to determine if and how microbes release ancient carbon.
“Ultimately, we aim to confirm whether this process is occurring throughout the Central Basin and to 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, T.W., Hemingway, J.D., Barthel, M., De Clippele, A., Haghipour, N., Wabakanghanzi, J.N., Van Oost, K., and Six, J. (2026). Thousand-year-old peat carbon degassed by the great humic lakes of the Congo Basin. Natural geosciences. https://doi.org/10.1038/s41561-026-01924-3
