'Blackwater' lakes and rivers are releasing ancient carbon into the atmosphere

 Congo based 'Blackwater' lakes and rivers are releasing ancient carbon into the atmosphere 

Deep in the Congo Basin, vast peatlands quietly store enormous amounts of Earth’s carbon, but new research suggests this ancient vault may be leaking. Scientists studying Africa’s largest blackwater lakes discovered that significant amounts of CO2 bubbling into the atmosphere come not just from recent plant life, but from peat that has been locked away for thousands of years. Carbon that has been buried in the Congo Basin's peatlands for millennia is seeping into lakes and rivers. Why this is happening remains unclear, but researchers warn that tropical peatlands could be nearing a tipping point. Blackwater lakes and rivers releasing carbon is up to 3,500 years old. Previously, scientists thought this carbon was safely stored in the surrounding peatlands, but the research reveals that's not the case. The finding contradicts the long-held assumption that old peat carbon remains trapped underground, suggesting that some tropical peatlands could switch from being carbon sinks to major carbon sources. "We are now faced with a 30-million-tonne question: we need to determine if this is just a small, natural leakage of ancient carbon, or the onset of broadscale destabilization," study lead author Travis Drake, a carbon biogeochemist at the Swiss Federal Institute of Technology Zurich (ETH Zurich), said.

At the confluence of the Fimi and Kasai rivers in the Democratic Republic of Congo, dark water from forest landscapes meets water from the savannahs, colored red by iron oxides. Tropical swamps and peatlands are critical players in Earth's carbon cycle and, by extension, the global climate. In regions such as the Amazon Basin, the Congo Basin, and the wetlands of Southeast Asia, thick layers of partially decomposed plant material build up over time. Together, these ecosystems lock away roughly 100 gigatonnes of carbon. At the center of Africa, the Congo Basin contains one of the largest and most significant of these carbon reserves. Although its peatlands and swamps cover just 0.3 % of the planet's land surface, they store about one third of all carbon held in tropical peatlands worldwide. Despite their importance, these remote ecosystems have not been extensively studied. Large parts of the central Congo Basin are difficult to reach, and travel to isolated lakes and swamps often requires boats or traditional pirogues. As a result, their influence on the global climate has remained uncertain. Drake and his colleagues have conducted 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 km's) region of forests and swamps in the Democratic Republic of the Congo which holds Earth's largest known tropical peatland complex. Situated in the heart and to the south of the Cuvette Centrale are two large blackwater lakes known as Lake Mai Ndombe and Lake Tumba, while a major blackwater river, the Ruki River, flows west-northwest across it to meet the Congo River.

Measurements show that substantial amounts of CO2 are escaping from both lakes into the atmosphere. However, the origin of that carbon was not what scientists anticipated. While some of the emissions come from recently grown plant material, up to 40 % of the CO2 originates from peat which accumulated thousands of years ago in nearby ecosystems. Researchers determined this by analyzing the age of the dissolved CO2 using radiocarbon dating (radiocarbon dating). "We were surprised to find that ancient carbon is being released via the lake," explains lead author Travis Drake. "The carbon reservoir has a leak, so to speak, from which ancient carbon is escaping," adds co-author Matti Barthel, research technician in SAE. 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, together with direct inputs of 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 emit enormous amounts of CO2 into the atmosphere. Crucially, however, none of the CO2 was previously thought to originate from the Cuvette Centrale's ancient peat, as these deposits, protected from decomposition by their oxygen-depleted, waterlogged environment, were believed to be highly stable. But Drake and his colleagues found otherwise. Their results showed that a significant proportion of the CO2 escaping the Cuvette Centrale's blackwater bodies is from peat carbon that is between 2,170 and 3,500 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 accessed Lake Mai Ndombe with small boats, which was difficult due to strong winds that almost capsized them, Drake said.

Most locations are almost impossible to reach by land. Therefore, small dinghy boats were used to access these remote sites in the central Congo Basin. "The ecosystems remain in relatively pristine condition," he said. "There are some small settlements and villages scattered around Lake Mai Ndombe, but they are far and few between". Over the past decade, a team led by ETH Zurich has been investigating the Congo Basin more closely. Their work has already revealed unexpected findings, including the Ruki River, one of the darkest blackwater rivers on Earth. Lake Mai Ndombe is more than four times larger than Lake Constance, and its water resembles strong black tea. It is bordered by vast swamp forests and largely undisturbed lowland rainforest growing atop deep peat deposits. As plant debris and soil organic matter wash into the lake from surrounding forests, they tint the water a deep brown. 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 lab, the researchers analyzed their samples with high-precision spectrometry to separate modern carbon from plants and older carbon from soils. "Because the organic carbon in the lake was modern, we assumed the inorganic carbon would be too, so we initially just 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 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 are breaking down peat carbon into CO2 and methane, which then seep into lakes and rivers before wafting into the atmosphere.

Previously, scientists believed that carbon stored in Congo Basin peat remained locked away for extremely long periods and would only be released under specific conditions such as extended drought. Exactly how this old carbon is being freed from undecomposed plant matter remains uncertain. Researchers also do not yet know the precise pathways which allow it to move from peat soils into lake water. Understanding whether this release signals a destabilizing shift or reflects a natural balance offset by new peat formation is now a key research question. The Cuvette Centrale is estimated to hold one-third of the carbon stored in tropical peatlands globally, equivalent to about 33 billion tons (30 billion metric tons). It's possible that recent losses of ancient peat carbon are linked to the formation of new peat deposits, in which case the phenomenon might be nature returning to a state of equilibrium. But it's also possible that climate change is destabilizing long-buried deposits and that the Congo Basin's peatlands are nearing a tipping point. Climate is not the only factor which could disrupt this system. Land use changes may pose an even greater threat. The population of the Democratic Republic of Congo is projected to triple by 2050, increasing demand for farmland and leading to further forest clearing. Deforestation can intensify drought conditions, potentially keeping lake levels persistently low. "We all know the analogy whereby forests are the green lungs of the Earth," says Barthel. "They are not only responsible for gas exchange like our lungs, however, but they also evaporate water through their leaves, thereby enriching the atmosphere with water vapor. This promotes cloud formation and precipitation, which in turn feeds rivers and lakes."

The researchers will analyze water trapped in the Congo Basin's peat to explore if and how microbes are releasing ancient carbon. "This pathway 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 into a major source to the atmosphere." The escape of ancient carbon could point to a broader concern. Environmental changes driven by climate change may be triggering processes which can increase carbon release. If conditions become drier, peat soils may dry out more frequently and for longer periods. This allows oxygen to penetrate deeper into the peat layers, accelerating microbial breakdown of once stable organic material. As decomposition speeds up, more CO2 from this enormous carbon store could enter the atmosphere. "Our results help to improve global climate models, because tropical lakes and wetlands have been underrepresented in these models so far," as stated by researcher. "Ultimately, we aim to confirm whether this process is happening across the entire Cuvette Centrale and quantify oxidation rates to determine if this leakage is a natural baseline or a sign of instability in this large carbon reservoir," Drake said.

Beyond CO2, the team also studied emissions of nitrous oxide and methane from Lake Mai Ndombe. In a parallel study, they found that water levels strongly influence how much methane escapes. When lake levels are high, microorganisms more effectively consume methane before it can reach the atmosphere. During the dry season, when water levels drop, methane is broken down less efficiently and larger amounts are released. "Our fear is that climate change will also upset this balance. If droughts become longer and more intense, the blackwater lakes in this region could become significant sources of methane that impact on the global climate," says ETH Professor Jordon Hemingway. "At present we do not know when the tipping point will be reached." These results sharpen our understanding of how tropical peatlands and blackwater lakes influence global climate dynamics. They also highlight the urgency of protecting Congo Basin wetlands and limiting greenhouse gas emissions. The research was conducted as part of the TropSEDs project led by ETH Zurich and funded by the Swiss National Science Foundation, in collaboration with scientists from the University of Louvain in Belgium and the Democratic Republic of Congo.