Congo Basin Peatlands Are Leaking Carbon That Is Thousands of Years Old
Beneath the remote swamp forests of the Congo Basin, packed into waterlogged soils that stretch across a territory the size of England, lies one of the planet's most important carbon stores. The peatlands here - saturated with partially decomposed plant material accumulated over millennia - hold roughly one-third of all the carbon stored in tropical peat worldwide, despite covering only 0.3 percent of Earth's land surface. The prevailing assumption has been that this carbon is stable: locked in place by the very water table that prevents decomposition.
New measurements from two lakes at the heart of this system suggest the store is not as static as assumed. Radiocarbon dating of dissolved carbon dioxide in Lac Mai Ndombe and neighboring Lac Tumba shows that a substantial fraction - up to 40 percent - of the CO2 being emitted from these lakes originated not from recently dead plant matter but from peat that accumulated thousands of years ago. The findings were published in Nature Geoscience by a team led by ETH Zurich. A companion study on methane and nitrous oxide from the same lakes appeared in the Journal of Geophysical Research.
The Blackwater Lakes of Central Africa
Lac Mai Ndombe is Africa's largest blackwater lake - its water the color of strong tea, stained by organic compounds leached from the surrounding peat and swamp forest. More than four times the area of Lake Constance, it sits at the center of a vast wetland landscape that receives input from virtually untouched lowland rainforest growing directly on thick peat. Lac Tumba lies nearby, smaller but similarly dark and similarly surrounded by swamp forest.
An ETH Zurich research group, part of the TropSEDs project funded by the Swiss National Science Foundation, has spent the last decade attempting to characterize the carbon dynamics of this region. Getting there requires boats and pirogues - roads into the central Congo Basin are few, and the remoteness of the area has limited scientific study considerably compared to the Amazon or the peatlands of Southeast Asia.
Old Carbon in a Young Process
The crucial measurement was radiocarbon dating of the CO2 dissolved in lake water. Radiocarbon - carbon-14 - is continuously produced in the atmosphere and absorbed by living plants, but it decays over time. Carbon from organic matter that died recently contains plenty of carbon-14; carbon from peat that has been sequestered for thousands of years is depleted in carbon-14. By measuring the carbon-14 content of the dissolved CO2, the researchers could estimate how old the carbon was when it entered the lake.
The expectation was that most of the carbon would be young - recently produced by decomposing leaf litter and plant roots in the surrounding forest. Instead, up to 40 percent of the carbon carried the isotopic signature of ancient peat.
"We were surprised to find that ancient carbon is being released via the lake," said lead author Travis Drake, a scientist in the Sustainable Agroecosystem group led by ETH Professor Johan Six. "The carbon reservoir has a leak, so to speak, from which ancient carbon is escaping," added co-author Matti Barthel.
The Mechanism Is Unknown
How the ancient carbon is being mobilized from the surrounding peat and transported into the lake is not yet established. The pathways - whether through groundwater, surface runoff, or some other route - are still unknown. It also remains unclear whether this release represents a natural, long-term equilibrium process that is balanced by new peat accumulation, or whether it signals an accelerating destabilization that could tip the system toward net carbon release.
That distinction matters enormously for climate projections. If the release of old carbon is simply how these systems function at a natural steady state - with new peat forming at the same rate that old peat dissolves and outgasses - then the net effect on the atmosphere may be small. But if climate change is accelerating peat decomposition, or if the current release rate reflects a departure from historical equilibrium, the implications for global carbon budgets could be serious.
Water Levels, Methane, and a Deeper Vulnerability
The companion study, published in the Journal of Geophysical Research, added another dimension. It found that water levels in Lac Mai Ndombe strongly influence methane emissions. When lake levels are high, aquatic microorganisms break down methane before it reaches the surface. When levels drop during dry seasons, less methane is oxidized, and larger quantities escape into the atmosphere.
The concern is that climate projections for the Congo Basin include more frequent and intense droughts. Prolonged lower water levels would simultaneously reduce methane oxidation and allow oxygen to penetrate deeper into peat layers, accelerating microbial decomposition of material that has been stable for millennia. "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," said ETH Professor Jordon Hemingway.
Land use change adds a separate pressure. The population of the Democratic Republic of Congo is projected to roughly triple by 2050, driving forest clearance for agriculture. Deforestation reduces evapotranspiration, lowering regional rainfall and lake levels - compounding the drought-related vulnerabilities. These research findings feed into global climate models that have historically underrepresented tropical lakes and wetlands, and they highlight a carbon system that remains poorly constrained even as pressure on it grows.