The waters of Lac Mai Ndombe resemble cold tea. Dark, almost non-transparent, tainted a deep brown by eons of decaying plant matter flowing in from the adjacent swamp forests — Africa’s largest blackwater lake lies in the center of the Democratic Republic of Congo, four times the size of Lake Constance, mostly unexplored, predominantly unmeasured. Reaching it necessitates boats and dugout canoes navigating through one of the most isolated stretches of rainforest on the planet. This partly clarifies why no one had posed the straightforward question: where, precisely, is all that CO₂ emanating from?
When Travis Drake at ETH Zurich and his team finally ventured onto the water and began gathering samples, they anticipated confirming what was already commonly believed. Lakes like this one emit carbon dioxide due to the surrounding forests losing organic material — leaves, roots, decomposing timber — that washes in and is decomposed by microorganisms. The carbon involved is recent; it has been photosynthesized in the last few decades, cycled through living flora, and returned to the atmosphere. A swift, tight loop.
However, the radiocarbon dating revealed a distinct narrative. The dissolved inorganic carbon in Lake Mai Ndombe had an average age of around 2,170 years. In Lake Tumba, the smaller counterpart to the north, the age was even greater: approximately 3,500 years. Carbon that last cycled through living plants during the time of the Roman Republic is presently escaping from these lakes into the atmosphere.
“We were taken aback to discover that ancient carbon is being released via the lake,” remarks Drake. His colleague Matti Barthel expresses it more strikingly: “The carbon reservoir has a leak, so to speak, from which ancient carbon is seeping.”
This leak is significant due to what lies beneath these lakes. The central depression of the Congo Basin — the Cuvette Centrale — houses the largest identified tropical peatland complex on the planet, containing an estimated 29 petagrams of carbon trapped in dense layers of compressed, water-saturated plant material accumulated over thousands of years. This is roughly equivalent to the total carbon dioxide emissions from global fossil fuels over about three years. Scientists had presumed this reservoir was, under typical circumstances, stable. The organic material in deep peat is ancient and already heavily decomposed; the reasoning suggested that microbes would preferentially decompose fresher, more readily available material at the surface, leaving the ancient stocks relatively untouched. Historically, the only time it was believed that old peat carbon could escape was during droughts when declining water levels expose peat to oxygen, accelerating decomposition dramatically.
The latest measurements, published in Nature Geoscience, indicate that this understanding is inadequate. Something is mobilizing carbon from deep within the peat and channeling it into the lakes, even without apparent disturbances. The exact mechanism remains unclear. The researchers’ best theory involves methanogenesis — anaerobic microbes deep within the peat producing CO₂ and methane as they metabolize ancient organic carbon, with the resulting gases being transported laterally through subsurface flows until they reach the open lake water, where they rapidly escape into the atmosphere. The lake, as described by Drake and his colleagues, functions as a chimney.
What complicates this issue is that the various forms of carbon in the water convey conflicting information. The dissolved organic carbon in both lakes — the actual pieces of plant matter and humic substances dissolved in the water — is effectively modern, with radiocarbon values nearing 1.0. The dissolved inorganic carbon, which is the carbon that escapes as CO₂, is composed of ancient material. These two pools seem largely disconnected, operating as a two-lane system where fresh surface carbon and ancient subsurface carbon navigate different routes to reach the lake. The team conducted 100,000 Monte Carlo simulations to ascertain the proportions, concluding that approximately 39 to 40 percent of the outgassed CO₂ originates from old peat across both lakes. Lake Mai Ndombe alone could be releasing over 150 gigagrams of ancient peat carbon annually.
Whether this signifies a natural equilibrium — ancient carbon escaping while new peat forms at a sufficient rate to balance — or the initial signs of a more troubling situation is, for now, an unresolved inquiry. The Congo peatlands are not pristine in any geological definition. Palaeoenvironmental research of peat cores reveals that previous drying events have led to significant carbon losses, and some researchers argue these systems may currently be close to a drought threshold influenced by climate change. If this is accurate, what the new study illustrates might represent one end of a spectrum that could shift rapidly under adverse conditions.
An accompanying study from the same group, published in the Journal of Geophysical Research, introduces another layer of apprehension. The volume of methane escaping from Lake Mai Ndombe turns out to be…