Humic Lakes in the Congo Basin Outgas Significant Amounts of CO2 from Nearby Peatlands: New Research Challenges the Idea of Peatlands as Secure Carbon Sinks

     This paper quantifies the carbon emissions of two lakes in the Congo Basin, lakes Mai Ndombe and Tumba, verifying that they are major CO2 sources. The researchers also discovered that the lakes were receiving significant amounts of CO2 from the adjacent peatlands and outgassing it. Peatlands are major carbon storers. They estimate that 39–40% of the lakes’ CO2 emissions derived from the adjacent peatlands. The researchers analyzed the isotopic composition of dissolved inorganic carbon (DIC) in the lakes. The age of that carbon was found to be much older than expected. Mai Ndombe Lake DIC was found to have a mean age of 2170 years via Carbon-14 dating, and Tumba Lake to have a mean age of 3515 years. They note that dissolved organic carbon (DOC) for both lakes and particulate organic carbon (POC) for Mai Ndombe (POC was not measured in Tumba) were both found to be modern in age.

“An aged DIC signature was also observed in the Fimi River, which receives the outflow from Lake Mai Ndombe. While the Fimi is a mixed system that also receives ~43% of its discharge from the Lukenie River, its similarly old DIC radiocarbon age still corroborates the lake DIC age and confirms the export of this ancient carbon from the lake system (Fig. 1b and Extended Data Table 1).”

     Depleted DIC was found in both lakes.

“The depleted lake DIC 13C signature, combined with the absence of carbonate geology in the lake catchments, indicates an origin from terrestrial C3 plant matter.”

“Even with this enrichment relative to DOC/POC and peat, this specific isotopic combination is, however, uncommon in the global context. While aged DIC is common globally, a recently compiled database shows it is typically associated with higher δ13C values, a pattern consistent with inorganic carbon inputs from carbonate weathering. The Congo lake DIC—being both aged and relatively 13C depleted—is a clear exception to this global trend (Extended Data Fig. 2). These results strongly suggest that the DIC partially originates from the decomposition of ancient peat, which is known to have similar isotopic characteristics.”

     The researchers used statistical simulations to estimate the amount of carbon derived from the peatlands. The model is shown in the figure below, and possible pathways for the peatland carbon to get into the lake as CO2 are quoted below.

“Our model outlines three potential subsurface pathways: (1) aerobic respiration, (2) direct CO2 production by acetoclastic methanogenesis and/or (3) hydrogenotrophic methanogenesis followed by CH4 oxidation. While the dominant pathway remains unconstrained, the predominantly anoxic nature of the peatlands suggests that methanogenesis (pathways 2 or 3) is more likely than aerobic respiration (1), as the latter would require extensive drainage to create oxic conditions. We acknowledge, however, that the precise hydrological pathways and fluxes that transport this respired CO2 are a key uncertainty and ongoing work to characterize the hydrology of the Congo peatlands will be crucial for resolving these complex dynamics. The proposed model framework is consistent with findings demonstrating a similar DOC and DIC age decoupling within boreal peatlands. There, porewater DOC was shown to be modern while porewater DIC and CH4 exhibited intermediate ages between the ancient peat and modern DOC. Similarly, the DOC from porewaters in tropical peatlands has been shown to be modern, even when the surrounding peat matrix is ancient. These studies lend support to our decoupling model and further suggest that the ancient DIC in the lakes is sourced from deep subsurface respiration (probably involving methanogenesis) of ancient peat.”

“These findings challenge the prevailing understanding that CO2 emissions from pristine humic lakes are derived from modern, rapidly cycling carbon.”

     More study of Congo Basin hydrology will be needed to better understand the transport and outgassing mechanisms and to determine whether other humic lakes with adjacent peatlands have similar mechanisms.

   

 

References:

 

Millennial-aged peat carbon outgassed by large humic lakes in the Congo Basin. Travis W. Drake, Jordon D. Hemingway, Matti Barthel, Antoine de Clippele, Negar Haghipour, Jose N. Wabakanghanzi, Kristof Van Oost & Johan Six. Nature Geoscience volume 19, pages 415–418 (February 2026). Millennial-aged peat carbon outgassed by large humic lakes in the Congo Basin | Nature Geoscience