­­­­Environmental evolution of fluvial peatlands in the Cuvette Centrale, Congo Basin

The Cuvette Centrale is one of the largest wetlands in the world covering about 30% of the Congo Basin, the second largest river system on the Earth. Below these wetlands, the world’s largest tropical peatlands store approximately 30 billion tons of carbon. Despite the enormous size and importance of these peatlands their development and vulnerability to past and future disturbances remain largely unknown.

The first studies from interfluvial basins revealed that peat formation started mainly during the last deglaciation, and that large domes developed. Between 7.5 and 2 ka BP an interval of intense peat decomposition occurred, potentially impacting the whole peat complex. However, along many left‑bank tributaries of the Congo River peat occurs in a different hydro-geomorphical setting spread along the dendritic hydrographic network. Thus, formation and stability of those peatlands potentially differ from the large interfluvial peatlands in the northern Cuvette Centrale.

Here, we assessed the development of such a fluvial peat deposit, located closely to the Momboyo River, a headwater tributary of the Ruki River (DRC Congo), and compared the characteristics of the peat formation and potential decomposition to the previously published peat record from the northern interfluvial peatland. We established an age model based on 15 radiocarbon ages and reconstructed the peat formation with a multiproxy approach employing bulk organic carbon and nitrogen concentrations and their stable isotopic composition, standard compositional and advanced thermal indices derived from Rock-Eval^® thermal analysis as well as plant-wax n‑alkane distributions and their carbon isotopic composition.

The lowermost sequence of the core is a sedimentary deposit dating back to 28 ka BP. Rock-Eval^® compositional parameters and n-alkane distributions reveal a substantial contribution of aquatic organic matter. Around 11 ka BP, the accumulation rate of organic matter increased markedly and a marshland developed indicated by Rock-Eval^® compositional parameters and bulk δ¹³C values. Rock-Eval^® compositional parameters still suggest a contribution of aquatic organic matter during this period whereas n-alkane distributions and n-alkane δ¹³C values around -33 ‰ point towards a rainforest vegetation. Around 9 ka BP this marshland evolved into a minerotrophic peatland, suggested by a strong shift in Rock-Eval^® compositional parameters indicating a fully terrestrial setting. A shift in n-alkane δ¹³C to -36 ‰ suggests a transition towards rainforest vegetation with a more closed canopy cover at that time. Within the uppermost sequence of the core a break in the age-depth relation occurs between 7 and 2 ka BP. Rock-Eval^® thermal parameters indicate very similar characteristics to those observed in the northern interfluvial peatland during the previously identified period of intense peat decomposition.

This study reveals that initation and development of the fluvial peatlands along the left-bank tributaries of the Congo River differs from the evolution of the northern interfluvial peatland and was strongly influenced by river flooding in its early phase. It also shows that the period of intense peat decomposition during the Mid- to Late Holocene was widespread in the Cuvette Centrale peatlands.