Sensitivity to climate and vegetation dynamics of a peatland record from central Cameroon during the African Humid Period

Abstract

Significant climatic and vegetation changes have occurred in tropical Africa over the Holocene, especially during the African Humid Period (AHP). However, the complexity of interpreting and comparing several proxies from diverse sites complicates the characterization and differentiation of climatic and environmental changes at local, regional and global scales. This study investigates a 6-m peat core from the Ngaoundaba maar volcanic crater (Northeastern Cameroon, later simply called Ngaoundaba), spanning the last 10 ka using pollen analysis and a large panel of lipid biomarkers. We produce new high-resolution, continuous, multiproxy records of vegetation, temperature, and precipitation spanning most of the Holocene. All of these proxies indicate a substantial transition approximately 5.7–5.6 ka cal BP, which is supported by cluster analyses and marks the end of the AHP. A shift from an open-water to a vegetated peatland, the disappearance of some wooded species, and the expansion of grass and sedge pollen all indicate significant local and regional changes. The gradual terrestrialization of peat surfaces also had an impact on lipid biomarker proxies. An unusual extensive variation in hydrogen isotopic composition (D/H) of long-chain n-alkanes during the Holocene, in contrast to other records from West and Central Africa, may be attributed to the increased contribution from local wetland plants, including sedges and grasses, which thrive in peat water that is more D-enriched than rainwater, peat water being the water accumulating in the wetland. Likewise, temperature variations reconstructed using bacterial branched glycerol dialkyl glycerol tetraethers (brGDGTs) are influenced by confounding factors, like changes in peat pH or moisture levels, which may be constrained using our multiproxy methodology. The temperature record from Ngaoundaba indicates a slight increase in temperature during the mid-Holocene relative to pre-industrial levels. The Ngaoundaba peat deposit documents a massive and abrupt shift in vegetation at the end of the AHP, linked with changes in precipitation amount and/or seasonality, which also significantly affected the peat microbial community. The Ngaoundaba peat record, because of its high sensitivity to climatic and environmental changes, is a crucial new source for understanding the end of the African Humid Period in Western Central Africa.