Holocene vegetation history of the Sahel: pollen, sedimentological and geochemical data from Jikariya Lake, north-eastern Nigeria

Aim: This study aims to separate regional and local controls on Holocene vegetation development and examine how well pollen records reflect climate change in a semi-arid region. The relative importance of climate and human activity as agents of vegetation change in the Sahel during the late-Holocene is also considered. Location: Jikariya Lake, an interdune depression in the Manga Grasslands of north-eastern Nigeria. Methods: Pollen and charcoal were used to provide a record of Holocene vegetation history. Palaeoclimate and hydrological changes were reconstructed from sedimentary and geochemical data. Regional and local influences were separated by comparing the evidence obtained from Jikariya Lake with previously published data from the Manga Grasslands. Results: The Manga Grasslands experienced a prolonged wet period during the early and mid Holocene during which swamp forest vegetation with Guinean affinities (Alchornea, Syzygium, Uapaca) occupied the interdune depressions. However, variation in the pollen records between sites suggests that their establishment was dependent upon conditions being locally favourable, rather than being directly coupled to regional climate. The pollen records from the Manga Grasslands are more consistent in suggesting the colonisation of the dunefields by trees associated with Sudanian savanna (Combretaceae, Detarium) c. 8700 cal. yrs BP. The Jikariya Lake pollen data are in accordance with the sedimentological and geochemical data from the region in indicating the onset of arid conditions occurred progressively during the late Holocene (from c. 4700 cal. yrs BP). Abrupt changes in pollen stratigraphy, recorded at other Manga Grasslands sites 3500 cal. yrs BP, appear to be the product of the local passing of ecological thresholds. The dunefield vegetation (Sahelian savanna) appears to have been resilient, or at least palynologically silent, to the climatic variability of the late Holocene. Main conclusions: While climate appears to have been the primary control on vegetation development in the Manga Grasslands during the Holocene, local conditions (particularly depression size and sand influx) had a strong influence on the timing of pollen stratigraphic changes. Anthropogenic influences are difficult to detect, even during the late Holocene