A late Holocene palaeoenvironmental ‘snapshot’ of the Angamma Delta, Lake Megachad at the end of the African Humid Period

During the African Humid Period (AHP) there was a large increase in the area of lakes and wetlands. Lake Megachad, one of several huge lakes, underwent dramatic fluctuations during the AHP prior to regression in the mid Holocene. However, the timing and nature of AHP termination has been disputed. We present evidence from sediments of the Angamma Delta, from the northern end of the palaeolake, for Lake Megachad lake-level fluctuations at the end of the AHP. Delta slope deposits were deposited around 7000 cal BP at the height of the AHP. Overlying bioclastic sediments, from 4300 to 4800 cal BP at an elevation of 285–290 m, lie below the palaeolake highstand (339 m) but close to the elevation of the Bahr el Ghazal sill, which divided the lake's two sub-basins. Ostracod δ18O values indicate that the waters of the northern sub-basin were evaporated to levels similar to modern Lake Chad. Palaeoecological evidence suggests that the lake was perennial and evaporative enrichment is attributed to restricted circulation of lake waters as the sill emerged. The age and elevation of the bioclastic sediment, coupled with published lake level reconstructions, suggest a complex lake-level history with a major regression at the end of the AHP, followed by a short lived, lake level rise and then a further regression. This new evidence for changes in lake level provides support for other geological records and some modelling experiments that suggest rapid fluctuations in hydroclimate at the end of the AHP.