The Holocene giant Lake Chad revealed by digital elevation models

The Chad Basin is a closed basin in the Central part of North Africa. A southern sub-basin, which is supplied by the humid tropics, includes the present-day Lake Chad. The northern sub-basin is presently dry, as it only receives drainage from Sahara rivers. In these areas, the existence of a giant lake known as the Lake Mega-Chad has been debated for a long time. Its level would be controlled by the Mayo Kebbi threshold in the southern part of the lake, which joined the Lake Mega-Chad and the Niger River via the Benoue Trough. Digital elevation models (the TOPO6 and GLOBE data sets) are used to characterise a well-defined shelf-like morphology, locally up to 50km wide, followed at a constant elevation over hundreds of kilometers. This strictly horizontal geomorphic feature is interpreted as a wave-cut lacustrine shoreline terrace. It is associated with a sandridge that represents a barrier-island system built by wave-generated currents. This study demonstrates that a Holocene Lake Mega-Chad was present from 11°N to 18°N, across the southern and northern sub-basins. The influence of such a water vapour source must be considered in palaeoclimatological reconstructions.