Late Holocene plant and climate evolution at Lake Yoa, northern Chad: pollen data and climate simulations

The discovery of groundwater-fed Lake Yoa (19.03◦N, 20.31◦E) in the hyperarid desert of northern Chad by the German research team ACACIA headed by S. Kropelin provides a unique, continuous sedimentary se- quence of late Holocene age available in the entire Saharan desert. Here we present pollen data and climate simulations using the LMDZ atmospheric model with a module repre- senting the climatologically-relevant thermal and hydrologi- cal processes occurring above and beneath inland water sur- faces to document past environmental and climate changes during the last 6000 cal yr BP. Special attention is paid to wind strength and direction, length and amplitude of the rainy season, and dry spell occurrence, all of which are of pri- mary importance for plant distribution and pollen transport. In addition to climate changes and their impact on the natu- ral environment, anthropogenic changes are also discussed. Two main features can be highlighted: (1) the shift from an earlier predominantly monsoonal climate regime to one dominated by northern Mediterranean fluxes that occurred after 4000 cal yr BP. The direct consequence of this was the establishment of the modern desert environment at Yoa at 2700 cal yr BP. (2) Changes in climate parameters (simu- lated rainfall amount and dry spell length) between 6 and 4000 cal yr BP were comparatively minor. However, changes in the seasonal distribution of precipitation during this time interval dramatically affected the vegetation composition and were at the origin of the retreat of tropical plant communities from Lake Yoa.