Impact of the Megalake Chad on climate and vegetation during the late Pliocene and the mid-Holocene.

Given the growing evidence for megalakes in the geological record, assessing their impact on climate and vegetation is important for the validation of paleoclimate simulations and therefore the accuracy of model/data comparison in lacustrine environments. Megalake Chad (MLC) occurrences are documented for the mid-Holocene but also for the Mio-Pliocene (Schuster et al., 2009). The surface covered by water would have reached up to ~350 000 km² (Ghienne et al., 2002; Schuster et al., 2005; Leblanc et al., 2006) making it an important evaporation source, possibly modifying the climate and vegetation in the Chad basin. We investigated the impact of such a giant continental water area in two different climatic backgrounds within the Paleoclimate Model Intercomparison Project phase 3 (PMIP3): the late Pliocene (3.3 to 3Ma, i.e. the mid-Piacenzian warm period) and the mid-Holocene (6 kyr BP). In all simulations including a MLC, precipitation is drastically reduced above the lake surface because deep convection is inhibited by colder air above the lake surface. Meanwhile, convective activity is enhanced around the MLC, because of the wind increase generated by the flat surface of the megalake, transporting colder and moister air towards the eastern shore of the lake. Effect of the MLC on precipitation and temperature is not suffcient to widely impact vegetation patterns. Nevertheless, tropical savanna is present in the Chad Basin in all climatic configurations, even without the MLC presence, showing that the climate itself is the driver of favourable environments for sustainable hominid habitats. [ABSTRACT FROM AUTHOR]