Megalake Chad impact on climate and vegetation during the late Pliocene and the mid-Holocene

C Contoux,A Jost,G Krinner,G Ramstein,P Sepulchre,M Schuster

doi:10.5194/cp-9-1417-2013

Abstract

Abstract. Given the growing evidence for megalakes in the geological record, assessing their impact on climate and vegetation is important for the validation of palaeoclimate simulations and therefore the accuracy of model–data comparison in lacustrine environments. Megalake Chad (MLC) occurrences are documented not only for the mid-Holocene but also for the Mio-Pliocene (Schuster et al., 2009). At this time, 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 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 3 Ma, i.e. the mid-Piacenzian warm period) and the mid-Holocene (6 kyr BP). In all simulations including MLC, precipitation is drastically reduced above the lake surface because deep convection is inhibited by overlying colder air. Meanwhile, convective activity is enhanced around 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. The effect of MLC on precipitation and temperature is not sufficient to widely impact vegetation patterns. Nevertheless, tropical savanna is present in the Chad Basin in all climatic configurations, even without MLC presence, showing that the climate itself is the driver of favourable environments for sustainable hominid habitats.

Highlights

The Chad Basin is a vOastceenadonrheSiccbiaesnincloecated in northcentral Africa, between 5 and 25◦ N (Fig. 1), covering approximately 2.5×106 km2
The present study aims at investigating the contribution of Megalake Chad (MLC) on climate and vegetation in two different contexts: the mid-Holocene and the late-Pliocene warm period
The Chad Basin is still influenced by southwesterly monsoon winds in summer (Fig. 5, left) and northeastern Harmattan wind in winter for all simulations (Fig. 5, right)

Summary

Introduction

The Chad Basin is a vOastceenadonrheSiccbiaesnincloecated in northcentral Africa, between 5 and 25◦ N (Fig. 1), covering approximately 2.5×106 km. A Harmattan wind pattern is inferred from coastal morphosedimentary structures for the mid-Holocene (Bouchette et al, 2010). Present-day Lake Chad is mainly fed by the Chari–Logone river system coming from the southern part of the basin (82 % of total inputs, Leblanc et al.,T2h01e1)C. BreycaoussepthheeCrhead Basin is endorheic and relatively flat, climatic variations have an important impact on the lake extent, as evidenced by seasonal fluctuations as well as by historical and geological archives. Since 1960, Lake Chad has shrunk from 25 000 to 6000 km on average, mainly because of a decrease in rainfall and associated Chari–Logone river flow to the lake (Olivry et al, 1996). The presence of Megalake Chad (MLC) in the mid-Holocene is widely attested from lake deposits and from coastal morphosedimentary structures

Results

Discussion

Conclusion