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

A.-M Lézine,G Krinner,P Braconnot,W Zheng

doi:10.5194/cp-7-1351-2011

Abstract

Abstract. 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. Kröpelin provides a unique, continuous sedimentary sequence 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 representing the climatologically-relevant thermal and hydrological processes occurring above and beneath inland water surfaces 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 primary importance for plant distribution and pollen transport. In addition to climate changes and their impact on the natural 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 (simulated 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.

Highlights

The timing and amplitude of the transition between the “green Sahara” and the present-day hyperarid desert at the end of the African Humid Period (AHP) is still a matter of debate
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 between 6 and 4000 cal yr BP were comparatively minor
The Lake Yoa pollen record shows the progressive retreat of tropical plant communities, which were widely spread throughout North Africa (Watrin et al, 2009) during the Holocene, and their replacement by herbaceous-dominated desert ecosystems (Fig. 8)

Summary

Introduction

The timing and amplitude of the transition between the “green Sahara” and the present-day hyperarid desert at the end of the African Humid Period (AHP) is still a matter of debate. Several authors have discussed the role of the vegetation feedback in triggering the long-term southward shift of the boreal summer limit of monsoon flow It is widely admitted, following Charney et al (1975), that changes in plant cover and related land-surface albedo have had considerable effects on monsoon fluctuations in the past and that a positive feedback between climate and vegetation has lead to amplification of orbital forcing. De Menocal et al (2000) suggested that this transition occurred abruptly, as recorded by the dramatic increase in terrigenous (eolian) sediment deposition in ODP core 658 at 5.5 ka BP This was interpreted as reflecting the decrease in grass cover on the nearby continent and the drying out of the Saharan desert. Local records of abrupt environmental changes, as observed in the western Sahel (Lezine, 1988), probably accounted for specific environmental (hydro-geological) settings, as Published by Copernicus Publications on behalf of the European Geosciences Union

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Conclusion