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19 July 2011
Ethiopian lake sediments reveal history of African droughts

A new survey of Lake Tana in Ethiopia - the source of the Blue Nile - suggests that drought may have contributed to the demise of the ancient Egyptian 'Old Kingdom', around 4200 years ago. A team led by the University of Aberystwyth (Wales) used seismic surveys and sediment cores to work out how the lake's water level has varied over the past 17,000 years and linked this to evidence for global climate change.
     Understanding how and why rainfall patterns change is particularly important for sub-Saharan Africa, where prolonged droughts have such serious social and economic consequences. The climate here is dominated by the African-Asian monsoon and the movements of the Inter-Tropical Convergence Zone (ITCZ) - an area of erratic weather patterns, where winds from the northern and southern hemispheres meet close to the equator. Sailors know it as the Doldrums.
     Seasonal movements of the zone can affect the strength of the monsoon, and be traced in ancient lake sediments. Lake Tana is particularly good for this kind of research because it's close to the northern limit of the zone, so even slight a southward movement of the zone is reflected in the lake's geological history.
     "Finding a distinct dry period around the time of decline of the Old Kingdom is complicated by the fact that the climate was becoming drier overall during that time anyway," explains Dr Michael Marshall, of Aberystwyth University.
     Nevertheless, the researchers' analysis of the sediments did reveal a distinct dry episode around 4200 years ago. This would have lowered water levels in Lake Tana and reduced the flow of the Nile, interrupting the regular supply of fertile sediment to the Nile delta. Archaeological evidence shows that the Old Kingdom was already beginning to wane; reduced Nile flow could have contributed to its demise.

Edited from PhysOrg (12 July 2011)

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