Precipitation estimates for the eastern Saharan palaeomonsoon based on a water balance model of the West Nubian Palaeolake Basin

During the Holocene, the eastern Sahara underwent a drastic climatic change: in less than 6000 years, the climate changed from hyperaridity to semiaridity and back to its current hyperarid state. The West Nubian Palaeolake Basin in NW Sudan (ca 18.5 degreesN/25.5 degreesE) contains widespread lake carbonates, which indicate development of a freshwater lake between approximately 9500 and 4000 years BP. The size of the lake has been estimated as between 1100 and 7000 km(2). Water balance calculations and a dense drainage net surrounding the terminal lake indicate predominance of superficial and minor importance of groundwater inflow. A lake with an area of 1100 km(2) would require an annual precipitation of approximately 500 mm to balance the respective aerial evaporation. Similar precipitation values are currently observed at a latitude of 12 degreesN, some 600 km farther south, while the West Nubian Palaeolake Basin receives an annual rainfall of less than 15 mm/year. A lake size of 7000 km(2) would have required 900 mm of annual rainfall, an amount characteristic of sub-tropical conditions. This value could indicate inhomogenities in the wet phase climate, which could strongly influence the water level of a terminal lake located in a flat terrain. The highly depleted oxygen isotope values of the lake carbonates indicate that the Holocene wet phase was characterized by intense tropical summer (monsoonal) rainfall with heavy thunderstorms. Thus, an intensified southwest palaeomonsoon apparently transported large quantities of moisture to the eastern Sahara, producing the necessary conditions for the persisting existence of freshwater in the West Nubian Palaeolake Basin. The reconstructed palaeoclimate implies high groundwater levels and corresponds to precipitation rates from other sites in North Africa. (C) 2000 Elsevier Science B.V. All rights reserved.