Abrupt return of the summer monsoon 15,000 years ago: new supporting evidence from the lower White Nile valley and Lake Albert

The Last Glacial period ended with an abrupt return to warmer and wetter conditions at a number of sites in intertropical Africa, Asia, Australia and South America ∼15,000 years (15 ka) ago. Similar abrupt warming is also evident at this time in high latitudes and in air trapped in Greenland ice. We here report new supporting evidence of this event from a climatically sensitive region in Africa: the White Nile valley of the central Sudan and Lake Albert in the Uganda headwaters. During the Last Glacial Maximum the White Nile valley was even more arid than it is today, with desert dunes active as far south as latitude 12°S. The sudden overflow of Lake Victoria in the Ugandan headwaters of the White Nile ∼14.5 ka, confirmed here by Sr-isotope statigraphy, caused extensive flooding in the lower White Nile valley and severe flooding in Egypt. The volume of fresh water flowing into the eastern Mediterranean at this time ultimately resulted in accumulation of highly organic sediments (Sapropel 1) on the floor of the Mediterranean Sea. Flooding in the central Sudan attained an elevation of 382 m along a north-south distance of ∼400 km. The recessional shoreline of the 382 m White Nile is indicated by concentrations of now buried freshwater gastropod shells and beach sands dated, respectively, by calibrated AMS radiocarbon and OSL to ∼15-16 ka. This mega-flood event marks the abrupt return of the summer monsoon and may reflect a globally synchronous event.