A river flowing through a desert: late Quaternary environments in the Nile basin – current understanding and unresolved questions

Abstract

Late Quaternary environments in the Nile Basin reflect the influence of the African summer monsoon upon plant cover, sediment yield and flood discharge in the Ethiopian and Ugandan headwaters of the Nile. Intervals of prolonged and very high Nile flow coincide with times of stronger summer monsoon and have been dated using a combination of 14C, OSL and 10Be methods. Periods of high Nile flow into the eastern Mediterranean coincide with the formation of highly organic sedimentary layers termed sapropels. Ages obtained so far for these times of sustained middle to late Pleistocene high flow in the Blue and White Nile are broadly coeval with sapropel beds S8 (ca 217 ka), S7 (ca 195 ka), S6 (ca 172 ka), S5 (ca 124 ka), S4 (ca 102 ka) S3 (ca 81 ka), S2 (ca 55–50 ka) and S1 (10–6.5 ka). Sapropel 5 (ca 124 ka) was synchronous with extreme Blue Nile floods and the formation of the 386 m lake in the lower White Nile Valley, as well as with a prolonged wet phase in the eastern Sahara. Fluctuations in Nile flow and sapropel formation reflect the influence of the precessional cycle upon the East African monsoon. Between 75 ka and 19 ka the climate in the Nile headwaters region became progressively colder and drier. During the Last Glacial Maximum, Lake Tana in Ethiopia and Lake Victoria in Uganda became dry, flow in the White Nile was reduced to a trickle, and the Blue Nile and Atbara became highly seasonal bed–load rivers. The return of the summer monsoon at 14.5 ka ushered in extreme Blue Nile floods, widespread flooding across the Nile Basin and the formation of the 382 m lake in the lower White Nile Valley. There was a brief return to aridity during the Younger Dryas (12.8–11.5 ka), after which the climate again became wetter and widespread flooding in the Nile Valley resumed. The early Holocene floods were later followed by incision and creation of the modern relatively narrow flood plain.