Williams

Ice, wind and water: Late Pleistocene environments in the main Nile, Atbara, Blue and White Nile basins (75-15 ka)

Department of Earth Sciences, Faculty of Science, University of Adelaide, Australia

Before regulation, the White Nile, Blue Nile and Atbara provided 10%, 68% and 22% of Nile peak monthly flow and 83%, 17% and 0% of Nile low monthly flow, respectively. Their respective contributions to total Nile sediment load were 3.5%, 72% and 24.5%. In short, the Blue Nile and Atbara provided most of the sediment and flood discharge but the White Nile provided most of the water during the time of minimum annual flow. When flow in the White Nile was cut off, as it was during 25-15 ka, the Nile would likely have ceased flowing all year. Episodes of very high Nile flow coincide with intervals of sapropel accumulation in the E Mediterranean, the most recent having ages of 81 ka (S3), 55 ka (S2) and 13.5-6.5 ka (S1).

Late Pleistocene climates in the Nile basin have fluctuated between two extremes: cold and arid or warm and wet. Between 75 and 15 ka the regional climate was mostly dry (apart from a moist phase at 55-50 ka) with active dunes encroaching on the Nile. In the Ethiopian headwaters of the Atbara and Blue Nile ³⁶Cl exposure ages for glacial moraines in the Semien Highlands range from 70 to 15 ka, after which the ice melted as the climate grew warmer.

During the Last Glacial Maximum (24-18 ka), temperatures in the Nile headwaters were 4-8°C lower than today. Periglacial processes were active down to 3000 m, slopes were unstable, and the tree line was 800-1200 m lower than today. The Blue Nile was then a highly seasonal river with a bed-load of sand and gravel, much of which was deposited by the main Nile in northern Sudan and southern Egypt. The ecotone between savanna and desert extended 300-500 km further south and the Atbara ceased to flow for much of the year. The abrupt return of the summer monsoon at 14.5 ka caused the hitherto closed lake basins in the Ugandan headwaters of the White Nile to fill and overflow, creating a shallow seasonal lake up to 25 km wide and over 400 km long south of the Blue and White Nile confluence of that time.

Until 14.5 ka, sand-bearing Blue Nile distributary channels radiated across the surface of the Gezira alluvial fan to join the White Nile up to 120 km south of its present confluence. Later Blue Nile incision beheaded these channels, reduced seasonal flooding, and caused desiccation of the wetlands along the Nile and its major tributaries. Episodic late Pleistocene channel incision along the main Nile converted former flood plains into alluvial terraces suitable for prehistoric occupation.

References:

Adamson, D.A., Gasse, F., Street, F.A. and Williams, M.A.J. (1980). Late Quaternary history of the Nile. Nature 287, 50-55.
Williams, M.A.J., Adamson, D., Cock, B. and McEvedy, R. (2000). Late Quaternary environments in the White Nile region, Sudan. Global and Planetary Change 26, 305-316.
Talbot, M.R., Williams, M.A.J. and Adamson, D.A. (2000). Strontium isotopic evidence for Late Pleistocene re-establishment of an integrated Nile drainage network. Geology 28 (4), 343-346.
Williams, M., Talbot, M., Aharon, P., Abdl Salaam, Y., Williams, F. and Brendeland, K.I. (2006). Abrupt return of the summer monsoon 15, 000 years ago: new supporting evidence from the lower White Nile valley and Lake Albert. Quaternary Science Reviews 25, 2651-2665.
Woodward, J.C., Macklin, M.G., Krom, M.D. and Williams, M.A.J. (2007). The Nile: Evolution, Quaternary river environments and material fluxes. In: A.Gupta (ed.), Large Rivers: Geomorphology and Management. John Wiley & Sons, Chichester, pp. 261-292.
Williams, M.A.J. (2009). Late Pleistocene and Holocene environments in the Nile basin. Global and Planetary Change 69, 1-15.
Williams, M.A.J, Williams, F.M., Duller, G.A.T., Munro. R.N., El Tom, O.A.M., Barrows, T.T., Macklin, M., Woodward, J., Talbot, M.R., Haberlah, D. & Fluin, J. (2010). Late Quaternary floods and droughts in the Nile Valley, Sudan: New evidence from optically stimulated luminescence and AMS radiocarbon dating. Quaternary Science Reviews29, 1116-1137.
Williams, M.A.J., Duller, G.A.T., Williams, F.M., Macklin, M.G., Woodward, J.C., El Tom, O.A.M., Munro, R.N., El Hajaz, Y. and Barrows, T.T. (2015). Causal links between Nile floods and eastern Mediterranean sapropel formation during the past 125 kyr confirmed by OSL and radiocarbon dating of Blue and White Nile sediments.Quaternary Science Reviews, 89-108, http://dx.doi.org/10.1016/j.quascirev.2015.05.024
Woodward, J.C., Williams, M.A.J., Garzanti, E., Macklin, M.G. and Marriner, N. (2015). From source to sink: Exploring the Quaternary history of the Nile. Quaternary Science Reviews, 3-8. http://dx.doi.org/10.1016/j.quascirev.2015.10.022.