NILE SEDIMENT BUDGET DEFICIT: A SOURCE-TO-SINK APPROACH

Sediment budget from source to sink is a critical step in successful modeling of clastic depositional systems. However, the estimate and timing of eroded sediment volume from a source region are often associated with large uncertainties. This study combats this challenge in a GIS-based analysis using DEM and geochronological data, and identifies a huge mismatch between sediment production and deposition volumes in the Nile system.

3D incision mapping of the Blue Nile on the Ethiopian Plateau allowed us to calculate that ~100,000 km³ of rocks were removed from the plateau since ~30 Ma. Using isotopic ages of various volcanic markers, we partitioned this erosion and sediment production in time: ~30,000 km³ of sediments were produced from 30-10 Ma, ~15,000 km³ from 10-6 Ma, and ~50,000 km³ during the last 6 Ma. As the Ethiopian Plateau supplies ~96% of the Nile sediment load through the Blue Nile and Tekeze drainage systems, these eroded sediment volumes should match the deposited sediment volumes down the Nile system up to the Nile deep-sea Fan. Although the existence of a north-flowing, continental-scale Nile system in the geologic past has been controversial, many workers agree the appearance of such a Nile system since at least the Messinian salinity crisis. Hence, the past 6 Ma sediment-yield from the Ethiopian Plateau is more relevant for the evolution of Nile Delta and deep-sea fan.

Based on published data of depositional areas and sediment thicknesses, we estimate sediment volumes, distributed by the Nile system, for various depositional systems. Over the past 6 Ma and from south to north, ~5,000 km³, ~150,000 km³, and ~200,000 km³ of sediments were deposited in alluvial fans of Sudan, Nile Delta, and in the Nile deep-sea fan, respectively. ~50,000 km³ of sediment-production from the Ethiopian Plateau source over the past 6 Ma is seven times lower than the total of ~355,000 km³ of sediments deposited during the same time in various sinks. This challenges the long-held notion that the Ethiopian Plateau supplies 96% of the Nile sediment, and rather indicates that huge amount of sediments were supplied from unrecognized continental sources. Lake Plateau region of East Africa (source of White Nile), and/or now subdued regions of north-central highlands and western flank of the Red Sea Rift are some the possible source areas.