Geology http://doi.org/q2k (2014)
The eastern Sahara Desert once hosted a large freshwater lake. Cosmogenic nuclide exposure dating and an analysis of ancient shorelines reveal that such a lake probably formed during the last interglacial period, more than 100,000 years ago, and was broadly similar in surface area to today's largest freshwater lakes.
Timothy Barrows at the University of Exeter, UK, and colleagues measured the amount of the isotope beryllium-10 in palaeolake shoreline deposits preserved in Sudan, eastern Sahara. Beryllium-10 accumulates in rocks when they are hit by cosmic radiation, so its abundance can be used to calculate the amount of time rocks or sediments have been exposed at the surface of the Earth. With the help of this method, the researchers date the palaeo-shoreline sediments to an age of about 109,000 years. Using a high-resolution digital elevation model to assess the regional topography today, and assuming that the topography at the time was not too dissimilar from now, they show that the lake could have covered an area of more than 45,000 km2.
The lake could have formed when the White Nile River overflowed during seasonal floods under monsoon conditions that were more intense than those of today.
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Whitchurch, A. Saharan lake. Nature Geosci 7, 82 (2014). https://doi.org/10.1038/ngeo2089