The arid White Nile Valley in Africa was not just a lot wetter in the past, it was home to one of the largest freshwater lakes in the world, report scientists in a newly released study.
If they are right the lake last existed before the start of the most recent ice age, or glacial period, and extended 100 miles farther south than proposed by previous researchers. It also held twice as much water as previously thought and rivaled the size of today's Great Lakes of North America.
The White Nile connects the lower Nile to its source at Lake Victoria to the south. For years scientists have been curious about what appear to be ancient lake shores way up on the hills around the current White Nile Valley. Previous research was limited to rough estimates of the size of the lake based on maps that hadn't very accurate elevation information. There were also no clues to how long ago the lake existed.
The new study, published in the January issue of the journal Geology, solves all that by using highly accurate data from the Shuttle Radar Topography Mission to map out the lake level all around the valley.
To date the shoreline, as well as river channels that helped the nearby Blue Nile back-up and fill the lake, the scientists employed a rare but naturally occurring soil element -- beryllium-10 -- which is created at a known rate over the millenniums by cosmic rays raining down into Earth's atmosphere. That pegged the date of the lake to at least 109,000 years ago.
"What I think really clinches it is that the Blue Nile was further south than it is now," said the University of Exeter's Timothy Barrows, the lead author on the paper.