Sediment study gives clues to ancient climate shift

A shift to a wetter and more stable climate in tropical Africa about 70,000 years ago may have contributed to our ancestors' world-colonizing migration out of Africa.

A shift to a wetter and more stable climate in tropical Africa about 70,000 years ago may have contributed to our ancestors' world-colonizing migration out of Africa.

That's the speculation of researchers, including some with University of Minnesota Duluth connections, on an international team that is examining sediment samples collected from east Africa's Lake Malawi in 2005.

While the sediments tell researchers nothing directly about early Homo sapiens, they do indicate what environmental conditions our ancestors had to endure, said Syracuse University Earth sciences professor and UMD alumnus Christopher A. Scholz, the project's principal investigator.

Initial studies of Lake Malawi's sediments dating back 150,000 years reveal a period of climate swings and extreme mega-droughts earlier than 70,000 years ago, according to an article published earlier this month in the Proceedings of the National Academy of Sciences.

During that time, our forbearers would have struggled to stay alive from generation to generation, Scholz said. Geneticists have seen DNA evidence that suggests early human populations went through a bottleneck, with numbers down to about 10,000 people.


"The end of the bottleneck was reached somewhere between 60,000 and 80,000 years ago," Scholz said. "The population expansion and subsequent spreading out of Africa may have been aided by the newly stabilized climate."

Before the Lake Malawi research, the prevailing wisdom was that over the past million years African droughts followed the planet's ice ages, which occurred on a roughly 100,000-year cycle. Sediments collected in the past from the bottom of the Atlantic and Indian oceans supported that view.

"The biggest surprise to us was the severity of the African droughts at times that did not correspond to major ice ages," said team member and UMD geological sciences professor Thomas Johnson. "In fact, these dry periods were far, far more severe at times in between the major ice ages."

The droughts occurred, in fact, on roughly a 20,000-year cycle. And those could be severe. During the worst mega-droughts, the team thinks, Lake Malawi may have held only 5 percent of the water compared to what it does today.

"That's going to dramatically change the vegetation," Scholz said. "The area would have likely become a semi-desert."

About two-fifths the size of Lake Superior, modern Lake Malawi is the third-largest lake in the Southern Hemisphere. It was created more than 7 million years ago by the forces that are slowly ripping Africa apart. Careful examination of fossils and chemicals in the 2,044 feet of core samples that Scholz's team collected in 2005 will reveal past climates dating back at least a million years.

"The sediment cores from Lake Malawi are the longest continuous record of climate change available from the continental tropics," Paul Filmer, program director in the National Science Foundation's Division of Earth Sciences, which helped pay for the research, said in a news release. "The link between the signals of East African moisture levels in core samples from the lake and a critical stage in human evolution is an important discovery."

After finding that tropical Africa's climates did not follow the ice ages, the team began speculating why that was the case. They think that changes in the Earth's orbit and its tilt in space are responsible.


"The Earth's orbital geometries are not as constant as you might think," Scholz said. "The Earth does this kind of crazy, wobbly dance. What we see coming out of this initial study is that you get real extremes in weather in tropical Africa when the Earth's orbit is more elliptical."

STEVE KUCHERA can be reached weekdays at (218) 279-5503 or by e-mail at .

Steve Kuchera is a retired Duluth News Tribune photographer.
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