Humans May Have Played Role In Loss of African Rainforests
February 12, 2012

Humans May Have Played Role In Loss of African Rainforests

An influx of humans into the lush rainforests of Central Africa, and the deforestation that followed, may have played a role in changing the landscape of the region into the savannas and grasslands that exist there today, a team of French scientists claim in a new study.

According to Rachel Nuwer of Science Now, lead author Germain Bayon, a geochemist at the French Research Institute for Exploration of the Sea in Plouzané, and colleagues originally intended to study the relationship between precipitation and the chemical breakdown of earth and rocks in the area.

Bayon and his team analyzed various types of marine mineral cores collected from the Congo River, seeking elements such as hydrogen that typically "leave distinct signatures in sediment," Nuwer said.

Likewise, the mineral potassium tends to be washed away rapidly, while aluminum tends to remain for longer periods of time, meaning more that the latter element will be present during chemical weathering than the former, Amina Khan of the Los Angeles Times reported on Friday.

"Looking back 20,000 years, these ratios seemed in sync with the rainfall levels, with amounts of rain and evidence of chemical weathering rising or falling together," Kahn wrote. "But that tidy relationship fell apart about 3,000 years ago. Though the climate had been growing drier for some time, the ratio of aluminum to potassium in the soil sediment rose significantly – and there was no plausible natural cause for the sudden change."

Bayon told the Times that the researchers were "puzzled" when they first saw those results, but then they learned that the Bantu people began migrating into rainforests around that time.

The researchers reportedly discovered that members of this African ethnic group used tools created through iron-smelting in order to cut down trees and clear the area in order to plant crops. Iron artifacts discovered at various archeological sites corroborated that theory, as did linguistic studies, according to both the Times and Science Now.

"Archeologists have shown that the Bantu brought agriculture to the region, growing crops such as pearl millet and yams. But in order for pearl millet to grow, seasonality, or distinct wet and dry seasons, is necessary. In other words, climate shifts toward more pronounced seasonality paved the way for agriculture," Nuwer wrote.

"To cultivate crops, the Bantu had to cut down stretches of forest, exposing the soil to weathering. Such intensive land use can lead to dramatically higher rates of chemical alteration, the researchers say, which would explain the sudden shift in weathering patterns 3000 years ago," she added.

Peter deMenocal, a marine geologist at the Columbia University Lamont-Doherty Earth Observatory in Palisades, New York, told Nuwer that it was a "very compelling study," and William Ruddiman, a geologist and climate scientist at the University of Virginia, told Kahn that Bayon and his colleague make "a really convincing case" for their theory.

Neither deMenocal nor Ruddiman were involved with the research project, which was published online Thursday in the journal Science. DeMenocal told Science Now that while the scientists have assembled "a really solid example" of the massive impact that mankind's activities can have on their environment, that more research is needed to prove just how much impact people had on the loss of Central Africa's rainforests.


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