Are early agriculturalists to blame for Africa’s lost rainforests?

ScienceNOW reports: About 3000 years ago, Central Africa was a landscape in transition. Lush evergreen forests were gradually giving way to savannas and grasslands as regional climate change pushed the formerly humid weather patterns toward drier, slightly warmer conditions. But climate was not the only factor at play. According to a new study, an influx of humans into the region at this time may have helped drive some of the original rainforests into oblivion.

The paper’s results, published online today in Science, came as a surprise to the researchers. “To be honest, at the beginning we were not at all aware of this human issue,” says lead author Germain Bayon, a geochemist at the French Research Institute for Exploration of the Sea in Plouzané.

He and his colleagues originally set out to investigate the relationship between precipitation and chemical weathering, or the breakdown of soils and rocks. They analyzed marine sediment cores collected near the mouth of the Congo River, where thousands of years of runoff have accumulated. Because rocks are composed of different minerals, Bayon explains, those materials that are more susceptible to weathering will more readily erode away, eventually washing into the ocean and forming layers of clay on the bottom. By analyzing clay’s composition, scientists can reconstruct the intensity of past weathering and infer environmental conditions.

The researchers analyzed the cores for elements like hydrogen that leave distinctive signatures in sediment. These geochemical markers correspond with past precipitation levels, which influence weathering. They also examined ratios of aluminum and potassium, which indicate weathering intensity, because potassium is a highly mobile element whereas aluminum is one of the most immobile. As expected, the weathering patterns closely followed precipitation levels — that is, until about 3000 years ago. At that point, Bayon says, the pattern became completely different. The sediment appeared to have undergone intense chemical weathering, which the climate alone could not explain. So the team began suspecting another factor was responsible.

As it turns out, around this time Bantu farmers — an African ethnic group — had begun a large-scale expansion across Central Africa and settled in the rainforest. Linguistic studies and archeological evidence, such as stone tools and iron artifacts, support this event. Perhaps most importantly, 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. 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.

“Climate did play an important role in the arrival of agriculture,” Bayon says. “But what we show is that the impact of those people developing and introducing agriculture probably had a quite significant impact on soil erosion.”

“This is a very compelling study,” says Peter deMenocal, a marine geologist at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, New York, who was not involved in the work. Even when compared with the traces natural climate change leaves, he says, “the human footprint on the environment can be very large.”

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