Some 3000 years ago the sediments of the Congo river's mouth were suddenly altered in an unexpected way: it is the signature of the first forest agriculture, which implied opening patches of the rainforest, allowing weathering of the soil.
Germain Bayon, Intensifying Weathering and Land Use in Iron Age Central Africa. Science 2012. Pay per view.
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.
Hat tip to Stone Pages' Archaeonews.