Late Holocene linkages between soil erosion, phosphate bioavailability and lake productivity in Central Africa

Abstract

Major vegetation shifts and large-scale erosion events occurred in Central Africa during the late Holocene in response to hydroclimate change and enhanced land use related to the Bantu expansion. On the Adamawa Plateau, a highland in Central Cameroon that experienced abrupt forest collapse about 3000 years ago, recent works showed that lake primary productivity markedly increased during the same period. Yet, the causes of increased productivity in Adamawa plateau lakes and its link to the Central African rainforest crisis still remain unclear. To address this issue, we investigated lake sediment records from this region, using radiogenic isotope proxies for sediment provenance and chemical weathering to complement previously published mineralogical and organic geochemistry data. Our results indicate that increased biological productivity in Adamawa Plateau lakes was linked to enhanced erosion of highly weathered lateritic soils. An examination into possible causes leads us to propose that phosphate desorption following erosion of kaolinite, iron (goethite) and aluminium (gibbsite) oxide-bearing soils may have played a key role in stimulating lake productivity at that time, together with degradation of terrestrial organic matter. These findings emphasize the importance of deforestation and land use on soil-P cycling and lake ecosystems in tropical regions during the Holocene.