Interaction of Holocene infilling processes between a tropical shallow lake system (Lake Ossa) and a nearby river system (Sanaga River) (South Cameroon)

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The valley of the Sanaga River, notably in its lower course, experienced a marked alluvial accumulation (7–12 m thick) when the Holocene sea-level and thus the base level of the river approached the present zero. This resulted in an increased height of the floor of the streambed, which facilitated the overflowing of the floods over the top of the river banks. The sediments of Lake Ossa, situated approximately 35 km from the Atlantic shoreline, but the margins of which extend to 1–2 km from the Sanaga River, were studied on the basis of five sediment cores. This lake basin remained isolated from the river until 6500 cal. BP, from when we note the first alluvial accumulation, especially in its southern part. A more easily submersible segment of the lake shoreline about 2 km long was identified. Several specific mineralogical markers of the sandy fraction and the clayey fraction allow us to distinguish autochthonous materials emanating from proximal hillsides upstream of the river, and then to follow the increase of these allochthonous particles during the second half of the Holocene. The first important introduction of alluvial materials in the lake, notably sands, was associated with an acceleration of the accumulation rate. These sedimentary processes seem to have intervened independently of the climate change of the region; they began in a dense forest wet phase and continued during a period of relative opening of the landscape corresponding to more contrasted seasons. The accumulation of coarse alluviums in the lake occurred according to an approximately south–north axis, while that of the suspended load developed with a wider fan-like geometry. These sedimentary processes are diachronous according to the distance from the Sanaga River and so lead us to reconsider partly the palaeoenvironmental meaning of the assemblages of pollen and possibly also of diatoms from previous studies. These vegetal markers were previously considered to have been deposited in the lacustrine system, in restricted isolation.