Nouvelle séquence pollinique du lac Tanganyika: 30,000 ans d'histoire botanique et climatique du bassin Nord

Abstract

This paper presents the pollen analysis of a new core from the Northern basin of Lake Tanganyika (4°30′S, 29°20′E, 773 m) which documents for the first time the uninterrupted botanical and climatic history of this region from 32,000 yr BP until today. Rich and well diversified pollen spectra give evidence of important changes in the vegetational communities of this basin. Before 15,000 yr BP, poorly diversified Zambezian woodlands occurred while numerous swampy ericaceous shrublands developed along the shoreline indicating cooler and drier climatic conditions than today. Between 15,000 and 10,000 yr BP, a great change occurred marked by the retreat of the Ericaceae. However, the continuation of open woodlands, the occurrence of cold and dry tolerant elements on the plateaus suggest that climate was still cool and dry. From 10,000 yr BP, the development (in extension and diversity) of wetter Zambezian communities including Guinean components, the dissppearance of many cold high altitude taxa imply an increase in rainfall and temperature, well documented until 2600 yr BP. This optimum climatic phase is interrupted by a distinct hydrological episode dated between 6800 and 6700 yr BP, by palynology and sedimentology previously interpreted as a sudden and temporary overflow of Lake Kivu via the Rusizi river into North Lake Tanganyika. After 2600 yr BP, a deterioration of the arboreal cover and the extension of grasses indicate the progressive establishment of the modern climatic conditions in the basin. The occurrence of pollen grains of Ricinus communis and Elaeis guineensis could indicate a recent human impact on the local vegetation. These reconstructions confirm those previously proposed for the Holocene period in the Northern Tanganyika basin, but also complete those proposed for the Last Glacial period in the Mpulungu basin, 400 km southward.