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In northern intertropical Africa, many geological data have documented several abrupt climatic fluctuations taking place during the time of the last deglaciation. One of them is an aridity event, generally placed in the interval 11,000-10,000 BP, and synchronous with the Younger Dryas cold episode of the temperate regions, that has been explained by the influence of the Atlantic Ocean circulation on global climate.

In this paper we present high resolution (50 to 150 years) studies of organic matter and pollen undertaken on a new core from the Rusaka peat bog (3 degrees 26’ South, 29 degrees 37’ East, 2070 m altitude), dated by the conventional (12 dates) and the AMS methods (18 dates obtained on bulk sediment and macroremains). Although the sediment at the bottom of the core was deposited during the last glacial period prior to 21,000 BP, the dating provides a reliable chronology for the last 12,000 BP only. Several vegetational changes are documented in the interval 12,000-9000 BP, but their chronological placement can only be achieved with a few years of uncertainties. A great dispersion of AMS dates is attributed to contamination by charcoal dated between 11,700 and 11,500 BP, and to a possible age plateau at ca. 10,000 BP. Therefore the results are preferably presented versus depth to keep synchronism in the stratigraphic placement of the successive events.

At Rusaka, increase in carbon storage, C/N ratio for terrestrial plants, and tree cover began at ca. 12,000 BP, soon after the swamp had been established. This date is in good agreement with a major increased hydrological balance well-documented throughout the intertropical region. The forest installation is progressive, rather complex and seen as a two step process. The first forest phase (11,700 BP to 10,600 BP) is dominated by Hagenia, a montane taxon, in good correspondance with previous records from other authors in East African mountains. The second forest regeneration took place post 10,000 BP, after a clearly reversed trend of tree cover from 10,600 to 10,000 BP, ending with a short episode of minimum trees lasting no more than about 200 years. But there is no indication of cooling whatsoever, and the opening of the forest is better explained by increased aridity, rather than normal pattern of forest succession. This will remain a controversal interpretation among palynologists until high resolution pollen data can be obtained for the 13,000 BP and earlier period. Nevertheless, the timing of the arid episode at Rusaka is in good agreement with other evidence mentioned from highland and lowland pollen data. A detailed comparison with 25 available pollen diagrams is limited by poor or inconsistent dating or low resolution of the pollen data. Nevertheless pollen from four of them document an arid event in good agreement with its timing at Rusaka and high resolution isotopic results from two sites in equatorial lowlands and from the Sahel. This work shows that the arid Younger Dryas occurred at the same time both in lowlands and highlands, and therefore was most likely a precipitation signal rather than a response to temperature decrease.