GLACIAL INTERGLACIAL RECORD FROM INTERTROPICAL AFRICA, HIGH-RESOLUTION POLLEN AND CARBON DATA AT RUSAKA, BURUNDI/

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 plac ed in the interval 11,000-10,000 BP, and synchronous with the Younger Dryas cold episode of the temperate regions, that has been explained b y the influence of the Atlantic Ocean circulation on global climate. I n this paper we present high resolution (50 to 150 years) studies of o rganic matter and pollen undertaken on a new core from the Rusaka peat bog (3 degrees 26' South, 29 degrees 37' East, 2070 m altitude), date d by the conventional (12 dates) and the AMS methods (18 dates obtaine d on bulk sediment and macroremains). Although the sediment at the bot tom of the core was deposited during the last glacial period prior to 21,000 BP, the dating provides a reliable chronology for the last 12,0 00 BP only. Several vegetational changes are documented in the interva l 12,000-9000 BP, but their chronological placement can only be achiev ed 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, incre ase in carbon storage, C/N ratio for terrestrial plants, and tree cove r began at ca. 12,000 BP, soon after the swamp had been established. T his date is in good agreement with a major increased hydrological bala nce well-documented throughout the intertropical region. The forest in stallation is progressive, rather complex and seen as a two step proce ss. The first forest phase (11,700 BP to 10,600 BP) is dominated by Ha genia, a montane taxon, in good correspondance with previous records f rom other authors in East African mountains. The second forest regener ation took place post 10,000 BP, after a clearly reversed trend of tre e cover from 10,600 to 10,000 BP, ending with a short episode of minim um trees lasting no more than about 200 years. But there is no indicat ion of cooling whatsoever, and the opening of the forest is better exp lained by increased aridity, rather than normal pattern of forest succ ession. This will remain a controversal interpretation among palynolog ists 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 highla nd and lowland pollen data. A detailed comparison with 25 available po llen diagrams is limited by poor or inconsistent dating or low resolut ion of the pollen data. Nevertheless pollen from four of them document an arid event in good agreement with its timing at Rusaka and high re solution isotopic results from two sites in equatorial lowlands and fr om the Sahel. This work shows that the arid Younger Dryas occurred at the same time both in lowlands and highlands, and therefore was most l ikely a precipitation signal rather than a response to temperature dec rease.