ABSTRACT
Palaeoenvironmental reconstruction of the inter-tropical regions of Africa have been carried out based on palynological studies. However, there are little or no palaeoenvironmental information on the areas west of Lake Kivu in spite of several approaches over the last twenty years. Sedimentary deposit of the Cishaka sequence was sampled on the foot of Mount Kahuzi at 2260 m a.s.l. in the Kahuzi-Biéga National Park (KBNP) located on the western Rift Valley flank of the Lake Kivu basin (D.R. Congo) and subjected to palynological investigation. The palynological data allowed the reconstruction of 35,000 years of regional palaeoenvironmental history. About 60 organic sediment samples derived every 10 cm from a 6 m depth core in the Cishaka sequence were palynologically analysed after extraction of fossils by acetolysis procedure. The sediment samples are mainly dark grey coloured clays. The base of the core is poor in pollen grains and has been radiocarbon dated up to 38,800 ± 4100 BP.
Based on different forest taxa, nine phases or zones of forest evolution were established. The dynamics of the main components of the forest taxa within the zones was used to subdivide the first zone into three subzones and the zone 7 into two subzones. During the Upper Pleistocene (“Kalambo Interstadial”, 32,000–26,000 years BP), a very heterogeneous forest occupies the area under a moderately hot and humid climate. During the regressive phase of the “Hypo- thermal” of Mount Kenya (25,000–15,000 years BP, LGM), vegetation cover shows an important extension of herbaceous open environment that witnesses a distinct dryness.
Subsequently, the restoration of an Afromontane forest with an Afro-subalpine connotation, point towards cold and humid climatic conditions which was temporarily interrupted by a renewed extension of more open environments attesting a dry throbbing that can be linked to the Younger Dryas (YD). During the African Humid Period (AHP, 10,000–7000 years BP) precipitation increased. Lake Kivu showed an important rise in sea level, whereas the depressions in altitude were transformed into shallow lakes. The subsequent progression of Afromontane rainforest that probably followed has not been archived because of an insufficient sedimentological record in the Cishaka sequence. Around 6500 years BP, the first swampy deposits of the Holocene reveal the existence of an Afromontane mixed rainforest with a mesophilic character. Around 4000 years BP, an important shift in natural climatic conditions (not anthropogenically driven!) caused open environments, 146highland grasslands at high altitude, and savannas on lower slopes. This indicates a hydrous deficit that can be put in relation with the African global period of aridity to this time (“First Millennium Crisis”). After an instable climatic situation till 2000 years BP, the ecosystem gained stability, like today’s conditions which allowed forests during the first millennium AD to recolonize the area.
