New evidence on palaeoenvironmental conditions in SW Cameroon since the Late Pleistocene derived from alluvial sediments of the Ntem River
- An interior delta in the lower course of the Ntem River near the sub-prefecture Ma’an was identified after interpretation of satellite images, topographical maps of SW Cameroon and geological as well as hydrological references and a reconnaissance fieldtrip to the study area. Here neotectonic processes have initiated the establishment of a ‘sediment trap’ (step fault), which in combination with environmental changes strongly generated the fluvial morphology. It transitionally led to temporary lacustrine and palustrine conditions in parts of this river section. Inside the interior delta an anastomosing multi-branched river system has developed, which contains ‘stillwater locations', periodically inundated sections, islands and rapids. Following geomorphological, physiogeographical and sedimentological research approaches, the alluvial plain has been prospected and studied extensively. 91 hand-corings, including three NE–SW transects, were carried out on river benches, levees, cut-off and periodical branches, islands as well as terraces throughout the entire alluvial plain and have unveiled multi-layered, sandy to clayey alluvia reaching up to 440 cm depth. At many locations, fossil organic horizons and palaeosurfaces were discovered, containing valuable palaeoenvironmental proxy data. At these sites, through additional detailed stratigraphical analysis (close-meshed hand-coring and exposure digging) a comprehensive insight into the stratification (lamination) of the alluvia could be gained, clarifying processes and conditions that prevailed in the catchment area during the period of their deposition. 32 Radiocarbon data of macro-rests (leafs, wood), charcoal and organic sediment sampled from these horizons provided ages between 48.230 ± 6.411 and 217 ± 46 years BP (not calibrated). This constitutes the importance of the alluvia as an additional, innovative palaeoarchive for proxy data contributing to the reconstruction of palaeoenvironment and palaeoclimate in western Equatorial Africa. The further examination of the alluvia will not only provide additional information on the dynamics of vegetation, climate and hydrology (esp. fluvial morphology) in SW Cameroon since the ‘First Millennium BC Crisis’ (around 3.000 years BP), the main focus of the DFG-research project, but also on conditions prevailing since the Late Pleistocene, during the Last Glacial Maximum (~18.000 years BP), the Younger Dryas impact (~11.000 years BP) and the ‘Humid African Period’ (~9.000–6.000 years BP). Delta13C-values (–31,4 to –26,4‰) evidence that at the particular drilling sites rain forest has prevailed during the corresponding time period (rain forest refuge theory). The sampled macrorests all indicate rain forest dominated ecosystems, which were able to persist in fluvial habitats, even during arid periods.