ABSTRACT
The ReSaKo-project has proved the significance of Central African, tropical alluvial sediments for palaeoenvironmental and palaeohydrological studies (Runge et al., 2006; Sangen 2008). Across selected reaches of anastomosing, anabranching, and meandering
river sections valuable sediment archives with embedded palaeosurfaces were uncovered, yielding maximum ages around 50 ka BP. Significant grain-size fluctuations as well as pedological and sedimentological characteristics of alluvial deposits indicate major modifications of run-off and sedimentation behaviour, most likely related to climatic fluctuations and internal fluvial reorganizations. First results from the Ntem and Nyong catchments (Runge et al., 2006; Sangen, 2009) show high correlation with findings from hemi-pelagic (e.g. Schneider et al., 1997; Zabel et al., 2001; Adegbie et al., 2003; Weldeab et al., 2007) and lacustrine (e.g. Street-Perrott and Perrott, 1990; Maley and Brenac, 1998; Nguetsop et al., 2004; Gasse et al., 2008; Giresse et al., 2008) data archives covering various sediment-stratigraphic periods. Fluvial-morphological response is highly dependent on monsoonal, hydrological and ecological feedbacks to climatic changes. The latter are basically determined by orbital parameters (Imbrie et al., 1984; Berger and Loutre, 1991) and global boundary conditions as well as inherited teleconnections (Blunier and Brook, 2001; Vidal and Arz, 2004). Various proxy data recovered from hemi-pelagic archives (Congo, Niger, Nyong and Sanaga deep-sea fans) manifest variations in the intensity of terrestrial erosion, weathering and fluvialmorphological activity, which are linked to climatic and monsoon-intensity fluctuations (Schneider et al., 1997; Zabel et al., 2001; Adegbie et al., 2003; Holtvoeth et al., 2005; Weldeab et al., 2007). These findings have been corroborated by various studies on marine pollen (e.g. Marret et al., 1999; Dupont et al., 2000; Lézine and Cazet, 2005), phytoliths (Abrantes et al., 2003) and diatoms (Pokras, 1987; Jansen and van Iperen, 1991). The climatic oscillations in western and central monsoonal Africa show high correlation with periodic orbital and insolation cycle changes (Rossignol-Strick, 1983; Pokras and Mix, 1987; Prell and Kutzbach, 1987; DeMenocal et al., 2000; Lézine et al., 2005). Extraterrestrial forced climate variability induces major global modifications of the atmospheric, oceanic, hydrological and ecological cycles, which in turn imply far reaching, complex feedbacks and impacts on global circulation processes (Barker et al., 2004; Vidal and Arz, 2004). As a consequence, the western tropical African region experienced major environmental changes which are linked to ITCZ displacements and variations of the monsoon/harmattan forcing (Rossignol-Strick, 1983; Pokras and Mix, 1987; Chiang et al., 2003; Broccoli et al., 2006). These climatic and environmental oscillations are known as hypothermal (cool and semi-arid to arid; Maluékien ∼70-40 ka BP, Léopoldvillien ∼27-14 ka BP) and hyperthermal (humid and moist; Njilien ∼40-27 ka BP, Kibangien A ∼12-5 ka BP) phases following early climatic stratigraphies postulated by van Zinderen Bakker (1962; 1967) and De Ploey (1964; 1965) based on sedimentary records from Angola and the Malebo (Stanley) Pool in Congo. Thus striking climatic and environmental changes in western Central Africa occurred during the Maluékien/Njilien transition (OIS 3), Last Glacial Maximum (LGM) and the Pleistocene/Holocene transition. Shorter climatic fluctuations occurred during several stages of the Holocene (e.g. 8.2 ka event, First Millennium BC Crisis, Little Ice Age; Mayewski et al., 2004; Wanner et al., 2008). Most of these modifications have been verified by palynological (Dupont et al., 2000; Maley, 2001; Lézine and Cazet, 2005) and lacustrine sediment archives (Barker et al., 2004; Elenga et al., 2004; Gasse et al., 2008). Nevertheless, the impact of Late Quaternary climate change on tropical African fluvial systems is still not well understood, although a lot of progress has been made (Neumer, 2007; Gasse et al., 2008; Lespez et al., 2008; Sangen, 2008, 2009; Thomas, 2008). In spite of increasing contributions palaeoenvironmentally evaluable sediment archives from alluvial rivers in Africa are very limited. Most of the palaeoenvironmental and palaeoclimatic proxy data from Southern Cameroon originate from lacustrine (e.g. Servant and Servant-Vildary, 2000; Elenga et al., 2004; Gasse et al., 2008), hemipelagic (Schneider et al., 1997; Zabel et al., 2001; Schefuß et al., 2005; Adegbie et al.,
2003; Weldeab et al., 2005, 2007) and other terrestrial (Kadomura, 1995; 2000; Thomas, 2000, 2004, 2008; Thomas and Thorp, 2003; Runge, 2008) sediment archives.
