Sedimentary environmental changes and millennial climatic variability in a tropical shallow lake (Lake Ossa, Cameroon) during the Holocene

The Holocene environmental evolution of the shallow Ossa lacustrine complex in Cameroon is analysed on the basis of the present-day sedimentary dynamics and, principally, by the study of five well-dated cores. Sediment accumulation rates, lithofacies and mineral content changes and various tracers of flux provenance are considered in connection with the pollen data. At the shallower sites (1-2-m-water depth during the dry season), a moderate rise in lake level may have induced the transition from hydromorphic marshy conditions (gleys) to lacustrine conditions (muds) ca. 8000 cal BP at the easternmost site. A moderate fall may have generated the opposite process, ca. 7000 cal BP in the near northern part. A marked decrease in sediment accumulation rate between 5000 and 3000 cal BP is apparent in some cores, coinciding with an increasing discontinuity of the particulate supply. During this interval, the weakening of monsoon rainfall is associated to an increasingly seasonal regime characterized by lower and sporadic fluxes. This decrease corresponds to a northern shift of equatorial climatic conditions with the main dry season occurring during the boreal summer. During the time interval between ca. 2800 and 2000 cal BP, the return of tropical climatic conditions induced the opening of the vegetation associated with more intensive erosion. Increasing Cyperaceae may indicate flooding of areas suitable for colonisation until 2800 cal BP. The first overflow of the Sanaga River into Lake Ossa occurred after ca. 5800 cal BP with a NE-SW depositional axis of coarse material. Transport of suspended matter in the surface waters northward was probably induced by prevailing SW winds. The timing of this climatic evolution is independently supported by other lake sediment records from the forest domain of West Africa and can be compared with other cyclical phenomena, such as Bond and Dansgaard-Oeschger, as well as solar oscillations. (c) 2004 Elsevier B.V. All rights reserved.