Transport and stratigraphy of pollen in Lake Malawi, Africa
This project represents the first comprehensive study of the pollen record of Lake Malawi in South Central Africa. In the first phase of the study, surface sediment samples from Lake Malawi and its catchment were analyzed for pollen to determine the patterns of pollen and spore provenance and transport in the lake. Riverine input of pollen is important, particularly around river deltas, and the spore to pollen ratio decreases with distance from river inflows. High pollen concentrations in the middle of the lake probably result from a lack of clastic sediment input. The primary transport vector varies among pollen types. Gramineae and possibly montane forest taxa seem to be primarily wind transported, whereas evergreen forest taxa, woodland taxa, Typha, and Cyperaceae seem to be transported primarily by water. Because of these variations in transport vectors, no single pollen source area can be defined for the lake. It is likely that the entire catchment is represented to some extent and vegetation variations in the catchment are reflected in the lake only at the coarsest scale, a conclusion that is supported by detrended correspondence analysis. Because of the lack of a definable source for the pollen deposited in the lake, multivariate statistical techniques calibrated to local pollen samples are not valid in large lakes and marine systems.
In the second phase of the study, two 10 meter cores were analyzed and a 37,500 year pollen
stratigraphy was established for Lake Malawi. The Holocene is characterized by climate and
vegetation quite similar to today, with indications of slightly wetter conditions between
6,150 and 3,000 BP and slightly drier conditions between 8,000 and 6,150 BP. During the
Last Glacial Maximum montane forest was widespread in the catchment, though woodlands
apparently persisted at low altitudes. Between 34,000 and 26,400 BP warm and dry conditions
prevailed. This warm period is contemporaneous with the Denekamp Interstadial of Europe.
From 35,900 to 34,000 BP montane forests were again widespread. The end of this second
montane forest episode at 34,000 BP is contemporaneous with Heinrich event 4. From the
bottom of the core (37,500 BP) to 35,900 BP conditions were extremely dry.