ENVIRONMENTAL CHANGES AT LAKE CHESHI, ZAMBIA, SINCE 40,000 YEARS B.P. WITH ADDITIONAL INFORMATION FROM LAKE VICTORIA, EAST AFRICA (DIATOM, POLLEN, PALEOECOLOGY).

摘要

Part 1. Lake Cheshi. Microfossil and mineralogical analyses of a 7.5 meter core from Lake Cheshi revealed a history of environmental change covering the last 40,000 years. The lake formed around 35,000 BP from climatic or tectonic causes. Subhumid conditions prevailed from 35,000 BP to the close of the Pleistocene, while unstable lake levels contributed to the spread of Nymphaea-Cyperaceae swamps around the lake margins. Around 15,000 BP the lake shrank and became chemically concentrated. The presence of gypsum in the late Pleistocene sediments probably reflects mineral precipitation under evaporative concentration. Lake levels rose again after 12,000 BP, reaching maximum levels between 10,000 BP and 5,000 BP. Two arid phases at 3,700 BP and 3,000 BP brought the pluvial period to a close and probably reflected environmental changes of global extent. Conditions have been relatively stable for the last three millenia, with the possible exception of a moderately arid phase around 2,000 BP. Human activity in the lake basin over the last 3,000 years contributed to the spread of fire, increased sedimentation rates and nutrient enrichment of the lake.; The ratio of sponge spicules to diatom valves provided a useful indicator of water clarity, and a new method of quantitative subsampling of diatomaceous sediments was developed and applied to the Cheshi core.; Part 2. The Diatom Record of Lake Victoria, East Africa: The Last 17,000 Years. Diatom analysis of a 9.9m core from the Damba Channel of Lake Victoria extends the microfossil record of that lake back to 17,300 BP. Numerous sponge spicules and the scarcity of diatoms in the lower portion of the core suggest that lake levels were low during the late Pleistocene. Melosira and Stephanodiscus increased in abundance as lake levels rose during the Holocene pluvial. M. nyassensis replaced M. granulata and M. ambigua during the late Holocene. Around 4,000-3,000 BP diatom abundances declined and Nitzschia became relatively more important, possibly reflecting the onset of human activities in the region.