TERRESTRIAL AND AQUATIC ECOSYSTEM CHANGE IN TROPICAL EAST AFRICA SINCE THE LAST GLACIAL MAXIMUM: THE BIOMARKER RECORD FROM LAKE MALAWI

The role of the tropics in global climate change is a topic of considerable debate. The idea that global climatic change is driven by the high-latitudes has recently come into question by an increasing number of studies that have provided strong evidence for low-latitude climatic events leading climatic events in the high-latitudes. The intertropical convergence zone (ITCZ) is a dominant driver of modern tropical climatic variability and variations in its strength and position (leading to major hydrological fluctuations) is also recognized as a main driver of Holocene tropical climatic variability. Lake Malawi, situated at the southernmost extent of the ITCZ, offers a unique opportunity to examine both past ITCZ variability and the response of tropical ecosystems to global climate change. Lake Malawi contains a continuous and high resolution sedimentary record, and is located in an ecologically sensitive location near the boundary of tree savannah (C₃ dominated) and grass savannah (C₄ dominated) vegetation zones. Previous studies of Lake Malawi have demonstrated a strong response to global climatic events (Filippi and Talbot, in press; Johnson et al., 2002); however, in some cases these studies have produced ambiguous results from failing to adequately distinguish between terrestrially and aquatically derived organic matter. Molecular analyses are therefore essential to accurately determining the paleoenvironmental history of Lake Malawi.

In this study molecular biomarkers and compound-specific carbon isotopes are used to: 1) examine the delivery of terrestrial organic matter to Lake Malawi since the LGM, 2) examine the response of the Lake Malawi algal community to climatic and hydrological fluctuations, 3) examine vegetation changes in the Lake Malawi basin (C₃ vs. C₄ plants) since the LGM, and, 4) examine contributions to sedimentary organic matter from bacterial sources. Results of this study indicate that significant environmental variability has occurred in Lake Malawi since the LGM. Major periods of variability are noted in nearly all biomarker records (algal, terrestrial, and bacterial biomarkers) during the Younger Dryas, at ~8 cal ka, at 5 cal ka, and from ~1710-1740 AD.