ENVIRONMENTAL VARIABILITY IN TROPICAL EAST AFRICA DURING THE LAST GLACIAL/INTERGLACIAL TRANSITION: THE LAKE MALAWI BIOMARKER RECORD

In this study, lipid biomarkers and compound-specific carbon isotopes are used to investigate terrestrial and aquatic ecosystem change in East Africa since the Last Glacial Maximum (LGM). Lake Malawi, located in the southern hemisphere between 9° and 14° S, provides a particularly interesting and sensitive location to examine the response of terrestrial and aquatic ecosystems to climatic change as the lake has been shown to respond to both southern and northern hemisphere climatic forcings, including the Antarctic Cold Reversal, the Younger Dryas, and the 8.2 cal ka cold event (Powers et al., 2005; Filippi and Talbot, 2005; Johnson et al., 2002). Climatic variability in East Africa is often expressed as hydrological fluctuations (Gasse, 2000), which typically lead to changes in the primary productivity of both terrestrial and aquatic ecosystems. Here, molecular reconstructions are used to provide information on the response of Lake Malawi's algal community to hydrological variations resulting from both global climate events and smaller regional events, such as fluctuations in position and intensity of the Inter-Tropical Convergence Zone (ITCZ). Lake Malawi has a simple algal community structure consisting of diatoms, cyanobacteria, green algae, and dinoflagellates. As each of these algal groups possesses characteristic biomarkers, from the molecular record we are able to examine changes in algal primary production and community dominance within Lake Malawi. Lake Malawi also provides a sensitive location to examine the response of terrestrial vegetation to climatic fluctuations as the lake is presently situated within a C₃ dominated vegetation zone (tree savannah) but C₄ dominated vegetation zones (grass savannah) are present to both the north and south of the lake. Compound-specific carbon isotopes on terrestrial plant biomarkers (n-alkanes, n-alkanols) are used to examine changes in the inputs of C₃ vs. C₄ biomass to Lake Malawi over the past 23 cal ka. Results of this study indicate significant environmental variability in both the terrestrial and aquatic ecosystems of Lake Malawi since the LGM, with periods of major change noted during the Younger Dryas, at ~8 cal ka, and at 5 cal ka.