The West African monsoon is an important component of the Earth’s atmospheric system because the monsoon redistributes heat and moisture in the tropics. In addition, West Africa is densely populated and has an ecosystem controlled by monsoon rainfall. Therefore, a better understanding of past monsoon variability has social relevance. The hydrologically-closed Lake Bosumtwi occupies a 1.07 Ma meteorite impact crater located in Ghana, West Africa. The lake lies beneath the seasonal passage of the ITCZ; hence, the lake’s sediment record is well suited for studies of past monsoon variability. This thesis research identifies down-core mineralogic variations in a 291-m long sediment core from Lake Bosumtwi using X-Ray diffraction (XRD). Scanning electron microscope and calcium carbonate measurements were also performed to support the XRD results. X-ray diffraction measurements were performed on more than 410 samples at a 1-meter spacing; and at a resolution of 50-cm or less, above 67 m.
Lake Bosumtwi mineralogy is shown to provide an interpretable proxy of paleoclimate variability. During several well-documented lake level lowstands (at 3.2 kyr and 16.3 to 20 kyr when the lake was 30 m and 60 m below the present lake level, respectively), increases in calcite, Mg-calcite and total carbonate content indicate carbonate precipitation by evaporative enhancement during arid climate conditions. During the lake-level highstand of the Holocene African Humid Period, calcite is completely absent and Mg-calcite is present in low abundance reflecting the diluted lake water during this moist climate period. In general, detrital (i.e., quartz), carbonate (i.e., calcite, Mg-calcite, ankerite, dolomite) and diagenetic (i.e., analcime) minerals are more abundant during times of low boreal summer insolation when the West Africa summer monsoon is weak. The source of quartz to Lake Bosumtwi include both the bedrock of the crater walls and wind-blown dust from the Sahel, both of which increase during arid periods. Spectral analysis of the quartz record reveals both obliquity and precession periods suggesting that the West African monsoon is influenced by both high and low latitude climate processes. During the last glacial period, pronounced climate events of the North Atlantic (Heinrich events) correspond to increases in calcite, Mg-calcite, aragonite, siderite and quartz. These mineralogic changes are interpreted to reflect arid conditions in West Africa and suggests connection between high and low latitude components of the climate system. Lastly, this research estimated that the variability of mole % MgCO3 in calcite of Lake Bosumtwi sediment can change the δ18O isotope signal by about 0.47 to 1.15 ‰. Hence, carbonate mineral variations need to be considered in stable isotope measurements of Lake Bosumtwi sediments.