XRD-MINERALOGY OF LAKE BOSUMTWI (GHANA) SEDIMENTS AS A PROXY FOR PAST HYDROLOGIC VARIABILITY OF WEST AFRICA

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. The purpose of this study is to identify down-core mineralogic variation over the last 150 kyr in a 65 m long sediment core from Lake Bosumtwi using X-Ray diffraction (XRD). A corundum standard is added to the sediment samples in order to semi-quantitatively determine mineral abundance. Different carbonate (e.g., calcite, ankerite, Mg-calcite, dolomite), detrital (e.g., quartz, clinochlore) and diagenetic (e.g., phillipsite and analcime) mineral assemblages, accumulating under different lake-level conditions, can indicate varying hydrologic and climatic conditions. During periods of low lake level, increased calcite, Mg-calcite, dolomite, analcime and phillipsite may be preserved. In contrast, the reduction or absence of these minerals may indicate higher lake levels. Preliminary data reveals that between 38 – 65 m (ca. 77 – 150 kyr) intervals of increased abundance of analcime, ankerite and dolomite suggest frequent pronounced low lake levels and more arid conditions. Whereas, above 38 m analcime, ankerite, and dolomite are less abundant. At times corresponding to the ages of the North Atlantic Heinrich events H0 through H6, there is an increased abundance of Mg-calcite in the Bosumtwi core. The increase in Mg-calcite is interpreted to reflect more arid conditions when the strength of West Africa summer monsoon was reduced and the lake was lower. These results indicate that continued XRD-mineralogic study is producing an interpretable paleoenvironmental proxy from Lake Bosumtwi.