ORBITAL AND NON-ORBITAL FORCING OF HYDROLOGIC AND VEGETATION CHANGE IN WEST AFRICA SINCE THE LGM AS REVEALED BY GEOCHEMICAL RECORDS AND HYDROLOGIC MODELING OF LAKE BOSUMTWI, GHANA

Hydrologic and vegetation changes in tropical Africa and South America are believed to have played an important role in the global climate changes during glacial to interglacial climate transitions, particularly through their influence on variations in atmospheric methane and the global hydrologic cycle. Here we present a multiproxy record of hydrologic and vegetation change in West Africa, which is based on a combination of isotopic and geochemical measurements on lake sediments and hydrologic modeling of well-dated paleo-beaches from Lake Bosumtwi, Ghana. Modeling results suggest that lake level changes since the LGM reflect the combined influence of precipitation, temperature, windiness and cloud cover, resulting from insolation-induced changes in the West African monsoon. Geochemical records support the dominant influence of low-latitude insolation forcing of the monsoon on hydrologic changes in West Africa, but also suggest that higher frequency changes related to tropical sea surface temperature variations played a key role in controlling glacial and Holocene climate changes. Molecular fossil indicators of terrestrial and aquatic organic matter suggest a complex biological response to hydrologic changes, but one that is consistent with both a direct vegetation response to changes in the monsoon and a West African source of atmospheric methane variations.