Diatom record of Late Quaternary climatic change in the eastern equatorial Atlantic and tropical Africa

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Abstract
Over the last 160,000 years, diatom productivity in the equatorial Atlantic has been high during glacial stages 2, 4, and 6, and moderate during parts of substages 5b and 5d (approximately 96,000 and 117,000 years B.P.). Productivity of diatoms is particularly low in substage 5e and in the middle of stage 1. Higher productivity implies more vigorous trade winds during glacial intervals, leading to stronger upwelling and enhanced lateral advection of surface waters. The influence of the Benguela Current near the equator was strongest during stage 2; a separate regime of coastal upwelling may have existed in the Gulf of Guinea at this time. Upwelling and advection may not always respond to the same forcing as sea surface temperature. Results support the hypothesis that Ethmodiscus rex oozes result from high diatom productivity. Runoff from the Zaire and Niger rivers was highest at the stage 2/1 transition and just prior to the stage 6/5 transition, due to intensification of the African Monsoon. Several other times of increased runoff are indicated. Periods of increased surface water circulation in the eastern equatorial Atlantic appear to correspond to times of aridity in tropical Africa. Conversely, intervals of decreased circulation in the eastern equatorial Atlantic coincide with times of humidity in tropical Africa. Precessional forcing of continental climate is suggested; however, precessional forcing of surface ocean circulation can be neither confirmed nor refuted based on the records examined here. Influx of Antarctic Bottom Water to the eastern equatorial Atlantic has been insignificant throughout the last 160,000 years.