Diatom response to oceanographic and climatic changes in the Congo fan area, equatorial Atlantic Ocean, during the last 190Â ka BP

Changes in siliceous productivity in the eastern Equatorial Atlantic Ocean, off the Western African margin, over the last several glacial cycles have been either related to global-to-regional oceanographic changes (upwelling intensity) or climate changes (precipitation and river discharge). Based on diatom assemblages in core KZAI-02, located to the south of the mouth of the Congo River, integrated with a selection of geochemical proxies, we show that siliceous productivity in the southeastern Angola Basin responded to non-linear interactions between both oceanographic and climate changes over the last 190,000 years. High diatom accumulation rates were recorded in the middle part of MIS 6, in cold substage MIS 5d and in MIS 3-2. During these intervals, high diatom productivity was sustained essentially by nutrients, including dissolved silica, injected by the Congo River into the ocean. The highest productivity was observed during MIS 3, when nutrients were sourced both from the river and regional upwelling. Low diatom accumulation rates were recorded during early and late MIS 6, MIS 5e, early MIS 4 and during the Holocene. These resulted either from low river discharge and overall low nutrient stocks in the Angola Basin (despite evidence for upwelling) or from extremely high river discharge. In the case of the latter, the terrigenous load drastically increased the turbidity of the surface waters in the southeastern Angola Basin and lowered phytoplankton productivity despite the presence of sufficient dissolved silica.