Monsoon related variations in Zaire (Congo) sediment load and influence of fluvial silicate supply on marine productivity in the east equatorial Atlantic during the last 200,000 years

Two sediment cores from the Zaire Fan and the Angola Margin have been investigated for their composition of terrigenous and biogenic constituents, respectively. For the late Quaternary, kaolinite/feldspar ratios and variations of terrigenous element ratios of Zr, Ti, K, Rb, and Al reveal that the composition of Zaire River sediment load has fluctuated in tune with precessional variations of boreal summer insolation. In particular, the correspondence of high kaolinite/ feldspar and Al/K ratios with low-latitude insolation maxima strongly corroborates the assumption that west African monsoonal precipitation and chemical weathering was enhanced during periods of increased central African heating. The most striking feauture observed is that opal accumulation has been 2 to 10 times higher in Zaire Fan sediments than outside in continental margin sediments off Angola, although biogenic Ba and C-org fluctuations from both areas indicate that changes of total paleoproductivity were of the same magnitude in the Zaire River plume and off Angola. From this we infer that the contribution of biogenic opal production to total paleoproductivity has been significantly higher within the Zaire River plume than in the oceanic upwelling regime farther to the south off Angola over the last 200,000 years. The pattern of opal accumulation rates with respect to that of marine organic carbon implies that enhanced opal production off the Zaire River to a great extent was the result of additional fluvial supply of dissolved silica during humid climates characterized by more intense chemical weathering on the continent, while total paleoproductivity created by oceanic upwelling was high in periods of increased zonal trade wind intensity at precessional insolation minima and during cold, more arid glacial climate conditions. We presume that paleoproductivity off the Zaire was controlled by the following two sources of nutrients: (1) marine nutrients including nitrate and phosphate as well as the uptake of Ba on particulate C-org caused by upwelling, and (2) silica, mainly delivered by the Zaire River dissolved silicate. Hence our results underline the importance of dissolved silicate in large rivers for marine silicate cycling in the tropical to subtropical Atlantic at a millenial timescale.