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Mineralogy, opal, and manganese of Middle and Late Quaternary sediments of the Zaire (Congo) deep-sea fan: origin and climatic variation
Van der Gaast, S.J.; Jansen, J.H.F. (1984). Mineralogy, opal, and manganese of Middle and Late Quaternary sediments of the Zaire (Congo) deep-sea fan: origin and climatic variation. Neth. J. Sea Res. 17(2-4): 313-341
In: Netherlands Journal of Sea Research. Netherlands Institute for Sea Research (NIOZ): Groningen; Den Burg. ISSN 0077-7579, more
Peer reviewed article  

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  • Van der Gaast, S.J.
  • Jansen, J.H.F.

    X-ray diffraction and X-ray fluorescence measurements on 5 piston cores provided data on the mineralogy and chemistry of sediments of the last 500.000 years. Opal contents are up to 60%. They increase and opal accumulation rates decrease with increasing water depth. Downcore opal variations, with maxima in glacial intervals of both contents and accumulation rates, form a combined signal of ocean fertility and opal preservation, and are controlled by climatic variations. XRD measurements, with an internal standard, show parallel distribution patterns of the major minerals. This demonstrates that the components are dominated by varying contributions of hardly detectable poorly crystallized smectites. The supply from the Zaire river consists of kaolinite, poorly crystalized smectite, quartz and minor amounts of illite and gibbsite. The continental shelf is the source of well crystallized smectite and admixtures of zeolite and cristobalite coming from Tertiary opal-rich layers. The river contribution is largest in interglacial sediments and the shelf contribution in glacial sediments. Illite is indicative of aridity, and its distribution points to expansion of the African deserts during the glacial stages 2 and 4 and the interstadial around 150.000 yBP. A decrease in supply of shelf minerals, coinciding with a general decrease in sedimentation rate ~350.000 yBP, suggests that a long-term climate warming occurred at that time. High smectite crystallinity values and the presence of reworked pyrite in a continental slope core show the occurrence of downslope transport. Turbidites in the upper fan are mainly of river origin, but minor contributions of cristobalite and zeolite originate from shelf transport intercepted by the Zaire canyon. All cores contain autigenic pyrite, the turbidites also reworked pyrite.In the pelagic and hemipelagic cores roughly 60% of the SiO2 is opal. The remainder forms part of the clay minerals together with Al. Fe and Ti are associated with the clay minerals. Mn is concentrated in subsurface Mn spikes. Fossil spikes occur in a core at 5500m and were buried by turbidites. Mn burial rates are high in glacial intervals. Among other things this may be due to climate controlled variations in the composition of the accumulating sediments, or to increased bioturbation related to increased plankton production. In all core samples of the anaerobic zone small amounts of MnCO3 are present, in one sample 12% of MnCO3 was found.

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