Aluminium-rich opal: an intermediate in the preservation of biogenic silica in the Zaire (Congo) deep-sea fan

Abstract

Diatom frustules, skeletons of radiolarians and silicoflagellates, and also phytoliths in the sediments of the Zaire deep-sea fan have a crumbly appearance, very spongy ultrastructures, and have very high atomic Al/Si ratios between 0.13 in the inner parts and 0.16 in the outer parts of the fan. Also small amounts of Ca and K are always found. The high Al content is related to a shift of the amorphous Silica bulge in XRD spectra from 0.41 to 0.36–0.37 nm, which shows that Al has substituted Si throughout the entire test of the great majority of the siliceous microfossils. This substitution is related to a low apparent solubility of about 350 mmol m −3 of H 4SiO 4, and to a low Silica release-rate constant of 1–2 × 10 −12 cm s −12. The high content of Al in biogenic Silica is caused by a combination of selective dissolution of Silica and precipitation of Al dissolved from minerals in the sediment. The Zaire deep-sea fan combines several environmental factors favourable for the formation of Al-rich opal, viz. a large production of diatoms in offshore waters, a high concentration of dissolved Al in plume waters, and the presence of kaolinite and gibbsite (both minerals which easily release Al) in the sediment supplied by the Zaire River.