11. LATE QUATERNARY POLYCYSTINE RADIOLARIANS AND SILICOFLAGELLATES OF A DIATOMACEOUS SAPROPEL FROM THE EASTERN MEDITERRANEAN, SITES 969 AND 9711Taniel Danelian2 and Dimitris Frydas3 |
ABSTRACTWell-preserved radiolarians and silicoflagellates discovered in the diatomaceous Sapropel S5 were studied and quantitatively analyzed. The silicoflagellate assemblage is almost exclusively composed of warm-water Dictyocha species. The radiolarian assemblage is very diverse and dominated by the counting-group Arachnocorallium, an indicator of eutrophication. Paleoecological evaluation of the fauna indicates that polycystin radiolarians occupied various ecological niches. Radiolarian taxa, known to be abundant in association with a subsurface layer of chlorophyll-a maxima are much better represented in the sapropel S5 than above it, suggesting that this deep layer of primary productivity was much stronger and better developed during accumulation of the sapropel. Deep-dwelling, detritivore radiolarians are also present and well-represented in the sapropel, contrary to the assemblage that is above it, in which these taxa appear to be absent. In the latter sample, surface-dwelling, symbiont-bearing radiolarians are the dominant group. If the Nile river was the main cause of eutrophication in the Eastern Mediterranean, excessive riverine discharge would have supplied abundant nutrients (in particular, phosphorus and silica) to sustain the bloom of the siliceous phytoplankton (diatoms and silicoflagellates). Eutrophication would have encouraged radiolarians to occupy ecological niches at various depths of the water column. Excessive export of biogenic silica (diatom shells in particular) diminished the corrosive action of the Mediterranean sea water and preserved siliceous microfossils. After the decrease of riverine discharge, which brought an end to the formation of Sapropel S5, silica was probably not available in sufficient quantities to sustain siliceous phytoplankton growth far beyond the coasts of Egypt, and settling of organic matter would have been reduced to levels below the critical values necessary for the sustenance of the detritivore radiolarian niches. This change in eutrophication did not affect the surface-dwelling radiolarians, whose needs are substantially supported by their symbionts. |
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