Cenozoic radiolarians from the Brazil Basin and Rio Grande Rise.

Abstract

The Neogene of the SW Atlantic is virtually barren of biogenic silica. Of the four sites drilled on Leg 72, only two contained identifiable radiolarian specimens. In the SW Brazil Basin (Site 515), radiolarians are present only from the upper Oligocene to the middle Miocene. On the Rio Grande Rise (Site 516), radiolarians are present only within a short interval of the lower Miocene. Possible mechanisms are discussed. Discrepancies suggest that the presence of calcareous microfossils may act to enhance post-depositional dissolution of biogenic silica tests by elevating the pH of the surrounding pore waters. Carbonate-free clays, by contrast, may provide a more favorable chemical environment for silica preservation.-from Author